Showing papers on "Transmission electron microscopy published in 2002"

Sub-ångstrom resolution using aberration corrected electron optics

Abstract: Following the invention of electron optics during the 1930s, lens aberrations have limited the achievable spatial resolution to about 50 times the wavelength of the imaging electrons. This situation is similar to that faced by Leeuwenhoek in the seventeenth century, whose work to improve the quality of glass lenses led directly to his discovery of the ubiquitous "animalcules" in canal water, the first hints of the cellular basis of life. The electron optical aberration problem was well understood from the start, but more than 60 years elapsed before a practical correction scheme for electron microscopy was demonstrated, and even then the remaining chromatic aberrations still limited the resolution. We report here the implementation of a computer-controlled aberration correction system in a scanning transmission electron microscope, which is less sensitive to chromatic aberration. Using this approach, we achieve an electron probe smaller than 1 A. This performance, about 20 times the electron wavelength at 120 keV energy, allows dynamic imaging of single atoms, clusters of a few atoms, and single atomic layer 'rafts' of atoms coexisting with Au islands on a carbon substrate. This technique should also allow atomic column imaging of semiconductors, for detection of single dopant atoms, using an electron beam with energy below the damage threshold for silicon.

Cerium oxide nanoparticles: Size-selective formation and structure analysis

Abstract: Nanoparticles of cerium oxide with a narrow size distribution (±15%) are prepared by mixing cerium nitrate solution with an ammonium reagent. High-resolution transmission electron microscopy (TEM) indicates that over 99% of the synthesized particles are single crystals. TEM and photon absorption are used to monitor particle size. The lattice parameter increases up to 0.45% as the particle size decreases to 6 nm, as observed with x-ray diffraction. Raman spectra also suggest the particle-size effect and concomitant lattice expansion. The lattice expansion can be explained by increased concentrations of point defects with decreasing particle size.

Molecular Junctions by Joining Single-Walled Carbon Nanotubes

Abstract: Crossing single-walled carbon nanotubes can be joined by electron beam welding to form molecular junctions Stable junctions of various geometries are created in situ in a transmission electron microscope Electron beam exposure at high temperatures induces structural defects which promote the joining of tubes via cross-linking of dangling bonds The observations are supported by molecular dynamics simulations which show that the creation of vacancies and interstitials induces the formation of junctions involving seven- or eight-membered carbon rings at the surface between the tubes

Synthesis of hcp-Co nanodisks

Abstract: hcp Co disk-shaped nanocrystals were obtained by rapid decomposition of cobalt carbonyl in the presence of linear amines. Other surfactants, in addition to the amines, like phosphine oxides and oleic acid were used to improve size dispersion, shape control, and nanocrystal stability. Co disks are ferromagnetic in character and they spontaneously self-assemble into long ribbons. X-ray and electron diffraction, electron microscopy, and SQUID magnetometry have been employed to characterize this material.

Regularly shaped, single-crystalline ZnO nanorods with Wurtzite structure

Abstract: A convenient route at ambient conditions was employed to prepare narrow-dispersed ZnO nanorods in terms of size and morphology. Transmission electron microscopy and X-ray diffraction were used to characterize the structurally uniform and well-proportioned products. The as-prepared specimen exhibits strong ultraviolet exciton emission at 385 nm and disappearance of visible defect emission.

Tin Oxide Nanowires, Nanoribbons, and Nanotubes

Abstract: Nanowires, sandwiched nanoribbons, and nanotubes of SnO2 are synthesized using elevated temperature synthesis techniques, and their structures are characterized in detail by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). In addition to the normal rutile structured SnO2, it has been possible to form an orthorhombic superlattice-like structure in the present study. The orthorhombic structure can form in a thin nanowire, coexist with the normal rutile structured SnO2 in a sandwiched nanoribbon, or occur in the form of nanotubes. This result is distinct from that for bulk SnO2 where pressures in excess of 150 kbar are required to form the orthorhombic form. The orientation relationship between the orthorhombic SnO2 and the rutile structured SnO2 is determined to be [001]o || [102]t and (100)o || (010)t for the nanowires and sandwiched nanoribbons, and [001]o || [3 ]t and (110)o || (451)t for the nanotubes. Although the growth direction of the rutile structured SnO2 nanowires i...

