Cu2−xS nanocrystal synthesis: a chemical toolbox for controlling nanocrystal geometry, phase and plasmonic behavior

Abstract

Cu_2−xS nanocrystals (NCs) have been recently exploited in several fields, ranging from energy conversion to biomedical applications, due to their intriguing geometry and phase-dependent semiconductor and near-infrared plasmonic properties. Although advances have been made in their synthesis by hot-injection at the present stage of research, the unexpected sizes and shapes of the Cu_2−xS NCs and broad polydispersity still represent critical issues that must be avoided, as they are responsible for the unpredictable optical response and undesirable broadening of the spectroscopic feature. This study intends to explore the influence that the reactants used in the Cu_2−xS NC synthesis have in the modulation of size, shape and phase. Furthermore, we aim to provide an effective toolbox for a judicious choice of synthetic conditions towards the production of monodispersed and uniform NCs. The way how, the precursor nature, tied up with the composition of amphiphilic molecules, controls the final NC geometry (e.g., size, shape and size/shape distribution), phase and plasmonic properties, is discussed on the basis of the Hard-Soft Acid-Base theory. In this regard, Cu_2−xS NCs prepared by means of various typically used reactants (CuCl, CuCl₂, Cu(acetylacetonate)₂, Cu(acetate)₂, S₈, tert-dodecanthiol (tDT) or dibuthyldisulfide (DBDS)) and amphiphilic molecules (oleic acid and oleylamine) are, in this work, extensively investigated. The valence of copper ions, copper counterion, molar ratio of copper to sulphur precursors, and coordinating agents demonstrate the essential factors to achieve a qualitative estimation of the nucleation and growth rate, and an effective elucidation of the NC growth mode and hence of the final NC plasmonic behavior.