Acoustic Manipulation and Alignment of Particles for Applications in Separation, Micro-Templating, and Device Fabrication

This dissertation studies the manipulation of particles using acoustic stimulation for applications in microfluidics and templating of devices. The term particle is used here to denote any solid, liquid or gaseous material that has properties, which are distinct from the fluid in which it is suspended. Manipulation means to take over the movements of the particles and to position them in specified locations. Using devices, microfabricated out of silicon, the behavior of particles under the acoustic stimulation was studied with the main purpose of aligning the particles at either low-pressure zones, known as the nodes or high-pressure zones, known as anti-nodes. By aligning particles at the nodes in a flow system, these particles can be focused at the center or walls of a microchannel in order to ultimately separate them. These separations are of high scientific importance, especially in the biomedical domain, since acoustopheresis provides a unique approach to separate based on density and compressibility, unparalleled by other techniques. The study of controlling and aligning the particles in various geometries and configurations was successfully achieved by controlling the acoustic waves. Apart from their use in flow systems, a stationary suspended-particle device was developed to provide controllable light transmittance based on acoustic stimuli. Using a glass compartment and a carbon-particle suspension in an organic solvent, the device responded to acoustic stimulation by aligning the particles. The alignment of light-absorbing carbon particles afforded an increase in visible light transmittance as high as 84.5%, and it was controlled by adjusting the frequency and amplitude of the acoustic wave. The device also demonstrated alignment memory rendering it energy-efficient. A similar device for suspended-particles in a monomer enabled the development of electrically conductive films. These films were based on networks of conductive particles. Elastomers doped with conductive metal particles were rendered surface conductive at particle loadings as low as 1% by weight using acoustic focusing. The resulting films were flexible and had transparencies exceeding 80% in the visible spectrum (400-800 nm) These films had electrical bulk conductivities exceeding 50 S/cm.

Cascade artificial neural networks technique for solving ellipsometry problems

Ellipsometry is a well known optical technique used for the characterization of reflective surfaces in study and films between two media. It is based on measuring the change in the state of polarization that occurs as a beam of polarized light is reflected from or transmitted through the film. Measuring this change can be used to calculate parameters of a single layer film such as the thickness and the refractive index. However, extracting these parameters of interest requires significant numerical processing due to the noninvertible equations. Typically, this is done using least squares solving methods which are slow and adversely affected by local minima in the solvable surface. This thesis describes the development and implementation of a new technique using only Artificial Neural Networks (ANN) to calculate thin film parameters. The new method offers a speed in the orders of magnitude faster than preceding methods and convergence to local minima is completely eliminated.

Influência da desordem estrutural nas propriedades magnéticas de filmes finos de Co2FeAl

Thin films of Co2FeAl (CFA) and trilayers with CFA/M/CFA, where M is Au or Ag, produced by magnetron sputtering onto glass and oriented (MgO (100)) substrates were investigated. The structural, magnetic static and magnetic dynamics properties were analyzed by distinct experimental techniques. Through X-ray diffraction was observed an A2 phase for the samples (completely disordered), where the atoms are randomly located in the lattice. The static magnetic behavior, in some samples, reveal a plateau behavior generated by a biphasic system. The magnetoimpedance measurements were performed by varying the angle between the external magnetic field and current with respect of anisotropy direction. For this reason, the MI results show a asymmetric magnetoimpedance (AMI) behavior. For the single and trilayers samples with 500 nm-thick, the AMI effect is more evident in comparison with samples with 1000 nm-thick. Therefore, in this work was stablished a route to produce Heusler alloy samples with A2 phase in thin film geometry onto amorphous and oriented substrates, and due to structural disorder was possible to study the hysteretic and MI asymmetric effects.

