Letter pubs.acs.org/NanoLett
Enhancement of Multiphoton Emission from Single CdSe Quantum Dots Coupled to Gold Films Sharonda J. LeBlanc,†,‡ Mason R. McClanahan,† Marcus Jones,‡ and Patrick J. Moyer*,† †
Department of Physics and Optical Science, University of North Carolina at Charlotte, 9201 University City Boulevard, Charlotte, North Carolina 28223, United States ‡ Department of Chemistry, University of North Carolina at Charlotte, 9201 University City Boulevard, Charlotte, North Carolina 28223, United States S Supporting Information *
ABSTRACT: Single molecule time-resolved fluorescence spectroscopy of CdSe/ZnS core−shell quantum dots (QDs) localized near a rough gold thin film demonstrates significant enhancement of multiphoton emission while at the same time showing a decrease in single photon emission. A rigorous analysis of time-resolved photon correlation spectroscopy and fluorescence lifetime data on single quantum dots at room temperature reveals an increase in radiative recombination rate of multiexcitons that is much higher than expected and, perhaps more significantly, is not correlated with concomitant increases in single exciton recombination rates. We believe that these results confirm a stronger coupling of multiexcitons to plasmon modes via a coupling to plasmon multipole modes. KEYWORDS: Nanocrystal, quantum dot, biexciton, plasmon, photon correlation, antibunching
N
shells in nanocrystals specifically designed to suppress the Auger rate,15,17−20 sometimes achieving BX:X quantum yield ratios of ∼0.6.14 Kambhampati provides excellent accounts of the current understanding of multiexciton dynamics via experimental results and corresponding modeling, with particular emphasis on the specific mechanisms and recombination pathways accessible to energized charges in nanocrystals.21,22 Herein, we report the direct observation of enhanced multiphoton emission from 80 single CdSe/ZnS quantum dots coupled to a nanostructured gold film using photon correlation spectroscopy. Our experiment provides a powerful route to direct observation of multiexciton dynamics in single quantum dots. Coupling of quantum dot (QD) excited states to nanostructured metals has recently become an area of intense research owing to the distinct, yet complementary optical properties of both nanomaterials. While quantum dots exhibit tunable absorption and emission for superior light-harvesting capabilities, nanostructured metals are capable of localizing electromagnetic energy, thereby enhancing excitation or emission fields.23 Exploiting their combined properties can lead to ultimate control of light−matter interactions at the nanoscale.24 These types of interactions have been shown to modify the excited state dynamics and emission properties of
anoscale emitters such as semiconductor nanocrystal quantum dots (QDs) exhibit complex excited state dynamics. Among the most intriguing phenomena observed in quantum dots as a result of these dynamics are fluorescence blinking,1 spectral diffusion,2,3 and multiple exciton dynamics.4 Of particular interest in the current work is the simultaneous existence of multiple excitons in a single quantum dot, which can be achieved in one of two ways: (i) from absorption of a single photon of energy many times the band gap energy or (ii) from absorption of more than one lower-energy photon from a single laser pulse. For solar cell applications, the generation of multiple excitons from a single high-energy photon and subsequent extraction of several carriers would ultimately improve the efficiency of third-generation solar cells,5−7 while emission from high-order excitons is valuable for lasing or lightemitting diode applications incorporating quantum dots.8,9 Multiple exciton emission from quantum dots has been inferred from ensemble spectroscopic techniques10,11 or observed directly from photon correlation measurements utilizing single molecule techniques.12−14 Using photon correlations, the ratio of the biexciton (BX) to exciton (X) fluorescence quantum yield can be readily determined.13 This method of observing emission from multiexcitons (MX) is distinct from low-temperature single molecule spectroscopic techniques.15 However, the quantum yield of biexcitons and higher order excitons is typically low due to efficient nonradiative Auger recombination.16 Methods to improve the quantum yield of multiple excitons include growth of thick © 2013 American Chemical Society
Received: January 10, 2013 Revised: March 15, 2013 Published: March 19, 2013 1662
