Title: Study of cerium oxide thin films for biosensing applications Author: Yuliia Kosto Department: Department of Surface and Plasma Science Supervisor: Prof. RNDr. Vladimír Matolín, DrSc. Abstract: The presented scientific work was conducted in two main directions. The first one is an investigation of the simple biomolecules (glycine and sarcosine) bonding to cerium oxide model films by surface science techniques: photoelectron and near-edge X-ray absorption spectroscopies. Adsorption chemistry and thermal stability of the molecules on the oxides were studied in relation to the oxidation state of ceria cations, film morphology, and molecular deposition method. The oxygen vacancies in the oxide were shown to affect the adsorption geometry of glycine and stimulate molecular decomposition. The polycrystalline oxide morphology provided stabilizing effect on the glycine adlayer. Sarcosine deposited in vacuum formed densely packed adlayer with the molecules directed outwards. Interestingly, the results revealed that molecular film deposited from the aqueous solution, in contrast to deposition in vacuum, induces continuous reduction of the cerium oxide during thermal annealing. The second part is a study of polycrystalline cerium oxide thin films as an electrode for electrochemical and electrochemiluminescent...

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This dissertation tested the hypothesis that energy transferred from a plasma or plume can be used to optimize the structure, chemistry, topography, optical and electrical properties of pulsed laser deposited and sputtered thin-films of ZnO, a-BOxNy, and few layer 2H-WS2 for transparent electronics devices fabricated without substrate heating or with low substrate heating. Thus, the approach would be compatible with low-temperature, flexible/bendable substrates. Proof of this concept was demonstrated by first optimizing the processing-structure-properties correlations then showing switching from accumulation to inversion in ITO/a-BOxNy/ZnO and ITO/a-BOxNy/2H-WS2 transparent MIS capacitors fabricated using the stated processes. The growth processes involved the optimization of the individual materials followed by growing the multilayer stacks to form MIS structures. ZnO was selected because of its wide bandgap that is transparent over the visible range, WS2 was selected because in few-layer form it is transparent, and a-BOxNy was used as the gate insulator because of its reported atomic smoothness and low dangling bond concentration. The measured semiconductor-insulator interfacial trap properties fall in the range reported in the literature for SiO2/Si MOS structures. X-ray photoelectron spectroscopy (XPS), Hall, photoluminescence, UV-Vis absorption, and X-ray diffraction (XRD) measurements investigated the low-temperature synthesis of ZnO. All films are nanocrystalline with the (002) XRD planes becoming more prominent in films grown with lower RF power or higher pressure. Low power or high chamber pressure during RF magnetron sputtering resulted in a slower growth rate and lower energetic conditions at the substrate. Stoichiometry improved with RF power. The measurements show a decrease in carrier concentration from 6.9×1019 cm-3 to 1.4×1019 cm-3 as power increased from 40 W to 120 W, and an increase in carrier concentration from 2.6×1019 cm-3 to 8.6×1019 cm-3 as the deposition pressure increased from 3 to 9 mTorr. The data indicates that in the range of conditions used, bonding, stoichiometry, and film formation are governed by energy transfer from the plasma to the growing film. XPS characterizations, electrical measurements, and atomic force microscopy (AFM) measurements reveal an increase in oxygen concentration, improved dielectric breakdown, and improved surface topography in a-BOxNy films as deposition pressure increased. The maximum breakdown strength obtained was ~8 MVcm-1, which is comparable to a-BN. Metal-Insulator-Metal (MIM) structures of a-BOxNy grown at 10 and 15 mTorr suggest a combination of field-enhanced Schottky emission and Frenkel-Poole emission are likely transport mechanisms in a-BOxNy. In comparison, better fitted data was gotten for field enhanced Schottky emission which suggests the more dominant mechanism. The static dielectric constant range is 3.26 – 3.58 for 10 and 15 mTorr films. Spectroscopic ellipsometry and UV-Vis spectroscopy measured a bandgap of 3.9 eV for 15 mTorr grown a-BOxNy. 