Langmuir 2000, 16, 5347-5353
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Effect of an Oxidized Gold Substrate on Alkanethiol Self-Assembly John T. Woodward,*,† Marlon L. Walker,‡ Curtis W. Meuse,† David J. Vanderah,† G. E. Poirier,§ and Anne L. Plant† Biotechnology Division, Surface and Microanalysis Science Division, and Process Measurements Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 Received December 22, 1999. In Final Form: April 7, 2000 UV-cleaned gold substrates incubated in solutions of alkanethiol show islands on the monolayer surface when imaged with noncontact atomic force microscopy (AFM). The height of the islands above the monolayer is approximately twice the height of the alkanethiol monolayer, and the diameter of the islands is 20-200 nm. These islands are easily pushed aside during contact mode AFM imaging without damaging the underlying monolayer. Islands are observed on gold substrates exposed to solutions of octadecane-, hexadecane-, and dodecanethiol and 1′-thiahexa(ethylene oxide)-1-octadecane at 0.01-1.0 mM concentrations in ethanol and hexadecane. AFM on samples with submonolayer coverage shows that the islands are not observed until the late stages of monolayer formation. Islands are not observed on freshly deposited gold substrates or on UV-cleaned gold substrates that are exposed to ethanol for longer than 10 min prior to incubation in alkanethiol solutions. We conclude that the island formation is associated with oxidation of the gold surface and that the islands are primarily composed of alkanethiol. We hypothesize that the stability of these structures may be due to the formation of multimolecular complexes of alkanethiols.
Introduction 1
Since 1983 alkanethiol monolayers on gold have been employed in studies of self-assembly,2,3 electron transport,4 and optical constants5 as well as for applications in surface modification, biosensors,6,7 and lithography.8 The characteristics of alkanethiols, such as self-assembly, strong adherence to metals, and regular and reproducible monolayer formation have contributed to their importance in studies of physical and chemical phenomena. Some of the appeal of using alkanethiol monolayers on gold in devices is that the monolayers are easy to prepare. A clean gold substrate is simply left in a 0.01-1.0 mM9 solution of alkanethiol in ethanol for 2-48 h and rinsed with ethanol or hexane to eliminate any unbound alkanethiol. Generally, either substrates are prepared by depositing gold immediately before immersion of the substrate in alkanethiol solution or previously deposited gold films are cleaned in a UV/ozone cleaner10,11 or with oxidizing solutions4,12 to remove organic surface contaminants prior * Corresponding author. Phone: 301-975-5495. Fax: 301-9758246. E-mail:
[email protected]. † Biotechnology Division. ‡ Surface and Microanalysis Science Division. § Process Measurements Division. (1) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (2) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (3) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (4) Miller, C.; Cuendet, P.; Gratzel, M. J. Phys. Chem. 1991, 95, 877. (5) Parikh, A. N.; Allara, D. L. J. Chem. Phys. 1992, 96, 927. (6) Cornell, B. A.; Braac-Maksvytis, V. L. B.; King, L. G.; Osman, P. D. J.; Raguse, B.; Wieczorek, L.; Pace, R. J. Nature 1997, 387, 580. (7) Crooks R. M.; Ricco, A. J. Acc. Chem. Res. 1998, 31, 219. (8) Kumar, A.; Biebuyck, H. A.; Whitesides G. M. Langmuir 1994, 10, 1498. (9) The accepted SI unit of concentration, mol/L, has been represented by the symbol M in order to conform to the conventions of this journal. (10) Saman, M. G.; Brown, C. A.; Gordon, J. G. Langmuir 1991, 7, 437. (11) Ron, H.; Rubinstein, I. Langmuir 1994, 10, 4566. Ron, H.; Matlis, S.; Rubinstein, I. Langmuir 1998, 14, 1116.
10.1021/la991672+
to immersion in alkanethiol solution. King has shown using variable-angle X-ray photoelectron spectroscopy (XPS) that UV cleaning can cause a gold oxide layer up to 2 nm thick to form on a gold surface.13 Krozer and Rodahl exposed gold samples to UV radiation and ozone under ultrahigh vacuum (UHV) and observed heterogeneous oxidation of the surface; XPS indicated an average thickness equivalent to several gold oxide monolayers.14 Saliba et al. observe an “oxidic” oxygen peak with XPS upon exposing gold to ozone in UHV.15 Ron et al. have shown using XPS that both UV cleaning and O2 plasma treatments can oxidize the gold surface and that this affects the self-assembly process.11 They find that contact AFM images of octadecanethiol monolayers on oxidized gold show 4.6 nm high islands on the surface. They attributed the islands to gold oxide that is covered by alkanethiol and thus sequestered from the ethanol, preventing the ethanol from reducing the oxide. Yan et al. have also used XPS, as well as near edge X-ray adsorption fine structure, to study alkanethiol adsorption on oxidized gold surfaces.16 They find that monolayers formed on gold oxidized by atomic oxygen have a higher density than those on bare gold and that there is oxygen buried under the monolayer. Here we present noncontact AFM data that indicate that, on oxidized gold, alkanethiols form monolayers with isolated multilayer islands. Materials and Methods Octadecanethiol, hexadecanethiol, and dodecanethiol were used as received (Aldrich Chemical Co., Milwaukee, WI)17 except as noted below. For one set of experiments octadecane disulfide (12) Evans, S. D.; Sharma, R.; Ulman, A. Langmuir 1991, 7, 156. (13) King, D. E. J. Vac. Sci. Technol., A 1995, 13, 1247. (14) Krozer, A.; Rodahl, M. J. Vac. Sci. Technol., A 1997, 15, 1704. (15) Saliba, N.; Parker, D. H.; Koel, B. E. Surf. Sci. 1998, 410, 270. (16) Yan, C.; Go¨lzha¨user, A.; Grunze, M.; Wo¨ll, Ch. Langmuir 1999, 15, 2414. (17) The mention or use of products in this manuscript is not meant as an endorsement by NIST nor as an indication that they are the best available.
