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Structural Analysis of Saturated Alkanethiolate Monolayers on Cu(100): Coexistence of Thiolate and Sulfide Species Stefan Vollmer, Gregor Witte,* and Christof Wo¨ll Lehrstuhl fu¨ r Physikalische Chemie I, Ruhr-Universita¨ t Bochum, Universita¨ tsstrasse 150, D-44801 Bochum, Germany Received May 29, 2001. In Final Form: August 30, 2001 The adsorption of alkanethiols [CH3(CH2)n-1SH] of various chain lengths (n ) 1, 2, 4, 7, 10) on a Cu(100) surface has been studied by means of low-energy electron diffraction, He atom scattering , thermal desorption spectroscopy, and X-ray photoelectron spectroscopy. Highly ordered thiolate films were prepared by vaporphase deposition which formed, at room temperature, an intermediate (2 × 2) phase and, upon further exposure, a c(6 × 2) saturation structure. An extended XPS analysis unveiled that, in contrast to the intermediate phase, the saturation phase is metastable and decomposes upon partial desorption of alkyl chains into a previously unobserved thermodynamically stable mixture of thiolate and sulfide species under retention of its c(6 × 2) structure. As a consequence, the density of the alkyl chains is significantly reduced, demonstrating that such alkanethiolate monolayers on copper are not as densely packed as in the case of gold.
Introduction Ultrathin organic films of self-assembled molecules (SAMs) have attracted considerable interest because of their promising technical applications in different fields such as lubrication, lithography, and corrosion protection.1 Whereas the vast majority of studies on SAMs have been focused on alkanethiolate films on gold surfaces because they are considered as prototype systems, which can be easily prepared by the immersion of a gold sample into thiol solution,2,3 the structure and properties of SAMs on other substrates are, by far, less intensely studied. Especially for the case of transition metals, where a corrosion protection by thiol films is reported,4,5 there is a great interest in the microstructure and stability of such films. The preparation of highly ordered SAMs on substrates other than gold is, however, complicated by their reactivity and, therefore, requires an alternative preparation by vapor-phase deposition. For the case of alkanethiols adsorbed on copper, quite recently several studies have been reported6-16 which allow * To whom correspondence should be addressed.
[email protected]. (1) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: Boston, MA, 1991. (2) Ulman, A. Self-Assembled Monolayers of Thiols. Thin Films; Academic Press: San Diego, CA, 1998; Vol. 24. (3) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151. (4) Laibinis, P. E.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 114, 9022. (5) Jennings, G. K.; Munro, J. C.; Yong, T.-H.; Laibinis, P. E. Langmuir 1998, 14, 6130. (6) 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. (7) Rieley, H.; Kendall, G. K.; Chan, A.; Jones, R. G.; Lu¨decke, J.; Woodruff, D. P.; Cowie, B. C. C. Surf. Sci. 1997, 392, 143. (8) Imanishi, A.; Takenaka, S.; Yokoyama, T.; Kitajima, Y.; Ohta, T. J. Phys. IV 1997, 7, C2-701. (9) Imanishi, A.; Isawa, K.; Matsui, F.; Tsuduki, T.; Yokoyama, T.; Kondoh, H.; Kitajima, Y.; Ohta, T. Surf. Sci. 1998, 407, 282. (10) Kariapper, M. S.; Grom, G. F.; Jackson, G. F.; MConville, C. F.; Woodruff, D. P. J. Phys.: Condens. Matter 1998, 10, 8661. (11) Tsuduki, T.; Imanishi, A.; Isawa, K.; Terada, S.; Matsui, F.; Kiguchi, M.; Yokoyama, T.; Ohta, T. J. Synchrotron Radiat. 1999, 6, 787.
