Durable solgel antireflective films with high laser ... - OSA Publishing

Xu et al.

Vol. 22, No. 4 / April 2005 / J. Opt. Soc. Am. B

905

Durable solgel antireflective films with high laser-induced damage thresholds for inertial confinement fusion Yao Xu, Lei Zhang, Dong Wu, and Yu Han Sun State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China

Zu Xing Huang, Xiao Dong Jiang, and Xiao Feng Wei Laser Fusion Research Centre, Chinese Academy of Engineering Physics, Mianyang 621900, China

Zhi Hong Li, Bao Zhong Dong, and Zhong Hua Wu Beijing Synchrotron Radiation Laboratory, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100039, China Received August 16, 2004; accepted October 21, 2004 We tested the use of two hydrophobic methyl-substituted silane precursors, methyltriethoxysilane and dimethyldiethoxysilane, to synthesize methyl-modified silica sols by a two-step method and a cohydrolysis method to produce durable antireflective films with high laser-induced-damage thresholds (LIDTs). Using smallangle x-ray scattering technology, we obtained details of the microstructure of clusters in sol and found various double fractal structural characteristics in the methyl-modified silica clusters; our findings were confirmed by transmission-electron micrographs. Through a 29Si magic-angle spin nuclear magnetic resonance study of the corresponding xerogels, we determined the double-fractal microstructure, which we then related to the LIDTs of AR films. The distribution configuration of methyls in clusters determined the double-fractal microstructure of clusters and then the LIDTs of AR films. The LIDTs of films produced by the cohydrolysis method (the highest was 38 J/cm2 for 1-ns, 1064-nm laser action) were much higher than those from the two-step method because of the loose netlike clusters in the former configuration. During the 220-day aging, the transmittance of hydrophobic AR film decreased ⬃0.2%. So it is practicable to prepare durable AR films with higher LIDTs than those of normal AR SiO2 films only by introducing hydrophobic methyls into a Si–O–Si matrix of clusters if an appropriate hydrophobic precursor is chosen. © 2005 Optical Society of America OCIS codes: 160.6060, 160.6030, 310.1210, 310.1620, 140.3330, 240.6490.

1. INTRODUCTION The objective of inertial confinement fusion is to produce a significant capability for fusion in the laboratory. The potential applications of inertial confinement fusion are broad and numerous, ranging from basic and applied science to energy production and future power and propulsion in outer space. In those high-power laser fusion drivers, more than 5000 m2 of optical surfaces were used, including lenses, mirrors, polarizers, rod amplifiers, Faraday rotators, and frequency-conversion crystals; each component creates a potential optical loss at a one- or two-sided optical surface. So, to reduce surface reflective loss, antireflective (AR) films must be coated onto every transmissive component. The traditional optical films are usually made by physical deposition (e.g., physical vapor deposition, chemical vapor deposition, and ionassisted deposition). Coatings that are physically produced have good optical, mechanical, and chemical properties for numerous applications. Nevertheless, not only do large-area physical vapor deposited optical coatings require expensive vacuum chamber equipment but also the fabrication procedure is lengthy, which makes it 0740-3224/2005/040905-08$15.00

difficult to use such coatings in the volume production of inertial confinement fusion optical films. The methods of preparing AR films have been reviewed in some papers.1–3 Since Geffken and Berger prepared single oxide coatings by the solgel process 60 years ago, considerable effort has been and is still being directed to the production of optical coatings by such a liquid deposition method.1 Through the chemical sol-gel method,4 nearly all kinds of film with all apertures can be obtained by spin coating, dip coating, or meniscus coating. Now several laboratories, such as the Lawrence Livermore National Laboratory in the United States and the Center d’Etudes de Limeil-Valeenton in France, are involved in research and development of optical films. But two needs that must be met are enhancement of the laser-induced-damage threshold (LIDT) and improvement of the durability or environmental compatibility of optical films. Generally the LIDT of a physical film is considerably affected by defects in the film, and the LIDT of solgel film is thought to be affected by residual organic impurities, solvent trapped in the film, and environmental contamination. The study of laser-induced damage to solgel films is far

