Scanning Tunneling Microscope Observation of Current Rectification by Self-Assembled Monolayers of Conjugated Organic Molecules

The current ($I)$ voltage ($V)$ characteristics of HS-C$_{6}$H$_{4}$-CH=N-C$_{6}$H$_{4}$-N=CH-C$_{6}$H$_{4}$-SH$_{,}$ 4,4'-[1,4-phenylenebis(methylidynenitrilo)]bisbenzenethiol (PMNBT), molecules adsorbed on Au (111)/Mica substrate has been determined by scanning tunneling microscope (STM) measurements. The self-assembled monolayers (SAMs) of PMNBT molecules exhibit rectification of the tunneling current. In contrast, SAMs of $\sigma $-bonded dodecanemonothiolate molecules do not exhibit rectification. The observed rectification in the case of PMNBT SAMs is attributed to a combination of (i) charge transfer from Au to molecule at Au-S interface and (ii) a highly delocalized $\pi $-electron charge in the donor level (highest occupied molecular orbital) of the molecule.