Role of Li+ ion in the luminescence enhancement of lanthanide ions

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REVIEW

Role of Li+ ion in

APtvMH(Ua(TtH

India. His research interests incluand application of lanthanide-basenanostructure and quantum dotsreviewed international journal paan Indian Patent.

aDepartment of Physics, Banaras Hindu Un

[email protected] de Ciencias Fısicas, Universi

Cuernavaca, Morelos C.P. 62210, MexicocDepartment of Physics, Indian Institute of

Varanasi-221005, India

Cite this: RSC Adv., 2014, 4, 27039

Received 6th February 2014Accepted 29th April 2014

DOI: 10.1039/c4ra01055h

www.rsc.org/advances

This journal is © The Royal Society of C

the luminescence enhancement oflanthanide ions: favorable modifications in hostmatrices

A. K. Singh,*ab S. K. Singhc and S. B. Raia

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.

khilesh Kumar Singh is aostdoctoral Fellow in the Insti-uto de Ciencias Fısicas, Uni-ersidad Nacional Autonoma deexico, Cuernavaca, Mexico.e holds an M. Sc. in Physics2004) from V.B.S. Purvanchalniversity, Jaunpur, India andn M. Tech. (2007) and Ph.D.2012) in Materials Science andechnology from Indian Insti-ute of Technology (Banarasindu University) Varanasi,de synthesis, characterizationd novel luminescent materials,. He has published 18 peer-pers and is a co-investigator of

iversity, Varanasi-221005, India. E-mail:

dad Nacional Autonoma de Mexico,

Technology (Banaras Hindu University),

hemistry 2014

1. Introduction

Lanthanide ions exhibit fascinating optical properties,including the ability to convert near infrared photons to ultra-violet/visible photons (through an upconversion process) andvice versa (quantum-cutting process).1–5 Due to their unique

Sunil K Singh, born in 1982,received doctoral degree inPhysics in 2011 from the Bana-ras Hindu University, India.Currently, he is in the DST-INSPIRE faculty at Indian Insti-tute of Technology (BanarasHindu University), Varansi,India. His research interestincludes the physics of multi-modal luminescence (upconver-sion, downconversion/quantum-cutting, downshiing) in

lanthanide and/or semiconductor quantum dot doped nano-structures. 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 abook chapter and more than 30 articles in peer-reviewed journals,and his current h-index is 9 (April 2014).

RSC Adv., 2014, 4, 27039–27061 | 27039

RSC Advances Review

optical features, many recent advances and emerging applica-tions such as lighting devices (light emitting diodes, econom-ical luminescent lamps), optical bres, display devices, lasersand many biological investigations are heavily dependent onthem.1–5 Fundamentally, lanthanide ions have essentially threetypes of optical transitions: intra-congurational 4f–4f transi-tion, inter-congurational 5d–4f transition and the chargetransfer transition (ligand to metal or metal to ligand).5,6 Ce3+

and Eu2+ are widely used activator ions for phosphor applica-tions.7–19 Their characteristic transition is 4f–5d, which is anallowed transition and has a lifetime on the order of nano-second. Since the 5d orbitals are strongly affected by the crystaleld and polarizability of the host crystal, these transitions(4f–5d transitions) can be easily tuned to the visible region bychanging 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 aforbidden transition, gives sharp and intense spectral lines(from UV to infrared region) with a lifetime on the order ofmicrosecond.20–30 Further, as the 4f orbital remain shielded by5s and 5p orbitals, 4f–4f transitions are much less inuenced bythe external environment (crystal eld of host matrices).20–30

In recent years, research of lanthanide-based luminescencematerials has been focused to exploit the unique features oflanthanides in various other novel applications, viz. for multi-modal emission (downconversion (DC)/quantum-cutting (QC),down-shiing (DS) and upconversion (UC) processes) basedimaging, in increasing the efficiency of solar cells (by using anadditional DC/UC layer in the solar cell to minimize the thermaland sub-bandgap losses), as contrast agents in magnetic reso-nance imaging, various biological applications, etc.31–35 Thepragmatic implication of luminescence materials for theseapplications demands efficient materials in many ways,including an increase in the emission intensity. Therefore,continuous active research effort in this eld is desirable toenhance the luminescence intensity of the lanthanide ions.

There are two fundamental ways to enhance the lumines-cence intensity of the lanthanide ions in a given host. The rst is

S. B. Rai has been a professor inthe Department of Physics,Banaras Hindu University, Vara-nasi, India since 1994. He gainedhis Ph.D. (Physics) in 1974 fromBanaras Hindu University andvisited Germany as an Alexandervon Humboldt fellow in 1982–84and several times later on. Hiscurrent research interests includemultifunctional materials dopedwith rare-earth ions e.g. glasses,ceramics, phosphors, quantum

dots, polymers, hybrid nanostructures, etc. and their applicationsin various emerging elds. He has edited three books and severalbook chapters, and authored/co-authored around 300 articles inpeer-reviewed national and international journals.

27040 | RSC Adv., 2014, 4, 27039–27061

to use a suitable sensitizer, which can absorb UV and/or NIRradiation effectively and can transfer the energy to the centrallanthanide ions.1–3,36–42 In the literature, different sensitizationstrategies are reported for increasing the luminescence oflanthanide ions, viz. by forming lanthanide ion complexes withorganic ligands, which can strongly absorb UV radiation andtransfer efficiently to the central lanthanide ion; by excited stateenergy transfer from the charge transfer band to the lanthanideion; by using co-dopant and thereby energy transfer amonglanthanide themselves, etc.24,38,41 The second way to enhance theluminescence of lanthanide ions is to modify the host matricesin such a way that it favors transitions of lanthanide ions. Dueto its smaller ionic radius alkali ions, the Li+ ion in particular isvery frequently used nowadays for this purpose.7–19,56–142 Li+ ionsare easily accommodated in host matrices, which tailors thelocal crystal eld around the lanthanide ions.

Recently, our group has also worked in this area and shownthat the use of Li+ ion enhances the photoluminescence (PL) oflanthanide ions in YPO4 and Y2O3 hosts.74,81,84 In the literature,several models have been proposed by different groups toexplain the luminescence enhancement in the lanthanide ionsin different host matrices.56–142 However, to the best of ourknowledge, there is no review article that summarizes acomprehensive view of how the alkali ionmodies a host matrixresulting in enhanced luminescence. The prime objective ofthis review is to ll this gap and highlight various contributingfactors that contribute towards luminescence enhancement.Along with this, since different types of host matrices have beenreviewed individually through the previous literature, thepresent article could also be very useful in studying/visualizingthe role of the matrix on the luminescence intensity of lantha-nide ions in general. This review is written in such a way that itcould be useful for a broad audience of chemists/biochemistsand materials scientists as well.

2. Basics of lanthanide luminescence

The term lanthanide originated from the Greek word lantha-neien meaning “lying hidden”. The lanthanides are a family of15 chemically similar elements, from lanthanum to lutetium(atomic number 57 to 71), which possess electronic congura-tion [Xe] 4fn5d0,16s2 (where ‘n’ varies from 0–14).5,6 Scandium(21) and yttrium (39) (d-block elements) are found in naturealways with lanthanides; therefore, lanthanides and these twoelements altogether are known as the rare-earth elements.Lanthanides exist usually in the +3 oxidation state (Ln3+), whichis their most stable oxidation state (though Eu2+, Sm2+ and Cr4+

also exist) and most of the optical properties of lanthanides areexplored in this oxidation state. As the lanthanide ions featurean electron conguration of 4fn, which offers large number ofpossible arrangements, there are rather large numbers of elec-tronic levels.

The promotion of a 4f electron into the 5d sub-shell inlanthanide ions is allowed by the parity rule. These transitionsare quite energetic and observed only in Ce3+, Pr3+ and Tb3+

ions. Since the 5d orbitals are external and interact directly withthe ligand orbitals, their energy largely depends on the metal

This journal is © The Royal Society of Chemistry 2014

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environment.7–19 In Ce3+ ions, luminescence is observed from2D3/2 levels to

2F5/2,2F7/2, whereas

2D5/2 lies at high energy andluminescence from this level is not generally observed. Theluminescence 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 allowedby Laporte's selection rule. The other and most studied transi-tion in lanthanides is the 4f–4f transition. According toLaporte's rule, electric dipole transitions with the same parityare not allowed. By virtue of that, the 4f–4f transition inlanthanides is forbidden.5,6 However, when these ions aredoped in a suitable host matrix, due to inuence of the ligand-eld, non-centrosymmetric interactions, a mixing of the wave-functions of the states of opposite parity with 4f-wavefunctions,takes place. This relaxes somewhat the parity selection rule and4f–4f transitions become partially allowed.5,6 Furthermore, thecrystal eld destroys the degeneracy of its energy levels. In fact,the ligand/crystal generates a ligand/crystal electrostatic eldwhich, in turn, interacts with the 4f electrons of the lanthanideions generating a ligand eld/crystal eld (or Stark) splitting ofthe spectroscopic levels.5,6 The 4f orbitals of the lanthanide ionsremain shielded by 5s and 5p orbitals, and therefore promotionof an electron into a 4f sub-orbital of higher energy does notchange the binding pattern signicantly. Thus, the inter-nuclear distance remains almost unchanged in the excitedstate, which produces narrow emission lines. The long lifetimeis because of the fact that the 4f–4f transitions in the lanthanideions are parity forbidden.5,6 Table 1 summarizes the energy ofprincipal 4f–4f transitions of lanthanide ions coming throughDS, QC and UC processes.

The 4f–4f transition in lanthanide ions facilitates UC andDC/QC as well as DS processes, which may allow for facile

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

Ln Transition Em

Pr 3P2 /3H4 440

3P0 /3H4 480

1D2 /3F4 103

Nd 2P3/2 /4I11/2 410

2P3/2 /4I13/2 452

4F3/2 /4I11/2 106

Sm 4G5/2 /6H7/2 601

Eu 5D0 /7F0,1,2,3,4 570

Tb 5D4 /7F6,5,4,3 480

Dy 4F9/2 /6H15/2 486

4F9/2 /6H13/2 575

Ho 5S2,5F4 /

5I8 5405F5 /

5I8 6445I6 /

5I8 118Er 4S3/2 /

4I15/2 5454F9/2 /

4I15/2 6654I13/2 /

4I15/2 154Tm 1D2 /

3F4 4503H4 /

3H6 800Yb 2F5/2 /

2F7/2 980

a QC – quantum-cutting, DS – down-shi, UC – upconversion, NIR – near-are taken from ref. 31 and 52.


photon management.43–51 In the DC/QC process, an incidenthigher-energy photon splits into two (or more) lower energyphotons with a conversion efficiency of more than 100%.46–49

DS is similar to QC but it is a single photon process thatinvolves the transformation of one absorbed high-energyphoton into one low-energy photon and so its conversionefficiency does not exceed 100%.50,51 Meanwhile, on the otherhand, UC is a nonlinear process that involves two or more lowenergy photons (usually NIR) to emit a single photon ofhigher energy (usually visible or ultraviolet).43–45 To generatepractically useful UC emission, the energy difference betweeneach excited level and its lower-lying intermediate level(ground level) should be close enough to facilitate photonabsorption and energy transfer steps involved in UCprocesses. Er3+, Tm3+, and Ho3+ typically feature such ladder-like arranged energy levels and are thus frequently usedactivators for UC.

