Synthesis and electrical characterization of bismuth ferrite thin films

Pure single phase BiFeO3 and Gd doped BiFeO3 with different Gd doping levels were synthesized through a metalorganic route. Quasi-epitaxial (columnar) BiFeO3 films were fabricated on the top of SrTiO3 substrates with preferred orientation. The rectifying properties of Nb:SrTiO3-BiFeO3-Pt structures, in which the BiFeO3 layer was doped with Gd (0 %; 5 %; and 10 %), were investigated by measuring current-voltage characteristic at different temperatures. It was found that the structures show a diode-like behavior with reverse bias for negative polarity and forward bias for positive polarity applied on the top Pt contact. The potential barrier was estimated for negative polarity assuming a Shottkylike thermionic emission with injection controlled by the interface and the drift controlled by the bulk. It was found that the height of the potential barrier is dependent on the Gd doping, being 0.32 eV for zero doping, 0.45 eV for 5 % doping and 0.60 eV for 10 % doping. The result is explained by the partial compensation of the p-type conduction induced by Bi volatility with Gd doping. The Fermi level moves upward as the doping concentration increases leading to a higher potential barrier for holes

Combined Intrinsic Caloric Effects in Ferroelectrics

There exist multiple driving forces in solid–state materials that can be utilized for entropy changes and hence stronger caloric response. In multiferroic materials, adiabatic temperature changes (ΔTad) can be obtained via combined application of electric, stress, and magnetic fields. These external stimuli provide additional channels of entropy variations resulting in a multi–caloric response. In ferroelectric (FE) materials, caloric responses can be obtained with the application of electric and mechanical fields. Here, we compute the intrinsic electrocaloric and elastocaloric of prototypical FE materials using Landau–Devonshire theory of phase transformations with appropriate electrical and electro–mechanical boundary conditions. Also, the flexocaloric response of FE material systems are computed due to generation of strain gradient induced misfit dislocations. Our electrocaloric calculations indicate that the intrinsic ΔTad in relaxor FEs are substantial and do not vary much over a large temperature interval. Also, we show that an elastocaloric ΔTad of 12.7 ◦C can be obtained in PbTiO3 with the application of uniaxial tensile stress of 500 MPa near its Curie point. Moreover, flexocaloric ΔTad exceeding 1.81 °C can be realized in 20 nm thick barium titanate films. We show a strong link between strain relaxation and strain gradients in epitaxial films and their caloric response. These findings indicate that caloric responses in ferroic materials can be deterministically controlled and enhanced by utilizing a variety of external stimuli. Our results suggest a promising perspective to find solid–state systems with giant caloric responses to be used as alternatives for conventional refrigeration technologies

Potential barrier increase due to Gd doping of BiFeO3 layers in Nb:SrTiO3-BiFeO3-Pt structures displaying diode-like behavior

The rectifying properties of Nb:SrTiO3-Bi1-xGdxFeO3-Pt structures (x = 0, 0.05, 0.1) displaying diode-like behavior were investigated via current-voltage characteristics at different temperatures. The potential barrier was estimated for negative polarity assuming a Schottky-like thermionic emission with injection controlled by the interface and the drift controlled by the bulk. The height of the potential barrier at the Nb:SrTiO3-Bi1-xGdxFeO3 interface increases with Gd doping. The results are explained by the partial compensation of the p-type conduction due to Bi vacancies with Gd doping in addition to the shift of the Fermi level towards the middle of the bandgap with increasing dopant concentration

Strong smearing and disappearance of phase transitions into polar phases due to inhomogeneous lattice strains induced by A-site doping in Bi(1-x)A(x)FeO(3) (A: La, Sm, Gd)

Doping of ferroics is often intended to generate new functionalities or enhance the already existing properties, but it comes at the expense of local structural distortions around dopants in the lattice. We report on the effect of A-site doping and their effect on the phase transition temperatures of sol-gel synthesized Bi-1 (x)A(x)FeO(3) (A: Gd, Sm, La) powders as a function of dopant type and concentration. A clear direct correlation between structural parameters and transition temperatures was noted as a function of ionic radii of dopants for any given concentration, implying the effect of inhomogeneous lattice strains around dopants. There is a dramatic reduction in the phase transition temperatures of BiFeO3 upon doping determined with differential thermal analysis. This is accompanied by a partial volume of the grains gradually shifting from the bulk rhombohedral towards a higher symmetry structure particularly in Sm and Gd doped powders while this change is minimal in La doped powders as evidenced by X-ray diffraction and Raman spectroscopy. We find that a phase mixture forms in powders whose fraction is a strong function of dopant radius for a given concentration. Moreover, there is a direct correlation between the ionic radius and the extent of reduction in the transition temperature of the polar phase in the mixture for a given dopant concentration. We suggest a mechanism to explain the inhomogeneous nature of the transition of the sol-gel synthesized powders where the dramatic reduction in the transition temperatures of Sm and Gd doped BiFeO3 is due to local lattice strains around unitcells containing dopant ions that create gradients in polarization leading to internal depolarizing fields, possibly stabilizing non-polar phases. We conclude that local disappearance of stereochemical activity of Bi3+ due to lone pairs is not sufficient to explain the dramatic changes in phase transition temperatures because of strong dependence on ionic radii of dopants

Toward High-Performance Triboelectric Nanogenerators by Engineering Interfaces at the Nanoscale: Looking into the Future Research Roadmap

To meet the future need for clean and sustainable energies, there has been considerable interest in the development of triboelectric nanogenerators (TENGs) that scavenge waste mechanical energies. The performance of a TENG at the macroscale is determined by the multifaceted role of surface and interface properties at the nanoscale, whose understanding is critical for the future development of TENGs. Therefore, various protocols from the atomic to the macrolevel for fabrication and tuning of surfaces and interfaces are required to obtain the desired TENG performance. These protocols branch out into three categories: chemical engineering, physical engineering, and structural engineering. Chemical engineering is an affordable and optimal strategy for introducing more surface polarities and higher work functions for the improvement of charge transfer. Physical engineering includes the utilization of surface morphology control, and interlayer interactions, which can enhance the active interfacial area and electron transfer capacity. Structural engineering at the macroscale, which includes device and electrode design/modifications has a considerable effect on the performance of TENGs. Future challenges and promising research directions related to the construction of next-generation TENG devices, taking into consideration “interfaces” are also presented