Hydrophobic Antireflective Silica Coatings via Sol-gel Process

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1 ISSN MATERIALS SCIENCE (MEDŽIAGOTYRA). Vol. 14, No Hydrophoic Antireflective Silic Cotings vi Sol-gel Process Jurgis PHILIPAVIČIUS 1, Ivn KAZADOJEV 1, Aldon BEGANSKIENĖ 1, Andrius MELNINKAITIS 2, Vlds SIRUTKAITIS 2, Aivrs KAREIVA 1 1 Deprtment of Generl nd Inorgnic Chemistry, Vilnius University, Nugrduko 24, LT Vilnius, Lithuni 2 Lser Reserch Center, Vilnius University, Sulėtekio l. 10, LT Vilnius, Lithuni Received 01 June 2008; ccepted 08 Septemer 2008 The opticl properties nd structure of hydrophoic ntireflective cotings (AR) deposited from silic sol nd HMDS nd MTMS were chrcterized in detil in this study. The influence of sol modifiction prmeters on the ntireflective ehviour of cotings hs een investigted. For the chrcteriztion of hydrophoic cotings the wter contct ngle mesurements, UV-visile spectroscopy nd tomic force microscopy were used. HMDS modified silic coting show the highest contct ngle (165 ) due to its etter hydrophoic covering. It ws determined tht the hydrophoic HMDS modified cotings reduced reflectnce s well s unmodified silic AR cotings. The Nd:YAG lser dmge threshold of AR modified silic coting exceeded 35 J/cm 2 t 1064 nm. Keywords: hydrophoic, ntireflection cotings, colloidl silic nnoprticles, sol-gel synthesis, lser dmge threshold. 1. INTRODUCTION The reflection loss of n opticl surfce is relted to the difference etween the refrctive indices of environment nd the opticl mteril. Due to their ility to decrese the reflectnce nd increse the trnsmittnce of light nd remove ghost imges, ntireflective (AR) cotings hve recently ttrcted much interest for their pplictions in displying devices nd opticl lenses systems [1 3]. The refrctive index (n c ) for n idel homogeneous AR coting meets the condition of n c = (n n s ) 1/2, reflection will e decresed t the wvelengths ner the qurterwvelength opticl thickness, where n nd n s re the refrctive indices of the ir nd the sustrte, respectively. For glss sustrte (n s ~1.5), the refrctive index of AR mteril should e ~1.22. However, nture mterils with such low refrctive index re either rre. An effective method to otin cotings with reduced refrctive index is to introduce nnopores during the coting friction. The refrctive index of porous cotings cn e well tilored y controlling the percentge of pores introduced, in which lrger percentge of pores leds to lower refrctive index [4 7]. The most populr methods of preprtion porous films re chemicl vpour deposition, chemicl etching nd sol-gel processes [2, 8, 9]. The silic nnoprticles (20 nm 40 nm) mde y the Stoer sol-gel process is convenient for use s ntireflective (AR) cotings for optics in high power lser system [10 14]. The AR cotings of porous silic re very ttrctive, due to low refrctive indexes, low sctter nd high lser dmge thresholds. However, such AR cotings hve one wekness the wter condenstion on the surfce of hydrophilic silic coting, wht very importnt for the moisture sensitive optic elements or crystls s potssium dihydrogen phosphte. Becuse of sorption of wter pour into the hydrophilic pores of norml porous silic AR film cn increse n c nd then lowers the ntireflection, nd it is necessry to mke the AR films tht Corresponding uthor. Tel.: , fx.: E-mil ddress: ldon.egnskiene@chf.vu.lt (A. Begnskienė) possess hydrophoicity. Therefore, the friction of AR cotings with wter-repellent properties is quite importnt. The wettility of liquid to solid surfce is governed y the chemicl properties of solid surfce nd its surfce morphology. So, commonly the cquirement of hydrophoic surfce is out of two pproches chemicl modifiction to surfce or roughening the surfce. Chemicl modifiction is the wy to chieve sustitution of the hydrogen of hydroxyl group y ttching some lkyl or fluorlkyl groups. The modifiction of silic sol hs een performed using hexmethyldisilzne (HMDS), trimethyllkokxysilne nd dimethyllkokxysilne [15 17]. The methyl or trimethylsilyl (TMS) functionliztion of silic colloids gretly modified the wter dsorption isotherms, mking them significntly more resistnt to wter sorption. In this study, we demonstrte the preprtion of hydrophoic sol-gel AR cotings on glss sustrtes y spin-coting technique showing the dependence of vrition of the AR ehviour on chemicl modifiction prmeters. The surfce of silic nnoprticles ws modified y dding different rtion of regents hexmethyldisilzne (HMDS) or methyltrimethoxysilne (MTMS) to colloidl silic sol. 2. EXPERIMENTAL The prticle size ws determined from the microgrphs otined from TEM mesurements. Trnsmission electron microscopy ws performed on the PEM-100 electron microscope. A copper grid with holey cron film ws dipped in the sol, dried t room temperture nd nlyzed in TEM. The results summrized from over hundred prticles were used for the clcultion of the verge prticle size nd stndrd devitions of ech smple. The coting trnsmittnce nd reflectnce of normlly incident light ws mesured using UV-vis spectrophotometers Perkin- Elmer Spectrum Lmd 19 nd LOMO over the spectrl rnge of 350 nm 900 nm. The ngle of incidence ws fixed t 70 (from the norml), nd the spectrl rnge proed ws 350 nm 850 nm. The AFM imges of the 283

