Encapsulation of methane molecules into carbon nanotubes

Methane gas (CH₄) is a chemical compound comprising a carbon atom surrounded by four hydrogen atoms, and carbon nanotubes have been proposed as possible molecular containers for the storage of such gases. In this paper, we investigate the interaction energy between a CH₄ molecule and a carbon nanotube using two different models for the CH₄ molecule, the first discrete and the second continuous. In the first model, we consider the total interaction as the sum of the individual interactions between each atom of the molecule and the nanotube. We first determine the interaction energy by assuming that the carbon atom and one of the hydrogen atoms lie on the axis of the tube with the other three hydrogen atoms offset from the axis. Symmetry is assumed with regard to the arrangement of the three hydrogen atoms surrounding the carbon atom on the axis. We then rotate the atomic position into 100 discrete orientations and determine the average interaction energy from all orientations. In the second model, we approximate the CH₄ molecule by assuming that the four hydrogen atoms are smeared over a spherical surface of a certain radius with the carbon atom located at the center of the sphere. The total interaction energy between the CH₄ molecule and the carbon nanotube for this model is calculated as the sum of the individual interaction energies between both the carbon atom and the spherical surface and the carbon nanotube. These models are analyzed to determine the dimensions of the particular nanotubes which will readily suck-up CH₄ molecules. Our results determine the minimum and maximum interaction energies required for CH₄ encapsulation in different tube sizes, and establish the second model of the CH₄ molecule as a simple and elegant model which might be exploited for other problems.     

The effect of the target structure and composition on the ejection and transport of polymer molecules and carbon nanotubes in matrix-assisted pulsed laser evaporation

Matrix-assisted pulsed laser evaporation (MAPLE) is a prominent member of a broad and expanding class of laser-driven deposition techniques where a matrix of volatile molecules absorbs laser irradiation and provides the driving force for the ejection and transport of the material to be deposited. The mechanisms of MAPLE are investigated in coarse-grained molecular dynamic simulations focused on establishing the physical regimes and limits of the molecular transfer from targets with different structures and compositions. The systems considered in the simulations include dilute solutions of polymer molecules and individual carbon nanotubes (CNTs), as well as continuous networks of carbon nanotubes impregnated with solvent. The polymer molecules and nanotubes are found to be ejected only in the ablation regime and are incorporated into matrix-polymer droplets generated in the process of the explosive disintegration of the overheated matrix. The ejection and deposition of droplets explain the experimental observations of complex surface morphologies in films deposited by MAPLE. In simulations performed for MAPLE targets loaded with CNTs, the ejection of individual nanotubes, CNT bundles, and tangles with sizes comparable or even exceeding the laser penetration depth is observed. The ejected CNTs align along the flow direction in the matrix plume and tend to agglomerate into bundles at the initial stage of the ablation plume expansion. In a large-scale simulation performed for a target containing a network of interconnected CNT bundles, a large tangle of CNT bundles with the total mass of 50 MDa is separated from the continuous network and entrained with the matrix plume. No significant splitting and thinning of CNT bundles in the ejection process is observed in the simulations, suggesting that fragile structural elements or molecular agglomerates with complex secondary structures may be transferred and deposited to the substrate with the MAPLE technique.     

Metallization of the semiconducting carbon nanotube by encapsulated bromine molecules

We use ab initio density-functional calculations to investigate the electronic structure of the bromine-adsorbed carbon nanotubes. When a Br₂ molecule is inside the (10,0) carbon nanotube, a trace of electron charge transfers from the nanotube to the Br₂ adsorbate, resulting in an increased Br–Br bond length. When the supercell contains two Br₂ molecules, total energy calculations reveal the formation of a linear chain of bromine atoms inside the carbon nanotube. Electron transfer from the nanotube to the atomic chains of the bromine adsorbates is much enhanced even in large-diameter nanotubes. We suggest that an exposure of the tip-opened carbon nanotube samples to a modest Br₂ partial pressure could result in a strong hole-doping of the nanotube, which makes the semiconducting nanotubes nearly metallic.     

Towards selective detection of chiral molecules using SWNT sensors

Calculations are performed to demonstrate that chiral molecules should in principle be selectively detected on chiral semi-conducting single-walled carbon nanotubes. These nanotubes, when they are used in a resonator configuration, could discriminate the two enantiomers (left and right handed species) of an amino acid through the adsorption energy difference ΔW and through the resonance frequency shift ΔΔf directly connected to the permittivity change of the tubes due to the molecule adsorption. Application to the calculation of ΔW and ΔΔf for the adsorption of l and d-alanine on various chiral nanotubes defines the parameters which are crucial for an optimized detection.     

