Langmuir 2000, 16, 3561-3563
High-Yield Luminescence from Cadmium Sulfide Nanoclusters Supported in a Poly(ethylene glycol) Oligomer Vlasoula Bekiari and Panagiotis Lianos* Engineering Science Department, University of Patras, 26500 Patras, Greece Received September 27, 1999. In Final Form: December 1, 1999
Cadmium sulfide nanoclusters are among the most extensively studied materials in the last 20 years. Following the original studies,1-4 a multitude of publications have reported their synthesis in a great variety of host environments, including reverse micelles,5-8 zeolites,9-10 sol-gel silica matrixes and glasses,11-12 organic polymeric matrixes,13-17 and so on. Of particular interest are recent communications and reviews dealing with nanoengineering of composite nanoparticles that involve CdS or similar semiconductors.18-24 The making of CdS nanoclusters (quantum dots) in different sizes and host matrixes is now considered routine work. However, making a luminescent material out of CdS nanoclusters, particularly an efficient one, is not easy. Quenching of luminescence, which usually originates from cadmium deficiency sites,25 is the rule rather than the exception in these materials. Nev* To whom correspondence should be addressed. Tel: 30-61997587. FAX: 30-61-997803. E-mail:
[email protected]. (1) Papavassiliou, G. C. J. Solid State Chem. 1981, 40, 330. (2) Kuczynski, J. P.; Milosavljevic, B. H.; Thomas, J. K. J. Phys. Chem. 1983, 87, 3368. (3) Rossetti, R.; Nakahara, S.; Brus, L. E. J. Chem. Phys. 1983, 79, 1086. (4) Fojtik, A.; Weller, H.; Koch, V.; Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1984, 88, 969. (5) Meyer, M.; Wallberg, C.; Kurihara, K.; Fendler, J. H. J. Chem. Soc., Chem. Commun. 1984, 90. (6) Lianos, P.; Thomas, J. K. Chem. Phys. Lett. 1986, 125, 299. (7) Modes, S.; Lianos, P. J. Phys. Chem. 1989, 93, 5854. (8) Motte, L.; Petit, C.; Boulanger, L.; Lixon, P.; Pileni, M. P. Langmuir 1992, 8, 1049. (9) Herron, N.; Wang, Y.; Eddy, M. M.; Stucky, G. D.; Cox, D.; Moller, K.; Bein, T. J. Am. Chem. Soc. 1989, 11, 350. (10) Chen, W.; Xu, Y.; Lin, Z.; Wang, Z.; Lin, L. Solid State Commun. 1998, 105, 129. (11) Modes, S.; Lianos, P. Chem. Phys. Lett. 1988, 153, 351. (12) Juodkazis, S.; Bernstein, E.; Plenet, J. C.; Bovier, C.; Dumas, J.; Mugnier, J.; Vaitkus, J. V. Thin Solid Films 1998, 322, 238. (13) Kuczynski, J. P.; Milosavljevic, B. H.; Thomas, J. K. J. Phys. Chem. 1984, 88, 980. (14) Kane, R. S.; Cohen, R. E.; Silbey, R. Langmuir 1999, 15, 39. (15) Huang, J.; Lianos, P.; Yiang, Y.; Shen, J. Langmuir 1998, 14, 4342. (16) Sooklal, K.; Hanns, L. H.; Ploehn, J.; Murphy, C. J. Adv. Mater. 1998, 10, 1083. (17) Herron, N.; Thom, D. L. Adv. Mater. 1998, 10, 1173. (18) Colvin, V. L.; Goldstein, A. N.; Alivisatos, A. P. J. Am. Chem. Soc. 1992, 114, 5221. (19) Correa-Duarte, M. A.; Giersig, M.; Liz-Marzan, L. M. Chem. Phys. Lett. 1997, 286, 497. (20) Alivisatos, A. P.; Barbara, P. F.; Castleman, A. W.; Chang, J.; Dixon, D. A.; Klein, M. L.; McLendon, G. L.; Miller, J. S.; Ratner, M. A.; Rossky, P. J.; Stupp, S. I.; Thompson, M. E. Adv. Mater. 1998, 10, 1297. (21) Bruchez, M.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science 1998, 281, 2013. (22) Li, Y.-D.; Liao, H. W.; Ding, Y.; Qian, Y.-T.; Yang, L.; Zhon, G. E. Chem. Mater. 1998, 10, 2301. (23) Wang, W.; Geng, Y.; Qian, Y.; Ji, M.; Liu, X. Adv. Mater. 1998, 10, 1479. (24) Guo, S.; Popovitz-Biro, R.; Weissbuch, I.; Cohen, H.; Hodes, G.; Lahav, M. Adv. Mater. 1998, 10, 121. (25) Spanhel, L.; Haase, M.; Weller, H.; Henglein, A. J. Am. Chem. Soc. 1987, 109, 5649.
