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Jan 15, 2019 - The cytotoxicity of the codoped nanophosphor system was evaluated using WI-38 cell lines. This optimized codoped nanophosphor material ...
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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Investigation of Upconversion Photoluminescence of Yb3+/ Er3+:NaLaMgWO6 Noncytotoxic Double-Perovskite Nanophosphors K. Naveen Kumar,*,† L. Vijayalakshmi,‡ and Jungwook Choi*,† School of Mechanical Engineering and ‡Department of Automotive Engineering, Yeungnam University, Gyeongsan, Gyeongbuk 38541, Republic of Korea

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ABSTRACT: Bright fluorescent rare-earth-ion-doped upconversion nanomaterials are attractive choices for photonic devices. A remarkable green upconversion emission has been obtained by the sensitizing effect of Yb3+ in a Yb3+/Er3+:NaLaMgWO6 (NLMWO) nanophosphor under near-infrared (NIR) excitation. A citrate sol−gel method was employed to synthesize the nanophosphor samples. The lack of a secondary phase in the X-ray diffraction pattern confirms that the Er3+ and Yb3+ ions are incorporated in the ordered double-perovskite structure. Surface analysis and particle evaluation are performed by field-emission scanning electron microscopy and transmission electron microscopy analysis. Upconversion and downconversion emission performances were systematically studied by varying the dopant concentrations. A strong upconversion green emission can be observed with the naked eye, and it resembles the upconversion spectra of Er3+-doped phosphors. Remarkably, because of an energy-transfer process, the green upconversion emission can be converted into a strong red emission by codoping with Yb3+ ions. We observed the color tuning effect from green to red, which can be controlled by varying the Yb3+ concentration in the codoped phosphors during NIR excitation. A systematic investigation of the upconversion mechanism from Yb3+ to Er3+ doubly doped NLMWO nanocrystals is demonstrated. The upconversion mechanism was evaluated only by varying the excitation power of the laser as well. A strong NIR emission at 1.57 μm corresponding to Er3+ can be significantly enhanced by increasing the codoping concentration of Yb3+ ions. The energy migration pathway is accurately presented. The Commission internationale de l’éclairage color coordinates were analyzed for singly and doubly doped nanophosphors. The cytotoxicity of the codoped nanophosphor system was evaluated using WI-38 cell lines. This optimized codoped nanophosphor material is noncytotoxic; thus, it can be useful for in vitro studies in biological studies. On the basis of the obtained results, the NLMWO:Yb3+/Er3+ nanophosphors can be a promising choice for novel upconversion photonic applications.

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INTRODUCTION The process of converting a near-infrared (NIR) excitation into a shorter visible or ultraviolet emission via a sequential multiphoton absorption process can be considered as an upconversion mechanism, which was first identified in the field of photonics in the pioneering work by F. Auzel in 1966.1 Over the last few decades, upconversion luminescent materials have attracted extensive research interest in science as well as in industry, owing to their exclusive characteristics, such as narrow emission bandwidths, large anti-Stokes shift, multicolor emission, low autofluorescence background, long lifetime, high penetration depth, and high photochemical stability.2 These unique characteristics enable their application in several optical and optoelectronic systems in the fields of communication, sensing, photodynamic therapy, photonics, photovoltaics, and bioimaging and analysis.3−5 However, the upconversion © XXXX American Chemical Society

mechanism is still limited to conventional experimental trial schemes without specific directions because of the different energy-transfer pathways. Because the capability of upconversion fluorescence strongly depends on the excited-state dynamics of dopants and the interaction between the host matrices and dopants,6 the proper combination of dopant ions and host materials can result in strong upconversion emission and provide a better understanding of its mechanism. Considerable efforts have been devoted to developing upconversion luminescence processes with active modulations by incorporating rare-earth ions in various inorganic and organic systems.7 Elements from lanthanum to lutetium and their trivalent ions (Ln3+) in the periodic table have filled 4f Received: October 22, 2018

