Oxygen-Vacancy-Induced Midgap States Responsible for the

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Oxygen-vacancy-induced midgap states responsible for the complex luminescence of the inverse spinel Mg(Mg,Sn)O4 Gaganpreet Behrh, Masahiko Isobe, Florian Massuyeau, Hélène Serier-Brault, Elijah E Gordon, Hyun-Joo Koo, Myung-Hwan Whangbo, Romain Gautier, and Stéphane Jobic Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b03906 • Publication Date (Web): 03 Jan 2017 Downloaded from http://pubs.acs.org on January 7, 2017

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Chemistry of Materials

Oxygen-vacancy-induced midgap states responsible for the fluorescence and the long-lasting phosphorescence of the inverse spinel Mg(Mg,Sn)O4 Gaganpreet Behrha, Masahiko Isobeb, Florian Massuyeaua, Hélène Serier-Braulta, Elijah E. Gordonc, Hyun-Joo Kood, Myung-Hwan Whangboc, Romain Gautiera, and Stéphane Jobica,* a

Institut des Matériaux Jean Rouxel (IMN), Université de Nantes, CNRS, 2 rue de la Houssinière, BP 32229, 44322 Nantes Cedex 3, France

b

Max Planck Institute for Solid State Research, Heisenbergstrasse 1, 70569 Stuttgart, Germany

c

Department of Chemistry, North Carolina State University, Raleigh, NC 27695-8204, USA

d

Department of Chemistry and Research Institute for Basic Sciences, Kyung Hee University, Seoul 130-701, Republic of Korea

ABSTRACT: Samples of inverse spinel Mg2SnO4 were prepared using a ceramic method, their phosphorescence phenomenon was probed by optical measurements, and its cause was explored on the basis of density functional theory calculations for model structures of Mg2SnO4 with oxygen vacancies VO. Mg2SnO4 exhibits long-lasting luminescence at two different wavelength regions, peaking at ∼498 nm and ∼755 nm. A Sn-VO-Sn defect plus a Mg vacancy VMg away from the VO generates the empty midgap states, σSn-Sn and σ*Sn-Sn, localized at the Sn-VO-Sn defect, while an oxygen vacancy VO between adjacent Sn4+ and Mg2+ sites creates a filled midgap state Sn2+ (5s2 lone pair) lying below the σSn-Sn level. The longlasting luminescence at two different wavelength regions and the up-conversion photo-stimulated luminescence observed for undoped Mg2SnO4 are well explained by considering the σ*Sn-Sn level as the trapping level for a photo-generated electron.

INTRODUCTION A luminescence phenomenon of a condensed matter at room temperature, in which light emission continues for several minutes or hours after stopping the light excitation, is commonly referred to as phosphorescence, longlasting luminescence, or long-lasting afterglow. This phenomenon originates from a photo-excitation that generates electron-hole pairs and a subsequent capture of the charge carriers (i.e., electrons or holes) at certain defect sites, for example, intrinsic sites (anion or cation vacancies) or extrinsic sites (reducible or oxidizable dopant sites). This photo-generated meta-stable state may last until a stimulus de-traps the captured charge hence releasing the stored energy. The stimulus for phosphorescence is the thermal energy at room temperature, so phosphorescence is a phenomenon of thermallystimulated luminescence. The phosphor, SrAl2O4:Eu,Dy, possesses an outstanding duration of long-lasting luminescence and is the most studied one of phosphorescent materials.1-2 Its green luminescence can be seen with naked eyes for more than ten hours. This exceptional prop-

erty arises from the photo-oxidation of dopant cations Eu2+ of the aluminate host lattice into Eu3+ ions, with concomitant capture of the photo-generated electrons either by oxygen-vacancy sites or by Dy3+ codopants.3-5 The exact chemical nature of this electron trap is still under debate.6 The trap depth of ∼0.6 eV is estimated to be the optimal activation energy necessary for high phosphorescence performances. More recently, new long lasting phosphorescent materials have emerged from spinels AB2O4, which crystallize in the cubic crystal system with space group Fd-3m (Z = 8). Typically, the structure of a normal spinel is made up of [BO6] octahedra containing B3+ ions and [AO4] tetrahedra containing A2+ cations. Every four [BO6] octahedra form an edge-sharing tetrahedral cluster (Fig. 1a), which may be represented by a [B4O4] “cube” (Fig. 1b). Such B4O4 cubes are arranged to form a pyrochlore lattice of B3+ ions (Fig. 1c), with each [AO4] tetrahedron sharing its oxygen corners with [B4O4] cubes (Fig. 1d) such that each O2anion is coordinated to one A2+ and three B3+ cations. A unit cell consists of 16 B3+ and 8 A2+ cations (Fig. 1e). In

