An Insight into the Various Defects-Induced ... - ACS Publications

Article pubs.acs.org/JPCC

An Insight into the Various Defects-Induced Emission in MgAl2O4 and Their Tunability with Phase Behavior: Combined Experimental and Theoretical Approach Nimai Pathak,*,† Partha Sarathi Ghosh,‡ Santosh Kumar Gupta,† Saurabh Mukherjee,† Ramakant Mahadeo Kadam,† and Ashok Arya‡ †

Radiochemistry Division and ‡Materials Science Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India S Supporting Information *

ABSTRACT: The present work describes various defects-induced tunable emission behavior of MgAl2O4 compounds obtained after annealing at different temperatures through a sol−gel combustion route. Multiple defect centers, such as F, F2, F+, and F22+ and different shallow and deep defects were found to be present inside the band gap, as confirmed by the lifetime and time-resolved emission spectroscopy (TRES) studies. The tunable emission characteristic at different annealing temperatures could be linked with the phase behavior of the spinel. Excitation wavelength variation suggested that a photoconversion process of F to F+ centers was involved with λex = 250 nm, followed by a trapping−de-trapping mechanism of the released electrons within different trap states. An exchange mechanism of electrons in between conduction band and shallow states was also observed at room temperature, which was absent at low temperature, as indicated by the emission profile. These observations render it to be a potential opticalbased thermal sensor material. DFT-based calculations were carried out for both pure and various oxygen-vacancy-introduced spinel phases in order to characterize the different defect states inside the band gap. Finally, on the basis of theoretical and experimental results, a model has been proposed to explain the mechanisms related to emission tunability.

1. INTRODUCTION Defects are typically considered as imperfections in an inorganic material, which may drastically degrade its various physical and chemical properties (optical, electrical, magnetic, etc.). Normally, during synthesis of any material, one always tries to minimize the amount of both intrinsic as well as extrinsic defects in order to get a good-quality powder or crystal. However, defects are not always deleterious, and sometimes they can have positive implication on materials properties. Recently, defect-induced properties in materials is a hot research area such as photocatalysis in TiO2,1 electronic and magnetic properties in graphene,2,3 optoelectronic properties in indium sulfide,4 plasmonic properties in silicon nanocrystal and CdO:Dy,5,6 electrochemical properties in LiFePO4,7 optical properties in activator free Ag2Se, and MoS2,8,9 and electric and ferroelectric properties in CuO:Fe,10 among others. As far as defects-induced optical properties in oxide materials are concerned, different types of electronic states inside the band gap of the material are responsible for the emission behavior, whereas the single electron trapped in these vacancies that results paramagnetic species such as F+ centers are responsible for the magnetic properties. Defect-induced photoluminescence (PL) is observed in various activator free materials such as ZnO, TiO2, SnO2, CuI, Al2O3, and MgO,11−16 among others. Recently alumina has been the focal point for © 2016 American Chemical Society

different oxygen-vacancy-induced optical properties such as photoluminescence (PL), thermoluminescence (TL), and optically stimulated luminescence (OSL), both in pure and doped α-Al2O3.15,17−20 In all these works, the presence of various defect centers such as F centers (oxygen vacancies occupied by two electrons or VO0), F+ centers (oxygen vacancies occupied by one electron or VO+1 or VO·), their aggregates such as F2 centers (two associated oxygen vacancies occupied by four electrons), and the F2+ centers (two associated oxygen vacancies occupied by three electrons) are reported to be responsible for the observed emission characteristics in α-Al2O3, and they are interconvertible using light irradiation. As discussed in various reports, MgO is also rich in different type of F centers, which may give rise to various kinds of luminescence and associated properties.16 Magnesium aluminate (MgAl2O4) is supposedly the best representative material from the wide range of available oxides with a crystalline spinel structure. The formula MgAl2O4 may be assumed to be a combination of MgO and Al2O3. The emission characteristic will therefore be always a complex one. Received: December 3, 2015 Revised: January 15, 2016 Published: January 22, 2016 4016

DOI: 10.1021/acs.jpcc.5b11822 J. Phys. Chem. C 2016, 120, 4016−4031

Article

The Journal of Physical Chemistry C

to different shallow and deep states in the visible and nearinfrared (NIR) regions are yet to be explored. The annealing temperature also plays an important role particularly in combustion synthesized spinels in terms of optimization of degree of cation orderings. In that context, Dwibedi et al.29 have observed high degree of inversion in MAS at low annealing temperature, which decreases at higher annealing temperature. Isostructural ZnAl2O4 is also reported to exhibit a similar trend with varying annealing temperature,30,31 which makes it a potential candidate for optical based thermal history sensor because of its tunable emission behavior with annealing temperature.32 Another isostructural ZnGa2O4 spinel also shows tunable optical emission behavior in the blue, violet, and red regions,33 depending on regular and distorted GaO6 and the paramagnetic single-charged oxygen vacancies (VO+1) associated with it. Henceforth, the percentage of intrinsic defects present in this kind of system can be controlled by annealing temperature, which will in turn control the optical properties of the system. Typically, such spinel shows optical emission in blue− green−yellow (BGY) and red−NIR regions under UV excitation. One can tune the emission of these spinels from blue to red region or to further longer wavelength such as the NIR region by different thermal treatments. Such study is of important because NIR spectroscopy has potential application in various areas such as NIR heater and pyrometer, night-vision devices, hyperspectral imaging, infrared homing, and NIR laser. NIR laser can also be used for cancer therapy.34 Therefore, NIR-emitting material has potential application in both technological and medical fields. Such reports pertaining to tunable optical properties of MAS in visible and NIR regions with varying thermal treatment is lacking in the literature. In the present work, we report a detailed investigation on multicolor emission of MAS compound, derived through sol− gel−combustion route at different annealing temperatures. This is the first report of its kind, where by using photon lifetime measurement and time-resolved emission spectroscopy (TRES) we were able to characterize and isolate different color components from the complex emission spectra, which arise due to different types of defect centers. CIE chromaticity coordinates of the emission spectra at different annealing temperatures were also evaluated to understand the tunable color-emitting characteristic. To correlate the oxygen vacancy (OV) with the emission behavior, the compound was further annealed under vacuum atmosphere. Earlier reports on different matrixes showed that a change in the metal ion’s composition ratio may lead to a change in the PL behavior.35 In the present case, to investigate any change in the PL behavior, we have slightly changed the Mg ratio such as 0.98 and 1 from

Magnesium aluminate spinel (MAS) has received significant interest both from academia as well as industry because of its various favorable properties such as high melting point (2135 °C), high hardness (16 GPa), relatively low density (3·58 g/ cm3), high mechanical strength at both room temperature (135−216 MPa) and elevated temperatures (120−205 MPa at 1300 °C), high resistance to chemical attack and thermal shock, wide band gap, high electrical resistivity, and relatively low thermal expansion coefficient (9 × 10−6 °C1− between 30 and 1400 °C),21 and so on. It is considered one of the most promising advanced ceramic materials and has been explored for various technological applications, such as a luminescence host, potential core material in fusion reactor, electronic humidity sensor, integrated electronic devices, electrode materials, and catalysis, among others.21−23 Sickafus et al.24 has proposed the structure of spinel a decade ago. MAS crystallizes into a close-packed face-centered cubic (fcc) structure with Fd3̅m symmetry. In an elementary cell, there are 64 tetrahedral and 32 octahedral sites to be occupied by 8 Mg2+ and 16 Al3+ ions, respectively, which corresponds to eight units per cubic cell [(Mg)8(Al)16O32]. In an ideal spinel structure, which is known as normal spinel, with the formula (Mg)tetra[Al2]octaO4 (where parentheses () and brackets [] are used to denote tetrahedral (Td) and octahedral (Oh) sites), divalent cations (Mg2+) present in tetrahedral voids, and trivalent cations (Al3+) present in octahedral voids. However, in both natural and synthetic spinels, few percentage of Mg2+ in octahedral sites and Al3+ in tetrahedral sites are generally observed, which leads to inverse spinel structure with general formula (Mg1−xAlx)tetra[MgxAl2−x]octaO4, where x stands for inversion parameter and is defined as the fraction of Al3+ in tetrahedral sites. For normal spinel, x = 0, and for inverse spinel, x = 1. However, in mixed spinel, 0 < x < 1. In MAS, it is reported that various factors such as stoichiometry, structural disorder, impurities, thermal/mechanical treatment, and sintering or annealing at different atmosphere can lead to creation of various kinds of defects.25 In pure stoichiometric MAS, there can be three types of intrinsic defects: misplaced atoms, Schottky and Frenkel defects, and approximately up to 30% of cation antisite disorder.26,27 Thus, both in inverse and mixed spinel, there will be antisite defects, which can be expressed according to Kroger−Vink as [Mg xMg] + [AlxAl] = [Al•Mg ] + [Mg′Al]

