Sulfidation of 2D transition metals (Mo, W, Re, Nb, Ta): thermodynamics, processing, and characterization

Abstract

Sulfidation of selected transition metal thin films (Mo, W, Re, Nb, Ta) was combined with thermodynamic calculations to study the synthesis of transition metal dichalcogenides (TMDCs) and understand variations among the metals as well as processing atmosphere. Metal seed layers were prepared by DC magnetron sputtering and sulfidized using sulfur vapor and H2S. Surface chemistry, structure, and morphology of the films were investigated using X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and atomic force microscopy (AFM), respectively. XPS analysis revealed that after treatment with sulfur vapor (p (S2) = 1–10 Torr), Mo, W, and Re films were transformed into MoS2, WS2, and ReS2, respectively. However, Nb and Ta films changed little, and Nb2O5 and Ta2O5 remained the predominant components. Alternatively, conversion of Nb and Ta films to NbS2 and TaS2 was feasible under H2S. Raman spectroscopy also revealed improved crystallinity for Mo, W, and Re sulfidized under H2S. Isobaric and isothermal stability diagrams were calculated to identify feasible processing conditions (sulfur partial pressure and temperatures) for the sulfidation of all of the metals, and our findings were in good agreement with the XPS and Raman results. It was found that for Mo, W, and Re a p (S2) = 10−5 bar is sufficient for the metals to be converted to sulfide phases at 750 °C. On the other hand, due to very high stability of Nb2O5 and Ta2O5, even at very low p (O2), a sulfur partial pressure of 103–104 bar is required to make NbS2 and TaS2, respectively. Nevertheless, thermodynamic calculations confirmed that Nb and Ta could be transformed to NbS2 and TaS2 under 760 Torr H2S. AFM analysis revealed very smooth films for MoS2, WS2, and NbS2 films, but dewetting of TaS2, and ribbons for ReS2. These results provide guidance for designing new processes for synthesizing 2D TMDCs.