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The birth of nanotechnology era has revolutionized materials science, catalysis and field of optoelectronics. Novel and unique phenomena emerge when material dimensions are reduced to ultra-small size regime and enter nanometre (2-100 nm) realm. Such novel materials are expected to replace bulk materials, offering lower cost of manufacturing and enabling progress in many areas such as solar cell, drug delivery, quantum communication and computing, catalysis and sensing applications. With the progress in nanomaterial synthesis and fabrication, the need for the state-of-art characterization techniques became obvious; such techniques help to establish a complete understanding of the nature and interactions of nanosized materials. In this thesis, the first part focuses on the synthesis of gold and ruthenium clusters, namely Au8, Au9, Au101, Ru3, Ru4 and AuRu3, using the well-established synthetic protocols in the literature. Apart from the standard lab-based characterization techniques such as nuclear magnetic resonance (NMR), UV-visible spectroscopy (UV-vis) and Fourier Transform Infra-red (FTIR), a less explored but useful technique far infra-red (far IR) spectroscopy, available at the Australian Synchrotron (AS), was employed to investigate the vibrational modes in these clusters. Peaks in the experimental far IR spectra were assigned unambiguously to specific vibrations by comparing with the ones generated via DFT calculations with the help of collaborators, group of Professor Gregory Metha, University of Adelaide. For the Au9 cluster, three significant gold core vibrations are observed at 157, 177 and 197 cm-1 in the experimental spectrum. In the case of the Ru3 cluster, only a single ruthenium core vibration is identified within the spectrum, at 150 cm-1 with the calculated force constant, k = 0.33 mdyne/Å. The Ru4 cluster exhibits two metal core vibrations at 153 and 170 cm-1 with force constants of 0.35 and 0.53 mdyne/Å, respectively. Substitution with a gold atom yielding a mixed metal AuRu3 cluster shifts the core transitions toward higher wavenumbers at 177 and 299 cm-1 with an increase in force constants to 0.37 and 1.65 mdyne/Å, respectively. This is attributed to the change in chemical composition and geometry of the metal cluster core. A combination of the DFT calculations and high quality synchrotron-based experimental measurements allowed the full assignment of the key transitions in these clusters. Next, these clusters were fabricated into heterogeneous catalysts by depositing on different metal oxide nanopowders. Synchrotron X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) studies were performed at the Australian Synchrotron and the Photon Factory synchrotron in Japan to investigate the electronic structure of Au8, Au9 and Au101 on TiO2 catalysts. The XPS analysis reveals that “as-deposited” Au8 and Au9 retain some un-aggregated clusters while Au101 show bulk-like gold. These findings are in line with TEM observations, where the aggregates (large particles, > 2 nm) of Au8, Au9 and Au101 are hardly seen under HRTEM. UV-visible diffuse reflectance spectroscopy (UV-vis DRS) studies show the absence of localised surface plasmon resonance (LSPR) peaks in these “as-deposited” clusters, suggesting they are below 2 nm in size. Importantly, the XAS spectrum of “as-deposited” Au9 clusters estimates that 60% of pure, un-aggregated Au9 clusters and 40% of bulk gold in the sample. Upon calcination under O2 and combined O2 and H2 (O2-H2), Au8, Au9 and Au101 clusters form larger nanoparticles (> 2 nm) with the appearance of LSPE peak in UV-vis DR spectra. In addition, majority of the phosphine ligands (that stabilise the gold core) dislodge and form phosphine oxide-like species by interacting with oxygen on the TiO2 surface. The third part focused on testing the catalytic performance of the supported Au8, Au9, Au101, Ru3, Ru4 and AuRu3 clusters on different TiO2, SiO2, ZnO and ZrO2 in benzyl alcohol oxidation. Au101-based catalysts display the highest catalytic activity with a turn-over frequency (TOF) up to 0.69 s-1. The high catalytic activity is attributed to the formation of large Au nanoparticles (> 2 nm) that coincides with the partial removal of capping ligands. Au8 and Au9 clusters which contain NO3- counter anions are found to be inactive in benzyl alcohol oxidation. Further work shows that the presence of NO3- species diminishes the catalytic activity. Monometallic ruthenium clusters, Ru3 and Ru4, are found to be inactive yet the bimetallic AuRu3 clusters are active in benzyl alcohol oxidation, suggesting the synergistic effect between ruthenium and gold metal. Investigation of catalytic testing parameters reveals that tuning selectivity of the product is possible through manipulating the reaction temperature. Finally, a joint experiment with Prof. Wojtek Wlodarski’s group at RMIT, Melbourne was undertaken to test the sensing ability of Au9 clusters for hydrogen detection. Au9 clusters were deposited onto radio-frequency (RF) sputtered WO3 films at two different concentrations; 0.01(S1) and 0.1(S2) mg/mL. It was found that the optimal temperatures for sensor S1 and S2 were 300 °C and 350 °C, respectively. The sensor with lower Au9 concentration (S1) displays a faster response and recovery time, and a higher sensitivity toward H2. HRTEM studies reveal that the sensor S1 contain a significant population of sub-5 nm Au nanoparticles which might be responsible for a faster rate of H2 adsorption and dissociation. The key finding in this study suggest that the addition of catalytic layer such as ultra-small Au9 clusters results in improved sensitivity and dynamic performance (response and recovery time) of H2 sensors. In summary, this thesis demonstrated that cluster-based nanomaterials have wide range of applications spanning from catalysis to sensing. Further improvements in material synthesis and use of multiple complimentary characterization techniques allowed better understanding of the nature of the key active species (metal nanoparticles) assisting design of catalysts and sensors with enhanced performance.

