Enhanced ferroelectric and improved leakage of BFO-based thin films through increasing entropy strategy

BiFeO_(3)(BFO)has received considerable attention as a lead-free ferroelectric film due to its large theoretical remnant polariza-tion.However,BFO suffers from a large leakage current,resulting in poor ferroelectric properties.Herein,the sol-gel method was used to deposit a series of BFO-based thin films on fluorine-doped tin oxide substrates,and the effects of the substitution of the elements Co,Cu,Mn(B-site)and Sm,Eu,La(A-site)on the crystal structure,ferroelectricity,and leakage current of the BFO-based thin films were invest-igated.Results confirmed that lattice distortion by X-ray diffraction can be attributed to the substitution of individual elements in the BFO-based films.Sm and Eu substitutions contribute to the lattice distortion in a pseudo-cubic structure,while La is biased toward pseudo-tet-ragonal.Piezoelectric force microscopy confirmed that reversible switching of ferroelectric domains by nearly 180°can be realized through the prepared films.The ferroelectric hysteresis loops showed that the order for the polarization contribution is as follows:Cu>Co>Mn(B-site),Sm>La>Eu(A-site).The current density voltage curves indicated that the order for leakage contribution is as follows:Mn<Cu<Co(B-site),La<Eu<Sm(A-site).Scanning electron microscopy showed that the introduction of Cu elements facilitates the formation of dense grains,and the grain size distribution statistics proved that La element promotes the reduction of grain size,leading to the increase of grain boundaries and the reduction of leakage.Finally,a Bi_(0.985)Sm_(0.045)La_(0.03)Fe_(0.96)Co_(0.02)Cu_(0.02)O_(3)(SmLa-CoCu)thin film with a qualitative leap in the remnant polarization from 25.5(Bi_(0.985)Sm_(0.075)FeO_(3))to 98.8µC/cm^(2)(SmLa-CoCu)was prepared through the syner-gistic action of Sm,La,Co,and Cu elements.The leakage current is also drastically reduced from 160 to 8.4 mA/cm^(2)at a field strength of 150 kV/cm.Thus,based on the increasing entropy strategy of chemical engineering,this study focuses on enhancing ferroelectricity and decreasing leakage current,providing a promising path for the advancement of ferroelectric devices.

Molecular Modulation of Structure and Ferroelectric Performance of Poly(vinylidene fluoride)Free Standing Films from Aspects of Molecular Weight and Crystallization Temperature

Ferroelectric polymer poly(vinylidene fluoride)(PVDF)has received great research interest because of its special electroactive properties which are strongly dependent on the crystalline structures and in turn processing conditions.The effect of molecular weight and crystallization temperature on the micro-structure and macro-properties of PVDF films casted from dimethyl sulfoxide(DMSO)solvent is investigated.The results demonstrated that a low molecular weight(180 kg/mol)and a low evaporation temperature(50℃)favored the formation of polarγ-phase,while a high molecular weight(1000 kg/mol)and a high evaporation temperature(125℃)made PVDF crystallize intoα-phase.Compared with films casted at 50℃,films casted at 125℃exhibited higher dielectric loss at a low electric field and less loss conductivity at a high electric field,which was due to their low degrees of crystallinity and fine evaporation of the solvent,respectively.PVDF with a molecular weight of 180 kg/mol casted at 125℃exhibited the highest remnant polarization(0.062 C/m^(2))among all of the solution-processed films,being a result of high chain mobility resulted from the low molecular weight.

Effects of annealing temperature on microstructure and ferroelectric properties of Bi_(0.5)(Na_(0.85)K_(0.15))_(0.5)TiO_3 thin films

Bi0.5(Na0.85K0.15)0.5TiO3(BNKT15) thin films were synthesized by metal-organic decomposition(MOD) at annealing temperatures of 650,680,710 and 740℃,and the effects of annealing temperature on the microstructure,dielectric properties,remnant polarization(2Pr) and leakage current density were studied with X-ray diffractometer,atomic force microscope,precision impedance analyzer,ferroelectric analysis station and semiconductor parameter tester.The results show that the thin film annealed at 710℃ exhibits a typical perovskite structure without predominant orientation and a smooth surface with evenly distributed grains.2Pr value(67.4 μC/cm2 under 830 kV/cm) and the leakage current density(1.6×10-6 A/cm2 at 170 kV/cm) for BNKT15 thin film annealed at 710℃ are better than those for thin films annealed at other temperatures.

Thermal and piezoelectric properties of Bi_(3.15)Nd_(0.85)Ti_3O_(12) thin film prepared by metal organic decomposition

Bi3.15Nd0.85Ti3O12 （BNT） powder and thin film were prepared by metal organic decomposition （MOD） method. The heat flow curve of BNT powder was measured with a modulated temperature differential scanning calorimeter, and thermal physical parameters such as thermal conductivity coefficient and thermal diffusion coefficient were obtained from the heat flow curve. The phase identification, ferroelectric, and piezoelectric properties of BNT thin film annealed at 700℃ were investigated with X-ray diffractometer, ferroelectric analyzer, and scanning probe microscope. The results show that the thin films consisting of a single phase of bismuth-layered perovskite are polycrystalline, without a preferred orientation. Remnant polarization 2Pr is 63.2 μC/cm2 under 530 kV/cm applied field, and the effective piezoelectric coefficient d33 is 30 pm/V.

Dielectric Characteristics of Ba_(0.65)Sr_(0.35)TiO_3 Thin Films by Sol-Gel Method

Ferroelectric Ba0.65Sr0.35TiO3 （BST） thin films on the Pt/Ti/SiO2/Si substrate have been successfully prepared by sol-gel. Such films have approximately 300 nm thicknesses with a remnant polarization of about 2.95 μ℃/cm^2 and a coercive field of about 21.5 kV/cm. The investigations of X-ray diffraction and atomic force microscopy show that the BST films annealed at 650 ℃ exhibit a tetragonal structure and that the films dominantly consist of large column or grains of about 89 nm in diameter. The curves of the temperature dependence of dielectric coefficient in different frequencies display the curie transition at the temperature around 23 ℃. The dielectric loss tangent of BST thin fdms at 100 kHz is less than 0.04. As a result, the BST thin films are more applicable for fabrication of infrared detector compared with the BST thin films reported previously.