Surface elastic properties of sol-gel derived porous nanosilica optical coatings were determined using scanning force microscopy. Silica nanocoatings prepared under acid and base catalyzed sol-gel process exhibited varying surface morphology, particle size and porosity. Force-distance spectroscopy measurements were conducted on these coatings using scanning force microscopy, and their elastic moduli were obtained by applying Hertz model. The elastic modulus of the coatings varied from 2.4 to 13.4 GPa depending on the nature and concentration of the catalyst used

Babu, S.

Seal, S.

Vincent, A.

English

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University of Central Florida STARS Faculty Bibliography 2000s Faculty Bibliography 1-1-2007 Surface elastic properties of porous nanosilica coatings by scanning force microscopy A. Vincent University of Central Florida S. Babu University of Central Florida S. Seal University of Central Florida Find similar works at: https://stars.library.ucf.edu/facultybib2000 University of Central Florida Libraries http://library.ucf.edu This Article is brought to you for free and open access by the Faculty Bibliography at STARS. It has been accepted for inclusion in Faculty Bibliography 2000s by an authorized administrator of STARS. For more information, please contact STARS@ucf.edu. Recommended Citation Vincent, A.; Babu, S.; and Seal, S., "Surface elastic properties of porous nanosilica coatings by scanning force microscopy" (2007). Faculty Bibliography 2000s. 7754. https://stars.library.ucf.edu/facultybib2000/7754 Appl. Phys. Lett. 91, 161901 (2007); https://doi.org/10.1063/1.2799249 91, 161901© 2007 American Institute of Physics.Surface elastic properties of porousnanosilica coatings by scanning forcemicroscopyCite as: Appl. Phys. Lett. 91, 161901 (2007); https://doi.org/10.1063/1.2799249Submitted: 01 September 2007 . Accepted: 25 September 2007 . Published Online: 15 October 2007A. Vincent, S. Babu, and S. SealARTICLES YOU MAY BE INTERESTED INElastic modulus of polystyrene film from near surface to bulk measured by nanoindentationusing atomic force microscopyApplied Physics Letters 89, 031925 (2006); https://doi.org/10.1063/1.2234648Calibration of atomic-force microscope tipsReview of Scientific Instruments 64, 1868 (1993); https://doi.org/10.1063/1.1143970Calibration of rectangular atomic force microscope cantileversReview of Scientific Instruments 70, 3967 (1999); https://doi.org/10.1063/1.1150021Surface elastic properties of porous nanosilica coatings by scanningforce microscopyA. Vincent, S. Babu, and S. SealaAdvanced Material Processing and Analysis Center (AMPAC), Nanoscience Technology Center,Mechanical, Materials, and Aerospace Engineering Department, University of Central Florida,Orlando, Florida 32816, USAReceived 1 September 2007; accepted 25 September 2007; published online 15 October 2007Surface elastic properties of sol-gel derived porous nanosilica optical coatings were determinedusing scanning force microscopy. Silica nanocoatings prepared under acid and base catalyzedsol-gel process exhibited varying surface morphology, particle size and porosity. Force-distancespectroscopy measurements were conducted on these coatings using scanning force microscopy, andtheir elastic moduli were obtained by applying Hertz model. The elastic modulus of the coatingsvaried from 2.4 to 13.4 GPa depending on the nature and concentration of the catalyst used. © 2007American Institute of Physics. DOI: 10.1063/1.2799249Influence of porosity in mechanical properties such aselastic modulus and Poisson’s ratio has been a subject ofinterest of many researchers as the elastic modulus decreaseswith an increase in porosity1 and a decrease in density.2 Ear-lier, we have reported porous silica coatings for antireflectivecoatings with 99% and above transmittance on boron silicateglass substrate.3 We found that the refractive index of thesecoatings can be tuned by varying the particle size and poros-ity. However, an increase in particle size and porosity canaffect the bonding between the particles and modify the me-chanical strength.4 Hence, it is necessary to understand theinfluence of porosity and particle size on the mechanicalproperties of these films.Mechanical response of the surfaces to forces at nanos-cale and nanometer penetration depth levels has been pos-sible with scanning force microscope SFM,5–14 where theapplied load can be controlled in nano- to piconewtons bychanging the tip curvature radius and cantilever stiffness.7Herein, we report the surface elastic modulus obtained as afunction of microstructure and porosity by