A-B二元系纳米管填充和掺杂改性的第一性原理研究

【作者】 王素芳；

【导师】 张建民；

【作者基本信息】 陕西师范大学 ， 声学， 2013， 博士

【摘要】 纳米管具有较大的比表面积,很多物理和化学过程比如表面扩散、吸附、氧化、催化、腐蚀等等的发生都与表面有关。通过在纳米管的内外表面上掺杂吸附来修饰改性,可以大大扩展纳米管的应用范围。本文采用第一性原理方法,重点研究了过渡金属填充、吸附和替代掺杂A-B二元系纳米管的几何结构、电、磁、光学性质,主要得到以下重要的结论：(1)对于BCC结构Fe纳米线填充扶手椅型(8,8)A-B二元系纳米管(用Fen@(8,8)AB表示),当内部纳米线与外部纳米管之间的相互作用以物理方式(库仑力)为主要作用机制时,无论是纳米线还是纳米管均保持与它们初始相似的形貌特征；当内部纳米线与外部纳米管之间的相互作用以化学方式(杂化成键)为主要作用机制时,整个纳米缆结构将会发生较严重的结构扭曲,因而限制了复合体系某些方面的优越性质。结合能计算结果表明,我们所讨论的Fen@(8,8)AB纳米缆结构的形成过程均是放热的,而且,在以库仑相互作用为主的纳米缆结构中,Fe。纳米线的稳定性有所增强。关注Fe5@(8,8)AB系统的导电能力时发现,GeC纳米管和BN纳米管包裹Fe5纳米线后,纳米缆结构的导电能力相比于Fe5自由纳米线不变或有所增强,而SiN纳米管包裹Fe5纳米线使得复合结构的导电能力有所下降。不仅如此,在Fe5@(8,8)AB纳米缆结构(尤其Fe5@(8,8)GeC和Fe5@(8,8)BN)中,电子在费米能级处还保持了相对较高的自旋极化。因而,复合系统不仅能够应用在纳米微电子电路连接方面,还可能会在电子自旋注入方面得到较好的应用,从而实现了材料的多功能化。(2)对于HCP结构Fe纳米线填充锯齿型BN纳米管和C纳米管(分别用Fen@(m,0)BN和Fe@(n,0)C表示)而言,形成能计算结果建议Fe@(m,0)BN(8≤m≤15)系统中只有Fe@(8,0)BN系统的形成过程是吸热过程,其他系统都是放热的。而Fe@(n,0)C(8≤n≤14系统中也是只有Fe@(8,0)C系统的形成过程是吸热过程。最稳定的Fe@(13,0)BN系统的形成能小于最稳定的Fe@(12,0)C系统的形成能,说明BN纳米管在包裹保护HCP结构铁磁金属纳米线方面可能比C纳米管的包裹更加稳定。考查系统电子自旋极化发现,所有Fe@(n,0)C系统的自旋极化都非常高,而Fe@(m,0)BN系统中除Fe@(8,0)BN系统之外其他系统也均保持了高的自旋极化。因此,这样的纳米缆结构可以被应用在自旋电子学的某些方面,如电子自旋注入和自旋输运等。磁性分析表明,相比于C纳米管,BN纳米管在屏蔽其内部金属纳米线的磁矩方面具有潜在的优势。更重要的是,低维Fe纳米线在化学性质稳定的C纳米管或BN纳米管保护下可以防止被氧化和退磁,进而能够在常规环境下稳定存在较长的时间。(3)对于3d过渡金属原子吸附BN纳米管而言,不同的吸附原子最稳定的吸附位置是不同的。