Gas-phase synthesis of nitrogen-containing carbon nanotubes and their electronic properties

Nitrogen-containing carbon nanotubes are synthesized using a gas-phase reaction. The synthesis of nitrogen-doped carbon nanotubes from 100 to 500 Å in diameter is accomplished through pyrolysis of acetonitrile (CH₃CN) at a temperature of 800°C. Cobalt and nickel metallic particles formed upon thermal decomposition of a mixture of maleate salts are used as catalysts. The materials synthesized are investigated by scanning and transmission electron microscopy. Analysis of the x-ray photoelectron spectra demonstrates that the content of nitrogen atoms in three nonequivalent charge states is approximately equal to 3%. A comparison of the CK _α x-ray fluorescence spectrum of the carbon nanotubes synthesized through electric-arc evaporation of graphite and the x-ray fluorescence spectrum of the nitrogen-containing carbon nanotubes prepared by catalytic decomposition of acetonitrile indicates that, in the latter case, the spectrum contains a certain contribution from the sp ³ states of carbon atoms. The temperature dependences of the electrical conductivity for different types of multi-walled carbon nanotubes are compared. The difference observed in the temperature dependences of the electrical conductivity is associated with the presence of additional scattering centers in nitrogen-containing carbon nanotubes.     

Characterization of fluorinated multiwalled carbon nanotubes by x-ray absorption spectroscopy

The C 1s and F 1s x-ray absorption spectra of fluorinated multiwalled carbon nanotubes with different fluorine contents and reference compounds (highly oriented pyrolytic graphite crystals and “white” graphite fluoride) were measured using the equipment of the Russian-German beamline at the BESSY II storage ring with a high energy resolution. The spectra obtained were analyzed with the aim of characterizing multiwalled carbon nanotubes and their products formed upon treatment of the nanotubes with fluorine at a temperature of 420°C. It was established that, within the probing depth (～15 nm) of carbon nanotubes, the process of fluorination occurs uniformly and does not depend on the fluorine concentration. The interaction of fluorine atoms with multiwalled carbon nanotubes in this case proceeds through the covalent attachment of fluorine atoms to graphene layers of the graphite skeleton and is accompanied by a change in the hybridization of the 2s and 2p valence electron states of the carbon atom from the trigonal (sp ²) to tetrahedral (sp ³) hybridization.     

Carbon systems of polymerized nanotubes: Crystal and electronic structures

Molecular dynamic calculations are carried out for the (P, T) phase diagram of a covalent compound of cross-linked carbon single-wall nanotubes (SWNT) and for the structures and electronic spectra of the novel crystals of polymerized carbon nanotubes. It is shown that the transformation of covalently bonded nanotubes in a close-packed conducting structure cardinally modifies their electronic properties. The P-SWNT crystal becomes semiconducting and, upon complete transformation of sp ²-hybridized carbon atoms into sp ³-hybridized ones, it becomes an insulator.     

Raman spectroscopy of graphene and carbon nanotubes

This paper reviews progress that has been made in the use of Raman spectroscopy to study graphene and carbon nanotubes. These are two nanostructured forms of sp² carbon materials that are of major current interest. These nanostructured materials have attracted particular attention because of their simplicity, small physical size and the exciting new science they have introduced. This review focuses on each of these materials systems individually and comparatively as prototype examples of nanostructured materials. In particular, this paper discusses the power of Raman spectroscopy as a probe and a characterization tool for sp² carbon materials, with particular emphasis given to the field of photophysics. Some coverage is also given to the close relatives of these sp² carbon materials, namely graphite, a three-dimensional (3D) material based on the AB stacking of individual graphene layers, and carbon nanoribbons, which are one-dimensional (1D) planar structures, where the width of the ribbon is on the nanometer length scale. Carbon nanoribbons differ from carbon nanotubes is that nanoribbons have edges, whereas nanotubes have terminations only at their two ends.     

