The Role of Anions in Rare-earth Activated Inorganic Host Materials for its Luminescence Characteristics

Abstract

This work is inspired from the comprehensive work done by our research team aimed at improving the efficiency of white light emitting diodes (LEDs) through improvements in the colour rendering index of the red light (CRI), one of the primary colours of white light. Such work is triggered through the incorporation of anions (BO₃^3-, PO₄^3-, SO₄^2-), either individually or as an integral part of dopant activated inorganic phosphor host materials. Numerous host materials such as ZnO, Y₂O₃, Ca₃(PO₄)₂, CaMoO₄, ABPO₄, ABSO₄ (where A represents alkali metals and B alkaline earth metals) have been considered ideal hosts materials for studying luminescence properties of materials (including other phosphors). In addition, red emitting dopants such as Sm³⁺, Eu³⁺ and Ce³⁺ have been incorporated into these host materials to achieve a higher CRI of red colour, an essential component of white light. The role anions in various materials is multifaceted; firstly, it acts as sensitizer whereby it absorbs excitation energy and transfers it non-radiatively to the dopants, secondly, it acts as a charge compensator to dopants with a charge of + 3, thirdly, it creates crystal fields that affects the electronic transitions of the dopants and fourthly, it creates a stable crystal structure that allows for dopant embedding. By understanding the exact role of these anions and their interactions with the host lattice and dopant ions, we could further optimize the luminescent properties of these activated host materials, which leads to higher efficiencies and performances in white light-emitting diodes and other lighting technologies. This work is a comprehensive review of the work undertaken by our research team aimed at enhancing the luminescent properties of WLEDs.

Keywords: Dopants; Energy transfer; Luminescence; Phosphor; WLEDs.

Fig. 1

XRD patterns of the NaMPO_4-Eu³⁺ phosphor materials with M²⁺ = Ca, Ba, Sr, and Mg [15] (Reused with permission from [15])

Fig. 2

FTIR spectra of the NaMPO₄-Eu³⁺ phosphor materials with M²⁺ = Ca, Ba, Sr, and Mg. The inset show the various series doped with Eu³⁺ ions [15] (Reused with permission from [15])

Fig. 3

PLE and PL spectra of a sampler NaBaPO₄:Eu³⁺ phosphor material is shown in figure (a) above, while the entire PL emission series for NaMPO₄:Eu³⁺phosphors is shown in (b) [15] (Reused with permission from [15])

Fig. 4

XRD patterns of the CaMoO₄-SO₄-xEu³⁺ (x = 0 to 2.5 mol%) phosphor materials [17] (Reused with permission from [17])

Fig. 5

FTIR spectra of the CaMoO₄-SO₄-Eu³⁺ phosphor material [17] (Reused with permission from [17])

Fig. 6

PLE spectra of the CaMoO₄-SO₄-xEu³⁺ phosphor material is shown in (a), while the PL emission spectra of the sample for concentration variations is shown in (b) [17] (Reused with permission from [17])

Fig. 7

XRD patterns of the NaMPO₄:Ce³⁺ (M²⁺ = Mg, Ca, Sr, and Ba) phosphor materials [19] (Reused with permission from [19])

Fig. 8

FTIR spectra of the NaMPO₄:Ce³⁺ (M²⁺ = Mg, Ca, Sr, and Ba) phosphor materials [19] (Reused with permission from [19])

Fig. 9

PLE excitation spectra for NaMPO₄:Ce³⁺ (M²⁺ = Mg, Ca, Sr, and Ba) phosphor materials are shown in (a), while the PL emission spectra is shown in (b) as a function of binding energy for different wavelengths [19] (Reused with permission from [19])

Fig. 10

XRD patterns of the Ca₃(PO₄)₂: Ce³⁺phosphor material co-doped with Gd³⁺ ions [20] (Reused with permission from [20])

Fig. 11

PLE excitation and PL emission spectra of Gd³⁺ co-doped Ca₃(PO₄)₂: Ce³⁺phosphor material, excited at different wavelengths in respective pairs at (a) and (b) [20] (Reused with permission from [20])

Fig. 12

XRD patterns of the ZnO: Ce³⁺phosphor material with SO₄^2- anionic incorporation in (a) and enlarged section in (b) showing the relative shifts in diffraction peaks [21] (Reused with permission from [21])

Fig. 13

FTIR spectra of the ZnO: Ce³⁺ phosphor materials with SO₄²⁻ incorporation [21] (Reused with permission from [21])

Fig. 14

PL emission spectra of the ZnO: Ce³⁺ phosphor materials, with SO₄^2- incorporations in (a), and (b) showing enlarged sections of (a), and (c) shows the trends of PL emission when SO₄^2- or Ce³⁺ ions are added to ZnO [21] (Reused with permission from [21])

Fig. 15

XRD spectra of the CaMoO₄:Eu³⁺ phosphor material with BO₃³⁻ incorporations [22] (Reused with permission from [22])

Fig. 16

FTIR spectra of the CaMoO₄:Eu³⁺ phosphor material with BO₃^3- incorporation [22] (Reused with permission from [22])

