Langmuir 2001, 17, 7605-7612
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Bonding and Orientational Ordering of Long-Chain Carboxylic Acids on Cu(111): Investigations Using X-ray Absorption Spectroscopy M. Wu¨hn,† J. Weckesser,‡ and Ch. Wo¨ll*,† Lehrstuhl fu¨ r Physikalische Chemie I, Ruhr-Universita¨ t Bochum, 44780 Bochum, Germany, and Max-Planck-Institut fu¨ r Festko¨ rperforschung, Heisenbergstrasse 1, 70569 Stuttgart, Germany Received February 14, 2001. In Final Form: August 15, 2001 The adsorption of various carboxylic acids with different chain lengths on Cu(111) has been investigated using near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and X-ray photoelectron spectroscopy (XPS). To systematically study the influence of the chain length, we used three different carboxylic acids in the range of 1-30 carbon atoms, namely, formic acid (HCOOH), hexanoic acid (C5H11COOH), and triacontanoic acid (C29H59COOH). Films with coverages ranging from the sub-monolayer to the multilayer regime are prepared by organic molecular beam epitaxy. The adlayers were characterized in situ by XPS and NEXAFS directly elucidating the nature of the chemical bond to the substrate and the degree of orientational order in the film. Highly oriented organic monolayers could be fabricated for all investigated carboxylic acids on the Cu(111) surface, but their adsorption geometries differ strongly. Whereas in the saturated monolayer the O-C-O plane of formic acid is significantly tilted, the alkyl chain and the O-C-O plane in hexanoic acid are oriented almost normal to the surface. For triacontanoic acid, a coverage driven reorientation of the molecular chain from flat lying to upright standing takes place upon completion of the first layer. In the saturated monolayer, the investigated carboxylic acids interact via two equivalent oxygen atoms, i.e., they form a bidentate carboxylate bonding to the copper surface.
1. Introduction In recent years, the spontaneous formation of wellordered, ultrathin organic films has attracted considerable interest. In particular, the Langmuir-Blodgett technique and the self-assembly of alkanethiols on Au substrates (SAMs, self-assembled monolayers) are widely used to fabricate well-ordered organic monolayers. Thiols are coupled to the substrate through the formation of a covalent Au-thiolate bond.1,2 In a recent study, it was demonstrated that vapor deposition of alkanethiols also allows fabrication of highly oriented films.3 Similar results have been reported for alkanethiols deposited on copper.4 In the present paper, another approach for anchoring organic molecules to a metal substrate is described, namely, the formation of carboxylate bonds. To gain more insight in the basic mechanisms of the self-assembly process, we have studied the interaction of formic acid (HCOOH), hexanoic acid (C5H11COOH), and triacontanoic acid (C29H59COOH) with the Cu(111) surface, where in previous work also the formation of highly oriented films has been reported.5,6 Furthermore, the bonding between carboxylic acids and copper surfaces plays an important role in catalytic processes. In the industrial methanol synthesis, which is based on Cu/ZnO/Al2O3 catalysts, the * Corresponding author tel: +49-234-3225529, fax: +49-2343214182, e-mail:
[email protected]. † Ruhr-Universita ¨ t Bochum. ‡ Max-Planck-Institut fu ¨ r Festko¨rperforschung. (1) Camillone, N.; Leung, T. Y. B.; Schwartz, P.; Eisenberg, P.; Scoles, G. Langmuir 1996, 12, 2737-2746. (2) Camillone, N.; Leung, T. Y. B.; Scoles, G. Surf. Sci. 1997, 373, 333-349. (3) Himmel, H.-J.; Wo¨ll, C.; Gerlach, R.; Polanski, G.; Rubahn, H.-G. Langmuir 1997, 13, 606-605. (4) Vollmer, S.; Fouquet, P.; Witte, G.; Boas, C.; Kunat, M.; Burghaus, U.; Wo¨ll, C. Surf. Sci. 2000, 462, 135-142. (5) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 45. (6) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 52.
