Langmuir 2001, 17, 3689-3695
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Structural Characterization of Organothiolate Adlayers on Gold: The Case of Rigid, Aromatic Backbones Claus Fuxen,† Waleed Azzam,† Ralf Arnold,† Gregor Witte,† Andreas Terfort,‡ and Christof Wo¨ll*,† Ruhr-Universita¨ t Bochum, Institut fu¨ r Physikalische Chemie 1, Universita¨ tsstrasse 150, D-44801 Bochum, and Universita¨ t Hamburg, Institut fu¨ r Anorganische und Angewandte Chemie, Martin-Luther-King-Platz 6, D-20146 Hamburg Received December 26, 2000. In Final Form: March 5, 2001 Here, we present the results of a detailed characterization of rigid-rod thiolate adlayers on Au(111) made by self-assembly of organothiols containing a terphenyl backbone, namely, p-terphenylthiol (TPT, C6H5(C6H4)2-SH) and p-terphenylmethanethiol (TPMT, C6H5-(C6H4)2-CH2-SH) on Au(111). The results of low energy electron diffraction measurements reveal the presence of long-range order for adlayers prepared from both molecules. From the body of data obtained by X-ray photoelectron spectroscopy, infrared reflectionabsorption spectroscopy, near-edge X-ray absorption fine structure measurements, and scanning tunneling microscopy, a structure model for adlayers of TPT and TPMT on Au(111) is proposed.
Introduction Self-assembled monolayers have been the focus of considerable interest during the past 15 years. A precise knowledge of their microscopic structure plays an important role, because the molecular structure is directly related to the physical and chemical properties of the interface. The structure of alkanethiolate monolayers on welldefined substrates has been investigated in considerable detail during the past decade, and the present understanding of their structure has been obtained using a large variety of analytical techniques.1 Alkanethiols are known to adsorb on Au(111) resulting in well-ordered domains exhibiting a c(4x3×2x3)R30° superstructure.2,3 The alkane chains are tilted by about 30° with respect to the surface normal. Very little structural information is available, however, for ultrathin organothiol films on gold made from monomers containing backbones different from n-alkanes, although the importance of aromatic compounds as potential candidates for molecular electronics has been addressed in several studies.4-6 Such aromatic backbones are rather interesting, because the type of lateral (intermolecular) interaction is different from that of the n-alkanes. The type of organic surface formed by adsorption of organothiols containing aromatic systems will also be different from that formed from alkanethiols, because in the latter case the film is exposing an aliphatic CH3 group, whereas in the former case the adlayer will be terminated by an aromatic hydrocarbon. Furthermore, it is known from several structural studies that the introduction of functional groups at the chain termini can lead to severe disorder which is in most cases † ‡
Ruhr-Universita¨t Bochum. Universita¨t Hamburg.
(1) Ulman, A. Self-Assembled Monolayers of Thiols; Academic Press: London, 1998; Vol. 24. (2) Camillone, N.; Chidsey, C.; Liu, G.; Scoles, G. J. Chem. Phys. 1993, 98, 3503-3511. (3) Poirier, G. E.; Tarlov, M. J. Langmuir 1994, 10, 2853. (4) Haag, R.; Rampi, M. A.; Holmlin, R. E.; Whitesides, G. M. J. Am. Chem. Soc. 1999, 121, 7895-7906. (5) Vondrak, T.; Cramer, C. J.; Zhu, X. Y. J. Phys. Chem. B 1999, 103, 8915-8919. (6) Vondrak, T.; Wang, H.; Winget, P.; Cramer, C. J.; Zhu, X. Y. J. Am. Chem. Soc. 2000, 122, 4700-4707.
