Throughout an extensive lengthscale hierarchy, from transistors to packaging and beyond, progress in the materials research community is allowing us to think more concretely about fundamental limits for the thermal management of electronic systems. What is the highest power density that can be managed by an individual transistor, within a chip, or by a smartphone? The answers are evolving with breakthroughs in the synthesis, integration, and nanostructuring of materials.
This presentation takes a fresh look at cooling limits and how they are impacted by achievements in several specific domains of materials research. We will discuss recent progress at Stanford and by our collaborators on transistor-level and FinFET thermal transport, power transistor cooling using diamond composite substrates, as well as the latest microfluidic heat sink approaches that use nanostructured materials for wicking and phase separation.

The operating temperature of devices, such as heat assisted magnetic recording, are often dominated by the thermal interface resistance between a metal and a high thermal conductivity dielectric. We present a study of thermal transport across a metal-dielectric interface with the addition of a metal adhesion layer by measuring the thermal interface resistance of a metal-dielectric system as a function of the adhesion layer&’s thickness and metallic alloy concentration. Measurements of thermal resistance are made via frequency domain thermoreflectance (FDTR). The introduction of a chromium (Cr) adhesion layer between an aluminum nitride-gold interface reduces the thermal resistance of the interface by a factor of 3 when the Cr layer is just 2 nm thick. For plasmonic applications, where reduced thermal interface resistance is sought along with ideal plasmonic properties, alloys may provide a solution. To study the effects of the adhesion layer&’s alloy concentration on thermal interface resistance, alloys consisting of individual constituents with disparate Debye temperatures were sputtered onto sapphire substrates. Initial measurements of a CuPd-sapphire interface indicate that the thermal conductance of the metal-dielectric is reduced by as much as ~30-50% at low alloy concentrations. By the time of the conference a range of metallic alloy-sapphire interface resistances will be measured as a function of film thickness.

The thermal management challenge posed by gallium nitride (GaN) high electron mobility transistor (HEMT) technology has received much attention in the past decade. The peak amplification power density of these devices is limited by heat transfer at the device, substrate, package, and system levels. Thermal resistances within micrometers of the transistor junction can limit efficient heat spreading from active device regions into the substrate and can dominate the overall temperature rise. GaN composite substrates, which consist of a heteroepitaxial AlGaN/GaN structure with thickness of a few microns on a thicker non-GaN substrate, govern the thermal resistance associated with the “near-junction” region. As a substrate material, SiC (~400 W m^-1 K^-1) has been widely used, but the performance of GaN devices grown on SiC is still severely limited by thermal constraints and associated reliability issues. The importance of effective near-junction heat conduction has motivated the development of composite substrates containing high-thermal-conductivity diamond (~1500 W m^-1 K^-1), but these composite substrates require careful attention to the thermal boundary resistance between the GaN and diamond.
This presentation will describe our experimental and theoretical investigation of thermal conduction in GaN composite substrates containing SiC and diamond. We measure the thermal boundary resistance (associated with the transition layer) between the GaN layer and the substrate as well as the thermal conductivity of the GaN at temperatures relevant to the operation of power transistor devices, using time-domain thermoreflectance (TDTR). We employ the semiclassical phonon transport theory—which uses an approximate solution to the Boltzmann transport equation (BTE)—to examine the relevant phonon scattering mechanisms (i.e., phonon-defect scattering) responsible for the temperature trend of the GaN-substrate boundary resistance. The best available data in literature are also presented in comparison with our BTE model as well as our data.

Understanding of heat transport across an interface between a solid and liquid is a necessary first step in designing nanoscale heat sources for applications in nanoscience and nanotechnology. In a typical experiment, a metal nanoparticle or thin film is heated by a laser pulse and the decay of the temperature of the metal is analyzed to extract the thermal conductance of the interface with a liquid. Plasmon resonances of Au nanostructures are also sensitive to the index of refraction, and therefore temperature, of the layer of liquid near the interface. The increase in temperature of the liquid near the interface is a more sensitive probe of the interface conductance than the decay of the metal temperature. We are using Insplorion Au nanodisks (120 nm diameter x 20 nm height) on fused silica substrates as the heat source and temperature sensor in pump probe experiments on heat transport between Au, self-assembled-monolayers, and liquid mixtures. We developed an analytical model to analyze the heat flow from Au nanodisks to the surrounding fluid. The thermal conductance of the nanodisks / fluid interface for nanodisks coated with a hydrophilic self-assembled monolayer (SAM) of sodium 3-mercapto-1-propanesulfonate is 90 < G < 190 MW m^-2 K^-1 as the fluid mixture is varied from pure ethanol to pure water. We relate the changes in changes in thermal conductance to the variations in interfacial energy of binary fluid mixtures.

Nanomaterials show various interesting unique thermal characteristics such as size dependent melting temperature drop, which can be used to develop plastic compatible low temperature metal patterning process. Focused laser as a local heat source can further reduce the processing temperature or induced localized thermochemical reaction. In this talk, recent research development and trend in nanomaterial based low temperature laser thermal engineering as well as applications will be discussed.

Colloidal nanocrystals consist of an inorganic crystalline core with short ligand molecules bound to its surface. These nanocrystals can be made with excellent size and shape uniformity, thereby enabling the exploration of relationships between microstructure and thermal properties.
We first present our work on thermal transport in polycrystalline In₂Se₃ thin films with varying volume fractions of CdSe nanocrystals. In₂Se₃ has a layered crystal structure and we find that the presence of CdSe nanocrystals preferentially destroys periodicity along its c-axis. We find that these composites have low thermal conductivities on the order of 10^-1 W/m-K and that the thermal conductivity increases as the CdSe nanocrystal volume fraction increases.
We also present our work on colloidal nanocrystals as phase change thermal storage materials. A desirable phase change material should have a large energy density as well as fast thermal charging/discharging rates. To accomplish this, we create composites consisting of phase change Bi nanocrystals embedded in a solid Ag matrix. Our use of an Ag matrix yields composite thermal conductivities that are 2 - 3 orders of magnitude larger than typical thermal storage materials, and consequently these composites have very fast thermal charging/discharging times. In addition, our use of colloidal nanocrystals enables us to use size-dependent melting as a new design variable that decouples melting temperature from chemical composition.

Light absorbed by plasmonic nanoparticles is very efficiently converted into heat and particle temperatures of several hundred degrees centigrade can be reached within nanoseconds when they are irradiated with a focused laser beam. A single gold nanoparticle can thus be used as a fine tool to apply heat to a nanosopic area which opens the door to exciting experimental applications were temperature sensitive processes can be studied in detail and with unprecedented resolution.
Here, we demonstrate how plasmonic heating can be applied to control the permeability of cell membranes. Lipid bilayers are almost impermeable to charged molecules and ions which, under normal circumstances, can only pass the membrane barrier with the help of specialized transport proteins. The physical properties of lipid membranes, however, are also affected by local temperature changes. In our experiment, we probed the temperature dependent permeability of a synthetic bilayer membrane in absence of any carrier proteins or ion channels with a planar patch-clamp device on an optical microscope. In this setup, gold nanoparticles that were bound to the membrane were used as small, localized heat sources where the particle temperature could be controlled by the intensity of the laser beam. Overall, we found that the bilayer resistance and ion current across the membrane could be altered reversibly by adjusting the nanoparticle temperature. Shifting to real cells, we could show that gold nanoparticles can also be injected into living cells by a combination of plasmonic heating and optical force.
In summary, these results illustrate how plasmonic heating can be employed to achieve nanoscale control over biological systems and to actively deliver nanoscopic objects into living cells, which paves the way for future biomedical applications in nanotheranostics and drug delivery.

Hydrophobic surfaces can be used to promote condensation in the dropwise mode, which is significantly more efficient than the common filmwise mode. However, limited longevity combined by low thermal conductivity of hydrophobic surface modifiers has prevented their wide spread use in industry. For example, to endure in a steam environment within a power plant condenser, a PTFE film must be at ~20 to 30 um thick [1]. Unfortunately, the additional thermal resistance added by this film negates any heat transfer benefits attained by promoting dropwise condensation. Recently, metal matrix composites (MMC) with microscale hydrophobic particles dispersed in hydrophilic metal matrix have been proposed as durable and self-healing alternative to hydrophobic surface coatings [2]. While dispersion of the hydrophobic graphite microparticles in MMC lead to a macroscopically hydrophobic surface, such composites are likely flooded by water condensate. In turn, the effect dispersion of hydrophobic nanoparticles (HNPs) with size comparable to water nuclei critical radii and spacing is not obvious. To this end, we fabricated highly ordered arrays of Teflon nanospheres on silicon and copper substrates that mimic the top surface of the MMCs with dispersed HNPs. We used light and electron microscopy to observe breath figures resulting from condensation on these surfaces at varied degrees of subcooling. We experimentally derive nanosphere diameters and spacing required for promoting dropwise condensation at different levels of surface subcooling. We translate the necessary hydrophobic nanoparticle surface distribution to a volumetric particle distribution within a metal composite matrix. Furthermore, we describe how addition of this secondary phase (e.g. hydrophobic ceria nanoparticles [3]) within the metal matrix (e.g. copper), impacts the effective thermal conductivity of the composite and overall heat transport during the condensation process.
[1] Rose, J. Proc.Instn. Mech. Engrs. 216 (2002)
[2] Nosonovsky, M. et al. Langmuir 27 (2011).
[3] Azimi, G. et al. Nature Materials (2013)

Energy generation through thermal processes still accounts for more than 80% of the world primary energy generation. While most of the thermal energy generation processes are efficient, there is still challenge of mismatch between production and demand. Thermal energy storage is one of solutions to such challenge and can help balance the energy generation during off-peak hours. Among various thermal energy storage approaches, solar charged thermal energy storage attracts great attention due to the increased investment in solar power plants. In solar charged thermal energy storage system, solar energy is absorbed by a solar absorber and then transported into the thermal energy storage media through thermal diffusion. The key challenge in such thermal energy storage system is the low thermal conductivity of the storage media, especially the organic material based storage media. The low thermal conductivity shortens the thermal diffusion length and results in slow charging and also limited storage capability. Most current approaches addressing such challenge rely on the increase of the thermal conductivity through doping the thermal energy storage media with materials that have high thermal conductivity. Such approaches require substantial doping to raise the thermal conductivity of the media, which not only makes the dopant unstable in the storage media, but also adversely reduces the thermal energy storage capacity of the system. In this talk, I will present a direct light charging approach to improve the efficiency of the solar thermal storage system. Metallic nanoparticles with intense and efficient photothermal plasmonic heating effect were doped into transparent storage media to optically charge such storage system. The nanoparticles were surface modified to maintain the good dispersion and stability during the charging and recharging process. The plasmonic nanocomposites demonstrated a much faster heating rate and higher charging efficiency than the approaches involving the doping of materials with high thermal conductivity. The thermal storage properties of such plasmonic nanocomposite system can be fine-tuned and optimized by engineering the nanoparticles to match the broad band solar spectrum. In the presentation I will also discuss the localized nanoscale heat generation and transportation process that enables the efficient solar thermal energy storage.

A challenging research area in nanoscale heat transfer is the investigation of highly non-equilibrium thermal systems created by ultrafast laser excitation. It has been observed that noble metal nanoparticles, illuminated by the plasmon resonance wavelength, can act as localized heat sources at nanometer length scales. This has several important applications in the field of medicine where intense heating selectively damages biological tissues close to the particles due to not only the high temperature in the surrounding medium but also the pressure transients induced by localized explosive boiling of the liquid.
Though several studies have investigated the formation of vapor bubbles around nanoparticles under continuous-wave and pulsed laser excitation, there has been difficulty in characterizing the exact properties (temperature, pressure, bubble size etc.) of the liquid-vapor phase transition near the nanoparticle surface. While, designing experiments to capture all these properties is a challenging task, suitable simulation models offer an alternative. The molecular dynamics (MD) technique offers a powerful tool to investigate the cavitation dynamics, wherein the relevant properties can be precisely known without the simplifying assumptions required by continuum-level model formulations.
In this work, we present results from a series of non-equilibrium MD simulations performed on an intensely heated nanoparticle immersed in a Lennard-Jones liquid. Specifically, we investigate the temporal evolution of vapor nanobubbles that form around the solid nanoparticles heated over ps time scale and provide a detail description of the following vapor formation and collapse. To the best of our knowledge, this work is the first time cavitation phenomenon has been observed via MD. Here, we demonstrate the existence of a nanoparticle-size dependent threshold temperature for vapor cavitation. Additionally, we link this cavitation threshold to the onset of spinodal decomposition 0.5-1 nm away from the particle surface. Finally, a detailed analysis of the bubble dynamics indicates adiabatic formation followed by an isothermal final stage of growth and isothermal collapse.

Magnetic nanoparticles (MNPs) have been intensely investigated in recent years, mainly on account of their attractive and unique physical and chemical properties. Moreover, MNPs are a class of materials that could be manipulated under the influence of an external magnetic field. Iron oxides nanoparticles attract increasing attention due to their immense broad range of applications and their low cost. Among the applications, magnetic hyperthermia has awaked particular interest during the last years. Magnetic hyperthermia is a cancer treatment method which uses the ability of magnetic nanoparticles to generate heat when exposed to alternating magnetic field [1,2]. In this work, we Mwill present a recent works on the synthesis, magnetic characterization and heating efficiency of γ-Fe₂O₃ nanopaticles [3]. Sol-gel technique is used for the synthesis of the sample with particle sizes above 10 nm for the aim of obtaining systems with appropriate sizes for local heating purposes. X-ray diffraction points out that the obtained nanoparticles are mainly composed of maghemitephase (γ-Fe₂O₃) with a particle size that ranges between 14 and 30 nm depending on the synthesis conditions. Mössbauer spectroscopy reveals the existence of two magnetic phases: maghemite, which is the majority phase, plus magnetite as a residual phase. For determining the usefulness of the nanoparticles for local heating purposes, the heating efficiency has been investigated for each particle size. The heating efficiency of the magnetic nanoparticles in the presence of an AC magnetic field is defined by specific absorption rate (SAR) which is the amount of heat generated per unit gram of magnetic material per unit time. SAR values are affected by several parameters such as the sample preparation method, structural and magnetic properties of the nanoparticles, the amplitudes and frequency of the applied field.
The heating efficiency of the obtained nanoparticles was studied under an alternating magnetic field and as a function of size, frequency and amplitude of the applied magnetic field. The results show the increase in temperature for colloidal samples when submitted to an alternating magnetic field.The dependence of SAR with field amplitude and frequency deviate from linear response theory (LRT) and the deviation is analyzed in terms of particle size and magnetic properties.
References:
[1] A. Jordan, P. Wust, H. Fahling, W. John, A. Hinz and R. Felix, International Journal of Hyperthermia 9 (1), 51-68 (1993).
[2] J. Carrey, B. Mehdaoui and M. Respaud, Journal of Applied Physics 109 (8) (2011).
[3] O.M. Lemine , K. Omri , L. El Mir , M Iglesias, V Velasco, P Crespo, P de la Presa ,Houcine Bouzid, Ali A. Yousifshy; and A.Hajry, Journal of Alloys and Compounds 607 (2014) 125-131

Nanofluids (NFs) are solid-liquid composites prepared by stabilizing nanostructured materials in a base liquid, such as water, ethylene glycol (EG) and engine oil. They have exhibited some potential to replace with conventional heat transfer fluids to enhance their thermal characteristics. NF has complex system and its thermo-physical properties including TC and viscosity can be influenced by several factors such as base liquid. The effect of base liquid on thermo-physical properties of NFs is not well studied. In this work this effect has been studied by comparing the TC and viscosity of a commercial silver (Ag) NFs with lab-made water and EG base Ag NFs. For this purpose, at first commercial water based Ag suspension with 1wt% Ag nanoparticles (NPs) was used. The original suspension was centrifuged and the obtained Ag NPs was re-dispersed in EG to prepare EG based Ag NF (1wt%). In order to study the effect of Ag NP concentration on thermo-physical properties, NFs with various concentrations (1, 1.5 and wt%) were fabricated from the original samples containing 1wt% Ag NP. For this end NFs with higher NP concentrations were fabricated by centrifuging and concentrating these samples in base liquids. Therefore, six NFs with different base liquids (water and EG) and various Ag NP concentrations (1, 1.5 and 2wt%) were prepared and studied. The physicochemical properties of Ag NP/NF were characterized by using various techniques and instruments including Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Electron Diffraction (ED), Dynamic Light Scattering (DLS) and Fourier Transform Infrared Spectroscopy (FT-IR). Ag NP displayed an average primary particle size of 40±5 nm from SEM micrographs. The thermo-physical properties of NFs including TC and viscosity of NFs were measured at different temperatures from 20 to 40 ^oC. The Newtonian behavior was observed for NFs with both water and EG base liquids. Moreover, viscosity of all NFs was increased by increase of Ag NP concentration and decreased by increase in temperature. The TC of NFs was measured and analyzed by Transient Hot Wire (THW) method at the same temperature range. The TC tests showed increase of TC with the increase of Ag NPs concentration as well as increase in temperature. As a result and compared to the water based suspensions, NFs with EG base liquid revealed higher TC enhancement and lesser viscosity increase. EG based NFs seem to be more beneficial for heat transfer applications. Among all NFs, a maximum TC enhancement of ~12.2 % was obtained for EG based NF with 2wt% Ag NP at 40 ^oC while maximum increase in viscosity of ~ 4 % was observed for the same NF showing the capability of utilizing this NF as heat transfer fluid. Finally and in order to compare the experimental results with the estimated data, proper predictive models/correlations were applied for NFs with both water and EG base liquids and our finding are presented in detail.

Nanofluids (NFs), suspension of nanoparticles (NPs) in conventional fluids such as water and ethylene glycol (EG) have been found capable of providing enhanced heat transfer compared to pure heat transfer fluids. Among several factors influencing thermo-physical properties of NFs including thermal conductivity (TC) and viscosity, particle size plays an essential role. Due to limited literature data on the impact of particle size on thermo-physical properties of NFs as well as inconsistent results in the literature, there is a serious need to perform a detailed study on the influence of particle size on thermo-physical properties of NFs. For this purpose a study was carried out using copper nanoparticles (Cu NPs) and copper microparticles (Cu MPs) to investigate, experimentally and theoretically, the effect of Cu NPs and MPs on thermo-physical properties of Cu NFs and Cu microfluids (Cu MFs). A series of stable Cu NFs and MFs with various NP/MP concentration (from1 wt% to 3 wt%) were fabricated by dispersing Cu NPs and Cu MPs in EG. The use of additives/surfactants was avoided to study the real impact of Cu NP/MP. The physico-chemical properties of Cu NFs and MFs were analyzed by various techniques and instruments including Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM) and Dynamic Light Scattering (DLS) and Infrared Spectroscopy (FT-IR). The thermo-physical properties (TC and viscosity) of NFs and MFs were measured at different temperatures between 20 and 40 ^oC. All Cu NFs and MFs showed higher TC and viscosity compared to the EG base liquid. The viscosity test results showed Newtonian behavior for NFs and MFs. Our study exhibited that NFs with Cu NPs revealed higher TC than those containing Cu MPs at the same particle concentration and temperature. For TC, Maxwell predictive equation and for viscosity of NFs/MFs, Kriger-Dougherty correlation was applied to compare the experimental results with the estimated values. Our findings on the physic-chemical, thermal and thermo-physical properties of the NFs/MFs, containing Cu NPs/MPs are presented in detail.

We introduce a novel doped hybrid glass, which serves as a Heat Generator and Energy Storage glass. Heat in the hybrid glass gets transferred into expansion or compression wave very effectively. Therefore, it can be also used as heat or energy storage material based on nano-structures. A alkylene-bridge sol-gel monomer was molecularly designed and then synthesized for the creation of new properties in the glassy host. It was prepared by a copolymerization of alkylene-bridged sol-gel monomer and a sol-gel processable chromium precursor. The resulting hybrid glass doped with chromium nano-particles shows an unusual property. TEM images revealed substantial regions of dark contrast, a highly periodic nano-scale structure of alkyl-spacers. We believe that the periodic nano-structures are sustained over substantial domains and appear to arise from alkyl chains of the glassy lattice fringes. From the electron diffraction pattern corresponded to the nano-fringe patterns, a lattice space of the structure was calculated about 50 Å from a distance between two diffraction spots in two sets of diffraction patterns. In NLO experiments, the doped glass shows a new optical property, a generation of huge acoustic wave. When a laser beam goes through a solid media, the density wave is usually linear. However, the doped glass shows a strong ‘acoustic response,&’ which was as strong as liquid. Interestingly, the diffraction efficiency (45 %) of the doped glass was higher than that of methanol (25 %), which meant the compressibility of the doped hybrid glass was as effective as the liquid.

The heat dissipation and hence the thermal conductance from optically excited gold nanowires into the surrounding water is measured in pure water and aqueous solutions of ionic solute molecules. The nanowires are lithographically fabricated on an Al_0.94Ga_0.06N thin film embedded with Er³⁺ which uses temperature dependent photoluminescence from thermally coupled Er³⁺ energy states for determination of temperature change of the nanowire excitation. The thermal conductance from heated nanowires into the pure water is extremely low (with the value of 3 ± 0.7 MW/m²-K) indicating the hydrophobic nature of an interface, and this low thermal conductance is believed to be caused by native surface charges that interact repulsively between the gold/water adhesion layer. Addition of ionic solute molecules at this interface increases the heat dissipation into the surrounding water with increasing concentration of the solute molecules before attaining saturation in thermal conductance. The results obtained for different ionic solutes follows the same trend when compared the thermal conductance values in terms of the respective ionic strength values for different solutions. The results for low molar concentration (0.1 moles per liter and lower) and hence lower ionic strength are consistent with the DLVO model for the interaction of two planar surfaces which suggests that an increase in ionic strength screen the electric double layer repulsion which leads to an increase in the interaction energy and consequently an increase in thermal conductance. Saturation in thermal conductance is observed for salt concentrations greater than or equal to 0.1 moles per liter. This saturation is consistent with previous observed high salt concentration break down of the DLVO model due to dispersion forces altering the interface distribution of ions.

The fundamental mechanisms driving phonon scattering at interfaces and boundaries have been the focus of an extremely active area of research over the past few decades, mainly driven by nanotechnology and thermal engineering of nanosystems. At interfaces comprised of a non-metal, the transport across this interface is dominated by phonon-phonon interactions. Via the mismatch theories, the phonon interfacial transmission is intimately related to the phonon bandwidth in the material comprising the interface. Therefore, the role of ballistic phonon transport on thermal conduction, which differs depending on the phonon spectra of the materials, should also lead to different values of thermal boundary conductance depending on the material. For example, the mean free paths of phonons on either side of an interface will drive the degree of non-diffusive transport in each material, and this in turn could affect the thermal boundary conductance between two solids. Motivated by this hypothesis, we present a series of thermal boundary conductance measurements between various metals and native oxide/Si substrates using time domain thermoreflectance. We vary both the pump and probe spot sizes along with pump modulation frequency during these measurements to investigate the role of non-diffusive transport in the metal and substrate on measured thermal boundary conductance. Furthermore, we study the direct correlation between measured thermal conductivity of silicon substrates and boundary conductance across Al/Si interfaces via TDTR measurements of Al/Si systems with varying degrees of interfacial disorder. This gives insight into the intertwined nature of thermal boundary conductance, "near interface" local thermal conductivity, phonon mean free path, and the non-diffusive nature of phonon transport in nanosystems.

The rapid emergence of nanostructured materials, which tend to have thermal properties vastly different from their bulk counterparts, has stimulated their utilization for heat dissipation and thermoelectric applications. However, the underlying physical mechanisms that dictate heat conduction still remain unclear. In fact, phonon transport can be impeded by a plethora of possible phonon scattering mechanisms, including phonon-phonon, phonon-boundary, and phonon-impurity scattering, which are difficult to individually characterize experimentally. In this work, we use classical molecular dynamics to explore the thermal conductivity of various nanostructures, including silicon-germanium nanowire alloys and three-dimensional sp² carbon polymorphs with negative curvature. We will present the physical origins of the thermal conductivities of these materials by means of the relative contributions of relevant phonon scattering events. Finally, we will discuss materials design rules to tune the thermal conductivity of these low-dimensional nanomaterials.

We have developed a platform based device through hybrid material integration that can achieve highly controlled separation and capture of rare cells based on the generation of microscale thermal gradients within microfluidic channels.
The separation, analysis, and isolation of rare cells currently presents a great challenge for researchers. Rare cells include cells such as circulating tumor cells (CTCs) which have been the focus of several cancer studies for their role in the spread of cancer. CTCs have proven useful for the detection and diagnoses of cancer. Rare cells are not limited only to CTCs, however, and also include fetal cells in maternal blood, stem cells, and infected cells. Because of the importance of being able to separate rare cells from a complex mixture, such as whole blood, for analysis and isolation, many studies have been carried out to find techniques which are capable of performing these tasks in a rapid and highly selective manner. The techniques currently in use suffer from issues such as sample loss during the process and a lack of the high specificity needed to identify rare cells in a heterogeneous mixture of cells. The work presented here demonstrates the development of a microfluidic lab-on-a-chip system for the rapid and highly specific separation, analysis, and isolation of rare cells.
First, separation of cells from a complex mixture is achieved through the design of the microfluidic channel geometry along with the incorporation of thermophoresis. Thermophoresis allows the manipulation of particle movement by the simple application of a thermal gradient. The creation of a non-uniform thermal field produces a thermal dipole and this interfacial thermal gradient is defined by the composition of the cell, which allows for thermal gradient based cell separation. Thermal gradients are created in the microfluidic channels by the effects of Joule heating in microelectrodes patterned using standard photolithography techniques. The combination of the thermophoretic effect and the channel design allows for the rapid separation of the cells from the sample. After separation the cells are then analyzed in order to identify the rare cells. This analysis is done by creating a thermal map of the cells using a microfabricated device within the microfluidic system that employs the Seebeck effect to register a potential that correlates to the temperature of the cell.
Cells have varying inherent temperatures based on the physiological processes taking place within the cell. Because of this, cells such as cancer cells have been shown to display higher temperatures then healthy cells. We leverage this intrinsic condition of the cells to identify the rare cells and then isolate them from the healthy cells in the sample. The ability of this device to isolate rare cells from a sample provides a versatile platform which has the potential to be employed not only for diagnostics, but also the study of rare cells and drug development.

A new, high resolution, scanning optical probe thermometry technique utilizing an optically trapped nanoparticle has been developed. While trapped and scanning over the surface of the substrate, the nanoparticle reports temperature that is resolved below the diffraction limit of light. The spatial resolution is now limited by the size of the trapped particle and not the structure of the irradiation source. This new technique has been applied to two new systems of nanoheaters to investigate the high resolution properties of this imaging method and to measure the temperature of hot nanoparticles through a phase transition. The high resolution experiment has shown resolved temperature decay on the order of 50 nm, significantly higher resolved than traditional optical methods. The phase transition experiment has shown nucleation of hot water to occur above the boiling point. More significantly, the temperature of the gold nanoparticle has been shown to experience extreme temperatures, over 1000 K, when insulated from the thermal reservoir by the vapor envelope of the nucleated bubble. #8203;

Hydrogen bonding is ubiquitous in nature and plays a critical role in modulating the orientation of π-conjugated organic semiconductors to facilitate charge transport. Furthermore, hydrogen bonding is believed to be responsible for the superior mechanical strength of proteins such as spider silk, and plays a crucial role in water&’s high thermal conductivity (0.6 W/mK) relative to other non-metallic liquids. Despite the importance of hydrogen bonding in polymers, proteins, and other organic materials, the mechanisms for thermal transport in hydrogen bonded materials are poorly understood. Small molecule organic semiconductors are auspicious candidates for devices including organic light emitting diodes due to their mechanical flexibility, potential for low production cost, and high carrier mobilities. However, the poor thermal stability of this class of compounds leads to deterioration in device performance. As such, thermal management of these materials is critical. Here we report the thermal conductivity of π-conjugated pentacene and its hydrogen bonded analogues, quinacridone and indigo. Ordered pentacene, quinacridone, and indigo thin films 20-100 nm in thickness were synthesized via vacuum sublimation and the thermal conductivities were measured using frequency domain thermoreflectance. The results of this study have important implications for the thermal management and successful incorporation of hydrogen bonded organic materials in electrical devices.

