High-Efficiency Two-Step Spin-Exchange Auger Photoemission from Manganese-Doped Quantum Dots

Photoemission is a process wherein a material emits free electrons upon its illumination with electromagnetic radiation. Typically, it requires energetic ultraviolet or even X-ray photons. Here we show that using CdSe colloidal quantum dots (CQDs) heavily doped with manganese (Mn), we can realize this process with visible light. The observed effect is enabled by extremely fast (<300 femtoseconds) spin-exchange Auger energy transfer from excited Mn ions to an intrinsic CQD exciton. Since the rate of this process outpaces that of intra-band cooling, the high-energy ‘hot’ electron produced by the first Auger-excitation step can be efficiently promoted further into the external ‘vacuum’ state via one more Mn-to-CQD energy-transfer step. This CQD ionization pathway exploits exceptionally large up-hill energy gain rates associated with the spin-exchange Auger process (>10 eV ps^-1) and leads to photoemission efficiencies of more than 3%, orders of magnitude greater than in the case of undoped CQDs. We demonstrate that using this phenomenon, we can achieve high-yield production of solvated electrons (>3% internal quantum efficiency), which makes it of considerable utility in visible-light-driven reduction photochemistry. Other potential applications include low-cost, visible-light-driven photocathodes and advanced photoconversion.