Scanning Tunneling Microscopic Characterization of an Engineered Organic Molecule

Surface topology and electron transport properties of self-assembled monolayer SAM of an engineered molecule 4,4-1,4-phenylenebismethylidynenitrilobisbenzenethiol PMNBT and 1-dodecanethiol dDT adsorbed on Au substrates have been investigated by scanning tunneling microscopy STM at ambient conditions. The electrical measurements of hexadecanethiol hDT, which is similar in length to PMNBT, have also been compared. The pi-bond dominated PMNBT molecule was engineered using first-principle ab initio molecular orbital theory. The estimated conductance, dIdVv0.75 123.91nS for PMNBT, is over an order of magnitude larger than the corresponding value 7.15nS for dDT and two orders of magnitude larger 0.078nS than hDT. The tunneling current I as a function of the applied bias V between STM tip and SAM of PMNBT exhibits asymmetric behavior. A combination of electronic and geometrical effects in the molecule and at the molecule-metal interface is proposed to be responsible for the observed asymmetric I-V characteristics. The increased conductance in PMNBT is also explained in terms of its nearest available electronic states.