2H-WS2 films were grown on both quartz and a-BOxNy which revealed that the XRD (002) planes became more prominent as substrate temperature increased to 400 oC. AFM shows nano-grains at lower growth pressure. Increasing the growth pressure to 1 Torr resulted in the formation of larger particles. XPS chemical analysis reveals improved sulfur to tungsten ratios as pressure increased. Sulfur deficient films were n-type, whereas sulfur rich conditions produced p-type films. Frequency dependent C-V and G-V measurements revealed an interface trap concentration (Nit) of 7.3×1010 cm-2 and interface state density (Nss) of 7.5×1012 eV-1cm-2 for the transparent ITO/a-BOxNy/ZnO MIS structures, and approximately 2 V was required to switch the a-BOxNy/ZnO interface from accumulation to inversion. Using 2H-WS2 as the channel material, the ITO/a-BOxNy/2H-WS2 required approximately 4 V to switch from inversion to accumulation in both n and p-channel MIS structures. Interface trap concentrations (Nit) of 1.6×1012 cm-2 and 3.2×1010 cm-2, and interface state densities (Nss) of 1.6×1012 eV-1cm-2 and 6.5×1012 eV-1cm-2 were calculated for n and p-channel 2H-WS2 MIS structures, respectively. The data from these studies validate the hypothesis and demonstrate the potential of ZnO, a-BOxNy, and few layer 2H-WS2 for transparent electronics.

The main automotive catalytic media, precious metal-doped ceria-zirconia oxides, fundamental system character is unknown. Understanding the catalytic system properties should allow for the production of an optimal model catalyst. This goal of this study is to gain understanding of ceria-zirconia-palladium systems and to determine a reproducible method for preparing catalysts with minimal surface-carbon. This study investigates ceria-zirconia-palladium catalyst preparation via aqueous chemistry methods and a sputter deposition technique for doping ceria-zirconia oxide mixtures. Prepared catalysts are characterized by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). XPS allows the surface species, after catalyst doping and annealing, to be evaluated. Prepared catalyst EM analysis allows for surface morphology and particle characteristic evaluation. Prepared catalysts are exposed to UHV conditions, palladium sputtered-coated, and annealed at various temperatures. Temperature dependency is observed in both percentage of carbon, metal, and oxygen species present. This study led to a reproducible, low-carbon content, doping method for use in future pollutant reaction studies.

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Recent developments of X-ray photoelectron spectroscopy using excitation energies different from the usual lab-sources Mg Kα and Al Kα, thus covering larger and different kinetic energy ranges, require more flexible approaches for determining the transmission function than the well-established ones using reference spectra. Therefore, the approach using quantified peak areas (QPA) was refined allowing a more precise estimation of the transmission function. This refinement was tested by comparing the results obtained with the new version with former calculations. Furthermore, the obtained transmission function was validated by comparing the results with a transmission function using the reference spectrum of polyethylene. Additionally, an ionic liquid was used as reference for estimating the transmission function at the energyresolved HE-SGM beamline at BESSY II. Comparison between the measured and stoichiometric composition shows that a transmission function was determined, which allows a reasonable quantification.

