Langmuir 1992,8, 80-89
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Silver Metalization of Octadecanethiol Monolayers Self -Assembled on Gold Michael J. Tarlov Process Measurements Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 Received April 15,1991.I n Final Form: July 17, 1991 The interaction between evaporated silver and self-assembled monolayers of octadecanethiol (ODT) [CH3(CH2)17SH] ongoldwasstudiedat temperaturesof 300and90KusingX-ray photoelectronspectroscopy (XPS),ultraviolet photoelectron spectroscopy (UPS), and ion scattering spectroscopy (ISS). Equivalent Ag coverages ranging from submonolayersto multilayers were examined. Ag deposited at 300 K penetrates the ODT monolayer and resides at the ODT/gold interface. The attenuation behavior of the XPS C 1s and Au 4f signals reveals that Ag nucleates as clusters underneath the ODT monolayer. UPS and ISS data are in accord with XPS results and, furthermore, indicate that the deposited Ag does not significantly alter the ODT monolayer film structure. In addition, an unusual shift to higher binding energy is observed for both the C 1s core level and the hydrocarbon valence band with increasing Ag coverage. It is suggested that the shift is due to a work function induced change whereby the photoemission of ODT molecules is referenced to the local vacuum level, not the metal Fermi level. In contrast to the behavior observed at 300 K, results from Ag deposition at 90 K indicate that Ag forms clusters on the ODT monolayer surface. In this case the Ag 3d core level exhibits a shift to higher binding energy relative to that of metallic silver that is attributable to a final state Coulombic effect characteristic of metal clusters on relatively poorly conducting substrates. There is apparently little chemical interaction between Ag and the hydrocarbon chains at both 90 and 300 K, although at 300 K Ag 3d XPS data suggest that Ag bonds to sulfur head groups. The low-temperature Ag/ODT/Au structure is apparently a metastable phase because warming of the sample to room temperature results in the migration of the Ag to the ODT/Au interface and possibly further clustering. ODT monolayer defects may play an important role in passage of Ag through the film.
Introduction Molecular monolayers formed by the process of selfassembly are the subject of much current interest and investigation. These investigations have been prompted by the potential importance of monolayer films in applications involving sensing, electrooptics, molecular electronics, microelectronics, tribology, and corrosion.lS2 Selfassembled monolayers consisting of N-alkanethiols and disulfides chemisorbed on Au have received the most attention, primarily because of their relative ease of preparation and the high quality of the resulting films. A wide variety of analytical techniques have been used to characterize these structures, including ellipsometric, electrochemical, and contact angle measurements, and Xray photoelectron and infrared spectroscopies. The cumulative results from these investigations demonstrate conclusively that the films are comprised of densely packed crystalline-like structure^.^-^ Indeed the presence of longrange order in these films has been established by electrong and atomlodiffraction studies that show that n-alkanethi01s form hexagonally arrayed domains on Au( 111)surfaces. In addition, it has been shown that the identity of the terminal functional group can be varied with little per-__ (1)S d e n , J. D.;Allara, D. L.; Andrade, J. D.; Chandross, E. A.; Garoff, s.;Israelachvili, J.;McCarthy, T. J.; Murray, R.; Pease,R. F.; Rabolt, J. F.; Wynne, K. J.; Yu, H. Langmuir 1987,3,932-950. (2) Fuchs, H.; Ohst, H.; Prass, W. Ado. Mater. 1991,3, lC-18. (3)Nuzzo, R. G.;Allara,D. L.J. Am. Chem. SOC.1983,105,4481-4483. 1987, (4)Nuzzo, R. G.;Fusco, F. A,; Allara, D. L. J. Am. Chem. SOC. 109,2358-2968. (5)Porter, M. D.;Bright, T. B.; A h a , D.L.; Chidsey, C. E. D. J.Am. Chem. SOC.1987,109, 3559-3568. (6) Bmn, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.;Nuzzo, R. G. J. A n . Chem. SOC.1989,111,321-335. (7) Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990,6 , 682-691. (8) Whitesides, G. M.; Laibinis, P. E. Langniuir 1990,6, 87-96. (9)Strong, L.; Whitesides, G. M. Langmuir 1988,4, 546-558. (10)Chidsey, C.E. D.; Liu, G. Y.; Rowntree, P.; Scoles, G. J. Chem. Phys. 1989,91, 4421-4423.
