Near‐Unity and Zero‐Thermal‐Quenching Far‐Red‐Emitting Composite Ceramics via Pressureless Glass Crystallization

Abstract

Laser-driven (LD) lighting is emerging as the next-generation high-power solid-state lighting technology. All-inorganic color converters with high quantum efficiency (QE), small thermal quenching, high thermal conductivity, and high thermal and chemical stabilities are crucial to coping with the enormous heat generated in LD lighting. Although luminescent translucent ceramics are the most promising class of color converters, only green/ yellow-emitting ones with satisfactory performance are developed before. Here, a far-red-emitting composite ceramics Y 3 Al 5 O 12 (YAG)-Al 2 O 3 :Cr 3+ with near-unity internal QE and zero-thermal quenching are prepared via pressureless glass crystallization, where the inside light scattering is finely tuned by simply manipulating the temperature to induce controllable grain growth. The commonly used inert matrix Al 2 O 3 here becomes an optically active component as YAG, thus alleviating the undesired concentration quenching while maintaining strong light absorption. Therefore, a high-power LD far-red lighting source with luminescence saturation threshold up to 21.4 W mm −2 is demonstrated, which may find broad applications in plant growth lighting, solar simulators, and phototherapy.

Glass‐Crystallized Luminescence Translucent Ceramics toward High‐Performance Broadband NIR LEDs

“…Transparent glass precursors with the compositions of Y 3− x Ca x Al 5− x − y Cr y Si x O 12 ( x = 0.0–1.4, y = 0.00–0.08) were obtained using an aerodynamic levitation furnace equipped with a laser heating system (see Experimental Section and Figure S1 , Supporting Information), which enables containerless melting and rapid cooling at high temperature (up to 2000 °C) without contamination. [ 19 , 21 ] To transform YAG into a glass‐forming material, we introduced the glass network former SiO 2 to establish the required interconnectivity. [ 24 ] Then, CaO was added to ensure the charge balance within the resulting garnet crystals by cosubstitution of (Ca 2+ —Si 4+ ) for (Y 3+ —Al 3+ ), [ 23 , 25 ] avoiding unexpected defects as well.…”

“…Since the small octahedral Al 3+ site for Cr 3+ in YAG has strong crystal field on Cr 3+ , [ 21 , 23 ] upon violet (430 nm) excitation YAG:Cr 3+ emits narrowband far‐red light originating from the 2 E → 4 A 2 transition ( Figure 2 a and Figure S6 , Supporting Information). With the cosubstitution of (Y 3+ —Al 3+ ) by (Ca 2+ —Si 4+ ), the emission band of Cr 3+ undergoes a gradual redshift from far‐red to NIR range with FWHM increased from 39.5 to 160 nm (Figure S6 , Supporting Information), indicating a weakening crystal field imposed on Cr 3+ according to the Tanabe‐Sugano diagram (Figure S7 , Supporting Information).…”

“…One reason is that larger grain size at higher annealing temperature ensures better crystallinity and thus fewer surface defects which usually act as luminescence killers. [ 21c ] Besides, the reduction of Cr 4+ to Cr 3+ can be completed only at higher temperatures, as indicated by the disappearance of the characteristic absorption band (Figure 1a ) of Cr 4+ around 1000 nm. [ 9 , 21 ] The IQE of YCAS: y Cr 3+ ceramics annealed at 1250 °C remains to be 90.1% at the optimum Cr 3+ doping concentration ( y = 0.04) (Figure 2d ).…”

“…[ 21c ] Besides, the reduction of Cr 4+ to Cr 3+ can be completed only at higher temperatures, as indicated by the disappearance of the characteristic absorption band (Figure 1a ) of Cr 4+ around 1000 nm. [ 9 , 21 ] The IQE of YCAS: y Cr 3+ ceramics annealed at 1250 °C remains to be 90.1% at the optimum Cr 3+ doping concentration ( y = 0.04) (Figure 2d ). Remarkably, the AE of translucent YCAS:0.04Cr 3+ ceramic reaches up to 66.0%, leading to a record EQE of 59.5% for broadband NIR emission (see Table S6 , Supporting Information, for detailed comparison).…”

“…[ 17 , 18 ] Instead, we adopted a much simpler and pressureless method, that is, full crystallization of amorphous materials (glasses) into crystalline ones (ceramics), [ 19 ] which has succeeded in developing transparent ceramics without cubic symmetry via introducing structural disorder [ 20 ] and efficient RGB multi‐phase translucent composites. [ 21 ] Considering the large flexibility of garnet crystals in composition, [ 22 ] we selected the well‐known Y 3 Al 5 O 12 (YAG) garnet as the starting material. Then we designed a series of garnet solid solutions via dual‐function cation cosubstitution of Ca 2+ —Si 4+ for Y 3+ —Al 3+ , by which narrowband far‐red emission of Cr 3+ was largely regulated to broadband NIR one due to the weakened crystal field of the octahedrally coordinated Cr 3+ , [ 23 ] and more importantly, YAG:Cr 3+ can be engineered into a glass‐forming material.…”