Cr-doped ZnGa₂O₄: Simple synthesis of intense red-NIR emitting nanoparticles with enhanced quantum efficiency

Red-NIR emitting ZnGa_2-xCr_xO₄ nanoparticles were synthesized using simple co-precipitation, and the effect of annealing was studied (600–1200 °C). Upon annealing, crystallite size increased (12 nm–73 nm), and color tone turned pink beyond 1000 °C, inferring dopant diffusion. Blue, green absorption bands in diffuse-reflectance spectra revealed Cr³⁺ in octahedral crystal field. Photoluminescence excitation spectra showed characteristic Cr³⁺ bands (∼400 nm and ∼550 nm), and the asymmetric 400 nm band suggested splitting the ⁴T_1g(F) state due to trigonal distortion. Emission spectra consisted of a zero-phonon line (∼688 nm), along with associated multi-phonon sidebands. ZnGa_1.98Cr_0.02O₄ showed intense red emission with λ_exc = 420 nm, while the tri-exponential fitting of the excited-state lifetime implied multiple decay processes due to Cr³⁺ occupying different environments. The internal quantum efficiency of the 1200 °C annealed sample was impressive (48%). Both UV and visible light up to 600 nm could induce persistent-luminescence (λ_{em =} 688 nm) for longer durations. The smaller size, intense emission, higher quantum yield, and visible light activation suggest that the nanoparticles are suitable for bioimaging applications.