Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session B13: Materials (General) -- Modeling and CharacterizationFocus
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Sponsoring Units: DMP Chair: Dmitry Ovchinnikov, University of Washington Room: BCEC 153B |
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B13.00001: Model of the piezoelectric coefficient of hexagonal two-dimensional materials Lok Lew Yan Voon, Morten Willatzen, Zhong-Lin Wang A quantum-mechanical microscopic model of the piezoelectric effect in two-dimensional materials is developed. The piezoelectric coefficient requires the calculation of an internal atomic displacement and an effective piezoelectric charge. The internal displacement is obtained from minimizing the strain energy given by a Keating-like model, while the effective charge takes into account the atomic displacements and also a redistribution of the electronic charge; a bond-orbital model is used to compute the latter. The final theory only requires atomic energies and the elasticity constants of the materials as input parameters. The piezoelectric coefficients of a number of II-V, III-V and IV-IV materials that could stably form in the planar hexagonal structure are computed; results for the IV-IV materials are obtained for the first time. |
Monday, March 4, 2019 11:27AM - 11:39AM |
B13.00002: Nucleation Kinetics of Structural Phase Changes in Two-Dimensional Transition Metal Dichalcogenides Aditi Krishnapriyan, Qian Yang, Yao Zhou, Ekin D Cubuk, Evan Reed Predictive capabilities for kinetic processes in materials are in their infancy, but kinetics are critical for a spectrum of energy applications ranging from phase change materials, catalysis, materials synthesis, and combustion. The structural phase transition between the metallic 1T or 1T’ and semiconducting 2H structures in two-dimensional transition metal dichalcogenide materials is important to understand for synthesis and may provide exciting new opportunities for energy-efficient electronic and optical devices. However, very little is known about the mechanisms and kinetics of these phase changes or how to engineer the kinetics. We propose a novel electronic structure based method to determine the nucleation kinetics and timescales of this phase change. Furthermore, we discuss the curious fact that the interface energies between phases for this challenging problem are mathematically ill-defined. We also point to strategies on the engineering of kinetics in these phase change materials that take into account nucleation barriers and nucleation time. |
Monday, March 4, 2019 11:39AM - 11:51AM |
B13.00003: Effective k.p models for phosphorene including the interband spin-orbit coupling Paulo De Faria Jr., Marcin Kurpas, Martin Gmitra, Jaroslav Fabian Phosphorene is a two-dimensional semiconductor with direct band gap and promising spin-dependent properties[1,2]. In this study, we investigate effective k.p models for phosphorene including the interband spin-orbit coupling[3], a term previously overlooked in the literature. The inclusion of such interband spin-orbit coupling provides a reliable description of the anisotropic dipole transition and of the effective g-factors. We also investigate excitonic effects and found very good agreement with reported results in the literature. To obtain reliable k.p parameters we use a robust fitting approach that takes into account not only the band structure but also the k-dependence of the effective mass. With the increasing interest in two-dimensional systems, our effective k.p Hamiltonians can be combined with other already available models to investigate novel physical phenomena in van der Waals heterostructures. [1] M. Kurpas, M. Gmitra and J. Fabian, PRB 94, 155423 (2016). [2] A. Avsar et al., Nat. Phys. 13, 888 (2017). [3] P. E. Faria Junior et al., PRB 93, 235204 (2016). |
Monday, March 4, 2019 11:51AM - 12:27PM |
B13.00004: Spin-orbit-torque magnetic manipulation with 2D materials Invited Speaker: Daniel Ralph Current-induced spin-orbit torques provide a promising strategy for efficient manipulation of nonvolatile magnetic memory and logic technologies. For best performance, both the layer of material that provides the spin-orbit torque and the magnetic layer being manipulated should be as thin as possible. Here we discuss experiments which probe the ultimate limits in which 2D materials are used for either the spin-orbit layer or the magnetic layer. |
