Deposition of thin films using pulsed vapor fluxes can provide additional kinetic handles on the early stages of film growth, enabling the tuning of film microstructure. This tuning becomes possible when the time domain of the pulsed flux is comparable with the characteristic time scales for adatom diffusion during film nucleation and island growth as well as island coalescence during film coarsening. Such time scale interactions have been studied before with kinetic Monte Carlo (KMC) simulations resembling film growth by state-of-the-art deposition methods, [1,2] though these methods typically do not allow independent control of the pulse width, frequency and amplitude of the flux. Recent developments in physical vapor deposition manifested by a pulsed plasma technique termed high power impulse magnetron sputtering (HiPIMS) [3] have made this control possible [4] providing a new tool to study experimentally the effect of pulsed vapor fluxes on film microstructural evolution [5]. These developments create an incentive to refine the theory behind film growth by pulsed vapor fluxes.
In this work, using as reference experimental data [5], we use KMC simulations to study the effect a of a deposition flux with the above described characteristics on the dynamics of nucleation and coalescence during Volmer-Weber growth (typical for metallic films deposited on amorphous substrates). We perform a systematic investigation of the effect of pulse width, pulsing frequency, deposition rate per pulse, adatom surface diffusion and island coalescence rates on the so-called elongation transition thickness (ΘElong), which signifies when the substrate is predominately covered by elongated structures as opposed to single isolated islands [2]. The results show multiple significantly different growth regimes represented by an exponential scaling relation between ΘElong and the frequency f, with the particular exponents, 0.0 (no f-dependence), -0.1, -0.4, and also the theoretically predicted -0.33 in common with continuous deposition. These growth regimes are explained with the aid of existing atomistic nucleation theory by clarifying the direct dependence of island growth and coalescence on nucleation density, as well as the effects of unbalancing the interplay between island growth and coalescence due to deposition. From these investigations, analytical expressions based on deposition and material parameters have been developed that are able to predict how one can access each specific growth regime.
[1] P Jensen, B Niemeyer, Surf. Sci. 384 (1997) L823.
[2] J M Warrender, M J Aziz, Phys. Rev. B 76 (2007) 045414.
[3] K. Sarakinos, J. Alami, and S. Konstantinidis, Surf. Coat. Technol. 204 (2010) 1661.
[4] D. Magnfält, V. Elofsson, G. Abadias, U. Helmersson, and K. Sarakinos, J. Phys. D: Appl. Phys. 46 (2013) 051910.
[5] V. Elofsson, B. Lü, D. Magnfält, P.E. Münger, and K. Sarakinos, manuscript in preparation.

A phase field model for representing fabrication process of Ag₂Ga alloy nanoneedle has been proposed. Meticulous analysis of Ag₂Ga needle growth is explained by using a model of phase transformation. Experimental observations expound a detailed process of silver-coated tip dipped into a drop of liquid gallium at room temperature, and then Ag₂Ga needle is formed spontaneously. The phase field model is employed to describe the fabrication and then supply reliable information on material science. A convenience of the phase field model is expressing a microstructure evolution in terms of both temporal and spatial dependence of continuum functions. In addition to these improvements, the computational method can provide a general framework for simulating many complicated micro- or nano-structure formations in real alloys. The proposed model has the phase field foundation and incorporates the phenomenological phase transformation. Changes of the free energy of the system is driving force for the phase transformation and mechanism of diffusion is considered to be important factor on influencing micro- or nano-structural evolution of the needle. The evolution of needle-shaped precipitates is modeled based on the free energy increase or the Gibbs-Thomson effect at the end of the needle. The excess free energy sets up a diffusive flow of solute along the axis of the needle that sustains the lengthening. Therefore, a growth rate and morphology of the needle is conceived base on the mathematical models to quantify and optimize the needle growth process. Quantitative observations develop a clear understanding of the growth mechanism as well as an improved control of the growth in a desired direction, the length and diameter of the needle. The computational method aims to investigate mechanical behaviors and test self-assembly process of special materials into useful micro- or nano- structures. An objective of detailed analysis of Ag₂Ga needle growth in a selected orientation based on the phase field model and driven by the phase transformation has profound significance for the process of alloy solidification and growth. The theoretical model provides an efficient verification on the experimental work and encourages the current approach to define the geometries required in actual practice.

Extremely thin single-crystal sheets, including semiconductor nanomembranes (NMs), graphene, MoS2 and other exfoliated layer compounds, show great promise as a platform for new nanotechnologies. Their properties (mechanical, electronic, charge transport, etc.) differ significantly from those of bulk counterparts. The primary drivers in these differences are 1) the extremely low flexural stiffness of sheets relative to bulk material, 2) their large surface-to-volume ratio, and 3) their capacity for accepting high degrees of strain, factors that influence many functional properties of the material relative to those of the bulk counterpart. These sheets are, however, seldom found free-standing, but rather as part of a composite, be it a support substrate or a material that becomes part of the functional properties.
We discuss several examples of composite thin-sheet/bulk substrate (or multiple-sheet composites) in which the surface, one or more of the interfaces, or the substrate properties affect the functional properties of the thin sheet, with or without the presence of strain. These include charge carrier mobility of graphene on Ge, mechanically straining a Ge NM bonded to polyimide, self-organized buckling of a strained Si NM bonded to highly elastomeric PDMS, surface transport in a clean Si NM on oxide, and strained Si/SiGe/Si NM trilayer packages that may serve as the basis for growing better strained-Si two-dimensional electron gases (2DEGs). Strain is of fundamental importance in many of these problems, in causing both mechanical and electronic changes. Additionally the two interfaces of the sheet may find themselves in different environments that may contribute differently to the properties. All of these examples represent situations that are not fully understood and may make good topics for theoretical contributions.
Research supported by DOE and NSF.

The material properties of nanofilms with complex hybrid interfaces were numerically investigated by using molecular dynamics and finite element method. The interfacial morphology in nano length scale is generally complex patterned such as self-affine fractal geometry. Numerical analysis on the layered structures of the nanostructured materials was performed to address the influence of interfacial geometry to mechanical property. Theoretical analysis using homogenization and contact mechanics was also done to compare with numerical simulation result. The complex geometry of the interfaces was modeled by self-affine fractals and was realized in atomic level and continuum limit as well. Stress-strain curves under the external deformations were obtained for two-phased nanofilms with various interfacial morphology. Interfacial regions with finite thickness can be constructed by the roughness of the interface and were observed to affect their mechanical properties. Current study provides a fundamental understanding of the mechanical characteristics of hybrid multilayered films.

Recently, global environmental consciousness is growing up all over the world, and it causes a growing concern about the clean energy, especially solar energy conversion by photovoltaic solar cells (PVSCs). However, for the establishment of PVSCs as a major source of the clean energy, there are so many problems to be solved technologically, such as the low energy-conversion efficiency, the high production cost, etc. In this paper, we propose the computational nano-materials design for the realization of high-efficiency II-VI-based PVSCs with a self-organized quasi-ono-dimensional nano-structure caused by the two-dimensional spinodal nano-decomposition [1].
For the present simulations we use the multi-scale simulation, which is based on the first-principles electronic structure calculations of the mixing energy and chemical pair-interactions, and Monte Carlo simulation (MCS) of the two-dimensional crystal growth. Since we have to treat alloy semiconductors, we employ the Korringa-Kohn-Rostoker coherent potential approximation (KKR-CPA) method for the calculation of the electronic structure. We use the KKR-CPA program package MACHIKANEYAMA2002 developed by Akai [2]. By performing the MCS of the two-dimensional layer-by-layer crystal growth, we have designed the self-organized quasi-one-dimensional nano-structures (Konbu-Phase [3]) fabricated by two-dimensional spinodal nano-decomposition for high-efficiency photovoltaic solar cells (PVSCs) in Cd(Te1-xSx ), and Cd(Te1-xSex ). The Konbu-Phase enhances the nano-scale electron-hole separation in PVSCs due to their Type II band alignment. The Konbu-Phase also increases the efficiency of PVSCs by multi-exciton formation using the inverse Auger effect in the self-organized quasi-one-dimensional nanostructures. We also discuss how to fabricate Konbu-Phase starting from the uniform nano-particles made by the photo-chemical reactions.
References:
[1] M. Oshitani, K. Sato, H. Katayama-Yoshida, Applied Physics Express 4 (2011) 022302.
[2] H. Akai, http://sham.phys.sci.osaka-u.ac.jp/kkr/
[3] Y. Tani et al., Appl. Phys. Express 3 (2010) 101201. Jpn. J. Appl. Phys. 51 (2012) 050202.

Physical vapor deposition provides a controllable means of growing crystalline metallic one-dimensional nanorods in addition to two-dimensional thin films. While a solid theoretic framework exists for the growth of crystalline metallic thin films, their counterpart for the growth of crystalline metallic nanorods is absent, in either theoretical or conceptual form. Here we present such a theory and puts it in historical perspective of a broader field of crystal growth. By recognizing (1) the diffusion barrier of adatoms down multiple-layer surface steps, and (2) the formation and stability of multiple-layer surface steps, this framework divides metallic nanorods growth in two modes. In mode I, the growth takes place on wetting substrates and nanorods have the shape of a tower. The step dynamics on the sides defines the diameter of nanorods. In mode II, the growth takes place on non-wetting substrates and nanorods have the shape of a cylinder. Because of the complete, or nearly complete, dominance of multiple-layer surface steps over monolayer surface steps, growth model II results in the smallest diameter of nanorods using physical vapor deposition. A closed-form theory that defines this diameter has been reported. Further, we verify the theory using lattice kinetic Monte Carlo simulations, and validate the theory using published experimental data. Finally, we carry out a series of theory-guided experiments to grow metallic nanorods of ~10 nm in diameter, which is the smallest ever reported using physical vapor deposition.

Barium titanate (BTO) nanoparticles exhibit intriguing size-dependent structural and dielectric properties which make them a candidate for use in novel capacitor technologies. We are developing a method for determining the permittivity of BTO nanoparticles in stable dispersions. As part of this technique, we are employing both analytical and computer models to relate the permittivity of the particles themselves to the effective permittivity of the particle dispersion for various volume fractions and dispersion media. We have demonstrated that particle permittivity can be determined more precisely when the particles are dispersed in a medium with a high dielectric constant compared to a dispersion in a medium with a low dielectric constant. We have also discovered that though the models agree with one another at low particle loadings, they diverge significantly after a particular threshold (approximately 30 percent by volume). In addition, we are using computer models to investigate the effect of agglomeration on the relationship between nanoparticle permittivity and the effective permittivity of the stable dispersion. Furthermore, we are modeling core-shell geometries to explore the impact of surface effects on this particle-dispersion permittivity relationship, including the effects of different ligands on the nanoparticles.
Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy&’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

Among many metals, copper and copper based compounds are the attractive materials due to their catalytic, optical, and electrical properties. Copper nanoparticles were synthesized successfully through chemical reduction of different copper salts stabilized by Ocimum Leaf Extract, a natural biopolymer. The resulting copper nanoparticles were characterized by UV Visible Absorption Spectrometer, X-Ray Diffraction (XRD), Fourier Transform Infrared (FTIR), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) experiments.
X-ray diffraction pattern exhibits the presence of pure Cu powder. The lattice parameter ‘a&’ has been calculated by using this profile is 3.61505 Å. The average size of copper nanoparticles is around 20 nm. The FTIR spectrum analysis has confirmed the presence of functional groups of stabilizer Ocimum leaf extract in capping the copper nanoparticles. The broad and strong bands are observed at around 3480 cm-1, 1627 cm-1 and 622 cm-1 corresponding to the O-H stretching frequency. The broad band observed at around 3480 cm-1 and 622 cm-1 illustrates the stretching frequency of hydroxyl group (OH group) present in the surface of the copper nanoparticle. It characterizes stretching vibrations of O-H bonds in H2O molecules absorbed on the nanoparticle surface. But it could be related also with the copper hydroxide presence. The band observed at around 1627 cm-1, which is characteristic for bending vibrations of O-H bonds in OH groups.
Copper nanoparticles prepared displayed an absorption peak at around 558 nm. Copper nanoparticles have been used to prepare substrates for Surface Enhanced Raman Scattering measurements. These results show that the surface enhancement of copper nanoparticles is good. Novelty of this present study is that the plant extract (Ocimum leaf extract) is very cost effective and eco-friendly. Therefore, this can be economic and effective alternative for the large scale synthesis of copper nanoparticles.

We have computationally predicted the formation process as well as the electronic properties of sponge-like Si/SiO2 nanocomposites, and we have verified both experimentally [1-3]. These nanoscale materials are promising candidates as absorber layers for next generation solar cells as they exhibit a widened band gap due to quantum confinement and electrical interconnectivity due to percolation of the nanostructured Si.
The thermally activated spinodal decomposition was performed by rapid thermal processing and partly using very rapid thermal processing by scanning a diode laser line source with a dwell time in the ms range, which is necessary for an industrial fabrication process.
The spinodal phase separation of homogeneous SiOx precursor materials into Si/SiO2 nanocomposites has been simulated by a bit-coded kinetic 3D lattice Monte Carlo method [1]. Employing large scale atomistic pseudopotential computations, these as yet unexplored nanostructures have been investigated in the subsystem level, as well as in full assembly in order to predict the band gap and the band offsets [3].
Experimentally, the nanostructure of the Si/SiO2 nanocomposites have been studied by cross-sectional energy-filtered transmission electron microscopy (EFTEM), and the 3D morphology has been investigated by EFTEM tomography as well as by atomic probe tomography (ATP) [2]. The band gap has been determined via light absorption measurements.
Additionally, 2D EFTEM images have been simulated in dependence on the thickness of the sample, which allows, by comparison with experimental images, conclusions about the 3D morphology too.
The predicted morphology of the sponge-like Si/SiO2 nanocomposite appears to be almost identical the measured ones. The predicted scaling behavior of the coarsening of the nanostructure during thermal treatment has been seen in experiments too, at least qualitatively.
The predictive computational studies have been proven to be very important to optimize the design and the fabrication in particular for Si/SiO2 nanocomposites, where experiments based mainly on EFTEM are costly and very time consuming.

