Charge Transport through Single Molecules Connected to Semiconductor Electrodes

Abstract

Single molecule electrical measurements have revolutionised our understanding of how charge flows through individual molecules and biomolecules in a wide variety of environments and are important for applications ranging from energy technology, DNA sequencing and interfacial electrochemistry. Such measurements have been typically made with metallic contacts but lately we have managed to extend this to the semiconductor gallium arsenide. This advance is important since future applications of molecular electronics will rely on integrating molecules alongside conventional semiconductor electronic technologies. In a recent publication in Nano Letters [1] we have shown that it is possible to make measurements of single molecules connected at one end to gallium arsenide and at the other end to a gold scanning tunnelling microscope tip. Using this methodology we can record current-voltage response of semiconductor – molecule – metal devices and measure the electrical conductance of single molecules in such junctions. For these measurements of charge transfer at the at the semiconductor-molecule interface the gallium arsenide surfaces are modified with organic self-assembled monolayers. Semiconductor / molecule(s) / metal junctions are formed by bringing a gold STM tip into contact with the molecular monolayer. Such junctions behave as a Schottky diode with the rectification ratio of the fabricated device being dependent on both the nature of the organic backbone and the electronic properties of the semiconductor. As well as recording I-V characteristics with the STM tip contacting the molecular monolayer, by using a time-dependent STM technique we were also able to detect the contribution of a single molecule to charge transport across the molecular junction. As well as showing that it is possible to form single molecule devices contacted to the semiconductor gallium arsenide we have also recently demonstrated that such single molecule devices show a strong photocurrent response.[2] Gallium arsenide is able to efficiently absorb visible light as it is a III-V direct bandgap (1.42 eV at room temperature) semiconductor and illumination of the molecular junction led to large changes in the current-voltage response. The junction configuration is schematically illustrated in the figure from reference [2] which shows a molecule bridging between the GaAs surface and the gold STM tip. The hemispheres in the GaAs substrate in this illustration depict the space charge layer (SCL) from which the charge carriers are collected. The resulting photo-generated charge carriers are transported to the metal electrode (STM tip) through the bridging organic dithiol giving the photo-current. Importantly, the photo-current response in these hybrid junctions can be controlled through the choice of molecular bridge as well as the doping density of the semiconductor amd the light intensity and wavelength.[2] In the dark high current rectification (> 103) is achieved with low-doping GaAs and saturated alkyl bridges and appreciable photocurrent is achieved upon illumination of these junctions. The response of the device is greatly influenced by molecular orbital alignment and this can be controlled by switching from saturated molecular bridges to conjugated ones with lower HOMO-LUMO gaps. Good photodiode functionality can be achieved with a space charge layer of only a few nanometres. Since the response of the molecular junctions is controlled by the combined properties of the semiconductor and the molecular wire this significantly extend of the single-molecule electronics “tool-box” as well as providing new methodologies for studying charge transfer at semiconductor-molecule interfaces. The presentation concludes with discussion of the perspective of single molecule photo-electrochemistry.