Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session X40: 2D Materials - Dielectrics and Stacking |
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Sponsoring Units: DMP Chair: Nancy Sandler, Ohio Univ Room: LACC 501C |
Friday, March 9, 2018 8:00AM - 8:12AM |
X40.00001: Synergetic Conductance Behavior in Graphene-SrTiO3 Heterostructure Kyeong Tae Kang, Haeyong kang, Jeongmin Park, Dongseok Suh, Woo Seok Choi Synergetic behaviors in heterostructures composed of 2D van der Waals materials and transition metal oxides (TMO) have attracted a lot of interest. However, difficulties to fabricate high-quality graphene and SrTiO3 (STO) heterostructure has largely prevented from observing the combined phenomena, despite the intrinsically intriguing properties such as quantum Hall effect and large dielectric constant of each constituent. In this study, we successfully observed the quantum conductance behavior of monolayer graphene on epitaxial STO thin film grown on Nb-doped STO single-crystalline substrate. Our device shows a narrow gate voltage sweep range compared to similar devices on a silicon substrates owing to the high-k nature of STO [1]. In addition, oxygen vacancies in the STO layer could be controlled by applying a wider range of gate-voltage, resulting in a systematic development of hysteresis of conductance on top of quantum Hall nature [2]. Considering the generation and annihilation of oxygen vacancies by electric field, we could construct a simple mechanism to figure out the hysteresis and a quantitative relationship among the dielectric properties of STO with oxygen vacancies. |
Friday, March 9, 2018 8:12AM - 8:24AM |
X40.00002: Non-Thermionic Switching in an Atomically Thin WSe2 Transistor with the Phase-Change Material VO2 Contact Mahito Yamamoto, Teruo Kanki, Azusa Hattori, Ryo Nouchi, Kenji Watanabe, Takashi Taniguchi, Keiji Ueno, Hidekazu Tanaka By using the metal-insulator transition material VO2 (TC ~ 340 K) as a contact electrode, we demonstrate an atomically thin WSe2 transistor whose subthreshold swing (SS) is limited not by the Boltzmann limit but by the abruptness of the phase transition of VO2. Polycrystalline VO2 films wtith thicknesses of 50 nm were grown on Al2O3 by the pulsed layer deposition method and etched to 1 μm in width. Few-layer WSe2 was transferred onto the VO2 wire so that the VO2 served as one of the contact electrodes. After defining another electrode with Ti/Au, a gate dielectric of hBN was transferred. The transistor is on the off-state, with the VO2 electrode being in the insulating phase at room temperature. However, the drain current shows abrupt increase at a given gate voltage, suggesting that the pahse transition in VO2 is induced by Joule heating. We observe a relatively small value of 150 mV/dec. for the SS, but this is still larger than the thermal limit of 60 mV/dec. We expect that the SS could be further improved by employing single crystalline VO2 that shows an abrupt transition. |
Friday, March 9, 2018 8:24AM - 8:36AM |
X40.00003: Abstract Withdrawn
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Friday, March 9, 2018 8:36AM - 8:48AM |
X40.00004: Record-high drain current in p-type 2D transistors with high-k dielectric and accumulation-type contacts Adam Charnas, Gang Qiu, Mengwei Si, Yixiu Wang, Wenzhuo Wu, Peide (Peter) Ye 2D van der Waals materials are promising candidates for ultimate device miniaturization. However, high-performance 2D p-type transistors are an important missing piece in potential 2D-based CMOS technology. Here we demonstrate that with a novel high-mobility narrow-bandgap p-type material tellurium, high-performance FETs can be realized for low-power ultra-fast electronic applications. Unprecedented high on-state current beyond 1 A/mm is achieved with advanced dielectric and contact engineering. The large EOT/breakdown field of the ALD-grown high-k dielectric allows a stronger displacement field which induces higher carrier density. Meanwhile we revisited the fundamental difference of metal-to-semiconductor interface between van der Waals materials and 3D bulk materials. The Fermi pinning effect, which governs the contact behavior in most traditional semiconductors, was significantly reduced in our device and for the first time accumulation-type Ohmic contacts are achieved on p-type 2D semiconductor tellurium. |
Friday, March 9, 2018 8:48AM - 9:00AM |
X40.00005: Study of Dielectric Breakdown Mechansim in Molecular Beam Epitaxial Ultrathin Hexagonal Boron nitride Zhenjun Cui, Alireza Khanaki, Hao Tian, yanwei He, Wenhao Shi, Jianlin Liu Hexagonal boron nitride is promising as a dielectric layer for two-dimensional (2D) electronic devices. Nevertheless, the studies on failure mechanism and reliability of ultrathin h-BN are quite limited. In this presentation, we report detailed investigation of metal-insulator-metal devices based on our MBE grown h-BN ultrathin films (0.3-2 nm). Current- Voltage (IV) and Time Dependent Dielectric Breakdown (TDDB) measurements were carried out. The breakdown dielectric field of ultrathin h-BN can be as high as 12 MV/cm. Besides hard breakdown, soft breakdown is also responsible for the failure mechanism but can be self-healed after the stress is relieved. Under bias, the defects which are close to each other start trapping electrons and pile up, creating a path that leads to soft breakdown and prevents hard breakdown. As the bias is relieved, electrons detrap and the insulating property of the film recovers. Soft breakdown phenomenon can also occur many times before the h-BN film is totally worn out. Subsequent TDDB result under different stress voltages not only confirms soft breakdown mechanism, but also reveals the potential applications of untrathin h-BN for Resistive Random Access Memories (RRAM). |
