Oxide Surfaces with Tunable Stiffness

Article pubs.acs.org/JPCC

Oxide Surfaces with Tunable Stiffness Katya Gotlib-Vainshtein, Olga Girshevitz, and Chaim N. Sukenik* Department of Chemistry and Institute of Nanotechnology & Advanced Materials, Bar Ilan University, Ramat-Gan 52900, Israel

David Barlam Department of Mechanical Engineering, Ben Gurion University, Beer Sheva, Israel

Estelle Kalfon-Cohen and Sidney R. Cohen* Department of Chemical Research Support, Weizmann Institute of Science, Rehovot 76100, Israel ABSTRACT: An important challenge of modern materials science and nanoscience is to develop ways to alter the mechanical properties of an interface in a controlled fashion. Doing this while preserving the bulk properties of a material and maintaining a fixed chemical composition and reactivity of the interface is particularly attractive. In this work, the creation of substrates with tunable stiffness has been achieved by coating a soft polymer with an adherent, crack-free oxide overlayer whose thickness is varied from 8 to 70 nm. Specifically, amorphous titania with controlled, variable, thickness was deposited on polydimethylsiloxane (PDMS), and the surface mechanical properties were characterized using atomic force microscope (AFM)-based nanoindentation. The force/ deformation curves can be quantitatively reproduced using a finite element analysis (FEA) modeling protocol. The FEA modeling facilitates predictability and enables the design of surfaces with independently customized chemical and mechanical properties.

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previously reported9 the deposition of titania on polydimethylsiloxane (PDMS) using liquid phase deposition (LPD),10 the approach proposed and demonstrated herein extends that earlier work by controlling the thickness of the titania through variation of the LPD deposition time. The expectation is that this should provide the control element needed for modulating surface stiffness. The relatively compliant nature of a polymer like PDMS and the stiffness of an oxide like titania provide a good testing ground for the range of stiffnesses over which meaningful modulation should be possible. This approach to tuning of the surface stiffness carries the additional advantage of independent control of the surface chemical behavior. Furthermore, the specific case of titania-coated PDMS is particularly attractive due to the biocompatibility of titania and the possible applications for such hard-on-soft composites in biotechnology.11 The ability to tune the mechanical properties of a surface must be accompanied by a reliable and precise method for monitoring these changes. Nanoindentation is commonly used

evelopment of materials with tunable surface stiffness presents significant technical challenges, while offering interesting opportunities. These range from controlling the growth and differentiation of stem cells by varying the effective elastic modulus of the substrate;1−3 to studying the behavior of polymers for electronic device encapsulation, where changes in their modulus can affect the behavior of the electronics;4 to applications in microfluidics.5,6 The most common strategy for controlling the mechanical properties of polymeric interfaces is to change their monomer/ cross-linker ratio.3,7 The disadvantage of this approach is that varying cross-linking changes the composition and chemistry of both the polymer bulk and its surface. An additional way to change mechanical properties of polymers is to prepare nanocomposites of nanoparticles and polymers in different ratios. In this case, the mechanical properties would vary as a function of the type and concentration of nanoparticles.8 However, here too, the presence of the nanoparticles changes the properties of the bulk. The guiding strategy behind the efforts presented herein is to make surfaces with tunable stiffness through the controlled deposition of stiff oxide films onto a soft polymer substrate. We posited that a very thin (a few nanometers), conformal, oxide film would form a composite only minimally stiffer than the underlying polymer substrate and that progressively thicker oxides would then provide progressively stiffer surfaces. Having © XXXX American Chemical Society

Special Issue: Ron Naaman Festschrift Received: January 2, 2013 Revised: January 31, 2013

