High-efficiency photoemission from magnetically doped quantum dots driven by multi-step spin-exchange Auger ionization

Abstract

Researchers show that up-conversion in manganese-doped CdSe colloidal quantum dots enables efficient electron photoemission. The effect is exploited for high-yield production of solvated electrons, demonstrating photochemistry applicability. Materials displaying electron photoemission under visible-light excitation are of great interest for applications in photochemistry, photocathodes, advanced electron beam sources and electron microscopy. We demonstrate that in manganese-doped CdSe colloidal quantum dots (CQDs), two-step Auger up-conversion enables highly efficient electron photoemission under excitation with visible-light pulses. This effect is enabled by extremely fast, subpicosecond Auger-type energy transfer from excited manganese ions to an intrinsic CQD exciton. Since the rate of this process outpaces that of intraband 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 manganese-to-CQD energy-transfer step. This CQD ionization pathway exploits exceptionally large uphill energy gain rates associated with the spin-exchange Auger process 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.