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Highly efficient Förster resonance energy transfer between CdTe nanocrystals and Rhodamine B in mixed solid films
Chemical Physics Letters 388 (2004) 100–104 www.elsevier.com/locate/cplett
€rster resonance energy transfer between Highly efficient Fo CdTe nanocrystals and Rhodamine B in mixed solid films E. Alphandery a
a,*
, L.M. Walsh a, Y. Rakovich a, A.L. Bradley a, J.F. Donegan a, N. Gaponik b
Semiconductor Photonics Group, Physics Department, Trinity College, Dublin 2, Ireland b Institute of Physical Chemistry, University of Hamburg, 20146 Hamburg, Germany Received 19 December 2003; in final form 16 February 2004 Published online: 18 March 2004
Abstract We report evidence for highly efficient F€ orster resonance energy transfer between CdTe nanocrystals (donors) and Rhodamine B (acceptors) in mixed solid films. Enhancement of the acceptor photoluminescence intensity and a decrease of the donor decay time are observed in the films containing nanocrystals mixed with Rhodamine B. For a wide range of films with a ratio between the amount of acceptors and the amount of donors of 0.2–5, an efficiency of energy transfer of more than 20% is estimated from time resolved measurements. 2004 Elsevier B.V. All rights reserved.
1. Introduction F€ orster resonance energy transfer (FRET) [1] can be used to measure the distance between two types of molecules, when the distance between them is less than 2R0 , where R0 is the so-called F€ orster distance [2]. FRET has been applied extensively to measurements on biological samples, because it does not cause them to deteriorate, in contrast with other techniques such as transmission electron microscopy. Until recently, FRET studies have mostly been carried out using dye molecules conjugated to biological samples, such as proteins [2]. However, the use of dyes presents a few drawbacks. Firstly, the sensitivity of FRET is often limited by the relatively short excited state lifetimes in most dyes, which are similar to the decay times of the autofluorescence signal from biological samples. Secondly, dyes have non-tunable absorption and emission spectra, which makes it difficult to find a suitable FRET pair with sufficient spectral overlap between the donor emission spectrum and the acceptor absorption spectrum. *
Corresponding author. Fax: +353-16-71-1759. E-mail address: [email protected] (E. Alphandery).
0009-2614/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.02.080
In order to improve the sensitivity and the efficiency of FRET, the use of semiconductor nanocrystals (NCs) has recently been proposed. The NCs have a number of advantages over dyes such as tunable absorption and emission wavelengths, better photostability and longer excited state lifetimes. Despite a considerable effort towards the development of close-packed NC films to investigate energy transfer between NCs of different sizes [3–7], only a few studies have been published on energy transfer between NCs and dyes [8,9]. In the NC to NC transfer case, simultaneous excitation of donors and acceptors is inevitable and FRET analysis can be prone to complications, due to the strong overlap between the absorption spectra of the NCs of different sizes. By contrast, in the NC to dye transfer case, the overlap between the absorption spectra of the NCs and dyes can be avoided providing more accurate and easier estimation of the FRET parameters [8,9]. In this Letter, we study FRET between CdTe NCs and Rhodamine B (RhB) in randomly mixed closepacked solid films. The first advantage of this system comes from the solubility of CdTe NCs in water. By contrast to CdSe NCs synthesized in organic solvents [3,4,7], the highly luminescent CdTe NCs studied here are directly produced in water [10] and can therefore be
E. Alphandery et al. / Chemical Physics Letters 388 (2004) 100–104
2. Experimental The CdTe NCs are stabilized by thioglycolic acid and synthesized in aqueous solution by reaction of cadmium perchlorate with H2 Te gas [10]. Two types of NCs are investigated, small NCs with emission wavelengths in the region of RhB absorption at 525–550 nm and large NCs with emission wavelength above this region at 650 nm. The aqueous solutions of NCs are either used on their own or mixed in different proportions with a solution of RhB in water. These solutions are deposited on top of a 1 1 mm2 glass substrate and dried for 12 h under ambient conditions. The solutions are used to
produce either films containing NCs on their own or randomly mixed NCs with RhB. In the second case the concentrations of NCs, CD , where the suffix D designs donors and of RhB, CA , where the suffix A designs acceptors, are varied between 1.1 · 104 and 2.5 · 104 M and between 0.5 · 104 and 5.5 · 104 M, respectively. The experimental results are presented as a function of the ratio between the concentration of acceptors and the concentration of donors, 0:2 < CA =CD < 5. The PL measurements are performed at room temperature with a Perkin Elmer LS-55 luminescence spectrometer, using a Xenon lamp as an excitation source. Absorption spectra are recorded using a Shimadzu UV–VIS spectrometer. Luminescence lifetimes measurements are performed with a Pico Quant pulsed laser (kexc ¼ 408 nm, 10 MHz repetition rate, 0.3 mW maximum average power) and a Jobin Yvon HR320 spectrometer combined with a single photon counting system used for light detection, providing a typical instrument response of 100 ps.
