Role of Li⁺ ion in the luminescence enhancement of lanthanide ions: favorable modifications in host matrices

Abstract

Lanthanide based materials are preferred over other luminescent materials for various applications. Current focus in this area is to exploit the unique luminescence features of lanthanide-based materials for multidisciplinary research and novel applications. Furthermore, efforts are going on to enhance the luminescence of lanthanide ions for better performance. In a broader sense, there are two ways to enhance the luminescence of lanthanide ions. The first is to use a suitable sensitizer, which can absorb excitation energy efficiently, and can transfer it to the lanthanide ions. This method has been known for a long time and is well documented in the literature. The second way is to modify host matrices in such a way that it favors radiative transitions. It is widely reported in the literature that the presence of alkali ions, particularly Li⁺ ions, in a matrix enhances the luminescence of lanthanide ions significantly. But there are no comprehensive reports available in the literature that summarize how alkali ions help in the luminescence enhancement of lanthanide ions in various host matrices. The prime objective of this review is to highlight various contributing factors that help in the luminescence enhancement of lanthanide ions in the presence of alkali ions.

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A. K. Singh

Akhilesh Kumar Singh is a Postdoctoral Fellow in the Instituto de Ciencias Físicas, Universidad Nacional Autónoma de México, Cuernavaca, México. He holds an M. Sc. in Physics (2004) from V.B.S. Purvanchal University, Jaunpur, India and an M. Tech. (2007) and Ph.D. (2012) in Materials Science and Technology from Indian Institute of Technology (Banaras Hindu University) Varanasi, India. His research interests include synthesis, characterization and application of lanthanide-based novel luminescent materials, nanostructure and quantum dots. He has published 18 peer-reviewed international journal papers and is a co-investigator of an Indian Patent.

S. K. Singh

Sunil K Singh, born in 1982, received doctoral degree in Physics in 2011 from the Banaras Hindu University, India. Currently, he is in the DST-INSPIRE faculty at Indian Institute of Technology (Banaras Hindu University), Varansi, India. His research interest includes the physics of multimodal luminescence (upconversion, downconversion/quantum-cutting, downshifting) in lanthanide and/or semiconductor quantum dot doped nanostructures. In terms of applications, his focus is on bio-imaging, increasing the conversion efficiency of photovoltaic cells, sensors, and use for security purposes, etc. He has authored/co-authored a book chapter and more than 30 articles in peer-reviewed journals, and his current h-index is 9 (April 2014).

S. B. Rai

S. B. Rai has been a professor in the Department of Physics, Banaras Hindu University, Varanasi, India since 1994. He gained his Ph.D. (Physics) in 1974 from Banaras Hindu University and visited Germany as an Alexander von Humboldt fellow in 1982–84 and several times later on. His current research interests include multifunctional materials doped with rare-earth ions e.g. glasses, ceramics, phosphors, quantum dots, polymers, hybrid nanostructures, etc. and their applications in various emerging fields. He has edited three books and several book chapters, and authored/co-authored around 300 articles in peer-reviewed national and international journals.

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1. Introduction

Lanthanide ions exhibit fascinating optical properties, including the ability to convert near infrared photons to ultraviolet/visible photons (through an upconversion process) and vice versa (quantum-cutting process).^1–5 Due to their unique optical features, many recent advances and emerging applications such as lighting devices (light emitting diodes, economical luminescent lamps), optical fibres, display devices, lasers and many biological investigations are heavily dependent on them.^1–5 Fundamentally, lanthanide ions have essentially three types of optical transitions: intra-configurational 4f–4f transition, inter-configurational 5d–4f transition and the charge transfer transition (ligand to metal or metal to ligand).^5,6 Ce³⁺ and Eu²⁺ are widely used activator ions for phosphor applications.^7–19 Their characteristic transition is 4f–5d, which is an allowed transition and has a lifetime on the order of nanosecond. Since the 5d orbitals are strongly affected by the crystal field and polarizability of the host crystal, these transitions (4f–5d transitions) can be easily tuned to the visible region by changing the crystal composition and structure.^7–19 In contrast, the 4f–4f transition in lanthanide(III) ions (viz. Pr³⁺, Nd³⁺, Ho³⁺, Er³⁺, Sm³⁺, Eu³⁺, Tb³⁺, Dy³⁺, Tm³⁺ and Yb³⁺), which is a forbidden transition, gives sharp and intense spectral lines (from UV to infrared region) with a lifetime on the order of microsecond.^20–30 Further, as the 4f orbital remain shielded by 5s and 5p orbitals, 4f–4f transitions are much less influenced by the external environment (crystal field of host matrices).^20–30

In recent years, research of lanthanide-based luminescence materials has been focused to exploit the unique features of lanthanides in various other novel applications, viz. for multimodal emission (downconversion (DC)/quantum-cutting (QC), down-shifting (DS) and upconversion (UC) processes) based imaging, in increasing the efficiency of solar cells (by using an additional DC/UC layer in the solar cell to minimize the thermal and sub-bandgap losses), as contrast agents in magnetic resonance imaging, various biological applications, etc.^31–35 The pragmatic implication of luminescence materials for these applications demands efficient materials in many ways, including an increase in the emission intensity. Therefore, continuous active research effort in this field is desirable to enhance the luminescence intensity of the lanthanide ions.

There are two fundamental ways to enhance the luminescence intensity of the lanthanide ions in a given host. The first is to use a suitable sensitizer, which can absorb UV and/or NIR radiation effectively and can transfer the energy to the central lanthanide ions.^{1–3,36–42} In the literature, different sensitization strategies are reported for increasing the luminescence of lanthanide ions, viz. by forming lanthanide ion complexes with organic ligands, which can strongly absorb UV radiation and transfer efficiently to the central lanthanide ion; by excited state energy transfer from the charge transfer band to the lanthanide ion; by using co-dopant and thereby energy transfer among lanthanide themselves, etc.^24,38,41 The second way to enhance the luminescence of lanthanide ions is to modify the host matrices in such a way that it favors transitions of lanthanide ions. Due to its smaller ionic radius alkali ions, the Li⁺ ion in particular is very frequently used nowadays for this purpose.^{7–19,56–142} Li⁺ ions are easily accommodated in host matrices, which tailors the local crystal field around the lanthanide ions.

Recently, our group has also worked in this area and shown that the use of Li⁺ ion enhances the photoluminescence (PL) of lanthanide ions in YPO₄ and Y₂O₃ hosts.^74,81,84 In the literature, several models have been proposed by different groups to explain the luminescence enhancement in the lanthanide ions in different host matrices.^56–142 However, to the best of our knowledge, there is no review article that summarizes a comprehensive view of how the alkali ion modifies a host matrix resulting in enhanced luminescence. The prime objective of this review is to fill this gap and highlight various contributing factors that contribute towards luminescence enhancement. Along with this, since different types of host matrices have been reviewed individually through the previous literature, the present article could also be very useful in studying/visualizing the role of the matrix on the luminescence intensity of lanthanide ions in general. This review is written in such a way that it could be useful for a broad audience of chemists/biochemists and materials scientists as well.

2. Basics of lanthanide luminescence

The term lanthanide originated from the Greek word lanthaneien meaning “lying hidden”. The lanthanides are a family of 15 chemically similar elements, from lanthanum to lutetium (atomic number 57 to 71), which possess electronic configuration [Xe] 4fⁿ5d^0,16s² (where ‘n’ varies from 0–14).^5,6 Scandium (21) and yttrium (39) (d-block elements) are found in nature always with lanthanides; therefore, lanthanides and these two elements altogether are known as the rare-earth elements. Lanthanides exist usually in the +3 oxidation state (Ln³⁺), which is their most stable oxidation state (though Eu²⁺, Sm²⁺ and Cr⁴⁺ also exist) and most of the optical properties of lanthanides are explored in this oxidation state. As the lanthanide ions feature an electron configuration of 4fⁿ, which offers large number of possible arrangements, there are rather large numbers of electronic levels.

The promotion of a 4f electron into the 5d sub-shell in lanthanide ions is allowed by the parity rule. These transitions are quite energetic and observed only in Ce³⁺, Pr³⁺ and Tb³⁺ ions. Since the 5d orbitals are external and interact directly with the ligand orbitals, their energy largely depends on the metal environment.^7–19 In Ce³⁺ ions, luminescence is observed from ²D_3/2 levels to ²F_5/2, ²F_7/2, whereas ²D_5/2 lies at high energy and luminescence from this level is not generally observed. The luminescence in Ce³⁺ can be tuned from about 290 to 450 nm, depending on the matrix into which the metal ion is inserted. The charge transfer transition in lanthanide ions is also allowed by Laporte's selection rule. The other and most studied transition in lanthanides is the 4f–4f transition. According to Laporte's rule, electric dipole transitions with the same parity are not allowed. By virtue of that, the 4f–4f transition in lanthanides is forbidden.^5,6 However, when these ions are doped in a suitable host matrix, due to influence of the ligand-field, non-centrosymmetric interactions, a mixing of the wavefunctions of the states of opposite parity with 4f-wavefunctions, takes place. This relaxes somewhat the parity selection rule and 4f–4f transitions become partially allowed.^5,6 Furthermore, the crystal field destroys the degeneracy of its energy levels. In fact, the ligand/crystal generates a ligand/crystal electrostatic field which, in turn, interacts with the 4f electrons of the lanthanide ions generating a ligand field/crystal field (or Stark) splitting of the spectroscopic levels.^5,6 The 4f orbitals of the lanthanide ions remain shielded by 5s and 5p orbitals, and therefore promotion of an electron into a 4f sub-orbital of higher energy does not change the binding pattern significantly. Thus, the inter-nuclear distance remains almost unchanged in the excited state, which produces narrow emission lines. The long lifetime is because of the fact that the 4f–4f transitions in the lanthanide ions are parity forbidden.^5,6 Table 1 summarizes the energy of principal 4f–4f transitions of lanthanide ions coming through DS, QC and UC processes.

Table 1 Principal luminescent transitions in lanthanide ions (Ln³⁺)^a

Ln	Transition	Emission wavelength (nm)	Remarks
a QC – quantum-cutting, DS – down-shift, UC – upconversion, NIR – near-infrared, P – phosphoresce, F – fluorescence. Most of the data presented are taken from ref. 31 and 52.
Pr	^3P2 → ^3H4	440	Weak, QC
	^3P0 → ^3H4	480	Strong, UC and QC
	^1D2 → ^3F4	1037 (P, NIR)	Medium, UC and QC
Nd	^2P3/2 → ^4I11/2	410	Strong, UC and QC
	^2P3/2 → ^4I13/2	452	Strong, UC and QC
	^4F3/2 → ^4I11/2	1064 (F, NIR)	Strong, QC and DS
Sm	^4G5/2 → ^6H7/2	601 (P, orange)	Strong, DS
Eu	^5D0 → ^7F0,1,2,3,4	570–720 (P, orange)	Strong, UC and DS
Tb	^5D4 → ^7F6,5,4,3	480–650 (P, green)	Strong, UC, DS and QC
Dy	^4F9/2 → ^6H15/2	486	Medium, DS and QC
	^4F9/2 → ^6H13/2	575 (P, yellow-orange)	Strong, DS and QC
Ho	^5S2, ^5F4 → ^5I8	540 (F, green)	Strong, UC and QC
	^5F5 → ^5I8	644 (F, red)	Strong, UC and QC
	^5I6 → ^5I8	1180 (NIR)	Strong, QC
Er	^4S3/2 → ^4I15/2	545 (F, green)	Strong, UC and QC
	^4F9/2 → ^4I15/2	665 (F, red)	Strong, UC and QC
	^4I13/2 → ^4I15/2	1540 (NIR)	Strong, QC
Tm	^1D2 → ^3F4	450 (blue)	Medium, UC and QC
	^3H4 → ^3H6	800 (NIR)	Strong, UC and QC
Yb	^2F5/2 → ^2F7/2	980 (F, NIR)	Strong, UC, QC and DS

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The 4f–4f transition in lanthanide ions facilitates UC and DC/QC as well as DS processes, which may allow for facile photon management.^43–51 In the DC/QC process, an incident higher-energy photon splits into two (or more) lower energy photons with a conversion efficiency of more than 100%.^46–49 DS is similar to QC but it is a single photon process that involves the transformation of one absorbed high-energy photon into one low-energy photon and so its conversion efficiency does not exceed 100%.^50,51 Meanwhile, on the other hand, UC is a nonlinear process that involves two or more low energy photons (usually NIR) to emit a single photon of higher energy (usually visible or ultraviolet).^43–45 To generate practically useful UC emission, the energy difference between each excited level and its lower-lying intermediate level (ground level) should be close enough to facilitate photon absorption and energy transfer steps involved in UC processes. Er³⁺, Tm³⁺, and Ho³⁺ typically feature such ladder-like arranged energy levels and are thus frequently used activators for UC.

