Wide-coverage and Efficient NIR Emission from Single-component Nanophosphors through Shaping Multiple Metal-halide Packages

Wide-coverage near infrared (NIR) phosphor-converted LEDs possess promising potential for practical applications, but little is developed towards the efficient and wide-coverage NIR phosphors. Here, we report the single-component lanthanide (Ln³⁺ ) ions doped Cs₂ M(In_0.95 Sb_0.05 )Cl₆ (M=alkali metal) nanocrystals (NCs), exhibiting emission from 850 to 1650 nm with high photoluminescence quantum yield of 20.3 %, which is accomplished by shaping the multiple metal halide octahedra of double perovskite via the simple alkali metal substitution. From Judd-Ofelt theoretical calculation and spectroscopic investigations, the shaping of metal halide octahedra in Cs₂ M(In_1-x Sb_x )Cl₆ NCs can break the forbidden of f-f transition of Ln³⁺ , thus increasing their radiative transition rates and simultaneously boosting the energy transfer efficiency from host to Ln³⁺ . Finally, the wide-coverage NIR LEDs based on Sm³⁺ , Nd³⁺ , Er³⁺ -tridoped Cs₂ K_0.5 Rb_0.5 (In_0.95 Sb_0.05 )Cl₆ NCs are fabricated and employed in the multiplex gas sensing and night-vision application.

A solid-solution modulation strategy in trivalent bismuth-doped gallate phosphors for single substrate tunable emission

The synthesis conditions of most phosphors doped with lanthanide ions with d-f transition require a reducing atmosphere. The doping Bi³⁺ ions selected in this study perfectly avoid this requirement, and they are environmentally friendly and safe. Nevertheless, the spectral tuning of Bi³⁺ is a great challenge that limits its application. Herein, by regulating the value of x in the new solid solution Sr_2+xLa_1-xGaO_5-xF_x, the luminescence of Bi³⁺ is well promoted. Through an excitation-driven strategy, the emission peak position of Bi³⁺ is redshifted, and the luminescence of trivalent bismuth is successfully adjusted, which can also be applied to anti-blue lighting. In addition, we constructed a Bi³⁺-Eu³⁺ dual luminescence system, and, regardless of changes in the Bi³⁺/Eu³⁺ concentration or excitation wavelength, a single matrix white light phosphor was realized. Through calculations, the activation energy of the phosphor doped with 2.5%Eu³⁺ was found to be 0.257 eV, which is higher than the activation energy of some common compounds. This indicates that the phosphor has good application prospects in the field of solid-state lighting. It is worth noting that based on the different thermal response behaviors of Bi³⁺ and Eu³⁺, when the Eu³⁺ content is fixed at 1%, the maximum relative sensitivity of the optical thermometer based on its fluorescence intensity ratio is about 1.46% K^-1 at 383 K, which is higher than that of Bi³⁺ and Eu³⁺ co-doped phosphors previously reported. We also obtained a high absolute sensitivity of 0.00139 K^-1 at 403 K. Therefore, we also studied the thermal sensitivity of Bi³⁺ and Er³⁺ co-doped solid solutions. The results show that this solid-solution phosphor has far-reaching application prospects in the temperature sensing field.

Efficient energy transfer from Bi3+ to Mn2+ in CaZnOS for WLED application

A series of Bi³⁺ and Mn²⁺ co-doped CaZnOS phosphors with a tunable emission color have been synthesized by a high temperature solid-state reaction method. Their crystal structure, spectroscopic properties, energy transfer and thermal quenching have been investigated systematically. An intense blue-green emission band at 485 nm and a red emission band at 616 nm were observed at an excitation wavelength of 375 nm, owing to the ³P_1,0→¹S₀ transition of Bi³⁺ and the ⁴T₁(⁴G) →⁶A₁(⁶S) transition of Mn²⁺, respectively. The tunable color from blue-green, white light to red light can be obtained by varying the Mn²⁺ ion concentration from 0.005 to 0.015 in CaZnOS:Bi³⁺. The decay time decreased from 642 to 273 ns with the Mn²⁺ ion concentration x increasing from 0.005 to 0.015, and the energy transfer efficiency η_T can reach up to 65% in the CaZnOS:Bi³⁺,0.015Mn²⁺ phosphor. As the temperature increases from 300 to 420 K, the emission intensity is maintained at 67%, and the activation energy E_a is estimated to be 0.28 eV. An LED fabricated using CaZnOS:Bi³⁺,0.01Mn²⁺ exhibited the chromaticity coordinates and corrected color temperature (CCT) of (0.338, 0.364) and 4655 K, respectively. These results validate the promising applications of the CaZnOS:Bi³⁺,Mn²⁺ phosphor in UV white LEDs.

Crystallization and visible-near-infrared luminescence of Bi-doped gehlenite glass

Gehlenite glass microspheres, doped with a different concentration of Bi³⁺ ions (0.5, 1, 3 mol%), were prepared by a combination of solid-state reaction followed by flame synthesis. The prepared glass microspheres were characterized from the point of view of surface morphology, phase composition, thermal and photoluminescence (PL) properties by optical and scanning electron microscopy (SEM), X-ray diffraction (XRD), differential scanning calorimetry (DSC) and PL spectroscopy. The closer inspection of glass microsphere surface by SEM confirmed a smooth surface. This was further verified by XRD. The basic thermal characteristics of prepared glasses, i.e. T _g (glass transition temperature), T _x (onset of crystallization peak temperature), T _f (temperature of the inflection point of the crystallization peak) and T _p (maximum of crystallization peak temperature), were estimated from the DSC records. High-temperature XRD experiments in the temperature interval range 600-1100°C were also performed. The PL emission properties of prepared glasses and their polycrystalline analogues (glass crystallized at 1000°C for 10 h) were studied in the visible and near-infrared (NIR) spectral range. When excited at 300 nm, the glasses, as well as their polycrystalline analogues, exhibit broad emission in the visible spectral range from 350 to 650 nm centred at about 410-450 nm, corresponding to Bi³⁺ luminescence centres. The emission intensity of polycrystalline samples was found to be at least 30 times higher than the emission of their glass analogues. In addition, a weak emission band was observed around 775 nm under 300 nm excitation. This band was attributed to the presence of a minor amount of Bi²⁺ species in prepared samples. In the NIR spectral range, the broad band emission was observed in the spectral range of 1200-1600 nm with the maxima at 1350 nm. The chemistry of Bi and its oxidation state equilibrium in glasses and polycrystalline matrices is discussed in detail.