V. Yeh

Uniform island height selection in the low temperature growth of Pb/Si(111)-(7×7)

Self-organized, uniform-height, Pb islands with flat tops and steep edges form on Si(111)-(7 x 7) at low temperatures 120<T<250 K. Islands of heights differing by bilayer height increments are observed depending on growth conditions. The formation of these structures is highly unusual since at low temperatures thermal diffusion is suppressed. The origin of the regular structures is believed to be quantum size effects (i.e. effects related to the quantization of the electron energy levels in the islands). We have studied with two complementary techniques (i.e. high resolution spot profile analysis low energy electron diffraction and variable temperature scanning tunneling microscopy) how the preferred island heights depend on the growth parameters (i.e. temperature, coverage, kinetic pathway etc.). We have constructed a kinetic phase diagram in the coverage-temperature plane which indicates the type of islands formed under different growth conditions. The phase diagram can be used as a guide so the island height can be easily controlled.

Direct observation of short-circuit diffusion during the formation of a single cupric oxide nanowire

Short-circuit diffusion was observed in a single CuO nanowire synthesized using a thermal oxidation method. The confocal Raman spectra of a single CuO nanowire permit direct observation of the nature of an individual CuO nanowire. The parameter order obtained from the inverse Raman Bg2 peak linewidth results in the length dependence of the linewidth and a short-circuit diffusion length of 3.3 µm. The observed structural information is also consistent with the energy dispersive x-ray spectroscopic mapping. The results confirm that the growth of CuO nanowires occurs through the short-circuit diffusion mechanism.

The Higher Energy Components in Ti2p Xps Spectrum of Ga Doped Barium Titanate

The X-ray photoelectron spectroscopy was applied to study a series of room temperature stabilised hexagonal barium titanate (h-BaTiO3), which have been synthesized with solid-state reaction method by adding Ga^(3+) ions into the process of producing BaTiO3. The doping levels vary from 5% to 25% (0.05 ≤ x ≤ 0.25). The photoelectron spectra collected at different spots of the samples showed inhomogeneous occurrences of the atomic replacement. The inhomogeneity increases significantly at and beyond 20% of doping levels (x ≥ 0.2). The expected appearance of Ti^(3+) ions, as a result of replacing Ti^(4+) with Ga^(3+), was observed only when the Ga concentration is 20% or higher. In all of the samples we observed unexpected components at higher binding energies than the expected Ti^(4+)2p3/2 and Ti^(4+)2p1/2 peaks in BaTiO3. These components stayed weak until the doping level reaches 20% and higher, and became stronger together with Ti^(3+) components (at lower binding energies than the corresponding Ti^(4+) peaks). These unexpected components may be attributed to the internal compressive stress due to the coexistence of Ba2TiO4 and BaTiO3 as described in [J. Appl. Phys. 95, 219 (2004)].

Stabilization of the Pb/Si(1 1 1)-(7×7) uniform height islands to higher temperatures with oxygen adsorption

We have studied how oxygen adsorbed on top of the uniform height Pb islands, (grown on Si(1 1 1)-(7×7) at low temperatures Tl<190 K) extends the temperature range of their stability. The evolution of the island height and size with temperature is monitored with SPA-LEED. The presence of oxygen suppresses Pb diffusion to higher levels and results in sharper island height distribution, when compared to the height distribution on the clean Pb/Si(1 1 1)-(7×7) system. Most likely this is because the barrier at the island edges, which controls the transfer of atoms from lower levels to the top of the islands, is increased with the adsorption of oxygen.

Raman scattering and growth disorders in single as-grown TiO2 nanowires

An oxidation procedure has been developed to grow single-crystalline TiO(2) nanowires of the pure rutile phase, allowing subsequent characterizations of SEM, XRD, Raman, and TEM without any post-growth preparations. TEM observations support that the 1D anisotropic growth is dominated by oriented attachment processes, leading to typical growth-induced defects in the nanowires. Spatial variations of the rutile E(g) and A(1g) Raman modes were unambiguously revealed on single nanowires while scanned along the growth direction parallel to the rutile [110]. Symmetry-sensitive deviations were identified by comparing the Raman data with the spatial correlation model calculations based on realistic dispersion relations of the rutile, reflecting morphology-correlated defect distributions along single nanowires. This work provides an efficient, non-destructive in situ characterization approach for guiding growth design in future nanotechnology.