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microscope. Excitation with a 470 nm pulsed diode laser (PicoQuant) was focused using an oil immersion microscope objective (Zeiss, 1.25 NA, 100×) in an inverted geometry. The pulse repetition rate was 10 MHz with a 90 ps pulse width. Fluorescence was collected by the same objective and directed to avalanche photodiode detectors (PicoQuant). Photon correlation and lifetime decay data were collected by timecorrelated single photon counting (TCSPC) using a TimeHarp200 PCI card (PicoQuant) in time-tagged-time-resolved (TTTR) mode. Photon correlation data was collected in Hanbury Brown-Twiss (HBT) geometry with a resolution of 900 ps.44 Photon flux within the QD/PMMA layer was determined after considering the attenuation of the beam due to the ∼10 nm chromium layer. The skin depth of Cr is 20 nm, so a 10 nm layer reduces the field strength by e−1/2, and therefore the light intensity drops by e−1. The HBT setup is a start−stop technique that enables correlation of time delays between photons emitted from a fluorescent sample. A 50/50 beam splitter divides the fluorescence signal from the sample into two single photon counting detectors, one of which is designated as the start channel and the other as the stop channel. The detection of a photon at one detector starts an internal clock, which stops when a photon registers at the other detector. The time delays (τ) between start and stop photon events are correlated in a histogram, with zero time delay indicating simultaneous detection of photons at both detectors. The absence of a peak at zero time delay (τ = 0) denotes photon antibunching behavior. Photon correlation spectroscopy of single CdSe quantum dots has revealed strong photon antibunching.45−48 If a peak appears at τ = 0, this indicates either (a) there are two or more photon emitters present or (b) multiple excitons are excited and multiple photons are emitted from the same fluorophore during one laser pulse. Side peaks occur at the repetition frequency of the laser, indicating consecutive single photon emission. This method of photon correlation is a powerful technique for characterizing excited state dynamics of single emitters. Representative photon correlation data of quantum dots from a single stock solution cast onto a Cr-coated glass coverslip are shown in Figure 2. Data were recorded for samples either without (Figure 2a) or with (Figure 2b) a sputtered Au film. The sample was excited with a laser power density of 1.5 kW/cm2. At this power density, the average number of excitons generated per QD per pulse, ⟨N⟩, is estimated at 1.5 (see Supporting Information, Section A for calculation). QDs in control samples (without gold) rarely exhibit multiphoton emission, evidenced by the almost complete absence of a peak at τ = 0 in Figure 2a. The absence of a central peak (antibunching) in the control data, despite a high average number of photogenerated excitons per QD per pulse, is well-known and results from fast Auger recombination that almost eliminates the possibility of multiphoton emission. In contrast, we observe two key phenomena in the photon correlation curves of QDs near to the gold film: (i) a central peak at τ = 0, indicative of multiphoton emission and (ii) a marked narrowing of both the central band (CB) and sideband (SB) peaks compared to the control QDs, which indicates a shortened excited state lifetime. These phenomena are likely caused by a combination of nonradiative energy transfer to the metal and exciton coupling to the plasmon modes of the rough gold film. It is important to note that we also performed the experiments with a chromium (Cr) film on the top so as to rule
single emitters. Enhanced absorption cross sections, increased radiative rates, and energy transfer have been observed in the weak coupling regime, which involves interaction of the excited state dipole with an electromagnetic field localized at the metal surface.25−30 This highly localized optical field is termed a surface plasmon (SP), generated by resonant oscillations of the surface electron density. Nanoparticles of gold and silver readily interact with optical frequencies, leading to strong surface plasmon resonances in the visible spectrum. SPs can also be generated in metallic films at metal/dielectric interfaces. Exciton−plasmon interactions proceed through coupling of the excited state to electronic states within the nearby metal surface. Coupling of single quantum dots to rough metal films and nanoparticles has revealed suppression of blinking dynamics, enhancement and quenching of fluorescence emission, increased spectral shifting, and reduced excited state lifetimes.31−40 A variety of other platforms for exciton− plasmon coupling have been investigated, including nanocrystal-metallic nanorod/nanowire,41,42 and J-aggregate-metal nanoshell constructs.43 In the current work, we present a simple architecture incorporating conventional CdSe/ZnS nanocrystals for the study of multiexciton−plasmon interactions. A schematic diagram of the sample construct used in these experiments is shown in Figure 1. This architecture was
Figure 1. A schematic diagram of the sample architecture: a dilute solution of QDs in PMMA/toluene is spin-cast onto a chromiumcoated glass coverslip, and then a gold film is sputtered directly on top of the QD/PMMA layer.