[1] Liedke, Heinig, Mücklich, Schmidt, Appl. Phys. Lett. 103, 133106 (2013)
[2] Friedrich, Schmidt, Heinig, Liedke et al. Appl. Phys.Lett. 103, 131911 (2013)
[3] Keles, Liedke, Heinig, Bulutay, Appl. Phys.Lett., submitted (2013)
The financial support by Germany&’s Federal Ministry of Education and Research and the Turkish funding organization TÜBITAK is acknowledged.

We utilize first principles density functional method and MPWorks, a python based high throughput framework, to compute the monovacancy formation energies in alkali, alkaline earth, and transition metals. Three different exchange correlation (xc) functionals, PBE, PW91 and LDA are evaluated with respect to the computed formation energies. Bulk and defect structures are relaxed using a mesh of 108000 k-points X atoms to achieve an accuracy of 10 meV or better. Due to the presence of void internal surface, DFT results for vacancy formation energies are typically not as accurate as the cohesive energies. Of the three functionals, LDA gives better results compared to the PBE and PW91 (GGA) functionals due to the cancellation of exchange and correlation error. PBE and PW91 predict noticeably different vacancy formation energy values even though the lattice constants and cohesive energies predicted by them are very close. Applying the surface error correction scheme proposed by Nazarov et. al. (Phys. Rev. B 85, 144118, 2012) brings the formation energies computed with the three functionals closer to the experimental data. The surface error correction term is in general small for LDA and bigger for GGA functionals. Meta-GGA functionals are expected to predict better surface energies and hence better vacancy formation energies. We report the performance of one such meta-GGA functional, revTPSS.

Kinetics of migration of chemisorbed hydrogen on a graphene sheet is studied with density-functional-theory (DFT) method. The chemisorbed H atoms interact strongly with each other through the carbon sheet. This energetically forbids independent migration of individual H atoms at room temperature, because the H migration would encounter a chemisorption state 1.6 eV/H2 above the gas state. However, we found that the pathways do exit whereby highly cooperative migration can circumvent the energetically unfavorable chemisorption sites so that all the encountered states fall below the energy level of hydrogen gas state. Mediated by water, the kinetic barriers to the cooperative migration can be reduced to 0.8 eV. These results indicate that trace mounts of moisture could play a critical role in future optimization of the spillover hydrogen storage at ambient conditions.

Nanoporous silicate glasses have recently emerged as one of the most promising candidates for the ultra-low-k inter-metal insulating materials of the next technology node primarily due to their superior mechanical and dielectric properties as compared with other low-k materials. At present, however, little is known about the details of their molecular and topological structures as well as their influence on the mechanical and dielectric properties of the insulating materials. In this study, we have performed first principles calculations in conjunction with the classical force field modeling to investigate the mechanical and dielectric properties of the organosilicates hybrid glasses and the nanoporous amorphous silica for their potential application to the interconnect technology in the advanced microelectronic devices. Based on our newly developed computational approach to effectively generate realistic structure models of the nanoporous silicate glasses, we have developed a detailed atomistic understanding regarding the influence of the carbon content, porosity, and the network connectivity on the mechanical and dielectric properties of the ultra-low-k glasses. Our calculated results show that the organic cross-linking fragments do not show any clear enhancement in the mechanical strength of the low-k glasses but simply lower down the elastic moduli as the carbon content increases in the silicate bond network. Our calculations further show that the mechanical strength of the nanoporous silicate glasses appear to be a lot more dependent on the network connectivity than on the porosity of the material matrix. In addition, we found that different types of terminal groups (e.g. -CH3 or OH) may induce different degree of porosity in the amorphous bond network primarily dependent on their interactions with the constituent atoms in the material matrix, so as to change the mechanical properties of the silicate glasses. On the other hand, our results also revealed that the dielectric constants of the nanoporous silicate glasses are not necessarily lowered down by the replacement of the O atoms with the methyl- or ethyl-bridging units. Controlling the porosity of the material matrix seems to be a much more effective way toward the synthesis of the ultra-low-k silicate glasses than by changing their chemical composition.

Laterally-offset graphene layers are known to exhibit oscillatory motion up to GHz frequency, which is studied in this work using a bilayer graphene model system via molecular dynamics (MD) simulations and density-functional theory (DFT). The maximum velocity predicted by the MD simulations is consistent with previously-reported experimental measurements, when the dependence of graphene roughness is taken into account, and can be correlated with the friction between layers. The atomic-scale features of the potential energy surface determine the lateral force, which in turn affects the trajectory of the top layer. Also, the maximum velocity is affected by the initial lateral offset, but independent of graphene layer dimensions. The findings provide insight into the mechanisms of graphene oscillation, and potentially enable the development of graphene-based nanoelectromechanical devices such as nanoswitches, oscillators, and nanomotors.

We report an extensive study of the mechanical properties of carbyne using first-principles calculations. We investigate carbyne&’s mechanical response to tension, bending, and torsion deformations. Under tension, carbyne is about twice as stiff as the stiffest known materials and has an unrivaled specific strength of up to 7.5×10^7 Nm/kg, requiring a force of ~10 nN to break a single atomic chain. Carbyne has a fairly large room-temperature persistence length of about 14 nm. Surprisingly, the torsional stiffness of carbyne can be zero but can be ‘switched on&’ by appropriate functional groups at the ends. We reconstruct the equivalent continuum-elasticity representation, providing the full set of elastic moduli for carbyne, showing its extreme mechanical performance (e.g. a Young&’s modulus of 32.7 TPa with an effective mechanical thickness of 0.772 Å). We also find an interesting coupling between strain and band gap of carbyne, which is strongly increased under tension, from 2.6 to 4.7 eV under a 10% strain. Finally, we estimate its chemical stability against self-aggregation, finding an activation barrier of 0.6 eV for the carbyne-carbyne cross-linking reaction and an equilibrium cross-link density for two parallel carbyne chains of 1 cross-link per 17 C atoms (2.2 nm).

The catalyst-mediated vapor-liquid-solid (VLS) process is a widely used method for growing crystalline nanowires. Often during the growth nanowires develop kinks and branches. To grow nanowires of desired morphology, size and composition a detailed understanding of the growth mechanism and geometry of the solid-liquid interface is necessary. Despite recent advances in experimental characterization, the full three dimensional structure of the solid-liquid interface remains incomplete. We perform 3d molecular dynamic (MD) simulations of nanowires wetted by liquid to understand the structure of the non-planar solid-liquid interface and the contact geometry at the solid-liquid-vapor triple line. We investigate how the nanowire diameter and size of the liquid droplet affects equilibrium temperature and wetting geometry. We also discuss the nature of a tilting instability observed in the MD simulations that shares features in common with kinking phenomena observed in nanowire growth.

Thermal transport in thin-film and low-dimensional superlattices has been the subject of intense scrutiny over the last 30 years. In this work, using nonequilibrium molecular dynamics simulation (NEMD) and lattice dynamics, we calculate phonon-mediated thermal conductivity of graphene-boron nitride superlattices (G/h-BN) with varying period and interface topology. We observe minimum thermal conductivities at specific period length for ideal superlattices, which is believed to result from a complex interplay between various competing mechanisms. To interpret the nonmonotonic dependence of the conductivity on the period, we implement a coarse-graining analysis and consider superlattice as a crystalline material. By isolating local interface effects, we only consider Bloch phonons propagating globally and predominating energy transport, while effects of filtering and tunneling due to interfaces are reflected implicitly in the resultant dispersion relation of crystalline superlattices. Within this framework, the occurrence of minimum conductivities can be simply ascribed to the competition between phonon softening and anharmonic effects. Nonetheless, in contrast to computational predictions, the minimum in thermal conductivity vs. period has not been observed for thin-film superlattices experimentally. This is often attributed to interfacial disorder or defects. For the 2D systems, we demonstrate instead that only ±1 atom-layer disorder in period can also effectively account for the absence of minimum values. On the contrary, the inclusion of periodic interface defects does not erase the conductivity valleys, nor does it lead to a systematic reduction in lattice thermal conductivity.

Fischer-Tropsh synthesis (FTS) has been known as a method for production of hydrocarbons from mixture gas of CO and H2. In particular, high temperature Fischer-Tropsch (or HTFT) reaction has been commercially operated at temperatures of 300°C-350°C by using an iron-based catalyst, mainly producing gasoline and light olefins. Various carbide phases (e.g., ε-Fe2C, εprime;-Fe2.2C, chi;-Fe5C2, theta;-Fe3C, etc.) have been produced and identified under FTS conditions. However, among the various carbide phases, the active phase during FTS might be the chi;-Fe5C2 phase. Alkali metals are excellent promoters of catalytic surface reactions because they are good electron donors. we have prepared a catalyst (Fe5C2) that is potentially a good candidate for FTS. The CO adsorption and dissociation reaction properties were observed on a clean surface using detailed theoretical DFT calculations. The Fe5C2 surface can be divided into Fe-terminated and C-terminated surfaces. To confirm the effects of K on the catalytic FTS activity, we compared the changes in the catalytic performance during CO adsorption and dissociation as the K surface coverage on the Fe5C2 surface was increased. In addition, the changes in the charge distribution were computed to analysis the Bader charge.

The present work arouse from the idea to generate some nanostructure compound which leaded us to minimize locally the lead obtained on public distributed potable water, delivered directly by the outside house water distribution network. Most the time, lead (Pb) can be also obtained directly from house&’s brass faucets, bathroom accessories and copper piping welds. Usually potable water is used as it is as toiletries, for drinking and cooking by common people.
It is important to get rid of lead since is heavy metal and harmful for human health as many others metals; moreover, lead accumulation over time in the body can cause serious illness.
For this reason we were working to find a given nanostructure capable to capture Pb metal in sense to purify the water in situ. The simulation work was carried out using computational software, Material Studio, looking for correspondent repulsion Pb-Pb energies and binding, adsorption and chemisorptions energies between the lead and nanostructure surface. We also made and report some others physical measurements to establish what other organic and inorganic substances are present in some Mexican region in the potable water. At the Active Mexican Norm the Pb must be less than 0.01 mg/L of water to be considered as potable water.
Finally, this investigation can be very important since distribution and supplying potable water is a huge problem for environmental and people health. This is not a particular problem of Mexico since it is also global around the world.
The author greatly acknowledge the financial support to this work given by UACM, CONACyT, CONACyT-SNI, ICyT, and to the Secretaría de Ciencia, Tecnología e Innovacioacute;n del Distrito Federal by a financial project.
Keywords: repulsion, chemisorption energy, lead, potable water, nanostructure.

As “smaller is stronger”, nanostructured materials such as nanowires, nanotubes, nanoparticles, thin films, atomic sheets etc. can withstand non-hydrostatic (e.g. tensile or shear) stresses up to a significant fraction of its ideal strength without inelastic relaxation by plasticity or fracture. Large elastic strains can be generated by epitaxy, or by static or dynamical external loading on small-volume materials, and can be spatially homogeneous or inhomogeneous. This leads to new possibilities for tuning the physical and chemical (e.g. electronic, optical, magnetic, phononic, catalytic, etc.) properties of a material, by varying the 6-dimensional elastic strain as continuous variables. By controlling the elastic strain field statically or dynamically, one opens up a much larger parameter space for optimizing the functional properties of materials, which gives a new meaning to Feynman&’s 1959 statement “there's plenty of room at the bottom”. The roadmap for rational ESE will be addressed. These include precisely applying and measuring large elastic strain (AFM, nanomechanics, microscopy and spectroscopy), predicting what strain does to physical and chemical properties (ab initio to continuum scale modeling), tailoring (sometimes via in situ experiments) quantitatively the properties in desired directions, and understanding how large an elastic strain can be sustained for how long (mechanisms of plastic deformation, defect evolution and failure in small-volume materials).
[1] Zhu, Li, "Ultra-strength materials," Progress in Materials Science 55 (2010) 710-757.
[2] J. Feng, Qian, Huang, Li, "Strain-engineered artificial atom as a broad-spectrum solar energy funnel," Nature Photonics 6 (2012) 865-871.
[3] Hao et al, "A Transforming Metal Nanocomposite with Large Elastic Strain, Low Modulus, and High Strength," Science 339 (2013) 1191-1194.

