Research On Field Emission Mechanism Of Carbon Nanometer Films

Many studies have indicated that the field emission F-N curves of carbon nanometer films manifest themselves the non-linearity. The standard F-N theory should be ideally adapted to flat cathode surfaces of metals with a sufficient number of free electrons. Thus, it may not be applicable for various carbon nanometer films that have complex structures with irregular shapes where the emission sites and electrical conductivity are significantly different from those of pure metal plates. Moreover, the number of field emission sites and parameters may not be constant when the high electric field of about 107 V/cm was applied to samples. Under the extremely high field, the field emission current was. limited by space charge limited current effect, screening effect, etc. So, in order to reasonably explain the field emission characteristics of carbon nanometer films, it may be necessary to modify the F-N theory.Taking carbon nano-needle films as the example, the discussion on the field emission mechanism of carbon nanometer films has been done. Thus, the Electric field region is divided into three regions:the low electric field region (unsaturated current density region), the middle electric filed region (partly-saturated current density region) and the high electric filed region (fully-saturated current density region). The field emission mechanism of carbon nanometer films in the low, middle and high E regions were reasonably interpreted by using modified classic F-N theory and space charge limited current model.In the low electric field region, the field emission current density steeply increased with increasing electric field. A modified F-N equation considering the Gauss distribution in structure of the field emission tips was obtained by the relation: IFN=CNE2exp[-Dr0/E+σ2(D/E)2/2]. The experimental data in the low electric field region were reasonably explained by using the modified F-N equation. In the high electric field region, the field emission current density tended to saturation with increasing electric field. An equation considering the space charge limited current (SCLC) effects represented in terms of the Child-Langmuir law was attained by the relation:ISC=KSC(E-Eh)3/2. It was shown that the theory equation well accorded with the experimental curves by curve fitting. In the middle electric field region, the field emission current density increased slowly with increasing electric field and showed the parted-saturated behavior. An equation considering the contribution of both IF-N controlled by the F-N mechanism and ISC controlled by the SCLC mechanism was obtained by the relation:I= k1IFN+k2ISC(0≤k1≤1,0≤k2≤1, 1≤k1+k2<2). The theory equation was well accorded with the experimental plots by adjusting the two parameters values.