High-Quality ZnS Shells for CdSe Nanoparticles - American Chemical

UniVersity of East Anglia (UEA), Norwich NR4 7TJ, U.K., and Federal Institute for Materials Research and Testing (BAM), Richard-Willstaetter-Strasse 1...
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Langmuir 2007, 23, 7751-7759

7751

High-Quality ZnS Shells for CdSe Nanoparticles: Rapid Microwave Synthesis Jan Ziegler,†,‡ Alexei Merkulov,† Markus Grabolle,§ Ute Resch-Genger,*,§ and Thomas Nann*,†,‡ Laboratory for Nanosciences, Freiburg Material Research Centre (FMF), Albert Ludwig UniVersity Freiburg, Stefan-Meier Strasse 21, 79104 Freiburg, Germany, School of Chemical Sciences and Pharmacy, UniVersity of East Anglia (UEA), Norwich NR4 7TJ, U.K., and Federal Institute for Materials Research and Testing (BAM), Richard-Willstaetter-Strasse 11, 12489 Berlin, Germany ReceiVed December 6, 2006. In Final Form: March 13, 2007 Using a domestic microwave oven and new, inexpensive precursors, a rapid and reliable synthesis of highly luminescent CdSe/ZnS NPs was developed. To evaluate the quality of our core/shell particles for varying shell thickness in comparison to that of CdSe/ZnS nanoparticles obtained commercially, the parameter fluorescence quantum yield is been used as well as a new, straightforward, thiophenol-based shell-quality test as a tool to ensure a dense ZnS shell without holes and cracks, which is a prerequisite for high luminescence and stability.

1. Introduction Semiconductor nanoparticles (NPs) or so-called quantum dots (QDs) are of increasing importance in a wide range of technological applications ranging from the labeling of biological samples1-3 to the construction of solar cells4-9 and light-emitting devices10-15 to optoelectronics.16 Groundbreaking work by Murray et al., Kortan et al., and Katari et al.17-19 on the synthesis of NPs with tunable sizes, narrow size distributions, and controlled surface chemistry paved the way for these applications. Further improvement by Peng et al.20 led to efficient routes to strongly luminescent monodisperse QDs with special emphasis dedicated to CdSe. The latest developments in the synthesis of CdSe NPs * Corresponding authors. E-mail: [email protected], [email protected]. † Albert Ludwig University Freiburg. ‡ University of East Anglia (UEA). § Federal Institute for Materials Research and Testing (BAM). (1) Larson, D. R.; Zipfel, W. R.; Williams, R. M.; Clark, S. W.; Bruchez, M. P.; Wise, F. W.; Webb, W. W. Science 2003, 300, 1434-1436. (2) Gao, X. H.; Chan, W. C. W.; Nie, S. M. J. Biomed. Opt. 2002, 7, 532-537. (3) Fan, H. Y.; Yang, K.; Boye, D. M.; Sigmon, T.; Malloy, K. J.; Xu, G. F.; Lopez, G. P.; Brinker, C. J. Science 2004, 304, 567-571. (4) Kumar, S.; Nann, T. J. Mater. Res. 2004, 19, 1990-1994. (5) Sun, B.; Marx, E.; Greenham, N. C. Nano Lett. 2003, 3, 961-963. (6) Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Science 2002, 295, 24252427. (7) Cava, R. J. et al. Solid State Chem. 2002, 30, 1-101. (8) Nozik, A. J. J. Physica E 2002, 14, 115-120. (9) Huynh, W. U.; Dittmer, J. J.; Libby, W. C.; Whiting, G. L.; Alivisatos, A. P. AdV. Funct. Mater. 2003, 13, 73-79. (10) Daboussi, B. O.; Bawendi, M. G.; Onitsuka, O.; Rubner, M. F. Appl. Phys. Lett. 1995, 66, 1316-1318. (11) Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P. Nature 1994, 370, 354357. (12) Tessler, N.; Medvedev, V.; Kazes, M.; Kan, S. H.; Banin, U. Science 2002, 295, 1506-1508. (13) Sunder, V. C.; Eisler, H. H.; Deng, T.; Chan, Y.; Thomas, E. L.; Bawendi, M. G. AdV. Mater. 2004, 16, 2137-2141. (14) Eisler, H. J.; Sunder, V. C.; Bawendi, M. G.; Walsh, M.; Smith, H. I.; Klimov, V. Appl. Phys. Lett. 2002, 80, 4614-4616. (15) Chan, Y.; Steckel, J. S.; Snee, P. T.; Caruge, J. M.; Hodgkiss, J. M.; Nocera, D. G.; Bawendi, M. G. Appl. Phys. Lett. 2005, 96, 073102. (16) Willner, I., Willner, B. Pure Appl. Chem. 2002, 74, 1773-1783. (17) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706. (18) Kortan, A. R.; Hull, R.; Opila, R. L.; Bawendi, M. G.; Steigerwald, M. L.; Carrol, P. J.; Brus, L. E. J. Am. Chem. Soc. 1990, 112, 1327. (19) Bowen, Katari, J. E.; Colvin, V. L.; Alivisatos, A. P. J. Phys. Chem. 1994, 98, 4109. (20) Qu, L.; Peng, Z. A.; Peng, X. Nano Lett. 2001, 1, 333.