The combination of low density, high elastic modulus, and low thermal conductivity is highly desirable for applications such as thermal protection systems on space vehicles, but rarely realized in common materials due to the correlation between these physical properties. Nanotrusses, which consist of hollow nanoscale beams architected into a periodic three-dimensional truss structure, could overcome this limitation by decoupling the length scales governing mechanical deformation and thermal transport. However, while the mechanical properties of these nanostructures has been reported, thermal transport remains poorly understood. This work investigates the thermal conductivity of nanotrusses to assess their viability as a multifunctional material. We use time domain thermoreflectance experiments to directly measure thermal conductivity along with Monte Carlo simulations of the Boltzmann transport equation to explore the physical mechanisms behind these measurements. Preliminary results demonstrate potential for hierarchical structure and size effects to yield a high performance material with both high elastic modulus and low thermal conductivity.

We examine the dependence of thermal conductivity on crystal coherence length in MgO thin films. Sputter deposited films were prepared on (100) silicon substrates and then annealed to vary the microstructure. The film crystalline coherence was characterized using x-ray diffraction line broadening and the roughness was assessed using atomic force microscopy. We measure the thermal conductivity via time domain thermoreflectance (TDTR) and use the traditional formulation for thermal conductivity, κ = #8531;C#159;v#159;l. We found no obvious change in density or porosity distribution as a function of annealing temperature. This indicates that there is no change in heat capacity, C, from sample to sample. Additionally, we measured the phonon group velocity, v, via picosecond acoustic measurements. Similar to the heat capacity, there was no change in the phonon group velocity over temperature. As a result, we are able to narrow the relevant term in the formulation for thermal conductivity down to the scattering length scale of the material. Interestingly, we find that the measured thermal conductivity of the MgO films varies proportionally with crystal coherence length. This implies that the driving force behind the change in thermal conductivity is linked to the inherent scattering lengths present in the films based on their various crystalline coherences.

Recently, organic electronic devices are drawing a lot of attention in the research community due to not only their promising flexibility, stretchability or lighter weight, but also compatible with large area fabrication and low fabrication cost. The active layer formed by the organic semiconductor will determine the overall performance of these devices, which is closely related to the crystallinity and molecular alignments of the organic molecules. Thermal conductivity is a nontrivial property to investigate the thermal properties of organic semiconductors. Here we apply 3-omega; method to study the thermal conductivity of polycrystalline molecular thin films and single crystal thin films which are grown under different conditions. For polycrystalline molecule thin films, such as, dinaphtho-[2,3-b:20,30-f]-thieno-[3,2-b]-thiophene (DNTT) and pentacene, they are deposited at different substrate with different surface treatments (i.e. surface energy) by thermal evaporation and annealed at different temperatures afterwards. For single crystal 2,9-di-decyl-dinaphtho-[2,3-b:20,30-f]-thieno-[3,2-b]-thiophene (C₁₀-DNTT) studied, single crystals are grown on different kind of substrates with different self-assembly monolayer and oxygen plasma treatment. The 3-omega; measurement findings show that both substrate surface energy and annealing temperatures have significant influence on the thermal conductivity of DNTT and pentacene thin films. For example, thermal conductivity of 50 nm DNTT thin film after annealing process shows a 25% increase in thermal conductivity from 0.357 W/m-K to 0.445 W/m-K. Meanwhile, C₁₀-DNTT developed on oxygen plasma treated substrate has a different surface morphology from the untreated substrate and the thermal conductivity is also different. The atomic force microscopy (AFM) and x-ray diffraction (XRD) results indicate that lower substrate temperature and annealing process would change the grain size of the thermal evaporated organic thin films. While oxygen plasma treatment increases the surface energy of silicon dioxide substrate, which facilitate the crystallization of large C₁₀-DNTT single crystals and closer packing of the crystal. The good crystallinity may decrease the disordered phonon scattering in thin films and improve the thermal transport in organic semiconductors.

We study the effects of grain boundary size on electron-phonon coupling in thin metal films. Specifically, we study 20 nm Au films deposited on glass as well as 42 and 81 nm nickel films deposited on Si substrates with a 8nm Ti wetting layer. Using time-domain thermoreflectance (TDTR), we calculate the effective electron-phonon coupling factor using a parabolic two step model for nonequilibrium temperatures induced by short (femtosecond) laser pulses in TDTR. During TDTR, these pulses excite electrons, which subsequently scatter with other electrons, phonons, and defects/boundaries. We systematically alter the grain size in our samples by annealing the Au and Ni films. Films as deposited offer the smallest grains and the grain size increases with increased annealing temperature. By increasing the grain size in our samples we reduce the number of grain boundaries available to provide thermal resistance to the electrons, thus changing the rate at which electrons scatter with phonons. For Au, resistance to electrons due to its lattice is very high, resulting in a low electron-phonon coupling factor. We find that introducing additional resistance due to grain boundaries in Au results in little change to the measured effective electron-phonon coupling factor; this is due to Au&’s large intrinsic lattice resistance. We hypothesize that altering the density of grain boundaries in our Ni films will directly influence the effective electron-phonon coupling factor due to the relatively low intrinsic lattice resistance found in Ni. Thus, we characterize the effects of multiple grain sizes on the effective electron-phonon coupling factor in Ni.

Widespread application of polymers, such as in packaging for high energy density material, is limited by their poor thermal transport properties. Much work has gone into embedding thermally conductive additives into polymer matrices to enhance the overall thermal conductivity (k). However, predicting k has proven to be difficult given the limited understanding of interface effects on thermal transport in such systems.
Using surface initiated atom transfer radical polymerization (SI-ATRP), we synthesized particle brushes comprising silica nanoparticles grafted with well defined polymer ligands of known length, architecture, and identity. By comparing isolated parameters such as graft architecture and graft-matrix interactions, we isolated the influence of the graft-matrix interface on thermal transport in such systems. We found that particle brushes with enthalpically favorable interactions at the brush-matrix interface have higher thermal conductivity than conventional binary nanocomposite systems and even predictions from effective medium approximations. The effect of chain confinement (or elongation) within the grafts on k was also probed by independently studying polymer brushes on flat substrates with different grafting densities and heights.

We investigate experimentally and theoretically the acoustic phonon propagation in two-dimensional phononic crystals. Solid-air and solid-solid phononic crystals were made of square lattices of air holes/Au pillars in/on 250 nm thick single crystalline Si membrane, respectively. The hypersonic phonon dispersion was investigated using Brillouin light scattering. Volume reduction (holes) or mass loading (pillars) accompanied with second-order periodicity and local resonances are shown to significantly modify the propagation of thermally activated GHz phonons. We use numerical modelling based on the finite element method to analyse the experimental results and determine polarisation, symmetry or three-dimensional localisation of observed modes.

The development of high power GaN transistors has led to increasing demands on thermal management technology due to the intense heating in the channel of the devices. The localized heating which is concentrated on the drain side edge of the gate, is dissipated by flowing through the GaN layer and across the supporting substrate. Attempts have been made to replace SiC with diamond layers in order to advance the thermal dissipation near the junction of GaN based HEMT devices. While diamond has a higher thermal conductivity than SiC, complications in integrating diamond layers can occur from interfacial thermal resistance and the phonon scattering that can occur in the nucleation region of the diamond layers and there non-homogenous microstructures where the GaN or other electronics may be attached.
In this work, we will present transient domain thermoreflectance measurements and Raman spectroscopy measurements of the anisotropic thermal conductivity and the thermal boundary resistance of diamond layers grown on various semiconductor substrates (GaN, Si, and SiC). Both the thermal boundary resistance and thermal conductivity were measured as a function of temperature, film thickness, and grain size. Data showed that the thermal boundary resistance and thermal conductivity were strong functions of grain size which is controlled by the nucleation process for the diamond films. The film composition in terms of sp2/sp3 bonding ratios were determined by XPS and FTIR analysis which showed a direct correlation to the thermal properties. In comparing TDTR and Raman methods, it was found that similar properties were obtained by both methods, thus allowing for good cross correlation of results. Finally, a model will be presented which relates grain structure to the thermal properties seen in the diamond films.

High gas adsorption capacity of metal organic frameworks (MOFs) has made it one of the ideal candidates for natural gas storage. Though, a challenge with the MOF-based adsorbed natural gas (ANG) storage is the slow fill times due in large part to the significant heat generated during gas adsorption. Theoretically, designing MOF-based ANG systems which can dissipate heat more quickly could allow for faster filling times. However, the heat transfer mechanisms for a MOF/natural gas system are poorly understood. In this work, we discuss the mechanisms of heat transfer in different regions of a MOF/natural gas system including: (a) the region inside the MOF particles with gas molecules in the pores, (b) the interface between the MOF particles and bulk gas in the storage vessel, and (c) bulk gas. Mainly, we address the heat transfer components in region (a) and the interfacial thermal conductance in region (b). In region (a), we study the gas molecules convection heat transfer, heat conduction in MOF by phonons and the interaction between gas molecules and phonons of MOF. For the calculations, we use equilibrium and non-equilibrium classical molecular modeling.

Monodisperse hollow silica nanoparticles are well-defined porous colloidal building blocks, which can be assembled into highly ordered three-dimensional structures. The resulting hollow silica sphere colloidal crystals represent a hierarchical material class with a well-defined and remarkably precise structural control. This offers the opportunity to study nanoscale thermal transport through such a porous material in great detail.
Here, we present the synthesis and detailed characterization of hollow silica nanoparticles with a diameter of 316 nm and a shell thickness of 44 nm. The focus will be laid on the dependency of the internal structure of the hollow spheres on the thermal transport properties. For the synthesis, polystyrene template particles were coated with a silica shell. Subsequent calcination of the core/shell structure led to the formation of hollow silica spheres. We produced large colloidal crystal monoliths via evaporation induced self-assembly. By varying the calcination temperature from 500 °C up to 950 °C, the internal microstructure could be easily and significantly tuned. To characterize the structural features from macro- to nanoscale, a range of techniques was used. SAXS, electron and optical microscopy showed the monodispersity and structural integrity even after calcination up to 950 °C, whereas solid-state NMR and N₂ sorption measurements demonstrated considerable changes on the local microscale. These changes translate into an increase in thermal conductivity of up to 100 %, measured by Xenon flash analysis (XFA).
We identified two different contributions to the increase in thermal conductivity. The first one can be assigned to the loss of scattering centers within the silica shell by closure of the micropores. This change is accompanied by a large reduction in surface area as well as an increase in the amount of Q⁴ Si atoms. The second contribution is given by a strengthening of the interfacial bonding between the individual spheres. Overall, combining low interfacial bonding and high microporosity leads to a remarkably low thermal conductivity of only 71 mWm^-1K^-1 for a silica material of density of 1.04 gcm^-3.
This study shows that colloidal crystals of hollow silica spheres represent a promising thermally insulating material class. Moreover, the well-defined structure enables the understanding of thermal transport processes in amorphous silica materials in great detail.

Colloidal crystals consisting of monodisperse spheres have drawn great attention due to their potential application in various technical fields. Whereas the photonic properties of this class of material had been extensively studied, colloidal crystals also provide great potential in the field of thermal insulation and heat management. Thinking about thermal insulation, colloidal crystals combine two common approaches to reduce the thermal transport through a given material. Firstly, they possess a high amount of interfaces, and secondly they feature a well-defined porosity, both being based on the fcc structure of the crystals. In this contribution we report the thermal conductivity properties of three-dimensional polystyrene (PS) colloidal crystals, consisting of PS spheres with a diameter of 366 nm. We show that these crystals possess a very low thermal conductivity κ of 0.047 W/Km (κ of bulk PS ~ 0.13 W/Km) in vacuum at room temperature. This is mainly due to the high amount of interfaces within the crystals. Remarkably, this low thermal conductivity is reached with a polymer material of comparatively high density of 0.75 g/cm³.
We further investigated the influence of the surrounding atmosphere on the overall thermal transport in such an open porous structure. We found that the thermal conductivity is hardly affected by the ambient atmosphere, which we changed from vacuum to air and helium. This can be rationalized by the size of the pores present within colloidal crystals. For the particular case, these range from 57 nm to 151 nm. Thus, the size of the pores is below or close to the mean free path of the tested gases. This leads to a strong reduction of the additional thermal transport through the gas phase.
Finally, the structure of the colloidal crystal can be manipulated by the external temperature due to the glass transition temperature of the PS nanospheres. By exceeding the T_g of the PS particles, the particles soften, leading ultimately to a continuous film. The crystalline structure vanishes resulting in a drastically increased heat transport through the material. This manifests by more than a doubling of the thermal conductivity of the specimen. Such a behavior enables new functional materials, which strongly increase their thermal conductivity beyond a certain temperature.

We report the investigation of thermophones (devices producing sound via the thermoacoustic effect) consisting of arrays of ultra-long (mm scale) polycrystalline gold nanowires. Arrays of 4000 linear gold nanowires are fabricated at 5 mu;m pitch on glass surfaces using lithographically patterned nanowire electrodeposition (LPNE). The properties of nanowire arrays for generating sound are evaluated as a function of frequency (from 5 - 120 kHz), angle from the plane of the nanowires, input power (from 0.30 - 2.5 W), and the width of the nanowires in the array (from 270 to 500 nm.) Classical theory for thermophones based upon metal films accurately predicts the measured properties of these gold nanowire arrays. Angular nodes for the on-axis sound pressure level (SPL) versus frequency data, predicted by the directivity factor, are faithfully reproduced by these nanowire arrays. The maximum efficiency of these arrays (10^-10 at 25 kHz), the power dependence, and the frequency dependence are independent of the lateral dimensions of these wires over the range from 270 to 500 nm.

While thermoelectric devices have remained largely a niche market due to their low efficiency, the introduction of nano-patterning drastically improved this by reducing greatly the thermal conductivity, thus increasing the Figure of Merit. Studying phonon transport in various nanostructures will help to increase the Figure of Merit. Periodic patterning can be one of the methods that can be useful to reduce thermal conductivity, using the wavy nature of phonons.
We fabricated different air-bridge Si phononic crystal (PnC) nanostructures and measured their thermal conductivities at room temperature. Samples were comprised of membranes, nanowires and fishbone-type 1D PnC structures. The fishbone structures consist of periodically patterned fins on both sides of a central nanowire. All the samples were processed from a commercial SOI wafer, with a 145nm-thick top Si layer, using lithography and etching techniques. The measurements were performed with our original optical system based on the time domain thermoreflectance method and fitted with simulation results from FEM simulations. Results are also compared with thermal conductivities obtained by solving Boltzmann transport equation by Monte-Carlo method. We discuss transport properties based on this comparison.
Room temperatures measurements of Si nanowires show a reduced thermal conductivity with decreasing width, down to below 40 W/mK for a 68-nm-wide nanowire. The 1D PnC structures exhibit better rigidity compared to nanowires with a similar neck size and a thermal conductivity of 27 W/mK was observed, for a neck size of 50 nm, a period of 300 nm, and large fins with a width between 50 and 250 nm parallel to the heat propagation and a total span over 300 nm perpendicular to it. Shape dependence show that the neck width controls most of the heat flow. The fin shape can be modified to decrease the thermal conductivity beyond just reducing the neck size. Experimental measurements and Monte-Carlo simulations are in fairly good agreement and both depict a shape dependence of the thermal conductivity in the diffusive regime. Adjustable specularity shows that room temperature transport is nearly exclusively diffusive. The reduced thermal conductivity of the PnC structures by a factor of three compared to an un-patterned membrane can be mostly explained by the Monte-Carlo simulation, which does not include phononic effect. While it is true that some part of the reduction might also be attributed to reduced group velocities, it seems reasonable to conclude that the main mechanism of the observed reduction is backscattering of phonons in the fins.

Engineering of nanostructured materials with very low thermal conductivity is a necessary step towards the realization of efficient thermoelectric devices. Superlattices, either in the form of thin films or nanowires, are promising candidates with a considerably high ZT coefficient, as their thermal conductivity can be dramatically reduced. However, the thermal conductivity of superlattices cannot be reduced further as periodic length drops below a certain value (usually about a few nanometers), due to the presence of coherent phonons facilitating the heat conduction of some specific phonons. We present here a new superlattice structure to effectively block the transport of coherent phonons by performing non-equilibrium molecular dynamics (NEMD) simulation. We explore the behind mechanism by comparing the cases between traditional superlattices and the new structures. Our study provides a new route for manipulating phonons in superlattices, which could be beneficial for enhancing energy conversion performance of thermoelectric materials

We consider energy transfer between non-equal nanoparticles mediated by a quantum system. The nanoparticles are considered as thermal reservoirs having finite numbers of atoms. Our approach is based on the generalized quantum Langevin equation. The thermal reservoirs are described as ensembles of finite numbers of oscillators within the Drude-Ullersma model having mode spacings Δ₁ and Δ₂. The quasi-static energy transport between the thermal reservoirs is investigated. As is shown, the double degeneracy of the mode frequencies, which occurred in the previously considered case when Δ₁ = Δ₂, is removed in the present case of non-equal mode spacings. Equations describing long-time (t sim;1/Δ_1,2) relaxation for the mode temperatures of the nanoparticles and the resulting heat current are derived and solved.

Phonon transmission across interfaces of dissimilar materials has been studied intensively in recent years by using atomistic simulation tools owing to its importance in understanding the physics of interfacial phonon scattering and designing the nanostructures with the desirable thermal performance using mesoscopic modeling tools. Atomistic Green&’s function (AGF) method with first-principles (FP) interatomic force constants evolves to be a promising approach to study phonon transmission in many not well-studied materials. However, extracting interatomic force constants directly from FP calculations becomes challenging or even impossible because the large unit cell is required to model the interfacial region, where lattice mismatch and species mixing occur. Here, we propose an integrated FP-AGF approach to predict phonon transmission, in which the harmonic force constants are extracted based on higher-order force constant (HOFC) model, which is originated from virtual crystal approximation. While the HOFC model avoids the large-scale FP calculations to model the interfacial region, the harmonic force constants from the HOFC model are of the same accuracy as those from the FP calculations with the standard routines. As a demonstration of the method proposed, we study the phonon transmission in the Mg₂Si-Mg₂Si_xSn_1-x systems. By comparing the phonon transmission calculated from the HOFC-AGF approach with that from the widely used mass approximation based AGF approach, the importance of local force field difference on the phonon transmission is identified. Also, the effects of alloy composition on the phonon transmission and interfacial thermal conductance are explored.

Thermal conductance across an organic/ inorganic interface is a topic of active interest. The two major factors believed to shape the conductance are the chemistry mediated bonding and the overlap in density of states. What is not as well explored is the role of anharmonicity in opening new transport channels. We combined Density Functional Theory (DFT) with three phonon scattering processes within the Non-equilibrium Green&’s Function (NEGF) for phonon transport, to elucidate the role of scattering at organic-inorganic interfaces.
We present first principle calculations of thermal conductance across Au(111)-Alkane-Au(111) interfaces with different end groups (-S, -NH₂) and gold surface roughness (unreconstructed vs reconstructed). The NEGF formalism allows us to turn on and off anharmonicity processes to separate out harmonic and anharmonic contributions. With different surface roughnesses and end groups, we quantify the modification of anharmonicity to the ballistic interfacial impedance. We find that anharmonicity has competing contributions; it helps phonon transport across the interface for weak bonding and at the same time impedes the phonon transport for strong bonding. Those results are elucidated using frequency resolved transmissions that highlights the inelastic scattering processes. We also consider the ballistic and anharmonic contributions to conductance as a function of the molecular length. We show the transition from coherent/ballistic to diffusive transport and its relation with anharmonic strength. We also show an added rectification at the Au (111)-Alkane interfaces due to the temperature dependence of anharmonic processes.

A mesoscopic model has been developed to investigate the effective thermal conductivity and interfacial thermal resistances of polymer composites containing carbon nanotubes (CNTs) and inorganic nanoparticles by the means of Off-Lattice Monte Carlo method. By taking into account the interfacial thermal resistance (R_bd) between any two phases and the synergistic effect of CNTs and nanoparticles, our model could predict the effective thermal conductivity (K_eff) of 3-phase composites more accurately than those predicted by Effective Medium Theories (EMTs). In our model, complex morphology of CNTs (CNT diameter and length, volume fraction, orientation and CNT bundle) and heat transfer phenomena at interfaces (interfacial thermal resistance) were quantified to study their effect on K_eff of multiphase composites. Moreover, by fitting the simulated K_eff with the experimentally measured K_eff of composites, interfacial thermal resistances of polymer-CNT and polymer-nanoparticle can be estimated, which are difficult to be determined by experiments. The simulation results showed that K_eff of multiphase composites increased as CNT fraction increased and as R_bd of polymer-nanofillers (CNTs and inorganic nanoparticles) decreased. CNT bundle structures were built in the model to investigate their effect on K_eff of the composite, which had not been considered in EMTs. It was found that CNT bundle exhibited different effect on K_eff of composites with different oriented CNTs. With the increase of CNT bundles, K_eff decreased in composites with random and parallel CNTs, whereas, K_eff increased in those with perpendicular CNTs. These findings not only fill a niche to bridge the macroscopic Finite Element methods (FEMs) and the microscopic molecular dynamics (MD) approaches, but also provide a paradigm to fabricate multiphase composites with high thermal conductivity.

In organic/metal hybrid materials, the thermal boundary conductance across the metal/organic interface plays a significant role in overall thermal conductivity of the film. Here by using an intermixing layer to simulate the Ag/ Dinaphtho[2,3-b:2&’,3&’-f]thieno[3,2-b]thiophene (DNTT) interface, we apply lattice Boltzmann method (LBM) to evaluate the thermal boundary conductance (TBC) of Ag/DNTT thin film. Simultaneously, 3-omega; method is used to experimentally measure the effective conductivity of the thin film and the results are compared with the modeling. By carefully controlling the concentration of the silver nanoparticles, an interface model involving the interfacial bonding and composition are developed and we show that interfacial bonding has a significant influence on the overall thermal conductivity thin film. For a thin layer of Ag and intermixed with DNTT (10% volume ratio), the thermal conductivity decrease from 0.363W/m-K (pure DNTT) to 0.305W/m-K which shows the importance of the thermal boundary conductance. Furthermore, we measured the activation energy (E_act) of the hybrid thin films and utilize the relationship between the temperature and the electrical conductivity of the film to form thermistors. By inserting up to 20% volume ratio Ag NPs into DNTT matrix, the activation energy was enhanced from temperature insensitive pure DNTT to 420 meV. Based on the hybrid organic/metal thermistor, an integrated temperature sensor array consists of thermistors and transistors is fabricated on flexible substrate with an low operating voltage of 5V. The low voltage flexible thermal sensor array is suitable for portable electronic devices and potentially scale up for electronic skin applications. Other application directions such as health monitoring or use as surgery tools can be achieved.

We used molecular dynamics simulations and a new formalism for calculating the modal contributions to thermal conductivity to study individual polythiophene chains. The simulations suggest that it is possible to achieve divergent/infinite thermal conductivity (e.g., thermal superconductivity) in individual polythiophene chains. The new modal analysis method allowed for exact pinpointing of the modes responsible for the anomalous behavior, which turned out to be transverse vibrations in the plane of the aromatic rings at low frequencies ~ 0.05 THz. Within the 5 ns of integration time, one mode in particular exhibits a thermal conductivity contribution greater than 100 W m^-1 K^-1, which is larger than many 3D bulk materials that consist of a large multitude of modes. Further investigation showed that the divergence arises from persistent correlation between the three lowest frequency modes on chains that have exact multiples of 30 unit cells in length. Sonification of the superconducting mode heat fluxes indicated distinct patterned differences between the convergent and divergent simulations, which suggests the phenomena may differ from previous models and a new explanation of the anomalous behavior may be required for polymers.

Two-dimensional (2D) layered materials have attracted great interest due to their different types of interlayer (cross-plane) and intralayer (in-plane) bonding, which results in unusual thermal and electrical transport properties. Due to the inherent layered structure, the ability to intercalate guest ions into the van der Waals gap of 2D layered materials offers the opportunity to engineering both electronic and thermal properties of such materials. In this work, we demonstrate the structural tuning of MoS₂ through electrochemical intercalation of Li⁺ ions. The amount of the intercalated Li⁺ is controlled by scanning the Li intercalation potential from high to low, and the resulting changes in both interlayer and intralayer thermal conductivity are characterized using time-domain thermoreflectance (TDTR) method. We show that Li intercalation could have profoundly different impacts on interlayer and intralayer thermal conductivities as a result of competitions between factors including expanding of van der Waals gap and changes in electronic structures.

Metal-organic-framework materials (MOFs) are nanoporous materials with amazing potential for applications in gas storage and gas separation. MOFs are also poor thermal conductors and in some cases this limits their gas storage utility. Heat conduction in MOFs is also unusual with heat transported both by propagating lattice modes and non-propagating molecular modes. Here we show that intercalating a gas into the framework of MOF-5 can produce a more than 25-fold increase in the MOF&’s thermal conductivity, although the gas itself is condensed on the framework and carries little heat. This is brought about because the intercalated gas provides coupling between linker arms thereby altering the thermal transport topology of the network. This result suggest exciting mechanisms for externally tuning thermal conductivity in these materials.

We use non-equilibrium molecular dynamics simulations to investigate heat dissipation during oscillatory shear of viscoelastic materials. We study viscous energy loss in two classes of viscoelastic materials: (a) Silicon - phenolic resin polymer nanocomposite and (b) high and low cross-linked polymer composite. During oscillatory shear deformation, the viscous loss manifests as a steady temperature and energy increase. This rate of increase agrees well with a simple prediction from macroscopic level analysis. We characterize the spatial distribution of heat generation in these heterogeneous structures, focusing on the role of interfacial shearing. The frequency dependent damping losses in these composites indicate a possibility of designing materials capable of strong damping of intermediate and high frequency phonons.

In the last years many materials for thermal insulation in buildings have been studied and tested. Among them are mineral wool, expanded polystyrene, extruded polystyrene, and polyurethane foam, with thermal conductivity values in the range between 30 - 50 mW / (m×K). Further developments have led to “super-insulation” involving vacuum insulation panels (VIP) and aerogels. These deliver values as low as 5 - 12mW / (m×K). In such composites interfaces generally present a resistance to heat flow in addition to the resistance of bulk materials. This, due to the high density of interfaces, can be particularly limiting to heat flow in nanocomposite materials. Common for these materials is that the heat transport is suppressed by pores/voids in the solid. Consequently, their mechanical stability is typically low.
In this paper we present our work on new nanoparticle-based composites with good thermal insulation values. The samples were obtained by mixing different powders through the combination of phonon acoustically diverse materials. Depending on the acoustic impedance of adjacent components, the heat conduction varies, due to phonon scattering at the particle interfaces. Thermal conductivity, scanning electron microscopy (SEM), and mechanical tests are shown and explained.

Thermal transport can be greatly influenced by the presence of nano- and mesostructures, as well as the interfaces, which exist in such materials. This structuring can be employed to specifically tailor the phonon spectrum and may ultimately lead to phononic devices, such as a thermal diode.^[1]
Colloidal materials represent an ideal platform to access well-defined devices, which can be structured on hierarchical length scales. Colloidal superstructures, which can be amorphous or crystalline, cover size ranges from a few ten nanometers up to micrometers and beyond.^[2] Additionally, one can easily implement different kinds of materials and can also fabricate heterostructured composites.^[3]
In this contribution, we demonstrate the power of colloidal superstructures to tailor and understand nanoscale thermal transport. To this end, we focus on two distinct material classes, which are polymer and silica based. We show how silica hollow spheres can be tuned to rival the insulating performance of silica aerogels. Concomitantly, these amorphous silica materials allow for a precise control of the periodicity and size of the pores.
Furthermore, polymer based colloidal crystals and glasses - known as photonic crystals - represent an efficient insulation structure with open pores. We assess the influence of the crystal symmetry and demonstrate how gas thermal transport can be suppressed in these systems. By changing the interfacial contact between the colloidal spheres one can drastically increase the thermal conductivity through such colloidal ensembles. We show how this switching behaviour can be tuned to specific temperatures.
[1] M. Maldovan, Nature 2013, 503, 209.
[2] M. Retsch, U. Jonas, E. L. Thomas, Abstracts of Papers of the American Chemical Society 2010, 240.
[3] M. Retsch, U. Jonas, Adv. Funct. Mater. 2013, 23, 5381.

