Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session S1: Focus Session: Beyond Graphene - Atomic Scale Properties |
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Sponsoring Units: DMP Chair: Abhay Pasupathy, Columbia University Room: 001A |
Thursday, March 5, 2015 8:00AM - 8:12AM |
S1.00001: Unravelling Electronic Band Structures of Transition Metal Dichalcogenides Chendong Zhang, Yuxuan Chen, Amber Johnson, Ming-Yang Li, Jing-Kai Huang, Lain-Jong Li, Chih-Kang Shih Accurate knowledge of the electronic structures of materials is the key enabler for the advancement of science and technology based on such materials. Recent emergence of transition metal dichalcogenides (TMDs) as potentially transformative 2D electronic and photonic materials has triggered intensive research activities to investigate their electronic structures. Compared with the previous experimental efforts such as optical spectroscopies and angle resolved photoemission, the scanning tunneling spectroscopy (STS) has the unique advantages in probing the quasiparticle band structures of the TMD samples with a limited lateral size. However, the STS investigations thus far have not yield consistent results even for the measurement of the quasiparticle band gap. Here we present a new comprehensive methodology of scanning tunneling spectroscopy, which contains the ability not only to probe electronic structures, but also to extract the information on their origins in the Brillouin zone. Thus, we map out, for first time, the critical point energy locations in both the valence and conduction bands of TMD compounds. This capability also allows us to unravel the systematic trend in the critical point energy locations as a function of the cation-anion orbital coupling, the spin-orbital coupling, as well as the interlayer coupling. Such knowledge is critical for the flourishing field of TMDs as emerging atomically thin 2D electronic and photonic materials. [Preview Abstract] |
Thursday, March 5, 2015 8:12AM - 8:24AM |
S1.00002: Real-space methods for calculating the electronic response of 2D materials Benjamin Garrett, James Chelikowsky We study the effects of applied fields on several 2D materials including graphene and metal dichalcogenides. We use a real space density functional method with 2D periodic boundary conditions. This negates the need for a supercell and allows us to directly simulate a transverse electric field. The dielectric properties can be calculated without layer-layer interactions. Changes in band structure in response to imposed fields are also discussed. [Preview Abstract] |
Thursday, March 5, 2015 8:24AM - 8:36AM |
S1.00003: Scanning tunneling microscopy/spectroscopy study of graphene -- MoS$_{2}$ heterojunction Shivani Rajput, Yaoyi Li, Dushyant Tomer, Lian Li Atomic scale topographic fluctuations are relevant to a number of graphene applications including graphene / semiconductor Schottky diodes [1,2]. In this work, we investigate the atomic structures and electronic properties of~graphene-MoS$_{2}$ heterojunction fabricated by transferring chemical vapor deposited monolayer graphene onto mechanically exfoliated multilayer MoS$_{2}$. Scanning tunneling microscopy reveals the formation of Moir\'{e} patterns with corrugations $\sim$ 0.03 nm, but no ripples, in contrast to graphene-SiC junctions [1,2]. Scanning tunneling spectroscopy further indicates that the periodic modulations of the Moir\'{e} pattern do not influence the electronic properties of the junction. Additional states near the Fermi level are also observed, likely due to impurities trapped at the interface during graphene transfer. These results and their impact on the properties of the van der Waals graphene-MoS$_{2}$ heterojunction will be discussed at the meeting. \\[4pt] [1] S. Rajput \textit{et al.} Nat. Commun. \textbf{4,} 2752 (2013).\\[0pt] [2] D. Tomer \textit{et al.} Appl. Phys. Lett. \textbf{105,} 021607 (2014). [Preview Abstract] |
Thursday, March 5, 2015 8:36AM - 8:48AM |
