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Langmuir 1994,10, 4566-4573
Alkanethiol Monolayers on Preoxidized Gold. Encapsulation of Gold Oxide under an Organic Monolayer Hannoch Ron and Israel Rubinstein* Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot 76100, Israel Received August 1, 1994@ The adsorption of dodecyl mercaptan (DM) monolayers onto plasma preoxidized gold was studied. The degree of preoxidation was determined quantitatively using electrochemical stripping analysis. It is shown that (i)alkanethiolsform oriented monolayers on gold oxide surfaces. (ii)The contact angles and ellipsometric parameters of DM monolayers on gold vary systematicallywith the degree ofpreoxidationofthe evaporated gold substrate. (iii) The monolayer effectively isolates the oxide from its surrounding, thus preventing chemical and electrochemical reduction of the inherently unstable gold oxide. The existence of a layer of encapsulated gold oxide under the organic monolayer is verified by X-ray photoelectron spectroscopy (XPS) and combined electrochemical-depth profiling measurements.
Introduction Self-assembled (SA) monolayers chemically adsorbed onto solid surfaces have been of growing interest in recent years, from both basic and technological points of view. Organosulfur amphiphiles form SA monolayers on as well as on ~ i l v e f land ~-~~ o p p e f l ~ surfaces; J ~ , ~ * ofthese, gold has the unique advantage of chemical inertness. Indeed, gold is by far the most commonly used metal for self-assembly. Of particular interest is the fact that a gold surface is not oxidized under ambient condition^,^^" while its high surface energy is lowered by adsorption of organic molecules from its surrounding.60 Hence, the cleanliness of the gold surface plays a major role in the self-assemblyprocess, and most published studies contain,
* Author to whom correspondence should be addressed. Abstract published in Advance ACS Abstracts, November 1, 1994. (1)Nuzzo, R. G.; Allara, D. L. J . Am. Chem. SOC. 1983,105,4481. (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. (3)Li, T.-T.-T.; Weaver, M. J. J . Am. Chem. SOC. 1984,106,6107. (4)(a) Allara, D. L.; Nuzzo, R. G. Langmuir 1986,1,45.(b) Allara, D. L.; Nuzzo, R. G. Langmuir 1986,1,52.(c) Nuzzo, R. G.; Fusco, F. A.; Allara, D. L. J . Am. Chem. SOC. 1987,109,2358.(d) Nuzzo, R. G.; 1987,109,733. (e) Zegarski, B. R.; Dubois, L. H. J . Am. Chem. SOC. 1990, Dubois, L. H.; Zegarski, B. R.; Nuzzo, R. G. J . Am. Chem. SOC. 112. 570. (0Nuzzo. R. G.: Dubois. L. H.: Allara. D. L. J.Am. Chem. Soc.’ 1990,112,558.’(g) Nuzzo, R. G.; Korenic, E: M.; Dubois, L. H. J. Am. Chem. SOC. 1990,93,767. (5)Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J . Am. 1987,109,3559. Chem. SOC. (6)Harris, A. L.; Chidsey, C. E. D.: Levinson. N. J.: Loiacono, D. N. Chem. Phys. Lett. 1987,141,350. (7)(a)Sabatani, E.; Rubinstein, I.; Maoz, R.; Sagiv, J.J . Electroanal. Chem. 1987,219,365.(b) Sabatani, E.;Rubinstein, I. J . Phys. Chem. 1987,91,6663. (c) Rubinstein, I.; Steinberg, S.; Tor, Y.; Shanzer, A.; Sagiv, J. Nature 1988,332,426. (8)Troughton, E.B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D. L.; Porter, M. D. Langmuir 1988,4,365. (9)(a) Bain, C. D.; Whitesides, G. M. J . Am. Chem. SOC.1988,110, 1988,110, 3665. (b) Bain, C. D.; Whitesides, G. M. J.Am. Chem. SOC. 6560. (c)Bain, C. D.; Whitesides, G. M. Science 1988,240,62.(d) Bain, C. D.; Whitesides, G. M. Angew. Chem., Int. Ed. Engl. 1989,28,506. (e) Bain, C. D.; Whitesides, G. M.; Evall, J . J . Am. Chem. SOC. 1989,111, 7155. (fl Bain, C. D.; Whitesides, G. M.; Evall, J. J.Am. Chem. SOC. 1989,111,7164.(g) Bain, C. D.; Whitesides, G. M. Langmuir 1989,5, 1370. (h) Bain, C. D.; Whitesides, G. M. J.Phys. Chem. 1989,93,1670. (i) Bain, C. D.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1989,5, 723. (10)Whitesides, G. M.; Laibinis, P. E. Langmuir 1990,6, 87. (11)Strong, L.; Whitesides, G. M. Langmuir 1988,4,546. (12)Prime, K. L.; Whitesides, G. M. Science 1991,252, 1164. (13)Hickman, J. J.;Laibinis, P. E.; Auerbach, D. I.; Zou, C.; Gardner, T. J.; Whitesides, G. M.; Wrighton, M. S. Langmuir 1992,8, 357. (14)Stefely, J.; Markowitz, M. A.; Regen, S. L. J.Am. Chem. SOC. 1988,110,7463. (15)Bravo, B. G.; Michelhaugh, S. L.; Soriaga, M. P. J . Electroanal. Chem. 1988,241,199. @
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or refer to, a certain procedure of surface treatment prior to monolayer absorption. Some of the common procedures for cleaning gold surfaces include exposure to powerful oxidizing agents, such as oxygen p l a ~ m aU , ~V l ~ z o n eand , ~ ~ a mixture of concentrated sulfuric acid and hydrogen peroxide (“piranha” solution).21 (Electrochemical oxidation pretreatment is different in that the oxidation is usually followed (16)Hautman, J.; Mien, M. L. J . Chem. Phys. 1989,91,4994. (17)Ulman, A. Introduction to Ultrathin Organic Films; Academic Press: New York, 1991. (18)Ulman, A. Fourier Transform Infrared Spectroscopy in Colloid and Interface Science; Scheuing, D. R., Ed.; ACS Symposium Series 447,American Chemical Society: Washington, DC, 1990;pp 144-159. (19)Ulman, A. J. Mater. Educ. 1989,11, 205. (20)Ulman, A.; Eilers, J. E.; Tillman, N. Langmuir 1989,5,1147. (21)Evans, S. D.; Sharma, R.; Ulman, A. Langmuir 1991,7, 156. (22)Widrig, C. A.; Alves, C. A,; Porter, M. D. J.Am. Chem. SOC.1990, 113,2805. (23)Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. 1990,112,4301. M. J . Am. Chem. SOC. (24)Chidsey, C. E. D.; Liu, G.-Y.; Rountree, P.; Scoles, G. J . Chem. Phys. 1989,91,4421. (25)Chidsey, C. E. D. Science 1991,251,919. (26)Trevor, D. J.;Chidsey, C. E. D.; Loiacono, D. N. Phys. Rev. Lett. 1989,62,929. (27)Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990,6, 709. (28)Camillone, N., III; Chidsey, C. E. D.; Liu, G.