- Email: [email protected]
Upconversion luminescence of colloidal solution of Y2O3 nano-particles doped with trivalent rare-earth ions
ARTICLE IN PRESS
Journal of Luminescence 128 (2008) 1267–1270 www.elsevier.com/locate/jlumin
Upconversion luminescence of colloidal solution of Y2O3 nano-particles doped with trivalent rare-earth ions Daisuke Matsuuraa,, Tatsuya Ikeuchia, Kohei Sogab a
Research and Development Center, Dai Nippon Printing Co., Ltd., 250-1 Wakashiba, Kashiwa 277-0871, Japan Department of Materials Science and Technology, Tokyo University of Science, Noda, Chiba 278-8510, Japan
b
Received 3 August 2007; received in revised form 30 November 2007; accepted 4 December 2007 Available online 15 December 2007
Abstract We have studied upconversion luminescence of colloidal solution of Y2O3 nano-particles codoped with 1 mol% Er3+ and 5 mol% Yb3+. Y2O3 nano-particles codoped with 1 mol% Er3+ and 5 mol% Yb3+ show sintering and agglomeration, because they are synthesized by firing a hydroxy carbonate precursor. Colloidal solution of Y2O3 nano-particles codoped with 1 mol% Er3+ and 5 mol% Yb3+ is prepared through two-step dispersion process and the average diameter of the primary nano-particles is about 50 nm. Under excitation with 980-nm laser diode, upconversion luminescence of colloidal solution of the primary Y2O3 nano-particles codoped with 1 mol% Er3+ and 5 mol% Yb3+ in methyl isobuthyl ketone strongly appeared near 660 nm and weakly near 550 nm. r 2007 Elsevier B.V. All rights reserved. Keywords: Upconversion luminescence; Colloidal solution; Nano-particle
1. Introduction Nano-sized materials doped with trivalent rare-earth (RE) ions have attracted much attention due to their optical properties [1–5]. Recently, several research groups have reported on nano-sized upconversion luminescent materials, which provide the emission of radiation at higher energy than excitation wavelength [6–9]. Upconversion luminescent materials play a significant role in display phosphors and solid-state laser materials. Furthermore, nano-sized upconversion luminescent materials have been investigated for using as fluorescent probes in biological staining and diagnostics due to their photochemical stability compared to conventional fluorescent organic molecules [10–12]. Excitation in the near-infrared light induces only a very weak auto-fluorescence background and avoids photodegradation in bio-tagging applications. It is possible to simplify the detection of the labeled target molecules and increase the sensitivity of the method.
Corresponding author. Tel.: +81 4 7134 0630; fax: +81 4 7133 9290.
E-mail address: [email protected] (D. Matsuura). 0022-2313/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2007.12.024
RE-doped materials with low phonon energies are known to show high-efficient upconversion luminescence. Upconversion luminescence has been observed and investigated in many doped bulk materials, mostly in oxides and halides because many host materials with low phonon energies exist in oxides and halides [13]. Several recent publications have reported upconversion luminescence in transparent colloids of nano-sized halides doped with RE ions [8,9]. In comparison to halides, oxides have high chemical durability, thermal stability and are suitable for mass production due to ease of preparation. However, nano-sized oxides doped with RE ions have been prepared as the state of sintering between nano-particles and agglomeration, because they have been crystallized due to high-temperature reaction. Therefore, it is difficult to prepare colloidal solution of nano-sized oxides doped with RE ions. In this work, we have studied upconversion luminescence of colloidal solution of Y2O3 nano-particles codoped with 1 mol% Er3+ and 5 mol% Yb3+ (abbreviated as Y2O3: Er3+, Yb3+ nano-particles). Er3+ is an excellent candidate for upconversion, since the 4f electronic levels provide intermediate levels, which closely match one of the most attractive laser diode (LD) wavelength, 980 nm. Yb3+ also
ARTICLE IN PRESS 1268
