A systematic approach has been used to optimise the fabrication process of pentacene-based nonvolatile organic thin film memory transistors (OTFMTs) operating at low programming voltages. In the first part of this work, reliable, reproducible and hysteresis free organic metal-insulator-semiconductor (OMIS) devices and organic thin film transistors (OTFTs) were fabricated and characterised. All devices were based on poly(methyl methacrylate) (PMMA) and poly(vinyl phenol) (PVP) as the organic insulators. The second part of this work focused on optimising the evaporation parameters to fabricate high-performance pentacene-based devices. About 50 nm thickness of pentacene film with a deposition rate of 0.03 nm s-1 on ~ 300 nm of PMMA was found to produce large, uniform and condense grains leading to high quality devices. OTFTs with high mobility of 1.32 cm2 V−1 s−1, on/off current ratio of 106, and negligible hysteresis and leakage current were demonstrated. The effect of the environment on the OTFTs obehaviour was also investigated. The bias stress effect was also investigated in terms of threshold voltage shift ΔVT at various conditions and times. The results show ΔVT increases with the increase of stress voltage. A negligible hysteresis is evident between the forward and reverse direction of the transfer characteristics and the shape of the transfer characteristics does not change with the bias stress. Floating gate memory structures with thin layer of gold, gold nanoparticles (AuNPs) and single walled carbon nanotubes (SWCNTs) were fabricated and characterised during this investigation. Hysteresis in memory structures was a clear indication of the memory effect and charge storage in these devices. Also, the hysteresis was centred close to 0 V for SWCNTs-based structures, which indicate that a low operation voltage is needed to charge the devices. A memory window of about 40 V was observed for AuNPs-based memory devices based on PVP; while the memory windows for devices based on PMMA with thin layer of Au and AuNPs floating gates were 22 V and 32 V, respectively. The electrical properties of the OTFMTs were improved by the use of the Au nanoparticles as the floating gate compared with that of an Au thin film. Using appropriate negative or positive voltages, the floating gate was charged and discharged, resulting in a clear shift in the threshold voltage of the memory transistors. Negative and positive pulses of 1 V resulted in clear write and erase states, respectively. Additionally, these organic memory transistors exhibited rather high carrier mobility of about μ = 0.319 cm2 V-1 s-1. Furthermore the data retention and endurance measurements confirmed the non-volatile memory properties of the memory devices fabricated in this study.

Building functional thin-film-transistor (TFT) circuits is crucial for applications such as wearable, implantable and transparent electronics. Therefore, developing a compact model of an emerging semiconductor material for accurate circuit simulation is the most fundamental requirement for circuit design. Further, unique analogue building blocks are needed due to the specific properties and non-idealities of TFTs. This dissertation reviews the major developments in thin-film transistor (TFT) modelling for the computer-aided design (CAD) and simulation of circuits and systems. Following the progress in recent years on oxide TFTs, we have successfully developed a Verilog-AMS model called the CAMCAS model, which supports computer-aided circuit simulation of oxide-TFTs, with the potential to be extended to other types of TFT technology families. For analogue applications, an accurate small signal model for thin film transistors (TFTs) is presented taking into account non-idealities such as contact resistance, parasitic capacitance, and threshold voltage shift to exhibit higher accuracy in comparison with the adapted CMOS model. The model is used to extract the zeros and poles of the frequency response in analogue circuits. In particular, we consider the importance of device-circuit interactions (DCI) when designing thin film transistor circuits and systems and subsequently examine temperature- and process-induced variations and propose a way to evaluate the maximum achievable intrinsic performance of the TFT. This is aimed at determining when DCI becomes crucial for a specific application. Compensation methods are reviewed to show examples of how DCI is considered in the design of AMOLED displays. Based on these design considerations, analogue building blocks including voltage and current references and differential amplifier stages have been designed to expand the analogue library specifically for TFT circuit design. The $V_T$ shift problem has been compensated based on unique circuit structures. For a future generation of application, where ultra low power consumption is a critical requirement, we investigate the TFT’s subthreshold operation through examining several figures of merit including intrinsic gain ($A_i$), transconductance efficiency ($g_m/I_{DS}$) and cut-off frequency ($f_T$). Here, we consider design sensitivity for biasing circuitry and the impact of device variations on low power circuit behaviour.