Selection of an appropriate host matrix for the preparation ofa lanthanide-based luminescence material is an essentialrequirement. As the luminescence of lanthanide ions is verysensitive to the local crystal eld environment, the host matrixshould have close lattice matches to the dopant ions.52–54 Failingto achieve this can create a large distortion in the matrix, whichstrongly inuences the luminescence of lanthanide ions. Inparticular, the 5d–4f transitions of the lanthanide ions arestrongly inuenced (both position and intensity) by the crystaleld of the matrix.7–19 Since the lanthanide(III) ions exhibitsimilar ionic size and chemical properties their inorganiccompounds are ideal hosts for lanthanide ion doping. Inaddition, alkaline earth ions (Sr2+, Ca2+, and Ba2+) and sometransition metal ions (Zr4+ and Ti4+) also exhibit close ionic sizeto lanthanide ions and are frequently used as host materials for

ission wavelength (nm) Remarks

Weak, QCStrong, UC and QC

7 (P, NIR) Medium, UC and QCStrong, UC and QCStrong, UC and QC

4 (F, NIR) Strong, QC and DS(P, orange) Strong, DS–720 (P, orange) Strong, UC and DS–650 (P, green) Strong, UC, DS and QC

Medium, DS and QC(P, yellow-orange) Strong, DS and QC(F, green) Strong, UC and QC(F, red) Strong, UC and QC0 (NIR) Strong, QC(F, green) Strong, UC and QC(F, red) Strong, UC and QC0 (NIR) Strong, QC(blue) Medium, UC and QC(NIR) Strong, UC and QC(F, NIR) Strong, UC, QC and DS

infrared, P – phosphoresce, F – uorescence. Most of the data presented

RSC Adv., 2014, 4, 27039–27061 | 27041

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:Eu2+,Li+ (ref. 7) Improves radiation hardness By reducing the X-ray induced vacancy centersLi2CaSiO4:Eu

2+ (ref. 8a) Small Stokes shi Smaller ionic radius of Li+ constrains thedistortion of the excited state

CaS:Ce3+,Pr3+,Li+ (ref. 9) 3 fold enhanced aerglow intensity and 4 foldenhancement in aerglow time

New cation vacancy formed, which works as anelectron trap center

CaO:Ce3+,Li+ (ref. 10) 1.88 fold enhancement in luminescenceintensity

Increased absorbance to the excitation photons

SrAl2O4:Eu2+,Ce3+,Li+ (ref. 11) Enhanced luminescence of Eu2+ Charge compensation

NaCaPO4:Eu2+,Li+ (ref. 12) Enhanced luminescence of Eu2+ Change in spin–orbit coupling and coordination

of Eu2+ ionsSr2–2xLiSiO4F:xCe

3+,xLi+ (ref. 13) Shi in peak position Ce3+ occupying different Sr2+ siteSr3SiO5:Ba

2+,Ce3+,Li+ (ref. 14) Li+ co-doping helps to incorporate Ce3+ ions onSr2+ site

By charge compensation

Lu2SiO5:Ce3+,Li+ (ref. 15) 2.2 fold enhancement in luminescence intensity Change in crystal eld around Ce3+ ions

Sr3Si2O4N2:Eu2+,Li+ (ref. 19) Red shi in emission band Re-absorption in Eu2+ ions

Sr3Si2O4N2:Ce3+,Li+ (ref. 19) No prominent red shi

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lanthanide ion doping. Further, the ideal host matrix shouldalso have low lattice phonon energies, to minimize non-radia-tive loss and maximize the radiative emission. The dopantconcentration, which determines the average distance between

Fig. 1 (a) CIE coordinates of Ca0.98SiN2�2d/3Od:0.01Ce3+/0.01Li+ and pho

PL emission spectra of Na1�yLiyCa0.99PO4:Eu0.01 phosphors sintered inconcentrations. (c) PL spectra of CaO:Ce3+ and CaO:Ce3+,Li+. The inset pCaO:Ce3+ is normalized. Photographs of: (d) an as-prepared CdSe QD- alight-emitting LED operated at 5 mA. (a) Reproduced from ref. 17 with pewith permission from ref. 12. Copyright © 2012 Elsevier B. V. All rights rese2013 Elsevier B. V. All rights reserved. (d and e) Reprinted (adapted) with pCo. KGaA, Weinheim.

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neighboring dopant ions, has a strong inuence on the opticalproperties of the lanthanide-based luminescence materials.52–54

In certain host matrices, luminescence of lanthanide ionsincreases appreciably with increasing crystallinity. This is

tos of the powder samples under daylight and 365 nmUV excitation. (b)argon initially and later in N2H2 atmosphere with varying lithium ionresents a histogram of the integrated PL intensity, where the intensity ofnd Sr3SiO5:Ce

3+,Li+ phosphor-based white LED and (e) the same whitermission from The Royal Society of Chemistry. (b) Reprinted (adapted)rved. (c) Reprinted (adapted) with permission from ref. 10. Copyright ©ermission from ref. 18. Copyright © 2008 WILEY-VCH Verlag GmbH &


Table 3 Luminescence enhancement (and their explanation) in various activators during down-shifting and upconversion processes in Y2O3 andGd2O3 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 andincreased optical phonon energy

Y1.9Li0.05Eu0.05O3 (ref. 57) DS 2.5 fold (CL) Charge compensationGd1.92Li0.03Eu0.05O3 (ref. 57) DS 3 fold (PL) Lowering of local symmetryGd1.9Li0.06Eu0.04O3 (ref. 58) DS 4 fold (PL) Charge compensation and lowering of local symmetryGd1.84Li0.08Eu0.08O3 (ref. 59) DS 2.3 fold (PL) Improved crystallinity and higher surface roughnessGd1.87Li0.06Yb0.06Tm0.01O3 (ref. 60) UC 10 fold (PL) Lowering of symmetry around Tm3+ and decrease in OH

concentrationGd1.935Li0.04Yb0.02Ho0.005O3 (ref. 61) UC 10 fold (PL) Lowering of symmetry around Ho3+

Y1.935Li0.05Nd0.015O3 (ref. 62) UC 2 fold (PL) Changed morphology, modication of local symmetry anddecrease in OH concentration

Y1.92Li0.05Yb0.02Er0.01O3 (ref. 63) UC 30 fold (PL) Prolong lifetime of their intermediate statesY1.92Li0.05Yb0.02Er0.01O3 (ref. 64) UC 25 fold (PL) Change in crystal eld, prolong lifetime of intermediate

states, increased optical active sites and dissociation ofclusters

Y1.94Li0.04Er0.02O3 (ref. 65) UC 10 fold (PL) Change in crystal eld 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 sizeY1.94Li0.05Er0.01O3 (ref. 67) UC 45 fold (PL) Tailored lifetime of intermediate levelsY1.84Li0.08Eu0.08O3 (ref. 68) DS 1.2 fold (EL) Improved crystallinity and higher surface roughnessY1.70Li0.1Eu0.2O3�d (ref. 69) DS 1.2 fold (PL) Improved morphologyY1.9455Li0.05Yb0.05Tm0.0025O3

(ref. 71)UC 15 fold (PL) Change in crystal eld, decrease in OH concentration and

dissociation of clustersY1.9Li0.05Yb0.05O3 (ref. 72) DS 12 fold (PL) Change in crystal eldY1.93Li0.05Tm0.02O3 (ref. 73) UC 19 fold (PL) Increase in lifetime of intermediate levelsY1.917Li0.05Yb0.03Er0.003O3 (ref. 74) UC 3 fold (PL) Change in crystal eld and decrease in OH concentration

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because of the fact that as the crystallinity increases, the particlesize also increases, which reduces surface quenching centers(surface to volume ratio decreases). The increased particle sizeis not required in many technological applications, which havebeen prompted due to the advancement in nanotechnology;therefore, researchers usually like to use a passive layer, byforming a core–shell structure, to reduce the surface quenchingcenters.54 The co-doping of alkali ions in the matrix stronglyaffects the crystal structure, crystallinity, grain size, surfacemorphology, quenching centers (OH�, NO3

�, etc.) and createdistortion in the matrix.56–142 Therefore, it would be interestingto study the effect of alkali ions, particularly the Li+ ion, on theluminescence properties of lanthanide ions doped in variousmatrices and try to nd out some meaningful information tounderstand the underlying mechanism, and also to explore thefuture possibilities. The following sections aim to investigatethese aspects in detail.

3. Effect of Li+ ion on theluminescence of Ce3+ and Eu2+ (5d–4ftransition) ions

As discussed in the preceding sections, the origin of lumines-cence from Ce3+ and Eu2+ ions is different from the character-istic luminescence of lanthanide(III) ions involving 4f–4ftransitions. The luminescence in Ce3+ and Eu2+ ions is due to4fn�15d1 to 4fn (ground state) transitions (where n ¼ 7 for Eu2+


and n ¼ 1 for Ce3+).7–19 These transitions are allowed and havelifetimes on the order of nanosecond. They exhibit broad-bandluminescence spectra, with their maxima lying in the yellowregion, and are very useful for lighting applications (LEDs,automobile lamps, backlighting, etc.). As the 5d orbitals areexternal, the position of these levels, and thereby the excitationand emission bands, strongly depend on the local environmentof the ions (crystal elds of the host matrices). Dorenbos55 hasmade a clear correlation between the coordination polyhedron(in which Eu2+ or Ce3+ lies) and the crystal eld splitting. Theyhave shown that crystal eld splitting tends to be the largest foroctahedral coordination followed by cubal, dodecahedral, and itis the smallest for tricapped trigonal prism and cuboctahedroncoordination. Thus, through appropriate host selection (withdifferent crystal eld splitting of the 5d band), the lumines-cence of Eu2+ and Ce3+ can be tuned according to the require-ment. Park et al. have shown that Eu2+ doped Sr2SiO4 andSr3SiO5 are stronger yellow emitting phosphors than theconventional light-conversion phosphor YAG:Ce3+.8b Co-dopingof Li+ ion with Eu2+/Ce3+ not only improves the luminescenceefficiency but brings out changes even in the excitation andemission spectral ranges, so both these activators ions havebeen effectively studied in presence of Li+ ions. Table 2 gives abrief overview of different host and dopant ions that have beenstudied to visualize the effect of Li+ co-doping.

