Theoretical Studies on the Scanning Tunneling Microscope

Abstract

The thesis explores calculating tunneling current densities between planar conducting electrodes in an STM. It considers factors like bias voltages and the separation between electrodes, using Fermi energy and work functions. Pauli blocking effects on forward and reverse current densities are introduced, comparing Airy function solutions with WKB results. A 'Russell Potential' is defined for field lines, considering non-linearities and image force effects. A multi-slice method using the transfer matrix approach calculates tunneling currents for the Russell Potential and trapezoid + image force potentials. The Simmons image potential's unreasonable enhancement of tunneling currents prompts the construction of models with distributed charge, which show negligible image effects compared to trapezoidal potentials. Tunneling currents increase with bias voltage, decrease exponentially with tip-sample distance, and rise with increasing tip curvature radius. The resolving power of the STM degrades with blunter tips, higher bias voltages, and increased tip-sample distances. In essence, the thesis covers calculating tunneling current densities in STM, exploring different potential models and their effects. It discusses factors influencing tunneling behavior, like bias voltage, tip-sample distance, and tip curvature, revealing limitations in imaging capabilities under certain conditions.