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dx.doi.org/10.1021/jp4000335 | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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to study the mechanical properties of surfaces.12−15 This technique is well-suited for characterization of elasto-plastic hard materials.16 In recent years, it has become increasingly popular for the study of soft polymeric materials.7,17 Nanoindentation has also been applied to the characterization of the mechanical properties of hard overlayers on softer substrates. However, these measurements have been typically done on substances like metals and glasses, i.e., cases where both materials are relatively stiff.18,19 Multilayers created using layerby-layer deposition is an interesting, but isolated, example of a system that mixes a hard material like an oxide with a soft organic material for which an effective modulus for the composite system has been measured by nanoindentation. In that unique case, the organic material seemed to increase hardness and Young’s modulus relative to a pure titania film.20,21 Layer-by-layer deposition has also been used to deposit titania on a softer substrate, polycarbonate, but in this case, mechanical properties for this system were only measured for a model titania film deposited on silicon.22 Despite this recent progress, nanoindentation measurements of hard films on a very soft base layer needs substantial further development in order to be able to provide broadly applicable and quantitatively meaningful results. Indentation analyses are commonly based on one of a number of contact-mechanics models. The most commonly used model in nanoindentation was developed by Oliver and Pharr.23 It is based on a Herzian model developed by Sneddon24 for indentation of an elastic half-space by rigid indenters of various geometries. Other, contact mechanicbased, approaches include effects of adhesion.25 A key element in these analyses is the assessment of the area of full contact between the indenter and the sample. When that contact area, or the stressed volume underneath the indenter, encompasses two or more different materials, the meaning of a material modulus is unclear. The canonical definition of elastic modulus, stress/strain, is unreliable for characterizing composite material in general and nanoscale multilayer composites in particular: such a value would clearly be dependent on the ultimate depth into the surface sensed, and the specific position probed. This would not be a good predictor of mechanical behavior and would not provide meaningful insight into the source of observed mechanical properties of the composite. A highly studied example is that of thin films, where a blind application of those common nanoindentation relations without relating to vertical compositional gradients would give very different modulus values for different deformations or indenter size. Methods exist for deducing the elastic modulus of the pure film material,26,27 but for some cases, as for that at hand, it is of interest to understand the overall mechanical response of the entire composite system, wherein the soft substrate tempers the stiffness of the overlayer according to the film thickness. In other words, for a thin film, the modulus of the film alone does not describe the system response to stress. Stiffness thus becomes a more tractable term, being an experimental observable and not subject to interpretation or assumptions. Modeling, in this case by finite element analysis (FEA), can then be used to gain microscopic insights into the mode of deformation at the nanometer scale and to understand the contribution of each component to the overall observed stiffness. The work presented below presents a well-defined approach to the preparation of surfaces with tunable mechanical properties along with the tools and techniques needed to

measure the stiffness and to assess the contribution of each of the components. Overall, it is a system whose characteristics can be modified in a controlled fashion and wherein analytical modeling gives it a predictability that is particularly intriguing.

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EXPERIMENTAL AND COMPUTATIONAL METHODS Preparation of Titania−PDMS Surfaces. Experiments were performed on silicon wafers (n-type), which were cleaned using chloroform, acetone, and ethanol, drying with a nitrogen stream after each solvent. PDMS was spin-coated (Spin-Coater Model KW-4A, ChemSols Corp.) onto these wafers by applying a mixture of Sylgard silicon elastomer 184 and curing agent (ratio 10:1 w/w). The spin coating of the PDMS was done at 5000 rpm and spinning the sample for 30 s. The elastomer was then cured overnight at 100 °C. Before the titania coating, the surface of the PDMS was pretreated with an air plasma (Model PDC-3XG, Harrick) at a pressure of 0.3 mmHg and 18 W power for 5 min. Immediately after plasma activation, samples were placed in a room temperature titania deposition solution (0.3 M H3BO3 and 0.1 M (NH4)2TiF6 in water10) that had been first aged for 6 h and then filtered through a 0.45 μm, 7 bar max filter (Schlecher and Schuele). The deposition was allowed to proceed for 2−6 h. The aged/ filtered solution gave smooth coatings whose thicknesses are attenuated by varying the deposition time. The stability of the coating was confirmed by sonication in water for 10 min. The TiO2-coated PDMS was rinsed, first in water and then in methanol, and then dried in a controlled humidity chamber.28 Surface Characterization. The PDMS samples, with and without the titania overlayers, were characterized using the following analyses. Scanning Electron Microscopy (SEM). The surface morphology of the samples was assessed by SEM (Inspect, FEI), an accelerating voltage of 30 kV with surface gold coatings of approximately 30 nm thickness. Rutherford Backscattering Spectrometry (RBS). The thicknesses of the TiO2 layers were measured by RBS (Tandem Pelletron Ion Accelerator 5SDH, NEC, USA) using a 2.0 MeV beam of α-particles. The backscattered particles were detected using a silicon surface barrier detector with 30 keV resolution. The beam current was measured on the target and kept around 13 nA. The RBS data was analyzed using RUMP software. The atomic composition of the surface of each sample was determined by X-ray photoelectron spectroscopy (XPS) (Kratos AXIS-HS spectrometer) using a monochromatized Al Kα source. Survey scans were run at 75 to 150 W. All data acquisition was done in a hybrid mode (using electrostatic and magnetic lenses) and detection pass energies of 40−80 eV. All XPS measurements were carried out at room temperature, under a vacuum of 1.0−3.0 × 10−9 Torr. Unless otherwise indicated, the spectra were acquired with an electron flood gun for charge neutralization. The spectrometer energy scale is routinely calibrated according to the ISO TC/201 SC7 international procedure for binding energy (BE) with Au 4f7/2 = 83.98 and Cu 2p3/2 = 932.67. Data processing was done with VISION 2.1 software (Kratos) using sensitivity factors for quantification. In most cases, a Shirley background was used. Curve fitting was performed using a 80/20 Gian/Lorentzian line shape. Regularly, 100−1000 iterations were used to reach the best fitting. Atomic Force Microscopy (AFM). All scanning probe microscopy was done using a MultiMode AFM with NanoB