Sonochemical Deposition of Silver Nanoparticles on Silica Spheres

Abstract: Silver nanoparticles with an average size of ∼5 nm were deposited on the surface of preformed silica submicrospheres with the aid of power ultrasound. Ultrasound irradiation of a slurry of silica submicrospheres, silver nitrate, and ammonia in an aqueous medium for 90 min under an atmosphere of argon to hydrogen (95:5) yielded a silver−silica nanocomposite. By controlling the atmospheric and reaction conditions, we could achieve the deposition of metallic silver on the surface of the silica spheres. The resulting silver-deposited silica submicrosphere samples were characterized with X-ray diffraction, transmission electron microscopy, differential scanning calorimetry, energy-dispersive X-ray analysis, high-resolution transmission electron microscopy, high-resolution scanning electron microscopy, photoacoustic spectroscopy, and Fourier transform infrared, UV−visible, and X-ray photoelectron spectroscopy.

Large scale rapid oxidation synthesis of sno2 nanoribbons

Abstract: One-dimensional nanostructures of SnO2 with a ribbonlike morphology have been prepared in large scale via rapid oxidation of elemental tin at 1080 °C. The products were characterized with scanning electron microscopy, X-ray powder diffraction, transmission electron microscopy, Raman scattering, and photoluminescence spectroscopy. The as-synthesized SnO2 nanoribbons appeared to be single crystals and had preferred [110] and [203] growth directions. The lengths of the nanoribbons were up to several hundreds of micrometers, and the typical width and thickness were in the range of 30−150 nm and 10−30 nm, respectively. The strong photoluminescence of the nanoribbons in the visible region suggested possible applications in nanoscaled optoelectronic devices. A possible growth mechanism for the SnO2 nanoribbons was proposed.

Heterostructures of ZnO–Zn coaxial nanocables and ZnO nanotubes

Abstract: The heterostructures of Zn–ZnO coaxial nanocables and ZnO nanotubes with an average diameter of 30 nm have been synthesized by simple pyrolysis of zinc acetylacetonate. High-resolution transmission electron microscopy analyses reveal that the Zn core and the ZnO sheath of the nanocables have an epitaxial relationship with their longitudinal axis oriented along the 〈001〉 direction. ZnO nanotubes with a wall thickness of 4 nm possess a single-crystal structure and appear to be the extension of the ZnO sheath of the coaxial nanocables. It is suggested that the ZnO nanotubes are formed by partial evaporation of Zn core of the Zn–ZnO coaxial nanocables.

Facile preparation of transition metal nanoparticles stabilized by well-defined (co)polymers synthesized via aqueous reversible addition-fragmentation chain transfer polymerization.

Abstract: Herein we disclose a facile method for the preparation of (co)polymer-stabilized transition metal colloids, via the “grafting-to” approach. The method takes advantage of the fact that (co)polymers synthesized in aqueous media, in a controlled fashion, via reversible addition-fragmentation chain transfer bear thiocarbonylthio end groups. These are readily reduced to yield (co)polymers with thiol end groups. When the reduction is performed, with NaBH4, for example, in aqueous media in the presence of an appropriate transition metal species, (co)polymer-stabilized metal nanoparticles are formed in which the size and size distribution are dependent upon the individual transition metals. Colloid formation is confirmed by transmission electron microscopy and UV−vis spectroscopy.

Quantification of ferrous/ferric ratios in minerals: new evaluation schemes of Fe L23 electron energy-loss near-edge spectra

