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Origin of Hydrophobicity in FIB-Nanostructured Si Surfaces Alberto Rota,*,†,‡ Manoj Tripathi,‡,§ GianCarlo Gazzadi,§ and Sergio Valeri†,‡,§ †

Centro Interdipartimentale per la Ricerca Applicata e i Servizi nel settore della Meccanica Avanzata e della Motoristica - Università di Modena e Reggio Emilia, Via Vignolese, 905/b - 41125 Modena, Italy ‡ Dipartimento di Scienze Fisiche, Informatiche e Matematiche - Università di Modena e Reggio Emilia, Via Campi 213/A − 41125 Modena, Italy § CNR - Istituto di Nanoscienze, Centro S3, Via Campi 213/A − 41125 Modena, Italy S Supporting Information *

ABSTRACT: Surface morphology has been demonstrated to influence the tribological properties at different scales, but the phenomena which occur at the nanoscale have not been completely understood. The present study reports on the effect of focused ion beam nanopatterning on coefficient of friction (CoF) and adhesion of Si(001) surface covered by native oxide. Regular arrays of nanogrooves reduce both CoF and adhesion, related to a hydrophobic character of the patterned surface, but this effect disappears as soon as the separation among the nanostructures approaches the microscopic scale. The dependence of this hydrophobic effect on the pitch is not linked to the corresponding contact area. It has been found that each nanostructure is surrounded by a low-friction region which extends some hundreds of nanometers from it. For pitches of 125 and 250 nm these low-friction regions completely overlap, generating a consistent decrease in CoF and adhesion, while for pitches of 500 and 1000 nm their effect is negligible. The low-friction regions were not observed in humidity-free ambient, indicating that they are the origin of patterns of hydrophobicity.

I. INTRODUCTION The present-day interest in nanotechnologies is mainly related to the different physical and chemical properties of materials when they are confined to the nanoscale. Also in the case of tribology, scaling the contact area and the applied load down to the nanoscale, novel phenomena occur. These phenomena are strictly dependent on adhesion between mating surfaces, which is usually negligible for a load in the micro/macroscale, but that plays a dominant role in the nanoscale load range. The topography has been demonstrated to play an important role in the tribological properties of surfaces and most of the studies found a close connection between adhesion and contact area.1−6 The modification of the surface morphology has been approached by different techniques, depending on the material and on the size and geometry of the desired pattern: photolithography,2,4 replica molding,1,3,5 colloidal lithography,6 focused ion beam (FIB).7,8 The main conclusions that could be drawn are: (1) adhesion and friction decrease with the contact area,1−5(2) patterning is able to enhance the hydrophobicity in already hydrophobic surfaces and hydrophilicity in already hydrophilic ones,1,4 in agreement with the Wenzel model,9 and (3) in the nano-Newton load range, different tribological properties appear with respect to micro/macro-Newton ones,2 related to the role played by adhesion. Many of the cited researches investigated the wettability of the materials and its dependence on roughness, but a clear connection between wettability and adhesion is still lacking. In © XXXX American Chemical Society

fact, in some works, wettability shows the same dependence on the roughness observed for adhesion1−3,5 and in others it behaves differently.4 In particular, in this last reference, the authors show how wettability can be described by Wenzel, hemiwicking, or Cassie−Baxter models, depending on the relative values of the geometrical parameters. In apparent contrast with conclusion (1), in our previous studies,7,8 we showed that adhesion and CoF reduction of the patterned Si(001) surface does not depend on the contact area. In fact, through tests in air and in high vacuum, we concluded that the lower adhesion and CoF of the patterned regions are related to a hydrophobic effect induced by nanopatterning. These conflicting results are probably related to a nonexhaustive knowledge of tribology at the nanoscale and, in particular, of the relation among wettability, adhesion, and morphology. The ambiguity probably has origins in the different size and geometry of the studied patterns, which sensibly change in the related literature. In their work, Pham et al.5 concluded that the lower wettability of patterned PMMA with respect to pristine surface, as well as for adhesion, is related to the lower contact area. However, the authors also reported on an anisotropic behavior of wettability with respect to the orientation of the pattern, composed by parallel grooves. Received: November 26, 2012 Revised: March 28, 2013

