Symposium Organizers
Subhash L. Shinde Sandia National Laboratories
David H. Hurley Idaho National Laboratory
Gyaneshwar P. Srivastava University of Exeter
Masashi Yamaguchi Rensselaer Polytechnic Institute
W1: Phonon Modes and Dispersion Relations
Session Chairs
Subash Shinde
Gyaneshwar Srivastava
Monday PM, November 28, 2011
Room 313 (Hynes)
9:00 AM - **W1.1
Inelastic Neutron Scattering Measurements and Lattice Dynamics Simulations of Phonon Dispersion and Lifetimes in UO2.
Judy Pang 1 , William Buyers 2 , Alexsandr Chernatynskiy 3 , Mark Lumsden 4 , Doug Abernathy 4 , Bennett Larson 1 , Simon Phillpot 3
1 Materials Science & Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States, 2 National Research Council, Chalk River Laboratories, Chalk River, Ontario, Canada, 3 Department of Materials Science & Engineering, University of Florida, Gainesville, Florida, United States, 4 Neutron Scattering Science Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractUnderstanding lattice thermal conductivity in oxides requires a correct accounting for a wide range of phonon scattering processes, including anharmonic phonon-phonon, phonon-point defect, and phonon-impurity scattering. Uranium oxide (UO2) has a low thermal conductivity as a result of these scattering processes. Over five decades there has been remarkably little high-temperature research on phonons in UO2 to underpin its widespread use as a reactor fuel. We have used reactor and spallation based inelastic neutron scattering measurements of phonon dispersion, group velocity, lifetime, and phonon density of states in UO2 at ambient (295 K) and high temperature (1200 K). High-resolution phonon linewidth measurements made along the [001] direction in UO2 on the HB-3 beamline at the High Flux Isotope Reactor (HFIR) showed that the lifetimes at both ambient and 1200 K (1) depend strongly on the phonon wave vector, (2) do not scale with energy within phonon branches, and (3) do not scale uniformly with temperature. Lattice dynamics third-order anharmonic simulations based on a rigid-ion model (with the short-range part modeled by a Buckingham potential parameterized by Catlow) found qualitatively similar effects, but were found to predict a larger linewidth increase with temperature than observed experimentally. These results will be discussed in relation to the initial momentum and energy constraints on phonon decay into a two-phonon continuum. Estimates of thermal conductivity for UO2 at 1200 K for individual phonon branches based on experimentally measured phonon group velocities and linewidths showed that the transverse acoustical (TA) and longitudinal optical (LO) branches transport heat as efficiently as the longitudinal acoustical (LA) branch. This experimental result will be compared with our lattice dynamics simulations, which predicted only the LO and LA branches to conduct heat significantly. The experimental results will also be compared with a recent first principles calculation that predicted only the LA phonons to contribute significantly to thermal transport in UO2 at high temperature. This research was supported as part of the Center for Materials Science of Nuclear Fuel, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science.
9:30 AM - W1.2
Finite-Size Effects in the Phonon Density of States of Nanostructured Germanium: A Comparative Study of Nanoparticles, Nanocrystals, Nanoglasses and Bulk Phases.
Daniel Sopu 1 , Karsten Albe 1
1 , TU-Darmstadt, Darmstadt Germany
Show AbstractMolecular dynamics simulations are used for studying finite-size effects in the phonon density of states (PDOS) of nanostructured germanium materials such as nanoparticles, nanocrystals, embedded nanoparticles and nanoglasses.By comparing with the PDOS of single crystalline and amorphous structures the physical origins of additional or vanishing vibrational modes or frequency shifts are identified.The changes in the PDOS are mostly due to three size effects: structural discontinuities such as surfaces, grain boundaries and interfaces, tensile surface stresses and phonon confinement due to the finite particle size.Each of these size effects is systematically studied for the different nanostructured materials, separately.In addition, the extension of phonon modes across and along glass-crystal interfaces is studied.Our findings provide a general view on the interplay of nanostructural features and lattice vibrations.
9:45 AM - W1.3
The Vibrational Properties of Ultrananocrystalline Diamond Based on Molecular Dynamics Simulations.
Shashishekar Adiga 1 , Vivekananda Adiga 2 , Robert Carpick 3 , Donald Brenner 4
1 Kodak Research Laboratories, Eastman Kodak Company, Rochester, New York, United States, 2 School of of Applied and Engineering Physics, Cornell University, Ithaca, New York, United States, 3 Department of Mechanical Engineering and Applied Mechanics, University of Pennsylvania, Philadelphia, Pennsylvania, United States, 4 Department of Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina, United States
Show AbstractWe investigate the phonon properties of ultrananocrystalline diamond (UNCD) using molecular dynamics simulations. The UNCD model was prepared by generating grains of random orientations and locations of average size 4 nm using the Voronoi construction. We have characterized the structure in terms of grain boundary and core atoms based on the local structure and the dynamical properties of atoms. We have computed the velocity autocorrelation function and phonon spectra to obtain the specific heat as a function of temperature. The specific heat of UNCD showed enhancements over that of single crystal diamond, and we found that this enhancement is approximately 20% at 300 K. The excess specific heat in UNCD in comparison to single crystal diamond is found to be maximum at approximately 350 K. Further, our calculations of the specific heat of the grain boundary and core atoms show that the excess specific heat arises predominantly due the grain boundary atoms. We will discuss the implications of these findings to thermal and mechanical properties of UNCD.
10:00 AM - W1.4
Phonons in Clathrate Crystals: Harmonic and Anharmonic Modes.
Katsumi Tanigaki 1 2 , Jingtao Xu 1 , Gang Mu 2 , Jiazhen Wu 2 , Satoshi Heguri 2 , Khuong Huynh 2 , Quynh Phan 2 , Youichi Tanabe 1
1 WPI-AIMR, Tohoku University, Sendai Japan, 2 Department of Physics, Graduate School of Science, Tohoku University, Sendai Japan
Show AbstractPhonons as well as electrons and magnons play a very important role as quantized quantities for controlling physical properties of materials. Lattice phonons have been the important issue for understanding temperature evolution of both thermal and electric transports in terms of electron and phonon scatterings for long years. Recently, the concept of phonon is broadly generalized ranging from a collective mode to a localized mode. The former is the typical Debye-mode phonons with a linear κ-ω phonon dispersion, while the latter is the Einstein-mode phonons with the same frequency of all oscillators. Both are generally categorized in the harmonic phonons. Other types of phonons, possibly created from nano-structure materials, have begun to be considered to play an essential role [1-4] and even more importantly such phonons can provide a way of tailoring phonons as phonon engineering. Historically, such typical examples can be found in crystals constructed from the building blocks of clusters as well as molecules consisting of carbon. Thanks to the inner nano spaces in polyhedral network crystals that can accommodate atoms, a large degree of freedom in motions of atomic oscillations can be generated and this is recently drawing much attention as a new mode of phonons to be categorized as anharmonic oscillations. Clathrate crystals have nano cage structure consisting of IVth group elements with face shared. Therefore, these crystals have sufficiently large inner spaces for atomic elements to be confined. Such endohedral atoms move under the anharmonic potentials made by cages and give rise to unique phonons recently known as rattling phonon. These phonons are greatly different from the conventional lattice phonons and therefore produce unique electron-phonon interactions [2,3]. In this talk, we would like to describe how electron-phonon interactions are modified and how heat transport can be influenced. Both of these factors are very important for designing excellent thermoelectric material, a class of materials that convert heat to energy, and this has recently become a subject of intensive scientific investigation. We will also discuss how these phonons can be tailored by changing the elements of the cage.[1] K. Tanigaki, et al, Nature Materials, 2, 653 (2004). [2] Y. Kohama, T. Rachi, Ju Jing, Z. Li, J. Tang, R. Kumashiro, S. Izumisawa, H. Kawaji, T. Atake, H. Sawa, Y. Murata, K. Komatsu, and K. Tanigaki, Phys. Rev. Lett., 102, 013001-013004 (2009).[3] J.Tang, J-T Xu, S. Heguri, H. Fukuoka, S. Yamanaka, K. Akai, and K. Tanigaki, Phys. Rev. Lett., 105, 176402 (2010).[4] J-T Xu, J. Tang, K. Sato, Y. Tanabe, H. Miyasaka, M. Yamashita, S. Heguri, and K. Tanigaki, Phys. Rev.B, 82, 085206 (2010).
10:15 AM - W1.5
Phonon Engineering in Silicon Nanowires Using Stable Isotopes.
Uri Givan 1 , Oussama Moutanabbir 1
1 , Max Planck Institute of Microstructure Physics, Halle Germany
Show AbstractSilicon nanowires have attracted a great deal of attention as powerful nanotechnological building blocks with a potential impact on various fields such as thermoelectrics, photovoltaics, biosensing, and quantum information to name a few. Natural silicon (NATSi) is composed of three stable isotopes: 28Si, 29Si, and 30Si with abundances of 92.23%, 4.67%, and 3.10%, respectively. Several physical properties of semiconductor crystals can be significantly influenced by their isotopic composition. A number of isotope effects are related to the change in the properties of phonons with atomic mass. The most drastic phonon-related isotope effect is found for thermal conductivity resulting from the role of isotope “impurities” as phonon scatterers [1, 2]. In this presentation, we exploit these effects to enable phonon engineering in silicon nanowires using enriched Si isotopes. Here silicon nanowires were grown through metal-catalyzed vapor phase epitaxy using isotopically enriched (>99.9 %) monsilane precursors 28SiH4, 29SiH4, and 30SiH4 [3]. By controlling the isotopic content during the growth process, monoisotopic and isotopically disordered (28Six30Si1-x) nanowires were synthesized. Vibrational spectroscopy was employed to probe the phonon behavior and heat transport in this novel family of nanowires. The implication of our work for silicon nanowire-based thermoelectrics will be discussed.References:[1] I. Pomeranchuk, J. Phys. (Moscow) 5, 237 (1942).[2] P. G. Klemens, Proc. Phys. Soc. London, Sect. A 68, 1113 (1955).[3] O. Moutanabbir et al., Nano Today 4, 393 (2009).
10:30 AM - W1.6
Phonons in Ab Initio Generated Nanoporous Carbon.
Ariel Valladares 1 , Cristina Romero 1 , R. Valladares 2 , Alexander Valladares 2
1 Estado Solido y Criogenia, Instituto de Investigaciones en Materiales, UNAM, Mexico, D.F. Mexico, 2 Departamento de Fisica, Facultad de Ciencias, UNAM, Mexico, D.F. Mexico
Show AbstractWe investigate the vibrational density of states (vDOS) in nanoporous carbon (np-C) obtained from a crystalline diamond-like supercell with 216 atoms. These crystalline structures were subjected to an ab initio molecular dynamics process at constant temperature (1000 K) after the supercell edges were lengthened until we got a density of 1.38 g/cm3, following the procedure implemented by Valladares et al. [1] to generate amorphous porous materials. The density mentioned above represents a porosity of 61% for this diamond-like structure. The resulting sample is fundamentally amorphous and the pore sizes fall into the nanometer length scale [2]. The vDOS obtained is compared to the experimental results for amorphous samples, since no experimental determinations for porous materials exist. Our phonon results are also compared to simulational results for crystalline, amorphous and porous samples found in the literature. [1] Valladares, A.A.; Valladares, A.; Valladares, R.M. Computer modeling of nanoporous materials: An ab initio novel approach for silicon and carbon. Mater. Res. Soc. Symp. Proc. 988 (2007) 97-102.[2] Romero, C.; Valladares, A.A.; Valladares, R.M.; Valladares, A.; Calles, A. Ab initio computationally generated nanoporous carbon and its comparison to experiment. Mater. Res. Soc. Symp. Proc. 1145 (2008) 69-74.
11:15 AM - **W1.7
Phonons in Quantum Dots and Quantum Wires.
Michael Stroscio 1 2 3 , Mitra Dutta 1 3 , Banani Sen 1 , Sushmita Biswas 1
1 ECE, Univ. of IL at Chicago, Chicago, Illinois, United States, 2 BioE Department, Univ. of IL at Chicago, Chicago, Illinois, United States, 3 Physics Department, Univ. of IL at Chicago, Chicago, Illinois, United States
Show AbstractThis presentation will cover recent experimental findings and supporting theory for phonons in nanowires as well as in colloidal quantum dots. The experimetal studies have determined the phonon frequencies in polycrystalline ZnO nanowires as well as confined and interface phonons in selected CdS and CdSe colloidal quantum dots. The supporting theory is based on the continuum model [1-2] and provides comparisons with experimentally determined phonon frequencies for interface and confined phonon modes in quantum dots. Theoretical results also include comparisons of piezoelectric interactions in wurtzite and zincblends wires. References[1] Michael A. Stroscio and Mitra Dutta, Phonons in Nanostructures, (Cambridge University Press, Cambridge, 2001), ISBN: 0521792797; also published in a Russia Language Edition by Dauka Press (2006) and in a Chinese Language Edition, Beijing (2005).[2] Vladimir V. Mitin, Viatcheslav V. Kochelap, and Michael A. Stroscio, Quantum Heterostructures for Microelectronics and Optoelectronics, (Cambridge University Press, Cambridge, 1999). ISBN: 0521631777.
11:45 AM - W1.8
Time-Resolved Optical Spectroscopy of the Vibrational Dynamics of Individual Gold Nanorings.
Renaud Marty 1 2 , Arnaud Arbouet 1 2 , Christian Girard 1 , Adnen Mlayah 1 2 , Vincent Paillard 1 2 , Vivian Kaixin Lin 3 , Siew Lang Teo 3 , Sudhiranjan Tripathy 3
1 , CEMES-CNRS, Toulouse France, 2 , Université de Toulouse, Toulouse France, 3 , Institute for Materials Research and Engineering, A*STAR, Singapore Singapore
Show AbstractIn noble metal nanoparticles (NPs), the existence of localized surface plasmon resonances (LSPR) is responsible for the strong enhancement of their linear and non-linear optical properties, and their ability to concentrate the electromagnetic energy at the nanometer scale. Such NPs are promising for applications in integrated optics, biosensing and nanomedecine. Due to the strong dependence of the LSPR on the size and shape of the metal NPs, surface plasmons are sensitive probes of the mechanical properties of NPs. To overcome the averaging in NPs assembly, high sensitivity optical spectroscopy experiments have been developed to measure the extinction cross-section of individual NPs, as well as perform time-resolved optical spectroscopy.Using femtosecond pump-probe spectroscopy, we investigated the acoustic vibrations of isolated gold nanorings fabricated by electron beam lithography. The transient probe transmission displays clear oscillations, allowing the precise determination of the period and damping times of the vibration modes. The measured periods are compared to those calculated using elastic theory and assuming a continuous elastic medium. Good agreement is found for the lower order axisymetric vibration mode. Finite element numerical simulations confirm this identification and allowed us to study the influence of the shape of the nano-object and the substrate on the detected vibration mode.The damping time of the acoustic vibrations provides a unique opportunity to get information on the mechanical coupling and nanoparticle/environment interface quality. Contrary to ensemble experiments affected by both size and shape distributions, our data on isolated objects allow to get rid of the inhomogeneous contribution, for an unambiguous investigation of the coupling between the object and its environment. The measured damping times vary significantly on the probed nanoring, although the e-beam lithography allows fabrication with unprecedented control. These fluctuations have been ascribed to fluctuations of the mechanical coupling between the nano-object and the substrate. Finally, by changing the environment of the nanoring, we provide a clear evidence of the impact of the surrounding medium on the damping of the acoustic vibrations and compare the strength of the mechanical coupling between the metal nanoparticle and the susbtrate to a reference elastic medium.Reference: Marty et al, Damping of the acoustic vibrations of individual gold nanoparticles, Nanoletters 2011, accepted.
12:00 PM - W1.9
Phonon Confinement in Semiconducting Nanostructures.
Pedro Alfaro 1 , Rodolfo Cisneros 1 , Montserrat Bizarro 1 , Miguel Cruz-Irisson 2 , Chumin Wang 1
1 Instituto de Investigaciones en Materiales, UNAM, Mexico D.F. Mexico, 2 ESIME-Culhuacan, Instituto Politécnico Nacional, Mexico D.F. Mexico
Show AbstractDuring the last decade great research efforts have been focused on the nanostructured semiconductors, such as porous semiconductors and semiconducting nanowires. These nanomaterials possess two unusual features: (1) an extremely high ratio of surface area per unit volume, which could significantly modify the boundary condition of phonon states through its surface reconstruction and atoms adsorbed on the surface, and (2) an important reduction of system size to nanometer scale, becoming the energy levels of most elementary excitations discrete enough to be measured at macroscopic scale. In this work, we study the confinement of optical phonons in nanostructures by using the Raman spectroscopy. In particular, a microscopic theory based on the local bond-polarizability model is presented and applied to the analysis of porous silicon, porous germanium, and nanowires. Within the linear response approximation, the Raman shift intensity is calculated by means of the displacement-displacement Green’s function and the Born model, including central and non-central interatomic forces [1]. For porous systems, the supercell method is used and ordered pores are produced by removing columns of Si or Ge atoms from their crystalline structures. This microscopic theory predicts a remarkable shift of the highest-frequency of first-order Raman peaks towards lower energies, in comparison with the crystalline case. This shift is discussed within the quantum confinement framework and quantitatively compared with the experimental results obtained from porous silicon samples, which were produced by anodizing p-type (100)-oriented crystalline Si wafers in a hydrofluoric acid bath [2]. A good agreement is observed between the theoretical and experimental data, showing a possible phonon confinement in semiconducting nanostructures [3].[1] C. Wang and R. A. Barrio, Phys. Rev. Lett. 61, 191 (1988).[2] R. Cisneros, H. Pfeiffer, and C. Wang, Nanoscale Res. Lett. 5, 686 (2010).[3] P. Alfaro, R. Cisneros, M. Bizarro, M. Cruz-Irisson, and C. Wang, Nanoscale 3, 1246 (2011).
12:15 PM - W1.10
Phonons in Crystals of Nano Materials: How We Separate Phonon Terms in Heat Capacity.
Gang Mu 1 , Jingtao Xu 1 , Jiazhen Wu 1 , Satoshi Heguri 1 , Khuong Huynh 1 , Quynh Phan 1 , Youichi Tanabe 1 , Katsumi Tanigaki 1
1 WPI-AIMR/Department of Physics, Tohoku University, Sendai Japan
Show AbstractPhonons play an important role for controlling physical properties, such as a charge density of wave phase transition as well as a phonon mediated BCS superconductivity. The important information on phonons is generally extracted from specific heat capacity (Cp) measurements and neutron diffraction experimental data. In Cp experiments, both phonons and electrons are generally involved in the entire temperature range. Therefore, the term of electrons can be extracted only by separating the phonon contributions supposing the Debye model and/or the Einstein model in the category of harmonic phonons. In the case of a conventional crystal, this generally gives a good estimate. However, this sometimes can no longer be a reasonable estimate in such special materials as glass and nano structure materials of clathrates (polyhedral network materials) and Fe pnictide (quasi two-dimensional materials) [1-3]. Especially in the case of complex glass structure, many potentials are ordinarily made as minima of the complex energetic curvature and the tunneling mode of electrons or particles should seriously be taken into account because the temperature (T) evolution of Cp becomes T-linear as suggested by Anderson. When the phonon density of states (PDOS) is available experimentally or theoretically, a good estimate of the phonon term in Cp can be evaluated and accordingly one can separate the electron terms accurately for describing the intrinsic electronic contributions. In this meeting, we would like to present how we can describe the phonon contribution in the case of nano materials by employing some examples of clathrate thermoelectrics and Fe pnictide superconductors. [1] J.Tang, J-T Xu, S. Heguri, H. Fukuoka, S. Yamanaka, K. Akai, and K. Tanigaki, Phys. Rev. Lett., 105, 176402_1-4 (2010).[2] J-T Xu, J. Tang, K. Sato, Y. Tanabe, H. Miyasaka, M. Yamashita, S. Heguri, and K. Tanigaki, Phys. Rev.B, 82, 085206_1-6 (2010).[3] G. Mu et al., condmat (2011).
12:30 PM - W1.11
Lattice Dynamics of Polonium: Symmetry Breaking Phase Transitions and Surface Phonons.
Matthieu Verstraete 1 2 , Giorgio Benedek 3 4
1 Physics, University of Liege, Liege Belgium, 2 , ETSF, Louvain la Neuve Belgium, 3 Materials Science, Universita Milano Bicocca, Milan Italy, 4 DIPC, Universidad del Pais Vasco, San Sebastian, Pais Vasco, Spain
Show AbstractThe physics of phase transitions in pure elemental phases is regularly revived by the discovery of strange phenomena in simple systems. Polonium shows very delicate physics, being the only simple cubic element at low temperature and pressure, due to the interplay between relativistic and chemical bonding effects. We show that this balance is broken at higher temperatures, where entropy trumps relativistic effects, exceptionally reducing symmetry upon increasing temperature. The strange vibrational structure of Po carries over to its surface, which also shows an interplay of spin-orbit coupling and surface nesting to stabilize the cubic [100] surface, a signature which should also appear in neutron or alpha particle scattering.
12:45 PM - W1.12
Hypersonic Properties of Polymer Films and Multi-Layers.
Paul Walker 1 , Eric Young 1 , Andrey Akimov 1 , James Sharp 1 , Vitaly Gusev 2 , Anthony Kent 1
1 School of Physics and Astronomy, University of Nottingham, Nottingham United Kingdom, 2 Laboratoire de Physique de l’Etat Condensé, Université du Maine, Le Mans France
Show AbstractPicosecond acoustic measurements were performed on ultrathin polymer films and thin film multilayers of polystyrene and polyvinylpyrollidone supported on silicon (Si) substrates using a state of the art THz acoustic technique. In these experiments, a femtosecond pulsed laser is used to excite picosecond duration strain pulses in an aluminium film evaporated on the reverse side of the Si substrate. These strain pulses then propagate through the substrate and interact with the polymer film/multi-layer. Vibrations in the film are detected optically using an optical probe pulse which is split form the pump laser beam, passed through an optical delay line and reflected from the surface of the polymer film/multi-layer. The reflected beam is detected using a photodiode and lock-in amplifier referenced to an optical chopper in the pump path.Ultrathin films of polystyrene and a styrene-butadiene-styrene block copolymer were found to exhibit quantized closed-pipe organ like modes in the 0- 50 GHz regime that were attributed to vibrations of the entire polymer film [1]. Thin film multilayer structures were found to display folded phonon dispersion curves that are characteristic of super-lattice structures [2]. These structures have potential applications in GHz and THz optical switching and biosensing applications.[1] A. V. Akmov, E. S. K. Young, J. S. Sharp, V. Gusev and A. J. Kent: "Coherent hypersonic closed-pipe organ like modes in supported polymer films", Appl. Phys. Lett., in press (2011).[2] P. M. Walker, J. S. Sharp, A. V. Akimov and A. J. Kent: "Coherent elastic waves in a one dimensional polymer hypersonic crystal", Appl. Phys. Lett. 97, 073106 (2010).