We demonstrate the application of a single nanowire resistive nano-heater to achieve fabrication of heterogeneous hierarchical nanowires (NWs) by an all solution, non-vacuum and low-temperature process, without any photolithographic steps. The resistive nano-heater induces a highly localized induced temperature field and a thermochemical reaction in the adjacent precursor laden medium. By means of selective laser sintering of silver (Ag) nanoparticle (NP) ink to pattern contacts, electrical current is applied through a selected single Ag NW to create a highly confined and controllable temperature field. This functionality was confirmed by scanning thermal microscopy (SThM) measurement in conjunction with numerical conductive heat transfer simulation. Moreover, through the precise thermal control in the nano-heater, hierarchical heterojunctions of ZnO NWs on the Ag NW could be grown by localized hydrothermal growth. The energy dispersive x-ray spectroscopy (EDS) analysis shows the formation of the metal and metal-oxide heterojunctions connecting the Ag and ZnO NWs. This work suggests a new approach for selectively integrating heterogeneous hierarchical NWs by nanoscale heat transport, that could have high potential as a novel fabrication method for all NW electronics.

The thermal boundary resistance (TBR) between dissimilar materials strongly influences the temperature rise and temperature distribution in integrated nanoscale structures. There has been previous research on thermal resistance at thin film interfaces between planar films, but these approaches are becoming less relevant for the non-planar interface geometries between metals, semiconductors, and dielectrics in modern semiconductor technologies. The complexity of phonon and electron scattering, nonequilibrium, and unusual defect distributions may cause these resistances to differ strongly from those measured in planar geometries.
Here we develop measurements that extract the thermal properties of nonplanar metal-dielectric interfaces that resemble those in interconnect-passivation geometries. We have designed and fabricated a lateral multilayer structure with alternating layers of high aspect ratio silicon dioxide and aluminum to measure the TBR between the nanoscale layers. The width of both the silicon dioxide and aluminum layers varies from 100 nm to 300 nm, and the thickness of the layers is 100 nm. These nano-grating measurement structures have one electrical heater and four resistive thermometers, and the effective thermal resistance of different number of layers is determined using steady state electrical thermometry measurements while applying Joule heating. The sidewall TBR between silicon dioxide and aluminum is extracted by extrapolating the thermal resistance of multilayers with varying interface density. The sidewall TBR between aluminum and silicon dioxide is found to be as large as 40 m²K/GW and is compared with data for planar interfaces. Transmission electron microscopy reveals that sidewall interfaces have rougher surfaces than planar interfaces due to inherent edge effects during reactive-ion etching, and the increased roughness contributes to the higher TBR. Furthermore, temperature-dependent sidewall TBR measurements down to 30K are conducted to amplify the importance of sub-continuum effects. [1,2]
[1] Chen, Gang. "Thermal conductivity and ballistic-phonon transport in the cross-plane direction of superlattices." Phys. Rev. B 57, 14958 (1998)
[2] Sood, Aditya, et al. "Thermal Conduction in Lattice-Matched Superlattices of InGaAs/InAlAs." Appl. Phys. Lett., 105, 051909 (2014)

Since their discovery, fullerenes and fullerene derivatives have been utilized in organic power electronics, photovoltaic and thermoelectric applications due to their unique structures, chemical functionality, and beneficial transport properties. For example, doped fullerene have demonstrated potential as thermoelectric thin films, and with the recent realization of the exceptionally low thermal conductivity of fullerene derivatives, high thermoelectric figures of merit could ensure from these carbon structures. In solution, they have also been used as a component of contrast agents for multiple medical imaging techniques. Furthermore, theThe relaxation of photo-excited fullerenes and the characterization of their thermal properties have been the focus of considerable research in the last decade, yet limitations in these scattering processes based on thermal properties still remain unknown. The lattice vibrations of fullerenes consist of predominantly localized modes (commonly referred to as Einstein oscillations), which can results in an ultra-low thermal conductivity in solid fullerene derivative films. However, the fundamental vibrational physics driving fullerene energy coupling remains relatively unknown. To understand the role of fullerene and molecular energy transport on thermal conductance, we study the exchange of energy of these localized oscillations with an aqueous environment using time domain thermo-transmittance. With Kapitza Conductance as a metric, we show that the molecule-liquid energy exchange depends on the overlap in vibrational modes of the fullerene and liquid molecules. This is supported by the fact that the type of functional group (i.e. ring structure vs. ester) is the determining factor for thermal transport efficiency, causingand can cause a change in thermal boundary conductance of up to ~40% for fullerenes of the same molecular weight. Vibrational mode overlap is determined by infrared radiation spectroscopy. This study provides experimental evidence supporting previous molecular dynamics studies which suggest that vibrational cooling of a molecule in a liquid depends on overlap of density of states between the molecule and liquid. Furthermore, our work elucidates the role of the chemical functionalization in vibrational energy transport mechanisms in fullerene-based systems and applications.

We study the electron and phonon thermal coupling mechanisms at interfaces between gold films with and without Ti adhesion layers on various substrates via pump-probe time-domain thermoreflectance. The coupling between the electronic and the vibrational states is increased by more than a factor of five with the inclusion of an ~3 nm Ti adhesion layer between the Au film and the non-metal substrate. Furthermore, we show an increase in the rate of relaxation of the electron system with increasing electron and lattice temperatures induced by the laser power and attribute this to enhanced electron-electron scattering, a transport channel that becomes more pronounced with increased electron temperatures. The inclusion of the Ti layer also results in a linear dependence of the electron-phonon relaxation rate to temperature which we attribute to the coupling of electrons at and near the Ti/substrate interface. This enhanced electron-phonon coupling due to electron-interface scattering is shown to have negligible influence on the Kapitza conductances between the Au/Ti and the substrates at longer time scales when the electrons and phonons in the metal have equilibrated. These results suggest that only during highly nonequilibrium conditions between the electrons and phonons (Te #8811; Tp ) does electron-phonon scattering at an interface contribute to thermal boundary conductance.

The ability to shape interfacial thermal conductance has many technological applications for thermal management at the nanoscale. It has already been shown that inserting a material at an interface can increase its thermal conductance. This counter intuitive result is often qualitatively explained as a consequence of an increment in density of states overlap. However a quantitative picture of the conductance enhancement, critical to choose the material properties of the junction, is still missing. We argue that the overlap of density of states is only part of the story, and needs to be supplemented with (1) the role of random atomic mixing that breaks perpendicular momentum conservation, and anharmonicity that opens new transport channels through inelastic jumps. Furthermore, there is (2) a strong dependence of the transmission on the polarizations of the incident and transmitted phonons (i.e. directivity) as well as the detailed structure of the interface. Those concepts are demonstrated on Si/alloy/Ge systems using ‘first principles&’ DFT based transport simulations. We employ the Non Equilibrium Green&’s function (NEGF) formalism with self-consistent Born approximation that allows us to isolate and separately include random atomic mixing and anharmonicity and explore their individual roles.
To quantify the enhancement of thermal conductance we use the Landauer-Buttiker formalism and its direct relation with NEGF. Within this formalism, we can establish that (1) the number of transport channels is a function of the degree of atomic mixing and the degree of anharmonicity, while (2) the transmission per channel depends on the polarization and wavelength of the involved phonons, on the atomic mass differences and on the granularity of the mixing distribution. Our results are compared to existing experiments on disorder interfaces that show increase in Thermal Boundary Conductance. This effort could guide the development of models for interfacial thermal conductance that depend explicitly on interfacial properties. In this way, the models can overcome the known drawbacks of the Acoustic and Diffusive Mismatch Models.

It has been proposed by several studies in the literature that diameter modulated nanowires can increase the thermoelectric efficiency. Within this framework, sawtooth roughness on a nanowire has been studied and the further reduction of the thermal conductivity compared to the smooth surface nanowire has been explained with the backscattering of phonons at the constrictions¹. K. Saha in his PhD suggested that asymmetric sawtooth nanowire surfaces can further cause phonon rectification, making the axial thermal conductance of the nanowire direction dependent. The phonon backscattering and rectification effects can be employed to enhance the thermoelectric figure of merit of nanowires².
With molecular dynamics simulations and the resolution of the Boltzmann transport equation for phonons in the framework of the relaxation time approximation by Monte Carlo simulations we studied the geometric and temperature effect on asymmetric sawtooth nanowires. Due to the back scattering that is much stronger towards only one direction, we have evidences of phonon rectifications in such structures. The physical insights of the phenomena will be given.
¹ A. Moore, S. Saha, R. Praseher and L. Shi, “Phonon backscattering and thermal conductivity suppression in sawtooth nanowires”, Appl. Phys. Lett.93, 083112 (2008)
² S. K. Saha PhD “Theoretical Study of Thermal Transport at Nano Constrictions and Nanowires with Sawtooth Surface Roughness”, Un. Texas at Austin, May 2007

In this contribution, we will discuss two issue related to interfacial heat transport in the vicinity of solids. We first discuss the effect of interfacial roughness on the thermal boundary conductance between two solids, on the basis of molecular dynamics simulations [1]. We report a transition between two behaviors, depending on the interfacial roughness. When the roughness is smaller than the phonon wavelength, the thermal conductance is found to be constant, and takes values
close to the conductance of a planar interface. On the other hand, when the interfacial roughness is large, the interfacial conductance becomes proportional to the true interfacial area and increases with the roughness. The transition between these two limiting behavior may be described using a simple model, which involves the fraction of phonon modes having a wavelength greater than the roughness.
We also show that the thermal conductance between dissimilar materials may be enhanced by a factor larger than three by playing with the interfacial slope. Finally, we discuss the effect of the shape of the interface, and conclude that at fixed roughness wave like interfaces may display the highest conductance, because of their large true interfacial area.
The other issue concerns heat transport in the vicinity of liquid/solid interfaces, a topic where fundamental understanding is partial. We discuss a viscoelastic model to predict the value of the thermal boundary resistance at the interface between liquid water and solid. The model is a generalization of the acoustic model of Prasher, which accounts for the finite bonding strength between two materials [2]. We generalize this model through two aspects: first, we differentiate the longitudinal and transverse polarizations, and more importantly we account for the viscoelastic
properties of the liquid at high frequencies (Thz). We unveil the important role of the high frequency viscoelastic properties of liquids. In particular, we show that acoustic models which do not account for the viscoelastic character of high frequency vibrations in liquids under predict the thermal boundary conductance by one order of magnitude [3]. On the other hand, a viscoelastic generalization of the
acoustic model may provide a good description of the experimental and simulation data available [3]. Finally, if time permits I will briefly discuss the effect of interfacial electron-phonon coupling in the heat transport across metal/dielectrics interfaces [4].
References
[1] S. Merabia and K. Termentzidis, Phys. Rev. B 89, 054309 (2014)
[2] R. Prasher, App. Phys. Lett., 94 041905 (2009)
[3] S. Merabia, J. Lombard and A. Alkurdi, submitted (2014)
[4] J. Lombard, F. Detcheverry, S. Merabia, J. Phys. Cond. Matt. In press

Phonon transport in silicon is a topic of great practical and fundamental interest. Several measurement structures using silicon-on-insulator wafers have been developed to measure the thermal conductivity of silicon thin films. However, in most of these structures, amorphous oxide layers are in parallel with the silicon film and thus heat is conducted in both crystalline silicon layer and amorphous oxide layers. These oxide layers include intentionally deposited electrical insulation layers and unwanted residual buried-oxide layers. Relatively large uncertainty in quantifying the silicon thermal conductivity results due to (a) the uncertainty in thermal conductivity of these oxide layers and (b) poor understanding of interface thermal resistances. The uncertainty in thermal conductivity measurements makes characterization of the size effect in these thin silicon layers challenging.
To better understand phonon transport in pure silicon thin film, we develop a nano-fabricated suspended structure to measure the thermal conductivity. As opposed to most other techniques, our measurement device has a limited oxide layer. Specifically, oxide is deposited under the resistive thermometers, but not across the whole silicon layer. Comparing results with and without the oxide layer sheds light on the impact of silicon-silica interface. In addition, although the thermal conductivity of silicon films with thickness 10-1000 nm have been measured previously, there is a lot of variation in these data, which is partly due to the different measurement techniques and samples used by each group. A series of experimental data of silicon thin films across a range of dimensions (20-1000 nm) measured with the same fabrication and experimental technique excludes these effects.
Experimental data indicates a large reduction in the thermal conductivity of silicon films due to phonon-boundary scattering. Solutions of Boltzmann transport equation are often used to evaluate these effects in particular for thicknesses less than tens of nanometer. But this method cannot be used to investigate the impact of the amorphous oxide layer on phonon transport in the silicon layer. In this work, we apply a variance-reduced Monte Carlo (VRMC) method to study phonon transport in silicon-silica layers. The silicon-silica interface is modelled as a diffusely reflecting boundary, which is a reasonable initial approximation as the microscopic details of phonon scattering at interfaces is poorly understood.
In summary, we develop a suspended structure to measure the thermal conductivity of silicon thin films across a range of dimensions, while using a VRMC model to study how boundary scattering and the silicon-silica interface impact phonon transport. Combining the results of the VRMC model and the thermal conduction experiments allows detailed investigation of the phonon physics and thermal transport in thin silicon films.

High-energy optical phonons are preferred in phonon-absorbing transitions such as phonon-assisted photon absorption and direct phonon absorption. In this research, based on the interaction kinetics of the low-energy phonon upconversion, we study the efficient supply of the optical phonons which have higher energy and lower entropy. The interaction kinetics is dependent on the phonon distribution, and here we consider nonequilibrium native phonon distribution (generated by phonon flux in homogeneous system) and one by heterojunction transmitted phonons. For heterojunction, steady phonon flux from low cut-off layer (e.g., Ge) is transmitted to high cut-off layer (e.g., Si), creating nonequilibrium population of low-energy phonons for upconversion, and its phonon distribution is calculated using the quantum spectral phonon transmission.
Using the first-principles calculations of the phonon interaction kinetics, we identify the high conversion efficiency channels, i.e., mode and Brillouin zone (BZ) location. Junction-transmitted phonons despite suffering from reflection and spreading interactions with the equilibrium native phonons can be targeted for their high upconversion rate to BZ boundary optical phonons. The nonequilibrium native phonons are efficiently upconverted over most of the zone, with high rates in BZ locations not covered by the transmitted-phonon upconversion. So, depending on the harvested optical phonon, one of these nonequilibrium phonons can be selected for efficient upconversion rate. The approaches to phonon kinetics and distribution suggested in this work can be applied to various phonon engineering applications and interface thermal transport analyses.

Nanoscale heat transport phenomena in colloidal gold ensembles subjected to femtosecond laser heating
There are several potential industrial, medical, and environmental applications of metal nanorods that rely on the physics and resultant kinetics and dynamics of the interaction of these particles with light. While melting of single particles in solution has been extensively studied, the melting and heat transport phenomena in an ensemble of nanoparticles has not been explored to-date. We report a surprising kinetics transition in the global melting of femtosecond laser-driven gold nanorod aqueous colloidal suspension. We find, for the first time, a remarkable transition in the melting kinetics of gold nanorods: as the peak laser power is raised, the ensemble melting behavior transforms from stretched to compressed exponential kinetics. This transition is analogous to that in jamming systems such as granular materials but the physical origins are radically different, as shown by our extensive molecular dynamics simulations and analysis. It is found the relative formation and reduction rate of intermediate shapes play a key role in the transition. Supported by both molecular dynamics simulations and a kinetic model, the behavior is traced back to the persistent heterogeneous nature of the shape dependence of the energy uptake, dissipation and melting of individual nanoparticles. These results could have significant implications for various applications such as water purification and electrolytes for energy storage that involve heat transport between metal nanorod ensembles and surrounding solvents.

Modern society desires faster and higher performance devices, with consumers demanding for lightweight and space-efficient products. The heat generated as a result has increased exponentially. The need for effective solutions to deal with the large amount of heat produced becomes ever more critical. Metal foams produced by space-holder methods- Lost Carbonate Sintering (LCS) in particular, are ideal candidates for thermal management solutions (e.g. heat exchangers) due to their high surface area and high fluid permeability for fluids, allowing combined conduction and convection heat transfer power. The unique internal architecture of LCS foams dictates the fluid flow within the structure and therefore greatly affects the heat transfer performance. Firstly, the strong bonding of small metal particles allows efficient heat conduction. Secondly, the micro-pores within the LCS foams are individual spherical pores randomly distributed in a metal matrix and interconnected through windows much smaller than the pores. This allows high permeability for fluids hence increased heat transfer by conduction and convection. In this paper, we demonstrate the heat transfer capabilities of LCS copper foams. The effects of the unique microstructure and structural parameters (i.e. pore size and porosity) on the heat transfer performance of the LCS copper foams are also discussed. More importantly, promising results showing the effects of hybrid combinations and distribution of porosities on the overall heat transfer performance of the LCS copper foams are explored.

Creating thermal devices similar to a diode/rectifier and to transistors could lead to a technological revolution similar to the one undergone by electronics since the 1950s. Asymmetric configurations, such as non-uniform mass and periodic arrangements, and nonlinear lattices in 1D geometry have been investigated. Other works observed thermal rectification in confined objects without analyzing it in details. It has been proposed to use a column of asymmetric objects, such as triangles, as an acoustic rectifier. The reason is that acoustic waves sent perpendicularly to the sides of a periodic column of triangles can be reflected in a stronger way than the wave sent to the vertices, which leads to the fact that the transmittivity is different in these two cases. However, Maznev et al. have recalled that reciprocity should be fulfilled in linear systems, so that this acoustic phenomenon is more a “differential filtering effect” than proper rectification. Hence, if all angles of incidence are excited and their contributions summed up as they are in thermal processes, no thermal rectification should be observed. Our work deals with asymmetric phononic crystals.
In a first step, we consider a linear medium and do not investigate rectification. Instead, we analyze how a column of periodic objects set perpendicularly to the heat flux direction can be considered as a thermal barrier associated to a thermal resistance. We study numerically the transmission of the phonons through crystals with various geometries, including triangular-hole based crystals. This is realized by solving the elastic equation in 2D with the Finite Element Method. In contrast to previous acoustic works, we excite not only the acoustic waves that are perpendicular to the periodic direction, but also oblique waves, which are also required for thermal processes. The total transmission coefficients are calculated for the two following cases: (i) wave is sent to the bases and (ii) to the vertices of triangles. We verify numerically that transmissions in both directions are equal for a given geometry. We finally compute the thermal resistance associated to the thermal barrier, as a function of the shape of the objects, such as asymmetric triangles and symmetric circular holes.
In a second step, the non-linear acoustic effect is introduced to achieve rectification in this 2D structure. We observe that the non-linear property modulates the wave amplitudes and lead to a difference of transmissions which depends on the frequencies. The range of frequencies that can be used in the structure and the rectification coefficients are defined for various shapes of crystals at each frequency. We analyze especially the interplay between the asymmetric configuration and the strength of the non-linearity and provide guidelines for improved thermal rectification in solids, which requires the integration over the whole thermal spectrum.

We present some of the elements governing heat transport in bulk cellulose nanocrystal (CNC) films using molecular dynamics simulations and experimental evidence. A multiscale description is achieved by introducing the properties of a single cellulose crystal (e.g., d~ 5 nm, L~100 nm, high crystallinity, rod-like shape, populated surface chemistry) into phenomenological models containing crystalline orientation given by the order parameter. Although, CNC interfaces seem to limit the phonon propagation (i.e. CNC lateral dimension lower than phonon mean free path), the interfacial thermal resistance estimated between chemically unmodified CNCs is significantly lower than that exhibited by other materials with much higher thermal conductivity. New opportunities for enhanced control over the heat transport in advanced CNC-based materials can arise from a synergistic tuning of their unique nanoparticle features (e.g. surface bonding strength, orientation) and their liquid crystalline lyotropic behavior.

Near-field thermal radiation between a tip and a substrate has recently received keen attention to measure the electromagnetic local density of states (De Wilde et al., Nature 444, 740, 2006) and the nanoscale infrared spectrum of materials (Jones et al., Nano Lett. 12, 1475, 2012). In order to properly interpret the experimental observations of the tip-substrate near-field interactions, several models have been suggested to date. While a spherical point dipole over a semi-infinite medium has been considered as a base model (Mulet et al., Appl. Phys. Lett. 78, 19, 2001), the shape dependence of the tip was discussed by implementing a polarizability tensor for a spheroid dipole (Huth et al., Eur. Phys. J. Appl. Phys. 50, 10603, 2010), and self-interactions of a spherical dipole in proximity to the substrate was taken into account for more accurate modeling (Joulain et al., JQSRT 136, 1, 2014). However, there is no simplified model that includes both tip shape effect and dipole-surface interactions, while maintaining the simplicity of the dipole-substrate model.
In order to consider the shape effect of the dipole, an anisotropic polarizability tensor which accommodates directional dependence of the dipole polarization will be implemented to the dipole-substrate model. The Clausius-Mossotti polarizability for a spherical dipole can be viewed as a special case when the polarizability tensor becomes isotropic. The polarizability tensor can be further modified to take the dipole-surface interactions into consideration. To date, dipole-surface interactions have been considered for the spherical dipole case by formalizing an effective polarizability tensor from the dyadic Green&’s function (Joulain et al., JQSRT 136, 1, 2014). Similarly, we will compute the general polarizability tensor with dipole-surface interactions to compute near-field thermal radiation between a spheroid dipole and a semi-infinite medium. Four models (i.e., sphere/spheroid dipole-substrate models with/without dipole-surface interactions) will then be compared to examine the effect of the dipole shape and dipole-surface interactions to the tip-substrate near-field thermal radiation. These models will also be compared with the thermal discrete dipole approximation model (Edalatpour and Francoeur, JQSRT 133, 364, 2014).
This work will lead to a better understanding of near-field interactions between a tip and a substrate when thermal radiation is involved. The gap dependence of tip-substrate near-field thermal radiation will be correlated to far-field scattering, which is easier to measure and spectroscopically analyze for experiments. The developed model will be used for our future experiments to measure the spectrum of near-field thermal radiation and its relation to near-field thermophotovoltaic power generation.

For decades, there have been substantial demands of efficient thermal management at nanoscale for improving energy consumption and dissipation in IT devices, lab-on-a-chips and energy harvest/conversion systems. For many of these applications, it is much desired to have a structure that reversibly switches thermal conduction with high ON/OFF ratios and at high speed. Existing thermal switches use liquids or membranes actuated by electrowetting or electrostatic forces, all imposing limitations in different aspects. Here we report design and implementation of a novel solid-state thermal switch based on structural modulation of a van der Waals interface (vdW) with the phase transition in vanadium dioxide (VO₂) thin films. We design and fabricate VO₂ -based thin film stack and investigated its out-of-plane thermal property using the 3omega; method. Comparing to control devices, the thermal switch devices demonstrate a change in thermal conduction with high ON/OFF ratios. As the phase transition in VO₂ can be driven thermally, electrothermally and photothermally, it is expected that with external stimuli such asenvironment temperature change, electrical current or laser heating, the device would enable thermal switch with high performance, reliability, speed, and energy efficiency in a wide range of applications such as in temperature-sensitive systems, rapid temperature cycling, pulsed thermal power operation, or heat-based logic circuits.

The ultrafast nature of the Metal-Insulator transition (MIT) and structural phase transition (SPT) in VO₂ near room temperature (T_c ~ 340K) makes it an attractive material for applications in electronics and optical devices. However, the accompanying drastic changes in thermo-physical properties and their potential applications have rarely been reported. In this study, we investigate the thermal and electronic properties of VO₂ thin films across the transition temperature and explore their potential application in VO_2-based thermal switches in thermal devices. In addition, we report on the doping of VO₂ films with transition metals and its effect on the shifting of the transition temperature and the accompanying change in thermal conductivity across the MIT Thermal conductivities in VO₂ thin films are measured using the time-domain thermoreflectance (TDTR) method. First principles density functional theory (DFT) calculations are performed in conjunction with Boltzmann transport calculations to model the electronic transport properties of pure and doped VO₂. We will discuss the interplay of the phononic and electronic components of the thermal conductivity across the MIT in VO₂ films in light of the Wiedemann-Franz law.
Use of the Center for Nanoscale Materials was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

Dynamic control of thermal transport in solid-state systems is a transformative capability with the promise to propel technologies including phononic logic, thermal management, and energy harvesting. To date, a purely solid-state solution to rapidly manipulate phonons has escaped the scientific community. In this work, we study the phonon transport processes and thermal conductivity of various ferroelectric oxide nanostructures to demonstrate: 1) static control of the phonon transport based on coherent interfaces, and 2) active and reversible tuning of thermal conductivity by engineering the concentration of mobile interfaces (i.e., the ferroelastic domain structure of a ferroelectric film) with applied electric fields. We measure the thermal conductivity of various ferroelectric thin films with time domain thermoreflectance. Our experimental results focus on: (i) phonon/grain boundary scattering in nanograined strontium titanate (SrTiO₃) and barium titanate (BaTiO₃) (Appl. Phys. Lett. 101, 231908 (2012), Appl. Phys. Lett. 105, 082907 (2014)), (ii) coherent phonon transport in strontium titanate/calcium titanate (SrTiO₃/CaTiO₃) superlattices (Nat. Mat. 13, 168 (2014)), and (iii) phonon scattering with ferroelectic domain walls in bismuth ferrite (BiFeO₃) and lead zirconate titanate (PbZr_xTi_1-xO₃) (Appl. Phys. Lett. 102, 121903 (2012)). Regarding the phonon-domain boundary interaction, we demonstrate that 30 nm thick epitaxial films of ferroelectric BiFeO₃ with differing densities of 71° domain walls showed ~30% differences in thermal conductivity at temperatures up to 400 K, indicating domain wall phonon scattering effects may be observed at non-cryogenic temperatures. Reconfiguring this nanoscale domain structure and domain wall density with an applied field can create a simple and integrable thermal switch that can operate over a broad temperature range. In the final part of this talk, we demonstrate active and reversible tuning of thermal conductivity by engineering the ferroelastic domain structure of a ferroelectric film with applied electric fields. Using Pb(Zr_0.3Ti_0.7)O₃, piezoresponse force microcopy- and scanning electron microscopy-based domain imaging show that the domain wall densities increase after and during the application of a DC field With sub-second response times, the room temperature thermal conductivity was modulated by 11% in Pb(Zr_0.3Ti_0.7)O₃ from this density change, induced by an electric field. By altering the domain landscape, one can either enhance or subdue phonon scattering, which opens a new pathway for electrically controlled phonon thermal switching.

We present temperature dependent measurements for two types of “thermal switches” in which the thermal conductivity of the film is reversibly controlled via the application of an external electric field. Both switches operate on the principle of modulating the ferroelastic domain structures within the films, which affects the rate of phonon scattering at domain boundaries within the film. The bilayer film (30/70 tetragonal PZT top, 70/30 rhombohedral PZT bottom) exhibits a response where the domain wall density increases with increasing electric field magnitude, resulting in an up to ~12% decrease in the thermal conductivity under fields of up to ~500kV/cm. The strainshy; released films (52/48 PZT) exhibit an opposite response where the thermal conductivity increases with increasing electric field magnitude. Temperature dependent measurements were performed to elucidate the nature of the phonon scattering in these differing domain structures. This work introduces several new ideas for future research frontiers, including electrothermal devices and thermal management technologies at the nanoshy;scale.

