Exploring Defect-Induced Emission in ZnAl2O4: An Exceptional Color

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

pubs.acs.org/IC

Exploring Defect-Induced Emission in ZnAl2O4: An Exceptional ColorTunable Phosphor Material with Diverse Lifetimes Nimai Pathak,*,†,⊥ Partha Sarathi Ghosh,‡ Suryansh Saxena,§ Dhanadeep Dutta,† Ashok Kumar Yadav,∥ Dibyendu Bhattacharyya,∥ Shambhu Nath Jha,∥ and Ramakant Mahadeo Kadam†,⊥ †

Radiochemistry Division, Bhabha Atomic Research Centre, Mumbai 400085,India Materials Science Division, Bhabha Atomic Research Centre, Mumbai 400085, India § Integrated Science Education and Research Centre, Visva Bharati, Santiniketan, West Bengal, India ∥ Atomic & Molecular Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India ⊥ Homi Bhabha National Institute, Mumbai 400094, India ‡

S Supporting Information *

ABSTRACT: Activator-free zinc aluminate (ZA) nanophosphor was synthesized through a sol−gel combustion route, which can be used both as a blue-emitting phosphor material and a white-emitting phosphor material, depending on the annealing temperature during synthesis. The material also has the potential to be used in optical thermometry. These fascinating color-tunable emission characteristics can be linked with the various defect centers present inside the matrix and their changes upon thermal annealing. Various defect centers, such as anionic vacancy, cationic vacancy, antisite defect, etc., create different electronic states inside the band gap, which are responsible for the multicolor emission. The color components are isolated from the complex emission spectra using timeresolved emission spectroscopy (TRES) study. Interestingly, the lifetime values of the various defect centers were found to change significantly from milliseconds to microseconds upon thermal annealing, which makes the phosphors more diverse (i.e., either long-persistent blue-emitting phosphors or short-persistent white-emitting phosphors). Fourier transform infrared (FTIR) and diffuse reflectance spectroscopy (DRS) confirmed the presence of antisite defect centers such as AlZn+ or ZnAl− in the matrix. X-ray absorption fine structure (EXAFS) study showed that the spinel structure was more disordered in nature for low-temperature-annealed compounds. Electron paramagnetic resonance (EPR) and positron annihilation lifetime spectroscopy (PALS) studies were also carried out in order to characterize various anionic and cationic vacancies and their clusters present in the compounds. Antisite defect centers such as AlZn+ or ZnAl−, which act as an electron or hole trap, were found to be responsible for the diverse lifetime behavior. To gain insight about the electronic states inside the band gap, density functional theory (DFT)-based calculations were performed for both pure and various vacancy-introduced spinel structures. Finally, based on the theoretical and experimental results, for the first time, a detailed investigation of various defect-induced emission behavior in ZA is presented, which also explains the mechanism of color tunability and dynamic lifetimes.

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phosphor hosts,4 UV-optoelectronic devices,5 electroluminescent displays, or optomechanical stress sensors and stress imaging devices.6,7 ZA is an appropriate host matrix in the field of photoemission application and recent literature survey have shown that both the undoped and doped ZA with different activator ions such as Eu3+, Dy3+, Tb3+, and Mn2+ exhibits excellent phosphor characteristics with various optical and medical applications.1,8−13 In the past few decades, activator-free (e.g., rare-earth or transition-metal) defect-related luminescent materials are

INTRODUCTION ZnAl2O4 (ZA), which is a naturally available mineral known as gahnite, belongs to the spinel-aluminate family. ZA is a direct wide-band-gap semiconductor material with an optical bandgap value of ∼3.8 eV.1 Properties such as wide energy band gap (Eg), high mechanical resistance, high chemical and thermal stability, high fluorescence efficiency, high photocatalytic activity, and low surface acidity make this compound a suitable material for a wide range of applications, e.g., photoelectronic devices, catalysts, electroluminescence displays, stress imaging devices, optical coatings, and highly efficient phosphors.2,3 The material is transparent to wavelengths >320 nm, which enables it to be used in several lighting and sensing applications, such as © XXXX American Chemical Society

Received: January 19, 2018

A

DOI: 10.1021/acs.inorgchem.8b00172 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

include Zn2SiO4:Mn2+,As (green) and Y2O2S:Eu3+ (red phosphors).27 In televisions, short-persistent phosphors are used to avoid smearing on moving images. For cases of longpersistent phosphors, the slow decay phenomena can be valuable for the examination of low-frequency and transient phenomena. In earlier days, radar screens used long-persistent phosphor tubes. An intermediate characteristic of these two types is also possible by mixing the two types of phosphor materials; these are known as intermediate persistent phosphors. The mechanism of long-persistent phosphors is slightly different from the phosphorescence phenomena of luminescence centers, where the slow lifetime is due to the forbidden triplet-to-singlet transition. Here, the delocalized charge carriers (electrons or holes), which are generated upon excitation, are captured on the trapping states, thereby delaying the electron−hole recombination process.28 Reports in this context are still missing in the literature, and such study may open a new avenue for this material for phosphor application. In the present report, we have synthesized ZA nanoparticles via sol−gel combustion routes at 750 °C, which were further annealed at higher temperature (such as 1000, 1200, and 1300 °C). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies were performed for particle size determination and morphological investigation. Various spectroscopic studies, such as Fourier transform infrared spectroscopy (FTIR), diffuse reflectance spectroscopy (DRS), X-ray absorption spectroscopy (EXAFS and XANES), electron paramagnetic resonance (EPR), and positron annihilation lifetime spectroscopy (PALS) study, have helped a great deal to characterize various defects centers, such as antisite defects, anionic and cationic vacancies, and their cluster present in the matrix. Photoluminescence (PL) study has shown that ZA-750 is a blue-emitting phosphor material while the ZA-1300 is a white-emitting phosphor. Lifetime study of various defect centers showed a drastic decrease, from the order of several milliseconds to microseconds, when annealed at higher temperature, making ZA compounds more diverse for phosphor applications. This change in lifetime was correlated to the positively charged antisite defect centers AlZn+, which act as electron traps. The existence of AlZn+ is directly related to the indirect phase of the spinel. Furthermore, by performing the time-resolved emission spectroscopy (TRES) study at different annealed temperatures, it is possible to isolate the different defect-related emission components from complex emission spectra. Finally, with the help of DFT calculation, an understanding about the electronic structure inside the band gap, because of various defect centers, has been achieved that helps to characterize the different emission components in the complex emission spectra.

