Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session R30: Transition Metal Dichalcogenides:Structure and DefectsFocus
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Sponsoring Units: DMP Chair: Jie Shan, Penn State University Room: 293 |
Thursday, March 16, 2017 8:00AM - 8:12AM |
R30.00001: Electrostatic Screening of Charged Defects in 2D Materials Timothy Atallah, Melissa Bosch, Jue Wang, Dongjea Seo, Osman Moneer, Justin Zhu, Demi Ajayi, Jenny Ardelean, Haiming Zhu, Monica Theibault, James Hone, Xiaoyang Zhu Two dimensional (2D) semiconductors, such as transition metal dichalcogenides (TMDCs) monolayers, are an emerging class of materials that show promise in the development of highly efficient nanoscopic optoelectronic devices. A major factor hampering TMDC monolayers device performance is the inconsistent quality of TMDCs due to presence of defects/traps. Here we show that charged defects can be electrostatically screened/passivated. We find that contacting TMDC monolayers with an ionic liquid reduces charge carrier trapping and non-radiative recombination. This effect is reversible and can increase photoluminescence yield by as much as two-orders of magnitude while suppressing the photoluminescence emission from trions. We propose that the mobile ions electrostatically screen local charged vacancies within the TMDC monolayer, thus protecting excitons from scattering with the defect sites. [Preview Abstract] |
Thursday, March 16, 2017 8:12AM - 8:24AM |
R30.00002: Phase transition in MoS$_{\mathrm{2}}$ monolayers through alloying Priyanka Manchanda, Shize Yang, Y.Y. Zhang, Yongji Gong, P.M. Ajayan, M. Chisholm, S.T. Pantelides, Wu Zhou Alloying of 2D monolayers can produce materials with different properties than either component. MoS$_{\mathrm{2}}$ is known to have a stable hexagonal 1H structure, but the alternative tetragonal DT structure has been stabilized in multilayer form by Li intercalation. It is also known that the stable structure of monolayer ReS$_{\mathrm{2}}$ is DT. In this work, we use electron microscopy and density functional theory calculations to demonstrate that an H-to-T phase transition can be achieved in MoS$_{\mathrm{2}}$ by Re doping. Both the microscopy images and the calculations find that the phase transition occurs at a Re concentration of just below 50{\%}. In contrast to the phase transition by lithium intercalation which is induced by the electron doping effect, the calculations find that electron doping alone, compensated by a uniform positive background, shift the critical concentration to 75{\%}, which indicates that changes in chemical bonding facilitate the transition. At 50{\%} alloying, the energy gap of the DT material is only 0.2 eV, compared with 1.8 eV in 1H MoS$_{\mathrm{2}}$. [Preview Abstract] |
Thursday, March 16, 2017 8:24AM - 8:36AM |
R30.00003: Tuning Local Electronic Structure of Monolayer MoS$_{2}$ through Defect Engineering Shengxi Huang, Yan Chen, Xiang Ji, Kiran Adepalli, Xi Ling, Mildred Dresselhaus, Bilge Yildiz, Jing Kong Two-dimensional molybdenum disulfide (MoS$_{2})$ has shown promising applications in electronics, photonics, energy and electrochemistry, and defects have shown to play an essential role in altering the performance of MoS$_{2}$. However, the mechanism of defects in affecting the MoS$_{2}$ properties is unsettled. In this work, we perform a systematic study on the effect that MoS$_{2}$ defects play on the electronic structure and electrochemical reactivity. Using chemical-vapor deposited monolayer MoS$_{2}$ combined with thermal driving and ion irradiation, we fabricate monolayer MoS$_{2}$ with different defect densities on various substrates. We reveal that the electronic state of MoS$_{2}$ is sensitive to both substrates and defects, supported by our X-ray photoelectron spectroscopy, Raman and photoluminescence spectroscopies, and scanning tunneling microscopy/spectroscopy. We further found that the defect density in MoS$_{2}$ can effectively tune the hydrogen evolution reactivity. Our findings provide useful guidance for defect engineering in MoS$_{2}$ and show the potential application of such defect engineering in using MoS$_{2}$ for a clean and effective energy source. [Preview Abstract] |
Thursday, March 16, 2017 8:36AM - 8:48AM |
