Organic mediated synthesis of highly luminescent Li⁺ ion compensated Gd₂O₃:Eu³⁺ nanophosphors and their Judd–Ofelt analysis

Abstract

Highly luminescent red emitting Gd₂O₃:Eu³⁺, Li⁺ nanophosphor has been synthesized by the solvothermal combustion of the metal–citrate complex in diethylene glycol medium. The morphology and luminescence properties of these nanophosphors are found to be highly sensitive to the extent of lithium ion compensation. It is found that lithium ions promote grain growth and alter the morphology of the Gd₂O₃:Eu³⁺ nanophosphor from nearly spherical to cobblestone like. A significant enhancement in intensity of luminescence and quantum efficiency is observed in lithium compensated nanophosphors. The highest emission intensity is observed for the Gd_1.75Eu_0.1Li_0.15O₃ nanophosphor, about 1.83 times that of Gd_1.9Eu_0.1O₃ and is attributed to the enhanced intra 4f–4f emission transitions arising from the modifications of the crystal field and distortion of the local symmetry around the europium ions. The luminescence decay profiles are found to be single exponential in nature and the lifetime measured was 1.36 ms for the Gd_1.75Eu_0.1Li_0.15O₃ nanophosphor. The chromaticity coordinates of these nanophosphors indicated high colour purity. Judd–Ofelt intensity parameters indicated that lithium compensation increases the polarization of the local environment and an increase of covalency and asymmetry around the europium ions.

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1. Introduction

Investigations on rare earth based inorganic nanophosphors have attracted significant scientific interest because of their excellent emission features and their tremendous demand for potential applications in displays, lighting, lasers, optical communication, finger print sensing, bio-labels and bio-imaging.¹ Inorganic luminescent materials with long-term stability, enhanced brightness, faster responses and better quantum efficiency are needed to meet the requirements of these devices. In this direction, extensive research has been carried out on rare earth ion activated oxide based phosphors, which is motivated by their excellent colour purity, long chemical durability, lack of photobleaching and high thermal stability. The 4f–4f electronic transitions of rare earth ions produce narrow emissions with relatively long lifetime, which is due to the shielding effect of its partially filled 4f-shell electrons by the filled 5s and 5p electrons.² The intensity, colour purity and quantum efficiency of the emission transitions from the rare earth activators depend on the nature of the host, interaction between the rare earth ions and the host, symmetry around the activator, concentration and distribution of activator ions and the effectiveness of energy transfer from the host or sensitizer to the activators.¹

Trivalent rare earth ions such as Eu³⁺, Tb³⁺, Sm³⁺, Er³⁺, Tm³⁺ and Ho³⁺ can act as excellent luminescent centres in inorganic phosphors because of their rich electronic energy levels for radiative transitions and their suitability of excitation in the ultraviolet, visible and near infrared regions. Among the different rare earth ions, trivalent Eu³⁺ activated phosphors are technologically important for photonic applications due to the simple energy level structure of Eu³⁺ ions, which produce intense, narrow red emissions by the intra 4f–4f ⁵D₀–⁷F_J (J = 0 to 6) optical transitions. Suitable host material for the rare earth ions should possess large optical band gap combined with good thermal and chemical stability. In order to improve the luminescence from Eu³⁺, it should be incorporated into host materials having low phonon energy. Gadolinium oxide (Gd₂O₃), a rare earth sesquioxide can perform as an excellent luminescent host material for the rare earth ions because of its better chemical durability, good thermal stability, high density, ability of easy incorporation with rare earth ions, high refractive index and low phonon energy of 600 cm⁻¹.^3–8 Moreover, the ionic radius of Eu³⁺ (0.0947 nm) is close to that of Gd³⁺ (0.0938 nm), which makes the incorporation of Eu³⁺ into the host lattice, Gd₂O₃ easier. Gd₂O₃:Eu³⁺ is a paramagnetic efficient red emitting phosphor that has been widely employed in fluorescent lamps, lighting, photonic displays, security inks, cathode-ray tubes, contrast agent in magnetic resonance imaging and bio-related applications.^1,9–11 At lower temperature, Gd₂O₃ crystallizes in the cubic bixbyite structure (space group: Ia3̄ (206), Th^​7) and two distinct cationic sites are available for the rare earth ions – a non-centrosymmetric site, C₂ and a centrosymmetric site, S₆ with an occupancy ratio of 3 : 1. The occupancy of Eu³⁺ ions in these sites is reflected in the luminescent and radiative properties of the synthesized phosphors.

The intra 4f–4f electronic transitions of Eu³⁺ ions are strongly dependent on the crystal structure of the host and are sensitive to the local structural environment around it. When Eu³⁺ is located at a non-centrosymmetric site, it can generate high quality red emission corresponding to the ⁵D₀–⁷F₂ electronic transition. Recent studies by several groups have revealed that the incorporation of rare earth ions together with alkali metal ions in host matrices can effectively enhance the emission intensity and quantum efficiency of nanophosphors. Alkali metal ions can distort the host lattice, alter the host–activator interactions, modify the energy absorption and transfer behaviours, inducing enhancement of emission properties. Moreover, alkali metal ions can play a crucial role to control the crystallinity and morphology of the nanophosphors. The alkali metal, lithium (Li⁺) ion has the least cationic radius making both its movement and site occupation substitutionally or interstitially in the host lattice easier, which will reduce the symmetry of the crystal field around the Eu³⁺ activators and results in enhanced emission properties. Several groups have studied the effect of monovalent lithium (Li⁺) incorporation on the structure, morphology and luminescence properties of YBO₃:Eu³⁺,¹² Y₂O₃:Eu³⁺,^13–15 Gd₂O₃:Eu³⁺,^16–19 GdVO₄:Eu³⁺,²⁰ YVO₄:Eu³⁺,²¹ YNbTiO₆:Eu³⁺,²² SrTiO₃:Pr³⁺,²³ CaTiO₃:Eu³⁺,²⁴ and BaMoO₄:Sm³⁺,²⁵ phosphors. These studies indicated the importance of evaluating the luminescence dynamics of lithium compensated rare earth based inorganic phosphors for practical applications. In the Li⁺ compensated Gd₂O₃:Eu³⁺ nanophosphors, Li⁺ is employed to act as a flux as well as a sensitizer of luminescence by reducing the symmetry around the Eu³⁺ activators.

