Magnetization of carbon nanotubes

For narrow-gap carbon nanotubes, the curvature effect due to the misorientation of 2p_z orbitals dominates over electronic structures and thus magnetic properties. It significantly changes magnetization and magnetic susceptibility, and creates special structures in them. There exists a critical field direction in changing magnetism. The critical angle strongly depends on the chiral angle, the nanotube radius, and the temperature. One-dimensional carbon nanotubes are quite different from zero-dimensional carbon tori, such as in terms of special structures in magnetization, the strength of the magnetic response, and the critical angle.     

Visible and near ultraviolet photocurrent generation in carbon nanotubes

Multiwall carbon nanotubes are found to generate photocurrent in the visible and near ultra violet spectral range using a photoelectrochemical technique. Peaks in the photocurrent are observed at excitation energies in the visible region. Their electron energy loss spectra exhibit the π plasmon feature, typical of graphite layers, and a peak at lower energy. Features at energies between 0 and 4 eV have been already observed for single wall carbon nanotubes and ascribed to interband electronic transitions due to the reduced dimensionality of these systems. The present measurements suggest that the usual identification of multiwall carbon nanotubes electronic density of states with that of graphite layers is not sufficient and more theoretical investigations are necessary to shed light on this point.     

Optical absorption by atomically doped carbon nanotubes under strong atom–field coupling

We analyze optical absorption by atomically doped carbon nanotubes with a special focus on the frequency range close to the atomic transition frequency. We derive the optical absorption line-shape function and, having analyzed particular achiral nanotubes of different diameters, predict the effect of absorption line splitting due to strong atom–vacuum–field coupling in small-diameter nanotubes.     

Gas phase kinetic and optical emission spectroscopy studies in plasma-enhanced hot filament catalytic CVD production of carbon nanotubes

Optical emission spectroscopy (OES) is used as the main experimental tool for comparison with simulations of the plasma and gas phase composition during plasma-enhanced hot filament catalytic chemical vapor deposition (PE HF CCVD) growth of carbon nanotubes (CNTs). Calculated concentration of more than 45 species in model of the CVD reactor is acquired by Chemkin™ software. Study of different conditions is performed and a close relationship can be found between the nature and the growth rate of carbon nanostructures and the concentration of the active gas phase species. Moreover it is shown that significant changes in the density and morphology of the CNTs grown in the presence of NH₃ could be mainly explained by the gas phase formation of CN and HCN.     

Magneto-optical spectroscopy of carbon nanotubes

We review our recent optical experiments on single-walled carbon nanotubes in high magnetic fields. The data revealed magnetic-field-induced optical anisotropy as well as broadening, splittings, and shifts of interband absorption and photoluminescence peaks. Quantitative comparison with theoretical predictions based on the Aharonov–Bohm effect is presented.     

Optical and photoconductivity studies of Cu2O nanowires synthesized by solvothermal method

The solvothermal method has been employed to synthesize cuprous oxide (Cu₂O) nanowires using a precursor of cupric acetate monohydrate (CuAc₂) and ethylene glycol (EG) as the solvent. By optimizing the reaction temperature and reaction time, we have prepared Cu₂O nanowires with a diameter of approximately 7 nm and a length of several nanometers. The UV-visible absorption spectrum of the nanowires shows obvious blueshift compared to the bulk Cu₂O, which arises from the quantum confinement effect of the excitonic transition expected for Cu₂O nanowires. Here we also report the role of different excitation energies on the photoluminescence (PL) properties of the Cu₂O nanowires by steady-state and time-resolved PL spectroscopy. The decay times vary from nanoseconds to picoseconds. Decay kinetics indicates that the average lifetime 〈τ〉 of the nanowires increases with increasing excitation energy. The current-voltage (I-V) curves of the nanowires give the photocurrent density 16 times larger than the dark current density.     

Crystalline perfection, optical and dielectric studies on l-histidine nitrate: A nonlinear optical material

Single crystals of l-histidine nitrate (LHN), a recently investigated nonlinear optical material, were grown by conventional solution technique. Crystal structure and vibrational modes of the grown crystals were confirmed by powder X-ray diffractometry and FT-Raman spectrometry, respectively. Crystalline perfection of the grown crystals was evaluated by employing an in-house developed high-resolution X-ray diffractometer (HRXRD) and it was found that the grown crystals were free from structural grain boundaries and the perfection was reasonably good. However, HRXRD could reveal the fact that the crystals contain predominantly the interstitial point defects. The birefringence was measured over a range of wavelength between 5480 and 5630 Å and it was found that its value is nearly constant and 10 times higher than that of KDP. The optical band gap was found to be ∼3.73 eV. The photoluminescence excitation and emission spectra for single crystals were recorded. The SHG efficiencies of LHN samples of different particle sizes were measured by the Kurtz and Perry technique and they removed the ambiguity in the values reported differently in the literature. Dielectric properties were studied as a function of temperature over a wide range of frequency. The optical and dielectric studies along with the crystalline perfection reveal that the LHN crystal could be a good candidate for nonlinear optical devices.     

