Semiconductor surfaces modified with organic self-assembled monolayers (SAMs) are of technological relevance in nanoscience, and their study can provide important insights into charge transport and charge generation occurring at the molecular interface. We prepared α,ω-dithiol SAMs on Gallium Arsenide (GaAs) substrates and used a scanning tunneling microscope (STM) to characterize their properties. The STM tip is brought into contact with the SAM to provide a semiconductor / molecule(s) / metal junction, behaving as a Schottky diode. The nature of the organic backbone is strongly correlated to the rectification ratio of the fabricated device, and we were also able to detect the contribution of a <i>single</i> molecule to the overall charge transport using time-dependent STM techniques. As GaAs is a III-V direct bandgap (1.42 eV at room temperature) semiconductor, it also efficiently absorbs light to generate a photocurrent, which is transported to the STM tip through the bridging organic dithiol. Fine control of the optoelectronic response of the device can therefore be exerted through appropriate choice of semiconductor type and doping density, nature of the molecular bridge, and light intensity and wavelength. For instance, alkyl bridges on low-doping GaAs are highly rectifying devices in the dark (&gt; 10<sup>3</sup>) and display appreciable photocurrent upon illumination. Charge carrier trapping at the semiconducting interface can also be observed. Molecular orbital alignment greatly influences the response of the device, and conjugated molecular bridges give enhanced photocurrent compared with saturated molecular wires. The portion of the semiconductor responsible for the observed behavior is the space charge region, only a few tens of nanometers in size, and semiconducting nanoelectrodes could therefore be used to probe molecular properties, while the possibility of optical spin-polarization of the charge carriers expands the range of applications of these devices to the field of spintronics.

Molecular photoelectronics at a rectifying gallium arsenide interface

255th National Meeting (2018)

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