rate-determining step is believed to be the hydrogenation of adsorbed formate (HCOO-) species.7 On the basis of infrared reflection absorption spectroscopy, Allara and Nuzzo5,6 proposed that carboxylic acids with different chain lengths (nC ) 16-22) adsorb from solution onto oxidized aluminum surfaces by deprotonation and the formation of strong carboxylate bonds to the substrate. Dubois et al.8 investigated the adsorption of carboxylic acids with shorter chain lengths (nC ) 1-4, 6) from the gas phase on Cu(100) by means of EELS (electron energy loss spectroscopy) and X-ray photoelectron spectroscopy (XPS). Two different bonding types of the carboxylate group to the substrate have been proposed, namely, a bidentate bond through two equivalent oxygen atoms at low coverages and a monodentate (with two inequivalent oxygen atoms) at higher coverage. These observations are insofar surprising; as in many other previous investigations, no evidence for monodentate formation as a result of carboxylic acid interaction with other Cu surfaces has been reported (Cu(100),9 Cu(110),10 Cu(111)11). To resolve this apparent inconsistency, we have studied the adsorption of three carboxylic acids with different chain lengths, containing 1, 6, and 30 carbon units, which enable us to characterize the bonding mechanisms in a more comprehensive way. To avoid complications caused by the presence of an oxide adlayer on the substrate, the experiments have been performed under ultrahigh vacuum conditions. (7) Asgaard, T. S. J. Catal. 1995, 156, 229-242. (8) Dubois, L. H.; Zegarski, B. R.; Nuzzo, R. G. Langmuir 1986, 2, 412-417. (9) Sto¨hr, J.; Outka, D. A.; Madix, R. J.; Do¨bler, U. Phys. Rev. Lett. 1985, 54, 1256-1259. (10) Karis, O.; Hasselstrom, J.; Wassdahl, N.; Weinelt, M.; Nilsson, A.; Nyberg, M.; Petterson, L. G. M.; Sto¨hr, J.; Samant, M. G. J. Chem. Phys. 2000, 112, 8145. (11) Sotiropoulos, A.; Milligan, P. K.; Cowie, B.; Kadodwala, M. Surf. Sci. 2000, 444, 52-60.
10.1021/la010237o CCC: $20.00 © 2001 American Chemical Society Published on Web 11/03/2001
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Figure 1. XP spectra of formic acid on Cu(111). In the left and the right panels, the data of the C1s and O1s regions are presented, respectively. The energetic positions of the peaks and the shift between monolayer and multilayer are marked.
2. Experimental Section All experiments were carried out at the Berlin synchrotron radiation facility BESSY I. The near-edge X-ray absorption fine structure (NEXAFS) data were recorded at the beamline HETGM-2 with a resolution E/∆E ) 580.12 The main chamber (base pressure 100 Å). Subsequent heating of the sample leads to desorption and hence a decreasing coverage. After the sample was annealed to 280 K, two peaks at 532.5 and 534.0 eV can be distinguished. They are attributed to molecules in the multilayer and in direct contact with the surface, respectively. While the multilayer peak is rather broad (full width at half-maximum (fwhm) ) 2.7 eV, as expected from the presence of two different oxygen atoms in the carboxylic acid unit), the monolayer peak is significantly smaller with a fwhm of only 1.5 eV, indicating
the presence of a single type of oxygen atom. In the spectrum recorded after the sample was heated to the highest temperature (320 K), a sharp instead of a broad peak is seen at 534.0 eV. This peak is clearly not related to a multilayer, and this O1s signal is thereforesin accord with previous work15sassigned to oxygen atoms not directly bound to the Cu substrate in a decomposition product. Figure 4 shows the C1s (left) and O1s (right) XP spectra recorded for triacontanoic acid on Cu(111) (increasing coverage from top to bottom). In the C1s region, a single peak is observed, which shifts continuously from 284.6 eV for the sub-monolayer film (2 Å) to 285.9 eV for the multilayer film (100 Å). This shift can again be explained in terms of the relaxation effect, a well-known final state effect resulting from the screening of the core hole generated in the photoionization process by the metal electrons.3,16,17 In contrast, the energetic position of the O1s peak at 531.5 eV does not initially change with increasing coverage. Only after the onset of multilayer growth is the peak position shifted by 2.4 to 533.9 eV. NEXAFS Data. Figure 5 compares NEXAFS data measured for formic acid, hexanoic acid, and triacontanoic acid in the multilayer regime with a saturated hydrocarbon, i.e., a multilayer hexatriacontane (taken from ref 18). All spectra are dominated by two resonances labeled “A” and “B” and exhibit a strong similarity to those reported in previous work.10 The spectra of hexanoic acid and triacontanoic acid are fully consistent with the presence of a carboxyl group (resonance A) and saturated hydrocarbons (resonance B).16 Table 1 summarizes the energetic positions of the different observed peaks. Their assignment is taken from ref 18 (resonance A) and ref 16 (resonance B). Note that also for short-chain alkyl chains (15) Wegner, H.; Weiss, K.; Wo¨ll, C. Surf. Rev. Lett. 1999, 6, 183192. (16) Weiss, K.; Weckesser, J.; Wo¨ll, C. THEOCHEM 1998, 458/1-2, 143-150. (17) Witte, G.; Wo¨ll, C. J. Chem. Phys. 1995, 103, 5860-5863. (18) Outka, D. A.; Sto¨hr, J.; Madix, R. J.; Rothermund, H. H.; Hermsmeier, B.; Solomon, J. Surf. Sci. 1987, 185, 53.
