Field-effect transistors (FET) based on graphene as channel has extraordinaryproperties in terms of charge mobility, charge carrier density etc. However, there aremany challenges to graphene based FET due to the fact graphene is a monolayer ofatoms in 2-dimentional space that is strongly influenced by the operating conditions.One issue is that the Dirac point, or K-point, shifts to higher gate voltage whengraphene is exposed to atmosphere. In this study graphene field-effect transistors(GFET) based on commercially available CVD graphene are electrically characterizedthrough field effect gated measurements. The Dirac point is initially unobservable andlocated at higher gate voltages (>+42 V), indicating high p-doping in graphene.Different treatments are tried to enhance the properties of GFET devices, such astransconductance, mobility and a decrease of the Dirac point to lower voltages, thatincludes current annealing, vacuum annealing, hot plate annealing, ionized water bathand UV-ozone cleaning. Vacuum annealing and annealing on a hot plate affect thegated response; they might have decreased the overall p-doping, but also introducedDirac points and non-linear features. These are thought to be explained by localp-doping of the graphene under the electrodes. Thus the Dirac point of CVDgraphene is still at higher gate voltages. Finally, the charge carrier mobility decreasedin all treatments except current – and hot plate annealing, and it is also observed that charge carrier mobilities after fabrication are lower than the manufacturer estimatesfor raw graphene on SiO2/Si substrate.

My master thesis is focused on design and realization fully automated system which will be used for the characterization of the organic FET structure, based on DPP derivatives with follow optimization of the characterization process. Program „MeasFET“ has been created at the LabVIEW surroundings that drives hardware gadgets was build-up during last year. Furthermore, initial tests were taken for optimalization process of charge carrier mobility measurements in derivatives DPP.

No description available.

One of the major challenges in the field of organic semiconductors is to develop molecular design rules and processing routes which optimise the charge carrier mobility, whilst independently controlling the radiative and non-radiative processes. To date there has existed a seeming trade-off between charge carrier mobility and photoluminescence efficiency, which limits the development of some devices such as electrically pumped laser diodes. This thesis investigates fluorescence enhancement strategies for high-mobility polymer semiconductor systems and the mechanisms by which they currently display poor emission properties. Four independent approaches were taken and are detailed as follows. 1. Solubilising chain engineering It is shown that for the high mobility polymer poly(indacenodithiophene-co-benzothiadiazole), the addition of a phenyl- initiated side chain can enhance the solid-state fluorescence quantum yield, exciton lifetime and exciton diffusion length significantly in comparison to that without phenyl-addition. 2. Energy transfer to a highly fluorescent chromophore It is shown that for the high mobility polymer poly(indacenodithiophene-co-benzothiadiazole) efficient energy transfer to a more emissive squaraine dye molecule is possible despite fast non-radiative decay short exciton diffusion lengths. This results in a significant fluorescence enhancement, which in turn facilitates an order of magnitude increase of the efficiency of polymer light emitting diodes made from this material combination. 3. Energy gap engineering The well known Energy Gap Law predicts an increase in the non-radiative rate as the optical bandgap of an organic chromophore decreases in energy. In combination with this, almost all polymer semiconductors reported to date with high charge carrier mobility have low optical bandgaps. Therefore, molecular design principles which act to increase the optical bandgap of polymer semiconductors whilst retaining a high mobility were sought out. One specific system was successfully identified and showed a significant fluorescence enhancement compared to is predecessor poly(indacenodithiophene-co-benzothiadiazole) in both the solution and the solid state. It is found that the Frenkel exciton lifetime in this new system is a factor of four larger which also results in a significantly increased exciton diffusion length. An inter-chain electronic state is also identified and discussed. 4. Hydrogen substitution For some low-bandgap material systems such as erbium chromophores, high energy vibrational modes such as the C-H stretching mode can act as non-radiative pathways. The effect of hydrogen substitution with deuterium and fluorine was therefore investigated in a series of polythiophene derivative families. It was found that in the solid state, fluorescence and exciton lifetime enhancement occurred when the backbone hydrogen atoms were replaced with fluorine. However, evidence is given that this was not owing to the initial hypothesis, and is more likely owing to structural differences which occur in these substituted material systems.

