Year of Publication
Doctor of Philosophy (PhD)
Materials Science and Engineering
Dr. Bruce J. Hinds
A new class of electrodes with nanometer-scale contact spacing can be produced at the edge of patterned metal/insulator/metal this film structures. A key challenge is to produce insulator layers with low leakage current and have pristine metal contacts for controlled molecular contacts. Atomic layer deposition of high quality Al2O3 thin films onto Au electrodes was enabled by surface modification with a self-assembled monolayer of -OH groups that react with a monolayer of trimethylaluminum gas source. Ar ion milling was then used to expose the edge of the Au/dielectric/Au structure for molecular electrode contacts. The junctions are characterized by atomic force microscope and tunnel current properties. The Au/self-assembled monolayer/Al2O3/Au tunnel junction, with a very thin oxide insulator layer (15.4 Å), is stable and has a small tunneling current density of about 0.20 ~ 0.75 A/cm2 at 0.5 V. Organometalic cluster molecules were attached to bridge the electrodes. Through tunnel current modeling, low temperature and photo current measurements, molecular current was found to be consistent with direct tunneling through the organic tethers to available states at the metal center.
This novel electrode was also used to study the efficiency of organic conducting thin films where the photovoltaic efficiency can be improved when the electrode separation distance is below the exciton diffusion length. Copper (II) phthalocyanine (CuPc) was thermally evaporated between the nano-gap electrodes formed by Au/Al2O3/Au tunnel junctions. A large photocurrent enhancement over 50 times that of bulk CuPc film was observed when the electrode gap distance approached 10 nm. CuPc diffusion length is seen to be 10 nm consistent with literature reports. All devices show diode I-V properties due to a large Schottky barrier contact resistance between the small top Au electrode and the CuPc film.
To add another dimension of nm-scale patterning, nanowires can be used as line-of-sight shadowmasks provided that nanowire location and diameter can be controlled. Lateral ZnO nanowires were selectively grown from the edge of a Si/Al2O3/Si multi-layer structure for potential integration into devices utilizing Si processing technology. Microstructural studies demonstrate a 2-step growth process in which the tip region, with a diameter ~ 10 nm, rapidly grew from the Al2O3 surface. Later a base growth with a diameter ~ 22 nm overgrew the existing narrow ZnO nanowire halting further tip growth. Kinetics studies showed surface diffusion on the alumina seed surface determined ZnO nanowire growth rate.
Hu, Bing, "FABRICATION AND STUDY OF MOLECULAR DEVICES AND PHOTOVOLTAIC DEVICES BY METAL/DIELECTRIC/METAL STRUCTURES" (2011). University of Kentucky Doctoral Dissertations. 222.