Electronic structure and ferroelectricity in SrBi2Ta2O9

The electronic structure of SrBi2Ta2O9 is investigated from first-principles, within the local density approximation, using the full-potential linearized augmented plane wave (LAPW) method. The results show that, besides the large Ta(5d)-O(2p) hybridization which is a common feature of the ferroelectric perovskites, there is an important hybridization between bismuth and oxygen states. The underlying static potential for the ferroelectric distortion and the primary source for ferroelectricity is investigated by a lattice-dynamics study using the Frozen Phonon approach.Comment: 17 pages, 7 figures. Phys. Rev. B, in pres

Efficient Deep Neural Network Accelerator Using Controlled Ferroelectric Domain Dynamics

The current work reports an efficient deep neural network (DNN) accelerator where synaptic weight elements are controlled by ferroelectric domain dynamics. An integrated device-to-algorithm framework for benchmarking novel synaptic devices is used. In P(VDF-TrFE) based ferroelectric tunnel junctions, analog conductance states are measured using a custom pulsing protocol and associated custom circuits and array architectures for DNN training is simulated. Our results show precise control of polarization switching dynamics in multi-domain, polycrystalline ferroelectric thin films can produce considerable weight update linearity in metal-ferroelectric-semiconductor (MFS) tunnel junctions. Ultrafast switching and low junction current in these devices offer extremely energy efficient operation. Through an integrated platform of hardware development, characterization and modelling, we predict the available conductance range where linearity is expected under identical potentiating and depressing pulses for efficient DNN training and inference tasks. As an example, an analog crossbar based DNN accelerator with MFS junctions as synaptic weight elements showed ~ 93% training accuracy on large MNIST handwritten digit dataset while for cropped images, a 95% accuracy is achieved. One observed challenge is rather limited dynamic conductance range while operating under identical potentiating and depressing pulses below 1V. Investigation is underway for improving the dynamic conductance range without losing the weight update linearity

Lead-free piezoceramics - Where to move on?

Lead-free piezoceramics aiming at replacing the market-dominant lead-based ones have been extensively searched for more than a decade worldwide. Some noteworthy outcomes such as the advent of commercial products for certain applications have been reported, but the goal, i.e., the invention of a lead-free piezocermic, the performance of which is equivalent or even superior to that of PZT-based piezoceramics, does not seem to be fulfilled yet. Nevertheless, the academic effort already seems to be culminated, waiting for a guideline to a future research direction. We believe that a driving force for a restoration of this research field needs to be found elsewhere, for example, intimate collaborations with related industries. For this to be effectively realized, it would be helpful for academic side to understand the interests and demands of the industry side as well as to provide the industry with new scientific insights that would eventually lead to new applications. Therefore, this review covers some of the issues that are to be studied further and deeper, so-to-speak, lessons from the history of piezoceramics, and some technical issues that could be useful in better understanding the industry demands. As well, the efforts made in the industry side will be briefly introduced for the academic people to catch up with the recent trends and to be guided for setting up their future research direction effectively.ope

Magnetic materials for magnetoelectric coupling: An unexpected journey

Magnetic materials for magnetoelectric coupling are reported. After an introduction of magnetoelectric effect and materials, an historical on the main developments in this field are presented. Then, the main concepts related to multiferroic and magnetoelectric materials are introduced, together with the description of the main types of magnetoelectric materials and structures. Finally, the magnetic materials used the development of magnetoelectric composites are presented and discussed, highlighting their main physico-chemical characteristics and processing methods. In this way, a complete account on concepts, materials and methods is presented in this strongly evolving research field, with strong application potential in the areas of sensors and actuators, among others.FCT- Fundação para a Ciência e Tecnologia- for financial support in the framework of the Strategic Funding UID/FIS/04650/2020 and under projects PTDC/BTM-MAT/28237/2017 and PTDC/EMD-EMD/28159/2017. P.M., A.C.L. and N.P. also support from FCT (for the contract under the Stimulus of Scientific Employment, Individual Support – 2017 Call (CEECIND/03975/2017, for the SFRH/BD/132624/2017 and for the SFRH/BD/131729/2017 grant, respectively). Finally, the authors acknowledge funding by the Spanish State Research Agency (AEI) and the European Regional Development Fund (ERFD) through the project PID2019-106099RB-C43/AEI/10.13039/501100011033 and from the Basque Government Industry and Education Department under the ELKARTEK, HAZITEK and PIBA (PIBA-2018-06) program

Switchable and Tunable Ferroelectric Devices for Adaptive and Reconfigurable RF Circuits.