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inverse spinels B(AB)O4, half the B3+ cations occupy the tetrahedral sites

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when Mg2SnO4 is doped with Li.14 Undoped Mg2SnO4 has been reported to exhibit up-conversion photo-stimulated luminescence; stimulated by 980 nm infrared laser after UV irradiation, Mg2SnO4 shows photo-stimulated emission band covering 470–550 nm.13,15 In this article we probe the origin of long-lasting luminescence and the upconversion photo-stimulated luminescence phenomena observed for undoped Mg2SnO4 by preparing samples of Mg2SnO4 under several different synthesis conditions and characterizing their optical properties, and by performing electronic band structure calculations for model Mg2SnO4 structures with oxygen vacancies. Our work shows that undoped Mg2SnO4 exhibits long-lasting luminescence at two different wavelength regions, and suggests strongly that both the long-lasting luminescence and the upconversion photo-stimulated luminescence arise from the midgap states generated by oxygen vacancies.

EXPERIMENTAL SECTION

Figure 1. Construction of a normal cubic spinel structure AB2O4 in terms of [AO4] tetrahedra and [BO6] octahedra. (a) a [B4O16] cluster made up of four edge-sharing [BO6] octahedra, (b) a representation of a [B4O16] cluster in terms of a [B4O4] cube, (c) the arrangement of [B4O4] cubes to form a pyrochlore structure in AB2O4, (d) the arrangements of the [AO4] tetrahedra among the [B4O4]cubes in AB2O4, and (e) the constituents of a unit cell of AB2O4 containing eight A2+ and 16 B3+ cations. while all the A2+ cations and the remaining B3+cations occupy the octahedral sites. Consequently, the oxygen environments are much more varied in inverse spinels than in normal spinel since O2- anions may be, statistically speaking, coordinated to three A2+ cations and one B3+ cation, as well as four B3+ and any intermediate configurations. Then, in a real structure with defects, many local distortions around an oxygen vacancy will exist. Recent studies on chromium doped zinc gallate, ZnGa2O4:Cr3+, and its modification Zn1+xGa23+ 7-12 illustrate that a normal spinel can 2x(Ge,Sn)xO4:Cr , generate persistent phosphorescence in near-infrared (NIR), which is useful for in vivo bioimaging. The origin of the phosphorescence is not yet fully understood but traps would be related to the existence of antisites (ZnGa and GaZn) and the ability of the spinel structure to stabilize cations in a metastable state with an uncommon oxidation states. Undoped inverse spinels can also bring about persistent luminescence. In the inverse spinel, Mg2SnO4, namely, Mg(Mg,Sn)O4, both Mg2+ and Sn4+ cations occupy the octahedral sites. Recently, Mg2SnO4 has received much attention for its white-greenish phosphorescence, which can be observed with naked eyes for about 5 hrs.13 The duration of this luminescence is extended further

Chemicals. In general, the amount of defects such as oxygen vacancies Vo in a given oxide sample depends on the sample-preparation condition. To ensure that our general conclusions do not reflect results from a particular sample, we prepared three different samples of Mg2SnO4 by employing quite different conditions of the ceramic route. Sample 1: MgO (99.995%, Carl Roth) and SnO2 (99.9%, Alfa Aesar) were intimately mixed together to form a pellet, which was heated in air at 1000°C for 24 hrs. The calcined sample was reground, pelletized and heated at 1500°C for 12 hrs. Powder X-ray diffraction (XRD) analyses show that it consists of Mg2SnO4 with a slight amount (2.0(3) weight %) of MgO. Sample 2: this was prepared as done for sample 1, but the calcination temperature was set at 1200 °C for 5 days. The sample was found to be almost pure, but a slight amount of MgO (2.1 (3) weight%) was identified by XRD analyses. Sample 3: MgO (99.95%, Strem Chemical) and SnO2 (99.996%, Alfa Aesar) were ground all together and heated at 1400 °C for 72 hrs in one step. XRD analyses did not reveal any byproduct. X-ray diffraction. X-ray diffraction patterns were recorded at room temperature with a Bruker AXS D8 diffractometer in the Bragg-Brentano geometry with Cu K-L3 radiation (germanium monochromator) operated at 40 mA and 40kV. The phase identification was based on Inorganic Crystal Structure Database. The data was collected in 5-90° 2θ with the step size of 0.013°. Photoluminescence. Photoluminescence spectra were recorded with a Spex Fluorolog-3 spectrofluorometer (Instruments Jobin Yvon). The excitation source was a 450-W Xe light used at room temperature. Band structure calculations. All atom positions were relaxed in a 2 × 1 × 1 supercell on the basis of non-spinpolarized density functional theory (DFT) calculations by employing the projector augmented wave method encoded in the Vienna ab initio simulation package16-18 and the generalized gradient approximation of Perdew, Burke and