(1)

Few other defects in MAS do exist because of cation disorder and can be described by following equation:26 ‴ ] = [Mg′Al] + [V Mg ″ ] [Mg xMg] + [V Al

(2)

″ ] = [Al•Mg ] + [V Al ‴] [AlxAl] + [V Mg

(3)

Al

2

1.96

that ideal 1/2 ratio in the pure spinel. Large variation of the Mg Al ratio from the ideal ratio may lead to formation of binary systems such as MgO−Al2O3 composite or a mixture consists of several phases such as MgAl2O4, MgO, Al2O3, and so on.36,37 Electron paramagnetic resonance (EPR) studies were carried out to identify the paramagnetic defect centers. We have also observed a photoconversion process of F to F+ centers during excitation wavelength variation in the MAS compound. PL measurement at higher excitation wavelength, revealed a kind of trapping−de-trapping mechanism of the trapped electrons in shallow and deep defects. Low-temperature measurement of the PL spectra also showed a tunable characteristic and brought out the fact of electron exchange in between the conduction

These vacancies may leads to formation of various kinds of defect centers, viz., V center (negatively charged cationic vacancies), F center, and F+ center. These defects can act as color centers by means of trapping of charge carriers (electrons and holes) in the compound. The optical properties due to these defect centers in insulators are of renewed interest because of the emergence of color-center lasers. Only a single report exists on visible emission in MAS due to cations disorder and oxygen vacancy (F+ center).28 However, emissions due to F, F22+, and F+ center surrounding each of the cations and due 4017


Article


Figure 1. XRD patterns of MAS after annealing at (a) 600 °C, (b) 800 °C, and (c) 1000 °C and with Mg/Al composition ratios (d)

0.98 2.00

and (e)

1 . 1.96

2. EXPERIMENTAL SECTION 2.1. Synthesis. In the present sol−gel−combustion method, citric acid (C6H8O7·H2O) (99.7%, AR-grade) was used as a fuel, whereas stoichiometric amounts of required magnesium carbonate [Mg(CO3)2] (99%, AR-grade) and aluminum nitrate [Al(NO3)3·9H2O] (98%, AR-grade) were used as starting materials for the synthesis of MgAl2O4. Magnesium carbonate and citric acid were taken in the molar ratio 1:10. First, required amounts of the mentioned metal carbonate and metal nitrate were dissolved in quartz-doubledistilled (QDD) water. The carbonate salt was dissolved in QDD by slight addition of pure HNO3. The prepared solutions of carbonate and nitrate salts were then mixed in a beaker and kept stirring by a magnetic stirrer for 1 h. After getting a clear

band and shallow states of MAS at room temperature. To understand the change in the electronic structures due to presence of different type of oxygen-vacancy-induced defects (neutral and charged), density functional theory (DFT) calculations were carried out using projector-augmented wave potential and generalized gradient approximation. The DFTcalculated electronic density of states (DOS) were used to understand the location and basic origin of the defect states in the band gap region, which otherwise is not easily possible by experimental means. Finally, on the basis of the DFT calculated results and experimental observations, a phenomenological model is proposed that can explain the mechanisms related to emission tunability. 4018


Article


crystalline nature of MAS samples. The two MAS compounds with Mg/Al composition ratio 0.98 and 1 (Figure 1d,e) do

solution, citric acid solution (prepared in QDD water) was added to the mixture with vigorous stirring. The solution mixture was then heated at 80 °C with continuous stirring for 5 h, until a highly viscous gel formation was observed. The gel was heated at 150 °C for 3 h, after which an ash-like product was obtained. The ash-like product was then ground and kept for annealing at 600 °C in air atmosphere for 12 h, after which a fine white powder was obtained. The phase purity of the prepared MAS compound was confirmed by X-ray diffraction (XRD). The compound was further annealed at different higher temperatures, viz., 800 and 1000 °C. Some amount of the asprepared MAS compound was also annealed in vacuum atmosphere at 800 °C for 15 h in order to compare its emission behavior with that of air-annealed MAS. In another lot, two MAS compounds with Mg/Al molar ratios 0.98 and 1 2.00

2.00

1.96

not show any impurity peaks, which indicates the absence of any binary systems with these compositions. 3.2. Photoluminescence. 3.2.1. Excitation, Emission, and Lifetime Spectroscopy. Figure 2 shows the excitation spectra at

1.96

were also prepared following the same procedure with the required amount of the precursor materials (magnesium carbonate and aluminum nitrate molar ratios 0.98 and 1 , 2.00

1.96

respectively). Photoluminescence (PL) and EPR spectra were recorded after each heating treatment. The instrument details have been given in the Supporting Information. 2.2. DFT Calculation Methodology. The MgAl2O4 fcc normal and inverse spinel phases are studied using the Vienna ab initio simulation package (VASP),38,39 which calculates the Kohn−Sham eigenvalues within the framework of DFT. The calculations have been carried out with the use of the generalized gradient approximation (GGA), whereas the exchange and correlation energy per electron have been described by the Perdew−Burke−Ernzerhof (PBE) parametrization.40 The interaction between electrons and atoms are described by means of the projector augmented-wave (PAW) method41 using Mg (3s − 2 valence electrons), Al (3s, 3p − 3 valence electrons), and O (2s, 2p − 6 valence electrons) as implemented in the VASP package. For normal and inverse cubic spinel unit cell as well as structures composed of oxygen vacancy (neutral and charged), optimization was carried out with respect to cutoff energy (Ecut) and k-point meshes to ensure convergence of total energy within a precision 0.1 meV/ atom. A Monkhorst−Pack42 k-space sampling of 13 × 13 × 13 for normal spinel and 7 × 7 × 7 for inverse spinel in reciprocal space for the brillouin zone integration and Ecut of 500 eV for the plane wave basis set was used. The total energy of normal and inverse cubic spinel unit cell as well as structures composed of oxygen vacancy (neutral and charged) were optimized with respect to volume (or lattice parameter) and atomic positions. Conjugate gradient algorithm was used for the unit-cell relaxations until the residual forces and stress in the equilibrium geometry were on the order of 0.005 eV/Å and 0.01 GPa, respectively. The final calculations of total electronic energy and density of states (DOS) were carried out using the tetrahedron method with Blöchl corrections.43

Figure 2. Excitation spectra of MAS compound observed after annealing at 600 °C with different emission wavelength.

different emission wavelengths of the MAS obtained after annealing at 600 °C. The excitation spectra consist of different bands with maxima at 230, 235, 245,250, 260, 273, 287, 298, 311, and 326 nm, and so on, which indicate the presence of different electronic states in the band gap of MAS. Although there are many approximations associated with the DFT calculation (discussed in section 3.3), one thing can be easily concluded from this calculation: because MAS is a wide-band semiconductor material, the excitation peaks observed in Figure 2 must be due to the transition of the electrons from the field defect states within the band gap to either the excited states or to the conduction band (Figure 14). The peaks in the 230−245 nm regions are attributed to F+ center, whereas those in the 250−260 nm regions are due to F centers. The bands in the 270−298 nm regions can be attributed to charge transfer between Al3+ at octahedral sites and its surrounding O2− ions.28 Figure 3 represents the room-temperature PL spectra of the MAS compounds prepared at different annealing temperatures. Because the luminescence processes involve Gaussian line broadening mechanism, the emission curves of MAS compounds were deconvoluted.45,46 After Gaussian peak fitting, the spectra showed 10 different emission components in the UV−visible−NIR region (UV−vis−NIR). They are represented in the Figure 3a−c by P1, P2, P3, P4, P5, P6, P7, P8, P9, and P10 at different emission wavelengths (nm): the UV component (P1) at λmax ≈ 375 nm, violet (P2) at λmax ≈ 410 nm, blue (P3) at λmax ≈ 460 nm, bluish-green (P4) at λmax ≈ 490 nm, green (P5) at λmax ≈ 530 nm, yellow (P6) at λmax ≈ 570 nm, orange (P7) at λmax ≈ 610 nm, red (P8) at λmax ≈ 720 nm, and the NIR components at λmax ≈ 780 nm (P9) and λmax ≈ 850 nm (P10) respectively. The existence of these emission components in the complex spectra will be confirmed by the TRES study later in this section. These multicolor emissions indicate the presence of various defect states within the band gap of the

