| Abstract Scope |
An unexplored gap exists between electronic transport through a single molecule and a large ensemble of molecules. The results cannot be obviously transposed between both types of molecular junctions. Here we show, by using an array of nanometric gold dots as electrodes, that we can easily get statistics on the conductance of thousands of molecular junctions composed of self-assembled monolayers with less than two hundred molecules. We demonstrate the formation of sub-10 nm single-grain Au dots (down to 5-8 nm) with small dispersion in size and perfect alignment by using e-beam lithography process. An annealing process converts dots from amorphous to single crystals. These nanodots are successfully used as electrodes for chemical and electrical characterization of organic self-assembled monolayers (SAMs) made of less than 200 molecules. The structure of these nanodots are revealed by physical measurements, such as atomic force microscope (AFM), atomic resolution scanning transmission electron microscopy (corrected-STEM) and by chemical techniques using energy dispersive x-ray spectroscopy (EDX). These Au nanodots are used for the fabrication of a large number of a metal-molecule-metal (MMM) nano-junctions. We formed SAMs of alkylthiols (C<SUB>n</SUB>H<SUB>2n+1</SUB>-SH, with n = 8, 12, 18) on these nanodots, and we measured current-voltage characteristics by C-AFM. We analyze conductance characteristics statistically over thousands of junctions using only a single C-AFM image, and this approach gives very significant and valuable statistical data on the electronic transport properties of these molecular devices, in a quicker and easier way compared to other approaches. On the amorphous nanodots, we observe two peaks of conductance whatever the chain length. The two peaks are observed at all applied voltages, but the peak separation is reduced at V > 0.7 V. The two peaks become closer when the chain length increases. On perfect single crystal nanodots, only a single peak is observed, except for small molecules (n=8). By stoping the tip over a nanodot, we recorded the I-V curves for representative dots belonging to each conductance peak family. The I-V curves are dominated by a tunneling behavior as usually observed in the MMM junctions of alkyl chains. The TVS (transient voltage spectroscopy) analysis shows lower transient voltages (-0.6 V and 0.7 V, e.g. for C<SUB>12</SUB>) for the "high" conductance peak than for the "low" conductance peak (-1.2 V and 1.4 V). These results are discussed and related to differences in the molecular organization of the SAMs as function of the chain length and structure of the underlying nanodot. In conclusion, we have demonstrated that these arrays of Au single crystal nanodots are very efficient test-beds for molecular electronics. |