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Langmuir 1992,8, 854-861
Consequences of Microscopic Surface Roughness for Molecular Self -Assembly Stephen E. Creager,. Lisa A. Hockett, and Gary K. Rowe Department of Chemistry, Indiana University, Bloomington, Indiana 47405 Received October 2, 1991.I n Final Form: December 4, 1991 Gold electrodes prepared by wet chemical etching of bulk polycrystalline gold and by vacuum evaporation onto silicon and heated mica were used as substrates for self-assembly of monolayers of n-alkanethiols. Defects in the resulting monolayers were characterized by studying the inhibition of interfacial electron transfer from the redox-active solute (hydroxymethy1)ferroceneand by following the exchange of bound n-alkanethiolsfor tagged alkanethiolspresent in a contactingsolution. A relative inabilityto block electron transfer and a tendency toward exchange of bound thiol for free thiol in solution were taken as evidence of a monolayer with a larger number of defects. Defectiveness in the monolayers correlatesstrongly with the method used to prepare the substrate; bulk gold Substratesthat were chemically etched with aqua regia prior to self-assemblyyielded monolayers with significantlyfewer defects than those prepared on evaporated thin film substrates, even when the evaporated films had been annealed prior to monolayer formation. Scanning electron microscopy and scanning tunneling microscopy were used to characterize macroscopic and microscopic surface roughness on the various gold surfaces. Microscopic roughness and the presence of crystal grain boundaries are concluded to be very important to the quality of the monolayers, while macroscopic roughness, even when quite prominent, appears to be less important.
Introduction The chemistry of self-assembled monolayer films has received considerable attention. The well-ordered, closepacked nature of many of these films makes them ideal model systems for testing new surface analytical techniques that are sensitive to molecular pr0perties.l They also offer an attractive means of building chemical functionality into an interface, with the eventual goal of defining and controlling interfacial reactivity patternsS2The attribute of these films that has proved most difficult to characterize is the defect sites; many potential applications of molecular thin films require that they be extremely free of defects, yet there are often no reliable diagnostics for determining what the nature and density of defect sites are, what causes them, and how to prevent them. This is a difficult analytical problem, since a very small number of defect sites could have a major impact on the behavior of the monolayer. Much of the ongoing research in this area has involved searching for new ways to prepare monolayers in which the density of defect sites is very low. A particularly attractive and heavily studied system is that of alkanethiols and related molecules on gold surface^.^-'^ Almost all work to date on this system has (1) In-situ Characterization OfElectrochemical Processes; Publication NMAB 438-3; National Academy Press: Washington, DC, 1987. (2) (a) New Horizons in Electrochemical Science and Technology; Publication NMAB 438-1; National Academy Press: Washington, DC, 1986. (b) Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, E. A.; Garoff, S.; Isrealachvili, J.; McCarthy, T. J.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wynne, K. J.; Yu, H. Langmuir 1987, 3, 932. (3) (a) Porter, M. D.; Bright, T.; Allara, D.; Chidsey, C. E. D. J. Am. Chem. SOC.1987,109,3559. (b) Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990,6,682. (c) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J . Am. Chem. SOC.1990,112,4301. (d) Chidsey, C. E. D. Science 1991, 251, 919. (4) (a) Finklea, H. 0.; Avery, S.; Lynch, M.; Furtsch, T. Langmuir 1987,3,409. (b) Finklea, H. 0.;Snider, D. A.; Fedyk, J. Langmuir 1990, 6. 371. (5) (a) Rubinstein, I.; Steinberg,S.;Tor, Y.; Shanzer, A.; Sagiv,J. Nu ture 1988,332,426. (b) Sabatani, E.; Rubinstein, I. J. Phys. Chem. 1987,91, 6663. (6) (a) Bain, C. D.; Evall, J.; Whitesides, G. M. J . Am. Chem. SOC.1989, 111, 7155. (b) Bain, C. D.; Whitesides, G. M. J. Am. Chem. SOC.1989, 111, 7164. ( c ) Bain, C. D.; Whitesides, G. M. Angew. Chem., Int. Ed. Engl. 1989,28,506. (d) Bain, C. D.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1989,5, 723. (e) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87.