include the use of single-source precursors,21-23 nonsolvents24 and alternative educts,25 microfluidic and flow reactors,26-32 ultrasound,33 and, most interestingly, microwave-supported synthesis.34-39 Organic ligands or inorganic shells can be used to protect sensitive CdSe cores and to saturate defect states and dangling bonds on the surface, which favor undesired nonradiative recombination and trapped emission.40 Because organic ligands face problems in simultaneously passivating both anionic and cationic surface sites, inorganic high-band-gap materials are usually favored. Typically, CdSe is covered with a ZnS shell according to a method originally developed by Murray et al. and Katari17,41 to enhance its quantum yield (QY) for radiative band (21) Cumberland, S. L.; Hanif, K. M.; Javier, A.; Khitrov, G. A.; Strouse, G. F.; Woessner, S. M.; Yun, C. S. Chem. Mater. 2002, 14, 1576-1584. (22) Revaprasadu, N.; Malik, M. A.; Carstens, J.; O’Brien, P. J. Mater. Chem. 1999, 9, 2885-2888. (23) Malik, M. A.; Ravaprasadu, N.; O’Brien, P. Chem. Mater. 2001, 13, 913-920. (24) Peng, Z. A.; Peng, X. G. J. Am. Chem. Soc. 2001, 123, 183-184. (25) Battaglia, D.; Peng, X. G. Nano Lett. 2002, 2, 1027-1030. (26) Xue, Z.; Terepka, A. D.; Hong, Y. Nano Lett. 2004, 4, 2227-2232. (27) Chan, E. M.; Mathies, R. A.; Alivisatos, A. P. Nano Lett. 2003, 3, 199201. (28) Wang, H. Z.; Li, X. Y.; Yamaguchi, Y.; Nakamura, H.; Miyazaki, M. P.; Shimizu, H.; Maeda, H. Chem. Commun. 2004, 48-49. (29) Comer, E.; Organ, M. G. J. Am. Chem. Soc. 2005, 127, 8160-8167. (30) Nakamura, H.; Yamaguchi, Y.; Miyazaki, M.; Maeda, H.; Uehara, M.; Mulvaney, P. Chem. Commun. 2002, 2844-2845. (31) Omata, T.; Nose, K.; Otsuka-Yao-Matsuo, S.; Nakamura, H.; Madea, H. Jpn. J. Appl. Phys. 2005, 44, 452-456. (32) Wang, H.; Nakamura, H.; Uehara, M.; Yamaguchi, Y.; Miyazaki, M.; Maeda, H. AdV. Funct. Mater. 2005, 15, 603-608. (33) Murcia, M. J.; Shaw, D. L.; Woodruff, H.; Naumann, C. A.; Young, B. A.; Long, E. C. Chem. Mater. 2006, 18, 2219-2225. (34) Gerbec, J. A.; Magana, D.; Washington, A.; Strouse, G. F. J. Am. Chem. Soc. 2005, 127, 15791-15800. (35) Verma, S.; Joy, P. A.; Khollam, Y. B.; Potdar, H. S.; Deshpande, S. B. Mater. Lett. 2004, 58, 1092-1095. (36) Bensebaa, F.; Zavaliche, F.; L’Ecuyer, P.; Cochrane, R. W.; Veres, T. J. Colloid Interface Sci. 2004, 277, 104-110. (37) Rogach, A. L.; Nagesha, D.; Ostrander, J. W.; Giersig, M.; Kotov, N. A. Chem. Mater. 2000, 12, 2676-2685. (38) Zhu, J. J.; Palchik, O.; Chen, S. G.; Gedanken, A. J. Phys. Chem. B 2000, 104, 7344-7347. (39) Murugan, A. V.; Sonawane, R. S.; Kale, B. B.; Apte, S. K.; Kulkarni, A. V. Mater. Chem. Phys. 2001, 71, 98-102. (40) Peng, X.; Schlamp, M. C.; Kadavanich, A. V.; Alivisatos, A. P. J. Am. Chem. Soc. 1997, 119, 7019-7029. (41) Bowen Katari, J. E.; Colvin, V. L.; Alivisatos, A. P. J. Phys. Chem. 1994, 98, 4109.

10.1021/la063528b CCC: $37.00 © 2007 American Chemical Society Published on Web 06/07/2007

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gap recombination and to protect the core against photo-oxidation as well as chemical and physical stress. This method has been widely used in various modifications, and several attempts have been reported to increase the luminescence quantum yield further and obtain smaller size distributions.42 The optimum thickness for enhanced QYs is reported to be between 0.4043 and 0.68 nm for ZnS.44 Recently, layer-by-layer approaches to passivate CdSe successively with CdS and ZnS were independently published by Xie and Talapin.45,46 These methods are based on Peng’s modifications of Nicolau’s SILAR technique,47-49 in which the middle layer of CdS reduces the lattice mismatch between CdSe and ZnS and thus enhances the core luminescence by decreasing the number of defect states responsible for the radiationless recombination of the charge carriers or excitons formed upon NP photoexcitation. The search for less time-consuming and more straightforward methods to synthesize ZnS shells led to synthesis in ultrasonic baths,33 the use of single molecular precursors,50-52 thiols,53 and numerous other approaches. For many applications within reach, the currently achievable QYs and size distributions of NPs are sufficient, and the point of interest is focused more and more on the long-term stability and robustness of the particles. Also, the toxicity of precursors and the industrial suitability of the synthetic procedure (i.e., upscaling strategies) are gaining importance. At present, however, all of the reported synthetic and surface modification strategies introduce new ways to increase the QY and to narrow the size distribution of CdSe/ZnS NPs, but most of them use toxic precursors such as hexamethyldisilathiane[bis(trimethylsilyl)sulfide, (TMS)2S] or thiols. Moreover, the eventually desired widespread application of CdSe/ZnS NPs as labels in bioanalysis and luminescent materials in photonic devices requires the stable and reliable protection of the sensitive and toxic CdSe core by the passivating ZnS shell to yield reliable data and to prevent the leakage of toxic cadmium ions. Accordingly, tools are also needed to evaluate shell quality. Despite the known strong influence of the shell on the optical properties and the photochemical and thermal stability of core/ shell NPs, little work has been dedicated to establishing methods and tools for quality control of the passivating shell (i.e., its closeness and density). For CdSe NPs without a ZnS shell, several groups reported the fluorescence quenching effect of thiols.54,55 And with electron acceptors such as methylviologen (for CdS),56 quinones (for CdS),57 and pyridine (for CdSe),42 a decrease in (42) Hines, M. A.; Guyot-Sionnest, P. J. Phys. Chem. 1996, 100, 468-471. (43) Dabbousi, B. O.; Rodriguez-Viejo, J. F.; Mikulec, V.; Heine, J. R.; Mattoussi, H.; Ober, R.; Jensen, K. F.; Bawendi, M. G. J. Phys. Chem. B 1997, 101, 9463-9475. (44) Baranov, A. V.; Rakovich, Yu. P.; Donegan, J. F.; Perova, T. S.; Moore, R. A.; Talapin, D. V.; Rogach, A. L.; Masumoto, Y. Phys. ReV. B 2003, 68, 165306. (45) Xie, R.; Kolb, U.; Li, J.; Basche´, T.; Mews, A. J. Am Chem. Soc. 2005, 127, 7480-7488. (46) Talapin, D. V.; Weller, H. J. Phys. Chem. B 2004, 108, 18826. (47) Nicolau, Y. F. Appl. Surf. Sci. 1985, 22/23, 1061. (48) Li, J. J.; Wang, Y. A.; Guo, W. Z.; Keay, J. C.; Mishima, T. D.; Johnson, M. B.; Peng, X. G. J. Am. Chem. Soc. 2003, 125, 12567-12575. (49) Battaglia, D.; Li, J. J.; Wang, Y. J.; Peng, X. G. Angew. Chem., Int. Ed. 2003, 125, 13559-13563. (50) Cumberland, S. L.; Hanif, K. M.; Javier, A.; Khitrov, G. A.; Strouse, G. F.; Woessner, S. M.; Yun, C. S. Chem. Mater. 2002, 14, 1576-1584. (51) Revaprasadu, N.; Malik, M. A.; Carstens, J.; O’Brien, P. J. Mater. Chem. 1999, 9, 2885-2888. (52) Malik, M. A.; Ravaprasadu, N.; O’Brien, P. Chem. Mater. 2001, 13, 913-920. (53) Jun, S.; Jang, E. Chem. Commun. 2005, 4616-4618. (54) Bullen, C.; Mulvaney, P. Langmuir 2006, 22, 3007-3013. (55) Quener, C.; Reiss, P.; Bleuse, J.; Pron, A. J. Am. Chem. Soc. 2004, 126, 11574-11582. (56) Hasselbarth, A.; Eychmueller, A.; Weller, H. Chem. Phys. Lett. 1993, 203, 271. (57) Hagfeldt, A.; Graetzel, M. Chem. ReV. 1995, 95, 49.

