Source: https://www.european-mrs.com/nanostructures-phononic-applications-emrs
Timestamp: 2019-04-20 12:27:24+00:00

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Heat and vibrations have traditionally been regarded as sources of loss. Today, however, phonons can be controlled and manipulated, particularly in nanoscale materials. This symposium aims at addressing fundamental issues related to phonon transport and the design of nanostructures for phonon manipulation.
Recent years have witnessed an enormous progress in the growth and design of nanostructures and now materials with unprecedented level of purity and structural quality are available. Present experimental capabilities are such that nanostructured features of the same characteristic length of phonons, the quantized vibrations of the crystal lattice, can be obtained. This enhanced degree of control in material design opens the way to a wealth of new strategies to control and manipulate phonon transport. The thermal conductivity of a material can be purposely suppressed, to engineer an efficient thermoelectric; thermal budget, which otherwise can be the bottleneck of the performance of many nanoelectronic devices, can be lowered; phonons can be used to encode logic function in devices analogous to their electronic counterparts, such as diodes and transistors; mechanical waves with frequencies within a specific range are not allowed to propagate within the periodic structure in phononic crystals. On the other hand, nano-mechanical vibrations can also be thought of as standing acoustic waves and hence as discretized, low frequency acoustic phonon modes. Additionally, cavity optomechanics explores the parametric coupling of a mechanical resonator to an optical cavity mode.
The progress in nanoscale thermal transport strongly depends on the development of reliable methods to precisely determine all the relevant parameters, ideally at the level of the individual nanostructure. The most pressing issues involve the precise measurement of the thermal conductivity and the determination of contact thermal resistances. Within this scenario, the predictive power of the state-of-the-art theoretical methods is becoming increasingly important, both to asses and help interpreting the results of the measurements and possibly providing guidelines for the design of new experiments. These include solution from first-principles of the Boltzmann Transport equation, for a quantitative prediction of the phononic properties of bulk materials and molecular dynamics calculations, which, despite capturing often only qualitative trends, allow addressing distinctive features of the nanostructuring, such as complex interfaces, or surface roughness.
Resume : Although classical size effects on phonon heat conduction are now well-established and understood, manipulating phonon heat conduction via waves is still a dream to be realized due to the broadband and short wavelength nature of phonons. In this talk, I will show, however, the wave effects on heat conduction can be observed and exploited to manipulate phonon heat conduction. In superlattice structures, ballistic phonon transport across the whole thickness of the superlattices implies phase coherence. We observed this coherent transport in GaAs/AlAs superlattices by fixing the periodic thickness but varying the number of periods. Simulations show that although high frequency phonons are scattering by roughness, remaining long wavelength phonons maintain their phase and traverse the superlattices ballistically. Accessing the coherent heat conduction regime opens a new venue for phonon engineering. We show further that phonon heat conduction localization happens in GaAs/AlAs superlattice by placing ErAs nanodots at interfaces. This heat-conduction localization phenomenon is confirmed by nonequilibrium atomic Green’s function simulation. These ballistic and localization effects can be exploited to improve thermoelectric energy conversion materials via reducing their thermal conductivity. This material is based upon work supported as part of the “Solid State Solar-Thermal Energy Conversion Center (S3TEC), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number: DE-SC0001299/DE-FG02-09ER46577.
Resume : The manipulation of phonons holds the promise of a quantum-mechanical manipulation of sound and heat . Furthermore, the quantum-mechanical behavior of phonons may enable the control of coherent phonon transport, which is of interest for fundamental research and applications [1, 2]. Phonon engineering leads to a controlled modification of phonon dispersion, phonon interactions, and phonon transport . We have investigated the phononic properties of twin superlattice (SL) GaP nanowire (NWs) by Raman spectroscopy. Contrary to a crystal phase SL, which is obtained by alternating crystal structures (e.g. wurtzite, WZ, and zincblende, ZB), a twin SL is obtained by periodically alternating ZB segments along  direction, and each segment is rotated by 60° about the <111> with respect to the previous/next segment . The existence of a twin SL results in the appearance of new phonon modes besides the longitudinal and transverse optical modes existing also in the bulk material with ZB symmetry. We developed a theoretical model able to reproduce the frequency of these new modes at the Γ point of the Brillouin zone in a GaP crystal with a unit cell as long as the SL period. The number of modes can be decided à la carte by tuning the SL period, thus allowing a controlled design of NW phononic properties. These results represent the first demonstration of phonon engineering in NWs and are an important step in the investigation of the quantum-mechanical nature of phonons. 	M. Maldovan, Nature 503, 209 (2013). 	S. Voltz et al., Eur. Phys. J. B 89, 15 (2016). 	A. A. Balandin and D. L. Nika, Materials Today 15, 266 (2012). 	R. E. Algra et al., Nature 456, 369 (2008).
