Increased Laser-Damage Resistance of Sol–Gel Silica Coating by

Article pubs.acs.org/JPCC

Increased Laser-Damage Resistance of Sol−Gel Silica Coating by Structure Modification Xiaoguang Li,†,‡ Mark Gross,‡ Bob Oreb,‡ and Jun Shen*,† †

Shanghai Key Laboratory of Special Artificial Microstructure Materials and Technology, Tongji University, Shanghai, China CSIRO Australian Centre for Precision Optics, Sydney, Australia

‡

ABSTRACT: Sol−gel silica coatings prepared for high-power laser applications were optimized by a two-step structure modification through treatment of the sol with poly(ethylene glycol) (PEG), followed by treatment of the coating with ammonia. The effect of each treatment on the laser-induced damage threshold (LIDT) at 1064 nm wavelength was studied, mainly in terms of the change in the coating’s structure. Both of these treatments increased the LIDT, and the greatest LIDT was achieved by combining the two treatments. The LIDT increase can be attributed to the decrease in structural defects such as nodular defect and the variety of pore-distribution, resulting from PEG and ammonia treatments, respectively. The procedure of producing coated glass with high LIDT for 1064 nm laser irradiation was suggested, involving substrate etching, sol modification, and coating treatment.

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INTRODUCTION

The as-prepared coating from basic Stöber process features mechanically very weak coating strength, but it can be increased by ammonia−water treatment of the coating or polymer addition to the sol. It has been found that most of the time, coatings with increased strength, either by sol or coating modification, also possess an increased LIDT.3,4,8 However, it is not clear whether and how the strength increase contributes to the LIDT increase. Belleville and Floch8 once suggested that the improved particle-to-particle adhesion due to ammonia treatment may help offset mutual Coulombic repulsion effects induced by surface photoelectronic mechanisms during laser irradiation, but no relevant evidence was provided. Because the strength increase is also accompanied by changes in coating structure, which could also impact the LIDT, researchers gradually turned to the study of structure modification in such silica systems and focused, especially on polymer modification in the solution stage.4,5,14 However, the discussions on polymer modification are mostly focused on how it impacts the coating strength or surface roughness but are barely related to structural defects, the important factor in laser damage. Likewise, the effect of ammonia treatment on structure modification has been largely ignored, perhaps due to its impressive and clear hardening effect. In our opinion, whatever the modification is, its effect on LIDT should be considered in terms of the defects and coating structure, not just from the coating strength and microroughness standpoint. In this work, we employed an organic binder (PEG) and ammonia to treat the sol and coating, respectively,

High-power laser systems, such as those at NIF, LMJ, and SG3, are the core devices in inertial confinement fusion (ICF) research. The maximum output power available from these systems is decisive to their success but is typically limited by laser-induced optical damage. Sol−gel silica coatings have been well-studied and applied to the transmissive optical components in high-power laser systems for their antireflection (AR) properties and high LIDT.1−10 This kind of coating is a product of silica sol prepared using the Stöber process,11 in which silicon alkoxides hydrolyze and condense in alcohol solvents in the presence of water and a catalyst (e.g., NH3). Silica particles and clusters are formed and extend to a network in the aged sol, thus giving rise to a porous structure in the resulting gelcoating, in which interparticle and particle interior pores both contribute to the porosity.10 The porosity of the coating is typically ∼50%, which results in not only an ideal refractive index for single-layer AR coatings on glass substrates but also a relaxed texture for laser-damage resistance. It is known that the laser-induced damage is always caused by various defects in the coating or the substrate.3,12,13 Unlike a PVD (physical vapor deposition) coating, a sol−gel coating is usually free of substoichiometric defects, but like a PVD coating, nodular defects are a common and serious source of damage in the sol− gel coatings.12 Therefore, the LIDT-increase could be realized by making the stacking silica colloids more homogeneous with fewer structural defects. Besides, because the relaxed porous texture of the coating is beneficial for resisting laser-induced damage, optimization of the porous structure could also lead to increased LIDT. © 2012 American Chemical Society

