Intertube Distance Research Articles

Modeling and simulation for the field emission of carbon nanotubes array

To optimize the field emission of the infinite carbon nanotubes (CNTs) array on a planar cathode surface, the numerical simulation for the behavior of field emission with finite difference method was proposed. By solving the Laplace equation with computer, the influence of the intertube distance, the anode–cathode distance and the opened/capped CNT on the field emission of CNTs array were taken into account, and the results could accord well with the experiments. The simulated results proved that the field enhancement factor of individual CNT is largest, but the emission current density is little. Due to the enhanced screening of the electric field, the enhancement factor of CNTs array decreases with decreasing the intertube distance. From the simulation the field emission can be optimized when the intertube distance is close to the tube height. The anode–cathode distance hardly influences the field enhancement factor of CNTs array, but can low the threshold voltage by decreasing the anode–cathode distance. Finally, the distribution of potential of the capped CNTs array and the opened CNTs array was simulated, which the results showed that the distribution of potential can be influenced to some extent by the anode–cathode distance, especially at the apex of the capped CNTs array and the brim of the opened CNTs array. The opened CNTs array has larger field enhancement factor and can emit more current than the capped one.

Field-enhancement factor for carbon nanotube array

To estimate the apex field-enhancement factor associated with an array of carbon nanotubes (CNTs) on a planar cathode surface, the model of the floating spheres between parallel anode and cathode plates was proposed. An approximate formula for the enhancement factor was derived showing that the intertube distance of a CNT array critically affects the field emission. When the intertube distance is less than the height of the tube, the enhancement factor decreases rapidly with decreasing distance in qualitative agreement with the numerical simulation. Considering the field-emission current density, the field emission can be optimized when the intertube distance is comparable with the tube height, in accordance with the results from the experiments. Finally, the influence of the anode-cathode distance on the enhancement factor was also discussed, showing that the anode-cathode distance has little effect on the field emission from a CNT array. Thus, we can reduce the theshold voltage to some extent by decreasing the anode-cathode distance.

Single-Walled Carbon Nanotube Purification, Pelletization, and Surfactant-Assisted Dispersion: A Combined TEM and Resonant Micro-Raman Spectroscopy Study

Resonant Raman spectroscopy and transmission electron microscopy were used to characterize the structural changes of three single-walled carbon nanotube samples processed with purification, pelletization, and surfactant-assisted dispersion. A two-stage purification process selectively removes metallic tubes as well as small-diameter ones, enriching large-diameter semiconducting tubes. Pelletizing reduces the intertube distance but greatly increases the intensity ratio of the D band to the G band. Single-walled nanotube (SWNT) bundle size decreases during ultrasonication dispersion aided by a surfactant. SWNT bundles composed of large-diameter tubes are prone to debundling.

Modeling and calculation of field emission enhancement factor for carbon nanotubes array

To estimate the apex field enhancement factor associated with carbon nanotubes (CNTs) array on a planar cathode surface, the image model of floated sphere between parallel anode and cathode plates was proposed. Firstly, the field enhancement factor of individual CNT was given as the following expression, β 0 = h / ρ + 3.5 , where h is the height and ρ is the radius of CNTs. Then the field enhancement factor of CNTs array was discussed and the above expression was modified to be β = h / ρ + 3.5 - W , in which W is the function of the intertube distance R and represents the coulomb field interaction between the CNTs. All results show that the intertube distance of CNTs array critically affects the field emission. When the intertube distance is less than the height of tube, the field enhancement factor will decrease rapidly with decreasing the intertube distance. According to the calculated results and considering the field emission current density, the filed emission is optimal theoretically when the intertube distance is comparable with the height of CNTs.

Electronic properties of potassium-intercalatedC60peapods

We present a study of the electronic structure of potassium-intercalated ${\mathrm{C}}_{60}$-filled single-wall carbon nanotubes (SWCNT's), so-called peapods, in comparison to the corresponding reference SWCNT's. The structural changes and the variation of the electronic properties were characterized by electron energy-loss spectroscopy in transmission. The analysis of the $\mathrm{C}1s$ core-level excitations shows that the doping level is nearly the same for peapods and the reference SWCNT's and that a competitive charge transfer of the $\mathrm{K}4s$ electrons to both the ${\mathrm{C}}_{60}$ peas and the SWCNT pods occurs. The intercalation process causes an expansion of the intertube distance in the bundle lattices and a decrease of the intermolecular distance between the ${\mathrm{C}}_{60}$ peas in the doped peapods. Regarding the optical properties, the charge transfer to the peapods (SWCNT's) yields the formation of a free-charge-carrier plasmon at about 1.3 eV (1.45 eV). An analysis by an effective Drude-Lorentz model shows that the lower plasmon energy in the doped peapods can be explained by a higher effective screening in these hybrid compounds.

Comparison of the field emissions between highly ordered carbon nanotubes with closed and open tips

We have studied the field emission from the closed and open tips of highly ordered carbon nanotubes fabricated on porous anodic aluminum oxide templates by changing the tube height. Due to the field-screening effect provoked by the proximity of the neighboring tubes, the field emission from both kinds of the tips was critically affected by the tube height that protruded from the surface. The field emission optimizes when the tube height is similar to the intertube distance for both kinds of tips. The field emission from the closed tips is much more efficient than that from the open ones.

Load transfer mechanism in carbon nanotube ropes

We used molecular mechanics and molecular dynamics to study the nature of load transfer in a single walled carbon nanotube (SWCNT) bundle consisting of seven (10,10) SWCNTs: one core tube surrounded by six tubes on the perimeter. The surface tension and the inter-tube corrugation are identified as the two factors that contribute to load transfer. The surface tension effectively acts over a “line” (roughly over the circumference of each tube). The inter-tube corrugation scales linearly with respect to the contact surface area, and increases non-linearly as the inter-tube distance decreases. Relaxation in the nanotube cross-section leads to better inter-tube load transfer as a slight “faceting” develops; the tubes appear to be partially polygonized, rather than perfect cylinders. Compared with parallel bundles, twisting can significantly enhance the load transfer between neighboring tubes; this has been computed as a function of twist angle for this nanotube bundle system.

Study of the field-screening effect of highly ordered carbon nanotube arrays

We have studied the field-screening effect provoked by the proximity of neighboring tubes by changing the tube height of highly ordered carbon nanotubes fabricated on porous anodic aluminum oxide templates. The field emission was critically affected by the tube height that protruded from the surface. The field emission was optimal when the tube height was similar to the intertube distance. The intertube distance to the tube height for maximum field emission is about one half the intertube distance predicted by Nilsson et al. [Appl. Phys. Lett. 76, 2071 (2000)].