We investigated the self-assembly of trimesic acid (TMA) at the solution–graphite interface by using scanning tunneling microscopy (STM). We show that the polymorphism of the adsorbate structures of TMA can be controlled by the substrate temperature during the deposition of the molecules out of the solution for various solvents of different polarity. TMA was dissolved in phenyloctane, octanoic acid, and undecanol. At elevated substrate temperatures, various periodic assemblies of TMA could be obtained. By increasing the temperature of the preheated substrate, the specific 2D supramolecular network structure and the corresponding packing density can be precisely tuned in each kind of the solvents studied. The results found by STM are explained by the increased concentration of the solution at the preheated substrate as well as the higher mobility of the solute molecules increasing the opportunity of interactions between the molecules, in particular different hydrogen bonding motifs. Our interpretation is supported by simulations for each structure using the semiempirical quantum-chemical method PM6-DH+.

Highly efficient magnetic spin memory devices operating at ambient temperatures were recently realized via self-assembled molecular layers of an enantiomeric phase of polyalanine (PA). However, the repeatability and yield of such molecular devices rely sensitively on the layer quality. In this work, we grew molecular layers of α-L-PA (36 monomers of αamino acid, 5.4 nm) on highly oriented pyrolytic graphite surfaces and investigated the structure by scanning tunneling microscopy under ambient conditions at the liquid−solid interface, complemented by X-ray photoelectron and Raman spectroscopies. Our results suggest that interdigitation of adjacent molecules takes place, which leads to a packing density of 0.8 PA/nm 2 , which is more than a factor of 2 higher than that of the closed-packed structure. The interdigitation not only stabilizes the PA film by additional H bonds but also ensures chiral alignment between adjacent molecules, which may be a prerequisite for a coherent electron transmission.

Polyalanine molecules (PA) with an α-helix conformation have recently attracted a great deal of interest, as the propagation of electrons through the chiral backbone structure comes along with spin polarization of the transmitted electrons. By means of scanning tunneling microscopy and spectroscopy under ambient conditions, PA molecules adsorbed on surfaces of epitaxial magnetic Al 2 O 3 / Pt/Au/Co/Au nanostructures with perpendicular anisotropy were studied. Thereby, a correlation between the PA molecules ordering at the surface with the electron tunneling across this hybrid system as a function of the substrate magnetization orientation as well as the coverage density and helicity of the PA molecules was observed. The highest spin polarization values, P, were found for well-ordered selfassembled monolayers and with a defined chemical coupling of the molecules to the magnetic substrate surface, showing that the current-induced spin selectivity is a cooperative effect. Thereby, P deduced from the electron transmission along unoccupied molecular orbitals of the chiral molecules is larger as compared to values derived from the occupied molecular orbitals. Apparently, the larger orbital overlap results in a higher electron mobility, yielding a higher P value. By switching the magnetization direction of the Co layer, it was demonstrated that the non-spin-polarized STM can be used to study chiral molecules with a submolecular resolution, to detect properties of buried magnetic layers and to detect the spin polarization of the molecules from the change in the magnetoresistance of such hybrid structures.

The self-assembly process of racemic mixtures of right (L)-and left (D)-handed α-polyalanine molecules (DL-PA) with a length of 4 nm (27 alanine units) and a helix pitch of λ = 0.54 nm on HOPG was investigated by scanning tunneling microscopy and spectroscopy under ambient conditions. While enantiopure PA molecules adsorb in a hexagonally close-packed structure, we found heterochiral dimers with a rectangular unit cell for DL-PA. Despite the steric hindrance, the packing density of the DL-PA heterophase is increased by 25% compared to the enantiopure PA structure. Apparently, this is achieved by shifting D-and L-PA along their helical axis by λ/2 to enable the formation of intermolecular hydrogen bonds. Our results support Wallach's empirical rule also for molecular 2D layers on surfaces, where intermolecular interactions are mediated by dipolar forces and Hbonds rather than dispersing forces. Moreover, the α-helix structure of the PA molecules seems to be preserved; thus, electrostatic forces indeed play an important role for the formation and stabilization of the helical structure.

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