Magnetic impurities turn colloidal quantum dots into emitters of free electrons

Colloidal Quantum Dots (CQDs) are solution-based nanoscale semiconductors with a wide range of applications in lighting, displays, solar energy harvesting and optoelectronic devices. Under optical excitation, those objects host excitons (electron-hole pairs). The dynamics evolution of those excitons is studied in our lab using ultrafast spectroscopy techniques.

Auger recombination is a non-radiative process wherein an electron recombines with a hole, while another ‘energy-acceptor’ electron captures the released energy and thereby is excited to a higher energy state. This process has been extensively studied for CQDs, in particular because it is detrimental for light-emitting applications.

Interestingly, uphill energy transitions associated with Auger recombination can be exploited to increase carrier energies at the expense of their number. This ‘up-conversion’ process could be useful in, e.g., advanced photovoltaics, photochemistry, and photoemission. The efficiency of this pathway is nevertheless limited in CQDs by the very fast intraband relaxation of hot carriers that outpaces Auger lifetime by an order of magnitude.

Using Mn-doped CdSe CQDs, we demonstrate that the Auger process can be accelerated circa 100-fold, thanks to spin-exchange processes between the CdSe excitons and Mn dopant states. These spin-exchange Auger interactions are due to the formation of hybrid multiexciton states, where the excitonic population is split between CdSe band-edge and Mn ions, owing to ultrafast (<100 fs) spin-exchange exciton transfer. In these doped CQDs, Auger lifetimes are of ~200 - 300 fs, which allow for the energy gain rates to outpace the energy loss rates by a factor of ~7. As a result, a band-edge electron can experience multiple steps of uphill Auger re-excitation and thereby reach a vacuum state outside the dot.

We practically demonstrate high efficiency photoemission via two-step spin-exchange Auger recombination using visible light pulses. Further, we take advantage of this ionization pathway to achieve high-yield generation of reactive solvated electrons (free electrons stabilized by water molecules) with unprecedented internal quantum efficiency of >3%. These results demonstrate great utility of the discovered effects in practical photoemission and photoconversion technologies.