force-distancespectroscopy on porous silica coatings prepared by acid andbase catalyzed sol-gel process.The details of silica sol-gel synthesis and coating prepa-ration are reported elsewhere.3 Using the acid catalysis, lessporous and highly dense coatings can be achieved while theuse of base catalyst results in highly porous and less densesilica coatings. In our experiments, depending on the natureof the catalyst acid or base, the samples were coded as A orB. It is to be noted that only one molar ratio of acid catalyzedsilica sample was chosen tetraethyl orthosilicate TEOS toHNO3 molar ratio of 1:0.025, while two different molarratios of base catalyzed silica samples TEOS to NH4OHmolar ratio of 1:1 and 1:3 were used for the experiment, asthe variation in molar ratio of acid catalyst did not result inmarked variation on surface morphology and mechanicalproperties among other acid catalyzed samples. The coatingswere deposited on boron silicate glass by dip coating.Figure 1 shows the force-distance F-d curves acquiredon a relatively smooth area selected based on atomic forcemicroscopy AFM image of the individual coating samplesas well as on the glass substrate at a load of 2.0 N usingscanning force microscopy Solver EC, NT-MDT, Russia.Acquisition of the force curve at several points was con-ducted for better uniformity and repeatability. All measure-ments were made with a silicon cantilever having a tip sizeof 20 nm in diameter. These curves are then compared withthe curve on the blank glass substrate. Since the silicon tipcould not penetrate the hard glass substrate AFM imagesshows no sign of indentation on glass substrate, the z axismovement of the piezostage linear solid curve, G in Fig. 1represents only cantilever deflection and no indentationwhile collecting force curves on the blank substrate. Consid-ering the F-d curves of the coatings, there is a clear trend ofdecrease in slope as well as deviation in the linear nature ofthe curve due to tip indentation. The difference in the piezo-height between the substrate and the silica coatings providesthe depth of indentation. The cantilever deflection D innanoampere can be converted into the applied load F inaElectronic mail: sseal@mail.ucf.eduFIG. 1. Force-distance curves obtained from base catalyzed silica coatingsB1 and B2, acid catalyzed silica coating A, and a bare glass substrateG. Inset shows the corresponding AFM images of the samples. Particlesize increases with an increase in the basic nature of the catalyst. The meanparticle size was found to be 5, 30, and 100 nm for A, B1, and B2,respectively.APPLIED PHYSICS LETTERS 91, 161901 20070003-6951/2007/9116/161901/3/$23.00 © 2007 American Institute of Physics91, 161901-1nanonewton using the relation F=kD /S, where k is thespring constant in nN/nm determined using the method ofSader et al.15 and S is the sensitivity in nA/nm obtainedfrom the slope of the F-d curve on the glass substrate. Basecatalyzed silica coatings showed lower resistance to indenta-tion as indicated by the decrease in the slope of the F-dcurves, and hence exhibits lower mechanical strength higherdepth of indentation in comparison to the acid catalyzedsilica coating. It is now possible to calculate the tip indenta-tion z into the soft sample as the difference in the respec-tive piezoheight and z positions relative to the blank glassreference. The elastic deformation has been defined throughHertz equation9–14 for a parabolic indenter parabolic in-denter is the most reasonable approximation for an SFM tip,as F= 4/3ErR1/23/2, where Er is the reduced elastic modu-lus also called as effective elasticity, R is the radius of theindenter, and  is the indentation depth. The elastic modulusof the sample Es can be obtained from the sample tip inter-action and is given by, 1 /Er= 1−t2 /Et+ 1−s2 /Es, where is the Poisson’s ratio, and Et and Es is the elastic modulusof the tip and sample, respectively. The elastic modulus andthe Poison ratio of silicon tip were taken from the literatureEt=130 GPa and t=0.27.16 Poisson’s ratio of 0.22 wasassumed for silica film and it was reported to be not affectedby the variation in film density.17 Hertzian model can beapplied only when EtEs.11 The model is valid only forelastic deformation and assumes no significant adhesion be-tween the tip and the sample.18 It is important to avoid anycontribution from inelastic deformation and tip-sample adhe-sion during the analysis of the load displacement curve. Theadhesion between contacting bodies becomes more signifi-cant at nanoscale.19 We have observed adhesion between thetip and silica coatings during unloading. Also in the case