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Correlation between the luminescence properties and the surface structures of submicron silica particles
Journal of Non-Crystalline Solids 353 (2007) 510–513 www.elsevier.com/locate/jnoncrysol
Correlation between the luminescence properties and the surface structures of submicron silica particles Seiichiro Inai, Akira Harao, Hiroyuki Nishikawa
*
Department of Electrical Engineering, Shibaura Institute of Technology, 3-7-5 Toyosu, Kouto-ku, Tokyo 135-8548, Japan
Abstract We investigated the correlation between the luminescence properties and the surface structures of submicron silica particles prepared by the Sto¨ber method. After annealing in a non-oxidizing atmosphere, the submicron-sized silica particles show a broad photoluminescence (PL) band at 500–540 nm by excitation at an ultraviolet wavelengths (254 and 365 nm), and the one at the 600 nm by excitation an Ar+ laser (488 nm). The PL appeared to result from the removal of impurities and subsequent formation of several luminescent structures on the internal surface of the primary particles by thermal annealing. Ó 2007 Elsevier B.V. All rights reserved. PACS: 65.60.+a; 73.20.Hb; 78.55.m Keywords: Optical properties; Photoluminescence
1. Introduction Since silica (or amorphous SiO2) has a high chemical stability and superior optical properties, it bears a role as a basic material in electronics and photonics. Using tetraethoxysilane (TEOS) as a starting material, submicronsized silica particles with mono dispersion in diameter, can be synthesized with the Sto¨ber method [1]. The mono-dispersed submicron silica particles can be selfassembled into three dimensional periodic structures, socalled ‘synthetic opal’. It has been used as a template for Si-photonic crystals which has a periodic structure at the scale of wavelength of light [2]. It is well known that the structure of silica synthesized from TEOS by the sol–gel method is porous with high-surface to volume ratio before anneal. The luminescence properties of the internal surface of mesoporous silica are reported [3–5]. The luminescence properties of nanometer-sized silica particles, called fumed silica, as starting materials to prepare bulk silica glass were also reported *
Corresponding author. Tel.: +81 3 5859 8217; fax: +81 3 5859 8201. E-mail address: [email protected] (H. Nishikawa).
0022-3093/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2006.10.051
[6]. Therefore, it is expected that the submicron silica particles with a high-surface to volume ratio, show a similar luminescence characteristics. In fact, we observed luminescence from submicron-sized silica particles synthesized from TEOS after anneal in non-oxidizing conditions [7]. These submicron silica particles can open a new possibility of non-doped luminescent silica without incorporation of active ions such as Eu+. In this paper, we studied the correlation between the luminescence properties and the surface structures of submicron silica particles. Based on optical absorption, photoluminescence, FT-IR and Raman spectroscopy, we investigate the origin of the luminescent centers in the submicron-sized silica particles. 2. Experimental methods We prepared silica particles with 500 nm by the Sto¨ber method. The submicron silica particles of mono dispersed were synthesized by the hydrolysis and condensation polymerization reactions of TEOS, as shown in Eqs. (1) and (2). Si(OC2 H5 )4 + 4H2 O ! Si(OH)4 + 4C2 H5 OH
ð1Þ
S. Inai et al. / Journal of Non-Crystalline Solids 353 (2007) 510–513
and
Photon Energy (eV) 2.6
ð2Þ
3. Results 3.1. Optical properties of submicron silica particles Luminescence of silica particles was observed by excitation at an ultraviolet lamp (254, 365 nm), and an Ar+ laser (488 nm) after annealing at 700 °C for 1 h in nitrogen, as shown in Fig. 1. The PL spectra are broad and spread from 450 to 650 nm by excitation at an ultraviolet (254 and 365 nm), and the one at 500–800 nm by excitation at an Ar+ laser (488 nm). The PL spectra are the peak wavelength red-shifts from 500 nm to 600 nm with increasing excitation wavelength from 254 nm to 488 nm. It suggests that silica particles of after annealing have the several luminescent structures. Absorption spectra of silica particles before and after annealing (700 °C, nitrogen) are shown in Fig. 2. Before annealing, absorption does not appear. The optical absorption of the silica particles after annealing shows broad spectrum with several overlapping components. 3.2. Structure of silica submicron particles Fig. 3 shows expanded FT-IR spectra around 1100 cm1 before and after the heat treatment at 700 °C in nitrogen
2.4
2.2
2
1.8
1.6
254nm excitation 365nm excitation
PL intensity (arb.units)
