M.Sc. (Nanoscience) / Please refer to full text to view abstract

Molecular fluorophores and semiconductor quantum dots (QDs) have been used as cellular imaging agents for biomedical research, but each class has challenges associated with their use, including poor photostability or toxicity. Silicon is a semiconductor material that is inexpensive and relatively environmental benign in comparison to heavy metal-containing quantum dots. Thus, red-emitting silicon nanoparticles (Si NPs) are desirable to prepare for cellular imaging application to be used in place of more toxic QDs. However, Si NPs currently suffer poorly understood photoinstability, and furthermore, the origin of the PL remains under debate. This dissertation first describes the use of diatomaceous earth as a new precursor for the synthesis of photoluminescent Si NPs. Second, the stabilization of red PL from Si NPs in aqueous solution via micellar encapsulation is reported. Thirdly, red to blue PL conversion of decane-terminated Si NPs in alcohol dispersions is described and the origins (i.e., color centers) of the emission events were studied with a comprehensive characterization suite including FT-IR, UV-vis, photoluminescence excitation, and time-resolved photoluminescence spectroscopies in order to determine size or chemical changes underlying the PL color change. In this study, the red and blue PL was determined to result from intrinsic and surface states, respectively. Lastly, we determined that the blue emission band assigned to a surface state can be introduced by base addition in originally red-emitting silicon nanoparticles, and that red PL can be restored by subsequent acid addition. This experimentally demonstrates blue PL is surface state related and can overcome the intrinsic state related excitonic recombination pathway in red PL event. Based on all the data collected and analyzed, we present a simple energy level diagram detailing the multiple origins of Si NP PL, which are related to both size and surface chemistry.

Information processors that harness quantum mechanics may be able to outperform their classical counterparts at certain tasks. Quantum information processing (QIP) can utilize the quantum mechanical phenomenon of entanglement to implement quantum algorithms. Endohedral fullerenes, where atoms, ions or clusters are trapped in a carbon cage, are a class of nanomaterials that show great promise as the basis for a solid-state QIP architecture. Some endohedral fullerenes are spin–active, and offer the potential to encode information in their spin-states. This thesis addresses the challenges of how to engineer the components of a scalable QIP architecture based on endohedral fullerenes. It focuses on the synthesis and characterization of molecules which may, in the future, permit the demonstration of entanglement; the optical read-out of quantum states; and the creation of quasi-one-dimensional molecular arrays. Due to its long spin decoherence time, N@C₆₀ is the selected as the basic molecular unit for ‘coupled’ fullerene pairs, molecular systems for which it may be possible to demonstrate entanglement. To this end, isolated fullerene pairs, in the form of spin-bearing fullerene dimers, are created. This begins with the processing of N@C₆₀ at the macroscale and leads towards the synthesis of ¹⁵N@C₆₀-¹⁵N@C₆₀ dimers at the microscale. High throughput processing is introduced as the most efficient technique to obtain high purity N@C₆₀ on a reasonable timescale. A scheme to produce symmetric and asymmetric fullerene dimers is also demonstrated. EPR spectroscopy of the dimers in the solid-state confirms derivatization, whilst permitting the modelling of spin–spin interactions for 'coupled' fullerene pairs. This suggests that the optimum inter–spin separation for which to observe spin–spin coupling in powders is circa 3 nm. Motivated by the properties of the trivalent erbium ion for the optical detection of quantum states, optically–active erbium–doped fullerenes are also investigated. These erbium metallofullerenes are synthesized and isolated as individual isomers. They are characterized by low temperature photoluminescence spectroscopy, emitting in the infra- red at a wavelength of 1.5 μm. The luminescence is markedly different where a C₂ cluster is trapped alongside the erbium ions in the fullerene cage. Er₂C₂@C₈₂ (isomer I) exhibits emission linewidths that are comparable to those observed for Er³⁺ in crystals. Finally, the discovery of a novel praseodymium-doped fullerene is reported. The balance of evidence favours the structure being assigned as Pr₂@C₇₂. This novel endohedral fullerene forms quasi-one-dimensional arrays in carbon nanotubes, which is a useful proof-of-principle of how a scaled fullerene-based architecture may be achieved.

