Nitrogen doping in carbon nanotubes

Nitrogen doping of single and multi-walled carbon nanotubes is of great interest both fundamentally, to explore the effect of dopants on quasi-1D electrical conductors, and for applications such as field emission tips, lithium storage, composites and nanoelectronic devices. We present an extensive review of the current state of the art in nitrogen doping of carbon nanotubes, including synthesis techniques, and comparison with nitrogen doped carbon thin films and azofullerenes. Nitrogen doping significantly alters nanotube morphology, leading to compartmentalised 'bamboo' nanotube structures. We review spectroscopic studies of nitrogen dopants using techniques such as X-ray photoemission spectroscopy, electron energy loss spectroscopy and Raman studies, and associated theoretical models. We discuss the role of nanotube curvature and chirality (notably whether the nanotubes are metallic or semiconducting), and the effect of doping on nanotube surface chemistry. Finally we review the effect of nitrogen on the transport properties of carbon nanotubes, notably its ability to induce negative differential resistance in semiconducting tubes.     

Modeling of the anodic arc discharge and conditions for single-wall carbon nanotube growth

A model of the arc discharge used for a single wall carbon nanotube (SWNT) synthesis is developed. Coupling solution of the non-equilibrium, Knudsen layer, with hydrodynamic layer and discharge column provides self-consistent solution for the ablation rate and plasma parameter distribution. It is predicted that the interelectrode gap decreases with the background pressure increase. Conditions for single wall carbon nanotube formation in the arc discharge method of nanotube synthesis are described. Carbon nanotube seed formation and charging in the interelectrode gap are found to be very important effects that may alter carbon nanotube formation in the cathode region. This model predicts that the long carbon nanotubes in the high pressure Helium environment can be deposited on the cathode surface. Model predictions are found to be in agreement with experiment.     

Computational Studies on Mechanical and Thermal Properties of Carbon Nanotube Based Nanostructures

The excellent set of properties of carbon nanotube and carbon nanotube-based nanostructures has been established by various studies. However the claimed property values and trends have not been unanimously agreed upon. Using state of the art molecular dynamics and ab initio methods, we have extensively studied the mechanical, thermal and structural properties of carbon nanotubes and carbon nanotube based nanostructures. Additionally this study aims to address the approaches used in various studies to assess the validity and influence of various definitions used for determining the physical properties as reported in earlier experiments and theoretical calculations. We have come up with equations, which quantitatively address the wide differences in trend and values of nanotube axial modulus available across the literature. Applying a novel bond rearrangement scheme, we have found similar values in twist modulus of zigzag and armchair nanotubes. This opposes the claim of difference that was shown to be valid only at finite limit in our study. We have shown that the contribution of van der Waals energy in a multi-wall nanotube is powerful enough to make it hexagonal in shape but negligible in affecting the axial modulus. These insights will also help in designing micromechanics model of materials made from carbon nanotube or nanotube like structures. In particular, we have calculated the mechanical properties (young modulus, bending modulus and twist modulus) of isolated and bundled nanotubes, single and multi-wall nanotubes and single and multi-wall carbon nanotube based tori. We also report studies on thermal variation of moduli and thermal expansion of nanotubes. The result obtained by first principles calculation based interatomic potential agrees well with the experimental results.     

Consideration of critical axial properties of pristine and defected carbon nanotubes under compression

Carbon nanotubes are hexagonally configured carbon atoms in cylindrical structures. Exceptionally high mechanical strength, electrical conductivity, surface area, thermal stability and optical transparency of carbon nanotubes outperformed other known materials in numerous advanced applications. However, their mechanical behaviors under practical loading conditions remain to be demonstrated. This study investigates the critical axial properties of pristine and defected single- and multi-walled carbon nanotubes under axial compression. Molecular dynamics simulation method has been employed to consider the destructive effects of Stone-Wales and atom vacancy defects on mechanical properties of armchair and zigzag carbon nanotubes under compressive loading condition. Armchair carbon nanotube shows higher axial stability than zigzag type. Increase in wall number leads to less susceptibility of multi-walled carbon nanotubes to defects and higher stability of them under axial compression. Atom vacancy defect reveals higher destructive effect than Stone-Wales defect on mechanical properties of carbon nanotubes. Critical axial strain of single-walled carbon nanotube declines by 67% and 26% due to atom vacancy and Stone-Wales defects.     

