Structure of Thioaromatic Self-Assembled Monolayers on Gold and

Structure of Thioaromatic Self-Assembled Monolayers on Gold and Silver ..... Structural Investigation of 1,1′-Biphenyl-4-thiol Self-Assembled Monola...
0 downloads 0 Views 118KB Size
Download PDF
2408

Langmuir 2001, 17, 2408-2415

Structure of Thioaromatic Self-Assembled Monolayers on Gold and Silver S. Frey, V. Stadler, K. Heister, W. Eck, M. Zharnikov,* and M. Grunze Angewandte Physikalische Chemie, Universita¨ t Heidelberg, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany

B. Zeysing and A. Terfort Anorganische und Angewandte Chemie, Universita¨ t Hamburg, 20146 Hamburg, Germany Received November 3, 2000. In Final Form: February 6, 2001 Self-assembled monolayers (SAMs) formed from thiophenol, 1,1′-biphenyl-4-thiol, 1,1′;4′,1′′-terphenyl4-thiol, and anthracene-2-thiol on polycrystalline Au and Ag were characterized by X-ray photoelectron spectroscopy and angle-resolved near-edge X-ray absorption fine structure spectroscopy. With the exception of the poorly defined thiophenol film on Au, all thioaromatic molecules were found to form highly oriented and densely packed SAMs on both substrates. The molecular orientation and orientational order of the adsorbed thioaromatic molecules depends on the number of aromatic rings, the substrate, and the rigidity of the aromatic system. The molecules, which on average are slightly inclined with respect to the surface normal, show a less tilted orientation with increasing length of the aromatic chain, and as observed for aliphatic SAMs, they exhibit smaller tilt angles on Ag than on Au. However, the difference in the tilt angles for aromatic SAMs on Au and Ag is smaller than that observed in the aliphatic films. A comparison of the monolayers formed from p-terphenylthiol and anthracenethiol films suggests that a higher molecular rigidity has only a slight effect on the final molecular orientation within the respective SAMs.

1. Introduction Self-assembled monolayers (SAMs) are polycrystalline films of chainlike or rodlike molecules that are chemically anchored to a suitable substrate. During the past two decades, these systems attracted considerable attention because of the possibility of tailoring the surface properties with respect to wetting, adhesion, lubrication, and corrosion.1-3 The SAM-forming molecules consist generally of three parts: a headgroup that binds strongly to the substrate, a tailgroup that constitutes the outer surface of the film, and a spacer that connects head- and tailgroups. The balance between the headgroup/substrate and the intermolecular interactions is of crucial importance for the molecular structure in the SAMs. In the case of the most extensively studied n-alkanethiols (AT) SAMs, the headgroup/substrate interaction plays a predominant role in this balance4 and the structure of these films is mainly determined by the choice of the substrate.1-3 In the less studied but practically important thioaromatic SAMs, a different relation between the intermolecular and headgroup/substrate interactions can occur. As compared to the aliphatic chains, aromatic rings are characterized by larger van der Waals (vdW) dimensions and interact with each other through both the vdW and electrostatic forces.5 According to the melting points of benzene C6H6 at 5.5 °C and hexane n-C6H14 at -95.3 °C as well as biphenyl C12H10 at 69.0 °C and dodecane n-C12H26 at -9.6 °C,6 the intermolecular forces between the phenyl * Corresponding author. urz.uni-heidelberg.de.

E-mail:

Michael.Zharnikov@

(1) Ulman, A. An Introduction to Ultrathin Organic Films: LangmuirBlodgett to Self-Assembly; Academic Press: New York, 1991. (2) Ulman, A. Chem. Rev. 1996, 96, 1533. (3) Thin films: self-assembled monolayers of thiols; Ulman, A., Ed.; Academic Press: San Diego, CA, 1998. (4) Zharnikov, M.; Frey, S.; Rong, H.; Yang, Y.-J.; Heister, K.; Buck, M.; Grunze, M. Phys. Chem. Chem. Phys. 2000, 2, 3359. (5) Lii, J.-H.; Allinger, N. L. J. Am. Chem. Soc. 1989, 111, 8576.

rings in aromatic systems can be stronger than the vdW forces between the alkyl chains in aliphatic systems. However, this difference in the melting points may be largely related to entropic contributions, because the respective heats of sublimation are close to each other.5 The fundamental understanding of the balance between the chain/chain and headgroup/substrate interactions in thioaromatic films is an important step toward tailoring the properties of these systems for specific applications such as chemical lithography7,8 and molecular electronics.9,10 Such an understanding is, however, not available at present because relatively little work has been done in the field of thioaromatic SAMs, especially on Ag substrates. For the simplest member of the arylthiols, thiophenol (TP), the detailed structure on the respective SAM on Au surfaces has not been unambiguously determined. Both well-ordered monolayers with an upright11,12 or strongly inclined13 adsorption geometry of the phenyl rings as well as poorly defined films14-16 were found after immersing Au(111) surfaces into the TP solution. On Ag(111) sur(6) CRC Handbook of Chemistry and Physics, 72nd ed.; Lide, D. R., Ed.; CRC Press: Boston, 1991-1992. (7) Geyer, W.; Stadler, V.; Eck, W.; Zharnikov, M.; Go¨lzha¨user, A.; Grunze, M. Appl. Phys. Lett. 1999, 75, 2401. (8) Eck, W.; Stadler, V.; Geyer, W.; Zharnikov, M.; Go¨lzha¨user, A.; Grunze, M. Adv. Mater. 2000, 12, 805. (9) Bumm, L. A.; Arnold, J. J.; Cygan, M. T.; Dunbar, T. D.; Burgin, T. P.; Jones, L., II; Allara, D. L.; Tour, J. M.; Weiss, P. S. Science 1996, 271, 1705. (10) Tour, J. M.; Jones, L., II; Pearson, D. L.; Lamba, J. S.; Burgin, T. P.; Whitesides, G. M.; Allara, D. L.; Parikh, A. N.; Atre, S. V. J. Am. Chem. Soc. 1995, 117, 9529. (11) Whelan, C. M.; Barnes, C. J.; Walker, G. H.; Brown, N. M. D. Surf. Sci. 1999, 425, 195. (12) Carron, K. T.; Hurley, L. G. J. Phys. Chem. 1991, 95, 9979. (13) Szafranski, C. A.; Tanner, W.; Laibinis, P. E.; Garrell, R. L. Langmuir 1998, 14, 3570. (14) Sabatani, E.; Cohen-Boulakia, J.; Bruening, M.; Rubinstein, I. Langmuir 1993, 9, 2974. (15) Tao, Y.-T.; Wu, C.-C.; Eu, J.-Y.; Lin, W.-L. Langmuir 1997, 13, 4018.