Terahertz applications of carbon nanotubes

We formulate and justify several proposals utilizing unique electronic properties of carbon nanotubes for a broad range of applications to THz optoelectronics, including THz generation by hot electrons in quasi-metallic nanotubes, frequency multiplication in chiral-nanotube-based superlattices controlled by a transverse electric field, and THz radiation detection and emission by armchair nanotubes in a strong magnetic field.     

The importance of solvent properties for optical limiting of carbon nanotube dispersions

The optical limiting performances of single-walled carbon nanotube dispersions in N-methyl-2-pyrrolidone were enhanced significantly by blending a range of organic solvents or by increasing the temperature of the dispersions up to 100 °C. While both nanotube bundle size and various solvent parameters have an influence on the optical limiting responses, we verified experimentally that the surface tension of the solvent plays a more important role than the viscosity or boiling point; the appropriate solvent properties contribute to the nonlinear scattering dominated optical limiting phenomenon more than the bundle size.     

Preparation and characterization of amphiphilic multi-walled carbon nanotubes

Multiwalled carbon nanotubes (MWCNTs, average diameter 8 nm) functionalized by N-vinyl pyrrolidone (NVP) were synthesized by radical polymerization and characterized by Fourier transform infrared and Raman spectroscopies, thermogravimetric analysis and transmission electron microscopy. These NVP–MWCNTs exhibit remarkable solubility in water, ethanol and dimethyl formamide. The polyvinyl pyrrolidone can be attached onto the surface of the MWCNTs and the degree of functionalization is affected by NVP content. The functionalization causes possible grafting reaction and solid physical coating between MWCNTs and PVP.     

Strong effects of substitute nitrogen-doping on linear optical absorption spectra of zigzag carbon nanotubes

We theoretically study effects of substitute nitrogen-doping on linear optical properties of zigzag carbon nanotubes using tight-binding model and gradient approximation. It is found that, generally, nitrogen-doping reduces the value of absorption peaks and creates a number of new absorption peaks. In addition, near the low energy range, linear optical absorption spectra of nitrogen-doped (m,0) zigzag tubes are remarkably dependent on whether m mod 3 (the remainder of dividing m by 3) is equal to 0, 1 or 2.     

Electric field-induced unzipping of hydrogenated carbon nanotubes into graphene nanoribbons

We have investigated the electric field effect on horseshoe-shape carbon nanotubes (CNTs) resulting from hydrogen adsorption on the single-wall armchair (n,n)CNTs with 6 ≤ n ≤ 16 by using the density functional theory calculations. The horseshoe-shape CNT is completely unzipped into a graphene nanoribbon upon applying a critical electric field, which decreases with increasing CNT diameter, thus enabling one to select a nanoribbon width. A simple model based on the tensile force exerted on the tube walls by the applied electric field was introduced to understand the CNT-diameter dependence of the critical field.     

Synthesis and characterization of carbon nanotubes on carbon microfibers by floating catalyst method

In this paper, carbon nanotubes were synthesized on carbon microfibers by floating catalyst method with the pretreatment of carbon microfibers at the temperature of 1023 K, using C₂H₂ as carbon source and N₂ as carrier gas. The morphology and microstructure of carbon nanotubes were characterized by field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The composition of carbon nanotubes was determined by energy dispersive X-ray spectroscopy (EDX). The results showed that the surface of treated carbon microfibers was thickly covered by carbon nanotubes with diameters of about 50 nm. EDX image indicated that the composition of carbon nanotubes was carbon. In comparison with the sample grown on untreated carbon microfibers surface, it was found that after carbon microfibers were boiled in the solution of sulfur acid and nitric acid (VH₂SO₄:VHNO₃ = 1:3) and immersed in the solution of iron nitrate and xylene, carbon nanotubes with uniform density can be grown on carbon microfibers surface. Based on the results, we concluded that the pretreatment of carbon microfibers had great effect on the growth of carbon nanotubes by floating catalyst method.     

