Deposição de filmes do sistema (Bi,Lu)-Fe-O e estudo da sua potencial utilização como biossensores
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2018-12-03
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Os óxidos cerâmicos multiferróicos constituem um pequeno subgrupo dos óxidos isolantes ou semicondutores que têm despertado elevado interesse devido às suas propriedades ferroelétricas, ferromagnéticas e ferroelásticas. Alguns destes óxidos também têm apresentado interessantes propriedades óticas, como o efeito magnetoótico ou o fotovoltaico. Estas caraterísticas potenciam a sua utilização em memórias, spintrónica ou sistemas fotovoltaicos ferroelétricos e justificam o elevado interesse da sua investigação, em particular sob a forma de filmes finos. O sistema Lu-Fe-O apresenta diversas fases, cujas propriedades têm vindo a ser estudadas: a ferrite hexagonal (h-LuFeO3), a perovesquite (o-LuFeO3), LuFe2O4, Lu2Fe3O7 e a granada (Lu3Fe5O12). Os filmes finos da ferrite hexagonal apresentam comportamento multiferróico. A perovesquite demonstra propriedades magnetocalóricas, magnetorresistivas e multiferróicas. Os filmes finos da fase LuFe2O4 são superparamagnéticos e, eventualmente, multiferróicos. A granada apresenta efeito magnetoótico (rotação de Faraday) conseguindo controlar a polarização eletromagnética devido às suas propriedades óticas no visível e infravermelho próximo. A valência mista do ferro presente na granada justifica as propriedades magnéticas e elétricas únicas. Neste trabalho pretendeu-se estudar a estabilidade termodinâmica e algumas propriedades de filmes finos do sistema Lu-Fe-O, depositados sobre substratos de vidro de sílica. Para estudar a termodinâmica do sistema, testaram-se diferentes condições de deposição, diferentes composições, diferentes temperaturas e durações de tratamento térmico ex situ. Para o efeito foram preparados precursores metalorgânicos de Lu(tmhd)3 e Fe(tmhd)3 e depositados filmes finos pela Deposição Química de Vapores Metalorgânicos (MOCVD) assistida por Aerossol. Posteriormente, também foram depositados filmes em substratos de vidro de sílica e fibra ótica funcionalizada (LPG) por vaporização catódica com RF assistida por magnetron (RF-Magnetron Sputtering). Previamente foi necessário sintetizar um cerâmico maciço de Lu3Fe5O12 pela técnica do estado sólido, partindo de uma mistura estequiométrica de Lu2O3 e de Fe2O3. Os filmes depositados foram caraterizados por difração de raios-X, microscopia eletrónica de varrimento (SEM) com EDS, magnetometria em SQUID, microscopia de força atómica (AFM) e por algumas técnicas óticas (UV-Vis, UV-Vis-NIR, FTIR e espetroscopia de fluorescência). Nos filmes finos depositados por RF-Magnetron Sputtering foram medidos os ângulos de contacto da gota para determinar se a superfície do filme era hidrofílica ou hidrofóbica. O comportamento dos filmes finos, como sensores de índice de refração, foi testado após a deposição em fibra ótica funcionalizada (LPG). Com este estudo concluiu-se que os filmes de Lu-Fe-O depositados em substrato de vidro de sílica, por ambos os métodos, necessitam de tratamentos térmicos (superiores a 750ºC) para cristalizar uma ou mais fases do sistema Lu-Fe-O. Nos filmes depositados por MOCVD, em função da composição destes e da temperatura de tratamento térmico (de 700 até 1000°C), podem cristalizar várias fases deste sistema: granada, perovesquite, hexagonal ou hematite. No filme com a fase da granada, verificou-se a presença de histerese ferromagnética à temperatura ambiente, com Hc = 85 Öe, Ms=3.98 emu/g e Mr=2.1 emu/g. Nos filmes depositados por sputtering apenas se verificou a cristalização da fase hexagonal, com tratamentos térmicos até 850°C. Os filmes depositados por MOCVD possuem uma espessura entre 200 e 350 nm, para 1 hora de deposição. Os filmes depositados por sputtering, durante 45 minutos, possuem uma espessura aproximada entre 200 e 220 nm. Pela análise dos espetros de transmitância UV-Vis-NIR determinaram-se os hiatos de banda direto e indireto dos diversos filmes depositados por MOCVD, sujeitos a tratamentos térmicos. Conclui-se que o aumento da temperatura de tratamento térmico provoca a diminuição do hiato de banda direto, de cerca de 3.65 eV em filmes depositados a 500°C, para 2.0 eV para filmes com tratamento térmico a 950°C. No entanto os hiatos de banda indiretos ( 1.7 e 1.1 eV) não parecem ser afetados pelas temperaturas de tratamento térmico. Verificou-se que os filmes depositados por sputtering à temperatura ambiente apresentavam caráter hidrofílico, mas os tratamentos térmicos a 800°C alteravam o comportamento para hidrofóbico. A medição da sensibilidade de uma fibra LPG (na qual foi depositada por sputtering, à temperatura ambiente, um filme fino de Lu-Fe-O), ao índice de refração de soluções de sacarose, forneceu um valor de -59.4 nm/R.I.U. Este valor é 16.7% superior aos valores de referência da literatura (-50.9 nm/R.I.U.) para uma fibra LPG sem filme depositado. No entanto, quando foi aplicado o tratamento térmico a 700°C à fibra LPG com o filme depositado, verificou-se uma sensibilidade de apenas -6.7 nm/R.I.U., devido ao caráter hidrofóbico do filme.
