MoS2 Transition Metal Dichalcogenides Research Articles

Intrinsic Electronic Transport Properties of High-Quality Monolayer and Bilayer MoS2

We report electronic transport measurements of devices based on monolayers and bilayers of the transition-metal dichalcogenide MoS2. Through a combination of in situ vacuum annealing and electrostatic gating we obtained ohmic contact to the MoS2 down to 4 K at high carrier densities. At lower carrier densities, low-temperature four probe transport measurements show a metal-insulator transition in both monolayer and bilayer samples. In the metallic regime, the high-temperature behavior of the mobility showed strong temperature dependence consistent with phonon-dominated transport. At low temperature, intrinsic field-effect mobilities approaching 1000 cm(2)/(V·s) were observed for both monolayer and bilayer devices. Mobilities extracted from Hall effect measurements were several times lower and showed a strong dependence on density, likely caused by screening of charged impurity scattering at higher densities.

Low-Frequency Electronic Noise in Single-Layer MoS2 Transistors

Ubiquitous low-frequency 1/f noise can be a limiting factor in the performance and application of nanoscale devices. Here, we quantitatively investigate low-frequency electronic noise in single-layer transition metal dichalcogenide MoS2 field-effect transistors. The measured 1/f noise can be explained by an empirical formulation of mobility fluctuations with the Hooge parameter ranging between 0.005 and 2.0 in vacuum (<10(-5) Torr). The field-effect mobility decreased, and the noise amplitude increased by an order of magnitude in ambient conditions, revealing the significant influence of atmospheric adsorbates on charge transport. In addition, single Lorentzian generation-recombination noise was observed to increase by an order of magnitude as the devices were cooled from 300 to 6.5 K.

Realization and electrical characterization of ultrathin crystals of layered transition-metal dichalcogenides

Ultrathin crystals of the layered transition-metal dichalcogenide MoS2 (semiconducting) and TaS2 (metallic) were obtained by mechanical peeling or chemical exfoliation techniques and electrically contacted using electron-beam lithography. The MoS2 devices showed high field-effect mobility in the tens of cm2∕Vs and an on/off ratio higher than 105. The TaS2 devices remained metallic despite the fabrication process and showed an enhancement of the superconducting transition temperature and disappearance of the charge density wave phase anomaly at low temperature.

Inorganic solution-processed hole-injecting and electron-blocking layers in polymer light-emitting diodes

The use of the solution-processed layered transition metal dichalcogenide (LTMDC) MoS2 as a hole-injecting electrode in polymer light-emitting diodes (LEDs) is reported. MoS2 functions as a very high work function metal and, in combination with an electron-blocking layer in the form of MoO3, provides good LED performance. In this study we investigated model LED devices with a single semiconductor layer, namely, the electron transporting polymer poly-[2,7-(9,9′-di-n-octylfluorene)-3,6-benzothiadiazole]. LED operation was successfully modeled using experimentally determined work functions, carrier mobilities, and barrier properties. Good agreement between experiment and model allows us to demonstrate that the MoS2 and the MoO3 layers act as a high work function hole-injection layer (MoS2) and an electron extraction barrier layer (MoO3), respectively. They improve device performance by allowing the buildup of electron density at the oxide/emissive layer interface which generates a local field, enhancing hole injection and recombination. Furthermore, the model shows the importance of controlling the thickness of the blocking layer to optimize device performance. The wide variety of polymeric emitters available and the range of electronic properties displayed by the LTMDC family and their corresponding oxides, provides the potential to tailor device performance through the selection of suitable organic and inorganic components.

Surface structures of compound semiconductors

A survey is given of surface-structure determinations for the low index faces of compound semiconductors via analyses of elastic low-energy electron diffraction intensities. The (100) face of (cubic-NaCl structure) MgO is thought to exhibit a surface structure that differs little from that of a truncated bulk crystal. Small deviations from the truncated bulk geometry have been reported for the layered transition-metal dichalcogenides MoS2 and NbSe2. The cleavage faces of tetrahedrally coordinated compound semiconductors [i.e., zincblende (110), wurtzite (101̄0), and wurtzite (112̄0)] exhibit the same space-group symmetry as the bulk crystal. The positions of the species in the uppermost atomic layer may be altered substantially from their bulk values, however, with the anion shifted outward, the cation inward, and the whole layer relaxed toward the bulk crystal. For the materials examined to date [i.e.,GaAs(110), ZnSe (110), ZnO(101̄0) and ZnO(112̄0)] the shifts in atomic position vary in magnitude but can exert profound effects on the electronic structure and chemical behavior of the associated surfaces.

Abstract: Surface structure determination of the layered compounds MoS2 and NbSe2 by the dynamical LEED approach

The surface structures of 2H polytypes of the layered transition metal dichalcogenides MoS2 and NbSe2 are studied by analyzing LEED intensity–energy (IV) data using the dynamical approach.1 We investigate contraction and expansion of the top interplanar and intersandwich distances and the stacking sequence at the surface of these compounds. Using a convergent layer-iteration method, LEED intensity profiles are calculated and compared to experimental data. Excellent agreement in the IV profiles is obtained between theory and experiment. From the comparison, we find that 2H–NbSe2, which has metallic properties, maintains the bulk spacing up to the surface plane. The semiconductor 2H–MoS2, on the other hand, exhibits a slight contraction of the top interplanar spacing of 5% or less. It is also found that lateral reconstructions that alter the bulk stacking sequence at the surface do not occur for these two compounds.