1 Capturing Wetting States in2 Nanopatterned Silicon3 XiuMei Xu,†,* Guy Vereecke,† Chang Chen,†,‡ Geoffrey Pourtois,†,§ Silvia Armini,† Niels Verellen,†,‡

4 Wei-Kang Tsai,^ Dong-Wook Kim, ) Eunsongyi Lee, ) ChangYou Lin,‡ Pol Van Dorpe,†,‡ Herbert Struyf,†

5 Frank Holsteyns,† Victor Moshchalkov,‡ Joseph Indekeu,‡ and Stefan De Gendt†,‡

6†IMEC, Kapeldreef 75, Leuven 3001, Belgium, ‡KU Leuven, Leuven 3001, Belgium, §Department of Chemistry, PLASMANT, University of Antwerp, B-2610 Wilrijk, Belgium,

7^University of Southern California, Los Angeles, California 90089, United States, and )Department of Physics, Ewha Womans University, Seoul 120-750, Korea

89

The wettability of a surface is often

10 characterized by water contact an-11 gles. On an ideal smooth surface, the12 static water contact angle depends on the13 interfacial energy balance and is well de-14 scribed by Young's equation.1�6 On the15 other hand, on a rough substrate, the wet-16 ting behavior can be quite different and17 varies from superhydrophilic to superhydro-18 phobic.2,3,5,7�11 Among numerous applica-19 tions of different wetting regimes, reliable20 wet cleaning of nanopatterned Si wafers is21 an extremely important issue in modern22 technology. In the end, it is one of the23 essential factors defining the limits of24 the powerful modern Si technology as it25 has been advancing and scaling along26 Moore's law. The realization of the hydro-27 philic regime plays a crucial role here and28 potential incomplete wetting of the nano-29 patterned Si surfaces could impede the30 applicability of the Si technology due to

31the absence of reliable wet cleaning at the32nanoscale.33The influence of topography and artificial34surface modulation on wetting is normally35studied by plotting the apparent contact36angle on a rough substrate (θ*) as a function37of the contact angle on a flat surface (θ).38Three distinct wetting states have been39observed:7,12 (i) the superhydrophilic hemi-40wicking state in which water spontaneously41penetrates the grooves and travels beyond42the apparent drop;5,13,14 (ii) Wenzel's model43describing another complete wetting state44in which water fills the surface landscape45underneath the drop;3,13�15 (iii) the super-46hydrophobic Cassie�Baxter state, some-47times referred to as the lotus effect, and48often characterized by a large apparent49contact angle and small hysteresis due to50air trapping underneath the drop.3,5,9,11,12,15

51Superhydrophobic properties are desired52for applications such as self-cleaning or

* Address correspondence toxiumei@imec.be.

Received for review October 28, 2013and accepted December 31, 2013.

Published online10.1021/nn405621w

ABSTRACT Spectacular progress in developing advanced Si

circuits with reduced size, along the track of Moore's law, has been

relying on necessary developments in wet cleaning of nanopat-

terned Si wafers to provide contaminant free surfaces. The most

efficient cleaning is achieved when complete wetting can be

realized. In this work, ordered arrays of silicon nanopillars on a

hitherto unexplored small scale have been used to study the wetting

behavior on nanomodulated surfaces in a substantial range of

surface treatments and geometrical parameters. With the use of

optical reflectance measurements, the nanoscale water imbibition depths have been measured and the transition to the superhydrophobic Cassie�Baxter

state has been accurately determined. For pillars of high aspect ratio (about 15), the transition occurs even when the surface is grafted with a hydrophilic

functional group. We have found a striking consistent deviation between the contact angle measurements and the straightforward application of the

classical wetting models. Molecular dynamics simulations show that these deviations can be attributed to the long overlooked atomic-scale surface

perturbations that are introduced during the nanofabrication process. When the transition condition is approached, transient states of partial imbibition

that characterize intermediate states between the Wenzel and Cassie�Baxter states are revealed in our experiments.

