Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session H17: Transition Metal Dichalcogenides: Defects and DegradationFocus
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Sponsoring Units: DMP Chair: Chongwu Zhou, University of Southern California Room: 316 |
Tuesday, March 15, 2016 2:30PM - 3:06PM |
H17.00001: TMD 2D Materials: Defects, Passivation, Functionalization and Device Impact Invited Speaker: Robert Wallace Transition metal dichalcogenides (TMDs) such as MoS2 have become popular in ``beyond CMOS'' device concepts and research due to their band structure in two-dimensional layers -- viz. a significant band gap. Various device demonstrations have been reported utilizing exfoliated and synthesized single/few layer TMDs for possible electronic and photonic applications. The performance of such devices will also necessarily depend upon the TMD layer quality. The impact of defects and impurities on device transport characteristics is of interest, as well as methods to passivate and minimize their effects. The interaction of the TMDs with component materials, such as dielectrics and contacts, is also an important aspect. This talk will present our recent work using in-situ and ex-situ methods to understand the physics and chemistry of TMDs and their associated interfaces. [Preview Abstract] |
Tuesday, March 15, 2016 3:06PM - 3:18PM |
H17.00002: Effects of High-Energy X-Ray Radiation on MoS2 FETs Amritesh Rai, Laxman Thoutam, Wei Zhang, Kiran Kovi, Sanjay Banerjee, Saptarshi Das FETs based on semiconducting MoS$_{\mathrm{2\thinspace }}$nanosheets are currently being extensively explored for various nanoelectronic device applications. In real-life, several of these applications mandate the exposure of devices to X-ray radiation. In this study, we investigate the effects of high-energy X-ray radiation on few-layer MoS$_{\mathrm{2}}$ transistors. Back-gated MoS$_{\mathrm{2}}$ FETs on SiO$_{\mathrm{2}}$ substrates were fabricated and exposed to X-ray radiation in an enclosed X-ray tube utilizing tungsten as the X-ray source. The devices were exposed to successive radiation doses up to a cumulative dose of 1500 kilorads (Krads). Even after high radiation doses, the devices maintained acceptable electrical performance with high I$_{\mathrm{ON}}$/I$_{\mathrm{OFF\thinspace }}$ratios and good current saturation. The subthreshold swing remained similar to initial values. There was, however, a slight reduction in the ON-currents after each successive radiation, concomitant with a positive threshold voltage shift that can be attributed to the formation of negative-fixed charges in the$_{\mathrm{\thinspace }}$substrate. Moreover, the maximum transconductance (g$_{\mathrm{m}})$ of the devices decreased slightly with increasing radiation dose. Finally, Raman spectroscopy revealed practically no change in the in-plane and out-of-plane Raman modes of MoS$_{\mathrm{2}}$ after radiation. [Preview Abstract] |
Tuesday, March 15, 2016 3:18PM - 3:30PM |
H17.00003: Raman shifts and in situ TEM electrical degradation of electron-irradiated monolayer MoS2. William M. Parkin, Adrian Balan, Liangbo Liang, Paul Masih Das, Michael Lamparski, Carl Naylor, Julio A. Rodriguez-Manzo, Alan T. Johnson, Vincent Meunier, Marija Drndic We report how the presence of electron-beam-induced vacancies affects first-order Raman modes and correlate this effect with the evolution of in situ TEM two-terminal conductivity of monolayer MoS$_2$ under electron irradiation. We observe a redshift in the E' Raman peak and a less pronounced blueshift in the A'$_1$ peak with increasing electron dose. Using energy-dispersive X-ray spectroscopy, we show that irradiation causes partial removal of sulfur and correlate the dependence of the Raman peak shifts with S vacancy density (a few percent), which is confirmed by first-principles density functional theory calculations. $\textit{In situ}$ device current measurements show exponential decrease in channel current upon irradiation. Our analysis demonstrates that the observed frequency shifts are intrinsic properties of the defective systems and that Raman spectroscopy can be used as a quantitative diagnostic tool to accurately characterize MoS$_2$-based transport channels. [Preview Abstract] |
