The mechanical tools currently used in orthopedic and dental surgery are imprecise and may cause heat damage. Ultrashort pulse lasers are a promising replacement, but their ablation efficiency must be improved. The goal of this thesis was to achieve high ablation efficiency, precision, and minimal collateral damage using an ultrafast laser on bovine hard tissue. This work used two types of lasers: a Ti:Sapphire laser (210 fs, 800 nm, 1 kHz) and a fiber laser (1 ps, 1035 nm, 100 kHz - 1 MHz). This thesis begins with a review of the literature on laser-tissue interactions and the effect of certain laser parameters on the ablation process. The next section uses a Ti:Sapphire laser and bovine bone to explore the properties of laser-tissue interactions, including ablation threshold and incubation coefficient. Results showed that as the number of incident pulses goes up, ablation threshold goes down. The threshold range went from 1.08 ± 0.15 J/cm2 at 25 incident pulses to 0.73 ± 0.12 J/cm2 at 1000 pulses. The incubation coefficient, S, was calculated to be 0.90 ± 0.02. The relationship between ablation depth and fluence, scanning speed, and number of successive passes was characterized as a first step towards preparing large-cavity with high removal efficiency using a Ti:Sapphire and fiber lasers. Depth increased with fluence and number of passes, but it decreased with scanning speed. The influence of environmental conditions including air, compressed air flow, still water and flowing water on cavity ablation depth, and rate was investigated using a Ti:Sapphire laser with aim to enhance ablation efficiency. Findings showed that the deepest cavities and fastest ablation rates were achieved with compressed air flow. Air flow also resulted in the most precise cuts, the smoothest surfaces, and the absence of microcracks. This thesis also used a fiber laser to explore the effect of fluence and repetition rate on removal rate and ablation quality. Results indicated that ablation rate increases with fluence and pulse rate. When the repetition rate exceeded 600 kHz, the laser caused thermal and mechanical damage, indicated by the presence of amorphous carbon. The effect of environmental conditions and laser parameters such as repetition rate provide valuable insights into the ultrafast laser ablation mechanisms for medicine and biology field. / Thesis / Doctor of Philosophy (PhD)

ABSTRACT The focus of this work was the study possible ways to improve the efficiency of solar cells. To this end, the main aim was to investigate the deposition process of Indium Tin Oxide (ITO), Titanium Dioxide (TiO2), multi-layers ITO/TiO2 on quartz SiO2 substrates under different conditions (oxygen pressure, laser fluence and wavelength, and temperature) and later gold nanoparticles by the Reactive Pulsed Laser Ablation Deposition (RPLAD) technique. It was intended to investigate their electrical structural and optical properties under selected conditions for possible application to Dye Sensitised Solar Cells (DSSC). Under optimised conditions, maximum deposition rates of 12nm/min for ITO and 21nm/min for TiO2 thin films were achieved. Rutherford Backscattering Spectrometry (RBS) with 2MeV He+ ions was used to measure the films thickness. Uniform thicknesses over a large area were found to be about 400nm and 800nm for ITO and TiO2 films, respectively. Crystalline properties were studied via x-ray diffraction and Raman spectroscopy. X-ray Diffraction (XRD) analysis revealed that the ITO films are highly orientated nanocrystals with their a-axis normal to the glass substrate surface. The average particle size of the precipitated nanocrystals was calculated to be 10-15nm. The structure of the films was characterised via Atomic Force Microscopy (AFM) imaging of the top surface of the film. The films have a rough surface with average roughness of 26-30nm. Pores were observed with a density of 144 and 125 pores/mm2 and average size of 150 and 110nm for ITO films deposited at 200 and 400°C, respectively. TiO2 films deposited on the prepared ITO films