Opal-Hematite and Opal-Magnetite Films: Lateral Infiltration

Oct 28, 2008 - Laboratory of Amorphous Semiconductors, Department of Solid State Electronics, Ioffe Physico-Technical Institute, Russian Academy of Sc...
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J. Phys. Chem. C 2008, 112, 17855–17861

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Opal-Hematite and Opal-Magnetite Films: Lateral Infiltration, Thermodynamically Driven Synthesis, Photonic Crystal Properties Sergei A. Grudinkin, Saveliy F. Kaplan, Nelly F. Kartenko, Dmitry A. Kurdyukov,* and Valery G. Golubev Laboratory of Amorphous Semiconductors, Department of Solid State Electronics, Ioffe Physico-Technical Institute, Russian Academy of Sciences, 26 Polytekhnicheskaya, St. Petersburg 194021, Russia ReceiVed: April 29, 2008; ReVised Manuscript ReceiVed: September 12, 2008

Hematite (R-Fe2O3) was infiltrated into opal film pores without depositing the oxide onto the outer film surface by the developed method of lateral infiltration of an aqueous solution of iron nitrate under capillary forces. The thermodynamically driven syntheses of either Fe3O4 (magnetite) or R-Fe were performed in the opal pores using R-Fe2O3 as a precursor. The opal-R-Fe2O3 and opal-Fe3O4 films were characterized by scanning electron microscopy, X-ray diffraction, reflectance, and transmittance measurements. The photonic band gap red-shifted gradually as the number of lateral infiltration runs increased. The maximum filling degrees both of opal-R-Fe2O3 and of opal-Fe3O4 films, calculated from the reflection spectra, appeared to be similar and equal to ∼55% of the pore volume because of very close values of molar volumes of the oxides. It was found that the reversible chemical transformation of fillers in opal pores R-Fe2O3 T Fe3O4 T R-Fe2O3 changes only the filler dielectric constant but does not practically produce structural defects that could affect the photonic crystal properties of the composite. A combination of lateral infiltration technique and thermodynamically driven synthesis is a cost-effective approach to produce three-dimensional photonic crystals based on opal films filled with A3B5, A2B6, and A4B6 semiconductors, oxides, and metals. Introduction Films of three-dimensional (3D) photonic crystals (PCs) are needed to develop all-optical microdevices (integrated circuits, filters, waveguides, superprisms, etc.). Synthetic opal films are regarded as promising and cheap templates for 3D PCs based on direct and inverted opal-filler composites.1-3 At present, the most common filling techniques for opal films are thermal chemical vapor deposition (thermal CVD),4 atomic layer deposition (ALD),5 and electrodeposition (electrolysis,6 electroless plating7). Some of these methods (thermal CVD, ALD) require the use of costly equipment and reagents. In contrast, a fairly inexpensive and versatile technology of pore filling in opals, porous glass, zeolytes, and so forth is the infiltration from aqueous solutions. One example is chemical bath deposition (CBD) and its modifications. These methods have been employed for filling bulk opals with metals,8,9 semiconductors,8,10-14 and oxides.15 Using neutron and synchrotron diffractometry, it was previously16-18 shown that solution methods could synthesize a nanocomposite material in amounts up to several cubic centimeters, providing a uniform distribution and a constant phase composition of the filler in the template pores. Solution methods, however, have not found a wide application for infiltration of opal films, because they involve several immersion runs which lead to accumulation of a substance to be infiltrated on the sample surface preventing further penetration of the precursor into the pores.14,19 The removal of the unwanted coating from the film surface without damaging it is a complicated task. Some researchers,20 for example, minimized the formation of the sample surface coating in the CBD method by promotion of heterogeneous, rather than homogeneous, nucleation of filling material (CdS, TiO2). Other workers21 * Corresponding author. E-mail: [email protected].