Tuesday, March 15, 2016 3:30PM - 3:42PM |
H17.00004: Spin- and Valley-Polarized Transport across Line Defects in Monolayer $\mbox{\boldmath MoS$_2$}$ Artem Pulkin, Oleg Yazyev We address ballistic transmission of charge carriers across ordered line defects in monolayer transition metal dichalcogenides. Our study reveals the presence of a transport gap driven by spin-orbit interactions, spin and valley filtering, both stemming from a simple picture of spin and momentum conservation, as well as the electron-hole asymmetry of charge-carrier transmission. Electronic transport properties of experimentally observed ordered line defects in monolayer $\mbox{\boldmath MoS$_2$}$, in particular the vacancy lines and inversion domain boundaries, are further investigated using first-principles Green's function methodology. Our calculations demonstrate the possibility of achieving nearly complete spin polarization of charge carriers in nanoelectronic devices based on engineered periodic line defects in monolayer transition metal dichalcogenides, thus suggesting a novel practical scheme for all-electric control of spin transport. [Preview Abstract] |
Tuesday, March 15, 2016 3:42PM - 3:54PM |
H17.00005: \textbf{Electronic and Optical properties of Vacancy Defects in two dimensional monolayer Transition metal Dichalcogenides} Mahtab Khan, Mikhail Erementchouk, Michael Leuenberger Defects play an important role in tailoring electronic and optical properties of two-dimensional monolayer transition metal dichalcogenides (TMDCs). Recently it has been shown that the presence of vacancy defects (VDs) in two-dimensional monolayer MoS\textunderscore 2 induces localized states which give rise to extra resonance peaks in both in-plane $\chi _{\parallel } $ and out-of-plane $\chi_{\bot } $ susceptibilities.$^{1}$ In-plane $\chi_{\parallel } $ and out-of-plane $\chi_{\bot } $ susceptibilities are related to the presence of even and odd states with respect to the Mo plane, respectively$^{1}$. Moreover, monolayer TMDCs have a large spin orbit coupling (SOC), originating from d-orbitals of heavy transition metals and being of the order of a few 100 meV. We present a more general picture of the electronic and optical properties of defected monolayer TMDCs. In particular, we consider MoS$_{2}$, MoSe$_{2}$, WS$_{2}$ and WSe$_{2}$ with three types of VDs (i) Mo, W vacancy, (ii) S$_{2}$, Se$_{2}$ vacancy, and (iii) S, Se vacancy. In addition, we investigate the effects of SOC on the band structures and the optical susceptibilities of VDs in TMDCs. 1. Mikhail Erementchouk, M. A. Khan, and Michael N. Leuenberger, Phys. Rev. B 92, 121401(R) (2015). [Preview Abstract] |
Tuesday, March 15, 2016 3:54PM - 4:06PM |
H17.00006: Ab-initio study of gold nanoparticles supported on defect-laden single-layer MoS2 Takat B. Rawal, Duy Le, Talat S. Rahman We have investigated the geometry, electronic structure, and catalytic properties of gold nanoparticles on defect-laden single-layer MoS$_{2}$ using density functional theory (DFT) based calculations with semi-empirical van der Waals interaction (DFT-D3). Our results show that the two-dimensional planar structure, the most favorable one for unsupported Au$_{13}$ nanoparticle, transforms into a distorted three-dimensional (3D) structure when supported on single-layer MoS$_{2}$ with single S-vacancy which is more favorable than the icosahedral, decahedron and cuboctahedron forms. The MoS$_{2}$ support substantially alters the electronic structure of Au$_{13}$ nanoparticle near the Fermi level, owing to the strong interaction of MoS$_{2}$ support with Au$_{13}$ in the presence of an S-vacancy. The modified electronic structure remarkably affects the catalytic activity of the MoS$_{2}$-supported Au$_{13}$, offering enhanced activity towards methanol synthesis reaction via CO hydrogenation reaction - a contrast from that of titania-supported Au$_{13}$ nanoparticle [1] which promotes methanol decomposition. [1] S. Hong and T. S. Rahman, JACS, 135, 7629 (2013) [Preview Abstract] |
Tuesday, March 15, 2016 4:06PM - 4:18PM |
