J. Phys. Chem. C 2010, 114, 18435–18438
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Temperature and Excitation Wavelength Dependence of Surface-Plasmon-Mediated Emission from CdSe Nanocrystals Liu Lu,† Daifen Chen,‡ Guangming Zhao,† Xifeng Ren,*,† and Guangcan Guo† Key Laboratory of Quantum Information, UniVersity of Science and Technology of China, Hefei 230026, China, and Hefei National Laboratory for Physical Science at the Microscale, UniVersity of Science and Technology of China, Hefei 230026, China ReceiVed: July 31, 2010; ReVised Manuscript ReceiVed: September 14, 2010
The authors demonstrate the temperature-dependent behavior of surface plasmon (SP) coupling emission of CdSe nanocrystals (NCs) on Au colloids at different excitation wavelengths. It is found that the photoluminescence (PL) enhancement factor of CdSe NCs on Au colloids increases with the measuring temperature from 8 to 300 K. Such a trend is independent of the excitation wavelength, which whereas greatly influences the PL enhancement factor and the critical temperature for PL quenching switching to enhancement. We attribute the temperature-dependent PL enhancement factor to the increase of the SP coupling rate kSP and the red shift of the PL energy of CdSe NCs with the rising temperature. Both the PL intensity and the energy of CdSe nanocrystals are modeled against the temperature (7-300 K). Compared with the bare nanocrystals, the nanocrystals on Au colloids have an equal activation energy, a larger temperature coefficient, and a temperature-dependent red shift of the PL energy. Introduction The resonant coupling of an emitter to surface plasmon (SP) has attracted much attention for its potential to increase emission efficiencies of light-emitting devices and nanoscale optical emitters.1-6 A large quenching or enhancement of the SP coupling emission can be observed depending on exact conditions, and this can be understood within classic electromagnetic theory.7,8 Recently, the temperature was observed to influence the behavior of SP coupling emission.3,9 The SP coupling efficiency was found to increase with the rising temperature in a Ag-coated InGaN/GaN quantum well,3 whereas the opposite result was observed in an Al-capped ZnO film.9 Therefore, more research is needed on temperature-dependent SP coupling emission. Recent advances in luminescent colloidal semiconductor nanocrystals (NCs) technology have expanded the range of their applications in biosensors and bioimaging.7,10-15 Furthermore, the SP coupling with the fluorescence of NCs is expected to increase the sensitivity of fluorescence-based sensors and image probes.16,17 In reality, temperature has a pronounced impact on the performance of the sensors and the probes. Consequently, it is also imperative to understand the behavior of the temperature dependence of SP coupling with the emission of semiconductor NCs. In this paper, we investigate the temperature-dependent behavior of SP coupling emission of CdSe NCs on Au colloids at different excitation wavelengths in the range of 7-300 K. The temperature dependences of photoluminescence (PL) intensities and energies of CdSe NCs on bare glass and Au colloids are also modeled to gain a deep insight into the temperature dependence of SP coupling emission. Herein, several physical parameters, including the activation energy, PL * To whom correspondence should be addressed. E-mail: renxf@ ustc.edu.cn. † Key Laboratory of Quantum Information. ‡ Hefei National Laboratory for Physical Science at the Microscale.
