Subnanometer Resolution and Enhanced Friction Contrast at the Surface of Perylene Diimide PDI8-CN​₂ Thin Films in Ambient Conditions​

We report high-resolution surface morphology and friction force maps of poly­crystalline organic thin films derived by deposition of the n-type perylene diimide semi­conductor PDI8-CN₂. We show that the in-plane molecular arrange­ment into ordered, cofacial slip-stacked rows results in a largely anisotropic surface structure, with a characteristic sawtooth corrugation of a few Ångstroms wavelength and height. Load-controlled experi­ments reveal different types of friction contrast between the alternating sloped and stepped regions, with transitions from atomic-scale dissi­pative stick-slip to smooth sliding with ultralow friction within the surface unit cell. Notably, such a rich pheno­menology is captured under ambient conditions. We demonstrate that friction contrast is well reproduced by numerical simu­lations assuming a reduced corrugation of the tip-molecule potential nearby the step edges. We propose that the side alkyl chains pack into a compact low-surface-energy overlayer, and friction modu­lation reflects periodic hetero­geneity of chains bending properties and sub­surface anchoring to the pery­lene cores.