In Situ investigation of Interfacial Transformations Between Oriented Transition Metal Dichalcogenides and Metals

Abstract

Layered materials, such as transition metal dichalcogenides (TMDCs), are attractive for energy storage, nanoelectronics, and electrochromic devices. Layered TMDC materials feature a strongly anisotropic crystal structure, with basal planes and edge sites within the structure exhibiting divergent chemical and bonding properties. The interaction of external metals and ions with TMDCs is critical for their use in batteries and electronic devices; metal ions are reversibly intercalated between TMDC layers when used as battery electrodes, and thin film metal electrical contacts must be interfaced with TMDCs in next-generation electronics. In both cases, the crystallographic orientation of the TMDC likely influences the interfacial reaction and/or ion intercalation pathways and kinetics, but the effect of crystallography on these phenomena is not well understood. Here, we use in situ x-ray photoelectron spectroscopy (XPS) to determine how the crystallography and structure of TMDCs (MoS2 and TaS2) influence interfacial transformation pathways and kinetics when interacting with different metals of interest for energy storage and electronic contacts. This work is enabled by the synthesis of MoS2 and TaS2 with controllably aligned crystal structure: thin films with vertically aligned layers are grown via a sulfidation process, and single crystals with horizontally aligned layers are also utilized. In situ XPS is carried out by sputter depositing thin metal films (lithium, silver, or gold) onto the oriented TMDC material within an XPS instrument, and photoelectron spectra are acquired at different times to probe near-surface chemical changes and reaction kinetics in the TMDC material. The results show that lithium reacts with both edge planes and basal planes in MoS2 at similar rates to form reduced Mo and S species. This indicates that the conversion reaction of MoS2 in Li-ion batteries is not strongly dependent on the crystalline anisotropy of the lattice. In contrast, less reactive metals (silver and gold) were observed to intercalate in low concentrations, and this intercalation behavior is strongly dependent on the orientation of MoS2­ and TaS2. Interestingly, despite the relatively unreactive nature of Ag and Au, these materials also induce slight chemical reduction at the TMDC interface. This study emphasizes the importance of using in situ experiments for understanding interfacial reaction mechanisms and kinetics, which are generally difficult to study in buried interfaces. Future work will focus on tuning interfacial interactions via crystallographic control.