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Figure 4. XP spectra of triacontanoic acid on Cu(111). In the left and the right panels, the XPS data of the C1s and O1s region are presented, respectively. The coverages are indicated and decrease from 100 (bottom) to 2 Å (top). The shifting of 1.3 eV of the peak position is indicated. While the C1s peak shifts continuously to higher binding energies with increasing coverage, the O1s peak remains constant except for the thickest multilayer film (100 Å).
Figure 5. NEXAFS spectra recorded for multilayers of different organic compounds containing alkane chains. All multilayers were deposited on a Cu(111) substrate. Table 1. Energetic Positions of the Observed Resonances and Their Assignments Taken from Refs 18 and 16 resonance A B
Eformic acid [eV]
Ehexanoic acid [eV]
Etriacontanoic acid [eV]
288.3
287.3 288.6
287.3 287.7
assignt π*CdO R
the R resonance (here labeled B) is located at almost the same energy as seen for the long-chain alkanes (see ref 19 for the case of hexanethiolate). (19) Imanishi, A.; Isawa, K.; Matsui, F.; Tsuduki, T.; Yokoyama, T.; Kondoh, H.; Kitajima, Y.; Ohta, T. Surf. Sci. 1998, 407, 282-292.
A comparison of these multilayer data reveals that only the spectrum of formic acid is dominated by the π*CdO resonance (labeled A), which is attributed to an excitation of a C1s core electron into the lowest unoccupied molecular orbital (1sfπ*CdO). For the acids with longer hydrocarbon chains, the relative contribution of this resonance decreases and the Rydberg R resonance (labeled B) becomes dominant (cf. refs 20 and 21). To gain insight into the chemical bonding to the copper substrate, we now focus on the regime where the coverage does not exceed one monolayer. At the top of Figures 6 and 7, the C1s spectra of formic acid and hexanoic acid are shown; in the diagram below, the corresponding O1s spectra are displayed. Monolayer spectra were recorded at grazing (30°, dotted line) and normal (90°, straight line) incidence; corresponding multilayer spectra (55°) are provided for comparison. In the case of formic acid (Figure 6), the C1s spectra of the monolayer film exhibit a dichroism with a maximum intensity of the π*CdO resonance (288.4 eV) at grazing incidence. A quantitative analysis21 yields a tilt angle of 35 ( 5° between the transition dipole moment (TDM) and the surface normal. At the O1s edge (Figure 6, bottom), a corresponding analysis of the dominating π*CdO resonance (533.2 eV) yields a tilt angle of the corresponding TDM of 49 ( 10°, 14° larger than the angle for the π*CdO TDM. The TDM of the π*CdO resonance is oriented perpendicular to the molecular O-C-O plane. Because the signal-to-noise ratio is significantly better for the C1s data than for the O1s data, in the following, only the orientation determined from the C1s NEXAFS data will be used. At the C1s edge, the energetic position of the dominating π*CdO resonance remains nearly constant for multilayer (288.3 eV) and monolayer (288.4 eV) coverages. At the (20) Bagus, P. S.; Weiss, K.; Schertel, A.; Wo¨ll, C.; Braun, W.; Hellwig, C.; Jung, C. Chem. Phys. Lett. 1996, 248. (21) Sto¨hr, J. NEXAFS Spectroscopy; Springer-Verlag: Berlin/ Heidelberg, 1996.
