Chunshan He

Screening effects on field emission from arrays of (5,5) carbon nanotubes : Quantum mechanical simulations

The simulation of field electron emission from arrays of micrometer-long open-ended (5, 5) carbon nanotubes is performed in the framework of quantum theory of many electrons. It is found that the applied external field is strongly screened when the spacing distance is shorter than the length of the carbon nanotubes. The optimal spacing distance is two to three times of the nanotube length, slightly depending on the applied external fields. The electric screening can be described by a factor that is a exponential function of the ratio of the spacing distance to the length of the carbon nanotubes. For a given length, the field enhancement factor decreases sharply as the screening factor larger than 0.05. The simulation implies that the thickness of the array should be larger than a value but it does not help the emission much by increasing the thickness a great deal.

The roles of apex dipoles and field penetration in the physics of charged, field emitting, single-walled carbon nanotubes

A 1 μm long, field emitting, (5, 5) single-walled carbon nanotube (SWCNT) closed with a fullerene cap, and a similar open nanotube with hydrogen-atom termination, have been simulated using the modified neglect of diatomic overlap quantum-mechanical method. Both contain about 80 000 atoms. It is found that field penetration and band bending, and various forms of chemically and electrically induced apex dipole play roles. Field penetration may help explain electroluminescence associated with field emitting CNTs. Charge-density oscillations, induced by the hydrogen adsorption, are also found. Many of the effects can be related to known effects that occur with metallic or semiconductor field emitters; this helps both to explain the effects and to unify our knowledge about FE emitters. However, it is currently unclear how best to treat correlation-and-exchange effects when defining the CNT emission barrier. A new form of definition for the field enhancement factor (FEF) is used. Predicted FEF values for these ...

Anode Distance Effect on Field Electron Emission from Carbon Nanotubes: A Molecular/Quantum Mechanical Simulation

Field electron emission from single-walled (5,5) carbon nanotubes was simulated with a quantum chemistry method, emphasizing the effect of distance between the anode and apex. The emission probability and the field enhancement factor were obtained for different anode-apex separations with two representative applied macroscopic fields. The quantum chemistry simulation was compared to the classical finite element calculation. It was found that the field enhancement factor was overestimated by about a factor 2 in the classical calculation (for the capped carbon nanotube). The effective work function lowering due to the field penetration into the apex has important contribution to the emission probability. A peculiar decrease of the effective work function with the anode-apex separation was found for the capped carbon nanotube, and its quantum mechanical origin is discussed.

Work Function Of Boron Carbide: A Dft Calculation

The work functions of (001) and (00-1) surfaces of B4C are investigated with density functional theory and symmetry slab model. These two surfaces are found to be almost nonpolarized and their work functions are 5.15 eV and 5.46 eV, respectively.

Quantum-Mechanical Investigation on Field Emission Properties of Carbon Nanotube Array

This study investigates quantum mechanically the effect of spacing of aligned carbon nanotubes on field emission properties. It is known that for a given length of CNTs, it is beneficial for electron emission as the spacing of CNTs are increased. As the spacing of CNTs reduces, the effect of screening becomes stronger and current density become smaller. It is very useful to know the relationship between current density and the spacing of CNTs quantitatively

Effects of Anode Locations on the Field Electron Emission from CNTs

The modified neglect of diatomic overlap semiempirical quantum mechanical method, a multiscale quantum/classical hybrid method, is used to simulate the field electron emission in single-wall carbon nanotubes. The classical analysis and quantum computations have shown that a larger separation of the anode from the emitter enhances the field electron emission

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

The apex structure of the SWCNT has been optimized 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 FE characteristics of open-ended SWCNTs with their open ends terminated by -O, -BH and -NH, respectively were compared. The simulations have shown that the -BH and -NH adsorbates have much larger binding energy than both oxygen saturated and hydrogen, two adsorbates should be superior for providing stable FE current. The -NH adsorbate is superior in FE characteristics than -O adsorbate, and -BH is the best among the three

Simulations on and Theoretical Understanding to the Field Emission from Carbon Nanotubes

Summary form only given. We have reviewed the recent progresses in simulations on the field electron emission (FE) from carbon nanotubes of physical sizes. We propose a theoretical model which reveals the origin of the low turn-on field of FE of carbon nanotubes