| Semiconductor nanostructures have been received by people’s extensive attention because of its unique physical properties and potential applications of photoelectric field. A lot of novel low dimensional nanomaterials such as nanotubes, nanowires, nanobelts, nanofilms and nano coaxial-cable have been prepared with the development of nanotechnology. GaN nanowire and AlN nanotube are both the important semiconductor materials which can be used extensively in the fields of spintronics, hydrogen storage, photoelectric conversion, and catalysis. Magnetic origins of exploring nanostructures doped metal systems has been magnetic field of electronics research hot topic, while looking for hydrogen storage capacity, and easy to release hydrogen new nano structure material is the key to hydrogen large scale application. However, due to the complication of the semiconductor nanostructures, it is very difficult to get detailed information of these systems experimentally. On the other hand, the significantly enhanced computational capability and the well developed first-principles calculation method in the past decades provide us the powerful tools to carry out the pertinent studies. In this thesis, we mainly focus on: （1） vacancies induce magnetism in GaN nanowires, in order to compare the calculation result, we calculated the vacancies in the GaN three-dimensional bulk and two-dimensional film; （2） a tomic and molecular hydrogen adsorption on outside and inside the zigzag and armchair AlN nanotubes; （3） the adsorption of the Mn atoms and Nb clusters on the Si surface. The following innovative results are obtained:1、The Ga vacancies can be formed in the GaN bulk, film and nanowire. For the GaN（ 112 0） film, the formation energies for the surface vacancies are less than those for the subsurface vacancies. The formation energies of the Ga vacancies inside the nanowire are almost the formation energies in the bulk and subsurface. When the Ga vacancies are on the surface in the nanowire, the formation energies of the Ga vacancies decrease dramatically, which indicates formation of Ga vacancies on the surface is easier than that in bulk or in surface. This because there is less number of bonds that need to be broken on the surface than inside nanowire. We note that per Ga vacancy induces 3.0μB irrespective of the Ga vacancy site in the bulk. For the bare and passivated nanowires of the Ga vacancies on the surface, per Ga vacancy products the total magnetic moment of 1.0μB, while those inside of the nanowires can lead to the formation of a net moment of 3.0μB. Although the total magnetic moment for Ga vacancy inside of the nanowire is larger than that on the surface, however, the Ga vacancy prefers to reside on the nanowire surface, which results in a magnetic nanowire. we find that per Ga vacancy develops with a 3.0μB magnetic moment irrespective of the Ga vacancy site in the bare nanowires without geometry optimization. We note that the relaxation only affects the magnetic properties when the Ga vacancies occupy on the surface sites. The calculated magnetic moment due to Ga vacancies in GaN system was mainly contributed by the the unpaired 2p electrons of N in the nearest-neighbor of the Ga vacancies. The magnetic coupling between Ga vacancies and we showed that the magnetic coupling between these defect is long-range ferromagnetic coupling.2、We have shown that the single H atom is preferentially adsorbed on the top of N atom. The adsorption energies on the surface sites are larger than that of corresponding the adsorption sites inside the nanotube. When the H atom adsorbed on the Al atom in the AlN nanotubes, all adsorption configurations are magnetic and the total magnetic moment is 1.0μB .The origin of magnetism results from the spin polarization of the H atom and the nearest N atoms. The energy barriers for all adsorption configurations are all small which indicates the adsorption configurations of a hydrogen atom adsorbed on the Al atom are less stable. We consider adsorption behavior of hydrogen on the （8, 0） and （5, 5） AlN nanotubes with 25%, 50%, 75%, 100%, 133%, and 200% coverages. For the adsorption configurations of the high hydrogen coverages on the （8, 0） and （5, 5） AlN nanotubes, The most favorable adsorption configurations are 100% hydrogen coverages with the adsorption energies of -2.355 and -2.305 eV, respectively. The H2 molecule is physisorbed on the outside and inside the AlN nanotubes. Each Al atom can bind one hydrogen molecule which leads to 8.89 wt% hydrogen gravimetric density exceeding the 2010 DOE target of 6 wt%. Moreover, the adsorption configuration corresponding to 8.89% hydrogen gravimetric density retains perfect tubular structures after full geometry optimization and the adsorption energy of hydrogen lies between physisorption and chemisorption energies which is about 0.2-0.4 eV.3、For the adsorption of closely spaced rows of Mn atoms and Nb4 on Si surface, the results indicate that the Si–Si dimer bond length of the Mn adsorption models is considerably elongated and the asymmetry in the dimer is significantly suppressed as compared with the clean Si surface. For 0.5 ML coverage, i-site and for 1 ML coverage, h + i model are found to be the minimum energy configurations. The spin magnetic moment of b site is the largest in all possible adsorption sites. The silicon surface was found to be metallic for Mn adsorption i and h + i sites. For the adsorption of Nb4 on Si （001）（4×2） surface, The present calculations show that the ordered tetrahedron- and rhombus-Nb4 clusters can both be stably adsorbed on the Si（001） surface, while the adsorption with the rhombus configurations of Nb4 is more stable. The energy barriers between the stable adsorption sites are estimated to be high, indicating that the Nb4 clusters would prefer these stable sites under moderate temperatures. The high stability of the Nb4 clusters on the Si（001） surface may have important applications, such as the ultrahigh density memories, catalysis and nanostructure device technology. |
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