Molecular orientation and bonding of terthiophene-thiol self-assembled on Au(1 1 1): A combined NEXAFS and XPS study

Self-assembled monolayer of 2,2':5',2'-terthiophene-5-methanethiol (5mTP) on Au(1 1 1) has been investigated with synchrotron-based X-ray photoelectron spectroscopy (XPS) and carbon K-edge near edge X-ray absorption fine structure (NEXAFS) spectroscopy. XPS data show that 5mTP bonds to Au(1 1 1) via an S-H bond cleavage while leaving the terthiophene intact. The binding energies of S 2p3/2 core levels for thiolate and terthiophene are located at 162.1 and 163.8 eV, respectively. The broad C 1s core level is centered at a binding energy of 285.2 eV. Intensity analysis of the S 2p peaks reveals a preferential alignment of the terthiophene with its long axis oriented near the surface normal. Carbon K-edge NEXAFS spectra show the presence of sharp, polarization-dependent, 1s → π* resonances located at energies of 285.60 and 287.54 eV. From the E-vector polarization analysis, the aromatic plane of the 5mTP is concluded to be tilted at 25 deg. from the surface normal