Effects of Cs treatment on field emission properties of capped carbon nanotubes (original) (raw)
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This study employs first-principles calculations to investigate the effects of alkali-metal Cs atom adsorption on the work functions and field enhancement factors of finite-length (3,3) and (5,0) single-walled carbon nanotubes (CNTs) with capped ends. It is found that the work functions of both capped tubes decrease in the presence of an adsorbed Cs. The modified work functions are primarily due to the effect of a surface dipole at the tip of the tube. Moreover, the field enhancement factor of capped (3,3) and (5,0) tubes can be increased with this Cs adsorption, with enhancement greater in the (3,3) tube than the (5,0) tube, similar to the tendency found in pristine capped (3,3) and (5,0) tubes. Such a phenomenon can be qualitatively explained by analyzing the geometrical shape of the Cs-adsorbed nanotube system, which reveals an increase in the field enhancement factor for the Cs adsorbed system because its sharper tip causes a higher surface charge density distribution. Our findings indicate that the CNT field emission properties can be modulated more efficiently just by providing a suitable electronic source.
Effect of the in situ Cs treatment on field emission of a multi-walled carbon nanotube
Chemical Physics Letters, 2002
The effect of an in situ Cs treatment on the field emission of a multi-walled carbon nanotube (MWNT) was investigated. The field emission of as-grown MWNT shows two Fowler-Nordheim (F-N) slopes in current-voltage characteristics. Both slopes in the two regions (high voltage and low voltage) were lowered as the result of the Cs treatment. The turn-on voltage was significantly decreased by a factor of about 1.3 and the total emission current was increased by over one order of magnitude after Cs deposition. In addition, the emission current was decreased beyond the optimum Cs deposition time. A stable emission current was observed after Cs deposition and during the no-Csdeposition state. A decrease in work function is the major cause of the electron field enhancement as the result of the formation of a surface dipole layer induced by Cs deposition. Ó
Applied Surface Science, 2004
The effect of field penetration induced charge redistribution on the field emission properties of carbon nanotubes (CNTs) have been studied by the first-principle calculations. It is found that the carbon nanotube becomes polarized under external electric field leading to a charge redistribution. The resulting band bending induced by field penetration into the nanotube tip surface can further reduce the effective workfunction of the carbon nanotubes. The magnitude of the redistributed charge DQ is found to be nearly linear to the applied external field strength. In addition, we found that the capped (9, 0) zigzag nanotube demonstrates better field emission properties than the capped (5, 5) armchair nanotube due to the fact that the charge redistribution of p electrons along the zigzag-like tube axis is easier than for the armchair-like tube. The density of states (DOS) of the capped region of the nanotube is found to be enhanced with a value 30% higher than that of the sidewall part for the capped (5, 5) nanotube and 40% for the capped (9, 0) nanotube under an electric field of 0.33 V/Å. Such enhancements of the DOS at the carbon nanotube tip show that electrons near the Fermi level will emit more easily due to the change of the surface band structure resulting from the field penetration in a high field.
Journal of Chemical Information and Modeling
In many field electron emission experiments on single-walled carbon nanotubes (SWCNTs), the SWCNT stands on one of two well-separated parallel plane plates, with a macroscopic field FM applied between them. For any given location "L" on the SWCNT surface, a field enhancement factor (FEF) is defined as F L /F M , where F L is a local field defined at "L". The best emission measurements from small-radii capped SWCNTs exhibit characteristic FEFs that are constant (i.e., independent of F M). This paper discusses how to retrieve this result in quantum-mechanical (as opposed to classical electrostatic) calculations. Density functional theory (DFT) is used to analyze the properties of two short, floating SWCNTS, capped at both ends, namely a (6,6) and a (10,0) structure. Both have effectively the same height (∼ 5.46 nm) and radius (∼ 0.42 nm). It is found that apex values of local induced FEF are similar for the two SWCNTs, are independent of F M , and are similar to FEF-values found from classical conductor models. It is suggested that these induced-FEF values relate to the SWCNT longitudinal system polarizabilities, which are presumed similar. The DFT calculations also generate "real", as opposed to "induced", potential-energy (PE) barriers for the two SWCNTs, for FM-values from 3 V/µm to 2 V/nm. PE profiles along the SWCNT axis and along a parallel "observation line" through one of the topmost atoms are similar. At low macroscopic fields the details of barrier shape differ for the two SWCNT types. Even for F M = 0, there are distinct PE structures present at the emitter apex (different for the two SWCNTs); this suggests the presence of structurespecific chemically induced charge transfers and related patch-field distributions.
