Effect of oxygen on the electrical characteristics of field effect transistors formed from electrochemically deposited films of poly(3-methylthiophene)

Field effect devices have been formed in which the active layer is a thin film of poly(3-methylthiophene) grown electrochemically onto preformed source and drain electrodes. Although a field effect is present after electrochemical undoping, stable device characteristics with a high modulation ratio are obtained only after vacuum annealing at an elevated temperature, and only then if the devices are held in vacuo. The polymer is shown to be p type and the devices operate in accumulation only. The hole mobility in devices thermally annealed under vacuum is around 10 -3 cm 2 V -1 s -1. On exposure to ambient laboratory air, the device conductance increases by several orders of magnitude. This increase may be reversed by subjecting the device to a further high-temperature anneal under vacuum. Subsidiary experiments show that these effects are caused by the reversible doping of the polymer by gaseous oxygen.

Electronic transport in field-effect transistors of sexithiophene

The electronic conduction of thin-film field-effect-transistors (FETs) of sexithiophene was studied. In most cases the transfer curves deviate from standard FET theory; they are not linear, but follow a power law instead. These results are compared to conduction models of "variable-range hopping" and "multi-trap-and-release". The accompanying IV curves follow a Poole-Frenkel (exponential) dependence on the drain voltage. The results are explained assuming a huge density of traps. Below 200 K, the activation energy for conduction was found to be ca. 0.17 eV. The activation energies of the mobility follow the Meyer-Neldel rule. A sharp transition is seen in the behavior of the devices at around 200 K. The difference in behavior of a micro-FET and a submicron FET is shown. (C) 2004 American Institute of Physics.

Étude sur les propriétés physicochimiques et électrochimiques des liquides ioniques redox et leur application en tant qu’électrolyte dans les supercapaciteurs

Ce mémoire porte sur les recherches et les développements dans le domaine des électrolytes à base de liquide ionique redox. Une nouvelle famille de liquide ionique redox basée sur le ferrocenylsulfonyl(trifluoromethylsulfonyl) (FcNTf) a été développée et étudiée pour la première fois afin de démontrer le potentiel de ces liquides ioniques dans les dispositifs de stockage d’énergie. En premier lieu, les liquides ioniques redox (RILs) composés de l’anion électroactif et du cation d’alkylimidazolium sont synthétisés et caractérisés. L’impact de la variation des chaînes alkyles du cation sur les propriétés physicochimiques et électrochimiques du RIL a été étudié. À une faible concentration en solution, l’impact du cation a peu d’influence sur l’ensemble des propriétés. Cependant, à haute concentration (>50 % massique) et sans électrolyte de support, la formation de films en oxydation a été observée à l'électrode positive. Ce point est intéressant pour les futures recherches et développements dans le domaine, puisque la variation des chaînes alkyles du cation des liquides ioniques redox et la formation de films lors de l’oxydation du FcNTf est peu connue et comprise en littérature. De plus, l’optimisation des conditions de solution d'électrolyte RIL dans les supercapaciteurs est aussi présentée. En deuxième lieu, la mise en application des RILs dans les supercapaciteurs a été testée. La performance énergétique et le mécanisme d’autodécharge ont été ciblés dans cette étude. En présence de l’électrolyte redox, la contribution des réactions faradaiques permet d'accomplir un gain énergique de 287 % versus les systèmes purement capacitifs. À cause de la formation de film à l’électrode, l’électrolyte redox FcNTf joue un rôle primordial dans la prévention de l’autodécharge versus les liquides ioniques qui étaient connus jusqu’à présent. Finalement, ce mémoire a permis de mieux comprendre les effets structure-propriétés relative aux modifications du cation chez les liquides ioniques redox.

CO-ELECTROPHORETIC DEPOSITION OF LIQUID METAL AND SILICON FOR LITHIUM-ION BATTERY APPLICATION

A low cost electrophoretic deposition (EPD) process was successfully used for liquid metal thin film deposition with a high depositing rate of 0.6 µ/min. Furthermore, silicon nano-powder and liquid metal were then simultaneously deposited as the negative electrode of lithium-ion battery by a technology called co-EPD. The liquid metal was hoping to act as the matrix for silicon particles during lithium ion insertion and distraction. Half-cell testing was performed using as prepared co-EPD sample. An initial discharge capacity of 1500 mAh/g was reported for nano-silicon and galinstan electrode, although the capacity fading issue of these samples was also observed.

