Vladimir Labunov

Growth of few-wall carbon nanotubes with narrow diameter distribution over Fe-Mo-MgO catalyst by methane/acetylene catalytic decomposition

Few-wall carbon nanotubes were synthesized by methane/acetylene decomposition over bimetallic Fe-Mo catalyst with MgO (1:8:40) support at the temperature of 900°C. No calcinations and reduction pretreatments were applied to the catalytic powder. The transmission electron microscopy investigation showed that the synthesized carbon nanotubes [CNTs] have high purity and narrow diameter distribution. Raman spectrum showed that the ratio of G to D band line intensities of IG/ID is approximately 10, and the peaks in the low frequency range were attributed to the radial breathing mode corresponding to the nanotubes of small diameters. Thermogravimetric analysis data indicated no amorphous carbon phases. Experiments conducted at higher gas pressures showed the increase of CNT yield up to 83%. Mössbauer spectroscopy, magnetization measurements, X-ray diffraction, high-resolution transmission electron microscopy, and electron diffraction were employed to evaluate the nature of catalyst particles.

Structure, composition and magnetic properties of carbon nanotubes doped by Fe during the growth process

The results of complex investigations of the crystalline structure, composition and specific magnetization of the multi-wall carbon nanotubes (CNTs) filled by magnetic nanocomposite are performed. CNT arrays have been synthesized by the high temperature pyrolysis of fluid hydrocarbon - p-xylole [C8H10] in the presence of volatile catalyst - ferrocene [Fe(C5H5)2] at the walls of tubular-type quartz reactor of specially constructed equipment. It was revealed that the obtained CNTs constitute complex nanocomposite: C - Fe3C - Fe5C2 - Fe. The magnetic properties of such CNTs in the temperature region of 78≤T≤1060 K are conditioned by the ferric carbide (in the form Fe3C H Fe5C2) and Fe.

Manifestation of coherent magnetic anisotropy in a carbon nanotube matrix with low ferromagnetic nanoparticle content

The influence of the magnetic medium can lead to peculiar interaction between ferromagnetic nanoparticles (NPs). Most research in this area involves analysis of the interplay between magnetic anisotropy and exchange coupling. Increasing the average interparticle distance leads to the dominant role of the random magnetic anisotropy. Here we study the interparticle interaction in a carbon nanotube (CNT) matrix with low ferromagnetic NP content. Samples were synthesized by floating catalyst chemical vapor deposition. We found that below some critical NP concentration, when NPs are intercalated only inside CNTs, and at low temperatures, the extended magnetic order, of up to 150 nm, presents in our samples. It is shown by analyzing the correlation functions of the magnetic anisotropy axes that the extended order is not simply due to random anisotropy but is associated with the coherent magnetic anisotropy, which is strengthened by the CNT alignment. With increasing temperature the extended magnetic order is lost. Above the critical NP concentration, when NPs start to be intercalated not only into inner CNT channels, but also outside CNTs, the coherent anisotropy weakens and the exchange coupling dominates in the whole temperature range. We can make a connection with the various correlation functions using the generalized expression for the law of the approach to saturation and show that these different correlation functions reflect the peculiarities in the interparticle interaction inside CNTs. Moreover, we can extract such important micromagnetic parameters like the exchange field, local fields of random and coherent anisotropies, as well as their temperature and NP concentration dependencies.

The effect of gas‐dynamic factors on selective carbon‐nanotube synthesis by injection CVD method for field‐emission cathodes

— Reversible selective growth of carbon-nanotube (CNT) arrays on Si/SiO2 topologies was investigated for field-emission-display applications. The method used was that of high-temperature pyrolysis of fluid hydrocarbon (p-xylene [C8H10]) in a mixture with volatile catalyst (ferrocene [Fe(C5H5)2]) using Ar as the gas carrier. The synthesized CNT arrays were analyzed by SEM, TEM, Raman, and TGA analyses. Reversible CNT growth both on Si and SiO2 surfaces was found to be sensitive to the gas-carrier flow rate and the catalyst/hydrocarbon solution injection rate into the synthesis zone. This phenomenon can be explained by inverse domination of active sites on Si and SiO2 surfaces at different flow rates of gas mixture, causing different types of catalyst precipitation followed by subsequent CNT growth. In principle, the possibility of growing CNTs using the proposed technology will allow the creation of precise geometries of field-emission cathodes excluding the step of catalyst localization.

Nitrogen-doped twisted graphene grown on copper by atmospheric pressure CVD from a decane precursor

We present Raman studies of graphene films grown on copper foil by atmospheric pressure CVD with n-decane as a precursor, a mixture of nitrogen and hydrogen as the carrier gas, under different hydrogen flow rates. A novel approach for the processing of the Raman spectroscopy data was employed. It was found that in particular cases, the various parameters of the Raman spectra can be assigned to fractions of the films with different thicknesses. In particular, such quantities as the full width at half maximum of the 2D peak and the position of the 2D graphene band were successfully applied for the elaborated approach. Both the G- and 2D-band positions of single layer fractions were blue-shifted, which could be associated with the nitrogen doping of studied films. The XPS study revealed the characteristics of incorporated nitrogen, which was found to have a binding energy around 402 eV. Moreover, based on the statistical analysis of spectral parameters and the observation of a G-resonance, the twisted nature of the double-layer fraction of graphene grown with a lower hydrogen feeding rate was demonstrated. The impact of the varied hydrogen flow rate on the structural properties of graphene and the nitrogen concentration is also discussed.