Abstract: Determination of Fe3+/ΣFe in minerals at submicrometre scale has been a long-standing objective in analytical mineralogy. Detailed analysis of energy-loss near-edge structures (ELNES) of the Fe L 23 core-loss edges recorded in a transmission electron microscope (TEM) provides chemical information about the iron oxidation state. The valence-specific multiplet structures are used as valence fingerprints. Systematic investigations on the Fe L 23 ELNES of mono and mixed-valence Fe-bearing natural minerals and synthetic solid solutions of garnets (almandine-skiagite and andradite-skiagite), pyroxenes (acmite-hedenbergite) and spinels (magnetite-hercynite) are presented where the presence of multiple valence states is distinguished by a splitting of the Fe L 3 edge. We demonstrate the feasibility of quantification of the ferrous/ferric ratio in minerals by analyzing the Fe L 23 ELNES as a function of the ferric iron concentration resulting in three independent methods: (1) The method of the modified integral intensity ratio of the Fe L 23 white lines employs two 2-eV-wide integration windows centring around both the Fe L 3 maximum for Fe3+ and the Fe L 2 maximum for Fe2+. This refined routine, compared to the previously published quantification method of the ferrous/ferric ratio in minerals, leads to an improved universal curve with acceptable absolute errors of about ±0.03 to ±0.04 for Fe3+/ΣFe ratios. (2) The second method uses a simple mathematical description of the valence-dependent splitting of Fe L 3 ELNES by fitting several Gaussian functions and an arctan function. The systematic analysis of the integral portions of the individual Gaussian curves for different mineral groups provides a further Fe3+/ΣFe quantification method with an absolute error of about ±0.02 to ±0.03. (3) The Fe L 3 ELNES can also be modelled with the help of reference spectra, whereby the Fe3+/ΣFe ratio can be determined with an absolute error of ca. ±0.02.

Effects of Nb doping on the TiO2 anatase-to-rutile phase transition

Abstract: We study the influence of Nb doping on the TiO2 anatase-to-rutile phase transition, using combined transmission electron microscopy, Raman spectroscopy, x-ray diffraction and selected area electron diffraction analysis. This approach enabled anatase-to-rutile phase transition hindering to be clearly observed for low Nb-doped TiO2 samples. Moreover, there was clear grain growth inhibition in the samples containing Nb. The use of high resolution transmission electron microscopy with our samples provides an innovative perspective compared with previous research on this issue. Our analysis shows that niobium is segregated from the anatase structure before and during the phase transformation, leading to the formation of NbO nanoclusters on the surface of the TiO2 rutile nanoparticles.

Field emission from open ended aluminum nitride nanotubes

Abstract: This letter reports the field emission measurements from the nanotubes of aluminum nitride which were synthesized by gas phase condensation using the solid-vapor equilibria. A dc arc plasma reactor was used for producing the vapors of aluminum in a reactive nitrogen atmosphere. Nanoparticles and nanotubes of aluminum nitride were first characterized by transmission electron microscope and tube dimensions were found to be varying from 30 to 200 nm in diameter and 500 to 700 nm in length. These tubes were mixed with nanoparticles of size range between 5 and 200 nm in diameter. Tungsten tips coated with these nanoparticles and tubes were used as a field emitter. The field emission patterns display very interesting features consisting of sharp rings which were often found to change their shapes. The patterns are attributed to the open ended nanotubes of aluminum nitride. A few dot patterns corresponding to the nanoparticles were also seen to occur. The Fowler–Nordheim plots were seen to be nonlinear in nature...

Needle-shaped silicon carbide nanowires: Synthesis and field electron emission properties

Abstract: Bunches of needle-shaped silicon carbide (SiC) nanowires were grown from commercially available SiC powders in thermal evaporation process and using iron as catalyst. Their structure and chemical composition were studied by Raman spectroscopy and high-resolution transmission electron microscopy. The powder of these nanowires may be easily dispersed, and was used to form samples of field electron emitters. The needle shape of individual nanowires is well-suited to field electron emission. Stable emission with current density of 30.8 mA/cm2 was observed at fields as low as 9.6 V/μm, and current density of up to 83 mA/cm2 was recorded.

Growth of multiwall carbon nanotubes in an inductively coupled plasma reactor

Abstract: A high density plasma from a methane–hydrogen mixture is generated in an inductively coupled plasma reactor, and multiwalled carbon nanotubes (MWNTs) are grown on silicon substrates with multilayered Al/Fe catalysts. The nanotubes are vertically aligned, and the alignment is better than the orientation commonly seen in thermally grown samples. A detailed parametric study varying inductive power, pressure, temperature, gas composition, catalyst thickness, and power to the substrate is undertaken. Transmission electron microscopy and Raman spectroscopy are used to characterize the nanotubes. Emission spectroscopy and a global model are used to characterize the plasma. The power in the lower electrode holding the substrate influences the morphology and results in a transition from MWNTs to nanofibers as the power is increased.

