Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session C8: Complex Structured Materials: Transport and Optical Characterization of Dichalcogenides |
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Sponsoring Units: DMP Chair: Nuh Gedik, Massachusetts Institute of Technology Room: 307 |
Monday, March 18, 2013 2:30PM - 2:42PM |
C8.00001: Transport Measurements of Multi-terminal MoS2 Devices Y. Yang, H.O.H. Churchill, B.W.H. Baugher, P. Jarillo-Herrero We report progress on the fabrication and measurement of multi-terminal devices based on few-layer MoS2. By using different contact metal recipes, we describe efforts to significantly decrease contact resistance and gain access to the intrinsic transport properties of MoS2. We measured four-terminal resistance of monolayer, bilayer, and trilayer MoS2 with Ohmic contacts to obtain the intrinsic field-effect mobility of these materials on SiO2 substrates at temperatures down to 4 K. We also probed Hall transport of MoS2 and extracted the temperature dependence of its Hall mobility. [Preview Abstract] |
Monday, March 18, 2013 2:42PM - 2:54PM |
C8.00002: High mobility ambipolar MoS$_{2}$ field-effect transistors Wenzhong Bao, Xinghan Cai, Do Hun Kim, Michael Fuhrer Unlike graphene, single and multilayer MoS2 have a 1-1.8eV band gap, which makes MoS2 an promising candidate for future semiconducting industry. However many groups have observed poor charge carrier mobility for thin MoS$_{2}$ crystals deposited on silicon dioxide substrates. Here we report on MoS$_{2}$ field effect transistors on SiO$_{2}$ and on polymethyl methacrylate (PMMA) dielectric. We measure the conductivity in a four-probe configuration as a funcation of carrier density controlled by the back gate electrode. For multilayer MoS$_{2}$ on SiO$_{2}$, the mobility is on order 10-60 cm$^{2}$/Vs, and independent of thickness (5-80 nm), and most devices exhibit unipolar n-type behavior. In contrast, multilayer MoS$_{2}$ on PMMA shows mobility increasing with thickness, up to 500cm$^{2}$/Vs (electrons) and 400 cm$^{2}$/Vs (holes) at thickness $\sim$50 nm. We observe activated temperature dependence of the resistance consistent with optical phonon scattering-limited resistance in the highest mobility devices. The dependence of the mobility on thickness for thicknesses up to 70 nm is unexpected, and points to a long-range dielectric effect of the bulk MoS$_{2}$ in increasing mobility. [Preview Abstract] |
Monday, March 18, 2013 2:54PM - 3:06PM |
C8.00003: The effect of the dielectric environment on electrical and optical properties of monolayer molybdenum disulfide Dhiraj Prasai, Alex Wynn, A.K.M. Newaz, Kirill Bolotin Monolayer molybdenum disulfide (MoS$_{2})$ is a two-dimensional atomic crystal characterized by a direct band gap, strong electron-electron and spin-orbit interactions. Electron transport in currently available monolayer MoS$_{2}$ devices is dominated by strong Coulomb scattering limiting carrier mobility to \textless\ 200 cm$^{2}$/Vs. Here, we explore possible routes towards increasing carrier mobility in MoS$_{2}$. First, we investigate suspended ($\sim$200nm above Si/SiO$_{2})$ monolayer MoS$_{2}$ devices by combining electron beam lithography and an isotropic sacrificial etching of the underlying substrate. Second, we explore the mobility of MoS$_{2}$ devices fabricated on highly uniform hexagonal boron nitride (h-BN) crystals as a substrate material. Initial results indicate an order of magnitude increase in the electrical mobility using both approaches. Finally, we study MoS$_{2}$ devices embedded in a dielectric material with high dielectric constant and explore the interrelation between carrier mobility and dielectric constant. [Preview Abstract] |
Monday, March 18, 2013 3:06PM - 3:18PM |
