Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session J6: Growth, Structure, Properties, and Defects |
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Sponsoring Units: DMP Chair: Paola Barbara, Georgetown University Room: 302 |
Tuesday, March 19, 2013 2:30PM - 2:42PM |
J6.00001: Temperature Dependence of the Raman Spectra of CVD-grown Monolayer MoS$_2$ A. Glen Birdwell, Frank J. Crowne, Terrance P. O'Regan, Pankaj B. Shah, Madan Dubey, Sina Najmaei, Zheng Liu, Pulickel M. Ajayan, Jun Lou, Rusen Yan, Huili Grace Xing We investigated the temperature dependence of the E$^{1}_{\mathrm{2g}}$ and A$_{\mathrm{1g}}$ peaks in the Raman spectra of monolayer MoS$_{2}$ grown by chemical vapor deposition (CVD) on Si/SiO$_{2}$ substrates. Micro-Raman spectroscopy was carried out using the 532 nm laser excitation over the temperature range from 30 to 175 $^{\circ}$C. The extracted values of the temperature coefficient of these modes are $\chi =$ -0.013 cm$^{-1}$/$^{\circ}$C and $\chi =$ -0.016 cm$^{-1}$/$^{\circ}$C, respectively. The obtained results may shed light on the anomalous behavior of these modes observed in laser power dependent studies of monolayer MoS$_{2}$. [Preview Abstract] |
Tuesday, March 19, 2013 2:42PM - 2:54PM |
J6.00002: Grains and grian boundaries in highly-crystalline monolayer molybdenum disulfide Arend M. van der Zande, Pinshane Y. Huang, Daniel A. Chenet, Yumeng You, Timothy C. Berkelbach, Gwan-Hyoung Lee, David R. Reichman, David A. Muller, Tony F. Heinz, James C. Hone Recent progress in large-area chemical vapor deposition (CVD) synthesis of monolayer molybdenum disulfide, a new two-dimensional direct-bandgap semiconductor, is paving the way for applications in atomically thin electronics. Little is known, however, about the microstructure of this material.~ Here we have refined CVD synthesis to grow highly crystalline islands of monolayer molybdenum disulfide up to 120 micrometers in size with optical and electrical properties comparable or superior to exfoliated samples. Using transmission electron microscopy, we correlate lattice orientation, edge morphology, and crystallinity with island shape to demonstrate that triangular islands are single crystals.~The crystals merge to form faceted tilt and mirror boundaries that are stitched together by lines of 8- and 4- membered rings.~ Density functional theory reveals localized mid-gap states arising from these 8-4 defects. The knowledge gained about grain structure enables systematic studies of the optical, mechanical, and electronic properties of grain boundaries. [Preview Abstract] |
Tuesday, March 19, 2013 2:54PM - 3:06PM |
J6.00003: Synthesis Single Layer Transition Metal Dichalcogenides with Chemical Vapor Deposition Yi-Hsien Li, Han Wang, Lili Yu, Wenjing Fang, Tomas Palacios, Lain-Jong Li, Jing Kong Recently, monolayers of layered transition metal dichalcogenides (LTMD), such as MX2 (M$=$Mo, W and X$=$S, Se), have been reported to exhibit significant spin-valley coupling and optoelectronic performances because of the unique structural symmetry and band structures. Monolayers in this class of materials offered a burgeoning field in fundamental physics, energy harvesting, electronics and optoelectronics. However, most studies to date are hindered with great challenges on the synthesis and transfer of high quality LTMD monolayers. Hence, a feasible synthetic process to overcome the challenges is essential. Here, we demonstrate the growth of high-quality MS2 (M$=$Mo, W) monolayers using ambient-pressure-chemical-vapor-deposition (APCVD) with the seeding of aromatic molecules. Electronic transport and optical performances of the as-grown MS2 monolayers are comparable to those of exfoliated MS2 monolayers. The growth of MS2 monolayer is achieved on various surfaces. Growth mechanism on the novel synthetic process is investigated. Understanding and better control of seeds for the novel growth on the class of materials may stimulate the progress in the emerging filed. [Preview Abstract] |
Tuesday, March 19, 2013 3:06PM - 3:18PM |