Resume : Fourier’s Law has traditionally been the cornerstone upon which the description of heat transport was built. However, as recent experiments have started probing heat transport at the nanoscale [1-4], it is seen that interpretation of the results in terms of a standard Fourier’s law leads to conclusions hard to justify on physical grounds, such as an anisotropic thermal conductivity for Si , or a phonon conductivity suppression function that selectively decreases the contribution to the thermal conductivity (kappa) from phonons having a mean free path within a predefined window . An obvious shortcoming of Fourier’s Law is its inability to describe ballistic transport, which takes over as sample size is reduced below the mean free path of an appreciable fraction of phonons. In the transition from ballistic to diffusive heat transport, a superdiffusive regime characterized by phonon Lévy flights makes its ap-pearance for some materials . There is yet another source of deviation from a purely diffusive behavior; namely, the fraction of current-conserving (ie normal) phonon scattering events with respect to the total. In bulk semiconductors, this gives rise to an enhanced contribution to kappa from a fraction of the phonons behaving in a collective manner, most no-tably in graphene, diamond and silicon. This is described by the Kinetic-Collective Model , based on the so-lution to the BTE by Guyer and Krumhansl . In the same work, Guyer and Krumhansl also showed that, when normal collisions dominate, hydrodynamic terms appear in the expression for the heat flux. We have shown that, in the general case with normal and resistive collisions present, these hydrodynamic terms are related to a characteristic length l. When experiments are carried out at length scales similar to or below l, hydrodynamic effects manifest. We have recently verified this experimentally , explaining thermoreflectance measurements in localized heat sources on InGaAs with characteristic widths down to 100 nm with a hydrody-namic extension to Fourier’s Law without resorting to altering the bulk kappa value. The measured temperature profiles are consistent with the heat flux acquiring vorticity over a length scale l. Finally, our approach also has the advantage that it can be used for complex geometries through finite elements at a huge computational ad-vantage over microscopic approaches, paving the way to heat transport engineering at the nanoscale. References:  Siemens, M. E. et al. Nat. Mater. 9, 26–30 (2010).  Wilson, R. B. & Cahill, D. G. Nat. Commun. 5, 5075 (2014).  Hoogeboom-Pot, K. M. et al. Proc. Natl. Acad. Sci. 112, 4846–4851 (2015).  Johnson, J. A. et al. Phys. Rev. Lett. 110, 25901 (2013).  Vermeersch, B., et al. Phys. Rev. B 91, 085203 (2015).  C. de Tomas, A. Cantarero, A. F. Lopeandia, and F. X. Alvarez, Proc. R. Soc. London A 470, 20140371 (2014).  R. A. Guyer and J. A. Krumhansl, Phys. Rev. 148, 778 (1966).  A. Ziabari, P. Torres, B. Vermeersch, Y. Xuan, X. Cartoixà, A. Torelló, J.-H. Bahk, Y. R. Koh, M. Parsa, P. D. Ye, F. X. Alvarez and A. Shakouri, Nat. Commun. 9, 255 (2018).
Thermal transport regimes in bismuth and thermal properties of bismuth nanostructures calculated ab initio.
Affiliations :  Ecole Polytechnique, Laboratoire des Solides Irradies, CNRS UMR 7642, CEA-DSM-IRAMIS, Universite Paris-Saclay, F91128 Palaiseau cedex, France,  IMPMC, UMR CNRS 7590, Sorbonne Universites - UPMC Univ. Paris 06, MNHN, IRD, 4 Place Jussieu, F-75005 Paris, France,  Dipartimento di Fisica, Universita di Roma La Sapienza, Piazzale Aldo Moro 5, I-00185 Roma, Italy.
Resume : In this talk we will present our recent results of the ab initio calculations of thermal transport properties in bismuth and bismuth nanostructures. We will first discuss the occurrence of the hydrodynamic heat transport regime in bismuth . Bismuth is one of the rare materials in which second sound has been experimentally observed. Our calculations predict the occurrence of the Poiseuille phonon flow in Bi between 1.5 K and 3.5 K for sample size of 3.86 mm and 9.06 mm, in consistency with the experimental observations. We will also discuss a Gedanken experiment allowing to assess the occurrence of the hydrodynamic regime in any bulk material. Then, we will discuss the heat transport reduction in Bi nanostructures. We have considered heat transport in polycrystalline thin films of Bi. The calculated lattice thermal conductivity is found in excellent agreement with the available experimental data , and the heat transport reduction is studied for various temperatures and nanostructure geometries.  M. Markov, J. Sjakste, G. Barbarino, G. Fugallo, L. Paulatto, M. Lazzeri, F. Mauri, N. Vast, PRL 2018, in print.  M. Markov, J. Sjakste, G. Fugallo, L. Paulatto, M. Lazzeri, F. Mauri, N. Vast, PRB 93, 064301 (2016).
Resume : Advances in nanotechnology demand a better understanding of heat transport in nanoscale systems. Graphene occupies a unique position because of its extremely high thermal conductivity, that can be also greatly reduced by rough edges, H-passivation and patterning. Among graphene structures, nanorings stand out because of the straightforward way in which they exploit electronic quantum interference effects, which could be used for designing new thermoelectric devices, as we demonstrated when heat is only carried by electrons . In this work, we address the contribution of the atomic lattice to heat transport in graphene nanoribbons and nanorings by using two different approaches. For temperatures above 100 K (roughly 1/3 of the Debye temperature), we use non-equilibrium molecular dynamics, while for lower temperatures we apply a combination of density functional-based tight-binding and Green’s functions. We consider nanoribbon widths up to 6 nm, several ring configurations as well as the effects of rough edges. Our results show that graphene nanorings can efficiently suppress the lattice thermal conductivity as compared to nanoribbons, especially at low temperatures. Furthermore, we demonstrate rough edges have a weaker impact on the heat current in nanorings .  M. Saiz-Bretín, A. V. Malyshev, P. A. Orellana and F. Domínguez-Adame, Phys. Rev. B 91, 085431 (2015).  M. Saiz-Bretín, A. V. Malyshev, F. Domínguez-Adame, D. Quigley and R. A. Römer, Carbon 127, 64 (2018).
Resume : Two-dimensional (2D) materials with graphene as a representative have been intensively studied for a long time. Recently, monolayer gallium nitride (ML GaN) with honeycomb structure was successfully fabricated in experiments, generating enormous research interest for its promising applications in nano- and opto-electronics. Considering all these applications are inevitably involved with thermal transport, systematic investigation of the phonon transport properties of 2D GaN is in demand. In this paper, by solving the Boltzmann transport equation (BTE) based on first-principles calculations, we performed a comprehensive study of the phonon transport properties of ML GaN, with detailed comparison to bulk GaN, 2D graphene, silicene and ML BN with similar honeycomb structure. Considering the similar planar structure of ML GaN to graphene, it is quite intriguing to find that the thermal conductivity (κ) of ML GaN (14.93 Wm/K) is more than two orders of magnitude lower than that of graphene and is even lower than that of silicene with a buckled structure. Systematic analysis is performed based on the study of the contribution from phonon branches, comparison among the mode level phonon group velocity and lifetime, the detailed process and channels of phonon–phonon scattering, and phonon anharmonicity with potential energy well. We found that, different from graphene and ML BN, the phonon–phonon scattering selection rule in 2D GaN is slightly broken by the lowered symmetry due to the large difference in the atomic radius and mass between Ga and N atoms. Further deep insight is gained from the electronic structure. Resulting from the special sp orbital hybridization mediated by the Ga-d orbital in ML GaN, the strongly polarized Ga–N bond, localized charge density, and its inhomogeneous distribution induce large phonon anharmonicity and lead to the intrinsic low κ of ML GaN. The orbitally driven low κ of ML GaN unraveled in this work would make 2D GaN prospective for applications in energy conversion such as thermoelectrics. Our study offers fundamental understanding of phonon transport in ML GaN within the framework of BTE and further electronic structure, which will enrich the studies of nanoscale phonon transport in 2D materials and shed light on further studies.
