Dissertations / Theses on the topic 'Colloidal Nanomaterisls'

Neste trabalho, foi realizado um estudo da obtenção de nanomateriais luminomagnéticos visando potenciais aplicações biológicas, a partir de dois diferentes tipos de estruturas, sendo elas: a formação de heteronanoestruturas luminomagnéticas de NPM de FePt/Fe3O4-CdSe recobertas com sílica; e a formação de nanomateriais luminomagnéticos por ligação covalente entre NPM de FePt/Fe3O4-Dopa-PIMA-PEG-NH2 e pontos quânticos de CdSe/ZnS-LA-PEG-COOH. Para o primeiro tipo de nanomaterial citado, foram testadas duas metodologias para obtenção das heteronanoestruturas: a mudança da estabilidade coloidal pela adição de pequenas quantidades de NaCl no meio contendo as NPM e os pontos quânticos previamente sintetizados; e o método de injeção a quente do precursor de selênio em um meio contendo as NPM como sementes, o precursor de cádmio e os agentes de superfície. O método de injeção a quente foi o que apresentou melhores condições para a formação das heteronanoestruturas. Para providenciar estabilidade coloidal em meio aquoso e superfície com biocompatibilidade, foi realizado o recobrimento com sílica na superfície das heteronanoestruturas luminomagnéticas com melhores condições. Para essa amostra, o tamanho médio obtido foi de 25,0 nm, com polidispersividade de 8,4 %, Ms = 11,1 emu.g-1 e comportamento superparamagnético, além de duas bandas de emissão (com excitação de 400 nm) centradas em 452 nm e 472 nm, respectivamente. Já para o segundo tipo de nanomaterial obtido neste trabalho, foram primeiramente obtidas NPM de FePt/Fe3O4 pelo método do poliol modificado acoplado à metodologia do crescimento, e pontos quânticos luminescentes de CdSe/ZnS pelo método de decomposição térmica de precursores organometálicos, sendo que ambas nanoestruturas apresentaram superfície hidrofóbica. Para a troca de ligantes para transferência das nanoestruturas para a fase aquosa e para providenciar biocompatibilidade visando aplicações biológicas, foram previamente preparados ligantes poliméricos de Dopa-PIMA-PEG-NH2 para recobrimento das NPM e de LA-PEG-COOH para recobrimento dos pontos quânticos. A conjugação química entre as nanoestruturas de FePt/Fe3O4-Dopa-PIMA-PEG-NH2 e CdSe/ZnS-LA-PEG-COOH foi realizada pelo método da carbodiimida em solução aquosa para a formação de uma ligação covalente amida entre os grupos amina e carboxilato em cada uma das nanoestruturas. Os nanomateriais luminomagnéticos obtidos apresentaram estabilidade coloidal em meio aquoso, com estreita distribuição de tamanho, apresentando RH de 79,96 nm, Ms de, aproximadamente, 10 emu.g-1 com coercividade e remanência quase nulos e intensa banda de emissão centrada em 580 nm. Espera-se que os nanomateriais obtidos neste trabalho possam ser promissores nanomateriais com propriedades multifuncionais para potenciais aplicações biológicas.
Here, luminomagnetic nanomaterials were obtained for potential biological applications. We have studied two different types of luminomagnetic nanomaterials, which are: formation of silica-coated FePt/Fe3O4-CdSe heteronanostructures; and formation of luminomagnetic nanomaterials from covalent bond between FePt/Fe3O4-Dopa-PIMA-PEG-NH2 magnetic nanoparticles and CdSe/ZnS-LA-PEG-COOH luminescent quantum dots. For the first type of luminomagnetic nanomaterials obtained, two methodologies were studied for formation of heteronanostructures, which are: modification of colloidal stability by addition of small amounts of NaCl into a solution with hydrophobic magnetic nanoparticles and luminescent quantum dots; and hot injection method of selenium precursor into a solution with magnetic nanoparticles seeds, cadmium precursors and surface agents. The hot injection method obtained better results than the other method studied for formation of heteronanostructures. To provide colloidal stability in aqueous solution and biocompatibility, the heteronanostructures were coated using silica shell. After silica coating, the heteronanostructures showed average diameter of 25 nm and polidispersivity of 8.4%, with Ms = 11.1 emu.g-1 and superparamagnetic behavior. Moreover, these nanomaterials showed two emission peaks centered at 452 and 472 nm. For the second type of nanomaterials obtained, FePt/Fe3O4 magnetic nanoparticles were synthesized by modified polyol method coupled to seeded-mediated growth, and CdSe/ZnS luminescent quantum dots were obtained by thermal decomposition of organometallic precursors. For the ligand exchange to transfer the nanostructures from organic media to aqueous solution, were used Dopa-PIMA-PEG-NH2 and LA-PEG-COOH polymers to provide colloidal stability and biocompatibility on magnetic nanoparticle surface and quantum dot surface, respectively. The chemical conjugation between FePt/Fe3O4-Dopa-PIMA-PEG-NH2 and CdSe/ZnS-LA-PEG-COOH nanostructures was obtained by EDC coupling in aqueous solution, which linked amine and carboxylate groups in each nanostructure to provide the formation of amide bond. The luminomagnetic nanomaterials obtained showed colloidal stability in aqueous solution, narrow size distribution, with RH equal to 79.96 nm, MS around 10 emu.g-1 with low coercivity and remanent magnetization, and intense emission peak centered at 580 nm. We expect these luminomagnetic nanomaterials be promisor nanomaterials with multifunctional properties for potential biological applications.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 2006.
Includes bibliographical references.
This thesis focuses on microfluidics based approaches for synthesis and surface-engineering of colloidal particles. Bottom-up assembly through colloidal nucleation and growth is a popular route to the controlled synthesis of nanomaterials. Standard bench-scale synthetic chemistry techniques often involve non-uniform spatial and temporal distributions of concentration and temperature, and are not readily scalable. Photolithography-based microfabrication enables the application of classical techniques of chemical reaction engineering to design chemical reactors that cannot be realized easily at the macroscale, and that closely approach theoretical 'idealized' reactor configurations. In addition, the microfluidic format allows precisely controlled reaction conditions such as rapid mixing, and concentration and temperature uniformity. The goal of this thesis was to design microfluidic reactors for synthesis of core-shell colloidal particles with tunable sizes. Microscale segmented gas-liquid flows overcome the large axial dispersion effects associated with single-phase laminar flows. Microchannel devices that yielded uniform, stable gas-liquid segmented flows over three orders of magnitude in flow velocity were first developed.
(cont.) Extensive experimental studies of the transport, dynamics and stability of such flows were then conducted with pulsed-laser fluorescent microscopy, optical stereomicroscopy and micro particle image velocimetry (-PIV). Flow segmentation not only reduces axial dispersion, but also allows rapid micromixing of miscible liquids through internal recirculations in the liquid phase. This added functionality is especially useful in syntheses involving colloidal particles that, due to inherently low diffusivity, cannot be rapidly mixed by laminar diffusive techniques. Continuous segmented flow reactors were then developed for the synthesis of colloidal silica and titania particles by sol-gel chemistry. Particle sizes could be tuned by varying the rates of flow of reactants, or by varying the chip temperature. Particle size distributions comparable to or narrower than the corresponding stirred-flask synthesis, with little agglomeration or shape distortion were obtained. Coating of colloidal particles with one or more layers of different materials is used to modify their optical, chemical or surface properties. Core-shell particles are often prepared by controlled precipitation of inorganic precursors onto core particles.
(cont.) Synthesis of such structures requires precise control over process parameters to prevent precipitation of secondary particles of shell material and agglomeration of primary particles. Particles coated with titania are exceptionally difficult to synthesize due to the high reactivity of the titania precursors, which makes controlled precipitation difficult. A novel continuous flow microfluidic reactor with sequential multi-point precursor addition was developed for colloidal overcoating processes. Silica particles were coated with uniform titania layers of tunable thickness by the controlled hydrolysis of titanium ethoxide, with no secondary particle formation or agglomeration. An integrated reactor for continuous silica synthesis and in-situ series overcoating with titania was then developed using a two-level stacked reactor fabrication process. Finally, multi-step nanomaterials synthesis and surface coating with incompatible chemistries requires the development of microfluidic 'unit operations' equivalent to particle filtration. In this context, rapid, continuous microfluidic particle separation was demonstrated using transverse free-flow electrophoresis.
by Saif A. Khan.
Ph.D.

This thesis focuses on different aspects of NCs colloidal synthesis, the exploration of the relevant surface chemistries that afford NC assembly and the NC implementation into porous nanomaterials. The work is divided into two blocks. The first block is devoted to developing and optimizing the synthesis of NCs followed by the examination of their suitability for potential applications in catalysis and photocatalysis. The second block is dedicated to establish procedures to fabricate single-component or multicomponent porous nanomaterials from NC building blocks. To embrace the use of the developed strategies in different application fields, several kind of materials were under research. Namely, metals (e.g. Au), metal oxides (e.g. CeO2, TiO2, Fe2O3), metal chalcogenides (e.g. In2S3, ZnS, PbS, CuGaS2 and Cu2ZnSnSe4), and their composites. CeO2 NCs synthesis was deeply investigated with the aim to achieve a proper control on the NCs morphology, facets exposed, crystal phase, composition, etc., required for application. Overall, CeO2 NCs with spherical, octapod-like branched, cubic hyperbranched, and kite-like morphology with sizes in the range 7 to 45 nm were produced by adjusting experimental conditions of the synthetic protocol. Branched and hyperbranched NCs showed higher surface areas, porosities and oxygen capacity storage values compared to quasi-spherical NCs. The NCs morphology-controlled synthesis has been extended to quaternary Cu2ZnSnSe4 (CZTSe). CZTSe NCs with narrow size distribution and controlled composition were produced. It was shown how off-stoichiometric CZTSe compositions were characterized by higher charge carrier concentrations and thus electrical conductivities. The strategy to functionalize the metal oxide NC surface composition by applying different ligands is proposed. This enables to develop a novel approach to assemble metal oxide NCs into porous gel and aerogel structures. Propylene oxide has been found to trigger the gelation process of glutamine functionalized NCs. The detailed investigation of the gelation mechanism is demonstrated for the case of ceria. The method is applied for NCs with different morphologies. Eventually, the versatility of the concept is proved by using of the proposed approach for the TiO2 and Fe2O3 nanocrystals. The assembly method has been extended to metal chalcogenides - In2S3 NCs - starting from the NCs synthesis, with further surface chemistry manipulation and eventually follows by the NC assembly into gels and aerogels. The optimization of NC surface chemistry was achieved by testing different ligand exchange approaches via applying short-chain organic and inorganic ligands. The assembly method based on ligand desorption from the NC surface and chalcogenide-chalcogenide bond formation has been established for In2S3. The comparison of the different ligands impact on the NC performance in colloidal form, when assembled into gels and when supported onto substrate is investigated towards photoelectrocatalysis. The oxidative ligand desorption assembly approach has been extended for multicomponent NCs for the case of CuGaS2 and CuGaS2-ZnS. Optimization of spin-coating process of the formed NCs inks followed by applying of sol-gel chemistry led to formation of highly porous layers from TGA-CuGaS2 and TGA-ZnS. Applied results of CuGaS2/ZnS nanocrystal-based bilayers and CuGaS2–ZnS nanocrystal-based composite layers have been shown by testing their photoelectrochemical energy conversion capabilities. The approach to adjust NC surface chemistry has been proposed and tested for performing multicomponent NC assemblies. Applying of different ligands for NC surface functionalization endows their surface with different charges which usually provides colloidal NCs stabilization. It has been found that mixing of oppositely charged NCs with certain concentration enabled their assembly/gelation via electrostatic interaction. The proposed approach has been applied and optimized to produce multicomponent NC gels and aerogels. The detailed investigation of the gelation mechanism is shown for combination of metal-metal oxide and metal oxide-metal chalcogenide NCs (Au-CeO2, CeO2-PbS). Applied results of the Au-CeO2 aerogels were demonstrated for CO-oxidation.
Esta tesis se centra en la síntesis coloidal de nanocristales (NCs), en la exploración de su química de superficie y en su ensabanado en nanomateriales porosos funcionales. Para demostrar la versatilidad de aplicación de dichas estructuras, en este estudio se han considerado NCs de distintos tipos de materiales: metales (Au), óxidos metálicos (CeO2, TiO2, Fe2O3), calcogenuros metálicos (In2S3, ZnS, PbS, CuGaS2,Cu2ZnSnSe4) y sus materiales compuestos. El trabajo se dividió en dos bloques. En el primero se desarrolló y optimizó la síntesis de NCs de óxidos y calcogenuros metálicos y se evaluó su potencial para aplicaciones de catálisis y fotocatálisis. Se investigó en profundidad la síntesis de NCs de CeO2, poniendo énfasis en controlar su morfología. Se consiguió producir NCs de CeO2 de forma controlada (esférica, octapodo ramificado, cúbico ramificado y romboidal) y con tamaño controlado (7-45 nm). Asimismo, se obtuvieron NCs de Cu2ZnSnSe4 con una fina distribución de tamaños y composición controlada. En el segundo bloque se establecieron y estudiaron procedimientos para fabricar nanomateriales porosos mono- o multicomponentes a partir del ensamblado de NCs. Se desarrolló una estrategia basada en el ajuste de la química de superficie de NCs de óxidos metálicos (CeO2, Fe2O3,TiO2) y de calcogenuros metálicos (In2S3, CuGaS2-ZnS) que permitió su ensamblaje controlado en estructuras porosas de tipo gel y aerogel. En el caso de los óxidos metálicos, se determinó que el ensamblado se inicia con la adición de un epóxido a NCs funcionalizados con glutamina, causando la gelación. La desorción oxidativa de ligandos basada en la formación de enlaces calcogenuro-calcogenuro se propuso como mecanismo de gelación en calcogenuros mono- (In2S3) y multicomponente (CuGaS2-ZnS). Se investigó el impacto del empleo de distintos ligandos en la eficiencia foto-electrocatalítica de NCs en forma coloidal, ensamblados en geles y soportados en sustratos. Se desarrolló y estudió el ajuste de la química de superficie de NCs para la obtención de ensamblajes multicomponente mediante interacción electrostática de coloides en suspensión. El mecanismo de gelación fue investigado al detalle para materiales compuestos de NCs de oxido metálico (CeO2) con NCs de óxido de calcogenuro (PbS-CeO2) y metálicos (Au-CeO2). Los aerogeles de Au-CeO2 demostraron potencial para la oxidación de CO.

