Tuning Surface Wettability at the Submicron-Scale: Effect of Focused

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Tuning Surface Wettability at the Submicron-Scale: Effect of Focused Ion Beam Irradiation on a Self-Assembled Monolayer Yutaka Yamada,† Koji Takahashi,*,†,‡,§ Tatsuya Ikuta,‡ Takashi Nishiyama,‡ Yasuyuki Takata,†,§,∥ Wei Ma,† and Atsushi Takahara†,⊥ †

International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, Fukuoka 819-0395, Japan Department of Aeronautics and Astronautics, Graduate School of Engineering, Kyushu University, Fukuoka 819-0395, Japan § Japan Science and Technology Agency (JST), CREST, Kyushu University, Fukuoka 819-0395, Japan ∥ Department of Mechanical Engineering, Graduate School of Engineering, Kyushu University, Fukuoka 819-0395, Japan ⊥ Institute for Materials Chemistry and Engineering, Kyushu University, Fukuoka 819-0395, Japan ‡

ABSTRACT: Realizing surface wettability tuning at the submicron-scale resolution is expected to enable the fabrication of micro/nano-structured fluidic devices and is particularly important in nanobiotechnology and high-resolution printing. Herein, we propose an approach to modify the wettability of self-assembled monolayer surfaces using focused ion beam (FIB) irradiation. The contact angle of the irradiated region changed from hydrophobic to hydrophilic by increasing the ion dosage. The chemical composition and associated depth profile of the sample surfaces were analyzed by glow discharge−optical emission spectroscopy. The results indicated that the content of fluorine at the surface decreased after FIB irradiation of the samples. A submicron-scale hydrophobic−hydrophilic hybrid surface was then fabricated by forming hydrophilic dots with diameters of ∼110 nm on a hydrophobic surface by FIB irradiation. The difference in wettability of the hydrophobic and hydrophilic areas on the surface was confirmed by microscale condensation and evaporation experiments. Condensed droplets with diameters of ∼300 nm appeared on the surface according to the fabricated pattern, thus suggesting that condensation preferentially occurred on the hydrophilic dots than on the hydrophobic surface. Furthermore, tiny droplets remained on the hydrophilic dots following evaporation of the larger droplets. The current approach provides a means to control wettability-driven phenomena.

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fabricate thin film or self-assembled monolayers (SAMs).9−11 Additionally, the surface wettability of macroscopic regions can be controlled by nano- and microscopic geometrical structures through the Wenzel and Cassie−Baxter states.12,13 Hydrophilic materials are used as antifogging surfaces because these materials form liquid films on surfaces and preserve light transmission.14 Boiling heat transfer can be enhanced by using hydrophilic surfaces, which delay vapor film formation.15 In contrast to surfaces with uniform wettability, which have been well studied, hydrophobic−hydrophilic hybrid surfaces have only recently attracted much attention16−20 owing to their potential in inkjet printing16 and biological applications such as cell adhesion.17 To date, spatial control of wettability at the microscale has been achieved by several fabrication techniques. For example, Wu et al.18 employed a photolithography technique to construct hydrophilic regions on hydrophobic surfaces by changing the surface potential. Varanasi et al.19 used a microcontact printing technique. By contacting a patterned mold coated with a hydrophobic material to a surface, the

INTRODUCTION Surface wettability plays an important role not only in determining solid−liquid interfacial properties,1 but also in phase change phenomena.2,3 Wettability is evaluated by measuring the static contact angle of a liquid droplet on a surface, as defined by Young’s equation, and is governed by the interfacial energy between solid, liquid, and gas phases. More detailed wettability characterization can be obtained by measuring the advancing and receding contact angles of a surface.4 In general, surfaces are simply classified as hydrophobic or hydrophilic. Particularly, those exhibiting extremely low and high wettability are termed as superhydrophobic and superhydrophilic, respectively. Such properties are important in various applications. For example, many insects and plants possess inherent superhydrophobic surfaces that enable removal of attached water droplets from their surface, as represented by the lotus effect.5,6 This knowledge has been applied to develop self-cleaning surfaces.7 Similarly, superhydrophilic surfaces are used as antifouling surfaces for organic fouling control.8 The hydrophobicity of a surface can be increased by using materials with low surface energies such as fluorine-based compounds (e.g., polytetrafluoroethylene). Such materials can be applied to a surface by dip or spray coating to © XXXX American Chemical Society