As wireless communication systems have become more prevalent, their role has broadened from simply a means of connecting individuals to one another to a means of connecting individuals to the vast information and social network of the Internet. The resulting exponential increase in the utilization of wireless communication systems, the fundamental limitation of the finite wireless spectrum, and the use of conventional wireless communication systems that are designed to operate at fixed predetermined carrier frequencies pose a significant challenge. One method to address this problem is to use adaptive and reconfigurable wireless communication systems that can change their frequency and mode of operation. Unfortunately, currently available RF and microwave circuit components cannot meet the frequency agility specifications, performance requirements, and cost constraints necessary for the widespread commercialization of such systems. This thesis explores how the multifunctional properties of ferroelectrics such as barium strontium titanate (BST) can be used to design switchable and tunable RF circuits for use in adaptive and reconfigurable wireless communication systems. In particular, the electric field dependent permittivity, electrostriction, and electric field induced piezoelectricity of BST are utilized for the design of electroacoustic resonators and filters. The main contribution of this thesis is the demonstration of several different intrinsically switchable, tunable, and reconfigurable resonator and filter designs. First, BST film bulk acoustic wave resonators (FBARs), which exhibit electric resonances that are controlled by an applied dc bias voltage, are designed, fabricated, and characterized. In addition, reconfigurable dual-frequency resonators that utilize intrinsically switchable and tunable BST FBARs are demonstrated for the first time. Second, intrinsically switchable and tunable ferroelectric FBAR filters with insertion losses as low as 4.1 dB at 1.6 GHz are presented. Furthermore, dual-band BST FBAR filters that exhibit two different pass band responses in the low GHz range are demonstrated for the first time. Third, intrinsically switchable and tunable lateral (contour) mode resonators with frequencies as high as 1.67 GHz are demonstrated for the first time. Last of all, an RF magnetron sputtering system dedicated to BST thin film deposition is designed, assembled, and configured for continuing the improvements in ferroelectric thin film performance.PHDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/107304/1/viclee_1.pd

Optimal Material Selection for Transducers

When selecting an active material for an application, it is tempting to rely upon prior knowledge or commercial products that fit the design criteria. While this method is time effective, it does not provide an optimal selection. The optimal material selection requires an understanding of the limitations of the active material, understanding of the function, constraints and objectives of the device, and rigorous decision making method to ensure rational and clear material selection can be performed. Therefore, this work looks into the three most researched active materials (piezoelectrics, magnetostrictives and shape memory alloys) and looks at how they work and their difficulties. The field of piezoelectrics is vast and contains ceramics, plastics and cellular structures that couple the mechanical and electrical domain. The difficulty with piezoelectric ceramics is their small strains and the dependence of their coefficients on the ferroelectric domains. Giant magnetostrictives materials couple the mechanical and magnetic domains. They are generally better suited for low-frequency operations since they properties deteriorate rapidly with heat. Shape memory alloys are materials that couple thermal and mechanical domains. They have large strain but are limited in their force output, fatigue life and cycle frequency. Optimal material selection requires a formalized material selection method. In mechanical material selection, the formal material selection method uses function, constraints and objectives of the designer to limit the parameter space and allow better decisions. Unfortunately, active materials figures of merit are domain dependent and therefore the mechanical material selection method needs to be adapted. A review of device selection of actuators, sensors and energy harvesters indicates a list of functions, constrains and objectives that the designer can use. Through the analysis of these devices figures of merit, it is realized that the issue is that the simplification that the figures of merit perform does not assist in decision making process. It is better to use decision making methods that have been developed in the field of operational research which assists complex comparative decision making. Finally, the decision making methods are applied to two applications: a resonant cantilever energy harvester and an ultrasound transducer. In both these cases, a review of selection methods of other designers provides guidance of important figures of merit. With these selection methods in consideration, figures of merit are selected and used to find the optimal material based upon the designer preference

Multiscale Simulations of Dynamics of Ferroelectric Domains

Ferroelectric materials exhibiting switchable polarization have been used as critical components in electronics, memory, actuators and acoustics, and electro-optics. The applications of ferroelectric materials heavily rely on the interactions between the polarization and external perturbations, such as electric field, stress, and temperature. It is therefore crucial to understand the dynamics of ferroelectric response at finite temperature. Despite the tremendous advance of computational power and the success of first-principles methods, large-scale simulations of dynamics in oxides at finite temperature can still only be performed using classical atomistic potential. We first develop a model potential based on principles of bond-valence and bond-valence vector conservation. The model potentials for PbTiO3 and BiFeO3 are parameterized using the results from first-principles calculations. The bond-valence-based force field allows for molecular dynamics simulations of ferroelectric response at large time and length scale. The intrinsic inertial response of ferroelectric domain walls is studied in PbTiO3. Examination of the evolution of the polarization and local structures of domain walls reveal that they stop moving immediately after the removal of the electric field, demonstrating that ferroelectric domain walls do not exhibit significant intrinsic inertial response. Taking the 90° domain walls in PbTiO3 as an example, we quantitatively estimate the domain wall velocity under a wide range of temperatures and electric fields. We find that many properties of ferroelectrics are dictated by the intrinsic nature of domain walls. We demonstrate that even in the absence of defects the intrinsic temperature- and field-dependence of the wall velocity can be described with a strongly non-linear creep-like region and a power-law depinning-like region. We propose a simple universal nucleation-and-growth-based analytical model that is able to quantify the dynamics of all types of domain walls in various ferroelectrics; this enables the prediction of the temperature- and frequency-dependence of coercive fields at finite temperature from first-principles. We also investigate the orientation-dependent evolution of nanoscale ferroelectric domain structures in PbZr0.2Ti0.8O3 films. Molecular dynamics simulations predict both 180° for (001)-/(101)-oriented films and 90° multi-step switching for (111)-oriented films, and these processes are subsequently observed in stroboscopic piezoresponse force microscopy. Finally, we investigate the domain walls in organometal halide perovskites. We find that organometal halide perovskites can form both charged and uncharged domain walls, due to the flexible orientational order of the organic molecules. The presence of charged domain walls will significantly reduce the band gap. We demonstrate that charged domain walls can serve as segregated channels for the diffusion of charge carriers