© 2005 Optical Society of America

906

J. Opt. Soc. Am. B / Vol. 22, No. 4 / April 2005

from complete, however, and the mechanisms of laserinduced damage are not clear, so a study of the correlation between solgel production and laser-induced damage appears necessary. A monodispersed silica colloidal suspension that is usually synthesized by the Sto¨ber method5 is always used to prepare porous AR film. But the adsorption of water in porous silica AR film so produced increases the refractive index of the film and then decreases the antireflectivity. Furthermore, the hydrophilic surfaces of such AR films attract a great deal of environmental contamination, which reduces the LIDT of film. It is known that the transmittance of an AR film with a hydrophilic porous surface decreases by ⬃0.01% per day. Thus the life of an AR film is less than 3 months, and a further deposition must be made. In fact, it is troublesome to redeposit so many films frequently. So it is important to coat transmissive optical elements with hydrophobic AR films in which special hydrophobic organic groups can greatly reduce the adsorption of water in film. The preparation of hydrophobic films has been described several times.6–13 But methods of prolonging the usable periods of AR films have seldom been reported. It is well known that the wettability of a liquid on a solid surface is governed by the chemical properties of the solid surface as well as by surface morphology.14 Recently, with fluoroalkyltrialkoxylsilane as a hydrophobic precursor, modification of a solid surface was used to form hydrophobic films in several studies.8–13 But the contact angle for water thus obtained was less than 120° if the surface roughness were small. So Minami et al.,7 Hong et al.,9 and Nakajima et al.10 have devoted their research to improving the hydrophobicity of film by increasing the surface roughness and have obtained good hydrophobic films; however, no antireflection was obtained on those films. In the research reported in this paper we utilized two different solgel methods to prepare four kinds of methylmodified silica sol and then hydrophobic AR silica films, using methyltriethoxysilane (MTES) or dimethyldiethoxysilane (DDS) as the hydrophobic methyl. By comparing the fractal growth of clusters in four methyl-modified silica sols we found that the spatial structure of sol clusters is the key to the LIDT of a film. Different modal modifications of methyls in silica clusters led to different cluster morphologies, porosities, and degrees of antireflection of film and consequently of LIDT. The configuration of methyls in clusters directly determined the hydrophobicity of the film. The film–water contact angles were less than 120° and reached 135° when MTES-modified and DDS-methyl-modified silica sols, respectively, were used. Through hydrophobic modification, the durability of AR films was enhanced extensively.

2. EXPERIMENTS A. Synthesis of Methyl-Modified Sols by the Two-Step Method Tetraethoxysilane, (TEOS; Acros, 99%), MTES (Acros, 99%), DDS (TCI, 99%), anhydrous ethanol, and pure water were used in the experimental preparations. In the first step, unmodified SiO2 sol (i.e., sample USiO) was pre-

Xu et al.

pared by the Sto¨ber synthesis,5 which consisted of ammonia-catalyzed hydrolysis of TEOS in ethanol at 20 °C. An aqueous ammonia solution (15 mol/L⫺1) was used to adjust the pH of USiO to 8. The concentration of SiO2 in sol USiO was 3 wt. %. The molar ratio of water to TEOS was 2:1. Then the sol was aged in a hermetically sealed chamber at 20 °C for 30 days. The second step involved the polymerization in ethanol of MTES or DDS, which was catalyzed by hydrochloric acid (12 mol L⫺1) at pH 4.0; the polymerization lasted for 30 days in a hermetically sealed chamber at 20 °C. The concentration of SiO2 in the polymer solution was also 3 wt. %. The molar ratio of water to MTES was 1.5:1, and the molar ratio of water to DDS was 1:1. After aging of the USiO and the polymer solution of MTES or DDS, the USiO was refluxed to remove ammonia and then mixed with the MTES or the DDS polymer solution to produce methyl-modified SiO2 sols (to which we refer as samples SM and SD, respectively). The final methyl-modified SiO2 sols had different molar percentages of hydrophobic precursor X: x ⫽ M X /(M X ⫹ M TEOS), where x is the molar percentage of hydrophobic precursor, X denotes MTES or DDS, and M X is the molar amount of X. The components are listed in Table 1. B. Synthesis of Methyl-Modified Sols by the Cohydrolysis Method TEOS and hydrophobic precursor X (MTES or DDS) were dissolved in ethanol and mixed at a special molar percentage of X: x ⫽ M X /(M X ⫹ M TEOS). An aqueous ammonia solution (15 mol L⫺1) was used to catalyze the hydrolysis and condensation of siloxane reactants at pH 8. The concentration of SiO2 in all the modified sols was 3 wt. %. The corresponding samples are referred to as ScM and ScD. These components are also listed in Table 1. C. Characterization of Films We used methyl-modified SiO2 sols to deposit dip-coated films onto well-cleaned optical glass substrates. The films were dried at 20 °C in air. The morphology of the dried films was tested by atomic-force microscopy (Nanoscope III␣, Digital Instruments). Thickness d of the films was adjusted such that 4n c d ⫽ ␭ 0 , where n c was the refractive index of the film and ␭ 0 (1064 nm) was the desired antireflective wavelength. The transmittance and the contact angle of water were measured with a UVTable 1. Components of Solsa Method of Production Two-Step

x (%) 10 20 30 40 50

Cohydrolysis

MTES and TEOS

DDS and TEOS

MTES and TEOS

DDS and TEOS

SM1 SM2 SM3 SM4 SM5

SD1 SD2 SD3 SD4 SD5

ScM1 ScM2 ScM3 ScM4 ScM5

ScD1 ScD2 ScD3 ScD4 ScD5

a x ⫽ M X /(M X ⫹ M TEOS), molar percentage of hydrophobic precursor X; X, MTES or DDS; M X , molar amount of X; M TEOS , molar amount of TEOS.