KEYWORDS: nanoscale wetting . transient states . Cassie�Baxter . Wenzel . hemiwicking . optical reflectance spectroscopy .molecular dynamics simulations

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CXXXX American Chemical Society

53 drag reduction.16,17 In contrast, for applications that54 require good wetting, mixing, or transport in between55 the structures, such as wafer cleaning in micro- and56 nanoelectronics fabrication processes,18 superhydro-57 phobicity should be avoided.19,20 Therefore, it is of58 utmost importance to identify the transition criteria59 between different wetting states. In thermodynamic60 equilibrium, the stable wetting state corresponds to61 the one with the minimum surface free energy. A62 simple calculation allows one to derive the transition63 thresholds between different wetting states as a func-64 tion of the geometrical factors.3,5,8,14,21 Equations 1�565 give the transition criteria for different wettingmodels.

66 Hemiwicking:

cosθ�h ¼ fs cosθþ 1 � fs, (0 < θ < θc(1)) (1)6768

69 Wenzel:

cosθ�W ¼ r cosθ, (θc(1) < θ < θc

(2)) (2)7071

72 Cassie�Baxter:

cosθ�C � B ¼ fs cosθ � 1þ fs , (θ > θc(2)) (3)

7374 where,

θc(1) ¼ cos�1((1 � fs)=(r � fs)) (4)

θc(2) ¼ cos�1((fs � 1)=(r � fs)) (5)

7576 For hemiwicking and Cassie�Baxter states, the ap-77 parent drop lies on top of a composite interface and78 the relevant geometrical factor is the top solid surface79 fraction, fs. For theWenzelmodel, the apparent contact80 angle depends on the ratio of the total surface area to81 the projected area, r.82 An analysis of contact angle measurements in terms83 of the classic wetting models requires the geometrical84 factors in eqs 1�5 to be given a priori, which is quite85 challenging for many natural22�24 or artificially synthe-86 sized surfaces with nonperiodic patterns.7,25,26 The87 development of microfabrication processes allows88 one to obtain artificial ordered arrays of structures with89 well-defined geometries.15,21,27,28 The reported con-90 tact angles are often observed to be higher than those91 predicted by the Wenzel model, and sometimes a92 Cassie�Baxter state is observed when the classic mod-93 els give a stable Wenzel state.26�28 The postulated94 reasons for these discrepancies are inaccurate estima-95 tions of geometrical factors,25,26 contact line pinn-96 ing,26,29 air trapping facilitated by the nonuniform97 structure profiles,30�32 or hierarchical surface topogra-98 phy.24,33 In a metastable Cassie�Baxter state, there is99 an energy barrier in the weakly hydrophobic regime,100 and the transition to the Wenzel state can be triggered101 spontaneously by capillary pressure34�36 or by external102 pressure.34 The investigation of the wetting transition103 is a very active research area, crucial, among other104 things, for modern nanotechnology. In this work, we105 use silicon nanopillars with well-defined profiles to

106study thewetting behavior influenced by nanopattern-107ing, and the wetting state is determined accurately by108using novel optical-based reflectance measurements.109Molecular dynamics simulations are carried out to110investigate the wetting mechanism.

111RESULTS AND DISCUSSION

112Structures. An ordered array of nanopillars is etched113into a 300 mm silicon wafer.37 The pillar arrays are114square packed with a 90 nm pitch (p) and the pillars115have diameters (d, 33�43 nm) and heights (h, 70�116460 nm) (Figure 1 F1). The uniform profiles allow us117to determine accurately the geometrical factors in118eqs 1�5) (cf. Supporting Information.

119Surface Treatments. Tomodify the intrinsic wettability120of the nanopillars, different treatments are used to121functionalize the silicon surface. All aqueous-based122treatments are ruled out because the high surface123tension of water can cause pattern collapse of the124nanopillars. Most of the surface functionalization is125performed in the vapor or gas phase, including UV126O3 cleaning, vapor HF etching, vapor phase deposition127of decamethyl tetrasiloxane, dodecamethyl pen-128tasiloxane and self-assembled monolayers of CUTS129(11-cyanoundecyltrichlorosilane), PETS (phenethyl-130trichlorosilane), BUTS (11-bromoundecyltrichlorosi-131lane) and FDTS (1H,1H,2H,2H-perfluoro-decyltrichloro-132silane). To have a well distributed coverage of the133range of water contact angles, monolayers of PEO134(methoxy(polyethyleneoxy)propyltrimethoxysilane),135MPTMS (3-mercaptopropyltrimethoxysilane) and APTMS136(3-aminopropyltrimethoxysilane) are also grafted on137nanopillars in anhydrous toluene. To minimize the138influence of surface contamination, all samples are139prepared freshly and the surface treatments and mea-140surements are performed in a class 1 cleanroom141environment at 35% humidity and 22 �C. Details of

