Owing to the small energy differences between its polymorphs, MoTe2 can access a full spectrum of electronic states, from the 2H semiconducting state to the 1T′ semimetallic state, and from the Td Weyl semimetallic state to the superconducting state in the 1T′ and Td phase at low temperature. Thus, it is a model system for phase transformation studies as well as quantum phenomena such as the quantum spin Hall effect and topological superconductivity. Careful studies of MoTe2 and its potential applications require large-area MoTe2 thin films with high crystallinity and thickness control. Here, we present cm 2 -scale synthesis of 2H-MoTe2 thin films with layer control and large grains that span several microns. Layer control is achieved by controlling the initial thickness of the precursor MoOx thin films, which are deposited on sapphire substrates by atomic layer deposition and subsequently tellurized. Despite the van der Waals epitaxy, the precursor-substrate interface is found to critically determine the uniformity in thickness and grain size of the resulting MoTe2 films: MoTe2 grown on sapphire show uniform films while MoTe2 grown on amorphous SiO2 substrates form islands. This synthesis strategy decouples the layer control from the variabilities of growth conditions for robust growth results, and is applicable to grow other transition metal dichalcogenides with layer control.
Charge transport in the wide band gap (Al,In)N/GaN heterostructures with high carrier density ~2 10 were investigated over a large range of temperature (280 mK < T < 280 K) and magnetic field (0 < B < 20 T). We observe the first evidences of weak localization in the two-dimensional electron gas in this system. From the Shubnikov-de Haas (SdH) oscillations a relatively light effective mass of ~0.23m e is determined. Furthermore the linear dependence with temperature (T < 20 K) of the inelastic scattering rate () is attributed to the phase breaking by electron-electron scattering. Also in the same temperature range the less than unit ratio of quantum lifetime than Hall transport time ⁄ 1 is taken to signify the dominance of small angle scattering. Above 20 K, with increasing temperature scattering changes from acoustic phonon to optical phonon scattering, resulting in a rapid decrease in carrier mobility and increase in sheet resistance. Suppression of such scatterings will lead to higher mobility and a way forward to high power and high frequency electronics. I.
We report electrical transport properties of carbon nanostructures with close-packed spherical voids. Under zero magnetic field, a non-metallic behavior is observed. With increasing magnetic field, magnetoresistance (MR) crosses over from quadratic to linear dependence. Longitudinal response, typically negligible in most materials, exhibits the same value and field-temperature dependence as transverse MR. At intermediate angles (0°–90°) MR is also found to be independent of the direction of magnetic field. It is reasoned that orientation-insensitive, linear MR is due to distorted current flow in the 3-dimensional porous structures of this system.
The optical absorption spectrums of nanomotors made from double-wall carbon nanotubes have been calculated with the time-dependent density functional based tight binding method. When the outer short tube of the nanomotor moves along or rotates around the inner long tube, the peaks in the spectrum will gradually evolve and may shift periodically, the amplitude of which can be as large as hundreds of meV. We show that the features and behaviors of the optical absorption spectrum could be used to monitor the mechanical motions of the double-wall carbon nanotube based nanomotor.
Quantum transport properties in monolayer graphene are sensitive to structural modifications. We find that the introduction of a hexagonal lattice of antidots has a wide impact on weak localization and Shubnikov-de Haas (SdH) oscillation of graphene. The antidot lattice reduces both phase coherence and intervalley scattering length. Remarkably, even with softened intervalley scattering, i.e., the phase-breaking time is shorter than intervalley scattering time, coherence between time reversed states remains adequate to retain weak localization, an offbeat and rarely reported occurrence. Whereas SdH oscillation is boosted by the antidot lattice, the amplitude of the SdH signal rises rapidly with the increasing antidot radius. But both effective mass and carrier density are reduced in a larger antidot lattice. A bandgap of ∼10 meV is opened. The antidot lattice is an effective dopant-free way to manipulate electronic properties in graphene.
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