Fluorescence Enhancement, Blinking Suppression, and Gray States of

Sep 8, 2010 - The optical properties of nanocrystals are drastically changed by the interaction with ... and chemical versatility, colloidal semicondu...
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Fluorescence Enhancement, Blinking Suppression, and Gray States of Individual Semiconductor Nanocrystals Close to Gold Nanoparticles Xuedan Ma, Hua Tan, Tobias Kipp,* and Alf Mews Institute of Physical Chemistry, University of Hamburg, Grindelallee 117, 20146 Hamburg, Germany ABSTRACT The optical properties of nanocrystals are drastically changed by the interaction with adjacent metal nanoparticles. By time-resolved photoluminescence spectroscopy, we investigate CdSe multishell nanocrystals coupled to self-assembled films of Au nanoparticles. The distance between emitter and metal is adjusted by coating the nanocrystals with silica shells. These NCs showed increased fluorescence intensity, a decreased fluorescence lifetime, strong blinking suppression, and fluorescence from gray states. These observations can be explained by the metal particle induced change of excitation and recombination rates. KEYWORDS CdSe nanocrystals, gold nanoparticles, silica coating, photoluminescence, blinking suppression

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ue to their unique spectral properties, photostability, and chemical versatility, colloidal semiconductor nanocrystals (NCs) are interesting for fundamental research and have become promising candidates for a wide range of electronic and optical applications, such as lightemitting diodes (LEDs),1,2 photovoltaics,3,4 and biological labeling.5 However, there are still several issues limiting their applications, in particular the universally observed fluorescence intermittency, also called blinking, which can be observed at the single nanocrystal level.6-8 The physical reasons for the blinking behavior are still controversially discussed, but it is generally accepted that excess charges within the particles lead to the dark states through fast Auger quenching of the exited state. Hence one strategy is to avoid charging by eliminating possible trap states on the NC surface or in its surrounding by modifying the NCs’ ligands or by growing thick semiconducting shells around core NCs.9-12 In fact, reducing those trap states leads to increased fluorescence lifetimes by suppressing nonradiative recombination channels and hence increases the fluorescence intensity.13 In another strategy the optical properties of fluorophores in general, and NCs in particular, can also be significantly modified by plasmons in nearby metallic nanostructures. Recent reviews on this topic are refs 14 and 15. The interaction strength of the NCs and metal structures depends on several parameters including the geometry, size, and composition of the metallic nanostructures, the distance and spectral overlap between the fluorophores’s emission or absorption and the metal’s plasmonic resonances. Basically

two effects have to be considered. On the one hand, the excitation rate of the fluorophore can be altered by a plasmon-induced local electric field. On the other hand, both the radiative and nonradiative decay rates of fluorophores can be modified by the fluorophore-metal interactions. Overall, the interaction can lead to a strong fluorescence enhancement but also to a complete quenching of the emission, depending on the particular fluorophore-metal system. In the case of NCs as fluorophores, their optical properties in the vicinity of (rough) metal films,16-22 of colloidal metal nanoparticles,23-32 or of lithographically prepared metal nanostructures33,34 have been investigated. Some of these experiments were performed on the single NC level.16,18-22,26,28,30,31,35 Besides the desirable knowledge and control of the structural properties of the particular metal films or nanostructures also the position of the NCs with respect to the metal, particularly their distance, should be controllable for a detailed analysis of the NC-metal interaction.20 In some of the above-mentioned experiments, NCs were directly brought on the metal surfaces;16-19,21,22,26 in others, polymer matrices were used to achieve a spatial separation between NCs and metal.27,29-31,33-35 Again, in other experiments, NCs and metals were linked by bioactive molecules like vitamins, proteins or peptides, and DNA.23,25,28 In this Letter we combine all aspects of the exciton plasmon interaction in detail by exploring the photophysical properties of CdSe multishell NCs on films of gold nanoparticles (AuNPs) at a single nanocrystal level. For this we use transparent films of well-ordered self-assembled AuNPs where the structural, plasmonic, and optical properties were determined by scanning electron microscopy (SEM), optical transmission spectroscopy and finite-difference time-domain (FDTD) simulations. In order to adjust variable fixed