Resume : Nano-phononic crystals and diameter-modulated nanowires have attracted significant attention due to their low thermal conductivities and their potential application as thermoelectric materials. Tuning the geometry of these nanostructures to change the scattering mechanics and thus engineering a lower thermal conductivity has been proven in recent publications. However, the modal phonon transmission coefficients across these geometrically irregular nanostructures and the effect of nanostructure geometry on thermal transport has not been fully understood. In this work, a harmonic lattice dynamics and scattering boundary method based atomistic modeling tool was created to calculate phonon spectra and modal phonon transmission coefficients in nano-phononic structures and diameter-modulated nanowires. Three key geometrical parameters including the size ratio of irregularity, the length of periodicity, and the number of irregularity have been studied in this work. The phonon transmission results in the three studies gave direction on how to achieve lower thermal conductivity in these nanostructures.
Resume : Thermal properties of nanostructures are of interest due to their applications in thermoelectric materials and energy harvesting. At small scales, the thermal properties of silicon nanostructures are significantly reduced compared to their bulk counterparts. In this light, Reverse Non-Equilibrium Molecular Dynamics (RNEMD) simulations are carried out on both silicon nanowires and silicon phononic crystals. Using the resulting RNEMD data, the thermal conductance and derived quantities are calculated using well known formulae. In addition, the thermal conductance results for different orientations of the phononic crystals are assessed. The thermal conductances of the nanowire systems are compared to bulk values for silicon and expected values for nanowires. Radial and length dependences of the thermal conductance for the nanowires are presented. The thermal conductances of the phononic crystals are compared to the results for nanowires of similar sizes, both in dimension and number of particles, from the previous simulation. Further comparison is done with nanowires of dimension similar to the phononic crystal structure to establish a relation between their thermal conductances.
Resume : We report calorimetric measurements of specific heat of surface-free zinc oxide nanowires (ZnONWs) and thermal conductance of interface between vertically aligned ZnONWs and silicon (Si) substrate. Heat-pulse calorimetric measurements were carried out on vertically aligned surface-free ZnONWs on Si substrate. The entire temperature response of the heat-pulse calorimeter was fitted to a model that takes into account the effect of the ZnONWs on the temperature rise and drop in a measurement cycle. Linear least squares method was used to determine the specific heat of the measured ZnONWs and the thermal conductance of the interface between ZnONWs and Si substrate from 1.8 to 300 K. It is found that at low temperatures (below 4 K) the ZnONW specific heat exhibits a clear contribution from an essentially two-dimensional crystal. From 4 up to 20 K, the ZnONW specific heat is lower than that of bulk ZnO. However, above 25 K, the specific heat of ZnONW exceeds that of bulk ZnO, and the enhancement factor increases as the temperature increases and the NW diameter decreases. The thermal conductance of the interface between ZnONWs and Si substrate is found to be orders of magnitude lower than that between bulk ZnO and Si substrate. Furthermore, the recorded data demonstrated transition between several regimes of phonon transport across the interface as the excited phonon wavelength changes.
Affiliations : 1 Instituto de Energía Solar & Dept. TFB. ETSI Telecomunicación, Universidad Politécnica de Madrid, Spain. 2 Instituto de Energía Solar & Dept. FAIAN, ETSI Aeronáutica y del Espacio, Universidad Politécnica de Madrid, Spain. 3 Catalonia Institute for Energy Research – IREC & ICREA, Barcelona, Spain.
Resume : High efficiency thermoelectric materials are characterized by high electrical conductivities and Seebeck coefficients and low thermal conductivities. In this work, we focus on Cu3SbSe4 and related compounds, which have become known as potential thermoelectric materials due to its excellent electrical transport properties, low thermal conductivity or cheap constituent elements. We explore electronic structures and thermoelectric transport properties for a set of doped-Cu3SbSe4 compounds at Sb site by means of Density Functional Theory (DFT) combined with Boltzmann transport theory. In addition, we also present a comparison between thermoelectric parameters obtained by using accurate DFT and experimental available data. Results here reported highlight the role of first principles calculations in the search of new materials with improved thermoelectric performance by modifying the electronic structure.
Resume : Control and improvement of interfacial thermal conductance between graphene nanosheets plays a crucial role with respect to the manufacturing of highly thermally conductive nanomaterials and devices. Chemical functionalization with organic molecules covalently attached to adjacent graphene sheets as a promising yet challenging approach for the enhancement of thermal conductance between them. This paper addresses the phonon transport and the thermal conductance through a range of different molecular junctions, including alkyl chains with variable length, aliphatic-aromatic structures and polyaromatic junctions. Phonon properties have been studied by Green’s functions phonon transport method through Density Functional based Tight Binding theory, implemented into DFTB package. The relation between lengths, mass, stiffness, and strain of the molecular junctions and the thermal conductance was found.
Resume : Recently, phonon relaxation dynamics in metallic nanostructures induced by hot electron relaxation has been explored very intensively by scientific community, because of its possible applications in many optoelectronic devices. The phonon excitation appears as one of relaxation stages of localized surface plasmons resonance (LSPR). LSPR relaxation typically consists of several steps: electron-electron (e-e) scattering, electron-phonon (e-p) coupling and phonon-phonon (p-p) coupling. Now days, the LSPR relaxation dynamics has been actively explored in noble metal nanoparticles and core-shell nanostructures. Transient absorption spectroscopy (TAS) was found to be an excellent tool to investigate phonon induced mechanical oscillation in these nanostructures. However, there are very few investigations related to LSPR relaxation dynamics in porous dealloyed metal film and porous nanostructures, despite TAS spectroscopy technique provides rich information about LSPR relaxation dynamics. We have observed and systematically studied e-p coupling and p-p coupling processes that resulted as energy transfer to Au lattice. Typical time intervals when the lattice heats up and it cools down were defined, i.e. ~1 ps and ~100 ps respectively. We demonstrate that ultrafast heating of the lattice leads to optomechanical oscillations driven by the formation of acoustic phonons. Deeper analysis of these acoustic phonons may impart information about the nanomaterial and its surroundings.
