不同电场下碳纳米管场致发射电流密度研究

Field-emission current densities of carbon nanotube under the different electric fields

本文运用密度泛函理论和金属电子论,深入研究了碳纳米管场致发射电流的变化规律.结果显示其发射电流密度取决于体系的态密度、赝能隙、管长和局域电场,在不同范围电场下的变化规律不同.在较低电场下,发射电流密度随电场增强而近似线性增大(对应的宏观电场须小于18 V·μm-1);但在较高电场下,发射电流密度随外电场增加呈现非周期性振荡增长趋势,碳纳米管表现为电离发射.本文进一步研究了金属性碳纳米管电导率在不同电场下的变化规律.

The field emission current variation law of carbon nanotube in a large electric field range （0–32 V·μm-1 is analyzed in depth by combining the density functional theory with metal electron theory. The results show that their emission current densities are determined by their densities of states, the pseudogap, the length and the local electric field, showing the different variation laws in the different electric field ranges. In the lower electric field （corresponding macroscopic field is less than 18 V·μm-1, when their density of states increases, their pseudogap decreases： the two trends are opposite, the former increases the number of electrons for emission, and the latter improves the ability to transfer electrons, they all turn to the increase of the emission current, so their field-emission current density increases linearly with increasing electric field in this range. But in the higher electric field （corresponding macroscopic field is less than 32 V·μm-1and more than 18 V·μm-1）, their densities of states and the pseudogaps take on the same decrease and increase, so do they in the opposite change case, therefore the emission current density behaves as a non-periodic oscillation in the increasing electric field, moreover the higher electric conductivity lead to the rising of current density, the combined effect of the emitter current density exhibits an oscillatory growth in this electric field range, and the carbon nanotubes behave as ionizing radiation. So the too high electric field may cause the emission current to be instable. The electric conductivity variation law of the metallic carbon nanotube is further studied in this paper. In the lower electric field （corresponding macroscopic field is less than 5 V·μm-1, the electric conductivity of CNT increases linearly with increasing electric field;when the macroscopic electric field increases up to a value in a range from 5 to 14 V·μm-1 the electric conductivity only changes like a slight concussion in （6.3–9.9） × 1017 S·m-1 range, when the macroscopic electric field increases to a value in a range from 16 to 32 V·μm-1 the electric conductivity appears as a sharp oscillation growth trend. Additionally, the specific binding energy of CNT is enhanced with increasing electric field, accordingly the structural stability turns better and the cone-capped carbon nanotubes could be used for emission cathode material. The calculation results are consistent with the experimental results of the literature.