Scanning Tunneling Microscopic Characterization of an Engineered Organic Molecule

Report No. ARL-TR-5639
Authors: Govind Mallick, Pamela Kaste, and Shashi P. Karna
Date/Pages: August 2011; 22 pages
Abstract: Surface topology and electron transport properties of self-assembled monolayer (SAM) of an engineered molecule 4,4'-[1,4-phenylenebis(methylidynenitrilo)]bisbenzenethiol (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 π-bond dominated PMNBT molecule was engineered using first-principle ab initio molecular orbital theory. The estimated conductance, (dI/dV)v=0.75 = 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.
Distribution: Approved for public release
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Last Update / Reviewed: August 1, 2011