Siu-Wai Chan

Cerium oxide nanoparticles: Size-selective formation and structure analysis

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.

Ceria nanoparticles: Size, size distribution, and shape

Nanocrystalline ceria particles have been prepared by mixing aqueous solutions of cerium nitrate and hexamethylenetetramine at room temperature. The smallest size of nanoparticles synthesized is 2 nm. For each batch prepared, a narrow size distribution is found with a standard deviation less than ±15%. A transmission electron microscopy (TEM) investigation shows that these particles are single crystals having either an octahedral shape with eight {111} surfaces, or with an additional {200} surface-truncated octahedral shape. In-situ ultraviolet-visible light absorption has been performed to measure the absorption edge and to monitor the growth of nanoparticles. The results from light absorption correlate well with those of the TEM images, providing an in-situ method to measure the particle size during synthesis.

Application of a near coincidence site lattice theory to the orientations of YBa2Cu3O7-x grains on (001) MgO substrates

Various orientations of YBa2Cu3O7−x grains in polycrystalline films prepared on (001)MgO substrates by in situ laser deposition were determined using electron diffraction. Eight different types of in‐plane orientations have been observed. These orientations agree well with the prediction of a simplified theory of near coincidence site lattice between YBa2Cu3O7−x and MgO. The YBa2Cu3O7−x grains were found to have a high probability of forming low angle or low Σ boundaries among themselves. These grain boundaries are of low energy and should exhibit a high connectivity of Cu‐O‐Cu chains. Therefore, YBa2Cu3O7−x thin films on MgO can attain a Tc of ∼90 K and a Jc of ∼106 A/cm2 at 77 K.

Epitaxy of Y‐Ba‐Cu‐O thin films grown on single‐crystal MgO

The epitaxy of a thin‐film Y‐Ba‐Cu‐O (YBCO) superconductor deposited on a single‐crystal [001] MgO substrate was examined by transmission electron microscopy. The large lattice mismatch (8–10%) in the basal plane of YBCO and MgO is accommodated mainly by the formation of a polycrystalline, mosaic structure. The grain boundaries correspond to unique crystallographic interfaces, determined by the crystal symmetry of the substrate and the thin film.

Study of energy vs misorientation for grain boundaries in gold by crystallite rotation method—I. [001] Twist boundaries

Abstract Small gold crystallites (~ 50–80 nm dia) were welded to thin film [001] single crystal gold substrates at a series of predetermined [001] twist angles in the range 0–45°. A pure [001] twist boundary therefore existed in each welded neck region which could be observed directly by transmission electron microscopy at normal incidence. Upon annealing, the crystallites rotated around [001] when the boundary energy varied with θ. The crystallites rotated into three misorientations corresponding to the special ∑1 and ∑5 misorientations and a symmetry related misorientation at θ = 45°. These results indicate the existence of grain boundary dislocation (GBD) related cusps on the boundary energy vs θ curve at ∑1 and ∑5. The rotations occurred conservatively by the motion of screw GBDs which could be observed directly by the transmission microscopy in certain regimes of θ. The results are relevant to recent calculations of the energies of [001] twist boundaries and the applicability of the GBD/structural unit model for grain boundaries.

Effect of the post-deposition processing ambient on the preparation of superconducting YBa2Cu3O7-x coevaporated thin films using a BaF2 source

We have investigated the effect of the post‐deposition processing ambient on the preparation of YBa2Cu3O7−x thin films from a BaF2 source. The role of H2O vapor during the high‐temperature anneal is understood through a thermodynamic analysis of the fluorine removal reaction. The role of a HF getter (e.g., SiO2) is understood through the same type of analysis. We have demonstrated that a zero resistance transition temperature at 77 K can be obtained for an annealing temperature as low as 690 °C for films deposited on SrTiO3 substrates by increasing the PH2O and decreasing PHF during the high‐temperature soak cycle.

Controlled synthesis of Co3O4 nanopolyhedrons and nanosheets at low temperature

Cobalt oxide (Co(3)O(4)) nanopolyhedrons and nanosheets were controlled synthesized at the low temperature of 80 degrees C via a novel, simple, aqueous method; the obtained nanosheets are only 2-3 nm thick.

