Transport in nanoscale devices

The drive toward further miniaturization of silicon-based electronics has led to renewed efforts to build molecular-scale components. A key step in using individual molecules as active circuit elements is the understanding of charge transport through metal-molecule-metal junctions. In our work, we focus on the two transport mechanisms in nanoscale devices, namely hopping and tunneling transport. The former is studied using polyaniline nanofiber with a about 2microm conduction path; the latter is studied using molecular rotor devices with a mono molecular layer of about 3nm. The major issues involved in the synthesis, device processing, and characterization are discussed.;Hopping transport is studied using interfacial synthesized polyaniline nanofibers doped with chloride acid (HCl). The investigation of the temperature dependence of the conductivity suggests that polyaniline nanofibers are more sensitive to temperature than conventionally synthesized ones. The sensitivity dependence of the conductance on the doping concentration was measured, showing a saturation point based on the material property. A three-dimensional conduction hopping model is proposed to explain the experimental results.;Tunneling transport is the emphasis of this thesis work since it is the dominant conduction mechanism in the nanoscale regime. Tunneling transport is studied using a molecule rotor device comprised of a monolayer of redox-active ligated copper compounds sandwiched between a gold electrode and a highly-doped P+ Si substrate as our model system because of its interesting optical, mechanical and electrical properties. A self-assembled chemical method of the molecule rotor layer was developed based on the strategy of a surface outward sequential synthesis that ensures the formation of Si-immobilized heteroleptic copper compounds. Both multilayer and monolayer molecular rotor devices were fabricated and the rotation of redox-dependent ligand rotation around the copper metal center was confirmed using optical absorption spectroscopy.;The switching effects of both multilayer and monolayer molecular rotor devices were characterized. We focus on the fabricated electrically driven sandwich-type molecular rotor device comprised of a monolayer of transition metal complexes containing redox-active pi-conjugated ligand subunits between a gold electrode and a highly-doped Si substrate. Our calculations predicted operational speeds in the picosecond timescale. Current-voltage spectroscopy curves (I-V) showed a negative differential resistance (NDR) associated with the devices, while reference samples of individual subunits, namely the redox-active pi-conjugated ligands and uncoordinated metal complexes alone, did not. Modeling of transverse molecular current conduction using time-dependent density function theory suggested the source of the observed NDR to be rotation of the ligand around Cu complexes. Optical absorption spectroscopy and the observed temperature dependence of the NDR behavior also support this hypothesis. The basic principle of the switching phenomenon as well as the band diagram is constructed to explain the electron transport behavior during the device operation. This is the first time a rotation-induced NDR effect on a solid support has been observed.;To study the scalability of the molecular rotor device, we extend our discussion to electron and ion transport and study their physical scalability. Furthermore, we describe the optimized chip architecture for integrating molecular switches with the conventional CMOS circuits to achieve a CMOL logic system.