Synthesis and characterization of oligomeric phenylsilsesquioxane-titania hybrid optical thin films

Materials Chemistry and Physics 83 (2004) 71–77

Synthesis and characterization of oligomeric phenylsilsesquioxane-titania hybrid optical thin films Wen-Chang Chen a,b,∗ , Wei-Chi Liu a , Pei-Tzu Wu a , Po-Fu Chen a b

a Department of Chemical Engineering, National Taiwan University, Taipei 106, Taiwan, ROC Institute of Polymer Science and Engineering, National Taiwan University, Taipei 106, Taiwan, ROC

Received 30 June 2003; received in revised form 25 August 2003; accepted 3 September 2003

Abstract In this study, oligomeric phenylsilsesquioxane (OPSQ)-titania optical thin films were synthesized and characterized. OPSQ with the end group of Si–OR was prepared first. Then, it was reacted with titanium (IV) n-butoxide, followed by spin-coating and multi-step curing to form the optical thin films. Highly homogeneous and transparent films were obtained at the titania content of 0–54.8 wt.%. The titania domain in the prepared films was estimated to be 8 nm from the TEM diagram. The optical properties of the prepared hybrid films could be tuned by the titania content. By increasing the titania content from 0 to 54.8 wt.%, the absorption edge and refractive index of the prepared film were increased from 277 to 322 nm and 1.527 to 1.759, respectively. These results could be explained from the growing size effect of the titania domain. The optical loss of the studied planar waveguide by using the OPSQ-titania as the core layer decreased from 0.568 to 0.415 dB cm−1 as increasing the titania content from 0 to 15.9 wt.%. It is resulted from the reduction of the C–H bonding density of the hybrid materials by increasing the titania composition. The prepared films could have potential applications for high refractive index coating or optical waveguides. © 2003 Elsevier B.V. All rights reserved. Keywords: Poly(silsesquioxanes); Titania; Nanocomposites; Optical properties

1. Introduction Poly(silsesquioxanes) have attracted extensive research interest because of their excellent thermal, mechanical, electronic, and optical properties [1–3]. Poly(hydrogen silsesquioxane) (HSSQ) and poly(methyl silsesquioxane) (MSSQ) have been recognized as a class of low dielectric constant materials for deep sub-micron IC processes [4–8]. Novel nanocomposites could be built up through the side chain or end group of silsesquioxanes. Functional side chain such as liquid crystalline mesogen or fluorescent moiety had been successfully attached to the cage structured silisesquioxane [3]. Our group is particularly interested in preparing poly(silsesquioxane) based materials for optoelectronic applications. Brown et al. [9] reported that low loss poly(phenylsilsesquioxane) (PPSQ) based planar optical waveguide for optical communication. Zhang and co-workers [10] success∗ Corresponding author. Present address: Department of Chemical Engineering, National Taiwan University, Taipei 106, Taiwan. Tel.: +886-2-23628398; fax: +886-2-23623040. E-mail address: [email protected] (W.-C. Chen).

0254-0584/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2003.09.039

fully attached nonlinear optical chromophores to the side chain of poly(silsesquioxanes), which had higher thermal stability than conventional organic polymers. We have developed new MSSQ-titania materials with high thermal stability, surface planarity and high optical transparence [11]. The MSSQ-titania materials could solve the poor thermal stability of acrylic-titania optical materials reported previously [12,13]. However, the low refractive index of the MSSQ moiety limits on tuning the refractive index range of the prepared MSSQ-titania materials. Hence, it will be interesting to develop the poly(silsesquioxane)-titania materials based on the high refractive index PPSQ. The synthesis and characterization of PPSQ have been reported by several groups [14–17]. It was synthesized from trichlorosilane or trialkoxysilane in the mixture of organic solvent and water. For the preparation of poly(silsesquioxane)-titania optical materials, large amount of water should be avoided because it could produce a large titania domain. Hence, preparation of the PPSQ in organic solvent for the PPSQ-titania materials is necessary. The Si–OR end group content is important for bonding with other moiety. Besides, high molecular weight PPSQ should be avoided for obtaining the homogeneous hybrid materials

