Ion beam applications research. A summary of Lewis Research Center Programs

A summary of the ion beam applications research (IBAR) program organized to enable the development of materials, products, and processes through the nonpropulsive application of ion thruster technology is given. Specific application efforts utilizing ion beam sputter etching, deposition, and texturing are discussed as well as ion source and component technology applications

The study and characterisation of plasma microfluidic devices

Controlling the behaviour of atmospheric pressure plasmas and their interaction with polymeric materials is of major interest for surface modification applications across multidisciplinary fields intersecting biomedical engineering, bio-nanoengineering, clinical/medical science, material science and microelectronics. The aim of the present work is to investigate the behaviour of atmospheric pressure dielectric barrier discharges in closed systems (microfluidic devices) and open systems (glass capillary devices) and their polymer-surface interactions. Atmospheric pressure microplasma jets operating in helium gas have been used to modify locally the surface energy of polystyrene (PS) and to interact directly with the surface of analytes using a novel plasma assisted desorption ionisation (PADI) method causing desorption and ionization to occur. Although atmospheric pressure micro-jets are now widely studied for the treatment of materials there is still a lack of understanding of the fundamental plasma-surface processes. A number of recent studies using plasma micro-jets for the surface modification of polymerics have used systems in which the emerging plume impinges directly the substrate head-on. Here, by placing the micro-jet side-on to the substrate we can observe how different flow regions of the jet affect the sample, allowing individual effects to be seen. In addition, this configuration may prove an efficient way of treating samples with reduced or no surface damage. These conclusions are considered to be an important contribution to the study of complex mechanisms underpinning the behaviour of radicals and reactive species in surface modification processes of polymeric materials. The study of the behavioural mechanism involved in the plasma was done using various diagnostic techniques such as electrical measurements, optical emission spectroscopy (OES), Time-averaged and time-resolved ICCD Optical Imaging and Schlieren Photography. The filamentary discharge mode was observed in bonded microchannels using metallic and liquid-patterned electrodes. The treated surfaces were characterised using various techniques such as X-ray photoelectron spectroscopy (XPS), Atomic Force Microscopy (AFM), Optical profiling measurements and Water Contact Angle (WCA) measurements. Schlieren photography has been used to indentify regions of laminar (pre-onset of visual instability) and turbulent flows (post-onset of visual instability) in the exiting gas stream and the nature of their interaction with the substrate surface. The length of both regions varies depending on operating parameters such as frequency, applied voltage and flow rate. WCA results from treated polystyrene (PS) samples exposed directly facing the microjet reveals a change from hydrophobic (high contact angle) to a hydrophilic (low contact angle) surface with substantial reductions in WCA (~ 50 to 60 °) occurring in downstream regions where the turbulent gas mixed with air impinges the substrate surface. In contrast, only small changes in WCA (~ 10 to 20 °) occur in regions where the gas flow is laminar. AFM imaging of treated PS samples reveal holes and ripple like effect with a much larger area than that of the capillary seen on treated samples positioned “head-on” and directly facing the sample but this was not seen using the side-on configuration. The results indicate that excited air species (either mixed or entrained in the He gas flow) which exist only in regions of turbulence are the main agents causing surface covalent bond breaking leading to surface modification. This thesis reports on atmospheric pressure microdischarges and their applications, a brief summary of work done so far including major results, using new and existing technologies including those under development in terms of design, properties and working conditions

The interaction of helium metastable atoms with metal surfaces

An investigation has been made of the interaction of helium metastable atoms (2³S and 2¹S) with metal surfaces in different degrees of contamination. In order to carry out this investigation, an ultra-high vacuum chamber has been constructed enabling the operator to clean the metal surfaces in situ, monitor the state of cleanliness of the surfaces with Auger electron spectroscopy and then perform experiments with a beam of metastable atoms. A new metastable atom source has been built to produce a reasonably intense beam of atoms in the metastable states. Experiments have been performed on a clean Ni (100) surface and this surface with adsorbed sulphur and oxygen. Further experiments have been performed on a polycrystalline tungsten surface both in the clean state and with adsorbed carbon monoxide. Secondary electron energy distributions resulting from the impact of excited atoms on these surfaces show little agreement with the results to be expected on the basis of presently accepted theories. It is found that, in general, the total yield of secondary electrons increases linearly with coverage. Extensive comparisons have been made between the experiments reported in this thesis and the results from INS and other experiments with metastable atoms. These comparisons lead to the conclusion that the interaction of an ion and that of an excited atom with metal surfaces are two different processes. A new model for the de-excitation of the excited atom has been suggested and the results to be expected from such a de-excitation mechanism examined

