J. Synchrotron Rad. (2001). 8, 1121±1123 # 2001 International Union of Crystallography  Printed in Great Britain ± all rights reserved 1121 short communications NEXAFS study of 1-butanethiol adsorbed on Cu(111) and √7 × √7 R19.1° S/Cu(111) Y. W. Yang, a* S. Venkatesh, a L. J. Fan, b T. E. Dann a and L. J. Lai a a Synchrotron Radiation Research Center, No 1, R&D Road, VI, Hsinchu, Taiwan 30077, R. O. C., b Department of Chemistry, National Tsing Hua University, Hsinchu, Taiwan 30043, R. O. C. Email: yang@srrc.gov.tw The adsorption of 1-butanethiol on Cu(111) and √7 × √7 R19.1° S/Cu(111) surfaces has been studied by S K-edge near edge X-ray absorption fine structure (NEXAFS) spectroscopy and thermal desorption spectroscopy. Upon adsorption on clean Cu(111) surface at room temperature, butanethiolate as well as atomic sulfur is formed. For the butanethiolate, the S-C bond is found predominately perpendicular to the surface as revealed by polarization analysis. In contrast, on √7 × √7 R19.1° S/Cu(111) surface, the S-H and S-C bonds of the butanethiol stay intact, resulting in a weakly chemisorbed butanethiol. Keywords: near edge X-ray absorption fine structure (NEXAFS); 1-butanethiol; Cu(111). 1. Introduction Ultrathin organic thin films of self-assembled monolayers have been the subject of intense research due to their potential applications in areas of lubrication, corrosion inhibitors, gas sensors (Ulman, 1991). In contrast with the extensively studied system of alkanethiols adsorbed on Au surfaces, there have been only a few reports of alkanethiols adsorption on Cu surfaces (Laibinis et al., 1991; Rieley et al., 1997; Hutt et al., 1998; Imanishi et al., 1998; Loepp et al., 1999; Floriano et al., 2000), presumably due to the difficulty of maintaining a clean Cu surface in the solution phase. In anticipation of the wider adoption of Cu-based interconnecting technology in the future generation of IC circuitry, an enhanced understanding of Cu- liquid interface and the corrosion protection of Cu metal becomes much more necessary. In the present study, we employed NEXAFS and TDS to study the adsorption of 1-butanethiol on both clean Cu(111) and √7 × √7 R19.1° S/Cu surfaces. The aims are focused on understanding the adsorption geometry, comparing the relative chemical bonding strength between two systems, and exploring the possibility of synthesizing the disulfide bond on the surface. 2. Experimental The experiments were carried out in a 12′′ mu metal sphere equipped with a sputtering ion gun, a LEED, an energy analyzer, and a microchannel plate (MCP). A Cu(111) single crystal was cut from a single crystal boule with a miscut angle determined via x-ray diffraction to be less than 0.5°. Standard sample sputtering/ anneal cycle was repeated until a sharp LEED pattern for the clean Cu surface was obtained and no contaminant was found with XPS. The partial electron yield detector was constructed from the MCP in conjunction with three Au meshes. For S K- edge NEXAFS measurements, a bias potential of 1900 V was chosen to retard the signals from Cu substrate and yet retain S Auger contribution. The measurements were performed at a DCM beamline (BL 15A) at SRRC, Taiwan (ring energy 1.5 GeV). The beamline resolution was better than 0.4 eV at the S K- edge. The polar angle of the incident x-ray was varied between 0° (normal incidence) and 70° (grazing incidence). The photoelectric current generated by a Cu mesh situated before the sample was used as the intensity of incident x-ray (I o ) for normalizing x-ray absorption spectrum. The 1-butanethiol was obtained from Aldrich Chemical Co and, after cycles of freezing/ thawing pretreatment, the purified 1- butanethiol was vacuum-dosed onto the surface. The dosage was measured in term of Langmuir (1 Langmuir = 10 -6 Torr x sec). The sample could be heated to 800 K by means of resistive heating and cooled down to 100 K by heat conduction to a liquid nitrogen reservoir external to the chamber. 3. Results and Discussion: Fig. 1 shows a series of S K-edge NEXAFS spectra acquired at two different x-ray incidence angles for 1-butanethiol adsorbed onto different surfaces. The curves labeled C4 ad refer to the spectra taken for 20 L butanethiol adsorbed at room temperature onto the clean Cu(111) surface. At grazing incidence condition (θ = 70°), a peak at 2469.9 eV is clearly noted. High-resolution synchrotron based S2p XPS data show that at room temperature the chemisorption of the butanethiol leads to coadsorbed thiolate and atomic sulfur species on the surface (Yang et al., 2000). The similar coexistence of thiolate and atomic sulfur species for methanethiol chemisorbed on Cu(111) has also been observed (Jackson et al., 2000). A previous XAS study unambiguously assigns the peak at 2469.9 eV to the atomically-adsorbed sulfur and the corresponding transition is originated from S 1s state to unfilled p-derived hybrid state of S and metal atoms (Stöhr et al., 1985). Figure 1 Polarization-dependent S K-edge NEXAFS spectra for (1) butanethiol/ Cu(111); (2) √7 × √7 R19.1° Sad; (3) butanethiol/ √7 × √7 R19.1° Sad/ Cu(111); (4) = (3) - (2). 2460 2470 2480 2490 2500 2510 C4/S ad - S ad 0 o 0 o 0 o 70 o 70 o 70 o 0 o 70 o C4/S ad - S ad C4/S ad C4/S ad S ad S ad C4 ad C4 ad Photon Energy (eV) Partial Electron Yield (a.u.)