Fabrication And Characterization Of Metal-Molecule-Metal Junctions

With the rapid development of nanoscience and nanotechnology,molecular electronics, aiming at molecular devices and molecular computers,has been the most active field.The main foci of recent research interest in molecular electronics are to fabricate molecular device,measure and control electron transport through a molecular device,and understand the relation between the structure and function of molecular devices.Metal/molecule/metal type molecular junctions is the most basic and widely studied structure in molecular device. However,it should be noted that there is no other effective technique for characterizing molecular junctions besides current-voltage(â… -â…¤) and inelastic electron tunneling spectroscopy(IETS).The present thesis is to construct metal-molecule-metal molecular junction and develop high sensitive spectroscopic methods to characterize molecular devices.The main results are listed as follows:1.Au nanowires were prepared by using a two-step alternative current electrodeposition (high voltage nucleation followed low voltage growth) inside the pores of porous alumina template.The molecules were then self-assembled on the wire surface inside the pores followed by electroless plating and electrodeposition of Au to produce Au nanowires with Au-molecule-Au junctions.The nanowires were then aligned by dielectrophoresis to bridge over two measuring nanoelectrodes for theâ… /â…¤curves characterization.2.A novel electrochemical method was developed to fabricate nanogap with metallic electrodes on silicon chips.First,a pair of facing gold electrodes with a gap and size at the micrometer scale were fabricated on a Si chip by a conventional lithography technique.Then,a chronopotential electrodepositing method was used to narrow the gap width down to the nanometer scale.With the electrode potential as the feedback controlled by a potentiostat,nanoelectrode pairs with a tunable gap width ranging from about ten nanometers down to few angstroms were obtained.3.A home-made mechanically controllable break junction(MCBJ) setup was constructed to allow us to precisely and continuously adjust the gap width of the nanogap on a chip.By self-assembling probe molecules with a proper length on the gap,we obtained metal-molecule-metal junctions with molecules bridging over the two electrodes and measured theâ… -â…¤curve of the molecular junctions.4.A method combining MCBJ and Surface-enhanced Raman spectroscopy(SERS) was developed to characterize the structure of the molecular junction.The optimized experimental conditions were systematically explored to obtain SERS from molecular junctions.The dependence of the SERS signal on the gap width was preliminarily studied by tuning the gap with the MCBJ setup.SERS spectra of some molecular junctions at different bias voltages were obtained.