S1.00004: Imaging and Manipulating Defects in Insulating Hexagonal Boron Nitride Using Scanning Tunneling Microscopy Dillon Wong, Jairo Velasco, Long Ju, Juwon Lee, Salman Kahn, Hsinzon Tsai, Chad Germany, Takashi Taniguchi, Kenji Watanabe, Alex Zettl, Feng Wang, Michael Crommie Scanning tunneling microscopy (STM) has a long history of visualizing individual defects in conductors and semiconductors, but such studies were previously not possible for intrinsic bulk insulators (due to lack of electrical continuity). Here we report STM imaging and control of point defects in a bulk insulator. This is accomplished by exploiting graphene's atomically thin nature to peer into defect phenomena that occur in an underlying bulk crystal of hexagonal boron nitride (BN). Using scanning tunneling spectroscopy (STS) in combination with a graphene/BN gated device, we obtain information on a variety of different localized BN defect structures. We also demonstrate the ability to manipulate these observed defects. The methods and analysis described here may also be used to investigate other defect/insulator systems. [Preview Abstract] |
Thursday, March 5, 2015 8:48AM - 9:00AM |
S1.00005: Electronics and atomic scale properties of defects and dopants in 2H-MoTe$_{2}$ Maria Longobardi, Alberto Ubaldini, Enrico Giannini, David R. Bowler, Christoph Renner We present a detailed STM/STS investigation and corresponding DFT modeling of native dopants and atomic scale defects and their influence on the local electron density of states of 2H-MoTe$_{2}$. Semiconducting transition metal dichalcogenides (TMDs) are attracting increasing interest in the field of electronics and optoelectronics owing to their layered structure and the indirect-to-direct band gap transition when approaching the single-layer limit. 2H-MoTe$_{2}$ is a semiconducting TMD with a bulk band gap of around 1.0 eV. This compound shows very high mobility at room temperature and strong absorption throughout the solar spectrum. Previous studies demonstrated the possibility to achieve gate-induced ambipolar transport at the surface [1]. 2H-MoTe$_{2}$ is thus an attractive candidate for novel optoelectronic devices such as light-emitting diodes, photo detectors and solar cell technology. Controlling the atomic nature and density of defects and dopants is crucial for the development of the aforementioned applications and devices. \\[4pt] [1] \textit{I. Guti\'{e}rrez Lezama et al. }2D Materials \textbf{1}, 021002 (2014) [Preview Abstract] |
Thursday, March 5, 2015 9:00AM - 9:12AM |
S1.00006: Simulated scanning tunneling microscopy of few-layer phosphorus allotropes through hexagonal boron nitride Pablo Rivero, Cedric Horvath, Zhen Zhu, Jie Guan, David Tomanek, Salvador Barraza-Lopez Four stable layered phosphorus allotropes that are almost degenerated in their configuration energy have been recently discussed [1]. Due to their high reactivity under ambient conditions, their exposed surfaces must be protected [2]. Here, we address the influence of~a capping monolayer of hexagonal boron nitride on the scanning tunneling microscopy images of few layered-phosphorus.\\[4pt] [1] J. Guan, and Z. Zhu, and D. Tomanek, \textit{Phys. Rev. Lett.} \textbf{113}, 046804 (2014). \newline [2] A. Castellanos-Gomez, L. Vicarelli, E. Prada, J.O. Island, K.L. Narasimha-Acharya, S.I. Blanter, D.J, Groenendijk, M. Buscema, G.A. Steele, J.V. Alvarez, H.W. Zandbergen, J.J. Palacios, and H.S.J. van der Zant.~ \textit{2D Materials} \textbf{1} 025001 (2014). [Preview Abstract] |
Thursday, March 5, 2015 9:12AM - 9:48AM |