energy, and temperature coefficient, of the CdSe NCs on bare glass and Au colloids are obtained and compared. Experimental Methods In our experiment, 65 nm Au colloids were prepared using a citrate seeding growth approach. A seed solution of 2.6 nm diameter Au colloids was first prepared according to the method described in ref 18. A 200 mL portion of 0.01% HAuCl4 · 3H2O was brought to boiling under microwave heating. To this solution was added 1 mL of freshly prepared seed colloids and 0.8 mL of 1% aqueous sodium citrate. Boiling continued for an additional 6 min in the microwave oven. The obtained 65 nm Au colloids were then self-assembled on 3-aminopropyltrimethoxysilane (APTMS)-coated glass slides. The as-grown Au colloid films were annealed at 550 °C for 5 min in a rapidly thermal annealing oven under a nitrogen atmosphere. The preparation of CdSe NCs doped with polymethyl methacrylate (PMMA) and the coating of the CdSe/PMMA composite on Au colloid films were similar to the method described elsewhere.19,20 The bare glass coated with the CdSe/PMMA composite was the reference sample. PL spectra were measured using a HITACHI F-2500 fluorescence spectrophotometer. For temperature-dependent PL measurement, the samples were loaded into an Oxford closed-cycle He cryostat, and the temperature was varied from 8 to 300 K. Results and Discussion Figure 1a shows the FESEM image of the annealed Au colloid film. Because most of Au NPs are not purely spherical, the mean particle diameter, d, herein is the average length of the major axis of all particles shown in the FESEM image. Figure 1b shows the absorption spectra of the 65 nm Au colloid film before and after PMMA capping. The absorption peak shifts from 545 to 575 nm when the Au colloid film is capped with the PMMA matrix. This phenomenon is due to the dielectric confinement effect caused by the change of the dielectric environment.
10.1021/jp107211n 2010 American Chemical Society Published on Web 10/13/2010
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Figure 1. (a) FESEM image of the Au colloid film. (b) Absorption spectra of Au colloid films (diameter ) 65 nm) with and without PMMA capping.
Figure 2. Temperature-dependent PL spectra of CdSe NCs on (a) bare glass and (b) the Au colloid film under exciting light at 370 nm.
Figure 2a,b shows the temperature-dependent PL spectra of the CdSe/PMMA composite on bare glass and the Au colloid film. The wavelength of the excitation light is 370 nm. For the CdSe/ PMMA on bare glass and that on the Au colloid film, it can be seen that, with the increasing of temperature, not only the PL intensity decreases but also the emission energy red shifts, and the spectra become broader. In fact, these behaviors are in qualitative agreement with the results known in the literature for CdSe NCs.21 It is noticed that with the decreasing temperature, the PL intensity of bare CdSe NCs increases more quickly than that of the CdSe NCs on Au colloids. This phenomenon relates to the introduction of Au colloids, the SP of which will mediate the emission of the CdSe NCs. To get a deep insight of this phenomena, the integrated PL intensities of CdSe NCs on bare glass (Ibare glass) and Au colloids (IAu colloids) are plotted as a function of the temperature, and the results are shown in Figure 3a. For the CdSe NCs on Au colloids, it is clear that the PL enhancement occurs in the range of 145-300 K, whereas the quenching occurs when the temperature is below 145 K. We also plot the enhancement factor f against the temperature in Figure 3d (curve, 370 nm excitation); the f is defined as the IAu colloids/Ibare glass. We attribute such a behavior to the coupling process of the SP and the emission of CdSe NCs. The increasing PL enhancement factor with the temperature indicates that the SP coupling efficiency increases with temperature. For the bare CdSe NCs, with the temperature increasing, the rate of the nonradiative recombination will increase due to the ionization of the localized carriers and their transport toward defects, whereas the radiative recombination rate will decrease. Apparently, these behaviors account for the great decrease of the PL intensity of bare CdSe NCs with the rise in temperature. For the CdSe NCs on Au colloids, because the density of states of the SP mode is much larger, the coupling rate (kSP) between the NCs and SP should be very fast, and this new path of recombination can increase the spontaneous emission rate. The coupling rate kSP, which contributes to the PL enhancement, also decreases with the temperature decreasing.3,5 It indicates