removed the excess LaF3:Tb colloidal solution from the opal film surface with a syringe. Another approach22 was evaporationdriven filling of opal films with a sol of germanium nanoparticles, but the nanoparticles could also be located on the surface. Since the solvent moves up to the film surface to be evaporated, it can cause nanoparticles to leave pores and migrate together with it. In this paper,23 the infiltration of opal film with a sol of CeO2 was described. It should be noted that the application of a colloidal solution as a precursor will obviously reduce the maximum filling, as compared with a true solution, because colloidal particles are much larger than solvated ions; therefore, the pore closure occurs at lower fill factors. Here, we describe a method of opal film infiltration with aqueous solutions of salts under capillary forces in a lateral direction. This practically excludes the presence of the solution on the outer film surface (except for a small region of contact between the film and the solution), thus preventing the formation of an unwanted surface coating. We have used this method to fill opal films with the iron oxides R-Fe2O3 (hematite) and Fe3O4 (magnetite). Recent interest in iron oxides has been due to their unique physical properties and wide potential applications. Fundamental studies are concerned with their optical, electrical, and magnetic behavior, as well as phase transitions. The ferrimagnetic oxide Fe3O4 is used in information recording and storage devices.24 Besides, Fe3O4 is considered to be a good candidate for application in spintronic devices25 owing to its high electric conductivity and electron-spin polarization, in addition to a high Curie temperature. Hematite is a perspective material for solar absorbers,26 catalysts,27 gas sensors,28 and chemical current sources.28 The magnetic materials find application in optical media such as magnetophotonic crystals29 which can control light beams when an external magnetic field is applied. Iron oxides can be synthesized in bulk opal templates

10.1021/jp8072443 CCC: $40.75  2008 American Chemical Society Published on Web 10/29/2008

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electrolytically,30 by immersion in a ferrofluid that contains nanometer-sized magnetite particles,31 and by a sol-gel procedure.19 Experimental Section Opal Films Preparation. The experimental opal films were grown from amorphous SiO2 spheres on substrates made from optically polished microscope glass of the Teget brand by using the vertical deposition technique.32 Spheres of 545 nm in diameter (size distribution e3%) were synthesized by slow base hydrolysis of tetraethyl ortosilicate Si(OC2H5)4.33 The opal film samples were annealed in air at the temperature of 750 K for 20 min. The typical dimensions of a sample were as follows: the substrate was 15 × 8 × 1.5 mm; the film thickness was 22 layers of a-SiO2 spheres, and the film surface was parallel to the (111) planes. Filling an Opal Film with r-Fe2O3 by the Lateral Infiltration (LI) Technique. The sample was immersed to a 1 mm depth into a 1 M aqueous solution of Fe(NO3)3 · 9H2O (Aldrich) at 293 K for 20 min. The sample was fixed vertically so that gravity prevented spreading of the solution on the film surface. Due to capillary forces, the entire film was infiltrated at a rate of 10-5 m · s-1, with the liquid transport occurring only through the pores (submicrometer capillaries) but not through the surface. The relative humidity in the setup was maintained at 97 ( 1%. The infiltrated sample was heated to 450 K at the rate of 10 K · min-1, stored at 450 K for 10 min, and cooled to room temperature at the same rate. This procedure was repeated up to 24 times (LI runs). The film portion which had been in contact with the solution and, hence, contained bulk material on it (about 3 mm film length) was cut off with a corundum disk. Reduction and Oxidation of Iron Oxides. The samples filled with R-Fe2O3 were placed in a fused silica tube, through which a hydrogen-argon mixture (the gas purity 10 ppm) was passed under the pressure of 1 bar with the hydrogen partial pressure of 0.2 bar. The gas flow rate was constant and equal to 1 sccm. The reduction of R-Fe2O3 to Fe3O4 was carried out at 630 K, and R-Fe was obtained at 850 K. The synthesis time in both cases was 15 h. The oxidation of Fe3O4 was performed in an oxygen flow (1 sccm, 1 bar) at 750 K for 20 h. Characterization. The SEM measurements were made by using a Camscan 4-88 DV 100 field-emission scanning electron microscope, and the initial bare opal film sample or the opalR-Fe2O3 and opal-Fe3O4 samples were not subjected to any surface treatment. The phase composition of the final samples was analyzed by a DRON-2 X-ray diffractometer (Cu KR radiation, Ni filter). The reflection and transmission spectra were measured by a Bruker IFS 113v Fourier infrared spectrometer in the nearinfrared region. The spectra were registered by using a cooled InSb detector, and a halogen lamp was a light source. The spectral resolution was 4 cm-1. The incident light was focused onto the sample within the solid angle of 10°. The transmission spectra were measured at 0° relative to normal to the surface. The incidence angle during the measurement of reflection spectra was 11° with respect to the surface normal. The reflection and transmission spectra were recorded from the sample area of 3 mm2. Single-domain spectroscopy described in our previous paper34 was used for reflection spectra measurements as a function of light incident angle. This technique enabled us to obtain spectra from exactly the same sample place at different incident angles. The spectra were recorded in the s-polarization by means of an

Figure 1. Schemes of the main steps of the composites fabrication process: (a) lateral infiltration by capillary forces. (b) Thermodynamically driven syntheses inside the opal pores (K-equilibrium constant): (1) reducing of hematite with hydrogen, (2) oxidation of magnetite with oxygen.

