Effect of reaction mechanism on precursor exposure time in atomic layer deposition of silicon oxide and silicon nitride

Atomic layer deposition (ALD) of highly conformal, silicon-based dielectric thin films has become necessary because of the continuing decrease in feature size in microelectronic devices. The ALD of oxides and nitrides is usually thought to be mechanistically similar, but plasma-enhanced ALD of silicon nitride is found to be problematic, while that of silicon oxide is straightforward. To find why, the ALD of silicon nitride and silicon oxide dielectric films was studied by applying ab initio methods to theoretical models for proposed surface reaction mechanisms. The thermodynamic energies for the elimination of functional groups from different silicon precursors reacting with simple model molecules were calculated using density functional theory (DFT), explaining the lower reactivity of precursors toward the deposition of silicon nitride relative to silicon oxide seen in experiments, but not explaining the trends between precursors. Using more realistic cluster models of amine and hydroxyl covered surfaces, the structures and energies were calculated of reaction pathways for chemisorption of different silicon precursors via functional group elimination, with more success. DFT calculations identified the initial physisorption step as crucial toward deposition and this step was thus used to predict the ALD reactivity of a range of amino-silane precursors, yielding good agreement with experiment. The retention of hydrogen within silicon nitride films but not in silicon oxide observed in FTIR spectra was accounted for by the theoretical calculations and helped verify the application of the model.

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

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

Ceramic Materials Development for Intermediate Temperature Solid Oxide Fuel Cell (IT-SOFC)

Solid oxide fuel cell (SOFC) is an electrochemical device that converts chemical energy into electric power with high efficiency. Traditional SOFC has its disadvantages, such as redox cycling instability and carbon deposition while using hydrocarbon fuels. It is because traditional SOFC uses Ni-cermet as anode. In order to solve these problems, ceramic anode is a good candidate to replace Ni. However, the conductivity of most ceramic anode materials are much lower than Ni metal, and it introduces high ohmic resistance. How to increase the conductivity is a hot topic in this research field. Based on our proposed mechanism, several types of ceramic materials have been developed. Vanadium doped perovskite, Sr1-x/2VxTi1-xO3 (SVT) and Sr0.2Na0.8Nb1-xVxO3 (SNNV), achieved the conductivity as high as 300 S*cm-1 in hydrogen, without any high temperature reduction. GDC electrolyte supported cell was fabricated with Sr0.2Na0.8Nb0.9V0.1O3 and the performance was measured in hydrogen and methane respectively. Due to vanadium’s intrinsic problems, the anode supported cell is not easy. Fe doped double perovskite Sr2CoMoO6 (SFCM) was also developed. By carefully doping Fe, the conductivity was improved over one magnitude, without any vigorous reducing conditions. SFCM anode supported cell was successfully fabricated with GDC as the electrolyte. By impregnating Ni-GDC nano particles into the anode, the cell can be operated at lower temperatures while having higher performance than the traditional Ni-cermet cells. Meanwhile, this SFCM anode supported SOFC has long term stability in the reformate containing methane. During the anode development, cathode improvement caused by a thin Co-GDC layer was observed. By adding this Co-GDC layer between the electrolyte and the cathode, the interfacial resistance decreases due to fast oxygen ion transport. This mechanism was confirmed via isotope exchange. This Co-GDC layer works with multiple kinds of cathodes and the modified cell’s performance is 3 times as the traditional Ni-GDC cell. With this new method, lowering the SOFC operation temperature is feasible.

Ceramic membranes for separation of proteins and DNA by in situ growth of alumina nanofibres with a nanomesh of metal oxide nanofibres

Ceramic membranes were fabricated by in situ synthesis of alumina nanofibres in the pores of an alumina support as a separation layer, and exhibited a high permeation selectivity for bovine serum albumin relative to bovine hemoglobin (over 60 times) and can effectively retain DNA molecules at high fluxes.

