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.     

Tubular fullerenes inside carbon nanotubes: optimal molecular orientation versus tube radius

We present an investigation of the orientations and positions of tubular fullerene molecules (C₉₀, ..., C₂₀₀) encapsulated in single-walled carbon nanotubes (SWCNT), a series of so-called fullerene nanopeapods. We find that increasing the tube radius leads to the following succession of energetically stable regimes: (1) lying molecules positioned on the tube's long axis; (2) tilted molecules on the tube's long axis; and (3) lying molecules shifted away from the tube's long axis. As opposed to C₇₀ and C₈₀ molecules encapsulated in a SWCNT, standing orientations do not develop. Our results are relevant for the possible application of molecular-orientation-dependent electronic properties of fullerene nanopeapods, and also for the interpretation of future experiments on double-walled carbon nanotube formation by annealing fullerene peapod systems.     

A Mössbauer study of iron and iron–cobalt nanotubes in polymer ion-track membranes

Iron and iron–cobalt nanostructures that were synthesized in polymer ion-track membranes have been studied via Mössbauer spectroscopy combined with raster electron microscopy, energy-dispersion analysis, and X-ray diffraction data. The obtained nanostructures are single-phase bcc Fe_1–xCo_x nanotubes with a high degree of polycrystallinity, whose length is 12 μm; their diameter is 110 ± 3 nm and the wall thickness is 21 ± 2 nm. Fe²⁺ and Fe³⁺ cations were detected in the nanotubes, which belong to iron salts that were used and formed in the electrochemical deposition. The Fe nanotubes exhibit eventual magnetic moment direction distributions of Fe atoms, whereas Fe/Co nanotubes have a partial magnetic structure along the nanotube axis with a mean value of the angle between the magnetic moment and nanotube axis of 34° ± 2°. Substituting the Fe atom with Co in the nearest environment of the Fe atom within the Fe/Co structure of nanotubes leads to a noticeable increase in the hyperfine magnetic field at the ⁵⁷Fe nuclei (by 8.7 ± 0.4 kOe) and to a slight decrease in the shift of the Mössbauer line (by 0.005 ± 0.004 mm/s).     

Conductance of a single-wall carbon nanotube in a one-parameter tight-binding model

The dependence of current-voltage characteristics of single-wall nanotubes on their radius and chirality is studied theoretically. It is shown that the conductance of a single-wall nanotube at low voltages can assume discrete values equal to zero for a dielectric tube and 4(e²/h) for a conducting tube (e is the electron charge, h is the Planck constant). The current-voltage characteristic of a nanotube exhibits kinks related to the discreteness of the electron spectrum. The behavior of the conductance of the nanotube at zero temperature is analyzed in a quantizing longitudinal magnetic field that changes the type of tube conduction. In a magnetic field, the conductance of a dielectric tube at low voltages can assume a value of 2(e²/h) in the region where the tube becomes conducting. In a weak magnetic field, a conducting tube becomes dielectric with an energy gap depending on the magnitude of the magnetic field. The conductance of a carbon nanotube is calculated as a function of the temperature and longitudinal magnetic field.     

A new class of MO<Subscript>2</Subscript> dioxide nanotubes (M=Si,Ge, Sn,Pb) composed of “square” lattices of atoms: Their structure and energy characteristics

New dioxide nanotubes are described. These nanotubes are rolled up of a “square” lattice of atoms differing from the conventional hexagonal lattice isoelectronic to graphite. The dependence of the strain energy on the nanotube diameter D departs from a 1/D² behavior, and the optimum shape at the same diameter corresponds to “zigzag” tubelenes. Two-layer nanotubes consisting of an MO₂ layer bonded to a carbon nanotube (CNT) are characterized by a considerably lower strain energy, which points to the possibility of using CNTs as a template for the synthesis of such MO₂ nanotubes.     

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.     

Magnetic properties of Fe nanoparticles trapped at the tips of the aligned carbon nanotubes

The magnetic properties of iron nanoparticles partially encapsulated at the tips of aligned carbon nanotubes have been studied. The carbon nanotube wall not only protects the metallic particles from oxidization, but also reduces the inter-particle dipolar interaction by non-magnetic separation. Magnetic characterizations performed in the temperature range of 5–350 K with magnetic field up to 3 T show that these carbon-nanotube-supported iron particles are good candidates for high-density magnetic recording media.     