* To whom correspondence should be addressed, [email protected]. Received for review: 07/14/2010 Published on Web: 09/08/2010 © 2010 American Chemical Society

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plasmons between closely packed AuNPs.36 The two AuNP films, named Au526 and Au590, will be used in the following for further NC deposition. The details of the semiconductor particles syntheses are outlined in the Supporting Information. Briefly, multishell NCs were produced by subsequently overgrowing nominally 2 monolayers (ML) CdS, 3.5 ML Cd0.5Zn0.5S, and 2 ML ZnS onto a 3 nm CdSe core via the standard SILAR method.37 These NCs had a PL peak at 600 nm and a fluorescence quantum yield larger than 80% in solution as compared to a standard dye solution (Rhodamin 6G). Silica shells of different thicknesses were coated onto the surface of the NCs by the microemulsion method.38 By variation of the reaction time and reactant ratios, differently sized silica shells were synthesized. The large majority of particles contained only one NC. Panels c and d of Figure 1 show exemplary transmission electron microscopy (TEM) images of silica-coated NCs (referred to as SiNCs) deposited on TEM grids. We emphasize that the emission wavelength of the NCs centered at about 600 nm (see black curve in Figure 1e) does not significantly change upon silica coating but the fluorescence quantum yield dropped to 30%. We mainly synthesized SiNCs with overall diameters of 20 and 40 nm to adjust the distance between NCs and the AuNPs. SiNCs were deposited onto the substrates with or without AuNPs by spin-coating dilute solutions. Figure 2a sketches a part of the photoluminescence microscopy setup. For excitation, the beam of a 470 nm pulsed diode laser with a repetition rate of 5 MHz (PDL 800D, LDH-D-C-470, PicoQuant GmbH) was diffraction-limited focused by a 100× air objective with 0.95 NA (MPLAPON, Olympus) onto the sample. For the following measurements a laser power of 30 nW measured behind the objective was used. The sample was scanned using a computer-controlled piezo stage. The emission from NCs was collected by the same objective, separated from the scattered laser light by long pass filters and guided to an avalanche photodiode (Micro Photon Devices) for scanned images. Time-tagged time-resolved (TTTR) measurements were performed using the reverse start-stop mode (PicoHarp 300, PicoQuant GmbH). The color plots in Figure 2 show scanned fluorescence images of 20 nm sized SiNCs on bare glass cover slides (Figure 2b) as well as on cover slides with Au526 (Figure 2c) and Au590 (Figure 2d) NP films. For each image the scan range is 10 µm × 7.5 µm, the step width in both directions is 100 nm, and for each step, the integration time of the photodiode was 25 ms. The fluorescence intensity is encoded in a color scale. The emission intensity of SiNCs on glass is rather small. In the depicted scan field we observe about seven emitting NCs, marked by circles for clarity. Their maximum intensity corresponds to about 500 counts/s. On the AuNP films the fluorescence intensity of SiNCs is strongly increased to maximum values of about 1200 and 2000 counts/s on Au526 and Au590, respectively. This increase

FIGURE 1. (a) SEM image of the Au526 NP film on glass. (b) SEM image of Au590 NP film on glass. (c, d) TEM images of (c) several 40 nm sized SiNCs and (d) one representative 20 nm sized SiNC on carbon support. (e) Measured extinction spectra of gold colloidal films shown in (a) and (b) (red solid, Au526; blue solid, Au590) and FDTD simulation (red dashed, Au526; blue dashed, Au590), together with the photoluminescence spectrum of silica-capped CdSe multishell NCs (black).

