title: Direct bandgap transition in many-layer MoS2 by plasma-induced layer decoupling.
abstract: 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 blueshift in the PL spectra and narrowing of linewidth are consistent with a transition of MoS2 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 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 that this method is robust and scalable. Monolayer MoS2 , 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 MoS2 , without reducing sample thickness, and hence optical density.
authors.affiliation: Ming Hsieh Department of Electrical Engineering, University of Southern California, Los Angeles, CA, 90089, USA; Center for Electron Microscopy and Microanalysis, University of Southern California, Los Angeles, CA, 90089, USA.
authors.forename: Rohan
authors.initials: R
authors.lastname: Dhall
authors.forename: Mahesh R
authors.initials: MR
authors.lastname: Neupane
authors.forename: Darshana
authors.initials: D
authors.lastname: Wickramaratne
authors.forename: Matthew
authors.initials: M
authors.lastname: Mecklenburg
authors.forename: Zhen
authors.initials: Z
authors.lastname: Li
authors.forename: Cameron
authors.initials: C
authors.lastname: Moore
authors.forename: Roger K
authors.initials: RK
authors.lastname: Lake
authors.forename: Stephen
authors.initials: S
authors.lastname: Cronin
keywords: density functional theory (DFT); intercalation; many-layer MoS2; photoluminescence; transition metal dichalcogenides