Turning Around a Diode at the Molecular Level
Christian A. Nijhuis
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore
117543. email: chmca@nus.edu.sg
Understanding the mechanisms of charge transport across molecules, or self-assembled
monolayers, is important in organic based photovoltaics, OLEDs, energy storage, bio-electronic
devices, etc. Issues regarding how molecular orbitals couple to the electrodes, or the influence of
subtle changes of the intermolecular interactions between the molecules in the SAMs, on the
performance of molecular electronic devices has rarely experimentally been addressed.
We identified a molecular diode: junctions with SAMs of S(CH2)11Fc, i.e., SAMs with
ferrocene at the top, on Ag bottom-electrodes contacted with EGaIn (an eutectic alloy of Ga and
In) top-electrodes rectified currents with a rectification ratio of 100, while those junctions with
SAMs of S(CH2)10CH3, i.e, SAMs without Fc units, did not rectify.
To study the influence of subtle changes in the intermolecular interactions between the
molecules in the SAMs, and the coupling of the molecular orbitals with the electrodes, we
fabricated devices with a series molecules of the type S(CH2)nFc (with n = 3, 4, …, 15) and
SCnFcCn-13 (with n = 1, 2, 3, …, 15), respectively. We found that devices with n is odd are ten
times better diodes than those devices with n = even. This so-called odd-even effect originates
from small changes in the structures of the SAMs, which, in turn, depends on the van der Waals
interactions between the molecules in the SAM. By controlling the coupling of the molecular
orbitals with the electrodes, we succeeded at “turning” around a diode at the molecular level.
Biography
Christian A. Nijhuis received his Masters degree in Chemistry from the University of
Groningen in 2002, and Ph.D. degree from University of Twente in 2006 (Cum Laude; top 5%).
Under the direction of Professor David N. Reinhoudt, his doctoral thesis included studies on the
surface chemistry of supramolecular assemblies and their use in bottom-up nano-fabrication. He
received the Simon Stevin Research award from the Netherlands Organization for Scientific
Research (NWO) in 2006 to conduct overseas research. In the group of Professor George M.
Whitesides, as a postdoctoral research fellow, he developed a platform for measurements of
charge transport across layers that are one molecule thick. In 2010, he received the NRF research
fellowship and he joined the Department of Chemistry at the National University of Singapore as
an Assistant Professor. He currently uses bottom-up nanofabrication techniques to construct selfassembled nano-electronic devices to open up and to solve key problems in physics.