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luminescence intensity as well as photo-oxidation45,58 of nanoparticles was observed. A different spectroscopic approach to measure shell quality is based on the assumption that surface (interface) modification during shell growth is accompanied by changes in the phonon frequencies and the line shape of Raman bands.44 The synthetic and analytical shortcomings mentioned above encouraged us to develop a new, rapid, less toxic way to synthesize ZnS shells for CdSe in a domestic microwave oven and systematically study the influence of the ZnS shell thickness on the resulting QY values. The stability of the CdSe/ZnS NPs was investigated with a straightforward thiophenol test that measures the closeness of the shell and the core protection. Additionally, we compare the stability and the QYs of these particles with those of commercial CdSe/ZnS NPs.

2. Results 2.1. Microwave Synthesis. CdSe Core Particles. Heating a mixture of cadmium stearate, trioctylphosphinoxide, trioctylphosphine, and selenium in a microwave oven allowed the synthesis of moderately luminescent CdSe NPs. To realize stringent conditions, the NPs were studied only after exposure to air and daylight for at least 2 months. With increasing emission wavelength (from 545 to 620 nm), the QY of the NPs decreased (from 6 to 2%, Figure 1). Outside of this spectral region, the QY values dropped. The full width at half maximum (fwhm) of the emission band, which is a measure of the NP size distribution, was around 26 nm (570-610 nm) and increased to shorter (545 nm, fwhm 35 nm) and longer wavelengths (622 nm, fwhm 32 nm) within the emission region covered by CdSe of different sizes (Figure 1). CdSe/ZnS Core/Shell Particles. To grow a shell of ZnS around a CdSe core, highly toxic Zn and S precursors diethylzinc and hexamethyldisilathiane are commonly added to washed CdSe NPs. In our synthesis, we used less toxic precursors (zinc undecylenate and cyclohexylisothiocyanate). No attempts were made to remove the remaining precursors of the CdSe NP synthesis or excess ligands by prior washing steps. This one-pot approach not only is less laborious but also ensures the same (58) Nazzal, A. Y.; Wang, X. Y.; Qu, L. H.; Yu, W.; Wang, Y. J.; Peng, X. G.; Xiao, M. J. J. Phys. Chem. B 2004, 108, 5507-5515. (59) Cabral do Couto, P.; Cabral, B. J. C.; Simoes, J. A. M. Chem. Phys. Lett. 2006, 421, 504-507. (60) Riyad, Y. M.; Naumov, S.; Hermann, R.; Brede, O. Phys. Chem. Chem. Phys. 2006, 8, 1697-1706. (61) Ioele, M.; Steenken, S.; Baciocchi, E. J. Phys. Chem. A 1997, 101, 29792987. (62) Wang, Y.; Herron, N. J. Phys. Chem. 1991, 95, 525. (63) Brus, L. J. Phys. Chem. B 1986, 90, 2555-2560. (64) Yu, W. W.; Qu, L.; Guo, W.; Peng, X. Chem. Mater. 2003, 15, 28542860. (65) Burda, C.; Green, T. C.; Link, S.; El-Sayed, M. A. J. Phys.Chem. B 1999, 103, 1783-1788. (66) Riyad, Y. M.; Maumov, S.; Hermann, R.; Brede, O. Phys. Chem. Phys. 2006, 8, 1697-1706. (67) Alam, M. M.; Ito, O. J. Org. Chem. 1999, 64, 1285. (68) Thyrion, F. C. J. Phys. Chem. 1973, 77, 1478. (69) Nakamura, M.; Ito, O.; Matsuda, M. J. Am. Chem. Soc. 1980, 102, 698. (70) Dean, J. A. Lange’s Handbook of Chemistry, 15th ed.; McGraw-Hill: New York, 1999; Table 4.11. (71) Micic, O. I.; Sprague, J.; Li, Z.; Nozik, A. J. Appl. Phys. Lett. 1996, 68, 3150. (72) Micic, O. I.; Fu, Cheong, H.; Zunger, A.; Sprague, J. R.; Mascarenhas, A.; Nozik, A. J. J. Phys. Chem. B 1997, 101, 4904. (73) Micic, O. I.; Jones, K. M.; Cahill, A.; Nozik, A. J. J. Phys. Chem. B 1998, 102, 9791. (74) Talapin, D. V.; Gaponik, N.; Borchert, H.; Rogach, A. L.; Haase, M.; Weller, H. J. Phys. Chem. B 2002, 106, 12659-12663. (75) Li, J. J.; Wang, A.; Guo, W.; Peng, X. J. Am. Chem. Soc. 2003, 125, 12567-12575. (76) Manna, L.; Scher, E. C.; Li, L. S.; Alivisatos, A. P. J. Am. Chem. Soc. 2002, 124, 7136-7145.

High-Quality ZnS Shells for CdSe Nanoparticles

Figure 1. Dependence of the width of the emission band (fwhm as a measure for of the size distribution, 2, left scale) and the fluorescence quantum yield (QY, 9, right scale) on the emission wavelength λem for CdSe NPs. As indicated by the lines that are only guides to the eye, QY decreases at longer emission wavelengths. Minimum in fwhm and thus size distribution results for particles emitting between 570 and 590 nm.

