Elucidating the structural properties of gold selenide nanostructures

Noble transition metal chalcogenide gold selenide is a relatively unexplored layered material. Herein, we report on the synthesis and characterization of polymorphic mixed-valence AuSe (Au1+Au3+Se2) by varying the sequence of the addition of the precursors in a colloidal synthesis. Despite the variations, all produced materials showed the co-existence of - and -AuSe. Although both polymorphs were observed, XRD showed that the addition of the gold precursor at higher temperatures resulted in -AuSe being the dominant phase while the addition at lower temperatures resulted in -AuSe being preferred. The crystal structures of both - and -AuSe consist of repeating units of a linearly bonded Au1+ ion to two Se atoms and a Au3+ ion bonded to four Se atoms in a square planar geometry. The Au4f core level spectrum of XPS showed only the Au+1 oxidation state, however, using the Se3d core level spectrum, the formation of AuSe (Au1+Au3+Se2) was evident. Using DFT calculations, the Raman spectra of - and -AuSe were simulated and only the square planar geometry was found to be Raman active. The square planar geometry (Au3+Se4)(-) ions belonging to the D-4h point group produced three Raman active vibrational modes, namely, a symmetric stretch (A(1g)), a planar bend (B-1g) and an asymmetric stretch (B-2g) for -AuSe as well as A(1g) and B-1g for -AuSe. Experimentally, all samples showed Raman vibrational lines from both phases. Moreover, Raman spectroscopy confirmed the presence of Au3+ in AuSe which was not detected using XPS. From the TEM and SEM results, it was evident that the morphologies of the predominantly -AuSe samples were nanobelts while the predominantly -AuSe samples showed plate-like structures. The predominantly -AuSe samples showed a broad absorption band with a maximum at 853 nm while the predominantly -AuSe samples showed evidence of absorption however with no defined excitonic peak.