Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session H34: Thermal Transport - Photonic and Nano EffectsFocus
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Sponsoring Units: DMP GERA Chair: Yi Xia, Argonne National Lab Room: 297 |
Tuesday, March 14, 2017 2:30PM - 2:42PM |
H34.00001: Effect of photo-excited carriers on the thermal conductivity of silicon Jiawei Zhou, Doug Shin, Alexei Maznev, Bolin Liao, Keith Nelson, Gang Chen Thermal conductivity is a critical quantity for heat dissipation in microelectronic and optoelectronic device, and has also been widely explored to enhance material's thermoelectric efficiency. Among these cases, the thermal transport often happen when the electron carriers also exist, which are either excited by light or introduced by dopants. Understanding how these carriers affect the thermal transport can lead to better device designs or material optimizations. Despite past modeling work and recent experiments on the interaction between electrons and the heat carriers - phonons, it is still not clear how the carriers affect the thermal conductivity in a quantitative way, mainly because the dopants will introduce defect scattering which is hard to separate from the effect of carriers alone, while the photo-excited carrier density is usually limited by the pulse energy and not significant enough to impact the thermal transport. In this work, we use the transient thermoreflectance set up with a high pulse energy to monitor the thermal conductivity change in silicon as a function of generated carrier density. We show that, the moderately generated carrier density has a clear influence on the thermal conductivity. These results also demonstrate the ability of utilizing the electron-phonon coupling to tune material's thermal transport. This work is supported by DOE EFRC (Grant No. DE-SC0001299). [Preview Abstract] |
Tuesday, March 14, 2017 2:42PM - 2:54PM |
H34.00002: Coherent Manipulation of Phonons at the Nanoscale Shangjie Yu, Min Ouyang Phonons play a key role in almost every physical process, including for example dephasing phenomena of electronic quantum states, electric and heat transports. Therefore, understanding and even manipulating phonons represent a pre-requisite for tailoring phonons-mediated physical processes. In this talk, we will first present how to employ ultrafast optical spectroscopy to probe acoustic phonon modes in colloidal metallic nanoparticles. Furthermore, we have developed various phonon manipulation schemes that can be achieved by a train of optical pulses in time domain to allow selective control of phonon modes. Our theoretical modeling and simulation demonstrates an excellent agreement with experimental results, thus providing a future guideline on more complex phononic control at the nanoscale. [Preview Abstract] |
Tuesday, March 14, 2017 2:54PM - 3:06PM |
H34.00003: High Temperature Optical Properties of Semiconductors for Thermophotovoltaics Chloe Doiron, Gururaj Naik According to the US Department of Energy, the US industrial sector generates over 1.5 PWh of unrecovered waste heat each year. A significant portion of this waste heat is at reservoir temperatures below 900K. Thermophotovoltaics (TPV) has the potential to capture waste heat for electricity generation. Previous work focused on TPV applications with emitter temperatures greater than 1000K. For low temperature TPV applications, emitters fabricated from semiconductors are exciting because of lower losses, tunable optical properties including losses, and ease of fabrication. However, optical constants of semiconductors are not measured at high temperatures. We will demonstrate methods to predict the optical properties of semiconductors at high temperatures. Using these models, we will discuss the role that optical loss plays in designing efficient thermal emitters. Additionally, the physical models elucidate material properties and carrier physics to consider when selecting the ideal semiconductor for a specific emitter temperature. We will compare semiconductors with metals showing that lossy dielectrics can have significant advantages. With this knowledge, we designed and simulated nanophotonic thermal emitters capable of achieving high TPV efficiencies at temperatures below 900K. [Preview Abstract] |
Tuesday, March 14, 2017 3:06PM - 3:42PM |
H34.00004: Greatly Enhanced Photothermoelectric Voltage in Plasmonic Au Nanowires and Nanogaps Invited Speaker: Pavlo Zolotavin We report a study of the thermoelectric effects in plasmonic Au bowtie devices with nanoscale junctions. A laser scanning microscope is used to locally heat the metal nanostructure via excitation of a plasmon resonance and direct absorption at room temperature and at 5 K. The increase of the local temperature of the unbroken nanowire is quantified by a bolometric approach. Finite element modeling of the heat dissipation reveals that the local temperature increase of the nanowire at temperatures below 50 K is determined by the thermal boundary resistance of the metal--substrate interface. A study of spatial dependence of the photothermoelectric response in this system revealed a number of unexpected results. In nanowires shorter than the spot size of the laser beam we observe a thermoelectric voltage distribution that is consistent with the local Seebeck coefficient being spatially dependent on the width of the nanowire. In longer structures we observe extreme variability of the net thermoelectric voltage as the laser spot is scanned along the length of the nanowire, including multiple sign reversals and sensitivity to the metal grain structure and surface conditions. In the nanowires with tunneling nanogaps, we observe a 1000x increase in the photothermoelectric voltage up to tens of mV. We discuss a range of possible explanations for the extraordinarily enhanced thermoelectric voltage focusing on the role of nonequilibrium, "hot" carriers generated upon the plasmon excitation. [Preview Abstract] |
