2, University of Antwerp, Antwerp, , Belgium
Ternary CuInX2 (X= S, Se, Te) nanocrystals (NCs) have attracted increasing attention as promising alternatives for CdX or PbX NCs due to their low toxicity, large absorption cross-sections across a broad spectral range and wide photoluminescence (PL) tunability (~550 to ~1100 nm for X= S). To achieve properties that are inaccessible to single component CuInX2 NCs (e.g., high PL quantum yields, stability, carrier localization regime), researchers have been synthesizing colloidal CuInX2-based hetero-NCs (HNCs) (e.g., CuInS2/ZnS concentric core/shell HNCs, CuInS2/CdS tetrapods, CuInSe2/CuInS2 dot-in-rod HNCs). Anisotropic CuInX2-based HNCs are particularly interesting, since they are expected to exhibit novel properties, such as polarized NIR PL and spatial charge separation, which are attractive for many applications (e.g., polarized LEDs, photocatalysts, luminescent solar concentrators, solar cells).
Nevertheless, reports on the synthesis of anisotropic CuInX2-based HNCs are scarce. This is probably related to the difficulty in balancing the reactivities of multiple precursors and the high solid-state diffusion rates of all the cations involved in the CuInX2 lattice. In this work, we report a two-step pathway that yields CuInS2/ZnS dot core/ rod shell HNCs via a seeded growth approach. The wurtzite CuInS2 NCs used as seeds are obtained by cation exchange in template Cu2-xS NCs. The CuInS2 NC seeds are injected together with the S precursor in a hot solution of Zn precursor and suitable coordinating ligands, which leading to heteroepitaxial growth of ZnS primarily on single polar facet of the seeds, as demonstrated by high-angle annular dark-field scanning transimission electron microscopy and electron tomography. These colloidal wurtzite CuInS2/ZnS dot-in-rod HNCs have large molar extinction coefficients, PL in the NIR (~800 nm) with PL quantum yield ~20%, and are readily dispersible in a variety of solvents.