Xu et al.


907

the xerogels to determine the covalent modification of methyls to SiO2 particles. We also analyzed the xerogels by differential scanning calorimetry (Model STA449C Netzsch calorimeter) in air to study the contributions of methyls to SiO2 clusters.

3. RESULTS Fig. 1. Schematic of optics for testing the LIDT. B, C, calorimeter and photodiode, respectively; M1, M2, mirrors.

visible spectrometer (Lambda-9, Perkin-Elmer) and a contact-angle meter (CA-A, Kyowa), respectively. The transmittance value used was that value at the designed antireflective wavelength ␭ 0 . The value of the contact angle was the average of at least ten droplets at different positions on the film. To determine the optical durability of AR films, we measured the transmissive spectra of all films in a home environment at intervals of 10 days. D. Testing of Laser-Induced Damage The LIDT was tested with the optical setup route shown in Fig. 1. A Q-tuned Nd:YAG laser was used to provide a nearly Gaussian-type pulse beam (spatially and temporally) at 1064-nm wavelength. A fixed energy attenuator was installed in the beam’s path to provide energy control. The maximum output energy was 800 mJ. Two wedges were used to pick off portions of the beam for a standard set of beam diagnostics that included a calorimeter (energy measurement), a photodiode (temporal), and a CCD camera (spatial profile). The laser spot on the testing film was ⬃0.58 mm in diameter (width at 1/e 2 of the pulse peak), and the pulse width was ⬃1 ns. The R-on-1 testing procedure (R-on-1: raising the laser energy irradiated on one spot until damage occurs) was carried out at 30 locations that were arranged into a 6 ⫻ 5 array. According to International Organization for Standardization standard ISO11145, the distance between two close spots was 5 mm, sufficient to prevent the spots from disturbing each other. By increasing the energy irradiated onto the film in 0.3-mJ steps, we shot the films until they were damaged. The amount of damage to the film was estimated by visual inspection of a plasma flash. The damage morphology of film was recorded by an optical microscope (Olympus AX80) because a damage spot several millimeters in diameter went beyond the scale of the atomic-force microscope. To make the damage morphology clear, we blew vapor onto the film. The change in hydrophilicity within or outside the damage region made the damage spot obvious. E. Characterization of Xerogels Small-angle x-ray scattering (SAXS) of xerogels was performed with a long-slit collimation system at the 4B9A beam line at the Beijing Synchrotron Radiation Facility. The incident x-ray wavelength was 0.154 nm. Scattering angle ␪ was approximately 0–3°. With SAXS data, the fractal structure of methyl-modified clusters was calculated. We used the 29Si magic-angle spin nuclear magnetic resonance (MAS-NMR; MSL-400, Bruker) spectra of

A. Optical Properties of AR Film Figure 2 shows the transmittance of AR film versus x, where x ⫽ M X /(M X ⫹ M TEOS) means the molar percentage of hydrophobic precursor X, X is MTES or DDS, M X is the molar amount of X, and M TEOS is the molar amount of TEOS. The transmittance of USiO was 99.8%. The transmittances of the SM and SD series increased slowly and then decreased quickly. The highest transmittance reached 100% (zero reflection on a glass substrate) at 20% x and 30% x for SM and SD series, respectively. But the transmittance of the ScM series and the ScD series decreased with increasing x. The transmittance of the ScD series decreased more quickly than that of the ScM series when x was larger than 30%. From Fig. 2, the additive hydrophobic precursor had different effects on the transmittance of AR film. In an appropriate added range of hydrophobic precursors (x ⭐ 30%), the transmittance of AR film was influenced slightly. But it decreased markedly when x was larger than 30%. B. Hydrophobicity of AR Film Figure 3 shows the water–film contact angle of AR film versus x. On the whole, the contact angle increased with x, beginning at 38° of USiO. The contact angle of the SM series underwent a short and speedy climb to 114° at 20% of x and a flat change from then on. The largest contact angle was 118° for SM4 (x ⫽ 40%). The SD series and the ScM series had continuous increases of contact angle, and the largest ones were 125° for SD5 and 117° for ScM5. The contact angle of the ScD series rapidly increased to 125° only at x ⫽ 10% and then underwent a slow increase to 135° at x ⫽ 40% and a final small decrease to 128° at x ⫽ 50%. From Fig. 3, the most effective hydrophobic film should be the ScD series, the other three series had approximately the same effect on enhancing the contact angle.

