The Ordered Metal Clusters On Surfaces: Electronic Properties And Structural Stabilities

The ordered arrays of identical metal clusters on surfaces have very important applications in the ultrahigh density recordings, next-generation micro-electronics and surface catalysis. In recent years, experimentally, the ordered arrays of several kinds of metal clusters have been successfully fabricated on semiconductor substrate, using the "module + the magic cluster" method. Therefore, it becomes very important to study the interactions between the substrate and the clusters as well as the interactions between the clusters theoretically, which is crucial for understanding the physical chemistry of the system. This dissertation employs the first-principles calculations to study the structural stabilities and electronic structures of periodically two-dimensional arrays of Nb₄ clusters on surfaces, such as the metal surfaces Cu(111), Cu(100), the semiconductor surface GaN(0001), and the insulator surface NaCl(100). The main results are shown as follows.Our calculations show that the Nb₄ clusters with both the tetrahedron (3D) and quadrangle (2D) configurations can be stably absorbed on the Cu(111), Cu(100), GaN(0001), and NaCl(100) surfaces. On the Cu(111), Cu(100), and GaN(0001) surfaces, the adsorptions of 2D-Nb₄ clusters are more "stable" than the 3D-Nb₄ ones (the better stability means higher binding energy). The energy barriers for the 3D-Nb₄ adsorptions to the 2D ones are relatively high. Electronic structure calculations suggest that the adsorption of Nb₄ on these surfaces causes significant charge redistributions between the surface layers and the Nb₄ as well as remarkable changes on the electronic structures of the surfaces, implying strong interactions between the clusters and the substrates. The fact that Nb₄ clusters can be stably adsorbed on these surfaces with extremely high density may have important applications. On the NaCl(100) surface, on the contrary, the binding energies of the Nb₄ adsorptions are small, and the charge redistributions between the surface layers and the Nb₄ adsorbates are tiny. These facts mean that the interactions between the clusters and the NaCl substrate are weak, which is due to the character of the insulating NaCl(100) surface. The binding energy difference between the 3D- and 2D-Nb₄ (2×2) arrays on NaCl(100) is small.Furthermore, for the (2×2) arrays of Nb₄ on the GaN(0001), the calculations on the difference of electron densities show that there are charge density overlaps between the neighboring Nb₄ clusters. However, for the (3×3) arrays, the charge density overlap disappears. Therefore, the (3×3) array of Nb₄ on the GaN(0001) is the one with maximum density for the separated quantum dots on this surface, which provides an important data for the experiment. Finally, by comparing all the systems, we conclude that Nb atoms within the Nb₄ clusters remain strong covalent bonding when adsorbed on all the surfaces studied, which contribute most significantly to the stability of Nb₄ clusters on these surfaces. The interactions between the clusters and the substrates also impact the stability of the clusters on the surfaces.