Near-Infrared-Emitting CuInS₂/ZnS Dot-in-Rod Colloidal Heteronanorods by Seeded Growth

Abstract

Synthesis protocols for anisotropic CuInX2 (X = S, Se, Te)-based heteronanocrystals (HNCs) are scarce due to the difficulty in balancing the reactivities of multiple precursors and the high solid-state diffusion rates of the cations involved in the CuInX2 lattice. In this work, we report a multistep seeded growth synthesis protocol that yields colloidal wurtzite CuInS2/ZnS dot core/rod shell HNCs with photoluminescence in the NIR (∼800 nm). The wurtzite CuInS2 NCs used as seeds are obtained by topotactic partial Cu+ for In3+ cation exchange in template Cu2–xS NCs. The seed NCs are injected in a hot solution of zinc oleate and hexadecylamine in octadecene, 20 s after the injection of sulfur in octadecene. This results in heteroepitaxial growth of wurtzite ZnS primarily on the Sulfur-terminated polar facet of the CuInS2 seed NCs, the other facets being overcoated only by a thin (∼1 monolayer) shell. The fast (∼21 nm/min) asymmetric axial growth of the nanorod proceeds by addition of [ZnS] monomer units, so that the polarity of the terminal (002) facet is preserved throughout the growth. The delayed injection of the CuInS2 seed NCs is crucial to allow the concentration of [ZnS] monomers to build up, thereby maximizing the anisotropic heteroepitaxial growth rates while minimizing the rates of competing processes (etching, cation exchange, alloying). Nevertheless, a mild etching still occurred, likely prior to the onset of heteroepitaxial overgrowth, shrinking the core size from 5.5 to ∼4 nm. The insights provided by this work open up new possibilities in designing multifunctional Cu-chalcogenide based colloidal heteronanocrystals.

Interplay between Surface Chemistry, Precursor Reactivity, and Temperature Determines Outcome of ZnS Shelling Reactions on CuInS2 Nanocrystals

“… 2 However, under seeded growth conditions, the crystal structure of the CIS seed NCs has a dramatic impact on the morphology of the product CIS/ZnS hetero-NCs, leading to CIS/ZnS dot core/rod shell heteronanorods, if wurtzite CIS NCs are used as seeds and the [ZnS] monomer concentration is allowed to build-up by delaying the injection of the seeds with respect to the injection of the Zn- and S-precursors. 78 This recent study clearly demonstrates the crucial role of the monomer formation rates in determining the outcome of shelling reactions, in agreement with the mechanism proposed above. There are also studies that used conditions that favor cation exchange (e.g., addition of only Zn-precursors), resulting in (CIS, ZnS) alloy NCs with high Zn-content and consequently very large blue-shifts (340 meV).…”

Optical Properties, Synthesis, and Potential Applications of Cu-Based Ternary or Quaternary Anisotropic Quantum Dots, Polytypic Nanocrystals, and Core/Shell Heterostructures

“…Xia and Winckelmans were the first who have produced a CIS/ZnS dot-in-rod heterostructure via a seeded growth method [163]. WZ CIS seeds were synthesised first, that was followed by growth of ZnS tips (Figure 11).…”

“…Scheme of the synthesis of the CIS/ZnS dot-in-rod heterostructure. Reproduced with permission from [163]. Copyright American Chemical Society, 2018.…”

The Photoluminescence and Biocompatibility of CuInS2-Based Ternary Quantum Dots and Their Biological Applications

“…Proper control over composition can lead to the formation of a different crystal structure (cubic to wurtzite structure) [28][29][30][31]. Several reports have been published on the synthesis of CIS NCs such as synthesis in solid phase [32,33], hydrothermal (including pressure cooker) [18,[34][35][36][37], decomposition of single-source precursors [28,[38][39][40][41] and the hot injection [42][43][44][45] method (Table 1). Hirpo et al [46] reported the conversion of (Ph 3 P) 2 CuIn(SEt) 4 and (Ph 3 P) 2 CuIn(SBu) 4 to CIS NCs.…”

“…To improve the PL emission of the CIS QDs, the most common strategies employed are the coating of shells and alloying strategies [13][14][15]18,42,43,55,56,58,59,61,65,66,[68][69][70]72,74,75,78,[87][88][89][90]. Earlier studies revealed that stoichiometry control is important in obtaining highly luminescent CIS QDs [20,21].…”