Grandhe et al.12 investigated the effect of Li+ ions on theemission characteristics of Eu2+ emission in phosphate matrix(Na1�yLiyCa1�xPO4:xEu

2+) synthesized by a conventional solid

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Fig. 2 X-ray diffraction (XRD) patterns of Y1.95�xLixO3:Eu0.05 and Gd1.95�xLixO3:Eu0.05 phosphor. Reprinted (adapted) with permission from ref. 57Copyright © 2005 Elsevier B. V. All rights reserved.

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state reaction method. The excitation spectra of NaCaPO4:Eu2+

phosphors revealed a broad excitation band ranging from 250 to450 nm with a maximum intensity at 373 nm. Upon 373 nmexcitation, the phosphor exhibits an intense bluish-greenemission band centered at 505 nm. Fig. 1(b) shows that theNa1�yLiyCa0.99PO4:0.01Eu

2+ phosphor containing 4 mol% of Li+

ions (y ¼ 0.04) exhibits maximum luminescence intensity. It issuggested that the difference in the ionic radii probably givesrise to diversity in the crystal lattice around the luminescencecenter. This may in turn inuence the spin–orbit coupling andthe crystal eld around the Eu2+ ions. Liu et al.8 have synthe-sized the Li2CaSiO4:Eu

2+ phosphor by a conventional solid statereaction method and found that the Li+ ion changes the exci-tation and emission characteristics of Eu2+ effectively in silicatematrix also. It was again concluded to be due to the change inhost lattice/crystal eld due to the involvement of Li+ ions in thecrystal structure. They proposed that the Eu2+ ions go to dis-torted dodecahedral Ca sites, which causes strong crystal eldsplitting leading to a broad excitation band extending from theUV to visible regions. Also, the high concentration of Li+ ions inthe structure constrained the distortion of the emission center(Li+ ions, which are behind the oxygen ions surrounding theemission centers, will restrict the expansion of the emissioncenters), which results in a smaller Stokes shi.

A similar study for Ce3+ doped into a silicate matrix has alsobeen presented by Zhu et al.16 The authors prepared aSr3SiO5:Ce

3+,Li+ phosphor, by a solid state reactionmethod, andshowed that under near-UV excitation at 415 nm, the phosphoremits a bright greenish-yellow color centered at 532 nm (origi-nating from the 5d to 4f (2F7/2 and 2F5/2) transition of Ce3+).Upon substitution of Sr2+ ions with Mg2+ (smaller ionic radiusthan Sr2+) and Ba2+ (larger ionic radius than Sr2+) ions, the peakposition of the PL spectrum shis towards higher and lowerenergy, respectively. This is because of the fact that as Mg2+ ions

27044 | RSC Adv., 2014, 4, 27039–27061

substitute Sr2+ ions the degree of covalence in the Ce–O bonds isdecreased, and consequently less negative charge transfers toCe3+ ions, thus increasing the difference between the 4f and 5dlevels (causes blue-shi in the emission band). In contrast, thesubstitution of Sr2+ with the larger Ba2+ increases the degree ofcovalence in the Ce–O bonds, which causes red-shi in theemission band. Thus, the emission color of Sr3SiO5:Ce

3+,Li+ canbe tuned by partial replacement of Sr2+ with Mg2+ or Ba2+, whichmakes it applicable to a variety of LEDs (from 410–450 nmchips). In another similar article by Shen et al.,14 the role of Li+

has been proposed in the same host in more detail, andexplores that the Li+ codoping compensates the charge differ-ence between the Ce3+ and Sr2+ ions and thus helps to incor-porate the Ce3+ into Sr2+ sites. The phosphor (Sr3SiO5:Ce

3+,Li+)shows high luminous efficiency under near-UV/blue light exci-tation, and the obtained emission is comparatively broaderthan that of the Eu2+-activated yellow phosphor. To improve thecolor rendering properties of the Sr3SiO5:Ce

3+,Li+ phosphor,Jang et al.18 have used additional Pr3+ ions (to enhance the red-emitting component) along with Ce3+ ions. Under blue lightexcitation, energy transfer from Ce3+ to Pr3+ ions takes place,which gives a shoulder at 619 nm (of Pr3+ ions) in the PLspectrum of Sr3SiO5:Ce

3+,Pr3+,Li+. The energy transfer has beenproved by steady-state and time domain luminescencemeasurements. Further, by making use of monodisperse CdSeQDs and the phosphor on a blue LED chip, they produced whiteLEDs with excellent color rendering properties (see Fig. 1(d) and(e)).

Wang et al.17 have prepared CaSiN2 powders at 1550 �C in aN2/H2 (6%) atmosphere by the solid-state reaction methodusing Ca3N2 and Si3N4 as the starting materials. The lumines-cence measurements on Ce3+/Li+ co-doped CaSiN2�2d/3Od showsa broad yellow band peaking at 530 nm. The maximum PLintensity was attained for the sample with the Ce3+ composition


Fig. 3 Upconversion emission in (a) ultraviolet-blue, and (b) green-red regions in Y2O3 nanocrystals doped with 1 mol% Er3+ ions and co-dopedwith 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 4I11/2 /4I15/2 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 4I11/2 / 4I15/2 transition in the range of 1000–1050 nm under 970 nm diode laserexcitation. (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, AIPPublishing LLC.

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x ¼ 0.01, above this concentration quenching occurs. Theoxygen content in the sample was analyzed and is found to be dz 0.2. Further, the chromaticity coordinates (0.362, 0.571) alsodemonstrate that CaSiN2�2d/3Od:Ce

3+/Li+ is a good yellowphosphor (see Fig. 1(a)). Hao et al.10 studied the PL properties of

Table 4 Luminescence enhancement (and their explanation) in varioustype of host in presence of Li+ ions

Sample DS/UC Luminescence enhance

Ca0.8Li0.1Er0.02Yb0.08MoO4 (ref. 76) UC 83 fold (PL)Ca0.95MoO4:0.05Dy

3+,0.05Li+ (ref. 77) DS 1.3 fold (PL)Ca0.95MoO4:0.05Tb

3+,0.05Li+ (ref. 78) DS — (PL)Li0.05Eu0.05La0.9PO4 (ref. 79) DS 2 fold (PL)La0.93Eu0.07VO4:0.25 wt% Li+ (ref. 80) DS — (PL)YPO4:2 at% Dy,7 at% Li+ (ref. 81) DS — (PL)YPO4:5% Eu3+,5% Li+ (ref. 82) DS 2.5 fold (PL)YPO4:5% Eu3+,3% Li+ (ref. 83–85) DS — (PL)YVO4:0.03% Eu3+,2% Li+ (ref. 86) DS 1.43 (PL)YVO4:0.03% Eu3+,1% Li+ (ref. 87) DS 1.7 (PL)


the CaO:Ce3+,Li+ phosphor. It is found that, upon 474 nmexcitation, the Li+ ion co-doped phosphor shows a 1.88 foldenhancement in PL intensity (see Fig. 1(c)). The XRD resultsshow the presence of a small amount of CeO2, a startingmaterial, in the CaO:Ce3+ phosphor, which suggests low

activators during down-shifting and upconversion processes in ABO4

ment Remarks

Local crystal eld distortion around Er3+ ionCharge compensationOxygen vacancyDecrease in interstitial oxygen and reduced internal reectionsOxygen vacancy, increased grain sizeImproved crystallinityIncreased crystallinity and grain sizeIncreased crystallinityIncreased crystallinity and surface roughnessIncreased crystallinity and surface roughness

RSC Adv., 2014, 4, 27039–27061 | 27045

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 0–15mol% Li+) near 2Q ¼ 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) PLspectra of YVO4:Eu

3+ ceramics with different Li+ ion content. (e) High resolution transmission electronmicroscopy image of the 500 �C annealedsample of 5 at% Li+ co-doped YPO4:5Eu. (a) and (b) Reprinted (adapted) with permission from ref. 76. Copyright © 2012 Elsevier B. V. All rightsreserved. (c) and (d) were published in ref. 86 and are reprinted with permission. Copyright © 2010 Elsevier Masson SAS. All rights reserved. (e) istaken from ref. 84; used in accordance with the Creative Commons Attribution 3.0 Unported License.

RSC Advances Review

solubility of Ce3+ in CaO. Upon Li+ ion co-doping the intensity ofthis peak decreases, indicating that more CeO2 is converted toCe3+ ions for effective doping into the CaO host lattice. Thisresults in increased absorption of the excitation photons inCaO:Ce3+,Li+ causing PL enhancement. This might be becauseof the charge compensation due to the presence of Li+ ions andthe effect of Li+ as a ux to increase diffusivity of the rawmaterials, which causes more Ce3+ ions to go into CaO lattice.Jia et al.15 have prepared the Lu2SiO5:Ce

3+,Li+ phosphor byPechini sol–gel chemistry. They optimized the concentration ofCe3+ and Li+ ions, which was found to be 0.006 wt% and 0.02wt%, respectively. The PL intensity of 0.006 wt% Ce3+, 0.02 wt%Li+ co-doped phosphor sample was 2.2 times higher than the0.006 wt% Ce3+ doped phosphor. It is suggested that the Ce3+

ions occupy two different crystallographic sites (with coordi-nation number 6 or 7) in the monoclinic lattice (space group C2/c) of Lu2SiO5.143 This causes two luminescence centers Ce1 andCe2 in this host. The peaks at 3.16 and 2.95 eV originate fromthe emission of the Ce1 center, while another peak at 2.74 eVcorresponds to the Ce2 center. With the increase of Li+ ion co-doping concentration, initially the intensity of the Ce1 emissionincreases and then decreases, while the Ce2 emission intensity

27046 | RSC Adv., 2014, 4, 27039–27061

always decreases. This suggests energy transfer from Ce2 to Ce1centers, which was demonstrated by time-domain measure-ments. Thus the enhanced luminescence of Lu2SiO5:Ce

3+

phosphors by Li+ ion co-doping is mainly regarded as the resultof the production of the local distortion in host lattice, whichalters the crystal eld around activator and results in the energytransfer from Ce2 to Ce1 centers.15

The effects of Li+ ions have also been investigated forpersistent/aerglow emission. Kojima et al.9 have studied theaerglow luminescence properties of green emitting Ce3+ andPr3+ co-doped CaS phosphors. The aerglow time of CaS:-Ce3+,Pr3+ was relatively short (about 10 min). When Li+ isintroduced in the matrix, the aerglow intensity and aerglowtime of CaS:Ce3+,Pr3+ both increase, by three and four fold,respectively. It is shown that the Li+ ions go to the interstitialspaces in the crystal lattice and increase the lattice constant.When a Li+ ion is incorporated into the crystal lattice of CaS, anew cation vacancy is formed for the charge compensation inthe Ca2+ site, which increases the lattice strain. This cationvacancy working as an electron trap can capture some of theexcited electrons and thereby increase the charging process.Chen et al.11 have prepared SrAl2O4:Eu

2+,Ce3+,Li+, another


Fig. 5 (a) Excitation and emission spectra of CaAl2O4:Eu3+,R+ (R ¼ Li,

Na, K) under 254 nm excitation and monitored at 615 nm. (b)Upconversion emission spectra of SrAl2O4:0.01Ho

3+,0.20Yb3+,yLi+.Inset shows the picture of the emission of SrAl2O4:0.01Ho

3+,0.20Y-b3+,0.06Li+ ceramics under excitation wavelength 980 nm. (a)Reprinted (adapted) with permission from ref. 89. Copyright © 2010Elsevier. All rights reserved. (b) Reprinted (adapted) with permissionfrom ref. 90. Copyright © 2012 Elsevier B. V. All rights reserved.

