ZnTe

15548

J. Phys. Chem. C 2009, 113, 15548–15552

Resonant Energy Transfer between CdSe/ZnS Type I and CdSe/ZnTe Type II Quantum Dots Chun-Hsiung Wang,† Chih-Wei Chen,† Chih-Ming Wei,† Yang-Fang Chen,*,† Chih-Wei Lai,‡ Mei-Lin Ho,‡ and Pi-Tai Chou‡ Departments of Physics and Chemistry, National Taiwan UniVersity, No.1, Sec. 4, RooseVelt Rd., Da-an District, Taipei 106, Taiwan ReceiVed: May 11, 2009; ReVised Manuscript ReceiVed: July 19, 2009

Fluorescence resonant energy transfer (FRET) has been clearly demonstrated between CdSe/ZnS type I quantum dots and CdSe/ZnTe type II quantum dots by photoluminescence, time-resolved photoluminescence (TRPL), and photoluminescence excitation (PLE) measurements. The result of PLE experiment provides us concrete evidence that the emission of CdSe/ZnTe quantum dots is indeed influenced by CdSe/ZnS quantum dots. It is found that by changing the size of CdSe/ZnS quantum dots, the emission spectra of CdSe/ZnTe quantum dots can be selectively enhanced in a particular wavelength range and the emission intensity of type II quantum dots can be greatly enlarged by up to four times. Together with the large tunability of the emission energy in the infrared region, our finding provides an opportunity for creating highly efficient optoelectronic devices and bioimaging labels derived from type II quantum dots. We stress here that the large difference in emission energies between donors and acceptors in our studied system is especially useful for the development of biosensors. Introduction In the ensemble of the nanocrystals, fluorescence resonant energy transfer (FRET) is the most common effect of mutual interdot interaction.1-3 The donor particles could transfer their exciton energy into the acceptor particles by nonradiative dipole-dipole interaction. In the past decade, FRET has been extensively studied by using inorganic type I quantum dots1,4,5 (such as CdSe, CdTe and PbS, etc.) and organic dyes6 due to the good design of structure with reduced donor-acceptor separation and the improved spectral overlap of the acceptor absorption and donor emission spectra. Due to these excellent experiments of FRET, there are more and more applications appearing in optoelectronic devices2 and bioimaging labels.7,8 Recently, type II quantum dots,9 such as CdSe/ZnTe and CdTe/CdSe, have attracted a lot of interest for their potential use in solar cells10 and biosensors11 due to the spatial separation of electrons and holes and tunable near-infrared (NIR) band gap. Briefly, in type II quantum dots, one photoexcited carrier is confined in the core while the other is confined in the shell.9 However, both of the carriers of type I quantum dots are confined in the core. In spite of the unique optical properties and potential applications of type II quantum dots, there has been no report concerning the FRET process in them. This paper is aimed as investigating the FRET effect between CdSe/ZnS type I and CdSe/ZnTe type II quantum dots. We clearly demonstrate that the FRET process can be well controlled between type I and type II quantum dots based upon a suitable design of the band alignment between two constituent quantum dots. Accordingly, the emission intensity of type II quantum dots can be enhanced by up to four times. With the high efficiency of the FRET process and the tunability of the emission * To whom correspondence should be addressed. Fax: +886-2-23639984. Phone: +886-2-33665125. E-mail: [email protected]. † Department of Physics. ‡ Department of Chemistry.

