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Design of photonic structures by sol–gel-derived silica nanospheres
Journal of Non-Crystalline Solids 353 (2007) 674–678 www.elsevier.com/locate/jnoncrysol
Design of photonic structures by sol–gel-derived silica nanospheres A. Chiappini a,*, C. Armellini b, A. Chiasera b, M. Ferrari b, Y. Jestin b, M. Mattarelli a, M. Montagna a, E. Moser a, G. Nunzi Conti c, S. Pelli c, G.C. Righini d, M. Clara Gonc¸alves e, Rui M. Almeida e a Dipartimento di Fisica, CSMFO group, Universita` di Trento, via Sommarive 14, 38050 Povo-Trento, Italy CNR-IFN, Istituto di Fotonica e Nanotecnologie, CSMFO group, via Sommarive 14, 38050 Povo-Trento, Italy CNR-IFAC, Optoelectronics and Photonics Department, Via Madonna del Piano 10, 50019, Sesto Fiorentino (Firenze), Italy d CNR, Department of Materials and Devices, via dei Taurini 19, 00185 Roma, Italy e Depart. Eng. de Materiais/ICEMS, Instituto Superior Te´cnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal b
c
Available online 14 February 2007
Abstract Sol–gel processing was used to obtain monosized silica spheres of 270 nm in diameter. Starting from these spheres, two different systems have been fabricated: (i) 3D Photonic Crystals by means of vertical deposition and evaporation-assisted sedimentation deposition methods; (ii) core-shell-like Er3+-activated silica spheres, where the core is the silica sphere and the shell is an Er2O3–SiO2 coating. Optical and spectroscopic assessment, as well as morphological and structural characterization of the systems, have been performed. Ó 2007 Elsevier B.V. All rights reserved. PACS: 42.70.a; 42.70.Qs; 78.55.m; 81.20.Fw; 82.70.Dd Keywords: Photonic bandgap; Colloids; Nanoparticles; Luminescence; Optical spectroscopy; Time resolved measurements; Silica
1. Introduction Monodisperse colloidal spheres in solution can selforganize into an ordered structure if their size is adequate and their size polydispersivity is low enough, yielding a periodic photonic bandgap structure, or Photonic Crystal [1]. These structures are commonly called three-dimensional Photonic Crystals (3D-PCs), or Opals. 3D-PCs and their properties largely derive from the structure, rather than from the material itself. Opals have only a relatively recent history as photonic bandgap materials and have received a strong thrust from their adequacy as scaffoldings [2]. These structures lead to photonic devices such as switches, mirrors, filters and superprisms [3]. In particular, in the last decade, significant attention has been paid to
*
Corresponding author. Tel.: +39 0461881695; fax: +39 0461881696. E-mail address: [email protected] (A. Chiappini).
0022-3093/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2006.10.034
various routes to fabricate these 3D-PCs. Furthermore, monodisperse colloidal spheres of predictable size and shape, activated with a controllable concentration of rare earth (RE) ions like Er3+ or Eu3+, have significant potential for use in optical devices such as micro-lasers, integrated optics structures, luminescent markers or nanosensors and active photonic band gap materials. In particular, in order to use RE-activated colloids in photonic crystals, it is well known that the size polydispersivity of the particles needs to be low and controllable. Typically, monosize silica particles are synthesized via base-catalyzed hydrolysis of TEOS [4]. However, the incorporation of RE ions, into the silica spheres, by dissolving a RE salt in ethanol, fails for the base-catalyzed reaction because the RE ion immediately forms an insoluble RE hydroxide [5]. Moran et al. have shown that it is possible to incorporate lanthanide ions during the growth of silica particles, using acid catalysis [6]. In this work, we present the details of the procedure used to synthesize monosize silica spheres and
A. Chiappini et al. / Journal of Non-Crystalline Solids 353 (2007) 674–678
