Sulphur-promoted growth of Mo6S2I8 nanowires via a metastable MoI2−xSx intermediate

Abstract

Sulphur incorporation plays a crucial role in the formation of Mo–S–I nanostructures, but its effect on phase stability and morphology has remained unclear. Here, we show that trace sulphur stabilizes a metastable MoI_2−xS_x phase that grows as high-aspect-ratio nanowires (NWs), in contrast to the low-aspect-ratio prisms of pure MoI₂. These intermediate NWs subsequently transform into Mo₆S₂I₈ NWs, revealing a sulphur-promoted growth pathway. Structural and electronic characterization using XRD, TEM, SEM, UV-Vis, Raman, UHV AFM/KPFM, and STM/STS clarifies the ambiguous role of MoI₂. The MoI_2−xS_x NWs show diameters of 100–300 nm, lengths up to 20 µm, and a nominal composition of 7.5% S, 38% Mo, and 54.5% I. Work function measurements indicate a progressive shift from 4.6 ± 0.1 eV in the intermediate phase to 5.0 ± 0.1 eV in the final Mo₆S₂I₈ NWs, while density-of-states analysis reveals a U-shaped band gap of ∼1.2 eV in the NW cores. Our results establish a general concept: minor compositional tuning can stabilize metastable intermediates as templates for controlled nanowire morphology and function, opening pathways for optoelectronic, nanoelectronic, and composite applications.