3. Results and discussion Fig. 1 shows the absorption spectra of three samples containing either NCs on their own, RhB on its own or NCs mixed with RhB. The optical density of all samples is well below 0.1, which guarantees that reabsorption effects are negligible. The CdTe NCs show an absorption peak at 475 nm and RhB shows two absorption peaks at 525 and 560 nm. Fig. 1 shows that in the mixed sample, the absorption spectrum is dominated by the absorption of the CdTe NCs below 500 nm and by the absorption of RhB above 500 nm. This provides a favorable condition for FRET, because it enables us to excite the CdTe NCs at a wavelength, for example 400 nm, where the absorption of RhB is weak. 0.025 560 525
0.020
Optical density
used for FRET studies on biological samples without any further treatments. The second advantage of this system lies in the efficiency of the transfer, which is expected to be high. This is due on the one hand to the relatively high fluorescence quantum yields of our NCs (20–40% in aqueous solution) and to the short-chain thiol capping, which provides a close proximity between NCs and dye molecules. In addition, the excited state lifetimes of RhB, 2 ns [11], and of the CdTe NCs, 6–8 ns in films, lie within the same order of magnitude of one another which make it possible for very efficient energy transfer to occur. At the same time, the excited state lifetimes are also sufficiently different that any decrease in the excited state lifetime of the NCs can be unambiguously detected in the presence of RhB acceptors. The third advantage of this system is the high efficiency of the transfer that is expected because of the strong overlap between the emission spectra of the NCs and the absorption spectrum of RhB, which can be optimized by adjusting the position of the NC emission peak. The fourth advantage is the avoidance of simultaneous excitation of donor and acceptor by selecting the NC excitation wavelength in the region of weak RhB absorption. The Letter is organized as follows: experimental results of a series of samples, containing either donors or acceptors on their own or donors mixed with acceptors are considered first. The absorption spectra are shown to confirm that the system of RhB and CdTe NCs presents favorable conditions for energy transfer. Photoluminescence (PL), photoluminescence excitation (PLE) and lifetime measurements are then described in order to study the interaction between the electronic states of NCs and those of dye molecules. Next, a series of samples with different ratio between the amount of acceptors and the amount of donors are studied. The efficiency of energy transfer is estimated for these samples from the lifetime measurements. Finally, an estimate of the F€ orster distance is made, which further supports the picture of a highly efficient energy transfer between CdTe NCs and RhB.
101
475
0.015 0.010 0.005 0.000
400
500 600 Wavelength (nm)
700
Fig. 1. The absorption spectra of three typical samples containing NCs on their own (dashes), RhB on its own (dots), and NCs mixed with RhB (line). In the mixed film, the ratio between the amount of acceptors and the amount of donors is CA =CD ¼ 0:5.
E. Alphandery et al. / Chemical Physics Letters 388 (2004) 100–104
(a)
PL Intensity (a.u.)
Normalised
1800
Fig. 2 shows the PL spectra of two series of samples, containing either small nanocrystals emitting in the region of RhB absorption at 525 nm (Fig. 2a) or large nanocrystals emitting above this region at 650 nm (Fig. 2b). The excitation wavelength is fixed at 400 nm in order to have a stronger absorption of the NCs relative to that of RhB and the spectra are normalized by the absorption of the NCs measured at 400 nm (Fig. 1). Fig. 2a shows that the intensity of the acceptor peak, observed at 580 nm, is enhanced between the film containing RhB on its own and the film containing small NCs mixed with RhB. In contrast to this behavior, no enhancement of the acceptor emission is observed in the film containing large NCs mixed with RhB in Fig. 2b. The results presented in Fig. 2 provide strong support for energy transfer between CdTe NCs and RhB, which should result in the increase of the acceptor PL intensity [2]. The fact that the increase of the acceptor PL intensity is only observed for nanocrystals having a good emission spectral overlap with the acceptor absorption (those emitting at 525 nm) indicates that it can only be associated with energy transfer between NCs and RhB. We now examine the PLE spectra of a series of three samples, two films containing either nanocrystals on their own (Fig. 3a) or NCs mixed with RhB (Fig. 3b) and one solution containing nanocrystals on their own (Fig. 3c). Fig. 3a–d show that the nanocrystal peak is systematically shifted towards longer wavelength as the detection wavelength is scanned through the nanocrystal emission peak, from 500 to 590 nm in solution (inset of
Film of RhB Film of RhB and NCs Film of NCs
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λdet = 630 nm
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(a)
λdet = 540 nm
Intensity (a.u.)
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Fig. 2. The PL spectra excited at 400 nm of two series of samples containing nanocrystals emitting at either 525 (a) or 650 nm (b). The symbols design the films containing NCs on their own (dashes), RhB on its own (dots), and NCs mixed with RhB, (line). The PL intensities of the films containing NCs on their own are multiplied by 0.35 (a) and 0.3 (b), so that the spectra of these films can appear on the same scale as the other ones.