distances between emitter and metal the NCs are coated with silica shells of different thicknesses. Time-resolved confocal photoluminescence (PL) microscopy reveals a fluorescence intensity enhancement, a decrease of the fluorescence lifetime, a strong blinking suppression, and the existence of gray states instead of off-states of the coupled NC/ AuNP systems. We will show that the fluorescence enhancement can mainly be attributed to an off-resonant excitation field enhancement due to the AuNPs. Furthermore, we will show that the decrease of fluorescence lifetime induced by interaction of the NC emitter with adjacent AuNPs can also be a reason for a further emission enhancement, the blinking suppression, and the occurrence of gray states. These findings are valuable not only for the development of strongly luminescent nanocrystals for possible applications but also for the understanding of the charge carrier dynamics in semiconductor NCs in general. Panels a and b of Figure 1 show SEM images of the two types of AuNP films used in this work. The AuNPs were prepared by standard chemical procedures36 and had an average diameter of 17 nm. The substrate shown in Figure 1a was prepared by self-assembling the AuNPs with citrate ligands onto precleaned glass substrates via bifunctional aminosilane molecules (see Supporting Information). This procedure leads to well-isolated AuNPs on the substrate and to an extinction spectrum with a relatively sharp peak at 526 nm (solid red line in Figure 1e). In order to tune the plasmon band, a similar substrate as shown in Figure 1a was treated first with a dithiol solution and again immersed in the solution of AuNPs. As a result the submonolayer of AuNP aggregates shown in Figure 1b is formed, and the peak is shifted to 590 nm (blue line in Figure 1e). It is red-shifted, broadened, and increased because of the coupling of surface © 2010 American Chemical Society

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performed similar experiments as shown in Figure 2 for 40 nm sized SiNCs. Here, the fluorescence intensity of the NCs is nearly independent of the substrate, showing that for the larger SiNCs the distance between the emitter and the AuNPs is too large for an effective interaction (see Supporting Information). On the other hand we could not observe any fluorescence for CdSe multishell NCs without silica shells spin-coated directly on gold films, which may be attributed to an ultrafast electron transfer from NCs to AuNPs. In the following, we therefore will concentrate only on the 20 nm sized SiNCs. After the samples were scanned over several larger areas than shown in Figure 2, the emission properties of many (approximately 50 for each substrate) single SiNCs have been investigated in detail. We restrict our further discussion on three typical SiNCs on the three different substrates which represent the multitude of investigated NCs, as is proved in the Supporting Information (cf. Figure S1). Figure 3a-c shows a set of fluorescence time traces from the single SiNCs on the three different substrates. These traces give the temporal evolution of the fluorescence intensity measured in consecutive 50 ms time bins. For the SiNC on glass (Figure 3a), the typical blinking behavior accompanied with relatively long off-times is observed. If we define the on- and off-states by choosing a threshold that is twice the mean background level,39 we calculate the fraction of on-times to be 60%. Figure 3d shows the corresponding histogram of photon counts per time bin. It can be roughly approximated by the sum of two Poisson distributions (black curve), one of them with a mean number of about 1 count/(time bin) describing the off-state quite accurately and the other with a mean number of about 32 counts/(time bin) describing the on-state. Deviations from the Poissionian approximation can be explained by a distribution of radiative states, as discussed below. If we define the threshold between on- and off-states as the minimum between the two distinct peaks in the histogram40 (dashed line in Figure 3a) we obtain the same value for the on-time fraction as given above. Measured under the same experimental conditions, the time traces and photon-count histograms for SiNCs on Au526 (Figure 3b,e) and Au590 (Figure 3c,f) significantly differ from the ones on glass. The most frequent maximum PL intensity is increased to about 55 counts/(time bin) and 104 counts/ (time bin), respectively. Each histogram still shows two peaks, which are again approximated by the sum of two Poissonians (red and blue curve). Interestingly, the peak which usually is attributed to the off-state shifts to about 8 counts/(time bin) and 31 counts/(time bin) for Au526 and Au590, respectively. We separately measured the background signal on each substrate under the same experimental conditions but at positions without SiNCs. It showed a perfectly Poissonian behavior with mean numbers of about 1, 3, and 14 counts/(time bin) for bare glass, Au526, and Au590, respectively. For Au590, the background time trace and its count rate histogram together with its Poissonian fit

FIGURE 2. (a) Sketch of the experimental setup (not to scale). A microscope objective focuses the excitation light and collects the emission. The sample can be laterally scanned in order to image the fluorescence. (b-d) Fluorescence images with a scan range of 10 × 7.5 µm2 of 20 nm sized SiNCs on bare glass, on Au526, and on Au590. The fluorescence intensity is encoded in a color scale from blue to yellow for low count rates up to 500 counts/s and red to black for count rates above 500 counts/s.

is attributed to the interaction of NCs with the nearby metal particles, as will be elaborated below. The increase of the background level and noise for AuNP films compared to the bare cover glass is caused by an increase of the scattered laser light for rough surfaces and an imperfect filtering. We © 2010 American Chemical Society

