Two types of luminescence blinking revealed by spectroelectrochemistry of single quantum dots
(10 November 2011)
20 April 2011
19 September 2011
09 November 2011
Photoluminescence blinking—random switching between states of high (ON) and low (OFF) emissivities—is a universal property of molecular emitters found in dyes
, polymers 1 , biological molecules 2 and artificial nanostructures such as nanocrystal quantum dots, carbon nanotubes and nanowires 3 . For the past 15 years, colloidal nanocrystals have been used as a model system to study this phenomenon 4 , 5 , 6 . The occurrence of OFF periods in nanocrystal emission has been commonly attributed to the presence of an additional charge 5 , 6 , which leads to photoluminescence quenching by non-radiative recombination (the Auger mechanism) 7 . However, this ‘charging’ model was recently challenged in several reports 8 . Here we report time-resolved photoluminescence studies of individual nanocrystal quantum dots performed while electrochemically controlling the degree of their charging, with the goal of clarifying the role of charging in blinking. We find that two distinct types of blinking are possible: conventional (A-type) blinking due to charging and discharging of the nanocrystal core, in which lower photoluminescence intensities correlate with shorter photoluminescence lifetimes; and a second sort (B-type), in which large changes in the emission intensity are not accompanied by significant changes in emission dynamics. We attribute B-type blinking to charge fluctuations in the electron-accepting surface sites. When unoccupied, these sites intercept ‘hot’ electrons before they relax into emitting core states. Both blinking mechanisms can be electrochemically controlled and completely suppressed by application of an appropriate potential. 9 , 10
Figures at a glance
Figure 1: Conventional charging model: A-type blinking and flickering.
a, In the conventional photoluminescence (PL) blinking model, ON and OFF periods correspond to a neutral nanocrystal ( X ) and a charged nanocrystal ( 0 X ), respectively. − b, Schematic photoluminescence decay of the ON and the OFF states on a logarithmic scale. The dynamics of the ON state is dominated by the radiative rate γ . In the charged state, the increase in the number of recombination pathways leads to a higher radiative rate, 2 r γ , responsible for the higher emission intensity at short delays. Simultaneously, the onset of three-particle Auger recombination with the rate r γ A γ opens a new, non-radiative, channel, leading to faster photoluminescence decay and reduced photoluminescence quantum yield. r c, When the timescale of charging and discharging is longer than the experimental binning time, binary blinking is observed. d, For fluctuations much faster than the bin size, a continuous distribution of intensities and lifetimes is obtained, often referred to as flickering. The insets in c and d show corresponding schematic fluorescence lifetime–intensity distributions (FLIDs).
Figure 2: Experimental set-up and electrochemical charging of an individual nanocrystal.
a, Set-up of a single-nanocrystal spectroelectrochemical experiment. APD, avalanche photodiode; BS, 50/50 beam splitter; ITO, indium tin oxide; TCSPC, time-correlated single-photon counting. b, Series of photoluminescence decays for a single nanocrystal for increasingly negative applied potentials. The thin grey lines show the best global triple-exponential fits with the shared time constants, yielding the lifetimes τ = 2 d ns, τ = 5 s ns and τ = 24 n ns. Top inset: the second-order photoluminescence intensity correlation function measured for this nanocrystal indicates that g (0) = 0.08. Bottom inset: residuals of the global fit indicate very high fidelity of the fitting procedure, with deviations within the noise level and below 1% of the maximum photoluminescence signal. 2
Figure 3: Correlated photoluminescence intensity and lifetime fluctuations: A-type blinking and flickering.
a, Photoluminescence intensities (black lines) and average lifetimes (red lines), and corresponding FLIDs, for the nanocrystal shown in Fig. 2 at three different potentials. Binary blinking seen at V = 0 V is largely suppressed at V = +0.6 V, whereas electron injection is achieved at V = −0.6 V. In the FLID colour scale, red corresponds to the most frequently occurring lifetime–intensity pair, and probabilities less than 1% of this maximum are indicated by dark blue. A linear scaling from blue to red is used between these extremes. b, Data from the same nanocrystal, acquired on a different day, display continuous photoluminescence intensity and lifetime fluctuations, typical of flickering. At V = −1.1 V, we observe emission from a doubly charged exciton, X . All data were analysed with a bin size of 50 2− ms. Full time trajectories for a and b are shown in Supplementary Fig. 1.
Figure 4: Photoluminescence intensity fluctuations without lifetime changes: B-type blinking.
a, Photoluminescence intensities (black lines) and average lifetimes (red lines), and corresponding FLIDs, for a nanocrystal showing the B-type OFF state; analysis done with a 10-ms bin. Full time trajectories are shown in Supplementary Fig. 6. b, The model of B-type blinking. The B-type OFF state is due to the activation of recombination centres ( R) that capture hot electrons at a rate, γ , that is higher than the intraband relaxation rate, D γ (the ground and the excited electron states are shown as 1 B S and 1 e P , respectively; 1 e S is the band-edge hole state). The position of the Fermi level, h E , relative to the trap energy, F E , is determined by the electrochemical potential and controls the occupancy of the surface trap R R. This, in turn, allows for electrochemical control of B-type blinking.
Figure 5: Electrochemically controlled switching between distinct statistics for ON and OFF times in the same nanocrystal, accompanying the transition from B-type to A-type blinking.
a, FLIDs indicating a nanocrystal switching from B-type blinking at −0.8 V (left) to A-type blinking at −1 V (right). Details of the analysis are given in Supplementary Fig. 9. b, Statistics for ON (red circles) and OFF (black squares) times for the FLIDs in a, in the log–log representation. At −0.8 V (B-type blinking), the data can be fitted to a power-law distribution, ∝ t, with − α = 1.17 for the ON times (red line) and α = 1.00 for the OFF times (black line). At −1 α V (A-type blinking), this description is no longer valid; however, the data can be closely fitted by introducing an exponential cut-off such that the distribution is ∝ texp(− − α t/ t ), where c = 0.54 and α t = 73.4 c ms for the ON times (red line) and = 0.37 and α t = 70.8 c ms for the OFF times (black line).