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shell quantum dots using nucleation-doping strategy
Optical Materials 31 (2008) 455–460
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Synthesis and photoluminescence of water-soluble Mn:ZnS/ZnS core/shell quantum dots using nucleation-doping strategy Qi Xiao *, Chong Xiao School of Resources Processing and Bioengineering, Central South University, Changsha 410083, China
a r t i c l e
i n f o
Article history: Received 14 April 2008 Received in revised form 13 June 2008 Accepted 18 June 2008 Available online 3 August 2008 Keywords: Mn:ZnS/ZnS (core/shell) quantum dots Nucleation-doping strategy Water-soluble Photoluminescence
a b s t r a c t The water-soluble Mn:ZnS/ZnS (core/shell) quantum dots were synthesized through nucleation-doping strategy in aqueous solutions in air, and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and photoluminescence (PL) spectra. It was found that the Mn2+ 4T1 ? 6A1 emission intensity of Mn:ZnS d-dots significantly increased with the increase of Mn2+ concentration, and showed a maximum when Mn2+ doping content was 1.0%. If Mn2+ concentration continued to increase, namely more than 1.0%, the Mn2+ 4T1 ? 6A1 emission intensity would decrease. By the growth of an additional ZnS shell on Mn:ZnS d-dots, the emission intensity of Mn2+ in Mn:ZnS/ZnS core/shell d-dots were enhanced due to the elimination of the surface defects. Mn:ZnS/ZnS (core/shell) d-dots were not at all air sensitive, while Mn:ZnS d-dots were strongly air sensitive. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Semiconductor quantum dots, or quantum dots (q-dots), with CdSe ones as the workhorse, have been widely explored as biomedical labeling reagents since 1998 [1–4]. However, the small-ensemble Stokes shift of intrinsic q-dot emitters made self-quenching. In addition, experimental results indicated that any leakage of cadmium from the quantum dots would be toxic and fatal to a biological system [5], and cadmium-containing products were eventually environmentally problematic. Recently, Peng et al. [6–8] reported that doped quantum dots (d-dots) (e.g., Mn:ZnSe d-dots) could not only replace cadmium in CdSe quantum dots with zinc, but also overcome a couple of intrinsic disadvantages of undoped quantum dots (q-dots) emitters, that is, strong self-quenching caused by their small ensemble stokes shift (energy difference between absorption spectrum and emission band) [9,10] and sensitivity to thermal, chemical, and photochemical disturbances [11,12]. Mn2+-doped ZnS quantum dots have been extensively investigated for use in various applications other than biomedical labeling, such as displays, sensors, and lasers [13–15]. In addition, the luminescent lifetime of Mn2+-doped ZnS quantum dots was ca. 1 ms. Such a long lifetime made the luminescence from the nanocrystal readily distinguishable from the background luminescence. Therefore, Mn2+-doped ZnS quantum dots could be potential candidates as fluorescent labeling agents, especially in biology [16]. Many approaches have been explored for the preparation of Mn2+-doped ZnS quantum dots (Mn:ZnS d-dots), including organo* Corresponding author. Tel.: +86 731 8830543; fax: +86 731 8879815. E-mail address: [email protected] (Q. Xiao). 0925-3467/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2008.06.010
metallic method [17], chemical precipitation method [18], microemulsion technique [19], reverse micelle method [20], etc. The traditional synthesis strategies of Mn:ZnS d-dots were based on a reaction system with both dopant ions and competitive host ions in it, which was difficult to control and optimize. To meet this challenge, Peng et al. [6–8] recently developed a new synthetic strategy, nucleation-doping, to prepare Mn:ZnSe d-dots through organometallic routes. The key feature of the nucleation-doping strategy was decoupling the doping from nucleation, which allowed for the growth of d-dots in a controllable fashion, thus yielding d-dots with pure dopant emission at a high efficiency. To be a suitable bio-labeling agent, the quantum dots should have high luminescent efficiency, water-solubilization, and biocompatibility. Water-soluble semiconductor quantum dots could be obtained mainly by two different methods. The first way was to replace the surface-capping molecules on the particles prepared by the TOPO (trioctylphosphine oxide) method with water-soluble thiols [21]. However, after the substitution of the surface-capping molecules by hydrophilic molecules, the nanoparticle photoluminescence decreased markedly [21]. The second method was to directly synthesize semiconductor quantum dots in aqueous solution using water-soluble stabilizers [22]. The second method became a popular recipe for making water-soluble nanoparticles. Therefore, a facile aqueous nucleation-doping strategy to obtain water-soluble Mn:ZnS d-dots were highly desired. On the other hand, the passivation of surface was of crucial importance for the applications of semiconductor quantum dots. In order to get a better passivation, inorganically passivated (or core/shell structured) quantum dots have been developed and shown dramatically enhanced properties [23]. Photoenhanced
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luminescence was observed in ZnS/CdS/ZnS quantum well [24], ZnS:Mn/ZnS nanoparticles [20], ZnS:Mn/ZnO nanophosphors [25], and ZnS:Mn/Zn(OH)2 nanocrystal [26], etc. Especially, formation of ZnS:Mn2+/ZnS core/shell nanocrystals resulted in obvious enhancement in the PL intensity with respect to that of bare ZnS:Mn2+ nanocrystals due to the good match between core (Mn:ZnS) and shell (ZnS) lattice constants and the effective elimination of the surface defects [20]. In this paper, we introduced the nucleation-doping strategy to synthesize Mn:ZnS/ZnS (core/shell) quantum dots in aqueous solution in air. By the growth of an additional ZnS shell on Mn:ZnS d-dots, the emission intensity of Mn2+ in Mn:ZnS/ZnS core/shell d-dots were enhanced due to the elimination of the surface defects. In addition, Mn:ZnS/ZnS (core/shell) d-dots were not at all air sensitive, while Mn:ZnS d-dots were strongly air sensitive. 2. Experimental section 2.1. Chemicals All chemicals used were of analytical grade. Zn (CH3COO)2 2H2O, Mn(CH3COO)2 2H2O, Na2S 9H2O, and thioglycolic acid (TGA) were obtained from Shanghai Chemical Reagents Company and used as received. High-purity water with a resistivity of 18.2 MX/cm was used for preparation of all aqueous solutions. 2.2. Synthesis 2.2.1. Synthesis of Mn:ZnS d-dots TGAZn and TGAMn stock solutions were prepared by adding Zn (CH3COO)2 2H2O and Mn (CH3COO)2 2H2O into 100 ml of TGA (0.12 M) aqueous solution, respectively. Then the different volume of TGAMn stock solution was quickly added into the Na2S solution under vigorously stirring, the reaction mixture was stirred at 80 °C for 20 min to form small-sized MnS core. Next, the TGAZn stock solution was quickly injected into the above reaction mixture under vigorously stirring, and then the reaction mixture was heated at 80 °C for 12 h. The final molar ratio of Mn/Zn was varied from 0 to 3%. The molar ratio between S-precursor and Zn and Mn precursors was equal to 1.5. The Mn:ZnS d-dots solid powders were obtained by adding excess ethanol to the solutions and then dried in vacuum.
