Structural characteristics of GaSB / GaAs nanowire heterostructures grown by metal-organic chemical vapor deposition

Highly lattice mismatched (7.8%) GaAs/GaSb nanowire heterostructures were grown by metal-organic chemical vapor deposition and their detailed structural characteristics were determined by electron microscopy. The facts that (i) no defects have been found in GaSb and its interfaces with GaAs and (ii) the lattice mismatch between GaSb/GaAs was fully relaxed suggest that the growth of GaSb nanowires is purely governed by the thermodynamics. The authors believe that the low growth rate of GaSb nanowires leads to the equilibrium growth. (c) 2006 American Institute of Physics.

Identification of industrial sources of airborne heavy metal pollution in urban areas

The research examines the deposition of airborne particles which contain heavy metals and investigates the methods that can be used to identify their sources. The research focuses on lead and cadmium because these two metals are of growing public and scientific concern on environmental health grounds. The research consists of three distinct parts. The first is the development and evaluation of a new deposition measurement instrument - the deposit cannister - designed specifically for large-scale surveys in urban areas. The deposit cannister is specifically designed to be cheap, robust, and versatile and therefore to permit comprehensive high-density urban surveys. The siting policy reduces contamination from locally resuspended surface-dust. The second part of the research has involved detailed surveys of heavy metal deposition in Walsall, West Midlands, using the new high-density measurement method. The main survey, conducted over a six-week period in November - December 1982, provided 30-day samples of deposition at 250 different sites. The results have been used to examine the magnitude and spatial variability of deposition rates in the case-study area, and to evaluate the performance of the measurement method. The third part of the research has been to conduct a 'source-identification' exercise. The methods used have been Receptor Models - Factor Analysis and Cluster Analysis - and a predictive source-based deposition model. The results indicate that there are six main source processes contributing to deposition of metals in the Walsall area: coal combustion, vehicle emissions, ironfounding, copper refining and two general industrial/urban processes. |A source-based deposition model has been calibrated using facctorscores for one source factor as the dependent variable, rather than metal deposition rates, thus avoiding problems traditionally encountered in calibrating models in complex multi-source areas. Empirical evidence supports the hypothesised associatlon of this factor with emissions of metals from the ironfoundry industry.

Ionized cluster beam deposition, and thin film analysis

In 1972 the ionized cluster beam (ICB) deposition technique was introduced as a new method for thin film deposition. At that time the use of clusters was postulated to be able to enhance film nucleation and adatom surface mobility, resulting in high quality films. Although a few researchers reported singly ionized clusters containing 10$\sp2$-10$\sp3$ atoms, others were unable to repeat their work. The consensus now is that film effects in the early investigations were due to self-ion bombardment rather than clusters. Subsequently in recent work (early 1992) synthesis of large clusters of zinc without the use of a carrier gas was demonstrated by Gspann and repeated in our laboratory. Clusters resulted from very significant changes in two source parameters. Crucible pressure was increased from the earlier 2 Torr to several thousand Torr and a converging-diverging nozzle 18 mm long and 0.4 mm in diameter at the throat was used in place of the 1 mm x 1 mm nozzle used in the early work. While this is practical for zinc and other high vapor pressure materials it remains impractical for many materials of industrial interest such as gold, silver, and aluminum. The work presented here describes results using gold and silver at pressures of around 1 and 50 Torr in order to study the effect of the pressure and nozzle shape. Significant numbers of large clusters were not detected. Deposited films were studied by atomic force microscopy (AFM) for roughness analysis, and X-ray diffraction.^ Nanometer size islands of zinc deposited on flat silicon substrates by ICB were also studied by atomic force microscopy and the number of atoms/cm$\sp2$ was calculated and compared to data from Rutherford backscattering spectrometry (RBS). To improve the agreement between data from AFM and RBS, convolution and deconvolution algorithms were implemented to study and simulate the interaction between tip and sample in atomic force microscopy. The deconvolution algorithm takes into account the physical volume occupied by the tip resulting in an image that is a more accurate representation of the surface.^ One method increasingly used to study the deposited films both during the growth process and following, is ellipsometry. Ellipsometry is a surface analytical technique used to determine the optical properties and thickness of thin films. In situ measurements can be made through the windows of a deposition chamber. A method for determining the optical properties of a film, that is sensitive only to the growing film and accommodates underlying interfacial layers, multiple unknown underlayers, and other unknown substrates was developed. This method is carried out by making an initial ellipsometry measurement well past the real interface and by defining a virtual interface in the vicinity of this measurement. ^