R30.00004: Structural Analysis of MoS$_{2}$ and other 2D layered materials using LEEM/LEED-I(V) and STM Maxwell Grady, Zhongwei Dai, Wencan Jin, Jerry Dadap, Richard Osgood, Jerzy Sadowski, Karsten Pohl Layered two-dimensional materials, such as molybdenum disulfide, MoS$_{2}$, are of interest for the development of many types of novel electronic devices. To fully understand the interfaces between these new materials, the atomic reconstructions at their surfaces must be understood. Low Energy Electron Microscopy and Diffraction, LEEM/$\mu$LEED, present a unique method for rapid material characterization in real space and reciprocal space with high resolution. Here we present a study of the surface structure of 2H-MoS$_{2}$ using $\mu$LEED intensity-voltage analysis. To aid this analysis, software is under development to automate the procedure of extracting I(V) curves from LEEM and LEED data. When matched with computational modeling, this data provides information with angstrom level resolution concerning the three dimensional atomic positions. We demonstrate that the surface structure of bulk MoS2 is distinct from the bulk crystal structure and exhibits a smaller surface relaxation at 320K compared to previous results at 95K. Furthermore, suspended monolayer samples exhibit large interlayer relaxations compared to the bulk surface termination. Further techniques for refining layer thickness determination are under development. [Preview Abstract] |
Thursday, March 16, 2017 8:48AM - 9:00AM |
R30.00005: Defect band emission enhancement and inhomogeneous linewidth narrowing of monolayer WSe$_{\mathrm{2}}$ on Ag nanotriangle arrays Alexander D Johnson, Fei Cheng, Yutsung Tsai, Chih-Kang Shih The photoluminescence of CVD grown monolayer WSe$_{\mathrm{2}}$ flakes was modified by transferring them onto an array of Ag nanotriangles that were created using colloidal lithography. Using low temperature (7K) micro-PL mapping and correlating it with SEM images, a 10 fold enhancement of the defect band emission and a suppression of the exciton emission of the WSe$_{\mathrm{2}}$ flakes on the Ag nanotriangles was observed compared to the flakes on the bare substrate, showing that the dynamics of the free and defect bound excitons within the material respond differently to interactions with the plasmonic structures. Furthermore, a significant decrease in the inhomogeneous linewidth of the enhanced defect band emission was seen, resulting from the limited amount of overlap between the defect-laden regions of the WSe$_{\mathrm{2}}$ and the plasmonic modes of the Ag nanotriangles. Due to the scalability of both the CVD growth of the WSe$_{\mathrm{2}}$ and the fabrication method of the Ag nanotriangle array, many flakes were able to be studied and the consistency of these results was demonstrated. [Preview Abstract] |
Thursday, March 16, 2017 9:00AM - 9:12AM |
R30.00006: Bound excitons at the edges in monolayer tungsten disulfide Victor Carozo, Yuanxi Wang, Kazunori Fujisawa, Bruno R. Carvalho, Chanjing Zhou, Simin Feng, Zhong Lin, Amber McCreary, Nestor Perea-Lopez, Ana Laura Elias, Bernd Kabius, Vincent H. Crespi, Mauricio Terrones Defects play a significant role in tailoring the optical properties of two-dimensional materials. Optical signatures of defect-bound excitons are important tools to probe defective regions and thus interrogate the optical quality of as-grown semiconducting monolayer materials. We have performed a systematic study of defect-bound excitons using photoluminescence spectroscopy combined with atomically resolved scanning electron microscopy and first-principles calculations. Spatially resolved photoluminescence spectroscopy at low temperatures revealed bound excitons that were present only on the edges of the triangular islands and not in the interior. Atomic-resolution images reveal that the areal density of mono-sulfur vacancies is much larger near the edges ($0.92\pm0.45$ nm$^{-2}$) than in the interior ($0.33\pm0.11$ nm$^{-2}$). First-principles calculations confirm that sulfur mono-vacancies introduce mid-gap states that host optical transitions with finite matrix elements. These results demonstrate that bound exciton emission induced by mono-sulfur vacancies is concentrated near the edges in as-grown monolayer tungsten disulfide. [Preview Abstract] |
Thursday, March 16, 2017 9:12AM - 9:24AM |
