碳纳米管异型结构以及生长机理的理论研究
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摘要
因其独特的电子传输、机械、气体吸附等性质,碳纳米管已在多种领域得到广泛的应用。通过介入五元、七元或者八元等非六边形环连接而成的Y、T、X、I或者L型碳纳米管异质结,可用于制造二极管和晶体管等电子器件,也已经在纳米功能材料和纳米电子领域显示出良好的应用前景,引起物理、化学及材料等学科界的极大兴趣。但由于碳纳米管及异质结的纳米级结构,限于目前实验方法和实验条件的不足,碳纳米管以及碳纳米管异质结的合成控制和性质研究仍存在许多未知问题。而理论研究,可以在原子水平上直观有效地模拟碳纳米管的结构、性质和形成过程,对实验现象进行合理的解释,并为相关的机理研究提供研究手段和理论依据。
本论文应用Dmol~3、Gaussian 03程序包中的密度泛函理论,以及紧束缚势能函数以及Brenner势能函数对碳纳米管生长过程、碳纳米管异质结的性质进行了研究,主要内容包括:
1.综述了碳纳米管的结构、性质和生长机理,异型碳纳米管的合成方法,结构和稳定性的关系,电学、力学、热学、光学性质以及相关的分子模拟方法在异型碳纳米管研究中的应用进展,并简要介绍了其在电子器件,储氢材料以及其它功能复合材料方面的应用。最后,讨论了目前研究中存在的问题并展望了该领域今后的发展趋势。
2.构建了具有不同缺陷排列的X,T和Y型碳纳米管异质结,采用经验势能函数,讨论了最为稳定的缺陷排列方式并得出了异质结稳定性同管径大小的关系。结果表明,碳纳米管分支的直径、螺旋型和缺陷排列都是影响异型碳纳米管稳定型的重要因素,最稳定的Y,T和X型碳纳米管分别包括6,6和12个七元环。锯齿型异质结的稳定性高于扶手椅型,异质结原子的平均能量和管径存在一定的函数关系。
3.采用密度泛函理论B3LYP方法结合6-31G(d)基组,优化了两组弯曲型和直型碳纳米管异质结(3,3)-(6,0)和(4,4)-(8,0),并在此基础上比较了这两种典型异质结的几何特性、稳定性、电荷分布及电子结构的不同。结果表明,弯曲型异质结的稳定性要高于直型异质结,而且弯曲型和直型异质结的前线轨道具有明显不同的分布,HOMO和LUMO轨道能级差也具有很大的不同。为得到反铁磁性的自旋极化基态,同时采用自旋非限制性方法进行计算,结果表明直型(4,4)-(8,0)存在反铁磁性的自旋极化基态,自旋极化发生在锯齿型分支和缺陷环,并引起了电子结构的极大变化。另外,采用密度泛函理论GGA近似方法结合PW91函数,基于周期性边界条件的能带结构的计算结果表明弯曲型异质结的能带宽大于直型异质结的能带宽,说明直型异质结的导电性有强于弯曲型异质结的趋势。
4.运用密度泛函理论的GGA/PW91方法对有限长Y型碳纳米管的结构和性质进行了研究。结果表明,由于缺陷环的影响,Y型碳纳米管具有与直型管明显不同的性质。而且,Y型碳纳米管结构和性质同分支管长度有一定的关系。当分支管长度大于10(?)时,Y型碳纳米管的结构,能隙和电学性质均出现了周期性振荡变化的趋势。
5.采用密度泛函理论研究了金属催化剂颗粒存在下的碳纳米管生长过程中吸附于管壁的碳原子如何迁移到碳纳米管靠近金属颗粒表面的底部,转化为碳纳米管完整格点的过程。所有的吸附原子均能同已经生成的碳纳米管壁结合成强的共价键,说明碳纳米管吸附原子的转移是碳纳米管生长的重要因素。通过分析碳原子吸附在各个位点的能量,得到最为稳定的吸附点在碳纳米管和催化剂的连接处。由于催化剂的存在,吸附原子的迁移呈现高度的各向异性。尽管Cr、Mn、Fe、Co、Ni、Cu和Zn都可以用作化学气相沉积生产碳纳米管的催化剂,但是Co、Ni和Zn不仅使碳纳米管生长速率加快,而且生成的碳纳米管结构较完美,缺陷少,更适合于作催化剂。这个结果是碳纳米管根部增长机理的有益补充。以Co为催化剂,计算了持续添加碳原子,碳纳米管可能的生长路径。最后,讨论了催化剂在碳纳米管生长过程中的作用,主要表现在改变已经生成的碳纳米管壁的电荷分布,从而有利于碳原子的吸附、迁移最终生成完美的结构。
Carbon nanotubes (CNTs) have been widely applied in many fields owing to unique electron transport, mechanics, and gas adsorption properties. The straight carbon nanotubes can be connected into Y-, T-, X-, I- and L-type multi-terminal heterojunctions (MTHJs) by the introduction of defect rings to the perfect hexagonal lattice. Since the potential applications for the miniaturization of electronic devices such as diode and transistor, MTHJs have also attracted the attention of many researchers focusing on the preparations and properties both experimentally and theoretically. Due to the structures of CNTs and MTHJs with nanometer size, which limited in experimental methods and conditions, many unclear or unknown problems still exist in the studies of their syntheses and properties. The structures and properties of CNTs and related MTHJs, however, can be investigated using theoretical methods, which are expected to explain and predict the experimental work.
