Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session R16: Transport in Nanostructures -- Electron-phonon Coupling and Phonon Transport in Nanostructures and HeterostructuresFocus
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Sponsoring Units: DMP Chair: Charles Harris, Sandia National Laboratories Room: BCEC 155 |
Thursday, March 7, 2019 8:00AM - 8:12AM |
R16.00001: Spatially-Resolved Non-Equilibrium Phonon Transport Across Nanoscale Interfaces Georgios Varnavides, Adam Jermyn, Polina Anikeeva, Prineha Narang Understanding phonon-mediated heat transfer at the nanoscale, to explain heat-dissipation in nanoelectronics, local temperature effects, and new phenomena probed my ultrafast coherent dynamics, presents a theoretical challenge. Further, nanoscale interfaces pose an exciting experimental frontier, with diverse applications from interface-engineering for thermoelectrics to catalytic interfaces and nanotheranostic agents. Despite the ubiquity of these applications, an accurate microscopic description of thermal interface resistance (TIR) remains elusive. To address this, we introduce a new theoretical and computational framework for semi-classical transport with position-, momentum-space, and scattering event resolution. We demonstrate the recursive formalism for phonon transport in a spherical nanoparticle, and highlight new insights made possible using this multi-dimensional resolution. We extend the formalism to handle interfaces explicitly to quantitatively capture TIR for the technologically relevant Si-Ge heterostructure. Finally, we will present preliminary results in coherent and driven phonon effects, now accesible via ultrafast spectroscopies. |
Thursday, March 7, 2019 8:12AM - 8:24AM |
R16.00002: Unconventional Impact of Interfacial Thermal Coupling in Film-On-Substrate Systems Kartik Kothari, Abhinav Malhotra, Martin Maldovan An accurate determination of thermal transport in thin film-on-substrate (FOS) architectures is crucial to optimum performance of nanostructured optoelectronic devices. A rigorous treatment of the nanoscale interfacial coupling between materials accounting for dispersion mismatch, interfacial roughness and shadowing effects is imperative in studying the impact of substrates on thin-film heat conduction. In a unique finding, we discover an increase in thermal conductivity with a reduction in thin-film thickness attributed to phonon injection from the substrate layer. We examine the in-plane and cross-plane configurations of thermal conduction in Ge and Al0.1Ga0.9As thin-films mounted over Si and GaAs substrates respectively. We provide an extensive analysis of phononic coupling and contrast the results with bulk and isolated thin film values. We present a detailed microscopic and spectral analysis by investigating the spatial thermal flux distribution, modal thermal conductivity, and mean free path and frequency spectra in the FOS architecture. We demonstrate how interlayer phonon coupling opens new avenues for thermal conductivity manipulation in nanostructures and achieves desired thermal properties for rational thermal material design in microelectronics and optoelectronics. |
Thursday, March 7, 2019 8:24AM - 8:36AM |
R16.00003: Modeling phonon transport in nanowires with rough surfaces Sid Abhinav, Khandker A Muttalib We study phonon transmission in thin silicon nanowires where surface-roughness dominates over bulk disorder. Our previous study was based on an exact mapping of surface disorder, modeled as a random distribution of localized phonons1. Here we extend the idea to study its effects on transport of propagating acoustic phonons.We characterize the localized phonons by surface roughness parameters and study the effects using NEGF techniques. Non-linear heat current is evaluated for various couplings between the propagating phonons from the leads and the localized phonon in the central device. Numerical evaluation gives frequency dependence of heat current at various finite temperatures, where the difference in temperature between the two leads can be large under non-equilibrium conditions. We show that our simple model captures the qualitative features obtained in experiments, and suggests combination of surface parameters that can lead to a substantial reduction in thermal conductivity, as required for applications in thermoelectricity. |
Thursday, March 7, 2019 8:36AM - 8:48AM |
R16.00004: Phonon-dominated thermal transport in an ultrathin Au-Ni bilayer Alexei Maznev, Jan-Etienne Pudell, Marc Herzog, Matthias Kronseder, Christian Back, Gregory Malinowski, Alexander von Reppert, Matias Bargheer Thermal conductivity in metals is normally predominantly electronic. However, at small distances, the phonon contribution to thermal transport becomes increasingly important, especially in metals with weak electron-phonon coupling such as Au. In this report, we show that thermal transport in an Au-Ni bilayer becomes predominantly phononic when the Au thickness is ~6 nm or less. We analyze recent ultrafast x-ray diffraction measurements of heat transport in an ultrathin metal bilayer consisting of 5.6 nm Au and 12 nm Ni [1] together with earlier measurements on a thicker bilayer [2] and show that in the experiment [1] more than 50% of the heat flow from the Ni lattice to Au lattice was carried by phonons, despite a high thermal boundary resistance for phonon heat transport due to a large mismatch of the phonon spectra. The measurements reported in Ref. [1] offer a way to study heat transport by phonons in multilayer structures on the single digit nanometer scale and could be used for direct testing of non-equilibrium molecular dynamics simulations. |