fabricate two different systems: (i) 3D Photonic Crystals, using two different colloidal routes; (ii) core-shell-like structures of Er3+-activated silica spheres, using a seed growth method. Optical and spectroscopic assessment, as well as morphological and structural characterization of these systems is reported. 2. Experimental Silica particles were prepared following the Stober method [4]. In particular tetraethyl orthosilicate (TEOS, Aldrich), ethanol (Merck), concentrated ammonia (Fluka) and distilled water were used as reagents. With the Stober method, the formation of silica spheres takes place by the hydrolysis and condensation–polymerization reactions, using TEOS as starting material, in an alcoholic solution (ethanol) of water and ammonia. Ammonia plays the role of catalyst [1]. Furthermore, in order to obtain monosize silica spheres, it is important to use the following working conditions: constant temperature reaction and appropriated molar concentrations of the reagents [7]. Therefore, the reactions were realized in a ‘clean-room’ and using the following molar concentrations: TEOS, 0.22 M, distilled water (DW), 15 M and NH3, 1 M. Two mother solutions were prepared, containing TEOSethanol and ammonia–water–ethanol, respectively, and then they were quickly combined in a reaction vessel. The mixture was stirred for 24 h with a magnetic stirrer. Subsequently, the silica suspensions were centrifuged at 3000 rpm for 30 min and washed with water. The centrifuging/washing procedure was repeated six times and finally the particles were dried at 80 °C overnight. The core-shell-like Er3+-activated silica spheres, where the core is the silica sphere and the shell is an Er2O3– SiO2 coating, were prepared using the following protocol: (i) the core was realized using Stober method, previously described; (ii) the shell was obtained by a seeded growth method [7]. Briefly, 150 mg of silica spheres of 270 nm diameter, obtained by the Stober method, were added to a solution with the molar ratio TEOS:CH3COOH:H2O of 1:8:8, plus 0.2 wt% ErCl3. The mixture was stirred for 45 min with a magnetic stirrer. After synthesis, the SiO2 particles were separated from the solution by centrifuging at 1000 rpm and washed at least twice with pure ethanol. Subsequently, the core-shell-like Er3+-activated silica spheres were heat treated at 950 °C for 30 min [5]. For the formation of opals, we have used two different methods: vertical deposition [8] and evaporation-assisted sedimentation (EAS) deposition method [9]. Details on the two deposition methods are reported in [1]. The particle sizes were determined from electron micrographs taken with a scanning electron microscope, SEM (JEOL-JSM 6300). The diameters of over a hundred particles were used in calculations of the average size and standard deviation d of each sample. Also the structural characterizations of the opals were performed using the
675
same SEM apparatus. Optical properties and quality of the opal structures were evaluated by transmittance and reflectance measurements at different angles, using a double beam Varian spectrophotometer. Photoluminescence (PL) spectroscopy on core-shell like structures, in the region of the 4I13/2 ! 4I15/2 transition of Er3+ ions, was performed using the 514.5 nm line of an Ar+ ion laser and the 980 nm line of a Ti:Sapphire laser as excitation sources. The luminescence was dispersed by a 320 mm single-grating monochromator with a resolution of 2 nm. The light was detected using a Si/InGaAs two color photodiode and standard lock-in technique. Decay curves were obtained by recording the signal by a digital oscilloscope. The measurements were performed on pressed KBr pellets, containing 5% of doped silica spheres. 3. Results In this work we report only the results related to opals obtained using vertical deposition method because equivalent to those obtained by EAS deposition method. In Fig. 1, we present an SEM image from the top surface of an opal obtained by vertical deposition method. A wellordered close packed structure is evident. The average diameter of the silica spheres, is 270 nm and the standard deviation (d) of these spheres was estimated at less than d < 5%. Fig. 2 shows the normalized reflectance spectra obtained at different angles of an opal formed using spheres of 270 nm. In Fig. 3 we present the optical transmission measured at normal incidence, where a pronounced attenuation dip in transmission at around 564 nm is observed. Fig. 4 shows an SEM image of the core-shell-like Er3+activated silica spheres, obtained using a seeded growth method on the 270 nm monosize silica spheres.
Fig. 1. SEM micrograph of the top surface of the opal formed by the vertical deposition method of 270 nm diameter silica spheres.