were less crystalline. Annealing was performed at 300 and 500°C for 3 consecutive hours and the XRD results show that the transformation of TiO2 film into anatase phase was almost complete with a crystal size of ~ 6-7nm. Scanning Transmission Electron Microscopy (STEM) of the surfaces was also performed. The TiO2 films deposited onto the prepared ITO films present a relatively high pore size with an average pore diameter of ~ 40nm and excellent uniformity. It is interesting to note that the pores are randomly arranged. The random arrangement of the pores network may actually be beneficial for producing a uniform electrode. In addition, STEM cross-sectional analysis of the films showed a columnar structure but no evidence of voids in the structure. The large surface area produced suggests applications in DSSC. The electrical properties of the films were investigated and an estimation of resistivity and Hall mobility was made. Low values of resistivity and high values of mobility were observed for ITO films. The resistivity of the film increases with increasing thickness while it decreases when increasing the deposition temperature. The lowest value was found to be 1.5x10-6Ωm for ITO films deposited at 400°C. Hall mobility was found to increase with substrate temperature. In this investigation, the highest Hall mobility at room temperature was estimated to be 22.3cm2/Vs under ambient O2 pressure (PO2) of 1Pa and 52.1 and 51.3cm2/Vs for films deposited at 200 and 400°C, respectively. But the best ITO film was deposited at 200°C, since this film combines good resistivity, good Hall mobility and good transmittance. UV-VIS-IR transmission spectra were recorded on a Perkin Elmer Lambda 900. From the transmission data, the energy gap as well as the optical constant was estimated. A high transmission for ITO films in the visible (Vis) range was observed which was above 88% for films produced at room temperature and above 95% for those deposited at 200°C. The transmission for the films produced in oxygen was about 90% above 400nm, whereas it lies between 70 and 80% for films produced in rare gases. An increase in the band gap was observed by increasing the oxygen pressure and substrate temperature for ITO films. Increasing the quartz SiO2 substrate temperature from room temperature to 400 °C resulted in an increase of the transmission of TiO2 films, mostly in the Visible Near Infrared (Vis-NIR) from about 70% to 92%. After annealing at 500°C for 3 consecutive hours, the transmission of TiO2 film further sharply decreases toward shorter wavelengths. Analysis of the transmittance curve of TiO2/Au shows a decrease of about 6% of the transmission in the Ultraviolet Visible (UV-Vis) range. Optical absorption edge analysis showed that the optical density could be used to detect the film growth conditions and to correlate the film structure and the absorption edge. The TiO2 films deposited present a direct band gap at 3.51eV and 3.37eV for TiO2 as deposited and after annealing, respectively, while the indirect band gap was found to be 3.55eV and 3.26eV for TiO2 films as deposited and after annealing, respectively. There was a shift of about 0.1eV between as deposited ITO monolayer films and ITO/TiO2 bilayers deposited at 200°C. A small shift towards shorter wavelengths has been observed for multilayer ITO/TiO2/Au. In this case, the increase of Eg was ascribed to a reduction of the oxygen vacancies with increasing substrate temperature at which the ITO film was deposited. The change in the shape of the fundamental absorption edge is considered to reflect the variation of density and the short range structural modifications undetected by structural characterisations. Enlargement of band-gap energies of semiconductors may be advantageous when used in DSSC to suppress the charge recombination between the reduced electrolytes and the photo-excited holes in the valence band of TiO2 substrates and enhance the open-circuit potential of the cell. When ITO/TiO2 bilayers were annealed before depositing Au, the gap energy remained constant.