Effect of electric field on the electronic structures of carbon nanotubes
Applied Physics Letters, 2001
We have investigated the electronic structures of a capped single-walled carbon nanotube under the applied electric field using density functional calculations. The capped tube withstands field strengths up to 2 V/Å. When the electric field is applied along the tube axis, charges are transferred from the occupied levels localized at the top pentagon of the cap, and not from the highest occupied level localized at the side pentagon, to the unoccupied levels. We find that the charge densities at the top of the armchair cap show two-or five-lobed patterns depending on the field strength, whereas those of the zigzag cap show a three-lobed pattern. The interpretation for the images of the field emission microscope is also discussed.
Gas molecule effects on field emission properties of single-walled carbon nanotube
Diamond and Related Materials, 2004
The effective workfunctions of single-walled carbon nanotubes (5,5) (SWNTs) with various geometries and adsorbates under external electric field have been calculated by the ab initio plane-wave, pseudopotential method. The capped, open-ended, and close-ended nanotubes show the workfunctions of 4.8 eV, 4.43 eV and 3.75 eV, respectively, and these results exhibit a good agreement with experiments. Under external electric field, the effective workfunction is further reduced due to the charge redistribution at the nanotube tip. In addition, the effects of participation of foreign adsorbates on the nanotube surface both physically and chemically on the variations of workfunctions have also been studied. In the physisorption process, the electrostatic interaction between adsorbates and nanotubes plays an important role under external electric field. The polar molecules like water have a large binding energy with the nanotube under electric field. These molecules act as tunneling states for electrons emitting from the nanotube tip into the vacuum. In the chemisorption process, the variations of effective workfunctions can be understood in terms of the surface dipole of the terminated bond due to the different electronegativity between nanotubes and adsorbates.
Applied Surface Science, 2005
Cs intercalation has demonstrated experimentally a significant reduction of the work function of carbon nanotubes, thus improving field emission properties. In this paper, we report a density functional theory (DFT) study within the generalized gradient approximation (GGA), regarding the effects of Cs on field emission of single-wall carbon nanotubes (SWCNTs). Specifically, a comprehensive examination was carried out to investigate the effects of Cs adsorbed on capped and open-ended C(5,5) armchair SWCNT tips. Our calculations showed a reduction in the ionization potential (IP) upon Cs physisorption, thus improving the emission properties of carbon nanotubes, as reported experimentally. The structure of the adsorbed Cs-cluster, the corresponding adsorption energies, and the IPs, were altered upon the inclusion of a local electric field in the calculations to mimic the emission environment.
Characterization of the field emission properties of individual thin carbon nanotubes
Applied Physics Letters, 2004
Electron emission measurements were conducted on individual carbon nanotubes. The nanotubes had a closed end and their surfaces were thoroughly cleaned. It is shown conclusively that individual carbon nanotube electron emitters indeed exhibit Fowler-Nordheim behavior and have a work function of 5.1± 0.1 eV for the nanotubes under investigation, which had diameters of 1.4 and 4.9 nm.
Analytical model for electron field emission from capped carbon nanotubes
Journal of Vacuum Science & Technology B, 2004
This article presents a model of electron field emission from quantum states arising from the tight confinement of quasi-free electrons on a nanotube hemispherical cap. The model outlines the possibility of inhomogeneous electron field emission for very thin carbon nanotubes at high emission levels and the appearance of peculiar ring-shaped field emission images. The conclusions qualitatively agree with existing experimental evidence, therefore supporting the hypothesis that part of the electrons on the cap of the emitter may behave as quasi-free in a high emission level/high-temperature regime.
2006
A 1 mu\mumum 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 MNDO (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 to explain electroluminescence associated with field emitting carbon nanotubes. 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 field electron 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 SWCNTs are significantly less than values predicted by simple classical formulae. The FEF for the closed SWCNT decreases with applied field; the FEF for the H-terminated open SWCNT is less than the FEF for the closed SWCNT, but increases with applied field. Physical explanations for this behavior are proposed. Curved Fowler-Nordheim plots are predicted. Overall, the predicted field emission performance of the H-terminated open SWCNT is slightly better than that of the closed SWCNT, essentially because a C-H dipole is formed that reduces the height of the tunnelling barrier. In general, the physics of a charged SWCNT seems more complex than hitherto realised.