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Ernzerh for the exchange-correlation functionals.19 We also carried out hybrid-DFT calculations20 for each optimized structure obtained from these DFT calculations because the latter underestimate band gaps. (Our calculations employed a set of 1×2×2 k-points, the plane wave cutoff energy of 520 eV, threshold energy of 10-6 eV for self-consistent-field energy convergence, and the residual force of 0.02 eV/Å for the geometry optimization.)

RESULTS AND DISCUSSION Long-lasting luminescence at two different wavelength regions. Fig. 2a shows the intensity and the wavelength of the photo-luminescence measured for sample 1 as a function of time, after irradiating the sample at 254 nm for 10 min (Prolabo lamp, 30 W). A white-bluish intense phosphorescence is observed with naked eyes for several minutes. The white glow is related to a broad emission band peaking at ∼498 nm (∼2.5 eV) (band I) ranging between ∼400 to ∼700 nm. This band is much less intense in sample 2 (Fig. 2b) and sample 3 (Fig. 2c). In sample 2 and 3, band I is accompanied by a second one peaking at ∼755 nm (∼1.6 eV) in NIR (band II), which extends up to at least 900 nm. The existence of the photoluminescence band II in Mg2SnO4 is reported here for the very first time. Sample 3 exhibits the same luminescence characteristics as does sample 2. However, band II is more intense than band I in sample 2, but the opposite is the case for sample 3. The decay curves for samples 1 – 3 at 498 nm and those for samples 2 and 3 at 757 nm were fitted by using two exponential functions with three fitting constants A0, A1 and A2, I(t) = A0 + A1 exp (-t/τ1) + A2 exp (-t/τ2) where I(t) is the luminescence intensity at time t after ceasing the excitation, while τ1 and τ2 are decay times associated with each exponential function. The τ1 and τ2 values are summarized in Table 1. The decay times change somewhat from one sample to another, indicating that the nature of defects or the surroundings of the defects responsible for the phosphorescence phenomenon are slightly different. As the white phosphorescence (band I) is detected for more than 5

hours with naked eyes for some samples, it would be important to optimize the luminescence in the NIR region (band II) for potential application in medicine for instance. Hereafter, the luminescence peaking at ∼2.5 eV will be referred to as the white luminescence, and the one at ∼1.6 eV as the NIR luminescence. Table 1. Decay times τ1 and τ2 (in seconds) of the long persistent phosphorescence of samples 1 – 3 at 498 and 757 nm (after excitation at 254 nm for 10 minutes). Estimated standard deviations are given in parentheses. Samples

λem

τ1

τ2

1

498 nm

105 (2)

704 (10)

757 nm

-

-

2

498 nm

80 (5)

701 (46)

757 nm

131 (4)

950 (27)

3

498 nm

174 (6)

1147 (45)

757 nm

152 (6)

965 (34)

The width of the white emission is much narrower for sample 3 than for samples 1 and 2, and strongly resembles the one observed for Mg2SnO4:Mn2+.21 The NIR emission (band II) is similar to the one suggested for Mg2GaO4:Mn,22 i.e., the emission from two Mn2+ ions residing at adjacent tetrahedral and octahedral sites sharing an oxygen corner. One might speculate if bands I and II originate from the presence of Mn2+ ions are unintentionally doped via starting materials used for the synthesis. However, this hypothesis is highly improbable. One might speculate if oxygen vacancies reduce Sn4+ cations into Sn2+, because the 5s15p1 → 5s2 (3P0,1 → 1S0) transition at Sn2+ sites might account for band II. This possibility is quite plausible, but it has been difficult to detect the presence of Sn2+ cations in Mg2SnO4 samples on the basis of NMR, XPS and Auger spectroscopy experiments, for instance. To explain the origin of the white and NIR luminescence, it is necessary to examine the electronic structures of Mg2SnO4 with defects (see below).

