Precursor stoichiometry controls dopant structure, defect chemistry, and valence in Mn-doped ZnGa${}_2$O${}_4$ nanocrystals

Abstract

The luminescence of doped nanocrystals is usually optimised through the activator; here we show that the host cation ratio is an equally decisive, and largely overlooked, design variable. Mn-doped ZnGa${}_2$O${}_4$ nanocrystals (Mn:Zn = $4\times 10^{-4}$) were grown hydrothermally at nominal Zn/Ga ratios between 0.30 and 0.75 (stoichiometric: 0.50) and post-annealed at 400--1100${}^{\circ }$C, and the resulting composition--temperature matrix was mapped by X-ray- and UV-excited luminescence, EPR, Rietveld-refined X-ray diffraction, Raman spectroscopy and SEM. The as-grown crystals are phase-pure spinel at every composition and accommodate their off-stoichiometry internally as a point-defect inventory, reflected in a 0.4\% variation of the lattice parameter; on annealing, the excess gallium is exsolved as $\beta$-Ga${}_2$O${}_3$ in amounts set by the solution composition. The Mn${}^{2+}$ emission separates into two optima, near-stoichiometric crystals annealed at 400--600${}^{\circ }$C and Zn-excess crystals annealed at 1100${}^{\circ }$C, the latter being twice as bright under X-ray excitation. Under X-ray and 365~nm excitation the two optima are separated by a minimum at 800${}^{\circ }$C that coincides with a collapse of the Mn${}^{2+}$ EPR population and has no counterpart in the Raman linewidths, identifying transient, defect-compensated oxidation of the dopant rather than lattice degradation as its origin. The most Zn-rich composition shows neither Mn${}^{2+}$ nor Mn${}^{4+}$ emission at any annealing stage, whereas its trace Cr${}^{3+}$ emission is strong, indicating that Mn is rejected already during growth. Solution stoichiometry therefore acts on two levels, dopant capture and native-defect inventory, while annealing controls dopant valence and lattice order. The brightest high-temperature phosphor is thus not the nominally stoichiometric one, but the composition whose excess is consumed on its way to stoichiometry.

arXiv comment30 pages: 9 pages main text with 5 figures, 21 pages Supporting Information with 18 figures and 1 table