The Gd3+ doping induced improvement of broadband NIR emission of NaScGe2O6:Cr3+ nanophosphors for contrast imaging lighting

Abstract

Herein, nano-sized NaScGe₂O₆:Cr³⁺ phosphors (NSGO:Cr³⁺, 16–35 nm) were successfully synthesized via the sol-gel method. Cr³⁺ ions preferentially substitute for the Sc³⁺ sites by occupying octahedral sites with a weak crystal field strength (Dq/B = 2.00). A broadband emission was observed within the first Near-infrared biological transparency window (NIR-I), located at 893 nm, which can be assigned to ⁴T₂→⁴A₂ transition of Cr³⁺ in NSGO:Cr³⁺. The full width at half maximum of the emission peak is approximately 150 nm. When doping concentration of Cr³⁺ reached 0.003, its NIR emission intensity became the strongest. The corresponding Huang-Rhys factor (0.44) is less than 1, indicating relatively weak phonon-electron coupling. Subsequently, the luminescence intensity of NSGO:Cr³⁺ was increased ∼20 % via incorporating Gd³⁺ into NSGO:Cr³⁺(NSGO:Cr³⁺,Gd³⁺) phosphors. Their phonon energy decreased from 0.131 eV to 0.125 eV. Meanwhile, the thermal stability of the luminescence in NSGO:Cr³⁺,Gd³⁺ was enhanced, resulting in an increase in thermal activation energy from 0.255 eV to 0.275 eV. The emission intensity of NSGO:Cr³⁺,Gd³⁺ at 150 °C retained 26.3 % of that at 20 °C. By combining blue LED chip with NSGO:Cr³⁺,Gd³⁺ phosphors, a NIR-I phosphor conversion light-emitting diode (NIR-I pc-LED) was fabricated and utilized as a backlighting source for contrast imaging. Both NIR contrast imaging photos of plant and animal tissues were successfully acquired using a NIR camera and the NIR-I pc-LED.