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Ph Ph

+ H2O, H , 60oC

Si(OCH3)3

THF/MIBK, 3 hrs

RO

Si

Ph O

O RO

Si Ph

Si

Ph O

O O

Si

OR

O

Si

O

Ph

Si OR n Ph

OPSQT0

H2O in THF

Ti(OBu)4 in THF Addition for 1 hr

Addition for 1 hr

O

O

O

O

O Ti O

O

Ti O O OH

O O Ti O Ti O Ph Ph Ph Ti Ti O O Ti O Ti O O Ti O O O Si O Si O Si O O O O O O Ti O O Ti OH O O Ti Ti O Ti O O O Ti O Si O Si O nSi O OH O O O O Ti O Ph Ph Ph O Ti O O O HO O Ti Ti Ti O OH O O O O

Concentration to desired solid content Multi-step curing

O O Ti

Spin coating

OPSQ-titania hybrid films(OPSQT10-OPSQT175)

Fig. 1. Synthetic scheme of OPSQT0 and OPSQ-titania hybrid materials (R = H or CH3 ).

since it reduces the bonding density between the PPSQ and titania. Hence, synthesis of oligomeric phenylsilsesquioxane (OPSQ) with functional end groups is important for the preparation of homogeneous hybrid materials. In this study, OPSQ-titania optical thin films were synthesized according to the reaction scheme in Fig. 1. An OPSQ precursor solution (OPSQT0) with the end group of Si–OR was prepared from phenyl trimethoxysilane first. Then, it was reacted with different ratio of titanium (IV) n-butoxide, followed by spin-coating and curing to become OPSQ-titania optical films. The structures of the prepared OPSQ-titania thin films were studied by FTIR, FE-SEM, TEM, and AFM. The effects of the titania content on the optical properties of the OPSQ-titania optical films and corresponding waveguides were investigated, including absorption band edge, refractive index, and optical loss.

2. Experimental 2.1. Materials Phenyltrimethoxysilane (PTMS, 97%, Aldrich), tetrahydrofuran (THF, 99%, stabilized and anhydrous, Acros), methyl isobutyl ketone (MIBK, Acros), titanium (IV) n-butoxide (Ti(OBu)4 , 99%, Acros), and hydrochloric acid (35%, Yakuri) and chloroform-d (99.8%, CIL) were used as received. PPSQ (Mw = 1200–1600, average OH content = 5.5%, Gelest) was used as reference in estimating the Si–OH content of the OPSQT0.

2.2. Synthesis of oligomeric phenylsilsesquioxane solution (OPSQT0) The synthetic approach for preparing the OPSQ was similar to that for preparing O-MSSQ precursor in our previous report [8]. However, the mole ratio of H2 O to PTMS and pH in this study was adjusted to 2.0 and 1.0, respectively. With this reaction condition, 6.61 g of PTMS and 9.33 g of MIBK were added to a three-necked, round-bottom flask of 100 ml and immersed in an ice bath. De-ionized water of 1.03 g with 0.26 g of HCl in 5.13 g of THF was added drop-wise over a period of 30 min with rigorous stirring. The reaction flask was then allowed to warm to room temperature and was immersed in silicon oil at 60 ◦ C. The hydrolysis and condensation reaction lasted for 3 h under a nitrogen atmosphere and reflux to obtain the OPSQT0. 2.3. Synthesis of OPSQ-titania hybrid materials and their optical thin films Different amounts of Ti(OBu)4 were added into the precursor solution OPSQT0 for the preparation of OPSQ-titania materials, OPSQTX. X is the ratio of the mole percent of Ti(OBu)4 over PTMS, which is 10–175 in this study. The preparation is described below using OPSQT60 as an example. Ti(OBu)4 of 6.81 g was diluted in 24.81 g of THF added drop-wise to OPSQT0 at the temperature of 60 ◦ C and maintained for 1 h. An additional 0.06 g of de-ionized water in 8.75 g of THF was added drop-wise into the reaction mixture and maintained for another 1 h to assure

