Boosting light emission from nanocrystals
It could well be that in your living room you have billions of semiconductor nanocrystals without even knowing.
These crystals, called “quantum dots” due to the quantum nature of their light emission, are found in high-end television screens, which use them to generate very brilliant and vivid colors. The seminal breakthrough of their discovery and synthesis was awarded the 2023 Nobel Prize in chemistry. Analogous to an oscillating guitar string emitting a sound wave, the electrons in semiconductor crystals can oscillate and emit a light wave. The frequency of the sound can be increased (resulting in a higher tone) by putting a finger on the string and thereby reducing the length that is free to vibrate. Similarly, the frequency of the light wave can be increased (higher energy i.e., a bluer color) by constraining the oscillating electrons to a smaller space, using tiny semiconductor crystals that are just a few nanometers wide. One nanometer is about 50,000 times smaller than the diameter of a single hair.
But without a guitar body, a single guitar string would produce a meager sound. Its small vibrating motion needs to be imposed on the wooden walls of the resonating body to amplify the sound tremendously. Correspondingly, the intensity of the light wave emitted by a single nanocrystal is limited. But new research published today in Nature shows that with perovskite nanocrystals, it’s possible to amplify light’s intensity.
The work was carried out by a group of researchers from ETH Zurich, Empa, the US Naval Research Laboratory, the Center for Hybrid Organic Inorganic Semiconductors for Energy (CHOISE), and IBM researchers in Zurich, all working together. We found that by enlarging the nanocrystals beyond the size required for their characteristic light frequency, which represents a lower frequency limit given by the material, they can be made to dissipate their energy much faster — or shine brighter.
To observe this effect, which is called “single photon superradiance,” it has to be very cold. Below -180°C, and ideally just a few degrees above absolute zero (-250°C), we saw that you can achieve about 10 times higher photon emission rate. Only then the electrons further out than the so-called exciton Bohr radius (3 nm) can chime together in to the “tune” of the originally oscillating electrons. Effectively, the quantum mechanical wave becomes delocalized over the whole, greater nanocrystal volume, acting as a large ensemble of coherent oscillators. At higher temperature, the atoms in the nanocrystal increasingly wobble around uncontrollably, and this motion disturbs the superradiance “concert.” Furthermore, making the nanocrystals larger and larger does not enhance the emission rate infinitely – the current limit seems to be around 10 times the Bohr radius, or about 30 nm.
These new, fundamental insights can help in the development of much brighter light sources used for quantum applications, such as quantum communication or optical quantum computing. By coupling many nanocrystals together, one can even further enhance their emission, which could be useful for ultrabright LEDs and screens. Eventually, perovskite nanocrystals could allow to build more efficient and faster optical components for data communication, and even all-optical computing devices.