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 (MoS 2 ) nanoparticles. The study was carried out by annealing the nanoparticles under Hydrogen (H 2 ) 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 ( T th ), for which maximum current response was observed. On maintaining the sample at constant temperature slightly above T th , the current was found to decrease exponentially with time. T th , therefore, divides the conducting regime into two segments; T < T th , where conduction is dominated by electrons activated from S-vacancy and T > T th , where the regime is dominated by chemisorbed Oxygen (O 2 ) depleting the electrons from the S-vacancy. After O 2 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 MoS 2 phase beneath the O 2 rich surface. In the case of H 2 annealed sample, conductivity increased monotonously with temperature even beyond the T th . This is due to molecular H 2 creating more S vacancy and introducing additional donor levels near the conduction bands with increasing temperature. At any given temperature ( T < T th ), the sample conductivity under H 2 ambience is found to be 20 times greater than the conductivity observed in air annealed samples. From our sensing measurements, we also show that T th sets the maximum temperature limit for MoS 2 nanoparticle-based H 2 sensor. Structurally H 2 annealed sample transformed from polycrystalline to a single crystalline phase with 002 preferential orientations.