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Figure 1. FEA model for 8 nm film, 1/4 model shown. Gray elements are substrate, red the film. Right, zoom near the origin, showing 1/4 of spherical indenter.

load. From the knowledge of the probe signal sensitivity (nm/ V) and spring constant (nN/nm), raw deflection vs sample displacement plots were converted into force vs deformation plots for comparison with finite element analysis (FEA). Finite Element Analysis. The FEA model chosen to model the experiment used a stiff upper layer of varying thickness, representing TiO2, glued to a softer substrate, representing the activated PDMS. An example for the 8 nm TiO2 layer is shown in Figure 1. The substrate is modeled as a cube with sides ranging between 180−300 nm. Three different thicknesses of the stiff upper layer were modeled8, 14, and 22 nm. Elastic modulus of the layer was the only variable parameter and was tuned to match experimentally measured stiffness of the relevant samples. The indenter was a nondeformable spherical tip with radius of 10 nm as determined by SEM images of the tip used for the experiments. Contact conditions are defined only between the indenter tip and the TiO2 layer. The FE model comprises about 150 000 eight-node hexahedral elements, 125 000 in the substrate and 10 000−30 000 in the layer. The size of the elements varied in the xy plane linearly, with the largest elements, at the outer edge, being 3.3 times larger than the smallest element under the center of the indenter at the model origin. Regardless of model size, there are 50 elements in each of x, y, and z directions in the substrate, while for the layer in the z direction, the number of elements in z is 4 for the 8 nm layer, 7 for the 14 nm layer, and 12 for the 22 nm layer. Because of the symmetry, a quarter of the model was used with the proper symmetric conditions. The simulation was executed by MSC.MARC FE code. Both substrate and layer are assumed to be 3D isotropic. For the PDMS layer, a Young’s modulus of 20 MPa and Poisson’s ratio ν = 0.4 are used. For TiO2, ν was assigned a value of 0.28,33 and the modulus was a fit parameter found by varying the modulus iteratively until the simulation and experimental curves converged. Indentations were run to a deformation of 10−15 nm. The choice of overall dimensions of the model and size of mesh were taken as the smallest size (i.e., most efficient use of computer resources) that was insensitive to boundary conditions.