Ocean Optics NIR-256 near-infrared spectrometer from the sample area of 0.01 mm2. Results and Discussion Figure 1a shows schematically the processes occurring in the pores and on the surface of an opal film immersed into the iron nitrate water solution: (1) lateral infiltration due to the Laplace pressure (capillary) force; (2) hydraulic resistance, which arises because of interaction of the flowing viscous liquid with the pore walls; (3) the solvent evaporation and capillary condensation, whose rates depend on the setup humidity; and (4) the liquid surface bending at the contact with the film (meniscus). The meniscus height at the contact with a vertical plane in a fully wetting liquid is equal to the capillary length.35 The capillary length for water under normal conditions is ∼2 mm. Therefore, the lower surface film portion in contact with the salt solution was about 3 mm long (including the immersed 1 mm length). The capillary water pressure in the pores of an opal sample consisting of 545 nm spheres was found from Laplace’s equation to be 14 bar at the temperature of 293 K (the minimum capillary pressure was in octahedral pores with the maximum size of ∼220 nm). This calculation was made on the assumption that the solution completely wetted the hydrated surface of SiO2 spheres. The calculation from Poiseuille’s equation of hydraulic resistance in the pores of this size, ignoring bendings and constrictions, showed that the pressure drop of 14 bar would be reached at the sample length of ∼1 m at the filling rate of 10-5 m · s-1 (the rate value was found experimentally). For the liquid to remain inside the opal pores, the condensation and evaporation rates must be the same. If the evaporation rate is higher, there will be an uncontrollable drying of the upper film portion. If the condensation rate becomes greater because the ambient vapor pressure is higher than the saturated vapor pressure in the pores, liquid droplets will appear on the film because of the condensation in the intersphere spaces. Using Kelvin’s formula, we calculated the saturated vapor pressure at the narrowest sites of contact between the tetrahedral and the octahedral pores with the minimum size of ∼70 nm and found it to be 3% lower than that over the plane surface. For this reason, the relative humidity in the setup was maintained at a value of 97 ( 1% to keep up a dynamic equilibrium between the processes of solvent evaporation and the capillary condensation in the pores.

Opal-Hematite and Opal-Magnetite Films

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Figure 2. SEM images: (a) bare opal, the inset shows the enlarged top view. (b) Opal infiltrated with R-Fe2O3 (24 LI runs). The image in the inset demonstrates that the SiO2 spheres are covered with R-Fe2O3 nanoparticles. (c) The composite after reducing of R-Fe2O3 to Fe3O4. (d) The enlarged fragment of the silica sphere surface with Fe3O4 nanoparticles.

We can conclude from the above estimates of effects occurring in the liquid inside pores and on a surface of a vertical opal film that the LI technique is capable to provide an effective filling of films having several tens of centimeters in length without bulk material being deposited on their surface. We carried out 24 LI runs, each followed by thermal decomposition of iron nitrate. It is known36 that the intermediate products of Fe(NO3)3 · 9H2O decomposition are base nitrates and iron hydroxide, which completely transform to hematite at 428 K. There was no salt solution on the film; therefore, the solid decomposition products were contained only in the template pores. It is seen from Figure 2b that the pores are filled uniformly. Some material on the surface was found only on the film portion of ∼3 mm long, which had been in contact with the solution. The rest of the film surface (∼12 mm length) did not contain R-Fe2O3, as evidenced by the SEM data. The composite X-ray diffraction pattern (XRD) presented in Figure 3 (curve 3) clearly shows diffraction maxima from nanocrystalline R-Fe2O3. The determined unit cell parameters of R-Fe2O3 (a ) 0.500(6) nm, c ) 1.362(7) nm) are in agreement with those of bulk R-Fe2O3 (JCPDS 33-663). Magnetite was synthesized in opal pores by hematite reduction with hydrogen under thermodynamic equilibrium conditions (Figure 1b), using argon as a carrier gas. To find the basic

