Tunnelling conductance of vectorial porphyrin monolayers

Abstract

A method for antipodal attachment of sulfur linkers to a nickel chelated porphyrin has been developed to facilitate robust chemisorption at gold electrodes. At sufficiently high molecular coverage ellipsometric, infrared and electroanalytical data are supportive of a close-to-vertical molecular orientation with respect to the substrate. Molecular conductance was assessed both within neat porphyrin monolayer films and on individual molecules isolated within an ordered saturated alkyl thiol (1-tetradecanethiol) self-assembled monolayer and then capped with chemisorbed gold nanoparticles. In the former, three distinct groups of current–voltage (I–V) curves were observed both by scanning tunnelling microscopy (STM) and conductive probe atomic force microscopy (CP-AFM), corresponding to the conductance of one, two, and three molecules, which were quantizable as integer multiples of a fundamental molecular conductance (1.37 ± 0.25 nS). This value compares well with that obtained on isolated molecules and is significantly higher than the host matrix. In all cases I–V trends are well described by a non-resonant tunnelling model.