11511
2007, 111, 11511-11515 Published on Web 07/18/2007
Time-Resolved Fo1 rster Energy Transfer from Individual Semiconductor Nanoantennae to Single Dye Molecules Daniel Soujon, Klaus Becker, Andrey L. Rogach, and Jochen Feldmann Photonics and Optoelectronics Group, Physics Department and CeNS, Ludwig-Maximilians-UniVersita¨t Mu¨nchen, Amalienstrasse 54, 80799 Munich, Germany
Horst Weller Institute of Physical Chemistry, UniVersity of Hamburg, Grindelallee 117, 20146 Hamburg, Germany
Dmitri V. Talapin The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720
John M. Lupton* Department of Physics, UniVersity of Utah, Salt Lake City, Utah 84112 ReceiVed: May 9, 2007; In Final Form: June 29, 2007
Single semiconductor nanocrystals can be used as nanoscale optical antennae to photoexcite individual dye molecules in an ensemble via Fo¨rster energy transfer. Energy transfer on the microscopic level between a single donor and acceptor depends on the individual spectral overlap of absorption and emission of the single entities as well as on their spatial separation. We recently demonstrated how the spectral overlap between a single donor and a single acceptor can be tuned electrically using the Stark effect. Here, we investigate the effect of the average donor-acceptor spacing on the time-resolved fluorescence dynamics of single donoracceptor pairs. The single nanocrystal donor luminescence is completely quenched for an average intermolecular separation of the dye of 9 nm. As the dye acceptor concentration decreases, both the number of donors observed and the average donor intensity increase due to an increase in nanocrystal fluorescence lifetime. At the same time, a temporal rise in the acceptor luminescence becomes discernible. The scatter in donor lifetimes observed increases with increasing acceptor concentration and is attributed to spatial disorder controlling the microscopic energy transfer rates.
Fo¨rster-type energy transfer by resonant nonradiative dipoledipole coupling is fundamental to many processes in nature1-3 and provides an important mechanism for applications as diverse as tracking conformational dynamics of biomolecules4-7 and tuning the color of light-emitting devices.8-10 A precise understanding of the microscopic nature of Fo¨rster energy transfer is imperative to exploiting the process in applications. As Fo¨rster transfer is an inherently microscopic process and occurs on the length scale of a few nanometers, it is important to realize that it is primarily the spectroscopic properties of the individual molecular entities which control the coupling strength.11 The efficiency of energy transfer depends on donor-acceptor separation, spectral overlap between donor emission and acceptor absorption, and relative transition dipole orientation,1 parameters which are not always accessible individually. Energy transfer within hybrid systems in which the individual components have very different functionality is of particular interest, as the combination of different classes of materials can * To whom correspondence should be addressed. E-mail: lupton@physics. utah.edu.
10.1021/jp073556i CCC: $37.00
potentially enable functionalities not found in either of the constituents. Semiconductor nanocrystals and dye molecules constitute such a combination of inorganic and organic materials, which have recently attracted considerable attention.6,7,11-20 Here, we focus on the role of average donor-acceptor separation in energy transfer and study this in the time domain using timecorrelated single-photon counting (TCSPC). As the energy transfer rate for point dipoles scales inversely with the sixth power of the donor-acceptor separation,1 an important question is how more distant dye molecules can affect the (potentially long-lived) emission of nanocrystal donors. The donor fluorescence lifetime therefore constitutes an ideal and sensitive probe of the single nanocrystal-dye coupling strength, which allows us to uncover the spatial disorder in energy-transfer rates. Strongly absorbing CdSe/CdS core-shell semiconductor nanocrystals consisting of a spherical CdSe core connected on one end to a rod-like CdS structure21 allow us to pick out individual dye molecules within a seemingly closed film of the dye by acting as an optical nanoantenna and funneling the absorbed light energy to exactly one adjacent molecular unit.11 Here, we demonstrate that the dye acceptors constitute strong © 2007 American Chemical Society
11512 J. Phys. Chem. C, Vol. 111, No. 31, 2007 quenchers of the single nanocrystals. This is witnessed by the significant reduction in fluorescence lifetime, which depends on the dye acceptor concentration and therefore on the average intermolecular spacing of the dye molecules. Concomitantly, energy transfer is observed directly in the time domain in terms of a rise in the fluorescence of the dye acceptor. Figure 1a illustrates the concept of energy transfer from a single semiconductor nanocrystal dispersed in a polystyrene polymer matrix blended with an accepting dye (a Cy5 derivative).11 The dye concentration is much larger than the nanocrystal concentration so that under direct excitation single dye molecules are not discernible in our far-field fluorescence microscope.11 Excitation of an individual dye molecule is only possible by energy transfer from an adjacent nanocrystal. We spin-coated ∼50 nm thin films of nanocrystal-dye-polystyrene blends onto silicon substrates covered with 200 nm of thermal oxide, mounted these in a He cold finger cryostat, and excited them with a frequency-doubled Ti:Sapphire laser of 74 MHz repetition rate at a wavelength of 400 nm and an intensity of 50 W/cm2. At this wavelength, the molar extinction coefficient of the dye is approximately 30,000 times smaller than that of the nanocrystals. We note that at this low excitation power, the formation of multiple excitations on one nanocrystal is extremely unlikely.22 The luminescence was collected by a long working distance microscope objective in epifluorescence configuration, optionally dispersed in a 25 cm monochromator, and detected using a Peltier-cooled charge-coupled device camera, typically integrating over 2 s per image. Further details of the materials such as ensemble absorption and PL excitation spectra are reported in refs 11 and 21. Figure 1b-f shows the change of the detected