Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session R15: 2D Materials (Semiconductors) -- Processing and CharacterizationFocus
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Sponsoring Units: DMP DCOMP Chair: Han Wang, University of Southern California Room: BCEC 154 |
Thursday, March 7, 2019 8:00AM - 8:12AM |
R15.00001: WITHDRAWN ABSTRACT
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Thursday, March 7, 2019 8:12AM - 8:24AM |
R15.00002: Real-space Observation of Charge Separation in Mixed Dimensional InSe-C70 van der Waals Heterojunctions Shaowei Li, Chengmei Zhong, Alex Henning, Mark Hersam Two-dimensional InSe is a direct bandgap semiconductor promising for high performance photodetectors and solar cells. Mixed dimensional p-n heterojunction between InSe and organic molecules integrates desirable properties of both. Electronic structure of surface states is critical for optimal band offsets at the heterojunction. Although there is no consensus on the type of intrinsic doping, few-layered InSe field-effect transistors showed exclusively n-type behavior. Here, we report that the atomically clean surface of mechanically exfoliated InSe is unintentionally p-doped. STS taken over the native surface selenium vacancies resolves occupied defect states near the valance band edge. These defect states lead to an upward band bending near the InSe surface, rendering InSe surface p-type. In prototype InSe/C70 heterojunction, PL spectroscopy and STS both indicate formation of a type-II heterojunction. Electrons can transfer between InSe and C70 evidence from mutual PL quenching of both InSe and C70. STS in vacuum and KPFM in the ambient consistently reveal the electron transfer from InSe to C70 resulting in a photovoltage of ~0.25 eV. This study reveals the unusual electron donor behavior of InSe in an organic-inorganic heterostructure. |
Thursday, March 7, 2019 8:24AM - 8:36AM |
R15.00003: Atomically thin dichalcogenide heterostructures and interfaces measured via cathodoluminescence in scanning transmission electron microscope Akshay Singh, haeyeon Lee, silvija gradecak Atomically thin transition metal dichalcogenides (TMDs) and associated heterostructures have distinct opto-electronic properties including enhanced luminescence and high on-off current ratios. However, optoelectronic properties measured using conventional methods are limited to micro-milimeter scale and with no direct structural correlation, despite the fact that relevant property fluctuations can be caused by significantly smaller structural features. We use cathodoluminescence (CL) in a scanning transmission electron microscope (STEM) as a nanoscale probe that offers direct structure-optical correlation with high spatial resolution. |
Thursday, March 7, 2019 8:36AM - 8:48AM |
R15.00004: Carrier density tuning in single layers of WS2 via photochlorination George Kioseoglou, Ioanna Demeridou, Ioannis Paradisanos, Panos Patsalas, Emmanuel Stratakis Monolayers of Transition Metal Dichalcogenides (TMDs) of the form MX2 (M=Mo or W and X=S or Se) are promising semiconducting materials for future 2D nanoelectronics due to their unique properties. Carrier modulation and doping reversibility are very important issues in the study of the electronic properties of TMDs and at the heart of many applications. We demonstrate electron density control in chlorine-doped WS2 monolayers by pulsed laser irradiation in a precursor gas atmosphere [1]. The increase of the photochlorination time gives rise to a systematic red-shift in the PL energy of the neutral exciton indicating a reduction in the electron density. This electron withdrawing process enabled also the determination of the trion binding energy of the intrinsic crystal, found to be 20 meV, in accordance to theoretical predictions. At the same time, it is found that the effect can be reversed upon cw laser rastering of the monolayer in air. SAM and XPS reveal that chlorine physisorption is responsible for the e-density modulation induced by the pulsed laser photochemical reaction process and confirmed by DFT calculations. [1] I. Demeridou et al, 2D Mater. 6 (2019), 015003 |
Thursday, March 7, 2019 8:48AM - 9:00AM |
