Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session F51: Focus Session: Beyond Graphene: Synthesis, Defects, Structure, and Properties IV |
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Sponsoring Units: DMP Chair: Junhao Lin, Vanderbilt University Room: Mile High Ballroom 1E |
Tuesday, March 4, 2014 8:00AM - 8:36AM |
F51.00001: Disorder in 2D Materials Invited Speaker: Manish Chhowalla Heterogeneity and aperiodicity in materials is typically viewed as undesirable but recent developments have shown that disorder in materials can lead to interesting and unexpected effects and that disorder and defect engineering are fundamental pathways for tailoring material properties. Towards this end, we utilize chemically exfoliated two-dimensional materials as model systems to study disorder. Chemical exfoliation leads to highly modified materials that are structurally and chemically heterogeneous, unlike the structurally pristine material obtained by mechanical exfoliation or chemical vapor deposition. In this talk, I will describe how several different structural phases with disparate properties in transition metal dichalcogenide (TMD) nanosheets such as MoS$_{\mathrm{2}}$ and WS$_{\mathrm{2}}$ are possible and how their concentrations can be controlled. I will also demonstrate metal-semiconductor transition in 2D material by phase transformation and how the metallic phase of 2D TMDs can be used to improve their catalytic activity for making hydrogen. [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 8:48AM |
F51.00002: First-principles study on native defects and dopant impurities in single-layer MoS2 Ji-Young Noh, Minkyu Park, Hanchul Kim, Yong-Sung Kim We have carried out first-principles calculations on the atomic and electronic properties of a single layer MoS2 with various native defects and substitutional dopants (V, Nb, Ta, N, P, As, Sb for n-type and Mn, Tc, Re, F, Cl, Br for p-type). For charged defects, various supercell sizes are considered to investigate the finite-size supercell effects, and we apply the electrostatic energy correction and level alignment to obtain the formation energies and transition levels of the isolated defects. We find that the S-vacancy and S-interstitial on top of a S atom have low formation energies among the native defects. The S-interstitial is found to be a neutral defect, while the S-vacancy is a deep acceptor. We discuss possible origins of the natural n-type doping in exfoliated single-layer MoS2 based on the substitutional dopant impurities. [Preview Abstract] |
Tuesday, March 4, 2014 8:48AM - 9:00AM |
F51.00003: Intrinsic Magnetism of Grain Boundaries in Two-dimensional Metal Dichalcogenides Zhuhua Zhang, Xiaolong Zou, Vincent H. Crespi, Boris I. Yakobson In two-dimensional (2D) atomic crystals, ubiquitous grain boundaries (GBs) have been shown to cause considerable degradation in material properties. Using first-principles calculations, we show that dislocations and GBs in 2D metal dichalcogenides MX$_{2}$ (M$=$Mo,W; X$=$S,Se) exceptionally exhibit substantial magnetism, in sharp contrast to other 2D materials. All dislocations are shown to have a high magnetic moment of 1.0 Bohr magneton, mainly contributed by the Mo 4d orbitals. GB composed of pentagon-heptagon pairs shows ferromagnetic spin ordering and undergoes transitions from semiconductor to half-metal and to metal as tilt angle increases; when the tilt angle is over 47$^{\circ}$, GB prefers square-octagon pairs and turns to antiferromagnetic semiconductor. A novel mechanism based on interplay between dislocation-induced localized states and local unbalanced stoichiometry of GB is revealed for elucidating the magnetic behavior. Our findings suggest that purposeful engineering of topological GBs can upgrade 2D MX$_{2}$ into promising magnetic semiconductors for spintronic applications. [Preview Abstract] |
Tuesday, March 4, 2014 9:00AM - 9:12AM |
F51.00004: Monolayers of MoS2 and WS2 as oxidation protective nanocoating materials Huseyin Sener Sen, Engin Durgun, Hasan Sahin, Francois Peeters First-principles simulation techniques are employed to analyse the interaction of oxygen with MoS$_2$ and WS$_2$ monolayers. Our calculations show that while oxygen atoms are strongly bound on top of sulphur atoms, oxygen molecule only weakly interact with the system. The penetration of oxygen atom and molecule through MoS$_2$ monolayer require a very high energy barrier indicating that MoS$_2$ can serve as protective layer from oxidation. Not only ideal structures but also possible defect formations are considered and penetration/diffusion barriers of oxygen are calculated for each case. The study is extended for WS$_2$ as well, and obtained results are compared. Our predictions indicate that ideal and/or defected MoS$_2$ and WS$_2$ monolayers can improve the oxidation and corrosion-resistance of the covered surface and can be used as an efficient nanocoating material. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F51.00005: Stability and Electronic properties of Ultra-thin Metallic nanowires on MoS$_{2}$ monolayer Ashok Kumar, Xiaoliang Zhong, Sanjeev K. Gupta, P.K. Ahluwalia, Shashi P. Karna, Ravindra Pandey MoS$_{2}$ has emerged as a promising 2D nanomaterial for several technological applications. It has recently been shown that the highly capacitive Au nanoparticles raised the effective gate voltage for the MoS$_{2}$ based device by an order of magnitude (Nano Lett. 13, 4434-41, 2013). In this talk, we examine stability and electronic properties of commensurable ultra-thin noble-metal nanowires (Cu, Ag, Au, Pt) on MoS$_{2}$ monolayer. Results based on density functional theory will be presented to determine the preferred configuration for nanowires on the monolayer together with the enhancement in the conductivity of the composite system considered. [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F51.00006: Understanding the intrinsic water wettability of graphite, graphene, and 2D materials Andrew Kozbial, Zhiting Li, Jianing Sun, Xiao Gong, Feng Zhou, Yongjin Wang, Haochen Xu, Haitao Liu, Lei Li Adsorption of airborne contaminants onto high energy surfaces can mask the intrinsic material properties and cause wettable surfaces to appear hydrophobic. We report the effect of airborne hydrocarbon contamination on the water wettability of graphite and its 2D counterpart, graphene. The WCA of HOPG was 64.4 $\pm$ 2.9$^{\circ}$ when measured within 10 seconds after exfoliation in air and increased to $\sim$90$^{\circ}$ after 15 minutes. Ellipsometry measurement showed growth of an adsorptive layer on exfoliated HOPG and ATR-FTIR data indicated that the layer is airborne hydrocarbon. Analogous experimental evidence on graphene indicated that a mildly hydrophilic and clean graphene surface with WCA of 44$^{\circ}$ (monolayer graphene on Copper) and 59.6$^{\circ}$ (2-3 layer graphene on Nickel) adsorbed airborne hydrocarbons resulting in a hydrophobic surface with WCA of 80$^{\circ}$. This indicates that graphite and graphene are intrinsically mildly hydrophilic and that surface adsorbed airborne hydrocarbon is the source of hydrophobicity. The results are extended to evaluating other 2D materials - MoS$_{2}$, WS$_{2}$, BN - to further elucidate the effect of hydrocarbon contamination. [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F51.00007: Dirac fermions in monolayer TiB$_{2}$ Lizhi Zhang, Shixuan Du, Hongjun Gao, Feng Liu Monolayer TiB$_{2}$ sheet ($m$-TiB$_{2}$), a two-dimensional metal-diboride, is investigated by first-principles calculations. We demonstrate that $m$-TiB$_{2}$ maintains isotropic Dirac cones near the Fermi level, having a Fermi velocity about one-half of the Fermi velocity of graphene. Different form graphene, these Dirac cones are located between K and $\Gamma $ point in the Brillouin zone, and have primarily the transition metal \textit{d-orbit} characters. Further analysis illustrates that the $d$-band Dirac cones arise from the hybridization of B $p $and Ti $d$ orbitals. Calculations of adsorption of the m-TiB$_{2}$ on hexagonal BN ($h$-BN) substrate reveal a negligible influence of the $h$-BN substrate to the electronic properties of $m$-TiB$_{2}$. Our findings extend the Dirac-band materials to metal-diborides. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F51.00008: Effect of Defects on Doping, Magnetism, and Reactivity in Hexagonal Boron Nitride / Graphene Layered Systems Alejandro Suarez, Thomas L. Reinecke Boron nitride is a promising substrate for graphene devices due to its ultra-flat and insulating characteristics. However, the interactions between defects within a hexagonal boron nitride (h-BN) substrate and a graphene layer are not yet well understood. Using ab-initio methods, we calculate the ground state energies of h-BN/ graphene bilayer systems with a number of defects including vacancies, substitutions, and interstitials. Lattice distortion, charging, and magnetism due to defects are reported and compared with literature on boron nitride bilayers. We also model the adsorption of hydrogen and fluorine atop the various defect configurations. Differences in adsorption energy, bonding geometry, and density of states of such adsorbates help elucidate which defects may be desirable for controlling graphene reactivity. [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F51.00009: Diffusion kinetics and capacity of defected and doped graphene in Li-ion battery Rahul Hardikar, Deya Das, Seungchul Kim, Sang Soo Han, Kwang-Ryeol Lee, Abhishek Singh Graphene, with high surface area, electrical conductivity, robust mechanical integrity, has been intensively studied as an anode material for Li ion batteries (LIBs). Better kinetics and reversible storage of Li are desirable characteristics in a LIB. Using first principles calculations, we study the diffusion of Li \textbf{through} and \textbf{across} the basal plane of defected and doped graphene. The di-vacancy graphene gives the lowest energy barrier of $1.34$ eV for Li to diffuse through the layer, while mono-vacancy B doped gives an energy barrier of $0.31$ eV for diffusion of Li across the basal plane, indicating across diffusion as the possible mechanism. It is also seen that the capacity of