Single-layer Molybdenum Disulfide Research Articles

Low-Resistance Contacts for Single-Layered MoS2 n-FETs By Hydrogen Plasma

Achieving low Schottky barriers interface is important for future electronic devices based on transition metal dichalcogenides. In this study, we demonstrate that the electronic properties of single-layer molybdenum disulfide (MoS2) can be tuned from semiconducting to metallic via controlling its defect level (sulfur vacancy) by using hydrogen plasma. Raman, Photoluminescence, Atomic Force Microscopy, and X-ray photoelectron spectra suggest that the top layer sulfur in SL-MoS2 can be totally removed and stabilized in ambient environment. Field-effect transistors (FETs) with highly sulfur vacancies/Ti electrodes fabricated and exhibited high performance n-FETs. The results provide a promising route for low Schottky barrier contacts devices with graded junction interface.

Atomically thin optical lenses and gratings.

Two-dimensional (2D) materials have emerged as promising candidates for miniaturized optoelectronic devices due to their strong inelastic interactions with light. On the other hand, a miniaturized optical system also requires strong elastic light–matter interactions to control the flow of light. Here we report that a single-layer molybdenum disulfide (MoS2) has a giant optical path length (OPL), around one order of magnitude larger than that from a single-layer of graphene. Using such giant OPL to engineer the phase front of optical beams we have demonstrated, to the best of our knowledge, the world’s thinnest optical lens consisting of a few layers of MoS2 less than 6.3 nm thick. By taking advantage of the giant elastic scattering efficiency in ultra-thin high-index 2D materials, we also demonstrated high-efficiency gratings based on a single- or few-layers of MoS2. The capability of manipulating the flow of light in 2D materials opens an exciting avenue towards unprecedented miniaturization of optical components and the integration of advanced optical functionalities. More importantly, the unique and large tunability of the refractive index by electric field in layered MoS2 will enable various applications in electrically tunable atomically thin optical components, such as micro-lenses with electrically tunable focal lengths, electrical tunable phase shifters with ultra-high accuracy, which cannot be realized by conventional bulk solids.

Molecular dynamics simulation of effects of temperature and chirality on the mechanical properties of single-layer molybdenum disulfide

Recently, the effect of temperature on the mechanical property (the Young's modulus) of the single-layer molybdenum disulfide (SLMoS2) is shown to be insignificant, which is obviously incompatible with the previously published result, i. e. the Young's modulus of SLMOS2 decreases monotonically as temperature increases. Aiming at clarifying the relationships between the mechanical properties of the single-layer molybdenum disulfide (SLMoS2) along the armchair (AC) and zigzag (ZZ) directions and the temperature, classical molecular dynamics (MD) simulations are performed to stretch the SLMoS2 along the AC and ZZ directions at the temperatures ranging from 1 K to 800 K by using the Stillinger-Weber (SW) interatomic potentials in this paper. The mechanical properties of SLMoS2 at the temperatures ranging from 1 K to 800 K, including ultimate strength, ultimate strain, and Young's Modulus, are calculated based on the stress-strain results obtained from the simulations. Results are obtained and given as follows. (1) The mechanical properties of the SLMoS2, including the ultimate strength and Young's modulus, are found to monotonically decrease as temperature increases. Increasing the temperature, the ultimate strength of SLMoS2 in the AC direction drops faster than in the ZZ direction, whereas the Young's modulus of SLMoS2 in the ZZ direction decreases quicker than in the AC direction, which means that the chirality effect on the ultimate strength is remarkably different from the Young's modulus of SLMoS2. However, the ultimate strain in the ZZ direction at the temperatures in a range from 1 K to 800 K is close to that in the AC direction, which means that the effect of chirality on the ultimate strain is insignificant. (2) Unlike the published results in the literature, the phase transition of SLMoS2 is found to only occur at a temperature of 1 K and at the moment of initial crack formation as tensiled along the ZZ direction, and the new phase of quadrilateral structure keeps stable after unloading. (3) The linear thermal expansion coefficients along the ZZ and AC directions are also measured, the magnitudes of which are found to be consistent with the published experimental results. Our simulation results support the viewpoint that the effect of the temperature on the mechanical property of SLMoS2 is significant, and the SLMoS2 can be regarded as an anisotropic material as the chirality effect cannot be ignored. The linear thermal expansion coefficients obtained with MD simulation are all in good agreement with the experimental data.

