Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session X56: Measuring Structure of 2D Materials in Real and Reciprocal SpaceLive
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Sponsoring Units: DMP Chair: Robert Hovden, University of Michigan |
Friday, March 19, 2021 8:00AM - 8:12AM Live |
X56.00001: Local structure changes in 1T-TaS2-2xSe2x systems Sharon Philip, John Schneeloch, Despina A Louca The transition metal dichalcogenides (TMD) materials are prototypical examples of charge density wave (CDW) instability where the CDW state is in close proximity to superconductivity. In this project, we investigate the CDW phases in 1T-TaX2 (X=S, Se) class of TMDs. It is reported that 1T-TaX2 forms a √13×√13 super cell, with a large distortion, about 10% of the lattice constant, in the commensurate CDW (CCDW) state. 1T-TaS2 undergoes a series of phase transitions upon cooling and achieves a CCDW phase below 183K. Simultaneously, electron correlation effects set in and localize the unpaired electron at the center of star, leading to a Mott insulating state. 1T-TaSe2 on the other hand, undergoes CCDW transition at a higher temperature of 473K and remains metallic upon cooling to low temperatures. No metal-to-insulator transition is observed in 1T-TaSe2 which implies that the spin on the unpaired electron is not localized within the star-of-David and implies a different distortion is at play, even though the average symmetries and periodic modulations are the same for TaS2 and TaSe2. We investigate the local atomic correlations in 1T- TaS2 and TaSe2 which is very important to study the lattice, spin and electronic correlations in this system. |
Friday, March 19, 2021 8:12AM - 8:24AM Live |
X56.00002: Direct, in-situ spatial imaging of van der Waals heterostructures Andrey Sushko, Kristiaan De Greve, Trond I Andersen, Giovanni Scuri, You Zhou, Kenji Watanabe, Takashi Taniguchi, Philip Kim, Hongkun Park, Mikhail Lukin The optical and electronic properties of stacked van der Waals (vdW) heterostructures depend strongly on the atomic stacking order of the constituent layers. This is exemplified by Mott insulator states in ABC-stacked graphene, and topologically protected states along AB/BA boundaries in bilayer graphene. Moreover, periodic variation of local atomic registry, known as moiré patterns, has given rise to superconductivity in twisted bilayer graphene and exotic exciton states in TMD heterobilayers. However, the nm-scale moiré structure is typically indirectly deduced, because the conventional imaging technique, transmission electron microscopy (TEM), requires sample preparation that is incompatible with most optical and transport measurements. We present a method to directly image the local stacking order in complete vdW devices, including hBN-encapsulation, top- and bottom- graphite gates, and standard Si-substrates. Using this method, we demonstrate imaging of reconstructed moiré patterns in stacked TMDs, ABC/ABA stacking order in graphene multilayers, and AB/BA boundaries in bilayer graphene. Furthermore, we show that the optical properties of the TMD are conserved after imaging, enabling correlation of local stacking order with optical and electronic properties. |
Friday, March 19, 2021 8:24AM - 8:36AM Live |
X56.00003: In-situ TEM study of graphene-encapsulated thin CrCl3 Koichi Tanaka, Belinda Zhen, Colin Ophus, Jim Ciston, Carlos Gonzalez, Zhehao Ge, Arthur Ramirez, Jason C. Lashley, Alex K Zettl, Jairo Velasco Jr., Aiming Yan Chromium Chloride (CrCl3) has recently attracted great interest owing to its unique properties as a 2D magnet.1,2 In this study, we investigate the crystal structure evolution of thin CrCl3 at different temperatures using in-situ transmission electron microscopy (TEM). In order to protect thin CrCl3 from degradation in ambient environment, we encapsulate the CrCl3 flake with exfoliated single- or bi-layer graphene inside a glovebox and transfer the heterostructure onto a TEM grid. In-situ TEM is used to study the phase transition in these CrCl3 TEM samples at low temperatures and structural evolution at high temperatures. Both electron diffraction and high-resolution scanning TEM imaging are carried out for this study. We observe a lack of phase transition in the encapsulated thin CrCl3 even below the phase transition temperature (240K) and robust crystal structure at high temperatures up to 500 °C. |
Friday, March 19, 2021 8:36AM - 8:48AM Live |
X56.00004: Varying Anisotropy in Oxidation of Two-Dimensional Transition Metal Dichalcogenides Myeong In Song, Sunmin Ryu In-plane anisotropy of two-dimensional crystals is not only intriguing but also of potential use for many applications. In this work, we studied how the structural anisotropy influences the thermal oxidation of single and few-layer transition metal dichalcogenides (TMDs) in the form of MX2, where M = Mo and W; X = S and Se. The reaction was initiated from randomly distributed reaction centers and led to triangular oxides (TOs) and triangular etch pits (TEs) of varying roundness that were revealed by AFM phase and height images, respectively. To quantify a given triangular object's roundness, we determined an isotropy index (R) defined as the radius ratio between its inscribed and circumscribed circles. TOs of WX2 showed larger isotropy values than TEs and TOs in MoX2, indicating that more anisotropic reactions occur in the latter. The material-dependent anisotropy was verified with the reaction rates at edges that increased in the order of M-zigzag, armchair, and X-zigzag. The shape evolution of the triangular objects was modeled by kinetic Wulff construction based on the experimental rates for the edges. |