D. Matsuura et al. / Journal of Luminescence 128 (2008) 1267–1270
exhibits a specifically large absorption cross-section under this wavelength. 2. Experimental Y2O3:Er3+, Yb3+ nano-particles were synthesized as described recently and showed strongly upconversion luminescence [7]. A methyl isobuthyl ketone (MIBK; Kanto Chemical X99.5%) solution mixed with the dispersant (BYK Chemie Disperbyk-161) was put in a conventional glass vessel. Y2O3:Er3+, Yb3+ nano-particles were milled in the glass vessel with 0.05-mm zirconia balls by paint-shaker (ASADA) for an hour. Y2O3:Er3+, Yb3+ nano-particles were characterized structurally on a Hitachi S-5000H scanning electron microscope (SEM), a Hitachi H-8100 transmission electron microscope (TEM). Upconversion luminescence spectra were measured at room temperature. A 980-nm LD (Coherent) was used as the excitation source. The upconversion luminescence signals were dispersed by a 25-cm single monochromator using 500-nm blazed 1200 grooves/mm gratings and detected by a photomultiplier (Hamamatsu R-928). The upconversion luminescence signals were amplified and analyzed with a lock-in amplifier (NF Electronic Instruments 5600A). 3. Results and discussion The characterization of Y2O3:Er3+, Yb3+ nano-particles is summarized in Fig. 1. The SEM observation (Fig. 1(a)) shows that the size distribution is broad and the average diameter is about 50 nm. A high-resolution transmission electron microscopy image (Fig. 1(b)) of one of the Y2O3:Er3+, Yb3+ nano-particles shows crystalline particle. Y2O3:Er3+, Yb3+ nano-particles show sintering and agglomeration, because they are synthesized by firing a hydroxy carbonate precursor. In order to obtain the colloidal solution of Y2O3:Er3+, Yb3+ nano-particles, it is necessary to disperse Y2O3:Er3+, Yb3+ nano-particles in solvent.
Colloidal solution of Y2O3:Er3+, Yb3+ nano-particles is prepared through two-step dispersion process (see Fig. 2). In the first step, the Y2O3:Er3+, Yb3+ nano-particles are generally milled to pulverize sintered nano-particles and the dispersant is used to inhibit agglomeration. The liquid solution mixes an MIBK solvent and the Disperbyk-161 dispersant and is put into a conventional glass vessel. An MIBK solvent is evaporated easily because of the high volatility and the Disperbyk-161 dispersant solves easily in MIBK. The colloidal solution of Y2O3:Er3+, Yb3+ nanoparticles (1 wt%) is separated from zirconia balls using nylon mesh after Y2O3:Er3+, Yb3+ nano-particles are milled in the glass vessel with zirconia balls. In the secondstep, the small size nano-particles are separated from nanoparticles of several sizes by decantation to isolate the primary nano-particles. In the decantation method, the colloidal solution of Y2O3:Er3+, Yb3+ nano-particles in the glass vessel is gently placed on a flat table. Fig. 3 displays SEM image of Y2O3:Er3+, Yb3+ nano-particles after decantation. We have obtained the primary Y2O3:Er3+, Yb3+ nano-particles through two-step dispersion process. SEM image shows that the primary nanoparticles have approximately uniform particle size and are spherical. Milling techniques introduce defects into surface of the oxide phosphor crystal. Defects have serious implications for luminescent materials, as they have non-radiative recombination routes for electrons and holes. In particular, upconversion luminescence is susceptible to non-radiative recombination. However, upconversion luminescence intensity would slightly decrease through dispersion process, provided milling time is short due to the choice of dispersants. Fig. 4(a) shows a photograph of visible upconversion luminescence in colloidal solution of the primary Y2O3:Er3+, Yb3+ nano-particles (1 wt% or less) under excitation with 980-nm LD (5 kW/cm2). The colloidal solution of the primary Y2O3:Er3+, Yb3+ nano-particles is prepared through the above-mentioned two-step dispersion process. The average diameter of the primary Y2O3:Er3+,
Fig. 1. Characterization of Y2O3:Er3+, Yb3+ nano-particles by scanning electron microscopy and transmission electron microscopy: (a) SEM image and (b) TEM image.
