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Enzymes are the most efficient catalysts known to date. However, decades of research have failed to fully explain the catalytic power of enzymes, and most of the current attempts to uncloak the details of atomic motions at active sites remain incomplete. Here, a straightforward manner for understanding the interplay between the complex or irregular enzyme topology and dynamical effects at catalytic sites is introduced, by revealing how fast localized vibrations form spontaneously in the stiffest parts of the scaffold. While shedding light on a physical mechanism that allowed the selection of the picosecond (ps) timescale to increase the catalytic proficiency, this approach exposes the functional importance of localized motions as a by-product of the stability-function tradeoff in enzyme evolution. From this framework of analysis—directly accessible from available diffraction data—experimental strategies for engineering the catalytic rate in enzymatic proteins are proposed.

Enzymes speed up biochemical reactions at the core of life by as much as 15 orders of magnitude. Yet, despite considerable advances, the fine dynamical determinants at the microscopic level of their catalytic proficiency are still elusive. In this work, we use a powerful mathematical approach to show that rate-promoting vibrations in the picosecond range, specifically encoded in the 3D protein structure, are localized vibrations optimally coupled to the chemical reaction coordinates at the active site. The universality of these features is demonstrated on a pool of more than 900 enzyme structures, comprising a total of more than 10,000 experimentally annotated catalytic sites. Our theory provides a natural microscopic rationale for the known subtle structural compactness of active sites in enzymes., 10 pages, 9 figures

The influence of vacancy defects on the electronic and phonon properties of graphene is studied with the models based on a unit cell of 180 carbon atoms and of 1, 2, 3, 6, and 24 vacancies. Ordered, with one defect per unit cell, and non-ordered (randomly arranged) vacancies are calculated with first-principle and molecular dynamics methods. Randomly oriented vacancies lead to a creation of characteristic V1(5-9) defects and the amorphization of graphene with different rings. Electronic and phonon densities of states are analyzed. The switching of the electrical conductivity from metal to semiconductor type is observed when increasing the defect sizes from a single vacancy to large clusters. The characteristic phonon modes are found in all these cases for their future experimental identification. Novel types of devices are proposed via doping of defective graphene.

International audience; Coherent phonon transport is regarded as a promising strategy for controlling thermal properties in solids using the wave nature of phonons. However, no clear distinction between the spatial and temporal phonon coherence has been accounted for and a formalism that quantifies these two effects is still to be found. In this work, we propose a statistical approach for calculating the spatial and temporal coherence spectra using molecular dynamics simulations. We provide a microscopic assessment of these properties and we theoretically demonstrate that, while temporal and spatial coherence can be analytically related under specific conditions, they represent two characteristic lengths that set apart different physical effects. The former is associated with the phonon mean free path while the latter can be regarded as a measure of localization, representing the spatial extension of phonon wave packets. This provides a framework to engineer heat conduction in solids by quantitatively revealing the wave/particle nature of the vibrational modes.

The ability of metallic nanoparticles to supply heat to a liquid environment under exposure to an external optical field has attracted growing interest for biomedical applications. Controlling the thermal transport properties at a solid–liquid interface then appears to be particularly relevant. In this work, we address the thermal transport between water and a gold surface coated by a polymer layer. Using molecular dynamics simulations, we demonstrate that increasing the polymer density displaces the domain resisting to the heat flow, while it does not affect the final amount of thermal energy released in the liquid. This unexpected behavior results from a trade-off established by the increasing polymer density which couples more efficiently with the solid but initiates a counterbalancing resistance with the liquid.

[*] Dr. S. Volz, Y. Chalopin Laboratoire d’Energetique Moleculaire, Macroscopique et Combustion Ecole Centrale Paris, CNRS UPR288, PRES UniverSud Paris Grande Voie des Vignes, 92295 Châtenay-Malabry Cedex (France) E-mail: volz@iis.u-tokyo.ac.jp Dr. S. Volz Laboratory for Integrated Micro-Mechatronic Systems Institute of Industrial Science, University of Tokyo, CNRS UMI2820 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505 (Japan)

The adsorption of the cis-1,2-dicyano-1,2-bis(2,4,5-trimethyl-3-thienyl)ethene (CMTE) molecule on the clean Si(100)-2 × 1 surface was studied using scanning tunneling microscopy at room temperature. These molecules belong to the class of photochromic dye molecules that undergo reversible photoisomerization between the open and closed isomers when irradiated with light. This is the first observation of individual photochromic molecules on a surface where it is possible to identify the two photoisomers. By changing the ambient lighting conditions prior to deposition, the relative proportion of the two forms of molecules observed on the silicon surface can be changed. The deduction is that the triangular form corresponds to the closed isomer, and the three-lobed form corresponds to the open isomer.