undisturbed ligand coverage and homogeneous distribution of NPs in the matrix for each synthesis. These equal starting-point conditions seem to be essential for reproducibly encapsulating CdSe NPs with a ZnS layer of the desired thickness. The mixture was exposed to a special heat treatment in a microwave oven. The resulting CdSe/ZnS NPs revealed emission wavelengths between 580 to 635 nm, a fwhm of 30 to 37 nm, and QY values of up to 50%. The amounts and ratio of Zn and S precursors added during the shelling process controlled the ZnS shell thickness, the resulting red shift of the absorption and emission wavelengths, and the fwhm values of the emission band of the CdSe core particles. A table with all the analytical and spectroscopic data is provided in Supporting Information. A representative overview of the data is given in the following paragraphs. Shell Thickness. In principle, the thickness of the ZnS shell can be calculated from the difference in the particle radii measured before and after the shelling procedure (Figure 2). Both values can be obtained from TEM measurements. However, possible core growth due to remaining core precursor in the reaction mixture is not taken into account by this approach. Therefore, the shell thickness is overestimated in this case. Peng’s method of correlating the wavelength of the first absorption maximum λabs with the particle radius77 gives access to the size of the CdSe core in a CdSe/ZnS core/shell particle (Figure 2), but tunneling of the exciton into the ZnS shell and the formation of mixed crystals layers (CdaSebScZndSe) are neglected. A comparison of the ZnS shell thickness, which would be expected from the amount of added Zn and S precursors, with the values derived from TEM and absorption measurements is given in Table 1. The shell thicknesses derived from both methods were usually much smaller than that expected from the amounts of precursors used. A possible reason for this discrepancy could be the formation of byproducts such as CdS and ZnS NPs in solution (Figures 3 and 4) as well as the formation of CdS or mixed CdS/ZnS layers on the surface of the CdSe particles. In the following text, we use the shell thickness calculated from the difference between the total CdSe/ZnS particle radius that is measured by TEM (r3, Figure 2) and the CdSe core radius derived from the absorption measurement (r2, Figure 2). Tunneling of the charge carriers into the ZnS shell and the formation of mixed crystals layers have been neglected for simplicity.43 (77) Yu, W. W.; Qu, L.; Guo, W.; Peng, X. Chem. Mater. 2003, 15, 2854.

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Figure 2. Schematic illustration of the core growth during the shelling procedure due to remaining core precursor and the determination of the ZnS shell thickness. CdSe radius r1 and CdSe/ ZnS radius r3 can both be measured with transmission electron microscopy (TEM) whereas r2 can be obtained only from absorption spectra. Using only TEM data overestimates the ZnS shell thickness: (r3 - r1) > (r3 - r2). The combination of TEM measurements with absorption data allows the determination of the size of the new CdSe core in the CdSe/ZnS core/shell particle. ashell) r3 - r2 equals the thickness of the ZnS shell used by us. The size of the CdSe NP before and after shelling are chosen not to reflect the real sizes but to illustrate changes accompanying the shelling procedure. Table 1. ZnS Shell Thickness ashella λem (nm)

ashell calcd from precursors (nm)b

ashell from ∆rTEM (ashell ) r3 - r1) (nm)c

ashell from rTEM - rabs (ashell ) r3 - r2) (nm)d

602 578 582 582 595 582 587 594 580 623 635 595

0.4 0.8 1.1 1.5 1.7 1.9 2.3 2.3 2.3 2.3 2.3 2.3

0.6 0.6 0.1 0.1 nd 0.3 0.4 nd nd 0.4 0.3 nd

0.3 0.5 0.1 0.4 0.5 0.2 0.6 0.7 0.9 0.7 -0.2 0.5

a The molar ratio of Zn to S precursor was 1:1.3. nd means not determined. b Obtained from a calculation using the amounts of added Zn and S precursor assuming 100% yield for the decomposition of precursor and growth on existing CdSe core particles (Experimental Section). c Obtained from the difference in radii derived from TEM measurements before and after the shelling procedure (ashell ) r3 - r1). d Obtained from the difference in radii obtained from the TEM and absorption measurements performed on shelled particles (ashell ) r3 r2).

2.2. Thiophenol Test (TP Test). QDs are increasingly used as fluorophores in bioanalytics, imaging applications, and LEDtype and photovoltaic devices. Therefore, their fluorescence efficiency or fluorescence quantum yield and stability are important. These include any change in luminescence behavior during permanent irradiation with light or exposure to chemically active substances such as oxygen under application-relevant conditions because such effects influence the interpretation of the experiments and hamper quantification. Moreover, in the case of in vivo imaging applications, these also determine to a considerable extent the cytotoxicity of such particles via release of toxic components such as cadmium. This encouraged us to find a parameter or quantity to describe the short- and long-term stability of our QDs and to compare the effect of particle stability on luminescence behavior for different particles. The addition of thiophenol (TP) to unshelled CdSe NPs immediately leads to a complete loss of luminescence upon

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Figure 3. Emission spectrum (uncorrected of CdSe/ZnS NPs in chloroform (λex ) 363 nm). The emission band at 470 nm indicates the presence of luminescent CdS particles in the reaction solution. We assume the formation of CdS NPs as a byproduct during the shelling procedure of CdSe with ZnS. Potentially formed ZnS NPs could not be excited at the chosen excitation wavelength.

excitation at 312 nm. A dense shell of an inert material such as ZnS, however, is expected to protect the CdSe core from fluorescence quenchers. Therefore, the time dependence of the emission intensity of the CdSe/ZnS NPs, which are continuously illuminated at 312 nm in the presence of TP, was used as a tool or criterion for the quality of the ZnS shell. The chosen illumination wavelength of 312 nm corresponds to the absorption maximum of TP and provides enough energy to photochemically create thiophenol radicals. (The homolytic bond dissociation enthalpy59-61 for thiophenol S-H is 347 kJ mol-1.) To monitor the corresponding changes in NP luminescence, CdSe/ZnS emission spectra were recorded for an excitation wavelength of 480 nm. In the first step in establishing the TP test, parameters such as TP concentration, illumination wavelength, illumination intensity, and choice of solvent were optimized with exemplary selected CdSe NPs. We observed the best results in chloroform. TP had reduced quenching capability in heptane and was not able to quench the luminescence of the shelled particles in toluene (no decrease in luminescence even after several days). Moreover, TEM measurements showed an increased particle diameter after the TP test in toluene. To quantify the short- and long-term stability of the NPs, relative fluorescence intensities Rf1 and Rf2 were calculated from the ratio of the emission intensity at the first minimum (occurring normally within the first 5 min of NP illumination) and after 30 000 s (ca. 8 h), respectively, to the initial emission intensity (I0s), measured before the addition of TP. Figure 5 depicts the fluorescence time traces of four exemplary chosen CdSe NPs in chloroform as measured during the TP test that is exemplary for four different CdSe NPs in chloroform, short-term stability Rf1 and long-term stability Rf2 are defined as

Rf1 )

I1.minimum I0s

(1)

I8h I0s

(2)

Rf2 )

For long-term stability, t ) 8 h was chosen because the emission intensity of particles of excellent stability reached a plateau after that time. A comparison of the TP test results obtained with different CdSe NPs is summarized in Table 2.

