Theoretical Study Of The Property Of Wide-bandgap Diluted Magnetic Semiconductor Nanocrystals

Diluted magnetic semiconductors and nanomaterials have attracted considerable world-wide attention. At present there has been much focus on the diluted magnetic nanocrystals due to their unusual properties. From a fundamental and technological standpoint, it is very important to explore the various novel properties and manipulating methods.In this thesis we have briefly reviewed the research history and present status of spintronics, the diluted magnetic semiconductors(DMS) and wide-band gap DMS including their low-dimensional cases; The Density Functional Theory (DFT), Discrete Variational Method (DVM) and ADF are introduced in detail. The Atomistic Simulation Method is also inlcuded.We employ first-principles calculations to investigate the electronic and magnetic properties of the wide-band gap DMS Mn/Cr-GaN nanocrystals systematically. The symmetry, surface adsorption and magnetic properties are analyzed intensively. Finally the defect distribution characteristics of Co/Fe-doped ZnO nanocrystals with different sizes have been studied using the atomistic simulation.Our calculated results show that all the total magnetic moments(MMs) are 3μ_B in Ga₁₂MnN₁₄H₂₄ , Ga₁₂MnN₁₄H₃₂ and Ga₁₂MnN₁₄H₃₈ rocksalt-type nanoclusters with Oh symmetry, in which two H atoms are adsorbed on each Ga atom and each N atom at apical and face center sites has no or one H atom leading to the H coverage of 44%, 59% and 70%. In the Cr-doped cases, all the total MMs are 2μ_B.In Ga₁₂Mn(Cr)N₁₄H₄₈ rocksalt-type nanoclusters with O_h symmetry and the H coverage of 89%, there are 2 hydrogen atoms adsorbed on each Ga atom and 3 hydrogen atoms on each nitrogen atom at the apex sites. For the Mn-doped case, we find that there exist one low-spin (LS) ground state and one high-spin(HS) metastable state with the total MMs of 1μ_B and 5μ_B. The energy difference between the two states is 0.03eV and the crossover energy barriers are 0.17eV, 0.14eV respectively. However, for the Cr-doped one, there is only ground state with the total MM of 3.8μ_B. The H coverage of Ga₁₂MnN₁₄H₅₄ with one H atom adsorbed on each N atom at face centers is 100% , resulting in one HS ground state of 5μ_B .Ga12Mn(Cr)N14H40 has D4h symmetry with the H coverage of 74%, in which there are 2 H atoms bonded to each apical N atom. One LS ground and two HS metastable states with the total MMs of 1μB, 3μB and 5μB are found in the Mn-doped case while one LS ground and two metastable ones with 0.2μB, 2μB, 4μB are for the Cr-doped case. The multiple magnetic states mainly result from the various ferromagnetic or antiferromagnetic couplings between the Mn/Cr and neighboring N atoms.In Ga12MnN16H36 zincblende-type nanocrystal with Td symmetry, one H atom is bonded to each Ga atom and two H atoms are bonded each surface N atom. The final total MM is 5μB .Finally we present an atomistic simulation of the Co/Fe doping distribution in the ZnO nanocrystals with the particle diameters of 1.5nm, 2.0nm, 2.5nm and 3.0nm. The dopants tend to take uniform core-doping distributions, which is in good agreement with the experimental observation of D.R. Gamelin et al.