Nanotube field and one-dimensional fluctuations of C<Subscript>60</Subscript> molecules in carbon nanotubes

C₆₀ molecules encapsulated in carbon nanotubes interact by van der Waals forces with the tube walls. The nanotube field leads to orientational confinement of the C₆₀ molecules which depends on the nanotube radius. In small tubes with radius R_T≤7 ? a fivefold symmetry axis of the molecule coincides with the tube axis, the center of mass of the molecule being located on the tube axis. The interaction between C₆₀ molecules encapsulated in the nanotube is then described by a O₂-rotor model on a one-dimensional (1-d) liquid chain with coupling between orientational and displacive degrees of freedom but no long-range order. This coupling leads to a temperature-dependent chain contraction. The structure factor of the 1-d liquid is derived. In tubes with larger radius the molecular centers of mass are displaced off the tube axis. The distinction of two groups of peapods with on- and off-axis molecules suggests an explanation of the apparent splitting of A_g modes of C₆₀ in nanotubes measured by resonant Raman scattering.     

Preferred orientations of encapsulated C60 molecules inside single wall carbon nanotubes

A systematical study of the orientational behavior of C60 molecules in single wall carbon nanotubes (SWCNTs) with different chirality and diameter has been performed by using a model of an infinite long nanotube filled with two C60 (denoted as C60-1 and C60-2) molecules. We studied the preferred orientation of the C60-1 molecule when the neighboring C60-2 molecule was fixed at the pentagon, double-bond, and hexagon orientations respectively. Our results showed that the C60-1 molecule prefers the pentagon (hexagon) orientation when the tube diameter is smaller (larger) than 1.31nm (1.36nm). For the tube diameter in between, the preferred molecular orientation of C60-1 changes from pentagon to hexagon with the increasing tube diameter when the neighboring C60-2 molecule is fixed at the pentagon or double-bond orientation. A novel vertex orientation for the C60-1 molecule has been found when the C60-2 molecule is fixed at the hexagon orientation.     

Scattering of linear molecules from solid surfaces

Scattering of linear molecules, which are internally deactivated initially, from solid surfaces is investigated classical mechanically. The solid is modeled with a three-dimensional isotropic Debye solid whose surface is essentially flat but thermally roughened by lattice vibration, and the molecule is described as a rigid body, which interacts with the solid via an exponential potential wall and a stationary attractive potential well. A variation in the interaction energy due to thermal roughening of the surface and due to nonsphericity of the molecule being regarded as perturbations on the system, a second order perturbation theory is developed to construct a Gaussian velocity distribution function of molecules scattered off the exponential wall and still moving inside the potential well. Among the molecules in the distribution, those with sufficient energy to escape from the potential well are regarded as being scattered into the free space. This model shows that the major differences between the scattering of monatomic molecules and that of linear ones result from less normal momentum transfer to the linear molecules than to the monatomic ones.     

First principles study of titanium-coated carbon nanotubes as sensors for carbon monoxide molecules

In this work, the electronic properties of the system composed by the CO molecules adsorbed on Ti-coated single-wall carbon nanotubes (SWNTs) are studied through first principles calculations. The changes in the electronic properties of the interaction of the CO molecules with a linear Ti wire covering an (8, 0) semiconductor SWNT are analyzed for different CO concentrations. A strong interaction between CO molecules and the SWCT/Ti system is observed, which decreases when the concentration of CO molecules increases. The resulting system are shown to be very sensitive to the CO concentration adsorbed on the tube/Ti system, making that the SWNT, which is originally semiconductor and becomes metallic after Ti covering, to recover the semiconductor behavior again when enough high concentrations of CO molecules is adsorbed on the SWNT/Ti system. These three distinct steps (semiconductor/metallic/semiconductor) constitute the basis for a feasible, flexible and efficient sensor device for CO molecule recognition.     

Theoretical studies of chemisorption of NO2 molecules on SiC nanotube

Silicon carbide (SiC) nanotubes have attracted extensive attention due to the unique properties. Modifying the electronic properties of SiC nanotubes is helpful for further widening their potential applications. In this paper, we have studied the chemisorption of NO₂ molecules at different coverage on a series of SiC nanotubes through density functional theory (DFT) calculations. The results indicate that changes in energetic, structural and electronic properties of the SiC nanotubes are significantly dependent on the coverage of adsorbed NO₂ molecules: (1) a nitrite-like structure is obtained for an odd number of NO₂ molecules adsorption on the SiC nanotube, while an even number of NO₂ molecules adsorption leads to a nitro-like configuration; (2) the adsorption energy per NO₂ molecule for even number adsorption is larger than that of odd number, suggesting that the NO₂ groups prefer the pair arrangement due to the coupling of two radicals; (3) with the increase of the coverage of the adsorbed NO₂, the band-gaps of SiC nanotubes are decreased, thus leading to the enhancement of the electro-conductivity of SiC nanotubes. Our results might provide an alternative strategy to modify the properties of SiC nanotubes, which might be useful for the design of SiC nanotubes-based nanodevices.     

