Photoluminescence enhancement in monolayer molybdenum disulfide by annealing in air

Abstract

Monolayer molybdenum disulfide is a novel two-dimensional material beyond graphene. It is a direct band gap semiconductor with excellent electrical and optical properties, promising wide application in nanoelectronics and optoelectronics, thus has drawn much attention recently. In this paper, we investigate the enhancement of photoluminescence of monolayer molybdenum disulfide by annealing in air. Monolayer molybdenum disulfide samples were prepared by mechanical exfoliation and chemical vapor deposition with molybdenum oxide and sulfur as sources, and argon as carrier gas. We found that air annealing for several minutes can distinctly enhance the photoluminescence intensity of A exciton by an order of magnitude, which is much better than annealing in argon. The blue shift of A exciton peak is observed after air annealing for all the monolayer molybdenum disulfide samples prepared by different methods above. We also found that this phenomenon widely exists in samples with different substrates like silicon dioxide and sapphire. Electrical transport measurements were carried out and indicate that the carrier mobility of monolayer molybdenum disulfide is largely reduced after annealing in air, which might mean the formation of considerable defects. This phenomenon is believed to be due to the doping effect caused by adsorption of oxygen, which is bonded to the defects after annealing. As acceptors, these oxygen dopants change the distribution of trions and neutral excitons, resulting in less tritons but more neutral excitons. Neutral excitons yield much higher quantum efficiency. As a comparison, we carried out control experiments in which annealing was in NH3 atmosphere. We found that photoluminescence is red shifted and can be quenched. This research results give a simple and effective method to enhance the photoluminescence of monolayer molybdenum disulfide.