The Si(OH)4 fragments produced by the reaction (1) are so rich in reactivity that the condensation polymerization of the Eq. (2) leads to form primary particles with diameters of 16–20 nm, which was confirmed by a small angle X-ray scattering measurements (SAXS) [8]. These primary particles further assemble into the secondary submicronsized particles of SiO2. Silica particles were synthesized from the mixture of 100 ml of ethanol and 8 ml of TEOS and 14 ml of ammonia and 2 ml of water. The temperature of the solution is 30 °C. Annealing of the silica particles were carried out at 700 °C for 1 h under flowing nitrogen. We evaluated submicron-sized silica particle by a scanning electron microscope (SEM) (SIMADZU, SSX-550) in terms of the size distribution. The PL spectra of silica particles in water were measured by a spectrometer with a CCD detector (Steller-net EPP2000) under the illumination of an ultraviolet lamp (254 and 365 nm excitation). Also, the PL from the silica particles on a silicon substrate was measured with a spectrometer equipped with a confocal microscope [TII Nanofinder, Ar+ laser (488 nm)]. All PL measurements were performed at room temperature. Optical absorption spectra in the visible and ultraviolet region were measured by a conventional spectrometer (Shimadzu, UV2450) equipped with an integrating sphere. Structures and impurities of silica particles were evaluated by an FT-IR spectrometer (JASCO, FT/IR-460plus). Raman scattering was measured by a micro-Raman spectrometer (JASCO, NS2100) under excitation at 488 nm.
488nm excitation
450
500
550
600
650
700
750
800
Wavelength (nm) Fig. 1. PL spectra of silica particles with a diameter of 500 nm after annealing at 700 °C for 1 h in nitrogen by excitation at 254, 365 and 488 nm.
Photon Energy (eV) 2.5
5 4.5
4
3.5
3
2.5
2
before annealing after annealing
Absorbance (arb.units)
BSiAOH + HOASiB ! BSiAOASiB + H2 O
511
2.0
1.5
1.0
0.5
0.0 240
320
400
480
560
640
720
800
Wavelength (nm) Fig. 2. Absorption spectra of silica particles with a diameter of 500 nm before and after annealing (700 °C, nitrogen).
atmosphere of the submicron silica particles. The spectrum of thermal oxide of silicon is also shown for reference. The spectral shape of the peaks at 1100 cm1, which is due to an Si–O–Si asymmetric stretching mode. After annealing, the peak of 1100 cm1 shifts to higher wavenumber. It means that silica particles are unstable inner surface structure. Especially a broad peak at the higher wavenumber of 1200 cm1 is peculiar to submicron particles with high specific surface area. Similar spectrum with luminescent mesoporous silica was reported [4,5]. It suggests that the porous structure is responsible for the luminescence in the submicron silica particles.
512
S. Inai et al. / Journal of Non-Crystalline Solids 353 (2007) 510–513
a Absorbance (arb.units)
Thermal oxide of Si before annealing after annealing
1000
1050
1100
1150
1200
1250
1300
-1
Wavenumber (cm ) Fig. 3. FT-IR spectra of silica particles with a diameter of 500 nm and 100-nm thick thermal oxide film on Si. The annealing was carried out at 700 °C for 1 h in nitrogen.
Figs. 4(a) and (b), respectively, show Raman spectra of silica particles before and after annealing and a schematic illustration of change in the surface structure of the primary silica particle by thermal annealing. From the Raman spectra in Fig. 4(a), the surface structure of the primary particle is in the form of a planar four-membered ring, as can be seen in the D1 band at 490 cm1. After annealing, the D1 band decreased and the D2 band at 606 cm1 due to a planar three-membered ring appears. In addition, the removal of H2O, OH, and CHX was observed [9–11]. Correlation of the luminescent structures will be discussed later. 4. Discussion As experimentally confirmed by SAXS [8], the submicron silica particles are expected to have a high-surface structure comprising of 16–20 nm primary particles. Fig. 4(b) illustrates the change in the surface structure of the silica particles and generation of the surface luminescent species by thermal annealing. First, the removal of impurities adsorbed on the surface of the primary particles occurs by annealing. Then, the three-membered rings are formed by the dehydroxylation reaction at the surface of the primary particles by annealing. This structural model is consistent with the appearance of the D2 band due to the three-membered rings in the Raman spectrum shown in Fig. 4(a). According to the report on the surface defects in silica [12], there are several luminescent structures, such as non-bridging oxygen hole centers (NBOHCs) and oxygen deficient centers (ODC(II)s). As shown in Fig. 2, the submicron silica particles show strong absorption peak at 250 nm and weak shoulders at about 260 and 650 nm.