International audience; The high thermal conductivity of SiC prevents the improvement of its thermoelectric figure of merit, although excellent power factor has been achieved. Here we propose a different type of SiC superlattice, i.e., antiphase superlattice (APSL) nanowires (NWs), composed of only SiC components but with different stacking sequences. Our molecular dynamics simulations show that the thermal conductivity of period modulated APSL NWs can be significantly reduced by up to a factor of two at room temperature compared to the one of pristine NWs. The phonon density of states reveals that new vibrational modes emerge on the interfaces due to the formation of Si-Si and C-C bonds. We identify the increased phonon interfacial scattering as the predominant factor that hinders the thermal transport along the wires with period Lp>6 nm. Phonon coherent transport is also observed in the structures with period Lp

We investigate in detail the mechanisms of heat relaxation at a gold:water interface and address the effect of a polymer (DHLA-Jeffamine) coated at the surface. We demonstrate that above a low surface density limit, the adjunction of polymers significantly enhances the interface conductance from the metal to the fluid. From molecular dynamics simulations, we have computed the corresponding increase of thermal conductance. The physical origin of this effect is analyzed in terms of transmission spectra. Transient non-equilibrium simulations finally compare the profile of the thermal field generated with and without the polymer.

The thermal interface conductance between Al and Si was simulated by a non-equilibrium molecular dynamics method. In the simulations, the coupling between electrons and phonons in Al are considered by using a stochastic force. The results show the size dependence of the interface thermal conductance and the effect of electron–phonon coupling on the interface thermal conductance. To understand the mechanism of interface resistance, the vibration power spectra are calculated. We find that the atomic level disorder near the interface is an important aspect of interfacial phonon transport, which leads to a modification of the phonon states near the interface. There, the vibrational spectrum near the interface greatly differs from the bulk. This change in the vibrational spectrum affects the results predicted by AMM and DMM theories and indicates new physics is involved with phonon transport across interfaces., United States. Dept. of Energy. Office of Science (Solid-State Solar-Thermal Energy Conversion Center Award DE-SC0001299/DE-FG02-09ER46577), National Natural Science Foundation (China) (11334007), National Natural Science Foundation (China) (11204216)

We have calculated the electric polarizability of two types of lipid bilayers, formed by 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) lipids. We demonstrate that despite the very similar chemical structures, the bilayers exhibit dramatically different terahertz infrared (IR) properties. We demonstrate that the chemical structure of the lipids influences the morphology of the bilayers, which in turn impacts their IR responses interestingly, both structures exhibit a non-uniform absorption. For the case of DPPC, the infrared absorption is mostly driven by the hydrophilic heads, whereas for the POPC lipids, the absorption occurs predominately at the hydrophobic tails. (C) 2015 AIP Publishing LLC.

We report the layer-number dependence of the averaged interlayer thermal resistances (${R}_{\mathrm{int}}$) of the suspended and supported few-layer graphene (FLG), simulated by equilibrium molecular dynamics (EMD). The existence of a silicon dioxide substrate significantly decreases the ${R}_{\mathrm{int}}$ of FLG at low layer number. We use the model of long-wavelength dynamics of a nanolayer adsorbed on a deformable crystal [Kosevich and Syrkin, Phys. Lett. A 135, 298 (1989)] to explain the appearance of the substrate-induced gaps in the FLG dispersion curves and phonon radiation into the deformable substrate from these gap modes. The enhanced thermal conductance in the cross-plane direction is ascribed to the phonon radiation from FLG into the deformable substrate, which partially transfers the flow of phonon energy in FLG from the in-plane to the cross-plane direction and to the substrate. To confirm this, we calculate the cross-plane thermal resistance of three-layer graphene supported by an effective SiO${}_{2}$ substrate in which atomic masses are increased by a factor of 1000. This makes the substrate almost immovable and suppresses phonon radiation from the supported FLG by complete phonon reflection at the interface. The cross-plane thermal resistance of three-layer graphene supported on such a substrate is found to be the same as its suspended counterpart.

