Tunable Luminescence Contrast of Na0.5Bi4.5Ti4O15:Re (Re = Sm, Pr, Er) Photochromics by Controlling the Excitation Energy of Luminescent Centers

How to Realize Ultrahigh Photochromic Performance for Real-Time Optical Recording in Transparent Ceramics

A combination of transparency and photochromic (PC) properties in ferroelectrics has promising application potential in smart windows and optical storage/imaging. Nonetheless, limited by understanding the underlying PC mechanism, a splendid PC performance is rarely achieved in transparent ferroelectrics. Here, a strategy to construct deep-lying traps by ion-doping induced defect engineering in (K_0.5Na_0.5)NbO₃-based ferroelectric ceramics is proposed. Based on the improved density functional theory simulations, a high concentration of vacancy defects can be realized by codoping 1 mol % Pr and 4 mol % Ba in (K_0.5Na_0.5)NbO₃, which helps achieving deep-lying traps and then superior PC performance. Through traditional pressureless sintering, highly transparent ceramics with designed optimal composition have been fabricated in a wide sintering temperature range (1170-1210 °C), exhibiting an ultrafast PC feature, i.e., 0.1 s response time (by illumination of 400 nm light), along with high PC efficiency (5.8 cm²·W^-1) and PC rate (7.1 s^-1), preeminent among reported inorganic PC transparent materials. Additionally, the ceramics have been utilized for real-time optical recording, displaying unambiguous patterning with long-time preservation (21 days). This research supplies a paradigm for designing high-performance PC transparent materials in optical applications and helps deepen the comprehensive understanding of the PC mechanism.

Photochromic and Electric Field-Regulating Luminescence in High-Transparent (K,Na)NbO3-Based Ferroelectric Ceramics with Two-Phase Coexistence

Rare earth Tb³⁺ doped (K_0.465Na_0.465Li_0.07)(Nb_0.93Bi_0.07)O₃ (KNNLB-x%Tb) lead-free transparent ferroelectric ceramics were designed and prepared. The effects of Tb³⁺ on phase structure, microstructure, optical transmittance, photoluminescence, and photochromic behaviors were studied. Although two ferroelectric phases coexist, the optical transmittance can reach the high value of 74% in the visible light region because of the fine grains, dense ceramic microstructure, large optical energy band gap, and relatively high symmetry of coexisting ferroelectric phases. In addition, Tb³⁺ works as a luminescent center, and the reversible photochromic modulation is achieved by alternate stimulation of illumination and heat treatment. Meanwhile, the luminescence contrast can be improved under in situ electric field stimulation due to the easy change of lattice symmetry in coexisting ferroelectric phases. The generation of color centers after illumination and the local crystal field around the luminescent center caused by in situ electric field contributes to above phenomena. These ceramics exhibit the great potential in optical data storage and anticounterfeiting.

Supramolecular Assembly-Driven Color-Tuning and White-Light Emission Based on Crown-Ether-Functionalized Dihydrophenazine

The development of color-tunable white-light-emitting systems is significant for artificial smart materials. Recently, a set of conformational dependent fluorophores N,N'-diaryl-dihydrodibenzo[a,c]phenazines (DPACs) have been developed with unique photoluminescence mechanism vibration-induced emission (VIE). DPACs can emit intrinsical blue emission at a bent excited state and abnormal orange-red emission at a planar excited state, which are due to the varied π-conjugation via excited-state configuration transformation along the N-N' axis from bent to planar form. Herein, a novel VIE-active compound DPAC-[B15C5]₂ is designed and synthesized with two wings of benzo-15-crown-5. The excited-state vibration of the DPAC moiety can be modulated by tuning the supramolecular assembly and disassembly via chelation competition of K⁺ between 15-crown-5 and 18-crown-6, and hence, a wide-color-tuning emission is achieved from blue to orange-red including white. Dynamic light scattering and transmission electron microscopy experiments were conducted to exhibit the supramolecular assembling process. Additionally, the moisture detection in organic solvents is realized since the water could dissociate the supramolecular assembly.

Room-Temperature Large and Reversible Modulation of Photoluminescence by in Situ Electric Field in Ergodic Relaxor Ferroelectrics

Ferroelectric oxides with luminescent ions hold great promise in future optoelectronic devices because of their unique photoluminescence and inherent ferroelectric properties. Intriguingly, the photoluminescence performance of ferroelectric ceramics could be modulated by an external electric field. However, researchers face a current challenge of the diminutive extent and degree of reversibility of the field-driven modification that hinder their use in room-temperature practical applications. Within the scope of current contribution in rare-earth-doped bismuth sodium titanate relaxors, the most important information to be noted is the shifting of the depolarization temperature toward room temperature and the resulting considerable enhancement in ergodicity, as evidenced by the dielectric properties, polarization, and strain hysteresis, as well as the in situ Raman/X-ray diffraction studies. After the introduction of 1 mol % Eu, the induced composition and charge disorders disrupt the original long-range ferroelectric macrodomains into randomly dynamic and weakly correlated polar nanoregions, which facilitates a reversible transformation between polar nanoregions and unstable ferroelectric state under an electric field, engendering a large strain. By virtue of this, both the extent and degree of reversibility of photoluminescence modulation are enhanced (∼60%) considerably, and room-temperature in situ tunable photoluminescence response is then achieved under electric field. These should be helpful for the realization of regulating the physical couplings (photoluminescent-ferroelectrics) in multifunctional inorganic ferroelectrics with a high ergodic state by reversibly tuning the structural symmetry.