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Conventional upconversion thermometry strategy, based on the thermally coupled levels (TCL) of lanthanide ions, confronts a dilemma in simultaneously achieving high absolute/relative temperature (T) sensitivities (Sa and Sr) and good signal discriminability. Herein, a novel thermometry strategy by utilizing the synergistic effect of dual-wavelength lasers to manipulate electron thermal distribution is proposed to go beyond the limitation of the TCL-based strategy. In the case of NaGdF4: 20%Yb3+, 2%Er3+ nanoparticles upon 980 & 1530 nm dual-excitation, it is found that more efficient utilization of excited photons results in a higher electron concentration in ladder-like energy levels of Er3+, owing to the diverse pumping routes, which increases transition rate of the T-dependent phonon-assisted cross-relaxation process, and in turn establishes a thermally-sensitive electronic connection between Er3+: 2H11/2,4S3/2 and Er3+: 4F9/2. Remarkably, not only a highest record value of Sa (0.0365 K−1) for the Er3+ doped host materials in the physiological temperature range (303–343 K) is achieved, but also a high Sr (1.29% K-1) and an excellent signal discriminability (Δλ = 112 nm) are obtained. Combining a much intensified upconversion (UC) signal and a good size/shape homogeneity in nanometer scale, the investigated NaGdF4: 20%Yb3+, 2%Er3+ upon dual-wavelength excitation is potentially applicable in the intracellular thermal sensing and imaging. This work exploits an effective way to develop high-performance T-sensors, and the proposed thermometry strategy can be extended to surges of the other lanthanide ions doped systems pumped by multiple-wavelength lasers.

Compared to the conventional opaque persistent luminescence (PersL) materials, transparent PersL glass and glass ceramic are highly desired for night-vision illumination and indication applications. Herein, we report a new transparent oxyfluoride glass and glass ceramic with multi-color persistent luminescence lasting for ∼1 h visible by the naked eye. By adjusting the crystallization duration, controlled growth of 5–20 nm α-Zn2SiO4 nanocrystals in a glass matrix is achieved. Evidently, the coordination environment of Mn2+ evolves from octahedra in the glass host to tetrahedra in the α-Zn2SiO4 nanophase, resulting in the PersL color changing from red to yellow, and then to green, as the crystallization of Mn2+:α-Zn2SiO4 proceeds. The trap statuses in the glass and glass ceramic are studied in detail aided by EPR and TL measurements, demonstrating that the trapped holes with a trap depth distribution of 0.099–0.789 eV for the glass, and 0.094–0.631 eV for the glass ceramic, determine the PersL behaviors. It is noteworthy that this is the first report that oxyfluoride glass and glass ceramic serve as efficient PersL hosts.

The development of a single-ion activated single-phased white-light phosphor is of great importance in the field of near ultraviolet based w-LED lighting. In this work, a Ba2Y3(SiO4)3F:Eu fluoride apatite, which shows a broad emission band covering the entire visible region with tunable color rendition, was successfully synthesized via a high-temperature solid-state route. The microstructure and composition of the phosphor were carefully examined with the aid of XRD Rietveld refinement, HRTEM and SEM analyses, as well as XPS measurement. Spectroscopic studies revealed the site occupancy conditions of Eu2+, i.e., the Eu2+(I) band centering at 470 nm originates from the [BaO9] 4f site, while the Eu2+(II) one peaking at 600 nm comes from the [BaO6F] 6h site. Interestingly, the obtained white light can be readily tuned from cool to warm just by varying the Eu doping content. The brightest luminescence was achieved when the Eu concentration reached 1 mol%, beyond which the d–d interaction-based energy transfer between Eu2+ ions would result in concentration quenching. The underlying mechanism of the incomplete conversion from Eu3+ to Eu2+ under a reducing atmosphere was explored, which was believed to be caused by the rigid framework of the crystal structure. After coupling Ba2Y3(SiO4)3F:Eu with a commercial 3W 370 nm UV chip, the constructed w-LED yielded warm white light with a CIE coordinate of (0.402, 0.371), CCT of 3530 K, and CRI of 83.5, upon being driven by a 350 mA forward-biased current.

