The research work in this thesis has been devoted to understand the formation mechanism of single-crystalline and textured polycrystalline nanoparticles and nanoflakes of SmCo5 and Nd2Fe14B prepared by surfactant-assisted (SA) ball milling and to study their microstructural and magnetic properties. The nanoparticles and nanoflakes are promising candidates to be used as hard magnetic phase for the fabrication of high-energy-density exchange-coupled nanocomposite magnets. The influence of a range of different surfactants, solvents and milling parameters on the characteristics of ball-milled powder has been systematically investigated. Small fraction (~10 wt.%) of SmCo5 nanoparticles of average diameter 15 nm along with textured polycrystalline nanoflakes of average diameter 1 µm and average thickness of 100 nm were obtained after SA – ball milling of SmCo5 powder. Isolated single-crystalline particles (200-500 nm) and textured polycrystalline flakes (0.2-1.0 µm) of Nd2Fe14B have been prepared in bulk amount (tens of grams), after SA – ball milling of dynamic-hydrogen-disproportionation-desorption-recombination (d-HDDR) processed Nd2Fe14B powder. These single-crystalline Nd2Fe14B particles are promising for their microstructure for the fabrication of exchange-coupled nanocomposite permanent magnets. The SmCo5 and Nd2Fe14B flakes and particles were well aligned in magnetic field: the former showed [001] out-of-plane orientation while the latter showed [001] in-plane orientation. A maximum degree of texture values of 93 % and 88 % have been obtained for the magnetically-oriented SmCo5 flakes and Nd2Fe14B single-crystalline particles, respectively. SA – ball milling resulted in an increase of coercivity of SmCo5 particles from 0.45 T for un-milled powder to a maximum value of 2.3 T after 1 h of milling, further milling resulted in a decrease of the coercivity. The coercivity of SA – ball-milled Nd2Fe14B particles decreased drastically from 1.4 T for un-milled d-HDDR powder to 0.44 T after 0.5 h of milling, isolated single-crystalline particles (200-500 nm size) obtained after 4 h of SA – ball milling showed a coercivity of 0.34 T. The drastic decrease in coercivity of ball-milled Nd2Fe14B particles is attributed to the morphological change because the demagnetization in Nd2Fe14B magnets is governed by nucleation mechanism. A remarkable enhancement in coercivity from 0.26 T to 0.70 T for ethanol-milled sample and from 0.51 T to 0.71 T for oleic-acid-milled samples has been obtained after an optimum heat-treatment at 400 0C. An increase of α-Fe and Nd2O3 phase contents and a sharp change of lattice parameter c of Nd2Fe14B was observed when heat-treating above 400 0C. The change in lattice parameter at higher temperature is thought to be due to partial substitution of carbon atoms present in the surfactant or solvent, for boron atoms. / Das Ziel dieser Arbeit ist es, den Mechanismus der Herstellung von einkristallinen und texturierten polykristallinen