J. Phys. Chem. B 2006, 110, 10855-10861
10855
Alkylphosphonate Modified Aluminum Oxide Surfaces E. Hoque,*,† J. A. DeRose,‡ G. Kulik,§ P. Hoffmann,§ H. J. Mathieu,† and B. Bhushan| Laboratoire de Me´ tallurgie Chimique, Institut des Mate´ riaux, Laboratoire de Transfert de Chaleur et de Masse, Institut des Sciences de l’EÄ nergie, and Laboratoire d’Optique Applique´ e, Institute d’Imagerie et d’Optique Applique´ e, Ecole Polytechnique Fe´ de´ rale de Lausanne, CH-1015 Lausanne, Switzerland, and Nanotribology Laboratory for Information Storage and Micro-/Nano-electromechanical Systems, Ohio State UniVersity, 650 Ackerman Road, Suite 255, Columbus, Ohio 43202 ReceiVed: March 2, 2006; In Final Form: April 7, 2006
The surface properties of aluminum, such as chemical composition, roughness, friction, adhesion, and wear, can play an important role in the performance of micro-/nano-electromechanical systems, e.g., digital micromirror devices. Aluminum substrates chemically reacted with octadecylphosphonic acid (ODP/Al), decylphosphonic acid (DP/Al), and octylphosphonic acid (OP/Al) have been investigated and characterized by X-ray photoelectron spectroscopy (XPS), contact angle measurements, and atomic force microscopy (AFM). XPS analysis confirmed the presence of alkylphosphonate molecules on ODP/Al, DP/Al, and OP/Al. No phosphonates were found on bare Al as a control. The sessile drop static contact angle of pure water on ODP/Al and DP/Al was typically more than 115° and on OP/Al typically less than 105° indicating that all phosphonic acid reacted Al samples were highly hydrophobic. The root-mean-square surface roughness for ODP/Al, DP/Al, OP/Al, and bare Al was less than 15 nm as determined by AFM. The surface energy for ODP/Al and DP/Al was determined to be approximately 21 and 22 mJ/m2, respectively, by the Zisman plot method, compared to 25 mJ/m2 for OP/Al. ODP/Al and OP/Al were studied by friction force microscopy, a derivative of AFM, to better understand their micro-/nano-tribological properties. ODP/Al gave the lowest coefficient of friction values while bare Al gave the highest. The adhesion forces for ODP/Al and OP/Al were comparable.
Introduction With the increasing miniaturization of electronics down to the atomic scale and advances in micro-/nano-electromechanical systems (MEMS/NEMS), self-assembled monolayers (SAMs) on aluminum surfaces are the subject of intensive studies due to their wide range of applications. Self-assembly means that no preassembly of the adsorbate molecules is carried out; the substrate is exposed directly to the adsorbate molecules dissolved in a solvent or evaporated onto the substrate, and an oriented arrangement of the adsorbate molecules takes place naturally. Typical SAMs, such as alkanethiols on gold and alkylsilanes on silica, have been studied extensively as demonstrated by the numerous reports recently reviewed in the literature.1 An important class of self-assembling organic molecules, phosphonic acids, which have not been well studied, are becoming of great practical interest because of their ability to form SAMs on a range of metal oxide surfaces.2 Studies of the properties of n-alkanoic acids on oxidized aluminum surfaces also can be found in the literature.3,4 However, not much information2,5-7 is available on the behavior of phosphonic acid SAMs. The friction properties of alkanethiols8 and alkylsilanes9 have been measured as a function of the alkyl chain length with the conclusion that shorter-chain SAMs exhibited higher friction * Author to whom correspondence should be addressed. Phone: +41 693 2972. Fax: +41 21 693 3946. E-mail:
[email protected]. † Institut des Mate ´ riaux, Ecole Polytechnique Fe´de´rale de Lausanne. ‡ Institut des Sciences de l’E Ä nergie, Ecole Polytechnique Fe´de´rale de Lausanne. § Institute d’Imagerie et d’Optique Applique ´ e, Ecole Polytechnique Fe´de´rale de Lausanne. || Ohio State University.
