Physical studies of porphyrin-infiltrated opal crystals

Materials Science and Engineering C 27 (2007) 985 – 989 www.elsevier.com/locate/msec

Physical studies of porphyrin-infiltrated opal crystals J. Sabataityte a,⁎, I. Simkiene a , G.-J. Babonas a , A. Reza a , M. Baran b , R. Szymczak b , R. Vaisnoras c , L. Rasteniene c , V. Golubev d , D. Kurdyukov d a

Semiconductor Physics Institute, Gostauto 11, LT 01108 Vilnius, Lithuania b Institute of Physics, PAN, PL 02668, Warsaw, Poland c Vilnius Pedagogical University, LT 08106, Vilnius, Lithuania d Ioffe Physico-Technical Institute RAS, 194021, S.Petersburg, Russia

Received 5 May 2006; received in revised form 28 September 2006; accepted 28 September 2006 Available online 30 November 2006

Abstract Artificial opals made of silica spheres and infiltrated with aqueous solution of iron porphyrin (FeTPPS) possessing the absorption band in a visible spectral range were studied. The structural, optical and magnetic properties of composite structures were investigated. Bulk samples of opal structure were obtained by sedimentation technique from colloidal solution of SiO2 spheres of diameter 240 and 245 nm. The structure of the samples was examined by atomic force microscopy. The properties of photonic crystals were demonstrated by optical measurements in transmission and reflection modes. The stop band was observed in the region 510–550 nm. In samples annealed at 900 °C the width of the stop band increased to ∼70 nm. Aqueous solutions of FeTPPS of concentration ∼1.0 mM and various pH-values were used for infiltration. The infiltration has led to a change of photonic characteristics, position of the stop band and dependence on light incidence angle. The absorption bands typical of FeTPPS were observed in the vicinity of the stop band. The photonic properties of infiltrated opal structures were determined to depend on the acidity of aqueous solution, which was used in technological procedure. Magnetic properties of FeTPPS-infiltrated opal samples, which have been studied at 5–300 K in magnetic fields up to 5 T, were discussed. From magnetic measurements it followed that magnetic Fe–Fe interactions have practically vanished in hybrid samples and Fe centers should be treated as isolated ones. © 2006 Elsevier B.V. All rights reserved. PACS: 81.07; 78.67-n Keywords: Opal; Hybrid structures; Optical properties

1. Introduction Recently, an artificial opal made of silica spheres has received a great attention as a template to load with another material and to obtain the structure with a complete photonic band gap [1,2]. In order to control the structure of stop band, opals were infiltrated with different materials like metals [3], liquid crystals [4], molecular aggregates [5], organic molecules and semiconductors [6], ferroelectrics [7], etc. It was demonstrated [7] that SiO2 colloid crystal infiltrated with ferroelectric BaTiO3 can be used for thermal tuning of the photonic band gap due to the sensitivity of ferroelectric microstructure on temper-

⁎ Corresponding author. E-mail address: [email protected] (J. Sabataityte). 0928-4931/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2006.09.048

ature. Photoswitchable photonic crystals were fabricated [4] by infiltrating liquid crystal into inverse opal structure and making use of photoinduced nematic–isotropic phase transition. When embedded in opal-based photonic crystals, organic molecules were shown [6] to possess a modified fluorescence spectra and photon trapping effect was observed [8] in dye-doped opal structures. It was also shown [9] that intragap light propagates in 3D photonic crystals infiltrated with highly polarizable medium due to the interaction between the Bragg gap and polariton gap. Tunability of photonic band gap upon applied electric field was found [10] to be effective in opal and in particular in inverse opal with liquid crystals possessing high optical anisotropy. Three-dimensional magnetophotonic crystals possessing typical magnetic properties were fabricated [1] on the base of artificial opals impregnated with different types of magnetic materials. A large enhancement of linear and