scope V electronics (Bruker AXS SAS, Santa Barbara, CA). The root-mean-square roughness (Rq) was calculated from 2 × 2 μm2 micrographs. All titania-coated samples were scanned in tapping mode using a LTESP silicon probe (force constant of 20−45 N/m, Bruker). PDMS samples were scanned using a FESP silicon probe (force constant 1−5 N/m, Bruker). The fast scan direction was perpendicular to the cantilever long axis, and the images were captured in the retrace direction with a scan rate of 1 Hz at pixel resolution of 512 samples/line. Before analysis of the images, first order “flatten” and “planefit” functions were applied to each image. The roughness was determined by Nanoscope analysis software. Nanoindentation. Mechanical measurements were done in the AFM nanoindentation mode. The spring constant of every probe was calibrated by the Sader method,29 an approach that models the cantilever as a beam oscillating in a fluid (air) and calculates the spring constant based on the length and width of the cantilever, the frequency of the oscillation, and quality factor Q. These indentations were designed to probe the first few nanometers of deformation in accordance with the low stiffness of the cantilever. For the hard on soft films, LTESP tips described above were applied. FESP probes were used to measure the modulus of the activated PDMS substrate. Preliminary work showed that the activated PDMS was significantly stiffer than the native PDMS. For these calibrations, indentations were limited to small depths (up to 10 nm) using FESP tips to provide a modulus value for input into the calculations. For these samples, at low loads, contact adhesion must be taken into account.7 Derjaguin, Muller, Toporov (DMT) analysis30,31 was applied to yield the result of 20 (±20%) MPa for this activated film. Finally, the modulus of the TiO2 films was measured using instrumented nanoindentation (Agilent, XP Nanoindenter) on a control sample, consisting of a 180 nm thick TiO2 LPD film on a silicon wafer. Measurements were made using both quasistatic indentation and continuous stiffness measurement (CSM). The latter imposes a small oscillation on the force and gives continuous reading of E and H with depth.16,32 In both cases, E was calculated using Oliver and Pharr analysis,23 and the value reported is the average for the depth range of 40−60 nm. Batches of independently prepared LPD TiO2-activated PDMS samples were evaluated by AFM nanoindentation. On each sample, 32 indentations were done with a 20 nN ultimate

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RESULTS AND DISCUSSION Titania−PDMS: Fabrication and Characterization. An initial air plasma treatment of the PDMS samples (∼10 μm C



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thickness of each sample takes into account the sample roughness, averaged over the entire spot size. Tapping mode AFM topographical images were used to study the TiO2 surface nanomorphology. The roughness of the TiO2 film was greater than that of the air-plasma-treated PDMS substrates for all film thicknesses studied. The treated PDMS had an rms roughness of Rq = 0.17 nm over a scan size of 2 × 2 μm2. RMS roughness of the TiO2 coating on this substrate was ∼6 nm, for all thicknesses (Figure 5). Despite the fact that this roughness represents a significant fraction of the overall film thickness, there are no evident bare patches of exposed PDMS, as verified by SEM and AFM analyses (except for the cracks mentioned above). Furthermore, the roughness does not vary with film thickness, as demonstrated in Figure 4. Thus, the surfaces obtained by depositing LPD titania on PDMS are sufficiently uniform and sufficiently comparable to each other so as to constitute a set of substrates whose mechanical properties can be compared in a meaningful way. Modeling. The FEA model was designed based on the physical properties of each of the components of the composite structure. Nanomechanical characterization of the substrate allowed assignment of elastic modulus, while Poisson ratio was estimated from the literature.34 Because of the strong bond between film and substrate in the real sample, the two components could be considered glued to one another. This is an important constraint since the stressed model clearly shows strain at the interface along the surface direction, and this must be included in the force balance. The parameter that remained somewhat uncertain was the elastic modulus of the TiO2 film, particularly since it is known to have a degree of porosity that may also depend on substrate type and film thickness. Nanoindentation on a relatively thick (180 nm) LPD titania film on a hard Si substrate provided a starting reference value and served to check the reliability of the FEA model. The FEA modeling treated the modulus of the film as an adjustable parameter, which was the only parameter modified during fitting of the experimental curves. The values obtained for modeling this derived film modulus for different film thicknesses were internally consistent, giving confidence in the suitability of the model. Optimizing the FEA modeling required satisfying both the demands of fidelity to the true system and efficiency of computing resources. Figure 6a shows a FEA model that extends 500 nm outward along the surface from the center of indentation. This size allows an overall view of the indentation profile but, to conserve computing time, used a mesh of low spatial density. It is seen that deformation of the layer begins about 200 nm from the center and occurs almost entirely in the substrate with no obvious deformation of the TiO2 film. This being the case, it can be presumed that the region outside a radius of 200 nm from the indentation center contributes very little to the overall force balance. The model size used in the final analysis ranged between 180 nm breadth for the 8 nm film to 300 nm for the 22 nm thick film. To test the effect of underestimating the model size, when it is reduced from 180 to 120 nm on the 8 nm film, the stiffness falls by about 3%, which translates to a drop of about 8% in associated film modulus as determined from the FEA simulations. In addition, an exaggerated bulging of the substrate is seen at the boundaries. Figure 6b shows a deformed surface under these model conditions. The Poisson ratio υ represents the transverse expansion perpendicular to an axial compression, and hence, a mismatch

