Synthesis And Field Emission Properties Of Carbon Based Films

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.