Chalcogen Incorporation Process during High-Vacuum Conversion of Bulk Mo Oxides to Mo Dichalcogenides

The optical, electronic, catalytic, and mechanical properties of transition-metal dichalcogenide (TMD) bulk crystals and monolayers have been examined extensively. TMD materials can be prepared through exfoliation, deposition from elemental and molecular precursors, and chemical conversion of corresponding transition metal oxides. Nevertheless, approaches for large-scale synthesis of high-quality TMD crystals are needed to ensure the continued utilization of these materials within optoelectronic and energy conversion applications. Moreover, studies examining the principal stages of TMD nucleation and growth are comparatively rare and require further attention. We report a method for the conversion of bulk MoO3 to MoX2 (X = S, Se) under high-vacuum conditions and use this approach to isolate and examine intermediates formed during the chalcogenation process. Micron-scale Mo sub-oxide deposits are readily converted to MoS2 when exposed to an atmosphere of S at 40 Torr and 300 °C. Notably, we show through elemental mapping of the cross-section of a partially converted single deposit that MoS2 is present at higher concentrations nearest the deposit–substrate interface. We also note the formation of either distinct or alloyed TMD phases depending on whether a stepwise or concurrent chalcogenation, respectively, is performed. Our results provide an effective route toward conversion of transition metal oxides to bulk TMDs and provide insights into the mechanism of this scalable conversion process.