W2: Phonon Scattering
Session Chairs
David Hurley
Masashi Yamaguchi
Monday PM, November 28, 2011
Room 313 (Hynes)
2:30 PM - **W2.1
Understanding the Lattice Thermal Conductivity in Real Materials: Phonon-Phonon and Phonon-Defect Interactions from Atomistic Simulations.
Aleksandr Chernatynskiy 1 , Judy Pang 2 , Bowen Deng 1 , Ben Larsen 2 , William Buyers 3 , Simon Phillpot 1
1 Materials Science and Engineering, University of Florida, Gainesville, Florida, United States, 2 , ORNL, Oak-Ridge, Tennessee, United States, 3 National Research Council, Chalk River Laboratories, Chalk River, Ontario, Canada
Show AbstractDespite tremendous progress in understanding lattice thermal conductivity, there are still many open questions. Advances in the computer power and simulation techniques allow to address those from the fundamental level of the interactions between individual atoms. From the microscopic perspective the interactions of phonons with each other and with lattice defects of control thermal transport. In this talk we will discuss techniques for the calculation of the thermal conductivity and demonstrate their applications. We describe spectral analysis of phonon/phonon interactions, using fluorite-structured UO2 as a prototype. This analysis is performed using Lattice dynamics/Boltzmann Transport Equation based techniques. These methods also allow us to determine phonon lifetimes, which are compared with experimental results on single crystals. The effects of microstructural defects, including dislocations on the thermal-transport properties are analyzed, from non-equilibrium molecular dynamics simulations. Finally, we characterize the mechanistic details of phonon-defect interactions for the specific case of the Si/SiO2/Si interfacial system from phonon wave packet simulations. This work was co-authored by a subcontractor (SRP) of the U.S. Government under DOE Contract No. DE-AC07-05ID14517, under the Energy Frontier Research Center (Office of Science, Office of Basic Energy Science, FWP 1356). Accordingly, the U.S. Government retains and the publisher (by accepting the article for publication) acknowledges that the U.S. Government retains a nonexclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for U.S. Government purposes.
3:00 PM - W2.2
Predicting the Thermal Conductivity of Defected Systems Using the Spectral Energy Density.
Jason Larkin 1 , Alan McGaughey 1
1 , Carnegie Mellon, Pittsburgh, Pennsylvania, United States
Show AbstractAccurately predicting the thermal conductivity of a dielectric or semiconducting material requires the properties of phonons from the entire Brillouon zone. Of particular importance are the phonon lifetimes, which are not accessible in experiment. Common theoretical techniques (e.g., lattice dynamics calculations) require the use of a perfectly periodic crystal to predict the required phonon properties. These techniques, however, break down when the system’s periodicity is broken (e.g., through point defects or the presence of an external fluid). The spectral energy density technique, where the atomic velocities are projected onto traveling waves, can predict the phonon properties of systems with a small perturbation from periodicity. In this study, we demonstrate that the spectral energy density technique can be used to model A_{1-x}B_x Lennard-Jones alloys with mass and bond point defects up to concentrations of x=0.1. The phonon lifetimes of these defected systems show a fourth-power scaling with the inverse of the phonon frequency, consistent with Rayleigh scattering theory. The phonon dispersion is found to agree with predictions from a virtual crystal approximation.
3:15 PM - W2.3
Thermal Transport in Zinc Antimonides.
Lasse Bjerg 1 3 , Georg K. H. Madsen 2 , Jeffrey Grossman 3 , Bo Iversen 1
1 iNANO & Department of Chemistry, Aarhus University, Aarhus C Denmark, 3 Department of Materials Science & Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 ICAMS, Ruhr-Universität Bochum, Bochum Germany
Show AbstractThermoelectric materials are capable of interconverting heat and electricity. To obtain high conversion efficiency, it is desirable to have as low a thermal conductivity as possible in thermoelectric compounds. Zinc antimonides are, generally, interesting as thermoelectric materials. The electronic structure, phonon dispersion, and elastic constants of ZnSb have been calculated using DFT methods. An atomistic potential has been produced and compared to the DFT results for the harmonic energy surface in zinc antimonides. The use of this potential for the calculation of anharmonic effects and thermal conductivity is discussed.
3:30 PM - W2.4
Coherent Acoustic Vibrations and Electron-Phonon Interactions in Nanostructured Copper Arrays.
Andrej Halabica 1 , Jianxun Liu 1 , Pei-I Wang 1 , Masashi Yamaguchi 1 , Gwo-Ching Wang 1 , Toh-Ming Lu 1
1 Physics, Applied Physics & Astronomy, RPI, Troy, New York, United States
Show Abstract Nanoscale confinement effects on phonon properties are critical elements for the thermal and electrical transport properties. Nanoscale copper materials are important ingredient of nanoscale devices, and the electron-phonon interactions in nanoscale copper and related electrical resistivity have been a topic of extensive studies. In this presentation, we report the results of ultrafast spectroscopic study of coherent acoustic vibrations and electron-phonon interactions in copper nanostrips. The samples are periodic copper strips with periods of 100 -150 nm and various strip width of 10-100 nm. Both of polycrystalline and epitaxial single crystal nanostrips are grown on a high resistivity silicon substrate by e-beam lithography. Electron-phonon interactions in copper nanostripes were studied using ultrafast transient reflectivity measuremnets. Electrons near the Fermi surface were excited with optical pump pulses with wavelength of 800 nm to above the Fermi surface. Temporal change of the electron distribution due to the electron-phonon coupling following the excitation pulse was detected by the probe pulses with the energy close to the resonance to d-band to near Fermi surface transition. Electron energy decay was modeled using the two-temperature model.1 At a relatively high pump power, the signal decay profile of the transient reflectivity signal showed dependencies on the pump power and relative orientation of the stripes to the pump pulse polarization. However, at the low pump power limit, these signals taken with different polarization converge to the same decay time and the anisotropy disappears for the sample with the 40 nm width. These power and orientation dependences are due to the nonlinear temperature dependence of the reflectivity and temperature dependent electron heat capacity. In addition, convergence of the signal of different orientation agrees with the fact that the measurement is in the thermal regime within the electron system. Coherent acoustic vibrations were excited with pump pulse with various wavelengths and probed at 800 nm, which is off resonant to the d-band to Fermi energy transition. Two kinds of oscillations with frequencies of 6.0 and 60.2 GHz were observed. Amplitudes of both vibrational components depend on the relative directions of the probe polarization and long axis of the strips, suggesting that the detection mechanism of the vibration is strain induced reflectivity change. While the ratio of the vibrational modes and thermal background does not change with the pump pulse power density, the ratio strongly depends on the excitation pump wavelength. The results suggest that the excitation of the vibrational modes depend on the excited electron energy, and the excitation is not a simple photothermal process.1.M. I. Kaganov, I. M. Lifshitz and L. V. Tanatarov, "Relaxation between Electrons and the Crystalline Lattice", Sov. Phys. JETP, 4, 173 (1957).
4:15 PM - **W2.5
Interaction of Elastic Waves with Dislocations.
Agnes Maurel 1 , Fernando Lund 1 , Pagneux Vincent 1 , Barra Felipe 1
1 Agnes Maurel, LOA/Institut Langevin, Paris France
Show AbstractAn overview of recent work on the interaction of elastic waves with dislocations is given. The perspective is provided by the wish to develop nonintrusive tools to probe plastic behavior in materials. For simplicity, ideas and methods are first worked out in two dimensions, and the results in three dimensions are then described. These results explain a number of recent, hitherto unexplained, experimental findings. The latter include the frequency dependence of ultrasound attenuation in copper, the visualization of the scattering of surface elastic waves by isolated dislocations in LiNbO3, and the ratio of longitudinal to transverse wave attenuation in a number of materials.Specific results reviewed include the scattering amplitude for the scattering of an elastic wave by a screw, as well as an edge, dislocation in two dimensions, the scattering amplitudes for an elastic wave by a pinned dislocation segment in an infinite elastic medium, and the wave scattering by a sub-surface dislocation in a semi-infinite medium. Also, using a multiple scattering formalism, expressions are given for the attenuation coefficient and the effective speed for coherent wave propagation in the cases of anti-plane waves propagating in a medium filled with many, randomly placed screw dislocations; in-plane waves in a medium similarly filled with randomly placed edge dislocations with randomly oriented Burgers vectors; elastic waves in a three-dimensional medium filled with randomly placed and oriented dislocation line segments, also with randomly oriented Burgers vectors; and elastic waves in a model three-dimensional polycrystal, with only low angle grain boundaries modeled as arrays of dislocation line segments.The theory suggests a non-intrusive way of measuring dislocation density in materials, which is confirmed with Resonant Ultrasound Spectroscopy (RUS) experiments using aluminum.
4:45 PM - W2.6
Soft Mode Behavior of Interface-Related Modes in AlGaN/GaN HEMT Structures Probed by UV Raman Scattering.
Esther Alarcon Llado 1 2 3 , Vivian Lin 3 , Surani Dolmanan 3 , Siew Lang Teo 3 , Alois Krost 4 , Armin Dadgar 4 , Sudhiranjan Tripathy 3
1 , Lawrence Berkeley National Laboratory, Berkeley, California, United States, 2 , Ecole Polytechniqu Federale Lausanne, Lausanne Switzerland, 3 , Institute of Materials Research and Engineering, A*STAR, Singapore Singapore, 4 , Otto von Guericke Universitat, Magdeburg Germany
Show AbstractIn the AlGaN/GaN system, a strong bending of the electronic bands leads to the formation of a highly dense and confined electron gas in the GaN, very close to the heterointerface. In such systems, thus, the electronic transport mainly takes places at the interface between AlGaN and GaN. The importance of phonons and their interactions in electronics and optoelectronics is well known. In the case of AlGaN/GaN based devices, interface-related phonons may also play an important role in electrical properties and can provide information about interface properties.In this work we provide a temperature-dependent Raman study of the interface properties of AlGaN/GaN HEMT based structures with high Al contents. AlGaN very thin films (<20nm) grown by different systems (metal-organic chemical vapor deposition and molecular beam epitaxy) and on different kind of substrates were probed. TEM measurements show smooth interfaces for both samples. Raman spectra were recorded in a backscattering configuration using the 325 nm line of a He-Cd laser as excitation source. Under such conditions, the Raman signal is basically confined at the interface due to the strong resonance with GaN.At room temperature, the typical Raman spectrum of such structures shows four main intense features. Apart from the well-known E2h and A1(LO) modes from the GaN buffer, three additional peaks at around 610 (IF1), 703 cm-1 (IF2) and 720 cm-1 (IF3) are observed for both samples. While IF2 has been observed in bulk material and is attributed to a disorder-activated mode characteristic of the one-phonon density of states of the III-N system, IF1 and IF3 have never been reported before and we assign them to interface modes (IF). A low-temperature stage was used to vary the sample temperature from 80 to 450 K. The GaN-related modes, as well as IF2 and IF3, show a typical temperature-induced red-shift. By contrast, the IF1 Raman feature blue-shifts with increasing temperature by more than 12 cm-1 from 80 to 450 K. This soft mode behavior suggests that the mode is strongly related to the interface.By a voltage-dependent study, we also show the large polar character of the IF modes. We observe that the intensity of IF1 and IF2 significantly drops with increasing voltage. We suggest that these additional modes are related to and/or confined in the AlxGa1-xN/GaN interface. Their spectral weight should decrease with increasing film thickness. The very small thickness of the present AlxGa1-xN layers, the strong electric fields at the interface due to the high Al contents, and the use of near-resonant excitation, might be the reason that these modes are observed in our experiments. Finally, we discuss the possible effects of the presence of such modes on AlGaN/GaN-based devices.
5:00 PM - W2.7
Diffuse Scattering of X-Rays in Nanoscale Silicon.
Gokul Gopalakrishnan 1 , Josef Spalenka 1 , David Czaplewski 2 , Martin Holt 2 , Tobias Schulli 3 , Paul Evans 1
1 Materials Science and Engineering, University of Wisconsin, Madison, Wisconsin, United States, 2 Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois, United States, 3 , European Synchrotron Radiation Facility, Grenoble France
Show AbstractThe emerging ability to engineer thermal properties using nanostructures arises in large part from confinement effects on the phonon dispersion. While a number of theoretical predictions have been made for the modification of the phonon band structure due to quantum confinement, so far, only the high energy regime of optical phonons near the Brillouin Zone center have been accurately measured in nanoscale samples. While conventional techniques such as Inelastic x-ray or neutron scattering have been successfully used to probe large wave vectors in macroscopic samples, the signals become impractically weak in small ensembles of nanoscale objects. Advances in high luminosity x-ray sources at synchrotron facilities have revived interest in the technique of Thermal Diffuse Scattering, which collects information from elastic scattering of x-rays by phonons.In this talk we describe results of x-ray diffuse scattering measurements performed on silicon membranes of varying thickness. The membranes were fabricated by selective wet etching of silicon-on-insulator samples releasing suspended silicon windows which were further thinned by reactive ion etching. Samples of thickness from 5 microns down to 50 nanometers were studied using a zone plate focused microbeam of 11 keV x-rays in a transmission measurement geometry. Details of the fabrication process and results of the scattering measurements will be presented.
5:15 PM - W2.8
Manipulation of the Cooling Behavior on the Nanoscale.
Anja Hanisch-Blicharski 1 , Simone Wall 1 , Tim Frigge 1 , Friedrich Klasing 1 , Annika Kalus 1 , Martin Kammler 1 , Horn-von Hoegen Michael 1
1 Department of Physics, University of Duisburg-Essen, Duisburg Germany
Show AbstractHow does a nanoscale thin film on a substrate cool? In order to investigate this seemingly simple question we have used ultrathin epitaxial Bi(111) films on a silicon substrate as a model system to explore the limits of the well accepted and commonly used acoustic mismatch model (AMM). AMM describes the thermal boundary conductance σtbc of films with an abrupt and smooth interface. In the framework of AMM the energy is carried by phonons which were treated as elastic waves. Most of the phonons are trapped in the Bi film due to total internal reflection. Only a small fraction of phonons with incident angles smaller than the critical angle can escape the film and contribute to the heat transport resulting in a low σtbc of only 1400 W/(cm2K). We extended the study to film thicknesses as low as 2nm. σtbc is determined from the cooling rate of the Bi film upon heating with a fs-laser pulse in a pump-probe experiment. The transient temperature was determined via the Debye-Waller effect employing surface sensitive ultrafast electron diffraction in reflection geometry. For films thinner than the mean free path of the phonons, we found that the thermalisation and repopulation of phonons smaller than the critical angle causes the bottleneck for the phonon transmission through the interface. This effect slows down the cooling of the film considerably, by 250%.
5:30 PM - W2.9
Elastic Anomaly of Pd Thin Films at Low Temperatures Studied by Picosecond Ultrasounds.
Kenichi Tanigaki 1 , Naoki Omori 1 , Tatsuya Kusumoto 1 , Hirotsugu Ogi 1 , Nobutomo Nakamura 1 , Masahiko Hirao 1
1 Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka, Japan
Show AbstractThe elastic constants of solids generally increase with cooling down from room temperature, and they remain unchanged below about 100 K since invariability of interatomic distances caused by the zero-point energy. However, at low temperatures, the elastic constants of Pd show unusual temperature dependences in spite of their normal thermal-expansion behavior. Contribution of the change in electron band structure is suggested as one possible cause. However, its intrinsic mechanism remains unclear because it is hard to separate contributions of electron and phonon (band structure and lattice vibration).In this study, we investigate the elastic properties of Pd thin films using the picosecond laser ultrasound (PSLU) method at cryogenic temperatures. The out-of-plane atomic distance of Pd thin films become very short at low temperatures because of restriction of the in-plane deformation by the substrate, and the resultant elastic strain can be significant, which is never achieved for bulk materials. On the other hand, the band structure of thin films is commonly similar to that of bulk materials. Thus, with larger elastic strain, we can estimate the contribution of phonons through the unharmonicity of interatomic potential. We develop an optical system for PSLU measurements at low temperatures, which is equipped with a cryostat vacuumized by a turbo molecular pump. The specimen is set in the cryostat and cooled by liquid He through a Cu heat exchanger. We can conduct the PSLU measurements through a quartz window of the cryostat. A titanium-sapphire pulse laser at 800 nm wavelength and 140 fs pulse width is separated into the pump light and the probe light. The former is used to generate an ultrahigh-frequency (~50 GHz) acoustic pulse through an instantaneous thermal expansion, and the latter used is for detection. The reflectivity change in probe light shows multiple pulse echoes or acoustic-phonon resonances depending on film thickness, which is determined accurately by X-ray reflectivity measurements. They give us the sound velocity along the film thickness direction and eventually the out-of-plane elastic stiffness of films. The elastic modulus of Pd thin films increases remarkably compared with the predicted elastic constant of the textured Pd using its bulk elastic constants at low temperatures. This result indicates the phonon’s contribution is normal. Thus, the electron band structure is considered to play the dominant role in Pd’s elastic anomaly. Furthermore, Ab-initio calculations of the effect of Fermi-Dirac broadening on elastic constants are performed to enhance understanding of the experimental results.
5:45 PM - W2.10
Low Phonon Energy BaCl2 Nanocrystals in Nd3+-Doped Fluorozirconate Glasses and Their Influence on the Fluorescence Properties.
Charlotte Pfau 1 , Ulrich Skrzypczak 1 , Manuela Miclea 1 , Paul-Tiberiu Miclea 2 3 , Christian Bohley 1 , Stefan Schweizer 1 2
1 , Centre for Innovation Competence SiLi-nano®, Martin Luther University of Halle-Wittenberg, Halle (Saale) Germany, 2 , Fraunhofer Center for Silicon Photovoltaics, Halle (Saale) Germany, 3 , Institute of Physics, Martin Luther University of Halle-Wittenberg, Halle (Saale) Germany
Show AbstractMulti-phonon relaxation (MPR) is one of the major fluorescence quenching mechanisms in rare-earth doped glasses. The MPR rate is significantly reduced in hosts providing low phonon frequencies like fluorozirconate (FZ) glasses (< 590 cm-1 [1]), which are based on the standard ZBLAN formulation made from Zr, Ba, La, Al, and Na fluorides [2]. The rare-earth ion Nd3+ shows emissions in FZ-based glasses from energy levels that would be quenched in high-phonon energy glasses. The FZ glasses were additionally doped with Cl ions by partial substitution of the fluorides BaF2 and NaF for BaCl2 and NaCl, respectively. The Cl substitution itself does not change the phonon frequency of the base FZ glass significantly, but subsequent annealing leads to the formation of BaCl2 nanocrystals in the glass. BaCl2 has a distinct lower phonon frequency (< 200 cm-1 [3]) than the FZ base glass. An interaction of the Nd3+ ions with the low frequency phonons of BaCl2 should result in significant influence on the Nd3+ fluorescence. For a deeper understanding, the phonon spectra and the fluorescence properties of the Nd3+- and chlorine-doped fluorozirconate glasses are investigated. Raman measurements of the heat-treated glasses show additional phonon bands at low phonon energies, this can be attributed to BaCl2 nanocrystals. The phonon spectra of the nanocrystals change with the annealing conditions due to a structural phase transition of the BaCl2 nanocrystals. For comparison, the phonon spectra of BaCl2 in different structural phases are calculated from first principles. Furthermore, the influence of the nanocrystals on the fluorescence properties is analyzed and interpreted with respect to a modified MPR. Time-resolved spectroscopy is applied to determine the fluorescence lifetime of the excited Nd3+ levels in different environments. In addition, temperature dependent lifetime measurements are performed to obtain information on multi-phonon relaxations and the phonons involved in the relaxation process. It will be shown that there is a strong interaction between the Nd3+ ions and the phonons of the BaCl2 nanocrystals. The formation of the nanocrystals increases the fluorescence efficiency of several Nd3+ transitions significantly.[1] S. Aasland, M.-A. Einarsrud, and T. Grande, J. Phys. Chem. 100, 5457 (1996).[2] I. D. Aggarwal and G. Lu, Fluoride Glass Fiber Optics (Academic, London, 1991).[3] A. Sadoc and R. Guillo, C. R. Acad. Sci., Ser. B 273, 203 (1971).
Symposium Organizers
Subhash L. Shinde Sandia National Laboratories
David H. Hurley Idaho National Laboratory
Gyaneshwar P. Srivastava University of Exeter
Masashi Yamaguchi Rensselaer Polytechnic Institute
W3: Phonon Transport-Bulk Materials
Session Chairs
Anthony Kent
Ali Shakouri
Tuesday AM, November 29, 2011
Room 313 (Hynes)
9:00 AM - **W3.1
Experimental and Theoretical Studies on Phonon Transport: From Bulk Materials to Nanostructures.
Gang Chen 1
1 , MIT, Cambridge, Massachusetts, United States
Show AbstractIn this talk, we will present recent progress in experimental and theoretical investigationsof phonon transport in bulk materials, and across single interfaces and superlattices.We investigate spectral distribution of phonon mean free path in bulk materials viacombined theoretical and experimental studies. Theoretically, we use first-principlecalculations to extract anharmonic force constants, and compute the phonon relaxationtime due to phonon-phonon scattering. Experimentally, we extend an optical pump-and-probe technique to measure contributions of phonons with different mean free paths tothermal conductivity via systematically changing the size of the heated regions. Thepump-probe experimental system is also used to probe phonon transport across a singleinterface and superlattices. Our experiment design identifies coherent phonon transportin superlattices.This work is supported by S3TEC, a DOE BES funded EFRC.
9:30 AM - W3.2
A Detailed Theoretical Study of the Thermal Conductivity ofBi2(Te0.85Se0.15)3 Single Crystals.
