Selective cap opening in carbon nanotubes driven by laser-induced coherent phonons

We demonstrate the possibility of a selective nonequilibrium cap opening of carbon nanotubes as a response to femtosecond laser excitation. By performing molecular dynamics simulations based on a microscopic electronic model we show that the laser-induced ultrafast structural changes differ dramatically from the thermally induced dimer emission. Ultrafast bond weakening and simultaneous excitation of two coherent phonon modes of different frequencies, localized in the spherical caps and cylindrical nanotube body, are responsible for the selective cap opening.     

Growth of carbon nanotubes on metal nanoparticles: a microscopic mechanism from ab initio molecular dynamics simulations

We report on ab initio molecular dynamics simulations of the early stages of single-walled carbon nanotube (SWCNT) growth on metal nanoparticles. Our results show that a sp2 bonded cap is formed on an iron catalyst, following the diffusion of C atoms from hydrocarbon precursors on the nanoparticle surface. The weak adhesion between the cap and iron enables the graphene sheet to "float" on the curved surface, as additional C atoms covalently bonded to the catalyst "hold" the tube walls. Hence the SWCNT grows capped. At the nanoscale, we did not observe any tendency of C atoms to penetrate inside the catalyst, consistent with total energy calculations showing that alloying of Fe and C is very unlikely for 1 nm particles. Root growth was observed on Fe but not on Au, consistent with experiment.     

Structural transformation of vapor grown carbon nanofibers studied by HRTEM

Vapor grown carbon nanofibers have been extensively manufactured and investigated in recent years. In this study commercially available vapor grown carbon nanofibers subjected to different processing and post processing conditions were studied employing high resolution TEM images. The analysis showed that the fibers consist primarily of conical nanofibers, but can contain a significant amount of bamboo nanofibers. Most conical nanofibers were found to consist of an ordered inner layer and a disordered outer layer, with the cone angle distribution of the inner layers indicating that these cannot have a stacked cone structure but are compatible with a cone-helix structure. Fibers that have been heat treated to temperatures above 1,500 °C undergo a structural transformation with the ordered inner layers changing from a cone-helix structure to a highly ordered multiwall stacked cone structure. The bamboo nanofibers were found to have a tapered multiwall nanotube structure for the wall and a multishell fullerene structure for the cap of each segment, surrounded by a disordered outer layer. When these fibers are heat treated the disordered outer layers transform to an ordered multiwall nanotube structure and merge with the wall of each segment. The end caps of each segment transform from a smooth multiwall fullerene structure to one consisting of disjointed graphene planes. A reaction-diffusion mechanism is proposed to explain the growth and structure of the bamboo nanofibers.     

The role of precursor gases on the surface restructuring of catalyst films during carbon nanotube growth

Catalyst films undergo considerable surface morphology restructuring prior to carbon nanotube nucleation, deeply influencing the nanostructures obtained. Here we study the influence of different gaseous atmospheres on the structure of thin Fe films. The morphology is influenced by process temperature and substrate interactions and varying the gas type and pressure can control the average catalyst island height.     

碳纳米管对接成异质结器件的计算模拟

基于Stone-Wales缺陷演变理论与分子动力学、Monte Carlo计算方法, 进行了碳纳米管(CNTs)对接成异质结器件的计算模拟.首先, 提出了一种模拟CNTs端帽位置变化的新算法, 并计算模拟了单根CNT的端帽从开口到闭合的过程. Stone-Wales缺陷演变被设计模拟这些端帽变化的跃变过程, 以模拟C–C键的生成与断裂, 而分子动力学则作为跃变后构型弛豫的渐变模拟. 同时, 研究了不同管型CNTs的端帽打开并对接形成异质结的过程.研究结果显示, 对接初期在对接处先产生大量的缺陷, 以促进反应的发生. 这些缺陷趋向于演变成稳定的六元环结构, 或者五元环/七元环的结构, 使异质结趋于稳定. 关键词： 碳纳米管 Monte Carlo Stone-Wales缺陷 分子动力学     