Ferroelectric : CNTs structures fabrication for advanced functional nano devices

Doutoramento em Ciência e Engenharia de MateriaisThis work is about the combination of functional ferroelectric oxides with Multiwall Carbon Nanotubes for microelectronic applications, as for example potential 3 Dimensional (3D) Non Volatile Ferroelectric Random Access Memories (NVFeRAM). Miniaturized electronics are ubiquitous now. The drive to downsize electronics has been spurred by needs of more performance into smaller packages at lower costs. But the trend of electronics miniaturization challenges board assembly materials, processes, and reliability. Semiconductor device and integrated circuit technology, coupled with its associated electronic packaging, forms the backbone of high-performance miniaturized electronic systems. However, as size decreases and functionalization increases in the modern electronics further size reduction is getting difficult; below a size limit the signal reliability and device performance deteriorate. Hence miniaturization of siliconbased electronics has limitations. On this background the Road Map for Semiconductor Industry (ITRS) suggests since 2011 alternative technologies, designated as More than Moore; being one of them based on carbon (carbon nanotubes (CNTs) and graphene) [1]. CNTs with their unique performance and three dimensionality at the nano-scale have been regarded as promising elements for miniaturized electronics [2]. CNTs are tubular in geometry and possess a unique set of properties, including ballistic electron transportation and a huge current caring capacity, which make them of great interest for future microelectronics [2]. Indeed CNTs might have a key role in the miniaturization of Non Volatile Ferroelectric Random Access Memories (NVFeRAM). Moving from a traditional two dimensional (2D) design (as is the case of thin films) to a 3D structure (based on a tridimensional arrangement of unidimensional structures) will result in the high reliability and sensing of the signals due to the large contribution from the bottom electrode. One way to achieve this 3D design is by using CNTs. Ferroelectrics (FE) are spontaneously polarized and can have high dielectric constants and interesting pyroelectric, piezoelectric, and electrooptic properties, being a key application of FE electronic memories. However, combining CNTs with FE functional oxides is challenging. It starts with materials compatibility, since crystallization temperature of FE and oxidation temperature of CNTs may overlap. In this case low temperature processing of FE is fundamental. Within this context in this work a systematic study on the fabrication of CNTs - FE structures using low cost low temperature methods was carried out. The FE under study are comprised of lead zirconate titanate (Pb1-xZrxTiO3, PZT), barium titanate (BaTiO3, BT) and bismuth ferrite (BiFeO3, BFO). The various aspects related to the fabrication, such as effect on thermal stability of MWCNTs, FE phase formation in presence of MWCNTs and interfaces between the CNTs/FE are addressed in this work. The ferroelectric response locally measured by Piezoresponse Force Microscopy (PFM) clearly evidenced that even at low processing temperatures FE on CNTs retain its ferroelectric nature. The work started by verifying the thermal decomposition behavior under different conditions of the multiwall CNTs (MWCNTs) used in this work. It was verified that purified MWCNTs are stable up to 420 ºC in air, as no weight loss occurs under non isothermal conditions, but morphology changes were observed for isothermal conditions at 400 ºC by Raman spectroscopy and Transmission Electron Microscopy (TEM). In oxygen-rich atmosphere MWCNTs started to oxidized at 200 ºC. However in argon-rich one and under a high heating rate MWCNTs remain stable up to 1300 ºC with a minimum sublimation. The activation energy for the decomposition of MWCNTs in air was calculated to lie between 80 and 108 kJ/mol. These results are relevant for the fabrication of MWCNTs – FE structures. Indeed we demonstrate that PZT can be deposited by sol gel at low temperatures on MWCNTs. And particularly interesting we prove that MWCNTs decrease the temperature and time for formation of PZT by ~100 ºC commensurate with a decrease in activation energy from 68±15 kJ/mol to 27±2 kJ/mol. As a consequence, monophasic PZT was obtained at 575 ºC for MWCNTs - PZT whereas for pure PZT traces of pyrochlore were still present at 650 ºC, where PZT phase formed due to homogeneous nucleation. The piezoelectric nature of MWCNTs - PZT synthesised at 500 ºC for 1 h was proved by PFM. In the continuation of this work we developed a low cost methodology of coating MWCNTs using a hybrid sol-gel / hydrothermal method. In this case the FE used as a proof of