Figure 1. SEM images of the silicon nanopillars. (a) Tiltedview of the square packed pillar arrays; a top view is shownin the inset. (b�f) Cross-sectional SEM images of nanopillarsof aspect ratio (AR) 2, 3, 7, 13 and 15, respectively. The scalebars are 50 nm.

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142 the surface treatments are given in Supporting143 Information.

144 Contact Angle Measurements. The topographic influ-145 ence on wetting is shown in Figure 2F2 , where the146 apparent water contact angles on the nanopillars (θ*)147 are plotted against the contact angles on flat silicon148 wafers with the same surface functionalization (θ). The149 theoretical curves of the hemiwicking (solid curves),150 Wenzel (dashed curves) and Cassie�Baxter (dotted151 curves) states are plotted using the geometrical factors152 listed in Table S1 in Supporting Information. Our data153 show three wetting regimes. I: for contact angles (θ)154 less than 55�, the data follow the trend of hemiwicking155 wetting. Since the top solid surface fractions are similar156 for all arrays, the apparent contact angles are alike for157 pillars of different heights. II: for 55� < θ < 80�, there is a158 sharp increase in the apparent contact angles. Wen-159 zel's model predicts that the increase in the total160 surface area should enhance wetting. However, the161 apparent contact angles we measured are consistently162 much higher than those on flat surfaces. In this regime,163 the pillar height strongly affects the contact angle data,164 but there is a trend break in its effect on wetting.165 For 55� < θ< 65�, wetting is enhanced by an increase in166 the surface area and for taller pillars the apparent167 contact angles are lower, while for 65� < θ < 80�, the168 wettability diminishes with increasing pillar height. III:169 for θ > 80�, the apparent contact angles exceed 140�,170 and there is good agreement with the Cassie�Baxter171 model.172 Remarkably, the dependence of θ* on θ in Figure 2173 is insensitive to the surface treatments that have been174 used. When comparing the self-assembled monolayer175 deposition of CUTS with vapor HF etch treatment, we

176find that both of them give very similar results on177planar silicon wafers as well as on nanopillars despite178the very different surface functionalization mechan-179ism. Therefore, we can rule out the influence of chain180length and orientation of the self-assembled mono-181layer molecules on the contact angle measurements.182In our experiments, the pillars have uniform profiles183and there is no apparent surface roughness in SEM184images with subnanometer pixel resolutions. In order185to determine whether the deviations observed in the186contact angles are due to air trapping underneath the187water drops, some quantitative inspection techniques188are needed. In the literature, measurements of contact189angle hysteresis are themost commonly usedmethods190to distinguish Cassie�Baxter from Wenzel states of191wetting.38�41 However, there is no agreed-uponmodel192for the contact angle hysteresis, and the reported193hysteresis can vary quite a lot depending on the sur-194face topography and treatments.24,39�41 Interference195microscopy has been used to measure the liquid196penetration and the meniscus profiles underneath197the drop,42 but this technique is applicable only when198the substrate is made of transparent material and the199patterned structures are on the micrometer scale.200Acoustic reflection techniques enable quantitative201wetting depth measurement for micrometer size202pillars.43 To the best of our knowledge, there are no203inspection techniques that could be used for quanti-204tative wetting depth measurements in nanostructures.205Optical Reflectance Measurements. Nanostructures of-206ten exhibit unique optical properties due to the in-207herent light confinement, e.g., silicon nanopillars of208different dimensions can display a variety of colors in209air due to the variations in the guided light modes.44