Unusual isotope effect on thermal transport of single layer molybdenum disulphide

Thermal transport in single layer molybdenum disulfide (MoS2) is critical to advancing its applications. In this paper, we use molecular dynamics simulations with first-principles force constants to study the isotope effect on the thermal transport of single layer MoS2. Through phonon modal analysis, we found that isotopes can strongly scatter phonons with intermediate frequencies, and the scattering behavior can be radically different from that predicted by conventional scattering model based on perturbation theory, where Tamura's formula is combined with Matthiessen's rule to include isotope effects. Such a discrepancy becomes smaller for low isotope concentrations. Natural isotopes can lead to a 30% reduction in thermal conductivity for large size samples. However, for small samples where boundary scattering becomes significant, the isotope effect can be greatly suppressed. It was also found that the Mo isotopes, which contribute more to the phonon eigenvectors in the intermediate frequency range, have stronger impact on thermal conductivity than S isotopes.

In site preparation of Pd(II)–MoS2 complex: A new high-efficiency catalyst for alkenylation of heteroaromatics by direct C[sbnd]H bond activation

Pd(II)–MoS2 catalyst was designed by affording Pd2+ salt on the surface of single-layered molybdenum disulfide nanosheets.. Based on the dispersible template, this metal salt and molybdenum disulfide complex offered several advantages, including easy preparation, low cost, and capability to prepare excellent dispersible metal ion cluster nanostructures. The application of this complex in alkenylation of heteroaromatics was reported. Direct CH activation of indoles, furans and thiophenes was carried out to combine with several kinds of terminal olefins. This catalyst also performed high efficiency in several other kinds of coupling reactions. Thus, a universal suitable catalyst for CH activation and CC bond formation was invented by a convenient in situ preparation.

Water desalination with a single-layer MoS2 nanopore.

Efficient desalination of water continues to be a problem facing the society. Advances in nanotechnology have led to the development of a variety of nanoporous membranes for water purification. Here we show, by performing molecular dynamics simulations, that a nanopore in a single-layer molybdenum disulfide can effectively reject ions and allow transport of water at a high rate. More than 88% of ions are rejected by membranes having pore areas ranging from 20 to 60 Å2. Water flux is found to be two to five orders of magnitude greater than that of other known nanoporous membranes. Pore chemistry is shown to play a significant role in modulating the water flux. Pores with only molybdenum atoms on their edges lead to higher fluxes, which are ∼70% greater than that of graphene nanopores. These observations are explained by permeation coefficients, energy barriers, water density and velocity distributions in the pores.

Hot Electron-Based Near-Infrared Photodetection Using Bilayer MoS2.

Recently, there has been much interest in the extraction of hot electrons generated from surface plasmon decay, as this process can be used to achieve additional bandwidth for both photodetectors and photovoltaics. Hot electrons are typically injected into semiconductors over a Schottky barrier between the metal and semiconductor, enabling generation of photocurrent with below bandgap photon illumination. As a two-dimensional semiconductor single and few layer molybdenum disulfide (MoS2) has been demonstrated to exhibit internal photogain and therefore becomes an attractive hot electron acceptor. Here, we investigate hot electron-based photodetection in a device consisting of bilayer MoS2 integrated with a plasmonic antenna array. We demonstrate sub-bandgap photocurrent originating from the injection of hot electrons into MoS2 as well as photoamplification that yields a photogain of 10(5). The large photogain results in a photoresponsivity of 5.2 A/W at 1070 nm, which is far above similar silicon-based hot electron photodetectors in which no photoamplification is present. This technique is expected to have potential use in future ultracompact near-infrared photodetection and optical memory devices.

Charge Mediated Semiconducting-to-Metallic Phase Transition in Molybdenum Disulfide Monolayer and Hydrogen Evolution Reaction in New 1T′ Phase

The phase transition of single layer molybdenum disulfide (MoS2) from semiconducting 2H to metallic 1T and then to 1T′ phases, and the effect of the phase transition on hydrogen evolution reaction (HER) are investigated within this work by density functional theory. Experimentally, 2H-MoS2 has been widely used as an excellent electrode for HER and can get charged easily. Here we find that the negative charge has a significant impact on the structural phase transition in a MoS2 monolayer. The thermodynamic stability of 1T-MoS2 increases with the negative charge state, comparing with the 2H-MoS2 structure before phase transition and the kinetic energy barrier for a phase transition from 2H to 1T decreases from 1.59 to 0.27 eV when 4e– are injected per MoS2 unit. Additionally, 1T phase is found to transform into the distorted structure (1T′ phase) spontaneously. On their activity toward hydrogen evolution reaction, 1T′-MoS2 structure shows comparable hydrogen evolution reaction activity to the 2H-MoS2 struct...