Intensity (a.u.)
102
480 (d)
Film of NCs and RhB Film of NCs solution of NCs
500
600 Wavelength (nm)
Fig. 3. The PLE spectra of a film containing NCs on their own (a), of a film containing NCs mixed with RhB (b) and of a solution containing nanocrystals on their own (c). The detection wavelength is increased by increment of 10 nm from 540 to 630 nm in (a) and (b) and from 500 to 590 nm in (c). (d) The variation of the nanocrystal peak position as a function of detection wavelength. The inset of (a–c) shows the PL spectra of the film containing NCs on their own, the film containing NCs mixed with RhB and the solution containing NCs on their own.
E. Alphandery et al. / Chemical Physics Letters 388 (2004) 100–104
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(a)
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curve of the mixed film is almost identical to that of the film containing NCs on their own. The fact that the decrease of the donor lifetime between the film containing NCs on their own and the mixed film is only observed in the samples containing small NCs indicates that it is due to energy transfer between NCs and RhB [2]. Next, we study the range of ratios between the amount of acceptors and the amount of donors, CA =CD , which is expected to produce a highly efficient energy transfer in the mixed samples containing small NCs. This study is motivated by previous reports, which have shown that the efficiency of FRET was dependent on the amount of acceptors relative to the donors [15–17]. The efficiency of energy transfer is estimated from the measurements of the PL decay time in a series of samples with CA =CD varied between 0.2 and 5. Given that the efficiency of energy transfer can be expressed as E ¼ 1 hsDA i=hsD i, it can be deduced from an estimate of the mean PL decay time of the NCs in the absence, hsD i, and in the presence, hsDA i, of RhB [1]. In turn, the mean PL decay times of the NCs, sD and sDA , are estimated from the fitting of the decay curves with stretched exponential functions [2,8,18] of the forms ID ðtÞ ¼ I0 expðt=sD Þb for the samples containing NCs on their own and IDA ðtÞ ¼ I0 expðt=sDA Þb for the samples containing NCs mixed with RhB, where b is a factor, which reflects the distribution in PL decay times and takes into account the non exponential shape of the NC PL decay curves [19]. The mean PL decay times of NCs hsD i and hsDA i are then estimated by multiplying sD and sDA by Cð1=bÞ=b, where C represents the gamma function [8,18,19]. The mean PL decay times of the NCs are found to decrease from hsD i 6–8 ns in the samples containing NCs on their own down to hsDA i 3–5 ns in the mixed samples. The efficiency of the energy transfer is shown in Fig. 5 as a function of CA =CD . It lies above
0
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FRET efficiency (%)
Normalised PL Intensity (a.u.)
Normalised PL Intensity (a.u.)
Fig. 3c), or from 540 to 630 nm in films (inset of Fig. 3a and b). The magnitude of this shift is much smaller in films (5–10 nm) than in solution (50 nm). For the nanocrystals in solution, the large shift of 50 nm of the nanocrystal peak has previously been reported and interpreted in terms of a size-selective absorption mechanism, taking place in a system of noninteracting nanocrystals [12,13]. For the nanocrystals in films, the small shift of 5–10 nm of the nanocrystal peak suggests a system of interacting nanocrystals, in which the energy is transferred from the small to the large nanocrystals [14]. Considering now the film containing nanocrystals mixed with RhB, it is not possible to determine from the data presented in Fig. 3b and d whether the small shift of 10 nm of the nanocrystal PL peak is due to energy transfer between nanocrystals of different sizes or to energy transfer between NCs and RhB but it is reasonable to assume that NC to dye transfer is indeed occurring. This assumption is strongly supported by the enhancement of the acceptor PL as shown in Fig. 2a. The last series of measurements supporting energy transfer are presented in Fig. 4a and b, which show the PL decay curves of two series of samples, containing either small NCs emitting at 540 nm (Fig. 4a) or large NCs emitting at 650 nm (Fig. 4b). In Fig. 4a, a much faster decay curve is observed in the film containing RhB mixed with NCs than in the film containing NCs on their own. In contrast, Fig. 4b shows that the decay
103
50 40 30 20 10
Time (ns) Fig. 4. PL decay curves of the small nanocrystals emitting at 540 nm (a) and of the large nanocrystals emitting at 650 nm (b). The spectra are detected at the NC PL peak emission wavelength for the samples containing NCs on their own (1) or NCs mixed with RhB (2) and at the RhB PL peak emission wavelength for the sample containing RhB on its own (3).
0 0.1
1 10 Acceptoramount / Donoramount(CA/CD)
Fig. 5. The efficiency of FRET as a function of the ratio between the amount of acceptors and the amount of donors, CA =CD . The line represents a polynomial fit to the data points (j).