Received: July 26, 2012 Published: August 6, 2012 18367

dx.doi.org/10.1021/jp307390u | J. Phys. Chem. C 2012, 116, 18367−18371

The Journal of Physical Chemistry C

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Figure 1. (a) Schematic of laser damage testing. (b) Example of LIDT calculation. The value of 15.2 J/cm2 corresponds to the x of the linear fitting result y = −1.815 + 0.119x when y = 0.

index of the coating was determined by modeling of the transmittance spectrum using WVASE32 software. Laser damage testing was performed in the 1-on-1 regime (i.e., one pulse per location on the sample) according to ISO standard 11254-1, using a Q-switched Nd:YAG pulsed laser with a laser wavelength of 1064 nm and a pulse length of 10 ns. Ten sites on the sample were exposed at the same fluence in each step, and the fraction of the sites that were damaged was recorded. The testing was carried out at 100 different sitelocations (10 steps in all) that were arranged into a 10 × 10 array. The LIDT was defined as the maximum incident pulse energy density for which the possibility of damage is 0%. The schematic of the damage test is displayed in Figure 1, and the testing apparatus used in our work was introduced in detail in ref 15.

and analyzed their individual contributions to LIDT, mainly in terms of structure modification. The greatest LIDT was achieved by combining the two treatments without reducing optical performance. This combination revealed a methodology for increasing a porous coating’s LIDT by taking two routes, that is, treating the sol and coating separately. In particular, the LIDT increase due to ammonia treatment is studied with respect to the change in pore distribution, in contrast with the strength discussion in other published reports.

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EXPERIMENTAL SECTION Chemicals and Reagents. Tetraethylorthosilicate (TEOS, 98%), ammonia (25%), and ethanol (100%) of reagent grade were purchased from Sigma-Aldrich. PEG 200 of synthesis grade was purchased from Scharlau. All of the reagents were used as received without further purification. Preparation of PEG-Modified Silica Sol. The PEGmodified silica sol was synthesized with the composition of TEOS/EtOH/NH3/H2O/PEG = 1:38:0.54:1.53:0.1 (molar ratio). First, half of the EtOH was mixed with TEOS, and the other half was mixed with ammonia (the water for reaction came from the ammonia), and the two solutions were stirred separately for 15 min. Hydrolysis and condensation were then carried out by adding, dropwise, the NH3 solution into the TEOS solution while stirring. When this was finished, a known volume of PEG 200 was rapidly added into the mixed solution, and the solution was stirred for another 2 h. Finally, the solution was sealed in a glass container and aged at room temperature for a minimum of 5 days to form a silica sol. Preparation of the Silica AR Coating. BK7 glass substrates were etched to a depth of ∼500 nm before use (9 min etching in 3 wt % NH4HF2 aqueous solution). The etching itself also acted as an excellent cleaning process, so no further cleaning was needed. The substrates were then dip-coated using the silica sol, and the thickness of the coating was controlled by the withdrawal speed to match the desired peak position of the transmittance spectrum. To carry out the NH3-treatment, the coated samples were sealed in a container along with a dish of ammonia−water solution and were kept there for 5 h. Characterization. The coating surfaces were characterized using a field-emission scanning electron microscope (SEM, Zeiss) and an atomic force microscope (AFM, Veeco Dimension 3100). Transmittance spectra were recorded on a UV−vis-NIR spectrophotometer (Cary 5000). The refractive

