Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session H13: 2D Materials (General) -- DefectsFocus
|
Hide Abstracts |
Sponsoring Units: DMP DCOMP Chair: Victor Brar, University of Wisconsin - Madison Room: BCEC 153B |
Tuesday, March 5, 2019 2:30PM - 2:42PM |
H13.00001: WITHDRAWN ABSTRACT
|
Tuesday, March 5, 2019 2:42PM - 2:54PM |
H13.00002: Observation of superlattice periodicity on the surface of highly-doped MoS2 Steven Schofield, Mohammed Bin Subhan, Asif Suleman, Gareth Moore, Peter Phu, HIdekazu Kurebayashi, Chris A. Howard We have used low temperature (~5 K) scanning tunnelling microscopy (STM) and spatially-resolved tunnelling spectroscopy (STS/CITS) to investigate the surface of highly-doped molybdenum disulphide (MoS2). In agreement with previous STM investigations of intrinsic MoS2 we find the surface exhibits a large density of charged point defects [e.g., ACS Appl. Mater. Interfaces, 9, 19278 (2017)]. Fourier analysis of our atomic-resolution imaging and spectroscopy data from the doped MoS2 surface reveals two separate superlattice periodicities in addition to the 1x1 sulphur lattice, one of which is enhanced in the vicinity of the charged defects. We present an interpretation of the observed superlattices consistent with prior DFT and ARPES data. |
Tuesday, March 5, 2019 2:54PM - 3:06PM |
H13.00003: Exotic hydrogenic impurity states in h-BN: the role of the structure and intervalley interactions Saif Ullah, Marcos Menezes, Rodrigo Capaz, Fernando Sato In this work, we employ a combination of theoretical calculations to explore the electronic properties of donor and acceptor states of substitutional impurities in h-BN, a wide-gap 2D insulator. We find that the structure of the impurity levels that appear inside the electronic gap strongly depends on the atom replaced by the impurity. For instance, when an acceptor replaces a N atom, the intervalley interaction induced by the impurity is found to be strong and the resulting level structure consists only of non-degenerate levels with strong valley and spin splittings. On the other hand, when an acceptor replaces a B atom, the intervalley interaction is much weaker and a near-valley-degenerate ground state is found. Donor impurities behave on a similar fashion. We show that the differences between these level structures can be traced to the peculiar sublattice-resolved electronic structure of pristine h-BN. In fact, these exotic impurity levels may also be present in other 2D semiconductors and insulators with similar bandstructures, such as other hexagonal III-V compounds, and their optical properties may be engineered for applications in future optoelectronic devices. |
Tuesday, March 5, 2019 3:06PM - 3:42PM |
H13.00004: Controlling and Exploiting Defects in Synthetic Two-Dimensional Materials Invited Speaker: Mark Hersam Defects including vacancies, dislocations, and grain boundaries play a fundamental role in determining the properties of materials. With fewer degrees of freedom, the effects of defects are particularly significant in two-dimensional (2D) materials [1]. For example, in chemical vapor deposited monolayer MoS2, the interplay between sulfur vacancies and grain boundaries lowers the barrier for vacancy motion, thus enabling the realization of field-driven vacancy motion. Since sulfur vacancies act as n-type dopants, field-driven vacancy motion leads to reconfigurable doping profiles and memristive charge transport [2]. This memristive charge transport can be further modulated with a gate potential, resulting in a hybrid device that combines the attributes of a memristor and transistor (i.e., a memtransistor) [3]. As a second example, molecular beam epitaxy allows the control of defect structures for 2D boron (i.e., borophene) [4]. In particular, mixed phases of borophene have been achieved that each consist of periodically ordered vacancies [5]. The linear defects in these mixed phases self-assemble into spatially periodic superlattices, which modulate correlated electron phenomena such as charge density waves, as observed at the atomic scale with scanning tunneling microscopy and spectroscopy [6]. |
Tuesday, March 5, 2019 3:42PM - 3:54PM |
H13.00005: Quantum dynamics of point defects in hexagonal boron nitride with variable temperature Benjamin Vest, Hamidreza Akbari, Pankaj Jha, Cora Went, Wei-Hsiang Lin, Harry Atwater Defects in solids strongly affect its optical, electrical, mechanical, and thermal properties. In particular, point defects in semiconductors have garnered great interest because they can serve as single-photon sources, which are the building blocks for photonic quantum technologies. Recently, point defects in hexagonal boron nitride have emerged as excellent candidates for single-photon sources. |
Tuesday, March 5, 2019 3:54PM - 4:06PM |
