pubs.acs.org/Langmuir © 2009 American Chemical Society
Correlation of Effective Dispersive and Polar Surface Energies in Heterogeneous Self-Assembled Monolayer Coatings Yan Xin Zhuang*,† and Ole Hansen‡ †
Key Laboratory of Electromagnetic Processing of Materials, Ministry of Education, Northeastern University, Shenyang 110004, P.R. China, and ‡Department of Micro- and Nanotechnology, DTU Nanotech and CINF, Technical University of Denmark, Building 345 east, DK-2800 Kgs. Lyngby, Denmark Received December 30, 2008. Revised Manuscript Received March 17, 2009
We show, theoretically, that the measured effective dispersive and polar surface energies of a heterogeneous surface are correlated; the correlation, however, differs whether a Cassie or an Israelachvili and Gee model is assumed. Fluorocarbon self-assembled monolayers with varying coverage were grown on oxidized (100) silicon surfaces in a vapor phase process using five different precursors. Experimentally, effective surface energy components of the fluorocarbon self-assembled monolayers were determined from measured contact angles using the Owens-Wendt-Rabel-Kaelble method. We show that the correlation between the effective surface energy components of the heterogeneous surfaces coated with fluorocarbon self-assembled monolayers is in agreement with the Cassie model.
1. Introduction In many cases, surface properties play a very important role in the performance of materials and devices. Growth of self-assembled monolayers (SAMs), which are highly ordered single molecular organic films spontaneously formed on material surfaces, is a very powerful tool to modify surface properties.1,2 Up to now, SAMs have attracted considerable scientific and industrial interest including formation mechanisms, stability, physical and chemical properties, degradation mechanisms, and other aspects.3-11 Applications range from functionalization of surfaces for biotechnology applications to control of adhesion, friction, and wear in microdevices and in nanoimprint lithography (NIL).12-17 Surface energy, defined as the amount of energy per area required to reversibly create an infinitesimally small unit surface, is an important surface property of a material. Methods for direct measurement of the solid surface energy are not readily available. *To whom correspondence should be addressed. E-mail: yxzhuang@ epm.neu.edu.cn. (1) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151. (2) Ruckenstein, E.; Li, Z. F. Adv. Colloid Interface Sci. 2005, 113, 43. (3) Bush, B. G.; Frank, W. D.; Opatkiewicz, J.; Maboudian, R.; Carraro, C. J. Phys. Chem. A 2007, 111, 12339. (4) Frechette, J.; Maboudian, R.; Carraro, C. Langmuir 2006, 22, 2726. (5) Zhuang, Y. X.; Hansen, O.; Knieling, T.; Wang, C.; Rombach, P.; Lang, W.; Benecke, W.; Kehlenbeck, M.; Koblitz, J. J. Microelctromech. Syst. 2007, 16, 1451. (6) Zhuang, Y. X.; Hansen, O.; Knieling, T.; Wang, C.; Rombach, P.; Lang, W.; Benecke, W.; Kehlenbeck, M.; Koblitz, J. J. Micromech. Microeng. 2006, 16, 2259. (7) Knieling, T.; Lang, W.; Benecke, W. Sens. Actuators, B 2007, 126, 13–17. (8) Ashurst, W. R.; Carraro, C; Maboudian, R. IEEE Trans. Device Mater. Reliab. 2003, 3, 173. (9) Gnanappa, A. K.; O’Murchu, C.; Slattery, O.; Peters, F.; O’Hara, T.; Aszalos-Kiss, B.; Tofail, S. A. M. J. Phys. Chem. C 2008, 112, 14934. (10) Flater, E. E.; Ashurst, W. R.; Carpick, R. W. Langmuir 2007, 23, 9242. (11) Gorham, J. M.; Stover, A. K.; Fairbrother, D. H. J. Phys. Chem. C 2007, 111, 18663. (12) Baker, M. A.; Li, J. Surf. Interface Anal. 2006, 38, 863. (13) Sugimura, H.; Ushiyama, K.; Hozumi, A.; Takai, O. Langmuir 2000, 16, 885. (14) Cox, J. D.; Curry, M. S.; Skirboll, S. K.; Gourley, P. L.; Sasaki, D. Y. Biomaterials 2002, 23, 929. (15) Kim, G. M.; Kim, B.; Liebau, M.; Huskens, J.; Reinhoudt, D. N.; Brugger, J. J. Microelectromech. Syst. 2002, 11, 175. (16) Ding, J. N.; Wong, P. L.; Yang, J. C. Wear 2006, 200, 209. (17) Lee, N.; Choi, S.; Kang, S. Appl. Phys. Lett. 2006, 88, 073101.