Fig. 2. Transmittance of hydrophobic AR films versus x; x is the molar percentage of hydrophobic precursor.

908


Xu et al.

Fig. 3. Water–film contact angle versus x; x is the molar percentage of hydrophobic precursor.

Fig. 4. AFM morphologies of hydrophobic AR films. R q is the average roughness in the direction perpendicular to the surface.

C. Morphology of AR Film Figure 4 shows the typical AFM morphologies of AR films from unmodified USiO, two-step synthesized SM5 and SD5 series, and cohydrolysis synthesized ScM5 and ScD5 series. Obviously the surface of film USiO was composed of uniform discrete particles ⬃20 nm in diameter and had abundant pores among these particles, which was the crucial factor for its antireflective property. The average roughness in the direction perpendicular to the surface, Rq, was only 3.699 nm. No matter which methylmodified SiO2 sol we used to obtain hydrophobic AR film, the atomic-force microscopy morphology of the film displayed an intense change. Typically, as shown in Fig. 4, films SM5, SD5, ScM5, and ScD5 all had many fewer pores than USiO; simultaneously the pores were larger than those in USiO. Apart from the decreased porosity, Rq of these films also increased, to 12.885, 9.658, 7.557, and 15.376 nm for SM5, SD5, ScM5, and ScD5, respectively. It is interesting that the film with larger Rq had correspondingly worse antireflection properties. So the antireflection of these hydrophobic films was much worse than that of USiO, in agreement with the results shown in Fig. 2.

D. LIDT and Damage Morphology of AR Film The LIDTs tested by a 1064-nm laser with 1-ns pulse width are listed in Table 2. We found that the LIDTs of AR films obtained from cohydrolysis were much higher than those obtained from the two-step method. Besides, the LIDTs of films with DDS as a hydrophobic precursor, e.g., SD3 and ScD3, were higher than those with MTES as a hydrophobic precursor, e.g., SM3 and ScM3. With respect to the films prepared by the two-step method, the LIDT decreased with increasing amounts of hydrophobic precursor, no matter whether MTES or DDS was used. The LIDT increased, however, with increasing amounts of hydrophobic precursor for the films prepared by cohydrolysis, no matter whether MTES or DDS was used. The LIDTs the from two-step method were lower than that of USiO, but they were higher for LIDTs from cohydrolysis. The damage morphologies of films are shown in Fig. 5 at a magnification of 28. The laser-induced damage to film USiO was typical fusion damage. No delamination damage or plasma scald damage was observed in USiO. But the other four films had widely different damage morphologies: The coating within the damage region seemed to be completely removed, so a clear boundary was left behind. In fact, because of destruction by the laser, the hydrophobicity within the damage region could not remain, and the surface became hydrophilic. Out of the damage region, the dense small dots were water drips conglomer-

Table 2. LIDT of Filmsa Two-Step Method

Cohydrolysis Method

Unmodified, USiO

SM1

SM3

SD1

SD3

ScM1

ScM3

ScD1

ScD3

23.09

13.79

11.99

19.32

13.91

19.34

27.32

32.16

38.43

a

In units of joules per square centimeter.

LIDT was tested with a 1064-nm laser at 1-ns pulse width.

Xu et al.


909

99.8% to 98.3%. Simultaneously the contact angle of SM2 has increased from the previously obtained 115° to 123° during 110 days’ aging of the film and maintained this level from then on. The increase of contact angle should be attributed to the long drying time of the film and removal of the polar solvent from the film.

4. DISCUSSION

Fig. 5. Laser-induced damage morphologies of hydrophobic AR films. The magnification is 28.

Fig. 6.

The extrinsic properties of the film, such as transmittance, hydrophobicity, and LIDT, should have an ultimate influence on the formation of sol and gel that determines the microstructure and surface characteristics of the sol particles that constitute the AR film. So, unlike the crucial factors in the preparation of physical vapor deposition films, the composition and the manner of preparation of sol must be considered carefully in relating the extrinsic properties to the internal structure. Figure 7 shows the 29Si MAS-NMR spectra of xerogels. To assign the 29Si MAS-NMR chemical shift to various silicate species we adapted traditional notation15: Q n , T n , and D n denote the tetrafunctional Si nucleus, the trifunctional Si nucleus, and the difunctional Si nucleus produced by reactant TEOS, MTES, and DDS, respectively, and n denotes the number of Si—O—Si bridges attached to the Si nucleus. The larger n is, the larger the Si— O—Si bridge produced by polycondensation between two silanols is and therefore the greater the extent of the polycondensation. In Fig. 7, only T 3 and D 2 can be observed in SM3 or SD3, but nearly all the other Si nuclei can be observed in ScM3 and ScD3. Hence in SM3 or SD3 the extent by which MTES or DDS was condensed was greater than in ScM3 or ScD3. The smaller extent of condensation of ScM3 or ScD3 implies a more branched structure of the sol cluster. This deduction was confirmed by observation with a transmission-electron microscope, as shown in Fig. 8. In Fig. 8 the monodispersed spheric SiO2 particles are typical of Sto¨ber synthesis. But when they were modified with a hydrophobic precursor, the particles formed clusters of different shapes and sizes. Obviously the clusters of ScM3 and ScD3 were more branched and netlike, and no separate particles

