Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session M21: Focus Session: Beyond Graphene Devices: Function, Fabrication, and Characterization IV |
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Sponsoring Units: DMP Chair: Darshana Wickramaratne, University of California, Riverside Room: 406 |
Wednesday, March 5, 2014 11:15AM - 11:27AM |
M21.00001: First Principles Study of Electronic Properties of MoS$_{2}$/HfO$_{2}$ Interface Santosh KC, Roberto C. Longo, Robert M. Wallace, Kyeongjae Cho Monolayer MoS$_{2}$ is direct band gap two dimensional (2D) semiconductor which has been recently investigated for low-powered field effect transistors and shown promising performance of high on/off current ratio (10$^{8})$ and a carrier mobility $\sim$ 200 cm$^{2}$/Vs with a high-k gate dielectric [1]. For a detailed understanding of the MoS$_{2}$ electronic devices, it is important to examine the detailed atomic and electronic structures of the MoS$_{2}$/HfO$_{2}$ interface. We have developed a lattice matched MoS$_{2}$/HfO$_{2}$ interface model, and investigated the interface atomic structures and the corresponding electronic structures using the density functional theory (DFT) calculations. The model interface was extensively investigated as a function of oxygen and hydrogen incorporation representing different HfO$_{2}$ growth conditions on MoS$_{2}$. The interface formation energies show strong effects of interfacial oxygen content and the valence band offset. \textit{In situ} XPS study of HfO$_{2}$ ALD on MoS$_{2}$ shows that the experimental MoS$_{2}$/HfO$_{2}$ interface properties are consistent with DFT results [2]. These studies can be extended to other TMDs in an effort to identify most promising candidates for electronic device applications. \\[4pt] [1] B. Radisavljevic \textit{et}.al, \textit{Nat. Nanotechnol}. \textbf{6}, 147 (2011). \\[0pt] [2] S. McDonnell \textit{et}. al. \textit{ACS Nano}~\textbf{(}Just Accepted). [Preview Abstract] |
Wednesday, March 5, 2014 11:27AM - 11:39AM |
M21.00002: ARPES studies of transition metal dichalcogenides MoS2 and MoSe2 Guang Bian, Nasser Alidoust, Suyang Xu, Raman Sankar, Chang Liu, Ilya Belopolski, Madhab Neupane, J.D. Denlinger, F.C. Chou, M.Z. Hasan Transition metal dichalcogenides have attracted much attention recently due to their potential applications in nanoelectronics and photonics, as a result of the desirable changes in their electronic band structure upon moving from the bulk limit to few layers and monolayer limit. Here we report our high resolution angle-resolved photoemission spectroscopy study on MoS2 and MoSe2. We, for the first time, resolve the two distinct bands at the Brillouin zone corner of the bulk MoSe2. By depositing potassium on the cleaved surface of MoSe2, we demonstrate the formation of a nearly free 2D electron gas on top of MoSe2. Moreover, the electronic structure of CVD-grown monolayer MoSe2 is carefully examined by ARPES. [Preview Abstract] |
Wednesday, March 5, 2014 11:39AM - 11:51AM |
M21.00003: Electronic properties of misoriented bilayer transition metal dichalcogenides Supeng Ge, Darshana Wickramaratne, Mahesh Neupane, Shanshan Su, Roger Lake Motivated by a growing interest in vertically stacked van-der-Waal heterostructures, we explore the effect of misorientation in bilayers of MoS2, MoSe2, WS2 and WSe2 on their electronic properties. Mechanical stacking of individual monolayers or chemical and epitaxial growth of this family of layered materials often leads to misoriented interfaces between individual monolayers. Isolated monolayers of MoS2, MoSe2, WS2 and WSe2 exhibit a direct band gap between 1 - 2 eV. The band gap transitions from direct to indirect when the film thickness increases from a monolayer to a bilayer. The question we address is ``Does misorientation between semiconducting TMD bilayers electronically decouple them, as is observed in misoriented bilayers of graphene?'' Using density-functional-theory we investigate the effect of different commensurate rotation angles, stacking order and displacements on the electronic structure of these materials. The effect of these atomic structural variations on the inter-layer coupling, band gaps and effective masses is presented and compared to the equivalent monolayer and bilayer properties for each material. [Preview Abstract] |
Wednesday, March 5, 2014 11:51AM - 12:03PM |
M21.00004: Twisted MoS2 Bilayers Mohammad Gani, Yudistira Virgus, Christopher Triola, Enrico Rossi Research interest in novel two-dimensional materials has grown rapidly recently because of their potential use for electronic and spintronic applications. Two-dimensional transition metal dichalcogenides are promising compounds for these applications since they possess a bandgap and a strong spin orbit interaction. One of the dichalcogenides that has been studied extensively is MoS2. In this talk I will present the results of our theoretical study of the electronic structure of twisted MoS2 bilayers formed by two single MoS2 layers stacked with a relative twist angle. Our results suggest that the twist angle can be used effectively to tune the electronic properties of MoS2. [Preview Abstract] |