Figure 2. Phosphorescence diagrams collected for (a) sample 1, (b) sample 2, and (c) sample 3 after excitation for 10 minutes at 254 nm in ambient conditions. The color changes from violet to red for emission intensity ranging from 0 to 1 on an arbitrary scale (0 = no luminescence, 1 = maximum of luminescence).

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We now examine the intensity of the fluorescence at the emission wavelengths (λemis.) as a function of the excitation wavelengths (λexci.). The results of this investigation obtained for samples 1 – 3 are summarized in Figure 3, which confirm the existence of bands I and II in all fluorescence spectra, even for sample 1.

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from high (red) to low (blue). The CIE x,y chromatic parameters for different excitation wavelengths are given on the right side with colored squares deemed to represent the emission color as perceived by naked eyes. xy chromaticity coordinates are calculated with the flat illuminant E using the CIE 1931 2° standard observer. xy coordinates are then converted in RGB values in order to obtain printable color rendering. For the latter, band II was not observable from the phosphorescence plot of Figure 2a due probably to the very strong intensity of band I. The relative intensities of these two emission bands depend strongly on the examined samples, indicating that the local environments of the luminescent centers differ slightly from one sample to another. This raises the question of reproducing the optical properties since the complex nature of the defects can be strongly influenced by a slight change in the synthesis conditions. Nevertheless, the coexistence of bands I and II with different intensities and different peak profiles fully explains the large dispersion of CIE x,y parameters (CIE = Commision Internationale de l’Eclairage) found for Mg2SnO4 materials; different colors were observed depending on both the samples and/or the excitation wavelengths as revealed by the CIE parameters and the color rendering at the right side of each plot. Midgap states of Mg2SnO4 arising from oxygen vacancy and their role. It is of interest to probe whether the optical properties of undoped Mg2SnO4 described in the previous section can be explained by its midgap states generated by defects. From the viewpoint of ionic bonding, Mg2SnO4 is described by (Mg2+)2(Sn4+)(O2-)4. Due to the relative energy ordering of the states O 2p < Sn 5s < Mg 3s, the valence band maximum (VBM) of Mg2SnO4 are represented largely by the O 2p states, and the conduction band minimum (CBM) of Mg2SnO4 largely by the Sn 5s states that make antibonding interactions with the O 2p orbitals and with some hybridization of Sn 5p orbitals. An oxygen vacancy, VO, reduces this antibonding and hence would lower the 5s level of the Sn adjacent to the VO, thereby generating a midgap state. It is of interest to examine the midgap states of Mg2SnO4 arising from defects such as oxygen vacancy VO and magnesium vacancy VMg, etc. The structure of Mg(Mg,Sn)O4 is quite complex because the Mg2+ and Sn4+ ions occupy the octahedral sites randomly and can also occupy the interstitial sites. Thus there are a huge number of possible defect structures to consider, but we limit our study only to the question concerning what kinds of midgap states are generated by the most probable defect, VO, and whether or not these states are responsible for the observed optical properties of undoped Mg(Mg,Sn)O4.

Figure 3. 2D excitation- emission maps of samples 1 – 3 at room temperature with intensity of the emission ranging

An oxygen vacancy VO introduced between two adjacent Sn sites leads to a Sn-VO-Sn defect. If the VO is accompanied by a magnesium vacancy VMg away from the Sn-VOSn defect, the following charge balance is achieved:

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Sn4+-O2--Sn4+ + O2--Mg2+-O2- → Sn4+-VO-Sn4+ + O2--VMg-O2+ ½ O2 + Mg (1) It is noted that Mg and O2 can recombine to give rise to MgO, as observed in samples 1 and 2. MgO could also be regarded as an unreacted precursor, which is a residue due to the fact that the “Mg2SnO4” material has the propensity to adopt an off-stoichiometry in magnesium and oxygen (in samples 1 and 2, this off-stoichiometry would not exceed a few percent). If the VO is not accompanied by a magnesium vacancy VMg (sample 3 for instance), the charge balance is written as Sn4+-O2--Mg2+ → Sn2+-VO-Mg2+ + ½ O2

(2)