Using analytical methods and numerical simulations, we demonstrate the possibility of one-way phonon transport in thin plate waveguides. This isolation is enabled via spatio-temporal modulation of the waveguide material properties, which permits symmetry breaking such that a vibrational mode entering the waveguide from one direction is converted into a different vibrational mode while the same mode entering from the other direction propagates unchanged.

A comprehensive understanding of acoustic phonon dynamics and thermal conductivity in silicon structures is essential for engineering thermal transport at the nanoscale. Phonon lifetimes are typically limited by intrinsic scattering due to the anharmonicity of the lattice and extrinsic scattering mechanisms such as scattering by impurities, defect, or boundaries (surfaces / interfaces) of the material. In conjunction with the group velocities, the lifetimes define the mean free paths of the phonons. Despite the fundamental importance of these parameters, accurate measurements of high frequency phonon lifetimes are challenging, and their values are still unknown for most materials. Even in the technologically most relevant case of Si, measurements of phonon lifetimes are scarce [1, 2] and the impact of stress or the modification of the dispersion relation by e.g. phononic crystals are largely unknown.
In this work, we investigate the influence of tensile stress on the thermo-mechanical properties of free-standing single crystalline Si nano-membranes with thicknesses between 8 and 100 nm. The membranes were fabricated utilizing silicon-on-insulator substrates. The stress was introduced by the drum-head straining technique where tensile SiN frame stretches the Si membrane in controlled manner [3]. The magnitude of the stress in the membranes as well as their spatial homogeneity was investigated using micro-Raman spectroscopy. A homogenous stress distribution was observed by Raman mapping of the membranes. The influence of stress on the lifetimes of the first order dilatational mode (D₁) was measured using an ultra-fast asynchronous optical sampling (ASOPS) technique. We observe that the lifetime of the D₁ phonons depends on the stress in the membranes with shorter lifetimes for higher tensile stress values. The influence of stress on the frequency of this mode is suitably explained using the elastic continuum model and finite-element simulations [4]. Furthermore, we study the modification of the lifetimes and the dispersion relation of phononic crystals based on patterned Si membranes using ASOPS and Brillouin scattering. Finally, we investigate the thermal properties of the stressed membranes and phononic crystals by two-laser Raman thermometry [5]. While the acoustic phonon dynamics strongly dependence on the stress in the membranes, the thermal conductivity is to a first approximation unaffected in the low stress regime. The combination of these techniques provides novel insight regarding the correlation of acoustic phonon dynamics, dispersion relation, and thermal conductivity in stressed and patterned Si nano-membranes.
[1] B. C. Daly et al., Phys. Rev. B 80, 174112 (2009)
[2] J. Cuffe et al., Phys. Rev. Lett. 110, 095503 (2013)
[3] A. Shchepetov et al., Appl. Phys. Lett. 102, 192108 (2013)
[4] B. Graczykowski et al., New. J. Phys. 16, 073024 (2014)
[5] J. S. Reparaz et al., Rev. Sci. Instr. 85, 034901 (2014)

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The aim of this talk is to introduce an atomistic description of thermal transport from an unified approach which is based on microscopic correlations. We will mostly discuss the radiative and conductive properties of different classes of nano-materials (semiconductors, oxydes, soft-matter..) by demonstrating that novel thermal properties of matter can be investigated by tracking the momentum and displacement fluctuations of atomic nucleus. The outline of the presentation is constructed in three parts : The first discusses infrared absorption in dielectric nano-objects and the violation of the classical electrodynamics at specific length scales. The next part introduces the Kapitza problem and the spectral decomposition of the thermal conductance at solid:solid but also solid:fluid interfaces. Finally, we present an estimator -so-called the coherence length - which unveils the particle versus wave nature of the phonons and consequently allows to taylor thermal properties of matter with an original and novel approach.
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Our inability to reliably and quantitatively calculate the conductance of different phonons across an interface between two dissimilar materials has been one of the major unresolved questions in thermal transport physics for the last century. Solving this problem can have a significant impact on microelectronics thermal management and other applications where thermal interface resistance is limiting and can aid in the rational design of thermal interface materials. A number of theories have been presented in this regard, but their predictive power is limited. A new formalism for extracting the modal contributions to thermal interface conductance with full inclusion of temperature dependent anharmonicity and all of the atom level topography is presented. The results indicate that when two materials are joined a new set of vibrational modes are required to correctly describe the transport across the interface. The new set of vibrational modes is inconsistent with the physical picture described by the ubiquitously applied phonon gas model (PGM). This issue arises because some of the most important modes are localized and non-propagating and therefore do not have a well-defined velocity nor do they impinge on the interface. Among these new modes, certain classifications emerge, as most modes extend at least partially into the other material. Localized interfacial modes are also present and exhibit a high conductance contribution on a per mode basis by strongly coupling to other types of vibrational modes. We apply our formalism to different realistic interfaces and for each case present thermal interface conductance accumulation functions, two-dimensional cross-correlation matrices to study the strength of interaction/correlation between different modes, and a quantitative determination of the contributions arising from inelastic effects. The provided new perspective on thermal transport across interfaces can open new gates towards deeper understanding of the physics of phonon-phonon as well as electron-phonon interactions around interfaces.

Advancing the understanding of thermal transport across interfaces has become indispensable to improve nearly all fields of nanoscience. We analytically derive and experimentally measure a thermal boundary conductance accumulation function. We validate this derivation by measuring the interface conductance between nine different metals on silicon substrate and five different metals on sapphire substrates. Measurements were performed at room temperature using time-domain thermoreflectance, an optical pump-probe technique for thermal characterization. The phonon cutoff frequencies in the metal films vary between 13.5 and 60 Trad s^-1 providing a variable bandwidth of phonons incident on the interface. The measurement results on Kapitza conductance across these interfaces can be compared to the accumulation of thermal boundary conductance and thus provide a direct method to experimentally validate the theory. In our formalism, we express the accumulation function as the product of an average interfacial phonon transmission function and the accumulation of the temperature derivative of the phonon flux incident on the interface. This allows for the determination of the average interfacial phonon transmission from the measured interface conductance, a quantity that has never been measured above cryogenic temperatures.

A multi-temperature Boltzmann transport equation method is built which accounts for the thermal transport by electrons and different phonon modes, and is used to model thermal transport in metal-dielectric heterojunctions. The phonon-phonon scattering rates between different phonon modes as well as the electron-phonon coupling rates between electron and different phonon modes are computed from first-principles methods. Specifically, both the electron-phonon coupling inside the metal and across the interface are included in our approach. Spectral phonon transmission across the interface is computed using the phonon wave-packet method. Our model is capable of capturing the mode-wise coupling and nonequilibrium between different heat carriers, and can predict the temperature fields of electrons and phonons in nanosized metal-dielectric heterojunctional systems. Au/Si and Au/SiO₂ are studied in this work as model systems, and both transient (mimic the ultra-fast laser pump-probe experiment) and steady-state cases are studied. It is found that the nonequilibrium between carriers play an important role in limiting the heat dissipation across the interface, and reducing such nonequilibrium or enhancing phonon transmission across the interface are beneficial to the heat dissipation from the metal to the dielectric substrate. This work can be important for thermal management in nanophotonic and nanoelectronic devices.

Using molecular dynamics simulations and a model GaN substrate - AlN adlayer interface we demonstrate that the thickness-dependent Kapitza resistance, R_K, between a substrate and thin (nanoscopic) film is a combined effect of internal and surface phonon scattering. In particular, when the phonon mean free path is larger than film thickness and the film surface is atomistically smooth, phonons transmitted from the substrate can travel ballistically in the thin film, be scattered specularly at the surface, and return to the substrate without energy transfer, thus reducing the effective phonon transmission coefficient. If the thin film surface scatters phonons diffusely, which is characteristic of rough surfaces, R_K is independent of film thickness and is equal to R_K that characterizes smooth surfaces in the limit of large film thickness. Our result is useful in calculation of the R_K of isolated interfaces. With very rough surfaces, the size effect on R_K can be essentially eliminated and a good estimation of R_K of an isolated interface can be obtained with system sizes much smaller than phonon mean free path.

Single-layer molybdenite (MoS₂) is a two-dimensional material with a band gap that renders it attractive for applications in logic devices and photonics. As with other 2-D materials, a key challenge in understanding fundamental transport and realizing applications is deciphering the interactions of the material with the substrate. We discuss recent and ongoing work in understanding electrothermal transport in single-layer molybdenite. Specifically, we discuss the Seebeck coefficient and thermoelectric power factor of SL-MoS₂ [1], phonon relaxation physics from ab initio calculations and measurements of substrate-film interaction [2].
1. H. Babaei, J. Khodadadi, S. Sinha, "Large Theoretical Thermoelectric Power Factor of Suspended Single-Layer MoS₂", Applied Physics Letters, 2014.
2. D. Ganta, S. Sinha and R.T. Haasch, “2-D Material Molybdenum Disulfide Analyzed by XPS”, Surface Science Spectra, 21, 19 (2014).

Two-dimensional (2D) transition metal dichalcogenides (TMDs) are an interesting class of materials that are receiving increasing attention due to their unique electronic, optical and thermal properties. The layered crystal structure of these materials allows for the insertion of chemical species into the inter-layer space. This could provide a reversible method to modify their properties at the nanoscale, and a rare opportunity to controllably tune their thermal conductivity (particularly in the cross-plane direction) depending on the specific application.
In this work, we examine the impact of intercalated lithium (Li) ions on the thermal properties of thin films of MoS₂ down to the order of tens of nanometers in thickness. We use in-situ time-domain thermoreflectance (TDTR) to monitor changes in the thermal conductivity of MoS₂ films (k_MoS2) while simultaneously performing intercalation and de-intercalation of Li⁺ ions. Changes in k_MoS2 are dynamically measured by recording changes in the ratio of the in-phase to out-of-phase voltage components of the reflected probe intensity at fixed, short delay times. This in-situ experiment is enabled by the fabrication of planar MoS₂ nanobatteries, where single flakes of MoS₂ are used as an anode to a Li reference electrode, and are encapsulated inside a transparent electrochemical cell filled with electrolyte. The transparent viewing window on the top-side also allows for in-situ optical observations of morphological changes in the MoS₂ flake, as well as spectroscopic measurements to help understand the physical and chemical changes occurring during the intercalation process.
This ability to reversibly tune thermal conductivity via electrochemical modulation at the nanoscale could enable new applications in energy conversion and thermal management, where the dynamic control of heat transfer is a potentially useful feature. Furthermore, these types of experiments in 2D systems could be a new way to study directly how thermal transport at nanometer length scales is affected by changes in the physical and chemical properties of a material.

We use pump-probe metrology based on the magneto-optic Kerr effect to measure the anisotropic thermal conductivity of a (001)-oriented MoS₂ single crystal. A asymp;20 nm thick CoPt multilayer with perpendicular magnetization serves as the heater and thermometer in the experiment. The low thermal conductivity and small thickness of the CoPt transducer improve the sensitivity of the measurement to lateral heat flow in the MoS₂ crystal. The in-plane thermal conductivity of CoPt transducer is measured to be 27±4 W m^-1 K^-1. The thermal conductivity of MoS₂ is highly anisotropic with basal-plane thermal conductivity varying between 85-110 W m^-1 K^-1 as a function of laser spot size. The basal-plane thermal conductivity is a factor of asymp;50 larger than the c-axis thermal conductivity, 2.0±0.3 W m^-1 K^-1. The measurement on the anisotropic thermal conductivity of MoS₂ single crystal provides a baseline value for further studies of thermal conductivity of dichalcogenide 2D materials and thermal management of 2D electronic devices.

Thermal conductivity values of MoS₂ have recently been reported^{[1, 2]}. In these previous works, thermal conductivity values were evaluated from Raman spectroscopy measurements, but without care for the measurement environment and optical absorbance. As a result, the reported values for MoS₂ suffer from overestimated optical absorbance and parasitic effects of the ambient measurement environments.
In this presentation, we suggest an improved approach for measuring thermal conductivity of a suspended MoS₂ film. The mono-, bi- and multi-layer MoS₂ synthesized by chemical vapor deposition (CVD) films were placed over holes in a gold thin film substrate. We measured and analyzed the temperature-dependent Raman spectra of in-plane E¹_2g and out-of-plane A_1g Raman modes for the suspended MoS₂ films in the range of 300~420 K using confocal Raman microscopy with a vacuum environment.
The numerical calculations of heat diffusion equations were performed to extract the room temperature values for mono-, bi and multi-layer MoS₂. For evaluating thermal conductivity, we use information from the local temperature rise at the excitation laser spot, which is monitored by the Raman shift change of in-plane E¹_2g and out-of-plane A_1g Raman modes, while also taking into account measured optical absorbance values for the number of MoS₂ layers.
We find that the room temperature value of thermal conductivity for mono-layer MoS₂ from our evaluation was lower than that of previous reported papers^[1]. Evaluated results of thermal conductivity for CVD grown mono-, bi- and multi-layer MoS₂ will be shown and discussed in the context of use to measurement environment and optical absorbance examined by Comsol multiphysics simulation.
This work was supported by IBS-R011-D1
[1] R. Yan, et al., ACS Nano, 8, 986 (2014).
[2] S. Sahoo, et al., J. Phys. Chem. C, 117, 9042 (2013).

Wearable electronic is now the upcoming trend for consumable electronics and the development of substrate material has been focusing on flexibility, stretchable, electrically insulative and moldable. While these basic characteristics can be largely fulfilled by various polymer based platforms, more advanced characteristics that are needed includes insulating materials with EM compatibility, high thermal dissipation and other properties tuning capabilities for catering to a wide diverse market. Although the infusion of highly thermal conductive nanofillers within the polymer matrix, such as graphene, carbon nanotube and metallic nanoparticles have shown improvements of the overall conductivity, there are still considerable challenges, such as inhomogeneous distribution of the nanofiller, aggregation and low filling fraction. Another critical concern is the poor long range thermal conduction seen in many of these composites as only a fraction of these individual nanomaterials are coupled together (weakly with Van der Waals forces) and most of the fillers are generally encapsulated entirely by the polymer matrix.
On the other hand, the recent reported synthesize of three-dimensional (3D) foam-like materials has demonstrated a new way of interconnecting these nanomaterials into 3D space. These 3D porous materials retain many of the unique properties of its constituent 2D material while creating a bulk, low density, flexible and ultralight version of its “bulk” form. The interconnected structure also prevents the inhomogeneous distribution, aggregation that is commonly faced by nanofillers and enhances the electrical and thermal conductivity of the whole structure.
Here, we would like to present the use of our 3D-Boron Nitride- Carbon (BNC) within conventional polymers used for flexible electronics. It is a 3D-interconnected material that we have developed recently through controllable hybridization of 3D-carbon foam and 3D hexagonal-boron nitride (3D-BN) into a wholly new 3D-BNC network with different C:BN composition [1]. The 3D composite itself has already showed highly tunable electrical conductivity and EMI shielding effectiveness while maintaining a high thermal conductivity. By infusing 3D-BNC into polymers, we could see an improvement of the overall thermal conductivity by a few orders and a wide range of tunable electrical conductivity and EMI shielding effectiveness. The mechanical strength and robustness of these materials will also be discussed.
This new 3D infused polymeric material could be ideal for electronics requiring a wide range of specific performance matrix (i.e. thermal, electrical, EM) and addressing the thermal related challenges of current flexible substrates.
1. Loeblein, M., et al., Configurable Three-Dimensional Boron Nitride-Carbon Architecture and Its Tunable Electronic Behavior with Stable Thermal Performances. Small, 2014. 10(15): p. 2992-2999. (Featured in the front cover (inner) of Small, Issue No. 10, Volume 15)

We measured the thermal conductance of a self-assembled monolayer (SAM) junction using frequency domain thermoreflectance experiments. Junctions having the configuration Au-SAM-X (X = Au, Ag, Pt or Pd) were fabricated, allowing for examination of the role played by the mismatch of the vibrational density of states between the leads and the SAM. We find that elastic scattering events dominate thermal transport at the interfaces, leading to a decrease in junction thermal conductance with an increase in lead vibrational mismatch. Molecular dynamics simulations were performed to further probe the thermal transport through these junctions. We resolved a long-standing discrepancy between experimental measurements and simulation predictions of junction thermal conductance by employing a contact mechanics model for rough surfaces to predict the percentage contact area of the SAM junctions. This analysis was informed by atomic force microscopy (AFM) measurements of the surface profile of the metal leads and models for the adhesion behavior of the SAM with Au.

This presentation will discuss our recent work examining large thermal conductivity enhancements in polymer blends. We show that a dense and homogeneous thermal network can be formed with certain pairs of polymers that have strong hydrogen bonds which form very close to the polymer backbones. We also summarize our computational studies of heat transfer between two polymer chains.

When the size of semiconductors is smaller than the phonon mean free path, phonons can carry heat with no internal scattering. Ballistic phonon transport has received great attention for understanding heat conduction in nanomaterials and managing heat dissipation in nanoelectronics. A recent experiment showed the ballistic thermal conductivity in SiGe nanowires in the length scale of 1-8 µm. However, the thermal conductivity demonstration in the length scale of 10-100 nm range, or below the phonon mean free path in silicon, is not available despite its direct relevance to nanoscale transistors and future phononic devices. Here we show ballistic phonon transport prevails in the cross-plane direction of holey silicon in the length scale of 35-200 nm. The thermal conductivity scales linearly with the length (thickness) even though the lateral dimension is as narrow as 20 nm. Ballistic phonon transport dominates heat conduction across the holey silicon nanostructures. We assess the impact of long-wavelength phonons and predict a transition from ballistic to diffusive regime using scaling models. The frequency dependent boundary scattering, accounting for specular scattering with surface disorder, is responsible for the strong size effect and the non-classical low temperature dependence. These results are particularly important for managing heat generation in silicon transistors that are scaling much below the phonon mean free path. Our results can also provide new pathways for manipulating phonons in thermoelectric energy-harvesting and phononic information-processing applications.

The thermal conductivity of superatom molecular solids composed of fullerene molecules and similarly sized inorganic clusters was measured. Electron-poor fullerenes were mixed with different electron-rich superatoms [e.g., Co₆Se₈(PEt₃)₆, Cr₆Te₈(PEt₃)₆, Ni₆Te₈(PEt₃)₈] to create ionic molecular crystals. Prior studies show that these crystals exhibit moderate electrical conductivity (resistivity ~10 ohm-cm) with activated electronic transport (activation energy ~ 100 meV). Single crystals ~100 mm in size were coated with Au and their thermal conductivity was measured using frequency domain thermoreflectance. Low thermal conductivities ranging from 0.1-0.3 W/m-K were measured at room temperature. Measurements from 80-300 K showed decreasing thermal conductivity with increasing temperature, consistent with crystalline behavior. With comparable thermal conductivities to pure fullerene solids and a vastly tunable electronic structure due to the versatility of the inorganic cluster, these molecular solids offer promise for thermoelectric energy conversion.

Metal-organic frameworks (MOFs) are extended, crystalline compounds consisting of metal ions interconnected by organic ligands such as benzene carboxylates, forming a crystalline, nanoporous structure. The low atomic density and long, bridge-like bonding characteristic implies that MOFs should exhibit low thermal conductivity, which is an attractive feature for thermoelectric energy conversion. However, most MOFs are electrical insulators due to the non-conjugated character of the organic ligands and poor overlap between their pi orbitals and the valence orbitals of the metal ions. Recently, we discovered that infiltrating the pores of the copper-containing MOF Cu₃(BTC)₂ with redox-active guest molecules TCNQ (7,7,8,8-tetracyanoquinododimethane) increases the electrical conductivity of thin film devices by as much as seven orders of magnitude [1]. Density functional theory indicates that electrical conductivity results from a donor-bridge-acceptor geometry, in which TCNQ binds to two Cu(II) dimer units within the MOF pore. In this paper we report the first thermoelectric properties characterization of a MOF thin film which yield a positive Seebeck coefficient of ~400 uV/K, in qualitative agreement with DFT calculations which also indicate that holes should be the dominant charge carriers in TCNQ@Cu₃(BTC)₂. Finally, we present results of molecular dynamics simulations which indicate that the phonon thermal conductivity of Cu₃(BTC)₂ MOF is indeed low, thus further suggesting that conducting MOFs are promising materials for thermoelectric energy conversion.
[1] A. A. Talin, A. Centrone, M. E. Foster, V. Stavila, P. Haney, R. A. Kinney, V. Szalai, F. El Gabaly, H. P. Yoon, F. Léonard, M. D. Allendorf, Science 343, 66 (2014)

Molecular junctions hold significant promise for efficient and high power output thermoelectric energy conversion. Recent experiments have probed thermoelectric properties of molecular junctions. However, active control of thermoelectric properties—which is critical for deeper insights—has not been possible due to technical challenges. In this talk we will describe first how by careful thermal design and nanofabrication of devices extremely large temperature gradients, exceeding 10⁹ K/m, can be established in nanoscale gaps bridged by molecules. Next we will describe how the electronic structure of molecules can be tuned via a gate electrode. Finally, we will present results from studies of prototypical Au-biphenyl-4,4&’-dithiol-Au and Au-fullerene-Au junctions that unambiguously demonstrate that the Seebeck coefficient and the electrical conductance of molecular junctions can be simultaneously increased by electrostatic control. Further, from our studies of fullerene junctions we will describe how the thermoelectric properties can be dramatically enhanced when the dominant transport orbital is located close to the chemical potential.

A precise determination of the thermal conductivity (κ) of a given material is usually a difficult task since the heat per unit time flowing in a certain spatial direction (Q equiv; Pabs) must be precisely determined. Contrary to the analogous case of electrons (or holes) propagating in a metal, where the electric current can be easily measured using a galvanometer in the magnetic field created by the current, there is no direct method to measure heat currents. Instead, the heat flux per unit time is usually inferred considering the geometry of the system and the excitation source; the temperature gradient (part;T/part;r) is measured in the heat flow direction and, finally, the thermal conductivity is obtained through Fourier&’s law [κ=Pabs/(part;T/part;r)]. Optical methods to determine the thermal conductivity have recently attracted considerable attention since most of them are contactless and, thus, require little or no sample preparation.
Here, we investigate the thermal properties of free-standing Si membranes [1][2] in the range between 8 and 1500 nm in thickness using a novel optical contactless technique recently developed for such purpose: 2-laser Raman thermometry (2LRT) [2]. Through investigating the dependence of the thermal decay with the thickness of the membranes we estimate the average thermal phonon mean free path in the range between 100 and 200 nm at room temperature in good agreement with previous determinations. These observations arise from the deviations observed from the spatial logarithmic thermal decay expected in two dimensional systems originating from diffusive thermal transport as explained by the Fourier's law in membranes with thicknesses below 200 nm. We also show that in ultra-thin membranes with 8 nm in thickness, for which the thermal phonon mean free path is well above their thickness, the thermal decays cannot be explained using this simple approach. We propose a model which account for the interaction of the thermal phonons with the surface of the membranes through the specularity parameter to determine the thermal bandwith of the phonon spectra which contribute most efficiently to heat transport. As a striking result we show that the thermal conductivity of such membranes is reduced below 2 W/mK at 750 K, i.e., approximately a 50-fold reduction with respect to its value at room temperature, which has a huge impact on the thermoelectric performance.
References:
[1] A. Shchepetov, M. Prunnila, F. Alzina, L. Schneider, J. Cuffe, H. Jiang, E. Kauppinen, C. M. Sotomayor Torres and J. Ahopelto, Appl. Phys .Lett. 102, 192108 (2013)
[2] E. Chávez-Ángel, J. S. Reparaz, J. Gomis-Bresco, M. R. Wagner, J. Cuffe, B. Graczykowski, A. Shchepetov, H. Jiang, M. Prunnila, J. Ahopelto, F. Alzina, and C. M. S. Torres, APL Mater. 2, 012113 (2014)
[3] J. S. Reparaz, E. Chavez-Angel, M. R. Wagner, B. Graczykowski, J. Gomis-Bresco, F. Alzina and C. M. Sotomayor Torres, Rev. Sci. Instr. 85, 034901 (2014)

Direct measurements of thermal transport and the mean free paths of acoustic phonons that contribute to it are crucial for fundamental understanding at a microscopic level and for applications ranging from thermoelectrics to nanoelectronics in which either very slow or very fast transport is desired. We have conducted transient grating measurements of free-standing submicron silicon membranes, in which spatially periodic heating in the plane of the sample is followed by monitoring of thermal transport from the heated to unheated regions [1]. The spatial period defines the length scale over which thermal transport is measured. When the spatial period is reduced below 15 microns, clear deviations from diffusive kinetics are observed, revealing the role of phonons with mean free paths comparable to or exceeding the thermal transport length scale. The transport kinetics are also influenced by the silicon layer thickness due to surface scattering effects on the phonon mean free paths. These effects were seen directly in separate photoacoustic measurements of coherent 270-GHz phonons [2]. Measurements of the thermal transport kinetics as a function of membrane thickness allowed detailed elaboration of the contributions of different parts of the phonon spectrum to thermal transport [3] and facilitated comparison to first-principles theoretical calculations [4]. In separate work that will be discussed briefly, transient grating measurements of acoustic phonons in silica microsphere monolayers on an Al-coated fused silica substrate were conducted and revealed the mixing of surface acoustic waves with a contact-based resonance of the spheres [5]. This opens the door to study of contact mechanics and adhesion through measurements of the acoustic behavior in linear and nonlinear response regimes. 1. "Direct measurement of room-temperature nondiffusive thermal transport over micron distances in a silicon membrane," J.A. Johnson, A.A. Maznev, J. Cuffe, J.K. Eliason, A.J. Minnich, T. Kehoe, C.M. Sotomayor Torres, G. Chen, and K.A. Nelson, Phys. Rev. Lett. 110, 025901 (2013). 2. “Lifetime of high-order thickness resonances of thin silicon membranes,” A.A. Maznev, F. Hofmann, J. Cuffe, J.K. Eliason, and K.A. Nelson, Ultrasonics 56, 116-121 (2015). 3. “Reconstructing phonon mean free path contributions to thermal conductivity using nanoscale membranes,” J. Cuffe, J.K. Eliason, A.A. Maznev, K.C. Collins, J.A. Johnson, A. Shchepetov, M. Prunilla, J. Ahopelto, C.M. Sotomayor Torres, G. Chen, and K.A. Nelson, arXiv:1408.6747 (2014). 4. “Heat transport in silicon from first-principles calculations,” K. Esfarjani, G. Chen, and H. Stokes, Phys. Rev. B 84, (2011). 5. "Interaction of a contact resonance of microspheres with surface acoustic waves," N. Boechler, J.K. Eliason, A. Kukmar, A.A. Maznev, K.A. Nelson, and N. Fang, Phys. Rev. Lett. 111, 036103 1-5 (2013).

The blossoming of nanotechnology involving the miniaturization of devices with enhanced rates of operation requires a profound understanding and optimization of their thermal performance. This is particularly critical in nanomaterials, which undergo a reduction of their thermal conductance as the size is scaled down. Surface phonon-polaritons (SPPs) have a propagation length much longer than that of phonons and hence they could have the potential to enhance the energy transport through these materials [1]. However, the contribution of these polaritons to the heat transport is not well understood to date, especially in absorbing media.
In this work, the SPP contribution to the heat conduction along nanofilms and nanowires of different polar materials is investigated in detail. Based on the Maxwell equations, the Boltzmann transport equation, and the Landauer formalism, it is shown that: (1) the SPP thermal conductivity of a nanofilm increases as its thickness reduces, and it can be higher than the one of phonons. A SPP thermal conductivity of 2.5 W/m.#159;K is obtained for a 30 nm-thick film of SiO₂ at room temperature, which is about 1.8 times larger than its phonon counterpart [1]. (2) The SPP thermal conductance of polar nanowires is independent of the material characteristics and is given by π²k_B²T/3h, where k_B and h are the Boltzmann's and Planck's constants, respectively and T is the temperature. This universal quantization holds not only for a temperature much smaller than 1 K, as is the case of electrons and phonons, but also for temperatures comparable to room temperature [2]. (3) Simple and symmetrical Fresnel-like formulas for the reflection and transmission coefficients of a SPP propagating along the surface of a nanofilm and crossing the interface of two dielectric media are analytically determined [3]. These formulas are consistent with the principle of conservation of energy and they can be used to quantify the SPP energy distribution at dielectric interfaces. The set of obtained results could have great applications in the thermal management of nanoscale electronics, phononics, and photonics.