Estudo da sinterização e oxidação anódica na produção de um capacitor eletrolítico de nióbio

It seeks to find an alternative to the current tantalum electrolytic capacitors in the market due to its high cost. Niobium is a potential replacement for be lighter and cheaper than tantalum. They belong to the same table group periodically and thus exhibit several physical and chemical properties similar. Niobium is used in many technologically important applications, and Brazil has the largest reserves, around 96%. These electrolytic capacitors have high specific capacitance, so they can store high energy in small volumes compared to other types of capacitors. This is the main attraction of this type of capacitor because is growing demand in the production of capacitors with capacitance specifies increasingly high, this because of the miniaturization of various devices such as GPS devices, televisions, computers, phones and many others. The production route of the capacitor was made by powder metallurgy. The initial niobium poder was first characterized by XRD, SEM and laser particle size to then be sieved into particle size 400mesh. The powder was then compacted at pressure of 150MPa and sintered at 1400, 1450 and 1500°C using two sintering time 30 and 60min. Sintering is an important part of the process as it affects properties as porosity and surface cleaning of the samples, which greatly affected the quality of the capacitor. After sintering the samples were underwent a process of anodic oxidation (anodizing), which created a thin film of niobium pentoxide over the whole surface of the sample, this film is the dielectric capacitor. The anodizing process variables influenced a lot in film formation and consequently the capacitor. The samples were characterized by electrical measurements of capacitance, loss factor and ESR (equivalent series resistance). The sintering has affected the porosity and in turn the specific area of the samples. The capacitor area is directly related to the capacitance, that is, the higher the specific area is the capacitance. Higher sintering temperatures decrease the surface area but eliminate as many impurities. The best results were obtained at a temperature of 1400°C with 60 minutes. The most interesting results were compared with the specific capacitance and ESR for all samples.

Estudo da sinterização e oxidação anódica na produção de um capacitor eletrolítico de nióbio

It seeks to find an alternative to the current tantalum electrolytic capacitors in the market due to its high cost. Niobium is a potential replacement for be lighter and cheaper than tantalum. They belong to the same table group periodically and thus exhibit several physical and chemical properties similar. Niobium is used in many technologically important applications, and Brazil has the largest reserves, around 96%. These electrolytic capacitors have high specific capacitance, so they can store high energy in small volumes compared to other types of capacitors. This is the main attraction of this type of capacitor because is growing demand in the production of capacitors with capacitance specifies increasingly high, this because of the miniaturization of various devices such as GPS devices, televisions, computers, phones and many others. The production route of the capacitor was made by powder metallurgy. The initial niobium poder was first characterized by XRD, SEM and laser particle size to then be sieved into particle size 400mesh. The powder was then compacted at pressure of 150MPa and sintered at 1400, 1450 and 1500°C using two sintering time 30 and 60min. Sintering is an important part of the process as it affects properties as porosity and surface cleaning of the samples, which greatly affected the quality of the capacitor. After sintering the samples were underwent a process of anodic oxidation (anodizing), which created a thin film of niobium pentoxide over the whole surface of the sample, this film is the dielectric capacitor. The anodizing process variables influenced a lot in film formation and consequently the capacitor. The samples were characterized by electrical measurements of capacitance, loss factor and ESR (equivalent series resistance). The sintering has affected the porosity and in turn the specific area of the samples. The capacitor area is directly related to the capacitance, that is, the higher the specific area is the capacitance. Higher sintering temperatures decrease the surface area but eliminate as many impurities. The best results were obtained at a temperature of 1400°C with 60 minutes. The most interesting results were compared with the specific capacitance and ESR for all samples.

Synthesis and characterization of some carbon based nanostructures

The aim of present paper is to present the latest results on investigations of the carbon thin film deposited by Thermionic Vacuum Arc (TVA) method and laser pyrolysis. X-ray photoelectron spectroscopy (XPS) and X-ray generated Auger electron spectroscopy (XAES) were used to determine composition and sp2 to sp3 ratios in the outer layers of the film surfaces. The analyses were conducted in a Thermoelectron ESCALAB 250 electron spectrometer equipped with a hemispherical sector energy analyser. Monochromated Al K X-radiation was employed for the XPS examination, at source excitation energy of 15 KeV and emission current of 20 mA. Analyzer pass energy of 20 eV with step size of 0.1 eV and dwell time of 100 ms was used throughout. © 2010 SPIE.