In this dissertation, the structures, properties of MTHJs, and growth mechanism of CNTs are studied using Dmol~3, Gaussian 03 passage, as well as some other program using tight-binding and REBO potential functions. The main contents include:
1. The structures, properties and growth mechanisms of the CNTs, as well as the synthetic methods of MTHJs, the relationship between the structure and stability, their electrical, mechanical, thermal and optical properties, and the applications of molecular simulation methods in the formation processes of MTHJs, are reviewed. In addition, their possible applications in nano-electronic devices, hydrogen storage and functional materials are summarized. Furthermore, some questions and the research trends of MTHJs are discussed.
2. A variety of multi-terminal junctions, including Y-, T- and X-type carbon nanotubes, were generated by connecting individual single-walled carbon nanotubes with different size and helicity. By optimizing these hetero-structures with different geometric features, the energetically favored structures were proposed. The corresponding energy features of the junctions indicate that the tube diameter, the helicity, and the distribution of defects, play an important role on the stability of junctions. The most stable Y-, T- and X-junctions contain six, six, and twelve heptagons, respectively. These defect rings tend to distributed at the corners of the junctions. The zigzag-type junctions are more stable than the armchair ones. Moreover, the average energy of an individual atom for Y-, T-, and X- junction is found to be a function of the tube diameter.
3. The properties of four finite-lengh bent and straight intramolecular junctions (IMJs) connecting two armchair and zigzag single-walled carbon nanotube segments, viz. (3,3)-(6,0) and (4,4)-(8,0), were investigated. Their structures were calculated using the density functional theory (DFT) methods at the B3LYP/6-31G(d) level of theory. The results indicate that the bent junctions are more stable than the straight ones due to the energetically favored defect structures. Remarkable differences of the HOMO and LUMO orbitals appear between the straight and the bent IMJs. The spin-unrestricted calculations at the same level of theory were also performed to obtain the antiferromagnetic-type ground state, suggesting that the spin polarization mainly occur on the zigzag edge and the defect rings of the straight (4,4)-(8,0) IMJ, and induce marked changes of the electronic structures. Additionally, the energy band structures of the four junctions with periodic boundary conditions were calculated based on DFT calculations using generalized gradient approximation (GGA) with the Perdew and Wang function (PW91). The calculated band gaps suggest that the conductance of the straight IMJs is higher than the bent ones.
4. The geometry and electronic structure of finite-length (4,4) Y-shaped CNTs were investigated using DFT with GGA/PW91 method. The results indicate that the difference between the Y-shaped CNTs and the pristine one is remarkable due to the influence of the defects in the junctions of the former. Furthermore, the structures and properties of the Y-shaped CNTs are found to be related to the length of the CNT branch. By comparing the properties of the Y-shaped CNTs with different lengths, the length-dependent oscillation behavior including structure, energy gap and electrical property were observed when the length is longer than 10(?).
5. The formation processes of the CNTs with metal catalyst were simulated to decipher the underlying growth mechanism. The metal catalyst is modeled by a cluster with 13 metal atoms. The DFT method is used to investigate the migration of carbon atoms from adsorption sites on the surface to the end of the nanotube, in the process of catalytic growth. All the adsorbed carbon atoms are found to form strong bonds with the nanotube, indicating that adsorptions are an important factor for carbon nanotube growth. By studying the total energy of one carbon atoms adsorbed at all possible sites, it is found that the energy favorable sites are at the interacttion region between carbon nanotube and the discussed catalyst. Calculated migration energies for the adatoms show the migration is highly anisotropic due to the influence of catalyst. Cr, Mn, Fe, Co, Ni, Cu, and Zn are all found to favor the CNT growth, moreover, the Co, Ni and Zn appear to be better than the others. The results may be beneficial supplements for the root-growth mechanisms of carbon nanotubes. The possible growth process rooted on a Co particle was calculated by adding carbon atoms continually. Furthermore, the role of the metal catalyst was discussed, indicating that it will affect the charge distribution of the end part of the nanotube, conducive to the adsorption and migration of the carbon atom to form a perfect tube.