La investigación de esta tesis se centra en la producción de nanoestructuras metálicas dentro de aceites orgánicos y nano-ensambles por ablación láser en líquidos para resolver los mayores problemas en su producción por métodos convencionales: poca estabilidad, producción de residuos químicos y reacciones químicas sin control debido a problemas de pureza. En particular, las mayores contribuciones son, la síntesis de nanofluidos basados en nanopartículas de oro que pueden ser utilizados como absorbentes volumétricos de luz e intercambiadores de calor. La fabricación de un nanofluido con una mejora de conductividad térmica de 4,06% sobre un fluido de transferencia de calor comercial, una mezcla eutéctica de óxido de bifenilo y difenilo, y la mejor estabilidad coloidal reportada en la literatura usando estos materiales. Y, por último, la demostración de la reducción parcial de láminas de óxido de grafeno y su decoración con nanopartículas de oro con ligandos libres, en un solo paso.
The research conducted during this thesis work is focused on producing metal nanostructures inside organic oils and nano-essambles by Pulsed Laser Ablation in Liquids (PLAL) to solve the biggest issues on their production by conventional approaches: Poor stability, production of chemical waste and uncontrolled chemical reactions due to purity problems. In particular, the biggest contributions achieved on the present work, lies on the experimental demonstration of the synthesis of gold nanoparticles-based aqueous nanofluids that can be used as both volumetric light absorbers and heat exchangers. The fabrication of a nanofluid with a thermal conductivity enhancement of 4.06% over a commercial heat transfer fluid, an eutectic mixture of biphenyl and diphenyl oxide, and the best colloidal stability reported in the literature using these materials. And finally, demonstration of partial reduction of graphene-oxide sheets and its decoration with ligand-free gold nanoparticles, in a single reaction stage avoiding the production of chemical waste.

We have developed the technology of using the colloidal solution of metal nanoparticles as fertilizers, which characterized by easiness to use, environmental safety and absence of corrosive properties. Colloidal solutions of biogenic metals, water-based, such as Fe, Mn, Zn, Mo, Co, Cu, and Ag, produced by a patented method of bittern natural colloidal solutions of the above metals were used. Seed treatment with colloidal solution of metal nanoparticles stored genetic purity grade, increased plant immune status via regulation of oxidative metabolism, photosynthetic activity, resistance to pathogens, and optimization of water regime of various winter wheat ecotypes during ontogenesis. Results of industrial tests proved that it is environ-mentally safe and economically feasible, since the cost of one liter of colloidal solutions of nanoparticles of metals ranges from 50-70 USD providing 500% level profitability. So, for the first time managed to opti-mize the function of biogenic metals through the use of physical and chemical characteristics of colloidal nanoparticle solutions to realize the productive potential of plants. When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/35441

Permanent magnets are used heavily for multiple applications in industry and current electronic technologies. However, the current permanent landscape is muddled by high cost of materials and insufficient magnetic or thermal properties. The primary focus of this dissertation work is the synthesis and optimization of a new permanent magnetic material, in the form of cobalt carbide nanomaterials. The optimization revolved around controlling the crystal phase and particle shape of synthesized cobalt carbide particles; these parameters have significant impact on the observed magnetic properties of magnetic nanoparticles. Co3C was identified to be the preferred crystal phase, leading to better magnetic properties. Cobalt Fumarate was found to be the ideal precursor to synthesize anisotropic Co3C particles and enhance magnetic properties of the synthesized cobalt carbide particles. Lastly, an ethanol based reduction system was employed to develop the greener synthesis of Co and Ni magnetic particles.

The work developed during this PhD has embraced several topics that I divide in three blocks. Each block contains two chapters in this dissertation. Additionally, a general introduction of the different topics is provided (Chapter 1). The first block corresponds to the study of colloidal synthetic routes to produce functional nanoparticles (Chapter 2 and 3). In the second block the developed nanoparticles are used to produce bulk nanostructured materials. The functional properties of the nanomaterials are also characterized in this second block. As the paradigmatic application for such bottom-up assembled nanostructured materials I considered thermoelectricity (Chapter 4 and 5). In the last block, I go one step beyond and design and prepare multiphase nanoparticles as building blocks for the bottom up production of nanocomposites with improved thermoelectric performance (Chapter 6 and 7).
El treball desenvolupat durant aquesta tesi doctoral engloba diverses temàtiques que s’han dividit en tres blocs. Cada bloc conté dos capítols. A més a més, com a Capítol 1 s’ha inclòs una introducció general de cadascuna de les temàtiques tractades. En el primer bloc, Capítols 2 i 3, s’estudien diferents síntesis col·loïdals per produir nanopartícules funcionals. En el segon bloc, Capítols 4 i 5, les nanopartícules desenvolupades s'utilitzen per produir materials nanoestructurats en bulk a partir del seu assemblatge. Les propietats funcionals d’aquests nanomaterials es caracteritzen també en aquest segon bloc. Com a aplicació paradigmàtica s’ha considerat la termoelectricitat. En l'últim bloc, Capítols 6 i 7, es va un pas més enllà i es dissenyen nanopartícules heterogènies com blocs de construcció per a la producció de nanocompostos amb millor rendiment termoelèctic.

Los polímeros de coordinación y su diseño racional permiten la formación de materiales nanoestructurados con una amplia variedad de propiedades. Las múltiples combinaciones entre iones metálicos y ligandos orgánicos como precursores de materiales autoensamblados, han fascinado a los científicos durante décadas. La aplicación de la química de coordinación a nanoescala se considera uno de los enfoques más versátiles para el desarrollo de nuevos materiales nanoestructurados debido a las infinitas posibilidades para alcanzar propiedades sin precedentes. Además, el desarrollo de sistemas metal-orgánicos ha despertado en una gran cantidad de ejemplos para su uso en una amplia gama de aplicaciones. En esta Tesis hemos estado particularmente interesados ​​en el ajuste controlado de las propiedades de los materiales nanoestructurados basados ​​en polímeros de coordinación que se obtuvieron a través de diferentes rutas sintéticas. El método de síntesis, la selección adecuada de precursores y el estudio de las propiedades finales han centrado el trabajo realizado. Además, la formación de suspensiones coloidales estables en agua se estableció como un requisito principal para su potencial aplicación. Para eso, fue necesaria una sinergia multidisciplinaria con el objetivo de buscar la aplicación final de los nuevos materiales nanoestructurados desarrollados. El logro de este objetivo fue posible gracias a un diseño adecuado de la estrategia seguida junto con la caracterización completa de las nanoestructuras preparadas. En una primera parte de esta Tesis, la nanoestructuración de sistemas conmutables basados ​​en Fe(II) con comportamiento de entrecruzamiento de espín (SCO por sus siglas en inglés) se logró siguiendo dos estrategias diferentes. Por un lado, se aplicó una metodología descendente (top-down) basada en la exfoliación en fase líquida para el aislamiento de láminas 2D de cristales multilaminares. Por otro lado, a través de un enfoque ascendente (bottom-up), la síntesis de nuevas nanopartículas fue posible modulando la difusión de la reacción utilizando metodologías basadas en microfluidica. En ambos casos, los materiales nanoestructurados se integraron en matrices poliméricas para evaluar su aplicación potencial como películas termocrómicas para su prueba de concepto. En la segunda parte de la Tesis, se estableció una nueva familia de polímeros de coordinación a nanoescala (NCP por sus siglas en inglés) basados ​​en Fe(III), Gd(III), Mn(II), In(III) y Cu(II) a través de su síntesis racional usando una reacción en un sola etapa. Las nanopartículas obtenidas se validaron mediante pruebas preclínicas in vivo que muestran un rendimiento interesante como posibles agentes teranósticos para la obtención de imágenes (resonancia magnética, tomografía por emisión de positrones y tomografía computarizada por emisión de fotón único) y pretratamiento potencial de glioblastoma y enfermedades pulmonares.
Coordination polymers and its rational design let the formation of nanostructured materials with a broad variety of properties. The multiple combinations between metal ions and organic ligands as precursors of self-assembled materials have fascinated scientists for decades. The application of coordination chemistry at the nanoscale is considered one of the most versatile approaches for the development of new nanostructured materials due to the infinite possibilities for reaching unprecedented properties. Furthermore, the development of metal-organic systems has aroused in a plethora of examples for their use in a wide range of applications. In this Thesis we have been particularly interested in the fine tune of the properties of nanostructured materials based on coordination polymers whose were obtained through different synthetic routes. The method of synthesis, the properly selection of precursors and the study of the final properties has centred the work carried out. Additionally, the formation of water-stable colloidal suspensions was stablished as a main requirement for their potential application. For that, a multidisciplinary synergy was necessary with the aim to pursue the final application of the novel nanostructured materials developed. Achieving this objective was possible thanks to a properly design of the strategy followed together with complete characterization of the nanostructures prepared. In a first part of this Thesis, the nanostructuration of Fe(II)-based switchable systems with spin crossover behaviour was achieved by following two different strategies. On the one hand, a top-down methodology based on liquid-phase exfoliation was applied for the isolation of 2D flakes from the bulk crystal. On the other hand, through a bottom-up approach, the synthesis of novel nanoparticles was possible by modulating the reaction diffusion using microfluidic based methodologies. In both cases, the nanostructured materials were integrated in polymeric matrices to evaluate their potential application as proof-of-concept thermochromic films. In the second part of the Thesis, a novel family of nanoscale coordination polymers (NCPs) based on Fe(III), Gd(III), Mn(II), In(III) and Cu(II) was stablished through its rational synthesis by using one-pot reaction. The nanoparticles obtained were validated by pre-clinical in vivo tests showing interesting performance as potential theranostic agents for imaging (Magnetic resonance imaging, positron emission tomography and single-photon emission computed tomography) and potential pre-treatment of glioblastoma and lung diseases.

