Studies On Preparation And Properties Of Multiferroic Oxides With Narrow Band Gap

Multiferroics have attracted much attention due to the unique physical properties and immense potential for non-volatile memory and integrated devices applications.Recently,the discovery of large photovoltages beyond band gap value in multiferroics films has attracted increased attention to ferroelectrics photovoltaics.However,most of the current ferroelectric oxides have wide band gap?E_g>3.0 eV?that are beyond the visible-light range and thus allow the use of only 8%?20%of solar spectrum,which may be one of the major obstacles that limits the photovoltaic application of ferroelectrics.Therefore,identifying approaches to reducing the band gap of ferroelectrics without losing the useful ferroelectricity will be highly significant.We take the thought of the "a strategy to reducing the band gap of multiferroic oxides" as a guide to carry out the experimental work.In our work,a series of multiferroics,such as Bi_1-xTb_xFe0₃?xBTFO?,[K_0.5Na_0.5NbO₃]_1-x[BaNi_O.5Nb_0.5O_3-?]_x?KNBNNO?and KBiFe₂O₅ were prepared.The results show that wide-band gap oxide can be experimentally implemented to narrow the band gap,which provides new ideas for photovoltaic applications of multiferroic oxides.The innovative results are displayed as follows:?1?The introduction of transition metal element Tb in BFO crystal lattice induced a structural transition from the rhombohedral to pseudotetragonal.The control of the higher ferromagnetism and the lower band gap by Tb doping has been realized.The transition metal element Tb was doped in BFO films successfully by sol-gel technology.The effects of Tb content in xBTFO films on structural phase transition,ferromagnetic and optical properties have been investigated.We found that the xBTFO films grown on quartz substrate preferred to the pseudotetragonal structure and?110?preferred orientation.The xBTFO films grown on quartz substrate,the Tb-doping results in improved film quality.The ferromagnetism of xBTFO films was increased linearly and the band-gap was decreased linearly.However,the xBTFO films grown on FTO substrate were inclined to keep rhombohedral structure,which turn into pseudotetragonal structure with the Tb content increased.The"strain-induced structural phase transition" was used to explain this phenomenon.The variation trend of the lattice constant,crystal cell volume and characteristic peaks of Raman spectra further corroborates this phase transition.The competition between strain-induced and doping-effect was used to explain the nolinear higher ferromagnetism and the lower band gap,which is accompanied by phase transition.?2?The KNBNNO films have been fabricated on different substrates using sol-gel method.The microstructure,surface morphology,optical property,ferromagnetism and substrate effects of KNBNNO films were assessed.Notably,the absorption behavior in visible light wave band for KNBNNO films have been realized,namely,we make it possible to use KNBNNO films for ferroelectric photovoltaic absorbing layer.The KNBNNO films have been fabricated on quartz and Pt?111?/Ti/SiO₂/Si?100?substrates by sol-gel method.We found that the KNBNNO films undergo a phase transition process from pseudo-cubic structure to cubic structure with increasing BaNi_0.5Nb_0.5O_3-?content?x?from 0 to 1,which mainly stems from the Ni²⁺doping-induced local lattice strain.The band gap of the KNBNNO films shows a non-linear variation,namely,a premier sharp decline and a remarkable elevating.The minimum value of the band gap was found at x = 0.3.The magnetism transition behavior from diamagnetism to ferromagnetism for KNBNNO films may originate from ferromagnetic exchange interactions between the magnetic Ni²⁺ ions and a spin-polarized electron trapped at oxygen vacancy.With the increasing x from 0 to 0.3,the quality of the film improved obviously and the band gap decreased linearly.Moreover,with increasing x from 0 to 0.3,the magnetism transition process of the samples from diamagnetism to ferromagnetism may originate from the competition between ferromagnetic exchange interactions in Ni²⁺-Vo^2-Ni²⁺ and superexchange interactions in Ni²⁺-Ni²⁺.Notably,the absorption behavior in visible light wave band for KNBNNO films have been realized,namely,we make it possible to use KNBNNO films for ferroelectric photovoltaic devices.?3?Multiferroics KBiFe₂O₅ ceramics,which has a narrow band gap 1.65 eV,was prepared by a solid-state reaction method.The KBiFe₂O₅ films were grown on different substrates by sol-gel method.The films grown on Pt?111?/Ti/SiO₂/Si?100?substrates have a strong absorption behavior in visible light wave band,which show that KBiFe₂O₅ maybe suitable for ferroelectric photovoltaic absorbing layer.The KBiFe₂O₅ ceramics were prepared by a solid-state reaction method,850 ?is the optimum synthesis temperature for the KBiFe₂O₅ ceramics.The samples obtained at 850 ? have a narrower band gap 1.65 eV,which is due to the stronger covalent in character of Fe-O in[FeO₄]than that in[FeO₆].The magnetic transition from paramagnetism to ferromagnetism and the magnetic moment modification in ferromagnitic phase in KBiFe₂O₅ could be correlated with different spin states of Fe³⁺in KBiFe₂O₅.The KBiFe₂O₅ films were grown on quartz and Pt?111?/Ti/SiO₂/Si?100?substrates by sol-gel method.The synthesized temperature?750 ??and the molar ratios of K/Bi?K:Bi =1.05?were main effecting factors on the phase formation of KBiFe₂O₅ films.The band gap of KBiFe₂O₅ films grown on quartz substrates was 2.43 eV and the band gap of KBiFe₂O₅ films grown on Pt?111?/Ti/SiO₂/Si?100?substrates were 1.10 and 1.90 eV,which show that KBiFe₂O₅ maybe suitable for ferroelectric photovoltaic absorbing layer.