The electronic charge density distribution or the electrostatic atomic potential of a solid or molecule contains information not only on the atomic structure, but also on the electronic properties, such as the nature of the chemical bonds or the degree of ionization of atoms. However, the redistribution of charge due to chemical bonding is small compared with the total charge density, and therefore difficult to measure. Here, we demonstrate an experimental analysis of charge redistribution due to chemical bonding by means of high-resolution transmission electron microscopy (HRTEM). We analyse charge transfer on the single-atom level for nitrogen-substitution point defects in graphene, and confirm the ionicity of single-layer hexagonal boron nitride. Our combination of HRTEM experiments and first-principles electronic structure calculations opens a new way to investigate electronic configurations of point defects, other non-periodic arrangements or nanoscale objects that cannot be studied by an electron or X-ray diffraction analysis.

A bisterpyridine based molecule, 3,3′-BTP, shows a variety of adlayer structures at the interface between highly oriented pyrolytic graphite (HOPG) and the liquid depending on the concentration in solution. Three closely related linear and one hexagonal 2D patterns are found. Comparison with the self-assembly at the HOPG|gas interface shows that in the absence of the solvent one of the linear and the hexagonal structures can be found. The concentration dependent order of appearance of the different surface structures is rationalized by a thermodynamic model. In the adlayer unit cell, the hexagonal phase offers a central void which is mostly filled with a seventh 3,3′-BTP molecule. In the presence of the solvent, those molecules are presumably rotating, whereas at the HOPG|gas interface no clear rotation can be observed.

The competition between intermolecular interactions and lateral variations in the molecule-substrate interactions has been studied by scanning tunneling microscopy (STM), comparing the phase formation of (sub)monolayers of the organic molecule 2,4'-BTP on buckled graphene/Ru(0001) and Ag(111) oriented thin films on Ru(0001). On the Ag films, the molecules form a densely packed 2D structure, while on graphene/Ru(0001), only the areas between the maxima are populated. The findings are rationalized by a high corrugation in the adsorption potential for 2,4'-BTP molecules on graphene/Ru(0001). These findings are supported by temperature programmed desorption (TPD) experiments and theoretical results.

SummaryThe competition between intermolecular interactions and long-range lateral variations in the substrate–adsorbate interaction was studied by scanning tunnelling microscopy (STM) and force field based calculations, by comparing the phase formation of (sub-) monolayers of the organic molecules (i) 2-phenyl-4,6-bis(6-(pyridin-3-yl)-4-(pyridin-3-yl)pyridin-2-yl)pyrimidine (3,3'-BTP) and (ii) 3,4,9,10-perylene tetracarboxylic-dianhydride (PTCDA) on graphene/Ru(0001). For PTCDA adsorption, a 2D adlayer phase was formed, which extended over large areas, while for 3,3'-BTP adsorption linear or ring like structures were formed, which exclusively populated the areas between the maxima of the moiré structure of the buckled graphene layer. The consequences for the competing intermolecular interactions and corrugation in the adsorption potential are discussed and compared with the theoretical results.

Now you see it, now you don't: A molecular monolayer polymorph of an oligopyridine forms a host–guest network with copper(II) phthalocyanine at the solid–liquid interface with controllable phthalocyanine occupation. The slow movement of the phthalocyanine guest molecules and the competition between oligopyridine and phthalocyanine guest molecules make it possible to manipulate individual guest molecules in analogy to “writing” and “erasing”.