While nanocrystals offer color tunability and are used in various technologies, achieving different colors requires using different nanocrystals for each color, and dynamic switching between colors has not been possible. A team of researchers at the Institute of Chemistry and the Center for Nanoscience and Nano­technology at the Hebrew University of Jerusalem, including graduate student Yonatan Ossia with seven other members, and led by Uri Banin, have now come up with an innovative solution to this problem. By developing a system of an artificial molecule made of two coupled semi­conductor nano­crystals which emit light in two different colors, fast and instan­taneous color switching was demonstrated.

Upon taking color emitting semiconductors to the nanoscale, quantum confinement comes into play: changing the size of the nanocrystal modifies the color of the emitted light. Thus, bright light sources can be obtained covering the entire visible spectrum. Due to the unique color tunability of such nano­crystals, and their facile fabri­cation and manipulation using wet-chemistry, they are already widely used in high-quality commercial displays, giving them excellent color quality along with significant energy saving characteristics. However, to this day, achieving different colors required the use of different nano­crystals for each specific color, and dynamical switching between the different colors was not possible.

Although color tuning of single colloidal nanocrystals which behave as artificial atoms has been previously inves­tigated and implemented in prototype opto­electronic devices, changing colors actively has been challenging due to the diminished brightness inherently accom­panying the effect, which only yielded a slight shift of the color. The research team overcame this limitation, by creating a novel molecule with two emission centers, where an electric field can tune the relative emission from each center, changing the color, yet, without losing brightness. The arti­ficial molecule can be made such that one of its constituent nano­crystals is tuned to emit green light, while the other red light.

The emission of this new dual color emitting artificial molecule is sensitive to external voltage inducing an electric field: one polarity of the field induces emission of light from the red center, and switching the field to the other polarity, the color emission is switched instan­taneously to green, and vice versa. This color switching phenomena is reversible and immediate, as it does not include any structural motion of the molecule. This allows to obtain each of the two colors, or any combination of them, simply by applying the appro­priate voltage on the device.  

This ability to precisely control color tuning in optoelectronic devices while preserving intensity, unlocks new possibilities in various fields including in displays, lighting, and nanoscale opto­electronic devices with adjustable colors, and also as a tool for sensitive field sensing for biological appli­cations and neuroscience to follow the brain activity. Moreover, it allows to actively tune emission colors in single photon sources which are important for future quantum communication techno­logies.

Uri Banin explained, “Our research is a big leap forward in nanomaterials for opto­electronics. This is an important step in our exposition of the idea of nanocrystal chemistry launched just a few years ago in our research group, where the nanocrystals are building blocks of artificial molecules with exciting new func­tionalities. Being able to switch colors so quickly and efficiently on the nanoscale as we have achieved has enormous possibilities. It could revolutionize advanced displays and create color-switchable single photon sources.”

By utilizing such quantum dot molecules with two emission centers, several specific colors of light using the same nanostructure can be generated. This approach opens doors to developing sensitive techno­logies for detecting and measuring electric fields. It also enables new display designs where each pixel can be indi­vidually controlled to produce different colors, simpli­fying the standard RGB display design to a smaller basis of pixels, which has the potential to increase the resolution and energy savings of future commercial displays. This advancement in electric field induced color switching has immense potential for transforming device customi­zation and field sensing, paving the way for exciting future innovations. (Source: Hebrew U.)

Reference: Y. Ossia et al.: Electric-field-induced colour switching in colloidal quantum dot molecules at room temperature, Nat. Mat., online 3 August 2023; DOI: 10.1038/s41563-023-01606-0

Link: Center for Nanoscience and Nanotechnology, Hebrew University of Jerusalem, Jerusalem, Israel