Near-Infrared Cu–In–Se-Based Colloidal Nanocrystals via Cation Exchange

Abstract

We developed a three-step colloidal synthesis of near-infrared (NIR) active Cu–In–Se (CISe)-based nanocrystals (NCs) via a sequential partial cation exchange realized in one pot. In the first step, binary highly copper deficient Cu2–xSe NCs were synthesized, followed by a partial cation exchange of copper to indium ions, yielding CISe NCs. This reaction allows for a precise control of the composition of the resulting NCs through a simple variation of the ratio between guest-cation precursors and parent NCs. To enhance the stability and the photoluminescence (PL) properties of the NCs, a subsequent ZnS shell was grown in the third step, resulting in CISeS/ZnS core/shell particles. These core/shell hetero-NCs exhibited a dramatic increase in size and a restructuring to trigonal pyramidal shape. The shell growth performed at a relatively high temperature (250 °C) also led to anion exchange, in which sulfur replaced part of selenium atoms close to the surface of the NCs, forming alloyed CISeS core structure. This efficient anion exchange is rarely reported for I–III–VI-based nanomaterials. Furthermore, we demonstrated that at higher reaction temperature, it is possible to obtain In-rich CISeS/ZnS NCs whose emission was shifted to the visible region. Therefore, a careful tuning of the reaction parameters, such as the Cu:Se and Cu:In ratios, temperature, and time, enables a distinct control over the size and composition of the NCs while maintaining their crystal structure. By varying the size of the CISeS/ZnS NCs from 9 to 18 nm, the PL spectra could be tuned, covering a wide NIR range with maxima from 990 to 1210 nm. Thus, in these experiments, we demonstrated a clear dependence of the optical properties of these materials on their size and extended the PL range of CISe-based NCs further to the infrared part of the spectrum. The results obtained might be important not only to elucidation of fundamentals of ion exchange reactions but also may provide a general preparative approach to a wide variety of copper chalcogenide-based NCs with well-controlled size, shape, composition, and even crystal structure.