Direct Bandgap Transition in Many‐Layer MoS 2 by Plasma‐Induced Layer Decoupling

We report a robust method for engineering the optoelectronic properties of many‐layer MoS 2 using low‐energy oxygen plasma treatment. Gas phase treatment of MoS 2 with oxygen radicals generated in an upstream N 2 –O 2 plasma is shown to enhance the photoluminescence (PL) of many‐layer, mechanically exfoliated MoS 2 flakes by up to 20 times, without reducing the layer thickness of the material. A blueshift in the PL spectra and narrowing of linewidth are consistent with a transition of MoS 2 from indirect to direct bandgap 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 many‐layer MoS 2 flakes (2–15 layers) tested, indicating that this method is robust and scalable. Monolayer MoS 2 , while direct bandgap, has a small optical density, which limits its potential use in practical devices. The results presented here provide a material with the direct bandgap of monolayer MoS 2 , without reducing sample thickness, and hence optical density.