"In the past decade nanomaterials have attracted the interest of scientists and engineers all over the world due to their unique properties. Through their devoted experimental efforts, limited advances have been made on the synthesis of nanomaterials, the integration of nanomaterials into the structures of larger scales, and the property study of nanomaterials to explore possible applications. Despite the huge amount of money, resources, and effort invested in nanomaterials, several challenges still remain as obstacles on the way towards the successful large scale use of nanomaterials to benefit human life and society. For example, the need for low-cost, robust, and highly productive manufacturing methods and the demand for efficient integration of nanomaterials with materials and devices of larger length scales are still left unmet. The objective of this work was to utilize cost-efficient nanofabrication methods such as template-assisted fabrication, electrodeposition, and chemical vapor deposition to fabricate nanomaterials, integrate nanomaterials with larger structures to form a hierarchical composite, and explore the application of unique nanostructured electrode in lithium-ion batteries. Thus the thesis consists of three main parts: (1) fabrication of one-dimensional inorganic nanomaterials such as metal nanowires, metal nanorods, and carbon nanotubes with good control over shape and dimension; (2) synthesis of hierarchical carbon nanofibers on carbon microfibers and/or glass microfibers; and (3) development of nanostructured anodes to improve high-rate capability of lithium-ion batteries by adapting nanorod arrays as miniature current collectors. "

No presente trabalho é apresentado o estudo sistemático das interações hiperfinas, em compostos de óxido de háfnio (HfO2) dopados com silício (Si), ferro (Fe), ítrio (Y) e lantânio (La) em porcentagens de 5% e 10%. A técnica aplicada para esse estudo foi o de correlação angular gama-gama perturbada (CAP) utilizando o núcleo de prova 181Hf(181Ta). Além disso, o estudo também foi estendido para o háfnio (Hf) na estrutura de ortosilicatos (HfSiO4) dopado com 20% Fe e na forma de filmes finos de HfO2. As amostras foram produzidas pelo método sol gel e para os filmes finos foi utilizado a técnica de spin coating. A caracterização estrutural destas amostras foi pela técnica de difração de raios-X e para morfologia foi utilizada as microscopias eletrônicas de varredura e transmissão. O núcleo de prova 181Hf(181Ta) presente na rede cristalina de todos os compostos forneceu os resultados da frequências de quadrupolo elétrico para o sitio monoclínico do óxido de háfnio (m- HfO2) bem caracterizado e um segundo sítio relacionado as vacâncias de oxigênio e defeitos na rede cristalina do HfO2. Além disso, as medições CAP que foram realizadas para as amostras de HfO2 dopadas, apresentam a formação de um terceiro sítio que está relacionado com o tamanho da partícula e a dopagem. Para o composto de HfSiO4 os resultados CAP indicam a temperatura de difusão do silício (Si), por volta de 700 °C e para o Fe- HfSiO4 mostra a influência do ferro na nucleação do composto que é superior em 30% em relação ao HfSiO4. Para as amostras de filmes finos os resultados CAP evidenciam os efeitos de superfície observado pelo surgimento de um terceiro sítio, ao longo do tratamento térmico 200 - 900 °C durante a medida. Este sítio também foi observado em temperaturas ambiente. / In this study, the development of a methodology for the synthesis of powder samples of Si-, Fe-, Y-, and La-doped hafnium oxide (HfO2) with concentrations of 5% and 10% is presented as well as the synthesis of orthosilicates (HfSiO4) samples doped with 20% of Fe. In addition, a procedure to produce HfO2 thin films using the spin coating method was also developed. All samples were characterized by usual techniques, such as X-ray diffraction, for structural verification and transmission and scattering electron microscopy, to study the size and morphology. Also a non-usual technique, perturbed angular correlation (PAC) was used to perform a systematic investigation of the hyperfine interactions in the doped samples, and at the Hf positions in the HfSiO4:Fe and HfO2 thin film samples. For PAC measurements 181Hf(181Ta) probe nuclei were used. The benefit in using 181Hf(181Ta) probe nuclei in these measurements is that the 180Hf isotope is naturally present in these samples, thus allowing the acquisition of 181Hf acquisition by the activation of the 180Hf in the IEA-R1 nuclear reactor. This method guarantees that probe nuclei are at Hf sites in the samples. The presence of 181Hf in the crystalline structure in all the compounds provided the hyperfine interaction results, such as electric quadrupole frequency, asymmetry parameter and delta, for the monoclinic site of the hafnium oxide (m-HfO2), and a second site related to the oxygen vacancies and defects in samples crystalline structure. Moreover, PAC measurements performed with the doped HfO2 samples revealed that probe nuclei occupy a third site related to the presence of the dopant. PAC measurements for HfSiO4 samples indicate the temperature of the silicon diffusion around 700 °C, and for the iron doped sample show the influence of iron atoms in the compound nucleation, which is 30 % higher when comparing to pure HfSiO4. For the thin film samples PAC results indicate the presence of surface defects, confirmed by the occurrence of a third site during a 200 - 900 °C annealing made during the measurement. This site has also been noted after a room temperature measurement after annealing.