designed for electric field modulation measurements, the results of which will be reported in a separate article. A thin layer of chromium (∼ 10 nm in thickness) was evaporated onto a clean glass substrate using an electron beam evaporator to act as the lower electrode for those experiments, but the chromium (Cr) electrode on the bottom is not used in these experiments. However, it is important to note that our control experiments later are conducted with the Cr film and without the top Au film so as to provide a valid control experiment, thus confirming the effect of the Au film. Commercially available CdSe/ZnS quantum dots (Evident Technologies, 617 nm emission) were diluted to 10−11 M in a poly(methyl methacrylate) (PMMA)/toluene (2.5 wt %) solution and spin-cast onto the chromium-coated glass substrate. The thickness of the QD/PMMA layer was ∼150 nm, measured using an Alpha-step surface profiler. Regions of the QD/ PMMA layer were masked, and a layer of gold was sputtered on top of the QD/PMMA at 100 W for 10 min. The gold layer covered the entire top surface of the PMMA film into which the individual quantum dots are imbedded. Individual quantum dots were imaged using a home-built laser scanning confocal 1663
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Figure 2. Representative photon correlation histograms of (a) three QDs in PMMA and (b) three QDs in PMMA near a gold film. Black lines are best fits using two-sided mono- and biexponential functions (eq 1). (c) Integrated areas under the fitted CB (filled symbols) and SB (open symbols) correlation peaks are plotted versus emission rate for QDs near a gold-film (red) and with no gold (blue). CB points are split into slow (round markers) and fast (square markers) rate components. Black lines are power-law fits to each data set. (d) Ratio of BX to X emission quantum yields for QDs near gold calculated from the photon-correlation data.
g(2) 0 can be used to measure the relative quantum yield of BX emission (ϕBX) to X emission (ϕX);13 however, this is valid only in the limit when N → 0, a condition that is not met in this case. Instead we have developed a new method to estimate the ratio between X and BX emission quantum yields, for QDs near gold. For this analysis we consider the slow SB and CB components, which have relatively long lifetimes (∼4−12 ns). As we will demonstrate later in this article, these slow components include at least one photon emitted from an X state. General expressions for these correlation components, denoted, GsSB(τ) and GsCB(τ), are given in eqs 2 and 3.
out the possibility of charge transfer to metallic atoms that may have diffused into the PMMA film during deposition of the top metal film. In Section B of the Supporting Information, we show the photon correlation results of the experiments performed with a top film of Cr. We note here the absence of a central peak in those experiments, thus favoring a QDplasmon coupling mechanism when the rough gold film is present. In this article we will try to unravel these contributions and present evidence for significant differences in the way that single excitons and multiple excitons interact with local surface plasmon modes. We fit the correlation curves using a sum of two-sided exponential functions (eq 1).