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Aqueous solutions of Fe(III)meso-tetra(4-sulfonatophenyl) porphine (FeTPPS) with concentration ∼ 1 × 10− 3 M were prepared. By adding the appropriate amount of 0.1 M NaOH and 0.1 M HCl, the acidity of FeTPPS aqueous solutions was changed. The solutions with pH-values 1 and 11 were prepared. Immediately before the infiltration procedure, opal samples were washed in alcohol in ultrasound bath for 10 min to improve the wetting conditions for water and then dried at room temperature. Opal samples were immersed into 2 ml of FeTPPS aqueous solution and placed into ultrasound bath at ∼35 °C for 1 h. The obtained composite samples were dried at 95 °C for 0.5 h. Fig. 1 illustrates the morphology of obtained composite opal structures infiltrated with FeTPPS. A regular opal-like structure was observed in an area of at least 10 μm. As is seen from Fig. 1, aggregates of FeTPPS organic molecules were intercalated into the voids of opal structure and were also located on the surface of SiO2 spheres. The self-assembled aggregates of FeTPPS were distributed non-homogeneously within opal crystal. 3. Results and discussion

Fig. 1. AFM micrographs of area 2 × 2 μm2 (a) and 0.7 × 0.7 μm2 (b) of opal samples of the second series infiltrated with FeTPPS.

nonlinear magneto-optical response was determined [2] in 1D, 2D and 3D magnetophotonic crystals. Thus, infiltrated photonic crystals enable one to control the light propagation making use of external fields, and from this point of view the structures under consideration are perspective for development of future optoelectronic devices [8]. The goal of present studies was to design the procedure for infiltration of anionic iron porphyrin (FeTPPS) into opal structure and to investigate the morphology, optical response and magnetic properties of obtained composite structures. 2. Experimental The silica spheres were synthesized by hydrolysis of tetraethoxysilane (TEOS) ethanol solution in the presence of ammonia (Stöber technique) [11]. The reaction was carried out slowly to provide for low size distribution of the spheres. Then, the suspension was separated by slow sedimentation of the spheres. The fractions having the highest monodispersity of the spheres have been chosen. Making use of two different equipment, two series of artificial opals were grown by gravitational sedimentation technique from aqueous colloidal suspension of monodispersed SiO2 microspheres of 240 and 245 nm in diameter on almost vertical (80°) and horizontal substrates, respectively. Selforganized close-packed 3D crystalline opal structures were obtained. The second series samples were annealed in vacuum for 8 h at 900 °C, and the mechanical stability of the crystals was enhanced. The samples of thickness 200–300 μm were cut parallel to (111) plane of opal fcc lattice.

Fig. 2 presents the optical transmission spectra of opal crystals. As is seen, the stop band manifests itself in the visible spectral range. The center of the stop band is located at 522 and 540 nm for the samples of the first and second series, respectively. The width of the stop band (∼ 70 nm) was significantly larger for second series annealed crystals than for the samples of the first group. It should be noted that a small optical anisotropy was observed in the bulk opal samples of the second series (Fig. 3). The optical anisotropy was observed as the effective birefringence Δnd, where Δn is birefringence and d is the thickness of the sample. The effective birefringence manifests itself as a nonzero oscillating with wavelength optical transmission of the sample placed between crossed polarizer and analyzer when the axis of optical anisotropy is aligned at 45° with respect to polarizer and analyzer. The axis of optical anisotropy is approximately oriented along the boundaries of (111) planes revealed on the sample front surface, when surface normal is not strictly parallel to b111N axis. The value of effective birefringence was measured making use of additional quartz

Fig. 2. Transmission spectra of opal bulk crystals of the first (1) and second (2) series.

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Fig. 3. Effective birefringence Δnd in opal bulk crystal (1) (the curve is only a guide) and transmission spectra of the sample placed between crossed polarizer and analyzer (2) and in unpolarized light (3).

plate. The birefringence Δn∼ 2–5 × 10− 4 was evaluated for the samples of thickness d∼ 200–300 μm in the long wavelength region. In the region of the stop band the interference pattern of optical transmission was not observed due to a strong absorption. The Δnd value at wavelengths shorter than a lower edge of the stop band was estimated assuming that the order of interference fringes due to additional quartz plate combined with opal sample was the same as that for quartz plane alone. The spectral dependence of the effective birefringence Δnd(λ) calculated in this model has shown a

Fig. 4. Transmission spectra of opal crystals of the first (a) and second (b) series before (1) and after (2) infiltration with FeTPPS basic aqueous solutions (pH 11). Reflection spectrum (3) is shown for the sample of the second series.