In the first part of this thesis, CaCO3 (calcite) powder ball-milled to micro/nano scale were subjected to isothermal firing in the temperature range of 450-600 ¢XC in an open air furnace in order to study specific-surface area reduction as a result of early-stage sintering/coarsening/coalescence/repacking (denoted as SCCR process) of the fine particles. The surface area and pore size distributions were obtained from the BET and BJH methods, respectively. The H1 type adsorption/desorption hysteresis loop of the type IV isotherm was used as an indicator of cylindrical pore formation upon dry pressing and firing of the powder. The apparent activation energy for the onset SCCR of the dry-pressed calcite powder turned out to be 57.5¡Ó1.0 kJ/mol based on t0.5, i.e. time for 50 % specific surface area reduction. The minimum temperature for such an incipient SCCR process was estimated to be 590K (317¢J) by extrapolating the specific-surface-area reduction rate to null. The mechanism of specific surface area change includes the Brownian motion, coarsening and coalescence/repacking of calcite particles besides sintering via synchronizing diffusion of calcium ion and carbonic acid ion along grain surface and boundary. In the second part of this thesis, pulsed laser ablation on micrometer-sized calcite (type I) powder in liquid H2O (PLAL) was conducted to study the structure and optical property change of calcium carbonate under a dynamic high-temperature. high-pressure aqueous condition. X-ray diffraction (XRD) indicated the fragmented calcite I powder via such a PLAL process change predominantly into a metastable CaCO3 II phase presumably by a displacive type transformation from calcite I and/or nucleated from atom clusters. The refined XRD lattice parameters indicate a significant internal compressive stress (up to 1.5 GPa) was retained for the predominant CaCO3 II nanoparticles having well-developed (013), (010) and (013) faces as revealed by transmission electron microscopy (TEM). Minor calcium carbonate nanoparticles were also identified by TEM to be other high-pressure polymorphs (type III and aragonite), hydrated (monohydrocalcite, ikaite), amorphized (amorphous calcium carbonate), and even decomposed as cubic lime (CaO). Monohydrocalcite occasionally occurred as epitaxial intergrowths within the predominant CaCO3 II matrix. Vibrational spectroscopy (Raman and FTIR) indicated the structure units of the overall nanoparticles by the PLAL process were considerably modified as a combined results of size miniature, protonation and internal compressive stress. The UV-visible absorption results further indicate that the minimum band gap of the colloidal solution was narrowed down to ca. 5 eV and 3 eV for the predominant CaCO3 II and minor accessory phases, respectively, thus shedding light on their potential opto-catalytic applications.

Thesis submitted in partial fulfilment of the requirements for the degree Doctor of Technology: Electrical, Electronic and Computer Engineering in the Faculty of Engineering at the Cape Peninsula University of Technology / This research investigates the effects of sub-10 MeV protons on coatings obtained from the pulsed laser ablation of W2B5/B4C. This is in an attempt to extend the bullet proof applications of W2B5/B4C to space radiation shielding applications, offering low cost and low mass protection against radiation including X-rays, neutrons, gamma rays and protons in low Earth orbit. The focus in this research, however, is on low energy protons. The associated problems addressed in this work are solar cell degradation and Single Event Upsets in high density semiconductor devices caused by low energy protons. The relevant constraints considered are the necessity for low cost, low mass and high efficiency solutions. The work starts with a literature review of the space environment, the interaction of radiation with matter, and on pulsed laser deposition as a technique of choice for the coating synthesis. This paves the way for the pulsed laser ablation of W2B5/B4C. The resulting coating is a solid solution of the form WC1-xBx which contains crystalline and amorphous forms. Two proton irradiation experiments are carried out on this coating, and the resulting effects are analysed. The effects of 900 keV proton irradiation were the melting and subsequent growing of nanorods on the surface of the coating, the lateral transfer of the proton energy across the coating surface, and the lateral displacement of matter along the coating surface. These effects show that the coating is a promising cost effective and low mass radiation shield against low energy protons. The effects of 1 MeV protons on this coating are the three-stage melting of rods formed on the coating surface, and further evidence of lateral transfer of energy across the coating surface. Optical measurements of this coating show that it is about 73% transparent in the Ultraviolet, Visible and near Infrared range. This allows it to be used as radiation shielding for solar cells, in addition to high density semiconductor devices, against low energy protons in low Earth orbit. Simulations show that based on coulombic interactions alone, the same level of protection coverglass offers to solar cells can be achieved with about half the thickness of WC1-xBx or less.

No description available.

Nanodiamonds are a promising platform for quantum technologies due to the combinations of their inherent properties and the properties of the fluorescent color centers hosted in diamond. They can be employed as quantum sensing devices with spatial resolution in the range of the nanometer and capable of withstanding harsh conditions while also being biocompatible, allowing applications with sensitive biological systems; but they also find application in quantum computing and photonics fields. For all these applications the central features are the properties of the photoluminescent color centers employed, the color centers on which this thesis is focused are the Nitrogen-Vacancy (NV) and Silicon-Vacancy (SiV) centers of diamond. Both centers are of high interest due to spin dependent properties of their fluorescent emission which can be accessed at room temperature. The development of quantum technologies based on such fluorescent nanodiamonds is stifled by the the lack of production techniques that can be easily scaled to industrial levels. In fact most of the more prominent techniques found in literature exhibit drawbacks both in terms of control of particle properties and of scalability. This thesis focuses on the synthesis of nanodiamonds by Pulsed Laser Ablation in Liquid, with particular interest in the possibility of producing continuously nanodiamonds containing NV and SiV centers. For the NV center the technique of choice have been Pulsed Laser Ablation in liquid nitrogen focusing on the yield of the process as the technique has already been experimentally validated. For the SiV centers the ablation process was performed in water and the graphite precursor have been substituted for a composite graphite and silicon carbide precursor.

