| In this paper, we firstly introduce the recent development of molecular devices, molecular rectifiers, and theoretic basis of molecular electronics. On this basis, we concentrate on the theoretical analysis of transport properties and rectifying performances of molecular rectifiers. Using the first-principles method ,we design the A-R rectifier based on the D-σ-A molecules to examine the rectifying performances. The calculated results show that the electronic structures for all of our systems perfectly match the A-R rectifier, as expected, but their rectifying direction is very strikingly opposite and working mechanism is completely different. This behavior can be rationalized through an asymmetrical shift of molecular levels under biases of different polarities, which is because of always-existing intrinsic asymmetrical coupling effects of molecular levels to electrodes. Detailed analysis demonstrates that the rectifying direction induced by this mechanism is always in opposition to that induced by A-R mechanism. It means that there always exist two types of intrinsic mutual competing rectifying mechanisms: A-R mechanism and asymmetrical potential drop mechanism. The resulting rectifying direction completely depends on which mechanism is a determinate factor. Next, we investigate rectifying performances of D-π-A molecules based on cyano-vinyl aniline derivatives. The calculated results also show that the rectifying direction of our models is opposite to the A-R mechanism due to the asymmetric shift of molecular levels under biases of different polarities. In addition, we constructed molecular devices based on a molecule coupling with both metal electrodes with different materials and investigated their transport properties by the first-principles method. The result shows that such devices can generate two asymmetrical Schottky barriers at contacts; the current rectification thus is created. This rectification can be also fully rationalized by the calculated transmission spectra and spatial distribution of the LUMO and HOMO states. Our study suggests that it is a very important way for both electrodes using different materials to realize a molecular rectification. At last, we investigate the electronic transport properties and rectifying performance of three different molecular devices based on different molecular configurations resulted from the same molecule. The results show that rotation of a mid-benzene ring (bonding bridge–πbridge) can change the delocalization of a molecular orbital and thus change their transport properties and rectifying performance. This finding suggests that a variation of the bonding bridge orientation is able to control the rectifying performance of a molecular device effectively. It is of important significance for designing a novel molecular rectifier. |