This thesis studies lead suphide (PbS) colloidal quantum dots and their photovoltaic applications. Different sizes of PbS QDs were synthesised and characterised using absorption spectroscopy and transmission electron microscopes. PbS QD Schottky junction devices were fabricated with AM1.5 power conversion efficiency up to 1.8 %. The Schottky junction geometry limits the device performance. A semiconductor heterojunction using ZnO as an electron acceptor was built and the device efficiency increased to 3%. By studying the light absorption and charge extraction profile of the bilayer device, the absorber layer has a charge extraction dead zone which is beyond the reach of the built-in electric field. Therefore, strategies to create a QD bulk heterojunction were considered to address this issue by distributing the junction interface throughout the absorber layer. However, the charge separation mechanism of the QD heterojunction is not clearly understood: whether it operates as an excitonic or a depleted p-n junction, as the junction operating mechanism determines the scale of phase separation in the bulk morphology. This study shows a transitional behaviour of the PbS/ZnO heterojunction from excitonic to depletion by increasing the doping density of ZnO. To utilise the excitonic mechanism, a PbS/ZnO nanocrystal bulk heterojunction was created by blending the two nanocrystals in solution such that a large interface between the two materials could facilitate fast exciton dissociation. However, the devices show poor performance due to a coarse morphology and formation of germinate pairs. To create a bulk heterojunction where a built-in electric field could assist the charge separation, a TiO₂ porous structure with the pore size matching with the depletion width was fabricated and successfully in-filled by PbS QDs. The porous device produces 5.7% power conversion efficiency, among one of the highest in literature. The enhancement comes from increased light absorption and suppression of charge recombination.

This dissertation presents results of a project bringing Scanning Tunneling Microscope (STM) into a regime of unlimited operational time at cryogenic conditions. Freedom from liquid helium consumption was achieved and technical characteristics of the instrument are reported, including record low noise for a scanning probe instrument coupled to a close-cycle cryostat, which allows for atomically resolved imaging, and record low thermal drift. Subsequent studies showed that the new STM opened new prospects in nanoscience research by enabling Scanning Tunneling Spectroscopic (STS) spatial mapping to reveal details of the electronic structure in real space for molecules and low-dimensional nanomaterials, for which this depth of investigation was previously prohibitively expensive. Quantum-confined electronic states were studied in single-walled carbon nanotubes (SWCNTs) deposited on the Au(111) surface. Localization on the nanometer-scale was discovered to produce a local vibronic manifold resulting from the localization-enhanced electron-vibrational coupling. STS showed the vibrational overtones, identified as D-band Kekulé vibrational modes and K-point transverse out-of plane phonons. This study experimentally connected the properties of well-defined localized electronic states to the properties of associated vibronic states. Electronic structures of alkyl-substituted oligothiophenes with different backbone lengths were studied and correlated with torsional conformations assumed on the Au(111) surface. The molecules adopted distinct planar conformations with alkyl ligands forming cis- or trans- mutual orientations and at higher coverage self-assembled into ordered structures, binding to each other via interdigitated alkyl ligands. STS maps visualized, in real space, particle-in-a-box-like molecular orbitals. Shorter quaterthiophenes have substantially varying orbital energies because of local variations in surface reactivity. Different conformers of longer oligothiophenes with significant geometrical distortions of the oligothiophene backbones surprisingly exhibited similar electronic structures, indicating insensitivity of interaction with the surface to molecular conformation. Electronic states for annealed ligand-free lead sulfide nanocrystals were investigated, as well as hydrogen-passivated silicon nanocrystals, supported on the Au(111) surface. Delocalized quantum-confined states and localized defect-related states were identified, for the first time, via STS spatial mapping. Physical mechanisms, involving surface reconstruction or single-atom defects, were proposed for surface state formation to explain the observed spatial behavior of the electronic density of states. This dissertation includes previously published co-authored material.

The bottom-up engineering of nanomaterials using solution-processing strategies is of particular interest for reducing cost and optimizing the performance of TE materials and devices. This thesis focuses on the development of scalable methods for the production of TE nanomaterials with optimized performance. The thesis is divided into 5 chapters. Chapter 1 introduces solution-based approaches for producing functional nanomaterials and the general state of the art in the field of thermoelectricity. Chapter 2 and chapter 3 present a fast and simple molecular ink-based method to produce low cost and crystallographically textured SnSe2 and SnSe nanomaterials. Molecular ink printing techniques could offer a scalable approach to fabricate TE devices on flexible substrates. In these chapters, I proved that cost-effective p-type SnSe NPLs could be produced by a molecular ink-based strategy that allowed introducing controlled amounts of Te to achieve unprecedentedly high TE figure of merit. On the other hand, n-type SnSe2 nanomaterials were also intentionally produced from the same strategy to complement an all Sn-Se based device. Both of the bulk nanomaterials displayed significant crystallographic texture after hot pressing, resulting in highly anisotropic charge and heat transport properties. Different approaches were applied to optimize their TE performance: SnSe2 NPLs were blended with metal NPs to produce a metal-semiconductor NC. The electrical conductivities of the blends were significantly improved with respect to bare SnSe2 bulk nanomaterial and a three-fold increase in the TE figure of merit was obtained, reaching unprecedented values up to ZT = 0.65 for SnSe2 material. For SnSe nanomaterials, I demonstrate that the introduction of small amounts of tellurium in the precursor ink allowed reducing the band gap, increasing both charge carrier concentration and mobility, especially cross plane, with a minimal decrease of the Seebeck coefficient. This strategy translated into record out of plane ZT values at 800 K, ZT=1.05 Chapter 4 and chapter 5 describe two different strategies to produce Bi2Te3-Cu2-xTe NCs based on the consolidation of nanostructured building blocks. I first detail a two-step solution-based process to produce the Bi2Te3-Cu2-xTe heteronanostructures, based on the growth of Cu2-xTe nanocrystals on the surface of Bi2Te3 nanowires. The transport properties of the NCs are investigated as a function of the amount of Cu introduced, which reveal that the presence of Cu decreases the material thermal conductivity through promotion of phonon scattering, modulates the charge carrier concentration through electron spillover, and increases the Seebeck coefficient through filtering of charge carriers at energy barriers. These effects result in an improvement of over 50% of the TE figure of merit of Bi2Te3. As comparison, I produced Bi2Te3-Cu2-xTe NCs by directly mixing proper ratio of individual Bi2Te3 nanowires with Cu2-xTe nanocubes and consolidating the resulting NP mixture by hot-press. A significant difference of transport properties was detected when compared with NCs fabricated by hot-pressing heterostructured Bi2Te3-Cu2-xTe nanowires. On the contrary to the composite obtained from hetero- nanostructures, the presence of Cu2-xTe nanodomains did not lead to a significant reduction of the lattice thermal conductivity of the reference Bi2Te3, which is already very low here, but it resulted in a nearly threefold increase of its power factor. Additionally, the presence of Cu2-xTe resulted in a strong increase of the Seebeck coefficient. This increase is related to the energy filtering of charge carriers at energy barriers within Bi2Te3 domains created by the accumulation of electrons in the regions nearby Cu2-xTe/Bi2Te3 junctions. Overall, a significant improvement of figure of merit, up to a 250%, was obtained with the suitable combination of Cu2-xTe NPs and Bi2Te3 nanowires. Finally, the main conclusions of this thesis and some perspectives for future work are presented.
La ingeniería de nanomateriales a partir del procesado en solución es de particular interés para optimizar el rendimiento de los materiales y dispositivos termoeléctricos. . Esta tesis estáse centra en el diseño y el ensamblaje racional de nanomateriales termoeléctricos de alto rendimiento a través de procesado en solución. La tesis se divide en 5 capítulos. El Capítulo 1 aborda la introducción fundamental del enfoque sintético para producir nanomateriales funcionales. Los capítulos 2 y 3 presentan un método rápido y simple basado en soluciones para producir nanomateriales SnSe2 y SnSe con textura cristalográfica. Dado que los calcogenuros de estaño son materiales especialmente interesantes para la conversión de energía termoeléctrica, se sintetizaron nanoplacas SnSe y SnSe2 controlables por forma mediante una estrategia basada en tinta molecular para lograr una figura de mérito termoeléctrica sin precedentes por dopaje con Te/Cu. Ambos nanomateriales mostraron una textura cristalográfica significativa después del prensado en caliente, lo que dio como resultado unas propiedades de transporte de carga calor altamente anisotrópicas. Los capítulos 4 y 5 describen dos estrategias diferentes para producir nanocompuestos Bi2Te3-Cu2-xTe basados en la consolidación de nanoestructuras. La presencia de Cu2-xTe da como resultado un fuerte aumento del coeficiente de Seebeck. Este aumento está relacionado con el filtrado de los portadores de carga en función de su energía en las barreras de energía dentro de los dominios Bi2Te3 creados por la acumulación de electrones en las regiones cercanas a las uniones Cu2-xTe / Bi2Te3. En general, se obtiene una mejora significativa de la figura de mérito con nanocompuestos Bi2Te3-Cu2-xTe. Finalmente, en el último capítulo se presentan las principales conclusiones de esta tesis y algunas perspectivas para trabajos futuros.

Monodispersed microgels composed of poly-acrylic acid (PAAc) and poly(N-isopropylacrylamide) (PNIPAM) interpenetrating networks were synthesized by 2-step method with first preparing PNIPAM microgel and then polymerizing acrylic acid that interpenetrates into the PNIPAM network. The semi-dilute aqueous solutions of the PNIPAM-PAAc IPN microgels exhibit an inverse thermo-reversible gelation. Furthermore, IPN microgels undergo the reversible volume phase transitions in response to both pH and temperature changes associated to PAAc and PNIPAM, respectively. Three applications based on this novel hydrogel system are presented: a rich phase diagram that opens a door for fundamental study of phase behavior of colloidal systems, a thermally induced viscosity change, and in situ hydrogel formation for controlled drug release. Clay-polymer hydrogel composites have been synthesized based on PNIPAM gels containing 0.25 to 4 wt% of the expandable smectic clay Na-montmorillonite layered silicates (Na-MLS). For Na-MLS concentrations ranging from 2.0 to 3.2 wt%, the composite gels have larger swelling ratio and stronger mechanical strength than those for a pure PNIPAM. The presence of Na-MLS does not affect the value of the lower critical solution temperature (LCST) of the PNIPAM. Surfactant-free hydroxypropyl cellulose (HPC) microgels have been synthesized in salt solution. In a narrow sodium chloride concentration range from 1.3 to 1.4 M, HPC chains can self-associate into colloidal particles at room temperature. The microgel particles were then obtained in situ by bonding self-associated HPC chains at 23 0C using divinyl sulfone as a cross-linker. The volume phase transition of the resultant HPC microgels has been studied as a function of temperature at various salt concentrations. A theoretical model based on Flory-Huggins free energy consideration has been used to explain the experimental results. Self-association behavior and conformation variation of long chain branched (LCB) poly (2-ethyloxazoline) (PEOx) with a CH3-(CH2)17 (C18) modified surface are investigated using light scattering techniques in various solvents. The polymer critical aggregation concentration (cac) strongly depends on solvent polarity, decreasing as the solvent becomes more hydrophobic.

Noble metal nanoparticles such as gold and silver are of interest because of the unique electro-optical properties (e.g., localized surface plasmon resonance [LSPR]) that originate from the collective behavior of their surface electrons. These nanoparticles are commonly developed and used for biomedical and industrial application. A recent report has predicted that the global market for gold nanoparticles will be over 12.7 tons by year 2020. However, these surface-functionalized nanoparticles can be potential environmental persistent contaminants post-use due to their high colloidal stability in the aquatic systems. Despite, the environmental risks associated with these nanoparticles, just a few studies have investigated the effect of nanofeature factors such as size and shape on the overall fate/transport and organismal uptake of these nanomaterials in the aquatic matrices. This study presents a comprehensive approach to evaluate the colloidal stability, fate/transport, and organismal uptake of these nanoparticles while factoring in the size and shape related properties. We demonstrate the importance and effect of anisotropicity of a gold nanoparticle on the colloidal behavior and interaction with ecologically susceptible aquatic biota. We also show how readily available characterization techniques can be utilized to monitor and assess the fate/transport of this class of nanoparticles. We further describe and investigate the relationship between the aspect ratio (AR) of these elongated gold nanoparticles with clearance mechanisms and rates from the aquatic suspension columns including aggregation, deposition, and biopurification. We illustrate how a fresh water filter-feeder bivalve, Corbicula fluminea, can be used as a model organism to study the size and shape-selective biofiltration and nanotoxicity of elongated gold nanoparticles. The results suggest that biofiltration by C. fluminea increases with an increase in the size and AR of gold nanoparticle. We develop a simple nanotoxicity assay to investigate the short-term exposure nanotoxicity of gold nanoparticles to C. fluminea. The toxicity results indicate that for the tested concentration and exposure period that gold nanoparticles were not acutely toxic (i.e., not lethal). However, gold nanoparticles significantly inhibited the activities of some antioxidant enzymes in gill and digestive gland tissues. These inhibitions could directly affect the resistance of these organisms to a secondary stressor (temperature, pathogens, hypoxia etc.) and threaten organismal health.
Ph. D.