concept was BT. BT is a well-known lead free perovskite used in many microelectronic applications. However, synthesis by solid state reaction is typically performed around 1100 to 1300 ºC what jeopardizes the combination with MWCNTs. We also illustrate the ineffectiveness of conventional hydrothermal synthesis in this process due the formation of carbonates, namely BaCO3. The grown MWCNTs - BT structures are ferroelectric and exhibit an electromechanical response (15 pm/V). These results have broad implications since this strategy can also be extended to other compounds of materials with high crystallization temperatures. In addition the coverage of MWCNTs with FE can be optimized, in this case with non covalent functionalization of the tubes, namely with sodium dodecyl sulfate (SDS). MWCNTs were used as templates to grow, in this case single phase multiferroic BFO nanorods. This work shows that the use of nitric solvent results in severe damages of the MWCNTs layers that results in the early oxidation of the tubes during the annealing treatment. It was also observed that the use of nitric solvent results in the partial filling of MWCNTs with BFO due to the low surface tension (<119 mN/m) of the nitric solution. The opening of the caps and filling of the tubes occurs simultaneously during the refluxing step. Furthermore we verified that MWCNTs have a critical role in the fabrication of monophasic BFO; i.e. the oxidation of CNTs during the annealing process causes an oxygen deficient atmosphere that restrains the formation of Bi2O3 and monophasic BFO can be obtained. The morphology of the obtained BFO nano structures indicates that MWCNTs act as template to grow 1D structure of BFO. Magnetic measurements on these BFO nanostructures revealed a week ferromagnetic hysteresis loop with a coercive field of 956 Oe at 5 K. We also exploited the possible use of vertically-aligned multiwall carbon nanotubes (VA-MWCNTs) as bottom electrodes for microelectronics, for example for memory applications. As a proof of concept BiFeO3 (BFO) films were in-situ deposited on the surface of VA-MWCNTs by RF (Radio Frequency) magnetron sputtering. For in situ deposition temperature of 400 ºC and deposition time up to 2 h, BFO films cover the VA-MWCNTs and no damage occurs either in the film or MWCNTs. In spite of the macroscopic lossy polarization behaviour, the ferroelectric nature, domain structure and switching of these conformal BFO films was verified by PFM. A week ferromagnetic ordering loop was proved for BFO films on VA-MWCNTs having a coercive field of 700 Oe. Our systematic work is a significant step forward in the development of 3D memory cells; it clearly demonstrates that CNTs can be combined with FE oxides and can be used, for example, as the next 3D generation of FERAMs, not excluding however other different applications in microelectronics.Este trabalho é sobre a combinação de óxidos ferroelétricos funcionais com nanotubos de carbono (CNTs) para aplicações na microeletrónica, como por exemplo em potenciais memórias ferroelétricas não voláteis (Non Volatile Ferroelectric Random Access Memories (NV-FeRAM)) de estrutura tridimensional (3D). A eletrónica miniaturizada é nos dias de hoje omnipresente. A necessidade de reduzir o tamanho dos componentes eletrónicos tem sido estimulada por necessidades de maior desempenho em dispositivos de menores dimensões e a custos cada vez mais baixos. Mas esta tendência de miniaturização da eletrónica desafia consideravelmente os processos de fabrico, os materiais a serem utilizados nas montagens das placas e a fiabilidade, entre outros aspetos. Dispositivos semicondutores e tecnologia de circuitos integrados, juntamente com a embalagem eletrónica associada, constituem a espinha dorsal dos sistemas eletrónicos miniaturizados de alto desempenho. No entanto, à medida que o tamanho diminui e a funcionalização aumenta, a redução das dimensões destes dipositivos é cada vez mais difícil; é bem conhecido que abaixo de um tamanho limite o desempenho do dispositivo deteriora-se. Assim, a miniaturização da eletrónica à base de silício tem limitações. É precisamente neste contexto que desde 2011 o Road Map for Semiconductor Industry (ITRS) sugere tecnologias alternativas às atualmente em uso, designadas por Mais de Moore (More than Moore); sendo uma delas com base em carbono (CNTs e grafeno) [1]. Os CNTs com o seu desempenho único e tridimensionalidade à escala nanométrica, foram considerados como elementos muito promissores para a eletrónica miniaturizada [2]. Nanotubos de carbono possuem uma geometria tubular e um conjunto único de propriedades, incluindo o transporte balístico de eletrões e uma capacidade enorme de transportar a corrente elétrica, o que os tornou de grande interesse para o futuro da microeletrónica [2]. Na