Long-Chain Carboxylic Acids on Cu(111)
Figure 6. NEXAFS spectra of formic acid on Cu(111) in the mono- and multilayer regime. The dichroism of the spectra recorded at the C1s and O1s edge yield a tilt angle of the π*CdO orbital of 35 ( 5° and 49 ( 10° with respect to the surface normal, respectively. In the O1s region, a small shift is observed for the leading π* resonance at different coverages.
O1s edge, the resonance of the monolayer film is located at slightly higher photon energies, 533.2 eV as compared to 532.8 eV. The slight shift of 0.4 eV between monolayer and multilayer is attributed to a chemical interaction of the monolayer carboxylate groups with the copper surface atoms. In the top panel of Figure 7, the C1s NEXAFS spectra of hexanoic acid on Cu(111) are shown. Again, the monolayer spectra exhibit a pronounced dichroism, indicating the presence of a highly oriented film. The evaluation of the dichroism of the dominating R resonance (labeled B) results in a TDM tilt angle of 30 ( 2° with respect to the surface normal. The TDM of this resonance is oriented along the molecular alkyl chain. At the O1s edge, the spectra of hexanoic acid are very similar to those obtained for formic acid (cf. Figure 6). The analysis of the dichroism of the π*CdO resonance yields a TDM tilt angle of 60 ( 2° for the monolayer. Taking into account that the π*CdO resonance TDM is oriented perpendicular to the R resonance TDM of the hydrocarbon chain, these results indicate that the complete molecule is tilted by 30 ( 2° with respect to the surface normal. At the C1s edge, the energetic position of the R resonance shows nearly no variation with coverage. In contrast, the π*CdO resonance at the O1s edge is slightly shifted by 0.5 eV toward higher energies in the monolayer. This again indicates a significant chemical interaction of the molecular carboxylate groups with the copper surface atoms. In Figure 8, C1s NEXAFS spectra of triacontanoic acid on Cu(111) are displayed (coverages increasing from top to bottom). For the film with the lowest coverage (2 Å), no R resonance is observed. Instead, two peaks located at
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Figure 7. NEXAFS spectra of hexanoic acid adlayers on Cu(111) in the mono- and multilayer regime. The dicroism of the spectra recorded at the C1s and O1s edge yields a tilt angle of the carbon chain and the π*Cd0 orbital of 30 ( 2° and 60 ( 2° with respect to the surface normal, respectively. In the O1s region, a small shift is observed for different coverages.
Figure 8. NEXAFS C1s spectra of triacontanoic acid on Cu(111) for coverages ranging from sub-monolayer (2 Å) to multilayer (100 Å) coverages. In the sub-monolayer film, the typical alkane resonances are quenched resulting in two new resonances, “β” and “γ”. By comparison with hexatriacontane, a flat adsorption geometry can be deduced. With increasing coverage, a reorientation takes place and the dichroism of the 16 Å film yields a tilt angle of 23 ( 5° with respect to the surface normal, i.e., nearly upright standing molecules.
287.0 eV (labeled “β”) and 289.0 eV (labeled “γ”) are present. The origin of these features, commonly observed
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for hydrocarbons in direct contact with metal surfaces,16 is presently only poorly understood. Although resonances β and γ do not allow for a direct determination of the molecular orientation, by comparison with other longchain saturated hydrocarbons,15 an adsorption geometry with the molecular chain oriented almost parallel to the surface plane can be deduced. At higher coverages, the expected R resonance at 287.7 eV appears. Resonance R is at the same position, where in alkane chains in direct contact with a Cu(111) surface, a M* resonance has been seen.16 In the present data, however, the fact that the feature is also seen for the thicker adlayers suggests small effects related to radiation damage. With increasing coverage, the situation changes drastically, and for the 16 Å film, the R resonance is highest at normal incidence, thus revealing an orientation of the alkyl chain almost normal to the surface. The strong angular dependence is characteristic for a highly oriented film. A quantitative analysis of the R resonance dichroism yields a tilt angle of 23 ( 5° with respect to the surface normal (for a detailed description of the procedure, cf. ref 22). In the multilayer regime (100 Å), the angular dependence of the R resonance vanishes almost completely, indicating the presence of a disordered multilayer. 4. Discussion In the present work, the adsorption behavior of carboxylic acids with different alkyl chain lengths (n ) 1, 6, and 30) on the Cu(111) surface is investigated. For the various chain lengths, significant differences in molecular orientation are observed. In the monolayer, the XP spectra of the C1s region show only one peak located at 290.3 eV in the case of formic acid (Figure 1), while in the case of hexanoic acid (Figure 2) there are two energetically separated peaks located at 285.6 and 289.1 eV. In the case of triacontanoic acid (Figure 4), only one peak located at 285.5 eV (16 Å film) is seen. According to the molecular structure and stoichiometry of these acids, the assignment of the signals can be made as follows: the peak observed at 290 eV is assigned to the C atom of the carboxyl group. In