Growth Dynamics and Electron Reflectivity in Ultrathin Films of Chiral Heptahelicene on Metal (100) Surfaces Studied by Spin‐Polarized Low Energy Electron Microscopy

Abstract

Molecular layers on surfaces have been widely used to study effects occurring at molecule/substrate interfaces. By controlling kinetic and thermodynamic conditions during the assembly of nano-objects, in particular molecules and atoms, various patterns of self-assembled, metalcoordinated or covalently linked structures have been obtained by careful selection of the components and surface symmetry. [1] Not only are these patterns interesting for their topologies, they also were found to induce interesting interface phenomena. For example, (sub)monolayer films of pentacene molecules on Cu(110) were introducing significant surface state shifts dependent on the coverage. [2] In addition, Co phthalocyanine molecules were used to manipulate the topological interface of Bi 2 Se 3 and Mn phthalocyanine molecules induced formation of Yu-Shiba-Rusinov states when adsorbed on superconducting Pb(111) substrate. [3,4] Assembled in checkerboard manner Mn and Fe phthalocyanine molecules formed a 2D ferrimagnet via Ruderman-Kittel-Kasuya-Yosida interactions through the surface state of Au(111). [5] Molecules were also shown to induce magnetism at Au interface "creatio ex nihilo". [6] Recently, it was demonstrated that molecular layers of Buckminsterfullerenes can induce ferromagnetism in Cu and Mn by beating the Stoner criteria. [7] Molecular layers of chiral molecules, on the contrary, showed the so-called chirality-induced spin selectivity (CISS), an effect of chirality-dependent transmission of electrons through the molecules. [8] Chiral molecules where shown to switch magnetization of underlying ferromagnetic Ni substrate. [9] And there are even examples of enantioseparation due to differences in interaction with underlying magnetic substrates. [10,11] Depending on the magnetization direction, differences of %80 meV in work function of chiral peptide layer on top of an out-of-plane ferromagnetic substrate were reported. [12] Helical aromatic hydrocarbons, so-called helicenes, have played an important role in understanding chiral crystallization when studied at surfaces by scanning tunneling microscopy (STM). [13] At coverages above a monolayer of rac-[7]H, enantiospecific separation from zigzag rows with alternating enantiomers into the separate layers occurred leading to a layered racemate

Enantiomer Adsorption in an Applied Magnetic Field: D‐ and L‐Aspartic Acid on Ni(100)

“…The most obvious difference may be the fact that, while Naaman has studied adsorption from solution phases, we have studied adsorption from a collisionless UHV environment. A recent study performed by Ernst [36] and also conducted under UHV conditions reported the absence of the CISS effect or spin selective transport through a homochiral helicene film. Under UHV conditions, the Asp is sublimated from the glass evaporator at $${T=438\ K}$$ and travels ballistically to the surface without experiencing any collisions with other species or a barrier to adsorption at the surface.…”

Enantioselective Adsorption on Magnetic Surfaces

“…In recent studies of chiral molecules on ferromagnetic Ni(100), neither enantiomeric separation nor differences in adsorption kinetics between magnetic domains were observed. [24,25] Here, we report enantiospecific adsorption of heptahelicene, a chiral helical polyaromatic molecule, on ferromagnetic singlecrystal cobalt surfaces in ultrahigh vacuum by means of spinpolarized scanning tunneling microscopy (SP-STM). Lateral enantioselection onto domains of opposite out-of-plane magnetization occurs in a transient physisorbed precursor state before final chemisorption.…”

“…In recent studies of chiral molecules on ferromagnetic Ni(100), neither enantiomeric separation nor differences in adsorption kinetics between magnetic domains were observed. [ 24,25 ]…”