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FIGURE 3. (a-c) Typical single dot PL intensity time traces and (d-f) corresponding count rate histograms of 20 nm sized SiNCs on (a, d) glass substrate, (b, e) Au526, and (c, f) Au590. The dashed lines are the thresholds and the curves in the histogram diagrams are the sum of two Poisson distributions. In (c) and (f) a background time trace and its count rate histogram together with its Poissonian fit is additionally shown. (g-l) Probability density of (g-i) on- and (j-l) off-times for the time traces shown in (a-c) plotted on a log-log scale.

is also shown in parts c and f of Figure 3, respectively. Obviously, the shifting, most prominent on Au590, cannot be solely explained by the increased background signal. We deduce that for SiNCs on AuNP films low-emitting periods occur instead of off-periods without emission. These lowemitting periods can be assigned to so-called gray states as will be discussed later. Defining again the threshold as the minimum between the two distinct peaks in the histograms (dashed lines), the on-time fraction of single SiNCs on Au526 and Au590 are calculated as 77 and 94%, respectively. This drastic increase of the on-time fraction comes along with a significant decrease in the durations of the low-emitting periods. The double logarithmic diagrams in panels g-i and j-l of Figure 3 show the probability density41 of, respectively, on- and off/gray-state duration times determined from the corresponding time traces a-c. Regardless of the substrate, both classes of states follow a universal power-law distribution P(t) ∼ t-m well-known for the blinking of single NC emitters. The values of the exponent are affected by some uncertainties because of the rather small statistical basis within the 300 s time traces. The analysis of further time traces reveals values between 0.9 and 2.1, for on-times typically smaller than for off/gray-times. Figure 4a shows background-subtracted PL decay curves of the very same NCs investigated in Figure 3. The instrument response function (IRF) had a fwhm of 128 ps. The decay curves were fitted with a stretched exponential function by deconvoluting the IRF signal:13,42

that was symmetrical about the zero axis. For single SiNCs on bare glass we determine values of τs ) 19.9 ns and β ) 0.84, giving an average lifetime42 of 〈τs〉 ) 21.7 ns, consistent with previously reported room temperature PL lifetime measurements.13,43 For SiNCs on different gold substrates, we observe τs ) 12.7 ns and β ) 0.83 for Au526, and τs ) 0.4 ns and β ) 0.44 for Au590. From those values the average lifetimes 〈τs〉 for single SiNCs on Au526 and Au590 are derived as 14.0 and 1.1 ns, respectively, much shorter than the lifetime of NCs on bare glass substrates. In principle the stretched exponential function can be considered as arising from a superposition of exponentials

I0 exp(-t/τs)β )

where the lifetime distribution function Fs(τ) can be calculated by a direct inverse Laplace transformation of the fitting function.42 In the inset of Figure 4a the corresponding decay rate distributions Fs(1/τ) are depicted, showing that the most probable decay rate and the width of the rate distribution increased considerably from the single SiNCs on glass substrate over those on Au526 to those on Au590. In the following we want to correlate the transient fluorescence intensities with the florescence lifetimes. In general, we measured for each time bin i a fluorescence decay curve and fitted it with a single exponential function

Ii(t) ) Ai exp(-t/τi) Simple scatter plots of the lifetime-intensity distribution constructed from the fitting results show that the lifetimeintensity pairs are broadly distributed and that long lifetimes are correlated with large fluorescence intensities (see Figure S2, Supporting Informaion).44,45 This finding is valid for SiNCs on bare glass and on AuNP films. In Figure 4b-d scatter plots are shown in which each lifetime-intensity data point is weighted with the number of photons Aiτi as calculated from each corresponding decay curve. Compared to the simple scatter plots, in this case, the integral over a

I(t) ) I0 exp(-t/τs)β with β (0 < β e 1) a constant that determines the rate distribution. A smaller β means a broader rate distribution while β ) 1 corresponds to a single exponential decay. The quality of fitting for the decay curves was estimated by making sure a χ2 value of less than 1.2 and a residual trace © 2010 American Chemical Society

