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Glycidylmethacrylate and propylene oxide were used in the epoxide initiated formation of titanium oxide sols which were spun to form thin films. Glycidylmethacrylate can be used to tune the refractive index of the resulting composite and allowed us to photo-pattern the material. The refractive index of the films can be controlled between 1.76 and 2.05 at 589 nm. The thicknesses of the films ranged between 80 and 200 nm and the rms roughness below 2 nm. The films were characterized by atomic force microscopy (AFM), electric force microscopy (EFM), x-ray photoelectron spectroscopy (XPS) and ellipsometry, among other techniques.
©2011 Optical Society of America
1. Introduction
Epoxides, common monomers for preparing polyethers, have recently been shown to moderate sol-gel polymerization of metal salts to afford metal oxides as monolithic gels and homogeneous coatings [1]. The strained epoxide group is thought to work by acting as a proton sponge and a ligand to slow the hydrolysis and condensation sufficiently to avoid precipitation. First described in a 1969 patent [2], this technique has been applied to making metal oxide gels from across the periodic table including aerogels and thin films from iron [3], chromium [3] uranium [4], aluminum [5], yttrium and zirconium salts [6]. Photocatalytic titania gels have been prepared from the propylene oxide mediated polymerization of titanium tetrachloride [7].
Titanium oxide is well-known for its high refractive index and has been used for this reason in the formation of sol-gel materials for optical applications quite extensively [8–11]. Recently, the use of sol-gel derived titanate for photonic nanosheets has been demonstrated [12]. Due to the reactivity of the titanium alkoxides, most approaches use sol-gel polymerizations of alkoxides in the presence of chelating additives or pre-formed titanium oxide nanoparticles dispersed into an organic polymer matrix. Chelating agents often add undesired properties to the materials limiting overall control on the optical properties and are therefore most often removed by a high temperature baking process. In general, nanoparticle composites are known to show scattering loss at high loading and are difficult to process.
Here we report the first glycidyl methacrylate (GLYME) mediated sol-gel polymerization of titanium tetrachloride, in a simple one-pot synthesis to prepare high quality thin films of organically modified titanium oxide with tunable refractive index. The refractive index of the films can be controlled by substituting some of the GLYME with propylene oxide (PO). The methacrylate functionality in GLYME allowed us to directly photopattern the obtained films and we were able to achieve a tunability of the refractive index at 589 nm (sodium D-line) from roughly 1.76 to 2.05. As such, the material is a strong candidate for applications in compact integrated photonics, since its high index allows for tight bends, while the index tunability provides for good coupling to optical fiber and compatibility with a wide range of other photonic materials.
2. Synthesis and processing
The films were prepared by dissolving titanium tetrachloride (0.36 M, 99% Aldrich) in ethanol (200 proof, anhydrous, 99.5%, Aldrich). This has to be done very carefully and inside a fume hood, due to the exothermic nature of the mixing process. The mixture is shaken for 5 sec. After the mixture has cooled down to room temperature an excess of water (1.2 M) is added and the mixture is shaken again for 5 sec. GLYME or a mixture of GLYME with propylene oxide (PO) (1.3 M) is added last. After a final vigorous shaking the mixture is aged at room temperature for a week before spinning films. By keeping the concentration of epoxide below 1.3 M, stable sols were obtained which, as pointed out above, is ordinarily difficult for titania-based sol-gels. Films were prepared by spincleaning a silicon substrate with water, acetone and 2-propanol. The sol to be spincoated is filtered through a 100nm polytetrafluorroethylene membrane syringe filter directly onto the substate. The films are spin-coated between 1000 and 4000 RPM without a spread cycle, followed by baking for 30 sec at 80°C, for 10 min at 120°C on a hotplate and then for 18h at 150°C in an oven. Heat treatment at 150°C for 18 h evaporated the residual ethanol and the lighter by-products of the epoxide ring opening producing mechanically stable, hard, transparent, colorless films. Typical film thicknesses obtained by this process are between 70 and 200 nm. For photopatterning 2% (with respect to solid content) Irgacure 369 (Ciba) was added to the sol prior to spincoating. The film was exposed in a MJB3 mask aligner with a UV 400 filter for 1 min. The power level was 8 mW/cm2 measured at 365nm. After a post exposure bake at 80°C for 20 sec the sample was developed in a 1:4 mixture of acetone and 2-propanol.
