Monolayer WS2 Nanosheets Passivated with HfO2 for Enhanced Photodetectors

Two-dimensional transition metal dichalcogenides (TMDCs) are widely used in electronic and optoelectronic devices. However, the conventional chemical vapor deposition (CVD) method is difficult to synthesize large-area monolayer WS2 nanosheets stably, which limits the application of WS2 in the field of photoelectric detection. In this work, we propose an innovative NaCl-assisted CVD method that allows freely adjustable substrate positions for synthesizing monolayer WS2 nanosheets. The obtained maximum grain size of the monolayer WS2 nanosheets is up to 30 mu m. Subsequently, we investigated the effect of the HfO2 passivation layer on the performance of the metal- semiconductor-metal (MSM) WS2-based photodetectors. The HfO2 passivation layer brought an overall improvement to the performance of the fabricated photodetectors, exhibiting a high responsivity of 1093.1 AW-1, a high specific detectivity of 2.6 x 1012 Jones, and a high external quantum efficiency of 2.1 x 105%. Furthermore, the physical mechanism of the fabricated photodetectors has been discussed to explain how the HfO2 passivation layer takes effect in the improvement of the WS2-based photodetectors. This result can accelerate the development of optoelectronic devices based on TMDCs.