parameters of the reduction process (temperature and hydrogen partial pressure), we computed the equilibrium composition of the mixture of Fe3O4, R-Fe2O3, R-Fe, γ-Fe2O3 (maghemite), FeO (wustite), R-FeOOH (goethite), γ-FeOOH (lepidocrocite), H2(g), H2O(g), and Ar(g) by using the VCS (Villars-Cruise-Smith) algorithm.37 We minimized the Gibbs energy min ∑ini · µi (ni is the number of moles of the ith substance, and µi is its chemical potential) at constant temperature and total pressure, taking into account the constraints imposed by the material balance equations A · b n) b b (A is the matrix of stoichiometric element numbers for the substances, b n is the mole numbers vector of the substances, and b b is the mole numbers vector of the elements). The values of standard chemical potentials were taken from the Ivtantermo database.38 The calculations show that R-Fe2O3 completely transforms to Fe3O4 at the hydrogen partial pressure of 0.2 bar, the partial pressure of water vapor of 10-4 bar, and the total pressure of 1 bar at temperatures below 650 K; the amount of impurities was below 0.01% mol. In the temperature range of 650-800 K, R-Fe and Fe3O4 may be present simultaneously in different proportions depending on the temperature, in addition to a small amount of FeO (from below 1% to several percent). At temperatures above 800 K, the reaction goes on until elemental iron is formed.

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Figure 3. XRD patterns: (1) bare opal; (2) R-Fe2O3, JCPDS 33-663; (3) opal filled with R-Fe2O3 (24 LI runs); (4) Fe3O4, JCPDS 19-629; (5) the composite after reducing of R-Fe2O3 to Fe3O4; (6) R-Fe, JCPDS 6-696; (7) the piece of the composite after reducing of R-Fe2O3 to R-Fe. Small reflections of R-cristobalite (JCPDS 39-1425) and R-tridymite (JCPDS 18-1170) are also seen in the patterns of the composites.

Grudinkin et al. nanocrystal size and the material distribution across the opal sphere surfaces are nearly the same before and after the reduction reaction. First, this is due to the fact that the filler molar volume remains practically constant during the reaction (the molar volumes of magnetite and hematite at 293 K recalculated per one mole of iron are 14.8 and 15.2 cm3 · mol-1, respectively). Second, the reaction temperature of 630 K seems to be too low for the filler substance to be recrystallized during the reduction process. The average size of Fe3O4 and R-Fe2O3 crystals in the pores is found by the SEM and XRD data to be about 20 nm. The particles of larger size seen in Figure 2b-d appear to be aggregates of smaller crystallites. An essential feature of the opal-iron oxide composites obtained is the manifestation of typical properties of photonic crystals.1 We could control and estimate the hematite filling degree from the dip position in the transmission spectra and the corresponding maximum in the reflection spectra caused by Bragg diffraction from the (111) planes of the face-centered cubic (fcc) lattice of the composite (Figure 4). The position of the extremum of the diffraction line λ111 can be described by Bragg’s formula

λ111 ) 2d(111)√〈ε〉 - sin2 θ

(1)

where d(111) is the interplane distance, θ is the light incident angle, 〈ε〉 is the average dielectric constant of the composite34,39

〈ε〉 )

Figure 4. (a) Transmission spectra of the opal-R-Fe2O3 composite on a glass substrate at selected LI runs: (1) 0, (2) 2, (3) 4, (4) 7, (5) 9, (6) 13, (6*) additionally filled with glycerol, (7) 18, (8) 24. (b) Reflection spectra: (1) bare opal, (2) opal-R-Fe2O3 (24 LI runs), (3) opal-Fe3O4. Dashed spectra in the both panels (curves 9 and 4) belong to the composite produced by annealing of opal-Fe3O4 in oxygen.

