Conformation, packing, defects, and molecular dynamics in monolayers of dialkyl-substituted benzenes

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Abstract
Monolayers of the two homologues dihexadecylbenzene and didodecylbenzene were absorbed at the interface between highly oriented pyrolytic graphite (HOPG) and an organic solution. The derivatization of a small molecule, in this case benzene, with long alkyl sidechains is shown to be a method to immobilize various molecular entities at the interface between HOPG and an organic solution. In situ STM studies at the internal interface revealed that the first adsorbed monolayer of the dialkyl-substituted benzenes exhibits a lamellar phase with the molecules extended and oriented parallel to the basal plane of graphite. Bias-dependent experiments allowed to either image preferentially the underlying substrate (at low bias) or the molecular adsorbate (at higher bias), without disrupting the adsorbate layer. From such experiments it was concluded that the alkane sidechains tend to orient parallel to a graphite lattice axis. High resolution images revealed different types of packing within a lamella, which were separated by sharp domain boundaries. Also packing defects involving individual molecules were observed. Most interesting, fast image recording on video tape allowed to directly follow the dynamics of such individual defects on the time scale between 100 ms and several minutes, i.e. the relevant timescale for the slow dynamics in crystalline organic phases.

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