PDMS spin-coated onto a single crystal silicon substrate) improves the adhesion of the titania to the PDMS. XPS measurements showed that the plasma-treated PDMS contains two chemical states of silicon. The spectrum, shown in Figure 2, can be deconvoluted to give two different types of silicon,

Figure 2. XPS spectra of silicon 2p spectral region (including curve fitting) of (a) spin-coated PDMS and (b) after treatment by oxygen plasma. The peak at 102.5 eV is that expected for the silicon in PDMS, while the peak at ∼103.7 eV corresponds to SiO2.

those associated with unaltered silicone in PDMS at 102.5 eV and a second peak from an oxidized, silica-like silicon at 103.7 eV. Integration of these spectra shows a silica to silicone ratio after plasma treatment of 81:19. This signal provides a useful analytical marker for use as a metric in making reproducibly activated PDMS surfaces. This chemical modification is not just a surface skin layer, but extends two micrometers into the surface as determined by RBS. The plasma-treated PDMS could be coated uniformly with adherent, amorphous, TiO2 by liquid phase deposition (LPD).9 Figure 3 shows SEM and AFM micrographs of different thicknesses of titania on the PDMS. Although both SEM and AFM images show good overall surface uniformity and coatings below 30 nm thickness are crack-free, higher resolution SEM images reveal some surface cracking on the thicker coatings. It is important to note that, even in the cracked samples, there are crack-free domains hundreds of square micrometers in size; these were used for measuring the mechanical properties. The reported thickness values are based on RBS measurements that show a steady change in titania thickness as a function of deposition time (Figure 4). RBS provides an average thickness of the titania within the beam diameter of 300 μm. The D



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Figure 3. Representative SEM and AFM images of titania coatings on the PDMS: 8 (a,b) and 70 (c,d) nm film thicknesses shown.

Figure 4. RBS signal intensity of the Ti peak as a function of deposition time. Concentration correlates with height and width of peak.

Figure 6. (a) Large (500 nm, x,y; 200 nm, z) stressed FEA model showing the full profile of indentation for 15 nm deformation on 8 nm thick titania on activated PDMS. The red band represents the titania layer, and the light blue line shows the position of the undeformed surface. (b) Small (120 × 120 × 150 nm, x,y,z) model leading to bulging at the borders.

when υ approaches 0.5, the response is of ideal rubber, and there is no volume change under load (infinite bulk modulus).Varying the value by ±10% around the value used here (0.4) led to variations of less than 10% in the best-fit modulus value found for the titania. Still, the mismatch of Poisson ratios at the interface is an important component in determining the force balance. In the FEA, the influence of the relatively high value of υ can be observed in the final mesh as a lateral motion of the elements. Some care in tuning of substrate and overlayer may be required for enhanced robustness of the model, though the variations on the order of 10% indicated here are within the experimental uncertainty levels. Nanoindentation. There are several issues to be considered when using AFM-based nanoindentation for semiquantitative information as reported herein. One consideration is that of dynamic range. For a given cantilever with a specific spring constant, the range of surface stiffnesses, which can be reliably measured, limits the range of elastic moduli,

Figure 5. RMS roughness of the titania-coated samples as a function of layer thickness, taken from 2 × 2 μm2 AFM images. Error bars represent standard deviations of 3−4 measurements made on each sample.

between υ of the material below and that above the interface should lead to tangential interfacial stress. The influence of the value of υ assigned to the PDMS was also checked. Increasing the Poisson ratio increases resultant stiffness in the FEA model: E



