Photochromic Color Tuning of Copper-Doped Zinc Sulfide Nanocrystals by Control of Local Dopant Environments

Inorganic photochromic materials offer several advantages over organic compounds, including relatively inexpensive and higher thermal stability. However, tuning their color with the same component has remained a significant challenge. In this study, we demonstrate that the photochromic color of Cu-doped ZnS nanocrystals (NCs), which is initially pale yellow before light irradiation, can be tuned from gray to brown by adjusting the surface stoichiometry of Zn and S, which is controlled through the use of thiol and non-thiol ligands. Several experiments and quantum chemical calculations using model clusters revealed that the color change is determined by the distribution of Cu, which significantly contributes to the coloration, specifically whether it resides on the Zn-rich or S-rich surface. In contrast, particle size and Cu concentration were found to have little effect on the photochromic color. This study expands the diversity of photochromic responses in inorganic NCs and marks an important step toward the development of further advanced photochromic nanomaterials.