turbation to hydrocarbon chain structure.l' These two properties, the ability to manipulate the surface functionality and the highly ordered nature of the films, have encouraged groups to use alkanethiol self-assembled monolayers as well-defined organic surfaces in studies of chemical r e a ~ t i v i t y , ~and J ~ electron-transfer reaction~.~J~-~~ In addition, self-assembled monolayer films are being employed as well-defined organic surfaces, much as metal or semiconductor single-crystal surfaces have been used, in fundamental studies of adsorption of rare gas atom and molecular over layer^.'^^^^ These studies have exploited the well-characterized structure of self-assembled monolayers, together with the ability they offer to isolate a particular functional group a t the monolayer surface. We are interested in using self-assembled monolayers as substrates in a similar manner to study the interaction of evaporated metals with organic surfaces. Interest in the reactivity of metals with organic surfaces has been spurred in large part by the growing abundance and technological importance of metal/polymer interfaces in microelectronic devices. The many investigations of metall (11)Nuzzo, R.G.; Dubois, L. H.; Allara, D.L. J.Am. Chem. SOC. 1990, 112,558-569. (12)Troughton, E.B.; Bain, C. D.; Whitesides, G. M.; Nuzzo,R. G.; A h a , D. L.; Porter, M. D. Langmuir 1988,4,365-385. (13)Ulman, A.; Tillman, N. Larigmuir 1989,5,1418-1420. (14)De Long, H.C.; Buttry, D. A. Langmuir 1990,6,1319-1322. (15)Finklea, H.0.; Robinson, L. R.; Blackburn, A.; Richter, B.; Allara, D.; Bright, T. Langmuir 1986,2, 239-244. (16)Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. SOC.1990,112,4301-4306. (17)Creager, S. E.; Collard, D. M.;Fox, M. A. Langmuir 1990,6,16171620. (18)Tarlov, M. J.; Bowden, E. F. J.Am. Chem. SOC. 1991,113,18471849. (19)Dubois, L. H.;Zegarski, B. R.; Nuzzo, R. G. J.Am. Chem. SOC. 1990,112,570-579. (20)Chidsey, C. E. D.; Liu, G. Y.; Scoles, G.; Wang, J. Langmuir 1990, 6, 1804-1806.
This article not subject to U.S.Copyright. Published 1992 by the American Chemical Society
Silver Metalization of Octadecanethiol Monolayers polymer interfacial reactions have provided great insights into the molecular scale events that determine such macroscopic properties as adhesion and long-term stability.21 We believe, however, that self-assembled monolayers offer two significant advantages over polymer surfaces for the study of metal/organic interfaces. First, the identity and concentration of surface functional groups can be controlled to isolate and better examine their reactivity with metals. Second, the ordered nature of monolayer surfaces might simplify the interpretation of structural changes accompanying metal organic complex formation. Although there have been several reports of metal deposition on Langmuir-Blodgett mono- and multilayers, these studies have been concerned primarily with making macroscopic electrical contacts for capacitance, conductivity, and electron tunneling measurement^.^^-^' T o our knowledge, systematic studies of the interaction of metals in coverages ranging from isolated atoms to multilayers with organic monolayer films have not been undertaken. Some of the questions we hope to answer in these investigations are the following: T o what extent do the evaporated metals chemically interact with the thiol molecules? Is the integrity of the monolayer film preserved when metals are deposited or is the molecular packing significantly disrupted? Can continuous metal overlayers be formed on self-assembled monolayer surfaces? In this paper we report results from an investigation of the interaction of vapor-deposited silver with selfassembled monolayers of n-octadecanethiol (ODT) [CHs(CH&7SH] chemisorbed on smooth Au thin film substrates. We chose to study the deposition of Ag on ODT/ Au because of the desire to examine first a relatively simple chemical system. On the basis of chemical intuition, Ag should interact weakly with monolayer films comprised of straight-chain hydrocarbons. The surface-sensitive techniques of X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and ion scattering spectroscopy (ISS) were used to characterize Ag coverages ranging from submonolayers to multilayers deposited on the ODT films. In this paper we will show that remarkably different Ag growth modes are obtained depending on whether depositions are performed with the ODT/Au sample held at 300 or 90 K.