These calculations were supported by experimental data. The sample reduced at 630 K was found by XRD analysis to contain only one crystalline substance, magnetite (Figure 3, curve 5), whose lattice parameter was 0.839(1) nm, in agreement with the reported value of 0.8396 nm (JCPDS 19-629). The sample reduced at 850 K contained only R-Fe (Figure 3, curve 7). These findings indicate that it was thermodynamically driven synthesis of magnetite and metallic iron that occurred in the opal pores. The comparison of the SEM image of the opal-magnetite composite (Figure 2c,d) and the micrograph of the initial opal-hematite composite (inset in Figure 2b) shows that the

∑ εifi

(2)

where εi and fi are the dielectric constant and the volume fraction of the ith constituent, respectively. In addition to the Bragg diffraction line, the transmission spectra contained interference fringes because of light reflection from two plane-parallel film surfaces. The film thickness was found from the positions of the interference fringes in the long wavelength region (relative to the Bragg line) of an unfilled film spectrum.40 The film appeared to be 22 layers thick (the dielectric constant of the SiO2 spheres was taken to be 1.96). This value is in agreement with SEM data performed on a cleaved edge of the sample. As the number of LI runs increased, the amount of filling material in the pores became larger, making the Bragg diffraction line shift to the long wavelength region (Figure 4). Simultaneously, the dielectric contrast of the opal-hematite composite reduced, which was assumed to be q ) |1 - s/p|, where s is the dielectric constant of spheres and p is the average dielectric constant of pores. The fact that the dielectric contrast tends to be zero is the reason for the Bragg line to become narrower and for the transmittance to be larger at the Bragg wavelength.13,41 After 7 LI runs, there were no Bragg lines in the spectra (curves 4,5 in Figure 4). However, further increase in the filling degree gave rise to an appearance of Bragg line again (curve 6 in Figure 4, after 13 LI runs). The average dielectric constant of pores became greater than that of SiO2 spheres. Each subsequent LI run led to increasing the dip depth and the width because of the increasing dielectric contrast of the composite (Figure 4a, curves 7 and 8).41 The observable decrease in the filled opal film transmittance with decreasing wavelength (Figure 4a) may be associated with Raleigh’s (diffusive) light scattering by bodies much smaller than the incident light wavelength.42,43 In our case, Raleigh’s scattering seems to be due to R-Fe2O3 nanocrystals formed on the surface of SiO2 spheres. In order to test this hypothesis, we filled the sample after 13 LI runs with an immersion liquid, glycerol with the dielectric constant of 2.16 (Figure 4a, curve

Opal-Hematite and Opal-Magnetite Films 6*). The dielectric constant ratio of hematite-glycerol is smaller than that of hematite-air, so the Raleigh scattering cross section44 for a sample filled with glycerol should be smaller. A glycerol-filled composite was found to have a higher transmittance over the whole spectral region (Figure 4a, curve 6*). The Bragg line shift to the long wavelength region for glycerolfilled composite is due to its higher average dielectric constant 〈ε〉, while the greater dip at the Bragg wavelength is accounted for by a greater dielectric contrast of the composite.41 The filling degree values of opal film with hematite after 13 LI runs, calculated from the reflection spectra of the sample before and after infiltration with glycerol using eqs 1 and 2, appeared to be similar and equal to ∼28% of the pore volume. Although the pore filling degree with R-Fe2O3 increased after 18 LI runs, the composite transmittance in the long wavelength region began to increase. A possible reason for this is a reduced incoherent scattering of radiation by individual R-Fe2O3 nanocrystals because of their aggregation into larger structures (see Figure 2). We can conclude from this evidence that it is diffusive scattering, rather than hematite absorption, which is largely responsible for the decrease of transparency of the opal-hematite composite out of the Bragg line spectrum. After the reduction of the filler to Fe3O4, we failed to measure the transmission spectrum of the composite because of a considerable magnetite absorption in this spectral region (the absorption coefficient is ∼105 cm-1).26 For this reason, the photonic crystal properties of the opal-magnetite composite were determined from the reflection spectra (Figure 4b). The reflection spectra were measured on the film side and on the back side through the glass substrate. The Bragg line positions and full widths at the half-maxima (fwhm) are the same in both cases, indicating a uniform pore infiltration of the film thickness. The filling degrees of opal-hematite after 24 LI runs and of opal-magnetite, calculated from the reflection spectra using the above Bragg formula, appeared to be similar and equal to ∼55% of the pore volume. The dielectric constant values for R-Fe2O3 and Fe3O4 used in the calculations were 6.76 and 4.1, respectively.26,45 The good agreement between the filling values is evidence for a practically complete transformation of hematite to magnetite in the opal pores during the reduction in hydrogen. Therefore, by varying the hematite filling degree from 0 to 55 vol %, one can essentially modify the photonic crystal properties of the composite. After an opal-Fe3O4 sample was annealed in oxygen, we again detected R-Fe2O3 in the opal pores. The transmission and reflection spectra measured following the oxidation are shown in Figure 4a (curve 9) and Figure 4b (curve 4). The Bragg lines in both spectra are seen to be shifted toward shorter wavelengths as compared with those for the opal-R-Fe2O3 composite after 24 runs. It is quite likely that the long-term annealing in oxygen (750 K, 20 h) somewhat decreased the interplane distance d(111). The measurement of reflection spectra as a function of light incident angle has been undertaken to provide an additional characterization of the photonic crystal structure of the final opal-hematite film (Figure 5). The angular dependence of the Bragg diffraction peak positions is shown in Figure 6. In the range of incident angles 10°-40°, the positions of the Bragg diffraction peaks could be described by Bragg formula (eq 1).46 The solid curve in the inset in Figure 6 was obtained by fitting the calculated curve derived from eq 1 to the experimental points, using the least-squares method. The fitting procedure allowed the reveal of values of the parameters d(111) and 〈ε〉. It was established that the interplane distance between the (111)

