Photoconductance of organic single-molecule contacts

We study the dc conductance of organic single-molecule contacts in the presence of external electromagnetic radiation (photoconductance). In agreement with previous predictions, we find that the radiation can lead to large enhancements of the conductance of such contacts by bringing off-resonant levels into resonance through photoassisted processes. In our analysis, we make use of the simplifying fact that, under certain assumptions, the photoconductance can be expressed in terms of the transmission function in the absence of the radiation. The conductance enhancement is demonstrated for oligophenylene molecules between gold electrodes, whose electronic structure is calculated based on density-functional theory. It is shown that the exponential decay of the conductance with the length of the molecule can be replaced by a length-independent value in the presence of radiation.

Figure 1

(Color online) (a) The four molecular contacts R1–R4 we have studied, containing oligophenylenes with one to four phenyl rings and coupled to Au [111] pyramids through sulfur atoms. (b) Our model assumes the induced ac voltage $V_{\mathit{ac}}$ to drop in a double-step manner, as illustrated here schematically for contact R4.

Figure 2

(Color online) (a) Transmission versus energy $\left[T\left(E\right)\right]$ for the contacts R1–R4 in Fig. 1 (dash-dot-dotted, dash-dotted, dashed, and solid lines, respectively). [(b)–(e)] The photoconductance versus external frequency $\omega$ for the contacts R1–R4, respectively. For each case, the results for the following values of $\alpha$ are shown: 0.2, 0.6, 1.0., 1.4, and 1.8, in order of increasing conductance. (f) The dc conductances in the absence ($G_1$, dots) and presence ($G_2$, crosses) of radiation with $\hslash \omega =1.5\mathrm{eV}$ and $\alpha =1.8$ for an increasing number $n$ of phenyl rings. The gray line is a fit of the $G_1$ results to an exponential law (see text).