Made available in DSpace on 2014-06-11T19:33:31Z (GMT). No. of bitstreams: 0 Previous issue date: 2012-01-27Bitstream added on 2014-06-13T19:44:22Z : No. of bitstreams: 1 magnajunior_jp_dr_prud.pdf: 3558640 bytes, checksum: fe0b1f329eb24d9ac82adc5a8e9815c8 (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / O avanço das técnicas de posicionamento, sobretudo do posicionamento por satélites artificiais, impulsionou os processos de atualização da estrutura geodésica fundamental em diversos países. No Brasil, a mais recente mudança foi a adoção do SIRGAS2000 em fevereiro de 2005, se tornando o terceiro referencial adotado oficialmente pelo Sistema Geodésico Brasileiro. A mudança de referencial faz com que produtos cartográficos possam ter suas coordenadas associadas a diferentes referenciais e/ou realizações. Portanto, tornam-se necessários processos de transformação de coordenadas entre sistemas e/ou realizações, que possam modelar as distorções existentes nas materializações e garantir a integridade dos dados. A evolução das técnicas de posicionamento e a atualização dos sistemas de referência são processos dinâmicos, portanto, os métodos para mudança de coordenadas são uma necessidade atual e constante. Nesta pesquisa é apresentado um método para transformação de coordenadas tridimensionais entre realizações de referenciais geodésicos com modelagem de distorções baseado em Thin-Plate Splines (TPS). Pretende-se explorar a capacidade da técnica TPS em modelar dados provenientes de uma transformação linear, juntamente com distorções de natureza... / The advances in the positioning techniques, especially in the satellite positioning, drove the updating process in the fundamental geodesic network in several countries. In Brazil, the most recent change was the adoption of the SIRGAS2000 in February, 2005, as the third official referential adopted by the Brazilian Geodetic System. The change of reference system let cartographic products with his coordinates associated to different reference systems and/or frames. So, processes of coordinates change between reference systems and/or frames are necessary, which are able to model the distortion in the reference frames and guarantee the data integrity. The evolution of the positioning techniques and the updating of reference frames are dynamic processes, so, the methods of coordinates change are an actual and continuous necessity. In this research a method is presented for transformation of threedimensional coordinates between reference frames with modeling of distortions based on Thin-Plate Splines (TPS). It is explored the capacity of the TPS in modeling data originated from a linear transformation, together... (Complete abstract click electronic access below)

