The model system offered by state-of-the-art free-standing ultra-thin silicon membranes [1] is used to study the contribution of acoustic phonons to the thermal conductivity in silicon with view to its applications as a thermoelectric material. Our understanding so far is based on measured and modelled acoustic phonon dispersion relations [2], their lifetime [3], the transport in the diffusive and ballistic regimes [4], the role of the native oxide [5], the role of 2-D phononic crystals in the membranes [6] and direct in-plane thermal conductivity measurements based on laser-Raman thermometry [7-8].
The dispersion relations of membranes of thickness down to 8 nm are measured and simulated and found to be discretised flexural and dilatational modes which deviations from the linear behaviour close to q=0. The group velocity is obtained from these measurements and shown to decrease by a factor larger than 20. The lifetimes of the experimentally accessible first order dilatational mode is observed to decrease over one order of magnitude compared to bulk values. The data are used to model the thermal conductivity using a modified Akhieser mechanism incorporating the discrete and projected acoustic modes [9] and are compared to measurements based on thermal decay of a hot spot in the membranes. Boundary scattering and normal processes are found to contribute to the thermal transport as well as the native oxide.
We will discuss our results in the light of thermodynamic models, non-equilibrium molecular dynamics calculations and the modified Akhieser approach modified to include the interplay of confinement and real surfaces and interfaces.
[1] A. Shchepetov et al., Appl. Phys. Lett. 102 192108 (2013).
[2] J. Cuffe et al., Nano Lett., 12 3569 (2012).
[3] J. Cuffe et al., Phys. Rev. Lett. 110 095503 (2013).
[4] J. A. Johnson et al., Phys. Rev. Letts. 110 025901 (2013).
[5] S. Neogi et al., under review, ACS Nano.
[6] B. Graczykowski et al., Phys. Rev. B 91 075414 (2015).
[7] E. Chavez Angel et al., Appl. Phys. Lett. Materials 2 012113 (2014).
[8] J. S. Reparaz et al., Rev. Scien. Instruments 85 034901 (2014).
[9] E. Chávez-Ángel et al., Semicond. Scie. Tech. 29 124010 (2014).

Two-dimensional (2D) materials, such as graphene, molybdenum disulfide (MoS2), and hexagonal boron nitride (h-BN), have attracted extensive research because of their unique electronic and thermal properties. However, when in contact with other materials, the intrinsic properties of these 2D materials are modified by interface interactions. For example, the thermal conductivity of graphene is reduced by ~10 times or more when it is supported or encased by SiO2 and the addition of a monolayer graphene would significantly suppress the heat flow across typical metal-dielectric contacts. Therefore, it is important to understand how interfaces affect both in-plane and cross-plane heat transfer in 2D material systems. Here, we demonstrate simultaneous measurements of thermal conductivity and thermal boundary conductance of graphene, MoS2, and h-BN on various surfaces such as Mica, PET and glass, using frequency domain thermoreflectance. We investigate the effect of surface morphology on heat transfer in 2D materials. Our results have implications for phonon interactions in 2D material systems, and suggest that locally modified surface morphology could be used to optimize thermal transport in devices based on 2D materials.

Understanding thermal transports of thin films in the nanometer range is essential for both fundamental and applied perspectives of modern electronic devices. While there is extensive experimental work on lateral (in-plane) heat transport in thin films, perpendicular (out-of-plane) thermal transport is far less explored. [1,2].
By means of dynamic Scanning Near-field Thermal Microscopy (SThM), we evidence experimentally for the first time the transition from diffusive to ballistic out-of-plane heat transport through ultra-thin metal-oxide films, prepared by atomic layer deposition. Using the 3omega;-technique [3], dynamic probe temperature variations (ΔT) are measured for thin films (down to 10 nm) of Al₂O₃, TiO₂, and Al₂O₃/TiO₂ nano-laminates, in reference to Si-substrates.
The properties of diffusive dynamic heat transport will be discussed from the engineering point of view taking equivalent electrical circuits into account. The measured ΔT between probe and substrate saturates with increasing film thickness and can be fitted with an exponential function [4]. We did not find any change in the out-of-plane thermal conductivity for a film thickness larger than 100 nm.
For a film thickness below 100 nm, a description considering the probability of phonon-scattering within the film is found by the phonon Boltzmann transport equation from the ballistic to diffusive transport limits [5,6]. This model will be shown to fail once the Casimir thickness limit of the respective material is reached and a ballistic heat transport of blackbody phonon radiative transfer has to considered [4].
In this work, we provide experimental evidence for this ballistic transport for the first time. A mean free path length (l) of 25 nm, 22 nm, and 17 nm, in the Al₂O₃, TiO₂, and Al₂O₃/TiO₂ nano-laminate, is determined, respectively. Thus, for a thickness < l, a frequency independent ΔT is found [7], which depends on the film material. The out-of-plane thermal conductivity decreases linearly as the film thickness is reduced [8,9].
The decrease of thermal conductivity with decreasing film thickness has motivated the claim of thin films being thermal insulators. In view of our results this concept has to be revised, as thin films are very well able to transport high amounts of thermal energy due to ballistic out-of-plane heat transport.
[1]Marconnet A.M. et al., Journal of Heat Transfer 135, (2013), 061601
[2]Gomes C.J. et al., Journal of Heat Transfer 128, (2006), 1114-1121
[3]Cahill D.G. and Pohl R.O., Phys. Rev. B 35, (1987), 4067-73
[4]Heiderhoff R. et al., Microelectronics Reliability 53, (2013), 1413-1417
[5]Majumdar A., Journal of Heat Transfer 115, (1993), 7-16
[6]Maassen J. and Lundstrom M., J. Appl. Phys. 117, (2015), 135102
[7]Lee S.-M. and Cahill D.G., J. Appl. Phys. 81, (1997), 2590-2595
[8]Yamane T. et al., J. Appl. Phys. 91, (2002), 9772-9776
[9]Feng X.-L. et al.,” Microscale Thermophys. Eng. 7, (2003), 153-161

Silicon is currently the major semiconductor material for ICT and will probably sustain the Moore&’s law the coming decades in the integrated circuits and memories. As thermoelectric material silicon has suffered from the relatively high thermal conductivity, preventing successful exploitation of silicon in energy harvesting and in thermal management, i.e., like cooling. The recent experimental and theoretical results have shown that in ultra-thin silicon membranes the propagation of phonons can be largely blocked while maintaining the good electrical properties, high conductivity and Seebeck coefficient.[1]
In this presentation we will discuss the potential of silicon membranes as thermoelectric material in the light of the recent findings of controlling the behaviour of phonons in the nanoscale membranes. The simulations show that cooling of several tens of degrees from the room temperature is possible with the membrane devices.
[1] S. Neogi, J. S. Reparaz, LF C. Pereira, B. Graczykowski, M. R. Wagner, M. Sledzinska, A. Shchepetov, M. Prunnila, J. Ahopelto, C. M. Sotomayor-Torres, and D. Donadio, Tuning Thermal Transport in Ultrathin Silicon Membranes by Surface Nanoscale Engineering, ACS Nano 9 (2015) pp 3820-3828.

The unusual properties of phonons in layered materials have attracted much attention in recent years. In the past few years graphene has served as a prototype material for phonon studies despite the unusual properties of the electron-phonon interaction in graphene. Whereas the electronic properties of layered compounds differ greatly between graphene, hexagonal boron nitride, the various metal dichalcogenides, and black phosphorene, the phonon properties are far less diverse. However, the diversity between the different materials&’ phonon properties nevertheless gives opportunities for different applications for the various layered materials, which will be the focus of this talk.

Hexagonal BN (h-BN) displays a preferential phonon transport along the a-axis with reported in-plane thermal conductivity as high as 390 W.m^-1.K^-1, while its c-axis conductivity remains low, in the range of 2 W.m^-1.K^-1. In comparison, the thermal conductivity of amorphous BN (a-BN) is in the order of 1 W.m^-1.K^-1. The anisotropic physical properties of h-BN and in particular of its high in-plane thermal conductivity originate from the orientation of its hexagonal structure, and consequently, the ability to control the orientation of these basal planes would unveil opportunities for applications requiring directional phonon transport.
Here, we present a novel way of controlling the growth of h-BN nanocrystals with preferential vertical orientation (as opposed to h-BN with horizontal alignment) and study its cross plane thermal conductivity. Vertically self-ordered Nanocrystalline Boron Nitride (ordered BN) is a highly ordered material similar to hexagonal BN, with its planar structure perpendicularly oriented to the substrate. The ordered BN thin films were grown using a High Power Impulse Magnetron Sputtering (HiPIMS) system with a lanthanum hexaboride (LaB₆) target reactively sputtered in nitrogen gas. Growth parameters were optimized to get the best vertical alignment of the BN planes and the effects of that preferential ordering on the film&’s thermal conductivity where studied using Time-Domain Thermoreflectance (TDTR). Owing to the intrinsic capability of h-BN to facilitate the propagation of phonons along its basal planes, we observed a significant increase in the cross plane thermal conductivity of ordered BN with a 5-fold increase (5 W.m^-1.K^-1) compared to amorphous BN.
The ability to direct phonon transport vertically with oriented BN, combined with its low electrical conductivity and low dielectric constant makes it the perfect candidate for top thermal dissipation on electronic devices. Its highly anisotropic heat transfer property could be used to create directional heat guides, where the phonon would propagate along the hexagonal planes straight to a heat sink.

Black phosphorus (BP), the stable allotrope of phosphorus at ambient temperature and pressure, has attracted great interest as a novel two-dimensional electronic material. It has a puckered honeycomb structure, which leads to strong in-plane anisotropy in transport properties along the two high-symmetry in-plane directions. We measured the thermal conductivities of BP along the three axes of the orthorhombic crystal at room temperature using conventional time-domain thermoreflectance (TDTR) and beam-offset TDTR. The BP samples are in the form of exfoliated flakes, ~500 nm thick, and encapsulated by a few nm of alumina deposited by atomic-layer deposition. The thermal conductivities are 86±9 W m^-1 K^-1 in the zigzag direction, 35±5 W m^-1 K^-1 in the armchair direction, and 3.4±0.4 W m^-1 K^-1 in the through-plane direction. The longitudinal speed of sound is 5.0±0.3 nm ps^-1 in the through-plane direction.

Successful thermal management is essential for designing thermoelectric cells, next generation integrated circuits and three-dimensional electronics.^[1] While the emerging class of 2D materials shows promise in their unprecedented carrier mobility, their potential tunability in thermal transport remains to be explored. To accelerate this process, multi-scale simulations have been implemented to design novel nanostructures to control both electronic and thermal properties. In particular, a recent theoretical study suggests that appropriate chemical functionalization may lead to efficient graphene-based thermoelectric materials.^[2]
Here we consider the prospect of a chiral architecture based on 2D materials, using a prototype system of a graphene nanoribbon (GNR) wrapping around a Si nanowire. The phonon thermal conductivity was calculated by non-equilibrium classical molecular dynamics, while the electronic properties were computed by Density functional theory and a Boltzmann transport approach. The results indicate that the phonon thermal transport may be modified by self-interaction between neighboring turns with the electronic properties nearly unaffected, due to the different characteristic lengths of phonon and electron. On the one hand, the attraction between turns of GNR with its edge passivated by H induces long-range disorder along the spiral axis, which suppresses the thermal contribution from phonons with long wavelengths, and thus leads to anomalous length independent phonon thermal transport in the quasi-1D system. According to our simulations, this may provide substantial improvement in the thermoelectric efficiency compared to planar structures. On the other hand, with appropriate organic ligands, the phonon thermal conductivity may be enhanced compared to the planar architecture, due to the additional transport paths through ligands that are close to or inter-connected to each other. Such effects may be beneficial for producing nanosolenoids for magnetic applications.
[1] Balandin, A. A. et al., Nat. Mater., 2011, 10, 569.
[2] Kim, J. et al., Nano Lett., 2015, 15, 2830.

Thermal properties of graphene reveal some unique features related to the specifics of the two-dimensional (2-D) phonon transport [1-2]. It is expected that phonon scattering due to point defects in 2-D systems may also differ from that in bulk crystals. In this presentation we will report the results of our investigation of thermal conductivity of suspended single layer graphene with defects introduced using the low-energy electron beam irradiation. For this study, the chemical vapor deposition (CVD) grown graphene was suspended over gold transmission electron microscopy (TEM) grid. The Raman D to G peak ratio was used to quantify the amount of defects after each electron beam irradiation step. The samples were exposed to the low energy (20 KeV) electron beam for a certain period of time in several steps, to obtain a certain defect density. For each step, the thermal conductivity was measured using the optothermal Raman technique [1]. For these measurements the gold TEM grid acted as a heat sink. The suspended graphene grains were illuminated at the center with laser light that provided local heating. The temperature rise was determined from the temperature shift of the Raman G peak of SLG. The thermal conductivity was obtained by solving the heat-diffusion equation with the help of COMSOL package. The thermal conductivity decreased with the increasing defect density. At the defect density of 1.5 × 10¹¹ cm^-2 we observed a change of the slope followed by the saturation behavior in the thermal conductivity. The non-monotonic decrease can be related to the interplay of the point defect and grain boundary scattering appearing due to defect clustering. The exact mechanism is currently under investigation.
The work at UC Riverside was supported, in part, by the National Science Foundation (NSF) project ECCS-1307671 Two-Dimensional Performance with Three-Dimensional Capacity: Engineering the Thermal Properties of Graphene.
[1] A.A. Balandin, Nature Materials, 10, 569 - 581 (2011).
[2] D.L. Nika and A.A. Balandin, Journal of Physics: Condensed Matter, 24, 233203 (2012).

