Single-walled carbon nanotubes formation with a continuous CO2-laser: experiments and theory

It has been proved [1] that the use of a CO₂-laser system operating in continuous wave mode (cw) can be efficiently used for the production of carbon single-walled nanotubes (SWNTs). In this article we first describe in detail the variable experimental conditions (different ambient gases, static gas pressure, and gas flow) for SWNT formation and summarize the results of the characterization studies of the synthesized materials. Second, we analyze the influence of the different experimental conditions on the SWNTs formation process. We show that the heat transport, kinetic, and diffusion processes allow us to explain seemingly different formation conditions in a qualitative and semi-quantitative agreement with the experimental results. The presented self-consistent scenario for nanotube formation in a gas phase allowed us to propose future experiments on testing the mechanism of nanotube formation.     

Front and back surface cw CO2-laser annealing of arsenic ion-implanted silicon

The annealing behavior of arsenic-implanted silicon under scanned cw CO₂-laser irradiation from front and back surfaces is investigated. Ellipsometry, Hall effect, Rutherford backscattering measurements and neutron activation analysis indicate an enhancement of annealing efficiency by laser irradiation from the back surface, which provides complete recovery of crystal damage, high substitutionality and electrical activation of implanted arsenic atoms without redistribution of concentration profile. The enhancement of annealing efficiency under back-surface irradiation is explained by the difference in laser reflection from the front and back surface of silicon wafers. No differences in the results are found for scanned and static annealing.     

Influence of cw CO2-laser radiation on the amorphous-to-microcrystalline phase transition in a-Si:H films: a Raman spectroscopic study

A detailed Raman spectroscopic study of the amorphous-to-microcrystalline phase transition in hydrogenated silicon thin films on glass substrates is presented. Crystallization is induced by exposing the film surface to a continuous wave CO₂-laser working at 10.6 μm, at constant irradiation time/variable power density and at constant power density/variable irradiation time. The induced crystallization is followed quantitatively by analyzing the Raman spectra of the exposed area. The crystallite size distribution and the film stress are then estimated using a parametric fitting procedure. The pertinent microcrystal geometry of the samples after CO₂-laser treatment has been directly correlated to Raman spectroscopic data. Variations of several spectral features, such as bandwidths and band frequencies have been interpreted in terms of three different mechanisms including the local heating due to the CO₂-laser heating, the reduction of crystallite size and the tensile stress of μc-Si:H films. The results are discussed in the context of recent experimental and theoretical works concerning the bond polarizability.     

单壁碳纳米管吸附酞菁类有机物的研究

利用苯氧基酞菁修饰单壁碳纳米管,并利用透射电子显微镜、紫外可见吸收光谱、荧光光谱以及拉曼光谱进行了表征分析.在透射电镜下观察到结合物呈现糖葫芦状,吸附后吸收光谱以及荧光光谱中峰的强度明显下降说明单壁碳纳米管吸附了大量的苯氧基酞菁,通过拉曼光谱发现吸附后单壁碳纳米管的拉曼光谱中主要峰的位置向长波数方向移动,原因是单壁碳纳米管吸附苯氧基酞菁前后状态的改变导致的.     

Characteristics of composite propellant ignition by a CO2-laser

The delay time of ignition of a composite propellant based on ammonium perchlorate by a CO₂-laser is measured at various pressures and radiant flux densities. The characteristics of the propellant at the moment of ignition are calculated. The obtained rate constants of the reactions in the propellant are compared to the respective rate constants of the reactions in the c-phase occurring during the combustion of the composite propellant.     

Experimental study of cw carbon monoxide flame chemical laser of the CS2/O2/CO2 type

The performance of a continuous-wave (cw) CO flame chemical laser (FCL) of the CS₂/O₂/CO₂ type is presented. The laser gives up to 0.7 W cw output power on a number ofP _v (J) lines corresponding to 1110, ..., 76 vibrational bands of CO molecule. The measured values of chemical efficiency based on the reaction O+CSCO^*(v)+S and the specific power are 0.1% and 0.7J/g, respectively. The spectral composition of the CO FCL of the CS₂/O₂/CO₂ type shows lasing in the region from 5.194 to 5.573 m. All experimental measurements are conducted with a nondispersive optical cavity.     

Phase conjugation of CO2-laser radiation by a three-beam interaction

The distortion correction of a CO₂-laser radiation wavefront is studied by using forward resonant parametric scattering in gaseous SF₆ and retrace after retroreflection from a mirror.     

