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Article Cite This: ACS Omega 2018, 3, 788−800
Neutralizing the Charge Imbalance Problem in Eu3+-Activated BaAl2O4 Nanophosphors: Theoretical Insights and Experimental Validation Considering K+ Codoping Rituparna Chatterjee,†,∥ Subhajit Saha,†,∥ Dipayan Sen,† Karamjyoti Panigrahi,† Uttam Kumar Ghorai,§ Gopes Chandra Das,† and Kalyan Kumar Chattopadhyay*,†,‡ †
School of Materials Science and Nanotechnology and ‡Department of Physics, Jadavpur University, Kolkata 700032, India § Department of Industrial Chemistry & Swami Vivekananda Research Centre, Ramakrishna Mission Vidyamandira, Belur Math, Howrah 711202, India S Supporting Information *
ABSTRACT: In recent years, rare-earth-doped nanophosphors have attracted great attention in the field of luminescent materials for advanced solid-state lighting and high-resolution display applications. However, the low efficiency of concurrent red phosphors creates a major bottleneck for easy commercialization of these devices. In this work, intense red-lightemitting K+-codoped BaAl2O4:Eu3+ nanophosphors having an average crystallite size of 54 nm were synthesized via a modified sol−gel method. The derived nanophosphors exhibit strong red emission produced by the 5D0 → 7FJ (J = 0, 1, 2, 3, 4) transitions of Eu3+ upon UV and low-voltage electron beam excitation. Comparative photoluminescence (PL) analysis is executed for Eu3+-activated and K+-coactivated BaAl2O4:Eu3+ nanophosphors, demonstrating remarkable enhancement in PL intensity as well as thermal stability due to K+ codoping. The origin of this PL enhancement is also analyzed from first-principles calculations using density functional theory. Achievement of charge compensation with the addition of a K+ coactivator plays an important role in increasing the radiative lifetime and color purity of the codoped nanophosphors. Obtained results substantially approve the promising prospects of this nanophosphor in the promptly growing field of solid-state lighting and field emission display devices. blue phosphors that can collectively produce white light.6,7 However, the subpar efficiency of the present-generation red phosphors creates a major bottleneck for their easy deployment for commercial purposes.8 To tackle this problem, currently, a great deal of research effort is being directed to developing highly luminescent red phosphors that can be efficiently excited in the n-UV regime. A recent development that has revolutionized the domain of solid-state lighting technology is the so-called nanophosphors. It is well established that by reducing the size of a crystallite, novel surface electronic states can be engineered. These surface states can drastically alter the optical properties, especially when the dimension of the crystallites encroaches the nanoregime. Because the intensity of scattered light is directly proportional to the sixth power of particle diameter, nanophosphors demonstrate less scattering loss of emitted radiation as compared to that in bulk phosphors when utilized in an LED device, leading to an enhanced external quantum efficiency.9,10 Also, it is well
1. INTRODUCTION The ever-growing impact of solid-state lighting technology on household usage in the form of white-light-emitting diodes (WLEDs) and high-resolution displays has, in recent times, initiated a pan-global hunt for improvement and optimization of brightness and color purity. WLEDs, the fourth-generation lighting source, are already exhibiting phenomenal advantages in efficiency, operational lifetime, form factor, and environmental impact and are well on the way to replace incandescent and fluorescent lights.1−3 Most widely deployed protocols to develop WLEDs adopt a “phosphor conversion” route, i.e., yellowemitting Y3Al5O12:Ce3+ (YAG:Ce) phosphor is coated on a blueemitting (460 nm) InGaN chip to mix yellow and blue light for generating white light. However, most of the current-generation WLEDs, even though called “white”, in effect, emit “cool white” light with low color rendering index (CRI < 80) and high correlated color temperature (CCT ∼ 7750 K)4,5 because of the lack of red components in their spectra, which further limits their widespread commercialization for indoor lighting purposes. To circumvent this problem, an alternate route is designed, which incorporates a combination of near-ultraviolet (n-UV) lightemitting diode (LED) chip (340−420 nm) with red, green, and © 2018 American Chemical Society
Received: October 10, 2017 Accepted: January 4, 2018 Published: January 22, 2018 788
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ACS Omega reported that films containing nanophosphors show greater transmittance than that from those containing bulk phosphors,9 which is very much beneficial for extracting an efficient emission output. Moreover, decreasing particle size also has an additional, much sought-after, effect, as in the nanoform, they can be coated over a substrate significantly higher times than their bulk counterparts to achieve a desired phosphor layer thickness. This results in meticulous control over thickness as well as homogeneity, which is a key perquisite for developing nextgeneration technology such as even higher resolution displays. Consequently, in the recent times, search for newer and novel nanophosphors is occupying a prime slot in both experimental and theoretical materials science. Recent developments in this emerging field have established a low-temperature protocol for realizing rare-earth-doped nanophosphors, which further highlights their versatility in contrast to that of the bulk phosphors. Continuing along this direction, in the current work, we aim to design a suitable, highly luminescent red nanophosphor with enhanced brightness and stability to address the aforementioned problems. Till date, the most prevalent protocols to realize brilliant luminescence involve rare-earth (RE) element-based activators11−13 as the partly occupied 4f orbitals of RE ions are largely screened from the contiguous chemical surroundings by completely occupied 5s and 5p orbitals. Consequently, the luminescence originating from the 4f → 4f transition usually consists of sharp lines with comparatively high lifetimes. Following this route, the de facto choice for realizing bright red luminescence is a Eu3+ activator as its all emission peaks lie in the red region of the visible spectra. Thus, the characteristic 5D0 → 7F2 transition of trivalent europium (Eu3+) is currently the most exploited protocol to achieve highest-quality red phosphors and is utilized extensively in numerous commercial applications ranging from display to solid-state lighting. However, it must be noted that the nature of this transition line strongly depends on the local environment and site symmetry; hence, locating a suitable host material that can facilitate extraction of rich luminescence from the activator constructs the next important phase of the problem. The most significant role in modulating the brightness of a phosphor is played by migration of energy from the host to the RE activator; thus, a host system with a large band gap and lower vibrational energy is incumbent to design the optimum solution.14,15 Following the above mandate, among a huge abundance of inorganic hosts, alkaline earth aluminates demand a closer scrutiny owing to various indigenous advantages such as low chemical toxicity and high thermal and chemical stabilities.16,17 Specifically, BaAl2O4 is a noteworthy material in this context for its excellent pyroelectric and dielectric properties and its suitability toward coating as well as refractory applications.18,19 Most importantly, the wide band gap (∼4 eV) of BaAl2O4 ensures negligible absorption in the visible region and thereby minimizes emission loss, rendering it an ideal sensitizer in nanophosphors. The stable crystal structure of BaAl2O4 is another advantage of using this material as a host. BaAl2O4 belongs to the family of stuffed tridymites. In the tridymite structure, the distorted AlO4 tetrahedral clusters share the corners, which connect together to form six-membered rings. Ba2+ cations are situated inside the channels of this structure.20 These Ba2+ sites construct highly suitable sites for Eu3+ doping as (a) their lower symmetry is highly favorable for the Eu3+ 4f → 4f transition and (b) the ionic radii of Eu3+ is comparable to that of Ba2+. Hence, motivated by these favorable aspects, till now,
various research groups have attempted to develop rare-earthdoped BaAl2O4 nanophosphors using various synthesis protocols. There are quite a few such reports regarding Eu2+ doping in BaAl2O4 nanophosphors;16,17,21,22 however, reports of successful doping of Eu3+ in a BaAl2O4 matrix are remarkably sparse. Wiglusz et al. had recently adopted a sol−gel method for developing Eu3+-doped BaAl2O4 nanophosphors and investigated their structural changes and luminescence properties;23 however, the maximum photoluminescence (PL) brightness obtained by them is still not up to the mark and leaves much to be desired. A highly effective strategy to improving the PL brightness in a case like the above might lie in codoping alkali metal ions along with the rare-earth ions in the nanophosphor matrix. The rationale behind this approach can be elaborated as follows: Doping only rare-earth ions imparts a charge imbalance in the crystal; however, concurrent doping of alkali metal ions actively counteracts the charge imbalance. This approach had been successfully employed by various research groups recently, which yielded significant improvement of the PL brightness for various types of rare-earth-doped nanophosphors.24−27 In the current work, following the above strategy, we have synthesized BaAl2O4:Eu3+ nanophosphors by the sol−gel route and investigated photoluminescence properties by varying the K+ coactivator concentration, which, to the best of our knowledge, hitherto have not been attempted.
