Structure of self-assembled and surface-frozen monolayers of n-silanes and alcohols on metal oxides

by Hans-Georg Steinrück (Universität Erlangen-Nürnberg)

Wednesday 17 Jun 2015, 15:00 → 16:30 Europe/Berlin

Room 109 in bldg. 25b (DESY Hamburg)

Self-assembled monolayers (SAMs) are utilized in applications ranging from thin film field-effect-transistors, over optoelectronics to lubricants. But, an open question in basic science concerns their internal structure and the correlation with the substrate. In alkyl-silane SAMs on amorphous SiO2 (native oxide of Si, denoted Si/SiO2), the chain-chain interaction seems to dominate, whereas the strong covalent bond of the head group to the crystalline substrate promotes epitaxial ordering for alkyl-thiols on gold. Thus, the intriguing question arises how alkyl-silanes behave on a crystalline substrate. Our investigations of OTS (C18-silane) on sapphire (001) reveal a novel structure denoted now as pseudo-rotational epitaxy, which comprises a hitherto unreported relation between the SAM’s crystallites’ size and their respective orientation [1]. Another important topic explored is the effect of varying the ratio between molecule-molecule and molecule-substrate interaction on the structure of the SAM. This was achieved by choosing n-alkylsilanes with varying chain lengths on Si/SiO2 or by exchanging the trichlorosilane head groups with weaker binding ones, e.g. hydroxyl groups a well-defined model system. Our results show partial vertical crystallinity for the silanes due to the strain introduced near the head group region, resulting from the size mismatch between lateral cross-links and the chain-chain packing. For the alcohols, a novel structural phase transition was elucidated, starting with the proliferation of Gauche-conformations, and ending with a collective domain size reduction. Furthermore, an unprecedentedly detailed structural picture of the junction of crystalline silicon and its native amorphous oxide will be presented [2]. [1] H.-G. Steinrück, A. Magerl, M. Deutsch, and B. M. Ocko, PRL 113, 156101 (2014). [2] H.-G. Steinrück et al., ACS Nano 12, 12676 (2014).