verdade, os CNTs podem ter um papel fundamental na miniaturização das memórias ferroelétricas não voláteis (NV-FeRAM). A mudança de uma construção tradicional bidimensional (2D) (ou seja, a duas dimensões, como são os filmes finos) para uma construção tridimensional 3D, com base num arranjo tridimensional de estruturas unidimensionais (1D), como são as estruturas nanotubulares, resultará num desempenho melhorado com deteção de sinal elétrico optimizada, devido à grande contribuição do elétrodo inferior. Uma maneira de conseguir esta configuração 3D é usando nanotubos de carbono. Os materiais ferroelétricos (FE) são polarizados espontaneamente e possuem constantes dielétricas altas e as suas propriedades piroelétricas, piezoelétricas e eletroópticas tornam-nos materiais funcionais importantes na eletrónica, sendo uma das suas aplicações chave em memórias eletrónicas. No entanto, combinar os nanotubos de carbono com óxidos FE funcionais é um desafio. Começa logo com a compatibilidade entre os materiais e o seu processamento, já que as temperaturas de cristalização do FE e as temperaturas de oxidação dos CNTs se sobrepõem. Neste caso, o processamento a baixa temperatura dos óxidos FE é absolutamente fundamental. Dentro deste contexto, neste trabalho foi realizado um estudo sistemático sobre a fabricação e caracterização estruturas combinadas de CNTs – FE, usando métodos de baixa temperatura e de baixo custo. Os FE em estudo foram compostos de titanato zirconato de chumbo (Pb1-xZrxTiO3, PZT), titanato de bário (BaTiO3, BT) e ferrite de bismuto (BiFeO3, BFO). Os diversos aspetos relacionados com a síntese e fabricação, como efeito sobre a estabilidade térmica dos nanotubos de carbono multiparede (multiwall CNTs, MWCNTs), formação da fase FE na presença de MWCNTs e interfaces entre CNTs / FE foram abordados neste trabalho. A resposta ferroelétrica medida localmente através de microscopia de ponta de prova piezoelétrica (Piezoresponse Force Microscopy (PFM)), evidenciou claramente que, mesmo para baixas temperaturas de processamento óxidos FE sobre CNTs mantém a sua natureza ferroelétrica. O trabalho começou pela identificação do comportamento de decomposição térmica em diferentes condições dos nanotubos utilizados neste trabalho. Verificou-se que os MWCNTs purificados são estáveis até 420 ºC no ar, já que não ocorre perda de peso sob condições não isotérmicas, mas foram observadas, por espectroscopia Raman e microscopia eletrónica de transmissão (TEM), alterações na morfologia dos tubos para condições isotérmicas a 400 ºC. Em atmosfera rica em oxigénio os MWCNTs começam a oxidar-se a 200 ºC. No entanto, em atmosfera rica em árgon e sob uma taxa de aquecimento elevada os MWCNTs permanecem estáveis até 1300 ºC com uma sublimação mínima. A energia de ativação para a decomposição destes MWCNTs em ar foi calculada situar-se entre 80 e 108 kJ / mol. Estes resultados são relevantes para a fabricação de estruturas MWCNTs - FE. De facto, demonstramos que o PZT pode ser depositado por sol-gel a baixas temperaturas sobre MWCNTs. E, particularmente interessante foi provar que a presença de MWCNTs diminui a temperatura e tempo para a formação de PZT, em cerca de ~ 100 ºC comensuráveis com uma diminuição na energia de ativação de 68 ± 15 kJ / mol a 27 ± 2 kJ / mol. Como consequência, foi obtido PZT monofásico a 575 ºC para as estruturas MWCNTs – PZT, enquanto que para PZT (na ausência de MWCNTs) a presença da fase de pirocloro era ainda notória a 650 ºC e onde a fase de PZT foi formada por nucleação homogénea. A natureza piezoelétrica das estruturas de MWCNTs - PZT sintetizadas a 500 ºC por 1 h foi provada por PFM. Na continuação deste trabalho foi desenvolvida uma metodologia de baixo custo para revestimento de MWCNTs usando uma combinação entre o processamento sol – gel e o processamento hidrotermal. Neste caso o FE usado como prova de conceito foi o BT. BT é uma perovesquita sem chumbo bem conhecida e utilizada em muitas aplicações microeletrónicas. No entanto, a síntese por reação no estado sólido é normalmente realizada entre 1100 - 1300 ºC o que coloca seriamente em risco a combinação com MWCNTs. Neste âmbito, também se ilustrou claramente a ineficácia da síntese hidrotérmica convencional, devido à formação de carbonatos, nomeadamente BaCO3. As estruturas MWCNTs - BT aqui preparadas são ferroelétricas e exibem resposta electromecânica (15 pm / V). Considera-se que estes resultados têm impacto elevado, uma vez que esta estratégia também pode ser estendida a outros compostos de materiais com elevadas temperaturas de cristalização. Além disso, foi também verificado no decurso deste trabalho que a cobertura de MWCNTs com FE pode ser optimizada, neste caso com funcionalização não covalente dos tubos, ou seja, por exemplo com sodium dodecyl sulfate (SDS)