Thursday, March 7, 2019 8:48AM - 9:00AM |
R16.00005: Beyond diffusive scattering: Phonon coupling to reduce thermal conductivity Abhinav Malhotra, Kartik Kothari, Martin Maldovan Diffusive scattering has remained the only mechanism to reduce thermal conduction at the nanoscale. In this talk, we show that phonon coupling in layered nanomaterials (bi-layers and tri-layers) can be engineered to reduce the thermal conductivity of a silicon thin-film below its free-standing value. We present a methodology to quantitatively evaluate the impact of phonon coupling on each layer in layered nanostructures. We evaluate the dependence of resultant thermal conductivity modulations on structural parameters and find that they are critically dependent on layer spacings and interface properties. The results of this work open new avenues within the rational thermal design by elucidating a new method that can be used to reduce thermal conductivities beyond the traditional diffusive scattering based approaches. The prospects of being able to modulate the thermal conductivity can radically change how we control heat flow in electronic, optoelectronic, and thermoelectric materials. |
Thursday, March 7, 2019 9:00AM - 9:36AM |
R16.00006: Electronic Thermal Transport in h-BN/Graphene/h-BN heterostructures Invited Speaker: Li Shi Using a senstive differential electro-thermal measurement technique, we observe that the Lorenz number for the electornic thermal conductivity of h-BN/graphene/h-BN heterostructures is considerablly lower than the Sommerfeld value in the single-band regime at room temperatrue and below. In the bi-polar regime near the Dirac point, a Lorenz peak well above the Sommerfeld value is observed at room temperatrue, and decreases with decreasing temperature. The deviation from the Wiedemann-Franz law and its temperature dependence provide insight into the unique transport behavior of Dirac Fermions in this system. |
Thursday, March 7, 2019 9:36AM - 9:48AM |
R16.00007: An Onsager reciprocity relation for ballistic phonon heat transport in anisotropic thin films of arbitrary orientation Geoff Wehmeyer, Andrea D Pickel, Chris Dames A classic Onsager reciprocity relation for Fourier heat conduction states that the thermal conductivity tensor in bulk anisotropic solids is symmetric. However, since Fourier’s law fails in thin dielectric films due to ballistic phonon transport effects, it is natural to ask whether an analogous Onsager relation can be identified for thin films. To answer this question, we solve the Boltzmann transport equation (BTE) under the relaxation time approximation for in-plane and cross-plane heat transport in thin films with anisotropic phonon dispersion relations and scattering rates. These BTE solutions show that the effective thermal conductivity tensor of thin films is symmetric from the diffusive through the boundary scattering regime. We validate the BTE solution against previous atomistic simulations of arbitrarily aligned graphite thin films, and use published first-principles calculations to model anisotropic heat flow in black phosphorus thin films. This derivation shows how Onsager reciprocity for anisotropic heat conduction extends to the boundary scattering regime, and reduces the number of independent measurements required to characterize heat transport in anisotropic thin films. |
Thursday, March 7, 2019 9:48AM - 10:00AM |
R16.00008: Surface Acoustic Wave Generation and Detection on LaAlO3/SrTiO3 Dengyu Yang, Yun-Yi Pai, Yuhe Tang, Yang Hu, Hyungwoo Lee, Jungwoo Lee, Chang-Beom Eom, Patrick Irvin, Jeremy Levy We aim to generate and detect surface acoustic waves (SAW) in LaAlO3/SrTiO3 heterostructures. Using a well-developed conductive-AFM lithography technique [1], we “sketch” interdigitated transducers (IDT) on LaAlO3/SrTiO3 “canvases”, which convert electronic signals into SAW and vice-versa. Two sets of IDTs are written on the structure to function as generator and detector. SAW can be used to generate dynamic potentials based on piezoelectric properties of LaAlO3/SrTiO3, and have the potential to drive electrons through nanostructures, a property that could be useful for quantum information applications. |
Thursday, March 7, 2019 10:00AM - 10:12AM |
R16.00009: Electron Phonon Coupling in Metallic Superlattices Andrius Bernotas, Brian Donovan, Ronald Warzoha, Patrick Hopkins Superlattices offer potential improvement in microelectronic transistor design as diffusion barriers between the silicon active layer and copper interconnects. While these materials provide a novel path forward in terms of electronic and atomic transport, more work must be done to understand potential thermal bottlenecks that could arise from their use in transistors. A major source of thermal buildup in microelectronic devices is the interaction between electrons and the surrounding atomic species in the form of electron-phonon coupling. In this project we explore the coupling between electrons and phonons as heat carriers in conductive multilayer superlattices of copper with tantalum, tungsten, and tantalum nitride. We use time domain thermoreflectance measurements and the two temperature thermal model to better understand the relationship between the electron phonon coupling in these materials and the physical parameters of the system. A greater understanding of this relationship will allow for greater control of the thermal properties of transistors, hopefully leading to increased thermal efficiency in microelectronics. |