Resume : Nanostructuration of materials is a key-issue to tailor and tune their properties at macroscale with a lot of applications in electronic, energy, spintronic, etc. This is particularly true for electrons and phonons transport in semiconductors where a lot of efforts were done to synthetize nanostructures with properties radically different from their bulk counterpart. In the case of Ge-based semiconductors a lot of work was done on the direct tailoring of the material geometry (nanowires, phononic crystals, nanoporous materials, etc.) achieving for example materials with ultra-low thermal conductivity. Such a behavior can be also observed by doping Ge with transition metal like manganese. This has been done experimentally by molecular beam epitaxy coupled to thermal annealing; allowing the elaboration of crystalline Ge thin film with embedded MnGe spherical nano-inclusions. The latter structures exhibit low thermal conductivity that can be controlled through size and dispersion of the inclusions. Despite the important application interest, there are few theoretical work on the physical properties of the MnGe compounds, and especially for phonon transport properties. Here, lattice thermal conductivities of MnGe compounds with different MnGe stoichiometries are calculated by classical MD simulations with the use of Neural Network potential. The obtained results are compared with those of DFT calculations. Furthermore, the phonon transport in MnGe nanoinclusions embedded in Ge matrix and MnGe/Ge superlattices were also studied. In addition to this specific compound, obtained results and the proposed modeling will be also helpful for other type of materials with nano-inclusions.
Resume : Heat conduction in nanostructures is a topic of great interest for various applications as thermoelectricity or integrated circuits. At the nanoscale, heat transport may diverge from classical physics. In semi-conductors (Si, Ge, …), heat is mostly carried by phonons (lattice vibrations) and the thermal conductivity (TC) can be tuned with nanostructuration while preserving the electrical properties. Recently, new types of nanostructures, which consist of interconnected nanowires forming a 2D or 3D network, have been experimentally elaborated. In this work, the TC of such nanostructures is investigated in order to understand heat transport at the nodes (interconnections of the nanowires). The TC is calculated with Equilibrium Molecular Dynamics. The time integration is performed with Verlet algorithm and the interatomic forces are computed from the Stillinger-Weber potential. After an equilibration of 200 ps at 300 K under NVT ensemble, the TC is calculated from the Green-Kubo formula. TC variations depending on network period (distance between two nodes) and nanowire diameter were investigated. The TC increases with the diameter of the nanowire and decreases when the distance between the nodes increases. As the nanostructures are extremely porous (80-98%) and their surface-to-volume ratio is huge, the TC is often ultra-low (<1 W/m.K). Interestingly, the 3D mesh always has a lower TC than 2D mesh, despite its lower surface-to-volume ratio.
Authors : Israel González, Alejandro Trejo, Miguel Cruz-Irisson.
Resume : In the last years the investigations about Germanium nanowires (GeNWs) have intensified due to their intrinsic properties like extraordinary flexibility and high carrier mobility, which could be used in devices such as solar cells and FETs; however, for these applications the study of its properties with different surface passivations is crucial. Many theoretical studies have been developed for GeNWs, focused mostly on the study of its electronic properties and seldom on its vibrational properties, Raman and IR spectrums although the study of the vibrational spectroscopies and phonon properties would be important for the non-destructive characterization of materials and other properties such as heat capacity. Hence, in this work, the effects of different surface passivations on the vibrational properties, IR and Raman spectrums of GeNWs was studied using the DFPT and the supercell scheme. The nanowires are modelled by removing atoms outside a circumference from a perfect Ge crystal on the  direction and the surface was passivated with H atoms, and subsequently replaced by Cl and F. The results show that due to the higher mass of F and Cl, the expected shift of the highest optical vibrational mode and the Raman spectrum to lower frequencies is lower compared to the H passivated nanowires. These results could be important for the characterization of these structures and to explore the stability of different surface passivations for applications such as in microelectronics.
Resume : In the last years, the thermal conductivity of bulk and 2D samples have been calculated with a great level of accuracy thanks to first principles formalisms. From the results, it has been demonstrated that heat transport in some materials like graphene could be described by the appearance of a hydrodynamic behavior. The main ingredient in this case is that the assumption of phonon mode independence is destroyed, that is, each mode has no longer an independent mean free path (MFP) and group velocities. In 2D materials this effect is very large and can be easily observed from ab initio calculations, but in some materials this contribution can be more subtle and can remain hidden inside the calculations or even numerically removed. The Kinetic Collective Model (KCM) has been especially developed to understand the transition between these two regimes and provides the hydrodynamic contribution even in materials where these effects are not dominant. From our formalism, it can be shown that the apparent resistive role of normal scattering or the enhancement of the contribution of the optical modes in thermal conductivity are related to the appearance of a hydro-dynamic behavior emerged from the collective regime. The richness of this new framework provides new physical insight on heat transport from microscopic to macroscopic scales with impact in applications.
Resume : The thermal conductivity of a silica-gallium nitride nanocomposite is investigated by means of Scanning Thermal Microscopy and predicted using Molecular Dynamics Simulations. In this work we report on the effect of several geometric parameters, as the volume fraction of the nanoinclusions, the total interfacial area and the surface/volume ratio of the nanoinclusions on the thermal properties of the nanocomposite. Upon increasing the size and the volume fraction of the crystalline nanoinclusions, the thermal conductivity increases rapidly even if there are no interconnections among the crystalline islands and it can reach values up to 40% of the thermal conductivity of bulk crystalline GaN. On the contrary, increasing the number of nanoinclusions does not lead to enhancement of the thermal transport. These results are discussed and physical insights are presented.