Desenho de materiais funcionais 2D para futuras aplicações em microeletrónica

Doutoramento em Ciência e Engenharia de MateriaisDevido à redução de dimensões e ao aumento da velocidade de processamento de dados nos dispositivos microeletrónicos baseados em semicondutores convencionais, estão a ser exploradas abordagens inovadoras envolvendo novos materiais tais como óxidos funcionais. Com o rápido desenvolvimento da indústria eletrónica existe uma maior necessidade de elevado desempenho, de elevada fiabilidade, e de componentes eletrónicos miniaturizados integrados em vários dispositivos. A fim de tornar os dispositivos amplamente acessíveis e de fácil utilização, requisitos adicionais devem ser considerados: o tamanho e peso desejados, o custo reduzido, o baixo consumo de energia e a portabilidade. Materiais funcionais de baixa dimensionalidade são muito promissores para cumprir essas exigências. Em particular, os ferroeléctricos de filmes finos bidimensionais (2D) têm recebido grande atenção devido à sua crescente utilização em memórias não voláteis, detectores piroelétricos, transdutores piezoeléctricos miniaturizados e dispositivos sintonizáveis de micro-ondas. A temperatura de cristalização é um parâmetro chave na preparação de ferroelétricos 2D. Muitos filmes finos ferroelétricos são cristalizados a temperaturas >600 °C. Esses valores estão acima da temperatura que certos elementos do dispositivo funcional podem suportar. Recentemente, este facto tornou-se ainda mais importante, devido às promissoras aplicações que podem ser consideradas caso os ferroeléctricos 2D sejam compatíveis com substratos poliméricos flexíveis de baixo custo e de baixo ponto de fusão. A compatibilidade de filmes finos ferróicos com estes últimos tipos de substratos é muito difícil, mas se conseguida pode ampliar acentuadamente a gama de aplicações para os mais recentes requisitos de eletrónica flexível e microeletrónica, onde dispositivos leves e baratos são exigidos. Neste trabalho, é implementada uma combinação da modificação da química de precursores e assistência por luz UV, com promoção simultânea da cristalização pela introdução de sementes nanocristalinas na solução precursora, para a fabricação de filmes finos ferróicos sem chumbo - Método de Precursores Fotossensíveis Semeados. Neste contexto, o principal objetivo deste trabalho foi fabricar filmes finos sem chumbo BiFeO3 (BFO) e Na0.5Bi0.5TiO3 (NBT) a baixas temperaturas (~300 °C) com uma resposta ferroelétrica competitiva. Além disso, a investigação do efeito do elétrodo-base sobre as propriedades dielétricas e ferroelétricas de filmes finos de BFO foi levada a cabo, e a comparação entre o comportamento de condensadores de BFO com base em IrO2, LaNiO3 (LNO) e Pt foi estabelecida. Adicionalmente, os efeitos dos vários eléctrodos sobre a microestrutura de filmes finos ferroeléctricos de BFO foram estudados por microscopia eletrónica de transmissão (TEM) de alta resolução. Primeiramente, filmes finos finos de perovesquite BFO e NBT foram preparados sobre substratos de silício revestidos com Pt, por deposição de solução química. Os filmes finos de BFO foram preparados a temperaturas na gama de 400-500 °C, a partir de soluções de precursores estequiométricas e com excesso de Bi. Os filmes de BFO cristalinos foram obtidos a 400 °C, o limite inferior de temperatura. Os filmes preparadas com excesso de Bi possuem curvas de histerese ferroelétrica mais definidas do que aqueles sem qualquer excesso, para filmes com espessuras ~150 nm. Uma vez que as densidades de corrente de fuga nos filmes finos diminuem com a diminuição da temperatura de processamento, a polarização de filmes finos de BFO preparados com excesso Bi e recozidos a 400 e 450 °C pode ser efetivamente comutada à temperatura ambiente. Obtiveram-se valores de polarização remanescente de Pr ~10 e ~60 μC/cm2 com campos coercivos de EC ~ 205 e 235 kV/cm para os filmes finos preparados a 400 e 450 °C, respectivamente. Os filmes finos de NBT foram preparados a temperaturas entre 400 e 650 °C. As propriedades estruturais e ferroelétricas dos filmes foram examinadas. A constante dieléctrica observada e as perdas dieléctricas a 100 kHz são 616 e 0,032, respectivamente, enquanto que a polarização remanescente observada e o campo coercivo são Pr ~ 24 μC/cm2 e EC ~ 215 kV/cm, respectivamente para o filme de NBT recozido a 650 °C. O recozimento térmico, em atmosfera de oxigénio após cada camada de revestimento, é eficaz na promoção da cristalização do filme na fase de perovesquite romboédrica a uma baixa temperatura de 400 °C. No entanto, obteve-se um ciclo P-E quase linear para os filmes NBT cristalizados a 400 °C devido à sua incipiente cristalinidade. Os filmes finos de BFO foram depositados numa gama de elétrodos para determinar o seu papel no controlo da formação de fases e da microestrutura. A cristalização em elétrodos de óxido seguiu a sequência: amorfa → Bi2O2(CO3) → perovesquite, enquanto que nos elétrodos de Pt cristalizaram diretamente a partir da fase amorfa. Os elétrodos de IrO2 promoveram a formação da fase de perovesquite à temperatura mais baixa e o LNO induziu adicionalmente o crescimento epitaxial