Thursday, March 7, 2019 10:12AM - 10:24AM |
R16.00010: Tuning the thermal boundary conductance at metal-dielectric interfaces by varying interlayer thicknesses Shany Mary Oommen, Simone Pisana Interfaces play a significant role in the heat transport across boundaries at sub-micron length scales. Interfacial adhesion and phonon matching are important factors in determining the thermal boundary conductance, and the addition of an interlayer can be used to tune the heat dissipation. In this study, we analyze the modification of the thermal boundary conductance at metal-dielectric interfaces by insertion of metal interlayers with varying thicknesses from 2.5Å to 100Å. We show that the insertion of a tantalum interlayer at Al/Si and Al/sapphire interfaces hinders the phonon transmission across the interfaces and it plateaus at ~20Å. We found that the addition of a nickel interlayer significantly increased the thermal interfacial conductance at both the Al/Si and the Al/sapphire interfaces. The nickel interlayer, having an intermediate Debye temperature as compared to the Aluminum layer and the substrates, increases the phonon transmission across the boundary. Thermal property measurements were performed through time domain thermo-reflectance, and are in good agreement with a formulation of the diffuse mismatch model based on real phonon dispersions, accounting for anharmonic phonon scattering and phonon confinement within the interlayer. |
Thursday, March 7, 2019 10:24AM - 10:36AM |
R16.00011: Lone-pair Electrons do not Necessarily Lead to Low Lattice Thermal Conductivity: an Exception of Two-dimensional Penta-CN2 Huimin Wang It has long been documented in literature that, the lone-pair electrons (LPE) are generally thought to lead to low lattice thermal conductivity (κL) of bulk materials by inducing strong phonon anharmonicity. Herein, we show an exceptional case of two-dimensional (2D) penta-CN2 that possesses LPE but exhibits more than doubled κL (660.71 Wm-1K-1) than the LPE free counterpart of penta-graphene (252.95 Wm-1K-1), which is unexpected and contradictory to the traditional theory of LPE leading to low κL. Based on the comparative study of four 2D systems possessing LPE and their respective LPE free counterparts (planar C3N vs. graphene and penta-CN2 vs. penta-graphene), the underlying mechanism is found lying in the bonds homogenization in penta-CN2 due to the wide spatial extension of the non-symmetrically distributed LPE, which compensates the lattice anharmonicity due to LPE and is responsible for the opposite tendency of LPE affected κL in the four 2D systems. |
Thursday, March 7, 2019 10:36AM - 10:48AM |
R16.00012: Tuning thermal transport in nanostructures and nanostructured materials Konstantinos Termentzidis Due to the rapid evolution of nanomaterials elaboration the last decade a serie of nanostructures and nanostructured material like phononic-like crystals or nanowire networks are easily fabricated today. New physical phenomena are observed in such structures as ballistic phonon transport, phonon focusing, phonon tunneling and coherence effects. In general, nanostructured materials have a much lower thermal conductivity compared to bulk materials, due to phonon confinement and boundary scattering. Such phenomena will be presented with the present work. |
Thursday, March 7, 2019 10:48AM - 11:00AM |
R16.00013: Calculating lattice thermal conductivity: A comparative study on carbon nanotubes. Daniel Bruns, Joerg G Rottler, A. Srikantha Phani, Alireza Nojeh Carbon nanotubes (CNTs) are commonly utilized in nanoscale devices. High structural order, rigid sp2-bonds and a low atomic mass result in an exceptionally high lattice thermal conductivity (TC), which motivates their use in applications demanding efficient heat removal. However, pinpointing exact TC values of individual defect-free CNTs remains a challenge both experimentally and computationally. The thermal transport properties of ideal CNTs are dominated by phonon-phonon scattering, and a theoretical prediction of TC has to include anharmonic terms in the interatomic potential energy which give rise to phonon-phonon interaction. Here we compare two computational frameworks that take into account lattice anharmonicity to predict TC: classical molecular dynamics (MD) vs. anharmonic lattice calculation (ALC). Taking the same empirical interaction potential as input to both MD and ACL, we contrast phonon-phonon scattering rates and TC results as a function of temperature. This comparison also allows us to critically evaluate several assumptions in the different methods, namely the description of anharmonicity by a truncated Taylor expansion of the interaction potential in ALC, the use of classical phonon statistics in MD, and the importance of Umklapp processes for the TC. |
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