ofbase catalyzed silica films, cracking and pileup of materialwere observed during indentation due to plastic deformation.Hence, only the initial elastic deformation region of the load-ing curve was analyzed for elastic modulus determination.Nanoindentation curves obtained on base and acid cata-lyzed silica coatings are shown in Fig. 2. The effective elas-tic modulus of the coatings can be obtained from the inden-tation curve through power law regression analysis using thefollowing equation, =aFb, where  is the indentation depth,and a and b are the fitting parameters. The value of power bis fixed as 2/3 by applying the Hertz model, and the value ofcoefficient a can be obtained by power law regression fittingvalues of coefficient a are listed in Table I. From the para-bolic tip model, the reduced elastic modulus Er can be cal-culated as Er=3/ 4a3/2R1/2, where R is the tip radius.The elastic modulus calculated for acid and base cata-lyzed coatings are listed in Table I. Among all the coatings,acid catalyzed silica coatings showed the highest elasticmodulus 13.4 GPa and is close to the earlier reported val-ues for sol-gel silica coating.20,21This higher value of elasticmodulus compared to base catalyzed silica is due to the lowporous nature 5% and higher density 2.18 g/cc of thecoatings. Unlike polycrystalline materials whose mechanicalproperties largely depends on the nature of grain boundaries,mechanical properties of glassy materials such as silica de-pend more on composition and structure of the bondingnetwork.22 Elastic modulus value calculated for base cata-lyzed silica coating B1 is 1.63 GPa, and is close to the re-ported values for porous silica.22 The lower value of elasticmodulus is due to the presence of large number of pores inthe film as the B1 silica film is approximately 49% porous innature and the density is very low 1.17 g/cc. Due to theirhigh porous nature, porous silica coatings often exhibit verylow density23 and their elastic modulus can be even 102–104smaller than that of silica glass.24 Coatings obtained fromporous silica sol-gel contain roughness in the form of asperi-ties. Presence of these asperities will increase the surfacearea over which the tip and the sample make the effectivecontact. This leads to a reduced contact pressure for a givenload and results in a lower value of elastic modulus.10 Inorder to determine the significance of surface roughness onelastic modulus determination, a nondimensional parameter has to be considered. =sR /a02=s16REr2 /9F21/3,where s is the rms roughness obtained experimentally anda0 is the contact radius for smooth surfaces under the load F.The Hertz theory for smooth surfaces is valid only if theparameter  is less than 0.05.25 The value of  calculated foracid catalyzed silica coating with a low surface roughness of0.3 nm is less than 0.05 which is within the error limit.FIG. 2. Load-Indentation profile obtained from base catalyzed and acidcatalyzed silica coatings. The power law fitting coefficient for each of thecurves is listed in Table I. Inset shows the three dimensional surface profileof an indent obtained on an acid catalyzed coating.TABLE I. Structural and elastic properties of acid and base catalyzed silica coatings.ThinfilmFilmthicknessnmaParticlediameternmaRoughnessnmaPoreradiusnmaPorosity%aDensityg/ccaPower lawcoefficient aElasticmodulusGPaA 635±2 5.9±1.1 0.3 1.7 5.0 2.18 0.00007 13.40B2 105±2 99.5±20.7 3.0 14 36.1 1.47 0.00017 5.02B1 110±2 33.5±8.8 4.5 4.7 49.1 1.17 0.00027 2.38aRef. 3.161901-2 Vincent, Babu, and Seal Appl. Phys. Lett. 91, 161901 2007Hence, no roughness correction is required for acid catalyzedsilica coating. However, the value of  calculated for basecatalyzed silica coating B1 with a surface roughness of4.5 nm is 0.23 0.05, and the value of value of  calcu-lated for base catalyzed silica coating B2 with a local clustersurface roughness of 3 nm is 0.26 0.05. A second nondi-mensional parameter , defined by Greenwood and Tripp26is given by = 8/3ss2R /	s1/2, where s is the asperitydensity and 	s is the mean curvature of the summits of as-perities. Greenwood and Tripp obtained two values of ,which is bracketed by a wide range of practical rough sur-faces. Using these two parameters,  and , the correctionrequired for elastic modulus is determined. The ratio of ef-fective contact radius experimental, a* to the Hertz radiustheoretical, a0 are influenced by the surface roughness, asshown in Fig. 3. Since E is proportional to 1/a3, the elasticmodulus value after the roughness correction for base cata-lyzed silica coating B1 is approximately 2.38 GPa. In thecase of base catalyzed silica