Photon Factory Activity Report 2004 #22 Part B (2005)
Surface and Interface

7A/2001S2-003

Adsorption of hexanethiolate on Au(111) in the ‘striped’ phase
Toru SHIMADA, Hiroshi KONDOH, Ikuyo NAKAI, Masanari NAGASAKA
Reona YOKOTA, Kenta AMEMIYA and Toshiaki OHTA*
Department of Chemistry, Graduate School of Science, The University of Tokyo,
Hongo, Tokyo 113-0033, Japan
Introduction
Self-assembled monolayers (SAMs) have attracted
wide attention in recent years because they have potential
applications in such fields as corrosion inhibition,
tribology, molecular recognition and biosensors, wherethorough understanding of the molecular structure ofSAMs is necessary for effective applications. In this work,
the adsorption structure of the ‘striped’ phase ofhexanethiolate on Au(111) was studied by near-edge Xray absorption fine structure (NEXAFS), X-ray
photoelectron spectroscopy (XPS) and scanned-energy
photoelectron diffraction (PD).
Experimental
The experiments were performed in ultrahigh vacuum-10(UHV) chamber (the base pressure of less than 2 × 10Torr) which was equipped with a hemispherical analyzer.
All the measurements were carried out at the soft X-ray
beamline BL-7A. An Au(111) surface was cleaned byrepeated cycles of Ar ion sputtering and annealing at 820
K. To prepare the hexanethiolate ‘striped’ phase, the cleansurface was exposed to 200 L gaseous hexanethiol atroom temperature. All the C K-edge NEXAFS and S 2p
PD measurements were performed at 300 K. The C Kedge NEXAFS spectra were recorded in the partial
electron yield mode with a retarding voltage of -200 V.
The molecular structure was determined by the R-factor
analyses.
Results and discussion
In S 2p XPS, only one S component was observed with
spin-orbit split peaks, 2p3/2 and 2p1/2 at 161.8 and 163.0 eV,
respectively, which agreed well with those foralkanethiolate. This shows that no disulfide-like structure
was formed with two different adsorption sites.
The polarization dependence of C K-edge NEXAFS
spectra from hexanethiolate adsorbed on Au(111)indicates that the -C-C-C- plane is almost parallel to thesubstrate.
Figure 1 shows a modulation of the normal emission
S 2p peak as a function of the wavenumber in the range-1of 3-10 Å . To determine the adsorption structure, theexperimental curve was compared with theoreticalsimulations, using the multiple scattering calculation ofdiffraction (MSCD) package developed by Chen and van
Hove [1]. The reliability factor (R-factor) analyses were
performed for several possible adsorption sites to obtain
the optimum structure model. Among several highsymmetry adsorption sites, the atop site gives theminimum of the R-factor at the S-Au distance of 2.40 ±0.05 Å, which agrees well with the previous observation[2]. This finding provides firm evidence for the atopadsorption, in agreement with the previous X-ray standing
wave (XSW) study for octylthiolate adsorbed on Au(111)in the lying down phase. The S-Au distance also agrees
with that reported value in XSW study (2.50 ± 0.05 Å) [3].

Fig. 1 Normal emission S 2p PD modulation (solidline). Dashied line is a simulation curve for the bestfit atop adsorption model.

Fig. 2 Schematic view of the best-fit adsorption
geometry of C6H13S and its arrangement on Au(111).

References[1] Y. Chen et al., MSCD package, Lawrence Berkeley
National Laboratory (1998)[2] T. Hayashi et al., J. Chem. Phys. 114, 7615 (2001).
[3] M.G. Roper et al., Chem. Phys. Lett. 389, 87 (2004)* ohta@chem.s.u-tokyo.ac.jp

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