Only recently has the prediction of the thermal conductivity on the basis of the first principles calculations become possible. Implementation of the methodology, however, is technically challenging and requires a lengthy development process. We thus introduce the Phonon Transport Simulator (PhonTS), a Fortran90, fully parallel code to perform such calculations. PhonTS possesses a large array of options and returns the thermal conductivity tensor together with related quantities, such as spectral thermal conductivity, phonon lifetimes, mean free paths and Grüneisen parameters. It can also determine the Raman and infra-red response of materials. First principles calculations are implemented via convenient interfaces to widely-used third-party codes, such as VASP, while many classical potentials are included in PhonTS itself. The code is carefully validated against data published in the literature from various thermal conductivity computational techniques and against experimental data. Examples of insights obtained from PhonTS are given, including fluorite-structured materials and Mg₂X (X=C, Si, Ge, Sn and Pb) family.

Colloidal nanocrystals consist of an inorganic crystalline core with short ligand molecules bound to its surface. These nanocrystals can be made with excellent control over size, shape, and composition, thereby enabling the exploration of relationships between microstructure and thermal transport. Furthermore, these nanomaterials are made via inexpensive solution-phase processing and can be easily transformed into nanoparticle-in-matrix composites. The excellent microstructural control and inexpensive processing of these materials make them an attractive candidate for thermal properties exploration.
In the first part of this talk, we discuss thermal transport in PbS nanocrystals that are assembled into thin films. We systematically study the effect of nanocrystal diameter (3 - 9 nm) and surface chemistry (oleic acid, ethanethiol, ethylamine, and tetrabutylammonium iodide). Our measurements find that these colloidal nanocrystal films have very low thermal conductivities ranging from 0.1 - 0.5 W/m-K. We find that larger nanocrystals and shorter ligands lead to higher thermal conductivities. Importantly, we find that surface chemistry can have a larger impact on thermal conductivity than nanocrystal size.
In the second part of this talk, we use a modular process to create nanoparticle-in-matrix composites and then study the corresponding composite thermal conductivity. We create the composites by mixing soluble metal-chalcogenide precursors with colloidal nanocrystals in the solution-phase. We then thermally transform the metal-chalcogenide precursors into a polycrystalline matrix that encapsulates the nanocrystals. Specifically, we focus on polycrystalline In₂Se₃ matrices with varying volume fractions of embedded CdSe nanocrystals. We find that the thermal conductivity of these composites is very low over the entire nanocrystal volume fraction range, on the order of 10^-1 W/m-K. We also find that the presence of CdSe nanocrystals strongly effects the formation of the In₂Se₃ matrix (i.e. grain orientation and size as well as ternary phase formation). These structural changes lead to competing effects on thermal transport relative to the cases of 0% and 100% nanocrystal volume fractions.

There is a recent growth in the demand and availability of analytical solutions to the Boltzmann transport equation. With experiments that probe small length scales, such as transient grating experiments, or fast time scales, such as the pump-probe thermo reflectance experiments, there is a clear need to understand the deviation of thermal transport from the diffusive limit. However, the BTE is notoriously difficult to solve and analytical solutions exist only for very special geometries and considerations. In this study we present an optimization technique to approximate the temperature profile from the BTE that is then used to extract suppression functions analytically which are accurate both at the diffusive limit and the ballistic limit. The utilization of the expected temperature behavior at the diffusive and ballistic limits allows one to solve the BTE with great accuracy and minimal computation effort over the entire range of possible length scales. The analytical suppression functions we obtain can then be used to study various materials and understand non-diffusive transport. We utilize these suppression functions to understand the size effects that come into effect at small length scales and to provide more accurate mean free path reconstruction. This work is supported by Solid State Solar-Thermal Energy Conversion Center (S³TEC), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, under Award Number: DE-SC0001299/DE-FG02-09ER46577.

Fluorite-based oxides (e.g. CeO₂, ZrO₂ and UO₂) are of technological importance to many applications such as fuel cells, thermal barrier coating materials, and nuclear fuels. The thermal transport property in these oxides is an important performance metrics for their applications. Impurities in these oxides, either doped or produced by reaction, can affect the phonon transport behavior in the oxides significantly. Here we use five different trivalent cations doped CeO₂ as model systems to study the roles of ionic radius and ionic mass of doped cations on the thermal transport in oxides. As these cations have the same charge state, they induce the same amount of oxygen vacancies. Therefore, the effects of ionic radius and ionic mass on thermal transport can be compared for different doped cations. Using non-equilibrium molecular dynamics simulations, the thermal conductivities of these doped systems are calculated for a wide range of concentrations of doped ions. We found that the bulk thermal conductivity decreases as the ionic radius increases while such a correlation does not exist for ionic mass. When these doped cations segregate to grain boundaries, they increase the grain boundary thermal resistance (Kapitza resistance) and the increment also correlates strongly with the ionic radius rather than ionic mass. Finally, using these atomistic results as inputs for an analytical model, the effect of segregation of doped cations to grain boundaries on the thermal transport in nanocrystals and polycrystals is studied for different doped ions and at different grain sizes. Our results show that segregation of cations to grain boundaries increases the thermal conductivity of nanocrystals significantly and the smaller ionic radius increases the thermal conductivity more.

With the strong needs for high-performance organic electronics, improving the electrical and thermal properties of organic semiconductors draws a lot of attentions. Due to the intrinsic amorphous nature of the material, the crystallinity, grain size, roughness, dislocation of organic semiconductor thin films significantly rely on the growth of organic semiconductors. Up to now, different techniques, including self-assembled monolayers (SAMs), elevated substrate deposition temperature, can be used to modify the properties of organic semiconductors. Here we particularly focus on investigating the post processing thermal annealing effect on the electrical and thermal properties of a small molecule organic semiconductor, dinaphtho[2,3-b:2&’,3&’-f]thieno[3,2-b]thiophene (DNTT). DNTT thin films are firstly deposited by thermal evaporation at room temperature (24 °C) and then annealed for 1 hour at different temperatures (24 °C to 140 °C) under nitrogen environment. From atomic force microscopy (AFM) and high resolution transmission electron microscope (HRTEM) images, it can be noticed that DNTT molecules will start to aggregate together and thus the crystal size and roughness will increase after thermal annealing. For the sample annealed at 140 °C, the selected area electron diffraction (SAED) pattern clearly indicates a single crystal diffraction pattern while the film annealed at 24 °C shows a polycrystal diffraction ring pattern. We further measure the cross plane space-charge-limited current (SCLC) mobility of DNTT thin films and the results indicates that the mobility firstly goes up and then drops down while the annealing temperature increases. At 80 °C, the mobility obtains a maximum value of 1.11×10^-3 cm²/V-s which gives a threefold increase in comparison with room temperature, 3.6×10^-4 cm²/V-s and this trend is correlated to crystallinity improvement of DNTT thin films. Other than electrical characterizations, we apply 3-omega; method to measure thermal conductivity of DNTT thin films. We find that thermal conductivity shows a much weak dependence on thermal annealing temperature and thermal conductivity at 80 ^oC just has a 17% enhancement in comparison with 24 °C. At higher temperature (ge;100 °C), thermal conductivity also shows a decrease trend. We also attribute the increase of thermal conductivity to the crystallinity improvement after annealing. As shown in HRTEM images, because thermal annealing generates larger crystal size, DNTT with larger crystal size can have a higher phonon mean free path, which enhances thermal conductivity. However, when the annealing temperature is higher than 100 °C, due to the serious aggregation of DNTT grains, larger roughness leads to the stronger charge and phonon scattering, which results in the decrease of SCLC mobility and thermal conductivity. Our findings show that thermal annealing could be an efficient and facile method to improve the electrical and thermal properties of organic semiconductors.

Recently, we have shown [1] that the lattice thermal conductivity of InSb is sensitive to an external magnetic field and that this effect arises from the diamagnetic moment induced in the samples by a high magnetic field. Our density-functional theory calculations showed that the atomic displacements that constitute a phonon result in a local distortion of the valence band structure. Since the valence band determines the diamagnetic susceptibility of a solid, this means that there is a local variation of the diamagnetic moment in the presence of an external applied magnetic field. While the spatial variation of the moment is small, its spatial derivative, which determines the magnetic force, is not. This force still results in only a small perturbation of the phonon frequencies, but has a much bigger influence on the Grüneisen parameters when the correct temperature range is chosen. These parameters determine the probability of phonon-phonon interactions. Experimentally, the effect is measured on a specially prepared sample in which the contribution of phonon-phonon scattering is amplified compared to that of all other phonon properties. An external field of 7 T is measured to affect the phonon-phonon Umklapp processes by 12% at 4K. The effect can be modeled in the absence of any adjustable parameter from band structure and phonon dispersion calculations, and the model explains the experimental results. In this talk, the consequences of this observation will also be discussed. For example, the presence of a phonon magnetoresistance implies the existence of a phonon Hall effect. Preliminary attempts at observing and calculating this new effect will be presented.
[1] Hyungyu Jin, Oscar D. Restrepo, Nikolas Antolin, Stephen R. Boona, Wolfgang Windl, Roberto C. Myers and Joseph P. Heremans, Nature Materials 14, 601-606 (2015) (DOI:10.1038/nmat4247)

In a conventional time-domain thermoreflectance (TDTR) measurement, the sample is first coated with an optically opaque metal film, typically Al, that acts as the optical transducer in the experiment. The transducer layer converts the incident pump optical pulses to heat and reports changes in temperature of the surface of the sample through changes in the intensity of the reflected probe. In TDTR, the transducer must be optically opaque to suppress contributions to the changes in the intensity of the reflected probe that would otherwise come from the temperature dependence of the optical constants of the materials under study. We have found that the rotation of the polarization of the probe created by the magneto-optical Kerr effect (TR-MOKE) of a thin ferromagnetic film provides robust alternative measurement of temperature that is both ultrafast and high signal-to-noise even when the magnetic layer is <10 nm thick. We will discuss how the reduction in thickness of the transducer layer in a TR#8209;MOKE experiment by an order of magnitude in comparison to TDTR enables improved measurements of i) lateral heat flow in thin films and crystals; ii) the thermal conductance of interfaces with low thermal conductivity materials and iii) the additional thermal resistance of metal/dielectric interfaces that is generated by the mismatch in the spectrum of phonons that carry heat across the interface versus the spectrum of phonons that carry heat in the crystal.

Indium gallium zinc oxide (IGZO) thin films have been attracting attention due to their outstanding electrical properties which enable applications such as display backplanes, smart windows, and low-cost flexible electronics. On the other hand, thermal properties of IGZO thin films are just as important in designing efficient thermal management, optimizing IGZO-based thermoelectric applications and understanding fundamental phonon behavior, but data until now has been lacking. Since IGZO can be grown by various techniques leading to different degrees of porosity over a range of morphologies from crystalline to amorphous, we report the temperature-dependent thermal conductivity of IGZO thin films representing all these morphologies, and observe that the thermal conductivity in certain cases changes over year-long time scales indicating morphological instability.
IGZO films with different morphologies were grown by both physical deposition and chemical synthesis on sapphire substrates. Pulsed laser deposition (PLD), sputtering, and combustion solution process were conducted to fabricate amorphous IGZO (a-IGZO) films, and additionally semi-crystalline (semi-c) and crystalline (c-) films were obtained by post-annealing after PLD. The crystallinity and film quality was confirmed by X-ray diffraction (XRD) and grazing incidence wide-angle X-ray scattering (GIWAXS).
The thin-film thermal conductivity k was measured with the 3omega; method from 18 K to room temperature 300 K. Dependences are described below:
(1) Porosity: All the dense films (by PLD, sputtering and spray-combustion) follow the same empirical power law of k ~T^0.6 throughout the measured temperature range showing consistency with the Cahill-Pohl model, while the porous spin-combustion k decreases significantly below 60 K. Pores acting as voids in phonon transport may play a more important role in suppressing low-frequency phonons at low temperatures, or residual chemical impurities may scatter long-wavelength phonons at low temperatures.
(2) Crystallinity: For PLD-grown films, the amorphous and crystalline films have similar low thermal conductivity. The disordered structure in a-IGZO and acoustic impedance variation in the layered structure in c-IGZO might both scatter phonons similarly. The semi-c film exhibits the highest thermal conductivity among the three. This may be caused by higher in-plane thermal conductivity for tilted grains.
(3) Shelf-life: After being stored in vacuum for months, the thermal conductivities of a- and c-IGZO films remain the same, while that of semi-c IGZO thin film reduces significantly at all temperatures but still follows the law k ~T^0.6. Consequently, the semi-c structure may be meta-stable between amorphous and crystalline phases, and it may have changed its morphology in ambient conditions. Storage in vacuum may also induce more oxygen vacancies, altering the mesoscopic structures and affecting phonon transport.

Introducing nanoscale inhomogeneity into semiconductor alloys is a known route to enhance the scattering of long-wavelength phonons and to subsequently reduce thermal conductivity. While extensive modeling exists for layered and nanoparticulate media, less focus has been directed toward fibrous nanostructures. Using a continuum mechanical treatment, we develop an analytic model for the phonon scattering cross-section of a cylindrically shaped elastic discontinuity in an isotropic medium, where incident phonons can have arbitrary polarization, angle of incidence, and wavelength. A Boltzmann transport theory framework is used to simulate the anisotropic thermal conductivity tensor of composites with fiber networks composed of aligned fibers, fiber mats, and randomly oriented fibers. We find that for fixed fiber volume fraction, there is an optimal fiber radius that produces lowest thermal conductivity, and that the optimal fiber radius can achieve lower thermal conductivity than a similarly optimized composite using spherical scatterers. Specific results are given for the case of silicide fibers embedded in silicon-germanium alloys. We also investigate the breakdown of the continuum treatment of phonon scattering cross-section using an atomistic model.

