Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session G53: 2D Synthesis and CharacterizationFocus
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Sponsoring Units: APS Chair: Fei Hui, Technion - Israel Institute of Technology Room: Mile High Ballroom 1F |
Tuesday, March 3, 2020 11:15AM - 11:51AM |
G53.00001: Towards room-temperature v magnetic semiconductors Invited Speaker: Young-Hee Lee The ferromagnetic state in van der Waals two-dimensional (2D) materials has been reported recently in the monolayer limit. Intrinsic CrI3 and CrGeTe3 semiconductors reveal ferromagnetism but the Tc is still low below 60K. In contrast, monolayer VSe2 is ferromagnetic metal with Tc above room temperature but incapable of controlling its carrier density. Moreover, the long-range ferromagnetic order in doped diluted chalcogenide semiconductors has not been demonstrated at room temperature. The key research target is to realize the long-range order ferromagnetism, Tc over room temperature, and semiconductor with gate tunability. Here, we unambiguously observe a ferromagnetic hysteresis loop together with magnetic domains above room temperature in diluted V-doped WSe2, while maintaining the semiconducting characteristic of WSe2 with a high on/off current ratio of five orders of magnitude. Some examples of other magnetic 2D materials are also presented. |
Tuesday, March 3, 2020 11:51AM - 12:03PM |
G53.00002: Polymer-Free Transfer of 2D Materials onto III-V Semiconductor Surfaces for Scanning Probe Microscopy and Molecule-Encapsulation Experiments Lihy Buchbinder, Sara Mueller, Jay A Gupta A tabletop, dry touch-transfer method that employs van der Waals forces was used to transfer multi- to few-layer flakes of hexagonal boron nitride (hBN) and graphite onto cleaved, atomically flat GaAs (110) and InSb (110) surfaces. This method offers a cleaner alternative to current polymer-involved transfer methods, which leave polymer traces on the sample surface and require an extensive cleaning process for scanning probe microscopy (SPM) study. |
Tuesday, March 3, 2020 12:03PM - 12:15PM |
G53.00003: Improved contacts to two-dimensional materials for extracting emergent properties Curt Richter, Siyuan Zhang, Nicholas B. Guros, Son Le, Arvind Balijepalli, Christina Hacker It is critical to control the properties of electrical contacts to two-dimensional (2D) material systems such as twisted bilayer graphene or superconducting 2D systems in order to probe and properly extract their emergent quantum properties. We present here effective routes for contact engineering. It has been demonstrated that TMD monolayers can be effectively doped by using molecular reductants and oxidants introduced onto the surface from solution. [1] In this study, we investigated the effects of molecular doping and contact engineering on the 2D/metal interfaces and charge transport properties of MoS2 devices. We show that MoS2 p-type FETs can only be achieved through combining molecular doping and high-work function metals (e.g. Pd) [2]. We have demonstrated an exposed material forming gas cleaning anneal that effectively decreases deleterious organic contamination arising during fabrication as evidenced by uniform and improved field-effect transistor (FET) performance metrics [3] such as field-effect mobility and threshold voltage. |
Tuesday, March 3, 2020 12:15PM - 12:27PM |
G53.00004: Fabrication and Characterization of h-BN capped Exfoliated Multi-layer Graphene U. Kushan Wijewardena, Tharanga Nanayakkara, Sajith Withanage, Annika Kriisa, Ramesh Mani As a 2D material, Graphene has attracted significant interest in recent years. Observation of unconventional superconductivity in twisted bilayer graphene, topological quantum phases, and observation of even denominator FQHE states are some of the exciting new findings [1]. The challenge lies in obtaining large enough single layers of graphene that can be used for device fabrication. Since graphene is sensitive to the surrounding environment, leaving the sample at ambient conditions would heighten the interactions with water and oxygen, increasing impurities and defects. Encapsulating graphene provides significant improvement to the quality of samples. Here we discuss the fabrication and characterization of graphene samples covered from h-BN prepared by mechanical exfoliation and dry transfer method. Optical microscope and AFM imaging were employed in determining the thickness of samples. We studied the transport properties of several µm scale graphene devices with a different number of layers and different geometries. Further, we examine the effect of current annealing on such devices. |
Tuesday, March 3, 2020 12:27PM - 12:39PM |