The viscosity mesurement ws mde using viscometer (Anton Pr) t sher rte of 200 s 1 nd constnt 25 C temperture.

2 silic cotings on glss were performed on Multimode Scnning Proe Microscope (Digitl Instruments). For the chrcteriztion of surfce properties, the mesurements of wter contct ngle on KVS Instrument CAM 100 were recorded. The viscosity mesurement ws mde using viscometer (Anton Pr) t sher rte of 200 s 1 nd constnt 25 C temperture. The lser dmge tests were crried out ccording (ISO stndrd) y Nd:YAG lsers (the output lser pulse durtion is 3.41 ns t 1064 nm, frequency 10 Hz) using spot size of pproximtely 586 μm dimeter. The lser dmge threshold, i. e. the lowest intensity to cuse on irreversile chnge, of the cotings ws evluted using high power lser system. Light scttering losses were mesured on pprtus for totl scttering mesurements t Lser Reserch Center of Vilnius University. Spin-coting methods on glss (lime, Bk-7) for producing AR cotings were used. In our investigtions, for the preprtion of thin colloidl silic films SCS P 6708 (Specilty Coting Systems) ws used. The cotings were used to deposit spinning-coting on well clened opticl glss sustrtes t 2500 rpm 3000 rpm. The cotings were dried t room temperture. Sol-gel synthesis of colloidl SiO 2 nnoprticles ws performed in non-queous system of TEOS. The precursor of SiO 2 colloidl sol ws prepred y the se ctlyzed hydrolysis of tetrethylorthosilicte (Fluk, 99 %) y the following method of preprtion of Stöer silic. The required mount of mmonium hydroxide (33 %, Riedel-de Hen) ws dded to hlf of the required volume of nhydrous ethnol. The lkline solution ws dded to the solution of TEOS in ethnol with continuous stirring t 25 C temperture. The otined rection mixture ws stored for 14 dys t room temperture to llow hydrolysis s much s possile. The finl product consisted of colloidl suspension of 3 % SiO 2 nnoprticles in n nhydrous solvent. Methyl-modified SiO 2 sols were prepred y dding different mount: 0.05, 0.1, 0.25, 0.5, 1.0 p.p.vol. of hexmethyldisilzne (HMDS) or methytrimethoxysilne (MTMS) to the 3 % colloidl silic suspension. The modified sols were ged 7 dys t 25 C temperture nd for vrition contct ngle with ging time smple of HMDS (0.25 p.p.vol.) sol ws ged from 1 to 50 dys. 3. RESULTS The monodispersed silic colloidl suspension tht ws mostly synthesized from modified Stöer method [10] ws used to prepre single-lyer porous AR films. The colloidl silic prticles formed vi the se-ctlyzed hydrolysis nd condenstion of tetrethylorthosilicte (TEOS) in non-queous mixture contining ethnol nd mmoni were found to e colloidly stle, hving nrrow size (25 nm 35 nm) distriution. After, the surfce of silic nnoprticles ws modified y dding different rtio of modifiction regents hexmethyldisilzne (HMDS) or methytrimethoxysilne (MTMS) to colloidl silic sol. The colloidl silic prticles were covered y hydroxyl groups, fter HMDS or MTMS ddition, some of the hydroxyl groups were replced y methyl groups. The TEM imges of prticles otined from rection mixtures 3 % SiO 2 sol nd modified with HMDS nd MTMS re shown in Fig. 1. Fig. 1. TEM imges of prticles otined from rection mixtures 3 % SiO 2 sol nd: HMDS (0.25), MTMS (0.25) TEM imges showed tht prticles modified with MTMS were connected into lrge nd irregulr clusters. After mixing the unmodified silic sol nd MTMS, the mono-dispersed prticles linked nd hydrophoic methyl group were introduced into clusters. The HMDS modified prticles were sphericl, esides in etter dispersion thn modified with MTMS. The methyl group could esily replce the Si-OH group on SiO 2 prticles surfce. So, the surfce morphology of HMDS modified ws composed of sphericl smll prticles (25 nm 35 nm). Viscosity mesurements re lso of fundmentl interest ecuse they cn furnish quntittive informtion concerning the structure of the colloid suspension nd hence the specific interctions mong prticles. Fig. 2 shows the chnge in viscosity with time for two modifiers HMDS nd MTMS ddition. As seen, the viscosity incresed with incresing ging time, while the chnges of viscosity of HMDS modified sol ws smll. Fig. 2. Vrition of viscosity with time of modified sol with MTMS (0.25) nd HMDS (0.25) sol Atomic force microscope (AFM) ws used for the chrcteriztion of surfce morphology of silic cotings. AFM imges exhiited direct reltion etween the surfce nd morphology with nture nd concentrtion of modified regents. The AFM imges of unmodified nd modified with MTMS nd HMDS cotings with 1.5 nd 0.1 rtio re shown in Figs. 3 nd

And, for the HMDS modified coting/wter contct ngle hs reched 140 fter 14 dys. The mximl vlue 165 of the coting prepred from modified with HMDS fter 40 d.

Contct ngles with modifier concentrtion for cotings fter 14 dy ging Fig. 4. AFM imges of surfce morphology of cotings otined from sol: modified HMDS (0.25), HMDS (0.

8 nm), rther thn typicl prticle-ump morphology of unmodified silic coting (R q 3.2 nm). The verge surfce roughness R q of HMDS modified film increses with the modifier content.

