A. N. Obraztsov

Chemical vapour deposition: Making graphene on a large scale

Graphene samples with areas of several square centimetres and excellent electrical and optical properties have been fabricated using chemical vapour deposition.

A novel method for metal oxide nanowire synthesis.

Nanowires (NWs) of metal oxides (Fe(2)O(3), CuO, V(2)O(5) and ZnO) were grown by an efficient non-catalytic economically favorable method based on resistive heating of pure metal wires or foils at ambient conditions. The growth rate of iron oxide NWs exceeds 100 nm s(-1). Produced NWs were typically 1-5 microm long with diameters from 10 to 50 nm. The produced metal oxide NWs were characterized by means of SEM, TEM, EDX, XPS and Raman techniques. The field emission measurements from the as-produced CuO NWs were found to have a threshold field as low as 4 V microm(-1) at 0.01 mA cm(-2). The formation mechanism of the NWs is discussed.

Invited Article: Electric solar wind sail: Toward test missions

The electric solar wind sail (E-sail) is a space propulsion concept that uses the natural solar wind dynamic pressure for producing spacecraft thrust. In its baseline form, the E-sail consists of a number of long, thin, conducting, and centrifugally stretched tethers, which are kept in a high positive potential by an onboard electron gun. The concept gains its efficiency from the fact that the effective sail area, i.e., the potential structure of the tethers, can be millions of times larger than the physical area of the thin tethers wires, which offsets the fact that the dynamic pressure of the solar wind is very weak. Indeed, according to the most recent published estimates, an E-sail of 1 N thrust and 100 kg mass could be built in the rather near future, providing a revolutionary level of propulsive performance (specific acceleration) for travel in the solar system. Here we give a review of the ongoing technical development work of the E-sail, covering tether construction, overall mechanical design alternatives, guidance and navigation strategies, and dynamical and orbital simulations.

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.

Broadband Light-Induced Absorbance Change in Multilayer Graphene

We report the ultrafast light-induced absorbance change in CVD-grown multilayer graphene. Using femtosecond pump-probe measurements in 1100-1800 nm spectral range, we revealed broadband absorbance change when the probe photon energy was higher than that of the pump photon. The observed phenomenon is interpreted in terms of the Auger recombination and impact ionization playing a significant role in the dynamics of photoexcited carriers in graphene.

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.

Giant optical rectification effect in nanocarbon films

We present observations of the optical rectification effect in the nanocarbon film, which is excited by nanosecond pulses of a Nd:YAG laser in the absence of an external electric field. Effective second order susceptibility of the film material is found to be 10−6 CGSE, which is higher than that of conventional noncentrosymmetric crystals. The measured ratio of the dc voltage to the laser power is 500 and 650 mV/MW at the wavelengths of 1064 and 532 nm, respectively. This makes the nanocarbon materials a promising alternative to conventional semiconductor-based terahertz radiation sources.

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.