(k = 0.15418 nm). Transmission electron microscopy (TEM) images were obtained using a JEM 3010 high-resolution transmission electron microscope. UV–Vis absorption spectra were recorded with a UV–Vis scanning spectrophotometer (Shimadzu UV-2450). The photoluminescence (PL) spectra of the samples in solid form were recorded with a fluorescence spectrophotometer F-4500. 3. Results and discussion 3.1. XRD analysis The XRD patterns of Mn:ZnS d-dots were shown in Fig. 1. These diffraction features appearing at 28.5°, 47.5°, and 56.3° corresponded to the (1 1 1), (2 2 0), and (3 1 1) planes of cubic zinc blende structure, which was very consistent with the values in the standard card (JCPDS No. 77-2100). No characteristic peaks of impurity phases are observed in the XRD pattern, indicating the high purity of the final products. In addition, the broadening of the diffraction peaks of all the Mn:ZnS d-dots were obvious, which was characteristic of nanocrystal. The averaged crystallite sizes D was determined according to the Scherrer equation D = kk/bcos h [27], where k was a constant (shape factor, about 0.9), k was the X-ray wavelength (0.15418 nm), b was the full width at half maximum (FWHM) of the diffraction line, and h was the diffraction angle. Based on the FWHM of (1 1 1) zinc blende reflection, the averaged crystallite sizes of Mn:ZnS d-dots doped with different Mn2+ concentration (0%, 1% and 3%) were estimated to be 3.96, 4.08 and 4.11 nm, respectively. 3.2. TEM analysis Fig. 2 showed the TEM results of the as-prepared 1% Mn doped ZnS/ZnS core/shell quantum dots (ZMS-25). A typical TEM image in low magnification was shown in Fig. 2a. It could be seen that the resultant quantum dots were uniform spherical-shaped particles with a very narrow size distribution. High-resolution transmission electron microscopy (HRTEM) image was shown in Fig. 2b. It demonstrated the high crystallinity of the as-prepared quantum dots, and the distances (0.3174 nm) between the adjacent lattice fringes were the interplanar distances of ZnS (1 1 1) plane, agreeing well with the (1 1 1) d spacing of the literature value, 0.312 nm (JCPDS No. 77-2100). However, the core and the shell of the quantum dots
2.2.2. Synthesis of Mn:ZnS/ZnS (core/shell) d-dots TGAZn and TGAMn stock solutions were prepared by adding Zn (CH3COO)2 2H2O and Mn (CH3COO)2 2H2O into 100 ml of TGA (0.12 M) aqueous solution, respectively. Then the TGAMn stock solution was quickly added into the Na2S solution under vigorously stirring, the reaction mixture was stirred at 80 °C for 20 min to form small-sized MnS core. In order to obtained Mn:ZnS/ZnS core/shell d-dots with different ZnS shell thickness, the TGA-zinc aqueous solution was injected into the MnS core solution at twostep. At the first step, 100%, 75%, 50%, and 25% of TGAZn stock solution was injected into the MnS core solution under vigorously stirring respectively, and heated at 80 °C for 10 h. At the second step, residual TGAZn stock solution was injected and heated at 80 °C for another 2 h. The Mn/Zn molar ratios in the four samples were fixed at 1%. The molar ratio between S-precursor and Zn&Mn precursors was equal to 1.5. The Mn:ZnS/ZnS core/shell quantum dots were obtained by adding excess ethanol to the solutions and then dried in vacuum, and the four different samples with different ZnS shell thickness were marked as ZMS-0, ZMS-25, ZMS-50 and ZMS-75. 2.3. Characterization The X-ray diffraction (XRD) patterns of the synthesized samples were obtained by a D/max-cA diffractometer using Cu Ka radiation
Fig. 1. X-ray diffraction patterns of Mn:ZnS d-dots with different Mn/Zn molar ratio.
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Fig. 2. The high-resolution TEM (HRTEM) images of as-prepared d-dots (ZMS-25).
had similar electron densities and lattice parameters, and the image contrast could not be used to distinguish the shell and core. 3.3. Control of the size of the MnS core In the nucleation-doping strategy, small-sized MnS core was the first key issue for Mn:ZnS d-dots [8]. Fig. 3 showed the UV–Vis absorption spectra of MnS nanocrystals with different S/Mn molar ratio. It was found that the absorption edge gradually shifted from
260 nm to 275 nm with the increase of reaction times when S/Mn molar ratio was 50 (shown in Fig. 3a), indicating that MnS nanocrystals continuously growed. When S/Mn molar ratio was higher than 100, the absorption edge of the MnS nanocrystals were steady around 265 nm (shown in Fig. 3 b, c). In addition, it was found that the large excess of S-precursor should be more suitable for the formation of smaller MnS nanocrystals (shown in Fig. 3d). According to Brus equation [28], the size of MnS nanocrystal was estimated to 1.2 nm when the S/Mn molar ratio was 150.
Fig. 3. Time evolution of the UV–Vis spectra of MnS core with different Mn/S molar ratio. (a) Mn:S = 1:50, (b) Mn:S = 1:50, (c) Mn:S = 1:50 (d) UV–Vis spectra of MnS with different Mn/S molar ratio heated at 20 min.
