Using shape to turn off blinking for two-colour multiexciton emission in CdSe/CdS tetrapods

Abstract

Semiconductor nanostructures capable of emitting from two excited states and thereby of producing two photoluminescence colours are of fundamental and potential technological significance. In this limited class of nanocrystals, CdSe/CdS core/arm tetrapods exhibit the unusual trait of two-colour (red and green) multiexcitonic emission, with green emission from the CdS arms emerging only at high excitation fluences. Here we show that by synthetic shape-tuning, both this multi-colour emission process, and blinking and photobleaching behaviours of single tetrapods can be controlled. Specifically, we find that the properties of dual emission and single-nanostructure photostability depend on different structural parameters-arm length and arm diameter, respectively-but that both properties can be realized in the same nanostructure. Furthermore, based on results of correlated photoluminescence and transient absorption measurements, we conclude that hole-trap filling in the arms and partial state-filling in the core are necessary preconditions for the observation of multiexciton multi-colour emission.

Figure 1. Electron microscopy images and ensemble optical properties for a nanoengineered tetrapod shape series.

(a–d) Transmission electron microscopy (TEM) images of four tetrapods (also shown schematically) differing in arm diameter (measured at the base of 3 arms for 30–40 tetrapods in each case) and arm length (also measured for 3 arms of 30–40 tetrapods for each of the tetrapod geometries), respectively—(a) TP1: 6.3 nm±0.5 nm and 24.8 nm±2.3 nm; (b) TP2: 10.9 nm±0.8 nm and 27.6 nm±2.9 nm; (c) TP3: 8.2 nm±1.3 nm and 40.7 nm±3.8 nm; (d) TP4: 10.6 nm±2.4 nm and 41.3 nm±4.6 nm (scale bar, 20 nm (in a applies also to b–d)) (e) Absorbance and PL spectra for each of the tetrapod geometries (TP1 (magenta), TP2 (ochre), TP3 (turquoise) and TP4 (blue)) showing the large effective Stokes shift between absorption positions and PL maxima (∼160 nm; indicated by arrow). (f) Ensemble PL lifetime measurements for red emission (∼650 nm) in the CdSe core (same colour scheme as e).

Figure 2. Dependence of on-time statistics and photobleaching on tetrapod shape from widefield microscopy.

Tetrapod (TP) on-time fraction histograms: (a) TP1 (thin/short arms), (b) TP2 (thick/short arms), (c) TP3 (thin/long arms), and (d) TP4 (thick/long arms). Insets show ensemble photobleaching behaviour for each type of tetrapod.

Figure 3. Single-photon counting experiments for different tetrapod shapes.

Representative intensity-time traces (normalized to photons emitted per exciton generated to facilitate comparisons between tetrapods) and fluorescence lifetime-intensity distribution (FLID) diagrams for single tetrapods reveal a range of blinking behaviours from on/off to on/grey-state fluctuations: (a) TP1 (thin/short arms), (b) TP2 (thick/short arms), (c) TP3 (thin/long arms) and (d) TP4 (thick/long arms).

Figure 4. Emergence of two-colour emission as a function of tetrapod geometry and pump fluence in single-tetrapod spectra.

(a) TP1 (thin/short arms), (b) TP2 (thick/short arms), (c) TP3 (thin/long arms) and (d) TP4 (thick/long arms). Both raw and fitted data are shown. Smooth lines represent the best fit to the experimental data and dotted lines are the individual Gauss-Lorentzian peaks comprising the best fit to the experimental data. PL, photoluminescence.

Figure 5. Fate of excited-state carriers generated in the CdS arms revealed.

(a) TA spectra of the thick/short-arm TP2 CdSe/CdS tetrapods in hexane recorded with λ_pump=400 nm (3.1 eV) and pump fluence j=250 μJ cm⁻² per pulse. The main panel shows the spectra recorded at pump-probe delay Δt=0–2 ps, with ∼150 fs steps. The inset shows the dependence of the spectra on the pump fluence in the 500–750 nm range at Δt=2 ps. (b) TA relaxation dynamics recorded at λ_mon=640 nm at multiple pump fluences. The colour coding is consistent with the legend in the inset of a. The points are the experimental data and the solid lines are the best multiexponential fits to the experimental data. Both the experimental data and the fits were normalized at Δt=300 ps. The inset shows the relaxation dynamics recorded at λ_mon=470 nm, at multiple pump fluences. The kinetic traces were normalized at Δt=2 ps.

Figure 6. Blinking-suppressed multi-colour multiexcitonic emission from long-arm tetrapods.

(a) Single-tetrapod PL/time trace for red core emission. (b) Single-tetrapod PL/time trace for green CdS arm emission and accompanying g⁽²⁾ trace (inset). (c) g⁽²⁾ trace for red CdSe core emission before and after applying time-gating. (d,e) Pump fluence-dependent emission spectra. (a–d) Data obtained for same single TP4 tetrapod. (e) Data obtained for a single TP3 tetrapod.