steadily emerging as promising alternatives to traditional phosphors in many applications, because of various advantages, such as low toxicity, stability, tunable emission color, and low cost.14−17 Although the origins of emission behaviors in these activator-free luminescent materials are very complex in nature and not yet entirely understood, mounting evidence suggests that the presence of special defects such as vacancies, impurities, radical impurities, donor−acceptor pairs, etc. impart these materials with excellent photoluminescent behaviors.18−20 These defect centers give rise to different electronic states inside the band gap of the material, which are responsible for the fascinating emission characteristics. Thus, defect centers are the precursors for various luminescence centers in activator-free luminescent materials. Very recent reports on various oxidebased semiconductors, such as ZnO, MgO, TiO2, SnO2, and Al2O3, showed that defects such as different type of F-centers or oxygen vacancies, as well as interstitial and cationic vacancies, are responsible for the observed defect-induced photoluminescence (PL).21−25 As far as rare-earth-free ZA phosphors are concerned, very few reports exist in the literature. In this context, Wang et al. have reported ZA phosphors to be blue-emitting phosphors prepared from different aluminum salts.2 Cornu et al. have observed a change in the emission profile of ZA compound when it was annealed at higher temperature, preparing through the coprecipitation synthetic route, which makes ZA a potential sensor material.8 Although these emission profiles are attributed to different defect centers (e.g., oxygen vacancies, antisite defects, and interstitial Zn), concrete evidence is still lack in the literature. As stated earlier in our literature,13 the ZA spinel has two different types of structures: “normal” and “inverse”. In the normal structure, the Zn2+ ions occupy the tetrahedral sites while the Al3+ ions occupy the octahedral sites. In the inverse spinel structure, all of the Zn2+ ions and half of the Al3+ ions occupy the octahedral sites, while the other half of the Al3+ ions occupy tetrahedral sites. Normally, in most of the cases, the spinel compounds have both the structures present with their respective percentages and they are called mixed spinel. Thus, the defect structures in ZA is directly linked with the phase or structure of the spinel and any inversion will lead to the presence of antisite defect centers such as AlZn+ or ZnAl−. These antisite defects may also lead to the formation of various cationic and anionic vacancies for charge neutrality. Thus, compared to other defect-related oxide materials, in spinelbased material, it is very easy to control the defect structures by controlling the phase of the spinel by means of thermal annealing.13 This, in turn, will help a great deal in controlling various defect-related physical properties including the optical property. Reports in this context are lacking in the literature. Furthermore, the most important criteria while choosing a phosphor for practical applications is the lifetime of the excited states. Some phosphors exhibit a very rapid decay of light output when the excitation source is switched off; these are termed “short-persistent phosphors”. In contrast, some phosphors glow for seconds to minutes to hours; these are known as “long-persistent phosphors”.26 The short-persistent and long-persistent phosphors are selected by considering their luminescence, brightness, and flicker. Generally, short-persistent phosphors are brighter than the long-persistent phosphors. Examples of short-persistent phosphors include ZnS:Cu,Al or ZnS:Au,Cu,Al, which are widely used in color televisions as green phosphors; examples of long-persistent phosphors

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EXPERIMENTAL SECTION

Synthesis. Zinc nitrate [Zn (NO3)2·6H2O] (99.0%, analytical reagent (AR) grade, s.d. Fine-Chemicals, Ltd.), aluminum nitrate [Al(NO3)3·9H2O] (98%, AR grade, s.d. Fine-Chemicals, Ltd.), citric acid (C6H8O7·H2O) (99.7%, AR grade, Chemico Fine Chemicals) (used as a fuel here) are the precursor materials and taken in the molar ratio 1:2:10 for the synthesis of the ZnAl2O4 compound. The procedure is similar to that of Fe3+ doped ZnAl2O4.13 Initially, separate solutions of the metal nitrates and citric acid were prepared by dissolving stoichiometric quantities in quartz double-distilled (QDD) water. These solutions then were mixed in a beaker with constant stirring with a magnetic stirrer for ∼1 h, after which a clear solution was observed. The citric acid solution was then added to this mixed solution under vigorous stirring and subjected to heating at 80 °C for 5 B


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Inorganic Chemistry h, after which a highly viscous gel formation was observed. The gel was then heated on a heating mantle at a temperature of 150 °C, where the combustion occurred, after which an ashlike product was obtained. The ashlike product was then ground in an agate mortar and subjected to calcining at 750 °C for 8 h; afterward, a fine white powder (ZA750) was obtained. The phase purity of the compound obtained at 600 °C was confirmed from X-ray diffraction (XRD) study. After confirmation of the pure product formation, the compound was further annealed at higher temperatures, viz., 1000 °C (ZA-1000), 1200 °C (ZA-1200), and 1300 °C (ZA-1300), which were subjected to characterization via different physical methods, such as FTIR, DRS, PL, PALS, EXAFS and EPR after each heating treatment. Since we have observed similar types of PL results for ZA-1200 and ZA-1300 compounds, we have concentrated our discussion in the present manuscript mainly on ZA-750, ZA-1000, and ZA-1300. Details regarding the instrumental techniques have been given in the Supporting Information. Density Functional Theory (DFT) Calculation Methodology. The normal phases of ZnAl2O4 (ZA) face-centered cubic spinel are studied using the spin-polarized plane wave-based density functional theory (DFT), as implemented in the Vienna Ab-intio Simulation package (VASP).29,30 The electron−ion interactions are described using the projector augmented wave (PAW) method,31 which includes the valence states of Zn (3d, 3p (12 valence electrons)), Al (3s, 3p (3 valence electrons)), and O (2s, 2p (6 valence electrons)), as implemented in the VASP package. The exchange-correlation potential is parametrized using the Perdew−Burke−Ernzerhof (PBE)32 form within the generalized gradient approximation (GGA). The unit cell of the ZA structure contains 56 atoms, and the integration over the Brillouin zone is carried out on 13 × 13 × 13 and 7 × 7 × 7 k-point meshes generated using the Monkhorst−Pack33 method for normal and inverse spinel structures, respectively. A cutoff energy (Ecut) of 500 eV for the plane wave basis set is used and the choice of Ecut and kpoint mesh are sufficient for energy convergence of 10 μs, it can be assumed that all the emitting colors due to different defect states will be present in the spectra at the very beginning of the decay time, such as at 2 or 5 μs. For the lowtemperature-annealed ZA-750 compound in Figure 8a, it can be observed that very little change is observed when the delay time is increased from microseconds to milliseconds. This is because, in ZA-750, most of the emitting components have lifetimes in the milliseconds region. The short-lived components with lifetimes in the microseconds range will decay rapidly, while the long-lived components with higher lifetime values (in the milliseconds range) will decay slowly. Thus, in the 5−500 μs range, if we subtract the TRES spectra at a 5 μs delay time from that at a 500 μs delay, the subtracted spectra will be composed of the decayed components, as shown in Figure 9a. On the other hand, in the range from 500 μs to 5 ms, the components exhibit signals at 420 and 850 nm, as shown in Figure 9b. At higher delay times (such as 60 ms), two components with higher lifetimes (with signals at 450 and 490 nm) still exist, as shown in Figure 10a. Now, in the time interval of 5−60 ms, the P3component (i.e., at 450 nm) will decay faster than the P4 component (490 nm), as shown by the TRES spectra in Figure 10b. The other two components that also decayed in this time interval are P7 (690 nm) and P8 (780 nm), as predicted by their lifetime values given in Table 1. Now, when the compound was annealed at higher temperature (such as 1300 °C), the lifetimes of most of the components decrease drastically from the order of milliseconds to microseconds for ZA-1300, as given in Table 1. From Figure 8b, it is clearly visible that, even at microsecond-scale delay times (such as 500 μs), most of the components vanished. Another interesting observation for this ZA-1300 compound is