As mentioned previously, unshelled CdSe NPs suffered a complete and permanent loss of luminescence immediately after the addition of thiophenol (Figure 5a). Accordingly, both Rf1 and Rf2 were 0, and no photobrightening could be observed. Without the addition of TP, the luminescence remained constant (Rf1 ) Rf2 ) 1). The best shell quality for our particles was achieved for a molar ratio of the zinc- and sulfur-containing precursors (Zn(Un)2 and CySCN) of 1:1.3 and a ZnS shell thickness of ashell > 0.5 nm. The best particles displayed an Rf1 value of 0.4 and an Rf2 value of 1.3. Here, even after several days of UV irradiation, thiophenyl radicals could not quench the luminescence of particles. Less than 0.5 nm ZnS shell did not have a protective effect during this test. Ratios of Zn(Un)2 to CySCN of 1:1 and 1:3 resulted in similar Rf1 values but significantly lower Rf2 values. A 3-fold excess of Zn(Un)2 did not lead to protective shells (Rf1 ) 0). After 1 year of storage in the dark, commercial CdSe/ZnS NPs from Evident Technologies (λem ) 615 nm, fwhm ) 28 nm, QY ) 16%; old Evidots in Table 2) showed medium short-term stability and no long-term stability (Rf1 ) 0.22, Rf2 ) 0), revealing immense photobrightening in between (Figure 5c). Recently obtained “fresh” CdSe/ZnS NPs from Evident Technologies (λem ) 627 nm, fwhm ) 27 nm, QY ) 12%; new Evidots in Table 2) displayed improved short-term stability but also no long-term stability (Rf1 ) 0.82, Rf2 ) 0; Figure 5d). Accordingly, we chose Rf1 > 0.3 and Rf2 > 0.5 as criteria for sufficient shell stability. Figure 6 illustrates the correlation between the thickness of the ZnS shell and the stability of the particles against TP and the fluorescence quantum yield of the NPs. The CdSe particles were synthesized according to our standard shelling procedure (1:1.3 Zn(Un)2/CySCN) and share a comparable core size (r ) 1.85 ( 0.15 nm). The best core protection and enhanced QY values were achieved for ashell ≈ 0.6 nm.

3. Discussion 3.1. Microwave Synthesis. CdSe. The CdSe synthesis in a microwave oven was based on a procedure by Strouse et al.34 In contrast to the classical wet-chemical synthesis, the MWpower and thus the total energy introduced into the reaction mixture can be adjusted very precisely. We confirm the discussed effects of time and MW power on the wavelength and the fwhm of the emission band. A fast synthesis at high MW power leads to luminescent CdSe NPs with a narrow size distribution. Low QYs of 2-6% can most likely be attributed to the long storage time (>2 months in air) prior to QY measurements and are consistent with literature data.45 The emission wavelength of the resulting CdSe cores could be easily adjusted from 547 nm (green) to 622 nm (red) by tuning the reaction time. Even though a domestic microwave is a multimode device and thus has no steady microwave field, the inevitable existence of hot and cold areas in the reaction chamber as well as the pulsed field did not have a significant influence on reproducibility and quality of our CdSe particles. This follows from the low standard deviation of eight syntheses: λem ) (576 ( 4 nm) and fwhm ) (26 nm ( 1 nm). CdSe/ZnS. The addition of the ZnS shell precursor to the CdSe cores without prior washing to remove remaining CdSe precursors led to stable CdSe/ZnS core shell NPs if a shell thickness of 0.5 nm was exceeded (Rf1 > 0.3, Rf2 > 0.5). This corresponds to roughly 2 monolayers (ML) of ZnS (1 ML of ZnS is 0.31 nm1). In accordance with data published by Xie et al.,45 the QY of our NPs reached their highest values for this shell thickness (Figure 6). Good Rf1 and Rf2 values did not necessarily accompany the highest QYs, as shown by the fact that orange particles (λem )

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Figure 4. TEM pictures of CdSe (left, λem ) 605 nm, fwhm ) 27 nm) and CdSe/ZnS (right, λem ) 623 nm, fwhm ) 36 nm, ashell ) 0.7 nm, Rf1 ) 0.3, Rf2 ) 0.5) NPs. The right picture shows other small particles (indicated by arrows) that might be attributed to the simultaneous formation of ZnS and CdS NPs during the shelling procedure. Further TEM material is available from the authors.

Figure 5. Time dependence of the emission intensity for four typical thiophenol (TP) tests. The short-term stability is expressed by the value Rf1 ) I1.minimum/I0s, and the long-term stability, by Rf2 ) I8h/I0s. The samples were continuously illuminated at 312 nm to generate the quenching species, most likely TP radicals, and the NP emission was excited at 480 nm. At t ) 0 s, TP is added to (a) unshelled CdSe or CdSe/ZnS of poor shell quality, Rf1 ) Rf2 ) 0, (b) our CdSe/ZnS with optimum performance (that satisfied the quality criteria Rf1 > 0.3 and Rf2 > 0.5), (c) commercial CdSe/ZnS from Evident Technology after 1 year of storage (old Evidots), and (d) new commercial CdSe/ZnS from Evident Technology (new Evidots). Generally, small Rf values indicate low NP stability.

585 nm) with a thin, defect-free ZnS layer had the highest QY even though they had low Rf values. But for particles that passed the TP test, the maximum QY of almost 50% correlated with the maximum stability and an acceptable size distribution (fwhm 620 nm (ashell > 0.6-0.9 nm and ∼2 to 3 ML of ZnS, but data is not shown

in Figure 6 because their CdSe core was larger then 1.85 ( 0.15 nm after the shelling procedure) correlated with the decrease of QY in CdSe nanocrystals of longer (e.g., red) emission wavelength (Figure 1). Xie et al.45 observed a similar decline in luminescence for ZnS shells that are thicker than 0.6 nm (2 ML of ZnS). They assumed, that according to statistics, thicker shells inevitably have more defect states from lattice imperfections within the

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Table 2. Comparison of Short-Term (Rf1) and Long-Term (Rf2) Stability of CdSe/ZnS NPs with Different ZnS Shell Thicknessesa

TP Rf1

test Rf2

0.00 0.00 0.02 0.36 0.32 0.43 0.43 old Evidots 0.20 new Evidots 0.80

0.00 0.04 0.20 1.24 0.51 0.54 0.60 0.00 0.00

CdSe/ZnS

ashell ashell ashell ) rTEM - rabs ) ∆rTEM ) calcd from ) r3 - r2 ) r3 - r1 ZnS precursor (nm) (nm) (nm) 0.1 0.4 0.2 0.5 0.6 -0.2 0.5 0.6 0.0

0.1 0.2 0.3 0.4 0.4 0.3 nd nd nd

1.1 1.5 1.9 2.3 2.3 2.3 2.3 nd nd

a Small Rf values indicate poor ZnS shell quality. Rf1 > 0.3 and Rf2 > 0.5 were chosen as empirical quality criteria. The Zn/S precursor ratio was 1:1.3 (Experimental Section). nd means not determined.