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A similar phenomenon was observed by others authors.6 We think that this anisotropy is an indication that wettability cannot be related only to the contact area and that other morphology-dependent phenomena occur. In particular, we think that the dimensions of the nanostructures that compose the pattern (i.e., depth/height ratio, width, pitch) are crucial for the hydrophilic/hydrophobic properties of the surface. The present study aims to better define the borderline of nanotribological effects and the role of surface nanostructures in these phenomena. The system under investigation is an array of equally spaced parallel nanogrooves, 50 nm wide and some nm deep. Four different patterns were investigated, which differ from each other in the pitch (i.e., 125, 250, 500 and 1000 nm). In our previous research,7,8 we focused on a 125 nm pitch pattern, showing that it induces a hydrophobic character to the Si(001), a typical hydrophilic material in standard conditions due to the presence of native oxide on the surface.10 Here, we show that the periodicity of the pattern is crucial for its tribological behavior and that, at the nanoscale, the decrease in adhesion and CoF does not depend only on the contact area. In our last study, we have observed that the presence of the nanogrooves induces a hydrophobic character to the surface and that this effect disappears when the geometrical dimension of the array moves toward the microscale (i.e., increasing the pitch from 125 to 1000 nm). We have found that each nanostructure acts as a local “source” of hydrophobicity, and the interplay among all these structures leads to the resultant hydrophobicity on the patterns. Differently with respect to most of previous studies, all tribological tests were performed using a modified rounded Si tip, which better reproduces the operating conditions of Micro-Electro-Mechanical Systems (MEMS) and Nano-Electro-Mechanical Systems (NEMS) devices, where the typical coupling structures are both made of Si. This similarity should give more accurate information for the future development of such devices.

Figure 1. SEM images of (a) the patterns fabricated by means of FIB sculpting on Si(001) covered by native oxide (magnification 80k) and (b) the rounded AFM Si tip used for tribological tests (magnification 50k). spring constant) and 3.02 × 10−7 Nm (torsional spring constant) by following the procedure of Sader et al.11 B. Measurement Procedure. Nanoscale friction analyses were carried out using AFM in the friction detection mode (LFM), in a system equipped with a turbomolecular pump and an oil-free scroll pump for the measurement in humidity-free ambient. Before each test, the sample was cleaned in an ultrasonic bath for 10 min in acetone, followed by 10 min more in isopropanol. Thereafter, it was dried under nitrogen flux. The friction force was measured in contact mode at room temperature in ambient atmosphere and in a vacuum (10−5 Torr), with a scan speed of 10 μm/s and a sliding direction perpendicular to the orientation of the grooves. When dealing with a very small load and reduced contact area, friction tests are influenced by many hardly controllable phenomena, such as the contact area, temperature and humidity, sticking of impurities on the tip apex, cantilever mounting, position of the spot laser on the cantilever, tip−surface attack angle, etc. This leads to a pronounced variability in the experimental data, generating a very large uncertainty in the measurements, as evident in our previous publication.8 For this reason, we adopted a procedure to estimate adhesion and CoF, assuming the flat area which surrounds each pattern as a reference. In each friction test, the scanned area included the pattern and a portion of the neighboring untreated surface (see Figure SI.1 of the Supporting Information). In this way, in every horizontal line of the scan, the tip slides on both the patterned and unpatterned area, therefore it is possible to compare almost instantaneously the tribological properties of these regions, limiting the effect of boundary conditions. It is then possible to study the relative change of friction properties on the patterns with respect to the untreated Si surface. Along the slow scan direction, the applied load was decreased down to the detach point, and the corresponding torsion of the cantilever was measured (see graphs in Figure SI.1 of the Supporting Information), in order to obtain the lateral force versus applied load ( f riction curve). To further limit the error bars, each friction test was repeated many times, and the results were averaged using the weighted averaging procedure.

II. EXPERIMENTAL SECTION A. Sample and Tip Preparation. Si(001) single crystal was used as base material in the present study for its very large use in MEMS/ NEMS fabrication. The crystal was P doped, and the resistivity was 1500−3000 Ω cm. The surface was covered by native Si dioxide related to air exposure, and this layer was not removed intentionally to better reproduce the operative conditions of Si-based microdevices. For the present study, four different patterns were fabricated by means of focused ion beam (FIB) milling, using a 30 keV Ga+ ion beam at normal incidence; the ion dose was 2.25 × 1016 ions/cm2. See also ref 7 for more details. The patterns consist of parallel nanogrooves 50 nm wide with variable pitches of 125, 250, 500 and 1000 nm; the depth of the grooves range between 2 and 4 nm, depending on the pitch. The desired in-plane geometry was checked in situ by SEM analysis (Figure 1a), while three-dimensional (3D) analysis of the patterns were performed ex situ by AFM, using a standard tip (radius of curvature less than 10 nm) (refer to the Topographical Analysis for a detailed description). For tribological investigation, a commercially available Si probe (VEECO MPP-21100) was modified to increase the contact area. The tip was flattened and rounded by FIB, and then polished by repeated scans over MgO-cleaved surface, varying the load and scan direction. Figure 1b is the front view of the rounded tip used for tribological investigation, whose radius of curvature has been estimated to be 2.89 μm. The size of the apex provides a large contact area with respect to the width of the nanogrooves, to be more sensitive to the global effect of the array on the tribology of the surface and to limit the topographical contribution to the torsion of the cantilever. The spring constants of the cantilever were calculated to be 5.48 N/m (flexural