Selection of an appropriate host matrix for the preparation of a lanthanide-based luminescence material is an essential requirement. As the luminescence of lanthanide ions is very sensitive to the local crystal field environment, the host matrix should have close lattice matches to the dopant ions.^52–54 Failing to achieve this can create a large distortion in the matrix, which strongly influences the luminescence of lanthanide ions. In particular, the 5d–4f transitions of the lanthanide ions are strongly influenced (both position and intensity) by the crystal field of the matrix.^7–19 Since the lanthanide(III) ions exhibit similar ionic size and chemical properties their inorganic compounds are ideal hosts for lanthanide ion doping. In addition, alkaline earth ions (Sr²⁺, Ca²⁺, and Ba²⁺) and some transition metal ions (Zr⁴⁺ and Ti⁴⁺) also exhibit close ionic size to lanthanide ions and are frequently used as host materials for lanthanide ion doping. Further, the ideal host matrix should also have low lattice phonon energies, to minimize non-radiative loss and maximize the radiative emission. The dopant concentration, which determines the average distance between neighboring dopant ions, has a strong influence on the optical properties of the lanthanide-based luminescence materials.^52–54 In certain host matrices, luminescence of lanthanide ions increases appreciably with increasing crystallinity. This is because of the fact that as the crystallinity increases, the particle size also increases, which reduces surface quenching centers (surface to volume ratio decreases). The increased particle size is not required in many technological applications, which have been prompted due to the advancement in nanotechnology; therefore, researchers usually like to use a passive layer, by forming a core–shell structure, to reduce the surface quenching centers.⁵⁴ The co-doping of alkali ions in the matrix strongly affects the crystal structure, crystallinity, grain size, surface morphology, quenching centers (OH⁻, NO₃⁻, etc.) and create distortion in the matrix.^56–142 Therefore, it would be interesting to study the effect of alkali ions, particularly the Li⁺ ion, on the luminescence properties of lanthanide ions doped in various matrices and try to find out some meaningful information to understand the underlying mechanism, and also to explore the future possibilities. The following sections aim to investigate these aspects in detail.

3. Effect of Li⁺ ion on the luminescence of Ce³⁺ and Eu²⁺ (5d–4f transition) ions

As discussed in the preceding sections, the origin of luminescence from Ce³⁺ and Eu²⁺ ions is different from the characteristic luminescence of lanthanide(III) ions involving 4f–4f transitions. The luminescence in Ce³⁺ and Eu²⁺ ions is due to 4fⁿ⁻¹5d¹ to 4fⁿ (ground state) transitions (where n = 7 for Eu²⁺ and n = 1 for Ce³⁺).^7–19 These transitions are allowed and have lifetimes on the order of nanosecond. They exhibit broad-band luminescence spectra, with their maxima lying in the yellow region, and are very useful for lighting applications (LEDs, automobile lamps, backlighting, etc.). As the 5d orbitals are external, the position of these levels, and thereby the excitation and emission bands, strongly depend on the local environment of the ions (crystal fields of the host matrices). Dorenbos⁵⁵ has made a clear correlation between the coordination polyhedron (in which Eu²⁺ or Ce³⁺ lies) and the crystal field splitting. They have shown that crystal field splitting tends to be the largest for octahedral coordination followed by cubal, dodecahedral, and it is the smallest for tricapped trigonal prism and cuboctahedron coordination. Thus, through appropriate host selection (with different crystal field splitting of the 5d band), the luminescence of Eu²⁺ and Ce³⁺ can be tuned according to the requirement. Park et al. have shown that Eu²⁺ doped Sr₂SiO₄ and Sr₃SiO₅ are stronger yellow emitting phosphors than the conventional light-conversion phosphor YAG:Ce³⁺.^8b Co-doping of Li⁺ ion with Eu²⁺/Ce³⁺ not only improves the luminescence efficiency but brings out changes even in the excitation and emission spectral ranges, so both these activators ions have been effectively studied in presence of Li⁺ ions. Table 2 gives a brief overview of different host and dopant ions that have been studied to visualize the effect of Li⁺ co-doping.

Table 2 Effect of Li⁺ ion on the luminescence properties of Ce³⁺ and Eu²⁺ ions doped in different host matrices

Sample	Change in luminescence properties	Remarks
CaBr:Eu^2+,Li^+ (ref. 7)	Improves radiation hardness	By reducing the X-ray induced vacancy centers
Li2CaSiO4:Eu^2+ (ref. 8a)	Small Stokes shift	Smaller ionic radius of Li^+ constrains the distortion of the excited state
CaS:Ce^3+,Pr^3+,Li^+ (ref. 9)	3 fold enhanced afterglow intensity and 4 fold enhancement in afterglow time	New cation vacancy formed, which works as an electron trap center
CaO:Ce^3+,Li^+ (ref. 10)	1.88 fold enhancement in luminescence intensity	Increased absorbance to the excitation photons
SrAl2O4:Eu^2+,Ce^3+,Li^+ (ref. 11)	Enhanced luminescence of Eu^2+	Charge compensation
NaCaPO4:Eu^2+,Li^+ (ref. 12)	Enhanced luminescence of Eu^2+	Change in spin–orbit coupling and coordination of Eu^2+ ions
Sr2–2xLiSiO4F:xCe^3+,xLi^+ (ref. 13)	Shift in peak position	Ce^3+ occupying different Sr^2+ site
Sr3SiO5:Ba^2+,Ce^3+,Li^+ (ref. 14)	Li^+ co-doping helps to incorporate Ce^3+ ions on Sr^2+ site	By charge compensation
Lu2SiO5:Ce^3+,Li^+ (ref. 15)	2.2 fold enhancement in luminescence intensity	Change in crystal field around Ce^3+ ions
Sr3Si2O4N2:Eu^2+,Li^+ (ref. 19)	Red shift in emission band	Re-absorption in Eu^2+ ions
Sr3Si2O4N2:Ce^3+,Li^+ (ref. 19)	No prominent red shift

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Grandhe et al.¹² investigated the effect of Li⁺ ions on the emission characteristics of Eu²⁺ emission in phosphate matrix (Na_1−yLi_yCa_1−xPO₄:xEu²⁺) synthesized by a conventional solid state reaction method. The excitation spectra of NaCaPO₄:Eu²⁺ phosphors revealed a broad excitation band ranging from 250 to 450 nm with a maximum intensity at 373 nm. Upon 373 nm excitation, the phosphor exhibits an intense bluish-green emission band centered at 505 nm. Fig. 1(b) shows that the Na_1−yLi_yCa_0.99PO₄:0.01Eu²⁺ phosphor containing 4 mol% of Li⁺ ions (y = 0.04) exhibits maximum luminescence intensity. It is suggested that the difference in the ionic radii probably gives rise to diversity in the crystal lattice around the luminescence center. This may in turn influence the spin–orbit coupling and the crystal field around the Eu²⁺ ions. Liu et al.⁸ have synthesized the Li₂CaSiO₄:Eu²⁺ phosphor by a conventional solid state reaction method and found that the Li⁺ ion changes the excitation and emission characteristics of Eu²⁺ effectively in silicate matrix also. It was again concluded to be due to the change in host lattice/crystal field due to the involvement of Li⁺ ions in the crystal structure. They proposed that the Eu²⁺ ions go to distorted dodecahedral Ca sites, which causes strong crystal field splitting leading to a broad excitation band extending from the UV to visible regions. Also, the high concentration of Li⁺ ions in the structure constrained the distortion of the emission center (Li⁺ ions, which are behind the oxygen ions surrounding the emission centers, will restrict the expansion of the emission centers), which results in a smaller Stokes shift.

Fig. 1 (a) CIE coordinates of Ca_0.98SiN_2−2δ/3O_δ:0.01Ce³⁺/0.01Li⁺ and photos of the powder samples under daylight and 365 nm UV excitation. (b) PL emission spectra of Na_1−yLi_yCa_0.99PO₄:Eu_0.01 phosphors sintered in argon initially and later in N₂H₂ atmosphere with varying lithium ion concentrations. (c) PL spectra of CaO:Ce³⁺ and CaO:Ce³⁺,Li⁺. The inset presents a histogram of the integrated PL intensity, where the intensity of CaO:Ce³⁺ is normalized. Photographs of: (d) an as-prepared CdSe QD- and Sr₃SiO₅:Ce³⁺,Li⁺ phosphor-based white LED and (e) the same white light-emitting LED operated at 5 mA. (a) Reproduced from ref. 17 with permission from The Royal Society of Chemistry. (b) Reprinted (adapted) with permission from ref. 12. Copyright © 2012 Elsevier B. V. All rights reserved. (c) Reprinted (adapted) with permission from ref. 10. Copyright © 2013 Elsevier B. V. All rights reserved. (d and e) Reprinted (adapted) with permission from ref. 18. Copyright © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

A similar study for Ce³⁺ doped into a silicate matrix has also been presented by Zhu et al.¹⁶ The authors prepared a Sr₃SiO₅:Ce³⁺,Li⁺ phosphor, by a solid state reaction method, and showed that under near-UV excitation at 415 nm, the phosphor emits a bright greenish-yellow color centered at 532 nm (originating from the 5d to 4f (²F_7/2 and ²F_5/2) transition of Ce³⁺). Upon substitution of Sr²⁺ ions with Mg²⁺ (smaller ionic radius than Sr²⁺) and Ba²⁺ (larger ionic radius than Sr²⁺) ions, the peak position of the PL spectrum shifts towards higher and lower energy, respectively. This is because of the fact that as Mg²⁺ ions substitute Sr²⁺ ions the degree of covalence in the Ce–O bonds is decreased, and consequently less negative charge transfers to Ce³⁺ ions, thus increasing the difference between the 4f and 5d levels (causes blue-shift in the emission band). In contrast, the substitution of Sr²⁺ with the larger Ba²⁺ increases the degree of covalence in the Ce–O bonds, which causes red-shift in the emission band. Thus, the emission color of Sr₃SiO₅:Ce³⁺,Li⁺ can be tuned by partial replacement of Sr²⁺ with Mg²⁺ or Ba²⁺, which makes it applicable to a variety of LEDs (from 410–450 nm chips). In another similar article by Shen et al.,¹⁴ the role of Li⁺ has been proposed in the same host in more detail, and explores that the Li⁺ codoping compensates the charge difference between the Ce³⁺ and Sr²⁺ ions and thus helps to incorporate the Ce³⁺ into Sr²⁺ sites. The phosphor (Sr₃SiO₅:Ce³⁺,Li⁺) shows high luminous efficiency under near-UV/blue light excitation, and the obtained emission is comparatively broader than that of the Eu²⁺-activated yellow phosphor. To improve the color rendering properties of the Sr₃SiO₅:Ce³⁺,Li⁺ phosphor, Jang et al.¹⁸ have used additional Pr³⁺ ions (to enhance the red-emitting component) along with Ce³⁺ ions. Under blue light excitation, energy transfer from Ce³⁺ to Pr³⁺ ions takes place, which gives a shoulder at 619 nm (of Pr³⁺ ions) in the PL spectrum of Sr₃SiO₅:Ce³⁺,Pr³⁺,Li⁺. The energy transfer has been proved by steady-state and time domain luminescence measurements. Further, by making use of monodisperse CdSe QDs and the phosphor on a blue LED chip, they produced white LEDs with excellent color rendering properties (see Fig. 1(d) and (e)).

