1. Introduction
Tungsten disulfide is a layered transition metal dichalcogenide (TMD) semiconductor that has long been studied for electronics and lubrication catalysis Novel electronic properties that arise as these materials thin to a single or a few layers thick make them particularly attractive for application in next-generation electronics, photonics, and optoelectronics MoS2 and WS2 are similar: they both exhibit a trigonal prismatic structure, are van der Waals layered solids, and have an indirect bandgap in the bulk that becomes a direct bandgap in monolayer films While many of the properties of WS2 and MoS2 are very similar, a few key differences are emerging. The photoluminescence intensity of the edges of monolayer WS2 is 20 times higher than that from monolayer MoS2 and monolayer WS2 MOSFETs have higher theoretical ballistic performance than MoX2 (X = S, Se, and Te) [13]. Besides, the Fermi-level pining of WS2 is close to the middle of the bandgap, making it desirable for ambipolar conduction in field effect transistors (FETs) [8] and [14].
For electronic applications, the interactions between TMDs and transition metals are of great importance because contact metallurgy for TMDs influences the performance of electronic devices. Recent investigation by Hwang et al. [14] showed that Ti/Au contacts to few-layer WS2 exhibit Schottky behavior, which made extraction of device parameters more challenging; historically, Ag paint/epoxy and a Ga–In eutectic were reported to be Ohmic contacts to bulk WS2[15]. This paper expands upon our earlier work predicting and discussing transition metal (M)–MoS2 interactions and M–Mo–S ternary phase diagrams, as described in Domask et al. [16]. Using a similar approach, we studied the M–W–S systems, where M is a metal from group 4 to group 13, and we draw comparisons between the interaction of metals with MoS2 and WS2. Although there could be some differences in the phase equilibria for nanoscale materials, the bulk phase diagrams we have calculated and compiled can still inform future research on contacts to single and few-layer WS2.