1. Introduction

Materials with periodic variation of dielectric constant (or refractive index) on the length scales of optical region in 1D, 2D or 3D are called photonic crystals (PhCs) [1–4]. These materials are known to be optical analogue of usual semiconductor crystals, which contain periodic arrangement of atoms or molecules on atomic length scale with a definite band gap for the electron motion. Similarly, the property by virtue of which makes the new periodic dielectric materials or PhCs to possess a forbidden region for the propagation of electromagnetic waves of any polarization in any direction for some frequency range, known as complete photonic band gap (PBG) [1]. The ease of steering the light with these PhCs, for the optical wavelengths meeting the Bragg diffraction, has opened the field of photonics with the development of wide range of studies on fabrication of low loss optical waveguides [5], electrically pumped ultra-low threshold lasers [6], optical switches [7] and chemical and biochemical sensors [8] etc. In this regard, several lithographic and self assembly techniques are being under study since past few decades for the fabrication of these PhCs [4,9–11]. However, self assembly based on vertical deposition technique has been a great attention because of its significant low cost and ease of fabrication of high quality opal films with large area and well oriented crystal domains. In this technique, the structures are grown from their building blocks of submicron dimension spheres in uniform thin films of face centered cubic (fcc) lattice crystals [4,12,13] called colloidal crystals. These crystals are known to present only a pseudo PBG, a forbidden region for certain directions of electromagnetic wave propagation for a certain frequency range [1]. Nevertheless, it still has interesting optical properties that are responsible for the brilliant appearance of natural opals, because of which the colloidal crystals are called as opals. Moreover, it turns out that the inverse structure, with fcc patterned air holes in high dielectric with resultant high index contrast, can have a complete PBG. Theoretically it was predicted that to attain the complete PBG with an fcc lattice of air spheres in a dielectric medium, a refractive index contrast of ≥2.8 is required [13]. In this regard, several inverse opal structures were developed with air spheres in the backbones of high refractive index materials like Si, Ge, GaAs, GaP, and InP, which show a complete PBG for infrared wavelength region and SnS₂, Ta₃N₅ and Sb₂S₃ for visible range wavelengths [14–16]. There are few more reports on the experimental results regarding the fabrication of PhCs with the infiltration of highly luminescent materials like CdS, CdSe, ZnS, etc [17,18]. Some kind of binary matrix systems were also realized with a mixture of polystyrene, PMMA beads to obtain a highly porous material to have high index contrast. Wang and associates [19] have synthesized highly porous binary inverse opal with a combination of meso and macroporous cavities. Though the index contrast is not enough for these structures to open the complete PBG, various luminescent materials can be filtered in and achieve control over its emission features in these opal architects. On the other hand, Hwang and associates [20] had fabricated a multistack opal structure by controlled deposition of materials with various refractive indexes. The resulted structure can work as a broadband optical filter. One can also make use of the opals of the high porosity to incorporate noble metal nanoparticles like gold, silver and copper to study the interesting unified features of plasmonic and PBG features [12]. Moreover, the high surface area property of inverse opal structure also can facilitate the structure to behave as a good gas sensor for various gases like CO₂, N₂ and alcohol vapors, etc. Moreover, the use of colloidal lithography based on self-assembly via rapid convective deposition [21,22] had also resulted in ability to form monolayer 2-D hexagonal close-packed microlens arrays in organic LEDs [23] and GaN-based LEDs [23–25] leading to improved light extraction efficiency and power conversion efficiency. This method had led to uniform self-assembly colloidal deposition in wafer scale regions, which had been implemented in real device technologies [21–25].

In this communication, we focus on the synthesis and optical characterization of inverse opal structures of SiO₂ and materials with multifunctional features like conductivity, sensing, and photorefractive properties, which can facilitate the resultant system in the field of photovoltaics, sensors and integrated optics, etc. To meet these requirements SnO₂ and SiO₂-SnO₂ binary oxide inverse opal systems are developed by a simple two step infiltration and calcination process. The incorporation of maximum amount of SnO₂ in SiO₂ would be highly beneficial to employ these structures in the field of silica based integrated optics because of the photorefractive properties of SnO₂.

2. Experimental

The fabrication of inverse opal structure involves two steps. The first step involves the fabrication of polystyrene (PS) opal and the second, slow infiltration of the PS opal with the desired oxide sol and controlled calcinations of the PS beads to obtain the inverse opal structure of oxide matrix.