Loading effect in copper(II) oxide cluster-surface-modified titanium(IV) oxide on visible- and UV-light activities

Cu(acac)2 is chemisorbed on TiO2 particles [P-25 (anatase/rutile = 4/1 w/w), Degussa] via coordination by surface Ti–OH groups without elimination of the acac ligand. Post-heating of the Cu(acac)2-adsorbed TiO2 at 773 K yields molecular scale copper(II) oxide clusters on the surface (CuO/TiO2). The copper loading amount (Γ/Cu ions nm–2) is controlled in a wide range by the Cu(acac)2 concentration and the chemisorption–calcination cycle number. Valence band (VB) X-ray photoelectron and photoluminescence spectroscopy indicated that the VB maximum of TiO2 rises up with increasing Γ, while vacant midgap levels are generated. The surface modification gives rise to visible-light activity and concomitant significant increase in UV-light activity for the degradation of 2-naphthol and p-cresol. Prolonging irradiation time leads to the decomposition to CO2, which increases in proportion to irradiation time. The photocatalytic activity strongly depends on the loading, Γ, with an optimum value of Γ for the photocatalytic activity. Electrochemical measurements suggest that the surface CuO clusters promote the reduction of adsorbed O2. First principles density functional theory simulations clearly show that, at Γ < 1, unoccupied Cu 3d levels are generated in the midgap region, and at Γ > 1, the VB maximum rises and the unoccupied Cu 3d levels move to the conduction band minimum of TiO2. These results suggest that visible-light excitation of CuO/TiO2 causes the bulk-to-surface interfacial electron transfer at low coverage and the surface-to-bulk interfacial electron transfer at high coverage. We conclude that the surface CuO clusters enhance the separation of photogenerated charge carriers by the interfacial electron transfer and the subsequent reduction of adsorbed O2 to achieve the compatibility of high levels of visible and UV-light activities.

Adsorption Calorimetry for Energy Conversion Technologies: Applications in Organic Photovoltaics, Catalysis, and Atomic Layer Deposition

Thesis (Ph.D.)--University of Washington, 2016-07

Synthesis, characterization and chemical vapor deposition of transition metal di(tert-butyl)amido compounds