Effect of disorder on Raman scattering of single-layerMoS2

We determine the effect of defects induced by ion bombardment on the Raman spectrum of single-layer molybdenum disulfide. The evolution of both the linewidths and frequency shifts of the first-order Raman bands with the density of defects is explained with a phonon confinement model, using density functional theory to calculate the phonon dispersion curves. We identify several defect-induced Raman scattering peaks arising from zone-edge phonon modes. Among these, the most prominent is the $\mathrm{LA}(M)$ peak at $\ensuremath{\sim}227\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$ and its intensity, relative to the one of first-order Raman bands, is found to be proportional to the density of defects. These results provide a practical route to quantify defects in single-layer $\mathrm{Mo}{\mathrm{S}}_{2}$ using Raman spectroscopy and highlight an analogy between the $\mathrm{LA}(M)$ peak in $\mathrm{Mo}{\mathrm{S}}_{2}$ and the $D$ peak in graphene.

Single Layer Molybdenum Disulfide under Direct Out-of-Plane Compression: Low-Stress Band-Gap Engineering.

Tuning the electronic structure of 2D materials is a very powerful asset toward tailoring their properties to suit the demands of future applications in optoelectronics. Strain engineering is one of the most promising methods in this regard. We demonstrate that even very small out-of-plane axial compression readily modifies the electronic structure of monolayer MoS2. As we show through in situ resonant and nonresonant Raman spectroscopy and photoluminescence measurements combined with theoretical calculations, the transition from direct to indirect band gap semiconductor takes place at ∼0.5 GPa, and the transition to a semimetal occurs at stress smaller than 3 GPa.

(Invited) Anti-Ambipolar, Gate-Tunable p-n Heterojunctions

The recent emergence of two-dimensional (2D) materials has enabled the realization of atomically thin heterostructure devices [1]. Of particular interest are heterojunctions based on p-type and n-type semiconductors since these devices possess non-linear, gate-tunable transport characteristics. Here, we demonstrate a gate-tunable p-n heterojunction diode using thin films of semiconducting single-walled carbon nanotubes (s-SWCNTs) and single-layer molybdenum disulfide (SL-MoS2) as p-type and n-type semiconductors, respectively [2]. The vertical stacking of these two direct band gap semiconductors forms a heterojunction with electrical characteristics that can be tuned with an applied gate bias over a wide range of charge transport behavior ranging from insulating to rectifying with forward-to-reverse bias current ratios exceeding 104. The transfer characteristic of this p-n heterojunction shows a unique 'anti-ambipolar' behavior with two off-states at either extreme of the gate voltage range and a current maximum in the middle. The continuous transition from positive to negative transconductance in an anti-ambipolar characteristic enables operation of analog communication circuits with a reduced number of circuit components compared to unipolar transistors. Anti-ambipolarity can be widely generalized to heterojunctions of other materials such as s-SWCNTs and n-type amorphous indium gallium zinc oxide (a-IGZO), ultimately leading to all solution processed p-n heterojunctions at the wafer scale [3]. [1] D. Jariwala, V. K. Sangwan, L. J. Lauhon, T. J. Marks, and M. C. Hersam, "Emerging device applications for semiconducting two-dimensional transition metal dichalcogenides," ACS Nano, 8, 1102 (2014). [2] D. Jariwala, V. K. Sangwan, C.-C. Wu, P. L. Prabhumirashi, M. L. Geier, T. J. Marks, L. J. Lauhon, and M. C. Hersam, "Gate-tunable carbon nanotube-MoS2 heterojunction p-n diode," Proc. Nat. Acad. Sci. USA, 110, 18076 (2013). [3] D. Jariwala, V. K. Sangwan, J.-W. T. Seo, W. Xu, J. Smith, C. H. Kim, L. J. Lauhon, T. J. Marks, and M. C. Hersam, "Large-area, low-voltage, anti-ambipolar heterojunctions from solution-processed semiconductors," submitted, 2014.

