Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session E15: Coupled Electron and Phonon Dynamics at the NanoscaleFocus
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Sponsoring Units: DMP Chair: Pierre Darancet, Argonne Natl Lab Room: LACC 304C |
Tuesday, March 6, 2018 8:00AM - 8:36AM |
E15.00001: Dimensionally-controlled studies of heat and charge transport in 1D, 2D, and 3D nanoscale materials Invited Speaker: Jeff Urban
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Tuesday, March 6, 2018 8:36AM - 8:48AM |
E15.00002: Probing local heating and cooling at interfaces: a non-equilibrium Green's function study Qichen Song, Jiawei Zhou, Te-Huan Liu, Gang Chen Interface electrical resistance is a well-known phenomenon that leads to heating at interface. Yet, it is not clear where the heating exactly happens. Knowing where heating happens is particularly important for nanodevices, especially considering that at the interfaces, there is also an interfacial thermal resistance. Pinpointing where heat is dissipated is essential for determining the temperature drop caused by interfacial thermal resistance. To answer this question, we employ non-equilibrium Green's function calculation. In particular, the electron-phonon interaction is introduced through Büttiker probes. The electron-phonon scattering rates are obtained from electron-phonon Wannier interpolation on a very fine mesh. With our parameter-free model, we are able to calculate spatially varying energy flux and probe the local heating and cooling. Our calculation helps reveal electron behaviors across interface and improve the design of nanodevices. |
Tuesday, March 6, 2018 8:48AM - 9:00AM |
E15.00003: Heat and Electron Transport in Multi-Component Nanostructures with Imperfect Interfaces Sanghamitra Neogi, Vitaly Proshchenko Heat management in modern electronic devices is becoming increasingly important with computing demands posed by data-intensive applications. When the device size reaches the nanoscale, scattering at interfaces dictate the device functionality. Additionally, dimensional reduction significantly modifies the properties of carriers in the nanostructure. A complete treatment of transport in a multi-interface system requires solving the complex interplay between dimensional confinement and interface scattering. In this work, we investigate phonon and electron transport in layered Si/Ge superlattices with imperfect interfaces employing classical molecular dynamics and density functional theory in combination with semi-classical Boltzmann transport theory, respectively. We determine the extent of disruption of the "superlattice" phonons by investigating the MFP distribution combined with the thermal conductances of the confined layers. We discuss strategies to tune electron transport in imperfect interfaces with strain and/or containing interstitials. Our work illustrates the carrier transport size effects in multilayered systems and highlights the effect of local interfacial structures on global transport in multi-component systems. |
Tuesday, March 6, 2018 9:00AM - 9:12AM |
E15.00004: Electron-Induced Non-Equilibrium Phonon Dynamics in Two-Dimensional Materials Sridhar Sadasivam, Maria Chan, Pierre Darancet Non-equilibrium energy transfer between hot electrons and phonons plays an important role in the design and operation of photovoltaic and nanoelectronic devices. In this talk, I will show how high energy electrons in two-dimensional (2D) materials are conducive to non-equilibrium phonon distribution using a newly developed first-principles simulation framework to model coupled time-evolution of electron and phonon occupations. Our method [1] accounts for electron-phonon and phonon-phonon interactions on an equal footing. For materials such as graphene, silicene, germanene, and transition metal dichalcogenides, we find that electron-induced heating strongly differs from classical heat, inducing a long-lasting, non-equilibrium distribution between in-plane and out-of-plane vibrational modes. We propose a general, dimensionless material descriptor accounting for field gradients and crystal symmetry able to characterize the nature of electronically-driven phonon distributions in two dimensional materials. |
Tuesday, March 6, 2018 9:12AM - 9:24AM |