Thermal conductivity of thin films is sensitive to the presence of interfaces and grain boundaries. Self-assembled oxide nanocomposite films, in which one phase grows as columns within another phase and both phases are epitaxial with the substrate, provide a model system with well-defined vertical interfaces that allow the contribution of interfaces to thermal properties to be quantified. A spinel/perovskite system consisting of pillars of ferrimagnetic CoFe₂O₄ (CFO) in a matrix of ferroelectric BiFeO₃ (BFO) was grown on single crystal SrTiO₃ by pulsed laser deposition. The pillars were ~100 nm tall and ~30 nm in diameter, spaced ~100 nm apart. The pillars have a square or rectangular cross-section and the interface between the BFO and CFO consists of (110) planes. The thermal properties of the BFO/CFO nanocomposites were investigated using time-domain thermoreflectance (TDTR). The nanocomposites exhibited decreased thermal conductivity, 2.1 - 2.5 W/mK, compared to single phase CFO (3.3 W/mK) and BFO (5.0 W/mK) films. The volume fraction of CFO and the strain state of the film were varied by changing the deposition conditions and substrate type and the effects on the thermal conductivity in BFO/CFO nanocomposites will be discussed. The dependence of thermal conductivity on the presence of magnetic domain walls was also investigated by comparing CFO films after ac and dc demagnetization, but the magnetic domain walls had small effects compared to the physical interfaces. Based on the systematic study, we interpret the reduced thermal conductivity of BFO/CFO nanocomposites by evaluating the effects of phase boundaries, strain, and domain walls on phonon transport. Understanding of the way phonons transport in multiferroic nanocomposites is useful for developing multifunctional thermoelectric energy conversion devices and manipulating phonons via external means. This work is supported by S³TEC, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Basic Energy Sciences, under Award NO. DE-FG02-09ER46577.

The Boltzmann transport equation for phonons is recast directly in terms of the heat-flux by means of iteration followed by truncation at the second order in the spherical harmonic expansion of the distribution function. This procedure displays the heat-flux in an explicitly coordinate-invariant form, and leads to a natural decomposition into two components, namely the solenoidal component in addition to the usual irrotational component. The solenoidal heat-flux is explicitly shown to arise in a right-circular cylinder when the heating breaks the azimuthal symmetry. These findings are important in the context of phonon resonators that utilize the strong quasi-ballistic thermal transport reported recently in silicon membranes at room temperature.

In this talk I will discuss the ballistic, coherent, and hydrodynamic regimes of heat conduction in nanostructures. We develop phonon mean free path spectroscopy techniques by exploring quasi-ballistic heat transfer around nanoscale heat sources. Two-dimensional and one-dimensional metallic nanostructures serve as nanoscale heat sources with minimal substrate electron-hole generation in femtosecond, time-domain thermoreflectance pump-probe experiments that measure size dependent thermal conductivity. Algorithms are developed to extract phonon mean free path distributions based on the experimental results, first-principles simulations, and solutions of the Boltzmann equation. In superlattice structures, ballistic phonon transport across the entire thickness of the superlattices implies that phase coherence is maintained throughout the structures. We observed such coherent transport in GaAs/AlAs superlattices with fixed period thicknesses and a varying number of periods. This interpretation is supported by first-principles, Green&’s functions, and molecular dynamics simulations. Accessing the coherent heat conduction regime opens a new venue for phonon engineering. We will show further thermal conductivity reduction and signs of phonon localization in GaAs/AlAs superlattices with ErAs nanodots at the interfaces. Finally, we will discuss the phonon hydrodynamic transport regime in graphene, recently discovered via first-principle simulations. In this regime, phonons under a temperature gradient drift with an average velocity, a behavior similar to fluid flow under a pressure gradient in a pipe. Conditions for observing this phonon hydrodynamic regime will be discussed.
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.

Al_xGa_1-xN proves to be technologically important material for LED industry as well as high electron mobility transistors (HEMTs). Because of high power applications, it is important to study submicron thermal transport in thin film Al_xGa_1-xN structures. Previous literature focused on films that are 100&’s nm in thickness and this limited the study of long mean-free-path ballistic phonons. [1] [2] In this study, a high quality 1.2mu;m Al_0.1Ga_0.9N thin film sample was grown by using plasma-assisted molecular beam epitaxy (PAMBE). We report on measurements of the thermal conductivity of Al_0.1Ga_0.9N thin film by using time-domain thermoreflectance (TDTR) over the laser modulation frequency 0.8< f <10 MHz and temperature range 100 < T < 500 K. Al_0.1Ga_0.9N achieves highest thermal conductivity at a relative high temperature ~300K. Based on the temperature dependence, we analyze the role of the different scattering mechanisms. The measurements show pronounced boundary scattering at low temperature (100K and 150K). This is also reflected in flat plateau in the thermal conductivity versus modulation frequency. We also found a unique frequency-dependent thermal conductivity at 200K, where both Umklapp (U-type) and boundaries scattering contribute to the thermal transport. Thermal conductivity of Al_0.1Ga_0.9N drops up to 40% at 10MHz compare to the bulk thermal conductivity at room temperature. Similarities and differences with other semiconductor alloys, e.g. SiGe, InGaAs [3] will be highlighted. Preliminary study of the superdiffusive transport parameters and Lévy fractal dimension for phonon random walk will be also presented.
[1] W. Liu and A. A. Balandin. J. Appl. Phys, Vol. 97, 073710 (2005)
[2] B. C. Daly, H. J. Maris, A. V. Nurmikko, M. Kuball and J. Han, J. Appl. Phys. 92,3820 (2002)
[3] Y. K. Koh and D. Cahill, Phys. Rev. B, 76, 075207 (2007)

Nanomaterial interfaces and concomitant thermal resistances are generally considered as atomic-scale planes that scatter the fundamental energy carriers. Given that the nanoscale structural and chemical properties of solid interfaces can strongly influence this thermal boundary conductance, the ballistic and diffusive nature of phonon transport along with the corresponding phonon wavelengths can affect how energy is scattered and transmitted across interfacial region between two materials. In hybrid composites comprised of atomic layer building blocks of inorganic and organic constituents, the varying interaction between the phononic spectrum in the inorganic crystals and vibrionic modes in the molecular films can provide a new way to manipulate the energy exchange between the fundamental vibrational energy carriers across interfaces. Here, we demonstrate nearly perfect transmission of phonon energy across interfaces comprised of single monolayer of hydroquinone molecules. In multilayer composites of atomic- and molecular-layer-grown zinc oxide and hydroquinone, the ballistic energy transfer of phonons in the zinc oxide is limited by scattering at the zinc oxide/hydroquinone interface, and then significant portion of the phonons with wavelengths larger than the thickness of the molecular monolayer ballistically transmit into the subsequent zinc oxide layer. The transmission of energy across the molecular interface transitions from quasi-ballistic to diffusive for hydroquinone interfaces greater than or equal to three molecules thick. This marks a ballistic to diffusive crossover size for which molecular interfacial thermal conductance transitions from being limited by the inorganic phonon flux to intrinsic vibrational resistance within the linked molecules.

Many micro/nanoscale heat transport experiments are interpreted using phenomenologically adjusted Fourier theory. It was recently shown that this can severely misrepresent the internal processes. When temperature gradient is over micron distances, the energy dynamics are much better described as truncated superdiffusive Lévy flights instead of conventional Brownian motion. All essential physics of the nondiffusive transport are captured by the fractal dimension and exponential decay length of the stochastic process. We determine these two new material parameters experimentally for several semiconductor alloys using transient laser thermoreflectometry. Finally, nonlocal relation between the heat flux and the temperature gradient is quantified and the thermal conductivity kernel is described as a function of truncated Lévy parameters. Implications for the design of high power electronic and optoelectronic devices will be discussed.

In recent years, significant progress has been made in studying phonon-mediated thermal transport in nanostructured materials. However, many questions remain unanswered and some issues are still hotly debated. Are “phononic crystal” effects observed in the thermal conductivity of nanostructures at room temperature? What are the limits of the diffuse boundary scattering (Casimir) model? Can thermal conductivity of nanostructures be obtained from ab-initio calculations without fitting parameters as has been recently demonstrated for many bulk materials? Answering these and other questions poses a challenge both for the theory and for experimental techniques. Measuring thermal transport properties with high accuracy is hard enough even with bulk materials, and becomes much more challenging with nanostructured samples. In this talk, I will present an overview of the laser-induced transient grating (TG) technique in which a spatially sinusoidal temperature “grating” is created in the sample, and its decay measured by a laser probe provides information about the thermal transport. The TG method is noncontact and yields intrinsically high absolute accuracy as it is based on measuring the dynamics of the TG decay rather than absolute values of the temperature and heat flux. In practical terms, it is particularly well suited for measuring the thermal conductivity of thin films and near-surface layers of bulk samples. The factors of particular significance for studying the fundamentals of phonon transport are that the TG technique avoids heat transfer across interfaces (i.e. all thermal transport takes place inside the material of interest), and the simple geometry with the sinusoidal temperature profile, which greatly simplifies theoretical analysis. We will review the experimental methodology of the optically heterodyned TG technique and discuss its contribution to the progress in understanding micro/nanoscale thermal transport by phonons made in recent years. In particular, we will be discussing the direct observation of non-diffusive thermal transport in semiconductors at room temperature and high-accuracy measurements of the in-plane thermal conductivity of solid and nanostructured silicon membranes. One important message of these measurements is that the room temperature thermal conductivity of solid Si membranes ranging from 15-1500 nm in thickness matches first-principle-based theory assuming diffuse boundary scattering, providing an example of a nanostructure whose thermal conductivity can be said to be well understood. The discussion will be concluded by an overview of further challenges both in the experiment and in the interpretation of the micro/nanoscale thermal transport measurements.
This material is based upon work supported as part of the S3TEC, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award DE-SC0001299/DE-FG02-09ER46577.

Nanostructured materials could hold the key for better thermoelectrics because of their ability to suppress heat transport while leaving the electrical conductivity unaltered. Size effects are predominant when the characteristic length of the material becomes comparable with the phonon mean free path (MFP). For many good thermoelectric materials, such as bismuth telluride, this condition is hard to achieve due to the very small MFPs of heat-carrying phonons (e.g. 10 nm). However, in certain earth-abundant materials, such as Si, phonon MFPs can be as large as few microns, allowing for beneficial nanostructuring effects even with mesoscale characteristic lengths. The interaction between phonons and the material&’s boundaries depends on the Knudsen number, i.e. the ratio between the intrinsic MFP and the characteristic length. As each phonon branch has its own MFP distribution, the degree of phonon suppression generally depends on the phonon branch. As the phonon branches cannot be separately resolved in experiments, an accurate computational approach is needed in order to understand and ultimately engineer heat transport in nanomaterials. Using a recently developed multiscale method based on density functional theory (DFT) and Boltzmann transport equation (BTE) [1], we calculate the thermal conductivity of nanoporous silicon (np-Si) at different temperatures. We predict that at room temperature the boundaries can suppress heat transport by more than one order of magnitude, potentially increasing the thermoelectric efficiency by the same extent. Moreover, we show that heat carried by optical phonons can contribute roughly 20% to the thermal conductivity, compared to only 5% in bulk Si. We attribute this effect to the fact that while long-MFP acoustic phonons suffer strong size effects, optical phonons travel mostly diffusively due to their low Knudsen numbers [2]. These predictions are in qualitative agreement with previous studies based on the Casimir model [3]. Interestingly, we further predict that, over the temperature range 200 - 300 K, the thermal conductivity is constant with temperature. The reason for such behavior is that acoustic phonons, which travel ballistically, have their MFPs bounded by the characteristic length of the material and, as such, the heat carried by them does not depend on the temperature. This unexpected feature has important consequences for designing thermoelectric materials that work for a wide range of temperatures. Although our study focuses on np-Si, results obtained for this case can be generalized to any material that has a wide MFP distribution. Our findings can enable novel routes for engineering nanostructured materials for high-efficiency thermal energy conversion.
[1] G. Romano and J. C. Grossman. Journal of Heat Transfer 137.7 (2015): 071302.
[2] G. Romano et al. arXiv preprint arXiv:1505.06122 (2015).
[3] Z. Tian et al.Applied Physics Letters 99.5 (2011): 053122.

Thermal transport in solids changes its nature from phonon propagation that suffers perturbative scattering processes to thermally activated hops between localized vibrational modes as the level of disorder increases. Models have been proposed for these two distinct extremes that predict opposite temperature dependence of the thermal conductivity, but not the intermediate regime. Here we explore thermal transport in two-dimensional crystalline and amorphous silica with varying levels of disorder, α, by performing molecular dynamics simulations as well as analysis based on the kinetic and Allen-Feldman theories. We demonstrate the crossover between the crystalline and amorphous regimes at α ~ 0.3, as identified by a turnover of the temperature dependence in thermal conductivity, and explained by the dominance of thermal hopping processes. The determination of this critical disorder level is also validated by the analysis of the participation ratio of localized modes, as well as the occurrence of heat flux localization. These factors can be used as good indicators to characterize the transition in heat transport mechanisms.

Semiconductors and dielectrics' ability to conduct heat depends upon their phonon mean free paths that describe the average travelling distance between two consecutive phonon scattering events. Nondiffusive phonon transport is being extensively exploited to extract phonon mean free path distributions. Here, we describe an implementation of a thermal conductivity spectroscopy technique that allows for the study of mean free path distributions in optically absorbing materials at tens of nanometers, with relatively simple fabrications and straightforward inversion scheme. We pattern 1D metallic grating of various linewidths but fixed gap size on samples. The metal lines serve as both heaters and thermometers in time-domain thermoreflectance measurements. We demonstrate the viability of this technique by studying length-dependent thermal conductivities of silicon and half heuslers at various temperatures. The reconstructed thermal conductivity accumulation functions based on the measured linewidth-dependent thermal conductivities and the suppression function calculated from Boltzmann equation show excellent agreement with first-principle based predictions. The agreement directly validates the predictive power of density function theory to determine the phonon mean free path distributions. This table-top ultrafast optical phonon spectroscopy technique enables the study of mean free path spectra in technologically interesting materials.
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.