L’engouement pour l’originalité des propriétés physiques des nanoparticules s’est accompagné d’un développement de nombreuses méthodes de synthèse depuis un demi siècle. Parmi elles, l’ablation laser en liquide permet de produire des nanoparticules avec des surfaces libres de tout contaminant et ce pour une multitude de combinaisons de matériaux et de solvants. Cependant, la simplicité apparente de cette technique dissimule la complexité des mécanismes physico-chimiques, ce qui entraîne actuellement un manque de contrôle des objets synthétisés. Tout d’abord, nous nous sommes intéressés à la cinétique des bulles pour laquelle les conditionsextrêmes d’ablation laser en liquide présentent des cas originaux de cinétique dans le domaine de la mécanique des fluides. Puis, ce travail de thèse vise à donner de plus amples perspectives quant à une meilleure maîtrise de la structure cristalline des nanoparticules et de la stabilité colloïdale. Une manière plus directe de contrôler la taille, la phase cristalline et la stabilité colloïdale des solutions contenant des nanoparticules est d’ajouter des ligands. Nous avons donc étudié les mécanismes de stabilisation de ces solutions en utilisant des ions qui se complexent aux nanoparticules d’or. Nous avons aussi réussi à synthétiser des nanoparticules de rubis (alumine dopée chrome). La stabilisation de ces nanoparticules dans une phase métastable en utilisant des ligands organiques a été expliquée par une étude théorique / Laser generation of nanoparticles in liquids : new insights on crystal structure control and colloidal stability The great interest of nanoparticles for their original physical and an chemical properties has been supported by the development of numerous methods of synthesis. In the nineties, laser generation of nanoparticles in liquids appeared, including Pulsed Laser Ablation in Liquids (PLAL). The PLAL technique enables to produce surface free particles for plenty of material and solvent combinations. However, the apparent simplicity of its implementation hides complex physico-chemical mechanisms resulting in a lack of control of the final products. We firstly investigated the dynamics of the laser-generated bubbles for which the PLAL extreme conditions present new studied cases of bubbles dynamics not encountered in the field of fluid mechanics. Then, we aim to bring new insights into better control of the nanoparticles morphology and their colloidal stability. A straight way to tune sizes, crystal structures and the colloidal stability consists in the addition of stabilizing agents. Hence, we investigated the mechanisms of stabilization of colloidal gold using complexing ions. We also succeed to synthesis nano-rubies, i.e. chromium doped corundum alumina nanoparticles, unexpected at nanoscale. The stabilization of the metastable crystal structure using ligands is explained thanks to a comprehensive theoretical approach

Perovskite based artificial superlattices has recently been extensively investigated due to the immense promise in various device applications. The major applications include non-volatile random access memories, microwave devices, phase shifters voltage tunable capacitor applications etc. In this thesis we have taken up the investigation of two different types of symmetric superlattices, viz. BaZrO3/BaTiO3 and SrTiO3/BaZrO3, with possible applicability to voltage tunable devices. Chapter 1 deals with the introduction to the perovskite based functional oxides. Their various applications and the specific requirements for voltage tunable device applications has also been discussed in detail. The basic properties of BaTiO3 and SrTiO3, which are well documented in the literature, have been reviewed. The fundamental physics of interfacial interactions that influence the properties of superlattices is also discussed using existing models. The reason behind the choice of constructing artificial superlattices of BaZrO3/BaTiO3 and SrTiO3/BaZrO3 and the motivation behind this thesis is outlined. Chapter 2 gives a brief description of the basic characterization techniques that has been employed for studying the thin films. These include pulsed laser deposition of oxide thin films, structural characterization using X-Ray Diffraction and Atomic Force Microscope and electrical characterization of thin film metal-insulator-metal structures. The basic principle behind the techniques has also been included in various sections of this chapter. Chapter 3 introduces the reader to basic properties of the less studied perovskite material BaZrO3, one of the parent components