International audience; Heat transfer between two silica clusters is investigated by using the nonequilibrium Green's function method. In the gap range between 4 Å and 3 times the cluster size, the thermal conductance decreases as predicted by the surface charge-charge interaction. Above 5 times the cluster size, the volume dipole-dipole interaction predominates. Finally, when the distance becomes smaller than 4 Å, a quantum interaction where the electrons of both clusters are shared takes place. This quantum interaction leads to the dramatic increase of the thermal coupling between neighbor clusters due to strong interactions. This study finally provides a description of the transition between radiation and heat conduction in gaps smaller than a few nanometers.

Based on the Landauer formalism, we demonstrate that the thermal conductance due to the propagation of surface phonon-polaritons along a polar nanowire is independent of the material characteristics and is given by Pi^2kB^2T/3h. The giant propagation length of these energy carriers establishes that this quantization holds not only for a temperature much smaller than 1 K, as is the case of electrons and phonons, but also for temperatures comparable to room temperature, which can significantly facilitate its observation and application in the thermal management of nanoscale electronics and photonics., 5 pages, 4 figures

International audience; The effective thermal conductivity of a layered system due to the propagation of surface phonon-polaritons is studied. We analytically demonstrate that the thermal conductivity of a set of nanolayers can be described as one of a single layer with an effective permittivity, which does not ordinarily appear in nature and depends on the permittivities and thicknesses of the individual components. For a two-layer system of SiO 2 and BaF 2 surrounded by air, it is shown that: (i) the propagation length of surfaces phonon-polaritons can be as high as 3.3 cm for a 200 nm-thick system. (ii) The thermal conductivity of the system with total thickness of 50 nm is 3.4 W/mÁK, which is twice that of a single layer of SiO 2 , at 500 K. Higher values are found for higher temperatures and thinner layers. The results show that an ensemble of layers provides more channels than a single layer for the propagation of surface phonon-polaritons and therefore for the enhancement of the thermal conductivity of common polar materials. V C 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4864430]

International audience; By formulating a microscopic description of the non-local dielectric constant, we have investigated the mechanisms of infrared absorption in dielectrics thin films by molecular dynamics simulations. We found that light absorption in dielectric slabs does not occur predominantly at the polaritons resonances but through anomalous surface modes extremely confined in space. This demonstrate that any macroscopic description of electrodynamics in dielectrics breaks down at the nanoscale.

International audience; We demonstrate the existence of a coherent transport of thermal energy in superlattices by introducing a microscopic definition of the phonon coherence length. A criterion is provided to distinguish the coherent transport regime from diffuse interface scattering and discuss how these can be specifically controlled by several physical parameters. Our approach provides a convenient framework for the interpretation of previous thermal conductivity measurements and calculations; it also paves the way for the design of a new class of thermal interface materials.

1Laboratoire d’Energetique Moleculaire et Macroscopique, Combustion, UPR CNRS 288, Ecole Centrale Paris, Grande Voie des Vignes, 92295 Châtenay-Malabry, France. 2CIRMM, Institute of Industrial Science, University of Tokyo, Japan. 3Ecole Centrale Paris, Laboratoire de Photonique Quantique et Moleculaire, CNRS (UMR 8537), Ecole Normale Superieure de Cachan, Grande Voie des Vignes, F-92295 Châtenay-Malabry cedex, France.

International audience; While graphene and few layer graphene (FLG) are considered as having the highest thermalconductivity in their in-plane directions, our molecular dynamics (MD) simulations however showthat those systems are also characterized by a superior thermal contact resistance, which could belargely tuned with the layer number when in contact with a silica substrate. Taking advantages ofsuch a resistive interface, MD simulations show that SiO2/FLG superlattices have a thermalconductivity as low as 0.30W/mK, exhibiting a promising prospect in nano-scale thermalinsulation. These findings pave the way for an improved thermal management of nanoscalesystems such as thermal barrier coatings and phase change memory materials with atomic-scalesuper-insulators. VC 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4824013]

