Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session A12: Devices from 2D Materials -- Microscopy and SpectroscopyFocus
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Sponsoring Units: DMP Chair: Kyle Seyler, Univ of Washington Room: BCEC 153A |
Monday, March 4, 2019 8:00AM - 8:36AM |
A12.00001: ABSTRACT WITHDRAWN
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Monday, March 4, 2019 8:36AM - 8:48AM |
A12.00002: Visualizing and Controlling Coupled Graphene Quantum Dots Fereshte Ghahari, Daniel Walkup, Kenji Watanabe, Takashi Taniguchi, Nikolai Borisovich Zhitenev, Joseph A Stroscio Recent progress in creating graphene quantum dots (QDs) with fixed build-in potentials has enabled visualizing the confined electronic states employing local probe measurements. In these nanometer-sized circular QDs, the Dirac quasiparticles are confined by Klein scattering from p-n junction boundaries. In this talk, I will present the scanning tunneling measurements (STM) of such double graphene QDs where the size and coupling between QDs can be tuned by varying the back-gate voltage. At high magnetic fields, Landau levels (LLs) inside each QD form a series of metallic rings separated by highly insulating incompressible rings where each LL shows its own charging characteristics. We used Coulomb blockade spectroscopy to probe and visualize the transition from the double dot system to a single large dot. The transition is reflected in distinct anticrossing patterns between charging lines corresponding to LL’s electrons in different quantum dots. |
Monday, March 4, 2019 8:48AM - 9:00AM |
A12.00003: Writable and tunable Landau level quantum dots in graphene devices Daniel Walkup, Fereshte Ghahari, Cyprian Lewandowski, Christopher Gutierrez, Joaquin Rodriguez Nieva, Kenji Watanabe, Takashi Taniguchi, Leonid Levitov, Nikolai Borisovich Zhitenev, Joseph A Stroscio In graphene devices supported by hexagonal boron nitride (hBN), local charge pockets can be written by pulsing the bias voltage of a scanning tunneling microscope (STM) tip. In a strong magnetic field, discrete Landau levels (LLs) replace the continuous graphene density of states, and the charge pocket is characterized by a concentric series of compressible and incompressible rings, as successive LLs cross the Fermi energy. Here, Coulomb blockade peaks appear in the scanning tunneling dI/dV spectra, revealing the addition of single electrons to the confined Landau island, and encoding the capacitances between it and the tip and back-gate electrodes. When two highly-gapped LLs cross the Fermi energy, two series of peaks appear, experiencing avoided crossings within the dot. The observed avoidance pattern of these anti-crossings is unusual and defies conventional explanation. We present a new phenomenological electrostatic model, taking into account the self-contained geometry of this double-quantum dot system, and are able to reproduce the most striking features of the experimental data. |
Monday, March 4, 2019 9:00AM - 9:12AM |
A12.00004: Visualization and transport properties of complex graphene moiré superlattices Yibo Wang, Colin Woods, Zihao Wang, Kostya Novoselov Over the past few years, graphene deposited on hexagonal boron nitride (hBN) has gained a great level of attention thanks to the unique physical properties of such heterostructures. In particular, due to the comparable lattice constants of graphene and hBN, the latter two can be aligned with respect to each, that results in the formation of a superlattice potential, also known as a Moire pattern. Such a potential drastically affects the band structure of graphene enabling the observation of many spectacular phenomena, i.e. second generation of Dirac points, Hofstadter butterflies, Brown-Zak high-temperature oscillations, etc. In this presentation, we will present an approach that allows characterization of graphene-hBN superlattices by atomic force microscopy (AFM). We will show that the AFM operating in the peak-force mode accompanied by the Raman spectroscopy provides a clear visualization of the Moire potential enabling the measurements of its period and strain distribution. Last, we will discuss electronic properties of complex graphene/hBN superlattices formed by a dual-alignment of graphene with respect to two hBN slabs. |
Monday, March 4, 2019 9:12AM - 9:24AM |
A12.00005: Nano-imaging of heat dissipation from individual atomic defects in graphene Dorri Halbertal Energy dissipation is a fundamental process governing the dynamics of classical and quantum systems, even though direct imaging of dissipation in quantum systems remained out-of-reach experimentally until recently. We developed a scanning nanoSQUID with sub 50 nm diameter that resides at the apex of a sharp pipette [1] acting simultaneously as nanomagnetometer with single spin sensitivity and as nano-thermometer providing cryogenic thermal imaging with four orders of magnitude improved thermal sensitivity of below 1 µK/Hz1/2 [2]. Using this scanning nano-thermometry we visualize and control phonon emission due to inelastic electron scattering off individual atomic defects in graphene [3]. The inferred electron-phonon “cooling power spectrum” exhibits sharp peaks when the Fermi level comes into resonance with electronic quasi-bound states at such defects, a hitherto uncharted process. The atomic defects are very rare in the bulk but abundant at the edges, acting as switchable atomic-scale phonon emitters that establish the dominant dissipation mechanism in graphene. |
