Control of junction resistances in molecular electronic devices fabricated by FIB

• Molecules provide an opportunity to fabricate electronic devices with much smaller basic unit in size i.e. 1-5 nm as compared to today’s silicon based electronics. Furthermore, molecules can be synthesized withalmost unlimited variation of their electronic structure. Theoretically, molecules in various configurations were demonstrated as rectifiers, transistors or memories, but experimentally it is still very difficult to obtaina  stable and reproducible molecular based device [1]. A major hurdle to realize such devices is to make reliable electrical contacts to a single or a few molecules. Here, we show the first reproducible and systematic evaluation of a nanogap-nanoparticle bridge set-up that can be used as base for development of few molecule molecular electronics under ambient conditions. We have developed a nano-contact platform by top-down approach [2] with a gap size of 20-30nm using combined techniques of photolithography, electron beam lithography and focused ion beam milling (Fig 1). These gaps demonstrate excellent resistance in order of 1000 TΩ enabling us to carry out electrical characterization of highly resistive nanomaterials.However, compared to the size of molecules these gaps are quite big. In this study, we used metallic nanoparticles to bridge the gap and thus obtain electrical contacts with 1-2nm long molecules in the junction between the nanoelectrodes and the nanoparticles. The nanoparticles are assembled in the gap  by a bottom-up approach using dielectrophrosis trapping process. Prior to introduction of molecules in such devices, we found that the trapping of gold nanoparticles (AuNP) in between clean nanoelectrodes without presence of molecules often gave resistance in order of mega-ohms to giga-ohms due to presence of a non conductive barrier. However, it was observed that cleaning protocols of both the gold contacts and nanoparticles in solution lead to resistance of less than a few hundreds of ohms (Fig 2). Molecules were introduced both by functionalizing the electrode gap and the the nanoparticles and the results of both functionalisation protocols are compared. By optimizing the electrode cleaning as well as the functionalisation of the metallic surfaces, we obtain reproducible electrical measurements. We fabricated such devices either by depositing a Self Assembled Monolayer (SAM) of molecules on the nano-contacts and bridging the gap by AuNP or by bridging the clean nano-contacts with molecule-coated-AuNP (Fig 3). Here we utilized a model molecules octanethiol (OT), octanedithiol and biphenyldithiol in fabrication of devices and study of metal molecule junction resistance. IV characterization of OT molecules (Fig 4) showed linear response where current levels varied between picoamps and femtoamps with an applied voltage of 1-3V. OT in this setup had one physisorbed contact with gold, which resulted in much less wave function mixing at the molecule-metal interface, and consequently decreased the transmission probability at the molecule-electrode interface. As a result, in the evaluation of more than 50 devices, a considerable variation of resistance between different devices due to the lack of covalent binding, the variation in number of trapped AuNPs, incomplete coverage of OT on the uneven surface of nanoelectrodes and variation in contact surface geometry. Density functional theory is used to understand the origin of the resistance fluctuation. We were able to estimate the average resistance per octanethiol molecule for such device in order of 175GΩ, in good agreement with other published results. Our results with the measurements on OT in such devices demonstrate that it is possible to fabricate stable electronic devices having relatively small numbers of molecules with reliable metal molecule junction by combing top-down and bottom-up approaches. By functionalizing the nanoparticles, we obtained a strong decrease of the resistance spread of such devices from 3 orders of magnitude to about 1 order of magnitude, making this technology a potential approach for molecular devices operating at ambient conditions. 

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