Atomically flat metal ultrathin films grown on semiconductor substrates form a quasi-two-dimensional (2D) electronic system, which offers a great opportunity to explore novel properties induced by quantum size effects (QSE). In such a system, the motion of electrons in the film plane is essentially free, however, in the film normal direction it is confined, which leads to the quantized electronic states, i.e., quantum well states (QWS). The formation of QWS induces the redistribution of electrons and changes the electronic structure of the metal films and thus modifies their properties. By controlling the film thickness-the width of the confinement potential well, the QWS, together with the physical and chemical properties of the films can be engineered. In this article, we will summarize our recent studies on this problem in the Pb/Si(111) system, and discuss the perspectives in this area.