In this thesis, it is detailed the bottom-up production and characterization of thermoelectric (TE) nanomaterials with significant enhanced performance by using colloidal nanocrystals (NCs) as building blocks. The production of TE nanomaterials with significant improved figure of merit (ZT), has to do, not only with the precise control of the NCs properties, but also with the further fine control over the crystallographic alignment of nanograins of highly anisotropic materials. The first part of the thesis correspond to the study of synthetic routes to produce high quality chalcogenide NCs that are doped during the NC synthesis, in order to control the charge carrier concentration. The system studied was I−V−VI chalcogenide semiconductor, specifically it was produced the materials: AgSbSe2 and Cu3SbSe4. A low-cost, high-yield and scalable synthesis route to produce monodisperse of AgSbSe2 and Cu3SbSe4 NCs was obtained. After ligand displacement, the NCs were used as building blocks to produce TE nanomaterials. Additionally, by means of substitutional doping, a large increment in the power factor and relatively lower thermal conductivities were observed. The optimization of the doping concentration resulted in ZT values of 1.10 at 640 K for AgSb0.98Bi0.02Se2, and of 1.26 at 673 K for Cu3Sb0.88Sn0.10Bi0.02Se4, which represents a significant increase beyond the state of the art in Te-free multinary Ag/Cu-based chalcogenide materials. In the second part of the thesis, the work about PbS-metal (Cu and Sn) nanocomposites produced by blending procedure is presented. The low work function metal is able to inject electrons to the intrinsic PbS matrix, which is another strategy to control the charge carrier concentration. The power factor is dramatically enhanced due to the increase of the electrical conductivity in the nanocomposites. Consequently, the ZTmax was remarkably enhanced by two times as compared with the pristine PbS. Furthermore, we also compared the TE performance of microcrystalline composites with the same composition as in nanocrystalline composites; commercial PbS host with Cu particles. The results revealed that with the same metal addition, higher electrical conductivities were obtained in the nanocomposite, but higher Seebeck coefficients were maintained in the microcomposite. Moreover, higher thermal conductivities were also obtained in the microcomposite. Finally, the figure of merit ZT were higher for the microcomposite system in the low temperature range, but much lower in the higher temperature range compared with the nanocomposites system. In the last block, the process of production of crystallographically textured materials is presented. We face here the challenge of bottom-up approaches to control the crystallographic alignment of nanograins. The production of nanostructured Bi2Te3-based alloys is presented. This can be done with controlled stoichiometry by solution-processing, and crystallographic texture by liquid-phase sintering using multiple pressure and release steps at 480 °C, above the tellurium melting point. Additionally, we explain the possible mechanism to produce the highly textured nanomaterials. This strategy results in record TE figures of merit: ZT=1.83 at 420 K for Bi0.5Sb2.5Te3 and ZT=1.31 for Bi2Te2.7Se0.3 at 440 K when averaged over 5 materials in the c direction, respectively. These high figures of merit extended over a wide temperature range, which results in energy conversion efficiencies a 50% higher than commercial ingots in the similar temperature range. In summary, different strategies to improve the TE performance of bulk nanostructured materials produced by bottom-up engineering of NCs, have been studied and confirmed in this thesis. Additionally, it has been proven that the solution-processed synthesis approach is low-cost, compatible with the scale-up engineering, and also versatile in tuning the size, shape, composition, and microstructure, among others parameters of different nanomaterials to optimize their TE properties.
Los nanocristales (NCs) coloidales tienen excelentes propiedades para diferentes aplicaciones, como la conversión de energía, la catálisis, los dispositivos electrónicos y optoelectrónicos, entre otros. Así mismo, la síntesis coloidal de NCs tiene ventajas en el control del tamaño, forma y composición a nivel de la nanoescala; las bajas temperaturas de reacción; y la no necesidad de equipos especializados. Este proyecto se concentra en el diseño racional y la ingeniería de materiales termoeléctricos (TE) nanoestructurados de alta eficiencia, usando la estrategia del ensamblado ascendente (bottom-up) de NCs coloidales. Primero, se diseñó una ruta de síntesis de bajo costo, alto rendimiento, con la cual, se obtuvieron NCs de AgSbSe2 y Cu3SbSe4. La optimización de la concentración de dopaje resultó en valores para la figura de mérito TE, ZT, de 1.10 a 640 K para AgSb0.98Bi0.02Se2, y de 1.26 at 673 K para Cu3Sb0.88Sn0.10Bi0.02Se4. El material con mejores propiedades se usó para la producción de un generador TE en forma de anillo, para acoplarlo a los tubos de escape de gases, obteniendo una potencia eléctrica de 1mW por elemento TE con una diferencia de temperatura de 160 °C. En la segunda parte, se presenta el trabajo de la producción de nanocopuestos de PbS-metal (Cu y Sn) usando un procedimiento versátil de mezcla de NCs. La función de trabajo del metal es capaz de inyectar electrones a la matriz intrínseca de PbS. El factor de potencia TE, se ve dramáticamente incrementado debido al aumento en la conductividad eléctrica en los nanocompuestos TE. Consecuentemente, el valor máximo de ZT se vio excepcionalmente incrementado por el doble del valor comparado con el material original PbS. Finalmente, se presenta el proceso de producción de materiales texturizados cristalográficamente, produciendo materiales tipo p BixSb2-xTe3 y tipo n Bi2Te3-xSex. Se controló la estequiometria durante el procesamiento en solución y la textura cristalográfica, por medio de la sinterización en fase líquida con un procedimiento de múltiples pasos de presión y relajación a una temperatura de 480°C. Los valores de la figura de mérito TE presentan el record de: ZT=1.83 a 420 K para Bi0.5Sb2.5Te3 y ZT=1.31 para Bi2Te2.7Se0.3 a 440 K.

Metal oxide nanostructures characterized by multiple morphologies and structures are at the forefront of applications driven nanotechnology research. In particular, they represent a versatile solution for performance enhancement and applications in multifunctional devices and offer distinct advantages over their bulk counterparts. The current state in ZnO nanomaterials research and its impact in nanotechnology and modern engineering are discussed through the lens of con-tinuing technological advances in synthetic techniques allowing to obtain the material with predefined specific set of criteria including size, functionality, and uniqueness. Aim of this research activity is fabrication and study of the potential ap-plications as biomolecular nanoplatforms of ZnO nanostructures obtained using different synthetic techniques ranging from vapor phase deposition (Metal-Organic Chemical Vapor Deposition) to solution growth (Chemical Bath Depo-sition). Moreover, hybrid synthetic approaches are used to obtain complex hier-archical ZnO structures having dual or multiple morphologies. The non-covalent interaction of these inorganic nanosystems with organic molecules, having spe-cific chemical behavior, represents a strategy to obtain hybrid organic-inorganic nanomaterials, thus offering interesting potentiality for the design of high per-formance devices. In particular, it is demonstrated that integration of Metal-Organic Chemical Vapor Deposition and Chemical Bath Deposition strategies with Nanosphere Colloidal Lithography allows to define two-dimensional hybrid ZnO-SiO2 nanoarrays having great potential as innovative fluorescence sensing substrates with individual addressability and tuning of the biomolecular detec-tion capability. Combination of Metal-Organic Chemical Vapor Deposition with Electro-spinning leads to fabrication of core shell Zn-doped TiO2 ZnO nanofibers char-acterised by hierarchical growth of ZnO nanoneedles onto the TiO2 nanofiber surface. XRD measurements revealed that after ZnO deposition at T > 500 °C, the TiO2 nanofibers were composed of the anatase rutile mixed phases with dif-ferent fractions of rutile, modulated by the Zn dopant concentration. These com-posite nanomaterials may be intriguing to the future study of nanofiber photo-catalysts and sensors, and functional properties based on titanium dioxide.

In the world around us, it is easy to think in different situations in which there are temperature gradients available. These could be converted into a great source of energy if using the proper technology. Thermoelectric devices are environmentally friendly solid-state harvesters able to play this role by converting temperature differences into an electric voltage and vice-versa. These devices, besides being highly reliable since they have no moving parts, if engineered and fabricated in a shape-adaptable manner, are able to fit in countless industrial or domestic applications to improve their efficiency or power low-consumption devices like sensors. If, on top of it, the whole fabrication process is cost-effective and easily scalable, the outcoming thermoelectric devices could potentially reach numerous markets banned to date due to a mix of low efficiencies and high prices of the currently existing solutions. The first milestone towards cost-effective thermoelectric devices relies on the improvement of the thermoelectric conversion efficiency of the constituents materials. However, such improvement cannot be at all costs. New materials with significant improved performance need to be designed and engineered with relatively low production cost. In this framework, solution-processed techniques are an outstanding alternative for the production of thermoelectric materials and devices. In particular, the bottom-up assembly of colloidal nanoparticles, with controlled size, shape, crystal phase and composition, has no competing technology to precisely design functional metamaterials without the need of a high capital equipment or complex procedures, not only for thermoelectrics, but also for a wide range of applications. Nevertheless, some limitations still need to be overcome to exploit the full potential of solution-processed assembly technologies, and two different challenges should be addressed. The first one is regarding materials efficiency enhancement, and the second one to the device development itself. In this work, we undertake a journey from the material development to the engineering of the final device. The thesis is structured in 5 chapters, starting from the Chapter 0 or General Introduction that intends to situate the reader into the broader context of the technology and present the main objectives of the work. Chapter 1 presents a general view of the solution-processed route for the development of bottom-up engineered nanoparticle-based thermoelectric nanomaterials and devices. It is an extended and comprehensive text where main concepts, challenges, advantages and opportunities that the technology offers are exposed. Chapter 2 is built around 3 publications that cover the three different steps of the solution-processed nanomaterials preparation, and how the efficiency can be enhanced within each one. First article is focused on the synthesis stage, and presents the production of core-shell nanoparticles as a way to design nanocomposites. The second one is related to the purification step, showing how, taking advantage of the nanoparticle surface, an HCl-mediated ligand displacement is able to introduce controlled amounts of dopants in the nanoparticle. Last one, is focused on the final assembly phase, in which by properly assembling two different kinds of nanoparticles, a semiconductor and a metal, the efficiency could be greatly enhanced. In Chapter 3 a step is made towards the production of a ring-shape device, taking PbSe as a model material. These results have been submitted for their publication. Chapter 4, presents the integration of a thermoelectric device together with a nanoparticle-based temperature sensor. This integrated assembly, including an ultra-low-power electronic management, was implemented as an autonomous soil moisture sensor, and shows the great opportunity that both solution-processed techniques and thermoelectrics technology offer for the development of new applications. Finally, some conclusions over the presented project and future work are listed.
Al món que ens envolta és fàcil pensar en situacions en què hi ha gradients de temperatura disponibles. Aquests, es podrien convertir en fonts d’energia molt interessants mitjançant l’ús adequat de la tecnologia. Els dispositius termoelèctrics son conversors d’estat sòlid capaços de jugar aquest important paper, ja que son capaços de transformar diferències de temperatura en energia elèctrica i vice-versa. Poden ser instal·lats a qualsevol emplaçament si son adaptats a l’aplicació en qüestió, ja sigui a escala domèstica o industrial, per millorar la seva eficiència energètica o, per exemple, alimentar altres dispositius de baix cost. Si, a més a més, el conjunt del procés de fabricació és de baix cost i fàcilment escalable per la seva producció en massa, els dispositius termoelèctrics resultants tindran la possibilitat d’entrar dins de nous mercats, fins ara impossibles degut a una barreja fatal d’alts preus i baixes eficiències dels productes comercials disponibles actualment. El primer pas cap a la fabricació de mòduls termoelèctrics més efectius en tots els sentits, és la millora de la seva eficiència a través de la recerca de nous o més efectius materials dels quals estan constituïts. Tanmateix, però, aquesta millora no pot ser a qualsevol cost. És necessari que aquests nous materials mantinguin alhora l’eficiència i baix cost en la seva fabricació. En aquest sentit, les tècniques de processat en solució son una gran alternativa per la producció de materials i dispositius termoelèctrics, i, en particular, la utilització de nanopartícules col·loïdals, amb mida, forma, fase i composició controlada. No hi ha cap altra tecnologia que aconsegueixi el seu nivell de control sobre el disseny de materials funcionals sense la necessitat de costosos equipaments o procediments complexes, no només per termoelèctrics sinó per un ampli ventall d’aplicacions. No obstant això, algunes limitacions encara han de ser superades per tal de poder explotar plenament el potencial que les tècniques de processat en solució ofereixen. Els dos majors reptes als quals la tecnologia s’enfronta son: primer, millorar l’eficiència dels materials, i, segon, en el desenvolupament de nous models de dispositius. En aquest treball, fem un viatge des del desenvolupament del material fins la fabricació d’un dispositiu.

