Exploration of optical cooling in semiconductor quantum dots

Cooling systems are an essential part of many modern technologies because heat tends to wear materials and reduce performance in a variety of ways. However, cooling can often be an inconvenient and energy-intensive process. Therefore, scientists have been searching for innovative and efficient ways to cool materials.

Solid-state optical cooling is a notable example of exploiting a very unique phenomenon called anti-Stokes (AS) emissions. Typically, when a material absorbs a photon from incoming light, its electrons transition to an “excited” state.

Under ideal conditions, when the electrons return to their original state, some of this excess energy is released as light and the rest is converted into heat.

In materials subjected to AS radiation, electrons interact with crystal lattice vibrations called “phonons” and the emitted photons become more energetic than the photons of the incident light. If the luminous efficiency of AS is close to 100%, these materials could theoretically be cooled rather than heated when exposed to light.

In a study published in Nano Letters on August 29, 2024, a research team led by Professor Yasuhiro Yamada of the Graduate School of Science at Chiba University in Japan delved into this phenomenon in the structure of a promising perovskite-based material.

A team led by Ken Oki from the Chiba University Graduate School of Science and Engineering, Kazunobu Kojima from the Osaka University Graduate School of Engineering, and Yoshihiko Kanamitsu from the Institute of Chemical Research at Kyoto University developed a Cs4PbBr6 host crystal matrix (CsPbBr3/Cs4PbBr6 crystal).

“Efforts to realize optical cooling in semiconductors have faced several difficulties, mainly due to challenges in reaching near 100% luminous efficiency, making true cooling difficult,” Yamada explains. .

“Although quantum dots are expected to have high luminous efficiency, they are notoriously unstable, and their luminous efficiency decreases when exposed to air or continuously illuminated. focused on a stable structure known as a “dot-in-crystal” that has the potential to overcome These restrictions. ”

There are unresolved issues with the use of semiconductor quantum dots. When light hits a semiconductor, it creates excitons, which are pairs of electrons and positively charged “holes.” When excitons recombine, they typically emit light.

However, as the exciton density increases, a process called Auger recombination becomes more pronounced, which releases energy as heat rather than light. For semiconductor quantum dots, this process often results in heating rather than cooling when exposed to high-intensity light.

So the researchers used time-resolved spectroscopy to identify conditions under which Auger recombination occurs more frequently. These experiments show that heating is unavoidable even at moderate light intensities, implying that experiments under low-intensity light are required to observe true optical cooling. I am.

Unfortunately, when the intensity is low, optical cooling is less effective. Under the best conditions, their samples showed a theoretical cooling limit of about 10 K from room temperature.

Another focus of the study was to make more reliable temperature measurements than previously reported efforts. To this end, they developed a method to estimate the temperature of a sample with high luminescence efficiency by analyzing the shape of the emission spectrum.

True optical cooling was observed in multiple samples, and the researchers noted that a transition from cooling to heating occurs as the intensity of the excitation light increases.

“Previous reports on optical cooling in semiconductors have been unreliable, mainly due to deficiencies in temperature estimation. However, our work not only established a reliable method but also demonstrated that optical cooling through time-resolved spectroscopy “We have defined the possibilities and limits of cooling and achieved important results in this field,” Yamada said.

This work paves the way for future research focused on minimizing Auger recombination to improve the cooling performance of intracrystalline dot arrangements.

If optical cooling is significantly improved and widely implemented, it could form the basis of several energy-saving technologies and contribute to global sustainability goals.