R15.00005: Surface Oxidation Induced P-doping in Transition Metal Dichalcogenides Minju Kim, Junkyeong Jeong, Dongguen Shin, Jeehong Park, Jaehyun Yang, Sangwan Cho, Hyunbok Lee, Yeonjin Yi Transition metal dichalcogenides (TMDCs) has received tremendous attention due to their fascinating electrical properties, such as a high carrier mobility, high on/off ratio and tunable bandgap. However, TDMC-based electronic devices do not show their pristine properties due to the contact resistance between the metal electrode and TMDCs. Surface oxidation has been known to induce a p-doping and it improves the contact resistance. With optimum oxidation, highly improved contact was evidently observed in field-effect transistors, while it is rather worsen with non-optimum conditions. To elucidate the origin of improvement, the changes in the electronic structure of TMDCs upon the surface oxidation should be understood. In this regard, the electronic structure of TMDCs was studied with ultraviolet and x-ray photoemission and inverse photoemission spectroscopy (UPS/XPS/IPES). TMDCs were treated with UV-ozone and the changes in oxidation states of TMDCs were investigated with XPS. Then, the changes of work function and band edges were directly observed with UPS and IPES. The surface oxidation can control the position of band edges efficiently. |
Thursday, March 7, 2019 9:00AM - 9:12AM |
R15.00006: Modulation doping in hexagonal BN atomic-layer semiconductor Susumu Saito, Yuuto Matsuura, Yoshitaka Fujimoto We study the stable stacking sequences and the electronic properties of hexagonal BN (h-BN) trilayers within the framework of the density functional theory. Because of its lower-symmetry geometry of the h-BN sheet than that of graphene, there appear several different stable stacking sequences in the h-BN trilayers. Interestingly, most of stable stacking sequences of trilayers are found to be different from that of the bulk phase. It is also found that spatial distributions of both the valence-band top states and the conduction-band bottom states are rather confined to a certain specific layer in stable h-BN trilayers. By utilizing this fact, we design the C-doped h-BN trilayers where carriers conduct not in the doped layer but mostly in the undoped perfect layer. These systems are therefore considered to be the ultimate modulation-doped semiconductor heterostructure where both the doped region and the conducting region are only one atomic-layer thick. We also discuss several four-layer and five-layer h-BN systems which can host further clear spatial modulation doping. |
Thursday, March 7, 2019 9:12AM - 9:24AM |
R15.00007: Doping dependent magnetic properties in monolayer PdSe2 YoSep Cho, Hyoung Joon Choi In recent years, ferromagnetism in two-dimensional materials which is induced by charge doping drawn great interest. Density functional theory (DFT) calculations with changing the total number of electrons predict that increase of hole doping leads to ferromagnetism in monolayer (ML) PdSe<span style="font-size:10.8333px">2</span>. In this work, we investigate doping dependent magnetism in ML PdSe2 using supercells with differently substituted impurities. Moreover, we performed virtual crystal approximations (VCA) for uniform and non-uniform doping to see doping method dependence for the magnetism. To understand the magnetism in this material, we discuss local-moment interactions and Stoner-type mechanism. |
Thursday, March 7, 2019 9:24AM - 9:36AM |
R15.00008: All-optical quality assessment of 2D TMDs using polarization-resolved SHG Emmanuel Stratakis, Sotiris Psilodimitrakopoulos, Leonidas Mouchliadis, Ioannis Paradisanos, Andreas Lemonis, George Kioseoglou While large-area crystal growth techniques, such as chemical vapor deposition (CVD), are successfully used to produce 2D transition-metal dichalcogenides, the presence of grain boundaries, vacancies and arbitrarily oriented grains, substantially affect their crystal quality. We demonstrate a fast, high-resolution non-linear optical method for the quality control of WS2 monolayers. Polarization resolved second harmonic generation (PSHG) imaging reveals with high-precision the orientation of the main crystallographic axis (armchair). By performing a pixel-by-pixel mapping of the armchair orientations on a CVD-grown sample area, we can distinguish between different domains, locate their boundaries and reveal their detailed structure. We fit experimental PSHG images of sub-micron resolution into a generalized theoretical model and we acquire the armchair orientation for every pixel. This allows us to measure the mean orientational average of armchair angle distributions from specific regions of interest and consequently to define the standard deviation of these distributions as a crystal quality factor. [1] S. Psilodimitrakopoulos, et al., Light Science & Applications 7, 18005 (2018). |
Thursday, March 7, 2019 9:36AM - 10:12AM |