Monday, March 4, 2019 9:24AM - 9:36AM |
A12.00006: Two-dimensional ferroelectric van der Waals heterostructures and domain dynamics Leo McGilly, Daniel A Rhodes, Michael Grandel, James Hone, Abhay Pasupathy Layered ferroelectric materials such as CuInP2S6 (CIPS) offer a huge potential for adding functionality in van der Waals (vdW) heterostructures and furthermore the opportunity to investigate some fundamental physics related to confinement in two dimensions plus the effect of electrostatic doping through ferroelectric polarization modulation. In this study we investigate, through piezoresponse force microscopy (PFM), pure-phase CIPS flakes exfoliated by standard Scotch tape methods and incorporated into vdW stacks through a dry-transfer technique. We show that the vdW interface electrostatic environment has significant effects on the local domain structure. Local switching dynamics determined by PFM were also observed to depend on the exact device geometry. |
Monday, March 4, 2019 9:36AM - 9:48AM |
A12.00007: Sub-Micron Imaging of Encapsulated 2D layers of Graphene and Transition Metal Dichalcogenides by Conductive Scanning Probe Microscopy Michael Altvater, Tianhui Zhu, Junxi Duan, Guohong Li, Eva Andrei Encapsulation of 2D materials protects them from environmental disturbances and significantly improves their quality. However, these benefits are lost if impurities or structural defects become trapped within the encapsulating layers. It is therefore crucial to detect these prior to embarking on time-consuming device processing. While encapsulated flakes can be detected via post-processing of optical images or by confocal Raman microscopy, these techniques lack the sub-micrometer resolution to identify structural defects and charged impurities within the encapsulated layer. We demonstrate a facile technique to visualize charged contaminants within the heterostructure by measuring surface potential fluctuations using Kelvin probe force microscopy (KPFM). By applying a fixed tip bias larger than the surface potential fluctuations, the encapsulated flakes and their sub-micron structural defects, cracks, and bubbles can be detected through electrostatic force microscopy (EFM). We show that these methods, which are standard extensions of atomic force microscopy (AFM), are perfectly suited for imaging encapsulated conductors and their local charge environments. |
Monday, March 4, 2019 9:48AM - 10:00AM |
A12.00008: Graphene-based Hall sensors for scanning magnetic microscopy Brian Schaefer, Lei Wang, Alexander B Jarjour, Kenji Watanabe, Takashi Taniguchi, Paul L McEuen, Katja Nowack Incorporating few-layer graphite gate electrodes and hexagonal boron nitride gate dielectrics into graphene devices provides an accessible route to create two-dimensional electron systems with extremely low charge inhomogeneity. Here, we fabricate micrometer-scale graphene Hall sensors that can be tuned to a carrier density of ~2×109 cm-2 at cryogenic temperatures. This translates to a Hall coefficient exceeding 300,000 Ω/T, two orders of magnitude larger than previously reported in any other Hall sensor. The magnetic resolution of our devices is ultimately limited by low-frequency flicker (1/f) and random telegraph noise, and we reach a minimum magnetic field noise spectral density of 60 nT/Hz1/2 at 1 kHz. Together, our results suggest that these devices have the potential to outperform semiconductor-based Hall sensors. We will also discuss our progress towards fabricating submicron-scale graphene-based scanning Hall probes that function over a wide range of temperatures and magnetic fields. |
Monday, March 4, 2019 10:00AM - 10:12AM |
A12.00009: Inelastic Electron Tunneling Spectroscopy of Graphene/hexagonal Boron Nitride heterostructures Zhehao Ge, John L Davenport, Frederic Joucken, Eberth Quezada, Junyan Liu, Takashi Taniguchi, Kenji Watanabe, David Lederman, Jairo Velasco Jr. Inelastic Electron Tunneling Spectroscopy (IETS) has been used for several decades as an experimental probe to access the vibrational properties of low dimensional materials. Notably, the ability to investigate these properties could be leveraged to gain further insight into the recently reported unconventional superconductivity in twisted bilayer graphene. Currently, there exists several IETS measurements on heterostructures of graphene and hexagonal Boron Nitride (hBN). These studies employed different experimental approaches such as tunneling from an atomically sharp asperity with scanning tunneling spectroscopy (STS) and planar tunneling via tunneling field effect transistors (TFETs). Importantly, the IETS results from these different experimental approaches lack agreement. To address this disparity we provide a direct comparison between IETS measurements acquired from STS and TFETs on graphene/hBN heterostructures that underwent the same fabrication processes. Our work contributes towards benchmarking IETS of two-dimensional material heterostructures and will enable future application of this technique to the study of unconventional superconductivity in twisted bilayer graphene. |
Monday, March 4, 2019 10:12AM - 10:24AM |