P-120: High Efficiency Method of Selective CNT Arrays Growth on the Metal/Dielectric/Semiconductor Substrates for FEDs Application

Multi-wall carbon nanotube (CNT) arrays growth by the atmospheric pressure CVD process of thermal decomposition of fluid (o- and p-xylole C8H10) hydrocarbons in the presence of volatile catalysts (ferrocene Fe(C5H5)2) with the use of Ar as a gas-carrier on the top of different metal/dielectric/semiconductor (MDS) structures, in particular Si/SiO2, Ti/SiO2/Si, Al2O3 matrix/Ni catalyst has been investigated for the FEDs application. The obtained results are showing that the CNTs growth selectivity and density of CNTs packing can be managed under certain conditions. At the investigated types of surfaces the vertically aligned close-packed and preferably oriented low density CNT arrays have been obtained as well as single CNTs in the pores of nanoporous Al2O3 matrix with Ni catalyst.

Enhanced Carbon Nanotubes Growth Using Nickel/Ferrocene-Hybridized Catalyst

Tall, crystalline carbon nanotubes (CNTs) are desired to successfully integrate them in various applications. As the crystallinity of CNTs improves with increasing growth temperatures, higher growth temperatures are required to obtain crystalline CNTs. However, in a typical chemical vapor deposition (CVD) process, CNT growth rate reduces when the growth temperature exceeds a specific level due to the degradation of the catalyst particles. In this study, we have demonstrated the improved catalytic activity of nickel/ferrocene-hybridized catalyst as compared to sole ferrocene catalyst. To demonstrate this, CNTs are grown on bare silicon (Si) as well as nickel (Ni) catalyst-deposited substrates using volatile catalyst source (ferrocene/xylene) CVD at the growth temperatures ranging from 790 to 880 °C. It was found that CNTs grown on bare Si substrate experience a reduction in height at growth temperature above 860 °C, whereas the CNTs grown on 10 nm Ni catalyst-deposited substrates experience continuous increase in height as the temperature increases from 790 to 880 °C. The enhancement in the height of CNTs by the addition of Ni catalyst is also demonstrated on 5, 20, and 30 nm Ni layers. The examination of CNTs using electron microscopy and Raman spectra shows that the additional Ni catalyst source improves the CNT growth rates and crystallinity, yielding taller CNTs with a high degree of structural crystallinity.

Field emission characteristics of short CNT bundles

The outstanding field emission characteristics of Carbon Nanotubes (CNT) offer promising potential for a wide range of vacuum electronics applications. In this study, short (1 μm) CNT arrays were grown selectively on SiO2/Si substrates using ferrocene/xylene catalyst source. High field emission current density of 366.7 mA/cm2 was obtained. A common field enhancement factor (β) value of 326 was obtained for CNT bundles with various separation distances.

Specific Features of the Carbon Nanotubes Nucleation and Growth in the Porous Alumina Membrane

Aligned, highly uniform multiwall carbon nanotubes (MWCNT) in a porous anodic aluminum oxide (AAO) membrane were successfully grown by chemical vapor deposition (CVD). The effectiveness of MWCNT formation was studied with various synthesis parameters. It was found that high catalyst (ferrocene) concentrations led to formation of a thick layer of MWCNT arrays on top surface of AAO membranes, which led to decrease of the pores filling with nanotubes. It was shown that the growth mechanism of the nanotubes in the AAO pores by this method was not connected with the traditionally used transition metal catalysts, no matter whether they were in a deposited (localized catalyst) or volatile (injected catalyst) state. The pre-annealing process in air atmosphere inhibited the nanotubes formation in the AAO pores. We speculate that the formation of MWCNT s in the AAO pores is governed by the pore structure reconstruction (water desorption, phase transformation) during the high-temperature (870° C) CVD process, though this phenomenon needs further investigation. KeywordsChemical Vapour Deposition; Carbon Nanotubes; Porous Aluminum Oxide; Ferrocene; Xylene

Femtosecond laser modification of an array of vertically aligned carbon nanotubes intercalated with Fe phase nanoparticles

Femtosecond lasers (FSL) are playing an increasingly important role in materials research, characterization, and modification. Due to an extremely short pulse width, interactions of FSL irradiation with solid surfaces attract special interest, and a number of unusual phenomena resulted in the formation of new materials are expected. Here, we report on a new nanostructure observed after the interaction of FSL irradiation with arrays of vertically aligned carbon nanotubes (CNTs) intercalated with iron phase catalyst nanoparticles. It was revealed that the FSL laser ablation transforms the topmost layer of CNT array into iron phase nanospheres (40 to 680 nm in diameter) located at the tip of the CNT bundles of conical shape. Besides, the smaller nanospheres (10 to 30 nm in diameter) are found to be beaded at the sides of these bundles. Some of the larger nanospheres are encapsulated into carbon shells, which sometime are found to contain CNTs. The mechanism of creation of such nanostructures is proposed.