E15.00005: Imaging oxygen vacancy dynamics in nanoscale ReRAM devices William Hubbard, Jared Lodico, Brian Regan Non-volatile resistive memory (ReRAM) is considered a potential successor to flash memory, but important facts about the microscopic physics underlying this technology remain controversial. For instance, in valence change ReRAM (VCM), which is thought to switch through the motion of oxygen vacancies, it is not clear how the conducting path forms. We microfabricated clean, functional, transmission electron microscopy (TEM)-compatible Ti/HfO2/Pt VCM devices and cycled them numerous times in situ. Imaging these devices with standard scanning TEM (STEM), we found that the current pathways are effectively invisible. However, employing STEM electron beam induced current (EBIC) imaging made the switching processes obvious. Our STEM EBIC system can be configured to achieve sensitivity to EBIC signals arising from traditional sources such as electric fields, as well from novel sources related to temperature, electrical potential, and secondary electron emission. Cycling the device between the ON and OFF states, we mapped the conducting path morphology at various stages with STEM EBIC. |
Tuesday, March 6, 2018 9:24AM - 9:36AM |
E15.00006: Mapping current paths and temperature in thin film NbO2 selector devices B. Regan, Toyanath Joshi, Matthew Mecklenburg, Brian Zutter, Gurleen Bal, Jared Lodico, David Lederman, William Hubbard A metal-to-insulator transition (MIT) occurs in NbO2 at high temperature coincident with a subtle change in its crystal structure. Because a similar transition occurs when sufficient electrical current is driven directly through the material, NbO2 is being considered for use as a selector in crossbar memory arrays. Theoretical studies presently disagree as to whether the bias-induced transition is simply the MIT triggered by Joule heating, or a distinct transition triggered by electric field effects. Using novel scanning transmission electron microscopy (STEM) techniques based on electron beam induced current (EBIC) imaging, we map current paths, potential, and temperature in functioning, thin-film NbO2 devices. We observe the bias-induced switching process at various stages, including the high-resistance state, the intermediate negative differential resistance regime, and the low-resistance state both with and without thermal runaway. |
Tuesday, March 6, 2018 9:36AM - 9:48AM |
E15.00007: Analytic understanding of resistive switching in ordered solids Jong E Han, Jiajun Li, Camille Aron, Gabriel Kotliar Mechanisms for resistive switching in transition metal |
Tuesday, March 6, 2018 9:48AM - 10:00AM |
E15.00008: Direct Measurement of Grain-boundary Resistivity and Mobility in Millimeter-sized Graphene Lihong Bao Grain boundaries (GBs) in polycrystalline graphene scatter charge carriers, which reduces carrier mobility and limits graphene applications in high-speed electronics. Here we report the extraction of the resistivity of GBs and the effect of GBs on carrier mobility by direct four-probe measurements on millimeter-sized graphene bicrystals grown by chemical vapor deposition (CVD). To extract the GB resistivity and carrier mobility from direct four-probe intra-grain and inter-grain measurements, an electronically equivalent extended 2D GB region is defined based on Ohm’s law. Measurements on seven representative GBs find that the maximum resistivities are in the range of several kΩ μm to more than one hundred kΩ μm. Furthermore, the mobility in these defective regions is reduced to 0.4 - 5.9 ‰ of the mobility of single-crystal, pristine graphene. Similarly, the effect of wrinkles on carrier transport can also be derived. The present approach provides a reliable way to directly probe charge-carrier scattering at GBs and can be further applied to evaluate the GB effect of other two-dimensional polycrystalline materials, such as transition-metal dichalcogenides (TMDCs). |
Tuesday, March 6, 2018 10:00AM - 10:12AM |
E15.00009: Altered Photothermoelectric Effects in Au nanowires via Surface Modification Xifan Wang, Charlotte Evans, Jacob Ciszek, Douglas Natelson Controlling morphology and composition via nanoscale structuring gives opportunities to improve the thermoelectric properties of materials for energy conversion and photodetection. In a previous study, we reported the open circuit photothermoelectric voltage in thin-film Au nanowire devices as a function of the position of an optical heat source. A focused laser beam is used to locally heat the metal nanostructure via a combination of direct absorption and excitation of plasmon resonance in Au nanowires. We found that the sign and magnitude of the net thermoelectric voltage is sensitive to structural defects, metal grain structure, and surface passivation of the nanowire. This finding opens the possibility of improved local control of the thermoelectric properties at the nanoscale. Here we report preliminary data on the alteration of the thermoelectric properties of Au nanowires via surface modification, including self-assembled monolayer formation |