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The adhesion and durability of model epoxy/silane/SiO2/Si bonded systems were investigated under various conditions, including the type of surface preparation, pH of the environmental media, temperature, cyclic thermal stress, and external applied stress. The fundamental debond mechanism was studied for bonded systems exposed to selected environments. The bond failure mode was characterized by examining the failed bond surfaces using X-ray photoelectron spectroscopy. The effectiveness of combining the oxygen plasma treatment and silane coupling agent (SCA) derivatization in adhesion promotion for an epoxy bonded to a silicon surface was evaluated in this research. SCAs with different amine functionalities were studied. The oxygen plasma treatment time was varied systematically to achieve a different extent of oxidation on the Si wafer. The surface chemistry/composition of various silane derivatized Si surfaces was investigated. The studies revealed that SCA interaction with the Si surface was enhanced by the oxygen plasma pre-treatment of the Si substrates. XPS surface analysis results showed that the SCA/SiO2 ratio did not correlate strongly with the increase in oxygen plasma pretreatment time. However, for Si surfaces treated for longer oxygen plasma pretreatment times, more silanol groups may be available to interact with the hydrolyzed silanol groups on silane, resulting in a stronger SCA-Si attachment. Three different tests were employed to determine adhesion and durability of the model epoxy/SCA/SiO2/Si bonded specimens. The immersion test qualitatively evaluates the bond durability for various systems exposed to different chemical and thermal conditions. Second, a novel probe test was used to quantitatively determine adhesion under critical debonding conditions for bonded specimens with different SCA preparations. A general trend of bond durability varied in the manner SCA-2 > SCA-3 > SCA-1 > no silane. Bond durability also increased for samples: model epoxy/SCA modified/O2 plasma treated/Si as the oxygen plasma pre-treatment time increased. Third, bond durability was studied using the wedge DCB (double cantilever beam) test under subcritical debonding conditions with environment-assisted crack growth as a function of applied strain energy release rate. Higher crack velocity and the absence of a Gthreshold value were noted in tests at 70 oC. The Gthreshold value increased as the strength of the interface increased and as the chemical aggressiveness of the environment decreased. For tests involving 25 oC -70 oC thermal cycling, only limited crack growth was found. / Ph. D.

The transition-metal oxides (TMOs) are a material class host to a number of intriguing and potentially technologically useful phenomena as a result of many-body correlation effects, from superconductivity, magnetic and orbital ordering, to ferroelectricity and metal-insulator transitions. Here, materials with similar structures and seemingly equivalent electronic configuration often exhibit wildly different properties as a result of strong competition between different ground states from the many degrees of freedom, whose balance can be further tuned through the use of pressure, doping, magnetic fields or temperature. To investigate these materials, we make use of photoelectron spectroscopy (PES), probing elementary excitations possible in the material and thus providing linked information both about the ground state and possible excited states, closely related to the physical properties of a material such as its response to external fields. Angle-resolved PES (ARPES) provides additional momentum information and as a result, it is uniquely suited to investigate the character of the electronic structure of solids as it resolves the dispersion, meaningful in the independent-electron view where crystal momentum is a well-defined quantum number, but which can retain validity even in strongly correlated systems through the concept of quasiparticles. While ARPES is a well-established technique, it is rarely used in the soft X-ray regime (SX-ARPES) due to significant experimental challenges posed. However, the higher energies in SX-ARPES allow it to be significantly more bulk-sensitive, an extremely important fact since the properties of the bulk material and its surface are often extremely, or worse, subtly different. Critically, this permits measurements on single crystals of TMOs, whose surfaces may show roughness or reconstruction, for example as a result of a polar surface compensation, but whose bulk properties are well-defined in contrast to thin films which are additionally subject to substrate effects. We demonstrate on three rather different perovskite oxides, a three-dimensional class of TMOs, that is worthwhile to overcome these issues since it provides access to the true momentum-resolved bulk electronic properties of materials and allows filling noticeable gaps in literature of k-resolved electronic structure measurements for this class of compounds stemming from the impossibility of such measurements at lower energy. A commonality between the materials studied in this thesis is the absence of a strong electronic symmetry-breaking order, such as local-moment antiferromagnetism or charge ordering, that could suppress the existence of sufficiently long-lived quasiparticles to observe dispersion (or equivalently prevent a mobile photo-hole). We first establish that SX-ARPES