s GSB (τ ) =
2
G(τ ) =
∑ [αn(e−k |τ− τ | + e−k |τ+ τ |) + βne−j |τ|] n
n=1
0
n
0
∞
∫0 [∑ G(nX(t ), X(t + τ + Δt )) n>1
n
+
(1)
∑ G(X(t ), nX(t + τ + Δt )) n>1
In the absence of gold, single exponential fits (n = 1) were sufficient, and no CB peak was observed (β1 = 0); however, QDs near gold typically required two exponential components (n = 1,2) to adequately fit each peak. The integrated areas under CB and SB peaks are plotted versus emission rate (kn and jn) in Figure 2c. Note that the fast components of the SB peaks are not shown since they were usually a negligible proportion of the total SB intensity. The correlation between total peak areas and emission rate for both CB and SB peaks can be modeled using power law functions with exponents: −1.6 (CB, fast component), −1.8 (CB, slow component), −2.2 (SB with Au), and −4.4 (SB, no Au). This correlation is likely caused by energy transfer from QD to either of the metal layers (Cr or Au), and although it was not possible to determine the depth of the QDs in the PMMA layer, we expect that the QDs in close proximity to the metal films would exhibit curtailed emission due to rapid nonradiative energy transfer. Dividing the integrated intensity of the CB peak (τ ≈ 0) by the average integrated intensity of the SB peaks yields the relative ratio, g(2) 0 , shown for the QDs in Figure 2b. A histogram of this ratio for the 80 quantum dots studied is presented in the Supporting Information, Section C. Nair et al. suggested that
+ G(X(t ), X(t + τ + Δt ))]dt s GCB (τ ) =
(2)
0
∫∞ ∑ G(nX(t ), X(t + τ))dt n>1
(3)
where, for example, G(nX(t), X(t + τ + Δt)) is the probability that a photon is emitted from an n-exciton state, nX, and detected in channel 1 at time t, and then another photon is emitted from a single exciton state, X, and detected in channel 2 at time t + τ + Δt, where Δt is the time between pulses. Using a simple three-state kinetic scheme (BX, X, and ground state) we can derive an expression for the ratio between the integrated SB and CB peak areas, GsSB and GsCB (slow components only), in terms of the X and BX emission yields (ϕX and ϕBX) and the average number of excitons generated per pulse (λ ∼1.5 for these data): s ⎡1 ⎤ GSB 1 ϕX −λ ⎢ ⎥ = + λ λ + + λ e (2 ) (2 ) s GCB 4 ϕBX ⎣⎢ 2 ⎦⎥
(4)
Full details of the derivation of eq 4 are given in Supporting Information, Section D. In Figure 2d we present a histogram of 1664
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Figure 3. Fluorescence decays and blinking trajectories of single quantum dots. Red data are QDs near gold, and blue data are recorded in the absence of gold. (a) Representative fluorescence decays from three different QDs with average PL lifetimes calculated from multiexponential fits. Each QD was excited at 1.5 kW/cm2. (b and c) Blinking trajectories of QD1 (b) and QD3 (c) with thresholds indicating 90% of the maximum count rate. Shown below are corresponding fluorescence decays composed from photons collected during above-threshold intensity periods. Inset, plots of the 90% threshold levels versus fluorescence lifetime for a selection of randomly chosen QDs.
An effect that might explain a disproportionately larger increase in radiative rate of MX versus X states has recently been discussed by Toropov et al.50 They lowered a 150 nm gold nanoparticle attached to an NSOM probe over an InGaN film or ZnCdSe QDs and, in both cases, saw much greater PL enhancement over spectral ranges consistent with quadrupole and octupole plasmon resonances in the gold. It is possible that strong electron coupling in the CdSe/ZnS QDs could give rise to significant multipole components in MX transition matrix elements, and these could couple strongly to multipole plasmon modes associated with relatively large features on the rough gold films. Such enhancement would not be possible for singly excited X states in which dipole terms are expected to dominate radiative transition matrix elements. Consider, for example, the manner in which the two dipoles simultaneously present in a QD BX might preferentially antialign which could cause a rapid nonradiative recombination event due to the close proximity of the electron and hole wave functions. In the presence of a plasmonic structure, if this BX mode, which we can consider also as a quadrupole, couples strongly to a quadrupole mode of a plasmonic structure, then the two dipoles will no longer strongly antialign which can render photon emission more likely. This is just a conceptual picture of how the coupling of various MX modes with plasmon modes could affect radiative and nonradiative (e.g., Auger) recombination rates. Thus far, we have demonstrated an increase in multiphoton emission from single QDs in the presence of a rough gold film. Considering the increase in the emission rates and the fact that multiphoton emission in drastically increased relative to that of single QDs that are not in the presence of gold films, we propose that the plasmon-QD interaction is responsible for the substantial increase in radiative emission rate or a concomitant decrease in Auger recombination rates of multiexcitons in the presence of plasmonic structures. We will now estimate the
