The Investigations Of Mechanical-Electronic-Magnetic Coupling In Low-Dimensional Zinc Oxide And Graphene Nanostructures

Low-dimensional nanomaterials have attracted much attention owing to their outstandingproperties and potential applications in further nanodevices. At this scale, some unique functions ariseinduced by quantum confinement effect, which are not available in bulk counterparts. Although theground properties of nanomaterials have been extensively investigated recently, the study for couplingbetween materials and external field is still in infancy. The investigation formechanical-electronic-magnetic coupling is very important and critical to optimization of propertiesand function control. In this thesis, using first principles calculations based on density functionaltheory, we systematically study the physical properties of one-dimensional ZnO nanowires, ZnOnanotube, ZnO nanoribbons, and defective graphene and hybrid graphene nanoribbons. We haverevealed the variation of structures, electronic and magnetic properties of these materials underexternal field, such as strain and electric field. Ample phenomena from the coupling of strain,electronic and magnetic properties are unveiled. The main contents are listed below:(1) Study on magnetic/electronic modulation of one-dimensional ZnO nanoribbons by externalelectric field and edge absorption. Through calculations based DFT including spin orbits, we haveinvestigated and analysized the magnetic properties of Zigzag ZnO nanoribbons under applied electricfield. It is shown that the magnetic moment of1D zigzag ZnO nanoribbons can be efficientlymodulated by transversely applied field. Depending on the field direction, the total magnetic momentin a zigzag ZnO nanoribbon can be remarkably enhanced or reduced and even completely quenchedwith increasing field over threshold strength. However, in weak electric fields below the threshold, themagnetic moment in the zigzag ZnO nanoribbons nearly remains unchanged, which can be explainedin terms of intrinsic transverse electric polarization and quantum confinement effects. The thresholdelectric field required to modulate the magnetic moment decreases significantly with increasingribbon width, showing practical importance. However, for semiconducting armchair ZnO nanoribbons,we have investigated the electric-field-and H chemical-absorption-induced band manipulations ofarmchair ZnO nanoribbons using first-principles calculations. It is shown that the band gap of asemiconducting armchair nanoribbon can be reduced monotonically with increasing transverse fieldstrength, demonstrating a giant Stark effect. The critical field strength to completely close the bandgap decreases with increasing ribbon width, while it is almost independent of the stacking thickness. On the other hand, the nanoribbon with the edges fully passivated shows an enhanced gap but aslightly weaker Stark effect. We also observe hydrogen-termination-induced metalization of theribbons when only the edge O atoms are passivated, which results from a n-type doping effect. Theapplied electric field rises (or lows) the electrostatic potential in the edge of the ribbon, leading thesplitting of band-edge states, thus reducing the band gap. These findings suggest potential ways ofband engineering in armchair ZnO nanoribbons.(2) The charge separation in one-dimensional bare ZnO nanowires and zigzag ZnO nanotubesinduced by local strain. Here, we have examined the effect of strain on electronic properties ofone-dimensional ZnO nanowires and nanotubes. Through the analysis for the distribution ofband-edge states, we found that the valence band and conduction band have different spatialdistributions in pristine [0001]-oriented ZnO nanowires, namely a spontaneous charge carrierseparation. It is shown that the shrinking strain induced by surface reconstruction causes electrons andholes to separate and move toward the core and surface region, respectively, because of staggeredband arrangement. Such separation can be enhanced by axially applied tensile strain as a result of theenhancement of surface strain induced by the Poisson effect, and be suppressed by compressive axialstrain. Similar carrier separations are found in IIB-sulfides. The charge carrier separation induced bylocal strain on the surface is expected to shed light on solar cell designs and application. Meanwhile,we also found that in zigzag ZnO nanotube, the external applied non-uniform strain (uniaxial andradial strain) also could result in significant charge carrier separation and electronic modulation. Ourresults show that local strain or deformation can cause significant reduction of the band gap owing toquantum-confined Stark effect induced by the built-in electric polarization. Driven by this polarizationfield, the charge carriers are separated with hole and electron states localized on the opposite ends ofthe tube. In sharp contrast, uniform tensile strain tends to widen the band gap while compressivestrain and radial deformation have negligible effects on the band gap. The present results reveal thekey role of local strain as an effective tool in tuning the properties of zigzag ZnO nanotubes andnanowires. Such local strain induced electronic modulation suggests an effective approach to designand implementation of1D ZnO nanostructures in nanoscale devices.(3) Electronic and mechanical coupling in bent ZnO nanowires and strain gradient effect.Bending deformation is regarded as an efficient way to modulate the electronic properties. We foundthat the electronic properties of ZnO nanowires can be effectively modulated by bending strainthrough first principles calculations. Detailed analysis indicates that remarkable band-edge energy shifting and gap reduction should contribute to the presence of very large strain gradient(0.152%~2.47%nm-1) rather than local strain effect. Driven by axial strain gradient, the chargecarriers are separated with electrons in tensile strained region and holes in compressive strainedregion.(4) Magnetism modulation in defective graphene and electronic/magnetic properties of hybridW-shaped grahene nanoribbons Graphene and derived nanoribbons have recently attracted muchattention because of the intriguing magnetic properties and potential application in further nanodevices.Inspired by successful experimentally fabrication of graphene with topological line defects, we examinethe magnetic properties of two-dimensional defective graphene using first-principles calculations andpredict a weak ferromagnetic ground state with spin polarized electrons localized along the extendedline defect. Our results show that tensile strain along the zigzag direction can greatly enhance localmagnetic moments and ferromagnetic stability of the system. In sharp contrast, tensile strain appliedalong the armchair direction quickly diminishes these magnetic moments. A detailed analysis revealsthat this intriguing magnetism modulation by strain stems from the redistribution of spin-polarizedelectrons induced by local lattice distortion. It suggests a sensitive and effective way to control magneticproperties of graphene which is critical for its applications in nanoscale devices.On the other hand, since the zigzag and armchair graphene nanoribbons exhibit distinctelectronic and magnetic properties origining from edge states, investigation for a novel W-shapedgraphene nanoribbon (W-GNR) with hybrid armchair and zigzag edges will reveal new physicalmechanism. Here a comprehensive first-principles investigation of electronic and transport propertiesof W-GNRs is presented. It is found that the W-GNRs are energetically stable and exhibitsemiconducting nature with features of armchair and zigzag GNRs remaining in the correspondingsegments. When a vertical electric field is applied, the spin up and spin down band gaps have oppositeresponses due to the presence of zigzag GNR edge states, eventually leading to half-metallicity.Owing to the asymmetrical spin density of state near band-edge state, transport calculations showasymmetrical spin transmission coefficients and non-equal spin currents under large bias voltage.These findings suggest that such W-GNRs are promising materials for spintronic devices applications.The above work concerning about graphene, hybrid nanoribbons and ZnO nanotubes was doneduring visiting US from March to December of2010.