【作者】 韦建卫；

【导师】 胡慧芳；

【作者基本信息】 湖南大学 ， 材料物理与化学， 2008， 博士

【摘要】 当今科技发展的显著特征之一是电子器件微型化,这使得纳米电子学成为一门由凝聚态物理、量子化学以及材料学等学科相互交叉的学科。碳纳米管不仅可以作为研究低维量子系统中许多奇特的量子现象的一个理想对象,同时也在量子化学、材料学方面具有十分广泛的应用前景,特别是在量子导线、分子器件等方面潜力巨大。随着计算方法的发展以及计算能力的提高,基于第一性原理的密度泛函理论方法能够在目前实验技术或条件难以实现的原子层面的结构性能预测等方面得到广泛应用,已成为纳米电子学理论研究的重要手段。本文利用基于第一性原理的密度泛函理论、结合非平衡格林函数方法,对单壁碳纳米管中存在的取代掺杂、拓扑缺陷等异质掺杂体系进行了系统的研究,获得了一些有意义的结论。这对基于单壁碳管的电子器件的实际制备和开发具有重要意义。对不同的掺杂浓度和掺杂分布位型进行的模拟表明,氮掺杂显著改变了半导体型碳管的电子结构,例如碳管(8, 0)从半导体型转变为准金属型。其电流-电压曲线模拟结果也同时证明了这一点。随着电压的增大,氮掺杂碳管的电流呈现非线性变化。随着氮原子浓度增大,体系的电子输运有明显的提高。氮杂质浓度不变的情况下,最近邻杂质原子在距离3个碳原子以上时,对掺杂管小偏压下的电子输运性能尤为有利。对于单壁碳管中Stone-Wales缺陷和氮杂质原子共存的复合情况,本文也进行了系统的模拟。对于半导体型(8, 0)碳管,这种复合缺陷总体上对输运起到增强的作用。缺陷中掺杂位置的不同对碳管的电子输运影响不同。对于金属管(9, 0),复合缺陷使得其电流普遍下降。不同掺杂位置的碳管体系,其电流相对大小关系随偏压方向变化而改变。此外,氮原子和空位形成的复合缺陷对碳管的输运也有显著的影响,这种复合结构也对半导体型碳管输运性能有所提高,而对金属型碳管的输运表现出削弱的作用。类似于氮原子,硼原子也易于在碳管中发生取代掺杂,硼掺杂对碳管造成的的网格畸变不大,但随着硼原子浓度的增多,碳管的圆形截面逐渐转变为椭圆。半导体(8, 0)在硼掺杂后,随偏压和能量变化的体系总透射值由原来的带状结构变成离散的岛状。在小偏压区域出现了规则的菱形岛状透射平台。电流-电压曲线结果表明,硼掺杂后使得(8, 0)管由原来的半导体型转变为准金属型。随着掺杂分布的变化,体系的电流大小也明显不同。小偏压情况下杂质原子分散度越高,对应体系的输运性能就越好,而大偏压情况下输运对杂质分布并不敏感。对于金属型管(6, 0)管,硼掺杂使得体系电流下降。不同掺杂分布的金属型碳管的电流在大偏压时有明显的趋同倾向。而小偏压时,当超原胞内的杂质相隔2～3个碳原子时电流最大,距离为一个原子的情况电流最小。在门电压的作用下理想(6, 0)管透射系数的量子化特征消失,呈现为光滑曲线。在C47B1型取代掺杂中,大多数能量范围内的透射系数随着门电压的升高而下降。在两端加上偏压时,门电压对硼掺杂(6, 0)管的影响有所削弱。随着门电压的增强,电流的减小幅度逐渐增大。对浓度为C46B2的三种异构体的电流在小门电压情况下随门电压的增大而减小。但是随着门电压的继续增长,不同的杂质分布使得门电压的调节效果各不相同。(8, 0)管硼掺杂管的电流随门电压变化的幅度显然大于(6, 0),在门电压的作用下表现出较强的可调节特性。而负门电压情况下电流下降显著小于正偏压情况。而在偏压值较大时(包括正偏压和负偏压)门电压的影响都显著下降。利用掺杂来调制碳管的性能是非常有前途的一个方法,所以更加深入和广泛的研究碳管掺杂的性能以及器件制备是非常重要的研究课题。更多还原