Investigation of properties of Cu containing DLC films produced by PECVD process

Copper containing diamond like carbon (Cu-DLC) thin films were deposited on various substrates at a base pressure of 1×10^?3 Torr using a hybrid system involving DC-sputtering and radio frequency-plasma enhanced chemical vapor deposition (RF-PECVD) techniques. The compressive residual stresses of these films were found to be considerably lower, varying between 0.7 and 0.94 GPa and Cu incorporation in these films improve their conductivity significantly. Their structural properties were studied by Raman spectroscopy, atomic force microscopy, scanning electron microscopy, X-ray photoelectron spectroscopy and X-ray diffraction techniques that clearly revealed the presence of Cu in the DLC structure. Raman analysis yields that Cu incorporation in DLC enhances the graphite-like sp² bonding. However, the sp² bonding was found to continuously reduce with the increasing C₂H₂ gas pressure, this may be due to reduction of Cu nanocrystal at the higher pressure. FTIR results inferred various bonding states of carbon with carbon, hydrogen and oxygen. In addition, hydrogen content and sp³ and sp² fractions in different Cu-DLC films were also estimated by FTIR spectra and were correlated with stress, electrical, optical and nano-mechanical properties of Cu-DLC films. The effect of indentation load (4–10 mN) on nano-mechanical properties of these films was also explored.     

High-yield solvo-thermal synthesis of carbon nanotubes from sp 3 hydrocarbons

Carbon nanotubes have been successfully synthesized by a high-yield solvo-thermal process by using sp³-hydrocarbons as the carbon sources and solvents. Studies have shown that sp³-hydrocarbons used in the process not only act as the carbon sources and solvents, but also increase the yield of carbon nanotubes dramatically. Besides carbon nanotubes, some interesting carbon materials, such as carbon “olives”, carbon hollow spheres, carbon microtubes and “crossed” carbon nanotubes, were also obtained by the present route. PACS 61.10.Nz; 81.10.Dn     

Nanotube nanoscience: A molecular-dynamics study

Carbon nanotubes, fullerenes, and other nanostructured carbon materials are now the most important material phases in the field of nanoscience and nanotechnology. We study the structural stabilities and the interconversion of carbon nanotubes and various other carbon nanostructured phases at elevated temperatures as well as under high pressure using the molecular dynamics method combined with a newly parametrized transferable tight-binding model. The model can deal with not only sp² and sp³ covalent bonds but also the interaction between sp² layers, which plays an important role in the structural and electronic properties of carbon nanostructured materials. It is found that, during a thermal transformation process of carbon nanotubes with C₆₀ fullerenes trapped inside into double-walled carbon nanotubes, the outer carbon-nanotube wall is chemically active and forms covalent bonds with inner carbon atoms, and that most vacancies on the initially imperfect outer tube wall are eventually filled with atoms migrated from inner fullerenes. It is also found that external pressure of about 20 GPa induces a variety of structural transformations in carbon nanostructures. On the other hand, pressure of 30 GPa or higher usually results in sp³-rich amorphous carbon materials. Finally, the rotational interlayer friction force in double-walled carbon nanotubes is studied for the system of (4,4)@(9,9), and the torque of the friction force per unit area acting on each nanotube of the system is found to be as small as . This small value indicates the importance of carbon nanostuctured materials not only for nanoelectronics but also for nanometer-scale machines in the future.     

Nitrogen ion implantation in single wall carbon nanotubes

We report an X-ray photoemission and electron energy loss study on 3 keV ion implantation in single wall carbon nanotubes. Our results show that nitrogen atoms can bind to carbon in tetrahedral sp³, defects related pyridine-like, and triangular sp² configurations and such bondings are stable for annealing up to 650 K. Heating at higher temperatures results in preferential substitutional nitrogen doping. This technique opens a new channel for controlled doping in carbon nanotubes for device applications.     