Transmission electron microscopy observations of CNT morphology before and after heating in vacuum

Heating of multi-walled carbon nanotubes is often required to obtain clean patterns in the field electron emission microscope (FEEM). A transmission electron micrograph study of morphological changes in the cap structure of multi-walled carbon nanotubes due to heating in vacuum is presented. The lack of significant structural change in the cap structure when specimens were heated to 1925 K for 2.5 h determines an upper bound for the diffusivity of multi-walled carbon nanotube surface atoms, of the order of 10⁻¹⁶ cm²/s at 1925 K.     

Fine-structured field emission images originating from coherently scattering of electrons within a multi-walled carbon nanotube

High resolution field emission image of a single multi-walled carbon nanotube was studied by field emission microscopy. The images contain patterns consisting of rather ordered bright fringes. We propose a model based on coherent electron scattering to explain the observed field emission image. The emitted electrons will undergo coherent scattering within the cap region of a multi-wall carbon nanotube, which may be viewed as elastic scattering by a polycrystalline structure with an infinite size. This study is helpful for understanding the physical mechanism of field emission of carbon nanotube.     

外加电场作用下碳纳米管结构稳定性及结构修饰研究

利用外加电场的方法，对多壁碳纳米管的结构稳定性进行了研究.结果表明当场强达到30 V/nm时，碳纳米管端部的结构失稳，端部碳原子间的π键被打开，外部原子开始进入到碳纳米管的结构中.利用电子显微镜作为纳米加工仪器，通过外加电场的方法在多壁碳纳米管的端部制备了非晶态碳纳米线，形成碳纳米管-纳米线复合结构.碳纳米管和纳米线结合处的σ键作为绝缘界面，形成了电子输运的势垒.     

Chiral vector determination of carbon nanotubes by observation of interference patterns near the end cap

We have observed clear interference images near the end caps of semiconducting carbon nanotubes with quasiarmchair and zigzag chiral vectors, using a scanning tunneling microscope. We performed a simple tight-binding calculation to simulate the interference patterns, in which Bloch states with k and -k wave vectors were superimposed. The calculations were able to reproduce the observed interference patterns. In addition, we demonstrated that the interference patterns can be categorized by the positions of k_{min} in the Brillouin zone that yield minimum energy in the conduction band, and can give information on the chiral vector of a carbon nanotube.     

Selective growth of carbon nantoubes on SiO2/Si substrate

Three-step raising temperature process was employed to fabricate carbon nanotubes by pyrolysis of ferrocene/melamine mixtures on silica and single crystalline silicon wafers respectively. Then the morphologies, structures and compositions of obtained carbon nanotubes are investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscope (EDX) and electron energy-loss spectroscopy (EELS). TEM and SEM observation shows that on silica substrate, high-oriented carbon nanotube can grow compactly to form continuous film on both frontal and cross-section surfaces, but on silicon substrate, only can form on cross-section surface. These carbon nanotubes have much irregular cup-like structure, and with outer diameter varying from 25 nm to 35 nm. At the top end of carbon nanotube there is a catalyst particle. EDX analysis reveals that the particle are iron cluster, and EELS spectrum indicates that the nanotube is composed of pure carbon. Finally, the effect of substrate surface roughness on the growth behavior of carbon nanotubes has been discussed.     