for a comparison with the corresponding films on gold. These studies indicate that the structure of alkanethiolate films on copper is dominated by the interaction of the sulfur headgroup with the metal surface which gives rise to a large sticking coefficient and a low activation barrier for deprotonation (i.e., thiolate formation)7-13,22,23 as compared to the case of gold, where an exposure of several thousand Langmuir (1 L ≡ 10-6 Torr s) is required to prepare a saturated thiolate monolayer.18 The strong Cu-S interaction leads to formation temperatures for copperthiolate significantly below room temperature and to a pronounced upright orientation of the molecules even at low coverage.9,11 At elevated temperatures (J400 K), the S-C bond is cleaved and the alkyl chain desorbs, with the S atoms remaining on the surface. The strong sulfurcopper interaction also results in a substantial reconstruction of the Cu(111) surface yielding a pseudo-(100) layer after formation of an alkanethiolate monolayer.14 The tendency of sulfur atoms to occupy 4-fold coordinated sites has been invoked to explain such a reorientation of the Cu(111) surface which favors the Cu(100) surface as an ideal support for an alkanethiol SAM.14 In fact, the corresponding X-ray absorption measurements on Cu(100) reveal no evidence of any surface reconstruction, and only an occupation of 4-fold coordinated adsorption sites is reported for the thiolate.11,13 However, an analysis of the lateral film structure is still missing for alkanethiolates on Cu(100). This is partly due to the fact that electron diffraction methods (i.e., low-energy electron diffraction (LEED)) are believed to cause a rapid dissociation and, (12) Loepp, G.; Vollmer, S.; Witte, G.; Wo¨ll, Ch. Langmuir 1999, 15, 3767. (13) Kariapper, M. S.; Fisher, C.; Woodruff, D. P.; Cowie, B. C. C.; Jones, R. G. J. Phys.: Condens. Matter 2000, 12, 2153. (14) Driver, S. M.; Woodruff, D. P. Langmuir 2000, 16, 6693. Driver, S. M.; Woodruff, D. P. Surf. Sci. 2000, 457, 11. (15) Vollmer, S.; Fouquet, P.; Witte, G.; Boas, Ch.; Kunat, M.; Burghaus, U.; Wo¨ll, Ch. Surf. Sci. 2000, 462, 135. (16) Driver, S. M.; Woodruff, D. P. Surf. Sci. 2001, 488, 207. (17) Driver, S. M.; Woodruff, D. P. Surf. Sci. 2001, 479, 1. (18) Himmel, H. J.; Wo¨ll, Ch.; Gerlach, R.; Polanski, G.; Rubahn, H. G. Langmuir 1997, 13, 602.
10.1021/la0107852 CCC: $20.00 © 2001 American Chemical Society Published on Web 11/03/2001
Analysis of Saturated Alkanethiolate Monolayers
therefore, are considered to be not applicable for the study of such rather fragile films.14,17 The apparent lack of information about the saturation structure and, in particular, the question concerning the applicability of LEED have motivated the present study where the scattering of thermal energy He atoms (HAS) has been employed to characterize the structure of alkanethiolate monolayers prepared by vapor-phase deposition of alkanethiols [ CH3(CH2)n-1SH] of various chain lengths [n ) 1 (methanethiol), 2 (ethanethiol), 4 (butanethiol), 7 (heptanethiol), 10 (decanethiol)] on a Cu(100) surface and to compare these results with LEED measurements. Note that because of the small incident energy (typically 30 meV), any distortion of the adsorbate layer as a result of the impinging He atoms can be safely excluded.19 For all studied thiols, the formation of an intermediate (2 × 2) phase and a somewhat closer-packed c(6 × 2) saturation structure was identified. The corresponding X-ray photoelectron spectroscopy (XPS) measurements reveal an unexpected instability of the saturation phase where a spontaneous partial dissociation takes place even at room temperature leading to an effective reduction of the film packing, which has important consequences for the deterioration of corrosion protection layers. Experimental Section The present experiment was carried out in two stages. In the first part, He atom scattering20 was employed to analyze the lateral structure of the alkanethiolate monolayers. The UHV apparatus used for these measurements is described in detail elsewhere.21 Basically, a nearly monoenergetic beam (∆E/E ≈ 2%) of thermal-energy (E ) 15-80 meV) He atoms is directed at the surface, and the scattered atoms are detected at a fixed total scattering angle of θSD ) 90° with respect to the incident beam with an angular resolution of better than 0.1°. Rotating the sample about the axis perpendicular to the scattering plane allows for the recording of high-resolution He atom diffraction scans (angular distributions) with a wide range of accessible parallel momentum transfers ∆K ) ki[sin(θSD - θi) - sin(θi)], where θi denotes the angle of incidence with respect to the surface normal, and ki ) x2mHeE/p is the incident wave vector. In a second set of experiments, XPS was used to characterize the chemical state of the thiols and to determine the coverage. The latter instrument, which is described in detail elsewhere,12 is equipped with an X-ray source (VG XR3E2) together with a hemispheric electron energy analyzer (Leybold EA200) yielding an energy resolution of about 0.8 eV. Electron diffraction patterns were recorded with a microchannelplate LEED system (OCI Vacuum Microengineering Inc., Canada) which allows incident electron fluxes as low as 50-150 pA/mm2. A quadrupole mass spectrometer (Balzers QMS200) was used for measuring thermal desorption spectra. In both instruments, a Cu(100) crystal, which had been polished to within (0.2° of the desired orientation, was prepared by repeated cycles of Ar+ sputtering (800 eV) and subsequent annealing to 950 K until the X-ray photoemission or Auger electron spectra revealed no impurities above the detection limit, and a sharp diffraction pattern with a low diffuse elastic background signal was obtained by HAS or LEED. The alkanethiols (Fluka, purity 98%) were purified thoroughly by repeated freeze-pump-thaw cycles, except the gaseous methanethiol (Sigma Aldrich, 99.5%), and were dosed through a leak valve by backfilling the target chamber (base pressure 1×10-10 mbar) at pressures of 10-9-10-7 mbar. (19) Camillone, N. I.; Leung, T. Y. B.; Scoles, G. Surf. Sci. 1997, 373, 333. (20) Toennies, J. P. In Surface Phonons; Kress, W., de Wette, F., Eds.; Springer Series in Surface Science 21; Springer-Verlag: Heidelberg, Germany, 1991. (21) Fouquet, P.; Witte, G. Surf. Sci. 1998, 400, 140.
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Results As demonstrated in previous studies,12,13,22,23 the roomtemperature adsorption of alkanethiols on copper surfaces causes a deprotonation of the -SH thiol endgroup leading to the formation of a copper thiolate film. Since for technical applications copperthiolate films are usually prepared by immersion at room temperature,5 the present films were prepared by vapor deposition also at that temperature to characterize their saturation structure. Figure 1a displays a typical He angular distribution recorded for the [011] azimuth direction after exposing the copper crystal to about 3 L of ethanethiol. To reduce the Debye-Waller attenuation of the diffraction peaks, the sample was cooled to 120 K after preparation for the actual diffraction measurements. From this and further measurements along the other high-symmetry directions of the Cu substrate, this phase was identified as a (2 × 2) structure. This adsorbate structure obtained from the He atom scattering is fully consistent with the LEED data also shown in Figure 1. Upon further exposure to ethanethiol, the HAS diffraction peaks became broader, and after a total exposure of about 15 L, a c(6 × 2) structure appeared (see Figure 1b). This structure was found to be stable even after a dosage of an additional 50 L of ethanethiol and is, thus, considered as the room-temperature saturation structure. As depicted in Figure 1b, the same diffraction pattern was observed in the LEED measurements, where the simultaneous existence of two domains rotated by 90° can also be seen. Additional HAS angular distributions recorded for adlayers obtained by exposing to butanethiol and heptanethiol also revealed the presence of an intermediate (2 × 2) and a c(6 × 2) saturation structure. Both phases could be observed over a long time (∼15 min) without any noticeable degradation with the microchannelplate LEED system operating at a typical flux of 100 pA/mm2. Moreover, the same structures were also obtained in a more extensive set of LEED measurements carried out for different alkanethiols (ranging from n ) 1 to 10). These latter measurements further revealed an increasing attenuation of the diffraction spot intensities as the length of the alkyl chains increases. In the case of decanethiol, LEED spots could not be seen. Additional adsorption experiments at various substrate temperatures indicate that the c(6 × 2) saturation phase could not be prepared at temperatures below 200 K, where even after an exposure of 100 L, only the (2 × 2) LEED pattern was obtained. Previous studies have shown that heating the copper thiolate films above room temperature causes a S-C bond breaking and a desorption of the entire alkyl chain.10,12 Figure 2 displays a typical thermal desorption spectrum recorded for a saturated ethanethiolate monolayer on Cu(100) after preparation at room temperature. Because of the dissociative desorption mentioned above, signals could be observed only for fragments such as the most probable fragment, C2H3 (m/z ) 27). The corresponding desorption signal reveals a rather broad distribution with an intermediate maximum at about 400 K and a main desorption maximum at 465 K. Very similar desorption spectra were also measured for the longer alkanethiols, indicating that the desorption kinetic is rather independent of the chain length which will be discussed elsewhere in detail.30 By applying the Redhead formula24 with a typical preexponential factor of 1 × 1013 s-1, dissociation enthalpies (22) Sexton, B. A.; Nyberg, G. L. Surf. Sci. 1986, 165, 251. (23) Bao, S.; McConville, C. F.; Woodruff, D. P. Surf. Sci. 1987, 187, 133. (24) Redhead, P. Vacuum 1962, 12, 203.
