Addition of nanopacticles in a base solvent leads to suspensions with enhanced physiochemical properties, compared to base solvent. This new type of suspensions is called nanofluids, with applications ranging from biomedicine to automotives. As a consequence extensive research is being conducted in the field, in particular, on the evaporation of these fluids as it leads to well-defined and highly ordered coffee-rings. However, the exact physics governing this phenomenon remain elusive. The goal of this experimental investigation is to elucidate how various parameters affect the progression of nanofluid coffee-stain formation. Examination of the coffee-ring structuring, produced by the free evaporation of sessile droplets containing nanoparticles, revealed an unexpected, disordered region at the exterior edge of the ring. A self-assembly mechanism with two components, particle velocity and wedge constraints, was proposed to describe the deposition of particles at contact lines of evaporating drops. Environmental pressure was used as a method to control particle crystallinity in the coffee-rings. Essentially, evaporation rate and pressure were found to be inversely proportional. Macroscopically, lowering pressure led to a transition from “stick-slip” to constant pinning. Nanoscopically, lowering pressure promoted crystallinity. Findings supported the proposed, in this thesis, particle self-assembly mechanism. Particle aspect ratio and flexibility were subsequently examined. Pinning strength was found to be a function of particle aspect ratio and rigidity, leading to constant pinning. The proposed, in this thesis, particle self-assembly mechanism was found to be applicable to a variety of aspect ratios and flexibilities. Lastly, particulate crystals grew following different pathways depending on particle flexibility.

In this study we looked at several different silicon nanostructures grown for the purpose of optimizing anodes for lithium ion batteries. We primarily focused on two distinct types of structures, nanospirals, and Rugate structures. The samples were designed to have the mechanical robustness to endure the massive expansion caused by lithiation of silicon. All of the samples were grown using an electron beam evaporator. Scanning electron microscope images show that we have achieved the desired structural growth. The spirals were shown to have an average diameter of 343 nm on polished copper, and 366 nm on unpolished copper. The Rugate structures had two distinct sample sets. The first mimicked the design of a thin film. The other formed distinct pillars that grouped into islands. The tops of the islands had an average diameter of 362 nm, while the pillars had an average width varying between 167 nm and 140 nm.

The thesis is mainly focused on the investigation of macroscopic and microscopic magnetic properties of selected nanomaterials containing cobalt and iron, and also the capability of our new device, scanning probe microscope Multimode V by Veeco, to directly visualise morphology and magnetic structure of these samples (Magnetic Force Miscroscopy, MFM). Investigated materials, such as CoFe2O4 nanoparticles and SiO2_Co_Si(111) thin films and multilayers are in general promising materials in many fields. In the medicine, the nanoparticles are used as the drug targets or contrast agents whereas in electronics, the (nano)granular thin films are the starting point in fabrication of high density storage media. The macroscopic magnetic properties of our samples are discusses in a view of superparamagnetic phenomena. The interactions in systems of nanoparticles are presented theoretically within the up-to date knowledge and also experimentally by demonstrating the behavior of the strongly-interacting, uper-spin-glass system. The thin films are studied in term of their granular structure and magnetic anisotropy. The morphology and the microscopic domain structure, respectively, are studied with use of the MFM. The first successful results obtained in our lab by this method are presented.