Flame synthesis of single-walled carbon nanotubes

Flames offer potential for synthesis of carbon nanotubes in large quantities at considerably lower costs than that of other methods currently available. This study aims to examine conditions for carbon nanotube formation in premixed flames and to characterize the morphology of solid carbon deposits and their primary formation mechanisms in the combustion environment. Single-walled nanotubes have been observed in the post-flame region of a premixed acetylene/oxygen/15 mol% argon flame operated at 6.7 kPa with Fe(CO)₅ vapor used as a source of metallic catalyst necessary for nanotube growth. Thermophoretic sampling and transmission electron microscopy were used to characterize the solid material present in the flame at various heights above burner (HAB), giving a resolution of formation dynamics within the flame system. Catalyst particle formation and growth is observed to dominate the immediate post-flame region (10–40 mm HAB). Nanotubes were observed to be present after 40 mm HAB with nanotube inception occurring as early as 30 mm HAB. Between 40 and 70 mm HAB, nanotubes are observed to coalesce into clusters. A nanotube formation ‘window’ is evident with formation limited to fuel equivalence ratios between 1.5 and 1.9. A continuum of morphologies ranging from relatively clean clusters of nanotubes to amorphous material is observed between these lower and upper limits. High-resolution TEM and Raman spectroscopy revealed nanotube bundles with each nanotube being single-walled with diameters between 0.9 and 1.5 nm.     

MEAM study of carbon atom interaction with Ni nano particle

Carbon, Ni, and C-Ni alloy modified embedded atom method (MEAM) potentials were developed to study the initial process of carbon nanotube growth on Ni catalyst particles. The MEAM potentials were used to study the atomistic interaction between a carbon atom and a fcc Ni nano particle, both on the particle surfaces and inside the Ni nano particles. The result shows that surface carbon atom is more stable than those in the bulk and sub-surface interstitial positions. Carbon atoms are expected to diffuse from the bulk to the surface, and the single walled and double-walled carbon nanotubes would be more favorable to form on Ni nano particle catalyst. The carbon and Ni nano particle interaction calculation shows that the corner and the edge of the particle are the energetically more favorable sites for the carbon adatom. The carbon nanotube may grow from the corner and edge of the particle.     

A continuous synthesis of carbon nanotubes by dc thermal plasma jet

In this contribution we present a dc thermal plasma jet route for the continuous synthesis of single- and multi- walled carbon nanotubes. Our findings show the as produced product to be dependent on the plasma atmosphere and catalyst. Multi walled carbon nanotubes can be synthesized without a catalyst. Single walled carbon nanotubes require the presence of a catalyst (Ni-Ce) and the addition of hydrogen to the buffer gas. Increasing the amount of hydrogen added to the reaction significantly improves the nanotube yield. PACS 81.07.De; 36.40.Gk; 63.50.+x; 68.37.Lp; 68.37.Hk     

Growth of carbon nanotubes from C<Subscript>60</Subscript>

Carbon nanotubes can be obtained from a multitude of molecular precursors in chemical vapor deposition (CVD) processes. Here we demonstrate that the use of C₆₀ as the carbon feedstock gas in an iron-catalyzed thermal CVD experiment leads to the formation of films of multi-walled carbon nanotubes. The critical role of the diameter of the catalyst particles in determining the efficiency of nanotube growth is clearly demonstrated. Electron microscopy and Raman spectroscopy were employed for the characterisation of the nanotube material. The structural properties of the individual nanotubes show distinctive differences to acetylene-grown multi-walled nanotubes. PACS 81.07.De; 81.10.Bk     

In situ measurements and modeling of carbon nanotube array growth kinetics during chemical vapor deposition

Direct measurements of carbon nanotube growth kinetics are described based upon time-resolved reflectivity (TRR) of a HeNe laser beam from vertically aligned nanotube arrays (VANTAs) as they grow during chemical vapor deposition (CVD). Growth rates and terminal lengths were measured in situ for VANTAs growing during CVD between 535 °C and 900 °C on Si substrates with evaporated Al/Fe/Mo multi-layered catalysts and acetylene feedstock at different feedstock partial pressures. Methods of analysis of the TRR signals are presented to interpret catalyst particle formation and oxidation, as well as the porosity of the VANTAs. A rate-equation model is developed to describe the measured kinetics in terms of activation energies and rate constants for surface carbon formation and diffusion on the catalyst nanoparticle, nanotube growth, and catalyst over-coating. Taken together with the TRR data, this model enables basic understanding and optimization of growth conditions for any catalyst/feedstock combination. The model lends insight into the main processes responsible for the growth of VANTAs, the measured number of walls in the nanotubes at different temperatures, conditions for growth of single-wall carbon nanotube arrays, and likely catalyst poisoning mechanisms responsible for the sharp decline in growth rates observed at high temperatures. PACS  61.46.+w; 81.07.De; 81.16.Hc     