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RESULTS AND DISCUSSION PEG is one of the popular organic modifiers for adjusting the network structure of a silica sol. The interaction between PEG and the chemicals in a sol is very complicated and is impacted by its own molecular weight, additive amount, the time at which it is added, water quantity in the solution, and so on. For example, as indicated by Vong,16 PEG tends to be hydrated in the initial stage of the sol−gel process if the water quantity is relatively rich, causing a reduction in the water available for TEOS hydrolysis, whereas in the case of a low water level, PEG tends to prereact with TEOS forming Si−O−C bonding. In a comparative study of polymer addition in the silica sol, carried out by Tian et al.,17 PEG was found to improve the condensation of TEOS, that is, to increase the average number of bridging Si−O−Si bonds around a Si atom. These effects can definitely impact the formation and the growth of the primary silica particles. Besides, PEG also orients the formation of the clusters and the network, that is, the final texture of the silica sol, either by hydrogen bonding or by forming Si−O−C bonds with silica particles.13,16,18 Consequently, the pore size, porosity, and specific surface area of the resulting coatings are all adjusted. Likewise, ammonia treatment also changes the porosity, and hence the refractive index, of the coating. So, optical performance should be considered at the same time as the LIDT increase. As shown in Figure 2, the addition of PEG increased the refractive index of the coating (related to the porosity19) from 1.140 to 1.254 at 1064 nm wavelength, whereas ammonia treatment increased the indices of both coatings (with and 18368

dx.doi.org/10.1021/jp307390u | J. Phys. Chem. C 2012, 116, 18367−18371

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Table 1. Optical Properties and LIDTs of the Bare and the Coated Glasses with Different Treatments samples BK7 glass coated glass with basic silica sol coated glass with basic silica sol + NH3 treatment coated glass with PEG-modified silica sol coated glass with PEG-modified silica sol + NH3 treatment

n of the coatings

transmittance at 1064 nm

LIDT (J/cm2)

1.140 1.143

91.9% 99.0% 99.0%

80 58 73

1.254

99.9%

66

1.256

99.9%

81

has a much greater LIDT (80 J/cm2) than the coated glass from the basic silica sol (58 J/cm2), which means that there is substantial room for improvement in the coating’s LIDT before the etched substrate has an impact. (Note: the substrate without etching possesses an LIDT of only 64J/cm2.) The addition of PEG increased the LIDT from 58 to 66 J/cm2, indicating that the sol-modification is quite useful, whereas ammonia post-treatment had even better performance, with an increase of ∼15 J/cm2 to both the standard coating and PEG coating. The constructive effects of PEG and ammonia are both in accordance with published work,3,8,17 and it is exciting that, as testified in this work, the combination of PEG and ammonia can result in even higher LIDT (compared in Table 1), with a very high transmittance at the same time. Two-dimensional AFM surface topography images of a standard coating and a PEG-coating are displayed in Figure 4a,b. The rms microroughness decreased from 5.87 to 2.70 nm after PEG modification, meaning the arrangement of the colloids became more regular and homogeneous. The reason can be attributed to the structure modification in the sol. To be exact, the network structure is more homogeneous with a reduced number of big clusters. As illustrated in Figure 4c, the linear PEG molecule (average molecular weight of 200) has a length comparable to the silica particle and can orient the growth of the cluster and the formation of the final network by the hydroxyl groups at the two opposite ends, making the network more regular and continuous through these linkages. The fact that the network becomes more continuous has been verified through transmission electron microscopy (TEM) images of the silica sols, by other researchers.5,14,17 In their work, a typical ring-like structure was present in the PEGmodified silica sol. When the two silica particles are connected by the linear molecule instead of Si−O−Si bonding, the

Figure 2. Refractive indices of the standard and PEG-modified coatings before and after ammonia treatment.

without PEG) by a very small factor. To place the peak position of the transmittance spectrum at 1064 nm, the thickness of the coating should satisfy the equation nd = λ/4, where n and d refer to the refractive index and the thickness of the coating, respectively, and λ refers to the desired center wavelength. The ammonia treatment can decrease the coating thickness by reducing the particle spacing, so the original thickness of the coating should be made thicker, which can be achieved by increasing the withdrawal speed during dip-coating. As shown in Figure 3, ammonia treatment for 5 h substantially moved the peak positions of the transmittance spectra of both the standard coating and PEG-coating but by a different amount, and the corresponding decrease in coating thickness is about 61 and 48 nm, respectively (calculated with the above equation). The PEG-modified coating resulted in a transmittance-increase of ∼0.9% compared with the standard coating, due to its refractive index being closer to the square root of the refractive index of the BK7 substrate. (The refractive index of BK7 glass is about 1.506 at 1064 nm.) Also notable is that although it is difficult to make the thickness of a sol−gel coating as accurate as for a PVD coating, the deviation of the peak transmittance position does not materially affect the transmittance at the desired wavelength (due to the broad-band nature of the peak). As shown in Figure 3b, the peak transmittance of the PEG-coating after ammonia treatment is 99.90% at 1077 nm, whereas the transmittance at the desired wavelength 1064 nm is 99.87% − a reduction of only 0.03%. The LIDTs and optical properties of different coatings are summarized in Table 1. The chemically etched BK7 substrate