Wang et al.¹⁷ have prepared CaSiN₂ powders at 1550 °C in a N₂/H₂ (6%) atmosphere by the solid-state reaction method using Ca₃N₂ and Si₃N₄ as the starting materials. The luminescence measurements on Ce³⁺/Li⁺ co-doped CaSiN_2−2δ/3O_δ shows a broad yellow band peaking at 530 nm. The maximum PL intensity was attained for the sample with the Ce³⁺ composition x = 0.01, above this concentration quenching occurs. The oxygen content in the sample was analyzed and is found to be δ ≈ 0.2. Further, the chromaticity coordinates (0.362, 0.571) also demonstrate that CaSiN_2−2δ/3O_δ:Ce³⁺/Li⁺ is a good yellow phosphor (see Fig. 1(a)). Hao et al.¹⁰ studied the PL properties of the CaO:Ce³⁺,Li⁺ phosphor. It is found that, upon 474 nm excitation, the Li⁺ ion co-doped phosphor shows a 1.88 fold enhancement in PL intensity (see Fig. 1(c)). The XRD results show the presence of a small amount of CeO₂, a starting material, in the CaO:Ce³⁺ phosphor, which suggests low solubility of Ce³⁺ in CaO. Upon Li⁺ ion co-doping the intensity of this peak decreases, indicating that more CeO₂ is converted to Ce³⁺ ions for effective doping into the CaO host lattice. This results in increased absorption of the excitation photons in CaO:Ce³⁺,Li⁺ causing PL enhancement. This might be because of the charge compensation due to the presence of Li⁺ ions and the effect of Li⁺ as a flux to increase diffusivity of the raw materials, which causes more Ce³⁺ ions to go into CaO lattice. Jia et al.¹⁵ have prepared the Lu₂SiO₅:Ce³⁺,Li⁺ phosphor by Pechini sol–gel chemistry. They optimized the concentration of Ce³⁺ and Li⁺ ions, which was found to be 0.006 wt% and 0.02 wt%, respectively. The PL intensity of 0.006 wt% Ce³⁺, 0.02 wt% Li⁺ co-doped phosphor sample was 2.2 times higher than the 0.006 wt% Ce³⁺ doped phosphor. It is suggested that the Ce³⁺ ions occupy two different crystallographic sites (with coordination number 6 or 7) in the monoclinic lattice (space group C2/c) of Lu₂SiO₅.¹⁴³ This causes two luminescence centers Ce1 and Ce2 in this host. The peaks at 3.16 and 2.95 eV originate from the emission of the Ce1 center, while another peak at 2.74 eV corresponds to the Ce2 center. With the increase of Li⁺ ion co-doping concentration, initially the intensity of the Ce1 emission increases and then decreases, while the Ce2 emission intensity always decreases. This suggests energy transfer from Ce2 to Ce1 centers, which was demonstrated by time-domain measurements. Thus the enhanced luminescence of Lu₂SiO₅:Ce³⁺ phosphors by Li⁺ ion co-doping is mainly regarded as the result of the production of the local distortion in host lattice, which alters the crystal field around activator and results in the energy transfer from Ce2 to Ce1 centers.¹⁵

The effects of Li⁺ ions have also been investigated for persistent/afterglow emission. Kojima et al.⁹ have studied the afterglow luminescence properties of green emitting Ce³⁺ and Pr³⁺ co-doped CaS phosphors. The afterglow time of CaS:Ce³⁺,Pr³⁺ was relatively short (about 10 min). When Li⁺ is introduced in the matrix, the afterglow intensity and afterglow time of CaS:Ce³⁺,Pr³⁺ both increase, by three and four fold, respectively. It is shown that the Li⁺ ions go to the interstitial spaces in the crystal lattice and increase the lattice constant. When a Li⁺ ion is incorporated into the crystal lattice of CaS, a new cation vacancy is formed for the charge compensation in the Ca²⁺ site, which increases the lattice strain. This cation vacancy working as an electron trap can capture some of the excited electrons and thereby increase the charging process. Chen et al.¹¹ have prepared SrAl₂O₄:Eu²⁺,Ce³⁺,Li⁺, another persistent luminescence material, by a solid-state reaction method using H₃BO₃ as flux. Li⁺ ions compensate the charge defects caused by the non-equivalent substitution of Sr²⁺ with Ce³⁺. Thus, the luminescence intensity of SrAl₂O₄:Eu²⁺ is significantly enhanced when Ce³⁺ and Li⁺ both are present in the sample.

4. Effect of Li⁺ ion on the luminescence of lanthanide(III) ions through 4f–4f transitions

4.1. Effect of Li⁺ ion on the luminescence of lanthanide ions in yttrium oxide (Y₂O₃) and gadolinium oxide (Gd₂O₃) host matrices

Y₂O₃ and Gd₂O₃ are well explored host matrices for luminescence studies owing to their higher chemical and mechanical durability, higher thermal stability, excellent optical properties and relatively low phonon cutoff energy (400–500 cm⁻¹) in comparison to other host matrices.^144–150 Another advantage of using Y₂O₃ and Gd₂O₃ host matrices is that one can easily prepare phase pure samples by conventional solid state reaction (at lower temperatures) methods and also by many other routes (such as solution combustion technique, sol–gel technique, hydrothermal technique, etc.).^144–150 Furthermore, particle size, shape and surface morphology can be tuned/controlled easily by controlling different synthesis parameters. Moreover, the ionic radii of Y³⁺ and Gd³⁺ is similar to the other activator lanthanide ions; thus they can easily be accommodated in the matrix.^144–150 Gd₂O₃ exhibits paramagnetic behavior; therefore, the phosphor prepared using this host is important for bio-imaging, MRI contrast reagents, etc.^148–150

Recently, several research groups, including our own, have used Li⁺ ions to further enhance the luminescence of lanthanide ions doped in this matrix.^56–74 Table 3 summarizes the concentrations of different activator (lanthanide) ions and Li⁺ ions to obtain the optimum luminescence. The table also describes the mode of luminescence measurement (e.g. DS, UC), quantitative information about luminescence enhancement and the explanation given by different research groups for the luminescence enhancement. It is evident from the Table 3 that different research groups have proposed different mechanisms for luminescence enhancement of lanthanide activator ions in presence of Li⁺ ions. A detailed description of the work done so far and their salient features are summarized below, including a detailed description of the representative works in the area concerned.

Table 3 Luminescence enhancement (and their explanation) in various activators during down-shifting and upconversion processes in Y₂O₃ and Gd₂O₃ in the presence of Li⁺ ion

Sample	DS/UC	Luminescence enhancement	Remarks
Gd1.84Li0.08Eu0.08O3 (ref. 56)	DS	2.3 fold (PL)	Improved crystallinity, higher surface roughness and increased optical phonon energy
Y1.9Li0.05Eu0.05O3 (ref. 57)	DS	2.5 fold (CL)	Charge compensation
Gd1.92Li0.03Eu0.05O3 (ref. 57)	DS	3 fold (PL)	Lowering of local symmetry
Gd1.9Li0.06Eu0.04O3 (ref. 58)	DS	4 fold (PL)	Charge compensation and lowering of local symmetry
Gd1.84Li0.08Eu0.08O3 (ref. 59)	DS	2.3 fold (PL)	Improved crystallinity and higher surface roughness
Gd1.87Li0.06Yb0.06Tm0.01O3 (ref. 60)	UC	10 fold (PL)	Lowering of symmetry around Tm^3+ and decrease in OH concentration
Gd1.935Li0.04Yb0.02Ho0.005O3 (ref. 61)	UC	10 fold (PL)	Lowering of symmetry around Ho^3+
Y1.935Li0.05Nd0.015O3 (ref. 62)	UC	2 fold (PL)	Changed morphology, modification of local symmetry and decrease in OH concentration
Y1.92Li0.05Yb0.02Er0.01O3 (ref. 63)	UC	30 fold (PL)	Prolong lifetime of their intermediate states
Y1.92Li0.05Yb0.02Er0.01O3 (ref. 64)	UC	25 fold (PL)	Change in crystal field, prolong lifetime of intermediate states, increased optical active sites and dissociation of clusters
Y1.94Li0.04Er0.02O3 (ref. 65)	UC	10 fold (PL)	Change in crystal field and dissociation of clustering.
Y1.94Li0.05Er0.01O3 (ref. 66)	UC	45 fold (PL)	Tailored lifetime of intermediate levels, suppressed cross relaxation and enlarged particle size
Y1.94Li0.05Er0.01O3 (ref. 67)	UC	45 fold (PL)	Tailored lifetime of intermediate levels
Y1.84Li0.08Eu0.08O3 (ref. 68)	DS	1.2 fold (EL)	Improved crystallinity and higher surface roughness
Y1.70Li0.1Eu0.2O3−δ (ref. 69)	DS	1.2 fold (PL)	Improved morphology
Y1.9455Li0.05Yb0.05Tm0.0025O3 (ref. 71)	UC	15 fold (PL)	Change in crystal field, decrease in OH concentration and dissociation of clusters
Y1.9Li0.05Yb0.05O3 (ref. 72)	DS	12 fold (PL)	Change in crystal field
Y1.93Li0.05Tm0.02O3 (ref. 73)	UC	19 fold (PL)	Increase in lifetime of intermediate levels
Y1.917Li0.05Yb0.03Er0.003O3 (ref. 74)	UC	3 fold (PL)	Change in crystal field and decrease in OH concentration

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Dhananjaya et al.⁵⁸ studied the effect of Li⁺ ions on the PL of Eu³⁺ in Gd₂O₃ host. They proposed that when the concentration of Li⁺ ions in the Gd₂O₃ host is low (<4 mol%), substitution of Gd³⁺ ions with Li⁺ ions induces oxygen vacancies. This causes a crystal field distortion around the activator ions (Eu³⁺), which results in increased PL. The oxygen vacancies also act as a sensitizer for activator lanthanide ions, by the involvement of an effective energy transfer from the strong charge transfer states (which come into picture due to the vacancy itself) to lanthanide ions. Further, they explore that as the Li⁺ concentration increases, along with substitutional Li⁺ sites, interstitial sites of Li⁺ ions are also present, which cause Gd³⁺ ion vacancies. This as a result produces more defects around the activator lanthanide ions. In this case the Stark level in the emission spectrum shows larger splitting. In a similar study, Shin et al.⁵⁷ have monitored cathodoluminescence (CL) of Eu³⁺ ions in Gd₂O₃ and Y₂O₃ as host matrices in the presence of Li⁺ ions, and they also proposed the charge compensation mechanism for CL enhancement of Eu³⁺ ions in both hosts, while for the case of Gd₂O₃ host the group additionally put forward other mechanisms also described in the subsequent paragraph.

Along with charge compensation mechanisms, several other mechanisms for the luminescence enhancement have also been reported. The cubic structure of Y₂O₃ and Gd₂O₃ offers two different crystallographic sites for activator lanthanide ions: one with C₂ (without center of inversion) and the other with S₆ (with center of inversion (C_3i)) symmetry.⁵⁷ The transitions in the lanthanide ions are affected effectively, as far as the intensity and splitting is concerned, by the symmetry of the crystal field of the host. For example, when a Eu³⁺ ion occupies S₆ symmetry, the magnetic dipole transition ⁵D₀ → ⁷F₁ is the dominant one. In case of Eu³⁺ doped material the occupancy ratio of two different sites (ORS) i.e. local symmetry around Eu³⁺ ion, can be easily obtained by taking the intensity ratio of ⁵D₀ → ⁷F₂ (∼611 nm) to ⁵D₀ → ⁷F₁ (∼590 nm) transitions.⁵⁷ Ionic radii of Y³⁺, Gd³⁺ and Li⁺ ions are 0.89, 0.94 and 0.76 Å, respectively.^57,58 Thus, Li⁺ ion substitution deforms the Gd₂O₃ lattice more as compared to Y₂O₃ lattice, and the statistical distribution of lanthanide (Eu³⁺) and Li⁺ ions at the C₂ and S₆ sites in the Gd₂O₃ lattice becomes more intricate. Shin et al.⁵⁷ have also reported that changes in ORS after Li⁺ ion incorporation is larger in Eu³⁺ doped Gd₂O₃ than the Y₂O₃ matrix. Based on this information, Shin et al. proposed that the mechanism of luminescence enhancement in Li⁺ ion doped Gd₂O₃ might be different to that in the Y₂O₃ host. X-ray diffraction (XRD) patterns (shown in Fig. 2) recorded by Shin et al.⁵⁷ reveal that Y_1.95−xLi_xO₃:Eu_0.05 phosphor samples have cubic structure even at higher doping concentration of Li⁺ ions. In contrast, Gd_1.95−xLi_xO₃:Eu_0.05 phosphor samples show monoclinic to cubic transformation as the doping concentration of Li⁺ increased. This gradual change from monoclinic to cubic lattice is also confirmed by Dhananjaya et al.⁵⁸ This change from monoclinic to cubic lattice in Gd₂O₃ host is also one of the reasons for increased luminescence.^57,58

Fig. 2 X-ray diffraction (XRD) patterns of Y_1.95−xLi_xO₃:Eu_0.05 and Gd_1.95−xLi_xO₃:Eu_0.05 phosphor. Reprinted (adapted) with permission from ref. 57 Copyright © 2005 Elsevier B. V. All rights reserved.