Review RSC Advances

persistent luminescence material, by a solid-state reactionmethod using H3BO3 as ux. Li+ ions compensate the chargedefects caused by the non-equivalent substitution of Sr2+ withCe3+. Thus, the luminescence intensity of SrAl2O4:Eu

2+ issignicantly enhanced when Ce3+ and Li+ both are present inthe sample.

Table 5 Luminescence enhancement (and their explanation) in varioperovskite host in presence of Li+ ions

SampleDS/UC

Luminescenceenhancement Re

BaTiO3:2%Er3+,3%Li+ (ref. 92) UC 56 fold LoBaTiO3:1%Er3+,5%Yb3+,7%Li+ (ref. 93) UC 10 fold LoCaTiO3:Pr

3+,1%Li+ (ref. 94 and 95) UC 3.5 fold ImCaZrO3:5%Eu3+,10%Li+ (ref. 96) DS 3 fold ChSrZnO3:Eu

3+,M+ (Li, Na, K)97 DS — Ch


4. Effect of Li+ ion on theluminescence of lanthanide(III) ionsthrough 4f–4f transitions4.1 Effect of Li+ ion on the luminescence of lanthanide ionsin yttrium oxide (Y2O3) and gadolinium oxide (Gd2O3) hostmatrices

Y2O3 and Gd2O3 are well explored host matrices for lumines-cence studies owing to their higher chemical and mechanicaldurability, higher thermal stability, excellent optical propertiesand relatively low phonon cutoff energy (400–500 cm�1) incomparison to other host matrices.144–150 Another advantage ofusing Y2O3 and Gd2O3 host matrices is that one can easilyprepare 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 easilyby controlling different synthesis parameters. Moreover, theionic radii of Y3+ and Gd3+ is similar to the other activatorlanthanide ions; thus they can easily be accommodated in thematrix.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, haveused Li+ ions to further enhance the luminescence of lantha-nide ions doped in this matrix.56–74 Table 3 summarizes theconcentrations of different activator (lanthanide) ions and Li+

ions to obtain the optimum luminescence. The table alsodescribes the mode of luminescence measurement (e.g. DS,UC), quantitative information about luminescence enhance-ment and the explanation given by different research groups forthe luminescence enhancement. It is evident from the Table 3that different research groups have proposed different mecha-nisms for luminescence enhancement of lanthanide activatorions in presence of Li+ ions. A detailed description of the workdone so far and their salient features are summarized below,including a detailed description of the representative works inthe area concerned.

Dhananjaya et al.58 studied the effect of Li+ ions on the PL ofEu3+ in Gd2O3 host. They proposed that when the concentra-tion of Li+ ions in the Gd2O3 host is low (<4 mol%), substitu-tion of Gd3+ ions with Li+ ions induces oxygen vacancies. Thiscauses a crystal eld distortion around the activator ions

us activators during down-shifting and upconversion processes in

marks

cal crystal eld distortion around Er3+, reduction in OH concentrationscal crystal eld distortion around Er3+ ionsproved crystallinity, increase in surface roughnessarge compensationarge compensation

RSC Adv., 2014, 4, 27039–27061 | 27047

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:Pr

3+

phosphors. (c) Upconversion emission spectra of BaTiO3:2Er3+ co-doped with 1–6mol% 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 4S3/2 /4I15/2 and

4F9/2 /4I15/2

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:Pr

3+ phosphors. (a and b) and (e–h) are published in ref. 94 and reprinted withpermission. Copyright © 2011 Elsevier Masson SAS. All rights reserved. (c) Reprinted (adapted) with permission from ref. 92. Copyright © 2011Elsevier B. V. All rights reserved. (d) Reprinted with permission from ref. 93. Copyright © 2010 Elsevier B. V. All rights reserved.

RSC Advances Review

(Eu3+), which results in increased PL. The oxygen vacanciesalso act as a sensitizer for activator lanthanide ions, by theinvolvement of an effective energy transfer from the strongcharge transfer states (which come into picture due to thevacancy itself) to lanthanide ions. Further, they explore that as

Table 6 Luminescence enhancement (and its explanation) in differentoxide nanoparticle hosts in the presence of Li+ ions

Sample DS/UC Luminescence enhancem

MgO:1.5%Dy3+,20%Li+ (ref. 98) DS 4 fold (PL)

SnO2:Eu3+,Li+ (ref. 99) DS 14 fold (PL)

TiO2:Er3+,Yb3+,Li+ (ref. 101) UC 215 fold (PL)

ZnO:Er3+,Li+ (ref. 102) UC PL switching (red to gre

ZnO:Dy3+,Li+ (ref. 103) DS 10 fold (PL)

ZnO:Er3+,Li+ (ref. 104) UC 120 fold (PL)

ZrO2:Er3+,Yb3+,Li+ (ref. 105) UC 1.5 fold (PL)

27048 | RSC Adv., 2014, 4, 27039–27061

the Li+ concentration increases, along with substitutional Li+

sites, interstitial sites of Li+ ions are also present, which causeGd3+ ion vacancies. This as a result produces more defectsaround the activator lanthanide ions. In this case the Starklevel in the emission spectrum shows larger splitting. In a

activators during down-shifting and upconversion processes in metal

ent Remarks

Improved crystallinity, increased grain size, change in size,change in crystal eld around Dy3+ ionsImproved crystallinity, increased grain size, increased oxygenvacanciesChange in local crystal eld around Er3+ ion

en) Change in local crystal eld around Er3+ ion, increasedcrystallinityRadiative recombination of the large amount of trappedcarriers excited from ZnO host, improved crystallinityChange in local crystal eld around ions, reduced OH groupconcentrationChange in local crystal eld around ions, increased grain size


Fig. 7 (a) Emission spectra of SnO2:Eu and Sn0.8Li0.2O2�d:Eu under the excitation wavelength of 300 nm. Room temperature upconversionspectra of (b) Er3+ doped and (c) Er3+–Li+ (1.0 mol%) co-doped ZnO nanoparticles under 980 nm diode laser excitation. (d) Powder X-raydiffraction 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 emissionspectra of ZrO2:Er

3+/Li+ (0 or 5 mol%) nanocrystals under 976 nm excitation. The inset shows the ln–ln plot of intensity versus excitation powerdependence of the green and red emissions. (a) Reprintedwith 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.

Review RSC Advances

similar study, Shin et al.57 have monitored cath-odoluminescence (CL) of Eu3+ ions in Gd2O3 and Y2O3 as hostmatrices in the presence of Li+ ions, and they also proposedthe charge compensation mechanism for CL enhancement ofEu3+ ions in both hosts, while for the case of Gd2O3 host the

Table 7 Effect of Li+ ion on the UC luminescence of lanthanide ions do

Sample Change in luminescence proper

NaYF4:Yb3+/Er3+,K+/Li+ (ref. 107) UC emission intensity decreases

doping, whereas it increases up

GdF3:Yb3+/Er3+,Li+ (ref. 108) Change in UC emission color fro

red emission is 8 times higher tNaGdF4:Yb

3+/Er3+

NaYbF4:Yb3+/Er3+,Li+ (ref. 109) UC color tunability

NaYF4:Yb3+/Tm3+,K+/Li+ (ref. 110) Decrease in concentration quen

NaGdF4:Yb3+/Er3+,Li+ (ref. 111) Enhanced UC luminescence (47

and 23 for ions, increased crystared), increased paramagnetismstate, decrease in distance betw(for increased paramagnetism)

ErF3:Er3+,Li+ (ref. 112) Four fold enhancement in UC e

Lu6O5F8:Yb3+/Er3+,Li+ (ref. 113) Increased UC emission

NaYF4:Yb3+/Er3+,K+/Li+ (ref. 114) Color tunability of UC emission


group additionally put forward other mechanisms alsodescribed in the subsequent paragraph.

Along with charge compensation mechanisms, several othermechanisms for the luminescence enhancement have also beenreported. The cubic structure of Y2O3 and Gd2O3 offers two

ped in fluoride host matrices

ties Remarks

upon Li+

on K+ dopingChange in morphology and crystal structureupon Li+ doping and lowering of crystalsymmetry around Er+ ions in K+ doping

m yellow to red,han in

Change in crystal symmetry around Er3+ ions,back energy transfer

Change in crystal symmetry around Er3+ ionsand weakened polarization effect

ching Crystal distortion and increased crystal sizefold for greenl size, increasedlifetime of 4S3/2een Gd3+ ions

Change in crystal symmetry around Er3+

mission Change in crystal symmetry around Er3+ ionsChange in crystal symmetry around Er3+ ions,increased crystallinity, reduced quenchingcenters (OH�)Change in crystal structure and lowering ofcrystal symmetry around Er+ ions

RSC Adv., 2014, 4, 27039–27061 | 27049

Fig. 8 (a) Schematic illustration of the formation process of LixNa1�xYF4 with various morphologies. (b) UC luminescence spectra of the as-prepared LixNa1�xYF4:Yb

3+/Er3+ products under 980 nm excitation at room temperature (x ¼ 0, 0.15, 0.30, 0.50, 0.70, 0.85 are denoted by S0 toS5). (c) Schematic illustration for the possible formation process of KyNa1�yYF4:Yb

3+/Er3+ with various morphologies. (d) UC luminescencespectra of the as-prepared KyNa1�yYF4:Yb

3+/Er3+ products under 980 nm excitation at room temperature (y ¼ 0.15, 0.30, 0.50, 0.70, 0.85 aredenoted by S6 to S10). The inset shows the integral intensity of green and red emission as a function of the concentration of K+ ions. Reprintedwith permission from ref. 107 Copyright © 2013 Elsevier B. V. All rights reserved.