Epitaxial growth of visible to infra-red transparent conducting In2O3 nanodot dispersions and reversible charge storage as a Li-ion battery anode

Unique bimodal distributions of single crystal epitaxially grown In2O3 nanodots on silicon are shown to have excellent IR transparency greater than 87% at IR wavelengths up to 4 μm without sacrificing transparency in the visible region. These broadband antireflective nanodot dispersions are grown using a two-step metal deposition and oxidation by molecular beam epitaxy, and backscattered diffraction confirms a dominant (111) surface orientation. We detail the growth of a bimodal size distribution that facilitates good surface coverage (80%) while allowing a significant reduction in In2O3 refractive index. This unique dispersion offers excellent surface coverage and three-dimensional volumetric expansion compared to a thin film, and a step reduction in refractive index compared to bulk active materials or randomly porous composites, to more closely match the refractive index of an electrolyte, improving transparency. The (111) surface orientation of the nanodots, when fully ripened, allows minimum lattice mismatch strain between the In2O3 and the Si surface. This helps to circumvent potential interfacial weakening caused by volume contraction due to electrochemical reduction to lithium, or expansion during lithiation. Cycling under potentiodynamic conditions shows that the transparent anode of nanodots reversibly alloys lithium with good Coulombic efficiency, buffered by co-insertion into the silicon substrate. These properties could potentially lead to further development of similarly controlled dispersions of a range of other active materials to give transparent battery electrodes or materials capable of non-destructive in situ spectroscopic characterization during charging and discharging.

Selective Preparation of Semiconducting Single-Walled Carbon Nanotubes: From Fundamentals to Applications

Aligned single-walled carbon nanotubes (SWNTs) synthesized by the chemical vapor deposition (CVD) method have exceptional potential for next-generation nanoelectronics. However, there are considerable challenges in the preparation of semiconducting (s-) SWNTs with controlled properties (e.g., density, selectivity, and diameter) for their application in solving real-world problems. This dissertation describes research that aims to overcome the limitations by novel synthesis strategies and post-growth treatment. The application of as-prepared SWNTs as functional devices is also demonstrated. The dissertation includes the following parts: 1) decoupling the conflict between density and selectivity of s-SWNTs in CVD growth; 2) investigating the importance of diameter control for the selective synthesis of s-SWNTs; 3) synthesizing highly conductive SWNT thin film by thiophene-assisted CVD method; 4) eliminating metallic pathways in SWNT crossbars by gate-free electrical breakdown method; 5) enhancing the density of SWNT arrays by strain-release method; 6) studying the sensing mechanism of SWNT crossbar chemical sensors.

The morphology of ordered block copolymer patterns as probed by high resolution imaging

The microphase separation of block copolymer (BCP) thin films can afford a simple and cost-effective means to studying nanopattern surfaces, and especially the fabrication of nanocircuitry. However, because of complex interface effects and other complications, their 3D morphology, which is often critical for application, can be more complex than first thought. Here, we describe how emerging microscopic methods may be used to study complex BCP patterns and reveal their rich detail. These methods include helium ion microscopy (HIM) and high resolution x-section transmission electron microscopy (XTEM), and complement conventional secondary electron and atomic force microscopies (SEM and TEM). These techniques reveal that these structures are quite different to what might be expected. We illustrate the advances in the understanding of BCP thin film morphology in several systems, which result from this characterization. The systems described include symmetric, lamellar forming polystyrene-b-polymethylmethacrylate (PS-b-PMMA), cylinder forming polystyrene-b-polydimethylsiloxane (PS-b-PDMS), as well as lamellar and cylinder forming patterns of polystyrene-b-polyethylene oxide (PS-b-PEO) and polystyrene-b-poly-4-vinylpyridine (PS-b-P4VP). Each of these systems exhibits more complex arrangements than might be first thought. Finding and developing techniques whereby complex morphologies, particularly at very small dimensions, can be determined is critical to the practical use of these materials in many applications. The importance of quantifying these complex morphologies has implications for their use in integrated circuit manufacture, where they are being explored as alternative pattern forming methods to conventional UV lithography.

