Charge transfer complexes in molecular electronics: Approaching metallic conduction

Recent advances in the field of molecular electronics include the measurement of the conductance of single molecules. These experiments and the accompanying theoretical work have shown that the resistance of a single molecule is very high, on the order of MO to GO. Since molecules behave like insulators, a method of increasing the conductance of single molecules is needed to forward the field of molecular electronics. This dissertation presents the discovery of a method of increasing the conductance of a single molecule near the regime of metallic conductors.; Increasing the conductance in a single molecule is achieved by the introduction of a partial charge to a delocalized pi orbital in the molecule through the formation of a charge-transfer (CT) complex. The effect of forming a CT complex introduces a partial charge to a delocalized pi orbital in the molecular system. Molecules favor being neutral or fully ionized, so introducing a carrier to a molecule typically forms an ion (- for electrons and + for holes), which, in turn, serves to trap charges rather than transport them. Molecular doping via CT complex formation introduces some density of states (new molecular levels) near the Fermi energy of the electrodes in metal-molecule-metal system. This increase in the density of states causes the conductance of the molecule to increase.; The conductances of single molecules are measured and compared to the conductances of CT complexes. The CT complex is 50 times more conducting than the undoped molecule. Furthermore, the conductance of the CT complex is 3.2% of the quantum of conductance (∼77muS, 2e2/ h), and is the highest conductance measured to date for a single molecule or molecular complex.; An infrared spectroscopic method for determining the degree of charge transferred in CT complexes is also presented. The degree of charge transferred (amount of molecular doping) has a bearing on the conductances of the CT complexes. Surface attachment of CT complexes to silicon is presented as are synthetic methods for tuning molecular conductance.