Nanopartikeln und Nanoflakes aus SmCo5 und Nd2Fe14B durch Tensid-gestütztes Kugelmahlen zu verstehen. Des Weiteren soll deren Gefüge und magnetische Eigenschaften untersucht werden. Die Nanopartikel sind vielversprechende Kandidaten zur Verwendung als hartmagnetische Phase in hochentwickelten, austauschgekoppelten Nanokomposit-Magneten. Der Einfluß der Art der verwendeten Tensid, Lösungsmittel sowie Mahlparameter auf die Eigenschaften der kugelgemahlenen Pulver wurde systematisch untersucht. Ein kleiner Anteil (~10 m.%) von SmCo5 Nanopartikeln mit mittlerem Durchmesser von 15 nm zusammen mit texturierten polykristallinen Plättchen mit mittlerem Durchmesser von 1 µm und mittlerer Dicke von 100 nm wurden nach Tensid-gestütztes Kugelmahlen erzeugt. Alleinstehende einkristalline Partikel (200-500 nm) und texturierte polykristalline Plättchen (0,2-1,0 µm) aus Nd2Fe14B wurden in größeren Mengen (einige 10 g) hergestellt. Das verwendete Ausgangspulver wurde hierbei durch dynamisches-Hydrierung-Disproportionierung-Desorption-Rekombination (d-HDDR) hergestellt und anschließend Tensid-gestütztes Kugelmahlen. Die genannten einkristallinen Nd2Fe14B Partikel sind ebenfalls vielversprechend als hartmagnetischer Bestandteil von austauschgekoppelten Nanokomposit-Magneten. Die SmCo5- und Nd2Fe14B-Plättchen und -Partikel wurden alle in einem Magnetfeld ausgerichtet: erstere zeigten aus der Ebende herauszeigende und letztere in der Ebene liegende [001]-Orientierung. Ein maximaler Texturgrad von 93% wurde für im Magnetfeld ausgerichtete SmCo5 flakes bzw. 88% für einkristalline Nd2Fe14B Partikel erzielt. Tensid-gestütztes Kugelmahlen führte zu einem Anstieg der Koerzitivfeldstärke von SmCo5 Partikeln von 0,45 T für ungemahlenes Pulver auf 2,3 T nach einer Mahldauer von 1 h. Weiteres Mahlen führte zu einem Abfall der Koerzitivfeldstärke. Die Koerzitivfeldstärke von Tensid-gestütztes Kugelmahlen Nd2Fe14B Partikeln verringerte sich stark von 1,4 T von ungemahlenem d-HDDR Pulver auf 0,44 T nach 0,5 h Mahlen. Freistehende einkristalline Partikel (200-500 nm groß), welche nach 4 h Tensid-gestütztes Kugelmahlen erhalten wurden, zeigten eine Koerzitivfeldstärke von 0,34 T. Der starke Abfall der Koerzitivfeldstärke von gemahlenen Nd2Fe14B Partikeln wird die morphologischen Veränderungen zurückgeführt, da die Ummagnetisierung nukleationsgesteuert ist. Ein bemerkenswerter Anstieg der Koerzitivfeldstärke von 0,26 T auf 0,70 T wurde für eine in Ethanol gemahlene Probe verzeichnet, sowie ein Anstieg von 0,51 auf 0,71 T für eine Probe, welche mit einer Zugabe von Oleinsäure gemahlen wurde. Beide Proben wurden einer optimierten Wärmebehandlung bei 400°C unterzogen. Bei höheren Temperaturen wurde für Nd2Fe14B ein Anstieg der Menge an α-Fe und Nd2O3 gefunden und eine sprungartige Veränderung des Gitterparameters c der Nd2Fe14B Phase. Die Veränderung des Gitterparameters wird auf die partielle Substitution von Kohlenstoffatomen des Tensid oder Lösungsmittels gegen Boratome zurückgeführt.

