【作者】 徐乐；

【导师】 于文学； 郑伟涛；

【作者基本信息】 吉林大学 ， 材料物理与化学， 2009， 博士

【摘要】 目前场发射冷阴极材料的研究与应用主要集中在碳基材料包括金刚石、非晶碳和碳纳米管等领域。其中非晶碳和碳纳米管制备温度较低且场发射性能更加优异,这使它们成为理想的场发射材料。然而对于他们的商业化应用,发射的强度和发射密度仍是一个主要问题。因此寻找能够提高和改善非晶碳和碳纳米管场发射性能的方法是十分重要和必要的。针对目前非晶碳及碳纳米管的场发射特性研究中的存在的问题。本文采用等离子体增强化学气相沉积方法(PECVD),合成了多壁碳纳米管薄膜,系统研究了催化剂(Fe,Co,Ni)、金属过渡层(Nb、Ni、Al、Ti)对碳纳米管结构及场发射性能的影响。结果表明,Fe,Co,Ni不同催化剂对合成碳纳米管的场发射性能有较大影响,由于Fe在三种催化剂中具有最低的功函数,因此由Fe合成的碳纳米管有最好的场发射性能;在Nb,Ti上生长的碳纳米管与过渡层金属可以形成稳定的共价键,这有利于碳纳米管场发射性质的提高。利用PECVD合成了内壁结晶、外壁非晶的碳纳米管薄膜并研究了其场发射特性,由于非晶的外壁可以引入大量缺陷,因此这种新型新型碳纳米管的场发射性能优于纯结晶碳纳米管与非晶碳纳米管。采用磁控溅射方法在硅衬底上沉积了非晶碳薄膜,研究了沉积条件、过渡层(Nb、NbC)及退火对其结构及场发射性能的影响。铌过渡层的引入增大了非晶碳薄膜的几何增强因子,高温退火可以使铌层与非晶碳层的界面处形成碳化铌,这都有利于提高非晶碳薄膜的场发射性能。立方相的碳化铌比六方相的碳化铌更有利于非晶碳薄膜场发射性能的提高,碳化铌过渡层内的Nb-C键与Nb-Nb键含量的比值越大,越有利于提高非晶碳薄膜的场发射性能。本论文对碳纳米管与非晶碳的场发射性质做了很好的基础研究,对合成和制备碳纳米管和非晶碳场发射材料有重要的理论指导意义。更多还原

【Abstract】 Field electron emission from carbon related materials such as diamond, carbon nanotubes （CNTs）, and amorphous carbon （a-C） films has been widely studied. These carbon-based emitters have been reported to show good electron field emission properties with a moderately low electric field because of their unique properties such as low work function and outstanding chemical inertness. a-C films is a promising field emitter that is very favorable to enhancing field emission. Some efforts have been made to improve the emission properties of a-C films and most work has focused on the surface treatment, doping and the quality of the films in order to improve their field emission properties. However, so far, there have been only a few reports about how buffer layer between a-C film and substrate does influence the field emission of carbon films. Even though a few mechanisms have been proposed, the influence of buffer layer on the field emission for a-C film is not well understood and the phenomena that are widely observed experimentally are not well explained yet. Nanotube production involves the widespread use of transition metal （TM） catalysts such as Ni, Co, or Fe, and also, many papers about electron field emission properties of carbon nanotubes have been published, in which carbon nanotubes demonstrated various excellent field-emission characteristics such as a low turn-on field, threshold field and field enhancement factor. However, there is no systematic study and comparison of electron field emission from carbon nanotubes prepared using different catalysts. Many reports have only attributed the field-emission performance of carbon nanotubes to their high aspect ratio （size effect）. Aside from the size effect, there is still relatively little known about the other factors affecting the emission properties of carbon nanotubes. Catalytic metal residue on nanotube may cause the change of the electronic states at the nanotube tips and then affect the field emission property of CNTs. In addition, the introduction of buffer layer may be a effective method to enhance the field emission properties of CNTs.In this work, we carry out the following investigation in order to improve the field emission properties of CNTs and a-C films.1. Synthesis and field emission properties of CNTs filmsWe have synthesized multi-walled carbon nanotubes on silicon substrates using plasma-enhanced chemical vapor deposition （PECVD） via decomposing methane using Fe, Co and Ni as the catalyst, and explore their field electron emission properties. It is found that the nanotubes synthesized using Fe catalyst have better field emission properties, compared with those synthesized using Ni and Co catalyst, which can be due to that the Fe catalyst has the lower work function than either Ni or Co catalyst. Futhermore, we find that the Ti or Nb buffer layer can substantially improve the electron field emission properties of CNTs, which is attributed to the formation of conductive Ti-C or Nb-C bonds, decreasing the interface barrier and improving the back contact between buffer layer and CNTs. This investigation suggests that the contact between buffer layer and CNTs plays an important role in the field emission properties of CNTs. In addition, the well-aligned CNTs growth on Ti buffer layer has the best field emission properties due to the high value ofβ.2. Synthesis and field emission properties of CNTs with cystalline inner and amorphous outer walls （C/A-CNTs）.we synthesize the carbon nanotubes with cystalline inner and amorphous outer walls （C/A-CNTs） on nickel catalyst using PECVD in a mixture of H₂ and CH₄ discharge gas, and find that that the structures of CNTs are strongly dependent on the gas flow rate ratio （R） of H₂/CH₄, in which lower R favors the formation of crystalline CNTs, whereas higher R promotes the growth of C/A-CNTs due to the larger size and lower activity for nickel catalyst as R is higher. The C/A-CNTs exhibits better field emission properties, compared to those amorphous or crystalline ones, which can be ascribed to lowering the work function by defect states induced by amorphous outer walls grown on the crystalline inner walls.3. Synthesis and field emission properties of a-C films We have prepared amorphous carbon （a-C） films on Si （100） substrates by radio frequent （RF） magnetron sputtering and investigated their field electron emission. We find that RF power can significantly affect the field emission of a-C films, in which the proper RF power （～200W） is more favourable to the enhancement in the field emission due to the higher disordered in the a-C films grown under 200W RF power.Nb buffer layer can substantially improve the electron field emission properties of the a-C films, which can be attributed to an increase in the enhancement factorβon the surface of the a-C films after insertion of the Nb layer. Moreover, the electron field emission can be further enhanced by annealing a-C/Nb/Si, which can be ascribed to the formation of NbC phase at the interface between a-C and Nb layer, revealed by X-ray diffraction for annealed a-C/Nb/Si. The first-principles calculated results show that the formation of NbC can lower the interface barrier and improve the back contact between Nb and a-C films, enhancing the field electron emission of a-C.NbC buffer layer with different phase structures and chemical bonding can substantially change the electron field emission properties of a-C films. The work function of NbC is lower than that of Nb₂C, which favors enhancing the electron emission properties of a-C. Moreover, the high ratio of Nb-C/Nb-Nb bonds in the NbC buffer layer is good for the field emission enhancement of a-C films, which is attributed to the lower work function of NbC than that of Nb.更多还原