Effect of Li⁺ doping on the quenching concentration of ²H_11/2/⁴S_3/2 level of Er³⁺

Abstract

Li + and Er 3+ codoped Y 2 O 3 powders are synthesized via sol-gel method. The effects of Li + doping on the optical properties of Er 3+ :Y 2 O 3 are investigated under 980 nm excitation. The emission intensity of Er 3+ is obviously improved after Li + doping. Moreover, the quenching concentration of 2 H 11/2 / 4 S 3/2 level of Er 3+ is 3.0 mol% for the samples without Li + but it decreases to 2.0 mol% after introducing 5.0 mol% Li + . This is because Li + doping shrinks the host lattice and decreases the Er 3+ -Er 3+ distance, which contribute to the formation of Er 3+ clusters and thus induces the quenching concentration of 2 H 11/2 / 4 S 3/2 level to decrease. Int. J. Mod. Phys. B 2014.28. Downloaded from www.worldscientific.com by Mr. du zhong on 02/03/15. For personal use only.

Down-/Up-Conversion Emission Enhancement by Li Addition: Improved Crystallization or Local Structure Distortion?

“…To assess the effect of Li addition on the local structure of Ln–Y 2 O 3 , we select Eu (1%), Sm(0.1%), Tb(1%), and Dy (1%) as the luminescence probes. ,, Selection of Li concentration at 5% is close to the value reported as optimal, above which a decline in the emission intensity of Ln activators is typically observed. ,,− ,,,,, Depending on the synthesis procedure, the type of lanthanide (co)­dopants, and the way the emission was induced (down- or up-conversion excitation), smaller (2, 3%) ,,,,, or greater (10%) optimal Li concentrations were also determined. To this point, we note that in most studies, the actual Li concentration is not confirmed by quantitative analysis (i.e., inductively coupled plasma mass spectrometry), ,, and thus the value may be lower than the nominal one.…”

“…The absorption and emission transition probabilities of Ln are governed by site-specific selection rules: for C 2 sites, both magnetic (MD) and electric dipole (ED) transitions are allowed, while for S 6 sites only the magnetic dipole transitions are allowed, leading generally to emissions with order(s) of magnitude less intense than those of C 2 sites . According to the literature, upon Li codoping of Ln–Y 2 O 3 , the small ionic radius of Li (0.76 Å compared to 0.9 Å for Y, in 6-fold coordination) enables facile insertion into Y 2 O 3 lattice, either substitutionally ,,,,, or both substitutionally and interstitially ,,,,,,,− generating strain and charge imbalance. Several scenarios have been advanced to explain the emission enhancement by Li addition: (i) the oxygen vacancies resulted from the charge-compensation can remove the inversion symmetry of S 6 site, and thus the forbidden electric dipole transitions become allowed, leading to an increased number of optically active lattice sites; ,,− , (ii) Li preferentially substitutes for C 2 site, further reducing the local symmetry; (iii) Li addition can reduce the local symmetry at both sites; − ,− ,,,− and (iv) the Ln–Ln interactions get weaker via breaking the Ln–Ln clusters in addition to reduced local symmetry. ,,, …”

“…Due to its facile synthesis in the nanometer regime and favorable physical properties such as a high melting point, phase stability, low phonon energy, and low thermal expansion, Y 2 O 3 represents an excellent host material for the luminescent lanthanide ions. − Li addition to Ln–Y 2 O 3 is considered as an effective optimization strategy that reportedly boosts the emission intensity from a few times up to 2 orders of magnitude, depending on the type of Ln (co)­dopants and concentrations, synthesis methods, Li concentration, and the mode emission excitation (down- or up-conversion). − Despite that almost 100 publications have been published in the last two decades on Ln,Li–Y 2 O 3 , there is still no consensus being reached on the causes of Li-induced emission enhancement. Several causes for emission enhancement are being considered, such as especially the tailoring of crystal-field via lowering of the local symmetry ,,,,,,,, or/and improved crystallization, − ,− ,,, but also changes of morphology, ,, reduction of surface OH defects, − and sensitization via oxygen vacancies induced by charge compensation. ,, The nature of Li precursors as well as the preparation method were found to also play a key role on the final emission properties. , Larger Ln-Y 2 O 3 crystallites are usually obtained upon addition of Li as evidenced by X-ray diffraction (XRD) patterns − ,− …”

“…Intuitively, crystallite size enlargement is expected to increase the contribution of core Ln at the expense of surface Ln that is exposed to efficient nonradiative emission quenching due to defects and impurities, thus leading to more intense emissions. On the other side, reduction of symmetry around a Ln activator is considered to arise from the size mismatch between ionic radii of Li and Y (0.76 to 0.9 Å, 6-fold coordination) and oxygen vacancies following aliovalent insertion of Li, which is considered to occur either substitutionally ,,,,, or interstitially or both. ,,,,,,,− Since lowering of the local symmetry around the Ln activator leads to the enhancement of f–f radiative transitions, local symmetry distortion by Li is a mechanism at hand that explains the emission enhancement in Y 2 O 3 but also in a wide range of Ln-doped oxide and fluoride host materials. ,, Besides the alteration of the emission shapes, distortion/lowering of the local symmetry would necessarily result in shortening of the emission decays. Yet, few reports have measured the effect of Li addition on the excited-state lifetimes of Ln in Y 2 O 3 , ,,, and these basically evidenced lengthening of the emission decays rather than their acceleration.…”