Tuesday, March 14, 2017 3:42PM - 3:54PM |
H34.00005: Solvent-dependent thermal effects on conductance across a series of molecular wires. Giacomo Lovat, Andrew Pinkard, Xavier Roy, Latha Venkataraman Charge transport through a molecular wire is a function of not only the molecular junction itself, but also the environment of that junction. As electronics continue to miniaturize, elucidating this connection will lead to devices with greater versatility and functionality, while also gaining a more fundamental understanding of principles that govern molecular electronics. In this context, thermal effects on conductance in single-molecule junctions have been the focus of a relatively small number of studies. Here, we examine the temperature-driven conductance changes in single-molecule junctions fabricated with the scanning tunneling microscope break-junction (STM-BJ) technique as a function of 1) molecular length 2) backbone structure, 3) charge carrier type and 4) solvent polarity. We demonstrate that tunneling conductance across single-molecule junctions varies with temperature and we attribute this observation to temperature-dependent polarity of the environment. We argue that the reorganization of the solvent dipoles in the vicinity of the electrode surface alters the alignment of the frontier molecular orbitals relative to the metal Fermi level and has a significant impact on molecular conductance. [Preview Abstract] |
Tuesday, March 14, 2017 3:54PM - 4:06PM |
H34.00006: Coherent Phonons in Semiconductor Nanowires Pierre-Adrien Mante, Yu-Chen Liu, Nicklas Anttu, Sebastian Lehmann, Magnus T. Borgstrom, Kimberly A. Dick, Arkady Yartsev The low dimensionality of nanowires drastically modifies phonon propagation and scattering. As a consequence nanowires has been intensively studied for various applications: size dependent thermal transport, thermoelectricity or nano-sized acoustic transducer. Despite these breakthroughs, there are still questions surrounding phonon transport in nanowires, from scattering to velocity. It is thus critical to achieve an understanding of the frequency dependent behaviour of phonons in these structures to get insights into thermal transport. After giving a brief introduction of the picosecond acoustic technique, we will present observations of propagating acoustic phonons in nanowires superlattices that highlight the strong modifications of phonon dispersion relations. We will then focus on the modified coupling of light and sound in nanowires and on the possibility to generate and detect phonon of specific polarization and frequency. Finally, we will demonstrate the possibility to perform the complete elasticity tensor characterization of nanowires thanks to the control over phonon generation. Our results provide a novel method to explore the frequency dependent phonon scattering processes and thus obtain microscopic insights into thermal transport. [Preview Abstract] |
Tuesday, March 14, 2017 4:06PM - 4:18PM |
H34.00007: Direct optical probing of non-equilibrium thermal transport length scales in hexagonal boron nitride Sean Sullivan, Kevin Olsson, Annie Weathers, Elaine Li, Li Shi The Fourier's law for diffusive heat conduction loses significance for thermal transport over small scales, especially in materials whose thermal conductivity is predominated by low-energy carriers with long mean free paths. One such example is the two dimensional (2D) layered compound hexagonal boron nitride (h-BN). h-BN achieves a fairly high in-plane thermal conductivity contribution from acoustic phonons and their weak interaction with high-frequency optical branches. We study the non-equilibrium thermal transport over submicron length scales in bulk hBN using combined micro-Raman and Brillouin light scattering (BLS) spectroscopies as temperature sensors for the optical and acoustic phonons, respectively. A focused laser acts as a small heating spot on the sample, coupling strongly to high frequency optical phonons. If the length scale of the heating zone is smaller than the distance over which the hot optical phonons give up their energy to lower frequency acoustic phonons, the optical phonons remain out of equilibrium with respect to the other phonon populations. Simultaneous measurement of both acoustic and optical phonon populations over these small length scales provides insight into the non-equilibrium spectral transport processes at play in this high thermal conductivity material, which is actively sought after for heat spreading applications in novel nanoelectronic devices where such length scales become important. [Preview Abstract] |
Tuesday, March 14, 2017 4:18PM - 4:30PM |