2. EXPERIMENTAL SECTION 2.1. Synthesis of BaAl2O4:Eu3+ and K+-Codoped BaAl2O4:Eu3+ Nanophosphors. The customized Pechinitype sol−gel route was adopted to synthesize Ba(1−x)Al2O4:Eux (x = 0, 0.005, 0.01, 0.02, 0.05, 0.07) and Ba(1−x−y)Al2O4:Eux, Ky (x = 0.05, y = 0.01, 0.02, 0.03, 0.04, 0.05) nanophosphors. The actual synthesis procedure is parallel to the one mentioned in our earlier report.28 The precursors used for the synthesis were Ba(NO3)2 (Merck 99.9%), Al(NO3)3·9H2O (Merck, 99.9%), Eu(NO3)3 (prepared by making a solution of Eu2O3 (SigmaAldrich 99.99%) in HNO3), KNO3, citric acid (Merck 99.9%), and ethylenediamine (Merck 99.9%), all of which were of analytical reagent grade. The typical synthesis procedure is described below. Initially, stoichiometrically weighed barium nitrate, aluminum nitrate, europium nitrate, and potassium nitrate were dissolved in 35 mL of deionized water and the solution was stirred in a magnetic stirrer for 1 h. Then, the aqueous solution of citric acid was added dropwise to the prepared metal nitrate solution in Cit3−/(Ba2+ + Al3+) = 1 molar ratio and then again stirred in a magnetic stirrer for another 30 min to make a homogeneous transparent solution. Afterward, ethylenediamine was added in the transparent solution to adjust the pH to 6. The solution was then put in a water bath and slowly evaporated, which converted it into a highly viscous colloidal gel. In the next step, the gel was dried in an oven at 120 °C overnight and then calcined at 700 °C for 6 h for removing the organics. At last, the porous material was ground using a mortar and annealed at 1000 °C for 5 h in a muffle furnace to obtain ultrafine Eu3+- and K+-doped BaAl2O4 powders. For Eu3+-doped BaAl2O4 nanophosphors, we have followed the same process except KNO3 was not added in that case. 2.2. Characterization. The phase purity and structural parameters of the as-synthesized samples were investigated through X-ray diffraction (XRD), with a Rigaku-Ultima-III XRay powder diffractometer using Cu Kα radiation (λ = 1.5404 Å). The Fourier transform infrared (FTIR) spectra were 789
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Figure 1. (a) XRD pattern of BaAl2O4:Eu3+ nanophosphors. (b) Demonstration of higher angle shifting of the (202) peak with an increase in Eu3+ concentration. (c) XRD pattern of K+-codoped BaAl2O4:5% Eu3+ nanophosphors for various concentrations of K+. (d) Demonstration of the BaAl2O4 host structure having a distorted AlO4 tetrahedral network. Nine-fold coordination of the Ba2+ site is also represented. (e) Transmission electron microscopy (TEM) image presenting the distribution of the nanorod-like structure of the BaAl2O4 crystals. (f) HRTEM image depicting the interplanar spacing of 0.342 nm corresponding to the (202) plane in BaAl2O4:5% Eu3+, 4% K+ nanophosphors.
recorded using a Shimadzu IRPrestige-21 spectrometer in the mid-IR wavenumber range, i.e., 4000−400 cm−1. The shape and crystalline structure of the nanocrystals were analyzed with highresolution transmission electron microscopy (HRTEM, JEOL JEM 2100). The morphologies of the sample were investigated by FESEM Hitachi S-4800. Diffuse reflectance spectra were acquired using a Shimadzu 3600 UV−vis−NIR spectrophotometer with BaSO4 as a reference. The steady-state photoluminescence spectra were recorded on JASCO FP-8300, whereas the time-resolved photoluminescence spectra were recorded on an Edinburgh FLSP-980 luminescence spectrometer using a microsecond flash lamp as the source of excitation. The absolute photoluminescence quantum yield of the nanophosphors was measured using a 120 mm diameter integrating sphere whose inner face is coated with BENFLEC for sufficient scattering of incoming light, as supplied by Edinburgh instruments. The quantum efficiency (η) can be calculated using the following expression η=
functional within the generalized gradient approximation (GGA) was used to describe the exchange and correlation terms. Plane wave basis up to an energy cut off of 400 eV was utilized in all calculations. Because the current computations involve fairly large supercells, a Γ point centered, 2 × 2 × 2 k point grid was deployed to perform Brillouin zone integrations. During structural optimization, all systems were allowed to fully relax until the change in the total (free) energies was smaller than 1 × 10−3 eV. Bader charge analysis29i was used to quantitatively estimate the charge transfer between various species. A 2 × 2 × 2 supercell of pristine BaAl2O4 (space group P6322 (D6-6), IT#: 182) containing 16 Ba, 32 Al, and 64 O atoms (relaxed lattice parameters: a = 10.047 Å, b = 10.047 Å, c = 17.957 Å) was used as the initial system in which substitution of one Ba atom amounted to 6.250% of doping. Subsequently, in the above structure, (a) one Ba atom was substituted by a Eu atom and (b) two adjacent Ba atoms were substituted by a Eu and a K atom and fully (both atomic coordinates and lattice vectors) relaxed to obtain the Eu-doped and Eu−K-codoped BaAl2O4 structures. For the doped and codoped systems, while describing the Eu atoms, [Kr] 4d10 was considered as the core and the shell 5s25p64f76s2 containing 17 electrons was considered as the valance shell to facilitate delocalization of the Eu f orbital. To ensure fast convergence in these cases, one-center PAW charge densities up to l = 3 were passed through the charge density mixer and smaller mixing parameters were used.