Effect of pulsed ion irradiation on the electronic structure of multi-walled carbon nanotubes

The effect of pulsed ion irradiation and vacuum annealing on the ratio of sp ²- and sp ³-hybridized orbitals of carbon atoms in the layers of oriented multi-walled carbon nanotubes has been studied by analyzing the photoemission spectra of the C1s core level and the valence band of carbon, which were obtained using the equipment of the BESSY II Russian-German beamline of synchrotron radiation and a Riber analytical system. It has been shown that the ion irradiation leads to a significant decrease in the fraction of atoms with the sp ³ hybridization of electrons. On the contrary, the annealing reduces the fraction of the sp ³-component in the spectra of carbon. Typical features of the valence band of multi-walled carbon nanotubes in the annealed and irradiated states have been established.     

Carbon–metal interfaces analyzed by aberration-corrected TEM: How copper and nickel nanoparticles interact with MWCNTs

Experimental confirmation for the stronger interaction of Ni with multi-walled carbon nanotubes (MWCNTs) compared to Cu with MWCNTs is presented. The interfaces between Cu (Ni) nanoparticles side-on oriented onto MWCNTs are analyzed with high spatial resolution electron energy-loss spectroscopy (EELS) of the carbon K-edge. The EEL spectra reveal a rehybridization from sp² to sp³ hybridized carbon of the outermost MWCNT layer at the Ni interface, but no such rehybridization can be observed at the Cu interface. The EELS results are supported by transmission electron microscopy (TEM) images, which show a better wetting behavior of Ni and a smaller gap at the Ni–MWCNT interface, as compared to the corresponding Cu interfaces. The different behavior of Cu and Ni can be explained in terms of differing valence d-orbital occupancy. For the successful experimental demonstration of this effect the use of a soft chemical metal deposition technique is crucial.     

高温退火对非晶CNx薄膜场发射特性的影响

采用射频磁控溅射方法在纯N₂气氛中沉积了非晶CN_x薄膜样品，并 在真空中退火至900 ℃.对高温退火引起的CN_x薄膜化学成分、键合结构及其场发射特性方面的变 化进行研究.用傅里叶变换红外光谱和x射线光电子能谱分析样品的内部成分及键合结构的变化，其中sp²键及薄膜中N的含量与薄膜的场发射特性密切相关.退火实验的结果表明 高温退火可以导致CN_x薄膜中N含量大量损失，并在薄膜中形成大量sp^{2< 关键词： x薄膜')" href="#">CN_x薄膜 化学键合 退火温度 场致电子发射}     

X-ray photoelectron spectroscopic study of nitrogen incorporated amorphous carbon films embedded with nanoparticles

The effect of substrate bias on X-ray photoelectron spectroscopy (XPS) study of nitrogen incorporated amorphous carbon (a-C:N) films embedded with nanoparticles deposited by filtered cathodic jet carbon arc technique is discussed. High resolution transmission electron microscope exhibited initially the amorphous structure but on closer examination the film was constituted of amorphous phase with the nanoparticle embedded in the amorphous matrix. X-ray diffraction study reveals dominantly an amorphous nature of the film. A straight forward method of deconvolution of XPS spectra has been used to evaluate the sp³ and sp² contents present in these a-C:N films. The carbon (C 1s) peaks have been deconvoluted into four different peaks and nitrogen (N 1s) peaks have been deconvoluted into three different peaks which attribute to different bonding state between C, N and O. The full width at half maxima (FWHM) of C 1s peak, sp³ content and sp³/sp² ratio of a-C:N films increase up to −150 V substrate bias and beyond −150 V substrate bias these parameters are found to decrease. Thus, the parameters evaluated are found to be dependent on the substrate bias which peaks at −150 V substrate bias.     

XPS, XAES, and TG/DTA characterization of deposited carbon in methane dehydroaromatization over Ga-Mo/ZSM-5 catalyst

The catalytic dehydroaromatization of methane over 3.0%Mo/HZSM-5 and 0.1%Ga-3%Mo/HZSM-5 catalysts were reported. The nature of deposited carbon was characterized by means of XPS, XAES, TG/DTG, HRTEM, and DTA techniques. In XAES study, the type of deposited carbon was characterized based on the fine structures of dN/dE spectra. The results confirm that the sp²/sp³ bonding ratio of coke species increases with on-stream time, suggesting that the deposition of heavier aromatic-type carbon is the main reason for catalyst deactivation. The addition of gallium to 3.0%Mo/HZSM-5 enhances the adsorption of CO_x species as well as the formation of alkene (such as ethylene). Therefore, the Ga-modified 3%Mo/HZSM-5 catalyst is more active and stable than the unmodified one.     