b
Fig. 4. (a) Raman spectra of silica particles before and after annealing and (b) a schematic illustration of the change in the surface structure of the primary silica particle by thermal annealing.
The 250 nm band is in good agreement with the absorption band reported for the ODC(II)s in surface silica [12]. This assignment is supported by the fact that the 250 nm band in the N2-annealed submicron silica particles disappeared by subsequent O2 annealing (data not shown). However, observed PL peak excited at 254 nm was 510 nm, which is not consistent with PL peak at 460 nm reported for the ODC(II)s in surface silica [12]. There have been some reports on the PL in similar highsurface silica. Glinka et al. reported a broad 530 nm PL which was assigned to hydrogen-related species [13]. Uchino et al. observed a broad PL band at 400–500 nm for fumed silica and assigned this to a pair of dioxasilirane @Si(O2) and silylene (@Si:) based on a theoretical calculation [14]. The dehydroxylation reaction proposed by Uchino et al. is consistent with our experimental data in Fig. 4(a), showing that the luminescent nature of silica particles appears after the removal of OH groups. Detailed
S. Inai et al. / Journal of Non-Crystalline Solids 353 (2007) 510–513
structure of the submicron silica particles is under investigation. 5. Summary We studied the correlation between luminescence properties and surface structures of submicron silica particles prepared by the Sto¨ber method. The PL spectra of submicron silica particles after annealing are broad and spread from 450 to 650 nm by excitation at an ultraviolet (254 and 365 nm), and the one at 500–800 nm by excitation at 488 nm. Based on the FT-IR and Raman spectra, the removal of impurities and consolidation of primary particles are responsible for the formation of luminescent centers induced by oxygen deficiency in the submicron-sized silica particles. Acknowledgment This work is partly supported by a grant-in-aid from Nippon Sheet Glass Foundation.
513
References [1] W. Sto¨ber, A. Fink, E. Bohn, J. Colloid Interf. Sci. 26 (1968) 62. [2] J.D. Joannopoulos, Nature 414 (2001) 257. [3] Y. Zhang, F. Phillipp, G.W. Meng, L.D. Zhang, C.H. Ye, J. Appl. Phys. 88 (2000) 2169. [4] H.J. Chang, Y.F. Chen, H.P. Lin, C.Y. Mou, Appl. Phys. Lett. 78 (2001) 3791. [5] J.L. Shen, P.N. Chen, Y.C. Lee, C.F. Cheng, Solid State Commun. 122 (2002) 65. [6] T. Uchino, T. Yamada, Appl. Phys. Lett. 85 (2004) 1164. [7] A. Harao, S. Inai, H. Nishikawa, in: Proceedings of 2005 International Symposium on Electrical Insulating Materials, 2005, pp. 672. [8] H. Boukari, G.G. Long, M.T. Harris, J. Colloid Interf. Sci. 229 (2000) 129. [9] F.L. Galeener, J. Non-Cryst. Solids 49 (1982) 53. [10] J. Bjo¨rnstro¨m, A. Martinelli, J.R.T. Johnson, A. Matic, I. Panas, Chem. Phys. Lett. 380 (2003) 165. [11] S. Takada, N. Hata, Y. Seino, K. Yamada, Y. Oku, T. Kikkawa, Jpn. J. Appl. Phys. 43 (2004) 2453. [12] L. Skuja, J. Non-Cryst. Solids 179 (1994) 51. [13] Y.D. Glinka, S.H. Lin, Y.T. Chen, Phys. Rev. B 66 (2002) 035404. [14] T. Uchino, N. Kurumoto, N. Sagawa, Phys. Rev. B 73 (2006) 233203.