Wednesday, March 5, 2014 12:03PM - 12:15PM |
M21.00005: Dimensionality-dependent Electronic and Optical Properties of MoS$_{2}$ Georgios Kopidakis, Daphne Davelou, Aristea E. Maniadaki, George Kioseoglou, Ioannis N. Remediakis We present theoretical calculations based on Density-Functional Theory (DFT) for MoS$_{2}$, a layered material which can be shaped into single-layer and several other nanostructures with unique catalytic, mechanical, electronic and optical properties. We consider ribbons, single-layer, bilayer and bulk structures, at equilibrium and under hydrostatic strain. We calculate the electronic band structure and use linear-response theory to obtain the imaginary part of the dielectric function. Other optical properties, such as absorption and reflectivity, are also calculated. Strain changes dramatically the electronic structure, as it induces changes in the location of both the conduction band minimum and the valence band maximum. Single layer MoS2 becomes an indirect-gap semiconductor while a direct gap is observed at zero strain. The results of the simulations are in good agreement with experimental measurements of energy-dependent reflectivity and photoluminescence spectra. We compare the dielectric properties of bulk (3D), single-layer (2D) and ribbons (1D) of MoS2 and discuss general trends of the macroscopic dielectric constant as a function of dimensionality. Some closely related dichalcogenides will also be discussed. [Preview Abstract] |
Wednesday, March 5, 2014 12:15PM - 12:27PM |
M21.00006: Direct observation of the indirect to direct band gap transition in epitaxial monolayer MoSe$_{2}$ film Yi Zhang, Tay-Rong Chang, Bo Zhou, Yong-Tao Cui, Hao Yan, Zhongkai Liu, Felix Schmitt, James Lee, Rob Moore, Yulin Chen, Hsin Lin, Hong-Tay Jeng, Sung-Kwan Mo, Zahid Hussain, Arun Bansil, Zhi-Xun Shen As a class of graphene-like two-dimensional materials, the layered metal dichalcogenides MX$_{2}$ (M $=$ Mo, W; X $=$ S, Se, Te) have gained significant interest due to the indirect to direct band gap transition in monolayer. Because of this direct band gap, monolayer MX$_{2}$ is favorable for optoelectronic applications. Here we report the direct observation such band gap transition by using angle-resolved photoemission spectroscopy on high-quality thin films of MoSe$_{2}$, with variable thickness from monolayer to 8 monolayer, grown by molecular beam epitaxy. The experimental band structure indicates a stronger tendency of monolayer MoSe$_{2}$ towards direct band gap, and with larger gap size, than theoretical prediction. Moreover, we observed a significant band splitting of $\sim$ 180 meV at valence band maximum of a monolayer MoSe$_{2}$, which was theoretically predicted to be 100{\%} spin-polarized. This spin signature gives the layered MoSe$_{2}$ great application potential in spintronic devices, as well as a new playground to investigate spin-obit physics beyond the topological insulators. [Preview Abstract] |
Wednesday, March 5, 2014 12:27PM - 12:39PM |
M21.00007: Optical pump-THz probe measurements of self-assembled h-BN/G heterostructures Bala Murali Krishna M, Catherine C, T. Harada, Soumya V, J. Taha-Tijerrina, P. Nguyen, P. Chang, P.M. Ajayan, N.T. Narayanan, K.M. Dani Two dimensional materials have attracted significant interest in recent times due to properties like large electron mobility, extreme thermal conductivity and high young's modulus. The potential of combining different two-dimensional materials to form new heterostructures with new functionality offers intriguing possibilities. Here, we study the opto-electronic properties of new types of solids consisting of randomly stacked layers of hexagonal boron nitride (h-BN) and graphene (G). We prepare these artificially stacked h-BN/G solids with different ratios of h-BN and G by mixing dispersions of exfoliated h-BN layers and graphene in different concentrations and allowing the exfoliated flakes to form the h-BN/G solids via van der Waals interaction. We study the ultrafast photocarrier dynamics in these solids by pumping with femtosecond visible-near-infrared pulses of light, and probing the transient photo-conductivity with sub-picosecond Terahertz pulses. As we tune the ratio of h-BN and G in the new h-BN/G solids, we not only observe opto-electronic properties that tune from the insulating h-BN phase to semi-metallic G phase, but we also see unique behavior, distinct from either phase, for certain h-BN/G ratios in between the two extreme phases. [Preview Abstract] |
Wednesday, March 5, 2014 12:39PM - 12:51PM |
