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Co-precipitation preparation, characterization and optical properties of blue CaSb2O6: Bi3+ nano-phosphor
Materials Letters 102–103 (2013) 59–61
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Co-precipitation preparation, characterization and optical properties of blue CaSb2O6: Bi3+ nano-phosphor Liumin Chen, Yumei Long, Yumin Qin, Weifeng Li n College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China
art ic l e i nf o
a b s t r a c t
Article history: Received 30 January 2013 Accepted 22 March 2013 Available online 30 March 2013
Bi3+ doped CaSb2O6 nanoparticles were prepared by the co-precipitation technique followed by heattreatment. The as-prepared samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and photoluminescence (PL) spectra. It was found that the CaSb2O6:xBi nano-crystals exist in the hexagonal phase, and show sphere-like shape with diameters of 60–80 nm. The CaSb2O6:xBi exhibited a strong emission band with peak wavelength of 437 nm at room temperature, which can be effectively excited both at 295 and 336 nm. It is believed that the formation of defect clusters such as ½ðBiCa Þ−ðV Ca Þ′ and/or ½ðBiCa Þ−ðV Ca Þ−ðBiCa Þ would play an important role on the optical properties of CaSb2O6:xBi phosphors. The intense blue emission and a higher quenching concentration suggested the potential application of nano-scaled CaSb2O6:xBi for lighting and display devices. & 2013 Elsevier B.V. All rights reserved.
Keywords: CaSb2O6:Bi Nanoparticles Phosphors Defects
1. Introduction The development of lighting and display systems with higher resolution and higher luminous efficiency continues to spur the search for novel luminescence materials. Great efforts have been dedicated to rare-earth ions as optical activators that present stable emission due to f–f electron transition. As a matter of fact, main group metal ions with s2 configuration are also excellent luminescence centers and attractive luminescence properties have been found [1,2]. The Bi3+ ion is of great importance as a luminescence activator [2–5] and its emission is attributed to the electron transition between the 6s2 ground states and the 6s6p excited states. Because of the strong interaction with the surrounding lattice and energy transfer from Bi3+ to Bi3+ or to another activator in different host lattices, the emission spectra of Bi3+ ions presents a broad band and the peak position of the band varies from the UV region to the red region with differing host lattices [4–7]. It is therefore expected that favorable optical properties can be achieved by doping Bi3+ ion into appropriate matrixes. The preparation and luminescence properties of MSb2O6:Bi (M ¼Ca, Sr) have been reported previously [8–10]. The only reported synthetic method is the solid state reaction. Due to the volatility of Bi2O3 and the tendency of Bi3+ to be oxidized to Bi5+ in the conventional sintering process, the visible emission quenching of MSb2O6:Bi (M ¼Ca, Sr) at room temperature occurred [8,11].
n
Corresponding author. Tel.: +86 512 65880089. E-mail address: [email protected] (W. Li).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.03.109
This work reported on the synthesis of CaSb2O6:Bi3+ phosphors by the co-precipitation method, which claims for low calcination temperature, short reaction time and mass-produced [12,13]. The optical properties of the as-synthesized samples were studied and the luminescence mechanism was also discussed.
2. Experimental The CaSb2O6:xBi (x ¼0, 0.0025, 0.005, 0.0075, 0.01, 0.02, 0.06, and 0.08) nanoparticles were prepared using the co-precipitation method. The starting materials were Bi2O3, SbCl3, CaCl2 2H2O, NaOH, and Na2C2O4, and they were all in analytical grade. In a typical synthesis procedure of CaSb2O6:0.01Bi phosphor, 0.0117 g Bi2O3 was dissolved in 25 mL hydrochloric acid with stirring and moderate heating. Then 0.7241 g of CaCl2 2H2O and 2.2812 g of SbCl3 were added sequentially into the obtained BiCl3 solution under stirring to form a clear and homogeneous mixture. The mixture solution was added dropwise into 80 mL of aqueous solution containing 3.52 g of NaOH and 0.737 g of Na2C2O4 with continuous stirring, and the white precipitation was obtained. After aging, filtration, washing and drying, the white precursor was obtained. Finally, the precursor was calcined at 1000 1C for 4 h in air. The structure of the samples was characterized by XRD on a X'Pert Pro MPD diffractometer (Pananalytical, Holland) using CuKα radiation (λ¼1.5406 Å). The SEM images were taken on Hitachi s-4700. The excitation and emission spectra were recorded on an F-2500 fluorescence spectrometer (Hitachi, Japan) at room temperature.