∫0∞ exp(-t/τ)Fs(τ) dτ

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FIGURE 5. Schematic model of excitation, recombination, and blinking of NCs. Neutral NCs are associated with on-states. Charged NCs are typically associated to off-states because of dominant nonradiative Auger recombinations.

values are in good agreement with those obtained from the stretched exponential fits of the overall decay curves. For a further comparison of both fitting approaches, in Figure 4e-g, the weighted lifetime distribution function τFs(τ) as obtained from the stretched exponential approximation is also plotted. Interestingly, we find a good correlation between the two methods proving that the assumption of single-exponential decays on short time scales such as the binning time is reasonable. Our main experimental findings can be summarized in four points: (I) fluorescence enhancement, compared to the situation on bare glass, the maximum PL intensity of the onstate of 20 nm sized SiNCs on Au526 and Au590 is increased by factors of about 2 and 3, respectively; (II) lifetime reduction, the measured fluorescence lifetime of the on-state of SiNCs on AuNP films is decreased, on Au590 by about one order of magnitude, compared to the case of bare glass; (III) occurrence of gray states, for SiNCs on Au526 and, even more explicit, on Au590, low-emitting periods with nonvanishing intensity, so-called gray states, occur instead of nonemitting off-states as on bare glass; (IV) blinking suppression, SiNCs on AuNP films show a suppressed blinking behavior compared to the case of bare glass. This manifests itself in an increase of the on-time fraction from 60% on bare glass to 94% on Au590. In the following, we want to address each of these four main experimental findings within a common model of a NC sketched in Figure 5. For the NCs, we generally distinguish between on-states, which allow for the efficient emission of photons, and low-emitting states, which typically are off-states but may also become gray states when the NC is coupled to adjacent AuNPs. The exact nature of these states and particularly their changeover mechanism is still strongly discussed. However, it is widely accepted that on- and offstates can be assigned to neutral and charged NCs, respectively. A NC in its uncharged ground state |1〉 is optically excited into |2〉 by the creation of an exciton with a rate of kexc. Experimentally, since the excitation laser wavelength (470 nm) is smaller than the emission wavelength (≈ 600

FIGURE 4. (a) PL decay curves of a single SiNC on bare glass (black), Au526 (red), and Au590 (blue) together with the IRF (green) plotted on a linear scale. The inset shows the corresponding rate distributions. (b, c, d) Scatter plots of weighted lifetime-intensity data points calculated by fitting decay curves from each time bin with single exponential functions from SiNCs on (b) glass substrate, (c) Au526, and (d) Au590. In (b), a bin time of 100 ms was used instead of 50 ms because of the comparably low PL intensity. (e, f, g) Corresponding weighted lifetime distributions derived from the single exponential fits for each time bin (bars) and from the stretched exponential fit (lines).

scatter plot would give a measure of the total fluorescence intensity instead of the total number of evaluated decay curves. From these scatter plots, histograms of the weighted lifetimes can be constructed, as shown in Figure 4e-g, which represent the discretized weighted lifetime distribution functions τ × F(τ) as obtained from the single exponential fit approach. From this the average lifetime can be calculated as

〈τi〉 )

∫ τF(τ) dτ/ ∫ F(τ) dτ

For the SiNC on bare glass, on Au526, and on Au590 we obtain, respectively, 〈τi〉 ) 18.4, 12.5, and 2.7 ns. These © 2010 American Chemical Society

