Abstract: Rapid-darkening and wavelength-tunable photochromic compositions and systems are disclosed. Nanoparticles photochromically darken in response to incident light by photo-oxidizing a dopant to reversibly create an optically absorbing mid-gap electronic state. The wavelengths that trigger photochromic darkening as well as the absorbed wavelengths may be tuned by changing the nanoparticle composition, the nanoparticle size, and/or the dopant. The time for reversal of darkening may be tuned using hole-accepting ligands attached to the nanoparticles as well as hole-accepting molecules adjacent to the nanoparticles. Some variations provide a photochromic composition comprising: semiconductor nanoparticles, such as PbS or Ag2S, that have an average nanoparticle effective diameter from about 1 nanometer to about 20 nanometers; and transition-metal dopant particles capable of undergoing reversible oxidation and reduction. In the disclosed systems, there is an on/off switch for the photochromic function.

Abstract: Some variations provide a photochromic heterostructure comprising: first semiconductor nanoparticles that have an average first-nanoparticle effective diameter from 1 nanometer to 20 nanometers, wherein the first semiconductor nanoparticles are not doped; and second semiconductor nanoparticles that have an average second-nanoparticle effective diameter from 1 nanometer to 60 nanometers, wherein the second semiconductor nanoparticles are doped with one or more transition metals, wherein the second semiconductor nanoparticles have a bandgap energy that is higher than the bandgap energy of the first semiconductor nanoparticles, and wherein the first semiconductor nanoparticles and the second semiconductor nanoparticles are in physical contact with each other. The disclosed photochromic heterostructure is especially useful when lower-energy light excites lower-energy-bandgap first nanoparticles to cause higher-energy-bandgap, doped second nanoparticles to photodarken.