III. RESULTS AND DISCUSSIONS A. Topographical Analysis. The AFM imaging inspection of the patterns, by means of a standard tip, reveals that the border of each groove swelled up, a well-known effect related to FIB-induced Si amorphization.12,13 The combination of swelling and ion milling generates a periodic topography composed by alternation of grooves and protrusions, which depends on the separation among the grooves. Figure 2 shows the average height profile of each pattern; the apex of an ideal spherical tip (spherical cap) with curvature B

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Figure 2. Vertical line profile relative to the different kind of pattern: 125 nm (black line), 250 nm (red line), 500 nm (blue line), and 1000 nm (purple line). The bottom of the grooves has been set to zero on the vertical axis. The lateral section of a spherical cap with radius of curvature 2.89 μm has been reported as tip approximation (black thick line). It is important to point out the different vertical and horizontal scales.

Figure 3. Graphs of lateral force vs load on patterns of different pitch (red ○) and on the corresponding flat regions (black □), acquired in air.

radius of 2.89 μm is also plotted. Related to the very different xand y-axis scales (about 3 orders of magnitude), the contour of the tip appears elongated. In Figure SI.2 of the Supporting Information, the same graph is shown, keeping the same scale for the x and y axis, in order to evidence the shape of the tip. Focusing on the profile of the 1000 nm pitch pattern, it is clear how the swelling effect occurs: at the very early stage of ion processing, the crystal is subject to amorphization and this lower-density region is pushed by the surrounding higherdensity crystalline material, inducing the formation of a protrusion; the further ion irradiation leads to surface milling and the consequent generation of the grooves, in the middle of the swollen region. The in-plane extension of swelling (30 nm) on the sides of each groove is related to the extension of the ion beam tails in the current intensity profile. The final shape of the pattern, as evident from the graph, is an array of grooves with upturned borders. For the 1000 nm pitch, the depth of the grooves, measured from the top of the borders, is 2 nm and the borders are about 1.6 nm high with respect to the untouched area. When the pitch of the pattern decreases from 1000 to 125 nm, the above-described topography evolves, due to the superposition of ion-induced swollen areas at the borders of each groove. In the case of 500 nm-pitch the grooves are 3 nm deep with 1.5 nm high borders; for the 250 nm pitch they are 3 nm deep with 0.5 nm high borders, and finally, for 125 nmpitch the borders are no longer recognizable due to their superposition and the grooves result 4 nm deep. In consideration of the radius of curvature, it is evident that the tip is not able to enter into the grooves. It is important to underline that in the case of 500−1000 nm pitches, the rounded tip is able to discriminate the topography related to the presence of the protrusions, but already at 250 nm (and consequently for 125 nm) it is not so and the resultant topographical image appears almost flat. B. Lateral Force Analysis. In Figure 3, the typical graphs of the lateral force versus applied load are reported for all patterns. The starting load is between 27 and 134 nN and progressively decreases down to the detach point, which is different for all the patterns depending on both the corresponding adhesion and CoF. The load range intentionally falls in the adhesive regime, in order to be more sensitive to capillary forces and, consequently, to the hydrophobic effect of FIB patterning already discussed in our previous work.8 In addition, the selected load range should limit the wear of the

patterns. From the graphs, the lateral force appears linearly dependent on the applied load in all cases, evidence of the multiasperity contact regime. It is important to notice that the friction curves on the flat regions show a pronounced variability, concerning both the slope and the vertical coordinate. We have attributed this variability to the difficulty in reproducing exactly the same testing conditions in terms of contact area between the tip and the surface, apex cleanness, humidity and temperature. In particular the tip−surface contact is considered to be the major source of uncertainty because it can change during the measurement campaign or even during the same scan. For this reason, each friction test on a pattern has always been referred to the corresponding surrounding flat region (see the description of the testing procedure in Experimental). For this reason, we have calculated at every load, the reduction of the LF of the patterned region with respect to the corresponding flat one, for every pattern: Lateral force reduction = [1 − LFpattern /LFflat]