Received: September 16, 2015 Revised: December 8, 2015

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DOI: 10.1021/acs.jpcc.5b09019 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C hydrophobic material at the contacting region was transferred to the surface. The fabricated hybrid surfaces examined to date feature not only line-type patterns, but also several other types of patterns, such as ring and regular lattice.21−24 However, the width of each region obtained using these methods is usually several tens of micrometers. Alternatively, dip-pen nanolithography can be employed to realize nanoscale-resolution patterning.25,26 In this technique, the required materials are deposited on a surface using a cantilever tip. However, such a technique is not suitable for thermal applications as the deposited materials will act as a thermal resistance. Therefore, a considerably finer patterning procedure that does not necessitate material deposition is more desirable for application in thermal engineering including submicron-scale droplet research, high-resolution printing, nanobiotechnology, and electronics fabrication. Accordingly, the present paper describes an approach to construct submicron-scaled hydrophilic regions on hydrophobic surfaces using focused ion beam (FIB) irradiation. This procedure has been used to modify the morphology and mechanical property of surfaces by sputtering.27−30 Additionally, it has been used to create patterns on SAM surfaces without any masks.31−33 For example, Rieke et al. performed SAM layer etching and deposited different types of SAMs and minerals on the etched region.31 Droplet condensation at the 10 μm-scale on the surface was additionally observed. However, the full potential of such an approach in producing much finer patterns without redeposition of materials is yet to be explored. Furthermore, such a technique can afford near-flat patterning unlike conventional fabrication techniques, and importantly, does not require a template. The formation of the submicronscaled hydrophilic regions is confirmed by the occurrence of preferential condensation of water droplets on these regions.

Figure 1. (a) Chemical structure of FOPA. (b) Formation of a SAM. (c) Scanning electron microscopy (SEM) image of a region irradiated with focused ion beam (FIB) (bright area) at a dosage of 4.1 × 1015 ions/cm2; the scale bar is 500 μm. (d) Configuration of grid pattern formed by FIB irradiation.

15 nA. The effect of irradiation voltage on wettability was investigated by preparing two additional samples that were irradiated at voltages of 5 and 1 kV using ion currents of 0.48 and 6.7 nA, respectively. The irradiation dosage used for all these additional samples was 0.4 × 1015 ions/cm2. All prepared samples were stored in a desiccator prior to further investigation. Surface wettability of the samples was evaluated by measuring the static contact angle θ of a droplet using the sessile droplet method. A droplet of deionized water (less than 100 nL), obtained from a water purifier system (RFP742HA, Advantec, Japan), was dropped from a needle tip on the pristine and FIB-irradiated FOPA surfaces. Side-view images of the droplets were captured by a charge-coupled device camera and θ was estimated using eq 1 as follows:

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EXPERIMENTAL SECTION To investigate surface wettability, a surface with very low roughness was used because wettability is affected by surface roughness and structure. The samples were prepared as follows: Silicon substrates with a few nanometer-thick naturally oxide layer (Sumitomo Material Co., Japan) were cleaned by ultrasonication in acetone for 30 min, dried in a stream of dry air, and treated with O2 plasma (PlasmaPrep2, Gabler Labor Instrument GmbH, Germany) at 50 W for 40 min. The flow rate of O2 was set at 70 sccm. The substrates were then dipped in a 1 mM solution of 1H,1H,2H,2H-perfluoro-noctylphosphonic acid (FOPA; Dojindo Laboratories, Japan) in ethanol (99.5%) for 1 h and then annealed at 120 °C for 1 h under atmospheric conditions. FOPA, the chemical structure of which is shown in Figure 1a, combined with the oxide substrate to form a SAM, as illustrated in Figure 1b. Then, the prepared samples were irradiated using a Ga+ FIB system equipped to a scanning electron microscope (Versa 3D, FEI, The Netherlands) at a FIB dosage of 0.01 × 1015, 0.15 × 1015, 0.4 × 1015, 2.4 × 1015, or 4.1 × 1015 ions/cm2. The irradiation voltage of Ga+ ions was set at 30 kV, and the ion current was set at 100 pA except for the dosage of 0.01 × 1015 ions/cm2 where the ion current was set at 49 pA. Figure 1c shows a representative region irradiated with FIB. The region was constituted of 630 rectangles of 60 × 40 μm2, which were scanned by the ion beam. The size of the entire irradiated area was 1.26 × 1.2 mm2, which was adequate for contact angle measurements. To determine the effect of irradiation current on wettability, two additional samples were prepared with ion currents of 0.5 and