involved self-assemblyonto either nominally flat gold thin films prepared by thermal evaporation or sputtering of gold onto either silicon or glass or, less frequently, onto bulk gold single crystals prepared by sputtering and annealing in ultrahigh vacuum. Our interest in expanding the scope of the gold/thiol system has led us to explore several alternative procedures for preparing polycrystalline bulk gold substrates prior to monolayer deposition. It seemed reasonable that if evaporated or sputtered thin films are suitable as substrates, then a suitably prepared bulk gold substrate would also work and might have the additional advantage of being more convenient in certain applications. We report here on the “defectiveness” of monolayers of alkanethiols on polycrystallinegold pretreated by oxidative chemical etching in aqua regia. We have characterized the gold substrates by lead underpotential deposition, gold oxide stripping, scanning electron microscopy, and scanning tunneling microscopy. The quality of a monolayer is judged by its ability to inhibit electron transfer between the underlying gold electrode and a redox-active molecule present in the contacting solution, and by monitoring exchange of chemisorbed alkanethiols,present after a first coating step, with other “tagged” alkanethiol molecules from solution. The tagged molecules have a redox-active ferrocenemoiety linked to an alkanethiol to facilitate quantitation of the exchange reaction. We find that, after appropriate chemical pretreatment, bulk gold electrodes are in fact quite suitable as substrates for self-assembly and are in many ways superior to the more conventional evaporated gold thin film substrates. Microscopic rough(7) Fabianowski, W.; Coyle, L. C.; Weber, B. A.; Granata, R. D.; Dastner, D. G.; Sadownik, A.; Regen, S. L. Langmuir 1989, 5 , 35. (8)Miller, C.; Cuendet, P.; Gratzel, M. J. Phys. Chem. 1991,95, 877. (9) BundingLee, K. A. Langmuir 1990, 6, 709. (10) (a) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. SOC. 1990, 112,558. (b) Dubois, L. H.; Zegarski, B. R.; Nuzzo, R. G. J. Am. Chem. SOC.1990,112, 570. ( c ) Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H. J. Am. Chem. SOC.1987,109, 733. (11)Widrig, C. A.; Alves, C. A.; Porter, M. D. J . Am. Chem. SOC.1991, 113, 2805. (12) Thomas, R. C.; Sun, L.; Crooks, R. M.; Ricco, A. J. Langmuir 1991, 7, 620. (13) Evans, S. D.; Sharma, R.; Ulman, A. Langmuir 1991, 7, 156. (14) Nordyke, L. L.; Buttry, D. A. Langmuir 1991, 7, 380.
0 1992 American Chemical Society
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Surface Roughness and Molecular Self- Assembly
Langmuir, Vol. 8, No. 3, 1992 855
ness appears to be much more critical than macroscopic roughness in determining how substrate morphology affects the properties of self-assembled monolayers.
Experimental Section Materials. Bulk gold electrodes were from Johnson-Matthey (99.9985% ) and were cut into 8 X 8 X 1mm squares, annealed at -lo00 "C in air, polished to mirror smoothness by standard metallographicmethods,and etched in aqua regia (3:l HC1:HNOs; CAUTION) to remove the damaged layer left from the polishing procedure. Subsequent surface preparation steps generally involved briefly repolishing and re-etching the electrode. Silicon wafers (2 in. diameter, Polishing Corporation of America) were pretreated as described below. Mica (UnimicaCorp.) was cleaved with the aid of a water-filled syringe needle prior to use. Alkanethiols (Aldrich) and (3-mercaptopropyl)trimethoxysilane (Petrarch) were used as received; (hydroxymethy1)ferrocene (Strem) was kept as a 0.2 M stock solution in acetone and was added to aqueous electrolytes as needed via microliter syringe. This was done to alleviate problems associated with slow dissolution of solid (hydroxymethy1)ferrocenein water. Control experiments showed that acetone itself had no effect on the electrochemical behavior. Water was purified with a Barnstead Nanopure system. Preparation of Gold Electrodes. Thin-film electrodes were prepared by evaporation of 2000 A of gold either onto silicon wafers that had been pretreated by etching in piranha solution (1:lH2SOd:H202; CAUTION) or onto freshly cleaved mica. Mica substrates were typically heated to 300 "C during deposition to promote epitaxial formation of crystallites with preferential exposure of the Au(ll1) face. Two different methods were used to promote adhesion of gold to silicon. In the first method, a 7-A chromium underlayer was deposited by vacuum evaporation prior to depositing the gold. (Cyclicvoltammetry showed no evidence of chromium on the exposed electrode surface after gold deposition. Use of thicker chromium layers was avoided as voltammetry then showed chromium to be present on the electrode surface.) In the second method, oxidized silicon wafers were coated with a monolayer of (3-mercaptopropyl)trimethoxysilane (MPS) by the method of Goss and co-workers.15 Briefly, this involved two treatments of the wafer with a hot solution of MPS and water in 2-propanol (1:1:40 (w/w)). Each treatment was for approximately 10min, followed by 10min of curing in a glassware oven at 115 "C. Wafers were then mounted in the vacuum chamber and coated with gold. In one instance, a coated wafer was then annealed a t 200 "C for 24 h prior to removal from vacuum. Bulk gold electrodes were polished by hand on a soft cloth with an aqueous slurry of 0.3-pm alumina and briefly sonicated in soapy water followed by distilled water to remove the alumina. They were then etched in either concentrated aqua regia (3:l HCl:HN03) or dilute aqua regia (3:1:6 HCLHN03:H20) in a quartz test tube, at room temperature, for 5 min. Preparation of Monolayers. After appropriate surface treatment, electrodes were rinsed with water followed by ethanol and immersed in an alkanethiol coating solution for not less than 12 h. Coating solutions were 0.10 M C12H26SH90.020 M C16H&H, and 0.020 M C&7SH, all in absolute ethanol. Neither the concentration of thiol nor the coating time appeared to be particularly critical, although monolayers prepared from brief (