Synthesis of Ag-loaded Sb₂WO₆ microsphere with enhanced photocatalytic ability for organic dyes degradations under different light irradiations

AbstractIn this study, the Ag-loaded Sb2WO6 photocatalysts have been successfully synthesized by a facile solvothermal method. The as-prepared Ag-loaded Sb2WO6 microspheres were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), high-resolution transmission electron microscopy (HRTEM), high angle annular dark-field transmission electron microscopy (HAADF-STEM), energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), UV–Vis diffuse reflectance spectroscopy (UV-DRS) and the Brunauer–Emmett–Teller (BET) method. Results indicate that metallic Ag was successfully grown on Sb2WO6. The photocatalytic activities of the Ag-Sb2WO6 photocatalysts were evaluated by degradations of MO, RhB and MB under UV light and visible light. It is found that the photocatalytic activity is excellent and superior after the decoration of the metallic Ag nanoparticles. A possible photocatalytic mechanism was proposed.Abstract In this study, the Ag-loaded Sb2WO6 photocatalysts have been successfully synthesized by a facile solvothermal method. The as-prepared Ag-loaded Sb2WO6 microspheres were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), high-resolution transmission electron microscopy (HRTEM), high angle annular dark-field transmission electron microscopy (HAADF-STEM), energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), UV–Vis diffuse reflectance spectroscopy (UV-DRS) and the Brunauer–Emmett–Teller (BET) method. Results indicate that metallic Ag was successfully grown on Sb2WO6. The photocatalytic activities of the Ag-Sb2WO6 photocatalysts were evaluated by degradations of MO, RhB and MB under UV light and visible light. It is found that the photocatalytic activity is excellent and superior after the decoration of the metallic Ag nanoparticles. A possible photocatalytic mechanism was proposed

An instrumental evaluation of selected metal functionalised semiconductors in the facilitation of photo-organic transformations.

Doctoral Degree. University of KwaZulu-Natal, Pietermaritzburg.Throughout this research study, various strategies to design, synthesise and test the photo-reactivity of attenuated wide band gap semiconductors in alcohol oxidation studies have been explored. An alizarin red-sensitised zinc oxide photocatalyst which was stabilised in a silver-sodium electrolyte effectively facilitated a broad aromatic and aliphatic alcohol oxidation table with reported conversions ranging from 10 to ≥ 99 %. A systematic characterisation of the alizarin red-sensitised zinc oxide investigated the photoelectronic migrations across the alizarin red–zinc oxide interface and detected the transfer of electrons from the highest occupied molecular orbital of alizarin to the defect site of zinc oxide at 507 nm. Further studies were directed towards the development of a novel titanium dioxide semiconductor that was activated by visible light. Three attempted strategies (pseudo perovskites [Cu3TiO5, Ni3TiO5, and Mn3TiO5], silver functionalised cadmium sulfide and a heterojunction between cadmium sulfide and titanium dioxide) explored the possibility of lowering the band gap potential of wide band gap semiconductors through metal ion functionalisation (Cu, Ni, Mn, and Ag) and heterojunction principles for the purpose of finding applications in mediating alcohol oxidations. Whilst the three strategies were unable to demonstrate viable photocatalytic properties, the instrumental insight obtained during the process identified a suitable three-component semiconductor system (Cu/Pd-N-TiO2). Cu/Pd-N-TiO2 was extensively characterised with an array of instrumental techniques, thus developing an in-depth understanding of the photophysical properties that governed the photo-oxidative transformation of a range of cyclic alcohols and in the remediation of two dyestuffs typically associated with environmental contamination