Ruthenium oxide (RuO2) nanorods are an emergent technology in nanostructure devices. As the physical size of electronics approaches a critical lower limit, alternative solutions to further device miniaturization are currently under investigation. Thin-film nanorod growth is an interesting technology, being investigated for use in wireless communications, sensor systems, and alternative energy applications. In this investigation, self-assembled RuO2 nanorods are grown on a variety of substrates via a high density plasma, reactive sputtering process. Nanorods have been found to grow on substrates that form native oxide layers when exposed to air, namely silicon, aluminum, and titanium. Samples were analyzed with Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) techniques. Conductive Atomic Force Microscopy (C-AFM) measurements were performed on single nanorods to characterize structure and electrical conductivity. The C-AFM probe tip is placed on a single nanorod and I-V characteristics are measured, potentially exhibiting rectifying capabilities. An analysis of these results using fundamental semiconductor physics principles is presented. Experimental data for silicon substrates was most closely approximated by the Simmons model for direct electron tunneling, whereas that of aluminum substrates was well approximated by Fowler-Nordheim tunneling. The native oxide of titanium is regarded as a semiconductor rather than an insulator and its ability to function as a rectifier is not strong. An electronic model for these nanorods is described herein.

Modification of nanostructures via laser processing is of great interest for a wide range of applications such as aerospace and the storage of nuclear waste. The primary goal of this dissertation is to improve the understanding of nanostructures through two primary routes: the modification of aerogels and pulsed laser ablation in ethanol. A new class of materials, patterned aerogels, was fabricated by photopolymerizing selected regions of homogeneous aerogel monoliths using visible light. The characterization and fabrication of functionally graded, cellular and compositionally anisotropic aerogels and ceramics is discussed. Visible light was utilized due to it’s minimal absorption and scattering by organic molecules and oxide nanoparticles within wet gels. This allowed for the fabrication of deeply penetrating, well resolved patterns. Similarly, nanoporous monoliths with a typical aerogel core and a mechanically robust exterior ceramic layer were synthesized from silica aerogels cross-linked with polyacrylonitrile. Simple variations of the exposure geometry allowed fabrication of a wide variety of anisotropic materials without requiring layering or bonding. Nanoparticle solutions were prepared by laser ablation of metal foils (Fe and Mo) in ethanol. Ablation of Fe generated Fe3O4 and Fe3C nanoparticles which were superparamagnetic with a saturation magnetization Ms = 124 emu/g. Zero field cooled (ZFC) measurements collected at an applied field of 50 Oe displayed a maximum magnetic susceptibility at 120 K with a broad distribution. Field cooled (FC) measurements showed a thermal hysteresis indicative of temperature dependent magnetic viscosity. Pulsed laser ablation of a Mo foil in ethanol generated inhomogeneous nanoparticles where Mo and MoC coexisted within the same aggregate. Formation of these unique nanoparticles is likely due to phase separation that occurs when a high temperature carbide phase cools after the laser pulse terminates. Similarly, magnetic nanoparticle suspensions were generated by pulsed laser ablation of Fe and Mo in ethanol. The formation of several carbide phases with no discernable alloy formation was seen. A decrease in magnetization with a decrease in Fe concentration was seen which was reconciled with the decreased Fe content in the system. However, at Fe concentrations below ~ 40%, an increase in Ms and Hc was observed which was reconciled with the disappearance of the ε–Fe3C. TEM analysis showed the formation of core-shell nanoparticles and Energy Filtered TEM showed the distribution of Fe-based nanoparticles in the suspensions.

Electrospinning is a process by which polymer fibers can be produced using an electrostatically driven fluid jet. Electrospun fibers can be produced at the micro- or nano-scale and are, therefore, very promising for air filtration applications. However, because electrospun fibers are electrically charged, it is difficult to control the morphology of filtration media. Fiber size, alignment and uniformity are very important factors that affect filter performance. The focus of this project is to understand the relationship between filter morphology and performance and to develop new methods to create filtration media with optimum morphology. This study is divided into three focus areas: unimodal and bimodal microscale fibrous media with aligned, orthogonal and random fiber orientations; unimodal and bimodal nanoscale fibers in random orientations; bimodal micrometer and nanometer fiber media with orthogonally aligned orientations. The results indicate that the most efficient filters, which are those with the highest ratio of particle collection efficiency divided by pressure drop, can be obtained through fabricating filters in orthogonal layers of aligned fibers with two different fiber diameters. Moreover, our results show that increasing the number of layers increases the performance of orthogonally layered fibers. Also, controlling fiber spacing in orthogonally layered micrometer fiber media can be an alternative way to study the filtration performance. Finally, such coatings presented throughout this research study can be designed and placed up-stream, down-stream, and/or in between conventional filters.

