Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session H15: Phonon Dynamics in Nanomaterials |
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Sponsoring Units: DMP Chair: Yi Xia, Argonne National Laboratory Room: LACC 304C |
Tuesday, March 6, 2018 2:30PM - 2:42PM |
H15.00001: Observation of THz range propagating elastic waves in amorphous materials using inelastic X-ray scattering. Jaeyun Moon , Austin Minnich The thermal atomic vibrations of amorphous solids can be distinguished by whether they propagate as elastic waves or do not propagate due to lack of atomic periodicity. In a-Si, prior works concluded that non-propagating waves are the dominant contributors to heat transport, with propagating waves being restricted to frequencies less than a few THz and scattered by anharmonicity. Our recent numerical study of a-Si supports a qualitatively different picture in which propagating elastic waves dominate the thermal conduction and are scattered by local fluctuations of elastic modulus rather than anharmonicity. Here, we present transient grating spectroscopy (TG) thermal conductivity measurements and dynamic structure factor measurements of a-Si and a-SiN with inelastic X-ray scattering (IXS). We explicitly demonstrate that propagating elastic waves exist up to around 10 THz despite the lack of periodicity in a-Si and are scattered by elastic fluctuations rather than anharmonicity in amorphous materials. |
Tuesday, March 6, 2018 2:42PM - 2:54PM |
H15.00002: Polarization-Resolved Study of Phonon Scattering from Embedded Nanoparticles Rohit Kakodkar , Joseph Feser Nanoparticle-in-alloy material systems are promising candidates for high efficiency thermoelectric materials, due to their greatly reduced lattice contribution to thermal conductivity. In this talk, we use a recently developed frequency-domain perfectly matched layer computational technique to calculate the scattering cross sections of embedded nanoparticles across the entire Brillouin zone and for all phonon modes including transverse acoustic phonons and optical phonons for the first time. For acoustic modes, we compare the computational results against previously used results from continuum mechanics and find excellent agreement so long as the Mie regime is accurately represented within the continuum mechanics models. Interestingly, we find that the interaction of optical phonons is remarkably different compared to its acoustic counterparts, with scattering efficiencies of optical phonons in the Raleigh regime observed to be up to 10-fold higher than acoustic phonons. Furthermore, we show that an interdiffused nanoparticle/matrix is more effective at scattering phonons compared to solids nanoparticle with the same net impurity concentration, with scattering efficiencies 2-fold higher in the dominant heat carrying regions. |
Tuesday, March 6, 2018 2:54PM - 3:06PM |
H15.00003: Field-Effect Doping and Thermoelectric Properties of PbS Colloidal Quantum Dot Solids Satria Bisri , Sunao Shimizu , Maria Ibanez , Maksym Kovalenko , Yoshihiro Iwasa Nanostructuring is one promising way to overcome the challenges to develop high-performance thermoelectric devices. Quantum confinement effect in colloidal quantum dots (QDs) leads to the formation of quasi-atomic discrete energy levels, which is beneficial for thermoelectric. The solution processability of the colloids allows us to fabricate well-ordered monolayers of QD arrays. Furthermore, the smaller than phonon mean-free-path of the QD size may decouple thermal and electronic transport in the arrays. Here we show high thermoelectric power factor in ultra-thin-film PbS QD solids. The utilization of field-effect doping using electrolyte gating allow us to accumulate a very high carrier density in the QD solids; thus significantly enhance the electrical conductivity by orders of magnitudes. Concomitantly, the preservation of quantum confinement of the QD in the array and the capability to access those discrete energy levels lead to the observation of high Seebeck coefficient values at room temperature. These demonstrations pave ways to develop new approaches to solve many bottlenecks to discover new systems for room temperature thermoelectric devices. |
Tuesday, March 6, 2018 3:06PM - 3:18PM |
H15.00004: Silicon Nanoparticle Thermometers in the Transmission Electron Microscope Brian Zutter , Matthew Mecklenburg , B. Regan Since silicon is the primary constituent of most microelectronic devices, a technique with high spatial resolution capable of local temperature measurements in silicon would be extremely useful. Here we describe applying plasmon energy expansion thermometry (PEET) to perform non-contact temperature measurements on a silicon nanoparticle. In PEET, a transmission electron microscope (TEM) equipped with electron energy loss spectroscopy (EELS) is used to measure a sample’s bulk plasmon energy. The plasmon energy is related to valence electron density, which in turn is related to temperature via the sample’s coefficient of thermal expansion (CTE). Over a range from room temperature to 1250 °C, temperature measurements of a 100 nm silicon nanoparticle performed with PEET agree with those of the co-located, 300-micron wide, calibrated Joule heater/thermometer. The results are a promising first step towards mapping temperature in silicon nanowires and operating, highly-scaled transistors. |
Tuesday, March 6, 2018 3:18PM - 3:30PM |
H15.00005: Artificial Thermal Anisotropy of Van der Waals Heterostructures Ruiqiang Guo , Austin Minnich Van der Waals (vdW) heterostructures are rapidly emerging as a central focus of material research, where novel properties and unique functionalities can be created by stacking atomically thin layers with selected materials and sequence. Owing to highly anisotropic crystal structures, vdW heterostructures open up new avenues to develop artificial thermal anisotropy that is of both fundamental and practical importance. Here we provide a principle for the design of thermal anisotropy using vdW heterostructures based on mass ratio. Our first-principles calculations indicate that the increase of mass difference between neighbouring layers changes the shape of nonspherical isoenergy surfaces, resulting in enhanced thermal anisotropy. Meanwhile, ultralow cross-plane thermal conductivity can be achieved combined with phonon zone folding while high in-plane thermal conductivity can be preserved by considering phonon scattering channels. The present results demonstrate how novel materials that do not occur naturally can be created using vdW heterostructures. |