nanocrystal luminescence upon varying the dye concentration in the film and keeping the nanocrystal concentration constant. The effective dye concentration is best visualized in the average spacing between dye molecules. As the average spacing decreases from 35 to 9 nm, the luminescence from the single nanocrystals at 50 K is strongly quenched. Quenching is manifested by a decrease in the density and the intensity of fluorescent spots (e.g., in the transition from panel c to d). The decrease in density is a consequence of complete energy transfer from the nanocrystal to the dye, whereas the reduction of the average photoluminescence (PL) intensity (, given in the righthand panels) arises due to partial PL quenching by partial energy transfer. This is an interesting case as it highlights the statistical nature of energy transfer, implying that the excitation energy is only transferred for a fraction of the photons absorbed and otherwise decays unperturbed on the nanocrystal. A few faint spots can be made out in panel e, which most likely originate from nanocrystals exhibiting incomplete energy transfer to adjacent molecules. No nanocrystal luminescence is discernible for a mean dye molecule spacing of 9 nm (panel f). The continuous variation of donor density and intensity with acceptor concentration provides strong evidence that both the donor and acceptor are uniformly spaced within the film and do not aggregate. The effect of energy transfer to dye molecules on the single nanocrystal PL can be quantified by plotting histograms of the intensities of the individual nanocrystal spots, as shown in panels g-k. The dye-free sample displays a significant scatter in intensity, which is most likely due to different orientations of the nanocrystals as well as short-term PL blinking. As the dye concentration is increased, the histograms first shift to lower intensities and narrow, suggesting that most particles are partially quenched by energy transfer. However, the overall number of
Letters fluorescence spots observed decreases too, indicating that some nanocrystals are completely quenched by the dye molecules. In this case, the close proximity of the donor and acceptor and the appropriate transition dipole alignment evidently give rise to efficient energy transfer. Efficient quenching necessitates both the donor-acceptor spacing requirement and the spectral overlap condition be met. While we have previously shown that not all individual close donor-acceptor pairs satisfy the spectral resonance condition at 50 K for more dilute samples,11 the results illustrate that for a mean separation between dye molecules of 9 nm, a suitable acceptor is always within reach of the nanocrystal donor. Studying the single-nanocrystal density and intensity only provides rudimentary insight into the way adjacent dye molecules affect the nanocrystal luminescence. We expect that the microscopic distances between the donor and acceptor in a pair are random, which should lead to a distribution of transfer efficiencies when comparing different pairs. Evidence for such a distribution in energy-transfer rates comes from the fluorescence lifetime of the individual nanocrystals. Figure 2 shows histograms of the single-particle luminescence decay times τ determined by performing TCSPC using a PicoQuant TimeHarp 200 TCSPC board with a Perkin-Elmer avalanche photodiode detector (600 ps time resolution). Typical decay traces are shown in the insets for different concentrations of the dye acceptor (the triangle symbols indicate the nanocrystal-only case). The initial decay of the luminescence can be described by an exponential in order to extract a characteristic lifetime.23 We note that the decay deviates from single-exponential behavior at longer times due to the detrapping-induced luminescence, which leads to a complex power-law time dependence.24 The histogram of the PL lifetime τ in Figure 2a shows that the singleparticle decay times scatter by over a factor of 2 for the dyefree sample. As the dye concentration increases, energy transfer becomes a competing decay channel to spontaneous emission, and the distribution of nanocrystal lifetimes shifts to shorter times and broadens somewhat. The broadening can be quantified by considering the ratio of the standard deviation σ to the mean value of the lifetime values measured. Values of 0.25, 0.31, and 0.44 are obtained for σ/ for the dye-free sample, the ) 16 nm sample, and the ) 12 nm sample, respectively. The broadening of the τ histogram with increasing dye concentration can be interpreted as being a result of the scatter in energy-transfer efficiencies due to the distribution in potential donor-acceptor separations being superimposed on the intrinsic spread of fluorescence lifetimes of the single nanocrystals. Although the decay transients of a single particle correlate with the emission intensity as it changes in time,23 a direct connection between different particles, for example, between Figures 1g and 2a, was not observed. This is most likely due to the scatter in nonradiative rates between particles as well as the efficiency with which the nanocrystals are excited in the laser field. A consequence of energy-transfer excitation of single dye molecules is the modification of the acceptor luminescence kinetics. In ensemble measurements, we have identified a general lengthening of the dye luminescence decay time under energy-transfer conditions (not shown). This arises because energy transfer from the nanocrystal to the dye continues beyond the effective dye lifetime and is linked to delayed emission from the nanocrystals.24 Single dye molecules excited by energy transfer can be identified at 50 K by shifting the spectral detection window outside of the emission range of the nanocrystals to wavelengths >675 nm. Figure 3a shows a fluores-
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J. Phys. Chem. C, Vol. 111, No. 31, 2007 11513
Figure 2. Fluorescence decay and fluorescence lifetimes of single nanocrystals (NCs) embedded in a polystyrene matrix with different concentrations of dye acceptors measured at 50 K. (a) No dye molecules; (b) mean dye molecule separation of approximately 16 nm; (c) separation of approximately 12 nm. The insets show typical nanocrystal decays without surrounding dye molecules (blue triangles) and with dye acceptors (red circles).