∫ LS ∫ ER − ∫ ES
where LS is the emission spectrum of the sample and ES and ER the spectra of the excitation light with and without the sample in the integrating sphere, respectively. Cathodoluminescence (CL) spectra were recorded using a Gatan Mono CL3 device attached to the field emission scanning electron microscope with a beam accelerating voltage of 5 kV. All first-principles calculations were carried out using the Vienna Ab initio Simulation Package (VASP).29a−d The ion cores were described using the projector-augmented wave (PAW) method,29e,f and the Perdew−Burke−Ernzerhof (PBE)29g,h
3. RESULTS AND DISCUSSION 3.1. Structural, Compositional, and Microstructural Analysis. The XRD patterns of BaAl 2 O 4 :Eu 3+ and BaAl2O4:Eu3+, K+ nanophosphors are presented in Figure 1a,c, 790
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Table 1. Variation of Full Width at Half-Maximum (FWHM) of the (202) Peak, Crystallite Size, Lattice Parameter, and Unit Cell Volume with Different Doping Concentrations of Eu3+ and K+ sample name 3+
BaAl2O4:0.5% Eu BaAl2O4:1% Eu3+ BaAl2O4:2% Eu3+ BaAl2O4:5% Eu3+ BaAl2O4:7% Eu3+ BaAl2O4:5% Eu3+, 1% K+ BaAl2O4:5% Eu3+, 2% K+ BaAl2O4:5% Eu3+, 3% K+ BaAl2O4:5% Eu3+, 4% K+ BaAl2O4:5% Eu3+, 5% K+
FWHM of (202) peak (deg)
crystallite size from Scherrer formula (nm)
a
c
unit cell volume (Å3)
0.162 0.175 0.185 0.197 0.202 0.174 0.16 0.153 0.137 0.147
56 52 50 46 45 52 58 59 61 60
10.455 10.453 10.45 10.448 10.447 10.45 10.459 10.461 10.475 10.452
8.8 8.797 8.796 8.788 8.784 8.807 8.82 8.824 8.826 8.818
833 832.4 831.83 830.75 830.22 832.87 835.53 836.23 838.66 834.23
again charge imbalance is created and these surplus K+ ions generate defects in the system, which eventually reduce the crystallinity of the nanophosphors. Moreover, incorporation of K+ in the site of Ba2+ always gives rise to oxygen vacancies in the BaAl2O4 lattice. Beyond 4% concentration of K+, several oxygen vacancies are created in the crystal, which are also responsible for the decrease in crystallinity of the nanophosphor. This observed degradation of crystallinity with substitution of alkaline metal ions in high concentration is also evident in prior studies with different hosts.33,28 Analysis of FTIR spectra (shown in Figure S2, SI) also confirms the formation of pure-phase K+-codoped BaAl2O4:Eu3+ nanophosphors. The presence of sharp peaks within 500−900 cm−1 and at 1443 cm−1 corresponding to the stretching-mode vibrations of the AlO4 unit confirms the formation of the BaAl2O4 tridymite structure.34 Because the regular morphology and enhanced crystallinity of the host−activator system result in efficient energy transfer from the host to the activator, in the present work, we have analyzed microstructural characteristics of the nanophosphors. The TEM and HRTEM images of the K+-codoped BaAl2O4:Eu3+ nanophosphor are shown in Figure 1e,f. The low-magnification TEM image demonstrates a rodlike structure of the nanophosphors with a diameter of ∼35−40 nm and length of ∼500−600 nm, which is in well correspondence with the field emission scanning electron microscopy (FESEM) image of the sample, as shown in Figure S3 in SI. A closer observation also reveals that the surface of each nanorod also consists of tiny one-dimensional nanostructures (shown in Figure 1f), developed due to the secondary growth of the nanocrystals. The typical HRTEM image clearly shows well-resolved lattice fringes with the estimated interplanar spacing of 0.342 nm, which is consistent with the (202) plane of K+-codoped BaAl2O4:Eu3+ nanophosphors with the hexagonal stuffed tridymite structure. X-ray photoelectron spectroscopy (XPS) is an imperative technique for probing the chemical composition of the developed nanophosphors. The X-ray photoelectron spectra of the synthesized nanophosphors are presented in Figure 2. Each spectrum has been charge-corrected using the C 1s line at 284.6 eV, which appeared due to the presence of adventitious carbon on the sample surface during the exposure to atmosphere. The survey scans of BaAl2O4:Eu3+ and K+-codoped BaAl2O4:Eu3+ nanophosphors are shown in Figure 2a, which strongly indicate the presence of Ba, Al, O, and Eu in every synthesized nanophosphor. In addition to that, the presence of K is also detected in the survey scan of the codoped nanophosphor. Moreover, the confirmation of the exact valence state of each constituent element was probed through its corresponding highresolution (HR) scan. Figure 2b represents the high-resolution
respectively. The peak positions and intensities are in good agreement with the ICDD PDF card no. 17-0306, which are indexed as (201), (202), (220), (222), (004), (402), (204), (224), and (422) in Figure 1a,c. There are no impurity peaks in the XRD pattern, which confirms the construction of a singlephase tridymite structure with space group P6322. The best way to visualize this kind of stuffed tridymite structure is a threedimensional framework of corner-sharing distorted Al−O4 tetrahedra, which is shown in Figure 1d. Ba2+ cations take the position within this ring of such six tetrahedral networks. From Figure 1b, it can be noted that because of the increase in Eu3+ concentration the (202) XRD peaks shift toward a higher angle side. Generally, Eu3+ occupies the position of Ba2+ in the BaAl2O4 lattice and the ionic radii of Ba2+ and Eu3+ are 1.36 and 0.95 Å, respectively. Hence, during substitution of Eu3+ in Ba2+ sites, the unit cell shrinks, leading to the shift of XRD peak toward a higher 2θ value.30,31 Doping of a Eu3+ ion in the Ba2+ site creates Ba2+ vacancies in the crystal, resulting in a charge imbalance problem. The obtained higher angle shift in the XRD peak can also be evidenced from the calculated values of lattice parameters (a,b,c) and unit cell volume (V) detailed in Table 1. For all compositions, the crystallite size of the prepared phosphors sticks within the range of 45−60 nm, ensuring the formation of crystals in the nanoscale regime. Table 1 also illustrates that with the increasing K+ doping concentration the crystallite size and lattice