Friday, March 19, 2021 8:48AM - 9:24AM Live |
X56.00005: Atomic resolution characterization of 2D materials with scanning transmission electron microscopy Invited Speaker: Nasim Alem The potential applications of two-dimensional (2D) transition metal dichalcogenides (TMDCs) critically depend on the atomic structures and the defects in them. Prof. Alem's group perform atomic resolution characterizations of 2D TMDCs with scanning transmission electron microscopy explore various atomic structures in 2D TMDCs and associate their stuctural properties to electronic properties. |
Friday, March 19, 2021 9:24AM - 9:36AM Live |
X56.00006: Nanoimaging of surface waves in 2D materials Dmitri Voronine Propagating surface waves such as surface-plasmon polaritons and exciton-polaritons have recently been of interest in a variety of optical, electronic and quantum communication applications. Nano-optical imaging of surface waves in two-dimensional (2D) transition-metal dichalcogenide (TMD) materials has been realized in several cavity-coupled systems. In this work, new nanoimaging schemes based on tip-enhanced photoluminescence (TEPL) and Kelvin-probe force microscopy (KPFM) are presented, revealing the optical and electric components of surface waves in 2D TMD heterostructures, respectively. These results could be used in novel nanophotonic coupling schemes for quantum emitters in 2D materials. |
Friday, March 19, 2021 9:36AM - 9:48AM Live |
X56.00007: Preferential hole defect formation in monolayer WSe2 by electron-beam irradiation Donghan Shin, Gang Wang, Andrew O'Hara, Mengjiao Han, Junhao Lin, Sokrates T Pantelides Monolayer transition-metal dichalcogenides (TMDs) have been studied extensively. Scanning transmission electron microscopy (STEM) has been used to both image and generate chalcogen vacancies, which then agglomerate into different multivacancy structures or linear defects. Density-functional-theory (DFT) calculations have been used to describe the formation of select such defects in several TMDs. Here we demonstrate that, in WSe2, an initial formation of particular multivacancy structures gradually leads to a high density of 10-, 12-, 14-, and 16-member-ring round holes. In contrast, in WS2, chalcogen vacancies agglomerate only into line defects. Time-lapse images and DFT calculations are used to track the agglomeration of chalcogen vacancies and identify the atomic-scale processes that energetically favor hole formation in WSe2 and linear structures in WS2. The demonstrated control of high-density round holes in WSe2 has potential for novel applications such as atomic and molecular sieving. |
Friday, March 19, 2021 9:48AM - 10:00AM Live |
X56.00008: Large-area single-crystal sheets of borophene on various metal surfaces Rongting Wu, Stephen Eltinge, Ilya K Drozdov, Adrian Gozar, Percy Zahl, Jerzy T. Sadowski, Sohrab Ismail-Beigi, Ivan Bozovic Borophene, monolayer boron, as a new two-dimensional (2D) quantum material, |
Friday, March 19, 2021 10:00AM - 10:12AM Live |
X56.00009: Facile and Non-Destructive Probe of Hydrogen Content in Hydrogenated Graphene: A Neutron Scattering Study Alexander Daykin, Sudhir Ravula, Helmut Kaiser, Thomas W Heitmann, Alessandro Mazza, Gary A. Baker, Paul F Miceli Defects and functionalization can be used to modify graphene’s physical properties such as changing its conductivity or inducing ferromagnetism, which could be of interest for spintronics. Here we investigate hydrogenated graphene nano-powder that has been produced by Birch reduction and is comprised of 3-4 layer flake thicknesses. One significant challenge in assessing these materials is the ability to determine the amount of hydrogen that has been incorporated into the graphene. We show that neutron incoherent scattering is an effective and non-destructive means to make this determination. Two methods are compared, the scattering and transmission of neutrons, which yield a consistent H:C ratio of 35% for our Birch-reduced hydrogenous graphene. Neutron and x-ray powder diffraction show that the layer spacing of 3.85 Å between the graphene sheets is significantly expanded compared to pristine graphene, indicating that H attaches onto the graphene planes. Our study demonstrates a simple, effective, and nondestructive method of characterizing H content in bulk graphene materials. |
Friday, March 19, 2021 10:12AM - 10:24AM Live |
