A molecular-scale portrait of domain imaging in organic surfaces

Progress in the general understanding of structure-property relationships in organic devices requires experimental tools capable of imaging structural details, as molecular packing or domain attributes, featuring ultra-thin films. An operation mode of scanning force microscopy, related to friction force microscopy (FFM) and known as transverse shear microscopy (TSM), has demonstrated the ability to reveal the orientation of crystalline domains in organic surfaces with nanometer resolution. In spite of those promising results, numerous questions remain about the physical origin of the TSM domain imaging mechanism. Taking as benchmark a PTCDI-C8 sub-monolayer, we demonstrate experimentally and theoretically that such mechanism is the same atomic scale stick-slip ruling FFM leading to the angular dependence of both signals. Lattice-resolved images acquired on top of differently oriented PTCDI-C8 molecular domains are crucial to permit azimuthal sampling, without the need of sample rotation. The simulations reveal that, though the surface crystallography is the direct cause of the FFM and TSM signals, the manifestation of anisotropy will largely depend on the amplitude of the surface potential corrugation as well as on temperature. This work provides a novel nanoscale strategy for the quantitative analysis of organic thin films based on their nanotribological response.