RSC Advances Review

different crystallographic sites for activator lanthanide ions:one with C2 (without center of inversion) and the other with S6(with center of inversion (C3i)) symmetry.57 The transitions inthe lanthanide ions are affected effectively, as far as the inten-sity and splitting is concerned, by the symmetry of the crystaleld of the host. For example, when a Eu3+ ion occupies S6symmetry, the magnetic dipole transition 5D0 / 7F1 is thedominant one. In case of Eu3+ doped material the occupancyratio 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 radiiof Y3+, Gd3+ and Li+ ions are 0.89, 0.94 and 0.76 A, respec-tively.57,58 Thus, Li+ ion substitution deforms the Gd2O3 latticemore as compared to Y2O3 lattice, and the statistical distribu-tion of lanthanide (Eu3+) and Li+ ions at the C2 and S6 sites inthe Gd2O3 lattice becomes more intricate. Shin et al.57 have alsoreported that changes in ORS aer Li+ ion incorporation islarger in Eu3+ doped Gd2O3 than the Y2O3 matrix. Based on thisinformation, Shin et al. proposed that the mechanism ofluminescence enhancement in Li+ ion doped Gd2O3 might bedifferent to that in the Y2O3 host. X-ray diffraction (XRD)patterns (shown in Fig. 2) recorded by Shin et al.57 reveal thatY1.95�xLixO3:Eu0.05 phosphor samples have cubic structure evenat higher doping concentration of Li+ ions. In contrast,Gd1.95�xLixO3:Eu0.05 phosphor samples show monoclinic tocubic transformation as the doping concentration of Li+

27050 | RSC Adv., 2014, 4, 27039–27061

increased. This gradual change frommonoclinic to cubic latticeis also conrmed by Dhananjaya et al.58 This change frommonoclinic to cubic lattice in Gd2O3 host is also one of thereasons for increased luminescence.57,58

Fan et al.62 have reported the effect of Li+ ion in enhancingthe PL of Nd3+ ions doped into Y2O3 samples. They proposedthat among two different crystallographic sites (C2 and C3i

symmetry, the ratio of C2 to C3i sites is 3 : 1) of cubic Y2O3,electric dipole transitions of lanthanide ions are forbidden forC3i sites. Thus, when Li+ ions occupy the C3i sites, it destroys theinversion symmetry and the forbidden electric dipole transi-tions of lanthanide ions (Nd3+) become partially allowed.Further, when Li+ ions occupy the C2 sites with higher pop-ulation in the lattice, the reduced symmetry of these sites againresults in the enhancement of PL of lanthanide ions (Nd3+).Thus, in both cases enhanced PL is observed. When the Li+ ionconcentration is too high it might cause a large local distortionaround optically active centers (lanthanide ions), which canlead to the PL quenching.

In similar context, several studies have also been reported forthe enhancement of UC emission. A study by Sun et al.60 reportsthe UC phenomenon in a silica coated Tm3+/Yb3+/Li+ ion dopedGd2O3 matrix, and found �10 fold enhancement of UC emis-sion in the sample doped with 6 mol% of Li+. They have sug-gested that the enhancement in PL is due to a change in thelocal asymmetry around Tm3+ ions. On further increasing the


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 4I11/2/4I13/2 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 placedinto a quartz vessel and excited under a 980 nm laser to visually demonstrate the naked-eye-visible brilliant red light. (a and b) Reproduced fromref. 111 with permission from The Royal Society of Chemistry. (c–g) Reprinted with permission from ref. 108 Copyright © 2012Wiley-VCH VerlagGmbH&Co. KGaA, Weinheim.

Review RSC Advances

Li+ ion concentration (up to 10 mol%) the UC emissiondecreases substantially. They have reported that at higher Li+

ion concentrations, the local crystal eld around Tm3+ ions yetagain becomes symmetric, which is unfavorable for UC emis-sion. 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+) havealso studied the effect of Li+ ions on the enhancement of UC/PLof lanthanide ions in Y2O3 and Gd2O3 host matrices and sug-gested the change in crystal symmetry around the lanthanideions as one of the reasons for PL enhancement.

The other contributing factors in the PL enhancement oflanthanide ions in these hosts are change in morphology,crystallinity and grain size of materials.56,59,68,69 Atomic forcemicroscopy (AFM), which is used to study the surfacemorphology, results reported by Jeong et al.56 shows that Li+ iondoping increases the grain size and roughness of theGd2O3:Eu

3+lms. Larger grain size reduces the density of the

grain boundaries, which in turn might be responsible forreduced adsorption and/or scattered light generated inside thelm, which favors PL enhancement. Furthermore, increasedcrystallinity due to Li+ ion doping in samples causes higheroscillating strength for optical transitions, which also favors PLenhancement.56 Yi et al.59 (Gd2O3:Eu

3+), Fan et al.62 (Y2O3:Nd3+),

Yi et al.68 (Y2O3:Eu3+) and Sun et al.69 (Y2O3:Eu

3+) have also


proposed that increased grain size, roughness and crystallinityof materials, in the presence of Li+ doping, is among one of thepotential reasons for luminescence enhancement of lanthanideions.

Time-domain studies have also been exploited to search forthe possible reasons behind the enhancement of PL/UC emis-sion. It has been observed that changes in lifetime of interme-diate state/states participating in energy transfer process alsoplay a crucial role. Chen et al.64 suggested that due to changes inthe local crystal eld of the Y2O3 matrix around the lanthanideions, observed lifetimes of intermediate states, 4I11/2 (Er

3+) and2F5/2 (Yb3+), can change. This can also modify the theoreticallifetimes of lanthanide ions by slightly changing their wave-functions. They monitored decay proles of 4I11/2 / 4I15/2transitions of Er3+ ions (at 1015 nm) in Y2O3 nanocrystals in thepresence of 0–15 mol% Li+ ions (see Fig. 3(d)).66,67 The lifetimeof the intermediate state 4I11/2 was found to be 0.8(2) ms and2.7(3) ms for 0 and 3 mol% Li+ ions, respectively, and it is about3.4(3) ms for higher Li+ ions concentrations (5–15 mol%). Thissuggest that the local crystal eld around Er3+ ions is graduallytailored 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 thelifetime increases. Fig. 3(a) and (b) show UC emission spectra of

RSC Adv., 2014, 4, 27039–27061 | 27051

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

Sample DS/UCLuminescenceenhancement Remarks

Ba2GdNbO6:Eu3+/Dy3+,Li+ (ref. 115) DS — Improved crystallinity, increased grain size

BaZn2(PO4)2:Sm3+,R+ (Li, Na, K)116 DS — Charge compensation

Ca3B2O6:Dy3+,Li+ (ref. 117) DS — Charge compensation, ux effect, change in

local environment around Dy3+ ionsCaSO4:Tm

3+/Dy3+,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 distortionYNbTiO6:Eu

3+,Li+ (ref. 120) DS 3 fold Improved crystallinity, increased grain sizeYBO3:Eu

3+,Li+ (ref. 121) DS Improved crystallinity, increased grain sizeZn2SiO4:Yb

3+,Er3+,Li+/Bi3+ (ref. 122) UC — Change in local environment around Er3+

SrB4O7:Eu2+, Li+ (ref. 123) DS — Change in electric eld distribution in lattice structure

Lu2SiO5:Ce,Li+ (ref. 125) DS 2.2 fold Change in crystal eld around Ce3+

Y2MoO6:Eu3+,Li+ (ref. 126) DS — Improved crystallinity

NaSrBO3:Tb3+,Li+ (ref. 127) DS — Charge compensation

Sr2SiO4:Eu3+,(Li+, Na+, K+)129 DS — Charge compensation

CaSnO3:Tb3+,(Li+, Na+, K+)131 DS — Charge compensation

Gd6MoO12:Li+/Ln3+ (ref. 132),

(Ln3+ ¼ Yb3+/Er3+/Tm3+)UC — Change in crystal symmetry around Ln3+

Y2SiO5:Pr3+,Li+ (ref. 133) UC 9 fold Change in crystal symmetry around lanthanide ions,

reduction in activator ions clusteringY2Zr2O7:Dy

3+,Li+ (ref. 134),(Ln3+ ¼ Yb3+/Er3+/Tm3+)

UC 2 fold Improved crystallinity

LixCa1�2xEuxSiyMo1�yO4 (ref. 136) DS — Charge compensationCa2BO3Cl:Sm

3+(Li+, Na+, K+)139 DS — Charge compensationCaSi2O2N2:Eu

2+,Dy3+,Li+ (ref. 140) DS — Charge compensation

RSC Advances Review

Er3+ doped Y2O3 in the absence and presence of Li+ ions. In ananother work, Fan et al.73 monitored the decay proles of 4I13/2/ 4I15/2 transition (at 1530 nm) of Er3+ ions in Y2O3 matrix inthe presence of 0–6 mol% of Li+ ions under 976 nm excitation.They also observed an increase in the lifetime with increasingconcentration of doped Li+ ions; the lifetime varies from 3.14ms to 3.28 ms for 5 mol% Li+ ions. The study concludes andproposes that the Li+ ion tailors the local crystal eld around theEr3+ ion, and therefore modies the theoretical lifetime byslightly changing the wavefunction, somewhat similar to theearlier reports.

Furthermore, some of the studies interestingly report thatthe doping of Li+ ions suppresses the PL quenching entities e.g.molecules with OH�, NO3

�, COx, etc. groups having highvibrational frequencies. During phosphor synthesis by chemicalroutes (viz. combustion, sol–gel methods) incomplete combus-tion of organic fuel introduces luminescence quenchingcenters, which decrease the PL intensity. Sun et al.60

(Gd2O3:Tm3+/Yb3+), Fan et al.62 (Y2O3:Nd

3+), Li et al.71

(Y2O3:Tm3+/Yb3+) and Mishra et al.74 recorded FTIR (Fourier

transform infrared) spectra of phosphor materials without andwith Li+ ion doping in various hosts and found that the broadabsorption bands peaking at 1626 and 3454 cm�1, owing tovibrational features of O–H groups, are suppressed signicantlyin the presence of Li+ ions (see Fig. 3(c)).74 In addition, Mishraet 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 dueto the fact that the Li+ ion neutralizes the hydroxyl (and other)

27052 | RSC Adv., 2014, 4, 27039–27061

groups present in the phosphor. Thus, the addition of Li+ ionsreduces the non-radiative transition probability and henceenhances the PL efficiency.

Thus in summary, Li+ ion doping in Gd2O3 and Y2O3 hostssignicantly affects both the structural and optical parameterssuch as charge compensation, change in crystal symmetryaround activator ions, increased crystallinity, increased grainsize, change inmorphology (higher surface roughness), increaseof lifetime of intermediate levels of activator ions, decrease innon-radiative channels (OH�, NO2

� concentrations), etc., whichaltogether enhances the optical emission of lanthanide ions.

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

Next to Y2O3 and Gd2O3 host matrices, another important classof compounds for PL studies is ABO4 (where A is an alkalineearth metal or trivalent lanthanide ion and B is a hexavalent orpentavalent transition metal ion) family. There are some wellknown hosts in this family of compounds, viz. CaMoO4, YVO4,GdVO4, YNbO4, YPO4, CaWO4, MgWO4, ZnWO4, etc., which haveattracted enormous interest in preceding years.83–85,151–154 Thecharacteristic feature of these compounds is the existence of astrong ligand (oxygen) to metal charge transfer band in UVregion.151–154 Under this charge transfer band excitation theyexhibit broadband PL, mostly in the blue region. An additionalfeature of this type of compound is that they serve as sensitizersfor the activator lanthanide ions by involving an energy transferfrom charge transfer state to the resonant level of lanthanide


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ions, so they do not need a co-dopant for the sensitizationprocess.24 They are stable at high temperatures. Lanthanidedoped YVO4 has already been explored for the fabrication oflasers. Recently, different research groups across the world havebeen studying the role of Li+ on the PL enhancement oflanthanide ions in these hosts (along with other hostmatrices).75–88 Table 4 gives some brief information aboutdifferent hosts in this family, activator ions, maximum PLenhancement and proposed mechanisms to explain the PLenhancement. The following discusses different mechanisms.