undesirable.7 It is expected that the stronger π-π interaction and the rigidity of the backbone will reduce the ability of terminal groups to affect the film order and therefore improve the integrity of the film. Previous scanning tunneling microscopy (STM) measurements have indicated the presence of long-range order for films made from different organothiols containing phenyl units within the backbone. In particular, Bumm et al. used a thiol with ethynyl linked phenyl units embedded in a dodecanethiolate monolayer film to study the electrical conductivity of the different molecules.8 For this organothiol, Dhirani et al. proposed a (2x3×x3)R30° structure on Au(111) in a later STM study.9 In a recent study on organothiols with a biphenyl backbone by Leung and co-workers,10 a (x3×x3)R30° structure for 4-methyl4′-mercaptobiphenyl has been seen with grazing-incidence X-ray diffraction and He-atom scattering. For organothiols with a longer aryl-containing backbone, Ishida et al. found a (x3×x3)R30° structure for p-terphenylmethanethiol (TPMT) on Au(111) in their STM measurements.11 These STM investigations are inherently restricted to small areas, and an unambiguous proof of long-range ordering for oligophenyl units with more than two phenyl units has still been lacking. The results from our present low-energy electron diffraction (LEED) studies reveal that SAMs made from organothiols with long rigid-rod backbones do exhibit a two-dimensional (2D) lateral ordering comparable to that seen for alkanethiolate adlayers. Experimental Section Chemicals. p-Terphenylthiol (TPT) and TPMT were synthesized using a previously described procedure.7 Ethanol (99.8%, Baker) and dodecanethiol (98%, Aldrich) were used as received. (7) Himmel, H.-J.; Terfort, A.; Wo¨ll, C. J. Am. Chem. Soc. 1998, 120, 12069-12074. (8) Bumm, L. A.; Arnold, J. J.; Cygan, M. T.; Dunbar, T. D.; Burgin, T. P.; Jones, L.; Allara, D. L.; Tour, J. M.; Weiss, P. S. Science 1996, 271, 1705. (9) Dhirani, A.; Zehner, R. W.; Hsung, R. P.; Guyot-Sionnest, P.; Sita, L. R. J. Am. Chem. Soc. 1996, 118, 3319-3320. (10) Leung, T.; Schwartz, P.; Scoles, G.; Schreiber, F.; Ulman, A. Surf. Sci. 2000, 458, 34-52. (11) Ishida, T.; Mizutani, W.; Akiba, U.; Umemura, K.; Inoue, A.; Choi, N.; Fujihira, M.; Tokumoto, H. J. Phys. Chem. B 1999, 103, 16861690.
10.1021/la0018033 CCC: $20.00 © 2001 American Chemical Society Published on Web 05/11/2001
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Langmuir, Vol. 17, No. 12, 2001
Sample Preparation. For the structural studies, an Au single crystal exhibiting a (111) surface was used, which was mechanically polished to within 0.2° of the desired orientation. After transfer into ultrahigh vacuum (UHV), it was subjected to sputtering (500 eV, Ar+) and annealing (700-800 K) cycles until a LEED pattern consistent with the presence of a perfect Au(111) surface12 was observed and no contamination could be seen within the detection limit of X-ray photoelectron spectroscopy (XPS) (∼5% of a monolayer). After this cleaning procedure, the crystal was removed from the UHV system and immediately immersed into a 10 µM ethanolic solution of the appropriate thiol for a period of between 15 and 24 h. Immersion was carried out at either room temperature (290 K) or 330 K. After removal from the solvent, the crystal was rinsed with the pure solvent to remove physisorbed overlayers and then immediately transferred back into the UHV system in order to minimize any effect of contamination from the ambient. The measurements were then carried out within the next 24 h. For the XPS, infrared reflection-absorption spectroscopy (IRRAS), and near-edge X-ray absorption fine structure (NEXAFS) investigations, polycrystalline Au substrates were prepared by evaporating 5 nm of titanium (99.8%, Chempur) and subsequently 100 nm of gold (99.995%, Chempur) onto polished silicon wafers (Wacker silicone) in an evaporation chamber operating at a base pressure of about 10-7 mbar. These substrates were stored in a vacuum desiccator until the adsorption experiments were carried out. Additional STM measurements have been carried out on substrates which have been prepared by evaporating 100 nm of Au onto freshly cleaved mica, which had been heated to about 600 K for 3 days in the evaporation chamber. After the metal evaporation, the substrates were cooled and the chamber was backfilled with nitrogen. The substrates