Controlling superstructure of binary nanoparticle (NP) mixtures in three dimensions (3D) from self-assembly opens enormous opportunities for the design of novel materials with unique properties for a variety of applications, e.g. energy related, photonic, and phononic, applications. Here, we present a synthetic approach toward such materials from bottom-up type block copolymer (BCP)—metal nanoparticle (NP) co-assembly. A BCP was used as a structure-directing agent for controlling spatial arrangement of metal NPs. Structure control of functional NPs at the nanoscale, mediated by BCP micro-phase separation, provides facile routes to nanostructured materials. In order to efficiently predict nanostructures of such materials, a novel theoretical approach was developed, allowing a level of complexity usually reserved to more costly molecular simulation treatments. The theory exhibits quantitative agreement with the experiment of a highly ordered 3-dimensional (3D) chiral metal nanoparticle (NP) network, synthesized via triblock terpolymer / ligand-stabilized NP self-assembly, and provides design criteria for controlling a range of NP based nanomaterial structures. The intimate coupling of synthesis, in-depth electron tomographic characterization, and a recently developed field theory enables exquisite control of superstructure in highly ordered porous 3D continuous networks from single and binary mixtures of metal NPs. Quantitative analysis provided insights into short- and long-range NP-NP correlations, and local and global contributions to structural chirality in the networks. Results provide design rules for next generation mesoporous network superstructures from binary NP mixtures for potential applications in areas including catalysis and plasmonics

Understanding grain boundary (GB) migration mechanisms plays a key role in understanding the deformation mechanics of nano-crystalline materials. Despite the many theories have been proposed, there still exists widespread disagreement in the research community. For instance, the normal direction diffusion model is often assumed in conventional grain growth models, but recent studies have suggested that shear-coupled grain boundary migration is the dominate mechanism for low or high angle structures during stress driven dynamics. This study addresses the competition between the two mechanisms in nano-crystalline copper by using molecular dynamics simulations to characterize symmetric tilt grain boundary migration in response to an external driving force. The fundamental idea is to first determine an order parameters using principal coordinate analysis and then find the reaction pathways under different simulation conditions by minimum free energy path (MFEP) search techniques. Once the MFEP is found, the free energy profile for GB migration can be computed from thermodynamic integration. Our preliminary results show that migration behavior of a symmetric tilt grain boundary with various misorientation angles can be well represented by two order parameters, and surprisingly the MFEP for most misorientation GBs has a zigzag shape instead of the commonly observed a smoothed interface.
This work was supported by a grant from the Office of Naval Research.

Stress-driven grain boundary (GB) migration is frequently observed during plastic deformation of nanocrystalline (NC) metals at room temperature. Migrating GBs can generate disclinations, which in turn may influence the mechanical properties of materials through their interactions with other crystal defects. We illustrate this by investigating the interaction of disclinations with nanocracks in 3D NC metals via molecular dynamics simulations. We found that the generation of disclinations due to GB migration can either heal or propagate nanocracks. The potential influence of this mechanism on the fracture behavior of NC metals will be discussed. This work was funded by the BP Materials and Corrosion Center at MIT.

First principles harmonic stability analyses predict many important high-temperature crystal phases to be mechanically unstable, necessitating the development of new atomistic methods to rigorously account for anharmonic degrees of freedom at finite temperature. In this talk we present the recently-developed anharmonic potential cluster expansion framework and describe its use within Monte Carlo simulation to predict finite-temperature thermodynamic properties, mechanical properties, and structural phase transitions in strongly anharmonic and mechanically unstable phases.
This framework allows a crystal's Born-Oppenheimer potential energy surface to be written as a polynomial expression that is invariant to finite rigid-body rotation and translation of the crystal, has been greatly simplified using the symmetries of a high-symmetry reference crystal, and can be parameterized from first principles electronic structure calculations, yielding an accurate, compact, and arbitrarily improvable model Hamiltonian. We have applied our method to ZrH₂, which exhibits a high-temperature cubic phase that, upon cooling, undergoes a symmetry-breaking second-order transition to one of three equivalent tetragonal phases. We find via Monte Carlo simulation that cubic ZrH₂, predicted by DFT to be dynamically unstable at 0K, can be anharmonically stabilized at high temperature, and our predicted cubic-to-tetragonal transition temperature is in good agreement with experiments. We also present calculated finite-temperature free energies and mechanical properties for both cubic and tetragonal ZrH₂, which can be used to parameterize continuum-scale constitutive models.

The rapid development of synthesis and characterization of materials with feature sizes at nanoscale as well as unprecedented computational power have brought forth a new era of materials research in which experiments, modeling and simulations are performed side by side to probe the unique mechanical properties of nanostructured materials. Here we report a number of recent studies on deformation and fracture mechanisms in nanotwinned materials, including the maximal strength of nanotwinned materials with equi-axed grains, crack bridging by nanotwins in thin films, twin-spacing-induced ductile-brittle transition in nanotwinned nanopillars, and plastic anisotropy in columnar-grained nanotwinned materials. In each study, there has been a strong synergy between theory and experiment, with new experimental findings driving advances in modeling and simulations, and new theoretical insights suggesting new experimental studies. The discussions will be organized around the current understandings based on existing experimental and theoretical efforts, as well as the outstanding questions that require further studies in the future.

Molecular dynamics (MD) is a powerful condensed matter simulation tool for bridging between macroscopic continuum models and quantum models (QM) treating a few hundred atoms, but it is limited by the accuracy of the interatomic potential. Sound physical and chemical understanding of these interactions have resulted in a variety of concise potentials for certain systems, but it is difficult to extend them to new materials and properties. The solution is obvious but challenging: develop more complex potentials that reproduce large QM datasets. In this talk I will discuss two different ways that we are pursuing this goal. The first approach uses the ReaxFF family of potentials that reproduces known chemical reaction pathways in small clusters of atoms, while still allowing molecular dynamics simulations of millions of atoms undergoing chemical reaction. The second approach, SNAP, is a very general machine-learning approach for automated generation of interatomic potentials from large QM datasets.
I. Initiation in energetic materials is fundamentally dependent on the interaction between a host of complex chemical and mechanical processes, occurring on scales ranging from intramolecular vibrations through molecular crystal plasticity up to hydrodynamic phenomena at the mesoscale. A variety of methods (e.g. quantum electronic structure methods (QM), non-reactive classical molecular dynamics (MD), mesoscopic continuum mechanics) exist to study processes occurring on each of these scales in isolation, but cannot describe how these processes interact with each other. In contrast, the ReaxFF reactive force field, implemented in the LAMMPS parallel MD code, allows us to routinely perform multimillion-atom reactive MD simulations of shock-induced initiation in a variety of energetic materials.
II. The growing availability of large QM data sets has made it possible to use automated machine-learning approaches for interatomic potential development. Bartok et al. demonstrated that the bispectrum of the local neighbor density provides good regression surrogates for QM models using Gaussian process regression (GAP). We adopt a similar bispectrum representation within a linear regression scheme that we call SNAP. We have produced potentials for tantalum and indium phosphide. Results will be presented demonstrating the accuracy of these potentials relative to the training data, as well as their ability to accurately predict material properties not explicitly included in the training data. Comparing to recent QM calculations of screw dislocation cores in BCC tantalum, we observe that the SNAP potential gives the correct core structure and the correct energy barrier for screw dislocation motion, unlike existing EAM and ADP potentials.

Semi-coherent interfaces are key structural features in a wide range of engineering materials. Such interfaces arise, for example, in epitaxial layers, precipitation, and both diffusional and diffusionless displacive phase transformations. Structures and properties of misfit dislocations and their intersections (nodes) in semi-coherent interfaces have been experimentally demonstrated to significantly affect the thermal and mechanical stability of interface. Employing atomistic simulations, we reveal that misfit dislocation lines can exhibit a spiral pattern (SP) with change in dislocation character or remain straight without change in dislocation character at their intersection. By analyzing nodes formation processes in terms of kinetics and energetics, we found that the appearance of SP is dependent on the competition between core energy of misfit dislocation and excess energy of coherent interface. Both of them are related to lattice mismatch, the separation distance between nodes, and interface chemical potential. It is also discovered in the MD simulations that the structure of the nodes can alter between the condensed and expanded forms upon mechanical loading or point defect absorption. In addition, the point defect formation energies at nodes are significantly reduced due to the switching of the node structures.

Liquid metal embrittlement (LME) is the transition from ductile to brittle fracture when a metallic material is stressed in contact with a liquid metal. The LME atomic scale mechanism is still not well understood. We will review experimental evidence of LME for the Cu/Hg couple taken as a model system (as received standard high purity OFHC copper and grain boundary (GB) engineered copper with a high fraction of sum;3 grain boundaries). We have investigated LME induced fracture of the symmetric high energy sum;5(210) GB by density functional theory (DFT) modeling. The solid-liquid interface Cu(210)/Hg has been modeled using ab-initio molecular dynamics. Such modeling shows that spontaneous grain boundary wetting is excluded (2γSL- γGB>0) and that there is an energy barrier to fracture in a Griffith sense. A cohesive zone model of fracture of the sum;5(210) GB will be presented that incorporates the effect of an additional disjoining pressure induced by nanosized capillary layer. This description, a long standing issue in solid-liquid interfaces, is supported by DFT and provides a possible mechanism to explain the important loss of cohesion seen in LME.

Mechanical properties of nanoscale objects can differ significantly from those of macroscopic objects of the same material, leading to many opportunities for new applications in nanotechnology. However, the measurement of these properties at the nanoscale remains in itself a significant challenge.
Within the framework of the European Metrology Research Programme (EMRP) project ‘Traceable measurement of mechanical properties of nano-objects&’, we performed atomistic molecular dynamics simulations of AFM indentation experiments on gold nanorods and nanoparticles with diamond AFM tips, as conducted at MIKES. Emphasis was placed on matching the experimental size and shape of both the nano-objects and the AFM tip apex.
We find that plastic deformations in nanoscale gold objects occur at smaller indentation depths than in bulk surfaces. The crystallographic properties of the gold nanorods (single crystal or penta-twinned) strongly affect the deformation mechanism on the atomic scale. We also discuss the effect of size and shape of the nano-object, as well as the effect of humidity in the AFM chamber on the reproducibility of the measurement.

Amorphous silica glass is a representative material for investigating structural properties of disordered systems. Although widely used in many applications, full understanding the structural characteristics of silica is still not achieved. This study investigates the structural transformations and properties of silica glass nanowires under tensile loading, via molecular dynamics simulations using the BKS interatomic potential. It is found that silica nanowires possess unique surface states reflected by a secondary peak in the bond angle distribution below the major peak positions. This unique secondary peak contributes to the abnormal mechanical response of silica nanowires under tension, particularly when the silica nanowire diameters are 4 nm or less. We use ring size analysis to unveil the underlying structural transformation mechanisms. Our analysis indicates that the key ring sizes responsible for the structural modifications are 3-, 4-, 5- and 6-membered rings. Within the elastic region, the ring size distributions remain relatively unchanged, while within the plastic region, ring size distributions change dramatically and reach plateaus when the nanowires are broken. Interestingly, for nanowires with diameters less than 4 nm, the variation of ring size distributions shows a distinct trend with respect to tensile strain, indicating that the surface states play a key role in both modifying the mechanical properties and structural characteristics. Our findings provide key insights into novel properties of nano-sized amorphous materials and are expected to inspire further experimental efforts.

In this talk, I will first give a brief review on the most recent development of quantitative TEM deformation technologies. Following this, I will report our applications of these novel technologies on hexagonal-close-packed (HCP) structured single crystal metals. It was found that when the sample size of a <0001> oriented titanium alloy single crystal is reduced down to 1 micro meter or so, the deformation mechanism will change from deformation twinning dominated plasticity to ordinary dislocation dominated plasticity. At the same time, the stress-strain curves characterized by significant strain bursts for larger sample size will be overtaken by the much smoother curves for smaller sample size (Yu et al, Nature, 2010). For magnesium and many HCP metals, twinning on the {10-1 2} plane is a common mode of plastic deformation. In addition, deformation twinning is usually thought to be accomplished by lateral gliding of twinning dislocations or disconnections in its twining plane. Recently, inspired by the computational simulation works, we studied the deformation of submicron-sized single-crystal magnesium compressed normal to its prismatic plane and found a brand new non-shear accompanied deformation mechanism. This newly discovered deformation mode is distinct from conventional deformation twinning and enriches the known repertoire of plasticity.

Experimental characterizations and molecular dynamics simulations have revealed several twinning-associated boundaries that play crucial roles in nucleation, growth, and interactions of deformation twins in HCP metals. Molecular dynamics simulations have been an alternative approach to provide insight into the microscopic mechanism for nucleation and growth of twins. However, accurate empirical potentials are not available for most of HCP metals. Here, we present first-principles density functional theory (DFT) based calculations to estimate the energies of twinning-associated boundaries: coherent twin boundaries for twins (i.e. 10-11, 10-12, 10-13), coherent prismatic||basal interfaces (i.e. (0001)||(10-10), coherent pyramidal||basal interfaces (i.e. (0001)||(10-1 1)) in HCP metals (Mg, Ti, Zr, Zn, Cd, Be). A theoretical model based on the energy balance between coherent twin boundaries and the two coherent interfaces was developed to predict kinetics in association with nucleation and growth of deformation twinning in HCP metals and the results are in agreement with the experimental observations.