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L. Chen et al. / Materials Letters 102–103 (2013) 59–61
3. Results and discussion Fig. 1 shows the XRD patterns of the as-prepared samples doped with Bi3+ at different concentrations. All peaks of the asprepared samples agree well with the hexagonal structure (JCPDS Card no. 46-1496), indicating that the doped Bi3+ does not change the crystal structure or induce a new phase. The diffraction peaks shift to lower theta values with introducing Bi (inset of Fig. 1) because of the larger radius of Bi3+ (1.03 Å) compared to that of Ca2+ (1 Å). Furthermore, it seems that Bi3+ ions have a larger solubility in CaSb2O6 prepared by co-precipitation here than that by the solid state reaction [8]. Fig. 2a and b presents SEM images of
Fig. 1. XRD patterns of CaSb2O6:xBi (x ¼ 0, 0.0025, 0.0075, 0.02, and 0.08). Inset is the amplification of XRD patterns around 26.51.
Fig. 2. SEM images of (a) pure CaSb2O6 and (b) CaSb2O6:0.0075Bi.
the pure and 0.75 mol% Bi-doped CaSb2O6 samples, respectively, which display that both the samples have uniform sphere-like shape with particle size of 60–80 nm. Excitation spectra of CaSb2O6:xBi phosphors via monitoring 437 nm emission are presented in Fig. 3. The excitation spectra of Bi3+-doped samples consist of three broad bands and they are located at 244, 295 and 336 nm. These absorption bands can be attributed to the Bi3+-related absorption due to the their absence in undoped CaSb2O6 sample. The intensities of all the excitation bands are dependent on the Bi3+ concentration (inset of Fig. 3). The ground state of Bi3+ is 1S0 and its excited states are 3P0, 3P1, 3P2 and 1P1 in order of increasing energy [14]. Due to the complete spin forbiddance of the transition from 1S0 to 3P0 and 3P2, optical absorption is ascribed to the electron transition from 1S0 to 3P1 and 1 P1 [15]. It is reported that the weak excitation band peaking at 244 nm is assigned to the 1S0–1P1 transition [8]. In this work, the visible emission can effectively be excited at both 295 and 336 nm at room temperature, which is different from previous reports [8,10]. The excitation band at 295 nm is readily assigned to the host lattice excitation because it agrees well with the band gap of CaSb2O6. The appearance of the excitation band of the host lattice in the excitation spectrum of Bi3+ indicates that there exists energy transfer from host lattice to Bi3+ ions. The dominant bands peaking at 336 nm have bigger half-widths than those of higherenergy excitation bands and their peaks shift toward lower energy with increasing Bi3+ concentration. This absorption band is attributed to 1S0–3P1 transition. Here, it is worth noting that the optimal excitation intensities were achieved in CaSb2O6:0.06Bi and CaSb2O6:0.0075Bi corresponding to the excitation wavelengths of 295 and 336 nm, respectively. Fig. 4 shows the emission spectra of CaSb2O6:xBi (0≤x≤0.08) phosphors excited at 336 nm, which exhibit strong blue emission bands centered around 437 nm. The visible emission was ascribed to 3P1–1S0 transition of Bi3+ pairs [8]. In fact, here the visible emission is a superposition of several emission bands. The inset of Fig. 4 displays two Gaussian components for CaSb2O6:0.0075Bi with approximate peak wavelengths of 435 and 464 nm. The maximal emission intensity is observed in the phosphor doped with 0.75 mol% Bi3+. It is believed that defects may play an important role on the optical properties of CaSb2O6:xBi phosphors. As for the compound CaSb2O6, there is only one crystallographic Ca2+ site for Bi3+. The defect of ðBiCa Þd will be formed when Bi3+ was incorporated on Ca2 + site. Considering the charge compensation of ðBiCa Þd , some other
Fig. 3. Excitation spectra of CaSb2O6:xBi via monitoring 437 nm emission.