10.1021/la001540c CCC: $20.00 © 2001 American Chemical Society Published on Web 03/24/2001

Thioaromatic SAMs on Gold and Silver

Langmuir, Vol. 17, No. 8, 2001 2409

chain (TP vs BPT vs TPT), the rigidity and the character of the aromatic chain (AnT vs BPT and TPT), and the substrate (Au vs Ag). In this way, both the intermolecular and adsorbate/substrate interactions are varied and their effect on the film structure is monitored. In the following section, we describe the experimental procedure and techniques. The results are presented and briefly discussed in section 3. An extended analysis of the data is given in section 4, followed by a summary in section 5. 2. Experimental Section Figure 1. Schematic drawing of the thioaromatic molecules in an artificial upright adsorption geometry including the respective numbers of carbon atoms and the length of the aromatic chains (in parentheses). For TP, BPT, and TPT SAMs, the lengths of the aromatic chains were taken from previous works (refs 14 and 20), and for AnT films the value was estimated by crystal structure data of the bulk materials (refs 22 and 23).

faces, TP molecules were found to adsorb nearly perpendicular to the substrate surface.17,18 With a growing number of phenyl moieties in the aromatic chain, the monolayers become both more densely packed and wellordered,7,10,14-16,19,20 which can be associated with the increase of the intermolecular interactions mediated by the aromatic entities. For the well-oriented p-biphenylthiol and p-terphenylthiol monolayers on Au, average tilt angles of 15 ( 5° 7 and 27 ( 5° 20 were determined by angleresolved near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The discrepancy of the tilt angles for the chemically quite similar homologue molecules is presumably caused by different data evaluation procedures (see subsection 3.2), so that the twist angle between the phenyl rings of two neighboring molecules was neglected or not exactly considered. This angle should be, however, important because theoretical simulations20,21 imply that a herringbone arrangement is favored over a face-stacked structure for biphenyl and naphthalene mercaptan monolayers. The herringbone packing is believed to be nearly the same on Au(111) and Ag(111) surfaces which should result in similar monolayer structures.21 In this paper, we studied SAMs formed from TP, 1,1′biphenyl-4-thiol (BPT), 1,1′;4′,1′′-terphenyl-4-thiol (TPT), and anthracene-2-thiol (AnT) on polycrystalline Au and Ag surfaces by using X-ray photoelectron spectroscopy (XPS) and NEXAFS spectroscopy. Among these systems (see a schematic sketch in Figure 1), the AnT monolayers, which are believed to be very important for potential applications as molecular wires and lithographic resist material, are investigated for the first time. The choice of these particular systems enables a simultaneous variation of the most important parameters affecting the structure of thioaromatic SAMs, namely, the length of the aromatic (16) Dhirani, A.-A.; Zehner, W.; Hsung, R. P.; Guyot-Sionnest, P.; Sita, L. J. Am. Chem. Soc. 1996, 118, 3319. (17) Gui, Y. T.; Lu, F.; Stern, D. A.; Hubbard, A. T. J. Electroanal. Chem. 1990, 292, 245. (18) Stern, D. A.; Wellner, E.; Salaita, G. N.; Laguren-Davidson, L.; Lu, F.; Batina, N.; Frank, D. G.; Zapien, D. C.; Walton, N.; Hubbard, A. T. J. Am. Chem. Soc. 1988, 110, 4885. (19) Ishida, T.; Choi, N.; Mizutani, W.; Tokumoto, H.; Kojima, I.; Azehara, H.; Hokari, H.; Akiba, U.; Fujihira, M. Langmuir 1999, 15, 6799. (20) Himmel, H.-J.; Terfort, A.; Wo¨ll, Ch. J. Am. Chem. Soc. 1998, 120, 12069. (21) Chang, S.-C.; Chao, I.; Tao, Y.-T. J. Am. Chem. Soc. 1994, 116, 6792.