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Figure 5. Bragg reflection spectra for various incident angles of s-polarized light for the opal-R-Fe2O3 composite produced by annealing of the opal-Fe3O4 composite in oxygen. Angle of registration relative to the normal to the (111) surface of the samples: (1) 10°, (2) 20°, (3) 35°, (4) 45°, (5) 55°, (6) 60°, (7) 65°, (8) 70°, (9) 75°.

Figure 6. Angular dependence of the reflection Bragg peak positions for the opal-R-Fe2O3 composite produced by the annealing of the opalFe3O4 composite in oxygen. The Bragg diffraction peak positions are shown by circles, and the positions of reflection minima in the doublets are indicated by squares. The solid curve shows the calculated peak positions for the reflection by the (111) planes parallel to the sample surface. Inset shows fitting results for the peak positions in the range of incidence angles 10-40° calculated by Bragg formula.

planes of final opal-hematite film decreased by 2% relative to one of the bare opal films. The filling degree of the final opal-hematite film calculated from eq 2, taking into account the average dielectric constant 〈ε〉 obtained from the angular dependence, appeared to be ∼53%. This value of filling degree is in a good agreement with that calculated from the Bragg peak position using eq 1. In the range of incident angles 45°-70°, the spectra of the opal-hematite film contain a well-defined doublet structure (Figure 5). The appearance of the doublet structure in the spectra at oblique incidence is associated with multiple diffraction on the {111} family of planes nonparallel to the sample surface.46,47 Rotation of the samples around the normal to the surface (changing of the azimuth angle) at fixed incidence angle changes the relation between doublet components. The most pronounced doublet structure with a comparable intensity of the doublet components is observed at the azimuth angles of Bragg diffraction when the wavevectors of the incident and reflected beams lie in the plane with the points, K, L, and U in the Brillouin zone.46 All presented experimental results were measured at such azimuth angles. The effect of opal sphere deformation due to heating in syntheses can lead to the deviation of the opal structure from fcc,47 and the uniaxial deformation of SiO2 spheres along the [111] growth axis transforms spheres to ellipsoids.46,47 The approach developed earlier46,47 was applied to study the uniaxial deformation. The lengths of ellipsoidal axes of close-packed ellipsoids can be denoted as D⊥ and D|, assuming them to be spheroidal and the D| axis coinciding with the rotation axis

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normal to the reflecting plane. The distance a00 between the centers of neighboring spheroids having a point contact and located in the lateral plane is taken to be D⊥. Quantitatively, this kind of deformation can be described by the anisotropic compression coefficient η ) D|/ D⊥. In Figure 6, the circles indicate the Bragg peak positions, and the squares stand for the minima in doublets at various incidence angles. The interplane distance between the (111) planes in the direction normal to the surface is described as

d(111) ) a00η√2 ⁄ 3

(3)