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Figure 1. Comparison of He atom scattering and LEED pattern for (a) the (2 × 2) and (b) the c(6 × 2) saturation structure of ethanethiol on the Cu(100) surface. For clarity, the primitive unit cell (dashed line) as well as the rectangular c(6 × 2) unit cell are shown in (b). The light gray balls indicate the presence of additional molecules within the unit cell of which the exact position is not known. Both He angular distributions were recorded at an incident energy of Ei ) 32.6 meV along the [011] direction as indicated by the black arrows in the LEED pattern which were taken at Ei ) 109 eV. For comparison, a LEED pattern of the atomic sulfur superstructure appearing after heating the surface to 550 K (Ei ) 67 eV) is also shown in panel c. To enhance the contrast, all measurements were recorded at 120 K.
of 102 and 116 kJ/mol, respectively, have been estimated from these desorption temperatures. For comparison, a further desorption spectrum for a saturated ethanethiolate monolayer on Cu(110) is also shown in Figure 2 (lower panel) which is identical to those reported previously for heptanethiol on Cu(110).12 Compared to Cu(100), the desorption peak on the Cu(110) surface is much sharper and its maximum appears at a reduced temperature of
Vollmer et al.
410 K which suggests a somewhat different S-Cu interaction on both substrates. Interestingly, the positions of the LEED diffraction spots did not change upon desorption of the alkyl groups, but they sharpen up significantly which is particularly pronounced in the case of the longer alkanethiols. Heating the resulting sulfur structure above 500 K causes a drastic change in the corresponding diffraction pattern showing ring-like structures rather than sharp spots as depicted in Figure 1c. This observation indicates a substantial surface reconstruction which has been studied previously in detail for atomic sulfur on various copper surfaces.17,25 We would like to point out that the present structure is different from the (x17 × x17)R ( 14° superstructure obtained for the sulfur-induced reconstructed Cu(100) surface25 which appears only at a fairly high sulfur coverage of θ ≈ 1/2. Thus, the observed LEED pattern gives some evidence that the sulfur coverage remaining after desorption of the alkyl chains is actually below 1/2. To compare the relative coverage of both phases and to determine the saturation coverage of the thiolate adlayers, the intensity of the S 2p XP lines were measured for the (2 × 2) and the c(6 × 2) structures (see Figure 3a). To reduce the attenuation of the sulfur signal through the alkyl chains, such data were first recorded for methanethiol. To determine the peak intensities, the S 2p3/2, 2p1/2 doublets were fit to the experimental data using two Gaussians with a fixed intensity ratio (I3/2/I1/2 ) 2:1) as well as a fixed energy separation of 1.3 eV and a Gaussian for the C 1s and Cu 2p3/2 peaks after a linear background subtraction, and finally the peak area was integrated. In this way, a ratio of the sulfur peak intensities relative to the Cu 2p3/2 peak of Ic(6×2)/I(2×2) ) 1.2 ( 0.08 was obtained. This value is in reasonable agreement with the ratio of the relative coverages, θc(6×2)/θ(2×2) ) 1.33, expected for a primitive c(6 × 2) unit cell containing two molecules as shown schematically in Figure 1b. The corresponding analysis of the relative C 1s and S 2p3/2 peak intensities at room temperature yielded a ratio of 1.05 ( 0.1 for the intermediate (2 × 2) phase, but surprisingly only a value of about 0.52 ( 0.1 for the saturated methanethiolate film. Since such a striking reduction of the carbon signal was also found for the saturated monolayer of the longer alkanethiols studied here, we have measured a comprehensive set of XPS data for methanethiol to again minimize the attenuation effect through the alkyl chain. Figure 3b displays the development of the S 2p3/2, C 1s, and Cu 2p3/2 peak intensities as a function of the methanethiol exposure recorded at a constant pressure of 3.3 × 10-9 mbar (≡0.2 L/min) at room temperature. The sulfur signal is found to further increase after completion of the (2 × 2) structure until it is saturated at about 15 L, whereas the C signal remains constant