Highly ordered structural organization of organic semiconductor monolayers on HOPG and Au(111) — STM studies of alkylphenyl N-substituted perylene diimide at liquid–solid interface

Scanning tunneling microscopy was used to investigate self-organization in mono- or multilayers of an organic semiconductor used for the fabrication of n-channel field effect transistors, namely N,N′-bis(4-n-butylphenyl)-perylene-3,4:9,10-tetracarboxylic-diimide (4-n-BuPh-PTCDI). Two types of substrates were used: HOPG and reconstructed Au(111). All investigations were performed at liquid–solid interface in a trichlorobenzene solution. Monolayers deposited directly on the HOPG show a brick-wall organization with individual molecules lying flat on the substrate surface (the planes of aromatic cores being parallel to this surface). Monolayers obtained by controlled dissolution of a previously deposited multilayer show a different 2D organization in which individual molecules are in the edge-on position i.e. stand perpendicular to the substrate surface with their long axis parallel to it. This finding clearly demonstrates the effect of the upper-lying layers on the first monolayer which changes the balance in the molecule–substrate and molecule–molecule interactions, leading to a rotation of the molecules in the first layer. A very similar structure of densely packed molecules, oriented perpendicular to the substrate, is also found for the monolayer deposited on the Au(111) surface.