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Among the different indirect approaches for determination of solid surface energies, the contact angle method is believed to be the simplest and hence a widely used approach. Based on Young’s equation, various methods have been developed to derive the surface energy of an ideal, rigid, flat, and chemically homogeneous solid surface from contact angles of several liquids.18-20 The surface energy component approach is often used, where the surface energy of a solid or a liquid is expressed as a sum of a dispersive surface energy and a polar surface energy. The dispersive surface energy results from molecular interaction due to London forces, while the polar surface energy comprises all other interactions due to non-London forces. However, most real surfaces are rough and chemically heterogeneous and thus need special attention. Several models21-23 have been suggested to describe the contact angles of the composite surfaces. Assuming that a heterogeneous surface is composed of patches of two distinct regions, type 1 and type 2, Cassie21 developed a simple equation to evaluate the contact angle (θ) for a liquid at a composite solid surface. For a two-component surface, the phenomenological Cassie equation is cos θ ¼ Γ cos θ1 þ ð1 -ΓÞcos θ2
ð1Þ
where Γ is the fractional area of type 1 region, and θ1 and θ2 are the contact angles on the pure homogeneous surfaces of 1 and 2, respectively. The Cassie equation forms the basis for explaining the effect of surface roughness and heterogeneity on contact angles. In the case of chemically heterogeneous patches of atomic or molecular dimensions, Israelachvili and Gee suggested that the contact angle of a heterogeneous surface should be obtained from 23 ð1 þ cos θÞ2 ¼ Γð1 þ cos θ1 Þ2 þ ð1 -ΓÞð1 þ cos θ2 Þ2
ð2Þ
:: (18) DSA V1.80 Drop Shape Analysis User Manual V021106; Kruss GmbH: Hamburg, Germany, 2002. (19) Adamson, A. W.; Gast A. P. Physical Chemistry of Surfaces, 6th ed.; John Wiley & Sons: New York, 1997. (20) Ozcan, C.; Hasirci, N. J. Appl. Polym. Sci. 2008, 108, 438. (21) Cassie, A. B. D. Discuss. Faraday Soc. 1948, 3, 11. (22) Drelich, J.; Miller, J. D. Langmuir 1993, 9, 619. (23) Israelachvili, J. N.; Gee, M. L. Langmuir 1989, 5, 288.
Published on Web 4/16/2009
DOI: 10.1021/la804318p
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which replaces the Cassie equation whenever the size of chemically heterogeneous patches approaches molecular or atomic dimensions. Perfect full coverage self-assembled monolayers are highly ordered nanostructures. However, defects unavoidably exist in the real SAM coatings, especially for the low coverage SAM films. The structure of these defects and how they affect the surface properties such as wettability of a material modified by a SAM coating is still an open question. Self-assembled monolayers grown from organosilanes are promising candidates as hydrophobic coatings due to their good bonding strength, low surface energy, low friction force, and good thermal stability. The use of, in particular, fluorinated SAM coatings for reduction of stiction, friction, and wear in microdevices makes detailed understanding of all issues related to growth and stability of these coatings of prime importance. The process parameters such as growth temperature, reaction time, partial pressure, and cleanliness of the preparation environment have a critical impact on the formation of SAM coatings. For example, the water contact angle of SAM coatings varies significantly depending on the process parameters. Growth of SAM coatings on SiO2 from organosilanes has been studied in much detail; however, SAMs grown from octadecyltrichlorosilane (OTS) in the liquid phase is by far the best understood system. Here, three distinct growth mechanisms are observed; these are island growth at low temperature, homogeneous growth at high temperatures, and a mixture regime at intermediate temperature.24 Film growth at low temperature results in a heterogeneous structure with islands of ordered, densely packed alkyl chains surrounded by the bare oxide surface. However, at higher preparation temperature, the films exhibit a homogeneous structure with no island observed, and at intermediate temperatures the films consist of densely packed islands surrounded by molecules with larger tilt angles or molecules lying down.1,3,24 Also, for fluorinated SAM coatings grown from organosilanes, experimental evidence for island growth is reported.25,26 Therefore, incomplete SAM coatings may simply be regarded