parameter increase gradually up to 4% and then decreases for 5%. However, FWHM of the (202) peak follows opposite trend i.e. up to 4% codoping it decreases and then increases for 5% K+ addition. Because of almost similar ionic radii of K+ and Ba2+ (1.38 and 1.36 Å, respectively),32 K+ efficiently substitutes Ba2+ in the BaAl2O4 lattice. This substitution easily eliminates Ba2+ vacancies from the crystal created as a result of the charge imbalance problem induced by the doping of Eu3+ in Ba2+ sites. Consequently, defects are reduced inside the crystal and crystallization is enhanced. Thus, curtailing of FWHM of the (202) XRD peak with an increase in K+ concentration clearly implies that K+ acts as a self-promoter for the improved crystallization of the Eu3+:BaAl2O4 lattice. Furthermore, a slightly high ionic radius of K+ as compared to that of Ba2+ leads to the expansion of unit cell, which effectively increases the crystallite size of the nanophosphors after codoping. The occurrence of this unit cell expansion can be traced from a gradual lower angle shift of the (202) XRD peak with the increasing concentration of K+ in the codoped nanophosphors (shown in Figure S1 of SI). However, the enriched crystallization sustains up to a particular K+ concentration (4% in the current work), and beyond this, further substitution of Ba2+ by K+ becomes very difficult. Thus, 791
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BaAl2O4, respectively.37,38 The characteristic peak of lattice oxygen is also observed in oxygen 1s core-level spectra at 530.4 eV. 3.2. Spectroscopic Analysis. The steady-state photoluminescence analysis of the BaAl2O4:Eu3+ and K+-codoped BaAl2O4:Eu3+ nanophosphors was conducted from PL excitation and emission spectra. Each measurement was carried out by taking identical amount of samples to estimate the intensity variation depending on the quality of samples only. Figure 3a represents PL excitation spectra of 5% Eu3+-doped BaAl2O4 and 5% Eu3+- and 4% K+-doped BaAl2O4 nanophosphors, which were recorded by monitoring the fixed emission of the Eu3+ ion at 614 nm. PL excitation spectra can be categorized into two segments: a broad peak and few sharp peaks. The observed broad peak at 256 nm can be directly assigned to the charge transfer band (CTB) arising due to electron transfer from the O2− (2p) orbital to the empty 4f orbital in Eu3+.39 Usually, such ligand−metal charge transfer is directly associated with the covalency between O2− and Eu3+ and the surrounding environment of Eu3+. It was interesting to note that the intensity of CTB increases in the case of K+-codoped samples. To find out the origin of this enhancement, Bader partial charges and charge density differences were computed using density functional theory. In the case of the Eu-doped system, the Eu atom is connected to the two neighboring Ba atoms by six oxygen atoms. The Ba/Eu atoms donate electrons and the O atoms receive them, rendering +ve and −ve charges to them, respectively. The calculated Bader partial charges of the Eu, Ba, and O atoms for this configuration are shown in Figure 4a,b. However, in the codoped system (Figure 4b), the Eu atom is straddled by a K atom and a Ba atom. Because the number of valence electrons in the K atom is less than that in the Ba atom, the number of electrons available for donation is also less. The first implication of the above can be directly observed in the Bader partial charges of the bridging O atoms as the Ba−O−Eu linkage has more negative charges than those of the K−O−Eu linkage. Second, the O atoms, in the codoped case, try to address this imbalance by grabbing even more electrons from the Eu atom, rendering more +ve charge to it. The schematic representation of this charge sharing is shown in Figure 4c. This also can be directly verified by noting that the Eu atom has a higher +ve Bader partial charge (+2.00 e) in the codoped case in contrast to that in the doped case (+1.67 e). The net effect is graphically illustrated in Figure 4d,e by plotting
Figure 2. (a) XPS survey spectra of the BaAl2O4:Eu3+ and BaAl2O4:Eu3+, K+ nanophosphors. (b) High-resolution core-level spectra of Eu 3d for BaAl2O4:Eu3+ (black line) and BaAl2O4:5% Eu3+, 4% K+ nanophosphor (red line). (c) K 2p core-level spectra of BaAl2O4:5% Eu3+, 4% K+ nanophosphor.
spectra of the Eu 3d core level for the K+-codoped BaAl2O4:Eu3+ nanophosphor. The HR scan consists of two strong core-level peaks at 1164.5 and 1134.6 eV, which can be designated as spin− orbit splitting components Eu 3d3/2 and Eu 3d5/2, respectively. The binding energy difference between these two peaks of Eu 3d is in good agreement with the earlier reports, confirming the presence of the 3+ valence state of Eu ions in the synthesized nanophosphors.35 The presence of monovalent K is also evident from the HR scan of K 2p, which shows distinct spin−orbit splitting components of 2p3/2 and 2p1/2 at 291.6 and 295.14 eV, respectively (shown in Figure 2c).36 Besides that for the activator and coactivator, the analysis of the HR scan is also carried out for other elements such as Ba, Al, and O, as shown in Figure S4 of the SI. The presence of spin−orbit split components in the core-level spectra of Ba 3d and Al 2p having separations of 15.21 and 1.02 eV confirms the +2 and +3 oxidation states of Ba and Al in
Figure 3. (a) PL excitation spectra of BaAl2O4:5% Eu3+ and BaAl2O4:5% Eu3+, 4% K+ nanophosphors observed at 614 nm emission. (b) Energy-level diagram representing the possible PL transition pathways in K+-codoped BaAl2O4:Eu3+ nanophosphors. (c) Diffuse reflectance spectra of undoped, Eu3+-doped, and Eu3+−K+-codoped BaAl2O4 nanophosphors. Signature of absorption corresponding to the Eu3+ f−f transition is shown in the inset. 792
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Figure 4. Bader partial charges over the constituent elements in the (a) Ba−Eu−Ba and (b) K−Eu−Ba configurations. (c) Schematic illustration of the charge transfer phenomenon. (d, e) Representation of the charge density difference in the doped and codoped systems.
