1. Introduction

Polymers containing azobenzene chromophore molecules or azobenzene-based photochromic polymers have been studied extensively by researchers due to their promising photonic applications including optical storage, optical switching, non-linear optics, and so on [1–4]. The desirable properties of these polymers are attributed to the trans-cis-trans photoisomerization phenomenon of the azobenzene molecule groups under photo-irradiation or heat-treatment in the material [5]. However, most of the photoactive organic molecular materials may not be stable in ambient environment for long time periods of time, low mechanical strength, and relatively low optical transparency as compared to an inorganic oxide [6]. Thus optically homogeneous and transparent organic-inorganic hybrid materials containing organic components are anticipated to be desirable photo-based materials.

Besides, photosensitive polymers have also been widely reported for fabrication of micro-optical elements in recent years [7–9]. It is mainly attributed to the intrinsic properties and photosensitivities of the photosensitive materials [10]. Thus, kinds of micro-optical elements including micro-lens arrays [11–14], micro-channel waveguides [15] and diffractive gratings [16] have been developed through different methods such as photo-lithography [17], electron-beam lithography [18], direct laser writing technique [19] and nanoimprint technique [20]. In fact, most of the photosensitive materials are based on the photosensitive organic materials or photo-resist, which are used as the transfer layers. Commonly, the micro-optical patterns were firstly fabricated on the photosensitive organic materials or photo-resist film, and then the patterning was transferred to the target material by an etching process, which makes the fabrication process complex.

Organically modified silanes (ormosils) based organic-inorganic hybrid materials prepared by a sol-gel process have been widely studied as a promising system for photonic applications recently [21–24]. Since the hybrid sol-gel method can provide a mixing of organic and inorganic materials at a nanoscale level and in some cases even on a molecular level, thus leading to optically homogeneous and transparent hybrid materials. Besides, organic components are integrated into the inorganic matrix and bulky organic components fill the pores between the inorganic oxide chains, making that a thick hybrid thin film with a few microns thickness can be easily obtained by s single spin-coating process at a low temperature. The incorporation of organic groups can also improve physical, chemical, and mechanical properties of the hybrid materials, and the modification of an inorganic network structure with organic groups gives larger space for the isomerization of organic photoactive molecules as compared to inorganic glasses. Thus, these organic-inorganic hybrid materials are anticipated as desirable materials for photonic applications, which can trap organic molecules. Due to these promising advances and potential photonic applications, many scientists have made studies and reported progress in such organic-inorganic hybrid systems [25–27]. TiO₂-based ormosils organic-inorganic hybrid materials have a significant potential for photonic applications and the preparation and optical properties have been reported in our previous work [28], but we think that it is of more interest to study the hybrid material doped with organic functional groups to achieve active properties.

Here, we report on the preparation and characterization of a novel multifunctional TiO₂/ormosil organic-inorganic hybrid materials containing both azobenzene small molecules and photosensitive groups for photonics and nanoimprint applications. Optical waveguide properties including film thickness, the refractive index, propagation loss and modes are studied by a prism coupling technique. The surface morphology and the structural properties of the hybrid films are characterized and investigated respectively by Atomic Force Microscope, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. Photo-responsive properties of the hybrid films are induced by a photo-irradiation of 365 nm UV light. Photosensitivities of the hybrid films are proved by building the micro-lens arrays on the hybrid films by using a UV soft imprint technique with a polystyrene (PS) template as the mother mold. In fact, micro-lens arrays are directly developed on this hybrid material without any etching process, that is, this hybrid material has excellent optical properties and can be used as the target material.