Although the transition metal chemistry of many dialkylamido ligands has been well studied, the chemistry of the bulky di(tert-butyl)amido ligand has been largely overlooked. The di(tert-butyl)amido ligand is well suited for synthesizing transition metal compounds with low coordination numbers; such compounds may exhibit interesting structural, physical, and chemical properties. Di(tert-butyl)amido complexes of transition metals are expected to exhibit high volatilities and low decomposition temperatures, thus making them well suited for the chemical vapor deposition of metals and metal nitrides. Treatment of MnBr₂(THF)₂, FeI₂, CoBr₂(DME), or NiBr₂(DME) with two equivalents of LiN(t-Bu)2 in benzene affords the two-coordinate complex M[N(t-Bu)₂]₂, where M is Mn, Fe, Co, or Ni. Crystallographic studies show that the M-N distances decrease across the series: 1.9365 (Mn), 1.8790 (Fe), 1.845 (Co), 1.798 Å (Ni). The N-M- N angles are very close to linear for Mn and Fe (179.30 and 179.45°, respectively), but bent for Co and Ni (159.2 and 160.90°, respectively). As expected, the d⁵ Mn complex has a magnetic moment of 5.53 μΒ that is very close to the spin only value. The EPR spectrum is nearly axial with a low E/D ratio of 0.014. The d⁶ Fe compound has a room temperature magnetic moment of 5.55 μΒ indicative of a large orbital angular momentum contribution. It does not exhibit a Jahn-Teller distortion despite the expected doubly degenerate ground state. Applied field Mössbauer spectroscopy shows that the effective internal hyperfine field is unusually large, Hint = 105 T. The magnetic moments of Co[N(t-Bu)₂]₂ and Ni[N(t-Bu)₂]₂ are 5.24 and 3.02 μΒ respectively. Both are EPR silent at 4.2 K. Treatment of TiCl₄ with three equivalents of LiN(t-Bu)2 in pentane affords the briding imido compound Ti₂[μ-N(t-Bu)]₂Cl₂[N(t-Bu)₂]₂ via a dealkylation reaction. Rotation around the bis(tert-butyl)amido groups is hindered, with activation parameters of ΔH‡ = 12.8 ± 0.6 kcal mol-1 and ΔS‡ = -8 ± 2 cal K-1 ·mol-1, as evidenced by variable temperature 1H NMR spectroscopy. Treatment of TiCl₄ with two equivalents of HN(t-Bu)₂ affords Ti₂Cl₆[N(t-Bu)₂]₂. This complex shows a close-contact of 2.634(3) Å between Ti and the carbon atom of one of the CH₃ substituents on the tert-butyl groups. Theoretical considerations and detailed structural comparisons suggest this interaction is not agostic in nature, but rather is a consequence of interligand repulsions. Treatment of NiI₂(PPh3)₂ and PdCl₂(PPh₃)₂ with LiN(t-Bu)₂in benzene affords Ni[N(t-Bu)₂](PPh₃)I and Pd₃(μ₂-NBut₂)2(μ₂-PPh₂)Ph(PPh₃) respectively. The compound Ni[N(t-Bu)₂](PPh₃)I has distorted T-shape in geometry, whereas Pd₃(μ₂-NBut₂)₂(μ₂-PPh₂)Ph(PPh₃) contains a triangular palladium core. Manganese nitride films were grown from Mn[N(t-Bu)₂]₂ in the presence of anhydrous ammonia. The growth rate was several nanometers per minute even at the remarkably low temperature of 80⁰C. As grown, the films are carbon- and oxygen-free, and have a columnar morphology. The spacings between the columns become smaller and the films become smoother as the growth temperature is increased. The composition of the films is consistent with a stoichiometry of Mn₅N₂.

CO-ELECTROPHORETIC DEPOSITION OF LIQUID METAL AND SILICON FOR LITHIUM-ION BATTERY APPLICATION

A low cost electrophoretic deposition (EPD) process was successfully used for liquid metal thin film deposition with a high depositing rate of 0.6 µ/min. Furthermore, silicon nano-powder and liquid metal were then simultaneously deposited as the negative electrode of lithium-ion battery by a technology called co-EPD. The liquid metal was hoping to act as the matrix for silicon particles during lithium ion insertion and distraction. Half-cell testing was performed using as prepared co-EPD sample. An initial discharge capacity of 1500 mAh/g was reported for nano-silicon and galinstan electrode, although the capacity fading issue of these samples was also observed.

Study of metal oxide-semiconductor capacitors with rf magnetron sputtering TiO(x) and TiO(x)N(y) gate dielectric layer

Metal oxide-semiconductor capacitors with TiO(x) deposited with different O(2) partial pressures (30%, 35%, and 40%) and annealed at 550, 750, and 1000 degrees C were fabricated and characterized. Fourier transform infrared, x-ray near edge spectroscopy, and elipsometry measurements were performed to characterize the TiO(x) films. TiO(x)N(y) films were also obtained by adding nitrogen to the gaseous mixture and physical results were presented. Capacitance-voltage (1 MHz) and current-voltage measurements were utilized to obtain the effective dielectric constant, effective oxide thickness, leakage current density, and interface quality. The results show that the obtained TiO(x) films present a dielectric constant varying from 40 to 170 and a leakage current density (for V(G)=-1 V, for some structures as low as 1 nA/cm(2), acceptable for complementary metal oxide semiconductor circuits fabrication), indicating that this material is a viable, in terms of leakage current density, highk substitute for current ultrathin dielectric layers. (C) 2009 American Vacuum Society. [DOI: 10.1116/1.3043537]