energy in the NIR region, our results should be useful for creating optoelectronic devices as well as bioimaging sensors based on type II quantum dots. Experimental Section The studied CdSe/ZnS and CdSe/ZnTe quantum dots were synthesized by the chemical colloidal method as reported before.9 For detailed preparation procedures, please refer to the previous report.12 The small (large) CdSe/ZnS type I quantum dots consist of CdSe with a diameter of 3.5 nm (3.9 nm) and ZnS with a thickness of about 1 nm. The CdSe/ZnTe type II quantum dots consist of CdSe with a diameter of 4.5 ( 0.3 nm and ZnTe with a thickness of about 1 nm. For all quantum dots, the ligands used are tri-n-octylphosphine oxide (TOPO), and the estimated diameters do include the length of TOPO. Both pure and hybrid films were prepared from the toluene solvent and then deposited by drop casting on the silicon substrate under ambient atmosphere. The pure film consists of either CdSe/ZnTe or CdSe/ZnS quantum dots and the hybrid film 1 (2) consists of 34% (by concentration) of CdSe/ZnTe quantum dots and 66% of large (small) CdSe/ZnS quantum dots. According to the crosssection scanning electron microscope (SEM) image as shown in Figure 1a, the film thickness was estimated to be 13.81 um. Figure 1b shows the top view of the SEM image of the hybrid film 1. According to the concentration of quantum dots used, the estimated separation between type I and type II quantum dots is about 6.3 nm for the hybrid film.13 For the NIR photoluminescence measurement, an Ar-ion laser with 514.5 nm wavelength was used as the excitation source. The photoluminescence spectra were recorded by a Triax 320 monochromator and detected by a InGaAs detector. For the visible photoluminescence measurement, a pulsed diode laser with 374 nm wavelength was used as the excitation source. The photoluminescence spectra were recorded by a Triax 320 monochromator and detected by a photomultiplier tube (PMT).

10.1021/jp904361a CCC: $40.75  2009 American Chemical Society Published on Web 08/07/2009

Resonant Energy Transfer between Quantum Dots

J. Phys. Chem. C, Vol. 113, No. 35, 2009 15549

Figure 2. (a) Steady state photoluminescence spectra of CdSe/ZnS (black solid curve) and CdSe/ZnTe quantum dots (red solid curve) and the absorption spectrum of CdSe/ZnTe quantum dots (black dashed curve). (b) The band alignment of CdSe/ZnS type I quantum dots and CdSe/ZnTe type II quantum dots. The arrows show the fluorescence resonant energy transfer process and the relaxation process of carriers in the conduction and valence band.

Results and Discussion

Figure 1. (a) The cross-section scanning electron microscope image of hybrid film 1. The measured film thickness of 13.81 µm is also shown in the image. (b) Top view of the scanning electron microscope image of the mixed quantum dots in hybrid film 1.

Time-resolved photoluminescence (TRPL) of donors was carried out by using the technique of time-correlated single-photon counting (TCSPC) and the same pulsed diode laser was used as the excitation source. The diode laser produces light pulses with a repetition rate of 5 MHz. The collected luminescence was directly projected into a Triax 320 monochromator and detected with a high-speed PMT. The photoluminescence signal is fed into a Time Harp counting card, which was triggered with a signal from the diode laser. In addition, for the TRPL measurements of acceptors in the nanomicrosecond region, a second harmonic of an Nd:YAG laser (532 nm, 8 ns, Continuum Surelite II) was used as an excitation source. Emission decay was then detected by a NIR sensitive photomultiplier tube (Hamamatzu R5509-72) operated at -80 °C, which was cooled by liquid nitrogen; the signal was sent through an oscilloscope (Model TDS 3012, Tektronix), which was then averaged over 512 shots for further analyses. For the photoluminescence excitation (PLE) measurement, a Xe lamp and Spectra Pro 300i monochromator were used as the excitation source. The visible and NIR quantum yield were measured by the comparative method,14 which used the standards of Fluorescein (with the quantum yield of 0.79 in 0.1 M NaOH)15 and NIR laser dye IR 125 (with the quantum yield of 0.11 in dimethyl sulfoxide),16 respectively.

Prior to the discussion of the experimental results, an illustration of the design principle is necessary. It is well established that there are three main requirements for the occurrence of FRET: (i) The donors should have larger emission photon energy than that of the acceptors and the donor emission and acceptor absorption spectra should have large spectral overlap.1 (ii) The distance between donors and acceptors should be under a decent range within about 2-10 nm.17 (iii) The transition dipole moment of donors and acceptors should lie on the correct orientation.1 To obtain a highly efficient FRET process in a hybrid film consisting of type I and type II quantum dots, we have grown CdSe/ZnS quantum dots, which have emission energy coinciding with the absorption spectrum of CdSe/ZnTe quantum dots as shown in Figure 2a. In addition, the broad photoluminescence line width of CdSe/ZnTe quantum dots is caused by the size distribution of the core of CdSe/ZnTe quantum dots. After energy transferred from CdSe/ZnS quantum dots to the core of CdSe/ZnTe quantum dots, the excited electrons and holes relax into the ground states of the conduction band in CdSe and the valence band in ZnTe, respectively, according to the type II band alignment, and then make a type II photoluminescence transition. The overall picture of our design principle is well described in Figure 2b. Figure 3a shows the photoluminescence spectra of CdSe/ZnS and CdSe/ZnTe quantum dots in pure films and hybrid film 1 under the same optical condition. It is found that the photoluminescence intensity arising from CdSe/ZnTe quantum dots in hybrid film 1 is four times larger than that in pure CdSe/ZnTe film. On the other hand, the photoluminescence intensity arising from CdSe/ZnS quantum dots in the hybrid film 1 is lower than that in pure CdSe/ZnS film. We have also performed a quantum

