Effect of the Elaboration Method on Structural and Optical Properties of Zn1.33Ga1.335Sn0.33O4:0.5%Cr3+ Persistent Luminescent Nanomaterials.

Near-infrared (NIR) persistent luminescence (PersL) materials have demonstrated promising developments for applications in many advanced fields due to their unique optical properties. Both high-temperature solid-state (SS) or hydrothermal (HT) methods can successfully be used to prepare PersL materials. In this work, Zn1.33Ga1.34Sn0.33O4:0.5%Cr3+ (ZGSO:0.5%Cr3+), a newly proposed nanomaterial for bioimaging, was prepared using SS and HT methods. The results show the crystal structure, morphology and optical properties of the samples that were prepared using both methods. Briefly, the crystallite size of the ZGSO:0.5%Cr3+ prepared using the SS method is ~3 µm, and as expected, is larger than materials prepared using the HT method. However, the growth process used in the hydrothermal environment promotes the formation of ZGSO:0.5%Cr3+ with more uniform shapes and smaller sizes (less than 500 nm). Different diameter ranges of nanoparticles were obtained using HT and ball milling (BM) methods (ranging from 25-50 nm) and by using SS and BM methods (25-200 nm) as well. In addition, the SS-prepared microstructure material has stronger PersL than HT-prepared particles before they go through ball milling to create nanomaterials. On the contrary, after BM treatment, ZGSO:0.5%Cr3+ HT and BM NPs present higher PersL and photoluminescence (PL) properties than ZGSO:0.5%Cr3+ SS and BM NPs, even though both kinds of NPs present worse PersL and PL compared to the original particles before BM. To summarize: preparation methods, whether by SS or HT, with additional grinding as a second step, can have a significant impact on the morphological and luminescent features of ZGSO:0.5%Cr3+ PersL materials.

Tm:CALGO lasers at 2.32†µm: cascade lasing and upconversion pumping.

We report on the first laser operation of a disordered Tm:CaGdAlO4 crystal on the 3H4 → 3H5 transition. Under direct pumping at 0.79 µm, it generates 264 mW at 2.32 µm with a slope efficiency of 13.9% and 22.5% vs. incident and absorbed pump power, respectively, and a linear polarization (σ). Two strategies to overcome the bottleneck effect of the metastable 3F4 Tm3+ state leading to the ground-state bleaching are exploited: cascade lasing on the 3H4 → 3H5 and 3F4 → 3H6 transitions and dual-wavelength pumping at 0.79 and 1.05 µm combining the direct and upconversion pumping schemes. The cascade Tm-laser generates a maximum output power of 585 mW at 1.77 µm (3F4 → 3H6) and 2.32 µm (3H4 → 3H5) with a higher slope efficiency of 28.3% and a lower laser threshold of 1.43 W, out of which 332 mW are achieved at 2.32 µm. Under dual-wavelength pumping, further power scaling to 357 mW at at 2.32 µm is observed at the expense of increased laser threshold. To support the upconversion pumping experiment, excited-state absorption spectra of Tm3+ ions for the 3F4 → 3F2,3 and 3F4 → 3H4 transitions are measured for polarized light. Tm3+ ions in CaGdAlO4 exhibit broadband emission at 2.3 - 2.5 µm making this crystal promising for ultrashort pulse generation.

Opportunities for Persistent Luminescent Nanoparticles in Luminescence Imaging of Biological Systems and Photodynamic Therapy.

The use of luminescence in biological systems allows us to diagnose diseases and understand cellular processes. Persistent luminescent materials have emerged as an attractive system for application in luminescence imaging of biological systems; the afterglow emission grants background-free luminescence imaging, there is no need for continuous excitation to avoid tissue and cell damage due to the continuous light exposure, and they also circumvent the depth penetration issue caused by excitation in the UV-Vis. This review aims to provide a background in luminescence imaging of biological systems, persistent luminescence, and synthetic methods for obtaining persistent luminescent materials, and discuss selected examples of recent literature on the applications of persistent luminescent materials in luminescence imaging of biological systems and photodynamic therapy. Finally, the challenges and future directions, pointing to the development of compounds capable of executing multiple functions and light in regions where tissues and cells have low absorption, will be discussed.

Abnormal co-doping effect on the red persistent luminescence SrS:Eu2+,RE3+ materials.

Persistent luminescence materials are a reality in several applications. However, there is still a lack of efficient red-emitting materials. SrS:Eu2+ phosphor is a potential candidate since its strong nephelauxetic effect shifts Eu2+4f65d1 → 4f7 to red, and its weak bond between strontium and sulphide, due to the soft base-hard acid character, generates a high number of intrinsic defects. SrS:Eu2+,RE3+ materials were efficiently prepared by two rounds of 22 min microwave-assisted solid-state synthesis. The highly crystalline purity and the material organization at the micro-scale were observed with X-ray powder diffraction and scanning electron microscopy, respectively. X-ray absorption spectroscopy revealed a low amount of Eu2+ compared to Eu3+ due to the efficient Eu2+ photo-oxidation by X-ray irradiation in the high storage capability SrS host matrix. Electron paramagnetic resonance spectra confirmed that at least 50% of Eu2+ ions in the material are photo-oxidized during excitation, reinforcing the previously established mechanisms. The RE2+ energy level positioned very close to or into the conduction band led to an abnormal co-doping effect, with similar effects independent of the co-dopant. The high concentration of intrinsic defects in SrS indicates that the soft-hard pair host is an excellent approach to develop efficient persistent luminescence materials.