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Table 1. Young’s Modulus Values of Activated PDMS and Titania Measured by Instrumented Nanoindentation (INI) and AFMBased Nanoindentation sample

method/tip

load (P)

indent depth

Young’s modulus (E)

activated PDMS TiO2/Si

AFM/FESP INI/Berkovich

20 nN 50−250 μN

10 nm 50 ± 10 nm

20 ± 4 MPa 66 ± 15 GPa

Table 2. Summary of Experimental and FEA-Computed Results for the Nanoindentation Experiments of the Samples Used in This Study sample deposition time titania thickness (nm) stiffness (N/m) FEA stiffness (N/m) Etitania* (GPa)

PDMS + 5 min plasma 0 0.72 ± 0.02

PDMS + TiO2

PDMS + TiO2

PDMS + TiO2

PDMS + TiO2

PDMS + TiO2

PDMS + TiO2

PDMS + TiO2

2h 8 1.52 ± 0.25 1.55 65

2.5 h 14 2.1 ± 0.5 2.2 65

3h 18 2.6 ± 0.2

3.5 h 22 4.7 ± 0.9 4.6 75

4h 36 5.5 ± 0.9

5h 60 6.6 ± 0.9

6h 70 8.5 ± 1.2

*

The value of E gives the indicated stiffness fit of FEA simulation to experiment.

performed (see experimental methods) at several spots on the sample. The average value is reported in Table 1. For thin films, substrate effects can be significant: their estimation is not simple and depends on a complex fashion on the film and substrate properties, as well as tip radius. For films below 200 nm thickness, these are difficult to quantify in a precise analytical fashion.15 The value reported in Table 1 refers to the measured modulus at an indentation depth of 50 nm. We expect the substrate effect to be small at this depth.35 Correction for substrate influence using a recently proposed model27 indicates that the substrate may lead to an overestimation of about 20% in the modulus value under these conditions. Since these films are much thinner than those discussed in the referenced work27 and since we must also consider the influence of surface roughness (which can lead to a significant lowering of the perceived modulus36,37), the modulus of these films is reported as measured, using standard Oliver and Pharr nanoindentation analysis, 23 with an uncertainty of 20%. Table 2 reports the measured stiffness obtained by averaging slopes of load−displacement curves from the nanoindentation. The experimental and associated FEA fit curves are shown in Figure 7. The increase in slope of the experimental curves as the thickness of the titania layer increases is striking. Table 2 also shows the values for E of the TiO2 layer derived from the

which can be detected for the given depth to values spanning approximately 2 orders of magnitude. The experimental stiffness depends on the elastic moduli, depth of deformation, and tip shape. A second issue is tip shape. Micromachined silicon probes typically have crystalline silicon tips whose very end shape is not well-defined. For purposes of nanomechanical measurements in AFM, this shape is often estimated as a semisphere. This approximation is only valid for shallow indentations, on the order of or below the tip radius. When a dedicated pyramidal tip is used at deeper indentations, the tip area function must be estimated.23 In this work, modulus values were measured for the titania film using dedicated instrumented nanoindentation and for the activated PDMS using AFM-based indentation. With these considerations in mind, AFM nanoindentation measurements were carried out as detailed in the experimental section. Force vs displacement curves measured on the composite surfaces were fit to a straight line to extract stiffness values, which were subsequently reproduced by FEA modeling. In addition, the modulus of the activated PDMS was measured as an input to the calculations. In order to measure the true surface mechanical properties and to prevent penetration of the indenter tip through the thin film, total deformations of the composite system were kept below 15 nm, which also kept the stiffness in a range appropriate for the spring constant of the cantilever used. The FEA simulations show that this deformation is primarily in the activated PDMS substrate, so the contact depth of the tip into the titania remains small, on the order of 1 nm or less. Table 1 records the moduli values determined for the substrate and film. Nanoindentations on the activated PDMS, limited to 10 nm indentation depth, provided a modulus value for input into the calculations. Independent determination of the elastic modulus of the LPD TiO2 film alone was performed by instrumented nanoindentation as a reference for comparison with the values subsequently estimated by FEA for the modulus of the titania film on the soft composite substrate. The solution deposition process is such that one might expect some minor variations in film properties for different substrates and different total film thickness. For these thin film samples, the ultimate contact depth is a trade-off between achieving sufficient depth to overcome local roughness and surface effects on the shallow extreme and limiting the influence of the Si substrate on the deeper extreme. Both CSM and quasistatic indentation were

