Quantum dots and nanorods are tiny particles of semiconductor material that can emit light of specific colors when exposed to an electric current. They are used in a variety of electronic devices, such as light-emitting diodes (LEDs) and lasers.
Researchers are always looking for ways to improve the efficiency of quantum dots and nanorods, as this would allow them to be used in a wider range of applications. The IMRE team's discovery could provide a way to do just that.
The researchers found that the key to bright light emission is the presence of "surface states" on the quantum dots and nanorods. Surface states are electron states that are located at the surface of a semiconductor material. They are created when atoms are missing from the surface of the material, leaving behind dangling bonds.
When an electric current is applied to a quantum dot or nanorod, the electrons in the surface states are excited and emit light. The more surface states there are, the more light the quantum dot or nanorod will emit.
The researchers found that quantum dots and nanorods made of pure semiconductors have more surface states than quantum dots and nanorods that contain impurities. This is because impurities can reduce the number of dangling bonds on the surface of the material.
The researchers' findings could lead to the development of more efficient quantum dots and nanorods for use in a variety of electronic devices.
Semiconductor nanocrystals (quantum dots, QDs) are promising candidates for future light-emitting devices due to their size-tunable emission and narrow emission linewidth. However, many of the synthesis methods for producing QDs also introduce a significant level of impurities, which often compromise the QD optical properties. Using theoretical calculations and experimental measurements, we demonstrate that these impurities quench the QD emission by providing alternative non-radiative decay channels for the photoexcited carriers. Furthermore, we unveil the critical role of surface states (dangling bonds) in enabling bright emission. We demonstrate that a higher density of surface states enhances the radiative decay and thus increases the emission quantum yield. For high-quality CdSe QDs capped with trioctylphosphine oxide (TOPO), we identify an optimal QD size (∼4.5 nm) that maximizes the number of surface states. This corresponds to the highest PL quantum yield, reaching 58%. Our findings provide guidelines for the purification of QDs that will greatly advance the applications of QD-based optoelectronic devices.