Regardless of whether the Mg is at an octahedral or a tetrahedral site, the Sn 5s orbital lies lower than the Mg 3s orbital. Since the Sn 5s orbital has a large orbital extension, the two Sn sites of either the Sn4+-VO-Sn4+ or the Sn3+-VO-Sn3+ (a delocalized description of the Sn4+-VoSn2+) defect will form a weak sigma bonding and a weak sigma antibonding levels, σSn-Sn and σ*Sn-Sn, respectively, the tin atoms relaxing towards the vacancy. The σSn-Sn and σ*Sn-Sn states of the Sn-VO-Sn defects are likely to lie below the CBM, because some O 2p – Sn 5s antibonding is removed due to the VO, and become midgap states. For the same reason, the 5s state of the Sn2+ ion in a Sn-VO-Mg defect is likely to lie below the CBM. To verify these expectations, it is necessary to determine the electronic structures of Mg2SnO4 with the defects on the basis of DFT electronic structure calculations. We note that the reduction of Sn4+ to Sn2+ cations is usually accompanied by a lower-coordinate environment, as achieved here the oxygen vacancy. For our DFT calculations, we construct a model structure of the inverse spinel Mg(Mg,Sn)O4 by modifying a normal spinel structure as follows: First, Mg2Sn2O4 cubes are formed by edge-sharing two MgO6 octahedra and two SnO6 octahedra, and then these Mg2Sn2O4 cubes are arranged in a (2a, b, c) supercell of a normal spinel as in Fig. 4, where each Mg2Sn2O4 cube is represented by a Mg2Sn2

tetrahedron for simplicity. Note that all Sn4+ ions are located in the octahedral sites, while Mg2+ ions are present in both octahedral and tetrahedral sites. On the basis of this model, we examine the electronic structures of the following five cases: (a) no defect (b) a Sn-VO-Sn defect with a VMg at an octahedral site far away from the VO (c) a Sn-VO-Sn defect with a VMg at a tetrahedral site far away from the VO (d) a Sn-VO-Mg defect with Mg at an octahedral site (e) a Sn-VO-Mg defect with Mg at a tetrahedral site

Figure 4. Model structure of the inverse spinel Mg(Mg,Sn)O4 employed for the DFT calculations. We form Mg2Sn2O4 cubes by edge-sharing two MgO6 octahedra and two SnO6 octahedra, and then arrange these cubes in a (2a, b, c) supercell of a normal spinel. For simplicity, each Mg2Sn2O4 cube is represented by a Mg2Sn2 tetrahedron (with cyan and blue circles for Sn4+ and Mg2+ ions, respectively), and the tetrahedral-site Mg2+ ions by yellow circles.

Figure 5. Dispersion relations obtained from the DFT+hybrid calculations for model inverse spinel Mg(Mg,Sn)O4: (a) no defect, (b) a Sn-VO-Sn defect with a VMg at an octahedral site far away from the VO, (c) a Sn-VO-Sn defect with a VMg at a

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tetrahedral site far away from the VO, (d) a Sn-VO-Mg defect with Mg at an octahedral site, and (e) a Sn-VO-Mg defect with Mg at a tetrahedral site. The red arrows point out the last occupied level. The dispersion relations of the bands obtained from the hybrid-DFT calculations along the ΓX, ΓY and ΓZ directions are presented in Fig. 5. The band structure with no defect (Fig. 5a) shows an calculated optical gap of ∼4.5 eV, which lies just in between the absorption pre-edge at 3.8 eV and the highest absorption peak at 5.3 eV reported by Zhang et al.13 for powdered Mg2SnO4. Our experiment found a rather small optical gap (i.e., 3.5 eV), which implies that our Mg(Mg,Sn)O4 samples contain deep donor and/or acceptor levels responsible for the observed small optical gap (see below). As expected, the Sn-VO-Sn defect with a VMg either at an octahedral or a tetrahedral site generates the empty σSn-Sn and σ*Sn-Sn states lying below the CBM but at ∼2.5 and ∼4.0 eV above the VBM (Fig. 5b,c), while the Sn-VO-Mg defect with Mg either at an octahedral or a tetrahedral site creates the filled Sn lonepair levels at ∼2.0 eV above the VBM (Fig. 5d,e), which constitute deep donor levels of Mg(Mg,Sn)O4. The calculated electronic structures of Mg(Mg,Sn)O4 with defects VO and VMg can be summarized as depicted in Fig. 6, which shows three midgap states, namely, the empty states σSn-Sn and σ*Sn-Sn states arising from a Sn-VO-Sn defect plus a VMg as well as the Sn2+ state arising from a Sn-VO-Mg defect. In this scheme, localized levels are assigned to defects due to their random distribution in the direct space.