ARTICLE IN PRESS D. Matsuura et al. / Journal of Luminescence 128 (2008) 1267–1270
sintering and agglomeration
milling
colloidal solution 4F 4 9/2 − I15/2
PL INTENSITY
decantation
1269
2H
Fig. 2. Illustration of two-step dispersion process: (a) first step and (b) second step.
11/2
/ 4S3/2 − 4I15/2
500
600 700 WAVELENGTH (nm)
Fig. 4. Photograph and spectrum of visible upconversion luminescence in colloidal solution of Y2O3:Er3+, Yb3+ nano-particles under excitation with 980-nm LD: (a) photograph of upconversion luminescence and (b) upconversion luminescence spectrum.
22 4
20
11/2
4
S3/2
4
F9/2
14
10 8 6
4I 9/2
550 nm
12
4
660 nm
Yb3+ nano-particles is about 50 nm, as shown in Fig. 3. Fig. 4(b) shows upconversion luminescence spectrum of colloidal solution of the primary Y2O3:Er3+, Yb3+ nanoparticles under excitation with 980-nm LD. The 4F9/2–4I15/2 transition of Er3+ appeared near 660 nm and the 2H11/ 4 4 4 2– I15/2 and S3/2– I15/2 transitions appeared near 530 and 550 nm, respectively. A typical energy-level diagram for the upconverted emission from a material codoped with Er3+ and Yb3+ under 980-nm light excitation is shown in Fig. 5. The excitation wavelength matches the absorption transition between the ground state (4I15/2) and excited level (4I11/2)
ENERGY (× 103 cm-1)
Fig. 3. SEM image of Y2O3:Er3+, Yb3+ nano-particles through two-step dispersion process.
16
530 nm
18
200 nm
F7/2
2H
2F 5/2
I11/2
4
I13/2
4 2 2F 7/2
4I 15/2
0 Er3+
Yb3+
Fig. 5. The energy-level diagrams for Er3+ and Yb3+ under 980-nm light excitation.
ARTICLE IN PRESS 1270
D. Matsuura et al. / Journal of Luminescence 128 (2008) 1267–1270
of Er3+ and the absorption transition between the ground state (2F7/2) and excited level (2F5/2) of Yb3+. After first-level excitation, two mechanisms would be considered as follows. The first possible one is simple excited-state absorption, where the same wavelength photons may pump up the excited atom from the 4I11/2 to the 4F7/2 level of Er3+. The second one is the energy transfer process from Yb3+ to Er3+. The red emission near 660 nm is the 4F9/2–4I15/2 transition of Er3+ and the green emissions near 530 and 550 nm are the 2H11/2–4I15/2 and 4 S3/2–4I15/2 transitions of Er3+, respectively.
4. Conclusions We have prepared colloidal solution of Y2O3:Er3+, Yb3+ nano-particles. Colloidal solution of Y2O3:Er3+, Yb3+ nano-particles is obtained through two-step dispersion process. Two-step dispersion process would be suitable for mass production but a method different from decantation would be considered for efficiently producing the primary nano-particles. The average diameter of the primary Y2O3:Er3+, Yb3+ nano-particles obtained through two-step dispersion process is about 50 nm. Upconversion luminescence of colloidal solution of the primary Y2O3:Er3+, Yb3+ nano-particles in MIBK strongly appeared near 660 nm and weakly near 550 nm under excitation with 980-nm LD.
