石墨烯的电场效应及不同浓度的氢吸附研究

发布时间：2018-05-15 00:20

本文选题：石墨烯 + 电场效应　；参考：《太原理工大学》2015年硕士论文

【摘要】：自2004年在实验中成功制备出石墨烯以来，由于其具有特殊的物理和化学性质，因此在复合材料、电子元件和晶体管、传感器、太阳能电池、吸附剂等方面都有广泛的应用前景。石墨烯是碳原子以六边形形式排列的超薄片状纳米材料，其厚度仅有碳原子大小，是到现阶段为止世界上已知的最薄、最坚硬的物质，它接近全透明，且电阻率与其他材料相比较低，电子迁移速度也相对较快。本文主要是利用基于密度泛函理论的第一性原理计算方法的CASTEP软件包，采用模守恒赝势和超胞模型，系统地研究石墨烯纳米带的电场效应以及石墨烯吸附不同浓度的氢原子的色散曲线和声子振动谱。具体完成的工作如下：首先，利用第一性原理计算含有三个或四个Z型碳链的超窄石墨烯纳米带的电场效应，其导电机制随所加的垂直电场的改变而变化。在电场效应下，超窄石墨烯纳米带的最高价带与宽型石墨烯纳米带类似，而最低导带却有两种情况：自旋退化和自旋劈裂，这两种最低导带在量子空间是独立的。随着电场强度的增加，导电机制由最低导带的自旋退化转化成自旋劈裂。利用LDA和GGA泛函可以得到相同的理论结果，在实际计算中LDA和GGA通常都低估带隙，而利用GGA计算得到的带隙比LDA大。其次，利用密度泛函微扰理论计算不同氢原子覆盖度下石墨烯的声子色散曲线和声子振动谱。根据声子色散曲线以及声子态密度的特征频率可以识别氢原子的覆盖度。由氢原子的化学性质可知，氢原子在石墨烯上最稳定的吸附位是碳原子正上方，即顶位。随着氢原子覆盖度的降低，高频特征频率值逐渐增大，振动强度逐渐减小，最后趋于无限大石墨烯吸附单个氢原子的情形。当覆盖度为50%时，由于氢原子间的相互作用强，使石墨烯的晶格结构发生大的形变，破坏了原有对称性，，改变了石墨烯的本征振动模式，出现了两支高频特征振动频率，这是超高覆盖度的特征。当覆盖度降低时，石墨烯自身结构没有大的形变，本征对称性基本保持不变，于是双特征频率恢复简并，双特征峰变成单特征峰。这一理论预言可以帮助指导实验中对石墨烯上氢原子覆盖度的测量和表征。最后，在附录部分我们基于第一性原理，研究了卤素原子轰击萘分子开环裂解成链、以及负电和溶液效应的反应动力学，分析了以产物的碳链长度、结合能、存在时间和燃烧热为参数的成本模型。结果表明：(1)萘分子和卤素原子之间的电荷转移是萘环开环裂解的物理原因；(2)对于不同的轰击位置，萘分子的碳碳键可以选择性断裂成不同长度的碳链，生成的碳链可以进一步通过加氢形成液体燃料；(3)负电效应可以降低成本，乙醚溶液效应对成本的影响是双重的。
[Abstract]:Because of its special physical and chemical properties, graphene has been widely used in composite materials, electronic components and transistors, sensors, solar cells, adsorbents and so on since it was successfully prepared in 2004. Graphene is an ultrathin sheet material with carbon atoms arranged in hexagonal form. Its thickness is only the size of carbon atoms. It is the thinnest and hardest material known in the world so far, and it is close to full transparency. The resistivity is lower than that of other materials, and the electron migration rate is relatively fast. In this paper, we mainly use the CASTEP software package based on density functional theory (DFT) to calculate the first principles, and adopt the modular conserved pseudopotential and supercell model. The electric field effect of graphene nanobelts and the dispersion curves and phonon vibrational spectra of different concentrations of hydrogen atoms adsorbed by graphene were studied systematically. The work accomplished is as follows: First, the electric field effect of ultranarrow graphene nanobelts containing three or four Z-type carbon chains is calculated by first principles, and the conduction mechanism changes with the change of the vertical electric field. Under the electric field effect, the highest valence band of ultra-narrow graphene nanobelts is similar to that of broad graphene nanoribbons, but the lowest conduction bands have two conditions: spin degradation and spin-splitting, which are independent in quantum space. With the increase of electric field intensity, the conduction mechanism changes from the spin degradation of the lowest conduction band to the spin splitting. The same theoretical results can be obtained by using LDA and GGA Functionals. In practical calculations, LDA and GGA usually underestimate the band gap, but the band gap calculated by GGA is larger than that by LDA. Secondly, the phonon dispersion curves and phonon vibration spectra of graphene with different hydrogen atomic coverage are calculated by density functional perturbation theory. According to the phonon dispersion curve and the characteristic frequency of phonon density of states, the covering degree of hydrogen atom can be recognized. According to the chemical properties of hydrogen atom, the most stable adsorption site of hydrogen atom on graphene is the top position of carbon atom. With the decrease of hydrogen atomic coverage, the high frequency characteristic frequency increases gradually, the vibration intensity decreases gradually, and finally tends to the case of infinite graphene adsorbing a single hydrogen atom. When the covering degree is 50, because of the strong interaction between hydrogen atoms, the lattice structure of graphene is deformed greatly, the original symmetry is destroyed, the intrinsic vibration mode of graphene is changed, and two high frequency characteristic vibration frequencies appear. This is the characteristic of high coverage. When the coverage degree decreases, there is no big deformation in graphene structure, and the intrinsic symmetry remains basically unchanged, so the double characteristic frequency is degenerate and the double characteristic peak becomes a single characteristic peak. This theoretical prediction can help to guide the measurement and characterization of hydrogen atomic coverage on graphene in experiments. Finally, in the appendix, based on the first principle, we study the reaction kinetics of halide atom bombarding naphthalene molecules by ring opening and cracking, as well as the reaction kinetics of negative charge and solution effect. The length of carbon chain and binding energy of the product are analyzed. Cost model with time of existence and combustion heat as parameters. The results show that the charge transfer between naphthalene molecules and halogen atoms is the physical reason for the ring-opening cracking of naphthalene rings. For different bombardment sites, the carbon bond of naphthalene molecules can be selectively broken into carbon chains of different lengths. The resulting carbon chain can be further hydrogenated to form a liquid fuel, the negative charge effect can reduce the cost, and the effect of ether solution on the cost is dual.
【学位授予单位】：太原理工大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TQ127.11;O647.3

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