Surface-Assisted Chemistry at Interfaces between Metals and Organic Thin Films

Interfaces between metals and organic thin films are of paramount importance for organic electronic devices. By photoemission spectroscopy (XPS), scanning tunneling microscopy/spectroscopy (STM/STS), temperature programmed desorption/reaction (TPD/TPR) and density functional theory calculations (DFT), chemical reactions and diffusion processes during the formation of interfaces/interphases, and their electronic structures are investigated. The presented thesis focuses on two distinct substance classes, arranged by the reacting functional groups of the involved organic phases. Various reaction systems including organic-on-metal and metal-on-organic interfaces are visited. In the first major topic, the surface coordination chemistry of thin films formed by porphyrinoid molecules with coinage metals - the latter either as crystalline substrate surface or dosed onto the films as atoms - is characterized. Molecular film thicknesses extend from incomplete layers, i.e., submonolayer coverages, to a few ten nanometers, i.e., multilayers. Metalation of 2H-tetraphenylporphyrin (2HTPP) at submonolayer coverages and 2H-phthalocyanine (2HPc) for up to multilayer coverages is found to be possible at elevated temperatures even from an atomically flat copper substrate. TPD/TPR studies reveal that the presence of an exchange mechanism could be responsible for the observed substrate based multilayer metalation. Besides the interfacial chemical bond, i.e., between the surface and the metal complex, secondary effects - such as charge transfer and weak band bending - are characterized in detail for cobalt phthalocyanine adsorbed on Cu(111). Individual molecules in porphyrin or phthalocyanine thin films are able to oxidize cobalt atoms into formal Co(II) species. By modification of the reaction center from porphin to corrole, i.e., essentially by the replacement of an iminic by a pyrrolic nitrogen in the macrocycle, the formation of Co(III) complexes is rendered possible. Moreover, multilayer metalation of 2HTPP molecules with deposited cobalt atoms is investigated in detail by both laboratory X-ray source (XPS) and synchrotronbased hard X-rays (HAXPES). Since the latter is a nondestructive method for bulk composition probing, chemical and physical properties can be investigated. This technique is also applied to obtain a depth profile of a layered battery cathode - initially comprising lithium-nickel-manganese-oxide and lithium-titanium-oxide - after ex situ electrochemical cycling. The second major topic is dedicated to the behavior of purposively designed terminal oligophenylene dibromides on the flat hexagonal single crystal surfaces of copper and silver. Submonolayer amounts of oligophenylene dibromides deposited at cryogenic temperatures on the copper substrate show bromine detachment upon moderate temperature increase along with an intermediate formation of metal-organic oligomers. Upon further annealing, the metal is eliminated and entirely organic covalent structures are formed on the surface in analogy to the Ullmann coupling. The here presented surface reaction allows in situ synthesis of giant hexagonal macrocycles consisting of thirty phenylene units. Macrocycles with altered numbers of members are also accessible with reduced yield. Since these square, pentagonal and heptagonal shapes are not fully compatible with the hexagonal motive of the oligophenylene monomers, they exhibit strained geometries leading to modified electronic structures. These huge molecules are further used as organic quantum corrals to achieve and analyze the induced surface state confinements. Similar precursor compounds are deposited on a silver substrate in order to increase the mobility of the organic molecules as well as to suppress the formation of covalent bonds with the metal. This enables the generation of equilibrium based, self-assembling structures. Utilizing building blocks with three-fold symmetry, defect-free molecular fractal-like patterns - resembling Sierpinski triangles - are obtained. Revealing the chemical and physical processes at the interfaces important for device performance is the intent of this thesis. By fine tuning various intrinsic and external conditions, structural and chemical control of two-dimensional supramolecular phases is achieved. Surface-assisted chemistry - here in situ metalation of porphyrinoid molecules, synthesis of giant honeycombene macrocycles, and two-dimensional molecular selfassembled networks - as well as the properties, e.g., adsorbate-substrate interaction, of formed, and in some cases buried, interfaces/interphases between metals and organic thin films are comprehensively studied with a wide range of complementary ultrahigh vacuum based surface science techniques. The results and conclusions of the therefrom emerged publications are summarized in this work

