Comparative Study of Tetrahydrothiophene and Thiophene Self Assembled Monolayers on Au(111): Structure and Molecular Orientation

Abstract

Surface structure and molecular orientation of self-assembled monolayers (SAMs) formed by the spontaneous adsorption of tetrahydrothiophene (THT) and thiophene (TP) on Au(111) were investigated by means of scanning tunneling microscopy (STM) and carbon K-edge near edge X-ray absorption fine structure (NEXAFS) spectroscopy. STM imaging revealed that THT SAMs have a commensurate (3 × 2√3) structure containing structural defects in ordered domains, whereas TP SAMs are composed of randomly adsorbed domains and paired molecular row domains that can be described as an incommensurate packing structure. The NEXAFS spectroscopy study showed that the average tilt angle of the aliphatic THT ring and π-conjugated TP ring in the SAMs were calculated to be about 30 o and 40 o , respectively, from the surface normal. It was also observed that the π* transition peak in the NEXAFS spectrum of the TP SAMs is very weak, suggesting that a strong interaction between π-electrons and the Au surface arises during the self-assembly of TP molecules. In this study, we have clearly demonstrated that the surface structure and adsorption orientation of organic SAMs on Au(111) are strongly influenced by whether the cyclic ring is saturated or unsaturated.

Thiophene Derivatives on Gold and Molecular Dissociation Processes

“…Similar XPS and NEXAFS results for thiophene reported by Noh et al. ,, showed an S 2p 3/2 peak at 162 eV with a smaller structure at about 161 eV. They concluded that thiophene SAMs form through chemical interaction between the sulfur headgroups in thiophene and the Au(111) surface, but we do not clearly interpret this as due to C–S bond breaking at room temperature.…”

“…At room temperature, STM imaging reveals interesting paired row ordered structures, suggesting differently coordinated molecules, which could explain the observation of the two CLBEs mentioned above . In their more recent work they concluded that although they do not observe a peak in NEXAFS at 285.6 eV the 162 eV CLBE corresponds to the bonding of an undissociated thiophene molecule. They affirm this on the basis of observation of ordered structures in STM for thiophene and tetrahydrothiophene (C 4 H 8 S), which has an aliphatic cyclic ring without a π conjugated system.…”

“…A number of problems subsist however and among these, there appeared one, relating to possible dissociation of these molecules at the electrode surface that involves S–C bond scission. ,− The existence of such dissociation processes leading to dehydrogenation and desulfurization is known to occur on transition metal surfaces, − which is not very surprising given their high reactivity. However, there are indications in recent literature ,,,,− that this may also occur on the less reactive coinage metal surfaces including gold, which is an important electrode material. This S–C bond scission leads to an opening up of the ring and observation of alkene thiol species.…”

“…Quite promising results have been obtained from a molecular electronics point of view. Many studies have therefore been devoted to the formation of multilayer and monolayer films of these molecules on various surfaces of interest as electrodes. ,− …”

Selenium, Benzeneselenol, and Selenophene Interaction with Cu(100)

“…The study of self-assembled monolayers (SAMs) has been an attractive field since SAMs allow tailoring of surface properties − such as adhesion, corrosion, and biocompatibility, and furthermore, these have been used in research on molecular electronic devices. − Historically work on SAMs involved much work on thiols ,,, with the S atom serving as linker to a substrate. Commonly investigated systems involve various thiols with alkane and phenyl groups ,,− and also thiophenes. − The interaction between the headgroup and substrate affects the characteristics of the SAMs such as orientation, structural configurations, and charge transfer properties and, to a large extent, determines the performance of SAMs. In recent years, therefore, interest in other headgroups as linkers, with possibly better characteristics for a given application, engendered research into molecules bearing heavier chalcogens − (Se, Te) as well as other atoms such as N and C. , Several investigations have thus been carried out on selenol SAMs and selenophene adsorption. − …”