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A. Chiappini et al. / Journal of Non-Crystalline Solids 353 (2007) 674–678 40º
1.0
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Reflectance (a.u.)
0.8
0.6
0.4
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0.0 460
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Wavelength (nm)
Fig. 2. Reflectance spectra at different incidence angles performed on the opal formed by the vertical deposition method of 270 nm diameter silica spheres.
Fig. 4. SEM images of the core-shell-like particles after seeded growth using the acid-based reaction.
80 70
stop band depth =40% Intensity (a.u.)
Transmittance %
60 50 40 30
b)
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0 400
1400 450
500
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Fig. 3. Transmission spectrum of the opal at normal incidence.
Fig. 5 represents the PL spectra for the core-shell like structure in the region of the 4I13/2 ! 4I15/2 transition of Er3+ ions, obtained upon excitation at 514.5 nm and at 980 nm. The decay curve presented in Fig. 6 shows a single exponential decay for the core-shell like structure.
4. Discussion The triangular arrangement observed in Fig. 1 can correspond to either a h1 1 1i surface of a face centred cubic (fcc) or a h0 0 1i surface of a hexagonal close packed (hcp) system. However, there are theoretical and experimental indications that the fcc is indeed the actual structure [10]. In Fig. 2, it is possible to identify the stop bands in the visible region and observe that increasing the angle of incidence the stop band position shifts to shorter wavelengths
1450
1500
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Wavelenght (nm)
Fig. 5. Room temperature photoluminescence spectra of 4I13/2 ! 4I15/2 transition of the Er3+ ions for the silica core-shell-like structure, (a) upon excitation at 514.5 nm and (b) upon excitation at 980 nm.
[11]. This behavior can be express by the modified form of Bragg’s law [12] qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 k ¼ 2 d ðneff Þ ðsin hÞ ; ð1Þ where k is the free-space wavelength of the light, d is the inter-planar spacing, neff is the effective refractive index and h is the angle measured from the normal to the planes. Assuming that the reflection occurs from the (1 1 1) planes of an fcc structure, the interplanar spacing, d, is related to the sphere diameter, D, by pffiffiffiffiffiffiffiffi d 111 ¼ 2=3 D: ð2Þ By a fit with curve (1) of the stop band wavelength, k, as a function of the incidence angle, h, neff and d are estimated. The fit gives D = 255 nm, in good agreement with the
A. Chiappini et al. / Journal of Non-Crystalline Solids 353 (2007) 674–678
Intensity (a.u.)
1
0.1
0.00
0.01
0.02
0.03
0.04
Time (s)
Fig. 6. Room temperature luminescence decay curve from the 4I13/2 state of Er3+ ions for the silica core-shell-like structure, upon excitation at 514.5 nm, after annealing for 30 min at 950 °C.
sphere size measured by SEM (D = 270 nm) and neff = 1.333. For a close packed structure, neff is given by n2eff ¼ n2spheres f þ n2medium ð1 f Þ;
ð3Þ
where nspheres and nmedium are the refractive indices of silica microspheres and the surrounding medium, respectively; and f is the filling factor (f = 0.74 for a close packed structure). We assumed a refractive index of the surrounding medium nmedium = 1. The refractive index of the spheres was measured with an index matching method [13] as nspheres = 1.425. The value of neff calculated using Eq. (3) is 1.327, very close to neff = 1.333 obtained from the fitted curve [11,14]. This is a further confirmation that the spheres are arranged in a close packed structure. From Fig. 3, it is possible to estimate the quality of the fabricated photonic crystal considering two different parameters: (i) the peak broadening effects [11,15]; (ii) the stop band depth [9]. The peak broadening effect at normal incidence, defined as Dk/kc, where Dk corresponds to the full width at half height of the transmittance spectrum and kc is the position of the transmittance minimum, gives a normalized measure of the photonic stop band. The stop band depth is defined as the dip in percent transmittance that occurs at the stop band peak for normal incidence (h = 0°) as measured from the lower plateau of the two arms [9]. From Fig. 3 a value of the stop band depth of 40% and an experimental value of peak broadening Dk/ kc = 0.08 are determined. These values can be compared to those published by Mc Comb et al. and Sheng et al. [9,15], who have obtained, on analogous samples, a value of peak broadening of 0.07 and a stop band depth of 26%. This confirms the high quality of the present opals in comparison to those reported in the current state of art literature [9,15].