Orientador: Augusto Batagin Neto / Resumo: Compostos baseados em carbono têm se mostrado materiais de grande interesse tecnológico, principalmente devido à sua alta flexibilidade de síntese, baixo custo relativo e propriedades únicas. Graças a isto, tem-se observado um número crescente de trabalhos teóricos e experimentais acerca da compreensão de características básicas destes sistemas e a proposição de novos compostos com propriedades otimizadas para aplicações específicas. No presente trabalho são apresentados estudos teóricos acerca de dois temas relacionados à reatividade e propriedades estruturais de materiais orgânicos e baseados em carbono: i) estudo e análise de reatividade de subunidades de melanina; e ii) estudo estrutural e de reatividade de nanoflocos (ou nanoflakes em inglês) de carbono para aplicações em sensores químicos. Melaninas são pigmentos naturais com propriedades biológicas e eletrônicas que as tornam promissoras para aplicações bio-eletrônicas. Contudo não há ainda entendimento pleno acerca de sua estrutura macromolecular e conexão entre suas unidades básicas. Neste estudo cálculos de estrutura eletrônica, combinados com análise de reatividade, foram realizados para melhor compreender os processos de oligomerização. Os resultados obtidos permitem propor as estruturas diméricas mais prováveis e identificar reações que ocorrem no processo de síntese de melanina. Pode-se também estabelecer uma ordem de dominância de reatividade entre as subunidades e identificar núcleos de polimerização, o que po... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: Carbon-based materials have been considered compounds of great technological interest, manly due to their flexibility of synthesis, relative low cost and unique opto-electronic properties. A crescent number of theoretical and experimental studies has been reported regarding the comprehension of basic features of these materials and the proposition of new compounds with optimized properties. This thesis presents studies about the reactivity and structural properties of carbon based and organic materials according to two themes: i) reactivity study and oligomerization analysis of melanin; and ii) reactivity and structural analysis of carbon nanoflakes and their application as chemical sensors. Melanins are natural pigments with biological and electrical properties that turn them promising compounds for bioelectronic applications. However, despite of their promising properties, up to date the macromolecular structure of melanin and the connection between its basic units have not been understood in detail. In this study, electronic structure calculations, combined with reactivity analysis were conducted to better understand the oligomerization process of this compound. The obtained results allow us to propose the most probable dimeric structures and identify relevant reactions that occur during melanin oligomerization process. Furthermore, it was possible to observe a dominance order in the reactivity of the subunits and identify possible nucleation centers of melanin polymerizat... (Complete abstract click electronic access below) / Mestre

Nanotechnology is an emerging inter-disciplinary paradigm which encompasses diverse fields of science and engineering converge at the nanoscale. This nanoscale science and nanostructure engineering have well demonstrated in the fabrication of sensors/transducers devices with faster response time and better sensitivity then the planer version of the sensor’s configurations. Nanotechnology is not just to grow/fabricate nanostructures by just mixing nanoscale materials together but it requires the ability to understand and to precisely manipulate and control of the developed nanomaterials in a useful way. Nanotechnology is aiding to substantially improve, even revolutionize, many technology and industry sectors like information technology, energy, environmental science, medicine/medical instrumentation, homeland security, food safety, and transportation, among many others. Such applications of nanotechnology are delivering in both expected and unexpected ways on nanotechnology’s promise to benefit the society. The semiconductor ZnO with wide band gap (~ 3.37 eV) is a distinguish and unique material and its nanostructures have attracted great attention among the researchers due to its peculiar properties such as large exciton binding energy (60 meV) at room temperature, the high electron mobility, high thermal conductivity, good transparency and easiness of fabricating it in the different type of nanostructures. Based on all these fascinating properties, ZnO have been chosen as a suitable material for the fabrication of photonic, transducers/sensors, piezoelectric, transparent and spin electronics devices etc. The objective of the current study is to highlight the recent developments in materials and techniques for electrochemical sensing and hetrostructure light emitting diodes (LEDs) luminescence properties based on the different ZnO nanostructures. The sensor devices fabricated and characterized in the work were applied to determine and monitor the real changes of the chemical or biochemical species. We have successfully demonstrated the application of our fabricated devices as primary transducers/sensors for the determination of extracellular glucose and the glucose inside the human fat cells and frog cells using the potentiometric technique. Moreover, the fabricated ZnO based nanosensors have also been applied for the selective determination of uric acid, urea and metal ions successfully. This thesis relates specifically to zinc oxide nanostructure based electrochemical sensors and photonic device (LED) applications.