Orientador: Michelle Leali Costa / Coorientador: Edson Cocchieri Botelho / Coorientador: Bruno Ribeiro / Banca: Luis Rogério de Oliveira Hein / Banca: Antonio Carlos Ancelotti Junior / Resumo: Os avanços tecnológicos na área dos compósitos poliméricos criaram novas oportunidades para estruturas de alto desempenho e com baixo peso, favorecendo o desenvolvimento de sistemas estratégicos em diversos setores, principalmente o aeronáutico. Dentro deste contexto os compósitos poliméricos nanoestruturados encontram-se em uma posição vantajosa em relação a outros materiais, pois seus constituintes podem agregar melhorias nos desempenhos mecânico, térmico e elétrico. Neste trabalho, compósitos nanoestruturados de poli(éter-imida) e nanotubos de carbono de paredes múltiplas (PEI/MWCNT) foram obtidos a partir da técnica de mistura em solução. Posteriormente, foi realizada a consolidação do compósito nanoestruturado reforçado com fibra de carbono (PEI/MWCNT/FC) via moldagem por compressão a quente. A partir das análises térmicas de termogravimetria (TGA) e dinâmico-mecânica (DMA) realizadas no compósito PEI/MWCNT, verificou-se uma melhoria na resistência térmica e nas propriedades viscoelásticas do material. Além disso, as melhorias nas propriedades físicas ocasionadas pela adição de MWCNT a matriz polimérica, influenciaram positivamente na qualidade de processamento dos laminados. Após os ensaios de ILSS e CST observou-se que a adição do nanoreforço gerou um incremento de 16% e 58%, respectivamente, sugerindo uma melhoria na adesão interfacial do compósito. O comportamento em tração não sofreu influência significativa a partir da adição de MWCNT, levando apenas uma melhoria d... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: Technological advances in polymer composites area have been created new opportunities for high-performance and lightweight structures, promoting the development of strategic systems in several sectors of industry, especially on the aerospace field. In this context, the nanostructured polymer composites are in an advantageous position compared to other materials, since its constituents may add improvements in mechanical, thermal and electrical performance. In this work, PEI/MWCNT nanostructured composites were obtained from solution mixing technique. Subsequently, the consolidation of the composite reinforced with carbon fiber, was performed by hot compression molding. TGA and DMA analyzes performed on PEI/MWCNT composite film revealed that there was an improvement in the thermal resistance and the viscoelastic properties of the material. In addition, the enhancement in physical properties due to the incorporation of MWCNT in polymer matrix had a positive role in the quality of the laminates. After ILSS and CST tests, it was observed that the addition of the nanofiller led to an increment of 16% and 58%, respectively, suggesting an improvement in the interfacial adhesion of the composite. The tensile behavior did not present a significant influence from the addition of MWCNT, leading to only a 5% improvement in tensile strength and 2% in the modulus of elasticity of the material. The addition of MWCNT did not significantly influence the fatigue strength of the laminates when a... (Complete abstract click electronic access below) / Mestre

by Guo Wenhua. / Thesis (Ph.D.)--Chinese University of Hong Kong, 1999. / Includes bibliographical references. / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Mode of access: World Wide Web. / Abstracts in English and Chinese.