This thesis presents a novel ZnO-hydrogel based fluorescent colloidal semiconductor nanomaterial system for potential bio-medical applications such as bio-imaging, cancer detection and therapy. The preparation of ZnO nanoparticles and their surface modification to make a biocompatible material with enhanced optical properties is discussed. High quality ZnO nanoparticles with UV band edge emission are prepared using gas evaporation method. Semiconductor materials including ZnO are insoluble in water. Since biological applications require water soluble nanomaterials, ZnO nanoparticles are first dispersed in water by ball milling method, and their aqueous stability and fluorescence properties are enhanced by incorporating them in bio-compatible poly N-isopropylacrylamide (PNIPAM) based hydrogel polymer matrix. The optical properties of ZnO-hydrogel colloidal dispersion versus ZnO-Water dispersion were analyzed. The optical characterization using photoluminescence spectroscopy indicates approximately 10 times enhancement of fluorescence in ZnO-hydrogel colloidal system compared to ZnO-water system. Ultrafast time resolved measurement demonstrates dominant exciton recombination process in ZnO-hydrogel system compared to ZnO-water system, confirming the surface modification of ZnO nanoparticles by hydrogel polymer matrix. The surface modification of ZnO nanoparticles by hydrogel induce more scattering centers per unit area of cross-section, and hence increase the luminescence from the ZnO-gel samples due to multiple path excitations. Furthermore, surface modification of ZnO by hydrogel increases the radiative efficiency of this hybrid colloidal material system thereby contributing to enhanced emission.

Les électrodes transparentes sont les composants indispensables de nombreux dispositifsoptoélectroniques commerciaux (cellules solaires, écrans plats, écrans tactiles ou encorediodes électroluminescentes). Elles sont constituées le plus souvent d’oxyde d’indium etd'étain (ITO). Les électrodes à base d'ITO sont produites par un procédé relativementcoûteux et sont très fragiles à la contrainte mécanique, ce qui limite leur utilisation au seinde dispositifs optoélectroniques flexibles. Des matériaux alternatifs, sans indium, à base deréseaux de nano-fils d’argent, font actuellement l'objet d'un grand nombre de recherches.Ces réseaux à base de nanostructures métalliques ont des propriétés opto-électroniquescomparables voire supérieures à celles de l’ITO. Ils sont adaptables à des substrats flexibleset sont compatibles avec les procédés de dépôt « roll to roll ». L'objectif de cette thèse estd'explorer de nouvelles voies de synthèse et de modification de surface de nanofils d'argentpour développer des électrodes transparentes plus performantes. De nouvelles nanostructuresmétalliques, différentes de celles commercialisées, ont été élaborées : (i) des fils d’argentultra-longs (ii) des fils d’argent présentant une architecture inhabituelle i.e avec desramifications. Des paramètres clés du procédé polyol ont été modifiés pour élaborer les filsà facteur de forme très élevé. Ils ont permis d'accroître les performancesrésistance/transparence des dispositifs conventionnels. Des nano-fils d’argent de forme « Y» ou « V » ont également été synthétisés en soumettant le milieu de croissance à des ultrasons.Ces nanostructures devraient permettre de limiter les problèmes de conduction quiapparaissent, à l'heure actuelle, au niveau des contacts entre les fils dans les dispositifsnanostructurés. Par ailleurs, des réseaux de fils d'argent modifiés en surface avec de l'acide11-mercaptoundecanoïque (MuA) ont été élaborés. Ils constituent une solution trèsintéressante pour améliorer la stabilité chimique des réseaux métalliques. Le MuA limite l'oxydation de surface du métal et permet aux électrodes de conserver leurs transparence etconductivité initiales
Transparent electrodes are a necessary component in a number of devices such as solar cells,flat panel displays, touch screens and light emitting diodes. The most commonly usedtransparent conductor, indium tin oxide (ITO), is expensive and brittle, the latter propertymaking it inappropriate for up-and-coming flexible devices. Films consisting of randomnetworks of solution-synthesized silver nanowires have emerged as a promising alternative toITO. They have transparency and conductivity values better than competing new technologies(e.g. carbon nanotubes films, graphene, conductive polymers, etc.) and comparable to ITO.Furthermore, these silver nanowire films are cheap, flexible, and compatible with roll-to-rolldeposition techniques. The main objectives of this PhD work are to improve the properties ofsilver nanowire electrodes and to study and solve issues that are currently hindering their usein commercial devices. Specifically, I studied the important areas of electrode conductivity andstability. To increase the conductivity of nanowire electrodes, two silver nanostructuresdifferent from what is commercially available were synthesized i) ultra-long nanowires and (ii)branched nanowires. Regarding (i), by using 1.2-propanediol as the medium rather than thetypical ethylene glycol in the polyol synthesis process, as well as the molecular weight of PVP,the temperature of the process, or the concentration of silver nitrate, we obtained silvernanowires with an aspect ratio between their lengths and diameters of 1050. Among all theultra-long silver nanowires elaborated in polyol process reported in the literature, they have themaximum length. The synthesis developed is also cheap and the reaction time takes less than2h. Moreover, they have a high yield of 2 mg/ml. Electrodes with a sheet resistance of 5 Ω/Sqfor a transparency of 94% were obtained (with post thermal treatment applied). However, thispost-deposition anneal is shown to have a small influence on the decrease of the sheetresistance. It is thus not required to elaborate electrodes with good performance, which is veryadvantageous for the elaboration of electrodes on plastic substrates. Regarding (ii), “V-like shape” or “Y-like branched” nanowires were elaborated thanks to the input of ultrasonicirradiation during the polyol process. Unfortunately, their length being short (6 μm), theirinterest is limited to enhance the performance of transparent electrodes. In addition, structuralanalyses of both branched and unbranched nanowires revealed the nanostructures notmonocrystalline. Concerning the stabilities issues, the thermal stability of silver nanowireelectrodes coated with graphene was investigated. This coating allows a better homogeneity ofthe heat through the network, decreasing the number of hot spots and thus increasing thelifetime of the electrodes. The corrosion of silver nanowire and the resulting electrode resistanceincrease over time is a severe problem hindering their use in commercial devices. 11-mercaptoundecanoic acid (MuA) was identified as a promising passivation agent of silvernanowires. Lifetime testing showed that the electrode resistance increased more slowly (12%)than any other passivated electrodes reported in the literature. Furthermore, unlike many otherpassivation methods, the MuA molecule itself does not negatively affect the conductivity ortransparency of the electrode and is very inexpensive, all contributing to the commercialviability of the passivation method

This thesis reports the successful rational design of three highly active photocatalytic semiconductor nanocrystal (SNC) systems by exploiting morphology effects and the electronic properties of type II semiconductor heterojunctions. Novel architectures of colloidal SNCs are produced with the aim of suppressing exciton recombination and improving charge extraction for the successful initiation of desirable redox chemistry. Rod-shaped niobium pentoxide Nb₂O₅ nanocrystals (NCs) are shown to exhibit significantly enhanced activity (10-fold increase in rate constant) relative to spherical-shaped NCs of the same material. The increase is attributed to Nb5⁺ Lewis acid site rich (001) surfaces, present in higher proportions in the rod morphology, which bind organic substrates from solution resulting in direct interaction with photogenerated charges on the surface of the NC. Building on the insights into morphology-activity dependence, type II semiconductor heterojunctions are exploited for their ability to increase exciton lifetimes and spatially separate charges. Two novel II-VI heterostructured semiconductor nanocrystals (HSNCs) systems are investigated: a series of CdX/ZnO (X = S, Se, Te) HSNCs and ZnS/ZnO HSNCs capped with two different surface ligands. In the first case, substantial photocatalytic activity improvement is observed for HSNCs (relative to pure ZnO analogues) according to the following trend: CdTe/ZnO > CdS/ZnO > CdSe/ZnO. The observed trend is explained in terms of heterojunction structure and fundamental chalcogenide chemistry. In the second case, both ZnS/ZnO HSNCs exhibit activity enhancement over analogous pure ZnO, but the degree of enhancement is found to be a function of surface ligand chemistry. Photocatalytic activity testing of all the materials investigated in this work is performed via the photodecomposition of methylene blue dye in aerated aqueous conditions under UVA (350 nm) irradiation. The synthetic techniques employed for the synthesis of colloidal SNCs investigated in this thesis range from chemical precipitation and solvothermal techniques to several different organometallic approaches. A wide variety of analytical techniques are employed for the chemical, structural and optical characterisation of SNC photocatalysts including: XRD, XPS, TEM, UV-vis absorption, PL spectroscopy and FTIR. Atom Probe Tomography (APT) is employed for the first time in the structural characterisation of II-VI heterojunctions in colloidal HSNCs. Overall, this thesis provides a useful contribution to the growing body of knowledge pertaining to the enhancement of photocatalytic SNCs for useful applications including: solar energy conversion to chemical fuels, the photodecomposition of pollutants and light-driven synthetic chemistry.

A study of three novel solar cells is presented, all of which incorporate a low-dimensional quantum confined component in a bid to enhance device performance. Firstly, intermediate band solar cells (IBSCs) based on InAs quantum dots (QDs) in a GaAs p-i-n structure are studied. The aim is to isolate the InAs QDs from the GaAs conduction band by surrounding them with wider band gap aluminium arsenide. An increase in open circuit voltage (V_OC) and decrease in short circuit current (J_sc) is observed, causing no overall change in power conversion efficiency. Dark current - voltage measurements show that the increase in V_OC is due to reduced recombination. Electroreflectance and external quantum efficiency measurements attribute the decrease in J_sc primarily to a reduction in InGaAs states between the InAs QD and GaAs which act as an extraction pathway for charges in the control device. A colloidal quantum dot (CQD) bulk heterojunction (BHJ) solar cell composed of a blend of PbS CQDs and ZnO nanoparticles is examined next. The aim of the BHJ is to increase charge separation by increasing the heterojunction interface. Different concentration ratios of each phase are tested and show no change in J_sc, due primarily to poor overall charge transport in the blend. V_OC increases for a 30 wt% ZnO blend, and this is attributed largely to a reduction in shunt resistance in the BHJ devices. Finally, graphene is compared to indium tin oxide (ITO) as an alternative transparent electrode in squaraine/ C₇₀ solar cells. Due to graphene’s high transparency, graphene devices have enhanced J_sc, however, its poor sheet resistance increases the series resistance through the device, leading to a poorer fill factor. V_OC is raised by using MoO₃ as a hole blocking layer. Absorption in the squaraine layer is found to be more conducive to current extraction than in the C₇₀ layer. This is due to better matching of exciton diffusion length and layer thickness in the squaraine and to the minority carrier blocking layer adjacent to the squaraine being more effective than the one adjacent to the C₇₀.

