p-State Luminescence in CdSe Nanoplatelets: Role of Lateral Confinement and a Longitudinal Optical Phonon Bottleneck.

We evidence excited state emission from p states well below ground state saturation in CdSe nanoplatelets. Size-dependent exciton ground and excited state energies and population dynamics are determined by four independent methods: time-resolved PL, time-integrated PL, rate equation modeling, and Hartree renormalized k·p calculations-all in very good agreement. The ground state-excited state energy spacing strongly increases with the lateral platelet quantization. Depending on its detuning to the LO phonon energy, the PL decay of CdSe platelets is governed by a size tunable LO phonon bottleneck, related to the low exciton-phonon coupling, very large oscillator strength, and energy spacing of both states. This is, for instance, ideal to tune lasing properties. CdSe platelets are perfectly suited to control the exciton-phonon interaction by changing their lateral size while the optical transition energy is determined by their thickness.