Figure 6. QY (b) and Rf values (Rf1, 9, Rf2, diamonds) as a function of ZnS shell thickness. The best core protection (high Rf1 and Rf2 values) was achieved for ashell ≈ 0.6 nm (2 ML of ZnS) and led to improved QY. The CdSe/ZnS particles were all of comparable CdSe core size (r ) 1.85 nm ( 0.15 nm as derived from λabs with literature methods).62-64

shell that allow radiationless recombination, and a thickness of 0.5 to 0.9 nm ZnS might be optimum, before QY is decreasing again. Shells thicker then 0.9 nm were not investigated. The emission wavelength region of our CdSe/ZnS NPs, which fulfilled our TP test quality criteria of Rf1 > 0.3 and Rf2 > 0.5 (fwhm of 30 to 37 nm), is limited to the region of 580-635 nm. Robust green-emitting CdSe/ZnS particles could not be obtained because a red shift of the emission upon shell synthesis could not be prevented. We believe that the red shift of the emission wavelength depended primarily on the growth of the CdSe core particle during the shelling procedure (Figure 7 and Table 2 in Supporting Information), whereas the thickness of the ZnS shell (and thus tunneling effects mentioned in the literature)43,45 did not play a significant role (Figure 8 and Table 2 in Supporting Information). These findings are only seemingly inconsistent with published data. In the literature,45 CdSe particles were synthesized from a molar ratio of 1:2 Cd/Se and were covered with CdS and ZnS shells of different thicknesses. CdSe/CdS NPs showed a larger red shift of the emission wavelength (in comparison to unshelled CdSe NPs) than CdSe/ZnS NPs, which was explained by the inverse relation between the energy barrier and tunneling probability of the charge carriers into the shell. Because the band gap difference for CdSe/CdS is lower than that for CdSe/ZnS, a larger red shift is expected for CdSe/CdS.43 However, the red shift in absorption and emission could also have occurred as a

Figure 7. Red shift of emission wavelength vs CdSe core growth during the ZnS shelling procedure. See Figure 2 for the calculation of the core growth. ∆rCdSe core ) r2 - r1 ) rabs(CdSe/ZnS) - rabs(CdSe). The line is only a guide to the eye to illustrate the correlation between the red shift and the core growth.

Figure 8. Red shift of the emission wavelength vs ZnS shell thickness. The thickness of the ZnS shell does not have a significant influence on the red shift of the emission wavelength during the shelling procedure.

result of CdSe core growth because the Cd precursor, which was added during the shelling process, could have reacted with the excess selenium precursor. If only a Zn precursor is added, then the CdSe core could grow only by the energetically less attractive combination of existing CdSe particles (Ostwald ripening). This also might have contributed to the red shift observed by Dabbousi et al.,43 where CdSe particles were heated for 5-10 min according to the desired ZnS shell thickness. Unfortunately, no data were available on the CdSe NP growth during this heating period in the absence of ZnS precursors. The width of the size distribution of our particles before and after the shelling procedure is randomly dependent on the change in the CdSe core size. The change in fwhm (i.e., ∆fwhm) seems to increase with the growth of the CdSe core ∆rCdSe (Table 2 and Figure 1 in Supporting Information), thus supporting the core growth theory as one cause of the red shift. The formation of ZnSe or mixed CdSe, ZnSe, and ZnS layers is also possible. To what extent excess Se is removed from the CdSe NPs during the washing procedure prior to the shelling step cannot be concluded from the literature data and needs further investigation. The influence of ligands on the tunneling probability could also have a significant influence and could lead to increased leakage of the CdSe exciton wave function in the surrounding sphere. The origin of the red shift could not absolutely be clarified by us and might be a mixture of both effects. We assume that the decomposition rate of the Zn and S precursors close to the nanoparticle surface is crucial to the

High-Quality ZnS Shells for CdSe Nanoparticles

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Figure 9. Schematic representation of the interaction between photochemically formed phenylthiyl radicals and CdSe (top), CdSe/ZnS with cracks and holes (middle), and CdSe/ZnS with a dense shell (bottom). If the phenylthiyl radical gets too close to the CdSe core, then the luminescence is quenched.

formation and quality of the ZnS shell. Because of the large lattice mismatch between CdSe and ZnS of 12%, the formation of the first ZnS layer needs increased activation energy in comparison to further growth of ZnS onto an existing ZnS layer. Ideally, the Zn and S precursors should adsorb onto the particle surface before decomposition, avoiding the formation of ZnS NPs as byproducts. Therefore, a balance has to be found between the high decomposition rate and the loss of precursor due to the formation of byproducts. For the formation of the first ZnS layer, a higher MW power and thus a higher temperature with a higher decomposition rate were favorable. In the following text, the MW power was reduced to avoid excessive conucleation. Heating in intervals was used to allow the precursor molecules to diffuse into the vicinity of the NPs. Nevertheless, only a moderate percentage of Zn and S precursors took part in the shelling process. To keep the concentration of the ZnS precursor low, experiments with alternating additions of Zn and S precursors between periods of heating were carried out (SILAR technique), but all failed to form shells or reduce conucleation. Because no washing step was carried out, the presence of the remaining CdSe precursor during the shelling process is likely. This could also have been advantageous because the formation of an intermediate CdS or ZnSe layer would ease the lattice strain between CdSe and ZnS. The use of CdSe NPs in their original ligand matrix provides the same starting conditions for each shelling procedure, which cannot be guaranteed after washing. The use of an MW instead of a conventional heating source allowed more precise control over the time and applied heating power. It should be possible to find the optimal balance between the power intake of the reaction mixture and the power consumption of the precursors to reduce CdS and ZnS NP formation dramatically. The tuning of the decomposition temperature and rate of the precursors could also

be achieved by modifying the precursor molecules. Further work in this area will show if zinc carboxylates and isothiocyanates are suitable for the generation of green- and blue-emitting CdSe/ ZnS NPs or for the shelling of other NPs. 3.2. Shell Quality (Thiophenol Test). A shell that is thick enough and free of cracks and holes protects the CdSe core against the quenching species (most likely a TP radical) (Figures 5, 6, and 9). The efficiency of the ZnS shell to withhold the quenching reagent from the luminescent core is correlated with the relative fluorescence values Rf1 and Rf2 defined by us, which provides the basis for the comparison of the quality of different shells after a certain illumination time (Rf1 as a minimum occurring within the first 5 min, Rf2 occurring after 8 h). The importance of a protecting shell follows immediately from the complete loss of luminescence for CdSe particles without shells. As revealed by the TP test, highly robust particles were obtained for a small excess of the S precursor (1:1.3 Zn/S) and a ZnS shell thickness exceeding 0.5 nm. Our NPs showed the same stability even after 8 months. The poor stability of similar particles that were smaller than 0.5 nm could be due to either holes in the shell or the fact that a certain thickness of ZnS may be needed to obtain the desired protective effect against TP. On the basis of our results concerning the phase transfer to water, values of Rf1 > 0.3 and Rf2 > 0.5 were chosen as empirical criteria for good NPs. Our particles display low stability for a ZnS shell thickness of less than 0.5 nm or unsuitable precursor ratios. Zn/S precursor ratios of 1:1 and 3:1 may not allow the formation of an intermediate CdS layer because S is scarce. A large excess of the S precursor might lead to fast homonucleation of ZnS and CdS. Accordingly, the resulting ZnS shell seems to be too thin and is formed with cracks or as a separated island. We can only speculate as to whether an intermediate stress-reducing CdS layer