104
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20% for a wide range of ratio CA =CD varied between 0.2 and 5. Considering the efficiency of the transfer as a function of CA =CD , it first increases from 10% upto 40– 50% when CA =CD is increased from 0.2 upto 1. It then saturates when CA =CD is further increased from 1 upto 3 and finally decreases when CA =CD is increased above 3. The increase followed by the saturation of the energy transfer efficiency with increasing number of acceptors is typical of FRET [15–17]. As the number of acceptors is increased, the probability of energy transfer from a donor to an acceptor is expected to become larger until it reaches saturation [17]. The decrease of the energy transfer efficiency in the high acceptor concentration limit is more unusual. It may be explained by the enhancement of either acceptor/acceptor interactions or interactions between acceptors and an environmental quencher such as oxygen. These interactions would indeed compete with and therefore weaken the energy transfer mechanism. Their presence is suggested by our lifetime measurements, which showed a faster decay curve in the film of RhB than in the solution of RhB. Further assessment of the high efficiency of FRET is made from an estimate of the F€ orster distance. Assuming that there are no loss channels, such as nonradiative recombination, of the donor excitation other than fluorescence and FRET, the efficiency of FRET can be expressed as E ¼ 1=½1 þ ðRDA =R0 Þ6 , where R0 , the F€ orster distance, is proportional to the NC fluorescence quantum yield and RDA is the distance between can donors and acceptors. The F€ orster distance, R0 (A), be estimated from the relation R0 ¼ 0:211ðk 2 QD Jn4 Þ1=6 [2], where k 2 is the orientation factor, QD is the NC fluorescence quantum yield, J is the overlap integral between the NC PL peak and the absorption spectrum of RhB and n is the refractive index of the medium between the NCs and RhB. The value of k 2 is chosen as 2/ 3, which assumes an isotropic orientation of the dipoles [3]. The value of the overlap integral, J, is estimated as between 4.62 · 1015 and 6.23 · 1015 M1 cm1 nm4 for the different samples, taking the molar extinction coefficient of RhB in solution into account [20]. Assuming that the quantum yield in the films of NCs is 10 times less than in solution [3], and that the refractive index of the surrounding medium is that of thioglycolic acid (n ¼ 1:505), a F€ orster distance of 3–3.5 nm is deduced, which is of the same order of magnitude as that reported for highly efficient FRET between dyes of different species [2] or NCs of different sizes [3], hence consistent with a highly efficient energy transfer.
4. Conclusion In conclusion, we have presented strong evidence of highly efficient resonance energy transfer between CdTe
NCs and RhB in mixed solid films. Experimental support for energy transfer is given by the change in the RhB PL peak intensity and NC lifetimes between the samples containing RhB mixed with NCs and those containing RhB or NCs on their own. The efficiency of energy transfer is estimated from the measurements of the NC decay times as lying above 20% for a wide range of samples with ratios between the amount of acceptors and the amount of donors, CA =CD , varied between 0.2 and 5.
Acknowledgements We thank Thomas Franzl and Andrey Rogach from the University of Munich and Yurii Gun’ko from the department of Chemistry at Trinity College for useful discussions as well as Enterprise Ireland for financial support under Grant No. IF/2002/656.
References [1] T. F€ orster, Ann. Phys. 2 (1948) 55. [2] B. van der Meer, G. Coker, S.Y. Simon Chen, Resonance Energy Transfer: Theory and Data, VCH Publishers, New York, 1994. [3] C.R. Kagan, C.B. Murray, M.G. Bawendi, Phys. Rev. B 54 (1996) 8633. [4] C.R. Kagan, C.B. Murray, N. Nirmal, M.G. Bawendi, Phys. Rev. Lett. 76 (1996) 1517. [5] O.I. Micic, K.M. Jones, A. Cahill, A.J. Nozik, J. Phys. Chem. B 102 (1998) 9791. [6] S. Wang, N. Mamedova, N.A. Kotov, W. Chen, J. Studer, Nanoletters 2 (2002) 817. [7] S.A. Crooker, J.A. Hollingsworth, S. Tretiak, V.I. Klimov, Phys. Rev. Lett. 89 (2002) 186802. [8] I.L. Medintz, A.R. Clapp, H. Mattoussi, E.R. Goldman, B. Fisher, J.M. Mauro, Nature Mater. 2 (2003) 630. [9] D.M. Willard, L.L. Carillo, J. Jung, A.V. Orden, Nanoletters 1 (2001) 469. [10] N. Gaponik, D.V. Talapin, A.L. Rogach, K. Hoppe, E.V. Shevchenko, A. Kornowski, A. Eychm€ uller, H. Weller, J. Phys. Chem. B 106 (2002) 7177. [11] W.T. Barnes, F.E. Lytle, Appl. Phys. Lett. 34 (1979) 509. [12] Y. Rakovich, L. Walsh, L. Bradley, J.F. Donegan, D. Talapin, A. Rogach, A. Eychm€ uller, Proc. SPIE 4876 (2002) 432. [13] M.V. Artemyev, U. Woggon, H. Jaschinski, L.I. Gurinovich, S.V. Gaponenko, J. Phys. Chem. B 104 (2000) 11617. [14] M. Achermann, M.A. Petruska, S.A. Crooker, V.I. Klimov, J. Phys. Chem. B 107 (2003) 13782. [15] B.P. Lyons, K.S. Wong, A.P. Monkman, J. Chem. Phys. 118 (2003) 4707. [16] J. Dong, P. Deng, J. Lum. 104 (2003) 151. [17] M. Inokuti, F. Hirayama, J. Chem. Phys. 43 (1965) 1978. [18] G. Schlegel, J. Bohnenberger, I. Potapova, A. Mews, Phys. Rev. Lett. 88 (2002) 137401. [19] J.L. Coffer, R.R. Chandler, C.D. Gutsche, I. Alam, R.F. Pinizzotto, H. Yang, J. Phys. Chem. 97 (1993) 696. [20] H. Du, R.A. Fuh, J. Li, A. Corkan, J.S. Lindsey, Photochem. Photobiol. 68 (1998) 141.
