Abstract As novel stress‐sensing materials, the reported mechanoluminescence (ML) phosphors work only at or above room temperature. Herein, the ML response to low temperatures (77 K) is extended by employing ultra‐shallow traps. Strong ML stimulated by handwriting force followed by persistent luminescence is observed in BaSi 2 O 2 N 2 :Eu 2+ (BSON) at 77 K. The UV pre‐irradiated BSON can still keep the characteristics of ML with 45% intensity after 300 min. Abundant ultra‐shallow traps with depth of ≈0.19 eV are found and revealed to be responsible for the low‐temperature ML and persistent luminescence. Manipulation of the ultra‐shallow traps is realized by doping Ge, Er, and Ce ions in BSON, leading to significant ML enhancement at 77 K. Together with ML, the ultra‐shallow traps also exhibit force memory ability to replicate the pre‐applied force pattern simply by afterglow. The finding advances the state‐of‐the‐art in force sensing under low temperature conditions.
As is well-known, the aliovalent substitution level is usually very limited due to the charge mismatch. Particularly, the single phase can hardly be obtained by solid-state reaction for the famous silicate garnet Ca3Sc2Si3O12 (CSS), even when the doping level of trivalent rare earth ion (RE3+) for Ca2+ in CSS is lower than 2 mol %, which largely restricts CSS to be an ideal host for RE3+-activated luminescence materials especially where high doping concentration is required. Herein, by using the strategy of multiple chemical unit cosubstitution, we obtained RE3+ heavily doped single-phase CSS via the sol–gel method followed by high-temperature sintering. Multiple chemical unit substitutions of [REO8], [AlO6], and [AlO4], respectively, for [CaO8], [ScO6], and [SiO4] polyhedra can act as charge compensators for each other to promote the doping level of RE3+ up to 20 mol %, which is high enough for most of the RE3+-doped luminescence materials. Moreover, intense cooperative upconversion (UC) luminescence (UCL) was observed in Yb3+ and Tb3+ codoped CSS, whose intensity is 37 times higher than that of the reported Y3Al5O12 with garnet structure as well, making it a potential candidate for optical applications like a tunable UC laser. The results show that the preferred formation of the Yb3+–Yb3+ pair in CSS can largely enhance the efficiency of the cooperative UC process. Besides, the UCL properties were investigated in detail to understand the UC processes and the underlying energy transfer mechanisms. It is confirmed that the multiple chemical unit cosubstitution is an effective strategy to promote the aliovalent substitution level or design solid solution materials to enhance or tune the luminescence properties where relatively high doping concentration is required.
Abstract Broadband near‐infrared (NIR) phosphor‐converted light emitting diode (pc‐LED) is demanded for wearable biosensing devices, but it suffers from low efficiency and low radiance. This study reports a broadband NIR Ca 3‐ x Lu x Hf 2 Al 2+ x Si 1− x O 12 :Cr 3+ garnet phosphor with emission intensity enhanced by 81.5 times. Chemical unit co‐substitution of [Lu 3+ −Al 3+ ] for [Ca 2+ −Si 4+ ] is responsible for the luminescence enhancement and further alters the crystal structure and electronic properties of the garnet. Using the optimized phosphor, a NIR pc‐LED with photoelectric efficiencies of 21.28%@10 mA, 15.75%@100 mA and NIR output powers of 46.09 mW@100 mA, 54.29 mW@130 mA is fabricated. The high power NIR light is observed to penetrate upper arms (≈8 cm). For application in NIR spectroscopy, the NIR pc‐LED is used as light source to measure transmission spectra of water, alcohol, and bovine hemoglobin solution. These results indicate the NIR garnet phosphor to be a promising candidate for NIR pc‐LED.
In this paper, the upconversion luminescent properties of Gd2O3:Er3+,Yb3+ nanowires as a function of Yb concentration and excitation power were studied under 978-nm excitation. The results indicated that the relative intensity of the red emission (F9∕24-I15∕24) increased with increasing the Yb3+ concentration, while that of the green emission (S3∕24∕H11∕22-I15∕24) decreased. As a function of excitation power in ln-ln plot, the green emission of S3∕24-I15∕24 yielded a slope of ∼2, while the red emission of F9∕24-I15∕24 yielded a slope of ∼1. Moreover, the slope decreased with increasing the Yb3+ concentration. This was well explained by the expanded theory of competition between linear decay and upconversion processes for the depletion of the intermediate excited states. As the excitation power density was high enough, the emission intensity of upconversion decreased due to thermal quenching. The thermal effect caused by the exposure of the 978-nm laser was studied according to the intensity ratio of H11∕22-I15∕24 to S3∕24-I15∕24. The practical sample temperature at the exposed spot as a function of excitation power and Yb3+ concentration was deduced. The result indicated that at the irradiated spot (0.5×0.5mm2) the practical temperature considerably increased.
Abstract K 2 Al 2 B 2 O 7 :Eu 2+ phosphor is prepared by reaction of a stoichiometric HNO 3 solution of Eu 2 O 3 , K 2 CO 3 , Al(NO 3 ) 3 , and H 3 BO 3 followed by drying and sintering in a reducing CO atmosphere (C sticks covered crucible, 1.
PbWO4: Er3+,Yb3+ nanocrystals (∼100 nm) were prepared by the hydrothermal method at different pH values (pH = 4, 7, and 9). Their structure and luminescence properties under 978-nm laser-diode excitation were studied. The results indicate that the practical ratio of W to Pb in the nanocrystals and the doping concentration of Yb3+ depended strongly on the pH value due to structure change. In upconversion, red (4F9/2→4I15/2) and green (2H11/2,4S3/2→4I15/2) emissions were observed, both of which occurred via a two-photon populating process. Biexponential upconversion dynamics were observed, which was attributed to luminescence centers surrounded by different local environments. The intensity ratio of 2H11/2→4I15/2 to4S3/2→4I15/2 (RHS) was explored to reveal the local thermal effect under the exposure of the laser diode, showing that the temperature at the exposed spot increased linearly with respect to excitation power density and Yb3+ concentration.
One-dimensional nanosized phosphors demonstrate special structural and photoluminescence properties, which have application potential in some optical fields. In this article, we present our recent progress on preparation and luminescence properties of some one-dimensional rare earth compounds and complexes, the core–shell oxide nanowires prepared by a two-step hydrothermal route, the nanowires of some inorganic compounds doped with rare earths and rare earth complexes/PVP composites prepared by the electrospinning method, and the rare earth complexes in the SBA-15 mesoporous molecule sieves. In these systems, some novel or improved photoluminescence properties are observed such as improved luminescence quantum yield, thermal stability and/or photostability, and depressed thermal effect in upconversion luminescence.
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