Upconversion luminescence for NaYF₄:Yb³⁺:Er³⁺ nanoparticles shows a marked decrease in quantum efficiency and much faster rates in the rise and fall of the luminescence decay curves as compared to bulk NaYF₄:Yb³⁺:Er³⁺. We propose that in moving from µm-sized to nm-sized β-NaYF₄:Yb³⁺:Er³⁺ particles, the effects observed in the decay curves can be mostly accounted for by the changes in only three rate constants, namely the non-radiative rate constant of Yb³⁺ (kYb_NR), the non-radiative rate constant for Er³⁺ from ⁴I_11/2 to ⁴I_15/2 (k_NR3) and the non-radiative rate constant for Er³⁺ from ⁴I_13/2 to ⁴I_15/2 (k_NR2). The increase in the effective values for kYb_NR and k_NR3 occurs because rapid energy migration among the levels emitting near 1 µm leads to equilibration between interior and surface sites, with a consequent increase over bulk values in the two rate constants relating to the non-radiative decay of these levels. The increase in k_NR3 also results from this rapid energy migration, since most of the Er³⁺: ⁴I_13/2 to ⁴I_15/2 emission comes from surface sites where ⁴I_11/2 is rapidly quenched to ⁴I_13/2. The changes that occur in the visible upconversion luminescence (red, green, and blue) are simply a reflection of the faster quenching at 1 µm. Computational analysis through a non-linear rate equation model of the upconversion process supports this hypothesis.

Characterizing and understanding the mechanical properties and deformation behavior are essential for optimizing the design and operating performance of advanced materials. Importantly, slight additions of impurities can result in significant change of macroscopic mechanical properties and deformation behavior. In titanium, which is a desirable structural material for a wide range of applications due to its high strength to weight ratio and corrosion resistance, small additions of oxygen result in a dramatic increase of its overall strength and a corresponding change in the slip behavior. However the intrinsic role of oxygen has not been systematically understood due to the complication of microstructure in macro-scale materials. Through quantitative in situ transmission electron microscopy (TEM) nanomechanical testing, we show that even though Ti-O precipitation does lead to Orowan strengthening, oxygen solid solution strengthening is the dominant strengthening mechanism in binary alpha-Ti-O alloys, which significantly multiplies strength. Dislocation core structures of high purity Ti samples with systematically varied oxygen concentrations were studied by aberration-corrected high-resolution (scanning) STEM imaging. Interestingly, oxygen atoms tend to segregate at dislocation cores with increasing oxygen concentration, thereby stabilizing the screw dislocations and maintaining the straightness of the dislocation lines. In contrast, screw dislocations tend to be bent with some edge component in materials with less oxygen, resulting in higher dislocation mobility. Theoretical simulations were also performed to study the influence of oxygen on the three-dimensional dislocation core structure at the atomic scale, where the results are well in agreement with the two-dimensional experimental observations.

Revealing the real-time atomic-scale deformation processes is central to understanding and controlling the mechanical degradation of engineering materials and devices. We have developed a novel in situ nanomechanical testing platform inside transmission electron microscope, which provides an unprecedented in-situ atomistically-resolved approach for discovering the previously unknown deformation mechanisms in metallic nanostructures. In this talk, we will present our recent progress in integrating the in situ nanomechanical testing with atomistic modeling for studying the deformation mechanisms and structure-mechanical properties relationship in Au nanowires. Firstly, we show that the surface-nucleated dislocations can strongly interact with each other inside the confined volume of Au nanowires. These interactions lead to a new type of dislocation-originated stacking fault tetrahedra (SFT), in distinct to the widely believed vacancy-originated SFT. Our work further reveals the complete atomic-scale processes of nucleation, migration and annihilation of dislocation-originated SFT, which shed new light onto the strain hardening and size effect on the mechanical behavior of small-volume materials. Secondly, we show a new type of size effect on nanotwinned Au nanowires. When the twin thickness is reduced to the smallest possible size of the angstrom scale, Au nanowires exhibit a remarkable ductile-to-brittle transition that is governed by the heterogeneous-to-homogeneous dislocation nucleation transition. Our quantitative measurements show that approaching such nanotwin size limit gives rise to the ultra-high tensile strength in Au nanowires, which is close to the strongest limit of the ideal strength of perfect Au crystals. The talk is based on our two recent publications in Nature communications (Nature Communications 4, 1742 (2013); Nature Communications 4, 2340 (2013)).

A well-defined array of Ge quantum structures possesses unique electronic properties for a variety of applications, including quantum-computers and infrared photodetectors. Herein, we use simulation to predict and experiment to demonstrate the compositional redistribution of Si and Ge in the near-surface region of Si_0.8Ge_0.2 substrates by applying a spatially structured compressive stress to the substrate and thermally annealing the substrate under stress. The primary advantage of the proposed approach is that a single, reusable template is used to induce the compositional variation for multiple substrates. The compositional redistribution of Ge is predicted under purely elastic deformation, using a kinetic Monte-Carlo simulation that accounts for the influence of composition, temperature, and stress on the diffusion kinetics of Ge in SiGe alloy. Atomistic stress field in a SiGe slab is computed using the Tersoff empirical potential and static relaxation. This compositional variation in turn can be used to selectively grow a 2D array of Ge quantum dots upon Ge exposure. To complement the computational prediction, the compressive stress is applied by pressing a 2D array of Si pillars against the Si_0.8Ge_0.2 substrate. Hertz contact model is used to calculate the compressive stress applied to the Si_0.8Ge_0.2 substrate under the Si nanopillars. We observe that the magnitude of the stress and annealing temperature determine the nature of deformation (elastic or plastic) in the Si_0.8Ge_0.2 substrate. Corresponding energy dispersive x-ray spectroscopy (EDS) shows that the compositional redistribution of Si and Ge in the near-surface region of Si_0.8Ge_0.2 substrates results from elastic deformation within a thermal annealing temperature range of 950 to 1000 ^omicron;C and an applied stress range of 15 to 18 GPa. Based on nano-probe energy dispersive x-ray spectroscopy, the elastically deformed compressed region shows Ge depletion by 85% and Si enrichment by 21% in atomic concentration. However, the temperature and stress exceeding the aforementioned ranges result in plastic deformation with no compositional variation. The plastic deformation depth is ~30 nm according to scanning transmission electron microscope (STEM) images. We attribute the plastic deformation to (1) the localized pressure applied to the substrate under the contact area, (2) the near-surface substrate stiffness at substrate temperature, and (3) the tensile biaxial stress under the compressed region due to different thermal expansion rates of Si vs. Si_0.8Ge_0.2.

A series of molecular dynamics simulations to study
adsorption of surfactants on solid surfaces were investigated.
Self-aggregation of sodium dodecyl sulfate (SDS)
on graphite and silica surfaces were conducted and it was
observed that SDS molecules aggregated in different ways,
depending on the properties of the surfaces. Whereas on the
graphite surface the SDS molecules were adsorbed by their tails
groups on the silica surface the molecules were adsorbed by both
head and tail groups.
Surfactant molecules and solid surfaces were constructed by an
atomistic model using force fields with classical potentials for the
inter and intra molecular interactions.
Structures of aggregated molecules and their formation were studied
in terms of density profiles of both head-groups and hydrocarbon
chains, coverage and geometric parameters. Adsorption studies and
how the molecules were attached to the solids were also studied.
The results were compared with those given by our own experiments
and similar data were obtained in terms of adsorption and structure of
the molecules.

The ability to tune electronic properties in photovoltaics and nanomaterials holds great promise for incorporating these materials in next-generation transistors, circuits, and nanoscale devices. In particular, the use of predictive first-principles calculations plays a vital role in rationally guiding experimental efforts to optimize energy harvesting in nanoscale and mesoscale materials. In this poster presentation, I will highlight recent work in using various quantum-mechanical approaches for understanding and predicting the electronic properties in light-harvesting materials, functionalized carbon nanotubes, and heterostructure nanowires. First, I will demonstrate that both the optical properties and excitation energies in photovoltaic molecules can be accurately predicted by constructing new exchange-correlation functionals for time-dependent density functional theory (DFT).[1] Next, the use of large-scale DFT calculations is presented to understand optical detection mechanisms in chromophore-functionalized carbon nanotubes. Through joint experimental-theoretical studies, I will show that a single-walled carbon nanotube functionalized with light-sensitive chromophores can function as a sensitive nanoscale color detector, where the chromophores serve as photoabsorbers and the nanotube operates as the electronic read-out.[2-3] Finally, a new theoretical approach is presented to understand electron localization effects in heterostructure nanowires. At nanoscale dimensions, the formation of mobile electron gases in AlGaN/GaN core-shell nanowires can lead to degenerate quasi-one-dimensional electron localization, in striking contrast to what would be expected from analogy with bulk heterojunctions. The reduction in dimensionality produced by confining electrons in these nanoscale structures results in a dramatic change in their electronic structure, leading to novel properties such as ballistic transport and conductance quantization.[4-5]

[1] M.E. Foster and B.M. Wong, Journal of Chemical Theory and Computation, 8 2682 (2012)
[2] X. Zhou, T. Zifer, B.M. Wong, K.L. Krafcik, F. Léonard, and A.L. Vance, Nano Letters 9, 1028 (2009)
[3] C. Huang, R.K. Wang, B.M. Wong, D.J. McGee, F. Léonard, Y.J. Kim, K.F. Johnson, M.S. Arnold, M.A. Eriksson, and P. Gopalan, ACS Nano 5, 7767 (2011)
[4] B.M. Wong, F. Léonard, Q. Li, and G.T. Wang, Nano Letters 11, 3074 (2011)
[5] A.W. Long and B.M. Wong, AIP Advances 2, 032173 (2012)

We investigate the stability and electronic structure of the single layer two dimensional (2D) BiSb sheet by using fully relativistic first-principles calculations. We demonstrate that 2D BiSb sheet hosts the giant Rashba spin splitting. The Rashba parameter is about 3.8 eVÅ, which is comparable with that in BiTeI. The Rashba-like spin splitting is originated from potential gradient introduced by the electronegativity difference between Bi and Sb. In additional, we find that the Rashba-like spin splitting can be modified by the external electric field and strain. The presence of giant and tunable Rashba-like spin splitting in 2D BiSb sheet offers tremendous opportunities for applications to various spin-dependent electronic functions.

Pipe diffusion in dislocation core has been long time discussed. It is widely believed that the fast diffusion channel can be found in the dislocation line. However, here we would report that carbon diffusion in iron is accelerated not in the dislocation line direction, but in a conjugate burgers vector direction c. This accelerated random walk arises from a simple crystallographic channeling effect [1].
With some crystallographic analysis, we found large avenues along conjugate burgers vector direction c in dislocation core which will cause fast diffusion in dislocation core in BCC, FCC, HCP&’s close-packed slip planes. Thus, diffusion along c looks like a common phenomenon in close-packed slip planes. To confirm that, we computed activation energy barriers along c for several solvent-solute pair using density functional theory. As a result, we conclude the conjugate burgers vector direction is actually fast diffusion path at least for all solvent-solute pairs we examined. On the other hand, for dislocation core on non-close packed slip planes, we couldn&’t find any large avenues. Thus, the fast diffusion in such dislocation cores may not occur.
[1] A.Ishii and J. Li and S. Ogata, PloS one, 8-4 , e60586 ,(2013).

Metamaterials are engineered materials with material properties which do not exist in nature. A major reason of the rare properties results from inner pattern rather than composition. Particularly, mechanical metamaterials have distinct characteristics, such as negative bulk modulus or Poisson&’s ratio.
Here, we propose a triply periodic bicontinuous structures for a novel structure of mechanical metamaterials. The triply periodic bicontinuous structures have an advantage to fabricate 3D photonic crystal structure due to its optical feature of wide and complete photonic band gaps. Moreover, the triply periodic bicontinuous structure is appropriate to laser interference lithography which assures rapid and low cost fabrication.
To describe the triply periodic bicontinuous structures, three different type of crystal structures are considered, such as simple cubic(P), diamond(D), and gyroid(G) structure, respectively. Additionally, an approximated level set approach is employed to inspect the effect of morphological conditions, such as a volume fraction and an aspect ratio. Threshold intensity and scale factor are selected for parameters to control the volume fraction and aspect ratio of unit cell. Those are closely related to the young&’s modulus and shear modulus.
A parametric study is performed with the various control parameters of threshold intensity, scale factor, and the kind of triply periodic bicontinuous structure. The elastic properties of each structure, which is a moduli ratio between young&’s modulus and shear modulus(E/G ratio), are calculated to estimate the possibility as mechanical metamaterials for the described triple periodic bicontinuous structure. The calculated ratios are compared with those of common materials which exist in nature. Furthermore, the stress distributions are analyzed to prove the effect of structural difference for the E/G ratio under compressive and shear load.
Finally, it is found that the simple cubic structure with low volume fraction and large aspect ratio has extremely high E/G ratio. We suggest the suitable crystal structure and morphological conditions for the 3D mechanical metamaterial.

Initiated Chemical Vapor Deposition (iCVD) is a well-known method for depositing polymers used in chemical, biological, and electrical applications. It is a variation of hot filament deposition and can used to produce conformal coatings of polymer films at relatively low reaction temperatures. It is also a solventless technique in which thin polymeric films are deposited by introducing controlled ratios of monomer and initiator gasses into the reaction chamber. Low temperatures in the reaction chamber allow the deposition of polymer films on a wide variety of substrates that include biological substrates.
The actual growth of iCVD polymer films involves the activation of the monomer precursor and an initiator over an array of resistively heated filaments. The mixture is heated in the hot-zone near the filament leading to the initiator reacting with the monomer. This larger molecule may be adsorbed by the substrate releasing the initiator back into the gas phase. The monomer may undergo surface diffusion and interact with another monomer unit forming a polymer island.