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Fig. 4 showed the photoluminescence excitation (PLE) spectra of Mn:ZnS d-dots doped with different Mn2+ doping concentration. It was found that the PLE spectra of all the doped samples showed only one emission band at about 340 nm. In addition, PLE intensity of Mn:ZnS d-dots significantly increased with the increase of Mn2+
concentration, and showed a maximum when Mn2+ doping content was 1%. If Mn2+ concentration continued to increase, namely more than 1%, the PLE intensity would decrease. Fig. 5 showed the room temperature photoluminescence (PL) spectra of Mn:ZnS d-dots doped with different Mn2+ concentration under 340 nm excitation. It was found that the PL spectra of undoped samples showed only one emission band at about 460 nm, which could be assigned to
Fig. 4. The photoluminescence excitation (PLE) spectra of Mn:ZnS d-dots doped with different Mn2+ doping concentration.
Fig. 5. Photoluminescence (PL) spectra of Mn:ZnS d-dots with different Mn/Zn molar ratio.
3.4. Photoluminescence (PL) spectra of Mn:ZnS d-dots
Fig. 6. The photoluminescence excitation (PLE) spectra of Mn:ZnS d-dots and Mn:ZnS/ZnS core/shell d-dots with different Mn doping concentration (a) 1.33%; (b) 2%; (c) 4%; (d) the photoluminescence excitation (PLE) spectra of Mn:ZnS d-dots with different ZnS shell thickness.
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the radiative recombination involving defect states in the ZnS quantum dots [29]. For all the doped samples, two different emission bands dominated the PL spectra. The first emission band at about 460 nm also existed in the PL spectrum of the undoped ZnS quantum dots, this emission band should indeed originate from the host ZnS but not from Mn2+ ions. The second emission band was centered at about 585 nm, which was due to 4T1 ? 6A1 transition within the 3d shell of Mn2+ [17]. When Mn2+ ions were incorporated into the ZnS lattice and substituted for host cation sites, the mixing between the s and p electrons of the host ZnS and the d electrons of Mn2+ occurred and made the forbidden transition of 4T1 ? 6A1 partially allowed, resulting in the characteristic emission of Mn2+ [17]. Sooklal et al. [30] found that Mn2+ incorporated into the ZnS lattice led to the Mn2+-based orange emission while ZnS with surface-bound Mn2+ yielded the ultraviolet emission. Thus, it could be concluded that the Mn2+ ions were indeed incorporated into the host ZnS quantum dots by using the nucleation-doping strategy, and when Mn2+ concentration reached a certain level, the concentration quenching was appeared. In addition, many researchers found that concentration quenching for the photoluminescence intensity of Mn2+-doepd ZnS nanocrystals. Sooklal et al. [30] found an optimal Mn2+ concentration of 2%. Khosravi et al. [31] observed a maximum luminescence at a doping concentration of 0.12 wt%. Leeb et al. [32] reported an optimal Mn2+ concentration of 1%. In our work, concentration quenching was also observed in our samples (shown in Fig. 5). It was found that the Mn2+ 4T1 ? 6A1 emission intensity of Mn:ZnS d-dots significantly increased with the increase of Mn2+ concentration, and showed a maximum when Mn2+ doping content was 1%. If Mn2+ concentration continued to increase, namely more than 1%, the Mn2+ 4T1 ? 6A1 emission intensity would decrease.
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3.5. Photoluminescence (PL) spectra of Mn:ZnS/ZnS (core/shell) d-dots 3.5.1. Effect of ZnS shell thickness Although the Mn/Zn molar ratios in the four samples were fixed at 1%, the actual Mn doping concentration in the Mn:ZnS d-dots core varied when the amount of TGA-zinc solution changed at the first step during the preparation of Mn:ZnS/ZnS (core/shell) d-dots with different ZnS shell thickness. The actual Mn doping concentrations were 1%, 1.33%, 2%, and 4% in the Mn:ZnS d-dots cores of the four different samples (ZMS-0, ZMS-25, ZMS-50, and ZMS-75), respectively. Fig. 6 showed the photoluminescence excitation (PLE) spectra of Mn:ZnS d-dots with different ZnS shell thickness. It was found that all of the Mn:ZnS/ZnS (core/shell) ddots with different ZnS shell thickness exhibited significantly increased PLE intensity with respect to that of Mn:ZnS d-dots with the corresponding Mn doping concentrations (shown in Fig. 6a– c), and ZMS-25 exhibited the most significantly increased PLE intensity with respect to that of ZMS-0 (shown in Fig. 6d). Fig. 7 shows the room temperature photoluminescence (PL) spectra of Mn:ZnS d-dots with different ZnS shell thickness under 340 nm excitation. All of the Mn:ZnS/ZnS (core/shell) d-dots with different ZnS shell thickness exhibited significantly increased photoluminescence intensity with respect to that of Mn:ZnS d-dots with the corresponding Mn doping concentrations (shown in Fig. 7a– c). It was assumed that there were a large amount of surface defects which served as nonradiative recombination paths for the excitation energy in bare Mn:ZnS d-dots, which would quench the emission of Mn2+ to some extent [33]. By the growth of an additional ZnS shell on Mn:ZnS d-dots, the emission intensity of Mn2+ in Mn:ZnS/ZnS core/shell d-dots were enhanced due to the elimination of the surface defects [34]. Especially, ZMS-25
Fig. 7. Photoluminescence (PL) spectra of Mn:ZnS d-dots and Mn:ZnS/ZnS core/shell d-dots with different Mn doping concentration (a) 1.33%; (b) 2%; (c) 4% and (d) PL spectra of Mn:ZnS/ZnS core/shell d-dots with different shell thickness.
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Fig. 8. Photoluminescence (PL) spectra of as-prepared d-dots exposure in air at different time: (a) ZMS-0 and (b) ZMS-25.
