Lee Chow

Data reduction methodology for perturbed angular correlation experiments

Data reduction schemes currently used for time differential perturbed angular correlation measurements are evaluated in terms of (i) their relative effectiveness in eliminating irrelevant variables, including single counter efficiencies and spectrum time shifts; and (ii) their effectiveness in putting data in a form that can easily be fitted by theoretical correlation functions. It is pointed out that erroneous conclusions may be derived from improperly reduced data, but that properly analyzed experiments performed at four angles allow a good determination of both the time-dependent and time-independent parts of the correlation function. Correction of data for accidentals, source self-absorption and spectrum livetime differences are discussed.

Optimization of Chemical Bath Deposited Cadmium Sulfide Thin Films

We report the optimization of CdS thin film grown by chemical bath deposition where homogenous reactions are minimized. The optimum parameters have enabled us to maximize the thickness of the deposited film in a single dip and to grow thicker films by periodically replenishing the concentration of reactants while the substrate remains continuously dipped in the reaction bath. Characterization results reveal the deposited CdS films exhibit improved optical and electrical properties.

A study of the effects of ammonium salts on chemical bath deposited zinc sulfide thin films

We have obtained further insights into the CBD growth mechanisms of ZnS thin films using an aqueous medium containing an ammonium salt. At room temperature the optimum concentration of this salt has enabled us to increase the thickness of the film by more than 400%. The optical transmission below the band edge of our best film is also found to be excellent.

Comparative study of CdS thin films deposited by single, continuous, and multiple dip chemical processes

We have used Rutherford backscattering spectrometry (RBS), X-ray diffraction (XRD), Raman, and photoconductivity to characterize CdS thin films grown by single, continuous, and multiple dip chemical processes. XRD has further shown, without ambiguity, that grown CdS films, independent of the process, in an almost homogeneous reaction free basic aqueous bath have a zincblende crystal structure where reflections from (111), (200), (220), and (311) planes are clearly identified. RBS, Raman, and photoconductivity confirm the high stoichiometry and excellent structural properties with low optically active trap state density of single and continuous dip CdS films. However, they collectively suggest that multiple dip CdS films suffer from defects that act as carrier traps and lead to prolong photoconductivity decay in these films.

Synthesis, characterization and DFT studies of zinc-doped copper oxide nanocrystals for gas sensing applications

Due to their unique properties, p-type copper oxide nanostructures have demonstrated promising potential for various applications, especially for the detection of ethanol vapour and other volatile organic compounds (VOCs). In this work a simple and cost-effective synthesis from chemical solutions (SCS) at low temperatures (≤80 °C) and rapid thermal annealing (RTA) process were used to grow zinc-doped copper oxide (ZnxCu1−xOy) nanostructures. The structural, morphological, vibrational, chemical, electronic and sensorial characteristics of ZnxCu1−xOy nanocrystallite layers obtained by using such an efficient approach based on both, the SCS and RTA processes, have been studied. The investigations demonstrated the possibility to tune sensitivity from VOC to H2, as well as an improved response and high selectivity with respect to hydrogen gas for ZnxCu1−xOy nano-crystalline thin films with x = 0.03. Density functional theory calculations showed that the charge transfer together with changes in the Fermi level facilitate H2 gas sensing, which is further enhanced by Zn doping. Hydrogen gas sensing with a high response and selectivity using p-type hybrid semiconductor nanostructures has been reported. An improved stability in humid air was observed by exposure of doped samples to rapid thermal annealing process for the first time. The experimental and calculation results provide an alternative to sensitive and selective detection of ethanol and hydrogen gases, which would be of particular benefit in the area of public security, industrial and environmental applications.