Solution combustion synthesis is a versatile, simple, cost effective rapid process to prepare a variety of nanosized materials. Recently, we have successfully synthesized Gd₂O₃:Eu³⁺ nanophosphors by the controlled solvothermal combustion in diethylene glycol medium.^26,27 Motivated by the attempts to develop efficient phosphors herein, synthesis of highly luminescent lithium compensated red emitting cubic Gd_1.9−xEu_0.1Li_xO₃ (x = 0, 0.05, 0.1, 0.15 and 0.2) nanophosphor by the solvothermal combustion of metal–citrates in diethylene glycol is reported. Possible reasons for the considerable enhancement in photoluminescence emission intensity by the incorporation of Li⁺ are discussed in detail. The compensation of lithium with ionic radius and charge states different from that of the host Gd³⁺ ions produced a remarkable effect on the growth process, crystallinity and crystal symmetry around the Eu³⁺ ions. A comprehensive investigation on the optical spectroscopy and luminescence dynamics of Gd₂O₃:Eu³⁺, Li⁺ was done from the emission spectrum by evaluating the Judd–Ofelt and radiative parameters.

2. Experimental

Gd_1.9−xEu_0.1Li_xO₃ (x = 0, 0.05, 0.1, 0.15 and 0.2) nanophosphors were synthesized by the solvothermal combustion of metal–citrate complex in diethylene glycol. The starting materials used were gadolinium oxide (Gd₂O₃, 99.99%, Aldrich), europium oxide (Eu₂O₃, 99.99%, Aldrich), lithium nitrate (LiNO₃, 99.9%, Merck), polyethylene glycol 200 (PEG, 99%, Merck), diethylene glycol (DEG, C₄H₁₀O₃, Merck, 99%), conc. HNO₃ (70%, Merck) and citric acid monohydrate (C₆H₈O₇·H₂O). Stoichiometric amounts of Eu₂O₃, Gd₂O₃ and LiNO₃ corresponding to the composition Gd_1.9−xEu_0.1Li_xO₃ (x = 0, 0.05, 0.1, 0.15 and 0.2) were weighed separately. Of these, Eu₂O₃ and Gd₂O₃ were dissolved in concentrated nitric acid and deionized water to prepare their fresh nitrate solutions and LiNO₃ is dissolved in deionized water. All the initial nitrate solutions were then homogeneously mixed by magnetic stirring and citric acid in diethylene glycol (citric acid to metal nitrates molar ratio as 2 : 1) was added drop-wise to the mixed nitrate solution to chelate the metal ions and to initiate the formation of metal–citrate complex. Consequently, about 2 ml of PEG is also added to this solution as a mineraliser. This homogeneously mixed solution was then kept at ∼100 °C in a water bath with continuous stirring until a highly transparent viscous solution is resulted. This viscous solution is then placed in a muffle furnace at 180 °C for one hour and then subjected to combustion at 400 °C. The well ground precursor powder was then subjected to heat treatment in a muffle furnace at 800 °C for 2 h to obtain the Gd_1.9−xEu_0.1Li_xO₃ nanophosphors.

X-ray diffraction (XRD) measurements were carried out on a X'pert Pro X-ray diffractometer (Philips PANalytical, Ni filtered CuKα: 1.54056 Å, 40 kV, 30 mA) scanning in the 10 to 60° 2θ range, employing X'Celerator and monochromator at the diffracted beam side. Low and high resolution transmission electron microscopy (TEM) were performed in a FEI Tecnai F20 electron microscope having a field emission gun operating at 200 kV and the images were collected digitally using a Gatan multipole CCD camera. Diffuse reflectance (DRS) measurements were recorded using a UV-visible spectrophotometer (JASCO V550) coupled with an integrating sphere attachment (ISV-469) and BaSO₄ is used as the reference for measurements. Raman studies were performed on a confocal microRaman spectrometer (Horiba Jobin-Yvon LABRAM-HR800) in the backscattering geometry with a 785 nm semiconductor diode laser (current of 198 mA) and employing a peltier cooled CCD detector. Photoluminescence emission and excitation spectra were recorded on a Jobin-Yvon Horiba Fluorolog (FL3-11) spectrofluorometer equipped with a 450 W xenon lamp as the excitation source and a photomultiplier tube in photon counting mode (Hamamatsu R928P) as the detector. Lifetimes were measured by ‘decay by delay method’ using a phosphorimeter (FL-1040) connected to the spectrofluorometer and equipped with a microsecond pulsed xenon lamp as the excitation source. The CIE chromaticity coordinates (x, y) and correlated colour temperature (CCT) of the nanophosphors were determined from the emission spectra using the 1931 Commission Internationale de l'Eclairage (CIE) 2° colour matching functions as the basis. Photoluminescent emission spectra and transient luminescence decay curves were used to evaluate the Judd–Ofelt intensity and radiative parameters of the synthesized phosphors.

3. Results and discussion

3.1 Phase analysis, crystal structure and morphology studies

X-ray diffraction technique was used to analyse the phase formation, crystal structure, crystallinity and crystallite size of the samples. Fig. 1 shows the X-ray diffraction (XRD) patterns of Gd_1.9−xEu_0.1Li_xO₃ nanophosphors as a function of Li⁺ content. All the diffraction peaks are indexed according to the cubic bixbyite structure of Gd₂O₃ (JCPDS file no. 12-0797) system. No secondary phase of europium or lithium is detected indicating that Li⁺ and Eu³⁺ are completely compensated in the Gd₂O₃ host lattice and does not cause any significant change to the crystal structure. However, with increase in lithium content, diffraction peaks are shifted to higher 2θ values indicating slight decrease in lattice parameters and cell volume due to the probable substitution of Li⁺ (ionic radius = 0.076 nm with a co-ordination of 6) in Gd³⁺ (ionic radius = 0.0938 nm with a co-ordination of 6) lattice sites. Also from the figure, it can be seen that the diffraction peak intensities increase and the width decrease with increase of Li⁺ content. This indicates that lithium acts as a self-promoting or flux agent for the growth of phosphors, consequently crystallinity and grain size get improved. The crystallite size (D) of the phosphors was determined from the diffraction data using the Debye–Scherrer equation²⁸where ‘λ’ is the wavelength of CuKα X-rays used (0.154056 nm), β is the full width at half maximum (FWHM) of the diffraction peaks in radian, θ is the Bragg diffraction angle, D_hkl represents the size along the specified (h k l) direction and K is a constant (nearly equal to 0.9). Fig. 2 depicts the variation of lattice constant, cell volume and crystallite size of the phosphors as a function of lithium content. It is found that the lattice constant and cell volume of Gd₂O₃:Eu³⁺ samples decrease and the crystallite size increases with increase of Li⁺ content.