A

DOI: 10.1021/acs.inorgchem.8b02990 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry shells. Lanthanide (Ln3+) ions typically exhibit several excited 4f energy levels apart from La3+, Ce3+, Yb3+, and Lu3+. Weak electron−phonon coupling and sharp/narrow f−f transition bands can be obtained from Ln3+ ions because of the shielding effect of the 4f−4f transitions of Ln3+ by 5s2 and 5p6 subshells. Moreover, the f−f transitions are Laporte-forbidden transitions, which can provide low transition probability and subsequent long-lived excited states. Hence, the upconversion process of rare-earth-ion-doped materials has become a promising choice for several photonic applications.8 Among the several available rare-earth ions, Yb3+ is typically used for sensitizing purposes when excited by a 980 nm laser, because of its favorable characteristic feature of a large absorption cross section compared with other rare-earth ions. Because of the appearance of an exact resonance between the 2 F7/2 → 2F5/2 transition of Yb3+ and f−f transitions of other activators, such as Ho3+, Tm3+, and Er3+ ions, efficient energy migration can occur from the Yb3+ ion to these activator ions.9 There have been numerous reports on the upconversion emission properties of several host materials doped with diverse combinations of rare-earth dopant ions, such as Er3+/ Yb3+, Tm3+/Yb3+, and Ho3+/Yb3+;10−13 however, the significant upconversion emission features have not been obtained yet. When the Yb3+ ion concentration increases in nanophosphors containing Er3+/Yb3+, there can be a considerable enhancement in the emission properties of the Er3+ ions compared to those of the emission from singly doped Er3+ compounds under the excitation of 980 nm. Because of the coincidence of the two energy levels 2 F 5/2 and 4 I 11/2 corresponding to Yb3+ and Er3+, respectively, there is a possible resonance condition between both of them, facilitating energy transfer from Yb3+ to Er3+.14 However, the Er3+ emission properties can also be reduced abruptly at a higher concentration of Er3+ ions because of the quenching effect of the concentration. Thus, an optimum concentration of the sensitizer and activator ions is required for stable, high upconversion signatures. In this study, we demonstrate strong upconversion photoluminescence of NaLaMgWO6 (NLMWO) nanophosphors codoped with Yb3+/Er3+ ions, which has not been systematically investigated yet. In addition to the presence of rare-earth ions and the dependence of their concentration, the selection of the host material is important in the production of upconversion emission. The phonon energy of the host matrix plays a pivotal in exhibiting good upconversion features. Generally, low-phonon-energy materials can exhibit a lower nonradiative transition rate, which enables reduced multiphonon relaxation, resulting in high upconversion luminescence efficiency.15 Recently, double perovskite tungstates, such as NLMWO, have emerged as promising host materials possessing high excitation efficiency, high energy-transfer possibility, and strong luminescence intensity.16 Moreover, their attracting double-perovskite structure can vary from various A and A′ cation pairs and exhibits a rock-salt ordering of B-site cations with a layered ordering of A-site cations.17 We can efficiently prepare a desired single-phase double-perovskite structure in NLMWO phosphors with easier synthesis processes over other phosphors of NaYF4, NaGdF4, and many others. Furthermore, the NLMWO host matrix is more compatible for the incorporation of rare-earth ions compared to NaYMgWO6. On the basis of these characteristics, rareearth ions such as Yb3+ and Er3+ were incorporated in the NLMWO host matrix in our present investigation. The

structure, morphology, and physical characteristics of Yb3+/ Er3+-doped NLMWO were studied using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Raman spectroscopy. By varying the Yb3+ and Er3+ concentrations, we found a remarkable color tuning effect from green to red under NIR excitation.

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EXPERIMENTAL SECTION

The NLMWO nanophosphor samples doped with Er3+/Yb3+ were synthesized by a citrate sol−gel method. Analytical-grade high-purity raw materials of Na2O4W·2H2O, La(NO3)3·6H2O, citric acid monohydrate, Er(NO3)3·5H2O, Yb(NO3)3·5H2O, and Mg(NO3)2· 6H2O were purchased from Sigma-Aldrich and used without further purification. Typically, stoichiometric amounts of precursor materials were prepared and dissolved in 200 mL of distilled water. Continuous stirring was maintained for 2 h to obtain a homogeneous mixture at room temperature. The citric acid monohydrate was prepared in an appropriate ratio (citric acid monohydrate to metallic ratio = 3:1) and added to this solution, followed by additional stirring for 1 h to obtain better homogeneity and a transparent solution. The pH value of the solution was maintained in the range of 6−8. Next, the solution was heated at 80 °C in a capped beaker in an oven for 1 h. Then, heating was continued for 6 h following the removal of the cap in the same oven under identical conditions to obtain the wet gel. The wet gel was placed in a heating oven at a temperature of 120 °C for 24 h to obtain a spumous and fluffy xerogel. Finally, the fluffy xerogel was kept heating in an oven at 800 °C for 4 h. The obtained mixture was calcined one more time at 1000 °C for 6 h. The final powder was removed from the oven for further measurements and analysis. The crystal structure of the prepared nanophosphor sample was analyzed by XRD (Seifert 303 TT X-ray diffractometer with Cu Kα radiation). The X-ray diffractometer was operated at a voltage of 40 kV and a current of 50 mA. The morphology of the optimized sample was studied by field-emission SEM (FE-SEM; JEOL JSM-7401F) at an accelerating voltage of 10 kV. Elemental analysis was performed using an energy-dispersive X-ray spectroscopy (EDX) recorder, integrated in the FE-SEM system. The binding energy study was carried out by XPS using an Al Kα X-ray source (Thermo Scientific KAlpha). An EO-SXB IR spectrometer was employed to perform Fourier transform infrared spectroscopy (FTIR) spectral analysis. The Raman spectra were recorded using a Horiba XploRA PLUS Raman microscope with a 532.14 nm laser as the light source. Highresolution field-emission TEM (JEOL JEM-2200FS with Cs-corrected TEM) was employed to evaluate the particle size of the optimized nanophosphor sample. The photoluminescence upconversion and NIR emission spectra of the prepared phosphor samples were recorded by an Edinburgh FLS980 fluorescence spectrometer with an excitation of 980 nm. The in vitro cytotoxicity assay of a codoped Er3+(6 mol %)/Yb3+(1 mol %):NLMWO nanophosphor sample was assessed in the WI-38 lung fibroblast using 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2Htetrazolium bromide (MTT) assay. Cells were seeded in a 96-well plate at a density of 1 × 104 cells well−1, using Eagle’s minimum essential medium (ATCC 30-2003) containing 10% fetal bovine serum, supplemented with antibiotics (50 IU mL−1 penicillin and 50 μg mL−1 streptomycin), and maintained at 37 °C in a 5% CO2humidified chamber overnight. Subsequently, the fibroblast cells were treated with calculated doses at various concentrations of 50, 100, 150, and 200 μg mL−1. After 48 h of treatment, the cells were washed twice with 100 μL of phosphate-buffered saline (PBS; pH 7.4), followed by the addition of 100 μL of MTT reagent (1.25 mg mL−1). The cells were then incubated in the dark for 4 h, and the formazan crystals formed within the cells were dissolved by using cell-grade dimethyl sulfoxide (100 μL well−1). The absorbance of samples was measured at 570 nm using a microplate reader (Multiskan EX, Thermo Scientific, USA), where the untreated cells were used as controls. The cell viability percentage was calculated as Asample/Acontrol B