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is formed, as is principally possible under our experimental conditions. Another reason for the low stability of these CdSe/ ZnS NPs might be an unsuitable ZnS shell composition because undecomposed and enclosed precursors could be washed out with time, leading to pinholes. Despite their relatively thick shell (ashell ) 0.6 nm), commercial CdSe/ZnS NPs from Evident Technologies had low Rf values (Rf1 ) 0.22; Rf2 ) 0) after 1 year of storage. We assume that the ZnS shell is porous or consists of ZnS particles that are adsorbed only onto the CdSe surface. In either case, TP can eventually approach the CdSe core and quench its luminescence. The immense photobrightening (Figure 5c) reveals the existence of a large number of defect states in the core and/or in the shell. Despite of the similarity of the QY values, the increased shortterm stability of new CdSe/ZnS NPs from Evident Technologies (Rf1 ) 0.82) suggests either improved washing or synthesis procedures or a dramatic degradation of the ZnS shell during 1 year of storage even in the dark. In any case, no improvement in long-term stability could be observed (Rf2 ) 0). The mechanism of the TP test is currently being investigated by us, and the suitability of alternative quenching reagents such as quinones for a ZnS shell test is being similarly tested. Thus, the different possible quenching mechanisms are only briefly discussed. A TP radical state in between the valence and conduction bands of CdSe could act as a fast radiationless pathway to transfer electrons from the conduction band of CdSe to the TP state and back to the valence band of CdSe. Similar behavior was reported for quinones and CdSe by El-Sayed et al.65 The TP state could act as an electron trap similar to methylviologen for CdS nanocrystals.56,66 Most likely, a photochemically formed phenylthiyl radical is the quenching species because TP has a strong absorbance peak around the UV wavelength used (312 nm) and the S-H bond readily undergoes homolytic dissociation (bond dissociation enthalpy BDE ) 347 kJ mol-1).59,66-69 These species most likely act as scavengers for charges localized on the CdSe core, thereby quenching the CdSe emission; this behavior is similar to that of other thiyl radicals. Further evidence for a radical mechanism is the failure of the TP test in toluene. The ability of toluene to form radicals easily (BDE ) 356 kJ mol-1)70 could lead to toluene functioning as a radical scavenger and thus depriving the solution of the quenching thiyl species. The existence of radicals could also explain or contribute to the observed photobrightening effects because similar effects were described for other etching substances such as oxygen and fluorines.71-76 The radicals do not seem to be able to dissolve the ZnS shell quickly because no decrease in particle size could be measured by TEM.

4. Conclusions We have developed a fast, reliable, and simple procedure for the preparation of highly luminescent CdSe NPs with ZnS shells in a domestic MW without using very toxic or expensive educts. The effects of synthesis parameters such as temperature (MW power; not shown), time (not shown), and shell thickness on the luminescence quantum yield and stability of the ZnS shell were optimized. As a criterion for shell quality, we introduced a thiophenol-based test and chose the values Rf1 > 0.3 and Rf2 > 0.5 as empirical values for sufficient short- and long-term stability. This TP test, which is currently being investigated by us, presents the first step toward a tool for the straightforward control of shell quality (e.g., the optical properties and cytotoxicity of such NPs). According to this test, a shell synthesized from a 1:1.3:1 molar ratio of Zn(Un)2/CySCN/octylamine for each additional formed ZnS layer of 0.31 nm thickness and a sufficient precursor for a

Ziegler et al.

shell of 2.3 nm (7.4 ML) thickness showed the required quality. This synthesis resulted in a shell thickness of 0.5 to 0.9 nm, roughly equaling 2 to 3 ML of ZnS. The QYs for robust CdSe/ ZnS particles emitting between 580 and 620 nm are 30-50%, 23% for λem ) 625 nm, and 10% for λem ) 635 nm. The QY of the red-emitting particles is similar to that of commercial particles, revealing only short-term stability (λem ) 615 nm, QY ) 16%). 5. Experimental Details Materials. Cadmium oxide (99%), stearic acid (99%), succinic acid (99%), tri-n-octylphosphine oxide (TOPO, 99%), hexadecylamine (HDA, 99%), selenium (99%), cyclohexyl isothiocyanate (CySCN, 98%), zinc undecylenate (Zn(Un)2, 98%), thiophenol (TP, p.a.), chloroform (p.a.), and methanol (p.a.) were purchased from Sigma-Aldrich. Tri-n-octylphosphine (TOP, 97% ABCR) was stored under nitrogen. All of the chemicals were used without further purification. For the spectroscopic measurements, all of the solvents were checked for fluorescent impurities prior to use. Instruments. A Eurostyle MWG 722 domestic microwave operating at 2.44 GHz, with a rated MW power output of 800 W and a cavity volume of 20 L, and a UV table from Vilbert Lourmat VL-TFX-20.MC (6 × 15 W; 312 nm tubes) were employed. Except for the QY measurements (see the next section), all of the emission and absorption spectra were recorded on a J&M FL3095 spectrometer. TEM measurements were made on a 120 keV instrument. The CdSe synthesis in a microwave oven was based on a procedure by Strouse et al.34 For the CdSe synthesis in the microwave, a stock solution of Se (2.174 g, 0.028 mol) in TOP (100 mL) was prepared. A stock solution of Cd(stearate)2/stearic acid was prepared by heating CdO (1.284 g, 0.010 mol) with stearic acid (10.242 g, 0.0360 mol) and a catalytic amount of succinic acid to 200 °C at 200 mbar of reduced pressure until the solution was clear. TOPO (3 g, 0.008 mol), HDA (1 g, 0.004 mol), and Cd(stearate)2/stearic acid (0.547 g) were heated in 100 mL tubes to 200 °C until a clear solution was obtained. After cooling in an oil bath (100 °C, 5 min), 2 mL of the TOP/Se stock solution was added. The tube was vigorously shaken in the oil bath for 1 min and then placed in a domestic microwave oven (MW). The sample was heated at 800 W for 30 to 120 s under moderate air cooling and atmospheric pressure, according to the desired size of the CdSe nanoparticles. The whole tube was immediately placed in an oil bath and cooled to 100 °C and then used in the shelling process. For the synthesis of green-emitting particles, toluene (3 mL) can be added before heating to dilute the mixture and to act as a cooling liquid and faciliate the synthesis. The resulting particles were characterized by TEM and absorption spectroscopy in chloroform.77 CdSe/ZnS. For the synthesis of the ZnS shell, the calculated amount of Zn(Un)2 and octylamine were heated to 200 °C until the solution was clear. After the addition of cyclohexyl isothiocyanate to this Zn precursor solution, the mixture was divided into two portions of equal size. The first was added to the prepared melt of CdSe/HDA/TOPO mentioned above. The shelling process was initiated at 800 W MW power for 30 s. The second half of the precursor mixture was added after a cooling period of 60 s. The mixture was shaken vigorously for 60 s, and then the following program was applied five times: 30 s at 600 W MW power followed by a 2 min break with shaking to avoid overheating. Calculation. (For further details, see Supporting Information.) To calculate the amount of precursors, the desired ratio of Zn and S precursors and octylamine in each monolayer (ML) and the envisaged shell thickness or number of MLs have to be considered. The shell volume and mass are calculated with the ZnS bulk parameters for lattice constant c (0.31 nm) and the density (4.1 g/cm3). Example: With λabs(CdSe) ) 571 nm, the particle radius is calculated to 1.62 nm. For a ZnS shell of 2.2 nm thickness and a ratio of Zn(Un)2 to CySCN to octylamine of 1:1.3:1 in each ZnS layer of 0.31 nm thickness, the following amounts are needed if the