Fan et al.⁶² have reported the effect of Li⁺ ion in enhancing the PL of Nd³⁺ ions doped into Y₂O₃ samples. They proposed that among two different crystallographic sites (C₂ and C_3i symmetry, the ratio of C₂ to C_3i sites is 3 : 1) of cubic Y₂O₃, electric dipole transitions of lanthanide ions are forbidden for C_3i sites. Thus, when Li⁺ ions occupy the C_3i sites, it destroys the inversion symmetry and the forbidden electric dipole transitions of lanthanide ions (Nd³⁺) become partially allowed. Further, when Li⁺ ions occupy the C₂ sites with higher population in the lattice, the reduced symmetry of these sites again results in the enhancement of PL of lanthanide ions (Nd³⁺). Thus, in both cases enhanced PL is observed. When the Li⁺ ion concentration is too high it might cause a large local distortion around optically active centers (lanthanide ions), which can lead to the PL quenching.

In similar context, several studies have also been reported for the enhancement of UC emission. A study by Sun et al.⁶⁰ reports the UC phenomenon in a silica coated Tm³⁺/Yb³⁺/Li⁺ ion doped Gd₂O₃ matrix, and found ∼10 fold enhancement of UC emission in the sample doped with 6 mol% of Li⁺. They have suggested that the enhancement in PL is due to a change in the local asymmetry around Tm³⁺ ions. On further increasing the Li⁺ ion concentration (up to 10 mol%) the UC emission decreases substantially. They have reported that at higher Li⁺ ion concentrations, the local crystal field around Tm³⁺ ions yet again becomes symmetric, which is unfavorable for UC emission. Chen et al.⁶⁴ (Yb³⁺/Er³⁺), Bai et al.⁶⁵ (Yb³⁺/Er³⁺), Li et al.⁷¹ (Yb³⁺/Er³⁺), Fan et al.⁷² (Yb³⁺) and Mishra et al.⁷⁴ (Yb³⁺/Er³⁺) have also studied the effect of Li⁺ ions on the enhancement of UC/PL of lanthanide ions in Y₂O₃ and Gd₂O₃ host matrices and suggested the change in crystal symmetry around the lanthanide ions as one of the reasons for PL enhancement.

The other contributing factors in the PL enhancement of lanthanide ions in these hosts are change in morphology, crystallinity and grain size of materials.^56,59,68,69 Atomic force microscopy (AFM), which is used to study the surface morphology, results reported by Jeong et al.⁵⁶ shows that Li⁺ ion doping increases the grain size and roughness of the Gd₂O₃:Eu³⁺ films. Larger grain size reduces the density of the grain boundaries, which in turn might be responsible for reduced adsorption and/or scattered light generated inside the film, which favors PL enhancement. Furthermore, increased crystallinity due to Li⁺ ion doping in samples causes higher oscillating strength for optical transitions, which also favors PL enhancement.⁵⁶ Yi et al.⁵⁹ (Gd₂O₃:Eu³⁺), Fan et al.⁶² (Y₂O₃:Nd³⁺), Yi et al.⁶⁸ (Y₂O₃:Eu³⁺) and Sun et al.⁶⁹ (Y₂O₃:Eu³⁺) have also proposed that increased grain size, roughness and crystallinity of materials, in the presence of Li⁺ doping, is among one of the potential reasons for luminescence enhancement of lanthanide ions.

Time-domain studies have also been exploited to search for the possible reasons behind the enhancement of PL/UC emission. It has been observed that changes in lifetime of intermediate state/states participating in energy transfer process also play a crucial role. Chen et al.⁶⁴ suggested that due to changes in the local crystal field of the Y₂O₃ matrix around the lanthanide ions, observed lifetimes of intermediate states, ⁴I_11/2 (Er³⁺) and ²F_5/2 (Yb³⁺), can change. This can also modify the theoretical lifetimes of lanthanide ions by slightly changing their wavefunctions. They monitored decay profiles of ⁴I_11/2 → ⁴I_15/2 transitions of Er³⁺ ions (at 1015 nm) in Y₂O₃ nanocrystals in the presence of 0–15 mol% Li⁺ ions (see Fig. 3(d)).^66,67 The lifetime of the intermediate state ⁴I_11/2 was found to be 0.8(2) ms and 2.7(3) ms for 0 and 3 mol% Li⁺ ions, respectively, and it is about 3.4(3) ms for higher Li⁺ ions concentrations (5–15 mol%). This suggest that the local crystal field around Er³⁺ ions is gradually tailored for lower doping concentration of Li⁺ ions (0–5 mol%) and becomes nearly constant for higher Li⁺ ion concentrations. The enhancement in UC emission has the same trend as the lifetime increases. Fig. 3(a) and (b) show UC emission spectra of Er³⁺ doped Y₂O₃ in the absence and presence of Li⁺ ions. In an another work, Fan et al.⁷³ monitored the decay profiles of ⁴I_13/2 → ⁴I_15/2 transition (at 1530 nm) of Er³⁺ ions in Y₂O₃ matrix in the presence of 0–6 mol% of Li⁺ ions under 976 nm excitation. They also observed an increase in the lifetime with increasing concentration of doped Li⁺ ions; the lifetime varies from 3.14 ms to 3.28 ms for 5 mol% Li⁺ ions. The study concludes and proposes that the Li⁺ ion tailors the local crystal field around the Er³⁺ ion, and therefore modifies the theoretical lifetime by slightly changing the wavefunction, somewhat similar to the earlier reports.

Fig. 3 Upconversion emission in (a) ultraviolet-blue, and (b) green-red regions in Y₂O₃ nanocrystals doped with 1 mol% Er³⁺ ions and co-doped with 0 or 5 mol% Li⁺ under 970 nm diode laser excitation. (c) Fourier transform infrared spectra of Er³⁺/Yb³⁺:Y₂O₃ doped with 0 or 5 mol% Li⁺ ions. (d) Decay profiles of the ⁴I_11/2 → ⁴I_15/2 transition of Er³⁺ in Y₂O₃ nanocrystals doped with 1 mol% Er³⁺ ions and 0, 3, 5, 7, 10, or 15 mol% Li⁺ ions. The inset shows the fluorescence spectrum of the ⁴I_11/2 → ⁴I_15/2 transition in the range of 1000–1050 nm under 970 nm diode laser excitation. (a) and (b) Reprinted (adapted) with permission from ref. 67. Copyright © 2008 Elsevier Ltd. All rights reserved. (c) Reprinted (adapted) with permission from ref. 74. Copyright © 2013 Elsevier Ltd. All rights reserved. (d) Reprinted with permission from ref. 66. Copyright© 2008, AIP Publishing LLC.

Furthermore, some of the studies interestingly report that the doping of Li⁺ ions suppresses the PL quenching entities e.g. molecules with OH⁻, NO₃⁻, CO_x, etc. groups having high vibrational frequencies. During phosphor synthesis by chemical routes (viz. combustion, sol–gel methods) incomplete combustion of organic fuel introduces luminescence quenching centers, which decrease the PL intensity. Sun et al.⁶⁰ (Gd₂O₃:Tm³⁺/Yb³⁺), Fan et al.⁶² (Y₂O₃:Nd³⁺), Li et al.⁷¹ (Y₂O₃:Tm³⁺/Yb³⁺) and Mishra et al.⁷⁴ recorded FTIR (Fourier transform infrared) spectra of phosphor materials without and with Li⁺ ion doping in various hosts and found that the broad absorption bands peaking at 1626 and 3454 cm⁻¹, owing to vibrational features of O–H groups, are suppressed significantly in the presence of Li⁺ ions (see Fig. 3(c)).⁷⁴ In addition, Mishra et al.⁷⁴ have also reported that the broad peak at ∼1369 cm⁻¹, due to stretching vibrations of NO in surface-adsorbed NO₃⁻ groups, is also suppressed in presence of Li⁺. This might be due to the fact that the Li⁺ ion neutralizes the hydroxyl (and other) groups present in the phosphor. Thus, the addition of Li⁺ ions reduces the non-radiative transition probability and hence enhances the PL efficiency.

Thus in summary, Li⁺ ion doping in Gd₂O₃ and Y₂O₃ hosts significantly affects both the structural and optical parameters such as charge compensation, change in crystal symmetry around activator ions, increased crystallinity, increased grain size, change in morphology (higher surface roughness), increase of lifetime of intermediate levels of activator ions, decrease in non-radiative channels (OH⁻, NO₂⁻ concentrations), etc., which altogether enhances the optical emission of lanthanide ions.

4.2. Effect of alkali ions on PL enhancement of lanthanide ions in ABO₄ type of compounds

Next to Y₂O₃ and Gd₂O₃ host matrices, another important class of compounds for PL studies is ABO₄ (where A is an alkaline earth metal or trivalent lanthanide ion and B is a hexavalent or pentavalent transition metal ion) family. There are some well known hosts in this family of compounds, viz. CaMoO₄, YVO₄, GdVO₄, YNbO₄, YPO₄, CaWO₄, MgWO₄, ZnWO₄, etc., which have attracted enormous interest in preceding years.^{83–85,151–154} The characteristic feature of these compounds is the existence of a strong ligand (oxygen) to metal charge transfer band in UV region.^151–154 Under this charge transfer band excitation they exhibit broadband PL, mostly in the blue region. An additional feature of this type of compound is that they serve as sensitizers for the activator lanthanide ions by involving an energy transfer from charge transfer state to the resonant level of lanthanide ions, so they do not need a co-dopant for the sensitization process.²⁴ They are stable at high temperatures. Lanthanide doped YVO₄ has already been explored for the fabrication of lasers. Recently, different research groups across the world have been studying the role of Li⁺ on the PL enhancement of lanthanide ions in these hosts (along with other host matrices).^75–88 Table 4 gives some brief information about different hosts in this family, activator ions, maximum PL enhancement and proposed mechanisms to explain the PL enhancement. The following discusses different mechanisms.

Table 4 Luminescence enhancement (and their explanation) in various activators during down-shifting and upconversion processes in ABO₄ type of host in presence of Li⁺ ions

Sample	DS/UC	Luminescence enhancement	Remarks
Ca0.8Li0.1Er0.02Yb0.08MoO4 (ref. 76)	UC	83 fold (PL)	Local crystal field distortion around Er^3+ ion
Ca0.95MoO4:0.05Dy^3+,0.05Li^+ (ref. 77)	DS	1.3 fold (PL)	Charge compensation
Ca0.95MoO4:0.05Tb^3+,0.05Li^+ (ref. 78)	DS	— (PL)	Oxygen vacancy
Li0.05Eu0.05La0.9PO4 (ref. 79)	DS	2 fold (PL)	Decrease in interstitial oxygen and reduced internal reflections
La0.93Eu0.07VO4:0.25 wt% Li^+ (ref. 80)	DS	— (PL)	Oxygen vacancy, increased grain size
YPO4:2 at% Dy,7 at% Li^+ (ref. 81)	DS	— (PL)	Improved crystallinity
YPO4:5% Eu^3+,5% Li^+ (ref. 82)	DS	2.5 fold (PL)	Increased crystallinity and grain size
YPO4:5% Eu^3+,3% Li^+ (ref. 83–85)	DS	— (PL)	Increased crystallinity
YVO4:0.03% Eu^3+,2% Li^+ (ref. 86)	DS	1.43 (PL)	Increased crystallinity and surface roughness
YVO4:0.03% Eu^3+,1% Li^+ (ref. 87)	DS	1.7 (PL)	Increased crystallinity and surface roughness

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Compounds in this family usually have relatively larger phonon cutoff ranges (∼800 cm⁻¹) than Y₂O₃ and Gd₂O₃ host matrices, and therefore, for achieving efficient UC emission, sensitization becomes utmost essential. Chung et al.⁷⁶ have studied the role of Li⁺ ions on UC emission of the Er³⁺/Yb³⁺ doped CaMoO₄ phosphor. They observed ∼83 fold enhancement in the green emission of Er³⁺ ions in the presence of 10 mol% of Li⁺ ion (see Fig. 4(b)). They attributed it as being due to a favorable structural modification (change in local crystal field symmetry) in the host lattice. The XRD patterns reveal that CaMoO₄ phosphor samples containing 2 mol% Er³⁺, 8 mol%Yb³⁺ and 0–15 mol% Li⁺ ion do not show any secondary phases. The diffraction peak (112) shifts gradually toward higher angle up to 5 mol% Li⁺ ion doping; however contrary to this, upon further increasing Li⁺ ion concentration it shifts toward smaller angles (see Fig. 4(a)). The gradual peak shift toward higher angle below 5 mol% suggests that the Li⁺ ions occupy the Ca²⁺ sites in lattice. However, upon further increasing the concentration of Li⁺ ions, the ions go to interstitial sites. This leads to the expansion of lattice. The occupation of Li⁺ ions in the interstitial sites distorts the local crystal field around Er³⁺ ions, and plays an important role in the enhancement of UC emission.