Correlation between the luminescence properties and the surface structures of submicron silica particles Seiichiro Inai, Akira Harao, Hiroyuki Nishikawa
*
Department of Electrical Engineering, Shibaura Institute of Technology, 3-7-5 Toyosu, Kouto-ku, Tokyo 135-8548, Japan
Abstract We investigated the correlation between the luminescence properties and the surface structures of submicron silica particles prepared by the Sto¨ber method. After annealing in a non-oxidizing atmosphere, the submicron-sized silica particles show a broad photoluminescence (PL) band at 500–540 nm by excitation at an ultraviolet wavelengths (254 and 365 nm), and the one at the 600 nm by excitation an Ar+ laser (488 nm). The PL appeared to result from the removal of impurities and subsequent formation of several luminescent structures on the internal surface of the primary particles by thermal annealing. Ó 2007 Elsevier B.V. All rights reserved. PACS: 65.60.+a; 73.20.Hb; 78.55.m Keywords: Optical properties; Photoluminescence
1. Introduction Since silica (or amorphous SiO2) has a high chemical stability and superior optical properties, it bears a role as a basic material in electronics and photonics. Using tetraethoxysilane (TEOS) as a starting material, submicronsized silica particles with mono dispersion in diameter, can be synthesized with the Sto¨ber method [1]. The mono-dispersed submicron silica particles can be selfassembled into three dimensional periodic structures, socalled ‘synthetic opal’. It has been used as a template for Si-photonic crystals which has a periodic structure at the scale of wavelength of light [2]. It is well known that the structure of silica synthesized from TEOS by the sol–gel method is porous with high-surface to volume ratio before anneal. The luminescence properties of the internal surface of mesoporous silica are reported [3–5]. The luminescence properties of nanometer-sized silica particles, called fumed silica, as starting materials to prepare bulk silica glass were also reported *
Corresponding author. Tel.: +81 3 5859 8217; fax: +81 3 5859 8201. E-mail address: [email protected] (H. Nishikawa).
0022-3093/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2006.10.051
[6]. Therefore, it is expected that the submicron silica particles with a high-surface to volume ratio, show a similar luminescence characteristics. In fact, we observed luminescence from submicron-sized silica particles synthesized from TEOS after anneal in non-oxidizing conditions [7]. These submicron silica particles can open a new possibility of non-doped luminescent silica without incorporation of active ions such as Eu+. In this paper, we studied the correlation between the luminescence properties and the surface structures of submicron silica particles. Based on optical absorption, photoluminescence, FT-IR and Raman spectroscopy, we investigate the origin of the luminescent centers in the submicron-sized silica particles. 2. Experimental methods We prepared silica particles with 500 nm by the Sto¨ber method. The submicron silica particles of mono dispersed were synthesized by the hydrolysis and condensation polymerization reactions of TEOS, as shown in Eqs. (1) and (2). Si(OC2 H5 )4 + 4H2 O ! Si(OH)4 + 4C2 H5 OH
ð1Þ
S. Inai et al. / Journal of Non-Crystalline Solids 353 (2007) 510–513
and
Photon Energy (eV) 2.6
ð2Þ
3. Results 3.1. Optical properties of submicron silica particles Luminescence of silica particles was observed by excitation at an ultraviolet lamp (254, 365 nm), and an Ar+ laser (488 nm) after annealing at 700 °C for 1 h in nitrogen, as shown in Fig. 1. The PL spectra are broad and spread from 450 to 650 nm by excitation at an ultraviolet (254 and 365 nm), and the one at 500–800 nm by excitation at an Ar+ laser (488 nm). The PL spectra are the peak wavelength red-shifts from 500 nm to 600 nm with increasing excitation wavelength from 254 nm to 488 nm. It suggests that silica particles of after annealing have the several luminescent structures. Absorption spectra of silica particles before and after annealing (700 °C, nitrogen) are shown in Fig. 2. Before annealing, absorption does not appear. The optical absorption of the silica particles after annealing shows broad spectrum with several overlapping components. 3.2. Structure of silica submicron particles Fig. 3 shows expanded FT-IR spectra around 1100 cm1 before and after the heat treatment at 700 °C in nitrogen
2.4
2.2
2
1.8
1.6
254nm excitation 365nm excitation
PL intensity (arb.units)
