Signatures of Exciton Dynamics and Carrier Trapping in the Time-Resolved Photoluminescence of Colloidal CdSe Nanocrystals

Abstract

Surface effects significantly affect the photoexcitation dynamics of colloidal nanocrystals, but their influence is hard to study because of sample complexity and the typically small extinction coefficient of trap states. Using temperature-dependent time-resolved photoluminescence (PL) measurements, we investigate the perturbations induced by surface-localized carrier traps on the exciton dynamics of the nanocrystals. We present a model of carrier trapping that is based on Marcus' electron-transfer theory and use it to accurately reproduce PL dynamics over a wide temperature range in five core-shell CdSe/CdZnS nanocrystal samples. The resulting pictures of carrier dynamics are then used to identify features in the PL data that may be used in subsequent experiments to reveal information about the energy and distribution of surface-localized trap states. We find that in certain cases, the shape of the ensemble distribution of trap energies can be accurately determined from data recorded at a single temperature that is easily identified from plots of average PL lifetime.

Enhanced Exciton–Phonon Coupling in Colloidal Type-II CdTe-CdSe Heteronanocrystals

“…57 Carrier trapping and PL quenching produce very clear signatures in PL decay curves, leading to deviations from a single exponential behavior. 69,74 We note that the CdSe QD samples investigated in this work have high room temperature PL QYs (>30%), and no trap related PL, even at 4.2 K. Moreover, the exciton lifetimes observed at 4.2 K are purely radiative (single exponential decays) and are consistent with values previously reported for similarly sized CdSe QDs. 68,69,80 Considering the discussion above, it is clear that the emission monitored while scanning the PLE spectra of CdSe QDs at 4.2 K originates primarily from the E ±2 fine-structure state of the 1S (e) 1S 3/2(h) exciton and that the involvement of surface states can be confidently excluded.…”

Temperature dependent recombination dynamics in InP/ZnS colloidal nanocrystals

“…The values of decay rates k 1 and k 2 are only weakly changing with temperature, while k 3 undergoes very strong oscillation around 50 K. This latter behaviour is very characteristic of traps in resonance with the core excitonic levels 4 which combined with low value of k 4 points to the external surface of the nanocrystal as the location of the traps. An amplitude of k 1 component of the decay curves decreases until at about 70 K and it becomes too small to be extracted by fitting.…”

“…A finite size as well as the quantisation of states in small nanocrystals modifies their band structure compared with bulk materials, often leading to different optical transitions compared with the ones familiar from the latter. 3 Radiative recombination of photo-excited carriers competes in NCs with trapping on surface states 4 and with Auger recombination. 5 Reduction of the parasitic recombination channels is of paramount importance for implementation of nanocrystals in devices and it requires detailed understanding of the carrier dynamics.…”

Colloidal synthesis and optical properties of type-II CdSe–CdTe and inverted CdTe–CdSe core–wing heteronanoplatelets

“…7. These biexponential PL decays result from an intercombination of the two lowest lying, emitting exciton states (dark and bright states) of CdSe 48 and CdTe platelets at the heterojunction. For the CdSe and CdSe-CdS we also tried biexponential fitting again taking the convolution with the instrument response into account.…”