The self-assembly of structured particles into monodisperse clusters is a challenge on the nano-, micro- and even macro-scale. While biological systems are able to self-assemble with comparative ease, many aspects of this self-assembly are not fully understood. In this thesis, we look at the strategies and rules that can be applied to encourage the formation of monodisperse clusters. Though much of the inspiration is biological in nature, the simulations use a simple minimal patchy particle model and are thus applicable to a wide range of systems. The topics that this thesis addresses include: Encapsulation: We show how clusters can be used to encapsulate objects and demonstrate that such `templates' can be used to control the assembly mechanisms and enhance the formation of more complex objects. Hierarchical self-assembly: We investigate the use of hierarchical mechanisms in enhancing the formation of clusters. We find that, while we are able to extend the ranges where we see successful assembly by using a hierarchical assembly pathway, it does not straightforwardly provide a route to enhance the complexity of structures that can be formed. Pore formation: We use our simple model to investigate a particular biological example, namely the self-assembly and formation of heptameric alpha-haemolysin pores, and show that pore insertion is key to rationalising experimental results on this system. Phase re-entrance: We look at the computation of equilibrium phase diagrams for self-assembling systems, particularly focusing on the possible presence of an unusual liquid-vapour phase re-entrance that has been suggested by dynamical simulations, using a variety of techniques.

ii Abstract Herein we report on the synthesis of MoSe2 nanomaterials using novel colloidal synthetic methods and their application as electrode materials in supercapacitors. Supercapacitors are energy storage devices with high power density and high cycle stability that can be used in applications that require rapid charge/discharge. The drawback in supercapacitors is their low energy density. Nanomaterials with high surface area are being explored as alternatives to activated carbon which has been a commonly used electrode material in supercapacitors. This is done to increase the energy density of supercapacitors. MoSe2 has been identified as an excellent candidate for use as an electrode material in supercapacitors because of its interesting properties. MoSe2 is a layered transition metal dichalcogenide (TMD) that is similar to graphene in structure and possesses interesting structural, optical and electronic properties. MoSe2 also has a high surface to volume ratio. A colloidal synthetic process was used for the synthesis of the MoSe2 nanomaterials, after which the effect of various reaction parameters was investigated. The reaction was run at 300 °C using oleylamine (OAm) as the solvent and surfactant. A time study on the synthesis of the nanomaterials revealed MoSe2 few-layer nanosheets grow from a flocculate formed in the initial stages of the reaction (30 min). At longer reaction times (e.g. 90 min) the flocculate is consumed to form wrinkled few-layer nanosheets. A variation of the metal precursor in the reaction results in changes to the morphology of the MoSe2 nanomaterials. The formation of the flocculate in the initial stages of the reaction when molybdenum hexacarbonyl is used as the metal precursor results in the formation of wrinkled few-layer nanosheets. The use of molybdic acid as the metal precursor results in the formation of nanoparticles with a central core which leads to the formation of MoSe2 nanoflowers. The effect of adding a co-surfactant to the reaction system was also investigated. The effect of adding oleic acid as a co-surfactant in the reaction along with oleylamine resulted in changes to the thickness of the nanosheets synthesized and slight changes in the morphology. The use of 1-octadecene as a co-surfactant resulted in the increased reactivity of the selenium precursor which increased the number of nanosheets growing per nanoflower. The electrochemical properties of the MoSe2 nanomaterials were investigated to determine the suitability of the nanomaterials for use as supercapacitor electrodes. The MoSe2 nanomaterials synthesized using colloidal synthesis exhibited electric double layer capacitance behaviour. The effect of the morphology on the electrochemical performance of the MoSe2 nanomaterials was investigated using MoSe2 nanoflowers and few-layer nanosheets. The MoSe2 nanoflowers were shown to have a higher specific capacitance at 81 Fg-1 than the few-layer nanosheets at 30 Fg-1. The nanoflowers also had better capacitance retention at higher current densities in the charge-discharge analysis compared to the few-layer nanosheets. The nanoflowers had higher capacitance retention of 68% compared to 20% for the few-layer nanosheets. The nanoflowers also had a lower equivalence series resistance (ESR) of 34.0 Ω compared to that of the few-layer nanosheets at 57.1 Ω.
EM2018

碩士
國立清華大學
化學工程學系所
105
We develop a new gas-phase quantitative approach for in-situ characterization of hybrid nanomaterial colloids using the concept of ion mobility instrumentation coupling. Electrospray-differential mobility analysis (ES-DMA), a gas-phase electrophoretic method, is used for quantifying particle size distributions and number concentrations of nanomaterial colloids. Aerosol particle mass analyzer (APM) and aerosol electrometer coupled to ES-DMA are used for in-situ measuring particle mass and surface area of size-classified aerosolized nanomaterial colloids, respectively. Transmission electron microscopy is employed complementary to provide the imagery of hybrid nanomaterial colloid. Combining the information of the mobility size distribution, aerosol surface area measurement, and the aerosol mass-based distribution, we can quantify (1) the kinetics of aggregation and surface dissolution of silver nanoparticles (AgNPs) conjugated with thiolated polyethylene glycol (SH-PEG) under an acidic environment; (2) The lateral size, surface area of the synthesized graphene oxide (GO) colloid prior to hybridization; (3) The electrostatic-directed assembly of AgNP@GO colloids and the corresponding stability. This study demonstrates a facile, prototype methodology to determine important formulation factors relevant to the formation of hybrid nanomaterial colloids and the performance in a variety of nanotechnological and bio-nanotechnological applications.

Miniaturisation of electronic and optoelectronic devices have enabled the realization of system-on-a-chip technology in modern image sensors, where the photo sensor arrays and the corresponding signal processing circuitry are monolithically integrated in a single chip. Apart from intrinsic advantages, the drive towards miniaturisation has been further fuelled by the exotic properties exhibited by semiconductor materials at the nano scale. As the dimension of the material is gradually reduced from the bulk, interesting physical and chemical properties begin to emerge owing to the increased confinement of charge carriers in different spatial dimensions. Solution-processed optoelectronics have revolutionised the field of device physics over recent years due to the superior performance, ease of processing, substrate flexibility, cost-effective production of large-area devices and other advantages associated with the technique. In the present work, solution-processed photo detectors have been fabricated on SiO2/Si substrate facilitating the ease of integration with conventional silicon CMOS technology. The present thesis deals with the successful exploitation of most common point defects in semiconductor nanostructures to reduce the overlap of hole wave function with the envelop wave function of the ground state electron to improve photoconduction. As a result of the investigation process, successful strategies have been devised for the improvement of photoconduction by engineering the defect states. In the first study, the intrinsic copper vacancies and the capping agent thiol have been employed to trap photo holes in photo detectors based on copper indium selenide nanoparticles, thereby allowing the photoelectrons to transit the device. In the second study, the optical excitation of charge carriers into the defect-related band originating from oxygen vacancies further raises the photoconductivity of molybdenum trioxide nanobelts based photodetectors. In the third study, the absence of photoconductivity in zinc selenide based quantum dots has been attributed to the radiative recombination of photogenerated carriers at the donor-acceptor states caused by the self-compensation of point defects in the dots. In the final study, the crucial role of the energy depth of trap states in determining the carrier relaxation dynamics (temporal response) of the photodetector based on SnO2 nanowires has been discussed in detail. .

Miniaturisation of electronic and optoelectronic devices have enabled the realization of system-on-a-chip technology in modern image sensors, where the photo sensor arrays and the corresponding signal processing circuitry are monolithically integrated in a single chip. Apart from intrinsic advantages, the drive towards miniaturisation has been further fuelled by the exotic properties exhibited by semiconductor materials at the nano scale. As the dimension of the material is gradually reduced from the bulk, interesting physical and chemical properties begin to emerge owing to the increased confinement of charge carriers in different spatial dimensions. Solution-processed optoelectronics have revolutionised the field of device physics over recent years due to the superior performance, ease of processing, substrate flexibility, cost-effective production of large-area devices and other advantages associated with the technique. In the present work, solution-processed photo detectors have been fabricated on SiO2/Si substrate facilitating the ease of integration with conventional silicon CMOS technology. The present thesis deals with the successful exploitation of most common point defects in semiconductor nanostructures to reduce the overlap of hole wave function with the envelop wave function of the ground state electron to improve photoconduction. As a result of the investigation process, successful strategies have been devised for the improvement of photoconduction by engineering the defect states. In the first study, the intrinsic copper vacancies and the capping agent thiol have been employed to trap photo holes in photo detectors based on copper indium selenide nanoparticles, thereby allowing the photoelectrons to transit the device. In the second study, the optical excitation of charge carriers into the defect-related band originating from oxygen vacancies further raises the photoconductivity of molybdenum trioxide nanobelts based photodetectors. In the third study, the absence of photoconductivity in zinc selenide based quantum dots has been attributed to the radiative recombination of photogenerated carriers at the donor-acceptor states caused by the self-compensation of point defects in the dots. In the final study, the crucial role of the energy depth of trap states in determining the carrier relaxation dynamics (temporal response) of the photodetector based on SnO2 nanowires has been discussed in detail. .