Predictions of a spiral diffusion path for nonspherical organic molecules in carbon nanotubes

The diffusive behavior of ethane and ethylene in single-walled carbon nanotubes is investigated using classical molecular dynamics simulations and density functional theory calculations. At low molecular densities, these nonspherical molecules follow a spiral path inside nanotubes with diameters of 13-22 A, which maximizes the interaction of molecular C-C bonds with the C-C bonds in the nanotubes. Spherical molecules, such as methane, are not predicted to follow a spiral diffusion path. This result quantifies the manner in which molecular shape and chemical bonding affects molecule-nanotube interactions and indicates the generality of spherical transport through nanotubes.     

Adsorption of hydrogen on normal and pentaheptite single wall carbon nanotubes

Density functional calculations of the physisorption of molecular hydrogen and the dissociative atomic chemisorption on the external surface of hexagonal and pentaheptite carbon nanotubes, have been carried out. Physisorption binding energies are near 100 meV/molecule and are similar on metallic and semiconducting nanotubes. Full coverage of the nanotube with one molecule per graphitic hexagon decreases the binding energy per molecule. Chemisorption binding energies per H atom are larger on pentaheptites than on hexagonal carbon nanotubes. The molecular physisorption and dissociative chemisorption states on pentaheptites have very similar total energies (some chemisorbed states are even slightly more stable than the physisorbed states), while on hexagonal carbon nanotubes molecular physisorption is more stable than dissociative chemisorption. However, a substantial energy barrier has to be overcome to go from physisorption to dissociative chemisorption in both types of nanotubes.     

The chiral effect of adsorption of univalent atoms and diatomic molecules on the surface of carbon nanotubes

Quantum-chemical semiempirical calculations were performed of the adsorption of fluorine and hydrogen atoms and molecules on the surface of single-layered carbon nanotubes with various diameters. Semiempirical quantum-chemical MNDO calculations were based on the model of a molecular cluster with boundary pseudoatoms. The energy characteristics of adsorption were determined. Changes in physical properties caused by the adsorption of atoms and diatomic molecules were analyzed.     

Nickel cluster functionalised carbon nanotube for CO molecule detection: a theoretical study

Using spin-polarised density functional theory calculation single-walled carbon nanotube (SWCNT) whose sidewall is functionalised with nickel cluster is studied for its possible application in CO molecule sensing. We have chosen (6,0) SWCNT functionalised with Ni₁₃ cluster as the model for nanotube-cluster system. Changes in the properties of nanotube-cluster system brought by the CO molecule are reported. The CO molecule binding is energetically more favourable to the nanotube-cluster system than the pristine nanotube. The electronic properties are investigated in terms of density of states and bandstructure calculations. Pristine carbon nanotubes are intrinsically non-magnetic but nanotubes functionalised with nickel cluster are observed to have a huge magnetic moment which reduced on adsorbing CO molecule. The change in magnetisation upon CO adsorption may be detected using a suitable magnetometer. This result suggests the possibility of using carbon nanotube-cluster system to detect CO molecules. Bader charge analysis shows that CO molecule withdraws electronic charge from the cluster atoms. Nature of chemical bonding is studied with crystal orbital Hamilton population (–COHP) analysis.     

Multiple photoionization of atoms and small molecules by synchrotron radiation

Absorption of one VUV photon by an atom or a molecule can induce the ejection of several electrons through different processes. Such multiple ionization processes, studied by coincidence electron spectroscopy, provide a wealth of information on electron correlations. A magnetic bottle electron time of flight spectrometer implemented on synchrotron radiation centers has allowed the efficient detection in coincidence of two, three and up to five electrons with good energy resolution. The branching ratios of the different processes are easily extracted from the experimental spectra due to the constant transmission of the spectrometer. Multiple Auger decay was observed in rare gases atoms after inner-shell ionization, while core-valence and core-core initial double ionization followed by Auger decay are other pathways to multiple ionization. For molecules, Coulomb explosion with energy released in ionic fragments may occur after multiple ionization, nevertheless, coincidence electron spectroscopy can also provide a clear interpretation for peculiar decay channels in molecules.     

Pumping of confined water in carbon nanotubes by rotation-translation coupling

Bidirectional single file water transport in a carbon nanotube is known to occur in "bursts" in short nanotubes. Here we show that in long carbon nanotubes, when the orientation of the water molecules is maintained along one direction, a net water transport along that direction can be attained due to coupling between rotational and translational motions. The rotations of the water molecules are correlated more with the translation of the neighboring water molecule with the acceptor oxygen than the neighbor with the donor hydrogen. This mechanism can be used to pump water through nanotubes.     

