Conjugated 12 nm long oligomers as molecular wires in nanoelectronics

Abstract

We demonstrate a generic synthetic approach to oligophenylenevinylene (OPV) derivative molecules with a molecular length of up to 12 nm and a relatively free choice of end group that can attach to different electrodes such as metallic gold or potentially transition metal oxide semiconductors. OPVs containing 3–19 phenyl units were synthesised by step wise HWE-reactions of a bifunctional OPV-monomer which allowed for complete control of the sizes of the OPVs. Workup and analysis (¹H- and ¹³C-NMR, mass spectrometry and size exclusion chromatography) of each step ensured a high purity of the final products. Final end group functionalities of the OPVs were introduced either as the first step (alcohol) or the last step (thioacetate). We further demonstrate a fabrication method for well defined nanogap electrode devices based on silicon-on-insulator technology, featuring a gap distance of down to 9 nm. Assembling the OPV derivatives onto these devices enabled preliminary investigations of their low-temperature transport properties, revealing a pronounced non-linear current–voltage characteristic at 4.2 K. We studied the electronic states of the molecule by Density Functional Theory (DFT) in order to show the effect of the ligands and of the gold contacts. By using the results of the DFT calculations in a non-equilibrium Green's function model, the current–voltage characteristics of OPVs have been analyzed, showing a good qualitative agreement with the experimental data.