3 And, for the HMDS modified coting/wter contct ngle hs reched 140 fter 14 dys. The mximl vlue 165 of the coting prepred from modified with HMDS fter 40 d. ging, while fter 50 ging dy smll decresing of hydrophoicity ws oserved (Fig. 7). Fig. 3. AFM imges of surfce morphology of cotings otined from sol: unmodified silic sol, MTMS (0.25) modified Fig. 6. Contct ngles with modifier concentrtion for cotings fter 14 dy ging Fig. 4. AFM imges of surfce morphology of cotings otined from sol: modified HMDS (0.25), HMDS (0.1) The surfce morphology of the cotings hs lrge effect on wter on wter repellence s well on light scttering on surfce. The MTMS modified coting hve smooth nd continuous surfce (roughness R q 1.8 nm), rther thn typicl prticle-ump morphology of unmodified silic coting (R q 3.2 nm). The verge surfce roughness R q of HMDS modified film increses with the modifier content. The coting otined from sol with highest HMDS (1.5 p.p.v.) content hve the iggest R q 3.5 nm, while with 0.1 p.v. of HMDS R q 2.8 nm. From the AFM imge we cn conclude tht just otined HMDS modified coting is composed of 40 nm silic prticles. This kind of morphology on cotings modified with HMDS generlly ment lots of mesopores in film, which ensured the low refrctive index n c needed for ntireflective ehviour of cotings. But, tht roughness is smll enough not to impct intence surfce light scttering, which reduces ntireflection. The hydrophoicity of coting ws shown y the wter/coting contct ngle s function of the rtion modifiction regents nd ging time of modified MTMS nd HMDS sols (Fig. 5 nd Fig. 6). The hydrophoicity ws not the sme for the coting from different methylmodified silic sols. Fig. 5. Imges of wter drops on cotings surfces: colloidl SiO 2 (21 ), HMDS modified (165 ) For the MTMS modified coting, the wter contct ngle incresed from 32 to 71 fter 14 dys ging of sol. Fig. 7. Vrition wter contct ngles with ging time for HMDS (0.25) modified silic cotings The ntireflection of modified one-side cotings is represented y the trnsmittnce shown in Figs. 8 nd 9. It is evident tht in oth cses of modifiction with HMDS nd MTMS the otined modified cotings visily reduced the reflectnce of the glss sustrte s compred to the unmodified colloidl SiO 2 coting. Trnsmittnce (%) SiO 2 glss HMDS modified Wvelength (nm) MTMS modified Fig. 8. Trnsmission spectr for BK-7 glss nd coting otined from sol: 3 % SiO 2, modified 0.25 HMDS, c 0.25 MTMS 285

4 As seen (Fig. 8), the trnsmission spectr show sinusoidl shpe with single mximum if the qurter-wve thickness occurred over the rnge of 400 nm 800 nm. Such ehviour is typicl nd expected for the single lyer cotings. The determined mximum of trnsmission for one side coted film otined from HMDS modified sol on glss ws 94.2 % (400 nm 550 nm), while for uncoted glss ws 91.3 %. The highest trnsmittnce (93.7 %) for MTMS modified coting ws chieved t 410 nm. The est AR ehviour showed the cotings otined from HMDS: silic sol (0.25 : 1) (Fig. 9). The highest solute trnsmittnce (95.0 %) ws chieved t 527 nm. The trnsmittnce of modified