H17.00007: Growth and optical characterization of distorted octahedral (T') WTe$_{2}$ and MoTe$_{2}$ Shao-Yu Chen, Thomas Goldstein, Jun Yan The polymorph of transition metal dichalcogenide (TMDC) has recently attracted great attention due to its novel physical properties. We grow distorted octahedral (T') WTe$_{2}$ and MoTe$_{2}$ by chemical vapor transport and rapid thermal quenching methods. The bulk and few layer samples exhibit distinct optical properties as compared with the the well-investigated semiconducting hexagonal (H) TMDCs. We observe sharp intralayer, as well as interlayer optical phonon modes, that display angular dependent intensities consistent with the estimation by Raman tensor calculation. We also demonstrate\textit{ in-situ} phase transition of MoTe$_{2}$ from H to T' \textit{via} rapid annealing in an electrically heated microfurnace. This enables ramping of the temperature from above 900C to room temperature within seconds. This well-controlled annealing process is promising for innovative fabrication of novel 2D materials devices. [Preview Abstract] |
Tuesday, March 15, 2016 4:18PM - 4:30PM |
H17.00008: Alloy Engineering of Defect Properties in Semiconductors: Suppression of Deep Levels in Transition-Metal Dichalcogenides Bing Huang, Mina Yoon, Bobby Sumpter, Suhuai Wei, Feng Liu Developing practical approaches to effectively reduce the amount of deep defect levels in semiconductors is critical for their use in electronic and optoelectronic devices, but this still remains a very challenging task. In this talk, we propose that specific alloying can provide an effective means to suppress the deep defect levels in semiconductors while maintaining their basic electronic properties. Specifically, we demonstrate that for transition-metal dichalcogenides, such as MoSe2 and WSe2, where anion vacancies are the most abundant defects that can induce deep levels, the deep levels can be effectively suppressed in MoWSe2 alloys at low W concentrations. This surprising phenomenon is associated with the fact that the band edge energies can be substantially tuned by the global alloy concentration, whereas the defect level is controlled locally by the preferred locations of Se vacancies around W atoms. Our findings illustrate a concept of alloy engineering and provide a promising approach to control the defect properties of semiconductors. [Preview Abstract] |
Tuesday, March 15, 2016 4:30PM - 4:42PM |
H17.00009: Focused helium-ion beam irradiation effects on electrical properties of multi-layer WSe2 Pushpa Raj Pudasaini, Michael Stanford, Nick Cross, Gerd Duscher, David Mandrus, Philip Rack Atomically thin transition metal dichalcogenides (TMDs) are currently receiving great attention due to their excellent opto-electronic properties. Tuning optical and electrical properties of mono and few layers TMDs, such as Tungsten diselenide (WSe2), by controlling the defects, is an intriguing opportunity to fabricate the next generation opto-electronic devices. Here, we report the effects of focused helium ion beam irradiation on structural, optical and electrical properties of few layer WSe2, via high resolution scanning transmission electron microscopy, Raman spectroscopy and electrical measurements. By controlling the ion irradiation dose, we selectively introduced precise defects in few layer WSe2 thereby locally tuning the electrically resistivity of the material. Hole transport in the few layer WSe2 is severely affected compared to electron transport for the same dose of helium ion beam irradiation studied. Furthermore, by selectively exposing the ion beams, we demonstrate the lateral p-n junction in few layer WSe2 flakes, which constitute an important advance towards two dimensional opto-electronic devices. [Preview Abstract] |
Tuesday, March 15, 2016 4:42PM - 4:54PM |
