Structures of Self-Assembled Monolayers of n-Alkanoic Acids on Gold

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Structures of Self-Assembled Monolayers of n-Alkanoic Acids on Gold Surfaces Modified by Underpotential Deposition of Silver and Copper: Odd-Even Effect Shu-Yi Lin, Tung-Ku Tsai, Chia-Mei Lin, and Chun-hsien Chen*,† Department of Chemistry, National Tsing Hua University, 101, Section 2 Kuang Fu Road, Hsinchu, Taiwan 30013, Republic of China

Yang-Chiang Chan and Hsiu-Wei Chen*,‡ Department of Chemistry, National Sun Yat-Sen University, 70 Lien Hai Road, Kaohsiung, Taiwan 80424, Republic of China Received December 17, 2001. In Final Form: April 30, 2002 The structures of n-alkanoic acid monolayers (CH3(CH2)mCO2H, m ) 5, 6, 11, 13-22) self-assembled on gold premodified by electrodeposition of a silver or copper adlayer are studied by contact angle measurements, IRAS (infrared reflectance-absorption spectroscopy), and XPS (X-ray photoelectron spectroscopy). Prior to immersion into the fatty acid-containing solutions, the substrates are prepared by underpotential deposition (upd), resulting in a sub-monolayer or full monolayer of silver or copper on gold. The adlayer promotes anchoring of carboxylate headgroup and assembly of n-alkanoic acids, which would otherwise exhibit no chemisorption on bare gold. The results show that the monolayers exhibit low wettability, all-trans methylene conformation, and a bidentate binding scheme of the carboxylate headgroups onto the upd-modified substrate. The odd-even effect of alternating peak intensity of the methyl-stretching modes is significant for monolayers on silver upd surface and, to a less extent, yet notably, for those on copper upd surface. These features are distinctly different from structures of n-alkanoic acid SAMs on bulk copper containing native oxides. We attribute the difference to the degree of surface oxide formation indicated by XPS which reveals that in ambient conditions the upd-modified surface is less oxidized by dioxygen than the corresponding bulk substrate.

Introduction One of the most significant advantages of self-assembled monolayers (SAMs) is the ease in engineering interfacial structures at the molecular level. The general preparation scheme for SAMs simply involves immersion of the substrate into deposition solutions containing amphiphilic molecules whose polar headgroups exhibit physi- or chemisorption toward the substrate.1 A repetitive desorption-readsorption mechanism of the amphiphiles, driven by thermodynamic equilibration between the solution phase and the solid-liquid interface, improves the interchain van der Waals attractions and thus spontaneously generates close-packed monolayer assembly. Surfaces with desired properties can be tailored by further derivatization at functional-group-terminated SAMs which are premodified with ω-functionalized amphiphilic compounds. Among a variety of self-assembling systems, the thiolbased SAMs on gold are the most comprehensively and intensively studied because of the excellent reproducibility in preparation of high-quality SAMs. This is in part due to the advantage of chemical inertness of gold compared to other materials where, under ambient conditions, surface oxides grow readily and obstruct the formation of densely packed organosulfur SAMs. However, from a different viewpoint as rationalized by Laibinis2 with * To whom correspondence should be addressed. † E-mail: [email protected]. Phone: (+886) 3-573-7009. Fax: (+886) 3-571-1082. ‡ E-mail: [email protected]. Fax: (+886) 7-525-3909. (1) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151-256. (2) Baker, M. V.; Jennings, G. K.; Laibinis, P. E. Langmuir 2000, 16, 3288-3293.