A12.00010: Complete Strain Mapping of Nanosheets of Transition Metal Chalcogenides Yue Cao, Tadesse Assefa, Soham Banerjee, Simon J L Billinge, Jedrzej Wieteska, Dennis Wang, Abhay Pasupathy, Xiao Tong, Yu Liu, Wenjian Lu, Yuping Sun, Yan He, Xiaojing Huang, Hanfei Yan, Yong S. Chu, Ian Keith Robinson Holding promise for future electronics because of their unique band structures, quasi-two-dimensional (quasi-2D) materials host electronic and mechanical properties sensitive to crystal strains in all three dimensions. Quantifying the crystal strain is a prerequisite to correlating it with the performance of the relevant device, and calls for fast characterization methods compatible with the potential devices and applications. Here we bridge this knowledge gap using fly-scan nano X-ray diffraction with strain sensitivity below 0.001 over sub 100 µm length scales. Coherent diffraction patterns were collected from a thin sheet of 1T-TaS2 using an area detector by scanning across and rotating the sample. Reconstructing from the resulting five-dimensional datasets yields information on the morphology of and the strain distribution around micron and sub-micron ‘bubbles’ which form spontaneously in the quasi-2D plane. Our studies thus open way to experimentally quantify local strains in these quasi-2D materials and will allow better understanding of strains in tuning material properties. |
Monday, March 4, 2019 10:24AM - 10:36AM |
A12.00011: Reversible Nanoscale Control of the Charge Neutrality Point in Graphene Using LaAlO3/SrTiO3 Heterostructures Jianan Li, Qing Guo, Jen-Feng Hsu, Shan Hao, Yang Hu, Hyungwoo Lee, Jungwoo Lee, Chang-Beom Eom, Brian R D'Urso, Patrick Irvin, Jeremy Levy The properties of graphene depend sensitively on doping with respect to the charge-neutrality point (CNP). Tuning the CNP usually involves electrical gating or chemical doping. Here, we describe a technique to reversibly control the CNP in graphene with extreme nanoscale precision, using LaAlO3/SrTiO3 (LAO/STO) heterostructures and conductive atomic force microscope (c-AFM) lithography. The conductivity of the LAO/STO interface can be tuned using a conductive AFM tip, even through graphene transferred on, affecting the LAO/STO interface conductive while shifting the position of graphene CNP. Here we demonstrate that edge state engineering can be achieved from this method using the quantum Hall effect. Clear quantized resistance at plateaus h/e2 and h/3e2 are observed in a split Hall device, demonstrating edge transport along the c-AFM written edge. This technique can be extended to many other device geometries. |
Monday, March 4, 2019 10:36AM - 10:48AM |
A12.00012: Probing band structure renormalization in 2D semiconductors induced by external dielectric screening through angle-resolved photoemission spectroscopy Lutz Waldecker, Archana Raja, Malte Roesner, Christina Steinke, Roland Koch, Aaron Bostwick, Chris Jozwiak, Takashi Taniguchi, Kenji Watanabe, Eli Rotenberg, Tim Wehling, Tony F Heinz We investigate the effect of dielectric screening by the external environment on the electronic states in the two-dimensional (2D) semiconductor WS2 using angle-resolved photoemission spectroscopy (ARPES). As has been previously reported from optical spectroscopy of monolayer WS2 [1], an increase in environmental screening from the presence of graphene can lead to a ~100 meV reduction of the bandgap. Our ARPES study of monolayer WS2 partially placed on h-BN and graphene, however, reveals that the dispersion of the valence band is essentially unchanged (< 10 meV shifts) when screening by graphene is present. Thus the screening-induced band renormalization appears to lead only to a rigid shift, rather than a restructuring of the valence band. We compare this experimental finding with theory based on GΔW calculations, including material-realistic frequency and momentum dependent screening due to the substrate. In agreement with experiment, theory predicts the primary effect of bandgap renormalization along with a scissor-like shift of all electronic states. We discuss the physical origin of this result and its implication for the use of controlled changes in the environment to tune the band structure of 2D semiconductors. |
Monday, March 4, 2019 10:48AM - 11:00AM |
A12.00013: Gating and superlattice effects in monolayer WSe2 devices in micro-ARPES Paul Nguyen, Natalie Teutsch, Abigail J Graham, Minhao He, Viktor Kandyba, Alexei Victorovich Barinov, Neil R Wilson, Xiaodong Xu, David Henry Cobden We investigate the effects of electrostatic doping and moiré superlattice potentials on monolayer WSe2 using micro-angle-resolved photoemission spectroscopy (micro-ARPES). By employing a local back gate of thin graphite under a hexagonal boron nitride dielectric support, we electrostatically populate the WSe2 conduction band during the photoemission measurements. We find the conduction band edge to be at the zone corner, or K point, confirming the direct gap nature of this material and yielding a measurement of the single-particle gap. As the electron doping is increased to a maximum of ~1.4E13 cm-2 the valence band shifts upwards towards the conduction band, corresponding to substantial band gap renormalization. At the highest doping levels, the conduction band minimum at the lower symmetry Q point is also populated, showing that it is within ∼30 meV of the minimum at K. In some devices we also observe folded copies of the hBN and WSe2 bands with intensities often comparable to the original bands, yielding information about moire superlattice effects. |
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