H17.00010: Interplay of magnetic order and defect modulation in mononlayer FeSe JUNQIANG LU, Pengfei Zhang, Jian Wu We investigate the role of the defects (vacancy and anti-site) at the Fe-site on the magnetic order in monolayer FeSe. Experimental STM studies of defect states reveal that two type dumbbell-like dimers are formed at the surface of monolayer FeSe. We perform first-principles calculations of the magnetic structure of FeSe monolayer in the presence of defects in order to identify the origin of the STM observations. We consider various distribution of the defects and compare the checkerboard and collinear antiferromagnetic orders. Our results show that a single defect can give a dimer in STM image. A preliminary analysis show that both dimers are centered at the defects with their bright ends positioned on two adjacent Se atoms. We show that the two magnetic orders give rise to two distinct dimers types, in agreement with experiments. [Preview Abstract] |
Tuesday, March 15, 2016 4:54PM - 5:06PM |
H17.00011: Spatially Correlated Disorder in Epitaxial van der Waals Heterostructures nouamane laanait, zhan zhang, Christian Schleputz, Ying Liu, Michael Wojcik, Rachael Myers-Ward, D. Kurt Gaskill, Paul Fenter, Lian Li The structural cohesion of van der Waals (vdW) heterostructures relies upon a cooperative balance between strong intra-layer bonded interactions and weak inter-layer coupling. The confinement of extended defects to within a single vdW layer and competing interactions introduced by epitaxial constraints could generate fundamentally new structural disorders. Here we report on the presence of spatially correlated and localized disorder states that coexist with the near perfect crystallographic order along the growth direction of epitaxial vdW heterostructure of Bi2Se3/graphene/SiC grown by molecular beam epitaxy. With the depth penetration of hard X-ray diffraction microscopy and high-resolution surface scattering, we imaged local structural configurations from the atomic to mesoscopic length scales, and found that these disorder states result as a confluence of atomic scale modulations in the strength of vdW layer-layer interactions and nanoscale boundary conditions imposed by the substrate. These findings reveal a vast landscape of novel disorder states that can be manifested in epitaxial vdW heterostructures. [Preview Abstract] |
(Author Not Attending)
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H17.00012: Defect-induced multiphoton absorption and photoluminescence in BN for bio-imaging Mehmet Karakaya, yongchang dong, Ramakrishna Podila, Apparao Rao Emerging two-dimensional materials are known for their excellent optical properties. Boron nitride (BN) is the only 2D material which exhibits multi-photon absorption. This combined with tunable defect-induced photoluminescence in BN could be used for multi-photon bio-imaging. Previously, a two-photon absorption process was proposed for explaining non-linear optical absorption in BN. However, as discussed in this talk, we show that defects (such as C and O) in BN result in mid-gap states that enable three-photon absorption in addition to tunable emission. The non-linear optical properties of BN could be used for bio-imaging at longer wavelengths which facilitate higher penetration depth and the resolution in vivo. [Preview Abstract] |
Tuesday, March 15, 2016 5:18PM - 5:30PM |
H17.00013: Impurity-mediated early charge density wave condensation in the oxygen-adsorbed In/Si(111)-(4$\times$1)/(8$\times$2) nanowire array Stefan Wippermann, Andreas Luecke, Wolf Gero Schmidt, Deok Mahn Oh, Han Woong Yeom The self-assembled In/Si(111)-(4x1) nanowire array is an extremely popular model system for one-dimensional electronic systems and features a reversible temperature-induced phase transition into a charge density wave (CDW) ordered ground state. While impurities have been widely known to affect this phase transition, the atomistic mechanisms have rarely been elucidated. Here we present a joint experimental and \emph{first principles} study, demonstrating oxygen impurity atoms to condense the In/Si(111) nanowire array locally into its CDW ground state, even above the transition temperature. Interestingly, CDW ordering is induced only by a concerted effect of multiple impurities. The mechanism is explained as a subtle interplay between coherent superposition of local impurity-induced lattice strain, a strong coupling between electronic and lattice degrees of freedom, and phononic effects on the free energy. Funding from DFG FOR1700 is gratefully acknowledged. [Preview Abstract] |
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