Fig. 5. Transmission spectra of opal crystals of the first (a) and second (b) series before (1) and after (2) infiltration with FeTPPS acid aqueous solutions (pH 1). Reflection spectrum (3) is shown for the sample of the second series.

strong dispersion in the vicinity of the stop band (Fig. 2). It is interesting to note that a strong dispersion in the region of stoop band was determined in the spectral dependence of the phase shift upon reflection from photonic crystal [12]. In addition, a strong enhancement of Faraday rotation was found [13] in the stop band of 3D photonic colloidal crystals impregnated with a Faraday active transparent liquid. The occurrence of optical birefringence can be related to the internal stresses in opal crystals and imperfections. It should be noted that the interference pattern was observed only in several samples under investigations and the polarization degree of light passed through the samples varied for different crystals under investigation. In the absorption spectra of FeTPPS in the spectral region under consideration, Q-bands are well resolved. In basic solutions a typical doublet structure at 577 and 612 nm were observed which are assigned [14] to electronic excitations in μ-oxo dimeric species, O-(FeTPPS)2 dimers. In acid aqueous solutions of FeTPPS the Q-bands are located at 532 and 680 nm. The latter bands are attributed to the excitations of FeTPPS in monomer form. The transformation of FeTPPS between monomeric and dimeric forms occurs in a relatively narrow range of pH-values from 6 to 7 [14]. The concentration of FeTPPS in aqueous solutions ∼1 mM was evaluated making use of the molar absorption coefficients for characteristic bands [14]. A steep rise of absorption in short wavelength region was observed which was due to the Soret or B-band at 392 and 415 nm for excitations of FeTPPS in monomer and dimer forms, respectively.

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The influence of infiltration with basic and acid FeTPPS aqueous solutions on the optical properties of opal crystals is shown in Figs. 4 and 5, respectively. As is seen, the transmission spectra of opal crystals impregnated with FeTPPS changes significantly in the vicinity of the stop band. The absorption bands of FeTPPS typical for basic aqueous solutions are clearly resolved in the spectra of infiltrated opal crystals (Fig. 4). As a result of infiltration, the higher stop band edge shifts towards long wavelength region. However, at the lower stop band edge the optical transmission decreased and the stop band became less pronounced. It is interesting to note the difference between the optical properties in the samples of two series. In the case of first-series samples (Fig. 4a), the stop band did not overlap with absorption bands of FeTPPS and the latter were observed in the region of a higher stop band edge. In contrast, in the second series samples, one Q-band of FeTPPS at 575 nm (Fig. 4b) fits to a close vicinity of the stop band. The effect of infiltration of opal crystals with FeTPPS acid aqueous solution was quite different (Fig. 5). The corresponding Q-band of FeTPPS at 532 nm was close and fits directly into the stop band of the infiltrated opal crystals of the first and second series, respectively. In the latter case (Fig. 5b), the optical transmission was only slightly decreased but the width of the stop band increased. The details of the behavior of the Bragg band of opal crystal and absorption band of FeTPPS can be seen from the dependence of transmission spectra on the angle of light incidence (Fig. 6). It should be noted that angle-tuning measure-

Fig. 7. Temperature dependence of magnetization at 6 kOe for second series samples of pure opal and opal infiltrated with aqueous solution of FeTPPS (pH 11).