Durability of films.

ated on the hydrophobic surface. We found that the damage spots of ScM3 and ScD3 were smaller than those of SM3 and SD3. E. Durability of AR Films With SM2, for example, the stabilities of the optical properties and the hydrophobicity of the AR film were examined in a home environment in which no special effort was made to control the environmental temperature and the humidity. The result is shown in Fig. 6. During the 220 days’ aging, the transmittance of SM2 decreased slightly, from 100% to 99.8%. By comparison, the unmodified USiO had an intense decrease in transmittance, from

Fig. 7.

29

Si MAS-NMR results for xerogels: ppm, parts in 106 .

910


Fig. 8. Transmission-electron microscope photographs of modified and unmodified SiO2 sols.

Xu et al.

mer, the second-level clusters are compact and irregular in SM3. Simultaneously, wormlike second-level clusters are formed in SD3 (Fig. 8). For the cohydrolysis method, the self-condensation of TEOS and the cocondensation of TEOS and MTES or DDS proceed together, so the formation of separate SiO2 particles is retarded and netlike clusters are formed. It is well known that SAXS technology has been successfully applied to the characterization of solgel and nanoscale microstructures.21–23 But until now there was no practicable way to investigate SAXS of optical films because of the films’ thinness. Despite that, some research revealed that the internal microstructure of a solgel film can be obtained by a study of the corresponding xerogels.24 So more-detailed structural information on these clusters was acquired through investigation of the SAXS of corresponding dried freely xerogels, and different double-fractal structures were discovered by the two-step and the cohydrolysis methods. The SAXS of a typical fractal system generally has an exponential decrease of scattering intensity in terms of I(q) ⬀ q ⫺Dm (1 ⭐ Dm ⬍ 3) or I(q) ⬀ q Ds⫺6 (2 ⬍ Ds ⬍ 3) for a mass-fractal system (fractal dimension, Dm) or a surface-fractal system (fractal dimension, Ds),25 respectively, where q ⫽ (4 ␲ /␭)sin(␪/2) is the scattering wave vector, ␭ is the wavelength of the incident x ray, and ␪ is the scattering angle. According to the studies of Craievich26 and Marliere et al.,27 the existence of a double-fractal structure, that is, a complex fractal structure including two fractal dimensions, is possible. For each fractal dimension, the spatial scale in which the fractal structure exists, L min –Lmax , can easily be defined by L min ⫽ 2␲/qmax and L max ⫽ 2␲/qmin , where q min –qmax is the linear range of fractal curve ln关I(q)兴 –ln(q).26,27 The calculated fractal data of SM and ScM series are collected in Table 3. Note from Table 3 that only one fractal dimension could be found for unmodified USiO but that two fractal dimensions were found for both the SM and the ScM series, thus indicating a more-complex internal structure in hydrophobic modified SiO2 clusters than in normal SiO2 particles. A common point is that both Dm1 and Dm2 are mass fractal and that 2 ⬍ Dm1 ⬍ 3 and 1 ⬍ Dm2 ⬍ 2. Dm1 of the USiO and SM series fall similarly at 2.2–2.4 and the corresponding spatial scale Table 3. Results Obtained from SAXSa

were seen. However, the clusters of SM3 and SD3 were conglomerations of particles. In the Sto¨ber process5 a SiO2 particle surface is covered by hydroxyl groups.16 MTES or DDS can hydrolyze rapidly into CH3 Si(OH) 3 or (CH3 ) 2 Si(OH) 2 in acidic conditions and then condense into several low-molecule polymers.17–20 It is commonly believed that a ladderlike polymer is produced by the polymerization of MTES and a helical linear polymer by the polymerization of DDS.17,21 Both MTES and DDS polymers have terminal Si—OH that can condense into Si—O—Si with Si—OH on a SiO2 particle. So, by mixing USiO with a MTES or DDS particle, we linked the monodispersed SiO2 particles into second-level clusters and introduced hydrophobic methyls into the clusters. Because the ladder-shaped MTES polymer is more compact than the helical, linear DDS poly-