DOI: 10.1021/acs.inorgchem.8b02990 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry × 100%, where A = absorbance at 570 nm. Similarly, optical images were also taken after 48 h of treatment with the same concentration. In addition, the in vitro cytotoxicity of a codoped Er3+(6 mol %)/Yb3+(1 mol %):NLMWO nanophosphor was also assessed using 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium (MTS) assay (Promega, USA). KB, A549, and WI-38 fibroblast cells were seeded in 96-well plates (1 × 104 cells well−1) and incubated for 24 h. Thereafter, fibroblast cells were treated with different concentrations (50, 100, 150, and 200 μg mL−1), incubated for an additional 48 h, washed twice with PBS, and then treated with a MTS solution. Untreated cells were taken as a control, and absorbance was measured at 493 nm using an automated microplate reader. The following equation was used to calculate the cell viability:

cell viability (%) = OD493(sample) − OD493(blank)

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/OD493(control) − OD493(blank) × 100

RESULTS AND DISCUSSION Morphology and Elemental Analysis. The crystalline structure of the NLMWO compound along the b crystallo-

Figure 2. XRD profiles of (a) NLMWO, (b) Er3+ (6 mol %):NLMWO, (c) Yb3+(1 mol %):NLMWO, and (d) Er3+(6 mol %)/Yb3+(1 mol %):NLMWO nanophosphors in the 2θ range from 10° to 80° (left side) and magnified profiles in the range between 30 and 35° (right side).

Figure 1. (a) Crystal structure of NLMWO compound formation along the b crystallographic axis, (b) FE-SEM image, (c) TEM image, and (d) EDX, (e) dark-field TEM image of the Er3+(6 mol %)/Yb3+ (1 mol %):NLMWO nanophosphor and elemental mapping of the Na, La, Mg, W,O, Er, and Yb elements.

graphic axis is shown in Figure 1a. The surface morphology of the optimized codoped nanophosphor was evaluated by FESEM. The semiagglomerated and crystallized NLMWO particles with doubly doped Er3+(6 mol %)/Yb3+(1 mol %) can be observed in the FE-SEM image, as shown in Figure 1b. The high-resolution TEM image of the doubly doped Er3+(6 mol %)/Yb3+(1 mol %):NLMWO nanophosphor system is shown in Figure 1c. The particle size is ∼105 nm, which is in

Figure 3. (a) FTIR and (b) Raman spectra of the (i) NLMWO, (ii) Er3+ (6 mol %):NLMWO, (iii) Yb3+(1 mol %):NLMWO, and (iv) Er3+(6 mol %)/Yb3+(1 mol %):NLMWO nanophosphors. C

DOI: 10.1021/acs.inorgchem.8b02990 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Functional Group Assignment for the NLMWO Phosphor vibrational frequency (cm−1)

functional group assignment

vibrational frequency (cm−1)

446

La−O

1428

612, 670, 748

W−O stretching vibrations CO32− stretching vibrations

3272

859

functional group assignment C−O asymmetric stretching vibrations O−H bending vibrations

agreement with the SEM observation. The obtained nanophosphors exhibit homogeneity with an average particle size of approximately 100−120 nm. The presence of elements in the phosphor, including Na, La, Mg, W, O, Er, and Yb, was confirmed by the EDX spectrum and TEM elemental mapping as shown in Figure 1d,e, respectively. XRD Analysis. The NLMWO host matrix belongs to the family of AA′BB′O6 perovskites, which possess simultaneous A- and B-site cation ordering. There are a number of factors that play an important role in determining the stability of the perovskite phase, including the size mismatch between the alkali-metal and rare-earth cations, tolerance factor, preferred oxidation state, and coordination environment of the B-site cation. The larger A-site cations can provide large tolerance factors, and such a criterion would predict that no hybrid perovskites are stable.18 Nevertheless, smaller A-site cations can yield a smaller tolerance factor, which leads to a larger domain size with high periodicity. This is a favorable scenario for the better stability of the perovskite structures.19 In our present work, the A-site cation of Na+ with smaller ionic radii on the order of 1.39 Å can contribute to a lower tolerance factor on the order of 0.952, which supports the stability of the present double-perovskite structure as described above. The crystalline structure of the prepared samples was evaluated by