Thermoremanence and zero-field-cooled/field-cooled magnetization study of Co x ( SiO 2 ) 1 − x granular films

Abstract: A systematic study of $\mathrm{Co}({\mathrm{SiO}}_{2})$ granular films by means of transmission electron microscopy (TEM), dc and ac initial magnetic susceptibility, and thermoremanent magnetization (TRM) is presented. The experimental results are compared with simulations of zero-field-cooled (ZFC) and field-cooled (FC) magnetization and TRM curves obtained using a simple model of noninteracting nanoparticles. The simulated ZFC/FC curves, using the actual parameters obtained from the TEM images, show a different behavior than the experimental magnetic data. The effect of the dipolar interaction among particles introduces a self-averaging effect over a correlation length \ensuremath{\Lambda}, which results in a larger average ``magnetic'' size of the apparent particles together with a narrower size distribution. The analysis of the ZFC/FC curves in the framework of independent ``particle clusters'' of volume ${\ensuremath{\Lambda}}^{3},$ involving about 25 real particles, explains very well the observed difference between the experimental data for the median blocking temperature $〈{T}_{B}〉$ and their distribution width with respect to the ones expected from the structural observations by TEM. The experimental TRM curves also differ from those obtained from the theoretical model, starting to decrease at a lower temperature than expected from the model, also indicating the strong influence of dipole-dipole interactions.

Electron diffraction and high-resolution transmission electron microscopy of the high temperature crystal structures of Gexsb2Te3+x (x=1,2,3) phase change material

Abstract: The crystal structures of GeSb2Te4, Ge2Sb2Te5, and Ge3Sb2Te6 were determined using electron diffraction and high-resolution transmission electron microscopy. The structure determined for the former two crystals deviates from the ones proposed in the literature. These crystal structures were developed jointly upon cooling of liquid Ge2Sb2Te5. A stacking disorder parallel to the basal plane was observed that increases with increasing cooling rates. For the GexSb2Te3+x (x=1,2,3) crystals it is shown that an a,b,c stacking holds with an alternating stacking of x GeTe double layers identically present in binary GeTe and one Te–Sb–Te–Te–Sb– repeat unit also present in binary Sb2Te3. A stacking disorder is a logical consequence of building crystals with these two principal units. On the other hand, it is likely that all stable crystals of the Ge–Sb–Te systems are an ordered sequence of these two units. Some of the implications of these findings of the stable and metastable crystal structures that develop from am...

Enhanced field emission from carbon nanotubes by hydrogen plasma treatment

Abstract: The field emission capability of the carbon nanotubes (CNTs) has been improved by hydrogen plasma treatment, and the enhanced emission mechanism has been studied systematically using Fourier-transform infrared spectroscopy, Raman, and transmission electron microscopy. The hydrogen concentration in the samples increases with increasing plasma treatment duration. A Cδ−–Hδ+ dipole layer may form on CNTs’ surface and a high density of defects results from the plasma treatment, which is likely to make the external surface of CNTs more active to emit electrons after treatment. In addition, the sharp edge of CNTs’ top, after removal of the catalyst particles, may increase the local electronic field more effectively. The present study suggests that hydrogen plasma treatment is a useful method for improving the field electron emission property of CNTs.

Purification of HiPCO carbon nanotubes via organic functionalization.

Abstract: We report a new method for the purification of HiPCO single-wall carbon nanotubes (SWNT), which consists of the following sequence: (a) organic functionalization of the as-produced nanotubes (pristine tubes, p-SWNT), (b) purification of the soluble functionalized nanotubes (f-SWNT), (c) removal of the functional groups and recovery of purified nanotubes (r-SWNT) by thermal treatment at 350 °C, followed by annealing to 900 °C. Each of these steps contributes to the purification, but only their sequential combination leads to high-purity materials. Organic functionalization makes the SWNT more easy to handle, which results in a better manipulation for potential practical uses. The electronic properties of the purified tubes are investigated via Raman and NIR spectroscopies along with transmission electron microscopy.