Photoferroelectric thin films for flexible systems by a three-in-one solution-based approach

The effective incorporation of (multi)functional oxides into next-generation flexible electronics systems requires novel fabrication technologies that enable the direct integration of crystalline oxide layers in them. Unfortunately, this is considerably challenging due to the thermal incompatibility between the crystallization temperatures of metal oxides (>600 degrees C) and the thermal stability of the flexible polymer substrates conventionally used (<400 degrees C). Here, it is shown that BiFeO(3)thin films can be grown on flexible plastic by solution processing involving three different but complementary strategies to induce the crystallization of the perovskite phase at a lower temperature limit of 325 degrees C. This "three-in-one" approach is based on the synthesis of tailored metal precursors i) with a molecular structure resembling the crystalline structure of the oxide phase, which additionally allows both ii) photochemical and iii) internal combustion reactions taking place in the thin films. The flexible BiFeO(3)thin films obtained from a specifically designed molecular complex withN-methyldiethanolamine yield a large remnant polarization of 17.5 mu C cm(-2), also showing photovoltaic and photocatalytic effects. This result paves the way for the direct integration of an interesting class of oxides with photoferroelectric properties in flexible devices with multiple applications in information and communication technology, and energy

Photochemical solution processing of films of metastable phases for flexible devices: the beta-Bi2O3 polymorph

The potential of UV-light for the photochemical synthesis and stabilization of non-equilibrium crystalline phases in thin films is demonstrated for the beta-Bi2O3 polymorph. The pure beta-Bi2O3 phase is thermodynamically stable at high temperature (450-667 degrees C), which limits its applications in devices. Here, a tailored UV-absorbing bismuth(III)-N-methyldiethanolamine complex is selected as an ideal precursor for this phase, in order to induce under UV-light the formation of a -Bi-O-Bi- continuous network in the deposited layers and the further conversion into the beta-Bi2O3 polymorph at a temperature as low as 250 degrees C. The stabilization of the beta-Bi2O3 films is confirmed by their conductivity behavior and a thorough characterization of their crystal structure. This is also supported by their remarkable photocatalytic activity. Besides, this processing method has allowed us for the first time the preparation of beta-Bi2O3 films on flexible plastic substrates, which opens new opportunities for using these materials in potential applications not available until now (e.g., flexible photocatalytic reactors, self-cleaning surfaces or wearable antimicrobial fabrics). Therefore, photochemical solution deposition (PCSD) demonstrates to be not only an efficient approach for the low temperature processing of oxide films, but also an excellent alternative for the stabilization of metastable phases

Láminas delgadas de (Bi0.5Na0.5)1-xBaxTiO3 en torno a la frontera de fase morfotrópica preparadas por métodos de depósito químico y fotoquímico de disoluciones

Esta tesis doctoral demuestra que el depósito químico de disoluciones (Chemical Solution Deposition, CSD) es una técnica viable para la preparación de láminas delgadas sin plomo de (Bi0.5Na0.5)1-xBaxTiO3 (BNBT), con un control preciso de su composición y estructura cristalina. Un estudio de estos materiales llevado a cabo con técnicas avanzadas de caracterización composicional y estructural ha permitido situar la frontera de fase morfotrópica (Morphotropic Phase Boundary, MPB) de estas láminas, así como establecer un perfil estructural y composicional de las mismas. Por otro lado, se ha desarrollado una nueva ruta sintética en disolución para materiales basados en bismuto, basada en reacciones fotoquímicas inducidas por luz UV, que ha permitido reducir notablemente la temperatura de procesado de las láminas delgadas de BNBT

Fotoactivación de precursors sol-gel para la preparación de láminas delgadas ferroeléctricas de PbTiO₃ a baja temperatura

