Nanostructured semiconducting, ferroelectric, and multiferroic crystals: synthesis, characterization and energy application

Organic solar cells (e.g., dye sensitized solar cells and organic-inorganic bulk heterojunction cells) are attracting considerable attention due to their low cost, easy processibility, and large scale fabrication capability. However, the solar energy conversion efficiency of these cells is low due to significant charge recombination and inefficient charge separation at the organic/inorganic interface. Thus, further improvement on the efficiency is necessary for the creation of low-cost devices. In this context, we focus on the synthesis of highly ordered TiO2 nanotube arrays, which was subsequently utilized as photoanode in dye sensitized solar cells and quantum dot solar cells. The highly ordered TiO2 nanotube arrays not only provide a large interface area where excitons, the bound electron-hole pairs, may effectively dissociate, but also have two separate channels for efficient electron and hole transport. Surface engineering, i.e., TiCl4 treatment and oxygen plasma exposure, was combined in the first time to improve the solar energy conversion efficiency of dye sensitized TiO2 nanotube solar cells. With a nanotube film thickness of 14 ym and optimized surface treatment, an overall power conversion efficiency of 7.37 % were obtained when a ruthenium dye N-719 was used as photosensitizer; this performance is among the best for TiO2 nanotube based solar cells. In addition, a number of functional nanocrystals, including semiconducting CdSe quantum dots, ferroelectric BaTiO3 and PbTiO3 nanocrystals, multiferroic BiFeO3 nanocrystals were synthesized and characterized, which possess potential applications in solar cells, light emitting diodes, biosensors, thin-film capacitors, pyroelectric detectors, electrooptic modulators, transducers, actuators, and magnetically recorded ferroelectric memory

Progress in Nanoporous Templates: Beyond Anodic Aluminum Oxide and Towards Functional Complex Materials

Successful synthesis of ordered porous, multi-component complex materials requires a series of coordinated processes, typically including fabrication of a master template, deposition of materials within the pores to form a negative structure, and a third deposition or etching process to create the final, functional template. Translating the utility and the simplicity of the ordered nanoporous geometry of binary oxide templates to those comprising complex functional oxides used in energy, electronic, and biology applications has been met with numerous critical challenges. This review surveys the current state of commonly used complex material nanoporous template synthesis techniques derived from the base anodic aluminum oxide (AAO) geometry

Synthesis and ferroelectric properties of multiferroic BiFeO₃ nanotube arrays

Author name used in this publication: J. Y. Dai2005-2006 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishe

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)

Synthesis and characterization of bismuth ferrite nanoparticles with sillenite phase via sol gel auto combustion route

Bismuth ferrite BiFeO3 (BFO) is one of the multiferroic materials that has high ferroelectric behavior and weak magnetic orders. The intrinsic coupling between the electric polarization and magnetic moments makes BFO potential material for advanced technological applications in different fields. The crystal structure plays very crucial role towards electric and magnetic properties of the nanoparticles. Determination and synthesis of the nanoparticles with specific phase is very important for specific applications. Substitution or doping of ferrites with other metal ions allows variations in their electric and magnetic properties which can be optimized for particular applications. Therefore, this research focuses on the synthesis and characterization of BFO nanoparticles with sillenite phase by sol gel auto combustion route. BFO is doped with cobalt, nickel, and magnesium to explore the role of dopants, concentration of dopants and annealing temperature especially for structural (phase) and electro-magnetic characteristics. Three different doping elements i.e. cobalt, nickel, and magnesium are used to study the role of dopants inside the BFO Bi1-xCoxFeO3, Bi1-xNixFeO3, and Bi1-xMgxFeO3 for five different concentrations 5%, 10%, 15%, 20% and 25% in context of structural and electro-magnetic characteristics. Structural and electro-magnetic characteristics of synthesized doped BFO nanoparticles are investigated using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), vibrating sample magnetometer (VSM) and energy dispersive X-ray (EDX). Results show that, the annealing temperature plays a crucial role towards the phase formation and crystallinity of BFO. At lower annealing temperature 400°C, all BFO nanoparticles possessed R3c rhombohedral phase with poor crystal structure. However, as the annealing temperature is increased, significant increase in the growth of sillenite phase is observed. Dopants such as Co, Mg, and Ni significantly improve the crystal structure, particle size and electro-magnetic characteristics of BFO. Cobalt-doped BFO nanoparticles have shown good ferromagnetic behaviour with high remnant magnetization and coercivity. Ni and Mgdoped BFO possess, super paramagnetic structure with low remnant magnetization and coercivity. Dopants significantly reduce the particle size of the host BFO. The increase in dopant concentration in BFO nanoparticle considerably promotes the formation of sillenite phase. Highly crystalline sillenite phase for BFO nanoparticle is obtained for 25% dopant concentration