The output of this procedure is shown in Figure SI.3 of the Supporting Information. The analysis of the curves shows that, practically for every load, the smaller the pitch the larger the LF reduction. From this first evidence, it is clear that the geometry of the pattern plays a crucial role in the tribological properties. However, even if the LF is the direct physical observable and the most important quantity in the practice, the study of the tribological properties through it can be misleading and sometime even erroneous because this quantity is dependent on both CoF and adhesion. For this reason, a detailed analysis of CoF and adhesion will be reported in the following. C. Adhesion Measurements. The load value at which the LF goes to zero in the friction curve corresponds to the Dynamic adhesion (see ref 14). In the present case, related to the simultaneous friction measurements on both the flat and patterned regions, the two friction curves are truncated at the same load value, which corresponds to the detachment on the regions which exhibits the lower adhesion. Consequently, to determine the dynamic adhesion, we have calculated the intercepts of the linear fits of the friction curves with the x axis. The corresponding dynamic adhesion versus pitch is reported in Figure 4a for the flat and for the patterned areas, C

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Figure 4. The graphs show the dynamic and static adhesion measured in air as a function of the pitch and the corresponding reduction refereed to the flat area: (a) dynamic adhesion, (b) dynamic adhesion reduction, (c) static adhesion, and (d) static adhesion reduction. In graph (d), the trend of the contact area is reported (continuous line), based on a simple model sketched in the inset and described in the text. In graph (c), the values related to a uniformly irradiated region and the corresponding flat, labeled “Ga+” and “Flat (Ga+)”, respectively, are reported.

progressively decreases with an increase in separation among the grooves, down to 8% at the 1000 nm pitch. On the basis of the topographic analysis of the patterns above-reported, the relative adhesion increment with pitch could be attributed to the related larger contact area, as depicted in most of the previous studies. However, the presence of nanostructures can induce a hydrophobic character to the surface as well, responsible for a further decrease in adhesion.2,3,8,15 The influence of surface morphology in hydrophilic/hydrophobic properties is also deducible in the work of Pham et al.,6 who showed that the wettability of nanogrooves on the PMMA surface is anisotropic with respect to the orientation of the grooves. In our opinion, the presence of this anisotropy indicates that the reduction in adhesion is not only related to the reduction of the contact area but also to the surface morphology itself, which prevents the formation of water capillary. To quantify the contribution of the contact area in our adhesion tests, we have modeled the system with a simplified morphology: the patterns have been approximated by boxlike grooves separated by flat regions and the tip apex by a flat square of 100 × 100 nm2 lateral dimensions (a sketch of the model is reported in the inset of Figure 4d). The dimensions of the tip in the model have been chosen based on the estimated dimensions of the real rounded tip during the experiments; the square shape has been chosen for simplicity. With the assumption that adhesion is only dependent on the contact area, the corresponding static adhesion reduction is represented by the continuous line in panel (d). The theoretical data does not match the experimental ones, being lower for 125 and 250 nm pitches and higher for 500 and 1000 nm pitches. This discrepancy could be related to the very raw simplification of the system, as both the patterns and the tip morphology are actually very different, but our opinion is that the trend of experimental data in panel (d) is related to the different hydrophobic character of the different patterns. In particular, results show that the pattern hydrophobicity decreases when the pitch moves from the nano- to the microscale. We have repeated the tests in humidity-free ambient to support this thesis. In Figure SI.4 of the Supporting Information, the friction curves for 250 and 500 nm pitches and the corresponding flat ones are reported, as examples. In

as the weighted average of many independent friction tests. It results evident that adhesion on the patterns is always lower than on the corresponding flat regions (note that adhesion is negative, as it is opposite to the applied load). It is important to notice that the value on the flat portions of the scanned area is not invariant during the experimental campaign. As already discussed, this is related to the variability in the tip−surface contact area and the reason why we have included the flat regions in each test. Taking the flat regions as reference, it is possible to calculate the dynamic adhesion reduction, as the relative decrease of adhesion when moving from the flat to the patterned surface: Dynamic adhesion reduction = [1 − Adh pattern /Adh flat]