Figure 11. Time-resolved emission spectra (TRES) of ZA-1300 in the time interval from 2 μs to 75 μs. Gaussian fitting in the wavelength scale has been carried out to show only the peak position of the emission components, wherein areas under the curves have not been considered for any discussion.

nm (P5), 720 nm (P7), and 780 nm (P8), will be decayed at higher delay times. In Figure 12, two TRES at two different delay times (viz. 400 μs and 1.7 ms) are presented, which are found to be composed of three components, viz, 530 nm (P5), 720 nm (P7), and 780 nm (P8). Among them, the 530 nm (P5) component is short-lived, which is why, at a decay time of 1.7 ms, it was almost decayed (Figure 8b). X-ray Absorption Spectroscopy. In the present study, we have performed temperature-dependent XAS (consisting of both XANES and EXAFS) measurements on ZA powder samples annealed at different temperatures at the Zn K-edge to probe the changes in the local structure surrounding Zn sites. The instrumental details are given in the Supporting Information.The normalized XANES spectra at the Zn Kedge is shown in Figure 13, along with Zn metal (0 oxidation state) foil and ZnO (+2 oxidation state) standards. The edge G


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Inorganic Chemistry

Figure 12. Time-resolved emission spectra (TRES) of ZA-1300 (a) at 400 μs delay and (b) at 1.7 ms delay. Gaussian fitting in the wavelength scale has been carried out to show only the peak position of the emission components, wherein areas under the curves have not been considered for any discussion.

Figure 14. Normalized EXAFS spectra of different ZA (ZnAl2O4) compounds at the Zn K-edge at different annealing temperatures. Figure 13. Normalized XANES spectra of different ZA (ZnAl2O4) compounds at the Zn K-edge for different annealing temperatures.

K=

μ(E) − μ0 (E) Δμ0 (E0)

ℏ2

(3)

where m is the electron mass. χ(k) is weighted by k2 to amplify the oscillation at high k and the χ(k)k2 functions are Fouriertransformed in R-space to generate the χ(R) vs R spectra, in terms of the real distances from the center of the absorbing atom. The set of EXAFS data analysis software available within the IFEFFIT software package have been used for EXAFS data analysis.38 This includes background reduction and Fourier transform to derive the χ(R) vs R spectra from the absorption spectra (using ATHENA software), the generation of the theoretical EXAFS spectra starting from an assumed crystallographic structure, and finally fitting of experimental data with the theoretical spectra using ARTEMIS software. The χ(R) vs R spectra for different annealing temperature are shown in Figure 15. The theoretical EXAFS spectrum is generated using the known crystal structure. As stated earlier, in the case of spinel structure of formula AB2O4, two types of crystal structures are present. One is the normal spinel structure, where A occupies the tetrahedral position and B occupies the octahedral position. However, in case of the inverse spinel structure, A occupies the octahedral position and B occupies the octahedral position, as well as the tetrahedral position. The beamline energy constraint (4−25 keV) allows us to perform the measurements at the Zn edge only. The bond distance, coordination number, and the Debye−Waller factor

position clearly shows that Zn is present in the +2 oxidation state as it coincides with the ZnO standard. Generally, 3d transition metals shows the pre-edge feature due to the transition of a 1s electron to unoccupied 3d states hybridized with O 2p states. However, since there are no empty d states for Zn2+, this leads to the absence of pre-edge features. The spectral features are marked “A” and “B” in the XANES spectra. It has been found that the intensity of the peak at A and B decreases as the annealing temperature increases. The normalized EXAFS spectra of the ZA sample annealed at different temperatures are shown in Figure 14 at the Zn K-edge. In order to take care of the oscillations in the absorption spectra, μ(E) has been converted to absorption function χ(E), which is defined as follows:37 χ (E ) =

2m(E − E0)

(2)

where E0 is the absorption edge energy, μ0(E0) the bare atom background, and Δμ0(E0) the step in μ(E) value at the absorption edge. The energy-dependent absorption coefficient χ(E) has been converted to a wavenumber-dependent absorption coefficient χ(k) using the relation H


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Inorganic Chemistry

spinel structure and is tetrahedral in the normal spinel structure. The partial oxygen coordination is an indication of the contribution of octahedral as well as tetrahedral coordination. This coordination number (4.84) obtained here is indicates the higher ratio of normal spinel in the ZA-750 sample. The coordination number decreases with the annealing temperature, which indicates the increasing contribution of tetrahedral coordination with the annealing temperature. The Zn−O bond length remains approximately the same; however, the Debye−Waller factor decreases as the annealing temperature increases. This decrease in the Debye−Waller factor is an indication of reduced disorder in the assumed structure with annealing temperature. The second broad peak, between 2.5 Å and 3.75 Å, is the contribution of Zn−Al, Zn−O, Zn−Zn coordination as well as multiple scattering paths. It was found that, in the case of the Zn−Al as well as the Zn−Zn paths, the bond distance increased marginally with annealing temperature. The Debye−Waller factors for Zn−Al and Zn−Zn show a decreasing trend with annealing temperature. Positron Annihilation Lifetime Spectroscopy (PALS). PALS measurements have been carried out at room temperature, using two BaF2 scintillation detectors connected to a fast−fast coincidence system. The 22Na positron source (∼10 μCi) deposited in a thin Kapton foil was kept inside the powder sample in an aluminum vial, which was kept between two BaF2 detectors. The resolving time measured with a 60Co source was 250 ps for the positron window settings, and the time calibration was 12.5 ps/channel. The spectrum with ∼2 × 106 counts was acquired for each measurement and data were analyzed using the PALSFIT analysis program.39 Figure 16 represents typical PALS spectra for ZA-750, ZA-1000, and ZA1200. The positron lifetime spectrum is a multiexponential decay curve given as

Figure 15. Fourier transform EXAFS spectra of (a) ZA-750, (b) ZA1000, and (c) ZA-1300 at the Zn K-edge. The spectra shown here is phase-uncorrected, which shows a peak at lower R than actual distances.