Investigation of chemical bath deposition of ZnO thin films using six different complexing agents

Chemical bath deposition of ZnO thin films using six different complexing agents, namely ammonia, hydrazine, ethanolamine, methylamine, triethanolamine and dimethylamine, is investigated. As-grown films were mainly ZnO2 with a band gap around 4.3?eV. Films annealed at 400??C were identified as ZnO with a band gap around 3.3?eV. X-ray diffraction and micro-Raman spectroscopy revealed that as-grown films consist mainly of cubic zinc peroxide that was transformed into hexagonal ZnO after annealing. Rutherford backscattering spectroscopy (RBS) detected excess oxygen content in ZnO films after annealing. Fourier transform infrared spectroscopy of as-grown films showed a broad absorption band around 3300?cm?1 suggesting that the as-grown films may consist of a mixture of zinc peroxide and zinc hydroxide. X-ray photoelectron spectroscopy multiplex spectra of the O 1s peak were found to be consistent with film stoichiometry revealed by RBS. High-resolution transmission electron micrographs showed small variations of the order of 10?nm in film thickness which corresponds to the average grain size. A carrier density as high as 2.24?1019?cm?3 and a resistivity as low as 6.48 ? 10?1???cm were obtained for films annealed at 500??C in argon ambient.

Self-assembly of densely packed and aligned bilayer ZnO nanorod arrays

We present a method of self-assembly of densely packed and aligned bilayer ZnO nanorod arrays in a hydrothermal synthesis process. The alkali hydrothermal environment first induced the growth of hydrotalcitelike zincowoodwardite plates, which provide a lattice-matched surface for the self-assembly of ZnO nanorod arrays. The high packing density of the ZnO nanorod arrays demonstrates efficient nucleation and growth processes of ZnO on the zincowoodwardite. The interfacial phenomena involved in the growth of ZnO and self-assembly are discussed. The two-dimensional arrays of ZnO nanorods may find future applications in nanoelectronics and nanophotonics.

Investigation of aluminium and indium in situ doping of chemical bath deposited CdS thin films

Aluminum and indium in situ doping of CdS using chemical bath deposition (CBD) is investigated. The effects of Al and In-doping on optical properties as well as on electrical properties, crystal structure, chemistry and morphology of CdS films are studied. Al doping of CdS using CBD is shown to be successful where a resistivity as low as 4.6 × 10−2 Ω cm and a carrier density as high as 1.1 × 1019 cm−3 were achieved. The bandgap of Al-doped films decreases to a minimum of 2.26 eV, then slightly increases and finally saturates at 2.30 eV as the [Al]/[Cd] ratio in solution increases from 0.018 to 0.18. X-ray diffraction studies showed Al3+ ions entering the lattice substitutionally at low concentration and interstitially at high concentration. Phase transition, due to annealing, and induced lattice damage, due to doping, were detected by micro-Raman spectroscopy. Film stoichiometry was found to be sensitive to Al concentration, while film morphology was unaffected by Al doping. Indium doping using CBD, however, was found to be highly unlikely due to the low solubility of indium sulfide. Instead, the formation of InS/In2S3 dominated the deposition process over CdS.

Practical adaptation in bulk superconducting magnetic bearing applications

Lifting capacities greater than 41 N/cm2 (60 psi) at 77 K have been achieved using a combination of permanent magnets and high quality melt‐textured YBa2Cu3O7−δ (YBCO). The key concept of this hybrid superconducting magnetic bearing (HSMB) is the use of strong magnetic repulsion and attraction from permanent magnets to support high loads in conjunction with flux pinning in a type II superconductor to counteract instabilities in a system consisting of magnets only. To illustrate this concept, radial and axial forces between magnet/superconductor, magnet/magnet, and magnet/superconductor/magnet, were measured and compared for the thrust and journal bearing configurations on a bearing prototype.

Electron transport through single carbon nanotubes

We report on the transport of energetic electrons through single, well aligned multi-wall carbon nanotubes (CNT). Embedding of CNTs in a protective carbon fiber coating enables the application of focused ion beam based sample preparation techniques for the non-destructive isolation and alignment of individual tubes. Aligned tubes with lengths of 0.7 to 3 mu m allow transport of 300 keV electrons in a transmission electron microscope through their hollow cores at zero degree incident angles and for a misalignment of up to 1 degree.