Unravelling elementary non-radiative effects in nanocrystal quantum dots

Abstract

Spectrally resolved fluorescence decay measurements reveal a non-Auger like dependence of the photoluminescence decay rate with excitation density. Our results suggest that Auger recombination accelerates phonon assisted ionisation that lead to the formation of dark, non-emissive nanocrystal states.The optoelectronic properties of semiconductor nanocrystal quantum dots (NQDs) bear great relevance to fundanental research in semiconductor physics [1,2]. Moreover, the tunability of their photoluminescence (PL) from ultraviolet to near infrared wavelengths and the large absorption cross-section under non-resonant excitation render these nanostructures promising elements of future applications, e.g. light-emitting diodes, biophysical fluorescence markers and for biomedical sensitisation purposes [3][4][5]. It is therefore important to understand the nature of spontaneous emission and competing non-radiafive decay channels in these materials. Fig. 1. Elementary non-radiative effects in core/shell NQDs with smaller band gap energy (Eg) than carrier ionisation energy (Es). A carrier from the ground state can be scattered to a trap by phonon assisted tunneling (left sketch) or to a higher energy state via an Auger recombination process (middle sketch). An excited carrier can relax energetically, or alternatively be scattered to a trap via phonon assisted tunnelling (right sketch). We show that the correlation between Auger recombination and phonon assisted tunnelling dominates the fluorescence decay dynamics in the high excitation density regime.In this paper, we present the experimental results of our study [6] on the interplay between Auger effects and ionisation processes in the limit of strong electronic confinement in core/shell CdSe/ZnS semiconductor nanocrystal quantum dots (NQDs). We show that cooperative non-radiative effects dominate the fluorescence decay dynamics in the high excitation density regime. Spectrally resolved fluorescence decay measurements reveal a monotonic increase of the photoluminescence (PL) decay rate with excitation density. In the usually studied regime of low excitation densities, PL decay rates remain virtually unaffected, whereas under high density excitation, in the multi e-h pair regime, PL decay rates clearly increase with increasing density. A kinetic analysis, pictorially described in Fig. 1, suggests that phonon assisted tunnelling is accelerated in the presence of the comparatively faster Auger recombination.In semiconductor nanostructures Auger effects strongly depend on the degree of spatial confinement [7]. Therefore, the distribution of NQD sizes in an ensemble impedes a quantitative interpretation of the recombination dynamics. Spectral filtering of the PL can substantially reduce the size distribution of NQDs involved in the dynamics. The distribution of sizes in the ensemble is mirrored in the inhomogeneously broadened PL specta of the film, as shown by the solid curve in the inset of Fig.2. Interferometric filters are usedto spectrally narrow the PL 546