H13.00006: Point defects in 1T'-MoS2 from first principles Michele Pizzochero, Oleg Yazyev Monolayers of group VI Transition Metal Dichalcogenides (TMDs) exist in either the semiconducting 2H phase or semimetallic 1T’ phase. While the stable 2H phase has been extensively investigated due to prospective optoelectronic applications, the metastable 1T’ phase has appeared in the spotlight only recently, mainly due to its topological properties. In this talk, I will provide an overview on the formation of point defects in 1T’-MoS2 within two experimentally-relevant situations, i.e. under thermodynamic equilibrium and under electron beam irradiation. First, I will address the stability of point defects, exploring several configurations of vacancy, adatom and antisite defects. All considered defects exhibit lower formation energies in the 1T’ phase compared to the 2H phase, suggesting that the 1T’ polymorph is more susceptible to lattice imperfections. Next, the response of 1T’-MoS2 to the electron irradiation will be examined. The range of electron beam energies needed to carry out imaging without inducing any damages in the sample and some guidelines for the controlled creation of defects in the electron microscope are discussed. Throughout my talk, I will draw comparisons between local disorder in the two phases to portray a complete picture of the role of defects in TMDs. |
Tuesday, March 5, 2019 4:06PM - 4:18PM |
H13.00007: Correlating Microscopic Electronic Features with Macroscopic Transport in Defective Graphene Jake Riffle, Caitlyn Meditz, Alana Gudinas, Shawna Hollen Past studies on graphene show that intervalley and intravalley scattering off atomic defects and impurities change the electronic transport properties and give rise to weak Anderson localization at low temperatures. We will present our studies on the interactions and correlations between electron-hole charge puddles and the weakly localized regions. Using a low temperature scanning tunneling microscope (STM) on graphene field effect transistors, we present experiments which aim to study the effects of point defects and to employ simultaneous STM and electronic transport measurements to directly associate microscopic electronic interactions with macroscopic transport. These experiments will show strides toward mapping out the phase space of defect-populated graphene in our investigation of the prospect of a metal-insulator quantum phase transition. |
Tuesday, March 5, 2019 4:18PM - 4:30PM |
H13.00008: Atomic manipulation of defects in the layered semiconductor 2H-MoTe2 Sara Mueller, Benjamin St. Laurent, Yaxian Wang, Wolfgang E Windl, Shawna Hollen, Jay A Gupta Here we demonstrate control over the charge state and layer position of individual defects in the layered semiconductor, 2H-MoTe2. Pristine surfaces were revealed at room temperature in ultrahigh vacuum by cleaving the top few layers from the crystal. STM images and spectroscopy were performed with a cut PtIr tip at 9K. Two classes of native defects were observed in large area STM images, appearing in both near-surface and sub surface layers with progressively fainter contrast. One class of defect images as a bright protrusion with a nm-scale fall-off in STM topography, indicative of band bending associated with a charged defect. Spectroscopic imaging reveals a ring-shaped feature associated with these defects, consistent with tip-induced ionization between two defect charge states. Consistent with studies in other semiconductor systems, the rings depend on the STM imaging conditions and tip apex. We find that subsurface defects exhibit discrete increases in apparent height following large positive voltage pulses, suggesting migration between layers near the surface. We compare with DFT calculations to identify these defects and estimate the energy barriers for inter-layer migration. |
Tuesday, March 5, 2019 4:30PM - 4:42PM |
H13.00009: Defect Characterization and Engineering in Black Phosphorus Ben St Laurent, Jake Riffle, Cameron C Flynn, Charlie Ayotte, Christine Caputo, Shawna Hollen A tunable band gap and high carrier mobility make black phosphorus (BP) attractive for device applications. To effectively engineer micron scale BP devices, it is essential to understand defects down to the atomic level. The most prominent defects on the surface of BP exhibit a large electronic signature in scanning tunneling microscopy (STM) images. We previously found that the predominant defects in BP are vacancies and are the source of p-doping in the material. These studies demonstrate the need for vacancy formation control during synthesis. Here, we will describe STM experiments of BP that further characterize the defects and explore defect control and creation through ultra high vacuum annealing. |
Tuesday, March 5, 2019 4:42PM - 4:54PM |