3. RESULTS AND DISCUSSION 3.1. Phase Purity: X-ray Diffraction. The XRD patterns of MAS samples annealed at different temperatures are shown in Figure 1. The figure shows that all the samples are very well consistent with the standard data for MAS phase (ICSD31373) and thereby confirmed the formation of the spinel 44 compound with space group Fd3m ̅ . The absence of impurity peaks in the XRD spectra (Figure 1a−c) also confirmed the formation of a single-phase spinel structure, whereas the strong and sharp diffraction peaks of the annealed samples indicate the 4019


Article


Figure 3. Emission spectra of the MAS compounds obtained after annealing at (a) 600 °C, (b) 800 °C, and (c) 1000 °C temperature with λex = 250 nm.

white to orange-red as the temperature is changed in the order 600 °C → 800 °C → 1000 °C, which renders this particular MAS compound as a potential material for optical thermometry. The drastic increase in the P7 and P8 bands’ intensity at higher annealing temperature, as shown in Figure 3, also render this particular compound as a potential NIR-emitting material. The detailed explanation of this annealing temperature emission behavior will be discussed in section 3.4.2. Figure 5 shows the emission spectra of the 1000 °C annealed MAS compound, at different excitation wavelengths. Two observations can be marked from this figure. First, the emission intensity in the red−NIR region has been found to increase sharply when the excitation wavelength was tuned to λex = 250 nm. The intensity of this region, which is composed of two different bands, viz., P8= 720 nm and P9 = 780 nm, has been found to increase continuously up to an excitation wavelength

material where each color arises due to a different electronic transition. The major change observed in the emission profile of the MAS compounds (Figure 3) prepared at different annealing temperature is that as the annealing temperature was increased from 600 to 1000 °C the intensity of the red and NIR bands (P8 and P9) was found to increase drastically in comparison to the bands present in the BGY regions. To denote specifically the color characteristics of MAS compounds formed at different annealing temperature, the CIE chromaticity coordinates were evaluated adopting standard procedures. The values of x and y coordinates for the MAS samples annealed at different temperatures are pictorially represented in Figure 4, where the color point is denoted by an asterisk (*). From the numbers and position of the asterisk, it can be easily observed that the color coordinate is changing from bluish to bluish4020


Article


Figure 4. CIE chromaticity diagram for the MAS phosphor at different temperature with λex = 250 nm.

Figure 6. Temperature-variation emission spectra of MAS synthesized after annealing at 1000 °C.

lifetime values were also found to increase accordingly. However, the most interesting observation of this lowtemperature measurement was that the emission intensity of the red−NIR region (650−900 nm) was lower in comparison to that of the BGY region (400−600 nm), as the temperature went down to 77 K or the intensity of the red−NIR region has not increased in the similar order as that of BGY region due to quenching of the nonradiative pathways at low temperature. Therefore, the emission can also be controlled by controlling the measurement condition, and the particular MAS compound can be used as a potential low-temperature sensor material. We will explain these two anomalous behaviors at different excitation wavelengths and at different low temperatures in detail by proposing a model in section 3.4.4. Figure 7 shows the emission spectra of the two MAS compounds with Mg/Al composition ratios 0.98 and 1 ,

Figure 5. Emission spectra of MAS derived after annealing at 1000 °C at different excitation wavelengths.

of λex = 250 nm. (At higher annealing temperature, the P10 = 850 nm band was found to disappear.) Beyond this wavelength, the intensity in red−NIR region started falling as evident in Figure 5. Second, at higher excitation wavelength, the peak maxima at ∼720 nm, which was evident up to λex = 280 nm, was red-shifted to 760 nm at λex = 320 nm (Figure S1). Emission spectra with different excitations for the MAS compounds annealed at 600 and 800 °C are shown in Figures S2 and S3, which also followed similar trend. Henceforth, we can say that as the excitation wavelength is increased to 330 nm the peak at 720 nm disappears and the spectrum in the red− NIR region consists of only one peak, i.e., ∼780 nm. Gaussian fit of the emission spectra in this region at 250 and 330 nm excitation wavelengths are also given in Figure S4. Figure 6 shows the low-temperature (77−300 K) emission spectra of the particular MAS compound, observed after annealed at 1000 °C. As the temperature is decreased, the overall intensity was found to increase. This is a general observation that the luminescence efficiency is higher at low temperatures due to quenching of the nonradiative pathways which are otherwise more efficient at higher temperatures. The

2.00

1.96

respectively, prepared after annealing at 600 °C. In both cases, it was found that there is an increase in intensity in the ∼410, 530,720, and 780 nm regions (shown by black arrows) compared to the spectrum o f the idea l spinel Mg Mg 1 0.98 composition Al = 2 . However, with Al = 2 , the intensity in the 410 nm region was found to increase more compared to that of the other regions, whereas in the other case with Mg 1 = 1.96 , the intensities of the 530, 720, and 780 nm regions Al were found to increase more than the 410 nm region. The increase in intensity in the 720 and 780 nm regions (red−NIR Mg 1 region) was also found to be more with Al = 1.96 compared to Mg

0.98

that with Al = 2 . Figure 8 shows the emission behavior of the pure MAS compounds after annealing in air and vacuum atmosphere. It can be seen that there is an overall increase in emission intensity (shown by black arrows) both in the BGY and red− NIR regions when the MAS compound was annealed in 4021


Article


where A0, A1, A2, and A3 are scalar quantities obtained from the curve fitting, t is the time, and τ1, τ2, and τ3 are decay time values for exponential components. The decay times (τ1, τ2, and τ3) with their respective percentage contributions of the different color components, viz., 375, 410, 460, 490, 530, 560, 610, 720, 780, and 850 nm, are given in Supporting Information Table 1. Because for most of the bands there is an overlap by either the preceding one or by the next band or by both, instead of getting a single lifetime we have obtained multiple lifetime values at all the emission maximums. Therefore, the lifetime value, which has the maximum contribution among all, can be assigned to the monitoring wavelength (emission peak under consideration). In this regard, from Supporting Information Table 1, the lifetime values of the respective emission wavelengths can be assigned as 14.23 μs for P1 (λem = 375 nm), 26.05 ms for P2 (λem = 410 nm), 11.90 μs for P3 (λem = 460 nm), 321.88 μs for P4 (λem = 490 nm), 126 μs for P5 (λem = 530 nm), 20 μs for P6 (λem = 560 nm), 170 μs for P7 (λem = 610 nm), 11.29 ms for P8 (λem = 720 nm), 51 μs for P9 (λem = 780 nm), and 191 μs for P10 (λem = 850 nm). 3.2.2. Time-Resolved Emission Spectroscopy. TRES spectra at different delay times, viz., 2 μs, 100 μs, 200 μs, 400 μs, 600 μs, 1 ms, 5 ms, 10 ms, and 50 ms, with a constant integration time of 80 ms are shown in Figure 10. In general, all the species are fully decayed at an average time that is approximately three times their respective lifetime (3τ). We can assume that at the very beginning of the decay time such as at 2 μs all the emitting colors due to different defect states will be present in the spectra because none of them have a lifetime below 2 μs (Supporting Information Table 1). The TRES spectra at 2 μs delay time is represented by Figure 10I. However, at longer time delays, such as at 100 μs, the species with lifetime values τ11= 14.23 μs (at λem = 375 nm), τ13 = 11.90 μs (at λem = 460 nm), and τ16 = 20 μs (at λem = 560 nm) will be completely decayed. This is also reflected by their decaying intensity at and around these wavelengths in Figure 10II at 100 μs delay time. Here, the spectra consist of other species except those with above-mentioned lifetime values. The other two species with slightly higher lifetime values such as τ19 = 51 μs (at λem = 780 nm) and τ110 = 36 μs (at λem = 850 nm) also started decaying in this time region (100 μs), and at longer delay times such as at 200 and 400 μs, they completely disappeared as seen in Figure 10III,IV. The next species in Supporting Information Table 1 with comparably higher lifetime value is that with τ25= 126 μs (at λem = 530 nm), and if we go to higher decay times such as ∼400 and 600 μs as represented by Figure 10IV,V, then a disappearance of the spectra in the 530 nm region was also observed. Therefore, we can say that in the time region of 2 μs −100 μs, the decaying species are those with lifetime values τ13 = 11.90 μs, τ11= 14.23 μs, τ16 = 20 μs, τ110 = 36 μs, and τ19 = 51 μs. Above 100 μs delay time and in the time region 200−400 μs, these species are completely decayed, and the new decaying species is τ25= 126 μs. Now if we subtract the TRES spectra at 2 μs delay time with that at 100 μs delay time, then it will give a spectra of mixed emission at 375, 460, 530, 560, 780, and 850 nm as shown in Figure 11A. In a similar manner, if we subtract the TRES at 200 μs delay time with that at 400 μs delay time, then we get a spectra consisting of emission at 530 nm as shown in Figure 11B. It is to be noted that in this time region (200−400 μs), along with the 530 nm emitting species, another two species with higher lifetimes τ27= 170.04 μs (at λem = 610 nm) and τ24= 321.88 μs (at λem = 490 nm) also started decaying, which can be seen by their weak emission peak at 610

Figure 7. Emission spectra of the MAS compounds with different Mg/ Mg Mg Mg 1 0.98 1 Al compositions: (a) Al = 1.96 , (b) Al = 2 , and (c) Al = 2 with λex = 250 nm.