as a mixture of patches of full coverage SAM coating and zero SAM coverage. Moreover, in order to understand and predict environmental degradation of the SAM coatings, which could limit the expected in-use lifetime of coated devices, a detailed knowledge of the degradation kinetics and the relation between the remaining SAM coverage on the degraded films and the effective surface energy of these must be established. It has been documented that the thermal degradation happens through loss of the full molecule except the silicon atom, meaning that the silicon surface is reclaimed during the degradation of the SAM films.4,7 In the process of modification of perfluorooctyltrichlorosilane SAMs by atomic hydrogen, Fairbrother et al.11 have shown a linear correlation between water contact angle and the CF3 and CF2 group coverage extracted from X-ray photoelectron spectroscopy, indicating that the wettability and surface energy are closely related to the film coverage. In this Letter, we report on experiments where fluorocarbon self-assembled monolayers were grown on oxidized silicon wafers from precursors in vapor phase using five different precursors. Some of these coatings were on purpose grown with (24) Carraro, C.; Yauw, O. W.; Sung, M. M.; Maboudian, R. J. Phys. Chem. B 1998, 102, 4441. (25) Onclin, S.; Ravoo, B. J.; Reinhoudt, D. N. Angew. Chem., Int. Ed. 2005, 44, 6282. (26) Banga, R.; Yarwood, J.; Morgan, A. M.; Evans, B.; Kells, J. Langmuir 1995, 11, 4393.
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low water contact angle (low surface coverage), while other initially high water contact angle (high surface coverage) coatings were thermally degraded to produce a lower final water contact angle. Both the as-prepared surfaces and the heat treated surfaces may be regarded composite surfaces consisting of perfect SAM areas (full SAM coverage) and oxidized silicon regions (zero SAM coverage); a priori, however, the dimensions of the patches are not known, and therefore, it is unclear whether a Cassie or an Israelachvili and Gee model would be appropriate for these heterogeneous surfaces. Using contact angle measurements with two or more test liquids, the effective surface energies of the coated surfaces were measured and separated into polar and dispersive components using the Owens-Wendt-Rabel-Kaelble method.27-29 We show, theoretically, that these measured surface energy components are correlated; the correlation, however, differs whether a Cassie21 or an Israelachvili and Gee23 model is assumed for the heterogeneous surface. The two models are tested using the experimental data.
2. Theoretical Basis 2.1. Surface Energy and Contact Angle. The equilibrium contact angle θ of a liquid droplet on an ideal smooth, homogeneous, rigid, and insoluble solid surface is related to the surface tensions by Young’s equation19 γlv cos θ ¼ γsv -γsl
ð3Þ
which is obtained from minimization of the free energy of the system. Here γlv, γsv, and γsl are the interface tension (surface energy) of the liquid-vapor, solid-vapor, and solid-liquid interfaces, respectively. The right-hand side originates from the change in system energy from the wetted surface. The work of cohesion Wa for the solid-liquid interface is30 Wa ¼ γlv þ γsv -γsl
ð4Þ
since two interfaces are lost and a new one created. When eqs 3 and 4 are combined, the Young-Dupree relation γlv ð1 þ cos θÞ ¼ Wa
ð5Þ
results. The interaction between the solid surface and the liquid and thus the work of cohesion can be assumed to be of dipole-dipole nature with additive polar Wpa and dispersive Wda contributions, Wa = Wpa + Wda. This assumption also leads to the existence of distinct polar (superscript p) and dispersive (superscript d) contributions to the surface tensions31 γlv ¼ γplv þ γdlv ,
γsv ¼ γpsv þ γdsv ,
and γsl ¼ γpsl þ γdsl ð6Þ
Contact angle measurements are often used to estimate the surface energy of the solid-vapor interface γsv by measurements using several liquids with known surface tensions, for example, using the Owens-Wendt-Rabel-Kaelble method.27-29 This is possible if additional assumptions about the work of cohesion are made. While a generally accepted equation of state is missing, we assume, justified by the dipole-dipole nature of the (27) (28) (29) (30) (31)
Owens, D. K.; Wendt, R. C. J. Appl. Polym. Sci. 1969, 13, 1741. Kaelble, D. H.; Uy, K. C. J. Adhes. 1970, 2, 50. Rabel, W. Farbe Lack 1971, 77, 997. Dupre, A. Th aorie Mecanique de la Chaleur; Gauthier-Villars: Paris, 1969. Fowker, F. M. Ind. Eng. Chem. 1964, 56, 40.