-Y.; Putvinski, T. M.; Scoles, G. J . Chem. Phys. 1991,94,8493. (29)(a) Finklea, H. 0.;Avery, S.; Lynch, M.; Furstsch, T. Langmuir 1987,3,409. (b) Finklea, H. 0.; Snider, D. A.; Fedyak, J. Langmuir 1990,6, 371. (c) Finklea, H. 0.; Hanshew, D. D. J . Am. Chem. SOC. 1992,114,3173. (30)Bunding-Lee, K. A. Langmuir 1990,6, 709. (31)(a) Delong, H. C.; Buttry, D. A. Langmuir 1990,6, 1319. (b) Delong, H. C.; Donohue, J. J.; Buttry, D. A. Langmuir 1991,7,2196. (32)Sun, L.; Johnson, B.; Wade, T.; Crooks, R. M. J . Phys. Chem. 1990,94,8869. (33)Sun, L.; Crooks, R. M. J . Electrochem. SOC. 1991, 138,L23. (34)Barner, B. J.; Corn, R. M. Langmuir 1990,6, 1023. (35)Gratzel, M. J . Phys. Chem. 1991,95,876. (36)Uosaki, K.;Sato, Y.;Kita, H. Langmuir 1991,7, 1510. (37)Rowe, G. K.; Creager, S. E. Langmuir 1991,7,2307. (38)Creager, S.E.; Hockett, L. A.; Rowe, G. R. Langmuir 1992,8, 854. (39)Saman, M. G.; Brown, C. A.; Gordon 11, J. G. Langmuir 1991, 7.437. (40)Kim, Y. T.; Bard, A. J. Langmuir 1992,8, 1096. (41)Buchholtz, S.;Fuchs, H.; Rabe, J. P. Adv. Mater. 1991,3,51. (42)Alves, C. A.; Smith, E. L.; Porter, M. D. J . Am. Chem. SOC. 1992, 114,1222. (43)Sondag-Huethorst, J . A. M.; Fokkink, L. G. J . Langmuir 1992, 8,2560. (44)Tarlov, M. J.; Newman, J . T. Langmuir 1992,8, 1398. (45)Kumar, A.; Biebuyck, H. A.; Abbott, N. L.; Whitesides, G. M. J. Am. Chem. SOC. 1992,114,9188. (46)Yunzhi, Li; Jingyn Huang; McIver, R. T., Jr.; Hemminger, J. C. J.Am. Chem. SOC. 1992,114,2428. (47)Laibinis, P. E.;Fox, M. A.; Folkers, J. P.; Whitesides, G. M. Langmuir 1991, 7,3167. ?
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Alkanethiol Monolayers on Preoxidized Gold by reduction.29a)These cleaning procedures have proven efficient in removing organic contaminations; what is hardly ever considered is the fact that the oxidizing treatments tend to oxidize the gold surface itself. For example, a gold surface is known to oxidize when treated with even mild oxygen plasma.2,61 Gold oxide is a thermodynamically unstable compound and is decomposed in the ambient.59bNonetheless, some attempts to self-assemble monolayers on gold surfaces oxidized prior to adsorption have been reported. Finklea et a1.62studied octadecyltrichlorosilane monolayers adsorbed onto anodically preoxidized gold surfaces. On the basis of cyclic voltammetry measurements, they concluded that the gold oxide is reduced by Sic13 moieties. Similar observations based upon XPS measurements were reported by Fischer et al.63in an attempt to adsorb 1,l’ferrocenediyldichlorosilane onto anodized gold surfaces. Nuzzo and Allara’ assembled monolayers of disulfides on zero-valent gold, which were then characterized by water contact angles, ellipsometry, and grazing angle Fourier transform infrared (GFTIR)spectroscopy. When adsorbed onto surfaces pretreated with oxygen plasma, the presence of a monolayer could not be detected. In another study, Hickman et al.64 reported successful formation of disulfide monolayers on oxygen plasma pretreated gold surfaces. XPS measurements could not detect any oxidized gold after monolayer adsorption. Bain et a1.2were unable to generate good n-alkanethiol monolayers consistently on gold surfaces cleaned by oxygen plasma, presumably due to the formation of a metastable gold oxide. It is therefore clear that, while oxidizing pretreatments are commonly applied in the study of SA monolayers on gold, the nature and consequences of adsorption of organosulfur monolayers onto gold surfaces pretreated by such procedures are not well understood. We have used dc oxygen plasma successfullyfor cleaning gold substrates prior to monolayer adsorption, as previously r e p ~ r t e d Reproducible .~ results were obtained only upon using short plasma cleaning under mild conditions. The ellipsometric parameters were measured before and after oxygen plasma cleaning, as well as after adsorption of the SA monolayer. ARer short oxygen plasma treatment the optical parameters of the gold slightly change and the water contact angle (CAI is effectively zero. This was interpreted as reflecting gold surface cleaning from organic contaminants. The monolayer thicknesses calculated using the gold parameters before and after mild plasma treatment were comparable and reproducible. When the exposure to oxygen plasma is prolonged, significant changes in measurable quantities are observed. (48) Laibinis,P. E.;Whitesides,G. M.;Allara,D. L.;Tao,Y.-T.;Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. SOC.1991,113, 7152. 1992,114, (49) Laibinis, P. E.; Whitesides, G. M. J . Am. Chem. SOC. 1990. (50)Widrig, C. A.; Chinkap, C.; Porter, M. D. J . electroanal. Chem. 1991, 310, 335. (51) Walczak,M. M.; Chinkap, C.; Stole, S. M.;Widrig, C. A.; Porter, 1991.113. 2370. M. D. J . Am. Chem. SOC. (52) Sandorf, C. J.; Garoff, S’.; Leung, K. P. Chem. Phys. Lett. 1983, 96, 547. (53) Arndt, Th.; Schupp, H.; Schrepp, W. Thin Solid Films 1989, 178, 319. (54) Harris, A. L.; Tothberg,L.; Dubois, L. H.; Levinson, N. J.; Dhar, L. Phys. Rev. Lett. 1990, 64, 2086. (55) (a) Sobocinski, R. L.; Bryant, M. A.; Pemberton, J. E. J . Am. 1990,112,6177. (b)Bryant, M. A.; Pemberton,J. E. J.Am. Chem. SOC. Chem. SOC.1991,113,3629. (c)Bryant, M. A.; Pemberton,J. E. J.Am. 1991,113,8284. Chem. SOC. (56) Fenter, P.;Eisenberger,P.;Jun Li; Camillone,N., 111;Bemasek, S.; Scoles, G.; Ramanarayanan, T. R.; Liang, K. S. Langmuir 1991, 7 , 2013. (57) (a) Blackman, L. C. F.; Dewar, M. J. S. J.Am. Chem. SOC.1957, 171. (b) Blackman, L. C. F.; Dewar, M. J. S.; Hampson, H. J.Appl. Chem. 1957, 7, 160.