exhibited the most significantly increased photoluminescence intensity (integrated area) by 45% with respect to that of ZMS-0 (shown in Fig. 7d). 3.5.2. Stability of Mn:ZnS/ZnS (core/shell) d-dots in air Peng et al. [6–7] reported that doped quantum dots (d-dots) (e.g., Mn:ZnSe d-dots) were much less sensitive to thermal and environmental variations. In this work, the stability of the prepared Mn:ZnS/ZnS (core/shell) d-dots solid powders were investigated in air. Fig. 8 showed the PL spectra of the as-prepared d-dots by exposure to air at different time. It was found that the PL gradually reduced in air (shown in Fig. 8a), which indicated that Mn:ZnS d-dots were strongly air sensitive. However, Mn:ZnS/ZnS (core/shell) ddots solid powders synthesized four months ago still retained their bright PL (shown in Fig. 8 b) and not at all air sensitive. 4. Conclusion The water-soluble Mn2+-doped ZnS quantum dots (Mn:ZnS ddots) were prepared through nucleation-doping strategy in aqueous solution in air. It was found that the Mn2+ 4T1 ? 6A1 emission intensity of Mn:ZnS d-dots significantly increased with the increase of Mn2+ concentration, and showed a maximum when Mn2+ doping content was 1.0%. If Mn2+ concentration continued to increase, namely more than 1.0%, the Mn2+ 4T1 ? 6A1 emission intensity would decrease. By the growth of an additional ZnS shell on Mn:ZnS d-dots, the emission intensity of Mn2+ in Mn:ZnS/ZnS core/shell d-dots were enhanced due to the elimination of the surface defects. Mn:ZnS/ZnS (core/shell) d-dots were not at all air sensitive, while Mn:ZnS d-dots were strongly air sensitive. Acknowledgements This work was supported by the Provincial Excellent Ph.D. thesis Research Program of Hunan (No.2004-141). References [1] M. Bruchez Jr., M. Moronne, P. Gin, S. Weiss, A.P. Alivisatos, Science 281 (1998) 2013.
[2] W.C.W. Chan, S. Nie, Science 281 (1998) 2016. [3] B. Dubertret, P. Skourides, D.J. Norris, V. Noireaux, A.H. Brivanlou, A. Libchaber, Science 298 (2002) 1759. [4] X. Michalet, F.F. Pinaud, L.A. Bentolila, J.M. Tsay, S. Doose, J.J. Li, G. Sundaresan, A.M. Wu, S.S. Gambhir, S. Weiss, Science 307 (2005) 538. [5] A.M. Derfus, W.C.W. Chan, S.N. Bhatia, Nano Lett. 4 (2004) 11. [6] N. Pradhan, D. Goorskey, J. Thessing, X. Peng, J. Am. Chem. Soc. 127 (2005) 17586. [7] N. Pradhan, D. Battaglia, Y. Liu, X. Peng, Nano Lett. 7 (2007) 312. [8] N. Pradhan, X. Peng, J. Am. Chem. Soc. 129 (2007) 3339. [9] C.R. Kagan, C.B. Murray, M. Nirmal, M.G. Bawendi, Phys. Rev. Lett. 76 (1996) 1517. [10] M. Achermann, M.A. Petruska, S.A. Crooker, V.I. Klimov, J. Phys. Chem. B 107 (2003) 13782. [11] S.A. Empedocles, D.J. Norris, M.G. Bawendi, Phys. Rev. Lett. 77 (1996) 3873. [12] J.J. Li, Y.A. Wang, W. Guo, J.C. Keay, T.D. Mishima, M.B. Johnson, X. Peng, J. Am. Chem. Soc. 125 (2003) 12567. [13] R.N. Bhargava, J. Lumin. 70 (1996) 85. [14] F. Parsapour, D.F. Kelley, S. Craft, J.P. Wilcoxon, J. Chem. Phys. 104 (1996) 4978. [15] K.E. Waldrip, J.S. Lewis III, Q. Zhai, M.R. Davidson, P.H. Holloway, S.S. Sun, Appl. Phys. Lett. 76 (2000) 1276. [16] J.Q. Zhuang, X.D. Zhang, G. Wang, J. Mater. Chem. 13 (2003) 1853. [17] R.N. Bhargava, D. Gallagher, X. Hong, A. Nurmikko, Phys. Rev. Lett. 72 (1994) 416. [18] J.H. Chung, C.S. Ah, D.J. Jang, J. Phys. Chem. B 105 (2001) 4128. [19] S.J. Xu, S.J. Chua, B. Liu, L.M. Gan, C.H. Chew, G.Q. Xu, Appl. Phys. Lett. 73 (1998) 478. [20] L.X. Cao, J.H. Zhang, S.L. Ren, S.H. Huang, Appl. Phys. Lett. 80 (2002) 4300. [21] H. Mattoussi, J.M. Mauro, E.R. Goldman, G.P. Anderson, V.C. Sunder, F.V. Mikulec, M.G. Bewendi, J. Am. Chem. Soc. 122 (2000) 12142. [22] D.V. Talapin, A.L. Rogach, E.V. Shevchenko, A. Kornowski, M. Haase, H. Weller, J. Am. Chem. Soc. 124 (2002) 5782. [23] H. Yang, P.H. Holloway, G. Cunningham, K.S. Schanze, J. Chem. Phys. 121 (2004) 10233. [24] L.X. Cao, S.H. Huang, S.Z. Lü, J.L. Lin, J. Colloid. Int. Sci. 284 (2005) 516. [25] N. Karar, H. Chander, S.M. Shivaprasad, Appl. Phys. Lett. 85 (2004) 5058. [26] D.X. Jiang, L.X. Cao, G. Su, H. Qu, D.K. Sun, Appl. Surf. Sci. 253 (2007) 9330. [27] H.P. Klong, L.F. Alexander, X-ray Diffraction Procedures for Crystalline and Amorphous Materials, Wiley, New York, 1954. [28] Louis Brus, J. Phys. Chem. 90 (1986) 2555. [29] A.A. Khosravi, M. Kundu, L. Jatwa, S.K. Deshpande, U.A. Bhagwat, M. Sastry, S.K. Kulkarni, Appl. Phys. Lett. 67 (1995) 2702. [30] K. Sooklal, B.S. Cullum, S.M. Angel, C.J. Murphy, J. Phys. Chem. 100 (1996) 4551. [31] A.A. Khosravi, M. Kundu, B.A. Kuruvilla, G.S. Shekhawat, R.P. Gupta, A.K. Sharma, P.D. Vyas, S.K. Kulkarni, Appl. Phys. Lett. 67 (1995) 2506. [32] J. Leeb, V. Gebhardt, G. Mu1ller, D. Haarer, D. Su, M. Giersig, G. McMahon, L. Spanhel, J. Phys. Chem. B 103 (1999) 7839. [33] H. Yang, P.H. Holloway, Adv. Funct. Mater. 14 (2004) 152. [34] Z.L. Wang, Z.W. Quan, P.Y. Jia, C.K. Lin, Y. Luo, Y. Chen, J. Fang, W. Zhou, C.J. O’Connor, J. Lin, Chem. Mater. 18 (2006) 2030.