C8.00004: Phonon Softening and Bandgap Engineering in Strained Monolayer MoS$_2$ Hiram Conley, Kirill Bolotin By straining monolayer MoS$_2$ with a 4 point bending apparatus, both phonon softening and a shrinking band gap were observed. Raman spectrum demonstrates phonon softening for both bi and single layer MoS$_2$ flakes, with a breaking of the E$^1_{2g}$ degeneracy at large strain. Photoluminescence data shows that the band gap of single layer MoS$_2$ decreases by ~50 meV per \% strain. The direct band gap of bilayer MoS$_2$ decreases by the same rate as for monolayer MoS$_2$ while the indirect band gap of bilayer MoS$_2$ decrease by ~120 meV \% strain. This work clearly demonstrates that MoS$_2$'s band gap and phonons are tunable by strain engineering suggesting a possibility of devices with mechanically tunable optical and electrical properties. [Preview Abstract] |
Monday, March 18, 2013 3:18PM - 3:30PM |
C8.00005: Electrical Control of Optical Properties of a Two Dimensional Material, Monolayer Molybdenum Disulfide (MoS$_2$) A.K.M. Newaz, D. Prasai, J.I. Ziegler, D. Caudel, S. Robinson, R.F. Haglund, K.I. Bolotin Materials with electrically controllable optical properties are long sought for uses in diverse applications ranging from electro-optical modulators to display screens. Here we demonstrate electrical control of photoluminescence quantum yield and absorption coefficient in the visible range for a different two-dimensional crystal, monolayer molybdenum disulfide (MoS$_2$). We investigate electrical gating of photoluminescence and optical absorption in monolayer MoS$_2$ configured in field effect transistor geometry. We observe an hundredfold increase in photoluminescence intensity and an increase in absorption at $\sim 660$ nm in these devices when an external gate voltage is decreased from +50 V to -50 V, while the photoluminescence wavelength remains nearly constant. In contrast, in bilayer MoS$_2$ devices we observe almost no changes in photoluminescence with gate voltage. We propose that the differing responses of the monolayer and bilayer devices are related to the interaction of the excitons in MoS$_2$ with charge carriers. [Preview Abstract] |
Monday, March 18, 2013 3:30PM - 3:42PM |
C8.00006: Excited-state interactions in monolayer MoS$_{2}$/graphene heterostructures Cyrielle Roquelet, Heather Hill, Arend van der Zande, Fan Zhang, James Hone, Louis E. Brus, Tony F. Heinz Recent progress in the formation of atomically thin 2-dimensional crystals by mechanical exfoliation and other synthetic techniques has led to the availability and study of various 2D materials other than graphene. Among them, molybdenum disulfide (MoS$_{2})$ has attracted particular attention. Although an indirect-gap material in the bulk, MoS$_{2}$ exhibits a direct gap in its monolayer form. Correspondingly, the material exhibits strong photoluminescence (PL), very sensitive to the environment. With the development of transfer techniques, it is now possible to create stacks of differing atomically thin materials. In this paper we apply this to investigate the influence of adjacent graphene layers on the PL of MoS$_{2}$ monolayers. Comparing the PL from MoS$_{2}$ on graphene with reference samples, we find that graphene induces strong quenching. Raman measurements of the graphene do not indicate the presence of any significant static charge transfer between layers. This suggests that the graphene layer provides efficient relaxation channels for the photoexcited MoS$_{2}$, rather than modifying its intrinsic properties. In this context, we discuss the relative contributions to PL quenching arising from excited-state charge and energy transfer processes. [Preview Abstract] |
Monday, March 18, 2013 3:42PM - 3:54PM |
