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 behaviour 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: non-conjugated 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 photo-generated 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 photo-generated charge carriers adds new functionality to single molecule nanodevices.
Vezzoli, A., Brooke, R.J., Higgins, S.J., Schwarzacher, W. and Nichols, R.J., 2017. Single-molecule photocurrent at a metal-molecule-semiconductor junction. Nano Letters.
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Categories: Semiconductor & Process Monitoring