The solid curves shown in Figure 6 were derived from eq 1 using the values of d(111) and 〈ε〉. The dashed curves connect the experimental points for the positions of the minima in the doublets. The interception of the curves corresponds to the wavelength (λ ) λ*) and the incidence angles (θ ) θ*); at that point, the diffraction conditions are fulfilled simultaneously for the (111) planes parallel to the sample surface and for the similar planes inclined to it. The wavelength λ* for this interception point must obey the relation46

λ* )

4a00√3 4 - η-2

sin θ*

(4)

By substituting the fitting values of d(111) and the experimental values of λ* and θ* into expressions 3 and 4, the value of η was calculated to be 0.98. For the final opal-hematite film, the shape of SiO2 spheres was close to spherical. Consequently, the applied methods of fabrication of hematite and magnetite did not result in deviation of the composite structure from fcc symmetry. The fwhm values of the Bragg lines in the opal-hematite spectra measured after the annealing in oxygen and of the initial opal-hematite composite are practically the same (Figure 4b, curves 2 and 4). Evidently, the thermodynamically driven processes of oxidation and reduction of iron oxides do not produce any structural defects in the composite, which might be responsible for the Bragg line broadening. It should be noted that the formation of the opal-hematite composite by annealing of the opal-magnetite composite in oxygen was accompanied by a significant decrease of fwhm of the reflection Bragg line (Figure 4b, curves 3 and 4). The Bragg line broadening for the opal-magnetite composite by over a factor of 2, as compared with the Bragg line for the opal-hematite composite, is likely to be due to a large value of the imaginary part of dielectric constant of magnetite (″) 4 for magnetite against ″) 0.51 for hematite at the Bragg line wavelength26,45). It should also be emphasized that, to our knowledge, we have first performed a reversible chemical transformation of fillers in opal pores: R-Fe2O3 f Fe3O4 f R-Fe2O3. A strong enhancement of the polar Kerr effect and a modification of the Faraday effect near the photonic band gap region were recently48 found in the duplicate opal-Fe3O4 composite fabricated by use of LI and TDS methods. Conclusions The method of lateral infiltration under capillary forces allows introduction of substances into opal film pores from a liquid precursor without depositing an unwanted substance on the outer film surface. The technique provides a uniform filler distribution in opal pores. Lateral infiltration has been used to produce 3D photonic crystal films based on an opal-hematite composite. It is shown that this approach makes it possible to control the photonic band gap position and the dielectric contrast in photonic

