Abstract
It has been recently shown that the strain gradient is able to separate the light-excited electron-hole pairs in semiconductors, but how it affects the photoelectric properties of the photo-active materials remains an open question. Here, we demonstrate the critical role of the strain gradient in mediating local photoelectric properties in the strained BiFeO3 thin films by systematically characterizing the local conduction with nanometre lateral resolution in both dark and illuminated conditions. Due to the giant strain gradient manifested at the morphotropic phase boundaries, the associated flexo-photovoltaic effect induces on one side an enhanced photoconduction in the R-phase, and on the other side a negative photoconductivity in the morphotropic Tprime-phase. This work offers insight and implication of the strain gradient on the electronic properties in both optoelectronic and photovoltaic devices.
Highlights
It has been recently shown that the strain gradient is able to separate the light-excited electron-hole pairs in semiconductors, but how it affects the photoelectric properties of the photo-active materials remains an open question
We have recently demonstrated that the strain gradient is able to separate photo-excited electron-hole pairs, similar to chemical gradients and electric fields, giving rise to a unique photovoltaic effect termed flexo-photovoltaic effect[15]
Due to the large lattice mismatch and epitaxial confinement, a giant strain as well as strain gradient is generated at the interface between these two morphotropic phases
Summary
It has been recently shown that the strain gradient is able to separate the light-excited electron-hole pairs in semiconductors, but how it affects the photoelectric properties of the photo-active materials remains an open question. Due to the flexoelectric effect, those strain gradients would alter domain structures or modify hysteresis curves of ferroelectric thin films and even induce a large electric polarization in paraelectric crystals[5,6,7,8,9,10] Apart from this electromechanical coupling, the strain gradient can affect electronic properties via redistributing charged ionic defects, such as oxygen vacancies in oxide materials[11,12,13,14]. By virtue of its moderate bandgap, BiFeO3 thin films can absorb visible light and generate photoconductive effects as well as more intriguing photoelectric effects, such as persistent conductivity[17] These morphotropic phases in strained BiFeO3 films with the same chemical composition provide an excellent platform to study the effects of strain gradient, especially the associated flexo-photovoltaic effect, on photoelectric properties in oxide materials. By directly characterizing the local electronic properties of these morphotropic phases with a nanometre resolution, we shed light on the critical importance of the strain gradient in mediating local photoelectric properties and elucidate the exact role of the morphotropic phase boundaries
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