Sample

Dm1

L1 min –L1 max (nm)

Dm2

L2 min –L2 max (nm)

USiO SM1 SM2 SM3 SM4 SM5 ScM1 ScM2 ScM3 ScM4 ScM5

2.30 2.22 2.29 2.21 2.26 2.23 2.27 2.31 2.55 2.48 2.56

8–13 6–11 9–13 8–14 12–16 13–16 8–14 7–13 7–14 9–15 8–15

none 1.24 1.39 1.50 1.69 1.82 none 1.01 1.07 1.08 1.14

none 37–72 32–77 28–73 30–67 31–81 none 51–103 67–118 69–114 60–103

a Dm1, Dm2, fractal dimensions; L1 min –L1 max and L2 min –L2 max , spatial scales that correspond to Dm1 and Dm2, respectively.

Xu et al.

Fig. 9. Differential scanning calorimetry thermal analysis results of xerogels.

L1 min –L1max located in a range of 10 ⫾ 5 nm. Meanwhile, Dm1 of the ScM series is a little larger than that of the USiO and SM series, but spatial scale L1 min –L1max is similar to that of the USiO and SM series From these results, the first fractal structure has a similar complexity to and nearly same spatial existence as the USiO, SM, or ScM series. Differently from Dm1, Dm2 increased with the additive hydrophobic precursor MTES no matter whether they were in the SM or the ScM series. And Dm2 of the SM series was larger than that of the ScM series, which disclosed a more compact cluster in the SM than in the ScM series. This was confirmed by transmission-electron microscopy photographs (Fig. 8). Furthermore, the fractal structure of the SM series exists on a spatial scale of 55 ⫾ 25 nm (Table 3), but the fractal structure of the ScM series exists on a larger spatial scale of 85 ⫾ 34 nm (Table 3). Together with the small Dm2 of the ScM series, the larger and wider fractal spatial scale of the ScM series implies looser clusters, which can be clearly seen from Fig. 8. In Fig. 8 the netlike clusters of ScM3 are obviously looser than the clusters of SM3. From completely similar analyses of SM and ScM series, the same trends in the fractal structures of the SD and ScD series were achieved, although they are not displayed here in detail. In both of the two sol synthesis methods Dm1 that characterizes the fractal growth of primary SiO2 particles is similar. The two routes. However, Dm2 characterizes the fractal construction of clusters by primary SiO2 particles that is crucial for the formation of various morphologies and the extrinsic properties of the film. Despite the above analysis, it was still difficult to discover how the different fractal structures were formed and how these fractal structures influenced the morphology and properties of the AR film. Above all, the mild cohydrolysis method has provided an appropriate situation for embedding of hydrophobic methyls into a Si—O—Si matrix of particle clusters; simultaneously the prepolymerized MTES or DDS precursors in the two-step method formed a linkage among the prepared SiO2 particles and consequently inserted many hydrophobic methyls into the interstices between SiO2 particles. Cohydrolysis caused the methyl modification to be carried out at the atomic level; meanwhile, the two-step method provided methyl modification at the particle level. So methyls were well


911

dispersed in ScM or ScD series and were concentrated in SM or SD series. Figure 9 shows the result of thermal analysis (differential scanning calorimetry in air) of SD3 and ScD3. The sharp exothermic peak of SD3 at 355 °C should be attributed to the combustion of methyls in air, which indicates a concentration of methyls. But no intensive exothermic peak was observed in ScD3, indicating the presence of dispersed methyls. It was the dispersion of methyls and their repulsion by polar solvent that caused netlike clusters to form in cohydrolysis systems. The clusters thus formed were certain to construct a film with a morphology similar to that of the clusters themselves, as can be seen from the atomic-force microscope morphologies shown in Fig. 4. No matter which AR film was taken into account, the antireflection was sure to decrease if a large amount of hydrophobic precursor was used, because of the decreased porosity. The hydrophobicity of a film is far more complex than its optical properties. Not only the chemical component but also the surface roughness of a film will play an important role in the contact angle. According to the Wenzel equation,14 the effect of the increased area caused by a rough surface can be taken into account as follows: cos ␪w ⫽ R cos ␪y , where R is the ratio between the effective area and the geometric area of a film, known as the roughness factor; ␪ w is the contact angle on the real surface; and ␪ y is the contact angle on the corresponding smooth surface. So some researchers have focused ways to enhance hydrophobicity by increasing surface roughness. Nakajima et al.,10 Tadanaga et al.,11 and Ogawa et al.12 reported that hydrophobicity increases with increasing surface roughness. However, it was impossible to apply their methods in the preparation of hydrophobic antireflective films. Moreover, the rougher the film is, the more intense the scattering of incident light and then the worse the antireflection will be. We note a strange phenomenon about the LIDTs of films, that is, the LIDT decreases with an increasing amount of additive hydrophobic precursor for the twostep method but increases with an increasing amount of additive hydrophobic precursor for the cohydrolysis method. In theory, netlike clusters have better energy transferability than discrete clusters, and laser energy can spread abroad efficiently along the film, so an AR film composed of netlike clusters should more easily produce a high LIDT. Experimentally, a LIDT has done so. The LIDTs of the films from cohydrolysis were much higher than those from the two-step method (Table 2). In the two-step method the discrete clusters became increasingly more irregular and compact when the amount of hydrophobic precursor increased, so the LIDT decreased. Although the LIDTs were higher with the cohydrolysis method, the laser damage spots were larger than with the two-step method (Fig. 5). This result also reveals the efficient transfer of laser energy of films obtained by the cohydrolysis method and consequently the higher LIDT. Another noticeable result is that the LIDTs of films when DDS was used as the modifier were always higher than the LIDTs of films when MTES were used. Because DDS has two methyls, more methyls were introduced into the Si—O—Si matrix of clusters and the clusters were looser than for MTES. Hence their LIDTs were higher.