The Si(OH)4 fragments produced by the reaction (1) are so rich in reactivity that the condensation polymerization of the Eq. (2) leads to form primary particles with diameters of 16–20 nm, which was confirmed by a small angle X-ray scattering measurements (SAXS) [8]. These primary particles further assemble into the secondary submicronsized particles of SiO2. Silica particles were synthesized from the mixture of 100 ml of ethanol and 8 ml of TEOS and 14 ml of ammonia and 2 ml of water. The temperature of the solution is 30 °C. Annealing of the silica particles were carried out at 700 °C for 1 h under flowing nitrogen. We evaluated submicron-sized silica particle by a scanning electron microscope (SEM) (SIMADZU, SSX-550) in terms of the size distribution. The PL spectra of silica particles in water were measured by a spectrometer with a CCD detector (Steller-net EPP2000) under the illumination of an ultraviolet lamp (254 and 365 nm excitation). Also, the PL from the silica particles on a silicon substrate was measured with a spectrometer equipped with a confocal microscope [TII Nanofinder, Ar+ laser (488 nm)]. All PL measurements were performed at room temperature. Optical absorption spectra in the visible and ultraviolet region were measured by a conventional spectrometer (Shimadzu, UV2450) equipped with an integrating sphere. Structures and impurities of silica particles were evaluated by an FT-IR spectrometer (JASCO, FT/IR-460plus). Raman scattering was measured by a micro-Raman spectrometer (JASCO, NS2100) under excitation at 488 nm.
488nm excitation
450
500
550
600
650
700
750
800
Wavelength (nm) Fig. 1. PL spectra of silica particles with a diameter of 500 nm after annealing at 700 °C for 1 h in nitrogen by excitation at 254, 365 and 488 nm.
Photon Energy (eV) 2.5
5 4.5
4
3.5
3
2.5
2
before annealing after annealing
Absorbance (arb.units)
BSiAOH + HOASiB ! BSiAOASiB + H2 O
511
2.0
1.5
1.0
0.5
0.0 240
320
400
480
560
640
720
800
Wavelength (nm) Fig. 2. Absorption spectra of silica particles with a diameter of 500 nm before and after annealing (700 °C, nitrogen).
atmosphere of the submicron silica particles. The spectrum of thermal oxide of silicon is also shown for reference. The spectral shape of the peaks at 1100 cm1, which is due to an Si–O–Si asymmetric stretching mode. After annealing, the peak of 1100 cm1 shifts to higher wavenumber. It means that silica particles are unstable inner surface structure. Especially a broad peak at the higher wavenumber of 1200 cm1 is peculiar to submicron particles with high specific surface area. Similar spectrum with luminescent mesoporous silica was reported [4,5]. It suggests that the porous structure is responsible for the luminescence in the submicron silica particles.
512
S. Inai et al. / Journal of Non-Crystalline Solids 353 (2007) 510–513
a Absorbance (arb.units)
Thermal oxide of Si before annealing after annealing
1000
1050
1100
1150
1200
1250
1300
-1
Wavenumber (cm ) Fig. 3. FT-IR spectra of silica particles with a diameter of 500 nm and 100-nm thick thermal oxide film on Si. The annealing was carried out at 700 °C for 1 h in nitrogen.
Figs. 4(a) and (b), respectively, show Raman spectra of silica particles before and after annealing and a schematic illustration of change in the surface structure of the primary silica particle by thermal annealing. From the Raman spectra in Fig. 4(a), the surface structure of the primary particle is in the form of a planar four-membered ring, as can be seen in the D1 band at 490 cm1. After annealing, the D1 band decreased and the D2 band at 606 cm1 due to a planar three-membered ring appears. In addition, the removal of H2O, OH, and CHX was observed [9–11]. Correlation of the luminescent structures will be discussed later. 4. Discussion As experimentally confirmed by SAXS [8], the submicron silica particles are expected to have a high-surface structure comprising of 16–20 nm primary particles. Fig. 4(b) illustrates the change in the surface structure of the silica particles and generation of the surface luminescent species by thermal annealing. First, the removal of impurities adsorbed on the surface of the primary particles occurs by annealing. Then, the three-membered rings are formed by the dehydroxylation reaction at the surface of the primary particles by annealing. This structural model is consistent with the appearance of the D2 band due to the three-membered rings in the Raman spectrum shown in Fig. 4(a). According to the report on the surface defects in silica [12], there are several luminescent structures, such as non-bridging oxygen hole centers (NBOHCs) and oxygen deficient centers (ODC(II)s). As shown in Fig. 2, the submicron silica particles show strong absorption peak at 250 nm and weak shoulders at about 260 and 650 nm.