Acknowledgment The authors thank Mr. M. Aso of Material Analysis and Research Center, Dai-Nippon Printing Co., Ltd., for the TEM and SEM observations. References [1] A. Konrad, T. Fries, A. Gahn, F. Kummer, U. Herr, R. Tidecks, K. Samwer, J. Appl. Phys. 86 (1999) 3129. [2] Y. Tao, G. Zhao, W. Zhang, S. Xia, Mater. Res. Bull. 32 (1997) 501. [3] R. Schmechel, M. Kennedy, H. von Seggern, H. Winkler, M. Kolbe, R.A. Fischer, L. Xaomao, A. Benker, M. Winterer, H. Hahn, J. Appl. Phys. 89 (2001) 1679. [4] T. Igarashi, M. Ihara, T. Kusunoki, K. Ohno, T. Isobe, M. Senna, Appl. Phys. Lett. 76 (2000) 1549. [5] G. Wakefield, E. Holland, P.J. Dobson, J.L. Hutchison, Adv. Mater. 13 (2001) 1557. [6] F. Vetrone, J.C. Boyer, J.A. Capobianco, A. Speghini, M. Bettinelli, J. Phys. Chem. B 107 (2003) 1107. [7] D. Matsuura, H. Hattori, A. Takano, J. Electrochem. Soc. 152 (2003) H39. [8] S. Heer, K. Kompe, H.U. Gudel, M. Haase, Adv. Mater. 16 (2004) 2102. [9] J.C. Boyer, F. Vetrone, L.A. Cuccia, J.A. Capobianco, J. Am. Chem. Soc. 128 (2006) 7444. [10] R.S. Niedbala, H. Feindt, K. Kardos, T. Vail, J. Burton, B. Bielska, S. Li, D. Milunic, P. Bourdelle, R. Vallejo, Anal. Biochem. 293 (2001) 22. [11] F. van de Rijke, H. Zijlmans, S. Li, T. Vail, A.K. Raap, R.S. Niedbala, H.J. Tanke, Nature Biotechnol. 19 (2001) 273. [12] B.A. Holm, E.J. Bergey, T. De, D.J. Rodman, R. Kapoor, L. Levy, C.S. Frend, P.N. Prasad, Mol. Cryst. Liq. Cryst. 374 (2002) 589. [13] K. Soga, W. Wang, R.E. Riman, J.B. Brown K. Mikeska, J. Appl. Phys. 93 (2003) 2946.
Journal of Luminescence 128 (2008) 1267–1270 www.elsevier.com/locate/jlumin
Upconversion luminescence of colloidal solution of Y2O3 nano-particles doped with trivalent rare-earth ions Daisuke Matsuuraa,, Tatsuya Ikeuchia, Kohei Sogab a
Research and Development Center, Dai Nippon Printing Co., Ltd., 250-1 Wakashiba, Kashiwa 277-0871, Japan Department of Materials Science and Technology, Tokyo University of Science, Noda, Chiba 278-8510, Japan
b
Received 3 August 2007; received in revised form 30 November 2007; accepted 4 December 2007 Available online 15 December 2007
Abstract We have studied upconversion luminescence of colloidal solution of Y2O3 nano-particles codoped with 1 mol% Er3+ and 5 mol% Yb3+. Y2O3 nano-particles codoped with 1 mol% Er3+ and 5 mol% Yb3+ show sintering and agglomeration, because they are synthesized by firing a hydroxy carbonate precursor. Colloidal solution of Y2O3 nano-particles codoped with 1 mol% Er3+ and 5 mol% Yb3+ is prepared through two-step dispersion process and the average diameter of the primary nano-particles is about 50 nm. Under excitation with 980-nm laser diode, upconversion luminescence of colloidal solution of the primary Y2O3 nano-particles codoped with 1 mol% Er3+ and 5 mol% Yb3+ in methyl isobuthyl ketone strongly appeared near 660 nm and weakly near 550 nm. r 2007 Elsevier B.V. All rights reserved. Keywords: Upconversion luminescence; Colloidal solution; Nano-particle
1. Introduction Nano-sized materials doped with trivalent rare-earth (RE) ions have attracted much attention due to their optical properties [1–5]. Recently, several research groups have reported on nano-sized upconversion luminescent materials, which provide the emission of radiation at higher energy than excitation wavelength [6–9]. Upconversion luminescent materials play a significant role in display phosphors and solid-state laser materials. Furthermore, nano-sized upconversion luminescent materials have been investigated for using as fluorescent probes in biological staining and diagnostics due to their photochemical stability compared to conventional fluorescent organic molecules [10–12]. Excitation in the near-infrared light induces only a very weak auto-fluorescence background and avoids photodegradation in bio-tagging applications. It is possible to simplify the detection of the labeled target molecules and increase the sensitivity of the method.