Tin disulfide (SnS2) is a two-dimensional (2D) material with excellent properties and high prospects for low-cost solutions to catalytic and optoelectronic applications. In this work, vertical nanoflakes of SnS2 have been synthesized using custom-designed close space sublimation (CSS) system and investigated for applications in photoelectrochemical (PEC) water oxidation and metal-semiconductor-metal (MSM) photodetector. For the PEC application, vertical SnS2 nanoflakes grown directly on transparent conductive substrates have been used as photoanodes, which produce record photocurrents of 4.5 mA cm−2 for oxidation of a sulfite hole scavenger and 2.6 mA cm−2 for water oxidation without any hole scavenger, both at 1.23 VRHE in neutral electrolyte under simulated AM1.5G sunlight, and stable photocurrents for iodide oxidation in acidic electrolyte. This remarkable performance has been attributed to three main reasons: (1) high intrinsic carrier mobility of 330 cm2 V−1 s−1 and long photoexcited carrier lifetime of 1.3 ns in the nanoflakes, (2) the nanoflake height that balances the competing requirements of light absorption and charge transport, and (3) the unique stepped morphology of these nanoflakes that improves photocurrent by exposing multiple edge sites in every nanoflake. In another application, these SnS2 nanoflakes have been used to enhance the performance of lead sulfide quantum dot (PbS QDs) photodetectors by providing a high-mobility channel for photoexcited charges from PbS QDs, which results in 2 orders of magnitude enhancement in responsivity. The physical models and experimental findings presented in this dissertation can help engineer more cost-effective solutions for PEC water splitting and optoelectronics based on 2D metal dichalcogenides.

The research work in this thesis has been devoted to understand the formation mechanism of single-crystalline and textured polycrystalline nanoparticles and nanoflakes of SmCo5 and Nd2Fe14B prepared by surfactant-assisted (SA) ball milling and to study their microstructural and magnetic properties. The nanoparticles and nanoflakes are promising candidates to be used as hard magnetic phase for the fabrication of high-energy-density exchange-coupled nanocomposite magnets. The influence of a range of different surfactants, solvents and milling parameters on the characteristics of ball-milled powder has been systematically investigated. Small fraction (~10 wt.%) of SmCo5 nanoparticles of average diameter 15 nm along with textured polycrystalline nanoflakes of average diameter 1 µm and average thickness of 100 nm were obtained after SA – ball milling of SmCo5 powder. Isolated single-crystalline particles (200-500 nm) and textured polycrystalline flakes (0.2-1.0 µm) of Nd2Fe14B have been prepared in bulk amount (tens of grams), after SA – ball milling of dynamic-hydrogen-disproportionation-desorption-recombination (d-HDDR) processed Nd2Fe14B powder. These single-crystalline Nd2Fe14B particles are promising for their microstructure for the fabrication of exchange-coupled nanocomposite permanent magnets. The SmCo5 and Nd2Fe14B flakes and particles were well aligned in magnetic field: the former showed [001] out-of-plane orientation while the latter showed [001] in-plane orientation. A maximum degree of texture values of 93 % and 88 % have been obtained for the magnetically-oriented SmCo5 flakes and Nd2Fe14B single-crystalline particles, respectively. SA – ball milling resulted in an increase of coercivity of SmCo5 particles from 0.45 T for un-milled powder to a maximum value of 2.3 T after 1 h of milling, further milling resulted in a decrease of the coercivity. The coercivity of SA – ball-milled Nd2Fe14B particles decreased drastically from 1.4 T for un-milled d-HDDR powder to 0.44 T after 0.5 h of milling, isolated single-crystalline particles (200-500 nm size) obtained after 4 h of SA – ball milling showed a coercivity of 0.34 T. The drastic decrease in coercivity of ball-milled Nd2Fe14B particles is attributed to the morphological change because the demagnetization in Nd2Fe14B magnets is governed by nucleation mechanism. A remarkable enhancement in coercivity from 0.26 T to 0.70 T for ethanol-milled sample and from 0.51 T to 0.71 T for oleic-acid-milled samples has been obtained after an optimum heat-treatment at 400 0C. An increase of α-Fe and Nd2O3 phase contents and a sharp change of lattice parameter c of Nd2Fe14B was observed when heat-treating above 400 0C. The change in lattice parameter at higher temperature is thought to be due to partial substitution of carbon atoms present in the surfactant or solvent, for boron atoms. / Das Ziel dieser Arbeit ist es, den Mechanismus der Herstellung von einkristallinen und texturierten polykristallinen Nanopartikeln und Nanoflakes aus SmCo5 und Nd2Fe14B durch Tensid-gestütztes Kugelmahlen zu verstehen. Des Weiteren soll deren Gefüge und magnetische Eigenschaften untersucht werden. Die Nanopartikel sind vielversprechende Kandidaten zur Verwendung als hartmagnetische Phase in hochentwickelten, austauschgekoppelten Nanokomposit-Magneten. Der Einfluß der Art der verwendeten Tensid, Lösungsmittel sowie Mahlparameter auf die Eigenschaften der kugelgemahlenen Pulver wurde systematisch untersucht. Ein kleiner Anteil (~10 m.%) von SmCo5 Nanopartikeln mit mittlerem Durchmesser von 15 nm zusammen mit texturierten polykristallinen Plättchen mit mittlerem Durchmesser von 1 µm und mittlerer Dicke von 100 nm wurden nach Tensid-gestütztes Kugelmahlen erzeugt. Alleinstehende einkristalline Partikel (200-500 nm) und texturierte polykristalline Plättchen (0,2-1,0 µm) aus Nd2Fe14B wurden in größeren Mengen (einige 10 g) hergestellt. Das verwendete Ausgangspulver wurde hierbei durch dynamisches-Hydrierung-Disproportionierung-Desorption-Rekombination (d-HDDR) hergestellt und anschließend Tensid-gestütztes Kugelmahlen. Die genannten einkristallinen Nd2Fe14B Partikel sind ebenfalls vielversprechend als hartmagnetischer Bestandteil von austauschgekoppelten Nanokomposit-Magneten. Die SmCo5- und Nd2Fe14B-Plättchen und -Partikel wurden alle in einem Magnetfeld ausgerichtet: erstere zeigten aus der Ebende herauszeigende und letztere in der Ebene liegende [001]-Orientierung. Ein maximaler Texturgrad von 93% wurde für im Magnetfeld ausgerichtete SmCo5 flakes bzw. 88% für einkristalline Nd2Fe14B Partikel erzielt. Tensid-gestütztes Kugelmahlen führte zu einem Anstieg der Koerzitivfeldstärke von SmCo5 Partikeln von 0,45 T für ungemahlenes Pulver auf 2,3 T nach einer Mahldauer von 1 h. Weiteres Mahlen führte zu einem Abfall der Koerzitivfeldstärke. Die Koerzitivfeldstärke von Tensid-gestütztes Kugelmahlen Nd2Fe14B Partikeln verringerte sich stark von 1,4 T von ungemahlenem d-HDDR Pulver auf 0,44 T nach 0,5 h Mahlen. Freistehende einkristalline Partikel (200-500 nm groß), welche nach 4 h Tensid-gestütztes Kugelmahlen erhalten wurden, zeigten eine Koerzitivfeldstärke von 0,34 T. Der starke Abfall der Koerzitivfeldstärke von gemahlenen Nd2Fe14B Partikeln wird die morphologischen Veränderungen zurückgeführt, da die Ummagnetisierung nukleationsgesteuert ist. Ein bemerkenswerter Anstieg der Koerzitivfeldstärke von 0,26 T auf 0,70 T wurde für eine in Ethanol gemahlene Probe verzeichnet, sowie ein Anstieg von 0,51 auf 0,71 T für eine Probe, welche mit einer Zugabe von Oleinsäure gemahlen wurde. Beide Proben wurden einer optimierten Wärmebehandlung bei 400°C unterzogen. Bei höheren Temperaturen wurde für Nd2Fe14B ein Anstieg der Menge an α-Fe und Nd2O3 gefunden und eine sprungartige Veränderung des Gitterparameters c der Nd2Fe14B Phase. Die Veränderung des Gitterparameters wird auf die partielle Substitution von Kohlenstoffatomen des Tensid oder Lösungsmittels gegen Boratome zurückgeführt.

No description available.