θ = 2 tan−1(h/r )

(1)

where h and r are the droplet height and contact radius between the liquid and solid phases, respectively.34 The surface structure and roughness of the samples before and after FIB irradiation were investigated using atomic force microscopy (AFM; SPM-8000FM, Shimadzu Co. Ltd., Japan), operating at a noncontact mode. The chemical composition and associated depth profile of the sample surface before and after FIB irradiation was characterized by glow discharge−optical emission spectroscopy (GD−OES; GD−Profiler2, Horiba Co. Ltd., Japan).35 Neon plasma was used to sputter atoms at the surface, and luminescence from the sputtered atoms was detected to characterize the elements. In this investigation, the sputtering rate for silicon was set at 6 nm/s, and FOPA layer was sputtered using the same plasma intensity. To investigate the submicron-scaled wettability patterning on the substrates, a grid pattern of hydrophilic dots on the FOPA surfaces were prepared by FIB irradiation. Such a pattern is the most simple shape among those reported in past research studies.21−24 The dosage, current, and acceleration voltage were set at 4.1 × 1015 ions/cm2, 1.6 pA, and 30 kV, respectively. During the fabrication process, the position of each irradiation dot was controlled within the nanoscale resolution by the operation software. The interval distance of the FIB irradiation was set at 500 nm (Figure 1d). Additionally, a point-by-point scanning FIB irradiation was performed, whereby irradiation is B

DOI: 10.1021/acs.jpcc.5b09019 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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ions/cm2; however, θ remained unchanged at ∼85° with further dosage increase (inset (ii) of Figure 2a). This value is larger than that displayed by SiO2 (i.e., ∼ 20°).37 These results suggest that part of the FOPA SAM remained after FIB irradiation. Thus, complete removal of the FOPA layer would require a higher ion dosage. Furthermore, the observed contact angles suggest that the chemical composition of FOPA SAM was modified upon collision with the high-energy ions.38 Figure 2b shows the relationship between θ and the ion beam current. As observed, θ was independent of the ion current and remained constant at 85−90° across the ion current range studied. The ion current reflects the irradiation energy per unit time, and thus the temperature of the surface is expected to rise as a result of ion collision.30 However, the obtained contact angles remained unchanged, thus suggesting that the temperature of the irradiated region did not rise to ∼500 °C, which is the onset temperature at which desorption of phosphonic acid from oxide surfaces is observed.39 Figure 2c depicts the dependence of θ on the acceleration voltage of the ion beam. As observed, θ decreased as the voltage increased, thereby indicating that ions irradiated at a higher voltage damaged the surface to a greater extent owing to their higher energy. The surface roughness, Ra, of the samples was investigated by AFM. Figure 3, parts a and b, shows topographic images of

performed at one specific region/dot at a time. Microscale condensation and evaporation experiments were then conducted using environmental scanning electron microscopy (ESEM; Versa 3D, FEI, The Netherlands). To control the sample temperature, the sample was attached to a copper sample holder and then mounted on a Peltier cooling stage. Before the condensation experiment, the water vapor pressure in the ESEM chamber was kept at less than 100 Pa. The temperature of the substrate surface was set at 0.0 °C. This condition was maintained for more than 10 min to ensure that the surface temperature was at the set temperature. Then, the vapor pressure was increased until droplets condensed on the surface. Evaporation behaviors were observed by decreasing the vapor pressure after the appearance of a droplet, which was large enough to cover the patterned surface. The sample surface was tilted at 80° relative to the horizontal plane to observe the tiny droplets. The electron beam voltage and current were set at 10 kV and 12 pA, respectively, to prevent sample heating.36