Encapsulation of a benzene molecule into a carbon nanotube

Aromatic hydrocarbon molecules encapsulated in carbon nanotubes have been proposed for applications as semiconductors. They can be formed by exploiting the van der Waals interaction as a simple method to incorporate molecules into carbon nanotubes. However, the existence of energy barriers near the open ends of carbon nanotubes may be an obstacle for molecules entering carbon nanotubes. In this paper, we investigate the encapsulation mechanism of a typical aromatic hydrocarbon, namely a benzene molecule, into a carbon nanotube in order to determine the dependence on radius of the tube. A continuous approach which assumes that the molecular interactions can be approximated using average atomic densities together with the semi-empirical Lennard–Jones potential function is adopted, and an analytical expression for the interaction energy is obtained which may be readily evaluated by algebraic computer packages. In particular, we determine the threshold radius of the carbon nanotube for which the benzene molecule will enter the carbon nanotube. The analytical approach adopted here provides a computationally rapid procedure for the determination of critical numerical values.     

On the microstructure of single wall carbon nanotubes reinforced ceramic matrix composites

A microstructural modelling of the microstructure in single wall carbon nanotubes reinforced alumina ceramics has been developed. The model accounts for the main microstructural features, being quite useful to describe the carbon nanotube distribution along the ceramic matrix. The microstructural analysis derived from this model is found to give a deeper insight into the high-temperature creep of these composites.     

A strategy for improving mechanical properties of a fiber reinforced epoxy composite using functionalized carbon nanotubes

Carbon fiber reinforced epoxy composite laminates are studied for improvements in quasi static strength and stiffness and tension-tension fatigue cycling at stress-ratio (R-ratio) = +0.1 through strategically incorporating amine functionalized single wall carbon nanotubes (a-SWCNTs) at the fiber/fabric-matrix interfaces over the laminate cross-section. In a comparison to composite laminate material without carbon nanotube reinforcements there are modest improvements in the mechanical properties of strength and stiffness; but, a potentially significant increase is demonstrated for the long-term fatigue life of these functionalized nanotube reinforced composite materials. These results are compared with previous research on the cyclic life of this carbon fiber epoxy composite laminate system reinforced similarly with side wall fluorine functionalized industrial grade carbon nanotubes. Optical and scanning electron microscopy and Raman spectrometry are used to confirm the effectiveness of this strategy for the improvements in strength, stiffness and fatigue life of composite laminate materials using functionalized carbon nanotubes.     

Contact Phenomena and Mott Transition in Carbon Nanotubes

We describe the transfer of electric charge in junctionsbetween a metal and carbon nanotube as well as betweenmetallic and semiconducting carbon nanotubes. The long rangeCoulomb interaction drastically modifies the charge transferphenomena in one-dimensional nanotube systems compared toconventional semiconductor heterostructures. Being broughtinto a contact with a metal, conducting nanotube accumulateselectric charge whose density decays slowly with the distancefrom the junction. The length of the Schottky barrier innanotube heterojunctions varies from the distances of theorder of the nanotube radius (nanometers) to the distances ofthe order of the nanotube length (microns) depending on adoping strength. The Schottky barrier height shows pronouncedasymmetry under the forward and reverse bias. This results inrectifying behavior of heterojunctions, in agreement withrecent experimental observations by Z. Yao et al. andP. McEuen et al. Finally, we discuss observability of recentlypredicted Mott insulating phase in metallic carbon nanotubes.     

Tunable field emission properties of carbon nanotube arrays by engineering Fe catalysts

High-quality carbon nanotube (CNT) arrays composed of nanotubes with different diameters and wall numbers were synthesized by water-assisted chemical vapor deposition (CVD) from engineered Fe catalysts. Interestingly, the distribution of nanotube diameter and wall number broadened over 2.5 times as the catalytic Fe thickness increased. The mean diameter and wall number of nanotubes increased monotonically with the Fe thickness, while the calculated CNT area density within an array dropped about 32 times. Field emission measurements revealed that the turn-on voltage for CNT arrays decreased from 3.5 to 2.5 V/µm with the increased catalytic Fe thickness. It was believed that the screening effect from the proximity of neighboring nanotubes has a dominant influence than the CNT diameter on the resulting turn-on voltage.     

