RESUMO
The phase stability and phase transition of transition metal dichalcogenide (TMD) monolayer materials have attracted tremendous attention due to their attractive diverse potential applications. Here, first-principles calculations based on density-functional theory are carried out to study the newly synthesized MoTe2 monolayer. A phase different from the semiconducting trigonal prismatic structure and octahedral coordinated structure is found to be stable at room temperature in a free standing state, as evidenced by phonon spectrum analysis and molecular dynamic simulation. Raman vibrations of all the possible phases are calculated to provide additional information for the distinction of different phases in the experiment.
RESUMO
Two-dimensional layered transition-metal dichalcogenides have attracted considerable interest for their unique layer-number-dependent properties. In particular, vertical integration of these two-dimensional crystals to form van der Waals heterostructures can open up a new dimension for the design of functional electronic and optoelectronic devices. Here we report the layer-number-dependent photocurrent generation in graphene/MoS2/graphene heterostructures by creating a device with two distinct regions containing one-layer and seven-layer MoS2 to exclude other extrinsic factors. Photoresponse studies reveal that photoresponsivity in one-layer MoS2 is surprisingly higher than that in seven-layer MoS2 by seven times. Spectral-dependent studies further show that the internal quantum efficiency in one-layer MoS2 can reach a maximum of 65%, far higher than the 7% in seven-layer MoS2. Our theoretical modelling shows that asymmetric potential barriers in the top and bottom interfaces of the graphene/one-layer MoS2/graphene heterojunction enable asymmetric carrier tunnelling, to generate usually high photoresponsivity in one-layer MoS2 device.