Monday, March 4, 2019 12:27PM - 12:39PM |
B13.00005: Strain-induced structural phase transformation in two-dimensional molydenum tungsten diselenide alloy Amey Anant Apte, Vidya Kochat, Pankaj Rajak, Aravind Krishnamoorthy, Praveena Manimunda, Jordan Hachtel, Juan Idrobo, Syed Asif Amanulla, Priya Vashishta, Aiichiro Nakano, Rajiv Kalia, Chandra Sekhar Tiwary, Pulickel M Ajayan Two-dimensional transition metal dichalcogenides (TMDCs) show intriguing mechanical properties compared to their bulk counterparts. This behavior can be modified in alloyed TMDCs due to doping and defects. In this work we develop CVD synthesis of large-area monolayer MoWSe2 alloy film on sapphire substrate. With Raman spectroscopy and mapping, we investigate the microscopic behavior of this alloy film transferred onto a flexible substrate as function of increasing bending strain. We observe the nature of crack propagation through the Mo and W-dominated regions in this 2D alloy film with differing stress concentration near the W-rich precipitates. Our molecular dynamics simulations illustrate how stress buildup at propagating crack tip in the film result in a 2H-1T phase transformation which is verified at micro- and nano-scale using Raman mapping and HAADF-STEM imaging. The MD simulations also predict increased crack resistance and healing in alloys compared to unalloyed pristine samples. |
Monday, March 4, 2019 12:39PM - 12:51PM |
B13.00006: Quantum plasmonic imaging of 2D transition metal dichalcogenides Alina Zhukova, Dmitri Voronine
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Monday, March 4, 2019 12:51PM - 1:03PM |
B13.00007: Missing links towards understanding equilibrium shapes of hexagonal boron nitride: algorithm, hydrogen passivation, and temperature effects Jingzhao Zhang, Wenjing Zhao, Junyi Zhu There is a large discrepancy between the experimental observations and the theoretical predictions in the morphology of hexagonal boron nitride (h-BN) nanosheets. Theoretically-predicted hexagons terminated by armchair edges are not observed in experiments; and experimentally-observed triangles terminated by zigzag edges are found theoretically unstable. There are two key issues in theoretical investigations, namely, an efficient and accurate algorithm of absolute formation energy of h-BN edges, and a good understanding of the role of hydrogen passivation during h-BN growth. Here, we first proposed an efficient algorithm to calculate asymmetric edges with a self-consistent accuracy of about 0.0014 eV/Å. This method can also potentially serve as a standard approach for other two-dimensional (2D) compound materials. Based on this algorithm, we constructed Wulff plot and discovered that only when edges are passivated by hydrogen atoms and temperature effects are taken into account can experimental morphology be explained. |
Monday, March 4, 2019 1:03PM - 1:15PM |
B13.00008: Plasmon excitations in trilayer AAB-stacked graphene Godfrey Gumbs, Chiun-Yan Lin, Bor-Luen Huang, Ming-Fa Lin We report on the fascinating electronic properties and distinct plasma excitations for low-symmetry trilayer AAB-stacked graphene. For the undoped structure, there are three pairs of unusual valence and conduction subbands which give rise to nine allowed interband excitations, for which the imaginary (real) part of the polarizability exhibits 1D square root asymmetric peaks and 2D shoulder structures (pairs of antisymmetric peaks and logarithm type symmetric peaks). The low frequency acoustic plasmon, appearing as a prominent peak in the energy loss spectrum, can survive in a narrow gap system with large density-of-states from the valence band. This type of plasmon mode is similar to that in a narrow gap carbon nanotube. The mechanism responsible for this plasmon is the intraband conduction state excitations. Its frequency, intensity and critical momentum exhibit a non-monotonic dependence on the Fermi energy. |
Monday, March 4, 2019 1:15PM - 1:27PM |
B13.00009: Correlated SPM–TERS Imaging: Revealing Unexpected Nanoscale Heterogeneities in 2D Semiconductors. Andrey Krayev Here I report on the application of scanning probe microscopy (SPM) cross-correlated with tip enhanced Raman scattering (TERS) and photoluminescence (TEPL) imaging for detection of unexpected heterogeneities in TMDs. |