Une étude de l’induction d’une anisotropie magnétique d’origine magnétostatique dans une couche mince magnétique (Ni81Fe19) à topologie modulée a été effectuée grâce aux observations réalisées à l’aide de mesures de magnéto-transport (AMR). La modulation à l’échelle nanométrique est obtenue avec une nouvelle approche basée sur la lithographie électronique. La présence d’une anisotropie uniaxiale orientée parallèlement à la modulation a été mise en évidence. Ce denier a permis la définition de deux axes orthogonaux d’anisotropie dans un même substrat et l’exploration de la faisabilité d’un capteur de champ magnétique bidimensionnel. Une géométrie de capteur à Effet Hall Planaire est proposée. Le développement et la fabrication d’un dispositif permettant l’étude de la propagation d’une paroi de domaines dans une piste submicronique effectué sur une vanne de spin ont été accomplis. Dans ces dispositifs, la position de la paroi est déterminée par la mesure de la magnétorésistance géante (GMR). L’étude systématique des champs d’injection et de dépiégeage a montré l’existence une asymétrie de la propagation de la paroi entre les transitions P ® AP et AP ® P. Il a été montré, grâce à une série de simulations micro magnétiques, que cette asymétrie résulte de l’influence du champ dipolaire créé par la couche fixe sur les configurations adoptées par la paroi dans la couche libre. Des mesures de relaxation de temps ont permis d’obtenir la probabilité cumulée de dépiégeage de la paroi à partir de la constriction. Une variété de comportements a été ainsi mise en évidence. / A study on the induction of a magnetic anisotropy of magnetostatic origin in a magnetic thin layer (Ni81Fe19) having a modulated topology was carried out by performing magneto-transport measurements (AMR). Modulation in the nanometric scale is obtained with a new approach based on the electronic lithography. The presence of a uniaxial anisotropy along the modulation was highlighted. This latter allowed the definition of two orthogonal axes of anisotropy in the same substrate and the exploration of the feasibility of a 2D dimensional magnetic sensor. A sensor geometry based on the Planar Hall effect is proposed. The development and the fabrication of a device allowing studying the domain wall propagation in a sub-micronic wire based on a spin-valve element have been accomplished. In such devices the domain wall position is determined by probing the Giant Magneto-Resistance (GMR). The systematic study of the injection and depinning fields showed the existence of an asymmetry in the propagation of the wall between the transitions P ® AP and AP ® P. It was shown, thanks to a series of magnetic micro simulations, that this asymmetry results from the influence of the dipolar field created by the fixed layer on the configurations adopted by the wall in the free layer. Relaxation time measurements made it possible to obtain the cumulated probability for the depinning of the domain wall from the constriction. A variety of behaviors was thus highlighted.

Mimicking the biological olfactory systems that consist of olfactory receptor arrays with large surface area and massively-diversified chemical reactivity, three dimensional (3D) metal oxide nanowire arrays were used as the active materials for gas detection. Metal oxide nanowire arrays share similar 3D structures as the array of mammal's olfactory receptors and the chemical reactivity of nanowire array can be modified by surface coatings. In this dissertation, two standalone gas sensors based on metal oxide nanowire arrays prepared by microfabrication and in-situ micromanipulation, respectively, have been demonstrated. The sensors based on WO3 nanowire arrays can detect 50 ppb NO2 with a fast response; well-aligned CuO nanowire array present a new detection mechanism, which can identify H2S at a concentration of 500 ppb. To expand the material library of 3D metal oxide nanowire arrays for gas sensing, a general route to polycrystalline metal oxide nanowire array has been introduced by using ZnO nanowire arrays as structural templates. The effectiveness of this method for high performance gas sensing was first investigated by single-nanowire devices. The polycrystalline metal oxide coatings showed high performance for gas detection and their sensitivity can be further enhanced by catalytic noble metal decorations. To form electronic nose systems, different metal oxide coatings and catalytic decorations were employed to diversify the chemical reactivity of the sensors. The systems can detect low concentrated H2S and NO2 at room temperature down to part-per-billion level. The system with different catalytic metal coatings is also capable of discriminiating five different gases (H2S, NO2, NH3, H2 and CO).