The importance of studying the physico-chemical properties of nano-dimensional materials has gained significant attention in the fields of semiconductors, pharmaceuticals, materials science, and atmospheric chemistry owing to the differences in physical properties between macro- and nano-dimensional solids. Nonetheless, direct studies of physical properties of materials at nanoscale is limited in part due to their inherent size constraints and experimental limitations. However, development of atomic force microscopy (AFM) led to the implementation of methods to characterize a wide range of physical properties, including – but not limited to – mechanical properties, electrical properties, viscoelastic properties, and surface tension. Herein, the dissertation focuses on AFM-based method development for characterization of atmospheric particles as well as understanding the relationship between structure and physical properties of organic solids at both macro- and nano-dimensions. In the atmospheric chemistry realm, the combined aerosol effect on the climate and environment has significant uncertainty in part due to lack of direct characterization of their physico-chemical properties. The difficulty in assessing the physical and chemical properties arises due to the presence of diversified aerosol sources, which in turn influences the size, morphology, phase states and chemical compositions. Sea spray aerosols (SSAs) are the second-largest source of aerosols in the atmosphere. Studying SSAs – especially in submicrometer-dimensions – requires high-resolution microscopy techniques such as AFM. AFM can be used for imaging of individual aerosols, quantifying organic volume fraction for core-shell morphologies, measuring water uptake, quantifying surface tension of individual droplets, and measuring mechanical and viscoelastic properties of materials. Herein, we employed AFM-based morphology and force spectroscopy studies to correlate the 3D morphology, phase state, and viscoelastic properties of selected single-component chemical systems found in sea spray aerosol (SSA). We established a quantitative framework toward differentiation of the solid, semisolid and liquid phase states of individual particles by utilizing both relative indentation depth (RID) and viscoelastic response distance (VRD) data obtained from the force−distance plots. Moreover, we established a semi-quantitative and quick phase assessment by measuring the aspect ratio (AR) that refers the extent of particle spreading as a result of impaction. Overall, the established AFM-based quantitative and semi-quantitative phase identification method can be utilized to assess the phases of aerosols irrespective of chemical identity. Next, we investigated the factors that may control the electrical and mechanical properties of pharmaceutical and organic semiconducting materials in nano- and macro-dimensions. Understanding the structure-property relationship of materials, especially in the nano-dimension, is necessary for proper drug design and development of organic semiconducting materials. In this context, cocrystals provide a means to modulate the physico-chemical properties of organic solids. For example, the modulation of the mechanical properties is important in the pharmaceutical industry for improving the tabletability. The mechanical properties may be affected by packing arrangement, interaction strength and type, and atomic and chemical composition. Herein, we report the influence of alkane and alkene functional groups on the mechanical properties of organic solids based on salicylic acid (SA). The approach affords both isostructural and polymorphic solids. The isostructural alkane functional solid exhibits a two-fold larger Young’s modulus (YM) compared to the cocrystal with the alkene, where the YM refers to the stiffness of the material. Here, the higher YM values are attributed to the presence of a bifurcated weak C-H···O interactions involving the alkane and neighboring SA molecules. On the other hand, in the case of alkene polymorphisms, molecular packing with column arrangement shows higher YM values compared to the herringbone arrangements. Thus, functional groups and crystal arrangements influence the stiffness of the solid organic cocrystals. Moreover, we report the modulation of mechanical properties of salicylic acid (SA) through cocrystallization by variation of propane and butane functionality with bipyridine coformers. We show that the variation of propane and butane functionality in bipyridine coformer with salicylic acid leads to synthesis of cocrystal and salt-cocrystal, respectively. The AFM nanoindentation study revealed that the Young’s modulus values follow the order salicylic acid < cocrystal << salt-cocrystal. The highest Young’s modulus values of the salt-cocrystal, among the studied systems, are attributed to the presence of strong N+–H···O– and O–H···O– interactions. On the other hand, higher Young’s modulus values of the propane-based cocrystal compared to the macro-dimensional salicylic acid are attributed to the stronger O–H ···N hydrogen bonding. Thus, homologous alkane functional groups can influence the mechanical properties of the organic solid crystals. Additionally, in situ solid-solid polymorphic phase transformation and nucleation of a metastable and elusive polymorph of SA cocrystals in combination with 4,4’-bipyridine were studied. Understanding the solid-solid phase transformations and nucleation mechanisms are important for proper control over the parameters associated with the synthesis of targeted crystalline solids with desired crystal structure. Using in situ powder X-ray diffraction (PXRD) and terahertz time domain spectroscopy (THz-TDS) data we showed that the Form II polymorph transforms to Form I over time. AFM imaging and nanoindentation techniques were utilized to follow and quantify in real-time the solid-solid polymorphic transformation of the metastable Form II to the thermodynamically stable Form I on a single crystal basis. AFM in situ single crystal data revealed that the metastable Form II has a rod-shaped morphology and relatively high elasticity (Young’s modulus), which transforms to prism-shaped nanocrystals of much smaller sizes with significantly reduced elasticity. The AFM imaging reveals that the single crystals on the order of 80-150 nm to undergo catastrophic changes in morphology that are consistent with cracking and popping owing to a release of mechanical stress during the transformation. The nucleation mechanism for the polymorphic transformation is not spatially localized and occurs over the entire crystal surface. The higher mechanical properties of the metastable Form II is due to the presence of the additional interlayer C-H···O interactions. Furthermore, we have studied the electrical properties of boron-based cocrystals. More specifically, cocrystallization of a nonconductive 2,4-difluorophenylboronic ester catechol adduct of a 4,4’-bipyridine (BEA) host with two aromatic semiconducting guests (pyrene and tetrathiafulvalene) generated conductive cocrystals with variable charge carrier mobilities. Charge carrier mobilities of the cocrystals with either pyrene or tetrathiafulvalene were measured using conducting probe AFM (CP-AFM). The incorporation of π-rich aromatic guests through face-to-face and edge-to-face π-contacts results in electrically conductive cocrystals. The cocrystal with tetrathiafulvalene as a guest shows approximately 7 times higher charge carrier mobility than the cocrystal with pyrene. Overall, the current dissertation demonstrates the AFM-based method development and applications towards materials characterization to measure the morphological, electrical, mechanical, and phase-states at both nano- and macro-dimensions. The high spatial precision of the methods developed enables us to better understand the controlling factors for materials design and processing across nano- and macro-dimensions.

