Optical detection and nanometer scale surface modification in an STM junction

The invention of the STM is a powerful tool for determining the surface structure and electronic properties on an atomic scale. With a point-contact-like structure and the nonlinear I-V characteristic, this device has a remarkably short response time allowing optical frequencies mixing, harmonic generation, and rectification. Additionally, with the ability to address the probe tip with nanometer precision and a finely focused electron beam, the STM also provides a powerful tool for nanometer scale surface modification. In this dissertation, the nonlinear I-V characteristic has been measured in an STM junction with a platinum tip and a gold sample, where both the surface modification and optical rectification effects have been experimentally examined.; An asymmetric I-V curve has been obtained when the gap distance is set to the field-emission region ({dollar}rm dge 15{dollar} A). A nonlinear but symmetric I-V characteristic has been observed in the transition region (10 A {dollar}le{dollar} d {dollar}le{dollar} 15 A) and a linear ohmic I-V measured in the tunneling region ({dollar}le{dollar}8 A). For optical rectification, experiments were designed to examine the mechanisms of geometric and thermal asymmetry. The dependence on the direction of incident laser field for surface modification on a gold substrate were studied. "Field evaporation" with or without enhanced laser field or the "chemical etching" effect from the laser ionized gas atoms in the STM junction results in surface morphology changes. The experiments of surface modification without enhanced laser field were carried out by applying a voltage pulse to the junction in the different gap distance regions, namely tunneling and field-emission. In the tunneling region, the formation of holes on a gold surface is independent of pulse polarity and mound deposition randomly occurs, while in the field-emission region, the controlled feature formation is dependent on pulse polarity. Field ionization followed by subsequent field evaporation of the electrodes' material is considered the main mechanism for the surface modification during the voltage pulse.; Development of nanofabrication techniques for super-miniaturized electronic circuits and optical detecting devices with ultra-fast switching times for optical computation and communication will be two major issues for tomorrow's industry. By using the STM, as discussed in this dissertation, a novel tool for atomic level manipulation is possible.