Pressure- and temperature-induced transformations in crystalline polymers of C60

The great advantage of the C₆₀ molecule is its potential for polymerization, due to which the molecule can be the building block of new all carbon materials. In addition, it contains, both sp ² and sp ³ hybridized carbon atoms, which allows synthesizing new carbon materials with desired physicochemical properties using both types of carbon bonding. The one- and two-dimensional polymeric phases of C₆₀ are prototype materials of this sort. Their properties, especially polymerization under pressure and room temperature via covalent bonding between molecules belonging to adjacent polymeric chains or polymeric layers, can be used for further development of new materials. The present review focuses on the study of the pressure-induced polymerization and thermodynamic stability of these materials and their recovered new phases by in-situ high-pressure Raman and X-ray diffraction studies. The phonon spectra show that the fullerene molecular cage in the high-pressure phases is preserved, while these polymers decompose under heat treatment into the initial fullerene C₆₀ monomer.     

Synthesis of nanocomposites based on nanotubes and silicates

In situ synthesis of nanocomposites based on carbon nanotubes and zeolite/montmorillonite was carried out in a hot filament CVD reactor where the precursors (methane and hydrogen) are activated by carbonized tungsten filaments heated up to 2200 °C. In nanocomposites formed both on zeolite and montmorillonite we observed cross-linking of the catalytic particles by nanotubes and creation of carbon nanotube bridges and three-dimensional networks. The length of nanotube bridges was in a range from several nm to nearly 10 μm. A high density of carbon nanotubes was observed in the whole volume of zeolite. The high catalytic efficiency of zeolite is most likely caused by its structure that allows anchoring of Fe³⁺ catalytic particles in the pores and prevents their migration from the sample. At the ends of the nanotubes grown on zeolite we observed particles of the catalyst. In montmorillonite, the particles catalyzing the growth of carbon nanotubes may be present not only on the external surface but also in the interlayer voids of the mineral. Its catalytic efficiency is enhanced as proved by the higher amount of CNTs and their bundles. In the course of CNTs synthesis probably also clumps of Fe³⁺ catalytic particles arise, which may be the reason for formation of bundles of nanotubes.     

Thermal conductive performance of deposited amorphous carbon materials by molecular dynamics simulation

A series of diamond-like carbon (DLC) films with different microstructure were prepared by depositing carbon atoms on diamond surface with incident energy ranging from 1 to 100 eV. The thermal conductivity of the deposited films and the Kapitza resistance between the film and the diamond substrate were investigated. Results show that the average density, the average fraction of sp³ bonding and the thermal conductivity of the DLC films increase first, reaching a maximum around 20–40 eV before decreasing, while the Kapitza resistance decreases gradually with increased deposition energy. The analysis suggests that the thermal resistance of the interface layer is in the order of 10^?10 m²K/W, which is not ignorable when measuring the thermal conductivity of the deposited film especially when the thickness of the DLC film is not large enough. The fraction of sp³ bonding in the DLC film decreases gradually normal to the diamond surface. However, the thermal conductivity of the film in normal direction is not affected obviously by this kind of structural variation but depends linearly on the average fraction of sp³ bonding in the entire film. The dependence of the thermal conductivity on the fraction of sp³ bonding was analysed by the phonon theory.     