Perylene-based materials, including charge-transport discotic liquid crystals and charge-transfer long-wavelength absorbing chromophores, for potential organic electronic and optoelectronic applications, were designed, synthesized and characterized. Two types of discotic liquid crystals, perylene diimides and coronene diimides, can form columnar liquid crystalline phases over a wide temperature range; many of them can have room-temperature liquid crystalline phases after cooling from isotropic liquid. Their charge transport properties were studied by space-charge limited current method; high charge carrier mobilities, with the highest being up to 6.6 cm2/Vs, were found in liquid crystalline phases of these materials under ambient conditions. Structural variables, including aromatic cores and side groups, were examined to get a certain degree of understanding of charge transport properties in these discotic liquid crystals. It was found that mesophase order can have an important effect on charge carrier mobilities. The discotic liquid crystals with high charge carrier mobilities are serious candidates for use in large-area low-cost applications such as solar cells. Long-wavelength, highly absorbing chromophores, featuring donor-substituted perylene diimides, were generated by a combination of charge-transfer process and conjugation extension. The charge-transfer chromophores are expected to lead to further investigation on their potentials as sensitizers in Grtzel solar cells.

Tato práce je zaměřena na studium vlastností organických polovodivých materiálů se zaměřením na jejich vodivost a pohyblivost nosičů náboje. Hlavním cílem této práce je objasnit vztah mezi chemickou strukturou organických polovodičů a jejich vlastnostmi. Teoretická část práce je zaměřena na základy organických polovodičů, transport náboje a přehled vlastností organických polovodivých materiálů, které vedly k jejich aplikaci v polních a elektrochemických tranzistorech. Experimentální část představuje přehled použitých materiálů, způsoby jejich přípravy a charakterizační metody. V rámci výsledkové části bylo vyvinuto nebo optimalizováno několik metod pro přípravu tenkých vrstev a následně byl studován jejich vliv na výkon organických polem řízených tranzistorů. Byly charakterizovány a diskutovány relevantní vlastnosti nových organických polovodivých materiálů se zaměřením na pohyblivost nosičů náboje. Byla charakterizována a diskutována biokompatibilita několika organických polovodičů. Elektrické vlastnosti, stabilita a biokompatibilita elektroaktivních polymerních inkoustů na bázi PEDOT:PSS byla charakterizována a diskutována s ohledem na jejich možné použití v bioelektronice. Nakonec byl zkonstruován organický bioelektronický senzor pro detekci fyziologické odpovědi kardiomyocytů na základě studovaných materiálů.

No description available.