R15.00009: Chemically Tunable 2D Layered Materials: Acoustic Phonons, Charge Density Waves, & Phase Transitions Invited Speaker: Kristie Koski I will present an innovative strategy to intercalate atomic species including heavy metals, semiconductors, and semimetals (Ag, Au, Bi, Cr, Cu, Ge, Mn, Mo, Ni, Os, Pb, Pd, Pt, Rh, Ru, Sb, W) into 2D layered materials. With intercalation of zero-valent elements, fundamentally new physical behaviors arise such as chemically-controllable commensurate and incommensurate charge density waves, altered acoustic phonons as measured with Brillouin scattering, and chemically-tunable pressure-dependent phase transitions. This strategy can be further used to achieve 2D metal-semiconductor heterostructures or 2D semiconductor heterostructures with unique physical properies. Chemically tunable physical properties of intercalated Bi2Se3, Si2Te3, GeS and MoO3 are demonstrated. |
Thursday, March 7, 2019 10:12AM - 10:24AM |
R15.00010: Understanding bound excitons in 2D monolayer semiconductors with nano-optical imaging and spectroscopy Nicholas Borys, Thomas Darlington, Jim Schuck, Andrey Krayev Bound excitons in 2D semiconductors are of interest as potential qubits as well as single-photon emitters for quantum computing and information applications. But our limited understanding of these states inhibits overcoming several challenges that they pose for integration into practical technological applications. One such issue is the suppression of bound excitons at temperatures above ~150 K. Employing a suite of time-resolved and nano-optical spectroscopy studies, we find that the thermally-activated process suppresses their formation and develop a nano-optical architectural motif to reverse this suppression and activate the states at room-temperature. Furthermore, using nano-optical imaging and spectroscopy, distinct bound exciton states are optically imaged within strain-induced potentials at resolutions down to 20 nm. This nanoscale resolution allows us to measure the extent to which these states are localized as well as identify how nanoscale strain controls their excited-state properties. From these studies, we can begin to envision how to use nano-optical antennas to integrate patterns of bound exciton states in two-dimensional monolayer semiconductors for model optoelectronic and room-temperature quantum devices. |
Thursday, March 7, 2019 10:24AM - 10:36AM |
R15.00011: Laser annealing for Large Excitonic Reflectivity from MoSe2 grown by Chemical Vapor Deposition Christopher Rogers, Dodd J Gray, Nate Bogdanowicz, Hideo Mabuchi We present data of a laser annealing procedure which drastically improves the quality of suspended monolayers of chemical vapor deposition grown MoSe2. Annealing with a green laser locally heats the suspended flake, which both removes contaminants and reduces strain gradients. At 4 K, we observe linewidths as narrow as 3.5 meV (1.6 nm) full width at half maximum for both photoluminescence and reflection. Large peak reflectances up to 47% are also observed. These values are comparable to those of the highest quality hexagonal boron nitride encapsulated samples. We demonstrate that this laser annealing process can yield spatially homogeneous samples where the length scale of the homogeneity is limited primarily by the size of the suspended area. Annealed regions are very stable, exhibiting negligible deterioration over 24 h at cryogenic temperatures. The annealing method is also very repeatable. |
Thursday, March 7, 2019 10:36AM - 10:48AM |
R15.00012: Spontaneous Curvature of Janus Transition Metal Dichalcogenide Nanoribbons: Effects on Optoelectronic, Magnetic and Mechanical Properties Jaron Kropp, Fatih Ersan, Can Ataca Janus transition metal dichalcogenides (TMDs) are two-dimensional materials of the form MXY (M = Mo, W; X, Y = S, Se, Te). These materials are similar to traditional TMDs with structure MX2, but with one chalcogen layer replaced by another chalcogen type. In this work, we study freestanding armchair and zigzag nanoribbons of the Janus materials Mo/WSSe, Mo/WSTe, and Mo/WSeTe using density functional theory. The lattice mismatch between the X and Y sides of the material causes curvature of the unsupported ribbons. We investigate the magnetic and electronic properties of these structures as a function of ribbon width and curvature angle. Zigzag nanoribbons are found to be metallic while armchair nanoribbons are semiconducting with a band gap that depends on the ribbon width. Additionally, all zigzag nanoribbons possess a magnetic moment while armchair ribbons may not. We also study the effects of edge passivation on both armchair and zigzag nanoribbons. Hydrogen passivation was found to increase the band gap, enhance the stability of nanoribbon edge and induce a magnetic moment in the armchair ribbons. This work sheds light on how the spontaneous curvature of the nanoribbons affect/enhance the optoelectronic, magnetic and mechanical properties. |
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