b
Fig. 4. (a) Raman spectra of silica particles before and after annealing and (b) a schematic illustration of the change in the surface structure of the primary silica particle by thermal annealing.
The 250 nm band is in good agreement with the absorption band reported for the ODC(II)s in surface silica [12]. This assignment is supported by the fact that the 250 nm band in the N2-annealed submicron silica particles disappeared by subsequent O2 annealing (data not shown). However, observed PL peak excited at 254 nm was 510 nm, which is not consistent with PL peak at 460 nm reported for the ODC(II)s in surface silica [12]. There have been some reports on the PL in similar highsurface silica. Glinka et al. reported a broad 530 nm PL which was assigned to hydrogen-related species [13]. Uchino et al. observed a broad PL band at 400–500 nm for fumed silica and assigned this to a pair of dioxasilirane @Si(O2) and silylene (@Si:) based on a theoretical calculation [14]. The dehydroxylation reaction proposed by Uchino et al. is consistent with our experimental data in Fig. 4(a), showing that the luminescent nature of silica particles appears after the removal of OH groups. Detailed
S. Inai et al. / Journal of Non-Crystalline Solids 353 (2007) 510–513
structure of the submicron silica particles is under investigation. 5. Summary We studied the correlation between luminescence properties and surface structures of submicron silica particles prepared by the Sto¨ber method. The PL spectra of submicron silica particles after annealing are broad and spread from 450 to 650 nm by excitation at an ultraviolet (254 and 365 nm), and the one at 500–800 nm by excitation at 488 nm. Based on the FT-IR and Raman spectra, the removal of impurities and consolidation of primary particles are responsible for the formation of luminescent centers induced by oxygen deficiency in the submicron-sized silica particles. Acknowledgment This work is partly supported by a grant-in-aid from Nippon Sheet Glass Foundation.
513
References [1] W. Sto¨ber, A. Fink, E. Bohn, J. Colloid Interf. Sci. 26 (1968) 62. [2] J.D. Joannopoulos, Nature 414 (2001) 257. [3] Y. Zhang, F. Phillipp, G.W. Meng, L.D. Zhang, C.H. Ye, J. Appl. Phys. 88 (2000) 2169. [4] H.J. Chang, Y.F. Chen, H.P. Lin, C.Y. Mou, Appl. Phys. Lett. 78 (2001) 3791. [5] J.L. Shen, P.N. Chen, Y.C. Lee, C.F. Cheng, Solid State Commun. 122 (2002) 65. [6] T. Uchino, T. Yamada, Appl. Phys. Lett. 85 (2004) 1164. [7] A. Harao, S. Inai, H. Nishikawa, in: Proceedings of 2005 International Symposium on Electrical Insulating Materials, 2005, pp. 672. [8] H. Boukari, G.G. Long, M.T. Harris, J. Colloid Interf. Sci. 229 (2000) 129. [9] F.L. Galeener, J. Non-Cryst. Solids 49 (1982) 53. [10] J. Bjo¨rnstro¨m, A. Martinelli, J.R.T. Johnson, A. Matic, I. Panas, Chem. Phys. Lett. 380 (2003) 165. [11] S. Takada, N. Hata, Y. Seino, K. Yamada, Y. Oku, T. Kikkawa, Jpn. J. Appl. Phys. 43 (2004) 2453. [12] L. Skuja, J. Non-Cryst. Solids 179 (1994) 51. [13] Y.D. Glinka, S.H. Lin, Y.T. Chen, Phys. Rev. B 66 (2002) 035404. [14] T. Uchino, N. Kurumoto, N. Sagawa, Phys. Rev. B 73 (2006) 233203.