Fig. 4 (a) X-ray diffraction patterns of Li⁺/Er³⁺/Yb³⁺ tri-doped CaMoO₄ upconversion phosphors (2 mol% of Er³⁺ and 8 mol% of Yb³⁺ and 0–15 mol% Li⁺) near 2Θ = 29° for (112) peak. (b) Photoluminescence spectra of CaMoO₄ phosphors fixed values of 2 mol% of Er³⁺ and 8 mol% of Yb³⁺ with various Li⁺ concentrations from 0 to 15 mol%. A comparison of the room temperature (c) photoluminescence excitation (PLE) and (d) PL spectra of YVO₄:Eu³⁺ ceramics with different Li⁺ ion content. (e) High resolution transmission electron microscopy image of the 500 °C annealed sample of 5 at% Li⁺ co-doped YPO₄:5Eu. (a) and (b) Reprinted (adapted) with permission from ref. 76. Copyright © 2012 Elsevier B. V. All rights reserved. (c) and (d) were published in ref. 86 and are reprinted with permission. Copyright © 2010 Elsevier Masson SAS. All rights reserved. (e) is taken from ref. 84; used in accordance with the Creative Commons Attribution 3.0 Unported License.

Li et al.^77,78 have also studied the effect of Li⁺ ions in the same host, CaMoO₄, but they have used Dy³⁺ and Tb³⁺ as the activator ions instead of Er³⁺. To explain the PL enhancement for Dy³⁺ doped CaMoO₄ in the presence of Li⁺ ions, they proposed that as the ionic radius in the case of the Dy³⁺ ion (0.091 nm) is similar to that of the Ca²⁺ ion (0.099 nm), it can easily be accommodated into the Ca²⁺ site. However, the Li⁺ ion, which has a considerably small radius (0.076 nm), will prefer to go into the interstitial spaces. The replacement of divalent calcium by trivalent dysprosium creates charge imbalance, and this imbalance is compensated by Li⁺ ion co-doping, which enhanced the PL intensity significantly. Further, in another work, Li et al.⁷⁸ extend their observation for the effect of Li⁺, Na⁺ and K⁺ on the PL enhancement of Tb³⁺ co-doped CaMoO₄ phosphors. A remarkable increase not only in PL, but also for X-ray excited luminescence is also attained for all three alkalis. The maximum PL was observed for 5 mol% Tb³⁺ and Na⁺ doped phosphor. The study proposes that the enhanced PL is due to the creation of oxygen vacancies after the occupation of Ca²⁺ sites by alkali ions.

Phosphor samples (LaVO₄:Eu³⁺) prepared by Park et al.⁸⁰ have both tetragonal and monoclinic phases. Of these two, the tetragonal phase of LaVO₄:Eu³⁺ showed higher PL than the monoclinic phase. Incorporation of Li⁺ (0.25 wt%) ions into the LaVO₄:Eu³⁺ (0.07 mol%) phosphor increases the PL remarkably. This is due to the fact that the presence of Li⁺ ions helps in the monoclinic to tetragonal phase transformation, which causes effective energy transfer from VO₄³⁻ to Eu³⁺. Yang et al.^86,87 prepared Li-doped (0 to 3 mol%) YVO₄:Eu³⁺ phosphors and observed that as the Li⁺ ion content increases from 0 to 2 wt%, the PL intensity was improved. The PL intensity of the 2 wt% Li-doped YVO₄:Eu³⁺ phosphor is ∼1.43 times greater than that of YVO₄:Eu³⁺ ceramic (see Fig. 4(c)). They proposed that the enhanced PL is due to improvement in crystallinity as well as due to the enhanced surface roughness.

Parchur et al.^81,83–85 in our group have also studied the effect of Li⁺ ions on PL properties of Dy³⁺ and Eu³⁺ doped YPO₄ phosphors. The Li⁺ ion co-doping improves the crystallinity of the material and reduces agglomeration among the particles. Upon addition of Li⁺ ions, the shift in the (200) diffraction peak towards a lower 2Θ value suggests that the Li⁺ ions occupy interstitial sites instead of Y³⁺/Eu³⁺ sites. Furthermore, the Li⁺ ion co-doping increases the crystallite size as well (crystallite size corresponding to Li⁺ = 0, 3, 5, 7 and 10 at% co-doped YPO₄:Eu³⁺ are found to be 20, 23, 36, 42 and 60 nm, respectively). In the high resolution transmission electron microscopy (HRTEM) image (see Fig. 4(e)) of Li⁺ and Eu³⁺ co-doped YPO₄ phosphor clear, damage free, visible lattice fringes indicate good crystallinity of the sample. This increased crystallinity and grain size causes significant enhancement in PL. Huang et al.⁸² have also proposed that the increased crystallinity is the main cause of PL enhancement in the Li⁺ and Eu³⁺ co-doped YPO₄ phosphor (2.5 fold enhancement in 5% Li⁺ and 5% Eu³⁺ co-doped YPO₄ compared with YPO₄:5%Eu³⁺).

Thus in summary, in Li⁺ doped ABO₄ type of compounds, the main mechanism involved in the PL enhancement is the change in local crystal field symmetry by modification of the crystal lattice, mostly due to interstitial occupancy of Li⁺ ions and development of vacancies. In addition to this, improvement in crystallinity, grain size, shape, etc. and energy transfer processes also have been accepted as potential mechanisms, as in other hosts too.

4.3 Effect of alkali ions on PL enhancement of lanthanide ions in calcium and strontium aluminates

Like Y₂O₃ and Gd₂O₃, calcium and strontium aluminates also have low phonon frequency, which makes them suitable for lanthanide ion doping.^150–160 Lanthanide doped phosphor materials prepared using these hosts exhibit good chemical stability and mechano-luminescence properties. Furthermore, aluminates are excellent hosts for the development of long persistence phosphors (CaAl₂O₄:Eu²⁺,Dy³⁺ and CaAl₂O₄:Ce³⁺), which makes them useful for display applications, delayed fluorescence, warning signals, emergency lighting and for other luminous products.^150–160 Very recently, a few publications have appeared on lanthanide doped aluminate hosts with Li⁺ ion co-doping that show an enhanced PL and decay time.^89–91

Recently, Guanghuan et al.⁸⁹ studied the effect of alkali ions (Li⁺, Na⁺, K⁺) on PL characteristics (both steady state and time-domain studies) of Eu³⁺ doped in CaAl₂O₄ phosphors. Optimum emission was obtained for 3 mol% of Eu³⁺. Addition of alkali ions was found to enhance the PL intensity significantly. The hypersensitive transition ⁵D₀ → ⁷F₂ evolved as the most intense peak, which was explained as Eu³⁺ ions occupying a low symmetry site. Since the ionic radii of Eu³⁺ (0.095 nm) is closer to that of Ca²⁺ (0.099 nm) than Al³⁺ (0.051 nm), it prefers to occupy the Ca²⁺ site rather than Al³⁺ site. It is suggested that the co-doping of alkali ions works as a charge compensator, induces a lattice distortion and lowers the lattice symmetry, which altogether favors the enhanced emission intensity of the ⁵D₀ → ⁷F₂ transition. The luminescence decay analysis of the CaAl₂O₄:Eu³⁺,Li⁺ phosphor, monitored at 615 nm under 254 nm excitation, reveals a long decay time: luminescence lasts for over 10 ms. It is also noted that the emission intensity is gradually enhanced as the ionic radii of the alkali ion decreases, i.e. minimum for K⁺ and maximum for Li⁺ ion. This has been explained as being due to a change in ionic radii of the alkali ions that influence the local structure around the luminescent center ions. This also influences the spin–orbit couplings and crystal field around the Eu³⁺ ions. The excitation spectra of CaAl₂O₄:Eu³⁺,R⁺ (Li, Na and K) shown in Fig. 5(a)⁸⁹ depict a broad band in the range of 200–300 nm, which is due to the charge transfer state from O²⁻ to Eu³⁺ ions. The incorporation of alkali ions in the CaAl₂O₄:Eu³⁺ phosphor results in increased CTS band excitation, which might be due to change in Eu–O distances by alkali ion doping.

Fig. 5 (a) Excitation and emission spectra of CaAl₂O₄:Eu³⁺,R⁺ (R = Li, Na, K) under 254 nm excitation and monitored at 615 nm. (b) Upconversion emission spectra of SrAl₂O₄:0.01Ho³⁺,0.20Yb³⁺,yLi⁺. Inset shows the picture of the emission of SrAl₂O₄:0.01Ho³⁺,0.20Yb³⁺,0.06Li⁺ ceramics under excitation wavelength 980 nm. (a) Reprinted (adapted) with permission from ref. 89. Copyright © 2010 Elsevier. All rights reserved. (b) Reprinted (adapted) with permission from ref. 90. Copyright © 2012 Elsevier B. V. All rights reserved.

Tang et al.⁹⁰ prepared Ho³⁺/Yb³⁺/Li⁺ co-doped SrAl₂O₄ phosphors and studied the effect of Li⁺ ion co-doping on the UC emission of Ho³⁺ ions. In the first step, the concentration of Yb³⁺ ions has been optimized to obtain maximum UC emission, and is found to be 0.20 mol fraction. It has been observed that a variation in Yb³⁺ content brings out significant structural changes in the host lattice. The XRD patterns reveal a pure monoclinic phase of SrAl₂O₄ for Yb³⁺ concentration <0.04 mol fraction, above which two new (minor) phases, namely YbAlO₃ and Yb₂O₃, were also identified. Upon increasing the Yb³⁺ concentration further, the diffraction peaks of the SrAl₂O₄ lattice shift towards higher angle, which is attributed due to the substitution of Sr²⁺ (1.12 Å) by Yb³⁺ (0.86 Å) with a comparatively smaller ionic radius. The presence of Li⁺ in the next step enhances the UC emission intensity significantly. An enhancement in green UC of up to six fold emission was found for Li⁺ ion concentration 0.06 (see Fig. 5(b)). This has been proposed both due to charge compensation and grain growth. Due to the low melting point of the lithium compound (Li₂CO₃, 723 °C), it may work as a flux to accelerate grain growth in the SrAl₂O₄. In a similar study, Chen et al.⁹¹ have prepared a Ce³⁺, Eu²⁺, Li⁺ co-doped SrAl₂O₄ phosphor. They found that the doping of Ce³⁺ and Li⁺ ion enhances the PL intensity of SrAl₂O₄:Eu²⁺ significantly.

4.4 Effect of alkali ions on PL enhancement of lanthanide ions in perovskite host matrices

Oxides with perovskite structure (chemical formula ABO₃) have large technological importance. They exhibit good piezoelectric, ferroelectric and magnetic properties.^161,162 Recently, due to their high dielectric constant, high charge storage capacity, good insulating property, good chemical and physical stability and relatively low phonon energy (∼700 cm⁻¹) these materials are being used for luminescence applications. Both UC and DS emission of lanthanide ions doped in perovskite hosts have been reported.^163,164 Further, the effect of Li⁺ ions in enhancing the luminescence efficiency of lanthanides has also been investigated.^92–97 Table 5 summarises the effect of alkali ions on PL enhancement and the possible mechanism for this.