Resume : Nanostructuring is a promising approach for next generation thermoelectric materials yielding ultra-low thermal conductivities and enhanced thermoelectric performance. More specifically, some of the lower thermal conductivities in nanocrystalline materials have been achieved in materials that include hierarchically sized structures, at the atomic size, the nanoscale, and mesoscale, which can scatter phonons of various wavelengths and reduce phonon transport throughout the spectrum. In this work, we describe the development of a large scale, comprehensive Monte Carlo simulator to model thermal transport in nanostructured materials with a large and arbitrary degree of hierarchical disorder. Geometry induced scattering of phonons on grain boundaries, surfaces, several defects, voids, and dislocations as in realistic nanocomposite which all contribute to reducing thermal conductivity, are investigated. Although this study focuses on Si-based materials, we discuss extensions of our simulator capabilities to other type of promising thermoelectric materials with various types of embedded nanoinclusions. For this, we discuss how we couple useful transport properties from molecular dynamics and ab initio calculations. We believe that this multi-physics/multi-scale approach could play a very useful part in optimizing thermal conductivity reductions not only in advanced new-generation thermoelectric materials.
Affiliations : Institute of Materials Chemistry, TU Wien, A-1060 Vienna, Austria.; LITEN, CEA-Grenoble, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France.
Resume : Since 2014 the layered semiconductor SnSe is known to be the most efficient thermoelectric material. At room temperature it crystalizes in the Pnma space group changing its phase at around 800 K to Cmcm. The character of the transition is not fully understood and the thermal conductivity of the high temperature phase has only been studied perturbatively. Using the Stochastic Self-Consistent harmonic Approximation (SSCHA) we show that it is a second-order structural phase transition driven by the anharmonic renormalization of the phonon spectrum. With the anharmonic force-constants calculated using SSCHA we show that non-perturbative effects are inherent of the system and yield a thermal conductivity in good agreement with experiments.
Resume : GeTe, a well-known ferroelectric and thermoelectric material, undergoes a structural phase transition from a rhombohedral to the rocksalt structure at ~600-700 K. We model this phase transition using density functional theory by minimizing the Helmholtz free energy using the elastic and quasi-harmonic approximations and Gruneisen theory. By accounting for up to the fourth order elastic constants and their temperature dependence, we obtain the temperature variation of the structural parameters of rhombohedral GeTe (the lattice constant, the angle between the primitive lattice vectors and the internal atomic displacement) in good agreement with experiment . From the calculated temperature dependence of the transverse optical (TO) mode, we extracted the critical temperature of 701 K and the critical exponent of 0.17, which are in good agreement with experiment . We find that the divergence of the thermal expansion coefficients near the phase transition in GeTe is induced by acoustic phonon coupling to soft TO modes.  T. Chattopadhyay et al, J. Phys. C 20, 1431 (1987)  E. F. Steigmeier and G. Harbeke, Solid State Commun. 8, 1275 (1970) This work is supported by Science Foundation Ireland PI Award 15/1A/3160.
Affiliations :  Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), 08193 Bellaterra, Spain.  Institut für Festkörperphysik, Technische Universität Berlin, Hardenbergstr. 36, 10623 Berlin, Germany.  Catalan Institute for Research and Advances Studies (ICREA), 08010 Barcelona, Spain.
Resume : We present a comprehensive study of the optical, vibrational, and thermal properties of Ga2O3 in the alpha-, beta-, gamma-, and epsilon-modification. The studied materials include beta-Ga2O3 in different crystal cut orientations grown by the Czochralski method, alpha-Ga2O3 and epsilon-Ga2O3 grown on sapphire, and epsilon-Ga2O3 grown on MgAl2O4 substrates. In addition, we also explore the influence of alloying Ga2O3 with Al in the whole compositional range. We will give a broad overview of the phononic structure of Ga2O3 polymorphs and alloys. The optical vibrational modes (Raman) of these structures will be discussed for different symmetries with special focus on addressing the components of the Raman tensor through polarized Raman measurements. I will discuss the temperature dependence of the thermal conductivity of several oxide polymorphs. The samples were investigated using the 3-omega method, although other state-of-the-art contactless experimental approaches based on Raman spectroscopy will be considered to address thermal anisotropy. The influence of size effects, composition, and temperature on the temperature conductivity reduction will be addressed for most cases.
Resume : According to our current understanding, thermal rectification in heterojunctions derives from the different temperature dependence of the thermal conductivity of the constituent materials. This view, however, neglects thermal boundary resistance, TBR, that plays a central role in nanoscale systems. Here we show that in nanoscale systems the physical mechanisms that result in heat rectification are more complex and include an active role of the interface through the temperature dependence of its TBR. In these conditions a suitably designed resistance mismatch, at odds with the matching requirement for bulk systems, can lead to a large heat rectification, exceeding the typical values obtained in bulk systems. Besides it can be much larger, it can even change sign with respect to the cases in which it can be neglected. We show, with theoretical calculations of a Si/GaP and a graphene/bilayer graphene heterojunction, that whenever the TBR of a heterojunction is large compared to the overall thermal resistance, a common situation in nanoscale systems, its temperature dependence can be exploited to engineer an interface-driven heat rectification. This effect is amplified in a suitable resistance mismatching condition, which results in very different interface temperatures, and thus TBR, under forward and reverse thermal bias. This is at odds with the design strategy of a bulk thermal diode, where the resistance matching condition maximizes the heat rectification. These findings reveal a new physical mechanism uderlying thermal rectification in nanoscale systems and open the way to a more flexible design of efficient thermal diodes.
Resume : When the characteristic length is shorter than the phonon mean free paths in nanostructured materials, the interface thermal resistance (ITR) will play a dominant component of the total thermal resistance. There are lots of factors affect the ITR and make the ITR prediction become a mathematical high dimensional problem. Here, we demonstrated by both experiments and data science approach an alternative strategy for achieving thermal insulating nanostructure. The BiSi system was selected from the machine learning prediction model and manufactured with various parameters by laboratory-built combinatorial sputtering system. The BiSi nanostructure is composed of crystallized Bi and amorphous Si without any additional phase. The Bi grain sizes, Bi/Si interface and Si/Bi volumetric ratio in the BiSi nanostructure have major effects on the thermal resistance. The ideal parameter range of Bi/Si interface design for the thermal insulating nanostructure is defined and an extremely low thermal conductivity, 0.21 W/mK, measured by frequency-domain thermoreflectance (FDTR) in a dense material is achieved. The power of data science in materials innovation is limited by data availability and reliability. “Data science + physics and chemistry “is a solution to obtain maximum effect with minimum data. By means of the machine learning prediction model, the potential systems for specific thermal management and application such as thermoelectric materials can be discovered and realized.