local. O LNO tem a estrutura de perovesquite com o parâmetro de rede a = 0.384 nm, compatível com o de BFO, a = 0.396 nm, e assim a epitaxia é mais provável. Todas as composições exibiram precipitados inteiramente coerentes ricos em Fe dentro do interior de grão da matriz de perovesquite, enquanto que a incoerente segunda fase de Bi2Fe4O9 foi também observada nos limites de grão de BFO crescido em eléctrodos de Pt. Esta última pode ser observada por difração de raios X, bem como TEM, mas os precipitados coerentes foram observados apenas por TEM, principalmente evidenciados pelo seu contraste Z em imagens de campo escuro anular. Estes dados têm consequências acentuadas permitindo alargar a utilização de filmes de BFO sob campo aplicado, a aplicações como atuadores, sensores e aplicações de memória. Em seguida, os filmes finos de BFO foram depositados em substratos de Si com elétrodos distintos, como Pt, LNO e IrO2, para investigar o efeito do elétrodo-base sobre o crescimento e as propriedades elétricas do BFO. Todas os filmes de BFO são compostos por grãos colunares cujo tamanho é dependente do elétrodo-base. Não se observou textura para filmes de 320 nm de espessura fabricados em Pt orientado (111). Os filmes sobre eléctrodos de óxido, em particular sobre LNO são altamente orientados no plano (012). A grande polarização remanescente em BFO/Pt e BFO/IrO2 é atribuída à alta contribuição de corrente de fuga. Os filmes BFO de 400 nm de espessura em LNO possuem uma baixa densidade de corrente de fuga ~4 × 10-6 A/cm2, uma grande polarização remanescente de 50 μC/cm2 e um pequeno campo coercitivo de 180 kV/cm à temperatura ambiente. Demonstramos que as camadas de LNO aumentam a cristalinidade e a orientação de filmes finos BFO, o que se reflete nas suas propriedades funcionais. Este estudo mostra que, além da simples necessidade de filmes monofásicos, os elétrodos de óxido de metal têm um impacto relevante no desenvolvimento de filmes finos BFO de alta qualidade fabricados por métodos químicos de deposição de solução. Estes resultados têm uma implicação grande para a fabricação de dispositivos BFO baseados em filmes finos. Finalmente, provamos que é possível fabricar diretamente filmes finos de BFO sem chumbo em substratos flexíveis de poliamida com funcionalidades ferroelétricas e magnéticas (multiferroicidade) à temperatura ambiente. O nosso método inovador, baseado em soluções de Precursores Fotossensíveis e nanosementes cristalinas, foi usado com sucesso para diminuir a temperatura de cristalização de filmes finos de BFO até uma temperatura tão baixa quanto 300 °C, a mais baixa temperatura reportada até agora para a preparação de filmes finos multiferróicos de BFO. Apesar deste excepcionalmente baixo nível térmico, obtém-se uma polarização remanescente Pr de 2.8 μC/cm2 para os filmes semeados + UV, com um campo coercitivo EC de 300 kV/cm. A estratégia de síntese baseada na utilização de precursores fotossensíveis sementados pode ser transferida para qualquer outra família de óxidos metálicos funcionais.With the dimensions reduction and data processing speeds increasing of conventional semiconductor based microelectronic devices, innovative approaches involving new materials such as functional oxides are being explored. With the rapid development of the electronics industry there is a need for high performance, high reliability and miniaturized electronic components integrated into various devices. In order to make the devices user friendly and widely accessible, additional requirements should be considered: the desired size and weight, low cost, low power consumption, and portability in addition to high levels of functionality. Low dimensional functional materials hold great promises to fulfil those requirements. In particular, two-dimensional (2D) thin film ferroelectrics have received wide attention because of their growing use as non-volatile memories, pyroelectric detectors, miniaturized piezoelectric transducers and tunable microwave devices. Crystallization temperature is a key parameter in preparation of 2D-ferroelectrics. Many ferroelectric thin films are crystallized at temperatures >600 °C. This is above the temperature that certain elements of the functional device can withstand. Recently it became even more important due to promising applications that can be envisaged if 2D-ferroelectrics will be compatible with low cost, low melting temperature flexible polymeric substrates. The compatibility of ferroic thin films with those last types of substrates can markedly widen the range of applications towards the most recent requirements of flexible electronics and microelectronics, where lightweight and cheap devices are demanded. In this work, a combination of the modification of precursor chemistry and the assistance of UV-light, with simultaneous promotion of crystallization by introducing nanocrystalline seeds in the precursor solution, is implemented to fabricate lead-free ferroic thin films - Seeded Photosensitive Precursor Method. Within this context, the main objective of this work was to fabricate lead-free BiFeO3 (BFO) and Na0.5Bi0.5TiO3 (NBT) thin films with a competitive ferroelectric