Conductance Measurement Of Molecular Junctions Based On Graphene Electrode

Traditional electronic devices are made of silicon semiconductor materials which has its limitation on ultimate miniaturization of electronic devices. Molecular electronics have attracted widely great attention for the propose of replacing traditional solid state electronics based on silicon semiconductor at the molecular level. A single or few molecules as the core component assembled to the molecular devices could act as microelectronics molecular devices, such as transistor, rectifier, molecular switch, and the negative differential resistance, which holds great promises for the emerging field of molecular electronics. The key issue of molecular electronics is to obtain the robust and stable connection on both ends of the electrode to form reproducible molecular junctions. Due to lack of proper techniques, the investigation of molecular electrical properties was impossible until the development of scanning probe techniques in 1980 s. The research scope of molecular electronics has been extended greatly since the development from initial mechanically-controllable break junctions(MCBJs) to a variety of methods based on scanning probe techniques. To date, most of the electrode are used metal material(Au, Ag, Pd) to construct molecular junctions, containing specific functional anchoring groups such as-HS,-COOH, NH2, pyridyl, etc. on each end to bridge the metal electrode. Especially, the gold electrode works as the ideal material due to its ability to form ordered self-assembled monolayers(SAM) with strong covalent bond to thiol. However, the drawbacks for gold are also obvious such as expensive cost, the atomic mobility of gold electrodes and its non-compatibility with complementary metal-oxide-semiconductor(CMOS) technology. Up to now, there are few reports on the use of the non-metallic electrode(mainly carbon-based materials, such as carbon nanotubes and graphene) in the molecular junction. Since the discovery of graphene in 2004, various structures and properties are exploring in recent years. Graphene can be wrapped up to zero-dimensional(0D) fullerenes, rolled into one-dimensional(1D) nanotubes or stacked into three-dimensional(3D) graphite, which shows many excellent physical and chemical properties, such as high mechanical strength, carrier mobility, good thermal conductivity and the quantum hall effect and so on.In this dissertation, graphene as one electrode is used to build alkanedicarboxylic acids with gold electrode. Based on scanning tunneling microscope(STM)-based matrix isolation I(s) method, a series of molecular conductance measurements were carried out. The conductance value of HOOC(CH2)nCOOH(n=2, 3, 4, 5, 6) is 15.6, 10.3, 5.1, 2.4 and 1.08 nS, respectively. Our measurements showed the mechanism of charge transport through the system of junctions is tunneling mechanism, and the tunneling decay constant ?N = 0.69 is smaller than the same molecular junctions built by symmetric Au, Ag, Pt electrodes. The great influence of electrode materials on molecular junctions has been demonstrated. In addition, the formations of molecular junctions(n=2, 4, 6) with graphene-Au electrodes and Au-Au electrodes were simulated by the density functional theory(DFT), and their conductance were calculated using Non-equilibrium Green method. The closest fit with experiment was found for EF=-0.3 e V relative to EF DFT with the attenuation coefficient ?N=0.82, whereas for graphene-molecule-Au junctions, the closest fit was found for EF =0.65 eV with the attenuation coefficient ?N=0.69 relative to EF DFT. For defective graphene-molecule-Au junctions, the attenuation coefficient ?N is 0.89 when EF=0 eV. The tendency of the local density functional approximation(LDA) underestimates the highest occupied molecular orbital(HOMO) and lowest occupied molecular orbital(LUMO) gap that results in an overestimated of the conductance which is greater than that of experimental values. The comparison of perfect graphene and defected graphene for the theoretical calculations demonstrated that the defect of graphene hinders the charge transport of electron in the molecular junction.