Thermal-Assisted Vertical Electron Injections in Few-Layer Pyramidal-Structured MoS 2 Crystals.

The interlayer screening effects and charge conduction mechanisms in atomically thin two-dimensional (2D) materials are crucial for the design and engineering of novel electronics and optoelectronics devices. However, such effects remain largely unexplored in highly-crystalline chemical vapor deposition (CVD) grown molybdenum disulfide (MoS2) crystals, which are critically important for large-scale fabrication. In this work, we report the controllable CVD-grown of monolayer MoS2 and layer-by-layer pyramidal-structured MoS2 crystals with an oxidized Mo foil as source precursor. The spatial variation interlayer screening effects and charge conduction mechanisms in the pyramidal-structured MoS2 crystals are studied using an electrostatic force microscopy (EFM) and a conductive atomic-force microscopy (C-AFM), respectively. Although the Fowler-Nordheim (FN) tunneling model is widely adopted to describe the vertical charge transport mechanism at the 2D-semiconductor/bulk-metal interface, we found that such mechanism cannot satisfactorily explain the electrical measurement obtained from our CVD-grown MoS2 samples. Instead, our analysis reveals the Richardson-Schottky (RS) emission, where charge conduction is mediated by thermionic and image-potential-induced barrier lowering effects, as the dominant transport mechanism when the bias voltage is less than 1 V. Our findings provide a fundamental understanding on the charge conduction mechanism in the CVD-grown MoS2 crystals, thus paving the important first-step towards the development of novel MoS2 electronics and optoelectronics devices.