coting decreses to with decresing HMDS mount. Trnsmitnce (%) glss Wvelenght (nm) Fig. 9. Trnsmission spectr for coting on lime glss otined from modified silic sol with different concentrtion HMDS The surfce topology of the sustrte nd the coting determine the qulity of the coting in terms of their opticl trnsmission. The surfce morphology of the cotings cn led to surfce scttering nd cn reduce the trnsmitted. The mp of light scttering loses for modified coting is presented in Fig. 10. Fig. 10. Light scttering losses mp of HMDS modified silic coting on BK-7 glss The igger light scttering losses round the smple sides were otined. The HMDS modified coting hs totl 0.02 % scttering losses. The lser-induced dmge threshold (LIDT) is very importnt prmeter of ntireflective cotings. To compre with the coting otined from unmodified sol nd HMDS modified sol were tested under the sme conditions. The lser dmge tests were crried out on Nd: YAG lsers t 1064 nm (1 H) (single-shot 3.4 ns, pulse repetition rte 10 Hz). The lser dmge threshold AR unmodified silic coting exceeded J/cm 2 t 1064 nm nd J/cm 2 t 355 nm [18], while modified HMDS 35 J/cm 2 t 1064 nm. 4. CONCLUSIONS The opticl properties nd structure of hydrophoic ntireflective cotings (AR) deposited from silic sol nd HMDS or MTMS s modifiers were chrcterized in detil. The contct ngle of wter incresed with incresing mount of HMDS or MTMS, ut HMDS modified coting hd much higher contct ngle. It ws determined tht the hydrophoic HMDS modified cotings reduced reflectnce s well s unmodified silic AR cotings. The HMDS modified smple hs totl 0.02 % scttering losses. The lser dmge threshold modified silic coting exceeded 35 J/cm 2 t 1064 nm The otined hydrophoic ntireflective cotings vi sol-gel method could e used for the coting opticl elements sensitive to humidity. Acknowledgments The finncil support from the Lithunin Stte Science nd Studies Foundtion under project LADA (No. B-07030) is grtefully cknowledged. REFERENCES 1. Chen, D. Antireflection (AR) Cotings Mde y Sol-Gel Process: A Review Solr Energy Mterils & Solr Cell : pp Nostell, P., Roos, A., Krlson, B. Opticl nd Mechnicl Properties of Sol-Gel Antireflective Films for Solr Energy Applictions Thin Solid Film : pp Nostell, P., Roos, A., Krlson, B. Antireflection of Glzings for Solr Energy Applictions Solr Energy Mterils nd Solr Cell : pp Gomert, A., Gluitt, W., Rose, K., Dreiholz, J., Blsi, B., Heinzel, A., Sporn, D., Doll, W., Wittwer, V. Suwvellenght-Structured Antireflective Surfce on Glss Thin Solid Films : pp Hmmrerg, E., Roos, A. Antireflection Tretment of Low Emitting Glzings for Energy Efficient Windows with High Visile Trnsmittnce Thin Solid Films : pp Thoms, I. Method for the Preprtion of Porous Silic Antireflection Cotings Vrying in Refrctive Index from 1.22 to 1.44 Applied Optics : pp Thoms, I. High Lser Dmge Threshold Porous Silic Antireflective Cotings Applied Optics : pp Bisws, P.K., Sujth Devi, P. Porous Anti-reflective Cotings with High Spectrl Coverge y Sol-Gel Spin Coting Technique Journl of Mteril Science Letters : pp

