Reactivity of contact metals on monolayer WS2

Incorporating two-dimensional transition metal dichalcogenides (TMDs) into electronic and optoelectronic applications requires a fundamental understanding of metal/TMD interactions. This work applies a fast and easy approach to observe reactivity between metal contacts and monolayer (1L) WS2 via Raman spectroscopy using both destructive and non-destructive methods. We compare findings from Raman spectra collected via a backside geometry and also from mechanically exfoliated metal/WS2 films after annealing with our previously published thermodynamic predictions for reactivity of bulk materials. The disappearance of the Raman-active phonon modes for WS2 suggests the consumption of WS2 through reactions with the continuous metal film, as observed completely for Ti upon deposition and nearly completely for Al after annealing at and above 100 °C. On the other hand, the persistence of multiple Raman-active phonon modes for WS2 confirms that Au, Cu, and Pd are unreactive with WS2 upon deposition and after cumulatively annealing for 1 h at 100, 200, and 300 °C, even though unreactive metal overlayers can shift some of the peaks in the spectrum. The metal/WS2 reactivity observed in this study is in excellent agreement with predictions from bulk thermodynamics, which can provide good guidance for studies of other metal/TMD systems. In addition, using a backside geometry for collecting Raman spectra can aid in fundamental studies of interfaces with TMDs.