We present numerical simulations of the thermal boundary (or Kapitza) resistance of single-wall carbon nanotubes (CNTs) embedded in a polyethylene (PE) matrix. The thermal boundary resistance (TBR) of the CNT-PE system is calculated via a lumped heat capacity method [1,2] using the temperature difference between the CNT and PE matrix obtained from non-equilibrium molecular dynamics simulations. The effects on phonon transport of both the structure of the PE matrix (amorphous or crystalline) and the degree of covalent crosslinking between PE and CNT are investigated. The results show that the boundary conductance is small (~ 12 MW m^minus;2 K^minus;1) for non-crosslinked matrices, and increases linearly for low degrees of crosslinking. For fully amorphous polymer matrices, crosslink densities of between 5-10% are required to achieve optimal interfacial transfer. However, the presence of a crystalline layer of polymer at the interface has a significant influence on TBR, and the implications for this these for the production of CNT-polymer composites with more efficient heat transfer at the interfaces are discussed.
[1] C.F. Carlborg, J. Shiomi, S. Maruyama Phys. Rev. B, 78, 205406 (2008)
[2] S. Hida, T. Hori, T. Shiga, J.A. Elliott and J. Shiomi Int. J. Heat Mass Transfer, 67, 1024-1029 (2013)

A variety of phonon-mediated processes centrally contribute to heat dissipation in colloidal quantum dot (QD) solids, and a method to tailor the QD vibrational spectrum may allow engineering of more efficient QD devices. Inorganic shells surrounding the quantum dot core, as well as organic ligands attached to the quantum dot surface, affect optical and electronic properties of quantum dots but their effect on the QD vibrational spectrum has been heretofore unknown. We use low-frequency non-resonant Raman spectroscopy to non-destructively probe the acoustic phonon vibrational structure of CdSe QD cores with a variety of different attached ligands and different shell thicknesses. We develop a mathematical model based on vibrations of an elastic sphere to understand shell- and ligand-dependent shifts in the QD Raman spectrum. These data further our understanding of the factors affecting phonon energies and heat transport in QD solids.

Pump-probe thermoreflectance techniques such as time domain thermoreflectance (TDTR) and frequency domain thermoreflectance (FDTR) have been widely used to study phonon transport and interactions in nanoscale materials. The analysis of these techniques is done through a least squares based inverse problem, minimizing the differences between the observed data and a heat transfer model, to extract unknown properties of interest. The obtained property values are affected by experimental noise and errors in controlled parameters. Here, we present an analytical model for predicting the uncertainty of thermal properties measured by thermoreflectance techniques. We derived the analytical solution of the standard deviation of parameters estimated by least squares based algorithms. Our model accounts for experimental noise in the collected data and errors in the controlled parameters. Examples of a bulk material and a thin film suggest that FDTR is overall better at determining multiple properties than TDTR but the latter technique is more reliable for measuring interface between metal and a low thermal conductivity material. In addition, we utilize a Monte Carlo method to validate our analytical model. Examples of measuring a gold film on a fused silica substrate show excellent agreement between property uncertainties predicted by our analytical model and those obtained by the Monte Carlo method. Our method can be a useful tool for analyzing measurement uncertainty of thermoreflectnce techniques.

We present a numerical approach to the solution of elastic phonon-interface and phonon-nanostructure scattering problems based on a frequency-domain decomposition of the atomistic equations of motion and the use of perfectly matched layer (PML) boundaries. Similar to the atomistic green's function (AGF) technique, the method models interactions between atoms as harmonic from the onset and reduces scattering problems to a system of linear algebraic equations via a sparse, tightly banded matrix. The formulation differs in that, in analogy with classical scattering problems, systems are modeled as a single-mode incident wave interacting with a nanostructure to produce a scattered wave. Having the exact knowledge of frequency of the incident wave requires that both the incident and scattered waves be infinitely extended in space.The PML's are used to absorb incident waves in a manner that minimizes reflections, thereby emulating an infinite system. Monitoring the energy dissipation inside the PML provides a simple interpretation of scattered energy flux.
The accuracy of the method is demonstrated on connected monoatomic chains, for which an analytic solution is known. The parameters defining the PML are found to affect the performance and accuracy of the method; guidelines for selecting optimal parameters are given. The method is used to study the energy transmission coefficient for connected diatomic chains over all available wavevectors for both optical and longitudinal phonons; it is found that when there is discontinuity between sublattices, even connected chains of equivalent acoustic impedence have near-zero transmission coefficient for short wavelengths. Statistically interdiffused interfaces between monatomic chains are also studied with the key observations being that (1) phonons have enhanced transmission coefficient for wavelengths comparable to the interdiffusion distance, with the mass variation acting as a phonon antireflection coating (2) disorder induced localization causes reduced transmission coefficient for short wavelength phonons, and (3) the widely applied diffuse mismatch model (DMM) is not recovered for interdiffused interfaces in one dimension. We also demonstrate the extention of the model to multiple-dimensions and investigate the breakdown of continuum theory

We present theoretical models and classical molecular dynamics (MD) simulations to predict the phonon transport properties in bulk single crystal and through grain boundary interface of uranium dioxide, which is commonly used as fuel material in modern nuclear power plants. Prediction of thermal transport properties is important for nuclear safety and industry. For single crystal of uranium dioxide, the calculations of thermal conductivity using MD simulations are based on the equilibrium Green-Kubo autocorrelation decay and the direct method (nonequilibrium), as a function of temperature. After removal of the size effect by extrapolation of results from a range of structure size, the MD simulations give results in agreement with the theoretical Slack relation and with experiments and other predictions, over a broad temperature range.
Then we use planar grains for the grain boundary effect on phonon transport. The effective thermal conductivity of the planar grain structure and the Kapitza thermal resistance at grain boundary are evaluated as parameters of the grain boundary effect using both the nonequilibrium MD simulation and theoretical models. The calculated thermal conductivity is compared with three theoretical models, namely the Klemens model, the localized continuum model, and the Callaway model. These models describe reduction of the thermal conductivity due to the phonon scattering at grain boundary in accordance with the grain size, temperature, the Kapitza length, reduction of phonon mean free path, and phonon scattering time. The thermal resistance of grain boundary is predicted using the diffuse mismatch model (DMM) and the grain region phonon density of states and speed from MD simulations. These are compared with predictions of other models and we show why the specific features of the grain boundary structure does not influence the grain boundary resistance.

Figure of merit which strongly governs the thermoelectric efficiency is defined as ZT = (S^2 σT)/K, where S is the Seebeck coefficient, σ is the electrical conductivity and K is the total thermal conductivity. Nanostructuring and alloying methods have been utilized to decrease thermal conductivity and boost ZT. Many efforts have been made to reduce the thermal conductivity of SiGe alloys and their nanostructures, with limited success due to relatively small mass difference. Approaching higher efficiencies motivates our introduction of a heavier group IV element, Sn, with specific properties.
Our model is based on solving phonon Boltzmann transport equation based on the improved Callaway model. We use the full phonon dispersion calculated from Weber&’s adiabatic bond charge model. In our calculation we have included normal and umklapp three-phonon scattering as well as impurity and alloy mass disorder scattering. As a result we calculated minimum thermal conductivity for bulk SiSn and GeSn to be 3 W/mKfor Sn composition of 0.51 and 5.86W/mK for Sn composition of 0.61 respectively. These values are far below the SiGe bulk in our calculation.
The nanostructured SiGeSn show a decrease of thermal conductivity with thickness. Thin layers of SiSn having thickness of 10nm produce a thermal conductivity of 0.7699 at Sn composition of 0.63 and GeSn has thermal conductivity of 1.244 at Sn composition of 0.59. These values are close to the minimum thermal conductivity, sometimes referred to as the amorphous limit, which we calculate from Cahill&’s minimum thermal conductivity model.
We conclude that SiSn has the lowest thermal conductivity among all the cases and its nanostructures reach very near the amorphous limit. Hence SiGeSn ternary alloys can be considered as an alternative to SiGe alloys, which, when coupled with the widely tuneable bandgap of SiGeSn alloys, may result in higher figure of merit ZT.

Electromechanical resonators are widely used structures which enable coherent phonons to be confined with small energy dissipation. A GaAs/AlGaAs heterostructure is one of the most ideal material systems for fabricating high performance electromechanical resonators, in which the phonon dynamics can be piezoelectrically controlled through external electrical signals. We recently demonstrated the coherent manipulation of phonon dynamics using the III-V compound semiconductors, where the phonon population can be transferred between different oscillation modes [1,2]. We also realized the dynamical control of phonon transport by constructing a phononic crystal waveguide, allowing the propagation dynamics to be switched through electrical signals [3]. The highly controllable phononic devices are promising for future applications, such like signal processing, ultrasonic imaging, functional sensing, and also the transduction of quantum information between different physical systems.
[1] I. Mahboob, K. Nishiguchi, H. Okamoto, and H. Yamaguchi, "Phonon-cavity electromechanics ", Nature Physics 8. 387-392 (2012).
[2] H. Okamoto, A. Gourgout, C.Y. Chang, K. Onomitsu, I. Mahboob, E.Y. Chang, and H. Yamaguchi, “Coherent phonon manipulation in coupled mechanical resonators” Nature Phys. 9, 480-484 (2013).
[3] D. Hatanaka, I. Mahboob, K. Onomitsu, and H. Yamaguchi, “Phononic crystal waveguides for electromechanical circuits”, Nature Nanotechnology 9, 520 (2014).

The manipulation of a macroscopic quantum phase (MQP), a collection of particles sharing a single macroscopic wave function, by external potentials, provides a way to implement quantum computation and simulation protocols. Examples of MQPs are atomic Bose-Einstein condensates in magneto-optical traps and supercurrents in superconducting circuits. MQPs may also exist in a semiconductor microcavity where photons couple with quantum-well (QW) excitons to form bosonic quasiparticles called polaritons. Being half-light, they possess a small mass, and so, a de Broglie wavelength of a few micrometres. Polariton MQPs form thus at low particle densities and high temperatures (up to 300 K) and can be manipulated by micrometric potentials. In this work, we demonstrate the modulation of polariton MQPs in an (Al,Ga)As microcavity by surface acoustic waves (SAWs) of wavelength of 8 mu;m, which create a travelling lattice-potential of tuneable amplitude.
The SAW strain field creates alternate regions of tension and compression at the crystal surface, modulating the cavity and the embedded QW properties, which induces the periodic potential. A square lattice is formed by intersecting two SAWs. A nice feature of polaritons is that their dispersion, and thus the lattice Brillouin zone (BZ), may be directly imaged by angle-resolved photoluminescence. Also, polaritons have strong repulsive interactions provided by their excitonic component, thus giving rise to strong non-linear effects. In particular, we observe that, in a shallow SAW lattice, the polariton MQP forms at the corners of the square BZ where the effective mass of the particles is negative. The MQP is thus, in this case, a self-localised wave packet, known as a variety of soliton, which results from the interplay of the negative effective mass and the dispersion due to the repulsive interactions. This gap-soliton (GS) is about 35 mu;m in diameter, its energy is within the lattice band gap and has zero group velocity with respect to the lattice.
By means of spectral tomography, we show that, at high particle number, the GS remains unaffected, but an ensemble of wire-like polariton MQPs with lower energy surrounds it in a concentric spatial arrangement. This complex behaviour is ascribed to the Gaussian energy profile induced by the polariton interactions in the laser spot. We also show that time-resolved imaging can directly detect the GS wave-packet in the moving SAW lattice. As expected, the GS is “carried” by the lattice, but surprisingly, it forms preferentially at specific positions within the laser spot. This is seen as time oscillations of the emission intensity and spectral linewidth with the SAW period, which could be due to the SAW-induced modulation of the resonant optical excitation. Our findings are supported by a theoretical model based on the variational solution of the Gross-Pitaevskii equation.
This work opens up an exciting possibility for the realization of “quantum chips” in semiconductors.

We study the non-linear interaction of optical phonons in semiconductors with high frequency surface acoustic wave (SAW) using Raman spectroscopy. The studies were carried out in Si and GaN substrates containing delay lines for the electrical excitation of SAWs with a wavelength of 5.6 mu;m. The SAW-induced changes, the differential Raman intensity (DR), are on the order of ~10^-3 and were detected using a modulation technique to improve the signal-to-noise ratio and compensate for temperature and laser power fluctuations. Analysis of the DR line shape in two materials reveals both local and non-local contributions. The former arises from the periodic modulation of the zone-center optical phonon energy by the SAW strain and results in a symmetric contribution to the DR line shape. In addition, the spatial strain distribution induces a coupling of optical phonon modes with different wave vectors. Due to the quadratic phonon dispersion, the latter introduces an asymmetric contribution to the DR line shape characterized by a tail towards lower frequencies. A model is presented to account for the modulation of the Raman response.