Fig. 1 X-ray diffraction patterns of Gd_1.9−xEu_0.1Li_xO₃ nanophosphors as a function of Li⁺ content (x): (a) 0, (b) 0.05, (c) 0.1, (d) 0.15 and (e) 0.2.

Fig. 2 Variation of lattice constant and cell volume of Gd_1.9−xEu_0.1Li_xO₃ nanophosphors as a function of lithium content [inset: crystallite size].

The morphology, grain size and lattice spacing of the phosphors were investigated by the low and high resolution transmission electron microscopic techniques. Fig. 3(a–f) present the typical TEM and selected area electron diffraction (SAED) patterns of Gd_1.9Eu_0.1O₃ and Gd_1.75Eu_0.1Li_0.15O₃ nanophosphors, respectively. Both the TEM images indicate that the synthesized nanophosphor is highly homogeneous in size and shape with diameters of about 25 nm and 65 nm, respectively, which are consistent with the results drawn from the X-ray diffraction study. The particles possess well defined boundaries with lesser agglomeration. TEM images of the lithium incorporated Gd_1.75Eu_0.1Li_0.15O₃ sample shows well connected particles with nearly cobblestone like morphology. Comparing the images of Gd_1.9Eu_0.1O₃ and Gd_1.75Eu_0.1Li_0.15O₃ nanophosphors, the morphology and size of the particles differ significantly indicating the flux effect and change in reaction kinetics, when compensated by lithium ions. The high resolution TEM images show clear lattice fringes indicating superior crystal qualities with nanocrystalline nature and no apparent defects or dislocations. Observed spacing between the adjacent lattice planes is about 0.312 nm, which corresponds to the interplanar spacing between the two (222) crystal planes of cubic structured Gd₂O₃. SAED patterns of both the samples show continuous dot rings, representing the polycrystalline nature and the indexed SAED patterns are in agreement with the standard JCPDS data of Gd₂O₃.

Fig. 3 Low and high resolution TEM images and selected area electron diffraction patterns of (a–c) Gd_1.9Eu_0.1O₃ and (d–f) Gd_1.75Eu_0.1Li_0.15O₃ nanophosphors, respectively.

3.2 Optical studies

Optical band gap energy (E_g) of the Gd_1.9−xEu_0.1Li_xO₃ nanophosphors were determined from the diffuse reflectance spectra employing the Kubelka–Munk method.^29,30 The expression connecting the diffuse reflectance of the sample (R_∞), absorption coefficient (K) and scattering coefficient (S) iswhere R_∞ = R_sample/R_reference, is the diffuse reflectance of the sample relative to the BaSO₄ reference at each wavelength, F(R_∞) is the Kubelka–Munk function. Band gap energy (E_g) of the samples were determined by plotting the variation of Kubelka–Munk function with photon energy according to the relation

F(R_∞)hν ∝ (hν − E_g)ⁿ (3)

here hν is the incident photon energy and the value of the exponent n, which depends on the type of optical transition triggered by photon absorption. Fig. 4 represents the typical Kubelka–Munk plots of Gd_1.9−xEu_0.1Li_xO₃ nanophosphors. Band gap energy values were determined by the extrapolation of the linear portion of the (F(R_∞)hν)² curve against the photon energy, hν to zero. From the figure, it can be seen that the direct band gap of all the lithium incorporated phosphors is higher than that of the non-substituted one. It was found that the band gap of Gd_1.9−xEu_0.1Li_xO₃ nanophosphors increases with Li⁺ content up to 0.1 and then it remains at 5.73 eV (±0.01 eV). The band structure and band gap of a compound are determined by the geometrical arrangement and the electronic configurations of the constituting elements. The observed variation in the band gap values of Gd_1.9−xEu_0.1Li_xO₃ nanophosphors can be expected to be due to a number of reasons, which include the degree of structural order–disorder in the host lattice by Li⁺ incorporation, extend of compressive strain produced in the lattice by the substitution of smaller Li⁺ ions in place of larger Gd³⁺ ions, charge imbalance, extend of oxygen vacancies and defects in the lattice. These effects have expected to change the energy level distribution and density of states in the host lattice. Some of the factors cause band gap widening whereas some results in band gap narrowing. Earlier studies have shown that compressive strain in the lattice causes band gap widening whereas oxygen vacancies results in narrowing of band gap.^18,31 For lower Li⁺ content, band gap widening can occur predominantly due to the compressive strain in the lattice. For higher Li⁺ content, large number of oxygen vacancies produced in the lattice can cause narrowing of band gap. Both these effects played their complementary roles in higher Li⁺ substituted phosphors and as a result band gap remains more or less constant for higher Li⁺ compensated phosphors.

Fig. 4 Kubelka–Munk plots of Gd_1.9−xEu_0.1Li_xO₃ nanophosphors with different Li⁺ content (x): (a) 0, (b) 0.05, (c) 0.1, (d) 0.15 and (e) 0.2.