The output of this calculation is shown in Figure 4b. The data in panel b are the results of weighted averaging of all the dynamic adhesion reduction values obtained from each independent friction test. With this procedure, the effect of contact variability has been limited. Adhesion reduction quantifies the effect of each pattern on adhesion and depends only on the pattern geometry. Its value is between 1 and 10%, but the uncertainties do not enable to speculate about the influence of the pitch. For this reason, we have measured the static adhesion as well, as the value of jump-off in the force−distance (FD) curve. A series of FD curves were collected on each pattern and in the surrounding flat region, each time without retracting the tip. This procedure was used in order to minimize the experimental error and the tip−surface contact variability. The corresponding average static adhesion values are reported as a function of pitch in Figure 4c. Also in this case, the value on the flat regions is always larger with respect to the corresponding pattern but not invariant. To quantify the contribution of each pattern in the decrease of adhesion, we have again calculated the static adhesion reduction. The output is reported in Figure 4d. In this case, the uncertainties are the results of error propagation from the absolute value of static adhesion displayed in Figure 4c. The reduced experimental uncertainties enable some considerations about the dependence of adhesion on the pattern geometry: it is evident how the effect of patterning on the adhesion reduction is more pronounced for the 125 nm pitch, being 20%; the effect D

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Figure 5. The graphs shows the dynamic and static adhesion measured in dry conditions as a function of the pitch, and the corresponding reduction refereed to the flat area: (a) dynamic adhesion, (b) dynamic adhesion reduction, (c) static adhesion, and (d) static adhesion reduction. In (d), the trend of the modeled contact area is reported (continuous line).

Figure 6. Coef f icient of friction vs pitch on the patterns (red ○) and on the surrounding flat regions (black □), measured in air and in dry conditions. The corresponding CoF reductions are reported in the bottom panels (blue △): (a) CoF absolute values in air, (b) CoF reduction in air, (c) CoF absolute values in dry conditions, and (d) CoF reduction in dry conditions. In (a), the values related to a uniformly irradiated region and the corresponding flat, labeled “Ga+” and “Flat (Ga+)”, respectively, are reported.

The static adhesion has been measured as well, and the results are reported in Figure 5c. The absolute values on the flat and patterned regions are very closed, except for the 125 nm pitch pattern. In Figure 5d, the static adhesion reduction calculated with respect to pristine areas is reported. The reduction is about 60% on a 125 nm pitch, dropping at 12% at 250 nm and stabilizing around 6−4% for higher pitches. This decreasing trend is well-fit by the simplified model of the contact area described above (continuous line), except for the 125 nm pitch. The agreement between the experimental data and the model, indicates that the adhesion on the patterns in vacuum is strictly related to the tip−surface contact area, as expected. We attribute the discrepancy observed for the 125 nm pitch to the simplification of the model, which is based on the assumption that the tip is flat and the patterns are composed by alternation of flat protrusion and flat grooves. The real morphology of the rounded tip and patterns, composed by alternation of rounded asperities and grooves, implies a smaller contact area, thus justifying the discrepancy. These tests and analyses confirm that the adhesion decrease on a nanostructured surface can be related, besides to a reduction of the contact area, to a hydrophobic character

this case, there is no effect of patterning on adhesion, as the curves of patterned and unpatterned regions almost overlap. Anyway, adhesion is still present, as it is necessary to apply a negative force to detach the tip from the surface as well, for all the patterns. This adhesion cannot be related to capillary forces because the vacuum condition ensures the almost total desorption of water from the surface. As described above, the linear fits of each friction curve and the corresponding intercepts with the x axis have been calculated to estimate the dynamic adhesion. The results are reported in Figure 5a. Comparing the absolute values obtained in dry conditions with that in air (Figure 4a), it is soon evident that in this second case, adhesion is about 1 order of magnitude lower. This is related to the absence of the water capillary, which enhances the adhesion between the tip and the surface in air. The absolute value on the patterns and on the surrounding flat regions is almost the same within the uncertainties, also evident from the calculation of the dynamic adhesion reduction in panel (b). However, the uncertainty in panel (b) does not enable to speculate about pitch dependence. E

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Figure 7. Average topographic line profiles on the 1000 nm pitch pattern (black line) and the corresponding TMR friction signal (blue line) related to: (a) air and (b) humidity-free ambient. The vertical red lines mark the correspondence between frictional and topographic features. The dashed circles mark the discontinuities in TMR, which correspond to the collision of the tip with the nanostructures. In the graphs, both TMR and height profiles have been vertically shifted in order to set to zero the level of the flat portion.