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RESULTS AND DISCUSSION Prior to evaluating the wettability of the FIB-irradiated surfaces, a pristine FOPA surface was characterized (inset (i) of Figure 2a). The contact angle, θ, of this surface was estimated as 104 ± 2°, which is characteristic of a hydrophobic surface. The effect of FIB irradiation dosage on θ is summarized in Figure 2a. As observed, θ decreased as the dosage increased to 0.4 × 1015

Figure 3. Atomic force microscopy (AFM) topographic images of (a) pristine FOPA and (b) FIB-irradiated FOPA surface. (c) AFM image of FIB-irradiated dots on a FOPA sample. A FIB dosage of 4.1 × 1015 ions/cm2 was used. In this image, the interval between each FIB irradiation spot was set at 1.5 μm as an example. The acceleration voltage was set at 30 kV for (b) and (c). The scale bars in (a), (b), and (c) are 200 nm, 200 nm, and 1 μm, respectively.

pristine FOPA SAM on a Si substrate and the sample following FIB irradiation at a dosage of 2.4 × 1015 ions/cm2, respectively. The estimated Ra value of the surface decreased from 0.57 to 0.25 nm following FIB irradiation. However, the observed difference in surface wettability would not be caused by changes in the surface geometry. Figure 3c shows the topographic image of the FIB-irradiated dots on the sample. The diameter of the dots, characterized by full-width half-maximum, was ∼110 nm. The FIB-irradiated area is a few nanometers higher than the surrounding surface. This height difference can also be observed at the edge of the FIB-irradiated surface in Figure 3b. This result may be attributed to trapped Ga+ ions and atomic vacancies, which are introduced into the SAM and/or substrate by FIB irradiation.40,41 The chemical composition and associated depth profile of the pristine and FIB-irradiated FOPA samples were investigated by GD−OES. Figure 4, parts a and b, depicts the GD−OES profiles of the samples. Though the vertical axis is related to the detected signal intensity, it cannot be used to assess the content of the different composition elements because the signal intensity from an atom depends on the type of element. The horizontal axis is the sputtering time, which corresponds to the

Figure 2. Dependence of the contact angle of the FIB-irradiated FOPA surface on (a) dosage, (b) ion current, and (c) acceleration voltage of the FIB. The insets in (a) show the droplets with the specified contact angles. C

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Figure 4. Glow discharge−optical emission spectroscopy (GD−OES) signals from (a) fluorine and phosphorus atoms of SiO2 (curves (i)), pristine FOPA (curves (ii)), and FIB-irradiated FOPA surface (curves (iii)). (b) Ga signals from pristine FOPA and FIB-irradiated FOPA surface detected by GD−OES. (c) Schematics of the surface structure of each sample.

sample surface (Figure 4c, image (iii)). Furthermore, the signals detected from the outermost surface were comparable in the pristine and FIB-irradiated FOPA samples. This result suggests that the irradiated Ga+ ions did not attach to the surface. The difference in wettability between the submicron-scale FIB-irradiated dots and pristine regions was confirmed by condensation experiments; droplet nucleation behaviors are known to differ on hydrophilic and hydrophobic surfaces. This effect is explained by the difference in the nucleation rate on different surfaces according to heterogeneous nucleation theory.43 Accordingly, the nucleation rate, I, is defined as follows:

depth profile. Fluorine and phosphorus atoms were detected on the surface of the sample owing to the coating of FOPA SAM on the Si substrate (Figure 4c, image (ii)). For the pristine FOPA surface, a fluorine peak appeared at ∼0.003 s followed by a phosphorus peak at ∼0.005 s (Figure 4a, curves (ii)). This result is consistent with the chemical composition and depth profile of FOPA, shown in Figure 1, parts a and b, as fluorine atoms present on the surface induce hydrophobicity, whereas the hydroxyl groups attached to phosphorus bind to the oxide surface.42 Here, because the sputtering rate of FOPA is unknown, determining the actual location of these atoms is difficult. However, the obtained result suggests that the sputtering rate of FOPA layer is in the order of 100 nm/s because signals corresponding to FOPA were detected promptly, and the thickness of the FOPA layer is expected to be 1−2 nm. The GD−OES signals of the FOPA surface irradiated at a dosage of 0.55 × 1015 ions/cm2 and acceleration voltage of 30 kV are also depicted in Figure 4a (curves (iii)). The peaks corresponding to fluorine and phosphorus were weaker than those observed in the nonirradiated sample. This result indicated that some of the fluorine and phosphorus atoms were sputtered by FIB irradiation. As the FIB dosage increased, more FOPA molecules were damaged, and the hydrophobicity of the surface decreased. When the FOPA film was completely damaged, its wettability did not change further even with further ion collisions. However, the intensity of those signals was considerably higher than that observed from the SiO2 surface (Figure 4a, curves (i)). This finding suggested that fluorine atoms remained on the surface, and the surface displayed increased hydrophobicity relative to the SiO2 surface. GD−OES was additionally used to determine the presence of Ga in the samples, as shown in Figure 4b. The time scale of the horizontal axis is 3 orders of magnitude larger than that in Figure 4a. A Ga signal in the sample, which was subjected to FIB irradiation, was detected within 2−10 s after the analysis was started. As mentioned above, the sputtering rate of Si is 6 nm/s. Thus, this finding implied that the irradiated Ga+ ions were located in the deeper region down to ∼60 nm from the

⎛ ΔG∗ ⎞ I = I0 exp⎜ − ⎟ ⎝ kT ⎠

(2)

where I0, k, and T are the kinetic constant, Boltzmann constant, and surface temperature, respectively. Parameter ΔG* is the free energy of the droplet at the critical nucleation radius r* and is defined as follows: ΔG∗ =

4 πγ r ∗2f (θ ) 3 LV

(3)

where γLV is the surface tension between the liquid and vapor phases. Parameter f(θ) is the function of the contact angle, defined as follows: f (θ ) =

(2 − 3 cos θ + cos3 θ ) 4

(4)

Using eqs 2−4, the nucleation rates of the samples were estimated, and they only depend on the contact angle, for a given temperature and γLV. The nucleation rate of the FIBirradiated FOPA surface (contact angle = 85°) was 9 orders of magnitude higher than that of the pristine FOPA surface (contact angle = 104°) assuming that r* = 1 nm. This estimation implies that droplet nucleation preferentially occurred on the hydrophilic FIB-irradiated dots than on the hydrophobic pristine FOPA surface. Figure 5a shows the condensed water droplets on the patterned surface. To observe D

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After the condensed droplets at the hydrophilic dots coalesced with each other and grew on the surface, evaporation of the droplets was observed. Figure 6a shows a grown droplet

Figure 5. (a) Condensation of droplets on the grid pattern observed by environmental scanning electron microscopy (ESEM). The surface was tilted at an angle of 80° relative to the horizontal axis; the scale bar is 2 μm. (b) Schematic of the side view of the patterned sample surface with condensed droplets.

tiny droplets, the vapor pressure was maintained at ∼560 Pa because a much higher pressure would induce fast condensation, thus causing the disappearance of the tiny droplets. The pressure employed is slightly lower than the saturation pressure at 0.0 °C (611 Pa). However, it is assumed that water molecules in the vapor phase are attracted to the hydrophilic dots and instigate condensation.44 In this experiment, the tilt angle of the sample substrate was kept at 80° relative to the horizontal plane as illustrated in Figure 5b. During the initial stage of condensation, droplets with diameters of ∼300 nm were observed on the FIB-irradiated dots only. And condensation did not occur on the hydrophobic surface, which was in close vicinity to the hydrophilic dots, while some droplets condensed randomly away from the grid pattern. In addition, droplets on the dots are much larger than droplets located on the hydrophobic surface, thereby indicating that condensation is promoted by these dots. The distance between each droplet condensed on the hydrophilic dots in the vertical direction appeared smaller than the distance in the horizontal direction (Figure 5a) because the patterned surface was tilted. The θ value of these tiny droplets was estimated from Figure 5a as ∼40°, which is markedly smaller than that of the macroscopic droplets (Figure 2). Here, we consider the pinning effect of the three-phase contact line on the observed droplet shape. In this observation, the most part of the threephase contact line of each droplet is assumed to be on the pristine FOPA SAM surface because the diameter of the observed droplet is larger than that of the hydrophilic dots, as illustrated in Figure 5b. Therefore, it is assumed that the contact line pinning effect at the boundary between the pristine and FIB-irradiated regions is negligible. Furthermore, if the contact angle during the condensation phase is static, the contact angle is expected to be ∼104°. Additionally, if contact line pinning due to surface roughness occurs, the contact angle should be larger than the static contact angle of 104°. However, as observed herein, the estimated angle of 40° is smaller than that of static contact angle. This finding suggests that the other force, i.e., the line tension τ, should be considered, even if there are no pinning effects due to roughness. Here, τ refers to the excess free energy in the vicinity of the three-phase junction.45 The effect of this force is evaluated by the shape of the droplet, of which the size is at the micro or submicron-scale. Accordingly, a modified Young’s equation applies as follows: τ cos θ = cos θ0 − rγLV (5)