Using the Monte Carlo method, we have simulated the growth of a monolayer of polymer films on two-dimensional surfaces. We saw the formation of polymer chains over a time scale on the order of microseconds to milliseconds. We have assumed the substrate to be at room temperature while the reactor pressure close to 1 torr.
The grid on which we have simulated this chain growth is a 100x100 matrix that is sent through a series of specialized functions with each time-step. These functions we have written can be divided into the categories of population, translation, and polymerization. The population functions apply the Monte Carlo method by using random numbers coupled with input pressure ratios to determine which of the grid cells are populated with initiator or monomer molecules. The translation functions apply the characteristic of surface migration of the molecules and small chains by allowing them to move within the grid. The polymerization functions apply the reaction probabilities to neighboring monomer and initiator molecules as well as a probability of each polymer chain growing or becoming inert.
We will also discuss strategies to extend our model to a three-dimensional grid to cover more complex surfaces. We will discuss the conditions that lead to the conformal growth of uniform films on these surfaces.

Physical vapor deposition (PVD) is one of many techniques used to produce thin-films. The conditions during PVD and the materials used will greatly influence the topology and microstructure for the depositing film. Experimental observations are useful in determining diagrams that relate deposition conditions to the thin-film structure but are costly, limited and very difficult for multiple phase materials. As such, a predictive model for microstructure formation and evolution during PVD is much needed.
This work has two main objectives. First, a phase-field model is developed to simulate vapor deposition and microstructure evolution in a multi-phase system. Phase-field simulations are a very popular technique for modeling and predicting the growth and evolution of the microstructure of a material. This model leverages previous phase-field modeling efforts on ballistic deposition of a single phase material and the development of a free energy functional for multi-phase systems. Field variables are introduced to describe the solid and vapor phases. The equations governing deposition couple the evolution of the thin-film field variable to the vapor phase field variable. In this manner, the growth of the thin film occurs at the expense of the incoming vapor flux. To allow for the deposition, nucleation and growth of multiple phases within the thin-film, a previously developed free energy functional that captures the evolution of multiple interacting phases is incorporated into the model. Therefore, this model will allow the study of deposition conditions on the thin-film microstructure and topology. Second, this newly developed multi-phase phase-field model is then used to simulate the vapor deposition of titanium to investigate the microstructure evolution and phase distribution of the α and β titanium phases that has direct experimental comparison.

Graphite has been used in the nuclear industry since its debut, and it remains a crucially important material for next-generation reactors. Nevertheless, predicting the thermal conductivity of graphite has proven difficult largely due to the variability in the morphology that arises during the manufacturing process, and because of the nanoscale morphological evolution that occurs in service as a direct result of radiation. To better the understanding and predictability of thermal conductivity in graphite, we map phonon-correlations around different defect types and numbers. Using the understanding of scattering properties of individual defect types, we then examine the additive behavior of defect ensembles.

Despite of many researches to unveil the nature and catalytic functions of hydrogen spillover phenomena, there have been still many controversies in the characterization of the hydrogen spillover phenomena, and even in its existence over the past few decades. Particularly, the possibility of long range hydrogen migration on the non-reducible substrates is considered as mostly questionable while the hydrogen spillover on the reducible substrates such as metals is regarded as relatively more feasible. This is because the reducible substrates enable the migration of hydrogen (H = H+ + e-) as a form of proton (H+) through quantum tunneling while the other constituent of hydrogen atom, i.e., the electron (e-) easily travels over the long range through the conduction band of the substrate. However, by using model catalysts, we recently showed the most direct evidence on the existence of hydrogen spillover particularly on the aluminosilicate surface, and also demonstrated the application of hydrogen spillover as hydro-/dehydrogenation catalysts with virtually switched-off activities in hydrogenolysis. To provide the mechanistic understanding on the observed spillover phenomena on non-reducible substrate, therefore, we perform a series of first principle density functional theory (DFT) calculations. Based on the DFT energetics, we propose a long-range migration mechanism where hydrogen is transferred via a proton relaying while the electron migrates through the non-reducible substrate (aluminoslicate) via polaron type conduction. In our proposing mechanism, we further discuss that the Bronsted acid sites (BAS) formed on the surface of aluminosilicates mediate the surface migration of activated hydrogen, which is consistent with the experimental observation on the dependency of catalytic activity on the surface hydroxyl concentration.

It has been experimentally reported that yield stress of Fe-Si alloys increases with increasing solute Si concentration at ambient temperature. However, the detailed effect of solute Si atoms on the yield stress has not been clear. To clarify the origin of plastic deformation properties of Fe-Si alloy, it is essential to get a detailed understanding of interaction between screw dislocation and a solute Si atom. In this study, we estimated the effect of solute Si atom on critical resolved shear stress (CRSS), which is one of the characteristic properties to evaluate the strength of metals. To estimate CRSS of bcc crystal from atomistic viewpoint, it is necessary to understand the glide behavior of screw dislocation, and the motion of screw dislocation is mainly driven by the thermal activation process such as double kink nucleation and migration. To obtain CRSS, we first carried out the atomistic analysis of single screw dislocation glide using nudged elastic band (NEB) method and estimated the energy barriers for kink nucleation and migration processes. In the present atomistic calculations, we used newly developed interatomic potential for Fe-Si interaction formulated by embedded atom method (EAM) potential, and parameters of the potentials are determined to fitting material properties of Fe-Si alloy obtained by first principle calculations. We estimated CRSS at given temperature, strain rate and solute Si concentration based on Orowan's equation, in which the velocity of screw dislocation was calculated by the obtained energy barriers. In addition, we also estimated athermal CRSS induced by frictional effect of solute Si atoms. It was found that the substituting Si atom in bcc-Fe decreases the energy barrier for kink nucleation and increases that for kink migration. Moreover, estimated CRSS of Fe-Si alloy becomes lower at low temperature and higher at high temperature than that of pure Fe, and this tendency becomes apparent as Si concentration increases. These results are consistent with the CRSS reported in experimental studies, in which solid solution softening at low temperature and strengthening at high temperature are observed. Based on these results, we will discuss the origin of plastic deformation properties of Fe-Si alloy from atomistic viewpoint.

Solvent-based thin-film deposition technologies are the most common techniques to fabricate organic thin film, because of their ease to scale-up for large commercial production. All solution-processing techniques involve preparing dilute solutions of two materials in a volatile solvent. After some form of coating onto a substrate, the thin film forms, solvent evaporates leading to intriguing structure formation. There are two main mechanisms structure can form: phase separation (spinodal decomposition) and crystallization. In some classes of polymers and small molecules, phases separation is dominant process (e.g.PFB:F8BT), while in others crystallization defines the final structure (e.g. P3HT:PCBM). The underlying physics is still largely unknown. In particular, what thermodynamic or kinetic parameters define the dominant mechanism is highly important question for rational design of manufacturing, yet it still remains open question.
In this work, we develop a diffuse interface-based computational framework to model morphology evolution during solvent-based fabrication of thin films. This model inherently allows to account for multi phase and multi components systems. In particular, the model is centered around the energy landscape formulation that depends on the thermodynamics parameters, e.g. solubility parameters or degree of polymerization. Using governing equation we explore this landscape to find the morphology evolution path under constrains prescribed by the kinetics of the solvent-based fabrication manufacturing. We specifically ask the question, when the crystallization or phase-separation-dominant path will be energetically preferred.

Hyperbolic metamaterials (HMMs), which could have large wavevectors and high photonic density of state, have attracted great attention recently for achieving novel optical applications and offering an exotic way to manipulate light. On the other hand, near-field thermal radiation between two planar surfaces separated by a vacuum gap smaller than the characteristic wavelengths of thermal radiation has been theoretically and experimentally demonstrated to exceed the blackbody limit by the strong coupling of evanescent waves across the nanometer vacuum gap. With coupling of surface plasmon or phonon polaritons (SPPs), near-field radiative heat transfer could be enhanced by several orders of magnitude higher than the far-field limit set by Planck&’s law. However, near-field radiation between nanostructures has not been well explored. Could the near-field thermal radiation be further enhanced with nanostructures such as HMMs?
In this study, we will theoretically investigate the near-field heat transfer between two semi-infinite HMMs made of metallic nanowires embedded in dielectric hosts. Fluctuational electrodynamics will be employed to calculate the near-field thermal radiation, and the nanowire-based HMM will be treated as an effective homogenous but uniaxial medium. Maxwell-Garnett effective medium theory will be used to approximate the effective dielectric functions of the HMMs with different filling ratios for both ordinary and extraordinary waves, and anisotropic wave propagation will be incorporated as well. Near-field radiation between metallic surfaces was previously shown not to have strong enhancement because of the wavelength mismatch between coupled SPPs and thermal radiation. However, when metallic nanowire-based HMM is considered, the effective plasmon frequency of the nanowire HMM will shift towards longer wavelengths due to less number density of electrons. As a result, the resonance wavelength of coupled SPPs between the nanowire HMMs matches better with that of thermal radiation, resulting in significant near-field heat transfer enhancement more than two orders of magnitude to that between bulk metals. Such strong enhancement will be demonstrated with silver nanowire HMMs. The dispersion relation for coupled SPPs with uniaxial media at different vacuum gaps will also be presented to help explain the underlying mechanism. Besides, the effect of epsilon-near-zero (ENZ) resonance mode from the HMMs on the near-field thermal radiation will also be studied for different filling ratios. A tungsten nanowire-based HMM will then be shown to achieve spectral heat flux enhancement above the bandgap of a photovoltaic cell due to the ENZ and SPP modes, which would potentially enhance the performance of near-field thermophotovoltaic devices with thermal emitters made of such nanowire-based HMMs.

The study of nanostructured energy materials offers great new opportunities in scientific areas and industries. One of the greatest challenges of the nanoscale is our ability to fully probe and fine tune the atomic details of structure and properties. Electron and atom/ion transport plays a crucial role in controlling the microstructural and local chemical evolution of nanostructured materials. Accordingly, developing the ability to accurately simulate these processes is pivotally important in order to develop the scientific underpinning for predictive models that can design nanostructured materials with superior thermal-mechanical properties and innate radiation resistance.
In this presentation, we will overview multiscale modeling bottom-up from first-principles, to molecular dynamics (MD) and kinetic Monte Carlo (KMC) simulations, all relevant to collective charge and ion transport. Such bottom-up multiscale framework aims to reveal the mechanisms that control the thermal and irradiation stability of energy materials at atomistic level. The development of dynamics charge transfer many-body potentials for MD simulations will also be presented.

Engelhard titanosilicate (ETS-10) is a synthetic microporous crystalline material (pore dimensions 4.9 and 7.6 Å) whose framework structure results from chains of corner-sharing TiO6 octahedra and SiO4 tetrahedra linked through bridging oxygen atoms generating a pore structure with 12-membered rings. Although the material is crystalline, its structural determination has proven to be challenging. An intriguing characteristic of ETS-10 is related to the fact that TiO6 octahedra are linked together to form -Ti-O-Ti-O-Ti- wires that run in the crystal [100] and [010] directions. These linear chains are effectively insulated by the silica matrix, and can be regarded as a 1-D quantum-confined form of titania whose band gap is related to the length of the crystals along the [110] direction. Understanding the extent of local structural changes that the -Ti-O-Ti-O-Ti- wire undergoes upon ion exchange provides a viable tool for tuning the material properties of ETS-10. Accordinlgy, we carried out a comprehensive study of the structural and electronic properties of ETS-10 and fully and partially ion-exchanged ETS-10, M+-ETS-10 (M+ = K+, Ca2+, Ag+, Zn+2, Ru3+, and Au3+) by using DFT on two ETS-10 model systems.
In order to elucidate the effect of ion-exchange on the local structural deformations induced into the -Ti-O-Ti-O-Ti- wire, Raman spectra were also taken. The obtained results suggest that the most significant changes into the -Ti-O-Ti-O-Ti- wire were induced upon M = Zn2+, Au3+, and Ru3+, with less pronounced damages formed upon M = Ca2+ and Ag+ cases. These observations were found to be in full correlation with the obtained computational results. Accordingly, the developed models were in full correlation with the Raman results, with the possibility to better interpret the framework behavior upon ion-exchange. It seems that the developed models can provide more detailed elucidation of changes induced into such framework moieties.

We propose that the production of blue and red upconversion luminescence in the popular NaYF4: Yb3+,Er3+ nanophosphor occurs via a very different mechanism than that which is commonly accepted. We find that both blue and red are produced through energy transfer upconversion from Yb3+ to Er3+ wherein the Er3+ is promoted from 2H11/2, 4S3/2 up to the high-lying 2K,4G manifold of states, which then rapidly populates the blue emitting state (through multiphonon relaxation) and the red emitting state (through back energy transfer). This new mechanism also impacts the green upconversion luminescence because it removes population from the 2H11/2, 4S3/2 green emitting states. Rate equations simulations based on this model give a quantitative match to experimentally measured luminescence decay curves, relative emission intensities, power dependences, and quantum efficiencies. The model also accurately reflects the optical changes which occur in going from the bulk phosphor to nanomaterials.