Coaxial Nickel Poly(Vinylidene Fluoride Trifluoroethylene) Nanowires for Magnetoelectric Applications

Magnetoelectric (ME) composite materials, in which the coupling between magnetostricitve and piezoelectric effects is achieved, are potential candidates for multifunctional devices where the interplay between electrical, magnetic and mechanical properties of these structures can be fully exploited. Nanostructured composites are particularly interesting due to the enhancement of ME coupling expected at the nanoscale. However, direct studies of ME coupling in nanocomposites by scanning probe techniques are rare due to the complex interplay of forces at play, including those arising from electrostatic, magnetic and electromechanical interactions. In this work, the ME coupling of coaxial nickel - polyvinylidene fluoride trifluoroethylene [Ni-P(VDF-TrFE)] composite nanowires, fabricated by a scalable template-wetting based technique, is studied using a systematic sequence of scanning probe techniques. Individual ME nanowires were subjected to an electric field sufficient for ferroelectric poling in piezo-response force microscopy (PFM) mode, while magnetic force microscopy (MFM) was used to measure localised changes in magnetization as a result of electrical poling. Kelvin probe force microscopy (KPFM) measurements of surface potential were conducted to eliminate for the effect of contact potential differences during these measurements. An inverse, static, magnetoelectric coupling coefficient of ~1 x 10-11 s m-1 was found in our coaxial nanocomposite nanowires, comparable to other types of planar composites studied in this work, despite having an inferior piezoelectric-to-magnetostrictive volume ratio. The efficient ME coupling in our coaxial nanowires is attributed to the larger surface-to-volume interfacial contact between Ni and P(VDF-TrFE), and is promising for future integration into ME composite devices such as magnetic field sensors or energy harvesters

Synthesis and Functional Properties of BiFeO₃ and Bi₂FeCrO₆ based Nanostructures and Thin Films.