Pearson’s principle3 of the relative stability of acid-base adducts, the softness of gold imposes the limitation of soft bases for the self-assembled amphiphiles. For example, for the formation of stable SAMs on gold, alkanethiols are particularly suitable and molecules with hard headgroups such as sulfonate and carboxylate are excluded. This is a drawback for SAMs on gold because, in comparison with thiols, the latter molecules have more variety and better availability for surface derivatization. Jennings and Laibinis have shown that electrodeposition of a copper or silver monolayer on gold modifies the properties of the substrate, suggesting that the limitation noted above can be overcome.4,5 By the electrochemical treatment of underpotential deposition (upd), in which adatoms are deposited at submonolayer coverage at potentials positive from the onset of bulk reduction, the adlayer is likely carrying partial charge6 and raises the hardness of the substrate. The upd adlayer can therefore promote anchoring of hard headgroups. For example, SAMs of alkanephosphonic2 and alkanoic acids7 on updmodified gold have been reported. Without upd modification, the phosphonate and carboxylate headgroups would otherwise exhibit no chemisorption on bare gold.8 (3) Pearson, R. G. J. Am. Chem. Soc. 1963, 85, 3533-3539. (4) Jennings, G. K.; Laibinis, P. E. Langmuir 1996, 12, 6173-6175. (5) Jennings, G. K.; Laibinis, P. E. J. Am. Chem. Soc. 1997, 119, 5208-5214. (6) Herrero, E.; Buller, L. J.; Abruna, H. D. Chem. Rev. 2001, 101, 1897-1930 and references therein. (7) Lin, S.-Y.; Chen, C.-h.; Chan, Y.-C.; Lin, C.-M.; Chen, H.-W. J. Phys. Chem. B 2001, 105, 4951-4955. (8) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: Boston, MA, 1991.

10.1021/la0157364 CCC: $22.00 © 2002 American Chemical Society Published on Web 06/14/2002

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Recently, we demonstrated that assembly of ω-alkanoic acids on Au/Ag(upd) and on Au/Cu(upd) is an alternative to the prevalent SAMs of ω-functionalized alkanethiols on gold.7 Experience in this laboratory has suggested that, within the potential windows of upd, high-quality SAMs are assembled most facilely on substrates modified at 350 and 0 mV versus potentials of bulk reduction for silver and copper, respectively. The characterization of SAMs prepared under these conditions concluded that the reproducibility of n-alkanoic acid SAMs on Au/Ag(upd) and on Au/Cu(upd) is superior to those on bulk silver and copper. In furthering our interests in this methodology, this paper presents that n-alkanoic acid SAMs exhibit the odd-even effect, a result of a highly crystalline methylene chains with the molecular axes tilted with respect to the surface normal.9-14 The number of the methylene units of the molecule dictates the orientation of the terminal group. The odd-even effect has been investigated by a series of detailed studies and is a subject of intense current interest due to the subtle but fascinating differences in friction,15 wetting,16 manipulating inter- and intrafacial bonds,17 and ionization reactivity for mass spectrometric measurements.11 Reported in the following sections are the results of infrared reflectance absorption spectroscopic (IRAS), contact angle characterizations of n-alkanoic acid SAMs (CH3(CH2)mCOOH, m ) 5, 6, 11, 13-22) on Au/Ag(upd) and Au/Cu(upd), and a correlation of the odd-even effects on the IR intensities of the terminal methyl group and on wetting properties. Experimental Section Preparation of UPD Modified Gold Surface. Substrates were 200-nm-thick gold films thermally evaporated onto glass slides precleaned with piranha solution which is a 1:3 (v/v) mixture of 30% H2O2 and concentrated H2SO4. This solution reacts violently with organic materials and should be handled with great care. The pressure in the bell-jar evaporator (a KV-301, KEY High Vacuum, Co., Nesconset, NY, and an Auto 306, Edwards High Vacuum International, West Sussex, U.K.) was nominally 3 × 10-6 Torr. A 10-nm Cr underlayer was used to enhance the adhesion of the gold film. Prior to vapor deposition, the glass slides were preheated at 250 °C in vacuo to thoroughly remove trace amounts of moisture. The gold substrate exhibited domains of flat terraces as revealed by scanning tunneling microscopy (NanoScope II, Digital Instruments, Santa Barbara, CA) and scanning electron microscopy (S-4700, Type II, Hitachi, Japan). A roughness factor of 1.4 ( 0.2 (n ) 15) was derived by experiments of Pb upd with 1 mM Pb(NO3)2 in a solution of 0.10 M NaClO4 and 0.10 M HClO4.18 The gold films were essentially polycrystalline but with characteristic Au(111) features,7 probably due to the thermal annealing at 250 °C during vapor deposition.19 The upd solutions were prepared from reagent grade chemicals and Millipore-Q purified water (18 MΩ cm-1). The solutions were 0.1 M H2SO4 containing 0.6 mM Ag2SO4 (Fisher Scientific) and (9) Tao, Y.-T. J. Am. Chem. Soc. 1993, 115, 4350-4358. (10) Smith, E. L.; Porter, M. D. J. Phys. Chem. 1993, 97, 8032-8038. (11) Angelico, V. J.; Mitchell, S. A.; Wysocki, V. H. Anal. Chem. 2000, 72, 2603-2608. (12) Walczak, M. M.; Chung, C.; Stole, S. M.; Widrig, C. A.; Porter, M. D. J. Am. Chem. Soc. 1991, 113, 2370-2378. (13) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y.-T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152-7167. (14) Ulman, A. Chem. Rev. 1996, 96, 1533-1554. (15) Wong, S.-S.; Takano, H.; Porter, M. D. Anal. Chem. 1998, 70, 5209-5212. (16) Shon, Y.-S.; Lee, S.; Colorado, R., Jr.; Perry, S. S.; Lee, T. R. J. Am. Chem. Soc. 2000, 122, 7556-7563. (17) Kim, H. I.; Houston, J. E. J. Am. Chem. Soc. 2000, 122, 1204512046. (18) Leopold, M. C.; Black, J. A.; Bowden, E. F. Langmuir 2002, 18, 978-980. (19) Chidsey, C. E. D.; Loiacono, D. N.; Nakahara, S.; Sleator, T. Surf. Sci. 1988, 200, 45-66.