Raman spectroscopy is a highly promising spectroscopic tool to investigate the vibrational phonon modes, lattice defects, dislocations, lattice strains, local cation distribution, charge-lattice and spin-lattice couplings of materials.^32,33 The normalised microRaman spectra of Gd_1.9−xEu_0.1Li_xO₃ nanophosphors excited by 785 nm diode laser are presented in Fig. 5(A). The host lattice Gd₂O₃ is cubic structured with space group Ia3̄ (Th^​7) and having Z = 16. So the expected optical (Γ_op) and acoustical (Γ_ac) phonon modes by factor group analysis³⁴ are expressed as

Γ_op = 4A_g + 4E_g + 14F_g + 5A_2u +5E_u + 16F_u, Γ_ac = F_u (4)

here A_g is the Raman active symmetric stretching mode, F_g is the Raman active triply degenerate symmetric stretching mode and E_g is the Raman active doubly degenerate symmetric bending mode. A_2u and E_u are both Raman and IR inactive and F_u is infrared (IR) active. As a result, 22 Raman vibrational modes of A_g, E_g, and F_g and 16F_u vibrational IR modes are expected. Gd_1.9−xEu_0.1Li_xO₃ nanophosphors showed Raman active vibration modes at 97, 120, 137, 146, 316, 362, 445 and 569 cm⁻¹ corresponding to cubic Gd₂O₃ and are in accordance with the values reported by Luyer et al.³⁵ The intense Raman band located at 362 cm⁻¹ is assigned as the combined F_g + A_g vibrational mode having large polarizability change. The phonon modes of materials are determined by the effective mass, bond type and symmetry of the constituting atoms in the primitive unit cell. A careful investigation of the Raman mode located at 362 cm⁻¹ indicated a shift to higher wave numbers (361.66 to 362.35 cm⁻¹) with increase in lithium content and is shown in Fig. 5(B). It is expected that lithium incorporation may lead to the shortening of bond lengths, change in force constant and local disorders. The average mass of the nanophosphor decreases with increase in lithium content, which also leads to the shift of phonon modes to higher frequency side.

Fig. 5 Normalised microRaman spectra of Gd_1.9−xEu_0.1Li_xO₃ nanophosphors with different lithium content (x): (A) – (a) 0, (b) 0.05, (c) 0.1, (d) 0.15 and (e) 0.2 and (B) magnified Raman spectrum in the neighbourhood of the peak at 362 cm⁻¹.

3.3 Luminescence dynamics, Judd–Ofelt and radiative analysis

Fig. 6 shows the excitation spectra of Gd_1.9−xEu_0.1Li_xO₃ nanophosphors monitoring the ⁵D₀ → ⁷F₂ emission at 612 nm of Eu³⁺. Excitation spectra of the samples consist of an intense high energy broad band with wavelength ranging from 220 to 300 nm and contain several sharp lines. Generally, charge transfer (CT) transitions occur when a valence electron is transferred from the ligand to the unfilled orbitals of the rare earth ion. The observed high energy broad excitation band is assigned as the charge transfer band (CTB), which originates from the transfer of an electron from the filled O²⁻ (2p⁶) ligand orbital to the 4f orbital of Eu³⁺.^1,36 Excitation lines observed at 276, 278 and 280 nm arise from the ⁸S_7/2 → ⁶I_7/2–17/2 and at 314 nm arise from the ⁸S_7/2 → ⁶P_3/2–7/2 intra f–f transitions of Gd³⁺ ion from its ground state of ⁸S_7/2. These transitions indicate the energy transfer between trivalent gadolinium and europium ions. Excitation lines in the lower energy region occur with lesser intensity compared to CT transition, which results from the intrinsic intra f–f transitions of Eu³⁺ from its ground state ⁷F₀ to the higher excited states and the observed electronic transitions are assigned as ⁷F₀ → ⁵H₆ at 323 nm, ⁷F₀ → ⁵D₄ at 364 nm, ⁷F₀ → ⁵G_2–6 at 383 nm, ⁷F₀ → ⁵L₆ at 395 nm, ⁷F₀ → ⁵D₃ at 416 nm, ⁷F₀ → ⁵D₂ at 467 nm and ⁷F₀ → ⁵D₁ at 533 nm.⁵ Even though the excitation spectra did not exhibit evident differences in the position and shape of transitions, intensity of the charge transfer band is found enhanced by the monovalent lithium incorporation. The occurrence of strong charge transfer band is favourable for the effective energy transfer from the host lattice to the activator and hence the luminescence of Eu³⁺ ion.³⁷ This indicate enhanced absorption by the Eu³⁺ ions in presence of Li⁺ ions and out of the compositions studied, intensity of the charge transfer band is the highest for Gd_1.75Eu_0.1Li_0.15O₃ phosphor. This makes excitation through the charge transfer band efficient. Position of the CT band in the phosphor is determined by the covalency of O²⁻ and Eu³⁺, Eu–O bond distance, charge of the ligand and coordination of the central Eu³⁺ ion.^1,38,39

Fig. 6 Excitation spectra of Gd_1.9−xEu_0.1Li_xO₃ nanophosphors (λ_em = 612 nm) with different Li⁺ content (x): (a) 0, (b) 0.05, (c) 0.1, (d) 0.15 and (e) 0.2.

The emission from inorganic phosphors originates from the complex interaction among the host matrix, activators, defects, sensitizers and interfaces. Luminescence of the trivalent europium ions results from the intra 4f–4f electron transitions. Since the 4f electrons are well shielded from the surrounding environment by the external electric fields of the outer closed 5s and 5p electrons, characteristic narrow line like emissions are produced. Fig. 7 shows the emission spectra of Gd_1.9−xEu_0.1Li_xO₃ nanophosphors under 265 nm charge transfer excitation. The emission spectra indicated no significant change in the position of the peaks, but the relative intensities of transitions changed considerably with lithium incorporation. Each emission spectrum consists of five groups of sharp emission peaks lying between 550 and 720 nm. These narrow and sharp transitions are indexed as the electronic transitions from the excited ⁵D₀ level to lower ⁷F_J (J = 0, 1, 2, 3 and 4) manifolds of Eu³⁺ ions and are assigned as the ⁵D₀ → ⁷F₀ (581 nm), ⁵D₀ → ⁷F₁ (588, 593, 599 nm), ⁵D₀ → ⁷F₂ (612, 630 nm), ⁵D₀ → ⁷F₃ (651 nm) and ⁵D₀ → ⁷F₄ (707 nm) transitions.⁵ Of these transitions, the ⁵D₀ → ⁷F₂ transition centred at 612 nm is the dominant one in the emission spectra of all the samples. The emission features of Eu³⁺ activated phosphors strongly depend on the crystal structure of the host and the site occupancy of Eu³⁺ ions in the host matrix.¹ In the body centred cubic Gd₂O₃ crystal structure, Eu³⁺ ions can occupy two crystallographically non-equivalent cationic sites with six-fold coordination viz., a 24d site having C₂ point non-inversion symmetry and 8b site having S₆ point inversion symmetry. The C₂ site lacks inversion centre, where as the S₆ site is inversion symmetric and the ratio of the probability of occupancy of Eu³⁺ ions in C₂ and S₆ sites is 3 : 1. The ⁵D₀ → ⁷F₂ transition is symmetry sensitive and parity forbidden forced electric dipole transition with the selection rule ΔJ = 2, which can be induced by the lattice sites C₂ of Eu³⁺ ions without inversion symmetry and the break of parity selection rules, whereas ⁵D₀ → ⁷F₁ is a structurally independent parity allowed magnetic dipole transition.^40,41 If more Eu³⁺ ions are occupied in S₆ sites with inversion centre, then the intensity of the pure magnetic dipole transition ⁵D₀ → ⁷F₁ with selection rule ΔJ = 1 is dominant. The electric dipole transition of ⁵D₀ → ⁷F₂ is dominant in the Gd_1.9−xEu_0.1Li_xO₃ nanophosphors suggesting that Eu³⁺ ions prefer to occupy the low symmetry site without an inversion centre.