High-flux ceramic membranes with a nanomesh of metal oxide nanofibers

Traditional ceramic separation membranes, which are fabricated by applying colloidal suspensions of metal hydroxides to porous supports, tend to suffer from pinholes and cracks that seriously affect their quality. Other intrinsic problems for these membranes include dramatic losses of flux when the pore sizes are reduced to enhance selectivity and dead-end pores that make no contribution to filtration. In this work, we propose a new strategy for addressing these problems by constructing a hierarchically structured separation layer on a porous substrate using large titanate nanofibers and smaller boehmite nanofibers. The nanofibers are able to divide large voids into smaller ones without forming dead-end pores and with the minimum reduction of the total void volume. The separation layer of nanofibers has a porosity of over 70% of its volume, whereas the separation layer in conventional ceramic membranes has a porosity below 36% and inevitably includes dead-end pores that make no contribution to the flux. This radical change in membrane texture greatly enhances membrane performance. The resulting membranes were able to filter out 95.3% of 60-nm particles from a 0.01 wt % latex while maintaining a relatively high flux of between 800 and 1000 L/m2·h, under a low driving pressure (20 kPa). Such flow rates are orders of magnitude greater than those of conventional membranes with equal selectivity. Moreover, the flux was stable at approximately 800 L/m2·h with a selectivity of more than 95%, even after six repeated runs of filtration and calcination. Use of different supports, either porous glass or porous alumina, had no substantial effect on the performance of the membranes; thus, it is possible to construct the membranes from a variety of supports without compromising functionality. The Darcy equation satisfactorily describes the correlation between the filtration flux and the structural parameters of the new membranes. The assembly of nanofiber meshes to combine high flux with excellent selectivity is an exciting new direction in membrane fabrication.

Synthesis and characterisation of cobalt hydroxide, cobalt oxyhydroxide and cobalt oxide nanodiscs

Cobalt hydroxide, cobalt oxyhydroxide and cobalt oxide nanomaterials were synthesized through simple soft chemistry. The cobalt hydroxide displays hexagonal morphology with clear edges 20 nm long. This morphology and nanosize is retained through to cobalt oxide Co3O4 through a topotactical relationship. Cobalt oxyhydroxide and cobalt oxide nanomaterials were synthesized through oxidation and low temperature calcination from the as-prepared cobalt hydroxide. Characterisation of these cobalt-based nanomaterials were fully developed, including X-ray diffraction, transmission electron microscopy combined with selected area electron diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, and thermal gravimetric analysis. Bonding of the divalent cobalt hydroxide from the oxyhydroxide and oxides by studying their high resolution XPS spectra for Co 2p3/2 and O 1s. Raman spectroscopy of the as-prepared Co(OH)2, CoO(OH) and Co3O4 nanomaterials characterised each material. The thermal stability of the materials Co(OH)2 and CoO(OH) were established. This research has developed methodology for the synthesis of cobalt oxide and cobalt oxyhydroxide nanodiscs at low temperatures.

Solid-state nanocrystalline dye-sensitized solar cell using 2-mercapto benzimidazole doped poly(ethylene oxide) as a new polymer electrolyte

New composite doped poly (ethylene oxide) polymer electrolyte was developed using 2-mercapto benzimidazole as plasticizer and iodide/triiodide as redox couple. The fabrication of the cell involves Poly(ethylene oxide)/ 2-mercapto benzimidazole / iodide/triiodide as polymer electrolyte in dye-sensitized solar cell fabricated with N3 dye and TiO2 nanoparticles as the photoanode and Platinum coated FTO (fluorine doped SnO2) as counter electrode. The current-volatage characteristics under simulated sunlight AM1.5 shows a short circuit current Isc of 8.7mA and open circuit photovoltage 508 mV. The conductivity measurements for the new polymer electrolyte and the photoelectrochemical measurments were carried out systematically. In 2-mercapto benzimidazole the electron rich sulphur and nitrogen atoms, act as pi-electron donors that form good interaction with iodine which plays a vital role in the performance of the fabricated dye-sensitized solar cells. The resonance effect increases the stability of the cell to a considerable extent. These results suggest that the new composite polymer electrolyte performs as a promising new doped polymer-electrolyte.

Synthesis and thermal stability of hydrotalcites containing manganese

The hydrotalcite based upon manganese known as charmarite Mn4Al2(OH)12CO3•3H2O has been synthesised with different Mn/Al ratios from 4:1 to 2:1. Impurities of manganese oxide, rhodochrosite and bayerite at low concentrations were also produced during the synthesis. The thermal stability of charmarite was investigated using thermogravimetry. The manganese hydrotalcite decomposed in stages with mass loss steps at 211, 305 and 793°C. The product of the thermal decomposition was amorphous material mixed with manganese oxide. A comparison is made with the thermal decomposition of the Mg/Al hydrotalcite. It is concluded that the synthetic charmarite is slightly less stable than hydrotalcite.