A Thesis submitted for the faculty of Science at the University of Witwatersrand Johannesburg, in the fulfilment for the degree of Doctor of Philosophy in Chemistry. Johannesburg, November 2016. / The synthesis of carbon spheres (solid and hollow) for application in organic photovoltaics and chemical sensors is a means of using inexpensive and readily processable carbons to eliminate global warming and to monitor harmful gases. The synthesis conditions used to make solid carbon spheres can also be used to tailor their structural, paramagnetic and thermal properties. More so, the ability to tailor the morphology, surface, structural and electronic properties of the hollow carbon spheres by a templating method is an added advantage to their applicability in electronic devices. Solid carbon spheres were synthesized by a vertically oriented chemical vapor deposition (CVD) reactor using acetylene as a carbon source and argon or hydrogen as the carrier gas. The flow rates of the acetylene or carrier gases determined the particle sizes of the carbon spheres. Annealing of carbon spheres in hydrogen resulted in an increase in thermal stability, fewer defects and narrower paramagnetic signals relative to the carbon spheres annealed in argon gas. In contrast, carbon spheres annealed in argon exhibited an increase in the number of defects, a decrease in thermal stability and broader paramagnetic signals. Doped carbon spheres portrayed an increase in ID/IG ratios, a decrease in thermal stability and stronger paramagnetic signals due to the presence of defects induced by nitrogen. The N doped carbon spheres synthesized in H2 comprised of 48% pyridinic-N, 22% pyrrolic-N and 24% quaternary -N while the N doped spheres obtained in the presence of Ar had 17% pyridinic- N, 20% pyrrolic-N and 49% quaternary-N. The presence of a higher percentage of pyridinic- N confirms the presence of more edge defects in carbon spheres synthesized under H2 gas corroborating with the stronger paramagnetic signal observed from the ESR spectra. Consequently, a higher N/C ratio was exhibited in the N doped CSs obtained in the presence of H2 (4.96) than in the presence of Ar (3.68). This could be attributed to the presence of edge defects in carbon spheres synthesized in the presence of H2 gas. The induction of edge defects in carbon spheres in the presence of H2 gas without the aid of a metal catalyst opens a platform for regulating surface and catalytic reactions using H2 gas. Pristine and mesoporous SiO2 spheres were synthesized using a modified Stober method. Carbonization of the pristine SiO2, pristine SiO2@PVP, mesoporous SiO2 and mesoporous SiO2@PVP spheres was carried out using a bubbling method with toluene as the carbon source and argon as the carrier gas in a CVD reactor for 1 h. Upon SiO2 removal, hollow carbon nanostructures of varying morphologies were obtained. The polyvinylpyrrolidone (PVP) adsorption time, PVP concentration, SiO2 mesoporosity, SiO2 particle size dispersion, and carbonization time played a role in the formation of unique hollow carbon nanostructures; complete HCSs, broken HCSs, deformed HCSs, edge connected, open ended, wormlike and bubble-like HCSs. The mesoporous broken HCSs and open ended HCSs portrayed a hierarchical structure with a bimodal pore size distribution. The surface area properties of these materials and the ease of control of the carbon morphology gives an insight into the application of these materials as dye adsorbents. The effect of the size dispersion of Au@SiO2 sphere templates for the synthesis of hollow carbon structures was evaluated using a CVD nanocasting method. The diameter of the template, the presence of the gold nanoparticles and the amount of PVP determined the size, thickness and shape of the synthesized carbon nanostructures. Carbonization (and SiO2 removal) of Au@polydispersed silica spheres for 1 h gave a graphene-like HCS layer while longer times (2-4 h) gave nanotube like (or worm like) HCSs. These results highlight the potential use of Au@carbon core shell structures for the generation of few layered graphene-like unusual nanostructures. As a proof of concept, the wormlike carbon structures were incorporated in organic solar cells and found to give a measurable photovoltaic response. The incorporation of Au nanospheres and nanorods in a hole transport layer (PEDOT:PSS) of a solar cell device increased the current density and the photo-conversion efficiency of the device due to the local surface plasmon resonance and enhanced light scattering effects of gold. However, high series resistance and leakage currents were obtained due to barrier centres created by uneven dispersion of Au nanaorods within the polymer matrix. The performance of bulk heterojunction organic photovoltaic cells based on poly(3-hexylthiophene- 2,5-diyl) (P3HT) and 