Eu3+-Doped Wide-Bandgap Zn2SnO4 Semiconductor Nanoparticles-Structure and Luminescence

Abstract

Zinc stannate (Zn2SnO4) is a transparent n-type semiconducting oxide with a diverse array of applications, such as in lead-free ferroelectrics, gas sensors, transparent conductors, lithium-ion batteries, dye-sensitized solar cells and photocatalysis. As a wide band gap material, it also has good potential for full color phosphors, however there are only few reports on the possibility of Zn2SnO4 hosting an activator ion for this kind of application. Among different possible dopants, the rare-earth elements are the most attractive because of their characteristic electronic transitions that could lead to sharp luminescence features from the ultraviolet (UV) to infrared (IR) range. Additionally, scalable methods of production are desired, as long as the material properties can be controlled. Thus the main objective of this work is to utilize a simple method of synthesis – mechanochemistry – in order to solve the comparatively complex and challenging task of nanocrystal doping. The mechanically initiated chemical reactions of oxides are typically accompanied by cation redistribution, formation of defect centers with unsaturated oxygen coordination, phase transformations and etc. This work investigates the structural and luminescence properties of mechanochemically synthesized Zn2SnO4 nanoparticles and the changes induced by europium doping. Nanocrystalline Zn2SnO4 owders doped with Eu3+ ions were synthesized via a mechanochemical solid-state reaction method followed by post annealing in air at 1200 oC. X-ray diffraction (XRD), energy-dispersive X-ray (EDX), Raman and photoluminescence (PL) spectroscopies provide convincing evidence for the incorporation of Eu3+ ions into the host matrix on non-centrosymmetric sites of the cubic inverse spinel lattice. Microstructural analysis shows that the crystalline grain size decreases with the addition of Eu3+ Formation of a nanocrystalline Eu2Sn2O7 secondary phase is also observed. Luminescence spectra of Eu3+ doped samples show several emissions, including narrow-band magnetic dipole emission at 595 nm and electric dipole emission at 615 nm of the Eu3+ ions. Excitation spectra and lifetime measurements suggest that Eu3+ ions are incorporated at only one symmetry site. According to the crystal field theory, it is assumed that Eu3+ ions participate at octahedral sites of Zn2+ or Sn4+ under a weak crystal field, rather than at the tetrahedral sites of Zn2+, due to the high octahedral stabilization energy for Eu3+ Activation of symmetry forbidden (IR-active and silent) modes is observed in the Raman scattering spectra of both pure and doped samples, indicating a disorder of the cation sublattice of Zn2SnO4 nanocrystallites. These results were further supported by the first principle lattice dynamics calculations. The spinel-type Zn2SnO4 shows effectiveness in hosting Eu3+ ions, which could be used as a prospective green/red emitter. This work also illustrates how sustainable and simple preparation methods could be used for effective engineering of material properties.