15550

J. Phys. Chem. C, Vol. 113, No. 35, 2009

Wang et al.

Figure 3. (a) Steady state photoluminescence spectra of pure CdSe/ZnTe film (black solid curve), hybrid film 1 in the NIR region (black dashed curve), hybrid film 1 in the visible region (red dashed curve), and pure CdSe/ZnS film (red solid curve) at 300 K. (b) Time-resolved photoluminescence spectra (detected at 2.175 eV of photon energy) of CdSe/ZnS quantum dots in pure film and hybrid film 1 at 300 K. The red solid curve shows the fitting results of each photoluminescence decay time. (c) Time-resolved photoluminescence spectra (detected at 1.405 eV of photon energy) of CdSe/ZnTe quantum dots in pure film and hybrid film 1 at 300 K.

efficiency measurement, and the obtained values were 0.7 and 0.01 for CdSe/ZnS and CdSe/ZnTe quantum dots, respectively. To gain insight into the relative emission intensities between the type I and type II quantum dots, we have scaled the data shown in Figure 3a with respect to the corresponding quantum efficiency. It is worth noting that because the size of the laser beam used in our measurement has a radius of about 1 mm, the variation of the quantum yield in the very small domain is negligible. We have repeated the photoluminescence measurements at different regions within the same sample and the result is reproducible. In addition, the experimental results for several different samples with the same size of quantum dots are also reproducible. To confirm that the underlying mechanism responsible for the interaction between CdSe/ZnS and CdSe/ ZnTe quantum dots is indeed dominated by the FRET process as proposed, we have measured the photoluminescence decay time of CdSe/ZnS and CdSe/ZnTe quantum dots in pure films and hybrid film 1. As shown in Figure 3b, the photoluminescence decay curve of CdSe/ZnS quantum dots in hybrid film 1 decays faster than that in pure CdSe/ZnS film, and both of the decay curves can be well fitted by the stretched exponential function:2,18

I(t) ) I0 exp[-(t/τ)β]

previously based on purely empirical findings.18 Furthermore, the corresponding average decay times 〈τ〉 can be estimated by the equation:2,18

〈τ〉 )

τ 1 Γ β β

()

(2)

where Γ(1/β)is the Γ function of stretching factor β. The obtained corresponding average decay times of CdSe/ZnS in hybrid film 1 and pure film are 7.5 (β ) 0.52) and 13.1 ns (β ) 0.73), respectively. The decrease of stretching factor β of hybrid film 1 reveals that the distribution of photoluminescence decay time of hybrid film 1 is broader (more relaxation channels) than that of the pure film. In addition, it is also found that the photoluminescence decay time of CdSe/ZnTe quantum dots in hybrid film 1 is longer than that in pure film as shown in Figure 3c. These behaviors can be well understood in terms of the energy transfer from CdSe/ZnS quantum dots to CdSe/ZnTe quantum dots through a FRET process.1,2 With the additional energy transfer channel, the relaxation dynamics of CdSe/ZnS quantum dots in hybrid film 1 can be described by the rate equation2

(1)

where I(t) is the photoluminescence intensity as a function of time, I0 is the initial photoluminescence intensity at t ) 0, τ is the time constant, and β is the stretching factor, which is related to the distribution of decay times. The relationship between stretching factor β and time constants has been discussed

1 τhybrid

)

1 τpure

+ ΓFRET

(3)

where 1/τhybrid is the photoluminescence decay rate of CdSe/ ZnS quantum dots in hybrid film 1, 1/τpure is the photoluminescence decay rate of CdSe/ZnS quantum dots in pure film,