nescence phenomena can be accounted for by considering the following series of energy transfer processes: (a) The photo-excitation from the filled 5s level of Sn2+ (associated with an oxygen vacancy) to the conduction band (CB) leads to Sn2+ + hν → Sn3+ + e. The appearance of Sn3+ cations can also be associated with the promotion of an electron from the valence band (VB) towards the CB, with capture of the generated hole by a Sn2+ cation. (b) The photo-generated electron e- in the CB cascades down to the CBM and is eventually trapped at the σ*Sn-Sn level, leading to e- + (σ*Sn-Sn)0 → (σ*Sn-Sn)1. To some extent, the levels associated with the Vo are made up of the Sn atomic orbitals, this can be regarded as an electron capture by the oxygen vacancy with reduction of its charge from +2 to +1 (Vo•• and Vo•, respectively, in the KrogerVink notation). This induces stabilization as the charge is decreased. (c) A thermal excitation of the trapped electron from the σ*Sn-Sn level provides an electron in the CBM. This electron can give rise to two kinds of luminescence. The white luminescence can be assigned to the transition associated with the Sn2+ level Sn3+ + e- → Sn2+ + hν1,

(3) 1

1

2

which may be described as the (5s 5p → 5s ) transition of the Sn2+ ion due to the hybridized nature of the CBM band orbitals (i.e., 5s/5p Sn orbitals). The NIR luminescence can originate from the CBM → σSn-Sn transition, (σ∗Sn-Sn)0+ e- → (σSn-Sn)1 + hν2.

(4a)

1

and the electron in the (σSn-Sn) level returns to the Sn3+ (i.e., the photo-ionized Sn2+) level in a radiation-less pathway. (σSn-Sn)1+ Sn3+ → (σSn-Sn)0 + Sn2+.

Figure 6. Schematic view of the midgap states of inverse spinel Mg(Mg,Sn)O4 generated by oxygen vacancies Vo, which are randomly distributed in the host lattice. The σSn-Sn and σ∗Sn-Sn levels associated with the Sn-VO-Sn defects result from weak overlap largely between the Sn 5s orbitals across the VO. Mechanism. It is most likely that the electron trap needed to explain the long-lasting white luminescence and the long persistent red luminescence is the σ*Sn-Sn state lying at ∼0.5 eV below the CBM. Then the white and NIR lumi-

(4b)

According to the estimates of the σSn-Sn, σ*Sn-Sn and Sn2+ levels discussed in the previous section, it is estimated that hν1 ≈ 2.5 eV and hν2 ≈ 2.0 eV. These results are consistent with the long-lasting white and NIR luminescence phenomena observed for the inverse spinel Mg2SnO4. We note that the light excitation giving rise to the white and NIR luminescence of Mg2SnO4 is the UV light that causes the valence band (VB) → CB transition, which generates a hole at the VBM. When this hole is filled by an electron from the Sn2+ cation, leading to a Sn3+ ion, then the outcome of the VB → CB transition is equivalent to that of the Sn2+ → CB transition. The cause for the NIR emission in Mg2GaO4:Mn was suggested to be the emission from Mn2+-Mn2+ dimers, Mn2+ ions residing at adjacent tetrahedral and octahedral sites sharing an oxygen corner.22 Given that the extent of Mndoping is not high, this possibility is quite remote. Formally, it is probable that oxygen vacancies in Mg2GaO4 generate empty σGa-Ga and σ*Ga-Ga levels in a manner simi-