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ertheless, an efficient blue-emitting CdS/dendrimer nanocomposite has been recently reported,16 while core-shell CdSe-CdS nanocrystals in a silica shell have been used as fluorescent labels in biological systems.21 In the present work, we are proposing a very simple and easy way of making luminescent CdS that can be useful to many laboratories. We have managed to produce high-yield luminescent CdS by reacting Cd2+ with S2- ions in the fluid environment of poly(ethylene glycol) (PEG) oligomers. The only parameter we then needed to control was reactant concentrations. Indeed, strongly luminescent CdS nanoclusters have been produced in a narrow concentration range, which is well defined under ambient conditions. We have recently shown that PEG oligomers constitute favorable host environments for luminescent ions such as lanthanides.26 The present extension to CdS nanoclusters may attract more attention to these solvents and increase the range of their applications. In this respect, we are not aware of any previous publication reporting synthesis of CdS in PEG oligomers. CdS nanoclusters have been synthesized by arrested precipitation of CdS in PEG-200 (Aldrich), at room temperature, by mixing two equimolar PEG-200 solutions, one containing Cd(NO3)2‚4H2O (Merck) and the other Na2S (Janssen), under stirring and flow of N2 and by using only freshly prepared solutions. Both solutions were prepared by directly dissolving solid into PEG-200. The reactor consisted of two conic flasks connected by a glass tube at their upper part. The two solutions were first separately put into each flask, and after a few minutes flow of nitrogen they were mixed in the flask containing Cd2+, by letting all the sulfur-containing solution flow through the connecting tube. All the measurements described henceforth have been made under ambient conditions. Only samples where cation and anion concentration was each in the range between 8 × 10-4 and 5 × 10-3 M were luminescent, while, for equimolar solutions, the maximum of luminescence was observed at 1 × 10-3 M. As will be discussed below, increase of Cd2+ concentration, without parallel increase of S2- concentration, had a noticeable effect on the structure of the luminescence spectrum. Figure 1 shows the absorption and luminescence spectra of the thus prepared CdS nanoclusters. The absorption onset was at 450 nm; however, most of the absorption lies below 400 nm and, for this reason, the CdS-containing samples are colorless. Small nanoclusters were then made at the above concentration range. At higher concentrations (i.e., >5 × 10-3 M) the solution was yellow corresponding to larger particles. The absorption onset followed suit; i.e., it appeared at longer wavelengths when the reactant concentration was higher. These larger yellow particles were not luminescent. The position of the absorption onset is a good index of the size of the nanoclusters according to the Brus effective mass model.27 The luminescence spectrum was relatively broad, indicating that more than one emitting species coexist in solution. Indeed, as seen in Figure 2, the emission spectrum changed in shape according to the excitation wavelength. Accordingly, the maximum of the excitation spectrum (not shown) shifted to longer wavelengths together with the emission wavelength. Apparently, size polydispersity in the nanoclusters formed in the above solution is responsible for the broadening of the luminescence spectrum. (26) Bekiari, V.; Lianos, P. Adv. Mater. 1998, 10, 1455. (27) Steigerwald, M. L.; Brus, L. E. Acc. Chem. Res. 1990, 23, 183.
10.1021/la991267i CCC: $19.00 © 2000 American Chemical Society Published on Web 02/15/2000
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Notes Table 1. Dependence of the Luminescence Decay Time τ of CdS Nanoclusters on Observation Wavelengtha emission wavelength (nm)
equimolar Cd2+ and S2τ (ns)
excess of Cd2+ τ (ns)
420 440 470 500 550
57 109 112 183 186
101 137 153 192 228
a Column 2 corresponds to nanoclusters made with 10-3 M Cd2+ and 10-3 M S2-. Column 3 corresponds to nanoclusters made with an excess of cadmium ions (2 × 10-3 M). Excitation wavelength 375 nm.
Figure 1. Absorption (1) and luminescence (2) spectrum of 10-3 M CdS in PEG-200. Curve 3 shows the luminescence spectrum of a solution containing 2 × 10-3 M Cd2+ and 10-3 M S2-.
Figure 2. Luminescence spectra of 10-3 M CdS in PEG-200, at four different excitation wavelengths: (1) 330 nm; (2) 365 nm; (3) 390 nm; (4) 420 nm. The corresponding luminescence maxima are found at (1) 458 nm, (2) 466 nm, (3) 493 nm, and (4) 510 nm.