Corresponding author. Tel.: +81 4 7134 0630; fax: +81 4 7133 9290.
E-mail address: [email protected] (D. Matsuura). 0022-2313/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2007.12.024
RE-doped materials with low phonon energies are known to show high-efficient upconversion luminescence. Upconversion luminescence has been observed and investigated in many doped bulk materials, mostly in oxides and halides because many host materials with low phonon energies exist in oxides and halides [13]. Several recent publications have reported upconversion luminescence in transparent colloids of nano-sized halides doped with RE ions [8,9]. In comparison to halides, oxides have high chemical durability, thermal stability and are suitable for mass production due to ease of preparation. However, nano-sized oxides doped with RE ions have been prepared as the state of sintering between nano-particles and agglomeration, because they have been crystallized due to high-temperature reaction. Therefore, it is difficult to prepare colloidal solution of nano-sized oxides doped with RE ions. In this work, we have studied upconversion luminescence of colloidal solution of Y2O3 nano-particles codoped with 1 mol% Er3+ and 5 mol% Yb3+ (abbreviated as Y2O3: Er3+, Yb3+ nano-particles). Er3+ is an excellent candidate for upconversion, since the 4f electronic levels provide intermediate levels, which closely match one of the most attractive laser diode (LD) wavelength, 980 nm. Yb3+ also
ARTICLE IN PRESS 1268
D. Matsuura et al. / Journal of Luminescence 128 (2008) 1267–1270
exhibits a specifically large absorption cross-section under this wavelength. 2. Experimental Y2O3:Er3+, Yb3+ nano-particles were synthesized as described recently and showed strongly upconversion luminescence [7]. A methyl isobuthyl ketone (MIBK; Kanto Chemical X99.5%) solution mixed with the dispersant (BYK Chemie Disperbyk-161) was put in a conventional glass vessel. Y2O3:Er3+, Yb3+ nano-particles were milled in the glass vessel with 0.05-mm zirconia balls by paint-shaker (ASADA) for an hour. Y2O3:Er3+, Yb3+ nano-particles were characterized structurally on a Hitachi S-5000H scanning electron microscope (SEM), a Hitachi H-8100 transmission electron microscope (TEM). Upconversion luminescence spectra were measured at room temperature. A 980-nm LD (Coherent) was used as the excitation source. The upconversion luminescence signals were dispersed by a 25-cm single monochromator using 500-nm blazed 1200 grooves/mm gratings and detected by a photomultiplier (Hamamatsu R-928). The upconversion luminescence signals were amplified and analyzed with a lock-in amplifier (NF Electronic Instruments 5600A). 3. Results and discussion The characterization of Y2O3:Er3+, Yb3+ nano-particles is summarized in Fig. 1. The SEM observation (Fig. 1(a)) shows that the size distribution is broad and the average diameter is about 50 nm. A high-resolution transmission electron microscopy image (Fig. 1(b)) of one of the Y2O3:Er3+, Yb3+ nano-particles shows crystalline particle. Y2O3:Er3+, Yb3+ nano-particles show sintering and agglomeration, because they are synthesized by firing a hydroxy carbonate precursor. In order to obtain the colloidal solution of Y2O3:Er3+, Yb3+ nano-particles, it is necessary to disperse Y2O3:Er3+, Yb3+ nano-particles in solvent.
