One Carbon Matters: The Origin and Reversal of Odd-Even Effects in Molecular Diodes with Self-Assembled Monolayers of Ferrocenyl-Alkanethiolates

We investigated the origin of odd-even effects in molecular diodes based on self-assembled monolayers (SAMs) of ferrocenyl-terminated n-alkanethiolates S(CH₂)_nFc with n = 6-15 on Ag or Au surfaces contacted with EGaIn top electrodes. These SAMs have different M-S-C bond angles of 180° when M = Ag and 104°when M = Au causing a multitude of odd-even effects in the performance of the diodes. By changing the M-S-C bond angles and using several characterization techniques, we were able to systematically identify and rationalize odd-even effects in the electronic structure of the device. Changing n from 6 to 15 resulted in an odd-even effect in the tilt angle of the Fc units (α), which, in turn, caused odd-even effects in the surface dipole, work function, and HOMO onset (HOMO = highest occupied molecular orbital). These odd-even effects caused an odd-even modulation of the tunneling current across the diode in the on state (the current that flows across the junctions when the diode allows the current to pass through). The current that flows across the diodes in their off state (the leakage current) also followed an odd-even effect that was related to an odd-even effect in the packing energy: SAMs with small tilt angles of the ferrocenyl units α (with the Fc units standing up) pack better than SAMs with large α values (with the Fc units in a parallel orientation with the plane of the electrode). All these odd-even effects were completely and consistently reversed when the Ag electrodes were replaced with Au electrodes proving they are induced by the M-S-C bond angle.