Novel metal insulator metal capacitors based on electrosprayed colloidal nanoparticles

Tesi en modalitat de compendi de publicacions. Aplicat embargament des de la data de defensa fins el 7/1/2022This work develops a novel capacitor device based on the use of nanotechnology. The device starts from the exiting metal-insulator-metal (MIM) concept, but instead of a continuous insulator layer, dielectric nanoparticles are used. Nanoparticles are mainly of silicon oxide (silica) and polystyrene (PS) and the diameter values are 255nm and 295nm respectively. The nanoparticles contribute to a very high surface to volume ratio and are easily available at low cost. The deposition technique developed in this work is the electrospray, which is a bottom-up fabrication technology that allows batch processing and achieves a good compromise between large area and low deposition time. With the objective of increasing the deposit surface, the electrospray set-up has been tuned to allow deposition areas from 1cm2 to 25cm2. The fabricated devices, the so called nanoparticles metal insulator metal (NP-MIM) capacitors, offer higher capacitance values than a similar conventional capacitor with a continuous insulator layer. In the case of silica NP-MIMs, a factor as high as 1000 of capacitance enhancement is achieved, whereas polystyrene NP-MIMs has capacitance gain of 11. In addition, silica NP-MIMs show capacitive behaviours in a certain frequency range which depends on the humidity and thickness of the nanoparticles layer, while polystyrene MIMs always maintain their capacitive behaviour. The fabricated devices have been characterized by scanning electron microscopy (SEM) measurements complemented with focusing ion beam (FIB) drilling to characterise the topography of the NP-MIMs. The devices have also been characterized by impedance spectroscopy measurements, at different temperatures and humidifies. The origin of the enhanced capacitance is associated in part to humidity in the nanoparticles interfaces. A circuital model based on distributed elements has been developed to fit and predict the electrical behaviour of the NP-MIMs. In summary, this thesis shows the design, fabrication, characterization and modelling of a new promising nanoparticles metal-insulator-metal capacitor that may pave the way to the development of a novel MIM-supercapacitor technology.Este trabajo desarrolla un novedoso dispositivo condensador basado en el uso de la nanotecnología. El dispositivo parte del concepto existente de metal-aislador-metal (MIM), pero en lugar de una capa aislante continua, se utilizan nanopartículas dieléctricas. Las nanopartículas son principalmente de óxido de silicio (sílice) y poliestireno (PS) y los valores de diámetro son 255nm y 295nm respectivamente. Las nanopartículas contribuyen a una alta relación superficie/volumen y están fácilmente disponibles a bajo costo. La técnica de depósito desarrollada en este trabajo es el electrospray, que es una tecnología de fabricación ascendente (bottom-up) que permite el procesamiento por lotes y logra un buen compromiso entre una gran superficie y un bajo tiempo de depósito. Con el objetivo de aumentar la superficie de depósito, la configuración de electrospray ha sido ajustada para permitir áreas de depósito de 1cm2 a 25cm2. El dispositivo fabricado, los llamados condensadores de metal aislante de nanopartículas (NP-MIM) ofrecen valores de capacitancia más altos que un condensador convencional similar con una capa aislante continua. En el caso de los NP-MIM de sílice, se alcanza un factor de hasta 1000 de mejora de la capacidad, mientras que los NP-MIM de poliestireno tienen una ganancia de capacidad de 11. Además, los NP-MIM de sílice muestran comportamientos capacitivos en un cierto rango de frecuencias que depende de la humedad y el grosor de la capa de nanopartículas, mientras que los MIM de poliestireno siempre mantienen su comportamiento capacitivo. Los dispositivos fabricados se han caracterizado por mediciones de microscopía electrónica de barrido (SEM) complementadas con perforaciones de haz de iones de enfoque (FIB) para caracterizar la topografía de los NP-MIMs. Los dispositivos también se han caracterizado por mediciones de espectroscopia de impedancia, a diferentes temperaturas y humedades. El origen de la capacitancia aumentada está asociado en parte a la humedad en las interfaces de las nanopartículas. Se ha desarrollado un modelo de un circuito basado en elementos distribuidos para ajustar y predecir el comportamiento eléctrico de los NP-MIMs. En resumen, esta tesis muestra el diseño, fabricación, caracterización y modelización de un nuevo y prometedor condensador nanopartículas metal-aislante-metal que puede allanar el camino para el desarrollo de una nueva tecnología de supercondensadores MIM.Postprint (published version

