Conductive and photoconductive AFM on exfoliated Mos2

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 optoelectrical device applications of M0S 2 requires a metal junction. Hence, a complete understanding of electronic band alignments and the Schottky barrier heights governing the transport through TMD-metal junction is critical. How ever it is unclear how energy band of different TM D layer aligns while in contact with a metal. In pursuit of removing this knowledge gap: we have studied conducting atomic force microscopy (CAFM) of atomically thin layered M 0S2 (1-5 layers) immobilized on atomically flat conducting Au surfaces (RMS surface roughness 0.2 nm) and indium tin oxide (ITO) substrate (RMS surface roughness 0 7 nm) forming a vertical metal (conductive-tip)-semiconductor-metal device. W e have observed that the current increases as the number of layers increases up to 5. 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 electron affinity (barrier height) increases (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. Using the lock-in technique, we are able to separate the DC current due to the constant applied voltage from the photocurrent (oscillating with modulation frequency ~9.8 kHz) due to the illumination. W e illuminated the samples using a laser (480 nm) with various intensities/voltage bias and observed the photocurrent responses of the samples under different conditions. Our study provides a fundamental understanding of the local electronic and optoelectronic behaviors of TMD-metal junction that are dependent on layer numbers and may pave the way toward developing nanoscale electronic devices with tailored properties of different layers.