Electron transport through OPE-based molecules in junctions formed by electromigration

We have studied the self-assembly and electron transport properties of (i) simple oligo(phenylene ethynylene) (OPE) chains (2.2 nm) and (ii) OPE based molecules with a naphthalene diimide acceptor group (4.5 nm). Both are capped with terminal isocyanide groups. The molecules are self-assembled on gold wires with a cross-section $\sim $ 20$\times $100 nm$^{2}$. The junctions are formed by electromigration and the transport measurements are carried out at 4.2 K. For simple OPE chains, more than 40{\%} of the junctions have shown non-linear I-V curves with resistance R in the range from M$\Omega $ to G$\Omega $, due to trapping of single or multiple molecules. In $\sim $10{\%} of the junctions (R $\sim $ a few G$\Omega )$, we observe I-V curves with discrete current steps due to electron transport through one or a few molecules. The histogram of the step voltages shows grouping at certain levels. These levels are in a semi-quantitative agreement with our calculations based on the general theory of single-electron transport. For long OPE molecules with acceptor groups, the yield is low ($<$15{\%}), and the data show Coulomb blockade with threshold voltages from 30 to 200meV. Currently we are working on different support nano-structures that will promise higher yield. This work is supported by AFOSR and NSF.