Friday, March 9, 2018 9:00AM - 9:12AM |
X40.00006: Anisotropic transport in graphene via 1D patterned dielectric superlattices Scott Dietrich, Carlos Forsythe, Shaowen Chen, Takashi Taniguchi, Kenji Watanabe, James Hone, Cory Dean Band structure engineering of two-dimensional materials entered the spotlight with the experimental discovery of Hofstadter's butterfly in graphene on hexagonal boron nitride. Recently it became possible to fabricate device structures with features beyond the limitations imposed by the naturally occurring moiré pattern through the use of patterned dielectric superlattices (PDSLs). In these devices, the dielectric between the back gate and the graphene is periodically structured to create a modulated carrier density on the order of tens of nanometers. We report the fabrication and measurement of 1D PDSLs that produce highly anisotropic transport in monolayer graphene. Resistivity measured perpendicular to the 1D superlattice shows the standard Dirac peak while resistivity measured parallel to the 1D superlattice demonstrates additional satellite peaks. Additionally, we observe Weiss oscillations when the cyclotron orbit of carriers is commensurate with the superlattice wavelength. These observations mark a crucial step for the in-situ tunable modification of graphene’s band structure in a highly anisotropic fashion that has applications in ballistic electron optics, van der Waals FETs, and plasmonics. |
Friday, March 9, 2018 9:12AM - 9:24AM |
X40.00007: Patterned Dielectric Superlattices on van der Waals 2D Materials Carlos Forsythe, Xiaodong Zhou, Takashi Taniguchi, Kenji Watanabe, Abhay Pasupathy, Pilkyung Moon, Mikito Koshino, Philip Kim, Cory Dean Patterned Dielectric Superlattices provide a powerful route towards modifying electron bands in 2D materials through external electric fields. We pattern dielectric layers in our devices using e-beam lithography to achieve lattice wavelengths as small as 35nm while maintaining high device mobility. Using this technique, we have realized clear Brillouin zone folding in graphene on boron nitride systems as well as fully developed Hofstadter fractal quantization of unmatched quality in any system with a fabricated superlattice. We present transport data from graphene systems under both triangular and square superlattice potentials, which exhibit fractal spectra under magnetic field unique to those specific lattice geometries. Additionally, we can tune the superlattice potential strength by varying relevant gating voltages, and even turn off the superlattice altogether. We further present our efforts in extending PDSLs to a wider variety of van der Waals materials and measurement techniques. |
Friday, March 9, 2018 9:24AM - 9:36AM |
X40.00008: Relation between the Stacking Shift and Electronic Structure in Layered
Materials: General Theory Ryosuke Akashi, Yo Iida, Kohei Yamamoto, Kanako Yoshizawa Atomically thin materials such as graphene, transition-metal dichalcogenide and phosphorene form various stacking polytypes and their electronic properties are sensitively dependent on the stacking pattern. Although there have been a variety of theoretical and experimental studies on such stacking-dependent properties, a general theory bridging individual materials has been lacking. Recently, we have found that the excitons in multilayered MoS2 can be protected from the interlayer hopping process when the system adopts a specific 3R stacking [RA et al., PRAppl. 4, 014002]. This protection is induced by an interference of the plane-wave part of the Bloch function. On top of this finding, we study such interference effect in general layered structures. We prove that the interference prohibits the interlayer hopping for the Bloch states at specific points in the Brillouin zone. The positions of such points respect only the interlayer shift and are applicable to any materials. We demonstrate by the first-principles calculations that this effect dominates the stacking dependence of the electronic band structure in graphene, boron nitride, transition metal dichalcogenide and phosphorene [RA et al., PRB 95, 245401]. |
Friday, March 9, 2018 9:36AM - 9:48AM |
X40.00009: Graphene Nanoribbon -- Transition Metal Dichalcogenide Heterostructures Satrio Gani, David Abergel, Enrico Rossi In this work, we study the electronic structure of heterostructures formed by graphene nanoribbons (GNRs) and monolayers of transition metal dichalcogenides (TMDs), such as MoS2. We study various stacking configurations between GNRs and TMDs and show the possibility to engineer the band-gap of the ribbon and remove its spin degeneracy. In particular, we find that in heterostructures in which the TMD is a semiconductor the strong spin-orbit coupling of the TMD can induce, via proximity effect, a significant enhancement of the spin-orbit coupling in the GNR's conduction and valence bands. We also study the case in which the TMD is NbSe2. This is an interesting case because NbSe2 is metallic at room temperature and superconducting at low temperatures. The strong enhancement of the spin-orbit coupling in the ribbon and the fact that one of the TMD monolayers, NbSe2, is superconducting at low temperatures make GNR TMD heterostructures a promising platform for the realization of quasi one-dimensional superconducting topological states supporting Majorana zero modes. |