210Since the diameter and the pitch of the pillars used in211this study are very small, the resonant guided mode212appears in the short wavelength range (<400 nm).213Thus, multiple beam interference between the top214and bottom interfaces of the pillar array dominates215the reflectance spectra in the visible range. When the216air surrounding the nanopillars is partially or comple-217tely replaced by the water, the difference in refractive218index can result in a big change in the optical reflec-219tance spectra. Figure 3 F3a shows the finite difference220time domain (FDTD) simulation results of the reflec-221tance response when the instantaneous water imbibi-222tion depth is assumed to be homogeneous and it223increases from 0% (Cassie�Baxter state) to 100%224(hemiwicking or Wenzel state). In the simulations, the225nanopillars are modeled as cylinders with dimen-226sions corresponding to AR 15. There are some reso-227nant modes present due to the constructive228interference of beams reflected at different inter-229faces (cf. Supporting Information). The resonant230wavelengths are very sensitive to any change in231refractive index, and thus can be used to determine232the water imbibition depths.

Figure 2. The apparent contact angle (θ*) on nanopillars asa function of the contact angle (θ) measured on a flat siliconwafer. Symbols are measurements for pillars with differentaspect ratios (AR) and curves are theoretical predictionsfor hemiwicking (solid lines), Wenzel (dashed lines) andCassie�Baxter models (dotted lines). Note the theoreticalcurves calculated for each AR are plotted with the samecolor as the corresponding symbols and some curves over-lap due to the closeness in the geometrical factors. Pictureson the right (i�vi) give the side view photographic images ofsessile drops (Φ∼ 1.5mm) on pillars of AR 15, correspondingto themarkedmeasurement points (blue squares) in the plot.

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233 Figure 3b shows themeasured reflection spectra for234 nanopillars of AR15 in different wetting regimes. Pillars235 with UV O3 cleaning have the most hydrophilic surface236 termination (Figure 2i), and thus are used as the237 reference for complete water penetration. There are238 two resonant dips in the reflectance spectra corre-239 sponding to the first and the second order modes.240 Compared with the FDTD simulations of complete241 wetting (dashed curve), there is a slight mismatch242 (about 10 nm) in the resonant wavelengths, which243 might be due to the curvature at the bottom of the244 pillars as can be seen from the SEM images (Figure 1).245 For pillars with MPTMS (Figure 2ii), CUTS (Figure 2iii)246 coatings, despite the very different apparent contact247 angles, the measured spectra are identical to the ones248 obtained with UV O3 treated pillars. This indicates that249 in wetting regime II, there is a Wenzel state although250 themeasured contact angles aremuch higher than the251 Wenzel predictions. In wetting regime III, both BUTS252 (Figure 2v) and FDTS (Figure 2vi) deposited pillars give253 apparent contact angles above 145�. In the reflectance254 spectra, as compared to the ones for pillars with255 hydrophilic functional groups, a red-shift (or blue-shift)256 is observed for the first (or the second) order inter-257 ference mode. There is excellent agreement with the258 simulated spectrum of 2% water imbibition (dash-259 dotted curve), and therefore a Cassie�Baxter state is260 indeed reached in wetting regime III.261 With this knowledge of the actual wetting states, it262 can be seen from Figure 2 that the measured Wenzel263 curves are shifted toward the hydrophilic regime. From264 the extrapolations of the contact angle measurements,

265the Wenzel to Cassie�Baxter transition happens when266the contact angle θ is around 80� for AR 15 pillars. Near267the transition, the free energies for Wenzel and Cassie�268Baxter states are close to each other and interest-269ing dynamic features could arise. By varying the de-270position conditions, the contact angles on a PETS271coated flat silicon wafer can be fine-tuned in the range272of 77�82�, which is close to the transition. The mea-273sured apparent contact angles on AR 15 pillars range274from 128� (Figure 2iv-1) to 146� (Figure 2iv-2). When275the apparent contact angles are low (Figure 2 iv-1), the276optical reflectance spectrum shows a Wenzel state of277wetting (as shown in Figure 3b). For PETSwith θ around27882� (Figure 2iv-2), although the apparent contact279angles are similar to those in the Cassie�Baxter states,280we observe a gradual breakdown of the superhydro-281phobic state after immersing the sample in water, and282the water imbibition process can take tens of minutes.283The inset of Figure 3c is a top view microscope image284showing the coexistence of different wetting states on285a larger length scale. The bright area in the image286corresponds to the place where the Cassie�Baxter287state is long-lived (as confirmed by the reflectance288spectrummeasurements), while in the dark area water289imbibition is taking place. Different degrees of water290penetration have been measured during the experi-291ments, from completewetting as in theWenzel state to292partial penetration. Figure 3c shows one of the partial293penetration spectra measured with PETS deposited294pillars, in which only one resonant dip is observed.295There is excellent agreement with the simulated spec-296trum for 22% water penetration.