Photocatalytic hydrogen production with iron oxide under solar irradiation

As solar hydrogen is a sustainable and environmental friendly energy carrier, it is considered to take the place of fossil fuels in the near future. Solar hydrogen can be generated by splitting of water under solar light illumination. In this study, the use of nanostructured hematite thin-film electrodes in photocatalytic water splitting was investigated. Hematite (á-Fe2O3) has a narrow band-gap of 2.2 eV, which is able to utilise approximately 40% of solar radiation. However, poor photoelectrochemical performance is observed for hematite due to low electrical conductivity and a high rate of electron-hole recombination. An extensive review of useful measures taken to overcoming the disadvantages of hematite so as to enhance its performance was presented including thin-film structure, nanostructuring, doping, etc. Since semiconductoring materials which exhibit an inverse opal structure are expected to have a high surface-volume ratio, unique optical characteristics and a shorter distance for photogenerated holes to travel to the electrode/electrolyte interface, inverse opals of hematite thin films deposited on FTO glass substrate were successfully prepared by doctor blading using PMMA as a template. However, due to the poor adhesion of the films, an acidic medium (i.e., 2 M HCl) was employed to significantly enhance the adhesion of the films, which completely destroyed the inverse opal structure. Therefore, undoped, Ti and Zn-doped hematite thin films deposied on FTO glass substrate without an inverse opal structure were prepared by doctor blading and spray pyrolysis and characterised using SEM, EDX, XRD, TGA, UV-Vis spectroscopy and photoelectrochemical measurements. Regarding the doped hematite thin films prepared by doctor blading, the photoelectrochemical activity of the hematite photoelectrodes was improved by incorporation of Ti, most likely owing to the increased electrical conductivity of the films, the stabilisation of oxygen vacancies by Ti4+ ions and the increased electric field of the space charge layer. A highest photoresponse was recorded in case of 2.5 at.% Ti which seemed to be an optimal concentration. The effect of doping content, thickness, and calcination temperature on the performance of the Ti-doped photoelectrodes was investigated. Also, the photoactivity of the 2.5 at.% Ti-doped samples was examined in two different types of electrochemical cells. Zn doping did not enhance the photoactivity of the hematite thin films though Zn seemed to enhance the hole transport due to the slow hole mobility of hematite which could not be overcome by the enhancement. The poor performance was also obtained for the Ti-doped samples prepared by spray pyrolysis, which appeared to be a result of introduction of impurities from the metallic parts of the spray gun in an acidic medium. Further characterisation of the thin-film electrodes is required to explain the mechanism by which enhanced performance was obtained for Ti-doped electrodes (doctor blading) and poor photoactivity for Zn and Ti-doped samples which were synthesised by doctor blading and spray pyrolysis, respectively. Ti-doped hematite thin films will be synthesised in another way, such as dip coating so as to maintain an inverse opal structure as well as well adhesion. Also, a comparative study of the films will be carried out.

Synthesis and characterisation of metal oxyhydroxide and oxide nanomaterials

In this work, a range of nanomaterials have been synthesised based on metal oxyhydroxides MO(OH), where M=Al, Co, Cr, etc. Through a self-assembly hydrothermal route, metal oxyhydroxide nanomaterials with various morphologies were successfully synthesised: one dimensional boehmite (AlO(OH)) nanofibres, zero dimensional indium hydroxide (In(OH)3) nanocubes and chromium oxyhydroxide (CrO(OH)) nanoparticles, as well as two dimensional cobalt hydroxide and oxyhydroxide (Co(OH)2 & CoO(OH)) nanodiscs. In order to control the synthetic nanomaterial morphology and growth, several factors were investigated including cation concentration, temperature, hydrothermal treatment time, and pH. Metal ion doping is a promising technique to modify and control the properties of materials by intentionally introducing impurities or defects into the material. Chromium was successfully applied as a dopant for fabricating doped boehmite nanofibres. The thermal stability of the boehmite nanofibres was enhanced by chromium doping, and the photoluminescence property was introduced to the chromium doped alumina nanofibres. Doping proved to be an efficient method to modify and functionalize nanomaterials. The synthesised nanomaterials were fully characterised by X-ray diffraction (XRD), transmission electron microscopy (TEM) combined with selected area electron diffraction (SAED), scanning electron microscopy (SEM), BET specific surface area analysis, X-ray photoelectron spectroscopy (XPS) and thermo gravimetric analysis (TGA). Hot-stage Raman and infrared emission spectroscopy were applied to study the chemical reactions during dehydration and dehydroxylation. The advantage of these techniques is that the changes in molecular structure can be followed in situ and at the elevated temperatures.

