Multiferroic Properties Of BiFeO₃Thin Films Grown By Inducement With Self-Assembled Monolayers And UV Irradiation

Since the multiferroic materials simultaneously possess reversible ferro-electric ordering and electric spin ordering, they have attracted considerableattention from researchers. They can solve the several bottleneck problems of theinformation industrial society, which is mainly composed of semiconductordevices, and are expected to become the main materials for preparing the nextgeneration information components. Especially, BiFeO₃is the only singlemultiferroic material which exhibits both spontaneous ferroelectric ordering andanti-ferromagnetic ordering at room temperature. It is favored by researchers dueto its wide industrial application prospect and has become a hot spot in theresearch field of multiferroic materials.In this topic, the BiFeO₃thin films were prepared by the graded reverseinducement adsorption with OTS self-assembled monolayer technology and UVmodification, using iron nitrate and bismuth nitrate as the raw materials, citricacid as the complexant and glacial acetic acid as the reaction controlagent. Thegrowth mechanism, phase structure, morphology and properties of the single-layer and multi-layer BiFeO₃thin films prepared by the graded reverseself-assembled monolayer technology were researched. The main contents andresults can be summarized as following:（1） The influences of the different deposition modes and depositionprocesses on single-layer BiFeO₃thin films are studied. The pure phase BiFeO₃thin films with the same phase structure can be prepared by different depositionmodes in the same process conditions. Compared with the positive deposition,the reverse adsorption technology adsorbes the small complex particles on thesurface of the substrate and prevents the adsorption of larger particles throughovercoming the effects of gravity. So the whole process is smooth and ordered soas to prepare thin films with a dense and smooth surface and smaller thickness.When citric and bismuth ratio is2:1, acetic acid content is2vol%, the deposition temperature is70℃and the deposition time is20h, the pure phase single-layerBiFeO₃thin film prepared by the reverse adsorption self-assembly monolayertechnology displays a dense and smooth surface and its thickness is about40nm.（2） The growth mechanism of the BiFeO₃thin film prepared by reverseadsorption self-assembled monolayer technology has been preliminarilydiscussed. The results indicate that the growth of BiFeO₃thin film can bedivided two processes. Firstly, the carboxyl in the complex particles and thehydroxyl groups on the functionalized self-assemble monolayer surface aredehydrated and condensed to form a monolayer film with2D reticular structureto completely cover the substrate surface. Subsequently, the film formationmainly comes from the van der waals force and the capillary condensation in themesh of the2D reticular structure. The film-forming mechanism illuminates thatthe reverse adsorption self-assembly monolayer technology has goodcontrollability for the thickness of the as-prepared thin film.（3） The effect of different modes of organic matter removal and differentnumbers of layers on the as-prepared BiFeO₃thin films have been studied. UVOtreatment can prevent the formation of cracks and increase the density ofas-prepared thin films. At the UVO treatment time of40min, the organic matterwithin the film is removed completely. Along with the increase of the numbers ofthe thin film layers, the interface between the film and the substrate becomesclear gradually and the interface phenomenon disappears. At the same time, thegrain size is increased gradually and the crystallization becomes better, but thecontents of Bi₂O₃and other impurity phases are also increased. The15-layer thinfilm oxidized by UVO treatment shows the minimum dielectric loss and smallerdielectric dispersion in the whole range of frequency. Its dielectric loss anddielectric constant are0.02and125, respectively at the frequency of10kHz. Thevoltage resistance strength is the maximum （150kV/cm） and the remanentpolarization is the maximum （0.15μC/cm²ï¼‰. The BiFeO₃thin films oxidized byhigh temperature treatment and UVO treatment, do not appear Fe2+but showweak ferromagnetic. The remanent magnetizations are6.4emu/cm³and7.3emu/cm³, the saturation magnetizations are42emu/cm³and58emu/cm³andthe coercive force fields are100Oe and80Oe, respectively.（4） Composition control of multi-layer BiFeO₃thin films through adjusting the Bi³⁺contents is studied. With the contents decrease of Bi³⁺, the contents ofBi₂O₃within the thin films are decreased and then increased, showing a V-typechange trend. Bi₂O₃could form DC conductivity effect, which is due to its highconductivity so that the BiFeO₃thin films have the high dielectric loss and thelow voltage resistance strength. The mass fraction of Bi₂O₃calculated by the Kvalue method is0.0329%when the reduction of Bi³⁺content is5%(comparedwith the original Bi³⁺quantity). Meanwhile, the dielectric loss is the minimum （Itis only0.02at the frequency of1MHz） and the dielectric constants is64showinga minuscule dielectric dispersion. The voltage resistance strength and theremanent polarization reach to the maximum, are375kV/cm and0.64μC/cm²,respectively.