Well-defined crystal surfaces are regarded as appropriate playgrounds for designing molecular devices and sensors based on the recent development of atomic level characterization techniques. For instance, thiol (R-SH) is well known as forming a strong chemical bond with Au surface through interaction of SH group and Au atoms. But little is known about the behavior of interface states between Au and S atoms. Thus the investigation of the metal-molecular interface is a key to understand the mechanism of chemical bond formations in order to design molecular devices.
In the present work, we have studied the chemical bonding states of interface between Au and S-containing molecules. As S-containing molecules, we concentrated on amino acids, i.e., L-cysteine [HSCH2CH(NH2)COOH], because SH exists the end of the carbon chains. For comparison, we also investigated thiophene [C4H4S] molecule because there is no S-H bond in contrast to L-cysteine. We have studied metal-molecular interface by using near edge X-ray adsorption fine structure (NEXAFS) at the S K-edge, and S 1s X-ray photoelectron spectroscopy (XPS). Three kinds of films were prepared, i.e., monolayer and multilayer of L-cysteine, and monolayer of thiophene.
For thiophene films, the S 1s →σ*(S-C) resonance peak was observed in the NEXAFS spectra at 2473.6 eV, which is lower by 8.0 eV than that for mono-layered L-cysteine. The energy of this resonance peak is rather close to that for multi-layered L-cysteine. This means that no charge transfer occurs at the Au-S interface for monolayer of thiophene. In the S 1s XPS spectra for thiophene, two peaks were observed at 2472.0 eV and 2473.4 eV. The energy of the first peak (2472.0 eV) is the same as that for the multi-layered L-cysteine, which confirmed that no charge transfer happens between S-Au interfaces for thiophene. On the other hand, the energy shift (ƒ¢E) of the second peak (2473.4 eV) is 1.4 eV, which is quite small compared with that observed for mono and multi-layered L-cysteine (ƒ¢E=8.2 eV), which suggests the formation of gold sulfide. On the basis of these results, it is elucidated that S atoms in SH molecules strongly donate electron to Au, while gold sulfides are formed at the Au-S interface for thiophene.
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