NONLINEAR EFFECTS IN MAGNETIC GARNET FILMS AND NONRECIPROCAL OPTICAL BLOCH OSCILLATIONS IN WAVEGUIDE ARRAYS

This dissertation presents detailed experimental and theoretical investigations of nonlinear and nonreciprocal effects in magnetic garnet films. The dissertation thus comprises two major sections. The first section concentrates on the study of a new class of nonlinear magneto-optic thin film materials possessing strong higher order magnetic susceptibility for nonlinear optical applications. The focus was on enlarging the nonlinear performance of ferrite garnet films by strain generation and compositional gradients in the sputter-deposition growth of these films. Under this project several bismuth-substituted yttrium iron garnet (Bi,Y) 3 (Fe,Ga)5 O12(acronym as Bi:YIG) films have been sputter-deposited over gadolinium gallium garnet (Gd 3 Ga5 O12 ) substrates and characterized for their nonlinear optical response. One of the important findings of this work is that lattice mismatch strain drives the second harmonic (SH) signal in the Bi:YIG films, in agreement with theoretical predictions; whereas micro-strain was found not to correlate significantly with SH signal at the micro-strain levels present in these films. This study also elaborates on the role of the film's constitutive elements and their concentration gradients in nonlinear response of the films. Ultrahigh sensitivity delivered by second harmonic generation provides a new exciting tool for studying magnetized surfaces and buried interfaces, making this work important from both a fundamental and application point of view. The second part of the dissertation addresses an important technological need; namely the development of an on-chip optical isolator for use in photonic integrated circuits. It is based on two related novel effects, nonreciprocal and unidirectional optical Bloch oscillations (BOs), recently proposed and developed by Professor Miguel Levy and myself. This dissertation work has established a comprehensive theoretical background for the implementation of these effects in magneto-optic waveguide arrays. The model systems we developed consist of photonic lattices in the form of one-dimensional waveguide arrays where an optical force is introduced into the array through geometrical design turning the beam sideways. Laterally displaced photons are periodically returned to a central guide by photonic crystal action. The effect leads to a novel oscillatory optical phenomenon that can be magnetically controlled and rendered unidirectional. An on-chip optical isolator was designed based on the unidirectionality of the magneto-opticBloch oscillatory motion. The proposed device delivers an isolation ratio as high as 36 dB that remains above 30 dB in a 0.7 nm wavelength bandwidth, at the telecommunication wavelength 1.55 μm. Slight modifications in isolator design allow one to achieve an even more impressive isolation ratio ~ 55 dB, but at the expense of smaller bandwidth. Moreover, the device allows multifunctionality, such as optical switching with a simultaneous isolation function, well suited for photonic integrated circuits.

Functionalized Carbon Micro/Nanostructures for Biomolecular Detection

Advancements in the micro-and nano-scale fabrication techniques have opened up new avenues for the development of portable, scalable and easier-to-use biosensors. Over the last few years, electrodes made of carbon have been widely used as sensing units in biosensors due to their attractive physiochemical properties. The aim of this research is to investigate different strategies to develop functionalized high surface carbon micro/nano-structures for electrochemical and biosensing devices. High aspect ratio three-dimensional carbon microarrays were fabricated via carbon microelectromechanical systems (C-MEMS) technique, which is based on pyrolyzing pre-patterned organic photoresist polymers. To further increase the surface area of the carbon microstructures, surface porosity was introduced by two strategies, i.e. (i) using F127 as porogen and (ii) oxygen reactive ion etch (RIE) treatment. Electrochemical characterization showed that porous carbon thin film electrodes prepared by using F127 as porogen had an effective surface area (Aeff 185%) compared to the conventional carbon electrode. To achieve enhanced electrochemical sensitivity for C-MEMS based functional devices, graphene was conformally coated onto high aspect ratio three-dimensional (3D) carbon micropillar arrays using electrostatic spray deposition (ESD) technique. The amperometric response of graphene/carbon micropillar electrode arrays exhibited higher electrochemical activity, improved charge transfer and a linear response towards H2O2 detection between 250μM to 5.5mM. Furthermore, carbon structures with dimensions from 50 nano-to micrometer level have been fabricated by pyrolyzing photo-nanoimprint lithography patterned organic resist polymer. Microstructure, elemental composition and resistivity characterization of the carbon nanostructures produced by this process were very similar to conventional photoresist derived carbon. Surface functionalization of the carbon nanostructures was performed using direct amination technique. Considering the need for requisite functional groups to covalently attach bioreceptors on the carbon surface for biomolecule detection, different oxidation techniques were compared to study the types of carbon–oxygen groups formed on the surface and their percentages with respect to different oxidation pretreatment times. Finally, a label-free detection strategy using signaling aptamer/protein binding complex for platelet-derived growth factor oncoprotein detection on functionalized three-dimensional carbon microarrays platform was demonstrated. The sensor showed near linear relationship between the relative fluorescence difference and protein concentration even in the sub-nanomolar range with an excellent detection limit of 5 pmol.