Friday, March 9, 2018 9:48AM - 10:00AM |
X40.00010: In-situ study of dynamics of symmetry-breaking stacking boundaries in bilayer MoS2 Aiming Yan, Chin Shen Ong, Diana Qiu, Colin Ophus, Jim Ciston, Christian Merino, Steven Louie, Alex Zettl Engineering symmetry in 2D materials has recently emerged as a promising way to achieve novel properties and functions. Here we report the in-situ integration of AA’ and AB stacked bilayer MoS2 with different inversion-symmetries by creating atomically sharp stacking boundaries between the differently stacked domains, via thermal stimulation and electron irradiation, inside an atomic-resolution scanning transmission electron microscope. The setup enables us to track the formation and atomic motion of the stacking boundaries in real time and with ultra-high resolution which enables in-depth analysis on the atomic structure at the boundaries. In conjunction with density functional theory calculations, we establish the dynamics of the boundary nucleation and expansion, and further identify metallic boundary states. Our approach provides a means to synthesize domain boundaries with intriguing transport properties, and opens up a new avenue for controlling valleytronics in nanoscale domains via real-time patterning of domains with different symmetry properties. |
Friday, March 9, 2018 10:00AM - 10:12AM |
X40.00011: Micrometer-Scale Stress from van der Waals Interactions in the Delamination of Graphene from Substrates Thanh-Tung Nguyen, Alberto Ambrosetti, Stéphane Bordas, Alexandre Tkatchenko Anomalous long-range non-bonded interactions have been experimentally observed in many different systems, e.g. interfaces between graphene and different substrate materials (Si, SiO$_2$, Cu foils). In this case, long-range forces are evidenced by measurements of non-vanishing stress that extends up to micrometer separations between graphene and the substrate. Existing classical approaches to describe adhesive properties are unable to explain these experimental observations, instead underestimating the measured distance range by a factor of 100 to 1000. Here we develop an analytical and numerical variational approach based on pairwise and/or many-body treatment of van der Waals dispersion interactions between two extended objects. A full relaxation of the coupled adsorbate/substrate geometry leads us to conclude that the wavelike deformation of carbon atoms in graphene (and possibly the substrate) is responsible for the observed ultra long-range stress in delamination of graphene from various substrates. Remarkably, the observed emergent stress seems to be a general phenomenon for stable deformable membranes and its correct description requires a quantum-mechanical many body treatment of interatomic interactions beyond the standard pairwise models for the van der Waals energy. |
Friday, March 9, 2018 10:12AM - 10:24AM |
X40.00012: Interdependence of Morphological Parameters in Self-Assembled Twisted Bilayers of Folded Graphene Johannes Rode, Dawei Zhai, Christopher Belke, Sung Ju Hong, Henrik Schmidt, Nancy Sandler, Rolf Haug Previous studies have shown the possibility of obtaining folded twisted bilayer graphene and graphene ribbons through spontaneous self-tearing and peeling from a substrate [1]. Here we investigate the morphology of spontaneously self-grown nanoribbon structures using atomic force microscopy. Data reveals similar twist angle dependence of the width and interlayer separation, as well as a width-dependent fold radius. These observations can be described by an energy minimization model that includes bilayer formation, bending, tearing and substrate peeling processes. The corresponding energy densities are parameters determined by comparison with experimental data, with the bilayer adhesion energy density modeled by a Morse potential. We obtain explicit expressions for the radius-width dependence that are in good agreement with experimental observations. These relations provide further evidence of a strong dependence of the bilayer adhesion energy density on the twist angle. |
Friday, March 9, 2018 10:24AM - 10:36AM |
X40.00013: Ultrasensitive measurement of interface adhesion by optically forged graphene blisters Pekka Koskinen, Karoliina Karppinen, Pasi Myllyperkiö, Vesa-Matti Hiltunen, Andreas Johansson, Mika Pettersson Although interface adhesion dominates the mechanical behavior 2D materials, its magnitude is notoriously difficult to measure. Here we measure the adhesion between SiO2 substrate and graphene by means of the topographies of round blisters created by a novel technique called optical forging [1]. The adhesion is quantified by comparing blister topographies from atomic force microscopy to ones simulated by thin sheet elasticity theory. In our sample, which consisted of SiO2 substrate and graphene grown by chemical vapor deposition, we obtained interface adhesion orders of magnitude smaller than typically found in layered van der Waals materials. The laser-induced blister formation process thus significantly broadens measurement ranges and enables ultrasensitive determination of interface adhesion. |
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