The luminescence quantum yield φ for 10-3 M CdS in PEG-200 was measured with reference to rhodamine B under the following procedure: Measurements were made about 90 min after mixing of the reactants, a time necessary for the completion of the reaction. φ for rhodamine B in ethanol, by excitation at its absorption maximum at 535 nm, was used as reference (φ ) 0.97). This value was reduced to 0.91 for excitation at 370 nm. Then the fluorescence quantum yield for rhodamine B in PEG-200, by excitation at 370 nm, was calculated and found equal to 0.86. Finally, the luminescence quantum yield of CdS in PEG-200 was calculated with reference to this last value and found to be φ ) 0.79. This value corresponds to reactant concentrations in the original solutions each equal to 1 × 10-3 M. This is the highest value for the luminescence quantum yield of CdS that
has, to our knowledge, been published so far (cf. ref 16) and shows off the beneficial properties of the PEG-200 environment. High quantum yield was accompanied by a relatively long luminescence decay time τ, as compared with values observed by previous authors.2,4 In fact, τ depends on emission wavelength, being longer at longer wavelengths, as previously observed.4 Indeed, as seen in Table 1, τ increased with the observation wavelength and, at all wavelengths, it was found much longer than previously reported with colloidal CdS suspensions4 but comparable with decay times of luminescent CdS nanoclusters entrapped in PMMA matrixes.15 The dependence of τ on observation wavelength could be related with size polydispersity. Larger particles, which, presumably, emit at longer wavelengths, are additionally qualified by longer decay times. The high φ value leaves little room for further increase in the presence of excess of Cd2+, contrary to what has been observed in other cases.25 The presence of an excess of Cd2+ has a rather insignificant effect on absorption spectrum but it does have an effect on luminescence spectral structure. In that case there is a tendency to emit at longer wavelength with enhancement of the yellow emission (cf. Figure 1), presumably by increasing the percentage of the larger CdS clusters. Luminescence spectra are in the present case more sensitive to size polydispersity than absorption spectra. However, the presence of an excess of Cd2+ had a dramatic effect on the luminescence decay time. As seen in Table 1, τ was then much longer than when equimolar quantities of reacting ions were used. The longer decay time in this case is, obviously, related with the enhanced emission at longer wavelength. The reason that PEG provides a favorable environment for suspending high-luminescence-yield CdS nanoclusters is related with the capacity of this oligomer to strongly bind positive ions. Cations are bound on ether oxygens. Binding of multivalent ions is strong, and this results in subsequent further binding of association complexes between ions and ligands,26 or binding of compounds ensuing from ionic attraction, as in the present case. There is strong indication of PEG clustering around the positive ions. This is based on previous studies28 and on our experimental results. We have employed the luminescent lanthanide cation, Eu3+, to probe association of cations with PEG chains. It is true that the trivalent Eu3+ should behave differently from the divalent Cd2+. However, the luminescence capacity of the lanthanide ion is useful in probing tendencies which should be observed by all cations. Europium(III) cations (Eu(NO3)3‚5H2O (Aldrich)) have been solubilized in water/PEG-200 mixtures, at a concentration of 40 mM, and their relative luminescence intensity and decay time have been measured. Water is (28) Cai, H.; Farrington, G. C. J. Electrochem. Soc. 1992, 139, 744.
Notes
a strong quencher of Eu3+ luminescence and this fact is reflected on both its luminescence intensity and decay time τ. Thus in pure water, τ ) 123 µs, but in pure PEG200, τ ) 568 µs. It is then interesting to follow the evolution of the decay time and the relative luminescence intensity with respect to the percentage of PEG-200 in water/PEG200 mixtures. Figure 3 shows this evolution. It is noted that the relative luminescence intensity increased linearly with PEG-200 content up to 80 wt % and then a plateau was observed. Correspondingly, the decay time increased slowly up to 80% PEG-200, but it jumped to much higher values at higher PEG-200 content. These results suggest that there exists a critical concentration of PEG necessary to protect Eu3+ from water quenching. In other words, PEG oligomer chains seem to organize themselves around the luminescent cations providing a protecting environment. This organization is realized by cross-links between PEG chains aided by cations. The above results are helpful in drawing conclusions by extrapolation to Cd2+, albeit of lower charge. Clustering of PEG chains around Cd ions, obviously, provides the appropriate environment also for stabilization of CdS nanoclusters. In conclusion, polymeric materials based on ethylene oxide monomers might be particularly interesting for synthesis and stabilization of high luminescence yield and long decay time CdS nanoclusters. Work is currently being carried on in our laboratory in an effort to produce highly
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Figure 3. Evolution of the relative luminescence intensity (1) and the luminescence decay time (2) of 40 mM Eu3+, solubilized in water/PEG-200 mixtures, as a function of PEG-200 content.
luminescent CdS, also in solid-state samples by employing poly(ethylene oxide) of much longer chain length. LA991267I
