Charge Transport through Silicon-Molecules-Silicon Nanowire Symmetric Junctions Anchored by Silicon-Carbon Covalent Bonds

This proposal aims at fabricating Si-C=C-(C6H6)n-C=C-SiNW symmetric junctions and studying electron transport through the junctions.

In molecular electronics, a basic unit consists of single or an assembly of molecules attaching to two electrodes at their both two ends simultaneously, for studying electron transport from one electrode to another through molecules. Au-S covalent bonds can be created spontaneously, hence are widely adopted to function as alligator clips for generating Au-S-molecules-S-Au symmetric junctions. However, it has been reported that Au-S contacts don’ have good mechanical stability and electrical reliability, and have high contact resistance to shield the electronic contribution of molecules. Alternatively, having a higher bonding energy than Au-S, Si-C covalent bonds may improve mechanical and electrical stability, and reduce contact resistance ascribed to the natural continuity from the C-based molecules. Hence it is desired to fabricate Si-C-molecules-C-Si symmetric junctions, in which Si-C replaces Au-S as the alligator clip. However, the recent development of nano engineering limits the generation to Si-C-molecules-metal asymmetric junctions, in which high resistance at the molecules-metal contacts and the “diode” effect derived from the asymmetric structure may mask the electronic signatures of molecules.

This project first proposes to replace the metal electrode in the asymmetric junctions with a silicon nanowire (SiNW), and create a junction Si-C=C-(C6H6)n-C=C-SiNW symmetrically anchored by Si-C bonds. The symmetric junctions are generated in two steps: immobilize the molecule C≡C-(C6H6)n-C≡C onto a Si wafer to generate Si-C=C-(C6H6)n-C≡C semi-junctions, and then graft SiNWs onto the free ends (C≡C) of the semi-junctions to create the symmetric junctions. Both two immobilizations are carried out by thermal hydrosilylation. Not only as an electrode, but also does a grafted SiNW act as a junction marker which can be readily located by a CAFM (conducting atomic force microscopy) tip for investigating charge transport through the symmetric junctions. It is elaborated how to reliably fabricate the junctions and evaluate molecular conductance, with consideration of the inevitable silicon oxidation and unpredictable contact/molecular configurations in the junctions. The mechanism of charge transport will be explored with applied bias, molecular length (n:1-3) and temperature.

We believe that the success of this project will provide a novel, reliable strategy to study molecular electronics without electrical and mechanical perturbation from the electrode- molecules contacts, potentially open a new door to hybridize molecular electronics with the mature silicon-based manufacturing technology and promote mass production, and establish for Hong Kong strong competitiveness in molecular electronics.