In this study, we designed a series of experiments to determine the factors governing ion permeation through individual carbon nanotubes (CNTs) less than 1.5 nm in diameter and 20 µm in length. We then rationalize the experimental results by using a model, which is drawn from previous literature on protein ion channels and is centered around a simplified version of the Gouy-Chapman theory of electrical double layer. Lastly, we experimentally demonstrate and discuss the general similarities in ion permeation characteristics between CNTs and biological ion-selective pores. The role of many potential factors influencing the ion transport is assessed by taking two experimental approaches: (1) studying the effect of electrolyte concentration and composition on channel conductance and reversal potential, and (2) examining a second type of nanochannel as a parallel ion conduction pathway within the same device architecture and measurement set-up, which we refer to as leakage devices. This helps to differentiate the effect of CNT on ionic transport from any other possible source. Taken together, these two experimental methods provide strong evidence that the electrostatic potential arising from ionized carboxyl groups at the nanopore entrance has a significant effect on ionic permeation in a manner consistent with a simple electrostatic mechanism.

Atomically thin quasi-two-dimensional materials like graphene and transition metal dichalcogenide (TMDC) layers exhibit extraordinary optical and electrical properties. They have not only been used as testing grounds for fundamental research but also show promise for their viability in optoelectronics, photovoltaics and photocatalysis, to name a few technological applications. In practice, seldom are these materials used in isolation. One often finds them as part of a multicomponent structure, or heterostructure. In a similar spirit as the influence of solvents on the properties of molecular complexes, nanomaterials are also affected by their dielectric environment. Engineering the effect of the surroundings on the excitations in these materials is both a challenge and an opportunity. Moreover, understanding the transport of energy and charge through these heterostructures is crucial for device design. In this dissertation I will explore the properties of excitations in zero-dimensional and two-dimensional nanostructures and their dependence on the details of the environment using optical spectroscopy. Here, I discuss three of the projects that I undertook during my graduate studies. The first project concerns the efficient near-field, non-radiative energy transfer (NRET) of photo-excited carriers from semiconductor nanocrystals to graphene and a TMDC, molybdenum disulfide. Photoluminescence quenching of single quantum dots and time-resolved photoluminescence were used to quantify the rate of energy transfer. The NRET rate exhibited surprisingly opposite trends with increasing number of layers of the acceptor 2D sheet. The rate increased with increasing thickness of adjacent graphene layers but decreased with increasing thickness of MoS₂. A model based on classical electromagnetism could successfully explain the countervailing trends in terms of the competition between the dissipative channels and reduction of the electric field within the 2D material. In the next project, the exciton binding energy and band gap in another TMDC, monolayer WS₂, were tuned via dielectric screening from the environment. Monolayers of WS₂ were capped with graphene layers of varying thickness (1 – 4 layers). The excitonic states of WS₂ in the resulting heterostructures were detected using reflectance contrast spectroscopy and theoretically studied by a semi-classical model. The binding energy of the exciton was halved to 150 meV by placement of a single layer of graphene adjacent to the WS₂. Furthermore, this dramatic decrease in the binding energy is accompanied by a reduction of the band gap by the same amount. Additionally, the average spacing between the graphene and WS₂ was also identified to be a critical parameter with respect to dielectric screening of the electron - hole interaction. This offers a flexible alternative for the external manipulation of the Coulomb interaction. In the final part, I study how excitons in WS₂ couple and scatter with the excitations of the lattice or phonons. The importance of this study stems from the contribution of the scattering rates to the spectral width of the excitonic feature, the dephasing dynamics and thermal transport. The transition from direct to indirect band gap semiconductor from mono- to bilayer is expected to add an additional scattering channel via phonon emission. Through temperature dependent reflectance contrast and photoluminescence spectroscopy, the scattering rate for the phonon emission and absorption processes have been quantified. Comparing the results to data reported in the literature, it is understood that the striking change for the scattering rates is expected only at the mono- to bilayer transition for WS₂. The results suggest material thickness as a handle for engineering exciton - phonon interactions at the nanoscale.