Figure 3. (a) FDTD simulations showing a contour map of the reflectance as a function of wavelength and the percentage ofwater imbibition depth, in which red denotes low reflectance and dark blue high reflectance as indicated in the color bar.A homogeneous instantaneous water penetration depth (hp) is assumed in the simulation as shown in the inset. (b) Stackedreflection spectra measured on aspect ratio 15 pillars with different surface treatments (solid lines i�vi). The correspondingside view images ofwater drops on such surfaces are given in Figure 2i�vi. The simulated spectra at 100% (dashed curve) and2% (dash-dotted curve)water imbibition are also plotted for reference. (c) Near the transition (the contact angle θ is about 82�in Figure 2), partial water penetration has been measured (solid line iv-2), and the dashed curve is a simulation of 22%waterpenetration (thin line in (a)). The inset is amicroscope image showing the coexistence of differentwetting states, in which thebright area corresponds to the Cassie�Baxter state and the dark area denoteswhere partial water imbibition takes place. Thescale bar is 50 μm. The array consists of 80 � 80 μm2 subarrays of nanopillars.

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297 This partial water penetration is transient and298 evolves to complete wetting in a time span of tens of299 minutes. Figure 4F4 a shows the reflectance spectra mea-300 sured at different locations after the PETS coated pillars301 (AR 15) are immersed in water. All the spectra are302 collected at locations where the color is uniform. Thus,303 good agreements with FDTD simulations, which as-304 sume homogeneous wetting depths, can be obtained.305 The lines in the FDTD simulations in Figure 4bmark the306 corresponding wetting depths for different spectra in307 Figure 4a.308 For pillars with lower aspect ratios (AR 2�7), the309 transition to the superhydrophobic Cassie�Baxter310 states occurs at larger contact angles (θ), as the critical311 angle in the transition criterion eq 5 increases with a312 decrease in surface area (r). For pillars of all aspect313 ratios, the measured transition criteria are consistently314 lower than the theoretical criteria in eq 5, as can be315 seen from the shift of themeasurements in theWenzel316 states (wetting regime II) in Figure 2.317 Molecular Dynamics (MD) Simulations. The optical reflec-318 tance measurements show that the transition to the319 Cassie�Baxter state actually happens before the the-320 oretical criterion eq 5 is met, and for AR 15 pillars it is321 even found to take place in the hydrophilic regime.322 Many publications report a metastable Cassie�Baxter323 state before the transition criterion is met, which is324 thought to be related to the energy barrier in the325 weakly hydrophobic regime.21,34,45 However, for struc-326 tures like ours, without reentrant profiles, there is no327 energy barrier in the hydrophilic regime that prevents328 water from penetrating and wetting most of the

329sidewall surfaces.21,31,40 In order to reveal the underly-330ing mechanism for the discrepancy between the the-331oretical models and the experiments, 3D molecular332dynamics simulations have been carried out using the333LAMMPS package.46

334A hexagonally packed lattice is employed to build335both planar and pillared surfaces.47,48 A hexagonal336honeycomb lattice is specifically chosen in order to337avoid building cylindrical pillars with many facets, as338corners will result inmore contact angle hysteresis. The339atoms on the substrates are fixed and separated by3401.42 Å, and the distance between adjacent layers is3413.4 Å. By tuning the amplitude of the Lennard-Jones342(LJ) potentials for the atoms in the solid (εs), the interac-343tion energy with water is modified and substrates with344different wettability can be simulated (cf. Supporting345Information). Contact angles on a smooth surface are346determined by fitting the periphery of an equilibrium347water drop in the density field.49 Cylindrical pillars with348different dimensions are packed in a square array, and349the pitch p is fixed to 30 Å. To avoid any influence from350the large capillary pressure of a tiny droplet (Φ ∼ 6 nm)351on the wetting behavior, a layer of equilibrated water352molecules is initially placed on top of the pillars and353periodic boundary conditions are applied to account for a354uniform array. Table 1 T1compares the equilibratedwetting355states after 10 ns obtained for pillars with different356diameters (d), heights (h) and wettability (θ). The theore-357tical transition criterion angle θc, obtained using eq 5, is358found to be in good agreement with the simulation.359The MD simulations reproduce the predictions of360classical models and show that the wetting transitions

