Bandgap-engineered ferroelectric ceramics for filterless visible-range color sensing and wavelength-selective photodetection
2Nano and Molecular Systems Research Unit, University of Oulu, Oulu, Finland
3Institute of Solid State Physics, University of Latvia, Riga, Latvia
Classic ABO3 perovskite ferroelectrics usually exhibit large photonic bandgaps, e.g. ~2.7 eV for rhombohedral BiFeO3 and >3 eV for BaTiO3 and the Pb(Zr,Ti)O3 family, which are beyond the majority of visible lights. Reducing the bandgaps deeply into the visible range (1.8-3.1 eV) may benefit many potential applications where interactions among light, conductivity and domain wall motion happen. It has not been an easy task to bring the bandgaps of classic ferroelectrics down to near 1.8 eV whilst maintaining a decent ferroelectricity including meaningful spontaneous and remanent polarizations, due to that both the band-band transition energy and spontaneous polarization are largely affected by the electronegativity in the B-O bond and they often evolve in the same direction. This talk will discuss a method of constructing a heterojunction between a polar perovskite phase and a polar tungsten bronze phase by introducing A-site vacancy into the Ba/Ni codoped (K,Na)NbO3 ceramics. Within the junction, the bandgap is minimized and the ferroelectricity is optimized, simultaneously resulting in a direct bandgap of ~2 eV and spontaneous polarization of 0.25 C/m2. The mechanism for bandgap reduction is identical to that for a semiconducting heterojunction, and the ferroelectricity optimization mimics what occurs in a morphotropic phase boundary. Thanks to this unique feature and thus the resultant opto-ferroelectric cumulative effect, the difference between the short-circuit and the biased photocurrents becomes considerably sensitive to the incident photon energy. Taking advantage of such a phenomenon, the functionality of detecting both the wavelength and intensity of the incident light will be demonstrated in this talk from a monolithic ceramic material without the need of auxiliary filters or multiple components which conventional semiconductor-based photodetectors rely on. This work provides a fundamentally different alternative for smart photosensors.
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