Low temperature trion formation in CdSe nanoplatelets

Negative trions are exciton-electron quasiparticles that have applications in optical qubits and other quantum protocols. Colloidal cadmium selenide nanoplatelets are candidate trion materials, since trions dominate their low temperature emission spectra; however, trion formation is not understood. We observe the absence of trion absorption and a finite trion formation lifetime, requiring an alternative explanation for trion generation. Using low-temperature transient absorption (TA) and time-resolved photoluminescence, we propose that trions form through biexciton hole trapping. This contrasts the previously assumed mechanism of charged ground state absorption.