Langmuir, Vol. 10, No. 12,1994 4567 The differences in the ellipsometric parameters before and after plasma cleaning increase, and hence the monolayer thicknesses calculated using the parameters of the gold before and &er oxygen plasma cleaning become substantially different. The CAS for the solvents used (water, bicyclohexyl, hexadecane) also change, and the ellipsometry and CA measurements become less reproducible. Hence, oxidizing pretreaments of gold prior to self-assembly,with all their clear merits, have one major drawback: The distinction between the conditions which produce an apparently “clean” surface and those which lead to erroneous results is not always clear and easy to identify. The cited literature reports and our own observations imply that the degree of gold oxidation prior to adsorption may influence strongly the characterization and possibly the properties of the SA monolayer, a topic well worth investigating. Moreover, adsorption of organosulfur monolayers on natively oxidized silver and copper surfaces has been receiving increasing attention r e ~ e n t l y . ~ ~The -~O influence of the oxide layer on the quality and the characterization of monolayers on active metals is far from being u n d e r s t ~ o dand ~ ~presents ,~~ substantial hindrance to such investigations. A quantitative study on the role of the oxide in monolayer formation and characterization in the case of metals such as silver, copper, or iron is complicated by the fact that controlling the amount of oxide on the surface of these metals under ambient conditions is practically impossible. Thus, absorption of a simple organothiol monolayer on a gold surface oxidized in a controlled manner may serve as a useful model and a starting point for the understanding of SA processes on natively oxidized surfaces. The first objective of the present work is therefore to investigate quantitatively the formation of SA monolayers on gold surfaces pretreated by oxidizing procedures prior to adsorption, and particularly the effect of surface preoxidation on monolayer characterization. The second objective concerns the fate of the thin gold oxide film following monolayer adsorption, i.e. whether or not the inherently unstable oxide persists under a single monomolecular layer. The latter question is of fundamental interest, as it would demonstrate (i) the exceptional blocking capability of alkanethiol monolayers and (ii)the possibility to “encapsulate7’an unstable compound under a single monolayer.
Experimental Section Materials. Dodecyl mercaptan (DM) (Merck, ’98%) was distilled under reduced pressure. Chloroform (Biolab, AR), bicyclohexyl (BCH)(Aldrich,AR), and hexadecane (HD) (Sigma, AR)were passed through a column of activated basic alumina (Alumina B, Akt. 1, ICN). Water was triply distilled. Ethanol (Merck, AR),acetone (Biolab,AR),&So4 (Merck, 95-97%), and LiC104 (Merck, AR) were used as received. Gases used were argon (99.996%),oxygen (99.5%),and dry purified air. (58) Laibinis, P. E.; Whitesides, G. M. J . Am. Chem. SOC.1992,114, 1990. (59) (a)Trapnell, B. M. W. Proc. R . SOC. London 1953,A218,566. (b) Somorjai, G. A. Chemistry in Two Dimensions: Surfaces; Comell University Press: Ithaca, NY,1981. (60) Smith, T. J . Colloid Interface Sci. 1980, 75, 51. (61) (a) Pireaux, J. J.; Liehr, M.;Thiry, P. A.;Delrue, J. P.;Caudano, R. Surf. Sci. 1984,141,211. (b) Pireaux, J. J.; Liehr, M.; Thiry, P. A.; Delrue, J. P.; Caudano, R. Surf. Sci. 1984,141, 221. (62) Finklea, H. 0.; Robinson, L. R.;Blackbum, A.;Richer, B.;Allara, D. L.; Bright, T. B. Langmuir 1986,2, 239. (63) Fischer, A. B.; Wrighton, M. S.; Umana, M.; Murray, R. W. J. Am. Chem. SOC. 1979,101, 3442. (64) Hickman, J. J.; Laibinis, P. E.;Auerbach, D. I.;Zou, C.; Gardner, T. J.; Whitesides, G. M.; Wrighton, M. S. Langmuir 1992, 8, 357. (65)Coburn, J. W. Plasma Etching and Reactive Ion Etching; American Vacuum Society Monograph Series, IBM Research Laboratory: San Jose, CA, 1982; pp 1-38
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4568 Langmuir, Vol. 10, No. 12, 1994
Preparation of Gold Substrates. (111)textured gold films were prepared in a cryogenically-pumped resistive evaporator. Gold (1000 A) (99.99%)was evaporated from a tungsten boat at 4x Torr, at a deposition rate of 1&s, onto glass microscope cover slides (Deckglass, Superior).@ Slides (22 x 11mm) were chemically cleaned with acetone followed by mild Ar plasma cleaning prior to gold evaporation. Monolayer Preparation on Bare Gold Substrates. Following gold evaporation, the chamber was opened by letting argon in. The ellipsometric parameters of no more than three slides were measured, and the slides were immediately immersed into the adsorption solution (2 mM solution of DM in ethanol) for 2 h. m e r removal from the solution the slides were rinsed twice with chloroform and ethanol and dried under a stream of dry air. Monolayer Preparation on Oxidized Gold Substrates. The plasma treatment was carried out in an Edwards S150B sputter-coater usingoxygen plasma at 1x 10-l Torr, 4 mA. Prior to oxidation the chamber was alternatingly purged 5 times with argon and oxygen. The amount of oxidation was controlled by controlling the duration of the plasma treatment. A n identical degree of oxidation was observed with slides that were successively oxidized under the same conditions. In a typical experiment two identical gold-coated slides were oxidized by 0 2 plasma under the same conditions and their optical constants measured. One of the slides was then used for DM adsorption, while the other slide was electrochemically stripped (see below) to determine the amount of oxide on the slides. The adsorption was carried out as above. Ellipsometry. Measurements were carried out before and immediately after plasma oxidation of the gold substrate (the gold oxide refractive index used t o estimate the oxide thickness is nf= 3.24, kf = 1.09, calculated from ref 671, as well as before and immediately aRer monolayer adsorption (the monolayer refractive index used to obtain the monolayer thickness is nf= 1.5, K f = O).2 A Rudolph Research Auto-EL IV ellipsometer with a monochromated tungsten-halogen light source was used, at an angle of incidence CD = 70" and a wavelength 1 = 6328 A. The same three points were measured on the bare and the monolayer covered slide. ContactAngle Measurements. Contact angles (advancing