6,6-phenyl-C61-butyric acid methyl ester (PCBM) processed from chlorobenzene solution can be enhanced by solution heat treatment of the blend. The morphology of films spin coated from the heat treated blend solution reveals a more favourable diffusion of PCBM into the P3HT matrix than heating of the individual solutions separately. The films obtained from heat treated P3HT and PCBM solutions had a more homogeneous dispersion and enhanced light absorption than those obtained from solutions heat treated separately. There was a significant improvement in the performance for devices made from a solution heat treated blends relative to the non-treated blend; a maximum power conversion efficiency of 3.5% and a fill factor up to 43% was achieved under Air Mass 1.5 at 100 mW/cm2 illumination. This study also reports on the sensing characteristics of ammonia in humid environment by hollow carbon spheres, hollow carbon spheres-polyvinylpyrrolidone composite and annealed hollow carbon spheres, at 20°C and 40°C. For device fabrication, a surfactant assisted method was used to homogeneously disperse the hollow carbon spheres, allowing their deposition onto an interdigitated electrode by casting. An enhanced response and recovery time of the devices was observed at the higher working temperature. Annealing of the hollow carbon spheres resulted in a tremendous decrease in the humidity dependent ammonia sensing due to a decrease in the number of the oxygenated groups and defects in their structure. The presence of hydroxyl groups on the pristine hollow carbon sphere surface resulted in an enhanced proton conductivity. However, the ammonia sensitivity at high relative humidity in the pristine hollow carbon spheres is negligible due to the inhibition of ammonia adsorption sites by the high concentration of water molecules. The sensor response was investigated by varying both ammonia concentration and relative humidity, determining the topology of the response as a function of these two variables, and applying a tristimulus analysis in an attempt to determine the ammonia concentration independently of the relative humidity. This study demonstrates the critical role played by humidity and surface chemistry in the ammonia sensing properties of hollow carbon spheres. The studies reveal the day to day application of ammonia sensors, with temperature and humidity playing a critical role in the carbon based sensor response and recovery of the materials. These carbon based sensors that simultaneously measure ammonia and relative humidity could be applied in agricultural industries to monitor ammonia concentration in soils, fishponds and in food industries to monitor meat spoilage. / LG2017

Thesis advisor: Michael J. Naughton / The nanoscale morphology of metals can enable special functionality in plasmonic and electrochemical devices, with applications in energy conversion and storage, sensors, and computers. In particular, sharp edges on metal nano and microstructures are understood to affect the density of electrons on the metal surface. The associated concentration of electric field can concentrate surface plasmon polaritons (SPPs) and enable waveguiding of the SPPs, as we show in this thesis for sharp ridges along aluminum nanowires. Also important is the presence of facets on the metal structures, which determines the orbitals that electrons occupy on the metal surface. Changes in both the electron density and orbitals can affect the binding of molecules to the metal, which can improve reaction kinetics in catalysis. We demonstrate this on gold dendrite and plate electrocatalysts for CO2 electrolysis. Regarding metal nanostructure fabrication, electrochemical deposition and corrosion have demonstrated promising control over the morphology, including the topography, crystallinity, grain boundaries, and crystal faceting. This is important, because existing methods for metal nanostructure fabrication can only produce a circumscribed assortment of morphologies. In contrast, semiconductors and insulators have many new deposition techniques that produce a wide range of controlled morphologies. Of further appeal, electrochemical techniques are solution-based and typically operate at room temperature and pressure, allowing facile scale-up to industrial production. Here we demonstrate and discuss the mechanisms of two new techniques, which produce the aluminum nanowires and gold dendrites and plates discussed above. / Thesis (PhD) — Boston College, 2018. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