We studied the heat propagation in Ti/Pt/Au micro-heater embedded thermal testing chips by computer simulations. Graphene was considered to be incorporated within the chips as a heat spreader in order to utilize its extremely high thermal conductivity. The classical heat conduction equation was solved numerically using the finite element analysis method. We found a linear relation between the temperature of the hot spot and the imposed heat flux, and a graphene spreader could effectively decrease the temperature of the micro-heater. These findings are in satisfying agreement with experimental measurements. In order to better understand the mechanisms behind these phenomena, the temperature distribution along the device surface was plotted and compared for systems with and without a graphene spreader. These results provide a better insight of graphene-based materials as heat spreaders and yield useful information to help improving heat removal from electronic devices.

International audience; The inter-layer resistance in few layer graphene (FLG) is an unknown intrinsic property that affectsthe heat removal efficiency of FLG-based thermal devices. Here we present data that demonstratesthe layer number dependence of the resistance between FLGs, by using molecular dynamicssimulations. The resistance was found to decrease as the layer number increases. FLGs with largerthicknesses are proposed to be advantageous in heat spreading owing to their lower contactresistances. The observed properties do not depend on temperature, which is crucial for FLG basedstructures to retain a stable heat removal efficiency while working at a large temperature range.VC 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4818341]

International audience; We present a microscopic approach for estimating the frequency vs. wave-vector dependentphonon transmission across a solid-solid interface. We show that the spectral properties of theheat flux can be generally deduced from the equilibrium displacements fluctuations of thecontact atoms. We have applied and demonstrated our formalism with molecular dynamicssimulations to predict the angular and mode dependent phonon transport in silicon andgermanium thin films. This notably unveils the existence of confined interface mode at thethermal contacts. VC 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4816738]

International audience; The thermal conductivity of diameter and polytype modulated SiC nanowires is predicted using nonequilibrium molecular dynamics. For the polytype modulated nanowires, the two main SiC polytypes, zinc blende (3C) and wurtzite (2H) were considered. We show that the thermal conductivity of the diameter modulated nanowires may be even smaller than that of the constant diameter nanowire with the small section. This remarkable reduction in thermal conduction is attributed to a significant thermal boundary resistance displayed by the constriction, as measured by independent molecular-dynamics simulations. The constriction resistance is related to the confinement of low-frequency modes, as shown by vibrational density-of-states calculations. We used Monte Carlo simulations to conclude that the value of the constriction resistance may be explained by the specular reflections of this class of modes on the surface surrounding the constriction.

International audience; We provide a derivation allowing the calculation of thermal conductance at interfaces by equilibrium molecular dynamics simulations and illustrate our approach by studying thermal conduction mechanisms in Si/Ge superlattices. Thermal conductance calculations of superlattices with period thicknesses ranging from 0.5 to 60 nm are presented as well as the temperature dependence. Results have been compared to complementary Green-Kubo thermal conductivity calculations demonstrating that thermal conductivity of perfect superlattices can be directly deduced from interfacial conductance in the investigated period range. This confirms the predominant role of interfaces in materials with large phonon mean free paths.

International audience; A large effect of pressure on the thermal conductance of silicon/carbon nanotube junctions is shown to result from induced anharmonicity at the interface. Through atomistic simulations, we demonstrate the opening of pressure induced inelastic phonon channels, which are responsible for a several fold increase of the thermal conductance.

In the ballistic regime of transport, we derive the thermal resistance of metal nanowires connected to two heat baths. We find that the thermal resistances for any metal nanowire remain in a narrow and temperature-dependent interval between 0.447/$T$ and 14/$T$ ${\mathrm{m}}^{2}$ K/W. For cross-section edges larger than 20 nm, the thermal resistance can actually be estimated from $\frac{{R}_{0}}{(3\ensuremath{\pi}{n}_{V}^{2}){}^{3}}$, where ${R}_{0}$ is the quantum of resistance and ${n}_{V}$ refers to the electron density. This prediction yields a contact resistance one order of magnitude larger than the one of previous estimations. Significant consequences for application fields such as nanoelectronics, where nanowire sizes involve ballistic transport, are expected.

