Studies On Synthesis And Properties Of Bismuth-Based Narrow Bandgap Oxides

Bismuth-based oxides already have a place in nowadays material society because of its low price,stability,innocuity and excellent performance.Bismuth-based oxides have kinds of stable crystal structures,which can not only change the intrinsic structural symmetry,spin configuration and energy band distribution by ion-doping in specific lattice position,but also can be combined with other materials to make multiple degrees of freedom coupling via elastic interaction.The modified bismuth-based oxides have more potential application possibilities in areas of non-volatile information storage,spin electronics and photoelectric energy conversion,that already have become one of the hottest materials in condensed matter physics and integrated information society.With the continuous progress of industrial development,the environmental energy problem become increasingly serious.Under this circumstances,ferroelectrics have been widely concerned by many scientists due to its unique bulk photovoltaic effect.In order to better match solar spectral band and enhance the absorption efficiency of sunlight,currently explorations of narrow bandgap ferroelectrics have been the most important research in the field of ferroelectric photovoltaic.On the basis of theoretical and experimental results reported by international authorities,this paper adopts the preparation methods from easy to difficult to study three kinds of bismuth-based materials with simple to complex structure:perovskite structure BiFeO₃?BFO?,Aurivillius phase layered structure Bi₆Fe₂Ti₃O₁₈?BFTO?and mullite structure Bi₂Fe₄O₉?B2FO?about the chemical modification and physical properties.Having found the phenomenon of crystal phase evolution,symmetry change,bandgap narrowing,magnetic enhancement and etc.,which provides a new design idea of practically assembling efficient photovoltaic devices and novel non-volatile data storage.The innovative achievements of this research can mainly be summarized as following points:1.Double-ion-doped Bi(Mg_3/8Fe_2/8Ti_3/8)O₃?BMFT?ceramic and its solid solution with CaTiO₃??1-x?BMFT-xCTO?have been prepared successfully by solid state reaction method.The bandgap and magnetic spin sequence of BMFT regulated by non-magnetic double-ion-doping have been studied using BFO ceramic as a comparative reference,and the effect of CTO content on the structure and physical properties of solid solution has also been explored,mainly focusing on the changes at the Morphotropic Phase Boundary?MPB?.Firstly,X-ray diffraction spectroscopy?XRD?has proved the polycrystalline perovskite structure of BFO,BMFT and?1-x?BMFT-xCTO ceramics sintered at high temperature,among which the disappearance,split and mergence of peaks are observed in the XRD and Raman spectra of?1-x?BMFT-xCTO.It is confirmed that the presence of component-controlled MPB in solid solution has been induced by CTO,which has rhombohedral phase of R3c space group?before phase transition?and orthorhombic phase of Pna2₁ space group?after phase transition?simultaneously at the MPB of x=0.075⁰.175 in?1-x?BMFT-xCTO.Scanning electron microscopy?SEM?result shows that the introduction of Mg²⁺and Ti⁴⁺inhibits the growth of BMFT grains,while the increase of CTO content changes the shape of solid solution grains from elliptical pebble to regular cube gradually.Besides,compared with the reported optical bandgap?E_g?of BFO,the substitution of Mg²⁺and Ti⁴⁺ions reduce the E_g of BMFT to 2.23 eV.Meanwhile,the E_g of?1-x?BMFT-xCTO changes non-linearly due to the influence of structure symmetry transformation after phase transition and increasing internal stress caused by ion-doping.At last,the results of electric hysteresis loop and magnetic hysteresis loop have verified the ferroelectricity and ferromagnetism of?1-x?BMFT-xCTO ceramics respectively.The appearance of ferromagnetism when x?0.15 can be attributed to the non-zero net magnetic moment,which results from the destruction of antiferromagnetic superexchange between adjacent Fe³⁺ions by non-magnetic ion substitution.However,when the content of CTO exceeds a certain thershold?x>0.15?,the existence of Fe³⁺in crystal can no longer induce long-term magnetic order.2.The synthesis of transition metal ion doped Bi₆Fe₂Ti₃O₁₈ ceramics and the investigation of magneto-optical properties at room temperature.Single Ni²⁺ion doped Bi6Fe?2-_x?Ni_x Ti3O18?xBFNTO?ceramics and Bi6Fe?2-_x?Co_x/2Ni_x/2Ti3O18?xBFCNT?ceramics which Fe is simultaneously replaced by Ni and Co have been prepared.Property characterizations show that the introduction of transition metal ions can effectively adjust the visible light response and spin structure of crystal.Microstructural testing methods have confirmed the polycrystalline orthorhombic structure of x BFNTO and xBFCNT ceramics.As the increase of doping ions with large radius,the diffraction peak of XRD shifts to lower angle,corresponding to the increase of crystal surface spacing.Plate-like grains can be clearly observed in SEM surface images which are well consistent with the typical morphology of bismuth-based Aurivillius layered materials.As increasing doping content,the grain arrangement of both xBFNTO and xBFCNT ceramics change from disorder to stacking growth in a same direction,which indicates that transition metal ions introduced in experiment can influence the growth and arrangement of grains directly.According to Williamson-Hall analysis,the internal stress of xBFCNT increased first and then decreased as the content of doping ions,which is in agreement with the change of magnetism measured at room temperature.This phenomenon results from the internal stress caused by random distribution of several ions with different radius in perovskite-like layers,which gives rise to lattice distortion and then makes the intrinsic spin sequence tilted,finally enhances the ferromagnetism of material through Dzyaloshinskii-Moriya?DM?interaction.Unfortunately,the accompanying aroused oxygen vacancies when Ni²⁺and