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Enhanced optical properties and the origin of carriers transport in BiFeO$_3$/TiO$_2$ heterostructures with 109$^{\circ}$ domain walls

The absorption performance in the BiFeO$_3$/TiO$_2$ bilayer film prepared by simple sol-gel method has been significantly improved in the ultraviolet and visible-light region, comparing with BiFeO$_3$ and TiO$_2$ films. Terahertz-radiation emission presents a direct evidence of photon-induced electrons and holes transport in the heterostructures. First-principles calculations agree well with the experiments and present an unambiguous explanation of charge carriers transport and enhanced absorbance which is induced by the large electrostatic potential drop in the interface of heterostructures with the 109$^{\circ}$ domain walls. This work provides a promising candidate toward designing novel photovoltaic BiFeO$_3$-based heterostructures with high efficiencies.