We report p-state excited state (ES) emission well below ground state (GS) saturation in CdSe nanoplatelets. Size dependent exciton ground state-excited state energies and dynamics are determined by three independent methods, time-resolved photoluminescence (PL), time-integrated PL 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. Our results suggest that the observed bi-exponential PL decay of CdSe platelets is governed by a phonon bottlenecksuppressing inter-relaxation of excited and ground state. This phonon Bottleneck is related to the reported low exciton phonon coupling in CdSe platelets and only observable as bimodal PL due to the very large oscillator strength and energy spacing of both states. As we can show that the bottleneck can be tuned by the lateral platelet size, the recombination dynamics can be tuned independently from the optical transition energies. The transition energies can be engineered by the strong transversal confinement of those colloidal quantumwells, whereas the effective transition rates can be tuned via the lateral size of the platelets. 

Due to the LO-bottleneck a dynamic thermal equilibrium between ES and GS states mediated by LO phonon scattering is observed. The existence of a phonon bottleneck between ES and GS is confirmed by three methods: A rate equation model for the temperature dependence, the temporal course of ES and GS emission and the observation of an ES/GS intensity ratio minimum in the time-integrated PL for size dependent ES-GS energyspacings resonant to the LO phonon energy of 25.4 meV in the CdSe nanoplatelets. We show that the observed high energy emission peak originates systematically from the NPL’s ES and correlates to the lateral confinement dependent ES–GS energy difference. Calculations and time integrated PL show a strong increase of the ES-GS energy spacing from about 18 to 36meV with increasing lateral quantization of the exciton wavefunction. The observed but yet not understood bi-exponential PL decay of nanoplatelets is therefore related to a phonon bottleneck between ES and GS populations allowing to observe ES luminescence. The presented existence of an excited p-state with high oscillatorstrength makes it necessary to reconsider several recently published results on CdSe nanoplatelets, e.g. with respect to lasing, FRET and other properties like field controlled modulators.

References: 1) Achtstein and Scott et al. PRL. 116, 116802(2016)                                                

                  2) Achtstein et al. Nano Lett. 12, 3151 (2012)