Abstract
Interaction of electrons with chiral matter gives rise to interesting phenomena such as the chirality‐induced spin selectivity. The interdependence of reflectivity of spin‐polarized low energy electrons and the absolute handedness of chiral molecules is investigated. First, the growth of homochiral films of helical aromatic hydrocarbons, so‐called helicenes, on a Cu(100) surface is studied by means of low energy electron microscopy and scanning tunneling microscopy in ultrahigh vacuum. As soon as the coverage exceeds one monolayer, double‐layer nucleation and growth is favored such that depletion in the first layer occurs. Spatially resolved work function measurements show that second‐layer patches have a lower work function than first‐layer areas. Reflectivity spectra of spin‐polarized electrons do not show any asymmetry between homochiral films of the enantiomers. Laterally resolved work function measurements do not confirm work function differences such as those reported earlier for photoelectron studies of chiral peptide films on ferromagnetic substrates.
Highlights
As soon as second-layer nucleation occurs, molecules from the first layer are transferred into the second layer, which leads to a depleted first layer at that stage
This phenomenon is captured by Low energy electron microscopy (LEEM) when the helicene growth is observed at Us 1⁄4 4.7 V, and it will be discussed in more details along with Figure 2
Increased electron reflectivity observed by spin-polarized electron beam is used for illumination (SPLEEM) in the first layer suggests a depletion of molecules in the first layer near second-layer islands which is confirmed by scanning tunneling microscopy (STM) studies at cryogenic temperatures
Summary
The deposition of molecules has been monitored in situ by electron reflectivity changes averaged on the complete LEEM field-ofview over time. First-layer depletion and preference of double-layer formation occur already in this highly dynamic film with enhanced molecular mobility Such behavior suggests rather entropic effects as origin of depletion instead of previously favored enthalpic and kinetic effects.[37] Once the second-layer islands are formed and stop growing, previously disordered bright areas in the first layer become filled and ordered and give gray contrast in LEEM again (Figure 2d). With respect to the clean Cu(100) surface, monolayers of pure [7]H enantiomers and rac-[7]H have roughly a 1 eV lower work function (Φ, Figure S2, Supporting Information) Such decrease is based on charge rearrangement in the substrate caused by Pauli repulsion, as observed for similar aromatic adsorbates.[41] Figure 3 shows a LEEM image (Figure 3a), a Φ-map (Figure 3b), and a line profile (Figure 3c), which highlight differences in Φ between the first and the second layer of (P)-7[H] on Cu(100). Conclusion derives values obtained by the complementary error function fittings of I–V curves
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