Design of Transparent Conducting Electrodes and Surface Relief Gratings for Plasmonic Polymer Solar Cells

As the concept of renewable energy becomes increasingly important in the modern society, a considerable amount of research has been conducted in the field of organic photovoltaics in recent years. Although organic solar cells generally have had lower efficiencies compared to silicon solar cells, they have the potential to be mass produced via solution processing. A common polymer solar cell architecture relies on the usage of P3HT (electron donor) and PCBM (electron acceptor) bulk heterojunction. One of the main issues with this configuration is that in order to compensate for the high exciton recombination rate, the photoactive layer is often made very thin (on the order of 100 $%). This results in low solar cell photocurrents due to low absorption. This thesis investigates a novel method of light trapping by coupling surface plasmons at the electrode interface via surface relief gratings, leading to EM field enhancements and increased photo absorption. Experimental work was first conducted on developing and optimizing a transparent electrode of the form &'()/+,/&'() to replace the traditional ITO electrode since the azopolymer gratings cannot withstand the high temperature processing of ITO films. It was determined that given the right thickness profiles and deposition conditions, the MAM stack can achieve transmittance and conductivity similar to ITO films. Experimental work was also conducted on the fabrication and characterization of surface relief gratings, as well as verification of the surface plasmon generation. Surface relief gratings were fabricated easily and accurately via laser interference lithography on photosensitive azopolymer films. Laser diffraction studies confirmed the grating pitch, which is dependent on the incident angle and wavelength of the writing beam. AFM experiments were conducted to determine the surface morphology of the gratings, before and after metallic film deposition. It was concluded that metallic film deposition does not significantly alter the grating morphologies.

Design of Transparent Conducting Electrodes and Surface Relief Gratings for Plasmonic Polymer Solar Cells

As the concept of renewable energy becomes increasingly important in the modern society, a considerable amount of research has been conducted in the field of organic photovoltaics in recent years. Although organic solar cells generally have had lower efficiencies compared to silicon solar cells, they have the potential to be mass produced via solution processing. A common polymer solar cell architecture relies on the usage of P3HT (electron donor) and PCBM (electron acceptor) bulk heterojunction. One of the main issues with this configuration is that in order to compensate for the high exciton recombination rate, the photoactive layer is often made very thin (on the order of 100 $%). This results in low solar cell photocurrents due to low absorption. This thesis investigates a novel method of light trapping by coupling surface plasmons at the electrode interface via surface relief gratings, leading to EM field enhancements and increased photo absorption. Experimental work was first conducted on developing and optimizing a transparent electrode of the form &'()/+,/&'() to replace the traditional ITO electrode since the azopolymer gratings cannot withstand the high temperature processing of ITO films. It was determined that given the right thickness profiles and deposition conditions, the MAM stack can achieve transmittance and conductivity similar to ITO films. Experimental work was also conducted on the fabrication and characterization of surface relief gratings, as well as verification of the surface plasmon generation. Surface relief gratings were fabricated easily and accurately via laser interference lithography on photosensitive azopolymer films. Laser diffraction studies confirmed the grating pitch, which is dependent on the incident angle and wavelength of the writing beam. AFM experiments were conducted to determine the surface morphology of the gratings, before and after metallic film deposition. It was concluded that metallic film deposition does not significantly alter the grating morphologies.

High-Performance WSe2 CMOS Technology and Integrated Circuits.

Because of their extraordinary structural and electrical properties, two dimensional materials are currently being pursued for applications such as thin-film transistors and integrated circuit. One of the main challenges that still needs to be overcome for these applications is the fabrication of air-stable transistors with industry-compatible complementary metal oxide semiconductor (CMOS) technology. In this work, we experimentally demonstrate a novel high performance air-stable WSe2 CMOS technology with almost ideal voltage transfer characteristic, full logic swing and high noise margin with different supply voltages. More importantly, the inverter shows large voltage gain (~38) and small static power (Pico-Watts), paving the way for low power electronic system in 2D materials.