Monday, March 4, 2019 1:27PM - 1:39PM |
B13.00010: Real-Space Mapping of Polaritons in 2D Materials Tobias Gokus, Stefan Mastel, Alexander Govyadinov The performance of the next-generation electronic devices based on graphene and other 2D materials is strongly influenced by the structure-function relationship. Scattering-type scanning near-field optical microscopy (s-SNOM) is the ideal technology to investigate such material systems at the nanoscale. s-SNOM combines the best of two worlds: (i) the high spatial resolution of Atomic Force Microscopy (AFM) and (ii) the analytical power of optical microscopy and spectroscopy. Achieving an unmatched spatial resolution below 10 nanometer this technology opens a new era for modern nano-analytical applications such as chemical identification, free-carrier profiling and plasmonic near-field mapping. Recent research highlights on graphene and other 2D materials include contact-free access to the local conductivity, the electron mobility, and the intrinsic electron doping by resolving propagating phonon-, plasmon-, and exciton-polariton directly in space and time. In this presentation we will introduce the basic principles of near-field microscopy for imaging and spectroscopy with 10 nanometer spatial resolution and address their impact and key applications in the field of 2D materials. |
Monday, March 4, 2019 1:39PM - 1:51PM |
B13.00011: Real time optical observation and control of transition metal dichalcogenide synthesis Talip Kasirga, Hamid Reza Rasouli, Naveed Mehmood, Onur Çakiroğlu Synthesis of atomically thin transition metal dichalcogenide (TMC) crystals have attracted tremendous amount of attention in the past few years. The common technique for the synthesis is to use a tubular split furnace with gas and exhaust connections. The precursors and the target substrate are placed in the hot zone of the furnace. The major shortcoming of the setup is the inaccessibility of the growth for real time characterizations. Thus, the growth has to be shut down blindly and the intermediate products that may lead to the growth has to be analysed later. Here, we developed a compact CVD chamber that allows real time optical observation of the crystal synthesis. Among our findings, for the first time we observed the formation of the atomically thin TMCs in real time, and characterized the various synthesis routes for several TMCs. By using the unique abilities we have with our chamber, we also observed the formation of the heterostructures. Furthermore, we demonstrated a directed synthesis of TMCs with prepatterned structures on a substrate. |
Monday, March 4, 2019 1:51PM - 2:03PM |
B13.00012: Oxidation effect on WTe2 using optical second harmonic generation Yujin Cho, Na Hyun Jo, Paul Canfield, Michael C Downer A transition metal dichalcogenide WTe2 has recently drawn many researchers’ attention because this material is Type-II Weyl semimetal in bulk1 and it is a topological insulator in a monolayer at low temperature2. |
Monday, March 4, 2019 2:03PM - 2:15PM |
B13.00013: Visualization of Photo-carrier Diffusion in Monolayer Transition Metal Dichalcogenides Zhaodong Chu, Jiamin Quan, Chun Yuan Wang, David Wannlund, Ali Han, Kevin Herrera, Di Wu, Lain-Jong Li, Chih-Kang Shih, Xiaoqin (Elaine) Li, Keji Lai Understanding of the spatial and temporal evolution of photo-generated charge carriers in atomically thin transition metal dichalcogenides (TMDs) is of critical importance for their application in optoelectronic devices. Using a light-assisted microwave impedance microscope (MIM), we demonstrate the first quantitative mapping of photo-carrier diffusion on monolayer TMDs. When the sample is illuminated by above-gap laser, the circular profile of local photoconductivity measured by the MIM is clearly broader than that of the laser spot. Numerical simulation shows that the free-carrier diffusion length of is on the order of micrometers, corresponding to a carrier lifetime of tens to hundreds of nanoseconds. Our work provides important insights to the remarkable electrical and optical properties of TMDs. |
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