Fabrication of Three-Dimensionally Ordered Nanostructured Materials Through Colloidal Crystal Templating

The void spaces in colloidal crystals (opals, three-dimensional (3D) close-packed arrays of silica nanospheres) and their replicas are used as templates in the fabrication of new nanostructured materials. 3D ordered nanomeshes and nanosphere arrays are readily obtained by chemical and/or electrochemical methods. Using silica opal templates, metals or polymers are infiltrated into the interstices between the silica nanospheres. Subsequent dissolution of the opals with HF solution produces open 3D mesh structures. Metal (such as Ni, Co, Fe, Pd, Au, Ag, and Cu) and conductive polymer (such as polyaniline) meshes are obtained by electrochemical deposition approach, while the nonconductive polymer (such as poly(methyl methacrylate) (PMMA)) meshes are synthesized by chemical polymerization method. Some new types of meshes are fabricated by the conversion of metal meshes and polymer meshes. NiO meshes are formed by oxidizing Ni meshes in the air. The NiO meshes exhibit higher volume occupation fraction than Ni meshes and the nanocrystalline sizes of NiO particles can be adjusted by the oxidation temperature. Due to the mechanical flexibility of polymer meshes, the compression of PMMA meshes produces deformed PMMA meshes which contain oblate pores. These meshes can be again served as templates to prepare new types of colloidal crystals (nanosphere arrays) and specific nanocomposites. By the use of poorly conductive NiO mesh or PMMA mesh arrays as templates, 3D periodic metal nanosphere arrays, such as those of Ni, Co, Au and Pd, are readily fabricated by the electrodeposition method. Metal/NiO or Metal/PMMA composites can also be obtained if the templates are left intact. The magnetic behavior of metal (such as Ni and Co) meshes and sphere arrays has been investigated. These nanoscale arrays show significantly enhanced coercivities compared with bulk metals, due to the size effect of the nanometer dimensions of the components in meshes and sphere arrays. Angle-dependent magnetic properties of Ni and Co sphere array membranes exhibit out-of-plane anisotropy

Fabrication of Three-Dimensionally Ordered Nanostructured Materials Through Colloidal Crystal Templating

The void spaces in colloidal crystals (opals, three-dimensional (3D) close-packed arrays of silica nanospheres) and their replicas are used as templates in the fabrication of new nanostructured materials. 3D ordered nanomeshes and nanosphere arrays are readily obtained by chemical and/or electrochemical methods. Using silica opal templates, metals or polymers are infiltrated into the interstices between the silica nanospheres. Subsequent dissolution of the opals with HF solution produces open 3D mesh structures. Metal (such as Ni, Co, Fe, Pd, Au, Ag, and Cu) and conductive polymer (such as polyaniline) meshes are obtained by electrochemical deposition approach, while the nonconductive polymer (such as poly(methyl methacrylate) (PMMA)) meshes are synthesized by chemical polymerization method. Some new types of meshes are fabricated by the conversion of metal meshes and polymer meshes. NiO meshes are formed by oxidizing Ni meshes in the air. The NiO meshes exhibit higher volume occupation fraction than Ni meshes and the nanocrystalline sizes of NiO particles can be adjusted by the oxidation temperature. Due to the mechanical flexibility of polymer meshes, the compression of PMMA meshes produces deformed PMMA meshes which contain oblate pores. These meshes can be again served as templates to prepare new types of colloidal crystals (nanosphere arrays) and specific nanocomposites. By the use of poorly conductive NiO mesh or PMMA mesh arrays as templates, 3D periodic metal nanosphere arrays, such as those of Ni, Co, Au and Pd, are readily fabricated by the electrodeposition method. Metal/NiO or Metal/PMMA composites can also be obtained if the templates are left intact. The magnetic behavior of metal (such as Ni and Co) meshes and sphere arrays has been investigated. These nanoscale arrays show significantly enhanced coercivities compared with bulk metals, due to the size effect of the nanometer dimensions of the components in meshes and sphere arrays. Angle-dependent magnetic properties of Ni and Co sphere array membranes exhibit out-of-plane anisotropy