is about 20−25%; the effect of patterning completely disappears at 500 and 1000 nm, as the CoF of the patterns is similar to that on the corresponding flat regions. This means that, similar to adhesion, the hydrophobic effect of patterning is strictly related to the dimensionality of the texture, being dominant when the pitch is far from the microscopic scale. However, differently with respect to the adhesion case, the value of CoF reduction does not sensibly change from 125 to 250 nm, and it drops at 500 nm around 0%, maintaining this value also for the 1000 nm pitch. In this case, we can identify a discontinuity in the CoF reduction between 250 and 500 nm, a sort of transition from the nano- to the microscale. To the best of our knowledge, this is the first time that the transition from the nano- to microtribology has been observed. Similar to that described above for adhesion, the effect of FIB patterning on the CoF is absent in a humidity-free ambient. In fact, as evident from the friction curves in SI.3 of the Supporting Information, the slope related to the patterns is the same for the corresponding flat regions. The measured CoF of the patterns in vacuum, plotted in Figure 6c, is between 2.9 and 3.6, varying the pitch. The corresponding graph of the CoF reduction is shown in Figure 6d. Within the experimental error, the CoF reduction is almost absent and does not depend on the pitch in humidity-free ambient, supporting what was already discussed about the hydrophobic effect of patterning. This is further evidence of the fact that nanopatterning is able to change the hydrophobic/hydrophilic properties of the SiO2− Si surface. E. Trace-Minus-Retrace Profile Analysis. To better understand this hydrophobic effect of the nanostructures and the role of the pitch, we have analyzed the trace-minus-retrace (TMR) signal on the patterns and the corresponding topographic profile acquired during the friction tests, averaged along all the applied loads. In Figure 7, the corresponding profiles for the 1000 nm pitch pattern have been reported for air (panel a) and dry (panel b) conditions. Focusing on panel (a), it is evident from the topographic profile that the rounded tip is not able to resolve the grooves and the resulting protrusions appear larger than their real width because of the convolution with the tip. The height of the protrusion is smaller with respect to what was measured in the topographic tests (see Figure 2), related to the wear induced by tip sliding. Although the tip has a quite large extension, it is able to resolve the flat regions among the

induced by surface morphology. In the presence of air, it is this second effect which dominates at the nanoscale and which induces a decrease in CoF and adhesion on the Si surface (see also refs 7 and 8). The possibility to generate this hydrophobicity probably depends on the chemical and crystallographic characteristics of the pristine surface, and we cannot exclude that it can be influenced by the patterning technique. The dependence of the induced lower adhesion on the separation among the nanostructures, indicates that the geometrical characteristics of the pattern play a fundamental role. For the studied geometry, the smaller the pitch the higher the hydrophobic effect. In fact, as soon as the pitch approaches the micrometric dimension, this effect progressively disappears. With respect to most of previous findings, where the waterrepellent effect of patterning was restricted to already hydrophobic materials, the present study shows that it is possible to inhibit the formation of a water capillary even on hydrophilic surfaces, such as native oxide-covered Si(001). D. Coefficient of Friction Analysis. The friction curves reported in Figure 3 show the linear dependence of LF on the load. The angular coefficient of the linear fit represents the CoF. The CoFs are plotted as a function of the pitch with those corresponding to the flat regions (Figure 6a). Each value is the result of the weighted-averaging statistical procedure of independent friction tests. The CoF of the patterned region is 1.4 for the 125 nm pitch; it decreases for the 250 and 500 nm pitches, and it increases again up to about 1.3 for the 1000 nm pitch. In analyzing this trend, similar to the case of adhesion, it is important to observe that the values of the flat regions sensibly vary. On the basis of this consideration, the absolute value of the CoF has a relative meaning, as its value can sensibly change in different experimental runs. Consequently, to bypass this complication, we have calculated the CoF reduction of each pattern with respect to the corresponding flat region, following the same approach used for LF and adhesion, for every individual friction test: CoF reduction = [1 − CoFpattern /CoFflat]

The output of this procedure is a series of single values of CoF reduction, one for every independent friction test. These values have been averaged by means of a weighted-averaging procedure, and the results are plotted in Figure 6b as a function of the pitch. At the 125 and 250 nm pitches, the CoF reduction F