Resonant Energy Transfer between Quantum Dots

J. Phys. Chem. C, Vol. 113, No. 35, 2009 15551

Figure 4. Photoluminescence excitation (PLE) spectra of pure CdSe/ ZnS film (red solid curve), pure CdSe/ZnTe film (black solid curve), and hybrid film 1 (green curve). The inset shows the PLE spectrum of hybrid film 1 and the fitting result based on the combination of PLE spectra of pure CdSe/ZnS and CdSe/ZnTe films.

and ΓFRET is the energy transfer rate. The energy transfer efficiency can be estimated by the equation17,19

E)

ΓFRET 1/τhybrid

(4)

On the basis of eqs 3 and 4, the deduced energy transfer efficiency of 43% is consistent with the magnitude of energy transfer efficiency in previously reported hybrid quantum dots systems.2,19 It is worth noting that the above results may also contain the contribution arising from the occurrence of energy transfer between type II quantum dots. Once the two quantum dots were mixed, the separation between type II quantum dots increases, which reduces the energy transfer efficiency between type II quantum dots. As a result, it increases the photoluminescence lifetime and enhances the emission of type II quantum dots. To justify the above viewpoint, we have made a comparison of the photoluminescence spectra of type II quantum dots in solid and solution states (not shown here). It is found that the photoluminescence peak position of type II quantum dots in the solid state does not show a red shift compared with that in the solution state. In addition, the absorption and emission spectra of type II quantum dots have no spectral overlap as shown in Figure 2a. Therefore, the energy transfer process between type II quantum dots does not play a significant role in our study. To provide further evidence for the FRET process, we have performed a PLE experiment. As shown in Figure 4, it is found that the PLE spectrum of hybrid film 1 monitored at the emission peak of CdSe/ZnTe quantum dots is different from that of pure CdSe/ZnTe film, while it is similar to that of pure CdSe/ZnS film. The similarities between PLE spectra of hybrid film 1 and pure CdSe/ZnS film reveal that the radiative recombination of CdSe/ZnTe quantum dots in hybrid film 1 is dominated by the absorption of CdSe/ZnS quantum dots.6 To verify our viewpoint, we plot the fitting curve by means of summing up the PLE spectra of pure films according to the photoluminescence intensity ratio of the enhanced spectrum of hybrid film 1. The enhanced photoluminescence intensity of hybrid film 1 consists of 75% of photoluminescence intensity from type I quantum dots and 25% from type II quantum dots, according to the four times photoluminescence enhancement of hybrid film 1 as shown in Figure 3a. As shown in the inset of Figure 4, the fitting

Figure 5. Steady state photoluminescence spectra of pure CdSe/ZnTe film (black curve), hybrid film 1 (orange curve), and hybrid film 2 (green curve) at 300 K. The arrows indicate the peak position of the spectra.

curve is consistent with the measured PLE spectrum of hybrid film 1. This behavior reflects the fact that the type II transition in CdSe/ZnTe quantum dots is indeed assisted by the absorption in CdSe/ZnS quantum dots through the FRET process. Figure 5 shows the photoluminescence spectra of pure CdSe/ ZnTe film and hybrid films 1 and 2, in which CdSe/ZnTe quantum dots were mixed with the CdSe/ZnS quantum dots having an emission energy of 2.175 (large dots) and 2.318 eV (small dots), respectively. Interestingly, the photoluminescence spectrum of hybrid film 2 shows a blue shift compared with the pure film, while that of hybrid film 1 reveals a red shift. This interesting behavior can be rationalized as follows: the efficient energy transfer occurs primarily between donors and acceptors having significant overlap in transition energy.1,20 As shown in Figure 2a, the absorption and emission spectra of type II quantum dots might contain many subsets of spectra of different sizes. Accordingly, the large (small) CdSe/ZnS quantum dots prefer transferring energy into the corresponding large (small) core of CdSe/ZnTe quantum dots with low (high) emission photon energy.20 Because the excited CdSe/ZnTe type II quantum dots with large core have smaller confinement energy, the emitted photon energy arising from type II transition is also smaller. Therefore, by changing the size of CdSe/ZnS quantum dots, one is able to selectively enhance the emission of ensemble CdSe/ZnTe quantum dots in a particular wavelength range. Conclusion In conclusion, we have clearly demonstrated the occurrence of the FRET process between CdSe/ZnTe type II and CdSe/ ZnS type I quantum dots. We have also found that by changing the size of CdSe/ZnS quantum dots, it is possible to selectively enhance the emission of ensemble CdSe/ZnTe quantum dots at different wavelengths. Compared with pure type II quantum dots, the emission intensity can be greatly enhanced by up to four times. With the tunability of the emission energy in the infrared range, our study shown here should be very useful for the application in bioimaging sensors derived from type II quantum dots. Besides, it is worth emphasizing here that the photon energy difference between the quenched emission of type I quantum dots and the enhanced emission of type II quantum dots is much larger than the Stokes shift found in most solids. This characteristic is especially beneficial to eliminate the contamination in the emission spectra of acceptors arising from donors in a FRET process. Acknowledgment. This work was supported by the National Science Council and the Ministry of Education of the Republic of China.