TP was obtained from Sigma-Aldrich (purity grade, minimum 99%) and used directly. The synthesis of BPT, TBM, and AnT is described elsewhere.20,24,25 Polycrystalline gold and silver substrates were prepared by thermal evaporation of 200 nm of gold or 100 nm of silver (99.99% purity) onto polished singlecrystal silicon (100) wafers (Silicon Sense) that had been precoated with a 5 nm titanium adhesion layer. The evaporation was performed at a pressure of 1.5 × 10-7 Torr and a deposition rate of 0.5 nm/s. Such films are commonly used as substrates for AT SAMs. They predominately exhibit the (111) orientation, which is, in particular, supported by the observation of the characteristic forward-scattering maxima in the angular distributions of the Au 4f and Ag 3d photoelectrons.26 The grain size of the Au and Ag films is 20-50 nm as observed by atomic force microscopy. The thioaromatic monolayers were prepared by immersion of the substrates in a 5 mmol/L solution of the corresponding thiols in analytical grade ethanol for 36 h at room temperature, followed by sonicating for 5 min in N,N-dimethylformamide (DMF), rinsing with ethanol, and drying in a nitrogen stream. Film preparation, handling, and storage were carried out under nitrogen. All systems were characterized by XPS and NEXAFS spectroscopy using a modified multitechnique ultrahigh vacuum (UHV) chamber27 attached to the HE-TGM 2 beamline28 at the German synchrotron radiation facility BESSY I in Berlin. The experiments were performed at room temperature under UHV conditions (base pressure better than 1.5 × 10-9 Torr). The time for the NEXAFS/XPS characterization was selected as a compromise between the spectral quality and the damage induced by X-rays.7,29,30 The XPS measurements were performed with a Mg KR X-ray source and a VG CLAM 2 spectrometer in normal emission geometry with an energy resolution of ≈0.9 eV. The X-ray source was operated at 260 W power and positioned ≈1 cm away from the samples. The energy scale was referenced to the Au 4f7/2 peak at 84.0 eV,31 which resulted in a binding energy of 368.2 eV for the Ag 3d5/2 peak in agreement with refs 31 (368.3 eV) and 32 (367.9 eV). For each sample, a wide scan spectrum and the C 1s, O 1s, S 2p, and Au 4f or Ag 3d narrow scan spectra were measured. No impurities, except for a small amount of surface oxide (less than 0.1 monolayer (ML) with respect to the monolayer of sulfur headgroups) for the TP and BPT layers on Ag surfaces, could be observed. The narrow scan spectra were normalized to (22) Kitaigorodskii, I. A. Organic Chemical Crystallography; Consultants Bureau: New York, 1961. (23) Cruickshank, D. W. J. Acta Crystallogr. 1956, 9, 915. (24) Newman, M. S.; Karnes, H. A. J. Org. Chem. 1966, 31, 3980. (25) Scharf, J.; Strehblow, H.-H.; Zeysing, B.; Terfort, A. J. Solid State Electrochem., in press. (26) Ko¨hn, F. Diploma Thesis, Universita¨t Heidelberg, Heidelberg, Germany, 1998. (27) Schaible, M.; Petersen, H.; Braun, W.; Koch, E. E. Rev. Sci. Instrum. 1989, 60, 2172. (28) Bernstorff, S.; Braun, W.; Mast, M.; Peatman, W.; Schro¨der, T. Rev. Sci. Instrum. 1989, 60, 2097. (29) Zharnikov, M.; Geyer, W.; Go¨lzha¨user, A.; Frey, S.; Grunze, M. Phys. Chem. Chem. Phys. 1999, 1, 3163. (30) Zharnikov, M.; Frey, S.; Heister, K.; Grunze, M. Langmuir 2000, 16, 2697. (31) Moulder, J. F.; Stickle, W. E.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy; Chastian, J., Ed.; PerkinElmer Corp.: Eden Prairie, MN, 1992. (32) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F.; Muilenberg, J. E. Handbook of X-ray Photoelectron Spectroscopy; PerkinElmer Corp.: Eden Prairie, MN, 1979.

2410

Langmuir, Vol. 17, No. 8, 2001

Frey et al.

the integral intensity of the wide scan spectra to correct for small differences in sample positions and X-ray source intensities33 and fitted by using a Shirley type background34 and a symmetric Voigt peak profile.35 The used normalization procedure works rather well in the case of SAMs on noble metal surfaces,29,30,33 because the vast majority of the primary and secondary electrons in the respective XPS wide scan spectra originates from the substrate. The NEXAFS measurements were carried out at the C 1s absorption edge in the partial electron yield mode with a retarding voltage of -150 V. Linear polarized synchrotron light with a polarization factor of ≈92% was used. The energy resolution was ≈0.65 eV. The incidence angle of the synchrotron light was varied from 90° (E-vector in surface plane) to 20° (E-vector near surface normal) to monitor the orientational order within the thioaromatic films. This approach is based on the strong dependence of the resonant photoexcitation process on the relative orientation of the light polarization and a molecular orbital of interest.36 The raw NEXAFS spectra were normalized to the incident photon flux by division through a spectrum of a clean, freshly sputtered gold sample. In the case of the thioaromatic SAMs on Ag, a spectrum of clean silver was subtracted from the raw SAM spectra before the normalization.30,33 The energy scale was referenced to the pronounced π1* resonance of graphite (highly oriented pyrolytic graphite) at 285.35 eV.30,33,37

3. Results 3.1. XPS Measurements. XP spectra provide quantitative information on the composition, chemical identity, and thickness of the thioaromatic films. The C 1s and S 2p spectra of TP, BPT, TPT, and AnT SAMs on Au and Ag are presented in Figure 2. To make a direct comparison between the spectra on both substrates, we have corrected them for the difference between the total electron yields from Au and Ag (these values were used for the spectral normalization, see previous section), which was roughly estimated using several reference samples of AT SAMs on these substrates. The lower (than in the case of Ag) signal-to-noise ratio of the C 1s spectra for the thioaromatic films on Au stems from a larger inelastic electron background originated from the Au 4f electrons with a binding energy (BE) below the C 1s ionization threshold. The relatively poor quality of the S 2p spectra is related to the attenuation of the corresponding signal by the aromatic overlayer and a relatively short acquisition time chosen to reduce possible X-ray-induced damage during the measurement.7,30 In the S 2p XP spectra of all thioaromatic systems in Figure 2c,d, a single S 2p3/2/S 2p1/2 doublet is observed on both substrates at approximately the same BE. The deconvolution of the S 2p3/2/S 2p1/2 doublets by two Voigt peaks with a fixed intensity ratio of 2 gives characteristic BEs of ≈162.0 eV (S 2p3/2) and ≈163.2 eV (S 2p1/2) associated with a thiolate species bonded to the metal surface.30,38-41 Thiolate formation requires the loss of the sulfhydryl hydrogen during adsorption.11,42 Note that the position and full width at half-maximum (fwhm) (1.2 eV) (33) Frey, S.; Heister, K.; Zharnikov, M.; Grunze, M.; Tamada, K.; Colorado, R., Jr.; Graupe, M.; Shmakova, O. E.; Lee, T. R. Isr. J. Chem. 2001, 40, 81. (34) Shirley, D. A. Phys. Rev. B 1972, 5, 4709. (35) Wertheim, G. K.; Butler, M. A.; West, K. W.; Buchanan, D. N. E. Rev. Sci. Instrum. 1974, 45, 1369. (36) Sto¨hr, J. NEXAFS Spectroscopy; Springer Series in Surface Science 25; Springer-Verlag: Berlin, 1992. (37) Batson, P. E. Phys. Rev. B 1993, 48, 2608. (38) Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H. J. Am. Chem. Soc. 1987, 109, 733. (39) Bain, C. D.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1989, 5, 723. (40) 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. (41) Himmelhaus, M.; Gauss, I.; Buck, M.; Eisert, F.; Wo¨ll, Ch.; Grunze, M. J. Electron Spectrosc. Relat. Phenom. 1998, 92, 139.