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swelling, based on some considerations and strongly supported by dedicated experimental tests, which are described in the following: (i) The observed low-friction regions extend some hundreds of nanometers from the swelled borders. In order to determine if there is an effect of ion bombardment in a region so far from the patterned area, we performed Monte Carlo simulations using the TRIM code and shooting 30 keV Ga ions perpendicular to a Si surface. The results show that both ion implantation and structural damage have lateral and in-depth extensions which do not exceed 40−50 nm in length, so they cannot be the origin of the observed low-friction regions. (ii) In addition, the implantation profile shows that ion concentration at the surface is negligible, while it has a maximum at about 25−30 nm below the surface. This suggests that the topmost layers of the flat and of the patterned regions should present similar chemical characteristics, being constituted by amorphous SiO2. (iii) The swelled regions clearly reveal that the FIB-treated regions are characterized by different mechanical properties, as their material density is lower than the untouched Si. However, as already described, the friction tests were performed in the adhesive regime, where the influence of the mechanical properties of the surface on tribology should be less important. (iv) If the different mechanical properties of the patterns would influence the tribological properties, this influence should be present both in air and in dry conditions. On the contrary, in dry conditions, both adhesion and CoF are the same on the patterns and on the flat surface. To support these statements a 10 × 10 μm2 square area of Si surface was uniformly irradiated at the same conditions used to fabricate the nanogrooves. The resulting surface consists of a square depression about 3.5 nm deep, reported in Figure SI.5 of the Supporting Information with the corresponding average line profile. The morphology of the treated area is almost flat (RMS = 0.4 nm), thus excluding any contribution of topography on the tribological properties. With the use of the same procedure described in Measurement Procedure, adhesion and CoF have been estimated and compared to the flat Si surface. The results show that both CoF and adhesion on the uniformly irradiated area do not differ with respect to the untreated region (see markers labeled “Ga+” and “Flat (Ga+)” in Figures 4c and in 6a. The results of these tests exclude the fact that the hydrophobic properties ascribed to the patterns can be related to any kind of secondary FIB effect, like Ga implantation or amorphization. To understand the origins of the observed phenomena further investigations are necessary, consisting in the study of the tribological properties of patterns fabricated with a different technique, such as Electron Beam lithography.

protrusions, as expected from its radius of curvature. In viewing the TMR profile (blue line), it is soon evident that in correspondence with each protrusion, the signal shows a reversed peak, which means a minimum in the friction force. What has to be noticed is that each TMR minimum is larger than the width of the corresponding protrusion: in fact, in proximity of the nanostructure, but still on the untreated surface, the friction signal starts to decrease. On the nanostructure, the TMR further decreases down to its minimum, which corresponds to the position where the tip is exactly on the groove. From this analysis, it follows that each protrusion is surrounded by a low-friction region which extends in the flat area and that cannot be related to any geometrical effect. The superposition of these regions generates a lowfriction level, which extends along the entire pattern. In fact, looking at the TMR profile in panel (a), the level on the untreated region on the very left (labeled A) is higher than the value found among the protrusions (labeled B). With careful analysis of each TMR reversed peak, a sort of discontinuity is evident at each left and right side (marked by the dotted circles), which corresponds to a sudden increase of friction. This discontinuity occurs at the position where the tip hits the border of the protrusion, generating the so-called “collision effect”,16 on the left during trace and on the right during retrace. This is further evidence that the low-friction region between two protrusion starts well before the tip encounters the nanostructures. The extension of this region is about 700 nm from the side of the swollen border. It is straightforward that when the pitch decreases, the coupling between neighboring low-friction regions becomes more and more effective, up to a saturation value that we expect not too far from what was observed for the 125 nm pitch pattern. The situation is different in the case of dry conditions, reported in Figure 7b. In this case, the average topographic profile still shows protrusions separated by flat regions, but in the corresponding TMR profile there is no evidence of the above-described low-friction regions surrounding the nanostructures. In this case, the lateral extremes of the reversed peaks in TMR exactly correspond to those of the protrusions and are related to the decrease in the contact area. In addition, the TMR level in the regions among the nanostructures is the same as that in the untreated region. Also in this case, but less evident due to the higher average friction, the discontinuity related to the collision effect is recognizable by the side of the reversed peaks. These detailed analyses of the TMR signal further support the thesis which relates the lower friction on the pattern to a hydrophobic effect induced by the nanostructures, where the contact area contribution is very limited. These results are not in contrast with what was already reported regarding the ability of patterns in decreasing the wettability related to air pocketing and well-modeled by Cassie−Baxter theory. However, we suggest that there are other phenomena that inhibit the adsorption of water and the formation of capillary layer at the surfaces. We have shown that these phenomena are strictly related to every single nanostructure and that the combination of them in an array can lead to an evident change in the tribological properties of materials. The physical−chemical phenomena which are at the basis of this hydrophobic character of the SiO2−Si(011) surface are not known at the moment. We can exclude the influence of secondary effects related to FIB patterning, such as ion implantation or ion-induced

