A. P. Volkov

Aligned carbon nanotube films for cold cathode applications

Thin film material of oriented multiwall carbon nanotubes was obtained by noncatalytical chemical vapor deposition in a glow-discharge plasma. The film phase composition, surface morphology, and structural features were studied by Raman and electron microscopy techniques. Low-voltage electron field emission of thin film nanotube material was obtained and examined in diode configuration. The I–V curves in Fowler–Nordheim coordinates were linear and the corresponding threshold average field was about 1.5 V/μm. The emission current density was up to 50 mA/cm2 at the field of 5 V/μm. The emission site density reached 107 cm−2 at the same value of electric field.

Structural measurements for single-wall carbon nanotubes by Raman scattering technique

The single-wall carbon nanotubes grown by different techniques have been investigated by Roman scattering and high resolution transmission electron microscopy (HRTEM). The tube diameter values and the tube distribution over diameter have been estimated from the position and shape of the low-frequency band in the Raman spectrum containing the ‘breathing” modes. The diameter-dependent enhancement of the Raman signals from the different nanotube fractions occurred not only due to optical resonance with the laser excitation energy, but also due to thermo-induced resonances. The low-field electron emission from the single-wall carbon nanotube material has been measured. The threshold fields were 0.75–2 V/μm, the emission current reached the value 15 mA/cm2 at fields of 10 V/μm.

DC discharge plasma studies for nanostructured carbon CVD

Abstract A synthesis of carbon films by d.c. discharge plasma-enhanced chemical vapor deposition using a hydrogen–methane gas mixture was investigated by optical emission spectroscopy and by measurements of current–voltage dependencies. The effects of gas composition and pressure on the characteristics of d.c. discharge in the methane–hydrogen gas mixture are studied. Variation of the deposition process parameters over a wide range allows us to obtain various carbon thin film materials, whose structure and composition were qualitatively characterized by Raman spectroscopy and electron microscopy. The data of optical emission spectroscopy show the presence in the discharge plasma of H, H2, CH and C2 activated species, which play a decisive role in nanostructured graphite-like carbon film formation and carbon condensation in the gas phase. We propose a model for the formation of graphitic nanostructured carbon films in plasma containing C2 dimers.

CVD growth and field emission properties of nanostructured carbon films

An investigation of the growth mechanisms, electronical and structural properties, and field emissions of carbon films obtained by chemical vapour deposition showed that field emissions from films composed of spatially oriented carbon nanotubes and plate-like graphite nanocrystals exhibit non-metallic behaviour. The experimental evidence of work function local reduction for carbon film materials is reported here. A model of the emission site is proposed and the mechanism of field emission from nanostructured carbon materials is described. In agreement with the model proposed here, the electron emission in different carbon materials results from sp3-like defects in an sp2 network of their graphite-like component.

Low-voltage electron emission from chemical vapor deposition graphite films

Thin film cold cathodes composed of a graphite-type carbon coating on Si substrate have been fabricated and tested. Electrons from the cathodes were emitted into vacuum when the average electric field exceeded 1.5 V/μm. The emission current density was more than 1 mA/cm2 and emission site density was higher than 107 cm2 at the electric field of 4 V/μm. The current–voltage dependencies were studied at different temperatures from 77 to 600 K and found to be typical for the field emission. We propose a mechanism of electron emission from carbon cold cathodes, based on the enhancement of electric field due to surface morphology and on the modification of electronic properties of carbon atoms localized on the surface of graphite-type material.

Field emission characteristics of nanostructured thin film carbon materials

Abstract Nanostructured carbon (nC) thin film materials were obtained by chemical vapor deposition (CVD) in dc discharge activated hydrogen–methane gas mixture. Film structure, surface morphology and phase composition was studied by Raman, electron microscopy and electron spectroscopy methods. A highly efficient cold electron emission was found for the films composed from graphite-like nano-structures including carbon nanotubes (CNT) and graphite nano-crystallites. Electron emission tests and analysis exhibit non-classical behavior of the nano-carbon cold cathodes. The statistical analysis of the cold emission show normal distribution of parameters of separate emission centers with narrow standard deviation. The possible mechanism of the cold emission is discussed. The highest efficiency of the nano structured carbon cathodes is demonstrated in the prototypes of vacuum cathodoluminescent lighting devices.

Non-classical electron field emission from carbon materials

Abstract The experimental current–voltage dependencies of vacuum diode with CVD film nano-carbon cold cathode have been analyzed by using classical Fowler–Nordheim approach in combination with statistical consideration for distribution of geometrical characteristics of emission sites. The adequate qualitative description of the current–voltage dependencies has been obtained. However, the numerical estimations performed for the emission sites geometrical characteristics lead to significant contradictions with basic assumptions of the classical theory. The possible non-metallic and non-classical nature of the nano-carbon emitters is discussed.

Role of the curvature of atomic layers in electron field emission from graphitic nanostructured carbon

Layers of oriented carbon nanotubes and nanometer-size plate-shaped graphite crystallites are obtained by chemical vapor deposition in a glow-discharge plasma. A structural-morphological investigation of a carbon material consisting of nanotubes and nanocrystallites is performed, and the field-emission properties of the material are also investigated. It is shown that electron field emission is observed in an electric field with average intensity equal to or greater than 1.5 V/μm. The low fields giving rise to electron emission can be explained by a decrease in the electronic work function as a result of the curvature of the atomic layers of graphitic carbon.

Mechanism of field emission from carbon materials

Field emission in diamond and graphite-like polycrystalline films is investigated experimentally. It is shown that the emission efficiency increases as the nondiamond carbon phase increases; for graphite-like films the threshold electric field is less than 1.5 V/μm, and at 4 V/μm the emission current reaches 1 mA/cm2, while the density of emission centers exceeds 106 cm−2. A general mechanism explaining the phenomenon of electron field emission from materials containing graphite-like carbon is proposed.

Defect induced lowering of work function in graphite-like materials

Abstract Different carbon materials have shown unusually high efficiency of electron field emission. The device application of the carbon cold cathodes requires a fundamental understanding of the emission mechanism. We describe the emission properties of carbon thin films grown by chemical vapor deposition. The structure, phase composition and electronic properties peculiar to the film material were investigated. New model of electron emission sites and mechanism of field electron emission are proposed. In accordance with the proposed model the electron emission in carbon materials occurs from sp 3 -like defects in an sp 2 network of graphite-like material. The corresponding mechanism of field emission consists of electrons escaping into vacuum by tunneling from the Fermi level of the graphite material through the atomically thin sp 3 -like layer and the energy barrier on its surface.