【作者】 张胜利；

【导师】 黄世萍；

【作者基本信息】 北京化工大学 ， 化学工程与技术， 2013， 博士

【摘要】 近年来,由于各种形貌ZnO材料的出现,以及在光、电、磁等方面展现许多特殊的性质,从而使其在化学化工、半导体物理、生物医药等领域发挥了重要的作用。另一种重要材料—碳,通过其丰富的sp、sp2和sp3杂化态,可以形成多种性质独特的碳结构。本论文主要基于密度泛函的第一性原理计算,针对ZnO、碳、及其碳纳米复合材料的结构和性质进行了系统的理论考察。一方面从ZnO和碳材料的设计和第一性原理计算入手,如对周期性的ZnO同质异形体(polymorphs)、低维ZnO、以及低维的sp-sp2杂化的碳纳米结构进行了预测并考察了其电子性质等；另一方面从定性到定量研究,考察了ZnO和碳纳米复合材料的场发射机理。主要工作概括如下：(1)预测了一种新颖的立方烷状氧化锌(CBE-ZnO)晶体结构。新颖的CBE-ZnO结构比前期理论和实验上报道的一些ZnO同质异形体,如CsCl-ZnO、BCT-ZnO和RS-ZnO晶体结构较稳定。通过弹性性质和声子色散曲线的计算,证实了这种结构在力学和动力学上都为稳定相。此外,我们给出了CBE-ZnO的XRD谱图、红外IR和拉曼Raman谱图,通过提供尽可能多的ZnO新相结构的信息,从而有助于实验上合成CBE-ZnO结构。(2)以高度稳定的(ZnO)12纳米笼为结构单元,组建了串珠状ZnO纳米结构。第一性原理的计算结果表明：当串珠状ZnO纳米结构的(ZnO)12基本单元数大于等于3时,其能隙对结构的尺寸增加变得不敏感。另外,随着ZnO纳米结构尺寸的增加,最高占据分子轨道的能级向价带顶移动,而最低未占据轨道向导带底移动,这充分解释了串珠状ZnO纳米结构随尺寸而呈现能隙减小的变化。通过综合离子势,电子亲和势和化学硬度的分析结果,可知(ZnO)12×n纳米结构随着尺寸的增加化学活性增强。(3)系统地考察了零维sp-sp2杂化的碳结构C24、C36、C48、C60、C72、 C108和C120模型,这些模型被我们统称为富勒炔。我们从多个方面考察了富勒炔的稳定性。特别地,Oh对称的C96结构的稳定性较好,计算结果显示C96富勒炔为半导体材料,其能隙值约为0.51eV。此外,通过结构的考察可知,C96富勒炔表面具有较大的孔,从而C96富勒炔在金属团簇或气体分子等的存储方面,如锂离子和氢气的存储等方面表现出潜在的应用价值。(4)基于第一性原理计算,我们详细地考察了外加电场下碳纳米管和氧化锌(CNT-ZnO)复合体系的电子态密度、最高占据轨道和最低未占据轨道的分布以及不同电场下电荷的分布。计算结果发现：CNT-ZnO结构的平均相互作用能随着碳层数的增加而增加,表明了CNT-ZnO的相对稳定性随着碳层数增加而增加；同时这些复合体系拥有较小的能隙,且能隙与碳层数的函数关系显示了一种振荡的趋势。计算其离子势发现,所有复合体系的离子势比氧化锌纳米团簇的离子势7.155eV要小。当碳层数大于2时,氧化锌和碳的复合材料的离子势在有外加电场和无外加电场情况下都展示了奇偶振荡行为；含四层碳的CNT-ZnO体系在0.2eV/A外加电场下,具有最小的离子势,其数值为3.625eV。(5)提出了石墨烯和氧化物复合结构模型,即Graphene-ZnO复合体系。详细地考察了外加电场下Graphene-ZnO复合体系的电子性质和功函数。随着外加电场的增加,最高占据轨道和最低未占据轨道分子能级向真空层方向迁移,这导致复合材料的场发射的势垒降低；同时体系的功函数和离子势随外加电场的增加呈现线性降低趋势。在未加电场的情况下,Graphene-ZnO复合体系的功函数为3.623eV,而ZnO团簇和石墨烯纳米盘的功函数分别为4.970和4.022eV。复合体系比单纯的ZnO和石墨烯具有更小的功函数,从而体现Graphene-ZnO具有优越的场发射性质,可以作为潜在的场发射器件材料。(6)利用第一性原理方法,我们详细地考察了新颖的岛状型半导体性的石墨烯和氮化硼(Graphene-BN)复合物的场发射机理。重要的一点是,我们定量地统计了岛状Graphene-BN复合物的场发射电流。同目前碳纳米管和氮化硼纳米锥相比,在同样的外加电场下该岛状复合物拥有更大的场发射电流。在0.5V/A的外加电场下,碳纳米管的总场发射电流为60.9μA,相应的4P-N氮化硼纳米锥构型的场发射电流为24.9μA。然而,岛状的Graphene-BN复合结构拥有最大的场发射电流,在0.5V/A外加电场下,场发射电流高达210.9μA。岛状的石墨烯和硼化氮复合物能够作为优越的场发射材料,本研究能够为未来制造高速电子器件提供一个崭新的锲机。更多还原

【Abstract】 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.更多还原