response at low temperatures. Moreover, investigations of the effect of the bottom electrode on the dielectric and ferroelectric properties of BFO thin films was conducted and the comparison between the behavior of IrO2, LaNiO3 (LNO) and Pt based BFO capacitors established. Additionally, the effects of these various bottom electrodes on the microstructure of BiFeO3 ferroelectric films was studied by high-resolution TEM. Firstly, BFO and NBT perovskite thin films were prepared on Pt-coated silicon substrates by chemical solution deposition. BFO was prepared at temperatures in the range 400-500 °C, and from stoichiometric and Bi excess precursor solutions. Crystalline BFO films were obtained at the lowest temperature limit of 400 °C. The films prepared with Bi excess possess more defined ferroelectric hysteresis loops than those without any excess; for films with thicknesses ~150 nm. As the leakage current densities in the films decrease with decreasing the processing temperature, polarization of BFO films prepared with Bi excess and annealed at 400 and 450 °C can be effectively switched at room temperature. Remanent polarization values of Pr ~ 10 and ~60 μC/cm2 with coercive fields of EC ~ 205 and 235 kV/cm were obtained for the films prepared at 400 and 450 °C, respectively. NBT thin films were prepared at temperatures from 400 to 650 °C. Structural and ferroelectric properties of the films were examined. The observed dielectric constant and dielectric losses at 100 kHz are 616 and 0.032, respectively, while the observed remanent polarization and coercive field are Pr ~ 24 μC/cm2 and EC ~ 215 kV/cm, respectively for the NBT film annealed at 650 °C. Thermal annealing in an oxygen atmosphere after each layer of coating is effective in promoting crystallization of the film into rhombohedral perovskite phase at a low temperature of 400 °C. However, almost linear, P-E loop was obtained for those NBT films crystallized at 400 °C due to incipient crystallinity. BFO thin films were grown on a range of electrodes to determine their role in controlling phase formation and microstructure. The crystallization on oxide electrodes followed the sequence: amorphous → Bi2O2(CO3) → perovskite, while those on Pt crystallized directly from the amorphous phase. IrO2 electrodes promoted perovskite phase formation at the lowest temperature and LaNiO3 additionally induced local epitaxial growth. LNO has the perovskite structure with lattice parameter a = 0.384 nm, compatible with that of BFO, a = 0.396 nm and thus epitaxy is more likely. It was observed for the first time that all compositions exhibited fully coherent Fe-rich precipitates within the grain interior of the perovskite matrix, whereas incoherent Bi2Fe4O9 second phase was also observed at the grain boundaries of BFO grown on Pt electrodes. The latter could be observed by X-ray diffraction as well as transmission electron microscopy (TEM) but coherent precipitates were only observed by TEM, principally evidenced by their Z contrast in annular dark field images. These data have pronounced consequences for the extended use of BFO films under applied field for actuator, sensor and memory applications. Then, BFO thin films were deposited on Si-based substrates with distinct electrodes, such as Pt, LNO, and IrO2, in order to investigate the effect of bottom electrode on the growth and electrical properties of BFO. All BFO films are composed of columnar grains which size is dependent on the bottom electrode. No texture was observed for 320 nm thick films fabricated on (111) oriented Pt. Films on oxide electrodes, in particular on LNO are highly (012) oriented. The large remanent polarization in BFO/Pt and BFO/IrO2 is attributed to the high leakage current contribution. 400 nm thick BFO films on LNO possess a low leakage current density ~4 × 10-6 A/cm2, a large remanent polarization of 50 μC/cm2 and a small coercive field of 180 kV/cm at room temperature. We demonstrate that LNO layers enhance the crystallinity and orientation of BFO thin films, which is reflected in their functional properties. This study shows that besides the simple need of monophasic films metal oxide electrodes have a relevant impact on the development of high quality BFO thin films fabricated by chemical solution deposition methods. These results have a broad implication for the fabrication of BFO thin film based devices. Finally, we prove that it is possible to directly fabricate lead-free BFO thin films on flexible polyamide substrates with ferroelectric and magnetic functionalites (multiferroicity) at room temperature. Our own proprietary novel solution-based Seeded Photosensitive Precursor Method was successfully used to decrease the crystallization temperature of BFO thin films down to a temperature as low as 300 °C, the lowest reported up to now for the preparation of multiferroic BFO thin films. Despite this exceptionally low thermal budget a remanent polarization Pr of 2.8 μC/cm2 is obtained for the seeded + UV films, with a coercive field EC of 300 kV/cm. The synthesis strategy based on the use of seeded photosensitive precursors can be transferred to any family of functional metal oxide