Well-constructed single-layer molybdenum disulfide nanorose cross-linked by three dimensional-reduced graphene oxide network for superior water splitting and lithium storage property.

A facile one-step solution reaction route for growth of novel MoS2 nanorose cross-linked by 3D rGO network, in which the MoS2 nanorose is constructed by single-layered or few-layered MoS2 nanosheets, is presented. Due to the 3D assembled hierarchical architecture of the ultrathin MoS2 nanosheets and the interconnection of 3D rGO network, as well as the synergetic effects of MoS2 and rGO, the as-prepared MoS2-NR/rGO nanohybrids delivered high specific capacity, excellent cycling and good rate performance when evaluated as an anode material for lithium-ion batteries. Moreover, the nanohybrids also show excellent hydrogen-evolution catalytic activity and durability in an acidic medium, which is superior to MoS2 nanorose and their nanoparticles counterparts.

Anisotropic bending behaviors and bending induced buckling in single-layered black phosphorus

The bending behavior of single-layered black phosphorus was systematically studied using density functional theory. Graphene, containing a single layer of molybdenum disulfide, was studied by considering changes in the bending strain energies as a function of the radius of the curvature. Bending-induced buckling in single-layered black phosphorus and its bent states were found along a zigzag direction as shown by the critical radius of the curvature. Combing the bending strain energies from first-principle calculations of the corresponding single-wall nanotubes of grapheme, single-layered black phosphorus and single-layered molybdenum disulfide with the strain energy densities using pure bending plate theory and by the introduction of a simple equivalent atomic area, the bending stiffnesses of three kinds of two-dimensional crystals were calculated and found to be in good agreement with experimental and empirical results. We also show that the bending stiffness is 5.336 eV under zigzag bending and 1.251 eV under armchair bending in single-layered black phosphorus. Apparent anisotropic bending behavior was found where high bending stiffness under zigzag bending leads to a buckling phenomenon. With insight into the physical mechanism of the anisotropic bending behavior, electron density distributions in the single-layered black phosphorus and the corresponding armchair (11,0) and zigzag (0,15) black phosphorus nanotubes with a similar curvature radius were further analyzed. We show that the anisotropic bending behavior can be attributed to repulsive phosphorus lone pair interactions, which are stronger under zigzag bending compared with armchair bending. When a critical curvature radius is attained, the hybridized sp 3 orbitals in single-layered black phosphorus will be affected by the lone pairs according to Pauli's exclusion principle. The local equilibrium conditions of the electron clouds are thus disrupted and bending-induced buckling finally occurs.

Rippling, buckling, and melting of single- and multilayerMoS2

Large-scale atomistic simulations using the reactive empirical bond order force field approach is implemented to investigate thermal and mechanical properties of single-layer (SL) and multi-layer (ML) molybdenum disulfide (MoS$_2$). The amplitude of the intrinsic ripples of SL-MoS$_2$ are found to be smaller than those exhibited by graphene (GE). Furthermore, because of the van der Waals interaction between layers, the out-of-plane thermal fluctuations of ML-MoS$_2$ decreases rapidly with increasing number of layers. This trend is confirmed by the buckling transition due to uniaxial stress which occurs for a significantly larger applied tension as compared to graphene. For SL-MoS$_2$, the melting temperature is estimated to be 3700~K which occurs through dimerization followed by the formation of small molecules consisting of 2 to 5 atoms. When different types of vacancies are inserted in the SL-MoS$_2$ it results in a decrease of both the melting temperature as well as the stiffness.

Van der Waals screening by single-layer graphene and molybdenum disulfide.

A sharp tip of atomic force microscope is employed to probe van der Waals forces of a silicon oxide substrate with adhered graphene. Experimental results obtained in the range of distances from 3 to 20 nm indicate that single-, double-, and triple-layer graphenes screen the van der Waals forces of the substrate. Fluorination of graphene, which makes it electrically insulating, lifts the screening in the single-layer graphene. The van der Waals force from graphene determined per layer decreases with the number of layers. In addition, increased hole doping of graphene increases the force. Finally, we also demonstrate screening of the van der Waals forces of the silicon oxide substrate by single- and double-layer molybdenum disulfide.

