| In recent years, various ZnO structures have been fabricated with unique optical, electrical, and magnetic properties. Thus, ZnO material plays an important role in chemistry and chemical engineering, semiconductor physics, biological medicine, and other areas. The other important material, carbon, also has formed diverse carbon structures by sp, sp2, and sp3hybridized carbon. In this study, we mainly use first-principle approaches to systemly study ZnO and carbon nanocomposites, and their electrical properties. On the one hand, we design ZnO polymorphs and low-dimensional carbon nanostructures, and then study their stabilities and electrical properties. On the other hand, from quality analysis to quantity analysis, we investigate carbon nanocomposites and their field emission mechanism. The main studies and findings are summarized as follows:(1) A novel cubane-type ZnO (CBE-ZnO) polymorph is predicted and studied by means of the first-principles density functional theory calculations. The new CBE-ZnO structure is also energetically more favorable than the previously predicted CsCl-ZnO, BCT-ZnO, and the synthesized RS-ZnO polymorphs. The results suggest that the CBE-ZnO polymorph is mechanically and dynamically stable. In addition, the new phase is identified and characterized by the simulated X-ray diffraction (XRD) patterns, infrared (IR), and Raman spectra to provide more information for possible experimental observation.(2) A beaded ZnO quasi-one-dimensional nanowire, as a novel stand-alone system, has been introduced by interconnecting different numbers of highly stable (ZnO)12basic units. The results indicate the energy gaps of the (ZnO)12×n nanostructures (n≥3) show a relatively slow decrease, indicating that the energy gaps are insensitive to the cluster size for the large clusters. The low-lying HOMO and LUMO are observed to shift to the top of the low energy levels and the bottom of the high energy levels, respectively, leading to the reduction of the energy gap. In addition, by calculating its energy gap, vertical ionization potential, adiabatic electron affinity and chemical hardness, we find that the beaded ZnO quasi-one-dimensional nanostructure has a high chemical reactivity during its growth process.(3) The sp-sp2hybridized carbon models are studied in detail, such as C24, C36, C48, C60, C72, C84, C96, C108, and C120zero-dimensional carbon allotrope, and we named the carbon allotrope "fullerenyne". The stabilities of sp-sp2hybridized fullerenynes are taken into account from four aspects. Especially, the spherical C96fullernenyne with Oh symmetry group exhibits exceptionally high stability. The sp-sp2hybridized C96fullerenyne exhibits semiconducting property with the calculated energy gap of0.51eV. In addition, the porous C96fullerenyne with sufficiently large holes would permit easy diffusion of these metal ions, even large metal clusters and gas molecules. Thus, the C96fullerenyne may be a promising candidate for metal-ion storage (e.g., lithium-ion storage) and molecule storage (e.g., hydrogen storage).(4) A density functional theory study is performed to understand electronic structures and field emission properties of CNT-ZnO nanocomposites. The calculated results show that the average interaction energies of CNT--ZnO nanocomposites monotonically increase with increasing numbers of carbon layers, indicating that addition of carbon layers between two ZnO nanocages could improve the stability of CNT-ZnO nanocomposites. These CNT-ZnO nanocompistes have small energy gaps. The energy gaps of the nanocompistes exhibit an oscillatory behavior as a function of the length of the carbon nanotubes. The ionization potentials of all CNT-ZnO nanocompistes are smaller than that of the ZnO nanocage of7.155eV. The ionization potentials of CNT-ZnO nanocontacts with more two carbon layers exhibit approximated odd-even oscillation in the absence and presence of an electric field. The CNT-ZnO nanostructure with four carbon layers has a smallest ionization potential of3.625eV under0.2eV/A external electric field.(5) A novel structural model of graphene-ZnO nanocomposite has been proposed. We have performed first-principles density functional calculations of electric structures and field emission properties of the hybrid graphene-ZnO. Effects of the applied electric field on the electronic structures of graphene-ZnO have been investigated. With increasing electric field, the HOMO and LUMO levels shift toward the vacuum level. As a result, it leads to a decrease in the potential barrier of graphene-ZnO and causes electrons to be emitted more easily. Both the work function and the ionization potential decrease linearly with increasing electric field, which confirm that graphene-ZnO has fairly good field emission properties. Without electric field, graphene-ZnO has a smaller work function of3.623eV, comparing with the work functions of pristine ZnO nanocage4.970eV and graphene nanodisk4.022eV. Therefore, graphene-ZnO may be a good field emission electron source material.(6) We propose an island-shape graphene-BN nanocomposite for potential applications of field-emission electron sources. By DFT calculations, we studied the field emission mechanism of graphene-BN in detail. Importantly we quantitatively calculate the field emission current of graphene-BN, and compare with these of carbon nanotubes and boron nitride nanocones. The results show that graphene-BN has a larger emission current from the individual orbital than carbon nanotubes and boron nitride nanocones have. The sum of the field current for carbon nanotubes is about60.9μA with an external electric field0.5V/A. Boron nitride nanocone has a small field current about24.9μA for4P-N configuration with an external0.3V/A electric field. However, the island-shape graphene-BN has a largest field current,210.9μA with an external0.3V/A electric field. The current work also opens new possibilities of doing further investigations on fabricating high-speed electronic devices. |
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