the X to BX emission yield ratios calculated in this way. The values span almost 2 orders of magnitude, consistent with a spread in QD positions and degree of coupling to the gold plasmons. The mean value obtained for ϕBX/ϕX is 0.2, which is remarkably high considering the fact that in the absence of gold, we see little or no BX emission. As has already been discussed, both X and BX undergo nonradiative energy transfer to the metal. In addition, Auger recombination opens up a fast nonradiative recombination pathway for the BX state, which explains the low BX quantum yields in the control QDs (ϕBX/ϕX ≈ 0). Assuming that energy transfer rates are similar for X and BX, the observation of significant BX quantum yields indicates that either (i) the BX Auger recombination rate is dramatically slowed by interaction with the gold film, or (ii) the BX radiative rate, kRBX, is enhanced by plasmon coupling to a much greater extent than is the X radiative rate, kRX. While Auger recombination rate reduction in nanocrystals has been observed in specially engineered nanocrystals,15,17,19,49 there are no reports that attribute modified Auger rates to interactions between MX states in colloidal nanocrystals and metal surface plasmon modes. As already discussed, however, there are many reports that demonstrate enhanced radiative recombination due to (multi)-exciton−plasmon coupling, making this a more likely scenario to explain our observations. In a simplistic model of band-edge radiative recombination, the MX radiative rate should scale as m2, where m is the number of excitons; however, kRBX must be enhanced significantly more than 4-fold from kXR to compensate for a likely Auger recombination rate between 109 and 1010 s−1. We therefore propose that coupling to surface plasmon modes on the gold film enhances kRBX significantly more than kRX in the QDs with ϕBX > ϕX. 1665
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Figure 4. (a) Photon correlation curves measured on a single CdSe/ZnS QD as a function of incident laser power. (b) Integrated areas of the fast and slow components derived from fitting the central and sideband peaks. (c) Power-dependent time-resolved fluorescence transients measured on a different CdSe/ZnS QD. Black lines are three-exponential fits.
When the QDs are in the presence of a gold film the 90% threshold fluorescence transient retains a short decay component and requires a two-exponential function to achieve a satisfactory fit (Figure 3c). Although there are too few counts in this decay (due to nonradiative energy transfer) to accurately determine the lifetime of the short component (reduced chisquared of fit, χ2red ≈ 0.5) it appears to be similar to the short components in Figure 3a and is likely due to MX emission. Since this decay is constructed from photons detected during the brightest emission periods, this observation confirms that ϕMX (likely ϕBX) is similar to ϕX for QDs near gold, as we found from the analysis of the photon correlation data in Figure 2. Nonradiative energy transfer and plasmon-coupled emission from QD excitons are the likely reasons that the longer decay component in Figure 3c is just 40% of the radiative lifetime of the control QD. Exciton emission lifetimes from 10 control QDs and 10 QDs near gold are plotted versus the 90% blinking threshold intensity in the insets to Figure 3b and c. For the control QDs there is little or no correlation with the 90% intensities, but for the QDs near gold we now find a much higher degree of correlation between the fluorescence lifetimes and intensities, with shorter lifetimes tending to correspond to less emission. This makes sense if energy transfer, which shortens fluorescence lifetimes and reduces emission yields, dominates exciton−plasmon coupling effects. Otherwise, emission intensities would be expected to increase with shorter lifetimes. Since the QDs were sampled randomly, they correspond to different heights within the PMMA film, and this accounts for the correlation between emission lifetime and threshold intensity. We can use this correlation to estimate the degree, αX, by which exciton radiative rates in QDs are enhanced by gold plasmon modes. From Figure 3c, the fluorescence yield (ϕAu QD) for QDs with an 11 ns lifetime (τAu = 11 ns) in the presence of f gold is approximately equal to the average intensity for QDs without gold (ϕQD), yet their fluorescence lifetime is about two-thirds of the nongold lifetimes (τf). We can therefore write the approximate relations: 2 Au Au ϕQD (τf = 11 ns) ≈ ⟨ϕQD⟩ and τfAu ≈ ⟨τf ⟩ (5) 3