(disorder factor) are used as the variables during the fitting. The fitting results are shown in Table 2. The single scatterings, as well as the high-amplitude double scattering paths, are used for fitting. The first peak at 1.5 Å in Fourier transform EXAFS spectra of ZA-750 sample is due to the contribution of the Zn−O coordination shell at ∼1.89 Å with coordination of ∼4.8 oxygen atoms. Please note that the spectrum shown in Figure 15 is phase-uncorrected spectra, which shows coordination peaks at lower distances than actual; however, the phase corrected spectra is used for fitting purposes, and the same is reported in Table 2. The coordination of Zn is octahedral in the inverse

k

F (t ) =

∑ i=1

Ii −t / τi e τi

(4)

Table 2. Bond Length, Coordination Number, and Disorder Factor Obtained by EXAFS Fitting at the Zn Edge Samples path

parameter

ZA-750

ZA-1000

ZA-1300

Zn−O

bond length, R (Å) coordination number, N disorder factor, σ2

1.89 ± 0.01 4.84 ± 0.45 0.0049 ± 0.001

1.89 ± 0.01 4.56 ± 0.40 0.0037 ± 0.001

1.87 ± 0.01 4.0 ± 0.28 0.0035 ± 0.0005

Zn−Al


3.20 ± 0.01 12 ± 1.08 0.0048 ± 0.001

3.21 ± 0.01 12 ± 1.04 0.0020 ± 0.0009

3.21 ± 0.01 12 ± 0.96 0.0015 ± 0.0005

Zn−O


3.07 ± 0.02 12 ± 1.08 0.0096 ± 0.0012

3.10 ± 0.03 12 ± 1.04 0.0068 ± 0.001

3.11 ± 0.02 12 ± 0.96 0.0033 ± 0.001

Zn−Zn


3.46 ± 0.01 4 ± 0.36 0.0014 ± 0.001

3.47 ± 0.01 4 ± 0.32 0.0011 ± 0.001

3.47 ± 0.01 4 ± 0.28 0.0010 ± 0.0009

Zn−O


4.20 ± 0.04 12 ± 1.08 0.0051 ± 0.0005

4.20 ± 0.04 12 ± 1.04 0.0034 ± 0.001

4.19 ± 0.01 12 ± 0.96 0.0028 ± 0.0008

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Inorganic Chemistry

Table 4. Average Lifetimes and Bulk Lifetimes for Different ZA Compounds

where F(t) represents the number of counts at time t, k is the number of exponential decay components, τi is the lifetime of the ith component, and Ii is the corresponding intensity. This multiexponential function is convoluted with instrumental resolution function, R(t) (250 ps). The lifetime spectra were fitted to four discrete components to obtain the best fit. Results are shown in Table 3. The average lifetime (τav) and the bulk lifetime (τB) (given in Table 4) were calculated as follows: I τ + I2τ2 τav = 1 1 I1 + I2 (5) I1 + I2 (I1/τ1) + (I2/τ2)

average lifetime, τav (ns)

bulk lifetime, τB (ns)

ZA-750 ZA-1000 ZA-1200

0.335 0.293 0.269

0.304 0.252 0.228

atures, the inversion rate is high, whereas at high annealing temperature, the inversion rate decreases.8 Theoretical calculation on various defects in ZA also showed that antisite defects have the lowest formation energy.46 In our present compounds, by means of FTIR, DRS, and EXAFS studies, we have obtained sufficient information about the presence of antisite defects centers. Thus, it is obvious that, for ZA-750, the concentration of AlZn antisite defects will be greater, while, for ZA-1200, they will be deactivated, because the Al atom will occupy its own site and thereby lead to a low inversion rate. Therefore, in the present case, it might be possible that, in the ZA-750 compound, bigger cluster vacancies such as 2AlZn−VZn are present, which gives rise to higher lifetime values. Now, upon thermal annealing, they may be reduced to smaller clusters, such as AlZn−VZn, since the Al atom in the Zn-site will now move to its own site. For further confirmation, EPR experiments were carried out, which can provide more evidence. The third (τ3) and fourth (τ4) lifetime components with very small intensity (1.5%−4.5%), which correspond to the formation of ortho-positronium (o-Ps), may be found in the large grain boundaries. The pick-of f annihilation of o-Ps with the surrounding electrons from the grain boundaries gives the long lifetime components. EPR Study. Figure 17 represents the EPR spectra of the ZA750, ZA-1000, ZA-1200, and ZA-1300 compounds, respec-

Figure 16. Typical PALS spectra for ZA-750, ZA-1000, and ZA-1200.

τB =

sample

(6)

The fitted τ1 value in the 750 °C annealed sample is very close to the theoretically calculated lifetime (254 ps) in a trivacancy consisting of two Zn vacancies plus one oxygen vacancy (2VZn−VO) in the lattice.40,41 After annealing at 1000 °C, the value of τ1 reduces to ∼182 ps, which is very close to Zn, as well as O mono-vacancy. After annealing at higher temperature at 1200 °C, it is suggested that the O vacancy population increases. The τ2 value corresponds to the multivacancy or vacancy clusters in the grain boundary or inside the particle. The τ2 value is reduced after annealing at 1000 °C, but its intensity is increased, which may indicate the declustering that occurs with increases in annealing temperature. It is also interesting to see that τav > τB, which is indicative of the presence of a vacancy in the lattice, even after annealing.42 The second positron lifetime value (τ2) might have been originated from Al-related vacancy. Literature survey on Al-doped ZnO samples showed that these compounds also consist of various AlZn antisite defect-related cluster vacancies, such as 2AlZn−VZn, AlZn−VZn, nAlZn−Oi.42−45 These AlZn antisite defects are very likely to exist in the spinel, since they are directly linked with inversion of the spinel. An earlier report on ZA prepared using the solution state method also showed that, for ZA compounds obtained at low annealing temper-