ments clearly showed that in these samples the b111N axis of opal crystals was at the angle ∼ 15° to the surface normal. The provided experiments have demonstrated that the composite system of opal crystals infiltrated with FeTPPS can be considered as a good model material for studying the interaction of absorption bands of different origin in photonic crystals. Fig. 7 shows the data for opal matrix with and without FeTPPS. The magnetization M at magnetic field 6 kOe for opal infiltrated with basic aqueous solution of FeTPPS is positive and significantly larger than negative magnetization of opal matrix. An increase of magnetization at low temperatures (T b 20 K) is most probably due to impurities in SiO2. The magnetization data indicates unambiguously the presence of FeTPPS in opal matrix. In higher magnetic fields up to 50 kOe the M-values increased up to ∼− 0.03 emu/g in the region of high temperatures, though the M(T) dependence for infiltrated samples was qualitatively unchanged. The temperature dependence of opal samples infiltrated with acid aqueous solution of FeTPPS was quite similar to that presented above. This observation allows one to assume that interaction between Fe ions in FeTPPS-infiltrated opal crystals was only weakly dependent on the acidity of FeTPPS aqueous solution. 4. Conclusions The obtained results have shown that using the designed procedure, opal crystals are efficient in retaining the solvents. This is in agreement with the results [15] obtained on sol–gel derived silica matrices impregnated with zinc sulfonated porphyrins. The aggregation state of FeTPPS-infiltrated in opal structure was the same as in acid or basic aqueous solutions. The optical spectra of opal crystals impregnated with FeTPPS underwent significant changes due to the overlapping of the stop band of opal crystals and absorption bands of FeTPPS. Acknowledgement

Fig. 6. Dependence of transmission spectra of opal crystal of the second series infiltrated with basic (a) and acid (b) aqueous solutions of FeTPPS on the angle of light incidence ϕ.

Support from EU project PHOREMOST (N 511616) is gratefully acknowledged.

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References [1] A.V. Baryshev, T. Kodama, K. Nishimura, H. Ucida, M. Inoue, J. Appl. Phys. 95 (2004) 7336. [2] M. Inoue, R. Fujikawa, A. Baryshev, A. Khanikaev, P.B. Lim, H. Uchida, O. Aktsiperov, A. Fedyanin, T. Murzina, A. Granovsky, J. Phys. D: Appl. Phys. 39 (2006) R151. [3] V. Kamaev, V. Kozhevnikov, Z.V. Vardeny, P.B. Landon, A.A. Zakhidov, J. Appl. Phys. 95 (2004) 2947. [4] S. Kubo, Z.-Z. Gu, K. Takahashi, Y. Ohko, O. Sato, A. Fujishima, J. Am. Chem. Soc. 124 (2002) 10950. [5] Z.-Z. Gu, S. Hayami, Q.-B. Meng, T. Iyoda, A. Fujishima, O. Sato, J. Am. Chem. Soc. 122 (2000) 10730. [6] S.V. Gaponenko, V.N. Bogomolov, E.P. Petrov, A.M. Kapitonov, A.A. Eychmueller, A.L. Rogach, I.I. Kalosha, F. Gindele, U. Woggon, J. Lumin. 87–89 (2000) 152.

989

[7] J. Zhou, C.Q. Sun, K. Pita, Y.L. Lam, Y. Zhou, S.L. Ng, C.H. Kam, L.T. Li, Z.L. Gui, Appl. Phys. Lett. 78 (2001) 661. [8] A. van Blaaderen, R. Ruel, P. Wiltzius, Nature 385 (1997) 321. [9] N. Eradat, A.Y. Sivachenko, M.E. Raikh, Z.V. Vardeny, A.A. Zakhidov, R.H. Baugham, Appl. Phys. Lett. 80 (2002) 3491. [10] H. Takeda, K. Yoshino, J. Appl. Phys. 92 (2002) 5658. [11] W. Stöber, A. Fink, E. Bohn, J. Colloid Interface Sci. 26 (1968) 62. [12] E. Istrate, E.H. Sargent, Appl. Phys. Lett. 86 (2005) 151112. [13] C. Koerdt, G.L.J.A. Rikken, E.P. Petrov, Appl. Phys. Lett. 82 (2003) 1538. [14] E.B. Fleisher, J.M. Palmer, T.S. Srivastava, A. Chatterjee, J. Am. Chem. Soc. 93 (1971) 3162. [15] G. Hungerford, M.I.C. Ferreira, M.R. Pereira, J.A. Ferreira, A.F. Coelho, J. Fluoresc. 10 (2000) 283.