Compounds in this family usually have relatively largerphonon cutoff ranges (�800 cm�1) than Y2O3 and Gd2O3 hostmatrices, and therefore, for achieving efficient UC emission,sensitization becomes utmost essential. Chung et al.76 havestudied the role of Li+ ions on UC emission of the Er3+/Yb3+

doped CaMoO4 phosphor. They observed �83 fold enhance-ment in the green emission of Er3+ ions in the presence of 10mol% of Li+ ion (see Fig. 4(b)). They attributed it as being due toa favorable structural modication (change in local crystal eldsymmetry) in the host lattice. The XRD patterns reveal thatCaMoO4 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) shis gradually toward higher angleup to 5 mol% Li+ ion doping; however contrary to this, uponfurther increasing Li+ ion concentration it shis toward smallerangles (see Fig. 4(a)). The gradual peak shi toward higherangle below 5 mol% suggests that the Li+ ions occupy the Ca2+

sites in lattice. However, upon further increasing the concen-tration of Li+ ions, the ions go to interstitial sites. This leads tothe expansion of lattice. The occupation of Li+ ions in theinterstitial sites distorts the local crystal eld around Er3+ ions,and plays an important role in the enhancement of UCemission.

Li et al.77,78 have also studied the effect of Li+ ions in the samehost, CaMoO4, but they have used Dy3+ and Tb3+ as the activatorions instead of Er3+. To explain the PL enhancement for Dy3+

doped CaMoO4 in the presence of Li+ ions, they proposed thatas the ionic radius in the case of the Dy3+ ion (0.091 nm) issimilar to that of the Ca2+ ion (0.099 nm), it can easily beaccommodated into the Ca2+ site. However, the Li+ ion, whichhas a considerably small radius (0.076 nm), will prefer to go intothe interstitial spaces. The replacement of divalent calcium bytrivalent dysprosium creates charge imbalance, and thisimbalance is compensated by Li+ ion co-doping, whichenhanced the PL intensity signicantly. Further, in anotherwork, 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+ dopedphosphor. The study proposes that the enhanced PL is due tothe creation of oxygen vacancies aer the occupation of Ca2+

sites by alkali ions.Phosphor samples (LaVO4:Eu

3+) prepared by Park et al.80

have both tetragonal and monoclinic phases. Of these two, thetetragonal phase of LaVO4:Eu

3+ showed higher PL than themonoclinic 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 themonoclinic to tetragonal phase transformation, which causeseffective energy transfer from VO4

3� 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:Eu

3+ phosphor is �1.43 times greater than that ofYVO4:Eu

3+ ceramic (see Fig. 4(c)). They proposed that theenhanced PL is due to improvement in crystallinity as well asdue to the enhanced surface roughness.

Parchur et al.81,83–85 in our group have also studied the effectof Li+ ions on PL properties of Dy3+ and Eu3+ doped YPO4

phosphors. The Li+ ion co-doping improves the crystallinity ofthe material and reduces agglomeration among the particles.Upon addition of Li+ ions, the shi in the (200) diffraction peaktowards a lower 2Q value suggests that the Li+ ions occupyinterstitial sites instead of Y3+/Eu3+ sites. Furthermore, the Li+

ion co-doping increases the crystallite size as well (crystallitesize corresponding to Li+ ¼ 0, 3, 5, 7 and 10 at% co-dopedYPO4:Eu

3+ are found to be 20, 23, 36, 42 and 60 nm, respec-tively). 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 indicategood crystallinity of the sample. This increased crystallinity andgrain size causes signicant enhancement in PL. Huang et al.82

have also proposed that the increased crystallinity is the maincause 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, themainmechanism involved in the PL enhancement is the changein local crystal eld symmetry by modication of the crystallattice, mostly due to interstitial occupancy of Li+ ions anddevelopment of vacancies. In addition to this, improvement incrystallinity, grain size, shape, etc. and energy transfer processesalso have been accepted as potential mechanisms, as in otherhosts too.

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

Like Y2O3 and Gd2O3, calcium and strontium aluminates alsohave low phonon frequency, which makes them suitable forlanthanide ion doping.150–160 Lanthanide doped phosphormaterials prepared using these hosts exhibit good chemicalstability and mechano-luminescence properties. Furthermore,aluminates are excellent hosts for the development of longpersistence phosphors (CaAl2O4:Eu

2+,Dy3+ and CaAl2O4:Ce3+),

which makes them useful for display applications, delayeduorescence, warning signals, emergency lighting and for otherluminous products.150–160 Very recently, a few publications haveappeared 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

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emissionwas obtained for 3mol%ofEu3+. Additionof alkali ionswas found to enhance the PL intensity signicantly. The hyper-sensitive 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+ siterather than Al3+ site. It is suggested that the co-doping of alkaliions works as a charge compensator, induces a lattice distortionand lowers the lattice symmetry, which altogether favors theenhanced emission intensity of the 5D0 / 7F2 transition. Theluminescence decay analysis of the CaAl2O4:Eu

3+,Li+ phosphor,monitored at 615 nm under 254 nm excitation, reveals a longdecay time: luminescence lasts for over 10 ms. It is also notedthat the emission intensity is gradually enhanced as the ionicradii of the alkali ion decreases, i.e. minimum for K+ andmaximum for Li+ ion. This has been explained as being due to achange in ionic radii of the alkali ions that inuence the localstructure around the luminescent center ions. This also inu-ences the spin–orbit couplings and crystal eld 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 200–300nm, 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, whichmightbe due to change in Eu–O distances by alkali ion doping.

Tang et al.90 prepared Ho3+/Yb3+/Li+ co-doped SrAl2O4 phos-phors and studied the effect of Li+ ion co-doping on the UCemission of Ho3+ ions. In the rst step, the concentration ofYb3+ ions has been optimized to obtain maximum UC emission,and is found to be 0.20 mol fraction. It has been observed that avariation in Yb3+ content brings out signicant structuralchanges in the host lattice. The XRD patterns reveal a puremonoclinic phase of SrAl2O4 for Yb3+ concentration <0.04 molfraction, above which two new (minor) phases, namely YbAlO3

and Yb2O3, were also identied. Upon increasing the Yb3+

concentration further, the diffraction peaks of the SrAl2O4

lattice shi towards higher angle, which is attributed due to thesubstitution of Sr2+ (1.12 A) by Yb3+ (0.86 A) with a comparativelysmaller ionic radius. The presence of Li+ in the next stepenhances the UC emission intensity signicantly. An enhance-ment in green UC of up to six fold emission was found for Li+

ion concentration 0.06 (see Fig. 5(b)). This has been proposedboth due to charge compensation and grain growth. Due to thelow melting point of the lithium compound (Li2CO3, 723 �C), itmay work as a ux to accelerate grain growth in the SrAl2O4. In asimilar 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+

signicantly.

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

Oxides with perovskite structure (chemical formula ABO3) havelarge technological importance. They exhibit good piezoelectric,ferroelectric and magnetic properties.161,162 Recently, due totheir high dielectric constant, high charge storage capacity,

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good insulating property, good chemical and physical stabilityand relatively low phonon energy (�700 cm�1) these materialsare being used for luminescence applications. Both UC and DSemission of lanthanide ions doped in perovskite hosts havebeen reported.163,164 Further, the effect of Li+ ions in enhancingthe luminescence efficiency of lanthanides has also beeninvestigated.92–97 Table 5 summarises the effect of alkali ions onPL enhancement and the possible mechanism for this.

Sun et al.92 have prepared BaTiO3 nanocrystals doped withEr3+ ions, and the effect of Li+ ion co-doping to enhance the PL ofEr3+ has been investigated. By using 3 mol% Li+ ions theyobserved a 56 fold enhancement (see Fig. 6(c)). The authorsattributed this enhancement to a change in crystal eldsymmetry around the Er3+ ion, which has been establishedthrough XRD measurements. XRD peaks show a gradual shiwith addition of Li+ ions up to 3 mol%, while above 3 mol%doping of Li+ ions a shi in XRD peak position is not observed.Authors suggest that since the Li+ ion has a small ionic radius, itcan t in any crystal site, such as substituting the Ba2+ ion oroccupying the interstitial sites. Substitution of divalent Ba by Li+

induces an oxygen vacancy in the matrix. In both the cases it isapparent that the local crystal eld around the Er3+ ion will bechanged. Another possibility of the Li+ ion doping at the Ti4+ sitehas been discarded in the study because of the large chargedifference between them. Furthermore, it has also been notedthat the addition of Li+ ions neutralizes OH groups and so alsodecreases the non-radiative channels. Thus, the enhanced PLhas been attributed to all these factors. In another work on Er3+/Yb3+ co-doped BaTiO3 by Chen et al.,93 time domain studies havealso been included to understand the PL enhancement onLi+ ionco-doping. Thedecay proles of the 4S3/2/

4I15/2 (at 548 nm) and4F9/2 / 4I15/2 (at 660 nm) transitions have been monitored indifferent compositions of thedoped ions e.g.BaTiO3:1%Er3+ and5%Yb3+ and BaTiO3:1% Er3+ and 5% Yb3+,7% Li+ nanocrystals(see Fig. 6(d)). They found that the lifetime of the 4S3/2 state ofBaTiO3:1% Er3+ and 5% Yb3+,7% Li+ nanocrystals (104.4 ms) islonger than BaTiO3:1% Er3+ and 5% Yb3+ nanocrystals (97.5 ms).This has been attributed due to a decrease in non-radiativechannels (OH group concentration) aer Li+ ion doping.

