Helical Ordering of α-l-Polyalanine Molecular Layers by Interdigitation

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 α-amino 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/nm2, 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.