Ovgu Yelgel 1 , Gyaneshwar Srivastava 1
1 Physics, University of Exeter, Exeter United Kingdom
Show AbstractWe present a theoretical investigation of the phonon conductivity of Bi2(Te0.85Se0.15)3 single crystals by using the Debye model within the single-mode relaxation-time approximation and a detailed account of alloy, electron-phonon, and three-phonon interactions [1]. Different levels of n-doping from SbI3 and CuBr dopants were considered. The total thermal conductivity was obtained by combining the electronic polar (κel), lattice (κph), and electronic bipolar (κbp) contributions. The κel contribution was calculated within the nearly-free electron model [2], and the κbp contribution was obtained by employing Price’s theory [3]. The ‘dip’ observed in the thermal conductivity-temperature curve for the alloy at around 350 K [4] is successfully explained to arise from the joint contribution from phonons, donor electrons, and electron-hole pairs. Fermi energy, Seebeck coefficient and electrical conductivity were calculated within the nearly-free electron approximation applying the Fermi-Dirac statistics. The computed results are used to explain the temperature variation of thermoelectric figure of merit (ZT) reported in the experimental measurements by Hyun et al. [4]. The ZT of the alloy doped with 0.1 wt.% CuBr is improved by more than 50% over the results obtained for the Bi2Te3 single crystal.[1] G. P. Srivastava, ‘The Physics of Phonons’, (Taylor and Francis Group, New York, 1990).[2] J. M. Ziman, ‘Electrons and Phonons’, (Clarendon Press, Oxford, 1960).[3] P. J. Price, Phil. Mag., 46, 1252 (1955).[4] D. B. Hyun et al., J.Mat.Sci., 33, 5595 (1998).
9:45 AM - W3.3
Thermal Conductivity of Gallium Arsenide from First-Principles.
Jivtesh Garg 1 , Tengfei Luo 1 , Keivan Esfarjani 1 , Junichiro Shiomi 1 2 , Gang Chen 1
1 Mechanical Engineering, MIT, Cambridge, Massachusetts, United States, 2 Mechanical Engineering, Tokyo University, Tokyo Japan
Show AbstractWe compute the thermal conductivity of GaAs from first-principles by using harmonic and anharmonic force constants which are derived from density-functional perturbation theory. Boltzmann transport equation is solved in the single-mode relaxation time approximation to obtain phonon relaxation times which are then used along with phonon frequencies, group velocities and Bose-Einstein populations to compute the thermal conductivity. The approach is found to yield excellent agreement with experimentally measured values. The predictive power of these first-principles calculations allows to access information of accumulative thermal conductivity with respect to phonon mean free path which can be used to lay out design rules for low thermal conductivity materials. For example at 300 K we show that 50% of the heat is conducted by phonons of mean free path larger than 150 nm. This information can be used to design nanostructured materials where additional scattering mechanisms introduced by the presence of grain boundaries or nanoparticles distributed around these optimal length scales can reduce the phonon mean free path and thus the thermal conductivity.(This material is based upon work supported as part of the S3TEC, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-FG02-09ER46577.)
10:00 AM - W3.4
Phonon Conduction in Lead Selenide from First Principles.
Zhiting Tian 1 , Keivan Esfarjani 1 , Takuma Shiga 2 , Jivtesh Garg 1 , Junichiro Shiomi 2 , Gang Chen 1
1 Mechanical Engineering, MIT, Cambridge, Massachusetts, United States, 2 , University of Tokyo, Tokyo Japan
Show AbstractLead selenide (PbSe) is much less frequently considered for thermoelectric than its sister material PbTe. Counter-intuitively, PbSe has even lower thermal conductivity despite the lighter mass. The low thermal conductivity of PbSe might lead to potential good thermoelectric performance. In this study, we performed first-principles calculations to detail the spectral phonon transport properties of PbSe and to understand the low thermal conductivity. We first extract harmonic and anharmonic force constants from density functional theory calculations within a supercell. After validating anharmonic force constants, perturbation theory is used to extract the phonon lifetimes and compute the thermal conductivity. The frequency and polarization dependent phonon lifetimes and thermal conductivity accumulation with phonon mean free paths will be presented. Compared to other modes, TO modes turn out to be highly anharmonic with very short lifetimes, especially near the center of the first Brillouin zone. Consequences related to thermal transport in PbSe will be discussed.This material is based upon work supported as part of the S3TEC, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-FG02-09ER46577
10:15 AM - W3.5
Influence of Impurities on Phonon Transport in ZnO and ZnS: Ab Initio Studies.
Michael Bachmann 1 , Michael Czerner 1 , Saeideh Edalati-Boostan 1 , Christian Heiliger 1
1 1.Physikalisches Institut, Justus-Liebig Universität, Giessen Germany
Show AbstractWe present ab initio calculations of ballistic phonon scattering at sulphide impurities in ZnO and of oxygen impurities in ZnS. The interatomic potential of the scatterer is calculated using density functional theory. In particular, the interatomic force constants are calculated, which are used in a atomistic Green’s function (AGF) method [1] to calculate the transmission function and the phonon density of states. Knowing the transmission function we calculate the thermal conductance within the linear response regime. We estimate the influence of such impurities on the conductance by comparing our results with calculation of pure ZnO and pure ZnS. In particular, we investigate the frequency dependence of the scattering.[1] W. Fisher, T. & Mingo, N.Numerical Heat Transfer, Part B: 2007, 51, 333
10:30 AM - W3.6
Effect of Mechanical Alloying on Lattice Thermal Conductivity of Uni-Axial Hot Pressed Chromium Disilicide.
Suresh Perumal 1 , Ujwala Ail 2 , Stephane Gorsse 2 , Arun Umarji 1
1 Materials Research Centre, Indian Institute of Science, Bangalore, Karnataka, India, 2 Institut de Chimie de la Matière Condensée de Bordeaux , ICMCB-CNRS, Bordeaux France
Show AbstractNowadays, searching for a renewable energy sources have been a major challenge for researcher working in a various fileds such as solar cell, fuel cells, hydrogen storage and so on. Among them, thermoelectric effect also contributes by converting waste heat into electricity with non-mechanical movable parts and receives much attention due to its thermal, chemical and mechanical stability. Transition metal silicides are being studied extensively these days due to its structural and thermal stability at high temperatures. Chromium disilicide (CrSi2) is degenerate semiconducting material with narrow band gap of 0.35eV and belongs to hexagonal C40 crystal structure with space group of P6222. CrSi2 can be a predominant material for high temperature thermoelectric applications because of its high electrical conductivity (σ = 1x105 S/m at 300K) and Seebeck coefficient (S = 100 µV/K at 300K) over a temperature range. However, its practical applications are limited due to high thermal conductivity (κ = 10 W/m.K). The total thermal conductivity can be reduced by decreasing the grain size which is done by mechanical alloying. Hence an attempt has been made for reducing lattice thermal conductivity by mechanical alloying which is largely contributing to total thermal conductivity in CrSi2. The stoichiometric composition of CrSi2 has been prepared by vacuum arc melting under argon atmosphere. Powders of ingots of CrSi2 have been subjected to mechanical alloying using Tungsten Carbide (WC) balls and vials with ball to powder ratio of 15:1. The milled nano crystallite size powders have been hot-pressed in graphite die under argon atmosphere with the pressure of 42 MPa at temperature of 1523K for the duration of 5 minutes. The dense pellets are subjected to transport studies in homemade apparatus such as electrical resistivity by Van der Paw method and Seebeck coefficient in the temperature range of 300K to 800K. Thermal conductivity measurement has been done by Laser-flash method and show substantial reduction in the lattice thermal conductivity due to mechanical alloying. The figure of merit of mechanically alloyed CrSi2 has been calculated and will be presented.
11:15 AM - **W3.7
Advanced Techniques for Nanothermal Metrology.
Stefan Dilhaire 1
1 LOMA, University of Bordeaux, Talence France
Show AbstractEnergy transport is fundamental to both improving our understanding of basic material properties and advancing the intelligent design of new and more optimally functional materials. Energy transport is mediated by various particles and their subsequent interactions, including electrons, photons, phonons, plasmons, spinons, and excitons. The precise nature of any material determines which of these are most important. A more complete understanding of materials currently in development is of critical importance for a wide range of applications ranging from energy harvesting photovoltaics and thermoelectrics to next generation molecular electronics and mechanisms of material failure in nanoelectronics. Currently, there are a number of unanswered questions that are impeding the advancements of many important real-world applications, including the development of green technologies. Our research goal is to answer some of these questions. For example, how does nano-structuring --- such as changes in dimensionality, layering, and nanoparticle impregnation --- change energy transport characteristics? What effects do chemical and structural modifications on the near-field scale have on far field measurements of energy transport? Is it possible to decouple electronic and thermal transport in materials? What is the source of heat generation in nano electronics systems? The goal of this work deals with the design of materials with more precise and efficient control of energy transport by managing thermal properties of those systems or make them work to our advantage.Currently, pump-probe spectroscopy is being successfully applied to energy transport measurements using techniques such as picosecond acoustics and time domain thermal reflectance with high temporal resolution [1]. These measurements, however, are limited by the diffraction to half the wavelength light. Quasiparticle excitations such as plasmon- [2] and phonon-polaritons [3,4] dominate near field, sub-wavelength effects, and high spatial resolution is required to probe them. Scanning-probe microscopy offers high spatial resolution and local probing of near field effects, but is wanting of temporal resolution. We will present different experimental approaches to measure thermal properties of nano structured materials taking example such as Nano-particles, nano wires, super-lattices, and thin films.[1] G. Pernot et al., Precise control of thermal conductivity at the nanoscale through individual phonon-scattering barriers, Nature Materials 9, 491 (2010)[2] S. Maier et al., Local detection of electromagnetic energy transport below the diffraction limit in metal nanoparticle plasmon waveguides, Nature Materials 2, 229 (2003)[3] S. Shen et al., Surface phonon polaritons mediated energy transfer between nanoscale gaps, Nano Lett. 9, 2909 (2009)[4] E. Rousseau et al., Radiative heat transfer at the nanoscale, Nature Photonics 3, 514 (2009)
11:45 AM - W3.8
Phonon Transport Model for Nanograin Polycrystalline Semiconductor Materials.
Martin Maldovan 1
1 Materials Science and Engineering, Massachusetts Institute of Technology (MIT), Cambridge, Massachusetts, United States
Show AbstractUnderstanding and controlling phonon transport in polycrystalline semiconductors is important for the efficiency of electronic devices and thermoelectric materials. In this paper, we study the effects of phonon grain boundary scattering on thermal energy transport in polycrystalline semiconductor materials across multiple grain sizes and temperatures. We calculate the reduction of phonon transport in polycrystals by using a novel theoretical approach based on the kinetic theory of transport processes. The approach involves an exact expression for the reduction of the phonon mean free paths that includes their directional, frequency, and polarization dependence. We show that phonon transport in polycrystalline materials is primarily controlled by the reduced thermal conductivity of the grain rather than the thermal boundary Kapitza resistance. We also show that in order to accurately calculate phonon transport in nanostructures, the proportion of heat transported by transverse and longitudinal phonons must be correctly taken into account. By using the model, we study grain boundary phonon scattering effects on the reduction of the thermal conductivity of polycrystalline silicon and silicon carbide. The calculated results are compared with experiments at different temperatures and grain sizes without using any free adjustable variable. The theoretical and numerical results provided in this paper help to understand and control the transport of phonons in polycrystalline semiconductors, which can be used to enhance the efficiency of electronic devices and thermoelectrics materials.
12:00 PM - W3.9
Onset of Size Effects in Lattice Thermal Conductivity and Lifetime of Low-Frequency Thermal Phonons.
Alexei Maznev 1
1 Chemistry, MIT, Cambridge, Massachusetts, United States
Show AbstractSignificant deviations of room temperature thermal transport from the diffusion model in semiconductors such as Si and GaAs have recently been observed at distances much larger than previously thought [1]. These observations, together with recent theoretical work [2,3], indicate the large role of low-frequency thermal phonons with mean free path >1 μm in room temperature thermal conductivity of single crystals. While rigorous analysis of phonon transport normally requires extensive numerical calculations, the onset of size effects lends itself to analytical treatment due to the fact that only low-frequency acoustic phonons are subject to size effects at the onset. Assuming a quadratic frequency dependence of phonon relaxation rates, we will derive simple closed-form formulae for the reduced effective thermal conductivity for two examples: (i) in-plane thermal conductivity of thin membranes, (ii) relaxation of a thermal grating. In either case, the thermal conductivity reduction scales as the square root of the characteristic length. The slow square root dependence leads to a prediction of measurable size-effects even at “macroscopic” distances exceeding ~100 μm. However, this prediction hinges on the extrapolation of the ω-2 dependence of phonon lifetimes to low frequencies, which is known to break down in the sub-THz range due to the transition from Landau-Rumer’s to Akhiezer’s mechanism of phonon-phonon relaxation [4]. The correction to thermal transport calculations due to Akhiezer’s relaxation will be discussed and estimated. This material is based upon work supported as part of S3TEC, a DOE BES EFRC under Award Number DE-SC0001299.[1] J.A. Johnson et al., in Proc. 2011 MRS Spring Meeting, to be published.[2] A. Henry and G. Chen, J. Comp. Theor. Nanosci. 5, 1 (2008).[3] A. Ward and D. A. Broido, Phys. Rev. B 81, 085205 (2010).[4] B.C. Daly et al., Phys. Rev. B 80, 174112 (2009).
12:15 PM - W3.10
High Interface Density Materials for Enhanced Thermal Insulation.
Dieter Brommer 1 , Kripa Varanasi 1
1 Mechanical Engineering, MIT, Cambridge, Massachusetts, United States
Show AbstractThermal contact resistance results in reductions in heat flux at material interfaces. This phenomenon has been exploited to create high performance insulators. The low thermal conductivity of aerogels, for example, is due to its structure consisting of interfaces with spacing of the dimensions of its particulate constituents. This work focuses on developing low-cost alternatives to aerogels with equivalent low thermal conductivity by utilizing interfaces at different length scales. We present a range of thermal conductivities based on the medium’s interface to volume ratio. By controlling the level of sintering of various nanopowders with an array of particle sizes, we modulate the volume to surface area ratio and show a correlation between insulator performance and the interface topology. We note a significant reduction in thermal conductivity when compared to bulk values and develop strategies for changing the performance of the proposed insulator by varying the composition of the medium between the particles; (i.e. gas, liquid, or solid). These studies provide pathways for inexpensive and scalable manufacturing of high performance thermal insulating coatings along with solutions for bulk insulation.
12:30 PM - W3.11
Influence of Embedded Nanoparticles on Frequency Dependent Thermal Conductivity of Alloy.
Gilles Pernot 1 , Laura Cassels 2 , Amirkoushyar Ziabari 1 , Bjorn Vermeersch 1 , Peter Burke 3 , Hong Lu 3 , Arthur Gossard 3 , Joshua Zide 2 , Ali Shakouri 1
1 Electrical Engineering, University of California Santa Cruz, Santa cruz, California, United States, 2 Department of Materials Science and Engineering, University of Delaware, Newark, Delaware, United States, 3 Materials Department, University of California Santa Barbara, Santa Barbara, California, United States
Show AbstractOver the past three decades, ultrashort laser pulses have been demonstrated to be a very powerful tool to investigate materials properties at the nanoscale. A key driving force is the high-time resolution required to study heat transfer across interfaces and in thin films. The Time-Domain Thermoreflectance (TDTR) is now widely used. This optical technique offers an interesting alternative to electrical approaches such as the 3ω method. In 2007, Koh and Cahill reported a reduction of the apparent thermal conductivity in semiconductors alloys at the MHz scale. They related this reduction to the thermal penetration length of the modulated heat source and claimed that phonons with a mean free path longer than the thermal penetration length do not contribute to the apparent thermal conductivity. However the mechanisms of this reduction and the influence of ballistic phonons on the total energy and heat transport in the layer are still unclear.We study a series of InGaAs alloy samples containing ErAs or TbAs nanoparticles (NP). Heat transport as a function of TDTR heat source modulation frequency is investigated. We show how the NPs concentration and composition affect the measured thermal conductivity and we report the first observation of a frequency independent thermal conductivity in crystalline alloy materials. The experimental results are compared with a theoretical model of heat transfer including both ballistic and diffusive phonons. These results highlight that Time Domain Thermoreflectance is a powerful technique to measure contribution of ballistic and diffusive phonons on heat transport.
W4: Phonon Transport-Thermoelectric Materials
Session Chairs
Stefan Dilhaire
Paulos Santos
Tuesday PM, November 29, 2011
Room 313 (Hynes)
2:45 PM - **W4.1
Ballistic and Diffusive Transport of Heat in Nanostructured Thermoelectric Materials.
Ali Shakouri 1 , Gilles Pernot 1 , Paul Abumov 1 , Bjorn Vermeersch 1
1 , Univ. of California Santa Cruz, Santa Cruz, California, United States
Show AbstractIn this talk we review recent advances in nanostructured thermoelectric materials based on metal semiconductor multilayers and alloys with embedded nanoparticles. Role of nanostructures in scattering mid and long wavelength phonons and reducing ballistic transport of heat will be discussed.
3:15 PM - W4.3
An Extensive Theoretical Study of the Phonon Conductivity and Thermoelectric Properties of SiGe Alloys.
Iorwerth Thomas 1 , Gyaneshwar Srivastava 1
1 School of Physics, University of Exeter, Exeter United Kingdom
Show AbstractWe present an extensive theoretical study of the phonon conductivity κ_ph of n-doped Si0.75Ge0.25 alloys over a wide temperature range. The phonon dispersion relations are obtained by employing the density-functional-perturbation scheme [1], and the required Brillouin zone summations are carried out using the special q-points scheme [2]. Thecubic anharmonic potential has been expressed by treating the Gruneisen constant as a semi-adjustable mode-average and temperature dependent parameter [3]. The total thermal conductivity κ is calculated by adding the electronic and bipolar contributions to κ_ph. Calculations are also performed within the nearly-free-electron approximation for the temperature variation of the Fermi energy, Seebeck coefficient S, and electrical conductivity σ. The results for σ are compared with the measurements made by Abeles et al [4], and the calculated values of S and σ are compared with experimental measurements by Meddins and Parrott [5]. We then compare our theoretically obtained results for the thermoelectric figure-of-merit ZT = S^2σT/κ with experimentally measured values [5]. By altering the strength of alloy scattering within a reasonable range, we provide a quantitative description of the role of phonon conductivity in enhancing ZT. [1] S. Baroni et al, Rev. Mod. Phys. 73, 515 (2001). [2] H. J. Monkhorst and J.D. Pack, Phys. Rev. B 13, 5189 (1976). [3] G. P. Srivastava, The Physics of Phonons (Adam Hilger, Bristol, 1990). [3] A.AlShaikhi and G. P. Srivasdtava, Phys. Rev. B 76, 195205 (2007). [4] B. Abeles, D. S. Beers, G. D. Cody, and J. P. Dismukes, Phys. Rev. 125, 44 (1962). [5] H. R. Meddins and J. E. Parrott, J. Phys. C: Solid State Phys. 9, 1263 (1976).
3:30 PM - W4.4
High Thermoelectric Figure of Merits of the Crystals of Halogen Doped In4Se3-xH0.03 (H = F, Cl, Br, I): The Effect of Peierls Distortion on the Phonon Scattering with Halogen Doping.
Kyunghan Ahn 1 , Jong-Soo Rhyee 2 , Kyu Hyoung Lee 1 , Sang Il Kim 1 , Eunseog Cho 1 , Sang Mock Lee 1
1 Advanced Material Research Center, Samsung Advanced Institute of Technology, Yongin-si Korea (the Republic of), 2 Department of Applied Physics, Kyung Hee University, Yongin-si Korea (the Republic of)
Show AbstractThermoelectric (TE) power generation is the focus of considerable attention because of the potential for environmentally benign and cost-effective conversion of waste heat to electricity. The search for high TE efficiency materials is a quest to maximize the dimensionless figure of merit ZT = (σS2/κ)T, where σ is electrical conductivity, S is Seebeck coefficient, and κ is thermal conductivity. Our recent study showed that a high ZT of ~1.5 at ~700 K for In4Se3-x crystals was achieved mainly due to Peierls distortion in a charge density wave (CDW) system. It has been found that a strong electron-phonon coupling in the CDW breaks the translational symmetry of lattice and thus the in-plane distortion by CDW of layered In4Se3-x crystals resulted in a very low thermal conductivity. The doping of chlorine in In4Se3-x crystals has been shown a significant enhancement in ZT over a wide temperature range compared to In4Se3-x crystals. In this study we explored the systematic investigation of halogen doping on TE properties of In4Se3-x crystals in an effort to clearly understand the origin of high thermoelectric performance of halogen doped crystals over a wide temperature range. We present detailed investigations of structural and spectroscopic data as well as transport properties including electrical conductivity, Seebeck coefficient, Hall effect, and thermal conductivity data on the crystals of In4Se3-xH0.03 (H = F, Cl, Br, I). An enhancement in ZT is observed and its origin will be discussed.
3:45 PM - W4.5
MBE-Grown IV-VI Semiconductor Structures for Thermal Conductivity Measurements.
Patrick McCann 1 , Leonard Olona 1 , Zhihua Cai 1 , James Jeffers 1 , Khosrow Namjou 1
1 ECE, University of Oklahoma, Norman, Oklahoma, United States
Show AbstractThermal conductivity reduction is one of the more promising approaches to improve the ZT figure of merit for thermoelectric materials. For example, Kanatzidis et al. reported a more than doubling of material ZT to 1.5 relative to a ZT value of 0.7 for bulk PbTe at 642 K due primarily to a low lattice thermal conductivity value of 0.38 W/mK based on flash diffusivity measurements of spinodally decomposed PbSnTe-PbS materials, more than 5 times lower than the 2.1 W/mK thermal conductivity value for bulk PbTe. Spinodal decomposition produces multilayered nanostructures where alternating layers with different compositions on the order of 10 nm thick can be effective in scattering or reflecting phonons. In this work, we report on the molecular beam epitaxial (MBE) growth and photoluminescence (PL) characterization of thin film IV-VI semiconductor heterostructures on silicon substrates. MBE growth allows deliberate synthesis of multilayered nanostructures that can be designed to reduce thermal conductivity. In addition, MBE-grown structures can also incorporate PL-emitting multiple quantum well (MQW) layers to allow hot spot temperature measurement within the epitaxial layer. Finite element analysis (FEA) is then used to extract the cross-plane thermal conductivity of various materials epitaxially grown on silicon.The IV-VI semiconductor structures used here consisted of a PL-emitting PbSrSe/PbSe MQW layer on top of an experimental thermoelectric (TE) material grown on (111)-oriented silicon using techniques described previously. The MQW and TE layers were on the order of 700 nm and 2 microns thick, respectively. PL measurements were performed at various heat sink temperatures and near-IR diode laser pumping levels producing absorbed optical power levels of between about 1 and 2 watts. Measured PL blue shifts were used to determine localized lattice temperatures within the MQW layer, and FEA was used to obtain the cross-plane thermal conductivity of the TE material. This PL temperature measurement technique, which has been used previously to demonstrate improved thermal management when transferring layers from a growth substrate to copper, is attractive because it can be performed immediately following MBE growth without any need for post-growth processing as is necessary for other thin film thermal conductivity measurement techniques such as three omega or time-domain thermoreflectance. A TE material consisting of a PbSnSe/PbSe superlattice (SL) with three different periodicities of 2.4 nm, 3.6 nm, and 4.8 nm revealed a cross-plane lattice thermal conductivity of 0.80 W/mK. This represents a 58% reduction as compared to the 1.9 W/mK value for bulk PbSe. Work is continuing on investigating other short-period SL materials to achieve further reductions and to help determine whether phonon scattering or phonon reflection is the dominant reason for this reduction.