coating B2, the coating wasmainly formed by individual silica agglomerates clusters ofsize closer to 100 nm AFM image of B2 in Fig. 1 andcontained larger size pores. However, B2 silica coatings wereless porous approximately 36% than that of B1 silica coat-ings. Due to the larger size of silica agglomerates comparedto SFM tip diameter, the indentation experiments conductedon these agglomerates particles only represents the mechani-cal properties of the skeletal silica agglomerates and not thatof the porous silica coating. The elastic modulus of indi-vidual silica agglomerates obtained after the roughness cor-rection is approximately 5.02 GPa. The amplitude contrastAFM images of B2 silica coatings inset of Fig. 3 showedthat these silica clusters are agglomerates of smaller size di-ameter of approximately 20 nm particles with a local clustersurface roughness of 3.0 nm. The bonding strength betweenthese individual particles determines the mechanical proper-ties of these bigger agglomerates. The reason for lower valueof elastic modulus of these silica agglomerates is mainly dueto the weaker bonding between the individual amorphousparticles.Porous silica optical coatings obtained by acid and basecatalyzed synthesis showed variation in elastic modulus de-pending on the microstructure and nature of the coatings.From SFM indentation curves, using the Hertz model, wehave obtained the elastic modulus of thin film silica opticalcoatings. Acid catalyzed silica coatings were denser in natureand exhibited higher elastic modulus than base catalyzedsilica samples. Among the two base catalyzed silica coatings,the particle size, microstructure and porosity greatly influ-enced the individual elastic modulus values.The authors thank Lockheed Martin and Florida HighTech Corridor for the funding.1B. B. Avar, N. Hudyma, and M. Karakouzian, Int. J. Rock Mech. Min. Sci.40, 919 2003.2H. Fan, C. Hartshorn, T. Buchheit, D. Tallant, R. Assink, R. Simpson, D.J. Kissel, D. J. Lacks, S. Torquato, and C. J. Brinker, Nat. Mater. 6, 4182007.3A. Vincent, S. Babu, E. Brinley, A. Karakoti, S. Deshpande, and S. Seal,J. Phys. Chem. C. 111, 8291 2007.4Y. Chen, K. Balani, and A. Agarwal, Appl. Phys. Lett. 91, 031903 2007.5G. M. Pharr and W. C. Oliver, MRS Bull. 17, 28 1992.6A. B. Nancy and J. C. Richard, J. Vac. Sci. Technol. A 7, 2906 1989.7K. Miyake, N. Satomi, and S. Sasaki, Appl. Phys. Lett. 89, 0319252006.8M. B. Salmeron, MRS Bull. 18, 20 1993.9A. J. Heim, W. G. Matthews, and T. J. Koob, Appl. Phys. Lett. 89, 1819022006.10E. P. S. Tan and C. T. Lim, Appl. Phys. Lett. 87, 123106 2005.11C. Munuera, T. R. Matzelle, N. Kruse, M. F. Lopez, A. Gutierrez, J. A.Jimenez, and C. Ocal, Acta Biomat. 3, 113 2007.12M. Salerno and I. Bykov, Microscopy and Analysis 20, S5 2006.13S. Garcia-Manyes, A. G. Güell, P. Gorostiza, and F. Sanz, J. Chem. Phys.123, 114711 2005.14A. Touhami, B. Nysten, and Y. F. Dufrene, Langmuir 19, 4539 2003.15J. E. Sader, J. W. M. Chon, and P. Mulvaney, Rev. Sci. Instrum. 70, 39671999.16W. A. Brantley, J. Appl. Phys. 44, 534 1973.17J. Gross, G. Reichenauer, and J. Fricke, J. Phys. D 21, 1447 1988.18M. Perkins, S. J. Ebbens, S. Hayes, C. J. Roberts, C. E. Madden, S. Y.Luk, and N. Patel, Int. J. Pharm. 332, 168 2007.19R. Gissi and P. Decuzzi, J. Appl. Phys. 98, 014310 2005.20C. M. Chan, G. Z. Cao, H. Fong, M. Sarikaya, T. Robinson, and L.Nelson, Mater. Res. 15, 148 2000.21S. Yu, T. K. S. Wong, X. Hu, and M. S. Yong, J. Sol-Gel Sci. Technol. 35,69 2005.22S. Takada, N. Hata, Y. Seino, K. Yamada, Y. Oku, and T. Kikkawa, Jpn. J.Appl. Phys., Part 1 43, 2453 2004.23M. Moner-Girona, A. Roig, E. Molins, E. Martinez, and J. Esteve, Appl.Phys. Lett. 75, 653 1999.24M. Gronauer and J. Fricke, Acustica 59, 177 1986.25K. L. Johnson, Contact Mechanics Cambridge University Press, Cam-bridge, 1985, pp. 397–423.26J. A. Greenwood and J. H. Tripp, J. Appl. Mech. 34, 153 1967.FIG. 3. Influence of surface roughness on the effective contact radius a*compared with the Hertz radius a0 for two values of  that encompass awide range of practical rough surfaces Ref. 26. For coating B1, =0.23and 417, and a* /a0 is between 1.12 and 1.15, and for coating B2,=0.26 and 417, and a* /a0 is between 1.13 and 1.17. Inset shows theamplitude contrast AFM image of base catalyzed silica coatings161901-3 Vincent, Babu, and Seal Appl. Phys. Lett. 91, 161901 2007

Surface elastic properties of porous nanosilica coatings by scanning force microscopy

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Surface elastic properties of porous nanosilica coatings by scanning force microscopy

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