Highly emissive multiexcitons in steady-state photoluminescence of individual "giant" CdSe/CdS Core/Shell nanocrystals

Abstract

The development of nanocrystal quantum dots (NQDs) with suppressed nonradiative Auger recombination has been an important goal in colloidal nanostructure research motivated by the needs of prospective applications in lasing devices, light-emitting diodes, and photovoltaic cells. Here, we conduct single-nanocrystal spectroscopic studies of recently developed core-shell NQDs (so-called "giant" NQDs) that comprise a small CdSe core surrounded by a 16-monolayer-thick CdS shell. Using both continuous-wave and pulsed excitation, we observe strong emission features due both to neutral and charged biexcitons, as well as multiexcitons of higher order. The development of pronounced multiexcitonic peaks in steady-state photoluminescence of individual nanocrystals, as well as continuous growth of the emission intensity in the range of high pump levels, point toward a significant suppression of nonradiative Auger decay that normally renders multiexcitons nonemissive. The unusually high multiexciton emission efficiencies in these systems open interesting opportunities for studies of multiexciton phenomena using well-established methods of single-dot spectroscopy, as well as new exciting prospects for applications, that have previously been hampered by nonradiative Auger decay.

Figure 1

PL spectra of two individual g-NQDs (‘a’ and ‘c’) measured at progressively increasing pump intensities (indicated in normalized units) of cw 532 nm excitation indicate a transition from single exciton (X₁) to biexciton (X₂) emission. The dependence of the amplitudes of these features on pump level is shown, respectively, in ‘b’ and ‘d’ (blue circles for X₁ and red triangles for X₂) together with the spectrally integrated PL signal (black squares).

Figure 2

Distributions of spectral shifts (ΔE) of the X₂ vs. X₁ band (histogram shown in red) and the X*₂ vs. X*₁ band (histogram shown in blue) derived from the analysis of the PL spectra of ~40 individual g-NQDs plotted in comparison to the expanded view of the ensemble PLE spectrum (black circles). The full ensemble PL and PLE spectra are shown in the inset.

Figure 3

PL spectra of two individual g-NQDs (‘a’ and ‘c’; the same dots as those in Figs. 1c and 1a, respectively) measured at progressively increasing pump intensities (indicated in normalized units) of cw 405 nm excitation that indicate two different types (A and B) of spectral features. The A-type g-NQD shows spectral peaks (in ‘a’) and PL pump-intensity dependences (in ‘b’) that are similar to those observed for cw 532 nm excitation and characteristic of emission from single excitons and biexcitons in neutral dots. The spectral features (in ‘c’) and pump-intensity dependences observed for the B-type g-NQD are consistent with emission from trions and charged biexcitons from charged nanocrystals.

Figure 4

Spectral features X₂ and X₁ in neutral (type-A) nanocrystals are due to emission from biexcitons and single excitons, respectively. The blue-shift of the X₂ emission energy with regard to the X₁ transition (not shown in the figure) is due to exciton-exciton repulsion, which develops in g-NQDs as a result of imbalance in spatial distributions of electron and hole wave functions. The decay constant of the biexciton (τ₂) is likely defined solely by radaitive transitions. In charged (type-B) g-NQDs, photoexcited biexcitons (charged biexcitons) emit due to both the band-edge transition [L₁(e) − L₁(h)] and the transitions involving a higher energy state [L₁(e) − L₂(h); shown by the arrow]. In the figure, for the purpose of example, we show a dot which contains an excess hole; however, based on the available data we cannot indentify with certainty the sign of the charge residing in the nanocrystal. Based on measured pump-dependent PL intensity, the decay of the charged biexciton (time constant τ_2*) is likely dominated by radiative processes.

Figure 5

(a) Time integrated PL spectra of the individual g-NQD measured using pulsed excited 405 nm excitation for different per pulse fluences (indicated in normalized units in the figure). In addition to the X₁ and X₂ bands, the spectra reveal four more higher-energy peaks (labeled X₃ –X₆) that are due to emission of triexcitons and other multiexcitons of higher order. (b) Pump-intensity dependence of the X₁ (black open circles) and the X₂ (blue open squares) features along with the spectrally integrated PL intensity (gray solid circles); dashed and solid lines correspond to linear and quadratic dependences, respectively. At low intensities, the measured dependence for the spectrally integrated PL signal is nearly linear but becomes sub-linear at j > 0.1. A progressive deviation of the measured data from the linear dependence with increasing j indicates that the decay of higher-order multiexcitons is contributed by Auger recombination, which leads to decreasing multiexciton emission yields with increasing exciton multiplicity. (c) PL dynamics measured at the positions of the X₂, X₃, and X₄ features for j = 1. The X₂ trace measured for j = 0.07 is also shown for comparison.