4:30 PM - W4.6
Phonon Transport and Thermoelectricity in Twinned InAs Nanowires.
Annie Weathers 1 3 , Arden Moore 1 3 , Jaehyun Kim 1 3 , Daniel Salta 2 3 , Kimberly Dick 4 5 , Lars Samuelson 4 , Heiner Linke 4 , Philippe Caroff 6 , Li Shi 1 2 3
1 Department of Mechanical Engineering, The University of Texas at Austin, Austin, Texas, United States, 3 Center for Nano and Molecular Science, The University of Texas at Austin, Austin, Texas, United States, 2 Material Science and Engineering Graduate Program, The University of Texas at Austin, Austin, Texas, United States, 4 The Nanometer Structure Consortium (nmC@LU) and Division of Solid State Physics, Lund University, S-22100 Lund Sweden, 5 Polymer & Materials Chemistry, Lund University, S-22100 Lund Sweden, 6 Institut d'Electronique, de Microelectronique et de Nanotechnologie, UMR CNRS 8520, F-59652 Villeneuve d'Ascq France
Show AbstractThere have been reports of improvements in the thermoelectric figure of merit through the use of nanostructured materials to suppress the lattice thermal conductivity. Here, we report on a fundamental study of the combined effects of periodic twinned planes and surface scattering on phonon transport and thermoelectric properties of twinned InAs nanowires. A microfabricated device is employed to measure the thermal conductivity and thermopower of individual suspended indium arsenide nanowires grown by metalorganic vapor phase epitaxy. The four-probe measurement device consists of platinum resistance thermometers and electrodes patterned on two adjacent SiNx membranes. A nanowire is suspended between the membranes, and electrical contact between the nanowire and the platinum electrodes is made with a Ni/Au deposition through a shadow mask. The exposed back side of the device substrate allows for characterization of the crystal structure of the suspended nanowire with transmission electron microscopy (TEM) following measurement. InAs nanowires (diameter 100-200 nm) were intentionally grown with zincblende structure with periodically spaced twin planes perpendicular to the nanowire growth direction, as confirmed by TEM analysis. Compared to bulk zincblende InAs, we find that phonon scattering at the twinned planes and at the nanowire surface reduces the thermal conductivity by more than a factor of two. The effect of twinned planes on the thermal conductivity is determined by comparing the conductivity of InAs nanowires with and without periodic twin defects.
4:45 PM - W4.7
Enhancement of Seebeck Coefficient in Bi0.5Sb1.5Te3 with High Density Tellurium Nanoinclusions.
Sang Il Kim 1 , Kyunghan Ahn 1 , Donghee Yeon 1 , Sungwoo Hwang 1 , Hyun Sik Kim 1 , Sang Mock Lee 1 , Kyu Hyoung Lee 1
1 Advanced Materials Research Center, Samsung Advanced Institute of Technology, Samsung Electronics, Yongin-si,, Gyeonggi-do, Korea (the Republic of)
Show AbstractThe authors have studied the enhancement of Seebeck coefficient by using Te nanoinclusions embedded in Bi0.5Sb1.5Te3 films made by using multi-target pulsed laser deposition technique. Bi0.5Sb1.5Te3 layers and layers of Te nanoparticles were alternatively deposited, constructing Bi0.5Sb1.5Te3 films with the relatively homogeneously dispersed ~15 nm diameter Te nanoparticles. As the amount of the tellurium nanoinclusions increases up to 15 vol %, the Seebeck coefficient was increased greatly from 169 to 248 μV/K. The authors conclude that the high density Te nanoinclusions in Bi0.5Sb1.5Te3 result in carrier energy filtering effect. Consequently, the thermoelectric power factor was enhanced over by 30% despite of reduction of electrical conductivity. The improvement of the power factor implies direct enhancement of the thermoelectric figure of merit ZT, providing the possibility of further ZT improvement by embedding Te nanoinclusions into Bi0.5Sb1.5Te3 bulk materials.
5:00 PM - W4.8
Enhancement of Thermoelectric Transport Properties in CoSb3–Based Skutterudites through Incorporation of Sn.
Si Hui 1 , Kevin Pipe 1 , Ctirad Uher 2
1 Mechanical Engineering, University of Michigan, Ann Arbor, Michigan, United States, 2 Physics, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractSkutterudites have shown promise as bulk materials for thermoelectric energy conversion at intermediate to high temperatures. Decades of research focused on n-type skutterudites has achieved high ZT in these materials through multiple-filling techniques and nanostructuring. However, p-type skutterudites have received relatively little attention even though they have equal importance in a thermoelectric device.In this work we examine the incorporation of Sn into CoSb3–based skutterudites, where it functions as both a p-type electronic dopant and a scattering center for phonons. Using a melting-annealing-sintering method, we synthesize CoSb3-xSnx skutterudites and use X-ray diffraction to determine the range of alloy fractions (x) over which stable Sn incorporation into the CoSb3 structure is possible and phase separation does not occur. We then perform measurements of thermal diffusivity, heat capacity, electrical conductivity, Seebeck coefficient, and Hall coefficient over the range of realizable alloy fractions across a wide temperature range.From measured reductions in thermal conductivity, we analyze the additional phonon scattering that takes place in CoSb3–based skutterudites upon Sn incorporation. We furthermore examine the possibility that Sn acts as a resonant dopant in CoSb3, distorting the density of states in the valence band near the Fermi energy and enhancing the Seebeck coefficient in a manner analogous to thallium dopants in p-type PbTe [1].[1] J. P. Heremans, V. Jovovic, E. S. Toberer, A. Saramat, K. Kurosaki, A. Charoenphakdee, S. Yamanaka, and G. J. Snyder, Science 321, 554 (2008)
5:15 PM - W4.9
Phonon Transport in Nanoporous and Nanostructured Bismuth Telluride.
Yanliang Zhang 1 , Rutvik Mehta 2 , Matthew Belley 1 , Ganpati Ramanath 2 , Theodorian Borca-Tasciuc 1
1 Mechanical, Aerospace, and Nuclear Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States, 2 Materials Science and Enginerring, Rensselaer Polytechnic Institute, Troy, New York, United States
Show AbstractThis work investigates thermal transport in nanoporous and nanostructured Bi2Te3 pellets with mass densities in the range of 75%-95% of bulk. The nanoporous pellets are fabricated by compacting and sintering 5-25 nm thick Bi2Te3 nanoplates prepared by surfactant-mediated microwave-stimulated synthesis. The lattice thermal conductivity and Lorentz number were experimentally determined from samples with various doping concentrations. We found a lattice thermal conductivity as low as 0.3 W/mK, a value ~75% lower than in single-crystal bulk Bi2Te3, and approaching the theoretically predicted lower limit. The mechanisms responsible for lattice thermal conductivity reduction were analyzed by fitting the experimental results with an integrated Debye-Callaway effective medium model. In addition, the measured electron mobility of the nanoporous Bi2Te3 structures was found to be comparable to single-crystal bulk counterparts of similar carrier concentrations. Since the material investigated here yields phonon-blocking electron-transmitting properties, it opens up new opportunities for obtaining high performance thermoelectric materials.
Symposium Organizers
Subhash L. Shinde Sandia National Laboratories
David H. Hurley Idaho National Laboratory
Gyaneshwar P. Srivastava University of Exeter
Masashi Yamaguchi Rensselaer Polytechnic Institute
W5: Phonon Transport - Nano Materials I
Session Chairs
Judy Pang
Clivia Sotomayor Torres
Wednesday AM, November 30, 2011
Room 313 (Hynes)
9:15 AM - **W5.1
Ab Initio Thermal Transport in Materials and Nanostructures.
Derek Stewart 1 , Natalio Mingo 2 , David Broido 3
1 Cornell Nanoscale Facility, Cornell University, Ithaca, New York, United States, 2 LITEN, CEA-Grenoble, Grenoble France, 3 Department of Physics, Boston College, Chestnut Hill, Massachusetts, United States
Show AbstractThe constant hum of laptop fans and the roar of cooling units in server farms highlights the critical and often unnoticed role of heat transfer in modern digital society. A clear understanding of thermal conductivity is crucial for several fields, including thermoelectrics, thermal barrier coatings, heat mitigation, and even the thermodynamics of planetary cores. However, developing an accurate theory of lattice thermal conductivity in materials has remained a long-standing and unsolved problem. To address this issue, we have developed a new ab-initio framework that accurately predicts thermal transport in both materials and nanostructures. This approach uses density functional theory to calculate harmonic and where necessary anharmonic force constants between atoms. In this talk, I will discuss two separate techniques for diffusive and ballistic phonon transport that build on this ab-initio foundation. By using these force constant terms in an iterative solution to the phonon Boltzmann transport equation, we can predict thermal conductivities for materials (i.e. Si, Ge, and diamond [1,2]) in excellent agreement with experiment and without the use of adjustable parameters. In addition, this Boltzmann approach is also well suited for examining nanocomposites, such as nanoparticle embedded in alloy thermoelectric (NEAT) materials for low nanoparticle concentrations. As one example, I will discuss how nanoparticle size and composition can dramatically affect thermal conductivity in SiGe alloys [3]. Ballistic transport of phonons dominates thermal transport at the nanoscale and requires a different transport approach. By using a Green’s function approach that builds on ab-initio interatomic force constants, we can study how disorder affects nanoscale thermal transport in the cases of phonon scattering from isotopes in boron nitride nanotubes[4] and isotope clusters in graphene[5].Research was supported in part by NSF Grants CBET-0651310, CBET-0651427, CBET-0651381, CBET-1066634, CBET-1066404, and by an European Union IRG grant. [1] D. A. Broido et al., Appl. Phys. Lett., 91, 231922 (2007).[2] A. Ward et al., Phys. Rev. B, 80, 125203 (2009).[3] A. Kundu et al., submitted to Phys. Rev. B, (2011).[4] D. A. Stewart, I. Savic, and N. Mingo, Nano Lett., 9, 81 (2009).[5] N. Mingo et al., Phys. Rev. B, 81, 045408 (2010).
9:45 AM - W5.2
High Thermal Conductivity in Silicon-Germanium Superlattices: A First-Principles Study.
Jivtesh Garg 1 , Nicola Bonini 2 , Nicola Marzari 2
1 Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Department of Materials, University of Oxford , Oxford, Oxford, United Kingdom
Show AbstractNanostructuring is often seen as a tool to decrease thermal conductivity, with fundamental applications in developing novel thermoelectric materials; it is rather more difficult to increase thermal conductivity of any given material. Here, we calculate the thermal conductivity of silicon-germanium superlattices fully from first-principles, and show how, as lattice period is decreased, a remarkable increase in thermal conductivity can be achieved, so that it becomes higher than either of the constituents.We present the microscopic origin of this effect, driven mostly by the increase in phonon relaxation times due to the opening of gaps in the dispersion relations, because of mass mismatch, and discuss comparison with experiments for III-V and IV semiconductors, and implications for high thermal conductivity materials.
10:00 AM - W5.3
Lifetimes of Confined Acoustic Phonons in Ultra-Thin Si Membranes.
John Cuffe 1 2 , Oliver Ristow 4 , Emigdio Chavez 1 3 , Jordi Gomis-Bresco 1 , Pierre-Olivier Chapuis 1 , Francesc Alzina 1 , Mike Hettich 4 , Andrey Shchepetov 5 , Mika Prunnila 4 , Jouni Ahopelto 5 , Thomas Dekorsy 4 , Clivia Sotomayor Torres 1 3
1 Phononic and Photonic Nanostructures Group, , Catalan Institute of Nanotechnology (ICN), Bellaterra, Barcelona, Spain, 2 Department of Physics, University College Cork, Cork Ireland, 4 Department of Physics, University of Konstanz, Konstanz, Germany, 3 Department of Physics, Universitat Autonoma de Barcelona, Bellaterra, Barcelona, Spain, 5 , VTT Technical Research Centre of Finland, Espoo Finland
Show AbstractThe acoustic properties of ultra-thin Si layers are important for many areas of nanotechnology, impacting on both structural integrity and thermal transport. The effect of phonon confinement is particularly important, affecting both heat dissipation and charge carrier mobility. The goal of this work is to obtain a deeper understanding of phonon confinement and propagation in materials with dimensions comparable to thermal phonon wavelengths. In this work we investigate the lifetimes of out-of-plane confined modes using the ultra-fast pump-probe technique of Asynchronous Optical Sampling (ASOPS). Free-standing Si membranes with thicknesses ranging from 8 to 400 nm were investigated. The frequencies of the confined modes were calculated from dispersion curves based on the continuum elasticity model for acoustic modes in an anisotropic membrane. The effect of the ~1 nm native oxide layer in such thin systems was also calculated using a three-layer model. The frequencies measured were found to be consistently lower than those predicted by continuum elasticity theory, which could not be explained simply in terms of error in the thickness measurements, the effect of a native oxide layer, or a temperature rise within the membranes. The lifetime of the first-order dilatational mode was found to decrease dramatically for the ultra-thin membrane, with the 8 nm membrane observed to have a decay time more than three orders or magnitude less than that of a 222 nm membrane. The contributions of various attenuation mechanisms are discussed to explain the observed trend. This work provides a basis to investigate the effect of acoustic phonon confinement on thermal transport and elastic constants in systems with sub- 50 nm dimensions.
10:15 AM - W5.4
A Hybrid Classical-Quantum Approach to Investigate Thermal Transport in Alloyed, Textured, or Nanostructured Materials.
Dmitri Volja 1 , Boris Kozinsky 2 , Jivtesh Garg 3 , Marco Fornari 4 , Nicola Marzari 5
1 DMSE, MIT, Cambridge, Massachusetts, United States, 2 Research and Technology Center, Robert Bosch LLC, Cambridge, Massachusetts, United States, 3 Department of Mechanical Engineering, MIT, Cambridge, Massachusetts, United States, 4 Department of Physics, Central Michigan University, Mt. Pleasant, Michigan, United States, 5 Department of Materials, Oxford Univeristy, Oxford United Kingdom
Show Abstract A whole spectra of intriguing physical properties appears in conventional materials when structural features reach nanoscale. Since thermal conductivity is controlled by the heat carriers' mean free paths, it becomes of paramount importance to understand and engineer the role of alloying, nanostructuring and grains' texture on transport coefficients. First-principles calculations often provide accurate microscopic parameters, but at significant computational costs even for ideal, perfect systems. Since thermal conductivity is dominated by harmonic terms (even lifetimes are constrained by conservation rules on energy and momentum), we introduce here a novel, hybrid approach to study thermal conductivity where we combine first-principles calculations of harmonic terms and force-field calculations of third-order derivatives. We validate our scheme for the case of SiGe superlattices,and discuss thermal transport in complex systems (e.g. in the presence of substitutional disorder). Lastly, we study the effects of crystalline texture and grain-size effects introducing boundary scattering terms and using Matthiessen's rule, and apply this to the case of the binary skutterudite CoSb3.
10:30 AM - W5.5
Anderson Localization of Phonons in Random Multilayer Thin Films.
Anthony Frachioni 1 , Bruce White 1
1 Department of Physics and the Materials Science Program, Binghamton University - State University of New York, Binghamton, New York, United States
Show Abstract1020 Joules of energy are generated by the United States each year; 60% of this energy is lost to heat. Thermoelectric based energy scavenging has tremendous potential for the recovery of significant quantities of this waste heat. However, utilization of thermoelectric devices is limited due to relatively low energy conversion efficiency and the utilization of relatively scarce materials. This work focuses on generating sustainable and efficient thermoelectric materials through modifications of the lattice vibrations of materials with excellent thermoelectric electronic properties (Seebeck coefficients larger than 500µV/K). In particular, Anderson localization of phonons in random multilayer thin films has been explored as a means for reducing lattice thermal conductivity to values approaching that of aerogels (~10mW/m-K). Silicon has been a sample of choice due to its high crust abundance and Seebeck coefficient. Reverse non-equilibrium molecular dynamics simulations have been employed to determine the thermal conductivity of structures of interest. Simulations with pure Lennard-Jones argon solids have been performed to establish a methodology and to characterize the effect of different kinds of mass disorder prior to the examination of silicon. The simulation results indicate that mass disorder confined to randomly selected planes is an effective way to reduce lattice thermal conductivity with the lattice thermal conductivity decreasing by a factor of thirty (to 4 mW/m-K) in the argon case and a factor of over ten thousand (to 15 mW/m-K) for silicon. Based on models in which the charge carrier mean free path is limited by scattering from the planes with mass disorder, the mobility of silicon is expected to reach values of 10 cm2/V-sec. At this mobility the thermoelectric figure of merit, ZT, (utilizing the Weidemann-Franz law to calculate the electronic thermal conductivity) is found to vary between 4.5 and 11 as the mass ratio of the disordered planes is varied from 4 to 10 in 20% of the lattice planes. These results indicate that the pursuit of nanostructured thermoelectric materials in the form of random multilayers may provide a path to efficient and sustainable thermoelectric materials.
10:45 AM - W5:PTNM1
BREAK
11:15 AM - **W5.6
Analytical Model of Raman Enhancement by Metal Nanoshells.
G. Sun 1 , J. Khurgin 2
1 , University of Massachusetts Boston, Boston, Massachusetts, United States, 2 , Johns Hopkins University, Baltimore, Maryland, United States
Show AbstractNanometer-scale metallic structures have been extensively studied as a way to dramatically modify optical properties of various optically active objects of similar dimensions such as atoms, molecules, or quantum dots that are placed in close proximity of these metallic structures. We have previously established an analytical model that unambiguously explained the enhancement of various optical processes including optical emission, absorption, and photoluminescence (PL) by single as well as coupled metal nanoparticles. This model has also allowed for fast optimization of the metal nanostructures. Building upon our earlier work, we present yet another analytical model for the enhancement of molecular Raman process by metal nanoparticles. Such an effect is known to be sensitive to the surface plasmon (SP) resonance associated with the collective oscillation of the conduction electrons confined in the metal, it is therefore desirable to achieve certain tuning range of the SP resonance by varying the nanoparticle geometry. To accomplish this, we study metal nanoparticles in the shape of a nanoshell that offer a wide range of resonance tuning by controlled variation of their inner and outer radii. The result of Raman enhancement is subsequently compared with that of a PL process. Although both processes are similar in the sense that they are both a two-photon process in which one photon is absorbed and another photon at a different frequency is emitted, the SP enhancement mechanisms for these two processes have some rather distinct features. In addition to stronger enhancement, the Raman process shows no sign of quenching ever taking place even when the molecule is placed right at the surface of the metal nanoshell – a situation that will lead to strong quenching effect in PL measurement. Significant advantages of our analytical approach include not just predications that are consistent with experimental observations, but rather a clear insight for the actual physical process at work.
11:45 AM - W5.7
Vacuum Phonon Tunneling.
Igor Altfeder 1 , Andrey Voevodin 1 , Ajit Roy 1
1 , Air Force Research Laboratory, Wright-Patterson AFB , Ohio, United States
Show AbstractNovel mechanism of interfacial thermal transport, field-induced phonon tunneling, has been revealed by STM and inelastic electron tunneling spectroscopy. We found that thermal vibrations of the last tip atom are effectively transmitted to sample surface despite few angstroms wide vacuum gap. We show that phonon tunneling is caused by interfacial electric field and thermally vibrating image charges, “thermal mirages”. By comparing experimental data and theory, we show that thermal energy transmitted through atomically narrow vacuum gap due to thermal vibration of image charges exceeds, by ten orders of magnitude, the Planck’s thermal radiation energy. The comparison of experimental data obtained on different material surfaces will be presented. References: I. Altfeder, A. A. Voevodin, A. K. Roy, Phys. Rev. Lett. 105, 166101 (2010); M. Prunnila, J. Meltaus, Phys. Rev. Lett. 105, 125501 (2010); G. D. Mahan, Appl. Phys. Lett. 98, 32106 (2011).
12:00 PM - W5.8
Weak Bonding Effect on the Ultralow Thermal Conductivity of Germanium Nanodot Arrays in Silicon.
Jean-Numa Gillet 1
1 Physics, University of Lille 1, Villeneuve d'Ascq cedex France
Show AbstractThe thermoelectric figure of merit ZT is inversely proportional to the thermal conductivity. Superlattices with periodic thin layers were studied to obtain ZT > 1 due to thermal conductivity reduction between their layers. Unfortunately, their synthesis with ZT higher than 1 is hazardous due to lattice mismatches forming dislocations and cracks. Nanowires with low dimensionalities were also proposed for phonon confinement. However, as the superlattices, they decrease the thermal conductivity in only one propagation direction. In experiments, these one-dimensional insulating materials usually fail to beat the lowest limit of amorphous Si (+/- 1 W/m/K) [1]. In this theoretical study, three-dimensional Ge quantum-dot arrays in Si are proposed to obtain an extreme thermal-conductivity reduction. Two decrease effects are shown from a molecular supercrystal model. First, low phonon group velocities are computed by lattice dynamics. Second, near-field scattering is exalted assuming weak interface bonding. This prediction can lead to a significant ZT increase. Indeed, for a broad temperature range from +/- 300 to 1200 K, Si-Ge supercrystals are modeled with various size parameters and present thermal conductivities equal or lower than that of air (+/- 0.025 W/m/K at 300 K) [2]. A thermal-conductivity global minimum K* is predicted for Si-Ge supercrystals with nanodot spacing higher than +/- 25 nm and Ge concentration of +/- 12.5 at%. The ultralow K* = 0.01 W/m/K is computed at 300 K assuming that all phonon modes are scattered by the weakly-bonded Si-Ge interfaces in the geometrical limit. Thermal conductivity evolution is analyzed with respect to the weakly-bonded nanodot density. [1] Gillet, ACS Appl. Mater. Interfaces 2, 3486 (2010). [2] Gillet, Appl. Phys. Express 4, 015201 (2011).