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nm), this excitation is not resonantly driven but is a combination of an excitation into higher excitonic states and a relaxation into the lowest excitonic state |2〉. kexc is proportional to the absorption cross section at the laser wavelength and the local excitation field intensity at the position of the NC. The neutral exciton in state |2〉 can efficiently recombine radiatively bringing the NC back into its uncharged ground state |1〉. The ratio of the radiative decay rate kr and the nonradiative decay rate knr accounting, e.g., for recombinations via surface states, determines the quantum efficiency of the on-state defined by ηon ) kr/(kr + knr). The NC can make a transition from the neutral excited state |2〉 to the charged ground state |1′〉 when either the electron or the hole is trapped outside the NC leaving its contrary charged counterpart delocalized back inside the NC. Then, a further optical excitation of an electron-hole pair brings the NC in its excited charged state |2′〉. From here, radiative recombination of the charged exciton is effectively inhibited by a very fast nonradiative Auger recombination process in which the electron-hole pair relaxation energy is transferred to the unpaired charge carrier. When k′nr . k′r, the NC appears to be dark. The radiative emission of the NC recovers when the trapped carrier returns into the NC restoring charge neutrality. Since the usually observed power-low behavior of onand off-states cannot be explained within the above simple model, at least not when the trapping and detrapping rates ktrap and kdetrap are assumed to be constant, several attempts have been made to modify this model, as has been recently reviewed in ref 8. We will discuss later how our results presented here may be used to further investigate the exact nature of blinking. In the above model, the effect of metal particles on nearby NCs is not included. It is the interaction of the metal NP plasmons with the excitation field and the NC emitter itself that can change all mentioned transition rates and thus alter the emission characteristics of the NCs. Fluorescence Enhancement. The PL intensity of the NCs can be altered by changing both the excitation and recombination rates, as is illustrated in Figure 5. The intensity of the on-state emission is proportional to the excitation rate kexc and the quantum efficiency ηon ) kr/(kr + knr). In a good approximation, we can treat absorption and emission independently, since we excite the NC far above its emission energy with excitation intensities below saturation. First, we deal with the excitation process. Since kexc is proportional to local field intensity at the position of the NC, we performed numerical two-dimensional FDTD simulations to estimate the local excitation field intensity (see Supporting Information). In a first step, we simulated the extinction spectra of Au526 and Au590 films on cover glasses and compared them to the experimental data in order to proof the validity of the simulations. For Au526, the values of size and edge-to-edge distance of AuNPs have been determined from SEM images (see Figure 1a) to be 17 and 15 nm, respectively. Figure 1e compares the measured extinction spectra of the gold film Au526 (solid, red) to the simulated © 2010 American Chemical Society

one of an array of 45 AuNPs (dashed, red). Both spectra exhibit a concordance of the extinction peak wavelength, while the broadening of the experimental spectrum can be explained by the nonuniform distribution of AuNPs on glass substrate and a variation in the incident direction of light. The Au590 film is not as easy to model as the Au526 film because of the distribution of different aggregates. In a first approximation we calculated the mean size of the metal particle islands and their average distance on the basis of the SEM images. In particular the Au590 aggregate film was approximated by an array of spheroids with equatorial radii of 30 nm parallel to the surface, a polar radius of 12 nm perpendicular to the surface, and an average edge-to-edge distance between neighboring AuNP islands of 15 nm. This leads to a simulated spectrum which is again in good agreement with the experimental one, as shown in Figure 1e (blue lines). In a next step, we calculated the local electric field enhancement factors rexc, which are defined as the ratio of the electric field intensity at the excitation wavelength 470 nm and the position of NCs, to the incident field value in the absence of the metal nanostructure. The results of these simulations are shown in Figure S5 in the Supporting Information. We note that the excitation wavelength is blueshifted compared to the extinction peak, thus, the change in the local field intensity is an off-resonance effect. Obviously, the local field intensity enhancement is strongly dependent on the NC location on the AuNP film, which is not a priori known. In the particular example depicted in Figure S5, the SiNCs are located within the gaps of neighboring AuNPs, and the FDTD simulations reveal similar enhancement values of rexc ) 1.9 and rexc ) 2.0 for NCs on Au526 and Au590, respectively. However, the visualization of the enhancement factor suggest values of up to 3.5 for the Au526 films and up to 5 for the Au590 film for different particle geometries. These ranges of values are in good agreement with the experimentally observed PL enhancement distributions on the respective AuNP films as shown in Figure S1. Hence, the simulations show that the experimentally observed PL enhancement can in large part, if not in total, be attributed to the excitation process. However, in principle also the transition rates of the recombination processes can affect the PL intensity. Lifetime Reduction. Metal NPs can act as optical antennasconvertingpartsoftheemitter’snearfieldintoradiation.15,46 The origin for the modulation of the NC emission dynamics is the interaction of the emitter with its own secondary field that was scattered in the local environment.15 The oscillating near field of the decaying NC generates localized surface plasmons in the adjacent metal particles, leading to an energy transfer from the NC to the metal particles. The scattered power in the environment originates mostly from the induced dipoles and, thus, is strongly dependent on the particular geometry of the metal particles. The energy transfer, dissipative losses inside the metal particles, and 4171