Surface acoustic waves (SAW) are a powerful tool for the study and manipulation of low-dimensional electron systems (low-DES). Recent work on the coupling of SAW transducers to qubits [1] emphasizes the intrinsic quantum nature of these interactions, which is often overlooked in the discussion of SAW interactions with extended quantum states such as quantum Hall states. But even a classical treatment of the SAW propagation provides a window into quantum behaviors, as seen in the ground-breaking experimental evidence for the composite fermion [2] provided by SAW transmission data. In our studies of single and bilayer systems, we use SAW transmission measurements in conjunction with electrical transport measurements to effectively discriminate between homo- and hetero-geneous quantum phases. We show that the bilayer excitonic condensate that forms in coupled GaAs quantum wells separated by a GaAs/AlAs superlattice when each layer is at filling factor 1/2 (known as the nu;_total = 1 state) is strongly homogeneous, demonstrating the zero longitudinal conductivity expected of an integer filling factor quantum Hall state. In contrast, the stripe and bubble phases near filling factor 9/2 in a single 2-DES, which are thought to be comprised of separated regions of integral filling factor, show unexpected evidence that these regions do not all have zero longitudinal conductivity. Our experiments make use of a bifrequency probe technique, where one set of low temperature coaxial leads controls two sets of transducers (at 232 and 343 MHz) that transmit along orthogonal directions. [3] An extension of this multifrequency technique with variable finger spacing across the transducer width would allow detailed spatial sampling and provide spectroscopic detail of the electron-phonon interactions in the 2-DES.
[1] R. Ruskov and C. Tahan DOI: 10.1126/science.1260180
[2] R. Willett et al. DOI: 10.1103/PhysRevLett.65.112
[3] M. Msall and W. Dietsche DOI:10.1088/1367-2630/17/4/043042

Interaction of light and sound in tiny optical cavities and waveguides is a vibrant topic nowadays, as for instance manifested in cavity optomechanics, but also in Brillouin scattering measurements in micro-wires and tapered optical fibers. Indeed, both optical and elastic wave fields are tightly confined to a very small volume and surface effects can become significant with the size reduction. Recent progress has shown that opto-acoustic interactions can benefit from the combination of the photoelastic and of the moving-interface effects. While the photoelastic effect is classical in bulk acousto-optics, the moving-interface effect is specifically driven by the vibrations of surfaces of a resonator or a waveguide. Assuming a photonic mode and a phononic mode are known beforehand, the diffraction efficiency for the creation of new photons can be estimated by overlap integrals. Reciprocally, the confined optical fields excert mechanical forces on the material composing the cavity. In order to describe this effect, we propose a variational principle describing the acousto-optical interaction. We construct a 3-wave Lagrangian describing the interaction of the original optical wave, the Doppler-shifted optical wave, and the acoustic phonons. The three waves are further phased-matched in waveguides. The Lagrangian exhibits both volume and surface contributions to the interaction energy. First, electrostriction results in a bulk force governed by the photoelastic tensor. Second, coupling of the electromagnetic field with the mechanical motion of the cavity further results in an effective surface force. We then solve the resulting elastodynamic equation subject to bulk and surface optical forces. A finite element model is specifically derived from the variational formulation. It is applied to acousto-optical (or optomechanical) interactions in a nanoscale silicon photonic cavity and in a silica micro-wire. The simulation results show that acoustic resonances can be excited all-optically in the multi-gigahertz range from infrared light. Significantly, the excited acoustic phonons are not necessarily unique normal modes of the structure, but their distribution is found to be governed by the spatial distribution of the optical forces. The finite element model is further applied to tiny optical waveguides and compared to experiments performed with silica micro-fibers, revealing the generation of surface acoustic waves propagation at the waveguide boundaries.

The study of acoustic wave propagation and phonon transport in phononic crystals and acoustic metamaterials has received significant interest in recent years. Within this field, two classes of material systems - granular media and locally resonant acoustic metamaterials - have shown tremendous promise in tailoring acoustic wave propagation owing to their dispersive properties and, in the case of granular media, their nonlinear response stemming from the Hertzian relationship between particles in contact. Recently, a self-assembled metamaterial comprised of a monolayer of micron-sized silica spheres adhered to an elastic substrate was shown to exhibit hybridization band-gaps at hundreds of Megahertz frequencies in the acoustic dispersion of guided acoustic waves, including both Lamb and surface acoustic waves (SAWs). In this type of metamaterial, the balance of adhesive and elastic forces between the microspheres and the substrate leads to a contact resonance that hybridizes with the waves giving rise to the metamaterial phenomena. As part of this study, we experimentally investigate methods to tune the response of these metamaterials. Multiple tuning methods will be discussed, which include modifying the microsphere geometry and sphere-substrate contact properties. The laser-induced transient grating technique is used to excite surface phonons and measure the dispersion in the metamaterial. This study may lead to the design of new types of metamaterials for SAWs with tunable functionalities.

The manipulation and control of phonons is important in understanding nonlinear propagation, caustic formation and shock interactions, as well as in applications ranging from sound insulation to ultrasonic imaging and shock dissipation. Unique to this challenge lies in the material&’s inherently nonlinear response, such as phonon-phonon scattering as well as amplitude dependent shock propagation which stems from the intriguing structure of the different materials across multiple length scales, from the atomic to the meso-scale. Phononic metamaterials (PMM) enables one to access exotic propagation behavior, such as super-tunneling, negative refraction and super-absorption, through deliberate structuring at a particular length scale. Furthermore, dynamic behavior in PMM may be exploited through affine deformation or elastic instabilities which alter the structural symmetry and hence the dispersion behavior. However, we propose that, by harnessing the intrinsic nonlinear responses in materials, (occurring at a particular length scale) together with the structural symmetry at targeted length scales, we can arrive at novel methods of controlling wave propagation behavior.
One surprising example of this comes from nature in the form of spider silk fibers, which possess macroscopically uniaxial symmetry. We theoretically and experimentally observed an indirect hypersonic phononic polarization band gap (30%) and importantly, negative index behavior; we further demonstrated that these properties can be dynamically and reversibly tuned with large amplitude strains (up to ±20%). The origin of this band gap is distinct from common mechanism attributed to scattering or hybridization while the negative index behaviour arises from the elastic nonlinearity, pointing the way forward to new methods of generating negative index behavior through nonlinearities; this reveals the major role of multilevel structural organization on elastic energy and the influence of nonlinearity in the mechanical behavior.
We further discuss other deliberate combinations of symmetry and material nonlinearity to demonstrate other tunable properties, such as multiple spectral gaps and phononic lens with tunable focal lengths
Hence, we believe that exploiting both these properties cooperatively provide avenues for novel dynamic systems with tailored and importantly, functionally optimized properties.

In the last two decades Phononic crystals (PnCs) have been an extremely active research field [1]. The principle of Bragg reflection on an artificially patterned crystal-like structure leading to additional spectral (band gaps, group velocity reduction), refractive (negative refraction, anisotropy) properties is scalable in any frequency range by a suitable choice of the crystal lattice constant.
During the last years, few works have illustrated how nanoscale PnCs represent a mean for reducing the thermal conductivity by modulating the thermal phonon propagation [2-3] without extremely degrading the electron conduction properties. Indeed, the lattice thermal conductivity can be reduced without affecting the electrical conductivity and thus increase the thermoelectric figure of merit zT. Such phononic engineered structures are promising candidates as efficient thermoelectric materials compatible with CMOS technology. The chosen material is Silicon, given its electrical conductivity, high Seebeck parameter, industrial compatibility, low cost and low toxicity.
In this work we investigate the thermal properties of phononic crystals fabricated in 60 nm thick silicon SOI membranes fully suspended between a central heater and two symmetric sensing serpentines [4]. The metrology devices are characterized under vacuum, to avoid air conduction and convection effects, making use of a 6 DC-probes measurement set up. Thanks to a careful thermal design and electro-thermal methodology, these micro-integrated platforms enable numerous and rapid thermal conductivity measurements over a 100K temperature window. The thermal conductivity of the patterned membranes is compared to plain membranes and literature results [5-6]. The results are discussed and compared to measurements performed by Raman thermometry and Scanning Thermal Microscopy (SThM) on similar membranes.
[1] M.S. Kushwaha et al, Phys. Rev. Lett. 71, 2022 (1993).
[2] J.-K. Yu et al, Nat. Nanotechnol.5, 718 (2010).
[3] P. E. Hopkins et al, Nano Lett.11, 107 (2011).
[4] M. Haras et al., IEDM 2014 IEEE International, 8.5. 1, (2014).
[5] E. Chávez-Ángel et al., APL Materials 2, 012113 (2014).
[6] M. Haras et al., Mater. Lett.157, 193 (2015).

Wavelength multiplexers are essential components in wavelength-division multiplexing (WDM) technology, which is widely used in nowadays fiber optic communications. However, most of the signal processing and routing is still performed electronically at the network nodes. In this way, a higher efficiency could be accomplished by replacing the electronic components, making use of the faster response time inherent to optical phenomena to control and process the signals. A promising proposal to realize compact and fast active devices consists of using coherent acoustic phonons, in the form of a surface acoustic wave (SAW), to modulate multiple optical waveguides (WGs) through the acousto-optical effect [1].
In this contribution, we demonstrate compact tunable wavelength-division multiplexers driven by SAWs in the low GHz range. Two different layouts are measured, in which multi-mode interference (MMI) couplers or free propagating regions (FPRs) are separately employed as couplers. A standing SAW modulates the refractive index of the arrayed WGs. In our approach, each wavelength component periodically switches paths between the preset output channel and the adjacent channels, at a fixed applied acoustic power. The fabricated multiplexers operate on five equally distributed wavelength channels, with a spectral separation of 2 nm. The operational period of the device is of approximately 2 ns, with a SAW-light interaction length of 120 microns. Finally, a design approach for compact SAW-driven light routers built upon MMI couplers is also presented. Very compact devices with a single input WG can be obtained by imposing restrictions on the modal excitation of the splitter MMI coupler. In this way, the experimental results for a symmetric-mode mixing three output WGs router are presented. The devices discussed have been monolithically processed on (Al,Ga)As, and this technology can be virtually implemented in any material platform.
[1] M. M. de Lima, Jr. and P. V. Santos, Rep. Prog. Phys. 68, 1639 (2005).

Thermal transport in nanocrystal arrays is believed to be regulated by the strength of vibrational coupling across the organic-inorganic interface at the surface of each nanocrystal. While much is known about the effect of molecular structure on the vibrational spectrum of organic ligands, comparatively little is known about what factors may influence vibrational frequencies within the nanocrystal cores. Using low-frequency nonresonant Raman scattering, we study the size-dependent acoustic phonons in colloidal semiconductor nanocrystals. Surprisingly, we find that a number of external variables can dramatically affect the eigenfrequencies of the CdSe core, including ligand molecular weight, chemical structure, the presence of an inorganic shell, and the surrounding matrix. We will show how these variables can be used to rationally tune the nanocrystal vibrational density of states without substantially altering electronic or optical properties.

It is becoming possible to conduct photoacoustic measurements across an increasingly broad swath of the Brillouin zone, in many cases with access to longitudinal and transverse waves as well as surface and waveguide acoustic modes. Experimental methods will be reviewed and recent results will be presented that illustrate acoustic phonon measurements in several contexts. Measurements of acoustic attenuation rates (i.e. mean free paths) and assessment of the underlying mechanisms due to either dynamic interactions or static heterogeneity in bulk materials and at interfaces will be presented, with emphasis on the mean-free-path-dependent contributions of acoustic phonons to thermal transport. Separate measurements of acoustic properties in glass-forming materials will be presented, illustrating how observations that cover an extremely wide frequency range can enable incisive tests of empirical models and first-principles theories of complex viscoelastic dynamics. Recent measurements of acoustic nonlinearities in liquids and granular solids also will be discussed, and novel methods for visualization of focusing shock waves will be described briefly. Finally the prospects for generation of coherent acoustic responses approaching the Brillouin zone boundary will be discussed.

Non-contact laser-based measurements of thermal conductivity in the form of time-domain thermoreflectance (TDTR) and frequency-domain thermoreflectance (FDTR) are widely used for the characterization of the thermal transport properties of materials. At high modulation frequencies where the penetration depth of the “thermal wave” into the material becomes comparable with the mean free path (MFP) of the dominant heat carriers, the break-down of Fourier&’s law for heat transport is expected. While limiting our capability of accurately measuring the thermal conductivity of very thin films and sub-surface layers, the break-down of Fourier&’s law offers an opportunity of gaining insight into heat carriers&’ MFPs. Recent experimental studies on semiconductor alloys such as SiGe have shown the dependence of the apparent thermal conductivity on the heating frequency in the tens of MHz range. However, for pure Si no general consensus on the occurrence of the frequency dependence of the effective thermal conductivity has been reached. In this study, we present the thermal conductivity measurement of Si and SiGe coated with an Al transducer film at heating frequencies up to several GHz, much higher than the current achievable upper frequency limit using FDTR, by Fourier transforming the data obtained by the TDTR method. No frequency dependence of Si thermal conductivity at room temperature is observed up to 5 GHz heating frequency range. For SiGe, the experimentally derived phase signal in the frequency domain significantly deviates from the diffusion model already in the 1 MHz - 15 MHz range. The implications of these findings for the phonon mean free path spectroscopy will be discussed. 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.

Recent progress in femtosecond laser technologies allows new physics to be probed at short time scale. Interesting and unexpected properties that do not exist in static equilibrium structures may be observed in an optically excited state. These processes are usually nonlinear due to mode-mode coupling in a strong laser field. Using density functional theory (DFT) calculations, we explore the nonlinear laser-lattice dynamics in the orthorhombic perovskites CaTiO_3 (band insulating d^0 electronic configuration) and LaTiO_3 (d^1 Mott insulator). Raman-IR mode coupling rooted in the ionic Raman scattering (IRS) process is observed, which can be described by a third-order coupling term. The coupling strength between A_g Raman modes and A_u and B_2g IR modes are computed from first principles. We find that the trends for the interaction strengths that couple these modes are similar regardless of the one-electron filling difference in the d-orbital manifold. This aspect is beneficial for the theoretical prediction of the general IRS coupling strength in oxides with stronger electron-electronic interactions, because it allows us to use knowledge of the uncorrelated d^0 system that is more easy to describe at the DFT level. Interestingly, by analyzing the role of electron correlation on the optical modes, we find that the on-site Coulomb interaction in LaTiO_3 has a substantial effect on the frequencies of three Raman modes. This is due to the electron-electron interactions within the t_2g orbital manifold lifting the orbital degeneracy. Furthermore, we explore how epitaxial strain can be used as an effective means to tune the coupling strength among modes. Here we find in LaTiO_3, the B_2g Raman mode can be used to manipulate the bandgap efficiently, which drives changes in the magnitude and character of the band gap: The indirect bandgap in LaTiO_3 is driven into a direct bandgap. We conclude by describing how the impulsive stimulated Raman scattering (ISRS) experiments could pump this mode and enable dynamic control over the band gap. By solving the ISRS equation of motion, the optimal duration of the pumping laser is calculated to be 137 fs with a pulse intensity of 0.5x10^16 W/cm^2. Such field strengths are accessible from current laser sources, albeit they may exceed the laser damage threshold of LaTiO_3.
This material is based upon work supported by the U.S. DOE, Office of Basic Energy Sciences (BES), under grant number DE-SC0012375.