Figure 5. PL emission spectra of (a) BaAl2O4:xEu3+ nanophosphors and (b) BaAl2O4:5% Eu3+, yK+ nanophosphors measured under 394 nm excitation. The insets in the lower left and right corners demonstrate the digital images of the samples (BaAl2O4:5% Eu3+ and BaAl2O4:5% Eu3+, 4% K+) kept under a UV lamp and their corresponding color coordinates, respectively.
charge density differences of the doped and codoped systems, respectively, with respect to pristine BaAl2O4. In the case of K codoping, the electron gain region (yellow isosurface) heavily populates the region between the O and Eu
atoms, highlighting a higher amount of charge transfer between them as a direct consequence of codoping. Thus, the above not only explains the root cause of enhanced charge sharing between Eu and O but also underpins the observation of stronger CTB in 793
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ACS Omega Eu3+−K+-codoped nanophosphors as compared to that in Eu3+doped counterparts from an atomistic point of view. Besides CTB, the excitation spectra consist of some other sharp lines in the wavelength range of 350−480 nm, whose appearance indicates the intraconfigurational 4f−4f transitions of Eu3+ in the BaAl2O4 lattice. Direct excitation of the Eu3+ ground state to the higher energy levels in the 4f6 configuration results in these sharp peaks, which can be indexed as 7F0 → 5D4 (362 nm), 7 F0 → 5L7 (380 nm), 7F0 → 5L6 (394 nm), 7F0 → 5D3 (415 nm), and 7F0 → 5D2 (464 nm) transitions. The whole excitation spectra follow a common trend, i.e., K+ codoping facilitates the increase in the intensity, which in turn amplifies the PL emission intensity also. The presence of strong peaks at 394 and 464 nm evidently validates the potential of the nanophosphor, which can be excited by both near-UV and blue LED chips. The excitation spectra are also found to be well corroborated with the diffuse reflectance spectra shown in Figure 3c. The signature of the f−f transition and CTB is again well identified in the reflectance spectra. The high reflectance value (∼95%) at higher wavelengths for both Eu3+-doped and Eu3+−K+-codoped nanophosphors strongly validates the brilliance of the synthesized samples.40 The optical band gaps of the produced nanophosphors were estimated from the absorption spectra by employing the Kubelka−Munk function, as mentioned in earlier reports,41 and the plots are shown in Figure S5. The band gaps of all of the samples were found to be greater than 3.2 eV, which is one of the most important criteria for a good phosphor. The observed decrease in band gap for the codoped nanophosphor is a consequence of the minor increase in crystallite size, as evidenced from Table 1.42 The room temperature PL emission spectra of BaAl2O4:Eu3+ and K+-codoped BaAl2O4:Eu3+ nanophosphors, excited at 394 nm, are presented in Figure 5a,b, respectively. Usually, it is seen that Eu3+ replaces nine coordinated Ba2+ in the BaAl2O4 lattice (also clear from Figure 1d). Such a coordination environment generates a strong crystal field over Eu3+, and the energy levels of Eu3+ get split consequently. Therefore, the PL emission spectra of both Eu3+ and Eu3+−K+-codoped nanophosphors comprise sharp lines within the 580−720 nm wavelength region. Mainly, five peaks are observed at 577, 590, 614, 654, and 702 nm, which can be attributed to the 5D0 → 7Fj (0−4) transitions of Eu3+ ions, respectively. Among these transitions, 5D0 → 7F1 lines arise due to the magnetic dipole (MD) transition, whereas 5D0 → 7F2 lines originate from the electric dipole (ED) transition. In general, the MD transitions do not depend on the site symmetry of the Eu3+ ions in the BaAl2O4 lattice. However, the ED transitions are hypersensitive and the PL emission intensity is hugely influenced by the site symmetry of Eu3+ ions in the host lattice. Also, it is well known that the ED transition dominates the PL spectra if Eu3+ occupies noninversion symmetric sites in the host lattice. Actually, the distorted Al−O4 tetrahedral network around Ba sites in BaAl2O4 creates large structural asymmetry.43 As Eu3+ occupies these noninversion symmetric sites, the ED transition dominates and, as a consequence of this, the 614 nm peak turns out to be the most pronounced in all doped and codoped samples. The possible luminescence mechanism in doped and codoped BaAl2O4 nanophosphors is schematically demonstrated in Figure 3b. In BaAl2O4, mainly the valence band is formed by oxygen 2p orbitals, whereas the conduction band is composed of the Ba 4d orbital. Transfer of electronic charge from the O 2p to the Eu 4f orbital creates strong CTB within the band gap. In this configuration, the electronic excitation can take place by two
alternate mechanisms: in one process, electrons from the valence band can jump directly to CTB (256 nm excitation), whereas in another process, they can be excited to even higher levels of Eu3+ (394 nm excitation). In both processes, the excited electrons quickly relax to the lowest excited state of Eu3+ (5D0) via nonradiative transitions and from there they can make five transitions to the ground state, giving rise to five lines in the emission spectra at 577, 590, 614, 654, and 702 nm. From the steady-state PL spectra of the BaAl2O4:Eu3+ nanophosphor, it is prominent that the PL intensity of ED transition increases gradually with the increasing Eu3+ concentration up to 5% and then it decreases for 7%. The nonradiative energy transfer between same types of RE ions is responsible for this type of concentration quenching phenomenon. It is already well known that nonradiative energy transfer can occur due to either exchange interaction or multipole−multipole interaction.44 For exchange interaction, if the Eu3+ concentration is such that the distance between two neighboring Eu3+ ions is adequately low, the nonradiative relaxation increases due to increased interaction between two identical Eu3+ ions and the PL intensity decreases consequently. The critical distance (RC) between the RE activator ions can be directly calculated by employing the Blasse equation, which can be expressed as45 ⎡ 3V ⎤1/3 R C = 2⎢ ⎣ 4πcN ⎥⎦