The symbols and special characters used in the original abstract could not be transcribed due to technical problems. Please use the PDF version to read the abstract. There is an increasing interest in developing and characterizing multiferroic materials, in which both ferromagnetic and ferroelectric orders coexist, as they exhibit rich physical properties and offer exciting opportunities for data storage, spintronics, sensors, electromagnets and photovoltaic (PV) applications. Among all multiferroic materials studied so far, BiFeO3 (BFO) has attracted considerable attention because it shows intrinsic ferroelectric (TC ~ 1103 K) and Gtype antiferromagnetic (TN ~ 643 K) orders simultaneously well above room temperature. In addition, multiferroic BiFeO3, with band gap energy of 2.2-2.8 eV, has been recently identified as a promising candidate for PV devices and photocatalysts in the visible range. Moreover, the coupling between ferroic orders in BiFeO3 materials offers new modes for investigating and controlling the PV effect, which may endow next generation solar and photoelectrochemical (PEC) cells with multiple functionalities. On the other hand, considerable interest has been attributed to multiferroic BiFeO3 nanostructures in the quest of miniaturizing devices and discovering interesting fundamental physics at nanoscale. BiFeO3 nanomaterials with various sizes and shapes such as nanoparticles, nanotubes, nanowires, and nano-/micro cubes have been reported so far and exhibit quite different physical and chemical properties compared to the bulk form of BiFeO3 crystals due to the nanosize effect. Therefore, the synthesis of multiferroic BiFeO3 nanostructures and investigation of their functional properties are considered important for both fundamental research as well as designing new multifunctional materials combining magnetic, ferroelectric and optoelectronic properties. Meanwhile, recent emergence of a novel double perovskites multiferroic material Bi2FeCrO6 (BFCO), with functional properties well above room temperature, opens new opportunities for practical applications of multiferroics. Bi2FeCrO6 has a similar crystal structure as BiFeO3 and exhibits a particularly Fe/Cr cationic ordering along the [111] pseudocubic direction. Recent works demonstrated that an ordered Bi2FeCrO6 phase can be obtained in both thin film and nanostructured form using pulsed laser deposition (PLD) technique. The reported Bi2FeCrO6 thin films possess a remnant polarization of about 55 C/cm2 along the [001] pseudocubic direction, and are ferrimagnetic with a magnetic moment depending on Fe/Cr cationic ordering, about 1.8 B per formula unit, far exceeding the properties of parent BiFeO3. In addition, theoretical studies showed that Fe and Cr mixed d orbital transition allow a small band gap around 2.3 eV. Therefore, Bi2FeCrO6 is expected to be a promising candidate for efficient PV devices and PEC cells using sun light. The work performed in this thesis was therefore driven by two main objectives: (1) synthesis and understanding the fundamental physical properties (i.e. ferroelectric and magnetic) of various low-dimensional BiFeO3 nanostructures; (2) design and investigate BiFeO3 and Bi2FeCrO6 based nanomaterials and thin film devices for high efficiency solar energy conversion (solar to chemical/electrical energy) applications. The results obtained in this work are resumed in two sections as follows: In the first section, we have synthesized and investigated the ferroelectric, magnetic and photocatalytic properties of BiFeO3 nanomaterials (1D nanowires and 2D nanoplates). We studied the ferroelectric properties of 1D single-crystalline BiFeO3 nanowires using piezoresponse force microscopy (PFM). PFM measurements demonstrated that the assynthesized BiFeO3 nanowires, down to 40 nm in diameter, have components of spontaneous polarization along both in plane and out of plane directions, thereby confirming the ferroelectric nature of the wires. We explained our results by estimating the shape of the piezoelectric tensor for the rhombohedral symmetry. We have also studied the photocatalytic solar water splitting properties of the BiFeO3 nanowires and discovered that the nanowires exhibit better visible-lightdriven photocatalytic activity for generation of O2 from water than other BiFeO3 materials (e.g. nanocubes) reported previously, which could be attributed to the unique morphology of the nanowires. To further enhance the photocatalytic activity, we designed and synthesized a hybrid Au/BiFeO3 nanocomposite photocatalyst consisting of single crystalline BiFeO3 nanowires and laser ablated Au nanoparticles by a functionalization-step-free solution process. We found that 1.0 wt% Au nanoparticle decorated BiFeO3 nanowires exhibit significantly higher photocatalytic activity (~30 times) of water oxidation for O2 than that of the parent wires during the first 4 h of the reaction. Their superior catalytic activity can be attributed to the role of the Au as electron trapping centers as well as the unique surface-chemistry features of the laser ablated Au nanoparticles that can strengthen the interaction and promote charge transfer. Meanwhile, we observed that the localized surface plasmonic resonance (LSPR) effect of Au nanoparticles could also contribute to the enhancement of the photoactivity. In addition, we developed a novel approach to synthesize (100) pseudocubic facets exposed 2D single crystalline BiFeO3 nanoplates, with thickness ranging from 20 to 120 nm and lateral size of sub-micrometers, via a rapid (1-2 min) microwave-assisted hydrothermal method. The BiFeO3 nanoplates exhibited weak ferromagnetic properties at room temperature, which we attribute to the size-confinement effect on magnetic ordering. The second section is focused on solar energy conversion (i.e. PV and PEC) applications of Bi2FeCrO6 thin film based cells. First, we presented the optical and PV properties of Bi2FeCrO6 epitaxial thin films grown on (100)-oriented SrTiO3 substrate buffered with SrRuO3 electrode deposited via PLD. In this part of work, we have achieved a wide band gap tunability from 1.4 to 2.5 eV in the epitaxial Bi2FeCrO6 films with significant polarization by tuning the ordering of transition-metal element Fe and Cr cations, which is remarkably large as compared with reported values from other doped ferroelectrics, opening up the possibility of discovering new narrow band gap multiferroic materials and designing high efficient oxide solar cells. With optimized PLD deposition conditions, we got a record power conversion efficiency of 3.3% under AM1.5G illumination (100 mW/cm2) in the Bi2FeCrO6 thin film based solar cells. Additionally, we demonstrated the use Bi2FeCrO6 epitaxial thin film as a new photocathode material for the visible-light-driven reduction of water to hydrogen. PEC measurements showed that the highest photocurrent up to −1.0 mA/cm2 at a potential of −1.0 V versus reversible hydrogen electrode (RHE) was obtained in p-type Bi2FeCrO6 thin film grown on CaRuO3/SrTiO3 substrate. For the positively poled Bi2FeCrO6 thin film, the photocurrent density was further enhanced by a factor of ~2, as a result of the modulation of the internal electric field gradient resulting from the ferroelectric polarization