Figure 5. (a) Upconversion emission spectra of Er3+:NLMWO nanophosphors. (b) Upconversion emission intensity with respect to different concentrations of Er3+ ions.

analysis of the XRD patterns. The XRD patterns of the pristine NLMWO, Er 3 + (6 mol %):NLMWO, Yb 3 + (1 mol %):NLMWO, and doubly doped Er3+(6 mol %)/Yb3+(1 mol

Figure 4. XPS survey spectra of codoped Er3+(6 mol %) + Yb3+ (1 mol %):NLMWO nanophosphor in the range of (a) 0−1300 eV, (b) 1075−1068 eV, (c) 860−830 eV, (d) 1307−1300 eV, (e) 43−28 eV, (f) 534−526 eV, (g) 205−188 eV, and (h) 170−162 eV. D

DOI: 10.1021/acs.inorgchem.8b02990 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 6. Upconversion emission spectra of Er3+ (6 mol %):NLMWO nanophosphors under a 980 nm excitation wavelength by varying the laser power.

Figure 8. (a) Upconversion emission spectra of codoped Er3+ (6 mol %)/Yb3+(0.5, 1, 2, 3, and 4 mol %):NLMWO nanophosphors. (b) Upconversion emission intensity with respect to different Yb3+ concentrations.

Figure 7. (a) NIR emission spectra of Er3+:NLMWO nanophosphors at different concentrations of Er3+ ions. (b) NIR emission intensity profile of the Er3+:NLMWO nanophosphors at different concentrations of Er3+ ions.

%):NLMWO nanophosphor samples are shown in Figure 2. It can be seen that all XRD patterns are in good agreement with the JCPDS 37-0243 of NLMWO, corresponding to the monoclinic double-perovskite structure.20 Other crystalline phases cannot be observed in the XRD patterns when rareearth ions were doped or codoped into the lattice, which implies that there are no structural changes of the basic crystal. Furthermore, XRD is sensitive to the presence of cation ordering, which can be regulated by the presence of reflections of the superlattice. The long-range ordering of B-site cations in a rock-salt model is confirmed by the existence of two XRD peaks at 19.6° (011) and 38.0° (211) within the structure. However, bonding instability could have resulted from A-site cations in layered ordering, which is confirmed by the peak at 25.5° (111). If the rock-salt form is dominant, then the layered ordering in the crystal as well as the peak from the layered ordering becomes negligible even at doping.21 This implies that the bonding stability in the structure was maintained appropriately after the doping as well. However, when doping and codoping is performed with Er3+ and Yb3+ ions, the major diffraction peak at 32.2° is slightly shifted to the higher angle side. Considering the ionic radii of the Er3+ and Yb3+ ions (R =

Figure 9. Upconversion emission spectra of Er3+ (6 mol %)/Yb3+(1 mol %):NLMWO nanophosphors under 980 nm excitation wavelength by varying the laser power.

1.01 Å; CN = 12) and the La3+ ion (R = 1.17 Å; CN = 12),22 the substitutional occupation of La3+ ionic sites by the Er3+ or Yb3+ ions can result in deterioration of the lattice parameters of the unit cell23 and in a peak shift. FTIR and Raman Analysis. The FTIR spectra of the NLMWO, Er 3 + (6 mol %):NLMWO, Yb 3 + (1 mol %):NLMWO, and doubly doped Er3+(6 mol %)/Yb3+(1 mol %):NLMWO nanophosphor samples were recorded from 4000 to 400 cm−1, as shown in Figure 3a and Table 1. The absorption band that can be observed at 1428 cm−1 is assigned E

DOI: 10.1021/acs.inorgchem.8b02990 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 10. (a) NIR emission spectra of codoped Er3+ (6 mol %)/Yb3+(0, 0.25, 0.5, 0.75, 1, 1.25, 1.5, and 1.75 mol %):NLMWO nanophosphors. (b) NIR emission intensity profile of Er3+(6 mol %)/Yb3+(0, 0.25, 0.5, 0.75, 1, 1.25, 1.5, and 1.75 mol %):NLMWO nanophosphors by varying the Yb3+ ion concentration. Figure 12. CIE chromaticity diagram of optimized Er3+(6 mol %):NLMWO and Er3+(6 mol %)/Yb3+(1 mol %):NLMWO nanophosphors.

to the C−O asymmetric stretching vibrations. The band appearing at 3272 cm−1 can be attributed to the O−H stretching and H−O−H bending vibrations. Three bands can be observed at 612, 670, and 748 cm−1, and they are assigned to the W−O stretching vibrations in the crystalline environment, while the CO32− stretching vibrational band is observed at 859 cm−1. The cutoff phonon frequency vibrations can be inferred from the band appearing at 446 cm−1, which is assigned to the La−O stretching vibrations. The existence of a low phonon frequency can support the least multiphonon relaxation in the phosphor sample.24,25 There are no significant changes in the FTIR spectral band positions of the host lattice when doping with Er3+ or Yb3+ and codoping with both of

them. The exceptionally low amount of hydroxyl groups (−OH) in all of the phosphor samples can result in high luminescence efficiency, suggesting that these materials can be suitable for practical device applications. The Raman spectra were recorded for the NLMWO, Er3+(6 mol %):NLMWO, Yb3+(1 mol %):NLMWO, and doubly doped Er3+(6 mol %)/Yb3+(1 mol %):NLMWO nanophosphor samples in the range of 100−1200 cm−1, as shown in Figure 3b. The ordered double-perovskite structure of

