Hot Electrons Generated from Semiconductor Nanocrystals

Hot charge carriers possessing high kinetic energy have drawn attention across a variety of research fields from its enhancement of electron transfer and injection in photovoltaic and photocatalytic applications. However, generating hot carriers from solar radiation is very difficult, requiring excess kinetic energy from high energy photons or a large photon flux. Metal nanoparticles were revealed as a potential hot carrier generation material, owing to their large absorption cross section and plasmon mediated hot electron generation mechanism already demonstrated in gold and silver nanoparticles. However, the excess kinetic energy is relatively low due to the low-lying Fermi level and fast carrier thermalization. The development of semiconductor quantum dots (QDs) has occurred over the past several decades resulting in the understanding of their photophysical properties and synthetic methods. One initial expectation was the slow carrier relaxation providing efficient hot electron generator due to a “phonon bottleneck”. Although the actual relaxation process is still fast considering Auger relaxation, QDs are still a promising hot electron generation material. Additionally, doped QDs provided an extended capability to manipulate charge carriers. Among all dopants, Mn^2+ and Cu^+ can store energy from the exciton or separate electrons from holes respectively. The long lifetime of excited Mn^2+ dopants (~ ms) and copper dopants with holes trapped around (~ μs) can realize back energy transfer from Mn^2+ dopants to band edge electrons producing high energy hot electrons. Given that Mn doped QDs showing strong ^4Tv1-^6Av1 transition of Mn^2+, It has the special capability to probe only energy transfer process in QD-reduced graphene oxide (RGO) structure. With the knowledge of energy transfer kinetics in RGO-QD composites, better photocatalyst could be designed to omit energy transfer as a pathway of carrier consumption. Then hot electron enhanced photocatalytic hydrogen production from water was demonstrated. This potentially proved the hot electron generation mechanism. Furthermore, hot electron was found to be much more capable to undergo electron transfer across insulating aluminum oxide layer which showed the potential of making hot electron based optoelectronic devices. To provide the benefit of hot electron generated from QDs, photoemission of electrons due to hot electron generation was studied as well. With ultra-high kinetic energy, hot electrons were even capable to escape QDs and emit into vacuum. Finally, highly uniformed perovskite QDs were made showing the potential of a new host of hot electron generation materials