Single-Molecule Photocurrent at a Metal–Molecule–Semiconductor Junction

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 (>10 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.

Charge transport at a molecular GaAs nanoscale junction

“…Heavily doped semiconductors will have a smaller SCL, which will allow for a more efficient tunnelling than the much larger SCL found in lightly doped GaAs. 8 It is interesting to note, however, that there is a key difference between p-and n-type GaAs, as the SCL size is also inuenced by the zero-bias Schottky barrier, which is lower for p-type (approx. 0.6 V) than for n-type (approx.…”

“…The bond formation/rupture process results in a change in the number of molecules bridging the tip-semiconductor junction, and the magnitude of the jump is therefore representative of the current owing through an integer number of molecular wires. 1,8,19 We collected for each sample current versus time traces containing hundreds of jumps in forward bias conditions, and analysed them statistically to quantify the single-molecule contribution to the overall current. Forward bias conditions were chosen simply because current is naturally larger than in reverse bias, allowing precise determination of single molecule events.…”

“…The photoelectric response is at the basis of Schottky photodiode behaviour, and we recently reported on this effect in single-molecule junctions, by measuring the reverse bias photocurrent through a molecular bridge. 8 We focussed our efforts on GaAs mainly because of its direct bandgap, which allows for efficient light absorption properties, and higher electron velocity than silicon. Furthermore, extensive literature shows that high-quality self-assembled monolayers (SAMs) can be prepared at a GaAs surface, [9][10][11][12] mainly through formation of As-S bonds.…”

Electrical and spin switches in single‐molecule junctions

“…This implies that a single‐molecule junction with an exciton binding energy similar in magnitude to the HOMO‐Fermi level energy difference may work as a photoswitch with excellent ON‐OFF ratio. Another new concept for a metal‐molecule‐semiconductor device employing Au tip/molecule/GaAs substrate was achieved by Nichols et al With low doped GaAs, molecular junctions displayed rectification ratios as particularly high as >10 3 in the dark and a high photocurrent in reverse biases. It should be noticed that the replacement of gold electrodes with conventional GaAs or silicon semiconductor materials reduces the gap between molecular electronic and traditional microelectronics .…”

Shaping the Atomic‐Scale Geometries of Electrodes to Control Optical and Electrical Performance of Molecular Devices

“…[5][6][7][8][9][10][11][12][13][14][15][16][17] In particular, scanning tunneling microscopy shows strong ability to form different types of molecular junctions (tip and substrate can be different materials), which makes it possible to realize new type of functional devices and to observe novel phenomenon. [18][19][20][21] Meanwhile, the effects of molecular anchors, [22][23][24][25] electrode material, [7,8] and external environment on the properties of single-molecule junctions have been investigated, [25][26][27][28] which is a large step toward the construction of functional molecular electronic devices. Nonetheless, a number of challenges still need to be overcome before single-molecule devices can be widely used as commercial products.…”