L. Chen et al. / Materials Letters 102–103 (2013) 59–61
61
which is one of the phenomenological characteristics of the variation of the crystal field strength [16].
4. Conclusions Nano-scaled CaSb2O6:xBi with sphere-like morphology has been successfully synthesized by the co-precipitation method. The as-prepared phosphors exhibit an intense blue emission at room temperature. The blue emission is a superposition of at least two emission bands and it could ascribe to the defect perturbation. The optimal excitation intensities of 295 nm and 336 nm are achieved in CaSb2O6:0.06Bi and in CaSb2O6:0.0075Bi, respectively. It is believed that the defect clusters such as ½ðBiCa Þ−ðV Ca Þ′ and ½ðBiCa Þ−ðV Ca Þ−ðBiCa Þ play important roles on the optical properties of CaSb2O6:Bi. The experimental results suggested that the co-precipitation method is favorable for preparing the CaSb2O6: Bi with desired optical properties.
Fig. 4. Emission spectra of CaSb2O6:xBi excited at 336 nm.
defects such as cation vacancies ðV Ca Þ″ have to be formed. Upon excitation of the host lattice (295 nm), the generated electrons and holes are trapped by a donor [like ðBiCa Þd ] and an acceptor [likeðV Ca Þ″], respectively. Following recombination, the energy is transferred from the donor-acceptor pair to Bi3+. Since defects have net charge, they tend to pair under thermal equilibrium and form ½ðBiCa Þ−ðV Ca Þ′ and/or ½ðBiCa Þ−ðV Ca Þ−ðBiCa Þ clusters. These defect-perturbed Bi3+ ions may result in the emission with multiple-peak structure of CaSb2O6:xBi phosphors. However, it needs further theoretical and experimental results to support. Red shift in the emission spectra excited at 336 nm and its corresponding excitation spectra are both observed for heavily Bi-doped (x≥0.02) samples. The change of crystal field strength could contribute to such a red shift. Due to the difference of ionic radius and charge between Bi3+ and Ca2+ ions, the incorporation of Bi3+ into the lattice would cause the expansion of the unit cell, and consequently changes in bond length, covalency, and possible symmetry. Fig. 3 shows the significant changes in the position and shape of the excitation band in the range of 300–400 nm,
Acknowledgments This work was supported by the National NSF of China (21005053) and the Priority Academic Program Development of Jiangsu Higher Education Institutions. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
Boutinaud P, Cavalli E. Chem Phys Lett 2011;503:239–43. Li JH, Liu J, Yu XB. J Alloys Compd 2012;509:9897–900. Angiuli F, Cavalli E, Belltti A. J Solid State Chem 2012;192:289–95. Dotsenko V, Efryushina N, Berezovskaya I. Mater Lett 1996;28:517–20. Xiao X, Yan B. J Alloys Compd 2006;421:252–7. Ju HD, Deng WP, Wang BL, Liu J, Tao XT, Xu SQ. J Alloys Compd 2012;516:153–6. Blasse G, Timmermans C. J Solid State Chem 1984;52:222–32. Xia YJ, Huang FQ, Wang WD, Wang AB, Shi JL. J Alloys Compd 2009;476:534–8. Pdrini C. J Chem Phys 1975;63:3085–94. Pedrini C, Boulon G, Gaum-mahn F. Phys State Solid A 1973;15:K15–8. Blank JD, Blasse G. Eur J Solid State Inorg Chem 1996;33:295–307. Ju XX, Li XM, Li WL, Yang WJ, Tao CY. Mater Lett 2011;65:2642–4. Su J, Zhang QL, Shao SF, Liu WP, Wan SM, Yin ST. J Alloys Compd 2009;470:306–10. Kellendonk F, Belt TVD, Blasse G. J Chem Phys 1982;76:1194–201. Zorenko Y, Gorbenko V, Voznyak T, Jary V, Nikl M. J Lumin 2010;130:1963–9. Xie RJ, Hirosaki N, Suehiro T, Xu FF, Mitomo M. Chem Mater 2006;18:5578–83.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Co-precipitation preparation, characterization and optical properties of blue CaSb2O6: Bi3+ nano-phosphor Liumin Chen, Yumei Long, Yumin Qin, Weifeng Li n College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China
art ic l e i nf o
a b s t r a c t
Article history: Received 30 January 2013 Accepted 22 March 2013 Available online 30 March 2013