Digital Fabrication of Transparent Electrodes for Simultaneously Optical and Electrochemical Biosensor Applications

This dissertation proposes a pioneering biosensor for detecting cancer biomarkers that combines electrochemical and optical recognition in the same analyzing spot, by taking advantage of laser direct writing to pattern transparent and flexible electrodes. The bi-omarker, the carcinoembryonic antigen (CEA), is detected by an antibody-like biomi-metic material as recognizing agent. The sensing film over indium tin oxide (ITO) coated glass substrate consisted of a molecularly imprinted layer of polypyrrole (PPy). The im-printed film is assembled on a three-dimensional photonic crystal composed of silica na-noparticles (NPs), allowing optical detection. The molecularly imprinted polymer was obtained through electropolymerization of pyrrole in the presence of the biomarker as template. The interaction between the biomarker and the sensing material produces elec-trochemical signals generating quantitative data. In addition, it triggers a difference in the reflectance of the sensing photonic film matrix. The analytical features of the biosensor were assessed in PBS buffer by electrochemical impedance spectroscopy (EIS) and by reflectance of the opal-based photonic crystal. The response of the proposed biosensor was in the range of physiological relevant levels of CEA (from 2.5 ng/mL to 10 ng/mL). This sensor was assembled on an ITO substrate as proof of concept, the best substrate among the several conductive material produced on glass support, even if the first part of the work focuses on the preparation of aluminum doped zinc oxide (AZO) thin films by RF sputtering and laser direct writing. These processes were optimized for the fabrication of a highly conductive and transparent oxide capable of replacing ITO in the future as a main component in most transparent and flexible electronics applications. The developed sensing device showed promising features to become a much simpler, faster and low-cost point-of-care (POC) portable solution for the detection of CEA when compared to conventional immunoassay approaches, due to its high sensitivity, stability and dual detection method. At the same time, it may open new doors for other applications and foreseen improvement concerning the early diagnosis of diseases

Probing Charge Generation in 3D Photonic Poly(3-hexylthiophene)/Titanium Dioxide Nanocomposites for Bulk Heterojunction Solar Cells

Organic photovoltaics (OPVs) are attractive for their inexpensiveness, large-scale fabrication methods, flexibility and semi-transparency. OPVs have lower efficiencies than conventional inorganic semiconductor-based solar cells, and hence methods to enhance light-harvesting properties are sought-after. Photonic crystals are unique nanomaterials that present the ability to enhance light-harvesting properties through electromagnetic field localization and slow photon effect. In this work, three-dimensional photonic crystals were successfully integrated into the active layers of bulk-heterojunction solar cells by fabricating a series of titanium dioxide inverse opals coated with poly(3-hexylthiophene). The optical, morphological, and charge generation properties of the nanocomposites were investigated. Transient photoinduced absorption spectroscopy showed enhanced charge generation due to a potential photonic enhancement and the increased interfacial area of the porous structure. This research serves as a proof of concept, where the photonic properties of inverse opals and their high surface area may be exploited with different materials in other solar cell systems

Optical and Photocatalytic Properties of Three-Dimensionally Ordered Macroporous Ta2O5 and Ta3N5 Inverse Opals