Owing to the magnetic dipole nature, the zero phonon line (ZPL) of Mn4+:2E → 4A2 transition is weak unless Mn4+ is situated at a site deviating from the centrosymmetric nature. Herein, we report a brand-new oxyfluoride, Na2WO2F4:Mn4+(NWOF:Mn4+), which shows unprecedented intense red ZPL at ∼620 nm along with relatively weak vibronic transitions under blue light excitation. This peculiar spectral feature is demonstrated to be originated from the highly distorted octahedral coordination environment in the C2v group symmetry surrounding Mn4+. High-resolution spectroscopic studies at 10 K disclose the fine structured electronic/vibronic transitions of Mn4+:2E, 2T1 → 4A2 and the weak electron–phonon interaction (Huang–Rhys factor S < 1) on the Mn4+ emissive state. Benefiting from the intense ZPL, an ultra-high color rendering index with Ra = 92.7 and R9 = 90.0 is achieved in the w-LED using YAG:Ce3+and NWOF:Mn4+ as color converters, and a wide color gamut of 107.1% NTSC in the w-LED using CsPbBr3 quantum dots and NWOF:Mn4+ is obtained. Herein, we first demonstrate that Mn4+-activated oxyfluorides have great potential in w-LED lighting and display applications. Our study can also enlighten researchers to design highly distorted octahedral sites for Mn4+ doping to achieve an ultra-intense ZPL.

Persistent luminescence (PersL) has long commanded the curiosity of researchers owing to the complicated and profound physics behind it. In this work, the PersL mechanism in a new kind of persistent garnet phosphors, Lu2CaMg2(Si1−xGex)3O12:Ce3+, is studied from the new perspective of a “solid-solution” scheme. Different from the conventional study in pursuit of long PersL, we focus on manipulation of afterglow to the millisecond range and tentatively demonstrate its potential to compensate the flickering of the alternating current driven LED (AC-LED) in every AC cycle. Evidently, the tailored host bandgap favors efficient electron charging and facilitates electron detrapping, as well as redeploying trap distribution, which results in a blue light activated afterglow in the millisecond time range, and subsequently a reduced percent flicker of 64.1% for the AC-LED. This investigation is the first attempt to establish the design guidelines for new PersL materials with an adjustable millisecond ranged afterglow, and, hopefully, it paves a pathway to the development of burgeoning low-flickering AC-LED technology.

The advent of wide color-gamut LED backlight technology greatly improves the user experience in visions. The bottleneck restricting progress of this technology is to develop red phosphor with satisfactory spectral features and low cost. Herein, a novel non-rare-earth K2XF7:Mn4+ (X = Ta, Nb) was successfully designed and synthesized, exhibiting an admirable quantum efficiency of ∼93.5%, an extremely narrow FWHM of ∼2.3 nm, and a high color purity of ∼99.6%. Unlike the previously reported Mn4+ activated fluorides, Mn4+ in K2XF7 suffers highly distorted octahedral environment in C1 (or C2v) group symmetry, enabling the appearance of intense zero phonon line and distinctive vibronic transitions. Systematical high-resolution spectroscopic analyses down to 10 K disclose the influences of strong crystal field, weak nephelauxetic effect, and intermediate electron-phonon coupling are responsible for the peculiar spectral features of Mn4+ in K2XF7. Impressively, a wide color-gamut reaching up to 86.7% NTSC is realized, demonstrating great validity of K2XF7:Mn4+ for LCD backlights. The present study not only brings a brand-new kind of Mn4+ activated host to the sight of phosphor community (the host can be extended to surges of tantalum/niobium fluorides), but also may enlighten researchers to design highly distorted environment to achieve unique spectral properties of Mn4+.