Blue and Orange Photoluminescence and Surface ... - ACS Publications

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Blue and Orange Photoluminescence and Surface Band-Gap Narrowing in Lithium-Doped MgO Microcrystals Haruka Soma and Takashi Uchino* Department of Chemistry, Graduate School of Science, Kobe University, Nada, Kobe 657-8501, Japan ABSTRACT: Structure and properties of Li-doped MgO have been extensively studied during the past decades in view of the effect of aliovalent doping on the catalytic, optical absorption and magnetic properties. However, the photoluminescence (PL) properties of Li-doped MgO have not been systematically investigated previously. In this work, we prepared micrometersized Li-doped MgO crystals using solid-phase redox reaction between B2O3 and metals of Mg and Li under argon atmosphere. The resulting Li-doped MgO microcrystals exhibit blue (2.8 eV or 440 nm) and orange PL (2.1 eV or 580 nm) emissions under excitation with photons of energies of ∼5 and ∼3 eV, respectively. The blue PL is attributed to the bulk oxygen vacancies, or F-type centers, whose emission energy levels are modified by the Li ions incorporated into the MgO structure. On the other hand, the orange PL presumably results from the oxygen vacancies at the near surface regions where a substantial bandgap narrowing down to visible range of ∼3 eV occurs. It is also found that the photoexcited electrons generated by surface interband transition can be transferred to the surface emission states via thermal activation, resulting in temperature antiquenching of the orange PL emission. Hence, the present method provides a simple and effective way to prepare visible luminescent Li-doped MgO microcrystals with an intriguing effect of aliovalent doping both on the bulk and surface electronic structures. centers, respectively,12−15 although recent theoretical calculations have posed a question about the conventional assignment for these PL bands.17,18 Note also that the photoexcitation of the F-type centers in MgO have attracted a renewed interest in terms of the thermal impact on spinpolarized tunneling.19 Thus, the structure, formation, and properties of F-type defects in MgO are still an active research topic in solid state physics and chemistry.20−24 In addition to the intrinsic defects, the structure and properties of the extrinsic defects in MgO have also been intensively investigated in the past decades.25−32 The most well-studied system is Li-doped MgO or MgO:Li, which is a potential catalyst for the oxidative coupling of methane.33−35 In MgO:Li crystals, surface [LiMg′OO• = Li+−O−] bonds were originally suggested to behave as chemically active catalytic sites.33,34 However, no direct proof for the [Li+−O−] center model has yet been given, and the structural origin of the active center is still a matter of discussion.36 As an alternative to the [Li+−O−] center, F-type centers and the related defect complexes are considered to be responsible for the catalytic reaction.37,38 According to recent density functional theory (DFT) calculations,39 various types of defect complexes with different charge states, e.g., [Li Mg V O]+, [Li Mg VO ], and [LiMgVO]− complexes, are likely to exist in Li-doped MgO.

1. INTRODUCTION Magnesium oxide (MgO), with a bandgap of 7.8 eV, has been one of the most extensively studied oxide-based crystals owing not only to its simple crystal structure of rock salt, but to versatile applicability and functionality in a diverse range of fields, including catalysis, electronics, spintronics, and photonics. For example, MgO and related MgO-based materials can be used as a catalytically active material,1,2 a tunneling barrier for magnetic tunneling junctions,3,4 strong pinning centers in high-temperature copper oxide superconductors,5 an electron emission source,6 ultraviolet (UV)7 and visible lasers.8,9 It has been well recognized that the electronic, optical and catalytic properties of MgO are strongly influenced by the type and amount of structural and surface defects, which arise both from intrinsic and extrinsic origin.10,11 Oxygen vacancies, which are usually called F-type (or VO in the Kröger−Vink notation) centers, are the most commonly observed intrinsic point defects in MgO.12−15 Thus, far, thermochemical reduction of MgO in a high-pressure vapor of Mg at high temperatures over 1800 °C is a well-recognized method to achieve a high concentration of F-type centers in the bulk MgO.12−16 It has been demonstrated that there exist two principal F-type centers, namely, the F+ and F centers, which are oxygen vacancies with one and two electrons, respectively. Both the F and F+ centers absorb practically the same energy of ∼5 eV, resulting in broad photoluminescence (PL) bands at 2.5 and 3.2 eV. It is commonly believed that the 2.5 and 3.2 eV PL bands result from the radiative recombination of the F and F+ © XXXX American Chemical Society