€rster resonance energy transfer between Highly efficient Fo CdTe nanocrystals and Rhodamine B in mixed solid films E. Alphandery a
a,*
, L.M. Walsh a, Y. Rakovich a, A.L. Bradley a, J.F. Donegan a, N. Gaponik b
Semiconductor Photonics Group, Physics Department, Trinity College, Dublin 2, Ireland b Institute of Physical Chemistry, University of Hamburg, 20146 Hamburg, Germany Received 19 December 2003; in final form 16 February 2004 Published online: 18 March 2004
Abstract We report evidence for highly efficient F€ orster resonance energy transfer between CdTe nanocrystals (donors) and Rhodamine B (acceptors) in mixed solid films. Enhancement of the acceptor photoluminescence intensity and a decrease of the donor decay time are observed in the films containing nanocrystals mixed with Rhodamine B. For a wide range of films with a ratio between the amount of acceptors and the amount of donors of 0.2–5, an efficiency of energy transfer of more than 20% is estimated from time resolved measurements. 2004 Elsevier B.V. All rights reserved.
1. Introduction F€ orster resonance energy transfer (FRET) [1] can be used to measure the distance between two types of molecules, when the distance between them is less than 2R0 , where R0 is the so-called F€ orster distance [2]. FRET has been applied extensively to measurements on biological samples, because it does not cause them to deteriorate, in contrast with other techniques such as transmission electron microscopy. Until recently, FRET studies have mostly been carried out using dye molecules conjugated to biological samples, such as proteins [2]. However, the use of dyes presents a few drawbacks. Firstly, the sensitivity of FRET is often limited by the relatively short excited state lifetimes in most dyes, which are similar to the decay times of the autofluorescence signal from biological samples. Secondly, dyes have non-tunable absorption and emission spectra, which makes it difficult to find a suitable FRET pair with sufficient spectral overlap between the donor emission spectrum and the acceptor absorption spectrum. *
Corresponding author. Fax: +353-16-71-1759. E-mail address: [email protected] (E. Alphandery).
0009-2614/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.02.080
In order to improve the sensitivity and the efficiency of FRET, the use of semiconductor nanocrystals (NCs) has recently been proposed. The NCs have a number of advantages over dyes such as tunable absorption and emission wavelengths, better photostability and longer excited state lifetimes. Despite a considerable effort towards the development of close-packed NC films to investigate energy transfer between NCs of different sizes [3–7], only a few studies have been published on energy transfer between NCs and dyes [8,9]. In the NC to NC transfer case, simultaneous excitation of donors and acceptors is inevitable and FRET analysis can be prone to complications, due to the strong overlap between the absorption spectra of the NCs of different sizes. By contrast, in the NC to dye transfer case, the overlap between the absorption spectra of the NCs and dyes can be avoided providing more accurate and easier estimation of the FRET parameters [8,9]. In this Letter, we study FRET between CdTe NCs and Rhodamine B (RhB) in randomly mixed closepacked solid films. The first advantage of this system comes from the solubility of CdTe NCs in water. By contrast to CdSe NCs synthesized in organic solvents [3,4,7], the highly luminescent CdTe NCs studied here are directly produced in water [10] and can therefore be
E. Alphandery et al. / Chemical Physics Letters 388 (2004) 100–104
2. Experimental The CdTe NCs are stabilized by thioglycolic acid and synthesized in aqueous solution by reaction of cadmium perchlorate with H2 Te gas [10]. Two types of NCs are investigated, small NCs with emission wavelengths in the region of RhB absorption at 525–550 nm and large NCs with emission wavelength above this region at 650 nm. The aqueous solutions of NCs are either used on their own or mixed in different proportions with a solution of RhB in water. These solutions are deposited on top of a 1 1 mm2 glass substrate and dried for 12 h under ambient conditions. The solutions are used to
produce either films containing NCs on their own or randomly mixed NCs with RhB. In the second case the concentrations of NCs, CD , where the suffix D designs donors and of RhB, CA , where the suffix A designs acceptors, are varied between 1.1 · 104 and 2.5 · 104 M and between 0.5 · 104 and 5.5 · 104 M, respectively. The experimental results are presented as a function of the ratio between the concentration of acceptors and the concentration of donors, 0:2 < CA =CD < 5. The PL measurements are performed at room temperature with a Perkin Elmer LS-55 luminescence spectrometer, using a Xenon lamp as an excitation source. Absorption spectra are recorded using a Shimadzu UV–VIS spectrometer. Luminescence lifetimes measurements are performed with a Pico Quant pulsed laser (kexc ¼ 408 nm, 10 MHz repetition rate, 0.3 mW maximum average power) and a Jobin Yvon HR320 spectrometer combined with a single photon counting system used for light detection, providing a typical instrument response of 100 ps.
