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Abstract: The structural and electronic properties of individual titanium oxide nanotubes have been studied using both empirical and ab initio calculations. Two different types of titanium oxide nanotubes (A-nanotube and B-nanotube) have been constructed and energy-minimized by molecular mechanics calculations. We found that the A-nanotubes are energetically more favorable than the B-nanotubes. The electronic band structure of the titanium oxide nanotubes was also calculated with respect to the tubule diameter and the tubule type using the ab initio method. The band gap of the A-nanotube was reduced by up to 60% as the tubule diameter decreases from 1.2 nm to 0.5 nm. [Copyright &y& Elsevier]

Abstract: The extinction and orientational dependence of electron diffraction from single-walled carbon nanotubes have been observed experimentally and investigated in detail theoretically using both algebraic analysis and numerical simulations. Electron diffraction from only achiral carbon nanotubes of zigzag or armchair structure shows observable orientational dependence and extinction of certain layer lines in experiment due to the interference of two primary Bessel functions of the same order that contribute to the scattering intensities on these layer lines. [Copyright &y& Elsevier]

Abstract: It is shown that the intensities of electron diffraction from a commensurate multiwalled carbon nanotube are sensitive to the angle of rotation of the nanotube about its axis. This dependence is usable to measure the handedness of the commensurate graphene shells of multiwalled carbon nanotubes. The period of intensity oscillation of the scattering intensity for graphene shells of the same handedness is much larger than that for nanotube shells of opposite handedness. By examining the angular dependence of the scattering intensities, the relative handedness, having the same or opposite handedness, of the commensurate shells can, therefore, be determined. [Copyright &y& Elsevier]

Abstract: High-resolution electron microscopy observation and energetic analysis have been performed on ropes formed from single-walled carbon nanotubes. When individual nanotubes are twisted, the nanotube ropes become energetically stable—a configuration that also offers better structural stability. Electron microscopic image simulations of an energetically-stable rope composed of seven single-walled carbon nanotubes have been carried out as well to elaborate the salient features that were observed experimentally. [Copyright &y& Elsevier]

Abstract: A direct method is described to determine the chiral indices (u, v) of a carbon nanotube from a nanobeam electron diffraction pattern of the nanotube with high accuracy. This method is a one-step procedure and allows rapid and precise assignment of the chiral indices of carbon nanotubes. Using a contemporary electron microscope, we have been able to achieve an accuracy that enables us to assign the chiral indices unambiguously up to the range of (30,30), corresponding to a diameter of 4.0nm. Examples of determination of the chiral indices for two single-walled carbon nanotubes are presented as an illustration of the method. [Copyright &y& Elsevier]

Abstract: A quantitative method for the structural determination by electron diffraction of nanotubes of elliptical cross-section is developed as a general case while the cylindrical nanotubes are treated as a special class. We found that the chiral indices of a carbon nanotube can always be measured from the electron diffraction pattern regardless if the nanotube is circular or elliptical. An experimental electron diffraction pattern from a partly-deformed carbon nanotube is also analyzed. Assisted with numerical simulations, it is determined that the observed carbon nanotube has chiral indices (15,7) with 8° tilt relative to the horizontal plane and eccentricity of 0.553. [Copyright &y& Elsevier]

Abstract: A practical procedure has been proposed to determine the handedness of a carbon nanotube from its electron diffraction patterns. When a nanotube is twisted, the diffraction layer line spacings change, although the intensity distribution on the layer lines stays unchanged. The handedness can be assigned by observing the directions in which the principal layer lines shift under known direction of twisting and the twisting angle can be derived directly from the ratio of the diffraction layer line spacings. Simulated electron diffraction patterns of a carbon nanotube twisted both clockwise and counterclockwise are also presented as an example to corroborate the theoretical analysis. [Copyright &y& Elsevier]

Abstract: Electron diffraction patterns of single-walled carbon nanotubes always have 2mm symmetry regardless if the nanotubes themselves have such symmetry. We here show that, for the case of multiwalled carbon nanotubes, the 2mm symmetry can break down when coherent interferences of the electron waves from two different shells take place such as when two shells of the same helicity but their chiral indices (u,v) have opposite evenness/oddity. Both an experimental electron diffraction pattern and analytic analysis are presented to demonstrate the conditions under which the 2mm symmetry breaks down in the electron diffraction patterns of multiwalled carbon nanotubes. [Copyright &y& Elsevier]

Microcomputed tomography is now widely used for in vivo small animal imaging for cancer studies. Achieving high imaging quality of live objects requires the x-ray source to have both high spatial and temporal resolutions. Preliminary studies have shown that carbon nanotube (CNT) based field emission x-ray source has significant intrinsic advantages over the conventional thermionic x-ray tube including better temporal resolution and programmability. Here we report the design and characterization of a CNT based field emission x-ray source that also affords a high spatial resolution. The device uses modified asymmetric Einzel lenses for electron focusing and an elliptical shaped CNT cathode patterned by photolithography. Stable and small isotropic x-ray focal spot sizes were obtained. [ABSTRACT FROM AUTHOR]

We report a method that allows direct, systematic, and accurate determination of the atomic structure of multiwalled carbon nanotubes by analyzing the scattering intensities on the nonequatorial layer lines in the electron diffraction pattern. Complete structure determination of a quadruple-walled carbon nanotube is described as an example, and it was found that the intertubular distance varied from 0.36 nm to 0.5 nm with a mean value of 0.42 nm. [ABSTRACT FROM AUTHOR]