Phase-Controlled Growth of CuInS₂ Shells to Realize Colloidal CuInSe₂/CuInS₂ Core/Shell Nanostructures

Synthetic routes to deposit CuInS2 (CIS) shells with either a cubic chalcopyrite (CP) or a hexagonal wurtzite (WZ) phase on trigonal pyramidal-shaped CuInSe2 (CISe) core nanocrystals (NCs) with a cubic CP crystal structure have been developed and governed by tuning the amount of the sulfur precursor tert-dodecanethiol. During the synthesis of CP-CIS/CP-CISe core/shell NCs, the CP-CIS shell initially starts to grow epitaxially in a uniform way, while the further addition of the CIS precursor induces islandlike growth, and finally a branched CIS shell is formed. In a stark contrast, when a WZ-CIS shell is deposited, it initially grows on a portion of each of the facets of the trigonal pyramidal-shaped CISe cores to form a monolayer, which then continues to increase in thickness and forms a multilayered WZ-CIS shell. Both CP-CISe/CP-CIS core/shell NCs and CP-CISe/WZ-CISe core/shell NCs exhibit rather low photoluminescence quantum yields (<10%), even with a smaller-sized CISe core, which calls for further refinements of the shell growth methods. Synthetic methods for the growth of CIS shells as described here allow for direct deposition of cadmium-free ternary compounds as shell materials and provide important insights into the different modes of growth of heterostructured NCs, ranging from epitaxial to island- and branched-like, as well to the facet-specific multilayer deposition.