G53.00005: Metallicity of 2H-MoS2 Induced by Au Hybridization Brandon Blue, Glenn Jernigan, Duy Le, Jose Fonseca, Stephanie Lough, Jesse E Thompson, Darian Smalley, Talat Rahman, Jeremy Robinson, Masa Ishigami The interaction between bulk metal contacts and two-dimensional (2D) transition metal dichalcogenides (TMDs) has a critical influence on overall device performance in a variety of potential applications. This interaction has been studied extensively in the past, especially in the case of moldybdenum disulfide (MoS2), with seemingly contradictory results. In this work, we directly exfoliate monolayer MoS2 onto bulk Au and anneal the resulting heterostructure under ultra-high vacuum (10-10 Torr, UHV) to 450 °C. After annealing, the MoS2 monolayer is observed under scanning tunneling microscopy/spectroscopy (STM/STS) to remain in its 2H lattice configuration, yet with regions ranging from fully gapped to metallic. This observation is corroborated by ex situ Raman and photoemission spectroscopies, which show no signs of the otherwise expected 1T or 1T' structural phase transitions. Theoretical calculations suggest the previous band-gap tuning observed in MoS2-Au heterostructures is due to hybridization and chemical bonding at the S-Au interface, which can completely close the band-gap of 2H-MoS2 at sufficiently short S-Au distances. |
Tuesday, March 3, 2020 12:39PM - 12:51PM |
G53.00006: Reversible hydrostatic strain in graphene/gold nanoparticles hybrid material induced by laser irradiation András Pálinkás Gold nanoparticles (NPs) were prepared on SiO2 substrate by local annealing of gold thin films using focused laser beam. CVD-grown graphene was transferred onto the prepared NPs. Subsequent Raman-spectroscopy measurements were performed on the samples using different laser powers. We used higher laser intensity (6 mW) to locally anneal the hybrid material. Low laser powers (0.6 mW) were used to characterize the doping and the strain formed in the same areas both before and after local heating. While we found that higher intensity laser irradiation increased gradually the doping and the defect concentration in SiO2-supported graphene, the same irradiation procedure did not induce such irreversible effects in the graphene supported by Au NPs. Moreover, the laser irradiation induced dynamic hydrostatic strain in the graphene on Au NPs, which turned out to be completely reversible. |
Tuesday, March 3, 2020 12:51PM - 1:03PM |
G53.00007: Environmental effects on optoelectronic properties of heterostructures of 2D materials Atikur Rahman, Vrinda Narayanan, Gokul M. A., Tamaghna Chowdhury We have developed a simple method to fabricate large-area MoS2 monolayer devices. We will discuss the fabrication of various heterostructures using monolayer MoS2 as one of the components. Due to the 2D nature of MoS2, the coulomb screening is less effective and the electronic and optoelectronic properties of these heterostructures depend strongly on the local environment. We will discuss how various characterization methods such as low-frequency noise, dielectric and transient photoresponse can be used to get a deeper understanding of the details of the charge transport mechanism in these systems. |
Tuesday, March 3, 2020 1:03PM - 1:15PM |
G53.00008: Electronic and Optoelectronic Properties of Indium Tin Selenide (In1-xSnxSe) Prasanna Patil, Robinson Karunanithy, Olli Pitkanen, Sidong Lei, Poopalasingam Sivakumar, Krisztian Kordas, Saikat Talapatra Selenide based binary- and ternary-layered compounds show exotic physical properties beneficial for developing them for future electronic & opto-electronics based applications.1 For example; few-layered Indium Selenide (InSe) has attracted attention due to presence of direct band gap and hence its impressive photo-responsive properties.1 It was also shown that ternary layered alloys such as Copper Indium Selenide (CuIn7Se11) possess properties that can be harnessed for developing fast photodetectors.2 In this talk, we will report on the electronic and optoelectronic properties of few-layered Indium Tin Selenide (In1-xSnxSe) presented. Specifically, a comparison of the key parameters associated with electronic and optoelectronic properties of In1-xSnxSe with x = 0, 0.05, 0.1 and 1 will be presented and discussed. |
Tuesday, March 3, 2020 1:15PM - 1:27PM |
G53.00009: Dispersive Thermometry with a Graphene Josephson Junction Raj Katti, Harpreet Singh Arora, Olli Saira, Ewa Rej, Matthew Matheny, Michael Roukes, Stevan Nadj-Perge Graphene, with its vanishing heat capacity and weak electron-phonon coupling at cryogenic temperatures, is a promising material for ultrasensitive calorimetry and single-photon detection. Here, we present dispersive thermometry measurements performed on a tunable graphene Josephson Junction (gJJ) integrated into a resonant microwave circuit. In contrast to DC detection methods that rely on the switching of the gJJ to its resistive state, this approach allows for continuous temperature readout. We will discuss device operation and show results in both the electron- and hole-doped regimes. Our results represent a step towards fast detection of low-energy photons and phonons. |