Received: December 3, 2016 Revised: January 4, 2017 Published: January 5, 2017 A

DOI: 10.1021/acs.jpcc.6b12177 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Thus, the observation and investigation of the F-type-centerrelated PL bands in MgO:Li will shed light on the electronic structure of the expected catalytic active centers. However, studies on the emission properties of MgO:Li have been rather limited.39−43 Furthermore, the PL spectra of the F-type centers from thermochemically reduced MgO:Li samples have not been reported previously.26,44 This is probably because most of the oxygen vacancies in Li-doped MgO are camouflaged as hydride ions (H−), which are formed by the capture of protons by the oxygen vacancies during the thermochemical reduction.44 Recently, we have proposed a simple but effective method to introduce a large amount of F-type centers in MgO at relatively low temperatures (∼700 °C) under highly reducing conditions.8,9,45 Our method utilizes the solid phase reaction between metallic Mg and a simple oxide, such as SiO2 and B2O3, under argon atmosphere. As for the Mg/B2O3 system, the reaction proceeds as follows: 4Mg + B2O3 → 3MgO + MgB2

(1)

During the reaction, substoichiometric MgOx (x < 1) species are preferentially sublimated and deposited as white powder with a size of several micrometers. The resulting MgO microcrystals show efficient PL emissions attributed to the F and F+ centers, even showing a broad-band laser emission under high-density photoexcitation.8,9,45 It should also be worth mentioning that unintentional hydrogen-related impurities, such as H− and OH−, hardly exist in the resulting MgO microcrystals.45 In this work, the above solid phase reaction is employed to synthesize the Li-doped substoichiometric MgO microcrystals by using metallic Li as well as Mg as reactants. We show that the resulting MgO:Li microcrystals exhibit a blue PL band at ∼2.8 eV, which is between the PL peak energies of the F and F+ centers in undoped MgO. In addition, substantial bandgap narrowing down to a value of ∼3 eV is shown to occur at the surface of the MgO:Li microcrystals. We also observe an orange PL emission at 2.1 eV under excitation with photons of energies above ∼3 eV. This orange PL most likely results from surface interband transition and the subsequent electron transfer to an oxygen-vacancy-related state via thermal activation.

Figure 1. Photographs of the as-prepared sample around Li grains in the crucible under the irradiation of a hand-held UV lamp of (a) 254 nm and (b) 365 nm. (c) XRD patterns of the deposited powder prepared in the absence of Li (lower pattern) and in the presence of Li (upper pattern). The insets in part c show the photographs of the respective powders under excitation of the designated wavelength.

nominally pure MgO. The MgO powder obtained in the absence of Li shows neither blue nor orange PL emission but shows green PL emission due to the F and F+ centers under irradiation of a UV lamp at 254 nm (see the lower inset of Figure 1c). Powder X-ray diffraction (XRD) patterns were obtained with a diffractometer (Rigaku, SmartLab) using Cu Kα radiation. Scanning electron microscopy (SEM) was conducted with a scanning electron microscope (JEOL. JSM-5610LVS). Inductively coupled plasma-atomic emission spectrometry (ICPAES) was performed with an ICP spectrometer (SII, SPS-3100) to estimate the concentration of Li in MgO. Diffuse reflectance spectra were measured using a spectrophotometer (Hitachi, UV-3500) equipped with an integrating sphere. Steady state PL spectra were recorded on a spectrofluorometer (JASCO, FP 6600) by using a monochromated xenon lamp (150 W). Timeresolved PL measurements were carried out with a gated imageintensified charge-coupled device (Princeton Instruments, PIMAX:1024RB) and 300 lines/mm grating by using the third harmonic (355 nm) or the fourth harmonic (266 nm) of a pulsed Nd:yttrium aluminum garnet (YAG) laser (Spectra Physics, INDI 40, pulse width 8 ns, repetition rate 10 Hz) as an excitation source. During the steady-state and time-resolved PL measurements, the sample temperature was controlled in a closed-cycle N2 cryostat in the temperature region from 78 to 400 K.