Granular media are known support a rich array of linear and nonlinear acoustic wave phenomena. However, many open questions remain regarding their contact-based dynamics, particularly at micron length scales and below. In this work, we explore the dynamics of monolayers of microspheres adhered to elastic substrates and their interaction with surface acoustic waves (SAWs) in the hundreds of MHz frequency range. We will present our measurements of the resonant transmission and reflection of SAWs by monolayers of microspheres adhered to elastic substrates. Utilizing a laser-induced transient grating technique, we observe resonant, narrowband attenuation through a 100 mu;m wide microsphere monolayer strip. Using a scanning photodeflection technique, we study transmission and reflection of broadband SAW pulses at an interface between a microsphere monolayer and plain-substrate regions. The scanning experiments reveal complex collective contact-based dynamics of the microspheres, including the presence of multiple contact resonances. The interaction of these resonances with SAWs leads to the resonant SAW attenuation that can be described within the concept of a locally resonant metamaterial. Besides revealing the intricate dynamics of the spheres, the results offer insight into adhesion forces between the microspheres and between the spheres and the substrate. Furthermore, microsphere-based metamaterials may lead to new types of linear and nonlinear SAW devices. Because of the advantages of self-assembly manufacturing, acoustic studies of microsphere monolayers can be extended into the nanoscale and may lead to phenomena affecting higher frequency phonon transport.

Exploiting the impact of terahertz (THz) and sub-THz coherent phonons on electrons in semiconductor nanodevices, for providing a control of electrical signals on picosecond and sub-micrometer temporal and spatial scales respectively, could represent a change of paradigm for high-frequency ultrasonics relevant for microwaves, quantum technologies and microscopy. In this talk we show how the advanced methods of picosecond acoustics for the generation and detection of high amplitude coherent phonons with a monochromatic spectrum lead to the development of new electro-phononic devices. Three types of experiments, which show new electro-phononic effects will be described:
(1) Heterodyne electro-phononic mixing. The flux of ~100 GHz coherent phonons and monochromatic microwave radiation simultaneously exciting a semiconductor Schottky diode result in the generation of an electrical signal, which possesses the harmonic spectrum centered at the frequency equal to the difference of phonon and microwave excitations.
(2) Harmonic phono-electrical transducer. While traveling along the axis of the weakly coupled n-doped semiconductor superlattice, the monochromatic coherent phonons affect the electron tunneling, and hence the electrical current through the device. The changes in the transport properties of the superlattices caused by the sub-THz phononic excitation are explained using the current-voltage relation of the unperturbed system.
(3) The modulation of coherent phonon flux by the applied electric field. Here we fabricate a phononic chip, which includes a generator of coherent monochromatic phonons with frequency 378 GHz, a sensitive coherent phonon detector, and an active layer: a doped semiconductor superlattice, with electrical contacts, inserted into the phonon propagation path. In the experiments, we demonstrate the modulation of the amplitude and phase of the coherent phonon flux by an external electrical bias applied to the active layer.
Our findings suggest applications of the semiconductor nanodevices as an instrument for THz and sub-THz electro-phononics.

A capability of altering the phonon dispersion by manipulating the geometry of the nanostructured materials offers potential benefits in tuning thermal, electrical, optical and mechanical properties [1-2]. Phononic crystals - materials with periodic variation in density and elastic properties - have attracted attention for exhibiting energy stop band and bandgaps where acoustic phonon of specific frequencies and wave vectors cannot propagate. This phenomenon can be used for controlling heat and sound propagation by imposing nanoscale periodic boundaries. In this presentation, we report results of our study of phonon dispersion in a set of nanoscale periodic arrays implemented with porous alumina. The hexagonally arranged pores of the two different samples had diameters of 25 nm and 40 nm. The inter-pore distances were 65 nm and 105 nm, respectively. The phonon dispersion close to the center of the Brillouin zone was directly measured using the Brillouin-Mandelstam light scattering spectroscopy (BMS). The phonon wave vector was controlled by changing the scattering geometry. The spectra were excited with solid-state diode-pumped laser operating at 532 nm wavelength. The BMS instrument allowed us to investigate phonon energies from a few to hundred GHz with spectral resolution of ~0.1GHz. We observed multiple phonon peaks in addition to the regular bulk acoustic phonon peak of alumina. The additional peaks were assigned to the phonon polarization branches that appear as a result of periodic arrangement of nanoscale pores. We have correlated the phonon dispersion with the pore size and inter-pore distance. The experimental data was interpreted with the help of COMSOL modeling. The possibility of modifying acoustic phonon dispersion in such structures provides a tool for tuning the thermal conductivity at nanometer scale by means other than phonon - boundary scattering.
This work was supported as part of the Spins and Heat in Nanoscale Electronic Systems (SHINES), an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES) under Award # SC0012670.
[1] A. A. Balandin, "Phonon engineering in graphene and van der Waals materials," MRS Bulletin, 39, 817 (2014).
[2] A.A. Balandin and D.L. Nika "Phonons in low-dimensions: Engineering phonons in nanostructures and graphene," Materials Today, 15 266 (2012).

The lattice thermal conductivity is an important factor in determining the suitability of materials for a number of applications, for example as thermoelectrics^[1] or heat-confining dielectric layers in phase-change memory.^[2] Understanding the relationship between atomic structure, lattice-dynamics and transport properties is therefore an important step toward the rational design and optimisation of materials tailored to specific applications and operating requirements. Ab initio lattice-dynamics calculations (e.g. within the density-functional theory formalism) are a powerful tool for studying the dynamical properties of materials, providing access to a wealth of information at moderate levels of theory. Lattice-dynamics within the (quasi-)harmonic approximation can be used quantitatively to model phonon frequencies,^{[3, 4]} thermodynamic potentials^[5] and the temperature dependence of material properties.^[6] More recently, developments in frameworks for studying phonon-phonon interactions from third-order force constants have enabled the theoretical study of phonon lifetimes and transport properties with similar levels of accuracy.^{[3, 6, 7]} This talk will present results from modelling the thermal conductivity across a range of materials, focussing on how insight from first-principles calculations can be used to devise strategies for engineering thermal transport by exploiting:
1. Compositional complexity.
2. Dynamical (soft-mode) instabilities at high temperature.
3. Nanostructuring and surface effects.
Computational studies such as these should add additional perspectives to current materials-design approaches, and provide possible new directions for the discovery and optimisation of e.g. new thermoelectric materials.
[1] J. R. Sootsman, D. Y. Chung and M. G. Kanatzidis, Angew. Chem. Int. Ed.48, 8161 (2009), DOI: 10.1002/anie.200900598
[2] D. Loke et al., Nanotechnology22, 254019 (2011), DOI: 10.1088/0957-4484/22/25/254019
[3] J. M. Skelton et al., APL Mat.3, 041102 (2015), DOI: 10.1063/1.4917044
[4] F. Brivio et al., arXiv:1504.07508v2
[5] E. L. da Silva et al., Phys. Rev. B91, 144107 (2015), DOI: 10.1103/PhysRevB.91.144107
[6] J. M. Skelton et al., Phys. Rev. B89, 205203 (2014), DOI: 10.1103/PhysRevB.89.205203
[7] A. Togo, L. Chaput and I. Tanaka, Phys. Rev. B91, 094306 (2015), DOI: 10.1103/PhysRevB.91.094306

Thermal properties of nanoscale materials are crucial for many of applications including nanoscale electronics.[1] The challenges of the fundamental understanding and the control of thermal transport come from the fact that thermal transport is an ensemble of phonon contributions at various frequencies, which have a large dispersion of transport properties. Particularly, in nanoscale materials, confinement effects on phonon transport depend on the relative sizes of the length scale (mean-free-path, wavelength) of phonons and feature size of the material. We believe that the acoustic transport in confined structures is a foundation to understand and control thermal properties in nanosclale materials. In this presentation, we report the transport properties of acoustic phonons along the long axes of nanorod arrays. The nanorods are grown vertical to the surface of the substrate, and are two dimensionally arrayed periodically on the surface. Acoustic pulses are generated using photoelastic effect in metallic transducer attached to the samples. The samples have 3 layers; Aluminum transducer layer, SiO₂ substrate layer and SiO₂ nanorod array layer. Aluminum and SiO2 layers are fabricated using e-beam assisted deposition and e-beam lithography, respectively. Optically generated acoustic pulses propagate through the substrate and then continue to propagate into the nanorods. The phonon wavelength at the peak of the generated phonon spectrum is around 200 nm, and we used the samples with rod diameter size smaller and larger than the wavelength. Multiple echoes of acoustic pulses propagated along the long axes of the nanorods were observed when the strain pulses were probed from the transducer side. In addition the echo contribution, vibrational contributions were observed when the signal was probed from the rod side. The frequencies of the vibrational components depend on the periods of the nanorod arrays, and the frequencies of the multiple components have a fixed ratio determined by the periods. We believe this contribution is due to the interference of the probe pulse reflected by the rod and the substrate layer, and the vibrational components were attributed to the acoustically excited rod motion induced by Raleigh wave on the surface of SiO2 layer. Such excitation of vibrational mode works as an additional channel for the energy dissipation of acoustic phonons in nanoscale materials.
Reference
1. D. G. Cahill, P. V. Braun, G. Chen, D. R. Clarke, S. Fan, K. E. Goodson, P. Keblinski, W. P. King, G. D. Mahan, A. Majumdar, H. J. Maris, S. R. Phillpot, E. Pop, and L. Shi, "Nanoscale thermal transport II 2003-2012," Applied Physics Reviews 1, 011305-011301-011345 (2014).

We report the thermal conductivity of misoriented bilayer graphene synthesized by hot filament chemical vapor deposition. The thermal conductivity was measured using a non-contact technique based on resonance Raman spectroscopy applied to a suspended sheet of bilayer graphene. The fast Fourier transform (FFT) of the HRTEM image shows that the graphene layers are rotated by 30 degrees with respect to each other. The estimated room temperature thermal conductivity and G peak temperature coefficient of the bilayer graphene are 1591 Wm^-1K^-1 and 1.4 10^-2 cm^-1K^-1, respectively, which are slightly lower than previously reported values. The small G peak temperature coefficient is mainly due to the defects in graphene while the lower thermal conductivity is attributed to weakly coupled monolayers and the Umklapp phonon scattering enhanced by the reduced Brillouin zone of the misoriented bilayer system.

Molybdenum disulfide (MoS₂) as one of the remarkable 2-D materials exhibits exceptional electrical properties compared to graphene. The large intrinsic bandgap and hexagonal planar lattice make it promising for flexible nano-electronic applications, such as field-effect transistors (FET) with a high on-off ratio.^[1] However, thermal management across the interface becomes a challenge for the performance and reliability of electronic devices. Phonons are expected to be the dominant energy carriers for the interfacial thermal transport. A fundamental understanding of phonon transport mechanism across interfaces is of great importance for improving heat dissipation. In this study, we investigate the interfacial phonon transport of MoS₂ with different metals by using first principle density functional theory (DFT) and atomistic Green&’s function (AGF) simulations.^[2,3] The second order interatomic force constants (IFCs) are calculated to describe the atomic interactions at interfaces. Phonon dispersion relations and phonon density of states of MoS₂ are calculated with the optimized structure in order to explore the phonon coupling mechanism at the interface. We perform atomistic Green&’s function simulations to predict the phonon transmission and thermal boundary conductance (TBC) across different metal/MoS₂/metal interfaces. The MoS₂/Scandium (Sc) interface reveals a better TBC because of the strong interaction between the MoS₂ and Scandium atoms. In addition, different lattice stacking configurations of MoS₂ on Sc are considered to elucidate the effect of these configurations on TBC. Results show that the lattice stackings have significant impact on the interaction at Sc/MoS₂/Sc interfaces, which could change the MoS₂/Sc interface from physisorption to chemisorption leading to significant enhancement in phonon transmission and TBC at the interface. This study will provide insights to understand the thermal transport mechanism at MoS₂/metal interface and to enhance the heat dissipation in MoS₂ based nano-electronic devices.
References
[1] B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti, and A. Kis, Nat. Nanotechnol. 6, 147 (2011).
[2] L. Chen, Z. Huang, and S. Kumar, RSC Advances 4, 35852 (2014).
[3] W. Zhang, T. S. Fisher, and N. Mingo, Journal of Heat Transfer 129, 483 (2006).

Thermal band gaps, the analogous to electronic band gaps in semiconductors, are frequencies ranges for which thermal vibrations are not allowed to propagate within certain periodic structures. Materials having thermal band gaps, also known as thermocrystals, create fundamental changes in the way we manipulate the flow of thermal energy. Their technological potential, such as enhancing thermoelectric energy conversion and improving thermal insulation, has recently fueled the search for highly efficient thermal band gap materials. Here we discuss recent developments in understanding and manipulating heat transport using wave interference and thermal band gaps. We show that rational design and fabrication of nanostructures provides unprecedented opportunities to create wave-like behavior of heat and band gap control, leading to a fundamentally new approach to manipulate the transfer of thermal energy.

Half-Heusler compounds are materials with promising properties for thermoelectric applications in an intermediate temperature range near 500 °C and they can be made from elements that are neither very rare nor toxic. However, these materials have a comparably high thermal conductivity. The reduction of the thermal conductivity is a crucial aspect in thermoelectric research. We prepared superlattices of TiNiSn and HfNiSn Half-Heusler compounds where we expect the isoelctronic substitution not to change the electronic properties, while the mass fluctuation scattering at the interface should efficiently scatter phonons.
We perform an intense structural characterization of our superlattices using atomic force microscopy, x-ray analysis and transmission electron microcopy. The superlattices grow epitaxially on MgO (001) oriented substrates, with a rotation of the in-plane cubic axes by 45° between film and substrate. The x-ray diffraction diagrams of the superlattices show a large number of satellites indicating an artificial layer structure. By simulation of the x-ray diffraction diagrams we can achieve a good agreement to the measurement and can determine the strain state for the out of plane lattice constant and determine the exact periodicity. Transmission electron microscopy measurements confirm the x-ray results.
We report a systematic and significant reduction of the cross-plane thermal conductivity in our model system consisting of DC sputtered TiNiSn and HfNiSn half-Heusler superlattices. All superlattice samples have a total thickness of 1000 nm but have different densities of interfaces, i.e. the TiNiSn/HfNiSn bilyar thickness varies. We implement the 3omega; method to determine the cross-plane heat conductivity κ. Down to superlattice periodicities of 20 nm the phonon spectrum mismatch between the superlattice components quantitatively explains the reduction of κ. For very thin individual layers the interface model breaks down as the typical phonon wavelength is larger than the layer thickness. In this limit the superlattice must be considered as an artificial crystal and shows an enhanced κ. In the intermediate thickness range for a superlattice with a periodicity of 3 nm we achieve a minimum of κ=1.2 W/mK. The reduction of κ leads to an enhanced figue of merit ZT for all investigated superlattices compared to the single TiNiSn and HfNiSn films.
Financial support by the DFG (SPP1386, Ja821/4-2), the Graduate School of Excellence Material Science in Mainz (GSC 266), and the DAAD (SpinNet) is gratefully acknowledged.