Figure 6. ESEM images of a droplet (a) on the patterned surface and (b) after evaporation. (c) High-magnification image of (b) (the surface tilt angle is 80°). (d) Schematic of tiny droplet formation during evaporation. The scale bar in (a), (b), and (c) are 10, 10, and 3 μm, respectively.

on the grid-patterned surface at 600 Pa (indicated by the arrow). This droplet was large enough to cover the patterned surface, and it was significantly larger than the other droplets observed on the sample because the hydrophilic dots promoted nucleation and condensation. Droplet evaporation proceeded as the vapor pressure in the ESEM chamber was reduced to 560 Pa. During the evaporation process, the droplet height decreased, and the three-phase contact line receded toward the center of the droplet. Then, the remaining thin droplet was believed to have split into several smaller liquid films, and the three-phase contact line of the split parts receded to form tiny droplets, or tiny droplets remained on the surface after the three-phase contact line of the large droplet receded (Figure 6b). Because most of these droplets were arranged regularly on the surface, as shown in Figure 6c (magnified image of Figure 6b), these droplets were believed to have formed on the hydrophilic dots (Figure 6d). The size distribution of the remaining droplets was wider when compared with that of the condensed droplets (Figure 5a). This phenomenon may be due to either the presence of impurities that pin the receding motion of the contact line of the droplet or the local temperature difference of the surface during evaporation. However, our result suggests that the hydrophilic dots can control the position of droplets formed following rupture of the liquid film or receding of the three-phase contact line. Thus, such a phenomenon may be useful for applications in highresolution inkjet printing and nanobiotechnology.

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CONCLUSIONS We proposed an approach to modify the surface wettability of FOPA SAM surfaces using FIB irradiation. The wettability of macro-scaled pristine and FIB-irradiated FOPA regions was characterized by contact angle measurements, which revealed that hydrophobicity diminished following FIB irradiation treatment. These findings were confirmed by GD−OES analysis, which showed that the amount of fluorine atoms at the FOPA surface decreased following FIB irradiation of the surface. Taking advantage of this phenomenon, FIB irradiation was used to fabricate a hydrophobic−hydrophilic-patterned surface with submicron-scaled resolution. The condensation

where θ0 represents the contact angle at the macroscale. Using this equation and the observed droplet base radius r, τ was estimated to be negative and in the order of 10−8 N. E

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behavior on the patterned surface was investigated by ESEM. Preferential nucleation was observed on the hydrophilic dots. Furthermore, the evaporation experiments revealed that tiny droplets remained on the hydrophilic dots at the end of the evaporation process of a large droplet that covered the entire patterned region. These results suggest that surface wettability can be successfully tuned at the submicron scale while maintaining a flat surface with a roughness of only a few nanometers.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (K.T.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI (Grant Nos. 24246038, 25420164, 26289047, and 26630067), JSTCREST, and a Grant-in-Aid for JSPS Fellows (25-4996).

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DOI: 10.1021/acs.jpcc.5b09019 J. Phys. Chem. C XXXX, XXX, XXX−XXX