that, as the temperature decreases, the PL intensity of bare CdSe NCs will increase more quickly than that on the Au colloids. It is noted that the quantum efficiency of NCs is proved to be temperature-dependent.22 This property may also influence the coupling rate, although the relationship between the quantum efficiency and the SP coupling emission is not clear.23,24 In addition, as shown in Figure 2b, the PL energy blue shifts from 590 to 578 nm with the decreasing temperature; however, the SP energy is almost independent of the decreasing temperature.25,26 This behavior is expected to affect the SP coupling efficiency due to the requirement of the energy match between the SP and the PL. To further demonstrate the influence of the blue shift of the PL energy on the enhancement factor, Figure 4 shows the PL spectra of CdSe NCs of different fluorescence wavelengths (532, 551, 572, and 593 nm) on bare glass and Au colloids. The inset is the PL enhancement factor as a function of the PL energy of NCs. It is clear that the PL enhancement factor decreases with the blue shift of the PL energy of CdSe NCs. Consequently, the behavior of the SP coupling efficiency decreasing with the drop of temperature is also influenced by the blue shift of the PL energy of CdSe NCs. The excitation wavelength is also altered to 470 and 530 nm, and the results are shown in Figure 3b,c, respectively. We can see that the shapes of the curves in Figure 3a-c are similar. However, the enhancement factor for the excitation wavelength at 530 nm is almost 2.6 times that for the 370 nm excitation at 300 K. The critical temperature Tf)1, at which the PL enhancement changes to PL quenching, drops from 145 to 24 K when the excitation wavelength is altered from 370 to 530 nm. This phenomenon can be explained from two aspects. On one hand, the exciting electromagnetic field is expected to be independent of temperature.5 In addition, the PL intensities under different exciting light all show an increase with the decreasing temperature. It indicates that the critical temperature Tf)1 almost depends on the initial enhancement factor at room temperature. On the other hand, the enhancement factor at room temperature is greatly affected by the energy match
SP-Mediated Emission from CdSe Nanocrystals
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Figure 3. (a-c) Temperature dependence on PL integrated intensities of CdSe NCs on bare glass (square) and the Au colloid film (circle). Panels a-c correspond to the different excitation wavelengths. (d) Temperature dependence on PL enhancement factors of CdSe NCs at different excitation wavelengths.
Figure 4. PL spectra of CdSe NCs of different fluorescence wavelengths (532, 551, 572, and 593 nm) on bare glass (solid line) and Au colloids (dashed line). The inset is the PL enhancement factor as a function of the PL energy of CdSe NCs. 20
between the incident light and localized surface plasmon. It is known that the electron oscillation in colloids accounts for the optical properties of metal NPs and the PL enhancement of NCs. However, the electron oscillation requires the energy match between the incident light and the SP of metallic NPs. For Au colloids, the exciting electromagnetic field, as well as excitation efficiency, under excitation at 530 nm is larger than those of 370 and 470 nm. Consequently, the enhancement factors for the excitation wavelengths at 530 nm are better, and the critical temperature at a 530 nm exciting wavelength is higher than those of 370 and 470 nm. As shown in Figure 2a,b, the PL intensities of CdSe NCs on bare glass and Au colloids decrease with the rising temperature. These behaviors can be attributed to the nonradiative relaxation increase with the temperature. Furthermore, it was reported that this nonradiative relaxation was the thermal escape from the NCs assisted by the phonon-electron coupling with the scatteing of LO phonons.27 To detailedly contrast temperature-dependent PL intensities of the CdSe NCs with and without the Au colloids,
Figure 5. PL integrated intensities of CdSe NCs on bare glass (empty square) and the Au colloid film (empty circle) as a function of 1000/T (K-1). The dotted line and the dashed line are the best-fit curves, as discussed in the text.
we modeled the temperature-dependent integrated PL intensity using the equation expressed as28
I(T) )
I0 (1 + Ce-Ea/KT)
(1)
and the result is shown in Figure 5. The activation energies Ea are 17.6 and 18.2 meV for the CdSe NCs on bare glass and Au colloids, respectively, consistent with the values (15-20 meV) described in refs 27 and 29. The margin of only 0.6 meV indicates that the energy necessary for activating the carrier localization in surface states is identical for CdSe NCs on Au colloids and bare glass. We also plot the PL energies as a function of temperature in Figure 6, which were fitted to the empirical Varshni relation as29
Eg(T) ) Eg(0) - R
T2 (T + β)
(2)
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Lu et al. wavelengths. The temperature-dependent behavior of SP coupling efficiency induces the PL enhancement changes to quenching as the temperature decreases. This phenomenon is independent of the excitation wavelength, which whereas greatly affects the enhancement factor and the critical temperature of PL enhancement and quenching due to the energy match between the incident light and the SP. In comparison with the bare CdSe NCs, the NCs on Au colloids had a similar value of activation energy ER, a larger temperature coefficient R, and a temperature-dependent red shift of PL energy (∆E).