Colloidal crystal templating is a simple yet remarkably versatile synthetic strategy toward inverse opal (IO) photonic crystals for optical sensing and catalytic applications. Herein, we report the successful fabrication of tantalum (V) oxide, Ta2O5, inverse opal thin films and powders using the colloidal crystal templating method, utilizing poly(methyl methacrylate) (PMMA) colloidal crystals as sacrificial templates and TaCl5 as the tantalum source. The Ta2O5 IO thin films and powders showed structural color at ultraviolet (UV) and visible wavelengths, with the photonic band gap (PBG) position along the [111] direction increasing linearly with the diameter of macropores (D) in the inverse opals and also the refractive index of the medium filling the macropores, in excellent accord with a modified Bragg’s law expression. Thermal ammonolysis of the Ta2O5 inverse opals at 700 °C yielded well-ordered Ta3N5 IO films and powders possessing high specific surface areas (37 m2 g–1) and a semiconductor band gap of 2.0–2.1 eV. A Pt/Ta3N5 IO photocatalyst delivered a H2 production rate of ∼300 μmol g–1 h–1 in aqueous methanol (10 vol % MeOH) under visible-light irradiation (300 W Xe lamp, λ ≥ 420 nm), approximately twice that achieved using conventional Pt/Ta3N5 powder photocatalysts (161 μmol g–1 h–1, 8.4 m2 g–1). Results demonstrate that inverse opal engineering is an effective approach for realizing Ta2O5 IO thin films for sensing applications and Ta3N5 IOs with enhanced photocatalyst performance

Effect of Ionic Liquid Electrolytes in DSSCs with Titanium Dioxide (TiO2) Inverse Opal Structures

Dye-sensitized solar cells (DSSC) are low-cost alternatives to conventional solar cells that can work well in low-light conditions. Despite considerable study on improving the efficiency of DSSCs, the current liquid electrolyte cell has plateaued at a conversion efficiency of ~ 12%. A major problem with these cells regarding their applicability is the low viscosity and high volatility of the toxic electrolyte, i.e., acetonitrile, which cause leakage and volatilization. We propose that using ionic liquids (ILs), which are more viscous, less volatile, and conductive, may be more suitable electrolytes. However, one unwanted side effect of the higher viscosity of the ILs may be an incomplete infiltration of the DSSC’s nanoporous TiO2 electrode. Here, we present a study of DSSCs with TiO2 inverse opal electrodes of controlled pore sizes (0.1-1 m) and ionic liquid derivatives of 1-alkyl-3-methylimidazolium tetrafluoroborate (alkyl: ethyl, butyl, and decyl) with viscosities ranging from 25.2 to 721 cP. The stability and functionality of the DSSCs is tested using electrochemical techniques that yield current-voltage and power curves

Doctor of Philosophy

dissertationThe synthesis, characterization, and nonclassical optical properties of photonic crystals (PCs) created from naturally occurring biological templates was studied. Biotemplated PCs were created from several different natural structures using sol-gel chemistry methods. PCs were characterized using a combination of reflection spectroscopy, SEM image analysis, three-dimensional structure modeling, photonic band structure calculations, and density of optical states calculations. The effect our PCs had on the density of optical states (DOS) was probed using time correlated single photon counting spectroscopy. By carefully controlling the sol-gel chemistry used in the templating process, it is possible to synthesize hollow silica inverse, solid silica inverse, hollow titania inverse, solid titania inverse, and solid titania replicate structures. The inverse-type structures have the advantage of being accessible through a single templating step, while the titania replica is capable of a predicted full photonic band gap. Each structure was investigated using methods mentioned above. The reliability of reflectance spectroscopy was investigated. It was found that in certain cases, a continuum of structural parameters yield reflections that match photonic band structure calculations. Methods to improve this situation are discussed. When applied to titania inverse opals, it was found that the refractive index could be determined to ±0.05 and the volume fraction to ±0.5%. Accurately determining the refractive index of inverse opals is useful in estimating the refractive index of other PCs made from the same sol-gel. Calculation of the DOS using a combination of MIT's photonic bands package and house-written software was applied to biotemplated photonic crystals. It was found that even partial band gap photonic crystals can greatly modify the DOS. Finally, the rate of spontaneous emission of quantum dots embedded in photonic crystals was measured to indirectly probe the DOS. Three different models were used to extract the lifetime from radiative decay curves. It was found that a log-normal distribution of lifetimes was the most meaningful model. The radiative lifetime of quantum dots embedded in titania photonic crystals replicated from Lamprocyphus augustus was modified by up to a factor of ten, an amount unprecedented in the photonic crystal literature