Figure 7. FEA simulations (solid lines) fit to experimental data for three different titania film thicknesses: 8 (black squares), 14 (red triangles), and 22 (blue diamonds) nm deposited on activated PDMS. F



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best fit of the FEA to experiment. Two important conclusions can be drawn from the data: (1) the value of E derived from the FEA conforms well to the value determined in the independent experiments by nanoindentation of thin TiO2 layer on Si. We therefore conclude that the simple two-component model is able to reproduce the physical film characteristics, in particular, the mechanical behavior, quite well. (2) The derived value for E of the hard film is relatively invariant for the different overlayer thicknesses, i.e., knowing only substrate and film modulus and film thickness should allow the prediction of composite stiffness for new films of different material and geometric characteristics. We also note that, even with careful AFM imaging after nanoindentation with the 20 nN load, there is no evident residual surface deformation after the indentation experiment. This is not surprising considering the small contact areas computed by FEA. At full contact for the maximum load used on the 8 nm film, this area is only 26 nm2. From this, we deduce that the final radius of the indentation imprint is on the order of 2 nm after accounting for elastic recovery. The modulus value of the titania film estimated here is much lower than that of bulk titanium oxide, 220−280 GPa,38 but falls closer to the range of that observed for porous films.39,40 This can be attributed to the particulate structure of the LPD titania film leading to reduced film density. The RBS measurements show that the LPD titania density (the amorphous titania) is 0.657 × 1023 atoms/cm3, while the reported density for anatase titania is 0.868 × 1023 atoms/cm3. The consistent values of the best-fit moduli computed for the different films indicates that the composite properties can be understood from a knowledge of the component properties and film thickness alone. This should allow design and preparation of different kinds of these composite structures exhibiting desired properties. Trial FEA simulations using higher substrate modulus (40 MPa, which could be achieved experimentally by varying cross-linking conditions) result in a best-fit modulus value of 25−30 GPa for the titania film when fit to these same experimental curves. This result highlights the suggested versatility in design for such systems. Stiffness, the property investigated herein, depends strongly on indenter size, shape, and indentation depth and thus is not a fundamental material property. Nonetheless, the conditions used here, nanometer-scale contacts in the nN force regime, are those found in many nanomechanical systems. It is of interest to note that the force−distance data obtained here, subject to DMT analysis,31 would give effective moduli ranging between 100 MPa for the thin film to 600 MPa for the thick composite film modulus. Although this contact−mechanics model should only be applied to homogeneous and not to layered surfaces, these ballpark figures suggest the utility of such surfaces in controlling chemical and mechanical functionality of substrates at the nanoscale.

unique and separate contributions of each component is the possibility for independent control of chemical and physical properties. For instance, by simply changing film thickness, a series of samples can be obtained that have the same bulk properties and surface chemistry but different stiffnesses. Surface chemistry, however, could be modified without changing stiffness by using thin-film coatings such as selfassembled monolayers. The limits of this approach both in terms of its extension to polymers with different intrinsic mechanical properties and the responsiveness of different oxides to such thickness changes are currently under investigation.

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AUTHOR INFORMATION

Corresponding Author

*(C.N.S.) E-mail: [email protected]. Tel: +972-35318072. (S.R.C.) E-mail: [email protected]. Tel: +972-8-9342703. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support from the Minerva Center for Biomaterial Interfaces and the Edward and Judith Steinberg Chair in Nanotechnology at Bar Ilan University, the ATOL Foundation, and the GMJ Schmidt Minerva Center for Supramolecular Structures at the Weizmann Institute of Science are gratefully acknowledged.

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CONCLUSIONS Nanometric oxide thin films deposited from aqueous solution are a powerful tool for controlling the chemical and physical characteristics of a polymer interface. This work indicates that the stiffness of the surface of PDMS can be significantly and controllably varied by applying titania coatings of different thickness. Nanoindentation, together with FEA, allows accurate determination of the compliance of hard-on-soft composite materials. These results show that the nanomechanical behavior of the composite material can be accurately predicted from the properties of each component. An important ramification of the G



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Oxide Surfaces with Tunable Stiffness

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