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Synthesis and photoluminescence of water-soluble Mn:ZnS/ZnS core/shell quantum dots using nucleation-doping strategy Qi Xiao *, Chong Xiao School of Resources Processing and Bioengineering, Central South University, Changsha 410083, China
a r t i c l e
i n f o
Article history: Received 14 April 2008 Received in revised form 13 June 2008 Accepted 18 June 2008 Available online 3 August 2008 Keywords: Mn:ZnS/ZnS (core/shell) quantum dots Nucleation-doping strategy Water-soluble Photoluminescence
a b s t r a c t The water-soluble Mn:ZnS/ZnS (core/shell) quantum dots were synthesized through nucleation-doping strategy in aqueous solutions in air, and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and photoluminescence (PL) spectra. It was found that the Mn2+ 4T1 ? 6A1 emission intensity of Mn:ZnS d-dots significantly increased with the increase of Mn2+ concentration, and showed a maximum when Mn2+ doping content was 1.0%. If Mn2+ concentration continued to increase, namely more than 1.0%, the Mn2+ 4T1 ? 6A1 emission intensity would decrease. By the growth of an additional ZnS shell on Mn:ZnS d-dots, the emission intensity of Mn2+ in Mn:ZnS/ZnS core/shell d-dots were enhanced due to the elimination of the surface defects. Mn:ZnS/ZnS (core/shell) d-dots were not at all air sensitive, while Mn:ZnS d-dots were strongly air sensitive. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Semiconductor quantum dots, or quantum dots (q-dots), with CdSe ones as the workhorse, have been widely explored as biomedical labeling reagents since 1998 [1–4]. However, the small-ensemble Stokes shift of intrinsic q-dot emitters made self-quenching. In addition, experimental results indicated that any leakage of cadmium from the quantum dots would be toxic and fatal to a biological system [5], and cadmium-containing products were eventually environmentally problematic. Recently, Peng et al. [6–8] reported that doped quantum dots (d-dots) (e.g., Mn:ZnSe d-dots) could not only replace cadmium in CdSe quantum dots with zinc, but also overcome a couple of intrinsic disadvantages of undoped quantum dots (q-dots) emitters, that is, strong self-quenching caused by their small ensemble stokes shift (energy difference between absorption spectrum and emission band) [9,10] and sensitivity to thermal, chemical, and photochemical disturbances [11,12]. Mn2+-doped ZnS quantum dots have been extensively investigated for use in various applications other than biomedical labeling, such as displays, sensors, and lasers [13–15]. In addition, the luminescent lifetime of Mn2+-doped ZnS quantum dots was ca. 1 ms. Such a long lifetime made the luminescence from the nanocrystal readily distinguishable from the background luminescence. Therefore, Mn2+-doped ZnS quantum dots could be potential candidates as fluorescent labeling agents, especially in biology [16]. Many approaches have been explored for the preparation of Mn2+-doped ZnS quantum dots (Mn:ZnS d-dots), including organo* Corresponding author. Tel.: +86 731 8830543; fax: +86 731 8879815. E-mail address: [email protected] (Q. Xiao). 0925-3467/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2008.06.010
metallic method [17], chemical precipitation method [18], microemulsion technique [19], reverse micelle method [20], etc. The traditional synthesis strategies of Mn:ZnS d-dots were based on a reaction system with both dopant ions and competitive host ions in it, which was difficult to control and optimize. To meet this challenge, Peng et al. [6–8] recently developed a new synthetic strategy, nucleation-doping, to prepare Mn:ZnSe d-dots through organometallic routes. The key feature of the nucleation-doping strategy was decoupling the doping from nucleation, which allowed for the growth of d-dots in a controllable fashion, thus yielding d-dots with pure dopant emission at a high efficiency. To be a suitable bio-labeling agent, the quantum dots should have high luminescent efficiency, water-solubilization, and biocompatibility. Water-soluble semiconductor quantum dots could be obtained mainly by two different methods. The first way was to replace the surface-capping molecules on the particles prepared by the TOPO (trioctylphosphine oxide) method with water-soluble thiols [21]. However, after the substitution of the surface-capping molecules by hydrophilic molecules, the nanoparticle photoluminescence decreased markedly [21]. The second method was to directly synthesize semiconductor quantum dots in aqueous solution using water-soluble stabilizers [22]. The second method became a popular recipe for making water-soluble nanoparticles. Therefore, a facile aqueous nucleation-doping strategy to obtain water-soluble Mn:ZnS d-dots were highly desired. On the other hand, the passivation of surface was of crucial importance for the applications of semiconductor quantum dots. In order to get a better passivation, inorganically passivated (or core/shell structured) quantum dots have been developed and shown dramatically enhanced properties [23]. Photoenhanced
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luminescence was observed in ZnS/CdS/ZnS quantum well [24], ZnS:Mn/ZnS nanoparticles [20], ZnS:Mn/ZnO nanophosphors [25], and ZnS:Mn/Zn(OH)2 nanocrystal [26], etc. Especially, formation of ZnS:Mn2+/ZnS core/shell nanocrystals resulted in obvious enhancement in the PL intensity with respect to that of bare ZnS:Mn2+ nanocrystals due to the good match between core (Mn:ZnS) and shell (ZnS) lattice constants and the effective elimination of the surface defects [20]. In this paper, we introduced the nucleation-doping strategy to synthesize Mn:ZnS/ZnS (core/shell) quantum dots in aqueous solution in air. By the growth of an additional ZnS shell on Mn:ZnS d-dots, the emission intensity of Mn2+ in Mn:ZnS/ZnS core/shell d-dots were enhanced due to the elimination of the surface defects. In addition, Mn:ZnS/ZnS (core/shell) d-dots were not at all air sensitive, while Mn:ZnS d-dots were strongly air sensitive. 2. Experimental section 2.1. Chemicals All chemicals used were of analytical grade. Zn (CH3COO)2 2H2O, Mn(CH3COO)2 2H2O, Na2S 9H2O, and thioglycolic acid (TGA) were obtained from Shanghai Chemical Reagents Company and used as received. High-purity water with a resistivity of 18.2 MX/cm was used for preparation of all aqueous solutions. 2.2. Synthesis 2.2.1. Synthesis of Mn:ZnS d-dots TGAZn and TGAMn stock solutions were prepared by adding Zn (CH3COO)2 2H2O and Mn (CH3COO)2 2H2O into 100 ml of TGA (0.12 M) aqueous solution, respectively. Then the different volume of TGAMn stock solution was quickly added into the Na2S solution under vigorously stirring, the reaction mixture was stirred at 80 °C for 20 min to form small-sized MnS core. Next, the TGAZn stock solution was quickly injected into the above reaction mixture under vigorously stirring, and then the reaction mixture was heated at 80 °C for 12 h. The final molar ratio of Mn/Zn was varied from 0 to 3%. The molar ratio between S-precursor and Zn and Mn precursors was equal to 1.5. The Mn:ZnS d-dots solid powders were obtained by adding excess ethanol to the solutions and then dried in vacuum.
