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

• DOI: 10.1021/acs.jpcc.5b04797
• Publication Date: 2015-07-09T17:51:11Z
• Authors: Li Yuan; Damien Thompson; Liang Cao; Nisachol Nerngcha...; Christian A. Nijhuis
• Keywords: 02 engineering and technology; 0210 nano-technology

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 ...