R30.00007: Interlayer breathing and shear modes in NbSe2 atomic layers Jeremiah van Baren, Rui He, Jia-An Yan, Xiaoxiang Xi, Zhipeng Ye, Gaihua Ye, I-Hsi Lu, S. M. Leong, C. H. Lui Atomically thin NbSe2 is a metallic layered transition metal dichalcogenide (TMD) with novel charge-density-wave (CDW) and superconductive phases. Properties of NbSe2 atomic layers are sensitive to interlayer coupling. We investigate the interlayer phonons of few-layer NbSe2 by ultralow-frequency Raman spectroscopy. We observe both the interlayer breathing modes and shear modes at frequencies below 40 cm-1 for samples of 2 to 15 layers. Their frequency, Raman activity, and environmental instability depend systematically on the layer number. In addition, the interlayer phonon modes evolve smoothly from T = 300 K to 8 K, with no observable response to the CDW formation in NbSe2. This finding indicates that the atomic registry between adjacent NbSe2 layers is well preserved in the CDW transition. [Preview Abstract] |
Thursday, March 16, 2017 9:24AM - 9:36AM |
R30.00008: Activation Energies of Rotational Defects in 2D Transition Metal Dichalcogenides Anthony Yoshimura, Michael Lamparski, Neerav Kharche, Vincent Meunier Owing to their outstanding electrical and optical properties, two-dimensional (2D) transition metal dichalcogenides (TMDs) are promising for a wide array of applications in electronics, photonics, and energy harvesting. As with any material, 2D TMDs are subject to structural defects, which can modify their physical properties. It is therefore important to understand the conditions in which these defects can occur. Here we examine a rotational defect in the pristine trigonal-prismatic TMD lattice, arising from a 60-degree rotation about the surface normal of the bonds of a transition metal atom. The resulting structure is a three-fold symmetric set of three 8-membered rings subtended by the central transition metal. This defect has been recently observed in WSe$_2$, WS$_2$ and MoSe$_2$. Using density functional theory (DFT), we calculate the activation energy for these defects in several TMDs: MoS$_2$, MoSe$_2$, MoTe$_2$, WS$_2$, WSe$_2$, and WTe$_2$. We find that the activation energy of the defect depends primarily on the chalcogen species, with sulfur having the largest and tellurium the smallest. Furthermore, we find that the chalcogen species also determines the defect’s thermodynamic stability, with tellurium being the most stable and sulfur the least. [Preview Abstract] |
Thursday, March 16, 2017 9:36AM - 9:48AM |
R30.00009: Explosive recombination-enhanced Re defect migration in MoS$_{\mathrm{2}}$~ Weiwei Sun, Shize Yang, Yuyang Zhang, Yongji Gong, Pulickel M. Ajayan, Matthew F. Chisholm, Wu Zhou, Sokrates Pantelides Via scanning transmission electron microscopy, substitutional Re impurities in MoS$_{\mathrm{2}}$ are observed to undergo exchanges with neighboring Mo atoms on a time scale of several seconds. Density functional theory calculations of multiple possible diffusion pathways predict~a large barrier that rules out thermal jumps and also rules out jumps induced by the electron beam because the energy transfer to Re is quite small. Microscopy further reveals that several S vacancies accompany Re atoms and that the initial and final configurations have different number and configurations of S vacancies. Density functional theory calculations find that the hydrogenic level of an isolated Re impurity becomes deep with small displacements of the Re atom and the S vacancies introduce additional deep levels so that the entire gap is filled with levels in constant motion. We propose that the observed jumps are the result of an ``explosive'' recombination-enhanced migration mechanism i.e., multiple electron-hole recombination events provide energy in multiples of the energy gap. The energy is transferred to local vibrations that ultimately cause the observed jumps. [Preview Abstract] |
Thursday, March 16, 2017 9:48AM - 10:00AM |
R30.00010: Spectroscopic Determination of Structural and Electronic Properties in Solution-Synthesized Tin Chalcogenide 2D Materials Adam Biacchi, Brian Alberding, Sugata Chowdhury, Sujitra Pookpanratana, Edwin Heilweil, Angela Hight Walker The vast majority of nanoscale 2D materials are synthesized by exfoliation or gas phase deposition techniques. Alternatively, bottom-up colloidal solution syntheses offer a scalable and cost-efficient means of producing 2D nanomaterials in high yield. However, routinely characterizing solution-based nanomaterials properties remain a substantial challenge due to their dimensions and the pervasive presence of surface-adsorbed stabilizing ligands. Here we present the synthesis of 2D tin chalcogenide nanomaterials and a thorough spectroscopic investigation of the inherent structural and electronic properties of