where V is the unit cell volume, c is the optimum concentration of the activator ion, and N is the number of ions present in the unit cell. In the current investigation, for the BaAl2O4 host, the value of critical distance, RC, was found to be 10.71 Å (by taking N = 6, c = 0.05, and V = 830.75). As this RC value is greater than 5 Å, it is confirmed that the exchange interaction between Eu3+ ions is not responsible for the phenomenon of concentration quenching, rather multipolar interaction is responsible for it.46 3.2.1. Effects of K+ Doping. The successive change in the emission spectra as a consequence of K+ codoping is depicted in Figure 5b. For all of the codoped samples, the Eu3+ concentration was kept fixed at 5%. Emission spectra of codoped nanophosphors reveal no changes in the peak position when the concentration of K+ was varied from 1 to 5%; however, as compared to that of the Eu3+-doped samples, a significant hike in the emission intensity is observed in the codoped nanophosphors. A careful observation of the emission spectra also discloses that as the concentration of K+ is increased from 1 to 4% the PL emission intensity increases gradually and beyond this concentration the emission intensity decreases. In fact, the 4% K+-codoped nanophosphor exhibits almost 2-fold enhancement in the emission intensity as compared to that of the most intense Eu3+-doped nanophosphor. For a lucid demonstration of this PL enhancement, the optical images of the nanophosphors illuminated under a UV lamp are presented in the left bottom insets of Figure 5a,b. A strong emission over a large area can be easily identified in the case of 4% K+-codoped BaAl2O4:Eu3+ nanophosphor. Usually, enhanced surface states of the nanocrystals often reduce the PL intensity in solid condition due to partial aggregation and energy transfer-mediated nonradiative recombination.47−49 Therefore, achieving high brightness in the solid condition defines a crucial benchmark for a good nanophosphor. From the optical images, it becomes clear that K+ codoping enhances the PL brightness significantly by overcoming the problems related to the enhanced surface states. 794
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ACS Omega There are several factors that are responsible for the enrichment of PL brightness with the incorporation of K+ in the BaAl2O4:Eu3+ nanophosphor. However, charge compensation plays the dominant role among them. In the BaAl2O4:Eu3+ nanophosphor, Eu3+ occupies the Ba2+ site. As a result of this kind of substitution, which involves two ions with different valence states, a net positive charge is developed in the system. Hence, if the concentration of Eu3+ increases further beyond the concentration of quenching limit, the repulsive force among the net positive charges increases so high that further substitution of Eu3+ ions in the lattice gets strongly obstructed. As a consequence, the Eu3+ ions start to accumulate instead of occupying a proper lattice site. These localized accumulation centers of Eu3+ ions serve as quenching sites, and they have enough capability to reducing the PL brightness via multipolarinteraction-coupled energy migration among them. Consequently, to design an electrically neutral solution, two Eu3+ ions must be incorporated into the lattice of three Ba2+ ions and thus a Ba2+ vacancy (VBa″) would be generated50−53 by the following reaction: 3BaBa → 2EuBa + VBa″. Ba2+ vacancies generated in the above process act like defect centers, which diminish the overall PL intensity due to efficient energy transfer to them from Eu3+.54 An effective countermeasure for this type of charge imbalance and vacancy generation problem lies in introducing alkali metal ions together with the rare-earth activators. For example, by incorporating K+ ions in the BaAl2O4:Eu3+ lattice, charge balance can be readily achieved by the following equation: 2VBa″ + Eu3+ + K+ → EuBa + KBa′. Thus, as demonstrated in Figure 5b, introducing K+ into the lattice significantly lowers the probability of nonradiative transition and dramatically boosts radiative transition. However, it must be noted that even though alkali metal ions provide charge compensation they cannot completely counter the volume imbalance that arises from the different ionic radii of Ba2+ and K+ ions. In the current case, the threshold up to which the volume imbalance problem can typically be ignored is ∼4% K+ concentration. Addition of K+ beyond this limitation however leads to severe deformation of the lattice and reduction of crystallinity, the net result of which will be manifested as subsequent quenching of the PL intensity. Another significant parameter that influences the PL brightness of the K+-codoped BaAl2O4:Eu3+ nanophosphors is the generation of oxygen vacancies developed by the substitution of Ba2+ by K+. Usually, these types of oxygen vacancies are formed on the nanophosphor surface, and they behave like sensitizers.55 These sensitizers assist efficient transfer of energy from the host material to the Eu3+ ions by creating a strong overlap with the charge transfer states. As a consequence of this, the oscillator strength of the radiative transition increases, leading to the subsequent enhancement in the PL intensity.56,57 However, if the concentration of K+ goes above a certain limit (4%), an enormous amount of oxygen vacancy is created in the host lattice. Because of this problem, the crystal lattice starts to collapse, leading to the subsequent degradation of PL intensity. The intensity of the 614 nm emission depends upon the site of the Eu3+ ions in Eu3+-doped as well as Eu3+−K+-codoped BaAl2O4 nanophosphors. If the Eu3+ ion is placed at a low symmetry site, then the ED transition dominates in the emission spectra, as represented in Figure 5a. Eventually, in the current study, it was noted that the emission intensity of the 614 nm peak was much higher than that of the emission peak at 590 nm of all of the doped and codoped nanophosphors. It is well known that the ratio of the intensities of the 5D0 → 7F2 and 5D0 → 7F1
transitions, commonly designated as the asymmetric ratio, essentially depicts the degree of deviation from the inversion symmetry in the vicinity of Eu3+ in the host lattice.58 For different concentrations of K+-codoped samples, the asymmetric ratios were estimated by respectively calculating the area under the emission peaks of 614 and 590 nm and taking their ratio. The graph depicting the asymmetric ratio versus the concentration of K+ is shown in Figure S6 of the SI. From the figure, it can be readily inferred that K+ codoping in the BaAl2O4:Eu3+ nanophosphor substantially increases the asymmetric ratio. Signature of this gradual increase in ED transition is also reflected from the Commission International del’Eclairage (CIE) chromaticity coordinates of the BaAl2O4:Eu3+ and K+-codoped BaAl2O4:Eu3+ nanophosphors shown in the bottom right insets of Figure 5a,b. An increase in the x coordinate along with subsequent decrease in the y coordinate becomes distinctly evident from the chromaticity diagram of the K+-codoped nanophosphor. This phenomenon also indicates an improvement of the color purity of the codoped nanophosphor. The actual estimation of this enhancement of color purity has been conducted by employing formula59 color purity =
(xs − x i)2 − (ys − yi )2 (xd − x i)2 − (yd − yi )2
× 100