Thesis advisor: Krzysztof Kempa / In my work presented in this dissertation, I have focused on simulation studies of light interaction with nanostructures made of metals and dielectrics. Of particular interest have been plasmonic effects. The structures included the wire and coaxial nanowaveguides, as well as periodic arrays of planar quasi-triangles, and periodic arrays of nanoholes in thin metallic films. In the nanowaveguides I focused on plasmon polariton modes which resemble the TEM modes propagating in the corresponding conventional radio transmission lines. This collaborative research, involving an experimental effort, showed how the nanoscopic plasmon polariton modes reduce in the retarded limit to the TEM modes, and in the non-retarded limit to the corresponding surface plasmon modes. My simulations explained details of recent experimental results involving plasmonic waveguiding in metallic nanowires. Similar results have been obtained for nanocoaxial waveguides. My simulations of the optical absorption in the arrays of nano quasi-triangles, recently observed experimentally, helped identify those as due to Mie plasmonic resonances in these nanoparticles. They also explained the peak shifts in terms of the 2D surface plasmon dispersion, and the plasmon momentum quantization. In the study of the arrays evolution from holes to quasi-triangles, my simulations provided the clue to the critical behavior of the peak position for structures approaching the percolation threshold (the transitional structure in the series, for which film resistance diverges), and allowed to identify the series of structures as an analog of the percolation threshold problem. Finally, I have simulated optical performance of nanorod arrays (or multi-core nanocoax), which have been employed as platform for novel solar cells. My simulations have been employed to predict and optimize these cells. My work resulted in 5 publications and 2 manuscripts in preparation. / Thesis (PhD) — Boston College, 2011. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

Avanços recentes na síntese e manipulação de nanofios semicondutores têm aberto novas oportunidades tecnológicas. Nanofios de silício (SiNWs) pertencem a uma classe única de nanofios semicondutores, pelo fato de que, em um futuro próximo eles possam ser utilizados como elementos de integração entre dispositivos dentro do contexto da tecnologia do silício convencionais. Também há outras aplicações, tais como nanosensores químicos e biológicos a nível atômico ou molecular, possibilitando aplicações e desenvolvimento de tecnologias de sensoriamento in vivo. Realizamos uma investigação teórica da estabilidade e plasticidade de nanofios de silício usando o estado da arte em simulações de dinâmica molecular e em potenciais interatômicos. Consideramos nanofios com as direções de crescimento h100i, h110i e h112i com diversos diâmetros e tipos de facetas. Encontramos que o perímetro, e não o diâmetro, é o parâmetro relevante para descrever as dimensões dessa classe de sistema. Verificamos a performance de diversos potenciais interatômicos para o silício, e encontramos que o EDIP fornece uma melhor descrição para nanofios de silício. Encontramos que as famílias de facetas de superfície desempenham um papel central na energia total do nanofio, que segue uma lei universal como função do perímetro. Também calculamos a resposta de um nanofio de silício a uma tensão uniaxial externa, que habilita-nos a sugerir um novo método de obter nanofios de silício ultrafinos por nanodeformação. Os resultados de estabilidade e plasticidade são comparados com dados experimentais e \'ab initio\' disponíveis na literatura. / Recent advances in synthesizing and manipulating semiconductor nanowires have opened new technological opportunities. Silicon nanowires (SiNWs) belongs a unique class of semiconductor nanowires, since they could be used in conventional silicon device technology in a near future. Additionally, there are other applications, such as chemical and biological nanosensors at atomic or molecular level, opening a new range of technological applications of in vivo sensoring. Here, we carried a theoretical investigation on the stability and plasticity of silicon nanowires using the state of art of molecular dynamics and interatomic potential. We considered nanowires with h100i, h110i and h112i growth directions with several diameters and facet configurations. We found that the perimeter, and not the diameter, is the relevant parameter to describe dimensions in this class of systems. We tested the reability of several interatomic potential for silicon, and found that the EDIP model provides the best description of silicon nanowires. We found that the surface facet family plays a central role on the nanowire total energy, which follows an universal scale law as a function of perimeter. We also computed the response of a silicon nanowire to external load, which allowed us to suggest a new method to obtain ultra thin silicon nanowires by nanodeformation. The results on stability and plasticity are compared to experimental and ab initio results available in the literature.