C8.00007: Ultrafast laser spectroscopy of exciton dynamics in CVD-grown monolayer MoS$_2$ Edbert Jarvis Sie, Yihua Wang, Yi-Hsien Lee, Jing Kong, Nuh Gedik Recently, much effort has been devoted to the spin-valley interplay in exfoliated monolayer MoS$_2$, yet the many-body interactions in this material are largely unexplored. In fact, monolayer MoS$_2$ offers a special platform in the study of many-body effects owing to its 2D nature with a large band gap and a giant exciton binding energy. Here, we use ultrafast laser spectroscopy to study the exciton dynamics of CVD-grown monolayer MoS$_2$. We observed a strongly non-linear fluence dependent behavior which indicates presence of many-body interactions in this material. [Preview Abstract] |
Monday, March 18, 2013 3:54PM - 4:06PM |
C8.00008: MoS$_{2}$ Field Effect Transistors with different polarity: study of electrode work functions Isha Dube, Anthony K. Boyd, Marcio Fontana, Igor Gayduchenko, Georgy Fedorov, Amy Liu, Makarand Paranjape, Paola Barbara The transfer characteristics of Molybdenum disulfide (MoS$_{2})$ field effect transistors (FETs) depend on the Schottky barrier formed between the metal electrode and the semiconducting MoS$_{2}$. We obtained p-type behavior for Pd-contacted MoS$_{2}$ FETs and n-type with both Au and Nb [1] contacts. We study the work function of these electrode metals to understand their effect on the Schottky barrier and therefore the polarity of the MoS$_{2}$ FETs. The work function of the above metals is measured using a non-contact Kelvin Probe technique under different ambient conditions. We will discuss the observed n-type and p-type behavior of MoS$_{2}$ FETs in relation to the measured metal work functions.\\[4pt] [1] M. Fontana, T. Deppe, A. Boyd, M. Rinzan, A. Liu, M. Paranjape, P. Barbara, Photovoltaic effect in gated MoS2 Schottky junctions, in, arXiv:1206.6125v1 [cond-mat.mtrl-sci] [Preview Abstract] |
Monday, March 18, 2013 4:06PM - 4:18PM |
C8.00009: Electronic Transport through Grain Boundaries in Monolayer Molybdenum Disulfide Grown by Chemical Vapor Deposition Daniel Chenet, Arend van der Zande, Pinshane Huang, Yumeng You, Timothy Berkelbach, Gwan-Hyoung Lee, David Reichman, David Muller, Tony Heinz, James Hone Monolayer molybdenum disulfide is a new direct bandgap semiconductor that has recently received significant attention for its potential utility in two-dimensional electronics. Recent advances in the large-area synthesis of this material by chemical vapor deposition are accelerating the device concept to realization process. However, little is currently known about the effect of growth defects on electronic transport in this material. Here, we have optimized the synthesis process to grow large single crystals up to 120 $\mu $m in size with electrical and optical properties comparable or superior to that of exfoliated samples. When these single crystals grow together to form large continuous sheets, the inevitable consequence is the formation of grain boundaries that should have different electrical properties than the bulk. With our ability to rapidly identify well-faceted single crystals and the boundaries between them by optical microscopy, we fabricate field effect transistors to measure the effects of individual grain boundaries on channel conductivity and mobility. [Preview Abstract] |
Monday, March 18, 2013 4:18PM - 4:30PM |
C8.00010: Electron-hole transport and photovoltaic effect in gated MoS$_{2}$ Schottky junctions Anthony Boyd, Marcio Fontana, Tristan Deppe, Mohamed Rinzan, Amy Liu, Makarand Paranjape, Paola Barbara Atomically thin molybdenum disulfide has emerged as an attractive material for novel nanoscale optoelectronic devices due to its reduced dimensionality and large direct bandgap. Since optoelectronic devices require electron-hole generation/recombination, it is important to be able to fabricate ambipolar transistors to investigate charge transport both in the conduction band and in the valence band. Although $n$-type transistor operation for single-layer and few-layer MoS$_{2}$ with gold source and drain contacts was recently demonstrated..., transport in the valence band has been elusive for solid-state devices. Here we show that a multi-layer MoS$_{2}$ channel can be hole-doped by palladium contacts, yielding MoS$_{2}$ $p$-type transistors [1]. When two different materials are used for the source and drain contacts, for example hole-doping Pd and electron-doping Au, the Schottky junctions formed at the MoS$_{2}$ contacts produce a clear photovoltaic effect [1]. \\[4pt] [1] M. Fontana, T. Deppe, A. Boyd, M. Rinzan, A. Liu, M. Paranjape, and P. Barbara, \textit{Photovoltaic effect in gated MoS}$_{2}$ \textit{Schottky junctions}, in, arXiv:1206.6125v1 [cond-mat.mtrl-sci] [Preview Abstract] |