Tuesday, March 6, 2018 10:12AM - 10:24AM |
E15.00010: Non-Equilibrium theory of hot electron generation in plasmonic nanostructures under illumination – thermal vs. non-thermal effects Yonatan Dubi, Yonatan Sivan Understanding the interplay between electrons, photons and phonons is a fundamental problem in physics. Recently, interest in this problem resurfaced in the context of non-equilibrium (hot) electron distributions, which are key for applications such as photo-catalysis, sensing etc. Here we report a formulation of the theory of hot electron generation in plasmonic nanostructures under continuous wave illumination, taking into account non-equilibrium and thermal effects. Specifically, we consider the effect of both photons and phonons on the electron distribution function, and calculate self-consistently the increase in electron and lattice temperatures above ambient conditions (as observed experimentally). This enables us to go well beyond the limits of existing theories, which are limited to low illumination intensities. We determine the electronic distribution and deviations from equilibrium under different conditions, and evaluate the rise in electron and lattice temperatures. Finally, we discuss the prospect of using the hot electrons for photocatalysis in light of recent experiments, and identify the efficiency and the photocatalytic performance. |
Tuesday, March 6, 2018 10:24AM - 10:36AM |
E15.00011: Plasmon Resonant Amplification of a Hot Electron-Driven Photodiode Lang Shen, Nirakar Poudel, George Gibson, Bingya Hou, Jihan Chen, Haotian Shi, Ernest Guignon, William Page, Arturo Pilar, stephen Cronin In this talk I will review our recent progress in the area of plasmon resonant excitation of hot electrons in a metal/oxide/metal (Au/Al2O3/graphene) heterostructure. In this device, hot electrons, excited optically in the gold layer, jump over the oxide barrier and are injected into the graphene layer, producing a photocurrent. To amplify this process, the bottom gold electrode is patterned into a plasmon resonant grating structure. The photocurrent produced is measured as a function of incident angle. We observe the maximum photocurrent at ±10° from normal incidence with p-polarized light (perpendicular to grating lines), and a constant (angle-independent) photocurrent with s-polaried light (parallel to the grating). These data show an amplification factor of 4.6× under resonant conditions. At the same angle (±10°),we also observe sharp dips in the photoreflectance corresponding to wavevector matching between the incident light and the plasmon mode in the grating, in agreement with predictions from our FDTD simulations. |
Tuesday, March 6, 2018 10:36AM - 10:48AM |
E15.00012: DNA condensation induced by plasmonic heating from array of silver domains Hitomi Sakai, Ryoko Shimada Different molecules in mixed solutions can be separated from each other along a temperature gradient. This phenomenon, so-called Soret effect, is quite important for molecular manipulation in various research fields. Typically, infrared laser has been utilized to obtain a temperature gradient in a certain area, but no large temperature gradient is made with this method. In contrast, plasmonic heating from a periodic array of metal domains is one of the effective ways to obtain a large local temperature gradient. In this work, we utilized the plasmonic heating from the array of silver (Ag) domains to attempt creation of such a large gradient for local periodic condensation (Soret effect) of DNA. In fact, a large, periodic temperature gradient with a magnitude of ~0.5K/µm was achieved through the excitation of the Ag array with blue light (400-440nm). In this temperature field, DNA molecules (5.6kbp), being labeled with fluorescent dyes (SYBR Gold) and mixed in a polyethylene glycol (PEG) solution, was found to be condensed at the edge of the heating domains: namely, a positive Soret effect was observed. Details of this experimental results will be discussed on site. |
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