A new study on the catalytic mechanism of Fe-based alloys in diamond formation by Mössbauer spectroscopy

M?ssbauer spectroscopy has been used to systemically study the catalytic mechanism of Fe-based alloys in diamond formation at high temperature–high pressure (HTHP) for the first time. M?ssbauer spectra reveal the magnetic state of the 3d electrons of a Fe atom in the Fe-based alloy catalyst during diamond formation at HTHP. During carburization at lower temperatures than that required for diamond formation and diamond formation in the diamond-stability region using Fe-based alloys as a catalyst, both the quadrupole splitting QS and the isomer shift IS change from negative to positive, especially reaching a state in which they are zero. It was indicated that the state of the 3d-shell electrons of the iron atom changes greatly during carburization and diamond formation and that the incomplete 3d sub-bands of Fe atoms in the catalyst alloys could be filled up in proper order by electrons of interstitial carbon atoms. During diamond formation, the unpaired 3d-shell electrons of an iron atom in the Fe-based alloy absorb and interact with 2P_z electrons of the carbon atoms. There exist a Fe–C bonding and an electron charge transfer stage. The 2P_z electrons of the carbon atoms could be dragged into the metal atoms in the catalyst alloy and would make a transition of triangular (sp²π) hybridization of valence electrons to tetrahedral (sp³) hybridization of valence electrons (a transition of sp²π bonds of graphite to sp³ bonds of diamond), resulting in a transition of graphite structure to diamond. Although the conclusion of this study is strictly applicable only to Fe-based alloy catalysts, it could be considered more general because of the chemical similarities between the transition elements used as solvent catalysts for diamond synthesis. Received: 2 March 2001 / Accepted: 20 August 2001 / Published online: 2 October 2001     

辉光放电等离子体辅助XeCl准分子激光溅射沉积碳氮薄膜

利用直流辉光放电等离子体辅助的脉冲激光沉积技术在Si衬底上生长了碳氮薄膜.通过扫描电子显微镜、X射线衍射、X射线光电子能谱、俄歇电子能谱等多种手段,对薄膜的形貌、成分、晶体结构、价键状态等特性进行了分析和确定.结果表明,沉积薄膜为含有非晶SiN和晶态氮化碳颗粒结构,晶态成分呈多晶态,主要为α-C₃N₄相、β-C₃N₄相,晶粒大小为40—60nm.碳氮之间主要以C-N非极性共价键形式相结合. 关键词： 脉冲激光沉积 直流辉光放电 碳氮薄膜     

Bonding character of lithium atoms adsorbed on a graphene layer

This work uses first-principles calculations to investigate the aspects of the bonding character of lithium atoms adsorbed on a graphene layer. The presented results are in contradiction to other results that have recently appeared in the specialized literature, although they confirm some previous claims. In particular, a discussion of the characteristics of the bonding between lithium and carbon atoms and whether they interact via an sp² or an sp³ hybridization is intended to clarify the problem. It is also found that the carbon-lithium bond is not purely covalent but instead presents a significant ionic character. The local geometry is governed by the π-acceptor character of lithium atoms which occupy reverse positions relative to the carbon atoms as compared to the positions of hydrogen in graphane.     

Wide wavelength-range optical studies of hydrogenated amorphous carbon films: from 700 nm to 10 μm

Hydrogenated amorphous carbon (a-C:H) films were deposited on double-side polished germanium substrates by RF plasma-assisted chemical vapour deposition method using benzene as a precursor, and their optical properties were investigated in the wavelength range from 700 nm to 10 μm. In particular, we used a dispersion-function-based non-linear regression to fit the reflectance in the range from 700 to 1800 nm and the baselines of the infrared transmittance in the range from 1000 to 5000 cm⁻¹, respectively. Optical constants determined in both ranges, together with their respective thickness values, showed overall consistency. As a function of self-bias, we found that the refractive index increased while the band gap decreased, which was indicative of film densification and increase of sp²-bonded carbons. Detailed information on the self-bias-dependent evolution of microscopic bonding structure in a-C:H films was revealed via the quantitative vibration-absorption spectra in the mid-infrared region, which was obtained after taking Urbach-tail-like electronic absorption into consideration. More specifically, it was found that as self-bias was increased hydrogen content decreased, olefinic sp²-bonding decreased while aromatic sp²-bonding increased, and more carbon bonding was distorted due to increased cross-linking. Maximum sp²-bonding was observed between −300 and −400 V of self-bias.