Fast T-Type Photochromism of Colloidal Cu-Doped ZnS Nanocrystals

This paper reports on durable and nearly temperature-independent (at 298-328 K) T-type photochromism of colloidal Cu-doped ZnS nanocrystals (NCs). The color of Cu-doped ZnS NC powder changes from pale yellow to dark gray by UV light irradiation, and the color changes back to pale yellow on a time scale of several tens of seconds to minutes after stopping the light irradiation, while the decoloration reaction is accelerated to submillisecond in solutions. This decoloration reaction is much faster than those of conventional inorganic photochromic materials. The origin of the reversible photoinduced coloration is revealed to be a strong optical transition involving a delocalized surface hole which survives over a minute after escaping from intraparticle carrier recombination due to electron-hopping dissociation. ZnS NCs can be easily prepared in a water-mediated one-pot synthesis and are less toxic. Therefore, they are promising for large-scale photochromic applications such as windows and building materials in addition to conventional photochromic applications. Moreover, the present study demonstrates the importance of excited carrier dynamics and trap depths, resulting in coloration over minutes not only for photochromic nanomaterials but also for various advanced photofunctional materials, such as long persistent luminescent materials and photocatalytic nanomaterials.