Figure 11. Partial energy-level scheme diagram of energy transfer from Yb3+ to Er3+. F

DOI: 10.1021/acs.inorgchem.8b02990 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 13. Cell viability assay of Er3+(6 mol %)/Yb3+(1 mol %):NLMWO nanophosphor in WI-38 fibroblast cells exposed for 48 h at different concentrations measured by (a) MTT and (b) MTS assay. Microscopic images of WI-38 cells after 48 h of treatment with the same sample at different concentrations: (c) control; (d) 50 μg mL−1; (e) 100 μg mL−1; (f) 150 μg mL−1; (g) 200 μg mL−1.

Raman band at ∼510 cm−1 can be a degenerate stretching mode, which affects the oxygen octahedral structure. The bending vibrations of O−W−O of the octahedra were observed in the range of 310−490 cm−1 with multiple peaks at 352, 436, and 460 cm−1. The A1g and T2g(1) vibrational modes have short- and long-range correlation radii, respectively. Typically, substitution can be observed by a change of the modes of T2g(1) and A1g when doping is performed with Er3+ and Yb3+ within the NLMWO lattice. We observed a slight intensity difference and a peak shift toward the lowerwavenumber side in the T2g(1) vibrational mode in the codoped sample compared with the other. There were no changes in the B-site substitution mode of A1g at 869 cm−1. This indicates that the bonding between the O and Eu3+ ions is stronger than that of O and Mg2+. Thus, we can affirmatively claim that rare-earth dopant ions are successfully substituted in the A site within the double-perovskite NLMWO structure.27 XPS Analysis. In order to evaluate the binding energy and electronic states of the elements inside the phosphor materials, we recorded the XPS spectrum for the codoped Er3+(6 mol %)/Yb3+(1 mol %):NLMWO nanophosphor system, as shown in Figure 4. It can be seen that the relevant elements of Na, La, Mg, W, O, Er, and Yb were identified in a wide-scan spectrum

NLMWO is similar to that of NaGdMgWO6, Sr2CaMoO6, and Ba2CaMoO6, which all possess double-perovskite structure with a rock-salt structure of B/B′ ions. Hence, they can have similar Raman spectra; however, the Raman spectrum of NLMWO can be more complex than those of the Sr2CaMoO6, Ba2CaMoO6, and Ba2CaWO6 structures because of its lower structural symmetry.26 Four active Raman vibrational spectral bands from the NLMWO phosphors were observed in the ranges of 100−200, 310−490, 500−600, and 790−900 cm−1, corresponding to the T2g(1), T2g(2), Eg, and A1g modes, respectively. The multipeaks in the Raman spectra in the ranges of 100−200 and 310−490 cm−1 could have resulted from splitting of the T2g(1) and T2g(2) modes because of the tilting junction of the (Mg/W)O6 octahedra and the layered order of Na/La. The vibrational mode of T2g(1) is due to Asite cations and coordinated octahedral atoms. This mode could be exhibited from translational vibrations of the A cations, and it is sensitive to A-site substitution, accordingly. However, the A1g mode could be exhibited because of the symmetric stretching vibrations of the WO6 octahedra, and it is sensitive to B-site substitution. The symmetric stretching vibrational bands attributed to O−W−O were observed at ∼869 cm−1 in all respective nanophosphor samples. The G