Figure 6. PL peak energies of CdSe NCs on bare glass (full circle) and the Au colloid film (empty circle) against the temperature. The dotted line and the dashed line are the best-fit curves with the model described in the text.
TABLE 1: Parameters Used in the Fit of the PL Integrated Intensity and PL Energy as a Function of Temperature with eqs 1 and 2, Respectively samples
Ea (meV)
Eg(0) (eV)
R (× 10-4 eV K-1)
β (K)
CdSe NCs on bare glass CdSe NCs on Au colloids
17.6 18.2
2.146 2.142
2.9 3.5
310 310
where Eg(0) is the energy gap at 0 K, R is the temperature coefficient, and β is a parameter related to the Debye temperature of the NCs. As shown in Figure 6, the CdSe NCs on bare glass and Au colloids all show a red shift of the PL energy with the rising temperature. This phenomenon is due to the temperaturedependent band-gap shrinkage of the NCs. The fitting results are show in Table 1. The best-fit curve for CdSe NCs on bare glass, with R ) 2.9 × 10-4 eV K-1 and β ) 310 K, reproduces well the experimental data. The values of R and β for bare NCs are consistent with the values for bulk CdSe, (2.8-4.1) × 10-4 eV K-1 and (181-315),21,27 demonstrating that the temperaturedependent behavior of the PL energy of CdSe NCs resembles that of the bulk CdSe, and the confinement energy for the carriers is weakly dependent on temperature. From Figure 6, it can be found that the CdSe NCs on Au colloids have a larger red shift of PL energy and a larger value of the temperature coefficient (R ) 3.5 × 10-4 eV K-1) than that on the bare glass. The red shift energy is defined as ∆E, which corresponds to the gap of the PL energies of the two samples. The different value of R indicates that the red shift energy ∆E is temperature-dependent. In detail, as the temperature increases up to about 75 k, the value of ∆E is about 4 meV and weakly dependent on temperature. A faster increase is then observed in the range of 75-300 K, and finally, it becomes about 10 meV at 300 K. As we know, dE/dT could be modified by the changes of Coulomb energy, the confinement energy, exciton-phonon scattering, and the thermal expansion of the lattice for a NC. Because the CdSe NCs on Au colloids and bare glass are the same, their Coulomb energies, as well as the confinement energies, should be identical. In addition, the independent boson model, which is commonly used to describe exciton-phonon interactions in NCs, suggests that the exciton-phonon scattering does not lead to a change in dE/dT.30 Therefore, the thermal expansion of the NC’s lattice induced by the introduction of Au colloids is possibly leading to the larger temperature coefficient. In fact, the temperature dependence of SP coupling emission would also contribute to the larger temperature coefficient. Conclusion In conclusion, we have reported the temperature dependence of SP coupling emission of CdSe NCs at different excitation