abstract: The electronic states of semiconductor interfaces have significant importance for semiconductor device performance, especially due to the continuing miniaturization of device technology. The application of ultra high vacuum (UHV) enables the preparation and characterization of fresh and cleaned interfaces. In a UHV environment, photoemission spectroscopy (PES) provides a non-destructive method to measure the electronic band structure, which is a crucial component of interface properties. In this dissertation, three semiconductor interfaces were studies to understand different effects on electronic states. The interfaces studied were freshly grown or pre-treated under UHV. Then in-situ PES measurements, including x-ray photoemission spectroscopy (XPS) and ultra-violet photoemission spectroscopy (UPS), were conducted to obtain electronic states information. First, the CdTe/InSb (100) heterointerface was employed as a model interface for II-VI and III-V heterojunctions. It was suggested that an interface layer formed, which consisted of In-Te bonding. The non-octal bonding between In and Te atoms has donor-like behavior, which was proposed to result in an electron accumulation layer in InSb. A type-I heterointerface was observed. Second, Cu/ZnO interfaces were studied to understand the interface bonding and the role of polarization on ZnO interfaces. It was shown that on O-face ZnO (0001) and PEALD ZnO, copper contacts had ohmic behavior. However, on Zn-face ZnO (0001), a 0.3 eV Schottky barrier height was observed. The lower than expected barrier heights were attributed to oxygen vacancies introduced by Cu-O bonding during interface formation. In addition, it is suggested that the different barrier heights on two sides of ZnO (0001) are caused by the different behavior for the ZnO (0001) faces. Last, a pulse mode deposition method was applied for P-doped diamond growth on (100) diamond surfaces. Pretreatment effects were studied. It is suggested that an O/H plasma treatment or a short period of H-plasma and CH4/H2 plasma could yield a higher growth rate. PES measurements were conducted on H-terminated intrinsic diamond surface and P-doped/intrinsic diamond (100) interfaces. It was suggested that electronic states near the valence band maximum caused Fermi level pinning effects, independent of the diamond doping. / Dissertation/Thesis / Doctoral Dissertation Materials Science and Engineering 2018

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Doctor of Philosophy / Department of Chemistry / Peter M.A. Sherwood / Thin films can be created on the surface of a metal, protecting it from oxidation and corrosion. Phosphate films have historically been a common choice for these corrosion resistant films. In this dissertation, the oxidation of vanadium metal by water and atmosphere is studied. Also, a series of phosphate films on the surface of vanadium metal were created and are studied as potential corrosion resistant films. Lastly, an independent study identifying the oxidation state of copper in a biological sample is carried out. To characterize these thin films, X-ray Photoelectron Spectroscopy (XPS) is employed. The reaction of vanadium metal with the atmosphere and distilled, de-ionized, water is studied. The core level and valence band results are explored and compared to calculated valence band spectra for some vanadium oxides. The etching of vanadium metal and reaction of the etched metal with a phosphoric acid solution are studied. Synthesized vanadium phosphate compounds serve as model compounds for the analysis of a phosphate coating created on the surface of vanadium metal by the reaction of vanadium metal with phosphoric acid by a newly developed bench top method. The core level and valence band regions for the compounds and coating are discussed along with cluster and band structure calculations for interpretation. The variation in the coating on vanadium metal by biasing the metal at different potentials during reaction is also studied. Coatings are also created on vanadium metal using different forms of phosphorus oxy-acid. An analysis of the various coatings is performed by XPS and accompanied by predictive calculations. In an additional study, the oxidation state of copper in a biological compound is identified. The analysis makes use of satellite features commonly seen in XPS to make the determination. A discussion of the origin of these features and the energy of the shifts is given, along with the results for the other core level XPS regions for the compound.

