Spin blockade and phonon bottleneck for hot electron relaxation observed in n-doped colloidal quantum dots

Understanding and manipulating hot electron dynamics in semiconductors may enable disruptive energy conversion schemes. Hot electrons in bulk semiconductors usually relax via electron-phonon scattering on a sub-picosecond timescale. Quantum-confined semiconductors such as quantum dots offer a unique platform to prolong hot electron lifetime through their size-tunable electronic structures. Here, we study hot electron relaxation in electron-doped (n-doped) colloidal CdSe quantum dots. For lightly-doped dots we observe a slow 1P_e hot electron relaxation (~10 picosecond) resulting from a Pauli spin blockade of the preoccupying 1S_e electron. For heavily-doped dots, a large number of electrons residing in the surface states introduce picosecond Auger recombination which annihilates the valance band hole, allowing us to observe 300-picosecond-long hot electrons as a manifestation of a phonon bottleneck effect. This brings the hot electron energy loss rate to a level of sub-meV per picosecond from a usual level of 1 eV per picosecond. These results offer exciting opportunities of hot electron harvesting by exploiting carrier-carrier, carrier-phonon and spin-spin interactions in doped quantum dots.