Electrical control of excitons and trions in MoS 2 monolayer and bilayer crystals

Ultrathin crystals based on transition metal dichalcogenides form a new fascinating class of materials. Large direct band gaps allow for optoelectronic device concepts, the sub-nanometer thickness is attractive for ultrathin flexible electronics and the unique band structure could evoke new components for spintronics or valleytronics [1]. MoS2 flakes that can be realized easily by mechanical exfoliation have seen intensive research during the last couple of years, yet main aspects are still unclear. A central question concerns the possibility to manipulate the emission properties by e.g. electric fields. Indeed, charged excitons (trions) that are well known from semiconductor quantum dots could be observed in electrically contacted flakes recently up to room temperature [2]. These findings are limited to monolayers up to now. However, few layer structures have seen growing experimental attention: They are a kind of a link between true two dimensional and bulk semiconductors, and some controversy exists whether significant photoluminescence can be expected from bilayers [3-5]. Up to now no detailed information is available at all on the exciton-trion control in MoS2 bilayers.