Bi3+ doped CaSb2O6 nanoparticles were prepared by the co-precipitation technique followed by heattreatment. The as-prepared samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and photoluminescence (PL) spectra. It was found that the CaSb2O6:xBi nano-crystals exist in the hexagonal phase, and show sphere-like shape with diameters of 60–80 nm. The CaSb2O6:xBi exhibited a strong emission band with peak wavelength of 437 nm at room temperature, which can be effectively excited both at 295 and 336 nm. It is believed that the formation of defect clusters such as ½ðBiCa Þ−ðV Ca Þ′ and/or ½ðBiCa Þ−ðV Ca Þ−ðBiCa Þ would play an important role on the optical properties of CaSb2O6:xBi phosphors. The intense blue emission and a higher quenching concentration suggested the potential application of nano-scaled CaSb2O6:xBi for lighting and display devices. & 2013 Elsevier B.V. All rights reserved.
Keywords: CaSb2O6:Bi Nanoparticles Phosphors Defects
1. Introduction The development of lighting and display systems with higher resolution and higher luminous efficiency continues to spur the search for novel luminescence materials. Great efforts have been dedicated to rare-earth ions as optical activators that present stable emission due to f–f electron transition. As a matter of fact, main group metal ions with s2 configuration are also excellent luminescence centers and attractive luminescence properties have been found [1,2]. The Bi3+ ion is of great importance as a luminescence activator [2–5] and its emission is attributed to the electron transition between the 6s2 ground states and the 6s6p excited states. Because of the strong interaction with the surrounding lattice and energy transfer from Bi3+ to Bi3+ or to another activator in different host lattices, the emission spectra of Bi3+ ions presents a broad band and the peak position of the band varies from the UV region to the red region with differing host lattices [4–7]. It is therefore expected that favorable optical properties can be achieved by doping Bi3+ ion into appropriate matrixes. The preparation and luminescence properties of MSb2O6:Bi (M ¼Ca, Sr) have been reported previously [8–10]. The only reported synthetic method is the solid state reaction. Due to the volatility of Bi2O3 and the tendency of Bi3+ to be oxidized to Bi5+ in the conventional sintering process, the visible emission quenching of MSb2O6:Bi (M ¼Ca, Sr) at room temperature occurred [8,11].
n
Corresponding author. Tel.: +86 512 65880089. E-mail address: [email protected] (W. Li).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.03.109
This work reported on the synthesis of CaSb2O6:Bi3+ phosphors by the co-precipitation method, which claims for low calcination temperature, short reaction time and mass-produced [12,13]. The optical properties of the as-synthesized samples were studied and the luminescence mechanism was also discussed.
2. Experimental The CaSb2O6:xBi (x ¼0, 0.0025, 0.005, 0.0075, 0.01, 0.02, 0.06, and 0.08) nanoparticles were prepared using the co-precipitation method. The starting materials were Bi2O3, SbCl3, CaCl2 2H2O, NaOH, and Na2C2O4, and they were all in analytical grade. In a typical synthesis procedure of CaSb2O6:0.01Bi phosphor, 0.0117 g Bi2O3 was dissolved in 25 mL hydrochloric acid with stirring and moderate heating. Then 0.7241 g of CaCl2 2H2O and 2.2812 g of SbCl3 were added sequentially into the obtained BiCl3 solution under stirring to form a clear and homogeneous mixture. The mixture solution was added dropwise into 80 mL of aqueous solution containing 3.52 g of NaOH and 0.737 g of Na2C2O4 with continuous stirring, and the white precipitation was obtained. After aging, filtration, washing and drying, the white precursor was obtained. Finally, the precursor was calcined at 1000 1C for 4 h in air. The structure of the samples was characterized by XRD on a X'Pert Pro MPD diffractometer (Pananalytical, Holland) using CuKα radiation (λ¼1.5406 Å). The SEM images were taken on Hitachi s-4700. The excitation and emission spectra were recorded on an F-2500 fluorescence spectrometer (Hitachi, Japan) at room temperature.