Resume : Elastic strain engineering utilizes stress to realize unusual material properties. For instance, strain is used to enhance the electron mobility in semiconductor thin films. In the context of nanomechanics, the pursuit of ultra-coherent resonators has led to intense study of ?dissipation dilution?, a technique where the stiffness of a material is effectively increased without added loss. Dissipation dilution causes the anomalously high quality factor of thin-film Si3N4 nanomechanical resonators; however, the paradigm has so far relied only on the strain produced during material deposition. Geometric strain engineering techniques?capable of producing local stresses near the material yield strength?remain largely unexplored. Here, we will present a spatially non-uniform phononic crystal pattern, used to co-localize the strain and flexural motion of a stoichiometric silicon nitride nanobeam, while increasing the former to near the yield strength. This combined approach produces string-like modes with Qf products approaching 10^15 Hz, exceeding previous values for a room-temperature mechanical oscillator of any size. The devices we have realized can have force sensitivities of aN/rtHz perform hundreds of quantum coherent cycles at room temperature, and attain Q>400 million at megahertz frequencies.
Resume : The capability to tune the acoustic phonon dynamics in technologically relevant group IV nanostructures provides a promising prospect to control the propagation of acoustic and thermal phonons with great implications on nanoscale hypersound and thermal transport. Despite their fundamental importance, accurate measurements of acoustic phonon lifetimes are challenging and their values are still unknown for most materials. Even in the case of the extensively studied group IV semiconductors, measurements of phonon lifetimes are scarce and the impact of strain engineering is not well understood. In this work, we address the influence of tensile stain on the thermo-mechanical properties of suspended Ge nano-bridges with thicknesses below 100 nm. It is shown that the acoustic phonon lifetimes can be tuned both by strain engineering of the suspended structures and strain modification by temperature variation in addition to a strong dependence on the thickness of the suspended structures . The combination of temperature dependent micro-Raman and femtosecond reflectivity measurements allows for a complete decoupling of the effects of temperature, geometry, and strain on the acoustic phonon dynamics [2, 3].  J. Cuffe et al., Phys. Rev. Lett. 110, 095503 (2013).  S. Neogi et al., ACS Nano 9, 3820 (2015).  M. R. Wagner et al., Nano Letters 16, 5661 (2016).
Resume : Progress in the last few decades in nano-scale thermal transport has enabled a significant degree of control over heat and sound propagation by lattice vibrations - phonons. The latest investigations on the thermal properties of silicon, the most common material in electronics, micro- and nano-electro-mechanical systems (MEMS and NEMS) and photonics, have pointed to nanostructuring as a highly efficient approach to acoustic phonon engineering [1-3]. Heat conduction in silicon can be effectively engineered by means of sub-micrometer porous thin free-standing membranes [1,4]. Tunable thermal properties make these structures good candidates for integrated heat management units such as waste heat recovery, rectification or efficient heat dissipation. However, possible applications require detailed thermal characterization at high temperatures which, up to now, has been an experimental challenge. In this work we use the contactless two-laser Raman thermometry to study heat dissipation in periodic porous membranes at high temperatures via lattice conduction and air-mediated losses [4,5]. We find the reduction of the thermal conductivity and its temperature dependence closely correlated with the structure feature size. Based on two-phonon Raman spectra, we attribute this behavior to diffuse (incoherent) phonon-boundary scattering. Furthermore, we investigate and quantify the heat dissipation via natural air-mediated cooling, which can be tuned by engineering the porosity. References: 1. M. Maldovan, Nature 503, 209 (2013). 2. M.R. Wagner, B. Graczykowski, J.R. Reparaz et al., Nanoletters 16(9), 5661–5668 (2016). 3. B. Graczykowski, M. Sledzinska, F. Alzina et al., Physical Review B 91, 075414 (2015). 4. B. Graczykowski, A. El Sachat, J.R. Reparaz et al. Nature Communications 8, 415 (2017). 5. J. Reparaz, E. Chavez-Angel, M. Wagner et al., Review of Scientific Instruments 85, 034901 (2014).
Resume : Nanoscale phononic crystals can be used in many areas including high-frequency signal processing and the control of thermal phonons. Technological applications of phononic crystals require in many cases a precise control of the vibrational band structure. Particularly important is the ability to control the frequency and width of vibrational band gaps. In this work, molecular dynamics simulations and finite element method calculations are used to study the vibrational band structures of nanoscale two-dimensional phononic crystals made from silicon. A comparison of results obtained with both methods shows that a simple two-dimensional linear-elasticity model is able to reproduce quantitatively the low-frequency part of the band structure obtained from computationally much more demanding molecular dynamics simulations of a three-dimensional atomistic model. The linear elasticity model is then used to study the effects of lattice symmetry, mass density distribution and grain boundaries on the vibrational band structures. The results show that these mechanisms affect the two lowest phononic band gaps in different manners. This makes it possible to control the frequency and width of the two band gaps independently.
Resume : We characterize nanometer-thin suspended silicon membranes in the pristine state or perforated in phononic crystals with periods below 100 nm, by means of scanning thermal microscopy (SThM) and Raman thermometry (RTh). Spatial resolution of SThM is shown to be similar to the one of RTh on these samples, once the air contribution is eliminated. Temperature distributions along the membranes are obtained, but require precise determination of the optical absorption for RTh as the membranes are also photonic objects. We find that the techniques are sensitive to cross-plane and in-plane heat conduction, and that the native oxide on both sides of the membranes cannot be neglected. The perforation depth of the holes is also a key quantity. For instance, the reduction of thermal conductivity in comparison to bulk silicon is 2.5 for a pristine membrane of 60 nm thickness, and limited to 6 for moderately-perforated arrays with pitch equal to thickness. The results are analyzed in light of phonon mean free path computations in complex geometries.
