Ferroelectrics have recently attracted attention as a candidate class of materials for use in photovoltaic devices due to their abnormal photovoltaic effect. However, the current reported efficiency is still low. Hence, it is urgent to develop narrow-band gap ferroelectric materials with strong ferroelectricity by low-temperature synthesis. In this paper, the perovskite bismuth ferrite BiFeO₃ (BFO) thin films were fabricated on SnO₂: F (FTO) substrates by the sol–gel method and they were rapidly annealed at 450, 500 and 550 °C, respectively. The microstructure and the chemical state’s evolution with annealing temperature were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS), and the relationship between the microstructure and electric, optical and photovoltaic properties were studied. The XRD, SEM and Raman results show that a pure phase BFO film with good crystallinity is obtained at a low annealing temperature of 450 °C. As the annealing temperature increases, the film becomes more uniform and has an improved crystallinity. The XPS results show that the Fe³⁺/Fe²⁺ ratio increases and the ratio of oxygen vacancies/lattice oxygen decreases with increasing annealing temperature, which results in the leakage current gradually being reduced. The band gap is reduced from 2.68 to 2.51 eV due to better crystallinity. An enhanced photovoltaic effect is observed in a 550 °C annealed BFO film with a short circuit current of 4.58 mA/cm² and an open circuit voltage of 0.15 V, respectively.

Inspired by the brilliant and tunable structural colors based on the large refractive index contrast (Δn) and non‐close‐packing structures of chameleon skins, ZnS–silica photonic crystals (PCs) with highly saturated and adjustable colors are fabricated. Due to the large Δn and non‐close‐packing structure, ZnS–silica PCs show 1) intense reflectance (maximal: 90%), wide photonic bandgaps, and large peak areas, 2.6–7.6, 1.6, and 4.0 times higher than those of silica PCs, respectively; 2) tunable colors by simply adjusting the volume fraction of particles with the same size, more convenient than the conventional way of altering particle sizes; and 3) a relatively low threshold of PC's thickness (57 µm) possessing maximal reflectance compared to that (>200 µm) of the silica PCs. Benefiting from the core–shell structure of the particles, various derived photonic superstructures are fabricated by co‐assembling ZnS–silica and silica particles into PCs or by selectively etching silica or ZnS of ZnS–silica/silica and ZnS–silica PCs. A new information encryption technique is developed based on the unique reversible “disorder–order” switch of water‐responsive photonic superstructures. Additionally, ZnS–silica PCs are ideal candidates for enhancing fluorescence (approximately tenfold), approximately six times higher than that of silica PC.

The conventional photonic crystals (PCs) are usually prepared by the self-assembly of silica or polystyrene particles. However, their applications are limited significantly due to the lack of the functions of the building blocks. Here, a new kind of photo-luminescent photonic crystals (PLPCs) with brilliant PL and structural colors were prepared by the self-assembly of dye-doped silica particles. The PL and structural colors of PCs can be well-controlled by altering the species of dyes and the size of the particles, respectively. Based on these advantages, PLPC patterns with encrypted information were fabricated through the combination of PLPCs and PCs with similar structural colors but diverse PL colors. These patterns can reversibly hide and display the encrypted information under sunlight and UV illumination, respectively. This work paves a new way for constructing functional PCs and will promote their applications in anti-counterfeiting, smart labels, and optical devices.

The phosphor Sr_0.97Al₂O₄:0.01Eu²⁺, 0.02Dy³⁺ was synthesized by a high temperature solid state reaction in a reducing atmosphere. After charging by ultraviolet radiation, the persistent luminescence decay was divided into different time ranges and well-fitted by a biexponential function, but the two lifetimes became longer with time of persistent luminescence decay. The fractional amplitude of the shorter lifetime increased with time, whereas that of the longer lifetime decreased. The change in lifetime has been associated with the emptying of a pseudo-continuum of trap states either from different traps or different levels of the same trap species. The investigation in photostimulated persistent luminescence indicated the sample could emit bright persistent luminescence again under infrared light excitation after the UV light-excited persistent luminescence had decayed completely. The NIR photostimulation exhibited a continuous broad band, with maximum at 760 nm, representing the emptying of filled traps into the conduction band. These results also infer that the traps are pseudo-continuous but the assignment of Dy²⁺ as the trapped species cannot be excluded. The read-in and write-out properties of the phosphor have been elucidated and these convey applications in information storage and retrieval.