Wetting Characteristics of CVD Grown 2D Monolayer WS2 supported on SiO2/Si substrate: Critically Important for Reliability & Performance of Next Generation Electronic Devices

Abstract TMDs composed of atomic layers of chalcogens (X = S, Se, Te) and group-VI transition metals (M = Mo, W). They are stable in the bulk form but also as few-layered as well as monolayers. Individual mono layer semiconducting TMDs (such as WS₂) exhibit a direct band gap which is not present in few-layered structures. As a result, monolayer TMDs exhibit remarkably enhanced interaction with visible light due to the indirect-to-direct bandgap conversion at the monolayer limit, thus making them versatile platforms for studying light-matter interactions. There are several emerging applications in which monolayer WS₂ has been utilized such as biosensors, optoelectronics, high frequency microelectronics, photo detectors etc. where the material inevitably comes in compact with aqueous media. Waterproofing is critically important to protect such optoelectronics and electronics components embedded in consumer electronics. Hydrophobic polymers such as epoxy resin, polydimethylsiloxane (PDMS) are usually used to protect microchips from water, however, these polymers have poor thermal conductivity hindering heat dissipation and degrade the optimal performance and reliability of the electronic devices. There are techniques to tune the wetting characteristics such as patterning with e-beam lithography, transferring films onto other flexible substrates (e.g. PDMS), but those techniques are complex and time-consuming. Controlling liquid-solid interface is extremely crucial for practical applications and these applications require air exposure stability of the surfaces. The important parameters for wetting characteristics are wettability of supporting substrate, crystallinity/morphology of a TMDs film and airborne contaminants. The research on the wetting behavior of monolayer TMDs is still limited. In this study, we have grown monolayer tungsten disulfide (WS₂) on SiO₂/Si substrate by the thermal reduction and sulfurization of WO₃. The monolayer WS₂ displayed a contact angle of ~ 71°. We also explored wetting as a function of aging. A clean monolayer WS₂ without any airborne contaminants demonstrated contact angle of ∼71°. However, over time, the sample ages as hydrocarbons and water present in air were adsorbed onto the clean WS₂. After ∼21 days, the aging process is completed and the contact angle of the aged monolayer WS₂ stabilizes at ~ 79°. These results suggest that clean and non-aged monolayer TMDs are less hydrophilic materials compared to bare SiO₂/Si substrate. Since such research is limited and has not been explored extensively, this study will open new pathways to investigate classical problems between TMDs and supported substrate interaction in surface science field.