Qiwei Zhang

A highly efficient, orange light-emitting (K0.5Na0.5)NbO3:Sm3+/Zr4+ lead-free piezoelectric material with superior water resistance behavior

Multifunctional luminescent materials based on rare earth doped ferro-/piezoelectrics have attracted much attention due to their potential applications in novel multifunctional devices. Currently, it remains a challenge to fabricate these materials with high photoluminescence quantum yields, comparable to values obtained for traditional phosphors. Herein, we reported a highly efficient, orange light-emitting material with a superior water resistance behavior based on a (K0.5Na0.5)NbO3 (KNN) matrix co-doped with Sm3+ and Zr4+. The phase structure, composition, photoluminescence properties, thermal quenching and water resistance behavior of the samples were systematically studied. A significantly enhanced orange light-emission at 597 nm originating from the 4G5/2 → 6H7/2 transition of Sm3+ was observed by the introduction of Zr4+ ions, which can be well explained by the exchange charge model of the crystal field. Particularly we achieve a photoluminescence quantum yield as high as 53% and superior water resistance property almost maintaining the same PL intensity as before immersion after 80 h water immersion time for the composition of (K0.5Na0.5)0.99Sm0.01Nb0.09Zr0.01O3 under 407 nm n-UV light excitation. The QY value can be comparable to some commercial phosphors, such as Y2O2S/Y2O3:Eu3+. These findings show the great potential of the Sm3+/Zr4+ co-doped KNN material for future applications in white LEDs and novel multifunctional devices.

Reversible Luminescence Modulation upon Photochromic Reactions in Rare-Earth Doped Ferroelectric Oxides by in Situ Photoluminescence Spectroscopy

Reversible luminescence photoswitching upon photochromic reactions with excellent reproducibility is achieved in a new inorganic luminescence material: Na(0.5)Bi(2.5)Nb2O9: Pr(3+) (NBN:Pr) ferroelectric oxides. Upon blue light (452 nm) or sunlight irradiation, the material exhibits a reversible photochromism (PC) performance between dark gray and green color by alternating visible light and thermal stimulus without inducing any structure changes and is accompanied by a red emission at 613 nm. The coloration and decoloration process can be quantitatively evaluated by in situ photoluminescence spectroscopy. Meanwhile, the luminescence emission intensity based on PC reactions is effectively tuned by changing irradiation time and excitation wavelength, and the degree of luminescence modulation has no significant degradation after several periods, showing very excellent reproducibility. On the basis of the luminescence modulation behavior, a double-exponential relaxation model is proposed, and a combined equation is adopted to well describe the luminescence response to light irradiation, being in agreement with the experimental data.

Dual-Mode Luminescence Modulation upon Visible-Light-Driven Photochromism with High Contrast for Inorganic Luminescence Ferroelectrics.

A luminescence ferroelectric oxide, Na(0.5)Bi(2.5)Nb2O9 (NBN), system with bismuth layer structure introduced by lanthanide ion (Er(3+)) has been demonstrated to exhibit reversible, high-contrast luminescence modulation (95%) and excellent fatigue resistance based on visible-light-driven photochromism (407 nm or sunlight). The coloration and decoloration process can be effectively read out by dual modes, upconversion and downshifting, and reversibly converted between green and dark gray by alternating visible light or sunlight irradiation and thermal stimulus. The luminescence modulation degree upon photochromic reactions is strongly dependent upon irradiation light wavelength and irradiation time. After undergoing several cycles, there are no significant degradations, showing high reversibility. Considering its high-contrast photoswitchable luminescence feature and intrinsic ferroelectricity of NBN host, NBN-based multifunctional materials can be suggested as a promising candidate for new potentials in photonic storage and optoelectronic multifunctional devices.

Nondestructive up-conversion readout in Er/Yb co-doped Na0.5Bi2.5Nb2O9-based optical storage materials for optical data storage device applications

Luminescence modulation based on photochromic reactions is always considered to be a promising method to achieve nondestructive readout in photochromic materials. Generally speaking, two conventional strategies have been widely used to achieve this target: tuning the absorption bands and adjusting luminescent quenching mechanisms. In this paper, we found a new strategy to improve effectively luminescence readout capability in Er/Yb codoped NBN-based solid-state inorganic photochromics by using a two-photon absorption mode of luminescent centers. Upon alternating visible light irradiation (407 nm) and the thermal stimulus, the materials exhibited a high luminescence switching contrast ratio (ΔRt = 86%), excellent reversibility, and significantly improved luminescent efficiency (22 times). Most importantly, the photochromic reaction process can be efficiently read out using the two-photon absorption (or up-conversion) mode without inducing any new reactions, showing extremely low destruction on information recording (destruction degree <11%), which is superior to other luminescence emission modes (down-shifting or down-conversion). These results could be used as a guide to tailor the luminescence modulation properties of photochromic materials to realize non-destructive readout in 3D optical data storage device applications.