12:15 PM - W5.9
Multiscale Modeling of Nanoporous Materials for Thermoelectric Applications.
Giuseppe Romano 1 , Aldo Di Carlo 2 , Jeffrey Grossman 1
1 , MIT, Cambridge, Massachusetts, United States, 2 Department of Electronics Engineering, University of Rome “Tor Vergata”, Rome Italy
Show AbstractNanoporous materials have gained considerable attention recently as attractive candidate materials for thermoelectric applications due to the substantially suppressed phonon transport [1,2]. Recent experiments demonstrated that silicon nanomeshes lower the phononic thermal conductivity by up to two orders of magnitude with respect to the bulk [3], confirming theoretical predictions based on molecular dynamics (MD) simulations [1]. Recently, MD was employed to show that heat transport in nanoporous materials is governed by the interplay between materials disorder at mesoscopic and atomic scales [4]. However, a predictive approach is still needed to understand and optimize heat transport that bridges the length scales from the atomic scale through the meso and macro scales, in order to engineer such materials for realistic devices. Here we combine atomistic and continuum models in order to investigate the scaling of materials properties to the device level, including the influence of the device architecture [5]. Thermal transport at the mesoscale is modeled by means of a simple model based on the Phonon Boltzmann Transport Equation (PBTE). Our results show that for lower pore spacing the surface-to-volume ratio, which correlates with phonon-pore scattering, increases and, consequently, the thermal conductivity decreases. Another way to decrease the thermal conductivity is to increase the pore size. Indeed, for large pore sizes, the porosity increases and phonons have less volume to travel through. In addition, a wide range of pore configurations has been explored and optimized in order to minimize the thermal conductivity. The PBTE results are combined with those obtained with molecular dynamic simulations in order to properly include atomistic disorder and phonon transmission across pore interfaces.[1] J.-H. Lee et al. NanoLetters 8, 3750 (2008).[2] J.-H. Lee et al. APL 91, 223110, (2007).[2] Jen-Kan Yu et al. Nat. Nanotech, 5, 718 (2010).[4] Yuping He et al. ACS Nano, 5 (3), pp 1839–1844. (2011).[5] M. Auf Der Maur et al. IEEE Trans. on Electron Devices 58, 5 (2011)
12:30 PM - **W5.10
Terahertz Acousto-Electric Effects in Semiconductor Nano Devices.
Anthony Kent 1
1 School of Physics and Astronomy, University of Nottingham, Nottingham United Kingdom
Show AbstractIn this talk, I will review recent experiments we have done in which (sub-) terahertz frequency bulk acoustic waves have been used to control the electronic transport properties of semiconductor nano devices on ultra-short timescales.Methods for the generation coherent acoustic phonons in the frequency range up to about a terahertz are now well established. These include: generation in thin metal films and semiconductor superlattices using femtosecond laser pulses and also active phononic devices, e.g. sasers. Potential applications of high-frequency coherent acoustic waves in areas such as generation and manipulation of terahertz electromagnetic radiation and high-speed electronics require devices whose electronic transport properties are able to respond to strain on ultra-short timescales.It has long been known that the transport properties of some electronic devices, e.g. junction diodes and bipolar transistors, can be modified by applying static, or slowly varying, strain. This is known as the piezojunction effect, and it is due to the effect of the lattice deformation on the semiconductor energy bands. More recently, we have shown that in p-n, p-i-n and Schottky diodes this process works on an ultra fast, picosecond, timescale [1, 2, 3].The experiments I will describe used picosecond-duration strain pulses generated by femtosecond laser excitation of a metal film deposited on the back side of the substrate on which the devices were fabricated. High-speed time-resolved measurements of the response of the devices to the strain pulses were made using a microwave electrical measurement system (p-n and Schottky diodes) or an optical pump-probe setup with slow electronics (p-i-n diode). The results will be explained in terms of deformation potential coupling of the strain with the electronic states in the structures.The studied devices were found to have a high sensitivity to strain, and may also form the basis of active strain-sensitive electronic components, e.g. mesfets or heterojunction bipolar transistors, which could be called phonotransistors. Such devices could find practical applications such as: the conversion of terahertz sound to electromagnetic radiation; mixer devices for high-resolution terahertz spectroscopy; sensitive transducers for nanoacoustics measurements; and high speed clock distribution. Some of these applications will be discussed in the talk.[1] Moss, D.M., Akimov, A.V., Campion, R.P., Kent, A.J.: Ultrafast strain-induced electronic transport in a GaAs p-n junction diode. Chin. J. Phys. 49, 499 (2011).[2] Moss, D.M., Akimov, A.V., Glavin, B.A., Henini, M., Kent, A.J.: Ultrafast strain-induced current in a GaAs Schottky diode. Phys. Rev. Lett. 106, 066602 (2011).[3] Moss, D., Akimov, A.V., Makarovsky, O., Campion, R.P., Foxon, C.T., Eaves, L., and Kent, A.J.: Ultrafast acoustical gating of the ptotocurrent in a p-i-n tunnelling diode incorporating a quantum well. Phys. Rev. B 80, 113306 (2009).
W6: Phonon Transport - Nano Materials II
Session Chairs
Harold de Wijn
Derek Stewart
Wednesday PM, November 30, 2011
Room 313 (Hynes)
3:00 PM - **W6.1
Thermal Conductance of Weak and Strong Interfaces.
David Cahill 1
1 , U. Illinois, Urbana, Illinois, United States
Show AbstractUnderstanding of the thermal conductance of interfaces is typically based on the acoustic-mismatch or diffuse-mismatch models that describe the transmission coefficients of phonons based on the sound velocities and densities of states of vibrational modes of the two materials on either side of the interface. Recent computational work and experiment are showing, however, that for interfaces with weak interfacial bonding, the thermal conductance is strongly suppressed. We are using the combination of time-domain thermoreflectance and high-pressure anvil cell methods to “pressure tune” the strength of weak, anharmonic interfacial bonds and study the consequent effects on thermal transport. For example, with increasing pressure towards 10 GPa, the thermal conductance of an Al/graphene/SiC interfaces increases by an order of magnitude and becomes nearly identical to the thermal conductance of a strong interface formed by in situ deposition of Al on SiC cleaned at high temperatures. The thermal conductance of interfaces formed by “transfer-printing” of metal films onto a variety of substrates are, in many cases, surprisingly large, comparable to the conductance of evaporated films, even when the transfer printed layers have significant roughness and therefore we would not expect that film and substrate would be in intimate contact. Layers of adsorbed hydrocarbons or water vapor may be playing a role in the anomalous heat conduction but measurements at elevated temperatures show that the relatively high thermal conductance is often stable to 600 K.
3:30 PM - W6.2
Thermal Conduction in Nanobeam Photonic Crystal Cavities.
Amy Marconnet 1 , Takashi Kodama 1 , Yiyang Gong 2 , Jelena Vuckovic 2 , Kenneth Goodson 1
1 Mechanical Engineering, Stanford University, Stanford, California, United States, 2 Electrical Engineering, Stanford University, Stanford, California, United States
Show AbstractNanobeam photonic crystal cavities have been investigated for achieving CMOS compatible, low-threshold lasers, and other active nanophotonic devices. Previous work has shown a strong correlation between cavity temperature and device performance and more work is needed to understand thermal conduction within the cavities. The cavities are formed by patterning air holes on a suspended nanobeam. For nanophotonic applications, the diameter of the air holes varies from the center to the edge of the beam. To better understand the phonon conduction within the nanophotonic devices, this work investigates thermal conduction in both nanobeams with air holes patterned for optimal photonic device operation and similar nanobeam with uniform diameter air holes across the entire beam. A steady-state four-probe measurement technique is used to extract thermal properties of the nanophotonic crystal cavities. A numerical model of thermal and electrical conduction within the structure is used to extract the effective thermal conductivity of the structure. This work considers the increased scattering rate of phonons at the air holes and surfaces of the nanostructured beam to explain the significant reduction in the thermal conductivity compared to bulk silicon.
3:45 PM - W6.3
Effect of Morphology on Thermal Conductivity of Single Crystalline Silicon Nanowires.
Kedar Hippalgaonkar 1 , Ming Zhi Li 1 , Peter Ercius 3 , Jongwoo Lim 2 , Renkun Chen 4 , Peidong Yang 3 , Arun Majumdar 5
1 Mechanical Engineering, UC Berkeley, Berkeley, California, United States, 3 National Center for Electron Microscopy, Lawrence Berkeley National Labs, Berkeley, California, United States, 2 Chemistry, UC Berkeley, Berkeley, California, United States, 4 Mechanical Engineering, UC San Diego, San Diego, California, United States, 5 Advanced Research Program Agency-Energy, Department of Energy, Washington DC, District of Columbia, United States
Show AbstractThe Casimir Limit of thermal conductivity in semiconductors and insulators has been the cornerstone of diffusive phonon transport. By changing the morphology in single crystalline Silicon Nanowires, the thermal conductivity can be tuned from close to the amorphous limit all the way to the Casimir limit. Electroless Etching allows for different changes in morphology including porosity, necking and roughness, each having a unique effect on the measured thermal conductivity. A detailed study of such an effect is performed.
4:30 PM - W6.4
Theoretical Investigation of Thermal Resistance at the Si/SiO2 Interface.
Evelyne Lampin 1 , Fabrizio Cleri 1 , Quoc-Hoang Nguyen 1 , Pierre-Arnaud Francioso 1
1 , IEMN, Lille France
Show AbstractIn advanced transistors fabricated from silicon on insulator (SOI) substrates, the heat conduction path from active device to heat sink crosses silicon (Si) /silicon dioxide (SiO2) interfaces. It is the purpose of the present work to investigate at the atomic scale the thermal resistance of this interface. The challenge is both to develop a realistic model of the interface and to extract relevant thermal characteristics from classical molecular dynamics simulations. Several kinds of atomic interfaces are constructed, including crystal/amorphous Si and crystal Si/ amorphous SiO2 in order to elucidate the respective role of the transition from order to disorder and the variation in chemistry. A particular attention is paid to the formation of a good quality amorphous SiO2 and of its interface with Si. For this purpose, interatomic potentials of the Tersoff family are tested [1]. In parallel, molecular dynamics simulations are carried out in various configurations : i) non-equilibrium molecular dynamics (NEMD) [2], where huge temperature gradients (K/nm) between restricted reservoirs are created, ii) equilibrium molecular dynamics (EMD) [3] where the temperature fluctuations in each side of the interface are measured after an equilibration phase and iii) approach-to-equilibrium molecular dynamics (AEMD) where the temperature transitory towards equilibrium is analyzed. Both EMD and AEMD have the advantage to rely on bulk definition of the temperature and do not require to estimate the absolute heat flux. The respective relevance of each method is discussed and general conclusions on the heat transport at the Si/SiO2 interface are raised.[1] Y. Umeno et al, Comput. Mater. Sci. 25 (2002) 447 ; S. Munetoh et al, Comput. Mater. Sci. 39 (2007) 334 [2] P. K. Schelling et al, Phys. Rev. B 65 (2002) 144306 [3] A. Rajabpour et al, J. Appl. Phys. 108 (2010) 094324
4:45 PM - W6.5
Thermal Transport in Oxide Superlattices.
Jayakanth Ravichandran 1 2 , Patrick Hopkins 3 4 , Pim Rossen 5 , Arun Majumdar 6 , R. Ramesh 5 7
1 Applied Science and Technology, Univ California Berkeley, Berkeley, California, United States, 2 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 3 Engineering Sciences Center, Sandia National Laboratories, Albuquerque, New Mexico, United States, 4 Department of Mechanical and Aerospace Engineering, University of Virginia, Charlottesville, Virginia, United States, 5 Department of Materials Science and Engineering, University of California, Berkeley, Berkeley, California, United States, 6 ARPA-E, Department of Energy, Washington, District of Columbia, United States, 7 SETP, Department of Energy, Washington, District of Columbia, United States
Show AbstractUnderstanding thermal transport across interfaces can have profound influence on designing materials for thermal management, thermoelectricity etc. Despite years of investigations, there are several open questions on the nature of thermal transport across solid-solid interfaces. The ability to control materials synthesis down to a monolayer has enabled study of interface phonon scattering in systems such as superlattices, heterostructures etc. Past studies on crystalline heterostructures and superlattices have largely centered on semiconductors. Given the suitability of oxides for applications in a wide range of temperatures, particularly high temperatures, it is essential to understand the limits of thermal transport across crystalline oxide interfaces. We chose perovskite titanates, which are excellent thermoelectric materials, as model systems to study interfacial thermal transport. Superlattices of SrTiO3 and CaTiO3, with period thicknesses of 2 – 176 monolayers (MLs), were grown using pulsed laser deposition in the layer-by-layer mode, monitored by in-situ RHEED. The interfaces were found to be atomically smooth as evidenced from STEM. We measured temperature dependent (100 – 400 K) cross-plane thermal conductivity of these superlattices using the time domain thermo-reflectance.[1] We do not observe a thermal conductivity minimum as a function of superlattice period thickness down to 2 ML period thickness at any temperature. The lowest thermal conductivity in these superlattices is a factor of two lower than the corresponding alloy at room temperature and comparable or lower than the theoretical amorphous limit for SrTiO3 and CaTiO3. Beating the amorphous limit in artificially engineered material systems opens up new opportunities for building efficient thermoelectric systems, thermal barrier coatings etc. [1] D. G. Cahill, Rev. Sci. Instrum. 75, 5119 (2004).
5:00 PM - W6.6
Phonon Transport across Si Nanomembrane Interfaces: Structure and Thermal Conductivity.
Daniel Schroeder 1 , Arnold Kiefer 1 , Deborah Paskiewicz 1 , Zlatan Aksamija 1 , Irena Knezevic 1 , Max Lagally 1 , Mark Eriksson 1
1 , University of Wisconsin-Madison, Madison, Wisconsin, United States
Show AbstractIt has recently become possible to form multilayers of single-crystal silicon and other materials by transfer and stacking of nanomembranes [1-3]. The thinness of these membranes makes it possible to envision materials and devices that make functional use of the interfaces between the membranes themselves or between a membrane and the underlying substrate. To explore the consequences of crystal misorientation on thermal transport through bonded interfaces, we describe a simple technique to fabricate interfaces between single crystals using thermal release tape transfer of silicon nanomembranes [4]. Typical interfaces are approximately 1 nm in thickness, comparable to high quality wafer bonded interfaces. We discuss measurements of the thermal conductivity normal to the plane of Si nanomembranes bonded directly to Si substrates through this extremely thin interface. Using the 3-omega method to measure the thermal conductivity [5], we observe clear shifts in plots of the 3-omega voltage vs. log frequency, indicating that the membrane and membrane/substrate interface increase the thermal resistance into the bulk. We discuss the results in the context of phonon scattering from both an amorphous Si interfacial layer, appropriate for samples with high twist angles between the membrane and the substrate, and a dislocation network at the interface, appropriate for samples with low twist angles. This research was primarily supported by AFOSR (Grant No. FA9550-08-1-0337). TEM and other structural characterization supported by DOE (Grant No. DE-FG02-03ER46028).[1] W. Peng et al., Appl. Phys. Lett. 90, 183107 (2007).[2] Y. F. Mei et al., ACS Nano 3, 1663 (2009).[3] J. A. Rogers, T. Someya, Y. G. Huang, Science 327, 1603 (2010).[4] A. Kiefer et al., ACS Nano 5, 1179 (2011).[5] D. G. Cahill, R. O. Pohl, Phys. Rev. B 35, 4067 (1987).
5:15 PM - W6.7
Factorial Increases in Thermal Conductance at a Metal-Dielectric Heterointerface Using a Nanomolecular Monolayer.
Peter O'Brien 1 , Sergei Shenogin 1 , Jianxun Liu 2 , Masashi Yamaguchi 2 , Pawel Keblinski 1 , Ganpati Ramanath 1
1 Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States, 2 Physics and Applied Physics, Rensselaer Polytechnic Institute, Troy, New York, United States
Show AbstractManipulating interfacial thermal transport is a compelling need for a number of technologies including nanoelectronics devices and packaging, solid-state lighting, energy generation, and nanocomposites. Here, we demonstrate that introducing a strongly-bonding organic nanomolecular monolayer (NML) at a metal-dielectric interface leads to a factor of four increase in the interfacial thermal conductance to values as high as 410 MW/m2-K. Decreasing the bond strength between the NML and the materials comprising the interface decreases the thermal conductance, and reveals a correlation between the interfacial bond strength and thermal conductance. Molecular dynamics simulations corroborate the experimental results and reveal the non-linear relationship between the interfacial bond strength and thermal conductance, which can be described in terms of a series thermal resistance model. Vibrational analyses of NML-tailored interfaces verify that this remarkable interfacial conductance enhancement is underpinned by overlapping of the low frequency vibrational density of states, which results from strong NML-silica and NML-metal bonding and facilitates efficient heat transfer through the molecules comprising the NML. These results provide, for the first time, a rational means of increasing heterointerfacial thermal conductance through molecular functionalization for a wide variety of material systems and applications.
5:30 PM - W6.8
Thermoelectric Transport under Large Temperature Gradients in Self-Heated Silicon Microwires.
Gokhan Bakan 1 , Ali Gokirmak 1 , Helena Silva 1
1 Electrical and Computer Engineering, University of Connecticut, Storrs, Connecticut, United States
Show AbstractSuspended highly doped nanocrystalline silicon (nc-Si) microwires are self-heated using microsecond voltage pulses1. The wires melt during the microsecond voltage pulsing and crystallize upon termination of the pulse. If the applied voltage pulse is insufficient to melt the entire wire, only a portion of the wire is melted and the melted section is consistently shifted from the center of the wire towards the carriers’ source terminal. Temperature gradients ~ 1 K/nm and current densities ~ 1-10 MA/cm2 are achieved in these experiments. Similar asymmetric self-heating of semiconductor microstructures has been observed by other groups and attributed to the heat carried by charge carriers (thermoelectric effects).Finite element modeling of the wires, including diffusive thermoelectric transport, is performed to understand the experimental results. Temperature profiles of the modeled wire show a shift in the hottest spot towards the source of the wire in agreement with the experimental observations. However, the magnitude of the shift is much smaller than in our experimental results. We are investigating two possible reasons for this discrepancy: 1) an enhanced form of phonon-drag due to stronger phonon-carrier interactions under large temperature gradients and current densities (in contrast to the known formulation of phonon-drag which predicts negligible effects at high temperatures, and also high doping levels2) and 2) non-local3 or ballistic4 heat transport under large temperature gradients which has been shown to explain suppressed heat conduction under large temperature gradients in laser annealing experiments.References 1. G. Bakan, N. Khan, A. Cywar, K. Cil, M. Akbulut, A. Gokirmak and H. Silva, Journal of Material Research 26, 1061 (2011). 2. C. Herring, Physical Review 96, 1163 (1954). 3. G. D. Mahan and F. Claro, Physical Review B 38, (1988). 4. G. Chen, Phys. Rev. Lett. 86, 2297 (2001).
5:45 PM - W6.9
Dependence of Thermal Transport on Surface Faceting and Twinning in Crystalline Silicon Nanowires.
Frederic Sansoz 1
1 School of Engineering, The University of Vermont, Burlington, Vermont, United States
Show AbstractSignificant experimental progress has been achieved in recent years in controlling the surface morphology of <111>-oriented Si nanowires (NWs) with sidewalls varying from atomically-smooth {112} planes to complex sawtooth faceting with either {111}/{100} facets or {111}/{113} facets. Also recent advances have enabled the synthesis of periodically-twinned Si NWs with either hexagonal or zigzag shapes. Control over phonon transport in these types of nanostructures is critical to the development of next-generation thermoelectric devices in semiconductors. However, our understanding of the effects of surface faceting and twinning on phonon transport and thermal conductivity in crystalline semiconductor NWs is still rather restricted. This talk examines the impact of NW morphology on thermal conductivity in crystalline <111> Si NWs with particular emphasis on circular, hexagonal and sawtooth-faceted shapes, by using non-equilibrium molecular dynamics (MD) simulations. A detailed analysis of vibrational density of states (VDOS) between core and surface atoms is also presented for different facets to gain insight into the surface scattering mechanism controlling thermal transport in this type of materials. From this study, 92% reduction of thermal conductivity is found in sawtooth-faceted {111}/{100} Si NWs compared to past experimental values obtained for bulk at the same temperature, and 47% reduction compared to NWs with smooth sidewalls and same size. This effect is shown to result from a strong dispersion of surface phonons in {100}-type facets compared to VDOS of atoms in the core. However, the simulations reveal that phonon dispersion is less pronounced with {113}-type facets and absent with {111}-type facets, which suggests that the facet contribution to surface modes is due to diffuse scattering rather than backward scattering. Also I show that some anomalous thermal transport characteristics exist in periodically-twinned Si NWs due to this unique surface faceting dependence on phonon modes. The conclusions of this study therefore provide a new approach to control phonon transport in nanoscale Si-based thermoelectric devices, as well as in other semiconductor NW systems where surface faceting is prominent.
W7: Poster Session: Phonons in NanoMaterials - Theory, Experiments and Applications
Session Chairs
David Hurley
Masashi Yamaguchi
Thursday AM, December 01, 2011
Exhibition Hall C (Hynes)
9:00 PM - W7.1
Seeing is Believing: Nanostructures in Alkali Metal Doped Lead Telluride.
Jiaqing He 1 2 , Ivan Blum 1 , John Androulakis 2 , David Seidman 1 , Mercouri Kanatzidis 2 3 , Vinayak Dravid 1
1 Department of Materials Science & Engineering, Northwestern University, Evanston, Illinois, United States, 2 Department of Chemistry, Northwestern University, Evanston, Illinois, United States, 3 Materials Science Division, Argonne National Laboratory, Argonne, Illinois, United States
Show AbstractThe thermoelectric material PbTe can be turned p-type by monovalent substitution on the Pb sub-lattice. A survey of the periodic table reveals that this can be achieved by the alkali metals (Li, Na, K, Rb, Cs, Fr), Ag and Tl. The size of alkali metals that are below K in the first column of the periodic table excludes them from efficiently doping PbTe because of their large size. On the other hand Tl is extremely toxic and Ag may exhibit ionic conductivity in PbTe. Thus, one is left with surprisingly few choices, out of which Na (and to a certain extend K) is the preferable dopant. Previously, Na-doped PbTe has been treated as a solid solution alloy phase and little attention has been given to its microstructural nature.In this contribution, we have studied a set of different X (X=Na, K, Na and K) doped PbTe thermoelectric materials employing advanced scanning transmission and transmission electron microscopy (S/TEM) and atom probe tomography (APT). For the first time, we find these samples to exhibit nanostructuring rather than solid solution behavior even at very low doping levels. The S/TEM experiments reveal that the nanostructures have platelet-like shapes with similar structure as the PbTe matrix, where Na and/or K partially replace Pb and form Pb1-x(Na, K)xTe. The calculated lattice thermal conductivity of Na and/or K doped PbTe is in good agreement with the measured data, indicating that the platelet-like precipitates do not strongly scatter phonons. The results and insights reported here may generate or stimulate new ideas on how to design more efficient p-type thermoelectric materials in the future. This may also open new windows to aim for further optimization of nanostructures for increasing the power factor even if it does not significantly lower the thermal conductivity.