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interaction at the interface qffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffi Wa ¼ 2 γplv γpsv þ 2 γdlv γdsv
should be averaged instead of the work of cohesion, such that the work of cohesion becomes ð7Þ Wa ðΓÞ ¼ 2
When this equation is combined with the Young-Dupree relation (eq 5), Owens-Wendt-Rabel-Kaelble method sffiffiffiffiffiffi qffiffiffiffiffiffi qffiffiffiffiffiffi γplv γlv ð1 þ cos θÞ qffiffiffiffiffiffi ¼ γdsv þ γpsv ð8Þ d 2 γ d lv γ lv
results, where the components of the solid-vapor surface energy (γsvp and γsvd) may be determined by linear regression if contact angle measurements using a number of liquids with known surface tension components are done. 2.2. Heterogeneous Surfaces. Cassie21 assumed that for a heterogeneous surface with, for example, patches of two distinct regions, type 1 and type 2, the work of cohesion should be averaged according to the fractional coverages Γ and 1 - Γ of the two regions, respectively Wa ¼ ΓWa1 þ ð1 -ΓÞWa2
ð9Þ
When this assumption is used in the Owens-Wendt-RabelKaelble method, the following relation qffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffi γlv ð1 þ cos θÞ qffiffiffiffiffi ffi ¼ Γ γdsv1 þ ð1 -ΓÞ γdsv2 þ 2 γdlv sffiffiffiffiffiffi qffiffiffiffiffiffiffiffi γplv qffiffiffiffiffiffiffiffi p Γ γ þ ð1 -ΓÞ γpsv2 ð10Þ sv1 γdlv results. Hence, extraction of effective surface energy components γdsveff and γpsveff of the heterogeneous surface results from the data analysis. It follows that the effective surface energy components are related to surface coverages and the surface energy components of the respective homogeneous surfaces qffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffi ð11Þ γdsveff ¼ Γ γdsv1 þ ð1 -ΓÞ γdsv2 qffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffi γpsveff ¼ Γ γpsv1 þ ð1 -ΓÞ γpsv2
rhffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi i ffi þ 2 Γγdsv1 þ ð1 -ΓÞγdsv2 γdlv
ð14Þ
When this assumption is used in the Owens-Wendt-RabelKaelble method, the equation below results γlv ð1 þ cos θÞ qffiffiffiffiffiffi ¼ 2 γdlv
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Γγdsv1 þ ð1 -ΓÞγdsv2 þ sffiffiffiffiffiffi ffi γplv qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi p p Γγ þ ð1 -ΓÞγ ð15Þ sv1 sv2 γdlv
Thus, the following effective surface energy components of the heterogeneous surface result from a measurement γdsveff ¼ Γγdsv1 þ ð1 -ΓÞγdsv2
ð16Þ
γpsveff ¼ Γγpsv1 þ ð1 -ΓÞγpsv2
ð17Þ
These surface energy components are correlated according to γdsveff -γdsv2 γdsv1 -γdsv2
¼
γpsveff -γpsv2 ¼Γ γpsv1 -γpsv2
ð18Þ
meaning that the effective dispersive surface tension is a linear function of the effective polar surface tension, while the film coverage is linearly related to the effective polar and dispersive surface tension. From eqs 13 and 18, it follows that it should be possible to evaluate the validity of the Cassie or Israelachvili and Gee assumptions for a given system if the effective surface tension components of heterogeneous surfaces with a range of different coverages are measured even though the coverages may not be known. It should be also possible to estimate the film coverage Γ by using eq 13 or 18 if the surface energy components of surface 1 and 2 are well-known.