Central to the implementation of colloidal nanomaterials in commercial applications is the development of high throughput synthesis strategies for technologically relevant materials. Solution based synthesis approaches provide the controllability, high throughput, and scalability needed to meet commercial demand. A flow through supercritical fluid reactor was used to synthesize silicon nanowires in high yield with production rates of ~45 mg/hr. The high temperature and high pressure of the supercritical medium facilitated the decomposition of monophenylsilane and seeded growth of silicon nanowires by gold seeds. Crystalline nanowires with diameters of ~25 nm and lengths greater than 20 [micrometers] were routinely synthesized. Accumulation of nanowires in the reactor resulted in deposition of a conformal amorphous shell on the crystalline surface of the wire. X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and energy dispersive X-ray spectroscopy were used to determine the shell composition. The shell was identified as polyphenylsilane formed by polymerization of the silicon precursor monophenylsilane. A post synthesis etch was developed to remove the shell while still maintaining the integrity of the crystalline silicon nanowire core. Subsequent surface passivation was achieved through thermal hydrosilylation with a terminal alkene. The development colloidal nanomaterials into commercial applications also requires simple and robust bottom-up assembly strategies to facilitate device fabrication. A Langmuir-Blodgett trough was used to assemble continuous monolayers of hexagonally ordered spherical nanocrystals over areas greater than 1 cm². Patterned monolayers and multilayers of FePt nanocrystals were printed onto substrates using pre-patterned polydimethylsiloxane (PDMS) stamps and a modified Langmuir Schaefer transfer technique. Patterned features, including micrometer-size circles, lines, and squares, could be printed using this approach. The magnetic properties of the printed nanocrystal films were also measured using magnetic force microscopy (MFM). Room temperature MFM could detect a remanent (permanent) magnetization from multilayers (>3 nanocrystals thick) films of chemically-ordered L1₀ FePt nanocrystals. Grazing incidence small angle X-ray scattering was used to quantitatively characterize the grain size, crystal structure, lattice disorder, and edge-to-edge spacing of the nanocrystal films prepared on the Langmuir-Blodgett trough both on the air-water interface and after transfer.
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The properties of bulk 3D materials of metals or semiconductors are manifested with various length scales(e.g., Bohr excitonic radius, magnetic correlation length, mean free path etc.) and are important in controlling their properties. When the size of the material is smaller than these characteristics length scales, the confinement effects operate reflecting changes in their physical behavior. Materials with such confinement effects can be designated as low dimensional materials. There are exceedingly large numbers of low dimensional materials and the last half a century has probably seen the maximum evolution of such materials in terms of synthesis, characterization, understanding and modification of their properties and applications. The field of” nanoscience and nanotechnology”, have become a mature field within the last three decades where, for certain application, synthesis of materials of sizes in the nanometer range can be designed and controlled. Interface plays a very important role in controlling properties of heterogeneous material of every dimensionality. For example, the interface forms in 2D thin films or interface of heterogeneous nanoparticles(0D). In recent times, a large number of remarkable phenomena have triggered understanding and controlling properties arises due to nature of certain interface. In the field of nanoparticles, it is well known that the photoluminescence property depends very strongly on the nature of interface in heterostructured nanoparticles. In the recent time a large variety of heterostructured nanoparticles starting from core-shell to quantum dot-quantum well kind has been synthesized to increase the photoluminescence efficiency up to 80%. Along with improvement of certain properties due to heterostructure formation inside the nanoparticles, the techniques to understand the nature of those interfaces have improved side by side. It has been recently shown that variable energy X-ray Photoemission Spectroscopy (XPS) can be employed to understand the nature of interfaces (internal structure) of such heterostructure nanoparticles in great detail with high accuracy. While most of the previous studies of variable energy XPS, uses photonenergies sensitive to smaller sized particle, we have extended the idea of such nondestructive approach of understanding the nature of buried interfaces to bigger sized nanoparticles by using photon energy as high as 8000eV, easily available in various 3rd generation synchrotron centers. The nature of the interface also plays an important role in multilayer thin films. Major components of various electronic devices, like read head memory devices, field effect transistors etc., rely on interface properties of certain multilayer thin film materials. In recent time wide range of unusual phenomenon such as high mobility metallic behavior between two insulating oxide, superconductivity, interface ferroelectricity, unusual magnetism, multiferroicity etc. has been observed at oxide interface making it an interesting field of study. We have shown that variable energy photoemission spectroscopy with high photon energies, can be a useful tool to realize such interfaces and controlling the properties of multilayered devices, as well as to understand the origin of unusual phenomenon exists at several multilayer interfaces. Chapter1 provides a brief description of low dimensional materials, overall perspective of interesting properties in materials with reduced dimensionality. We have emphasized on the importance of determining the internal structure of buried interface of different dimensionalities. We have given a brief overview and importance of different interfaces that we have studied in the subsequent chapters dealing with specific interfaces. Chapter 2 describes experimental and theoretical methods used for the study of interface and self-assembly reported in this thesis. These methods are divided into two categories. The first section deals with different experimental techniques, like, UV-Visible absorption and photoluminescence spectroscopy, X-Photoelectron Spectroscopy(XPS), X-Ray diffraction, Transmission Electron Microscopy(TEM) etc. This section also includes brief overview on synchrotron radiation and methods used for detail analysis of interface structure using variable energy XPS. In the second part of this chapter, we have discussed theoretical methods used in the present study. \ In Chapter 3A we have combined low energy XPS, useful to extract information of the surface of the nanoparticles, with high energy XPS, important to extract bulk information and have characterized the internal structure of nanoparticle system of different heterogeneity. We have chosen two important heterostructure systems namely, inverted core-shell(CdScore-CdSeshell) type nanoparticles and homogeneous alloy(CdSeS)type nanoparticles. Such internal structure study revealed that the actual internal structure of certain nanomaterial can be widely different from the aim of the synthesis and knowledge of internal structure is a prerequisite in understanding their property. We were able to extend the idea of variable energy XPS to higher energy limit. Many speculations have been made about the probable role of interface in controlling properties, like blinking behavior of bigger sized core-shell nanoparticles, but no conclusive support has yet been given about the nature of such interface. After successfully extending the technique to determine the internal structure of heterostructured nanoparticles to very high photon energy region, we took the opportunity to determine the internal structure of nanoparticles of sizes as large as 12nm with high energy photoemission spectroscopy for the first time. In Chapter 3B we emphasize on the importance of interface structure in controlling the behavior of bigger sized nanoparticles systems, the unsettled issues regarding their internal structure, and described the usefulness of high energy XPS in elucidating the internal structure of such big particles with grate accuracy to solve such controversies. The existence of high density storage media relies on the existence of highly sensitive magnetic sensors with large magnetoresistance. Today almost all sensor technologies used in modern hard disk drives rely on tunnel magnetoresistance (TMR) CoFeB-MgO-CoFeB structures. Though device fabrication is refined to meet satisfactory quality assurance demands, fundamental understanding of the refinement in terms of its effect on the nature of the interfaces and the MgO tunnel barrier leading to improved TMR is still missing. Where, the annealing condition required to improve the TMR ratio is itself not confirmatory its effect on the interface structure is highly debatable. In particular, it has been anticipated that under the proposed exotic conditions highly mobile B will move into the MgO barrier and will form boron oxide. In Chapter 4 we are able to shed definite insights to heart of this problem. We have used high energy photoemission to investigate a series of TMR structures and able to provide a systematic understanding of the driving mechanisms of B diffusion in CoFeBTMR structures. We have solved the mix-up of annealing temperature required and have shown that boron diffusion is limited merely to a sub-nanometer thick layer at the interface and does not progress beyond this point under typical conditions required for device fabrication. We have given a brief overview on the evolution of magnetic storage device and have described various concepts relevant for the study of such systems. The interface between two nonmagnetic insulators LaAlO3 and SrTiO3 has shown a variety of interface phenomena in the recent times. In spite of a large number of high profile studies on the interface LaAlO3 and SrTiO3 there is still a raging debate on the nature, origin and the distribution of the two dimensional electron gas that is supposed to be responsible for its exotic physical properties, ranging from unusual transport properties to its diverse ground states, such as metallic, magnetic and superconducting ones, depending on the specific synthesis. The polar discontinuity present across the SrTiO3-LaAlO3 interface is expected to result in half an electron transfer from the top of the LaAlO 3 layer to each TiofSrTiO3 at the interface, but, the extent of localization that can make it behave like delocalized with very high mobility as well as localized with magnetic moments is not yet clear. In Chapter 5 we have given a description of this highly interesting system as well as presented the outcome of our depth resolved XPS investigation on several such samples synthesized under different oxygen pressure. We were able to describe successfully the distribution of charge carriers. While synthesizing and understanding properties of nanoparticles is one issue, using them for device fabrication is another. For example, to make a certain device often requires specific arrangements of nanoparticles in a suitable substrate. Self-assembly formation can be a potential tool in these regards. Just like atom or ions, both nano and colloidal particles also assemble by themselves in ordered or disordered structure under certain conditions, e.g., the drying of a drop of suspension containing the colloid particles over a TEM grid. This phenomenon is known as self-assembly. Though, the process of assembly formation can be a very easy and cost-effective technique to manipulate the properties in the nano region, than the existing ones like lithography but, the lack of systematic study and poor understanding of these phenomena at microscopic level has led to a situation that, there is no precise information available in literature to say about the nature of such assembly. In Chapter 6 we have described experiments that eliminate the dependence of the self-assembly process on many complicating factors like substrate-particle interaction, substrate-solvent interaction etc., making the process of ordering governed by minimum numbers of experimental parameter that can be easily controlled. Under simplified conditions, our experiments unveil an interesting competition between ordering and jamming in drying colloid systems similar to glass transition phenomenon Resulting in the typical phase behavior of the particles. We establish a re-entrant behavior in the order-disorder phase diagram as a function of particle density such that there is an optimal range of particle density to realize the long-range ordering. The results are explained with the help of simulations and phenomenological theory. In summary, we were able to extend the idea of variable energy XPS to higher energy limit advantageous for investigating internal structure of nonmaterial of various dimensionalities and sizes. We were able to comprehend nature of buried interface indicating properties of heterostructures quantum dots and thin films. Our study revealed that depth resolved XPS combined with accessibility of high and variable energies at synchrotron centers can be a very general and effective tool for understanding buried interface. Finally, we have given insight to the mechanism of spontaneous ordering of nanoparticles over a suitable substrate.

The properties of bulk 3D materials of metals or semiconductors are manifested with various length scales(e.g., Bohr excitonic radius, magnetic correlation length, mean free path etc.) and are important in controlling their properties. When the size of the material is smaller than these characteristics length scales, the confinement effects operate reflecting changes in their physical behavior. Materials with such confinement effects can be designated as low dimensional materials. There are exceedingly large numbers of low dimensional materials and the last half a century has probably seen the maximum evolution of such materials in terms of synthesis, characterization, understanding and modification of their properties and applications. The field of” nanoscience and nanotechnology”, have become a mature field within the last three decades where, for certain application, synthesis of materials of sizes in the nanometer range can be designed and controlled. Interface plays a very important role in controlling properties of heterogeneous material of every dimensionality. For example, the interface forms in 2D thin films or interface of heterogeneous nanoparticles(0D). In recent times, a large number of remarkable phenomena have triggered understanding and controlling properties arises due to nature of certain interface. In the field of nanoparticles, it is well known that the photoluminescence property depends very strongly on the nature of interface in heterostructured nanoparticles. In the recent time a large variety of heterostructured nanoparticles starting from core-shell to quantum dot-quantum well kind has been synthesized to increase the photoluminescence efficiency up to 80%. Along with improvement of certain properties due to heterostructure formation inside the nanoparticles, the techniques to understand the nature of those interfaces have improved side by side. It has been recently shown that variable energy X-ray Photoemission Spectroscopy (XPS) can be employed to understand the nature of interfaces (internal structure) of such heterostructure nanoparticles in great detail with high accuracy. While most of the previous studies of variable energy XPS, uses photonenergies sensitive to smaller sized particle, we have extended the idea of such nondestructive approach of understanding the nature of buried interfaces to bigger sized nanoparticles by using photon energy as high as 8000eV, easily available in various 3rd generation synchrotron centers. The nature of the interface also plays an important role in multilayer thin films. Major components of various electronic devices, like read head memory devices, field effect transistors etc., rely on interface properties of certain multilayer thin film materials. In recent time wide range of unusual phenomenon such as high mobility metallic behavior between two insulating oxide, superconductivity, interface ferroelectricity, unusual magnetism, multiferroicity etc. has been observed at oxide interface making it an interesting field of study. We have shown that variable energy photoemission spectroscopy with high photon energies, can be a useful tool to realize such interfaces and controlling the properties of multilayered devices, as well as to understand the origin of unusual phenomenon exists at several multilayer interfaces. Chapter1 provides a brief description of low dimensional materials, overall perspective of interesting properties in materials with reduced dimensionality. We have emphasized on the importance of determining the internal structure of buried interface of different dimensionalities. We have given a brief overview and importance of different interfaces that we have studied in the subsequent chapters dealing with specific interfaces. Chapter 2 describes experimental and theoretical methods used for the study of interface and self-assembly reported in this thesis. These methods are divided into two categories. The first section deals with different experimental techniques, like, UV-Visible absorption and photoluminescence spectroscopy, X-Photoelectron Spectroscopy(XPS), X-Ray diffraction, Transmission Electron Microscopy(TEM) etc. This section also includes brief overview on synchrotron radiation and methods used for detail analysis of interface structure using variable energy XPS. In the second part of this chapter, we have discussed theoretical methods used in the present study. \ In Chapter 3A we have combined low energy XPS, useful to extract information of the surface of the nanoparticles, with high energy XPS, important to extract bulk information and have characterized the internal structure of nanoparticle system of different heterogeneity. We have chosen two important heterostructure systems namely, inverted core-shell(CdScore-CdSeshell) type nanoparticles and homogeneous alloy(CdSeS)type nanoparticles. Such internal structure study revealed that the actual internal structure of certain nanomaterial can be widely different from the aim of the synthesis and knowledge of internal structure is a prerequisite in understanding their property. We were able to extend the idea of variable energy XPS to higher energy limit. Many speculations have been made about the probable role of interface in controlling properties, like blinking behavior of bigger sized core-shell nanoparticles, but no conclusive support has yet been given about the nature of such interface. After successfully extending the technique to determine the internal structure of heterostructured nanoparticles to very high photon energy region, we took the opportunity to determine the internal structure of nanoparticles of sizes as large as 12nm with high energy photoemission spectroscopy for the first time. In Chapter 3B we emphasize on the importance of interface structure in controlling the behavior of bigger sized nanoparticles systems, the unsettled issues regarding their internal structure, and described the usefulness of high energy XPS in elucidating the internal structure of such big particles with grate accuracy to solve such controversies. The existence of high density storage media relies on the existence of highly sensitive magnetic sensors with large magnetoresistance. Today almost all sensor technologies used in modern hard disk drives rely on tunnel magnetoresistance (TMR) CoFeB-MgO-CoFeB structures. Though device fabrication is refined to meet satisfactory quality assurance demands, fundamental understanding of the refinement in terms of its effect on the nature of the interfaces and the MgO tunnel barrier leading to improved TMR is still missing. Where, the annealing condition required to improve the TMR ratio is itself not confirmatory its effect on the interface structure is highly debatable. In particular, it has been anticipated that under the proposed exotic conditions highly mobile B will move into the MgO barrier and will form boron oxide. In Chapter 4 we are able to shed definite insights to heart of this problem. We have used high energy photoemission to investigate a series of TMR structures and able to provide a systematic understanding of the driving mechanisms of B diffusion in CoFeBTMR structures. We have solved the mix-up of annealing temperature required and have shown that boron diffusion is limited merely to a sub-nanometer thick layer at the interface and does not progress beyond this point under typical conditions required for device fabrication. We have given a brief overview on the evolution of magnetic storage device and have described various concepts relevant for the study of such systems. The interface between two nonmagnetic insulators LaAlO3 and SrTiO3 has shown a variety of interface phenomena in the recent times. In spite of a large number of high profile studies on the interface LaAlO3 and SrTiO3 there is still a raging debate on the nature, origin and the distribution of the two dimensional electron gas that is supposed to be responsible for its exotic physical properties, ranging from unusual transport properties to its diverse ground states, such as metallic, magnetic and superconducting ones, depending on the specific synthesis. The polar discontinuity present across the SrTiO3-LaAlO3 interface is expected to result in half an electron transfer from the top of the LaAlO 3 layer to each TiofSrTiO3 at the interface, but, the extent of localization that can make it behave like delocalized with very high mobility as well as localized with magnetic moments is not yet clear. In Chapter 5 we have given a description of this highly interesting system as well as presented the outcome of our depth resolved XPS investigation on several such samples synthesized under different oxygen pressure. We were able to describe successfully the distribution of charge carriers. While synthesizing and understanding properties of nanoparticles is one issue, using them for device fabrication is another. For example, to make a certain device often requires specific arrangements of nanoparticles in a suitable substrate. Self-assembly formation can be a potential tool in these regards. Just like atom or ions, both nano and colloidal particles also assemble by themselves in ordered or disordered structure under certain conditions, e.g., the drying of a drop of suspension containing the colloid particles over a TEM grid. This phenomenon is known as self-assembly. Though, the process of assembly formation can be a very easy and cost-effective technique to manipulate the properties in the nano region, than the existing ones like lithography but, the lack of systematic study and poor understanding of these phenomena at microscopic level has led to a situation that, there is no precise information available in literature to say about the nature of such assembly. In Chapter 6 we have described experiments that eliminate the dependence of the self-assembly process on many complicating factors like substrate-particle interaction, substrate-solvent interaction etc., making the process of ordering governed by minimum numbers of experimental parameter that can be easily controlled. Under simplified conditions, our experiments unveil an interesting competition between ordering and jamming in drying colloid systems similar to glass transition phenomenon Resulting in the typical phase behavior of the particles. We establish a re-entrant behavior in the order-disorder phase diagram as a function of particle density such that there is an optimal range of particle density to realize the long-range ordering. The results are explained with the help of simulations and phenomenological theory. In summary, we were able to extend the idea of variable energy XPS to higher energy limit advantageous for investigating internal structure of nonmaterial of various dimensionalities and sizes. We were able to comprehend nature of buried interface indicating properties of heterostructures quantum dots and thin films. Our study revealed that depth resolved XPS combined with accessibility of high and variable energies at synchrotron centers can be a very general and effective tool for understanding buried interface. Finally, we have given insight to the mechanism of spontaneous ordering of nanoparticles over a suitable substrate.