9:00 PM - W7.10
Atomistic Green's Function Method Supported by ab initio Calculations: Application to Phonon Transport in ZnO and ZnS.
Michael Bachmann 1 , Michael Czerner 1 , Saeideh Edalati-Boostan 1 , Christian Heiliger 1
1 1.Physikalisches Institut, Justus-Liebig Universität, Giessen Germany
Show AbstractWe present an approach to calculate ballistic phonon transport that combines the atomistic Green's function (AGF) method [1] with ab initio results. The equilibrium positions of the atoms and the interatomic force constants(ifcs) are calculated using the ABINIT program package [2], which is based on density functional theory. Therefore, the presented approach is parameter free. From the Green's function of the system we determine the density of states as well as the transmission function. The thermal conductance is obtained within the linear response regime. We apply this approach to bulk ZnO and bulk ZnS. Transmission functions for different transport directions for each material are presented. A comparison of the transmission function shows, that a ZnO/ZnS interface could be a promising phonon blocker. Adding such interfaces in ZnO or ZnS based thermoelectric devices could therefore increase the figure of merit.[1] , W.; Fisher, T. & Mingo, N.Numerical Heat Transfer, Part B: 2007, 51, 333[2] http://www.abinit.org
9:00 PM - W7.12
Enhanced Thermopower in CNT Film with Nanopatterning.
Seungwoo Jung 1 , Gustavo Fernandes 2 , Jinho Kim 2 , Ki-Bum Kim 1 , Jimmy Xu 2 1
1 WCU Hybrid Materials Program, Department of Materials Science and Engineering, Seoul National University, Seoul Korea (the Republic of), 2 School of engineering, Brown University, Providence, Rhode Island, United States
Show AbstractA random network of carbon nanotubes is an interesting system for basic sciences and engineering. Many interesting questions remain open or underexplored, one of which is its thermoelectricity. A pristine metallic nanotube is expected to exhibit no thermoelectric power, yet experiments showed otherwise. What thermoelectric power can be expected of a thin carbon nanotube film is also an intriguing but more complex question. In this work, we studied two types of such films – one subjected to nano-patterning and the other not. In the latter we obtained a thermoelectric power of 30 μV/K, in agreement with that measured in a single nanotube [1]. In the former, a substantial increase, up to 30%, in thermoelectric power was measured. The nanotubes used here had a 1:2 semiconducting to metallic ratio. The films were formed by water-dispersible nanotubes then membrane filtration and transfer to fused silica plates. Nano-patterning was performed by RIE through a hexagonal nanopore array alumina membrane placed on the film. For future applications, we note both are scalable in size. The nano-pattern has 100 nm period and 50 nm pores.The 30% increase in thermoelectric power measured in the nano-patterned film is intriguing, if not a complete surprise, as it is hard to believe, given the 1:2 semiconducting-metallic ratio, that a semiconducting pathway resulted from imposing a large array of nanopores into the well-dispersed but densely-packed nanotube film. Analyzing the results with conventional models for bulk would lead to unphysical conclusions such as reduced effective mass by patterning. Raman spectral comparisons of the two samples, our earlier experimental finding of electron localization [2], and independent nanotube bundle study [1] suggest that the metallic and semiconducting tube remained contributing in parallel. But patterning induced electron localization in both, which itself is a result of multiple effects – SP2 to SP3 bond transformation, self-doping, and on-tube defects.[1] Hone, J. et al. Thermoelectric Power of Single-Walled Carbon Nanotubes. Phys. Rev. Lett. 80, 1042 (1998).[2] Rakitin, A., Papadopoulos, C. & Xu, J.M. Electronic properties of amorphous carbon nanotubes. Phys. Rev. B 61, 5793 (2000).
9:00 PM - W7.13
Phonon Transport in Segmented Nanowires.
Vicenta Sanchez 1 , Chumin Wang 2
1 Departamento de Fisica, Facultad de Ciencias, UNAM, Mexico D.F Mexico, 2 Instituto de Investigaciones en Materiales, UNAM, Mexico D.F. Mexico
Show AbstractNanowires are capable to link tiny organic and inorganic components in order to create devices at nanometer scale. In particular, segmented nanowires exhibit great advantages in the enhancement of thermopower properties, because the heterogeneous interfaces between the nanodots could obstruct the phonon transport along the wire axis leading to a higher thermoelectric figure of merit [1]. In this work, based on the Kubo-Greenwood formula, the transport of phonons in segmented nanowires is studied by means of a real-space renormalization plus convolution method [2]. This method has the advantage of being computationally efficient, without introducing additional approximations and capable to analyze aperiodic nanowires. For semiconducting nanowires, the Born model with central and non-central interatomic forces is used to calculate the lattice thermal conductivity [3]. In particular, single, periodic and quasiperiodic segmented nanowires are comparatively studied. For low temperature region, the lattice thermal conductance reveals a power-law temperature dependence, in agreement with experimental data [4]. [1] R. Venkatasubramanian, E. Siivola, T. Colpitts, and B. O’Quinn, Nature 413, 597 (2001).[2] V. Sanchez and C. Wang, Phys. Rev. B 70, 144207 (2004).[3] C. Wang, F. Salazar, and V. Sanchez, Nano Lett. 8, 4205 (2008).[4] D. Li, Y. Wu, L. Shin, P. Yang, and A. Majumdar, Appl. Phys. Lett. 83, 2934 (2003).
9:00 PM - W7.14
Experimental and Theoretical Investigation of Transient Thermal Grating Decay via Non-Diffusive Thermal Transport in Semiconductors.
Jeremy Johnson 1 , Alexei Maznev 1 , Jeffrey Eliason 1 , Keith Nelson 1 , Austin Minnich 2 , Keivan Esfarjani 2 , Tengfei Luo 2 , Jivtesh Garg 2 , Gang Chen 2 , John Cuffe 3 , Timothy Kehoe 3 , Clivia Sotomayor Torres 3
1 Chemistry, MIT, Cambridge, Massachusetts, United States, 2 Mechanical Engineering, MIT, Cambridge, Massachusetts, United States, 3 , Catalan Institute of Nanotechnology, Barcelona Spain
Show AbstractDeviation from the Fourier law at microscopic distances in phonon-mediated thermal conductivity is an issue of both fundamental and practical importance. The transient thermal grating technique, where crossed laser beams are used to create a spatially sinusoidal temperature profile and monitor its decay, offers a convenient method for studying thermal transport at distances in the 1-10 μm range. Experiments performed on thin free-standing Si membranes to ensure one-dimensional thermal transport revealed that the thermal grating decay time deviated from the expected quadratic dependence on the grating period, thus providing model-independent evidence of non-diffusive transport at room temperature and at micron distances, i.e. much larger than the frequently cited mean free path estimate of ~40 nm. Measurements on bulk intrinsic GaAs also indicated significant deviations from the diffusion model at thermal grating periods of a few microns. The simplicity of the transient grating geometry allows us to treat the non-equilibrium phonon transport problem analytically using the Boltzmann transport equation. We find that at small grating periods the effective thermal conductivity is reduced due to diminishing contribution of low-frequency phonons with a long mean free path. Using phonon lifetime data obtained from first-principles calculations we find that a significant effect is expected at grating periods of a few microns in both Si and GaAs, in agreement with the experiments. This material is based upon work partially supported through S3TEC, a DOE BES EFRC under Award Number DE-SC0001299.
9:00 PM - W7.15
Heat Conduction in High Thermal Conductivity Networked Silver/Epoxy Composite Films.
Hafez Fard 1 , Kamyar Pashayi 1 , Fengyuan Lai 2 , Joel Plawsky 3 , Theodorian Borca-Tasciuc 1
1 Mechanical, Aerospace, and Nuclear Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States, 2 Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States, 3 Chemical and Biological Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States
Show AbstractFast and efficient exchange of thermal energy plays a vital role in the thermal management of electronic and optoelectronic devices. Since the thermal path between the heat source and the heat sink is usually made of a complicated network of interfaces, thermal interface materials (TIMs) are considered to play an important role in thermal management problems. We recently demonstrated high k in bulk nanocomposites of silver nanoparticles dispersed in epoxy and cured at low temperature (150C). A nanocomposite with 30 vol. % 20nm particles exhibited κ ~30 Wm-1K-1. The process responsible for enhancing k was found to be the self-construction, through in-situ sintering, of high aspect ratio metallic networks inside the nanocomposite. Therefore these materials are named high thermal conductivity networked (HTCNet) composites. The objective of this study is to explore the thermal conductance of films of HTCNet composites for possible applications as Networked TIMs (Net-TIMs) and to understand if the high thermal conductivity observed in bulk samples can be extended to confined systems. Thermal characterization employs test structures composed of Net-TIMs sandwitched between silicon substrates and Joule heating thermometry with microfabricated heaters/thermistors.
9:00 PM - W7.2
Enhanced Thermoelectric Properties of Ba-Filled Skutterudites by Grain Size Reduction and Ag Nanoparticle Inclusions.
Xiaoyuan Zhou 1 , Guoyu Wang 1 , Long Zhang 2 , Jeff Sakamoto 2 , Ctirad Uher 1
1 Physics, University of Michigan, Ann Arbor, Michigan, United States, 2 Chemical Engineering and Materials Science, Michigan State University, East Lansing, Michigan, United States
Show AbstractBa-filled skutterudite compounds, Ba0.3Co4Sb12, with dispersed Ag nanoparticles have been synthesized by ball milling followed by hot pressing. The influence of Ag nanoparticles and their size distribution on electrical and thermal transport properties has been investigated in the temperature range from room temperature to 823 K. It was found that Ag nanoparticles in the Ba0.3Co4Sb12 matrix reduce thermal conductivity while also improving the electron transport. These concomitant effects result in an enhanced thermoelectric performance with the dimensionless figure of merit ZT of around 1.0 at 823 K, an improvement of more than 30% in comparison to the parent Ba0.3Co4Sb12. In addition, it is advantageous to employ a wider size distribution of Ag nanoparticles in order to further reduce the thermal conductivity by enhancing phonon scattering. These observations demonstrate an exciting scientific opportunity to raise the figure-of-merit of filled skutterudites.
9:00 PM - W7.3
Laser Induced Heating of Group IV Nanowires.
Julian Anaya 1 , Alonso Martin-Martin 1 , Juan Jimenez 1 , Andres Rodriguez 2 , Tomas Rodriguez 2
1 GdS optronlab, Universidad de Valladolid, Valladolid Spain, 2 Tecnología Electrónica, ETSIT, Universidad Politecnica de Madrid, Valladolid Spain
Show AbstractSemiconductor nanowires (NWs) are fundamental structures for the future electron¬ic and optoelectronic nanoscale devices. Thermal transport in these nanostructures is a big challenge for the performance of the devices based on them. The drastic reduction of the thermal conductivity when compared to their bulk counterparts is crucial for the thermal management of the devices based on NWs. The understanding of the interaction between NWs and light is essential for applications as nanophotonics and photovoltaics. The NWs under laser beams, or under electric bias will substantially enhance the temper¬ature, which shall have negative consequences on the device performance. Therefore, the study of the thermal behaviour of semiconductor NWs is a crucial issue for their future application. In this way, we present herein an analysis of the thermal transport in Si, Ge, and SiGe NWs under the influence of a laser beam. The temperature distribution inside the NWs under the laser beam excitation is modelled using finite element analysis for solving the heat transport equation inside the NW. Different parameters as the laser wavelength, laser power are considered. On the other hand the NW dimensions are also critical to the temperature reached by the NW. Finally, the NWs have diameters much smaller than the laser beam diameter, which results in very different energies absorbed by the NW depending on its position inside the laser beam. All these factors are considered for the estimation of the temperature reached by group IV NWs under excitation with laser beams.
9:00 PM - W7.4
Local Temperature Sensing in Noncontact Mode with a Scanning Thermal Microprobe.
Liang Han 1 , Yanliang Zhang 1 , Theodorian Borca-Tasciuc 1
1 Mechanical, Aerospace, and Nuclear Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States
Show AbstractQuantitative thermal measurements with contact mode scanning thermal probe microscopy (SThM) require control of the complex heat transfer pathways at the tip-sample contact, including solid conduction, air conduction, and liquid conduction. A noncontact SThM technique is explored in this work to eliminate uncertainties due to solid contact and liquid meniscus conduction, and circumvent probe wearing. Noncontact SThM could open new possibilities for fast and quantitative temperature imaging of microelectronic and optoelectronic devices. The work is based on a recent proof of concept demonstration by the authors of noncontact local thermal conductivity characterization of nanoporous thin films and nanostructured bulk thermoelectric materials using a heated Wollaston wire micro thermal probe. Experiments in contact and noncontact mode for sensing of local sample temperature are performed using a V-shaped Wollaston microprobe that serves as a thermometer when small electrical current is passed through it. The measured sample surface temperature is determined from a heat transfer model using the probe temperature and the calibrated thermal contact resistances. Experiments are performed using substrates with patterned microheaters of various widths.
9:00 PM - W7.5
Analysis of Light Emission from Self-Heated Nanocrystalline Silicon Microwires.
Niaz Khan 1 , Gokhan Bakan 1 , Ali Gokirmak 1 , Helena Silva 1
1 , University of Connecticut, Storrs, Connecticut, United States
Show AbstractPatterned nanocrystalline silicon (nc-Si) microwires are self-heated using low-frequency (~1 Hz) AC signals to study thermoelectric transport in the wires [1]. Asymmetric self-heating of the wires are observed through light emission from the wires stressed with an AC signal. The brightest spot on the wires alternates position, always being skewed towards the source terminal, as the current direction changes. Similar asymmetric self-heating of semiconductor microstructures has been observed by other researchers [2, 3] and attributed to thermoelectric effects. The peak intensity and its location, voltage, current, and power across the wires are extracted in the experimental work. The extracted parameters are compared for different lengths of nc-Si microwires and wires previously crystallized using a microsecond pulse technique [4]. Further analysis will allow for extrapolation of temperature and other thermoelectric parameters which are essential for studying thermoelectric transport in materials.3D finite element modeling of the wires is also accomplished using COMSOL Multiphysics with the current continuity and heat transfer equations solved self-consistently, including the diffusive thermoelectric model. Although the simulated direction in shift of the hottest spot is correct, the magnitude in shift is smaller than the experimental results. This is attributed to the nonlinear thermoelectric behavior under extreme temperature gradients (~ 1 K/nm) and current densities (> 1 MA/cm2). References [1] G. Bakan, N. Khan, A. Cywar, K. Cil, M. Akbulut, A. Gokirmak and H. Silva, "Self-heating of silicon microwires: crystallization and thermoelectric effects," Journal of Material Research, vol. 26, pp. 1061-1071, 2011. [2] C. H. Mastrangelo, J. H. J. Yeh and R. S. Muller, "Electrical and optical characteristics of vacuum-sealed polysiliconmicrolamps," IEEE Trans. Electron Devices, vol. 39, pp. 1363-1375, 1992. [3] O. Englander, D. Christensen and L. Lin, "Local synthesis of silicon nanowires and carbon nanotubes on microbridges," Appl. Phys. Lett., vol. 82, pp. 4797, 2003. [4] G. Bakan, A. Cywar, H. Silva and A. Gokirmak, "Melting and crystallization of nanocrystalline silicon microwires through rapid self-heating," Appl. Phys. Lett., vol. 94, pp. 251910, 2009.
9:00 PM - W7.7
Determining the Thermal Conductivity of Inhomogeneous Thin-Films: A Case Study with Cu-Carbon Nanotube Composite Films.
Stephen Kang 1 , Jung Joon Yoo 2 , Ho-Ki Lyeo 1 , Jae Yong Song 1 , Sungjun Lee 1 , Jin Yu 2
1 , Korea Research Institue of Standards and Science, Daejeon Korea (the Republic of), 2 , Korea Advanced Institute of Science and Technology, Daejeon Korea (the Republic of)
Show AbstractMany practical thin-film materials are microscopically inhomogeneous. Composite thin-films, for example, acquire functionality by mixing different materials. When it comes to measuring the thermal conductivity of these materials, however, inhomogeneity presents difficulties since most thin film measurements extract the property of a local region. Thus measured values may not be representative for these thin films. Further difficulties are caused, especially for contact methods, in the case where the thin-film has irregular surface features such as surface protrusions. In this presentation, we demonstrate a simple method for assessing the thermal conductivity of these kinds of non-ideal thin-films with significant inhomogeneity. By combining the time-domain thermoreflectance method with a microscopic mapping technique and statistical analysis, we show the procedure for determining the thermal conductivity of Cu composite films containing dispersed carbon nanotubes (CNTs). We find that even small variations within a sample cause appreciable microscopic non-uniformities in the thermal transport property of these Cu-CNT composites, which makes them a good example for our demonstration. The representative thermal conductivity of the composite decreased from 188 to 60 W/m-K as multi-walled CNTs were added from 0 to 1.8 wt.% into nanocrystalline Cu. Comparing the decreasing trend with that calculated from a scattering model shows that the CNTs scatter the heat carriers in Cu. We discuss the implications of our results in terms of thermal coupling across the interface between the Cu matrix and CNTs.
9:00 PM - W7.8
Resonant Raman Effects on InN Nanowires: Excitation Wavelength Dependence.
Nuria Domenech 1 , Ramon Cusco 1 , Thomas Stoica 2 , Raffaella Calarco 2 3 , Luis Artus 1
1 Inst. Jaume Almera, C.S.I.C., Barcelona Spain, 2 Institute of Bio- and Nanosystems, Research Center Jülich, Jülich Germany, 3 Paul-Drude-Institut für Festkörperelektronik, Research Center Jülich, Berlin Germany
Show AbstractThe development of catalyst-free synthesis of III-V nanowires by MBE has opened the possibility of obtaining high quality material with enormous potential in solid state lighting and solar cell applications. InN is a very interesting material and the least understood of the group III nitrides. In the last few years there has been a significant improvement in the quality of InN layers, although these still contain a high density of dislocations. InN nanowires (NWs) grow almost defect free and offer advantages relative to epitaxial layers. Raman spectroscopy is a powerful non-destructive technique widely used to characterize NWs that provides valuable information about the composition, the possible strain, the crystalline quality and the presence of free charge.1,2 In this work we investigate the excitation wavelength effects on the Raman spectra of InN NWs. The studied InN NWs were grown by plasma-assisted MBE along the c-axis under N-rich conditions. Raman spectra of the as-grown wires have been obtained in near-backscattering geometry for excitation wavelengths ranging from 488 to 820 nm. As most of the excitation light enters the NWs through lateral faces, the LO signal contains a strong component of the E1(LO) mode, which is observed by resonant Fröhlich mechanism. We find that the E1(LO) frequency considerably increases for longer excitation wavelengths. The frequency shift, which is much higher than that expected from the E1(LO) phonon dispersion, is attributed to the selective excitation of large wave-vector E1(LO) phonon through the double-resonant mechanism as it has been recently reported for InN thin films.3 This effect is related to the peculiar electronic bandstructure of InN and has not been reported in other III-V semiconductors.The E1(LO)/E2h intensity ratio increases with increasing excitation wavelength and becomes much higher than those reported for thin films. We analyze this behavior in terms of resonance effects of the LO modes and effective scattering configuration. In the case of NWs, depending on the tilt of the nanowires and the sample orientation, scattering through lateral faces can yield a significant contribution of the x(zz)-x scattering configuration in which the E2 mode is forbidden, leading to a further enhancement of the E1(LO)/E2h intensity ratio. The excitation wavelength dependence of the E1(LO) Raman peak found in InN NWs is relevant for the characterization of InN NWs samples by means of Raman scattering and should be taken into account to obtain reliable conclusions from the Raman spectra. The strain determination from the E1(LO) frequency can be severely affected by the E1(LO) frequency dispersion.References:[1] R. Cuscó et al. Applied Physics Letters 97, 221906 (2010)[2] F. Limbach et al. Journal of Applied Physics 109, 014309 (2011)[3] V. Yu. Davydov et al. Physical Review B 80, 081204(R) (2009)
9:00 PM - W7.9
Phonon Transmission across Silicon Grain Boundaries.
Zhiting Tian 1 , Keivan Esfarjani 1 , Gang Chen 1
1 Mechanical Engineering, MIT, Arlington, Massachusetts, United States
Show AbstractModeling phonon transmission is vital for multiscale modeling of heat transport in nanostructured materials. In this study, we calculate the phonon transmission in three-dimensions via Green's function method. It will be applied to a silicon sigma3 grain boundary for which the force constants will be calculated using either a semi-empirical potential developed by Stillinger and Weber (SW), or first-principles density functional theory (DFT) calculations. The coherent transmission is calculated as a function of incident phonon frequency and transverse momentum. Results between first-principles and the SW potential will be compared. It will now be possible to integrate the information on momentum and energy-dependent transmission and the bulk mean free paths, both calculated from first-principles DFT, to accurately model heat transport in complex nanostructured materials.This material is based upon work supported as part of the S3TEC, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-FG02-09ER46577
Symposium Organizers
Subhash L. Shinde Sandia National Laboratories
David H. Hurley Idaho National Laboratory
Gyaneshwar P. Srivastava University of Exeter
Masashi Yamaguchi Rensselaer Polytechnic Institute
W8: Phonon Transport - CNTs and Graphene
Session Chairs
Mauricio de Lima
Martin Kuball
Thursday AM, December 01, 2011
Room 313 (Hynes)
9:00 AM - **W8.1
Phonon Transport in Graphene and Topological Insulator Thin Films.
