Electrical characteristics of organic molecular wires by scanning probe microscopy

Reportedly, molecular wires which have intrinsically different characteristics from semiconductor nano wires show some unique transport properties. However, it seems that there is not so much understanding about their local properties. So, in this work, we characterized electrical properties of carotenoid which had differently substituted end-groups by using Scanning Probe Microscopy. To confirm the difference of the electrochemistry with substituted end-groups, we performed cyclic voltammetry in an aqueous solution. Electrochemical reaction was detected in the potential region between 0.5 and 0.75 V vs. Ag | AgCl | KCl (sat), whereas the conductance values were showed difference of differently substituted end-groups. Physisorptional effect between molecule and Au substrate was removed by introducing thiol[SH] group at each end of molecule resulting in covalent bond. After inserting these molecules into a 1-methylsulfanyl-octadecane monolayer pre-deposited on Au substrate, an Au nanoparticle is attached to the other end of each molecule via the protruding thiol group. To confirm the chemical conjunction between an Au nanoparticle and carotenedithiol molecule, we performed Electrostatic Force. These measurements allow us to simulate the transport property of carotenoid molecule. Followed I-V measurement reconfirmed the results.A PtIr coated AFM probe is used as an electrode to contact the molecule through the Au nanoparticle. They reveal that methoxy-phenyl substituted carotenedithiol molecules showed better conducting properties and bromo-phenyl substituted carotenedithiol molecules showed worse than phenyl-substituted one did. I also obtained that current-voltage (I-V) curves were quantized as integer multiples of one fundamental curve. These allow us to simulate the transport property of single carotenoid molecule.