Light Emission Intensities of Luminescent Y₂O₃:Eu and Gd₂O₃:Eu Particles of Various Sizes

Abstract

There is great technological interest in elucidating the effect of particle size on the luminescence efficiency of doped rare earth oxides. This study demonstrates unambiguously that there is a size effect and that it is not dependent on the calcination temperature. The Y₂O₃:Eu and Gd₂O₃:Eu particles used in this study were synthesized using wet chemistry to produce particles ranging in size between 7 nm and 326 nm and a commercially available phosphor. These particles were characterized using three excitation methods: UV light at 250 nm wavelength, electron beam at 10 kV, and X-rays generated at 100 kV. Regardless of the excitation source, it was found that with increasing particle diameter there is an increase in emitted light. Furthermore, dense particles emit more light than porous particles. These results can be explained by considering the larger surface area to volume ratio of the smallest particles and increased internal surface area of the pores found in the large particles. For the small particles, the additional surface area hosts adsorbates that lead to non-radiative recombination, and in the porous particles, the pore walls can quench fluorescence. This trend is valid across calcination temperatures and is evident when comparing particles from the same calcination temperature.

Keywords: Gd2O3:Eu; Y2O3:Eu; cathodoluminescence; fluorescence; luminescence; particle size effect; scintillation.

Figure 1

(a) Schematic structure of the X-ray measuring station for the spatially resolved determination of the X-ray excited light yield of scintillating materials; (b) Sample holder made of highly reflective Teflon containing a 0.5 mm thick scintillating powder layer (see the text for further details).

Figure 2

Average particle size d_SSA after calcination. (a) Peroxidic Precipitation (PP) particles as a function of the calcination temperature; (b) Urea based homogeneous precipitation (UBHP) particles after calcination at 850 °C as a function of the concentration ratio c_urea/c_metals and of the reaction medium (pure H₂O for points connected with the dotted line; others: H₂O plus EG = ethylene glycol, B = 1-butanol, P = 2-propanol, see Table 1. At c_urea/c_metals = 63 there are two overlapping data points).

Figure 3

Transmission electron microscope (TEM) images of the PP particles, (a) Y₂O₃:Eu calcined at 450 °C; (b) Gd₂O₃:Eu calcined at 450 °C; (c) Y₂O₃:Eu calcined at 1000 °C; (d) Gd₂O₃:Eu calcined at 1000 °C.

Figure 4

Environmental scanning electron microscope (ESEM) images [taken with the large field detector, exception: image (f)] of UBHP Gd₂O₃:Eu particles calcined at 850 °C; (a) NS141-850 (d_SSA = 86 nm); (b) NS130-850 (d_SSA = 147 nm); (c) NS126-850 (d_SSA = 244 nm); (d) NS133-850 (d_SSA = 326 nm); (e) NS133-850—view into a trench cut by Focused Ion Beam (FIB) into the pellet pressed for prior cathodoluminescence (CL) examination; (f) NS133-850—particle image, taken with the back-scattered electron (BSE) detector.

Figure 5

Particle size d_SEM and crystallite size d_XRD of the calcined UBHP particles, as a function of the size information d_SSA.

Figure 6

Luminescence spectra of the Gd₂O₃:Eu particles (no indication) and of the Y₂O₃:Eu particles (where indicated), (a) fluorescence excitation spectra (cut peak around 306 nm = 1st order diffraction from monochromator, i.e., spectrometer artefact); (b) fluorescence emission spectra; (c) cathodoluminescence emission spectra (taken from 577 to 636 nm).

Figure 7

Relative light yield of the Gd₂O₃:Eu particles (no indication) and of the Y₂O₃:Eu particles (where indicated), depicted in dependence of the particle size d_SSA. Particles with sizes up to 74 nm are from the PP synthesis, larger particles with data points in the ovals are from the UBHP synthesis. (a) For excitation with 250 nm UV light (second ordinate giving the absolute QY values); (b) for excitation with e-beam (cathodoluminescence); (c) for excitation with X-ray.