Heating of dielectric nanoparticles by black-body radiation is investigated by using molecular-dynamics simulation. The thermal interaction with the radiation is modeled by coupling the ions with a random electric field and including a radiation reaction force. This approach shows that the heat is absorbed by the polariton mode. Its subsequent redistribution among other vibration modes strongly depends on the particle size and on temperature. We observe energy trapping in a finite subset of vibrational modes and study the relaxation pathway of (MgO)${}_{4}$ by performing a selective excitation with a deterministic force.

International audience; Molecular dynamics (MD) is a numerical simulation technique based on classical mechanics. It has been taken for granted that its use is limited to a large temperature regime where classical statistics is valid. To overcome this limitation, the authors introduce in a universal way a quantum thermal bath that accounts for quantum statistics while using standard MD. The efficiency of the new technique is illustrated by reproducing several experimental data at low temperatures in a regime where quantum statistical effects cannot be neglected.

International audience; Using atomistic Green’s function calculations, we find that the phonon thermal conductivity of pellets composed of ∼μm long carbon nanotubes has an upper bound of a few W/m K. This is in striking contrast with the extremely high thermal conductivity of individual nanotubes (∼3000 W/m K). We show that, at room temperature, this upper bound does not depend on the nanotube diameter. Conversely, for low temperatures, an inverse proportionality with nanotube diameter is predicted. We present concrete results as a function of nanotube length and chirality, pellet density, and temperature. These results imply that carbon nanotube pellets belong to the category of thermal insulators, contrasting with the good conducting properties of parallel nanotube arrays, or individual nanotubes

International audience; Superlattices with thermal-insulating behaviors have been studied to design thermoelectric materials but affect heat transfer in only one main direction and often show many cracks and dislocations near their layer interfaces. Quantum-dot (QD) self-assembly is an emerging epitaxial technology to design ultradense arrays of germanium QDs in silicon for many promising electronic and photonic applications such as quantum computing, where accurate QD positioning is required. We theoretically demonstrate that high-density three-dimensional (3D) arrays of molecular-size self-assembled Ge QDs in Si can also show very low thermal conductivity in the three spatial directions. This physical property can be considered in designing new silicon-based crystalline thermoelectric devices, which are compatible with the complementary metal-oxide-semiconductor (CMOS) technologies. To obtain a computationally manageable model of these nanomaterials, we investigate their thermal-insulating behavior with atomic-scale 3D phononic crystals: A phononic-crystal period or supercell consists of diamond-cubic (DC) Si cells. At each supercell center, we substitute Si atoms by Ge atoms in a given number of DC unit cells to form a boxlike nanoparticle (i.e., QD). The nanomaterial thermal conductivity can be reduced by several orders of magnitude compared with bulk Si. A part of this reduction is due to the significant decrease in the phonon group velocities derived from the flat dispersion curves, which are computed with classical lattice dynamics. Moreover, according to the wave-particle duality at small scales, another reduction is obtained from multiple scattering of the particlelike phonons in nanoparticle clusters, which breaks their mean free paths (MFPs) in the 3D nanoparticle array. However, we use an incoherent analytical model of this particlelike scattering. This model leads to overestimations of the MFPs and thermal conductivity, which is nevertheless lower than the minimal Einstein limit of bulk Si and is reduced by a factor of at least 165 compared with bulk Si in an example nanomaterial. We expect an even larger decrease in the thermal conductivity than that predicted in this paper owing to multiple scattering, which can lead to a ZT much larger than unity.

Thermal conductivity ($\ensuremath{\kappa}$) of isolated carbon nanotubes (CNTs) is higher than the $\ensuremath{\kappa}$ of diamond; however, in this Letter we show that the $\ensuremath{\kappa}$ of a packed bed of three-dimensional random networks of single and multiwall CNTs is smaller than that of thermally insulating amorphous polymers. The thermoelectric power ($\ensuremath{\Sigma}$) of the random network of CNTs was also measured. The $\ensuremath{\Sigma}$ of a single wall nanotube network is very similar to that of isolated nanotubes and, in contrast with $\ensuremath{\kappa}$, $\ensuremath{\Sigma}$ shows a strong dependence on the tube diameter.