Figure 8. Emission spectra of the MAS compounds observed after annealing in (a) vacuum and (b) air atmospheres.

vacuum atmosphere. The detailed explanation of these emission behaviors with different compositions and different annealing atmospheres will be given later in this report. Figure 9 shows the luminescence decay profile of the MAS compounds prepared at 600 °C with λex = 250 nm and at different emission wavelengths, viz., λem = 410, 460, 530, and 720 nm. The decay curves recorded at 375,490, 560, 610, 780, and 850 nm emission peaks are displayed in Figure S5. All the decay curves were found to be multiexponential and could be fit into a sum of two or three exponentials. The decay curves at 400, 530, 720, 780, and 850 nm were fitted using the following biexponential decay. I(t ) = A 0 + A1 exp( −t /τ1) + A 2 exp( −t /τ 2)

(4)

whereas the decay curves at 460, 495, 570, and 610 nm were best fitted using the triexponential decay I(t ) = A 0 + A1 exp( −t /τ1) + A 2 exp( −t /τ 2) + A3 exp( −t /τ 3)

(5) 4022


Article


Figure 9. PL decay curves of MAS compound prepared at 600 °C with λex = 250 nm and λem = (a) 410, (b) 460, (c) 530, and (d) 720 nm.

i.e., τ28= 11.29 ms (at λem = 720 nm) and τ22 = 26.05 ms (at λem = 410 nm). As shown in Figure 10VI,VII,VIII,IX, after a delay time of about 1 ms, all the TRES consists of two prominent peak; one at 410 nm and other at 720 nm. However, at higher delay time such as at 10 ms, the intensity of the 720 nm peak kept on decreasing and disappeared at 50 ms delay time. After 50 ms delay, the spectra consists of a single component with τ22

and 490 nm in Figure 11B. For more clearance, we have provided the Gaussian fit of TRES spectra (Figure 11B) in the time region 200−400 μs in Figure S6, which clearly shows the existence of these two components. When we subtracted Figure 11B from Figure 11A, we observed a spectra that consists of emission at 375, 460, 560, 780, and 850 nm, as shown by Figure 11C. Next two species are with relatively higher lifetime values, 4023


Article


Figure 10. Time-resolved emission spectra (TRES) at different delay times.

space group Fd3̅m (Oh7). Crystal structure of normal spinel is represented by lattice constant (a0) and oxygen parameter (u). In the normal spinel type, Al3+ and Mg2+ ions are in octahedral (local point group symmetry D3d) and tetrahedral (local point group symmetry Td) coordination, respectively. In this spinel type, our DFT calculated a0, u, Mg−O and Al−O bond lengths are 8.16 Å, 0.263, 1.96, and 1.94 Å, respectively. On the other hand, in the inverse spinel type, Mg2+ and Al3+ ions occupy the octahedral sites in equal proportions. Our DFT calculated equilibrium volume of inverse phase is 9.4 Å3 (per unit cell) lower compared to normal phase. To study the effect of oxygen vacancy induced defects in normal spinel, an O atom was removed from the unit cell, situating at 0.5, 0.5, 0.75 position (in direct lattice coordinate), which is in first nearest neighbor to the Mg atom situating at 0.375, 0.375, 0.875 position and to the Al atom situating at 0.75, 0.5, 0.75 position respectively (in direct lattice coordinate). Similarly, for inverse spinel, an O atom was removed situating at 0.514, 0.51089, 0.7556 position (in direct lattice coordinate), which is in first nearest neighbor to the Mg atom situating at 0.5, 0.5, 0.5 position and to the Al atom situating at 0.75, 0.496, 0.749 position respectively (in direct lattice coordinate). 3.3.1. Normal Spinel. To investigate the change in electronic structure of MgAl2O4 normal spinel with the presence of oxygen defect, at first the total and angular momentum decomposed density of states (DOS) of pure normal spinel structure are calculated and plotted in Figure 12a. Lower part of the valence band (VB) is mainly composed of s-states of Al as well as Mg and upper part of VB comprises of p-states of Al, Mg and O. On the other hand, lower part of the conduction band (CB) is contributed by s and p states of Mg majorly as well as by s and p states of Al. The DFT calculated electronic band gap is 6.0 ev, which is lower compared to experimentally reported value of 7.8 eV (direct band gap at Γ point) measured from optical reflectivity experiment.47 Underestimation of band gap is a well-known limitation of the GGA.48,49 In this study we

Figure 11. Time-resolved emission spectra (TRES) of the respective species.

= 26.05 ms emitting at 410 nm. Subsequently, Figure 10IX represents the TRES of the species with lifetime, τ22 = 26.05 ms. As we go further to higher delay time, the intensity kept on decreasing. Now as we subtracted the TRES at 5 ms delay time with that at 50 ms, we got the TRES of the species with τ28= 11.29 ms as shown in Figure 11D, emitting at 720 nm. Thus, TRES study confirmed the presence of all these emission components as observed from Gaussian fitted spectra in Figure 3. 3.3. Electronic Structure and Band Gap Energy. MgAl2O4 spinel has a face-centered-cubic structure having 4024


Article


Figure 12. Total and angular momentum decomposed density of states (DOS) of MgAl2O4 normal spinel (a) without any oxygen vacancy, (b) with neutral oxygen vacancy (VO0), (c) with +1-charged O defect (VO+1), and (d) with +2-charged O defect (VO+2). Vertical lines at zero energy represent Fermi energy. Impurity states are marked by arrows in this figure.

inserted in the band gap. Presence of defect states just below the CB minima reduces the band gap by 0.8 eV. For this case, the energy difference between VB maximum and CB minimum (electronic band gap) is now 5.2 eV. Figure 12d shows the total and angular momentum decomposed DOS due to presence of O vacancy with charge +2 (VO+2). Overall nature of the VB remains unaltered but an impurity band appears above VB maximum in the band gap. The Fermi level is situated just above the VB maximum. An impurity state is present 3.4 eV above the VB maximum. Impurity levels are composed of s, p states of Mg 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 5.7 eV. 3.3.2. Inverse Spinel. Figure 13 (a) represents electronic DOS of defect free inverse spinel. Overall DOS features of the defect free inverse spinel is very similar to the defect free normal spinel with an exception of band gap width 4.95 eV, which is 1.05 eV lower compared to normal spinel. The reduction of electronic band gap is expected due to the atomic disorder present in the inverse spinel structure. Figure 13b−d shows electronic DOS of inverse spinel with neutral O defect (VO0), +1-charged O defect (VO+1) and +2charged O defect (VO+2), respectively. Overall DOS features of inverse spinel with different O defects are very similar to that of normal spinel with an exception of location of impurity states. For VO0, impurity band appears 2.2 eV ahead of VB maximum in the band gap and just below the Fermi level. This impurity band is mainly contributed by the Mg s and p and Al p states. For VO+1, impurity state appears due to spin-up component 1.8

focus on the change of the band gap due to presence of O vacancy (neutral and charged), so it is expected to cancel the GGA calculated band gap error during comparison. Figure 12b shows the total and angular momentum decomposed DOS due to presence of neutral O vacancy. The spin-up and spin-down components are shown separately in upper and lower panels, respectively. Overall nature of the VB remains unaltered but an impurity band appears 2.4 eV ahead of VB in the band gap and just below the Fermi level. This impurity band is mainly contributed by the Mg-s, p and Al-p states. Moreover, an impurity band also appears just below the CB, which is mainly contributed by Mg-s, p and Al-p states. The overall band gap was thus reduced by 0.5 eV compared to pure normal spinel. For this case, the energy difference between VB maximum and CB minimum (electronic band gap) now is 5.5 eV. Figure 12c shows the total and angular momentum decomposed DOS due to presence of O vacancy with charge +1 (VO+1). The spin-up and spin-down components are shown separately in upper and lower panels, respectively. Overall nature of the VB remains unaltered but two impurity bands appears above VB maximum in the band gap. Impurity levels are composed of s, p states of Mg and p states of Al in the spinup and spin-down components. Impurity state generated due to spin-up components is filled with electrons as it is situated just below the Fermi energy whereas impurity state generated due to spin-down components is empty as it is situated above the Fermi energy. Impurity bands appear just below the CB minimum are similarly composed of s, p states of Mg and p states of Al. Among them, Mg-p and Al-p states are deeply 4025


Article


Figure 13. Total and angular momentum decomposed density of states (DOS) of MgAl2O4 inverse spinel (a) without any oxygen vacancy, (b) with neutral oxygen vacancy (VO0), (c) with +1-charged O defect (VO+1), and (d) with +2-charged O defect (VO+2). Vertical lines at zero energy represent Fermi energy.

