Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session K36: 2D Materials -Role of DefectsFocus
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Sponsoring Units: DMP Chair: Kathleen McCreary, U.S. Naval Research Laboratory Room: LACC 410 |
Wednesday, March 7, 2018 8:00AM - 8:12AM |
K36.00001: Lateral band bending and defects in WS2 flakes Roland Koch, Christoph Kastl, Søren Ulstrup, Jyoti Katoch, Simon Moser, Christopher Chen, Tevye Kuykendall, Adam Schwartzberg, Shaul Aloni, Alexander Weber-Bargioni, Eli Rotenberg, Chris Jozwiak, Aaron Bostwick The properties of transition metal dichalcogenides are strongly influenced by changes of the electronic structure. Angular resolved photoemission spectroscopy (ARPES) is the ideal tool for obtaining precise information about the electronic state. At the nano-ARPES facility of the MAESTRO beamline at the Advanced Light Source, we combine ARPES with spatial resolution by focusing the illuminating photon beam down to a spot of 100 nm diameter. Nano-ARPES allows us to laterally map differences in band bending and changes in the spectral function of WS2 and hexagonal BN on different substrates. By combining the angular resolved data with laterally resolved core level spectroscopy, we pinpoint the observed changes to edges and/or defects in the materials. |
Wednesday, March 7, 2018 8:12AM - 8:24AM |
K36.00002: Direct Quantification of Defect Density in Monolayer WS2 and the Impact of Defect Density of Photoluminescence Matthew Rosenberger, Hsun-Jen Chuang, Kathleen McCreary, Connie Li, Berend Jonker Despite the importance of understanding defect-related phenomena in 2D materials, there remains a need for quantitative characterization of defect density over large areas in order to understand the relationship between defects and observed properties, such as photoluminescence (PL). We report direct observation of defects in monolayer WS2 with nanometer-scale precision over large length scales (up to 20 µm distances) using conductive atomic force microscopy (CAFM). CAFM enables precise identification of defect locations and direct quantification of areal defect density, which ranges from 2.3 x 1010 cm-2 to 4.5 x 1011 cm-2 in our samples. We observe a pronounced inverse relationship between PL intensity and defect density. We develop a model in which observed electronically active defects serve as non-radiative recombination centers, and obtain good agreement with experimental data. Our results provide important information for understanding the cause of spatial variations in WS2 properties, and are a critical demonstration of a technique for mapping defect density over length scales relevant for observed 2D material behaviors. |
Wednesday, March 7, 2018 8:24AM - 8:36AM |
K36.00003: Optical properties of Transition Metal Dichalcogenides in the Defect-Free Limit Drew Edelberg, Daniel Rhodes, Jue Wang, Bumho Kim, Amirali Zangiabadi, Chanul Kim, Alexander Kerelsky, Elton Santos, Chris Marianetti, Xiaoyang Zhu, Katayun Barmak, James Hone, Abhay Pasupathy Transition metal dichalcogenides (TMDs) of the form MX2 exhibit a direct optical gap in single layer with promising applications. Monolayer optical experiments are highly sensitive to defects, both those intrinsic to bulk, and those occurring in the substrate. While substrate defects can be passivated, intrinsic defects limit TMD performance. In this work, we quantify defect densities of TMD materials using scanning tunneling microscopy (STM). We show that the best crystals have a defect density of 0.01%, while typical crystals have a density of 0.1% or higher. Tunneling spectroscopy was then used to map the local bandgap on scales relevant for optical measurements. Based on STM results, the optical properties of exfoliated monolayers of known defect density were compared using photoluminescence (PL) spectroscopy. Our primary finding is that defects cause non-radiative decay of excitons, reducing the intensity of the observed PL by up to an order of magnitude at 0.1% defect concentrations. Spin splitting in the TMD conduction band additionally breaks the exciton into an optically bright and dark state. Since the PL is proportional to the population of excitons in each state, signatures of intervalley scattering to the dark exciton are observed in temperature-dependent PL spectra. |
Wednesday, March 7, 2018 8:36AM - 9:12AM |
K36.00004: Electronic properties of defects in single-layer MoSe2 Invited Speaker: Sara Barja Intrinsic properties of 2D transition metal dichalcogenide (TMD) semiconductors are highly sensitive to the presence of defects in the crystal structure. Understanding the defect electronic structure at the atomic scale will enable unprecedented control over material functionality. |
Wednesday, March 7, 2018 9:12AM - 9:24AM |
K36.00005: Characterizing and manipulating the charge state of individual defects in WSe2 Rui Zhang, Genevieve Clark, Xiaodong Xu, Pierre Darancet, Jeffrey Guest Defect structures in tungsten diselenide (WSe2) are drawing increasing attention because they play a role in the optical response and in some cases give rise to single-photon emission, providing new platforms for the application in quantum information processing1-4. To date, however, the understanding of the nature and electronic structure of such structural defects in WSe2 atomic layers is still lacking5. Here we describe the characterization of individual point defects in WSe2 atomic layers using scanning tunneling microscopy and spectroscopy. In bilayer WSe2 on graphene, we observe in-gap electronic states, and spatially dependent spectroscopy and dI/dV maps showed that the charge state of the defect can be manipulated. We also characterize other types of point defects in our measurements, including ones characterized by a very small energy bandgap (similar to that of a metallic phase of WSe26) and others which feature sharp electronic states near the valence band and a negative differential resistance (NDR), similar to that observed in single quantum states7,8. Finally, we propose a minimal single-electron transport model explaining these effects. |
