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J. Phys. Chem. C 2007, 111, 3956-3962
Alkylperfluorosilane Self-Assembled Monolayers on Aluminum: A Comparison with Alkylphosphonate Self-Assembled Monolayers E. Hoque,*,† J. A. DeRose,‡ P. Hoffmann,§ B. Bhushan,| and H. J. Mathieu† 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, 201 West 19th AVenue, Columbus, Ohio 43210 ReceiVed: September 18, 2006; In Final Form: December 21, 2006
The performance of micro- and nanoelectromechanical systems depends on the surface properties of the substrate material, such as chemical composition, roughness, friction, adhesion, and wear. Substrates of aluminum deposited onto Si (Al/Si) have been chemically reacted with perfluorodecyldimethylchlorosilane (PFMS), octadecylphosphonic acid (ODP), decylphosphonic acid (DP), octylphosphonic acid (OP), and perfluorodecylphosphonic acid (PFDP) and then characterized by X-ray photoelectron spectroscopy (XPS), contact angle measurements, and atomic force microscopy (AFM). PFMS/Al self-assembled monolayers (SAMs) were studied by friction force microscopy, a derivative of AFM, to better understand their microand nanotribological properties. The adhesion forces for PFMS/Al SAMs were found to be lower when compared to those of bare Al/Si; however, the coefficient of friction for both was comparable. XPS analysis revealed the presence of the corresponding alkyl chain molecules on PFMS/Al, ODP/Al, DP/Al, OP/Al, and PFDP/Al SAMs. The sessile drop static contact angle of pure water demonstrates that all the SAMs are extremely hydrophobic, giving contact angles typically >130° on PFDP/Al, ODP/Al, and PFMS/Al SAMs and >125° on DP/Al and OP/Al SAMs. The surface energy of PFMS/Al SAMs determined by the Zisman plot method is 16.5 ( 2 mJ/m2 (mN/m). The rms surface roughness of ODP/Al, DP/Al, OP/Al, PFMS/Al, and PFDP/Al SAMs, before exposure to warm nitric acid (pH 1.8, 30 min, 60-95 °C), as well as bare Al, is less than 40 nm as determined by AFM. The XPS data and stability against harsh chemical conditions indicate that a type of bond forms between a phosphonic acid or silane molecule and the oxidized Al/Si surface. Stability tests using warm nitric acid (pH 1.8, 30 min, 60-95 °C) show ODP/Al SAMs to be the most stable followed by PFDP/Al, DP/Al, PFMS/Al, and OP/Al SAMs.
Introduction Due to ever increasing miniaturization of electronics and advances in micro- and nanotechnological systems (MEMS/ NEMS), self-assembled monolayers (SAMs) on aluminum surfaces are one area of intense interest for a wide range of applications. Typical SAMs, such as alkanethiols reacted with gold and alkylsilanes with silica, have been studied extensively as demonstrated by numerous reports in the literature.1 Studies of the properties of n-alkanoic acids reacted with oxidized aluminum2,3 and alkyl phosphoric acids with tantalum oxide4,5 surfaces can also be found. However, not much information6-9 is available in the literature on phosphonic acid-based SAMs, which is one important class of self-assembling organic molecules due to their ability to react with a range of metal oxide surfaces.6 The friction properties of alkanethiols10 and alkylsilanes11 have been studied as a function of the alkyl chain * Corresponding author. Phone: +41 21 693 2972. Fax: +41 21 693 3946. E-mail:
[email protected]. † Laboratoire de Me ´ tallurgie Chimique, Ecole Polytechnique Fe´de´rale de Lausanne. ‡ Laboratoire de Transfert de Chaleur et de Masse, Ecole Polytechnique Fe´de´rale de Lausanne. § Laboratoire d’Optique Applique ´ e, Ecole Polytechnique Fe´de´rale de Lausanne. | Ohio State University.