Figure 17. Room-temperature EPR spectra for ZA-750, ZA-1000, ZA1200, and ZA-1300.

tively. From this figure, it can be seen that the EPR spectra of ZA-750 and ZA-1000 are different from that of ZA-1200 and ZA-1300. There is a huge change in intensity and a change in

Table 3. Positron Lifetimes of ZnAl2O4 (ZA) Compounds Annealed at Different Temperatures sample

τ1 (ns)

I1 (%)

τ2 (ns)

I2 (%)

τ3 (ns)

I3 (%)

τ4 (ns)

I4 (%)

ZA-750 ZA-1000 ZA-1200

0.254 ± 0.003 0.182 ± 0.003 0.170 ± 0.002

60.95 ± 1.56 46.89 ± 1.67 52.69 ± 0.94

0.488 ± 0.007 0.400 ± 0.005 0.388 ± 0.004

32.25 ± 1.54 49.13 ± 1.62 43.81 ± 0.90

5.21 ± 0.11 2.53 ± 0.18 2.03 ± 0.08

2.32 ± 0.03 1.46 ± 0.07 1.69 ± 0.06

56.88 ± 0.75 23.84 ± 1.08 19.88 ± 0.52

4.48 ± 0.03 2.52 ± 0.03 1.80 ± 0.17

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Inorganic Chemistry

Figure 18. Room-temperature EPR spectra for ZA-1000 and ZA-1200.

g = 2.30 might have originated from spin exchange interaction,49 which was absent for ZA-750 and ZA-1000, since the concentration of paramagnetic mono-vacancies V−Zn and V+O were less. In ZA-750, most of the vacancies are in cluster form, such as (2VZn−VO) and AlZn−VZn, which are diamagnetic. Only the g = 2.02 signal, which is due to V+O, was present in ZA-750, suggesting that some mono oxygen vacancies (V+O) were also present, along with the cluster vacancies. Electronic Structure and Band-Gap Energy. The crystal structure of ZA (ZnAl2O4) normal spinel (face-centered-cubic structure, space group Fd3̅m (Oh7)) is represented by lattice constant (a0) and oxygen parameter (u). In the normal spinel type, Al3+ and Zn2+ ions are situated in octahedral (local point group symmetry D3d) and tetrahedral coordination (local point group symmetry Td). Table 5 compares our DFT calculated a0,

the resonance profile when ZA was annealed at higher temperature (e.g., 1200 and 1300 °C). Figure 18 represents the EPR spectrum of ZA-1000 and ZA-1200 with characteristic g-values for different paramagnetic species. As seen from this figure, only one paramagnetic signal at g = 2.02 is present in ZA-1000, whereas, for ZA-1200, additional signals such as g = 2.07, g = 2.14, and g = 2.30 also appeared in the spectrum. Therefore, these signals must be the results of thermal annealing of the samples. Some new defect centers must have formed in the compound at higher annealing temperature. The g = 2.02 signal can be attributed to an electron trap at the oxygen vacancy.47−49 This signal exists among all the compounds and, upon thermal annealing of ZA, the intensity was found to increase, as shown in Figure 19. From positron

Table 5. DFT-GGA Calculated and Experimental Crystallographic Data for Normal Spinel ZA Oxide with Space Group Fd3̅m Value structural parameter Defect-Free a0 (Å) V0 (Å3) u bond distances (Å) Zn−O Al−O a

this study (DFT-GGA)

experiment

previous DFT

8.176 546.59 0.264

8.0912a 529.70a 0.2654a

8.0215 (LDA)b 516.14 (LDA)b 0.2638 (LDA)b

1.98 1.93

Data taken from ref 50. bData taken from ref 51.

Figure 19. Room-temperature EPR spectra for ZA-750 and ZA-1000.

u, Zn−O and Al−O bond lengths with experimental values and previous DFT calculated values. Analysis of Table 5 shows that the DFT-GGA calculated lattice parameter and equilibrium volume is overestimated by 1% and 3.2%, compared to experimental values. The oxygen internal parameter is also in good agreement with experimental values. Our DFT-GGA calculated values are in good agreement (within 5.5%) with previous DFT-LDA calculated values. In order to investigate the change in electronic band structure of ZA with the presence of O, Al, and Zn defects, the total and angular momentum decomposed density of states (DOS) of the defect-free spinel structure is calculated and

lifetime study, we have observed that the (2VZn−VO) vacancy dissociated into mono-vacancies (i.e., VZn and VO). Thus, it might be possible that the (2VZn−VO) cluster was first reduced to (VZn−VO) through deactivation of one Zn vacancy by one interstitial Zni and finally the (VZn−VO) vacancy dissociated into mono-vacancies (i.e., V−Zn and V+O). The signal at g = 2.07 can be attributed to VZn−. The signal was prominent for ZA1200 and ZA-1300. This correlates well with our assumption that, at higher annealing temperatures, the (VZn−VO) vacancy dissociated into mono-vacancies (i.e., V−Zn and V+O. The signal at K


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Inorganic Chemistry

Figure 20. Total and angular momentum decomposed density of states (DOS) of normal spinel: (a) pure ZA, (b) normal spinel with neutral oxygen vacancy (V0O), (c) +1 charge vacancy (V+O), and (d) +2 charge vacancy (V2+ O ). Vertical lines at zero energy represent the Fermi energy. O1 and O2 represent oxygen atoms bonded with Zn and Al atoms in its 1st nearest neighbor, respectively.

that is due to the presence of an O vacancy with charge +1 (V+O). The overall nature of the VB remains unaltered, but two impurity bands appear above the VB maximum in the band gap. Impurity levels are composed of the d states of Zn and the p states of Al in the spin-up and spin-down components. Impurity states generated due to spin-up components are filled with electrons, because it is situated just below the Fermi energy and impurity states generated due to spin-down components are empty. Impurity bands thatappear just below the CB minimum are similarly composed of d states of Zn and p states of Al. Among them, Zn-d and Al-p states are deeply inserted in the band gap. The electronic band gap in this case is increased by 0.50 eV, compared to the defect-free situation. Figure 20d shows the total and angular momentum decomposed DOS that is due to the presence of an O vacancy with charge +2 (V2+ O ). The overall nature of the VB remains unaltered, but an impurity band appears above the VB maximum in the band gap. The Fermi level is situated just above the VB maximum. An impurity state is present 3.04 eV above the VB maximum. Impurity levels are composed of d states of Zn and p states of Al in the spin-up and spin-down components. For this case, the energy difference between VB maximum and CB minimum (electronic band gap) is 4.87 eV, which is increased by 0.82 eV, compared to the defect-free case.

