D. X. Huang

Electrical properties of a highly oriented, textured thin film of the ionic conductor Gd :CeO2-δ on (001) MgO

Highly oriented ionic conductor gadolinium-doped CeO2−δ (Ce0.8Gd0.2O2−δ) thin films have been grown on single-crystal (001) MgO substrates by pulsed-laser ablation. The films are highly c-axis oriented with cube-on-cube epitaxy, as shown by x-ray diffraction and electron microscopy. The interface relationship is, surprisingly, found to be (001)film//(001)sub and [100]film//[100]sub with an extremely large lattice misfit of more than 28%. Ac impedance measurements in the temperature range of 500 to 800 °C reveal that electrical conductivity is predominantly ionic over a very broad oxygen partial pressure range from pO2 from 1×10−19 atm to 1 atm. The activation energy Ea for ionic conductivity measured on unannealed films is 0.86 eV, but after heat treatment, Ea decreases to 0.74 eV.

Strain relaxation by directionally aligned precipitate nanoparticles in the growth of single-crystalline Gd-doped ceria thin films

Transmission electron microscopy has been used to investigate the microstructure and epitaxial behavior of gadolinium-doped ceria (Ce0.8Gd0.2O2−δ) thin films on single crystal (001) LaAlO3. The results show that the films have single-crystal cubic structure and a sharp interface with an interface relationship of (001)film∥(001)sub and [100]film∥[110]sub. Accompanying the high film crystallinity, a directionally aligned, precipitated nanoparticle structure has been observed. The precipitated particles have an average size of ∼4 nm, a Ga-rich composition of Ce0.7Gd0.3O2−δ, a rhombic shape with mainly {111} facets, and are uniformly distributed over the entire film area. The nanoparticles contribute a uniform tensile strain to the film that effectively compensates the compressive film strain induced by the substrate, and also leads to a uniform relaxation of the residual film strain by generating misfit dislocations at the film/particle interfaces. The high film crystallinity is believed to result from this ...

Interface structures and periodic film distortions induced by substrate-surface steps in Gd-doped ceria thin-film growth

Gadolinium-doped ceria (Ce0.8Gd0.2O2−δ) thin films were grown on single-crystal (001) LaAlO3 (LAO) substrates by a pulsed laser ablation. The transmission electron microscope observation reveals a unique type of periodic film distortion along the film∕substrate interface. Each distorted film area is associated with a few substrate-surface steps and the spacing between these distorted areas is about 50μm. The distortion starts at the substrate-surface steps and extends into the film along one of the {111} planes at the step-forward direction. The {111} planar defects induced by the nearby steps can interact with each other to form a planar defect network. The structure of the (001) LAO surface, the structure of the film∕substrate interface, and the mechanism of the formation of these {111} planar defects have been analyzed using a high-resolution electron microscopy. Structural models for these planar defects and their interaction are suggested.

Organized two-dimensional Ti–SiO2 metal quantum dot composites induced by subplantation

Two-dimensional (2D) titanium nanodots were formed in the subsurface layer of single crystal SiO2, i.e., a 2D Ti metal quantum dot composite, by subplantation of 9 keV Ti+ ions. Transmission electron microscopy images show that the Ti nanodots have a uniform size distribution of ∼2 nm with very little deviation, almost constant edge-to-edge spacing (∼1 nm) between neighboring nanodots in the lateral direction, and a very narrow depth distribution. These nanodots exhibit no crystallinity and are smaller than our previously reported single crystalline Ti nanodots with body-centered-cubic β phase, indicating that the size and crystallinity of metal nanodots can be controlled through subplantation. A preliminary mechanism for the formation of 2D nanodots during subplantation is discussed.

Titanium metal quantum-dot composite induced by subplantation

Crystalline titanium nanodots have been formed in the subsurface layer of single-crystal SiO2, i.e., a Ti-based metal quantum dot composite, by subplantation of 9 keV Ti+ ions. Transmission electron microscopy images show that the Ti nanodots have a single, uniform size distribution of ∼3–4 nm, they are single crystals of mainly the Ti bcc β-phase, and their position in the subsurface is controllable through the ion energy. The unique features of subplantation for promoting the precipitation/clustering of crystalline Ti nanodots are discussed. These results confirm previous findings based on the linear optical properties of Ti in SiO2.

Ti–Sn alloy nanodot composites embedded in single-crystal SiO2 by low energy dynamic coimplantation

Organized extremely small Ti–Sn alloy nanodots have been formed in the subsurface of SiO2 by dynamic coimplantation of isotopic Ti+48 and Sn+120 at a low kinetic energy of 9keV into (0001) Z-cut quartz at different substrate temperatures. Transmission electron microscopy images show that the Ti–Sn alloy nanodots are single crystal and have been formed uniformly at room temperature. They are distributed in a two-dimensional array with similar size of ∼4nm and constant interdot spacing between each dot. The regions beyond and below the two-dimensional array are depleted of detectable nanodots. At high temperature, the distribution and crystallinity were destroyed with much smaller amorphous nanodots in a slightly deeper region. The implantation was carried out by dynamic coimplantation, rather than the commonly used sequential implantation. These results indicate that dynamic low energy coimplantation is capable of forming well-ordered two-dimensional array of alloy nanodots.

Epitaxial growth of multilayered YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta///Y/sub 0.7/Ca/sub 0.3/Ba/sub 2/Cu/sub 3/O/sub 7-/spl delta// films for high current application

Multilayered YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO)/Y/sub 0.7/Ca/sub 0.3/Ba/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YCBCO) films were epitaxially grown on [001] LaAlO/sub 3/ and [001] SrTiO/sub 3/ by using pulsed laser ablation. Constant-high critical current density (J/sub c/) of 106 A/cm/sup 2/ at 77 K has been achieved with the combination of multilayered YBCO and YCBCO layers for films with various thickness. The critical current density exhibits a high constant value for the multilayered structures indicating that the J/sub c/ in the superconductor films no longer decreases with the increase of the film thickness. The result verifies the model that interfaces can limit the density of dislocations that form in the film during the film growth. The artificial multilayered structures terminate the dislocations and the superconductive films at the optimal J/sub c/ conditions maintain a constant high J/sub c/ at various film thickness.Multilayered VBa 2 Cu 3 O 7-δ (YBCO)/Y 0.7 Ca 0.3 Ba 2 Cu 3 O 7-δ (YCBCO) films were epitaxially grown on (001) LaAlO 3 and (001) SrTiO 3 by using pulsed laser ablation. Constant-high critical current density (J c ) of 106 A/cm 2 at 77 K has been achieved with the combination of multilayered YBCO and YCBCO layers for films with various thickness. The critical current density exhibits a high constant value for the multilayered structures indicating that the J c in the superconductor films no longer dec with the increase of the film thickness. The result verifies the model that interfaces can limit the density of dislocations that form in the film during the film growth. The artificial multilayered structures terminate the dislocations and the superconductive films at the optimal J c conditions maintain a constant high J c at various film thickness.

Microstructure and Strain Relaxation of Single-crystal Gadolinium-doped Ceria Thin Films on LaAlO3 Substrates

Gadolinium-doped ceria (GDC) is very attractive as an electrolyte for solid oxide fuel cell (SOFC) applications at low operating temperature (~500 oC) due to its high ionic conductivity. The ohmic loss across the electrolyte can be further minimized by using a thin film of the electrolyte. Lower operating temperatures can lead to SOFC devices with reduced cost and higher long-term stability. For practical SOFC applications, therefore, fabricating high-performance GDC in thin film form is highly desirable.