Abundance variations in the globular cluster M71 (NGC 6838)

Context. Abundance variations in moderately metal-rich globular clusters can give clues about the formation and chemical enrichment of globular clusters. Aims. CN, CH, Na, Mg and Al indices in spectra of 89 stars of the template metal-rich globular cluster M71 are measured and implications on internal mixing are discussed. Methods. Stars from the turn-off up to the Red Giant Branch (0.87 < log g < 4.65) observed with the GMOS multi-object spectrograph at the Gemini-North telescope are analyzed. Radial velocities, colours, effective temperatures, gravities and spectral indices are determined for the sample. Results. Previous findings related to the CN bimodality and CN-CH anticorrelation in stars of M71 are confirmed. We also find a CN-Na correlation, and Al-Na, as well as an Mg(2)-Al anticorrelation. Conclusions. A combination of convective mixing and a primordial pollution by AGB or massive stars in the early stages of globular cluster formation is required to explain the observations.

The metal content of bulge field stars from FLAMES-GIRAFFE spectra - I. Stellar parameters and iron abundances

Aims. We determine the iron distribution function (IDF) for bulge field stars, in three different fields along the Galactic minor axis and at latitudes b = -4 degrees, b = -6 degrees, and b = -12 degrees. A fourth field including NGC 6553 is also included in the discussion. Methods. About 800 bulge field K giants were observed with the GIRAFFE spectrograph of FLAMES@VLT at spectral resolution R similar to 20 000. Several of them were observed again with UVES at R similar to 45 000 to insure the accuracy of the measurements. The LTE abundance analysis yielded stellar parameters and iron abundances that allowed us to construct an IDF for the bulge that, for the first time, is based on high-resolution spectroscopy for each individual star. Results. The IDF derived here is centered on solar metallicity, and extends from [Fe/H] similar to -1.5 to [Fe/H] similar to + 0.5. The distribution is asymmetric, with a sharper cutoff on the high-metallicity side, and it is narrower than previously measured. A variation in the mean metallicity along the bulge minor axis is clearly between b = -4 degrees and b = -6 degrees ([Fe/H] decreasing similar to by 0.6 dex per kpc). The field at b = -12 degrees. is consistent with the presence of a gradient, but its quantification is complicated by the higher disk/bulge fraction in this field. Conclusions. Our findings support a scenario in which both infall and outflow were important during the bulge formation, and then suggest the presence of a radial gradient, which poses some challenges to the scenario in which the bulge would result solely from the vertical heating of the bar.

First stars XIII. Two extremely metal-poor RR Lyrae stars

Context. The chemical composition of extremely metal-poor stars (EMP stars; [Fe/H] < similar to -3) is a unique tracer of early nucleosynthesis in the Galaxy. As such stars are rare, we wish to find classes of luminous stars which can be studied at high spectral resolution. Aims. We aim to determine the detailed chemical composition of the two EMP stars CS 30317-056 and CS 22881-039, originally thought to be red horizontal-branch (RHB) stars, and compare it to earlier results for EMP stars as well as to nucleosynthesis yields from various supernova (SN) models. In the analysis, we discovered that our targets are in fact the two most metal-poor RR Lyrae stars known. Methods. Our detailed abundance analysis, taking into account the variability of the stars, is based on VLT/UVES spectra (R similar or equal to 43 000) and 1D LTE OSMARCS model atmospheres and synthetic spectra. For comparison with SN models we also estimate NLTE corrections for a number of elements. Results. We derive LTE abundances for the 16 elements O, Na, Mg, Al, Si, S, Ca, Sc, Ti, Cr, Mn, Fe, Co, Ni, Sr and Ba, in good agreement with earlier values for EMP dwarf, giant and RHB stars. Li and C are not detected in either star. NLTE abundance corrections are newly calculated for O and Mg and taken from the literature for other elements. The resulting abundance pattern is best matched by model yields for supernova explosions with high energy and/or significant asphericity effects. Conclusions. Our results indicate that, except for Li and C, the surface composition of EMP RR Lyr stars is not significantly affected by mass loss, mixing or diffusion processes; hence, EMP RR Lyr stars should also be useful tracers of the chemical evolution of the early Galactic halo. The observed abundance ratios indicate that these stars were born from an ISM polluted by energetic, massive (25-40 M(circle dot)) and/or aspherical supernovae, but the NLTE corrections for Sc and certain other elements do play a role in the choice of model.

