Elucidating the structural properties of gold selenide nanostructures

Abstract

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 α- and β-AuSe. Although both polymorphs were observed, XRD showed that the addition of the gold precursor at higher temperatures resulted in α-AuSe being the dominant phase while the addition at lower temperatures resulted in β-AuSe being preferred. The crystal structures of both α- and β-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 α- and β-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 α-AuSe as well as A_1g and B_1g for β-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 α-AuSe samples were nanobelts while the predominantly β-AuSe samples showed plate-like structures. The predominantly α-AuSe samples showed a broad absorption band with a maximum at 853 nm while the predominantly β-AuSe samples showed evidence of absorption however with no defined excitonic peak.