Nano-materials are interesting material category with a single unit size between 1 and 1000 nanometers and possess unique mechanical, electrical, optical, and other physical properties that make them stand out from ordinary materials. With increasing demand for reduced size of electronic devices and integrated micro/nano-electro-mechanical systems (MEMS / NEMS), there is a high driving force in scientific research and technological advancement in nanotechnology. My research is about two popular novel nanomaterials: carbon nanotubes (1-dimensional material) and thin-layer transition metal dichalcogenides (2-dimensional materials). My first research direction is about the characterization of electrical properties of carbon nanotubes and using them as bio-sensors. Carbon nanotubes (CNTs), in general, are a material of great interest for many applications since their first discovery in 1991 [1], due to their unique structure, extraordinary electrical and mechanical properties, and unusual chemical properties. High-throughput fabrication of carbon nanotube field effect transistors (CNTFETs) with uniform properties has been a challenge since they were first fabricated in 1998. We invent a novel fabrication method to produce a 1×1 cm2 chip with over 700 CNTFETs fabricated around one single carbon nanotube. This large number of devices allows us to study the stability and uniformity of CNTFET properties. We grow flow-aligned CNTs on a SiO2/Si substrate by chemical vapor deposition and locate a single long CNT (as long as 1 cm) by scanning electron microscopy. Two photolithography steps are then used, first to pattern contacts and bonding pads, and next to define a mask to ‘burn’ away additional nanotubes by oxygen plasma etch. A fabrication yield of ~72% is achieved. The authors present statistics of the transport properties of these devices, which indicates that all the CNTFETs share the same threshold voltage, and similar on-state conductance. These devices are then used to measure DNA conductance by connecting DNA molecule of varying lengths to lithographically cut CNTFETs. While one single carbon nanotube is considered 1-dimensional material because it only has one side with “non-nano” length, the thin-layer transition metal dichalcogenides (TMDCs) are called the 2-dimensional materials since they have two sides of normal lengths and the other side of atomic size. Atomically thin materials such as graphene and semiconducting transition metal dichalcogenides have attracted extensive interests in recent years, motivating investigation into multiple properties. We use a refined version of the optothermal Raman technique [2][3] to measure the thermal transport properties of two TMDC materials, MoS2 and MoSe2, in single-layer (1L) and bi-layer (2L) forms. This new version incorporates two crucial improvements over previous implementations. First, we utilize more direct measurements of the optical absorption of the suspended samples under study and find values ~40% lower than previously assumed. Second, by comparing the response of fully supported and suspended samples using different laser spot sizes, we are able to independently measure the interfacial thermal conductance to the substrate and the lateral thermal conductivity of the supported and suspended materials. The approach is validated by examining the response of a suspended film illuminated in different positions in radial direction. For 1L MoS2 and MoSe2, the room-temperature thermal conductivities are (80±17) W/mK and (55±18) W/mK, respectively. For 2L MoS2 and MoSe2, we obtain values of (73±25) W/mK and (39±13) W/mK. Crucially, the interfacial thermal conductance is found to be of order 0.1-1 MW/m2K, substantially smaller than previously assumed, a finding that has important implications for design and modeling of electronic devices.

