Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session P57: Electronic and Optical Properties of 2D Materials IIIFocus
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Sponsoring Units: DMP Chair: Wencan Jin, Auburn University Room: Mile High Ballroom 3A |
Wednesday, March 4, 2020 2:30PM - 3:06PM |
P57.00001: Vallytronics and excitonics in 2D materials. Invited Speaker: Tony Heinz Selected by Focus Topic Organizer (Liuyan Zhao, Robert Hovden) |
Wednesday, March 4, 2020 3:06PM - 3:42PM |
P57.00002: High-temperature exciton condensation in two-dimensional atomic double layers Invited Speaker: Zefang Wang Bose-Einstein condensate is the ground state of matter of a dilute gas of bosons, such as atoms cooled to temperatures close to absolute zero. With a much smaller mass than atoms, excitons have been predicted to condense at significantly higher temperatures. The small exciton binding energy in conventional semiconductors has so far limited the condensation temperature to below 1 K. In the past several years, a new class of two-dimensional (2D) semiconductors with much larger exciton binding energy has emerged. In this talk, I will discuss the development of transition metal dichalcogenide double layer structures and electrical injection of interlayer excitons up to 1012 cm-2 in the system. We study the electron-hole double layers through the interlayer tunneling and electroluminescence (EL). We observe a threshold dependence of the EL intensity on exciton density, accompanied by a super-Poissonian photon statistics near the threshold, and a large EL enhancement peaked narrowly at equal electron-hole densities. These results provide compelling evidence for interlayer exciton condensation in atomic double layers. |
Wednesday, March 4, 2020 3:42PM - 3:54PM |
P57.00003: Comparative KPFM measurements on doped monolayer MoS2 Christina McGahan, Pin-Chun Shen, Yuxuan Lin, Ashley Cavanagh, Tomas Palacios, Jing Kong, Katherine Aidala Tuning the electrical properties of monolayer MoS2 through doping allows better control and flexibility over future device design. Here we present electrical measurements on sulfur-rich and oxygen-rich MoS2 flakes using kelvin probe force microscopy (KPFM) and a field effect transistor sample geometry, allowing us to resolve spatial variations. Preliminary KPFM measurements show that oxygen-rich MoS2 has a larger work function than sulfur-rich MoS2 and that the work function of monolayer MoS2 differs from that of multilayer MoS2. The trend in work functions between differently doped MoS2 flakes observed via KPFM is consistent with the trend in threshold voltages extracted from bulk current-voltage measurements on the flakes as well as the trend in work functions calculated via density functional theory. The sensitivity of the shift in work function of the MoS2 flakes from an applied gate voltage suggests that local density of states measurements can be extracted with KPFM to experimentally examine the energy levels of the differently doped monolayer MoS2 flakes. |
Wednesday, March 4, 2020 3:54PM - 4:06PM |
P57.00004: Tomonaga-Luttinger Liquid in MoSe2 Twin Domain Boundaries Yipu Xia, Junqiu Zhang, Yuanjun Jin, Wingkin Ho, Hu Xu, MAOHAI XIE In epitaxial MoSe2 monolayer grown by molecular-beam epitaxy, dense networks of twin domain boundaries (DBs) have been observed [1]. They behave as metals. Being sandwiched between semiconducting pristine MoSe2 domains, they represent an ideal one-dimensional (1D) system for studying the physics of 1D metals, such as the Peierls type instability and collective excitations of Tomonaga-Luttinger liquid (TLL). Indeed, there have already been reports of charge density waves (CDW) [2] and the TLL [3] in such systems. In this work, we report a low-temperature scanning tunneling spectroscopy study revealing the quantum confinement effect and signatures of the TLL in the 4|4P-type DB defects in MoSe2. Notably, CDW is not observed at temperatures down to ~ 5 K. This finding is consistent with a recent study of another type (4|4E) 1D defect in MoS2 [4]. |
Wednesday, March 4, 2020 4:06PM - 4:18PM |
P57.00005: Probing Excitonic Resonances of Metal Films on Suspended 2D Membranes Using Electron Energy Loss Spectroscopy Todd Brintlinger, Jose Fonseca, James Clifford Culbertson, Maxim Zalalutdinov, Rhonda Michele Stroud, Jeremy Robinson We characterize Oriented Porous mEtallic Networks (OPEN) with aberration-corrected scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS). These OPEN films are synthesized by annealing metal films on 2D layers, which causes lattice registry between the metal layer and the 2D materials. We are studying the morphological influence that MoS2 monolayers have on relatively thick (13-25nm) gold thin films during annealing. Ex-situ annealing experiments of MoS2/Au on SiO2 substrates shows that the Au films can become textured (oriented), as revealed through electron beam scattering diffraction, and that the Au film can locally dewet beneath the MoS2 layer to form a porous metal layer with suspended MoS2 membranes. Further, 60 kV STEM imaging and EELS show that the metal and 2D lattices are aligned, with several EELS resonances in the low loss range. One resonance at ~ 2 eV is associated with the so-called A and B excitons. We will discuss the origin of these resonances, and how both the metal overlayer and disordered carbon contamination affect them. |