degree by which radiative rates are enhanced by exciton− plasmon coupling in our systems via an analysis of single QD fluorescence decays. Figure 3a shows representative fluorescence decays measured on three individual QDs. QD1 (blue curve) was from a control sample (no gold), while QD2 and QD3 were both located near a gold film. The QD1 and QD2 decays were each fit to a biexponential function, and the QD3 decay was fit to a triexponential function. Average fluorescence lifetimes were 18 ns for QD1 and 8.8 and 7 ns for QD2 and QD3, respectively, and the contribution of a fast decay component (∼100 ps) increases from QD1 to QD2 and QD3.While the slow component of the decay can be reliably attributed to X emission, evidence from the photon-correlation experiments suggests that the fast decay components are due to MX emission. To determine the plasmon-coupled radiative rate enhancements we needed to determine kRX for isolated QDs and for QDs near gold. To do this we analyzed fluorescence trajectories from 10 control QDs and 10 QDs in the presence of the gold film. Figure 3b and c shows the blinking traces of QD1 (no gold) and QD3 (with gold) from Figure 3a. We note two observations from these data: a ∼50% decrease in the maximum count rate and a noticeable increase in gray state (neither off nor fully on) emission in the presence of the gold film, which is consistent with previous observations of CdSe blinking close to gold nanoparticles.37 Following literature methods,51,52 we generated fluorescence transients using only photons collected during periods when the emission rate was within 10% of the maximum. This procedure yielded the decays shown below each blinking trace in Figure 3, with average lifetimes of 18.1 and 6.0 ns for QD1 and QD3, respectively. For the control QDs (Figure 3b) the fluorescence decay is well-modeled using a single exponential. Fisher et al. suggested that QD fluorescence yields approach unity at the brightest points in their blinking trajectories.52 Following their analysis we assume that the top 10% fluorescence decays for the control QDs (no gold) are from periods of unity quantum yield, ϕ ≈ 1. Hence, the fluorescence lifetime of the control QDs is equal to their radiative lifetime (1/kR = 18.1 ns). 1666
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emission could be correlated with delayed QD fluorescence after the dot had discharged from a long-lived trap state. Total intensities of the fast components from the central and side bands are shown in Figure 4b. Both central and sideband peaks show a marked cubic dependence on laser power, which indicates that they are formed after emission from a triexciton (TX) state. Since the fast components have such a short lifetime compared with X recombination, the second photon likely comes from BX emission. While not all of the QDs exhibited such sharp features in their photon correlation curves, these data clearly show that gold can enhance MX radiative rates to such an extent that TX emission is observed. This further supports the assertion that interactions with gold enhance MX emission more than X emission; otherwise fast Auger recombination would be expected to eliminate TX states before radiative recombination could occur. In the central band peak, TX emission must precede BX emission, and following the convention introduced in eq 2, the correlation function can be denoted: G(TX, BX); however, the sideband peaks can arise from TX and BX emission occurring in any order (G(TX, BX), G(BX, TX), G(TX, TX), and G(BX, BX). This explains why the total intensity of the sideband fast components (yellow markers) is greater than that of the central fast components (green markers). It also explains why the lifetime of the sideband fast component (0.66 ns) is slightly longer than the central band fast component (0.57 ns) since we would expect that one of the contributing terms, G(BX, BX), decays slightly more slowly then G(TX, BX). Power-dependent time-resolved fluorescence was also recorded, and representative data (from a different QD) are plotted in Figure 4c. The lifetime data presented in Figure 4c include all collected photons, as opposed to the lifetime data shown in Figure 3c which only includes the photons collected in the top 10% of the collected photon intensity. We include all photons here to demonstrate that the overall average lifetime does indeed decrease as laser intensity increases thus supporting the MX influence on emission in the presence of a plasmonic structure. These transients show a rising contribution of a fast (
