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Gold Film Surface Preparation for Self-Assembled Monolayer Studies Jing Kang and Paul A. Rowntree* Department of Chemistry, UniVersity of Guelph, Guelph, Ontario, Canada N1G 2W1 ReceiVed July 11, 2005. In Final Form: September 14, 2006 Evaporated gold films are frequently used as substrates for the study of biomolecular adsorbates, nanoparticle systems, amd partial and full monolayer films. These studies often benefit from a predeposition cleaning of the surface that removes adventitiously adsorbed material from laboratory contaminants. Scanning tunneling microscopy (STM) is used in this study to explore the microscopic consequences of two pretreatment protocols used in literature reports of self-assembled monolayers, based on sulfochromic and piranha acid solutions. These measurements show that treatment of the Au/mica surface with piranha acid can lead to extensive and uncontrolled etching of the surface and severe disruption of the surface topography; extended exposure causes the precipitation of crystallites on the surface that are highly mobile during STM imaging processes. Exposure of Au/mica surfaces to sulfochromic acid leads to the formation of permanent etch pits of the surface that are exclusively one Au layer deep; extended exposure leads to progressive etching and oxidation of the surface, ultimately leading to the formation of 0.33-0.36 nm high islands on the otherwise flat Au/mica surface. The piranha acid solutions are significantly more likely to cause the Au film to delaminate from the mica support than are the sulfochromic acid solutions. These results show that sulfochromic surface preparation is a direct and reliable method for the elimination of organic residues from Au(111)-textured surfaces, while causing a minimum of structural and chemical surface damage.
Introduction Ultraflat metal surfaces have applications in a wide variety of technological and surface science related studies ranging from device fabrication to adsorption studies. The key requirements for these applications include surface uniformity, cleanliness, and structural integrity. Modern diffraction and scanning probe surface studies impose few constraints on the sample dimensions, and state-of-the-art measurements with these techniques often use single-crystal surfaces, with the resulting uniformity of surface structure that this entails. However, quantitative structural investigations of the surface and subsurface regions of adsorbed films that are performed with grazing incidence infrared reflection-absorption spectroscopy (IR-RAS) benefit by the use of relatively large samples extending along the direction of propagation of the IR beam; for example, with the commonly used incident angle of 85° from the surface normal, the full projection of a ∼7 mm diameter IR beam onto the substrate produces an illuminated ellipse with a long axis of ∼80 mm. In the case of IR-RAS spectra of weakly absorbing or submonolayer species, it is clearly advantageous to employ large samples of as high quality as possible to maximize the absolute differences in the reflected intensities caused by the presence of the adsorbates. In all quantitative analyses, it is absolutely essential to minimize the quantity of adventitiously adsorbed impurities on this surface, since such species may impede the adsorption of the film constituents, as well as interfere with the spectral analysis of the data. The study of self-assembled organic monolayers1,2 deposited onto Au(111) surfaces is a rapidly growing application of thin film substrates. In these studies, organic species are chemisorbed to the Au surface by a terminal thiol group, presumably with the simultaneous evolution of molecular hydrogen; although a * To whom correspondence should be addressed. E-mail: Rowntree@ UoGuelph.ca. (1) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (2) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321.