Structural studies of anodic films on pure aluminium

The morphology and composition of anodically formed alumina films on pure aluminium are well characterised, but their structure is not well known due to the long range order. Two systems have been looked at in this study; the nature of the structure of tungsten incorporated from tungstate electrolyte during polarisation of aluminium at 100 V, and the other system is the thin passivating layers formed anodically on aluminium surface by polarisation within the range -2.2 - +2.5 V in aqueous solutions at various pH's. Even though other techniques, such as, electron microscopy and XPS have been used to confirm the presence of anion species in alumina films only EXAFS can yield direct information about the structure of incorporated species. In this study, EXAFS has been used for the first time to resolve the structure of tungsten incorporated into anodic alumina. Accurate determination of structural parameters require the establishment of reliable phase shift. By fitting the EXAFS of tungsten foil and potassium tungstate, the reliability of the calculated phase shifts for oxygen and tungsten was established. Based on phase shift transferability, the phase shifts calculated for 0 and W were used in the fitting of fluorescence EXAFS of tungsten in alumina oxide films formed on aluminium in tungstate solution at 100V. The experimental EXAFS function was fitted to a single oxygen shell with four atoms at a distance of 1.79 ± 0.01 A. From the structural parameters for incorporated tungsten it is clear that it is in a W04 2- even though this can not be the entity which is outwardly mobile. It is suggested that the tungstate may be linked in some way to the alumina and that because aluminium cations are outwardly mobile so tungstate is dragged along. The other kind of anodic films on aluminium are those produced by polarising aluminium in aqueous solutions at various pH's to various voltages within the range -2.2 - + 2.5 V. In some instances, passivating films grow on aluminium causing aluminium protection against corrosion. The structure of these films is not defined and because of their existence on aluminium, and their small thickness, so their structure is not easy to be studied. Electron microscope and XPS techniques have been used to determine their morphology and composition. Some work on indirectly extracting the structure of these films from XPS data was reported and it was claimed that these films have a pseudoboehmite like structure. XPS results of these films reported in this study have confirmed that under open circuit conditions an oxide film forms, but to provide good corrosion protection aluminium is polarised at different potentials. The oxide film which forms in these conditions has been found to have both 4 and 6 coordiation. The Al K-edge EXAFS of these oxide films was fitted in the same way as those for the incorporated tungsten. In this case, the EXAFS of aluminium foil and alpha alumina was fitted by the use of phase shifts calculated in EXCURV92 for aluminium and oxygen. The oxygen and aluminium phase shifts calculated in EXCURV92 were reliable, so they were used to fit the EXAFS of the passivating films on aluminium of unknown structure. The EXAFS results in this study have confirmed that these oxides do not have a single phase crystalline structure like boehmite or any others. Their structure is of a short range order and it is characterised in terms of co-ordination number and shell distances. All of the results reported in the EXAFS section of this work indicate that these oxide films are relatively thin and that is indicated by a metallic aluminium signal present most of the time. The sampling depth of the x-ray photons is much greater than the thickness of the oxide film and for this reason a metallic signal was recorded. By comparing the bond length value to the relation between the bond length and co-ordination number, it is obvious that these oxides films have a mixture of 4 and 6 co-ordination. In some cases, like aluminium polarised to + 0.9 V in pH 10, most of the co-ordination of the oxide is 6 except a little of it in the 4 co-ordination. XANES spectra for the model compounds and those for oxide films on aluminium and their derivative have agreed with the EXAFS results about the structure of thin films formed on aluminium. Also these findings were complemented by the XPS data where the Al 2p of pseudoboehmite has a binding energy of 80.0 eV whilst that of the oxide films in general were - 74 eV. So, it is clear that the oxide films structure is not a psuedoboehmite, but it is a local one with different values for the bond length and as a consequence a mixture of different (4 and 6) co-ordination

Research Reports: 1983 NASA/ASEE Summer Faculty Fellowship Program

Thirty-five technical reports contain results of investigations in information and electronic systems; materials and processing; systems dynamics; structures and propulsion; and space sciences. Ecology at KSC, satellite de-spin, and the X-ray source monitor were also studied

