High-Efficiency Photoemission Due to Ultrafast Spin-Exchange Auger Interactions in Mn-Doped CdSe Quantum Dots

Auger recombination is a non-radiative process wherein an ‘energy-donor’ electron recombines with a hole, while another ‘energy-acceptor’ electron captures the released energy and whereby is excited to a higher energy state. This process has been extensively studied for colloidal quantum dots (CQDs), in particular because it is detrimental for light-emitting applications such as lasers and light emitting diodes (LEDs) [1].<br/>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 limited by intraband relaxation of hot carriers, which can be quantified in terms of an energy gain/loss-rate ratio [2]. In a typical CQD, the Auger lifetime is on the order of tens of ps, while energy losses due to phonon emission occur on a sub-ps time scale. As a result, the energy losses outcompete the energy gains by a factor ~3 [3,4].<br/>Using transient absorption spectroscopy to probe the band-edge dynamics of Mn-doped CdSe CQDs, we demonstrate that the Auger processes can be accelerated <i>circa </i>100-fold, thanks to spin-exchange processes between the CdSe excitons and Mn dopant states [5]. These Mn-assisted 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 (&lt;100 fs) spin-exchange exciton transfer [2,5]. 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 <i>via</i> two-step spin-exchange Auger recombination using band-edge excitation with 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 [6]) with unprecedented internal quantum efficiency of &gt;3%. These results demonstrate great utility of the discovered effects in practical photoemission and photoconversion technologies.<br/><br/>[1] Klimov et al. “Quantization of Multiparticle Auger Rates in Semiconductor Quantum Dots.” <i>Science</i> 287, 1011–13, <b>2000</b><br/>[2] Singh et al. “Hot-Electron Dynamics in Quantum Dots Manipulated by Spin-Exchange Auger Interactions.” <i>Nature Nanotechnology</i>, 1–7, <b>2019</b><br/>[3] Klimov et al. “Multicarrier Interactions in Semiconductor Nanocrystals in Relation to the Phenomena of Auger Recombination and Carrier Multiplication.” <i>Annual Review of Condensed Matter Physics</i> 5, 285–316, <b>2014</b>.<br/>[4] Alig et al. “Electron-Hole-Pair Creation Energies in Semiconductors.” <i>Physical Review Letters</i> 35, 1522–25, <b>1975</b>.<br/>[5] Livache et al. “High-efficiency photoemission from magnetically-doped quantum dots driven by multi-step, spin-exchange Auger ionization”, <i>submitted</i>.<br/>[6] Jou et al. “Temperature and Isotope Effects on the Shape of the Optical Absorption Spectrum of Solvated Electrons in Water.” <i>The Journal of Physical Chemistry</i> 83, 2383–87, <b>1979</b>.