J6.00004: Oxidation of atomically thin MoS$_2$ on SiO$_2$ Mahito Yamamoto, William Cullen, Theodore Einstein, Michael Fuhrer Surface oxidation of MoS$_{2}$ markedly affects its electronic, optical, and tribological properties. However, oxidative reactivity of atomically thin MoS$_{2}$ has yet to be addressed. Here, we investigate oxidation of atomic layers of MoS$_{2}$ using atomic force microscopy and Raman spectroscopy. MoS$_{2}$ is mechanically exfoliated onto SiO$_{2}$ and oxidized in Ar/O$_{2}$ or Ar/O$_{3}$ (ozone) at 100-450 $^{\circ}$C. MoS$_{2}$ is much more reactive to O$_{2}$ than an analogous atomic membrane of graphene and monolayer MoS$_{2}$ is completely etched very rapidly upon O$_{2}$ treatment above 300 $^{\circ}$C. Thicker MoS$_{2}$ (\textgreater\ 15 nm) transforms into MoO$_{3}$ after oxidation at 400 $^{\circ}$C, which is confirmed by a Raman peak at 820 cm$^{-1}$. However, few-layer MoS$_{2}$ oxidized below 400 $^{\circ}$C exhibits no MoO$_{3}$ Raman mode but etch pits are formed, similar to graphene. We find atomic layers of MoS$_{2}$ shows larger reactivity to O$_{3}$ than to O$_{2}$ and monolayer MoS$_{2}$ transforms chemically upon O$_{3}$ treatment even below 100 $^{\circ}$C. Work supported by the U. of Maryland NSF-MRSEC under Grant No. DMR 05-20741. [Preview Abstract] |
Tuesday, March 19, 2013 3:18PM - 3:30PM |
J6.00005: Comprehensive Studies of NbS$_{2}$ and the Affects of Nb Doping in the Layered Systems of WS$_{2}$ and MoS$_{2}$ Brian Cooper, Mauricio Terrones, Tom Mallouk, Camden Henderson, Nina I. Kovtyukhova Research on layered dichalcogenides (compounds of the form MX$_{2}$, with M as a metal and X as any member of group 16 in the periodic table) has picked up momentum due to a sympathetic reverberation created in response to the enormously prodigious research in graphene. Although much progress in graphene research has been made, there are still many hurdles to clear, and prudence has made requisite parallel courses in research. Layered dichalcogenides exhibit similar features to graphene; namely they are relatively easy to exfoliate, and have hexagonal symmetries, but unlike graphene, these compounds represent a spanning set of the materials under investigation in various scientific branches ($e.g.$ superconductors, semiconductors, topological insulators, \textit{etc.}). We have taken many approaches to the synthesis, manipulation, and device design of these materials. In our attempts to better understand the role of doping Nb into the MoS$_{2}$ and WS$_{2}$ systems, we serendipitously realized the merits, which previously lay quiescent, of studying NbS$_{2}$ itself. A metallic dichalcogenide, NbS$_{2}$ exhibits both charge density wave and superconducting phase transitions below respective appropriate temperatures. Studying mono, bi, and tri-layer geometries have afforded us the opportunities to probe not only the details of quantum confinement effects in the NbS$_{2}$ system, but also how these effects percolate through and affect the various properties of other dichalcogenidal systems. [Preview Abstract] |
Tuesday, March 19, 2013 3:30PM - 3:42PM |
J6.00006: Effects of strain on band structure and effective masses in MoS$_2$ Hartwin Peelaers, Chris G. Van de Walle Molybdenum disulfide (MoS$_2$) is a layered semiconductor that shows great promise for devices such as field-effect transistors. It has an important advantage compared to graphene, namely that it has a band gap. However, a lot of crucial information about the band structure and electronic properties of this material is still lacking, hampering interpretation of experiments and preventing accurate device modeling. Here we use hybrid density functional theory to calculate key materials parameters such as band gaps and effective masses, as well as to investigate effects of strain. We show how strain allows engineering the nature (direct vs. indirect) and size of the band gap and the magnitude of effective masses. In addition, insight into the fundamental physics is provided by considering the transition between the bulk and the monolayer as a function of tensile uniaxial stress. [Preview Abstract] |
Tuesday, March 19, 2013 3:42PM - 3:54PM |
