Red Shift of CT-Band in Cubic Y₂O₃:Eu³⁺upon Increasing the Eu³⁺Concentration

Abstract

In this article we describe the redshift of the charge transfer band of nanosized cubic (Y 1−x Eu x ) 2 O 3 upon increasing the Eu 3+ concentration. This redshift amounts to 0.43 eV (25 nm) in going from 0.1 Mol % Eu 3+ to 100 Mol % (which is pure Eu 2 O 3 ). The charge transfer band consists of two broad sub-bands; both shift almost parallel with the Eu 3+ concentration and are related to the two symmetry sites for the cation, C 2 and C 3i , in the bixbyite-type lattice. The area ratio of the bands is 3:1 and is the first direct evidence for the population of the two lattice sites by the Eu 3+ cations being in accord with the crystal structure ratio. A model is presented that quantitatively describes the redshift of the charge transfer band of (Y The red luminescent phosphor Y 2 O 3 :Eu 3+ has been studied extensively, because of its rather high efficiency and high stability to electron bombardment, UV-irradiation and ion bombardment. These attributes led cubic Y 2 O 3 :Eu 3+ to be widely employed in cathode ray tubes (projection tubes) and it is still used in fluorescent lamps. Many properties of this phosphor have been studied by photoluminescence (PL) and cathodoluminescence (CL) spectrometry and have been well documented, because of its industrial applications. Notwithstanding the extensive literature on Y 2 O 3 :Eu 3+ , voids still exist in our knowledge on this phosphor. One of these voids is related to the position of the charge transfer (CT) band in the excitation spectrum, although this subject has been widely studied. In the present study we shall focus on the position of the CT-band, which is located at 253 nm in Y 2 O 3 when doped with 0.1 Mol % Eu 3+ . 1 The wavelength of the maximum of the CT-band in Y 2 O 3 :Eu 3+ , indicated by the E CT in this study and expressed in nanometers (nm) or electron-volts (eV) depends on the concentration of the Eu 3+ dopant and the size of the phosphor particles.2-10 Upon increasing the Eu 3+ concentration between 1 and 15 Mol % a redshift of the E CT of about 6 nm has been observed, [2][3][4] whereas Kang et al. 5 did not observe a change of E CT in increasing the Eu 3+ dope from 2 to 10 Mol %. Some peculiar observations were reported on the effect of the particle size of Y 2 O 3 :Eu 3+ . Igarashi et al. 6 reported a blueshift of the E CT of 6 nm in reducing the particle size from 2.1 μm to 56 nm; Fu et al.7 measured a blueshift of 5 nm in reducing the particle size from 2.1 μm down to 10 nm. The opposite trend was found by others. Jia et al.8 measured a redshift of the E CT of 6 nm upon particle size reduction from 2-3 μm to 7 nm. Shang et al. 9 and Zhang et al. 10 reported large red shifts, 11 nm and 7 nm, in reducing the size of Y 2 O 3 :Eu 3+ nanoparticles from 40 nm to 9 nm and from 40 nm to 5 nm respectively. Semiconductor particles are expected to show a blueshift of their absorption peaks in reducing the particle size from 15 to 5 nm because of quantum confinement.11 The observed opposite trend indicates that quantum confinement cannot explain the reported ...

Why Do Relative Intensities of Charge Transfer and Intra-4f Transitions of Eu3+ Ion Invert in Yttrium Germanate Hosts? Unravelling the Underlying Intricacies from Experimental and Theoretical Investigations

“…Due to fundamental and technological importance, the charge-transfer (CT) phenomenon has been a key area of research. − For technologically important rare-earth ions like Eu 3+ , which inherently have poor excitation efficiency, charge transfer plays a central role in governing their luminescent properties . CTB is useful for sensitization of Eu 3+ luminescence, because it can act as an antenna to absorb incoming light and then transfer excitation energy to the Eu 3+ ion, in a way similar to sensitization by organic chromophores. − Common Eu 3+ phosphors have broad CTB of O 2– → Eu 3+ originating due to transfers of an electron from a neighboring oxygen 2p orbital to an empty 4f orbital of Eu 3+ ion. ,, Previously, many attempts have been made to find factors influencing CT energy (i.e., the position of CTB in the excitation spectra) both qualitatively and quantitatively. ,− In these investigations, CTB has been found to be dependent on host structure, sites occupied by Eu 3+ ion in the host matrix, ionicity/covalency of Eu–O bond, position of the valence band/electronic structure, etc.…”

Novel method to control initial crystallization of Eu³⁺ doped ZrO₂ nanophosphors derived from a Sol–Gel route based on HNO₃ and their site-selective photoluminescence

“…On the other hand, amorphous local structures around Eu 3+ provide lower excitation energy in CTB around 250 nm due to longer EuO bonding length. 20)…”

Nanosized (Y1−xGdx)2O2S:Tb3+ particles: synthesis, photoluminescence, cathodoluminescence studies and a model for energy transfer in establishing the roles of Tb3+ and Gd3+

“…In ref. 24 we have suggested that this could be caused by a difference in compressibility between the Y 2 O 2 S:Tb 3+ and the Gd 2 O 2 S:Tb 3+ lattices. In this reference we have already mentioned that the proposed electrostatic model in terms of the Madelung energy leads to a similar result as the chemical shi model proposed by Dorenbos.…”

“…Recently we have introduced an electrostatic model in terms of the Madelung energy of the transferred charge that could quantitatively describe the red shi of the CT-band in Y 2 O 3 :Eu 3+ . 24 This model will now be applied to the data presented in Fig. 6a 25 The charge transfer during excitation of a photon can be represented by the following reaction:…”

“…We have recently argued that red shi of the CT-band, represented by DE CT , can be described by the Madelung energy of the inserted eh-pair. 24 Eqn (5) implies that the Madelung energy of the insertion of 33 eh-pairs needs to be evaluated, in which a fraction of the electron is positioned at the Ln 3+ site and a (smaller) fraction of the hole at the O 2À and S 2À sites. The Madelung energy DEM eh (x) for inserting 33 eh pairs in the Ln 2 O 2 S lattice can be written as…”