Conversion of centimeter-scale amorphous niobium oxide thin films into crystalline niobium disulfide (NbS2): Synthesis and stability

Niobium disulfide (NbS₂) is a layered transition metal dichalcogenide (TMD) with metallic characteristics. While mono- and few-layer NbS₂ have been explored by various research groups, significant challenges remain in achieving continuous films. In this study, we present a procedure for converting centimeter-scale thin films of niobium oxide (Nb_xO_y), deposited using physical vapor deposition (PVD), into a continuous, crystalline, mixed-phase (2H/3R) NbS₂ film through sulfurization. We examine the influence of the initial Nb_xO_y film thickness on the NbS₂ conversion and stability. Adjusting the initial Nb_xO_y thickness results in distinct roughness profiles, electrical resistivities and stabilities against re-oxidation. The NbS₂ films demonstrate intrinsically low resistivity, measuring at 190 ± 23 Ω/sqr (80 ± 5 Ω/sqr) for the thinner (thicker) film. However, over a 21-day exposure to air, the long-term stability of these films varies with thickness. Thinner films show a significant increase in resistivity, rising by 367 %–699 ± 97 Ω/sqr. In contrast, the thicker film exhibits a modest increase of only 22 %, reaching 97 ± 5 Ω/sqr. Even after one year of air exposure, the thicker film remains conductive, and the initial characteristics of the converted films can be restored through the resulfurization process. This process enables the scalable production of large-area NbS₂ films, suitable for nanotechnological applications.