Figure 4. Partial and transient water penetration on PETS coated pillars (AR 15, Figure 2(iv-2)). (a) Stacked reflection spectrameasured at different locations after the sample is immersed in water. (b) Lines on FDTD simulationsmark the correspondingwetting depths for spectra in (a).

TABLE 1. Summary of the Equilibrium Wetting States Calculated with MD Simulations for Pillars of Different Size and

Wettability

d (Å) h (Å) p (Å) θc θ = 34� θ = 66� θ = 93� θ = 100� θ = 110� θ = 118�

20 90 30 96� Wenzel Wenzel Wenzel C�B C�B C�B10 100 30 102� Wenzel Wenzel Wenzel Wenzel C�B C�B

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361 are properly described by eq 5. In the experiments,362 there is, however, a consistent shift in the con-363 tact angles, implying an early transition to the Cassie�364 Baxter state. In theMD results given in Table 1, the pillar365 surface has the same atomic arrangement as the planar366 substrate; thus, the interaction energywith water is the367 same for both surfaces. On the other hand, during368 the fabrication, there are inevitable modifications of369 the pillar surface with respect to the planar silicon370 wafer. One possible source of surface alterations arises371 from the difference in crystal plane orientations, which372 can result in very different contact angles.50 Compared373 with the (100) plane on a planar silicon wafer, the374 surface of the studied nanopillars consists of mixed375 crystal planes such as (110), (111) etc., which are well-376 known to have different atomic densities and surface377 energies. Another possible surface modification of the378 pillars could reside in atomic-scale lattice defects379 induced by the fabrication process. Commercially380 available Si planar wafers are polished and the381 surfaces are characterized by atomically smooth382 terraces, on the other hand there are more surface383 defects on nanostructures due to random ion bom-384 bardments or atomic scale inhomogeneous chemi-385 cal etching. To account for these effects in MD386 simulations, we implement two types of surface387 alterations. One relates to the change in surface388 density of an atomically smooth surface and is389 realized by increasing (or decreasing) the in-plane390 lattice constant by 10%. The other surface alteration391 is introduced by randomly creating 20% (or 50%) of392 atomic vacancies in the top layer.393 The impact of these surface modifications on wet-394 ting is first studied on planar surfaces. To quantify their395 influence, we calculate the surface energy defined as396 the excess free energy per unit area of creating a397 surface from bulk material.51 Figure 5F5 shows the sur-398 face energy (a) and contact angles (b) calculated on399 planar substrates as a function of the LJ well depth εs.400 By increasing εs, the surface energy of the substrate as401 well as the water�substrate intermolecular interaction402 energy (εij, cf. Supporting Information) are augmented,403 which results in a lower contact angle and better

404wettability. For different atomic arrangements, due to405the variation in the effective surface density, the sur-406face energies are different. Increasing the lattice con-407stant or the number of atomic vacancies lowers the408effective surface density. As a result, the surface energy409andwettability are reduced as reflected by the increase410in contact angle. When the contact angles are plotted411as a function of the surface energy, the results for412different surface modifications fall on a universal curve413as shown in Figure 5c.414The aforementioned surface modifications also af-415fect wetting on pillars. In the MD simulations shown in416Table 2 T2, the Lennard-Jones potentials of the atoms in417the solid are kept constant and the atomic arrange-418ments are varied on the pillar surface. The surface419packing densities, atomic vacancies in the top layer,420surface energies and contact angles on the flat surface421with the same modifications are compared. Although422the implemented surface modifications are all at the423atomic scale, the impact on surface wettability is quite424significant as can be seen from the sessile drop profiles425and contact angles in Table 2. Snapshots showing the426evolution of the watermolecule positions on the pillars427are taken at fixed time intervals. In Figure 6 F6, the428complete wetting time is plotted as a function of (a)429surface energy and (b) contact angle on a flat surface430with the same atomic characteristics as the pillar sur-431face (θ). The vertical solid line in Figure 6(b) marks the432transition between the Wenzel and Cassie�Baxter433states calculated with eq 5. When the surface energy434and wettability of the pillar surface are reduced, the435wetting speed is lower due to a weaker driving force.436The wetting time increases exponentially rapidly437when the transition is approached. This is in line438with our experiments, in which a slow water imbibi-439tion is observed near the transition and a transient440intermediate state in between the Wenzel and441Cassie�Baxter states is captured by the optical reflec-442tance measurements.443In the experiments (Figure 2), planar silicon wafers444are used to characterize the contact angles on the445modified surface. However, as compared to planar446silicon wafers, additional modifications could be