J6.00007: Two-Dimensional Semiconductors From Theory to Experiments: MoS$_{2}$ and MoSe$_{2}$ Can Ataca, Jeffrey C. Grossman, Sefaattin Tongay, J. Zhou, K. Lo, Junqiao Wu After the synthesis of graphene, single layer transition metal dichalcogenides have been shown to possess superior optical properties than those of graphene. Until now, both theorists and experimentalists have mainly focused on the properties of single-layer MoS2. In this work, the first synthesis of single and few layers of MoSe2 are shown experimentally and are complemented by stability analysis through phonon and electronic structure calculations using density functional theory (DFT). The DFT calculations include van der Waals and spin-orbit interactions which are shown to play an important role in the geometric structure, electronic, magnetic and vibrational properties. Single-layer MoSe2 is measured and calculated as a direct band gap material, having band gap values suitable for solar cells and optical devices. Dimensionality effects predicted by DFT calculations such as variation of the energy band structures and Raman active vibrational modes are confirmed by experiments. Optical and electronic properties of single and few layers MoSe2 can be tuned by varying the temperature, number of layers and applying pressure to the samples. Single layer MoS2 and MoSe2 possess a number of properties that make them highly promising materials for future nanoscale applications. [Preview Abstract] |
Tuesday, March 19, 2013 3:54PM - 4:06PM |
J6.00008: ABSTRACT WITHDRAWN |
Tuesday, March 19, 2013 4:06PM - 4:18PM |
J6.00009: Electronic Structure of Defect-Single-Layer MoS2 Takat Rawal, Duy Le, Talat S. Rahman We present density functional theory based investigations of the electronic structural properties of a single-layer molybdenum disulfide (MoS$_{\mathrm{2}})$ with a sulfur vacancy row. We show that the vacancy row introduces a defect state in the band-gap of MoS$_{\mathrm{2}}$. This state is unoccupied and localized at the vacancy row. We also present detailed analysis of the density of states and charge density of the system. The defect state reduces the band gap of the system to 0.5 eV from 1.8 eV that we obtain for the clean single-layer. In particular, we find Kohn-Sham wave functions that are confined to the defective site are responsible for this particular energy band. We discuss the possible applications of this effect on other physical properties of the system. We also extend the calculations to the case of a MoS$_{\mathrm{2}}$ on Cu(111) for which experimental observations [1] suggest the presence of structures of the type under discussion here. [1] D. Kim, D. Sun, W. Lu, Z. Cheng, Y. Zhu, D. Le, T. S. Rahman, and L. Bartels, Langmuir 27, 11650 (2011). [Preview Abstract] |
Tuesday, March 19, 2013 4:18PM - 4:30PM |
J6.00010: The Incorporation of C in the Crystal Lattice of Metals; Its Role on the Structure and Properties of these New Materials Called Covetics Lourdes Salamanca-Riba, Romaine Isaacs, David Forrest, Azzam Mansour, Andrew Herzing, Melburne LeMieux, Jason Shugart Nanocarbon has been successfully incorporated in molten metals and metal alloys using a new method of manufacturing in which the molten metal (or metal alloy) acts as ionizing medium causing nanocarbon structures to form in-situ. C in concentrations up to $\sim$10{\%} weight was incorporated in Ag, Al and Cu. The bonding between the carbon and the metal is very strong and persists after re-melting and resolidification. These materials, called ``covetics,'' show improved properties over those of the host metal. For example, the thermal conductivity of Cu covetic is higher than pure Cu. The electrical conductivity of Al covetic is higher than for pure Al. The yield strength of Al and Cu covetics is higher than the pure metals. We have used X-ray diffraction, X-ray photoelectron spectroscopy, and Raman spectroscopy to investigate the incorporation of C in the metal. Scanning and transmission electron microscopy (TEM) were also employed along with energy dispersive X-ray spectroscopy and electron energy loss spectroscopy (EELS). The nanocarbons in the covetics are in the form of, graphene nanoribbons, and amorphous nanocarbon, and all are bonded to the metal. The C-K edge in the EELS, and the Raman spectra from these samples show signals characteristic of graphitic sp2 bonding. [Preview Abstract] |
Tuesday, March 19, 2013 4:30PM - 4:42PM |
J6.00011: Tuning Valley Magnetic Moment in Bilayer MoS2 via Symmetry Control Sanfeng Wu, Jason Ross, Gui-Bin Liu, Grant Aivazian, Aaron Jones, Zaiyao Fei, Wenguang Zhu, Di Xiao, Wang Yao, David Cobden, Xiaodong Xu Monolayer MoS2 provides the opportunity to explore the coupled spin-valley physics arising from broken inversion symmetry. Although inversion symmetry is present in pristine bilayer MoS2, it can be broken by applying a perpendicular electric field. It offers the remarkable possibility of switching on/off and continuously tuning the physical properties of the Dirac valleys, such as valley magnetic moment and Berry curvature, by reversible electrical control. In this work, we employ polarization-resolved photoluminescence (PL) to investigate this effect using bilayer and monolayer MoS2 field effect transistors. We find that in bilayer MoS2 the circularly polarized PL can be continuously tuned from -15{\%} to 15{\%} as a function of gate voltage, whereas in structurally non-centrosymmetric monolayer MoS2 the PL polarization is gate-independent. The observations are well explained as resulting from the continuous variation of orbital magnetic moments between positive and negative values via symmetry control. [Preview Abstract] |