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lar to undoped Mg2SnO4, and the CBM → σGa-Ga transition is responsible for the NIR emission of Mg2GaO4:Mn. The supposition that the σ*Sn-Sn level is the electrontrapping level also explains the observed up-conversion photo-stimulated luminescence; the UV irradiation excites an electron from a Sn2+ level generating a Sn3+ ion and eventually a trapped electron at a σ*Sn-Sn level. The latter is excited into the CB by the 980 nm infrared irradiation, creating eventually an electron at the CBM. The latter subsequently combines with the Sn3+ ion generating the emission band covering 470–550 nm. The above discussion strongly suggests that the σ*Sn-Sn level acts as the electron-trapping level. On the basis of this finding, we examine why the Li doping extends the duration of the phosphorescence of Mg(Mg,Sn)O4. The doped Li atoms exist as Li+ ions most probably at Mg2+ sites. Then, the charge balance for this substitutional doping is achieved by oxygen vacancy VO. 2 Li + 2 Mg2+ + O2- → 2 Li+ + 2 Mg + ½ O2 Namely, the Li-doping increases the extent of oxygen vacancy, which then would increase the Sn4+-VO-Sn4+ defects and hence the number of the electron-trap levels σ*Sn-Sn. The latter would enhance the phosphorescence duration. Our supposition of the σ*Sn-Sn level as an electron-trap level implicitly assumes that the electron trapped there does not cascade down to the σSn-Sn level. For this to be true, the transition dipole moment between the σ*Sn-Sn and σSn-Sn levels should almost vanish. This is likely because the major orbital contributions to these levels are the Sn 5s orbitals, which are combined in-phase and outof-phase in σSn-Sn and σ*Sn-Sn, respectively. Thus the transition dipole moment at each Sn site is proportional to r 5s r 5s , which is zero. The transition dipole moment 3+ r 5s r 5p is nonzero, so the CBM → σSn-Sn and CBM → Sn

transitions would be nonzero because the CBM has some contributions of S 5p.

CONCLUSION Our study shows that undoped Mg2SnO4 exhibits both white and NIR luminescence. Oxygen vacancies of Mg2SnO4 generate the empty midgap states, σSn-Sn and σ*Sn-Sn, as well as the deep donor state, Sn2+. It is most likely that the σ*Sn-Sn state is a trapping level for the photo-generated electron. The UV-light excitation of Mg2SnO4 generates an electron trapped at the σ*Sn-Sn level and a hole trapped at the donor site Sn2+. Thus, the longlasting white and NIR luminescence can be explained by the transition of the de-trapped electron to the holetrapped Sn2+ and σSn-Sn levels, respectively. Our study has taken into consideration only a few representative oxygen vacancy structures. The existence of many different oxygen-vacancy environments would be responsible for the widths of the white and red emissions as well as the afterglow peaks reported by Zhang et al.13

ACKNOWLEDGMENT This work used resources of the HPC Center of NCSU and those of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