and receding) of HzO, BCH, and HD were measured within 10 min after removal of the slides from the adsorption solution. Three measurements at different spots were carried out with each solvent. A Rame-Hart NRL Model 100 contact angle goniometer was used. The error in the contact angle measurements was f2" (for BCH and HD) and f 3 " (for water). Electrochemical Measurements. The electrochemical system included a potentiostat and an electrochemicalprogrammer, both from the Department of Chemistry, Technion, Haifa, and a Houston Instruments Model 100 x-y recorder. The counter and reference electrodes were, respectively, a Pt flag (1x 1cm2) and a mercurous sulfate reference electrode (MSE, +0.400 V vs KC1 saturated calomel electrode, SCE). For the sake of clarity, all potentials are reported with respect t o a SCE. Two types of measurements were carried out: (i) Electrochemical stripping of gold oxide: Immediately following ellipsometric characterization the slide was mounted as a working electrode in a three-electrode cell containing 0.1 M H2S04. The gold oxide was reduced by applying a potential scan from +1.025 to +0.200 V (vs SCE) at 0.1V/s (the same method was previously used to evaluate the amount of oxide produced by electrochemical oxidation of g01d).62,68369 The stripping was usually followed by an additional scan in the same voltage range, to verify the total removal of the oxide in the first scan. (ii)Open-circuit potential decay (OCPD) of preoxidized slides: This technique involves the measurement of the potential of an electrode (vs a reference electrode) as a function of time at open circuit (no applied potentialL70 The open-circuit potential oftreated electrodes was measured vs a SCE reference electrode (through a salt bridge, (66)Golan,Y.;Margulis, L.; Rubinstein, I. Su$. Sci. 1992,264,312. (67)Kim, Y. T.;Collins, R. W.; Vedam, K. Surf. Sci. 1990,233,341. (68)Rand, D.A. J.; Woods, R. J. Electroanal. Chem. 1971,31,29. (69)Oesch, U.;Janata, J. Electrochim. Acta 1983,28,1237. (70)Celdran, R.; Gonzalez-Velasco,J.J.Electrochem. SOC.1988,132, 2373.
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Figure 1. (a)(-) Cyclic voltammogram for the electrochemical oxidation-reduction of a gold electrode in 0.1 M HzS04 (scan rate, 100 mV/s, electrode area, 1.3 cm2). (- -) Same, in pH = 7.0 potassium phosphate buffer. (b) Linear potential scan in 0.1 M H2S04 for the stripping of gold oxide produced by exposure of a gold electrode to (-1 oxygen plasma for 2 min; (- -) UV/ ozone treatment; (. no pretreatment (scan rate, 100 mV/s; electrode area, 1.1cm2). e)
0.1 M LiC104) in either ethanol or in 0.1 M HzS04, using the potentiostat (in an open-circuit mode) and the recorder (in a y-t mode). X-rayPhotoelectron Spectroscopy (XPS). Measurements were carried out at the Facultes Universitaires Notre-Dame de la Paix, Laboratoire LISE, Namur, Belgium, using a Surface Science SSX-100 ESCA instrument. A beam size of 1000 pm was used for the survey scans and 600 pm for the narrow scans. Monolayer Etching. Argon ion etching of monolayers controlled by Auger electron spectroscopy(AES)was carried out at the Surface Analysis Laboratory, Technion, Haifa, using a scanning Auger spectrometer (Perkin-Elmer PHI model 590-A). UV/Ozone Treatment of Bare Gold. Gold-covered slides were treated 10 min with ozone in a UV/ozone cleaner (UVOCS, Inc., Model TlOxlO/OES-E). In the present study the amount of resultant oxide is qualitative and varies with the distance from the U V radiation source.
Results and Discussion Quantitative Analysis of Gold Oxide Using Controlled Electrochemical and Chemical Decomposition. Cyclic voltammetry (CV) a n d open-circuit potential decay (OCPD) were used to study quantitatively the oxidation of gold surfaces by reactive oxygen plasma. Figure l a (solid line) shows a voltammogram obtained with a gold electrode in 0.1 M HzS04 upon cycling the potential between +0.20 a n d $1.41 V (vs W E ) at a scan rate of 0.10 VIS. In this solution the anodic oxidation of gold takes place at potentials positive of $1.03 V, while the reduction of t h e gold oxide is observed upon reversing the scan direction, between $1.03 a n d +0.63 V. In pH = 7 buffer the anodic oxidation of gold occurs at potentials positive of +0.65 V, while the reduction of the gold oxide takes place between +0.65 and +0.20V, as shown in Figure l a (dashed line). The amount of oxide produced in the anodic oxidation of gold can be quantitatively determined by integrating the area under the reduction peak.68,69 Figure l b (solid line) shows a voltammogram for a similar gold-coated slide pretreated in a reactive oxygen
Alkanethiol Monolayers on Preoxidized Gold plasma, while the gold in Figure l b (dashed line) was pretreated in a UV/ozone chamber. Here the potential scan was initiated at 1.025 V, in the negative direction. When the potential is scanned from +1.025 to +0.200 V, sizable reduction peaks are observed, corresponding to reduction of gold oxide formed by the oxygen plasma and W/ozone treatments. Hence, gold oxide produced as a result of 0 2 plasma, as well as UV/ozone treatment, can be quantitatively reduced electrochemically (“stripped”). Note that when the slide is not preoxidized, or when it is exposed to argon plasma, no reduction peak is observed upon applying a stripping cycle, and a curve similar to the one shown in Figure l b (dotted line) is observed. Electrochemical stripping was not possible when goldcoated slides used in the present work were pretreated in “piranha” solution, as the gold films peeled off. Nevertheless, OCPD measurements described below indicate that the gold surface is oxidized t o some extent following treatment in “piranha” solution. The amount of oxide produced upon exposure of gold to oxygen plasma and ozone treatment can be quantitatively determined using electrochemical stripping, as shown above (Figure lb) and previously reported (for 0 2 plasma) by Armstrong and White.71 Note that Sondag-Huethorst and Fokkine3 concluded, based upon cyclic voltammetry measurements, that a gold surface is not oxidized upon exposure to UV/ozone. This seemingly erroneous observation appears to be due to the fact that they started the potential scan at a potential negative of the gold oxide reduction, where the oxide is reduced instantaneously. Figure l b shows unequivocally that gold surfaces