Bias and molecular-length dependent odd–even effect of rectification in 4′-methyl-2,2′-bipyridyl-terminated n-alkanethiolate single-molecule diodes

Abstract

Relating the electronic transport properties of single-molecule devices to the geometric and/or electronic structures of the active molecular components inside them has always been an appealing pursuit but a challenging task in molecular electronics. Here, using the first-principles calculations, the rectification performance in single-molecule diodes composed of a 4′-methyl-2,2′-bipyridyl-terminated n-alkanethiolate (SC_nBIPY, n = 6–14) has been theoretically investigated. Pronounced rectification has been found in these single-molecule diodes, which can be ascribed to the monotonic evolution of conducting molecular orbitals spatially governed by the BIPY terminal group under bias voltage. Interestingly, an odd–even effect has been demonstrated for the rectifying performance as the length of the alkyl chain (i.e., n) varies, which has also been observed in structure-similar ferrocene-alkanethiolate diodes. Moreover, it has been found that the odd–even effect of the rectification can be reversed by changing the external bias voltage and the length of the alkyl chain n. Further analysis reveals that the odd–even effect in SC_nBIPY is dominated by the behaviors of forward current against n under bias voltage, which is sharply different from the case of ferrocene-alkanethiolate diodes. The behaviors of forward current (and hence the rectification performance) are closely related to the alignment of conducting molecular orbitals with the Fermi energy and are substantially changed by whether the diodes are in the resonant or non-resonant charge transport regime, which can be switched by the bias voltage and also the length of the alkyl chain. The results here provide a further understanding of the rectification in such diodes or structure-similar analogues, which is essential for the future design of single-molecule diodes with high performance.