(k = 0.15418 nm). Transmission electron microscopy (TEM) images were obtained using a JEM 3010 high-resolution transmission electron microscope. UV–Vis absorption spectra were recorded with a UV–Vis scanning spectrophotometer (Shimadzu UV-2450). The photoluminescence (PL) spectra of the samples in solid form were recorded with a fluorescence spectrophotometer F-4500. 3. Results and discussion 3.1. XRD analysis The XRD patterns of Mn:ZnS d-dots were shown in Fig. 1. These diffraction features appearing at 28.5°, 47.5°, and 56.3° corresponded to the (1 1 1), (2 2 0), and (3 1 1) planes of cubic zinc blende structure, which was very consistent with the values in the standard card (JCPDS No. 77-2100). No characteristic peaks of impurity phases are observed in the XRD pattern, indicating the high purity of the final products. In addition, the broadening of the diffraction peaks of all the Mn:ZnS d-dots were obvious, which was characteristic of nanocrystal. The averaged crystallite sizes D was determined according to the Scherrer equation D = kk/bcos h [27], where k was a constant (shape factor, about 0.9), k was the X-ray wavelength (0.15418 nm), b was the full width at half maximum (FWHM) of the diffraction line, and h was the diffraction angle. Based on the FWHM of (1 1 1) zinc blende reflection, the averaged crystallite sizes of Mn:ZnS d-dots doped with different Mn2+ concentration (0%, 1% and 3%) were estimated to be 3.96, 4.08 and 4.11 nm, respectively. 3.2. TEM analysis Fig. 2 showed the TEM results of the as-prepared 1% Mn doped ZnS/ZnS core/shell quantum dots (ZMS-25). A typical TEM image in low magnification was shown in Fig. 2a. It could be seen that the resultant quantum dots were uniform spherical-shaped particles with a very narrow size distribution. High-resolution transmission electron microscopy (HRTEM) image was shown in Fig. 2b. It demonstrated the high crystallinity of the as-prepared quantum dots, and the distances (0.3174 nm) between the adjacent lattice fringes were the interplanar distances of ZnS (1 1 1) plane, agreeing well with the (1 1 1) d spacing of the literature value, 0.312 nm (JCPDS No. 77-2100). However, the core and the shell of the quantum dots
2.2.2. Synthesis of Mn:ZnS/ZnS (core/shell) d-dots TGAZn and TGAMn stock solutions were prepared by adding Zn (CH3COO)2 2H2O and Mn (CH3COO)2 2H2O into 100 ml of TGA (0.12 M) aqueous solution, respectively. Then the TGAMn stock solution was quickly added into the Na2S solution under vigorously stirring, the reaction mixture was stirred at 80 °C for 20 min to form small-sized MnS core. In order to obtained Mn:ZnS/ZnS core/shell d-dots with different ZnS shell thickness, the TGA-zinc aqueous solution was injected into the MnS core solution at twostep. At the first step, 100%, 75%, 50%, and 25% of TGAZn stock solution was injected into the MnS core solution under vigorously stirring respectively, and heated at 80 °C for 10 h. At the second step, residual TGAZn stock solution was injected and heated at 80 °C for another 2 h. The Mn/Zn molar ratios in the four samples were fixed at 1%. The molar ratio between S-precursor and Zn&Mn precursors was equal to 1.5. The Mn:ZnS/ZnS core/shell quantum dots were obtained by adding excess ethanol to the solutions and then dried in vacuum, and the four different samples with different ZnS shell thickness were marked as ZMS-0, ZMS-25, ZMS-50 and ZMS-75. 2.3. Characterization The X-ray diffraction (XRD) patterns of the synthesized samples were obtained by a D/max-cA diffractometer using Cu Ka radiation
Fig. 1. X-ray diffraction patterns of Mn:ZnS d-dots with different Mn/Zn molar ratio.
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Fig. 2. The high-resolution TEM (HRTEM) images of as-prepared d-dots (ZMS-25).
had similar electron densities and lattice parameters, and the image contrast could not be used to distinguish the shell and core. 3.3. Control of the size of the MnS core In the nucleation-doping strategy, small-sized MnS core was the first key issue for Mn:ZnS d-dots [8]. Fig. 3 showed the UV–Vis absorption spectra of MnS nanocrystals with different S/Mn molar ratio. It was found that the absorption edge gradually shifted from
260 nm to 275 nm with the increase of reaction times when S/Mn molar ratio was 50 (shown in Fig. 3a), indicating that MnS nanocrystals continuously growed. When S/Mn molar ratio was higher than 100, the absorption edge of the MnS nanocrystals were steady around 265 nm (shown in Fig. 3 b, c). In addition, it was found that the large excess of S-precursor should be more suitable for the formation of smaller MnS nanocrystals (shown in Fig. 3d). According to Brus equation [28], the size of MnS nanocrystal was estimated to 1.2 nm when the S/Mn molar ratio was 150.
Fig. 3. Time evolution of the UV–Vis spectra of MnS core with different Mn/S molar ratio. (a) Mn:S = 1:50, (b) Mn:S = 1:50, (c) Mn:S = 1:50 (d) UV–Vis spectra of MnS with different Mn/S molar ratio heated at 20 min.