Boundary scattering of phonons is an important physical process that remains poorly understood. Whether a phonon scatters specularly or diffusely from a rough material boundary has not been experimentally measured, and theoretical predictions are still based on Ziman&’s phonon specularity model introduced over 50 years ago. Here we report the first direct measurement of the wavelength-dependent phonon specularity parameter by performing transient grating experiment on free-standing silicon membranes with controlled surface roughness, from room temperature to cryogenic temperatures. Our work provides direct experimental insights into the roughness scattering mechanism for phonons that will inform applications such as the coherent manipulation of thermal phonons and heat dissipation in LEDs.

Surface phonon polaritons are coupled states of polar atomic vibrations (optical phonons) and electromagnetic waves that are known to enhance near-field thermal radiation from polar materials.Theory suggests that surface phonon polaritons could also increase the thermal conductivity of nanoscale materials under certain conditions. This concept, however, is difficult to demonstrate experimentally, especially at room temperature. Here we design, predict, and demonstrate a different approach to observe thermal energy conduction by surface phonon polaritons - that is, a surface phonon polariton crystal. Packed beds of silicon dioxide nanoparticles have ultralow thermal conductivity at room temperature and large internal surface area, such that, when several water molecules are adsorbed on the nanoparticles to increase the effective relative permittivity of their surrounding medium (a mix of air and water), thermal energy conduction from surface phonon polaritons can dominate that generated by phonons. Using this material, we resolve experimentally a surface phonon polariton thermal conductivity that is as high as 1.2 times the phonon value near room temperature. Long-range coupling of surface phonon polaritons in the ordered nanoparticle bed, analogous to the extension of phonons in an atomic crystal, enable this enhancement. In contrast to expectations, the surface phonon polaritons dictate the total thermal conductivity of the material due to an apparent quenching of phonon conduction. The effects of nanoparticle diameter and surrounding medium on thermal conductivity data are in excellent agreement with theory, providing an unambiguous case for significant thermal energy conduction by surface phonon polaritons. We anticipate that the theoretical, and practical, framework established here will enable heat conduction by surface phonon polaritons to be explored in detail experimentally, to build on recent interesting theoretical predictions, and to potentially provide an alternative route to engineer thermally conductive dielectric materials for thermal management.

Regarding such problems of conventional energy harvesting as materials choice and limited efficiency of modern devices, thermoelectricity may play an important role in future energy harvesting. Si is one of promising materials for thermoelectric applications due to its high compatibility with microelectronics and possibilities to improve its thermoelectric properties by nanopatterning. In the present work we aim to demonstrate the increase of figure of merit (ZT) in Si, by increasing the phonon scattering due to the polycrystallization and phononic patterning.
We fabricated three types of poly-Si air-bridged phononic nanostructures and measured their thermal conductivities at room temperature. The phononic structures were fabricated on an SOI wafer with a 145-nm-thick top undoped poly-Si layer frown by LPCVD, using lithography and etching processes. The grain sizes of the poly-Si was measured by TEM images and it ranges from several nm to 50 nm. Thermal conductivity of the samples was measured by originally developed micro time domain thermoreflectance method and a fitting by simulation data obtained by FEM.
The measured thermal conductivity of a poly-Si membrane was 11 W/mK, which is 15% of a single crystalline Si membrane with the same thickness. The reduction stems from mostly grain boundary scattering and increased surface scattering at the rough top surface. Three types of Poly-Si nanowires were prepared, with 172nm, 125nm and 106nm in width. Each of these wires displayed a thermal conductivity reduced even further than the membrane, down to 6.0, 5.7 and 5.2 W/mK respectively. These results show that the impact of the width is stronger in Poly-Si nanowires compared to their single-crystalline counterpart [1]. In a similar way, patterning 2D PnC with circular air-holes aligned in a square lattice (period: 200 nm, air-hole diameter: 150 nm) allowed us to reach the value of 1.5 W/mK, which is nearly on par with the amorphous SiO2 thermal conductivity of 1.4 W/mK. The reduction of the thermal conductivity for a poly-Si membrane compared to a single-crystalline one is due to increased phonon scattering rate by grain boundary and more diffusive scattering at the rough top surface. Further large reduction, from 11 to 1.5 W/mK, by phononic patterning is the main discussion of this work. In poly-Si, phonons with relatively short mean free path are already blocked by the grain boundary and main heat carriers are phonons with relatively large mean free path. The phononic patterning with a period of 200 nm functions as an efficient phonon scattering center. These two scattering sources with different scale effectively block phonon transport and resulted in very low thermal conductivity.
Reference
[1]#12288;J. Maire and M. Nomura, Jpn. J. Appl. Phys., 53 06JE09 (2014)

Vertically-aligned metal nanowire (NW) arrays can be effective thermal interface materials owing to the combination of high thermal conductivity and mechanical compliance. In the present work, copper NW arrays are synthesized via template-assisted electrodeposition, where a sacrificial porous membrane is used to mask a substrate while copper is electrochemically deposited into the pores. Both polycarbonate track-etched membranes and anodized aluminum oxide membranes are commonly used as templates due to their large areas and high degree of aligned pores with tunable properties without the need for lithographic processing. Using this method, NWs are fabricated with diameters ranging from 50-1000 nm and lengths up to 30 microns. The constituent NWs are synthesized to be nominally vertically-aligned and uniform over centimeter-sized length scales. The effective thermal conductivity of the NW array is measured using the 3-omega method, which is implemented in the present work by synthesizing the NW array directly above an electrically-isolated, microfabricated metal heater/thermometer. We report a measured cross-plane (i.e. parallel to the NWs) effective thermal conductivity of the array to be k_eff = 37 W m^-1 K^-1 for 12% dense copper NWs. These arrays are highly anisotropic due to the preferential vertical-alignment of the NWs, where cross-plane conduction is primarily facilitated by axial conduction along the NWs while in-plane conduction is predominately limited by interactions between adjacent NWs. Consequently, the lateral (i.e. perpendicular to the NWs) thermal conductivity is found to be nearly an order of magnitude smaller than the cross-plane conductivity. The effective thermal conductivity of a NW array is then correlated to both the morphology of the array and the thermal conductivity of the individual NWs, which includes contributions due to grain boundaries and size effects. This becomes particularly interesting as the NW diameters approach the room temperature mean free path of the conduction electrons in the bulk material (~40 nm in copper). Further consideration is then given to optimizing the transport properties of metal NW arrays for thermal interface applications, including the effects of array geometry, bulk metal conductivity, and interstitial matrix material. These results are compared to effective medium theory models to predict effective transport properties in composite media based on the individual properties of the constituent phases.

I will provide strong experimental evidence that micron-scale ballistic thermal conduction can be found in both homogeneously alloyed SiGe nanowires and heterogeneously-interfaced Si-Ge core-shell nanowires exhibiting low thermal conductivities. Because alloy scatterings in these materials filter out most high frequency phonons, an unexpected low percentage of phonons carries out the heat conduction process in these SiGe nanowires. The suppressed thermal conductivity is accompanied with an elongation of phonon mean free paths more than 5mu;m. Moreover, the ballistic thermal conduction is insensitive to twin-boundaries, defects, and local strain. Our result indicates that the heterogeneous interfaces can induce phonon localization in the body of a Si-Ge core-shell nanowire, much more effective than previous theoretical expectations. The discovery will help to realize wave-engineering of phonons at room temperature and open new paradigms for heat managements.

Heat transport properties in low dimensional structures are no longer the ones of the bulk state and they depend among other parameters on the geometry at the microscopic scale. For example, nano-devices like superlattices, thin films, nanotubes and nanowires exhibit outstanding phononic and electronic properties. Phonon engineering can be achieved by nanostructuration. In this study, we focus on the nano-constriction effect on the thermal transport properties of individual silicon nanowires. The purpose of our work is to appraise the thermal conductance G in these constrictions and to predict the variations of the overall thermal conductivity (TC) in modulated silicon nanowires. We will demonstrate that the shape of the constriction as well as its magnitude can significantly alter the thermal properties through the modification of the phonon mean free path (MFP) in the considered nanostructures [1]. All the thermal properties are derived from the resolution of the Boltzmann transport equation for phonons in the framework of the relaxation time approximation by Monte Carlo simulations [2,3].
An important parameter in our study is the shape of the constrictions (smooth and steep). We compare the thermal conductivity of the modulated nanowires to the one of a nanowire without constriction. The simulation data pointed out the fact, that the constriction shape notably modifies the nanowire thermal conductivity by the introduction of a new resistive process. In the case of a steep constriction, the TC is reduced when compared to the unconstricted nanowire because of the supplementary internal thermal resistance. This effect is even enhanced for smooth constriction and is also observed in corrugated nanowires. An explanation could be proposed considering the geometric MFP variations for steep and smooth constrictions as well as taking into account a new characteristic length that lies on the periodicity of modulation [4].
More details about the involved mechanisms and results for a broad range of constriction shapes will be given during the presentation of this study.
References
[1] R. Prasher, "Predicting the thermal resistance of nanosized constrictions", Nano Letters, 5 (11), 2155-2159, 2005
[2] D. Lacroix, K. Joulain, and D. Lemonnier, "Monte Carlo transient phonon transport in silicon and germanium at nanoscales", Phys. Rev. B 72, 064305, 2005
[3] J.-P. M. Péraud and N. G. Hadjiconstantinou, "Efficient simulation of multidimensional phonon transport using energy-based variance-reduced monte carlo formulations", Phys. Rev. B 84, 205331, 2011
[4] X. Zianni, V. Jean, K. Termentzidis, and D. Lacroix, "Scaling behavior of the thermal conductivity of width-modulated nanowires and nanofilms for heat transfer control at the nanoscale", Nanotechnology, in press, 2014

Nano-structuring has been proven to be one of the most effective approaches to engineer thermal conductivities of solids. Numerous studies have shown that that the phonon mean free path (MFP) is limited by the characteristic size of crystalline nanostructures, known as the boundary scattering or Casimir limit, whereas amorphous materials usually exhibit low thermal conductivity due to their disorder nature. Here, we show that crystalline Si nanotubes with shell thickness down to ~5nm, comparable to the dominant phonon wavelength in Si at room temperature, exhibit an extremely low thermal conductivity of ~ 1.1W/m-K, which is lower than the apparent boundary scattering limit and is even lower than that of amorphous Si nanotubes with comparable sizes. This finding is contrary to the prevailing general notion that amorphous materials represent the lower limit of thermal transport, but can be explained by the strong elastic softening effect that was observed in the crystalline Si nanotubes but was absent in amorphous ones. In addition, we also found a strong size effect on thermal conductivity of amorphous Si nanotubes, which suggests long mean free path associated with propagons in amorphous Si, as revealed by recent measurements and atomistic simulations.

Thermal conductivity of silicon nanowires has been studied in great detail in both smooth-surface Vapor-Liquid-Solid (VLS) nanowires as well as rough Electrolessly Etched (EE) nanowires. In this study, we use the newly developed electron-beam based micro-electrothermal device technique to study the length-dependent thermal conductivity of mesoporous silicon nanowires that have a single-crystalline scaffolding. We observe diffusive thermal transport along the length of the nanowire (with ~20nm resolution) and are able to determine the cross-sectional area of the nanowires directly using the e-beam absorption. The apparent thermal conductivity of the nanowires approaches the amorphous limit, with observed k < 1 W/m-K at 300K.

The remarkable and versatile properties exhibited by single-walled carbon nanotubes (SWCNTs) have led to interest in their use as active electronic materials in a variety of energy-related applications, such as photovoltaic and thermoelectric devices. Despite this drive toward their use in applied research there is still considerable effort devoted to a fundamental understanding of their physical properties, particularly in device-relevant architectures (e.g. thin films and composites).
We present a study focused on expanding the understanding of the electronic and thermal properties of carbon nanotube composites composed of enriched semiconducting SWCNT networks dispersed in a semiconducting polymer matrix. The s-SWCNT diameters and band gap distributions can be sensitively tuned through judicious choice of the semiconducting polymer, allow us to probe the effect of the electronic properties of the carbon nanotubes on the thermoelectric performance of the composites. We employ a stable charge-transfer dopant to impart fine control the density of carriers in the s-SWCNT networks, and demonstrate that their performance is comparable to neat p-type and n-type s-SWCNT networks doped by either nitric acid or hydrazine treatments, despite the presence of the semiconducting polymer.
Initially, we employ a suite of measurement techniques to probe the relationship between the electrical conductivity and Seebeck coefficient (thermopower) in the s-SWCNT networks as a function of the carrier density and position of the Fermi energy. At very low carrier densities we have measured a thermopower as high as ~2,500 µV/K, which is more than an order of magnitude larger than has been previously reported for SWCNT-based material systems. Although the thermopower does decrease with increasing carrier density we are able to maintain a value above 200 µV/K even up to conductivities of ~2,000 S/m, resulting in a thermoelectric power factor of ~100 µW/mmiddot;K². These studies suggest that the low dimensionality of the SWCNTs allows the electrical conductivity to be increased without significant detrimental impact on the thermopower, implying that the properties are less strongly coupled in these systems than is observed for compound inorganic semiconductors. Finally, we use a sensitive technique, based on a microfabricated silicon nitride thermal isolation platform, to probe the thermal conductivity in the s-SWCNT networks, allowing us to estimate the thermoelectric figure of merit (zT) for these materials to be ~ 0.01. Although the value is low these observations demonstrate the ability to exert exquisite control of the thermoelectric performance by tuning the carrier density and/or Fermi energy, and touts carbon nanotubes as an avenue for realizing thermally-stable, low-temperature thermoelectric devices fashioned from inexpensive and abundant organic constituents.

The understanding of energy conversion in complex systems is a fundamental problem, also of practical interest in connection with the
challenging task of developing high-performance thermoelectric heat engines and refrigerators. A rather abstract perspective of non-equilibrium statistical mechanics and dynamical system&’s theory is taken to view at this very practical problem. Recently discovered general mechanisms of optimizing the figure of merit of thermoelectric efficiency are discussed, also in connection to momentum-conserving interacting systems [1.2], to the breaking of time-reversal symmetry by an applied magnetic field [3-6], and to multiterminal steady-state quantum thermal machines [7].
References:
[1] G. Benenti, G. Casati and W. Jiao, Conservation laws and thermodynamic efficiencies, Phys. Rev. Lett. 110, 070604 (2013).
[2] G. Benenti, G. Casati and C. Mejia-Monasterio, Thermoelectric efficiency in momentum-conserving systems, New J. Phys. 16, 015014 (2014).
[3] G. Benenti, K. Saito and G. Casati, Thermodynamic bounds on efficiency for systems with broken time-reversal symmetry, Phys. Rev. Lett. 106, 230602 (2011).
[4] K. Saito, G. Benenti, G. Casati and T. Prosen, Thermopower with broken time-reversal symmetry, Phys. Rev. B 84, 201306(R) (2011).
[5] M. Horvat, T. Prosen, G. Benenti and G. Casati, Railway switch transport model, Phys. Rev. E 86, 052102 (2012).
[6] V. Balachandran, G. Benenti and G. Casati, Efficiency of three-terminal thermoelectric transport under broken time-reversal symmetry, Phys. Rev. B 87, 165419 (2013).
[7] F. Mazza, R. Bosisio, G. Benenti, V. Giovannetti, R. Fazio and F. Taddei, Thermoelectric efficiency of three-terminal quantum thermal machines, New J. Phys. 16, 085001 (2014).

A monolayer-scale (ML) chemically ordered semiconductor alloy could offer major improvements in thermoelectric properties. The “L1₁” ordered structure for Si_1-xGe_x alloy is double the unit cell size of the chemically-random alloy, with alternating {111} bilayers enriched in one or the other species. The ordering at this scale is expected to have electrical conductivity significantly superior to that of a random alloy, while retaining reduced thermal conductivity at elevated temperatures due to enhanced Umklapp scattering from the small Brillouin zone. Chemical ordering is known to occur in SiGe alloys due to specific surface interactions, but is metastable in the bulk. We synthesized Si_1-xGe_x alloys under various growth parameters to investigate and optimize the chemical ordering of the alloy in our hyper-thermal molecular beam epitaxy (MBE) system. All the samples are quantitatively characterized by single crystal x-ray diffraction (XRD) for the order parameter. In almost all cases investigated here, across a wide range of growth conditions, order parameters are low. The use of miscut wafers helped increase ordered domain size, but did not raise the overall volume-average order parameter. Recently, we found that growth on Ge (001) wafers leads to considerably enhanced ordering relative to growth on Si (001). Time domain thermo-reflectance (TDTR) technique is being used to measure the thermal properties of the MBE grown Si_1-xGe_x alloy samples, which is an optical pump-probe technique using a femtosecond pulsed laser. Our preliminary TDTR scan taken at 80K for 300nm thick samples on Si(001) substrate, suggests that the thermal conductivity decreases with increased ordering. Measurements on higher-ordered films grown on Ge will be reported. Electrical conductivity in the cross-plane direction will be obtained from differential I-V measurements from select thin film alloys to examine dependence on ordering. We acknowledge the support of this work from National Science Foundation (NSF).

As the major heat carriers in dielectrics and semiconductors, phonons are strongly scattered by boundaries and interfaces at the nanoscale, which can lead to a significantly reduced lattice thermal conductivity k_L. In recent years, such phonon size effects have been used to enhance the thermoelectric performance of various nanostructured materials. In physics, the effectiveness of a thermoelectric material is evaluated by its dimensionless figure of merit (ZT), defined as ZT=S²σT/(k_E+k_L), where S, σ, k_E, and T represent Seebeck coefficient, electrical conductivity, electronic thermal conductivity, and absolute temperature, respectively. Along this line, ZT around 0.4 was achieved at room temperature in Si membranes with hexagonal packed pores, mainly due to the two orders of magnitude k_L reduction from that for the bulk Si.¹
Despite these encouraging results, challenges still exist in the theoretical explanation of the observed low k_L values.^2-4 Existing studies mainly attribute the observed low k_L to 1) phonon bandstructure modification by coherent phonon processes in a periodic structure (phononic effects), and/or 2) pore-edge defects and surface damages. Further determination of these effects requires systematic theoretical and experimental studies on samples with different geometries, particularly over a wide temperature range. In this work, temperature-dependent k_L is measured for nanoporous Si films with different pore sizes and spacing to compare with model predictions. The results will provide guidance for phonon engineering in general materials with periodic interfaces or boundaries.
References:
1. J. Tang, et al., Nano Letters10, 4279-4283 (2010).
2. A. M. Marconnet et al., Journal of Heat Transfer135 (2013).
3. A. M. Marconnet et al., Nanoscale and Microscale Thermophysical Engineering16, 199-219 (2012).
4. D.G. Cahill et al., Nanoscale thermal transport. II. 2003-2012. Applied Physics Reviews 1, 011305/1-45 (2014).

Thermochemical reaction of adsorbed fuel can be controlled by anisotropic thermal properties of 1-dimensional or 2-dimensional carbon nanomaterial substrates [1, 2]. The reaction propagation velocity of fuel adsorbed on two-dimensional thin films of randomly-oriented or aligned carbon nanotubes could be enhanced by increasing surfacial heat transfer while suppressing perpendicular heat loss [1]. The reaction velocity of fuel coated around one-dimensional carbon nanotubes also increased along the length axis due to the high axial thermal conductivity [2]. The temperature gradient at the reaction front generated thermopower waves [2]. However, the small Seebeck coefficient of carbon nanotubes limited the induced maximum peak voltage. We could improve the Seebeck coefficient by hybridizing Sb₂Te₃ with carbon nanotubes [3] or graphene [4] increasing the peak voltage. The electrical conductivity and power factor of the heterostructures were also enhanced compared with those of pure Sb₂Te₃ materials [4]. When carbon nanomaterials were incorporated in composites, the phonon scattering at phase boundaries decreased thermal conductivity of the sintered composites, in spite of the high intrinsic thermal conductivity, enhancing thermoelectric figure of merit [5]. The recent progress with Bi_0.5Sb_1.5Te₃ and graphene composites will also be introduced. References: [1] Energy and Environmental Science, 4, 2045, 2011. [2] Nature Materials, 9, 423, 2010. [3] The Journal of Physical Chemistry C, 117, 913, 2013. [4] Physical Chemistry Chemical Physics, 14, 13527, 2012. [5] Journal of Materials Chemistry, 22, 21376, 2012.

Systems with reduced dimensionality display different phonon properties with respect to bulk materials, both in the low-frequency range of sound and ultrasound, and in the THz regime of thermal phonons. For example, dimensionality reduction gives rise to surface acoustic waves with quadratic dispersion relations at the center of the Brillouin Zone, with remarkable consequences on the speed of sound and on the density of states. In turn low-dimensional systems have also a very large surface-to-volume ratio, thus making it possible to manipulate phonon dispersion relations and propagation over the whole spectrum by surface modifications.
Here we demonstrate how surface structure and surface chemistry can be tailored to determine thermal transport in silicon membranes, thus proving by experiments and atomistic simulations that ultra-thin membranes, with thickness below 10 nm, may be exploited both as acoustic and as thermal metamaterials.

Single-layer graphene (SLG) is a unique material made up of a monolayer of sp²-hybridized carbon atoms that is capable of purely two-dimensional (2D) electrical and thermal transport. Graphene's strong carbon bonds lead to record high thermal conductivity, and lattice (phonon) thermal conductivity 1800-5000 W/m/K was experimentally measured and theoretically explained with both empirical and first principles models. More recent experiments demonstrated a logarithmic length divergence of intrinsic in-plane thermal conductivity in suspended graphene. This intriguing behavior in low dimensional materials cannot be captured by Fourier&’s law and was initially attributed to significant quasi-ballistic contribution from extremely long wavelength, low frequency acoustic phonons. However, recent experiments reported that this diverging behavior can be observed in ribbons as long as 16 um, nearly 20 times as long as the phonon mean free path (~775nm) in suspended graphene. Thus, it can be inferred that long wavelength acoustic phonons are not the only reason for this length divergence. Unlike traditional semiconductors such as Si and Ge, graphene has very high Debye temperature (~2000K). Because of this high Debye temperature, the contribution from normal processes also become very crucial and significantly enhances the intrinsic thermal conductivity. In our work we have taken these normal processes into account based on the improved Callaway model, recently proposed by Allen. We find that the non-resistive normal processes contribute significantly towards thermal conductivity in suspended graphene ribbons as size is increased and are the major cause of the logarithmically diverging behavior. We use a complete solution of phonon Boltzmann Transport Equation (pBTE) coupled with the full phonon dispersion and both resistive umklapp and non-resistive normal phonon-phonon scattering. For finite graphene flakes, we also include line edge-roughness scattering, while the length-dependence is capture by the ballistic term related to phonon velocity and length (v/L). In orderto compute intrinsic thermal conductivity in suspended graphene flakes, we increase length L and observe the length dependence of both the resistive and normal contributions to thermal conductivity. We find that the non-resistive normal processes (which aid to thermal conductivity) dominate over the resistive normal scattering mechanism (which suppresses heat conduction) in graphene large-L samples. Our results show that thermal conductivity due to resistive scattering mechanisms tends to converge when L becomes large compared to phonon&’s mean free path. On the other hand, the normal contribution continues to diverge logarithmically with L. Thus, it clearly elucidates that the length dependence of thermal conductivity in the intrinsic (very large L) regime mainly comes from the non-resistive normal contribution.

The intrinsic phonon mean free path of graphite in the c-axis direction has been commonly believed to be only a few nanometers at room temperature according to the simplified kinetic theory. In the past couple of years, several experimental and theoretical reports on graphitic materials suggested that it could be much longer. However, to date there has been no report of systematic experimental studies to provide direct evidence of the recently projected long phonon mean free path along the c-axis of graphite.
In this work, we report on experimental studies of the phonon mean free path in the c-axis direction of graphite through systematically measuring the cross-plane thermal conductivity of thin graphite flakes with thickness ranging from 24 nm to 714 nm via a differential three omega method. The measured c-axis thermal conductivity shows a strong thickness dependence for films < 356 nm thick due to the classical size effect. The intrinsic phonon mean free path in the c-axis direction of graphite is estimated to be 204 nm at room temperature, much larger than the commonly believed value of just a few nanometers. The measured thermal resistance of graphite films at 300 K roughly remains constant below 207 nm and increases linearly for thicker films, which verifies that the thickness dependence comes from the very long phonon mean free path but not the interfacial thermal resistance. Our study provides direct experimental evidence for the recently projected very long phonon mean free path along the c-axis of graphite. The large intrinsic phonon mean free path along the c-axis of graphite could have important implications in understanding the interlayer interactions of van der Waals solids and coupling between in-plane and cross-plane phonons.

Owing to its efficiency in transporting energy carriers, graphene continues to be pursued for numerous microelectronic and optoelectronic applications. Regardless of its final end, common to each pursuit is the practical necessity of interfacing this two-dimensional material (2D) into a three-dimensional (3D) world. Practically, such interfacing requires that the graphene be mechanically supported and electrically contacted. Accomplishing either task requires that dissimilar materials come into physical contact with the graphene. This contact, in turn, affects graphene's properties and thus the performance of any device based upon its use. Here, we examine the implications of these alterations on the self-heating of graphene devices. Specifically, we demonstrate using a combination of infrared (IR) thermography and Raman spectroscopic imaging that the interactions between graphene, the substrate supporting it, and the dielectric layers surrounding it dictate self-heating and the eventual failure induced by the heating.
Interlayer interactions were varied via the fabrication of graphene channels having differing surroundings but identical geometries. Specifically, graphene channels were made with: (1) epitaxial graphene synthesized atop SiC (2) mechanically transferred graphene atop SiC and (3) transferred graphene on top of a SiO₂/Si stack. By imaging the temperature distribution with IR-thermography for each of the devices and comparing it to the strain and carrier concentration distribution obtained via Raman imaging, we find that the enhanced coupling to the substrate in epitaxial graphene devices actually reduces the amount of power that a graphene device can dissipate before failing. It is instead only when a highly conductive substrate is utilized with mechanically transferred graphene - and thus lower interlayer interactions -- that self-heating is reduced as to allow for greater power dissipation.
Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy&’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

We predict the thermal conductivity of the two-dimensional materials black phosphorene and blue phosphorene using first principles calculations. Black phosphorene has a thermal conductivity anisotropy ratio of three, with predicted values of 110 and 36 W/m-K along its armchair and zigzag directions at a temperature of 300 K. For blue phosphorene, which is isotropic with a zigzag structure, the predicted value is 78 W/m-K. While strong anisotropy in thermal conductivity is observed for van der Waals layered materials when comparing the in-plane and cross-plane directions, no other covalently-bonded 2D or 3D materials show the in-plane anisotropy we predict for black phosphorene. The two allotropes show strikingly different thermal conductivity accumulation, with phonons of mean free paths between 10 nm and 1 mu;m dominating in black phosphorene, while a much narrower band of mean free paths (50-200 nm) dominate in blue phosphorene.