3. Experimental Details ð12Þ
Obviously, the effective surface energy components are correlated qffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffi ffi qffiffiffiffiffiffiffiffi γpsveff - γpsv2 γdsveff - γdsv2 qffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffi ¼ qffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffi ¼ Γ ð13Þ γpsv1 - γpsv2 γdsv1 - γdsv2 such that the square root of the effective dispersive surface tension is a linear function of the square root of the effective polar surface tension, and the film coverage is also a linear function of the square root of the effective polar and dispersive surface tension. Israelachvili and Gee23 argued that if the scale of the different patches was small enough, the dipole moments or polarizabilities Langmuir 2009, 25(10), 5437–5441
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi p Γγsv1 þ ð1 -ΓÞγpsv2 γplv
The fluorocarbon self-assembled monolayers were grown on monocrystalline silicon wafers from precursors in vapor phase using five different precursors with water as a catalyst at room temperature. The five precursors were tridecafluoro-1,1,2,2-tetrahydrooctyl trichlorosilane CF3(CF2)5(CH2)2SiCl3 (FOTS), tridecafluoro-1,1,2,2-tetrahydrooctyl triethoxysilane CF3(CF2)5(CH2)2Si (OC2H5)3 (FOTES), tridecafluoro-1,1,2,2-tetrahydrooctyl methyldichlorosilane CF3(CF2)5(CH2)2Si(CH3)Cl2 (FOMDS), tridecafluoro1,1,2,2-tetrahydrooctyl dimethylchlorosilane CF3(CF2)5(CH2)2Si (CH3)2Cl (FOMMS), and heptadecafluoro-1,1,2,2-tetrahydrodecyl trichlorosilane CF3(CF2)7(CH2)2SiCl3 (FDTS). The growth of the SAM coatings from these silanes proceeds through the formation of hydroxyl groups followed by anchoring of the molecules to hydroxyl groups on the hydroxylized substrate surface through hydrolysis.5-7,32,33 The self-assembled monolayers were deposited onto monocrystalline silicon (100) substrates using (32) Srinivasan, U.; Houston, M. R.; Roger, T. H.; Maboudian, R. J. Microelectromech. Syst. 1998, 7, 252. (33) Fadeev, A. Y.; McCarthy, T. J. Langmuir 2000, 16, 7268.
DOI: 10.1021/la804318p
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Letter Table 1. Surface Tension of Four Test Liquids Used and Expected Contact Angles on an Ideal Teflon-like Film and Oxidized Silicon, Which Are Derived from eq 8 by Using Known Surface Energy for Teflon-like Films (γdsv = 6 mJ/m2 and γpsv = 0 mJ/m2) and Oxidized Silicon (γdsv2 = 32.2 mJ/ m2 and γpsv2 = 31.2 mJ/m2 from ref 35) γlv (mN/m)
γdlv (mN/m)
γplv (mN/m)
θTeflon (deg)
θSiO2 (deg)
72.8 50.8 47.7 27.6
21.8 50.8 30.9 27.6
51.0 0 16.8 0.0
133.3 108.2 115.4 93.9
34.6 53.7 0 0
water diiodomethane ethylene glycol n-hexadecane
a special vapor phase coating setup. Single side polished, 100 mm diameter, (100) single-crystal silicon wafers (Okmetic) were used as substrates; the root-mean-square roughness of the substrate surfaces was approximately 0.2 nm as measured using atomic force microscopy (AFM) on the as-supplied surface, indicating an almost atomically flat surface with monatomic steps. Before the actual coating process, a O2 plasma pretreatment was applied to the substrates, which results in a highly hydrophilic surface as measured by a low static water contact angle. The quality of the SAM coatings depends on the process parameters and the nature of the precursors. In the vapor phase processes, the water vapor concentration in the reaction chamber must be carefully controlled. Otherwise, incomplete SAM coatings may be formed due to insufficient water content or a coating with many aggregations may be formed because of excess water. Unfortunately, in the deposition setup used, the water amount cannot be precisely controlled and measured. However, by carefully controlling the process and the time delay between the pretreatment and silanization, high quality SAM coatings have been achieved. In the present study, some of the coatings were grown in an optimized process, resulting in a static water contact angle of about 110° corresponding to a high surface coverage. Some of the coatings were on purpose grown with lower water contact angles corresponding to a lower surface coverage, while other initially high water contact angle coatings were thermally degraded to produce a lower final water contact angle and thus lower final coverage. After the deposition, the coatings were investigated using a commercial atomic force microscope (NanoMan, Digital Instruments) in tapping mode using commercial