The multiferroic ceramic oxides constitute a small subset of the insulating or semiconducting oxides that have aroused high interest due to their ferroelectric, ferromagnetic and ferroelastic properties. Some of these oxides have also presented interesting optical properties, such as the magneto optical kerr effect or photovoltaic effect. These characteristics enhance their use in memories, spintronics or ferroelectric photovoltaic systems and justify the high interest of their research, in particular in the form of thin films. The Lu-Fe-O system has several phases whose properties have been studied: hexagonal ferrite (h-LuFeO3), perovskite (o-LuFeO3), LuFe2O4, Lu2Fe3O7 and garnet (Lu3Fe5O12). Thin films of hexagonal ferrite present multiferroic behavior. The perovskite phase reveals magnetocaloric, magnetoresistive and multiferroic properties. The films of LuFe2O4 phase are superparamagnetic and multiferroic, eventually. The garnet demonstrates magneto optical Kerr effect (Faraday rotation) managing to control the electromagnetic polarization due to its optical properties in the visible and near infrared. The mixed valence of the iron present in the garnet justifies the unique magnetic and electrical properties. In this work we intend to study the thermodynamic and some properties of thin films of the Lu-Fe-O system deposited on silica glass substrates. To study the thermodynamic stability of the system, we tested different deposition conditions, compositions and ex situ heat treatments. For this purpose, we prepared Lu(tmhd)3 and Fe(tmhd)3 metalorganic precursors and deposited thin films by the Aerosol-assisted Metalorganic Chemical Vapour Deposition (MOCVD). Subsequently, films were also deposited on silica glass substrates and functionalized optical fiber (LPG) by RFMagnetron Sputtering. Previously, it was necessary to synthesize a ceramic of Lu3Fe5O12 by solid state technique, starting from a stoichiometric mixture of Lu2O3 and Fe2O3. The deposited films were characterized by X-ray diffraction, scanning electron microscopy (SEM) with EDS, magnetometry in SQUID, atomic force microscopy (AFM) and by some optical techniques (UV-Vis, UV-Vis-NIR, FTIR and fluorescence spectroscopy). In the thin films deposited by RF-Magnetron Sputtering the contact angles were measured to determine whether the surface of the film was hydrophilic or hydrophobic. The behavior of thin films, as refractive index sensors, was tested after deposition on functionalized optical fiber (LPG). We concluded that Lu-Fe-O films deposited on a silica glass substrate, by both methods, require thermal treatments (above 750ºC) to crystallize one or more phases of the Lu-Fe-O system. In the films deposited by MOCVD, depending on their composition and the heat treatment temperature (from 700 to 1000°C), several phases of this system can crystallize: garnet, perovskite, hexagonal or hematite. In the film with the garnet phase, the presence of ferromagnetic hysteresis was found at room temperature, with Hc=85 Öe, Ms=3.98 emu/g and Mr=2.1 emu/g. In the films deposited by sputtering only the hexagonal phase crystallizes with thermal treatments up to 850°C. The films deposited by MOCVD have a thickness between 200 and 350 nm for 1 hour of deposition. The films deposited by sputtering for 45 minutes have a thickness of approximately 200 up to 220 nm. By the analysis of the UV-Vis-NIR transmittance spectra the direct and indirect bandgap were determined for several films deposited by MOCVD, subject to thermal treatments. We concluded that the increase of the heat treatment temperature causes the reduction of the direct band gap, of about 3.65 eV in films deposited at 500°C, to 2.0 eV for films with heat treatment at 950°C. However, indirect bandgaps ( 1.7 and 1.1 eV) do not appear to be affected by the heat treatment temperatures. The films deposited by sputtering at room temperature were hydrophilic, but thermal treatments at 800°C changed the behavior to hydrophobic. Measurement of the sensitivity of an LPG fiber (in which a thin Lu-Fe-O film was deposited by sputtering at room temperature) to the refractive index of sucrose solutions gave a value of 59.4 nm/R.I.U. This value is 16.7% higher than the literature reference values (-50.9 nm / R.I.U.) for a LPG fiber without deposited film. However, when the heat treatment at 700°C was applied to the LPG fiber with the deposited film, a sensitivity of only -6.7 nm/R.I.U. was observed due to the hydrophobic character of the film.