912


The durability of AR films can be improved through hydrophobic modification, by either a two-step or a cohydrolysis method, of the SiO2 clusters that compose an AR film. The abundant pores in the AR film were covered by hydrophobic methyls, and wetting of the pores retarded. The existence of methyls in the pores, however, can change the conglomeration configuration of clusters and consequently affect the degree of antireflection of a film.

Xu et al.

7.

8.

9.

5. CONCLUSIONS It is practicable to prepare durable AR films with higher laser-induced damage thresholds than those of normal AR SiO2 films only by introducing hydrophobic methyls into a Si–O–Si matrix of clusters that constitute a film. If an appropriate hydrophobic precursor is chosen, the antireflection and the contact angle of the film can be improved at the same time. Use of too large a hydrophobic precursor will lead to a decrease of transmittance. The change of contact angle by the addition of a hydrophobic precursor is highly complex. But the form of distribution of methyls in clusters determines the double fractal microstructure of clusters and then the LIDT of AR film. As a result, the LIDTs of films produced by the cohydrolysis method were much higher than those from the two-step method because of the loose netlike cluster in the former configuration. During the 220 days’ aging, the transmittance of hydrophobic AR film decreased slightly, from 100% to 99.8%. Comparatively, the unmodified USiO had an intense decrease in transmittance, from 99.8% to 98.3%.

ACKNOWLEDGMENTS Financial support from the National Key Native Science Foundation, China (grant 20133040) is gratefully acknowledged.

10. 11.

12. 13.

14. 15. 16. 17.

18.

19.

Y. H. Sun’s e-mail address is [email protected]. 20.

REFERENCES 1. 2.

3. 4.

5. 6.

H. G. Floch and P. F. Belleville, ‘‘Optical thin films from the sol-gel process,’’ in Optical Interference Coatings, F. Abeles, ed., Proc. SPIE 2253, 764–790 (1994). D. R. Uhlmann, J. M. Boulton, G. Teowee, L. Weisenbach, and B. J. J. Zelinski, ‘‘Sol-gel synthesis of optical thin films and coatings,’’ in Sol-Gel Optics, J. D. Mackenzie and D. K. Ulrich, eds., Proc. SPIE 1328, 270–294 (1990). H. G. Floch, P. F. Belleville, and P. M. Peg˙on, ‘‘Sol-gel optical coatings for lasers. II,’’ Bull. Am. Ceram. Soc. 74, 84–89 (1995). H. A. Mcinnes, J. E. Andrew, N. J. Bazin, A. J. Morris, and K. J. Porter, ‘‘Ultra violet laser damage and optical properties of high refractive index single layers, multiple layers and high/low index multilayer stacks,’’ in Laser-Induced Damage in Optical Materials: 1997, G. J. Exarhos, A. H. Guenther, M. K. Kozlowski, and M. J. Soileau, eds., Proc. SPIE 3244, 500–508 (1998). W. Sto¨ber, A. Fink, and E. Bohn, ‘‘Controlled growth of monodisperse silica spheres on the micro size range,’’ J. Colloid Interface Sci. 26, 62–66 (1968). K. Tadanaga, N. Katata, and T. Minami, ‘‘Formation process of super-water-repellent Al2 O3 coating films with high

21.

22.

23. 24.