of Ba(Zr,Ti)O3 based ceramics for high frequency applications. BaZrO3 is the common material in both the types of superlattices studied in this thesis. Initially the growth of polycrystalline BaZrO3 on (111)Pt/TiO2/SiO2/Si has been elaborated in this chapter. After characterizing the crystalline quality of the films and optimizing the growth conditions, epitaxial BaZrO3 films has been grown on (001) SrTiO3 substates. Dielectric properties of epitaxial BaZrO3 film have been measured as a function of temperature and frequencies. The electric field tunability of BaZrO3 films has been calculated from capacitance-voltage data for comparison with superlattice structures. Chapter 4 deals with the basic considerations involving growth of artificial superlattices and multilayers using pulsed laser ablation technique. The fundamental differences between formation of multilayers and superlattices have also been discussed, and the basic considerations for optimizing growth parameters are analyzed in this chapter. X-ray θ-2θ and φ-scans have been performed to investigate crystal quality of superlattices. The growth rates calculated from the satellite reflections in X-ray θ-2θ scans indicate fair degree of control over the growth and φ-scans confirms epitaxial cube-on cube growth of both types of superlattices. Atomic Force microscopy has been used to hcaracterize the film quality and surface morphology of superlattice structures and it has been found that the films have a very smooth surface with rms roughness of the order of few nanometres. Chapter5 deals with the detailed electrical characterization of both types of superlattices structures. Dielectric response showed nearly temperature invariance for both types of superlattices. Polarization measurements show that the heterostructures are in paraelectric state. Even for paraelectric/ferroelectric BaZrO3/BaTiO3 superlattices, stress induced stabilization of the paraelectric state is exhibited in low period superlattices. Paraelectric/paraelectric-SrTiO3/BaZrO3 superlattices exhibited a tunability of ~20% at intermediate modulation periods and an extremely stable dissipation factor with respect to temperature which is very attractive for device application point of view. A maximum tunability of ~40% has been observed for lowest period BaZrO3/BaTiO3 superlattice. Relatively high Quality Factors has been observed for both type of superlattices and their dependence on the modulation periods has been analyzed. Dielectric relaxation data showed that Maxwell-Wanger type of behaviour is exhibited but the presence of a conductance component G had to be realized in the equivalent circuit representation, which originates from the observation of a square law dependence of the alternating current on the frequency. Finally DC electrical characteristics were investigated as a function of temperature to determine the type of conduction mechanism that is involoved. The data has been analyzed using existing theories of high field conduction in thin dielectric films and it has been found that at different temperature ranges, the conduction mechanism varied from bulk limited Poole-Frenkel to Space Charge limited conduction. The activation energy calculation indicate that the physical processes responsible for dielectric relaxation and dc conduction are identical.

Les nanoparticules métalliques (NPs) présentent des propriétés optiques (PO) uniques provenant de l’oscillation collective de leurs électrons. Cet effet se traduit par l'émergence d'une bande plasmon dont les caractéristiques peuvent être modulées par la taille, la forme, la nature des NPs et le milieu hôte. Il existe de nombreuses méthodes pour la préparation de ces NPs, l'une d'entre elles est l'ablation laser en milieu liquide (ALML). Cette technique offre certains avantages comme la simplicité, l’adaptabilité et des NPs dépourvues de contamination. Ses principaux inconvénients sont la productivité et le contrôle de la taille et de la forme des NPs. Ce travail est consacré à l'élaboration de NPs d’Ag par l’ALML et à l'étude théorique de leurs PO. Nous donnons dans ce manuscrit, les résultats de l'optimisation des paramètres d'élaboration conduisant à l'obtention de distributions en NPs reproductibles et contrôlées. Les PO de ces NPs sont mesurées et comparées à des modèles physiques spécifiques basés sur la théorie des milieux effectifs (EMT). L'EMT, telle que le modèle de Maxwell-Garnett, permet de décrire les PO de NPs monodisperses. Cependant, les voies de préparation classiques conduisent inévitablement vers des NPs montrant une distribution