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As for core-shell like structure (Fig. 4), shows that seeded growth in the Er solution occurs on individual particles that do not collapse in higher size clusters. PL measurements, obtained upon excitation at 514.5 nm and at 980 nm, confirm the incorporation of Er3+ ions in the silica shell; in fact, the shape of the emission band in Fig. 5 is typical of Er3+-activated silica glasses with a main emission peak at 1533 nm [16] and a spectral bandwidth, measured at 3 dB from the maximum intensity, of 27 ± 2 nm. The shape of the luminescence band is the same for 514.5 and 980 nm excitation. The decay curve of Fig. 6 exhibits a single-exponential behavior with a lifetime of 12.8 ± 0.1 ms. We can compare this result with the lifetime of 13.2 ms measured on similar samples by Dood et al. [5], which can be considered close to the radiative lifetime (srad) for erbium in silica. The measured lifetime (smeas) must be compared with the radiative lifetime, to obtain the quantum efficiency g defined by their ratio: g = smeas/srad. The obtained results demonstrate a quantum efficiency of 97% for the sol–gel-based core-shell silica spheres. 5. Conclusions The measured optical parameters and their comparison with those reported in the current literature indicate that high quality synthetic opal photonic crystals were formed by controlled self-organisation of colloidal silica spheres. A sol–gel fabrication protocol was elaborated, obtaining silica microspheres of 270 nm diameter with a polydispersivity of less than 5%. We have demonstrated that large well-ordered crystals of synthetic opal, which exhibit a photonic stopband, can be produced in a few days, starting from these spheres by vertical deposition or evaporationassisted sedimentation. The results related to opals obtained using EAS deposition method are equivalent to these obtained by vertical deposition. Reflectance measurements at different angles and SEM measurements indicate that the structure is a well ordered three dimensional face centered cubic structure. Transmission measurements permit an estimate of the peak broadening effect of Dk/kc = 0.08 and a value of the stop band depth of 40%, which are a further indication of the high quality of the opals prepared. Seeded growth was successfully applied to synthesize core-shell-like Er3+-activated silica spheres. Typical photoluminescence spectrum of Erbium, with a lifetime of 12.8 ms, was observed for the core-shell-like structure, after annealing for 30 min at 950 °C. Acknowledgements Authors acknowledge the financial support of MIURFIRB RBNE012N3X, PAT FAPVU 2004–2006, MIURPRIN 2004, ITPAR (2003–2006) and also the support of a GRICES-CNR collaborative grant for the period of 2005/2006.
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References [1] A. Chiappini, C. Armellini, S.N.B. Bhaktha, A. Chiasera, M. Ferrari, Y. Jestin, M. Mattarelli, M. Montagna, E. Moser, G. Nunzi Conti, S. Pelli, G.C. Righini, V.M. Sglavo, Proc. SPIE 6182 (2006) 454. [2] J.F. Galisteo, F. Garcı`a-Santamarı`a, D. Golmayo, B.H. Jua`rez, C. Lo`pez, E. Palacios, J. Opt. A: Pure Appl. Opt. 7 (2005) S244. [3] R. Fenollosa, M. Ibisate, S. Rubio, C. Lopez, J. Sanchez-Dehesa, J. Appl. Phys. 93 (2003) 671. [4] W. Stober, A. Fink, J. Colloid Interf. Sci. 26 (1968) 62. [5] M.J.A. de Dood, B. Berkhout, C.M. van Kats, A. Polman, A. van Blaaderen, Chem. Mater. 14 (2002) 2849. [6] C.E. Moran, G.D. Hale, N.J. Halas, Langmuir 17 (2001) 8376. [7] G.H. Bogush, M.A. Tracy, C.F. Zukoski IV, J. Non-Cryst. Solids 104 (1988) 95.