Made available in DSpace on 2014-06-11T19:35:08Z (GMT). No. of bitstreams: 0 Previous issue date: 2012-09-11Bitstream added on 2014-06-13T20:06:34Z : No. of bitstreams: 1 alencar_mas_dr_araiq.pdf: 4202119 bytes, checksum: 35fa6bb33ce6843d922c597e61ac48c8 (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Os vidros calcogenetos a base de telúrio vem despertando grande interesse acadêmico e tecnológico devido às suas versáteis propriedades ópticas, elétricas e físico-químicas, permitindo a sua aplicação em várias áreas da fotônica. Este trabalho descreve o estudo cinético de cristalização e dos fenômenos fotoinduzidos no sistema vítreo Ga-Ge-Te. Duas composições, Ga20Ge10Te70 (GGT7) e Ga10Ge10Te80 (GGT10), tiveram sua cinética de cristalização estudadas, nos vidros com tamanho de partículas de 53<x>45 μm e em monolito, analisando-se a fração cristalizada. Foi possível calcular a energia de ativação pelos métodos de Kissinger, Ozawa, e Augis e Benett e o expoente de Avrami, que fornece informações dos processos de nucleação e crescimento de cristais para ambas as composições. Os dados obtidos mostram que estas composições apresentaram altos valores de energia de ativação, característica interessante em aplicações onde a estabilidade da fase amorfa seja requerida. Na segunda parte do trabalho foram produzidos filmes finos por “Electron Beam Physical Vapor Deposition” utilizando as composições vítreas GGT7 e GGT10. Os efeitos fotoinduzidos foram avaliados nos filmes, onde as amostras foram irradiadas no comprimento de onda de 488 nm usando um laser contínuo (CW) de argônio, variando-se a potência de irradiação e o tempo de exposição. Nesse caso, foram observados a fotocristalização, onde a presença da fase de Te hexagonal foi observada por espalhamento Raman e Difração de Raios X. Esses efeitos mostraram-se reversíveis com o tratamento térmico das amostras em temperaturas próximas de sua temperatura de transição vítrea (Tg) / Chalcogenide glasses base of tellurium is attracting great academic and technological interest due to their versatile optical properties, electrical and physical and chemical properties, allowing use in several areas of photonics. This thesis presents the study of crystallization kinetics and photoinduced phenomena in glass system Ge-Te-Ga. The compositions Ga20Ge10Te70 (GGT7) and Ga10Ge10Te80 (GGT10) were carried out study the crystallization induced by thermal treatments performed on glasses with particle size of 53 < x > 45 μm and a piece are analyzing the fraction crystallized. This analysis relies on the crystallization peaks, obtained with the Differential Scanning Calorimetry (DSC) technique. The curves of the crystallized fraction and decreased crystallization time with increasing heating rate. The calculation of the crystallized fraction was realized, first, by considering the 0peaks of crystallization. The activation energy from Kissinger, Ozawa, and Mahadevan Augis and Benett method was obtained and Avrami exponent, which provides information on the processes of nucleation and crystal growth for both compositions. The data showed high values of activation energy, interesting feature in applications where stability of the amorphous phase is required. In the second part of the work were produced thin films by Electron Beam Physical Vapor Deposition, using the vitreous compositions GGT7 and GGT10. The effects were evaluated in the photoinduced films, where the samples were irradiated by wavelength of 488 nm using a continuous Argon laser (CW), varying the power of irradiation and exposure time. The photocrystallization were observed, where the presence of hexagonal Te phase were observed by Raman scattering and X-ray diffraction These effects were shown to be reversible by thermal treatment of the samples at temperatures close to their glass transition temperature (Tg)

This work presents photophysical and nonlinear optical properties of a novel Cd 2,3-[octakis{4-tert-butylphenoxyphthalocyanine}] (CdOtBPPc) and compared with those of Pb 2,3-[octakis{4-tert-butylphenoxyphthalocyanine}] (PbOtBPPc). For both the CdOtBPPc and PbOtBPPc, third order imaginary susceptibility and second order hyperpolarizability values were found to be within the limit set for good optical limiters. The Pcs were embedded in poly (methyl methacrylate) (PMMA) and poly(bisphenol A carbonate) (PBC) as thin films. The optical limiting values of the Pcs once embedded in film were found to be greatly improved and the limiting intensity of each film was well below the maximum threshold. Both PbOtBPPc and CdOtBPPc showed better optical limiting when embedded in PBC compared to PMMA. CdOtBPPc shows better nonlinear optical behaviour than PbOtBPPc in solution and as thin films, even though the former is aggregated in solution. Novel phosphorus triazatetrabenzcorroles (TBC) tetrasubstituted at the α- and β- and octa substituted at the β- positions of the peripheral fused benzene rings with t-butylphenoxy substituents were prepared and characterized. The effects of the substituents and the missing aza-nitrogen on the electronic structures and optical spectroscopy are investigated with TD-DFT calculations and MCD spectroscopy. The optical limiting properties were investigated to examine whether the lower symmetry that results from the direct pyrrole-pyrrole bond and hence the permanent dipole moment that is introduced result in higher safety thresholds, relative to the values that have been reported for phthalocyanines. The suitability of the compounds for singlet oxygen applications has also been examined. Novel phosphorus phthalocyanines, analogous to the triazatetrabenzcorroles were also investigated. Due to their high photodegradation quantum yield however, only the fluorescence quantum yields and lifetimes were able to be determined.