Table 5 Luminescence enhancement (and their explanation) in various activators during down-shifting and upconversion processes in perovskite host in presence of Li⁺ ions

Sample	DS/UC	Luminescence enhancement	Remarks
BaTiO3:2%Er^3+,3%Li^+ (ref. 92)	UC	56 fold	Local crystal field distortion around Er^3+, reduction in OH concentrations
BaTiO3:1%Er^3+,5%Yb^3+,7%Li^+ (ref. 93)	UC	10 fold	Local crystal field distortion around Er^3+ ions
CaTiO3:Pr^3+,1%Li^+ (ref. 94 and 95)	UC	3.5 fold	Improved crystallinity, increase in surface roughness
CaZrO3:5%Eu^3+,10%Li^+ (ref. 96)	DS	3 fold	Charge compensation
SrZnO3:Eu^3+,M^+ (Li, Na, K)^97	DS	—	Charge compensation

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Sun et al.⁹² have prepared BaTiO₃ nanocrystals doped with Er³⁺ ions, and the effect of Li⁺ ion co-doping to enhance the PL of Er³⁺ has been investigated. By using 3 mol% Li⁺ ions they observed a 56 fold enhancement (see Fig. 6(c)). The authors attributed this enhancement to a change in crystal field symmetry around the Er³⁺ ion, which has been established through XRD measurements. XRD peaks show a gradual shift with addition of Li⁺ ions up to 3 mol%, while above 3 mol% doping of Li⁺ ions a shift in XRD peak position is not observed. Authors suggest that since the Li⁺ ion has a small ionic radius, it can fit in any crystal site, such as substituting the Ba²⁺ ion or occupying the interstitial sites. Substitution of divalent Ba by Li⁺ induces an oxygen vacancy in the matrix. In both the cases it is apparent that the local crystal field around the Er³⁺ ion will be changed. Another possibility of the Li⁺ ion doping at the Ti⁴⁺ site has been discarded in the study because of the large charge difference between them. Furthermore, it has also been noted that the addition of Li⁺ ions neutralizes OH groups and so also decreases the non-radiative channels. Thus, the enhanced PL has been attributed to all these factors. In another work on Er³⁺/Yb³⁺ co-doped BaTiO₃ by Chen et al.,⁹³ time domain studies have also been included to understand the PL enhancement on Li⁺ ion co-doping. The decay profiles of the ⁴S_3/2 → ⁴I_15/2 (at 548 nm) and ⁴F_9/2 → ⁴I_15/2 (at 660 nm) transitions have been monitored in different compositions of the doped ions e.g. BaTiO₃:1% Er³⁺ and 5%Yb³⁺ and BaTiO₃:1% Er³⁺ and 5% Yb³⁺,7% Li⁺ nanocrystals (see Fig. 6(d)). They found that the lifetime of the ⁴S_3/2 state of BaTiO₃:1% Er³⁺ and 5% Yb³⁺,7% Li⁺ nanocrystals (104.4 μs) is longer than BaTiO₃:1% Er³⁺ and 5% Yb³⁺ nanocrystals (97.5 μs). This has been attributed due to a decrease in non-radiative channels (OH group concentration) after Li⁺ ion doping.

Fig. 6 (a) Excitation and (b) emission spectra of CaTiO₃:Pr³⁺ with different concentrations (0.5, 1, 2, and 5 wt%) of Li⁺ ion co-doped CaTiO₃:Pr³⁺ phosphors. (c) Upconversion emission spectra of BaTiO₃:2Er³⁺ co-doped with 1–6 mol% Li⁺ ions, under 976 nm laser excitation. The inset shows integral intensity of green and red emission as a function of the concentration of Li⁺. (d) Decay profiles of the ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions of Er³⁺ ions in BaTiO₃:1% Er³⁺,5%Yb³⁺ and BaTiO₃:1%Er³⁺,5%Yb³⁺,7%Li⁺ under 976 nm diode laser excitation. SEM images of (e) CaTiO₃:Pr³⁺ (f) 0.5 wt%, (g) 1 wt%, and (h) 2 wt%, Li-doped CaTiO₃:Pr³⁺ phosphors. (a and b) and (e–h) are published in ref. 94 and reprinted with permission. Copyright © 2011 Elsevier Masson SAS. All rights reserved. (c) Reprinted (adapted) with permission from ref. 92. Copyright © 2011 Elsevier B. V. All rights reserved. (d) Reprinted with permission from ref. 93. Copyright © 2010 Elsevier B. V. All rights reserved.

Yang et al.^94,95 prepared CaTiO₃:Pr³⁺ phosphors with different concentrations of Li⁺ ions (0.5 to 5.0 wt%). The maximum PL enhancement (∼3.5 times) was observed for the 1.0 wt% Li⁺ co-doped CaTiO₃:Pr³⁺ phosphor. The excitation spectrum observed by monitoring emission at 613 nm (see Fig. 6(a)) shows a band at 330 nm, which has been assigned as the transition from the valence band to the conduction band, O²⁻ (2p) → Ti⁴⁺ (3d). The other band at 380 nm has been attributed to the charge transfer (CT) from Pr³⁺ to metal ions, Pr³⁺/Ti⁴⁺ ↔ Pr⁴⁺/Ti³⁺. The emission spectrum recorded upon excitation with the charge transfer (CT) band shows high brightness (see Fig. 6(b)). The PL brightness first increases up to 1.0 wt% of Li⁺ then starts to decrease. The reason behind this has been explored by using XRD and SEM characterization techniques. The XRD analysis reveals that upon addition of Li⁺ ions up to 1.0 wt%, crystallinity increases, while above this, it starts decreasing. Also, for Li⁺ concentration of more than 2 wt% some impurity phase was also detected. The SEM images (see Fig. 6(e)–(h)) of CaTiO₃:Pr³⁺ doped with different concentrations of Li⁺ reveal that the grains of Li-doped CaTiO₃:Pr³⁺ phosphors are highly packed and have larger grain sizes than CaTiO₃:Pr³⁺. On the basis of these results, it has been concluded that the high density of packing and improved crystallinity leads to higher oscillating strengths for the optical transitions. Also, the addition of Li⁺ ions creates oxygen vacancies. Due to strong mixing of charge transfer states, the oxygen vacancies act as sensitizers for the energy transfer from host to Pr³⁺ ion. These factors together are responsible for enhanced photoluminescence of activator ions.

Du et al.⁹⁶ prepared Eu³⁺ and Li⁺ co-doped CaZrO₃ nanocrystals. The optimum doping level of Eu³⁺ is 5 mol%, above which the PL start decreasing due to concentration quenching. The closed packed structure of CaZrO₃ does not have space to accommodate interstitial atoms. Eu³⁺ ions substitute at Ca²⁺ sites. Because of the substitution of Ca²⁺ by Eu³⁺ there would be some charge difference. The addition of 10 mol% of Li⁺ ion into the CaZrO₃ lattice enhances the PL three fold and further increase in Li⁺ concentration decreases the PL intensity. The enhancement in the PL intensity has been attributed to the lattice distortion produced after Li⁺ ion doping. The lattice distortion reduces the symmetry around Eu³⁺ ions and increases the PL intensity of the ⁵D₀ → ⁷F₂ transition. Yu et al.⁹⁷ have prepared a series of luminescent materials, SrZnO₂:Eu³⁺,M⁺ (M = Li, Na, K). As the ionic radius of Eu³⁺ is closer to Sr²⁺ than Zn²⁺, it would prefer to occupy the Sr²⁺ site. The substitution of Eu³⁺ for Sr²⁺ results in charge imbalance. The incorporation of alkali metal ions enhanced the PL intensity, which has been attributed due to charge compensation.

4.5 Effect of Li⁺ ion on PL enhancement of lanthanide ions in metal oxide nanoparticles

Recent years have witnessed enormous interest in semiconductor nanocrystals due to their potential applications in photocatalysis, solar cells and various other optoelectronic devices.^165–167 Many research groups are working to further enrich the optical and other properties of these semiconductors by combining lanthanides with them. Out of a large number of semiconductor materials available, ZnO, ZnS, TiO₂, MgO and SnO₂ semiconductors have been widely used as hosts for lanthanide doping due to their large band gap, low phonon energy, excellent physical and chemical stability and environmentally friendly nature.^168–170 These semiconductors are very frequently used in electroluminescent devices. However, due to mismatch between the ionic radius of lanthanide ions (usually trivalent for optical applications) and Zn²⁺, Ti⁴⁺, Mg²⁺, Sn⁴⁺ metal ions, it is difficult to accommodate lanthanide ions in semiconductor matrices. Therefore, lanthanide doping creates charge imbalance and vacancies, which affect the luminescence yield significantly. Researchers are using Li⁺ ions to modify semiconductor host matrices and compensate the charge imbalance so as to improve the luminescence intensity.^98–105 Table 6 summarizes the effect of Li⁺ ions on PL properties of various lanthanide ion doped semiconductor host matrices.

Table 6 Luminescence enhancement (and its explanation) in different activators during down-shifting and upconversion processes in metal oxide nanoparticle hosts in the presence of Li⁺ ions

Sample	DS/UC	Luminescence enhancement	Remarks
MgO:1.5%Dy^3+,20%Li^+ (ref. 98)	DS	4 fold (PL)	Improved crystallinity, increased grain size, change in size, change in crystal field around Dy^3+ ions
SnO2:Eu^3+,Li^+ (ref. 99)	DS	14 fold (PL)	Improved crystallinity, increased grain size, increased oxygen vacancies
TiO2:Er^3+,Yb^3+,Li^+ (ref. 101)	UC	215 fold (PL)	Change in local crystal field around Er^3+ ion
ZnO:Er^3+,Li^+ (ref. 102)	UC	PL switching (red to green)	Change in local crystal field around Er^3+ ion, increased crystallinity
ZnO:Dy^3+,Li^+ (ref. 103)	DS	10 fold (PL)	Radiative recombination of the large amount of trapped carriers excited from ZnO host, improved crystallinity
ZnO:Er^3+,Li^+ (ref. 104)	UC	120 fold (PL)	Change in local crystal field around ions, reduced OH group concentration
ZrO2:Er^3+,Yb^3+,Li^+ (ref. 105)	UC	1.5 fold (PL)	Change in local crystal field around ions, increased grain size

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Gu et al.⁹⁸ have prepared Dy³⁺ and Li⁺ co-doped MgO nanocrystals. Li⁺ ion co-doping enhances the emission of Dy³⁺ ions, which has been concluded to be due to improved crystallinity and grain size and a change in symmetry around Dy³⁺ ions. The first conclusion has been drawn on the basis of XRD and TEM measurements, while the ratio of intensity of the magnetic and electric dipole allowed transitions (⁴F_9/2 → ⁶H_15/2 and ⁴F_9/2 → ⁶H_13/2 transitions) of Dy³⁺ has been calculated to give an idea of the site symmetry around the dysprosium ions. The XRD studies reveal a pure cubic structure for MgO:Dy³⁺ and Li⁺ co-doped samples. The incorporation of Li⁺ ions increases the crystallinity of the MgO nanocrystals. TEM images of MgO crystals exhibit cuboid lamellar morphology with a mean particle size of ∼20 nm, which remains unchanged upon addition of Dy³⁺ ions. However, in presence of Li⁺ ions, the crystallite size increases to 30–40 nm. They proposed that during the combustion process Li₂O may melt/or react with MgO and Dy₂O₃ and thus form a eutectic liquid, which helps to improve the crystallinity. The ⁴F_9/2 → ⁶H_15/2 and ⁴F_9/2 → ⁶H_13/2 transitions of Dy³⁺ ion are known as magnetically allowed (hardly vary with the crystal field strength around the dysprosium ion) and forced electric dipole transitions (being allowed only at low symmetries with no inversion center), respectively. The ratio of the intensities between the electric dipole and the magnetic dipole gives information about the site symmetry in which the dysprosium ion is situated. Under excitation at 273 nm, the asymmetry ratio of the Dy³⁺ doped MgO sample is 1.3, whereas for the 20% Li⁺ co-doped sample, it found to be 1.5 and also the PL increases four fold (see Fig. 7(a)). The variation of the asymmetry ratio indicates a change in symmetry and vibrational modes around the Dy³⁺ ions by Li⁺ ion doping.

Fig. 7 (a) Emission spectra of SnO₂:Eu and Sn_0.8Li_0.2O_2−δ:Eu under the excitation wavelength of 300 nm. Room temperature upconversion spectra of (b) Er³⁺ doped and (c) Er³⁺–Li⁺ (1.0 mol%) co-doped ZnO nanoparticles under 980 nm diode laser excitation. (d) Powder X-ray diffraction patterns of Er³⁺:ZrO₂ with and without Li⁺ ions and Er³⁺/Yb³⁺:ZrO₂ with and without Li⁺ ions in the range of 27–33°. (e) UC emission spectra of ZrO₂:Er³⁺/Li⁺ (0 or 5 mol%) nanocrystals under 976 nm excitation. The inset shows the ln–ln plot of intensity versus excitation power dependence of the green and red emissions. (a) Reprinted with permission from ref. 99. Copyright © 2005 Elsevier B. V. All rights reserved. (b) and (c) Reprinted with permission from ref. 102. Copyright © 2008 Elsevier B. V. All rights reserved. (d) and (e) Reprinted with permission from ref. 105. Copyright © 2011 Elsevier B. V. All rights reserved.

Zhang et al.⁹⁹ have prepared Eu³⁺ and Li⁺ co-doped SnO₂ phosphors. SnO₂ has a tetragonal structure (space group of P42/mnm), in which the Sn atom occupies a slightly distorted octahedral site. The Eu³⁺ ions go into the Sn⁴⁺ sites, therefore the local environment around Eu³⁺ ions should be of high symmetry, which weakens the ⁵D₀ → ⁷F₂ (613 nm) transition compared to ⁵D₀ → ⁷F₁ transition. Li⁺ doping increases the crystallinity and particle size (established through XRD measurements), which induces a remarkable increase in PL intensity. Further, the occupation of Sn⁴⁺ sites by Li⁺ ions would naturally give rise to a considerable number of vacant sites in the oxygen ion array and then expand the lattice to decrease crystal density, which may also promote increased PL.