This thesis focuses on the study of nanotechnology and its applications in immunology and mechanosensing using micro- and nano-scale topographies, such as gratings, grids, and pillar substrates. In the past five years, we have developed three types of platforms and explored the influence of nano-patterned substrates on cell morphology, proliferation, protein secretion, and mechanosensing. I will introduce the three generations of Integrated Mechanobiology Platform (IMP) for T cell study, including the fabrication process of each generation of IMP, their advantages and disadvantages, and the comparison with existing High Throughput Screening System (HTSS). For the applications of IMP, I will focus on grating and grid topographies with IMP generation 3 format, and study how these nano-patterned substrates affect T cell morphology, expansion, cytokine secretion, drug-topography combination effects on T cells and long-term expansion for adoptive immunotherapy. I will demonstrate how IMP enables such studies in a high throughput manner. I also will discuss how Multiple Stiffness Pillar Platform (MSPP) facilitates the study of mechanosensing in cells spanning across different rigidities. First, I will talk about how MSPP is different from existing dual stiffness platforms. Differences include flexibility in distribution of different rigidities, consistency in pillar dimensions and ease of controlling the stiffness fold increase. In the sections of MSPP fabrication and characterization, I will focus on measurements of stiffness change and surface chemistry uniformity. I will then discuss the Mouse Embryonic Fibroblast (MEF) mechanosensing study on dual stiffness pillar substrates, including the preferential localization of rigidity sensing associated proteins (myosin IIA, phosph-myosin, paxillin, and p130CAS), MEFs actomyosin network building, and adhesion formation. These studies revealed previously undiscovered results in MEF mechanosensing, and demonstrate the great potential of MSPP in this research discipline. In the last part of this thesis, I will present on the mass production of thermoplastic nanopatterned molds. The demonstrated technology can produce large batches of nanostructured molds with decreased fabrication time and expense. In this chapter, I will discuss the necessity of developing such a technology and platform, as well as the design, fabrication, and characterization of the thermoplastic nano-patterned molds.

When nature is observed at the nanoscale, quantum physics is typically the most accurate model to describe and predict its behavior. Furthermore, quantum effects are increasingly at the core of the operation of new advanced electronic and photonic devices, which, in some cases, are designed on the basis of controlling quantum systems. This thesis focuses on two such systems, united by the methods used to realize them. These methods represent the cutting-edge of nanofabrication, which is the structuring of matter at ultra-small dimensions with a degree of precision and control that has not been previously attained. Pushing these methods to their limits enables the emergence of unique phenomena in the quantum systems explored here. The first system involves the realization of artificial graphene in an AlGaAs/GaAs quantum heterostructure. The appearance of massless charge carriers in graphene, which are described by the relativistic Dirac equation, originates from the linear energy-momentum dispersion of the electronic states in proximity to the K and K’ points of the hexagonal Brillouin zone. This unique quantum behavior is a direct result of the honeycomb symmetry of the graphene lattice. The prospect of reproducing this physics in an adjustable, artificial honeycomb lattice, known as artificial graphene, offers a platform for the exploration of novel quantum regimes of massless Dirac fermions beyond the limits imposed by the inability to manipulate the lattice of the natural material. The electronic properties of a two-dimensional electron gas whose density is modulated by a periodic potential with honeycomb symmetry have been predicted to generate massless Dirac-fermions with tunable Fermi velocity. This thesis reports the observation of a graphene-like band structure in a modulation-doped AlGaAs/GaAs quantum well engineered with a honeycomb lateral surface superlattice. This was accomplished by reactive ion etching of the surface to within a few tens of nanometers from the quantum well. A metal hard-mask, patterned by electron beam lithography combined with metal deposition and lift-off, was used to form a honeycomb artificial lattice with a variable lattice period, down to 40 nm. This is a three-fold reduction with respect to the smallest reported to date in pertinent literature. The BCl3-based shallow etching produces cylindrical pillars below which the two-dimensional electron gas is expected to form quantum dots, where the electron density is higher than in the surrounding etched regions. Low-temperature resonant inelastic light scattering measurements reveal new electronic transitions. An accurate interpretation of these can be found in the joint density of states derived from the calculated graphene-like linearly-dispersed energy bands, induced by the honeycomb potential modulation. The second system comprises the nanoscale engineering of individual electron spin qubits in diamond. Spin systems in solid-state have been intensively investigated as an outstanding pathway towards quantum information processing. One of the advantages of solid-state spintronics is the possibility of applying nanofabrication techniques commonly used by the semiconductor industry to produce and integrate spin qubits. The negatively charged nitrogen-vacancy (NV-) center in diamond stands out because of its optically addressable spin, which shows long coherence time and viable spin initiation, manipulation and read-out. A central challenge has been the positioning of NV- centers with nanometer scale control, that would allow for efficient and consistent dipolar coupling and the integration within an optoelectronic device. I demonstrate a method for chip-scale fabrication of arrays of closely-spaced NV- centers with record spatial localization of approximately 10 nm in all three dimensions and controllable inter-NV spacing as small as 40 nm. This is the highest spatial resolution realized to date in positioning NV- centers at the nanoscale with high throughput, and approaches the length scale of strong dipolar coupling. This method used masked implantation of nitrogen in an ultra-pure CVD-grown diamond substrate through nano-apertures in a thin gold film, patterned via electron-beam lithography and dry etching. The high-density and high-atomic weight of gold results in a mask which is significantly thinner than polymer films used in other works, whilst still successfully impeding ion penetration, with a mask contrast of more than 40 dB. This process allows for the creation of apertures with lower aspect ratio which are therefore easier to pattern in close proximity to one another, i.e., within the dipolar coupling range. The position and spin coherence properties of the resulting near-surface NVs were measured through wide-field super-resolution optically detected magnetic resonance imaging, Hahn echo and CPMG pulsed microwave spectroscopy. The patterning methodology demonstrated here is optimally suited to functional integration with plasmonic nanostructures, which can enhance our ability to control single-photon emission with the prospect of creating near-surface nanoscale sensors of electric or magnetic fields and quantum optoelectronic devices.

