Band bending, surface photovoltage and tunnelling microscopy on WSe2:Rb

The interest in the photovoltaic properties of layered transition metal dichalcogenides has triggered several studies of alkali metal deposition onto these materials. We have studied cleaved, p-type semiconducting WSe2 crystals exposed to Rb at room temperature under UHV conditions. Rather than being intercalated in the van der Waals gap like the smaller alkali atoms Li and Na, Rb is adsorbed on the WSe2 surface. Photoemission measurements in the very first stage of submonolayer Rb adsorption reveal a rapid surface band bending of the order of the band gap, already saturating for ∼1 × 1012 Rb atoms cm−2. Topographic STM images clearly show the Rb induced defects, yielding an estimate of their area density. For low coverages, a strong surface photovoltage effect is observed which can be quantitatively explained in terms of a semiclassical semiconductor theory.