A. A. Zolotukhin

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.

Formation of nanostructured carbon films in gas-discharge plasmas

Deposition of carbon materials from methane-hydrogen gas mixtures in a DC gas discharge is investigated. Parameters ensuring stable discharge conditions and synthesis of diamond and graphite-like films are determined. Optical emission spectroscopy is used to analyze the composition of the activated gas phase in the course of carbon film deposition. Synthesis of graphite-like carbon nanotubes and nanocrystallites is shown to correlate with the presence of C2 dimers in the plasma. A noncatalytic mechanism of synthesis of nanostructured graphite in a carbon-containing gas phase is proposed.

In situ plasma diagnostics for chemical vapor deposition of nano-carbon thin film materials

We report on the study of nano-carbon film deposition by in situ measuring of the optical emission and electrical parameters of the d.c. discharge in a methane-hydrogen gas mixture. By changing the deposition process parameters over a wide range, we obtain a variety of carbon thin film materials, the structure and composition of which are qualitatively characterized by a number of methods including the Raman and electron microscopy. The obtained results show the presence of H-, H2-, CH-, and C2-activated species in the discharge plasma. These species play a decisive role in the formation of nano-carbon graphite-like films including carbon nanotubes, and in carbon condensation in gas phase. We propose a model that describes the formation of graphitic nanostructured carbon films in the C2-dimers enriched plasma.

Double resonant Raman scattering in nanographite films

Experimental results are presented on Raman scattering in graphite films produced by DC plasmaenhanced chemical vapor deposition from a methane-hydrogen gas mixture. Scanning electron and probe microscopy data show that, depending on substrate material and deposition time, the deposited film is either a mesoporous material consisting of graphite nanocrystallites with basal planes oriented perpendicular to the substrate surface or an atomically flat, nanometer-thick stack of graphene layers parallel to the substrate. A comparative Raman spectroscopy analysis is performed for film samples deposited on nickel and silicon substrates for 5 and 60 min, as well as for highly ordered graphite samples. The Raman spectra of the examined samples correspond to the double resonant scattering mechanism. The behavior of Raman peak position and intensity as functions of excitation wavelength suggests a high degree of structural order in the graphite films deposited on nickel for 5 min. The results obtained are used to show that the thickness of these films is 1.5 ± 0.5 nm.

Nanodimensional carbon materials formed in a gas discharge plasma

We have experimentally studied the formation of nanodimensional carbon materials from a methane-hydrogen gas mixture activated by a dc discharge. The range of discharge voltages and currents ensuring stable deposition of carbon films was determined. Data on the carbon-containing components of the activated gas phase were obtained by in situ optical emission spectroscopy of the gas discharge plasma. It is shown that the formation of nanodiamond and nanographite particles, as well as carbon nanotubes, in the deposited films is correlated with the presence of C2 carbon dimers in the gas phase. A mechanism of the noncatalytic formation of carbon nanotubes from platelike graphite nanoparticles is proposed.

Effect of electric field on the vapor-phase growth of carbon nanostructures

A mathematical model of dc gas discharge plasma has been developed in order to determine the electric field strength at a substrate surface during plasmachemical deposition of carbon nanostructures. A numerical solution of the model equations has been obtained using the experimentally determined boundary conditions and model parameters. A comparison of the solution to experiment confirms the adequacy of the proposed mathematical model, which provides the electric field profiles and the electron and ion density distributions near the substrate surface. Estimations show that, for carbon nanostructures with a characteristic size of about 30 nm, the electric field strength at the surface is sufficiently high to provide for their directional growth along the field.

Effect of substrate material on the structure of carbon films obtained by plasmachemical deposition

Carbon films were obtained on nickel and silicon substrates by plasmachemical deposition from a hydrogen-methane gas mixture activated by dc discharge. The deposits were characterized by Raman scattering and scanning electron microscopy. Nanographite films on nickel are formed at a significantly lower substrate temperature and methane concentration in the gas phase than on silicon.

Electric field induced alignment of carbon nanotubes grown by CVD

In this report, results of the experiments devoted to developing of the method for controllable growth of CNT aligned in different directions were presented. For this purpose the usual setup for CVD was modified to provide shielding of the substrate from electric field existing between the cathode and the anode when the high voltage is applied to activate dc discharge. A mesh mask is used as the electrostatic shield. Similar to the standard CVD process, generation of C/sub 2/ species occurs in the plasma, and then these species are deposited on the substrate surface via the holes in the mask. With appropriate conditions, CNT growth was obtained on various substrates (Ni plate, quartz substrate patterned with Ni thin film stripes). An application of bias voltage between the Ni stripes allows for the partial alignment of CNT along the direction of the electric field between the stripes. This result confirms a possibility to achieve controllable growth of the CNTs.