Sridhar, G and Hebbar, Deepak N and Lewis, Prinston and Choudhari, KS and Kulkarni, Suresh D (2022) Cr-doped ZnGa2O4: Simple synthesis of intense red-NIR emitting nanoparticles with enhanced quantum efficiency. Optical Materials, 123. pp. 1-9. ISSN 0925-3467

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Abstract

Red-NIR emitting ZnGa2-xCrxO4 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 4 T1g(F) state due to trigonal distortion. Emission spectra consisted of a zero-phonon line (~688 nm), along with associated multiphonon sidebands. ZnGa1.98Cr0.02O4 showed intense red emission with λexc = 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 (λem = 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.

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