Polarization-gradient KNbO3 film with a large photovoltaic current

Potassium niobate (KNbO3, KNO) has been intensively investigated for photovoltaic performance because of its non-toxicity and excellent nonlinear optical properties. The corresponding photovoltaic current density, however, remains very low due to a wide bandgap. Chemical doping and strain engineering strategies have been employed to tailor the band structure to enhance photovoltaic current density. Nevertheless, the original current density is still at a level of several tens of nA/cm2, significantly limiting device applications. In this work, we report a lattice-gradient KNO film on (100) single-crystal 0.7 wt. % Nb doped SrTiO3 (NSTO) substrate processed by annealing, generating a polarization-gradient that allows us to generate a large current density via a built-in field. The film exhibits a remarkable short-circuit current density (Jsc) of 58.63 µA/cm2 under the 375 nm irradiation of 500 mW/cm2 light intensity, where the corresponding responsivity (117.26 µA/W) is ∼3.82 times higher than those of reported KNO-based materials. It was revealed that the annealing process driven interfacial structure evolution from disorder to atomic-scale smoothness, accompanied by the transformation of the polarization shielding mechanism. After this process, an intriguing lattice-gradient throughout the film was established to have a uniform polarization direction, possibly accounting for the improved photovoltaic current density of KNO film. These findings may trigger interest in developing KNO as a potential key material for lead-free optoelectronic or photodetector devices

Solution epitaxy of polarization-gradient ferroelectric oxide films with colossal photovoltaic current

Abstract Solution growth of single-crystal ferroelectric oxide films has long been pursued for the low-cost development of high-performance electronic and optoelectronic devices. However, the established principles of vapor-phase epitaxy cannot be directly applied to solution epitaxy, as the interactions between the substrates and the grown materials in solution are quite different. Here, we report the successful epitaxy of single-domain ferroelectric oxide films on Nb-doped SrTiO3 single-crystal substrates by solution reaction at a low temperature of ~200 oC. The epitaxy is mainly driven by an electronic polarization screening effect at the interface between the substrates and the as-grown ferroelectric oxide films, which is realized by the electrons from the doped substrates. Atomic-level characterization reveals a nontrivial polarization gradient throughout the films in a long range up to ~500 nm because of a possible structural transition from the monoclinic phase to the tetragonal phase. This polarization gradient generates an extremely high photovoltaic short-circuit current density of ~2.153 mA/cm2 and open-circuit voltage of ~1.15 V under 375 nm light illumination with power intensity of 500 mW/cm2, corresponding to the highest photoresponsivity of ~4.306×10−3 A/W among all known ferroelectrics. Our results establish a general low-temperature solution route to produce single-crystal gradient films of ferroelectric oxides and thus open the avenue for their broad applications in self-powered photo-detectors, photovoltaic and optoelectronic devices

Single-Crystal BiFeO₃ Nanoplates with Robust Antiferromagnetism

Freestanding and single-crystal BiFeO₃ (BFO) nanoplates have been successfully synthesized by a fluoride ion-assisted hydrothermal method, and the thickness of the nanoplates can be effectively tailored from 80 to 380 nm by the concentration of fluoride ions. It is revealed that BFO nanoplates grew via an oriented attachment of layer by layer, giving rise to the formation of the inner interface within the nanoplates. In particular, antiferromagnetic (AFM) phase-transition temperature (Néel temperature, <i>T</i>_N) of the BFO nanoplates is significantly enhanced from typical 370 to ∼512 °C, whereas the Curie temperature (<i>T</i>_C) of the BFO nanoplates is determined to be ∼830 °C, in good agreement with a bulk value. The combination of scanning transmission electron microscopy, electron energy loss spectroscopy, and the first-principle calculations reveals that the interfacial tensile strain remarkably improves the stability of AFM ordering, accounting for the significant enhancement in <i>T</i>_N of BFO plates. Correspondingly, the tensile strain induced the polarization and oxygen octahedral tilting has been observed near the interface. The findings presented here suggest that single-crystal BFO nanoplate is an ideal system for exploring an intrinsic magnetoelectric property, where a tensile strain can be a very promising approach to tailor AFM ordering and polarization rotation for an enhanced coupling effect