A. V. Eletskii

Optimization of the parameters of a carbon nanotube-based field-emission cathode

A procedure for optimizing a field-emission cathode based on carbon nanotubes (CNTs) is developed. An array of identical equidistant vertical CNTs is considered. The optimization procedure takes into account the effect of screening of an electric field by neighboring nanotubes by solving a Laplace equation and the thermal instability of nanotubes, which limits the emission current density of a nanotube, by solving a heat conduction equation. The relation between the emission current and the applied voltage is described by the Fowler-Nordheim relationship containing the CNT tip temperature as a parameter. Upon optimization, the optimum distance between CNTs that ensures the maximum emission current density is calculated. The calculation results demonstrate that this parameter depends substantially on both the applied voltage and the nanotube geometry. These dependences are weakly sensitive to the choice of the transport coefficients (thermal conductivity, electrical conductivity) of nanotubes.

A comparative study of graphene materials formed by thermal exfoliation of graphite oxide and chlorine trifluoride-intercalated graphite

Graphene 3D materials GM1 and GM2 obtained by explosive exfoliation of graphite oxide and graphite intercalated with chlorine trifluoride, respectively, have been studied by elemental analysis, X-ray photoelectron spectroscopy, mass spectrometry, infrared and Raman spectroscopy, and scanning electron microscopy. The specific surface area, the pore size, and electrical conductivity of the materials have been measured. A comparative study has shown that the gas mixture produced during the preparation of GM1 is less hazardous than that in the case of GM2. However, GM2 exhibits a higher conductivity and a larger size of graphene crystallites. The feasibility of isolation of a suspension of graphene nanosheets from the test 3D materials has been demonstrated. Possible applications of these materials are discussed.

Influence of the electric field on the alignment of carbon nanotubes during their growth and emission

The problems of the electric field action on carbon nanotubes (CNTs) during their growth and under the electron field emission conditions are considered. The relations determining the growth rate of an extended structure under the action of the electric field are established. The relation connecting the angle of orientation of a CNT inclined to the substrate surface and the applied electric field is used for computing current-voltage characteristics of the cathode consisting of inclined CNTs. The degree of deviation of these characteristics from the Fowler-Nordheim classic dependence is determined, on the one hand, by the parameters characterizing the CNT spread over the angles of inclination and, on the other hand, by the value of the Young modulus characterizing the bending stiffness of a nanotube. It is shown that in zero external electric field, a certain effect on the CNT orientation can be produced by the CNT potential relative to the substrate, which is due to the effect of the contact potential difference.

Degradation of a carbon nanotube-based field-emission cathode during ion sputtering

The degradation rate of carbon nanotubes (CNTs) in an electron field emitter is calculated. The degradation mechanism is taken to be the sputtering of the CNT surface by the ions that result from the ionization of residual gas molecules by an electron impact. The degradation rate and the corresponding CNT lifetime are calculated as a function of the nanotube geometry, the applied voltage, the pressure and kind of a residual gas, the interelectrode gap, and the nanotube array density. The obtained strong dependence of the degradation rate on the applied voltage is caused by a sharp character of the I–V emission characteristic determined by the Fowler-Nordheim relationship. The dependence of the degradation rate on the interelectrode gap is induced by the corresponding dependence of the probability of reaching the CNT surface.

Degradation of a CNT-Based Field Emission Cathode due to Ion Sputtering

The rate of degradation of carbon nanotubes (CNT) comprising a field emission cathode is calculated. The sputtering of CNT material by incident ions formed through the electron impact ionization of the residual gas molecules is considered as the degradation mechanism. The degradation rate and the relevant lifetime of CNTs have been calculated as a function of the nanotubes geometry, applied voltage, inter-electrode distance and density of the array. It has been found the abrupt dependence of the degradation rate on the applied voltage which is caused by a sharp character of the Fowler–Nordheim dependence. The dependence of the degradation rate on the inter-electrode distance is caused by the corresponding dependence of the probability of an ion to reach the CNT surface.

Electrical Field Enhancement in Carbon Nanotube-Based Electron Field Cathodes

The problem of evaluating the field amplification factor in electron field emitters on the basis of carbon nanotubes (CNT) has been considered. The electrostatic problem of determination of the field amplification factor for CNTs having various structures of the tip, depending on the tilting angle in regard to the cathode plane as well as on the inter-electrode distance, has been resolved. The dependence of the field amplification factor for vertically aligned CNTs comprising an array on the intertube distance has been obtained. This dependence is used for evaluation of the optimal surface density of CNTs in the array providing the maximum emission current density. The current-voltage characteristic of an array calculated with taking into account the statistical spread of the orientation angle of CNTs is compared with that obtained recently with taking into account a spread in geometrical parameters of CNTs.

Fluorination of carbon nanostructures and their comparative investigation by XPS and XAES spectroscopy

Abstract X‐ray photoelectron (XPS) and Auger electron (XAES) spectroscopies have been used to investigate fluorine interaction with surfaces of nanocarbons. MWCNT, SWCNT and Fiber were fluorinated in F2 and HOPG in ClF3 atmosphere at room temperature. The performed measurements imply that the surface of the samples is inert to the environmental before and after fluorination. The XPS spectra show that MWCNT and Fiber after fluorination have similar composition, which includes CF, CF2 and CF3 states, but MWCNT includes CF4 state as well. During defluorination of the fluorinated HOPG sample with initial C‐F bonds the formation of CF2 and CF3 states has been observed. XAES spectra show the absence of C‐F bonding in the outer layers of fluorinated HOPG, MWCNT and Fiber, while C‐F bonding exists in SWCNT.

Characterization of carbon atoms chemical states in nanotubes containing soot materials and fullerene by XPS, XAES

X-ray excited Auger electron spectroscopy (XAES) and XPS were used to studying chemical states of carbon atoms in various carbon compounds. There have been shown clear distinction in the XAES of different compounds, which permits to identify carbon containing contaminations (CCC) on a surface of nanotubes and fullerene.

Nonlinear resistance of polymer composites with carbon nanotube additives in the percolation state

The electrical properties of a polymer composite with carbon nanotube additives have been analyzed. The state of the system near the percolation threshold, when charge is transferred along a single percolation path, has been considered. For this state, the current–voltage characteristics of a percolation chain made up of carbon nanotubes have been calculated under the assumption that the contact resistance between neighboring nanotubes is much higher than the intrinsic resistance of the nanotubes. According to recent data, the distance between neighboring (contacting) nanotubes has been assumed to be randomly distributed. It has been shown that, under the given conditions, the current–voltage characteristic is essentially nonlinear. This indicates the nonohmic conductivity of the composites. The dependence of the current–voltage characteristic on the spread of the contact distribution over distances has been discussed.

Effect of thermal motion of residual gas molecules on the degradation of the field emission cathode based on carbon nanotubes

The rate of degradation of carbon nanotubes in an electron field emission cathode is calculated taking into account thermal motion of the residual gas atoms. As the degradation mechanism, we consider the sputtering of the surface of carbon nanotubes by ions formed as a result of ionization of residual gas molecules by electron impacts. It is shown that the allowance for the initial thermal motion of atoms, the ionization of which is a source of ions, considerably reduces the degradation rate as compared to the results of approximate calculations based on the assumption of motionless atoms.