Nenhuma / Os nanotubos de carbono são estruturas cilíndricas formadas apenas por átomos de carbono arranjados em uma rede hexagonal, com diâmetros que podem chegar ao limite mínimo de 0,4 nm e cujo comprimento é ilimitado. Além da dimensão nanométrica e alta razão de aspecto, os nanotubos de carbono apresentam uma dualidade inédita em comportamento eletrônico (podem ser metálicos ou semicondutores), são ótimos condutores térmicos (um nanotubo de carbono isolado apresenta a maior condutividade térmica já observada para um material (1750-5800 W/mK)) e elétricos (densidade de corrente 1000 vezes maior que em metais como prata e cobre) e apresentam propriedades mecânicas excepcionais (resistência à tração ~ 30 vezes maior que a da fibra de carbono e módulo elástico de ~ 1 TPa). Sendo assim, desde a sua descoberta em 1991, estudos envolvendo nanotubos de carbono têm atraído cada vez mais a atenção da comunidade científica e do setor produtivo, com vistas em aplicações importantes principalmente nas áreas de eletrônica molecular, compósitos, geração e estocagem de energia e biomédicas. Os nanotubos de carbono como sintetizados apresentam-se sempre em meio a impurezas, agregados em feixes, os quais contêm tubos de diferentes diâmetros, comprimentos, simetrias e comportamento eletrônico, além de serem insolúveis. Um conhecimento íntimo da química da superfície dos nanotubos, sua reatividade e seletividade, tem sido portanto fortemente requerido e explorado, no sentido de favorecer a seleção dos nanotubos por tipos, o estudo fundamental de um tubo individual e a sua integração com diferentes meios (orgânico, inorgânico e biológico). Neste trabalho, uma das primeiras dissertações do Laboratório de Química de Nanoestruturas do CDTN/CNEN, tem-se como objetivo a obtenção de derivados quimicamente modificados que possam demonstrar ganhos em seletividade, cessibilidade e desempenho dos nanotubos de carbono, possibilitando assim sua manipulação e utilização em diversas aplicações. São relatadas aqui as experiências de obtenção de derivados funcionalizados com grupos oxigenados (-COOH), que foram, por sua vez, transformados em derivados hidrogenados (-H), amida (-CONHR), amina (- CH2NHR), tioésteres (-COSR) e tioéteres (-CH2SR), onde R = C18H37. De nosso conhecimento, os procedimentos aqui desenvolvidos para a obtenção do primeiro e dos três últimos derivados são inéditos na literatura. A escolha de tais derivados foi motivada pela possibilidade de sua utilização posterior em estudos de separação dos nanotubos por quiralidade e de ancoramento de, por exemplo, nanopartículas, sítios catalíticos e entidades biológicas. As técnicas de microscopia eletrônica de arredura, microanálise por espectrometria de energia dispersiva e termogravimetria foram utilizadas para a caracterização da morfologia e determinação quali- e quantitativa da composição e pureza de cada amostra. A análise termogravimétrica permitiu ainda a determinação do grau de funcionalização dos derivados contendo cadeias alquílicas longas e a caracterização das possíveis interações desses grupos com os nanotubos. Por espectroscopia Raman, foi caracterizada a qualidade estrutural dos nanotubos de carbono antes e após a etapa de purificação. Já as espectroscopias na região do infravermelho e de fotoelétrons excitados por raios X e análise titulométrica foram ferramentas poderosas na caracterização também quali- e quantitativa dos grupos funcionais adicionados aos nanotubos em cada etapa dos procedimentos de modificação química dos mesmos. Amostras de nanotubos com teor de pureza acima de 80% em massa foram obtidas. As funcionalizações ocorreram com sucesso e interações fortes entre os grupos funcionais e os nanotubos foram caracterizadas. No caso dos derivados contendo nitrogênio e enxofre, no entanto, a natureza da ligação, se covalente ou iônica, não foi evidenciada. Além disso, a presença de grupos adsorvidos sobre a superfície dos tubos também foi verificada. Os grupos -CONHR, - CH2NHR, -COSR e -CH2SR (R = C18H37) parecem se organizar de forma micelar ao redor dos nanotubos. A mudança da natureza química da superfície dos novos derivados foi caracterizada por testes de dispersabilidade em diversos solventes em uma faixa ampla de polaridade.