M21.00008: Electronic and Optical Properties of Atomically Thin PbI$_{2}$ Crystals Alexis Toulouse, Benjamin Isaacoff, Guangsha Shi, Marie Matuchov\'{a}, Emmanouil Kioupakis, Roberto Merlin Layered materials with weak inter-layer van der Waals bonds such as PbI$_{2}$ are of interest for the novel properties they can exhibit as their thickness is reduced to the monolayer limit. We present the results of a joint experimental and theoretical study of the optical and electronic properties of atomically thin samples of PbI$_{2}$. First-principles calculations based on density functional and many-body perturbation theory were performed for the electronic, excitonic, and optical properties of mono and few-layer structures. These results are compared with emission data from photoluminescence experiments performed on mechanically exfoliated samples ranging from bulk to a few monolayers. Our results show that despite a significant increase in the electronic band gap due to quantum confinement in ultrathin samples, the optical gap, defined by excitonic effects, remains unaffected by quantum confinement until its dimensions are reduced to one monolayer. Computational resources were provided by the DOE NERSC facility. [Preview Abstract] |
Wednesday, March 5, 2014 12:51PM - 1:03PM |
M21.00009: Coupling of MoS$_{2}$ thin films with different substrates probed by temperature dependent Raman Spectroscopy Liqin Su, Yong Zhang, Yifei Yu, Linyou Cao Few-layer MoS$_{2}$ is emerging as a new 2-D material beyond graphene, showing a number of interesting properties that could lead to applications in optoelectronics. In most if not all real applications, the MoS$_{2}$ films are expected to be supported by substrates, thus the film-substrate coupling is inevitable. In this work, we study the temperature-dependent Raman shifts of both in-plane (E$_{\mathrm{2g}}^{1})$ and out-of-plane (A$_{\mathrm{1g}})$ phonon modes for single-layer and bi-layer MoS$_{2}$ films on different substrates in a temperature range of 25 -- 500 $^{\circ}$C. By investigating the temperature dependence of Raman scattering, we show that, with increasing temperature, the chemical bonding between film and substrate introduces a damping to E$_{\mathrm{2g}}^{1}$ Raman temperature shift for the MoS$_{2}$ thin-film grown on sapphire by CVD, while the changes in the film morphology leads to significant nonlinear effects for the A$_{\mathrm{1g}}$ mode, such as nonlinear sometimes even non-monotonic temperature shift of Raman frequency and temperature dependence of Raman linewidth, for the transferred MoS$_{2}$ thin-film on SiO$_{2}$/Si substrate. [Preview Abstract] |
Wednesday, March 5, 2014 1:03PM - 1:15PM |
M21.00010: Tuning excitons in monolayer and few-layer MoS$_{2}$ Diana Y. Qiu, Felipe H. da Jornada, Steven G. Louie Our recent ab initio GW-BSE calculations showed that monolayer MoS$_{2}$ is a computationally challenging system, requiring a large number of empty bands and very fine k-point sampling to converge its quasiparticle band structure and optical properties. Careful convergence of a GW-BSE calculation reveals that MoS$_{2}$ has a large number of bound excitons with varying k-space characteristics. Specifically, there are two series of excitons: a low-energy series with k-space wavefunctions localized at the K/K$'$ valleys in the Brillouin zone and a higher energy series localized in a ring around the $\Gamma$ point. There is very little hybridization between these two exciton series in monolayer MoS$_{2}$, but changes in electronic structure and screening due to additional layers, strain, or doping can lead to changes in exciton binding energies, character, and hybridization. Thus, we have carried out ab initio GW-BSE calculations to study the excitonic properties of few-layer MoS$_{2}$. We find that layering and straining MoS$_{2}$ systematically changes the exciton binding energies, the peak positions and amount of absorbance in the optical spectrum, and the character and hybridization of the excitons near $\Gamma$. [Preview Abstract] |
Wednesday, March 5, 2014 1:15PM - 1:27PM |
M21.00011: Control of Two-Dimensional Excitonic Light Emission via Photonic Crystal Sanfeng Wu, Sonia Buckley, Aaron Jones, Jason Ross, Nirmal Ghimire, Jiaqiang Yan, David Mandrus, Wang Yao, Fariba Hatami, Jelena Vuckovic, Arka Majumdar, Xiaodong Xu Monolayers of transition metal dichalcogenides (TMDCs) exhibit many novel and outstanding photonics and optoelectronic behaviors in the two dimensional (2D) limit, such as rich spin-valley interplays, tunable excitonic effects, and strong light-matter interactions. Excitonic light emission is essential for many of these novelties and potential applications. However, the manipulation of its light emission is still undeveloped. Here we demonstrate the control of excitonic light emission from monolayer tungsten diselenide (WSe2) in an integrated photonic structure, achieved by transferring one monolayer onto a photonic crystal (PhC) with nanocavity. A greatly enhanced ($\sim$ 60 times) photoluminescence of WSe2 and an effectively coupled cavity-mode emission is observed in such systems. More importantly, we are able to redistribute the emitted photons in both polar and azimuthal directions in the far field through designing PhC structures. A 2D optical antenna is thus constructed in our hybids. Our work suggests a new way of manipulating photons in hybrid 2D photonics, important for future energy efficient optoelectronics and 2D nano-lasers. [Preview Abstract] |
