Bulk direct band gap MoS2 by plasma induced layer decoupling

We report a robust method for engineering the optoelectronic properties of many-layer MoS2 using low energy oxygen plasma treatment. Gas phase treatment of MoS2 with oxygen radicals generated in an upstream N2-O2 plasma is shown to enhance the photoluminescence (PL) of many-layer, mechanically exfoliated MoS2 flakes by up to 20 times, without reducing the layer thickness of the material. A blue shift in the photoluminescence spectra and narrowing of linewidth is consistent with a transition of MoS2 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 MoS2. 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 many-layer MoS2 flakes (2–15 layers) tested, indicating this method is robust and scalable. Monolayer MoS2, 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 MoS2, without reducing sample thickness, and hence optical density.