Orientador: Carlos Cesar Bof Bufon / Banca: Lucas Fugikawa Santos / Banca: Nilson Cristino da Cruz / Resumo: A técnica de autoenrolamento de nanomembrana foi utilizada neste trabalho para a fabricação de capacitores ultracompactos (UCCap), permitindo a caracterização de filmes finos de materiais orgânicos e híbridos. O método é conhecido como roll-up, e consiste na formação de uma nanomembrana tensionada, elaborada a fim de produzir estruturas autosustentadas, que promovem o enrolamento do sistema ao serem libertadas de um substrato, determinando uma arquitetura em 3D. Neste trabalho, a tecnologia de nanomembranas foi utilizada com o objetivo de determinar as propriedades elétricas e dielétricas, sob diferentes temperaturas, de camadas de moléculas semicondutoras (CoPc, CuPc e F16CuPc) e de camadas de estruturas híbridas metal-orgânicas (HKUST-1). A caracterização desses materiais em nanoescala foi possível por meio de sua incorporação em UCCap. Os dispositivos foram caracterizados por medidas de espectroscopia de impedância e corrente elétrica. Em filmes finos das ftalocianinas (= 5 nm) na temperatura ambiente (≈ 296 K), foram encontrados valores de 2,1 ± 0,5 para a constante dielétrica da CoPc (kCoPc), 3,1 ± 0,6 para a CuPc (kCuPc) e 1,2 ± 0,6 para F16CuPc (kF16CuPc). As propriedades elétricas / dielétricas dos filmes das ftalocianinas foram analisadas sob diferentes temperaturas e filmes de CoPc foram explorados na presença de campos magnéticos aplicados com valores de magnitude entre - 500 e + 500 mT. As camadas de HKUST-1 incorporadas ao UCCap possibilitaram a determinação do v... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: The rolled-up nanomembrane-based technique was used in this work to the manufacture of ultracompact capacitors (UCCap), allowing the characterization of thin films of organic and hybrid materials. The method, known as roll-up, consists in the formation of a strained nanomembrane, elaborated in order to produce self-supported structures that promote the winding of the system when released from a substrate, determining a 3D architecture. In this work, the nanomembrane technology was used to determine the electrical and dielectric properties, for different conditions of temperatures, in layers of semiconductor molecules (CoPc, CuPc and F16CuPc) and layers of hybrid metal-organic structures (HKUST-1). The characterization of these materials at nanoscale was possible by their incorporation into UCCap. Current-voltage and impedance spectroscopy measurements were used to characterize the devices. For thin films of the phthalocyanines (= 5 nm) at room temperature (≈ 296 K), values of 2,1 ± 0,5 were found for CoPc dielectric constant (kCoPc), 3,1 ± 0,6 for CuPc (kCoPc) and 1,2 ± 0,6 for F16CuPc (kF16CoPc). The electrical / dielectric properties of the phthalocyanine films were analyzed under different temperatures and CoPc films were screened in the presence of applied magnetic fields with magnitude values between - 500 and + 500 mT. The HKUST-1 layers incorporated into the UCCap allowed the determination of a value of 3,2 ± 1,6 for its dielectric constant (kHKUST-1) at ≈ 296 K. The v... (Complete abstract click electronic access below) / Mestre