3. Results and discussion Fig. 1 shows the absorption spectra of three samples containing either NCs on their own, RhB on its own or NCs mixed with RhB. The optical density of all samples is well below 0.1, which guarantees that reabsorption effects are negligible. The CdTe NCs show an absorption peak at 475 nm and RhB shows two absorption peaks at 525 and 560 nm. Fig. 1 shows that in the mixed sample, the absorption spectrum is dominated by the absorption of the CdTe NCs below 500 nm and by the absorption of RhB above 500 nm. This provides a favorable condition for FRET, because it enables us to excite the CdTe NCs at a wavelength, for example 400 nm, where the absorption of RhB is weak. 0.025 560 525
0.020
Optical density
used for FRET studies on biological samples without any further treatments. The second advantage of this system lies in the efficiency of the transfer, which is expected to be high. This is due on the one hand to the relatively high fluorescence quantum yields of our NCs (20–40% in aqueous solution) and to the short-chain thiol capping, which provides a close proximity between NCs and dye molecules. In addition, the excited state lifetimes of RhB, 2 ns [11], and of the CdTe NCs, 6–8 ns in films, lie within the same order of magnitude of one another which make it possible for very efficient energy transfer to occur. At the same time, the excited state lifetimes are also sufficiently different that any decrease in the excited state lifetime of the NCs can be unambiguously detected in the presence of RhB acceptors. The third advantage of this system is the high efficiency of the transfer that is expected because of the strong overlap between the emission spectra of the NCs and the absorption spectrum of RhB, which can be optimized by adjusting the position of the NC emission peak. The fourth advantage is the avoidance of simultaneous excitation of donor and acceptor by selecting the NC excitation wavelength in the region of weak RhB absorption. The Letter is organized as follows: experimental results of a series of samples, containing either donors or acceptors on their own or donors mixed with acceptors are considered first. The absorption spectra are shown to confirm that the system of RhB and CdTe NCs presents favorable conditions for energy transfer. Photoluminescence (PL), photoluminescence excitation (PLE) and lifetime measurements are then described in order to study the interaction between the electronic states of NCs and those of dye molecules. Next, a series of samples with different ratio between the amount of acceptors and the amount of donors are studied. The efficiency of energy transfer is estimated for these samples from the lifetime measurements. Finally, an estimate of the F€ orster distance is made, which further supports the picture of a highly efficient energy transfer between CdTe NCs and RhB.
101
475
0.015 0.010 0.005 0.000
400
500 600 Wavelength (nm)
700
Fig. 1. The absorption spectra of three typical samples containing NCs on their own (dashes), RhB on its own (dots), and NCs mixed with RhB (line). In the mixed film, the ratio between the amount of acceptors and the amount of donors is CA =CD ¼ 0:5.
E. Alphandery et al. / Chemical Physics Letters 388 (2004) 100–104
(a)
PL Intensity (a.u.)
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1800
Fig. 2 shows the PL spectra of two series of samples, containing either small nanocrystals emitting in the region of RhB absorption at 525 nm (Fig. 2a) or large nanocrystals emitting above this region at 650 nm (Fig. 2b). The excitation wavelength is fixed at 400 nm in order to have a stronger absorption of the NCs relative to that of RhB and the spectra are normalized by the absorption of the NCs measured at 400 nm (Fig. 1). Fig. 2a shows that the intensity of the acceptor peak, observed at 580 nm, is enhanced between the film containing RhB on its own and the film containing small NCs mixed with RhB. In contrast to this behavior, no enhancement of the acceptor emission is observed in the film containing large NCs mixed with RhB in Fig. 2b. The results presented in Fig. 2 provide strong support for energy transfer between CdTe NCs and RhB, which should result in the increase of the acceptor PL intensity [2]. The fact that the increase of the acceptor PL intensity is only observed for nanocrystals having a good emission spectral overlap with the acceptor absorption (those emitting at 525 nm) indicates that it can only be associated with energy transfer between NCs and RhB. We now examine the PLE spectra of a series of three samples, two films containing either nanocrystals on their own (Fig. 3a) or NCs mixed with RhB (Fig. 3b) and one solution containing nanocrystals on their own (Fig. 3c). Fig. 3a–d show that the nanocrystal peak is systematically shifted towards longer wavelength as the detection wavelength is scanned through the nanocrystal emission peak, from 500 to 590 nm in solution (inset of
Film of RhB Film of RhB and NCs Film of NCs
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600
700
PL Intensity (a.u.)