Gravimetric and density profiling using the combination of surface acoustic waves and neutron reflectivity

A new approach is described herein, where neutron reflectivity measurements that probe changes in the density profile of thin films as they absorb material from the gas phase have been combined with a Love wave based gravimetric assay that measures the mass of absorbed material. This combination of techniques not only determines the spatial distribution of absorbed molecules, but also reveals the amount of void space within the thin film (a quantity that can be difficult to assess using neutron reflectivity measurements alone). The uptake of organic solvent vapours into spun cast films of polystyrene has been used as a model system with a view to this method having the potential for extension to the study of other systems. These could include, for example, humidity sensors, hydrogel swelling, biomolecule adsorption or transformations of electroactive and chemically reactive thin films. This is the first ever demonstration of combined neutron reflectivity and Love wave-based gravimetry and the experimental caveats, limitations and scope of the method are explored and discussed in detail.

Étude sur les propriétés physicochimiques et électrochimiques des liquides ioniques redox et leur application en tant qu’électrolyte dans les supercapaciteurs

Ce mémoire porte sur les recherches et les développements dans le domaine des électrolytes à base de liquide ionique redox. Une nouvelle famille de liquide ionique redox basée sur le ferrocenylsulfonyl(trifluoromethylsulfonyl) (FcNTf) a été développée et étudiée pour la première fois afin de démontrer le potentiel de ces liquides ioniques dans les dispositifs de stockage d’énergie. En premier lieu, les liquides ioniques redox (RILs) composés de l’anion électroactif et du cation d’alkylimidazolium sont synthétisés et caractérisés. L’impact de la variation des chaînes alkyles du cation sur les propriétés physicochimiques et électrochimiques du RIL a été étudié. À une faible concentration en solution, l’impact du cation a peu d’influence sur l’ensemble des propriétés. Cependant, à haute concentration (>50 % massique) et sans électrolyte de support, la formation de films en oxydation a été observée à l'électrode positive. Ce point est intéressant pour les futures recherches et développements dans le domaine, puisque la variation des chaînes alkyles du cation des liquides ioniques redox et la formation de films lors de l’oxydation du FcNTf est peu connue et comprise en littérature. De plus, l’optimisation des conditions de solution d'électrolyte RIL dans les supercapaciteurs est aussi présentée. En deuxième lieu, la mise en application des RILs dans les supercapaciteurs a été testée. La performance énergétique et le mécanisme d’autodécharge ont été ciblés dans cette étude. En présence de l’électrolyte redox, la contribution des réactions faradaiques permet d'accomplir un gain énergique de 287 % versus les systèmes purement capacitifs. À cause de la formation de film à l’électrode, l’électrolyte redox FcNTf joue un rôle primordial dans la prévention de l’autodécharge versus les liquides ioniques qui étaient connus jusqu’à présent. Finalement, ce mémoire a permis de mieux comprendre les effets structure-propriétés relative aux modifications du cation chez les liquides ioniques redox.

Anomalous resistivity at the structural phase transition of polycrystalline SnTe

An excess resistivity has been observed in thin film polycrystalline samples of SnTe with low carrier concentration and is attributed to the additional scattering due to the phonon softening associated with the structural phase transition.

Sticking non-stick: surface and structure control of diamond-like carbon in plasma enhanced chemical vapour deposition

This short review article explores the practical use of diamond-like carbon (DLC) produced by plasma enhanced chemical vapour deposition (PECVD). Using as an example issues relating to the DLC coating of a hand-held surgical device, we draw on previous works using atomic force microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, scanning electron microscopy, tensiometry and electron paramagnetic resonance. Utilising data from these techniques, we examine the surface structure, substrate-film interface and thin film microstructure, such as sp2/sp3 ratio (graphitic/diamond-like bonding ratio) and sp2 clustering. We explore the variations in parameters describing these characteristics, and relate these to the final device properties such as friction, wear resistance, and diffusion barrier integrity. The material and device characteristics are linked to the initial plasma and substrate conditions.