Figure 1. (a) Energy transfer from an elongated semiconductor nanoantenna to a single adjacent dye molecule within an ensemble of dye molecules dispersed in a polystyrene film. (b)-(f) Quenching of the luminescence of single CdSe/CdS nanocrystals (NCs) in a polystyrene matrix mixed with dye acceptors, detected in a spectral window of 605 ( 15 nm at 50 K. As the concentration of the dye acceptor is increased while the concentration of nanocrystals remains constant, the density and intensity of emitting nanocrystals decreases. The approximate mean dye molecule spatial separations are given. (g)(k) Histograms of the PL intensities (in arbitrary units) of the individual nanocrystal spots observed. The mean PL intensities are indicated for each case.
cencemicroscope image detected in this spectral region for a sample of average dye spacing ) 14 nm at 50 K. Single spots are identified, which correspond to the emission of single dye molecules rather than single nanocrystals, as discussed in the following. In contrast to the nanocrystal images in Figure 1, a substantial background is observed in these images. This background originates from direct excitation of the dye molecules by the laser as these are present in greater concentration than the nanocrystals.11 The temporal profile of the background luminescence is given by the blue curve in panel b. It exhibits a prompt rise. The time traces of two single dye molecules marked in the image (panel a) are also shown (the points indicate the raw data, and the solid lines are a guide to the eye). These traces were corrected for the background and display a substantial luminescence rise over ∼2 ns and an increased luminescence decay time with respect to the dye background. Photopumping of single dye molecules via the semiconductor nanocrystal nanoantennae therefore substantially modifies the fluorescence kinetics by extending the luminescence decay rate and introducing a fluorescence rise. We note that such a rise will most likely only become visible for the longest living nanocrystal donors. A series of measurements of the acceptor fluorescence rise and decay of one single molecule provides direct evidence for temporal fluctuations in the energy-transfer
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Figure 3. PL of single dye molecules ( ) 14 nm) excited via energy transfer from adjacent nanoantennae and detected in a spectral window of wavelength >675 nm. (a) Fluorescence image. (b) Slow rise and decay of the fluorescence of two single dye molecules (background subtracted) following pulsed excitation of single nanocrystals at 50 K. The dye background excited directly by the laser exhibits a faster rise and decay (blue). The solid lines are best fits to the data and serve as a guide to the eye. (c) Time trace of the dye emission spectrum excited by energy transfer at 5 K, displaying spectral diffusion, a typical characteristic of single-molecule emission. (d) Polarization anisotropy of the dye emission photopumped by a single nanocrystal. The linear polarization supports the conclusion of emission occurring from a single dye molecule.