plotted in Figure 20a. The lower part of the valence band (VB) is generated from the hybridization of s-states of Al, Zn, and O. The upper part of VB consists of p-states of Al, d-states of Zn, and p-states of O. On the other hand, the lower part of the conduction band (CB) has major contributions from the s states of Zn, as well as the p states of Al and O. The DFT-GGA calculated electronic band gap is 4.05 eV, which is close to the experimentally reported value of 3.8−3.9 eV (the direct band gap at the Γ point), measured from optical reflectivity experiment.52 Thus, our DFT-GGA calculated band gap is in good agreement with experimental values. Therefore, DFTGGA description is sufficient to reproduce structural parameters and electronic structure of normal spinel ZA. Figure 20b shows the total and angular momentum decomposed DOS that is due to the presence of neutral O vacancy. The spin-up and spin-down components are shown separately in the upper and lower panels, respectively. The overall nature of the VB remains unaltered, but an impurity band appears 2.1 eV ahead of VB in the band gap and just below the Fermi level. This impurity band is mainly contributed by the Zn-d and Al-p states. Moreover, a shallow impurity band also appears just below the CB, mainly contributed by the Zn dstates and Al p-states. In this case, the electronic band gap increases by 0.4 eV, compared to the defect-free case. Figure 20c shows the total and angular momentum decomposed DOS L


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Inorganic Chemistry

Figure 21. Total and angular momentum decomposed density of states (DOS) of normal spinel ZA with (a) a neutral Al vacancy (V0Al), (b) a −3 0 2− charged Al vacancy (V3− Al ), (c) a neutral Zn vacancy (VZn), and (d) a −2 charged Zn defect (VZn ). Vertical lines at zero energy represent the Fermi energy. Arrows in this figure mark impurity states. O1 and O2 represent oxygen atoms bonded with Zn and Al atoms, respectively, in its first nearest neighbor at the defect site.

Figure 22. Total and angular momentum decomposed density of states (DOS) of normal spinel ZA with an antisite defect. Vertical lines at zero energy represent the Fermi energy. O1 and O2 represent oxygen atoms bonded with Zn and Al atoms in its first nearest neighbor at the defect site.

M


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Inorganic Chemistry

Figure 23. Total and angular momentum decomposed density of states (DOS) of normal spinel ZA with (a) neutral Zn interstitial defect and (b) +1 charged Zn interstitial defect. Vertical lines at zero energy represent the Fermi energy. O1 and O2 represent oxygen atoms bonded with Zn and Al atoms in its first nearest neighbor at the defect site.

3− Figure 24. Overall summary of the location of the defect states that arise due to O vacancies (V0O, V+O, and V2+ O ), an Al vacancy (VAl ), Zn vacancies 0 2− + + − (VZn,VZn ), interstitial Zn (Zni ), antisite defects (AlZn and ZnAl), etc. in ZA. Bands with blue colored circles are filled with electrons and that with white colored circles are empty.

the electronic band gap is reduced by 0.4 eV, compared to the defect-free case. Figures 21b and 21d show electronic DOS of ZA with −3 charge for the Al vacancy (V3− Al ) and −2 charge for the Zn vacancy (V2− Zn ). DOS features for this case are similar to the neutral Al and Zn vacancy cases. The impurity band is situated very close to the VB maxima and contributed by the Op minority spin states coming from the O atoms situated in the

Figures 21a and 21c show electronic DOS of ZA with neutral Al vacancy (V0Al) and neutral Zn vacancy (V0Zn), respectively. In both cases, the overall nature of the DOS is similar to that of the defect-free case, but an impurity band appears just above the VB maxima. This impurity band is contributed by the O minority spin states of p character, which is present in the first nearest neighbor of Al and Zn atoms, respectively. In this case, N


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Inorganic Chemistry

Figure 25. Probable electron transition of various color components: (a) 375 nm, (b) 420 nm, (c, d) 450 nm, and (e, f) 490 nm.

defect states are contributed by the Zn d-state and the O pstate. The electronic band gap in this case is 3.11 eV, which is 0.94 eV smaller, compared to the defect-free case. Thus, an overall summary of the electronic structures inside the band gap, as observed from the density of states (DOS) for different types of charged and uncharged vacancies and vacancy clusters, is pictorially represented in Figure 24, where the filled electronic states are represented by bold circles and the unfilled states are represented by white circles. The reported experimental band gap for ZA is 3.8 eV.52 However, the DFT-GGA calculated electronic band gap is 4.05 eV for defectfree normal spinel. Now, to correlate the experimental results with the theoretical results similar to our earlier case in MgO,22 we have multiplied all of the energy levels with a normalization factor so that all the electronic states can be represented within the scale of 3.8 eV. These electronic states arise due to different types of defect centers, which are shown by arrows on the righthand side of the figure. The electronic states are also marked as different levels, viz, L-1, L-2, L-3, etc.

defect site. The electronic band gap is 2.58 and 3.07 eV for 2− (V−3 Al ) and (VZn ), respectively. Figure 22 shows DFT-GGA calculated electronic DOS of ZA with an antisite defect. The antisite defect is produced by flipping over the position of one Zn and Al atom in 1 × 1 × 2 supercell of the ZA unit cell. An impurity band is generated just above the VB, because of the antisite defect. The impurity band is mainly contributed by the Zn d-state and the O p-states. Because of the antisite defect, the electronic band gap becomes 3.15 eV, which is 0.9 eV smaller, compared to the defect-free case. Figure 23a shows DFT-GGA calculated electronic DOS of ZA with neutral Zn interstitial. Presence of Zn interstitial creates two impurity bands near the CB minimum. In this case, the Fermi energy has shifted to the CB minimum and two impurity bands are present just below and above the Fermi energy. The defect states are contributed by the Zn d state, the Al p-state, and the O p-state. The electronic band gap in this case is 3.00 eV which is 1.05 eV smaller compared to the defect free case. Figure 23b shows DFT-GGA calculated electronic DOS of ZA with neutral Zn interstitial with charge +1. The presence of Zn interstitial creates impurity states near the CB minimum. In this case, the Fermi energy has shifted to the CB minimum and impurity states are present just above the Fermi energy. The

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DISCUSSION FTIR, DRS, and EXAFS studies confirmed the presence of an inverse phase of the spinel compounds obtained at low O