Acknowledgment. This work was funded by the National Basic Research Programme of China (Grant Nos. 2009CB929600 and 2006CB921900), the Innovation Funds from the Chinese Academy of Sciences, the National Natural Science Foundation of China (Grant Nos. 10604052 and 10904137), Anhui Provincial Natural Science Foundation (Grant No. 090412053), and the Science and Technological Fund of Anhui Province for Outstanding Youth (Grant No. 2009SQRZ001ZD). References and Notes (1) Kwon, M. K.; Kim, J. Y.; Kim, B. H.; Park, I. K.; Cho, C. Y.; Byeon, C. C.; Park, S. J. AdV. Mater. 2008, 20, 1253. (2) Okamoto, K.; Niki, I.; Scherer, A.; Narukawa, Y.; Mukai, T.; Kawakami, Y. Appl. Phys. Lett. 2007, 87, 071102. (3) Lu, Y. C.; Chen, C. Y.; Yeh, D. M.; Huang, C. F.; Tang, T. Y.; Huang, J. J.; Yang, C. C. Appl. Phys. Lett. 2007, 90, 193103. (4) Akimov, A. V.; Mukherjee, A.; Yu, C. L.; Chang, D. E.; Zibrov, A. S.; Hemmer, P. R.; Park, H.; Lukin, M. D. Nature 2007, 450, 402. (5) Toropov, A. A.; Shubina, T. V.; Jmerik, V. N.; Ivanov, S. V.; Ogawa, Y.; Minami, F. Phys. ReV. Lett. 2009, 103, 037403. (6) Chang, D. E.; Sorensen, A. S.; Demler, E. A.; Lukin, M. D. Nat. Phys. 2007, 3, 807. (7) Biju, V.; Itoh, T.; Anas, A.; Sujith, A.; Ishikawa, M. Anal. Bioanal. Chem. 2008, 391, 2469. (8) Ford, G. W.; Weber, W. H. Phys. Rep. 1984, 113, 195. (9) Li, J.; Ong, H. C. Appl. Phys. Lett. 2008, 92, 121107. (10) Biju, V.; Itoh, T.; Ishikawa, M. Chem. Soc. ReV. 2010, 39, 3031. (11) Frasco, M. F.; Chaniotakis, N. Sensors 2009, 9, 7266. (12) Ji, X. J.; Zheng, J. Y.; Xu, J. M.; Rastogi, V. K.; Cheng, T. C. D.; Frank, J. J.; Leblanc, R. M. J. Phys. Chem. B 2005, 109, 3793. (13) Vannoy, C. H.; Xu, J. M.; Leblanc, R. M. J. Phys. Chem. C 2010, 114, 766. (14) Biju, V.; Muraleedharan, D.; Nakayama, K.; Shinohara, Y.; Itoh, T.; Baba, Y.; Ishikawa, M. Langmuir 2007, 23, 10254. (15) Biju, V.; Mundayoor, S.; Omkumar, R. V.; Anas, A.; Ishikawa, M. Biotechnol. AdV. 2010, 28, 199. (16) Robelek, R.; Niu, L. F.; Schmid, E. L.; Knoll, W. Anal. Chem. 2004, 76, 6160. (17) Lakowicz, J. R. Plasmonics 2006, 1, 5. (18) Brown, K. R.; Walter, D. G.; Natan, M. J. Chem. Mater. 2000, 12, 306. (19) Lu, L.; Xu, X. L.; Shi, C. S.; Ming, H. Thin Solid Films 2010, 518, 3250. (20) Lu, L.; Chen, D. F.; Sun, F. W.; Ren, X. F.; Han, Z. F.; Guo, G. C. Chem. Phys. Lett. 2010, 71, 492. (21) Salman, A. A.; Tortschanoff, A.; Mohamed, M. B.; Tonti, D.; Mourik, F. V.; Chergui, M. Appl. Phys. Lett. 2007, 90, 093104. (22) Biju, V.; Makita, Y.; Sonoda, A.; Yokoyama, H.; Baba, Y.; Ishikawa, M. J. Phys. Chem. B 2005, 109, 13899. (23) Pompa, P. P.; Martiradonna, L.; Della Torre, A.; Della Sala, F.; Manna, L.; De Vittorio, M.; Calabi, F.; Cingolani, R.; Rinaldi, R. Nat. Nanotechnol. 2006, 1, 126. (24) Shimizu, K. T.; Woo, W. K.; Fisher, B. R.; Eisler, H. J.; Bawendi, M. G. Phys. ReV. Lett. 2002, 89, 117401. (25) Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 4212. (26) Kreibig, U. J. Phys. F: Met. Phys. 1974, 4, 999. (27) Valerini, D.; Creti, A.; Lomascolo, M.; Manna, L.; Cingolani, R.; Anni, M. Phys. ReV. B 2005, 71, 235409. (28) Wang, C. H.; Chen, T. T.; Tan, K. W.; Chen, Y. F.; Cheng, C. T.; Chou, P. T. J. Appl. Phys. 2006, 99, 123521. (29) Ehlert, O.; Tiwari, A.; Nann, T. J. Appl. Phys. 2006, 100, 074314. (30) Liptay, T. J.; Ram, R. J. Appl. Phys. Lett. 2006, 89, 223132.
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