coefficients than longer-chain ones due to a more disordered film structure. Recently, aluminum-based substrates have gained increasing interest, particularly after the development of the digital micromirror device (DMD).10,11 The efficiency, power output, and steady-state operation of MEMS/NEMS devices can be critically influenced by adhesion, friction, and wear.10,12-15 The necessity for an ultrathin, lubricant film to minimize adhesion, friction, and wear between surfaces in contact for MEMS/NEMS is clear. One of the lubricant systems used for this purpose is that of SAMs. A SAM is composed of a large number of molecules with a headgroup that chemisorbs on a substrate, a tail group that interacts with the outer surface of the film, and a spacer (backbone) chain group that connects the head and tail groups resulting in a coating.1,16 The use of SAMs is a powerful and highly flexible approach to the design of functional surfaces by attaching different reactive groups to the substrate of interest on a molecular scale.17,18 The optimal choice for each group will yield the SAM with the best performance. Recently, we compared the performances of perfluorinated phosphonic acids with alkylphosphonic acids reacted with aluminum films freshly evaporated onto silicon substrates.19 The roughness of the latter was slightly higher, and as a result, higher contact angles were obtained. An indepth study of relatively flat aluminum sheets (mirror finish) was carried out. Self-assembled monolayer formation of phosphonates on Al surfaces relies on the hydroxylation of the oxide (alumina) layer. Generally, chemisorption of alkylphosphonic acid occurs by proton dissociation to form an alkylphosphonate species. The phosphonic acids undergo a condensation reaction with surface-bound alumino-hydroxyl (-Al-OH) species to
10.1021/jp061327a CCC: $33.50 © 2006 American Chemical Society Published on Web 05/13/2006
10856 J. Phys. Chem. B, Vol. 110, No. 22, 2006 form alumino-phosphonates and H2O as a byproduct: R-PO(OH)2 + -Al-OH f R-(OH)OP-O-Al- + H2O. This method readily promotes the formation of spontaneously organized, oriented, self-assembled monolayer, thin films on Al.2 This paper presents results from the preparation and in-depth characterization of alkyl phosphonic acids reacted on oxidized Al substrates. Various surface properties, such as morphology, wetting properties, surface energy, root-mean-square (rms) roughness, adhesion, and friction of different chain length alkylphosphonate SAMs on Al are elucidated using surfacesensitive techniques, such as X-ray photoelectron spectroscopy (XPS), contact angle measurements (CAMs), atomic force microscopy (AFM), and friction force microscopy (FFM). Experimental Section Materials. All solvents and reagents used were standard commercial grade and no further purification was done. The solvents used to clean the aluminum substrates were 99.5% pure ethyl acetate (CH3COOC2H5) and 99.7% pure 2-propanol (CH3CHOHCH3), both purchased from Merck, Darmstadt, Germany. Tetrahydrofuran (THF) (C4H8O; Merck, Darmstadt, Germany) of purity 99.8% was utilized as the solvent for all n-alkylphosphonic acid (PA) solutions. The alkylphosphonic acid molecules utilized for Al surface modification were n-octadecylphosphonic acid (ODP) with 18 carbon atoms (H3C(CH2)17PO(OH)2) (Alpha Aesar, Karlsruhe, Germany, CAS 4724-47-4), n-decylphosphonic acid (DP) with 10 carbon atoms (H3C(CH2)9PO(OH)2) (ABCR, Karlsruhe, Germany; CAS 5137-70-2), and n-octylphosphonic acid (OP) with 8 carbon atoms (H3C(CH2)7PO(OH)2) (ABCR, Karlsruhe, Germany; CAS 4724-48-5). Substrate and Film Preparation. Substrates. Al pieces of approximately 10 × 12 × 0.5 mm3 were cut from a 300 × 210 × 0.5 mm3 specially produced, mirrorlike, polished Al plate (Alcan, Singen, Germany). The polymer protection layer was peeled off the Al samples; they were then degreased by immersion for 10 min in an ultrasonic bath containing ethyl acetate and finally blown dry with argon (Ar) gas. Subsequently, they were further cleaned with 2-propanol and deionized (DI) water for 10 min each to minimize any surface contamination. Between each ultrasonic cleaning step, samples were blown dry using Ar gas. The final cleaning step (and simultaneously surface oxidation) was accomplished by radio frequency (RF) oxygen plasma for 10 min with a power of 50 W and an O2 partial