Monday, March 18, 2013 4:30PM - 4:42PM |
C8.00011: Electrical control of truly two-dimensional neutral and charged excitons in monolayer MoSe$_2$ Jason Ross, Sanfeng Wu, Hongyi Yu, Nirmal Ghimire, Aaron Jones, Grant Aivazian, Jiaqiang Yan, David Mandrus, Di Xiao, Di Xiao, Xiaodong Xu Monolayer transition metal dichalcogenides (TMDs) have emerged as ideal 2D semiconductors with valley and spin polarized excitations expected to enable true valley-tronics. Here we investigate MoSe$_{2}$, a TMD which has yet to be characterized in the monolayer limit. Specifically, we examine excitons and trions (their singly charged counterparts) in the ultimate 2D limit. Utilizing high quality exfoliated MoSe$_{2}$ monolayers, we report the observation and electrostatic tunability of positively charged (X$+)$, neutral (Xo), and negatively charged (X-) excitons via photoluminescence in FETs. The trion charging energy is large (30 meV), enhanced by strong confinement and heavy effective masses, while the linewidth is narrow (5 meV) at temperatures below 55 K. This is greater spectral contrast than in any known quasi-2D system. Further, the charging energies for X$+$ and X- to are nearly identical implying the same effective mass for electrons and holes, which supports their recent description as massive Dirac fermions. This work demonstrates that monolayer MoSe$_{2}$ is an ultimate 2D semiconductor opening the door for the investigation of truly 2D exciton physics while laying the ground work necessary to begin valley-spin polarization studies. [Preview Abstract] |
Monday, March 18, 2013 4:42PM - 4:54PM |
C8.00012: The influence of composition and mechanical strain on the optoelectronic properties of transition-metal dichalcogenide monolayers Ashwin Ramasubramaniam Single and few-layer transition-metal dichalcogenides (TMDs) are of significant current interest for nanoscale optoelectronic applications. While these materials have been well characterized in their bulk form, a comprehensive understanding of their properties at the nanoscale is still emerging. We present studies of the quasiparticle band structures and optical properties of MoS$_{\mathrm{2}}$, MoSe$_{\mathrm{2}}$, MoTe$_{\mathrm{2}}$, WS$_{\mathrm{2}}$, and WSe$_{\mathrm{2}}$ monolayers using the GW approximation in conjunction with the Bethe-Salpeter equation (BSE). The inclusion of two-particle excitations in the BSE approach reveals the presence of two strongly bound excitons (A and B) below the quasiparticle absorption onset arising from vertical transitions between a spin-orbit-split valence band and the conduction band. The transition energies for monolayer MoS$_{\mathrm{2}}$, in particular, are shown to be in excellent agreement with available experiments. Excitation energies for the remaining monolayers are predicted to lie in the range of 1--2 eV. Systematic trends are identified for band gaps, transition energies, and exciton binding energies within as well as across the Mo and W families of dichalcogenides. Finally, we study the influence of homogeneous strains on the optoelectronic properties of TMD monolayers and demonstrate the potential for facile tuning of electronic and optical band gaps. Overall, the results suggest that quantum confinement of carriers within monolayers can be exploited in conjunction with chemical composition and mechanical strains to widely tune the optoelectronic properties of TMDs at the nanoscale. [Preview Abstract] |
Monday, March 18, 2013 4:54PM - 5:06PM |