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L. Chen et al. / Materials Letters 102–103 (2013) 59–61
3. Results and discussion Fig. 1 shows the XRD patterns of the as-prepared samples doped with Bi3+ at different concentrations. All peaks of the asprepared samples agree well with the hexagonal structure (JCPDS Card no. 46-1496), indicating that the doped Bi3+ does not change the crystal structure or induce a new phase. The diffraction peaks shift to lower theta values with introducing Bi (inset of Fig. 1) because of the larger radius of Bi3+ (1.03 Å) compared to that of Ca2+ (1 Å). Furthermore, it seems that Bi3+ ions have a larger solubility in CaSb2O6 prepared by co-precipitation here than that by the solid state reaction [8]. Fig. 2a and b presents SEM images of
Fig. 1. XRD patterns of CaSb2O6:xBi (x ¼ 0, 0.0025, 0.0075, 0.02, and 0.08). Inset is the amplification of XRD patterns around 26.51.
Fig. 2. SEM images of (a) pure CaSb2O6 and (b) CaSb2O6:0.0075Bi.
the pure and 0.75 mol% Bi-doped CaSb2O6 samples, respectively, which display that both the samples have uniform sphere-like shape with particle size of 60–80 nm. Excitation spectra of CaSb2O6:xBi phosphors via monitoring 437 nm emission are presented in Fig. 3. The excitation spectra of Bi3+-doped samples consist of three broad bands and they are located at 244, 295 and 336 nm. These absorption bands can be attributed to the Bi3+-related absorption due to the their absence in undoped CaSb2O6 sample. The intensities of all the excitation bands are dependent on the Bi3+ concentration (inset of Fig. 3). The ground state of Bi3+ is 1S0 and its excited states are 3P0, 3P1, 3P2 and 1P1 in order of increasing energy [14]. Due to the complete spin forbiddance of the transition from 1S0 to 3P0 and 3P2, optical absorption is ascribed to the electron transition from 1S0 to 3P1 and 1 P1 [15]. It is reported that the weak excitation band peaking at 244 nm is assigned to the 1S0–1P1 transition [8]. In this work, the visible emission can effectively be excited at both 295 and 336 nm at room temperature, which is different from previous reports [8,10]. The excitation band at 295 nm is readily assigned to the host lattice excitation because it agrees well with the band gap of CaSb2O6. The appearance of the excitation band of the host lattice in the excitation spectrum of Bi3+ indicates that there exists energy transfer from host lattice to Bi3+ ions. The dominant bands peaking at 336 nm have bigger half-widths than those of higherenergy excitation bands and their peaks shift toward lower energy with increasing Bi3+ concentration. This absorption band is attributed to 1S0–3P1 transition. Here, it is worth noting that the optimal excitation intensities were achieved in CaSb2O6:0.06Bi and CaSb2O6:0.0075Bi corresponding to the excitation wavelengths of 295 and 336 nm, respectively. Fig. 4 shows the emission spectra of CaSb2O6:xBi (0≤x≤0.08) phosphors excited at 336 nm, which exhibit strong blue emission bands centered around 437 nm. The visible emission was ascribed to 3P1–1S0 transition of Bi3+ pairs [8]. In fact, here the visible emission is a superposition of several emission bands. The inset of Fig. 4 displays two Gaussian components for CaSb2O6:0.0075Bi with approximate peak wavelengths of 435 and 464 nm. The maximal emission intensity is observed in the phosphor doped with 0.75 mol% Bi3+. It is believed that defects may play an important role on the optical properties of CaSb2O6:xBi phosphors. As for the compound CaSb2O6, there is only one crystallographic Ca2+ site for Bi3+. The defect of ðBiCa Þd will be formed when Bi3+ was incorporated on Ca2 + site. Considering the charge compensation of ðBiCa Þd , some other
Fig. 3. Excitation spectra of CaSb2O6:xBi via monitoring 437 nm emission.