the case of hexanoic acid, the additional peak located at about 285.6 eV is assigned to the alkyl chain. The energetic difference of 4.5 eV can be explained in terms of different chemical environments of a carbon atom in the alkyl chain and the carboxyl group resulting in different binding energies of the 1s electrons. In the case of triacontanoic acid, the peak of the alkyl chain is located at 285.5 eV and the signal of the carboxylic group is too small to be observed as expected from the molecular stoichiometry (29:1). The peaks of condensed multilayers are shifted toward higher binding energies as indicated in Figures 1, 2, and 4. Because there is no evidence for a chemical interaction between the carbon atoms and the copper surface, the lower C1s binding energy of the monolayer relative to the multilayer is attributed to the so-called relaxation effect (see above). Switching the focus to the O1s region (cf. Figures 1 and 2), a single peak is observed for the monolayer films as well as for the multilayer films, which is attributed to the 1s signal of the two carboxyl oxygen atoms. While for all multilayer films of the various carboxylic acids the O1s peak is located at about 534 eV, this peak is shifted toward lower binding energies in the monolayer regime. For all films in the monolayer regime, the fwhm of the O1s peak is smaller (fwhm ) 1.5 eV) than that for the multilayer (22) Kinzler, M.; Schertel, A.; Ha¨hner, G.; Wo¨ll, C.; Grunze, M.; Albrecht, H.; Holzhu¨ter, G.; Gerber, T. J. Chem. Phys. 1994, 100 (10), 7722.
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(fwhm ) 2.7 eV). These observations strongly suggest the formation of a bidentate bonding, where two equivalent oxygen atoms interact with the Cu surface. In contrast, the molecules in the multilayer are intact; i.e., two chemically different oxygen species exist in the carboxylic acid group and the O1s peak is thus considerably broader. In a previous study, Dubois et al.8 have investigated carboxylic acids with short chain lengths (one to four and six C atoms) adsorbed from the gas phase on Cu(100) by means of EELS and XPS. On the basis of XP spectra for different preparation conditions, two different bonding types of the carboxylate group to the surface are proposed. In these experiments, first a thick multilayer of acetic acid was adsorbed and subsequently heated to 220 K. This results in the appearance of a band in the EELS data at 1630 cm-1 and an increase of the line width of the XPS O1s peak. On the basis of these observations, the presence of two different types of O atoms is concluded and a monodentate bonding is proposed, where only one O atom interacts directly with the surface. In this model, the presence of carboxylic acid molecules asymmetrically tilted away from the surface (“canted”) is proposed to yield higher coverage. In our study, several attempts were undertaken to obtain the canted carboxylate species, as characterized by a rather large line width (resulting from the presence of two inequivalent O atoms) of the XPS O1s peak. All attempts failed; for film thicknesses in the monolayer regime, always a single sharp O1s XPS peak was observed for all three different carboxylic acids investigated here. At present, this inconsistency between our experiments and the previous data by Dubois et al.8 cannot be explained. We would like to note, however, that these differences are possibly related to the different surface structures of Cu(100) and Cu(111). In a recent study on formate and acetate adsorption on Cu(110), a bidentate species has been identified.10 It is interesting to mention that the second O1s peak at 533.9 eV observed by Dubois et al. for acetic acid actually coincides with the O1s peak position seen for the hexanoic acid multilayer. To gain more insight into the orientation of the alkyl chain and the carboxylate group, the presented NEXAFS spectra were analyzed further (for details of this procedure, cf. refs 21 and 22). The resonances of the multilayer spectra of formic acid, hexanoic acid, triacontanoic acid, and hexatriacontane are compared in Figure 5, and the dominating peak positions together with their assignment are summarized in Table 1. The spectra are dominated by a R (labeled B) and two σ* resonances (“C” and “D”), which are typical for saturated hydrocarbons.16 In contrast, the spectrum of formic acid is dominated by the π*CdO resonance (A) since this molecule contains only one carbon atom. Formic Acid. The analysis of the dichroism in the C1s NEXAFS spectra of formic acid yields average TDM tilt angles of 35 ( 5° with respect to the surface normal for the π system (cf. Figure 6). This indicates an average tilt angle of 55 ( 5° between the carboxylate plane and the surface normal. No significant difference in the energetic position of the dominating π*CdO resonance in the monolayer and multilayer regime is observed in the C1s region. In contrast, the O1s data reveal a small shift of 0.4 eV toward lower photon energies with increasing coverage. This shift is typical for the presence of a chemisorbed species and has been reported previously.23 Therefore, the adsorption model for formic acid on Cu(111) is proposed (cf. Figure 9) as follows: In the monolayer, the molecule (23) Ho¨vel, S.; Kolczewski, C.; Wu¨hn, M.; Albers, J.; Weiss, K.; Staemmler, V.; Wo¨ll, C. J. Chem. Phys. 2000, 112 (8), 3909-3916.
