Optical and electrical addressing in molecule-based logic circuits (2012)

By using state of the art industrial production methods it is possible to fabricate device geometries with lateral dimensions of 22 nm. A further reduction of these components is accompanied by a fundamental change of electronic properties, if a certain threshold is overcome.Organic molecules applied as functional components provide new possibilities in the nanometer scale. The first step, which is necessary for the development of molecular electronics, is the definition of reliable electrical contacts to individual molecules. It has been shown that this implicates major difficulties. Therefore, the aimed target of the presented experiments is to develop reliable contacts between molecular capped nanoparticles and metallic electrodes, and to mimic logic circuits in such an arrangement of nanoparticles and electrodes. In order to realize this aim nano-electrodes with a distance of only a few nanometers (3 ± 1 nm) were developed in an optimized electron-beam lithography process, using the vector graphics EBPG5000+ electron beam writer from Vistec. This precision was achieved by developing an optimized two layer E-beam process, in combination with a customized system of developers. Moreover, an optimal geometry of the electrodes was elaborated including short-range scattering effects in the considerations.In this work, individual nanoparticles with a diameter of a few nanometer were immobilized between electrodes with a shorter tip separation. Charged and uncharged nanoparticles were immobilized applying dielectrophoretic trapping, i.e. the application of an AC, or DC, voltage over the nanoelectrodes. In this configuration the obtained tunnel junctions are based on a few (5-10) molecules per tunnel junction. The focus of this work is the characterization of tunnel junctions and the thoroughly analysis of the transport mechanisms involved, such as Arrhenius, super-exchange, granular metal-based transport or tunneling. The total conductivity across the gap formed by the nanoelectrodes is shown to be critically dependent upon how perfectly the NP fills the gap. For nanogaps partially and completely filled with a nanoparticle, respectively, representative measurements are shown. From transport measurements characteristic molecule parameters as the tunneling barrier height of the BP3 molecules and the remaining vacuum gap-width are obtained by a combination of Simmons fit and transition voltage spectroscopy. Surprisingly, the transport properties of the ’electrode/ molecule-nanoparticle-molecule/electrode’-contact were dominated by molecular properties. Scanning tunneling microscopic measurements on the applied molecules have shown astonishing similarities in the I(U) characteristics. Furthermore, the observed energy levels could be assigned to molecular energy levels. Nanoparticles with a diameter of less than 5 nm have shown Coulomb blockade, an electrostatic phenomenon, already at room temperature. Coulomb blockade diamonds were observed in nanoparticle based devices with transistor geometry.Among others functionalized, photo-isomeric molecules, which reversibly change the atomic arrangement under irradiation of UV light, were applied. The two isomer conformations show dramatically different transport properties.Beyond the application as a light-controlled switch with such a type of device it is also possible to mimic, with a suitable choice of parameters, a complete full adder circuit. The aim in this work was not only a reduction of existing bricks of current CMOS devices, but the implementation of complex logic circuits in a single bricked unit.