2. Experimental

The multifunctional TiO₂/ormosils organic-inorganic hybrid films were prepared by combining the low temperature sol-gel process and the spin-coating technique. The precursor matrices were synthesized by three solutions. For solution I, 1 mol of tetrabutyl titanate was mixed with 4 mol of acetylacetone and stirred for about an hour until the homogenization was obtained. For solution II, 1 mol of γ-glycidoxypropyltrimethoxysilane (GLYMO) was mixed with 4 mol of absolute ethanol and 4 mol of deionized water, and after being stirred for about 30 min, 0.01 mol of hydrochloric was added into the solution as the catalyst. Then the solution was stirred for an hour again in air. For solution III, 1 mol of 3-methacryloxypropyltrimethoxysilane (MEMO) was mixed with 3 mol of isopropanol and 3 mol of deionized water, 0.01 mol of hydrochloric acid was added as the catalyst, and the solution was stirred for about an hour. Three solutions were then mixed to have a molar ratio of TiO₂: GLYMO: MEMO with 0.2: 0.4: 0.4. After the mixed solution was stirred for about 2 hours, the azobenzene powder with 3 wt. % was added into the system. Then the mixed solution was stirred for another 25 hours at room temperature. It should be stressed here that before the final solution being used, a photo-initiator in the form of bis(2,4,6-trimethylphenyl)phosphine with 4 wt. % was added into the final solution to enable the photo-polymerization at room temperature of the carbon-carbon double bond from MEMO under UV light irradiation. Silicon, silica-on-silicon and glass slides were used as the substrates and they were ultrasonically cleaned in acetone, ethanol and deionized water, respectively for 20 min, rinsed with de-ionized water and dried. Following the common practice for spin-coating, a 0.22 μm pore filter was attached to the syringe to remove the foreign particles before the solution was spin-coated onto the substrate. Each single layer of the hybrid film was spun onto the substrate at a spinning speed of 3500 rpm for 35 s, followed the coated hybrid films were baked in air for about 10 min at different temperatures of room temperature, 50, 80, 100, 150, 200, 300, 400 and 500 °C. Then the films were respectively irradiated under UV light at a wavelength of 365 nm for different time from 5 s to 30 min, to study the photo-responsive properties of the hybrid films.

The properties of the hybrid films were characterized by different techniques. The morphology of the hybrid films was observed by a Bruker Dimension FastScan Atomic Force Microscopy (AFM) using the tapping mode. The optical waveguide properties including the refractive index, film thickness, propagation loss and modes of the hybrid films were measured by an m-line apparatus (Metricon 2010) based on the prism coupling technique at a wavelength of 633 nm. The photo-responsive measurements of the hybrid films were carried out by a UNICO UV-2800A UV-VIS Spectrometer after being irradiated by UV light for various time intervals. The UV irradiation light was produced by a PLS-SXE xenon lamp with a peak emission wavelength at 365 nm and irradiance of 15 mW/cm². The XPS results of the hybrid film deposited on silicon substrate was also obtained by using Al Kαradiation as the X-ray source (ESCALAB MK-II model) which operates at 12.5 KV and 20 μΑ. Τhe operating vacuum pressure was 10⁻⁸ Pa. The FTIR spectra of the hybrid films deposited on silicon substrate were characterized by a FTIR (Perkin Elmer Two, with a resolution of ± cm⁻¹) spectrometer in the range of 4000-400 cm⁻¹ to monitor the photochemical activities of the hybrid films. Surface morphology of the micro-lens arrays were observed by scanning electron microscopy (SEM) (S-4800, Hitachi, Tokyo, Japan, 15 kV operating voltage).

3. Results and discussion

The surface morphology of the hybrid films doped with 3 wt. % azobenzene and baked at different temperatures were studied by using an atomic force microscopy technique. The hybrid films were spin coated on silicon substrates at a spinning speed of 3500 rpm for 35 s. Figure 1 shows an AFM image of the hybrid film obtained at room temperature. It can be observed from Fig. 1 that the film has a smooth, dense and micro-pore free surface morphology. When organic groups are integrated into the inorganic system, the shrinkage of the film is low because the bulky organic components fill the pores between the inorganic oxide chains. Thus, the film can reach its final density at a relatively low temperature. In fact, in our experiment, hybrid films with a dense and smooth surface can always be obtained when the baking temperature is below 200 °C. However, as the baking temperature further rises from 300 to 500 °C, some micro pores will appear on the surface of the hybrid films, which is related to the combustion and decomposition of the organic matter in the material. In addition, the value of the root mean square (RMS) surface roughness of the films was also examined over a 5 μm × 5 μm area, and the RMS surface roughness value of the films baking at different temperatures is below 0.3 nm, which is small enough for the optical waveguide and micro optical elements applications.

 

Fig. 1 AFM image of the hybrid film doped with 3 wt. % azobenzene and obtained at room temperature.

