Adsorbate Effects on Charge Distribution of Carbon Nanotube Simulated by a Multi-scale Method

摘要

The apex structure of CNT has impact influence to the FE process. It has been observed that the hydrogenation of the tube wall transforms a metallic CNT to a semiconducting one [1], that O{sub}2 exposure increases the turn-on field of SWCNTs and decreases the FE efficiency [2] and that the adsorption of H{sub}2O enhances the field emission current. However, experimental observations so far have not been conclusive [3-5]. To understand the dependence of FE upon the atomic structures of apexes of CNTs, careful simulations via the density functional method (DFT) have been done. There are contradictory conclusions about the effect of adsorbates. Zhou et al [6] and Kim et al [7] obtained the local density of states (LDOS) at the apex by ab initio methods; they found that the LDOS at the charge-neutrality level was suppressed by the hydrogen. They therefore concluded that hydrogen adsorption reduces FE current density. By contrast, Mayer et al. calculated the AVB using a dipole and point charge model [8]; they assumed that the apex-vacuum barrier was reduced by the presence of the hydrogen, and concluded that hydrogen adsorption enhances the FE current density. More careful studies on this topic would obviously be useful. Only recently, it is possible to tackle the realistic size of single-walled carbon nanotubes (SWCNTs) in field electron emission (FE) conditions by a multi-scale method involving quantum mechanics and molecular mechanics [9,10]. We adopted this method to simulate the FE from open-ended SWCNTs of realistic length (1μm) with atomic decorations. The purpose of this study is to optimize the apex structure of the SWCNT to achieve higher FE efficiency. Obviously the apex-vacuum barrier (AVB) and thereby the FE characteristic will be strongly affected by the electron transfer between carbons and adsorbates and among the adsorbate atoms. The atomic decoration and geometric symmetry breaking in the apex will lead to formation of dipoles and/or quadrupoles (referred to as spontaneous multipoles). If the dipole has its positive end outward to the vacuum (positive dipole), it tends to suppress the AVB; otherwise (negative dipole), it tends to raise the barrier. This simple argument suggests that the carbon dangling bonds in the opened end of the SWCNT should be saturated by atoms of lower electronegativity (denoted by X{sub}s) in contrast with carbons. The electron supply is another important component for FE current. For a given AVB, the emission current is proportional to the incident electron flux in the Fermi level. Could one suppress the AVB and have higher electron density in the apex in the same time? The simulation here suggested that a spontaneous quadrupole introduced by atomic decoration in the free end of the SWCNT would be helpful for this purpose.