MS-9-P-3146 Copper sulfide nanocrystals with tunable composition

Localized surface plasmon resonances (LSPRs) arise from the interaction of electromagnetic radiation with free charge carriers in nanoscale metals, which leads to coherent charge oscillations at optical frequencies. LSPRs of metallic nanocrystals (NCs) have drawn great attention due to their various applications. However, the occurrence of LSPR is not restricted only to nanostructured metals, but is also observable in various other nanomaterials, e.g. the binary copper chalcogenides. The interesting properties of Cu_2-xS, a well-known p-type semiconductor exhibiting stoichiometry-dependent bandgap [1], make its NCs appealing in very diverse fields. Thus, in recent years various studies on plasmonic behavior and different synthetic approaches with controlled Cu stoichiometry were proposed. NCs with compositions comprised between the limiting cases represented by CuS and Cu₂S (i.e. covellite and chalcocite, respectively) can be routinely made, although each one with a different synthetic procedure. This makes it difficult to compare their physical properties, since each sample has its own geometrical parameters and type of surface passivation.
Our approach, based on the reaction of the as-synthesized covellite NCs with a Cu(I) complex at room temperature, allows to access several stoichiometries in colloidal copper sulfide NCs, starting from CuS (covellite) NCs, up to Cu₂S. Thus, starting from a common sample, by this approach it is possible to access a wide range of compositions of NCs and study variations in their structure and plasmonic response: from the metallic covellite, with a high density of free carriers, up to Cu₂S NCs with no localized surface plasmon resonance (Figure 1). In all these NCs the valency of Cu in the lattice stays close to +1, while the mixed -1/-2 valency of S in covellite gradually evolves to -2 with increasing the Cu content, i.e. sulfur is progressively reduced. The addition of copper to covellite NCs is similar to the intercalation of metal species in layered transition metal dichalcogenides (TMDCs), i.e. the dichalcogenide bonds holding the layers are progressively broken to make room for the intercalated metals, while their overall crystal structure does not change much (Figure 2). However, differently from TMDCs, the intercalation in covellite NCs is sustained by a change in the redox state of the anion framework. Furthermore, the amount of Cu incorporated in the NCs upon reaction is associated with the formation of an equimolar amount of Cu(II) species in solution, so that the reaction scheme can be written as: CuS + 2γCu(I) → Cu_1+γS + γCu(II).
[1] Liu et al. Thin Solid Films, 431-432, (2003), 477.