Influence of filler alignment in the mechanical and electrical properties of carbon nanotubes/epoxy nanocomposites

In this work, we report the mechanical and electrical properties of carbon nanotubes/epoxy composites prepared with aligned and randomly oriented nanotubes as filler. The samples are disks of 30 mm in diameter and 3 mm in thickness. To obtain the carbon nanotubes alignment, an external electric field (250 VAC; 50 Hz) was applied through the thickness of the sample during all the cure process. The AC electrical current was measured, during the cure, as a strategy to determine the optimum time in which the alignment reaches the maximum value. DC conductivity measured after the cure shows a percolation threshold in the filler content one order of magnitude smaller for composites with aligned nanotubes than for composites with randomly oriented filler (from 0.06 to 0.5 wt%). In the percolation threshold, the achieved conductivity was 1.4×10⁻⁵ Sm⁻¹. In both cases, aligned and randomly distributed carbon nanotube composites, the wear resistance increases with the addition of the filler while the Rockwell hardness decreases independently of the nanotubes alignment.     

Electronic properties of alkali-metal intercalated single walled carbon nanotubes

We present a comparative study of the electronic properties of sodium and lithium intercalated single walled carbon nanotubes in a bucky paper sample by electron energy loss spectroscopy and photoemission spectroscopy. We have found that at room temperature both sodium and lithium rapidly diffuse into the bulk of the sample while different magnitudes of charge transfer from Na and Li to the nanotube bundles have been observed. The maximum observed displacement of the Fermi level is almost the same for both alkali although Na and Li induce quite different changes in the carbon nanotube electronic structure. We interpret our results as a more covalent character of the Li-carbon nanotube interaction with respect to the ionic character of the Na-carbon nanotube interaction; the localization of the charge density along the Li-C bond is responsible for an intertube interaction within the carbon nanotube ropes.     

Mechanical properties of nickel-coated single-walled carbon nanotubes and their embedded gold matrix composites

The effects of nickel coating on the mechanical behaviors of armchair single-walled carbon nanotubes (SWCNTs) and their embedded gold matrix composites under axial tension are investigated using molecular dynamics (MD) simulation method. The results show that the Young's moduli and tensile strength of SWCNTs obviously decrease after nickel coating. For armchair SWCNTs, the decreased ratio of the Young's moduli of SWCNTs with smaller radius is larger than that of SWCNTs with larger radius. A comparison is made between the response to Young's modulus of a composite with parallel embedded nanotube and the response of a composite with vertically embedded nanotube. The results show that the uncoated SWCNT can enhance the Young's modulus of composite under the condition of parallel embedment, but such improvement disappears under the condition of vertical embedment because the interaction between SWCNT and gold matrix is too weak for effective load transfer. However, the nickel-coated SWCNT can indeed significantly improve the composite behavior.     

Biological functionalization and fluorescent imaging of carbon nanotubes

Biofunctionalization and manipulating of carbon nanotubes (CNTs) is important for biomedical research and application. Cy5 labeled goat anti-rabbit IgG (anti-IgG-Cy5) is chemically bonded to CNTs via a two-step process of diimide-activated amidation. This process can avoid the intermolecular connection of proteins. Fluorescent imaging of CNTs in aqueous solution has been demonstrated using anti-IgG-Cy5 immobilized CNTs (IgG-CNTs) as the model. The biologically functionalized carbon nanotubes (f-CNTs) in solution have been observed successfully using fluorescence microscopy. The fluorescent image of highly oriented f-CNTs is obtained at first time. The influencing factors on fluorescent imaging including oxidation duration, background noise and reactant concentration are discussed.     

Damping of electron Zitterbewegung in carbon nanotubes

Zitterbewegung (ZB, trembling motion) of electrons in semiconductor carbon nanotubes is described taking into account dephasing processes. The density matrix formalism is used for the theory. Differences between decay of ZB oscillations due to electron localization and that due to dephasing are discussed.     

Synthesis and optical properties of Er and Eu doped SrAl2O4 phosphor ceramic

We report, for the first time on luminescence from a Er³⁺ doped SrAl₂O₄ phosphor. Effects of Eu³⁺ doping were also studied. The influence of rare-earth doping in crystal structure and its optical properties were analysed by means of X-ray diffraction (XRD), Raman scattering, optical absorption, excitation and emission (PL) spectroscopy, thermally stimulated luminescence (TSL) and scanning electron microscope (SEM). Luminescence spectra and luminescence decay curves for Er³⁺ transitions in the near infrared region were recorded. The PL maximum for Eu doped SrAl₂O₄ is obtained at 620 nm and corresponds to the orange region of the spectrum. Diffraction patterns reveal a dominant phase, characteristic of the monoclinic SrAl₂O₄ compound and the presence of dopants has no effect on the basic crystal structure of SrAl₂O₄. The shapes of the glow curves are different for each dopant irradiated with either a ⁹⁰Sr-⁹⁰Y beta source, or UV light at 311 nm, and in detail the TL signals differ somewhat between Er and Eu dopants.