S1.00007: Scanning Tunneling Spectroscopy of Transition Metal Dichalcogenides: Quasiparticle Gap, Critical Point Energies and Heterojunction Band Offsets Invited Speaker: Chih-Kang Shih As an emergent atomically thin electronic and photonic materials material, transition metal dichalcogenides (TMDs) has triggered intensive research activities toward understanding of their electronic structures. Here I will introduce a comprehensive form of scanning tunneling spectroscopy (STS) which allows us to probe details quasi-particle electronic structures of TMDs. More specifically, we show that not only the quasi-particle band gaps but also the critical point energy locations and their origins in the Brillouin Zone (BZ) can be revealed using this comprehensive form of STS. By using this new method, we unravel the systematic trend of the critical point energies for TMDs due to atomic orbital couplings, spin-orbital coupling and the interlayer coupling. Moreover, by combining the micro-beam X-ray photoelectron spectroscopy (micro-XPS) and STS, we determine the band offsets in planar heterostructures formed between dissimilar single layer TMDs (MoS2, WSe2, and WS2). We show that both commutativity and transitivity of heterojunction band offset hold within the experimental uncertainty. \\[4pt] Other Contributors: (i) Chendong Zhang, Yuxuan Chen, and Amber Johnson at the University of Texas at Austin; (ii) Ming-Yang Li, Jing-Kai Huang, Lain-Jong Li, Chih-Piao Chuu and Mei-Yin Chou at the Institute of Atomic and Molecular Sciences, Academia Sinica, Taiwan. [Preview Abstract] |
Thursday, March 5, 2015 9:48AM - 10:00AM |
S1.00008: Structural phase transitions in monolayer molybdenum dichalcogenides Duk-Hyun Choe, Ha June Sung, Kee Joo Chang The recent discovery of two-dimensional materials such as graphene and transition metal dichalcogenides (TMDs) has provided opportunities to develop ultimate thin channel devices. In contrast to graphene, the existence of moderate band gap and strong spin-orbit coupling gives rise to exotic electronic properties which vary with layer thickness, lattice structure, and symmetry. TMDs commonly appear in two structures with distinct symmetries, trigonal prismatic 2H and octahedral 1T phases which are semiconducting and metallic, respectively. In this work, we investigate the structural and electronic properties of monolayer molybdenum dichalcogenides (MoX2, where X $=$ S, Se, Te) through first-principles density functional calculations. We find a tendency that the semiconducting 2H phase is more stable than the metallic 1T phase. We show that a spontaneous symmetry breaking of 1T phase leads to various distorted octahedral (1T') phases, thus inducing a metal-to-semiconductor transition. We discuss the effects of carrier doping on the structural stability and the modification of the electronic structure. [Preview Abstract] |
Thursday, March 5, 2015 10:00AM - 10:12AM |
S1.00009: Scanning Tunneling Microscopy study on exfoliated single-layer MoSe$_{2}$ Xiaodong Zhou, Ali Dadgar, Frances M. Ross, Abhay N. Pasupathy Monolayer transitional metal dichalcogenides (TMDs) MX$_{2}$ (M $=$ Mo, W, Ti etc; X $=$ S, Se, Te) are a new platform for exploring new electronic and optical phenomena and functionality. However, much remains to be understood about their chemical and local electronic properties when taken to the monolayer limit. We will discuss a scanning tunneling microscopy (STM) study on exfoliated single-layer MoSe$_{2}$ using a 4-probe STM system. The ability to carry out scanning electron microscopy (SEM) in our system allows us to easily locate and measure single-layer MoSe$_{2}$ flakes that are mechanically exfoliated on a SiO$_{2}$/Si substrate and are only a few micrometers in lateral size. Using a combination of imaging and spectroscopy, we will discuss the chemical purity and nature of defect states in this monolayer material. Using a electrostatic back gate, we will describe measurements of the single-particle electronic bandgap as a function of the chemical potential. [Preview Abstract] |
Thursday, March 5, 2015 10:12AM - 10:24AM |
S1.00010: Defect states in monolayer transition metal dichalcogenides Mahtab Khan Monolayer transition metal dichalcogenides (TMDC) have attracted considerable attention in the past few years. They are direct bandgap semiconductors, with the conduction and valence band edges at the doubly degenerate corners ($\pm$ K points) of the hexagonal Brillouin zone. Recently, by using novel etching techniques, it was possible to remove a controlled number of atoms from monolayer MoS$_{2}$, thereby creating a hexagonally shaped pit. By solving the Dirac equation analytically, we show that a pit gives rise to bound states with interesting properties. In particular, the optical selection rules turn out to be very strict. We confirm our analytical results by means of numerical density functional theory (DFT) calculations. [Preview Abstract] |
