Abstract
In this thesis, low temperature scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) are utilized to study the quantum properties of Pb islands with flat top surface. There are two main topics: strength modulation of quantum-well states in Pb islands with periodic distortions on Si(111) and phase contribution of image potential on empty quantum-well states in Pb islands on the Cu(111) surface. In the first topic, we used STS to explore three-atomic-layer Pb islands with two types of pattern grown on Si(111) surface. Our results demonstrate that the pattern appearing on the island surface is the superposition of geometric corrugation and local variation of the electronic structure. The former originates from two kinds of interface relaxation, resulting in two types of periodic distortion of the lattice. The latter is due to the periodic strength modulation of quantum-well states in Pb islands, causing inhomogeneity in the integration of the density of states and the bias-dependent pattern. This strength modulation of the quantum-well states can be attributed to the electronic screening effect induced by the lattice distortion in Pb islands. In the second topic, we use STS to explore the quantum well states in the Pb islands grown on a Cu(111) surface. Our observation demonstrates that the empty quantum well states, whose energy levels lie beyond 1.2 eV above the Fermi level, are significantly affected by the image potential. As the quantum number increases, the energy separation between adjacent states is shrinking rather than widening, contrary to the prediction for a square potential well. By simply introducing a phase factor to reckon the effect of the image potential, the shrinking behavior of the energy separation can be reasonably explained with the phase accumulation model. The model also reveals that there exists a quantum regime above the Pb surface in which the image potential is vanished. Moreover, the quasi-image-potential state in the tunneling gap is quenched because of the existence of the quantum well states.