Figure 2. The C 1s (a and b) and S 2p (c and d) XP spectra of the TP, BPT, TPT, and AnT monolayers on Au (a and c) and Ag (b and d) surfaces. With the exception of TP on Au, all observed C 1s emission structures are fitted by two Voigt functions (solid lines). For TP on Au, four Voigt functions were used. The S 2p3/2/S 2p1/2 doublets were deconvoluted using two Voigt functions with the same fwhm and Gauss/Lorentz proportion. See text for details.

of the S 2p3/2 and S 2p1/2 peaks are nearly the same as for conventional alkanethiol SAMs on Au and Ag (≈162.0 and ≈161.8 eV, respectively, for the S 2p3/2 peak).30 Except for TP/Au, the intensities of the S 2p3/2/S 2p1/2 doublets for both substrates exhibit the expected reduction by going from TP to BPT and further to TPT, which is related to the increasing attenuation of the S 2p photoelectrons by the thicker aromatic overlayer. The small S 2p intensity for TP/Au can be associated with the lower packing density of possibly poorly defined TP layers on Au.14,15,17,18 As to the AnT SAMs, the S 2p intensity for AnT/Ag is similar to that for BPT/Ag, whereas the S 2p intensity for AnT/Au is closer to that for TPT/Au. Considering that the rigid AnT molecules with an off-axis thiol group need presumably more space than the flexible BPT or TPT molecules and that the AnT molecule has two carbon atoms more and four less than BPT and TPT, respectively, one would expect that the S 2p intensity for the AnT SAMs should be closer to that for the BPT films, as found on Ag. Consequently, a less dense packing of the AnT molecules is suggested for AnT/Au as compared to AnT/Ag. In the C 1s XP spectra in Figure 2a,b, a single C 1s photoemission maximum with a higher BE tail is observed for all investigated SAMs. Whereas the main C 1s emission can be unequivocally attributed to benzene-like carbon atoms, the minor component at a higher BE has been (42) Whelan, C. M.; Smyth, M. R.; Barnes, C. J. Langmuir 1999, 15, 116.

Thioaromatic SAMs on Gold and Silver

alternatively assigned to electron-deficient sulfur-bonded carbon atoms or to shake-up type excitations in the aromatic matrix.7,10,11,42,43 Recent high-resolution XPS measurements unequivocally identified the peak as a shake-up feature.44 For all SAMs, this component can be fitted by a single Voigt peak, except for TP/Au where several such peaks have to be used to fit the higher BE tail. This is a further indication for the different adsorption behavior of TP on Au as compared to the other investigated molecules. The positions of both C 1s features on Au and Ag substrates are slightly different. The BEs of the main C 1s emission for the TP, BPT, and TPT monolayers on Au and Ag substrates are ≈284.1 and ≈284.2 eV (a fwhm of ≈1.2 eV), respectively. For the AnT films, on both substrates this peak is shifted by ≈0.1 eV to a lower BE, but the same trend between the two substrates remains. A qualitatively similar BE difference was observed for conventional AT SAMs on Au and Ag20,30,45 and was associated with the different packing of the alkyl chains in the case of Ag.44,46 Also, for the small high-BE component the best fit gives a BE of 284.9-285.0 eV for TP, BPT, and TPT monolayers on Au and ≈284.8 eV for AnT/Au, whereas for the same films on Ag these values are larger by ≈0.1 eV. In the same manner as the S 2p emissions, the integral intensities of the C 1s features correlate with the molecular compositions of the respective SAMs assuming an exponential attenuation of the C 1s photoelectrons in the aromatic films. The total intensity of the C 1s emission increases with the number of aromatic carbon atoms in the molecules. The single exception is the AnT monolayers, which despite their higher carbon content per molecule exhibit a slightly smaller C 1s integral intensity than the BPT SAMs. This observation implies a reduced packing density of the AnT as compared to the BPM SAMs. In addition, the relation between the integral C 1s intensities for AnT/Au and BPT/Au (or TPT/Au) is smaller than that for AnT/Ag and BPT/Ag (or TPT/Ag). In line with the abovementioned behavior of the S 2p integral intensities, this is a further indication that AnT is more densely packed on Ag as compared to Au. 3.2. NEXAFS Measurements. NEXAFS experiments provide both chemical and structural information about the occurrence and average orientation of unoccupied molecular orbitals within organic films. The C 1s NEXAFS spectra of the TP, BPT, TPT, and AnT SAMs on Au and Ag acquired at several selected X-ray incidence angles are presented in Figures 3 and 4, respectively. All spectra exhibit a C 1s absorption edge related to C 1s f continuum excitations located at ≈287 eV7,20 and several pronounced π* and σ* resonances. At first sight, only one π* and one σ* resonance associated with the respective π* and σ* orbitals would be expected because of the equivalence of all carbon-carbon bonds within the phenyl rings. However, the spectra presented in Figures 3 and 4 exhibit several such resonances because of a strong interaction between the localized π* and σ* states producing a set of partly delocalized orbitals that are significantly separated in energy.36,47 (43) Beamson, G.; Briggs, D. High-Resolution XPS of Organic Polymers: The Scienta ESCA300 Database; John Wiley & Sons: Chichester, U.K., 1992. (44) Heister, K.; Johansson, L. S. O.; Zharnikov, M.; Grunze, M. J. Phys. Chem. B, submitted for publication. (45) Biebuyck, H. A.; Bain, C. D.; Whitesides, G. M. Langmuir 1994, 10, 1825. (46) Thome, J.; Himmelhaus, M.; Zharnikov, M.; Grunze, M. Langmuir 1998, 14, 7435.

Langmuir, Vol. 17, No. 8, 2001 2411

Figure 3. The C 1s NEXAFS spectra of TP (a), BPT (b), TPT (c), and AnT (d) on Au acquired at different angles of X-ray incidence. The characteristic absorption resonances are indicated by arrows.