already after an exposure of about 6 L. To rule out possible X-ray-induced damages of the thiolate films,28 the adsorption experiment was repeated by recording the XP spectra only every 10 L (filled symbols in Figure 2b) and switching off the X-ray source in between. Since identical curves were obtained in both cases, we feel that radiation damages can safely be excluded as the origin of the unexpectedly low C/S ratio. Furthermore, an enhancement (25) Colaianni, M. L.; Chorkendorff, I. Phys. Rev. B: Condens. Matter 1994, 50, 8798. (26) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F. In Handbook of X-ray Photoelectron Spectroscopy; Muillenberg, G. E., Ed.; Perkin-Elmer Corporation: Eden Prairie, MN, 1979. (27) Scofield, J. H. J. Electron Spectrosc. Relat. Phenom. 1979, 8, 129. (28) Ja¨ger, B.; Schu¨rmann, H.; Mu¨ller, H. U.; Himmel, H.-J.; Neumann, M.; Wo¨ll, Ch.; Grunze, M. Z. Phys. Chem. 1997, 202, 263.
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Figure 2. Thermal desorption spectrum for a saturated ethanethiol film on Cu(100) recorded for the C2H3 fragment (m/z ) 27) at a heating rate of 5 K/s. For comparison, a corresponding spectrum is shown below for a saturated ethanethiol film on Cu(110).
or attenuation of single XP lines due to photoelectron diffraction to cause this unexpected ratio can also be excluded on the basis of measurements for different electron emission angles relative to the surface normal which revealed only a variation of the C to S intensity ratio of less than 10%. Thus, the measurements clearly demonstrate that the stable monolayer film at room temperature actually consists of a mixture of sulfide and thiolate species forming a well-ordered c(6 × 2) structure. As we have shown in an earlier study of the chemisorption of heptanethiol on Cu(110),12 the thiolate formation is accompanied by a characteristic reduction of the S 2p3/2 core-level binding energy to about 162 eV. The present S 2p XP spectra (see Figure 3a), which have all been referenced to a Cu 2p3/2 peak energy of 932.4 eV,26 reveal such a shift for the (2 × 2) and the c(6 × 2) phases and, thus, indicate that the thiol molecules are completely deprotonated in both cases. After thermal desorption of the entire alkyl chain, an S 2p3/2 peak position of 161.3 eV has been measured for the remaining atomic sulfur (see last spectrum in Figure 3a) which is in close agreement with previous studies.12,13 A closer inspection of the XP spectra further indicates that the S 2p doublet of the stable c(6 × 2) saturation phase is somewhat broader as compared to that of the (2 × 2) phase which is consistent with the presence of additional atomic sulfur at the room-temperature saturation phase. Unfortunately, the present resolution was not sufficient to distinguish the different sulfur species. Preliminary results show, however, that they can be well distinguished in high-resolution XPS measurements when using synchrotron radiation.30 In contrast to the partial dissociation occurring during a slow adsorption as shown in Figure 3b, an intact c(6 × 2) saturation layer could be prepared at room temperature (310 K) by faster adsorption at a fairly high pressure of typically g3 × 10-7 mbar. This metastable state has been utilized to further study the temperature dependence of the degradation by rapid cooling of an intact methanethiolate film to temperatures of 250 or 200 K and, subsequently, by monitoring the C 1s and S 2p XP signals as a function of time and switching off the X-ray source in between. Figure 4 summarizes the evolution of the atomic ratio NC/NS which has been determined from the C 1s and
Figure 3. (a) XP spectra of the S 2p and C 1s peaks for the (2 × 2) and c(6 × 2) structures of methanethiol on Cu(100) and after desorption of the alkyl chains recorded at room temperature (310 K). Panel b displays the intensities of the Cu 2p3/2 (4), C 1s (0), and the S 2p3/2 (O) peaks during the exposure of the Cu(100) surface to methanethiol at a pressure of 3.3 × 10-9 mbar (≡ 0.2 L/min) at room temperature. The first set of data (open symbols) was recorded with the X-ray source operating continuously, whereas in a second run it was switched on only for the recording of the four single data points (denoted by the filled symbols).
