Bulk Direct Band Gap MoS$_{2}$ by Plasma Induced Layer Decoupling

We report a robust method for engineering the optoelectronic properties of few layer MoS$_{2}$ using low energy oxygen plasma treatment. Gas phase treatment of MoS$_{2}$ with an upstream N$_{2}$-O$_{2}$ plasma is shown to enhance the photoluminescence (PL) of few layer MoS$_{2}$ flakes by up to 20 times, without reducing the layer thickness. A blue shift in the photoluminescence spectra and narrowing of linewidth is consistent with a transition of MoS$_{2}$ from indirect to direct band gap material. Atomic force microscopy and Raman spectra reveal that the flake thickness actually increases as a result of the plasma treatment, indicating an increase in the interlayer separation in MoS$_{2}$. Ab-initio calculations reveal that the increased interlayer separation is sufficient to decouple the electronic states in individual layers, leading to a transition from an indirect to direct gap semiconductor. With optimized plasma treatment parameters, we observed enhanced PL signals for 32 out of 35 few layer MoS$_{2}$ flakes tested, indicating this method is robust and scalable. Monolayer MoS$_{2}$, while direct band gap, has a small optical density, which limits its potential use in practical devices. The results presented here provide a material with the direct band gap of monolayer MoS$_{2}$, without reducing sample thickness, and hence optical density.