treated under UV/ozone conditions are oxidized. Gold oxide is known to be an unstable compoUnd,G1and it decomposes rapidly in the ambient unless special care (such as reduced temperature or controlled environment) is taken. It should be noted, however, that the amount of oxide (as measured by voltammetric stripping) remains practically unchanged during the time needed to perform our experiments (i.e. within 15 min of the gold oxidation). The potentials where oxidatiodreduction of gold takes place depend upon the pH of the solution (Figure la). In addition, the peak shape and position vary with the composition of the gold oxide.69,72r73 Comparison of the voltammetric shape for the stripping of gold oxide (in 0.1 M H2SO4) produced by exposure to 0 2 plasma and UV/ ozone (Figure lb) with that for the anodic gold oxide (Figure l a , solid line) shows a marked similarity, suggesting that the oxide produced by the former treatments is also Au203. The latter conclusion is supported by XPS studies of gold oxide produced by anodization of gold in an aqueous solution, carried out by Dickinson et al.74as well as by Peuckert et al.,75and by Amstrong and White’s study71 concerning the exposure of gold to oxygen rf plasma. Comparison with XPS measurements of gold oxides produced upon exposure to 0 2 plasma by Pireaux et al.‘jlb shows again that gold oxidation by 0 2 plasma produces AU203. The thickness of the gold oxide produced by 0 2 plasma was measured ellipsometrically, as listed in Table 2 for various exposure times. The oxide thicknesses in Table (71) Amstrong, N.R.;White, J. R. J . Electroanul. Chem. 1982,131, 121. (72) Bruke, L.D.In Electrodes of Conductive Metallic Oxides, Part A, Trasatti, S.,Ed.; Elsevier: Amsterdam, 1980; p 141. (73) Angerstein-Kozlowska, H.; Conway, B. E.; Hamelin, A.; Stoicoviciu, L.J . Electround. Chem. 1987, 228, 429. (74) Dickinson, T.; Povey, A. F.; Shenvood, P. M. A. J . Chem. Soc., Faraday Trans. 1 1975, 7 1 , 298. (75) Peuckert, M . ; Coenen, F. P.; Bonzel, H. P . Surf. Sci. 1984,141, 515.
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20
Time [min]
Figure 2. Open-circuit potential decay of gold electrodes after the followingtreatments: (1)2 min in oxygen plasma (measured in pH = 7.0potassium phosphate buffer); (2) 2 min in oxygen plasma (measured in ethanol); (3) 6 min in oxygen plasma (measured in ethanol); (4) 0.5 min in “piranha” solution (measured in ethanol);( 5 ) 2 min in argon plasma (measured in ethanol). 2 were calculated using the refractive index of anodically grown gold oxide, derived from the results of Kim et al.42 In the above publication a thickness of 4 A corresponds to the stripping of 1400 ,uC/cmz, in noted disagreement with our present results, where a thickness of 4.2 A corresponds to 600 pC/cm2. Previous, unsuccessful attempts to determine optical properties of gold oxide were reported by Shironi and G e n ~ h a w as~well ~ as by Horkans et al.77 Our thicknesses are in good agreement with those calculated for the electrochemically-grown gold oxide in ref 69, where 4.7 A oxide (a monolayer of oxide) corresponds to 723 ,uC/cm2. It should be emphasized that the above studies involved gold oxide produced upon anodization of gold, while in our case the investigated oxide was produced upon exposure to 0 2 plasma. The influence of the aqueous environment on the composition (i.e. hydration) and optical parameters of gold oxide, as discussed p r e v i ~ u s l y ,is~still ~ , ~not ~ completely understood and may explain the disagreement in the results. The salient point in the present study is, however, that the changes i n Y and A (Table 2) show a systematic correlation between the thickness of the gold oxide and exposure time to oxygen plasma. Gold oxide is chemically reduced by alcohols, such as allyl alcohols70and various aliphatic alcohols.78 Celdran and GonzBles-Velas~o~~ carried out a comprehensive study where the OCPD technique was applied to follow the reduction ofgold oxide in aqueous solution of allyl alcohols. On the basis of their results, it is evident that Figure 2, lines 2 and 3, are characteristic of the potential decay resulting from gold oxide (as well as platinum oxide) reduction by alcohols. In particular, the final plateau potential reached (Figure 2, lines 2 and 3) is, according to C e l d r h and Gonzdes-Velasco, an indication of complete reduction of the oxide. Hence, OCPD is an efficient semiquantitative measure of the time needed for complete reduction of the gold oxide. For example, 10min in ethanol is sufficient to completely reduce the gold oxide produced by 6 min of 0 2 plasma oxidation (Figure 2, line 3). Stripping voltammetry carried out after this treatment shows a curve identical to the one in Figure l b (dotted line), i.e. no oxide reduction. (76) Shironi, R.S.;Genshaw, M . A. J . Electrochem. SOC.1970,117, 626. (77) Horkans, J.; Cahan, B. D.; Yeager, E. Surf. Sci. 1974, 46, 1. (78) Beltowaska-Brzezinska,M.EZectochim. Acta 1979,24, 409.
4570 Langmuir, Vol. 10, No. 12, 1994
Ron and Rubinstein
Table 1. Characterization of DM Monolayers Adsorbed onto Freshly Evaporated (FE) Gold Surface and onto Gold Surface Treated with 6 min of Oxygen Plasma and 10 min of Ethanol Dip (OP + Et) contact angle (deg)
HzO
ellipsometry substrate
FE OP
+ Et
-6A (deg)
- 6 (deg) ~
1.31 f 0.09 1.31 f 0.16
0.20 f 0.01 0.16 f 0.01
calcd thickness
(A)
12.5 12.5
BCH
HD
adv
rec
adv
rec
adv
rec
112 112
110 110
54 53
52 52
47 47
44 44
Table 2. Changes in the Ellipsometric 411 and A as a Result of Gold Surface Oxidation followed by Adsorption of a DM Monolayep ~~
(A)
plasma oxidation +DM adsorption (de@
- 6 after ~ plasma oxidation +DM adsorption (deg)
2.5 3.2 4.2 6.1 9.1 11.1
1.31 f 0.09 1.54 f 0.00 1.75 f 0.07 2.56 f 0.39 3.87 f 0.48 6.19 f 0.83 8.97 f 1.02
0.20 f 0.01 0.10 f 0.03 0.19 f 0.01 0.17 f 0.02 0.18 f 0.02 0.16 f 0.02 0.11 i 0.05
-6A after
slide
a
exposure to oxygen plasma (s)
amount of oxide stripped (uC/cm2)
0 20 30 60 120 240 360
0 320 460 600 830 1320 1640
plasma oxidation (ded
- 6 after ~ plasma oxidation (deg)
calculated oxide thicknessb
0.69 f 0.02 0.89 f 0.69 1.16 f 0.07 1.56 f 0.13 2.49 f 0.22 3.01 f 0.39
0.09 f 0.04 0.24 f 0.02 0.24 f 0.03 0.29 f 0.03 0.35 f 0.02 0.39 f 0.01
-dA after
The monolayer in slide 1was adsorbed onto freshly-evaporated gold (no preoxidation). Assuming that the oxide is At1203 (see text).