Figure 14. Overall summary of the location of the defect states arises due to VO0, VO+1, and VO+2 in the normal and inverse spinel of MgAl2O4. Lightgray bands are filled with electrons, and white bands are empty.

eV ahead of VB maximum and Fermi energy is situated just after this band. Another empty impurity band appears 2.9 eV ahead of VB maximum. For VO+2, impurity state appears just below (0.5 eV) the CB minimum is due to spin-up and spindown component. Fermi energy is situated just after VB maximum. In all these cases, overall electronic band gap remains almost unaltered.

Therefore, the following summary can be depicted pictorially in Figure 14, as observed from the DOS analysis based on DFT calculations on both normal and inverse spinel, with different kind oxygen vacancies having different charges. The calculations show a decrease in band gap energy for the inverse spinel compared to normal spinel, both with and without defect states. Since the oxygen vacancy (OV) can exist in all the 4026


Article


phase of MgAl2O4 (where both of the Al atoms have octahedral co-ordination), the blue shifting of the F center emission surrounding the Al−Oh (Oh = octahedral) compare to the Mg− Oh can be explained by considering the higher band gap energy of normal phase in compare to the inverse phase, shown in Figure 14. Longer lifetime value for F center surrounding Al− Oh than Mg−Oh can be explained on the basis of higher charge of Al3+ ion. The emission at 560 nm (P6) can be attributed to the F22+ centers52,53 surrounding the octahedral Al3+ ions which are nothing but a pair of F+ centers. The emission around 460 nm (P3) is due to the charge transfer transition between Al3+ at octahedral sites and its surrounding O2− ions33,34 and being allowed transition, the lifetime (τ12 = 11.90 μs) is also very less. The green emission peaks at λmax ≈ 490 nm (P4) can be assigned to transition of an electron to a hole trapped at Mg ion vacancy54 whereas the yellow-orange emission peak (P7) at λem = 610 nm can be assigned to the oxygen vacancies resulting from the existence of MgAl antisites,32 respectively. The most important task is to assign the P8 (720 nm), P9 (780 nm) and P10 (850 nm) emission components. Since the P10 (850 nm) band was found to disappear at higher annealing temperature (Figure 3), we believe this emission originates from some kind of surface defect state which is annealed at higher annealing temperature. Obviously these longer wavelength (P8 and P9) emissions are associated with some kind of defects which exist deeply inside the band gap. Kim et al. has also shown similar red emission at 680 nm in isostructural ZnGa2O4.33 They have shown that this emission with long decay time will only occur in case of distorted octahedral site (where there is an oxygen vacancy surrounding the Ga atom in octahedral site) and the electronic transition will be from O2− ion to the singly charged oxygen vacancy (VȮ or VO+1) confirmed by EPR spectroscopy. We have also observed an asymmetric signal at g ≈ 1.9967 in EPR spectroscopy as shown in Figure 15, due to paramagnetic

charge states (0, +1, and +2), all of these defects states arising due to different charges will have contribution to the emission profile. As shown in Figure 14, both for normal and inverse phase, two filled states due to the neutral (VO0) and singly positive charge OVs (VO+1) have appeared above the valence bond. On the other hand two shallow vacant states (they are very close to each other and thereby showing a single band) just below the conduction band have also appeared due to the singly and doubly positive charged OVs. In the middle of the band gap two vacant deep defect states due to VO+1 and VO+2 are found to be exist in case of normal phase. In case of inverse phase, the deep states due to VO+2 is found to be very close to the shallow states and almost merge with them, showing as a single broad band. This representation of defect states will be considered as basis to understand the PL profile. 3.4. Explanation of the Origin of the Different Color Components and Their Tunability with a Proposed Model. 3.4.1. Origins of Different Color Components. Next task is to explain the origin of each of these different defect emissions. Since, in the reported annealing temperature range, MAS mostly used to be formed as a mixed spinel, both Al and Mg atoms have tetrahedral and octahedral co-ordination and their respective percentage (at octahedral and tetrahedral coordination) will also vary, depending upon the annealing temperature. Therefore, a control over the inversion rate of the Mg2+ and Al3+ cations inside the spinel crystallographic network can be optimized by varying the synthesis condition and annealing temperature.29 As in the mixed spinel, fraction of the Mg2+ ions are in octahedral co-ordination (inverse phase), it is interesting to compare the emission characteristics of MAS in the inverse phase with that of MgO; where all the Mg2+ have octahedral co-ordination.50,51 This will help to understand the luminescence phenomenon which occurs surrounding the Mg2+ co-ordination environment. On the similar line; comparison of PL of MAS normal phase with α-Al2O3 gives information related to emission behavior surrounding the octahedral Al atoms in the mixed spinel.17 The emission at 375 nm (P1) cannot be due to exciton emission due to large band gap of MAS.21 However, it is in close resemblance with the F+ center emission in MgO.16 Its lifetime (τ11= 14.23 μs) value also indicates it to be due to singly ionized oxygen vacancy (VO+1) or F+ center49 and therefore P1 (λmax ≈ 375 nm) is attributed to the 2T1u-2A1g transitions of F+ center around the octahedral Mg atom. Similarly P5 (λmax ≈ 530 nm) can be attributed to the Fcenter surrounding the octahedral Mg atom.50 From Figure 14 also, it is clearly visible that the filled states due to the neutral F centers (VO0) is placed above the states due to the singly charged F+ centers (VO+1). The long-lived decay τ24= 126 μs (at λem = 530 nm) of this F-center compare to the F+ center at 375 nm can be explained in terms of forbidden transition of the F centers. The electronic structure of the F center can be treated as a helium atom where the ground-state configuration (ls)2 corresponds to the term 1SO and the excited states can be generated by promoting one electron into a 2s or a 2p state, resulting excited-state configurations 1s2s or 1s2p. These configurations correspond to both singlet and triplet states (1S, 3S, 1P, and 3P). The emission of F center is due to a 3P−1S transition that is forbidden.51 The emission at 410 nm (P2) can be attributed to the F center associated with the distorted Al octahedral sites. The long lifetime value τ21 = 26.05 ms (at λem = 410 nm) is also very similar to the one observed by Itou et al. in α-Al2O3.17 Since the co-ordination environment of the Al atoms in α-Al2O3 closely resemble with that in the normal

Figure 15. EPR spectra of MAS samples at different annealing temperatures.

VȮ . As stated earlier in the summery of DFT calculation (Figure 14), inside the band gap of the MAS compound, two vacant deep defect states and two vacant shallow defect states are present due to the singly and doubly positive charged oxygen vacancies. Electrons from the conduction band may also be trapped into these vacant shallow and deep states. Since the energy gap between these shallow and deep states are very close to the observed emission wavelengths in the red−NIR region, we believe transition between these states are responsible for 4027