Yang et al.94,95 prepared CaTiO3:Pr3+ phosphors with different

concentrations of Li+ ions (0.5 to 5.0 wt%). The maximum PLenhancement (�3.5 times) was observed for the 1.0 wt% Li+ co-doped CaTiO3:Pr

3+ phosphor. The excitation spectrum observedbymonitoring emission at 613 nm (see Fig. 6(a)) shows a band at330 nm, which has been assigned as the transition from thevalence band to the conduction band, O2� (2p)/ Ti4+ (3d). Theother band at 380 nm has been attributed to the charge transfer(CT) from Pr3+ tometal ions, Pr3+/Ti4+4 Pr4+/Ti3+. The emissionspectrum recorded upon excitation with the charge transfer (CT)band shows high brightness (see Fig. 6(b)). The PL brightnessrst increases up to 1.0 wt% of Li+ then starts to decrease. Thereason behind this has been explored by using XRD and SEMcharacterization techniques. The XRD analysis reveals that uponaddition of Li+ ions up to 1.0 wt%, crystallinity increases, whileabove this, it starts decreasing. Also, for Li+ concentration ofmore than 2 wt% some impurity phase was also detected. TheSEM images (see Fig. 6(e)–(h)) of CaTiO3:Pr

3+ doped with


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different concentrations of Li+ reveal that the grains of Li-dopedCaTiO3:Pr

3+ phosphors are highly packed and have larger grainsizes than CaTiO3:Pr

3+. On the basis of these results, it has beenconcluded that the high density of packing and improved crys-tallinity leads to higher oscillating strengths for the opticaltransitions. Also, the addition of Li+ ions creates oxygen vacan-cies. Due to strong mixing of charge transfer states, the oxygenvacancies act as sensitizers for the energy transfer from host toPr3+ ion. These factors together are responsible for enhancedphotoluminescence of activator ions.

Du et al.96 prepared Eu3+ and Li+ co-doped CaZrO3 nano-crystals. The optimum doping level of Eu3+ is 5 mol%, abovewhich the PL start decreasing due to concentration quenching.The closed packed structure of CaZrO3 does not have space toaccommodate interstitial atoms. Eu3+ ions substitute at Ca2+

sites. Because of the substitution of Ca2+ by Eu3+ there would besome charge difference. The addition of 10 mol% of Li+ ion intothe CaZrO3 lattice enhances the PL three fold and furtherincrease in Li+ concentration decreases the PL intensity. Theenhancement in the PL intensity has been attributed to thelattice distortion produced aer Li+ ion doping. The latticedistortion reduces the symmetry around Eu3+ ions andincreases the PL intensity of the 5D0 /

7F2 transition. Yu et al.97

have prepared a series of luminescent materials,SrZnO2:Eu

3+,M+ (M ¼ Li, Na, K). As the ionic radius of Eu3+ iscloser 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 inten-sity, which has been attributed due to charge compensation.

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

Recent years have witnessed enormous interest in semi-conductor nanocrystals due to their potential applications inphotocatalysis, solar cells and various other optoelectronicdevices.165–167 Many research groups are working to furtherenrich the optical and other properties of these semiconductorsby combining lanthanides with them. Out of a large number ofsemiconductor materials available, ZnO, ZnS, TiO2, MgO andSnO2 semiconductors have been widely used as hosts forlanthanide doping due to their large band gap, low phononenergy, excellent physical and chemical stability and environ-mentally friendly nature.168–170 These semiconductors are veryfrequently used in electroluminescent devices. However, due tomismatch between the ionic radius of lanthanide ions (usuallytrivalent for optical applications) and Zn2+, Ti4+, Mg2+, Sn4+

metal ions, it is difficult to accommodate lanthanide ions insemiconductor matrices. Therefore, lanthanide doping createscharge imbalance and vacancies, which affect the luminescenceyield signicantly. Researchers are using Li+ ions to modifysemiconductor host matrices and compensate the chargeimbalance so as to improve the luminescence intensity.98–105

Table 6 summarizes the effect of Li+ ions on PL properties ofvarious lanthanide ion doped semiconductor host matrices.

Gu et al.98 have prepared Dy3+ and Li+ co-doped MgOnanocrystals. Li+ ion co-doping enhances the emission of Dy3+


ions, which has been concluded to be due to improved crys-tallinity and grain size and a change in symmetry around Dy3+

ions. The rst conclusion has been drawn on the basis of XRDand TEM measurements, while the ratio of intensity ofthe magnetic and electric dipole allowed transitions (4F9/2 /6H15/2 and 4F9/2 / 6H13/2 transitions) of Dy3+ has been calcu-lated to give an idea of the site symmetry around the dyspro-sium ions. The XRD studies reveal a pure cubic structure forMgO:Dy3+ and Li+ co-doped samples. The incorporation of Li+

ions increases the crystallinity of the MgO nanocrystals. TEMimages of MgO crystals exhibit cuboid lamellar morphologywith a mean particle size of �20 nm, which remainsunchanged upon addition of Dy3+ ions. However, in presenceof Li+ ions, the crystallite size increases to 30–40 nm. Theyproposed 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 4F9/2 / 6H15/2

and 4F9/2 / 6H13/2 transitions of Dy3+ ion are known asmagnetically allowed (hardly vary with the crystal eld strengtharound the dysprosium ion) and forced electric dipole transi-tions (being allowed only at low symmetries with no inversioncenter), respectively. The ratio of the intensities between theelectric dipole and the magnetic dipole gives informationabout the site symmetry in which the dysprosium ion is situ-ated. Under excitation at 273 nm, the asymmetry ratio of theDy3+ 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 fourfold (see Fig. 7(a)). The variation of the asymmetry ratio indi-cates a change in symmetry and vibrational modes around theDy3+ ions by Li+ ion doping.

Zhang et al.99 have prepared Eu3+ and Li+ co-doped SnO2

phosphors. SnO2 has a tetragonal structure (space group ofP42/mnm), in which the Sn atom occupies a slightly distortedoctahedral site. The Eu3+ ions go into the Sn4+ sites, thereforethe local environment around Eu3+ ions should be of highsymmetry, which weakens the 5D0 / 7F2 (613 nm) transitioncompared to 5D0 / 7F1 transition. Li+ doping increases thecrystallinity and particle size (established through XRDmeasurements), which induces a remarkable increase in PLintensity. Further, the occupation of Sn4+ sites by Li+ ionswould naturally give rise to a considerable number of vacantsites in the oxygen ion array and then expand the latticeto decrease crystal density, which may also promoteincreased 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. PLstudies reveal that incorporation of Li+ induces a long lived PLon the order of millisecond time range, with a maximum at724 nm. As the Li+ and Y3+ concentration increases to 7% or10%, an increase in emission intensity is observed. Caoet al.101 have investigated 1 mol% Er3+, 10 mol% Yb3+ and 0–20mol% Li+ tri-doped TiO2 nanocrystals. The XRD patternreveals that 1Er:TiO2 has tetragonal rutile phase of TiO2 withsome impurity phase due to Er2Ti2O7. On addition of 10 mol%Yb3+ a new phase, Yb2Ti2O7, was observed along with rutileTiO2 and Er2Ti2O7 phases. From the shis in XRD peak

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positions of TiO2, it is revealed that for low Li+ ion concen-tration (1–2 mol%), Li+ occupies Ti4+ sites, whereas for higherLi+ concentration (5–20 mol%) it goes to interstitial sites.Upon Yb3+ and Li+ co-doping, the intensity of green and redbands of Er3+ increases signicantly. This has been attributedto the energy migration between the Er3+ and Yb3+, as well asthe distortion of crystal eld symmetry of Er3+ for lowerconcentrations 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 themain red emission band and the green band of Er3+ is seen(see Fig. 7(b) and (c)). They proposed that the change in UCemission behavior is due to the modication of the localcrystal eld around the Er3+ ions due to the introduction of Li+

ions. Fig. 7(b) shows the XRD patterns of the as prepared andannealed Er3+ doped samples. Gu et al.103 have prepared Dy3+

doped ZnO nanocrystals. It is observed that the Li+ co-dopingin ZnO:Dy3+ nanocrystals increases PL by 10 fold. Theyproposed that since the f–f absorption transitions in lantha-nide ions are forbidden, the number of carriers excitedthrough f–f transitions in Dy3+ ions is much less in comparisonto those excited through band gap excitation of ZnO. Thus, theimprovement in Dy3+ emission is mainly due to the radiativerecombination of the large amount of trapped carriers excitedfrom ZnO host. The increased recombination probabilityenhances the emission of Dy3+ by energy transfer process. Inaddition, the incorporation of Li+ ions can create oxygenvacancies, which might act as the sensitizer for the energytransfer to the rare earth ion due to the strong mixing ofcharge transfer states, resulting in the highly enhanced lumi-nescence. Further, they have reported reduction of the asym-metry ratio, which suggests a change of symmetry andvibrational modes around the luminescent Dy3+ centers by Li+

doping. Bai et al.104 have also studied the role of Li+ ions onEr3+ doped ZnO nanocrystals. They have also observedenhanced UC emission intensity of Er3+ ions upon Li+ co-doping. Though the Li+ ion does not have enough energy todestroy the ErO6 structure, yet, it may alter the local structurearound Er3+ which affects the 4f–4f transitions of Er3+ ion,which results in UC emission enhancement. Further, Li+ ionalso reduces the OH� group concentration, which is anotherreason 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 bandsof Er3+ by a factor of 1.93 and 1.65, respectively. The XRDpattern reveals that Er3+ doped ZrO2 samples have both tetrag-onal and monoclinic phases. Yb3+/Er3+ co-doping increases theformation of the tetragonal phase in the sample. Further, theincorporation of Li+ suppressed the tetragonal phase in theYb3+/Er3+ doped ZrO2 sample (Fig. 7(d)). Further, upon Li+

doping the slope of the excitation power versus UC emissionintensities curve decreases (see Fig. 7(e)), which suggests thatLi+ ions can tailor the local structure of the host lattice and thuscan improve energy transfer processes from Yb3+ to Er3+ ions.

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4.6 Effect of alkali ions on UC emission of lanthanide ions inuoride host matrices

Fluoride host matrices, particularly NaYF4 and NaGdF4, areknown to be excellent hosts for UC studies.54,106 Due to theirsimilar ionic radii, Na+ and RE3+ ions occupy the same sites inthe hexagonal NaYF4 lattice. These host matrices possesses lowphonon energies, which is an essential requirement forobtaining efficient UC emission. They show higher chemicalstability than other halides (chloride, bromide).54,106 The crystalstructure of these matrices highly inuences the UC properties,and crystal structure is quite sensitive to the alkali ions presentin the crystal. So, it is interesting to study the effect of alkali ionson the UC properties of uoride host matrices (Table 7).

