Anchoring structure of smectic liquid-crystal layers on MoS₂ observed by scanning tunnelling microscopy

Abstract

THE molecular alignment of liquid crystals on solid substrates is important both to fundamental issues in physics and biology and in practical applications such as optoelectronic devices. The structure of adsorbed phases is determined to a large extent by the balance of molecule–molecule and substrate–molecule forces at the solid-adsorbate interface. Scanning tunnelling microscopy (STM) now allows the possibility of direct observation of the interfacial structure with molecular resolution, and has been used to study positional and orientational order in monolayers of organic molecules on graphite^1–4. Here we report STM imaging of smectic liquid crystals (4'-n-octyl-4-cyanobiphenyl (8CB)) condensed on a molybdenum disulphide single crystal. The anchoring structure is different from that on graphite⁴, and is unusual in that the molecules are aligned so that intermolecular interactions are energetically unfavourable. The influence of interactions with the substrate is clearly important in this case, and this is further supported by an apparent degree of epitaxy in the adsorbed layer.