H13.00010: Direct correlation of defects with photoluminescence and electrical conductivity in monolayer transition metal dichalcogenides Matthew Rosenberger, Hsun-Jen Chuang, Kathleen McCreary, Saujan Sivaram, Connie Li, Berend Jonker Transition metal dichalcogenides (TMDs) are promising candidates for emerging applications such as transparent and flexible optoelectronics and electronics. Understanding the impact of defects on TMD properties is essential for the advancement of these materials. Here, we demonstrate the ability to observe electronically active defects in monolayer TMDs using conductive atomic force microscopy in ambient conditions, and we correlate defect density with local optoelectronic and electronic properties. We find that CVD-grown WS2 samples have up to an order of magnitude variation in defect density within a single triangular grain. We also find that photoluminescence (PL) intensity is inversely proportional to defect density. To investigate electronic properties, we use kelvin probe force microscopy to obtain spatial maps of electrostatic potential in operating TMD transistors. We find that regions with low PL intensities exhibit large potential gradients, corresponding to high resistivity. This suggests that the defects responsible for decreased PL intensity are also responsible for decreased electrical conductivity. |
Tuesday, March 5, 2019 4:54PM - 5:06PM |
H13.00011: Spatially Selective Enhancement of Photoluminescence in MoS2 by Photo-mediated Adsorption and Defect Passivation Saujan Sivaram, Aubrey Hanbicki, Matthew R. Rosenberger, Hsun-Jen Chuang, Kathleen McCreary, Berend T. Jonker Monolayers of transition metal dichalcogenides (TMD) are promising components for optoelectronic devices due to their direct band gap and atomically thin nature. Their photoluminescence (PL) is strongly dependent on mid-gap defects which serve as non-radiative recombination sites for excitons. We demonstrate up to a 200x increase in PL intensity by exposing MoS2 synthesized by chemical vapor deposition (CVD) to laser light in ambient. This spatially resolved passivation treatment is air and vacuum stable, which indicates strong bonding of moieties from ambient. A wavelength dependent study confirms that this PL brightening is concomitant with exciton generation in the MoS2; laser light below the optical band gap of MoS2 fails to brighten the TMD. We highlight the photo-sensitive nature of the process by successfully brightening with a broadband white light source (< 10 nW/mm2). We decouple changes in absorption from defect passivation by examining the degree of circularly polarized PL. This measurement, which is independent of exciton generation, confirms that the laser brightening reduces non-radiative recombination sites in the MoS2. We propose that H2O molecules passivate sulfur vacancies in the MoS2 but requires photo-generated excitons to overcome the adsorption barrier. |
Tuesday, March 5, 2019 5:06PM - 5:18PM |
H13.00012: Defect-related photoluminescence from networks of suspended 2D crystal membranes Andrew L Yeats, Jose Fonseca Vega, Joel Q Grim, Samuel Carter, Cory D Cress, James Clifford Culbertson, Maxim Zalalutdinov, Jeremy T Robinson Luminescent defects in 2D semiconductors hold promise for applications in photonics and quantum communication. For instance, strain-induced defects in WSe2 have attracted attention as narrow line-width single photon sources. We present a series of low-temperature photoluminescence (PL) microscopy studies on networks of suspended 2D crystal membranes formed by controlled dewetting and recrystallization of an underlying metal film. For monolayer membranes of WSe2 and heterojunctions of WSe2/MoS2 formed on a porous Au film, we find a strong (1000x) enhancement of PL intensity from the suspended regions, as well as the appearance of relatively sharp (< 1 meV) emission lines. The crystallographic texturing of the metal under-layer may also lend itself to low-loss propagation of surface plasmon polaritons (SPP), offering a means for energy transfer between discrete luminescent centers. We use a split excitation/collection imaging approach to characterize nonlocal luminescence in this unique material system, and discuss the outlook for studying networks of interconnected defects in 2D materials. |
Tuesday, March 5, 2019 5:18PM - 5:30PM |
H13.00013: Nano-imaging of local strain in hexagonal boron nitride Bosai Lv, Hongyuan Li, Lili Jiang, Wanfei Shan, Hans Bechtel, Michael Crocker Martin, Weidong Luo, Feng Wang, Zhiwen Shi Strain plays an important role in condensed matter physics. Strain effect becomes more interesting in two-dimensional materials, both because an unusually large strain can be achieved without breaking the material, and because the strain can lead to novel behavior such as the generation of pseudomagnetic field in graphene. Here, we report an ultra-sensitive nanometer scale mapping and a quantitative analysis of local strain field in atomically thin hexagonal boron nitride. |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700