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scattering lead to a modulation of both the radiative and nonradiative decay rates of a NC close to AuNPs as compared to the free space situation. Experimentally the change of the radiative and nonradiative rates for SiNCs on bare glass and on AuNP films can be estimated, if the respective quantum efficiencies are known. For SiNC in solution we measured a quantum efficiency of about 31%. However, for NCs without a silica shell, it has been shown in several examples that only a fraction of the individual NCs shows fluorescence at all.47,48 Taking also into account that these luminescent particles have an on-time fraction below 1, it has been argued that the maximum quantum efficiency of the on-state of NCs should be certainly higher than the quantum yield in solution,44 or even close to unity.48,49 However, the quantum efficiency of the on-state is not a priori known. If the on-state recombination of SiNCs is assumed to be indeed purely radiative (knr ) 0, ηon ) 1), the on-state intensity enhancement on AuNP films can be explained only by considering excitation enhancement, totally excluding emission enhancement. Then the measured rate for the glass SiNCs on glass should be equal to the radiative rate kmeasured glass -1 ) kr ) (18.4 ns) and the total quantum yield of an individual particle during the investigation should be equal to the on-time fraction of 60%. For the SiNC on Au590 the Au590 ) (2.7 ns)-1 is considerably measured decay rate kmeasured higher because of an energy transfer from the SiNC to the AuNPs which can lead either to scattering into the far-field or to dissipative losses. Thus, in general, the radiative as well as the nonradiative decay rate can be changed. The quantum Au590 would stay unity only in the efficiency of the on-state ηon scenario in which solely the radiative decay rate is changed and the nonradiative decay rate stays zero. Then, all the energy transferred from the NC to the metal particles is scattered into the far-field. In this case the total quantum yield of an individual SiNC coupled to Au590 should be equal to the on-time fraction of 94%. However, in general it is very unlikely that nonradiative processes can be totally excluded. First in the very likely case glass that the on-state quantum efficiency of SiNCs on glass, ηon is smaller than 1, part of the PL intensity enhancement might be attributed to emission enhancement due to interaction with the metal particles. Second, this metal particle interaction might open up additional nonradiative channels. In esscence there is a complicated interplay between the changes of radiative and nonradiative rates. FDTD simulations could help to quantify the change of emission dynamics by calculating the ratio of the total radiated power of a dipole source with and without adjacent AuNP films and identify it with the ratio of the radiative decay rate with and without adjacent AuNP films.50 However, first this method is valid only for emitters without nonradiative decay channels and second it is strongly depending on the exact three-dimensional structure of the system, which is not known at this point. Further models to simulate the radiative and nonra© 2010 American Chemical Society

diative decay rate modifications of fluorophores in proximity to a gold nanoparticle are given, e.g., in refs 15 and 51. Gray States. Usually it is assumed that in charged NCs the fluorescence is quenched because of a very fast Auger recombination process, as depicted in Figure 5 and explained above. However recently Spinicelli et al. reported for CdSe NCs coated with a rather thick (5 nm) CdS shell that this quenching is not complete.52 Instead of off-states without measurable photon emission, so-called gray states with nonzero fluorescence quantum efficiency have been observed. This was explained by a decreased Auger recombination rate particular for the investigated NC system, allowing the radiative rate of charged excitions to be competitive.52 While our SiNCs on bare glass show typical off-states with no significant photon emission, the situation changes when coupling the NCs to AuNP films. On Au590 the SiNCs exhibit low emitting periods with nonvanishing intensity clearly above the background. The probability density for the gray-state durations follows a power-law distribution as is well-known for off-states. Similarly as in the first report on gray states in NCs, we explain the occurrence of the gray states by competitive nonradiative and radiative rates for the charged exciton recombination. However, in contrast to the previous work, in our case the competitiveness is reached not by decreasing the Auger rate but by increasing the radiative rate due to the coupling to the AuNP film. Experimentally, we were not able to directly measure the recombination rates of the gray states because of the limited timeresolution of our setup and the only small number of photons emitted from these states. We can only correlate the quantum efficiencies of gray states and on-states by relating their Au590 Au590 background corrected mean count rates: ηgray /ηon ≈ 19%. Blinking Suppression. Our experimental results suggest a correlation between the above-discussed modulation of the recombination dynamics and the blinking behavior. Compared to the situation on bare glass, SiNCs on Au590 films reveal an increase of the on-time fraction from 60% to 94% and the fluorescence rate is increased by about 1 order of magnitude. Strictly speaking, since both the on- and off/gray time probabilities follow a power law, it is impossible to declare an average on- or off/gray time.7 Thus, also the declaration of an on-time fraction is not utterly correct, since a NC has a finite probability of being in a specific state for periods that are of the order of the measurement time. However, we use the on-time fraction as a figure of merit, since its increase for SiNCs on Au590 compared to on bare glass is a general finding and not just specific for the two SiNCs presented here in detail. One of the models which can successfully explain the power law behavior is the fluctuating tunneling barrier model of Kuno et al.41 Here we apply this model to our data, since it is congruent with the scheme sketched in Figure 5, if we claim that the trapping and detrapping occur via tunneling processes and that the tunneling barriers and, consequently, ktrap and kdetrap are fluc4172

