Interplay of anchoring and ordering in aromatic self-assembled monolayers

The structure and thermal stability of differently anchored aromatic self-assembled monolayers (SAMs) of benzenethiol (BT) and benzeneselenol (BS) on Au(111) have been studied in-depth by means of thermal desorption spectroscopy, X-ray photoelectron spectroscopy, near-edge X-ray absorption spectroscopy, and scanning tunneling microscopy. In contrast to long range ordered BS-SAMs consisting of closed packed, upstanding molecules forming a ((1)(7)-(4)(4)) structure which resembles the molecular packing of benzene in the crystalline phase, BT-SAMs are characterized by a loose packing of largely tilted molecules which are arranged in a ((6)(1)(24)) structure appearing only in small domains. Thermal desorption spectroscopy measurements clearly show that BS is less strongly bound to the gold substrate and moreover reveal different desorption peaks for each SAM, thus indicating the presence of different adsorption states. Heating both SAMs causes initially a partial desorption and formation of a diluted phase where the remaining molecules adopt a nearly flat laying adsorption geometry. The diversity of both molecular films is discussed in terms of the different substrate interaction of both anchoring groups.