Colloidal solution of Y2O3:Er3+, Yb3+ nano-particles is prepared through two-step dispersion process (see Fig. 2). In the first step, the Y2O3:Er3+, Yb3+ nano-particles are generally milled to pulverize sintered nano-particles and the dispersant is used to inhibit agglomeration. The liquid solution mixes an MIBK solvent and the Disperbyk-161 dispersant and is put into a conventional glass vessel. An MIBK solvent is evaporated easily because of the high volatility and the Disperbyk-161 dispersant solves easily in MIBK. The colloidal solution of Y2O3:Er3+, Yb3+ nanoparticles (1 wt%) is separated from zirconia balls using nylon mesh after Y2O3:Er3+, Yb3+ nano-particles are milled in the glass vessel with zirconia balls. In the secondstep, the small size nano-particles are separated from nanoparticles of several sizes by decantation to isolate the primary nano-particles. In the decantation method, the colloidal solution of Y2O3:Er3+, Yb3+ nano-particles in the glass vessel is gently placed on a flat table. Fig. 3 displays SEM image of Y2O3:Er3+, Yb3+ nano-particles after decantation. We have obtained the primary Y2O3:Er3+, Yb3+ nano-particles through two-step dispersion process. SEM image shows that the primary nanoparticles have approximately uniform particle size and are spherical. Milling techniques introduce defects into surface of the oxide phosphor crystal. Defects have serious implications for luminescent materials, as they have non-radiative recombination routes for electrons and holes. In particular, upconversion luminescence is susceptible to non-radiative recombination. However, upconversion luminescence intensity would slightly decrease through dispersion process, provided milling time is short due to the choice of dispersants. Fig. 4(a) shows a photograph of visible upconversion luminescence in colloidal solution of the primary Y2O3:Er3+, Yb3+ nano-particles (1 wt% or less) under excitation with 980-nm LD (5 kW/cm2). The colloidal solution of the primary Y2O3:Er3+, Yb3+ nano-particles is prepared through the above-mentioned two-step dispersion process. The average diameter of the primary Y2O3:Er3+,
Fig. 1. Characterization of Y2O3:Er3+, Yb3+ nano-particles by scanning electron microscopy and transmission electron microscopy: (a) SEM image and (b) TEM image.
ARTICLE IN PRESS D. Matsuura et al. / Journal of Luminescence 128 (2008) 1267–1270
sintering and agglomeration
milling
colloidal solution 4F 4 9/2 − I15/2
PL INTENSITY
decantation
1269
2H
Fig. 2. Illustration of two-step dispersion process: (a) first step and (b) second step.
11/2
/ 4S3/2 − 4I15/2
500
600 700 WAVELENGTH (nm)
Fig. 4. Photograph and spectrum of visible upconversion luminescence in colloidal solution of Y2O3:Er3+, Yb3+ nano-particles under excitation with 980-nm LD: (a) photograph of upconversion luminescence and (b) upconversion luminescence spectrum.
22 4
20
11/2
4
S3/2
4
F9/2
14
10 8 6
4I 9/2
550 nm
12
4
660 nm
Yb3+ nano-particles is about 50 nm, as shown in Fig. 3. Fig. 4(b) shows upconversion luminescence spectrum of colloidal solution of the primary Y2O3:Er3+, Yb3+ nanoparticles under excitation with 980-nm LD. The 4F9/2–4I15/2 transition of Er3+ appeared near 660 nm and the 2H11/ 4 4 4 2– I15/2 and S3/2– I15/2 transitions appeared near 530 and 550 nm, respectively. A typical energy-level diagram for the upconverted emission from a material codoped with Er3+ and Yb3+ under 980-nm light excitation is shown in Fig. 5. The excitation wavelength matches the absorption transition between the ground state (4I15/2) and excited level (4I11/2)
ENERGY (× 103 cm-1)
Fig. 3. SEM image of Y2O3:Er3+, Yb3+ nano-particles through two-step dispersion process.
16
530 nm
18
200 nm
F7/2
2H
2F 5/2
I11/2
4
I13/2
4 2 2F 7/2
4I 15/2
0 Er3+
Yb3+
Fig. 5. The energy-level diagrams for Er3+ and Yb3+ under 980-nm light excitation.