Figure 3. Transmittance spectra of the standard (a) and PEG-modified (b) coatings before and after ammonia treatment. The substrates are BK7 glasses. 18369

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bonding in resisting the heat generated during laser irradiation. Whereas this is not enough to explain the LIDT increase, because PEG is also an organic composition, the presence of which actually increases the LIDT. Accordingly, the structure change may play a more important role. Figure 5a,b displays the SEM images of the PEG-coating before and after ammonia treatment. As shown, the obvious

Figure 4. Two-dimensional AFM images of the coating surfaces from basic (a) and PEG-modified (b) silica sols. Schematic illustration of the mechanism of PEG modification in the silica sol (c).

distance between the particles is longer, and the connection is quite relaxed. Therefore, the probability of forming big clusters (that are tightly united by lots of particles through Si−O−Si bonding) decreases. This is very meaningful because the big cluster always becomes the origin of the nodular defects in sol− gel coatings,12 which is fatal to the LIDT. To summarize, it is the more continuous network and decreased big clusters that make the final texture of the sol more homogeneous, which results in a more homogeneous coating with fewer structural defects. Published work about ammonia treatment mostly relates to hardness enhancement. The as-prepared silica sol−gel coating is easily abraded because no chemical bond exists between the silica particles and the glass substrate. However, this is of no concern in high-power laser system applications and is really an advantage as it allows easy removal of the damaged coating layer without going through an onerous and time-consuming repolishing step.8 Indeed, ammonia-hardening was studied originally for realizing the ability to handle the coated articles easily and to withstand vigorous cleaning. However, ammonia treatment unexpectedly revealed its potential to increase the LIDT, and, moreover, it has been found that ammonia-treated coatings are more contaminant-resistant than as-prepared coatings, irrespective of the environment, atmosphere,7 or vacuum.9 To address these issues, the changes in chemical composition and structure of the coating after ammonia treatment should be considered. During ammonia treatment, the residual ethyoxyl groups in the coating hydrolyze with ammonia and water vapor, forming more hydroxyl groups, followed by self-condensation to Si−O−Si linkages. The transformation from physical interaction to Si−O−Si linkage between silica particles and with the substrate endows the coating with a much greater scratch-resistance. Hence, in terms of chemical composition, there is a decrease in the amount of the organic group −OCH2CH3, which is weaker than Si−O−Si

Figure 5. SEM images of the PEG-coatings before (a) and after (b) ammonia treatment. The red arrows refer to the premium-size pores and the green arrows refer to the large-size pores. Schematic illustration of the mechanism of ammonia−water vapor treatment to the porous silica coating (c).

difference is the pore-size distribution. Many large, medium, and small pores occurred after ammonia treatment, all generally uniformly distributed. In contrast, the coating before treatment mainly featured small pores and a small quantity of medium pores. The mechanism of the change in pore-size distribution is illustrated in Figure 5c. In brief, ammonia treatment accelerates the hydrolysis of the residual −OCH2CH3 groups and the selfcondensation between Si−OH groups. The condensation tends to occur between adjacent particles, so the pores produced by particles that are relatively far apart from each other would expand as the particles move outside. Accordingly, the size increases take place randomly in the coating, and the pore distribution is changed. Whereas it may not be clear how this structure change impacts the LIDT, there is a strong possibility that the pore redistribution contributes to the LIDT increase by resulting in a more relaxed coating structure that is more suitable for relieving the expansion of the coating skeleton when absorbing the laser energy. The effect of structure modification on laser-damage resistance still needs further study, and the finite element method could be recommended to study the thermalmechanical coupled process during laser irradiation, especially with respect to the pore-distribution change due to the ammonia treatment. Here the structure changes are explained, and a method to get high LIDT is provided. Figure 6 18370