Discovery of VVV CL001 A low-mass globular cluster next to UKS 1 in the direction of the Galactic bulge

Context. It is not known how many globular clusters may remain undetected towards the Galactic bulge. Aims. One of the aims of the VISTA Variables in the Via Lactea (VVV) Survey is to accurately measure the physical parameters of the known globular clusters in the inner regions of the Milky Way and search for new ones, hidden in regions of large extinction. Methods. From deep near-infrared images, we derive deep JHK(S)-band photometry of a region surrounding the known globular cluster UKS 1 and reveal a new low-mass globular cluster candidate that we name VVV CL001. Results. We use the horizontal-branch red clump to measure E(B-V) similar to 2.2 mag, (m - M)(0) = 16.01 mag, and D = 15.9 kpc for the globular cluster UKS 1. On the basis of near-infrared colour-magnitude diagrams, we also find that VVV CL001 has E(B-V) similar to 2.0, and that it is at least as metal-poor as UKS 1, although its distance remains uncertain. Conclusions. Our finding confirms the previous projection that the central region of the Milky Way harbours more globular clusters. VVV CL001 and UKS 1 are good candidates for a physical cluster binary, but follow-up observations are needed to decide if they are located at the same distance and have similar radial velocities.

Chemical evolution of the Galactic bulge as traced by microlensed dwarf and subgiant stars III. Detection of lithium in the metal-poor bulge dwarf MOA-2010-BLG-285S

Context. To study the evolution of Li in the Galaxy it is necessary to observe dwarf or subgiant stars. These are the only long-lived stars whose present-day atmospheric chemical composition reflects their natal Li abundances according to standard models of stellar evolution. Although Li has been extensively studied in the Galactic disk and halo, to date there has only been one uncertain detection of Li in an unevolved bulge star. Aims. Our aim with this study is to provide the first clear detection of Li in the Galactic bulge, based on an analysis of a dwarf star that has largely retained its initial Li abundance. Methods. We performed a detailed elemental abundance analysis of the bulge dwarf star MOA-2010-BLG-285S using a high-resolution and high signal-to-noise spectrum obtained with the UVES spectrograph at the VLT when the object was optically magnified during a gravitational microlensing event (visual magnification A similar to 550 during observation). The Li abundance was determined through synthetic line profile fitting of the (7)Li resonance doublet line at 670.8 nm. The results have been corrected for departures from LTE. Results. MOA-2010-BLG-285S is, at [Fe/H] = -1.23, the most metal-poor dwarf star detected so far in the Galactic bulge. Its old age (12.5 Gyr) and enhanced [alpha/Fe] ratios agree well with stars in the thick disk at similar metallicities. This star represents the first unambiguous detection of Li in a metal-poor dwarf star in the Galactic bulge. We find an NLTE corrected Li abundance of log epsilon(Li) = 2.16, which is consistent with values derived for Galactic disk and halo dwarf stars at similar metallicities and temperatures. Conclusions. Our results show that there are no signs of Li enrichment or production in the Galactic bulge during its earliest phases. Observations of Li in other galaxies (omega Cen) and other components of the Galaxy suggest further that the Spite plateau is universal.