The management of heat and the understanding of heat transfer are ubiquitous challenges in numerous sciences and technologies, from models of Earth&’s thermal history to managing local hot spots in microelectronics. For energy-related materials, the lattice thermal conductivity, k_L, is a fundamentally important parameter that determines the utility of a material for thermal transport, thermoelectric and nuclear applications. Until recently, however, theoretical approaches for examining k_L have been limited by a variety of approximations and the use of fitting parameters to match existing measured data. Such empirical techniques lack predictive capabilities, and typically give limited insight into the microscopic mechanisms that govern k_L in bulk and nanoscale systems.
With the development of advanced theoretical tools (e.g., density functional theory (DFT)) and improvements in computational power, it is now possible to examine k_L using microscopic first principles techniques, without the use of adjustable parameters. Theory can now confidently predict thermal transport properties of systems for which little is known. Here, I will discuss a first principles technique that combines a full solution of the Peierls-Boltzmann transport equation with interatomic forces determined from DFT. This approach is transferrable to a wide range of systems, and has demonstrated good agreement with measured k_L data for a number of materials, validating its predictive power. In particular, I will demonstrate the utility of this approach for examining k_L in nanoscale systems, e.g., wires, two-dimensional sheets and layered materials.
Discussion will focus on basic vibrational properties and phonon scattering mechanisms that determine k_L. A comparison of these properties for lower-dimensional nanoscale systems and their bulk counterparts will be given. Recent theoretical calculations for an array of nanoscale systems will be presented, and recent successes and continued challenges will be reviewed. Further, I will discuss the role of some extrinsic phonon scattering mechanisms in relation to intrinsic thermal resistance for engineering thermal transport properties in nanoscale materials.
L. L. acknowledges support from the Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division for work done at ORNL.

In this paper we develop a solar selective absorber and discus a new pulsed voltage technique in order to electrodeposit Nickel into porous anodic aluminum oxide films (Al₂O₃). We analyse the effects of a relaxation time (t_r) applied during the negative half cycle of Sine-Wave Alternate Current Voltage (SWACV). This analysis is made to correlate the optical properties of the composite Ni-Al₂O₃ film, namely, reflectance, absorptance (α) and infrared thermal emissivity (ε_T) with the relaxation time. The Al₂O₃ films are grown on 1050 aluminum (99.5 % Al) by anodization process. After made this process it was grown a pore aluminium oxide film with a thickness of 655±4.5 nm, and the pore diameter of 24.5 ± 1.3 nm, barrier layer thickness of 19.4±1.3 nm and pore density of 9.3±0.1X10¹⁰ Pores/cm². We apply a SWACV at 60 Hz (T/2=8.33 ms) and a t_r in the negative half cycle of the sinusoidal signal, which is modified using a microcontroller and a TRIAC (Triode for Alternative Current), as main control elements. Relaxation time varies from 1.3 to 4 ms. Nickel electrodeposition electrolytic solution bath was carried out in with NiSO₄, H₃BO₃, (NH₄)₂SO₄, and MgSO₄ in adequate composition at pH=4.2. The applied electrodeposition potential is varied from 9-12 ACV at fixed time of 2 min. We found that absorptance values in the visible solar spectrum are bigger than 90%, for all the used relaxation times. In particular, at 9 ACV and t_r =1.3 ms the reflectance spectrum of our samples resemble that of an ideal solar selective absorber coating. So that the coatings of metal-dielectric composite Ni-Al₂O₃ developed in this work by the new technique pulsed specifically with 9V AC t_r=1.3 ms could improve the efficiency of conversion of solar energy into useful heat in practical applications of solar heating because they have a sharp transition from low to high reflectance.

Coalescence of liquid droplets on a solid surface is a problem of fundamental importance in understanding dynamic droplet processes, and is explored for highly directional heat transfer in the heat pipe cooling technologies. Experimentally, it was found that coalescence of microsized droplets on a super-hydrophobic solid surface could result in departure of the coalesced droplet from the surface. Using molecular dynamics simulations, we simulated coalescence of nanoscale (a few tens of nanometers) liquid droplets on a hydrophobic surface, and observed departures of the coalesced droplet with velocities of several m/s. We systematically varied the solid-fluid interaction strength and diameter of droplets to investigate the influence of surface adhesion and droplet size on the departure velocity of coalesced droplet. The conversion between surface energy and kinetic energy of droplets during the coalescence process is analyzed. The application of coalescence-induced jumping droplets in phase-change based cooling element is discussed.

Recent experimental measurements with Time Domain Thermoreflectance (TDTR) have reported anisotropic and modulation-frequency dependent thermal conductivity in semiconductors. However, the fundamental origin of these observations remains poorly understood. Here we use efficient Monte Carlo simulations of the Boltzmann transport equation to investigate quasiballistic thermal transport under the exact experimental conditions of the TDTR experiments. In particular, we rigorously account for accumulation and modulation effects in our simulations. Our results provide insight into how the TDTR signal should be interpreted in the quasiballistic regime and specifically show how the measurements can be used to obtain phonon mean free path distributions.

The thermal conductivity accumulation function describes mean-free-path-dependent contributions to the bulk thermal conductivity of a solid.¹ Experimentally, the accumulation function can be found using nondiffusive measurements of thermal conductivity (e.g. using thermoreflectance techniques) and the experiment-specific suppression function.² This procedure has been used to find accumulation functions of materials where phonons dominate thermal transport.^3-5 The necessity of a metal transducer layer in thermoreflectance experiments, however, has motivated the study of nondiffusive electron transport and electron-phonon coupling in thermoreflectance experiments.
In this poster, we present nondiffusive measurements of thermal conductivity of metals vs. penetration depth by broadband frequency domain thermoreflectance. Our measurements of single crystal Au from 20-300 K compare favorably to predictions from the electron-phonon coupled, gray Boltzmann transport equation under the relaxation time approximation. Furthermore, we use a two-temperature model to interpret nondiffusive thermal conductivity measurements by thermoreflectance techniques to elucidate the effect of electron-phonon coupling in the metal transducer layer.
¹C. Dames and G. Chen, in Thermoelectrics Handbook: Macro to Nano, edited by D.M. Rowe (CRC Press, Boca Raton, FL, 2006).
²A.A. Maznev, J.A. Johnson and K.A. Nelson, Phys. Rev. B 84, 195206 (2011).
³K.T. Regner, A.J.H. McGaughey and J.A. Malen, Phys. Rev. B 90, 064302 (2014).
⁴K.C. Collins, A.A. Maznev, Z. Tian, K. Esfarjani, K.A. Nelson and G. Chen, J. Appl. Phys. 114, 104302 (2013).
⁵D. Ding, X. Chen and A.J. Minnich, Appl. Phys. Lett. 104, 143104 (2014).

In recent years, single-layer graphene has become a major subject of research owing to its excellent mechanical, electrical, and thermal properties. In particular, the use of graphene has been extensively explored for the thermal management of electronics due to its large thermal conductivity (which can be as high as 1500minus;5000 Wm^-1K^-1). However, the thermal conductivity tends to be suppressed once graphene is supported on a substrate; for example on amorphous SiO₂, the thermal conductivity is reduced to around 600 Wm^-1K^-1 at room temperature. While the mechanisms responsible for this suppression are still not clearly understood, several factors have been identified as possible means of tuning the thermal conductivity of supported graphene, including the adhesion strength and internal strain energy which both depend on the morphological differences between graphene and the substrate surface. In this talk, we present a first principles-based investigation on the relative contributions of these factors on the thermal conductivity reduction. Our findings show that the thermal conductivity is extremely sensitive to the surface roughness of the substrate, which we identify to be largely driven by the degree of inhomogeneity of the applied force on graphene.

In cryopreservation by vitrification, damaging ice crystal formation during cooling can be suppressed by glass formation. Cryoprotectants, such as dimethyl-sulfoxide(DMSO) can be added to a biological sample to aid in vitrification. Results in cryopreservation by vitrification are highly dependent on the thermal history of the sample during cooling and subsequent rewarming. Because of this, the thermal properties of cryoprotectants are useful in ensuring successful outcomes.
In this study, the transient hot wire technique was used to measure thermal conductivities of 2-10 M concentrations of dimethyl-sulfoxide solutions from 25° to -180°C. Samples were programmed to cool at -5°C/min and re-warm at +3°C/min. A scanning cryomacroscope was used to observe concentrations of 2-6 M DMSO crystallizing on cooling. Concentrations of 7.05-10 M DMSO were observed to vitrify. Vitrified samples showed monotonically decreasing thermal conductivity with decreasing temperature, behavior typical of other glasses. The minimum thermal conductivity of 10 M DMSO predicted by the Cahill-Pohl model showed good agreement with measured values. Crystalline samples showed increased thermal conductivity with decreasing temperature after phase transition. Thermal conductivity of crystalline samples in the mushy zone showed a strong dependence on solid fraction.
These differing behaviors of vitrified and crystalline samples result in large thermal conductivity differences, which start at phase transition of crystalline samples and increase as temperature decreases. There is roughly a tenfold difference at -180°C between thermal conductivity of 2 M and 7.05 M DMSO. This large difference can result in drastically different heat transfer scenarios in cryopreservation and the quantification of DMSO and other cryoprotectants is therefore critical to cryobiology.

Alloying a semiconductor introduces mass disorder and alters the bonding environment, leading to changes in the properties of the phonon modes responsible for thermal transport. We study the effects of alloying on mean free path dependent contributions to thermal conductivity using the thermal conductivity accumulation function, which describes the cumulative MFP-dependent energy carrier contributions to thermal conductivity. Silicon-germanium and aluminum-gallium-nitride alloys are studied using an integration of experiments and theory. By varying the heating frequency in a broadband frequency domain thermoreflectance experiment we create nondiffusive transport that can be interpretted by a solution to the Boltzmann Transport Equation to reconstruct the accumulation function. The experimental results are compared to first principles predictions of the accumulation function, where the alloying is included using the virtual crystal approximation.

The continued reduction in the size of electronic, photonic, and phononic devices has resulted in characteristic lengths on the order of the mean free paths of the relevant energy carriers. Thermal transport in these devices is dominated by interactions at the interfaces and is strongly influenced by the strength of the bond at the interface and the constituent materials forming the interface. The current study uses non-equilibrium molecular dynamics with the Lennard-Jones potential to explore the relative contributions of mass-difference and lattice-difference scattering to thermal boundary conductance. Results show that 92% and 90% reductions in thermal boundary conductance are achieved by mass and well depth difference ratio of 2 at 10 K, respectively. The maximum thermal boundary conductance is achieved, as expected, when the lattice and mass are matched, although the effective thermal conductivity is not necessarily a maximum. As the mass and interatomic potential depth ratio move away from unity, the thermal boundary conductance quickly decreases and saturates, meaning small differences in mass and interatomic potential depth are as significant as large differences. It is also found that Lorentz-Berthelot mixing rules do not contribute to thermal boundary conductance. In the low temperature limit, the thermal boundary conductance is a function of the frequency differences in the two constituent materials where at higher temperatures inelastic scattering begins to play a role, therefore reducing the impact of the frequency dependence. In addition to the role of the mass and interatomic potential depth, the interatomic separation, σ, is also studied and the individual and combined interactions at the interface between two dissimilar materials will be presented. Finally, the study will be generalized by using different interatomic potentials.

Two-dimensional materials have obtained great attention owing to their unique properties. To understand thermal transport in 2D materials, we developed a thermal conductivity measurement method based on the hot wire technique^{1, 2}. In the technique, generated Joule heating in a suspended line, which is used as a heater and a thermometer, is designed to dissipate through the 2D material. Average temperature rise depends on the thermal resistance of the 2D material. We characterized the measurement setup by measuring thermal conductivities of SiO₂ nanolayer, with thickness of 135 nm from 50 K to 500 K, and found that they are identical to those of bulk SiO₂; thermal conductivity of an amorphous bulk material should be comparable to that of its nanosized counterpart, because lattice vibration in the amorphous material could behave like an individual oscillator. To study the effect of thermal contact resistance, contact region was treated with liquid layer utilizing the “stiction problem” with deionized water, isopropyl alcohol, or by Pt/C deposition using the focused ion beam. We did not observe any difference in thermal conductivities among these contact treatments. Furthermore, we present sensitivity analysis in which we determine thermal conductance ranges of the 2D material through this technique. We believe that this technique should be useful to understand thermal transport physics in 2D materials.
Reference:
¹C. Dames, S. Chen, C. T. Harris, J. Y. Huang, Z. F. Ren, M. S. Dresselhaus and G. Chen Rev Sci Instrum 78, (2007).
²M. Fujii, X. Zhang, H. Q. Xie, H. Ago, K. Takahashi, T. Ikuta, H. Abe and T. Shimizu Phys Rev Lett 95, (2005).

Graphene-MoS₂ hybrid structures have recently gained a lot of attention owing to their excellent thermal and electronic properties. A thorough knowledge of energy transport through these materials is essential to employ them for potential applications in thermal management. We present thermal conductivity predictions of graphene/MoS₂/graphene hybrid structure using molecular dynamics simulations. The contributions of in-plane and flexural phonon modes to thermal transport are analyzed through the phonon density of states. The mismatch in the vibrational characteristics of successive layers of dissimilar materials in the structure contributes to reduce thermal conductivity through the hybrid material.

In this presentation, we report our recent experimental study on thermal rectification of the radiative heat flux in the far-filed regime. Experimental results on conductively coupled thermal rectifiers have been presented but no reports have been made on radiative rectifier but theoretical prediction. We experimentally observed a thermal rectification contrast ratio as large as two on a radiative thermal rectifier in the far-field regime by utilizing the phase-change property of vanadium dioxide (VO₂), which exhibits a reversible metal-insulator phase transition around 340 K. A thin film VO₂ on a silicon substrate at a lower temperature was used as a thermal receiver in the forward direction, whereas a fused quartz substrate at a higher temperature was an emitter. The radiative heat flux from the emitter was absorbed by the VO₂ film in the insulating state. In the reverse direction, on the other hand, the VO₂ film in the metallic state at a higher temperature was used as the emitter, thereby suppressing the radiative heat flux. The observed heat flux in the forward direction was three times as large as that in the reverse direction. The experimental results were cross-checked by a theoretical model based on fluctuational dissipation theorem. Furthermore, the operating temperature of the rectifier was found to be controllable by changing the doping concentration of tungsten into VO₂, showing more flexibility in device designs. To the best of our knowledge, this is the first experimentally confirmed result of the radiative thermal rectification. The rectification contrast ratio we reported is higher than the values ever measured on the conductive rectifiers. The developed rectifier leads to a new perspective on thermal management and thermal information processing.

Superionic conductors with liquid-like behaviors can be considered as phonon-liquid electron-crystal (PLEC) thermoelectrics. Copper selenide, Cu_2-xSe, is a representative material of PLEC thermoelectrics with of liquid-like behavior. In the Cu_2-xSe, Se atoms provide a crystalline pathway for semiconducting electrons and the Cu ions are superionic with liquid-like mobility, resulting in a low lattice thermal conductivity κ_L which enables ZT improvement. We prepared high-quality, nearly uniform-sized Cu_2-xSe nanocrystallites by colloidal method. The composition, size and morphology of the nanocrystallites were controlled by varying the starting materials and reaction time. We prepared Cu_2-xSe bulk ingot using spark plasma sintering (SPS) and evaluated the composition-dependent thermoelectric properties of Cu_2-xSe nanocrystals. The phase structure, microstructure, band gap, and thermoelectric properties of Cu_2-xSe can be tuned by the compositional optimization through controlling Cu contents.

Thermal conductivity measurements over variable lengths on nanostructures such as nanowires provide important information about the mean free paths (MFPs) of the phonons responsible for heat conduction. However, nearly all of these measurements have been interpreted using an average MFP even though phonons in many crystals possess a broad MFP spectrum. Here, we present a reconstruction method to obtain MFP spectra of nanostructures from variable-length thermal conductivity measurements. Using this method, we investigate recently reported length-dependent thermal conductivity measurements on SiGe alloy nanowires and suspended graphene ribbons. We find that the recent measurements on graphene imply that 70% of the heat in graphene is carried by phonons with MFPs longer than 1 micron.

Vanadium Dioxide is a correlated material and undergoes an electronic and structural phase transition at 340K. Below 340K, it is an insulator and the transition can be induced by voltage, temperature as well as light. We perform thermal and electrical conductivity, and Seebeck measurements to analyse the electrical contribution to thermal transport at the transition temperature. The total measured thermal conductivity does not change across the phase transition, while the electrical conductivity increases by 4 orders of magnitude. We explain the anomalous measured thermal conductivity as a possible violation of the Weidemann-Franz (WF) law. Tungsten-doped VO₂ nanobeams with varying W levels show recovery of the WF law that corroborates our hypothesis.

Efficient thermoelectric devices and thermal management systems consist of low dimensional materials and complex nanostructures in which interfaces play a major role. Phonon properties of nanomaterials are especially sensitive to the interfaces and the dimensionality of the materials because of phonon localization and confinement effects. Thermal transport in materials is largely determined by low-frequency phonons, their interactions with each other and the interfaces, and phonon confinement effects. In order to explore the application of nanomaterials to thermoelectric energy conversion and other devices, it is necessary to model the thermal transport at nanoscales. This would require an understanding of propagation of low frequency phonons in nanomaterial systems for which we need a robust and reliable mathematical model.
Modeling low frequency phonons involves an accurate knowledge of the temporal behavior of a many-body system over an extended time range (up to nano or even microseconds). This is a formidable task for conventional molecular dynamics even with modern computers. We use a new mathematical technique for simulation of phonon transport based upon the use of causal Green&’s functions (CGF). In this technique, we expand the Hamiltonian or the potential energy up to second order terms O(1/N²) in the field variables such as atomic displacements, where N asymp; 10⁶ is the number of the iterations. This is in contrast with the conventional molecular dynamics based models that retain only the first order terms O(1/N) in the Hamiltonian. The presence of the second order O(1/N²) terms improves the convergence of our technique by a factor of O(N^1/2). The power of the CGF technique arises from the fact that the corresponding temporal equations can be solved exactly even for quadratic terms. Further, the technique is especially suitable for representing phonons because it is precisely the second order terms that define the phononic characteristics of the system. This model also provides a convenient method for including the electromagnetic effects such as plasmon phonon coupling that may be important for electronic nanomaterials.
I will describe our current work on the modeling of phonon transport in nanomaterial systems, specifically in graphene and other two-dimensional (2D) materials. The effect of antidots and other possible discontinuities will also be discussed. The choice of 2D materials containing antidots is attractive for thermoelectric applications because of their reduced thermal conductivity as compared to normal materials. In fact the thermal conductivity of 2D materials can be tuned by appropriate phonon engineering by choosing the geometry of the dots and the array for specific phonon wave vectors. An array of metallic nanodots may also be placed on the materials with appropriately matched wave vectors for efficient plasmon-phonon coupling.

Phase-change processes and evaporation in particular are of significant interest in thermal management since they can accommodate large heat fluxes when utilizing the latent heat of phase change. Of these, thin film evaporation continues to be actively studied due to its many applications in thermal management systems such as evaporators and condensers. Numerical modeling of heat and mass transfer in thin evaporating films is a complicated multiscale problem where the intermolecular interactions between the substrate and the adsorbed thin film and within the thin film and their effect on the evaporation should be correctly accounted for in the macro-scale model of fluid flow. The accommodation coefficient which is usually an empirical parameter in the fluidic model of thin film evaporation can be inferred from the atomic-scale knowledge of the molecular structure and intermolecular interactions of the evaporating liquid. The accommodation coefficient at a liquid-gas interface is defined as the fraction of the gas molecules impinging the surface that gets adsorbed (or stuck) at the interface. At equilibrium, the evaporation and condensation are reversible processes and, therefore, evaporation and condensation coefficients are the same as the accommodation coefficient defined above. In this work, we compute the accommodation coefficient of liquid octane using a statistical approach and classical molecular dynamics simulations. The available force field parameters for modeling octane are compared and the temperature dependence of the computed accommodation coefficient is studied.

One approach to treating atrial fibrillation relies on freezing tissue of the heart wall. This surgical technology requires sub-millimeter spatial resolution when dynamically tracking the freezing of pulmonary vein; conventional techniques such as ultrasound lack the necessary precision. Here we use an electrothermal “3-omega” method to track propagating freezing fronts in nearly real time. The heater line is excited with multiple frequencies simultaneously, and the freezing front detected as it passes through the various penetration depths due to the contrast between thermal conductivities on either side of the front. Comparison of water freezing experiments with video images confirms the accuracy of the method. Analysis and experiments show how the uncertainty, time response, and measurement range depend on the frequencies and thermal conductivity contrast. Finally, the method is demonstrated on biological tissues as further proof of principle for medical applications.

Thermoelectric energy production has recently gained increased interest as an alternative energy resource, accompanied by the request for materials with enhanced thermoelectric properties. However, optimization of the efficiency of such materials is not trivial since the integrated parameters (Seebeck coefficient, electrical and thermal conductivity) are interdepending properties. Promising materials in this respect are (semiconducting) alloys and nanostructured materials such as superlattices, polycrystals or nanocomposites [1]. Maintaining a certain degree of crystallinity in such materials ensures a high electrical conductivity while the introduction of impurities (alloys) and nanostructures (e.g. grain boundaries) reduces the thermal conductivity. Both Si_1minus;xGe_x alloys [2] and nanostructures built from crystalline Si, Ge or SiGe alloys [2,3] have shown great potential in this respect. However, phonon transport and scattering effects on impurities and grain boundaries in such materials is still not well understood.
Here, we have used approach-to-equilibrium molecular dynamics [2,4] to study thermal conductivity of Si_1minus;xGe_x/Si_1minus;yGe_y superlattices, polycrystalline nanocomposite SiGe and amorphous SiGe. In our simulations we have addressed both the determination of an overall thermal conductivity and the investigation of specific interfacial thermal resistivity effects by simulating thermal transport through one particular interface [4]. More in detail, we have simulated several layers of Ge and Si_1minus;xGe_x as a function of the layer thickness and directly the interfacial thermal resistance focusing on the effect of the boundary thickness. Results of both methods have been compared giving insight to the underlying phonon scattering processes leading to reduced thermal conductivity. Introduction of nanostructures as present in such superlattices or nanocomposite SiGe resulted in a reduced thermal conductivity compared to the one in a crystalline SiGe alloy. These results offer the basis for the design of new nanostructured SiGe-based materials with tailored thermoelectric properties.
[1] A. J. Minnich et al., Energy & Environmental Science 2 (2009), 466.
[2] C. Melis and L. Colombo, Physical Review Letters 112 (2014), 065901.
[3] P. Chen et al., Physical Review Letters, 83 (2013), 4184.
[4] E. Lampin et al., Applied Physics Letters, 100 (2012), 131906.

Polymers are of substantial interest for thermoelectrics applications because of their low thermal conductivity and mechanical flexibility. However, studying thermoelectric transport in polymers remains challenging due to the difficulty of in-plane thermal measurements. Transient Grating spectroscopy (TG), an optical technique in which a sample is impulsively heated with a spatially sinusoidal pump beam, shows great promise for in-plane thermal conductivity measurements because it requires no metal transducer film and the only fitting parameter is in-plane thermal conductivity. However, this technique has been only minimally applied in the thermal sciences. Here, we report the application of TG to common polymers such as PMMA and polythiophene with thicknesses on the order of one micron. Our work demonstrates a new approach to thermal characterization of polymers that enables further improvement of their thermoelectric performance.

GaN-based wide band gap materials have shown great potentials for high temperature thermoelectric application due to their unique materials properties, including high Seebeck coefficient, electrical conductivity, and high thermal and mechanical stability. However, their inherent high thermal conductivity needs to be reduced without comprising their electric properties. Low dimensional nanostructured structures, such as superlattice and quantum wells, have recently demonstrated to be an effective approach to decouple the electron and phonon transport in several materials systems. In this work, temperature-dependent thermoelectric properties of MOCVD AlGaN superlattice are experimentally studied. Compared to that of bulk materials, the Seebeck coefficient, electrical conductivity and power factors of studied samples have increased due to the quantum confinement effect. Time-Domain Thermoreflectance method is used to understand the nanoscale heat transfer of AlGaN superlattice, and the effect of interface on phonon transport behavior.

Surface plasmon polaritons (SPPs), which are the coupled collective wave of electrons and photons, have drawn much attention for a long time because of its superb ability to greatly concentrate the electromagnetic field beyond the optical diffraction limit. In many important applications, plasmonic nanostructures have been used to focus optical energy to the critical dimensions far smaller than the wavelength, such as nanoscale imaging, absorption enhancement in solar cells, surface enhanced Raman scattering, near-field transducer (NFT) for heat assisted magnetic recording (HAMR), and plasmonic lithography. During the operation of plasmonic structures, the incident optical energy is converted into SPPs and the local light intensity can be enhanced by a few orders of magnitude of the incident. Such strong local field will inevitably lead to intense Joule heating in the metallic structure and significant device temperature rise. The resulted high temperature can cause unreliable device functions and short device lifetime. In the past, the optothermal processes for simple metallic structures have been numerically studied at different time and size scales. However, the optothermal process associated with plasmonic structures features nanoscale spatial field gradients in 3D, strong photon-electron-phonon couplings and non-equilibrium responses, which is yet to be studied at the device level.
Here we report our multi-physics study of the non-equilibrium heat generation and transport in nanoscale plasmonic structures. The dumbbell-shaped aperture plasmonic lens is chosen as an example to simulate the thermal transport process in the plasmonic structures at picosecond time scale. The lens metal is Chromium and a 12-picosecond pulsed 355-nm laser is used as light source. Three physical mechanisms are considered in this study including: (1) thermal energy deposition from SPPs to the electron system, (2) the energy exchange between electrons and the lattice, and (3) the propagation of energy in the structure. Our results indicate that at picosecond time scale the hyperbolic two temperature model is more reliable than the single temperature model to predict the maximum lattice temperature which is related the potential structure damage.

The interest for thermophysical properties of nanomaterials has been growing explosively worldwide. A lot of possible applications in various fields such as thermoelectrics and nanophononics are the reason behind this interest. Previous researches on nanoscale thermal transport have demonstrated that nanostructuring permits a large reduction of the effective thermal conductivity, which results from phonon-boundary scattering, particularly at low temperatures. The phenomenon has been demonstrated in nanowires and thin layers. The aim of this work is to study heat conduction from a surface micro- to nanoscale heat source in different transport regimes, and to analyze the results in light of the Boltzmann transport equation (BTE) for phonons. This can be seen as the analysis of a thermal constriction in the transition from the diffusive to the ballistic regime.
To do so, nanolines of width W acting both as the heat source of micrometric to nanometric size and the thermometer are deposited on top of flat silicon substrate, with various widths from 10 µm down to 20 nm. This is equivalent in cartesian geometry to recent experiments linked to mean free path spectroscopy in radial geometry [1], but with electrical devices that allow to reach smaller dimensions because they are not limited by diffraction. The mean free paths (MFPs) of phonons in the Si substrate, here denoted L, are comparable to this geometric dimension W. The ratio of the mean free path L to the width W is defined as the Knudsen number . Here we analyze the transition between two regimes, when the Kn < 1 (diffusive regime) and when Kn > 1 (ballistic regime), since the estimated average MFP of silicon at room temperature is close to 300 nm and it becomes larger when the substrate temperature decreases. The measurements are performed with the different temperature levels using a cryostat. This configuration provides various degrees of freedom (temperature, device size) that allow probing the range of Knudsen number Kn. The principle of our technique is similar to the one used to measure the thermal conductivity of materials in four-probe devices with the 3w method. In our case, measurement is performed in dc or ac mode to determine the thermal conductance of the small scale devices.
Our results are compared to previous experimental investigations with similar geometry involving ridges [2] and also to numerical simulations based on both Finite Element Method (FEM) and the BTE for phonons solved with the discrete-ordinate method (DOM).
[1] K. Regner et al., Nat. Commun., 4, 1640 (2013)
[2] P.O. Chapuis et al. , “Effect of phonon confinement on heat dissipation in ridges,” Proceedings of THERMINIC (2010).