Synthesis of single-walled carbon nanotubes in an expanding vapor-gas flow produced by laser ablation of a graphite-catalyst mixture

Single-walled carbon nanotubes (SWCNTs) are synthesized by the ablation of a catalyst-containing carbon target with a cw CO₂ laser. Emphasis is on ablation conditions that are favorable to self-organized SWCNT synthesis. It is shown that the graphite target intensely evaporates with the formation of fractal-like tubes at the edge of the jet when the laser power density exceeds 10⁵ W/cm². Still more favorable conditions for carbon nanotube synthesis are set if the power density lies within 2×10⁴–5×10⁴ W/cm². Under these conditions, both individual SWCNTs and their bundles of diameter from 1.1 to 1.5 nm are produced, as shown by Raman scattering and electron microscopy studies. In this series of experiments, the maximal fraction of SWCNTs reaches 20%. A mechanism of SWCNT fast growth in the laser torch is suggested.     

Carbon nanocrystals produced by pulsed laser ablation of carbon

Plasma laser ablation experiments were performed irradiating glassy-carbon targets placed in vacuum through a pulsed Nd:YAG laser operating at the second harmonic (532 nm), 9 ns pulse width and 10⁹ W/cm² density power.

Thin films of ablated carbon were deposited on silicon oxide substrates placed at different distances and angles with respect to the target.

The analysis of the deposited material was carried out by using surface profiler, scanning electron microscopy (SEM), Fourier transform infrared (FT-IR) and Raman spectroscopy.

Results show the evidence of carbon nanocrystals and nanostructures with dimension of the order of 100 nm deposited on the substrates together with a large amount of amorphous phase. The spectroscopic investigations and the SEM images indicate the formation of nanodiamond seeds as a nucleation process induced on the substrate surface. Nanostructures were investigated as a function of the laser intensity and angle distribution. Experimental results were compared with the literature data coming from nanodiamonds growth with different techniques.

Experiments performed at Instituto Nazionale di Fisica Nucleare-Laboratori Nazionali del Sud (INFN-LNS) of Catania (Italy) and data analysis conducted at Dipartimento di Fisica and DFMTA of the Università of Messina (Italy), CNR-ITIS of Messina and ST-Microelectronics of Catania will be presented and discussed.     

CO2-laser annealing of Al/a-Si:H contact

The paper concerns the possibility of using CW (continuous wave) CO₂-laser annealing (=10.6 m,P 100 W/cm²) for formation of a barrier in the Al/a-Si:H/SS (SS-stainless steel) structures with good rectifyingI–V characteristics. The infrared absorption spectra, photoelectric properties, temperature effect on the conductivity and saturation current were analyzed and various contact models are discussed.We thank P. imanec for useful and stimulating discussion, J. Stuchlík for kindly manufacturing the a-Si:H films for us and D. N. Goncharov for the aid with the measurement of the conductivity temperature dependences.     

Carbon nanotubes produced by tunneling of cobalt nanoparticles in carbon fibers

Carbon nanotubes (CNTs) have been produced by the tunneling of cobalt nanoparticles in carbon fibers that are derived from electrospun polyacrylonitrile (PAN) fibers. During annealing, the PAN fibers transform to a composite of cobalt nanodroplets and carbon fibers. Driven by the high chemical potential of wrinkled graphene platelets and amorphous carbon with respect to graphite, the cobalt nanodroplets are to tunnel in the carbon fibers. When cobalt nanodroplets have an elongated shape, carbon atoms dissolved in the droplets precipitate preferentially and completely at their lateral sides, producing perfect CNTs that form bulk structures.     

Effect of the manufacturing parameters on the structure of nitrogen-doped carbon nanotubes produced by catalytic laser-induced chemical vapor deposition

Series of self-assembled and mono-dispersed bovine serum albumin (BSA)-conjugated ZnS/CuS nano-composites with different Zn/Cu ratios had been successfully synthesized by a combination method of the biomimetic synthesis and ion-exchange strategy under the gentle conditions. High-resolution transmission electron microscopy observation, Fourier transform infrared spectra and zeta potential analysis demonstrated that BSA-conjugated ZnS/CuS nano-composites with well dispersity had the hierarchical structure and BSA was a key factor to control the morphology and surface electro-negativity of final products. The real-time monitoring by atomic absorption spectroscopy and powder X-ray diffraction revealed that the Zn/Cu ratio of nano-composites could be controlled by adjusting the ion-exchange time. In addition, the metabolic and morphological assays indicated that the metabolic proliferation and spread of rat pheochromocytoma (PC12) cells could be inhibited by nano-composites, with the high anti-cancer activity at a low concentration (4 ppm). What were more important, Zn and Cu in nano-composites exhibited a positive cooperativity at inhibiting cancer cell functions. The microscope observation and biochemical marker analysis clearly revealed that the nano-composites-included lipid peroxidation and disintegration of membrane led to the death of PC12 cells. Summarily, the present study substantiated the potential of BSA-conjugated ZnS/CuS nano-composites as anti-cancer drug.     