Lin et al. 1 mM CuSO4 (May & Baker), respectively, for Ag and Cu upd. Because the size of the gold substrates was 1 in. × 3 in., a 100mL beaker is used to hold the electrolytes. The solutions were used without deoxygenation. The reference electrode for Ag upd was a high-purity silver wire flame-annealed with a butane microtorch and quenched into pure water prior to upd experimentation. A high-purity copper wire (Aldrich) was utilized as the reference electrode for Cu upd and was cleaned by dipping into diluted nitric acid (Fisher Scientific) for a short period of time prior to each use. The potentials of the Ag and the Cu wires were 408 ( 2 and 36 ( 2 mV, respectively, against a commercially available Ag/AgCl reference electrode (model MF 2052, BioAnalytical System, West Lafayette, IN) in their corresponding upd solutions used here. Potential control was obtained with a PAR VersaStat (EG&G Instruments, Princeton, NJ) or a bipotentiostat (model AFCBP1, Pine Instrument, Grove City, PA). Typical cyclic voltammograms of these electrodes were reported elsewhere.7 The initial potentials for upd were 600 mV (vs EAg+/0) and 500 mV (vs ECu2+/0). The substrates were modified by holding the potentials at 350 and 0 mV, respectively, for Ag and Cu upd. The updmodified substrates were subsequently removed under potential control, rinsed with copious ethanol, blown dry in a stream of nitrogen, and transferred through air into solutions containing n-alkanoic acids.2,4,5,7,20-22 For experiments examining the effect of surface oxide on SAM structures, the upd-modified substrates were placed into a plasma generator (Harrick, PDC-32G, Ossining, NY). The pressure of the chamber was pumped down to 80 mTorr and then back-filled with oxygen to 800 mTorr. Surface oxide was grown under medium power (60 W) for 1 min. Preparation of Alkanoic Acid SAMs. The n-alkanoic acids (CH3(CH2)mCOOH, m ) 5, 6, 11, 13-22, Aldrich or Tokyo Chemical Industry, Co.) were used as received. Soaking solutions were 1 mM of the fatty acids prepared in n-hexadecane (Sigma), which was percolated through neutral alumina (Activity 1, Merck) twice. When temperature was colder than the melting point of n-hexadecane (18 °C), the solutions were warmed in an oven ( 15, the angle of alternation lies between 41° and 46°. For m ) 11-15, contact angle values decrease with shorter acids and still show an alternation trend. The alternation is attributed to the tilt of the alkyl chain from surface normal, which results in a mixture of CH2 and CH3 groups at the film terminus for odd number SAMs and CH3 only for even ones. At the hexadecane/film interface, as suggested by Lee et al.,16 the odd number SAMs expose more atomic contacts per unit area toward the probing liquid and thus exhibit stronger van der Waals interactions and smaller contact angles than the even SAMs do. In the case of SAMs (23) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321-335. (24) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7164-7175. (25) Tao, Y.-T.; Lee, M.-T.; Chang, S.-C. J. Am. Chem. Soc. 1993, 115, 9547-9555.