Co²⁺ions replace Fe³⁺ions simultaneously will gather in the domain boundary and block the exchange of ferroelectric domain,thereby weakening the ferroelectricity of xBFCNT,resulting in the failure to improve ferroelectricity and ferromagnetism at the same time.In addition,the optical bandgap of samples can be calculated by UV-vis-NIR diffuse reflection spectrum,and it has found that the single-ion-doping of Ni²⁺make the E_g of xBFNTO reduce from 2.26 eV to 2.06 eV,meanwhile double-element-doping further lower the bandgap of xBFCNT to 1.28 eV.The narrowing of bandgap can be attributed to the energy band distribution of BFTO affected by the splitting of d-orbital of transition metal ions in octahedral crystal field.3.Replacing elements in different lattice positions of Bi₆Fe₂Ti₃O₁₈ crystal has been achieved by Sol-Gel method,which Ni²⁺ion doped xBFNTO and La³⁺,Mn²⁺ions doped Bi_5.5La_0.5Fe₂Ti₃O₁₈?BLFTO?,Bi₆Fe_1.5Mn_0.5Ti₃O₁₈?BFMTO?and Bi_5.5La_0.5Fe_1.5Mn_0.5Ti₃O₁₈?BLFMTO?thin films have been prepared.Various property characterizations have verified that the introduction of different ions can adjust the ferromagnetic order and energy band structure of crystal in different degree.Based on the study of single-phase film,composite bilayer films xBi₆Fe₂Ti₃O₁₈/?1-x?CoFe₂O₄?xBFTO/?1-x?CFO?have been further synthesized.The opto-magnetic properties of composite can be continuously regulated by controlling the constant thickness of total film and changing the thickness of two component layers.Based on the study of x BFNTO ceramics,the Ni²⁺ion doped xBFNTO thin films have been further formed,and samples are determined as orthorhombic structure with B2cb space group according to the test results of microstructure and micromorphology.The doping of Ni²⁺changes the distortion degree of crystal,and then enhances the ferromagnetism of x BFNTO film through DM interaction.Besides,La³⁺and Mn²⁺are selected to replace Bi³⁺and Fe³⁺in BFTO respectively to prepare a series of doped films grown on the quartz substrate.The introduction of La³⁺with large ion radius and Mn²⁺with small ion radius lead to the expansion and contraction of crystal cells respectively,among them Mn²⁺has a great influence on the preferred direction of crystal growth and vibration mode of molecules.The Mn²⁺and La³⁺ions introduced into lattice have narrowed and expanded the bandgap of BFMTO and BLFTO correspondingly.Finally,BFTO and CFO are deposited by layer to synthesize xBFTO/?1-x?CFO composite bilayer films.XRD,Raman scattering spectra,SEM and AFM results have confirmed the independence of two components in composite films.The effective bandgap and room-temperature ferromagnetism of composites are analyzed by means of transmission spectrum and vibration magnetometer,which the remanent and saturation magnetization both increase linearly while the effective bandgap decreases with the increase of CFO.It means that the optical and magnetic properties of composite films can be regulated continuously by manually adjusting the relative content of two phases,so that xBFTO/?1-x?CFO composite films will get broader application prospect in the field of multi-functional devices of information society.4.The preparation and related properties of Bi₂Fe₄O₉?B2FO?-based ceramics and monocrystalline thin films which have strong absorption of visible light have been researched.The ceramics composed with spinel CFO or NiFe₂O₄?NFO?not only exhibit strong absorption of visible light at room temperature,but also display significantly enhanced ferromagnetic order,which have great potential application in optical and magnetic area.Moreover,the monocrystalline thin film of B2FO has been deposited on?100?SrTiO₃ substrate by high vacuum pulsed laser deposition?PLD?technology for the first time,and the ferroelectric single-crystal oxide thin film with strong ferromagnetism and narrow bandgap has been successfully synthesized under optimized process which is expected to be an appropriate material for absorbed layer in high efficiency ferroelectric photovoltaic devices.The B2FO ceramic with mullite structure has three electron transitions at room temperature,and two obvious absorption peaks among them result from the interband electron transition between metal ion and ligand and the intervalence charge transfer between Fe²⁺and Fe³⁺.The uneven distribution of positive and negative charge in part of crystal not only induces Fe³⁺to transform into Fe²⁺,but also leads to the formation of oxygen vacancy which causes another weak wide peak in near infrared region of absorption spectrum.After compositing with spinel phases,electron transitions between splitting d orbitals of metal ions in CFO and NFO has made the redshift and raise of absorption edge in composite samples.In the meantime,the appearance of ferromagnetic order in composite ceramics are on account of the intrinsic strong magnetism of CFO and NFO.What's more,the B2FO single-crystal thin film which grows epitaxially along?n00?direction has been deposited on the?100?SrTiO₃ substrate using PLD technology.The effects of substrate temperature,laser energy and oxygen partial pressure on composition,crystallinity,crystal structure and surface morphology of samples are studied by XRD and AFM characterizations.The photoluminescence spectrum has detected luminescence peaks caused by defects and interband electronic transition respectively.The calculated bandgap is about 1.62 eV,which is very close to the optimal value?1.4 eV?of absorbing materials in solar cell indicating that monocrystalline B2FO is expected to break the limitation of traditional ferroelectric oxides with wide bandgap and will become an appropriate candidate for ferroelectric photovoltaic devices.Finally,on the strength of size effect of nanomaterials,stress to thin films made by substrate and F-center exchange effect due to oxygen vacancy,this article has explained the significant enhanced ferromagnetism of B2FO thin film which also means great potential in the field of magnetic devices that other materials aren't able to have.