Figure 4. Evolution of the carbon to sulfur atomic ratio, NC/ NS, determined from the C 1s and S 2p XP intensities after room-temperature preparation of the methanethiol c(6 × 2) saturation structure on Cu(100) for various surface temperatures. The solid lines represent the result of an exponential fit for the various surface temperatures.
S 2p3/2 peak intensities in view of their specific sensitivity.27 At 310 K, this ratio is found to decrease to about 0.55 ( 0.1 within less than 40 min and stays constant at this value, whereas such a decay takes more than 300 min at 250 K. In contrast, at 200 K, no change in the stoichiometry of the film was observed within the experimental error
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Figure 5. (a) Schematic phase diagram showing the composition and stability of saturated alkanethiolate films on Cu(100) which consists of thiolate (denoted by a gray filling) and sulfide species (white filling). (b) Schematic representation of the thermodynamically stable c(6 × 2) room-temperature monolayer on Cu(100) revealing a partial dissociation (in the case of methanethiol, approximately every second molecule) and possibly a sulfur-induced surface reconstruction.
which again shows that radiation damages can be excluded. The observed temperature dependence of the alkyl abstraction and sulfide formation (solid lines in Figure 4) is consistent with an activation energy of about E ) 25 ( 10 kJ/mol when assuming a decay, ∝ exp(-t/λ), with an Arrhenius-type decay time, λ(T) ) λ0 exp(E/kBT). Moreover, from the activation energy, an effective decay time of about 106 s for a temperature of 150 K is estimated, indicating that such a decay is rather difficult to measure at low temperatures and has not been observed in previous experiments.8,11,13 Discussion The present measurements have shown that the coverage and composition of saturated alkanethiolate monolayers on the Cu(100) surface depend critically on the temperature. At temperatures below 200 K, only a (2 × 2) structure appears, whereas upon saturation of the surface at room temperature, a c(6 × 2) structure is formed. The observed structures are in close agreement with the results of a very recent room-temperature STM study by Driver and Woodruff.16 As evidenced by the XPS data, however, the c(6 × 2) phase is instable toward partial decomposition at room temperature leading to a mixed thiolate/sulfide adlayer. The stable mixed phase exhibits the same c(6 × 2) diffraction pattern as seen for the initially formed pure thiolate c(6 × 2) phase. This behavior is depicted schematically in the phase diagram in Figure 5a. The sulfide formation is found to start already at temperatures as low as 250 K, whereas upon further heating above about 400 K, all alkyl chains are completely desorbed. Note that the metastable room-temperature thiolate film, the mixed thiolate/sulfide film, and the pure sulfur film all reveal a c(6 × 2) structure, indicating that the periodicity of the S atoms remains unchanged upon the alkyl desorption. Heating the sample further above 500 K leads finally to a drastic sulfur-induced reconstruction of the surface. The appearance of the (2 × 2) phase after saturating the sample at low temperatures (
