Molecular diodes: an altered polarity for rectification in ultra-thin films where the orientation of the dipole is reversed

Langmuir-Blodgett (LB) monolayers of donor-(π-bridge)-acceptor dyes exhibit asymmetric current-voltage characteristics when aligned on gold-coated substrates and contacted by Au or PtIr probes. The molecules align with their hydrophobic alkyl tails pointing upwards and exhibit rectification in opposite quadrants of the I-V plot when substituted at the acceptor end [D-π-A-C_nH_2n+1] and donor end [A-π-D-C_nH_2n+1]. The polarity is dependent upon dipole orientation and, in each case, the electron flow at forward bias is from the cathode to acceptor on one side of the device and from the donor to anode on the other side. Self-assembled monolayers (SAMs) in which the acceptor end is chemisorbed to a gold substrate [Au-S-C_nH_2n-A-π-D] exhibit asymmetric I-V characteristics that comply with the above and the behavior is assigned to molecular rectification.