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tuating in time. In ref 53, time traces of nanocrystals have been calculated using Monte Carlo simulations. Following this reference, we performed similar simulations assuming a fixed excitation rate kexc ) 5 × 105 s-1 and randomly fluctuating uncorrelated tunneling rates between neutral and charged states of ktrap ) k21′ exp(-x) and kdetrap ) k1′2 exp(-x′), respectively, with k21′ ) 5 × 106 s-1, k1′2 ) 1 × 105 s-1 and exponentially distributed random numbers x and x′. For the recombination rate from state |2〉 to |1〉 we assumed various values from k21 ) 5 × 106 s-1 to k21 ) 5 × 109 s-1. These simulations reveal that indeed by increasing the recombination rate also the on-time fraction is increased (see Figure S6, Supporting Information). A recombination rate of k21 ) 5 × 107 s-1, which is of the same order as the measured one for SiNCs on bare glass, leads to a simulated time trace with an on-time fraction of about 60%. Increasing the recombination rate by 1 order of magnitude to k21 ) 5 × 108 s-1, like in the experiments for SiNCs on Au590, reveals an increased simulated on-time fraction of about 98%. Obviously, these simulations show that the experimentally observed increase of the on-time fraction from 60% for SiNCs on bare glass to 94% for SiNCs on Au590 can in principle be explained within the fluctuating barrier model. This model suggests that also long off-times would be possible, even for SiNCs on gold but that they occur only very rarely because the respective trapping rate competes with the high radiative recombination rate. For a further investigation of the exact blinking mechanism, additional experiments with a larger set of samples of NCs with different radiative lifetimes governed by systematically changing the interaction to adjacent metal particles, as well as longer observation and simulation times might be very useful. We note that the use of thiols during the preparation of the Au590 film might also have an impact on the observed blinking suppression since thiols on the surface of NCs are reported to suppress the blinking.9 However, we suppose this effect to be negligibly small because of the silica shell surrounding our NCs. In conclusion, we have synthesized CdSe multishell NCs coated with silica shells and coupled them to AuNPs prepared on glass cover slides. These NCs showed an average increase of the on-time fluorescence intensity by a factor of up to 3 and a decreased fluorescence lifetime by about 1 order of magnitude. In addition we observed fluorescence from gray states and a strong blinking suppression with an increase of the on-state fraction from 60% to more than 90%. These observations can be explained by the metal particle induced change of excitation and recombination rates, which finally leads to a more than 4-fold increase of the total photon yield from individual nanocrystals.

Supporting Information Available. Detailed experimental procedures, maximum PL intensity statistics, lifetimeintensity distributions, FDTD simulation results of gold films, Monte Carlo simulation of blinking. This material is available free of charge via the Internet at http://pubs.acs.org. REFERENCES AND NOTES (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26) (27) (28) (29)

Acknowledgment. The authors thank A. Kornowski for assistance with the TEM and SEM experiments. H.T. thanks the Humboldt Foundation for financial support. This work was supported by the DFG under ME 1380/13-1. © 2010 American Chemical Society

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