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Figure 9. Schematic growth model for the various carboxylic acids on Cu(111). Highly oriented saturated monolayers are formed for all three investigated molecules. For formic acid, the angle between the O-C-O plane and the surface normal amounts to 55° and the average tilt angle of the carbon chains decreases with increasing chain length from 55° for hexanoic acid to 23° for triacontanoic acid. For all systems, the carboxylic acid groups are in direct contact to the Cu(111) substrate interact via two equivalent oxygen atoms with the surface; that is, a bidentate bonding is formed. For triacontanoic acid, there is a coverage driven reoriention from flat lying molecular chains in the sub-monolayer range to nearly upright standing molecular chains in the saturated monolayer. In the multilayers, the intact molecules adsorb in a random fashion on the first layer and the order of the film is thus lost.
is chemisorbed via the two equivalent carboxylate oxygen atoms thus forming a bidentate bonding. In this configuration, the molecular O-C-O plane is tilted by 55 ( 5° with respect to the surface normal. Outka et al.18 investigated the adsorption of formic acid on Si(111)(7 × 7). They report a tilt angle of the O-C-O plane of 55° with respect to the surface normal. On Cu(110), the adsorption of formic acid has been studied intensively.21,24 The molecule is always found to be oriented upright and to interact with the substrate via the oxygen atoms of the carboxylic group with the O-C-O plane oriented parallel to the copper rows. In summary, on all different copper surfaces investigated so far, formic acid forms a bidentate carboxylate-copper bond. On Cu(111), the molecule is tilted, whereas on the (110) surface, it is oriented upright. On the surface of the highly oriented monolayer, further adsorption takes place in a random fashion thus yielding rather disordered multilayers of formic acid. Hexanoic Acid. The NEXAFS spectra of hexanoic acid on Cu(111) are dominated by the R resonance at about 288 eV at the C1s edge and by the π*CdO resonance at about 533 eV at the O1s edge (cf. Figure 7). A quantitative analysis of the dichroism yields a tilt angle of 30° with respect to the surface normal for the alkyl chain and the O-C-O plane. There is no significant shift of the R resonance in the C1s region when increasing the coverage from monolayers to multilayers. In contrast, as in the case of formic acid, the O1s spectra yield a shift of the π*CdO resonance by 0.5 eV toward higher photon energies for the monolayer. This again indicates a strong chemical bonding of the carboxylate group to the substrate. Thus, the adsorption model is proposed (cf. Figure 9) as follows: In the monolayer, hexanoic acid adsorbs via the carboxylic group forming a bidentate ligand and exhibits a molecular tilt angle of 30 ( 5° with respect to the surface normal. On the surface of this highly oriented film, further adsorption takes place in a random fashion thus yielding rather disordered hexanoic acid multilayers. Triacontanoic Acid. In Figure 8, a series of C1s NEXAFS spectra measured for triacontanoic acid on Cu(111) with coverages increasing from top (2 Å) to bottom (100 Å) are displayed. The structure and the dichroism of
the spectra reveal a significant dependence on the layer thickness. At low coverage (2 Å), the expected R resonance at about 288 eV is strongly quenched and splits into two components located at about 287 eV (β) and 289 eV (γ) making a quantitative analysis difficult. However, a comparison with corresponding data for long-chain alkanes on the same substrate16 indicates that the alkyl chains are oriented parallel to the surface. For higher coverages of 4 and 8 Å, the intensities of the two peaks β and γ decrease and the dichroism nearly disappears. The 16 Å film shows the typical NEXAFS features of long alkyl chains (R, σ*), indicating that the alkyl chain is no longer in direct contact with the substrate. As compared to the 2 Å film, the strong dichroism is reversed. A detailed quantitative analysis reveals a tilt angle of 23 ( 5° with respect to the surface normal for the hydrocarbon chain. At this coverage, the C1s NEXAFS spectra become similar to