Vapochromism and Enhanced Yellow Emission of CuI Under Ammonia Vapor

Copper(I) iodide (CuI) is a wide-bandgap p-type semiconductor and an important hole-transporting material in optoelectronic devices, yet its powder samples usually exhibit weak photoluminescence (PL). In this work, we demonstrate that exposure to ammonia vapor induces distinct vapochromism and markedly enhances the PL of CuI powders. Pristine CuI shows a weak emission centered at 703 nm. Upon exposure to NH₃, this red band gradually decreases, wheras a new yellow emission band at 615 nm emerges and becomes dominant, leading to an overall increase in the PL intensity and a clear color change. X-ray diffraction and Fourier transform infrared spectroscopy reveal the formation of copper(I)-ammonia complexes, indicating that the yellow emission may originate from Cu(I)-NH₃-I related emissive centers. Similar yellow emission ~615 nm is observed when CuI powders are exposed to various amino-containing organic molecules, suggesting that the formation of Cu(I)-amine surface complexes and the associated yellow emission could be a general phenomenon for nitrogen-coordinating species. These findings provide a simple vapor-assisted strategy to tune and enhance the emission of CuI powders and highlight their potential as vapochromic fluorescent probes for ammonia and amines, as well as phosphor candidates for low-cost light-emitting applications.