For the assignment of the observed resonances, we first consider the spectra acquired at the “magic” X-ray incident angle of 55°. At this particular experimental geometry, the NEXAFS spectra exclusively reflect the electronic structure of the unoccupied molecular orbitals of the investigated films and are not affected by the angular dependence of the absorption cross sections.36 The respective spectra of the TP, BPT, and TPT monolayers are quite similar (in agreement with literature data for benzene, BP, and p-TP),48 whereas the spectrum of the AnT film exhibits a more complex π* resonance structure. Similar to benzene47,49 and TP on Mo(110),36,50,51 the spectra of TP, BPT, and TPT films are dominated by a strong π1* resonance at 285.0 eV, accompanied by a weaker π2* resonance at 288.9 eV. The assignments of these resonances are given in Table 1 together with the positions of the σ* resonances and the respective data for benzene from refs 47, 49, and 51 (there is also an alternative σ(CH) assignment for the resonance at 288.9 eV).48,52 In the same manner as found for TP on Mo(110),36 no further splitting of the degenerated π1* and π2* resonances by symmetry reduction and/or chemical shifts associated with the substituent sulfur group was detected for the TP, BPT, (47) Horsley, J. A.; Sto¨hr, J.; Hitchcock, A. P.; Newbury, D. C.; Johnson, A. L.; Sette, F. J. Chem. Phys. 1985, 83, 6099. (48) Yokoyama, T.; Seki, K.; Morisada, I.; Edamatsu, K.; Ohta, T. Phys. Scr. 1990, 41, 189. (49) Weiss, K.; Gebert, S.; Wu¨hn, M.; Wadepohl, H.; Wo¨ll, Ch. J. Vac. Sci. Technol., A 1998, 16, 1017. (50) Friend, C. M.; Roberts, J. T. Acc. Chem. Res. 1988, 21, 394. (51) Sto¨hr, J.; Outka, D. A. Phys. Rev. B 1987, 36, 7891. (52) Ågren, H.; Vahtras, O.; Carravetta, V. Chem. Phys. 1995, 196, 47.

2412

Langmuir, Vol. 17, No. 8, 2001

Frey et al.

Table 1. Absolute Photon Energies (eV) and Assignments of the C 1s NEXAFS Resonances for TP, BPT, TPT, and AnT Monolayers on Au and Ag feature

assignment for benzenea,b

benzenea (gas phase) (0.1 eV

benzenea (solid phase) (0.5 eV

TP, BPT, TPT (0.2 eV

π1*

π* (e2u)

285.2

285.0

285.0

R*/C-S* π2* σ1* σ2*

π* (b2g) σ* (e1u) σ* (e2g + a2g)

288.9 293.5 300.2

288.9 293.3 300.1

287.4 288.9 ≈293.0c ≈300.0c

a

AnT (0.2 eV

}

π1a* ) 284.4 π1b* ) 285.7 mixed ≈293.0c ≈300.0c

References 47, 49, and 51. b Only the final orbital is listed in parentheses. c Main features may consist of several resonances.

Figure 4. The C 1s NEXAFS spectra of TP (a), BPT (b), TPT (c), and AnT (d) on Ag acquired at different angles of X-ray incidence. The characteristic absorption resonances are indicated by arrows.

and TPT SAMs within the experimental resolution. At the same time, the spectra of the AnT films in Figures 3d and 4d exhibit a pronounced splitting of the π1* resonance into the π1a* and π1b* resonances at 284.4 and 285.7 eV, which can be related to the chemical shift of the two symmetry independent carbon atoms of AnT with strong influence of excitonic effects; similar splitting is observed in the NEXAFS spectra of polyacenes.48,52 The excitonic effects should also be very strong in the NEXAFS spectra of TP, BPT, and TPT films,48,52,53 but a splitting of the π1* resonance does not occur because of a higher equivalence of the constituent carbon atoms.48,52 As to other spectral features, the resonance near the C 1s absorption edge at 287.4 eV, which is especially pronounced in the spectra of BPT/Ag in Figure 4b, can be assigned to both a Rydberg49,54 and/or a C-S σ* excitation.51 The broad (53) Oji, H.; Mitsumoto, R.; Ito, E.; Ishii, H.; Ouchi, Y.; Seki, K.; Yokoyama, T.; Ohta, T.; Kosugi, N. J. Chem. Phys. 1998, 109, 10409. (54) Bagus, P. S.; Weiss, K.; Schertel, A.; Wo¨ll, Ch.; Braun, W.; Hellwig, H.; Jung, C. Chem. Phys. Lett. 1996, 248, 129.

resonances at ≈293 and ≈300 eV are commonly related to the transitions into two σ* orbitals. The similarity of the C1s NEXAFS spectra for TP, BPT, and TPT SAMs is related to the localization of the π* orbitals by the creation of a C1s core hole.48 When comparing the intensities of the π1* and π2* resonances in the TP, BPT, and TPT SAMs, similar values are expected because of the normalization of the NEXAFS spectra to the height of the absorption edge and, consequently, to the number of carbon atoms. However, the relative intensities of these resonances for both substrates increase with the number of aromatic moieties. This phenomenon can probably be explained by the excitonic effects52,55 and by the difference in packing density of the thioaromatic moieties.14,15,19 Note that the increase of the oscillator strength of the resonance by going from benzene to BP and p-TP was obtained in the simulated C 1s NEXAFS spectra of these compounds.48 Finally, the reduced π* excitation intensities in AnT refer to the delocalization of the respective molecular states induced by a stronger π* conjugation than in TP, BPT, and TPT.52 A fingerprint of the orientational order in the thioaromatic films is the linear dichroism in the NEXAFS spectra, that is, the dependence of the intensity on the angle of light incidence.36 In our case, this dependence can be best monitored by comparing the C 1s NEXAFS spectra acquired at the X-ray incidence angles of 90° and 20° in Figures 3 and 4. Except for TP/Au, the intensities of the NEXAFS resonances vary significantly when the angle of X-ray incidence is changed, which implies on average welloriented and densely packed SAMs on both Au and Ag substrates. In the case of TP/Au, the relatively small angular dependence of the resonance intensity is a strong indication for a disordered molecular film. The intensity of a NEXAFS resonance depends on the orientation of the light polarization with respect to the probed molecular orbital and is maximal if the direction of the E-vector and the orbital are collinear. The spectra in Figures 3 and 4 (except for TP/Au) exhibit an increase of the π* resonance and a decrease of the σ* resonance intensity with increasing angle of the light incidence, which implies an upright orientation of the thioaromatic molecules in the respective SAMs. The reverse behavior of the σ* and π* resonance intensities is explained by the orthogonal orientation of the π* and σ* orbitals: whereas the latter orbitals are arranged in the ring plane, the π* orbital can be represented by vectors perpendicular to this plane.36 In the case of TP/Au, the decrease of the π* resonance intensity suggests a preferential orientation of the phenyl rings parallel to the substrate surface. A value of the average tilt angles of the thioaromatic molecules in the respective SAMs is obtained by a quantitative analysis of the angular dependence of the NEXAFS resonance intensities. For this analysis, the π1* (55) Carravetta, V.; Ågren, H.; Pettersson, L. G. M.; Vahtras, O. J. Chem. Phys. 1995, 102, 5589.