Alexander Balandin 1
1 Department of Electrical Engineering, University of California - Riverside, Riverside, California, United States
Show AbstractIt was recently discovered experimentally that suspended graphene reveals unusually high thermal conductivity, which can exceed that of bulk graphite basal planes [1-2]. The latter is achieved when the size of graphene layers is sufficiently large and the extrinsic phonon scattering mechanisms do not dominate the phonon transport [3-4]. We explained this phenomenon by logarithmic divergence of the intrinsic thermal conductivity in two-dimensional (2D) systems. The crossover of the phonon heat conduction in transition from 2D system such as graphene to 3D system such as graphite was observed directly using a set of suspended few-layer graphene samples [5]. As the number of layers continues to increase, the thermal conductivity of FLG approaches the bulk graphite limit of ~2000 W/mK. Our experimental results have been independently confirmed by a large number of independent molecular dynamics simulations [6]. The phonon transport is different in quasi-2D crystals of topological insulator thin films. We have recently demonstrated the first “graphene-like” mechanical exfoliation of topological films with the thickness down to a single quintuple [7]. The quintuples were selected using Raman nanometrology [8]. In topological insulator quintuples, the 2D limit is never achieved. Instead the thickness variation and defects lead to an additional decrease of thermal conductivity below the bulk level [9]. We show that such a decrease in thermal conductivity with preserved electronic properties leads to enhancement of the thermoelectric figure of merit. This work was supported, in part, by DARPA – SRC Center on Functional Engineered Nano Architectonics (FENA). More details can be found at group’s web-site at: http://ndl.ee.ucr.edu .[1] A.A. Balandin, et al., Nano Lett., 8, 902 (2008)[2] S. Ghosh, A.A. Balandin, et al., Appl. Phys. Lett., 92, 151911 (2008).[3] D.L. Nika, S. Ghosh, E.P. Pokatilov and A.A. Balandin, Appl. Phys. Lett., 94, 203103 (2009)[4] D.L. Nika, E.P. Pokatilov and A.A. Balandin, Phys. Rev. B, 79, 155413 (2009).[5] S. Ghosh, A.A. Balandin, et al., Nature Mat., 9, 555 (2010).[6] A.A. Balandin, Thermal properties of graphene and nanostructured carbon materials, Nature Mat. (2011); see also at arXiv: 1106.3789v1[7] D. Teweldebrhan, V. Goyal and A.A. Balandin, Nano Lett., 10, 1209 (2010).[8] K.M.F. Shahil, et al., Appl. Phys. Lett., 96, 153103 (2010).[9] V. Goyal, D. Teweldebrhan and A.A. Balandin, Appl. Phys. Lett., 97, 133117 (2010).
9:30 AM - W8.2
The Role of Three-Phonon Normal Processes in the Thermal Conductivity of Graphene.
Ayman Alofi 1 , Gyaneshwar Srivastava 1
1 School of Physics, University of Exeter, Exeter United Kingdom
Show AbstractWe have made a theoretical study of the thermal conductivity of a monolayer graphene. For this we employed analytical expressions for the phonon dispersion relations and the vibrational density of states based on the work by Nihira [1] within the continuum model. The conductivity expression has been evaluated within Callaway’s relaxation time approach [2] and the two-dimensional Debye scheme. It is found that consideration of the momentum conserving nature of three-phonon Normal processes is very important for explaining the magnitude as well as the temperature dependence of the experimentally measured results for CVD-grown graphene samples [3,4]. The N-drift contribution (the second term in Callaway’s theory) provides a significant addition to the result obtained from the single-mode relaxation time theory (the first term in Callaway's theory), clearly suggesting that the single-mode relaxation time approach is inadequate for describing the phonon conductivity of graphene. We further discuss our attempt to explain the experimental measurements for a mechanically exfoliated graphene sample [5].[1] T. Nihira and T. Iwata, Phys. Rev. B 68, 134305 (2003).[2] J. Callawy, Phy. Rev. 113, 1046 (1959).[3] W. Cai et al, Nano Lett. 10, 1645 (2010).[4] S. Chen et al, ACS Nano 5, 321 (2011).[5] J.-U. Lee et al, Phys. Rev. B 83, 081419(R) (2011).
9:45 AM - W8.3
Electron-Phonon Interactions and Room Temperature Transport in Graphene Nanoribbons with Defects: An Ab Initio Study.
Amir Farajian 1 , Kirti Kant Paulla 1
1 Mechanical and Materials Engineering, Wright State University, Dayton, Ohio, United States
Show AbstractWe investigate the effects of electron-phonon interactions on the conductance of graphene nanoribbons. The method is based on ab initio calculation of electron-phonon coupling and Green's function treatment of transport properties. We analyze the contributions from different phonon modes, and calculate the conductance of the system, in presence of possible defects, at room temperature. The results reveal the effects of finite temperatures and defects, two of the practical issues in transport through graphene nanoribbons.This study is supported by the National Science Foundation Grant ECCS-0925939.
10:00 AM - W8.4
Heat Transfer at Interfaces with Graphene.
Zhiping Xu 1
1 Engineering Mechanics, Tsinghua University, Beijing, Beijing, China
Show AbstractGraphene has an ultrahigh in-plane thermal conductivity (5500 W/mK), but simultaneously a much lower conductivity along the c-axis in graphite or at the interfaces with other materials. As graphene finds more and more applications in nanoelectronics and high-performance composites, these interfaces become critically important in defining their heat dissipation and conduction performance. Unlike conventional interfaces in materials such as grain boundaries, the interfaces with graphene can be tuned by chemically modifying the graphene monolayer or intercalating the interfaces. These nano-engineering proposals require fundamental understanding of the heat transfer mechanisms.In order to obtain some insights on the transfer processes of mechanical and thermal energy across these interfaces, we perform series of molecular dynamics simulations, in combination with theoretical analysis by considering the quasi-ballistic nature of phonon transport at nanoscale. The result shows that heat dissipation or transport can be divided into two stages, beginning with an interface-controlled process. The effects of interface structures and binding properties on the whole process will be covered in this talk, with several examples showing how the interfacial thermal transfer can be engineered.References[1] Z. Xu, Thermal dissipation through nanoscale interfaces (in submission)[2] Z. Xu and M. J. Buehler, J. Phys.: Condens. Matter 22, 485301 (2010)[3] Z. Xu and M. J. Buehler, ACS Nano 3, 2767 (2009)
10:15 AM - W8.5
Two-Dimensional Materials with Tunable Thermal Properties.
Krishna Muralidharan 1 , Robert Erdmann 1 , Pierre Deymier 1 , Keith Runge 1
1 Materials Science and Engineering, University of Arizona, Tucson, Arizona, United States
Show AbstractTwo-dimensional (2-D) high Debye temperature-materials such as graphene and BN sheets demonstrate significant temperature dependent harmonic and anharmonic regimes, and when suitably nanostructured, can exhibit interesting, tunable thermal properties. In this work, using equilibrium and non-equilibrium molecular dynamics (MD) in conjunction with phononic band structure calculations, we examine the ability to manipulate the thermal-phonon life-times and consequently the thermal properties of nanostructured graphene and 2-D BN sheets. The Tersoff-Brenner family of potentials is used to represent the interatomic interactions of graphene and BN.In nano-anti dot superlattices of graphene, competition between coherent Bragg scattering and inelastic scattering of thermal phonons leads to dramatic changes in the phonon lifetimes (and therefore the thermal conductivity) for filling fractions as low as 2.5% over a significant range of temperature, while for a range of filling fractions (5-10 %), the phonon life-times appear to be independent of temperature, indicating that the thermal properties of graphene may be suitably tuned to be temperature-independent.Recent experiments have shown that 2-D BN sheets and thin films contain non-periodic distributions of triangular nanometric holes. MD simulations clearly show that such systems exhibit spatially varying as well as temperature dependent thermal conductivity. These effects suggest that BN sheets containing oriented non-periodic triangular holes may be designed to function as thermal rectifiers or thermal diodes.
10:30 AM - W8.6
Progress in the Theory and Waves.
Joel Abrahamson 1 , Changsik Song 3 1 , Jennifer Hu 1 , Jared Forman 1 , Sayalee Mahajan 1 , Nitish Nair 1 , Wonjoon Choi 2 1 , Eun-Ji Lee 3 , Michael Strano 1
1 Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 3 Chemistry, Sungkyunkwan University, Suwon Korea (the Republic of), 2 Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractProblems involving chemical reaction coupled to thermal diffusion are central in the chemistry and physics of propulsion, self-propagating high-temperature synthesis (SHS) of materials, and new approaches to energy storage and power generation involving thermopower waves. We present an analytical solution to a one-dimensional Fourier description of heat propagation in a solid reactant, with a first-order chemical reaction of Arrhenius form providing a thermal source. A generalized logistic function can completely determine the form of the temperature and concentration profiles within the solid, as well as the velocity of the reaction wave under a wide range of conditions. This alternative to the common asymptotic and three-zone approximate solution methods in the literature requires fewer assumptions and is valid at all time and length scales. The solution is limited to cases where the reaction wave velocity is constant, but can be generalized to nth-order kinetics. We apply this solution to predict the velocity of self-propagating thermopower waves and demonstrate a new thermal conduit material: nitrobenzene-functionalized carbon nanotubes. We report the first synthesis of single-walled carbon nanotubes decorated with mono-, di-, and trinitrobenzenes via diazonium chemistry as a means of increasing their energy density. Differential scanning calorimetry confirms thermally initiated energy release from such systems, with no release from control materials. Analysis of calorimetric data shows a statistically lower value of activation energy at low conversion, providing evidence for nanotube-guided chain reactions. Although covalent functionalization introduces defects that tend to scatter electrons and phonons, reducing electrical and thermal conductivity, we show that thermopower waves are still able to rapidly propagate along such decorated nanotubes and produce electrical power. The results offer new ways of storing chemical energy within carbon nanotubes.
10:45 AM - W8.7
Thermal Transport Properties of Ag-Based Nanocomposites Containing MWCNTs.
Masahiro Inoue 1 , Yamato Hayashi 2 , Hirotsugu Takizawa 2
1 , ISIR, Osaka University, Ibaraki Japan, 2 , Tohoku University, Sendai Japan
Show AbstractElectrically conductive pastes composed of Ag nanoparticles and micro-particles have been studied as novel thermal interface materials (TIMs) for advanced microsystems. Because Ag particles in these pastes are sintered during bonding process, high thermal conductivities (above 100 W/mK) can be achieved. However, their thermal conductivity is limited depending on the electrical conductivity since the thermal transport property is almost determined by the electronic contribution. In the present work, variation in thermal conductivity of Ag-based TIMs by introduction of MWCNTs was investigated. The Ag/MWCNTs nanocomposite particles were prepared via a sonoprocess. The nanocomposite particles were successfully prepared when appropriate surfactants were used. The nanocomposite particles were sintered at 250-300 C. After sintering, the thermal conductivity of the composites was compared with the electronic contribution to thermal conductivity that was estimated from experimental values of the electrical conductivity. The thermal conductivity of Ag/MWCNTs nanocomposites was much higher than the electronic contribution. Therefore, the increase in thermal conductivity of the Ag-based composites is attributed to phonon transfer along the percolation network of MWCNTs. Relationship between the thermal conductivity and microstructure of the nanocomposites will be discussed.
11:30 AM - W8.8
Thermal Conductivity of Graphene Paper.
Hafez Fard 1 , Gongkai Wang 1 , Xiang Sun 1 , Jie Lian 1 , Theodorian Borca-Tasciuc 1
1 Mechanical, Aerospace, and Nuclear Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States
Show AbstractIn this work thermal properties of graphene paper is investigated using direct temperature mapping and four probe Joule heating thermometry measurements. A rectangular piece of graphene paper is suspended between 4 electrodes. Electrical current passing through outer electrodes is used to generate heating in the sample. In one thermal characterization method, a small thermocouple is mounted above the sample and swept longitudinally to get the sample temperature at different locations and determine the temperature profile along the graphene paper sheet. Fitting the experimental data with a heat transfer model, thermal conductivity of the graphene sheet is obtained. It was shown that for a 3 microns thickness graphene paper sheet the effective thermal conductivity along the sheet is ~1150 W/mK. A four probe DC Joule heating thermometry technique is used to cross-check the results.
11:45 AM - W8.9
A Universal Design Framework for PhoXonic Metamaterials: From Surface States to Topological Defects.
Cheong Yang Koh 1 , Edwin Thomas 1
1 Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractPhoXonic (X=t,n) crystals and metamaterials are artificially structured materials (at various length scales) that provide new avenues toward the development of unique phoXon propagation behavior, from hyperlensing to mediating photon-phonon interactions. Phoxonic behavior becomes especially important at the nanoscale when the length scales of these quasi-particles approach that of the structural length scales and the detailed spatial distributions of the eigenmodes can be then tailored by the geometric structure of the metamaterial. This affords exciting possibilities and avenues for tailoring interactions between specific eigenmodes within the nano-scaled metamaterials, suggesting new fundamental limits in performance by taking into account these newly achievable material length scales. The successful realization of these applications requires a framework that is scale-invariant, and guides the design of these metamaterials with regards to their intrinsic spatial dispersion as well as the practical aspects of their finite size, surfaces, interfaces and purposeful defects. This latter point is crucial as these generalized 'defects' are where the interactions occur between the metamaterials and the external environment. We present a design framework that allows us to exactly treat the vector nature of phonons, photons and other waves and control their propagation, unifying the design of phoXonic crystals, metamaterials, waveguides and numerous other structures, both infinite and finite, within the same framework. In particular, we demonstrate how to i) optimize surface and interface states for PhoXonic metamaterials, creating 'fast' waveguides in "slow" structures and 'slow' waveguides in 'fast' structures, through the correct choice of the physical lattice topology, ii) rationally engineer defect states (both their frequencies and spatial eigen-mode profiles) within these metamaterials and distinguish between topologically trivial and nontrivial defects, and iii) couple and decouple edge states in finite "thin" metamaterials and geometrically control their degree of spatial localization into the structure. We also show explicitly how to link the design of these different classes of generalized defect states (cavities, interfaces, edges etc) within the same symmetry framework, hence providing unprecedented control over the manipulation of phoXons through control over their propagation velocities, spatial interactions and localization. This design framework provides a promising route towards the design of new nano-scale devices that mediate the coupling of phonons with photons, magnons and other waves with high efficiencies through the explicit design of their dispersions relations.
12:00 PM - W8.10
Simulation of Raman Shifts for Strained [111] Silicon Nanowires.
Christian Tuma 1 , Alessandro Curioni 1
1 , IBM Research - Zurich, Rüschlikon Switzerland
Show AbstractThe ab initio derived Tersoff type interatomic potential [1] is employed to calculate vibrational spectra for a series of axially strained 1D-periodic [111] silicon nanowires (SiNW). Different SiNW cross sectional shapes (hexagonal, circular, quadratic) are combined with diameters ranging from 4.4 nm to 22 nm. Raman active modes are identified based on a criterion for maximum overlap with corresponding eigenmodes calculated for bulk silicon. Raman shifts are obtained for a strain interval ranging from -5 % (compressive) to +5 % (tensile). For non-zero strain a splitting of the 3-fold degenerate optical mode into one mode oriented longitudinal and two modes oriented perpendicular to the strain direction is observed for both the bulk and all SiNW models. Even without the application of external strain all SiNW models show a diameter dependent intrinsic strain and corresponding splittings of the Raman active modes. A second-order polynomial is used for each model to fit an analytical expression describing individual strain/raman-shift relations. Virtually independent of the SiNW diameter and, hence, similar to the bulk, in first approximation all SiNW models show the same curvature of the strain/raman-shift relation. The only diameter dependent term is an offset value related to the intrinsic strain of the SiNW. The smaller the SiNW diameter, the larger the intrinsic Raman red-shift and the more the type of SiNW surface termination becomes important. Bulk behavior is predicted for SiNW diameters larger than about 20 nm.In summary, the results of this work allow to map Raman shifts obtained experimentally for [111] SiNW of known size to corresponding strain values. This can be of great use for virtually all cases where a direct determination of strain in [111] SiNW is not feasible.[1] S. R. Billeter, A. Curioni, D. Fischer, W. Andreoni; Phys. Rev. B 73, 155329 (2006)
12:15 PM - W8.11
High Throughput Thermal Measurements of Rough Silicon Nanowire Arrays.
Joseph Feser 1 , Keng Hsu 2 , Swaroop Jyothi 2 , Jun Ma 2 , Bruno Azeredo 2 , Kyle Jacobs 2 , Placid Ferreira 2 , Sanjiv Sinha 2 , David Cahill 1
1 Materials Science and Engineering, University of Illinois, Urbana-Champaign, Urbana, Illinois, United States, 2 Mechanical Engineering, University of Illinois, Urbana-Champaign, Urbana, Illinois, United States
Show AbstractTime domain thermoreflectance is used to study the thermal transport properties of vertically-aligned roughened silicon nanowires. Using newly developed synthetic techniques which independently control diameter, length, and roughness, we perform a systematic investigation of thermal conductivity of similar sized nanowires (100-150nm) over a large range of roughness (0-3.5nm RMS). In contrast to previous techniques, this allows for the simultaneous sampling of >10,000 wires/measurement with minimal post-processing. Wires with small roughness are found to behave similarly to the Casimir limit, however wires with high roughness are found to have thermal conductivity <15W/m-K, below the expected Casimir limit, despite no observed defects in the nanowire core’s to-date.
W9: Phonon Techniques and Devices
Session Chairs
Alex Balandin
Agnes Maurel
Thursday PM, December 01, 2011
Room 313 (Hynes)
2:30 PM - **W9.1
Coupling of an Electric Circuit to Phonon and Photon Environments.
J. Pekola 1
1 Low Temperature Laboratory, Aalto University, Helsinki Finland
Show AbstractI focus on mesoscopic electronic circuits at low temperatures. The energy relaxation is determined primarily by coupling of the electrons to phonon and photon baths. I discuss both superconducting and normal circuits.
3:00 PM - **W9.2
Phonons in Electronics: A Powerful Magnifying Glass for Understanding Performance and Reliability of Electronics.
M. Kuball 1 , James Pomeroy 1
1 Center for Device Thermography and Reliability (CDTR), H.H. Wills Physics Laboratory, University of Bristol, Bristol United Kingdom
Show AbstractSemiconductor electronics is being innovated to be able to handle higher frequencies and higher powers than possible to date by using novel material systems such as GaN for a wide range of applications from radars, satellites to communication, as well as for enabling very high efficiency power conversion devices for green economy. New material and device systems, however, often suffer in the development phase from reliability challenges, i.e., electronic devices do not last long enough, or they underperform compared to what is expected or hoped for. In this presentation, we will illustrate how the analysis and probing of phonons in semiconductor electronic devices can be used as optical magnifying glass to help the development of novel electronic devices, from the nano to the microscale. Complementary electrical analysis techniques will also be discussed.
3:30 PM - W9.3
Coherent and Incoherent Contributions to Thermal Transport in GaAs/AlAs Superlattices.
Keivan Esfarjani 1 , Tengfei Luo 1 , Maria Luckyanova 1 , Austin Minnich 1 , Gang Chen 1
1 Mechanical Engineering, MIT, Cambridge, Massachusetts, United States
Show AbstractExperiments on heat transport in GaAs/AlAs superlattices, based on pump-probe technique have revealed both incoherent and coherent components of the thermal conductance of this system. We have used a coherent transport formalism forthese superlattice systems based on Green' function technique. Incoherency may also be included in the bulk of the systems using the mean-free paths of the bulk system. The latter is calculated using first-principles density functional theory (FP-DFT) methods, and was shown to produce a thermal conductivity in good agreement with experimental data. Transmission is calculated for a model system as a function of incident phonon energy and transverse momentum. The obtained thermal conductance versus superlattice length and temperature will be qualitatively compared to our experimental results.
4:15 PM - **W9.4
Phonon-Assisted Electron Relaxation in Multi-Valley Semiconductors.
Sergey Pavlov 1
1 , German Aerospace Center, Berlin Germany
Show AbstractElectronic decay in multi-valley semiconductors was a hot topic of semiconductor research in late 50ths and 60ths due to its direct relation to electronic distributions in semiconductor devices for electronics and optoelectronics. Different theoretical propositions have been explored in order to find an adequate description of experimentally observed anomalous fast rates of electronic capture onto charge centers, such as dopants, in semiconductors. Most of commonly accepted theories consider electronic capture as a multi-step, “cascade” process of phonon-assisted gradual relaxation of an electron through ladders of excited states of impurity centers. Individual steps in the cascade model are accompanied by emission of intravalley acoustic phonons and obey the energy and momentum conservation law, making relaxation steps between the closest in energy states as most probable. This point of view has been reconsidered recently due to experimental results indicating that the cascade model of intracenter relaxation fails for multi-valley semiconductors, such as silicon. In these media a dominant contribution in intracenter relaxation occurs from an electron interaction with intervalley phonons and enhanced around the energy resonances between the phonons and impurity states. This results in appearance of alternative relaxation stairs with different probabilities and in effect causes fast relaxation rates observed experimentally. Our experiments on determination of lifetimes of most longliving excited states of shallow donors in silicon show that the longest individual relaxation steps do not exceed a couple of hundreds ps. Specific combinations of significantly different relaxation rates cause existence of short- and longliving states of impurity centers in multi-valley semiconductors. The latter can be used for creation of inverted electronic distributions inside the centers with resulting amplification of light, dominantly in the terahertz frequency range, where intervalley phonons hold their frequencies. The experimentally observed relaxation rates of some excited states of shallow donor centers in silicon have been used for the proof of different theoretical descriptions of intracenter relaxation taking into account interaction with intervalley phonons.
4:45 PM - W9.5
Nano Superlattice-like Materials as Thermal Insulators for Phase Change Random Access Memory.
Desmond Loke 1 2 , Luping Shi 2 , Weijie Wang 2 , Rong Zhao 2 , Lung-Tat Ng 2 , Kian-Guan Lim 2 , Hongxin Yang 2 , Tow-Chong Chong 3 , Yee-Chia Yeo 4
1 NUS Graduate School for Integrative Sciences and Engineering, National University of Singapore, Singapore Singapore, 2 , Data Storage Institute, Singapore Singapore, 3 , Singapore University of Technology & Design, Singapore Singapore, 4 Department of Electrical and Computer Engineering, National University of Singapore, Singapore Singapore
Show AbstractPhase change random access memory (PCRAM) is one of the leading candidates for next generation data storage devices. These devices have to retain information without external power, read/ write at fast speeds and withstand high number of read/ write cycles. PCRAMs based on the reversible switching of phase change materials between the amorphous and crystalline states, can satisfy all of these criteria. However, the operating power remains high, making it a challenge to integrate PCRAM with small transistors. Difficulties also exist to achieve higher writing speed due to the trade-off between the speed and stability of phase change materials. Resolving these limitations is of great importance. It would pave the way for commercialization of PCRAMs.In this paper, the thermal properties of PCRAMs with nano superlattice-like (SLL) materials are investigated, and the correlations between the operating power, writing speed and size of the SLL layers are identified. When the size of the SLL layers is sufficiently small, much higher peak temperatures were achieved in PCRAMs with nano SLL materials compared to that of PCRAMs with bulk materials. Higher performances were also obtained with improved cell geometry and functional materials. This can be attributed to the greater interface phonon scattering effects in the in-plane and cross-plane directions at the nanoscale. The results are expected to boost the development of low power and high speed PCRAMs, which could replace multiple types of memories currently used in electronic devices.