Fig. 7 Emission spectra of Gd_1.9−xEu_0.1Li_xO₃ nanophosphors (λ_ex = 265 nm) with different Li⁺ content (x): (a) 0, (b) 0.05, (c) 0.1, (d) 0.15 and (e) 0.2.

The crystal field asymmetry of coordination polyhedron around the Eu³⁺ ions can be determined by evaluating the hypersensitive asymmetric ratio (A). It can also provide an estimate of the covalent nature and polarization of the surrounding of the Eu³⁺ ions by short range effects. Higher the asymmetric ratio, higher is the distortion from inversion symmetry and is evaluated as the ratio of the integrated emission intensity of electric (⁵D₀ → ⁷F₂) to magnetic (⁵D₀ → ⁷F₁) dipole transitions^1,42 as

This ratio depends on the cation site occupancies, bond lengths, bond angles, distortion of the lattice and nature of the host matrix.⁴³ The calculated asymmetric ratio and the integrated emission intensity of the nanophosphors with different Li⁺ content are shown in Fig. 8 and the partial energy level diagram illustrating the excitation and emission process in Gd_1.9−xEu_0.1Li_xO₃ nanophosphors is shown in Fig. 9. Initially, the asymmetric ratio increases with Li⁺ content and reaches a value of 5.955 for Gd_1.75Eu_0.1Li_0.15O_3, indicating high asymmetric environment of the coordination polyhedron around Eu³⁺ ion and then the value decreases. The observed increase in asymmetric ratio due to Li⁺ incorporation indicates reduction in crystal field symmetry around the Eu³⁺, which in turn increases the probabilities of the various intra 4f–4f transitions of Eu³⁺ ions. It is interesting to note that Li⁺ compensation enhances the intensity of photoemission, keeping the shape and position of the electronic transitions unaltered (Fig. 7). It can be seen that the emission intensity first increases with increase of Li⁺ content up to 0.15 and then decreases due to the increased rate of non-radiative energy transfer to quenching sites and surface defects. In the present study, the composition Gd_1.75Eu_0.1Li_0.15O₃ is found to be highly luminescent and its emission intensity is estimated to be 1.83 times that of Gd_1.9Eu_0.1O₃. There are several factors which may contribute to the improvement of photoluminescence intensity in these phosphors. Sohn et al.⁴⁴ reported that lithium incorporation may create a lower symmetry around the rare earth activator ions. In the present study, Li⁺ causes reduction in symmetry around Eu³⁺ as reflected in the asymmetric ratios of Gd_1.9−xEu_0.1Li_xO₃. Besides charge imbalance, the substitution of trivalent gadolinium by monovalent lithium can create oxygen vacancies in the lattice. Oxygen vacancies can act as a sensitizer to promote energy transfer from the host to the activator, up to a critical defect density¹⁷ and it produces an increase in distortion of the crystal field around the activator. Improved morphology, better crystallinity, larger grains, lesser defects, efficient energy transfer and reduced local crystal field symmetry around Eu³⁺ ions caused more optical activation in presence of Li⁺ ions, which lead to increase in emission intensity at lower lithium contents.^13,17,45 But excess lithium compensation causes oxygen vacancies to increase in the crystal lattice and results in luminescence quenching by energy transfer or migration to the quenching sites.^1,13,46

Fig. 8 Variation of integrated emission intensity and asymmetric ratio of Gd_1.9−xEu_0.1Li_xO₃ nanophosphors as a function of Li⁺ content.

Fig. 9 Schematic model of partial energy level diagram illustrating the excitation and emission process in Gd_1.9−xEu_0.1Li_xO₃ nanophosphors.

In general, the emission colour of any photoemitter can be represented by the (x, y) chromaticity coordinates in the Commission Internationale de l'Eclairage (CIE) 1931 diagram.⁴⁷ The chromaticity coordinates (x, y) of the phosphors were calculated from the corresponding photoemission spectrum based on the CIE 1931 colour matching functions. Table 1 lists the CIE chromaticity coordinate values of Gd_1.9−xEu_0.1Li_xO₃ phosphors and the corresponding 1931 CIE chromaticity diagram for different Li⁺ compensated phosphors is shown in Fig. 10. All these values are in the red region of the diagram and no significant change of chromaticity coordinates were observed. Colour purity of a particular dominant colour from a light source is measured as the ratio of the distance between the emission colour coordinates and the coordinates of equal energy point to the distance between the equal energy point and the dominant wavelength point in the CIE diagram.^48,49 The effect of Li⁺ on the colour purity of phosphor is analyzed using

where (x_d, y_d), (x, y) and (x_i, y_i) are the coordinates of the dominant wavelength, emission light and of the CIE white illuminant E (x_i = 0.3333, y_i = 0.3333), respectively. Colour purity of all the Gd_1.9−xEu_0.1Li_xO₃ phosphors was found to be more than 95%. Correlated colour temperature (CCT), a measure to characterize the quality of emission from photoemitters is determined by McCamy method⁵⁰ and is expressed as

CCT = 449n³ + 3525n² + 6823.3n + 5520.33 (7)

where n = (x − x_e)/(y_e − y) is the inverse slope line and (x_e, y_e) as (0.3320, 0.1858) is the epicentre (Table 1). The CCT values obtained for Gd_1.9−xEu_0.1Li_xO₃ phosphor were in the range, 2052–2130 K and are shown in the correlated colour temperature diagram (Fig. 10(B)).