Experimental Section Single-crystal (100) silicon wafers scribed into -2-cm squares were used as substrates for the Au films. The Si pieces were cleaned by sonicatingsequentiallyfor 30 min in analyticalreagent grade dichloromethane, methanol, and deionized (>15 M Cl cm) water. All water referred to in this study was obtained from a Millipore four-cartridge water purification system.28Following sonic cleaning, Si samples were placed in concentrated HF for 10s, rinsed with water, and blown dry with high-purity nitrogen. (21) Metallization of Polymers; Sacher, E., Pireaux, J. J., Kowalczyk,
S. P., Eds.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990, Vol. 440. (22) Shanley, R.; O’Beirn, B.; Casey, V.; McMonagle, J. B. Sens. Actuators, B 1990,2,57-62. (23) Polymeropoulos, E. E. J . Appl. Phys. 1977, 48, 2404-2407. (24) Polymeropoulos, E. E.; Sagiv, J. J. Chem. Phys. 1978,69, 18361847. (25) Leger, A.; Klein, J.; Belin, M.; Defourneau, D. Thin Solid Films 1971,8, R51-R54. (26) Tredao1d.R. H.; Jones, R. IEEProc.,PartI: Solid-StateElectron Deuices 198f, 128, 202-206. (27) Winter, C. S.;Tredgold,R.H.; Hodge, P.; Khoshdel, E.IEEProc., Part I Solid-state Electron Devices 1984,131, 125-128. (28) Certain commercial products and instruments are identified to adequately specify the experimental procedure. In no case does such identification imply endorsement by the National Institute of Standards and Technology.
Langmuir, Vol. 8, No. 1, 1992 81 The Si samples were then immediately placed in a liquid nitrogen trapped, diffusion-pumped, sputter-deposition chamber and Torr. A Cr layer of evacuated to a base pressure of -7.5 X -50 8, was sputter-deposited on Si substrates prior to Au deposition to improve adhesion of the Au film. Au thin films were deposited by dc sputtering of a 99.99%, 5-cm Au target in a pure Ar atmosphere at a pressure of 2.6 mTorr. The Au deposition rate was -7.5 A/s at a target to substrate distance of 10 cm, and a target bias and current of 500 V and 0.1 A, respectively. Film thicknesses were determined by stylus measurements. All Au films used in this study were nominally 2000 8,thick. X-ray diffraction patterns of samples prepared in this manner indicated that the Au films were polycrystalline and highly oriented in the (111)direction. The ODT (Aldrich) and absolute ethanol (Warner-Graham) were used as received to make up the adsorbate solutions.28All glassware used to contain and mix solutions was cleaned by soaking at least 24 h in chromic acid, followed by sequential rinsing with deionized water and absolute ethanol. Au thin films were immediately immersed in a M ODT-absolute ethanol solution followingremoval from the sputter-deposition chamber. Monolayer assembly times ranged from 1 to 5 days. No measurable differencein film composition or structure was found between samples equilibrated 1-5 days as monitored by ellipsometry, infrared reflection absorption spectroscopy (IRRAS), and XPS.% Samples to be used for Ag deposition experiments were removed from adsorbate solution, rinsed with copious amounts of absolute ethanol, blown dry with a stream of highpurity nitrogen, and immediately inserted in the rapidintroduction chamber of the surface analysis system. The total elapsed time between the deposition of Au films and the Ag deposition experiments was 14days or less. The diffusion of Cr through Au is well known; however, no Cr was detected with XPS or ISS profiling on any of the samples used in Ag deposition experiments. In addition to the techniques of XPS, UPS, and ISS,the quality of the ODT monolayers prepared in this laboratory was also judged using ellipsometry, electrochemical capacitance measurements, and IRRASZ9 The ODT/Au samples examined in these separate studies were prepared using the indentical procedures described above and in some instances were from the same sample sets used for Ag deposition experiments. The calculatedaveragethicknessfor six independently prepared ODT/ Au samples as determined by ellipsometry was 27 i 2 This value is in the range of those