Synthesis and crystallinity of carbon nanotubes produced by a vapor-phase growth method

We have studied the effect of temperature on the growth and crystallinity of carbon nanotubes (CNTs), synthesized by a vapor-phase growth method using a catalytic reaction of iron pentacarbonyl (Fe(CO)₅) and acetylene (C₂H₂) gas. By increasing the growth temperature from 750 °C to 950 °C, both the growth rate and the diameter of the CNTs increase. Moreover, the crystallinity of the graphite sheets improves progressively with increasing growth temperature. Adjustment of the growth temperature gives potential for controlled growth of CNTs in a large-scale synthesis of CNTs. PACS 61.46.+w; 68.37.-d; 81.07.De     

Transforming carbon nanotubes by silylation: an ab initio study

We use ab initio density functional calculations to study the chemical functionalization of single-wall carbon nanotubes and graphene monolayers by silyl (SiH(3)) radicals and hydrogen. We find that silyl radicals form strong covalent bonds with graphene and nanotube walls, causing local structural relaxations that enhance the s p(3) character of these graphitic nanostructures. Silylation transforms all carbon nanotubes into semiconductors, independent of their chirality. Calculated vibrational spectra suggest that specific frequency shifts can be used as a signature of successful silylation.     

GISAXS study of carbon nanotubes grown by CVD

We report a quantitative Grazing Incidence Small Angle X‐ray Scattering (GISAXS) study of a dense film of mutually oriented carbon nanotubes (CNTs) grown by a catalytically‐activated DC HF CCVD process after dispersion of metallic catalytic (Co) islands on SiO₂/Si(100) substrates. The GISAXS pattern analysis is expanded to non‐correlated surface science systems and is based on CNTs density, characteristic lengths, atomic Co dispersion throughout the CNTs and roughnesses of uncorrelated particles. The results are closely compared to SEM and TEM observations. The GISAXS patterns, even dominated by envelope features of disordered objects, provide significant complementary quantitative data about CNTs films. The results underline that cobalt continuously fills the nanotube in the course of the growth and that the CNTs experience a large tendency toward mutual alignment. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

CO2连续激光蒸发制备单壁碳纳米管及其Raman光谱的研究

讨论了在室温下用波长10.6μm的CO2连续激光制备单壁碳纳米管的工艺条件和生长机理.用大功率CO2连续激光蒸发制备单壁碳纳米管,所用激光功率400—900W,高分辨透射电镜观察表明单壁碳纳米管直径1.1—16nm,随着激光功率的增加稍微增加.本文还分别用波长为514.5nm和632.8nm激发光测量了CO2红外激光制备的单壁碳纳米管的一级和二级Raman光谱,发现在Raman特征峰的位置、强度上都存在差异.还将用CO2红外激光制备的单壁碳纳米管的Raman光谱与用YAG激光制备的单壁碳纳米管的Raman光谱进行了比较. 关键词： 单壁碳纳米管 CO2连续激光 Raman光谱     

Nanowire formation by coalescence of small gold clusters inside carbon nanotubes

The coalescence of Au₁₃, Au₅₅ and Au₁₄₇ icosahedral clusters encapsulated inside single walled carbon nanotubes (CNTs) of different diameters are investigated using molecular dynamics simulation with semi-empirical potentials. Three steps needed for the formation of encapsulated nanowires are followed in detail, namely, the penetration of clusters in CNTs, the coalescence between two clusters inside CNTs and their accumulation to form wires. It is suggested that no significant energy barrier is encountered during the penetration of free clusters into CNTs provided the CNT radius is large enough, that is, about 0.3 nm larger than the cluster radius. The relative orientation of clusters imposed by the CNT favors their spontaneous coalescence. After coalescence of two clusters, the Au atoms are rearranged to form new structures of cylindrical symmetry that may be seven fold, six fold, five fold, helical or fcc depending on the CNT diameter. The thermal stability of these structures is discussed and the structural properties of nanowires formed by accumulation of many clusters in CNTs are analyzed in detail. A geometrical method is presented which allows the prediction of the structure of multi-shell helical wires, when knowing only the CNT radius. These modeling results suggest the possibility of synthesizing metallic nanowires with controlled diameter and structure by embedding clusters into nanotubes with suitable diameters.     

Oxidation of carbon nanotubes by singlet O2

Chemisorption of singlet (1)Delta(g) O2 on single-walled carbon nanotubes is reexamined by first principles calculations, and the reaction barrier is substantially lower than previously reported when the spin on O2 is correctly treated. The process is initiated by the cycloaddition of a singlet O2 on top of a C-C bond and ended with an epoxy structure with each of the two oxygen atoms occupying a bridge position. The overall process is exothermic, with an activation barrier as low as 0.61 eV for the (8, 0) tube. Our results raise the possibility that carbon nanotubes with small diameters could be degraded after exposure to air and sunlight, similar to the degradation of natural rubber and synthetic plastics.