3. Measurements and results
Propylene oxide can react to form propylene glycol, 1-chloro-2-propanol and 2-chloro-1-propanol when used as a gelation agent with a metal chloride [1]. The boiling points of these compounds are between 133°C and 189°C at standard pressure [13], [14]. The corresponding reaction products of glycidylmethacrylate (3-chloro-2-hydroxypropyl methacrylate, boiling point 104-108°C at 5 torr [15], 2-chloro-3-hydroxypropyl methacrylate, boiling point 116-148°C at 5 torr [16] and 2,3 dihydroxypropylmethacrylate, boiling point 95-97°C at 0.05 torr for the racemate [17]) are much less volatile. In addition to the high boiling points of the reaction products of GLYME, an FTIR analysis indicated a potential coordination of the carbonyl oxygen to titanium. This is indicated by a peak in the infrared spectrum at 1520 cm−1. The spectra were recorded on a Thermo Electron Nicolet 6700 FT-IR spectrometer. For preparation of solid film samples the sol was drop cast onto a sodium chloride plate which was then baked in the same manner as the films for refractive index measurements. It is known that the coordination of a Lewis acid to carbonyl oxygen can cause the absorption band to shift into this region [18]. More indication for this coordination is that titanium oxy acetylacetonate and bis (tris- (acetalacetonato)) titanium hexachloro-titanate have both carbonyl oxygens coordinated to titanium and show peaks at 1525 cm−1 and 1534 cm−1, respectively [19]. We believe that in addition to the coordination of the carbonyl group, the diol of 2,3 dihydroxypropylmethacrylate and propylene glycol also coordinate to the titanium oxide particles formed during the sol-gel reaction. A schematic of a titanium oxide particle with a randomly oriented organic shell is shown in Fig. 1 together with the reaction of GLYME. Dynamic light scattering measurements showed the size of the particles in suspension to be app. between 3 and 10. A typical light scattering data plot is also shown in Fig. 1.
Fig. 1 Schematic of the ring opening reaction of GLYME and one particle as it might be present in the annealed film (left and above) and a typical dynamic light scatter data plot for determining the size distribution of particles in a sol synthesized with 75% GLYME and 25% PO (lower right).
An X-ray photoelectron spectroscopy (XPS) analysis of the films revealed that the percentage of carbon in the final samples depends on the amount of GLYME used in the synthesis. Two typical survey spectra are shown in Fig. 2 . Besides the expected elements, some residual Cl from GLYME reaction products was found in some samples. The area of the O 1s, Ti 2p, C 1s and if applicable of the Cl 2p peak were use for the quantitative analysis of the carbon content. This data was correlated with optical properties of the films measured with a GSE5 spectral ellipsometer from Sopra. Ellipsometry measures the intensity and phase difference of TE and TM light reflected from the film and the substrate. If other properties of the samples, like the optical properties of the substrate are known, this data can be used to determine a dispersion law that describes the refractive index of the film as well as the film thickness. In our case the titanium oxide films were measured on silicon wavers. Every substrate was first measured blank to examine the thickness of the native oxide layer. A Sellmeier model was used to describe the real part of the dielectric function of the thin films between 400 and 800 nm. Absorption was neglected for most of the films, because the quality of the fit did not increase if absorption was included into the model. A transmission measurement of two films on quartz did not reveal any absorption between 400 and 800 nm. As shown in Fig. 3 (upper right) the carbon content in the films changes linearly with the amount of GLYME used in the synthesis. For the same range (0 to app. 50% GLYME) refractive indices of the films also change approximately linearly with the GLYME concentration. To test if the refractive indices of the materials are dependent on the epoxide used for synthesis, we compared the refractive indices of films which were made from sols synthesized with mixtures of PO and GLYME with films obtained by mixing two sols solely made from either PO or GLYME. The two sols were mixed right before spin coating to obtain PO and GLYME concentrations similar to the sols synthesized with a mixture of PO and GLYME.The resulting refractive index data plotted against the GLYME content are also shown in Fig. 3. The x-axis indicates the amount of GLYME in the sol. For the materials synthesized with a mixture of GLYME and PO a value on the x-axis of for example 0.75 indicates that 75% of the total molar epoxy concentration in the sol comes from GLYME and 25% is from PO. In the case of films made from the mixtures of two sols, the x-axis values indicate the mixing ratio of the two sols. Again, 0.75 on the x-axis means that 75% of a GLYME sol and 25% of a PO sol were used. Every data point was measured for at least three different samples. Each synthesis was performed in sets of three. The change of refractive index for the synthesized and the mixed samples follows the same shape and no significant offset can be seen, indicating that the influence of GLYME during the synthesis is negligible and that GLYME is just as suitable as PO in the epoxy initiated formation of titanium oxide. This also implies that the coordination of the reaction products of GLYME to the titanium oxide particles happens during annealing and not during the particle formation. Overall, this data shows the facility of this method for preparing films with refractive indices between 1.76 to 2.05. The dispersion behavior of the materials is in the expected range for titanium oxide containing mixtures. An extrapolation of the fitted Sellmeier function into the near IR suggests that the refractive index of the materials at 1550nm is between 1.7 and 1.9. Annealing the materials at 550°C for 10 h increases refractive index of the films by approximately 0.3 at 800nm. This heat treatment also creates in the previously completely amorphous film a phase of anatase, as X-ray diffraction analysis showed. The domain size in the anatase was estimated to be around 3 nm.The methacrylate group in gylcidylmethacrylate can be photopolymerized, allowing us to demonstrate photopatterning. By adding Irgacure 369 as a photoinitiator and exposing through a photomask we were able to create a 120nm high pattern. The smallest feature on the mask was 4µm. A surface profile scan over a 300µm wide feature is shown in Fig. 4 . We believe that because of their high and adjustable index as well as their lithographic processability these materials can be very useful for the fabrication of compact photonics devices and structures such as couplers, ring resonators and gratings. By having control over the refractive index in this region is it possible to fine tune the transmission band of a waveguide Bragg grating, without changing the structure. This has a big cost benefit as the the photo mask or nano imprint stamp used to fabricate the structure does not have to be redesigned.
