Abstract
We demonstrate here a new concept for a metal-molecule-semiconductor nanodevice employing Au and GaAs contacts that acts as a photodiode. Current-voltage traces for such junctions are recorded using a STM, and the "blinking" or "I(t)" method is used to record electrical behavior at the single-molecule level in the dark and under illumination, with both low and highly doped GaAs samples and with two different types of molecular bridge: nonconjugated pentanedithiol and the more conjugated 1,4-phenylene(dimethanethiol). Junctions with highly doped GaAs show poor rectification in the dark and a low photocurrent, while junctions with low doped GaAs show particularly high rectification ratios in the dark (>103 for a 1.5 V bias potential) and a high photocurrent in reverse bias. In low doped GaAs, the greater thickness of the depletion layer not only reduces the reverse bias leakage current, but also increases the volume that contributes to the photocurrent, an effect amplified by the point contact geometry of the junction. Furthermore, since photogenerated holes tunnel to the metal electrode assisted by the HOMO of the molecular bridge, the choice of the latter has a strong influence on both the steady state and transient metal-molecule-semiconductor photodiode response. The control of junction current via photogenerated charge carriers adds new functionality to single-molecule nanodevices.
Highlights
The use of single molecules as active electronic components in a device has developed from a scientific curiosity to an important tool in the study of the fundamental quantum phenomena dominating charge transport at the nanoscale
In a metal−molecule− semiconductor (MMS) junction, the semiconducting electrode can impart a rectifying behavior because charge carrier depletion at the junction allows a larger charge flow when the device is biased in one direction than the other
Do we measure the reverse bias photocurrent through a single molecule for the first time, and we demonstrate a novel feature of this type of junction, namely that when a reverse bias is applied the photocurrent does not saturate as it would in a planar device
Summary
Letter electrode because it is straightforward to build densely packed self-assembled monolayers (SAMs) on its unoxidized surface[19−21] and because it has a high electron mobility. To characterize the photocurrent response further, with the feedback loop disabled and the tip in shallow contact with the monolayer, we used an optical chopper to alternate the junction between dark and illuminated conditions and obtained timedependent reverse bias photocurrent traces (Figure 4). 1,4-Phenylene(dimethanethiol) (1Ph1) on GaAsLD showed high rectification ratios in the dark, though lower than PDT (>102), but an even higher photocurrent response upon laser illumination, with values larger than 10 nA at 1.5 V reverse bias Both these results can be explained by the energy alignment between the frontier molecular orbitals and the metal Fermi level. Histograms compiled from sliced traces in (b) forward bias and (d) reverse bias in the dark or (f) under illumination, with relative density map (inset) showing the stability over time of a single-molecule junction. As the charge carriers could be spin-polarized by illuminating GaAs with circularly polarized light, our work paves the way to single-molecule photospintronics
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