Wednesday, March 7, 2018 9:24AM - 9:36AM |
K36.00006: Crystalline hydrogenation of graphene by scanning tunneling microscope tip-induced field dissociation of H2 Steven Tjung, Shawna Hollen, Grady Gambrel, Nancy Santagata, Ezekiel Johnston-Halperin, Jay Gupta Because of the sensitivity of 2D material surfaces, chemical functionalization can be exploited tune the electronic structure of these materials. For example, hydrogen bonding to carbon atoms in graphene tunes the material from a semi-metal to a wide-gap insulator. We developed a novel method for crystalline hydrogenation of graphene on the nanoscale. Molecular hydrogen was physisorbed at 5 K onto pristine graphene islands grown on Cu(111) in ultra-high vacuum. Field emission local to the tip of the scanning tunneling microscope dissociates H2 and results in hydrogenated graphene. At lower coverage, isolated point defects are found on the graphene and are attributed to chemisorbed H on top and bottom surfaces. Repeated H2 exposure and field emission yielded patches and then complete coverage of a crystalline √3 × √3 R30° phase, as well as less densely packed 3 × 3 and 4 × 4 structures. The hydrogenation can be reversed by imaging with higher bias. Scanning tunneling spectroscopy did not show the predicted band gap which we attributed to the interaction of the hydrogenated graphene with the conducting Cu substrate. |
Wednesday, March 7, 2018 9:36AM - 9:48AM |
K36.00007: Visualization of native defects in PdSe2 using scanning tunneling microscope Giang Nguyen, Liangbo Liang, Akinola Oyedele, Zheng Liu, Kai Xiao, An-Ping Li Palladium diselenide (PdSe2), a newly discovered 2D layered transition-metal dichalcogenide (TMD), have attracted great research interest recently due to its extraordinary high carrier mobility and interesting anisotropic properties. In contrast to most other TMDs with isotropic planar hexagonal structure, PdSe2 has a unique buckled pentagonal structure in the 2D layer. Here we report our recent study on PdSe2 bulk crystals cleaved in situ under ultra-high vacuum (UHV) using Scanning Tunneling Microscopy (STM). Atomic resolution images and scanning tunneling spectroscopy are acquired to reveal structural and electronic properties of the material. Interestingly, native defects in PdSe2 appear as a ringlike feature in STM differential conductance (dI/dV) maps. The observation is explained by the reversibly switched charge states of the defects controlled by the tip-induced band bending. The defect structure and defect states are resolved experimentally and corroborated by first-principle calculations. |
Wednesday, March 7, 2018 9:48AM - 10:00AM |
K36.00008: Effects of controlled doping and defects on the physical properties of Group-VI transition metal dichalcogenides Wei-Hsiang Lin, Robert M Polski, Marcus Teague, Wei-Shiuan Tseng, Marcin Konczykowski, Harry Atwater, Nai-Chang Yeh Group-VI transition metal dichalcogenides (TMDCs) are promising atomically thin semiconductors for nanoelectronic and nanophotonic devices. Intrinsic defects in these materials are well known to have profound influence on the electronic and optical properties. We have successfully synthesized high quality group-VI TMDCs materials by using the chemical vapor transport (CVT) method and have characterized these TMDCs samples at both atomic and macroscopic scales. Raman spectroscopy revealed a linear dependence of thickness with the growth time. X-ray photoelectron spectroscopy revealed a metal/chalcogenides atom ratio to be (1/2) for all thicknesses. Combining the ultraviolet photoemission spectra and scanning tunneling spectra, we successfully constructed the band diagram of these TMDCs materials. Additionally, by introducing controlled concentrations of point-like defects via 2.5MeV electron irradiation, the effects of irradiation on various physical properties of TMDCs can be understood systematically. We performed Raman spectroscopic studies to examine the disorder effects on phonon modes, and magneto-opto Kerr effect (MOKE) measurements to understand point defect-induced magnetism in TMDCs. |
Wednesday, March 7, 2018 10:00AM - 10:12AM |
K36.00009: Unraveling the Effect of Multiple Defect States in Synthetic Monolayer MoS2 Through Electronic and Optical Probes Pin-Chun Shen, Yuxuan Lin, Xingzhi Wang, Xi Ling, Tomas Palacios, Jing Kong Two-dimensional transition metal dichalcogenides (2D TMDs) has been proven to be ideal for post-silicon technology. However, the intricacy and diversity of the defects in 2D TMDs affect the electrical and optical properties in many different ways, some of which even contradict to our intuition. Several challenging issues including Femi level pinning at metal/2D TMD interfaces, unintentional doping, and non-radiative excitonic recombinations, etc. have been attributed to a considerable amount of sulfur vacancy in monolayer MoS2. On the other hand, specific types of defects, if controlled carefully, also offers the access to engineer the nature of monolayer MoS2, such as intentional doping and exciton reservoirs to prolong the excitonic lifetime. Here, we explored the correlation between the domain shapes and the presence of different types of defects in monolayer MoS2 grown by CVD through transport and spectroscopy measurements. Based on a two defect states model, the geometry-modulated behaviors of the MoS2-metal band alignments, mobilities, and photoluminescence spectra could be explained simultaneously. This work not only offers a strategy to engineer the nature of MoS2 from the synthesis perspective, but also pave a path to realize low-power MoS2 CMOS integrated circuits. |