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Figure 2 shows the dependence of the thickness and the refractive index of the hybrid films on the baking temperature at the wavelength of 633 nm. The films were deposited on the silicon substrates and respectively baked at room temperature, 50, 80, 100, 150, 200, 300, 400 and 500 °C for 10 min. It can be seen from Fig. 2 that as the baking temperature rises from room temperature to 500 °C, the film thickness reduces from 1.9382 μm to 0.6061 μm. It can be observed that the reduction in the film thickness is relatively slow (1.9382 μm to 1.8460 μm) in the temperature range from room temperature to 100 °C, which is due to the relatively small amount of evaporation of the organic solvent and water in the films. However, the reduction in the film thickness is relatively obvious (1.8460 μm to 1.6081 μm) from 100 to 200 °C, which is related to the evaporation of a large amount of organic solvent and water in the films. It should be noticed that as the temperature rises from 200 to 400 °C, the change in the film thickness is a lot more substantial, which is ascribed to the thermal decomposition and combustion of organic compounds in the films. With further increase the temperature to 500 °C, there is almost no film thickness change, indicating that the film becomes inorganic dense. It is worthy to note that the refractive index of the hybrid films increases slightly from room temperature to 80 °C, which is related to the more dense film due to the higher baking temperature. However, the refractive index decreases in some sort with an increase of the temperature from 80 to 200 °C. As is known, a dense hybrid film can be obtained at room temperature due to the full filling the pores among the inorganic oxide chains of a large amount of organic solvents and water. As the baking temperature rises from 80 to 200 °C, on the one hand, the film becomes porous due to the evaporation of a large amount of the organic solvents and water. On the other hand, the film becomes denser due to the photo-polymerization process [29]. In addition, the change of the mobility of the chromophore molecules of the hybrid films at an elevated temperature also can cause the change of the chemical composition of the hybrid films [30]. Thus, it is probably to induce a decrease of the refractive index in this temperature range. With further increase the baking temperature to 400 °C, the increase of the refractive index of the film should be owing to a complete photo-polymerization of the material, which can be proved by the complete vanishing of the C = C bonds at 400 °C in the FTIR spectra as shown in Fig. 7. It can be found that the increase of the refractive index of the film becomes more slowly in the temperature range from 400 to 500 °C, which is mainly because the film becomes almost completely inorganic dense.

 

Fig. 2 Dependence of the thickness and the refractive index of the hybrid films on the baking temperature. The error bars are standard deviation of five measurements and indicated by vertical lines.

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Figure 3 shows the optical guided TE modes of the hybrid film doped with 3 wt. % azobenzene based on the prism coupling technique at the wavelength of 633 nm. The hybrid film was spin coated on a silica-on-silicon substrate at a spinning speed of 3500 rpm for 30 s and baked at a low temperature of 80 °C for 10 min. It can be obviously seen from Fig. 3 that several mode dips in the intensity can be observed, indicating that the as-prepared hybrid film obtained by a single spin-coating process can support several TE modes at 633 nm. However, only the three modes of TE₀, TE₁ and TE₂ can be used as the guided modes of the film since the substrate index is 1.46 for a wavelength of 633nm, and only the three highest indices for each polarization are higher than this value. The other mode dips are attributed to the substrate modes due to their refractive indices smaller than the value of the substrate index. Additionally, the optical guided TE modes of the hybrid film at the wavelength of 1538 nm were also studied in our experiment, and only one mode of TE₀ can be used as the guided mode of the hybrid film at the wavelength of 1538 nm.

 

Fig. 3 Optical guided TE modes of the hybrid film doped with 3 wt. % azobenzene and baked at 80 °C at the wavelength of 633 nm.

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Figure 4 shows the optical propagation loss curve of the hybrid film sample in Fig. 3 for the TE₀ mode evaluated by a scattered-light measurement technique based on a fiber photometric detection at the wavelength of 633 nm. In our experiment, the optical propagation loss of about 5 cm long hybrid film was tested, and the optical propagation loss of the hybrid waveguide film can be calculated according to the formula below:

 

Fig. 4 Optical propagation loss curve of the hybrid film for the TE₀ mode at the wavelength of 633 nm.

