Investigation On The Field Emission Properties Of Carbon Nanotube

Since the discovery by Iijima in 1991, carbon nanotubes(CNTs) have attractedattention of scientists from all over the world. Much efforts have been devoted to thepreparation and characterization of these quasi one-dimensional structures. Previousstudies on the properties of CNTs demonstrated the promising potential applications,including nanoscale optoelectronic and photonic devices, field effect transistor, fieldemission devices, hydrogen storage, and sensors for biological and chemical systems.Of these areas, field-emission-based flat-panel display is the closest to realizing thefirst commercial application. Compared with conventional metallic field emissiontips, CNTs as field emission electron source have many advantages: unusually highaspect ratio (long tube length and small curvature radius on the tip), high chemicalstability, low extracting field, high current density, and long operating time as well.Most field emission studies on CNTs are performed by the Fowler-Nordheim(F-N) tunneling theory, which is successful in describing the field emission ofmicro-sized metallic tips. F-N tunneling theory suggests that CNT field emissionprocess includes two steps: electrons transporting along CNT body to its tip andelectrons emitting from the tip by tunneling through the surface potential barriers.However, some experiments reveal that the field emission mechanism of CNTs,which shows nonlinear I-V characteristics of the F-N plots at high current range, isquite different from that of the conventional metallic tips. The intrinsic mechanism ofCNTs field emission is more complicated and needed for further investigation.Despite the latest development of nanotechnologies, gaining a detailedunderstanding of nanoscale materials remains to be a challenge. Simulation becamedefinitely necessary because of the restriction of experimental conditions. As for thetheoretical investigations, several groups have reported the electronic structures ofCNTs and localized states on CNTs tips. The geometrical and quantum size effectson CNTs field emission properties are also investigated. The emission currents ofvarious types of CNTs have been calculated, and the results show that the localizedstates on CNTs tips play an important role in CNTs emission process. In addition,some simulations have exhibited that capped CNTs have better field emissionproperties than open ended CNTs. Based on first-principles calculations, Maiti et al.report that the field emission current is enhanced significantly by the adsorption ofwater molecules (large dipole moment molecules are preferred) on CNTs tips. Theeffects of other adsorbates on the field emission of CNTs have also been studied.These enhancements of the field emission properties are attributed to the adjustmentof the local electronic structures of the emitting regions of CNTs. However, no clearrelationships between water adsorbates and the electronic structures of CNTs underapplied electric field have been built, and the mechanism of the enhancement of fieldemission by water adsorbates is not well clearly revealed, which requires furtherinvestigations.In this thesis, we mainly investigate the electronic properties of carbonnanotubes, as well as carbon nanotubes adsorbed with H2O, CH2O or doped withnitrogen atoms using first-principles method, which is based on Density FunctionTheory (DFT). These are all operated on the console of DMOL3, which is one of themodulus of Material Studio. Local density approximation (LDA) and generalgradient approximation (GGA) functional methods for band structure calculation ofinsulating and metallic solids are used to calculate the systems' properties, such asenergy, Fermi level, energy gap, work function, ionization energy and so on.In the introduction, we briefly described the calculated methods and the purposeof our study. Then, the basic knowledge of carbon nanotubes are presented, includingits discovery, geometrical structure, categories, variety of properties and potentialapplications. The structure for SWCNTs makes it uniquely different from othermaterials. In the following, the first-principle computation software DMOL3, whichis based on the Density Function Theory (DFT), is introduced. We give a detailedexplanation of Kohn-Sham Function and Local density approximation (LDA) andgeneral gradient approximation (GGA). The mechanism of field emission is alsoreferred to since it well explains our results.The electronic structures of capped (5,5) single-walled carbon nanotubes withwater physisorption on the tip with and without the applied electric field have beeninvestigated using first-principles density-functional theory. It is found that thestructures of capped (5,5) single-walled carbon nanotubes with water adsorption arestable under the field emission conditions. The ionization energy, which representsthe emission properties qualitatively, becomes smaller as the electric field increases.The calculated work function shows that it is easier for electrons to emit from thesystem in higher electric field. The energy gap between the lowest unoccupiedmolecular orbital and the highest occupied molecular orbital decreases significantlywhen water adsorbates are present under the applied electric field. These resultselucidate that the field emission properties of carbon nanotubes are enhanced due tothe adsorption of water molecules, which are consistent with the experiments results.Formaldehyde adsorbed at the tip is also simulated. Different phenomenonappears since that the C=O in the molecule carries much more negative electrons.This blocks the transfer of electrons from the body to the tip. Some of the results areopposite to that of water adsorbates. For example, the ionization energy increases asthe applied electric field increases, and the Fermi level becomes lowercorrespondingly. All these means that not all the adsorbates might enhance carbonnanotube field emission.N doping SWCNTs leads to that there is little change in the geometry of thesystem. Binding energy becomes higher since the different size between N atom andC atom. The calculated work function indicates that doping significantly decreasesthe work function, and it is preferred for N atom to substitute at the body of carbonnanotube to enhance the emission properties. Charge distribution is mainly affectedby the negativity of N atom.Finally, we come to the conclusion that some of the adsorbates enhance the fieldemission, such as H2O, and some not, such as CH2O. This needs further research. Ndoping into SWCNTs can improve the properties of field emission.