Resume : Doubly-clamped pre-stressed silicon nitride string resonators excel as high Q nanomechanical systems enabling room temperature quality factors of several 100,000 in the 10 MHz eigenfrequency range. Dielectric transduction is implemented via electrically induced gradient fields, providing an ideal platform for actuation, displacement detection and frequency tuning. The two orthogonal fundamental flexural modes of a single string vibrating in- and out-of-plane with respect to the sample surface can be engineered to tune reversely. This allows bringing both modes into resonance where a pronounced avoided crossing is observed, indicating strong mechanical coupling and the formation of a mechanical two-mode system. Here we focus on the nonlinear dynamics of the string subject to a strong resonant or parametric actuation, and discuss the string’s response both far from the avoided crossing where the individual eigenmodes can be considered, and in the coupling region where the normal modes of the two-mode system have to be taken into account. Depending on the actuation amplitude and frequency, satellite resonances arise which enable deep insights into fundamental properties of the system.
Resume : Acoustic phonons in the sub-THz range have emerged as a suitable platform to study complex wave physics phenomena. This has spurred the development of a large bandwidth of versatile nanophononic devices for full control and manipulation of phonons on the few-nm length scale. Furthermore, the strong interactions between acoustic phonons and other excitations in solids extend the range of applications for nanophononic devices into other areas of research such as electronics and optomechanics. For example, recent advances in material science and fabrication techniques enabled the fabrication of nanometric devices in which photons (VIS-NIR) and phonons (GHz-THz frequencies) are simultaneously confined in a single resonant cavity giving rise to unprecedented large optomechanical coupling factors. In addition, the engineering of acoustic waves with GHz-THz frequencies is also at the base of the study of mechanical quantum phenomena and non-classical states of mechanical motion. In this work I will first introduce and compare strategies to generate, manipulate and detect ultra-high frequency acoustic phonons using either ultrashort laser pulses or high resolution Raman scattering. Second, I will describe the acoustic behavior of standard nanophononic Fabry-Perot resonators and finally present experimental and theoretical results on a series of novel nanomechanical devices able to control the interactions between light, sound and charge at the nanoscale.
Resume : In nano-phononics experimental studies and applications, features related to the phonon transport are entangled with other phenomena also due to the strongly confined heat sources which are needed to activate phonon transport. In order to correctly design devices and control experiments accurate modelling is needed, especially when the reference systems are complex 3D structures composite by ~nm wide objects with different shapes and made of different materials. We present a computation tool, which we have developed for the simulation of phononic systems' features. The code is designed to efficiently simulate 3D structures with TCAD capabilities for the design of the system. Coupling between system's regions ruled by advanced transport models (derived by the Boltzmann transport equation) and regions ruled by the conventional Fourier law is considered. Heating can be also evaluated by means of self consistent solutions of the Maxwell equations where optical constants are functions of the local average phonons' energy (temperature in quasi-equilibrium conditions). A preliminary calibration is provided for given materials/phases. Applications of the simulations will be discussed in order to demonstrate the potentiality of the method.
Affiliations : (1) Instituto de Investigaciones en Materiales, Universidad Nacional Autonoma de Mexico, Mexico; (2) Departamento de Fisica, Facultad de Ciencias, Universidad Nacional Autonoma de Mexico, Mexico.
Resume : The direct conversion between thermal and electrical energies by thermoelectric devices is becoming an important alternative for the clean energy generation. Nanowires seem to be promising candidates, whose efficiency is determined by the dimensionless thermoelectric figure-of-merit (ZT) that can be calculated by using the Boltzmann formalism . The inherent correlation between the thermoelectric quantities, such as electrical and thermal conductivities, makes difficult to improve the value of ZT. In this work, we study the thermoelectric properties of nanowires with periodic and quasiperiodically placed Fano defects  by means of a real-space renormalization plus convolution method developed for the Kubo-Greenwood formula , in which tight-binding and Born models are respectively used for the study of electric and lattice thermal conductivities . The results reveal an enhanced ZT when the quasiperiodicity is introduced, because it diminishes the thermal conduction of long wavelength acoustic phonons that are not easy to block their transmission since they do not feel local defects neither impurities. This work has been supported by CONACyT-252943 and UNAM-DGAPA-IN106317. Computations were performed at Miztli of DGTIC-UNAM.  J. E. Gonzalez, V. Sanchez, C. Wang, J. Electron. Mater. 46, 2724 (2017).  V. Sanchez, C. Wang, Phil. Mag. 95, 326 (2015).  V. Sanchez, C. Wang, Phys. Rev. B 70, 144207 (2004).  C. Wang, F. Salazar, V. Sanchez, Nano Lett. 8, 4205 (2008).
Resume : One intriguing feature of van der Waals materials is the layer thickness and misorientation angle dependence that involve stark optical gain and electrical transport modulation. Yet, the phase transformation modulated by the misorientation angle has never been accessible to date. Here, we report misorientation angle-dependent phase transformation of multilayer MoS2 via in situ electron beam irradiation. AA’ stacked-bilayer MoS2 undergoes structural transformation from 2H semiconducting to 1T’ metallic phase similar to monolayer MoS2, which is confirmed via in situ transmission electron microscopy. Meanwhile, non-AA’ stacking which has no local AA’ stacking order in Moire pattern does not reveal such a phase transformation. While collective sliding motion of chalcogen atoms easily occurs during transformation in AA’ stacking, such a collective motion in non-AA’ stacking is suppressed by weak van der Waals strength and furthermore by the interlocked chalcogen atoms at different orientations, which unfavor their kinetics by the increased entropy of mixing.
Resume : Silicon nanowires have attracted high attention for their possible application in nanoelectronics and thermoelectricity. Because of phonon boundary scattering, thermal transport in such structures strongly depends on the dimensions and the surface state (roughness) and is still not fully understood. With means of Non Equilibrium Molecular Dynamics and Monte Carlo resolution of Boltzmann Transport Equation, we studied the radial evolution of the heat flux inside silicon nanowires. It is found with both methods that the heat flux is maximum at the center of the nanowire, while it decreases when reaching the exterior surfaces of nanowires. Surprisingly, this phenomenon does not depend on the diameter of the nanowire as long as d < 500nm. For macroscopic diameters, the heat flux becomes constant as boundary scattering weakly affect thermal transport. Phonon Density of States (DOS) were computed by Molecular Dynamics for different regions in the structure. DOS close to the free surface is found to differ from the one at the center of the nanowire, revealing more populated modes at lower frequencies.
