Research On Surface Electronic Structure And Magnetism Of Several Kinds Of Nano-materials

The emergence of quartz optical fiber and GaAs laser promote the developmentof the fiber-optic communications and make our passage into the information ageduring the1970s. The development of nano material and nanotechnology, whichmakes people contral the new devices from the nano-level, has a profound effect onthe political, economic and military, and changes people life in the most radical ways.With the reduction of nano-material size, the surface effect of nano-materials resultsin the importance of surface characteristics in determining the physical porperties ofnano-materials, such as electrical, magnetism etc. In this dissertation, usingfirst-principles calculations, we have studied the electronic structures and magneticproperties of III-V semiconductor nanowires and oxide semiconductor quantum dot,such as GaAs, InSb nanowires and SnO2quantum dot, which have wide applicationprospects, and get some meaningful results.We have simulated a experimental result, which is that a consistent increase inCurie temperature (Tc) with decreasing (Ga,Mn)As wire width was observed in theexperiment. We calculate the formation energy of Mn interstitials located at differentsites in the GaAs nanostructure with different shapes, orientations, surface atoms andsizes. We nd that the Mn atom is located at the outmost tetrahedral interstitial site andthe As atoms occupy this tetrahedral outside surface, the formation energy is thesmallest regardless of the shape and orientation, but it increases with reducing size ofthe nanowire. Therefore, increasing the free surface by nanostructure engineeringallows the Mn interstitials to diffuse out at the sidewalls. Reducing theself-compensating Mn interstitials will effectively increase the Tcof the (Ga,Mn)Asmagnetic semiconductor, which gives a reasonable explanation to the previousexperimental observation.We have investigated the effect of surface passivation on the magnetic propertiesin (Ga,Mn)As nanowires. We find that the Mn atom prefers to stay near the surface in anonpassivated and halogen passivated GaAs nanowire. The Mn atom can stay close tothe center when the nanowire is passivated by pseudo-hydrogen (PH) atoms. Themagnetic coupling in the nanowires depend on the crystallographic structure andsurface passivation. The nonpassivated nanowire with zinc-blend structue has theferromagnetic and antiferromagnetic coupling. After the zinc-blend nanowire is passivated, the antiferromagnetic coupling is translated into ferromagnetic coupling.Both nonpassivated and passivated nanowire with wurtzite structure has theferromagnetic coupling, which is strengthened by surface passivation.We have investigated the surface passivation effects on the electronic propertiesof InSb nanowires. We find that surface passivation effects can make InSb nanowireexhibit intrinsic electronic properties. The band gap modulation is base on chargecompensation between passivation atoms and surface atoms, which is eliminating thesurface states. Surface passivation effects can enhance the carrier mobilities of InSbnanowires. Comparing with the PH passivation, halogen passivation is more distinct.We have studied the uniaxial strain effects on the electronic properties of InSbnanowires, We find that applying strain (compression or tension) can effectivelymodulate the electronic properties of nanowires with various size, direction and crystalstrucutre. After the compression is applied on the growth axie of nanowire, the bandgap of InSb zinc-blend(ZB) nanowires grown along the [111] direction experiences adirect-to-indirect transition, the band gap of InSb wurtzite(WZ) nanowires grownalong the [0001] direction experience a transition from semiconductor to metal.Effective mass of carriers in InSb nanowire can be modulated by strain. Tension canreduce the effective mass of holes and electrons and the compression can increase theeffective mass of holes and electrons in ZB nanowires. Both tension and compressioncan increase the effective mass of holes and electrons in WZ nanowires.The effect of surface dangling bonds on the magnetism of a SnO2quantum dot hasbeen investigated. We find that the spin splitting states of surface dangling bonds is asource of d0magnetism. The anionic surface states mainly are distributed above the topof valence band and are similar to accepter level providing holes, the cationic surfacestates mainly are distributed below the bottom of conduction band and are similar todonor level providing electrons. After the surface dangling bonds are fully passivatedby PH, the surface spin states are eliminated. Co-and Ni-3d states appear in band gap,and the surface states arising from dangling bonds also appear in the band gap. Thesestates couple with each other, and induce a large magnetic moment so that the SnO2quantum dot shows very abundant variation of the magnetic properties. The changes ofmagnetic moments can be explained well by a carrier modulation model.