crystals based on opal films by varying the degree of pore filling with hematite. The applied method of thermodynamically driven synthesis allowed us to realize for the first time the reversible chemical transformation R-Fe2O3 T Fe3O4 occurring in opal pores, which changed only the filler dielectric constant but did not practically produce structural defects and did not result in deviation of the composite structure from fcc symmetry that could affect the photonic crystal properties of the composite. A combination of lateral infiltration technique and thermodynamically driven synthesis is a cost-effective approach to produce 3D photonic crystals based on opal films filled with A3B5, A2B6, and A4B6 semiconductors, oxides, and metals. Acknowledgment. This work was supported by the Russian Academy of Sciences, the EC-funded project PHOREMOST (FP6/2003/IST/2-511616), the Russian Foundation for Basic Research (Project No. 08-02-00450), and the Dutch-Russian Cooperation Project (047.011.2005.026). References and Notes (1) Lo´pez, C. AdV. Mater. 2003, 15, 1679. (2) Xia, Y.; Gates, B.; Yin, Y.; Lu, Y. AdV. Mater. 2000, 12, 693. (3) Velev, O. D.; Kaler, E. W. AdV. Mater. 2000, 12, 531. (4) Vlasov, Y. A.; Bo, X.-Z.; Sturm, J. C.; Norris, D. J. Nature 2001, 414, 289. (5) King, J. S.; Gaillot, D. P.; Graugnard, E.; Summers, C. J. AdV. Funct. Mater. 2006, 18, 1063. (6) Wijnhoven, J. E. G. J.; Zevenhuizen, S. J. M.; Hendriks, M. A.; Vanmaekelbergh, D.; Kelly, J. J.; Vos, W. L. AdV. Mater. 2000, 12, 888. (7) Kulinowski, K. M.; Jiang, P.; Vaswani, H.; Colvin, V. L. AdV. Mater. 2000, 12, 833. (8) Stein, A. Microporous Mesoporous Mater. 2001, 44-45, 227. (9) Pokrovsky, A. L.; Kamaev, V.; Li, C. Y.; Vardeny, Z. V.; Efros, A. L.; Kurdyukov, D. A.; Golubev, V. G. Phys. ReV. B 2005, 71, 165114. (10) Davydov, V. Yu.; Golubev, V. G.; Kartenko, N. F.; Kurdyukov, D. A.; Pevtsov, A. B.; Sharenkova, N. V.; Brogueira, P.; Schwarz, R. Nanotechnology 2000, 11, 291. (11) Gadzhiev, G. M.; Golubev, V. G.; Zamoryanskaya, M. V.; Kurdyukov, D. A.; Medvedev, A. V.; Merz, J.; Mintairov, A.; Pevtsov, A. B.; Sel’kin, A. V.; Travnikov, V. V.; Sharenkova, N. V. Semiconductors 2003, 37, 1400. (12) Golubev, V. G.; Davydov, V. Yu.; Kartenko, N. F.; Kurdyukov, D. A.; Medvedev, A. V.; Pevtsov, A. B.; Scherbakov, A. V.; Shadrin, E. B. Appl. Phys. Lett. 2001, 79, 2127. (13) Blanco, A.; Mı´guez, H.; Meseguer, F.; Lo´pez, C.; Lo´pez -Tejeira, F.; Sa´nchez-Dehesa, J. Appl. Phys. Lett. 2001, 78, 3181. (14) Jua´rez, B. H.; Rubio, S.; Sa´nchez-Dehesa, J.; Lo´pez, C. AdV. Mater. 2002, 14, 1486. (15) Murzina, T. V.; Kim, E. M.; Kapra, R. V.; Moshnina, I. V.; Aktsipetrov, O. A.; Kurdyukov, D. A.; Kaplan, S. F.; Golubev, V. G.; Bader, M. A.; Marowsky, G. Appl. Phys. Lett. 2006, 88, 022501. (16) Golosovsky, I. V.; Mirebeau, I.; Fauth, F.; Kurdyukov, D. A.; Kumzerov, Yu. A. Solid State Commun. 2007, 41, 178. (17) Golosovsky, I. V.; Tovar, M.; Hoffman, U.; Mirebeau, I.; Fauth, F.; Kurdyukov, D. A.; Kumzerov, Yu. A. JETP Lett. 2006, 83, 298. (18) Golosovsky, I. V.; Mirebeau, I.; Andre, G.; Kurdyukov, D. A.; Kumzerov, Yu. A.; Vakhrushev, S. B. Phys. ReV. Lett. 2001, 86, 5783. (19) Kodama, T.; Nishimura, K.; Baryshev, A. V.; Uchida, H.; Inoue, M. Phys. Status Solidi B 2004, 241, 1597. (20) Zhou, L.; Boyle, D. S.; Govender, K.; O’Brien, P. J. Exp. Nanosci. 2006, 1, 221. (21) Aloshyna, M.; Sivakumar, S.; Venkataramanan, M.; Brolo, A. G.; van Veggel, F. C. J. M. J. Phys. Chem. C 2007, 111, 4047. (22) Shimmin, R. G.; Vajtai, R.; Siegel, R. W.; Braun, P. V. Chem. Mater. 2007, 19, 2102. (23) Waterhouse, G. I. N.; Metson, J. B.; Idriss, H.; Sun-Waterhouse, D. Chem. Mater. 2008, 20, 1183. (24) Prinz, G. A. Science 1998, 282, 1660. (25) Coey, J. M. D.; Chien, C. L. MRS Bull. 2003, 28, 721. (26) Karlsson, B.; Ribbing, C. G.; Roos, A.; Valkonen, E.; Karlsson, T. Phys. Scr. 1982, 25, 826. (27) Kesavan, V.; Dhar, D.; Koltypin, Y.; Perkas, N.; Palchik, O.; Gedanken, A.; Chandrasekaran, S. Pure Appl. Chem. 2001, 73, 85. (28) Wu, C.; Yin, P.; Zhu, X.; OuYang, C.;. Xie, Yi. J. Phys. Chem. B 2006, 110, 17806. (29) Inoue, M.; Uchida, H.; Nishimura, K.; Limb, P. B. J. Mater. Chem. 2006, 16, 678.

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