Normalised
700
0
(b)
Film of RhB Film of RhB and NCs Film of NCs
Intensity x 0.3
500
600
700
Wavelength (nm)
λdet λdet
500 600 700 Wavelength (nm)
λdet = 630 nm
500 600 Wavelength (nm)
700
(c)
PL of NC solution
λdet
λdet
500 600 700 Wavelength (nm)
λdet = 590 nm
500 600 Wavelength (nm)
700
540 λdet = 540 nm λdet
PL of film of NC and RhB
λdet
500 600 700 Wavelength (nm)
λ det = 630 nm
(b)
λdet = 500 nm
PLE Intensity (a.u.)
Intensity (a.u.)
PL of NC Film
500 600 Wavelength (nm)
700
Nanocrystal Peak Position (nm)
PLE Intensity (a.u.)
(a)
λdet = 540 nm
Intensity (a.u.)
PLE Intensity (a.u.)
Fig. 2. The PL spectra excited at 400 nm of two series of samples containing nanocrystals emitting at either 525 (a) or 650 nm (b). The symbols design the films containing NCs on their own (dashes), RhB on its own (dots), and NCs mixed with RhB, (line). The PL intensities of the films containing NCs on their own are multiplied by 0.35 (a) and 0.3 (b), so that the spectra of these films can appear on the same scale as the other ones.
Intensity (a.u.)
102
480 (d)
Film of NCs and RhB Film of NCs solution of NCs
500
600 Wavelength (nm)
Fig. 3. The PLE spectra of a film containing NCs on their own (a), of a film containing NCs mixed with RhB (b) and of a solution containing nanocrystals on their own (c). The detection wavelength is increased by increment of 10 nm from 540 to 630 nm in (a) and (b) and from 500 to 590 nm in (c). (d) The variation of the nanocrystal peak position as a function of detection wavelength. The inset of (a–c) shows the PL spectra of the film containing NCs on their own, the film containing NCs mixed with RhB and the solution containing NCs on their own.
E. Alphandery et al. / Chemical Physics Letters 388 (2004) 100–104
1
(a)
1 0.1
2 0.01
3 1E-3 1E-4
0
10
20
30
40
50
60
70
80
90
60
1
(b)
0.1
1 2
0.01 1E-3 1E-4
curve of the mixed film is almost identical to that of the film containing NCs on their own. The fact that the decrease of the donor lifetime between the film containing NCs on their own and the mixed film is only observed in the samples containing small NCs indicates that it is due to energy transfer between NCs and RhB [2]. Next, we study the range of ratios between the amount of acceptors and the amount of donors, CA =CD , which is expected to produce a highly efficient energy transfer in the mixed samples containing small NCs. This study is motivated by previous reports, which have shown that the efficiency of FRET was dependent on the amount of acceptors relative to the donors [15–17]. The efficiency of energy transfer is estimated from the measurements of the PL decay time in a series of samples with CA =CD varied between 0.2 and 5. Given that the efficiency of energy transfer can be expressed as E ¼ 1 hsDA i=hsD i, it can be deduced from an estimate of the mean PL decay time of the NCs in the absence, hsD i, and in the presence, hsDA i, of RhB [1]. In turn, the mean PL decay times of the NCs, sD and sDA , are estimated from the fitting of the decay curves with stretched exponential functions [2,8,18] of the forms ID ðtÞ ¼ I0 expðt=sD Þb for the samples containing NCs on their own and IDA ðtÞ ¼ I0 expðt=sDA Þb for the samples containing NCs mixed with RhB, where b is a factor, which reflects the distribution in PL decay times and takes into account the non exponential shape of the NC PL decay curves [19]. The mean PL decay times of NCs hsD i and hsDA i are then estimated by multiplying sD and sDA by Cð1=bÞ=b, where C represents the gamma function [8,18,19]. The mean PL decay times of the NCs are found to decrease from hsD i 6–8 ns in the samples containing NCs on their own down to hsDA i 3–5 ns in the mixed samples. The efficiency of the energy transfer is shown in Fig. 5 as a function of CA =CD . It lies above
0
10
20
30
40
50
60
70
80
90
FRET efficiency (%)
Normalised PL Intensity (a.u.)
Normalised PL Intensity (a.u.)