Alloy Plasmonics - Fundamentals and Application in Catalysis

Alloys have for a long time been important in the development of our society; from the bronze age, where man learned how to alloy copper with tin, to today, where many products are made of steel and aluminum alloys. Similarly, but maybe not as generally well known, also in heterogeneous catalysis alloys are explored to develop solutions to increase activity and selectivity of chemical processes. Furthermore, alloys have lately been proposed as a new paradigm in nanophotonics, as a means to tailor optical properties of nanomaterials that find applications within telecommunication, sensing, or biotechnology. Nanophotonics and catalysis, separately and in combination, are the focus of this thesis. Specifically, we have compiled a library of alloy complex dielectric functions for the late transition metals by utilizing time-dependent density-functional theory. The calculated dielectric functions were benchmarked by (i) nanofabricating series of alloy nanoparticle arrays with systematically varying composition, (ii) measuring their plasmonic properties, and (iii) comparing these properties with electrodynamic simulations of alloy nanoparticles, using the dielectric function library as the input. The second theme in this thesis is plasmon-enhanced catalysis. In this field there is a continuous discussion regarding the reaction enhancing mechanisms when noble metal catalyst nanoparticles are irradiated with visible light during a catalytic reaction. Here we investigated the role of photothermal enhancement of reactions by tailoring the catalytic activity of nanofabricated particles without radiation by means of alloying Pd with Au, while keeping the optical absorption cross-section constant, as confirmed by electrodynamics simulations using our dielectric function library as the input

Surface properties and electronic band structure of 2D materials and thin films

Two-dimensional (2D) semiconductors have the potential to drive significant advances in quantum science and technology. Atomically thin 2D materials play a dominant role in their electronic and optical properties due to their well controlled, tuneable and scalable band structure. This thesis explores the growth and electronic properties of GaSe layers on epitaxial graphene, with a focus on the evolution of the valence band maximum as the GaSe thickness increases at room temperature. The effects of oxygen and air exposure on monolayer GaSe, as well as the impact of thermal treatment, were systematically examined to understand the stability and electronic modifications induced by environmental interactions. Additionally, C60 monolayers and multilayer thin films were deposited on an Au(111) surface, and their electronic band structures were investigated. The findings provide valuable insights into the electronic behaviour of these materials, offering potential implications for future optoelectronic and nanotechnology applications. In this thesis, by utilizing angle-resolved photoelectron spectroscopy (ARPES), low energy electron diffraction (LEED) and scanning tunnelling microscopy (STM), we observed that atomically-thin layers of GaSe, grown by molecular beam epitaxy (MBE), align with the underlying graphene lattice in the layer plane. Our investigations reveal a transition from a ring-shaped to a parabolic valence band maximum as the GaSe layer thickness increases. In multilayers, the valence band maximum is centred at the Γ point, while in mono- and bilayers, it shifts along the ΓK direction, creating a ring-shaped maximum. This GaSe/graphene heterostructure features a charge dipole at the GaSe/graphene interface and a band structure that can be tuned based on the layer thickness. Our data shows that GaSe layers on graphene become n-type due to electron transfer from the n-type graphene, contrasting with the intrinsic p- type behaviour of GaSe. We investigated the stability of monolayer GaSe grown on epitaxial graphene in this thesis. GaSe demonstrates resilience to oxygen at room temperature, where the adsorption of O2 molecules on its surface effectively restores its original electronic properties. At temperatures above 450 ◦C, GaSe begins to react with oxygen, leading to the formation of gallium oxide (Ga2O3) with no selenium remaining. We further explore the effects of oxygen exposure, air exposure, and heat treatments on the electronic band structure of monolayer GaSe. While the band structure is preserved under these conditions, a notable shift towards the Fermi level indicates that the GaSe layer acts as an acceptor. Additionally, in the GaSe/graphene van der Waals heterostructure, the interaction between the two layers remains robust, highlighting the potential of this heterostructure for advanced device applications. Finally, we deposited C60 on an Au(111) surface via sublimation using a Knudsen cell (K-cell) to prepare C60 thin films adsorbed on the substrate. We employed angle-resolved photoelectron spectroscopy (ARPES) to investigate the band structure of the thin C60 film on Au(111), focusing on the delocalized π-like valence states. Specifically, we analyzed the highest occupied molecular orbital (HOMO) and the HOMO-1 states within the entire valence band structure of C60, a prototypical three-dimensional (3D) organic molecule. We found that the two frontier molecular orbitals, HOMO and HOMO-1, exhibit weak band dispersion, which we attribute to their pure π-orbital character that extends across adjacent molecules. In contrast, the sharper emission peaks at larger binding energies are attributed to σ orbitals, which remain localized on individual C60 molecules. This charge transfer leads to a bonding interaction between the molecule and the metal surface, often described as chemisorption. The electron density transferred to the C60 molecule stabilizes it on the surface, resulting in a stronger interaction compared to mere physisorption, which involves weaker van der Waals forces