Impact of heterocirculene molecular symmetry upon two-dimensional crystallization.

Despite the development of crystal engineering, it remains a great challenge to predict the crystal structure even for the simplest molecules, and a clear link between molecular and crystal symmetry is missing in general. Here we demonstrate that the two-dimensional (2D) crystallization of heterocirculenes on a Au(111) surface is greatly affected by the molecular symmetry. By means of ultrahigh vacuum scanning tunneling microscopy, we observe a variety of 2D crystalline structures in the coverage range from submonolayer to monolayer for D(8h)-symmetric sulflower (C16S8), whereas D(4h)-symmetric selenosulflower (C16S4Se4) forms square and rectangular lattices at submonolayer and monolayer coverages, respectively. No long-range ordered structure is observed for C(1h)-symmetric selenosulflower (C16S5Se3) self-assembling at submonolayer coverage. Such different self-assembly behaviors for the heterocirculenes with reduced molecular symmetries derive from the tendency toward close packing and the molecular symmetry retention in 2D crystallization due to van der Waals interactions.

Mapping the electronic surface potential of nanostructured surfaces.

We present a method for the quantitative determination of the surface potential landscape of nanostructured surfaces based on the local analysis of the lowest field emission resonances by scanning tunneling spectroscopy. The method has a lateral resolution of approximately 1 nm and is applied to elucidate the site-specific adsorption properties of the strain relief pattern formed by two monolayers of Ag on Pt(111). For the example of C60 fullerenes, we show that the surface potential difference of up to 0.35 eV is responsible for the site-selective immobilization on the strain relief pattern.

Fabrication of a well-ordered nanohole array stable at room temperature.

We report on the fabrication of a new type of nanotemplate surface consisting of a hexagonally well-ordered array of one monolayer deep holes with a tunable size of about 4 nm (2) and a fixed spacing of 7 nm. The nanohole array fabrication is based on the strain-relief trigonal network formed in the 2 monolayer Ag on Pt(111) system. Removing about 0.1 ML of the Ag top layer of this surface structure, for example, by He- or Ar-ion sputtering, leads to the formation of nanoholes at specific domains of the trigonal network, which are stable at room temperature.