Fig. 3c), or from 540 to 630 nm in films (inset of Fig. 3a and b). The magnitude of this shift is much smaller in films (5–10 nm) than in solution (50 nm). For the nanocrystals in solution, the large shift of 50 nm of the nanocrystal peak has previously been reported and interpreted in terms of a size-selective absorption mechanism, taking place in a system of noninteracting nanocrystals [12,13]. For the nanocrystals in films, the small shift of 5–10 nm of the nanocrystal peak suggests a system of interacting nanocrystals, in which the energy is transferred from the small to the large nanocrystals [14]. Considering now the film containing nanocrystals mixed with RhB, it is not possible to determine from the data presented in Fig. 3b and d whether the small shift of 10 nm of the nanocrystal PL peak is due to energy transfer between nanocrystals of different sizes or to energy transfer between NCs and RhB but it is reasonable to assume that NC to dye transfer is indeed occurring. This assumption is strongly supported by the enhancement of the acceptor PL as shown in Fig. 2a. The last series of measurements supporting energy transfer are presented in Fig. 4a and b, which show the PL decay curves of two series of samples, containing either small NCs emitting at 540 nm (Fig. 4a) or large NCs emitting at 650 nm (Fig. 4b). In Fig. 4a, a much faster decay curve is observed in the film containing RhB mixed with NCs than in the film containing NCs on their own. In contrast, Fig. 4b shows that the decay
103
50 40 30 20 10
Time (ns) Fig. 4. PL decay curves of the small nanocrystals emitting at 540 nm (a) and of the large nanocrystals emitting at 650 nm (b). The spectra are detected at the NC PL peak emission wavelength for the samples containing NCs on their own (1) or NCs mixed with RhB (2) and at the RhB PL peak emission wavelength for the sample containing RhB on its own (3).
0 0.1
1 10 Acceptoramount / Donoramount(CA/CD)
Fig. 5. The efficiency of FRET as a function of the ratio between the amount of acceptors and the amount of donors, CA =CD . The line represents a polynomial fit to the data points (j).
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E. Alphandery et al. / Chemical Physics Letters 388 (2004) 100–104
20% for a wide range of ratio CA =CD varied between 0.2 and 5. Considering the efficiency of the transfer as a function of CA =CD , it first increases from 10% upto 40– 50% when CA =CD is increased from 0.2 upto 1. It then saturates when CA =CD is further increased from 1 upto 3 and finally decreases when CA =CD is increased above 3. The increase followed by the saturation of the energy transfer efficiency with increasing number of acceptors is typical of FRET [15–17]. As the number of acceptors is increased, the probability of energy transfer from a donor to an acceptor is expected to become larger until it reaches saturation [17]. The decrease of the energy transfer efficiency in the high acceptor concentration limit is more unusual. It may be explained by the enhancement of either acceptor/acceptor interactions or interactions between acceptors and an environmental quencher such as oxygen. These interactions would indeed compete with and therefore weaken the energy transfer mechanism. Their presence is suggested by our lifetime measurements, which showed a faster decay curve in the film of RhB than in the solution of RhB. Further assessment of the high efficiency of FRET is made from an estimate of the F€ orster distance. Assuming that there are no loss channels, such as nonradiative recombination, of the donor excitation other than fluorescence and FRET, the efficiency of FRET can be expressed as E ¼ 1=½1 þ ðRDA =R0 Þ6 , where R0 , the F€ orster distance, is proportional to the NC fluorescence quantum yield and RDA is the distance between can donors and acceptors. The F€ orster distance, R0 (A), be estimated from the relation R0 ¼ 0:211ðk 2 QD Jn4 Þ1=6 [2], where k 2 is the orientation factor, QD is the NC fluorescence quantum yield, J is the overlap integral between the NC PL peak and the absorption spectrum of RhB and n is the refractive index of the medium between the NCs and RhB. The value of k 2 is chosen as 2/ 3, which assumes an isotropic orientation of the dipoles [3]. The value of the overlap integral, J, is estimated as between 4.62 · 1015 and 6.23 · 1015 M1 cm1 nm4 for the different samples, taking the molar extinction coefficient of RhB in solution into account [20]. Assuming that the quantum yield in the films of NCs is 10 times less than in solution [3], and that the refractive index of the surrounding medium is that of thioglycolic acid (n ¼ 1:505), a F€ orster distance of 3–3.5 nm is deduced, which is of the same order of magnitude as that reported for highly efficient FRET between dyes of different species [2] or NCs of different sizes [3], hence consistent with a highly efficient energy transfer.
4. Conclusion In conclusion, we have presented strong evidence of highly efficient resonance energy transfer between CdTe
NCs and RhB in mixed solid films. Experimental support for energy transfer is given by the change in the RhB PL peak intensity and NC lifetimes between the samples containing RhB mixed with NCs and those containing RhB or NCs on their own. The efficiency of energy transfer is estimated from the measurements of the NC decay times as lying above 20% for a wide range of samples with ratios between the amount of acceptors and the amount of donors, CA =CD , varied between 0.2 and 5.
Acknowledgements We thank Thomas Franzl and Andrey Rogach from the University of Munich and Yurii Gun’ko from the department of Chemistry at Trinity College for useful discussions as well as Enterprise Ireland for financial support under Grant No. IF/2002/656.
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