REFERENCES (1) Matsuzawa, T.; Aoki, Y.; Takeuchi, N.; Murayama, Y. A New Long Phosphorescent Phosphor with High Brightness, SrAl2O4:Eu2+,Dy3+. J. Electrochem. Soc. 1996, 143, 2670–2673 (2) Takasaki, H.; Tanabe, S.; Hanada, T. Long-Lasting Afterglow Characteristics of Eu, Dy Codoped SrO-Al2O3 Phosphor. J. Ceram. Soc. Jpn. 1996, 104, 322–326. (3) Clabau, F.; Rocquefelte, X.; Jobic, S.; Deniard, P.; Whangbo, M.H.; Garcia, A.; Le Mercier, T. Mechanism of Phosphorescence Appropriate for the Long-Lasting Phosphors Eu2+-doped SrAl2O4 with Codopants Dy3+ and B3+. Chem. Mat. 2005, 17, 3904– 3912. (4) Clabau, F.; Rocquefelte, X.; Le Mercier, T.; Deniard, P.; Jobic, S.; Whangbo, M.H. Formulation of Phosphorescence Mechanisms in Inorganic Solids Based on a New Model of Defect Conglomeration. Chem. Mater. 2006, 18, 3212–3220. (5) Dorenbos, P. Mechanism of Persistent Luminescence in Eu2+ and Dy3+ Codoped Aluminate and Silicate Compounds. J. Electrochem. Soc. 2005, 152, H107–H110. (6) Van den Eeckhout, K.; Smet, P. F.; Poelman, D. Persistent Luminescence in Eu2+-Doped Compounds: A Review, Materials 2010, 3, 2536-2566. (7) Bessière, A.; Jacquart S.; Priolkar, K.; Lecointre, A.; Viana, B.; Gourier, D., ZnGa2O4:Cr3+: a New Red Long Lasting Phosphor with High Brightness, Opt. Express, 2011, 19, 10131–10137. (8) Bessière, A.; Sharma, S. K.; Basavaraju, N.; Priolkar, K. R.; Binet, L.; Viana, B.; Bos, A. J. J.; Maldiney, T.; Richard, C.; Scherman, D.; Gourier, D. Storage of Visible Light for Long-Lasting Phosphorescence in Chromium-Doped Zinc Gallate, Chem. Mater. 2014, 26, 1365–1373. (9) Zhuang, Y.; Ueda, J.; Tanabe, S.; Dorenbos, P. Band-Gap Variation and Self-redox Effect Induced by Compositional Deviation in ZnxGa2O3+x:Cr3+ Persistent Phosphors, J. Mater. Chem. C 2014, 2, 5502–5509. (10) Pan, Z.; Lu, Y.; Liu, F. Sunlight-activated Long-Persistent Luminescence in the Near-Infrared from Cr3+-doped Zinc Gallogermanates, Nat. Mater. 2012, 11, 58–63. (11) Allix, M.; Chenu, S.; Véron, E.; Poumeyrol, T.; KouadriBoudjelthia, E.; Alahraché, S.; Porcher, F.; Massiot, D.; Fayon, F. Considerable Improvement of Long-Persistent Luminescence in Germanium and Tin Substituted ZnGa2O4, Chem. Mater. 2013, 25, 1600–1606. (12) Chenu, S.; Véron, E.; Genevois, C.; Garcia, A.; Matzen, G.; Allix, M. Long-Lasting Luminescent ZnGa2O4:Cr3+ Transparent Glass-Ceramics, J. Mater Chem. C, 2014, 2, 10002-10010. (13) Zhang, J.; Yu, M.; Qin, Q.; Zhou, H.; Zhou, M.; Xu, X.; Wang, Y. The Persistent Luminescence and Up-Conversion Photostimulated Luminescence Properties of Nondoped Mg2SnO4 Spinel, J. Appl. Phys. 2010, 108, 123518-7. (14) Torisaka, A.; Kaneko, H.; Nomizu, K.; Abe, S.; Ishigaki, T.; Toda, K.; Uematsu, K.; Sato, M. Synthesis and Luminescence Properties of Non-Doped Long Persistent Phosphors: Mg2SnO4, Key Engineering Materials, 2010, 421-422, 352-355. (15) Zhang, J.-C.; Qin Q.-S.; Yu, M.-H.; Sun, J.-Y.; Shi, L.-R.; Ma, X.-L., Up-Conversion Photostimulated Luminescence of Mg2SnO4 for Optical Storage, Chin. Phys. Lett. 2011, 28, 027802.

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Page 8 of 9

(16) Kresse, G.; Hafner, J. Ab-Initio Molecular Dynamics for Liquid Metals, Phys. Rev. B 1993, 47, 558-561. (17) Kresse, G.; Furthmüller, J. Efficiency of Ab-Initio Total Energy Calculations for Metals and Semiconductors Using a PlaneWave Basis Set, Comput. Mater. Sci. 1996, 6, 15-50. (18) Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab-Initio Total-Energy Calculations Using a Plane-Wave Basis Set, Phys. Rev. B 1996, 54, 11169-11186. (19) Perdew, J. P., Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple, Phys. Rev. Lett., 1996, 77, 3865-3869. (20) Heyd, J.; Scuseria, G. E.; Ernzerhof, M. Hybrid Functionals Based on a Screened Coulomb Potential, J. Chem. Phys. 2003, 118, 8207-8215. (21) Lei, B.; Li, B.; Wang, X.; Li, W. Green Emitting Long Lasting Phosphorescence (LLP) Properties of Mg2SnO4:Mn2+ Phosphor, J. Lumin. 2006, 118, 173-178. (22) Song, E. H.; Wang, J. L.; Yu, D. C.; Ye, S.; Zhang, Q. Y. Anomalous Tunable Visible to Near Infrared Emission in the Mn2+-Doped Spinel MgGa2O4 and Room-Temperature UpConversion in the Mn2+ and Yb3+ Codoped Spinel, J. Mater Chem. C, 2014, 2, 8811-8816.

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TABLE OF CONTENTS: Mg2SnO4 shows the long lasting luminescence in the visible (498 nm) and NIR (755 nm) regions. This long lasting luminescence at two different frequencies arises from the fact that the σ∗Sn-Sn state lying ∼0.5 eV below the conduction band minimum, generated by an oxygen vacancy, acts as an efficient trap for a photogenerated electron.

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Oxygen-Vacancy-Induced Midgap States Responsible for the

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