Tuesday, March 6, 2018 3:30PM - 3:42PM |
H15.00006: Thermal Phonon Wave Effects in Superlattice Heat Conduction: Thermal Band Gaps and Phononic Quantum Wells Kartik Kothari , Abhinav Malhotra , Martin Maldovan An accurate determination and precise control of thermal conduction at the nanoscale necessitate consideration of the wave and particle nature of thermal phonons simultaneously. However, current formulations are unable to combine both phonon transport phenomenon. In this talk, we incorporate the wave nature of phonons by considering that coherent interference of thermal phonons leads to two distinct wave phenomena – thermal band gaps and phononic quantum wells. We elucidate the particle formalism by employing a rigorous boundary scattering theory, extended from the Beckmann-Kirchoff formalism, to account for interface features, phonon coupling and shadowing effects, for predicting thermal phonon transport across an interface. We also investigate the conditions governing the wave effects and analyze their impact on thermal transport in Si-Ge, SiGe alloy and III-V semiconductor superlattices. A thorough spectral phonon transport analysis combining particle and wave effects paves the way for a fundamentally new approach for thermal manipulation. Analogous to photonic and electronic revolutions, thermal phonon wave effects can open new paradigms in rational thermal material and device design in the fields of optoelectronics, thermoelectrics and microelectronics. |
Tuesday, March 6, 2018 3:42PM - 3:54PM |
H15.00007: Engineering Thermal Conductivity Enhancement in Nanostructures Abhinav Malhotra , Kartik Kothari , Martin Maldovan The ability to manipulate thermal transport at nanoscale has been restricted to reducing the thermal conductivity via diffuse scattering of phonons at interfaces. Rational material design has consequently been limited to applications such as thermoelectrics. On the other hand, enhancing thermal conduction at nanoscales which holds significant potential to develop efficient electronic, and optoelectronic devices has been largely unexplored. In this talk, we will present a novel approach to enhance the thermal conductivity of semiconductor thin-films in layered nanostructures. Specifically, we will show that the thermal conductivity enhancement can be achieved in germanium films cladded by silicon. The observed enhancement is a consequence of the coupling of the phonon spectrum of germanium and silicon materials through the interfaces. We will also present the effect of various structural conditions such as layer thicknesses and surface roughnesses on the phonon spectral coupling effect and their influence on the resultant thermal conductivities, establishing a new method to control the flow of thermal energy at the nanoscale beyond the current limits. |
Tuesday, March 6, 2018 3:54PM - 4:06PM |
H15.00008: Evaluation of Thermal Transport Properties for Verification of Inter-atomic Potentials for WS_{2} and WSe_{2} Crystals Arash Mobaraki , Ali Kandemir , Haluk Yapicioglu , Oguz Gulseren , Cem Sevik Transition metal dichalcogenides (TMDs) have been emerging as a new class of two-dimensional layered materials due to their astonishing properties. For applications such as thermoelectric devices or overcoming general overheating issues, understanding and characterization of thermal transport is very important for efficient engineering of 2D TMD materials. Using particle swarm optimization (PSO), we obtain Stillinger-Weber type empirical potential parameters for single-layer WS_{2} and WSe_{2} crystals. Our results are quite consistent with first-principles calculations in terms of bond distances, lattice parameters, elastic constants and vibrational properties. The effect of temperature on phonon energies and phonon linewidth are investigated using spectral energy density analysis. Calculated frequency shift with respect to temperature is compared to experimental data, demonstrating the accuracy of the generated inter-atomic potentials. Also, the lattice thermal conductivities of these materials are evaluated by means of classical molecular dynamics simulations. The predicted thermal properties are in very good agreement with the ones calculated from first-principles. |
Tuesday, March 6, 2018 4:06PM - 4:18PM |
H15.00009: High Frequency Measurements of Thermo-Physical Properties of Thin Films Using a Modified Broad-Band Frequency Domain Thermo-Reflectance Approach Mohammadreza Shahzadeh , Mizanur Rahman , Simone Pisana We present a new method to perform high-frequency thermoreflectance measurements on thin films, Differential Broad-Band Frequency Domain Thermo-Reflectance (DBB-FDTR). Our method follows BB-FDTR developed previously [Rev. Sci. Instrum. 84, 064901 (2013)], but it does not require the use of expensive Electro-Optical Modulators. In DBB-FDTR, the thermal phase of interest is recovered by performing two measurements: either at different laser focal positions, or at different pump/probe laser offsets. The unwanted contributions to the phase signal are removed by subtracting the measured phases to calculate the Differential Thermal Phase (DTP). The DTP is then fitted to a solution of thermal diffusion equation to obtain the thermo-physical properties of interest. We also show how to mitigate coherent noise. Moreover, by reducing the laser spot size, the SNR can be increased by both increasing the thermal signal magnitude and reducing the coherent noise. |
Tuesday, March 6, 2018 4:18PM - 4:30PM |
H15.00010: Direct Entropy Measurement in a Mesoscopic Quantum System Nikolaus George Hartman , Christian Olsen , Saeed Fallahi , Michael Manfra , Joshua Folk Entropy measurements, typically derived from bulk properties (e.g. heat capacity), are difficult to access in mesoscopic samples. Taking advantage of a well-known Maxwell relation, we build a mesoscopic device in which it is possible to measure the entropy of quantum states down to the single electron level. To demonstate the efficacy of this method, we apply it to the first few-electron spin states in a gate-defined GaAs quantum dot. The entropy of a single spin (k_{B }ln 2) can be determined within 5% accuracy, as can the entropy at the singlet-triplet crossing for two electrons in a large magnetic field. Looking forward, this measurement approach will be applied to systems with less trivial ground states, such as one or two-channel Kondo systems. |
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