Yttrium Oxide Upconverting Phosphors. 5. Upconversion Luminescent Emission from Holmium-Doped Yttrium Oxide under 632.8 nm Light Excitation

Abstract

Both the anti-Stokes and Stokes emission properties of cubic Y 2 O 3 :Ho 3+ arising from excitation at a wavelength of 632.8 nm are reported over the concentration range 0.25-10 mol % Ho 3+ . Power dependence studies are used to assign the bands in the emission spectra. The most efficient yellow-green upconversion emission was observed at a Ho 3+ concentration of 0.5 mol %. This originates from an absorption process involving two photons and undergoes progressive quenching as the Ho 3+ concentration is increased. It is shown herein that as the concentration of Ho 3+ is increased, cross-relaxation processes become dominant. These processes involving Ho 3+ ions in the 5 F 5 levels and the metastable 5 I 7 states enable an alternative process for the population of the 3 K 8 , 5 F 3 , and 5 F 2 levels that gives rise to enhanced emission between 480 and 505 nm as the concentration of Ho 3+ ions is increased from 0.5 to 10 mol %. This cross-relaxation process is discussed in the text. In this work we also report the first discrimination between emission bands due to transitions originating from the 5 F 4 and 5 S 2 levels of Ho 3+ . The emission bands originating from the 5 F 4 level (viz. the yellow-green 530-570 nm and the near-infrared 740-780 nm bands) are shown (as expected) to have a similar dependence on the incident photon flux.

Preparation, characterization and luminescence of nanocrystalline Y2O3:Ho

“…Compared to other holmium doped materials a significant emission in the red spectral region due to the 5 F 3 , 5 F 2 , 3 K 8 → 5 I 7 transition was observed. In comparison with the work of Silver [14] the relative intensity of the ( 5 S 2 + 5 F 4 ) → 5 I 7 transition is much higher and there is also a discrepancy in relative intensity of other bands. This difference in distribution of relative intensities of emission lines can be attributed to the adsorption of atmospheric CO 2 and H 2 O on the surface of the material [15].…”

Spectroscopic Properties and Upconversion Studies in Ho3+/Yb3+ Co‐doped Calcium Scandate with Spectrally Pure Green emission

“…For Ho 3+ concentrations over 0.2 %, the intensity begins to diminish because of the cross‐relaxation processes of Ho 3+ 14. 21 Figure 3 b shows the relative intensity of the red emissions, which have a slight enhancement as the Ho 3+ concentration is increasing. The intensity ratio of green‐to‐NIR emissions does not vary with Ho 3+ concentration.…”

“…[15] All observed X-ray diffraction (XRD) peaks are obtained with the fit parameters of the profile R-factor (R p = 2.65 %) and the weighted profile R-factor (R wp = 4.35 %), indicating that a relatively pure phase was obtained. The inset of Figure 1 shows the schematic crystal structure of CaSc 2 [14,21] Figure 3 b shows the relative intensity of the red emissions, which have a slight enhancement as the Ho 3 + concentration is increasing. The intensity ratio of green-to-NIR emissions does not vary with Ho 3 + concentration.…”

Preparation and upconversion luminescence of Y2O3:Yb3+, Ho3+ nanocrystalline powders coated with SiO2

“…The upconversion mechanism in Yb-Ho system has been widely studied in many host materials [15][16][17]. Because of the character- istic energy levels of Ho 3+ ions and the excitation source, energy transfer for the sensitizing effect of Yb 3+ ions plays an important role for the resulting upconversion emissions.…”