Processing and characterisation of Mo6S2I8nanowires

Abstract

One-dimensional nanostructures based on the Mo–S–I system have recently aroused a lot of interest as a viable alternative to the ubiquitous carbon nanotube due to their uniform structure and electronic properties for a given composition. Previous research on the Mo₆S₃I₆ and Mo₆S_4.5I_4.5 stoichiometries has also shown them to be soluble in common solvents like water, acetone or isopropyl alcohol, and to debundle on dilution. Here, the solubility, debundling and composition of Mo₆S₂I₈ nanowires are presented. They were found to be most soluble in dimethylformamide, which retained 47 wt% of a 0.08 gl⁻¹ nanowire (NW) material dispersion as thin NW bundles after one week. Dispersions of 0.8 gl⁻¹ and 5 gl⁻¹ even retained 54 wt% and 66 wt%, respectively. However the NW material was completely insoluble in water, and the surface energy of Mo₆S₂I₈ NWs was deduced as 67 mJ m⁻², higher than for other Mo–S–I NWs. UV-vis-NIR spectroscopy showed nanowire peaks familiar from Mo₆S₃I₆ and Mo₆S_4.5I_4.5 spectra around 1.8 and 2.8 eV, as well as unforeseen ultraviolet peaks at 3.5 and 4.4 eV. These chemical differences suggest an alternate, more strongly bonded structure to that seen for Mo₆S₃I₆ and Mo₆S_4.5I_4.5 NWs. Films deposited from a range of concentrations were investigated using atomic force microscopy (AFM) to determine bundle diameter distributions. The average diameter and the spread in diameters were found to decrease somewhat with decreasing concentration. However extrapolation gave a finite bundle size at infinite dilution, and an extension of the existing debundling model is proposed to take this into account. To confirm the nominal stoichiometry of Mo₆S₂I₈, which does not follow the generic Mo₆S_xI_9−x formula of previous stoichiometries, EDX was carried out. The composition of nanowire bundles was found to be Mo₆S_2.3I_8.6, supporting the nominal composition.