DOI: 10.1021/acs.inorgchem.8b02990 Inorg. Chem. XXXX, XXX, XXX−XXX

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through the ETU process. The 4I13/2 energy level behaves as an acceptor and intermediate energy state because its lifetime is longer than that of the other energy levels. Bright-green upconversion emission can be obtained by the combination of GSA and ESA processes, which provides a sufficient amount of energy to be cross-relaxed to the 4S3/2 level. Figure 6 shows the upconversion emission of optimized Er3+(6 mol %):NLMWO phosphors with respect to the excitation laser power. By increasing the laser power, we observed an enhancement in the upconversion emission intensity, especially in the green and red regions. In spite of this, when increasing the excitation laser power, we were unable to identify any additional emission peaks, and the shapes of the upconversion spectra remained identical. These results suggest that the Er3+(6 mol %):NLMWO phosphor has good stability toward the laser excitation energy. The NIR emission spectra of the NLMWO phosphor samples doped with Er3+ ions as a function of the Er3+ ion concentration in the range of 1400−1750 nm are presented in Figure 7a. We observed NIR emission at 1.57 μm for all NLMWO phosphors containing Er3+. This could have originated from the electronic transition of 4I13/2 → 4I15/2.32 The NIR emission intensity is significantly enhanced by the increasing Er3+ ion concentration, and it shows prominent emission features at an Er3+ concentration of 6 mol %. It can be seen that the NIR emission intensities were decreased after 6 mol % Er3+ concentration, as shown in Figure 7b, due to a concentration quenching effect. Photoluminescence and Energy-Transfer Mechanism of Yb3+/Er3+:NLMWO. We recorded the upconversion luminescence spectra of doubly doped Er3+(6 mol %)/Yb3+(x mol %):NLMWO nanophosphor systems by varying the Yb3+ ion concentration under the excitation of 980 nm at room temperature, as shown in Figure 8a. The upconversion spectral features of the codoped nanophosphor samples are different from nanophosphors containing singly doped Er3+ with respect to the dominant emission intensity because of differences in the energy-transfer processes. The upconversion luminescence spectra composed of weak and strong green emissions were observed at 531 and 550 nm, together with a strong red emission centered at 663 nm. The obtained green and red emission bands correspond to 2H11/2 → 4I15/2, 4S3/2 → 4I15/2, and 4F9/2 → 4I15/2, respectively, which are intrinsic transitions of Er3+ ions. The green emission band is split into two peaks, and the broad red emission band can be seen as a combination of three peaks, which could have originated from the stark sublevels of 4S3/2 and 4F9/2, respectively, and induced by electronic interactions and spin−orbit coupling.33 Here, the concentration of the activator Er3+ was kept constant at 6 mol % and the sensitizer concentration of Yb3+ was varied from 0 to 2 mol % in the codoped NLMWO nanophosphor systems. The upconversion emission intensities of the codoped NLMWO nanophosphors were greatly increased by increasing the sensitizer concentration of Yb3+. This can be attributed to the effective energy transfer from Yb3+ to Er3+ in codoped nanophosphor systems. This energy-transfer process results from spectral matching between the 4I11/2 excited state of Er3+ and the 2F5/2 excited state of Yb3+ and from the fact that the absorption cross section of Yb3+ ions at 980 nm is larger than that of Er3+, which increase the possibility of energy transfer from Yb3+ to Er3+.34 At 1 mol % sensitizer concentration of Yb 3+ , the codoped NLMWO nanophosphor shows a predominant upconversion red emission performance over the other samples (Figure 8b). Therefore, the optimum

(Figure 4a). The peak appearing at 1071.1 eV originated from Na 1s, as shown in Figure 4b. We found two pairs of peaks at 854.61 and 850.49 eV and 837.84 and 833.59 eV, which could have resulted from the La 3d spectrum corresponding to the subsplitting of La 3d3/2 and La 3d5/2,28 as shown in Figure 4c. The peak appearing at 1303.38 eV can be attributed to Mg 1s, as shown in Figure 4d. In addition, the spin−orbit coupling of electrons results in the appearance of W 4f doublet peaks, and we observed two peaks at 34.92 and 36.75 eV, which can be attributed to W 4f7/2 and W 4f5/2, respectively (Figure 4e). The spectrum related to O 1s is presented in Figure 4f, exhibiting a peak at 531.08 eV, which can be assigned to O2−. We clearly observed the bands at 194.0 and 167.10 eV, which can be attributed to the binding energies related to Yb 4d and Er 4d, respectively,29 as shown in Figure 4g,h. From this XPS analysis, we confirm that the Yb and Er ions are successfully incorporated into the NLMWO lattice. There are no other noticeable impurities in the XPS spectrum, and it is in good agreement with the EDX results presented in Figure 1. Photoluminescence Analysis of Er3+:NLMWO Phosphors. The properties of upconversion emission were examined for Er3+-doped NLMWO nanophosphors as a function of the concentration of Er3+ ions under excitation by a 980 nm laser in the wavelength region of 400−780 nm, as shown in Figure 5a. The upconversion emission spectra exhibited three signatures, centered at 535, 556, and 663 nm. These emission bands resulted from the corresponding electronic transitions, which originated from several excited states of 2H11/2, 4S3/2, and 4F9/2, to the 4I15/2 ground state, respectively. Nevertheless, the prominent green emission intensity was observed at 556 nm, which can be assigned to the electronic transition of 4S3/2 →4I15/2. Furthermore, we observed that the emission intensities could be significantly enhanced by increasing the Er3+ ion concentration, and the predominant green upconversion emission features were observed at an Er3+ ion concentration of 6 mol % (Figure 5b). Nonetheless, the upconversion emission intensities were apparently diminished at Er3+ ion concentrations greater than 6 mol %, which could be due to enhancement in the nonradiative transition rate by a cross-relaxation process between Er3+ ions from 4S3/2 to 4F9/2,30 and it affected the radiative transition rate of 4S3/2 → 4I15/2. Similarly, the remaining relevant emission bands also decrease with an increase of the Er3+ ion concentration above 6 mol %, which could be attributed to a multiphoton or cross-relaxation process from 2H11/2 to 4S3/2. Thus, the optimum concentration of Er3+ ions for high upconversion luminescence is determined as 6 mol %. There are two well-supported upconversion mechanisms regarding Er3+ ions under excitation of NIR radiation: one mechanism is the excited-state absorption (ESA), and the other is energy-transfer upconversion (ETU). When excited with 980 nm NIR radiation, the ground-state energy level 4I15/2 of Er3+ absorbs the NIR energy and populates the 4I11/2 higherenergy state via a ground-state absorption (GSA) process. It also successfully populates the 4I13/2 energy level by a multiphoton relaxation process. The Er3+ ions can be further excited to 4F7/2 by energy migration between adjacent Er3+ ions following GSA. 2H11/2 and 4S3/2 can be populated by rapid nonradiative relaxation from 4F7/2, which generates green upconversion emission from phosphor materials containing Er3+.31 A certain amount of energy transition occurs, which is known as the ESA from the 4I11/2 level to the 4F7/2 higher level by absorbing or accepting energy from nearby Er3+ ions H