A importância da pesquisa em eletrônica orgânica, se comparada à microeletrônica convencional baseada principalmente em silício, surge pela presença de inúmeros semicondutores e técnicas de deposição de baixo custo e em grande superfície. Os Transistores de Filmes Finos Orgânicos (OTFTs, do inglês Organic Thin-Film Transistors) são a unidade fundamental em circuitos eletrônicos e, geralmente, apresentam a estrutura de um transistor de efeito de campo. Podem ser fabricados sobre substratos plásticos e oferecem grande número de aplicações como: mostradores, etiquetas de identificação por rádio frequência e eletrônica têxtil. Além disso, há demanda por componentes eletrônicos portáteis e baratos, principalmente como sensores em diagnósticos médicos e veterinários in-situ. A geometria de OTFT mais utilizada em sensores na atualidade é a bottom gate sobre substratos de silício altamente dopado e com óxido de porta inorgânico. Polímeros como poli(3-hexiltiofeno) (P3HT) vêm sendo amplamente utilizados pela comunidade científica, atestando o potencial comercial deste semicondutor em sensores. Neste contexto, esta tese apresenta o desenvolvimento de transistores à base de P3HT como sensores na detecção de analitos em fase vapor. O estudo é composto por uma etapa inicial de caracterização da mobilidade dos portadores de carga por técnicas de transiente de corrente, seguida pela otimização do desempenho de parâmetros elétricos do transistor através de alterações no processamento dos filmes dielétrico e semicondutor. Enfim, conclui-se a investigação através do entendimento dos fatores ligados à degradação do OTFT após exposição à atmosfera e sob estresse elétrico, além do detalhamento da sensibilidade e especificidade do sensor. Sensores de P3HT oferecem enorme potencial de detecção de amônia, cetonas e compostos organoclorados. Outros semicondutores poliméricos são provavelmente necessários para maior especificidade em relação a vapor dágua e álcoois. / Research on organic electronics, compared to conventional silicon-based microelectronics, is necessary as it offers plenty of semiconductors and low-cost deposition techniques that can be performed over wide surfaces. Organic Thin-Film Transistors (OTFTs) are the fundamental unity in electronic circuits and, usually, display the metal insulator semiconductor field-effect transistor (MISFET) structure. OTFTs can be processed over cheap plastic substrates and integrate a high number of applications as: flexible displays, radio frequency identification tags, textile electronics and sensors (e.g. chemical and biological compounds). Nowadays, consumers demand portable and low-cost electronic devices, mainly as sensors for in-situ medical and veterinarian diagnosis. The most widely used OTFT structure in sensing is the bottom-gate/bottom-contact FET over highly-doped silicon substrates and inorganic dielectrics. Polymers as poly(3-hexylthiophene) (P3HT) have found increasing acceptance by the scientific community, attesting their potential as semiconductors for commercial applications. In this context, the thesis lies in the development of organic transistors based in P3HT polymer for the detection of vapor-phase compounds. This study begins with transistor performance optimization through changes in dielectric and semiconductor processing. Thin-film thickness and P3HT cast solution drying time are the main studied parameters. It involves also the understanding of device performance degradation when exposed to atmosphere and under bias stress, before finally mapping sensitivity and specificity against gaseous analytes. P3HT-based sensors are potentially interesting for ammonia, ketones and organochlorides detection. Other polymeric semiconductors may be necessary to increase specificity against water steam and alcohol analytes.

The PHD project has two main goals. The first one is specifically related to investigations on molecular dynamics, structural conformations and packing of polyfluorene-based polymers. For this purpose, Wide Angle X-Ray Diffraction (WAXD), Solid-State Nuclear Magnetic Resonance (NMR) and Dynamical-Mechanical Thermal Analysis (DMTA) are being used as the main techniques. The second goal is to correlate molecular phenomena, as characterized in the first part, with opto-electronic properties of polyfluorene when used as active layer in an electronic device, such as a Polymer Light-Emitting Diode (PLED). In the second part, fabrication of devices and their electrical characterization as a function of temperature are the main objectives. Impedance Spectroscopy, Current-Voltage characterization of the devices and Time-Of-Flight (TOF) techniques are among the main techniques to be used in the second part of the project. Therefore, the project combines fundamental studies on molecular dynamics with technological performance of organic electronic. / O projeto de doutorado entitulado \"Correlação das Propriedades Óticas e Elétricas com a Estrutura Física e Dinâmica Molecular de Filmes e Dispositivos de Polifluorenos e Derivados\". O primeiro é especificamente ligado a investigação da dinâmica molecular, conformação estrutural e empacotamento de polímeros derivados do polifluoreno. Para isso, Difração de Raio-X de Alto Ângulo (WAXD)1, Ressonânica Magnética no Estado Sólido (RMN) e Análise Térmica Dinâmico Mecânica (DMTA) serão utilizadas como técnicas principais. O segundo objetivo é o de correlacionar, os fenômenos observados na primeira parte do projeto, com as propriedades opto-eletrônicas dos filmes poliméricos sendo utilizados como camada ativa em dispositivos eletrônicos do tipo Diodo Polimérico Emissor de Luz (PLED). Na segunda parte, a fabricação dos dispositivos e sua caracterização como função da temperatura serão os principais objetivos. Espectroscopia de Impedância, Corrente-Voltagem, Tempo de Vôo (TOF) e Photo-CELIV serão as principais técnicas de caracterização utilizadas. Dessa forma, o projeto combina estudos fundamentais de aspectos moleculares com o desempenho tecnológico de dispositivos optoeletrônicos.