Journal of Luminescence 226 (2020) 117482 Available online 28 June 2020 0022-2313/© 2020 Elsevier B.V. All rights reserved. A new microwave approach for the synthesis of green emitting Mn 2þ -doped ZnAl 2 O 4 : A detailed study on its structural and optical properties Samvit G. Menon a, * , Arup K. Kunti a , Suresh D. Kulkarni b , Raju Kumar c , Mayank Jain c , Dirk Poelman d , Jonas J. Joos d , Hendrik C. Swart a, ** a Department of Physics, University of the Free State, Bloemfontein, 9301, South Africa b Department of Atomic and Molecular Physics, Manipal Academy of Higher Education, Manipal, 576104, India c Department of Physics, Technical University of Denmark, Risø Campus, Roskilde, 4000, Denmark d LumiLab, Department of Solid State Sciences, Ghent University, Ghent, 9000, Belgium ABSTRACT A simple recipe for synthesizing green emitting Mn 2þ -doped ZnAl 2 O 4 phosphor has been developed. Metal-organic complexes, with their unique properties, were employed as precursors to obtain phase-pure, nanocrystalline material in the as-prepared form within just 5 min of microwave irradiation. The Mn 2þ doping concentration that showed the highest photoluminescence (PL) intensity was optimized and a comprehensive investigation of the structural and optical properties were made for various annealing temperatures. Rietveld refnement of the samples annealed at 1200 � C and 1400 � C, showed that the cationic inversion in the spinel decreased from 3.4 to 2.1% and this change was validated by the X-ray photoelectron spectroscopy results. XPS confrmed that the inversion for Zn 2þ , Al 3þ , and Mn 2þ cations decreased with annealing temperature, despite of which, inversion remained at 20%, 10%, and 15%, respectively for the sample annealed at 1400 � C, emphasizing the fact that synthesis plays an important role in controlling the amount of inversion in an otherwise normal spinel. Electron paramagnetic resonance spectra of the as-prepared and the samples annealed at high temperatures confrmed that the Mn 2þ hyperfne spectrum was not just a function of the crystal feld environment but also strongly depends on the doping concentration. The PL spectrum taken at different annealing temperatures, comprised of the characteristic 4 T 1 (G) → 6 A 1 (S) spin-forbidden Mn 2þ transitions, showed that the emission intensity depends on the material crystallinity. The sample annealed at 1400 � C displayed a signifcantly higher PL intensity compared to those annealed at lower temperatures. The variation of PL spectrum of this sample was investigated between 9 K and 300 K to determine the origins of the asymmetry at room temperature and the vibrational sidebands at lower temperatures. The energy levels of the Mn 2þ dopant, calculated theoretically and verifed experimentally, were used to determine the spectroscopic parameters such as the Racah B and C values and the crystal feld energy, Dq. These values showed that the Mn 2þ was in a weak tetrahedral feld. This work demonstrates a technologically important, green, and swift technique in synthesizing phosphors for various applications in displays, bioimaging, solid state lighting, etc. 1. Introduction Developing a simple and swift technique for the synthesis of doped and undoped wide bandgap semiconductors has always been chal- lenging. Additionally, in the case of Mn 2þ -doped metal oxides, pre- venting the oxidation of Mn 2þ into higher oxidation states is also challenging, particularly when used in applications such as solid-state lighting, sensing, communications, and bioimaging. Conventional methods used to synthesize Mn 2þ -doped oxides such as co-precipitation, hydrothermal, solid state, or combustion methods usually employ metal salts as precursors and consequently hold the risk of converting Mn 2þ into its higher oxidation states, unless reactions are controlled, and annealing is done under inert conditions. The presence of Mn in multiple oxidation states can affect the decay lifetime and quantum yield which can hamper the performance of the intended application or device. Be- sides, many of these methods require high temperatures, longer reaction durations, and adjusting the pH of the reaction mixture which involves high energy consumption or the addition of corrosive acids/alkalis that are environmentally and technologically undesirable. Thus, the chal- lenge of developing a swifter, low temperature, pH independent recipe for the synthesis of Mn 2þ -doped oxide phosphors with the ability to retain its þ2-oxidation state, despite annealing in air, has been accepted and addressed in this manuscript. Because of their low dissociation temperatures, metal organic com- plexes have been widely employed as precursors for fabricating metal oxide thin flms through various chemical vapour deposition techniques [1–3]. Even though the synthesis of metal oxide nanomaterials through microwave assisted solvothermal methods was pioneered by Markus * Corresponding author. ** Corresponding author. E-mail addresses: samvit.menon@gmail.com (S.G. Menon), swarthc@ufs.ac.za (H.C. Swart). Contents lists available at ScienceDirect Journal of Luminescence journal homepage: http://www.elsevier.com/locate/jlumin https://doi.org/10.1016/j.jlumin.2020.117482 Received 5 May 2020; Received in revised form 28 May 2020; Accepted 16 June 2020