2, CPT-CNRS, Marseille, , France
3, MOLTECH-CNRS, Angers, , France
4, Univ. of Mons, Mons, , Belgium
5, NTT-BRL, Kanagawa, , Japan
We present the properties of molecular junctions fabricated on a large array of sub-10 nm single crystal Au nanodot electrodes (nanodot-molecule junctions : NMJ), contacted by conducting AFM (C-AFM) [1-4] or interferometric scanning probe microscope (iSMM) . Each individual NMJ consists of less than one hundred molecules. Typically, 2000-4000 NMJs are measured in a few μm2 C-AFM image. Conductance histograms and 2D histograms of the current-voltage (I-V) curves are extracted from these measurements [2, 3]. We report a C-AFM study of π-π interactions from electrochemical and conductance measurements on a large array of ferrocene-thiolated gold nanocrystals . Despite substantial theoretical progress, a direct experimental measurement of the π-π electronic coupling energy parameter t has remained challenging due to molecular structural variability and the large number of parameters that affect the charge transport. We confirm the theoretical prediction  that t can be assessed from a statistical analysis of current histograms for thousands of nanocrystals. The extracted value of t ≈ 35 meV is in the expected range based on our density functional theory analysis. Furthermore, the t distribution is not necessarily Gaussian and could be used as an ultrasensitive technique to assess intermolecular distance fluctuation at the sub-angström level. Molecular electronics originally promised that molecule(s) bridging two or more electrodes would generate electronic functions and overcome scaling limits of conventional technology. However, so far, these molecular devices have been only demonstrated at low frequency. Here, we demonstrate molecular diodes operating up to 18 GHz with an estimated cut-off frequency of 520 GHz . With the iSMM, we demonstrate microwave molecular diodes. The DC and RF properties were simultaneously measured on a large array of NMJs with ferrocenyl undecanethiol molecules, contacted by the tip an interferometric scanning microwave microscope (iSMM). We show that the measured S11 parameters exhibit a diode rectification ratio up to 12 dB and up to 18 GHz. This feature can be directly related to the DC conductance and aF range fringing capacitance of the molecular junctions. The RF and DC data are combined using simple model from which we extract a high conductances up to 0.36 mS and aF range of fringe capacitances. From these results, we extrapolate a cut-off frequency of 520 GHz for the molecular diode .
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