This article not subject to U.S. Copyright. Published 2000 by the American Chemical Society Published on Web 05/19/2000
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Langmuir, Vol. 16, No. 12, 2000
(approximately 4% mol fraction; 400 MHz 1H NMR analysis) was removed from commercial octadecanethiol by chromatography on silica gel (J. T. Baker, 40 µm; column 33 cm × 3 cm), activated by heating (110 °C), using hexane as the eluant. 1′Thiahexa(ethylene oxide)-1-octadecane (THEO-C18) was synthesized as previously described.18 Solvents used for making solutions and rinsing samples were ethanol (200 proof anhydrous; Warner Graham Co., MD), hexane (95+%; Sigma-Aldrich), methanol (99%; Mallinckrodt, KY), tetrahydrofuran (100%; Mallinckrodt), chloroform (100%; Mallinckrodt), and hexadecane (99%; Aldrich). Water was from a NANOpure II (Barnstead, IA) filtration system. Gold substrates were prepared by depositing 1.5 nm of chromium and 5.0 nm of gold on polished silicon wafers using an ion-mill/evaporation technique.19 AFM images show our gold substrates have a root mean square roughness of 0.2 nm over a 1 µm × 1 µm area. Two procedures for cleaning substrates were used. For both procedures, substrates were placed for 10 min in a UV cleaner (Boekel Scientific, PA), rinsed with water and ethanol, and returned to the UV cleaner for 5 min. The substrates either were placed immediately in alkanethiol solution without further rinsing (procedure 1) or were submersed in pure ethanol for at least 10 min before being placed in alkanethiol solution (procedure 2). Solutions ranging in concentration from 0.01 to 1.0 mM of octadecanethiol were prepared in ethanol and hexadecane. Solutions from 0.1 to 1.0 mM hexadecanethiol and dodecanethiol were prepared in ethanol. Solutions of 0.1 and 1.0 mM THEO-C18 were prepared in hexadecane. Glassware was cleaned by soaking for 15-30 min in potassium persulfate/sulfuric acid (120 gm/L) solution followed by rinsing with NANOpure water. Upon removal from alkanethiol solution the samples were rinsed with solvent either by using a pipet or by submerging them in solvent for 5 min. Ethanol, methanol, hexane, THF, chloroform, water, and 2 wt % sodium dodecyl sulfate (SDS)/ water solution were used for rinsing. AFM imaging was performed with a Park Scientific Instruments model LS atomic force microscope (ThermoMicroscopes, CA) using Ultralever noncontact tips. Except where noted, all images were taken in noncontact mode. The z axis of the AFM was calibrated to the 5.5 nm step height of a cadmium arachidate Langmuir-Blodgett film.20 The island height was measured as the difference in height between peaks in the height histogram of plane subtracted images. The density of islands at the surface was measured from the histogram as the percentage of the surface with a height greater than the height half way between the two peaks in the histogram. Contact angles were measured using a Rame´-Hart model 100-00 contact angle goniometer (Rame´-Hart Inc., NJ) under ambient atmosphere. All contact angles reported are the contact angle of an advancing water drop. XPS was performed using a VG Escalab II (VG Scientific, West Sussex, U.K.) with a Mg anode operated at 12 kV and 20 mA.
Results and Discussion Panels a-d of Figure 1 show noncontact AFM images of gold substrates cleaned by UV/ozone treatment according to procedure 1 outlined above: after they were removed from the UV cleaner they were immediately added to a 0.1 mM ethanol solution of dodecane-, hexadecane-, or octadecanethiol or an 0.1 mM hexadecane solution of THEO-C18, respectively. Upon removal from the alkanethiol solution they were rinsed with ethanol using a pipet, submerged for 5 min in ethanol, submerged for 5 min in hexane, and blown dry with nitrogen. All four images clearly show the existence of islands on the surface. Cross sections from the images in Figure 1 are shown in Figure 2. For the series of alkanethiols examined, the islands are 20-50 nm in diameter and are frequently aggregated in clusters up to 200 nm laterally. The height of the islands above the background is uniform and roughly (18) Vanderah, D. J.; Meuse, C. W.; Silin, V.; Plant, A. L. Langmuir 1998, 14, 6916. (19) Pedulla, J.; Dealattes, R. D. Proc. SPIE 1993, 2011, 299. (20) Schwartz, D. K.; Viswanathan, R.; Zasadzinski, J. A. N. J. Phys. Chem. 1992, 96, 10444.
Woodward et al.
equal to twice the thickness of the monolayer. The island height as measured from AFM images of dodecanethiol is 2.6 nm, for hexadecanethiol it is 4.2 nm, and for octadecanethiol it is 4.8 nm, compared to monolayer heights of 1.5, 2.0, and 2.3 nm, respectively, as measured by ellipsometry.3,21,22 The THEO-C18 islands tend to be larger laterally (200-1000 nm) and have a height of 7.8 nm, again roughly twice the ellipsometric thickness of the monolayer of 4.1 nm.18 There are also a few islands (