length, with the conclusion that shorter chain SAMs exhibit higher friction coefficients than longer chain ones due to a more disordered film structure. No report in the literature was found concerning the study of SAMs formed by perfluorosilanization of Al. Recently, aluminum-based substrates have gained increasing interest, particularly after the development of the digital micromirror device.12,13 The efficiency, power output, and steady-state operation of MEMS/NEMS devices can be critically influenced by adhesion, friction, and wear.12,14-17 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 head group that chemisorbs onto 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. Interactions between spacer groups of different molecules, such as van der Waals forces and/or hydrogen bonding, can hasten SAM film formation and contribute to its stability.1,18 SAMs can be used to design functional surfaces at the molecular scale by attaching different reactive groups to the substrate of interest.19,20 The optimal choice for each group will yield the SAM with best performance. Recently, we compared the performances of perfluorinated phosphonic acids and alkylphos-
10.1021/jp066101m CCC: $37.00 © 2007 American Chemical Society Published on Web 02/20/2007
Alkylperfluorosilane SAMs on Aluminum phonic acids reacted with Al films deposited onto Si.21 An indepth study of alkylphosphonic acids on relatively flat Al sheets (mirror finished) was also carried out.22 SAM formation of alkyl silanes and 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 silanol (-Si-OH) and phosphonic acids undergo a condensation reaction with surfacebound aluminohydroxyl (-Al-OH) species to form siloxyaluminum and aluminophosphonate compounds with H2O as a byproduct: R3-Si-OH + -Al-OH f R3-Si-O-Al- + H2O and R-PO(OH)2 + -Al-OH f R-(OH)OP-O-Al+ H2O.6,23,24 The poor durability of most SAMs under ambient or more extreme conditions does not allow them to be used for many practical applications. The introduction of fluorocarbon groups in the alkyl chain of the silane or phosphonic acid molecule causes the SAM to be significantly more stable compared to one composed from the analogous hydrocarbon molecule. This report compares the properties of perfluoroalkylsilane/ Al and perfluoro- and non-perfluoroalkylphosphonate/Al SAMs (i.e., chemical stability, morphology, hydrophobicity, surface energy, rms roughness, adhesion, and friction). These properties are measured using surface-sensitive techniques, such as X-ray photoelectron spectroscopy (XPS), contact angle measurement (CAM), atomic force microscopy (AFM), and friction force microscopy (FFM). Experimental Section Materials. All the precursors and solvents used for this experiment were standard commercial grade. They were used as received without any further purification. All substrates used were cleaned with 99.5% pure ethyl acetate (CH3COOCH2CH3) and 99.7% pure 2-propanol [(CH3)2CH-OH], both purchased from Merck (Darmstadt, Germany). The alkylsilane and alkylphosphonic acid molecules utilized for Al/Si surface modification were 1H,1H,2H,2H-perfluorodecyldimethylchlorosilane (PFMS) with 10 carbon atoms [CF3(CF2)7(CH2)2(CH3)2SiCl; 97%; ABCR, Karlsruhe, Germany; CAS 74612-30-9]; noctadecylphosphonic acid (ODP) with 18 carbon atoms [H3C(CH2)17PO(OH)2; 100%; Alpha Aesar, Karlsruhe, Germany; CAS 4724-47-4]; 1H,1H,2H,2H-perfluorodecylphosphonic acid (PFDP) with 10 carbon atoms [CF3(CF2)7(CH2)2PO(OH)2; 95%; Unimatec, Ibaraki-Ken, Japan; CAS 80220-639]; n-decylphosphonic acid (DP) with 10 carbon atoms [H3C(CH2)9PO(OH)2; 98%; ABCR, Karlsruhe, Germany; CAS 5137-70-2]; and n-octylphosphonic acid (OP) with eight carbon atoms [H3C(CH2)7PO(OH)2; 99%; ABCR, Karlsruhe, Germany; CAS 4724-48-5]. The solvents used for making the alkylsilane and alkylphosphonic acid solutions were n-hexane [CH3(CH2)4CH3; 98.5%] and ethanol [CH3CH2OH; 99.8%], both purchased from Merck (Darmstadt, Germany), respectively. Substrate Preparation. Pure Al (99.98%) was DC sputtercoated (Edwards Coating System) onto Si pieces of approximately 10 × 15 × 0.5 mm3 in a high vacuum chamber (base pressure of 10-6 mbar). The sample holder was rotated, passing sequentially above the Al target to provide a uniform Al film approximately 1-µm thick. Then, Al/Si surface cleaning (and simultaneously surface oxidation) was accomplished by radio frequency 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., Petaluma, CA) followed by venting of the chamber with N2 gas. The same cleaning and oxidation method was applied for all Al/Si before modification with PFMS
J. Phys. Chem. C, Vol. 111, No. 10, 2007 3957 or any phosphonic acid (PA). The bare Al/Si was found to be hydrophilic with a water contact angle of 125° for DP/Al and OP/Al SAMs. The surface energy of PFMS/Al SAM determined by the Zisman plot method is 16.5 ( 2 mJ/m2 (mN/ m). PFMS SAM shows much lower adhesion than umodified Al, while the coefficient of friction values are comparable. XPS data indicate that PFMS and PFDP form a chemical bond with the oxidized Al, which accounts for the SAMs’ good stability. Stability tests based on resistance to exposure in warm nitric acid provide evidence that ODP produces a more densely packed phosphonate SAM on Al, while DP and OP form a less densely packed SAM. ODP/Al is the most stable SAM followed by PFDP/Al, DP/Al, PFMS/Al, and OP/Al. The robustness of SAMs on Al correlates with the akyl chain length and perfluorination. Acknowledgment. Financial support for this work was provided by the Swiss Technology Oriented Program of TOP Nano 21 (Contract CTI 5824.4), Bern, the U.S. National Science Foundation (Contract No. ECS-0301056), and J. Thome of LTCM, EPFL. The content of this information does not necessarily reflect the position or policy of the U.S. Government, and no official endorsement should be inferred. We thank M. Cichomski (Nanotribology Laboratory for Information Storage and MEMS/NEMS, Ohio State University) for the tribology measurements. References and Notes (1) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: Boston, 1991; Chapter 3. (2) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 45. (3) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 52. (4) Brovelli, D.; Ha¨hner, G.; Ruiz, L.; Hofer, R.; Kraus, G.; Waldner, A.; Schlo¨sser, J.; Oroszlan, P.; Ehrat, M.; Spencer, N. D. Langmuir 1999, 15, 4324. (5) Textor, M.; Ruiz, L.; Hofer, R.; Rossi, A.; Feldmann, K.; Ha¨hner, G.; Spencer, N. D. Langmuir 2000, 16, 3257. (6) Pellerite, M. J.; Dunbar, T. D.; Boardman, L. D.; Wood, E. J. J. Phys. Chem. B 2003, 107, 11726.
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