where (xs,ys) are the coordinates of a sample point, (xi,yi) are the coordinates of the illuminant point, and (xd,yd) are the coordinates of the dominant wavelength. In this work, taking (xi,yi) = (0.3101, 0.3162) for the illuminant point and (xd,yd) = (0.67, 0.32) for the dominant wavelength at 614 nm, the color purities of the 5% Eu3+ and 5% Eu3+-, 4% K+-codoped BaAl2O4 nanophosphors were estimated to be 90.4 and 93%, respectively. As a whole, after K+ codoping, the color coordinate reaches really near the ultimate red chromaticity for the National Television Standard Committee (NTSC) system and the characteristic index proves that these codoped phosphors have greater color saturation. Cathodoluminescence (CL) characteristics of the synthesized nanophosphors have also been examined to justify the suitability of the present samples for the application in field emission display. The CL spectra of 5% Eu3+ and 5% Eu3+-, 4% K+-doped nanophosphors are presented in Figure 6. Under the low-voltage electron beam excitation (5 kV), the CL spectra evidently follow
Figure 6. Cathodoluminescence spectra of 5% Eu3+-doped BaAl2O4 and 5% Eu3+-, 4% K+-codoped BaAl2O4 nanophosphors. 795
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multipolar interaction among Eu3+ ions turns out to be a significant cause of nonradiative transitions. The distance between two Eu3+ ions becomes a crucial parameter in this case in controlling the energy migration among the same type of activator ions. This phenomenon is theoretically studied by measuring the Eu−Eu distances for the Eu-doped and Eu−Kdoped systems, using larger, double-sized supercells of the relaxed configurations described earlier. For the Eu-doped system, the Eu−Eu distance was observed to be 10.257 Å, whereas the same increased to 10.311 Å in the Eu−K-codoped system (shown in Figure 7b,c) as a direct consequence of codoping. This enhanced distance between Eu atoms is favorable for destroying the energy transfer among same activators. As a result, with the incorporation of a K+ coactivator in the BaAl2O4:Eu3+ lattice, the rate of nonradiative transition decreases, which increases the radiative lifetime consequently. Also, it is well known that the efficiency of any radiative transition is always directly proportional to the decay time of that particular transition; hence, high lifetimes are always recommended for solid-state lighting applications. The millisecondorder lifetime of the codoped nanophosphors strongly indicates that the samples carry great promise for use in the field of display and lighting applications. 3.2.2. Thermal Stability and Electron−Phonon Coupling. It is often seen that the thermal stability of nanophosphors plays a vital role in uninterrupted operation of LEDs because the enhanced temperature at the phosphor−UV LED chip interface strongly affects the light output and the lifetime of the LED device.61 Accordingly, the high temperature stability of the doped and codoped phosphors has been investigated for better exploration of Eu3+ behavior in the corresponding hosts at high temperature. The temperature-dependent PL spectra of BaAl2O4:5% Eu3+ and BaAl2O4:5% Eu3+, 4% K+ nanophosphors are shown in Figure 8a. In both cases, the PL emission intensity decreases as the temperature elevates to 210 °C from room temperature, demonstrating a temperature quenching phenomenon. A careful inspection also clarifies that the PL emission intensity decays to 19 and 23% of the initial intensity at 210 °C for Eu3+doped and Eu3+−K+-codoped nanophosphors, respectively. Because during operation the temperature at the phosphor− chip interface increases to ∼150 °C,62 it is very important to investigate the performance of the nanophosphor at this temperature. At 150 °C, the emission intensity drops to 17 and 20% of the initial intensity for BaAl2O4:Eu3+ and BaAl2O4:Eu3+, K+ nanophosphors, respectively. The comparison of temperature quenching behaviors of the phosphors strongly reveals that incorporation of K+ enhances the thermal stability of the BaAl2O4:Eu3+ nanophosphor. The observed thermal quenching behavior of the nanophosphors can be explained with the help of a typical configuration coordinate model, which is represented in Figure 8b. At lower temperatures, the transition from the bottom of the 5 D excited state toward the 7F ground state is not dependent on temperature and gives radiative emission. However, at higher temperatures, the excited electrons at the 5D level become thermally promoted to the CTB via the 5D0-CTB crossover point and then relax nonradiatively to the 7F0 ground state of Eu3+ through the 7F0-CTB crossover point. The nonradiative transition becomes more prevalent with the increasing temperature and therefore diminishes the luminescence intensity. In this thermal quenching process, the excited electrons need to be activated by overcoming the energy barrier between the 5D
the same spectral properties that were achieved from the PL measurement. Furthermore, the CL intensity is also found to be enhanced because of the addition of K+ in the nanocrystal. The obtained strong red emission along with excellent color purity designates these K+-codoped nanophosphors to be promising candidates for solid-state lighting and field emission display applications. To confirm the decrease in nonradiative transition with the incorporation of a K+ coactivator, the PL decay profiles of 614 nm emission for both 5% Eu3+- and 5% Eu3+-, 4% K+-codoped BaAl2O4 nanophosphors have been examined under 394 nm UV excitation, as shown in Figure 7a. The luminescence decay curves
Figure 7. (a) PL decay curves of the 5D0 → 7F2 transition for BaAl2O4:Eu3+ and BaAl2O4:Eu3+, K+ nanophosphors excited via 394 nm UV light. (b, c) Representation of the Eu−Eu distance in a 2 × 2 × 2 supercell of Eu3+-doped and Eu3+−K+-codoped BaAl2O4 nanophosphors, respectively.
of the 5D0 → 7F2 transition for both the nanophosphors were well fitted by a biexponential function, which may be represented as follows ⎛ t ⎞ ⎛ t⎞ I(t ) = A1 exp⎜ − ⎟ + A 2 exp⎜ − ⎟ ⎝ τ2 ⎠ ⎝ τ1 ⎠
where A1 and A2 are the weighting parameters and τ1 and τ2 are the decay components of luminescence lifetimes. The average lifetime for the biexponential decay can be expressed as60 τav =
A1τ12 + A 2 τ22 A1τ1 + A 2 τ2
The calculated average lifetimes of 5% Eu3+-doped BaAl2O4 and 4% K+-codoped BaAl2O4:5% Eu3+ nanophosphor are 0.69 and 0.81 ms, respectively. Generally, at a higher concentration of the activator ion, energy transfer to the Ba2+ vacancies together with 796
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Figure 8. (a and b) Temperature-dependent PL spectra of BaAl2O4:5% Eu3+ and 4% K+-codoped BaAl2O4:5% Eu3+ nanophosphors. (c) Configuration coordinate diagram for the illustration of thermal quenching phenomena of the nanophosphors.