Figure 5. For planar substrateswith different surfacemodifications, the surface energy (a) and contact angles (b) can be quitedifferent even for the same atomic interaction energy εs. (c) The contact angles fall onto a single curvewhenplotted versus thecalculated surface energy.

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447 introduced to the pillar surface during the nanofabri-

448 cation process. These also affect the packing density of

449 the surface functional groups in the following surface

450 treatment steps. As a result, a shift in the measured

451 contact angles is observed in Figure 2.

452CONCLUSIONS

453For investigations ofwetting at the nanoscale, one of454the main drawbacks is the lack of a reliable metrology455to evaluate the actual wetting state in nanostruc-456tures. In this work, novel optical-based reflectance

TABLE 2. MD Simulations of Wetting on Pillars (d = 20 A, h = 90 A, p = 30 A) at Different Time Intervalsa

a The LJ potential well depth, εs = 0.05 kcal/mol. The surface lattice density, atomic vacancy fractions in the top layer, surface energy and contact angles on the modified flatsurfaces are compared.

Figure 6. In theMD simulations, for pillars (d, 20 Å; h, 90 Å; p, 30 Å) with different lattice constant scaling, the time needed forcomplete wetting is plotted against (a) the surface energies of the pillar surface in planar form, and (b) the contact anglesmeasured on a flat surface with the same lattice constant as the pillar surface. The theoretical transition criterion calculatedusing eq 5 is shown as the vertical solid line in (b).

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457 measurements are utilized to determine quantitatively458 instantaneous water imbibition depths and to define459 the actual wetting state on nanopillars. The wetting of460 nanopillars with different dimensions and surface461 functionalization is studied systematically. Three re-462 gimes are encountered, corresponding to the hemi-463 wicking, Wenzel and Cassie�Baxter states.464 A systematic deviation between the theoretical pre-465 dictions and the measurements is observed. In some466 cases, the transition to the superhydrophobic Cas-467 sie�Baxter state is found to happen even in the468 hydrophilic regime. Molecular dynamics simulations469 suggest the deviation could be due to atomic scale470 surface modifications of the pillar surface. In the nano-471 fabrication process, these are inevitable and include,472 among others, different crystal plane orientations or473 atomic-scale surface defects. Compared to a planar474 wafer surface, themodified pillar surface in planar form475 can have very different surface energy and wettability.476 The fabrication-induced surface modifications also477 affect the packing density of the surface functional478 groups in a consistent way. As a result, the dependence479 of θ* on θ is universal with respect to the surface480 treatments used. Near the transition between the481 Wenzel and Cassie�Baxter states, the wetting speed482 is significantly reduced, and transient intermediate483 states with partial water imbibition in nanostructures484 can, remarkably, be captured by optical reflectance485 measurements.486 Our results provide an incentive to revisit some of487 the early works in the light of the impact of surface488 modifications on wetting properties of micro- and489 nanomachined structures, as similar surface topography490 modifications can be introduced by etching (applicable491 for crystal structures and metals) or can be transferred492 from the template molds (applicable for soft materials).493 Awareness of such potential influence is particularly494 important for the study of the metastable Cassie�Baxter495 state and the forced transition to the Wenzel state.496 Conflict of Interest: The authors declare no competing497 financial interest.

498 Supporting Information Available: Details of structure di-499 mensions, contact angle measurements, optical reflectance500 measurements, FDTD simulations and molecular dynamics501 simulations are given. This material is available free of charge502 via the Internet at http://pubs.acs.org.

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