Structural,electronic and magnetic properties of Fe(1−x)Cox alloy nanowires encapsulated inside (10,0) boron nitride nanotube

The structural, electronic and magnetic properties of Fe–Co alloy nanowires encapsulated inside zigzag (10,0) boron nitride nanotube (BNNT) are investigated by ab initio calculations. Similar to pristine nanotube, the opposite directional relaxations for the N atoms (move outwards) and B atoms (move inwards) from their initial positions are observed for outside BNNT although with the Fe–Co alloy nanowires inside, but the outward relaxations of the N atoms bonding to the Fe or Co atoms are smaller due to their attractions. The combining processes of Fe–Co/BNNT composites are endothermic when Co concentration x≤0.6 and exothermic x>0.6, and the most stable Fe–Co/BNNT composite is at Co concentration x=0.8. So the semiconducting (10,0) BNNT can be used to shield the Co-rich Fe–Co nanowires. The charges are transferred from Fe–Co nanowires to BNNT and the formed Co–N bonds have covalent bond as well as slight ionic bond characters. Although (10,0) BNNT is nonmagnetic and a decrease in the magnetic moment is found after Fe–Co nanowires are encapsulated inside (10,0) BNNT, the Fe–Co/BNNT composites still have large magnetic moment, reflecting they can be utilized in magnetic storage and ultra high-density magnetic recording devices.     

Multiwalled carbon nanotubes effect on the bioavailability of artemisinin and its cytotoxity to cancerous cells

Artemisinin regarded as one of the most promising anticancer drugs can bind to DNA with a binding constant of 1.04 × 10⁴ M⁻¹. The electrochemical experiments indicated that for longer incubation time periods, the reduction peak current of artemisinin on carbon nanotube modified electrode increases. Therefore, the uptake of drug molecules from a solution into CNTs will be achieved automatically by adsorption of 88.7% of artemisinin onto carbon nanotubes surface without alteration in drug properties. Hence, capability of carbon nanotubes to have synergistic effect on the bioavailability of artemisinin was investigated. Experimental tests on K562 cancer cell lines growth by MTT assay proved that multi-walled carbon nanotubes can enhance the cytotoxity of artemisinin to the targeted cancer cells with unprecedented accuracy and efficiency. The IC₅₀ values were 65 and 35 μM for artemisinin and artemisinin loaded on multi-walled carbon nanotubes, respectively; demonstrating that artemisinin loaded on multi-walled carbon nanotubes is more effective in inhibition of cancer cell lines growth.     

Comparison of influence of incorporated 3d-, 4d- and 4f-metal chlorides on electronic properties of single-walled carbon nanotubes

In the present work, the channels of single-walled carbon nanotubes were filled with melts of ZnCl₂, CdCl₂, and TbCl₃ by a capillary method with subsequent slow cooling. The detailed study of electronic structure of filled nanotubes was performed using Raman, optical absorption, and X-ray photoelectron spectroscopy. The obtained data are in mutual agreement and it proves that the filling of carbon nanotube channels with all these salts leads to the charge transfer from nanotube walls to the incorporated compounds, thus acceptor doping of nanotubes takes place. It was found out that encapsulated terbium chloride has the largest influence on the electronic properties of carbon nanotubes.     

Orientation ordering of N<Subscript>2</Subscript> molecules in vertically aligned CN<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> nanotubes

Arrays of vertically aligned nitrogen-doped carbon (CN _x ) nanotubes have been synthesized by decomposition of aerosol mixture of acetonitrile and ferrocene at 850°C. Nitrogen concentration in the outer shells of the CN _x nanotubes was found from X-ray photoelectron spectroscopy (XPS) data to reach ∼6%. The XPS N 1s spectra and N 1s near-edge X-ray absorption fine structure (NEXAFS) spectra identified three chemical forms of nitrogen in the CN _x nanotube arrays: pyridine-like, graphitic, and molecular nitrogen. The π ^* resonance of molecular nitrogen showed clear polarization dependence that indicates predominant orientation of N₂ molecules along the nanotubes axis. The estimated range of the polar angle distribution of the N₂ molecules orientation in the CN _x nanotube array amounts to 15°.     