The infiltration of the oxide sol in the PS opal was carried out using a dip coater controlled with Newport ESP300 Universal Motion Controller. The calcination of the samples was performed in Lenton laboratory furnace. The optical transmission spectra and variable angle reflection spectra were measured using a double beam UV-Vis/NIR spectrophotometer (JASCO V-670). The structural parameters were investigated using Zeiss Ultra-55 field emission scanning electron microscope (FESEM). The X-ray diffraction studies were performed with INEL X-ray spectrometer using Co K_α radiation (λ = 1.7889 Å). TECNAI G2 FEI F12 transmission electron microscope (TEM) was used to observe the multilayer structure and amorphous or crystalline phase of the infiltrated oxide matrix of the colloidal crystals.

2.1 Fabrication of polystyrene opal structure

A bare PS opal was first grown by the vertical deposition technique [Figs. 1(a) & 1(b)] [26]. In this technique initially a 3 ml aqueous solution of homemade monosized 320 nm diameter PS spheres (Fig. 1(a) inset) of 1.9 mg/cc was taken in a 5 ml beaker. Then a clean and hydrophilized glass substrate of 3cm × 1cm was kept at 45° angle in PS spheres’ solution and left at a temperature of 50 °C and a relative humidity of 70%-80% for 48 hrs. During the process of water evaporation the spheres get pulled towards the meniscus due to the capillary forces and self-assemble into a fcc crystalline lattice. Though we can fabricate opals of nearly 30 layers over an area of 1 cm × 1 cm with very good uniformity, approximately 20 layers was chosen as it provided us optimum speed of growth, well pronounced stop band depth and transparency. The resulted structure was heat treated at 80 °C for 10 hrs to attain mechanical stability and small necks between the spheres.

 

Fig. 1 (a) Top view and (b) cross-sectional view of bare PS opal. (c) Top view and (d) cross-sectional view of ISO. The inset of Fig. 1(c) shows the high magnification of ISO with air spheres and triangular lattice holes (black arrow) which are resultant of former contacts between the spheres.

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2.2 Fabrication of oxide inverse opal structure

2.2.1 Fabrication of inverse silica opal

The silica solution was obtained by mixing tetraethylorthosilicate (TEOS), methanol, deionized water and hydrochloric acid (HCl) as a catalyst. It was prehydrolyzed for 1h at 65 °C. A molar ratio of 1:0.01:2:33.8, was used for the TEOS: HCl: H₂O: CH₃OH mixture [27,28]. The resulted sol was filtered using 0.22 μm filter. The sol was infiltrated into the 26% air volume fraction of the PS spheres’ fcc lattice by using a dip-coating technique. In this technique, the sample was immersed and withdrawn from the sol at a slow dip rate of 10 mm/min, and then it was dried at room temperature for 15 min prior to the next dip. The same procedure was repeated for total of 3 cycles so that the total bare PS opal structure got infiltrated with the silica solution. Prior to achieving the inverse opal structure the required annealing temperatures were determined by performing the thermogravimetric analysis of bare PS opals [12,29]. The thermogravimetric data showed us that PS spheres get completely evaporated by annealing the matrix at 450 °C for 30 min. So, the sol infiltrated structure was calcinated at 450 °C for 30 minutes and then brought it to the room temp at a ramp rate of 0.5 °C/min so that the PS spheres get evaporated and the silica matrix gets densified simultaneously. At the end, we will be left with ordered air cavities in the backbone of silica matrix by resembling a negative replica of PS opal [Figs. 1(c) &1(d)]. This type of structure is known as the inverse silica opal (ISO).

2.2.2 Fabrication of Tin oxide inverse opal

A transparent methanol colloidal suspension of SnCl₂.2H₂O precursor was prepared [27,28] with 0.448 M and infiltrated through the PS opal and calcinated by following the same fabrication protocol parameters like the ISO. The resulted structure with ordered air cavities in the SnO₂ matrix is shown in Fig. 2(a) . This inverse SnO₂ opal structure is labeled as ISnO in the following text.

 

Fig. 2 SEM image shows the (111) facet of (a) ISnO and (b) ISSnO. The white circled regions in (b) correspond to mesoporous cavities observed in case of ISSnO, the inset of Fig. 2(b) also shows the high magnification image of the mesoporous cavities.

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2.2.3 Fabrication of 75SiO₂-25SnO₂ inverse opal

The solutions of 75 mol % silica and 25 mol % tin oxide sols were refluxed for 16h at room temperature to obtain binary oxide 75SiO₂-25SnO₂ transparent sol. Then the filtered solution was infiltrated through the bare PS opal and calcinated, like the protocol followed for the ISO and ISnO, to obtain an inverse 75SiO₂-25SnO₂ opal (ISSnO).