pressure of 0.3 Torr (Plasmaline 415, Barrel Type Asher, Tegal Corp., Santa Barbara, CA) followed by venting of the chamber with N2 gas. The same cleaning method was applied to all Al substrates modified with PA. Preparation of Self-Assembled Films. Surface modification of Al was performed using the simple and cost-effective liquidphase deposition. PA solutions of 0.1% (wt) with THF as the solvent were prepared by dissolving the PA powder into THF during 15 min with agitation of the solvent by hand. Hydrated, oxidized Al substrates were reacted with a PA solution by immersion. The samples were kept in the solution for approximately 24 h at room temperature under ambient conditions and then removed. After each reaction, rinsing of the samples was done several times using fresh THF in a beaker via hand agitation for approximately 30 s to remove physisorbed molecules from the substrate surface. Then, the samples were blown dry carefully with Ar gas giving the final ODP/Al, DP/Al, and OP/Al SAM samples. Film Characterization. Surface Morphology. High-resolution topographic images were taken for Al, ODP/Al, DP/Al, and OP/
Hoque et al. Al over an area of 5 × 5 µm2 using a Park Scientific Instruments AFM to determine the rms surface roughness and morphology. The imaging was done at room temperature and atmospheric pressure in contact mode with a set point of 6 nN. X-ray Photoelectron Spectroscopy. X-ray photoelectron spectroscopy was carried out with a Kratos AXIS ULTRA system using a monochromatic Al KR X-ray radiation source. The surface chemical composition was determined for Al before and after PA modification. The source was operated at 15.0 kV and 150 W power in ultrahigh vacuum (UHV) (base pressure below 10-9 mbar). Samples were grounded (earthed) to prevent charging, and electron charge compensation was also applied. The survey scans were performed in an energy range of 0-1100 eV with a pass energy of 80 eV and an acquisition time of 240 s, while the core level single peaks were measured using a pass energy of 20 eV and an acquisition time of 180 s. The electron binding energy (BE) was calibrated against the alkyl C 1s photoelectron peak at 284.7 eV.20 The samples were also analyzed by angle-resolved XPS measurements carried out at takeoff angles of 0°, 15°, 30°, 45°, 60°, and 70° by tilting the sample holder with respect to the surface normal. The takeoff angle is defined as the angle between the direction normal to the detector and that normal to the sample surface. Quantitative analysis of the elements at the surface is derived from the peak areas of the XPS multiplex after background subtraction. Curve fitting of high-resolution XPS peaks has been done by a mixed Gaussian-Lorentzian fit after linear-type background subtraction using a standard CASA XPS processing software.21 The estimated relative error for all XPS data is (2%. Water Contact Angle Measurements. The wetting properties of unmodified and PA-modified Al were studied using a GBX Digidrop CAM apparatus. Sessile drop static contact angle and dynamic contact angle measurements were made with ultrapure H2O (Fluka) at 75-90% relative humidity (RH) and 23-25°C. CAM values reported are the average of at least five measurements. Drops with typical volumes of 0° on unmodified Al, then its surface energy must be between 28 and 72 mJ/m2. The surface energy for OP/Al was also not possible to measure, as hexadecane, which has the highest surface tension, had a very low contact angle on its surface, thus indicating the surface energy of OP/Al is approaching 27.43 mJ/m2. The surface energies of DP/Al and ODP/Al were determined to be 22 ( 2 and 21 ( 2 mJ/m2, respectively, as shown in Figure 5b. Although the surface energies for DP/Al and ODP/Al were comparable, a higher surface energy was observed for OP/Al, which explains why it gives lower water static contact angles. Dynamic Water Drop Contact Angles. The wetting properties of phosphonate SAMs were also determined by dynamic water contact angle measurements, which directly reflect the composition and structure of the probed surface.31 From Young’s equation
γsl + γlv cos(θo) ) γsv
Figure 5. CAM data: (a) water sessile drop static contact angle data for Al, OP/Al, DP/Al, and ODP/Al, respectively, and (b) histogram of the surface energies for OP/Al, DP/Al, and ODP/Al. All contact angles have an absolute error of (3°.