Recent advancements in nanotechnology and emerging applications of nanomaterials in various fields have stimulated interest in fundamental scientific research dealing with the size and structure controlled synthesis of nanoparticles. The unique properties of nanoparticles are largely size dependent which could be tuned further by varying shape, structure, and surface properties, etc. The preparation of monodisperse nanoparticles is desirable for many applications due to better control over properties and higher performance compared to polydispersity nanoparticles. There are several methods for the synthesis of nanoparticles based on top-down and bottom-up approaches. The main disadvantage of top-down approach is the difficulty in achieving size control. Whereas, uniform nanoparticles with controllable size could be obtained by chemical methods but most of them are difficult to scale up. Moreover, a separate step of size separation is necessary in order to achieve monodispersed which may lead to material loss. In this context, a post-synthetic size modification process known as digestive ripening is highly significant. In this process, addition of a capping agent to poly disperse colloid renders it highly monodisperse either under ambient or thermal conditions. In addition to size control, digestive ripening is also effective in controlling the structure of nanoparticles in colloidal solution comprising two different elements. Use of co-digestive ripening strategy in conjunction with solvated metal atom dispersion (SMAD) method of synthesis resulted in hetero structures such as core–shell, alloy, and composite nanoparticles. Despite the versatility of digestive ripening process, the underlying mechanism in controlling size and structure of nanoparticles are not understood to date. The aim of this thesis is to gain mechanistic insight into size control of digestive ripening as well as to investigate structure control in various binary systems. Objectives  Study digestive ripening of Au nanoparticles using various alkyl amines to probe the mechanism  Study co-digestive ripening of binary colloids consisting of two metals, Pd and Cu prepared separately by SMAD method  Study co-digestive ripening of binary colloids consisting of a metal (Au) and a semiconductor (CdS) prepared separately by SMAD method  Study vaporization of bulk brass in SMAD reactor and analyse phase, structure, and morphology of various Cu/Zn bimetallic nanoparticles obtained from bulk brass under various experimental conditions Significant results In chapter 1, fundamental processes of nanoparticle formation and common synthetic techniques for the preparation of monodisperse nanoparticles are briefly discussed. Chapter 2 presents a mechanistic study of digestive ripening process with regard to size control using Au nanoparticles as a model system. Three long chain alkyl amine molecules having different chain length were used as digestive ripening agents. The course of digestive ripening process was analysed by UV-visible spectroscopy and transmission electron microscopy. The experimental conditions such as concentration of digestive ripening agent, time, and temperature were found to influence the size distribution of nanoparticles. The average particle size was found to be characteristic of metal-digestive ripening agent combination which is considered as the optimum size preferred during digestive ripening under a given set of experimental conditions. This study discusses stabilization of optimum sized particles, surface etching, and reversibility in digestive ripening. Chapter 3 describes the synthesis and characterization of PdCu alloy nanoparticles by co-digestive ripening method. Syntheses of individual Pd and Cu colloids were carried out by SMAD method. Pd nanoparticles obtained using THF as solvent and in the absence of any capping agent resulted in an extended small Pd nanowire network assembly. Morphological evolution of spherical Pd nanoparticles from Pd nanowire network structure was observed with the use of capping agent, hexadecyl amine (HDA) in SMAD method. Co-digestive ripening of Pd and Cu colloids was studied at various temperatures. This study revealed temperature dependent diffusion of Cu atoms into Pd lattice forming PdCu alloy nanoparticles. Next, co-digestive ripening of a colloidal system comprising a metal and a semiconductor was explored. Au-CdS combination was chosen for this study owing to its interesting photocatalytic properties. Chapter 4 deals with the synthesis of Au and CdS nanoparticles by SMAD method and Au/CdS nanocomposite by co-digestive ripening. CdS nanoparticles of size 4.0 + 1.2 nm and Au nanoparticles of size 5.6 + 1.1 nm were obtained as a result of digestive ripening process. Au/CdS nanocomposite obtained by co-digestive ripening was characterized by a matrix-like structure made up of CdS nanoparticles in which Au nanoparticles were embedded. CdS nanoparticles were found to establish an intimate surface contact with Au nanoparticles and the matrix of CdS surrounding Au was developed via aggregation during digestive ripening. Chapter 5 describes a comprehensive study on various Cu/Zn bimetallic nanoparticles obtained from bulk brass. Vaporization of bulk brass in SMAD reactor led to a deploying process and further growth of nanoparticles from phase separated Cu and Zn atoms formed a composite structure. The characterization of Cu/Zn nanocomposite revealed covering of composite surface with Cu resulting in a core-shell structure, Cu/Zn@Cu. Post-synthetic digestive ripening of these core-shell composite particles showed diffusion of Zn atoms to the composite surface in addition to size and shape modification. Annealing of Cu/Zn nanocomposites prepared in THF resulted in α-CuZn alloy nanoparticles via sequential transformation through η-CuZn5, γ-Cu5Zn8, and β-CuZn (observed as marten site) phases.

Recent advancements in nanotechnology and emerging applications of nanomaterials in various fields have stimulated interest in fundamental scientific research dealing with the size and structure controlled synthesis of nanoparticles. The unique properties of nanoparticles are largely size dependent which could be tuned further by varying shape, structure, and surface properties, etc. The preparation of monodisperse nanoparticles is desirable for many applications due to better control over properties and higher performance compared to polydispersity nanoparticles. There are several methods for the synthesis of nanoparticles based on top-down and bottom-up approaches. The main disadvantage of top-down approach is the difficulty in achieving size control. Whereas, uniform nanoparticles with controllable size could be obtained by chemical methods but most of them are difficult to scale up. Moreover, a separate step of size separation is necessary in order to achieve monodispersed which may lead to material loss. In this context, a post-synthetic size modification process known as digestive ripening is highly significant. In this process, addition of a capping agent to poly disperse colloid renders it highly monodisperse either under ambient or thermal conditions. In addition to size control, digestive ripening is also effective in controlling the structure of nanoparticles in colloidal solution comprising two different elements. Use of co-digestive ripening strategy in conjunction with solvated metal atom dispersion (SMAD) method of synthesis resulted in hetero structures such as core–shell, alloy, and composite nanoparticles. Despite the versatility of digestive ripening process, the underlying mechanism in controlling size and structure of nanoparticles are not understood to date. The aim of this thesis is to gain mechanistic insight into size control of digestive ripening as well as to investigate structure control in various binary systems. Objectives  Study digestive ripening of Au nanoparticles using various alkyl amines to probe the mechanism  Study co-digestive ripening of binary colloids consisting of two metals, Pd and Cu prepared separately by SMAD method  Study co-digestive ripening of binary colloids consisting of a metal (Au) and a semiconductor (CdS) prepared separately by SMAD method  Study vaporization of bulk brass in SMAD reactor and analyse phase, structure, and morphology of various Cu/Zn bimetallic nanoparticles obtained from bulk brass under various experimental conditions Significant results In chapter 1, fundamental processes of nanoparticle formation and common synthetic techniques for the preparation of monodisperse nanoparticles are briefly discussed. Chapter 2 presents a mechanistic study of digestive ripening process with regard to size control using Au nanoparticles as a model system. Three long chain alkyl amine molecules having different chain length were used as digestive ripening agents. The course of digestive ripening process was analysed by UV-visible spectroscopy and transmission electron microscopy. The experimental conditions such as concentration of digestive ripening agent, time, and temperature were found to influence the size distribution of nanoparticles. The average particle size was found to be characteristic of metal-digestive ripening agent combination which is considered as the optimum size preferred during digestive ripening under a given set of experimental conditions. This study discusses stabilization of optimum sized particles, surface etching, and reversibility in digestive ripening. Chapter 3 describes the synthesis and characterization of PdCu alloy nanoparticles by co-digestive ripening method. Syntheses of individual Pd and Cu colloids were carried out by SMAD method. Pd nanoparticles obtained using THF as solvent and in the absence of any capping agent resulted in an extended small Pd nanowire network assembly. Morphological evolution of spherical Pd nanoparticles from Pd nanowire network structure was observed with the use of capping agent, hexadecyl amine (HDA) in SMAD method. Co-digestive ripening of Pd and Cu colloids was studied at various temperatures. This study revealed temperature dependent diffusion of Cu atoms into Pd lattice forming PdCu alloy nanoparticles. Next, co-digestive ripening of a colloidal system comprising a metal and a semiconductor was explored. Au-CdS combination was chosen for this study owing to its interesting photocatalytic properties. Chapter 4 deals with the synthesis of Au and CdS nanoparticles by SMAD method and Au/CdS nanocomposite by co-digestive ripening. CdS nanoparticles of size 4.0 + 1.2 nm and Au nanoparticles of size 5.6 + 1.1 nm were obtained as a result of digestive ripening process. Au/CdS nanocomposite obtained by co-digestive ripening was characterized by a matrix-like structure made up of CdS nanoparticles in which Au nanoparticles were embedded. CdS nanoparticles were found to establish an intimate surface contact with Au nanoparticles and the matrix of CdS surrounding Au was developed via aggregation during digestive ripening. Chapter 5 describes a comprehensive study on various Cu/Zn bimetallic nanoparticles obtained from bulk brass. Vaporization of bulk brass in SMAD reactor led to a deploying process and further growth of nanoparticles from phase separated Cu and Zn atoms formed a composite structure. The characterization of Cu/Zn nanocomposite revealed covering of composite surface with Cu resulting in a core-shell structure, Cu/Zn@Cu. Post-synthetic digestive ripening of these core-shell composite particles showed diffusion of Zn atoms to the composite surface in addition to size and shape modification. Annealing of Cu/Zn nanocomposites prepared in THF resulted in α-CuZn alloy nanoparticles via sequential transformation through η-CuZn5, γ-Cu5Zn8, and β-CuZn (observed as marten site) phases.