At low temperatures, thermal transport in single crystalline nanowires with sub-10-nm diameters is defined in terms of the universal quantum of conductance. In the case of a nanowire connected to plane substrates, additional conductances appear due to the contacts. We calculate the contact conductances and prove that they are much smaller than the conductance of the nanowire. The reason is that the number of excited modes per unit volume in the substrates becomes smaller than the one in the wire at low temperatures. The substrate then generates the predominant thermal resistance because its specific heat becomes smaller than the one of the wire. From these considerations, the wire-membrane and membrane-plane substrate thermal conductances can also be predicted.

Superlattices have been used to design thermoelectric materials with ultra-low thermal conductivities. Indeed, the thermoelectric figure of merit ZT varies as the inverse of the material thermal conductivity. However, the design of a thermoelectric material with ZT superior to the alloy limit usually fails with the superlattices because of two major drawbacks: First, a lattice mismatch can occur between the different layers of a superlattice as in a Si/Ge superlattice. This leads to the formation of defects and dislocations, which reduces the electrical conductivity and therefore avoids the increase of ZT compared to the alloy limit. On the other hand, the superlattices only affect heat transfer in one direction. To cancel heat conduction in the three spatial directions, we propose atomic-scale three-dimensional (3D) phononic crystals. Because the lattice constant of our phononic crystal is of the order of some nanometers, we obtain phonon confinement in the THz range and a nanomaterial with a very low thermal conductivity. This is not possible with the usual phononic crystals, which show band gaps in the sub-MHz range owing to their large lattice constant of the order of 1 mm. A period of our atomic-scale 3D phononic crystal is composed of a given number of diamond-like silicon cells forming a supercell. A periodic Si/Ge heterostructure is obtained since we substitute at each supercell center the Si atoms in a smaller number of cells by Ge atoms. The Ge atoms in the cells located at each supercell center form a box-like nanoparticle with a size that can be varied to obtain different atomic configurations of our nanomaterial. We also propose another design for our phononic crystal where we introduce a small number of diamond-like silicon cells at the center of a periodic supercell of diamond-like germanium cells. In this second design, we form box-like nanoparticles of Si atoms in a germanium matrix instead of boxlike nanoparticles of Ge atoms in a silicon matrix in the first design. With the dispersion curves computed by lattice dynamics and a general equation, we obtain the thermal conductivities of several atomic configurations of our phononic crystal. Compared to a bulk material, the thermal conductivity can be reduced by at least one order of magnitude in our phononic crystal. This reduction is only due to the phonon group velocities, and we expect a further decrease owing to the diminution of the phonon mean free path in our phononic crystal.Copyright © 2008 by ASME

Extensive research on semiconducting superlattices with a very low thermal conductivity was performed to fabricate thermoelectric materials. However, as nanowires, superlattices affect heat transfer in only one main direction, and often show dislocations owing to lattice mismatches when they are made up of a periodic repetition of two materials with different lattice constants. This reduces their electrical conductivity. Therefore it is challenging to obtain a thermoelectric figure of merit ZT superior to unity with the superlattices. Self-assembly with lithographic patterning and/or liquid precursors is a major epitaxial technology to fabricate ultradense arrays of germaniums quantum dots (QDs) in silicon for many promising electronic and photonic applications as quantum computing where accurate QD positioning and low degree of dislocations are required. We theoretically demonstrate that high-density three-dimensional (3-D) arrays of self-assembled Ge nanoparticles, with a size of some nanometers, in Si can also show a very low thermal conductivity in the three spatial directions. This property can now be considered to design new thermoelectric devices, which are compatible with new complementary metal-oxide-semiconductor (CMOS) processes. To obtain a computationally manageable model of these nanomaterials, we simulate their thermal behavior with atomic-scale 3-D phononic crystals. A phononic-crystal period or supercell consists of diamond-like Si cells. At each supercell center, we substitute Si atoms by Ge atoms in a given number of cells to form a box-like nanoparticle. According to our model, in an example 3-D phononic crystal, the thermal conductivity can be reduced to a value lower than only 0.2 W/mK or by a factor of at least 750 compared to bulk Si at 300 K. This value is five times smaller than the Einstein Limit of single-crystalline bulk Si. We considered the flat dispersion curves computed by lattice dynamics to obtain this huge decrease. However, we did not consider multiple-scattering effects as multiple reflections and diffusions of the phonons between the Ge nanoparticles. We expect a larger decrease of the real thermal conductivity owing to the reduction of the phonon mean free paths from these collective effects. We hope to obtain a large ZT in these self-assembled Ge nanoparticle arrays in Si. Indeed, they are crystalline with an electrical conductivity that can be also increased by doping using CMOS processes, which is not possible with other recently proposed materials.