silicon tips. Among the coatings with high water contact angle, only those without aggregations, with a root-mean-square roughness of less than 0.2 nm, were used for static contact angle measurements and determination of surface energies. The AFM characterization was not performed for the SAM coatings with lower water contact angle. The initially high water contact angle coatings were heated at different temperatures and times in air to produce surfaces with lower water contact angles. The samples were annealed in air (ordinary room ambient with a relative humidity of 40% at 20 °C) on a hot plate at a predefined temperature for a given time and then removed from the hot plate to a bulk aluminum plate and allowed to cool down to room temperature. The static contact angles using different test liquids were then measured at room temperature. The contact angle measurements were performed using the :: contact angle meter DSA10 from Kruss GmbH, Germany equipped with an automatic dispensing system for four liquids and a frame grabber. The four test liquids were deionized (DI) water, diiodomethane (Aldrich 99%), ethylene glycol (Aldrich 99.8%), and n-hexadecane (Fluka 99.9%) due to their wide range of surface tensions and ratios of dispersive-to-polar components. The surface tensions of the four test liquids used are given in Table 1, which are from the Kruss GmbH DSA I software database. The contact angles were determined using the drop :: shape analysis software from Kruss GmbH, Germany. Static contact angles were used to calculate the surface energy of SAM coatings. The static contact angle values were taken 5 s after deposition of the droplets on the surface to allow for droplet relaxation. The static contact angle values reported are the average of measurements on at least 10 droplets. However,
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Figure 1. Correlation of the effective dispersive and polar surface energies for as-deposited fluorocarbon self-assembled monolayers. The data for oxidized silicon are the average values calculated for more than 10 samples. The data for oxidized silicon found from ref 35 is also shown in the figure for comparison. Fits to the experimental data using the energy model (eq 13) and the dipole moment model (eq 18) are shown. Different surface energies for a full coverage SAM coating are used in dipole moment model 1 (γdsv = 6 mJ/m2 and γpsv = 0 mJ/m2) and dipole moment model 2 (γdsv = 9.0 mJ/m2 and γpsv = 0.9 mJ/m2). ethylene glycol and n-hexadecane are observed to spread on the oxidized silicon surface, which are also easily verified from known surface energies, meaning that the contact angles of these two liquids are useless in calculating the effective surface energy of some composite surfaces. The expected contact angles of test liquids on an ideal Teflon-like film (γdsv = 6 mJ/m2 and γpsv = 0 mJ/m2) and oxidized silicon (γdsv2 = 32.2 mJ/m2 and γpsv2 = 31.2 mJ/m2 from ref 35) derived from eq 8 are also listed in Table 1. Therefore, the surface energies of coatings were calculated from static contact angles of DI water and diiodomethane using the Owens-Wendt-Rabel-Kaelble method (refer to eq 8).
4. Results and Discussion Figure 1 shows the correlation between the effective dispersive and polar surface energies for various as-prepared fluorocarbon self-assembled monolayers. Data for the uncoated, oxidized silicon surface (γdsv2 = 34.8 ( 1.7 mJ/m2 and γpsv2 = 30.2 ( 3.2 mJ/m2) are included and serve as the zero coverage reference. Fits to the experimental data using the energy model (eq 13) and the dipole moment model (eq 18), respectively, are shown. The linear correlation (eq 13) intersects the dispersive energy axis at a value (γdsv= 6 mJ/m2 and γpsv = 0 mJ/m2) corresponding to an ideal Teflon-like surface;34 this, however, does not imply that perfect fluorocarbon SAM coatings have these surface energies. The experimental data from the lowest energy surfaces are approximately γdsv1= 9.0 mJ/m2 and γpsv = 0.9 mJ/m2. The surface energy of the oxidized silicon used in the models is the average value calculated from contact angles using Owens-Wendt-Rabel-Kaelble method for more than 10 oxidized silicon wafers treated with O2 plasma. The data for oxidized silicon given in ref 35 are also included in Figure 1 for (34) Hare, E. F.; Shafrin, E. F.; Zisman, W. A. J. Phys. Chem. 1954, 58, 236. (35) Lechner, H. Application note #219, Kruss GmbH: Germany.