ARTICLE IN PRESS 1270
D. Matsuura et al. / Journal of Luminescence 128 (2008) 1267–1270
of Er3+ and the absorption transition between the ground state (2F7/2) and excited level (2F5/2) of Yb3+. After first-level excitation, two mechanisms would be considered as follows. The first possible one is simple excited-state absorption, where the same wavelength photons may pump up the excited atom from the 4I11/2 to the 4F7/2 level of Er3+. The second one is the energy transfer process from Yb3+ to Er3+. The red emission near 660 nm is the 4F9/2–4I15/2 transition of Er3+ and the green emissions near 530 and 550 nm are the 2H11/2–4I15/2 and 4 S3/2–4I15/2 transitions of Er3+, respectively.
4. Conclusions We have prepared colloidal solution of Y2O3:Er3+, Yb3+ nano-particles. Colloidal solution of Y2O3:Er3+, Yb3+ nano-particles is obtained through two-step dispersion process. Two-step dispersion process would be suitable for mass production but a method different from decantation would be considered for efficiently producing the primary nano-particles. The average diameter of the primary Y2O3:Er3+, Yb3+ nano-particles obtained through two-step dispersion process is about 50 nm. Upconversion luminescence of colloidal solution of the primary Y2O3:Er3+, Yb3+ nano-particles in MIBK strongly appeared near 660 nm and weakly near 550 nm under excitation with 980-nm LD.
Acknowledgment The authors thank Mr. M. Aso of Material Analysis and Research Center, Dai-Nippon Printing Co., Ltd., for the TEM and SEM observations. References [1] A. Konrad, T. Fries, A. Gahn, F. Kummer, U. Herr, R. Tidecks, K. Samwer, J. Appl. Phys. 86 (1999) 3129. [2] Y. Tao, G. Zhao, W. Zhang, S. Xia, Mater. Res. Bull. 32 (1997) 501. [3] R. Schmechel, M. Kennedy, H. von Seggern, H. Winkler, M. Kolbe, R.A. Fischer, L. Xaomao, A. Benker, M. Winterer, H. Hahn, J. Appl. Phys. 89 (2001) 1679. [4] T. Igarashi, M. Ihara, T. Kusunoki, K. Ohno, T. Isobe, M. Senna, Appl. Phys. Lett. 76 (2000) 1549. [5] G. Wakefield, E. Holland, P.J. Dobson, J.L. Hutchison, Adv. Mater. 13 (2001) 1557. [6] F. Vetrone, J.C. Boyer, J.A. Capobianco, A. Speghini, M. Bettinelli, J. Phys. Chem. B 107 (2003) 1107. [7] D. Matsuura, H. Hattori, A. Takano, J. Electrochem. Soc. 152 (2003) H39. [8] S. Heer, K. Kompe, H.U. Gudel, M. Haase, Adv. Mater. 16 (2004) 2102. [9] J.C. Boyer, F. Vetrone, L.A. Cuccia, J.A. Capobianco, J. Am. Chem. Soc. 128 (2006) 7444. [10] R.S. Niedbala, H. Feindt, K. Kardos, T. Vail, J. Burton, B. Bielska, S. Li, D. Milunic, P. Bourdelle, R. Vallejo, Anal. Biochem. 293 (2001) 22. [11] F. van de Rijke, H. Zijlmans, S. Li, T. Vail, A.K. Raap, R.S. Niedbala, H.J. Tanke, Nature Biotechnol. 19 (2001) 273. [12] B.A. Holm, E.J. Bergey, T. De, D.J. Rodman, R. Kapoor, L. Levy, C.S. Frend, P.N. Prasad, Mol. Cryst. Liq. Cryst. 374 (2002) 589. [13] K. Soga, W. Wang, R.E. Riman, J.B. Brown K. Mikeska, J. Appl. Phys. 93 (2003) 2946.