Figure 2 compares the effect of oxygen plasma, argon plasma, and “piranha’) solution on gold surfaces. The oxidized gold maintains a characteristic positive opencircuit potential, which decays upon reduction of the oxide by ethanol. The open-circuit potential range for the gold oxide reduction in ethanol (Figure 2, line 2) is in qualitative agreement with the voltammetry in neutral solution (Figure la). Moreover, Figure 2 provides additional support for the stripping results, showing unequivocally that, while reactive 02 plasma produces oxidized gold, argon plasma leaves a reduced gold surface. It also indicates that treatment in “piranha” solution results in an oxidized gold surface and, in addition, that gold oxide is relatively stable in aqueous sulfuric acid. DMMonolayers on Gold Preoxidized by 0 2 Plasma and Reduced by Ethanol. The characterization of a DM monolayer adsorbed onto freshly-evaporated gold and onto gold oxidized for 6 min in oxygen plasma followed by 10 min immersion in ethanol (to remove the oxide) prior to DM adsorption is shown in Table 1. I t is clear that the two DM monolayers are remarkably similar as far as contact angles and ellipsometicparameters are concerned. This indicates clearly that efficient removal of the oxide just before monolayer adsorption leads to monolayers which are indistinguishable from ones adsorbed onto freshly evaporated gold. This result provides a highly effective and reproducible pretreatment procedure for gold substrates, which combines the virtues of dry (oxygen plasma) and wet (ethanol dip) treatments to produce welldefined, reduced gold surfaces appropriate for monolayer ad~orption.~~ DM Monolayers on 0 2 Plasma Preoxidized Gold Surfaces. The effect of the gold oxide on the characterization of DM monolayers adsorbed onto plasma preoxidized gold was studied. Based upon the results shown in Figure 2, one would intuitively assume that after 2 h adsorption in a solution of DM in ethanol the gold oxide would be completely reduced. This assumption was tested by carrying out a detailed analysis of DM monolayers adsorbed onto gold slides oxidized to different degrees prior to adsorption. The amount of oxide produced during oxygen plasma treatment can be controlled by varying the exposure time to the plasma, as shown in Table 2. When two gold films (79) Ron, H.; Rubinstein, I. In preparation.
are prepared and plasma oxidized under the same conditions, the difference in the amount of oxide stripped from the two is less than 5%(denoted “equivalent slides”). Hence, monolayer characterization was carried out with DM adsorbed slides, while the amount of preoxidation was determined in each case by oxide stripping voltammetry of the “equivalent slide”. Contact Angle Measurements. Advancing and receding contact angles (CAS) of HzO, BCH, and HD are plotted in Figure 3 as a function of the amount of oxide on the gold surface before adsorption (i.e. the amount of oxide stripped from the “equivalent slide”)for DM monolayers on gold preoxidized by OZ-plasma. The CAS of all three solvents are strongly influenced by the degree of preoxidation. BCH and HD CAS on DM monolayers adsorbed onto freshly-evaporated gold are smaller than 90” (see Table 1)and exhibit the same qualitative behavior when the monolayer is adsorbed onto preoxidized gold surfaces. A n increase in the amount of oxide leads to a gradual decrease in the advancing CAS and to a pronounced decrease in the receding CAS. Hence, the CA hysteresis increases with the degre&nf preoxidation. Water CAS of DM monolayers adsorbed onto freshlyevaporated gold are higher than 90” and preoxidation results in an increase in the advancing CAS. In contrast with the organic solvents, the water CA hysteresis reaches a maximum around 800,uC/cm2,beyond which the receding CAS begin to increase. The CA results, and most notably the water CA hysteresis behavior, are in excellent agreement with the theoretical (ref 80a, Figure 10) and experimental (ref 80b, Figure 3) results of Dettre and Johnson who studied the influence of surface roughness on CA measurements. Hence, the variations in the CAS with the degree of oxidation prior to DM adsorption are characteristic of increasing surface roughness, resulting from the plasma treatment. An attempt to assess the roughness scale by using visible light produced no interference fringes, indicating that the surface roughness of the plasma preoxidized gold is below the micrometer scale. It should be emphasized that a consistent modification in the contact angles of all three solvents such as that (80)(a) Dettre, R. H.; Johnson, R. E., Jr. In Contact Angle, Wettability and Adhesion; ACS Advances in Chemistry Series 43; American Chemical Society: Washington, DC, 1964; pp 112-135. (b) ibid., pp 136-144.