Thioaromatic SAMs on Gold and Silver

Langmuir, Vol. 17, No. 8, 2001 2413

resonance in the corresponding film was selected as the most intense and distinct resonance in the absorption spectra. To extract the π1* resonance intensities, each spectrum for a given thioaromatic SAM was fitted by the absorption edge and several symmetric Gaussian peaks with constant energy position and width for every resonance. The positions and widths of these peaks were determined from the difference spectra (“90°”-“20°”) following the procedure described in refs 56 and 57. For the thioaromatic molecules, the intensity I of the π1* resonance (vector-type orbital) is related to the average tilt angle R of the corresponding orbitals with respect to the surface normal and the X-ray incidence angle θ by51

I(R) ∝ 1 + 1/2(3 cos2 θ - 1)(3 cos2 R - 1)

(1)

The term cos2 R in eq 1 can be expressed through the twist angle ϑ of the aromatic rings with respect to the plane spanned by the surface normal and the molecular axis and through the average tilt angle φ of the molecular axis with respect to the surface normal by58

cos R ) cos ϑ sin φ

(2)

where a planar conformation of the thioaromatic molecules in the densely packed SAMs is assumed in accordance with literature data for crystalline BP5 and TP59 and TPT SAMs on Au.20 Consequently, the average tilt angle φ of the molecular axis for a known twist angle ϑ is defined by

I(ϑ,φ) ∝ 1 + 1/2(3 cos2 θ - 1)(3 cos2 ϑ sin2 φ - 1)

(3)

For a herringbone arrangement of the thioaromatic molecules in the respective SAMs,16,20,21 two different spatial orientations of the molecules with reverse twist angles ϑ1 ) -ϑ2 and the same tilt angles φ1 ) φ2 are expected. In this particular case, the contributions of each spatial orientation to the resonance intensity (3) are the same and eq 3 can be used for the data evaluation without any modification. However, for the absolute intensities we used instead the intensity ratios R ) I(θ)/I(20°), where I(θ) is the intensity of the π1* resonance at an X-ray incidence angle θ.36,51 The comparison of the experimental intensity ratios for TP (b), BPT (2), TPT (1), and AnT (() on gold and silver with theoretical intensity ratios calculated from eq 3 (dotted lines) is presented in Figure 5. For the theoretical curves, we assumed the same twist angles ϑ of 32° for the TP, BPT, and TPT films and 26° for the AnT SAMs as found for the respective bulk systems.22,23,60,61 This assumption is supported by the analysis of the calculated 2D molecular arrangements for biphenyl and naphthalene mercaptan on Au21 and by the experimental data for a series of oligo(phenylethynyl)benzenethiols16 and biphenyl-substituted alkanethiols.58 The determined average tilt angles for TP, BPT, TPT, and AnT SAMs are 49°, 23°, 20°, and 23° in the case of the Au substrate and 24°, 18°, 16°, and 14° in the case of the Ag substrate. The estimated accuracy of these values (56) Ha¨hner, G.; Kinzler, M.; Thu¨mmler, C.; Wo¨ll, Ch.; Grunze, M. J. Vac. Sci. Technol., A 1992, 10, 2758. (57) Outka, D. A.; Sto¨hr, J.; Rabe, J. P.; Swalen, J. D. J. Chem. Phys. 1988, 88, 4076. (58) Rong, H. T.; Frey, S.; Yang, Y.-J.; Zharnikov, M.; Buck, M.; Wu¨hn, M.; Wo¨ll, Ch.; Helmchen, G. Langmuir, in press. (59) Rietveld, H. M.; Maslen, E. N.; Clews, C. J. B. Acta Crystallogr. 1970, B26, 693. (60) Charbonneau, G.-P.; Delugeard, Y. Acta Crystallogr. 1976, B32, 1420. (61) Trotter, J. Acta Crystallogr. 1961, 14, 1135.

Figure 5. The angular dependencies of the π1* intensity ratios for TP (b), BPT (2), TPT (1), and AnT (() adsorbed on Au and Ag surfaces. For comparison, the theoretical dependencies for different tilt angles of the aromatic backbone are added as dotted lines.

is about (5° and is related to the uncertainty in the twist angles and the inaccuracy of the location of the absorption edge. The average tilt angle for BPT/Au coincides within the error bars with the respective value from a recent X-ray diffraction study,62 where a tilt angle of 19° was found for 4-methyl-4′-mercaptobiphenyl layers on Au(111). This supports our assumption that the twist angles in the thioaromatic SAMs are similar to the twist angles in the bulk materials. Note that the large average tilt angle of the TP molecules on Au in this study is consistent with a poorly defined monolayer. A tilt angle of 49° is very close to the magic angle of 55°, which can be in principle associated with a random molecular orientation. Otherwise, it is possible that the value of 49° reveals the real tilt angle of the homogeneously (but strongly) inclined TP molecules. 4. Discussion The presented NEXAFS and XPS data imply that the studied thioaromatic molecules, with the exception of the poorly defined TP/Au system, form highly oriented, densely packed SAMs on polycrystalline gold and silver substrates having a predominant (111) orientation. The intact molecules adsorb on the surface as thiolate species as indicated by a single S 2p3/2/S 2p1/2 doublet in the XP spectra at the same characteristic BE as for alkanethiol SAMs. (62) Leung, T. Y. B.; Schwartz, P.; Scoles, G.; Schreiber, F.; Ulman, A. Surf. Sci. 2000, 458, 34.

