Layer-Dependent Interfacial Transport and Optoelectrical Properties of MoS2 on Ultraflat Metals.

Layered materials based on transition-metal dichalcogenides (TMDs) are promising for a wide range of electronic and optoelectronic devices. Realizing such practical applications often requires metal-TMD connections or contacts. Hence, a complete understanding of electronic band alignments and potential barrier heights governing the transport through metal-TMD junctions is critical. However, it is presently unclear how the energy bands of a TMD align while in contact with a metal as a function of the number of layers. In pursuit of removing this knowledge gap, we have performed conductive atomic force microscopy (CAFM) of few-layered (1 to 5 layers) MoS2 immobilized on ultraflat conducting Au surfaces [root-mean-square (rms) surface roughness < 0.2 nm] and indium-tin oxide (ITO) substrates (rms surface roughness < 0.7 nm) forming a vertical metal (CAFM tip)-semiconductor-metal device. We have observed that the current increases with the number of layers up to five layers. By applying Fowler-Nordheim tunneling theory, we have determined the barrier heights for different layers and observed how this barrier decreases as the number of layers increases. Using density functional theory calculations, we successfully demonstrated that the barrier height decreases as the layer number increases. By illuminating TMDs on a transparent ultraflat conducting ITO substrate, we observed a reduction in current when compared to the current measured in the dark, hence demonstrating negative photoconductivity. Our study provides a fundamental understanding of the local electronic and optoelectronic behaviors of the TMD-metal junction, which depends on the numbers of TMD layers and may pave an avenue toward developing nanoscale electronic devices with tailored layer-dependent transport properties.