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Inorganic Chemistry

Figure 26. Probable electron transition of various color components: (a) 530 nm and (b, c) 600 nm.

annealing temperature, which subsequently leads to various antisite defect centers in the matrix. These studies also confirmed that, upon thermal annealing of the ZA compound, the inverse phase started to disappear and the crystal structure transformed to the more-ordered normal phase. From EPR and PALS study, it can be confirmed that the following defects (viz, O vacancy, Zn vacancy, antisite defects) are present in the ZA matrix, either in cluster form or single form. This cluster vacancy is nothing but a pair of two or three identical or different vacancies when they are neighboring each other. Such a cluster vacancy (e.g., VZn−VO can be formed when a pair of 2+ V2− Zn and VO exist side by side to each other. The similar vacancy can also be formed by a pair of V−Zn and V+O. In earlier reports on the ZA matrix, synthesized through the same sol−gel combustion method, various groups (including our group) concluded that, at lower annealing temperatures, the percentage of inverse phase is greater while its percentage decreases upon annealing the compound at higher temperature.13,53 Various other groups also discussed that, in this spinel compound, it is possible to control the inversion rate by means of thermal annealing,8 which led to interesting optical properties. During thermal treatment, there is a reciprocal exchange of Zn2+ ions and Al3+ ions between octahedral and tetrahedral positions, which leads to inverse phase of the spinel. Now this reciprocal exchange of cations creates the existence of [Zn2+Al]− or − [Al3+Zn]+ antisite defects. This negatively charged [Zn2+ Al ] can 3+ + acts as a hole trap while the positively charged [AlZn] act as an electron trap.54 Since Al3+ ions have low electron affinity, the captured electron may finally leave its trap and recombine with a trapped hole.54 Thus, by trapping the electron, these antisite defect centers delay the electron−hole recombination process, which is reflected in the higher lifetime values of the various emitting components of the ZA-750 compound, where the rate of inversion is high. At higher annealing temperatures, since the spinel compound becomes more and more ordered and the rate of inversion is low, the concentration of these antisite defect centers become low. Therefore, the electron and hole trapping

process via these defect centers becomes less significant, which is reflected in the drastic decrease in the lifetime values. From the DFT summary in Figure 24, it can be seen that there are electronic states inside the band gap arising due to various defect centers. Some are filled by electron (states below the Fermi energy level) and some are unfilled (states above the Fermi energy level). Since the excitation energy (250 nm, ∼4.95 eV) is more than the band-gap energy (3.8 eV), the electron from valence band (VB) can be prompt into the conduction band (CB), leaving a hole in the VB. This hole can be trapped into the electronic states close to the VB, such as L9, L-10, L-11, etc. Similarly, electrons from the filled states, such as L-5, L-6, L-7, L-8, etc., can also be prompted into the CB, leaving a hole in those states. The electrons from the conduction band can now be recombined with the holes at those levels or can be trapped into various vacant states, such as L-1, L-2, and L-3. Now, each and individual color components, because of various defects centers, will be explained by considering their electronic structure observed from DFT calculations. We have presented different band-gap models for clear understanding of the associated electron-hole recombination processes of these color components. P1 (375 nm), P2 (420 nm), and P3 (450 nm) Emission. As presented in the model in Figure 25, 375 nm is very close to the energy gap between the L-1 and L-12 levels, which arises because of antisite defect centers. Since the energy of the emission 375 nm (3.3 eV) is lower than the band gap (3.8 eV), there must be some shallow level. Also, in a earlier report,8 this band was observed where the authors attributed this band to the radiative recombination of a trapped electron below the CB with a hole in the valence band. Now, in the present case, as observed from our DFT calculation, these shallow states below the CB (L-1 and L-2) have contributions from the antisite defect and O-vacancy defects (see Figure 24). Thus, we can attribute the 375 nm emission to antisite defect states, which creates some shallow vacant states below the CB and some filled states just above the VB, as shown in Figure 24. Upon excitation, electrons may prompt into the CB or into the shallow states below the CB, as shown in Figure 25a, creating P


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Inorganic Chemistry

Figure 27. Probable electron transition of various color components: (a, b) 720 nm, (c) 780 nm, and (d) 850 nm.

nm emission can be linked to transition of an electron trapped in a deep level, such as L-3 to L-12, as shown in Figure 25e, where a hole has been created due to excitation. The 490 nm emission is also energetically close to the CB → L-5 transition, as shown in Figure 25f. In both cases, either the electron or hole is in the trapped state. The lifetime of this particular emission is the highest among others. Since the L-12 has direct contributions from the antisite defects, it might be possible that the 490 nm components are the result of the L-3 → L-12 transition. In Figure 6a, we can see that there is a decay in intensity in the 370−500 nm region when the ZA-750 compound was further annealed at higher temperatures (ZA1000 and ZA-1300). The changes in emission related to the antisite defect centers might be explained based on decreases in the percentage of inverse structure of the spinel compound upon thermal annealing. However, cationic and anionic vacancy-related emissions might be explained based on decay in concentration on the surface of the particle, because of decreases in the surface area, as observed in the SEM study. P5 (530 nm) and P6 (600 nm) Emission. The 530 nm emission can be correlated to the transition from the L-3 level to L-11, as shown in Figure 26a. L-11 has contributions from V−Zn while L-3 is linked to V2+ O . In the emission spectra in Figure 6a, it can be found that, by increasing the annealing temperature, the intensity around this region is enhanced. In the EPR spectra also, the resonance signal due to V+O and V−Zn

hole in the VB or L-12 level. The electron from CB can exchange into the L-1, followed by recombination with the hole in L-12. In an earlier report,2 emissions in 400−450 nm regions were attributed to oxygen vacancies. However, a clear understanding about the electronic transition involving those emissions is still required. In the present case, the 420 nm emission was found energetically close to the electronic transition L-1 → L-7, L-8, as shown in Figure 25b. The L-1 level has contributions from various oxygen-vacancy-related defect centers, while L-7 and L8 have contributions from cationic vacancies. For the P3 (450 nm) component, the transition from CB to the L-6 level is an energetically good match, as shown in Figure 25c. L-6 has contributions from the oxygen vacancy (VO). Emission in the same region was also observed by Cornu et al., and they have attributed it to interstitial Zn (Zni).8 We have also calculated the electronic structure due to Zni. From the energy matching criteria, it was found that the P3 (450 nm) component can be linked to the L-2 → L-7, L-8 transition, where the L-2 has contributions from Zni, as shown in Figure 25d. Furthermore, since the electronic states due to antisite defect centers very close to these level (in the case of the L-1 level, the energy level of both oxygen vacancy defect centers and antisite defect centers are very close and thereby represented by a single level, i.e., L-1), they may act as an electron and hole trap. This results in delay of the electron hole recombination process. The 490 Q


