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Electroless Gold as a Substrate for Self-Assembled Monolayers Zhizhong Hou, Nicholas L. Abbott,* and Pieter Stroeve* Department of Chemical Engineering and Materials Science, University of California at Davis, Davis, California 95616 Received December 3, 1997. In Final Form: April 9, 1998 We demonstrate that close-packed self-assembled monolayers (SAMs) can be formed from long chain alkanethiols on the surface of electroless gold. Gold films were deposited on glass microscope slides, high-index glass, and polycarbonate “track-etch” (PCTE) membranes using an electroless plating technique. The roughness of the surface of electroless gold was large but could be reduced to levels comparable to those for evaporated films by thermal annealing of samples supported on glass substrates. Although the largest peaks in X-ray diffraction patterns corresponded to Au(111), electroless gold has significant (200), (220), and (311) reflections and is, therefore, different from Au(111) textured gold films prepared by evaporation. Self-assembled monolayers formed from alkanethiols on electroless gold were characterized by contact angles of hexadecane, cyclic voltammetry, and grazing-angle FTIR spectroscopy. To form closepacked SAMs, it was necessary to apply postplating treatments to the electroless gold such as immersion in 25% HNO3 for membrane-supported gold and thermal annealing in combination with electrochemical cleaning for glass-supported gold. The coverage of SAMs on electroless gold, as estimated from cyclic voltammograms, was greater than 99.8%. Peak positions of C-H stretching modes in IR spectra were consistent with past measurements obtained using SAMs supported on evaporated films of gold. The IR spectra suggested, however, a smaller tilt angle (from the surface normal) of the alkyl chains on electroless gold than on evaporated gold, a conclusion that is consistent with the presence of Au(200) on the surface of electroless gold.
Introduction Electroless plating permits the deposition of metals from solution onto surfaces without the need to apply an external electrical potential.1,2 The method is based on the chemical reduction of metal salts to metals at surfaces and is easy to perform in wet-chemical laboratories. Furthermore, this method for the deposition of metals is not constrained by shape, size, or conductivity of the supporting substrate. Past uses of electroless plating include fabrication of printed circuits and hard disk memory.2 Recently, Martin and co-workers reported the use of electroless plating to deposit gold onto the walls of pores in polycarbonate “track-etch” (PCTE) membranes.3-5 Although the original size of the pores in the PCTE membranes was 30-50 nm, the inner diameter of the pores was reduced to a few nanometers by deposition of the gold.4,5 Gold “nanotubules” were thus formed. Such membranes were used to fabricate arrays of electrodes with nanometer-scale dimensions3 and to establish principles for tunable, ion-selective separations.4 In the latter case, application of a positive or negative electrical potential to the gold nanotubule resulted in selective permeation of anions and cations under a concentration gradient of ions across the membrane. Ion selectivity was changed reversibly by manipulation of the applied po* To whom correspondence should be addressed. E-mail:
[email protected]. Telephone: (530) 752-6527. Fax: (530) 7521031. E-mail:
[email protected]. Telephone: (530) 752-8778. Fax: (530) 752-1031. (1) Mallory, G. O., Hajdu, J. B., Eds. Electroless Plating: Fundamentals and Applications; American Electroplaters and Surface Finishers Society: Orlando, FL, 1990; Chapter 1. (2) Hajdu, J. B. Plating Surf. Finish. 1996, 83 (Sept.), 29-33. (3) Menon, V. P.; Martin, C. R. J. Anal. Chem. 1995, 67, 1920-1928. (4) Nishizawa, M.; Menon, V. P.; Martin, C. R. Science 1995, 268, 700-702. (5) Jirage, K. B.; Hulteen, J. C.; Martin, C. R. Science 1997, 278, 655-658.
tential. This capability offers the possibility of active control of separation processes and may also be useful as a model system for studies of the transport of ions across biological membranes. One problem encountered by Martin and co-workers was, however, that anions such as Cl-, Br-, and I- strongly adsorb to gold and thus decorate the surface of the gold nanotubules with excess negative charges. The adsorption of anions causes irreversible changes in ion selectivity. To avoid adsorption of anions and to maintain tunable ion selectivity, the gold was immersed into a solution of 1-propanethiol.4 Whether or not 1-propanethiol (or any alkanethiol) forms close-packed monolayers on the surface of electroless gold was, however, not examined. It is well-known that self-assembled monolayers (SAMs) can be formed from alkanethiols on the surface of gold.6-9 This system has become a widely used tool for the design of surfaces and for the study of phenomena on surfaces.10-20 Past studies have focused on SAMs formed on evaporated films of gold and Au(111) single crystals. X-ray diffraction (6) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 44814483. (7) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321-335. (8) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559-3568. (9) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87-96. (10) Ulman, A. CHEMTECH 1995, 25 (3), 22-28. (11) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437-463. (12) Ulman, A. An Introduction to Ultrathin Organic Films: from Langmuir-Blodgett to Self-Assembly; Academic Press: San Diego, CA, 1991; pp 279-298. (13) Rubinstein, I.; Steinberg, S.; Tor, Y.; Shanzer, A.; Sagiv, J. Nature 1988, 332, 426-428. (14) Abbott, N. L.; Folkers, J. P.; Whitesides, G. M. Science 1992, 257, 1380-1382. (15) Abbott, N. L.; Rolison, D. R.; Whitesides, G. M. Langmuir 1994, 10, 2672-2682. (16) Drawhorn, R.; Abbott, N. L. J. Phys. Chem. 1995, 99, 1651116515.
S0743-7463(97)01327-9 CCC: $15.00 © 1998 American Chemical Society Published on Web 05/20/1998
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has been used to show that the predominant crystallographic orientation of evaporated gold deposited on mica21 and silicon single crystals22 is Au(111), although quantitative analyses of the diffraction patterns do not appear to have been reported. The structure of SAMs formed on Au(111) from solutions of long chain alkanethiols [CH3(CH2)nSH, n g 9] has been extensively studied. Electron diffraction23 and scanning tunneling microscopy (STM)24 show that the sulfur atoms form a commensurate (x3 × x3)R30° structure with a nearest sulfur-sulfur spacing of 4.97 Å. Helium atom diffraction (temperature < 100 K),25-27 low-energy electron diffraction (LEED),28 grazing-angle X-ray diffraction,29 and atomic force microscopy (AFM)30 show that the alkyl chains also form hexagonal lattices with a unit mesh constant of ∼5 Å, which is consistent with the (x3 × x3)R30° structure of sulfur atoms (Figure 1a). The specific area of each alkyl chain within this structure is 21.7 Å2/molecule.11,27 Recent studies, however, also indicate the presence of unit cells containing more than one chain. Band splitting in infrared spectra at low temperatures (