Using two different model systems, this thesis considers the old, but fascinating question: how do atoms or particles possessing a particular set of individual characteristics combine to form assemblies with quite distinct, ensemble characteristics, and how do those characteristics evolve as a function of the size of the assembly? For the last thirty years, numerous experiments studying the emergence of collective material properties have focused on a class of semiconducting, colloidal nanocrystals commonly known as quantum dots, which are notable for the size-dependence of their optical properties. Despite years of effort, even the most uniform quantum dot samples possess some heterogeneity in size, shape, and composition, which has prevented complete structure determination and hindered understanding of structure-property relationships. Chapter 1 of this thesis presents an approach to overcoming this challenge and reports the synthesis of a set of four, new, atomically precise cadmium selenide nanocrystal samples, which we call CdSe(350 nm), CdSe(380 nm), CdSe(408 nm), and CdSe(435 nm) after their lowest energy absorption features. We determine their structures and formulas through a combination of single crystal and powder X-ray diffraction measurements, elemental analysis, and spectroscopy. We also describe the optical properties of these samples and their sensitivity to ligand coverage, compare them to other previously reported cadmium selenide nanomaterials, and discuss ongoing experiments. Because CdSe(350 nm), CdSe(380 nm), CdSe(408 nm), and CdSe(435 nm) are atomically precise, they allow us to correlate specific structural features with material properties, which is the focus Chapter 2. Here we present a series of Raman scattering experiments designed to probe the evolution of vibrational structure with size. We find that the Cd-Se stretching region of the Raman spectra exhibits two peaks, which are assigned to primarily surface-derived and interior-derived atomic motions using density functional theory calculations. By performing variable temperature measurements, we discover that the smallest sample, CdSe(350 nm), exhibits behavior that can be well-described using a model developed for small molecules while the vibrations of the largest measured cluster, CdSe(408 nm), are better described by a model developed for bulk materials. This observation is evidence that the transition to a more bulk-like vibrational structure occurs relatively rapidly when cadmium selenide materials are approximately 2 nm in size. The emergence of collective material properties is also the subject of Chapter 3, but the topic is approached from a different perspective. Instead of focusing on a series of atomically precise clusters that differ in size, Chapter 3 presents a series of molecules composed of atomically precise clusters. We prepare octahedral hexaruthenium carbonyl clusters, [Ru₆C(CO)₁₆]²⁻, and use them as building blocks to assemble oligomers linked by single metal atom bridges. We synthesize and structurally characterize a set of compounds varying in length (from monomer to trimer) and linker atom identity (cadmium and mercury) and study the effect on electronic structure using infrared and UV-Visible absorption spectroscopies and density functional theory calculations. With increasing oligomer length, the UV-Vis absorption profile changes and shifts to lower energy, which we attribute in part to the development of coupling between neighboring clusters. Our calculations show that the infinite polymer composed of [Ru₆C(CO)₁₆]²⁻ linked by Hg²⁺ would be a one-dimensional semiconductor with a 1.5 eV direct band-gap. More detailed abstracts can be found at the beginning of each chapter.