Integrated nanophotonics based on nanowire plasmons and atomically thin material

The continually increasing demands for higher-speed and lower-operating-power devices have resulted in the continued impetus to shrink photonic components. We demonstrate a primitive nanophotonic integrated circuit element composed of a single silver nanowire and single-layer molybdenum disulfide (MoS2) flake. Using scanning confocal fluorescence microscopy and spectroscopy, we find that nanowire plasmons can excite MoS2 photoluminescence and that MoS2 excitons can decay into nanowire plasmons. Finally, we show that the nanowire may serve the dual purpose of both exciting MoS2 photoluminescence via plasmons and recollecting the decaying exciton as nanowire plasmons. The potential for subwavelength light guiding and strong nanoscale light–matter interaction afforded by our device may facilitate compact and efficient on-chip optical processing.

Vapor Phase Synthesis of Conducting Polymer Nanocomposites Incorporating 2D Nanoparticles

The one step fabrication of nanocomposite films of conducting polymers with 2D nanoparticles is investigated in this study. Specifically, the inclusion of nanomaterials (single layer graphene, single layer molybdenum disulfide) within PEDOT is achieved using the vapor phase polymerization (VPP) technique. This facile process allows for the formation of thin films of the order of less than 200 nm, which display a wide range of enhanced properties (mechanical, optical, and electrochemical). Herein, in a typical example with added graphene (<0.003% w/w), the in-plane modulus of the film is increased to 145 GPa (ca. 65% increase above PEDOT–Tos) without any decrease in light transmission or lowering of conductivity. Furthermore, the nanocomposite outperforms both the PEDOT–Tos film and a Pt substrate in the reduction of oxygen when acting as an air-electrode.

Postgrowth Tuning of the Bandgap of Single-Layer Molybdenum Disulfide Films by Sulfur/Selenium Exchange

We demonstrate bandgap tuning of a single-layer MoS2 film on SiO2/Si via substitution of its sulfur atoms by selenium through a process of gentle sputtering, exposure to a selenium precursor, and annealing. We characterize the substitution process both for S/S and S/Se replacement. Photoluminescence and, in the latter case, X-ray photoelectron spectroscopy provide direct evidence of optical band gap shift and selenium incorporation, respectively. We discuss our experimental observations, including the limit of the achievable bandgap shift, in terms of the role of stress in the film as elucidated by computational studies, based on density functional theory. The resultant films are stable in vacuum, but deteriorate under optical excitation in air.

The effect of the substrate on the Raman and photoluminescence emission of single-layer MoS2

We quantitatively study the Raman and photoluminescence (PL) emission from single-layer molybdenum disulfide (MoS2) on dielectric (SiO2, hexagonal boron nitride, mica and the polymeric dielectric Gel-Film®) and conducting substrates (Au and few-layer graphene). We find that the substrate can affect the Raman and PL emission in a twofold manner. First, the absorption and emission intensities are strongly modulated by the constructive/destructive interference within the different substrates. Second, the position of the A1g Raman mode peak and the spectral weight between neutral and charged excitons in the PL spectra are modified by the substrate. We attribute this effect to substrate-induced changes in the doping level and in the decay rates of the excitonic transitions. Our results provide a method to quantitatively study the Raman and PL emission from MoS2-based vertical heterostructures and represent the first step in ad hoc tuning the PL emission of 1L MoS2 by selecting the proper substrate.

Electrical transport and low-frequency noise in chemical vapor deposited single-layer MoS2 devices

We have studied temperature-dependent (77–300 K) electrical characteristics and low-frequency noise (LFN) in chemical vapor deposited (CVD) single-layer molybdenum disulfide (MoS2) based back-gated field-effect transistors (FETs). Electrical characterization and LFN measurements were conducted on MoS2 FETs with Al2O3 top-surface passivation. We also studied the effect of top-surface passivation etching on the electrical characteristics of the device. Significant decrease in channel current and transconductance was observed in these devices after the Al2O3 passivation etching. For passivated devices, the two-terminal resistance variation with temperature showed a good fit to the activation energy model, whereas for the etched devices the trend indicated a hopping transport mechanism. A significant increase in the normalized drain current noise power spectral density (PSD) was observed after the etching of the top passivation layer. The observed channel current noise was explained using a standard unified model incorporating carrier number fluctuation and correlated surface mobility fluctuation mechanisms. Detailed analysis of the gate-referred noise voltage PSD indicated the presence of different trapping states in passivated devices when compared to the etched devices. Etched devices showed weak temperature dependence of the channel current noise, whereas passivated devices exhibited near-linear temperature dependence.