Physical effects of passivation and creation of sulphur vacancy in MoS2 nanoparticles

In this work, we investigated the physical effect of creating and passivating Sulphur (S) vacancy on the Molybdenum di-Sulphide (MoS2) nanoparticles. The study was carried out by annealing the nanoparticles under Hydrogen (H2) and air atmosphere and simultaneously recording the conductivity as a function of temperature (T). As the nanoparticles were heated in open-air condition, it was observed that there exists a threshold temperature (Tth), for which maximum current response was observed. On maintaining the sample at constant temperature slightly above Tth, the current was found to decrease exponentially with time. Tth, therefore, divides the conducting regime into two segments; T < Tth, where conduction is dominated by electrons activated from S-vacancy and T > Tth, where the regime is dominated by chemisorbed Oxygen (O2) depleting the electrons from the S-vacancy. After O2 passivation, the morphology of the sample transformed into a large sheet-like structure whose XRD pattern conforms to Molybdenum tri-oxide phase. Interestingly, the UV-visible spectrum of the sample showed absorbance at 653 and 713 nm indicating the presence of MoS2 phase beneath the O2 rich surface. In the case of H2 annealed sample, conductivity increased monotonously with temperature even beyond the Tth. This is due to molecular H2 creating more S vacancy and introducing additional donor levels near the conduction bands with increasing temperature. At any given temperature (T < Tth), the sample conductivity under H2 ambience is found to be 20 times greater than the conductivity observed in air annealed samples. From our sensing measurements, we also show that Tth sets the maximum temperature limit for MoS2 nanoparticle-based H2 sensor. Structurally H2 annealed sample transformed from polycrystalline to a single crystalline phase with 002 preferential orientations.