C8.00013: Photoluminescence mapping of grain boundaries in CVD-grown MoS$_{2}$ monolayers YuMeng You, Arend van der Zande, Daniel Chenet, Pinshane Huang, James Hone, David Muller, Tony Heinz Monolayer MoS$_{2}$ is an atomically thin 2-D material with a direct energy gap. Recently, rapid progress has been made in the growth of this material by chemical vapor deposition (CVD). Here we apply photoluminescence (PL) mapping to study monolayer MoS$_{2}$ samples prepared by CVD. For appropriate growth conditions, MoS$_{2}$ monolayers can be grown that exhibit well-defined boundaries between different crystal domains. Using electron microscopy, we have identified boundaries between crystals of different orientation and between mirror-twin crystals. PL mapping has been found to permit the ready identification of both of these boundaries through shifts in the strength and energy of the emission peaks. This sensitivity renders PL imaging a convenient tool for the identification of grain boundaries that remain hidden in conventional optical microscopy. The strong structural modification of material at a grain boundary extends for only around 1 nm. Thus only slight variation in the PL might be anticipated, given the excitation laser spot size of around 500 nm. We will discuss the possible physical origins of the strong contrast observed in the PL maps, including the role of exciton diffusion to the grain boundaries. [Preview Abstract] |
Monday, March 18, 2013 5:06PM - 5:18PM |
C8.00014: Metal Contacts on Semiconducting Two-Dimension Crystals Han Liu, Adam Neal, Yuchen Du, Peide Ye Semiconducting 2-D crystals, such as MoS$_{2}$, WSe$_{2}$, are viewed as promising candidates for electronic applications for their high carrier mobility, thermal stability, compatibility to CMOS process, and superior immunity to short channel effects. However, with the difficulty in ion implantation, the metal contacts on 2-D crystals are yet with large contact resistance, thus eliminates further device performance. We study different metal contacts from low work function to high work function metals on MoS$_{2}$ and WSe$_{2}$ crystals with various thicknesses and discuss the Fermi level pinning at the metal/semiconductor interface. Effective Schottky Barrier Heights (SBHs) are also measured. Molecular doping and dual-side contacts metals are performed as two tentative solutions to reduce the effective SBHs, and high-performance of field effect transistors are achieved by reduced contact resistance. [Preview Abstract] |
Monday, March 18, 2013 5:18PM - 5:30PM |
C8.00015: Electronic properties of bilayer mixtures of WS2 and MoS2 with different stackings Humberto Terrones, Ana Laura Elias, Nestor Perea-Lopez, Humberto R. Gutierrez, Ayse Berkdemir, Andres Castro-Beltran, Ruitao Lv, Florentino Lopez-Urias, Takuya Hayashi, Yoong Ahm Kim, Morinobu Endo, Mauricio Terrones Besides graphene and hexagonal boron nitride, transition metal chalcogenides (TMC) such as MoS2, WS2, NbS2 and WSe2 also exhibit a layered structure in which the layers weakly interact via Van der Waals forces, and for this reason these materials exhibit excellent lubrication properties. For TMC, the layers are formed by the transition metal atom sandwiched by the sulfur atoms. MoS2 and WS2 in bulk are indirect band gap semiconducting materials. However, an isolated sheet of MoS2 or WS2 becomes a direct gap semiconductor. This particular behavior makes them very attractive in terms of optical properties such as spin polarization, in which the lack of center of inversion of one layer plays a crucial role. Therefore, it is important to study the properties of different configurations of WS2 and MoS2 mixtures bi-layer TMC systems with different stackings. First principles calculations are carried out to study how the indirect and the direct gaps behave, thus shedding light in a new type of bi-layered material. [Preview Abstract] |
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