The multiferroic ceramic oxides constitute a small subset of the insulating or semiconducting oxides that have aroused high interest due to their ferroelectric, ferromagnetic and ferroelastic properties. Some of these oxides have also presented interesting optical properties, such as the magneto optical kerr effect or photovoltaic effect. These characteristics enhance their use in memories, spintronics or ferroelectric photovoltaic systems and justify the high interest of their research, in particular in the form of thin films. The Lu-Fe-O system has several phases whose properties have been studied: hexagonal ferrite (h-LuFeO3), perovskite (o-LuFeO3), LuFe2O4, Lu2Fe3O7 and garnet (Lu3Fe5O12). Thin films of hexagonal ferrite present multiferroic behavior. The perovskite phase reveals magnetocaloric, magnetoresistive and multiferroic properties. The films of LuFe2O4 phase are superparamagnetic and multiferroic, eventually. The garnet demonstrates magneto optical Kerr effect (Faraday rotation) managing to control the electromagnetic polarization due to its optical properties in the visible and near infrared. The mixed valence of the iron present in the garnet justifies the unique magnetic and electrical properties. In this work we intend to study the thermodynamic and some properties of thin films of the Lu-Fe-O system deposited on silica glass substrates. To study the thermodynamic stability of the system, we tested different deposition conditions, compositions and ex situ heat treatments. For this purpose, we prepared Lu(tmhd)3 and Fe(tmhd)3 metalorganic precursors and deposited thin films by the Aerosol-assisted Metalorganic Chemical Vapour Deposition (MOCVD). Subsequently, films were also deposited on silica glass substrates and functionalized optical fiber (LPG) by RFMagnetron Sputtering. Previously, it was necessary to synthesize a ceramic of Lu3Fe5O12 by solid state technique, starting from a stoichiometric mixture of Lu2O3 and Fe2O3. The deposited films were characterized by X-ray diffraction, scanning electron microscopy (SEM) with EDS, magnetometry in SQUID, atomic force microscopy (AFM) and by some optical techniques (UV-Vis, UV-Vis-NIR, FTIR and fluorescence spectroscopy). In the thin films deposited by RF-Magnetron Sputtering the contact angles were measured to determine whether the surface of the film was hydrophilic or hydrophobic. The behavior of thin films, as refractive index sensors, was tested after deposition on functionalized optical fiber (LPG). We concluded that Lu-Fe-O films deposited on a silica glass substrate, by both methods, require thermal treatments (above 750ºC) to crystallize one or more phases of the Lu-Fe-O system. In the films deposited by MOCVD, depending on their composition and the heat treatment temperature (from 700 to 1000°C), several phases of this system can crystallize: garnet, perovskite, hexagonal or hematite. In the film with the garnet phase, the presence of ferromagnetic hysteresis was found at room temperature, with Hc=85 Öe, Ms=3.98 emu/g and Mr=2.1 emu/g. In the films deposited by sputtering only the hexagonal phase crystallizes with thermal treatments up to 850°C. The films deposited by MOCVD have a thickness between 200 and 350 nm for 1 hour of deposition. The films deposited by sputtering for 45 minutes have a thickness of approximately 200 up to 220 nm. By the analysis of the UV-Vis-NIR transmittance spectra the direct and indirect bandgap were determined for several films deposited by MOCVD, subject to thermal treatments. We concluded that the increase of the heat treatment temperature causes the reduction of the direct band gap, of about 3.65 eV in films deposited at 500°C, to 2.0 eV for films with heat treatment at 950°C. However, indirect bandgaps ( 1.7 and 1.1 eV) do not appear to be affected by the heat treatment temperatures. The films deposited by sputtering at room temperature were hydrophilic, but thermal treatments at 800°C changed the behavior to hydrophobic. Measurement of the sensitivity of an LPG fiber (in which a thin Lu-Fe-O film was deposited by sputtering at room temperature) to the refractive index of sucrose solutions gave a value of 59.4 nm/R.I.U. This value is 16.7% higher than the literature reference values (-50.9 nm / R.I.U.) for a LPG fiber without deposited film. However, when the heat treatment at 700°C was applied to the LPG fiber with the deposited film, a sensitivity of only -6.7 nm/R.I.U. was observed due to the hydrophobic character of the film.
Descrição
Dissertação de mestrado em Eng Biomédica apresentada à Universidade de Trás-os-Montes e Alto Douro
Palavras-chave
MOCVD , RF-Magnetron Sputtering , granada , LPG , sensores