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Fig. 4 showed the photoluminescence excitation (PLE) spectra of Mn:ZnS d-dots doped with different Mn2+ doping concentration. It was found that the PLE spectra of all the doped samples showed only one emission band at about 340 nm. In addition, PLE intensity of Mn:ZnS d-dots significantly increased with the increase of Mn2+
concentration, and showed a maximum when Mn2+ doping content was 1%. If Mn2+ concentration continued to increase, namely more than 1%, the PLE intensity would decrease. Fig. 5 showed the room temperature photoluminescence (PL) spectra of Mn:ZnS d-dots doped with different Mn2+ concentration under 340 nm excitation. It was found that the PL spectra of undoped samples showed only one emission band at about 460 nm, which could be assigned to
Fig. 4. The photoluminescence excitation (PLE) spectra of Mn:ZnS d-dots doped with different Mn2+ doping concentration.
Fig. 5. Photoluminescence (PL) spectra of Mn:ZnS d-dots with different Mn/Zn molar ratio.
3.4. Photoluminescence (PL) spectra of Mn:ZnS d-dots
Fig. 6. The photoluminescence excitation (PLE) spectra of Mn:ZnS d-dots and Mn:ZnS/ZnS core/shell d-dots with different Mn doping concentration (a) 1.33%; (b) 2%; (c) 4%; (d) the photoluminescence excitation (PLE) spectra of Mn:ZnS d-dots with different ZnS shell thickness.
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the radiative recombination involving defect states in the ZnS quantum dots [29]. For all the doped samples, two different emission bands dominated the PL spectra. The first emission band at about 460 nm also existed in the PL spectrum of the undoped ZnS quantum dots, this emission band should indeed originate from the host ZnS but not from Mn2+ ions. The second emission band was centered at about 585 nm, which was due to 4T1 ? 6A1 transition within the 3d shell of Mn2+ [17]. When Mn2+ ions were incorporated into the ZnS lattice and substituted for host cation sites, the mixing between the s and p electrons of the host ZnS and the d electrons of Mn2+ occurred and made the forbidden transition of 4T1 ? 6A1 partially allowed, resulting in the characteristic emission of Mn2+ [17]. Sooklal et al. [30] found that Mn2+ incorporated into the ZnS lattice led to the Mn2+-based orange emission while ZnS with surface-bound Mn2+ yielded the ultraviolet emission. Thus, it could be concluded that the Mn2+ ions were indeed incorporated into the host ZnS quantum dots by using the nucleation-doping strategy, and when Mn2+ concentration reached a certain level, the concentration quenching was appeared. In addition, many researchers found that concentration quenching for the photoluminescence intensity of Mn2+-doepd ZnS nanocrystals. Sooklal et al. [30] found an optimal Mn2+ concentration of 2%. Khosravi et al. [31] observed a maximum luminescence at a doping concentration of 0.12 wt%. Leeb et al. [32] reported an optimal Mn2+ concentration of 1%. In our work, concentration quenching was also observed in our samples (shown in Fig. 5). It was found that the Mn2+ 4T1 ? 6A1 emission intensity of Mn:ZnS d-dots significantly increased with the increase of Mn2+ concentration, and showed a maximum when Mn2+ doping content was 1%. If Mn2+ concentration continued to increase, namely more than 1%, the Mn2+ 4T1 ? 6A1 emission intensity would decrease.
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3.5. Photoluminescence (PL) spectra of Mn:ZnS/ZnS (core/shell) d-dots 3.5.1. Effect of ZnS shell thickness Although the Mn/Zn molar ratios in the four samples were fixed at 1%, the actual Mn doping concentration in the Mn:ZnS d-dots core varied when the amount of TGA-zinc solution changed at the first step during the preparation of Mn:ZnS/ZnS (core/shell) d-dots with different ZnS shell thickness. The actual Mn doping concentrations were 1%, 1.33%, 2%, and 4% in the Mn:ZnS d-dots cores of the four different samples (ZMS-0, ZMS-25, ZMS-50, and ZMS-75), respectively. Fig. 6 showed the photoluminescence excitation (PLE) spectra of Mn:ZnS d-dots with different ZnS shell thickness. It was found that all of the Mn:ZnS/ZnS (core/shell) ddots with different ZnS shell thickness exhibited significantly increased PLE intensity with respect to that of Mn:ZnS d-dots with the corresponding Mn doping concentrations (shown in Fig. 6a– c), and ZMS-25 exhibited the most significantly increased PLE intensity with respect to that of ZMS-0 (shown in Fig. 6d). Fig. 7 shows the room temperature photoluminescence (PL) spectra of Mn:ZnS d-dots with different ZnS shell thickness under 340 nm excitation. All of the Mn:ZnS/ZnS (core/shell) d-dots with different ZnS shell thickness exhibited significantly increased photoluminescence intensity with respect to that of Mn:ZnS d-dots with the corresponding Mn doping concentrations (shown in Fig. 7a– c). It was assumed that there were a large amount of surface defects which served as nonradiative recombination paths for the excitation energy in bare Mn:ZnS d-dots, which would quench the emission of Mn2+ to some extent [33]. By the growth of an additional ZnS shell on Mn:ZnS d-dots, the emission intensity of Mn2+ in Mn:ZnS/ZnS core/shell d-dots were enhanced due to the elimination of the surface defects [34]. Especially, ZMS-25
Fig. 7. Photoluminescence (PL) spectra of Mn:ZnS d-dots and Mn:ZnS/ZnS core/shell d-dots with different Mn doping concentration (a) 1.33%; (b) 2%; (c) 4% and (d) PL spectra of Mn:ZnS/ZnS core/shell d-dots with different shell thickness.
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Fig. 8. Photoluminescence (PL) spectra of as-prepared d-dots exposure in air at different time: (a) ZMS-0 and (b) ZMS-25.