efficiency, as has previously been reported for energy transfer between single dye molecules.25,26 Because of this temporal fluctuation, the rise and decay of the acceptor luminescence is not simply described by a combination of donor and acceptor decay rates and one fixed time constant for the energy transfer. The time trace in Figure 3b therefore cannot simply be fitted by a straightforward product of exponentials. Final confirmation that the luminescence spots originate from single dye molecules rather than from multiple dye molecules surrounding a single nanocrystal donor comes from a consideration of the temporal dynamics of the emission and the emission polarization. Figure 3c shows a time trace of the dye emission spectra at 5 K, excited by energy transfer. As expected for a single molecule, the dye emission displays both fluctuations of the emission intensity (blinking) and a random change in the emission wavelength (spectral diffusion).27 Such random spectral fluctuations of the acceptor cannot originate from dynamics of the donor and constitute a signature of a single acceptor emitting. In addition, the acceptors generally display linearly polarized emission, as shown by the modulation of the PL intensity under rotation of an analyzer in the detection pathway (Figure 3d). Multiple acceptors would have slightly different dipole orientations in the vicinity of the much larger nanocrystal donor, leading to weakly polarized emission. The transient information on acceptor rise and decay can be used to derive a very rough estimate of the donor-acceptor separation, bearing in mind that the point dipole approximation may not be appropriate for the nanorods, necessitating a modification to Fo¨rster theory.28 The populations L of excited donors D and acceptors A are given by the two rate equations
dLD ) -(kD + kET)LD dt dLA ) kETLD - kALA dt
where k describes the rates of donor emission (D), acceptor emission (A), and energy transfer (ET). Solving the equations and differentiating the solution to find the time of maximum acceptor population tmax yields
tmax )
(
)
kD + kET 1 ln kD - kA + kET kA
Extracting a rise time of 2 ns (black curve) and assuming characteristic donor and acceptor lifetimes of τD ) 2.5 ns and τA ) 3 ns, respectively, as measured, leads to an energy-transfer time of τET ) 1/kET ) 3.2 ns. The energy-transfer efficiency is then given by η ) kET/(kD + kET) ≈ 44%. While we cannot measure the microscopic Fo¨rster radius from the spectral overlap of donor emission and acceptor absorption directly, the Fo¨rster radius of R0 ) 6.0 nm extracted from the ensemble data can provide a rough estimate.11 The observed kinetics then correspond to a donor-acceptor separation of ∼6.2 nm. An immediate conclusion of this rough consideration is that the particular dye molecules shown in Figure 3 cannot be located close to the nanocrystal donors, that is, dye and nanocrystals do not physically aggregate. Incorporating single semiconductor nanocrystals into a dense film of dye molecules allows us to pick out a single dye molecule by energy-transfer excitation.11 The efficiency of energy transfer scatters widely due to the distribution in local donor-acceptor separation and individual spectral overlap. Semiconductor nanocrystals can potentially be employed as nanoscale probes of molecular materials such as organic semiconductors. We expect to be able to pick out individual molecules in a closed film or blend of, for example, conjugated polymers and study the intrinsic optical properties such as fluorescence lifetime, blinking, spectral diffusion, and charging effects in situ under operating device conditions.29 Conjugates of nanocrystal donors and dye acceptors should provide exquisite sensors to spatial and electronic perturbations and may find
Letters applications in, for example, fluorescence-based actuators. In future experiments, we hope to be able to monitor acceptor rise and donor decay simultaneously by resorting to such welldefined energy-transfer couples. Acknowledgment. We thank W. Stadler and A. Helfrich for technical assistance and acknowledge financial support by the VW Foundation, the Elite Network of Bavaria, and the DFG through the SFB486, the Gottfried-Wilhelm-Leibniz Preis, and the Cluster of Excellence NIM. Work at the Molecular Foundry was supported by the Office of Basic Energy Sciences of the U.S. Department of Energy under Contract No. DE-AC0205CH11231. References and Notes (1) Fo¨rster, T. Discuss. Faraday Soc. 1959, 27, 7. (2) Kloepfer, J. A.; Cohen, N.; Nadeau, J. L. J. Phys. Chem. B 2004, 108, 17042. (3) Scholes, G. D. Annu. ReV. Phys. Chem. 2003, 54, 57. (4) Weiss, S. Science 1999, 283, 1676. (5) Gordon, G. W.; Berry, G.; Liang, X. H.; Levine, B.; Herman, B. Biophys. J. 1998, 74, 2702. (6) Clapp, A. R.; Medintz, I. L.; Mauro, J. M.; Fisher, B. R.; Bawendi, M. G.; Mattoussi, H. J. Am. Chem. Soc. 2004, 126, 301. (7) Hohng, S.; Ha, T. ChemPhysChem 2005, 6, 956. (8) Coe, S.; Woo, W. K.; Bawendi, M.; Bulovic, V. Nature 2002, 420, 800. (9) Ego, C.; Marsitzky, D.; Becker, S.; Zhang, J. Y.; Grimsdale, A. C.; Mu¨llen, K.; MacKenzie, J. D.; Silva, C.; Friend, R. H. J. Am. Chem. Soc. 2003, 125, 437. (10) Achermann, M.; Petruska, M. A.; Kos, S.; Smith, D. L.; Koleske, D. D.; Klimov, V. I. Nature 2004, 429, 642. (11) Becker, K.; Lupton, J. M.; Mu¨ller, J.; Rogach, A. L.; Talapin, D. V.; Weller, H.; Feldmann, J. Nat. Mater. 2006, 5, 777.
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