15552

J. Phys. Chem. C, Vol. 113, No. 35, 2009

References and Notes (1) Crooker, S. A.; Hollingsworth, J. A.; Tretiak, S.; Klimov, V. I. Phys. ReV. Lett. 2002, 89, 186802. (2) Lu, S.; Madhukar, A. Nano Lett. 2007, 7, 3443. (3) Kuhn, H. J. Chem. Phys. 1970, 53, 101. (4) Koole, R.; Liljeroth, P.; Donega´, C. de M.; Vanmaekelbergh, D.; Meijerink, A. J. Am. Chem. Soc. 2006, 128, 10436. (5) Clark, S. W.; Harbold, J. M.; Wise, F. W. J. Phys. Chem. C 2007, 111, 7302. (6) Zhang, Q.; Atay, T.; Tischler, J. R.; Bradley, M. S.; Bulovic´, V.; Nurmikko, A. V. Nat. Nanotechnol. 2007, 2, 555. (7) Zhag, J.; Campbell, R. E.; Ting, A. Y.; Tsien, R. Y. Nature ReV. Mol. Cell Biol. 2002, 3, 906. (8) Medintz, I. L.; Berti, L.; Pons, T.; Grimes, A. F.; English, D. S.; Alessandrini, A.; Facci, P.; Mattoussi, H. Nano Lett. 2007, 7, 1741. (9) Kim, S.; Fisher, B.; Eisier, H. J.; Bawendi, M. J. Am. Chem. Soc. 2003, 125, 11466. (10) Gur, I.; Fromer, N. A.; Geier, M. L.; Alivisatos, A. P. Science 2005, 310, 462.

Wang et al. (11) Kim, S.; Lim, Y. T.; Soltesz, E. G.; De Grand, A. M.; Lee, J.; Nakayama, A.; Parker, J. A.; Mihaljevic, T.; Laurence, R. G.; Dor, D. M.; Cohn, L. H.; Bawendi, M. G.; Frangioni, J. V. Nat. Biotechnol. 2004, 22, 93. (12) Cheng, C.-T.; Chen, C.-Y.; Lai, C.-W.; Liu, W.-H.; Pu, S.-C.; Chou, P.-T.; Chou, Y.-H.; Chiu, H.-T. J. Mater. Chem. 2005, 15, 3409. (13) Xu, L.; Xu, J.; Li, W.; Zhao, W. M.; Sun, P.; Ma, Z. Y.; Huang, X. F.; Chen, K. J. J. Mater. Sci. 2007, 42, 9696. (14) Williams, A. T. R.; Winfield, S. A.; Miller, J. N. Analyst 1983, 108, 1067. (15) Umberger, J. Q.; Lamber, V. K. J. Am. Chem. Soc. 1945, 67, 1099. (16) Benson, R. C.; Jues, H. A. J. Chem. Eng. Data 1977, 22, 379. (17) Lakowicz, J. R. Principles of Fluorescence of Spectroscopy, 2nd ed.; Kluwer Academic/Plenum: New York, 1999; p 368. (18) van Driel, A. F.; Nikolaev, I. S.; Vergeer, P.; Vanmaekelbergh, D.; Vos, W. L. Phys. ReV. B 2007, 75, 035329. (19) Chen, C.-W.; Wang, C.-H.; Chen, Y.-F.; Lai, C.-W.; Chou, P.-T. Appl. Phys. Lett. 2008, 92, 051906. (20) Franzl, T.; Klar, T. A.; Schietinger, S.; Rogach, A. L.; Feldmann, J. Nano Lett. 2004, 4, 1599.

JP904361A