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which can be attributed to the electronic transition of 4I13/2 → I15/2 under an excitation of 980 nm, as shown in Figure 10a. Initially, the excited state of 4I11/2 is relaxed to 4I13/2 by a nonradiative relaxation process, and then a radiative electronic transition occurs from 4I13/2 to 4I15/2 under an excitation of 980 nm, which can generate a 1.57 μm emission. This can be attributed to the possible energy-transfer process from Yb3+ to Er3+ ions within the NLMWO lattice. The energy-transfer mechanism and process are clearly illustrated in Figure 11. However, the predominant NIR emission features are observed at the sensitizing concentration of 1 mol % Yb3+ ions in the codoped nanophosphor system. Nevertheless, we have noticed that the NIR emission intensities at 1.57 μm are decreased by increasing the Yb3+ ion concentration beyond 1 mol %, as shown in Figure 10b. This could be attributed to the concentration quenching effect. Therefore, the optimized concentration for the NIR emission in the codoped nanophosphor system is determined as 1 mol % of Yb3+ ions. Analysis of the Commission internationale de l’éclairage (CIE) Chromaticity Coordinates. The color perceptions of phosphor samples of optimized Er3+(6 mol %):NLMWO and Er3+(6 mol %)/Yb3+(1 mol %):NLMWO nanophosphors were demonstrated by analysis of the CIE color coordinates, as illustrated in Figure 12. The chromaticity coordinates for these two optimized phosphor samples were evaluated from the upconversion emission spectra,38 and they were found as (0.2337, 0.3831) and (0.3918, 0.5963), respectively. It can be seen that the Er3+(6 mol %):NLMWO phosphor sample is spotted in the strong green region by its own emission, as was discussed in the upconversion emission analysis. However, the codoped Er3+(6 mol %)/Yb3+(1 mol %):NLMWO nanophosphor sample is located in the yellow region. We can observe that the color coordinates transformed from (0.2337, 0.3831) to (0.3918, 0.5963) as a result of codoping with Yb3+ ions. This indicates that the color hue is tunable from green to yellow by codoping with Yb3+ ions. The yellow color could have originated from the combination of green and red emission from its emission under 980 nm excitation. Cytotoxicity Assay. The cytotoxic response of the codoped Er3+(6 mol %)/Yb3+(1 mol %):NLMWO nanophosphor sample is shown in Figure 13. To evaluate the cytotoxicity, the WI-38 cell lines were used and exposed to the codoped Er3+(6 mol %)/Yb3+(1 mol %):NLMWO nanophosphors to a variety of sample concentrations of 50, 100, 150, and 200 μg mL−1 for 48 h. The result of the cell viability assays (Figure 13a,b) showed that there was no significant reduction in the percentage of viable cells after 48 h of exposure. Further, there were no obvious changes on the morphologies of WI-38 fibroblast cells (Figure 13c−g) even after 48 h of exposure with codoped Er3+(6 mol %)/Yb3+(1 mol %):NLMWO nanophosphors, attributable to the nontoxic nature of our nanophosphors. These noncytotoxic Er3+(6 mol %)/Yb3+(1 mol %):NLMWO nanophosphor materials can be a potential choice for practical devices.39