We will present the first scanning tunneling microscopy (STM) study of standing wave patterns produced by decoupled from bulk 2D phonons. Phonon interference patterns of double atomic size periodicity were observed by STM on quasi-freestanding WSe₂ monolayers prepared by metal organic chemical vapor deposition (MOCVD) technique [1]. In these samples, phonon standing waves develop around individual intercalating defects. The STM 2D Fourier analysis indicates that all optical phonons from the first Brillouin zone contribute to formation of observed standing waves. Our results reveal the unknown previously connection between phonon interference and quantum phase synchronization effects. Interference patterns of both synchronized and non-synchronized optical phonons, as well as symmetry breaking for individual phonon wave packets were experimentally observed. We show that crucial role in STM detection of surface phonons belongs to electron-phonon interaction.
References: [1] S. M. Eichfeld et al., ACS Nano 9, 2080 (2015)

Phonon scattering at surfaces and interfaces is of critical importance for determining the transport of thermal energy in semiconductor nanostructures. In particular, establishing the amount of phonon specular reflection and diffuse scattering at smooth and rough surfaces is a complex problem involving different physical properties. Although phonon surface scattering directly influences thermal conduction, understanding of such critical phenomenon has been largely limited. Here, we use perturbation theory and quasi-classical approximations to systematically analyze phonon surface scattering and its impact on thermal energy transport. A systematic and rigorous analysis is presented where the transport of both high and low frequency phonons is investigated in terms of the local/non-local character of their interactions with roughness. Resultant thermal conductivities are calculated based on these approaches and compared with experiments. To accurately predict nanoscale thermal transport, we found that current models in the literature need to be improved to include additional surface and phonon properties not considered in existing models.

The operating temperature of nanoscale devices can be influenced and often dominated by thermal resistance due to poor phonon transmission at metal-dielectric interfaces. Heat assisted magnetic recording is a new technology that exemplifies the need to minimize thermal interface resistance in concert with optimizing other interface functions. In the case of HAMR, the unique plasmonic properties of Au-dielectric interfaces are critical to operation, and metals with inherently lower thermal interface resistance are unsuitable. To remedy this, we are studying tradeoffs in thermal and plasmonic performance of Au-dielectric interfaces enhanced by adhesion layers or through alloying with other metals.
We first present a study of thermal interface resistance across a Au-sapphire interface with tapered Cu and Cr adhesion layers ranging from 0-5 nm in thickness. Thermoreflectance measurements of thermal interface resistance made as a function of the adhesion layer thickness demonstrate rapid reductions by factors of greater than 2 and 4, respectively, when the layer is just 1 nm thick. Predictions of phonon transmission based on the diffuse mismatch model cannot account for this behavior and offer evidence that interfacial adhesion, not just phonon matching on either side of the interface, are critical to phonon transmission in the presence of thin intermediate layers. We then present a second study of thermal resistance across alloy-sapphire interfaces for alloy films compositionally graded between pure Au and pure Cu, as well as between pure Au and pure Pd. For these metal pairs, which are miscible at all compositions, we find monotonically decreasing trends in thermal interface resistance as a function of Au atomic fraction. These trends can be readily explained using the diffuse mismatch model modified by the virtual crystal approximation. Alloys with more complex phase diagrams may not adhere to this simplistic behavior. While both adhesion layers and alloying reduce Au-sapphire thermal interface resistance, their as yet undetermined impact on plasmonic properties is equally important to HAMR.

Understanding the propagation and scattering of phonons across solid interfaces is fundamentally imperative due to its relationship with thermal transport in composite materials and relevance for nanoscale thermal management. The acoustic mismatch model (AMM) and the diffuse mismatch model (DMM) dominate the current conceptual landscape for understanding the phonon transmission physics, but yet these methods can neither accurately incorporate the atomistic structure and orientation of the interface nor include the full phonon dispersion of the constituent solids in the calculations. On the other hand, the commonly used atomistic Green&’s function (AGF) method, which provides an atomistic framework for computing the overall transmission, lacks a direct connection to the phonon dispersion of the bulk materials and does not allow us to spectrally resolve the contribution by individual modes.
In our talk, we discuss our reformulation of the AGF method [Z.-Y. Ong and G. Zhang, "Efficient Approach for Modeling Phonon Transmission Probability in Nanoscale Interfacial Thermal Transport," Phys. Rev. B 91, 174302 (2015)] in which we can now compute efficiently the transmission probability of individual acoustic and optical modes, allowing us to bridges the gap between the AGF method and the simpler AMM/DMM theories. With this new approach, we can easily calculate the transmission coefficients of all the individual modes along the various acoustic and optical phonon branches across the entire frequency range in one swoop, thus availing to us a level of detail previously inaccessible with other computational techniques. We also obtain the Brillouin zone composition of the strongly transmitted phonon modes and thus, a bird&’s eye view of how the overall distribution of transmitted modes depends on phonon momenta.
To illustrate its utility, we apply the method to the analysis of the graphene/ hexagonal boron nitride (Gr/h#8209;BN) lateral heterostructure. We show that its interfacial structure has profoundly different effects on the transmission of acoustic and optical phonons. We also find the phononic analogue of Snell&’s law predicted in the AMM but generalized for the non-isotropic phonon dispersion in graphene and h-BN. We also analyse how the phonon polarization affect its transmission across the interface and show that the interfacial conductance is dominated by flexural phonons. This new computational method provides us with fresh opportunities to study phonon scattering processes at the interface, setting the stage for further theoretical and computational advances in thermal transport and other related phenomena.

Thermal transport across the interfaces between few-layer graphene sheets and soft materials plays an important role in many applications of graphene. An intriguing anomaly emerging from recent studies of such thermal transport is that, when interpreted using the classical Kapitza model, the thermal conductance of the same interface depends very strongly on how the interfacial thermal transport is probed. One can differentiate two different modes of thermal transport, which lead to an apparent duality of interfacial thermal conductance. In the first “across” mode of thermal transport, heat enters graphene from one side of its basal plane and leaves through the other side. In the second “non-across” mode of thermal transport, heat enters or leaves graphene simultaneously from both sides of its basal plane. For a single-layer graphene immersed in octane fluid, our molecular dynamics simulations show that the interfacial thermal conductance is ~150 and ~5 MW/m²K, for the “across” and “non-across” modes of interfacial thermal transport, respectively. We clarify the physical mechanisms of the apparent duality of the thermal conductance of graphene-octane interfaces.
To help establish a coherent description of the thermal transport across graphene-soft material interfaces that is free of the above anomalies, we propose a nonlocal flux-temperature drop constitutive law for such thermal transport and suggest that the interfacial thermal transport properties of these interfaces can be characterized jointly by a quasi-local conductance and a nonlocal conductance instead of the classical Kapitza conductance. Using this nonlocal model, we rationalize anomalies of the thermal transport across embedded graphene straightforwardly. We also demonstrate a special case where the temperature jump across a zero flux interface could not be explained by the Kapitza model but is predicted by the nonlocal model. The implications of the nonlocal interfacial thermal transport at graphene-soft material interfaces in the development and characterization of graphene-based thermal management materials will be discussed.

The thermal interface conductance between several common metal silicides and single crystal silicon is studied. By growing silicides using various substrate orientations, both epitaxial and non-epitaxial interfaces were formed including PtSi/Si, NiSi/Si, and CoSi2/Si. Time-domain thermoreflectance measurements were used to determine the thermal interface conductance. For all material combinations studied, we find that the thermal interface conductances of the epitaxial interface and non-epitaxial interfaces are the same to within experimental resolution, indicating that surface structure is not a determining factor for thermal interface conductance. NiSi shows the largest room temperature thermal interface conductance of any known interface with silicon with G = 390 MW/m2-K.

A major obstacle to the technological application of single-layer graphene is its lack of an electronic band gap. Advances in chemical vapor deposition have enabled fabrication of in-plane structures built from graphene and other two-dimensional materials, including hexagonal boron nitride (hBN). These heterostructures (e.g., an in-plane graphene-hBN superlattice) allow for the creation and tuning of an electronic band gap, while still taking advantage of graphene's high thermal conductivity. Such materials hold promise for device applications, however, the influence of in-plane interfaces and the substrate on thermal transport must be quantified.
We apply the atomistic scattering boundary method to calculate the phonon transmission coefficients and thermal conductance of symmetrically-strained interfaces between graphene and hBN with and without a substrate. Force constants are obtained from both first principles density functional theory (DFT) calculations and from Tersoff potentials optimized to fit DFT-based phonon dispersions. Transmission coefficients are computed for phonon wave vectors from the entire first Brillouin zone and we quantify the effect of transverse-longitudinal and acoustic-optical mode conversion at the interface. We probe whether the effect of the substrate on the interface thermal conductance can be explained solely by its modification of the bulk phonon dispersions. We also quantify the impact of using force constants from DFT versus those from empirical potentials.

At the interfaces between dissimilar solids, thermal boundary resistance is the property that primarily dictates the transport of energy carriers. In micro and nanoscale systems such as in microelectronic and energy conversion devices, the interfaces become much more significant to the thermal management because they have very high heat dissipation densities. Thermo-reflectance based techniques are vastly used to measure the Interfacial Thermal Resistance (ITR). We present an experimental technique based on Infrared thermometry which exploits the size dependence of thermal resistance in solid thin films that magnifies the temperature drop. The interface between two thin films is prepared and made freestanding using standard nanofabrication techniques. An Infrared microscope is used to obtain the temperature map of the specimen and the temperature drop at the interface. The heat flux transported across the interface is obtained by modeling the heat conduction in the sample specimen as micro-fin geometry with heat loss to surrounding air. We present experimental results on metal-dielectric and semiconductor-dielectric interfaces involving Aluminum, Copper, Silicon, Silicon di-oxide and Hafnium oxide for validation of the technique

Structure-property-processing correlations are of increasing importance as atomic layer materials move towards practical device applications. As such, grain boundaries (GBs) are being widely known to govern the properties of atomically thin vapor deposited films. Graphene has served as the model 2D system for over a decade, and the effect of GBs on its electrical and mechanical properties are well investigated. However, no direct measurement of the correlation between thermal transport and graphene GBs has been reported so far. This is mainly because the direct experimental thermal transport study of GBs requires a measurement platform capable of segregating the contribution of the GB from the graphene lattice itself. Here, for the first time, we have been able to perform a simultaneous comparison of thermal transport in supported single crystalline graphene to thermal transport across an individual graphene GB. Our experiments show that thermal conductance (per unit area) through an isolated GB can be up to an order of magnitude lower than the theoretically anticipated values. Our measurements are supported by Boltzmann transport modelling which uncovers a new bimodal phonon scattering phenomenon initiated by the GB structure, in which the boundary roughness scattering dominates the phonon transport in low-mismatch GBs, while for higher mismatch angles there is an additional resistance caused by the formation of a disordered region at the GB. Non-equilibrium molecular dynamics simulations verify that the amount of disorder in the GB region is the determining factor in impeding thermal transport across GBs. Our results highlight the important structure-property-processing correlations of 1D defects on thermal transport in 2D crystals, emphasizing the importance of engineering such correlations in emerging 2D materials and devices.

Nitride material systems are promising wide-bandgap semiconductors for next-generation RF military electronics due to their high breakdown voltages and carrier densities. GaN-based high electron mobility transistors (HEMTs) in particular have attracted significant interest for high frequency RF devices. However, performance of these devices at high frequencies is limited by heat removal to the substrate. Silicon carbide (SiC) is used as the substrate for the vast majority of today's high power GaN-based devices because of its closely matched lattice spacing to GaN and its high thermal conductivity. We have measured the thermal boundary resistance of a heteroepitaxially grown GaN-on-SiC interface on 4H-SiC and 6H-SiC for the temperature range of 300-600K. Our results are in good agreement with a corrected diffuse mismatch model. This is the first experimental measurement of a GaN-on-SiC interface without a transition layer. Our results provide insight on factors determining heat dissipation in GaN devices.

Phonon transport theory represents an adequate paradigm to study the microstructure and stoichiometry effects on thermal transport phenomena in irradiated ceramics. In this regard, the difficulty of solving Boltzmann Transport Equation (BTE) exactly stimulated the interest in introducing relevant approximations. The Relaxation Time Approximation (RTA) is one of the most common approaches in this context. Under RTA all phonon physics are lumped together in one set of parameters called relaxation times that appears in the linearized form of BTE. Accordingly, relaxation time calculation is the most critical part in this approach. In earlier studies, the motivation of using RTA was to derive analytical expressions for phonon relaxation times to be used in estimating lattice thermal conductivity. Later on, further approximations were introduced regarding the shape of the Brillouin zone and dispersion relations, e.g.; long wave length approximation, isotropic continuum, neglecting the optical polarizations of phonon spectrum, or even assuming only one effective polarization branch (Debye model) or one mode (grey model). Due to fitting the adjustable parameters with experimentally measured thermal conductivity, most of these approximations seem capable of reproducing thermal conductivity with an acceptable accuracy (agreement in integral sense); however extending the use of these models to other regimes out of the ranges for which they were fit necessitates comprehensive understanding of how reliable their global behavior is compared with the original model and investigating the point-by-point correspondence with experimentally measured phonon linewidth. We present a quantitative assessment of different approximations used to calculate intrinsic relaxation time of phonons based on Fermi's golden rule for the case of UO₂, by comparison with a more rigorous procedure that accounts for the exact shape of the brillouin zone, all phonon polarization branches, and momentum and energy conservation rules. In addition, anisotropy in dispersion relations is fully resolved by considering the experimental data of phonon dispersion relations in high symmetry directions.

