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When photochromic materials are exposed to ultraviolet or visible light, their color and optical properties change reversibly. The material is made from organic compounds, which are often very expensive to synthesize. According to reports, scientists at Ritsumeikan University in Japan have discovered the phenomenon of rapid switching photochromism in cheap inorganic materials for the first time. The inorganic material is a copper-doped zinc sulfide nanocrystal. The results of this research pave the way for many potential applications, such as smart adaptive glass windows, sunglasses and anti-counterfeiting agents.

The glass windows of the office building can adaptively darken according to the intensity of sunshine, and the glasses can automatically turn into sunglasses when exposed to the sun and return to ordinary glasses after entering the building. This is all thanks to the invention of photochromic materials, and all of the above inventions are possible. Today, almost all fast-switching photochromic materials are made of organic compounds, which are expensive and complex to synthesize and cumbersome to process, making mass production difficult to achieve. Therefore, although the material has numerous potential applications, its commercial applications are limited. Finding an inorganic photochromic material that can be rapidly switched is very challenging to achieve commercialization.

(Source from:Ritsumeikan University)

In the study, a research team from Japan's Ritsumeikan University, led by Associate Professor Yoichi Kobayashi, found that zinc sulfide (ZnS) nanocrystals doped with copper (Cu) ions have unique photochromic properties. These crystals change from milky white to dark gray when exposed to ultraviolet and visible light (UV-Vis). Interestingly, after turning off the radiation source, it took about a full minute for the material to return to its original milky white color in the air, but only a few microseconds when immersed in an aqueous solution. The research team analyzed the material theoretically and experimentally and decided to explore the intricacies of this special photochromic behavior.

Why do copper-doped zinc sulfide nanocrystals change color when exposed to light, and why do they take so long to return to their original color? As the results show, this has a lot to deal with the dynamics of photoexcited charge carriers. When photons hit a material, the collision excites the electrons, causing them to deviate from their otherwise stable positions in the molecular orbitals. Losing an electron leaves behind a localized positive charge, which is called a hole in solid-state physics.

In most materials, electron-hole pairs exist for a short time before canceling each other out, thereby re-releasing some of the energy the electrons originally gained. However, in copper-doped zinc sulfide, the situation is quite different. Cu ions effectively trap holes created by Cu ions, while photoexcited electrons are free to jump to other molecules, thereby delaying the recombination process. Studies have shown that the presence of holes for a long time will change the optical properties of the material, causing a photochromic effect.

The first discovery of rapidly switching photochromic inorganic nanocrystals is a major advance in the field, especially for practical applications. Kobayashi said: "Zinc sulfide is relatively non-toxic, and its synthesis process is simple and cheap. We believe that our research results will promote the widespread use of fast-converting photochromic materials in daily life." Photochromic materials can usually be used in 3D TVs, smart phones, Advanced anti-counterfeiting agents for eyeglasses, vehicle and house windows, high-speed holographic memories, and important brands and pharmaceuticals.

In addition, this study may also benefit researchers in other areas of applied optical physics. Kobayashi said: "We have proven that the photochromic reaction of nanomaterials can be adjusted by controlling the lifetime of photo-excited carriers. The development of new nanomaterials with ultra-long-lived excited carriers is important for photochromic materials, and advanced optical Functional materials are very important, such as luminescent materials and photocatalysts." This research could pave the way for practical applications of photochromism, including adaptive lighting.

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