Structural and Electrical Investigation of C 60 –Graphene Vertical Heterostructures

Graphene, with its unique electronic and structural qualities, has become an important playground for studying adsorption and assembly of various materials including organic molecules. Moreover, organic/graphene vertical structures assembled by van der Waals interaction have potential for multifunctional device applications. Here, we investigate structural and electrical properties of vertical heterostructures composed of C60 thin film on graphene. The assembled film structure of C60 on graphene is investigated using transmission electron microscopy, which reveals a uniform morphology of C60 film on graphene with a grain size as large as 500 nm. The strong epitaxial relations between C60 crystal and graphene lattice directions are found, and van der Waals ab initio calculations support the observed phenomena. Moreover, using C60-graphene heterostructures, we fabricate vertical graphene transistors incorporating n-type organic semiconducting materials with an on/off ratio above 3 × 10(3). Our work demonstrates that graphene can serve as an excellent substrate for assembly of molecules, and attained organic/graphene heterostructures have great potential for electronics applications.

A boron nitride - graphene - C60 heterostructure

We have fabricated a new van-der-Waals heterostructure composed by BN/graphene/C60. We performed transport measurements on the preliminary BN/graphene device finding a sharp Dirac point at the neutrality point. After the deposition of a C60 thin film by thermal evaporation, we have observed a significant n-doping of the heterostructure. This suggests an unusual electron transfer from C60 into the BN/graphene structure. This BN/graphene/C60 heterostructure can be of interest in photovoltaic applications. It can be used to build devices like p-n junctions, where C60 can be easily deposited in defined regions of a graphene junction by the use of a shadow mask. Our results are contrasted with theoretical calculations.

Manifestation of Colossal Thermoelectric Power by Charge Ordered Small and Intermediate Bandwidth Manganites

Thermoelectric materials are revisited for various applications including power generation. The direct conversion of temperature differences into electric voltage and vice versa is known as thermoelectric effect. Possible applications of thermoelectric materials are in eco-friendly refrigeration, electric power generation from waste heat, infrared sensors, temperature controlled-seats and portable picnic coolers. Thermoelectric materials are also extensively researched upon as an alternative to compression based refrigeration. This utilizes the principle of Peltier cooling. The performance characteristic of a thermoelectric material, termed as figure of merit (ZT) is a function of several transport coefficients such as electrical conductivity (σ), thermal conductivity (κ) and Seebeck coefficient of the material (S). ZT is expressed asκσTZTS2=, where T is the temperature in degree absolute. A large value of Seebeck coefficient, high electrical conductivity and low thermal conductivity are necessary to realize a high performance thermoelectric material. The best known thermoelectric materials are phonon-glass electron – crystal (PGEC) system where the phonons are scattered within the unit cell by the rattling structure and electrons are scattered less as in crystals to obtain a high electrical conductivity. A survey of literature reveals that correlated semiconductors and Kondo insulators containing rare earth or transition metal ions are found to be potential thermoelectric materials. The structural magnetic and charge transport properties in manganese oxides having the general formula of RE1−xAExMnO3 (RE = rare earth, AE= Ca, Sr, Ba) are solely determined by the mixed valence (3+/4+) state of Mn ions. In strongly correlated electron systems, magnetism and charge transport properties are strongly correlated. Within the area of strongly correlated electron systems the study of manganese oxides, widely known as manganites exhibit unique magneto electric transport properties, is an active area of research.Strongly correlated systems like perovskite manganites, characterized by their narrow localized band and hoping conduction, were found to be good candidates for thermoelectric applications. Manganites represent a highly correlated electron system and exhibit a variety of phenomena such as charge, orbital and magnetic ordering, colossal magneto resistance and Jahn-Teller effect. The strong inter-dependence between the magnetic order parameters and the transport coefficients in manganites has generated much research interest in the thermoelectric properties of manganites. Here, large thermal motion or rattling of rare earth atoms with localized magnetic moments is believed to be responsible for low thermal conductivity of these compounds. The 4f levels in these compounds, lying near the Fermi energy, create large density of states at the Fermi level and hence they are likely to exhibit a fairly large value of Seebeck coefficient.