ground state and 5D-CTB crossover point. Hence, this energy barrier is called activation energy, which can be directly estimated from the modified Arrhenius equation63 IT =
Because the energy gap between the 5D2 and 5D3 transitions is larger than vibrational energies of most of the chemical bonds in inorganic compounds, any weak band situated between two consecutive transitions (7F0 → 5D2 and 7F0 → 5D3 in case of Eu3+) denotes phonon-assisted transition. The phonon-assisted transition was found to be situated at 1920 cm−1 for both the doped and codoped nanophosphors. The level up to which any electronic transition is coupled with such a phonon mode can be calculated from the electron− phonon coupling strength, which is expressed by the Huang− Rhys factor (S) as65
I0 1 + C e−Ea / KT
where I0denotes the initial PL emission intensity, T is the temperature in kelvin, IT represents the PL intensity at temperature T, C is a constant parameter, and k is the Boltzmann constant (8.629 × 10−5 eV K−1). The activation energy, Ea, can be obtained from the slope of the straight line generated by plotting the graph ln[(I0/IT) − 1] versus 1/kT (shown in S7 of SI). The activation energies were found to be 0.034 and 0.038 eV for the Eu3+-doped and Eu3+−K+-codoped BaAl2O4 nanophosphors. It is well known that the nonradiative decay and multiphonon relaxations of Eu3+ in the host matrix are highly dependent on the electron−phonon coupling strength. Phonon sideband (PSB) spectroscopy is a popular technique to determine the electron− phonon coupling strength of rare-earth ions in the host matrix.64 PSB spectra can be obtained by taking the barycenter wavenumber of the 7F0 → 5D2 ED transition of Eu3+ as a reference for the zero-phonon line. Figure 9 presents the PSB spectra of Eu3+-doped BaAl2O4 and K+-codoped BaAl2O4:Eu3+ nanophosphors under an emission wavelength of 614 nm.
S=
IPSB IZP
where IPSB and IZP are the integral intensity of the PSB and the zero-phonon line, respectively. The Huang−Rhys factors for the 5% Eu 3+-doped and 5% Eu 3+-, 4% K+ -codoped BaAl 2O 4 nanophosphors were estimated to be 0.106 and 0.082, respectively. This parameter further ascertains that addition of K+ leads to lower electron−phonon coupling and subsequent reduction in nonradiative transition, giving rise to the enhancement of PL intensity. 3.3. Comparison with the Commercial Phosphor. Furthermore, to check the performance of the nanophosphors with respect to commercial phosphors, PL characteristics of the synthesized nanophosphors are compared with those of commercial red-emitting Y2O3:0.08Eu3+ phosphors under 394 nm excitation (shown in Figure 10a). Steady-state PL spectra reveal that PL intensity of 5% Eu3+, 4% K+:BaAl2O4 nanophosphors is slightly less than that of the commercial phosphor. However, the color coordinate of the commercial phosphor is found to be (0.641, 0.358), which is even worse than that of the synthesized 5% Eu3+, 4% K+:BaAl2O4 nanophosphors. The calculated color purity of the commercial phosphor is 91.4%, which is less than that of the phosphors synthesized in the current work (93%). Moreover, the thermal stability of the commercial phosphor is depicted in Figure 10d, which shows that at 210 °C the phosphor undergoes significant quenching of PL (35% of initial PL intensity), whereas 5% Eu3+, 4% K+:BaAl2O4 nanophosphors exhibit only 19% decay of the initial PL intensity. Absolute quantum yield (QY) of the phosphors is also calculated using an integrating sphere, and the corresponding PL
Figure 9. Phonon sideband spectra of BaAl2O4:5% Eu3+ (green line) and BaAl2O4:5% Eu3+, 4% K+ (red line) nanophosphors. The magnified view of the phonon-assisted transition is shown in the inset. 797
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Figure 10. Comparison of (a) PL emission spectra, (b) CIE chromaticity diagrams, and (c) quantum yields of the synthesized BaAl2O4:5% Eu3+, 4% K+ nanophosphor with the commercial phosphor Y2O3:8% Eu3+. (d) Temperature-dependent PL spectra of the commercial phosphor Y2O3:8% Eu3+.
spectra of the commercial phosphor and 5% Eu3+, 4% K+:BaAl2O4 nanophosphors in the integrating sphere are depicted in Figure 10c. The QY value of the 5% Eu3+, 4% K+:BaAl2O4 nanophosphor is calculated to be 77.4%, which is closely comparable to that of the QY of commercial phosphor (84.2%). The obtained results strongly suggest that Eu3+−K+codoped BaAl2O4 nanophosphors hold great promise for competing commercial phosphors in lighting and display applications.
strongly demonstrate that K+-codoped BaAl2O4:Eu3+ nanophosphors could be promising candidates for use in solid-state lighting as well as field emission display devices.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.7b01525. Shift of the (202) XRD peak to the lower 2θ value with the increasing concentration of K+; FTIR spectra of K+codoped BaAl2O4:5% Eu3+ nanophosphors; low-expansion FESEM image of the 4% K+-codoped BaAl2O4:5% Eu3+ nanophosphor; high-resolution XPS spectra of Ba 3d, Al 2p, and O 1s for the 4% K+-codoped BaAl2O4:5% Eu3+ nanophosphor; band gap plot of the nanophosphors; variation of the asymmetric ratio with the K+ concentration in coactivated nanophosphors; and the activation energy plot of the doped and codoped nanophosphors (PDF)
4. CONCLUSIONS In summary, we have successfully presented a cost-effective citrate nitrate technique to synthesize Eu3+-doped and Eu3+−K+codoped BaAl2O4 nanophosphors for a wide range of Eu3+ and K+ concentrations. The derived nanophosphors entail bright red emission because of the 5D0 → 7FJ (J = 0−4) transitions of Eu3+ when excited under UV and low-voltage electron beam (5 kV) irradiation. PL and CL investigations of the nanophosphors reveal that the incorporation of a K+ coactivator remarkably enhances the luminescence intensity and subsequent color purity. The charge compensation phenomenon, which is observed with the addition of the K+ coactivator is responsible for the enhancement of crystallinity and the consequent increase in luminescence intensity. Incorporation of the K+ coactivator also results in a stronger charge sharing between Eu and O, which further intensifies the CTB in codoped nanophosphors. It is also established that K+ codoping increases the inter-Eu distance, which in turn increases the radiative lifetime of Eu3+ ions by reducing various nonradiative transitions and electron−phonon coupling strength. Enhancement of thermal stability is also achieved via K+ codoping. The strategy of K+ codoping successfully stands out as an effective route to enhancing the efficiency of BaAl2O4:Eu3+ nanophosphor by reducing the nonradiative transition centers in it. Finally, the obtained results
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Kalyan Kumar Chattopadhyay: 0000-0002-4576-2434 Author Contributions ∥
R.C. and S.S. contributed equally to this work.
Notes
The authors declare no competing financial interest. 798
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ACKNOWLEDGMENTS The authors sincerely acknowledge the financial help from the Department of Science & Technology (DST), the Government of India, for awarding an INSPIRE Fellowship during the execution of the work. They also wish to thank the Council of Scientific and Industrial Research (CSIR), the Government of India, and the University Grants Commission, for the University with Potential for Excellence (UPE-II) scheme.
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