Kallel et al.¹⁰⁰ have prepared Y³⁺ and Li⁺ co-doped TiO₂ nanoparticles, which have rutile phase. Traces of Y₂Ti₂O₇ anatase are also found for the Y³⁺ doped titania powder. PL studies reveal that incorporation of Li⁺ induces a long lived PL on the order of millisecond time range, with a maximum at 724 nm. As the Li⁺ and Y³⁺ concentration increases to 7% or 10%, an increase in emission intensity is observed. Cao et al.¹⁰¹ have investigated 1 mol% Er³⁺, 10 mol% Yb³⁺ and 0–20 mol% Li⁺ tri-doped TiO₂ nanocrystals. The XRD pattern reveals that 1Er:TiO₂ has tetragonal rutile phase of TiO₂ with some impurity phase due to Er₂Ti₂O₇. On addition of 10 mol% Yb³⁺ a new phase, Yb₂Ti₂O₇, was observed along with rutile TiO₂ and Er₂Ti₂O₇ phases. From the shifts in XRD peak positions of TiO₂, it is revealed that for low Li⁺ ion concentration (1–2 mol%), Li⁺ occupies Ti⁴⁺ sites, whereas for higher Li⁺ concentration (5–20 mol%) it goes to interstitial sites. Upon Yb³⁺ and Li⁺ co-doping, the intensity of green and red bands of Er³⁺ increases significantly. This has been attributed to the energy migration between the Er³⁺ and Yb³⁺, as well as the distortion of crystal field symmetry of Er³⁺ for lower concentrations of Li⁺, and phase transformation at higher Li⁺ concentration.

Han et al.¹⁰² have prepared Er³⁺ doped ZnO nanoparticles. Under 980 nm excitation, this produces strong red emission. Upon Li⁺ ion co-doping, a luminescent switching between the main red emission band and the green band of Er³⁺ is seen (see Fig. 7(b) and (c)). They proposed that the change in UC emission behavior is due to the modification of the local crystal field around the Er³⁺ ions due to the introduction of Li⁺ ions. Fig. 7(b) shows the XRD patterns of the as prepared and annealed Er³⁺ doped samples. Gu et al.¹⁰³ have prepared Dy³⁺ doped ZnO nanocrystals. It is observed that the Li⁺ co-doping in ZnO:Dy³⁺ nanocrystals increases PL by 10 fold. They proposed that since the f–f absorption transitions in lanthanide ions are forbidden, the number of carriers excited through f–f transitions in Dy³⁺ ions is much less in comparison to those excited through band gap excitation of ZnO. Thus, the improvement in Dy³⁺ emission is mainly due to the radiative recombination of the large amount of trapped carriers excited from ZnO host. The increased recombination probability enhances the emission of Dy³⁺ by energy transfer process. In addition, the incorporation of Li⁺ ions can create oxygen vacancies, which might act as the sensitizer for the energy transfer to the rare earth ion due to the strong mixing of charge transfer states, resulting in the highly enhanced luminescence. Further, they have reported reduction of the asymmetry ratio, which suggests a change of symmetry and vibrational modes around the luminescent Dy³⁺ centers by Li⁺ doping. Bai et al.¹⁰⁴ have also studied the role of Li⁺ ions on Er³⁺ doped ZnO nanocrystals. They have also observed enhanced UC emission intensity of Er³⁺ ions upon Li⁺ co-doping. Though the Li⁺ ion does not have enough energy to destroy the ErO₆ structure, yet, it may alter the local structure around Er³⁺ which affects the 4f–4f transitions of Er³⁺ ion, which results in UC emission enhancement. Further, Li⁺ ion also reduces the OH⁻ group concentration, which is another reason for the UC emission intensity enhancement.

Liu et al.¹⁰⁵ have demonstrated the UC emission in Er³⁺, Yb³⁺ and Li⁺ tri-doped ZrO₂ nanocrystals. The incorporation of Li⁺ ions increases the emission intensities of green and red bands of Er³⁺ by a factor of 1.93 and 1.65, respectively. The XRD pattern reveals that Er³⁺ doped ZrO₂ samples have both tetragonal and monoclinic phases. Yb³⁺/Er³⁺ co-doping increases the formation of the tetragonal phase in the sample. Further, the incorporation of Li⁺ suppressed the tetragonal phase in the Yb³⁺/Er³⁺ doped ZrO₂ sample (Fig. 7(d)). Further, upon Li⁺ doping the slope of the excitation power versus UC emission intensities curve decreases (see Fig. 7(e)), which suggests that Li⁺ ions can tailor the local structure of the host lattice and thus can improve energy transfer processes from Yb³⁺ to Er³⁺ ions.

4.6 Effect of alkali ions on UC emission of lanthanide ions in fluoride host matrices

Fluoride host matrices, particularly NaYF₄ and NaGdF₄, are known to be excellent hosts for UC studies.^54,106 Due to their similar ionic radii, Na⁺ and RE³⁺ ions occupy the same sites in the hexagonal NaYF₄ lattice. These host matrices possesses low phonon energies, which is an essential requirement for obtaining efficient UC emission. They show higher chemical stability than other halides (chloride, bromide).^54,106 The crystal structure of these matrices highly influences the UC properties, and crystal structure is quite sensitive to the alkali ions present in the crystal. So, it is interesting to study the effect of alkali ions on the UC properties of fluoride host matrices (Table 7).

Table 7 Effect of Li⁺ ion on the UC luminescence of lanthanide ions doped in fluoride host matrices

Sample	Change in luminescence properties	Remarks
NaYF4:Yb^3+/Er^3+,K^+/Li^+ (ref. 107)	UC emission intensity decreases upon Li^+ doping, whereas it increases upon K^+ doping	Change in morphology and crystal structure upon Li^+ doping and lowering of crystal symmetry around Er^+ ions in K^+ doping
GdF3:Yb^3+/Er^3+,Li^+ (ref. 108)	Change in UC emission color from yellow to red, red emission is 8 times higher than in NaGdF4:Yb^3+/Er^3+	Change in crystal symmetry around Er^3+ ions, back energy transfer
NaYbF4:Yb^3+/Er^3+,Li^+ (ref. 109)	UC color tunability	Change in crystal symmetry around Er^3+ ions and weakened polarization effect
NaYF4:Yb^3+/Tm^3+,K^+/Li^+ (ref. 110)	Decrease in concentration quenching	Crystal distortion and increased crystal size
NaGdF4:Yb^3+/Er^3+,Li^+ (ref. 111)	Enhanced UC luminescence (47 fold for green and 23 for ions, increased crystal size, increased red), increased paramagnetism lifetime of ^4S3/2 state, decrease in distance between Gd^3+ ions (for increased paramagnetism)	Change in crystal symmetry around Er^3+
ErF3:Er^3+,Li^+ (ref. 112)	Four fold enhancement in UC emission	Change in crystal symmetry around Er^3+ ions
Lu6O5F8:Yb^3+/Er^3+,Li^+ (ref. 113)	Increased UC emission	Change in crystal symmetry around Er^3+ ions, increased crystallinity, reduced quenching centers (OH^−)
NaYF4:Yb^3+/Er^3+,K^+/Li^+ (ref. 114)	Color tunability of UC emission	Change in crystal structure and lowering of crystal symmetry around Er^+ ions

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Mao et al.¹⁰⁷ have studied the effect of alkali ions (Li⁺ and K⁺) on crystal structure and UC emission of NaYF₄:Yb/Er crystals. It was observed that the Li⁺ ion co-doping in the NaYF₄:Yb/Er crystals affects the morphologies to a greater extent and significant changes from rod-like shape to disk shape and finally to polyhedron structure is attained. However, K⁺ ion co-doped NaYF₄:Yb/Er crystals do not show any change in morphology and they almost maintain the rod-like shape throughout (see schemes (a) and (c) in Fig. 8). The crystal structure analysis further reveal that not only the surface morphology, but also the phase transition from β-hexagonal to tetragonal (LiYF₄) in Li_xNa_1−xYF₄ crystals is initiated at x = 0.5, whereas the phase of K_yNa_1−yYF₄ remains stable up to y = 0.85. The UC emission studies show that as the Li⁺ content increases in the crystal, UC emission intensity decreases, whereas the UC emission intensity of NaYF₄:Yb/Er crystals increases (K_0.7Na_0.3YF₄:Yb, Er crystal shows 8 and 7 fold enhancement in green and red UC emission, respectively) when K⁺ ions are introduced into the crystal (see Fig. 8(b) and (d)). The change in UC emission is related to the change in crystal structure, surface morphology, and distance between alkali and fluorine atoms. In the Li⁺ doped crystal, the Li–F distance will be larger than the Na–F distance, which can intensify the interaction between F⁻ and Er³⁺ in the crystal lattice, which may enhance the phonon energy of Er³⁺ ions. On the other hand, substitution of Na⁺ ions with K⁺ ions in the lattice reduces the inter-quenching of Er⁺ and decreases the local crystal field symmetry, which increases UC intensity. In a similar work, Dou and Zhang¹¹⁴ have also studied effect of Li⁺ and K⁺ ions doping on crystal structure and UC emission behavior of NaYF₄:Yb/Er. The phase transition in NaYF₄:Yb/Er crystal, has also been observed by Dou and Zhang¹¹⁴ upon Li⁺ ion doping, but instead of hexagonal to tetragonal phase transition, as observed by Mao et al.,¹⁰⁷ they observed hexagonal to cubic phase transition. The UC studies reveal that the intensity ratios between the blue, green, and red emission peaks change upon Li⁺ and K⁺ doping in the crystal. They have proposed that the Li⁺ or K⁺ doping will slightly change the size of the unit cell in the crystal and then to change the Yb–Er distance, which can influence energy transfer process. On the other hand, the higher concentration of K⁺ ions in the crystal causes formation of a new K₂NaYF₆ phase. The coordination number of RE is 9 in hexagonal phase, 8 in cubic phase and 6 in K₂NaYF₆ phase. This suggests that the K₂NaYF₆ phase is less stable than the cubic and hexagonal phases. Further, Misiak et al.¹¹⁰ have shown that Li⁺ doping in the cubic NaYF₄:Yb³⁺/Tm³⁺ colloidal crystals significantly reduces the concentration quenching. Interestingly, the initial UC emission intensity decreases at first, and then starts increasing. This might be due to the fact that initially Li⁺ ions substitute the Na⁺ ions in the lattice, while further increase in the Li⁺ ion concentration results in the occupancy of Li⁺ ions in the interstitial sites, which results in increase in the crystal size. Further, substitution of Na⁺ by Li⁺ introduces distortion in the lattice, which prevents concentration quenching.

Fig. 8 (a) Schematic illustration of the formation process of Li_xNa_1−xYF₄ with various morphologies. (b) UC luminescence spectra of the as-prepared Li_xNa_1−xYF₄:Yb³⁺/Er³⁺ products under 980 nm excitation at room temperature (x = 0, 0.15, 0.30, 0.50, 0.70, 0.85 are denoted by S₀ to S₅). (c) Schematic illustration for the possible formation process of K_yNa_1−yYF₄:Yb³⁺/Er³⁺ with various morphologies. (d) UC luminescence spectra of the as-prepared K_yNa_1−yYF₄:Yb³⁺/Er³⁺ products under 980 nm excitation at room temperature (y = 0.15, 0.30, 0.50, 0.70, 0.85 are denoted by S₆ to S₁₀). The inset shows the integral intensity of green and red emission as a function of the concentration of K⁺ ions. Reprinted with permission from ref. 107 Copyright © 2013 Elsevier B. V. All rights reserved.