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Figure 2. Grazing incidence polarized infrared spectra for SAMs of n-alkanoic acids on Au/Ag(upd).

on Au/Cu(upd), contact angle measurements do not show an observable odd-even effect. Infrared Reflectance Absorption Spectroscopy (IRAS). Figures 2 and 3 show a representative series of IR spectra for SAMs of n-alkanoic acids on Au/Ag(upd) and on Au/Cu(upd), respectively. The assignment of the vibrational modes was described in detail elsewhere,9,26-28 and those for n-nonadecanoic acid (m ) 17) SAMs are summarized in Table 1. The important features revealed from the spectra are (1) νs(CO2-) at ∼1400 cm-1 and the lack of νa(CO2-) at 1550-1600 cm-1 show that the carboxylic acid headgroup forms a carboxylate species at both upd-modified surfaces with a bidentate binding scheme, (2) νa(CH2) at ∼2917 cm-1 and ω(CH2) at 13501200 cm-1 suggest that the long-chain alkanoic acids form a fully extended and crystalline-like assembly, and (3) δ(CH2) at ∼1470 cm-1, whose vibration mode is vertical to the molecular chain axis, indicates a tilted orientation of the alkyl chain (because IRAS is insensitive to transition dipole vertical to surface normal). The fact that the methylene scissors deformation does not exhibit splitting peaks suggests a triclinic packing for the alkanoic SAMs with only one type of alkyl chain per unit cell.29 At the high-frequency region, the peak intensity of methyl stretch modes alternates with the number of methylene units. The higher absorbance arises for the odd-number of chains with νa(CH3, ip) at ∼2965 cm-1 and with νs(CH3) at ∼2878 cm-1 for the even-number ones. This odd-even dependence of the methyl orientation at the chain terminus confirms an all-trans conformational arrangement for the alkyl chains. The methyl antisymmetric C-H stretch is composed of νa(CH3, ip) at 2965 cm-1 and νa(CH3, op) at 2957 cm-1, (26) Chen, S. H.; Frank, C. W. Langmuir 1989, 5, 978-987. (27) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 52-66. (28) Parikh, A. N.; Allara, D. L. J. Chem. Phys. 1992, 96, 927-945. (29) Snyder, R. G. J. Mol. Spectrosc. 1961, 7, 116-144.

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Figure 4. Absorbance of νa(CH3, ip) and νs(CH3, FR) versus number of methylene units for SAMs of n-alkanoic acids on (A) Au/Ag(upd) and (B) Au/Cu(upd). Figure 3. Grazing incidence polarized infrared spectra for SAMs of n-alkanoic acids on Au/Cu(upd). Table 1. Mode Assignments and Peak Frequency (cm-1) for n-Nonadecanoic Acid (m ) 17) Adsorbed on Au/ Ag(upd) and on Au/Cu(upd) vibrational mode νa(CH3, ip), asym stretch νs(CH3, FR), sym stretch νs(CH3), sym stretch νa(CH2), asym stretch νs(CH2), sym stretch νa(CO2-), asym stretch νs(CO2-), sym stretch δ(CH2), deformation δ(CH3), deformation ω(CH2), wagging a