General theories for the electrical transport properties of carbon nanotubes

We have shown that the general theories of metals and semiconductors can be employed to understand the diameter and voltage dependency of current through metallic and semiconducting carbon nanotubes, respectively. The current through a semiconducting multiwalled carbon nanotube (MWCNT) is associated with the energy gap that is different for different shells. The contribution of the outermost shell is larger as compared to the inner shells. The general theories can also explain the diameter dependency of maximum current through nanotubes. We have also compared the current carrying ability of a MWCNT and an array of the same diameter of single wall carbon nanotubes (SWCNTs) and found that MWCNTs are better suited and deserve further investigation for possible applications as interconnects.     

Eccentric compression stability of multi-walled carbon nanotubes embedded in an elastic matrix

This paper reports the results of an investigation on the eccentric compression stability of multi-walled carbon nanotubes embedded in an elastic matrix. Based on continuum modeling, a multilayer shell model is presented for the eccentric compression buckling of multi-walled carbon nanotubes embedded in an elastic matrix, in which the effect of van der Waals forces between two adjacent tubes is taken into account. The critical bending moment and the eccentric compression mode for three types of multi-walled carbon nanotubes with different layer numbers and ratios of radius to thickness are calculated. Results obtained show that the eccentric compression buckling mode corresponding the critical bending moment is unique, and is different from the purely axial compression buckling of an individual multi-walled carbon nanotube. For different types of multi-walled carbon nanotubes, the effect of matrix stiffness on the critical bending moment of multi-walled carbon nanotubes under eccentric compression loading is obviously different, and is dependent on the innermost radius and layer numbers of the multi-walled carbon nanotubes. The critical bending stress exerted on the center tubes of nearly solid multi-walled carbon nanotubes does not change as the ratio of the axial compression loading to the bending membrane force increases. The new features and meaningful numerical results in this paper are helpful for the application and the design of nanostructures in which multi-walled carbon nanotubes act as basic elements.     

Supported lipid bilayer/carbon nanotube hybrids

Carbon nanotube transistors combine molecular-scale dimensions with excellent electronic properties, offering unique opportunities for chemical and biological sensing. Here, we form supported lipid bilayers over single-walled carbon nanotube transistors. We first study the physical properties of the nanotube/supported lipid bilayer structure using fluorescence techniques. Whereas lipid molecules can diffuse freely across the nanotube, a membrane-bound protein (tetanus toxin) sees the nanotube as a barrier. Moreover, the size of the barrier depends on the diameter of the nanotube--with larger nanotubes presenting bigger obstacles to diffusion. We then demonstrate detection of protein binding (streptavidin) to the supported lipid bilayer using the nanotube transistor as a charge sensor. This system can be used as a platform to examine the interactions of single molecules with carbon nanotubes and has many potential applications for the study of molecular recognition and other biological processes occurring at cell membranes.     

Adhesion between two radially collapsed single-walled carbon nanotubes

Continuum mechanics analysis and molecular mechanics simulations are performed to study adhesion between two identical, radially collapsed single-walled carbon nanotubes. Not only the inter-adhesion energy between nanotubes but also the inner adhesion energy in a nanotube is considered. A closed-form solution to the adhesion configuration is achieved, which is well consistent with our molecular mechanics simulation. Comparing the potential energy of the adhesion structures formed by two identical single-walled carbon nanotubes, three types of configurations, i.e., circular, deformed, and collapsed shape, will be formed with increasing carbon nanotubes radius and separated by two critical radii of the single-walled carbon nanotube. Furthermore, it is found that the collapsed adhesion structure possesses the highest interfacial adhesion energy. The results demonstrate that, as a potential application in carbon nanotube reinforced composites, arrays formed by collapsed carbon nanotubes will be optimal due to the strong interface strength.     

Collapse and stability of single- and multi-wall carbon nanotubes

The collapse and stability of carbon nanotubes (CNTs) have important implications for their synthesis and applications. While nanotube collapse has been observed experimentally, the conditions for the collapse, especially its dependence on tube structures, are not clear. We have studied the energetics of the collapse of single-?and multi-wall CNTs via atomistic simulations. The collapse is governed by the number of walls and the radius of the inner-most wall. The collapsed structure is energetically favored about a certain diameter, which is 4.12, 4.96 and 5.76?nm for single-, double-?and triple-wall CNTs, respectively. The CNT chirality also has a strong influence on the collapsed structure, leading to flat, warped and twisted CNTs, depending on the chiral angle.     