En la presente tesis doctoral se han preparado y caracterizado, desde un punto de vista estructural, microestructural y eléctrico, láminas delgadas ferroeléctricas de titanato de plomo (PbTiO3, PT) a tan sólo 400°C mediante PCSD, diseñando para ello una nueva estrategia de síntesis de las disoluciones precursoras. La fabricación de las láminas se ha llevado a cabo mediante el depósito de las disoluciones por spin-coating sobre substratos de Pt/TiO2/SiO2/(100)Si seguido de un secado y un tratamiento térmico en un horno RTP combinado con irradiación UV para favorecer la eliminación de los orgánicos y la cristalización del óxido a baja temperatura. El titanato de plomo cristaliza con estructura perovsquita y es el ferroeléctrico por excelencia, presentando un alto valor de polarización espontánea Ps~50μC/cm2. Por este motivo, este óxido ha sido elegido como objeto de este estudio. El responsable de una reducción tan significativa en la temperatura de procesado de las películas de PT es un compuesto fotosensible, N-metildietanolamina (MDEA), utilizado por primera vez en la fabricación de láminas con este fin, que no presenta toxicidad y con un máximo de absorción muy próximo a la longitud de onda de la lámpara utilizada en la irradiación (222 nm). La caracterización de los soles pone de manifiesto que el MDEA forma complejos con los cationes metálicos presentes en el sistema, que son los responsables de una alta absorción en el UV a través de un proceso de transferencia de carga, un tipo de transición electrónica con coeficientes de absortividad molar muy superiores a los que presentan las transiciones π-π*, que son las que experimentan habitualmente las disoluciones precursoras fotoactivadas en la técnica PCSD. La reducción en la temperatura de cristalización del óxido se debe tanto al aumento de la fotosensibilidad de las disoluciones precursoras debido a la existencia de transferencias de carga en los complejos que forma el compuesto MDEA, como a un fenómeno de autoignición que tiene lugar en el sistema debido a la presencia del nitrógeno del MDEA junto a un alto contenido de orgánicos, que origina una gran cantidad de energía que el sistema consume en la cristalización del óxido, reduciéndose así la temperatura. De esta forma, MDEA proporciona unos resultados que otros fotoactivadores no permiten obtener. De este modo, se esperarían los mismos efectos para cualquier composición que contuviera al menos un catión de configuración d0 o d10, puesto que son los complejos que forma el MDEA con los cationes metálicos de este tipo los responsables del aumento de la fotosensibilidad de las disoluciones precursoras a través de las transferencias de carga. Como las principales familias de ferroeléctricos contienen cationes de este tipo, esta nueva estrategia de síntesis abre las puertas a la preparación de una gran variedad de composiciones de óxidos mixtos funcionales con gran interés aplicativo en dispositivos electrónicos

Láminas delgadas de (Bi0.5Na0.5)1-xBaxTiO3 en torno a la frontera de fase morfotrópica preparadas por métodos de depósito químico y fotoquímico de disoluciones

Esta tesis doctoral demuestra que el depósito químico de disoluciones (Chemical Solution Deposition, CSD) es una técnica viable para la preparación de láminas delgadas sin plomo de (Bi0.5Na0.5)1-xBaxTiO3 (BNBT), con un control preciso de su composición y estructura cristalina. Un estudio de estos materiales llevado a cabo con técnicas avanzadas de caracterización composicional y estructural ha permitido situar la frontera de fase morfotrópica (Morphotropic Phase Boundary, MPB) de estas láminas, así como establecer un perfil estructural y composicional de las mismas. Por otro lado, se ha desarrollado una nueva ruta sintética en disolución para materiales basados en bismuto, basada en reacciones fotoquímicas inducidas por luz UV, que ha permitido reducir notablemente la temperatura de procesado de las láminas delgadas de BNBT

Photoferroelectric thin films for flexible systems by a three-in-one solution-based approach

The effective incorporation of (multi)functional oxides into next-generation flexible electronics systems requires novel fabrication technologies that enable the direct integration of crystalline oxide layers in them. Unfortunately, this is considerably challenging due to the thermal incompatibility between the crystallization temperatures of metal oxides (>600 degrees C) and the thermal stability of the flexible polymer substrates conventionally used (<400 degrees C). Here, it is shown that BiFeO(3)thin films can be grown on flexible plastic by solution processing involving three different but complementary strategies to induce the crystallization of the perovskite phase at a lower temperature limit of 325 degrees C. This "three-in-one" approach is based on the synthesis of tailored metal precursors i) with a molecular structure resembling the crystalline structure of the oxide phase, which additionally allows both ii) photochemical and iii) internal combustion reactions taking place in the thin films. The flexible BiFeO(3)thin films obtained from a specifically designed molecular complex withN-methyldiethanolamine yield a large remnant polarization of 17.5 mu C cm(-2), also showing photovoltaic and photocatalytic effects. This result paves the way for the direct integration of an interesting class of oxides with photoferroelectric properties in flexible devices with multiple applications in information and communication technology, and energy

Low-temperature sol–gel methods for the integration of crystalline metal oxide thin films in flexible electronics