Au/Cu(upd)/ Au/Ag(upd)/ CH3(CH2)17COOH CH3(CH2)17COOH 2965 shoulder (2936) 2879 2917 2850 nda 1400 1470 shoulder (1383) 1350-1200

2966 shoulder (2936) 2878 2918 2851 nda 1412 1469 shoulder (1382) 1350-1200

nd, not detected.

while both modes are vertical to C-CH3, the former and the latter are defined, respectively, as parallel and vertical to the plane of the CCC backbone.30-33 Literature shows that at room temperature, the absorbance at 2957 cm-1 is about 40% of that of νa(CH3, ip),31 but the two vibrational modes are too close to resolve.33 Upon cooling to 150 K, the band splits and the intensities for both modes are similar, almost reaching three times that of νa(CH3, op) at 300 K.31 Although the intensity of νa(CH3, op) is heavily temperature dependent30-33 and the superimposition might influence the odd-even effect, false analysis due to temperature effect is unlikely because of the small variation in experimental conditions (293-298 K). (30) Chidsey, C. E. D.; Liu, G.-Y.; Scoles, G.; Rowntree, P. J. Chem. Phys. 1989, 91, 4421-4423. (31) Nuzzo, R. G.; Korenic, E. M.; Dubois, L. H. J. Chem. Phys. 1990, 93, 767-773. (32) Parikh, A. N.; Allara, D. L. J. Chem. Phys. 1992, 96, 927-945. (33) Bensebaa, F.; Ellis, T. H.; Badia, A.; Lennox, R. B. Langmuir 1998, 14, 2361-2367.

Peak intensities summarized in parts A and B of Figure 4 are, respectively, the odd-even dependence of the methyl stretching bands for n-alkanoic acid SAMs on Au/Ag(upd) and Au/Cu(upd), with the former more pronounced than the latter. For a rough estimation of the molecular orientation with respect to the surface normal, the angles are derived from the intensity ratio of methyl stretch (Iodd/ Ieven of νa(CH3, ip) and νs(CH3) peaks) and are 7-13° and ∼5° for fatty acids on Au/Ag(upd) and on Au/Cu(upd), respectively. Note that the computation is oversimplified. In principle, for an all-trans zigzag molecule aligned nearly vertical to the surface, the transition dipoles of δ(CH2) (∼1470 cm-1) and νs(CH2) (∼2850 cm-1) are parallel to the surface and insensitive to IRAS. In Figures 2 and 3 the presence of δ(CH2), the increase in νs(CH2) intensity associated with the number of methylene unit, and the odd-even effect indicate a tilted angle for fatty acids. Nevertheless, the broad picture that fatty acids on Au/ Cu(upd) are less tilted than those on Au/Ag(upd) is supported by their relative magnitudes in δ(CH2) intensity, the odd-even effect, and contribution in νs(CH2) and νa(CH2) intensity per methylene unit. The spectra of n-alkanoic acid SAMs on Au/Ag(upd) and on bulk silver are almost identical with difference in magnitude, whereas in the case of copper, the spectra are significantly different. On bulk copper, the presence of νa(CO2-) at ∼1550 cm-1 suggests that the direction of the transition dipole is not parallel to the surface and thus a monodentate binding scheme is concluded.9 There is no odd-even effect of the methyl stretch modes, and the intensity of methylene stretching modes, νs(CH2) and νa(CH2), does not increase with the methylene unit, indicative of a molecular orientation normal to the surface. Also noticed is the missing of the progressive ω(CH2) bands for fatty acid SAMs on bulk copper, suggesting that the crystallinity is not as good as those on Au/Cu(upd). The intensities and peak areas of νs(CO2-) measured on Au/Ag(upd) and on Au/Cu(upd) are about one-fourth weaker than those on bulk materials. The trends suggest

