One concept to build up hybrid electronic devices based on molecules or nanoparticles with rectifying properties is based on nanoscale objects that are immobilized between two electrodes composed of different metals forming asymmetric contacts. Following this concept, we introduce an optimized procedure to fabricate heterometallic nanoelectrodes with a separation of only 5 nm. Gold nanoparticles (AuNPs) with a diameter of 15 nm, stabilized with 4-mercaptophenylamine, were used to form electrode1molecule/AuNP/molecule-electrode2 devices comprising at most a small number of AuNPs. Immobilization was performed by dielectrophoretic trapping. The molecular properties of 4-mercaptophenylamine are reflected in transition voltage spectroscopy features of the device. Cyclic current−voltage measurements on 20 functional devices revealed distinct differences in conductivities based on minor differences in device geometry. Analysis of the electron transport characteristics discloses that under these experimental conditions an asymmetric contact configuration alone is not sufficient for building up a molecule-based rectifier.

Electronic transport measurements of single, systematically varied 4,4'-bis(mercaptoalkyl)-biphenyl derivatives (MABP) are performed in a controlled test-device. The molecules are composed of a central biphenyl unit (BP) carrying two mercaptoalkyl substituents with different chain lengths (m, n = number of CH(2)-units), in the para-position of the BP unit. The total length of both spacers is m + n = 10. The molecular conductance of these individual MABPs deposited on Au (111) substrates is studied using STM-based break junctions. It is shown that the molecular conductance depends on the relative position of the BP unit within the molecule. In the case of the symmetric derivative 5BP5 a value of 0.07 +/- 0.01 nS is obtained, while for 1BP9 the molecular conductance is doubled and a value of 0.17 +/- 0.03 nS results. This relatively high value of conductance for the single Au(tip)-1BP9-Au(substrate) junction is attributed to an increased coupling of the BP unit to the adjacent electrode, i.e. the STM-tip or the Au-substrate. We address the role of the specific contact situation (-S-Au) and of the position of the electrically active molecular moiety and thus come to a deeper understanding of the electronic transport properties of 4,4'-bis(mercaptoalkyl)biphenyl derivatives.

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The synthesis, adsorption behavior, surface structure, and the charge transport properties of a persulfurated coronene asterisk with a 3D-polyaromatic system, namely dodecakis(phenylthio)coronene (DPTC), deposited on HOPG(0001) and Au(111) surfaces, are investigated by means of scanning tunneling microscopy (STM) and spectroscopy (STS). DPTC molecules adsorbed on HOPG (0001) show an orbital mediated tunneling through mainly undisturbed frontier molecular states. DPTC molecules self-assemble on Au(111) into a highly ordered p-stacked columnar ''edge-on pattern. The columnar stacking is a gold surface mediated process, as ascertained by fluorescence correlation spectroscopy (FCS). DPTC was monomeric in the precursor solution before assembly. The tunneling spectra of ordered DPTC stacks on Au (111) show an energetic splitting of the frontier molecular states, indicating orbital overlap and supramolecular p-p interactions of adjacent molecules. DPTCs are sufficiently flexible to facilitate dense 1D stacks. The multiple aryl-sulfur bonds play a dominant role in the modulation of the self-assembly properties of the coronenes which in turn affect their electronic properties. Our results encourage further applications in dendrimer chemistry toward molecular electronics, by using the functionalized coronene core and its multidirectional 3D properties.

Magnetic-field-controlled switching of a nonlinear three-terminal nanojunction was studied. Noise-activated switching was observed, which depends sensitively on the strength of an external magnetic field. Such a functioning resembles a noise-activated nonlinear detector as proposed by Gammaitoni and Bulsara [Phys. Rev. Lett. 88, 230601 (2002)] for magnetic fields realized with a nanoelectronic device.