2414

Langmuir, Vol. 17, No. 8, 2001

Frey et al.

Figure 6. Thicknesses of the thioaromatic monolayers derived from the NEXAFS (b) and XPS (2) data. The theoretical values for an upright molecular orientation are depicted as dotted lines.

Film thicknesses were estimated from both the NEXAFS data based on the derived average tilt angles33 and from the XPS data considering the attenuation of the substrate photoelectrons by the hydrocarbon overlayer.63-65 Taking a theoretical length of the aromatic chain ltheo (see Figure 1) and a thickness dS ) 1.8 Å for the adsorbed sulfur unit,66,67 we can calculate the effective thickness deff of the thioaromatic molecules from the NEXAFS-derived average tilt angle φ by

deff ) ltheo cos(φ) + ds

(4)

From the XPS data, the thickness was estimated assuming an exponential attenuation of the substrate photoelectrons in the aromatic overlayer and the same attenuation lengths as for the aliphatic SAMs. On the basis of these assumptions, the standard expression63-65 for the case of normal emission was used to calculate the effective layer thickness:

( )

deff ) -λSub ln

ISub

I0Sub

(5)

where ISub and I0Sub are the photoemission intensities of the adsorbate-covered and clean substrates. The attenuation lengths λSub of the Au 4f and Ag 3d photoelectrons in the hydrocarbon overlayers were taken as 31 and 26 Å, respectively (Mg KR X-ray source).46,68 The resulting NEXAFS- and XPS-derived thicknesses are shown in Figure 6 by circles and triangles, respectively, together with the maximal possible layer thickness for upright oriented molecules (dotted lines). Despite the rather different approaches for the calculation of the layer thicknesses, the values agree well ((1 Å) with each other. Otherwise, the agreement of the NEXAFS- and XPSderived thicknesses can be considered as an indication that the attenuation lengths in the aliphatic and aromatic overlayers are quite similar although the expected denser (63) Bain, C. D.; Whitesides, G. M. J. Phys. Chem. 1989, 93, 1670. (64) Bain, C. D.; Whitesides, G. M J. Am. Chem. Soc. 1989, 111, 7164. (65) Laibinis, P. E.; Bain, C. D.; Whitesides, G. M. J. Phys. Chem. 1991, 95, 7017. (66) Sellers, H.; Ulman, A.; Shnidman, Y.; Eilers, J. E. J. Am. Chem. Soc. 1993, 115, 9389. (67) Dannenberger, O.; Weiss, K.; Himmel, H.-J.; Ja¨ger, B.; Buck, M.; Wo¨ll, Ch. Thin Solid Films 1997, 307, 183. (68) Harder, P.; Grunze, M.; Dahint, R.; Whitesides, G. M.; Laibinis, P. E. J. Phys. Chem. B 1998, 102, 426.

packing of AnT on Ag as compared to Au (see section 3.1) is not reproduced by the XPS-derived (from the Au 4f and Ag 3d spectra) values in Figure 6. The discrepancy is, however, within the experimental error. So far, we considered the XPS and NEXAFS data for all systems together and analyzed the chemical identity and the geometric and electronic structure of the thioaromatic SAMs. In the following, we discuss these data and the derived values in the context of the structuredetermining factors such as the length and rigidity of the aromatic chain as well as the substrate. 4.1. Influence of the Aromatic Chain Length. The chain length of the thioaromatic molecules was varied from one phenyl ring in TP to two phenyl rings in BPT and three phenyl rings in TPT and AnT. This increase in the aromatic chain length is monitored by the increase of the C 1s emission intensity (see Figure 2) and the effective film thickness (see Figure 6). The comparison of the chemical homologue TP, BPT, and TPT monolayers clearly shows that the molecular orientation becomes less tilted with increasing length of the aromatic chain. This tendency is observed on both Au and Ag substrates and can be explained within the assumption that the intermolecular forces responsible for self-assembling become stronger with increasing number of aromatic rings. In particular, weak intermolecular forces can be the reason for the poorly defined TP monolayers on Au. Note that the progressing upright molecular orientation with increasing number of phenyl rings in the aromatic chain correlates well with the increasing packing density, molecular order, and stability found previously for thioaromatic monolayers on Au(111).10,14-16,19 4.2. Influence of the Substrate. In the same way as for alkanethiol SAMs, thioaromatic monolayers exhibit smaller tilt angles with respect to the surface normal on Ag as compared to Au substrates, which can be presumably associated with the different corrugations of the headgroup-substrate binding energy surface (similar to AT SAMs).1,2,66 However, the absolute difference of the tilt angles on Au and Ag is smaller than that for the alkanethiols. Excluding the relatively short TP, the thioaromatic SAMs reveal an average tilt angle of ≈22° on Au and ≈16° on Ag compared to the long-chain alkanethiols with average tilt angles of ≈30° and ≈12°, respectively. We assume that the difference between the thioaromatic and thioaliphatic films is presumably related to a stronger influence of intermolecular interaction in the former systems. In the case of alkanethiol SAMs, we have recently shown that the headgroup-substrate interaction is probably the most essential factor in determining the final molecular orientation.4 Here, in the case of the thioaromatic films, we suppose that in the balance between molecule/substrate interaction and intermolecular forces the latter forces are more important. An additional support for this assumption is that in contrast to the chain length independent tilt angle of longchain alkanethiols the tilt angle of thioaromatic molecules in the respective SAMs varies slightly with the length of the aromatic chain. Nevertheless, the substrate is still an important factor for the structure of the aromatic films. First, despite the supposed stronger intermolecular forces (than in AT SAMs) there still is a difference in the average tilt angles on Au and Ag substrates. Second, for the TP monolayers on Au and Ag the effect of the substrate on the structure of the SAMs is crucial. Whereas the TP monolayer on Au is poorly defined, the same monolayer on Ag is well-ordered and densely packed. Considering the weaker intermolecular forces in the TP films, we suppose that relative to the molecules with a larger