Fig. 17

PLE excitation spectra of the CaMoO₄:Eu³⁺ phosphor material is shown in (a), while the PL emission spectra of the same sample is shown in (b) with BO₃^3- incorporations [22] (Reused with permission from [22])

Fig. 18

XRD pattern of the Y₂O₃-AG-Eu³⁺ phosphor materials, with AG = (BO₃³⁻), (PO₄³⁻), and (SO₄²⁻) additions, and transformed structures for borate and phosphate additions [23] (Reused with permission from [23])

Fig. 19

FTIR spectra of the Y₂O₃-AG-Eu³⁺ phosphor materials, with AG = (BO₃^3-), (PO₄^3-), and (SO₄^2-) [23] (Reused with permission from [23])

Fig. 20

PL excitation and emission spectra of the Y₂O₃-AG-Eu³⁺ phosphor materials, with AG =(BO₃^3-), (PO₄^3-), and (SO₄^2-) [23] (Reused with permission from [23])

Fig. 21

XRD patterns of the Na₂Ca (SO₄)₂: Sm³⁺/Eu³⁺ phosphor material is shown in (a), while figure (b) is an enlarged section of the (-2 2 1) plane [24] (Reused with permission from [24])

Fig. 22

FTIR spectra of the co-doped Na₂Ca (SO₄)₂: Sm³⁺/Eu³⁺ phosphor materials [24] (Reused with permission from [24])

Fig. 23

PL excitation and emission spectra of the co-doped Na₂Ca (SO₄)₂: Sm³⁺/Eu³⁺ phosphor materials [25] (Graph taken from reference [25], published in the Journal of Molecular Structure)

Fig. 24

XRD patterns of the Na₆Mg (SO₄)₄: Sm³⁺ phosphor material, co-doped with Eu³⁺ ions [25] (Graph taken from reference [25], published by Inorg. Nano-Metal Chem., 2022)

Fig. 25

FTIR measurements of the Na₆Mg (SO₄)₄: Sm³⁺ phosphor material, co-doped with Eu³⁺ ions [25] (Graph taken from reference [25], published by Inorg. Nano-Metal Chem., 2022)

Fig. 26

Full PLE and PL spectrum of the Na₆Mg (SO₄)₄: Sm³⁺ phosphor material is shown in (a), figure (b) shows the PLE spectrum of the phosphor material and c shows the PL emission spectra of the material, co-doped with Eu³⁺ ions [25] (Graph taken from reference [25], published by Inorg. Nano-Metal Chem., 2022)

Fig. 27

XRD spectra of the CaMoO₄-AG: Sm³⁺ phosphor materials, with AG = BO₃³⁻, PO₄³⁻ and SO₄²⁻ ions [3] (Reused with permission from [3])

Fig. 28

FTIR spectra of CaMoO₄ -AG: Sm³⁺ (1 mol%) phosphor materials with AG = BO₃^3-, PO₄^3- and SO₄^2- ions [3] (Reused with permission from [3])

Fig. 29

Figure a shows the PLE excitation spectra and figure b shows the PL emissions spectra of the CaMoO₄ -AG: Sm³⁺ (1 mol%) phosphor materials with AG = BO₃^3-, PO₄^3- and SO₄^2- ions [3] (Reused with permission from [3])

Fig. 30

XRD spectra of the Ca_3-xLi_x(PO₄)_2-x(SO₄)_x:Dy³⁺, Sm³⁺ phosphor materials [14] (Reused with permission from [14])

Fig. 31

PLE excitation and PL emission spectra of the Ca_3-xLi_x(PO₄)_2-x(SO₄)_x:Dy³⁺, Sm³⁺ phosphor materials [14] (Reused with permission from [14])

Fig. 32

XRD spectra of the Ca_10.5–0.5x(PO₄)_7-x(SO₄)_x and Ca_9.5–0.5xMg(PO₄)_7-x(SO₄)_x phosphor material for x = 1 mol% [26] (Reused with permission from [26])

Fig. 33

FTIR spectra of the Ca_10.5-0.5x(PO₄)_7-x(SO₄)_xand Ca_9.5-0.5xMg(PO₄)_7-x(SO₄)_x phosphor material for x = 1 mol% [26] (Reused with permission from [26])

Fig. 34

PLE excitations spectra of the prepared material is shown in (a), while the PL emission spectra is shown in figure (b) [26] (Reused with permission from [26])

Fig. 35

XRD spectra of the Eu³⁺ activated LiY(PO₃)_4(1-x)SO_4x phosphor materials for varying concentrations of SO₄^2- ions [13] (Graph taken from reference [13], published by ECS J Solid State Sci. Technol., 2023)

Fig. 36

PLE (first) and PL spectra (second) of the Eu³⁺ activated LiY(PO₃)_4(1-x)SO_4x phosphor materials for varying concentrations of SO₄^2- ions [13] (Graphs taken from reference [13], published by ECS J Solid State Sci. Technol., 2023)