W.-C. Chen et al. / Materials Chemistry and Physics 83 (2004) 71–77

2.4. Characterization Infrared spectra of OPSQ-titania thin films were obtained by using a DIGILAB Model FTS 3500GX spectrophotometer. The Si–OH content in OPSQT0 was determined as the following. The peak areas of 1450–1410 and 960–830 cm−1 in the FTIR spectrum of the OPSQT0 were calculated by an integral function, which were assigned to the Si–Ph and Si–OH groups, respectively. By comparison with a reference PPSQ sample from Gelest with a reported OH content of 5.5%, the approximate content of the Si–OH group in the synthesized OPSQT0 was estimated. 1 H NMR and 29 Si NMR spectra of OPSQT0 were obtained using a BRUKER AV500 spectrometer. Chloroform-d was used as the deuterated solvent. The molecular weight of the prepared OPSQT0 precursor was determined by GPC chromatography. The GPC system consisted of an elution column (PLgel 5 ␮m MIXED-C and D, Polymer Laboratories) and refractive index detector (RI2000, Schambeck SFD GmbH). It was operated at 40 ◦ C using THF elution at 1 ml min−1 . An atomic force microscope (Digital Instrument, Inc., Model DI 5000 AFM) was used to probe the surface morphology of the coated films. Root mean square roughness (Rq ) and average roughness (Ra ) of the studied films were determined. The microstructure of the prepared hybrid materials was further examined by field emission scanning electron microscope (FE-SEM, Hitachi, Model-4000) and transmission electron microscope (TEM, JOEL, Model-2000ES). For the thermal analysis of the OPSQT0, thermogravimetric analysis (TGA) was performed under nitrogen flow using a Dupont Model 951 thermogravimetric analyzer at a heating rate of 20 ◦ C min−1 . UV–Vis–NIR spectra of the OPSQ-titania thin films prepared on quartz were obtained by using a Jasco Model UV/VIS/NIR V-570 spectrophotometer in the wavelength range of 190–1600 nm at room temperature. The near infrared (NIR) absorption spectra of the hybrid films were obtained using a UV–Vis–NIR spectrophotometer (Jasco, Model No. V-570, resolution: 0.5 nm in the NIR region) in the wavelength range of 1000–1800 nm. The refractive indices at 632.8 nm and thickness of the OPSQ-titania thin

films were measured by using ellipsometer (GAERTNER, Model L116 D). The thickness of the OPSQT0–OPSQT30 thin films on the prepared optical planar waveguides were measured by using a prism coupler (Metrican Corporation, Model No. 2010). The optical losses of the prepared optical planar waveguides were measured by a designed optical system (manufactured by Center of Measurement and National Standards, Industrial Technology Research Institute, Hsinchu, Taiwan) using a cut-back method at 1310 nm [11].

3. Results and discussion 3.1. Characterization of the OPSQ precursor solution: OPSQT0 The FTIR spectrum of OPSQT0 is shown in Fig. 2. The absorption bands of Si–O–Si and Si–OH bond are observed at 1098 and 910 or 3400 cm−1 , respectively. The Si–Ph absorption bands are observed at 700, 740, 1135, 1460 and 1594 cm−1 . The peak positions are similar to those reported in the literatures [14–17]. A new peak at 920 cm−1 increases its intensity with increasing the titania content as shown in Fig. 2, which is due to the stretching of Ti–O–Si bonding [18]. This confirms the successful bonding between the OPSQT0 and titania. The Si–OH content in the synthesized OPSQT0 was estimated to be 5.62% from the comparison of the FTIR spectra between the OPSQT0 and the commercially available PPSQ. The 1 H NMR spectrum of OPSQT0 shows the following peaks: 7.63 ppm (Si–Ph), 3.48 ppm (Si–OCH3 ), and 3.73 ppm (Si–OH), respectively. The chemical shifts of OPSQT0 with one (T1 ), and two (T2 ) siloxane bonds (Si–O–Si) determined from 29 Si NMR were observed at −66 and −70 ppm, respectively. The chemical shifts of the Si–Ph, SiOCH3 , Si–OH, and Si–O–Si bonds in

