Turning Around a Molecular Diode
Abstract Details:
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 ultra-flat 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.1,2
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 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. Thus, we are able to control the direction of rectification at the molecular level.
References
1) Nijhuis, C. A.; Reus, W. F.; Barber, J.; Dickey, M. D.; Whitesides, G. M. Nano Lett. 2010, 10, 3611.2) Nijhuis, C. A.; Reus, W. F.; Siegel, A. C.; Whitesides, G. M. J. Am. Chem. Soc. 2011, 133, 15397
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