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. Itwas observed that the Li+ ion co-doping in the NaYF4:Yb/Ercrystals affects the morphologies to a greater extent andsignicant changes from rod-like shape to disk shape andnally to polyhedron structure is attained. However, K+ ion co-doped NaYF4:Yb/Er crystals do not show any change inmorphology and they almost maintain the rod-like shapethroughout (see schemes (a) and (c) in Fig. 8). The crystalstructure analysis further reveal that not only the surfacemorphology, but also the phase transition from b-hexagonal totetragonal (LiYF4) in LixNa1�xYF4 crystals is initiated at x ¼ 0.5,whereas the phase of KyNa1�yYF4 remains stable up to y ¼ 0.85.The UC emission studies show that as the Li+ content increasesin the crystal, UC emission intensity decreases, whereas the UCemission intensity of NaYF4:Yb/Er crystals increases(K0.7Na0.3YF4:Yb, Er crystal shows 8 and 7 fold enhancement ingreen and red UC emission, respectively) when K+ ions areintroduced into the crystal (see Fig. 8(b) and (d)). The change inUC emission is related to the change in crystal structure, surfacemorphology, and distance between alkali and uorine atoms. Inthe Li+ doped crystal, the Li–F distance will be larger than theNa–F distance, which can intensify the interaction between F�

and Er3+ in the crystal lattice, which may enhance the phononenergy of Er3+ ions. On the other hand, substitution of Na+ ionswith K+ ions in the lattice reduces the inter-quenching of Er+

and decreases the local crystal eld symmetry, which increasesUC intensity. In a similar work, Dou and Zhang114 have alsostudied effect of Li+ and K+ ions doping on crystal structure andUC emission behavior of NaYF4:Yb/Er. The phase transition inNaYF4:Yb/Er crystal, has also been observed by Dou andZhang114 upon Li+ ion doping, but instead of hexagonal totetragonal phase transition, as observed by Mao et al.,107 theyobserved hexagonal to cubic phase transition. The UC studiesreveal that the intensity ratios between the blue, green, and redemission peaks change upon Li+ and K+ doping in the crystal.They have proposed that the Li+ or K+ doping will slightlychange the size of the unit cell in the crystal and then to changethe Yb–Er distance, which can inuence energy transferprocess. On the other hand, the higher concentration of K+ ionsin the crystal causes formation of a new K2NaYF6 phase. Thecoordination number of RE is 9 in hexagonal phase, 8 in cubicphase and 6 in K2NaYF6 phase. This suggests that the K2NaYF6


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phase is less stable than the cubic and hexagonal phases.Further, Misiak et al.110 have shown that Li+ doping in the cubicNaYF4:Yb

3+/Tm3+ colloidal crystals signicantly reduces theconcentration quenching. Interestingly, the initial UC emissionintensity decreases at rst, and then starts increasing. Thismight be due to the fact that initially Li+ ions substitute the Na+

ions in the lattice, while further increase in the Li+ ionconcentration results in the occupancy of Li+ ions in the inter-stitial sites, which results in increase in the crystal size. Further,substitution of Na+ by Li+ introduces distortion in the lattice,which prevents concentration quenching.

Similar to NaYF4, co-doping of alkali Li+ brings out signif-icant changes in crystal structure and the optical properties aregreatly improved. Cheng et al.111 have studied the effect of Li+

ions on UC emission of b-NaGdF4:Yb3+/Er3+ nanoparticles. It is

found that the presence Li+ ions in the crystal enhanced UCemission drastically (nanoparticles doped with 7 mol% Li+

ions show 47 and 23 fold enhancement in green and red UCemission peaks, respectively). The enhanced UC emission isattributed to the distortion of the local asymmetry around Er3+

ions. The presence of Li+ in the nanoparticles also helps in thegrowth of the crystals, which is proved by XRD studies. Anothercontributing factor to the enhanced UC is the increased life-time of the 4S3/2 state of NaGdF4:Yb

3+/Er3+ NPs in the Li+ ionsdoped crystals (see Fig. 9) which causes increased populationin the 4S3/2 state, resulting in enhanced green emission.Furthermore, the presence of Li+ also increases paramagneticbehavior of the nanoparticles. This is attributed to the increasein Gd3+ molar concentration in NaGdF4 resulting due tointroduction of Li+ ions. Zhang et al.109 have also studied theeffect of Li+ ions on the UC emission behavior of NaYbF4:Ermicrocrystals. Interesting, they observed a blue shi in UCemission bands. The XRD studies show hexagonal to tetrag-onal phase transformation upon Li+ co-doping. In the LiYbF4crystal, each Yb3+ ion is coordinated to eight F� ions (formingthe YbF8 polyhedral units) and each Li+ ion is coordinated tofour F� ions (forming the LiF4 tetrahedral units). The crystal-lographic point site symmetry with S4 symmetry in LiYbF4 ishigher than that of NaYbF4 with D2d symmetry. This results ina more symmetrical distribution of electronic density and aweakened polarization effect of the local environment inLiYbF4 causes a blue shi in UC bands. Guo et al.113 havestudied the effect of Li+ ions on the UC emission behavior ofLu6O5F8:Yb

3+/Er3+ nanoparticles. They have studied UC, DCand CL properties of the materials and all these increases inthe presence of Li+ ions. They proposed that increased lumi-nescence properties are due to the increased crystallinity (byux action of Li), decreased absorption bands of the surfacecontaminants (OH and CO groups) and distorted localsymmetry around the Er3+ ions. Yin et al.108 have studied theeffect of Li+ on UC emission behavior of GdF3:Yb,Er nano-particles. In GdF3:Yb,Er nanoparticles, the intensity of UCemission in the red region is higher than that in the greenregion, which results in a bright yellow emission evenobservable by the naked eye. Upon Li+ doping (0 to 5, 10, and25 mol%) the green emission intensity gradually decreases andthe red emission is enhanced (see Fig. 9 (c)–(g)). This is


attributed to the change in local crystal eld around Er3+ ionsin the presence of Li+ ions in the GdF3:Yb,Er host lattice.

Thus in summary, it is pointed out that alkalis in uoridesand similar hosts effectively modify the crystal structure andaffect the surface morphology, phase, particle size, crystallinity,etc. In this way the crystal eld around the lanthanide ion isfavorably changed. Not only this, the doping of alkalis makes asignicant change in critical separation between lanthanideions, lanthanide-alkali ions, lanthanide-uorine ions, lantha-nide activator and sensitizers, etc., due to which energy transferprocesses become effective, giving rise to an increase in the UCemission. A change in the different components of the emissionviz. blue, green and red varies systematically, which createsopportunities for color tunability also. Above all this, a specicblue shi is also observed, which is peculiar in this host. Someinteresting observations, although beyond the scope of thisstudy, e.g. increase in the paramagnetic behavior, could be veryuseful for readers.

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

The preceding sections described the role of alkali ions, espe-cially Li+, in the luminescence properties in some well knownhost matrices viz. Y2O3 and Gd2O3, ABO4 type compounds,strontium and calcium aluminates, perovskite, oxide nano-particles and uoride host matrices. In addition to these, thereare several other host matrices viz. Ba2GdNbO6:Eu

3+/Dy3+,Li+,115

BaZn2(PO4)2:Sm3+,R+ (R ¼ Li, Na, K),116 Ca3B2O6:Dy

3+,Li+,117

CaSO4:Tm3+/Dy3+,Li+,118 (Y, Gd)BO3:Tb

3+,Li+,119 YNb-TiO6:Eu

3+,Li+,120 YBO3:Eu3+,Li+,121 Zn2SiO4:Yb

3+,Er3+, Li+/Bi3+,122

SrB4O7:Eu2+,Li+,123 Lu2SiO5:Ce,Li

+,125 Y2MoO6:Eu3+,Li+,126 NaSr-

BO3:Tb3+,Li+,127 Sr2SiO4:Eu

3+,(Li+, Na+, K+),129 CaSnO3:Tb3+(Li+,

Na+, K+),131 Gd6MoO12:Li+/Ln3+,132 Y2SiO5:Pr

3+,Li+,133 Y2Zr2O7:Dy3+,Li+,134 LixCa1�2xEuxSiyMo1�yO4,136 Ca2BO3Cl:Sm

3+(Li+, Na+,K+)139 and CaSi2O2N2:Eu

2+,Dy3+,Li+,140 etc. in which the role ofLi+ in luminescence properties has also been investigated.Table 8 gives brief information about these and also outlinesvarious possible mechanisms involved in the PL enhancement.It is basically due to increased crystallinity, grain size, chargecompensation, change in local crystal eld around the lantha-nide ions, etc. All these mechanisms are discussed in thepreceding sections in context of other well known matrices, sohere we will not discuss it again.

5. Conclusions

This review was planned with prime objective to explore theeffect of Li+ ion co-doping on the PL of lanthanide ions doped indifferent host matrices, and also to highlight various contrib-uting factors that plays important roles. The follow up of theliterature reviewed so far in this article unanimously depictsthat the co-doping of Li+ ions up to a certain concentration inlanthanide doped phosphors enhances the luminescence (bothfor UC and DC based emission) of lanthanide ions considerably.

The possible explanation for the enhanced PL intensity ofthe lanthanide ions in the presence of Li+ ions can be

RSC Adv., 2014, 4, 27039–27061 | 27057

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summarized as follows. When Li+ ion is introduced into anymatrix, because of its smaller ionic radius, it can eithersubstitute the cation of the matrix or occupy interstitial sites ofthe matrix, which usually depends on the concentration ofdoped alkali. As the Li+ ion concentration increases, the possi-bility to occupy the interstitial sites increases, which distorts thematrix signicantly. In a few hosts, changes even in the phaseand crystal structure are also noted. The change in crystalstructure can also change the occupancy site of the lanthanideions. Also, due to the lower melting temperature of the Li, it canwork as a ux and increase the crystallinity and the grain size ofthe materials. In addition, it is also found that Li+ ions decreasethe concentration of the inorganic/organic groups that work asnon-radiative channels and cause a decrease in the PL intensity.Further, when a trivalent lanthanide ion replaces a divalentcation it produces charge imbalance in the matrix that can becompensated by using alkali ions. All these factors favor lumi-nescence enhancement of the lanthanide ions. In the case ofCe3+ and Eu2+ ions, by changing the local crystal elds aroundthe activator ions, they can also be used for tuning the color ofthe luminescence materials. Furthermore, in uoride hostmatrices, co-doping of Li+ causes a change in separationbetween lanthanide ions, lanthanide-alkali ions, lanthanide-uorine ions, lanthanide activator and sensitizers, etc., whichstrongly inuences the energy transfer processes. Thus, it canbe concluded that the Li+ ions (alkali ions) modify host matricesin many ways, which favors radiative transitions, causingenhancement in the luminescence intensity. Since this reviewprovides in-depth case studies for various hosts, and describesthe effect of various matrix parameters (e.g. crystal structure,crystallinity, grain size, surface morphology, quenching enti-ties, etc.) on PL, therefore, it could also be useful as input fordesigning various novel lanthanide doped luminescentmaterials.

The effects of co-doping with Li+ ions have been used foralmost all different types of luminescence mechanismsincluding photoluminescence/downshiing, upconversion,thermo-luminescence/persistent emission, cathado-lumines-cence, etc. However, its use for one of the most important classof emissions, i.e. quantum-cutting, is still lagging and hasstrong scope for future studies.

Acknowledgements

S. K. Singh thankfully acknowledges the nancial support byDepartment of Science and Technology, New Delhi, India in theform of INSPIRE Faculty Award [IFA12-PH-21]. A. K. Singhacknowledges nancial support by UGC, New Delhi, India (DrD. 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 ofpostdoctoral fellowships.

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Role of Li+ ion in the luminescence enhancement of lanthanide ions

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