Two dimensional materials based on atomic monolayers are emerging as leading materials for future energy efficient and multifunctional electronics. Due to the single atom thickness of monolayers, their properties are strongly affected by interactions with the external environment. We study graphene on a silicon dioxide substrate as a prototypical example of a two dimensional material interacting with a common electronics environment. In order to assess its potential as an electronic switch—an application which is limited by heat dissipation—we study the thermal conductance between graphene and its substrate. A new model is developed to explain the heat transfer from the two dimensional graphene into a three dimensional substrate via weak van der Waals (vdW) bonding. We find the flexural phonon branch to contribute most to the thermal conductance in the cross plane direction. Results correlate well with experimentally found data [Z. Chen et al., Appl. Phys. Lett. 95, 161910 (2009).]. We also contrast our results against predictions by empirical models such as the vdW Acoustic Mismatch Model [R. Prasher, Appl. Phys. Lett. 94, 041905 (2009).] and the Diffuse Mismatch Model.

Tailoring thermal properties in nanostructured two-dimensional materials represents a potential route to innovation within a variety of applications, including thermoelectric energy conversion and microelectronic thermal management. Interfaces and defects in these atomically-thin films result in anomalous scattering of thermal energy carriers which gives rise to large variance in bulk thermal conductivity. Here, we describe the application of a correlative dark-field transmission electron microscopy and optothermal Raman spectroscopy technique to the study of thermal properties of suspended graphene membranes of varying grain structure. Structural information, including grain orientation and spatial arrangement determined with electron diffraction and real-space imaging, was linked to Stokes shifts observed with Raman spectroscopy and associated with local heating in the membranes. With these methods, we measured a five-fold reduction in thermal conductivity of the membranes as grain size decreased from a few micrometers to a few-hundred nanometers. Additionally, atomic thermal vibrations in individual grains induced via in-situ laser heating, and the commensurate Debye-Waller factor extracted from selected-area electron diffraction patterns, were quantified, and it was found that atomic root-mean-square displacements from equilibrium positions ranged from 4 to 7% of the carbon-carbon bond length in graphene for varying grains at peak-laser excitations. We discuss the implications of these results in the context of in-situ nanoscale thermometry of emerging two-dimensional materials such as the transition metal dichalcogenides and black phosphorus.

The mean free paths of transport in structurally disordered dielectrics are significantly reduced as compared with those of phonons in crystalline counterparts. Spatial and temporal confinement of vibrational modes, where the mean free path of transport is comparable with the vibrational wavelength and the lifetime compares with the vibrational period, generally results in modified thermal conductivity of disordered dielectric solids with sufficiently suppressed mean free paths, e.g., glasses, proteins, viscous liquids and nanoscale clustered arrangements. In this domain, there is fundamental uncertainty associated with the parameters in reciprocal space and time. In this article, thermal properties of amorphous silica, heat capacity and thermal conductivity, are predicted as a function of concomitant propagating and non-propagating vibrations. An effective energy dispersion for non-propagating vibrations is derived by relating the magnitude of uncertainty in frequency to the uncertainty of momentum in reciprocal space. Effective group and phase velocities for non-propagating vibrations are derived analogously to the definitions for plane waves. In this way, the harmonic thermal diffusivity is derived self-consistently.

Better understanding transport and thermodynamic behaviors in UO₂ is essential to predict fuel performance and design new fuel types. In spite of being important, it is very difficult to use first principles to predict phonon transport and interactions and resulting thermodynamic behaviors in UO₂, stemming from combining a strong electron correlation, multi-minima and relativistic effects of 5f valence electrons. In this work, we present results of a comprehensive first principles study of phonon transport and interactions of phonon-boundary, phonon-polarization and phonon-magnetization, as well as resulting thermophysical properties, in nanoscale UO₂, and compare with results of recent experimental studies. This work is supported by the Nuclear Energy Advanced Modeling and Simulation (NEAMS) program funded by the U.S. Department of Energy, Office of Nuclear Energy.

Diamond nanothreads (DNTs), a one-dimensional glassy carbon allotrope with sp³ hybridization and disordered Stone-Thrower-Wales (STW) defects, have recently been synthesized in the laboratory for the first time^1,2. In this work, using first-principles calculations and classical atomistic simulations, we determine the length and temperature dependence of the lattice thermal conductivity of DNTs and related materials. Since DNTs exhibit a distribution of STW defects, they can be considered to be a hydrogenated, disordered analog of (3,0) sp² carbon nanotubes. Assessment of the thermal conductivity offers an opportunity to determine the effects of disorder in one-dimensional systems. Therefore, we first consider regular, ordered (3,0) sp² carbon nanotubes (CNTs) and show that they are even two-fold less conducting than recently reported (2,1) CNTs³. The unique selection rule leads to long mean free path of flexural (X,Y) and torsional (Θ) modes, and the transport remains ballistic to lengths greater than 450nm. The gradual introduction of both sp³ hybridization via hydrogenation and ordered STW defects impedes phonon transport and further reduces thermal conductivity, but the thermal conductivity still grows up to lengths approaching 1 mu;m. Also we show the pronounced effects of localization that changes the behavior when STW defects are introduced in a disordered manner.
[1] T. C. Fitzgibbons, M. Guthrie, E. Xu, V. H. Crespi, S. K. Davidowski, G. D. Cody, N. Alem, and J. V. Badding, Benzene-derived carbon nanothreads, Nat. Mater. 14, 43 (2015).
[2] R. E. Roman, K. Kwan, and S. W. Cranford, Mechanical properties and defect Sensitivity of diamond nanothreads, Nano. Lett. 15, 1585 (2015).
[3] L. Zhu and B. Li, Low thermal conductivity in ultrathin carbon nanotube (2, 1), Sci. Rep. 4, 1 (2014).

Velocity saturation plays the most important role in determining operational characteristics of field effect transistors. While the causes of velocity saturation in GaAs and InP high electron mobility transistors (HEMT&’s) are well known to be band nonparabolicity and inter-valley transfer, for the GaN -based HEMT&’s the cause has not been indisputably determined. Moreover, the saturation velocity in GaN HEMT has been shown experimentally to be a strong function of carrier density, with higher doped GaN channels showing lower saturation velocity. The source of this dependence also remains unclear.
In this work, we show that the cause of velocity saturation in GaN may well be stimulated emission of LO phonons. In high fields the shift of the Fermi distribution of electrons can lead to the situation in which probability of stimulated emission of LO phonons for certain wave vectors exceeds the probability of absorption of these phonons, creating the phonon gain, analogous to the photon gain in a laser. This gain is proportional to the electron density. Since LO phonons in GaN are long-lived and Froelich interaction between phonons and electrons is strong, eventually the phonon gain exceeds the LO decay loss and the avalanche of phonons ensues, a situation resembling lasing threshold. Once the threshold is reach velocity saturates as all the additional kinetic energy acquired by electrons is lost via the “phonon lasing”.
Once the simulated emission is included into the Boltzmann transport model, the velocity saturation values for different carrier densities are obtained. These values show excellent agreement with experimental results, indicating that “phonon lasing” is a highly probable cause of velocity saturation in GaN HEMT&’s, and also shedding light on the way to increase the saturation velocity and thus the speed of the devices.

The insulator to metal transition, as well as the metal to superconducting transition of lead thin films have been studied extensively. Traditional methods of fabricating thin films typically are not able to quench-condense material for in-situ measurements while still maintain precise control over deposition conditions. Standard deposition techniques require a significant amount of power, making it difficult to keep the temperature of the substrate below the superconducting transition temperature during deposition. Annealing effects at low temperatures have been measured, hence minimizing the power required for thin film growth will result in highly quench-condensed thin films, which exhibit different electrical and structural characteristics from annealed thin films.
Based on the ‘Fab on a Chip&’ approach, we have developed a system of MEMS based micro sources and mass sensors, which enable us to quench-condense superconducting lead thin films with low heat generation and precise control over deposition rates. We have implemented a programmable thermal evaporation source by heating a micron-scale silicon plate using pulse width modulation to control the power. An on-chip MEMS mass sensor precisely monitors the deposition thickness. MEMS devices consume very low power during evaporation, thus significantly limit their impact on the cryogenic system. As a result, the thin film stays cold on the substrate, and is quench-condensed below the superconducting transition temperature.
We show in-situ transport experiments of quench-condensed superconducting lead thin films fabricated using our approach. The superconducting phase transition increases with thickness and is first observed at ~2 nm, indicating that a smooth and uniform quench-condensed film is already achieved at ~6 monolayers. The result shows that our devices are capable of fabricating high-quality quench-condensed thin films with fine control over deposition conditions.
We thank the NSF for their support of this research under grant No.1361948.

The widespread usage of silicon in modern technology makes the understanding of its thermodynamics and thermal transport of great importance. As the phonons dominate the total entropy as well as thermal transport properties, it is essential to map out the temperature dependent lattice dynamics accurately. Inelastic neutron scattering on a single crystal of silicon and silicon powder was performed at temperatures ranging from 100 to 1500 K. The single crystal was machined for optimal neutronics in the ARCS spectrometer at the SNS. These high temperature measurements of dispersions and densities of states are the first at these temperatures, as previous work had measured the phonon densities of states of pure silicon only up to 700 K and phonon dispersions up to 300 K. Large phonon anharmonicities were manifested by phonon energy shifts and broadenings at high temperatures in the phonon dispersions and in the phonon density of states. At 1500 K the anharmonicity contributes approximately 80% of the deviation from the harmonic vibrational entropy. These large effects are beyond the predictions of the quasiharmonic model, demonstrating that phonon anharmonicities are the cause of substantial changes in both vibrational entropy as well as phonon lifetimes. An update of our analysis of the lifetime broadenings of individual phonons will be presented at the meeting.

While zT above 300 K has seen significant enhancement recently, progress in zT at lower temperatures has been slow, mainly due to the relatively low Seebeck coefficient and the high thermal conductivity. It has been discovered that certain materials exhibit an “anomalous” Seebeck coefficient which increases as temperature decreases (the so-called “phonon drag” effect). It is therefore tempting to make use of this effect for higher-efficiency thermoelectrics via boosting the Seebeck coefficient. However, experimentally it is found that the phonon drag effect is reduced as the doping concentration increases, while good thermoelectric materials are usually heavily doped. Furthermore, a large phonon drag effect implies a high thermal conductivity, which is reflected in the experimental fact that the low-temperature peak of the phonon drag Seebeck coefficient usually coincides with the peak of the thermal conductivity. So far it is not clear to what extent one can retain a significant phonon drag Seebeck coefficient in a heavily doped material while reducing the lattice thermal conductivity, the latter of which has recently become a common strategy in increasing the thermoelectric efficiency. In this work we report, for the first time, first-principles simulation of the phonon drag effect in silicon and we examine the detailed phonon mode contributions. Our results are justified by comparing with experiments across a wide range of temperatures and carrier concentrations, which show that in heavily doped samples the phonon drag Seebeck coefficient is not negligible as previously thought. By analyzing the contributions from different phonon modes, we identify the phonons that contribute most to phonon drag which are spectrally distinct from those that carry heat. This provides the key to decouple the two properties and leads to the strategy of reducing the thermal conductivity without sacrificing the phonon drag Seebeck coefficient much, by selecting the “preferable” phonon modes and filtering out others. Based on this, an ideal phonon filter is demonstrated to increase zT in heavily doped n-type silicon to ~0.25 at room temperature, and the enhancement ratio reaches 70 times at 100 K. A practically feasible phonon filter based on nanocluster scattering is also discussed. We envisage that along this path more material systems can be systematically studied with quantitative understanding of their coupled electron-phonon transport. This approach can thus open up a new venue towards fabricating better themoelectric materials by harnessing non-equilibrium phonons.

Modern cryogenic engineering enables the development of cryotherapy, Bose-Einstein condensates, and superconductivity. To achieve solid-state cryocooler which has advantages of compact, high liability, no vibration, and no need for cryogenic fluid, laser cooling of solids have been intensively studied. Here we show the first time laser-refrigeration of Yb_shy;shy;shy;³⁺-doped NaYF₄ microcrystals in aqueous media with a single-beam near-infrared (NIR) laser trapping system. The local refrigeration of the individual microcrystals in aqueous media is quantified through the analysis of its cold Brownian motion. An individual NaYF₄ microcrystal is optically trapped by a NIR continuous wave laser source with an irradiance on the order of 1MW/cm². The individual NaYF₄ microcrystals show local cooling by 9°C with heat being extracted out of the lattice through anti-Stokes photoluminescence of Yb³⁺ excited-state-mediated optical phonon absorption, which suggests a range of potential future applications.

The ultrafast nature of the Metal-Insulator transition (MIT) and structural phase transition (SPT) in VO₂ near room temperature (T_c ~ 340K) makes it an attractive material for applications in electronics and optical devices. However, few works have been done to study the drastic phase change induced changes in thermo-physical properties and their potential applications. In this work, we investigate the respective phonon and electronic thermal conductivity changes across the phase transition in pristine and metal ion doped VO₂ thin films in both experiments and theory. The thermal conductivity of the VO₂ thin films are measured using the time-domain thermoreflectance (TDTR) method. First principles density functional theory (DFT) calculations are performed in conjunction with Boltzmann transport calculations to model the electronic and thermal transport properties of pristine and doped VO₂. We will also discuss the impact of electron-phone interactions on thermal conductivity for monoclinic and rutile phaseVO₂.