Shunfu Xu

Work functions of capped (5, 5) and (9, 0) single-walled carbon nanotubes adsorbed with alkali-metal atoms

The influence of alkali metal (Li or Cs) adsorption on the work functions of capped (5, 5) and (9, 0) single-walled carbon nanotubes (CNTs) was investigated using first-principles calculations. After Cs adsorption, the decrease in the work functions of (5, 5) and (9, 0) CNTs was more pronounced than that of Li-adsorbed CNTs. The decline in the work functions was due to the increase in Fermi levels and the decrease in vacuum levels induced by electrons transfer from the alkali atom to the CNTs. A vacancy defect raised the work functions of the pristine and alkali-metal-adsorbed CNTs.

Role of alkali metal adsorption and defect position on the work function of a (5, 5) capped single-walled carbon nanotube

The authors used first-principles calculations to investigate the influence of alkali metal (Li/Na/Cs) adsorption and defect position on the work function of a (5, 5) armchair single-walled carbon nanotube (CNT) with a capped edge. The atomic Cs adsorption can more effectively reduce the work function of the CNT than the atomic Li/Na adsorption. Adsorption positions have a measurable impact on the work function of the CNT. Any vacancy defect on the tip can raise the work function of the CNT regardless of whether or not an alkali metal atom is absorbed. The variations of work functions are mainly attributed to the change of Fermi levels induced by charge redistributions. The alkali metal adsorption can also transform the semiconducting CNT into a metallic tube, which is significant for the CNTs as a promising field emission cold cathode material.

Field emission characteristics of pristine and lithium-doped boron nanotubes: A theoretical study

First-principles calculations are used in order to investigate the electronic and field emission properties of capped (5, 5) and (9, 0) boron nanotubes (BNTs), which indicate that the electric currents of the (5, 5) and (9, 0) BNTs under an applied electric field are very close to those of carbon nanotubes, and pentagons and hexagons on the tips of the BNTs are the most possible spots for emitting tunneling electrons under an external electric field. In addition, the work functions of the (5, 5) and (9, 0) BNTs decrease linearly with applied electric fields. The significant influence of lithium adsorption on field emission characteristics of BNTs is also studied. The work functions of BNTs decrease distinctly after lithium adsorption, while the emission currents increase by a large margin. Moreover, the lithium adsorption can improve the electric conductivity of a mixture of BNTs.

Simulation of field enhancement and electron focusing performance of ZnO nano-cone arrays in planar gated emitter

The electric field and electron focusing performance of ZnO nano-cone arrays in planar gated emitter is mimulated by finite element method. The influences of the ZnO arrays with various geometric structures on surface field of the cone will be presented. Focusing performance of the electron beam is also discussed.

Modulation of the work function of fullerenes C60 and C70 by alkaline earth metal adsorption: A theoretical study

The significant influence of alkaline earth metal (Be/Mg/Ca/Sr/Ba/Ra) adsorption on work functions of fullerenes C60 and C70 was investigated by first-principles calculations. The work functions of fullerenes C60 and C70 with Ca/Sr/Ba/Ra adatoms decrease linearly with the electronegativities of the alkaline earth metals. The work functions are also affected considerably by adsorption positions. The variations of the work functions depend on the changes of Fermi level (which is attributed to charge transfer) and the changes of vacuum levels (which is attributed to induced dipole moments). Moreover, the alkaline earth metal adsorption can also improve the electric conductivity of a fullerene mixture.

First-principles study of interaction of lithium atoms with H-adsorbed (3, 3) single-walled carbon nanotube

In this paper, we have employed density functional theory to investigate the adsorption mechanisms of one lithium atom on sidewalls of H-adsorbed (3, 3) single-walled carbon nanotubes (SWCNTs) which have vacancy defects. An understanding of influence of hydrogen adsorption and vacancy defects on lithium adsorption is obtained.

Roles of alkali-metal adsorption and defect position on work functions of capped single-wall carbon nanotubes

Summary form only given. The influence of alkali-metal adsorption positions and defects positions on work functions of (5, 5) single-walled carbon nanotubes (CNTs) with a capped edge had been investigated by first-principles calculations. An single-walled armchair (5, 5) CNT with a capped edge was assumed. A single vacancy defect was created by removing a carbon atom from different atomic layers (which were labeled as T1-T4 in FIG. 1(a)). The alkali-metal adatoms (Li/Na/Cs) were located above the center of the pentagons or hexagons (which were labeled as P1-P4 in Fig. 1(b)) on the caps for the perfect CNT (P-CNT), while they were associated with defective CNTs (D-CNTs) on the vacancy defects. After Li/Na/Cs adsorption, the work functions of the Pand D-CNTs along the Z-axis and the X-axis (X-WF) decrease significantly. Compared with adsorption of one Li/Na atom, the work functions of CNTs in axial or radial directions decreased more obviously after Cs adsorption. For comparison purpose, FIG. 1(c) summarizes the work functions of P-CNTs and D-CNTs (Ti) with alkali-metal adatoms on the top, plotted against the electronegativity of Li, Na and Cs. All the axial and radial work functions of (5, 5) Pand D-CNTs with Li/Na/Cs on P1 increase linearly with the electronegativity. The curves for the axial or radial work functions are almost parallel to each other. We plot the work functions of the (5, 5) P-CNT with alkali-metal adatoms on different positions in FIG. 1(d)-(e). For the adatom-P-CNT systems, there is no significant difference between the work functions in radial direction (except lower work functions in P3), while the work functions in axial direction actually depend on the adsorption position of alkali-metal atoms. The adatom-P-CNT systems have the lowest work functions with alkali-metal adatoms on P1 in axial direction and on P3 in radial direction. The work functions of different D-CNTs and adatom-D-CNTs systems with one vacancy defect in different atomic layers are shown in FIG. 1(f)-(g). One vacancy defect could raise the work functions of the CNTs. For the adatom-D-CNTs systems, there is no visible trend between the work functions in axial direction, while the work functions in radial direction show a monotonously decrease from T1 to T4. The adatom-D-CNTs systems have the lowest work functions with the Li adatom on T4 and the Na/Cs adatom on T2 in axial direction and with alkali-metal adatoms on T3 in radial direction. Since the electronegativity of Li/Na/Cs is less than carbon, the Li/Na/Cs adatoms on the CNTs are easily ionized. The charge density redistributions or charge transfer will lead to increase of the Fermi levels of the Pand D-CNT. The variation of work functions can be induced by either an enhanced (reduced) surface dipole moments, or a lowering (rising) of its intrinsic bulk Fermi levels [1]. Our results show that the changes of the work functions mainly come from the shifts of Fermi levels. The induced dipole moments lead to a minor decrease in the work functions.