Thickness-dependent electrical conductivities and ohmic contacts in transition metal dichalcogenides multilayers

We report on the observation of the substantial thickness (t)-dependent electrical conductivity (sigma) at a wide thickness range for an MoSe2 layer semiconductor. The conductivity increases for more than two orders of magnitude from 4.6 to 1500 Omega(-1) cm(-1) with a decrease in thickness from 2700 to 6 nm. The conductivity was found to follow a nearly linear relationship with the reciprocal thickness, i.e. sigma proportional to 1/t. The temperature-dependent conductivity measurements also show that the MoSe2 multilayers have much lower activation energies at 3.5-8.5 meV than those (36-38 meV) of their bulk counterparts, indicating the different origins of the majority carrier. These results imply the presence of higher surface conductivity or carrier surface accumulation in this layer crystal. The fabrication of ohmic contacts for the MoSe2 layer nanocrystals using the focused-ion beam (FIB) technique was also demonstrated. This study provides a new understanding which is crucial for the development of flexible electronic devices and transparent conducting materials using ultrathin dichalcogenide layer materials.