OPSQT0 (spin-on)

Absorbance (Arbitrary Unit)

higher degree of condensation on Ti(OBu)4 . The precursor solution was then allowed to cool to room temperature and concentrated to a higher solid content. The concentrated solution was then spin-coated (3000 rpm/20 s) on singly polished silicon wafer or quartz. The thin films were baked at 60, 80, 120, 150 and 250 ◦ C for 10 min, respectively, and then cured at 400 ◦ C under nitrogen atmosphere for another 1 h. The prepared films were then used for the characterization of the molecular structures and properties. Planar waveguides were constructed with the structures of OPSQT0–OPSQT30 films on the top of a thermal oxide (refractive index ∼1.447) using silicon wafers as the substrate for optical loss measurement. Thick films were spin-coated at a low spin rate (1500 rpm/15 s) and cured to become films.

73

OPSQT0

OPSQT20 OPSQT40

OPSQT80

OPSQT100

OPSQT120

4000

3500

3000

2500

2000

1500

1000

500

-1

Wavenumber (cm ) Fig. 2. FTIR spectra of the prepared OPSQ-titania hybrid materials in the wavenumber range of 400–4000 cm−1 .

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W.-C. Chen et al. / Materials Chemistry and Physics 83 (2004) 71–77

Fig. 3. TEM diagram of the OPSQT50 film.

the prepared OPSQT0 are similar as those reported in the literatures [14–17]. The Si–OR content is very important for bonding with other moiety for preparing hybrid materials. The weight average and number average molecular weight determined from GPC were 718 and 653, respectively. The T3 peak is not observed in the 29 Si NMR spectrum. Hence, the prepared OPSQT0 is an oligomer from the results of GPC and 29 Si NMR. 3.2. Characterization of the OPSQ-titania hybrid materials The TEM diagram of the OPSQT50 is shown in Fig. 3. According to the diagram, the size of the titania domain in the OPSQ-titania materials is estimated to be 8 nm. The small titania domain could ensure that highly optical transparent hybrid materials could be obtained. The surface morphology of the OPSQT10–OPSQT175 was studied by AFM. Fig. 4 shows the AFM diagram of the OPSQT50 thin film. The Ra and Rq of the prepared films are 0.215 and 0.270 nm, respectively, which suggest the excellent surface planarity. The film thickness and surface roughness (Ra and Rq ) of the prepared OPSQ-titania thin films are shown in Table 1. For all the relative roughness in comparison with its film thickness is smaller than 0.4%. The result is consistence with the TEM diagram and demonstrates uni-

form film qualities. For the earlier report in literature [19], silica–titania thin films prepared by the sol–gel reaction had low surface planarity and serious phase separation. This might be due to the highly acidic reaction environment and high concentration of reactants to accelerate the titania growth to a large domain. For our earlier study on the MSSQ-titania hybrid materials [11], the homogeneous structure only could be obtained at low titania content and high pH value. There are two possible reasons for obtaining the homogeneous hybrid films with the nano-sized titania domain: (1) the steric effect of the phenyl group and (2) the efficient coupling between the OPSQT0 and titania because of the oligomeric characteristic of the OPSQT0. Hence, the growth of the titania domain was limited in the hybrid films and a small titania domain was obtained even at the conditions of high titania content and low pH value. The decomposition temperature, Td (95% of sample residue remains) of the prepared OPSQT0 determined by TGA was 544 ◦ C. Besides, the curing temperature for preparing the OPSQ-titania thin films was 400 ◦ C. This suggests that the prepared OPSQ-titania hybrid thin films had excellent thermal stability. Although we did not have suitable equipment for measuring the hardness of the prepared hybrid films, the parent PPSQ films was reported to have a hardness of 0.8 GPa [20]. Hence, the thermal and