Neutron reflectivity offers the opportunity for non-destructive characterisation of buried polymer interfaces. Specular neutron reflectivity is commonly used to characterise the structure of conjugated polymer interfaces found in organic electronic devices. However, detecting specularly reflected neutrons only allows the measurement of the root-mean-square (RMS) roughness of the interface averaged over a macroscopic lateral distance. There are two contributions to the RMS roughness of a polymer/polymer interface; i) the so-called 'intrinsic interfacial width' due to mixing of the polymers at a molecular level and ii) lateral roughness due to deviations of the interface position from the plane of the substrate. In this work a numerical model is developed to analyse experimental off specular reflectivity. The model is based on the distorted wave Born approximation (DWBA). Specular and off specular neutron reflectivity data is collected from a model conjugated polymer/amorphous polymer interface (poly(9,9-dioctylfluorene) (F8) on deuterated PMMA) and a conjugated poly- mer/fullerene interface (poly(3-hexylthiophene) (P3HT) polymer on [6,6]-phenyl C61-butyric acid methylester (PCBM)). This allows probing of the structure in the plane of the interfaces to distinguish the intrinsic interfacial width from the lateral roughness. The structure of the F8/dPMMA interface is studied by systematically varying the film thickness, which strongly impacts on the amplitude of the lateral interface roughness, and allows more complete analysis of the relative contributions of intrinsic mixing and lateral roughness. For comparison off specular measurements on amorphous/amorphous polymer (PMMA/polystyrene) interfaces are performed, which have been studied previously using specular neutron reflectivity and self consistent held theory. Fitting specular reflectivity using standard techniques and off specular reflectivity data using the model developed allow intrinsic and lateral roughness contributions for the F8/dPMMA system to be separated by direct measurement. The P3HT/PCBM interface exhibits no lateral roughness as the materials are found to be miscible.

No description available.

After the advent of nanotechnology, self-assembly has become an active area of research, as it being one of the few efficient methods to generate ensembles of nanostructures. In this thesis, we present studies on two dimensional self-assembly of polymer grafted nanoparticle (PGNPs) and thin film modelling approach to understand the physics involved in the self-assembly mechanism of polymeric nanoparticles. The two dimensional, hierarchical assemblies of PGNPs are created from evaporating solution films spread at the air-water interface using Langmuir-Blodgett technique. A transition in the patterns is observed with increase in concentration which is followed by a remarkable re-entrance of initial patterns with further concentration increment. The pattern is long length scale network type at low and high concentrations whereas it is short length scale distribution of clusters at intermediate concentrations. Clusters are composed of lateral arrangement of individual PGNPs. The characteristics of clusters are tailored by changing various experimental conditions such as molecular weight of the grafted chains, concentration, temperature and evaporation rate. The patterns are unaffected by the transfer surface pressure, suggesting that the self-assembly occurs in the presence of solvent via solution thin film instabilities and the resulting structures of PGNPs are frozen upon complete evaporation. Films of neat polystyrene also exhibit similar morphology and transitions in pattern length scales with initial solution concentration as observed in PGNP films. This confirms that the self-assembly of PGNPs is driven by the intrinsic nature of the grafted polymer chains. Gradient dynamics model is employed to study the stability and dynamics of polymer solution thin films by incorporating Flory Huggins free energy and concentration dependent Hamaker constant. Dispersion curves obtained from linear stability analysis of thin film equations show existence of bimodal instability in the film that corresponds to dewetting and decomposition. Phase diagram spanned by concentration and Flory parameter indicate that the thin film instability transits from dewetting to decomposition and then re-enters to dewetting with increase in concentration of the solution. Using the material parameters of the PGNP thin films for linear stability analysis, experimental observations of bimodal length scale of patterns and re-entrant nature are well explained. Nonlinear simulations which are performed to capture the evolution of patterns in the film show that the decomposition progresses through different pathways depending upon the concentration of the solution. This is explained by analyzing the local variation of spinodal parameter (curvature of the free energy per unit area) in the film. The gradient dynamics model is extended to study the stability and dynamics of evaporating solution thin films. Nonlinear simulations demonstrate that the film undergoes evaporative thinning without any significant growth of dewetting or decomposition instability initially and becomes unstable at a certain intermediate thickness where the spinodal parameter of dewetting or decomposition changes the sign. The rupture of the film (dewetting) or the phase segregation (decomposition) occurs explosively and subsequently evaporation progresses till the film attains chemical equilibrium with the ambient vapour phase. Rate of evaporation significantly affect the intermediate thickness at which the patterns emerge and thereby determines the length scale of initial patterns and instability growth rate. Quasi-steady analysis and nonlinear simulations show that the length scales of patterns of dewetting and decomposition decrease with evaporation rate and exhibit a power law behaviour. Thin films in which the solvent quality drops down with confinement due to evaporation are modelled by assuming a simple functional dependence of Flory parameter on mean film thickness. Quasi-steady analysis demonstrates that the dominating instability of such films switches from dewetting to decomposition and then returns to dewetting with increase in the initial concentration of the solution. We note that even though the functional form of Flory parameter with confinement is not exact, it represents the essential nature of the expected variation. We presume that the phenomenon discussed above is quite generic and may manifest itself in many situations where thin films of colloidal solutions undergo a decrease in the solvent quality due to confinement effects resulting in a competition between spinodal dewetting and decomposition instabilities. This will result in a competition and interplay of the different instability scales and by choosing appropriate control parameters novel self-assembled patterns can be created.