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(8) Belleville, P. F.; Floch, H. G. Proc. SPIE 1994, 2288, 25−32. (9) Li, X. G.; Shen, J. J. Sol-Gel Sci. Technol. 2011, 59, 539−545. (10) Zhang, X. X.; Ye, H. P.; Xiao, B.; Yan, L. H.; Lv, H. B.; Jiang, Bo. J. Phys. Chem. C 2010, 114, 19979−19983. (11) Stöber, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62−69. (12) Yang, F.; Shen, J.; Sun, Q.; Zhou, B.; Wu, G. M.; Mugnier, J. Proc. SPIE. 2006, 6034, 603410: 1−8. (13) Camp, D. W.; Kozlowski, M. R.; Sheehan, L. M.; Nichols, M.; Dovik, M.; Raether, R.; Thomas, I. M. Proc. SPIE. 1998, 3244, 356− 364. (14) Sun, J. H.; Fan, W. H.; Wu, D.; Sun, Y. H. Stud. Surf. Sci. Catal. 1998, 118, 617−624. (15) Hu, J. J.; Yang, J. X.; Chen, W.; Zhou, C. H. Chin. Opt. Lett. 2008, 6, 681−684. (16) Vong, M. S. W.; Bazin, N.; Sermon, P. A. J. Sol-Gel Sci. Technol. 1997, 8, 499−505. (17) Tian, H.; Zhang, L.; Xu, Y.; Wu, D.; Wu, Z. H.; Lv, H. B.; Yuan, X. D. Acta Phys.-Chim. Sin. 2012, 28, 1197−1205. (18) Ravaine, D.; Seminel, A.; Charbouillot, Y.; Vincens, M. J. NonCryst Solids 1986, 82, 210−219. (19) Vincent, A.; Babu, S.; Brinley, E.; Karakoti, A.; Deshpande, S.; Seal, S. J. Phys. Chem. C 2007, 111, 8291−8298. (20) Kamimura, T.; Akamatsu, S.; Horibe, H.; Shiba, H.; Motokoshi, S.; Sakamoto, T.; Jitsuno, T.; Okamato, T.; Yoshida, K. Jpn. J. Appl. Phys. 2004, 43, 1229−1231.

Figure 6. Procedure of producing coated glass with high LIDT for 1064 nm laser irradiation in this work.

summarizes the procedure for producing coated glass with high LIDT by using two modifications of a sol−gel coating on an etched substrate. The etching can increase the substrate’s LIDT to a point where the coating optimization becomes the key determinant of the LIDT for 1064 nm laser irradiation. (The etching-induced LIDT increase in the substrate is mainly due to the decrease of subsurface defects.13,15,20) We believe that the structure modifications that brought about the higher LIDT in this work will also result in higher LIDT for other wavelengths of illumination because the modifications brought about an improvement in the coating’s intrinsic quality.

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CONCLUSIONS PEG and ammonia were employed to modify a sol−gel silica coating’s structure, and they both contributed to an increase in the LIDT but in different ways. PEG modification can engender a more regular network in the sol stage, which endows the gel coating with a more homogeneous structure with fewer nodular defects. Likewise, ammonia treatment of the coating can act as a pore-modification mechanism during which relatively large pores are produced and the pore-size distribution is changed. The greatest LIDT was obtained by combining the two modifications. High transmittance was also achieved. These two modifications work in the sol and the gel stages, respectively; they both adjust the arrangement of the silica particles by making use of the hydroxyl groups, and they are considered to also be applicable to other porous materials containing hydroxyl groups.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Jingxin Yang and Junjiang Hu for their great help in the LIDT test. This work was supported by the International cooperation projects of Shanghai Science and Technology Commission (grant no. 10520706800) and National Nature Science Foundation of China (grant no. 11074189).

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REFERENCES

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