Plasmonic Cu(2-x)S nanocrystals: optical and structural properties of copper-deficient copper(I) sulfides.

Cu(2-x)S (x = 1, 0.2, 0.03) nanocrystals were synthesized with three different chemical methods: sonoelectrochemical, hydrothermal, and solventless thermolysis methods in order to compare their common optical and structural properties. The compositions of the Cu(2-x)S nanocrystals were varied from CuS (covellite) to Cu(1.97)S (djurleite) through adjusting the reduction potential in the sonoelectrochemical method, adjusting the pH value in the hydrothermal method and by choosing different precursor pretreatments in the solventless thermolysis approach, respectively. The crystallinity and morphology of the products were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM), which shows that most of them might be of pure stoichiometries but some of them are mixtures. The obtained XRDs were studied in comparison to the XRD patterns of previously reported Cu(2-x)S. We found consistently that under ambient conditions the copper deficient Cu(1.97)S (djurleite) is more stable than Cu(2)S (chalcocite). Corroborated by recent computational studies by Lambrecht et al. and experimental work by Alivisatos et al. This may be the reason behind the traditionally known instability of the bulk Cu(2)S/CdS interface. Both Cu(2)S and the copper-deficient Cu(1.97)S have very similar but distinguishable electronic and crystal structure. The optical properties of these Cu(2-x)S NCs were characterized by UV-vis spectroscopy and NIR. All presented Cu(2-x)S NCs show a blue shift in the band gap absorption compared to bulk Cu(2-x)S. Moreover the spectra of these Cu(2-x)S NCs indicate direct band gap character based on their oscillator strengths, different from previously reported experimental results. The NIR spectra of these Cu(2-x)S NCs show a carrier concentration dependent plasmonic absorption.