Table 1 Values of the CIE chromaticity coordinates (x, y), correlated colour temperature (CCT) and colour purity of Gd_1.9−xEu_0.1Li_xO₃ nanophosphors

Li content	Chromaticity coordinates		CCT (K)	Colour purity (%)
	x	y
0	0.6341	0.3530	2052	95.57
0.05	0.6351	0.3505	2103	95.72
0.10	0.6357	0.3497	2123	95.78
0.15	0.6361	0.3496	2129	95.91
0.20	0.6354	0.3492	2130	95.57

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Fig. 10 (A) CIE chromaticity diagram and (B) correlated colour temperature diagram of Gd_1.9−xEu_0.1Li_xO₃ nanophosphors with different Li⁺ content (x): (a) 0, (b) 0.05, (c) 0.1, (d) 0.15 and (e) 0.2.

Transient emission decay curves (Fig. 11) of 612 nm, ⁵D₀ → ⁷F₂ radiative transition with excitation at 265 nm were used to probe the luminescence dynamics of Eu³⁺ ions in the Li⁺ incorporated Gd₂O₃ host matrix. The normalized decay curves observed were fitted using the single exponential method as

where I(t) is the emission intensity at time t, I₀ is the initial emission intensity at t = 0 s and τ is the lifetime of the emitter. Monoexponential decay behaviour shows the possibility of only one type of emission centre in the phosphor. The lifetime values of Gd_1.9−xEu_0.1Li_xO₃ nanophosphors were 1.21716, 1.24565, 1.2771, 1.35888 and 1.2603 ms with Li⁺ content of 0, 0.05, 0.1, 0.15 and 0.2, respectively. It is found that the lifetime of the phosphor initially increases with Li⁺ content and then decreases due to increase in non-radiative energy transfer by energy migration to quenching sites. Consequently, shortening of lifetime and decrease of emission intensity was observed.

Fig. 11 Luminescence decay curves of Eu³⁺ (⁵D₀ → ⁷F₂ transition at 612 nm, λ_ex = 265 nm) in Gd_1.9−xEu_0.1Li_xO₃ with different Li⁺ content (x): (a) 0, (b) 0.05, (c) 0.1, (d) 0.15 and (e) 0.2 (experimental data (o) and fitting results (-)).

Detailed investigation on the site symmetry and luminescence dynamics of Eu³⁺ ions in Gd_1.9−xEu_0.1Li_xO₃ was evaluated by analysing the Judd–Ofelt intensity parameters. Judd–Ofelt analysis^51,52 is a powerful spectroscopic technique, which effectively describes the spectral behaviour in a specific coordination environment of rare earth ion doped single and polycrystalline materials, glasses and solutions. Through this analysis one can determine the rates of parity forbidden electric dipole transitions between different electronic levels of the rare earth ion, understand the local structural environment around the rare earth ion and can evaluate the bond covalency of rare earth and its associated ligands. The three Judd–Ofelt (J–O) intensity parameters, Ω_λ (λ = 2, 4 and 6) can act as a probe to identify the structural environment and symmetry of rare earth ions in host matrices. Detailed physical interpretation of these parameters was discussed by Jorgensen and Reisfeld.⁵³ These parameters were determined from the photoemission spectrum considering the intensity of ⁵D₀–⁷F₁ magnetic dipole allowed transition as the reference, since it is unaltered by the surrounding crystal field environment. Judd–Ofelt theory gives the spontaneous emission probability of ⁵D₀–⁷F₁ magnetic dipole transition (A₀₁) as

The electric dipole transition rates (A_0J) of ⁵D₀ → ⁷F_J (J = 2, 4 and 6) transitions are given by

here ‘e’ is the electron charge, ν_J is wavenumber of the respective electronic transition, (2J + 1) equals 1 for ⁵D₀ transitions, h is the Planck's constant, S_md is the magnetic dipolar line strength of the ⁵D₀–⁷F₁ transition, which is independent of the host matrix, equal to 9.6 × 10⁻⁴² units and ‘n’ is the effective refractive index of the nanophosphor.^54,55 Effective refractive index (n) of the nanophosphors was determined using the equation⁵⁵

n =xn_b + (1 − x)n_m (11)

where n_b is the refractive index of the bulk (n_b = 1.935), n_m is the refractive index of the surrounding medium (n_m = 1 for air) and ‘x’ is the optical filling factor. The filling factor represents the fraction of the space occupied by the nanophosphor. The emission lifetimes of the ⁵D₀ state in bulk (τ_b = 0.94 ms) and nanocrystals (τ_s) are related to the refractive index of the bulk (n_b) and refractive index of the sample (n_s) by

Solving this equation, the optical filling factor ‘x’ can be determined as

The squared reduced matrix element is independent of the surrounding environment of the Eu³⁺ ion and the values are 0.00324, 0.00229 and 0.00023 for J = 2, 4 and 6, respectively.⁶ Because of the selection rules and the unique nature of transition intensities of Eu³⁺ ions, each of the values determine the intensities of the corresponding transitions since the other two reduced matrix elements are zero. As a result the Judd–Ofelt intensity parameters can be determined from the ratio of the intensities of electric to magnetic dipole transitions given by