The development of low-temperature sol–gel (solution) processes for the fabrication of crystalline metal oxide thin films has become a key objective in the emerging Flexible Electronics. To achieve this target, crystalline oxide films need to be deposited on flexible substrates, which have degradation temperatures below 350 °C (e.g., polymers or textile). This achievement would be a step towards improving the performance of the flexible device, making feasible applications now restrained (e.g. smart-skin, flexible-displays or solar-cells) and whose performance is associated to the functional properties of the crystalline oxide (e.g., ferroelectricity, pyroelectricity or piezoelectricity). However, this is a challenge because the crystallization of these oxides usually occurs at high temperatures, over 600 °C. This paper shows an overview to the solution strategies devised in our group for the low-temperature fabrication of crystalline metal oxide thin films, mostly ferroelectric perovskites (e.g., BiFeO3, PbTiO3 or Pb(Zr,Ti)O3). We have made use of UV-light as an alternative energy source to the thermal energy conventionally used to obtain the crystalline oxide. High photosensitive sol–gel solutions have been synthesized and the solution-deposited layers irradiated with UV-excimer lamps. A precise control of the photoreactions occurring during the irradiation of these layers has been carried out with the aim of advancing the formation of a high-densified, defect-free amorphous metal oxide film that easily can be converted into crystalline at temperatures compatible with the use of polymer substrates

Low-temperature sol–gel methods for the integration of crystalline metal oxide thin films in flexible electronics

The development of low-temperature sol–gel (solution) processes for the fabrication of crystalline metal oxide thin films has become a key objective in the emerging Flexible Electronics. To achieve this target, crystalline oxide films need to be deposited on flexible substrates, which have degradation temperatures below 350 °C (e.g., polymers or textile). This achievement would be a step towards improving the performance of the flexible device, making feasible applications now restrained (e.g. smart-skin, flexible-displays or solar-cells) and whose performance is associated to the functional properties of the crystalline oxide (e.g., ferroelectricity, pyroelectricity or piezoelectricity). However, this is a challenge because the crystallization of these oxides usually occurs at high temperatures, over 600 °C. This paper shows an overview to the solution strategies devised in our group for the low-temperature fabrication of crystalline metal oxide thin films, mostly ferroelectric perovskites (e.g., BiFeO3, PbTiO3 or Pb(Zr,Ti)O3). We have made use of UV-light as an alternative energy source to the thermal energy conventionally used to obtain the crystalline oxide. High photosensitive sol–gel solutions have been synthesized and the solution-deposited layers irradiated with UV-excimer lamps. A precise control of the photoreactions occurring during the irradiation of these layers has been carried out with the aim of advancing the formation of a high-densified, defect-free amorphous metal oxide film that easily can be converted into crystalline at temperatures compatible with the use of polymer substrates

A UV-absorber bismuth(III)-N-methyldiethanolamine complex as a low-temperature precursor for bismuth-based oxide thin films

Novel synthetic methods in solution that reduce the formation temperature of bismuth-based electronic oxides are essential for their successful integration with substrates of low thermal stability within microand flexible-electronic devices. This has become crucial for these oxides, since they appear as promising low-toxic functional materials alternative to other electronic oxides containing heavy metals. However, this is a challenge, since the crystallization of bismuth oxides occurs at high temperatures. To overcome these problems, we synthesize here a UV-absorber charge transfer metal complex in solution between the Bi(III) ion and an alkanolamine, N-methyldiethanolamine (Bi(III)–mdea). We take advantage of the photoreactivity of this complex to prepare bismuth-based oxide thin films at low temperature, which cannot be achieved by traditional thermal processing methods. Room temperature stable oxide thin films of the high-temperature d-Bi2O3 phase are prepared from these solutions by UV-irradiation and annealing at 350 C. The efficiency of this synthetic strategy is additionally proven for the low temperature preparation of thin films of much more complex bismuth based functional oxides: the multiferroic bismuth ferrite, BiFeO3, and the relaxor-ferroelectric perovskite of bismuth, sodium and barium titanate, (Bi0.5Na0.5)0.945Ba0.055TiO3

A UV-absorber bismuth(III)-N-methyldiethanolamine complex as a low-temperature precursor for bismuth-based oxide thin films

Novel synthetic methods in solution that reduce the formation temperature of bismuth-based electronic oxides are essential for their successful integration with substrates of low thermal stability within microand flexible-electronic devices. This has become crucial for these oxides, since they appear as promising low-toxic functional materials alternative to other electronic oxides containing heavy metals. However, this is a challenge, since the crystallization of bismuth oxides occurs at high temperatures. To overcome these problems, we synthesize here a UV-absorber charge transfer metal complex in solution between the Bi(III) ion and an alkanolamine, N-methyldiethanolamine (Bi(III)–mdea). We take advantage of the photoreactivity of this complex to prepare bismuth-based oxide thin films at low temperature, which cannot be achieved by traditional thermal processing methods. Room temperature stable oxide thin films of the high-temperature d-Bi2O3 phase are prepared from these solutions by UV-irradiation and annealing at 350 C. The efficiency of this synthetic strategy is additionally proven for the low temperature preparation of thin films of much more complex bismuth based functional oxides: the multiferroic bismuth ferrite, BiFeO3, and the relaxor-ferroelectric perovskite of bismuth, sodium and barium titanate, (Bi0.5Na0.5)0.945Ba0.055TiO3