Thioaromatic SAMs on Gold and Silver

aromatic chain the balance between the headgroup/ substrate interactions and the intermolecular forces is shifted in favor of the former interactions. In that case, a poor quality of the TP monolayers on Au can probably be explained by steric constraints caused by the pinning of the TP molecules to a definite adsorption site (because of the large corrugation of the sulfur-metal binding energy surface2,66) and by an unfavorable bond angle at the sulfur atom, which is for bulk poly(p-phenylene sulfide) in a range of 101°-110°.69-72 Note that in contrast to the AT SAMs, the substrateS-phenyl bond angle, which is associated with a definite hybridization of the S atom (sp3 and sp for AT/SAMs on Au and Ag, respectively),4 can be rather similar in the thioaromatic films on both substrates. Along with a different intermolecular interaction, this similarity can be an additional factor responsible for the relatively small differences observed in the tilt angles of the thioaromatic molecules on Au and Ag. Generally, the surface bonding of a S atom at a phenyl ring cannot be easily described within either the sp3 or sp hybridization but can probably be associated with some intermediate state, which is not precisely known at present. An additional factor, which can be of importance for the structure and packing of the thioaromatic molecules in the respective SAMs, is the larger vdW dimension of the aromatic rings as compared to the aliphatic chains. In ref 62, this factor is believed to be solely responsible for the given tilt angle (less than 19°) of the aromatic chains in the BPT films. 4.3. Influence of the Molecular Rigidity. In comparison with the BPT and TPT molecules, the aromatic chain of the AnT molecules has a larger rigidity due to the conjugation of the phenyl rings. Furthermore, the thiol group in the AnT molecule is off the molecular axis, whereas the thiol group in the TP, BPT, and TPT molecules is located along this molecular axis. Taking into account these factors, a rather disordered monolayer was expected for the AnT molecules. However, the linear dichroism of the NEXAFS resonances in Figures 3d and 4d implies well-ordered and densely packed AnT monolayers on both Au and Ag surfaces. On these substrates, the average tilt angles are nearly the same as for the single covalentbonded thioaromatic counterparts. The pronounced difference of 9° in the average tilt angle on Au and Ag as compared to the difference of 4-5° for the other thioaromatic molecules can be a result of the lower packing density of the AnT molecules on the Au surfaces, as concluded from the C 1s and S 2p photoemission spectra in Figure 2. In the same way as for TP layers on Au, the lower packing density of the AnT layers on Au can probably be explained by steric constraints related to the substrate (see section 4.2). However, in contrast to the TP molecules with only one phenyl ring in the aromatic chain, the AnT molecules form highly oriented SAMs, which are presum(69) Napolitano, R.; Pirozzi, B.; Salvione, A. Macromolecules 1999, 32, 7682. (70) Yoder, G.; Dickerson, B. K.; Chen, A.-B. J. Chem. Phys. 1999, 111, 10347. (71) Tabor, B. J.; Magre, E. P.; Boon, J. J. J. Eur. Polym. 1971, 7, 1127. (72) Garbarczyk, J. Polym. Commun. 1986, 27, 335.

Langmuir, Vol. 17, No. 8, 2001 2415

ably a consequence of the stronger intermolecular interaction between the conjugated phenyl rings of the adjacent AnT chains. Consequently, the number of aromatic moieties affects the final molecular orientation within the layer to a larger extent than the rigidity of the molecule. 5. Summary The structure of self-assembled monolayers formed from TP, BPT, TPT, and AnT molecules on polycrystalline Au and Ag substrates was studied by XPS and NEXAFS spectroscopy. The number of aromatic moieties, the substrate, and the rigidity of the thioaromatic molecule were varied to monitor the effects of the substrate and the intermolecular interactions on the molecular orientation within the respective SAMs. With the exception of the poorly defined TP layers on Au, all thioaromatic molecules were found to form highly oriented and densely packed monolayers on both substrates. In the same way as for self-assembled aliphatic systems, the thioaromatic molecules in the respective SAMs are less tilted on Ag as compared to Au. Average tilt angles of ≈22° on Au and ≈16° on Ag with respect to the surface normal are determined for the SAMs formed from BPT and TPT molecules comprising two and three aromatic moieties. In contrast to the aliphatic films, the difference of the tilt angles on the Au and Ag substrates is smaller for the thioaromatic layers. In addition, an increase of the number of phenyl rings in thioaromatic molecules, associated with an increase of the intermolecular interactions, results in a less inclined orientation of these molecules in the respective SAMs. On the basis of both this finding and the comparison of the aliphatic and aromatic films, we suppose that in the case of thioaromatic SAMs the structure determining balance between the headgroup/substrate interactions and the intermolecular forces is shifted toward intermolecular forces. Nevertheless, the headgroup/substrate interaction cannot be neglected, because the average tilt angles for the long-chain thioaromatic SAMs on Au and Ag are still different. In addition, the effect of the headgroup/substrate interaction on the molecular orientation is supported by the substrate dependent orientation of the TP monolayers. Whereas these monolayers are poorly defined on Au, the TP monolayers are well-ordered and densely packed on Ag and exhibit an average tilt angle of ≈24°. Despite the larger rigidity, the AnT monolayers show similar average tilt angles as compared to the BPT and TPT SAMs on both surfaces. On Au, the AnT monolayers exhibit a slightly reduced packing density, which can probably be associated with the steric constraints provided by the Au substrate. These constraints hinder the rigid molecules from arranging as densely as in the case of the more flexible BPT and TPT molecules. Acknowledgment. We thank the BESSY staff, especially M. Mast, for technical help, G. Albert for preparation of the substrates, and Ch. Wo¨ll (Universita¨t Bochum) for providing us with experimental equipment. This work has been supported by the German Bundesministerium fu¨r Bildung und Forschung through Grant No. 05 SF8VHA 1 and by the Fonds der Chemischen Industrie. LA001540C