Over the last few decades the interest in nanoscale materials and their applications has been increasing because of their unique properties and the outstanding performance. Thermal management at the nanoscale is of great importance for the efficiency, functionality and stability of the structures. Heat transfer at this scale can differ significantly from that at the macroscale. A well-known example is the failure of the Fourier&’s law to capture ballistic nature of heat carriers when the characteristic size of the associated structure is comparable to the mean free path of heat carriers. Various models have been proposed to consider ballistic transport behaviors. Among these models, the Boltzmann transport equation (BTE) offers a general way to deal with this problem, but is limited to the cumbersomeness in its implementation. Different levels of approximations have been developed and a widely accepted approach is to split the carriers into ballistic and diffusive components and treat them separately. The advantage is that the ballistic component can be expressed explicitly given the heat source distribution. The scattered diffusive component is then solved in conjunction with proper boundary conditions, which is similar to the scheme used in solving the Fourier equation. Hence the numerical cost can be greatly reduced compared to solve BTE directly where time-consuming iterations are generally required. However, due to the intrinsic inconsistency between ballistic and diffusive expressions, total energy is not conserved, i.e., the sum of two components is not a constant at equilibrium. Energy conservation is an important rule to keep when accurate results are desired. Few studies have worked on this. Here we propose a self-consistent model which integrates the simplicity of ballistic-diffusive treatment and the energy conservation requirement. Inspired by dividing the total light intensity into collimated and diffusive parts, first we revisited the idea of splitting the carriers into ballistic and diffusive components from the radiative transfer perspective. The inconsistency caused by the diffusion approximation was illustrated in a special scenario where the constant thermal conductivity and heat capacity were assumed. Then we brought up the idea of spatially variant thermal conductivity in the governing equation for the diffusive component to compensate unbalanced local energy. The physical origin is that the thermal conductivity contributed by the ballistic component should not be counted again in the diffusive part. The effectiveness of this strategy was examined using the benchmark problem of phonon transport across thin film with two parallel isothermal boundaries. And its general applicability was demonstrated using a two-dimensional substrate with confined surface heating. Our results showed that by introducing the spatially variant thermal conductivity, energy conservation in the ballistic-diffusive approximation can be achieved.

Constant endeavor has been devoted to the reduction of thermal conductivity through phonon engineering for thermoelectric applications. Exploring new alternatives to introduce additional thermal barriers in solid and crystalline materials has hence become of primary importance. We propose to exploit a new physical mechanism based on two-path phonon interference effect to control the phonon distribution carrying the heat in crystals. The key point is to introduce guest atoms possessing strong long-ranged interactions that induce phonons to propagate simultaneously through both the nearest-neighbor bonds and the non-nearest-neighbor ones. The resultant phonon interference between the two phonon paths yields pronounced transmission reduction in the spectrum of long-wavelength phonons, even in the absence of local interference resonances. We use both Non-Equilibrium Green's Function and Molecular Dynamics technique to investigate this phenomenon and design the optimal configuration.
The reduction in the phonon transmission from such two-path phonon interference [1] is physically different than that reported in the superlattices and self-assembled monolayers (SAM) [2]. The interference in superlattices and SAM resulting from coherent multiple reflections of phonons at the layer interfaces requires only a single phonon path, which can be considered as a phonon analogy of the Fabry-Perot resonance in optics. The two-path phonon interference is superior since its manifestation is pronounced for a few unit cells, i.e. several nanometers, while the superlattice requires hundreds of nanometers to demonstrate an evident transmission reduction.
Recently long-range interaction of fourth-nearest neighbors were demonstrated to have comparable strength with the first-nearest neighbor bonds through resonant bonding in IV-VI, V2-VI3 and V materials. [3] The low thermal conductivity was attributed to the strong anharmonic scattering due to the resonant bonding.
We demonstrate that even in the harmonic approximation, the thermal conductivity can be effectively modulated by two-path phonon interference due to the presence of the long-range bondings. Our work highlights the importance of phonon wave effects and contribute to the understanding of phonon coherence in the heat conduction in solids.
[1] Haoxue Han, Lyudmila G. Potyomina, Alexandre A. Darinskii, Sebastian Volz, and Yuriy A. Kosevich. Phonon interference and thermal conductance reduction in atomic-scale metamaterials. Phy. Rev. B 89, 180301(R) (2014)
[2] Lin Hu, Lifa Zhang, Ming Hu, Jian-Sheng Wang, Baowen Li, and Pawel Keblinski. Phonon interference at self-assembled monolayer interfaces: Molecular dynamics simulations. Phy. Rev. B 81, 235427 (2010)
[3] Sangyeop Lee, Keivan Esfarjani, Tengfei Luo, Jiawei Zhou, Zhiting Tian, and Gang Chen. Resonant bonding leads to low lattice thermal conductivity. Nature Commun. 5, 3525 (2014)

Using first principles calculations, we predict the mode-dependent phonon-phonon scattering rates, electron-phonon coupling coefficients, and electron-phonon scattering rates in aluminum, gold, and beryllium. The phonon-phonon scattering rates are obtained from anharmonic lattice dynamics calculations by considering three phonon scattering processes. The electron-phonon coupling coefficients are initially obtained on a coarse grid using the density functional theory package Quantum Espresso and then interpolated to a finer grid using Wannier functions as implemented in the Electron-phonon Wannier package. By only considering phonon-phonon scattering, our initial calculations predict lattice thermal conductivities of 2.4, 9.0, and 147/210 W/m-K in aluminum, gold and beryllium (a axis/c axis) at a temperature of 300 K. The predicted lattice thermal conductivity in gold is dominated by phonons with mean free paths spanning two orders of magnitude (1-100 nm). In aluminum and beryllium, however, phonons with mean free paths spanning only one order of magnitude (2-20 nm in aluminum and 20-200 nm in beryllium) contribute to the lattice thermal conductivity.

Rare-earth (RE) pyrochlores exhibit anomalously low lattice thermal conductivities as shown in some experimental measurements. However the underlying structural origin of such low thermal conductivity is still unclear. Here we present the density functional theory (DFT) and quasi harmonic approximation (QHA) to study phonon properties of two sets of RE pyrochlores, Ln₂Zr₂O₇ (Ln = La, Nd, Sm, Gd) and Gd₂T₂O₇ (T = Zr, Hf, Sn, Pb). The phonon and corresponding Gruneisen-parameter dispersion curves are compared with respect to different Ln³⁺ and T⁴⁺. The thermal conductivities are calculated using Clarke&’s and Cahill&’s model as well as relaxation time approximation (RTA) within Debye approximation and the calculated results from RTA agree well with the experimental data. Finally the atomic vibrational patterns corresponding to the low-lying optical branches that strongly scatter the acoustic branches are found and thereby the structural origins of low thermal conductivity are made clear.

Conventional fluids, such as water (W), ethylene glycol (EG) and mixture of them (W/EG), are usually used as heat transfer fluids. Their poor heat transfer rate is an obstacle for enhancing efficiency of heat exchangers. A novel type of fluids called “nanofluids” (NFs) is recognized for improving the performance of heat transfer fluids. NFs are two phase fluids where solid nanoparticles (NPs) are dispersed in base liquids, which are expected to enhance the heat transfer properties of traditional fluids by improving their thermal conductivity (TC). In the last decade, NFs have achieved considerable attention due to their enhanced thermal conductivity. Mainly two methods are used to fabricate NFs: the two-step method wherein first NPs are synthesized and then dispersed in the conventional heat transfer fluids, while in the one-step method NPs are formed directly inside the base liquid. Due to large scale availability of commercial NPs, two-step method is the most commonly used technique. However, some issues such as presence of impurities or undesired phase on commercial NPs may influence the thermo-physical properties of NFs including TC and viscosity. In fact, making commercial NPs without any impurities is almost impossible and removal of undesired phases from the commercial NPs with no negative impact on NPs composition is also difficult. Hence, to study the real contribution of NPs on thermo-physical properties of NFs we performed a systematic experimental work using commercial SiC NPs with both α- and β crystal structure and silica (SiO₂) NPs as secondary phase to make SiC/SiO₂ composite material. The reason for selecting SiO₂ is due to our recent findings on commercial α- and β SiC NPs where analysis on NPs, particularly β phase, showed silica impurity phase. Presence of this impurity/undesired phase does not reflect the real heat transport property of SiC NPs. Thus we focused on fabrication of SiC/ SiO₂ composite material with different structures including core-shell structure (presence of silica phase as shell), functionalization of SiC NPs with SiO₂ phase as well as addition of different percentage of SiO₂ to SiC NPs as secondary phase. The obtained composite materials were used to fabricate W/EG based NFs with 9wt% NP concentration. Physicochemical properties of NPs/NFs were characterized by using various techniques. The thermo-physical properties of NFs including TC and viscosity were measured and analyzed at 20 ^oC. Our findings and results obtained from physico-chemical, thermo-physical and heat transport characteristics are presented in detail.

Efficient photothermal conversion of solar energy requires spectrally selective absorbing surfaces that simultaneously absorb visible light while emitting minimal infrared thermal radiation. Many modern selective absorbers are composed of metal dielectric photonic crystals that are stable at high temperatures but are still far from the ideal selective surface. Direct band gap semiconductors offer another approach to realize high spectral selectivity due to the decrease in absorption near the band edge. In fact, semiconductors have been investigated as selective absorbers since the mid 1970s, but recent advances in modern photonics offer opportunities for considerable improvement due new photonic design principles, fabrication techniques, and materials. Here we show how one-dimensional multi-layer stacks can yield surfaces with extremely high spectral selectivity. We identify a photonic crystal design that yields an optical-to-infrared absorption ratio of 33, more than a 4-fold increase over previous semiconductor absorbers and a 20-fold increase over metal-dielectric photonic crystals.

As the National Metrology Institute of the UK, NPL have recently started to develop new metrological capabilities to measure thermoelectric transport properties at a scale below 100 nm. In this contribution, we will present a series of uncertainty analysis and initial results related to the accurate measurement of key transport properties at the nanoscale: electrical resistivity, thermal conductivity and thermopower. These nanoscale measurements will be contrasted with traceable values obtained for macroscale samples.
The following nanoscale methods will be frist discussed: Current Atomic Force Microscopy and Scanning Spreading Resistance Measurement, where a pressure of a few GPa is applied to the tip, are used to measure the electrical resistivity of a material.
The core of our presentation will be based around the repeatability, accuracy and sensitivy of various methods used to measure heat transfer at the nanoscale or temperatature variation in heterostructure. For example where n active Scanning Thermal Microscope probe is used to measure the electrical to thermal transfer function either in DC mode or in the so-called 3-w mode. These methods are generally used in the literature to evaluate electrical and thermal conductivities.
We will argue that quantification is so far only possible indirectly, i.e when a series of calibration sample from the exact same materials are available. This is due to large uncertainties on the contact area between the tip and the samples, as well as uncertainties related to some key materials properties such as the mean-free path of key carriers. Procedures used by NPL to ensure consistent and reproducible results will be presented as well as key aspect of the instrumentation such as amplifier and feedback-loop.
Methods to measure thermopower, in direct contact or using Scanning Kelvin Probe will also be presented and discussed.

We calculate, by means of fluctuational electrodynamics, the thermal emission of an aperture filled by vacuum or a material at temperature T. We show that thermal emission is very different whether the aperture size is large or small compared to the thermal wavelength. Subwavelength apertures filled with vacuum (subwavelength blackbody) have their thermal emission strongly decreased compared to classical blackbodies. A simple expression of their emissivity can be calculated and their total emittance scales as T⁶instead of T⁴ for large apertures. Subwavelength apertures filled with an opaque material have generally a similar behavior. However, if the material support surface waves or exhibit resonances, thermally excited evanescent resonant modes are scattered by the aperture. In this case, the thermal emission tends to be monochromatic and is strongly enhanced in comparison to the Planck theory. We think this behavior could open the way to very useful applications such as very narrowband passive source.

Radiative heat transport between polar materials supporting surface-phonon polaritons has been shown to be greatly enhanced when two objects are placed at sub-wavelength separation due to the contribution of near-field surface modes. However, the enhancement is limited to extremely small distances due to the evanescent decay of the surface waves. In this work, we investigate how this limitation might be overcome using an active method to extract near-field surface modes to the far-field. By placing a gain medium in the near-field of the object and introducing external off-resonant optical pumping, the resonant surface mode can be emitted into the far-field in an anti-stokes fashion. We analyze the system with realistic parameters to assess the feasibility and efficiency of our approach.

Highly anisotropic materials like graphite are of fundamental and practical interest. While thermal transport in graphite has been studied for decades, the mean free paths (MFPs) of phonon responsible for heat conduction remain poorly understood. Here, we present an experimental measurement of the in-plane phonon MFP spectrum of graphene using the ultrafast transient grating technique. This optical pump-probe technique uses systematic variations in the grating period to measure the phonon MFP distribution. Our measurement provides insight about thermal phonons into materials with extreme thermal anisotropy.

High-performance thermoelectric materials development is one of the crucial aspects for direct thermal-to-electric energy conversion. Atomic scale point defect engineering and lowering the dimensionality of the structures are introduced as new strategies to optimize the electrical properties and lattice thermal conductivity of thermoelectric materials¹. In the case of Bismuth Telluride materials, they have been already reported in the literature with layered defects as the “septuple” Bi₃Te₄ related to dislocations in Bi₂Te₃ nanowires^2,3, or stacking faults⁴.
This strategy drove us to introduce two dimensional defects. We neglected whole monolayers, creating new binary Bi-Te composite systems. These hypothetical (up to now) structures have been studied with ab-initio simulations. Simulations showed that stable binary composites beside the classic Bi₂Te₃ exist. Most of them are metastable phases, but under specific conditions could be dynamically stable. We are aware that these structures are not stable in “normal conditions”, but there are evidences that partially these stoichiometries exist within the classical 2-3 stoichiometry. Preliminarily results on their structural and phonon properties of these structures will be presented. The elaboration methods are developing fast and probably in the future we could arrive to increase the volume of these defected stoichiometries and even to isolate them.
¹ L. Hu, T. Zhu, X. Liu and X. Zhao, “Point Defect Engineering of High-Performance Bismuth-Telluride-Based Thermoelectric Materials”, Adv. Functional Materials 24, 5211 (2014)
² Y. Jiang et al, “Direct atom-by-atom chemical identification of nanostructures and defects of topological insulators”, Nano Letters 13, 2851 (2013)
³ D. Medlin et al, “Dissociated dislocations in Bi2Te3 and their relationalship to seven-layer Bi3Te4 defects”, J. Materials Science 49, 3970 (2014)
⁴ L. Seixas et al, “Topological states ruled by stacking faults in Bi2Se3 and Bi2Te3”, J. Apllied Physics 113, 023705 (2013)

The last decade due to the development of the nanofabrication methods a renewed interest in increasing the thermoelectric performance of materials has emerged. Nanostructuring allows tailoring the thermal and electrical properties of materials through structural engineering at atomic level. Bi₂Te₃ is the best thermoelectric material with a figure of merit near to unity around room temperature. The main reason for this is its anisotropy, which is due to the weak Van der Waals bonds connecting adjacent Te atoms while strong covalent bonds between the Bi and Te atoms exist. As a consequence, Bi₂Te₃ has very low lattice thermal conductivity. With the nanofabrication development a strategy to increase further the figure of Merit is to decrease even more the thermal conductivity. In this framework defected bulk Bi₂Te₃¹ or lower dimensional structures (nanofilms², nanowires^3,4) have been proposed recently in the literature with promising results.
In this work, we study the effect of geometric and structural modulations of Bi₂Te₃ nanowires on thermal properties. The growth angle of the nanowires in respect to the c-direction, as well as stoichiometric variations are studied by means of ab-initio and molecular dynamics simulations. Preliminarily results show an important decrease of the thermal conductivity of the modulated nanowires, something that could explain discrepancies among measurements.
¹ K. Termentzidis et al, “Large thermal conductivity decrease in point defective Bi2Te3 bulk materials and superlattices”, J.Appl. Phys.113, 013506 (2013).
² Y. Zhao et al, “Te-seeded growth of few-quintuple layer Bi2Te3 nanoplates”, Nano Research (2014).
³ B. Qiu, L. Sun and X. Ruan, “Lattice thermal conductivity reduction in Bi2Te3 quantum wires with smooth and rough surfaces: A molecular dynamics study”, Phys. Rev. B 83, 035312 (2011).
⁴ C. Yu et al, “Thermal transport along Bi2Te3 topological insulator nanowirtes”, Appl. Phys. Lett.105, 023903 (2014).

Providing material specificity, sub-micron resolution and the ability to quantify in-plane properties, Raman thermal conductivity measurements are being increasingly utilized to probe low-dimensional materials in the form of nanowires and their two-dimensional analogs. Despite its burgeoning implementation, the technique is comparatively immature resulting in an incomplete understanding of error implicit with the measurement. In response, the sources and magnitudes of error in Raman thermal conductivity measurements are investigated via numerical simulation.
Raman thermal conductivity measurements were simulated for thermal conductivities ranging from 40-2500 W/mK in the two geometries most often employed -- joule heating of a wire and laser heating of a suspended wafer. Using a finite element thermal simulation of the experiment, a "measured" Raman temperature was calculated and then transformed into thermal conductivity by employing analytical models used in previous experimental reports. The deduced thermal conductivity was subsequently compared to the thermal conductivity input within the simulation to assess error. Using this methodology, error was examined as it manifests due to: (1) assumptions within the analytical model, (2) uncertainty in the quantification of heat flux and temperature and (3) the evolution of strain. Under ideal measurement conditions, errors of 10-20% in the conductivity are calculated. This level of error is comparable to more established techniques like 3omega; and time domain thermoreflectance (TDTR). If thermal stress develops as a consequence of the heating, however, error can grow to larger than 60%. Taken together, the results provide a means to compare the utility of Raman based conductivity measurements relative to more established techniques while at the same time identifying situations where its use is most efficacious.
Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy&’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

We analyze how to measure the effective local thermal conductivity of samples such as irradiated porous silicon (I-PSi) by using two techniques, Scanning Thermal Microscopy (SThM) and Raman thermometry. By irradiating the PSi samples with swift heavy ions (¹²⁹Xe), we had observed a sharp decrease in PSi thermal conductivity down to a factor 4.4 and 8.8 as acquired from, respectively, SThM and Raman measurements. This difference in the obtained thermal conductivity values requires more investigations regarding the different probing depth for both techniques, and regarding the differences between optical (Raman) and electrical (SThM) measurements. Our samples could appear as complex since PSi was formed by electrochemical etching of silicon wafers in HF based electrolytes, leading to mesoporous samples with pores on the order of ~10nm with some dispersion. Samples irradiation was then done at GANIL accelerator (Caen, France), at two different energies 90 MeV and 29 MeV, at 5 different fluences [3]. A partial amorphisation of the PSi skeleton appears while the fluence is increased, further complexifying the structure of the samples. A linear relationship between the amorphous fraction and the sample thermal conductivity had been evidenced.
A common experimental calibration of the two techniques was performed on both set-ups to eliminate possibilities of systematic errors. Finite-Element analysis of the heat transfer in the effective material was performed to account for the real geometry and the real constriction of the heat flux lines. Finally, our model could reconcile the data, showing also that simple analytical formulas are not always the most accurate way to analyze local thermal measurements.

We present recent studies on the in-plane heat transport in thin films of laser-sintered group-IV nanoparticles. These films, which are relevant for thermoelectric power conversion, are between 300nm and 1µm thick and exhibit a significantly lower thermal conductivity compared to bulk material, due to their meso-porous network¹. We study the measurement of the thermal conductivity of such thin films via microscopic infrared thermography. For this, a freestanding sample geometry is achieved by arranging the films on a substrate with 40µm wide trenches. Sapphire is used as a substrate to provide good thermal contact and transparency in the visual and near-infrared range. Illumination from below heats the freestanding part of the sample. With an infrared microscopy system based on an InSb focal plane array (FPA), sensitive in the 3-5µm wavelength range, we are able to qualitatively verify the analytically predicted parabolic temperature distribution over the trenches. With knowledge of the absorption of the film and its emissivity, the thermal conductivity can be calculated from this temperature distribution. The sensitivity of the system is increased by lock-in detection. The improvement of the image quality is also investigated by shifting the sample by subpixel distances and combining the resulting images to a super-resolution micrograph in order to effectively reduce the 1.5µm pixel pitch of the InSb-based microscope.
[1] B. Stoib et al., Appl. Phys. Lett. 104, 161907 (2014)

Stiction is a major problem during the fabrication and functioning of NEMS/MEMS devices. Indeed, the formation of menisci at small contacts due to the capillary condensation under ambient conditions leads to an apparent sticking of close surfaces. Atomic Force Microscope with its high spatial resolution has been used to study the dimensions and growth dynamics of water layers at nanoscale contacts. In order to explore the effect of thermally-controlled devices on stiction, we employ Scanning Thermal Microscope SThM, an AFM-based technique where an electrically heating resistive probe is used under ambient conditions.
First, a V-shaped Wollaston wire probe (15 µm in curvature radius and 5 µm in wire diameter) is used on a hydrophilic sample of germanium (Ge). As the capillary forces between the probe and the sample are dominant among the pull-off forces under ambient conditions, the pull-off forces are measured through force-distance curves. The decreasing of the pull-off forces measured as a function of the probe mean temperature (T_pm) indicates that the water meniscus evaporates progressively until a certain temperature limit beyond which the meniscus largely disappears. Taking into account the thermal conductances at the probe-water and water-sample interfaces, the thermal conductance describing the heat transferred from the probe to the sample through water is determined as a function of T_pm [1]. The results show that the meniscus thermal conductance is, at most, one order of magnitude lower than the conductance through air for the same probe [1].
Second, we present the results of similar experiments performed with a Pd/Si₃N₄ probe. This probe has a better force resolution and its radius of curvature is smaller than 100 nm. Measurements of the pull-off forces are performed on two hydrophilic samples of Ge and polyimide (Po) as a function of T_pm. While the pull-off forces data indicate that the water meniscus on the Po sample is largely reduced at T_pm around 100 °C, they indicate that a large amount of the meniscus is still remaining for the Ge sample when T_pm is close to 150 °C [2]. This is due to the difference between the thermal conductivities of the two samples. For a given value of T_pm,, the temperature at the tip apex increases when the sample thermal conductivity decreases [2]. The thermal conductance through the meniscus is evaluated for the two samples [2]. Our results consolidate and complete the approaches previously proposed for quantifying this conductance for tips with similar range of curvature radius [3]. Our global results allow an improved use of quantitative SThM. Moreover, they establish a new procedure towards reducing the stiction at the nano-scale using thermally-controlled devices.
[1] A. Assy, S. Lefèvre, P.-O. Chapuis and S. Gomès, JPhysD: Applied Physics 47 (44), 442001 (2014).
[2 ] A. Assy, S. Lefèvre, P.-O. Chapuis and S. Gomès, in preparation
[3 ] L. Shi, A. Majumdar, JHT, 124 (2), 329-337 (2002).

In this work, we address the question if it is possible to tune the thermal conductance through photoisomerism-capable molecular junctions. Using non-equilibrium molecular dynamics simulations, we study heat flow between two silicon leads connected via two photoisomeric molecules - (a) azobenzene and (b) Spiropyran (SP) - Merocyanine (MC) isomers. For the case of azobenzene, isomeric states different conformations are realized via mechanical strain, while in the case of SP-MC, via a hybridization change. We observe that the thermal conductance of both junctions is rather insensitive to the isomeric state, thereby rendering the tunability of molecular thermal devices unlikely. Consistent with these observations, the vibrational density of states (VDOS) for different configurations yields very similar spectra.

Electron-phonon interaction has been well known to create major resistance to electron transport in metals and semiconductors, whereas less studies were directed to its effect on the phonon transport, especially in semiconductors. We calculate the phonon lifetimes due to scattering with electrons (or holes), combine them with the intrinsic lifetimes due to the anharmonic phonon-phonon interaction, all from first-principles, and evaluate the effect of the electron-phonon interaction on the lattice thermal conductivity of silicon. Unexpectedly, we find a significant reduction of the lattice thermal conductivity at room temperature as the carrier concentration goes above 10¹⁹ cm^-3 (the reduction reaches up to 45% in p-type silicon at around 10²¹ cm^-3), a range of great technological relevance to thermoelectric materials. This work is supported partially by S³TEC, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Basic Energy Sciences, and partially by the Air Force Office of Scientific Research Multidisciplinary Research Program of the University Research Initiative (AFOSR MURI) via Ohio State University.

Understanding the thermal conductivity of bulk crystalline solids is essentially a solved problem and it is well described by the phonon gas model (PGM). The PGM treats individual phonons (e.g., quanta of lattice vibration energy) as particles, similar to gas molecules, in the sense that they carry energy at a certain speed for some averaged distance, termed the mean free path (MFP). This model does an excellent job at explaining the thermal conductivity of crystalline solids and due to advancements in modeling over the last decade, one can now calculate phonon energies, velocities and MFPs fully from first principles. This now allows one to predict the thermal conductivity of virtually any crystalline material with excellent agreement with experiments at virtually all temperatures of technological interest. By employing Monte Carlo methods or the Boltzmann Transport Equation, one can also accurately predict the thermal conductivity of micro and nanostructures due to quantum or classical size effects. As a result of the great success of this model, it has prevailed as the primary physical picture used to understand and interpret all phonon transport related phenomena. However, there are a number of technologically important material classes and molecules that are not well described by the PGM. This talk will discuss several instances where the PGM is inconsistent with the atomic level behaviors observed in molecular dynamics simulations. The talk will also cover several new theoretical modeling developments that offer a different perspective on phonon-phonon interactions and allows for direct calculation of phonon contributions to thermal conductivity and interface conductance.

In all substances, energy/heat is carried through atomic motions. In electrically conductive materials, electrons can become the primary heat carriers, but in all phases of matter, atomic motions are always present and contribute to the thermal conductivity of every object. In studying the physics of thermal conductivity, tremendous progress has been made over the last 20 years toward understanding lattice thermal conductivity in crystalline solids. However, most of the existing methods are based on “phonon gas model”, which is the dominant paradigm. It essentially treats vibrations as gas particles, which scatter with each other. This analogy works well for crystals, but it hinges on the assumption that particle velocity being well defined. Because amorphous materials and molecules lack periodicity, it is difficult to define the phonon velocity.
We used molecular dynamics simulations and a new formalism for calculating the modal contributions to thermal conductivity to study the amorphous materials, a-Si and a-SiO2. It is the first method that is able to obtain the modal details of phonon transport in amorphous materials including full anharmonicity. The simulation results are with excellent agreement with experimental results. This method offers a different perspective on phonon-phonon interactions and allows for direct calculation of phonon contributions to thermal conductivity, which will advance our understanding of the phonon transport mechanism and facilitate heat transfer applications in disordered solids and polymers.

Phonon scattering rates are fundamental to the study of many thermal transport problems; however, few methods are available to determine these rates and their energy dependency. Researchers often rely on heuristic models such as the relations proposed by Callaway and Holland for computational studies. Other methods have been developed using anharmonic lattice dynamics and fluctuation dissipation theorem to determine scattering rates. Results are presented from a simple method for determining phonon scattering rates using molecular dynamics. A standing wave is imposed in a structure allowing for the decay of the vibrational mode to be observed. The rate of transition to other modes is related to the phonon scattering rate. Additionally comparisons are made to anharmonic lattice dynamics methods and fluctuation dissipation theorem methods. The relative advantages and disadvantages of the methods for determining thermal conductivity are discussed.

In this talk, I will discuss a novel method - a dedanken experiment that can visualize the anharmonic phonons in nonlinear (anharmonic) lattices. The method uses an external driving force applies to the front particles of a slab of nonlinear lattices. Then one monitors the excited wave evlution by molecular dynamics. This allows us to determine, simultaneously, the existence of anharmonic phonon mean free paths and corresponding wavenumbers as a function of temperature. The scheme is tested by several well-known nonlinear lattices exihibint anomalous and normal heat conduction.

Achieving high thermoelectric figure of merit (ZT), especially peak ZT, has been the goal of the thermoelectric community. However, we found that a high peak ZT does not warrantee a high conversion efficiency, but a high average ZT does. Up to now, the conversion efficiency of any materials can only be calculated based on the temperature independent ZT, which is very much different from the actual efficiency that can be obtained if a device is made simply because of the ZT of any materials is very much temperature dependent. We have recently redefined a set of new parameters and found that they are very reliable on predicting the conversion efficiency of any materials when the temperature dependent electrical conductivity, Seebeck coefficient, and thermal conductivity is known. Another issue is that we found high conversion efficiency does not warrantee a high output power for a given device, which is because output power is not only determined by efficiency but also the power factor. I will show a few materials to support these findings.