Table 2 summarizes the Judd–Ofelt intensity parameters and emission intensity ratios of the major electronic transitions from the ⁵D₀ level of Eu³⁺ to ⁷F_J manifolds in Gd_1.9−xEu_0.1Li_xO₃ nanophosphors. The Judd–Ofelt intensity (J–O) parameters are important for investigating the local structure, local crystal symmetry and bonding in the vicinity of rare earth ions.^56,57 Wang et al.⁵⁸ suggested the dependence of Ω₂ parameter on the covalence of rare earth ions with coordinating ligands and the site symmetry of the local environment of Eu³⁺ ion. Jiang et al.⁵⁹ pointed out that if higher is the asymmetry around Eu³⁺ ion, higher will be the Ω₂ parameter. This means that Ω₂ parameter value is attributed to the covalency and structural changes in the vicinity of the Eu³⁺ ion exhibiting short range effect whereas the Ω₄ parameter is dependent on the viscosity and dielectric constant of the host causing long range effect.⁶⁰ Large values for Ω₄ represent low rigidity of the host matrices. The Ω₆ intensity parameter could not be calculated here because ⁵D₀–⁷F₆ emission located in the infrared region was quite weak and was not observed in the present case. For Gd_1.9−xEu_0.1Li_xO₃ nanophosphor, the Ω₂ value is increased with the increase of Li⁺ content up to 0.15, which indicates stronger covalence of Eu–O bonding and lower symmetry around the Eu³⁺ ion. Low symmetry is suitable for the enhanced emission of Eu³⁺ ions at 612 nm because the ⁵D₀–⁷F₂ electric dipole transition is highly sensitive to its local environment. Smaller values of Ω₄ obtained shows appreciable rigidity of the crystalline host matrix. Electronic transitions from the ⁵D₀ level to low-lying ⁷F_J manifolds with J = 0, 3 or 5 are electrically and magnetically forbidden by Judd–Ofelt theory, since their correlated reduced matrix elements are zero.^51,52 Possible reason for the observed weak emission transitions to these levels is attributed to the crystal field induced J-mixing, which borrows intensity from the allowed electric dipole transitions by the even crystal field interaction.⁶ The extent of this J-mixing was evaluated using the R₀₂ ratio given in Table 2 and the Judd–Ofelt intensity parameters were also determined by considering this J-mixing effect (see Table 2). The tendency of the J–O parameters in all the lithium compensated Gd_1.9−xEu_0.1Li_xO₃ nanophosphor is found to be in the order Ω₂ > Ω₄.

Table 2 Judd–Ofelt intensity parameters (Ω₂ and Ω₄) and ratio of integrated emission intensities of the ⁵D₀–⁷F₀ and ⁵D₀–⁷F₂ (R₀₂), ⁵D₀–⁷F₂ and ⁵D₀–⁷F₁ (R₂₁), ⁵D₀–⁷F₄ and ⁵D₀–⁷F₁ (R₄₁) transitions of Gd_1.9−xEu_0.1Li_xO₃ nanophosphors

Gd1.9−xEu0.1LixO3	Refractive index	Without J mixing		With J mixing		Intensity ratio
		Ω2 (×10^−20 cm^2)	Ω4 (×10^−20 cm^2)	Ω2 (×10^−20 cm^2)	Ω4 (×10^−20 cm^2)	R02	R21	R41
Gd1.9Eu0.1O3	1.7988	8.514	1.278	9.852	1.427	0.032	5.669	0.390
Gd1.85Eu0.1Li0.05O3	1.7868	8.751	1.175	10.126	1.313	0.036	5.809	0.358
Gd1.8Eu0.1Li0.1O3	1.7740	8.837	1.246	10.226	1.392	0.036	5.847	0.378
Gd1.75Eu0.1Li0.15O3	1.7421	9.071	1.309	10.496	1.462	0.039	5.955	0.394
Gd1.7Eu0.1Li0.20O3	1.7808	8.294	1.081	9.597	1.207	0.036	5.497	0.328

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Radiative parameters such as radiative transition probability, radiative lifetime, quantum efficiency, branching ratios and stimulated emission cross-sections for ⁵D₀–⁷F_J transitions of Eu³⁺ ions in Gd_1.9−xEu_0.1Li_xO₃ phosphor were determined from their emission spectra. The total radiative transition probability (A_R) is obtained by summing over all the radiative rates A_0J for each ⁵D₀–⁷F_J transition and is expressed as

where ν₀₁ and ν_0J are the energy barycentre of the ⁵D₀–⁷F₁ and ⁵D₀–⁷F_J transitions, A₀₁ is the Einstein's coefficient between ⁵D₀–⁷F₁ levels, I_0J is the integrated area of the emission spectrum corresponding to the particular ⁵D₀–⁷F_J transition.

The radiative lifetime (τ_rad) of the excited ⁵D₀ level is the reciprocal of the total radiative transition rate and is expressed as

The luminescence lifetime (τ_obs) of the Eu³⁺ first excited state, ⁵D₀ was measured at 612 nm with an excitation wavelength of 265 nm. This lifetime (τ_obs) of ⁵D₀ state is determined by the radiative, A_R and non-radiative, A_NR transition rates, which is closely related to the crystalline environment around the Eu³⁺ ions and is given by

where A_T is the total transition rate. The luminescence quantum efficiency (η) is the ratio of the number of photons emitted to the number of photons absorbed by the Eu³⁺ luminescent centres. Using the emission spectra and lifetime of the ⁵D₀ emission level, the quantum efficiency of the Eu³⁺ ion excited state can be determined. Assuming that non-radiative and radiative processes are involved in the depopulation of the ⁵D₀ state, η can be determined by

The relative intensity of a particular emission transition with respect to all other transitions from an excited electronic state is measured as the fluorescence branching ratio and this ratio (β_0J) for the emission from the ⁵D₀ level to a low lying level ‘J’ of Eu³⁺ is expressed as

Judd–Ofelt theory can also be used to evaluate the induced emission cross-sections of electronic transitions. The peak stimulated emission cross-sections have been calculated from the emission spectra by Fuchtbabauer–Landenburg formula⁶¹ and is related to the radiative transition rate (A_0J) as

where λ_p is the peak wavelength of emission band and Δλ_eff is the effective line width of the emission transition.