Li storage in doped and un-doped mono-vacancy graphene is several orders greater than its pristine counterparts. Through our study, we show that the defected and doped graphene structures emerge as promising anode materials for application in LIBs. [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F51.00010: Electronic properties of functionalized MoS$_{2}$ Jyoti Katoch, Simranjeet Singh, Duy Le, Daniel Chenet, Arend van der Zande, James Hone, Talat Rahman, Laurene Tetard, Masahiro Ishigami We have measured the impact of ad-atoms on MoS$_{2}$ using photoluminescence (PL) and Raman spectroscopy. We find that ad-atoms induce a new peak in the PL spectra, indicating that excitons are bound at the ad-atom sites. Our results will be discussed in light of recent density functional theory calculations. [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F51.00011: Origin and design of indirect-to-direct band gap transition in group-VIB transition metal dichalcogenide films and heterostructures Lijun Zhang, Liping Yu, Jun-Wei Luo, Alex Zunger Group VIB transition metal dichalcogenides (TMDs) often have indirect band gaps in bulk forms but become direct at monolayer thin films. This is an effect often associated with quantum confinement as demonstrated in semiconductor nanostructures. Using first-principle calculations with van der Waals interaction included, (i) we study the indirect-to-direct transition in films of a single TMD material as a function of its thickness for a series of TMDs (MX${_2}$, M = Cr, Mo, W; X = S, Se, Te). By systematic analysis of the obtained critical transition thickness, effective masses and energy evolution of the band edge states, we rule out the mechanism of (kinetic energy controlled) quantum confinement, in favor of an (potential energy controlled) inter-layer coupling. (ii) We explore the electronic structure of different layer stacking in the van der Waals heterostructures consisting of a few TMDs. We found in such multiple layered systems that certain stacking sequences result in a direct band gap, and thus accompanied by a remarkably different optical response. In some heterostructures, the behavior of charge separation, i.e. electron and hole in different layers, is observed. The results of our work provide new insight on engineering optoelectronic properties of TMDs. [Preview Abstract] |
Tuesday, March 4, 2014 10:36AM - 10:48AM |
F51.00012: Annealing and adsorption effects on MoS2 electronic properties Alaric Bergeron, Alexandre Favron, Richard Martel, Richard Leonelli, Sebastien Francoeur Monolayers of MoS$_2$ and other transition metal dichalcogenides open up a number of promising opportunities in the fields of spintronics and flexible electronics. However, the properties of these 2D semiconductors are often altered by interactions with their environment that modify significantly their electrical behavior and their optical response. In this work, we investigated the influence of 1) laser annealing, 2) adsorbed gas molecules and 3) substrate characteristics on electronic states and properties of MoS$_2$ monolayers through low-temperature spatially resolved photoluminescence. We find that laser annealing suppresses low-temperature bound exciton and trion photoluminescence, as well as decreases the relative intensity of the B-exciton photoluminescence. The use of a polymer substrate enables a permanent five-fold enhancement of the room-temperature photoluminescence of the monolayer by laser annealing. This suggests that adsorbed molecules have a significant effect on doping levels. Further studies through annealing in different atmospheres allowed us to link this effect to specific adsorbates. Finally, MoS$_2$ monolayers were encapsulated in polymer to maximize the photoluminescence enhancement by completely shielding the surface from surrounding gas molecules. [Preview Abstract] |
Tuesday, March 4, 2014 10:48AM - 11:00AM |
F51.00013: Synthesis and Photoresponse of Few Layer Liquid Phase Exfoliated Molybdenum Disulphide (MoS$_2$) Flakes Sujoy Ghosh, Baleeswaraiah Muchharla, Andrew Winchester, Simin Feng, Ana Laura Elias, Nestor Perea Lopez, Swastik Kar, Mauricio Terrones, Saikat Talapatra We report on the temperature dependent photo response of~thin films of MoS$_{2}$ consisting of few layered flakes obtained by liquid phase exfoliation of bulk MoS$_{2}$ powder. We found that under a constant laser power (wavelength $=$ 658 nm) the photocurrent (I$_{\mathrm{ph}}$) increases with increasing temperature and reaches a maximum value of I$_{\mathrm{ph(max)}}$ at T$=$T$_{\mathrm{m}}$ within the studied temperature range (330K \textless T \textless 25). Thereafter, I$_{\mathrm{ph}}$, decreases with further increase in temperature and also becomes temperature independent at low temperatures. Further, it was found that in such films I$_{\mathrm{ph}}$ $\sim$ (laser intensity)$^{\gamma}$ with 0.5 \textless $\gamma $ \textless 1.0. These findings will be presented and discussed under various available models related to photoconductivity in semiconductors. [Preview Abstract] |
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