Analyzing the Effect of Soiling on the Performance of a Photovoltaic System of Different Module Technologies in Kalkbult, South Africa

The fact that most of the large scale solar PV plants are built in arid and semi-arid areas where land availability and solar radiation is high, it is expected the performance of the PV plants in such locations will be affected significantly due to high cell temperature as well as due to soiling. Therefore, it is essential to study how the different PV module technologies will perform in such geographical locations to ensure a consistent and reliable power delivery over the lifetime of the PV power plants. As soiling is strongly dependent on the climatic conditions of a particular location a test station, consisted of about 24 PV modules and a well-equipped weather station, was built within the fences of Scatec’s 75 MW Kalkbult solar PV plant in South Africa. This study was performed to a better understand the effect of soiling by comparing the relative power generation by the cleaned modules to the un-cleaned modules. Such knowledge can enable more quantitative evaluations of the cleaning strategies that are going to be implemented in bigger solar PV power plants. The data collected and recorded from the test station has been analyzed at IFE, Norway using a MatLab script written for this thesis project. This thesis work has been done at IFE, Norway in collaboration with Stellenbosch University in South Africa and Scatec Solar a Norwegian independent power producer company. Generally for the polycrystalline modules it is found that the average temperature corrected efficiency during the period of the experiment has been 15.00±0.08 % and for the thin film-CdTe with ARC is 11.52% and for the thin film without ARC is about 11.13% with standard uncertainty of ±0.01 %. Besides, by comparing the initial relative average efficiency of the polycrystalline-Si modules when all the modules have been cleaned for the first time and the final relative efficiency; after the last cleaning schedule which is when all the reference modules E, F, G, and H have been cleaned for the last time it is found that poly3 performs 2 % and 3 % better than poly1 and poly16 respectively, poly13 performs 1 % better than poly15 as well as poly5 and poly12 performs 1 % and 2 % better than poly10 respectively. Besides, poly5 and poly12 performs a 9 % and 11 % better than poly7. Furthermore, there is no change in performance between poly6 and poly9 as well as poly4 and poly15. However, the increase in performance of poly3 to poly1, poly13 to poly15 as well as poly5 and poly12 to poly10 is insignificant. In addition, it is found that TF22 perform 7% better than the reference un-cleaned module TF24 and similarly; TF21 performs 7% higher than TF23. Furthermore, modules with ARC glass (TF17, TF18, TF19, and TF20) shows that cleaning the modules with only distilled water (TF19) or dry-cleaned after cleaned with distilled water(TF20) decreases the performance of the modules by 5 % and 4 % comparing to its respective reference uncleanedmodules TF17 and TF18 respectively.

Cálculo de constantes ópticas de películas delgadas de Cu3BiS3 a través del método de Wolfe

Se calculó la obtención de las constantes ópticas usando el método de Wolfe. Dichas contantes: coeficiente de absorción (α), índice de refracción (n) y espesor de una película delgada (d ), son de importancia en el proceso de caracterización óptica del material. Se realizó una comparación del método del Wolfe con el método empleado por R. Swanepoel. Se desarrolló un modelo de programación no lineal con restricciones, de manera que fue posible estimar las constantes ópticas de películas delgadas semiconductoras, a partir únicamente, de datos de transmisión conocidos. Se presentó una solución al modelo de programación no lineal para programación cuadrática. Se demostró la confiabilidad del método propuesto, obteniendo valores de α = 10378.34 cm−1, n = 2.4595, d =989.71 nm y Eg = 1.39 Ev, a través de experimentos numéricos con datos de medidas de transmitancia espectral en películas delgadas de Cu3BiS3.