【Abstract】 IV The miniaturization of electronic devices is the remarkable characteristic of the development of science and technology. This makes the nanoelectonics to become a cross-subject about condensed matter physics, quantum chemistry and material science. Carbon nanotubes not only are the perfect model used to investigate many peculiar quantum phenomenon of one dimensional system but also have extensive potential applications on condensed matter physics, quantum chemistry and materials science, especially on quantum wires and molecular devices. With the development of calculating methods and capability, the density functional theory based on the first principles is able to be extensive applied on the physical properties prediction of atomistic structures in which it is hardly caught by experiment. Moreover, it becomes one of important methods to investigate the nanoelectronic.Using the first principles density functional theory combined with noequilibrium green function technology, the paper systematically explores the impurity doping systems, such as substitutive doping, topological defect in single-walled carbon nanotubes, and gets some helpful results. It is significant for the practical preparation and development of the nanotubes-based devices.The simulations of the systems with different concentration and different distribution show that the nitrogen doping shifts the nanotube from semiconducting to quasi-metal. It is proved again by the later calculation of the current-voltage curves. With increasing voltage, the currents through the doped nanotubes are nonlinear variation. The transport properties of the system are improved with the increase of nitrogen concentration. The transport properties of system are optimum when the nitrogen atoms are separated by more than three carbon atoms.The paper also has simulated the situation that Stone-Wales defect and nitrogen substitutive doping present contemporarily. For semiconducting (8, 0) nanotube, the complex defects enhance the transport properties of the system as a whole. The transport properties of systems are sensitive to the position of nitrogen in SW defect. On the other hand, the complex defects reduce the current through the metal nanotubes (9, 0) universally. The different of the nitrogen position in SW defect has remarkable effect on the transport properties, and the order of currents of different models change with the direction of the bias. Moreover, the complex defects composed of nitrogen and vacancy are considered in the transport of nanotubes. These defects are favorable to the transport of semiconducting nanotube and unfavorable to the metallic one.Similar to nitrogen atom, boron atom is easy to substitutive dope in carbon nanotubes. Boron atoms doping cause few distortion of lattice, but the cross-session change from circle to ellipse with the increasing of boron concentration. After boron doping in semiconducting (8, 0), the total transmission coefficients as functions of energy and bias voltage change from banded structure to separated island. There are some regular rhombic high platforms when the bias is small. The results of I-V curves show that the boron doping shifts the nanotube from semiconducting to quasimetallic. The currents of the doped systems vary remarkably with the distribution of the boron atoms. The transport properties are better under small bias when the boron atoms decentralize better, whereas the transport properties of different kinds of distributions tend to be identical under high bias. For the metallic (6, 0), boron doping decreases the current of the systems. The transport properties of the doped nanotubes do not sensitive to the kinds of boron distribution. The doped systems have better transport properties when the boron atoms separated by 2 or 3 carbon atoms and the currents are the smallest when there is only one carbon atoms.The quantum characters of transmissions of (6, 0) disappear under the gate voltage stress and the curves show as sleek lines. For the models of C47B1, the values of transmission decrease with the adding of gate voltage in most of the area. The effect of the gate voltage on boron doped (6, 0) is weakened when the bias is added on the both leads. The amplitude of current decreasing increases with the adding of gate voltage. For the three isomers of C46B2, the current decreases with the increase of gate voltage when the gate voltage is small. With the continuous increase, the adjustment effects of the gate voltage are sensitive to the distribution models of boron atoms. Obviously, the adjustability of boron doped (8, 0) is better than that of doped (6, 0), and have a bigger amplitude of current decrease. The negative gate voltage has great effect on the current of the boron doped (8, 0). Nevertheless, the effect is weakened when the bias voltage is high.It is one of promiseful methods that to modulate the properties of nanotubes by impurity doping etc. So that it is an important subject to investigate the properties and the device fabrication of doped nanotubes more deeply and comprehensively.更多还原