Article


can be attributed to tuning of distortion around octahedral sites of the spinel. Now the explanation of emission behavior with different compositions and different annealing atmospheres can be given from the above conclusion. As stated earlier that the emission in the 410, 530, 720, and 780 nm regions are due to different type of oxygen vacancies, annealing of the MAS samples in vacuum atmosphere will definitely increase the intensity of these regions. This is what we have observed for the emission in case of the vacuum annealed MAS sample, shown in Figure 8. Now the explanation for the emission characteristics of different compositions of Mg and Al in the spinel such as with Mg/Al molar ratio 0.98/2.00 and 1/1.96 can be given on the basis of cationic vacancies. As with these composition ratio, the MAS compounds will be deficient of the Mg2+ (for 0.98/ 2.00) and Al3+ (for 1/1.96) cations, the new negatively charged cationic vacancies such as VMg// or VAl/// will lead to formation of positively charged oxygen vacancies for charge balance of the system. This is why an increase in intensity was observed at 410, 530, 720, and 780 nm regions for both Mg2+ (for 0.98/ 2.00) and Al3+ (for 1/1.96) cations-deficient MAS compounds in Figure 7. Because these cationic vacancies will lead to formation of oxygen vacancies in their nearest neighbor, a deficiency in Mg2+ ion from its lattice site will initiate the vacancy of the oxygen atoms from the Al3+ coordination and vice versa since these oxygen atoms are shared by both Mg2+ and Al3+ ions for their respective coordination. This is also reflected in the increasing intensity at 410 nm (Figure 7) with Mg 0.98 = 2 , which is due to F center emission surrounding the Al

the P8 (720 nm) and P9 (780 nm) bands. As seen in Figure 14, the vacant deep defect states due to VO+1 (or VȮ ) is placed below the vacant states due to VO+2. Therefore, the P8 (720 nm) band can be assign to the transition of electron trapped at the shallow states to the vacant deep sates arising due to singly charged paramagnetic VO+1. On the other hand the P9 (780 nm) band can be assigned to the transition of an electron trapped at the deep states of VO2+ to the vacant deep sates of paramagnetic VO+1. Now in the next section we will explain the annealing temperature effect in the emission profile of MAS compound. 3.4.2. Tunability with varying Annealing Temperature, Annealing Atmosphere and Mg/Al Composition. As shown in annealing temperature variation PL spectra in Figure 3, the intensity of the 720 and 780 nm band were increasing constantly, as the annealing temperature was increased from 600 to 1000 °C. We have already stated in the previous section that these two bands are linked to the paramagnetic singly charged oxygen vacancy, VO+1 or (VȮ ). Therefore, the concentration of VȮ has also to be increased with higher annealing temperature. One of the most common ways to identify and quantify the paramagnetic species like VȮ is Electron Paramagnetic Resonance (EPR) spectroscopy. Figure 15 showed the EPR spectrum of MAS annealed at different temperature. All the compounds showed an asymmetric signal at g ≈ 1.9967 which can be attributed to the paramagnetic singly charged oxygen vacancy (VȮ ).49,55 It is worth to be noted here that all the positively charged defect centers will get stabilized through formation of a nearby negatively charged cationic vacancy such as VMg//, VAl///, MgAl/ and Oi// etc. The figure also showed an increase in intensity of the resonance signal with increasing annealing temperature. This confirms that, this EPR signal which is being arisen out due to VȮ surrounding the distorted Al−Oh and Mg−Oh sites must be responsible for the red−NIR emission.33 In general, these types of spinel compounds synthesized at the mentioned annealing temperatures are mixed spinel in nature and they possess both the inverse and normal phase with their respective percentage. At low annealing temperature (≈ 600 °C), percentage of inverse phase is more as reflected by higher inverse parameter whereas at higher annealing temperature (≈ 1000 °C) percentage of normal phase is more as reflected by lower inverse parameter.19Since the sharing of the octahedral (Oh) sites by Al atom will be doubled in the normal phase than in the inverse phase, a transformation of inverse to normal form with annealing temperature will also be accompanied by a change in coordination of the tetrahedral (AlO4) to octahedral [AlO6]. Thus, the co-ordination around Al3+ ions will be 4≤ Alcoordination ≤6, through this path of transformation. Hence the Al atoms, which are newly placed at the octahedral sites, will not have perfectly octahedral symmetry or some of them will have distorted octahedral symmetry (as the co-ordination of Al will be ≤6). Reverse is also true for Mg atom. There will be a change in coordination of the octahedral [MgO6] to tetrahedral (MgO4) through this path of transformation and the perfectly octahedral [MgO6] will now become distorted. Thus, some oxygen vacancy will always be associated with the Al3+ and Mg2+ions at distorted Oh sites. Henceforth as the temperature is increased from 600 to 1000 °C, distortion around the Al−Oh and Mg−Oh sites is also increased, which is reflected in the sharp increase in intensity of the red−NIR band at higher temperature and also in EPR signal intensity. Therefore, the tuning of emission color of MAS compound

Mg

1

octahedral Al atom. Similarly with Al = 1.96 , an increase in intensity was also observed at 530 nm, which is due to the F center emission surrounding the Mg2+ atom. Again because the red−NIR emission is linked to the VO+1 surrounding the distorted Al−Oh and Mg−Oh sites, the intensity was increased in case of both compositions. However, a deficiency of Al atom Mg 1 with Al = 1.96 is more efficient in creating VO+1, as reflected by the higher intensity of the red−NIR emission compared to that Mg 0.98 with Al = 2 . 3.4.3. Photoconversion of F to F+ Centers: ExcitationTunable Emission Characteristics. The results of emission characteristics at different excitation wavelengths in Figure 5 clearly indicate that some kind of trapping−de-trapping mechanism is involved with the electrons within the different electron-trapped states inside the band gap. The continuous increase in intensity of the P8 = 720 nm band up to 250 nm excitation wavelength followed by a decay (Figures S1−S3) indicates that the electrons started getting more and more trapped into the states responsible for P8 = 720 nm emission as the excitation wavelength was tuning near to 250 nm. On the other hand the disappearance of the P8 = 720 nm band at and above 300 nm and the existence of only P9 = 780 nm band (Figure S4) indicates that de-trapping of the electrons from the P8 = 720 nm states has occurred followed by re-trapping of the same electrons into the states responsible for P9 = 780 nm band. The maximum intensity of these bands at 250 nm excitation wavelength indicated that a photoionization process might be involved with the 250 nm excitation wavelength, from which the free electrons are coming and being trapped into the different defect states in the band gap. 4028


Article


sates are energetically very close. Therefore, at room temperature the electrons from the conduction band may go to the shallow trap states and thereby increasing the red−NIR emission intensity compared to the BGY region. However, as the experimental temperature is reduced, this process becomes less pronounced and the slight increase in intensity in the redNIR region is due to quenching of the nonradiative pathways. From DFT calculation, it is clear that for both inverse and normal spinel compound that the band gap is more than 5.5 eV. The experimentally reported band gap value is even more than 6 eV (200 nm).47 Because we have not observed any excitation peak near to 200 nm, the electron transfer from valence band (VB) to conduction band (CB) is not possible in the present case. Other possibilities are electron transfer from VB to the defect states such as shallow and deep-trap states. VB to shallow-trap state transfer is not possible because they are energetically very close to the CB and therefore have a higher energy difference. Electron transfer from VB to the vacant deep-trap states is also less probable because they are placed around 3−4 eV (more than 300 nm) away from the VB and also no prominent excitation peak at or above 300 nm was observed. Transition of the trapped electrons in the defect states to the VB is also ruled out because we have not observed any emission with wavelength less than 350 nm in the UV region, whereas these trap states are placed more than 4 eV (309 nm) away from the VB.

A parallel comparison can be drawn with emission properties of MgO compound, where a photoconversion of F to F+ center at ∼250 nm is reported by various authors.56,57 At 250 nm excitation, the F center will be converted to F+ centers following the equation F + ℏυ ↔F+ + e−. Therefore, we believe that in MAS, the F centers surrounding the Mg atom (distorted octahedral) undergo a photoconversion at 250 nm excitation wavelength. The electron may be promoted to the conduction band or may be trapped into the vacant shallow and deep defect states arising as a result of VO+1 and VO+2, as observed from DFT calculation in Figure 14. 3.4.4. Proposed Model. To give a clear explanation about the trapping−de-trapping mechanism, a proper model has now been proposed in Figure 16 on the basis of all the theoretical

4. CONCLUSIONS We have successfully demonstrated the photoluminescence behavior of magnesium aluminate spinel (MAS) compound, synthesized through sol−gel−combustion route at different annealing temperatures. The resulting MAS compound was found to be a potential candidate for optical based thermal sensor material owing to its tunable emission characteristic in the blue-green and red−NIR region with varying annealing temperature during synthesis and in the low-temperature measurements. DFT-based calculations were carried out on both pure as well as different neutral and charged oxygen vacancy (VO0, VO+1, and VO+2) introduced phases of normal and inverse spinel. The calculations help to characterize different defects states present inside the band gap region of MAS that are responsible for the emission behavior. Gaussian fitting of the emission spectra showed the presence of 10 different type of color components, which can be linked to different types of defect centers, such as F, F2, F+, F22+ and different shallow and deep defects, within the band gap of the materials. The existence of these different defect states were supported by their lifetime values and time-resolved emission spectra (TRES). Annealing temperature tunability of the emission behavior can be explained by explaining the changing concentration of singly charged oxygen vacancy (or VO+1) surrounding the distorted octahedral sites. On the basis of our DFT calculated results and experimental observations, a suitable model has been proposed in order to explain the tunable emission behaviors at low temperatures and with different excitation wavelengths. At room temperature, the electrons from conduction band are found to be exchanged with the shallow-trap states, whereas at low temperature, the exchange interaction was ceased. In the case of excitation wavelength variation, it was found that a photoconversion process was involved at 250 nm excitation wavelength. The released electrons in this process were found to be trapped into both shallow and deep states. At higher excitation wavelengths