X56.00010: Moire engineering for grain boundary design in graphene Emil Annevelink, Zhu-Jun Wang, Guocai Dong, Harley T Johnson, Pascal Pochet Deterministic control of graphene atomic structure would enable the precise tuning of its electrical, mechanical, and thermal properties. Current synthesis methods like chemical vapor deposition, however, have limited atomic control due to a lack of mechanistic insight on defect formation due to synthesis conditions. Furthermore, it is impossible to observe atomic scale mechanisms during synthesis due to the incompatible time scales of growth and microscopy. In our work, we overcome these challenges by coupling atomic scale mechanistic models with moiré engineering to enable experimental observation of atomic scale defects during synthesis. We present an application of this technique to tailoring graphene grain boundaries during synthesis. Our method introduces a mechanistic atomic scale model for the formation of graphene grain boundaries during graphene nuclei coalescence. The atomic scale models are promoted to the nanoscale to create the moiré engineering technique that addresses the incompatible time scales and enables real-time observation of atomic scale defects. We showcase this technique using in-operando scanning tunneling microscopy data of graphene grown on rhodium and show how it can enable real-time decision making under scalable synthesis conditions of graphene. |
Friday, March 19, 2021 10:24AM - 10:36AM Live |
X56.00011: Probing Excitonic Resonances of Metal Films on Suspended 2D Membranes Using Electron Energy Loss Spectroscopy Todd Brintlinger, Jose Fonseca Vega, James Clifford Culbertson, Maxim Zalalutdinov, Rhonda Michele Stroud, Jeremy Robinson We characterize Oriented Porous mEtallic Networks (OPEN) using aberration-corrected scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS). These OPEN films are synthesized by annealing metal films on 2D layers, which causes lattice registry between the metal layer and the 2D materials, and we will examine the morphological influence that MoS2 monolayers have on relatively thick (13-25nm) gold thin films during annealing. Ex-situ annealing experiments of MoS2/Au on SiO2 substrates shows that the Au films can become textured (oriented), as revealed through electron beam scattering diffraction, and that the Au film can locally dewet beneath the MoS2 layer to form a porous metal layer with suspended MoS2 membranes. Further, using both STEM and EELS at 60 kV, we show the metal ordering is aligned with the 2D lattice and find EELS resonances at ~ 2 eV, which is associated with the so-called A and B excitons, as well as at several other energies. We will discuss the origin of these resonances, and how both the metal overlayer and disordered carbon contamination affect them. |
Friday, March 19, 2021 10:36AM - 10:48AM Live |
X56.00012: Scanning Tunneling Microscopy Study of S Vacancy Defect-Defect Interactions in Monolayer WS2 Madisen Holbrook, Chao Lei, Wei-Ting Hsu, Li-Syuan Lu, Wen-Hao Chang, Allan MacDonald, Feliciano Giustino, Philipp Ebert, Chih-Kang Shih Atomic defects in crystalline semiconductors strongly affect their electronic properties, such as electron transport and optical response. Defects in two dimensional (2D) semiconductors, such as transition metal dichalcogenides (TMD’s), have a more dramatic impact than bulk counterparts due to less screening and increased substrate interactions. In particular, different atomic vacancies have been reported, but their role in the electronic structure must be established. Deep in-gap states were predicted for different defect structures in 2D TMD’s, but direct experimental observation of defect structure and electronic properties is necessary to tailor TMD’s for device design. Here we report the creation of a high concentration of S vacancy defects in WS2 monolayers by vacuum annealing, and characterization of their electronic properties and structure using scanning tunneling microscopy and spectroscopy. We find that interaction of nearby S vacancies results in a rich variety of deep in-gap states, shedding light on the role of defects in TMD electronic properties. |
Friday, March 19, 2021 10:48AM - 11:00AM On Demand |
X56.00013: Revealing the nature of size and odd/even effect observed by NanoDSC in 2D organic metal chalcogenides: atomic resolution crystallography via diffraction and ab initio calculations Jie Zhao, Zichao Ye, Kisung Kang, Yu-Tsun Shao, Scott McCormack, Mikhail Efremov, Andre Schleife, Jian-Min Zuo, Waltraud Kriven, Leslie Allen Layered silver alkanethiolate (AgSCn) is currently of growing scientific interest as 2D organic metal chalcogenides. Exploring the structure of AgSCn with atomic resolution is critical to understand its size and odd/even effect observed in properties such as melting point/enthalpy (NanoDSC). Crystallography determination is often challenging for beam-sensitive hybrid materials with small crystal sizes. In this work, we innovatively overcome this obstacle by combining synchrotron powder X-ray diffraction, nanobeam electron diffraction, and ab initio calculations. Preliminary findings show AgSCn structure consisting of three segments: (i) Ag-S inorganic backbone, which undergoes deformation modulated by organic chain length due to the lattice mismatch between Ag-S framework and the epitaxial hydrocarbon chains; (ii) Interlayer interface: the structural origin of odd/even effect lies in the parity-dependent H-H interactions exhibited among the terminal methyl groups; (iii) Alkyl chains: the epitaxial mismatch between the backbone and the alkyl chains triggers the progression of distortion in the local alkyl configurations towards the optimal chain packing. |
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