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Gate-controlled scanning tunneling spectroscopy of CoPc molecules on graphene

Gate-controlled scanning tunneling spectroscopy of CoPc molecules on graphene

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BOUVRON, Samuel, 2014. Gate-controlled scanning tunneling spectroscopy of CoPc molecules on graphene [Dissertation]. Konstanz: University of Konstanz

@phdthesis{Bouvron2014Gatec-28793, title={Gate-controlled scanning tunneling spectroscopy of CoPc molecules on graphene}, year={2014}, author={Bouvron, Samuel}, address={Konstanz}, school={Universität Konstanz} }

2014-08-19T13:22:44Z Bouvron, Samuel 2014 2014-08-19T13:22:44Z Molecular electronics is based on the use of single molecules as elementary functional electronic components. It is considered to be a promising strategy for further miniaturization of electronic circuits and long-term replacement of semiconductor technology. Moreover the quantum mechanical nature of molecules may be used to design new logical functions, beyond the electron-charge based data processing of classical transistors. With the experimental probing and manipulation of electron and nuclear spin, and the observations of quantum interferences in such systems, this area of research has gained increasing attention in the last decade.<br /><br />Contacting individual molecules in order to perform transport experiments at such a small scale is challenging. Various methods based on the creation of metallic nano-contacts into which molecules are deposited have shown promising results, and already lead to the realization of single-molecule transistors.<br />On the other hand, scanning tunneling microscopy(STM) has also proven to be a successful method for studying single molecules on surfaces and probing some of their electronic properties. Due to geometrical constraints, it is extremely challenging to implement a third gate electrode in such an experiment. This is, however, a prerequisite for controlling the energy of the molecular level responsible for charge transport. The aim of this work was to use graphene on SiO_2/Si as a substrate, Si and graphene being independently electrically contacted and serving as drain- and gate-electrode. The very low density of states of graphene makes it screen only a fraction of the electric field generated by the applied potential on the silicon. A molecule deposited on the graphene is thus placed in a controllable electric field, so that the geometrical arrangement STM tip/molecule/graphene/SiO_2/Si forms a molecular transistor,<br />which makes it possible to tune the molecular levels contributing to charge transport in an STM.<br /><br />A newly acquired low temperature STM was first put into operation. Spatial and energy resolution of the device could be tested, before graphene deposited on various substrates was examined. This consists in a preliminary work laying the basis for the later deposition of molecules onto graphene. STM spectroscopy could furthermore demonstrate the emergence of Landau levels in graphene on NbSe_2 when a magnetic field is applied.<br /><br />The molecules investigated in this work are cobalt phthalocyanines (CoPc). They are planar organic molecules, which have found application in various areas. Since they are thermally stable, they can be easily deposited on various substrates and belong to the most studied molecules in surface science. The adsorption of CoPc molecules on metallic surfaces has been studied first, showing a strong interaction between the molecular orbitals and the substrate, leading to a charge transfer. The insertion of graphene between metallic substrate and molecule was then investigated as a possible buffer layer for decoupling of the molecular levels. STM spectroscopy studies indicate that the molecular energy levels of CoPc on graphene do in fact correspond to the ones of the gas phase. Finally, CoPc molecules on graphene on SiO_2 were studied and their electronic properties probed as a function of the potential applied to the gate. A control of the energy of molecular states contributing to the tunneling current could be achieved, with an efficiency similar to classical transport experiments. The possibility to combine the charge transport characterization of molecules by means of a gate electrode with the spatial resolution of a STM was thus demonstrated. This opens a door towards promising future experiments with a combined control over the spatial position of the source electrode in the picometer-range and over the electronic properties of the studied nanostructures. Gate-controlled scanning tunneling spectroscopy of CoPc molecules on graphene eng Bouvron, Samuel terms-of-use

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