Graphene has become one of the most interesting nanomaterials in the past 7 years because of its unique physical properties, which originate from low dimensionality. [ 1 , 2 ] From the point of 2D conductors, other layered nanomaterials including transition metal dichalcogenides (TMDs) are also expected to have great potential in nanoelectronics, sensing, and energy harvesting. When compared with the poor mobility of organic semiconductors, semiconducting TMD compounds were reported to have higher mobilities, [ 3 ] enlightening the wide area of the applications of organic semiconductors. As the layered TMDs can be made into ultrathin fi lms with structural fl exibility, thin-fi lm devices such as thin-fi lm transistors (TFTs), sensors, and diodes can be envisaged. Furthermore, the layered structures allow ionic or molecular intercalation, suggesting possible applications in ion batteries, [ 4 ] hydrogen storage, [ 5 ]

Electrical characteristics of molybdenum disulfide flakes produced by liquid exfoliation.

This work was supported by MICINN/MINECO (Spain) through the programs MAT2011-25046 and CONSOLIDER-INGENIO-2010 “Nanociencia Molecular” CSD-2007-00010, Comunidad de Madrid through program Nanobiomagnet S2009/MAT-1726 and the European Union (FP7) through the programs RODIN and ELFOS. E.C. acknowledges a Marie Curie Grant, PIEF-GA-2009-251904.

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Electric-Field Screening in Atomically Thin Layers of MoS2: the Role of Interlayer Coupling

The electrical conductivity and Hall coefficient perpendicular to the c-axis of hexagonal MoS2, MoSe2, and WSe2 are measured over the temperature ranges 120 to 1170 K and 140 to 820 K, respectively. The derived temperature-dependent carrier densities are described by n ∼ exp (–E/kT) for temperatures less than 800 K indicating (thermal) energy gaps of 0.38 eV in n-type MoS2, 0.27 eV in n-type MoSe2, and 0.19 eV in p-type WSe2. Above room temperature the carrier mobilities are given by μ ∼ T−α, where α ≈ 1.5 in MoS2, MoSe2 and 2.7 in WSe2. The high-temperature (>800 K) thermal activation energies of conduction correspond to energy gaps of 1.27, 0.95, and 1.33 eV in MoS2, MoSe2, and WSe2, respectively. Similar measurements on MoTe2 crystals grown by bromine vapour transport demonstrate the effects of included bromine on the low-temperature conduction processes.

Temperature dependence of the electrical conductivity and hall coefficient in 2H‐MoS2, MoSe2, WSe2, and MoTe2

The article is a review of band-structure calculations for transition metal compounds (carbides and nitrides, layer compounds, A15 structures, and others).

Band structure of transition metal compounds

Nanoparticles of zinc ferrite (ZnFe2O4) prepared by both wet- and dry-high-energy ball milling (HEBM), have been investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), atomic force microscopy (AFM), surface area and pore size distribution (BET) and wavelength-dependent diffuse reflectance and scattering turned into absorption coefficient estimation using the Kubelka–Munk theory. It was found that after 72 h of HEBM, the particle size was decreased from 220 nm for the initial material to 16.5 nm and 9.4 nm for the wet- and dry-milled samples, respectively. The optical absorption analysis revealed that the energy gap is increased (blue shift) for wet-milled and decreased (“anomalous” red shift) for dry-milled samples of ZnFe2O4 as the particle size decreased. The likely origins for the phenomena, namely the blue and red shifts as large as 0.45 eV and 0.14 eV, respectively, are proposed and discussed, and the effects are quantitatively estimated.

Optical absorption red and blue shifts in ZnFe2O4 nanoparticles

In the layer structure MoS2, the oscillatory inter-band magneto-optic absorption has revealed a mass ratio of the order of 0.01. Theory is presented to show that this mass may be associated with the H-point of the Brillouin zone. Moreover, it is proposed that the origin of the mass is an instability of the equally-spaced layer arrangement giving rise to a distorted form with alternate layers displaced in the axial direction.

http://iopscience.iop.org/article/10.1088/0022-3719/7/7/017/pdf

P G Harper

Papers

Anomalous effective mass in MoS2