[8] P. Jang, J.F. Bertone, K.S. Hwang, V.L. Colvin, Chem. Mater. 11 (1999) 2132. [9] Sheng-Yuan Hsiao, David Shan Hill Wong, Shish-Yuan Lu, J. Am. Ceram. Soc. 88 (2005) 974. [10] L.V. Woodcock, Nature 385 (1997) 141. [11] H. Miguez, C. Lopez, F. Meseguer, A. Blanco, L. Vazquez, R. Mayoral, M. Ocana, V. Fornes, A. Mitsud, Appl. Phys. Lett. 9 (1997) 1148. [12] R.M. Almeida, S. Portal, Curr. Opin. Solid State Mater. Sci. 7 (2003) 151. [13] F. Garcia-Santamaria, H. Miguez, M. Ibisate, F. Meseguer, C. Lopez, Langmuir 18 (2002) 1942. [14] H. Fudouzi, J. Colloid Interf. Sci. 275 (2004) 277. [15] D. McComb, B.M. Treble, C.J. Smith, R.M. De La Rue, N.P. Johnson, Mater. Chem. 11 (2001) 142. [16] W.J. Miniscalco, J. Lightwave Technol. 9 (1991) 234.
Design of photonic structures by sol–gel-derived silica nanospheres A. Chiappini a,*, C. Armellini b, A. Chiasera b, M. Ferrari b, Y. Jestin b, M. Mattarelli a, M. Montagna a, E. Moser a, G. Nunzi Conti c, S. Pelli c, G.C. Righini d, M. Clara Gonc¸alves e, Rui M. Almeida e a Dipartimento di Fisica, CSMFO group, Universita` di Trento, via Sommarive 14, 38050 Povo-Trento, Italy CNR-IFN, Istituto di Fotonica e Nanotecnologie, CSMFO group, via Sommarive 14, 38050 Povo-Trento, Italy CNR-IFAC, Optoelectronics and Photonics Department, Via Madonna del Piano 10, 50019, Sesto Fiorentino (Firenze), Italy d CNR, Department of Materials and Devices, via dei Taurini 19, 00185 Roma, Italy e Depart. Eng. de Materiais/ICEMS, Instituto Superior Te´cnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal b
c
Available online 14 February 2007
Abstract Sol–gel processing was used to obtain monosized silica spheres of 270 nm in diameter. Starting from these spheres, two different systems have been fabricated: (i) 3D Photonic Crystals by means of vertical deposition and evaporation-assisted sedimentation deposition methods; (ii) core-shell-like Er3+-activated silica spheres, where the core is the silica sphere and the shell is an Er2O3–SiO2 coating. Optical and spectroscopic assessment, as well as morphological and structural characterization of the systems, have been performed. Ó 2007 Elsevier B.V. All rights reserved. PACS: 42.70.a; 42.70.Qs; 78.55.m; 81.20.Fw; 82.70.Dd Keywords: Photonic bandgap; Colloids; Nanoparticles; Luminescence; Optical spectroscopy; Time resolved measurements; Silica
1. Introduction Monodisperse colloidal spheres in solution can selforganize into an ordered structure if their size is adequate and their size polydispersivity is low enough, yielding a periodic photonic bandgap structure, or Photonic Crystal [1]. These structures are commonly called three-dimensional Photonic Crystals (3D-PCs), or Opals. 3D-PCs and their properties largely derive from the structure, rather than from the material itself. Opals have only a relatively recent history as photonic bandgap materials and have received a strong thrust from their adequacy as scaffoldings [2]. These structures lead to photonic devices such as switches, mirrors, filters and superprisms [3]. In particular, in the last decade, significant attention has been paid to
*
Corresponding author. Tel.: +39 0461881695; fax: +39 0461881696. E-mail address: [email protected] (A. Chiappini).