Developing a composite material of polymers and micrometer-sized fillers with higher heat conductivity is crucial to realize modular packaging of electronic components at higher densities. The mechanisms of the enhanced heat conductance of the polymer-filler interfaces by adding the surface-coupling agent in such a polymer composite material, that is observed experimentally, are investigated through the non-equilibrium molecular dynamics and wave-packet dynamics simulation. A simulation system is composed of alpha-alumina as the filler, bisphenol-A (bisA) epoxy molecules as the polymers, and model molecules for the surface-coupling agent. The inter-atomic potential between the alpha-alumina and surface-coupling molecule, which is essential in the present simulation, is constructed to reproduce the calculated energies with the electronic density-functional theory.
In the non-equilibrium molecular dynamics, we produce heat flux that directs from the alpha-alumina to bisA regions by controlling the local temperatures in the bisA and alpha-alumina regions. We calculate the effective thermal conductivity of the interface area using the heat flux, surface area, and temperature profile in the system. In a simulation run we confirm that the system is in the steady state by comparing the input and output energies in the temperature-controlled areas. The heat flux and temperature profile are obtained as averages over 0.5 ns in the steady state. We thereby find that the effective thermal conductivity at the interface increases significantly by increasing either number or lengths of the surface-coupling molecules and that the effective thermal conductivity approaches to the theoretical value corresponding to zero thermal-resistance at the interface.
Detailed investigation of microscopic heat-transfer routes around the interface is performed using two different methods. In a method, we first calculate the time-averaged kinetic energies (or atomic temperatures) of atoms in the non-equilibrium run. The heat transfer routes are therewith identified under the assumption that the difference in the atomic temperatures of a pair of atoms gets smaller as the degree of thermal conductance between the two atoms becomes smaller. In the other method, we set a phonon wave-packet near the interface and then perform a molecular dynamics run to trace the time evolution of the kinetic and potential energies of each atom. We thereby identify the energy (or heat) transfer routes around the interface. Comparing those results obtained in the two methods we find key heat-transfer routes that involve the surface-coupling molecules.

A density functional theory and non-equilibrium Green&’s function method were used to
calculate the electron transmission function in the contact between two metallic single wall
carbon nanotubes (CNTs). It was shown that current-voltage (I-V) characteristics of the contact allow quasi-classical description of quantum electric transport in the contact through an effective resistance parameter. The calculations show small electron transmission through the contact at equilibrium distance, resulting in a large effective resistance of 0.5..1 MOmega; for a single contact in the “static” structure at 0 K. For higher temperatures, the statistical distribution of effective contact resistances that originates from thermal molecular mobility of polymer matrix was considered, reducing the problem to the Dynamic Percolation Model. Within the model, an effective medium approach was used to calculate I-V curves, and the temperature and frequency dependence of electric conductivity for CNT-polymer composites. The model predictions quantitatively agree with experimentally measured dependences of composite electric conductivity on temperature and frequency, offering the possibility to study charge transport mechanisms in percolated fiber composites using direct measurements of macroscopic material properties.

There have been many observations in recent decades of the remarkable response of materials driven from equilibrium to self-organize into mesoscale patterns with wavelengths in the range of typically 10-100 nanometers. Well-known examples include, for example, systems driven by irradiation, severe plastic deformation and laser pulses. As a specific example, we will discuss the well-known, but poorly understood irradiation-induced formation of voids and their subsequent self-organization into periodic void lattices. Empirical evidence suggests that such nanoporous materials are highly stable and tolerant to further irradiation. We outline an atomistically-informed mesoscale modeling approach based on phase-field theory in order to elucidate the mechanisms and underlying causes responsible for this remarkable self-organization phenomenon. The insights that can thus be gained enable development of a computational mesoscale materials design tool to guide the synthesis of novel nanostructured materials with designed functionality.

An important trend in electronics involves the development of materials, mechanical designs and manufacturing strategies that enable the use of unconventional substrates, such as polymer films, metal foils, paper sheets or rubber slabs. The last possibility is particularly challenging because the systems must accommodate not only bending but also stretching, sometimes to high levels of strain (>100%). Although several approaches are available for the electronics, a persistent difficulty is in energy storage devices and power supplies that have similar mechanical properties, to allow their co-integration with the electronics. We introduce a type of ultrathin deformable interconnect in the layout that use ‘self-similar&’ structures of wires in serpentine configurations to offer, simultaneously, high system-level stretchability, large filling ratio of active devices, and low interconnect resistances. Both the experiment measurement and finite element simulations reveal the unique mechanism of ordered unraveling for the self-similar interconnect under stretching, which is responsible for the ultra-high stretchability. This type of interconnect is adopted for developing a rechargeable lithium ion battery that exploits thin, low modulus, silicone elastomers as substrates, with a segmented design of the active materials. The result enables reversible levels of stretchability up to 300%, while maintaining capacity densities of ~1.1 mAh/cm^2.

Li-ion batteries are widely used for power source of portable devices and electric vehicles due to its high energy density. Many researchers study Li-ion diffusion in a Li-ion battery including cathode, anode and electrolyte. Because the Li-ion diffusion directly affects electron flows. Diffusion of Li-ion is faster in electrolyte than each electrode. The reason for these phenomena is Li-ion diffuses freely in electrolyte but hard to diffuse in cathode because intercalation sites are limited. Li-ion stagnation which induces disturbance of Li-ion intercalation occurs at an interface between electrode and electrolyte since the diffusivity of Li-ion is different.
The Li-ion intercalation and diffusion in the electrodes are primary depending on material properties of electrode. Moreover, other important factors such as phase transition and Li-ion distribution characteristics on affecting Li-ion intercalation have been proposed. The specific characteristics are different in each electrode. Generally, carbon is suggested for anode material but many transient metals are considered as candidates of cathode. Numerical study has been performed to analyze Li-ion intercalation in cathodes and evaluate performance of the Li-ion batteries with various transient metals. The both of phase transitions and Li-ion distributions are considered in the computational model.
The Li-ion distribution behaviors in cathode materials can be classified as continuous and discrete types. In the case of cathodes reside in the continuous distribution, the Li-ion concentration is gradual increasing or decreasing as Li-ion is intercalated or extracted. And ratio of each phase can be expressed as function of Li-ion concentration and changes smoothly. On the other hand, Li-ion concentration is also changed in the discrete case. The Li-ion concentration only exists at specific values to ensure that the structure is in the stable state. Li-ion concentration still exists at specific values and only ratio of each phase is changed when giving different Li-ion concentration. In the cathode, the specific Li-ion concentration corresponds to the phase, Li-ion concentration jumps between the stable values. Here, we construct phase field model of the Li-ion diffusion incorporate phase transition. The computational method provides a detailed understanding on the process of Li-ion intercalation and enhances the cathode structure for high capacity battery.
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Nanoporous graphene has emerged as a powerful alternative to conventional membrane filters and gained an appreciable popularity in a variety of applications due to its many remarkable and unique properties. Careful regulation of the size and density of nanopores can generate graphene membranes with controllable selectivity and flow rate, thereby greatly enhancing the potential marketability of graphene-based membranes. Here we employ molecular dynamic simulation to systematically investigate the mechanistic and quantitative effect of significant parameters such as temperature, impact energy, strain, and pore density on the nanopore morphology of graphene by impacting fullerenes into graphene sheet. Simulation results have demonstrated that nanopore size and morphology of graphene sheet can be tailored by carefully controlling the energy of the impact cluster, the temperature of the environment, and the strain applied on the graphene sheet, which serve the conceptual guideline to fabricate nanoporous graphene with desired size and patterns for a variety of implications such as DNA sequencing, water purification, and nanocomposites.

Physical Vapor Deposition (PVD) nano-multilayered composites of Copper and Niobium have demonstrated extraordinary ability to withstand both mechanical and radiation induced damage nucleation when the layer thicknesses are less than one micron. Although the bi-metallic interfaces are believed to play a role in the enhanced performance, we have yet to demonstrate thorough explanations for these observed performance enhancements. There is a desire to scale the manufacture of this class of materials to commercially viable processes. For this reason, the accumulated roll bonding process has been developed to manufacture these layered composite materials. During this process, the individual Copper (fcc) and Niobium (bcc) layer thicknesses begin at 1mm and continue until the layer thicknesses become on the order of 10&’s of nanometers. We present a new local single crystal model for the potential influence of the bi-material interface on dislocation motion in the near vicinity of the interface and apply this model to polycrystal multi-layer simulations in an attempt to predict the dominant experimentally observed orientation relationships across the interface. Simple compression and nano-indentation mechanical test results have been used to characterize each of the material model parameter sets. These simulations employ statistically equivalent polycrystal structures as representation of the experimentally characterized individual composite layers. Calculations for the purpose of predicting the evolution of crystallographic texture in these layered composite materials will be presented. Calculations of orientation relationship stability of experimentally observed dominant interface relationships will also be presented and potential hypotheses for these observations will be made. We will also present results of interface orientation relationships across the interface and their evolution. Direct comparison of numerical results to experimental results will be made to the fullest extent possible.

Dislocations in materials provide spatially heterogeneous strain fields that drive solute and vacancy flow. This can result precipitates near dislocation cores with nanometer sizes, but at concentrations where a solid solution is expected. Using first-principles methods with atomic and mesoscale modeling, we create predictive models of this behavior in two different systems: hydrogen in palladium, and silicon in nickel. The treatment of interstitial diffusion (for hydrogen) and vacancy-mediated diffusion (for silicon) requires different models for transport; with transition state theory to extend to appropriate time scales, and self-consistent mean-field theory to compute Onsager coefficients. For hydride formation, we can predict the formation of hydrides along with neutron scattering intensity which validates our model. For silicide formation, the strain-induced breaking of cubic symmetry leads to unusual flow patterns, and the coevolution of vacancy and silicon concentration profiles leading to nanoscale silicide formation.

Triple junctions and grain boundaries, owning to the disordered atom distribution and high potential energy, are thermodynamically instable which potentially contribute to grain growth in crystalline materials under heat treatment or applied stress especially when the grain size is reduced to nanoscale. Previous work on grain growth and microstructural evolution in nanocrystalline materials mainly concentrated on the mobility of individual grain boundaries. The mobility of triple junctions, on the other hand, was rarely studied, although triple junctions add strong constrains to the motion of grain boundaries and play an important role during the overall microstructural evolution in materials. In this study, we extended the interface random walk method originally developed to extract grain boundary mobility from molecular dynamics simulations to study triple junctions. For this purpose, four different methods were used to accurately keep track of the thermal fluctuation of triple junctions. The triple junctions showed similar random walk behavior to that found for grain boundaries due to purely thermal effects, which can be used to extract the triple junction mobility.

The phase-field crystal (PFC) model is an emerging tool for predicting phenomena relevant to nanoscale materials, which lie at the intersection of atomistic length scale and diffusive time scale. The original and simplest formulation of the PFC model by Elder et al. [K. R. Elder and M. Grant, PRE 70, 051605 (2004)] was limited to stable bcc and lamellar structures. However, a recent formulation by Greenwood et al. [Greenwood et al, PRL 105, 045702 (2010)] has led to PFC models that are also capable of producing stable simple-cubic, fcc, and hcp structures. Using the formulation by Greenwood, we introduce a PFC model that can stabilize a diamond-cubic structure, which is important for heteroepitaxial growth of semiconductor materials. We demonstrate that our model captures diamond-cubic/liquid phase coexistence, which allows us to analyze the thermodynamic and structural properties of the (100), (110), and (111) solid-liquid interfaces. The calculated interfacial properties are in agreement with those from molecular-dynamics simulations [A. A. Pankaj and X. C. Zeng, APL 92, 221903 (2008)].

This talk will review the results of recent atomistic computer simulations, investigating the structural phase transitions in metallic grain boundaries, induced by changes in temperature and composition. These first order GB transitions are accompanied by a change in the number of atoms in the GB plane and can also be induced by injecting interstitials/vacancies in the bicrystal. Our simulations demonstrate that GBs can readily absorb and/or reject large amounts of atoms by simply changing their structure, which may play an important role in recovery of materials after severe plastic deformation or radiation damage. We discuss implications of these transformations on GB mobility, segregation and diffusivity.

The electronic coupling (or transfer integral) and the reorganization energy are two key parameters that determine the rate of charge transfer, and therefore the overall charge mobility, in organic semiconductors. A first purpose of this work is to investigate the influence of the quantum-mechanical (QM) methodology in describing these two microscopic parameters, the understanding of which will help expand our insight into of one of the most crucial processes in organic photovoltaics, field-effect transistors, and light-emitting diodes.
A second part of the work deals with the role of polarization and relaxation on the structures and energies of cluster anions of naphthalene, anthracene, and tetracene that have been experimentally investigated via photoelectron (PE) spectroscopy.[1] The PE spectra for naphthalene clusters indicate a combination of localized and delocalized charged states. The impact of polarization on the energies of hole or electron charge carriers has already been investigated with the hybrid quantum-mechanics/molecular mechanics (QM/MM) approach at the DFT level, under the assumption that the charge carriers in all instances localize completely on a single molecule.[2] We show long-range corrected DFT optimized clusters (up to six naphthalenes) that compare well with experiment. However, if the charge carrier is delocalized over several sites (the so-called large polaron), the DFT approach is anticipated to no longer be appropriate because of the multi-reference nature of such electronic states delocalized over several molecules. Work will also be presented employing high-level multi-reference post Hartree-Fock methods to accurately model charge delocalization and polarization in model systems.
[1] Ando, N., Mitsui, M. & Nakajima, A. J. Chem. Phys. 128, 154318 (2008); Ando, N., Mitsui, M. & Nakajima, A. J. Chem. Phys. 127, 234305 (2007); Mitsui, M., Ando, N. & Nakajima, A. J. Phys. Chem. A 111, 9644 (2007).
[2] Norton, J. E. & Brédas, J. L. J. Am. Chem. Soc. 130, 12377 (2008).