determined by the applied methods. The effective film thicknesses for ODP/Al, DP/Al, and OP/Al determined by AR-XPS were 2.1, 1.6, and 1.5 nm, respectively, which agree with ellipsometric data already reported.2 Contact Angle Measurements. Static Water Drop Contact Angles. Figure 5a shows water sessile drop static contact angle measurements performed on Al, OP/Al, DP/Al, and ODP/Al. Water CAMs were done to determine the wetting properties of the SAM films (hydrophilic oxidized, unmodified Al versus hydrophobic alkylphosphonate modified Al). Contact angle measurements are also sensitive to the orientation of the molecules in the SAM. In well-ordered monolayers, the terminal methyl group of the alkyl chain is oriented outward, reducing access of the water drop to the Al surface. Contact angle measurement data verifies a large improvement in the hydrophobicity of phosphonate modified Al when it is compared to oxidized, unmodified Al; an H2O contact angle of 116° was
where γsl, γlv, and γsv are the interfacial tensions between the liquid and the solid, the liquid and the vapor, and the solid and the vapor, respectively, and θo is the equilibrium contact angle the drop makes with the surface, it appears that there is only one thermodynamic contact angle. However, daily experience shows that drops have a spectrum of contact angles ranging from the so-called advancing contact angle up to the so-called receding contact angle, which are the maximum and minimum contact angle values possible.32,33 Imperfections or defects (either chemical or structural) on the solid surface act to pin the contact line and it is this pinning effect that gives rise to contact angle wetting hysteresis between the advancing and receding angle (∆(cos θ) ) cos θa - cos θr). Hysteresis provides a measure of the degree of surface roughness or heterogeneity of the interface.31 The dynamic contact angle data measured for (a) ODP/Al, (b) DP/Al, and (c) OP/Al are summarized in Table 3 with their respective wetting hysteresis values. ODP/Al and DP/ Al show similar hysteresis, but OP/Al, which had the lowest water static contact angle and highest surface energy, shows the highest hysteresis. Water contact angle generally increases with a decrease in surface energy.34 A lower equilibrium surface density of the phosphonate film for OP/Al due to reduced vdW interactions is very likely the cause of higher surface energies, lower water contact angles, and higher wetting hysteresis. An earlier study demonstrated that the increased wettability of films formed from n-alkanethiols
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Hoque et al. of friction for ODP/Al was lowest, while for OP/Al it was higher; thus tribology data agrees with the XPS, CAM, and surface energy measurement data. Bhushan and Liu35 and Liu and Bhushan36 have used a “molecular spring” or “brush model” to explain why less compliant SAMs show larger friction. In terms of the chain length effect, it has been reported that the coefficient of friction for SAM surfaces decreases with the carbon backbone chain length (n) up to 12 carbon atoms (n ≈ 12).25,37,38 Conclusion In this paper, characterization of the properties of oxidized Al substrates and phosphonate SAMs on Al substrates has been presented. The hydrocarbon phosphonate SAMs have been studied by XPS, CAMs, and AFM. X-ray photoelectron spectroscopy and contact angle measurement data confirm the presence of alkyl phosphonate SAMs on oxidized Al. CAM data illustrates that ODP/Al and DP/Al are highly hydrophobic yielding a water sessile drop static contact angle >115°, dynamic contact angle hysteresis values of 19° and 21°, and surface energies of 21 and 22 mJ/m2, respectively. OP/Al is found to be less hydrophobic having water sessile drop static contact angles