0022-3093/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2006.10.034
various routes to fabricate these 3D-PCs. Furthermore, monodisperse colloidal spheres of predictable size and shape, activated with a controllable concentration of rare earth (RE) ions like Er3+ or Eu3+, have significant potential for use in optical devices such as micro-lasers, integrated optics structures, luminescent markers or nanosensors and active photonic band gap materials. In particular, in order to use RE-activated colloids in photonic crystals, it is well known that the size polydispersivity of the particles needs to be low and controllable. Typically, monosize silica particles are synthesized via base-catalyzed hydrolysis of TEOS [4]. However, the incorporation of RE ions, into the silica spheres, by dissolving a RE salt in ethanol, fails for the base-catalyzed reaction because the RE ion immediately forms an insoluble RE hydroxide [5]. Moran et al. have shown that it is possible to incorporate lanthanide ions during the growth of silica particles, using acid catalysis [6]. In this work, we present the details of the procedure used to synthesize monosize silica spheres and
A. Chiappini et al. / Journal of Non-Crystalline Solids 353 (2007) 674–678
fabricate two different systems: (i) 3D Photonic Crystals, using two different colloidal routes; (ii) core-shell-like structures of Er3+-activated silica spheres, using a seed growth method. Optical and spectroscopic assessment, as well as morphological and structural characterization of these systems is reported. 2. Experimental Silica particles were prepared following the Stober method [4]. In particular tetraethyl orthosilicate (TEOS, Aldrich), ethanol (Merck), concentrated ammonia (Fluka) and distilled water were used as reagents. With the Stober method, the formation of silica spheres takes place by the hydrolysis and condensation–polymerization reactions, using TEOS as starting material, in an alcoholic solution (ethanol) of water and ammonia. Ammonia plays the role of catalyst [1]. Furthermore, in order to obtain monosize silica spheres, it is important to use the following working conditions: constant temperature reaction and appropriated molar concentrations of the reagents [7]. Therefore, the reactions were realized in a ‘clean-room’ and using the following molar concentrations: TEOS, 0.22 M, distilled water (DW), 15 M and NH3, 1 M. Two mother solutions were prepared, containing TEOSethanol and ammonia–water–ethanol, respectively, and then they were quickly combined in a reaction vessel. The mixture was stirred for 24 h with a magnetic stirrer. Subsequently, the silica suspensions were centrifuged at 3000 rpm for 30 min and washed with water. The centrifuging/washing procedure was repeated six times and finally the particles were dried at 80 °C overnight. The core-shell-like Er3+-activated silica spheres, where the core is the silica sphere and the shell is an Er2O3– SiO2 coating, were prepared using the following protocol: (i) the core was realized using Stober method, previously described; (ii) the shell was obtained by a seeded growth method [7]. Briefly, 150 mg of silica spheres of 270 nm diameter, obtained by the Stober method, were added to a solution with the molar ratio TEOS:CH3COOH:H2O of 1:8:8, plus 0.2 wt% ErCl3. The mixture was stirred for 45 min with a magnetic stirrer. After synthesis, the SiO2 particles were separated from the solution by centrifuging at 1000 rpm and washed at least twice with pure ethanol. Subsequently, the core-shell-like Er3+-activated silica spheres were heat treated at 950 °C for 30 min [5]. For the formation of opals, we have used two different methods: vertical deposition [8] and evaporation-assisted sedimentation (EAS) deposition method [9]. Details on the two deposition methods are reported in [1]. The particle sizes were determined from electron micrographs taken with a scanning electron microscope, SEM (JEOL-JSM 6300). The diameters of over a hundred particles were used in calculations of the average size and standard deviation d of each sample. Also the structural characterizations of the opals were performed using the
675
same SEM apparatus. Optical properties and quality of the opal structures were evaluated by transmittance and reflectance measurements at different angles, using a double beam Varian spectrophotometer. Photoluminescence (PL) spectroscopy on core-shell like structures, in the region of the 4I13/2 ! 4I15/2 transition of Er3+ ions, was performed using the 514.5 nm line of an Ar+ ion laser and the 980 nm line of a Ti:Sapphire laser as excitation sources. The luminescence was dispersed by a 320 mm single-grating monochromator with a resolution of 2 nm. The light was detected using a Si/InGaAs two color photodiode and standard lock-in technique. Decay curves were obtained by recording the signal by a digital oscilloscope. The measurements were performed on pressed KBr pellets, containing 5% of doped silica spheres. 3. Results In this work we report only the results related to opals obtained using vertical deposition method because equivalent to those obtained by EAS deposition method. In Fig. 1, we present an SEM image from the top surface of an opal obtained by vertical deposition method. A wellordered close packed structure is evident. The average diameter of the silica spheres, is 270 nm and the standard deviation (d) of these spheres was estimated at less than d < 5%. Fig. 2 shows the normalized reflectance spectra obtained at different angles of an opal formed using spheres of 270 nm. In Fig. 3 we present the optical transmission measured at normal incidence, where a pronounced attenuation dip in transmission at around 564 nm is observed. Fig. 4 shows an SEM image of the core-shell-like Er3+activated silica spheres, obtained using a seeded growth method on the 270 nm monosize silica spheres.