W.-C. Chen et al. / Materials Chemistry and Physics 83 (2004) 71–77

75

Fig. 4. AFM diagram of the OPSQT50 film (scanning area: 5 ␮m × 5 ␮m).

mechanical properties of the prepared hybrid films have the potential to be used in optoelectronic devices. Fig. 5 shows the UV–Vis–NIR spectra of the prepared OPSQ-titania hybrid thin films in the wavelength range of 190–800 nm. Excellent transparence is shown in the visible range for all the prepared thin films. In fact, all films showed excellent optical transparence up to 1800 nm. There are two major absorption peaks in the spectrum of the OPSQT0 at 196 and 263 nm, which are resulted from the phenyl group. The absorption edges of the prepared hybrid films are summarized in Table 1. The band edges of the prepared

hybrid films are shifted from 277 to 322 nm as increasing the titania content. The trend is the same as our previous reports on other hybrid thin films [11–13,21]. The charge transfer effect of the Ti–O–Ti segment increases with its size and results in a reduction of energy gap as increasing the titania size. Such effects would be significant as the titania domain is less than 10 nm [22,23]. As discussed in Fig. 3, the sizes of the titania segment in the prepared hybrid films is estimated to be 8 nm. Therefore, the red shift of the band edge of the prepared hybrid films is due to the growing size effect of the titania segment.

Table 1 Properties of the prepared OPSQ-titania hybrid films and their corresponding waveguides Sample

OPSQT0 OPSQT10 OPSQT20 OPSQT30 OPSQT40 OPSQT50 OPSQT60 OPSQT80 OPSQT100 OPSQT120 OPSQT140 OPSQT175 a b

Titania content (wt.%)

Optical thin films n

Thickness (nm)

Ra (nm)

Rq (nm)

Band edgea (nm)

Thickness (nm)

Optical loss (dB cm−1 )

0.00 5.84 11.14 15.90 20.27 24.21 27.82 34.23 39.71 44.48 48.65 54.82

1.527 1.531 1.535 1.545 1.561 1.579 1.602 1.633 1.667 1.698 1.722 1.759

146.8 186.0 95.8 103.2 86.3 81.0 92.3 135.1 147.5 122.9 122.4 122.6

0.388 0.375 0.261 0.241 0.235 0.215 0.164 0.174 0.134 0.190 0.166 0.150

0.486 0.478 0.331 0.302 0.295 0.270 0.208 0.220 0.168 0.240 0.209 0.193

277 283 291 297 309 310 311 316 317 319 321 322

2173.0 675.5 659.0 486.8 –b –b –b –b –b –b –b –b

0.568 0.507 0.470 0.415 –b –b –b –b –b –b –b –b

From UV–Vis–NIR absorption spectrum. The thickness is too thin to measure the optical loss.

Planar waveguides

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W.-C. Chen et al. / Materials Chemistry and Physics 83 (2004) 71–77 5.2

Absorbance (Arbitrary Unit)

OPSQT0

5.1

OPSQT10

Log ( I0 / I ) x 10

OPSQT20 OPSQT30 OPSQT40 OPSQT50 OPSQT60 OPSQT80 OPSQT100 OPSQT120

Y = 0.4702X + 3.1908

5.0

2

R = 0.97 4.9 4.8 4.7 4.6

OPSQT140 OPSQT175

200

400

4.5

600

800

2.8

3.0

3.2

Wavelength (nm)

3.4

3.6

3.8

4.0

4.2

Length (cm)

Fig. 5. UV–Vis–NIR spectra of OPSQT0–OPSQT175 in the wavelength range of 190–800 nm.

