  Transition metal dichalcogenides (TMDs) are layered semiconducting van der
Waal crystals and promising materials for a wide range of electronic and
optoelectronic devices. Realizing practical electrical and optoelectronic
device applications requires a connection between a metal junction and a TMD
semiconductor. Hence, a complete understanding of electronic band alignments
and the potential barrier heights governing the transport through a
metal-TMD-metal junction is critical. But, there is a knowledge gap; it is not
clear 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 (1-5)
MoS2 immobilized on ultra-flat conducting Au surfaces (root mean square (RMS)
surface roughness <0.2 nm) and indium tin oxide (ITO) substrate (RMS surface
roughness <0.7 nm) forming a vertical metal (conductive-AFM
tip)-semiconductor-metal device. We have observed that the current increases as
the number of layers increases up to 5 layers. By applying Fowler-Nordheim
tunneling theory, we have determined the barrier heights for different layers
and observed that the barrier height decreases as the number of layers
increases. Using density functional theory (DFT) calculation, we successfully
demonstrated that the barrier height decreases as the layer number increases.
By illuminating the TMDs on a transparent ultra-flat 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 TMD-metal junction, and may pave an avenue toward developing nanoscale
electronic devices with tailored layer-dependent transport properties.