is indeed capable and suited to measure the bulk-representative electronic structure by measurements on the perfect cubic d1 perovskite ReO3. We present the first k-resolved electronic structure for this material which is rather well explained by band structure, especially close to the Fermi level. In particular, we show and quantify the impact of the significant spin-orbit coupling on the Fermi surface and bands. However, the oxygen bands are less well reproduced by calculations and are correctable by use of hybrid functionals, taken as a sign of spurious self-interaction effects - likely due to the large extent and density differences between delocalised Re 5d and more localised O 2p. We also show that there are signs of some hitherto unknown distortion in ReO3. We then turn to LaNiO3, a metallic oxide in a family of formally d7 rare-earth nickelates which otherwise all undergo metal-insulator and antiferromagnetic (AFM) transitions as well as oxygen bond disproportionation, with a strong competition between these ground states and possible exotic resulting states in the phase diagram. We are able to resolve the dispersion of the eg quasiparticle spectrum along high symmetry cuts of this material as well as its Fermi surface, the latter of which is accurately reproduced by band theory calculations. We investigate the influence of the rhombohedral distortion present in the material through unfolding methods to better compare their influence to measurement, and show how significantly it affects the dispersion, confirming again the importance of single crystals. Its effects are shown to be similar to correlation-induced mass enhancement and their effects are untangled with the help of first DFT+U and later rhombohedral multi-band dynamical mean-field theory (DMFT) calculations. Local correlation effects prove to be the dominant influence on the spectrum, although certain k-dependent mismatches remain, pointing to a possible simultaneous importance of non-local mechanisms. Finally, on the d6 system LaCoO3 that is close to a spin-state transition, we show that this method can also be applied to insulating oxides. Absent a Fermi surface, we naturally concentrate more on the full valence band, where we show that the observed dispersion is well-described by mean-field band methods in the low-spin (LS) regime of LaCoO3 provided that static energy corrections of DFT+U are accounted for (which show a good match to local LS many-body configuration interaction calculations), thus providing k-resolved evidence that one may effectively consider LS LaCoO3 a band insulator, despite possibly strong correlations. We further unveil clear evidence of crystal periodicity doubling by observation of a backfolded oxygen band, and show evidence of a significant asymmetry in the k-resolved lineshape in the valence band and lastly we take a look at the spin state of Co at the surface, which, contrary to prior results, appears to be the same as in the bulk, but which we show to be complicated by significant orbital-shape matrix element effects.

Un-doped, Tb3+ as well as Yb3+ doped ZnO nanocrystals with different concentrations of RE3+ (Tb3+, Yb3+) ions were successfully synthesized via sol-gel method to produce rare earth activated zinc oxide nanophosphors. The phosphor powders were produced by drying the precursor gels at 200˚C in ambient air. Based on the X-ray diffraction results, it was found that the pure and RE3+ doped ZnO nanophosphors were highly polycrystalline in nature regardless of the incorporation of Tb3+ or Yb3+ ions. Moreover, the diffraction patterns were all indexed to the ZnO Hexagonal wurtzite structure and belong to P63mc symmetry group. The Raman spectroscopy confirmed the wurtzitic structure of the prepared samples. Elemental mapping conducted on the as prepared samples using Scanning electron microscope (SEM) equipped with energy dispersive X-ray spectrometer (EDX) revealed homogeneous distribution of Zn, O, and RE3+ ions. The high resolution transmission electron microscope (HR-TEM) analyses indicated that the un-doped and RE3+ doped samples were composed of hexagonal homogeneously dispersed particles of high crystallinity with an average size ranging from 4 to 7 nm in diameter, which was in agreement with X-ray diffraction (XRD) analyses. ZnO:Tb3+ PL study showed that among different Tb3+ concentrations, 0.5 mol% Tb3+ doped ZnO nanoparticles showed clear emission from the dopant originating from the 4f-4f intra-ionic transitions of Tb3+ while the broad defects emission was dominating in the 0.15 and 1 mol% Tb3+doped ZnO. Optical band-gap was extrapolated from the Ultraviolet Visible spectroscopy (UV-Vis) absorption spectra using TAUC‟s method and the widening of the optical band-gap for the doped samples as compared to the un-doped sample was observed. The PL study of ZnO:Yb3+ samples was studied using a 325 nm He-Cd laser line. It was observed that the ZnO exciton peak was enhanced as Yb3+ions were incorporated in ZnO matrix. Furthermore, UV-VIS absorption spectroscopic study revealed the widening of the band-gap in Tb3+ doped ZnO and a narrowing in the case of Yb3+ doped ZnO system. X-ray photoelectron spectroscopy demonstrated that the dopant was present in the doped samples and the result was found to be consistent with PL data from which an energy transfer was evidenced. Energy transfer mechanism was evidenced between RE3+ and ZnO nanocrystals and was discussed in detail. / Physics / M.Sc. (Physics)