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Figure 5 shows the UV-visible absorption spectral change of the hybrid films doped with 3 wt. % azobenzene and baked at different temperatures after irradiation with non-polarized UV light of 365 nm for different irradiation time from 5 s to 30 min. As is known, azobenzene chromophores exist in both cis and trans isomers and are in the more stable tans state in the dark room temperature. It can be seen from the absorption spectra of the hybrid film obtained at room temperature as shown in Fig. 5(a) that there have been an intense absorption peak at 352 nm corresponding to the $\pi \to {\pi }^{\ast }$ electronic transition of the trans azobenzene side chain and a weak peak at about 435 nm which originates from the weak $n\to {\pi }^{\ast }$ electronic transition of the cis isomers. When the hybrid film is irradiated by the non-polarized UV light of 365 nm, azobenzene chromophores induces trans-cis photoisomerization in the azobenzene side chains of the hybrid film. The process demonstrates itself in the absorption spectra as a reduction in the trans isomers $\pi \to {\pi }^{\ast }$ peak and a concomitant increase in the weaker cis isomers $n\to {\pi }^{\ast }$ peak. As a result, the intensity of the absorption peak at 352 nm gradually decreases with increase the irradiation time. At the same time, the intensity of the absorption peak at 435 nm becomes more pronounced until an equilibrium state reaches, in which a trans isomer and a cis isomer coexist in the material. A similar trans-cis photoisomerization process can also be observed for the hybrid films baked at the temperatures from 50 to 200 °C as shown in Figs. (b-f). It can be easily seen from Fig. 5(b) that the absorbance change is relatively larger for the hybrid film baked at 50 °C than that of the hybrid film obtained at room temperature. It is probably related to the evaporation of some organic solvents and water, which makes the film become much denser and induces a relatively larger absorption. However, with further increase the baking temperature from 80 to 200 °C, the absorbance change at the two peaks gradually becomes much smaller. What is more, when the baking temperature rises to 200 °C, the weak absorbance peak at 435 nm almost disappears. It is mainly caused by the further evaporation of organic solvents and water and the decomposition of the azobenzene dye inside the hybrid film due to the higher baking temperature, which probably leads to some structure change of the hybrid film including appearance of some pin hole inside the film and the surface structure on the hybrid film [31]. All these results indicate that the hybrid film baked at 50 °C has the strongest peak absorption and thus the best optical switching characteristics.

 

Fig. 5 UV absorbance changes of the hybrid films doped with 3 wt. % azobenzene and baked at different temperatures of (a) room temperature, (b) 50, (c) 80, (d) 100, (e) 150 and (f) 200 °C upon irradiation under UV light of 365 nm.

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The chemical composition and banding states of the hybrid films were further characterized by XPS. Figure 6 shows the XPS spectrum of the hybrid film doped with 3 wt. % azobenzene and baked at 100 °C. Results indicate that O 1s and 2s; Ti 2s and 2p; Si 2s and 2p; C 1s and N 1s can be clearly observed from the measured hybrid film. It is noted that the C–C bonding appears at 283.58 eV as shown in Fig. 6. In addition, It is well known that the C 1s peak position of graphite (sp²C) is 284.15 eV, and the peak position of diamond (sp³C) is 285.50 eV. The C 1s peak position of our coating is 283.58 eV, which is close to that of graphite [32]. These results mean that the hybrid film mainly contains the sp² carbon phase.

 

Fig. 6 The XPS spectrum of the hybrid film doped with 3 wt. % azobenzene and baked at 100 °C.

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Figure 7 shows the FTIR absorption spectra of the hybrid films doped with 3 wt. % azobenzene, which are deposited on silicon substrates and baked at different temperatures. It can be easily found from Fig. 7 that there are several peaks for all the samples, and there is almost no change for the peaks in intensity when the baking temperature is below 200 °C. The main band peak at about 1110 cm⁻¹ is assigned to Si-O-R stretching vibrations of ethoxy groups directly bonded to silicon [29], with increase the baking temperature up to 400 °C, the band in intensity decreases obviously. The shoulder band peak at about 925 cm⁻¹, which is attributed to the stretching vibration of Si-OH or SiO-groups superimposed onto the by Si-O-Ti stretching [33,34], is present in every sample. The low peak at about 606 cm⁻¹ observed for all samples is from the substrate. The absorption peak at about 1290 cm⁻¹ is due to the symmetric deformation of Si-CH₃ bond and decreases gradually in intensity with increase the baking temperature and it almost vanishes when the baking temperature rises up to 300 °C. It can be observed that the weak peak centered at 2908 cm⁻¹ is assigned to -CH₂- symmetallic stretching, and the band in intensity decreases as the baking temperature rises, when the baking temperature rises up to 400 °C, the band almost vanishes. It should be mentioned that there are two peaks at about 1720 and 1638 cm⁻¹, which are attributed to the vinyl group C = C stretching mode and the carbonlyl ester groups, respectively. Obviously, both of them decrease in intensity with increase the baking temperature from room temperature to 500 °C, and the peak at 1638 cm⁻¹ almost vanishes as the baking temperature rises up to 300 °C. As is known, the photochemical properties of the hybrid films is due to the C = C stretching mode and C = O stretching mode [34]. It can be concluded based on these results that a photosensitive hybrid film can be obtained below a baking temperature of 300 °C and a polymerization of the hybrid film proceeds with the increases of the baking temperature.

