Carrier density tuning in single layers of WS₂ via photochlorination

Monolayers of Transition Metal Dichalcogenides (TMDs) of the form MX₂ (M=Mo or W and X=S or Se) are promising semiconducting materials for future 2D nanoelectronics due to their unique properties. Carrier modulation and doping reversibility are very important issues in the study of the electronic properties of TMDs and at the heart of many applications. We demonstrate electron density control in chlorine-doped WS₂ monolayers by pulsed laser irradiation in a precursor gas atmosphere [1]. The increase of the photochlorination time gives rise to a systematic red-shift in the PL energy of the neutral exciton indicating a reduction in the electron density. This electron withdrawing process enabled also the determination of the trion binding energy of the intrinsic crystal, found to be 20 meV, in accordance to theoretical predictions. At the same time, it is found that the effect can be reversed upon cw laser rastering of the monolayer in air. SAM and XPS reveal that chlorine physisorption is responsible for the e-density modulation induced by the pulsed laser photochemical reaction process and confirmed by DFT calculations. [1] I. Demeridou et al, 2D Mater. 6 (2019), 015003