Thursday, March 5, 2015 10:24AM - 10:36AM |
S1.00011: Thermoelectric imaging of grain boundaries in monolayer MoS2 on the atomic scale Sanghee Cho, Ho-Ki Lyeo, Lain-Jong Li, Yong-Hyun Kim, Chendong Zhang, Chih-Kang Shih We used scanning thermoelectric microscopy to investigate structural defects such as point defects, edge and grain boundary in ultrathin films of MoS$_{\mathrm{2}}$ grown on graphite. Such structural changes cause the variation in local electronic states, which can be detected by thermoelectric measurement that is differentially sensitive to the Fermi electronic states. Measured thermoelectric power increased with increasing thickness of MoS$_{\mathrm{2}}$ from monolayer to multilayer, which makes a different contrast in the images of thermoelectric measurements. The changes in thermoelectric power with varying thickness can be accounted for by the changes in energy band structure. This imaging method enabled us to identify the metallic edge states, which is similar to prior measurements from tunneling spectroscopy, at the boundaries between MoS$_{\mathrm{2}}$ and graphite. Moreover, grain boundaries appear with distinct contrast in thermoelectric measurements from micrometer to atomic scale, whereas the boundaries were subtle in topographic measurements. Simultaneous measurements of topographic and thermoelectric signal revealed the structural and electronic properties of grain boundaries on the atomic scale. [Preview Abstract] |
Thursday, March 5, 2015 10:36AM - 10:48AM |
S1.00012: Occupied and Unoccupied Electronic Structure of Na doped MoS$_2$(0001) T. Komesu, D. Le, X. Zhang, Q. Ma, E. F. Schwier, Y. Kojima, M. Zheng, H. Iwasawa, K. Shimada, M. Taniguchi, L. Bartels, T. S. Rahman, P. A. Dowben The influence of sodium on the band structure of MoS$_2$(0001) and the comparison of the experimental band dispersion with density functional theory (DFT) shows excellent agreement for the occupied states (angle-resolved photoemission), and qualitative agreement for the unoccupied states (inverse photoemission spectroscopy). We will show that Na-adsorption leads to charge transfer to the MoS$_2$ surface causing an effect similar to n-type doping of a semiconductor. Moreover, results of our simulations and measurements clearly indicate that the MoS$_2$ occupied valence band structure shifts rigidly to greater binding with little change in the occupied state dispersion and that the unoccupied states shift downward, approaching the Fermi level, yet the amount of the shift for the unoccupied states is greater than that of the occupied states, effectively causing a narrowing of the bandgap. At higher Na coverages MoS$_2$ surafce becomes metallic. Details of electronic band structure of Na/MoS$_2$(0001) will be discussed in light of the role of the frontier orbitals in facilitating chemical reactivity of the system. [Preview Abstract] |
Thursday, March 5, 2015 10:48AM - 11:00AM |
S1.00013: Effects of spatial averaging on the ARPES spectra of graphene Frederic Joucken, Nicolas Reckinger, Jose Avila, Maria Carmen Asensio, J\'er\^ome Lagoute, Jean-Fran\c{c}ois Colomer, Jacques Ghijsen, Robert Sporken We report on an ARPES nanoscope investigation (nano-ARPES) and demonstrate that spatial averaging of ARPES data, even on a single graphene domain, lead to apparent kinks in the dispersion relation as well as variations of the MDC widths with binding energy that do not appear in the spectra acquired on a very small spot (below 100nm X 100nm). At the same time, we show that the electronic dispersion relation of our graphene sample is perfectly linear while the MDC widths do not display a simple dependence with the binding energy. [Preview Abstract] |
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