Hot Electron Cooling in <i>n</i>‑Doped\nColloidal Nanoplatelets Following Near-Infrared Intersubband Excitation

Intersubband transition was recently\ndiscovered in colloidal nanoplatelets,\nbut the associated intersubband carrier relaxation dynamics remains\npoorly understood. In particular, it is crucial to selectively excite\nthe intersubband transition and to follow the hot electron dynamics\nin the absence of valence-band holes. This is achieved herein by exciting\nthe predoped electrons in CdSe/ZnS nanoplatelets using near-infrared\nfemtosecond pulses and monitoring nonequilibrium electron dynamics\nusing broad-band visible pulses. We find that the <i>n</i> = 2 electrons relax to the <i>n</i> = 1 subband and establish\na Fermi–Dirac distribution within 200 fs, and finally reach\nan equilibrium with the lattice within a few ps. The cooling dynamics\ndepend mainly on the excitation fluence but weakly on the doping density\nand the lattice temperature. These characteristics are well captured\nby our numerical simulation that explicitly accounts for the state\noccupation effect and optical phonon scattering.