13C NMR investigation of carbon nanotubes and derivatives

We report on nuclear magnetic resonance on single wall carbon nanotubes. Depending on the chemical preparation the electronic and dynamical properties of carbon nanotubes are presented and discussed. From a room temperature study of the spin lattice relaxation of carbon nanotubes prepared with various catalysts we clearly identified two components. In agreement with previous NMR studies and theoretical predictions, one-third of the intensity of the signal is found with a short relaxation time (about 5 s) attributed to metallic nanobutes while the rest of the signal presents a relaxation time of about 90 s corresponding to semiconducting nanotubes. In the case of oxidized or cut nanotubes only one relaxation time is observed with characteristics similar to the slow component. The disappearance of the fast relaxing component is associated with the absence of metallic nanotubes damaged by the chemical or mechanical treatments. In this case, the T dependence of the spin lattice relaxation reveals the effect of thermally activated small amplitude motions (twistons) of the nanotube in ropes. If diffusion of twistons might induce movement of ¹³C sites and local magnetic field fluctuations, orientational order could appear below the transition temperature of 170 K. In the last part, we present the theoretical predictions of chemical shift tensor in carbon nanotubes.     

First-principles study on structural and electronic properties of copper nanowire encapsulated into GaN nanotube

We present a systemic study of the structural and electronic properties of Cu_n nanowires (n=5, 9 and 13) encapsulated in armchair (8,8) gallium nitride nanotubes (GaNNTs) using the first-principles calculations. We find that the formation processes of these systems are all exothermic. The initial shapes are preserved without any visible changes for the Cu₅@(8,8) and Cu₉@(8,8) combined systems, but a quadratic-like cross-section shape is formed for the outer nanotube of the Cu₁₃@(8,8) combined system due to the stronger attraction between nanowire and nanotube. The electrons of Ga and N atoms in outer GaN sheath affect the electron conductance of the encapsulated metallic nanowire in the Cu₁₃@(8,8) combined system. But in the Cu₅@(8,8) and Cu₉@(8,8) combined systems, the conduction electrons are distributed only on the copper atoms, so charge transport will occur only in the inner copper nanowire, which is effectively insulated by the outer GaN nanotube. Considering the maximal metal filling ratio in nanotube, we know that the Cu₉@(8,8) combined system is top-priority in the ultra-large-scale integration (ULSI) circuits and micro-electromechanical systems (MEMS) devices that demand steady transport of electrons.     

Effect of thin aluminum interlayer on growth and microstructure of carbon nanotubes

The aluminum (Al) interlayer with various thicknesses ranging from 0.75 to 6 nm was deposited on silicon (Si) substrates prior to the deposition of ultra-thin iron (Fe) catalyst for the growth of carbon nanotubes. In this paper we report the effect of ultra-thin Al interlayer on the growth of multiwalled carbon nanotubes (MWCNTs). The SEM was used to examine the microstructures of nanotubes. We observed as the Al interlayer thickness increases the height of nanotube decreases. Raman spectra of MWCNT showed typical D and G peaks at ∼1345 cm⁻¹ and ∼1575 cm⁻¹, respectively. The XPS revealed the presence of Al and Fe on the top of CNT surface which were further supported by TEM. The high resolution TEM results also revealed bamboo like CNTs with diameter ∼10–40 nm.     

Structure and properties of BeO nanotubes

The structure of a new non-carbon (beryllium oxide BeO) nanotube consisting of a rolled-up graphene sheet is proposed, and its physical properties are described. Ab initio calculations of the binding energy, the electronic band structure, the density of states, the dependence of the strain energy of the nanotube on the nanotube diameter D, and the Young’s modulus Y for BeO nanotubes of different diameters are performed in the framework of the density functional theory (DFT). From a comparison of the binding energies calculated for BeO nanotubes and crystalline BeO with a wurtzite structure, it is inferred that BeO nanotubes can be synthesized by a plasma-chemical reaction or through chemical vapor deposition. It is established that BeO nanotubes are polar dielectrics with a band gap of ～5.0 eV and a stiffness comparable to that of the carbon nanotubes (the Young’s modulus of the BeO nanotubes Y _BeO is approximately equal to 0.7Y _C, where Y _C is the Young’s modulus of the carbon nanotubes). It is shown that, for a nanotube diameter D > 1 nm, the (n, n) armchair nanotubes are energetically more favorable than the (n, 0) zigzag nanotubes.     