25. 26. 27.

transparency by the sol-gel method,’’ J. Am. Ceram. Soc. 80, 3213–3216 (1997). T. Minami, N. Katata, and K. Tadanaga, ‘‘Preparation and characterization of super water-repellent Al2 O3 coating films with high transparency,’’ in Sol-Gel Optics IV, B. S. Dunn, J. D. Mackenzie, E. J. Pope, H. K. Schmidt, and M. Yanane, eds., Proc. SPIE 3136, 168–175 (1997). T. Nakagawa and M. Soga, ‘‘A new method for fabricating water repellent silica films having high heat-resistance using the sol-gel method,’’ J. Non-Cryst. Solids 260, 167–174 (1999). B. S. Hong, J. H. Han, S. T. Kim, M. S. Park, T. Dolukhanyan, and C. Sung, ‘‘Durable water-repellent glass for automobiles,’’ Thin Solid Films 351, 274–278 (1999). A. Nakajima, K. Abe, K. Hashimoto, and T. Watanabe, ‘‘Preparation of hard super-hydrophobic films with visible light transmission,’’ Thin Solid Films 376, 140–143 (2000). K. Tadanaga, J. Morinaga, and T. Minami, ‘‘Superhydrophobic–superhydrophilic micropatterning on flowerlike alumina coating film by the sol-gel method,’’ Chem. Mater. 12, 590–592 (2000). K. Ogawa, M. Soga, Y. Takada, and I. Nakayama, ‘‘Development of a transparent and ultrahydrophobic glass plate,’’ Jpn. J. Appl. Phys. 32, 614–615 (1993). Y. Aksmatsu, K. Makita, H. Inaba, and T. Minami, ‘‘Water-repellent coating films on glass prepared from hydrolysis and polycondensation reactions of fluoroalkyltrialkoxylsilane,’’ Thin Solid Films 389, 138–145 (2001). M. Morra, E. Occhiehho, and F. Garbassi, ‘‘Contact angle hysteresis in oxygen plasma treated poly(tetrafluoroethylene),’’ Langmuir 5, 872–876 (1989). C. J. Brinker and G. W. Scherer, Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing (Academic, San Diego, Calif., 1990), pp. 160–173. Ref. 15, pp. 97–99. B. M. De Witte, D. Commers, and J. B. Uytterhoeven, ‘‘Distribution of organic groups in silica gels prepared from organo-alkoxysilanes,’’ J. Non-Cryst. Solids 202, 35–41 (1996). Z. Y. Zhang and H. Wakabayashi, ‘‘Structural study of Al(acac) 3 catalyzed CH3 SiO3/2 gel,’’ J. Sol-Gel Sci. Technol. 12, 153–158 (1998). C. A. Capozzi, L. D. Pye, and R. A. S. Condrate, ‘‘Vibrational spectral/structural changes from the hydrolysis/ polycondensation of methyl-modified silicates. I. Comparisons for single monomer condensates,’’ Mater. Lett. 15, 130–136 (1992). S. Yoshiyuki, O. Seigo, and K. Kazuyuki, ‘‘29Si NMR study of hydrolysis and initial polycondensation processes of organoalkoxysilanes. I. Dimethyldiethoxysilane,’’ J. Non-Cryst. Solids 139, 25–34 (1992). R. Zhang, Y. Xu¨, Q. H. Meng, J. Wang, H. Zhao, and B. Z. Dong, ‘‘Small angle x-ray scattering study of microstructure changes of organic hydrogels from supercritical carbon dioxide drying,’’ J. Supercrit. Fluids 28, 263–276 (2004). Y. Xu, Z. H. Li, W. H. Fan, D. Wu, Y. H. Sun, L. X. Rong, and B. Z. Dong, ‘‘Density fluctuation in silica-PVA hybrid gels determined by small-angle x-ray scattering,’’ Appl. Surf. Sci. 225, 126–133 (2004). Y. Xu, Z. H. Li, W. H. Fan, D. Wu, Y. H. Sun, L. X. Rong, and B. Z. Dong, ‘‘Two fractal structures of methyl-modified silica gels by SAXS,’’ Acta Phys. Sin. 52, 442–447 (2003). C. Mutter, T. N. M. Bernards, and M. P. J. Peeters, ‘‘The effect of the solvent on the cross-link density of SiO2 coatings,’’ Thin Solid Films 351, 95–98 (1999). K. D. Keefer and D. W. Schaefer, ‘‘Growth of fractally rough colloids,’’ Phys. Rev. Lett. 56, 2376–2379 (1986). A. F. Craievich, ‘‘SAXS study of the porous fractal structure of tricalcuim silicate dry gels,’’ J. Appl. Crystallogr. 20, 327– 329 (1987). C. Marliere, F. Despetis, P. Etienne, and J. Phalippou, ‘‘Two fractal structures in aerogel,’’ J. Non-Cryst. Solids 285, 175–180 (2001).