Similar to NaYF₄, co-doping of alkali Li⁺ brings out significant changes in crystal structure and the optical properties are greatly improved. Cheng et al.¹¹¹ have studied the effect of Li⁺ ions on UC emission of β-NaGdF₄:Yb³⁺/Er³⁺ nanoparticles. It is found that the presence Li⁺ ions in the crystal enhanced UC emission drastically (nanoparticles doped with 7 mol% Li⁺ ions show 47 and 23 fold enhancement in green and red UC emission peaks, respectively). The enhanced UC emission is attributed to the distortion of the local asymmetry around Er³⁺ ions. The presence of Li⁺ in the nanoparticles also helps in the growth of the crystals, which is proved by XRD studies. Another contributing factor to the enhanced UC is the increased lifetime of the ⁴S_3/2 state of NaGdF₄:Yb³⁺/Er³⁺ NPs in the Li⁺ ions doped crystals (see Fig. 9) which causes increased population in the ⁴S_3/2 state, resulting in enhanced green emission. Furthermore, the presence of Li⁺ also increases paramagnetic behavior of the nanoparticles. This is attributed to the increase in Gd³⁺ molar concentration in NaGdF₄ resulting due to introduction of Li⁺ ions. Zhang et al.¹⁰⁹ have also studied the effect of Li⁺ ions on the UC emission behavior of NaYbF₄:Er microcrystals. Interesting, they observed a blue shift in UC emission bands. The XRD studies show hexagonal to tetragonal phase transformation upon Li⁺ co-doping. In the LiYbF₄ crystal, each Yb³⁺ ion is coordinated to eight F⁻ ions (forming the YbF₈ polyhedral units) and each Li⁺ ion is coordinated to four F⁻ ions (forming the LiF₄ tetrahedral units). The crystallographic point site symmetry with S₄ symmetry in LiYbF₄ is higher than that of NaYbF₄ with D_2d symmetry. This results in a more symmetrical distribution of electronic density and a weakened polarization effect of the local environment in LiYbF₄ causes a blue shift in UC bands. Guo et al.¹¹³ have studied the effect of Li⁺ ions on the UC emission behavior of Lu₆O₅F₈:Yb³⁺/Er³⁺ nanoparticles. They have studied UC, DC and CL properties of the materials and all these increases in the presence of Li⁺ ions. They proposed that increased luminescence properties are due to the increased crystallinity (by flux action of Li), decreased absorption bands of the surface contaminants (OH and CO groups) and distorted local symmetry around the Er³⁺ ions. Yin et al.¹⁰⁸ have studied the effect of Li⁺ on UC emission behavior of GdF₃:Yb,Er nanoparticles. In GdF₃:Yb,Er nanoparticles, the intensity of UC emission in the red region is higher than that in the green region, which results in a bright yellow emission even observable by the naked eye. Upon Li⁺ doping (0 to 5, 10, and 25 mol%) the green emission intensity gradually decreases and the red emission is enhanced (see Fig. 9 (c)–(g)). This is attributed to the change in local crystal field around Er³⁺ ions in the presence of Li⁺ ions in the GdF₃:Yb,Er host lattice.

Fig. 9 (a) UC luminescence spectra of NaGdF₄:Yb/Er/Li⁺ (0–15 mol%) nanoparticles under 980 nm excitation at room temperature. The inset shows the integral intensity of green and red emission as a function of the concentration of Li⁺ ions. (b) Decay profiles of the ⁴I_11/2/⁴I_13/2 transition in NaGdF₄:Yb³⁺/Er³⁺ NPs with 0–15 mol% Li⁺ under 980 nm excitation. (c to f) Luminescence photographs of calcined GdF₃:Yb,Er (20, 2 mol%) UCNPs (having 0, 5, 10 and 25 mol% of Li⁺) dispersed in deionized water. (g) Solid powder (0.20 g) of GdF₃:Yb,Er,Li (25 mol%) UCNPs was placed into a quartz vessel and excited under a 980 nm laser to visually demonstrate the naked-eye-visible brilliant red light. (a and b) Reproduced from ref. 111 with permission from The Royal Society of Chemistry. (c–g) Reprinted with permission from ref. 108 Copyright © 2012 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim.

Thus in summary, it is pointed out that alkalis in fluorides and similar hosts effectively modify the crystal structure and affect the surface morphology, phase, particle size, crystallinity, etc. In this way the crystal field around the lanthanide ion is favorably changed. Not only this, the doping of alkalis makes a significant change in critical separation between lanthanide ions, lanthanide-alkali ions, lanthanide-fluorine ions, lanthanide activator and sensitizers, etc., due to which energy transfer processes become effective, giving rise to an increase in the UC emission. A change in the different components of the emission viz. blue, green and red varies systematically, which creates opportunities for color tunability also. Above all this, a specific blue shift is also observed, which is peculiar in this host. Some interesting observations, although beyond the scope of this study, e.g. increase in the paramagnetic behavior, could be very useful for readers.

4.7 Effect of alkali ions on luminescence enhancement of lanthanide ions in miscellaneous host matrices

The preceding sections described the role of alkali ions, especially Li⁺, in the luminescence properties in some well known host matrices viz. Y₂O₃ and Gd₂O₃, ABO₄ type compounds, strontium and calcium aluminates, perovskite, oxide nanoparticles and fluoride host matrices. In addition to these, there are several other host matrices viz. Ba₂GdNbO₆:Eu³⁺/Dy³⁺,Li⁺,¹¹⁵ BaZn₂(PO4)₂:Sm³⁺,R⁺ (R = Li, Na, K),¹¹⁶ Ca₃B₂O₆:Dy³⁺,Li⁺,¹¹⁷ CaSO₄:Tm³⁺/Dy³⁺,Li⁺,¹¹⁸ (Y, Gd)BO₃:Tb³⁺,Li⁺,¹¹⁹ YNbTiO₆:Eu³⁺,Li⁺,¹²⁰ YBO₃:Eu³⁺,Li⁺,¹²¹ Zn₂SiO₄:Yb³⁺,Er³⁺, Li⁺/Bi³⁺,¹²² SrB₄O₇:Eu²⁺,Li⁺,¹²³ Lu₂SiO₅:Ce,Li⁺,¹²⁵ Y₂MoO₆:Eu³⁺,Li⁺,¹²⁶ NaSrBO₃:Tb³⁺,Li⁺,¹²⁷ Sr₂SiO₄:Eu³⁺,(Li⁺, Na⁺, K⁺),¹²⁹ CaSnO₃:Tb³⁺(Li⁺, Na⁺, K⁺),¹³¹ Gd₆MoO₁₂:Li⁺/Ln³⁺,¹³² Y₂SiO₅:Pr³⁺,Li⁺,¹³³ Y₂Zr₂O₇:Dy³⁺,Li⁺,¹³⁴ Li_xCa_1−2xEu_xSi_yMo_1−yO₄,¹³⁶ Ca₂BO₃Cl:Sm³⁺(Li⁺, Na⁺, K⁺)¹³⁹ and CaSi₂O₂N₂:Eu²⁺,Dy³⁺,Li⁺,¹⁴⁰ etc. in which the role of Li⁺ in luminescence properties has also been investigated. Table 8 gives brief information about these and also outlines various possible mechanisms involved in the PL enhancement. It is basically due to increased crystallinity, grain size, charge compensation, change in local crystal field around the lanthanide ions, etc. All these mechanisms are discussed in the preceding sections in context of other well known matrices, so here we will not discuss it again.

Table 8 Luminescence enhancement (and their explanation) in different activators during down-shifting and upconversion processes in miscellaneous host in presence of Li⁺ ions

Sample	DS/UC	Luminescence enhancement	Remarks
Ba2GdNbO6:Eu^3+/Dy^3+,Li^+ (ref. 115)	DS	—	Improved crystallinity, increased grain size
BaZn2(PO4)2:Sm^3+,R^+ (Li, Na, K)^116	DS	—	Charge compensation
Ca3B2O6:Dy^3+,Li^+ (ref. 117)	DS	—	Charge compensation, flux effect, change in local environment around Dy^3+ ions
CaSO4:Tm^3+/Dy^3+,Li^+ (ref. 118)	TL	—	Creation of charge trap centers
(Y, Gd)BO3:Tb^3+,Li^+ (ref. 119)	DS	—	Spherical and non-agglomerated, particles, strong lattice distortion
YNbTiO6:Eu^3+,Li^+ (ref. 120)	DS	3 fold	Improved crystallinity, increased grain size
YBO3:Eu^3+,Li^+ (ref. 121)	DS		Improved crystallinity, increased grain size
Zn2SiO4:Yb^3+,Er^3+,Li^+/Bi^3+ (ref. 122)	UC	—	Change in local environment around Er^3+
SrB4O7:Eu^2+, Li^+ (ref. 123)	DS	—	Change in electric field distribution in lattice structure
Lu2SiO5:Ce,Li^+ (ref. 125)	DS	2.2 fold	Change in crystal field around Ce^3+
Y2MoO6:Eu^3+,Li^+ (ref. 126)	DS	—	Improved crystallinity
NaSrBO3:Tb^3+,Li^+ (ref. 127)	DS	—	Charge compensation
Sr2SiO4:Eu^3+,(Li^+, Na^+, K^+)^129	DS	—	Charge compensation
CaSnO3:Tb^3+,(Li^+, Na^+, K^+)^131	DS	—	Charge compensation
Gd6MoO12:Li^+/Ln^3+ (ref. 132), (Ln^3+ = Yb^3+/Er^3+/Tm^3+)	UC	—	Change in crystal symmetry around Ln^3+
Y2SiO5:Pr^3+,Li^+ (ref. 133)	UC	9 fold	Change in crystal symmetry around lanthanide ions, reduction in activator ions clustering
Y2Zr2O7:Dy^3+,Li^+ (ref. 134), (Ln^3+ = Yb^3+/Er^3+/Tm^3+)	UC	2 fold	Improved crystallinity
LixCa1−2xEuxSiyMo1−yO4 (ref. 136)	DS	—	Charge compensation
Ca2BO3Cl:Sm^3+(Li^+, Na^+, K^+)^139	DS	—	Charge compensation
CaSi2O2N2:Eu^2+,Dy^3+,Li^+ (ref. 140)	DS	—	Charge compensation

---

5. Conclusions

This review was planned with prime objective to explore the effect of Li⁺ ion co-doping on the PL of lanthanide ions doped in different host matrices, and also to highlight various contributing factors that plays important roles. The follow up of the literature reviewed so far in this article unanimously depicts that the co-doping of Li⁺ ions up to a certain concentration in lanthanide doped phosphors enhances the luminescence (both for UC and DC based emission) of lanthanide ions considerably.

The possible explanation for the enhanced PL intensity of the lanthanide ions in the presence of Li⁺ ions can be summarized as follows. When Li⁺ ion is introduced into any matrix, because of its smaller ionic radius, it can either substitute the cation of the matrix or occupy interstitial sites of the matrix, which usually depends on the concentration of doped alkali. As the Li⁺ ion concentration increases, the possibility to occupy the interstitial sites increases, which distorts the matrix significantly. In a few hosts, changes even in the phase and crystal structure are also noted. The change in crystal structure can also change the occupancy site of the lanthanide ions. Also, due to the lower melting temperature of the Li, it can work as a flux and increase the crystallinity and the grain size of the materials. In addition, it is also found that Li⁺ ions decrease the concentration of the inorganic/organic groups that work as non-radiative channels and cause a decrease in the PL intensity. Further, when a trivalent lanthanide ion replaces a divalent cation it produces charge imbalance in the matrix that can be compensated by using alkali ions. All these factors favor luminescence enhancement of the lanthanide ions. In the case of Ce³⁺ and Eu²⁺ ions, by changing the local crystal fields around the activator ions, they can also be used for tuning the color of the luminescence materials. Furthermore, in fluoride host matrices, co-doping of Li⁺ causes a change in separation between lanthanide ions, lanthanide-alkali ions, lanthanide-fluorine ions, lanthanide activator and sensitizers, etc., which strongly influences the energy transfer processes. Thus, it can be concluded that the Li⁺ ions (alkali ions) modify host matrices in many ways, which favors radiative transitions, causing enhancement in the luminescence intensity. Since this review provides in-depth case studies for various hosts, and describes the effect of various matrix parameters (e.g. crystal structure, crystallinity, grain size, surface morphology, quenching entities, etc.) on PL, therefore, it could also be useful as input for designing various novel lanthanide doped luminescent materials.

The effects of co-doping with Li⁺ ions have been used for almost all different types of luminescence mechanisms including photoluminescence/downshifting, upconversion, thermo-luminescence/persistent emission, cathado-luminescence, etc. However, its use for one of the most important class of emissions, i.e. quantum-cutting, is still lagging and has strong scope for future studies.

Acknowledgements

S. K. Singh thankfully acknowledges the financial support by Department of Science and Technology, New Delhi, India in the form of INSPIRE Faculty Award [IFA12-PH-21]. A. K. Singh acknowledges financial support by UGC, New Delhi, India (Dr D. S. Kothari postdoctoral fellowship (no. F. 4-2/2006 (BSR)/13-906/2013 (BSR)) from April 2013 to February 2014) and UNAM, Mexico (DGAPA, started from March 2014) in the form of postdoctoral fellowships.

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