Neste trabalho foram estudadas as propriedades magnéticas e características estruturais de filmes finos bicamadas e multicamadas formados com Ni{x}Cu{1-x} (ferromagneto, 50 x 90 e níquel puro), NiO, FeMn e IrMn (antiferromagnetos). Ligas de níquel-cobre podem ser classificadas como fracamente ferromagnéticas. Ferromagnetismo fraco gera efeitos únicos em propriedades de transporte eletrônico, junções com supercondutores e sistemas de exchange bias. Sendo uma solução sólida extremamente simples, diversas propriedades do NiCu dependem linearmente da estequiometria da liga. Os filmes finos foram depositados via magnetron sputtering em substratos monocristalinos de Si (100) a temperatura ambiente. A estrutura cristalina foi caracterizada por difração de raios-X e a morfologia foi analisada por retroespalhamento Rutherford. Propriedades magnéticas foram medidas com um SQUID a temperaturas entre 5K e 300K (curvas ZFC/FC e de magnetização a diferentes temperaturas) e com um VSM a temperatura ambiente (curvas de magnetização em diferentes ângulos no plano do filme). As amostras com Ni{x}Cu{1-x} mais ricas em cobre apresentam os maiores campos coercivo e de exchange bias a baixas temperaturas, mas os menores em altas temperaturas, geralmente respeitando as temperaturas de bloqueio indicadas nas curvas ZFC/FC. O acoplamento na interface das bicamadas NiO/Ni{x}Cu{1-x} foi muito reduzido em temperaturas mais altas, não sendo observada anisotropia unidirecional nas medidas a temperatura ambiente. As constantes de interação J{int} foram calculadas para as bicamadas Ni{x}Cu{1-x}/FeMn e Ni{x}Cu{1-x}/IrMn a 5K e temperatura ambiente. O sistema Ni/IrMn apresentou o maior valor dessa constante em ambas as temperaturas, diminuindo muito com o acréscimo de cobre na camada Ni{x}Cu{1-x}. O sistema Ni{90}Cu{10}/FeMn apresentou os maiores valores em ambas as temperaturas (excluindo o de níquel puro), comparável até com Ni/IrMn no caso de 5K. Esse comportamento pode ser explicado por uma maior afinidade entre as estruturas do FeMn e Ni{x}Cu{1-x} na direção [111], o que favorece a formação da face (111) no FeMn, mais consistentemente do que no IrMn. Nenhuma das multicamadas estudadas se comportou como uma válvula de spin, mas foi observado um aumento na coercividade e supressão do exchange bias, provavelmente por causa de acoplamento entre as camadas ferromagnéticas. / In this work, a study of magnetic properties and structural characteristics of bilayered and multilayered thin films was carried out. These films were made with Ni{x}Cu{1-x} (ferromagnet, 50 x 90 and pure nickel) and NiO, FeMn or IrMn as antiferromagnets. Nickel-copper alloys may be classified as weakly ferromagnetic. Weak ferromagnetism generates unique effects in electron transport, junctions with superconductors and exchange bias systems. Being an extremely simple solid solution, many properties of the NiCu alloy have a linear dependence with its stoichiometry. The thin films were deposited via magnetron sputtering on Si (100) monocrystalline substracts at room temperature. The crystalline structure was characterized by X-ray diffraction, while the morphology was analyzed by Rutherford back-scattering. Magnetic properties were measured by SQUID at temperatures between 5K and 300K (ZFC/FC curves and magnetization curves at different temperatures) and by VSM at room temperature (magnetization curves at varying angles at the film plan). The samples with copper-richer Ni{x}Cu{1-x} showed the largest coercive and exchange bias fields in lower temperatures, but the lowest ones in higher temperatures, usually following the blocking temperatures denoted by the ZFC/FC curves. The interfacial coupling in NiO/Ni{x}Cu{1-x} was very weak in higher temperatures, not showing unidirectional anisotropy at room temperature. The J{int} exchange interaction constants were calculated for the Ni{x}Cu{1-x}/FeMn and Ni{x}Cu{1-x}/IrMn bilayers at 5K and room temperature. Ni/IrMn had the largest values at both temperatures, heavily decreasing with the copper content in the Ni{x}Cu{1-x} layer. Ni{90}Cu{10}/FeMn showed the largest values at both temperatures (except for the pure Ni sample), even reaching the values of Ni/IrMn at 5K. This behavior may be explaned by a greater matching between the FeMn and Ni{x}Cu{1-x} crystalline structures on the [111] direction, which favors the formation of the (111) FeMn face more consistently than the corresponding IrMn face. All the studied multilayers showed no spin valve behavior, but an increased coercivity and supressed exchange bias were observed, probably because of coupling between the ferromagnetic layers.