those of highly oriented alkanethiolate monolayers adsorbed on gold substrates.25 The spectra of the multilayer film (100 Å) show no dichroism, typical for a thick disordered film. Because of the low oxygen content in triacontanoic acid, i.e., a stoichiometric ratio of oxygen to carbon of 1:15, the NEXAFS spectra at the O1s edge suffer from a poor signal-to-noise ratio for this long-chain molecule and are not presented in this work. On the basis of the different results presented above, the growth model of triacontanoic acid on Cu(111) is proposed (cf. Figure 9) as follows: At low coverages (about 2-4 Å), the molecules are physisorbed and the alkyl chains are lying flat on the surface. With increasing coverage a reorientation of the molecular chains takes place, and at higher coverages (16 Å) the molecules exhibit an almost upright orientation. In contrast to the low coverage regime, the molecules are now bound to the copper surface through the carboxylate group and the same bidentate species as for shorter alkyl chain acids are observed. On the surface of this highly oriented film, further adsorption takes place in a random fashion thus yielding rather disordered multilayers of triacontanoic acid.
(24) Somers, J.; Robinson, A. W.; Lindner, T.; Ricken, D.; Bradshaw, A. M. Phys. Rev. B: Condens. Matter Mater. Phys. 1989, 40, 2053.
(25) Dannenberger, O.; Weiss, K.; Himmel, H.-J.; Ja¨ger, B.; Buck, M.; Wo¨ll, C. Thin Solid Films 1997, 307, 183.
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5. Summary For all three investigated carboxylic acids, adsorption on a Cu(111) surface was found to result in the formation of highly oriented organic monolayers. The NEXAFS spectra of a saturated formic acid monolayer show a large dichroism revealing molecules with the carboxylate plane tilted on average by 55 ( 5° with respect to the surface normal. For the multilayer films, the dichroism vanishes indicating the loss of orientational order. It is thus concluded that further arriving molecules adsorb with random orientation on top of the highly oriented saturated monolayer. This behavior is observed for all three investigated carboxylic acids. For the carboxylic acid with a longer alkyl chain, hexanoic acid, a similarly strong dichroism in the C1s NEXAFS spectra of the monolayer indicates again a high degree of orientational order. However, as compared to formic acid, the intensities are reversed revealing that for this molecule the molecular chains are oriented almost upright with an average tilt angle of 30 ( 2° with respect to the surface normal. No signs for a monodentate carboxylate species, as proposed by Dubois et al. for hexanoic acid on Cu(100),8 are found in our study. Our results are fully consistent with a symmetric bidentate bonding with two equivalent oxygen atoms of the carboxylate group interacting with the copper substrate as found in the bulk of previous work for carboxylates
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interacting with Cu surfaces. This bidentate bonding type is present in the saturated monolayers of all three investigated carboxylic acids. Increasing the length of the alkyl chain, i.e., going to triacontanoic acid, the NEXAFS spectra of the saturated monolayer lead to an even stronger dichroism yielding a tilt angle as small as 23 ( 5° with respect to the surface normal. In the sub-monolayer regime, the NEXAFS spectra resemble those reported recently for saturated hydrocarbons16 and a flat adsorption geometry is thus concluded. With increasing coverage, the alkyl chains start to tilt away from the surface plane. This reorientation is accompanied by the formation of the symmetric bidentate bonding. The present study demonstrates that it is possible to fabricate highly oriented monolayers by organic molecular beam epitaxy of carboxylic acids on Cu(111). These systems are therefore an interesting and promising analogy to the intensively studied SAMs of thiols on gold. Acknowledgment. This project was funded by the German BMBF (05625VHA3). The authors thank M. Mast and W. Braun at the Berlin synchrotron facility BESSY for the excellent technical and scientific support. We also acknowledge a careful reading of the manuscript by Thomas Strunskus. LA010237O