Fig. 1. SEM micrograph of the top surface of the opal formed by the vertical deposition method of 270 nm diameter silica spheres.
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A. Chiappini et al. / Journal of Non-Crystalline Solids 353 (2007) 674–678 40º
1.0
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20º 12.5º
Reflectance (a.u.)
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0.6
0.4
0.2
0.0 460
480
500
520
540
560
580
600
Wavelength (nm)
Fig. 2. Reflectance spectra at different incidence angles performed on the opal formed by the vertical deposition method of 270 nm diameter silica spheres.
Fig. 4. SEM images of the core-shell-like particles after seeded growth using the acid-based reaction.
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stop band depth =40% Intensity (a.u.)
Transmittance %
60 50 40 30
b)
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0 400
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500
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Wavelength (nm)
Fig. 3. Transmission spectrum of the opal at normal incidence.
Fig. 5 represents the PL spectra for the core-shell like structure in the region of the 4I13/2 ! 4I15/2 transition of Er3+ ions, obtained upon excitation at 514.5 nm and at 980 nm. The decay curve presented in Fig. 6 shows a single exponential decay for the core-shell like structure.
4. Discussion The triangular arrangement observed in Fig. 1 can correspond to either a h1 1 1i surface of a face centred cubic (fcc) or a h0 0 1i surface of a hexagonal close packed (hcp) system. However, there are theoretical and experimental indications that the fcc is indeed the actual structure [10]. In Fig. 2, it is possible to identify the stop bands in the visible region and observe that increasing the angle of incidence the stop band position shifts to shorter wavelengths
1450
1500
1550
1600
1650
1700
Wavelenght (nm)
Fig. 5. Room temperature photoluminescence spectra of 4I13/2 ! 4I15/2 transition of the Er3+ ions for the silica core-shell-like structure, (a) upon excitation at 514.5 nm and (b) upon excitation at 980 nm.
[11]. This behavior can be express by the modified form of Bragg’s law [12] qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 k ¼ 2 d ðneff Þ ðsin hÞ ; ð1Þ where k is the free-space wavelength of the light, d is the inter-planar spacing, neff is the effective refractive index and h is the angle measured from the normal to the planes. Assuming that the reflection occurs from the (1 1 1) planes of an fcc structure, the interplanar spacing, d, is related to the sphere diameter, D, by pffiffiffiffiffiffiffiffi d 111 ¼ 2=3 D: ð2Þ By a fit with curve (1) of the stop band wavelength, k, as a function of the incidence angle, h, neff and d are estimated. The fit gives D = 255 nm, in good agreement with the
A. Chiappini et al. / Journal of Non-Crystalline Solids 353 (2007) 674–678
Intensity (a.u.)