Langmuir, Vol. 10, No. 12, 1994 4571
Alkanethiol Monolayers on Preoxidized Gold
120
115
1
+
I
-1 A
t
A
4
BCH
o
50
'A
n
0
HD
t
0
n
A ,
5
0
DM adsorption onto the oxidized gold surfaces promotes two evident changes: (i)Y decreases, indicating weaker absorption of light. (ii)The decrease in A upon 0 2 plasma oxidation adsorption of a DM monolayer is much too large to be attributed to the DM monolayer alone. Moreover, the changes in A after DM adsorption correlate qualitatively with the amount of oxide on the slide prior to adsorption. The results in Table 2 suggest clearly that a layer of gold oxide remains under the DM monolayer and is responsible for the systematic variation in QA.An analogous case was previously reported by Ritter and Kruger,szwho performed ellipsometric measurements of an organic layer on an oxidized surface. Although their organic transparent layer (cellulose dinitrate, nf = 1.45, kf = 0) was much thicker than a monolayer, changes in A were interpreted as changes in the oxide layer (iron oxide, nf = 2.5, kf = 0.12). The abnormal differences between QA after 0 2 plasma treatment and dA of the same slides after DM adsorption (Table 2) may indicate changes in the gold oxide composition upon DM adsorption, as also suggested by the XPS results described below. Note also that adsorption of DM monolayers from BCH solution onto preoxidized gold surfaces has the same effect on the contact angles and ellipsometric measurements as the adsorption from ethanol. A comparison between the contact angles and ellipsometric parameters of a DM monolayer adsorbed onto a gold slide oxidized in oxygen plasma followed by 10 min of immersion in ethanol prior to adsorption (Table 1)and a slide oxidized similarly in oxygen plasma and adsorbed ~ ~ ~ ~ l ~ ~ l l ~ ~ ~ ~ l ~ without reduction of the' gold oxide (Figure 3 and Table 2) shows clearly that, unless the oxide is reduced before DM adsorption, the measured values are markedly different from those of the monolayer on freshlyevaporated gold. This, and the results ofRitter and Kruger discussed suggest that the DM monolayer inhibits the reduction of the gold oxide, maintaining a stable layer of the oxide buried under the monolayer. The amount of "encapsulated" oxide is related directly to the degree of preoxidation of the gold. Electrochemical Oxide Stripping from Preoxidized DM-Adsorbed Gold. The contact angle and ellipsometry results for DM monolayers adsorbed onto preoxidized gold slides provide indirect evidence of the existence of an oxide layer under the monolayer, effectively insulated from the ambient by the compact organic monolayer. Moreover, the "encapsulated" oxide layer is not electrochemically active, as shown in Figure 4a, presumably due to the effective insulating effect of the monolayer. In order to obtain more direct evidence, the experiment described below was designed to allow electrochemical stripping of oxide from a gold slide which was plasma preoxidized and then DM adsorbed, by causing controlled damage to the monolayer while maintaining at least some of the oxide underlayer intact. For this purpose the technique of controlled sputter depth-profiling combined with Auger electron spectroscopy (AES)was applied. Controlled sputtering of the sample was carried out using a 2.5-keV argon ion beam. During the depth profiling the Auger line intensities were followed with the primary beam scanned over the sample. The sputtering was stopped when the intensity of the (KLL)carbon line decreased to 80%-50% of its initial value. Once taken out from the vacuum chamber, the slides were electrochemically stripped within 30 min.
~
~
I
500
,
L
.
~
1000
l
L
~
1500
~
~
'
~
~
~
2000
Charge [pC/cm2] Figure 3. Advancing (0) and receding (A) contact angles of HzO, bicyclohexyl (BCH) and hexadecan (HD) for DM monolayers as a function of the amount of oxide on the gold surface prior to adsorption of the monolayer. shown in Figure 3 strongly suggests that the chemical nature of the surface remains unchanged upon increasing roughness ofthe surface. As shown above (Table l), when the oxide is reduced (by ethanol) before DM adsorption, the contact angles are remarkably similar to those of a DM monolayer on freshly-evaporated gold. Hence, the roughness which modifies the contact angles is related to the gold oxide and possibly to the existence of a layer of oxide under the DM monolayer. Ellipsometic Measurements. Table 2 summarizes the results of ellipsometric measurements on gold surfaces treated by 02 plasma before adsorption. Note that the contact angle results reported in Figure 3 were measured on the same slides as in Table 2. The DM monolayer in slide 1 in Table 2 was adsorbed on freshly-evaporated gold and measured before and after DM adsorption. All other slides were measured before and after plasma oxidation and after DM adsorption. The changes in the ellipsometric A and Y after plasma treatment (Table 2) are due to the formation of gold oxide on the surface.61bThe magnitude and sign ofthese changes are consistent with an increase in the amount of oxide at longer exposure times to the 0 2 plasma, as confirmed by the electrochemical stripping. The negative changes in A correspond to an increased film thickness, while the changes in Y correspond to energy loss due to formation of the absorbing oxide.s1 (81)Gottesfeld, S.In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1989; Vol. 15,pp 143-172.
~
I
(82)Ritter, J. J.; Kruger, J. Swf Sci. 1980,96, 364.
Ron and Rubinstein
4572 Langmuir, Vol. 10,No. 12, 1994 a J
. ..... , . . . , . ..,. ., .' . .. ... .. . . :
.-c Q c
*
"
C
V
0.20
0.60
1 .oo
1
6600
E [V vs. SCE]
Figure 4. Linear potential scan in 0.1 M HzS04 covering the gold oxide strippingregion (scan rate, 100mV/s; electrode area, 0.5 cm2),for gold electrodes after the following treatments: (a) 6 min in oxygen plasma followed by adsorption for 2 h in 2 mM DM in ethanol; (b) DM monolayer adsorbed onto freshlyevaporated gold, followed by argon ion beam sputtering; (c) 6 min in oxygen plasma followed by adsorption for 2 h in 2 mM DM in ethanol, followed by argon ion beam sputtering.
Figure 4c shows a linear potential scan for the stripping of gold oxide from an ion-beam damaged DM monolayer adsorbed onto a gold-coated slide treated 6 min in oxygen plasma prior to DM adsorption. The characteristic gold oxide reduction peak indicates the existence of a gold oxide layer that was protected from chemical and electrochemical reduction by the DM monolayer. Hence, electrochemical reduction becomes possible only upon partial removal of the monolayer. The detected gold oxide (Figure 4c) is not related to the argon ion etching, as argon plasma was shown above (see Figure 2) to be a nonoxidizing treatment. In addition, Figure 4b shows the results of a blank experiment with a DM monolayer adsorbed on a freshly-evaporated gold slide (not oxidized prior to adsorption) and similarly treated with the argon ion beam. No oxide is detected in this case upon scanning the potential. Since the technique of sputter depth-profiling does not ideally remove material layer-by-layer,49removal of the monolayer by argon ion beam sputtering is undoubtedly accompanied by some gold substrate and gold oxide removal. This would account for the relatively small oxide reduction peak in Figure 4. The results are nonetheless invaluable in verifying the existence of a gold oxide layer under the DM monolayer. X-ray Photoelectron Spectroscopy (XPS).Figure 5a (leR)shows an XPS survey spectrum of a DM monolayer adsorbed onto freshly-evaporated gold, while parts b and c of Figure 5 (left)present survey spectra of DM monolayers adsorbed onto gold preoxidized 6 min and 15 min, respectively. The sulfur 2p line is not seen in Figure 5a, while a clear sulfur peak with an increasing intensity is observed in parts b and c of Figure 5. Combined with the similar trend in the intensity of the carbon 1 s peak, the data indicate a gradual increase in the amount of DM molecules adsorbed on the surface with the degree of oxidation. The possibility that the growth in the sulfur
4400
220.0
OD 5370
533.0
.