exhibited the most significantly increased photoluminescence intensity (integrated area) by 45% with respect to that of ZMS-0 (shown in Fig. 7d). 3.5.2. Stability of Mn:ZnS/ZnS (core/shell) d-dots in air Peng et al. [6–7] reported that doped quantum dots (d-dots) (e.g., Mn:ZnSe d-dots) were much less sensitive to thermal and environmental variations. In this work, the stability of the prepared Mn:ZnS/ZnS (core/shell) d-dots solid powders were investigated in air. Fig. 8 showed the PL spectra of the as-prepared d-dots by exposure to air at different time. It was found that the PL gradually reduced in air (shown in Fig. 8a), which indicated that Mn:ZnS d-dots were strongly air sensitive. However, Mn:ZnS/ZnS (core/shell) ddots solid powders synthesized four months ago still retained their bright PL (shown in Fig. 8 b) and not at all air sensitive. 4. Conclusion The water-soluble Mn2+-doped ZnS quantum dots (Mn:ZnS ddots) were prepared through nucleation-doping strategy in aqueous solution in air. It was found that the Mn2+ 4T1 ? 6A1 emission intensity of Mn:ZnS d-dots significantly increased with the increase of Mn2+ concentration, and showed a maximum when Mn2+ doping content was 1.0%. If Mn2+ concentration continued to increase, namely more than 1.0%, the Mn2+ 4T1 ? 6A1 emission intensity would decrease. By the growth of an additional ZnS shell on Mn:ZnS d-dots, the emission intensity of Mn2+ in Mn:ZnS/ZnS core/shell d-dots were enhanced due to the elimination of the surface defects. Mn:ZnS/ZnS (core/shell) d-dots were not at all air sensitive, while Mn:ZnS d-dots were strongly air sensitive. Acknowledgements This work was supported by the Provincial Excellent Ph.D. thesis Research Program of Hunan (No.2004-141). References [1] M. Bruchez Jr., M. Moronne, P. Gin, S. Weiss, A.P. Alivisatos, Science 281 (1998) 2013.
[2] W.C.W. Chan, S. Nie, Science 281 (1998) 2016. [3] B. Dubertret, P. Skourides, D.J. Norris, V. Noireaux, A.H. Brivanlou, A. Libchaber, Science 298 (2002) 1759. [4] X. Michalet, F.F. Pinaud, L.A. Bentolila, J.M. Tsay, S. Doose, J.J. Li, G. Sundaresan, A.M. Wu, S.S. Gambhir, S. Weiss, Science 307 (2005) 538. [5] A.M. Derfus, W.C.W. Chan, S.N. Bhatia, Nano Lett. 4 (2004) 11. [6] N. Pradhan, D. Goorskey, J. Thessing, X. Peng, J. Am. Chem. Soc. 127 (2005) 17586. [7] N. Pradhan, D. Battaglia, Y. Liu, X. Peng, Nano Lett. 7 (2007) 312. [8] N. Pradhan, X. Peng, J. Am. Chem. Soc. 129 (2007) 3339. [9] C.R. Kagan, C.B. Murray, M. Nirmal, M.G. Bawendi, Phys. Rev. Lett. 76 (1996) 1517. [10] M. Achermann, M.A. Petruska, S.A. Crooker, V.I. Klimov, J. Phys. Chem. B 107 (2003) 13782. [11] S.A. Empedocles, D.J. Norris, M.G. Bawendi, Phys. Rev. Lett. 77 (1996) 3873. [12] J.J. Li, Y.A. Wang, W. Guo, J.C. Keay, T.D. Mishima, M.B. Johnson, X. Peng, J. Am. Chem. Soc. 125 (2003) 12567. [13] R.N. Bhargava, J. Lumin. 70 (1996) 85. [14] F. Parsapour, D.F. Kelley, S. Craft, J.P. Wilcoxon, J. Chem. Phys. 104 (1996) 4978. [15] K.E. Waldrip, J.S. Lewis III, Q. Zhai, M.R. Davidson, P.H. Holloway, S.S. Sun, Appl. Phys. Lett. 76 (2000) 1276. [16] J.Q. Zhuang, X.D. Zhang, G. Wang, J. Mater. Chem. 13 (2003) 1853. [17] R.N. Bhargava, D. Gallagher, X. Hong, A. Nurmikko, Phys. Rev. Lett. 72 (1994) 416. [18] J.H. Chung, C.S. Ah, D.J. Jang, J. Phys. Chem. B 105 (2001) 4128. [19] S.J. Xu, S.J. Chua, B. Liu, L.M. Gan, C.H. Chew, G.Q. Xu, Appl. Phys. Lett. 73 (1998) 478. [20] L.X. Cao, J.H. Zhang, S.L. Ren, S.H. Huang, Appl. Phys. Lett. 80 (2002) 4300. [21] H. Mattoussi, J.M. Mauro, E.R. Goldman, G.P. Anderson, V.C. Sunder, F.V. Mikulec, M.G. Bewendi, J. Am. Chem. Soc. 122 (2000) 12142. [22] D.V. Talapin, A.L. Rogach, E.V. Shevchenko, A. Kornowski, M. Haase, H. Weller, J. Am. Chem. Soc. 124 (2002) 5782. [23] H. Yang, P.H. Holloway, G. Cunningham, K.S. Schanze, J. Chem. Phys. 121 (2004) 10233. [24] L.X. Cao, S.H. Huang, S.Z. Lü, J.L. Lin, J. Colloid. Int. Sci. 284 (2005) 516. [25] N. Karar, H. Chander, S.M. Shivaprasad, Appl. Phys. Lett. 85 (2004) 5058. [26] D.X. Jiang, L.X. Cao, G. Su, H. Qu, D.K. Sun, Appl. Surf. Sci. 253 (2007) 9330. [27] H.P. Klong, L.F. Alexander, X-ray Diffraction Procedures for Crystalline and Amorphous Materials, Wiley, New York, 1954. [28] Louis Brus, J. Phys. Chem. 90 (1986) 2555. [29] A.A. Khosravi, M. Kundu, L. Jatwa, S.K. Deshpande, U.A. Bhagwat, M. Sastry, S.K. Kulkarni, Appl. Phys. Lett. 67 (1995) 2702. [30] K. Sooklal, B.S. Cullum, S.M. Angel, C.J. Murphy, J. Phys. Chem. 100 (1996) 4551. [31] A.A. Khosravi, M. Kundu, B.A. Kuruvilla, G.S. Shekhawat, R.P. Gupta, A.K. Sharma, P.D. Vyas, S.K. Kulkarni, Appl. Phys. Lett. 67 (1995) 2506. [32] J. Leeb, V. Gebhardt, G. Mu1ller, D. Haarer, D. Su, M. Giersig, G. McMahon, L. Spanhel, J. Phys. Chem. B 103 (1999) 7839. [33] H. Yang, P.H. Holloway, Adv. Funct. Mater. 14 (2004) 152. [34] Z.L. Wang, Z.W. Quan, P.Y. Jia, C.K. Lin, Y. Luo, Y. Chen, J. Fang, W. Zhou, C.J. O’Connor, J. Lin, Chem. Mater. 18 (2006) 2030.