Tailoring selenium vacancies in MoSe2 through oxygen passivation for room-temperature NO2 sensing enhancement

Vacancy defects are intrinsically present in the as-grown MoSe2, which may greatly affect its gas sensing performance. However, it remains ambiguous how their concentrations influence the sensing properties. The vacancy defects as active sites may facilitate gas adsorption, while as trapping and scattering sites they could lower electron density and mobility. It is therefore fundamentally important to elucidate their roles of double-edged-sword. Herein, MoSe2 nanoflowers with Se vacancies were first colloidally synthesized and the vacancies are identified using a transmission electron microscope, electron paramagnetic resonance spectra, and Raman spectra. The defect engineering of oxygen passivation was then carried out to tailor the vacancy density, through which the room-temperature NO2 sensing response is enhanced by a factor of 6.3. The density functional theory calculations reveal that the vacancies cause the increase of adsorption energy and the decrease of charge transfer during the NO2 sensing. Therefore, it is reasonable to expect that when the vacancy density is tailored to an optimal level, the role of the vacancies as active sites outweighs their role as electron trapping sites, and the sensing response can be maximized. The vacancy-mediated sensing mechanism is therefore proposed, and may offer insightful guidance for improving the performance of layered metal dichalcogenide based gas sensors.