Layer-Controlled CVD Growth of 2D Tungsten Disulfide and Its Lateral Heterostructure with Molybdenum Disulfide for Optoelectronic and Biosensing Applications

Abstract

This thesis presents a comprehensive study on the layer-controlled CVD growth and multifunctional applications of WS2, MoS2, and in-situ WS2-MoS2 lateral heterostructures (HS). The 1L-WS2 exhibited excellent PL emission at room temperature, which was employed as a sensitive tool for the selective detection of Staphylococcus aureus in PBS and urine samples. Functionalization of 1L-WS2 with ssDNA aptamers enabled high selectivity, even in the presence of E. coli. For 2L-WS2, photodetection performance was evaluated through asymmetric contact engineering using metals with work functions both lower and higher than that of 2L-WS2, leading to unidirectional carrier flow. Compared to symmetric configurations, photodetectors (PDs) with asymmetric contacts demonstrated superior responsivity and detectivity. Thereafter, the influence of metal (Cr/Au) and non-metal (Bi2Se3) contacts on 1L-MoS2 was investigated. Devices with ultrathin Bi2Se3 contacts exhibited cleaner contact interfaces with lower defect density, leading to improved performance, achieving an on/off current ratio of 108, two orders of magnitude higher than their metal-contacted counterparts. The flexible 1L-MoS2 PD incorporating Bi2Se3 contacts also showed notable enhancements in responsivity, detectivity, and external quantum efficiency. Controlled in situ growth of WS2-MoS2 lateral HS was achieved by tuning the CVD growth parameters, enabling modulation of the relative areas of MoS2 and WS2 domains within single flakes. The diffuse interface induced interfacial strain, resulting in a fivefold increase in PL intensity from 2L-WS2 near the junction. When integrated as channels in back-gated FETs, these HSs demonstrated superior electrical performance, including enhanced on/off ratios and improved subthreshold swings. Finally, WS₂ quantum dots (QDs), coupled with Bi2O2Se nanosheets, were utilized for the selective detection of S. aureus. The QDs, functionalized with ssDNA aptamers, enabled detection even in the presence of other analytes.