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Figure 2. Correlation of the effective dispersive and polar surface energies for as-deposited fluorocarbon self-assembled monolayers by plotting the data on linear energy scales. The data for oxidized silicon are the average values calculated for more than 10 samples. Fits to the experimental data using the energy model (eq 13) and the dipole moment model (eq 18) are shown. Different surface energies for a full coverage SAM coating are used in dipole moment model 1 (γdsv = 6 mJ/m2 and γpsv = 0 mJ/m2) and dipole moment model 2 (γdsv = 9.0 mJ/m2 and γpsv = 0.9 mJ/m2).
lowest energy surface measured may not have full coverage. The energy model is seen to agree very well with the experimental data, while the dipole moment model does not agree well with data. This observation is supported by plotting the same data with linear energy axes (as shown in Figure 2) where a linear fit cannot be justified given the experimental data. It may be concluded that the effective dispersive and polar surface energies for as-deposited fluorocarbon self-assembled monolayers are correlated according to energy model, that is, eq 13. Figure 3 illustrates the correlation between the effective dispersive and polar surface energies for the thermally annealed FOTS and FOTES SAM coatings. Data for annealed FOTS and FOTES, ideal Teflon-films, and oxidized silicon surfaces are shown in the figure. Fits to the experimental data using the energy model and the dipole moment model are shown using the same procedure as in Figure 1. It appears that the energy model agrees well with the experimental data, meaning that the effective dispersive and polar surface energies in the annealed fluorocarbon self-assembled monolayer are correlated according to the energy model, that is, eq 13, in agreement with the conclusion above.
5. Conclusions
Figure 3. Correlation of the effective dispersive and polar surface energies of the annealed fluorocarbon self-assembled monolayers. The data for oxidized silicon are the average value calculated for more than 10 samples. The data for oxidized silicon found from ref 35 as well as the data for a Teflon surface are also shown in the figure for comparison. Fits to the experimental data using the energy model (eq 13) and the dipole moment model (eq 18) are shown. Different surface energies for a full coverage SAM coating are used in dipole moment model 1 (γdsv = 6 mJ/m2 and γpsv = 0 mJ/m2) and dipole moment model 2 (γdsv = 9.0 mJ/m2 and γpsv = 0.9 mJ/m2).
comparison, where the surface energy and its components are derived from the contact angles of five test liquids. The surface energy measured in the present study agree well with the data in ref 35, suggesting that the data for the oxidized silicon used in the models are credible. The curves according to the dipole moment model (eq 18) are shown for two values of the full coverage films. The dipole moment model 1 results when a full coverage SAM coating is supposed to be an ideal Teflon-like film with γdsv = 6 mJ/m2 and γpsv = 0 mJ/m2, while the dipole moment model 2 represents a situation where the lowest energy surface with γdsv = 9.0 mJ/m2 and γpsv = 0.9 mJ/m2 is assumed to be a full coverage SAM coating; note that model 2 is the most favorable assumption possible for the dipole model, since even the
Langmuir 2009, 25(10), 5437–5441
The effective surface energy components of a heterogeneous surface, extracted from contact angle measurements, are in theory correlated; however, the correlation differs whether a Cassie or an Israelachvili and Gee model is assumed. Experimental data from fluorocarbon self-assembled monolayers of varying coverages show far better agreement with the energy model (Cassie) than with the dipole moment model (Israelachvili and Gee model) for the SAM coatings investigated. In summary, the effective dispersive and polar surface energies of the heterogeneous surfaces coated with fluorocarbon self-assembled monolayers are correlated according to the energy model, where a Cassie model is assumed. This observation is consistent with an island growth mode for the films and suggests that also the thermally degraded films consist of islands. The film coverage is linearly related to the square root of the effective polar and dispersive surface tension according to eq 13. Experimental support for the validity of Cassie’s model both for the system as grown and in a thermally degraded state is an important step toward understanding and modeling of the reliability of the SAM coatings in MEMS applications. Moreover, the validity of Cassie’s model for this system suggests that degradation studies to a large degree may be based on contact angle measurements supported by some surface science characterization for calibration and verification; this is important, since films degraded in real environmental conditions need cleaning before surface science characterization which might affect the results. Acknowledgment. Financial support from the Danish Research Council for Technology and Production Science is acknowledged; Center for Individual Nanoparticle Functionality (CINF) is sponsored by the Danish National Research Foundation. Y.X.Z. is thankful for the support from Northeastern University.
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