Anisotropic packing and one-dimensional fluctuations of C60 molecules in carbon nanotubes

The confinement of a C60 molecule encapsulated in a cylindrical nanotube depends on the tube radius. In small tubes with radius RT approximately < 7 A, a fivefold axis of the molecule coincides with the tube axis. The interaction between C60 molecules in the nanotube is then described by a O2-rotor model on a 1D liquid chain with coupling between orientational and displacive correlations. This coupling leads to chain contraction. The structure factor of the 1D liquid is derived. In tubes with a larger radius the molecular centers of mass are displaced off the tube axis. The distinction of two groups of peapods with on- and off-axis molecules suggests an explanation of the apparent splitting of Ag modes of C60 in nanotubes measured by resonant Raman scattering.     

Field emission microscopy of adsorption and desorption of residual gas molecules on a carbon nanotube tip

Field emission of electrons from multiwall carbon nanotubes has been investigated by field emission microscopy (FEM) in ultra-high vacuum. A carbon nanotube, at the end of which at least six pentagons exist to make a closed cap, gives an FEM pattern consisting of bright pentagonal rings if the nanotube surface is clean. Adsorption of residual gas molecules is observed as bright spots in the FEM pattern, giving rise to an abrupt increase in the emission current. Adsorbed molecules seem to reside preferentially on the pentagonal sites where the strong electric field is concentrated. A heat cleaning of the emitter at about 1300 K allows the molecules to desorb, and the nanotube emitter recovers its original clean surface. It has been revealed that the adsorption and desorption of gas molecules are responsible for stepwise fluctuation of the emission current.     

Strange non-chaotic attractors in a state controlled-cellular neural network-based quasiperiodically forced MLC circuit

The adsorption energies, bond order, atomic charge, optical properties, and electrostatic potential of nitrogen molecules of armchair single-walled carbon nanotubes (SWCNTs) and nitrogen-doped single-walled carbon nanotubes (N-SWCNTs) were investigated using density functional theory (DFT). Our results show that adsorption of the \(\hbox {N}_{2}\) molecules on the external wall of a nanotube is more effective than on the internal wall in SWCNTs. The results show that \(\hbox {N}_{2}\) molecule(s) are weakly bonded to SWCNTs and N-SWCNTs through van der Waals-type interactions. The interaction of \(\hbox {N}_{2}\) molecules with SWCNTs and N-SWCNTs is physisorption as the adsorption energy and charge transfer are small, and adsorption distance is large. The electronic transitions from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) (\(\hbox {H}\rightarrow \hbox {L}\)) have the maximum wavelength and the lowest oscillator strength. The potential sensor on the surface of pristine SWCNTs and N-SWCNTs for the adsorption of \(\hbox {N}_{2}\) molecule(s) is investigated. The N-loaded single-walled carbon nanotube is introduced as a better \(\hbox {N}_{2}\) molecule(s) detector when compared with SWCNTs.     

Adsorption and desorption of an O2 molecule on carbon nanotubes

Adsorption and desorption of an oxygen molecule on carbon nanotubes are investigated using density functional calculations. Several precursor states exist at the edge of armchair nanotubes, whereas an exothermic adsorption takes place at the edge of zigzag nanotubes. We also estimate desorption barriers of a CO molecule from nanotubes as well as fullerenes and amorphous phases. Our calculations suggest that carbon nanotubes can survive selectively during the oxidative etching process with a precise control of annealing temperature, in good agreement with experimental results of purification process of carbon nanotubes.     

Density and thermodynamics of hydrogen adsorbed on the surface of single-walled carbon nanotubes

A method is proposed for calculating the adsorption of hydrogen in single-walled carbon nanotubes. This method involves solving the Schrödinger equation for a particle (hydrogen molecule) moving in a potential generated by the surrounding hydrogen molecules and atoms forming the wall of the carbon nanotube. The interaction potential for hydrogen molecules is taken in the form of the Silvera-Goldman empirical potential, which adequately describes the experimental data on the interaction between H₂ molecules (including the van der Waals interaction). The interaction of hydrogen molecules with carbon atoms is included in the calculation through the Lennard-Jones potential. The free energy at a nonzero temperature is calculated with allowance made for the phonon contribution, which, in turn, makes it possible to take into account the correlations in the mutual arrangement of the neighboring molecules. The dependences of the total energy, the free energy, and the Gibbs thermodynamic potential on the applied pressure P and temperature T are calculated for adsorbed hydrogen molecules. These dependences are obtained for the first time with due regard for the quantum effects. The pressure and temperature dependences of the hydrogen density m(P, T) are also constructed for the first time.