One- and two-dimensional materials, such as single wall BN nanotube (BNNT) and monolayer hexagonal BN (h-BN), have demonstrated attractive physics and several potential applications in nano/micro devices. However, there is a high anisotropy associated with each of these BN allotropes as the excellent properties (e.g. strength) of BNNT and h-BN are along the tube axis and in-plane directions, respectively. In spite of promising future of BNNT and h-BN, there are currently no hybrid BN structures that encompass various BN allotropes to moderate anisotropic features and/or provide new functionalities. Here, we demonstrate a new series of 3D BN prototypes, namely Pillared Boron Nitride (PBN), by in-silico fusing of single wall BNNTs and monolayer h-BN allotropes aimed at filling this gap. We use Density Functional Theory (DFT) and Molecular Dynamics (MD) simulations to probe the diverse mechano and thermo-mutable properties of PBN prototypes, followed by 3D printing and tensile mechanical experiments. Unlike the stacked of h-BN sheets, which show weak out-of-plane properties, PBN provides substantial improvements in the out-of-plane direction. Our results demonstrate that the synergistic behavior of the junctions, tubes and sheets in PBN overcomes the intrinsic limitations of its constituents, and amplifies superior characteristics including 3D balance of strength, toughness and thermal transport, emergence of negative Poisson&’s ratio, and elimination of strain softening in the armchair direction. These features, combined with porous and lightweight structure, render PBN as a 3D multifunctional template for applications in graphene-based nanoelectronics, optoelectronics, gas storage and functional composites.

Plastic deformation is due to the collective motion of large numbers of dislocations. A new algorithm is presented to determine equilibrium configurations of dislocations as a function of temperature and strain rate. The equilibrium is achieved by minimizing the total interaction energy of the dislocations and then thermal activation mechanism is incorporated at appropriate time scale. This approach allows decoupling of time scales associated with dislocation glide and it gives access to strain rates that are of interest in practical applications. We verified our new quasi-static method by comparison with full dynamics simulations. Numerical examples show that the new model can capture rate effects over 6 orders of magnitude change in the applied strain rate, with negligible change in the simulation time.

Many device structures combine the functionality of materials with very different bonding types: metallic, ionic and covalent. Traditional empirical potentials have been designed to consider one type of bonding only. The Charge Optimized Many-Body (COMB) approach allows for the seamless simulation of structures composed of dissimilar materials. This is because COMB includes a charge equilibration method that allows each atom to autonomously and dynamically determine its charge, and a sophisticated description of bond order, by which the strength of an individual pair bond is modulated by the presence and strength of other local bonds. Simulations using COMB potentials are orders of magnitude faster than electronic-structure calculations, can consider much larger systems and can easily simulate dynamically behavior. The power of this approach is illustrated from problem of interest for various condensed phase systems including U/UO2, Zr/ZrO2/ZrH2 and Cu/SiO2.

Modified embedded atom method (MEAM) was developed by Baskes based on EAM potential [1, 2]. It has been broadly used to model metallic or covalent bonded materials. Throughout years of research, MEAM potential development remains an active research topic because of the increasing demand for large-scale simulation for advanced materials in the fields such as materials synthesis, catalyst, energy harvesting and storage, etc. In this work, we propose a new formalism of MEAM potential to enhance its accuracy and transferability. We will introduce a new functional form of the potential in comparison with other empirical many body potentials. Using magnesium silicide (Mg₂Si) as an example, we have developed Mg-Si potential based on first-principles calculation data. The bulk elastic properties and defect formation energy of Mg₂Si are well predicted with our model. As Mg₂Si is a semiconductor with interesting thermoelectric property, our developed Mg-Si potential can accurately model heat transfer in Mg₂Si. The impact of various nanostructure defects on thermal conductivity of Mg₂Si is also discussed.
II-VI support
References:
[1] M. I. Baskes, Modified embedded-atom potentials for cubic materials and impurities, Phys. Rev. B 46, 2727 (1992).
[2] M. S. Daw and M. I. Baskes, Embedded-atom method: Derivation and application to impurities, surfaces, and other defects in metals, Phys. Rev. B 29, 6443 (1984).

Molecular dynamics simulations are common in nanomechanics. For metals and alloys, these simulations generally rely on the embedded atom method (EAM) potentials. However, (1) the EAM imposes three constraints on crystalline elastic constants, which are not valid for all metals and alloys; and (2) the EAM is unable to describe surface inward/outward relaxations. With these two drawbacks, the validity of molecular dynamics simulations of metallic nanostructures is subject to question. In this talk, we will present a new method - Response EAM (R-EAM). The R-EAM eliminates the two drawbacks of EAM without much loss of computational efficiency, and will enable physical simulations of nanomechanics.
This talk is based on two recent publications: Zhou and Huang, Physical Review B 87 (2013) 45431; Zhou and Huang, ASME Journal of Engineering Materials and Technology 135 (2013) 11010.

The electronic properties of the interface between Rh clusters and CeO2(111), (110) and (100) surfaces were studied using an isothermal-isobaric (NPT) ensemble at 773 K and 101.343 kPa using the tight binding-quantum chemical molecular dynamics (TB-QCMD) method. The amount of electronic exchange by interaction at the interface between the supported Rh55 clusters and each CeO2 surface was investigated quantitatively. A comparison of the mean square displacement (MSD) showed that the topmost oxygens on the Rh-supporting CeO2 surface exhibited higher mobility than those of the bare CeO2 surface. Although the mobility of the topmost oxygens on the bare CeO2 surface was in the order (100) > (110) > (111), this sequence was altered by the presence of Rh, so that the oxygen mobility for the more open (110) surface was the largest. The amount of electron exchange that occurred between Rh and the CeO2(110) surface was also larger than for the (111) or (100) surface. The Ce 4f orbitals on the CeO2(110) surface exhibited the strongest mixing with Rh 4d orbitals, which simultaneously caused restructuring and instability of the topmost Ce-O bonds. This enhancement of oxygen migration in the presence of Rh was occurred together with an increase in the number of oxygen vacancies on the ceria surface. This was because the topmost oxygens was shifted to have a stronger affinity with Rh and thus formed stronger bonds with Rh than with Ce.

Since the discovery of the photoluminescence (PL) of porous silicon [1], and the possible usage of silicon nanocrystals (NC) in light emiting devices, the study of the PL properties of the Si NCs has been an active field of research. However, the PL is greatly influenced by the impurities on the surface of the Si NCs. Thus, we focus our study on the surface impurities and their effect on the electronic and optical properties of Si NCs. For the purpose of our study we considered spherical Si NCs with three different diameters: 1.0, 1.5 and 1.9 nm. We tested the influence of different impurities on the surface, as: OH, O, CH3, CH2, SiH3, SiH2, as well as the small molecule C2H5NH2. All of the systems are analyzed using the pseudopotential DFT package SIESTA, with SZP basis set for Si, and DZ basis set for O, C, N and H. The atom positions are relaxed until all the forces acting on the atoms were lower than 0.04 eV/Å. For each of the relaxed structures we calculated the density of states (DOS) and the absorption indices.
We analized the influence of the impurities on the DOS and the absorption indices in terms of the induced changes in the region around the gap. We see that the single bonded impurities (-OH, -CH3, -O-, -SiH3) induce much smaller changes compare to the ones influenced by the double bonded impurities (=O, =CH2, =SiH2). The results are compared with the previous DFT calculations [2-4]. Moreover, neighboring impurities induce bigger changes in the DOS and the absorption indices, compared to spatially separated impurities. This study is the first step into understanding of the PL properties of Si NCs passivated with different atom or molecules. One of those systems are Si NCs passivated with C2H5NH2 molecules, which has been shown to exibit interesting PL properties [5]. Therefore we passivated the surface of the Si NCs with only C2H5NH2 molecules, and we calculated the DOS and the absorption indices for the systems. We also looked into the layer resolved projected DOS and the spatial distribution of the wavefunctions of the states around the gap and their overlap integrals.
References:
[1] L. T. Canham, Appl. Phys. Lett. 57, 1046 (1990).
[2] A. Puzder, A. J. Williamson, J. C. Grossman, G. Galli, Phys. Rev. Lett. 88, 097401 (2002).
[3] T. J. Pennycook, G. Hadjisavvas, J. C. Idrobo, P. C. Kelires, S. T. Pantelides, Phys. Rev. B 82, 125310 (2010).
[4] R. Guerra, E. Degoli, M. Marsili, O. Pulci, S. Ossicini, Phys. Status Solidi B 247, 2113 (2010).
[5] K. Linehan, H. Doyle, Size Controlled Synthesis of Silicon Nanocrystals Using Cationic Surfactant Templates, Small (2013).

Ferroelastic domains observed in single-domain, epitaxial ferroelectric thin films are often associated with dislocations near the film/substrate interface in compressively strained films. The morphology of these domains depends on the film thickness and local stress; in relatively thin films the domains extend through the film while in relatively thick films these domains remain localized at the film/substrate interface. In this study phase field simulations with supporting TEM observations are used to investigate the transition between the observed ferroelastic domain morphologies as a function of film thickness or under a mechanical force or electric field in an epitaxial Pb(Zr_0.2,Ti_0.8)O₃ (PZT) film. Analyses of the thermodynamic state of the PZT film reveal the existence of discontinuous changes in the first derivatives of the free energy of the film and hysteretic transition behavior, indicating that the transition is first order.

Topological insulators (TI) are a new class of materials exhibiting unique quantum transport properties with potential applications in spintronics and quantum computing. To date, all of the experimentally confirmed TIs are inorganic materials. In this talk, I will present our recent theoretical studies of the two-dimensional organic TIs made by assembling organometallic compounds with strong spin-orbit coupling into hexagonal and kagome lattices. The realization of quantum anomalous Hall effect in magnetic organic TIs in both Dirac band and flat Chen band will also be discussed. We envision that organic topological materials will greatly broaden the scientific and technological impact of topological materials.
This work was supported by DOE-BES (No. DE-FG02-04ER46148) and NSF-MRSEC (No. DMR-1121252).

The synthesis of graphene usually relies on a chemical vapor deposition of a carbon bearing gas on the surface of a metal surfaces. We investigate the case of graphene on Ni where the epitaxial growth of graphene on (111) Ni surfaces is possible.
Using a tight-binding model developed to describe Ni-C interactions at atomic scale [1,2], we investigate the CVD synthesis of graphene on a Ni surface. Depending on the growth conditions, we show that variable amounts of C can be found in the subsurface layers and we correlate this to experimental data. Since the obtained graphene-like layer covering the Ni surface often presents defects (pentagons, heptagons, vacancies, hellip;), we also numerically study the healing mechanisms of these defects that are made more efficient in the presence of the metal surface layer [3]. Lastly, thermodynamic conditions are identified and proposed to obtain single layer of high quality graphene.

[1] H. Amara, J.-M. Roussel, C. Bichara, J.-P. Gaspard and F. Ducastelle Phys. Rev. B 79, 014109 (2009)
[2] M.-F. C. Fiawoo, A.-M. Bonnot, H. Amara, C. Bichara, J. Thibault-Pénisson and A. Loiseau, Phys. Rev. Lett. 108, 195503 (2012)
[3] S. Karoui, H. Amara, C. Bichara and F. Ducastelle, ACS Nano 4, 6114 (2010)
[4] M. Bahri, M. Diarra, H. Amara, C. Bichara and F. Ducastelle, (in preparation)

Because of the technological importance of commercial purity titanium, and the key benefits to strength and detriments to ductility of oxygen and similar interstitial alloying elements, a deeper understanding of the behavior of these interstitial components is needed. Recently, the Ti-rich phase diagram of Ti-O has been investigated by several groups using density functional theory, combined with cluster expansions. However, to date the role of coherency strains on the precipitation of sub-oxide compounds from the disordered solution has not been investigated. We present a prediction of the coherent phase diagram based on density function theory, and compare it with the incoherent phase diagram from the same model. We also examine the predicted shape of small coherent precipitates. To include long-range elastic interactions, we combine a microscopic elasticity model with a medium-ranged cluster expansion to represent the chemical interactions and correct the elastic model at short distances. This research was supported by the Office of Naval Research, Grant No. N00014-12-1-0413.

Half-metallic nanoscale materials are highly sought after for spintronic applications. Using first-principles calculations, we show that the half-metallic and magnetic properties of monolayered hexagonal SiC and AlN zigzagged nanoribbons can exhibit a large tunability under physical and chemical modifications. In a spin-resolved selective self-doping process, AlN ribbons can undergo a transition from half-metallic to fully-metallic or semiconducting behavior with the application of an electric field or uniaxial strain. Also, pristine zigzagged SiC nanoribbons with bare edges are identified as a metal-free material with intrinsic half-metallic behavior which may be tuned with O and S edge functionalization.

The resurgence in interest in the use of amorphous carbon as an active material for enhanced light absorption faces the challenge of dealing with the vast variation in composition and atomic structures in those materials. Using ab initio simulations, we demonstrate the possibility to associate the chemical composition and atomic scale structures of these materials to their UV/visible light adsorption properties at the micro-scale. The ensemble average of our simulation results agrees qualitatively with experimental observations.