Wednesday, March 7, 2018 10:12AM - 10:24AM |
K36.00010: In situ study of photo-induced chemical modification in transition metal dichalcogenides Tariq Afaneh, Prasana Sahoo, Igor Paschoalatte-Nobrega, Yan Xin, Humberto Gutierrez In this work, we performed an in situ Raman-PL study of the laser-induced chemical modification of transition metal dichalcogenides layers. Using a homemade sealed mini-chamber with a quartz optical viewport, a laser beam (532 nm) was focused onto the sample, consisting on MoSe2 or WSe2, while keeping a reactive sulfur-rich atmosphere within the chamber. The process can be tuned thereby choosing appropriate laser power, exposure time, and reactive gas environment. The time-dependent intensities of the Raman peaks were fitted to exponentially decaying functions. Depending on the reactive atmosphere, two different processes with distinct time constants can be identified for the creation of Se vacancies. The subsequent incorporation of sulfur atoms into the Se vacancy sites was also determined from time dependent Raman spectra. The optimization of this process will allow to develop techniques based on photo-induced chemical reactions for local doping, alloying and the fabrication of in-plane TMD heterostructure. |
Wednesday, March 7, 2018 10:24AM - 10:36AM |
K36.00011: First principles simulation of local response in two-dimensional transition metal dichalcogenides under electron irradiation Anthony Yoshimura, Vincent Meunier Electron beam irradiation by transmission electron microscopy (TEM) is a common and effective method for post-synthesis defect engineering in two-dimensional transition metal dichalcogenides (TMDs). Combining density functional theory (DFT) with relativistic scattering theory, we simulate the generation of such defects in monolayer group-VI TMDs, MoS2, WS2, MoSe2, and WSe2, focusing in on two fundamental TEM-induced atomic displacement processes: chalcogen sputtering and vacancy migration. Our calculations show that the activation energies of chalcogen sputtering depend primarily on the chalcogen species, and are smaller for TMDs containing Se. Meanwhile, vacancy migration activation energies hinge on the transition metal species, being smaller in TMDs containing Mo. Incorporating these energies into a relativistic, temperature-dependent cross section, we predict that, with appropriate TEM energies and temperatures, one can induce migrations in all four group-VI TMDs without simultaneously producing vacancies at a significant rate. This can allow for controlled manipulation of the TMD crystal for targeted functionality, without the risk of substantial collateral damage. |
Wednesday, March 7, 2018 10:36AM - 10:48AM |
K36.00012: Vacancy Controlled Contact Friction in Graphene Prakash Gajurel, Mina kim, Qiang Wang, Weitao Dai, Haitao Liu, Cheng Cen We have demonstrated that, during the controlled oxidation in oxygen plasma and subsequent reduction induced by high energy photons, the contact friction in Chemical Vapor Deposition (CVD) grown graphene is dominated by the vacancies formed instead of the bonding with C-O based add-atoms and functional groups. Presence of vacancies can make graphene sheet more flexible to pucker at the contact edge, which increase the contact area and as a result leads to a larger contact friction. Modified graphene with large contact friction has a large density defects, but remains a good electrical conductor, in which the carrier transport is strongly affected by quantum localization effects even at room temperature.It is found that the oxidation process in graphene is substrate-sensitive. Comparing to monolayer graphene on SiO2 substrate, the oxidation process progresses much faster when the substrate is SrTiO3, while bilayer graphene exhibits great oxidation resistance on both substrates. |
Wednesday, March 7, 2018 10:48AM - 11:00AM |
K36.00013: Fingerprints of Disordered Nanostructure Materials Chinedu Ekuma, Vladimir Dobrosavljevic, Daniel Gunlycke The need to design and discover energy-efficient materials is fundamental to modern technology. Alongside experiment and theory, computer simulations have provided insights into the properties of materials. Computational materials design aims to understand the fundamental origin of complex materials' behavior and use this information to make predictions. This has the potential to mitigate experimental risk, cost, and time and accelerates the discovery of materials with exotic properties before synthesis in a laboratory. Low-dimensional materials are promising for exploring these exotic physics because their properties emerge from an intricate interplay among the electronic degrees of freedom often on many length/energy scales that can be harnessed to improve device performance. Often, materials contain defects, which could be significant in low-dimensional materials due to quantum confinement. Using a first-principles-based, many-body, typical medium approach [1], we explore the role of atomic defects on the electronic and absorption spectra of low-dimensional materials. |
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