Figure 16. Proposed model for trapping−de-trapping mechanism.

and experimental results observed until now. In this figure, a photoionization of the F to F+ center at 250 nm excitation wavelength is shown following the equation F + ℏυ ↔ F+ + e−. The electron released in this photoconversation process may either go to the conduction band or may be trapped into the shallow and deep-trap levels, as shown by green arrows. The P8 = 720 nm emission band is shown by transition of electron trapped at shallow states to the vacant VO+1 at distorted octahedral sites, whereas in the case of the P9 = 780 nm band, the transition is from a deep-trap level. Now at 250 nm excitation, both of these states are populated by trapping of electrons coming from the photoconversion of F centers and thereby sharp increase in intensity in the red−NIR region was observed. However, when the excitation wavelength was tuned to 310 nm or above, the electrons trapped in the shallow states were getting de-trapped followed by re-trapping in the deep states responsible for 780 nm emission. This explains the presence of two emission bands at the 250 nm excitation wavelength, whereas at 310 nm excitation, only one band is exist. The low-temperature emission characteristic shown in Figure 6 can also be explained from this model. We can see from Figure 6 that at room temperature the intensity of the red−NIR region is more comparable to that of the blue-green region whereas at low temperature the trend is reversed. As observed from the DFT calculation, the conduction band and shallow 4029


Article


(10) Gaur, U. K.; Kumar, A.; Varma, G. D. Fe-induced Morphological Transformation of 1-D CuO Nanochains to Porous Nanofibers with Enhanced Optical, Magnetic and Ferroelectric Properties. J. Mater. Chem. C 2015, 3, 4297−4307. (11) Park, C.; Lee, J.; Chang, W. S. Geometrical Separation of Defect States in ZnO Nanorods and Their Morphology-Dependent Correlation between Photoluminescence and Photoconductivity. J. Phys. Chem. C 2015, 119, 16984−16990. (12) Li, J.; Wang, Z.; Zhao, A.; Wang, J.; Song, Y.; Sham, T. K. Nanoscale Clarification of the Electronic Structure and Optical Properties of TiO2 Nanowire with an Impurity Phase upon Sodium Intercalation. J. Phys. Chem. C 2015, 119, 17848−17856. (13) Kar, A.; Kundu, S.; Patra, A. Surface Defect-Related Luminescence Properties of SnO2 Nanorods and Nanoparticles. J. Phys. Chem. C 2011, 115, 118−124. (14) Saha, S.; Das, S.; Sen, D.; Ghorai, U. K.; Mazumder, N.; Gupta, B. K.; Chattopadhyay, K. K. Bane to Boon: Tailored Defect Induced Bright Red Luminescence from Cuprous Iodide Nanophosphors for On-Demand Rare Earth free Energy Saving Lighting Applications. J. Mater. Chem. C 2015, 3, 6786−6795. (15) Ikeda, S.; Uchino, T. Temperature and Excitation Energy Dependence of the Photoionization of the F2 center in α-Al2O3. J. Phys. Chem. C 2014, 118, 4346−4353. (16) Pikhitsa, P. V.; Kim, C.; Chae, S.; Shin, S.; Jung, S.; Kitaura, M.; Kimura, S.; Fukui, K.; Choi, M. Two-Band Luminescence from an Intrinsic Defect in Spherical and Terraced MgO Nanoparticles. Appl. Phys. Lett. 2015, 106, 183106. (17) Itou, M.; Fujiwara, A.; Uchino, T. Reversible Photoinduced Interconversion of Color Centers in α-Al 2O3 Prepared Under Vacuum. J. Phys. Chem. C 2009, 113, 20949−20957. (18) Lee, K. H.; Crawford, J. H. Electron Centers in Single-Crystal Al2O3. Phys. Rev. B 1977, 15, 4065−4070. (19) Draeger, B. G.; Summers, G. P. Defects in Unirradiated Al2O3. Phys. Rev. B: Condens. Matter Mater. Phys. 1979, 19, 1172−1177. (20) White, K. W.; Kelkar, G. P. Evaluation of the Crack Face Bridging Mechanism in a MgAl2O4 Spinel. J. Am. Ceram. Soc. 1991, 74, 1732−1734. (21) Ganesh, I. A Review on Magnesium Aluminate (MgAl2O4) Spinel: Synthesis, Processing and Applications. Int. Mater. Rev. 2013, 58, 63−112. (22) Kim, Y.; Lim, J.; Kang, S. Thermodynamic Investigation of Ti doping in MgAl2O4 Based on the First-Principles Method. J. Mater. Chem. C 2015, 3, 8970−8978. (23) Wiglusz, R. J.; Boulon, G.; Guyot, Y.; Guzik, M.; Hreniak, D.; Strek, W. Structural and Spectroscopic Properties of Yb3+-Doped MgAl2O4 Nanocrystalline Spinel. Dalton Trans. 2014, 43, 7752−7759. (24) Sickafus, K. E.; Wills, J. M.; Grimes, N. W. Structure of Spinel. J. Am. Ceram. Soc. 1999, 82, 3279−3292. (25) Morita, K.; Kim, B. N.; Yoshida, H.; Hiraga, K.; Sakka, Y. Spectroscopic Study of the Discoloration of Transparent MgAl2O4 Spinel Fabricated by Spark-Plasma-Sintering (SPS) Processing. Acta Mater. 2015, 84, 9−19. (26) Ting, C.-J.; Lu, H.-Y. Defect Reactions and the Controlling Mechanism in the Sintering of Magnesium Aluminate Spinel. J. Am. Ceram. Soc. 1999, 82, 841−848. (27) Schmocker, U.; Boesch, H. R.; Waldner, F. A Direct Determination of Cation Disorder in MgAl2O4 Spinel by ESR. Phys. Lett. A 1972, 40, 237−238. (28) Sawai, S.; Uchino, T. Visible Photoluminescence from MgAl2O4 Spinel with Cation Disorder and Oxygen Vacancy. J. Appl. Phys. 2012, 112, 103523. (29) Dwibedi, D.; Avdeev, M.; Barpanda, P. Role of Fuel on Cation Disorder in Magnesium Aluminate (MgAl2O4) Spinel Prepared by Combustion Synthesis. J. Am. Ceram. Soc. 2015, 98, 2908−2913. (30) Mathur, S.; Veith, M.; Haas, M.; Shen, H.; Lecerf, N.; Huch, V.; Hufner, S.; Haberkorn, R.; Beck, H. P.; Jilavi, M. Single-Source Sol-Gel Synthesis of Nanocrystalline ZnAl2O4: Structural and Optical Properties. J. Am. Ceram. Soc. 2001, 84, 1921−1928.

(>300 nm), the electrons trapped at shallow states were detrapped followed by re-trapping in the deep states.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b11822. Instrumental technique, emission spectra of MAS derived after annealed at at 600, 800, and 1000 °C at different excitation wavelengths, Gaussian fit of the emission spectra in red−NIR region at 250 and 330 nm excitation wavelengths, PL decay curves monitored at λex = 250 nm and λem = 375, 490, 560, 610, 780, and 850 nm, and Gaussian fit of TRES spectra in the time region 200−400 μs. (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] or [email protected]. Tel.: +91-22-25590715/0636. Fax: +91-22-25505151. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We are grateful to Dr. P. K. Pujari, Head, Radiochemistry Division at Bhabha Atomic Research Centre, for his keen interest and encouragement during the course of this work.