1
0.1
0.00
0.01
0.02
0.03
0.04
Time (s)
Fig. 6. Room temperature luminescence decay curve from the 4I13/2 state of Er3+ ions for the silica core-shell-like structure, upon excitation at 514.5 nm, after annealing for 30 min at 950 °C.
sphere size measured by SEM (D = 270 nm) and neff = 1.333. For a close packed structure, neff is given by n2eff ¼ n2spheres f þ n2medium ð1 f Þ;
ð3Þ
where nspheres and nmedium are the refractive indices of silica microspheres and the surrounding medium, respectively; and f is the filling factor (f = 0.74 for a close packed structure). We assumed a refractive index of the surrounding medium nmedium = 1. The refractive index of the spheres was measured with an index matching method [13] as nspheres = 1.425. The value of neff calculated using Eq. (3) is 1.327, very close to neff = 1.333 obtained from the fitted curve [11,14]. This is a further confirmation that the spheres are arranged in a close packed structure. From Fig. 3, it is possible to estimate the quality of the fabricated photonic crystal considering two different parameters: (i) the peak broadening effects [11,15]; (ii) the stop band depth [9]. The peak broadening effect at normal incidence, defined as Dk/kc, where Dk corresponds to the full width at half height of the transmittance spectrum and kc is the position of the transmittance minimum, gives a normalized measure of the photonic stop band. The stop band depth is defined as the dip in percent transmittance that occurs at the stop band peak for normal incidence (h = 0°) as measured from the lower plateau of the two arms [9]. From Fig. 3 a value of the stop band depth of 40% and an experimental value of peak broadening Dk/ kc = 0.08 are determined. These values can be compared to those published by Mc Comb et al. and Sheng et al. [9,15], who have obtained, on analogous samples, a value of peak broadening of 0.07 and a stop band depth of 26%. This confirms the high quality of the present opals in comparison to those reported in the current state of art literature [9,15].
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As for core-shell like structure (Fig. 4), shows that seeded growth in the Er solution occurs on individual particles that do not collapse in higher size clusters. PL measurements, obtained upon excitation at 514.5 nm and at 980 nm, confirm the incorporation of Er3+ ions in the silica shell; in fact, the shape of the emission band in Fig. 5 is typical of Er3+-activated silica glasses with a main emission peak at 1533 nm [16] and a spectral bandwidth, measured at 3 dB from the maximum intensity, of 27 ± 2 nm. The shape of the luminescence band is the same for 514.5 and 980 nm excitation. The decay curve of Fig. 6 exhibits a single-exponential behavior with a lifetime of 12.8 ± 0.1 ms. We can compare this result with the lifetime of 13.2 ms measured on similar samples by Dood et al. [5], which can be considered close to the radiative lifetime (srad) for erbium in silica. The measured lifetime (smeas) must be compared with the radiative lifetime, to obtain the quantum efficiency g defined by their ratio: g = smeas/srad. The obtained results demonstrate a quantum efficiency of 97% for the sol–gel-based core-shell silica spheres. 5. Conclusions The measured optical parameters and their comparison with those reported in the current literature indicate that high quality synthetic opal photonic crystals were formed by controlled self-organisation of colloidal silica spheres. A sol–gel fabrication protocol was elaborated, obtaining silica microspheres of 270 nm diameter with a polydispersivity of less than 5%. We have demonstrated that large well-ordered crystals of synthetic opal, which exhibit a photonic stopband, can be produced in a few days, starting from these spheres by vertical deposition or evaporationassisted sedimentation. The results related to opals obtained using EAS deposition method are equivalent to these obtained by vertical deposition. Reflectance measurements at different angles and SEM measurements indicate that the structure is a well ordered three dimensional face centered cubic structure. Transmission measurements permit an estimate of the peak broadening effect of Dk/kc = 0.08 and a value of the stop band depth of 40%, which are a further indication of the high quality of the opals prepared. Seeded growth was successfully applied to synthesize core-shell-like Er3+-activated silica spheres. Typical photoluminescence spectrum of Erbium, with a lifetime of 12.8 ms, was observed for the core-shell-like structure, after annealing for 30 min at 950 °C. Acknowledgements Authors acknowledge the financial support of MIURFIRB RBNE012N3X, PAT FAPVU 2004–2006, MIURPRIN 2004, ITPAR (2003–2006) and also the support of a GRICES-CNR collaborative grant for the period of 2005/2006.
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