Electron field emission properties of closed carbon nanotubes

Recent experiments have shown that carbon nanotubes exhibit excellent electron field emisson properties with high current densities at low electric fields. Here we present theoretical investigations that incorporate geometrical effects and the electronic structure of nanotubes. The electric field is dramatically enhanced near the cap of a nanotube with a large variation of local field distribution. It is found that deviation from linear Fowler-Nordheim behavior occurs due to the variation of the local field in the electron tunneling region. The maximum current per tube is of the order of 10 microA. Local and microscopic aspects of field emission from nanotubes are also presented.     

Nucleation of single-walled carbon nanotubes

The nucleation pathway for single-wall carbon nanotubes on a metal surface is demonstrated by a series of total energy calculations using density functional theory. Incorporation of pentagons at an early stage of nucleation is energetically favorable as they reduce the number of dangling bonds and facilitate curvature of the structure and bonding to the metal. In the presence of the metal surface, nucleation of a closed cap or a capped single-wall carbon nanotube is overwhelmingly favored compared to any structure with dangling bonds or to a fullerene.     

Growth of carbon nanotubes from ring carbon clusters

The possibility of growing single-wall carbon nanotubes from ring carbon clusters that appear at a certain stage of cooling carbon vapor is discussed. Such a technique could allow one to grow single-wall nanotubes without introducing a macroscopic amount of a catalyst and to retain nanotubes open during their growth. An analysis performed using semiempirical quantum-chemical methods shows that, when catalyst atoms interact with the edge of an already formed nanotube surface, the bonds of these atoms with carbon tend to occupy positions normal to the generatrix of the nanotube. This situation is natural for transition-metal atoms, since they favor the destruction of pentagonal cycles at the edge of the surface. The destruction mechanism consists in the fact that pentagons incorporate carbon atoms from the outside and become hexagons. The dependence of this tendency on the type of catalyst atom is considered.     

Monitoring structural defects and crystallinity of carbon nanotubes in thin films

We report the influence of catalyst formulation and reaction temperature on the formation of carbon nanotube (CNT) thin films by the chemical vapour deposition (CVD) method. Thin films of CNTs were grown on Fe-Mo/Al₂O₃-coated silicon wafer by thermal decomposition of methane at different temperatures ranging from 800 to 1000°C. The electron microscopic investigations, SEM as well as HRTEM, of the as-grown CNT thin films revealed the growth of uniform multi-walled CNTs in abundance. The intensity ratio of D-band to G-band and FWHM of G-band through Raman measurements clearly indicated the dependency of structural defects and crystallinity of CNTs in thin films on the catalyst formulation and CVD growth temperature. The results suggest that thin films of multi-walled CNTs with negligible amount of defects in the nanotube structure and very high crystallinity can be obtained by thermal CVD process at 925°C.     

Dependence of the electrical properties of defective single-walled carbon nanotubes on the vacancy density

The relationship between the electric properties and the vacancy density in single-walled carbon nanotubes has been investigated from first principles as well as the dependence of the influencing range of a vacancy in the nanotube on the nanotube chirality.Compared with the long-range interaction of the vacancies in a single-walled carbon nanotube with non-zero chiral angle,a much shorter interaction was found between vacancies in a zigzag single-walled carbon nanotube.In this study,we investigated the bandstructure fluctuations caused by the nanotube strain,which depends on both the vacancy density and the tube chirality.These theoretical results provide new insight to understand the relationship between the local deformation of a defective single-walled carbon nanotube and its measurable electronic properties.