In recent years, portable devices and larger display are the trend of market, so transparent conducting oxides (TCO) draw a considerable interest due to their unique characteristics and essential role in the technology of flat panel display. Indium tin oxide (ITO) is one of the most widely used TCO in the application of display technology. In this work, properties of ITO thin film as a function of dopant ratio and density of sputtering targets had been done. It is found that the mechanism of oxygen vaccines is more dominated in electrical conduction than dopant concentration. Meanwhile, the conductivity of ITO thin film depends on the sputtering condition than the target itself. Annealing process is one of the ways to enhance the optical properties of ITO thin film. This process can change the crystal structure of film from amorphous to crystalline but limited by the presence of the oxygen. Apart from the transitional single layer of ITO thin film, a modified structure had been done by inserting a thin layer of metal (Al/Ag) into ITO which provides a highway for carrier transparent. A modified structure with Ag is successfully demonstrated and well agrees with the theory. OLED application was also done by using sandwich structure. The key of sandwich structure is the metal layer, non-reactive and highly conducting material should be selected. Upper and bottom TCO layers are purpose-built for application without affect the properties of structure. This structure also shows more robust on the flexible substrate than single layer ITO.

Thesis (MTech (Science))--Peninsula Technikon, Cape Town, 1998 / The chemical system, Si < >1 Pd 1Cr, was investigated to study the formation of CrSil on polycrystalline PdlSi, formed on Si<IOO> and epitaxial PdlSi formed on Si<lll>. To ascertain the reaction mechanism during the formation, tantalum was used as an inert marker, since it does not participate in the reaction and is readily measured by Rutherford Backscattering Spectrometry (RBS). This investigation was performed in two parts. In the first part, the tantalum was inserted in the PdlSi layer to determine which species; palladium or silicon diffuses during CrSil formation. In the second part, the marker was inserted in the CrSiI layer to determine whether chromium or silicon moves. In addition, the effect of marker thickness on the growth of CrSiI was investigated. The samples were prepared by electron gun evaporation in vacuum, the elements being deposited on the particular silicon substrates. This was followed by the thermal treatment of the samples at temperatures of 400,425,450,475,500 and 550°C. Normal and dynamic Rutherford backscattering spectrometry was used to characterize the thin film structures. If the marker, when inserted in the PdISi layer, should move towards the PdISi I CrSil interface then, the formation of CrSil would be due to the dissociation of Pd2Si. In this case PdlSi dissociates into Pd and Si and the Si diffuses to the interface of CrSi2 and Cr to form CrSil, whereas the Pd diffuses to the Si < > I Pd2Si interface to regrow PdISi. However, if the marker position remains constant with respect to the Pd2Si layer it can be concluded that the formation of CrSiI is due to the movement of Si from the substrate to the interface of CrSi2 and Cr to form CrSiI. If the marker when inserted in the CrSi2 should move towards the sample surface then the chromium diffuses to the interface ofCrSi1 and PdlSi to react with the silicon, forming CrSi2 .