Research has found that copper doped zinc sulfide exhibits photochromic behavior and can be used for automotive glass

When photochromic materials are exposed to ultraviolet or visible light, their color and optical properties will undergo reversible changes. This material is made of organic compounds, and the synthesis of organic compounds is usually very expensive. According to reports, scientists from Ritsumeikan University in Japan have discovered for the first time the rapid conversion of photochromic phenomena in inexpensive inorganic materials. The inorganic material is a copper doped zinc sulfide nanocrystal. The results of this study pave the way for its potential applications in intelligent adaptive glass windows, sunglasses, and anti-counterfeiting agents.

The glass windows of the office building can adaptively dim according to the intensity of sunlight, and the glasses can automatically turn into sunglasses in the sun and return to regular glasses after entering the building. This is all thanks to the invention of photochromic materials, and all of these inventions have the potential to be realized. Nowadays, almost all fast converting photochromic materials are made of organic compounds, which are not only expensive and complex to synthesize, but also require complicated processing procedures, making it difficult to achieve large-scale production. Therefore, although this material has countless potential applications, its commercial applications are limited. Finding a rapidly convertible inorganic photochromic material for commercialization is highly challenging.

(Image source: Ritsumeikan University)

In this study, a research team led by Associate Professor Yoichi Kobayashi from Ritsumeikan University in Japan discovered that zinc sulfide (ZnS) nanocrystals doped with copper (Cu) ions exhibit unique photochromic properties. When exposed to ultraviolet and visible light (UV Vis), these crystals change from milky white to dark gray. It is interesting that after turning off the radiation source, the material takes about a full minute to recover to its original milky white color in the air, while it only takes a few microseconds when immersed in an aqueous solution. The research team analyzed this material both theoretically and experimentally, and decided to explore the complexity of this particular photochromic behavior.

Why do copper doped zinc sulfide nanocrystals change color when exposed to light, and why does it take a long time to restore their original color? As the research results show, this is closely related to the dynamics of photoexcited charge carriers. When photons collide with materials, the collision excites electrons, causing them to deviate from their originally stable positions on molecular orbitals. After losing electrons, a local positive charge is left, which is called a "hole" in solid-state physics.

In most materials, electron hole pairs exist for a short period of time before canceling each other out, thereby releasing a portion of the energy initially obtained by the electrons. However, the situation is quite different in copper doped zinc sulfide. Holes are effectively captured by Cu ions, while photoexcited electrons can freely jump onto other molecules, thereby delaying the recombination process. Research has shown that the prolonged presence of holes can alter the optical properties of materials, leading to photochromic effects.

The first discovery of inorganic nanocrystals that rapidly convert photochromic light is a significant advancement in this field, especially in practical applications. Kobayashi stated, "Zinc sulfide is relatively non-toxic, and its synthesis process is simple and inexpensive. We believe that our research results will promote the widespread use of fast conversion photochromic materials in daily life." Photochromic materials are commonly used in 3D TVs, smart glasses, vehicle and house windows, high-speed holographic memory, and advanced anti-counterfeiting agents for important brands and drugs.

In addition, this study is also beneficial for researchers in other fields of applied optical physics. Kobayashi stated, "We have demonstrated that the photochromic reaction of nanomaterials can be regulated by controlling the lifetime of photoexcited charge carriers. Developing new nanomaterials with ultra long lifetime excited charge carriers is crucial for photochromic materials and advanced photo functional materials, such as luminescent materials and photocatalysts." This research can pave the way for practical applications of photochromism, including adaptive lighting.