The decomposition of formic acid proceeds via two principal reaction pathways: dehydration and dehydrogenation. Mechanisms and reaction ratios depend on the nature of the catalysts used. This work provides mechanistic insight into the decomposition of formic acid on Co(0001) and a highly stepped cobalt surface. The catalytic systems were studied in ultra-high vacuum by XPS and temperature programmed desorption. On both surfaces, an overall reaction (1) was observed: 2 HCOOH→H_2 O+CO+H_2+CO_2 (1) The surfaces had differing reaction intermediates, reaction temperatures, and activation energies. On Co(0001), formate, carbon, and hydroxyl are intermediates and the reaction has an activation energy of 44.3 ± 0.6 kJ/mol, pre-exponential factor of 0.7 ± 0.05 mbar/s. On highly stepped cobalt, formate and formyl are intermediates and the reaction has an activation energy of 147.2 ± 2.0 kJ/mol and pre-exponential factor of 1011.3 ± 0.2 mbar/s. Desorption energies of observed species and mechanisms of observed reactions are reported. A detailed description and proof of concept of a PM-IRRAS reactor designed for this thesis is also presented.

In this work a reversible transition between CeO2/Cu(111) and Ce2O3/Cu(111) was studied by metalic ceria evaporation and oxygen exposition. Prepared layers were characterised by XPS, ISS (and its angle resolved modification), LEED and XPD combined with computer modelling using EDAC code. Four reconstructi- on were identified within the transition - ( √ 7 × √ 7)R19.1◦ , ( √ 3 × √ 3)R30◦ , 3 × 3 and 4 × 4 - for which structural models were suggested. Prepared layers of Ce2O3/Cu(111) exhibiting the 4×4 reconstruction were identified as a cubic pha- se of Ce2O3 by the combination of all methods. The photoelectron spectroscopy study of the chemical state of layers revealed that reduction proceedes from the surface and oxidation is realised by oxygen diffusion into the volume.

In this thesis, the growth of 2D materials, in particular graphene and FeO2 on crystalline supports, is studied by a multitude of surface-sensitive techniques. The mechanisms of graphene growth in ultra-high vacuum and high Ar pressure are explored, and a simple device for the manufacturing of high-quality, monocrystalline graphene on SiC is described. The electronic and chemical properties of B and N dopants on graphene are characterized by means of STM/AFM with CO-functionalized tips and supported by DFT calculations. The chemical interaction of a probe molecule (FePc) with doped graphene is also investigated. The long-standing controversy of the so-called "biphase" reconstruction of α - Fe2O3(0001) is resolved by the discovery of a complete FeO2 overlayer in this phase. The structure of this overlayer is investigated by means of STM, LEEM and DFT calculations. A thorough description of the routes to obtain single phases over the whole surface of α - Fe2O3(0001) is provided.

III-nitride semiconductors have garnered significant attention due to their diverse applications in the fields of optics and electronics. As GaN-based visible light-emitting diodes (LEDs) and laser technologies continue to advance, there has been a surge of interest in the development of ultraviolet (UV) devices. In order to explore the UV range, extensive research has been conducted on BN-based materials and their alloys with conventional III-nitrides, driven by the quest for materials exhibiting larger bandgaps and enhanced refractive index contrast. Additionally, the incorporation of boron into III-nitrides through alloying provides a promising avenue for effectively modulating lattice parameters and manipulating the crystalline structure. This offers a novel approach for strain engineering, lattice matching, and structural manipulation, facilitating the optimization of device performance and expanding the capabilities of III-nitride semiconductors in the realm of UV device development. In this work, we optimize and investigate the epitaxial growth of BGaN using metalorganic chemical vapor deposition, and characterize the physical and electronic properties of the grown films using several techniques such as X-ray diffraction, atomic force microscopy, UV-Visible spectroscopy, X-ray Photoelectron Spectroscopy (XPS), electron energy loss spectroscopy (EELS) and more. We have explored different metalorganic chemical vapor deposition techniques —such as continuous growth and pulsed-flow modulation, high temperature and low temperature growths, hydrogen-containing and hydrogen-free growths, trimethylgallium (TMG) and triethylgallium (TEG) sourced growths, Triethylborane (TEB) and borazine (BRZN) sourced growths— to grow BGaN alloys. Samples grown using continuous-flow method, low temperatures, TEG source and hydrogen-free carrier gas show higher boron content and better crystalline quality when having TEB as a boron source. BRZN is used to reduce carbon impurities for the purpose of film doping. With BRZN, TMG was found as a preferred gallium source. Additionally, we have characterized the electronic properties of the grown films in details using XPS, EELS and other related techniques. We have studied the band offset of BGaN with AlN using traditional methods. Furthermore, we have developed a statistical technique to find small offsets at interfaces at the precision of the measuring instrument.

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