reported by other researchers for the ODT/Au ~ y s t e m .The ~ . ~differential capacitance determined from cyclic voltammetry ranged from 0.8 to 1.1pF/cmZfor four ODT/Au samples (at 0.0 V vs NHE (normal hydrogen electrode), scan rate of 0.05 V/s,0.1 M KC1 supporting electrolyte) which is comparable to the value of -1.0 pF/cm2reported by Porter et. al for ODT/Au.S The IR C-H stretching region of alkanethi01s assembled on Au provides information about the alkyl chain environment and has been shown to be a valuable indicator of the monolayer structural IR spectra obtained from ODT/Au samples prepared in this laboratory show the relative peak intensities and positions of the CHz and CH3 stretching modes to be in excellentagreement with those reported by others6 for ODT/Au. Together, these data indicate that the ODT monolayers used in this study are densely packed, relatively low defect density assemblies that are compositionally and structurally similar to ODT monolayers examined by other researchers.5~6 All Ag deposition experiments were performed in an ultrahigh vacuum surface analytical system based on the VG ESCALAB MK I1 that consisted of an analysis chamber and a sample treatment chamber with an attached rapid sample introduction chamber.28The analysischamberhoused all the surfaceanalytical techniques, and was evacuated with a liquid nitrogen trapped diffusion pump. The sampletreatment chamberwas cryopumped (29) Tarlov, M. J. Unpublished results. Ellipsometry was performed at 632.8 nm at a 70’ angle of incidence using a Rudolph 423 null-point ellipsometer.28 The ODT film thickness was calculated by assuming a real refractive index of 1.45 for the ODT film and using the algorithm developed by McCrackin (McCrackin,F. L.; Passaglia, E.; Stromberg,R. R.; Steinberg, H. L. J.Res. Natl. Bur. Stand., Sect. A 1963,67,363-377). An Analect FTIR spectrometerBwas used in aglancing angle mode similar to that reported by others6 to obtain infrared spectra of the ODT/Au.
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and equipped with a metal evaporator. A long-throw, precision XYZ manipulator was used to translate the sample between the analysisand treatment chambers. All chambers could be isolated from each other by gate valves. The base pressure of the analysis and sample treatment chambers was 1 X 10-lo Torr. For experiments at 300 K, samples were transferred from the turbomolecular-pumped rapid introduction chamber to the XYZ manipulator using a magnetically coupled rod. For experiments at 90 K it was necessary to bring the sample treatment chamber up to atmospheric pressure to mount the sample on a copper cooling stage attached to the XYZ manipulator. Tantalum clips were used to secure the sample to the copper stage. A copper braid was used as a thermal connection between a liquid nitrogen reservoir and the copper stage. A W 5% Re/W 26% Re thermocouple held mechanically on the Au film was used to measure sample temperature. After installing the sample on the low-temperature mount, the preparation chamber was pumped for 12-14 days before performing low-temperature experiments to avoid the condensation of water on the sample. The sample treatment chamber was not baked during pump down because of possible thermal decomposition of the ODT monolayer. Ag was deposited at a rate of 2.0 0.5 A/min by resistive heating of a tantalum boat containing 99.999% Ag wire pieces. The amount of Ag deposited was monitored with a quartz crystal microbalance (QCM). The QCM was calibrated from stylus measurements on 300-500-A-thick Ag films assuming a bulk density of 10.5g/cm3for Ag. Coverages are reported in Ag atoms per centimeter squared so as not to imply layer-by-layergrowth of the Ag film. Coverages can be converted to equivalent angstroms by multiplying the coverages (Ag atoms/cm2)by 1.71 X 10-15. For experiments at 90 K the evaporator was outgassed for 7-12 days during the 12-14 days of pump down of the sample treatment chamber by heating the Ag to a point just below measurable deposition as monitored by the QCM (