3. Results and discussion

3.1 Structural properties

Figure 1 shows the SEM images of homemade PS beads, bare PS opal and ISO top and cross-sectional view. The size of the spheres is found to be ~330 (± 5) nm with a relative standard deviation of 2%. During the self assembly process by vertical deposition technique, the PS beads assemble in multilayer structure in (111) plane facet of fcc lattice at normal direction to the substrate surface [12]. This is evident from Figs. 1(a) & 1(b), where Fig. 1(a) shows the hexagonal arrangement of spheres at the top surface while Fig. 1(b) shows the ABCABC...… type layers pattern of the PS beads [4,26] along the thickness of the sample. It is also evident that the opals have attained higher dimension domains with very few structural defects like point and line defects, which are unavoidable during the colloidal crystal growth [6].

Figures 1(c) & 1(d) show the similar top and cross-sectional view of ISO. These images show a clear indication that the inverse opal structures are simply a negative replica of bare PS opals, where PS beads replaced by air spheres and air medium by infiltrated material. From the inset of Fig. 1(c), the higher magnification image of the selected region, the size of the air spheres is found to be ~280 (± 5) nm, which occupies the space of the PS beads before going to be burnt out during calcination. The size of the air spheres is found to be less than the size of the original PS beads. This is not surprising as the infiltrated medium and PS beads are mesoporous, and the water vapor gets released from these pores during the calcination leading to shrinkage and hence the size of the air cavity (D). The inverse opal structures are associated with additional air holes (Fig. 1(c) inset), which are formed at the former contact points (necks) between the polystyrene spheres of the template and their arrangement on a triangular lattice shows an fcc array of the whole ISO structure. These holes are represented with a black arrow in the high magnification image of ISO (Fig. 1(c) inset). The Fig. 1(c) also shows the high homogeneity of the ISO over more than 20 μm² area. Moreover these features indicate the high porosity of the structure of almost of 74% because of its solid PS spheres replacement by air spheres in its fcc lattice structure.

Figure 2 shows the inverse opal structures of SnO₂ and 75SiO₂-25SnO₂ compositions. In detail, the ISnO shown in Fig. 2(a) resembles the ISO structure with air spheres’ diameter of ~250 (± 5) nm. The reduction in cavity size compared to the ISO might be due to the differences in the expansion/compression levels of silica and SnO₂ materials.

Figure 2(b) shows the top surface view of 75SiO₂-25SnO₂ inverse opal structure like ISnO with air sphere diameter of ~285 (± 5) nm. The variation in size compared to both ISO and ISnO could be due to combinational physical properties of SiO₂ and SnO₂ matrices in this binary oxide 75SiO₂-25SnO₂ composition, especially the high porosity of the 75SiO₂-25SnO₂ medium which gets reduced during calcination. Moreover, in case of ISSnO, an additional triangular cavity is observed in between every three air spheres combination. These cavities are shown with white circled regions in Fig. 2(b). These additional mesoporous cavities show a possible increase in the porosity of the inverse opal structure compared to other two inverse opal structures of ISO and ISnO. The consequent optical properties of ISSnO are discussed in the next section 3.2 on optical characterization. The choice of 75SiO₂-25SnO₂ composition was opted from our earlier reports on fabrication of planar waveguide by sol-gel technique [27,28], as it supports a single mode for the C-band wavelength (1.54 μm) propagation with low propagation losses.

Figure 3 shows the TEM measurements performed on ISnO and ISSnO with a tilt of 25° to the electron beam incidence to the (111) facet of our opal’s fcc lattice. The multilayer pattern of crystal lattice can be observed very clearly. In the case of ISnO (Fig. 3(a)), the matrix SnO₂ is found to contain nanocrystals to a maximum extent with uniform distribution of nanocrystals throughout the matrix. Figure 3(b) shows high resolution TEM (HRTEM) image with lattice fringes corresponding to SnO₂ nanocrystals of ~4 (± 1) nm in diameter. The selected area electron diffraction (SAED) in the inset of Fig. 3(a) shows the evidence of glass-ceramic phase of the SnO₂, which was further confirmed by XRD studies. In the case of ISSnO, Fig. 3(c), there is no evidence of SnO₂ nanocrystals formation. The SAED pattern also emphasizes the pure amorphous phase of the ISSnO composition.