Transition rates (A_R, A_NR and A_T), lifetime (τ_rad and τ_obs), quantum efficiency (η), branching ratios (β₀₀, β₀₁, β₀₂, β₀₃ and β₀₄) and emission cross-sections (σ₀₁, σ₀₂ and σ₀₄) of Gd_1.9−xEu_0.1Li_xO₃ nanophosphors are summarized in Table 3. The branching ratios of emission transitions follow the trend as β₀₂ > β₀₁ > β₀₄ > β₀₃ > β₀₀ and emission cross sections follow the trend as σ₀₂ > σ₀₁ > σ₀₄ in all the samples. A comparison of the luminescence properties obtained in the present study with other reports available in the literature is presented in Table 4. Variation of lifetime and quantum efficiency of Gd_1.9−xEu_0.1Li_xO₃ nanophosphors with Li⁺ content is depicted in Fig. 12. With Li⁺ content, quantum efficiency and lifetime values increase and the highest quantum efficiency of 83.93% with a lifetime of 1.35888 ms is obtained for the Gd_1.75Eu_0.1Li_0.15O₃ nanophosphor. Thereafter quantum efficiency and lifetime values decrease due to the increased rate of non-radiative relaxations. Clearly, the incorporation of Li⁺ ions reduces the non-radiative relaxation and increases the quantum efficiency of Gd_1.9Eu_0.1O₃ phosphors.

Table 3 Radiative parameters such as transition rates (A_R, A_NR and A_T), lifetimes (τ_rad and τ_obs), quantum efficiency (η), branching ratios (β₀₀, β₀₁, β₀₂, β₀₃ and β₀₄) and stimulated emission cross-sections (σ₀₁, σ₀₂ and σ₀₄) of Gd_1.9−xEu_0.1Li_xO₃ nanophosphors

Radiative parameter	Gd1.9Eu0.1O3	Gd1.85Eu0.1Li0.05O3	Gd1.8Eu0.1Li0.1O3	Gd1.75Eu0.1Li0.15O3	Gd1.7Eu0.1Li0.20O3
AR (s^−1)	646.01	648.10	639.59	617.67	608.85
ANR (s^−1)	175.57	154.69	143.43	118.23	184.61
AT (s^−1)	821.58	802.79	783.02	735.90	793.46
τrad (ms)	1.54796	1.54297	1.56350	1.61899	1.64243
τobs (ms)	1.21716	1.24565	1.2771	1.35888	1.26030
η (%)	78.63	80.73	81.68	83.93	76.73
β00 (%)	2.336	2.636	2.618	2.695	2.612
β01 (%)	13.008	12.709	12.603	12.359	13.392
β02 (%)	76.113	76.187	76.053	76.158	75.980
β03 (%)	2.495	3.048	3.045	2.984	2.773
β04 (%)	6.048	5.420	5.681	5.804	5.243
σ01 (×10^−22 cm^2)	7.4297	7.3982	7.4107	7.5036	7.4222
σ02 (×10^−21 cm^2)	3.9801	4.7680	4.8833	4.9313	4.6002
σ04 (×10^−22 cm^2)	5.9459	5.7931	5.7455	5.9747	4.8699

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Table 4 Comparison of the luminescence properties of Gd₂O₃:Eu³⁺ based nanophosphors obtained in this work with other reports. (B_g-band gap, λ_ex-excitation wavelength, λ_em-dominant emission wavelength, τ-measured lifetime, η-quantum efficiency)

Composition	Synthesis method	Crystal structure	Morphology	Bg (eV)	λex (nm)	λem (nm)	τ (ms)	η (%)	Judd–Ofelt parameter (×10^−20 cm^2)		Intensity	Ref.
									Ω2	Ω4
Gd1.9Eu0.1O3	Solution combustion	Cubic	Nanoparticles	5.67	265	612	1.217	78.63	9.85	1.43	—	27
Gd1.8Eu0.2O3	Solution combustion	Cubic	Microparticles	—	355	611	1.41	23.6	5.28	1.66	—	62
Gd2O3:Eu (5 mol%)	Hydrothermal method	Cubic	Nanoparticles and nanorods	—	258	611	2.3	—	12.39	2.02	—	6
Gd2O3:Eu (4 mol%), Li^+ (6 mol%)	Solution combustion	Monoclinic and cubic	Nanoparticles	5.16	254	612	—	—	—	—	4 times that of Gd2O3:Eu (4 mol%)	18
Gd2O3:Eu^3+ (4 mol%), Li^+ (1 mol%)	Solution combustion	Monoclinic and cubic	Nanoparticles	5.49	243	612	—	—	—	—	1.64 times that of Gd2O3:Eu^3+ (4 mol%)	19
Gd1.75Eu0.1Li0.15O3	Solution combustion	Cubic	Nanoparticles	5.73	265	612	1.359	83.93	10.50	1.46	1.83 times that of Gd1.9Eu0.1O3	This work

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Fig. 12 Variation of lifetime and quantum efficiency of Gd_1.9−xEu_0.1Li_xO₃ nanophosphors as a function of Li⁺ content.

4. Conclusions

Highly luminescent Gd₂O₃:Eu³⁺, Li⁺ nanophosphors were successfully synthesized by the controlled solvothermal combustion in diethylene glycol medium. It is found that lithium compensation promotes grain growth resulting in better crystallization. Morphology of the Gd_1.9−xEu_0.1Li_xO₃ nanophosphors is found to be highly sensitive to the extent of compensation by the lithium ions. All the lithium compensated phosphors showed enhanced ⁵D₀–⁷F_J (J = 1–4) photoemissions of Eu³⁺ under charge transfer excitation. The improvement of emission intensity and quantum efficiency is attributed to the improved crystallinity and grain size leading to higher oscillator strengths for the 4f–4f electronic transitions and reduction of symmetry around the Eu³⁺ with lithium ion compensation. The highest emission intensity was observed for Gd_1.75Eu_0.1Li_0.15O₃ composition and was found to be 1.83 times that of Gd_1.9Eu_0.1O₃ phosphor. The luminescence decay profiles follow single exponential kinetics. Higher value of Judd–Ofelt intensity parameters indicate highly polarized local environment, higher covalency and more asymmetry around the europium ions. Strong visible emissions, large stimulated emission cross-sections, better quantum efficiency and higher branching ratio make these nanophosphors as a novel luminescent material. The present report highlights the use of monovalent lithium compensation as a strategy to enhance the emission features of rare earth activated nanophosphors.

Acknowledgements

R. G. Abhilash Kumar acknowledges University Grants Commission (UGC), India for the award of teacher fellowship under the Faculty Development Programme.

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