本论文应用Dmol~3、Gaussian 03程序包中的密度泛函理论,以及紧束缚势能函数以及Brenner势能函数对碳纳米管生长过程、碳纳米管异质结的性质进行了研究,主要内容包括:
1.综述了碳纳米管的结构、性质和生长机理,异型碳纳米管的合成方法,结构和稳定性的关系,电学、力学、热学、光学性质以及相关的分子模拟方法在异型碳纳米管研究中的应用进展,并简要介绍了其在电子器件,储氢材料以及其它功能复合材料方面的应用。最后,讨论了目前研究中存在的问题并展望了该领域今后的发展趋势。
2.构建了具有不同缺陷排列的X,T和Y型碳纳米管异质结,采用经验势能函数,讨论了最为稳定的缺陷排列方式并得出了异质结稳定性同管径大小的关系。结果表明,碳纳米管分支的直径、螺旋型和缺陷排列都是影响异型碳纳米管稳定型的重要因素,最稳定的Y,T和X型碳纳米管分别包括6,6和12个七元环。锯齿型异质结的稳定性高于扶手椅型,异质结原子的平均能量和管径存在一定的函数关系。
3.采用密度泛函理论B3LYP方法结合6-31G(d)基组,优化了两组弯曲型和直型碳纳米管异质结(3,3)-(6,0)和(4,4)-(8,0),并在此基础上比较了这两种典型异质结的几何特性、稳定性、电荷分布及电子结构的不同。结果表明,弯曲型异质结的稳定性要高于直型异质结,而且弯曲型和直型异质结的前线轨道具有明显不同的分布,HOMO和LUMO轨道能级差也具有很大的不同。为得到反铁磁性的自旋极化基态,同时采用自旋非限制性方法进行计算,结果表明直型(4,4)-(8,0)存在反铁磁性的自旋极化基态,自旋极化发生在锯齿型分支和缺陷环,并引起了电子结构的极大变化。另外,采用密度泛函理论GGA近似方法结合PW91函数,基于周期性边界条件的能带结构的计算结果表明弯曲型异质结的能带宽大于直型异质结的能带宽,说明直型异质结的导电性有强于弯曲型异质结的趋势。
4.运用密度泛函理论的GGA/PW91方法对有限长Y型碳纳米管的结构和性质进行了研究。结果表明,由于缺陷环的影响,Y型碳纳米管具有与直型管明显不同的性质。而且,Y型碳纳米管结构和性质同分支管长度有一定的关系。当分支管长度大于10(?)时,Y型碳纳米管的结构,能隙和电学性质均出现了周期性振荡变化的趋势。
5.采用密度泛函理论研究了金属催化剂颗粒存在下的碳纳米管生长过程中吸附于管壁的碳原子如何迁移到碳纳米管靠近金属颗粒表面的底部,转化为碳纳米管完整格点的过程。所有的吸附原子均能同已经生成的碳纳米管壁结合成强的共价键,说明碳纳米管吸附原子的转移是碳纳米管生长的重要因素。通过分析碳原子吸附在各个位点的能量,得到最为稳定的吸附点在碳纳米管和催化剂的连接处。由于催化剂的存在,吸附原子的迁移呈现高度的各向异性。尽管Cr、Mn、Fe、Co、Ni、Cu和Zn都可以用作化学气相沉积生产碳纳米管的催化剂,但是Co、Ni和Zn不仅使碳纳米管生长速率加快,而且生成的碳纳米管结构较完美,缺陷少,更适合于作催化剂。这个结果是碳纳米管根部增长机理的有益补充。以Co为催化剂,计算了持续添加碳原子,碳纳米管可能的生长路径。最后,讨论了催化剂在碳纳米管生长过程中的作用,主要表现在改变已经生成的碳纳米管壁的电荷分布,从而有利于碳原子的吸附、迁移最终生成完美的结构。
Carbon nanotubes (CNTs) have been widely applied in many fields owing to unique electron transport, mechanics, and gas adsorption properties. The straight carbon nanotubes can be connected into Y-, T-, X-, I- and L-type multi-terminal heterojunctions (MTHJs) by the introduction of defect rings to the perfect hexagonal lattice. Since the potential applications for the miniaturization of electronic devices such as diode and transistor, MTHJs have also attracted the attention of many researchers focusing on the preparations and properties both experimentally and theoretically. Due to the structures of CNTs and MTHJs with nanometer size, which limited in experimental methods and conditions, many unclear or unknown problems still exist in the studies of their syntheses and properties. The structures and properties of CNTs and related MTHJs, however, can be investigated using theoretical methods, which are expected to explain and predict the experimental work.