H34.00008: Grain size dependent thermal conductivity of polycrystalline twisted bilayer graphene by Raman spectroscopy Tej B. Limbu, Konstanze R. Hahn, Frank Mendoza, Satyaprakash Sahoo, Joshua J. Razink, Ram S. Katiyar, Brad R. Weiner, Gerardo Morell We report here the results of an investigation on the grain size dependent room temperature thermal conductivity of a polycrystalline tBLG by employing a noncontact optical technique based on micro-Raman spectroscopy. Polycrystalline tBLG sheets of different grain sizes were synthesized on copper by hot filament chemical vapor deposition. The measured thermal conductivity values are $1305\pm 122$,$971\pm 73$, and $657\pm 42$ $Wm^{-1}K^{-1}$ for polycrystalline tBLG with grain sizes of 54, 21, and 8 nm, respectively. Based on these thermal conductivity values, we also estimated the grain boundary conductance, $(14.43\pm 1.21)\times 10^{10}\,Wm^{-2}K^{-1}$, and the thermal conductivity for single crystal tBLG, $(1510\pm 103)\,\,Wm^{-1}K^{-1}$. Our results show that the relative degradation of thermal conductivity due to grain boundaries is smaller in bilayer than in monolayer graphene. This result has been supported by molecular dynamics (MD) simulations. The quantitative study of the grain size dependent thermal conductivity of polycrystalline BLG is valuable in technological applications as well as for fundamental scientific understanding. [Preview Abstract] |
Tuesday, March 14, 2017 4:30PM - 4:42PM |
H34.00009: Phonon dephasing in Monolayer MoS$_{\mathrm{2}}$ Liuyang Sun, Kha Tran, Sebastian Roesch, Eduardo Priego, Galan Moody, Junho Choi, Yu-Ming Chang, Kevin Silverman, Richard Mirin, Xiaoqin Li Phonons are coordinated lattice vibrations, the study of which is important for understanding optical, electrical, and thermal properties of materials. Raman spectroscopy has been applied extensively to investigate phonon modes in atomically transition metal dichalcogenides such as monolayer MoS$_{\mathrm{2}}$. In this work, we demonstrate that the linewidth of the Stokes peak does not represent instrinsic phonon dephasing times. We report phonon dephasing times in monolayer and bulk MoS$_{\mathrm{2}}$ at both room temperature and low temperatures. [Preview Abstract] |
Tuesday, March 14, 2017 4:42PM - 4:54PM |
H34.00010: Enhancement of thermoelectric efficiency by quantum interference effects in trilayer silicene flakes Luis Rosales, Natalia Cort\'es, Leonor Chico, M\'onica Pacheco, Pedro Orellana In recent years, the enhancement of thermoelectric efficiencies has been accomplished in nanoscale systems by making use of quantum effects. We exploit the presence of quantum interference phenomena such as bound states in the continuum and Fano antiresonances in trilayer silicene flakes to produce sharp changes in the electronic transmission of the system. By applying symmetric gate voltages the thermoelectric properties can be tuned and, for particular flake lengths, a great enhancement of the figure of merit can be achieved. We show that the most favorable configurations are those in which the electronic transmission is dominated by the coupling of bound states to the continuum, tuned by an external gate.\\ References: [1] N. Cort\'es et al., J. Phys.: Condens. Matter \textbf{29}, 015004 (2017). [Preview Abstract] |
Tuesday, March 14, 2017 4:54PM - 5:06PM |
H34.00011: Influence of nanoscale structure and phonon mean free path spectrum on thermal transport probed using tabletop coherent extreme ultraviolet light J. L. Knobloch, J. N. Hernandez-Charpak, T. D. Frazer, B. Abad, Weilun Chao, E. H. Anderson, K. M. Hoogeboom-Pot, D. Nardi, H. C. Kapteyn, M. M. Murnane As advances in nanofabrication push the characteristic dimension of nanosystems deep into the nano regime ($\ll$100nm), transport and other properties are often modified compared to bulk materials. However, our ability to fabricate nanosystems has outstripped our ability to characterize them. We have developed a nanometrology technique using tabletop high harmonic coherent extreme ultraviolet beams with wavelengths (10-30nm) and pulse durations ($\approx$10fs) that are well matched to the intrinsic length and time scales of functioning nanosystems. Previously, we observed both size- and spacing-dependent deviations from expected bulk thermal transport away from periodic arrays of nanolines. We found that collective phonon transport can counteract the decreased heat dissipation efficiency due to quasi-ballistic transport when the nanostructure spacing approaches the phonon mean free paths in the substrate. In this new work, we explore nanoscale thermal transport and validate our collective transport models by separately varying the linewidths and periodicities of both 1D and 2D nanostructure arrays on different substrates. Using effective theories and Boltzmann transport equation calculations, we analyze these experimental findings and compare them with new and existing models. [Preview Abstract] |
Tuesday, March 14, 2017 5:06PM - 5:18PM |
H34.00012: Measurements Of The Effects Of Grain Boundary And Alloy Scattering On Spectral Phonon Mean Free Path Distributions. Sean Lubner, Md. Imran Khan, Chris Dames In the electronics and clean energy fields, it is increasingly necessary to reliably model the dissipation of heat from micro and nanostructures or nanostructured materials such as in batteries, computer chips, and thermoelectrics. In these regimes where length scales are comparable to the mean free paths (MFPs) of energy carriers, the diffusion law of heat conduction begins to break down. In this talk, I present our recent results from using a time domain thermoreflectance (TDTR) technique with laser spot 1/e-squared radii less than 2 microns to measure sub-diffusion thermal transport in silicon, nanograined-silicon (ng-Si), and silicon germanium (SiGe) alloys. Our results experimentally demonstrate that alloy scattering skews phonon spectra toward longer MFPs, while nanostructuring skews phonon spectra toward shorter MFPs. As a consequence, we show that a significant fraction of the heat-carrying phonons in SiGe have MFPs greater than 10 microns at room temperature, and that the thermal conductivity of ng-Si overtakes that of SiGe after microstructuring. [Preview Abstract] |
Tuesday, March 14, 2017 5:18PM - 5:30PM |
H34.00013: Abstract Withdrawn |
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