sensitizer concentration for upconversion emission in the codoped nanophosphor system is determined to be 1 mol %. Moreover, the upconversion emission intensity in the red region was found to be more enhanced than that in the green region in the codoped sample compared with the single Er3+doped NLMWO phosphor samples discussed earlier. It was observed that the upconversion emission features at the red region were mostly increased by an increase of the sensitizer concentration in codoped phosphor systems. The upconversion emission intensities were diminished above the optimum value of the sensitizer concentration, which could have occurred because of concentration quenching.35 It can be seen that the red-to-green emission intensity ratio is greatly improved by increasing the sensitizer concentration in codoped nanophosphor samples, with red as the dominant emission, as shown in Figure 8a. The energy-transfer process from Yb3+ to Er3+ and enhancement in the upconversion emission in codoped systems can be explained with the energylevel structure as follows. The Er3+ ions are excited to the 4I11/2 energy level from the 4I15/2 energy level by absorbing the incident energy (980 nm) by the GSA process. Nevertheless, the energy migration process most likely occurs between Yb3+ and Er3+ ions because of the larger absorption cross section of Yb3+ and the existence of a resonance between the emission transition of 2F5/2 → 2F7/2 of Yb3+ and the absorption transition of 4I15/2 → 4I11/2 of Er3+. The 2H11/2 and 4S3/2 energy states of Er3+ can possibly be populated by successive energytransfer processes from the Yb3+ excited state 2F5/2 to the Er3+ ions.36 Because of energy transfer from the Yb3+ ions, first the energy states 4I11/2 and 4F7/2 of the Er3+ ions can be efficiently excited, and then they are relaxed nonradiatively to 2H11/2 and 4 S3/2 because of the minor energy band gap between both the 2 H11/2 and 4S3/2 energy states. However, the green emission with two relevant peaks centered at 531 and 550 nm can be radiatively generated by a faster relaxation to the 4S3/2 state. The Er3+ ions can also be relaxed nonradiatively to the 4F9/2 level, resulting in a strong red emission, as shown in Figure 8a. It should be noted that the amount of red emission and the emission bandwidth are significantly enhanced by increasing the codopant concentration of Yb3+ ions, compared with the phosphor samples containing singly doped Er3+. The relative changes in the green and red emission peak intensities were observed by varying the Yb3+ ion concentration. The crossrelaxation process 4F7/2 + 4I11/2 → 4F9/2 + 4F9/2 has more contribution to the strong red emission than the green emission in codoped nanophosphors.37 The cross-relaxation can promote the populated 4F9/2 states of the Er3+ ions, which can significantly enhance generation of the strong red emission. Among all prepared nanophosphor samples, the Er3+(6 mol %)/Yb3+(1 mol %):NLMWO nanophosphor sample exhibits a prominent red upconversion emission. The upconversion emission spectra, which depend on the laser excitation power, of the optimized Er3+(6 mol %)/Yb3+(1 mol %):NLMWO nanophosphor were recorded and are shown in Figure 9. We observed a significant enhancement in the upconversion emission spectral intensity of the Er3+(6 mol %)/Yb3+(1 mol %):NLMWO nanophosphor by increasing the laser excitation power, while the shape of the emission spectra remained the same. This suggests that the Er3+(6 mol %)/Yb3+(1 mol %):NLMWO nanophosphor is sufficiently stable toward the laser excitation energy.31 By increasing the concentration of Yb3+ ions in the codoped nanophosphor system, the NIR emission at 1.57 μm is remarkably enhanced,

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CONCLUSIONS We successfully synthesized NLMWO nanophosphors doped and codoped with Er3+, Yb3+, and Yb3+/Er3+ by a citrate sol− gel method. The double-perovskite structure of the prepared phosphor materials was confirmed by XRD analysis. The assignments of the vibrational bands and other functional group details are discussed based on the Raman spectral I

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2016R1D1A1B03932028) and by the Ministry of Science, ICT and Future Planning (Grant NRF-2017R1A4A1015581).

studies together with FTIR analysis. The results of surface morphological studies and particle-size evaluation were clearly verified by SEM and TEM, respectively. Verification of the presented elements in the optimized codoped nanophosphor and the ionic states of the elements was systematically done by EDS analysis and XPS, respectively. Strong upconversiongenerating green emission was observed from Er3+-doped NLMWO phosphor materials under an excitation of 980 nm because of the electronic transition of 4S3/2→ 4I15/2. A significant enhancement of the green upconversion emission was observed when the Er3+ ion concentration was increased up to 6 mol %. The green upconversion emission was found to be diminished above 6 mol % because of the concentration quenching effect. Thus, the optimized concentration for Er3+ ion green upconversion emission was determined as 6 mol %. Similarly, the NIR emission at 1.57 μm (4I13/2 → 4I15/2) was also identified and increased effectively by increasing the concentration of Er3+ ions under an excitation of 980 nm. Remarkably, the strong green upconversion emission was partially converted to strong red upconversion emission (7F9/2 → 4I15/2) when NLMWO nanophosphor systems were codoped with Yb3+ ions together with Er3+ ions. Moreover, the upconversion emission features were remarkably enhanced by increasing the concentration of Yb3+ ions up to 1 mol %, which can result from an energy-transfer mechanism from Yb3+ to Er3+ ions. Above 1 mol %, the upconversion emission properties of the Yb3+/Er3+:NLMWO nanophosphors were drastically reduced because of the concentration quenching effect. In addition, the NIR emission properties of codoped Yb3+/Er3+:NLMWO nanophosphors were systematically evaluated. The upconversion and NIR emission properties were significantly enhanced by increasing the concentration of the sensitizer Yb3+ ion in the codoped network. The upconversion emission spectra of the optimized codoped nanophosphor were evaluated by varying the excitation power of the 980 nm laser. A possible energy-transfer mechanism from Yb3+ to Er3+ ions and the cross-relaxation mechanism were clearly demonstrated by a partial energy-level scheme diagram. The CIE color coordinates were evaluated from their upconversion emission spectra and accurately analyzed. The predominant upconversion emission and NIR emission characteristics were observed from the optimized codoped Er3+(6 mol %)/Yb3+(1 mol %):NLMWO nanophosphor system. We evaluated the cytotoxicity of the optimized codoped Er3+(6 mol %)/Yb3+(1 mol %):NLMWO nanophosphor system. It was found that the presented optimized nanophosphor is a noncytotoxic material even at a higher concentration of 200 μg mL−1 with a cell exposed for 48 h.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Jungwook Choi: 0000-0001-5916-9714 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (Grant NRFJ

DOI: 10.1021/acs.inorgchem.8b02990 Inorg. Chem. XXXX, XXX, XXX−XXX

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