Study On Structures And Properties Of Two Kinds Of Mixed Atomic Clusters And GaN Monolayer Adsorbing Atoms

The size of nanomaterials is comparable to the electron coherent length, which leads to obvious surface effect, quantum size effect, quantum tunnel effect, and so on. Accordingly, they exhibit many unusual properties in the fields of thermodynamics, magnetism, optics and conductivity, etc, which are much different from those of corresponding bulk and atomic state. In the past few decades, nanomaterials based on molecules, clusters, nanowires, nanotubes or graphene developed rapidly, which bring about a revolutionary change in medical science, manufacturing industry, information communication, material science, and so on. The nanomaterials have not only wide application prospect but also a great fundamental importance of exploring the internal relations between macroscopic and microscopic systems.Clusters, as the bridge between microscopic atoms or molecules and macroscopic condensed matter, are ideal building blocks to design new nanomaterials. Before assembling new materials from clusters, we need to make it clear how the properties of clusters vary with their shapes and sizes, and how to modulate their properties for different purpose as well. Mixed atomic clusters, especially binary semiconductor clusters and alloy clusters, have drawn much attention, due to that their properties can be modulated easily by changing the composition. In addition, with the rapid development of electronic components and magnetic storage devices, the size of the components and electric circuit becomes smaller and smaller, and as a result the surface and interface get more and more important. The adsorption is a common surface phenomenon, which can exert a great influence on the activity of adsorbate, the magnetism and band structure of the materials, and so on. In this thesis, we use various methods from empirical to first-principles to study the structures and properties of medium-sized GanAsm clusters （n+m=17-24, n-m=0,±1）, Pdn clusters （n=3-19） doped by Mn atoms and graphene-like GaN monolayer adsorbing F, O and N adatoms.By using genetic algorithm（GA）, basin-hopping Monte Carlo method （BHMC） combined with Stillinger-Weber empirical potential and density functional theory （DFT） calculations, the geometries, electronic properties and polarizabilities of stoichiometric and nonstoichiometric GanAsm clusters （n+m=17-24, n-m=0,±） are investigated systematically, with emphasis on what determines the polarizabilities. All the lowest energy structures are hollow cages with a total number of bonds of about2n+m. The total polarizabilities of GanAsm clusters exhibit a linear dependence on their volumes despite of different composition. The polarizabilities are also strongly correlated to ionization potentials and composition. The nonstoichiometric GanAsm clusters tend to possess higher polarizabilities than stoichiometric ones, but this behavior is hard to be observed from experiment due to isomerization. Ga10As11, seen as a fragment of Ga12As12, has special HOMO-LUMO electron density distribution which accounts for its unusual electronic properties like stoichiometric clusters.The geometries and magnetic properties of both Pdn and Pdn-1Mn （n=3-19） clusters are calculated by using spin-polarized density functional theory approach combined with genetic algorithm and embedded atom method （EAM）.We emphatically discussed the magnetism evolution of the Mn-doped palladium clusters with their structures and size, and how the Mn element influences the magnetic behavior of palladium clusters as well. By comparing our geometries with early results, several new lowest energy structures of Pdn clusters （n=11,12,14,17） are found. The jump of the total magnetic moments of Pdn clusters is explained in terms of structures and hybridization. For large cluster size （n>9）, when a Mn atom is doped into the Pdn cluster, it tends to occupy an interior site of the cluster and the geometry reconstruction generally occurs, and the cluster is apt to form an icosahedron based structure for n>13. The doping of Mn atoms not only enhances the stability of Pdn clusters, but also increases their total magnetic moments by a magnitude of3-5μB. Moreover, the doping of Mn atoms causes the Pdn₁Mn clusters to give rise to an evident oscillation of step-like magnetic behavior, which is closely related to the Pd-Mn bond length and can be attributed to the d-d interaction between Pd and Mn atoms.The effect of adatoms （F, N and O） on the magnetism and band structure of graphenelike GaN monolayer （ML） at different coverages （1/2and1/8） is studied by spin-polarized density functional theory calculations. Adsorbing F adatoms at low coverage causes the GaN monolayer to be transformed from a nonmagnetic indirect-band-gap semiconductor into a magnetic half metal, whereas the monolayer turns back to a nonmagnetic semiconductor at high coverage. Using the Ising model and Monte Carlo simulation, we get an estimation of the Curie temperature of low coverage F-adsorbed GaN monolayer of260K. The GaN monolayer becomes a magnetic half metal after adsorbing N adatoms, no matter whether the coverage is high （1/2） or low （1/8）, and their Curie temperatures are estimated to be220K and50K. The magnetism mentioned above mainly comes from the spin splitting of N2p （especially N2pz） orbits. On the other hand, the adsorption of O does not induce magnetism but reduces the band gap of the GaN monolayer and the higher the coverage is the smaller the gap is.