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Subnanometer Resolution and Enhanced Friction Contrast at the Surface of Perylene Diimide PDI8-CN2 Thin Films in Ambient Conditions
01.03.2018 17:00

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.

Publication:

Renato Buzio, Andrea Gerbi, Mario Barra, Fabio Chiarella, Enrico Gnecco, Antonio Cassinese: "Subnanometer Resolution and Enhanced Friction Contrast at the Surface of Perylene Diimide PDI8-CN2 Thin Films in Ambient Conditions", Langmuir 34(10) (2018) 3207


Subnano-Resolution - Figure 1

(a) The PDI8-CN2 molecule. (b) Crystal structure of a surface bilayer, viewed along a-axis [100] direction. The side alkyl chains of the top layer (black ellipses) generate a pronounced surface corrugation in the b-axis [010] direction. Vertical arrows indicate the alternating positions of the „slippery” (blue) and dissipative (red) tip sliding motion (see text). (c) Space-filling model of the surface viewed along c* (the height direction of topographies). Alkyl chains (black ellipses) arrange according to an anisotropic oblique unit cell with a=5.019Å, b=8.84Å, and γ=100.15° and form lamellar stripes along [100]. Φ is the angle defined by the [100] direction and the horizontal FFM fast scan direction (dashed line).

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