Structure prediction for novel materials requires computationally inexpensive lattice relaxation methods. The most available family of methods, density functional theory using a semi-local approximation for the exchange-correlation functional, provides structural parameters accurate to within a few percent, but cannot predict band gaps. Band gap calculations, performed on DFT-relaxed structures, require much more computationally expensive methods such as hybrid functionals or the GW approximation.
The above accuracy-tiered method fails dramatically for Cu3PSe4. When the generalized gradient approximation (GGA) is used to relax the lattice and ions, band gaps calculated using both the single shot GGA+GW method and the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional method are a full 0.5 eV lower than the band gaps calculated for the unrelaxed, experimental structure. The GW and HSE methods predict accurate band gaps (within 0.2 eV of the experimental value 1.4 eV) only when used with the correct experimental structure. We show that in Cu3PSe4, the lowest conduction band state is primarily anti-bonding in character, and the energy level is very sensitive to the P-chalcogenide bond length, resulting in a strong linear dependence of the band gap to that bond length. Variations in the P-chalcogenide bond length on the order of a few percent lead to large (order of 30%) changes in the band gap. Small changes in the P-chalcogenide bond length can be affected by different lattice relaxation methods or by chalcogenide substitution. The linear bond length band gap relation holds regardless of the origin of the structural variation, and is unaffected by other structural parameters such as the Cu-chalcogenide bond lengths. A similar mechanism holds for many ternary copper chalcogenides, a material family of interest for solar applications. We examine the band gap-bond length dependence for several of these materials including CuInSe2 and CuGaSe2, and systematically analyze which computational methods are most promising as as input to band gap calculations.

Organic semiconductors have been studied extensively in the literature for use in organic electronic applications. Thousands of diverse structures have been synthesized using organic techniques and often the quest for higher performing materials requires an Edisonian approach. Successful structures are generally chosen for their favorable optical and electronic properties but increasingly, molecular shape and planarity are implicated as crucial parameters for dictating functionality in semiconductor devices. The structural units within the semiconductor, responsible for providing the optical and electronic properties, can also be used to control the overall shape and planarity through intramolecular interactions.
To investigate these intramolecular interactions prevalent in successful organic semiconductor structures, eight oligomers were synthesized to obtain systematic structural changes while maintaining good optoelectronic properties. Using single crystal x-ray crystallography, the conformation of each oligomer was determined. Density functional theory calculations were concomitantly performed for each molecule and the optimized geometry determined. Analysis of interactions within these optimized structures was then completed using rotational barrier calculations and the Natural Bond Orbital software suite. Three major types of intramolecular interactions were revealed: steric, electrostatic and donor-acceptor orbital interactions. Several heteroatomic interactions, including nitrogen—sulfur, nitrogen—hydrogen, and fluorine—sulfur, were found to have large enough energy to cause conformational rigidity, or “locks,” within the molecular structure. These conformational “locks” influence molecular shape, as well as help dictate planarity of the conjugated structures. Critically, these computational data are highly consistent with the observed crytstallographic conformation.
Through this work, it has become clear that computational approaches to modeling organic semiconductors complement the well-established techniques, such as X-ray crystallography, that chemists currently employ. Importantly, the computational approach provides some predictive capability, which may allow molecules to be screened without time consuming synthetic work. Using these approaches can help chemists design the next class of high performance materials for optoelectronic applications.

We consider finite size effects on energy transfer between nanoparticles mediated by quantum systems. The nanoparticles are considered as heat reservoirs with a finite number of modes. An expression for the quasi-static energy transport between the heat reservoirs having a finite mode spacing Δ is derived. The resulting equations describing long-term
(t ge;1/Δ) relaxation for the mode temperatures and the average temperatures of the nanoparticles are solved. The solutions, that depend on a small number of measurable parameters, show unusual peculiarities in their time dependencies. As is shown, Fourier&’s law in a chain of identical subsystems (nanoparticles) can be validated only on a short time scale. For a larger time scale, when t ~ 1/Δ, the temperatures of the different modes deviate from each other preventing thermal equilibrium in each subsystem and the validity of Fourier&’s law cannot be established. As is also found, the long-term asymptotic value of the mode temperatures does not depend on the mode number and is the same as in the case if Fourier&’s law were valid for all times.

In this work, we grow thin MoS2 films (50-150 nm) uniformly over large areas (>1 cm2) with strong basal plane (002) or edge plane (100) orientation to characterize thermal anisotropy. Measurement results are correlated with molecular dynamics simulations of thermal transport for perfect and defective MoS2 crystals. The correlation between predicted (simulations) and measured (experimental) thermal conductivity are attributed to factors such as crystalline domain orientation and size, thereby demonstrating the importance of thermal boundary scattering in limiting thermal conductivity in nano-crystalline MoS2 thin films. Furthermore, we demonstrate that the cross-plane thermal conductivity of the films is strongly impacted by exposure to ambient humidity.

Using a parameter free, self-optimizing functional (SOF) within the framework of density functional theory, we show that it is possible to attain the accuracy of fit-based functionals using an ab initio approach. Specifically, we compare predictions for the structural, electronic, and vibrational properties of a large number of molecules (the G2/97 test) using a self-optimized form of the LC-omega;PBE functional. We demonstrate its ability to predict ground state geometries, ionization energies, and zero point energies with high fidelity. Moreover, this SOF greatly improves accuracy of the electronic structure and naturally provides insight into the character of the electron gas.

Solar thermal fuels make use of molecules that undergo reversible photo-isomerization to store solar energy and convert it into thermal energy [Kucharski, T. J., et al. Energy Environ. Sci. 4, 4449 (2011).]. Because solar thermal fuels produce no emissions and can store and convert energy within one material, they are an attractive option for a renewable alternative energy source. Azobenzene, which undergoes trans- to cis- photo-isomerization, has drawn attention as a candidate material for solar thermal fuels. However, both isomers are photoactive in similar regions of the solar spectrum, and the metastable cis-isomer exhibits a significant absorption coefficient, leading to a photo-stationary state (dynamic equilibrium of the two directions of photoisomerization) with a significant amount of the lower energy trans isomer and a resulting low energy storage capacity. Using ab initio methods, we evaluated possible design strategies of (i) functionalizing azobenzene and (ii) designing close-packed semi-crystalline azobenzene/template nanostructures [Kolpak, A. M., et al. Nano Lett. 11, 3156-3162 (2011)., Kolpak, A. M., et al. J. Chem. Phys. 138, 034303 (2013).], which modifies absorption properties and isomerization of the two isomers. We aim to maximize the overlap of absorption spectrum of trans-isomer with the solar spectrum while simultaneously minimizing that of cis-isomer with solar spectrum. Using time dependent density functional theory (TDDFT), we calculated the absorption properties of the trans- and the cis-photoisomers and examined how much gain we can get in the storage efficiency of solar thermal fuels. We observed that cis-isomer content at the photostationary state varies from 68 to 91 percent depending on the electron-withdrawing character of functional groups and we examined the packing effect induced by templates.

In recent years, two-dimensional monolayers, such graphene and h-BN, have been studied intensively for their novel properties and potential applications in electronics and energy. Among them, many efforts have been deveoted to introduce various defects in two-dimensional monolayer to taliroing their optical, electronic, and magnetic properties. Here, on the basis of the first-principles calculations and quantum molecular dynamic simulations, we show the effect of localized defect and extended defects on the electronic and chemical reactivities of two-dimensional h-BN monolayer.I will mainly talk about the band gap and magnetism engineering, as well as chemical reactivity on the defected h-BN supported metal atom. At last, a new metallic h-BN monolayer based framework will be introduced.

Thermal management becomes an increasingly important issue for electronic devices, when it comes to downsizing: The smaller integrated devices, the larger the temperature gradients between their fractions. Nowadays CPUs made of nanometer-size transistors need miniaturized active cooling modules, which could ideally be thermoelectric (TE) solid-state Peltier elements. In spite of the advantages of being small, flexible and of involving no use of fluids, unfortunately the low performance of thermoelectric materials severely limits the application of Peltier devices.
Our work aims at the improvement of the efficiency of such devices by replacing the standard p-n junctions with a membrane-based layered structure. The membranes itself will have an elaborate design of nano-structured features embedded in a semiconducting matrix. The low dimensional character of the membranes as well as the nano-features will ensure a reduction of the phononic thermal conductivity κ, thus enhancing the thermoelectric figure of merit, ZT = S²σT/κ
Mn-Ge compounds turned out to be excellent candidates for nanostructuring due to the broad structural variety - 16 different phases have been identified as a function of the thermodynamic conditions and the relative concentration of Ge and Mn. Experimentalists could indeed grow Mn5Ge3 nano-clusters and Mn-Ge nano-columns in a Ge matrix [1,2]. For these structures a very low thermal conductivity is expected since these nano-inclusions serve as phonon scatterers.
We performed first-principles electronic structure calculations, in particular density functional theory, in order to characterize various smaller Mn-Ge compounds - including several MnxGey bulk species as well as different Ge/Mn-Ge superlattices.
Reaching larger length scales, however, constitutes a major obstacle in this project: Due to structural diversity, a standard empirical potential would not be suitable to describe the structural and vibrational properties of Mn-Ge compounds. Therefore we have constructed a transferable neural network potential [3,4], with which we can characterize larger superlattices and nanostructured membranes up to device-like size, and determine their thermal transport properties. This multiscale modeling approach is a powerful tool to design materials and devices with specifically engineered phonon properties, and enhanced thermoelectric performances.
This project is funded by the program FP7-ENERGY-2012-1-2STAGE under contract number 309150. For more details see www.merging.eu
[1] Jamet et al Nature Materials5, 653 (2006)
[2] Jain et al J. Appl. Phys.109, 013911 (2011)
[3] Behler, Parrinello Phys. Rev. Lett.98, 146401 (2007)
[4] Behler, Donadio, Parrinello Phys Status Solidi B245, 261 (2008)

Using non-equilibrium molecular dynamics simulations, we study the decrease in thermal conductivity (k) in isotopically impure carbon nanotubes (CNTs) and graphene nanoribbons. We observe that the decrease in k for the doped CNTs, developed from the zone-folding approach of graphene sheets, is more than that of the doped graphene nanoribbons since the edge scattering effects due to development of dangling bonds in graphene decreases the participation of phonon modes there. We analyze the vibrational density of states (DOS) both along and perpendicular to the direction of heat transfer. We find that the high energy modes of vibration shift to lower wave numbers and the highest peaks of the vibrational modes become shorter reflecting the strong influence of mass disorder that impedes formation of delocalized modes in impure materials. We analyse that in the lower as well as the higher wave number range, both the pure and doped graphene nanoribbons possess enhanced number of vibrational modes than the corresponding nanostructures. At lower wave numbers, the out-of-plane flexural acoustic modes in graphene nanoribbons and the delocalized transverse modes in CNTs play a more prominent role in the heat transfer process. The presence of isotopic dopants perturbs the DOS as well as the phonon dispersion curves thereby decreasing the overall phonon group velocity and reducing k.

We report a new form of crystalline order that has been found in ternary nanoparticles (two cations and one anion): an uninterrupted, perfect, global Bravais lattice, in which the two cations exhibit a wide array of distinct ordering patterns within the cation sublattice, forming interlaced domains and phases without any strain or structural defects. Partitioning into domains and phases can be performed in multiple ways and the corresponding boundaries entail no energy cost. We call this form of crystalline order “interlaced crystals” and present the example of hexagonal CuInS2 nanoparticles. Interlaced crystals are possible in I-III-VI2 and II-VI-V2 compounds. The key criterion is to have tetrahedral bonding and two electrons per bond, which is possible for example in I-III-VI2 only if each group-VI atom has two group-I and two group-III neighbors. The tetrahedral bonding enables strain-free, disorder-free, interlaced domains and phases at no energy cost.
We discovered the novel crystalline order during an investigation of hexagonal CuInS2 semiconductor nanoparticles. These hexagonal nanoparticles are believed to be single crystals with an ordered sulfur sublattice, but it has not been established whether the Cu and In atoms are randomly distributed or ordered. Conventional XRD cannot discern any small peaks and subtle changes in the pattern that would be diagnostic of the presence of one or several ordered phases. Instead, only the average lattice (wurtzite) is determined.
We employ density functional theory to determine the low-energy structures that are possible in hexagonal CuInS2 nanoparticles and use Z-contrast imaging on an aberration-corrected STEM to directly distinguish the In and Cu atoms in the nanoparticles. The two approaches independently yield the same result: the Cu and In cations are not randomly distributed in the cation sublattice of a wurtzite crystal structure. Instead the two cation species exhibit a range of ordered structures while the abstract cation sublattice retains its perfect structure. The calculations predict the existence of a family of ordered phases with essentially identical energies. The experimental images show a continuum of interlaced phases and domains that can be demarcated in a variety of ways. The calculations further find that phase and domain boundaries cost no energy so that the interlacing is strain free, with the global Bravais lattice kept intact. Similarly, strain analysis of the Z-contrast images reveals that there is no measurable strain.
The interlacing has little effect on electron transport as the domain and phase boundaries are free of defects and distortions, but reduces phonon transport due to the acoustic mismatch. Such features make interlaced crystals attractive for thermoelectric applications.
This work is supported by DOE grant DE-FG02-09ER46554, by DOE BES MSED, by ORNL&’s CNMS (DOE-BES), by a Wigner Fellowship of ORNL (DOE), and by NSF under TN-SCORE and DMR-0938330.