High-Quality ZnS Shells for CdSe Nanoparticles reaction yield is 100%: Zn(Un)2 (3.67 g), CySCN (1.87 mL), and octylamine (2.72 mL). TP Test. The time dependence of the emission intensity of CdSe/ ZnS NPs in the presence of photochemically excited thiophenol (TP) recorded on a J&M FL3095 spectrometer was used as a quality test for the ZnS shell in CdSe/ZnS NPs. Prior to the quality test, the CdSe/ZnS NPs were diluted with chloroform until they showed maximum luminescence when excited with 480 nm light in a Helma quartz cuvette transparent to UV light. This solution (or suspension or colloidal solution) of NPs (3 mL) was inserted into the measuring chamber and placed on a UV table with a 312 nm excitation wavelength. This wavelength corresponds to the absorption maximum of TP and ensures the formation of TP radicals. After the acquisition of a dark spectrum with an excitation wavelength of λex ) 480 nm, the UV table was switched on, and after 5 s, the thiophenol quenching reagent (120 µL, 1.17 mmol) was added. The cuvette and the measuring chamber were sealed, and the decrease in luminescence under continuous UV irradiation was recorded with λex ) 480 nm. Rf values were calculated from the first minimum and after 8 h. Quantum Yield Measurements. The nanocrystal stock solutions were diluted in chloroform (spectroscopic grade, Merck). Quartz glass cuvettes of 1 cm optical path length were used, and all measurements were performed at 25 °C. The absorption spectra were recorded on a Cary 5000 UV/vis/NIR spectrophotometer (Varian). The fluorescence spectra were measured with an SLM 8100 spectrofluorometer (Spectronics Instruments) with Glan Thompson polarizers placed in the excitation and emission channels set to 0 and 54.7°, respectively. The fluorescence spectra were corrected for the wavelength- and polarization-dependent spectral responsivity of the detection system as described in refs 78 and 79 For the fluorescence quantum yield measurements, fluorescein 27 in 0.1 N NaOH (QY ) 0.72 ( 0.03)80,81 was employed as a fluorescence standard. The excitation wavelength used was 477 nm for all samples. The optical density at the long-wavelength maximum of both the standard and the NC solutions was set to be always below 0.05 to minimize reabsorption effects. At the excitation wavelength (78) Resch-Genger, U.; Pfeifer, D.; Monte, C.; Pilz, W.; Hoffmann, A.; Spieles, M.; Rurack, K.; Hollandt, J.; Taubert, D.; Scho¨nenberger, B.; Nording, P. J. Fluoresc. 2005, 15, 315-336. (79) Pfeifer, D.; Hoffmann, K.; Hoffmann, A.; Monte, C.; Resch-Genger, U. J. Fluoresc., in press. (80) Demas, J. N. Measurements of Photon Yields. In Optical Radiation Measurements; Mielenz, K. D., Ed.; Academic Press, New York, 1982; Vol. 3, pp 195-248.

Langmuir, Vol. 23, No. 14, 2007 7759 (477 nm), the optical density of the standard and the probe was between 0.02 and 0.08. To check for concentration-dependent effects, for each standard probe pair the quantum yield was determined twice, using two different concentrations of the fluorescence standard and the nanocrystal solution. Prior to each QY measurement, the NC solutions were checked for a possible photobrightening effect. For this purpose, each sample was illuminated at the same wavelength with the same light intensity as used for the QY measurement, and a fluorescence time trace was collected at the fluorescence peak wavelength to track the possible increase in fluorescence intensity with increasing illumination time. No photobrightening was observable for illumination times needed to complete a quantum yield measurement. The reported fluorescence quantum yields QY (Table 1 in Supporting Information) were calculated from integrated, blank-, and spectrally corrected emission spectra on a wavelength scale using the following equation: QYp ) QYs

Asnp2 Apns

2

∫ I λ dλ ∫ I λ dλ p s

A is the absorbance at the excitation wavelength, n is the refractive index of the solvents used, I is the wavelength-dependent emission intensity, and λ is the emission wavelength. Subscripts p and s represent the sample and the standard, respectively. Typical uncertainties of fluorescence quantum yield values as derived from previous measurements are (5% (for QY > 0.4), (10% (for 0.2 > QY > 0.02), (20% (for 0.02 > QY > 0.005), and (30% (for 0.005 > QY).82

Acknowledgment. J.Z. acknowledges the support of the BMBF NanoLux project. M.G. gratefully acknowledges financial support from the Federal Ministry of Technology and Education (BMBF; grant FLUOPLEX; Biophotonics II). Supporting Information Available: Analytic data for CdSe and CdSe/ZnS NPs. Increase in fwhm as a function of the growth of a CdSe core particle during the shelling procedure. Radius of the CdSe core. TEM image of a CdSe/ZnS sample. This material is available free of charge via the Internet at http://pubs.acs.org. LA063528B (81) Velapoldi, R. A. in AdVances in Standards an Methodology in Spectrophotometry; Elsevier: Amsterdam, 1987; pp 175-194. (82) Rurack, K.; Bricks, J. L.; Schulz, B.; Maus, M.; Reck, G.; Resch-Genger, U. J. Phys. Chem. A 2000, 104, 6171.