Extensive research about superlattices with a very low thermal conductivity was performed to design thermoelectric materials. Indeed, the thermoelectric figure of merit ZT varies with the inverse of the thermal conductivity but is directly proportional to the power factor. Unfortunately, as nanowires, superlattices reduce heat transfer in only one main direction. Moreover, they often show dislocations owing to lattice mismatches. Therefore, fabrication of nanomaterials with a ZT larger than the alloy limit usually fails with the superlattices. Self-assembly is a major epitaxial technology to fabricate ultradense arrays of germaniums quantum dots (QD) in a silicon matrix for many promising electronic and photonic applications as quantum computing. We theoretically demonstrate that high-density three-dimensional (3-D) periodic arrays of small self-assembled Ge nanoparticles (i.e. the QDs), with a size of some nanometers, in Si can show a very low thermal conductivity in the three spatial directions. This property can be considered to design thermoelectric devices, which are compatible with the complementary metal-oxide-semiconductor (CMOS) technologies. To obtain a computationally manageable model of these nanomaterials, we simulate their thermal behavior with atomic-scale 3-D phononic crystals. A phononic-crystal period (supercell) consists of diamond-like Si cells. At each supercell center, we substitute Si atoms by Ge atoms in a given number of cells to form a box-like Ge nanoparticle. The phononic-crystal dispersion curves, which are computed by classical lattice dynamics, are flat compared to those of bulk Si. In an example phononic crystal, the thermal conductivity can be reduced below the value of only 0.95 W/mK or by a factor of at least 165 compared to bulk silicon at 300 K. Close to the melting point of silicon, we obtain a larger decrease of the thermal conductivity below the value of 0.5 W/mK, which is twice smaller than the classical Einstein Limit of single crystalline Si. In this paper, we use an incoherent-scattering approach for the nanoparticles. Therefore, we expect an even larger decrease of the phononic-crystal thermal conductivity when multiple-scattering effects, as multiple reflections and diffusions of the phonons between the Ge nanoparticles, will be considered in a more realistic model. As a consequence of our simulations, a large ZT could be achieved in 3-D ultradense self-assembled Ge nanoparticle arrays in Si. Indeed, these nanomaterials with a very small thermal conductivity are crystalline semiconductors with a power factor that can be optimized by doping using CMOS-compatible technologies, which is not possible with other recently-proposed nanomaterials.Copyright © 2008 by ASME

Owing to their thermal insulating properties, superlattices have been extensively studied. A breakthrough in the performance of thermoelectric devices was achieved by using superlattice materials. The problem of those nanostructured materials is that they mainly affect heat transfer in only one direction. In this paper, the concept of canceling heat conduction in the three spatial directions by using atomic-scale three-dimensional (3D) phononic crystals is explored. A period of our atomic-scale 3D phononic crystal is made up of a large number of diamond-like cells of silicon atoms, which form a square supercell. At the center of each supercell, we substitute a smaller number of Si diamond-like cells by other diamond-like cells, which are composed of germanium atoms. This elementary heterostructure is periodically repeated to form a Si/Ge 3D nanostructure. To obtain different atomic configurations of the phononic crystal, the number of Ge diamond-like cells at the center of each supercell can be varied by substitution of Si diamond-like cells. The dispersion curves of those atomic configurations can be computed by lattice dynamics. With a general equation, the thermal conductivity of our atomic-scale 3D phononic crystal can be derived from the dispersion curves. The thermal conductivity can be reduced by at least one order of magnitude in an atomic-scale 3D phononic crystal compared to a bulk material. This reduction is due to the decrease of the phonon group velocities without taking into account that of the phonon average mean free path.Copyright © 2007 by ASME