Fig. 2 XPS survey spectra of a material made only with PO and of one only made with GLYME. Peak assignments are shown in the graph. Note that the C 1s peak in the black curve is not visible at this magnification due to its small size.
Fig. 3 Inset: The carbon content of the films changes linearly with the amount of GLYME used in the synthesis. Main graph: The squares represent data from sols synthesized with mixtures of GLYME and PO. The concentration of epoxy groups, water and TiCl4 was kept constant for all the materials. The circles represent the films obtained from mixtures of two sols, synthesized separately with either PO or GLYME. The error bars show the standard error of the mean. Every data point represents at least 4 samples.
Fig. 4 Surface profile scan over a 300µm wide structure. The image in the middle shows a microscope picture of the sample.
To investigate the nanoscale distribution of titanium oxide in the composites, subsurface dielectric imaging of thin films was pursued using electrostatic force microscopy (EFM). EFM experiments were realized using a double pass method: during the first pass, the topographic image was acquired in normal tapping mode using a Veeco Innova atomic force microscope equipped with a Nanodrive controller. The second pass raised the conductive atomic force microscope (AFM) probe above the sample to a fixed distance and re-scanned the surface with a bias voltage applied between the probe and AFM stage, following the previously recorded topography to maintain a constant tip–sample separation. The amplitude and phase of the probe during the second pass were recorded as EFM signals. Two types of conductive AFM probes were used. One is a conventional platinum-iridium coated AFM probe (Bruker SCM-PIT). The other is a high aspect ratio antimony doped highly conducting Si probe (Bruker, FESP) with apex diameter < 8nm, resonance frequency at 77.25 kHz and force constant of 2.5 N/m. The bias voltage applied to the AFM probes ranged from −10 to + 10 V and the lift height was varied from 10 to 100 nm. The images reported here were recorded using FESP tips with a lift height of 18 ± 2 nm and a bias voltage of −5V applied through the substrate. The AFM height image in Fig. 5 shows a relatively smooth surface with an rms roughness of 0.6nm; the AFM phase image, which reflects the relative hardness of the material, indicates nanophase separation on the surface, possibly between titanium oxide (hard - dark) and organic rich (soft - white) domains. The average domain size is less than 20nm.
Fig. 5 AFM and EFM images of a titanium oxide - organic composite thin film with ~46 wt% of titanium oxide; upper left: height image, rms roughness 0.6 nm, upper right: AFM phase, scale 40°, lower left: EFM phase, scale 40°, lower right: Near bimodal distribution of relative dielectric constants computed from EFM image. In the lower right of the AFM and EFM phase image a zoomed on section is shown. The yellow line represents 100nm.
The EFM phase image shows the subsurface distribution of the domains with the darker parts indicating domains with higher dielectric constant. The dark areas account for approximately 46% of the area, which corresponds to 46 wt%. Both images show a homogenous distribution of organic and inorganic domains without large clusters. The relative static dielectric constants of the two phases were estimated from a series of EFM images recorded under changing biasing voltages based on the methodology of Zhao [20] and Riedel [21]. Relative static dielectric constants calculated from the EFM phase map show a nearly bimodal distribution of organic rich domains at 7.5 ± 3 and titanium oxide rich ones at 68 ± 12. For comparison, the static dielectric constants for rutile are 170 and 86 for the ordinary and extraordinary axis [22] and 3.6 for poly methylmethacrylate [23], respectively. Details of these measurements and quantitative estimation of local dielectric properties in thin films of the composite over the entire range of concentrations reported in the manuscript will be published elsewhere.
4. Conclusion
In summary, we presented a simple one-pot method for the fabrication of titanium oxide containing films with tunable refractive index. The tunability of the refractive index is achieved by the formation of an inorganic organic nanocomposite of the titanium oxide with the reaction products of the used epoxides. The materials show no measurable absorption between 400 and 800 nm. The particle size is small enough that scattering in the telecom windows in the near infrared should not be an issue. We note that there may be hydroxyl functionalities present from the organics as well as from the partially condensed amorphous titanium oxide phase, which could lead to moderate absorption in the C-band. We further showed for the first time that glycidyl methacrylate is suitable as an agent in the epoxy initiated synthesis of titanium oxide and enables one to directly photopattern films made from the resulting stable sols. This material shows significant potential for applications in compact photonics and nanophotonics devices.
Acknowledgments
The authors would like to acknowledge support from the National Science Foundation through MDITR (Grant#-0120967), CIAN (Grant#-EEC0812072) and GEO-photonics (Award#0946131), Canon, Inc., the Research Initiative Fund (TRIF) through the Photonics Initiative for graduate student support as well as Veeco through Energy Lab Grant 2010. The authors would also like to thank Kenneth Nebesny and Paul Lee for measuring the XPS spectra.
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