结合能计算结果显示,3d轨道未满的金属V、Cr和Mn原子能够强有力地吸附在扶手椅型(8,0)BN纳米管上(Eb>4.OeV);Sc、Ti、Co和Ni原子能够以化学方式吸附(1.OeV<Eb<2.0eV)；Fe和Cu原子只能以很小的结合力吸附(Eb<0.5eV)；Zn原子吸附是一个特例,即对于任意一个初始吸附位置,优化后计算的结合能几乎为零,意味着Zn原子不是以化学方式吸附在BN纳米管外表面上而是通过范德瓦尔斯力即物理方式吸附在管外壁上。特别指出的是,对于V、Mn和Fe原子吸附的(8,0)BN纳米管,只有一种类型的电子(自旋向上或自旋向下)穿越费米能级,意味着吸附系统具有半金属性,因而可以应用于生产近100%自旋极化电流的自旋电子学领域；对于Cr原子在洞位H吸附(8,0)BN纳米管,吸附系统即具有磁性又表现出半导体性,是一种潜在的稀磁半导体材料。(4)对于过渡金属原子M(M=V,Cr,Mn和Fe)替代掺杂BN纳米管,B19VN20和B19MnN20系统表现出半金属性特征。与Cr原子吸附BN纳米管相似,B19CrN20系统同样呈现出半导体和磁性双重特性,并且对于低浓度Cr替代掺杂BN纳米管,这种双重特性可能不依赖于或很少依赖于BN纳米管的手征性。光学介电函数的分析结果表明,M原子掺杂BN纳米管后虚部ε"(ω)出现明显的红移现象,导致了光吸收范围有所扩大。尤其是,B19CrN20系统在约0.3eV(红外区域)处显示了一个新的主吸收峰,意味着BN纳米管在外来Cr原子的调制作用下对红外光的吸收大大增强,因而B19CrN20系统在红外技术相关领域(如红外探测器、红外微波发射器等)可能具有潜在的应用。(5)受曲率的影响,完整的、Cd替代掺杂的和O缺失的ZnO纳米管的禁带带宽小于相同情况下类石墨烯结构ZnO单层的禁带带宽。对于ZnO单层和ZnO纳米管两种结构,Cd替代掺杂情况下的禁带带宽小于理想情况下的禁带带宽,相反地,O空位情况下的禁带带宽大于理想情况下的禁带带宽。特别地,Zn空位的出现为ZnO单层和ZnO纳米管引入了磁性。ZnO单层卷曲成纳米管结构后,曲率诱导的导带向费米能级的漂移使得光学吸收边红移,引起了理想和缺陷情况下ZnO纳米管结构对可见光的吸收。同时,电子在r点从价带顶激发到邻近导带上所需的光子能量减小(波长增大),相应的吸收峰向可见光区移动。不同于Cd替代掺杂和Zn空位,O空位的存在使得r处的电子可能更容易从价带顶激发到附近的导带上。这便导致在O空位情况下ZnO纳米管对可见光的吸收变得更强。我们所获得的结果对于改进太阳能辐射吸收新型材料的设计来说是一个有益的启发,为其提供了一个新思路。(6)对于提议的LiF纳米管状结构,虽然计算的平均原子结合能稍微小于体相结构LiF的结合能,但是较大的结合能仍然意味着LiF纳米管状结构存在的可能性。能带结构分析表明,锯齿型和扶手椅型LiF纳米管都具有绝缘特性,因此宽带隙的锯齿型和扶手椅型LiF纳米管可能在纳米光学及材料存储等方面有潜在的应用。当过渡金属M (M=V, Cr, Mn和Fe)原子吸附掺杂(8,0)LiF纳米管时,无论M原子在哪个初始吸附位置,优化后过渡金属原子几乎都位于其最近的F原子的顶位。特别是V原子、Cr原子和Mn原子吸附掺杂(8,0)LiF纳米管后,优化后的吸附系统均表现出半金属特征,因而有望应用在准一维自旋相关输运电子器件方面。更多还原