were stored under argon and flame-annealed in a butane/oxygen flame immediately before the adsorption experiments were carried out. This procedure yields Au substrates with large terraces (several hundreds of nanometers, as evidenced by STM) exhibiting a (111) surface. Structural Investigations. LEED data were obtained in a multichamber UHV system using a microchannelplate LEED system (OCI) and a CCD camera connected to a frame grabber card for image aquisition. This system is capable of recording LEED data with very low electron fluxes (50-150 pA/mm2).13 After insertion into the UHV system, the Au single crystal was heated to the appropriate temperature (see below) and then cooled to 110-120 K for the LEED measurements in order to reduce the inelastic background. This UHV system was also used to acquire the XP and the thermal desorption (TD) spectra. NEXAFS data were obtained at the synchrotron facility BESSY I in Berlin at the beamline HE-TGM2 using a multichamber UHV system equipped with a load lock system. IRRAS spectra were taken using a Biorad Excalibur Fourier transform infrared spectrometer (FTS 3000) equipped with a grazing incidence reflection unit (Biorad Uniflex) and a narrow band MCT detector. All spectra were taken with 2 cm-1 resolution at an angle of incidence of 80° relative to the surface normal and further processed by using boxcar apodization. Baseline correction was done using spline functions.14 STM data were obtained using a commercial Nanoscope IIIa microscope Multimode (Digital Instruments, Santa Barbara, CA) equipped with a type E scanner. The tips were prepared by cutting a Pt/Ir (80:20, Chempur) wire mechanically. All images were taken in air at room temperature.
Results XPS. XP spectra were recorded for TPT, TPMT, and dodecanethiolate adlayers on polycrystalline Au substrates. All three samples were mounted on a single sample (12) The clean Au(111) surface actually exhibits a (23×x3) reconstruction (ref 34). The additional, very closely spaced diffraction peaks could not be resolved with the present LEED system, but the shape of the diffraction spots was consistent with previous work. (13) Loepp, G.; Vollmer, S.; Witte, G.; Wo¨ll, C. Langmuir 1999, 15, 3767-3772. (14) WIN - IR Pro Software Package, version 2.96; Biorad, 1999.
Fuxen et al.
Figure 1. (a) XP spectra recorded for polycrystalline gold substrates showing the carbon 1s and gold 4f regions of TPT, TPMT, and DDT layers on Au(111). (b) Sulfur 2p region of a TPMT monolayer on Au(111).
holder to maintain the same geometric (i.e., distance and angles of X-ray gun and energy analyzer toward the sample) conditions during the measurements. The energy scales of all spectra were referenced to the Au 4f7/2 peak located at a binding energy of 84.0 eV. From the data depicted in Figure 1a, the film thickness was calculated using the relative intensities of the Au 4f and the C 1s peaks and by using dodecanethiol on Au as a reference system.15
IC (sample) IAu ) IC (reference) IAu
{
} {
}
-dsample -dreference exp λC λAu -dsample -dreference exp 1 - exp λAu λC
1 - exp
{
}
{
}
(1)
Assuming the same photoelectron escape depths of the gold (λAu ) 4.5 nm at a kinetic photoelectron energy of 1402 eV) and carbon (λC ) 3.5 nm at 1202 eV) photoelec(15) Himmel, H. J.; Weiss, K.; Ja¨ger, B.; Dannenberger, O.; Grunze, M.; Wo¨ll, C. Langmuir 1997, 13, 4943-4947.
Characterization of Organothiolate Adlayers on Au
trons for films of TPT, TPMT, and the alkanethiolate films,16 eq 1 was used for the calculation and yielded thicknesses of 2.10 nm for TPMT and 2.17 nm for TPT. The intensities were determined by fitting Gauss peaks after a Shirley type background subtraction. The thickness of the dodecanethiol film was assumed to be 1.64 nm, corresponding to a tilt angle of the molecules of 30° with respect to the surface normal and an Au-S distance of 0.2 nm. The value for the thickness is higher than the expected value of 1.6 nm for a TPT film, where the molecular axis is vertical with repect to the surface plane. This could be either a result of contamination or a result of different escape depths of the photoelectrons in TPT and TPMT films. In the TPT films, oxygen contamination with concentrations depending on the sample (C/O ratio between 5:1 and 2:1) could be detected, whereas in TPMT films the oxygen contamination was significantly lower and on some samples below the detection limit (