Tuesday, March 3, 2020 1:27PM - 1:39PM |
G53.00010: Characterization of nonlinear screening in ionic liquid gated graphene multilayers via infrared spectroscopy Jiho kim, Marcelo Kuroda, Jiwon Jeon, Byoungju Lee, Kwangnam Yu, Eunjip Choi We quantify charge distributions in turbostratic few-layer graphene combining broadband infrared transmittance spectroscopy and analytical models. We show that the non-invasive experimental technique provides layer-resolved charge density profiles in regimes of interest for energy storage applications accessed using ionic-liquid gating. More importantly, we obtain unambiguous evidence of nonlinear screening of graphene which varies with thickness and charge density. Our results present good agreement with our theoretical model that accounts for the electrostatic coupling between layers and quantum capacitance of graphene. The proposed capacitor network model suggests that the effective channel capacitance increases with multilayer thickness but saturates after three layers, underscoring graphene’s qualities for ultrathin charge storage applications. Our work suggests that the combination of ionic liquid gating and infrared transmission spectroscopy may prove useful to the study of charge distributions in two-dimensional material systems. |
Tuesday, March 3, 2020 1:39PM - 1:51PM |
G53.00011: Lithium intercalation of Black Phosphorous: Ex-situ and in-situ Raman Spectroscopy Study Manthila Rajapakse, Md Rajib khan Musa, Usman Onuminya Abu, Gamini U Sumanasekera, Ming Yu, Jacek Bogdan Jasinski Black phosphorous is emerging as a promising two-dimensional (2D) material which can be used as a host material for many intercalants. A systematic study on Li intercalation of black phosphorous was done using both in-situ and ex-situ electrochemical cells using black phosphorous as the cathode material. Galvanostatic discharge of dedicated in-situ electrochemical cell for Raman spectroscopy were used to study time evolution of vibrational modes under lithiation. Other than the peak broadening which is a result of structural expansion, it was observed that peaks corresponding to all three atomic vibrational modes A1g, B2g and A2g were red-shifted as a result of lithiation. Peaks B2g and A2g shifted more towards lower wavenumbers whereas peak A1g shifted only 50 % with respect to the other two, suggesting that Li ions prefer to be intercalated in-plane, along zig-zag and armchair directions than out-of-plane direction. Additional Transmission Electron Microscopy images were used to confirm the associated structural changes and Density Functional Theory (DFT) calculations were used to build a theoretical model for the phenomena. |
Tuesday, March 3, 2020 1:51PM - 2:03PM |
G53.00012: Study of Interface States and Dielectric Permittivity of Two-dimensional Tungsten Diselenide by Impedance Measurements FIDA ALI, Yang Zheng, Sekhar Babu Mitta, Won Jong Yoo The dielectric permittivity of material holds great importance for understanding the intrinsic material properties and meanwhile, it's a critical design parameter for the design of novel device structure. In this article, we studied the capacitance-voltage (C-V) and conductance-voltage (G-V) characteristics of 2-D semiconducting WSe2 based metal-insulator-semiconductor-metal capacitors under varying gate voltage (VG), frequency and temperature. The obtained C-V and G-V results show a strong dependence on applied VG, frequency (1 kHz to 1 MHz), and temperature (78K to 300K). From frequency-dependent C-V response, we extracted interface state density (Dit) by using the high-low frequency method, where Dit values ranging from 3.8 ×1010 to 3.6×1012 cm-2eV-1 in the depletion region. Furthermore, we calculated the real (ε′), imaginary (ε'') parts of the dielectric constant and loss tangent (tanδ) using C-V and G-V results. The results show that the values of ε′, ε'' and tanδ decrease with increasing frequency and increase with increasing temperature. |
Tuesday, March 3, 2020 2:03PM - 2:15PM |
G53.00013: Direct, in-situ spatial imaging of stacking order in van der Waals heterostructures Andrey Sushko, Kristiaan De Greve, Trond I Andersen, Giovanni Scuri, Takashi Taniguchi, Kenji Watanabe, Philip Kim, Hongkun Park, Mikhail Lukin The optical and electronic properties of 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 optical properties of the TMD are conserved after imaging, enabling correlation of local stacking order with optical and electronic properties. |
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