 

Fig. 7 FTIR absorption spectra of the hybrid films doped with 3 wt. % azobenzene and baked at different temperatures.

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In order to further prove the photosensitive properties of the as prepared hybrid film, micro-lens array was built in the hybrid film by using a UV nanoimprint technique, and below is the process details. The monolayer and close-packed PS template was used as the mother mold, which was obtained by combining a spin-coating and a drawing method according to the literature [35]. It should be mentioned here that the PS micro-spheres aqueous suspension with 10 wt. % was prepared by the emulsifier-free polymerization method with the PS micro-spheres diameter of 378 nm [36,37]. The PDMS soft mold was fabricated by a replication method. Firstly, the PDMS precursor polymer was obtained by mixing the elastomer base and curing agent (Sylgard 184, Dow Corning) at a mass ratio of 10:1 with an after-treatment in a Vacuum Oven for about 20 min to eliminate air bubbles inside the mixture. Then, the as prepared PDMS precursor was carefully cast against the PS template, following a heat treatment at 90 °C for 40 min. Finally, the obtained PDMS soft mold was peeled off from the PS template. The photosensitive hybrid film was spun onto the glass substrate at a spinning speed of 1000 rpm for 60 s and used as the imprint layer. Finally, the PDMS soft mold was imprinted into the photosensitive hybrid film and the obtained sample was then irradiated under UV light for 15 min by a PLS-SXE xenon lamp with a peak emission wavelength at 365 nm and irradiance of 15 mW/cm² so as to solidify the imprinted layer, after which the PDMS soft mold was peeled off from the hybrid film. Thus, the micro-lens array built in the photosensitive hybrid film was obtained. Figure 8 shows the SEM image of the micro-lens arrays built in the hybrid film. It can be seen that the micro-lens array has a clean and neat surface, indicating that those impurities are not brought during the whole experiment process. Moreover, the shape of the micro-lens built in the hybrid films is in a good evidence to prove a successful fabrication of the micro-lens array by the nanoimprint process. It is expected that the micro-lens arrays built in this photosensitive hybrid film have a potential application for optical micro-lens arrays.

 

Fig. 8 SEM image of the micro-lens arrays built in the hybrid film by using a UV soft nanoimprint technique.

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4. Conclusions

In conclusion, a new multifunctional TiO₂/ormosils organic-inorganic hybrid films with photoresponsive and phtotosensitive properties have been prepared by combining the low-temperature sol-gel process and spin-coating technique. The optical planar waveguide properties of the hybrid films including propagation modes, propagation loss, the refractive index and film thickness, have been measured. The surface morphology and the structural properties of the hybrid films were also characterized and investigated respectively by Atomic Force Microscope, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. Results indicate that a smooth and dense film can be easily obtained at a low temperature, and the roughness value of the hybrid film is below 0.3 nm which is small enough for the photonic application. The photo-responsive properties of the trans-cis-trans photoisomerization under the irradiation of UV light and the effects of the baking temperature on photo-responsive properties of the hybrid films have been studied. Results indicate that there is a most obviously photo-responsive properties for the hybrid film obtained at 50 °C. Finally, micro-lens array with a microlen diameter of about 378 nm were successfully built in the hybrid film, indicating the photosensitive properties of the hybrid films. All these results show that the multifunctional hybrid material reported in this paper are promising candidates for integrated optics and photonic applications, which would allow directly integrating onto a single chip, the optical micro-elements and the optical data storage and responsive devices.