New functional polythiophene based materials: fine-tuning of photovoltaic or chiroptical properties

There is a remarkable level of interest in the development of π-conjugated polymers (ICPs) which have been employed, thanks to their promising optical and electronic properties, in numerous applications including photovoltaic cells, light emitting diodes and thin-film transistors. Although high power conversion efficiency can be reached using poly(3-alkylthiophenes) (P3ATs) as electron-donating materials in polymeric solar cells of the Bulk-Heterojunction type (BHJ), their relatively large band gap limits the solar spectrum fraction that can be utilized. The research work described in this dissertation thus concerns the synthesis, characterization and study of the optical and photoactivity properties of new organic semiconducting materials based on polythiophenes. In detail, various narrow band gap polymers and copolymers were developed through different approaches and were characterized by several complementary techniques, such as gel permeation chromatography (GPC), NMR spectroscopy, thermal analyses (DSC, TGA), UV-Vis/PL spectroscopy and cyclic voltammetry (CV), in order to investigate their structural and chemical/photophysical properties. Moreover, the polymeric derivatives were tested as active material in air-processed organic solar cells. The activity has also been devoted to investigate the behavior of polythiophenes with chiral side chain, that are fascinating materials capable to assume helix supramolecular structures, exhibiting optical activity in the aggregated state.

A multi-methodological approach for the study of polymorphism in organic pigments and pharmaceuticals

The study of polymorphism has an important role in several fields of materials science, because structural differences lead to different physico-chemical properties of the system. This PhD work was dedicated to the investigation of polymorphism in Indigo, Thioindigo and Quinacridone, as case studies among the organic pigments employed as semiconductors, and in Paracetamol, Phenytoin and Nabumetone, chosen among some commonly used API. The aim of the research was to improve the understanding on the structures of bulk crystals and thin films, adopting Raman spectroscopy as the method of choice, while resorting to other experimental techniques to complement the gathered information. Different crystalline polymorphs, in fact, may be conveniently distinguished by their Raman spectra in the region of the lattice phonons (10-150 cm-1), the frequencies of which, probing the inter-molecular interactions, are very sensitive to even slight modifications in the molecular packing. In particular, we have used Confocal Raman Microscopy, which is a powerful, yet simple, technique for the investigation of crystal polymorphism in organic and inorganic materials, being capable of monitoring physical modifications, chemical transformations and phase inhomogeneities in crystal domains at the micrometre scale. In this way, we have investigated bulk crystals and thin film samples obtained with a variety of crystal growth and deposition techniques. Pure polymorphs and samples with phase mixing were found and fully characterized. Raman spectroscopy was complemented mainly by XRD measurements for bulk crystals and by AFM, GIXD and TEM for thin films. Structures and phonons of the investigated polymorphs were computed by DFT methods, and the comparison between theoretical and experimental results was used to assess the relative stability of the polymorphs and to assist the spectroscopic investigation. The Raman measurements were thus found to be able to clarify ambiguities in the phase assignments which otherwise the other methods were unable to solve.

Sensori elettrochimici a base di PEDOT:PSS stampati a getto di inchiostro

Conductive polymers (CPS) are a class of carbon-based materials, capable of conducting electric current, characterized by metallic properties in combination with the intrinsic properties of conventional polymers. The structural model of the CP consists of a system of double π-conjugated on the backbone (polyene structure) which can easily undergo reversible doping reaching a wide range of conductivity. Thanks to their versatility and peculiar properties (mechanical flexibility, biocompatibility, transparency, ease of chemical functionalization, high thermal stability), CPS have revolutionized the science of materials giving rise to Organic Bioelectronics, the discipline resulting from the convergence between biology and electronics. The Poly (3,4-ethylenedioxythiophene) : poly (styrenesulfonate) (PEDOT: PSS), complex polyelectrolyte, in the form of a thin film, currently represents the reference standard in applications concerning Bioelectronics. In this project, two types of electrochemical sensors ink-jet printed on a flexible polymeric substrate, the polyethylene terephthalate, have been developed and characterized. The Drop on Demand (DOD) inkjet technology has allowed to control the positioning of fluid volumes of the order of picoliters with an accuracy of ± 25μm. This resulted in the creation of amperometric sensors and organic electrochemical transistors (OECT) all-PEDOT: PSS with high reproducibility. The sensors have been used for the determination of Ascorbic Acid (AA) which is currently considered an important benchmark in the field of sensors. In Cyclic Voltammetry, the amperometric sensor has detected AA at potentials less than 0.2 V vs. SCE thanks to the electrocatalytic properties of the PEDOT: PSS. On the other hand, the OECT detected AA concentrations equal to 10 nanomolar in Chronoamperometry. Furthermore, a promising new generation of all-printed OECTS, consisting of silver metal contacts, has been created. Preliminary results are presented.