Optical spectroscopy of individual single-walled carbon nanotubes of defined chiral structure.

We simultaneously determined the physical structure and optical transition energies of individual single-walled carbon nanotubes by combining electron diffraction with Rayleigh scattering spectroscopy. These results test fundamental features of the excited electronic states of carbon nanotubes. We directly verified the systematic changes in transition energies of semiconducting nanotubes as a function of their chirality and observed predicted energy splittings of optical transitions in metallic nanotubes.

Inversion of two-band superconductivity at the critical electron doping of (Mg, Al)B2.

Electron energy-loss spectroscopy (EELS) was combined with heat capacity measurements to probe changes of electronic structure and superconductivity in Mg(1-x)Al(x)B(2). A simultaneous decrease of EELS intensity from sigma-band hole states and the magnitude of the sigma gap was observed with increasing x, thus verifying that band filling results in the loss of strong superconductivity. These quantities extrapolated to zero at x approximately 0.

Characterization of palladium nanoparticles by using X-ray reflectivity, EXAFS, and electron microscopy.

We compared the characteristics of dodecanethiolate palladium nanoparticles synthesized by two different techniques, a one-phase method and a two-phase method. From transmission electron microscopy (TEM), we determined that the particle sizes were 46 +/- 10 angstroms and 20 +/- 5 angstroms for the one- and two-phase particles, respectively. Electron diffraction confirmed that their structure was face-centered cubic (fcc).

Quasi-continuous growth of ultralong carbon nanotube arrays.

We report the growth of ultralong (>10 cm) multi-walled and single-walled carbon nanotubes such that the length is limited by the size of the furnace rather than by the termination of growth. The disturbance of microscale laminar flows results in disordered or shorter growth of carbon nanotubes. By downsizing reaction pipes, reaction gas flows are stabilized with low Reynolds numbers.

Assembly and interaction of Au/C core-shell nanostructures: in situ observation in the transmission electron microscope.

Using transmission electron microscopy, we identify the temperature-dependent interaction pathway of carbon-supported Au nanoparticles. At low temperature (room temp. to 400 degrees C), Au nanoparticles predominantly interact by coalescence initiated by an atomic Au bridge.

In-situ formation of ultrathin Ge nanobelts bonded with nanotubes.

A novel nanostructure of ultrathin Ge nanobelts bonded with nanotubes has been fabricated and characterized. Nanotubes (either carbon or BN) are first coated with amorphous germanium and then heated and observed by an in-situ TEM. The thickness, down to 2 nm, and the width of the Ge nanobelts are determined by the thickness of this amorphous Ge coating and the diameter of nanotubes, respectively.

Formation and oxidation state of CeO(2-x) nanotubes.

Well-crystalline CeO(2-x) nanotubes are synthesized via a mild hydrothermal reaction route using cerium nitrate and ammonium hydroxide as reactants. The CeO(2-x) nanotubes have the same structure as the bulk CeO2 but larger lattice parameter. The measurement of the ratio of M5/M4 edge shows the valence reduction of cerium ions for the CeO(2-x) nanotubes.

Copper oxide nanocrystals.

It is well-known that inorganic nanocrystals are a benchmark model for nanotechnology, given that the tunability of optical properties and the stabilization of specific phases are uniquely possible at the nanoscale. Copper (I) oxide (Cu(2)O) is a metal oxide semiconductor with promising applications in solar energy conversion and catalysis. To understand the Cu/Cu(2)O/CuO system at the nanoscale, we have developed a method for preparing highly uniform monodisperse nanocrystals of Cu(2)O.

An organometallic synthesis of TiO2 nanoparticles.

We report the synthesis of TiO2 nanoparticles that uses the low-temperature reaction of low-valent organometallic precursors. Bis(cyclooctatetraene)titanium reacts with dimethyl sulfoxide in organic solution at temperatures as low as room temperature to produce TiO2. In the absence of any supporting ligand, the reaction gives precipitation of amorphous TiO2 powder; however, in the presence of basic ligands such as tributylphosphine, tributylphosphine oxide and trioctylphosphine oxide, the precipitation is arrested, and chemically distinct, isolated, internally crystalline TiO2 nanoparticles are formed.

Direct evidence for negative grain boundary potential in Ca-doped and undoped YBa2Cu3O7-x.

Using electron holography in a transmission electron microscope, we obtained direct evidence for the reduction of negative charge at grain boundary dislocations in Ca-doped YBa2Cu3O7 (YBCO) when compared to undoped YBCO. Because of the finite width of the valence band in the superconducting CuO2 planes, the negative grain boundary charge can lead to a depletion of electron holes available for superconductivity. A significant reduction in the size of the perturbed region in the Ca-doped samples appears to be the principal mechanism for the improved interfacial superconductivity.

Zinc oxide quantum rods.

Nanoscale zinc oxide (ZnO) rods of diameters close to the Bohr-exciton radius ( approximately 2 nm) can be prepared from a simple acetate precursor, resulting in ligand-capped rods of ZnO, highly dispersible in nonpolar solvents. Zinc oxide, ZnO, is a wide band-gap semiconductor with applications in blue/ultraviolet (UV) optoelectronic devices and piezoelectric devices. We observe self-assembly into uniform stacks of nanorods aligned parallel to each other with respect to the long axis, and photoluminescence measurements provide evidence for one-dimensional quantum confinement.

Deterministic phase unwrapping in the presence of noise.

We present a new Fourier-based exact solution for deterministic phase unwrapping from experimental maps of wrapped phase in the presence of noise and phase vortices. This single-step approach has superior performance for images with high phase gradients or insufficient digital sampling approaching 2pi/pixel and therefore performs as a fast and practical solution for the phase-unwrapping problem for experimental applications in applied optics, physics, and medicine.

Accurate measurements of valence electron distribution and interfacial lattice displacement using quantitative electron diffraction.

We developed a novel electron-diffraction technique by focusing a small probe above (or below) the sample in an electron microscope to measure charge density and lattice displacement in technologically important materials. The method features the simultaneous acquisition of shadow images within many Bragg reflections, resulting in parallel recording of dark-field images (PARODI). Because it couples diffraction with images, it is thus suitable for studying crystals as well as their defects.

Fast phase unwrapping algorithm for interferometric applications.

A wide range of interferometric techniques recover phase information that is mathematically wrapped on the interval (-pi, pi). Obtaining the true unwrapped phase is a longstanding problem. We present an algorithm that solves the phase unwrapping problem, using a combination of Fourier techniques.