5:00 PM - W9.6
Microfabrication of a Phonon Spectrometer.
Jared Hertzberg 1 , Obafemi Otelaja 1 , Richard Robinson 1
1 Department of Materials Science and Engineering, Cornell University, Ithaca, New York, United States
Show AbstractWe describe a device for spectroscopic measurement of phonon modes transmitted through nanostructures. Decay of quasiparticles injected into a superconducting film excites phonons in a controllable, non-thermal spectral distribution. The phonons radiate into an adjacent microstructure and are detected after passing through it by a second superconducting transducer.[1] In initial investigations of this technique, we have employed aluminum superconducting tunnel junctions (STJs) attached to 30-micron silicon microstructures as generators and detectors of phonons. Modulating the generator's current causes modulation of the phonon flux for detection by lock-in amplification. We will describe the further development of this technique to extend the spectral range and improve the spectral resolution and signal-to-noise ratio. Spectral range is limited by phonon reabsorption in the generating superconductor and may be adjusted by using superconductors of higher bandgap such as granular aluminum. Spectral resolution may be improved by reducing superconducting bandgap inhomogeneity and modulation amplitude. Signal-to-noise ratio is improved by maximizing the phonon flux across the microstructure, by using low-noise detectors and by suppressing Josephson currents in the detecting STJ. We will also describe the micromachining of silicon nanostructures of sub-100 nm dimension in contact with the STJs, using electron-beam lithography, plasma etch and wet chemical etch methods. [1] W. Eisenmenger, A. H. Dayem, Phys. Rev. Lett. 18, 125 (1967).
5:15 PM - W9.7
Gauging the Influence of Proton Irradiation on Phonon Mediated Thermal Transport.
Marat Khafizov 1 , Anthony Shulte 2 , Mahima Gupta 2 , Zilong Hua 1 , Clarissa Yablinsky 2 , Todd Allen 2 , David Hurley 1
1 , Idaho National Laboratory, Idaho Falls, Idaho, United States, 2 , University of Wisconsin, Madison, Wisconsin, United States
Show AbstractThermal conductivity of metal oxide nuclear fuels is an important design parameter for efficient and safe operation of nuclear reactors. Thermal transport in oxide fuels is governed primarily by phonons and is strongly influenced by microstructure. During reactor operation, nuclear fuel undergoes drastic microstructural changes driven by large thermal gradients and neutron irradiation. This study addresses the relation between irradiation damage and thermal transport properties using proton irradiation. Proton irradiation creates similar defect microstructures to that produced by neutron irradiation. In addition, proton irradiation creates damage in a faster rate than typical reactor neutron damage allowing experiments to be completed in much shorter time periods. For protons in 2-4 MeV range the damage profile is roughly constant for 30-40 μm followed by a large, narrow damage peak due to ionic displacements.Thermal characterization of proton irradiated materials requires a measurement technique capable of providing depth resolved thermal conductivity measurements. A laser based thermal wave imaging technique that is well suited to measure thermal properties of thin proton irradiated layers was utilized. In this work, a test sample was proton irradiated using an ion-beam tandem accelerator and its microstructure was characterized using electron microscopy. To extract thermal conductivity of the damage layer, experimental results are compared to a model that accounts for depth dependent thermal properties. In the experiment, the probe depth is varied by changing the thermal wavelength. In this way, the thermal properties of the damage layer can be isolated from that of the undamaged base material. By comparing the thermal transport measurements to the electron microscopy results, conclusions about the relation between irradiation damage and thermal transport can be made.
5:30 PM - W9.8
Nanoferrofluid-Based Thermal Valves for Gating Heat Transport.
Alexander Gardner 1 , Indira Seshadri 1 2 , Ganpati Ramanath 2 , Theodorian Borca-Tasciuc 1
1 Mechanical, Aerospace, and Nuclear Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States, 2 Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States
Show AbstractHere we demonstrate a new concept of ferrofluid-based thermal valves based on ferromagnetic nanofluids. We show that the thermal conductance of a magnetite-based nanoferronanofluid can be increased by more than 100% by means of an external magnetic field with strength up to 685 Gauss, providing a basis for switching thermal conductance states through the use of a magnetic field. We will present our results of the time-response and cycling behavior of the conductivity switching at different magnetic fields to distinguish between effects arising from configurational entropy of the nanoparticles and heat capacity effects in the test systemjoule heating. We will show that the nanofluid thermal conductivity increases monotonically, but non-linearly, with the applied magnetic field. This behavior is explained in terms of the changes in the network connectivity of nanoparticles in the fluid due to magnetic-field-induced clustering that provide pathways for phonon transport. We also reveal the dependence of the thermal conductivity-magnetic field nexus on the nanoparticle size, composition, concentration, morphology and surfactant capping. These results open up a new way of gating heat transport in a variety of engineering systems and applications.
5:45 PM - W9.9
Development of a Phonon Spectrometer for Investigating Nanoscale Thermal Transport.
Obafemi Otelaja 1 , Jared Hertzberg 1 , Richard Robinson 1
1 Department of Materials Science and Engineering, Cornell University, Ithaca, New York, United States
Show AbstractUnderstanding nanoscale thermal transport will provide important insights necessary for the realization of efficient thermoelectric and phononic devices. The classical diffusion-based model for heat transport begins to fail at small length scales, and new phenomena become critically important: phonon scattering from surfaces and interfaces, reduction in modes due to confinement to 1 or 2 dimensions, changes in crystal anisotropy, and scattering, reflection and resonance due to step-changes in geometry. Thermal conductance measurements, although widely applied to nanostructures, remain an imperfect probe of these behaviors. We describe instead a more precise technique: a microscale phonon transmission spectrometer.[1] We have successfully fabricated Al superconducting tunnel junctions as generators and detectors of quasi-monochromatic phonons transmitted through a silicon microstructure. This device operates at cryogenic temperatures and has a spectral range from ~80 to ~200 GHz (wavelengths ~30 to ~70 nm), frequency resolution ~10 GHz and spatial resolution smaller than 1 micron. Measurements are made in the ballistic-transport regime where phonon-phonon interactions may be neglected. We will describe the use of this device to probe surface and acoustic confinement effects on phonon transport in silicon microstructures and nanostructures formed using micromachining methods.[1] M.N. Wybourne and J.K. Wigmore, Rep.Prog.Phys. 51, 923
Symposium Organizers
Subhash L. Shinde Sandia National Laboratories
David H. Hurley Idaho National Laboratory
Gyaneshwar P. Srivastava University of Exeter
Masashi Yamaguchi Rensselaer Polytechnic Institute
W10: Phononics and Coherent Acoustics
Session Chairs
Subash Shinde
Gyaneshwar Srivastava
Friday AM, December 02, 2011
Room 201 (Hynes)
9:30 AM - **W10.1
Control of Polariton Condensates by Acoustic Fields.
Paulo Santos 1 , Edgar Cerda-Mendez 1
1 , Paul Drude Institute for Solid State Electronics, Berlin Germany
Show AbstractMicrocavity (MC) exciton-polaritons are bosonic quasi-particles resulting from the strong coupling between excitons confined in a quantum well and photons in a semiconductor microcavity (MC) containing it. As a consequence of their photonic character, polaritons have a low effective mass and, therefore, de Broglie wavelengths l_dB of several mum. Furthermore, it has recently been demonstrated that polaritons undergo, at high densities, a transition to a Bose-Einstein-like state (a condensate) with extended temporal and spatial coherence.[1] Due the low effective mass, these solid-state condensate form at temperatures (of few K) orders of magnitude higher than for atomic BE-condensates. The exploitation of the unique quantum properties of polariton condensates requires processes for their manipulation and control. In this talk we present a novel approach based on the use of surface acoustic wave (SAW) fields. A SAW propagating on the surface of the MC produces a moving strain field, which periodically modulates both the photonic and excitonic polariton components. For SAW wavelengths l_SAW < l_dB, this lateral modulation leads to the formation of a tunable polaritonic crystal with grating constant and contrast given by the SAW period and amplitude, respectively.[2] Photoluminescence (PL) measurements of the polariton dispersion in (Al,Ga)As microcavities excited by SAWs with l_SAW of a few mum shows the folding of the polariton dispersion into mini-Brillouin zones bound by energy gaps.[3] At high particle excitation intensities, a polariton condensate is formed at the bottom of the lower polariton branch. This macroscopic quantum phase is characterized by an extended spatial coherence length (l_c, tens of microns) as well as by a long coherence time (t_c, of hundreds of ps). l_c and t_c have been measured using an interferometric setup.[3] The spatial modulation by a SAW does not destroy the condensed phase but divides it into an array of condensate wires aligned along the SAW wave fronts. This acoustically induced fragmentation is confirmed by the progressive reduction of l_c along the SAW propagation direction with increasing SAW amplitude. Similar results were obtained for the modulation by two orthogonal SAW beams, which lead to the formation of an array of polariton dots. The tunable modulation by a SAW provides an efficient mechanism for the formation of confined condensates and to control the coupling between them. These confined condensates open the way for the investigation of many body interactions (such as Josephson coupling) in non-equilibrium quantum phases as well as for the implementation of functionalities for quantum information processing.*
[email protected][1] J. Kasprzak et al., Nature 443, 409 (2006)[2] M. M. de Lima, Jr., Phys. Rev. Lett. 47, 045401 (2006).[3] E. A. Cerda-Méndez et al., Phys. Rev. Lett. 104, 126402 (2010)
10:00 AM - W10.2
Meso-Scale Surface Phononic Spectral Gaps in Hierachicial Nanostructured MetaMaterials.
Sisi Ni 2 1 , Yang Koh Cheong 1 2 , Markus Retsch 2 1 , Steve Kooi 1 , Edwin Thomas 2 1
2 , Institute for Soldier Nanotechnologies1, Cambridge, Massachusetts, United States, 1 Materials science and Engineering, MIT, Cambridge, Massachusetts, United States
Show AbstractSubwavelength bandgap formation in phononic metamaterials, such as locally resonant crystals, have received extensive attention due to their potential for designing compact form factor devices that manipulate longer wavelength phonons1. In this work, one and two-dimensionally periodic surface structures, infiltrated with “building blocks” consisting of soft polymers/oligomers loaded with glassy monodisperse spherical nanoparticles, were fabricated as an experimental platform to probe meso-scale band structure. We demonstrate the possibility of developing meso-scale gaps in sub-micron surface structures by combining geometrical induced surface band-gaps with resonant-like sub-wavelength spectral gaps due to avoided crossings between the intrinsic response of non-periodic resonant building blocks and the non-local surface modes of the entire system. By varying the plane group symmetries of the surface structure, and the controlled variation of the nanoparticle loading fraction and the degree of disorder of the spheres within the building blocks, the entire band dispersion, i.e. both position and width and of the spectral gaps, can be facilely manipulated for desired applications. Brillouin light scattering and Near-field Scanning Optical Microscopy (NSOM) were carried out to verify the theoretically designed dispersion relations to further develop understanding of the general design framework. 1.Sheng, P., Mei, J., Liu, Z., & Wen, W. (2007). Physica B: Condensed Matter, 394(2), 256-261.
10:15 AM - W10.3
Thermal Transport in Silicon Thin Films with Periodically Arrayed Nanoscale Perforations.
Tom Harris 1 , Bongsang Kim 1 , Charles Reinke 1 , Patrick Hopkins 1 , Roy Olsson III 1 , Ihab El-Kady 1 , Eric Shaner 1 , John Sullivan 1
1 , Sandia National Laboratories, Albuquerque, New Mexico, United States
Show AbstractThermal transport in submicron structures is a topic of significant importance in microelectronics, thermoelectrics, and in both micro- and nanoscale electromechanical systems. Although phonon size effects are known to influence thermal transport in low-dimensional structures, much remains to be understood regarding phonon scattering mechanisms in semiconductor thin films. In an effort to develop an improved understanding of phonon size effects on thermal transport, we have performed temperature-dependent thermal transport measurements on single-crystal silicon thin films with periodically arrayed nanoscale perforations. Silicon films were fabricated into freestanding membranes, which were formed from silicon-on-insulator wafers with 500nm-thick, p-type (~10-cm) device layers. Perforations in the silicon membranes consisted of cylindrical through-holes with 300nm diameters patterned in a square lattice arrangement across the extent of each membrane. To vary the void fraction (i.e., the porosity) of the silicon thin films, the minimum spacing between voids, for a given film, was adjusted from 200nm to 500nm. The microfabrication techniques employed in this work permit wafer-scale production of the aforementioned perforated silicon thin films. By implementing an AC resistive thermometry technique, the in-plane thermal conductance of these silicon films was measured from 20K – 295K. Based on these measurements, we discuss the influence of void fraction on the temperature-dependent behavior of thermal transport in thin film silicon. This work was performed, in part, at the Center for Integrated Nanotechnologies, a U.S. DOE, Office of Basic Energy Sciences user facility. Portions of this work were also supported by a Sandia National Labs LDRD project. Sandia is a multi-program laboratory operated by Sandia Corp., a wholly-owned subsidiary of Lockheed Martin Co., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.
10:30 AM - **W10.4
Probing Acoustic Fields in Tunable Coupled Phononic Cavities by Interferometry.
Mauricio de Lima 1 , Paulo Santos 2 , Andres Cantarero 1 , Yuriy Kosevich 1 3
1 Materials Science Institute, University of Valencia, Valencia, Valencia, Spain, 2 , Paul Drude Institute for Solid State Electronics, Berlin Germany, 3 Semenov Institute of Chemical Physics, Russian Academy of Sciences, Moscow Russian Federation
Show AbstractUnderstanding the physics of coupled cavities is fundamental for a large variety of wave phenomena. For instance, we have recently experimentally observed the surface acoustic analog of Bloch oscillations, Wannier-Stark ladders, and Landau-Zener tunneling by using strongly coupled surface acoustic cavities [1]. These structures were fabricated within a SAW delay line on a 128° rotated Y-cut LiNbO3 substrate. The bare-LiNbO3 surface acoustic cavities are placed in-between acoustic Bragg mirrors (BMs) that comprises λ/4 NiCr/Au stripes. The BM behavior is defined by the acoustic reflectivity of the metallic stripes, which is determined by the thickness (h) of the stripes (mass loading effect) as well as by the change in the SAW velocity generated by the screening of piezoelectric field under the metallic stripe. In this contribution, we introduce an approach for the electric tuning of the coupling between the acoustic cavities. Tuning is achieved by either keeping the metallic stripes electrically isolated from each other or by short-circuiting them all together. In the latter case, the piezoelectric potential vanishes under all stripes, whereas in the former the piezoelectric potential of the individual stripes float with respect to each other. The acoustic transmission through the coupled cavities has been measured by means of a network analyzer. In addition, the use of a planar geometry allows a direct access to the vertical component of the particle displacement distribution, which is probed by Michelson interferometry. These experiments demonstrate that, for the particular ratio h/λ ~ 1% used for the NiCr/Au stripes onto the LiNbO3 surface, the acoustic contrast is reduced when the piezoelectric field is screened. Therefore, the mass loading and the piezoelectric contributions to the acoustic impedance are opposite in sign. In particular, there is a π phase difference between the standing wave pattern for coupled cavities in shorted and floating configurations.[1] M. M. de Lima Jr., Yu. A. Kosevich, P. V. Santos, and A. Cantarero, Phys. Rev. Lett. 104, 165502 (2010).
11:30 AM - **W10.5
Confined Acoustic Phonons in Ultrathin Si Membranes.
Clivia Sotomayor Torres 1 2 6 , John Cuffe 1 , Emigdio Chavez 1 6 , P.-Olivier Chapuis 1 , El Houssain El Boudouti 4 , Francesc Alzina Sureda 1 , Bahram Djafari-Rouhani 5 , Andrey Shchepetov 3 , Mika Prunnila 3 , Jouni Ahopelto 3
1 Phononic and Photonic Nanostructures, Catalan Institute of Nanotechnology, Bellaterra Spain, 2 , Institució Catalana de Recerca i Estudis Avançats (ICREA), , Barcelona Spain, 6 Physics, UAB, Bellaterra Spain, 4 LDOM and FSO, University of Oujda , Oujda Morocco, 5 IEMN, Université de Lille 1, Lille France, 3 Microsystems and Nanoelectronics, VTT , Espoo Finland
Show AbstractThe acoustic properties of ultra-thin Si layers are important for many areas of nanotechnology, impacting on both structural integrity and thermal transport. The effect of phonon confinement is particularly relevant, affecting both heat dissipation and charge carrier mobility in the first instance. The goal of this work is to obtain a deeper understanding of phonon confinement and propagation in materials with dimensions comparable to thermal phonon wavelengths. Here we present the dispersion relations of confined acoustic modes obtained from Brillouin Light Scattering (BLS) spectroscopy. The thicknesses of the free-standing Si membranes measured ranged from 8 to 400 nm. Using an angle-resolved approach, flexural, dilatational, and shear modes propagating in the plane of the membrane have been investigated. The dispersion curves were calculated based on the continuum elasticity model for acoustic modes in an anisotropic membrane. The effect of the ~1 nm native oxide layer in such thin systems was also calculated using a three-layer model. Green’s function simulations were used to calculate the projected local density of states of the phonon modes at the surface (z = 0), for which the normal component was shown to be proportional to the intensity of light scattered via the ripple effect.A satisfactory agreement was found between the dispersion of the modes obtained from semi-analytic and Green’s function calculations with experiments confirming a small phase and group velocity for the first order flexural mode of the ultra-thin membranes, with a correspondingly large density of states. The frequencies measured were found to be consistently lower than those predicted by continuum elasticity theory for both the lowest order flexural mode and the first order dilatational mode. This observation could not be explained simply in terms of error in the thickness measurements, the effect of a native oxide layer, or a temperature rise within the membranes. This work provides a basis to investigate the effect of acoustic phonon confinement on thermal transport and elastic constants in systems with sub- 50 nm dimensions.
12:00 PM - **W10.6
The Ruby SASER as a Source of Coherent Acoustic Waves.
Harold de Wijn 1
1 Utrecht University, Debye Institute, Utrecht Netherlands
Show AbstractThe last decade has seen the realization of the acoustic analogue of the laser, which presently is referred to by the acronym SASER, standing for Sound Amplification through Stimulated Emission of (acoustic) Radiation. The first condition required for a SASER is the presence of a “hot zone”, a condensed ensemble of two-level quantum systems connected by a one-phonon transition, in the present case made up of ions in a solid. These systems are pumped into their “inverted” state, such that initially all reside in their upper level. While stimulated emission of phonons had been observed quite some time ago, a genuine SASER additionally requires that the generated phonons propagate in a collimated beam which repetitively passes through the inverted zone to engage in further amplification. This was first realized in dilute optically excited ruby [1].Various aspects of the ruby SASER will be discussed, namely the ruby level system, the geometry, the creation of the inverted hot zone by optical pumping, the detection of the generated phonons, and the phonon occupation reached. The stimulated emission and amplification need be described in terms of coherent Bloch-type equations of motion governing the level populations and the transverse spin polarization. Finally, the frequency spread and the angular divergence of the phonon beam have been determined.--- [1] L.G. Tilstra, A.F.M. Arts, and H.W. de Wijn, Phys. Rev. B 68, 144302 (2003); Phys. Rev. B 76, 024302 (2007); J. Phys. Conf. Ser. 92, 012011 (2007); Chin. J. Phys. 49, 184 (2011).
12:30 PM - W10.7
Generalized Phononic Networks: Controlling Complexity through Simplicity.
Cheong Yang Koh 1 , Edwin Thomas 1
1 Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractThe manipulation and control of phonons is extremely important from both a fundamental scientific and applied technological standpoint, providing applications ranging from sound insulation to heat management. Phononic crystals and metamaterials are artificially structured materials (at certain length scales) that provide promise in controlling the propagation of phonons in solids. However, the vector nature of the phonon makes the development of a governing framework with which to guide the design of these phononic metamaterials complicated and no coherent framework currently exists for the design of phononic structures. In this work, we utilize a combination of global symmetry principles, adopted from group theory and the theory of representations, together with conservation principles and broken symmetry concepts to formulate our generalized design framework. In particular, we are able to explain the choice of a particular physical topography for a desired phononic propagation behavior in a coherent fashion. In addition, we show how to explicitly control the dispersion relations of a phononic metamaterial in order to obtain a desired final band structure. Demonstrations range from a new polychromatic phononic metamaterial which possesses multiple complete in-plane spectral gaps totaling over 100% in normalized gap size to a phononic metamaterial which exhibits a single complete in-plane spectral gap of 102% and a complete spectral gap of 88%, both significant advancements over the state of the art. We show that only a few governing principles are required to design the desirable band dispersion relations of such phononic metamaterials. The generality of our framework allows extension to other vector and scalar waves, such as photonic, plasmonic and magnonic structures and provides a promising route forward to the development of integrated structured material platforms that allow for the rational manipulation and interactions of phonons with other waves, such as phonons and spin waves.
12:45 PM - W10.8
Softening of Copper Nanowires in Early Stage of Electromigration Studied by Picosecond Ultrasound Spectroscopy and Micromechanics Modeling.
Hirotsugu Ogi 1 , Akihiro Yamamoto 1 , Nobutomo Nakamura 1 , Teruo Ono 2 , Masahiko Hirao 1
1 , Osaka University, Osaka Japan, 2 , Kyoto University, Uji Japan
Show AbstractWe observed significant decrease in the elastic constant in a Cu nanowire during electromigration through the decrease in thickness resonance frequency of the nanowire. High-frequency vibrations related with Cu nanowires on a Si substrate were excited and detected by picosecond ultrasound through the reflectivity change in the probe light pulse. Width, thickness, and spacing of nanowires were varied to identify observed vibrational modes, and we find the thickness resonance of the nanowire at 75 GHz. It decreased as the progress of the electromigration while those of other modes (surface-wave resonances and Brillouin oscillations) remain nearly unchanged. We attribute the change in the resonance frequency to the stiffness decrease and present a model that vacancy diffusion occurs along grain boundaries to form thin defects, keeping total volume fraction unchanged. A micromechanics calculation combined with the electromigration-diffusion theory is proposed to predict the dynamic change in the stiffness of nanowire during electromigration.