DFT study on the chemical sensitivity of C3N nanotubes toward acetone

Potential application of single-walled C₃N nanotubes was investigated as chemical sensors for acetone molecules based on the density functional theory calculations. It was found that the pristine nanotube weakly adsorbs an acetone molecule with the adsorption energy of − 9.7 kcal/mol, and its electronic properties are not sensitive to this molecule. By replacing a C atom with a Si atom, the nanotube becomes a p-type semiconductor. The adsorption energy of the acetone molecule on the Si-doped nanotube becomes much more negative (E_ad=−67.4 kcal/mol). The adsorption process leads to a sizable increase in the resistance of the Si-doped tube, thereby, it can show the presence of acetone molecule, creating an electronic signal. Also, the sensitivity of these devices can be controlled by the doping level of Si atoms. By increasing the number of dopant atoms from 1 to 4, the sensitivity is gradually increased.     

Raman and SERS studies of carbon nanotube systems

In this paper, we report on Raman studies carried out on different carbon nanotube systems, namely single-walled and multi-walled carbon nanotubes and polymer/nanotube composites. We focus on different types of interactions which can take place in these materials. In single-walled nanotubes, the introduction of van der Waals interactions between tubes when arranged in bundles leads to an upshift of the radial breathing mode (RBM) ranging from 11 to 16 cm⁻¹ depending on the size of the bundle. In multi-walled carbon nanotubes, similar interactions between concentric tubes permit to interpret the low frequency Raman modes. In composites, PMMA/nanotubes, an upshift of the RBM is also observed, explained by the dynamical strain applied by the polymer on the bundles, in response to the breathing vibration. In addition, surface enhanced Raman scattering experiments have demonstrated the occurrence of interfacial reactions between the nanotubes and the metallic support. This is put in evidence by the degradation of tubes, especially metallic ones, and reconstruction of C₆₀-like molecules are in some cases observed.     

Encapsulation of lamivudine into single walled carbon nanotubes: A vdW-DF study

To explore a suitable carrier for lamivudine drug, the incorporation of lamivudine inside the single walled carbon nanotubes (SWCNTs) has been investigated by using first-principles van der Waals density functional (vdW-DF) calculations. The obtained binding energies reveal that lamivudine prefers to be encapsulated into the metallic nanotubes with diameter of about 13 Å. Semiconducting SWCNTs exhibit slightly weaker interaction strength with the lamivudine in comparison with the metallic counterparts. However, the calculated binding energies for both considered nanotubes are typical for the physisorption. The influence of nanotube length on the lamivudine incorporation inside the various considered nanotubes has also been investigated and the results show that it plays an important role in the encapsulation process. The electronic structures analysis for the energetically most favorable complexes reveal that incorporated lamivudine changes slightly the electronic properties of SWCNTs. This indicates that there is no considerable hybridization between the corresponding orbitals and the weak interaction obtained quantitatively in terms of binding energies.     

Phonon spectrum and interaction between nanotubes in single-walled carbon nanotube bundles at high pressures and temperatures

The Raman spectra of single-walled carbon nanotubes at temperatures up to 730 K and pressures up to 7 GPa have been measured. The behavior of phonon modes and the interaction between nanotubes in bundles have been studied. It has been found that the temperature shift of the vibrational G mode is completely reversible, whereas the temperature shift of radial breathing modes is partially irreversible and the softening of the modes and narrowing of phonon bands are observed. The temperature shift and softening of radial breathing modes are also observed when samples are irradiated by laser radiation with a power density of 6.5 kW/mm². The dependence of the relative frequency Ω/Ω₀ for G ⁺ and G ^? phonon modes on the relative change A ₀/A in the triangular lattice constant of bundles of nanotubes calculated using the thermal expansion coefficient and compressibility coefficient of nanotube bundles shows that the temperature shift of the G mode is determined by the softening of the C-C bond in nanotubes. An increase in the equilibrium distances between nanotubes at the breaking of random covalent C-C bonds between nanotubes in bundles of nanotubes is in my opinion the main reason for the softening of the radial breathing modes.