Tuesday, March 19, 2013 4:42PM - 4:54PM |
J6.00012: Electrical tunability and optical control of valley and spin in WSe2 Aaron Jones, Hongyi Yu, Nirmal Ghimire, Bo Zhao, Sanfeng Wu, Grant Aivazian, Jason Ross, Guibin Liu, Jiaqiang Yan, David Mandrus, Wang Yao, Di Xiao, Xiaodong Xu Monolayer group VI transition metal dichalcogenides have enormous potential for use in nano- and optoelectronic applications due to their reduced dimensionality and direct bandgap in the visible wavelength range. Their hexagonal structure is graphene-like, but with strong spin-orbit coupling effects. The interesting coupled spin-valley physics has been investigated both theoretically and experimentally based on the single particle picture. Here we investigate the physical properties of valley excitons in monolayer field effect transistor devices via photoluminescence measurements. By tuning the chemical potential to control exciton species, we are able to investigate the optical selection rules, photo-excitation energy dependence, and temperature dependence of individual excitons. These studies reveal the fine structures of valley excitons due to the electron-electron interactions, electron-phonon interactions, and coupled spin-valley degrees of freedom, which are important for the potential application of valleytronics/spintronics based on monolayer semiconductors. [Preview Abstract] |
Tuesday, March 19, 2013 4:54PM - 5:06PM |
J6.00013: Electronic Properties of Few-layer Black Phosphorus Likai Li, Yijun Yu, G.J. Ye, X.H. Chen, Yuanbo Zhang Black phosphorus is a layered allotropy of phosphorus that closely resembles graphite. In a single atomic layer, phosphorus atoms are covalently bonded into a puckered honey comb structure. All five valence electrons are localized, so unlike graphene monolayer black phosphorus is a semiconductor with a band gap of $\sim$ 2 eV. In a bulk crystal the interlayer coupling reduces the band gap to $\sim$ 0.3 eV. Using mechanical exfoliation method, we have successfully fabricated few layer black phosphorus field effect transistors. Our samples exhibit bipolar behavior with on-off ratio up to 10$^6$, and a low off-state current. Electronic mobilities up to $\sim$ 1000 cm$^2$V$^{-1}$s$^{-1}$ are currently achieved, with possibilities for further improvement. Such characteristics make black phosphorus a potential candidate for future nanoelectronic applications. [Preview Abstract] |
Tuesday, March 19, 2013 5:06PM - 5:18PM |
J6.00014: Bilayer Silicene: a first principles investigation Renato Borges Pontes, Jos\'e Eduardo Padilha, Aldaberto Fazzio, Ant\^onio J.R. da Silva By performing ab-initio total energy calculations we study the structural and electronic properties of a silicene bilayer. We show that the lowest energy configuration, contrary to the Bernal stacking of graphene, is when two silicene sheets are placed exactly one on top of the other. In this configuration, there is an energy gain if the system loses its $\pi$ cloud to create extra ($\sigma$-like) chemical bonds between the two layers. To minimize the total energy and the forces that arise due to these new connections made between the layers, the system increases the lattice constant, becoming planar and, consequently, loosing its buckled structure. Moreover, the bilayer of silicene on this planar configuration is a metal and it is insensitive to the presence of an applied external electrical field, a behaviour different from the single layer. We also discuss the role played by the unoccupied 3d-orbital of the silicon in the formation of this new structure. Theoretical STM calculations show excellent agreement with experimental images of silicene bilayers. [Preview Abstract] |
Tuesday, March 19, 2013 5:18PM - 5:30PM |
J6.00015: ABSTRACT WITHDRAWN |
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