The interaction of low-energy photoelectrons with well-ordered monolayers of enantiopure helical heptahelicene molecules adsorbed on metal surfaces leads to a preferential transmission of one longitudinally polarized spin component, which is strongly coupled to the helical sense of the molecules. Heptahelicene, composed of only carbon and hydrogen atoms, exhibits only a single helical turn but shows excess in longitudinal spin polarization of about P = 6 to 8% after transmission of initially balanced left- and right-handed spin polarized electrons. Insight into the electronic structure, that is, the projected density of states, and the spin-dependent electron scattering in the helicene molecule is gained by using spin-resolved density functional theory calculations and a model Hamiltonian approach, respectively. Our results support the semiclassical picture of electronic transport along a helical pathway under the influence of spin-orbit coupling induced by the electrostatic molecular potential.

Insight into recognition among helical molecules is highly relevant for understanding chiral separation or biomolecular activity. The two-dimensional selfassembly of enantiopure and racemic heptahelicene has been studied on the (111) surfaces of silver and gold by means of scanning tunneling microscopy. As found earlier for racemic heptahelicene on Cu(111), the racemate forms zigzag rows with alternating enantiomers. In contrast to Cu(111), no enantiomorphism in the form of oblique alignment of molecular adlattice vectors with respect to substrate lattice vectors is observed. That is, the zigzag rows run parallel to the high-symmetry directions of the substrate. Because of the larger interatomic distances, stress builds up in the racemic molecular layer on Ag(111), which leads to relaxation into different structures after every third or fourth zigzag row. For enantiopure (M)-heptahelicene, identical structures at monolayer saturation coverage are observed, showing a pronounced transmission of chirality via azimuthal alignment of adjacent molecules.

In this article, we discuss the recent progress on controlled structural modification of 2D materials by means of molecular functionalization, with a focus on scanning probe microscopy techniques for their characterization. For many practical applications of these novel materials, it is necessary to tune their electronic and optical properties, and molecule-based functionalization is a powerful approach to reach this. We discuss recent covalent and non-covalent approaches, for functionalization of graphene, transition metal dichalcogenides, black phosphorus, and hexagonal boron nitride. Nanostructuring approaches and their impact on 2D materials' properties are highlighted.

Two-dimensional (2D) nucleation and crystallization of the helical aromatic hydrocarbon heptahelicene on the single crystalline copper(100) surface has been studied with scanning tunnelling microscopy. In contrast to previously observed racemic 2D crystals on Cu(111), separation into homochiral domains is observed for Cu(100).