de forme et de taille qui induit des changements drastiques sur leurs PO. Le modèle SDEMT est proposée pour le calcul de la fonction diélectrique effective et du coefficient d'absorption de solutions colloïdales de NPs métalliques. Contrairement à Maxwell-Garnett, ce modèle donne une meilleure description des spectres d'absorption et d’ellipsométrie mesurés sur des échantillons contenant des NPs d’Ag et d’Au / Metal nanoparticles (NPs) exhibit unique optical properties (OP) coming from the collective oscillations of their electrons. This effect is translated by the emergence of a band of plasmon, the characteristics which can be modulated by the size, the shape and the nature of the NPs as well as by the environment of the host. There are many methods for the preparation of NPs, and one of them is the pulsed-laser ablation in liquid (PLAL). This technique offers some advantages such as simplicity, versatility and surface NP without contamination (reducing agent residues and/or stabilizers). Its main drawbacks are the lacks of productivity and control of the NP size and shape. This work is devoted to elaboration of Ag NPs by PLAL and theoretical investigation of their OP. We give here the results about the optimization of elaboration parameters leading to obtaining reproducible and controlled distributions of Ag NPs. The OP of these NPs are measured and compared to specific physical models based on the effective medium theory (EMT). Classical EMT such as Maxwell Garnett approximation allows describing the OP of monodisperse NPs. However, conventional preparation routes unavoidably conduct to NPs showing a shape and a size distribution which induces drastic changes in the OP. A SDEMT model which considers the shape dispersion is proposed for the calculation of the effective dielectric function and absorption coefficient of colloidal solution of metal NPs in water. Contrary to the conventional theory, this model gives a better description of the measured absorption and ellispometry spectra of sample containing Ag and Au NPs

[pt] A pesquisa aponta para a possibilidade de realizar a reação de redução de CO2 (CO2RR) para a formação de nanomateriais de carbono por ablação a laser pulsado(PLA) de alvos magnéticos de Ferro(Fe) e Níquel(Ni) em água pura deionizada. Os materiais coloidais sintetizados foram caracterizados por diferentes técnicas de espectroscopias ópticas (UVVis, ICP-MS, FTIR e Raman) e microscopia eletrônica de transmissão (TEM), revelando a presença de nanopartículas de óxidos e hidróxidos de metais de transição, junto com nanomaterial orgânico. Esse último, é bem visível por TEM, espectroscopia de raio-X por dispersão em energia (EDS), espectroscopia por perda de energia de elétrons (EELS), e espectroscopia Raman, que indica a presença de carbono amorfo grafítico e vibrações CH. No caso do nanomaterial obtido do Níquel, os resultados FTIR confirmam a presença da fase do hidróxido beta-Ni(OH)2, enquanto as medidas Raman e TEM sugerem também a presença de nano-folhas de Ni(HCO3)2. Os resultados experimentais foram enfim discutidos no contexto da origem e da evolução de moléculas simples e complexas de interesse astroquímico, com foco especial nas espécies potencialmente formadas na superfície de pequenos corpos metálicos do Sistema Solar e grãos de poeira cósmica do meio interestelar. / [en] The proposed research points to the possibility to perform CO2 reduction reaction (CO2RR) to solid carbon nanomaterials by the pulsed laser ablation (PLA) of magnetic target of iron (Fe) and nickel (Ni) in pure deionized water. The synthesized colloidal dispersions were characterized by different optical spectroscopies (UV-Vis, ICP-MS, FTIR and Raman) and transmission electron microscopy (TEM), revealing the presence of nanosized transition metal oxide and hydroxide nanoparticles, together with organic nanomaterial. The latter is well visible by TEM, energy-dispersive X-Ray spectroscopy (EDS), electron energy-loss spectroscopy(EELS), and Raman spectroscopy, which indicates the presence of amorphous graphitic carbon and CH vibrations. In the case of Ni derived nanomaterial, FTIR results confirm the presence of a beta-Ni(OH)2 hydroxide phase, while Raman and TEM measurements suggest also the presence of Ni(HCO3)2 nanosheets. The experimental results were finally discussed in the frame of the origin and evolution of simple and complex molecules of astrochemical interest, with special focus on those species potentially formed on the surface of metallic minor bodies in the solar system and cosmic dust grains in the interstellar medium(ISM).