IV. CONCLUSIONS The present study shows that regular arrays of FIB-patterned nanostructures, consisting of parallel nanogrooves with upturned borders, are able to modify the tribological properties of the Si(001) surface, depending on the periodicity of the grooves. In particular, when the separation among the grooves falls in the nanometric scales (i.e., 125 and 250 nm), both CoF and adhesion are lower with respect to the flat surface. This effect is related to a hydrophobic character of the patterned surface, as it has not been observed in humidity-free ambient. Approaching the microscale (i.e., for the pitches of 500 and 1000 nm), this effect disappears. In particular, for the CoF, the transition from the nano- to the microregimes is abrupt and takes place between 250 and 500 nm. The origin of these G

dx.doi.org/10.1021/la304684f | Langmuir XXXX, XXX, XXX−XXX

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(7) Marchetto, D.; Rota, A.; Calabri, L.; Gazzadi, G. C.; Menozzi, C.; Valeri, S. AFM Investigation of Tribological Properties of NanoPatterned Silicon Surface. Wear 2008, 265, 577−582. (8) Marchetto, D.; Rota, A.; Calabri, L.; Gazzadi, G. C.; Menozzi, C.; Valeri, S. Hydrophobic Effect of Surface Patterning on Si Surface. Wear 2010, 268, 488−492. (9) Wenzel, R. N. Resistance of Solid Surfaces to Wetting by Water. Ind. Eng. Chem. 1936, 28, 988−994. (10) Maboudian, R.; Howe, R. T. Critical Review: Adhesion in Surface Micromechanic Structures. J. Vac. Sci. Technol., B 1997, 15 (1), 1−20. (11) Sader, J. E.; Chown, J. W. M.; Mulvaney, P. Calibration of Rectangular Atomic Force Microscope Cantilevers. Rev. Sci. Instrum. 1999, 70, 3967−3969. (12) Rota, A.; Contri, S. F.; Gazzadi, G. C.; Cottafava, S.; Gualtieri, E.; Valeri, S. Focused Ion Beam Induced Swelling in MgO(001). Surf. Sci. 2006, 600, 3718−3722. (13) Lugstein, A.; Basnar, B.; Hobler, G.; Bertagnolli, E. Current Density Profile Extraction of Focused Ion Beams Based on Atomic Force Microscopy Contour Profiling of Nanodots. J. Appl. Phys. 2002, 92 (7), 4037−4042. (14) Carpick, R. W.; Agraıt, N.; Ogletree, D. F.; Salmeron, M. Measurement of Interfacial Shear (Friction) with an Ultrahigh Vacuum Atomic Force Microscope. J. Vac. Sci. Technol., B 1995, 14 (2), 1289−1295. (15) Cheng, Y.-T.; Rodak, D. E. Is the Lotus Leaf Superhydrophobic? Appl. Phys. Lett. 2005, 86, 144101−144103. (16) Sundararajan, S.; Bhushan, B. Topography-Induced Contributions to Friction Forces Measured Using an Atomic Force/Friction Force Microscope. J. Appl. Phys. 2000, 88 (8), 4825−4831.

effects is not related to the different contact area between the tip and the patterns but to the presence of low-friction regions which surround every single nanogroove, which disappears in humidity-free ambient. These hydrophobic regions completely overlap for 125 and 250 nm pitches, inducing an evident decrease of the CoF and adhesion. Tests on a uniformly FIBirradiated region, at the same conditions used for pattern generation, exclude a detectable contribution from secondary FIB effects, such as ion implantation or crystal amorphization.

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ASSOCIATED CONTENT

S Supporting Information *

Figure SI.1 shows the methodology used in the friction tests. Figure SI.2 represents the same graph reported in Figure 2, but the x- and y-axis scales are the same. The shape of the spherical cap, reported as tip approximation, is more appreciable. Figure SI.3 is the graph of the lateral force reduction versus load. Figure SI.4 reports the graphs of the lateral force versus load for 250 and 500 nm pitches in dry conditions. Figure SI.5 reports the AFM image of the region uniformly irradiated by FIB and the corresponding line profile. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. These authors contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present research was funded by Centro Interdipartimentale per la Ricerca Applicata e i Servizi nel settore della Meccanica Avanzata e della Motoristica - INTERMECH MO.RE., located at the Faculty of Engineering “Enzo Ferrari”, University of Modena and Reggio Emilia, and by Regione Emilia Romagna, Italy.

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REFERENCES

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dx.doi.org/10.1021/la304684f | Langmuir XXXX, XXX, XXX−XXX