 

Fig. 3 (a) TEM image of ISnO with SAED pattern in the inset; (b) HRTEM image indicates the nanocrystals of SnO₂ with ~4 (± 1) nm dimension; (c) TEM image of ISSnO with SAED pattern indicates the amorphous phase of the matrix; (d) XRD spectra of ISO, ISnO and ISSnO.

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From the XRD spectra shown in Fig. 3(d), the spectrum of ISnO composed of diffraction peaks corresponding to casseterite phase of SnO₂ (JCPDS card no. 77-0452) with the peak maxima at 31.2°, 39.1° and 63.8° corresponding to (110), (101) and (220) planes respectively. The presence of broad band centered at 30.12° along with broad diffraction peaks is an indication that a part of SnO₂ is still in amorphous phase, which is also evident from the TEM measurements shown in Fig. 3(a). The XRD spectrum corresponding to ISSnO simply consists of a broad band centered at 27.16° without any diffraction peaks indicating the purely amorphous nature of the 75SiO₂-25SnO₂ matrix. This is also evident from the TEM studies shown in Fig. 3(c). The XRD spectrum of amorphous ISO is also included in Fig. 3(d) to make the comparison with respect to ISSnO. The absence of SnO₂ nanocrystals in ISSnO is an indication that the formation of nanocrystals depends on the dispersion amount of SnO₂ in the silica phase. This is more evident from the reports of Bhaktha et al. [28] that the crystallization of SnO₂ in 75SiO₂-25SnO₂ systems was observed at the temperature of 900°C.

3.2 Optical characterization

In this section, the photonic band gap (PBG) properties of the colloidal crystals were studied by measuring its optical transmission spectra and the angle dependent reflection spectra, which are shown in Fig. 4 . Optical transmittance spectra (Fig. 4(a)) of bare PS opal and inverse opal structures were collected with 2 mm diameter spot size for the light beam and spectral resolution of 0.2 nm.

 

Fig. 4 (a) Optical transmittance of bare PS opal and inverse opal structures. (b) Reflectance spectra of inverse opal structures at 5° angle of incidence to the normal of (111) plane. (c) & (d) show an angle resolved reflectance of ISO and ISnO, where the behavior of (111) and $(\overline{1}11)$ planes along with its theoretical fit (thick line) for Eq. (1) and Eq. (3) respectively.

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The strong reduction in the optical transmission curves of respective bare PS and inverse opal structures correspond to the stop bands, the position of which can be calculated using the modified Bragg formula given by [12,13,17]

The behavior due to (hkl) planes of colloidal crystals other than (111) plane can be explained by the equation [26,30]

From Fig. 4(a), the depth of the PBGs for PS, ISO, ISnO and ISSnO are found to be 53% and 35%, 25% and 30% respectively. These are the best ever reported values of stop band depths for any inverse opal structure. Thus our simple two step process is an indicator for the fabrication of high quality opals with prominent stop band depths. Figure 4(b) shows the reflectance spectra of ISO, ISSnO, and ISnO measured at 5° angle of incidence to the sample surface. The reflection spectra of each opal associated with a band centered at certain wavelength is a resultant of diffraction by the set of planes, where the parameters of D, n_eff and θ satisfies the Bragg diffraction given in Eq. (1). The corresponding parameters for each opal are reported in the Table 1 . The refractive index contrast of the each opal structure was calculated by taking the ratio for mediums of high refractive index to the lower one.

Table 1. Optical and structural parameters of bare PS and inverse opal structures

View Table

Figures 4(c) & 4(d) show an angle dependent reflectance spectra of ISO and ISnO measured with white light beam incidence at θ^o to the (111) plane of fcc lattice varied from 5° to 60°. The angle dependent reflectance behavior was understood by fitting the experimental observations to the Eq. (1). The fitting resulted to the approximation of values like PS/air sphere diameter D and effective refractive index of the opal n_eff. A similar angle dependent reflectance behavior was observed in the case of ISSnO (not reported here) also, the approximated values of D and n_eff are reported in Table 1. The values of air sphere diameter (D) for the opals are in agreement with the values measured from SEM studies as shown in Figs. 1 & 2. Along with optical properties, a long range periodicity also can be observed from SEM studies which further prove the high structural and optical quality of our inverse opal structures.