4
529.0
Binding Energy [eV]
Figure 5. XPS results for DM monolayers adsorbed on gold
after various treatments, showing suvery spectra (left) and narrow scan spectraofthe oxygen 1s region (right): (a)freshlyevaporated gold; (b)6 min oxygen plasma oxidation; (c) 15 min oxygen plasma oxidation. and carbon peaks is due to the formation of more than one layer of DM is highly unlikely considering the chemical nature of DM molecules, the contact angle results (particularly for water), and the ultrahigh vacuum conditions Torr). The increase during the XPS measurements in the amount of adsorbed DM with the degree of plasma oxidation is, therefore, most likely the result of an increase in the surface area, i.e. in the surface roughness. The narrow spectra of the oxygen 1s region (Figure 5, right) show that no oxygen is detected when the monolayer is adsorbed onto freshly-evaporated gold. Sizable oxygen peaks are observed at 532.5 eV for gold oxidized 6 and 15 min in oxygen plasma prior to adsorption, with the amount of detected oxygen increasing with the degree of preoxidation. Note that the oxygen 1 s line is clearly observed in the survey spectrum in Figure 5c (left). The oxygen peaks (Figure 5, right) provide strong indication, but not conclusive evidence, of the presence of gold oxide. An unequivocal confirmation requires a careful analysis of the gold peaks. The narrow scans in the gold 4f region of DM-coated gold preoxidized 15 min in oxygen plasma prior to DM adsorption (Figure 6a) and of untreated, bare gold (Figure 6b) show a shift of about 0.5 eV of the former peaks to higher binding energies, accompanied by an increase of 0.4 eV in the peak width a t half-height (fwhm). Note that the gold 4f peaks observed with a DM monolayer adsorbed on freshlyevaporated gold are similar to those of bare gold. The XPS results can be understood upon comparison with data reported by Pireaux et a1.,61bwho published a comprehensive study of gold oxidation by 0 2 plasma. They showed that the gold 4f 712 peak a t 84 eV was shifted upon oxidation to a higher binding energy, while the fwhm was broadened. In addition, an oxygen 1s line was observed a t 530 eV with a 2.0-eV fwhm, corresponding to the formation of AuzO3 under the 0 2 plasma. Long exposure to X-ray and heating treatment was characterized by a gradual shift of the gold 4f 712 peak to lower binding energies (84 eV) and a gradual shift of the oxygen
Alkanethiol Monolayers on Preoxidized Gold
a
.. . ...,. .. ..............
b
.....
..............
T
?
3 .
.
.
..........................
:'
90.0
86.0
820
Binding Energy [eV]
Figure 6. Narrow scan XPS results showing the gold 4ffor (a) untreated "bare"gold and (b)DM monolayer adsorbed on gold after 15 min of oxygen plasma treatment.
Langmuir, Vol. 10, No. 12, 1994 4573 hydrogen sulfide in air. It comprised an electrochemical cell, where an applied potential kept a gold surface in an oxidized state. The determination of the amount of H2S was based upon a measured current. It is clear, though not stated in the invention, that H2S was detected upon reaction with the gold oxide. The authors stress that the apparatus detects mercaptans, too. Oxidation of thiols by lead dioxidea4might give a clue as to the possible reaction between gold oxide and thiols. The chemical reaction shown in eq ls4demonstrates the ability ofthiols to serve as reducing agents for metal oxides PbO,
+ 4RSH - Pb(SR), + RSSR + H,O
(1)
Thus, a similar reaction with gold oxide may be suggested, as shown in eq 2
A U , ~ ,+ 6RSH
-
2AuSR
+ 2RSSR + 3H,O
(2)
Note that ethanol (and possibly other solvents) may 1s peak to higher binding energies. This was attributed also cause a certain reduction of gold oxide during the t o a change in the chemical composition of the oxide, i.e. a gradual transformation from AuzO3 to A u ~ O .The ~ ~ ~ initial stages of monolayer adsorption. same change in the oxidation state of gold was mentioned Conclusions by Celdran and Gonzalez-Velas~o~~ as an intermediate Contact angle measurements, ellipsometry, electrostate in the chemical reduction of gold oxide upon exposure chemical stripping-depth profiling analysis, and XPS were of oxidized gold to allyl alcohol in aqueous solution. used to study the formation of DM monolayers on It can therefore be concluded that the XPS measurepreoxidized gold surfaces. The results show that (inherments confirm the observation that gold oxide, produced ently unstable) gold oxide is stabilized upon adsorption prior to DM adsorption, is stabilized upon adsorption of of a DM monolayer. The oxide is encapsulated by the a DM monolayer, and a considerable part of it is not close-packed alkanethiol monolayer adsorbed onto the gold reduced by the ethanol during the adsorption process. oxide, so that chemical and electrochemical reduction of The oxide is stable under the monolayer for a period of at the oxide is prevented. The results also indicate that least several weeks (the time passed between preparation contact angles and ellipsometric parameters of alkanethiol and XPS measurements), and under the X-ray beam. The monolayers adsorbed onto gold surfaces are sensitive to position of the gold 4f 712 peak and of the oxygen 1s peak oxidizing treatments of the gold surface prior to adsorption. in Figures 6b and 5c (narrow scans), respectively, may Thus, conclusions concerning structure, orientation, and also suggest changes in the composition of the oxide, as thickness of thiol monolayers on metal substrates which discussed by Celdran and Gonzales-Velas~o;~~ this, howoxidize spontaneously (such as silvers6),which are based ever, is yet to be verified. As noted above, the variations upon contact angles and ellipsometric measurements, in the ellipsometric Y and A upon adsorption of a DM must be considered with caution due to the possible monolayer on preoxidized gold (Table 2) also suggest influence of an underlying oxide. changes in the gold oxide composition. Interaction between Thiols and Gold Oxide. An Acknowledgment. We wish to thank Mr. Q.-T. Le interesting question concerns the mode of attachment of and Professor J. Verbist (University of Namur, Belgium) a thiol monolayer (e.g., DM) to gold oxide. To the best of for performing the XPS measurements and Dr. R. Brenner our knowledge, the interaction between gold oxides and (Technion, Haifa) for his assistance with the electrothiols has not been investigated, although indirect evichemical-depth profiling experiments. dence exists for a chemical reaction. Novack and TomaSupport of this work by the MINERVA Foundation, s0vicS3 described an apparatus for the detection of Munich, Germany, and the Israel Science Foundation, Grant No. 68193-1, are gratefully acknowledged. (83) Novac, R. L.; Tomasovic, B. A. U.S. Patent No. 4636294,January 1987.
(84)Mukaiyama, T.; Endo, T. Bull. Chem. SOC.Jpn. 1967,40,2388.