One of the objectives of nanotechnology is to develop ways to build functional nanoscale devices from nanostructures. Whether these nanodevices will constitute the basis for new technologies rests on the ability to precisely manipulate the nanostructures in such a way that large numbers of functional devices can be built in parallel, with each nanodevice precisely located and addressed. In this work nanostructures dispersed in solution are organized onto surfaces by means of molecular-scale directed assembly. This technique combines top down high resolution lithographic patterning to bottom up self-assembly: specific molecular interactions take place at locations precisely defined by lithography, resulting in the parallel assembly of an arbitrarily large number of devices into complex and precisely ordered arrangements. While different molecules are used in this study, DNA plays a key role throughout the work due to the specificity of its interactions, its programmability and outstanding chemical flexibility. Two approaches are developed to direct the assembly of nanostructures on a surface. The first involves the patterning and selective functionalization of metallic nanodots that are used as anchors for the attachment of DNA molecules, proteins, DNA nanostructures and single-wall carbon nanotube (SWCNT) segments wrapped by DNA. Different strategies are explored to maximize the yield of the desired assembly. This platform also allows the monitoring of DNA-protein interactions with single molecule resolution, which has many potential biomedical applications. In the second approach, lithographic patterning is used to define regions of high surface energy that promote the binding of DNA origami and SWCNT segments. The high patterning resolution again allows for single nanostructure manipulation. This method facilitates the assembly of SWCNT field effect transistors from DNA-wrapped SWCNT segments. The formation of multi-component nano-objects in solution, by directing the linkage of properly functionalized nanostructures, is also studied. The products of these reactions are suitable for surface placement with the developed directed assembly techniques, thereby resulting in a hierarchical directed assembly process. Among others, the synthesis of SWCNT-dsDNA heterostructures is described. These hybrid objects can be used to electrically probe dsDNA using the SWCNTs as electrodes, by assembling solid state devices by means of the directed assembly methods, and also by conductive AFM. The results of some electrical measurements of double stranded DNA are discussed. The techniques developed in this thesis are directly applicable to fundamental studies of electron transport in molecules and other nanostructures, but they also have utility in other fields, such as chemistry and biology, where single molecule resolution is required. In addition, the approaches developed in this work may facilitate the advancement of new electronics technologies, including, but not limited to, future circuits based on single-wall carbon nanotubes with specific electronic properties.

Biological systems are intricate self-assembled systems built from dynamic nanoscale components. These nanoscale components are responsible for many tasks, from subcellular (e.g. DNA replication, cytoplasmic streaming, intracellular transport) to organismal (e.g. intercellular signalling, blood circulation). At each level, biological materials demonstrate complex and dynamic behaviors which are still robust to many perturbations, requiring a balance of dynamism and stability. Being able to emulate biology by dynamically assembling complex systems and structures from nanoscale building blocks would greatly expand the types of materials and structures available, possibly leading to better smart, adaptive, self-healing materials in engineering. The overarching goal of this dissertation is to further the understanding of assembly in dynamic nanoscale systems. To this end, in vitro assays of kinesin motor proteins and microtubule cytoskeletal filaments are employed, providing a well-tested, minimalist, and convenient model system. In these assays, the kinesin motors are attached to the surface of the flow cell and the microtubule filaments are propelled over them. As the majority of past studies in active self-assembly of microtubules have been performed with biotin-labeled microtubules with streptavidin as a cross-linker (a "sticky" gliding assay), the first three parts of this dissertation focus on that system. In the first part, the adsorption kinetics of the streptavidin cross-linker onto the microtubule, which determines the interaction strength between microubule building blocks, is studied. The adsorption curve suggests that this is a negatively cooperative process, and here, the cause of the apparent negative cooperativity in the adsorption process is elucidated as a combination of steric and electrostatic interactions. In the second part, the difference between kinesin-propelled assembly and diffusion-driven assembly is investigated. While the kinesin-propelled microtubule assay has been used for over a decade, a control experiment comparing the active motor-driven system to a passive diffusion-driven system had never been performed. The control experiments showed conclusively that the passive system resulted in smaller and more disordered structures. Furthermore, these results fit well with existing models. The third part investigates the origins of microtubule spools observed in kinesin-propelled microtubule gliding assays, where the microtubules are allowed to cross-link via streptavidin and biotin. These microtubule spools have long been considered an example of a non-equilibrium structure which arises in motor-driven assembly. These spools exist in a dynamic state, having been observed to unwind in previous studies, and store large amounts of bending energy. Determining the origins of these spools is a first step towards understanding how to induce dynamically stable states. Finally, in the last part, a new dynamic system is engineered in which the microtubule assembles its own kinesin track as it moves along the surface while kinesin tracks which are not being used spontaneously disassemble. Thus, this system is stable enough to promote the motion of microtubules over the surface, but dynamic enough to allow for components to be recycled and assembled as needed. While such systems have been realized with mesoscopic to macroscopic components, such a system had not been realized in the nanoscale. As such, the realization of this system is the first step towards designing biomimetic active materials. Throughout this dissertation, the importance of short-range interactions on assembly kinetics is highlighted. The findings presented not only further the understanding and theory behind self-assembly in active nanoscale systems, but also further push the boundaries of experimentally realized systems.