Wednesday, March 5, 2014 1:27PM - 1:39PM |
M21.00012: Electron-Hole Asymmetry in WS$_2$ Revealed by Scanning Photocurrent Microscopy under Ionic-Liquid Gating Nicolas Ubrig, Sanghyun Jo, Helmuth Berger, Alberto F. Morpurgo, Alexey B. Kuzmenko We perform scanning photocurrent microscopy on WS$_2$-based ambipolar ionic liquid-gated field effect transistors with almost ideal transport characteristics. Both in the electron- and the hole-doping regimes, the photocurrent decays exponentially as a function of the distance between electrical contacts and the illumination spot, in agreement with a two-terminal Schottky-barrier device model. This allows us to compare the value and the doping dependence of the diffusion length of the minority electrons and holes on the same sample. Interestingly, the diffusion length of the minority electrons is several times larger than the one of the minority holes at the same doping concentration, which points to a strong intrinsic electron-hole asymmetry. [Preview Abstract] |
Wednesday, March 5, 2014 1:39PM - 1:51PM |
M21.00013: Electron-Phonon Coupling and Photoluminescence in monolayer MoS$_{2}$ Neha Nayyar, Volodymyr Turkowski, Duy Le, Talat Rahman We have carried out first principles calculations of the photoluminescent properties of monolayer MoS$_{2}$ using density functional theory. In particular, we have analyzed the role of electron-phonon interactions in the photoluminescence process. Phonon dispersion curves calculated using density functional perturbation theory served as the basis for the evaluation of the system electron-phonon coupling, which in turn was used to calculate electron self-energy and the electron spectral function within the Eliashberg approach. We find that the resulting photoemission spectrum is in good agreement with experimental data. We pay special attention to the ultrafast relaxation of the electron system as manifested by the electron-phonon coupling and evaluate the ultrafast photoluminescence of the excited system by using the two-temperature model. It is shown that similar to graphene, MoS$_{2}$ may demonstrate significant ultrafast photoluminescence. [Preview Abstract] |
Wednesday, March 5, 2014 1:51PM - 2:03PM |
M21.00014: Novel Exciton States in Monolayer MoS$_2$: Unconventional Effective Hamiltonian Felipe da Jornada, Diana Qiu, Steven Louie Recent well-converged ab inito GW-BSE calculations show that monolayer MoS$_2$ has a large number of strongly bound excitons with varying characters. We show that these excitonic states cannot be even qualitatively described by an effective mass hydrogenic model without a detailed understanding of the 2D screening. Additionally, we identify and analyze one exciton series having an unusually high binding energy, which originates around the $\Gamma$ point of the Brillouin zone. We show that this excitonic series arises from a fundamentally different effective Hamiltonian with a kinetic energy term resembling a Mexican hat in momentum space, which is responsible for the unusual ordering of the energy levels and distribution of oscillator strength. This work was supported by NSF grant No. DMR10-1006184 and the U.S. DOE under Contract No. DE-AC02-05CH11231. [Preview Abstract] |
Wednesday, March 5, 2014 2:03PM - 2:15PM |
M21.00015: Wavefunction Properties and Electronic Band Structures of High-Mobility Semiconductor Nanosheet MoS$_{2}$ Seung Su Baik, Hee Sung Lee, Seongil Im, Hyoung Joon Choi Molybdenum disulfide (MoS$_{2})$ nanosheet is regarded as one of the most promising alternatives to the current semiconductors due to its significant band-gap and electron-mobility enhancement upon exfoliating. To elucidate such thickness-dependent properties, we have studied the electronic band structures of bulk and monolayer MoS$_{2}$ by using the first-principles density-functional method as implemented in the SIESTA code. Based on the wavefunction analyses at the conduction band minimum (CBM) points, we have investigated possible origins of mobility difference between bulk and monolayer MoS$_{2}$. We provide formation energies of substitutional impurities at the Mo and S sites, and discuss feasible electron sources which may induce a significant difference in the carrier lifetime. This work was supported by NRF of Korea (Grant Nos. 2009-0079462 and 2011-0018306), Nano-Material Technology Development Program (2012M3a7B4034985), and KISTI supercomputing center (Project No. KSC-2013-C3-008). [Preview Abstract] |
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