Affiliations :  Catalonia Institute for Energy Research (IREC) Jardins de les Dones de Negre 1, 08930, Sant Adrià del Besòs, Spain;  Catalan Institution for Research and Advanced Studies (ICREA) Passeig Lluís Companys 23, 08010, Barcelona, Spain.
Resume : The high cost and complex production techniques restrict the use of the conventional thermoelectric (TE) generators. Providing a low-cost TE device with a respectable performance is even more important than providing a very high-performance TE device which would cost an unexpected price. Silicon meets the criteria of abundance and non-toxicity, but it is characterized by a poor TE efficiency with a low figure of merit, ZT. We have established a versatile, cost-effective, reproducible and scalable synthesis route for the fabrication of a thermoelectric metamaterial based on nanostructured fibres. The architecture developed is self-supported and can easily be moulded to any shape. For p-silicon gives very low thermal conductivity due to the influence of phonon boundary scattering at the surface which leads to a figure of merit as high as 0.34 at T=823K. The extension to other materials is also possible and will be shown. We finally propose cost-efficient and environment-friendly TE devices for middle and high operating temperatures by harnessing silicon and silicon technology. By using the silicon-based thermoelectric metamaterial, we greatly reduce the cost production of the thermoelectric generator module.
Resume : Silicon nanowires are promising candidates for thermoelectric harvesting since they combine an easy integration in electronic devices with an enhancement of thermoelectric performance conferred by nanostructuring. The measurement of the thermoelectric figures of these nanostructures is a complex task, requiring sophisticated strategies to obtain rigorous results. In this work, Si nanowires were integrated in different planar micromachined structures. Vapour-Liquid-Solid synthesis in Chemical-Vapour-Deposition reactors (CVD-VLS) was used to grow the nanowires, using gold particles as seeds. This bottom-up approach allows fabricating doped epitaxial nanowires with controlled properties (diameter, length, doping level and alloy composition) fully integrated in silicon microdevices, i.e. monolithically grown with low contact resistances. Different thermoelectric characterization silicon micromachined structures were used for the assessment of the thermal and electrical properties of the single nanowires and arrays. As a result, it was possible to determine ZT of a series of individual suspended nanowires from room temperature up to 350°C. The epitaxial nature of the growth allows overcoming the possible artefacts coming from contact resistances. The results arisen from this study demonstrate the feasibility of a practical implementation of this technology in integrated thermoelectric power generators.
Resume : Thermoelectric performance of Ag nanoparticle dispersed Indium selenide bulk materials Pallavi Dhama, Aparabal Kumar and P. Banerji Materials Science Centre, Indian Institute of Technology, Kharagpur 721302, India Direct conversion of thermal energy to electrical energy can be realised through the transmission and interaction of charge carriers and phonons in the solid by using thermoelectric effect. To tailor the thermoelectric properties of In4Se3, concept of energy filtering of the charge carriers applied in the present work. The energy barriers at the grain interfaces of the Ag and In4Se3 are supposed to control carrier scattering. Polycrystalline In4Se3:Ag samples were prepared by solid state reaction, ball milling and spark plasma sintering at 693 K for 5 min with a pressure of 70 MPa under vacuum condition. Structural characterizations were performed to study the phase purity, crystal structure and microstructure. To study thermoelectric properties of all the samples, temperature dependent electrical resistivity, Seebeck coefficient, Hall and thermal conductivity measurements were performed in the temperature range of 300 K – 700 K. All samples had negative thermopower which indicated that electrons transported the electric charge. In4Se3:Ag samples showed increased thermopower and decreased thermal conductivity which was attributed to the Ag nanoparticles as an effective phonon scattering centres.
Affiliations : School of Engineering and Materials Science, Queen Mary University of London, Mile End Road, London E1 4NS, United Kingdom.
Resume : Optimisation of thermoelectric materials requires a process of structural tuning in the nano/meso-scale regime to reduce thermal conductivity with minimal effect on electrical properties. Here we use in-plane thermal conductivity measurements which are particularly sensitive to thin film morphology, and apply this to several materials including conductive polymers and halide perovskites. Halide perovskite films of various thicknesses and morphologies were formed by thermal co-evaporation of methyl ammonium iodide with lead chloride. Despite an intrinsically low thermal conductivity in this material, we measure a sizeable Kapitza resistance (thermal boundary resistance) and find a further reduction in thermal conductivity when grain sizes are in the range 10-100nm. I will also present our results on anisotropic thermal conductivity in highly aligned polymer films, using doped polymers of interest for thermoelectric applications. An asymmetry in thermal conductivity of ~2 is observed between the directions parallel and perpendicular to the chain alignment, with implications for using highly oriented polymer films for thermoelectric applications.
Resume : Thermoelectricity is one of the best alternative sustainable energy sources to fulfill the global demand for energy with a rapidly increasing population as it converts waste heat into electricity. In the present work, Ge doped Cu3SbSe4 samples (Cu3Sb1-xGexSe4, x = 0 to 0.05) were made by melt growth, ball milling and spark plasma sintering. Structural and microscopic characterizations such as XRD, SEM and TEM were carried out to analyze the phase purity, crystal structure, crystallite size, morphology and microstructure of the samples. Electrical conductivity, thermal conductivity, and Seebeck coefficient measurements were performed in the temperature range 300 - 650 K. Enhanced power factor in Ge doped samples is attributed to the increased electrical conductivity via the increase in carrier concentrations while low thermal conductivity is due to the increased phonon-phonon interactions. Moreover, presence of nanograins in Cu3SbSe4 enhance the scattering of phonon by grain boundaries and defects. Transport mechanism involved in all the samples are explained well within the limit of acoustic phonon scattering approximations with the single parabolic band model. Our results show that Ge doping significantly increases the figure of merit (ZT) of Cu3SbSe4 due to low lattice thermal conductivity and high power factor with maximum ZT (~0.78) in Cu3Sb0.98Ge0.02Se4 at 625 K. Thus, the Ge doped p-type Cu3SbSe4 is a suitable material for green energy application.

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