Fig. 7. Variation of intensity ratio with the waveguide length for the planar waveguide based on the OPSQT20 film using the cut-back method.

Fig. 6 shows the variation of refractive index (at 632.8 nm) of the prepared OPSQ-titania thin films with the titania content. The refractive index increases from 1.527 to 1.759 with the titania content increases from 0 to 54.8 wt.%. This result suggests that incorporation of the titania into the OPSQ is an effective approach to increase the refractive index of the hybrid materials. The high refractive index of the prepared films could be compared to several different kinds of hybrid materials for optoelectronic applications [11–13,21,24,25]. Planar waveguides were prepared by using the prepared hybrid films and thermal oxide (refractive index = 1.447) as the guiding and cladding layers, respectively. The optical loss was obtained by using a cut-back method at 1310 nm.

Fig. 7 shows the variation of the intensity ratio with the waveguide length for the optical waveguide based on the OPSQT20. A linear relationship between the intensity ratio and the waveguide length suggests the accuracy of the measurement. The optical loss of the studied planar waveguide decreases from 0.568 dB cm−1 of OPSQT0 to 0.415 dB cm−1 of OPSQT30 as increasing the titania content, as shown in Table 1. Since the prepared OPSQ-titania films have excellent surface planarity and homogeneous structure, the scattering loss of the prepared films is not significant. Thus, the optical loss is probably resulted from the overtone vibration absorption band of the C–H bond of the phenyl group in the NIR region. Fig. 8 shows the NIR absorption spectrum of

1.80

1.75

Refractive Index

1.70

1.65

1.60

1.55

1.50 0

5

10

15

20

25

30

35

40

45

50

55

60

Titania Content (wt%) Fig. 6. Variation of the refractive index (at 632.8 nm) of OPSQT0–OPSQT175 thin films with the titania content.

W.-C. Chen et al. / Materials Chemistry and Physics 83 (2004) 71–77

77

Absorbance (Arbitary Unit)

applications for high refractive index coating or optical waveguides.

Acknowledgements We thank the National Science Council of Taiwan (contract no. 91-2216-E002-012 and NDL 91S-C057) and Ministry of the Economic Affairs (contract no. 91-Ec-17-A08-S1-0015) for the financial support of this work.

1310 nm

References 1000

1200

1400

1600

1800

Wavelength (nm) Fig. 8. NIR absorption spectrum of the OPSQT30 film in the wavelength of 1000–1800 nm.

OPSQT30. There are three major absorption bands shown in Fig. 8. The absorption bands in the range of 1100–1260, 1330–1540, and 1600–1800 nm are assigned to be the third (ν3 ), the combination bands of the second harmonic stretching vibration and bending vibration of the C–H bond (ν2 +δ) combined with the second harmonic stretching vibration of O–H bond (ν2 ), and (ν2 ), respectively. The overtone absorption bands shown in Fig. 8 are assigned to the C–H vibration band of the phenyl group in the OPSQ segment, which are similar to those reported in the literature [11]. The OH absorption might be from the absorbed moisture or the OH residue in the hybrid film. However, they are far from the measured wavelength of 1310 nm and thus the obtained loss at 1310 nm is mostly from the C–H vibration absorption. As increasing the titania content, the C–H number density from the phenyl group is decreased. Thus, it is observed that the reduction of the optical loss as increased the titania content if assuming the OH absorption is the same in the different OPSQ-titania film. The low optical loss of the prepared waveguide is comparable to the organic–inorganic hybrid material based waveguides from the sol–gel method [26–28].

4. Conclusions OPSQ-titania optical thin films were successfully synthesized from an OPSQ and titanium (IV) n-butoxide. The prepared hybrid films had homogeneous, transparent, and planar structures even at a high titania content. The optical properties of the prepared OPSQ-titania films could be controlled by adjusting the titania content, including refractive index, optical absorption edge, and optical loss. These were attributed to the growing size effect of the titania segment. The prepared films could have potential

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