(K0.5Na0.5)NbO3:Eu3+/Bi3+: a novel, highly efficient, red light-emitting material with superior water resistance behavior

Materials emitting red light (∼611 nm) under excitation with blue light (440–470 nm) are highly desired for fabricating high-performance white light-emitting diodes (LEDs). Conventionally used red light-emitting materials (e.g., Y2O3:Eu3+ or Y2O2S:Eu3+) exhibit a relatively poor blue light-absorption and a weak chemical stability. In this paper, we reported on a novel red light-emitting material based on a (K0.5Na0.5)NbO3 (KNN) matrix co-doped with Bi3+ and Eu3+ showing a strong absorption in the blue light region and superior water resistance properties. The crystal structure, photoluminescence, thermal stability, energy transfer mechanism and water resistance behavior of the samples were systematically investigated. A strongly enhanced red light-emission at 616 nm originating from the 5D0 → 7F2 transition of Eu3+ ions was observed after adding Bi3+ ions as an alternative to increasing the Eu3+ concentration due to the energy transfer from Bi3+ to Eu3+. After adding 0.05 mol of Bi3+ as sensitizer, the sample with the composition of (K0.5Na0.5)0.90Eu0.05Bi0.05NbO3 exhibited the strongest red light-emission and a high quantum yield under 465 nm excitation. Doping with Bi3+ also endowed the KNN:Eu3+ samples with a good thermal stability (83% of the initial intensity at 150 °C) and a superior water resistance behavior (94.3% of the initial intensity after 40 h of immersion). These results demonstrate the great potential of the Bi3+/Eu3+ co-doped KNN material for a future application in white LEDs and novel multifunctional devices.

Luminescence photoswitching of Ho-doped Na0.5Bi2.5Nb2O9 ferroelectrics: the luminescence readout process

Luminescent switching materials upon photochromic reactions have potential applications in optical switching and high-density optical data storage in optoelectronic devices. To avoid interference and destruction of information in practical data storage applications, a nondestructive luminescence readout is essential. However, it is still unclear how to select the optimized excitation and emission bands to avoid the photochromic reaction during the “reading” process while maintaining high luminescence contrast and stability in inorganic photochromic materials. On the basis of the nonradiative energy transfer mechanism, Ho3+ ions were introduced into the Na0.5Bi2.5Nb2O9 host to obtain efficient luminescence switching due to their special excitation (451 nm) and emission (547 nm) characteristics. Under 407 nm irradiation (“writing”), the photochromic phenomenon can be effectively read out by measuring the changes in the luminescence emission intensity. The luminescence switching contrast increased up to 94%. Importantly, the excitation and emission energies did not significantly induce new photochromic reactions, causing less destruction to the material and the luminescence readout process. This outcome is superior to our previously reported results. Furthermore, the luminescence switching properties exhibit hardly any degradation after undergoing several cycles of the “writing”, “reading” and “erasing” processes, indicating excellent reversibility.

(K,Na)NbO3 ferroelectrics: a new class of solid-state photochromic materials with reversible luminescence switching behavior

In this paper, we reported a new photosensitive material, Sm doped K0.5Na0.5NbO3 (KNN) ceramics, fabricated using a solid-sate reaction method, which exhibits both photochromism and luminescence switching properties. By alternating visible light irradiation (λ > 407 nm) and thermal stimulus, the samples show a reversible color change from the initial green to pale gray. Interestingly, luminescence emission intensity can be effectively tuned using photochromic reactions. Furthermore, the luminescence switching degree strongly depends on the firing temperature. These results suggest that KNN-based perovskite oxides with photochromism, luminescence switching and ferroelectric energy storage properties are particularly attractive for optical data storage applications as multi-functional materials.

Synthesis of 1,3-dicarbonyl-functionalized reduced graphene oxide/MnO2 composites and their electrochemical properties as supercapacitors

A novel 1,3-dicarbonyl-functionalized reduced graphene oxide (rDGO) was prepared by N-(4-aminophenyl)-3-oxobutanamide interacting with the epoxy and carboxyl groups of graphene oxide. The high-performance composite supercapacitor electrode material based on MnO2 nanoparticles deposited onto the rDGO sheet (DGM) was fabricated by a hydrothermal method. The morphology and microstructure of the composites were characterized by field-emission scanning electron microscopy, transmission electron microscopy, Raman microscopy and X-ray photoelectron spectroscopy. The obtained results indicated that MnO2 was successfully deposited on rDGO surfaces. The formed composite electrode materials exhibit excellent electrochemical properties. A specific capacitance of 267.4 F g−1 was obtained at a current density of 0.5 A g−1 in 1 mol L−1 H2SO4, while maintaining high cycling stability with 97.7% of its initial capacitance after 1000 cycles at a current density of 3 A g−1. These encouraging results are useful for potential energy storage device applications in high-performance supercapacitors.

Giant energy-storage density and high efficiency achieved in (Bi0.5Na0.5)TiO3–Bi(Ni0.5Zr0.5)O3 thick films with polar nanoregions

The development of electronic devices towards integration, miniaturization and environmental friendliness has propelled much recent research on lead-free dielectric capacitors for energy storage, however, high energy-storage density is still an extremely challenging objective for lead-free dielectric materials. Here, a novel lead-free relaxor ferroelectric (1 − x)(Bi0.5 Na0.5)TiO3–xBi(Ni0.5Zr0.5)O3 (BNT–xBNZ, x = 0–0.5) thick film (1 μm) was fabricated by a water-based sol–gel method. Doping of BNZ into the BNT host promoted the formation of polar nanoregions (PNRs), whose domain switching became easier, leading to an improved energy-storage performance. Surprisingly, an ultrahigh recoverable energy density of 50.1 J cm−3 and a high energy-storage efficiency of 63.9% under 2200 kV cm−1 were achieved simultaneously with x = 0.4, which are both more than 100% higher than those of the pure BNT sample. This excellent energy-storage performance can be perfectly comparable with that of lead-based films. Furthermore, the BNT–0.4BNZ thick film showed strong fatigue endurance after 6 × 107 cycles, and it possessed good thermal and frequency stability. The pulsed discharge current waveform demonstrated that the BNT–0.4BNZ thick film showed a very fast discharge speed (210 ns). This study shows that BNT-based materials have an unexpected role as a lead-free family in the field of energy storage and could stimulate the design and fabrication of BNT-based dielectrics with ultrahigh energy-storage performance.