In this paper, we investigate thermal transport in a heterogeneous nanocomposite of Na doeped PbTe for thermoelectric application. The heterogeneous nanocomposite is composed of nanodot nanocomposites with an average nanodot size of a few nanometers and nanograined nanocomposites with a grained size of around 10 nm. In this case, nanodots only form inside the nanodot nanocomposite and nanosized grains only exist in the nanograined nanocomposite. We found that the heterogeneous nanocomposite is a very efficient thermoelectric material; thermoelectric figure of merit, zT, of the material is around 2.0 at 773 K, which is 25 % increase over that of the homogenous nanodot nanocomposite. The main reduction comes from the thermal conductivity reduction - around 15% reduction compared with that of the homogeneous nanodot nanocomposite. The heterogeneous nanocomposite was acquired by putting excess amount of Na into the PbTe to precipitate nanodots and by decreasing quenching rate. We also present theoretical analysis on the lattice thermal conductivity of the heterogeneous nanocomposite, which suggests that reduced inter-dot distance and the nanograined region due to the inhomogeneity of the material reduced the phonon mean free path.

Anomalous, vacancy-induced anisotropic thermal conductivity is one of the important properties in the high-ZT In₄Se_3minus;x compound. We have investigated the lattice thermal conductivity of In₄Se_3minus;x using equilibrium molecular dynamics simulation, the point-defect model, and ab initio calculations. The charge density distribution shows highly anisotropic structure with strong bonding along In-Se-In chain direction. Se vacancy strongly suppresses the phonon propagation along the chain direction, with little change in other directions. We show that suppressed long-range acoustic phonon transport caused by the vacancy results in anisotropic change of lattice conductivity. We suggest controlling of vacancy in intrinsic low-dimensional compounds is critical in achieving optimal lattice thermal conductivity and other thermoelectric properties.

The lead-free compound tin telluride (SnTe) has recently been suggested to be a promising thermoelectric material because of its similar electronic band structure to the well-known lead telluride. Motivated by the recent success in enhancing thermoelectric properties through nanostructuring, we have performed systematic thermoelectric measurements on single crystalline nanowires of SnTe that were synthesized via an Au-catalyzed vapor-liquid-solid growth. Measurements of thermopower, electrical conductivity and thermal conductivity were carried out on the same individual nanowires to accurately determine the figure of merit ZT over a temperature range of 25 - 300 K. Thermopower and electrical conductivity measurements suggest that the SnTe nanowires are heavily p-type doped, arising from the high concentration of native Sn vacancies. While the electrical conductivity does not show a strong dependence on nanowire diameter, the thermopower increases by a factor of two when the diameter is decreased from ~ 913 nm to ~ 218 nm. The thermal conductivities of the measured NWs are lower than that of the bulk SnTe, which may originate from the enhanced phonon-grain boundary scattering and phonon-defect scattering. Temperature dependent figure of merit ZT was determined for individual nanowires and the achieved maximum value at room temperature is about three times higher than that in bulk samples of comparable carrier density.

This talk will discuss two aspects of thermal radiation. One is thermodynamics of thermal radiation, focusing on entropy of thermal radiation and its consequences on the limits of various energy conversion processes such as photovoltaics, thermophotovoltaics, and thermal upconverison. The other is on limits of radiative heat transfer in the near and far fields. When the spacing between two surfaces is less than the dominant wavelength of thermal radiation, heat transfer between the two surfaces can significantly exceed the blackbody limit. In the past, we have demonstrated experimentally that near-field thermal radiation can far-exceed the far-field blackbody limit and the experimental results are in agreement with the Rytov&’s fluctuating electrodynamics description. However, in the limit when two surfaces are in contact, heat transfer is described in terms of heat conduction and phonon transport. We develop an approach using lattice dynamics and the microscopic Maxwell equations to bridge the theories of conduction and radiation. Finally, we will discuss the limit of far-field thermal radiation.
This work is supported by DOE BES (DE-FG02-02ER45977)

We report on quantitative measurements of the heat flux between a gold coated near-field scanning thermal microscope tip and a planar gold sample at nanometer distances across a vacuum gap. After a precise calibration of our temperature sensor with a hot wire calibration standard we are able to perform quantitative measurements of the heat flux. We find an extraordinary large heat flux which is more than five orders of magnitude larger than black-body radiation and three orders of magnitude larger than the values predicted by conventional theory of fluctuational electrodynamics. Additionally, we compare our data with different theories of phonon tunneling which might explain a drastically increased heat flux, but are found not to be able to reproduce the distance dependence observed in our experiment. The findings demand modified, or even new models to describe the heat transfer across a vacuum gap at nanometer distances.

Near-field radiative heat transfer (NFRHT) has attracted considerable attention recently, with orders-of-magnitude heat transfer enhancement already demonstrated between bulk materials. Using custom-built experimental platforms we conducted experimental studies in the sphere-plate configuration and tip-plate configuration to probe NFRHT in the 30 nm - 10 micron range and 1 nm - 10 nm ranges, respectively. In our sphere-plate study, we systematically investigated the effect of film thickness on NFRHT. By studying thermal radiation between a hot silica microsphere and thin silica films of varying thicknesses (50 nm to 3 microns) as a function of gap size (30 nm to 10 microns), we found substantial enhancements in heat transport properties due to near-field effects, even for the thinnest films when the gaps size was comparable to the film thickness¹. Further, we find that at larger separations (~1 micron), the thicker films show substantially larger near-field enhancement than thinner films. These results provide first direct evidence of a distance-dependent penetration depth in thin films. In our experiments leveraging the tip-plate configuration we sought to understand NFRHT in the extreme near-field regime (gaps of 1 - 10 nm) using atomic force microscope (AFM)-based scanning probes with integrated nanoscale thermocouples, which were coated with dielectrics (SiO₂ or SiN_x) and metals (Au, Pt). Our measurements² of heat transport between the scanning probes and a flat substrate coated with dielectrics/metals, performed in an ultra-high vacuum environment, suggest that heat transport is dramatically enhanced in the near-field. This measured enhancement in heat flows was found to be in good agreement with computational predictions—thus establishing the validity of using fluctuational electrodynamics in modeling near-field heat transport even at single-digit nanometer separations.

State of the art techniques for temperature mapping at the nanoscale include scanning thermal microscopy (SThM) [1] and optical thermoreflectance [2], with the respective limitations of requiring physical contact and being diffraction limited. Scanning electron microscopy (SEM) is ubiquitous for topographical imaging and has the advantages of being both non-contact and high spatial resolution, but has not been used for thermal imaging. Here we demonstrate that this is possible. Electron beams can give rise to several types of temperature-dependent signals, including electron diffraction [3, 4] and cathodoluminescence [5]. There also have been several reports of temperature effects on secondary electron yield in metals [6], semiconductors, and insulators [7], which have been attributed to various mechanisms such as electron-phonon scattering [7] and work function effects [8, 9]. We have measured the temperature dependence of secondary electron yield from various group IV and III-V semiconductors, and found typical response coefficients of -1E-4 to -1E-3 per degree K. Current work aims to exploit this effect to obtain a high spatial resolution temperature map using SEM.
References
1. Kim K., Jeong W., Lee W., and Reddy P., ACS Nano 2012 6 (5), 4248-4257
2. Grauby S., Dilhaire S., Jorez S., and Claeys W., IEEE Electron Device Letters, 26(2), 2005.
3. Xiaowei Wu and Robert Hull, Nanotechnology 23 (2012) 465707.
4. Li He and Robert Hull, Nanotechnology 23 (2012) 205705.
5. Caldwell et al., J. Phys. Chem. C 114, 22064
6. Wooldridge, Phys. Rev. 58, 316; Beer, Ann. Physik 449, 201.
7. Johnson & Mckay, Phys. Rev. 91 582 and Phys. Rev. 93, 668.
8. Cho A.Y. et al., Phys. Rev. Lett. 22 (22) (1969)
9. Fischer, T.E. et al. Physical Review Letters 21(1), (1968)

Scanning Thermal Microscopy (SThM) with its submicrometric scale spatial resolution is a promising tool to determine the local thermal conductivity of sample materials. In this contribution, experimental results are shown with two kinds of resistive probe: V-shaped Wollaston wire probe (15 µm in radius of curvature and 5 µm in resistive wire diameter) and KNT probe (Pd/Si₃N₄ tip - with an apex radius smaller than 100 nm).
The thermal signal is measured first under ambient conditions at the contact between the tip and the sample as a function of the sample thermal conductivity k_s for the two probes. The results show that the k_s sensitivity of measurements is up to 10 W.m^-1.K^-1 for the Wollaston wire probe and around 1-2 W.m^-1.K^-1 for the KNT probe. Moreover, these measurements help quantifying the thermal conductance through air that is the dominant heat mechanism in the probe-sample thermal interaction under air conditions.
In order to reach a better sensitivity in the thermal conductivity calibration, measurements under vacuum conditions were performed with the two probes at probe temperatures where the water meniscus almost disappears [1]. Under these conditions, the heat is transferred to the sample through thermal radiation and the mechanical contact. The measurements as a function of the sample thermal conductivity with the two probes do not show the expected k_s dependence. That is related to the thermal resistance at the tip-sample interface. In these measurements, this resistance effect becomes highly important as the contact size becomes smaller or comparable to the mean free path of the materials in contact. It introduces an additive ballistic resistance at the tip-sample constriction. This resistance depends on the thermal properties of the two surfaces in contact but also on the quality of the contact (roughness of both the probe and the sample as an example). Boundary resistances are estimated for the contact between each of the used probe and various materials.
[1] A. Assy, S. Lefèvre, P.-O. Chapuis and S. Gomès, Journal of Physics D: Applied Physics 47 (44), 442001 (2014).

Advances in controlling and manipulating thermal energy have been hampered by a lack of control and sensitivity needed to measure thermal behavior at the meso and nanoscales. Here we introduce a new optical probe technique using a laser trapped nanoparticle that measures absolute temperature below the diffraction limit. The temperature spatial resolution is now dependent upon the size of the nanoparticle and not limited by the wavelength of the excitation laser. We apply this new technique to measure the temperature where vapor nucleation occurs for a 40 nm gold nanoparticle immobilized on silicon and immersed in water. Optical probe thermometry is capable of measuring the temperature through the entire nucleation event, including inside the vapor nucleation microbubble, where extreme temperatures (~1300 K) are observed initially after vapor nucleation.

Thermal Measurement Platform (TMP) has played an important role in probing heat transport at nanoscale in the past 15 years. A wide range nanostructures such as single and multi-walled carbon nanotubes and nanowires have been studied using TMPs, based on calorimeter devices with nanowatt resolution. However, a wide range of material systems with low thermal conductance have remained unexplored due to limitations on resolution of TMPs. Recent progress toward the development of calorimeter devices with picowatt resolution has led to a direct improvement on the Noise Equivalent Power (NEP) of the TMPs. However, NEP of the TMPs depends on NEP of calorimeter devices as well as the background thermal conductance between the two devices. Here, we present a new TMP platform that is based on an asymmetric design of heating and sensing devices that suppresses background thermal conductance and employs a differential measurement scheme that quantifies it. Furthermore, we employ resistive thermometers based on niobium nitride thin films with four times higher Temperature Coefficient of Resistance (TCR) which in combination with the differential measurement scheme pushes the calorimeter sensitivity to sub-picowatt resolution.

With the decrease of the thickness of silicon layer, the thermal problem of ULSI gets worsen due to the reduced thermal conductivity of the silicon layer and the low thermal conductivity of buried oxide. Therefore, it is important to observe the change in the temperature and the thermal conductivity variation from the device level at nanoscale. However, reliable methods and tools that can measure the temperature and thermal properties with nanoscale resolution are not available yet. For example, the use of conventional SThM is still quite limited due to the three main problems: (i) distortion of the measured signal due to heat transfer through the air, (ii) the unknown and variable value of the tip-sample thermal contact resistance, and (iii) perturbation of the sample temperature due to the heat flux through the tip-sample thermal contact.
Recently, we proposed null-point scanning thermal microscopy (NP SThM) as a way of overcoming these problems by tracking the thermal equilibrium between the end of the SThM tip and the sample surface, and demonstrated that NP SThM can profile temperature quantitatively, using the high performance SThM probe. We also showed that NP SThM can profile the spreading thermal resistance at least qualitatively. In this study, using NP SThM, we quantitatively profile the temperature and the spreading thermal resistance around 70 nm wide platinum nano-heater patterned on SOI wafer, the thickness of whose silicon layers are 50 nm and 100 nm.
The measured temperature profiles are compared with the modeling results from the finite element analysis that uses the reduced thermal conductivity of the thin silicon layer. When the temperature is close to room temperature, the measured profile fits quite well with the modeling result using the uniform reduced thermal conductivity. However, near the nano-heater, where the temperature is much higher (> 50 ^oC) than the room temperature, the measured value is noticeably higher than the modeling result. This seems caused by the further decrease of the thermal conductivity of silicon layer due to the increased temperature near the nano-heater. This local reduction of thermal conductivity can be also observed from the profile of the spreading thermal resistance measured simultaneously by NP SThM.
As we show in this study, since NP SThM can quantitatively and simultaneously profile the temperature and thermal conductivity around a nano-heater without the perturbation of the sample temperature owing to the heat flux through the tip-sample thermal contact, NP SThM will be used effectively in thermal characterization of not only for the nano-heater of simple structure used in this study but also for the nano-devices and materials with a more complex structure.

Quasiballistic phonon transport, in which some phonons do not scatter local to a heater, is of fundamental and practical interest. While optical methods have been successful in probing this transport regime, the smallest heating size is limited to 1 micron due to the diffraction limit. Here, we use TDTR to investigate quasiballistic heat conduction from nanoline heaters patterned using e-beam lithography to achieve the heater sizes ranging from tens of nanometers to tens of microns. Measurements were performed for materials including sapphire, SiC, MgO, and SrTiO₃ over temperatures from room temperature down to 20 K. Our work extends prior studies of heat transfer from nanolines at room temperature to cryogenic temperatures, for which quasiballistic effects are much stronger, and provides detailed insight into quasiballistic transport.

Critical applications including thermoelectrics for energy harvesting, nanoparticle-mediated thermal therapy, nano-enhanced photovoltaics, and thermal management in integrated circuits require a comprehensive understanding of energy flow at the nanoscale. However, a complete fundamental description of nanoscale thermal transport is still elusive and current theoretical efforts are limited by a lack of experimental validation.
In order to study the thermal transport properties unique to nanostructured systems, we use a newly developed pump-probe setup to directly observe their dynamics on the intrinsic time and length scales. Samples consist of periodic arrays of metallic nanostructures (as small as 20nm in linewidth), which were patterned on dielectric and semiconductor substrates. The metal structures absorb a 25-femtosecond infrared pump pulse, and the resulting thermal expansion and relaxation is probed using the diffraction of coherent extreme ultraviolet (EUV) light centered at a wavelength of 29nm, generated by high harmonic up-conversion of an 800nm Ti:sapphire laser [1]. Use of this short probe wavelength in an interferometric diffraction measurement enables sensitivity to picometer-scale deformations in the sample surface profile.
While past work has shown that Fourier&’s law for heat conduction dramatically over-predicts the rate of heat dissipation from isolated heat sources with dimensions smaller than the mean free path (MFP) of the dominant heat-carrying phonons [2], we demonstrate here that heat source size is not the only important dimension to consider [3]. A new regime of nanoscale thermal transport dominates when the separation between nanoscale heat sources is small compared with the dominant phonon MFPs. In this case, close proximity between neighboring heat sources can counteract the reduction usually observed in heat dissipation from individual nanoscale sources due to ballistic effects, to such an extent that the collective behavior restores heat transfer efficiency to near the diffusive limit. This finding suggests new strategies for optimizing thermal management in nanoscale systems.
Furthermore, we use this new phenomenon to extract the contribution to thermal transport from specific regions of the phonon MFP spectrum, opening up a new approach for thermal transport metrology and MFP spectroscopy. This is because by varying both nanostructure size and separation, an effective phonon filter is introduced that suppresses specific sections of MFP contributions to thermal conductivity. This unique capability is important, as the need for precise phonon MFP distributions in complex nanoscale systems becomes more pressing - for both fundamental understanding and to harness systems where modeling does not yet exist.
1. Popmintchev et al., Science336, 1287 (2012).
2. Siemens et al., Nature Mater.9, 26 (2010).
3. Hoogeboom-Pot et al., submitted, arXiv 1407.0658 (2014).

Phonon transport across interfaces is an important physical process for many materials such as superlattices, yet this process is poorly understood. Many studies assumed the transmissivity of phonons across the boundary to be constant in a gray model despite the predictions of atomistic calculations that show that transmissivity depends on phonon frequency. Nevertheless, there is only indirect experimental evidence to support those numerical results. Here, we demonstrate that measurements from time-domain thermoreflectance (TDTR) contain information on phonon transmissivity at interfaces. With our recent advances in solving the phonon Boltzmann transport equation, we are able to directly model the phonon transport in TDTR. Using the model to explain the experimental measurements, we explicitly calculate how much heat is carried by each phonon mode across the interface and hence directly extract the phonon transmissivity. We expect that this new knowledge of phonon transmissivity will prove useful to understand heat transport across interfaces.

We report the attenuation of asymp; 50GHz surface acoustic waves (SAW) in Si between room temperature and 600 K to gain insight on the intrinsic attenuation of transverse phonons in Si with energies <<k_BT where anharmonic three-phonon interactions are less important than dissipation by relaxational mechanisms, e.g., Akhieser damping. SAW was generated and detected by pump-probe metrology using an 700 nm period Al grating pattern fabricated on Si(100) oriented surfaces by nano-imprint lithography . To minimize the attenuation caused by mass loading of the Al metal lines, we fabricated a gap in the grating between the regions of grating used to generate SAW by the pump optical pulses and the regions of the grating used to detect SAW by the probe optical pulses. The propagation direction is <110> direction to prevent attenuation of pseudo-SAW. We fit the temperature dependent attenuation data to a model of Akhieser damping based on an rms average Gruneisen parameter and an average equilibration time of thermally excited phonons. We find that Akhieser damping of transverse modes is significantly weaker than it for longitudinal modes. The results help set an upper limit on the contribution to thermal conductivity by low frequency phonons with long mean-free-paths.

We report ultrafast pump-probe experiments on periodically patterned layered nanostructures. The film stack for all samples consists of 25 nm of TiN deposited on 180 nm of 12% porous, low-k dielectric (a-SiOC:H) deposited by PECVD. The TiN/low-k films were formed on top of 12 nm of a-SiOC:H etch stop deposited on 100 nm of CVD grown SiO₂ on Si (100). The TiN layer was dry plasma etched on all the samples to form lines of rectangular cross section with a pitch of 420 nm, 250 nm, 180 nm, and 168 nm.
Our work is motivated by interests in both high frequency surface acoustic waves (SAWs) in nanostructures and in the properties of technologically relevant materials. TiN has become an important material in both nanoelectronics, where it is used as a metal hard mask for patterning features in low-k/Cu interconnects, and in plasmonics where its compatibility with semiconductor fabrication techniques is seen as a potential advantage. Porous low-k a-SiOC:H is used as an inter-layer dielectric (ILD) in metal interconnects and can reduce capacitive signal delays and power loss in nanoelectronic devices.
The absorption of ultrafast pulses from a Ti:Sapphire oscillator operating at 800 nm caused the generation of surface waves that were detected by time-delayed probe pulses from the same oscillator via a reflectivity change (ΔR). In each of the four cases, one frequency component of ΔR was detected that depended on the pitch of the sample, and these ranged from 12 GHz - 54 GHz. Interestingly, the detected SAW frequency did not change linearly with pitch as had been seen in previous experiments of this sort.
Previous pulsed laser studies of SAWs on nanoscale patterned samples have shown detection of Rayleigh-like waves that have speed very close to the Rayleigh speed of the substrate. The literature describing SAWs on layered substrates indicates that for loaded substrates (where the sound velocity in the layer is slower than the sound velocity in the substrate) there are additional SAW modes that are possible which are akin to guided plate modes of the top layer. These modes are called Sezawa waves, and these have been reported in at least one paper in the few-GHz range.
We have compared our data to coarse-grained molecular dynamics simulations of the mechanical motion of the samples. We find that the type of surface oscillation to which we are sensitive changes depending on the sample pitch. In the 420 nm pitch case we appear to detect a Rayleigh-like mode at 12 GHz, while in the 250 nm case we appear to detect a Sezawa mode at 22.4 GHz. In the two smaller pitch cases we detect modes at 42 and 54 GHz, which compare closely only to simulated modes that have significant components propagating into the Si substrate indicating a radiative or leaky SAW.
We also report rigorous coupled wave analysis calculations of the electromagnetic field amplitude in the experiment in order to more fully explain the SAW generation and detection mechanism.

Recent first-principles calculations predict a major contribution of phonons with mean-free-paths >1 mu;m to heat conduction in Si and thus a substantial reduction in the thermal conductivity of micron-sized Si structures. This appreciable reduction in the thermal conductivity was, however, not observed in prior experiments. In this letter, we investigate the contribution of phonons with mean-free-paths >1 mu;m by accurately measuring the in-plane and cross-plane thermal conductivities of a wide range of crystalline silicon films with a thickness of 1-10 µm at temperatures of 100-300 K by time-domain thermoreflectance (TDTR). We employ a dual-frequency TDTR approach in our cross-plane thermal conductivity measurements to improve the accuracy. Our measurements indicate that the reduction of the thermal conductivity in Si thin films is noticeably less than the predictions by the first-principles calculations. Particularly, at 300 K, the thermal conductivity of Si thin films is reduced by 30% and 8% for characteristic thicknesses of 1 mm and 10 mm respectively, compared to reductions of 45% and 18% predicted by the first-principles calculations. By comparing our measurements to calculations of a model that includes Akheiser's damping for low-energy phonons, we show that Akheiser's damping could only explain the lower-than expected reduction of the thermal conductivity of Si thin films at room temperatures, but not at cryogenic temperatures. This work provides important experimental insights into the distribution of phonon mean-free-paths in crystalline silicon.

As one emerging plasmonic material, graphene can support surface plasmons at infrared and terahertz frequencies with unprecedented properties due to the strong interactions between graphene and low frequency photons. Since graphene surface plasmons exist in the infrared and terahertz regime, they can be thermally pumped (excited) by the infrared evanescent waves emitted from an object. Here, we show that thermal graphene plasmons can be excited with a remarkable efficiency of about 90% and have monochromatic and tunable spectra, thus paving a way to harness thermal energy for graphene plasmonic devices. We further demonstrate that we can potentially realize "thermal information communication" via graphene surface plasmons by effectively harnessing thermal energy from various heat sources, e.g., the waste heat dissipated from nanoelectronic devices. These findings open up a new avenue of thermal plasmonics based on graphene for different applications, ranging from infrared emission control, to information processing and communication, and to energy harvesting.

Understanding and controlling heat transfer at very short length scales, has become very crucial in the last decade due to the continuous development in nanotechnology and the rapid evolution in the synthesis and fabrication of different materials at a nanometer scale. At these scales, two heat transfer mechanisms become dominant, namely near field radiation (mediated by photons) and interface conduction (mediated by phonons) between two solid materials. As the distance between the two latter decreases tending to zero, there will be a natural transition from the radiative regime to the conductive regime of heat transfer when the two solids are in contact. The aim of the present work is to shed light on this specific natural transition by investigating the possibility of induced phonon transfer in vacuum. We describe the process in a general way assuming a certain phonon coupling mechanism between two identical nonmagnetic isotropic dielectric solid materials. Then we particularly illustrate the case of coupling through the Casimir force. We analyze how this mechanism of heat transfer compares and competes with the near field thermal radiation using a local model of the dielectric function.We show that the former mechanism can be very effective and even surpass the latter mechanism depending on the nature of the solid dielectric materials, the distance gap between them, as well as the operating temperature regime. This work will be a generalization of the approach recently presented by Budaev and Bogy.

The last few years have seen a growing interest in using plasmonic metallic nanoparticles (MNPs) to control temperature at the nanoscale. Under suitable optical radiation, MNPs can exhibit enhanced light absorption/scattering thus turning it into an ideal nano-source of heat. The photoexcitation of MPNs induces an electric-driven Joule heating with a consequent energy exchange with the surrounding medium. An important challenge in the field is the accurate ability of measuring the temperature variation at the surface of the MNPs under optical illumination. Investigation of the heat transport mechanism, from the heated GNRs to their surrounding medium, is a fundamental step in realizing nano-localized sources of heat for applications in nanotechnology and thermal-based therapies. A particular class of MNPs, gold nanorods (GNRs) have two LPRs due to transverse and longitudinal excitation modes, which can be tuned from visible to NIR as a function of particle geometry. We have advanced a breakthrough in the monitoring nanoscale temperature variations under optical illumination by combining GNRs properties and smart thermosensitive materials. Compared to previously reported techniques the proposed method offers an innovative non-invasive methodology wherein the properties of well known liquid crystalline materials can be used for monitoring photoinduced temperature variations around GNRs with high sensitivity. We present here the struture/property relationships of such a material system. MNPs can be potential key components in miniaturized all-optical computing nanocircuits due to their capability of confining light at the nanoscale by overcoming the diffraction limit of light. We also explore the utility of the same system as an optically controlled nanoscale RC circuit.

Although blackbody radiation described by Planck's law is commonly regarded as the maximum of thermal radiation, thermal energy transfer in the near-field can exceed the blackbody limit due to the contribution from evanescent waves. Here, we demonstrate experimentally a broadband thermal extraction device based on hyperbolic metamaterials that can significantly enhance near-field thermal energy transfer. The thermal extractor made from hyperbolic metamaterials does not absorb or emit any radiation but serves as a transparent pipe guiding the radiative energy from the emitter. At the same gap between an emitter and an absorber, we observe that near-field thermal energy transfer with thermal extraction can be enhanced by around one order of magnitude, compared to the case without thermal extraction. The novel thermal extraction scheme has important practical implications in a variety of technologies, e.g., thermophotovoltaic energy conversion, radiative cooling, thermal infrared imaging, and heat assisted magnetic recording.

Hybrid photovoltaic/thermal (PV/T) collectors have the potential to deliver greater solar efficiency than either technology alone. However, the thermal efficiency of PV/T collectors is currently limited by the low fluid temperatures required to prevent thermal degradation of the solar cells. Recent attempts to overcome this barrier use various beam-splitting mechanisms, including thin films and metamaterials, to more efficiently utilize the solar spectrum. However, these developments are not currently commercially viable in PV/T collectors due to high fabrication costs. One novel approach is the use of liquid optical filters, which can be tuned to absorb specific portions of the solar spectrum. This study investigates a few key nanofluid formulations which can flow through a channel placed between a light source and a PV cell. This nanofluid transmits only the optimal portion of the solar spectrum for electricity generation, while absorbing the remaining incident light for thermal energy delivery. To date, there have been no reported nanofluid optical filters used in such a system. The liquid filter requires only low particle volume fractions (<<1%), but the choice of particle material, size, and shape are critical. Furthermore, maintaining a well-dispersed nanofluid over the course of hundreds of thermal cycles and exposure to concentrated ultraviolet radiation presents an immense challenge.
In this study, we propose a mixture of multiple nanoparticles with different optical properties to absorb short (UV-Vis < 700nm) wavelengths as well as all long (IR > 1100nm) wavelengths. A mixture of multiwall carbon nanotubes (MWCNTs) and metallic nanoparticles has been nominated as the selectively-absorbing optical filter. Silver nanodiscs ranging from 10nm to 50nm serve as the short wavelength absorbers due to their strong plasmonic properties and low optical scattering in the visible spectrum whilst MWCNTs serve as the long wavelength absorbers because of their high thermal conductivity and strong IR absorption. Melting and subsequent agglomeration of the nanodiscs is prevented by a thin (<5nm), optically transparent, and chemically inert silica shell. The CNTs are functionalised by plasma treatment to remain well-dispersed within the fluid. If stable nanofluids can be produced, these systems have the potential to meet both industrial and domestic heat demands, thus significantly expanding the breadth of the current PV/T market with minimal additional cost.