Wednesday, March 4, 2020 4:18PM - 4:30PM |
P57.00006: Out-of-plane charge transport anomalies in polytypes of NbS2 Konstantin Semeniuk, Edoardo Martino, Carsten Putzke, David LeBoeuf, helmuth berger, Philip Moll, Laszlo Forro Tuning interlayer coupling in quasi-two-dimensional materials is a powerful tool for realising novel electronic states for potential applications in electronic and optical devices. Crucial information regarding the underlying interactions can be obtained by probing the c-axis resistivity1. We present the case of the 2H and 3R polytypes of NbS2, which have the same in-plane structure but different stacking of the layers and as a result exhibit very contrasting charge transport properties. By utilising focused-ion-beam-assisted sample preparation we were able to unambiguously probe the in-plane and out-of-plane resistivities of the compounds. The corresponding anisotropies are substantially lower than has been thought before. Furthermore, we show a number of previously unobserved features, including non-monotonic temperature dependence of resistivity and negative longitudinal magnetoresistance. We suggest that the stacking faults, occurring naturally in the studied compounds2, are the primary cause of the aforementioned behaviours. |
Wednesday, March 4, 2020 4:30PM - 4:42PM |
P57.00007: Identification of select populations of single photon emitters in hexagonal boron nitride based on their zero phonon line and linewidth broadening footprint Hamidreza Akbari, Wei-Hsiang Lin, Benjamin Vest, Harry Atwater Single photon emitters (SPE) in hexagonal boron nitride (hBN) have shown promising electrical and optical properties for utilization in quantum light source applications. The unpredictability of defect properties in hBN remains a significant challenge in order to reproducibly and more deterministically identify bright and robust emitters. |
Wednesday, March 4, 2020 4:42PM - 4:54PM |
P57.00008: Unraveling the Mystery of the 1.5 layer Raman Response in Exfoliated MoS2. Andrey Krayev, Matěj Velický Recently a new method of exfoliation of large area monolayers of transition metal dichalcogenides (TMDC) was proposed1. Confocal Raman characterization of exfoliated monolayers of MoS2 showed A’ peak splitting, the components of this spilt peak corresponding to a standard A’ peak of the monolayer and A1g peak of the bi-layer correspondingly. We further performed tip enhanced Raman Scattering (TERS) imaging of these exfoliated crystals. Thanks to greatly improved spatial resolution of TERS imaging, we discovered that some areas of what seemed to be a uniform monolayer, actually contained a network of tiny islands (few tens of nm across) having spectral signature closely matching TERS response of the bilayer while TERS spectra of areas in between these islands closely resembled standard Raman spectrum of the monolayer. Thanks to these observations we came to the conclusion that the gold-assisted exfoliation of MoS2 may result in inhomogeneous crystals featuring a network of tiny bilayer islands, and consequently, greatly increased crystal edge length per unit of area, a feature that may be useful for (photo)catalytic applications of TMDCs. |
Wednesday, March 4, 2020 4:54PM - 5:06PM |
P57.00009: Correlating 3D atomic defects and electronic properties of 2D transition metal dichalcogenides with picometer precision Xuezeng Tian, Dennis S Kim, Shize Yang, Christopher Ciccarino, Yongji gong, Yongsoo Yang, Yao Yang, Blake Duschatko, Yakun Yuan, Pulickel M Ajayan, Juan-Carlos Idrobo, Prineha Narang, Jianwei Miao The field of research in two-dimensional (2D) transition metal dichalcogenides (TMD), has experienced extraordinary growth. To understand the structure-property relationship of these materials at the fundamental level, we must know their 3D atomic structure with high precision. |
Wednesday, March 4, 2020 5:06PM - 5:18PM |
P57.00010: Transport Studies of Nanofabricated Tellurium Devices Xinxin Cai, Chenyu Yu, Prafful Golani, Yan Wang, Xinglong Ren, Steven J. Koester, C. Daniel Frisbie, Vlad S. Pribiag Tellurium is a narrow bandgap semiconductor with high hole mobility of several hundred cm2(Vs)-1. It has attracted great interest due to its unique crystal structure, consisting of 1D helical chains interconnected by van der Waals type bonds. A recent development is the successful synthesis of two-dimensional tellurium, or tellurene, using a solution-based process. Various interesting transport phenomena have been experimentally demonstrated since then, such as anisotropic magnetoresistance, quantum Hall effect and gate-tuned insulator-metal transition, revealing its potential for fundamental studies and future applications of nanoscale devices. We discuss the fabrication and electrical measurements of few-layer and bulk tellurium flakes grown by the solution method. We study anisotropic and nanoscale transport of the material. |
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