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Inorganic Chemistry were found to be more intense for the compound annealed at high temperature. In the PALS study, the cluster vacancies were found to be dissociated into mono Zn and O vacancies. This established the fact that the green component at 530 nm is directly linked with the Zn and O vacancy. An earlier report8 in ZA showed that the 600 nm emission is probably associated with some oxygen-vacancy-related intermediate levels. Through our DFT calculation, this 600 nm emission can be energetically linked to two transitions: (i) L-2 → L-5 and (ii) L-4 → VB, as shown in Figures 26b and 26c. Both L-4 and L-5 are linked to filled and unfilled O-vacancyrelated states. The enhanced intensity at higher annealing temperature can be explained on the basis of an increase in free V+O due to dissociation of the cluster vacancy. Red Near-IR Region: 720 nm (P7), 780 nm (P8), and 850 nm (P9). From TRES study, it was found that the 720 nm (P7) emission was absent for the ZA-750 compound. A group of lines with the greatest intensity at ∼690 nm was found for all heated samples, which might be due to the Cr3+ ion.8 However, a prominent peak ∼720 nm was observed for the ZA-1000 and ZA-1300 compounds. Thus, the transition might be associated with free O and Zn vacancies, which arise only at higher annealing temperatures. From the DFT-calculated electronic structure, this emission can be linked to two transitions: (i) L-3 → L-6 in Figure 27a and (ii) L-4 → L-11 in Figure 27b. In the first case, the transition levels are associated with the oxygen vacancies, whereas in the second case, L-4 is linked to V+O and L-11 to V−Zn. The second case is found to be more sound, since, from PALS and EPR studies, it was observed that the concentration of free V+O and V−Zn were greater at higher annealing temperatures. The 780 nm (P8) emission can be attributed to L-1 → L-4 transition, as shown in Figure 27c. On the other hand, the 850 nm (P9) emission can be assigned to the L-3 → L-5 transition (Figure 27d). Both transitions are linked to an O vacancy. Thus, slight enhancement of the emission intensity in the nearIR region can be due to the same region as that of 720 nm. Explanation of the Change in Color Coordinate from Bluish to Near-White. At low annealing temperatures, the bluish emission characteristics is due to strong intense violetblue emissions, as shown in Figure 6a, where the percentage of green and red emissions was low. Now, as the annealing temperature was increased, the percentage of green and red color was also increased, because of the increase in V+O and V−Zn defects, arising from the dissociation of cluster vacancy. In contrast, a decrease in intensity of the blue region was observed due to decreases in the surface-related defect and antisite defect centers. Thus, in a balance of all three colors (viz, blue, green, and red), the compound showed white emission characteristics, which is reflected in the color coordinate in Figure 6b.

This thermally tunable color characteristic also makes this material have potential for use in optically based thermal sensor materials. TRES study, in combination with decay kinetics, helped to isolate various color components from the complex emission spectra. The emitting components are ultraviolet (P1) at λmax ≈ 375 nm, violet (P2) at λmax ≈ 410 nm, blue (P3) at λmax ≈ 440 nm, cyan (P4) at λmax ≈ 490 nm, green (P5) at λmax ≈ 530 nm, yellow (P6) at λmax ≈ 590 nm, red (P7) at λmax ≈ 720 nm, and near-infrared at λmax ≈ 780 nm (P8) and λmax ≈ 850 nm (P9). Decay kinetics showed a higher lifetime value for the emitting components at low annealing temperatures, which was significantly reduced at higher annealing temperatures. FTIR, DRS, and EXAFS studies showed that, at low annealing temperatures, the percentage of inverse phase is greater in the ZA nanoparticles, whereas upon thermal annealing, this inverse structure transformed to the more-ordered normal phase. Since the inverse structure is directly linked with the antisite defect centers, such as Al+Zn or Zn−Al, it was confirmed that, at higher temperature, their concentration was greatly reduced. PALS showed that, in ZA-750 samples, bigger cluster vacancies, such as 2AlZn−VZn, were present. Now, upon thermal annealing, they may reduce to smaller clusters, such as AlZn−VZn. This observation was further supported by EPR study, which showed the presence of different resonance signals due to cationic and anionic vacancies and, upon thermal annealing, they changed significantly. DFT calculation was performed for the ZA compound with various defect centers to characterize the electronic structures inside the band gap of material. Finally, based on several theoretical and experimental results, various electron−hole recombination models are proposed to correlate the various emissions with the possible defect centers. The diverse lifetime behavior of ZA nanophosphors at various annealing temperatures are attributed to the antisite defect centers, such as Al+Zn or Zn−Al, which act as electron and hole trap centers.

CONCLUSION ZA phosphor material was synthesized via the sol−gel combustion route at different annealing temperatures. SEM and TEM studies showed the formation of single-phase ZA nanoparticles and particle size increased upon thermal annealing of the sample. The resulting ZA compounds showed a broad emission due to various defect centers. At low annealing temperature, ZA was found to be blue-emitting, whereas at higher annealing temperatures, the compound transformed to a near-white light-emitting material. Thus, the compound was found to be a potential tunable color-emitting phosphor material, because of its tunable color characteristic.

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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/acs.inorgchem.8b00172. Description of the instrumentation used; particle size calculations for different samples; Figures S1 and S2 (PDF)

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AUTHOR INFORMATION

Corresponding Author

* Tel.: +91-22-25590715/0636. Fax: +91-22-25405151. E-mail addresses: [email protected], [email protected].

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Nimai Pathak: 0000-0003-0880-6255 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to Dr. P. K. Pujari, Associate Director, RC & IG, BARC and Head, RCD, BARC for his continued support and encouragement during the course of this project. The authors thank Dr. Kathi Sudarshan, Head, Nuclear Probe Section, RCD, BARC for his help in the TEM sample preparation and other scientific discussions. The authors R


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acknowledge the contribution of Dr. S. Neogy, MMD, BARC for his kind help in measuring and analyzing the TEM and SAED images. The authors thank to Dr. Sabyasachi Patra, RCD, BARC, for his help while carrying out the diffuse reflectance spectroscopic measurements. The authors would like to thank Mr. Rahul Agarwal and Mr. Bal Govind Vats, FCD, BARC, for their help in SEM and FTIR measurements during the course of this project.

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