5 9. Hench, L. L, West, J. K. The Sol-Gel Process Chemicl Review : pp Stoer, W. Fink, A., Bohn, E. Controlled Growth of Monodisperse Silic Spheres in the Micron Size Rnge Journl Interfce Science : pp Okuder, H., Hozumi, A. The Formtion nd Growth Mechnisms of Silic Thin Film nd Sphericl Prticles through the Stoer Process Thin Solid Films : pp Green, D. L., Lin, J. S., Lm, Y., Hu, M. Z., Schefer, D. W., Hrris, M. T. Size, Volume Frction, nd Nucletion of Stoer Silic Nnoprticles Journl of Colloid nd Interfce Science : pp Bogush, G. H., Zukovski, C. F. Studies of the Kinetics of Precipittion of Uniform Silic Ptrticles throught the Hydrolysis nd Condenstion of Silicon Alkokside Journl of Colloidl nd Interfce Science : pp Xu, Y., Zhng, B., Fn, W. H., Sun, H. Y. Sol-Gel Brodnd Antireflective Single-Lyer Silic Films with High Lser Dmge Threshold Thin Solid Films : pp Surtwl, I., Hnn, L., Miller, E. L., Whitmn, P. K, Thoms, I. M., Ehrmnn, P. R. Surfce Chemistry nd Trimethylsilyl Functionliztion of Stoer Silic Sols Journl of Non-Crystline Solids : pp Xu, Y., Zhng, B., Fn, W. H., Sun, H. Y. Antireflective Silic Thin Films with Super Wter Repellence vi Sol-Gel Applied Optics : pp Xu, Y., Zhng, B., Fn, W. H., Sun, H. Y. Comprtive Study on Hydrophoic Antireflective Films from the Three Kinds of Methyl-Modified Silic Sols Journl of Non- Crystline Solids : pp Begnskiene, A., Melninkitis, A., Skirznovs, S., Kzdojev, I., Sirutkitis, V. Kreiv, A. Sol-Gel Derived Antireflective Coting with Controlled Thickness nd Reflective Index Journl Mteril Science Polnd : pp Presented t the 17th Interntionl Conference "Mterils Engineering 2008" (Kuns, Lithuni, Novemer 06 07, 2008) 287

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