individual crystals. First, we detail the development of a novel bottom-up, solution-based synthetic approach to produce nearly-monodisperse colloidal 2D metal chalcogenides of varying size and morphology. We then employ a variety of spectroscopies, ranging across the electromagnetic spectrum from X-ray to terahertz, to probe the crystallographic and electronic structure of the crystals, as well as carrier transport phenomena. These studies allow us to develop structure-property relationships among 2D materials of disparate size, morphology, and surface ligand composition when considering variances in measured band energies, interatomic vibrations, oxidation states, photoconductivity, and charge carrier mobility. [Preview Abstract] |
Thursday, March 16, 2017 10:00AM - 10:12AM |
R30.00011: Charging effect and midgap states at grain boundaries of MoS2 Xi Dong, Chenhui Yan, Michael Weinert, Lian Li Grain boundaries (GBs) are the inevitable extended defects in chemical vapor deposited (CVD) MoS2 single layer films. Here, we report the direct observation of atomic and electronic structures of GBs in CVD grown MoS2 on epitaxial graphene/SiC(0001) substrate using scanning tunneling microscopy/spectroscopy. Atomic resolution imaging indicates that GBs are mostly consisted of a linear array of dislocation cores such as 7-5 rings. Tunneling spectroscopy investigations further show the existence of mid-gap states at these dislocation cores, as well as charging effect that shifts both conduction and valence band edges. Our findings provide new insights into the nature of GBs in MoS2. [Preview Abstract] |
Thursday, March 16, 2017 10:12AM - 10:24AM |
R30.00012: A first-principles study of the role of defects and electron correlations in monolayer molybdenum disulfide Chinedu Ekuma, Daniel Gunlycke Low-dimensional materials, including monolayer MoS$_2$, are increasingly being explored for 21st-century device applications. These materials often contain intrinsic defects, which present both opportunities and challenges. To take advantage of such defects, we need a better understanding of the interplay between defects and electron interactions, which are significant in low-dimensional materials. Using our first-principles-based approach for electron localization, we have calculated the typical local density of states and charge susceptibility at various sulfur vacancy concentrations and electron-electron interaction strengths. The local density of states suggest the existence of a correlation-assisted insulator-metal transition. From the charge susceptibility, we have predicted the optical adsorption spectra in the presence of defects and electron interactions. These spectra show exciton peaks in good agreement with experiment. [Preview Abstract] |
Thursday, March 16, 2017 10:24AM - 11:00AM |
R30.00013: Phase Engineered 2D Transition Metal Dichalcogenides for Electronics Invited Speaker: Manish Chhowalla Two-dimensional transition metal dichalcogenides (2D TMDs) --- whose generalized formula is MX$_{\mathrm{2}}$, where M is a transition metal of groups 4--7 and X is a chalcogen --- consist of over 40 compounds. Complex metal TMDs assume the 1T phase where the transition metal atom coordination is octahedral. The 2H phase is stable in semiconducting TMDs where the coordination of metal atoms is trigonal prismatic. High performance of electronic and opto-electronic devices have been demonstrated with semiconducting TMDs while interesting condensed matter effects such as charge density waves and superconductivity have been observed in bulk metallic 1T phase TMDs. However, stability issues have hampered the study of interesting phenomena in two-dimensional 1T phase TMDs. Recently there has been a surge of activity in developing methodology to reversibly convert 2D 2H phase TMDs to 1T phase. In contrast with typical phase transformation conditions involving pressure and temperature, phase conversion in TMDs involves transformation by chemistry at room temperature and pressure. Using this method, we are able to convert 2H phase 2D TMDs to the 1T phase or locally pattern the 1T phase on 2H phase 2D TMDs. The chemically converted 1T phase 2D TMDs exhibit interesting properties that are being exploited for in applications such as high performance field effect transistors. In this contribution, I will summarize the key properties of 2D 1T phase TMDs and their applications for electronics. [Preview Abstract] |
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