L. Chen et al. / Materials Letters 102–103 (2013) 59–61
61
which is one of the phenomenological characteristics of the variation of the crystal field strength [16].
4. Conclusions Nano-scaled CaSb2O6:xBi with sphere-like morphology has been successfully synthesized by the co-precipitation method. The as-prepared phosphors exhibit an intense blue emission at room temperature. The blue emission is a superposition of at least two emission bands and it could ascribe to the defect perturbation. The optimal excitation intensities of 295 nm and 336 nm are achieved in CaSb2O6:0.06Bi and in CaSb2O6:0.0075Bi, respectively. It is believed that the defect clusters such as ½ðBiCa Þ−ðV Ca Þ′ and ½ðBiCa Þ−ðV Ca Þ−ðBiCa Þ play important roles on the optical properties of CaSb2O6:Bi. The experimental results suggested that the co-precipitation method is favorable for preparing the CaSb2O6: Bi with desired optical properties.
Fig. 4. Emission spectra of CaSb2O6:xBi excited at 336 nm.
defects such as cation vacancies ðV Ca Þ″ have to be formed. Upon excitation of the host lattice (295 nm), the generated electrons and holes are trapped by a donor [like ðBiCa Þd ] and an acceptor [likeðV Ca Þ″], respectively. Following recombination, the energy is transferred from the donor-acceptor pair to Bi3+. Since defects have net charge, they tend to pair under thermal equilibrium and form ½ðBiCa Þ−ðV Ca Þ′ and/or ½ðBiCa Þ−ðV Ca Þ−ðBiCa Þ clusters. These defect-perturbed Bi3+ ions may result in the emission with multiple-peak structure of CaSb2O6:xBi phosphors. However, it needs further theoretical and experimental results to support. Red shift in the emission spectra excited at 336 nm and its corresponding excitation spectra are both observed for heavily Bi-doped (x≥0.02) samples. The change of crystal field strength could contribute to such a red shift. Due to the difference of ionic radius and charge between Bi3+ and Ca2+ ions, the incorporation of Bi3+ into the lattice would cause the expansion of the unit cell, and consequently changes in bond length, covalency, and possible symmetry. Fig. 3 shows the significant changes in the position and shape of the excitation band in the range of 300–400 nm,
Acknowledgments This work was supported by the National NSF of China (21005053) and the Priority Academic Program Development of Jiangsu Higher Education Institutions. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
Boutinaud P, Cavalli E. Chem Phys Lett 2011;503:239–43. Li JH, Liu J, Yu XB. J Alloys Compd 2012;509:9897–900. Angiuli F, Cavalli E, Belltti A. J Solid State Chem 2012;192:289–95. Dotsenko V, Efryushina N, Berezovskaya I. Mater Lett 1996;28:517–20. Xiao X, Yan B. J Alloys Compd 2006;421:252–7. Ju HD, Deng WP, Wang BL, Liu J, Tao XT, Xu SQ. J Alloys Compd 2012;516:153–6. Blasse G, Timmermans C. J Solid State Chem 1984;52:222–32. Xia YJ, Huang FQ, Wang WD, Wang AB, Shi JL. J Alloys Compd 2009;476:534–8. Pdrini C. J Chem Phys 1975;63:3085–94. Pedrini C, Boulon G, Gaum-mahn F. Phys State Solid A 1973;15:K15–8. Blank JD, Blasse G. Eur J Solid State Inorg Chem 1996;33:295–307. Ju XX, Li XM, Li WL, Yang WJ, Tao CY. Mater Lett 2011;65:2642–4. Su J, Zhang QL, Shao SF, Liu WP, Wan SM, Yin ST. J Alloys Compd 2009;470:306–10. Kellendonk F, Belt TVD, Blasse G. J Chem Phys 1982;76:1194–201. Zorenko Y, Gorbenko V, Voznyak T, Jary V, Nikl M. J Lumin 2010;130:1963–9. Xie RJ, Hirosaki N, Suehiro T, Xu FF, Mitomo M. Chem Mater 2006;18:5578–83.