Surface oxidation study of single wall carbon nanotubes

Functionalization of single wall carbon nanotubes (SWCNTs) is desirable to enhance their ability to be incorporated into polymers and enhance their bonding with the matrix. One approach to carbon nanotube functionalization is by oxidation via a strong oxidizing agent or refluxing in strong acids. However, this approach can damage the nanotubes, leading to the introduction of defects and/or shorter nanotubes. Such damage can adversely affect the mechanical, thermal, and electrical properties. A more benign approach to nanotube functionalization has been developed involving photo-oxidation. Chemical analysis by XPS revealed that the oxygen content of the photo-oxidized SWCNTs was 11.3?at.% compared to 6.7?at.% for SWCNTs oxidized by acid treatment. The photo-oxidized SWCNTs produced by this method can be used directly in various polymer matrices or can be further modified by additional chemical reactions.     

Flame Synthesis of Coiled Carbon Nanotubes

A flame synthesis technique of coiled carbon nanotubes using acetylene jet flames and iron catalyst is presented. It was found that acetylene flames generate single wall coiled nanotubes within in a narrow pyrolysis zone above the burner for a limited range of fuel flow rates. The corresponding local convective time scale corresponding to the coiled nanotube formation zone is around 0.04 ms. The yield of the coiled carbon nanotubes was further increased by changing the local convective time scale with the addition of controlled turbulence.     

Investigation of the interactions between molecules of β-Carotene, Vitamin A and CNTs by MD simulations

Two typical phenomena (wrapping and filling), mainly about the interactions between biological molecules and carbon nanotubes (CNTs), were investigated by performing molecular dynamics (MD) simulations. We calculated the center of mass (COM) distance and the interaction energy between the biological molecules and single-walled nanotubes (SWNTs). The influence of nanotube wall number, chirality, radius and temperature was also investigated by a series of MD simulations. The results indicated that Vitamin A and β-Carotene were two promising biomaterials for decoration of CNTs. The interactions between biological molecules and CNTs could be influenced by those four factors. The general conclusions derived from this study may be of importance in medical and biological areas.     

Capillary absorption of metal nanodroplets by single-wall carbon nanotubes

We present a simple model that demonstrates the possibility of capillary absorption of nonwetting liquid nanoparticles by carbon nanotubes (CNTs) assisted by the action of the Laplace pressure due to the droplet surface tension. We test this model with molecular dynamics simulation and find excellent agreement with the theory, which shows that for a given nanotube radius there is a critical size below which a metal droplet will be absorbed. The model also explains recent observations of capillary absorption of nonwetting Cu nanodroplets by carbon nanotubes. This finding has implications for our understanding of the growth of CNTs from metal catalyst particles and suggests new methods for fabricating composite metal-CNT materials.     

Catalytic action of gold and copper crystals in the growth of carbon nanotubes

Multi-wall carbon nanotubes are grown in a chemical vapor deposition process by using bulk gold and copper substrates as catalysts. Nanotube growth starts from a nanometer-sized roughness on the metal surfaces and occurs in a mechanism where the catalyst particle is either at the tip (Au) or root (Cu) of the growing nanotube. Whereas Au leads to nanotubes with good structural perfection, nanotubes grown from Cu show a higher density of defects. High-resolution transmission electron microscopy shows the bonding between Au and carbon at the metal-nanotube interface whereas no bonds between Cu and carbon occur. Highly mobile Au or Cu atoms adsorb at the growing edge of a carbon nanotube from where diffusion along the nanotube wall can lead to the formation of Au or Cu nanowires inside the central hollow of carbon nanotubes.     

Effect of curvature on structures and vibrations of zigzag carbon nanotubes: A first-principles study

First-principles pseudopotential-based density functional theory calculations of atomic and electronic structures, full phonon dispersions and thermal properties of zigzag single wall carbon nanotubes (SWCNTs) are presented. By determining the correlation between vibrational modes of a graphene sheet and of the nanotube, we understand how rolling of the sheet results in mixing between modes and changes in vibrational spectrum of graphene. We find that the radial breathing mode softens with decreasing curvature. We estimate thermal expansion coefficient of nanotubes within a quasiharmonic approximation and identify the modes that dominate thermal expansion of some of these SWCNTs both at low and high temperatures.