Current rectification by molecules with asymmetric tunneling barriers

Abstract

A simple experimentally accessible realization of current rectification by molecules (molecular films) bridging metal electrodes is described. It is based on the spatial asymmetry of the molecule and requires only one resonant conducting molecular level (π orbital). The rectification, which is due to asymmetric coupling of the level to the electrodes by tunnel barriers, is largely independent of the work function difference between the two electrodes. Results of extensive numerical studies of the family of suggested molecular rectifiers HS-(CH2)m-C6H4-(CH2)n-SH are presented. The highest rectification ratio ∼ 500 is achieved at m = 2 and n = 10.

Controlling the direction of rectification in a molecular diode

“…, which gives the ratio of the voltage drop between the molecule and the right electrode 34,38 . In principle, rectification can only occur when Z V a½, that is, when the junction is asymmetric and the Fc units are close to the left or right electrodes.…”

The Porter-Whitesides Discrepancy: Revisiting Odd-Even Effects in Wetting Properties of n-Alkanethiolate SAMs

“…The ferrocene terminated alkanethiols are widely studied in charge transport as molecular diodes. In these molecules, however, a reduction in width of the tunneling barrier is only observed at forward bias, due to the fact that the highest occupied molecular orbital (HOMO) of the Fc falls between Fermi levels of both electrodes and, as such, charge can hop into the Fc units; at reserve bias, the tunneling barrier is larger since the charges cannot be injected into Fc frontier orbitals [35]. Such a difference between forward and reverse bias leads to current rectification, forming a molecular diode.…”

“…The origin of such an odd-even effect is significantly influenced by the large difference in dielectric constant for the odds (εr,odd) and the evens (εr,even), and is suggested to stem from the odd-even effect in the twist angle of the alkanethiol molecule, γ [35].…”

Nano-Molecular Junctions on STM Tips

“…For the FFBT and BPEBT molecules the preferred indentation for I-V measurement appears to be I t ∼ 5 nA, whereas at control currents I t =0 to ∼2 nA there is a vacuum barrier or weak mechanical contact (as discussed below for the IV data) and for I t > 10 nA the mechanical contact is excessive. Figure 4 shows three sets of I-V curves of the molecule FFBT which is expected to exhibit diode properties [22][23][24] in the absence of conformation change. The spectra are taken at different tip-sample separations on the corresponding I-Z curve of Fig.…”

“…Since the electrical contact appears, the rectification may arise from the different charge injection barriers for positive or negative sample polarity (as above) or from asymmetric alignment of energy levels across the molecule. To elaborate, we follow the discussion of Williams and co-workers [24] who outlined a simple model of molecular rectification, where a single electroactive unit was positioned asymmetrically with respect to electrodes and the HOMO and LUMO were positioned asymmetrically with respect to the Fermi level. In FFBT, the required conditions for rectification by an asymmetric barrier and asymmetric arrangement of the electrode Fermi energy with respect to the HOMO and LUMO are well met, since the HOMO and LUMO are 5.75 and 2.84 eV respectively, and the Au work function is 5.1 eV.…”