Resonant Tunneling and the Substituent Effect on Negative Differential Resistance in a Molecular Junction
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Resonant Tunneling and the Substituent Effect on Negative Differential Resistance in a Molecular Junction
Nikita Matsunaga* and Karl Sohlberg** * Department of Chemistry and Biochemistry, Long Island University, Brooklyn, NY 11201 ** Department of Chemistry, Drexel University, Philadelphia, PA 19104
ABSTRACT Recently there has been an explosion of interest in the potential use of individual molecules as electronic device elements. The electrical characteristics of molecular junctions, individual molecules spanning the gap between two metal electrodes, have been reported and certain molecular species have been found to exhibit highly nonlinear current versus appliedvoltage (I/V) properties. Intriguingly, these nonlinearities (pronounced peaks) in the I/V behavior are extremely sensitive to the functionalization of the molecule forming the junction. The substitution of a single functional group can completely eliminate the nonlinear behavior. Many have suggested that resonant tunneling could lead to the observed nonlinearities. Resonant tunneling requires a double potential barrier along the electron transfer coordinate. We propose a possible physical origin for such a double potential barrier and support the model with first principles electronic structure calculations. Next we discuss a quantum mechanical tunneling model for electron transport through the double potential barrier. The model gives insight into the origin of nonlinear I/V behavior in molecular junctions and the effect of substituent functional groups on the junction molecule. INTRODUCTION Conjugated organic molecules have been identified as a strong candidate for molecules that can serve as molecular electronic device elements [1,2]. Particularly promising are those molecules exhibiting negative differential resistance (NDR) since the property of NDR is critical to the function of traditional solid-state semiconductor devices [3]. Through a combination of self assembly techniques and a mechanical break junction, Reed et al [4] have demonstrated charge transport through single molecules. Through measurements on very small numbers of molecules, Chen et al. [3,5] have discovered molecular junctions exhibiting the property of NDR. To date, there has been considerable interest in the mechanism of NDR in molecular junctions. One possibility that has been proposed is that conduction arises from resonant tunneling [6,7]. A key observation is that in molecular junctions, NDR is highly dependent upon the functionalization of the molecule forming the junction [5]. This is strong evidence that the electronic structure of the molecule forming the junction is intimately tied to the NDR. Herein, we suggest a possible physical origin of the double potential barrier required for resonant tunneling. The model is then dressed with ab initio molecular electronic structure calculations. This model offers an explanation for the dependence of NDR on the junction molecule's functionalization.
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THEORY Electronic structure calculations were carried out for several
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