■

REFERENCES

(1) Bak, T.; Li, W.; Nowotny, J.; Atanacio, A. J.; Davis, J. Document Photocatalytic Properties of TiO2: Evidence of the Key Role of Surface Active Sites in Water Oxidation. J. Phys. Chem. A 2015, 119, 9465− 9473. (2) Gonzalez, J. W.; Rosales, L.; Pacheco, M.; Ayuela, A. Electron Confinement Induced by Diluted Hydrogen-Like Ad-Atoms in Graphene Ribbons. Phys. Chem. Chem. Phys. 2015, 17, 24707−24715. (3) Ito, Y.; Christodoulou, C.; Nardi, M. V.; Koch, N.; Kläui, M.; Sachdev, H.; Mullen, K. Tuning the Magnetic Properties of Carbon by Nitrogen Doping of Its Graphene Domains. J. Am. Chem. Soc. 2015, 137, 7678−7685. (4) Chaudhari, N.; Mandal, L.; Game, O.; Warule, S.; Phase, D.; Jadkar, S.; Ogale, S. Dramatic Enhancement in Photoresponse of βIn2S3 through Suppression of Dark Conductivity by Synthetic Control of Defect-Induced Carrier Compensation. ACS Appl. Mater. Interfaces 2015, 7, 17671−17681. (5) Kramer, N. J.; Schramke, K. S.; Kortshagen, U. R. Plasmonic Properties of Silicon Nanocrystals Doped with Boron and Phosphorus. Nano Lett. 2015, 15, 5597−5603. (6) Sachet, E.; Shelton, C. T.; Harris, J. S.; Gaddy, B. E.; Irving, D. L.; Curtarolo, S.; Donovan, B. F.; Hopkins, P. E.; Sharma, P. A.; Sharma, A. L.; et al. Dysprosium-Doped Cadmium Oxide as a Gateway Material for Mid-Infrared Plasmonics. Nat. Mater. 2015, 14, 414−420. (7) Amisse, R. M.; Sougrati, T.; Stievano, L.; Davoisne, C.; Dražič, G.; Budič, B.; Dominko, R.; Masquelier, C. Singular Structural and Electrochemical Properties in Highly Defective LiFePO4 Powders. Chem. Mater. 2015, 27, 4261−4273. (8) Ji, C.; Zhang, Y.; Zhang, T.; Liu, W.; Zhang, X.; Shen, H.; Wang, Y.; Gao, W.; Wang, Y.; Zhao, J.; et al. Temperature-Dependent Photoluminescence of Ag2Se Quantum Dots. J. Phys. Chem. C 2015, 119, 13841−13846. (9) Chow, P. K.; Jacobs-Gedrim, R. B.; Gao, J.; Lu, T.-M.; Yu, B.; Terrones, H.; Koratkar, N. Defect-Induced Photoluminescence in Monolayer Semiconducting Transition Metal Dichalcogenides. ACS Nano 2015, 9, 1520−1527. 4030


Article

The Journal of Physical Chemistry C (31) Pathak, N.; Gupta, S. K.; Sanyal, K.; Kumar, M.; Kadam, R. M.; Natarajan, V. Photoluminescence and EPR Studies on Fe3+ Doped ZnAl2O4: An Evidence for Local Site Swapping of Fe3+ and Formation of Inverse and Normal phase. Dalton Trans. 2014, 43, 9313−9323. (32) Cornu, L.; Gaudon, M.; Jubera, V. ZnAl2O4 as a Potential Sensor: Variation of Luminescence with Thermal History. J. Mater. Chem. C 2013, 1, 5419−5428. (33) Kim, J. S.; Kang, H. I.; Kim, W. N.; Kim, J. I.; Choi, J. C.; Park, H. L.; Kim, G. C.; Kim, T. W.; Hwang, Y. H.; Mho, S. I.; Jung, M.-C.; Han, M. Color Variation of ZnGa2O4 Phosphor by ReductionOxidation Processes. Appl. Phys. Lett. 2003, 82, 2029−2031. (34) Zhang, Z.; Wang, J.; Nie, X.; Wen, T.; Ji, Y.; Wu, X.; Zhao, Y.; Chen, C. Near Infrared Laser-Induced Targeted Cancer Therapy Using Thermoresponsive Polymer Encapsulated Gold Nanorods. J. Am. Chem. Soc. 2014, 136, 7317−7326. (35) Zhou, J.; Sun, G.; Zhao, H.; Pan, X.; Zhang, Z.; Fu, Y.; Mao, Y.; Xie, E. Tunable White Light Emission by Variation of Composition and Defects of Electrospun Al2O3−SiO2 Nanofibers. Beilstein J. Nanotechnol. 2015, 6, 313−320. (36) Farooq, M.; Ramli, A.; Subbarao, D. Physiochemical Properties of γ-Al2O3-MgO and γ-Al2O3-CeO2 Composite Oxides. J. Chem. Eng. Data 2012, 57, 26−32. (37) Kirszensztejn, P.; Przekop, R.; Szymkowiak, A.; Maćkowska, E.; Gaca, J. Preparation of MgO-Al2O3 Binary Gel System with Mesoporous Structure. Microporous Mesoporous Mater. 2006, 89, 150−157. (38) Kresse, G.; Furthmueller, J. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 11169−11186. (39) Kresse, G.; Furthmueller, J. Efficiency of Ab-Initio Total Energy Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set. Comput. Mater. Sci. 1996, 6, 15−50. (40) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (41) Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 50, 17953−17979. (42) Monkhorst, H. J.; Pack, J. D. Special Points for Brillouin-Zone Integrations. Phys. Rev. B: Condens. Matter Mater. Phys. 1976, 13, 5188−5192. (43) Blö chl, P. E.; Jepsen, O.; Andersen, O. K. Improved Tetrahedron Method for Brillouin-Zone Integrations. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 49, 16223−16233. (44) Wiglusz, R. J.; Boulon, G.; Guyot, Y.; Guzik, M.; Hreniak, D.; Strek, W. Structural and Spectroscopic Properties of Yb3+ - doped MgAl2O4 Nanocrystalline Spinel. Dalton Trans. 2014, 43, 7752−7759. (45) Strȩk, W.; Dereń, P.; Jeżowska-Trzebiatowska, B. Optical Properties of Cr3+ in MgAl2O4 Spinel. Phys. B 1988, 152, 379−384. (46) Lim, J. H.; Kim, B. N.; Kim, Y.; Kang, S.; Xie, R. J.; Chong, I. S.; Morita, K.; Yoshida, H.; Hiraga, K. Non-Rare Earth White Emission Phosphor: Ti-doped MgAl2O4. Appl. Phys. Lett. 2013, 102, 031104. (47) Bortz, M. L.; French, R. H. Optical Reflectivity Measurements Using a Laser Plasma Light Source. Appl. Phys. Lett. 1989, 55, 1955− 1957. (48) Gupta, S. K.; Ghosh, P. S.; Pathak, N.; Arya, A.; Natarajan, V. Understanding the Local Environment of Sm3+ in Doped SrZrO3 and Energy Transfer Mechanism Using Time-Resolved Luminescence: A Combined Theoretical and Experimental Approach. RSC Adv. 2014, 4, 29202−29215. (49) Pathak, N.; Gupta, S. K.; Ghosh, P. S.; Arya, A.; Natarajan, V.; Kadam, R. M. Probing Local Site Environments and Distribution of Manganese in SrZrO3:Mn; PL and EPR Spectroscopy Complimented by DFT Calculations. RSC Adv. 2015, 5, 17501−17513. (50) Choudhury, B.; Basyach, P.; Choudhury, A. Monitoring F, F+ and F22+ Related Intense Defect Emissions from Nanocrystalline MgO. J. Lumin. 2014, 149, 280−286. (51) Lee, K. H.; Crawford, J. H. Luminescence of the F center in Sapphire. Phys. Rev. B: Condens. Matter Mater. Phys. 1979, 19, 3217− 3221.

(52) Springis, M. J.; Valbis, J. A. Visible Luminescence of. Colour Centers in Sapphire. Phys. Status Solidi B 1984, 123, 335−343. (53) Evans, B. D.; Stapelbroek, M. Optical Vibronic Absorption Spectra in 14.8 MeV Neutron Damaged Sapphire. Solid State Commun. 1980, 33, 765−770. (54) Martínez-Boubeta, C. M.; Martínez, A.; Hernández, S.; Pellegrino, P.; Antony, A.; Bertomeu, J.; Balcells, L.; Konstantinović, Z.; Martínez, B. Blue Luminescence at Room Temperature in Defective MgO Films. Solid State Commun. 2011, 151, 751−753. (55) Gupta, S. K.; Pathak, N.; Ghosh, P. S.; Rajeshwari, B.; Natarajan, V.; Kadam, R. M. Temperature Dependent Electron Paramagnetic Resonance (EPR) of SrZrO3. J. Magn. Magn. Mater. 2015, 391, 101− 107. (56) Edel, P.; d'Aubigne, Y. M.; Romestain, R.; Henderson, B.; Kappers, L. A. Photoluminescence Properties of Additively Coloured MgO. I. Effects of Uniaxial Stress and ODMR. J. Phys. C: Solid State Phys. 1979, 12, 5245−5253. (57) Kappers, L. A.; Hensley, E. B. F+↔ F Center Conversions in Magnesium Oxide. Phys. Rev. B 1972, 6, 2475−2477.

4031


An Insight into the Various Defects-Induced ... - ACS Publications

Recommend Documents