Modifying SAM Structures

a lower molecular packing density for upd-modified substrates. The intensities of νa(CH2), however, are stronger on upd-modified substrates than on bulk materials. A plausible explanation for the opposite trend of νs(CO2-) and νa(CH2) intensities is a tilt angle (φ) of the plane of CCC backbone with respect to surface normal. The tilt is probably due to a result of relatively low packing density on upd-modified substrates. The νa(CH2) intensity of SAMs on Au/Cu(upd) is one-third stronger than those on Au/Ag(upd), suggesting a larger φ for the former. With a larger CCC plane tilt angle (φ), SAMs on Au/Cu(upd) should have a smaller intensity of νs(CH2) than those on Au/Ag(upd). However, the intensities of their νs(CH2) for SAMs with the same number of methylene chains are similar, suggesting a higher molecular packing density on Au/Cu(upd) than on Au/Ag(upd). The higher packing density on Au/Cu(upd) agrees with the higher coverage of upd adatoms. For SAMs on Au/Cu(upd) to hold a higher packing density and a larger φ, it would be necessary to adopt a smaller chain tilt angle, θ, with respect to the direction of C-C axis of C-CO2- headgroup. The larger φ and smaller θ for SAMs on Au/Cu(upd) result in a less distinct odd-even effect on alternating intensity of νa(CH3)/νs(CH3) than those on Au/Ag(upd). Although the effect of molecular twisting is to complicated to discuss here, the proposed structures are consistent with the relative intensities of νs(CO2-), νs(CH2), νa(CH2), νa(CH3), and νs(CH3).34 The difference in chain tilt angles, as one of the reviewers pointed out, could be a result of the difference in coverages of upd adatoms. The upd potentials employed in this study develop one-third of a monolayer and a full monolayer for upd adatoms on Au/Ag(upd) and Au/Cu(upd), respectively.6 The open structure of Au/Ag(upd) allows the carboxylate headgroup binding with both the underlayer Au and adlayer Ag sites. Therefore, the bidentate configuration would tilt the CCC plane with respect to the surface normal. Whereas on the full monolayer Au/Cu(upd), such a heterogeneous binding scheme is unlikely and thus the molecules exhibit a smaller chain tilt angle. X-ray Photoelectron Spectroscopy (XPS). XPS has been utilized to explore the degree of surface oxide formation on upd modified gold. Groups of Laibinis5 and Crooks6,20 reported that XPS reveals no oxide signal on Au/Ag(upd) and only a moderate amount of surface oxide on Au/Cu(upd). In the present study, oxygen plasma is utilized to deliberately oxidize the upd surface prior to its immersion in the deposition solution. For fatty acid SAMs on Au/Ag(upd), the spectra of XPS and IRAS are not affected by the treatment of oxygen plasma. However, a totally different scenario is observed in the case of Au/Cu(upd). Spectra A and B of Figure 5 are the XPS spectra of n-nonadecanoic acid (m ) 17) SAMs on Au/Cu(upd) without and with the pretreatment of oxygen plasma, respectively. Prior to the preparation of SAMs, Au/Cu(upd) exhibits the shake-up peaks (indicated by asterisks in Figure 5 at ∼942 and ∼963 eV) for Cu(2p3/2) and Cu(2p1/2) weaker than those of the characteristic four-peak pattern of copper oxides,35 similar to the XPS spectrum reported (34) The intensity of δ(CH2) for SAMs on Cu upd-modified substrates can be realized as the projection of the transition dipole onto the surface normal (i.e., x cos φ), after projected onto the direction of the C-C bond of C-CO2- (i.e., x sin θ). For the Cu upd case with a larger φ (plane tilt) and a smaller θ (chain tilt), it is likely that the evolution in δ(CH2), a weak absorption, is not as significant as it is on Ag upd-modified substrates. (35) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F. Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer CorporationsPhysical Electronics Division: Eden Prairie, MN, 1979.

