Tip-induced and electrical control of the photoluminescence yield of monolayer WS2
Résumé
Monolayer transition metal dichalcogenides (TMDs) are two-dimensional (2D) direct bandgap semiconductors that have the potential to lead to breakthrough applications in nanodevice technologies due to their unique optical and electronic properties[1]. The photophysics of monolayer TMDs is governed by the dynamics of bound electron-hole pairs (i.e., excitons) and their interactions with charge carriers[2]. Manipulating the creation, diffusion, and recombination of excitons is essential for the performance of monolayer TMD-based devices. In particular, locally controlling the radiative quantum yield and the formation of charged excitons (i.e., trions) in these materials has been a long-sought-after goal. Attempts to achieve such nanoscale control have been reported, for example using the plasmonic tip of an atomic force microscope[3]. Nevertheless, most of the techniques based on a scanning probe that are used to locally control or excite excitons in 2D materials do not provide any direct information about the local diffusion and recombination processes of these excitons, a key aspect for the integration of these materials into devices.
In this communication, we present a novel method to locally and electrically control the radiative quantum yield of monolayer TMDs on a transparent electrode (i.e., indium tin oxide-coated glass) using a scanning tunneling microscope equipped with a tungsten tip. Using a combination of scanning tunneling microscopy and wide-field laser-induced photoluminescence microscopy, we demonstrate and we spatially and spectrally resolve the reversible effects of the biased tip-sample junction on the excitonic properties of a WS2 monolayer[4]. The near-field non-radiative electromagnetic energy transfer from the excitons to the tungsten tip quenches the luminescence in a diffraction-limited area below the tip. Moreover, bias- and current-dependent luminescence quenching and enhancement occur in a micrometer-scale region around the tip position. These "long-range" effects result from the electron-tunneling-induced lateral spatial gradients of the charge carrier density in the monolayer, which modify the relative contributions of excitons versus trions and the radiative quantum yield of the excitons.
[1] Mak et al, Phys. Rev. Lett. 2010, 105, 136805
[2] Wang et al, Rev. Mod. Phys. 2018, 90, 021001.
[3] He, et al, Sci. Adv. 2019, 5, eaau8763.
[4] Peña Román et al, submitted