growing number of works employ other crystalline faces3-5 (and occasionally other substrate metals6,7), the vast majority of published studies are based on thiols adsorbed on Au(111) surfaces:
R-SH + Au(111) f R-S-Au(111) + 1/2H2 The most common preparative method involves immersion of the Au surfaces into ∼10-3 M solutions of the selected thiol in an appropriate solvent (usually ethanol or methanol) for ∼24 h. Working solutions at these concentrations contain many orders of magnitude of excess thiol with respect to the saturation surface coverage of 4.6 × 1014 molecules/cm2 (∼7.6 × 10-10 mol/cm2) calculated for a x3×x3R30° superlattice of the Au(111) surface structure. Self-assembled monolayers (SAMs) prepared in this manner have been shown to be highly structured and well ordered and have been extensively studied using infrared,1,8 scanning probe,9-11 electrochemical,12,13 and diffraction3,5,14-17 methods. Several thin film substrates have been employed for the study of the chemisorbed thiol systems, most notably Au/Cr/Si, Au/ mica, and Au/glass; Hegner et al.18 have shown that the surfaces (3) Camillone, N., III; Chidsey, C. E. D.; Liu, G.-Y.; Scoles, G. J. Chem. Phys. 1993, 98, 4234. (4) Poirier, G. E. J. Vac. Sci. Technol. 1996, B14, 1453. (5) Dubois, L. H.; Zegarski, B. R.; Nuzzo, R. G. J. Chem. Phys. 1993, 98, 678. (6) Bensebaa, F.; Ellis, T. H.; Badia, A.; Lennox, R. B. J. Vac. Sci. Technol. 1995, A13, 1331. (7) Bryant, M. A.; Pemberton, J. E. J. Am. Chem. Soc. 1991, 113, 8284. (8) Truong, K. D.; Rowntree, P. A. J. Phys. Chem. 1996, 100, 19917. (9) Widrig, C. A.; Alves, C. A.; Porter, M. D. J. Am. Chem. Soc. 1991, 113, 2805. (10) Poirier, G. E.; Tarlov, M. J. J. Phys. Chem. 1995, 99, 10966. (11) Kang, J.; Rowntree, P. Langmuir 1996, 12, 2813. (12) Chidsey, C. E. D. Science 1991, 251, 919. (13) Widrig, C. A.; Chung, C.; Porter, M. D. J. Electroanal. Chem. 1991, 310, 335. (14) Chidsey, C. E. D.; Liu, G.-Y.; Rowntree, P.; Scoles, G. J. Chem. Phys. 1989, 91, 4421. (15) Fenter, P.; Eberhardt, A.; Eisenberger, P. Science 1994, 266, 1216. (16) Camillone, N., III; Chidsey, C. E. D.; Liu, G.-Y.; Scoles, G. J. Chem. Phys. 1993, 98, 3503. (17) Camillone, N., III; Eisenberger, P.; Leung, T. Y. B.; Schwartz, P.; Scoles, G.; Poirier, G. E.; Tarlov, M. J. J. Chem. Phys. 1994, 101, 11031.
10.1021/la0518804 CCC: $37.00 © 2007 American Chemical Society Published on Web 12/08/2006
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of “template-stripped” Au/mica samples (in which the experimental surface is that which was deposited directly onto the mica, rather than the exterior surface of the ∼100-150 nm thick metal film) can also provide large flat surfaces for surface adsorption studies, most notably of biomolecular species such as DNA and proteins. Au/mica substrates remain widely used because of the ease of preparation, the extremely flat surfaces, and the absence of “binder” layers (e.g., Cr) that can diffuse into the Au films. In our efforts to optimize Au/mica substrate preparation methods, we have found that sample quality and reproducibility are sensitive to both the quality of the substrate and the presence of organic impurities on the surface. Shortchain alkanethiols are particularly sensitive to the substrate quality, due to the relatively weak intermolecular forces that stabilize the 3D structure of the system;19 our STM measurements of butanethiol/Au have shown that the lowest energy c(4 × 2) reconstruction of the monolayer structure19 requires the cooperative organization of at least 103 molecules in a single defect-free domain (∼300 nm2),20 which imposes severe constraints on the defect density of the substrate and on the sample preparation procedures. The susceptibility of Au substrates to organic contamination has been documented2 using ellipsometry; it is often presumed that weakly adsorbed organic species from the ambient environment of the laboratory will eventually be displaced2 by the strongly binding (∼44 kcal/mol21) thiolates. The situation is more complicated in the case of submonolayer film formation or studies of film deposition dynamics; in the case of deposition conditions without large excesses of thiol in solution, it is not certain that the thiol adsorption will quantitatively remove all contaminants from the surface, nor at what rate. In the study of the temporal evolution of the absorption spectra, the kinetics for thiol adsorption2,8,22,23 will be influenced by the kinetics for the displacement of the contaminants, thus affecting the reproducibility (and physical interpretation) of the time scales measured. Furthermore, in the case of submonolayers of shortchain alkanethiolates, the infrared spectrum for the chemisorbed adsorbates is necessarily weak (