International audience; The effects of the permittivity anisotropy and size of a nano thin film on its thermal conductivity due to surface phonon-polaritons are studied. We demonstrate that this thermal conductivity is a linear combination of the inverse first and third powers of the film thickness. For a 100-nm-thick film of α-quartz surrounded by air, the thermal conductivity along the optical axis is comparable to the phonon counterpart and equals 13 W m %1 K %1 , which is 25% higher than that along the perpendicular direction, at room temperature. Higher values are found for thinner films at higher temperatures. S urface phonon-polaritons (SPPs) are electromagnetic waves generated by coupling between photons and phonons at the interface between two media. 1–3) Over the past few years, various research groups have shown that these surface waves have promising applications for improving the thermal performance of nanoscale devices, 4–6) radiative heat transfer, 7–10) high-density infrared data storage , 11) surface infrared absorption, 12) coherent thermal emission, 13) and photonics. 14,15) In these cases, the operating principles are based on the fact that in nanomaterials, the surface effects predominate over the volumetric ones, so the energy transport by SPPs is particularly important. The SPP energy contribution increases as the material size is scaled down in the direction perpendicular to the propagation one. 4,6)

The surface phonon-polaritons contribution to the thermal conductivity of a nano thin film of silicon dioxide is investigated based on the Maxwell equations and the Boltzmann transport equation. It is shown that: (1) a small difference between the permittivities of the substrate and superstrate of the film can generate giant propagation lengths and therefore remarkably enhances its thermal conductivity with respect to values obtained for a freestanding one. (2) The propagation of surface phonon-polaritons is present in a broad band of frequencies and exhibits its largest propagation lengths at the frequency where the absorption of energy is minimal. (3) The increase of the thermal conductivity of the film as its thickness decreases is higher when it is deposited on potassium bromide instead of being suspended in air. The difference in the thermal conductivity for these two systems increases with increasing temperature and reducing the film thickness. A thermal conductivity as high as 2.5 W/m K is obtained for a 30 nm-thick thin film at room temperature, which is about 1.8 times larger than its bulk phonon value. The obtained results show that the propagation of surface phonon-polaritons has the potential not only to offset the reduction of the phonon thermal conductivity of a nano thin film, when its sizes are scaled down, but also to enhance it, by choosing properly the permittivity of its substrate.

The thermal conductivity of diameter modulated SiC nanowires is computed with the non-equilibrium Molecular Dynamics (NEMD) method. The two main polytypes 3C (zinc-blend) and 2H (wurtzite) of SiC nanowires are investigated, but also the superlattice SiC nanowires with shape modulation. For the case of the shape modulated nanowires the Kapitza resistance is calculated with both NEMD and Equilibrium MD (EMD) methods. This thermal resistance is related with the restrictions between two different cross sections. Finally, we proceed to the physical explanation of this phenomenon with the help of the partial densities of states of phonons.

Inter-plane thermal resistance in 5-layer graphene is calculated from equilibrium molecular dynamics (EMD) by calculating the autocorrelation function of temperature difference. Our simulated inter-plane resistance for 5-layer graphene is 4.83 × 10−9 m2K/W. This data is in the same order of magnitude with the reported values from NEMD simulations and Debye model calculations, and the possible reasons for the slight differences are discussed in details. The inter-plane resistance is not dependent on temperature, according to the results of the EMD simulation. Phonon density of states (DOSs) were plotted to better understand the mechanism behind the obtained values. These results provide a better insight in the heat transfer across a few layer graphene and yield useful information on the design of graphene based thermal materials.

Calculations of the dispersion relation and of the propagation length of surface phonon-polariton modes in a micrometric glass waveguide have been performed. The dispersion relation was solved in two ranges of frequency where SPP appear in glass. We succeeded in showing a maximal propagation length of 35 μm in a 10 μm radius waveguide with a 1 μm thick wall.

International audience; We prove that interfacial asperity sharpness allows for tuning superlattice in-plane thermal conductivity below or above the limit of high roughness derived from the Lucas-Ziman (LZ) model. Whereas LZ’s model predicts molecular dynamic (MD) results of Lennard-Jones superlattices for small asperities, it has to be modified with a roughness- and sharpness-dependent layer thickness to remain relevant at higher roughness. For the case of sharpest asperities, the modified LZ model still fails, and ray-tracing computations matching MD data reveal a phonon-trap effect in the asperity valleys. This behavior scales with the Knudsen number and should appear at the micron scale in large mean-free-path crystals, such as silicon.