From Figs. 4(c) & 4(d), the red filled circles represent the diffraction maxima due to $(\overline{1}11)$ plane of the crystal, which move towards longer wavelength with the increase of angle of incidence, the behavior of which can be understood by fitting the observations to the Eq. (3). A similar behavior of angle resolved reflection features due to multiple planes was observed in case of all our opal structures, but only the ISO and ISnO are reported here. In the case of ISnO (Fig. 4(d)), the angle dependent reflectance behavior is evident from initial angles to higher angles but in the rest of the opals the behavior is limited over some range. From the theoretical fitting of Eq. (3) to the $(\overline{1}11)$ plane, the values of d_hkl and β are estimated as 210 nm and 70° respectively. From Fig. 4(c), the observation of two overlapped reflection band maxima at 35° due to the diffraction by two planes (111) and $(\overline{1}11)$ is attributed to the phenomenon of wave coupling that occurs when the incident wave vectors reaches the U/K point of the reduced Brillouin zone of fcc lattice [26,30,31]. The same phenomenon of wave coupling was observed in case of ISSnO and ISnO at 36° and 37.5° respectively.

The refractive index of 1.52 was obtained for 75SiO₂-25SnO₂ composition by using Lorentz-Lorentz equation [27,28], with the values of _{$n_{Si{O}_2}$} = 1.45 and $n_{Sn{O}_2}$ = 1.72. From the theoretical fitting of Eq. (1) to the experimentally observed angle dependent reflectance measurement of ISSnO, the parameters of D and n_eff are approximated as 287 nm and 1.09 respectively. From Eq. (2), the value of n_eff for the ISSnO with n_medium = 1.52 and f = 0.26 is expected to be 1.16. The observed value for n_eff as 1.09 supports the change in the filling fraction value of medium. From Eq. (2), the filling fraction of air and dielectric medium are found to be 0.85 and 0.15 respectively, which supports the value of n_eff as 1.09. The increase in filling fraction from the usual 0.74 to 0.85 can be explained as due to the observation of additional triangular air cavity between every three air spheres. These additional air holes are highlighted by a black arrow in the high magnification FESEM image of ISSnO (Fig. 2(b) inset). This explains the high porosity of the ISSnO structure as well in comparison to ISO and ISnO.

A theoretical model was proposed by Tarhan and Watson [31] for the calculation of full width at half maximum (FWHM: Δλ) of the PBG, which depends on the refractive index contrast 'δn_c' and the volume fraction 'f' of material/background. The values of f and n from Table 1 were used for the theoretical calculation of FWHM for the PS opal, ISO, ISSnO and ISnO. The value of Δλ from the theoretical is consistent with the experimental value, which is tabulated in Table 1, by calculating the ratio of their respective Δλ for each opal. However, some small variations are expected due to the presence of defects such as cracks, dislocations and missing of spheres which are unavoidable during the fabrication of colloidal crystals [32]. These improved optical properties further confirm the opal structures grown by our simple two step fabrication process are high in optical and structural quality.

The nanocrystals of SnO₂ have indirect band gap [33] and help with possible energy transfer from ultraviolet region to nearby luminescent materials by the well-known Förster resonance energy transfer (FRET) mechanism [34], with application in the designing of solar cells. The SnO₂ nanocrystals are also known for its maximum phonon energy [28] of <630cm⁻¹, which favors the reduction of non-radiative relaxation of doped luminescent materials. In addition to the low phonon energy, photonic crystal architecture can also enhance the quantum yield of the doped luminescent material in these nanocrystals particularly at the photonic band edge. Recently it is also proposed by Lu et.al [35] that the property of randomly oriented SnO₂ NCs in 3D PhC can act like amplifying media to observe a new kind of lasing action called random lasing in these PhC structures. The presence of SnO₂ nanocrystals can also show the conductivity behavior, which can make the PhC architectures in the development of electroluminescence based applications. The SnO₂ is also known for its photorefractivity [36], the property of which can make these structures more promising in the field of integrated optics. The dispersion of such SnO₂ in silica PhCs like our 75SiO₂-25SnO₂ can make this system a potential candidate for integrated optics [27,28,35,36]. Moreover the high porosity of our opal structures, obtained by our simple two step infiltration and controlled calcinations process, can also find application in the development of highly durable gas sensors [37].

4. Conclusions

In conclusion, we have developed a simple two step sol-gel technique to fabricate ISO structures with highest ever reported stop gap depth of 35% and inverse opal structures of SnO₂ and its silica based binary oxide composites. The structural properties studied by FESEM, TEM and XRD reveal the high structural quality and porosity of these structures. The optical properties illustrate the interesting photonic crystal features and high porosity of the structures from 74%-85%. Moreover the observed high porosity of the structures indicates its applications in the biomedical area and sensors. The agreements of the experimental results with theoretical results imply the high optical quality and indicate the implementation of the opals in the development of sensors, novel laser systems and integrated optics.