Strong doped ions segregation in strain gradient of Y₂SiO₅:Pr³⁺ and YVO₄:Er³⁺,Yb³⁺ nanocrystals

The development of nanophosphors as promising materials for various modern light applications requires precise and controlled fabrication of nanostructures in a volume and on a surface. The optical and luminescence properties of nanophosphors as well as bulk phosphors are known to determine by the small portion of doped rare-earth (RE) or transition metal ions. The surface produced strain gradient penetrating of most part of nanocrystal could stimulate a total uphill diffusion of doped ions which are surrounded by own stress field.
So, the sufficient fraction of doped ions could be expected to segregate and elastically stretched region (nearby surface) of nanocrystal will be enriched with ions having radius greater in comparison with regular one and vice versa.

Our results:
We have observed abnormal low ¹D₂-³H₄ luminescence concentration quenching threshold for Y₂SiO₅:Pr³⁺ nanocrystals. The analysis of luminescence excitation spectra of Y₂SiO₅:Pr³⁺ nanocrystals with different Pr³⁺ concentrations have revealed the formation of Pr³⁺ pairs at low Pr concentration (fig.1). These effects have been explained by the effect of doped ion segregation within nanocrystal surface determined by size difference for impurity and regular ion.

fig.1. ³H₄-¹D₂ excitation spectra of Y₂SiO₅:Pr³⁺ nanocrystals at different Pr concentrations and ³H₄-¹D₂ absorption spectrum of Y₂SiO₅:Pr³⁺ bulk crystal with Pr concentration of 2 at.%. T = 77 K. The line assigned to the pair state is marked with the arrow.

To show generality and importance of the surface segregation phenomena in doped nanocrystals, well-known YVO₄:Er³⁺,Yb³⁺ crystal, which demonstrates an efficient up-conversion has been selected. Temperature treatment of YVO₄:Er³⁺,Yb³⁺ nanocrystals at 650 C leads to the significant increase of the “red-to-green ratio”, as if we deal with Er³⁺ concentration increasing (fig.2).

fig. 2. Up-conversion luminescence spectra of YVO₄:Er³⁺ (0.5 at.%), Yb³⁺ (10 at.%)
nanocrystals before and after annealing

Main publications:

• A.A.Masalov, O.G. Vyagin, I.I. Ganina, Yu.V. Malyukin. Strong quenching of praseodymium f – f luminescence induced by a surface of Y₂SiO₅:Pr³⁺ nanocrystal// Journal of Luminescence – 2009. – v.129., № 12. – P.1695-1697. abstract

• Yu.V. Malyukin, P.N. Zhmurin, N.V. Zhnamenski, T.G. Yurkina, Strong quenching of Y₂SiO₅:Pr³⁺ nanocrystal luminescence by praseodymium nonuniform distribution // Phys. Stat. Sol.(b) – 2007. – v.244., № 9. – P.3325-3332. abstract

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