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Figure 5. XPS spectra in the Cu(2p) region of n-nonadecanoic acid SAMs on Au/Cu(upd) (A) without and (B) with pretreatment by oxygen plasma. The asterisks indicate the shake-up peaks of copper oxides.

Figure 6. A typical IRAS spectrum of n-nonadecanoic acid SAMs on Au/Cu(upd) with pretreatment by oxygen plasma. The presence of νa(CO2-) at 1560 cm-1 indicates a monodentate binding scheme resulting from formation of surface oxides.

by Crooks and co-workers.20 Figure 5A shows that the shake-up peaks have completely disappeared after formation of the fatty acid monolayers and that the peak position of Cu(2p3/2) shifts from 933.4 to 932.2 eV. In the case of growth of surface oxide on Au/Cu(upd) by oxygen plasma, the XPS pattern (Figure 5B) resembles that of copper oxides.35 After the preparation of SAMs, the peak positions remain unchanged (934.0 eV for Cu(2p3/2)) and the intensities of the shape-up peaks decrease only slightly. The variation in chemical shifts shows that the coppercarboxylate interactions on copper oxides exhibit a relatively larger binding energy and thus suggests less ionic characteristics on Au/Cu(upd) than on copper oxides. It turns out that such oxygen-plasma-treated Au/Cu(upd) becomes relatively difficult to obtain highly crystalline SAMs because the νa(CH2) band often shifts toward higher wavenumbers, sometimes up to 2926 cm-1. Concomitantly, IR spectra (Figure 6) reveal the presence of νa(CO2-) at 1560 cm-1 and the shift of νs(CO2-) position from 1412 to 1441 cm-1, similar to that on copper with native oxides. The spectroscopic information confirms that the binding scheme of the carboxylate headgroup is affected by the degree of surface oxide. This finding is supported by literature reports that the carboxylate binds monodentately on native copper oxides for SAMs prepared under ambient conditions9,36 and that the carboxylate exhibits (36) Boerio, F. J.; Chen, S. L. J. Colloid Interface Sci. 1980, 73, 176185.

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the bidentate bonding on oxide-free copper via gas-phase adsorption in ultrahigh vacuum.37-42 In conclusion, the structures of n-alkanoic acid SAMs on Au/Ag(upd) and Au/Cu(upd) are examined and compared with those prepared on bulk silver and copper. The results show that the structures of SAMs on Au/Ag(upd), on Au/Cu(upd), and on bulk silver are similar, different (37) Dubois, L. H.; Zegarski, B. R.; Nuzzo, R. G. Langmuir 1986, 2, 412-417. (38) Haq, S.; Leibsle, F. M. Surf. Sci. 1996, 355, L345-L349. (39) Wuhn, M.; Weckesser, J.; Woll, C. Langmuir 2001, 17, 76057612. (40) Stohr, J.; Outka, D. A.; Madix, R. J.; Dobler, U. Phys. Rev. Lett. 1985, 54, 1256-1259. (41) Karis, O.; Hasselstrom, J.; Wassdahl, N.; Weinelt, M.; Nilsson, A.; Nyberg, M.; Pettersson, L. G. M.; Stohr, J.; Samant, M. G. J. Chem. Phys. 2000, 112, 8146-8155. (42) Sotiropoulos, A.; Milligan, P. K.; Cowie, B. C. C.; Kadodwala, M. Surf. Sci. 2000, 444, 52-60.

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only in the degree of the odd-even effect and the tilt angles of the molecular chains and the plane of the CCC backbone. The structural features are distinctly different from those of n-alkanoic acid SAMs on bulk copper, which show no odd-even effect, exhibit the monodentate bonding for the carboxylate headgroup, and are less crystalline. XPS and IRAS measurements suggest that the degree of surface oxide is an important factor affecting the headgroup binding scheme and the monolayer structure. Acknowledgment. The authors thank the National Science Council (R.O.C.), the Chemistry Department of National Sun Yat-Sen University, and National Tsing Hua University for a generous financial and research support. Finally, we thank the referees for their valuable comments. LA0157364