Functionalised zinc oxide nanowire gas sensors : enhanced NO2 gas sensor response by chemical modification of nanowire surfaces

Surface coating with an organic self-assembled monolayer (SAM) can enhance surface reactions or the absorption of specific gases and hence improve the response of a metal oxide (MOx) sensor toward particular target gases in the environment. In this study the effect of an adsorbed organic layer on the dynamic response of zinc oxide nanowire gas sensors was investigated. The effect of ZnO surface functionalisation by two different organic molecules, tris(hydroxymethyl)aminomethane (THMA) and dodecanethiol (DT), was studied. The response towards ammonia, nitrous oxide and nitrogen dioxide was investigated for three sensor configurations, namely pure ZnO nanowires, organic-coated ZnO nanowires and ZnO nanowires covered with a sparse layer of organic-coated ZnO nanoparticles. Exposure of the nanowire sensors to the oxidising gas NO2 produced a significant and reproducible response. ZnO and THMA-coated ZnO nanowire sensors both readily detected NO2 down to a concentration in the very low ppm range. Notably, the THMA-coated nanowires consistently displayed a small, enhanced response to NO2 compared to uncoated ZnO nanowire sensors. At the lower concentration levels tested, ZnO nanowire sensors that were coated with THMA-capped ZnO nanoparticles were found to exhibit the greatest enhanced response. ΔR/R was two times greater than that for the as-prepared ZnO nanowire sensors. It is proposed that the ΔR/R enhancement in this case originates from the changes induced in the depletion-layer width of the ZnO nanoparticles that bridge ZnO nanowires resulting from THMA ligand binding to the surface of the particle coating. The heightened response and selectivity to the NO2 target are positive results arising from the coating of these ZnO nanowire sensors with organic-SAM-functionalised ZnO nanoparticles.

Low temperature synthesis of carbon nanotubes on indium tin oxide electrodes for organic solar cells

The electrical performance of indium tin oxide (ITO) coated glass was improved by including a controlled layer of carbon nanotubes directly on top of the ITO film. Multi-wall carbon nanotubes (MWCNTs) were synthesized by chemical vapor deposition, using ultra-thin Fe layers as catalyst. The process parameters (temperature, gas flow and duration) were carefully refined to obtain the appropriate size and density of MWCNTs with a minimum decrease of the light harvesting in the cell. When used as anodes for organic solar cells based on poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM), the MWCNT-enhanced electrodes are found to improve the charge carrier extraction from the photoactive blend, thanks to the additional percolation paths provided by the CNTs. The work function of as-modified ITO surfaces was measured by the Kelvin probe method to be 4.95 eV, resulting in an improved matching to the highest occupied molecular orbital level of the P3HT. This is in turn expected to increase the hole transport and collection at the anode, contributing to the significant increase of current density and open circuit voltage observed in test cells created with such MWCNT-enhanced electrodes.

Microstructural investigation of Bi-Sr-Ca-Cu-oxide thick films on alumina substrates

The microstructure of Bi-Sr-Ca-Cu-oxide (BSCCO) thick films on alumina substrates has been characterized using a combination of X-ray diffractometry, scanning electron microscopy, transmission electron microscopy of sections across the film/substrate interface and energy-dispersive X-ray spectrometry. A reaction layer formed between the BSCCO films and the alumina substrates. This chemical interaction is largely responsible for off-stoichiometry of the films and is more significant after partial melting of the films. A new phase with fee structure, lattice parameter a = 2.45 nm and approximate composition Al3Sr2CaBi2CuOx has been identified as reaction product between BSCCO and Al2O3.