Visible thermal emission from sub-band-gap laser excited cerium dioxide particles

Cerium dioxide particles excited in air with sub-band-gap radiation emit very broad radiation in the visible spectrum above a threshold intensity that decreases with increasing ambient temperature. Concomitant with this emission is the near disappearance of the Stokes and anti-Stokes Raman scattering peaks. Both phenomena are reversible in air up to just above threshold, and are seen for nanoparticles and several-micron-diameter particles with particle diameter comparable to or smaller than the laser focus. Temperature estimates using the Stokes/anti-Stokes scattering intensity ratio suggest there is laser heating due to small intragap absorption and possible nonlinear processes, given the very slow thermal conduction. The broad emission in this loose powder may well be due to thermal emission, on the basis of spectral fitting of the high-energy part of the spectrum to a blackbody radiator at ∼1200–1400 °C, although luminescence from a new phase is a possibility. The sudden decrease in Raman scattering and increase in emission in air are consistent with a transition to a new, possibly luminescent, phase, as is the continued disappearance of the Raman peaks in forming gas when the laser power is reduced below the upstroke threshold. Oxygen point defects and their complexes may play an important role in many of these processes.

Scanning tunneling microscopy and theoretical study of water adsorption on Fe3O4: implications for catalysis.

The reduced surface of a natural Hematite single crystal α-Fe(2)O(3)(0001) sample has multiple surface domains with different terminations, Fe(2)O(3)(0001), FeO(111), and Fe(3)O(4)(111). The adsorption of water on this surface was investigated via Scanning Tunneling Microscopy (STM) and first-principle theoretical simulations. Water species are observed only on the Fe-terminated Fe(3)O(4)(111) surface at temperatures up to 235 K. Between 235 and 245 K we observed a change in the surface species from intact water molecules and hydroxyl groups bound to the surface to only hydroxyl groups atop the surface terminating Fe(III) cations. This indicates a low energy barrier for water dissociation on the surface of Fe(3)O(4) that is supported by our theoretical computations. Our first principles simulations confirm the identity of the surface species proposed from the STM images, finding that the most stable state of a water molecule is the dissociated one (OH + H), with OH atop surface terminating Fe(III) sites and H atop under-coordinated oxygen sites. Attempts to simulate reaction of the surface OH with coadsorbed CO fail because the only binding sites for CO are the surface Fe(III) atoms, which are blocked by the much more strongly bound OH. In order to promote this reaction we simulated a surface decorated with gold atoms. The Au adatoms are found to cap the under-coordinated oxygen sites and dosed CO is found to bind to the Au adatom. This newly created binding site for CO not only allows for coexistence of CO and OH on the surface of Fe(3)O(4) but also provides colocation between the two species. These two factors are likely promoters of catalytic activity on Au/Fe(3)O(4)(111) surfaces.

Study of energy vs misorientation for grain boundaries in gold by crystallite rotation method-II. Tilt boundaries and mixed boundaries

Abstract In Part I [ Acta metall . 33 , 1113 (1985)] small gold crystallites were welded to thin film single crystal gold substrates at different controlled misorientations to produce pure twist boundaries in the neck regions which could be observed directly by transmission electron microscopy. In the present work these experiments were extended to include pure tilt and also mixed (tilt/twist) boundaries. Upon annealing, the crystallites untilted and untwisted into several particular misorientations in order to reduce the boundary energy. The untilting and untwisting occurred by the observed climb of edge grain boundary dislocations (GBDs) and glide of screw GBDs respectively. The directions of the rotations were generally consistent with a boundary energy vs tilt/twist “surface” constructed on the basis of the structural unit/GBD model for grain boundaries. Kinetic models for the untilting indicated that the climbing edge GBDs acted as efficient line sources/sinks and that the diffusional transport was controlled by either lattice diffusion or edge GBD core diffusion for low angle pure tilt boundaries and by grain boundary diffusion for high angle mixed boundaries.