【Abstract】 Nanomaterials like nanotubes have the larger specific surface area, a lot of physical and chemical processes, such as surface diffusion, adsorption, oxidation, catalysis, corrosion and so on, are related to the surface. The modifications of doping on Inside and outside surface of the nanotubes can effectively expand their applications. In this study, Therefore, Using first-principles method, we pay special attention on the geometry structure, electronic, magnetic and optical properties of the transition metal filled, adsorbed and substitution doped A-B binary nanotubes. The important conclusions are shown as follows:(1) The structural, electronic and magnetic properties of single-wall (8,8) GeC, BN and SiN nanotubes filled with bcc Fen nanowires (n=5,9, and13)(denoted as Fen@(8,8)AB) have been investigated systematically using the first-principles PAW method. We find that the initial shapes of both wires and tubes are retained in the composite systems where the coulomb interaction is dominated between Fe nanowires and outer nanotubes, while those are badly distorted in the composite systems where the chemical bonding is dominated between Fe nanowires and outer nanotubes. The calculated binding energy of the Fen@(8,8)AB systems show that the forming progress of the considered composite is exothermic. Furthermore, the stability of the Fen nanowires is enhanced in the composite systems where the coulomb interaction is dominated between Fe nanowires and outer nanotubes. Concerning the conductivity of Fe5@(8,8)AB, compared with the corresponding free-standing Fe5nanowires, both Fe5@(8,8)GeC and Fes@(8,8)BN systems hold the same or higher conductivity, and Fe5@(8,8)SiN system holds lower conductivity. Additionally, the higher spin polarization at Fermi energy for the Fes@(8,8)AB composite systems is preserved, especially for the Fes@(8,8)GeC and Fes@(8,8)BN. Therefore, these composite systems can be used in the applications for the nanometer microelectronics circuit connection and spin injection, and thus the multi-functional materials occur.(2) The structural, electronic and magnetic properties of HCP Fe nanowire filled in (m,0) BN and (n,0) C nanotubes have been investigated using the first-principles PAW method. Among Fe@(m,0)BN (8≤m≤15) systems, only the Fe@(8,0)BN system is formed endothermically, the other larger systems are formed exothermically, while among Fe@(n,0)C (8≤n≤14) systems, only the Fe@(8,0)C system is also formed endothermically. Furthermore, the formation energy of the most stable Fe@(13,0)BN is smaller than that of the most stable Fe@(12,0)C, indicating BN nanotubes may be more appropriate to protect HCP ferromagnetic nanowires compared with C nanotubes. It is found that the all composite systems hold high spin polarization at Fermi energy with the exception of the Fe@(8,0)BN system. Hence, such nanocables have potential application in the spin inject and spin transport. The magnetic analysis shows that the BN nanotubes have potential advantage in shielding the magnetic moment of the inner metal nanowires. More important, the BN and C nanotubes can be protected Fe nanowires from oxidation and demagnetization, and thus these nanowires could be existed in the conventional environment for long time.(3) Adsorption configurations for ten kinds of3d transition metal M atoms adsorbed on the zigzag (8,0) BN nanotube at five different initial sites have been investigated using the first-principles PAW potential. The most stable adsorption sites are different for different M atoms. Partially filled3d metals V, Cr and Mn can bind strongly with zigzag (8,0) BN nanotube (Eb>4.0eV), and Sc, Ti, Co and Ni can be chemically adsorbed on the (8,0) BN nanotube (1.0eV<Eb<2.0eV). The binding between the Fe or Cu atom and the BN nanotube is only marginal (Eb<0.5eV). One unusual case is Zn. Its zero binding energy independent of the adsorption sites implies it can only physically adsorbed on the BN nanotube mainly stemmed from the van de Waals interaction. For V-, Mn-and Fe-adsorbed (8,0) BN nanotube, only one type of electrons (either spin up or spin down) passes through the Fermi level implies these adsorbed systems are all half-metals, suggest that these systems could be used in the field of spintronics devices for producing nearly100%spin polarized currents. The more stable possess magnetic and semiconducting characters, hence a promising diluted magnetic semiconductor.(4) The electronic structure, magnetic and optical properties of3d transition metal M (M=V, Cr, Mn and Fe) doped (5,5) boron nitride nanotube (B19MN20) are investigated by using the first-principles PAW potential. It is found that B19VN20and B19MnN20systems have half-metallic character. Similar to Cr-adsorbed (8,0) BN nanotube, the B19CrN20system also possess magnetic and semiconducting character that may be undependent or less dependent on the chirality of the BN nanotubes for low concentration Cr substitutional doped BN nanotubes. The analyses of optical dielectric functions show that in the ε"(ω) spectrum, the absorption edge of the pristine (5,5) BNNT is shifted towards the low energy after introducing M impurities, i.e., there is a significant redshifted phenomenon, and hence the light absorption range is increased. Remarkably, the ε"(ω) spectrum of B19CrN20system exhibits a new main peak at about0.3eV (infrared region), implying the absorption of infrared light is strongly enhanced, and thus may be utilized in the fields that are associated with the infrared technology, such as infrared detector, infrared maser and so on.(5) Attributing to the influence of curvature, the band gaps of the defect-free, Cd substitutional and O deficient ZnO nanotubes are decreased, as compared with the corresponding sheets. Moreover, for both ZnO sheets and ZnO nanotubes, the band gap of the substitutional Cd case is smaller than that of the defect-free case, by contraries, O vacancy case is larger. In particular, the Zn vacancy could introduce magnetism in both ZnO sheet and ZnO nanotubes. Curvature-induced drifting of the conduction bands towards the Femi level allows the red-shift of absorption edge, resulting in the absorption in visible region for both perfect and defective ZnO nanotubes. Meanwhile, the required energy is decreased when the electrons at valence-band maximum are excited to the adjacent conduction band at Γ-point, and the corresponding absorption peak shifts towards visible region. Different from the ZnO nanotubes with Cd impurity and Zn vacancy, at Γ-point the electronic excitation from valence-band maximum to adjacent conduction band may become easier in the case of O vacancy. This contributes strongly to the optical absorption in visible region. The results we obtain may be an inspiration for the design of new generation of materials with improved solar radiation absorption.(6) A systematic density functional theoretical study of the suggested single walled LiF nanotubes in armchair (n,n) and zigzag (n,0)(2<n<11) configurations is presented. Both zigzag and armchair LiF nanotubes can exist stably due to their large binding energies which are slightly smaller than the bulk LiF. Furthermore, the insulating character is observed for them, hence potential application in nanooptics and material storage. Given single transition metal M atom adsorbed stable (8,0) LiF nanotube, it is found that M atom almost locates on the top site of the nearest F atom, regardless of the initial sites. Dramatically, the half-metallic character is obtained for V-, Cr-and Mn-adsorbed (8,0) LiF nanotubes. Thus, these adsorbed systems are expected to be used in the field of quasi-one-dimensional spintronics devices.更多还原