In this dissertation, the structures, properties of MTHJs, and growth mechanism of CNTs are studied using Dmol~3, Gaussian 03 passage, as well as some other program using tight-binding and REBO potential functions. The main contents include:
1. The structures, properties and growth mechanisms of the CNTs, as well as the synthetic methods of MTHJs, the relationship between the structure and stability, their electrical, mechanical, thermal and optical properties, and the applications of molecular simulation methods in the formation processes of MTHJs, are reviewed. In addition, their possible applications in nano-electronic devices, hydrogen storage and functional materials are summarized. Furthermore, some questions and the research trends of MTHJs are discussed.
2. A variety of multi-terminal junctions, including Y-, T- and X-type carbon nanotubes, were generated by connecting individual single-walled carbon nanotubes with different size and helicity. By optimizing these hetero-structures with different geometric features, the energetically favored structures were proposed. The corresponding energy features of the junctions indicate that the tube diameter, the helicity, and the distribution of defects, play an important role on the stability of junctions. The most stable Y-, T- and X-junctions contain six, six, and twelve heptagons, respectively. These defect rings tend to distributed at the corners of the junctions. The zigzag-type junctions are more stable than the armchair ones. Moreover, the average energy of an individual atom for Y-, T-, and X- junction is found to be a function of the tube diameter.
3. The properties of four finite-lengh bent and straight intramolecular junctions (IMJs) connecting two armchair and zigzag single-walled carbon nanotube segments, viz. (3,3)-(6,0) and (4,4)-(8,0), were investigated. Their structures were calculated using the density functional theory (DFT) methods at the B3LYP/6-31G(d) level of theory. The results indicate that the bent junctions are more stable than the straight ones due to the energetically favored defect structures. Remarkable differences of the HOMO and LUMO orbitals appear between the straight and the bent IMJs. The spin-unrestricted calculations at the same level of theory were also performed to obtain the antiferromagnetic-type ground state, suggesting that the spin polarization mainly occur on the zigzag edge and the defect rings of the straight (4,4)-(8,0) IMJ, and induce marked changes of the electronic structures. Additionally, the energy band structures of the four junctions with periodic boundary conditions were calculated based on DFT calculations using generalized gradient approximation (GGA) with the Perdew and Wang function (PW91). The calculated band gaps suggest that the conductance of the straight IMJs is higher than the bent ones.
4. The geometry and electronic structure of finite-length (4,4) Y-shaped CNTs were investigated using DFT with GGA/PW91 method. The results indicate that the difference between the Y-shaped CNTs and the pristine one is remarkable due to the influence of the defects in the junctions of the former. Furthermore, the structures and properties of the Y-shaped CNTs are found to be related to the length of the CNT branch. By comparing the properties of the Y-shaped CNTs with different lengths, the length-dependent oscillation behavior including structure, energy gap and electrical property were observed when the length is longer than 10(?).
5. The formation processes of the CNTs with metal catalyst were simulated to decipher the underlying growth mechanism. The metal catalyst is modeled by a cluster with 13 metal atoms. The DFT method is used to investigate the migration of carbon atoms from adsorption sites on the surface to the end of the nanotube, in the process of catalytic growth. All the adsorbed carbon atoms are found to form strong bonds with the nanotube, indicating that adsorptions are an important factor for carbon nanotube growth. By studying the total energy of one carbon atoms adsorbed at all possible sites, it is found that the energy favorable sites are at the interacttion region between carbon nanotube and the discussed catalyst. Calculated migration energies for the adatoms show the migration is highly anisotropic due to the influence of catalyst. Cr, Mn, Fe, Co, Ni, Cu, and Zn are all found to favor the CNT growth, moreover, the Co, Ni and Zn appear to be better than the others. The results may be beneficial supplements for the root-growth mechanisms of carbon nanotubes. The possible growth process rooted on a Co particle was calculated by adding carbon atoms continually. Furthermore, the role of the metal catalyst was discussed, indicating that it will affect the charge distribution of the end part of the nanotube, conducive to the adsorption and migration of the carbon atom to form a perfect tube.
引文
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