2. EXPERIMENTAL PROCEDURES Pure Mg (99.9%), Li (over 99%), B2O3 (99.9%) were used as starting materials. The Mg/B2O3 mixture of molar ratio of 5:1 was thoroughly mixed, and ∼1 g of the mixture powder was spread evenly in a rectangular alumina crucible with a size of 90 × 90 × 50 mm. Then, several pieces of Li metal with a size of around 1 mm were put on the center of the Mg/B2O3 mixture powder. After being closed with a 4 mm thick alumina lid, the crucible containing Mg, B2O3, and Li was placed in an electric furnace. The furnace was evacuated down to 30 Pa before purging with argon. The temperature of the furnace was raised to 750 °C at a rate of ∼10 °C/min and kept constant at 750 °C for 3 h under flowing argon of 400 mL/min. After the heating process, the furnace was naturally cooled to room temperature. We found that white powder was deposited around the grains of originally metallic Li. The thus obtained white powder exhibits blue and orange PL emissions under irradiation of a hand-held UV lamp at 254 nm (4.9 eV) and 365 nm (3.4 eV), respectively (see Figure 1a,b and the upper insets of Figure 1c). For comparison, we also carried out the reaction without putting metallic Li on the Mg/B2O3 mixture powder to prepare

3. RESULTS 3.1. Structural and Morphological Characterization. Figure 1c shows the XRD patterns of the white powder prepared in the presence/absence of metallic Li. The XRD patterns of both samples show diffraction peaks that correspond only to MgO (periclase). We carried out an ICP elemental analysis of the white powder prepared in the presence of Li and found that the concentration of Li is 1.1 B


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The Journal of Physical Chemistry C ± 0.2 wt %. Thus, Li along with Mg will behave as a reducing agent toward B2O3, and the Li ions are doped into the crystal lattice of MgO during the sublimation and deposition process. In what follows, we will refer to the sample prepared in the presence (absence) of Li as MgO:Li (MgO). SEM images of the MgO and MgO:Li samples are shown in Figure 2. One sees from Figure 2a that the crystal sizes of the

3.2. Steady State PL Measurements. As mentioned in the Experimental Procedures section, the MgO microcrystals prepared in the absence/presence of Li exhibit different PL properties. We show in Figure 3a,b the contour plots of the PL intensity collected from the MgO and MgO:Li samples at room temperature as a function of excitation and emission wavelengths. Under excitation with photons of ∼5 eV (250 nm), the undoped MgO sample exhibits two broad PL bands peaking at 3.22 eV (385 nm) and 2.55 eV (490 nm) (see also the green dotted line PL spectrum in Figure 3c), which are conventionally attributed to the F+- and F-center emissions, respectively.13−15 When Li is introduced into the MgO structure, these two PL bands are replaced by a single broad PL band peaking at 2.81 eV (440 nm) under ∼5 eV excitation (see the blue solid-line PL spectrum in Figure 3c), accompanied by an energy shift in the peak energy of the corresponding PL excitation (PLE) spectra from 4.8 to 5.1 eV (compare the solidand dotted-line PLE spectra in Figure 3c). This 2.81 eV PL band will correspond to the blue PL emission under 254 nm (4.9 eV) excitation shown in Figure 1a. Furthermore, the MgO:Li sample shows an additional PL band at 2.14 eV (580 nm) under excitation with photons of energy higher than ∼3 eV (Figure 3b,d), in accordance with the orange PL emission under 365 nm (3.4 eV) excitation shown in Figure 1b. It should be noted that the PLE spectrum of this orange PL emission is characterized by a sharp energy onset at ∼3 eV, followed by a continuum falling off gradually in intensity up to 6 eV (Figure 3d). This PLE spectral feature suggests that the orange emission results not from a defect-to-defect transition but

Figure 2. SEM images of the deposited powder prepared (a) in the absence of Li and (b) in the presence of Li.

MgO powder prepared without the presence of Li are typically 1 to 2 μm. Note also that most of these micrometer-sized MgO crystals have a cubic shape with well-defined (001) surfaces. However, such smooth (001) surfaces are not seen in the SEM image of the MgO powder prepared in the presence of Li (Figure 2b); rather, a substantial roughening of the surface can be recognized. This is in agreement with the previous results showing that the Li addition on MgO leads to the formation of higher index planes.41,46,47

Figure 3. Contour plots of the PL intensity collected from (a) the MgO and (b) the MgO:Li as a function of excitation and emission wavelengths measured at room temperature. (c) Room temperature PL and PLE spectra of the MgO (green dotted lines) and MgO:Li (blue solid lines) samples. (d) Room temperature PL and PLE spectra of the MgO:Li. The inset in part d shows the PL spectra before (upper spectrum) and after (lower spectrum) etching with dilute hydrochloric acid. C


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Figure 4. (a) Temperature dependent PL spectra of the MgO:Li recorded with excitation at 5.1 eV. (b) Corresponding PLE spectra recorded with emission at 2.8 eV.

Figure 5. (a) Temperature dependent PL spectra of the MgO:Li recorded with excitation at 3.1 eV. (b) The corresponding PLE spectra recorded with emission at 2.16 eV. The insets in parts a and b show the temperature dependence of the energy-integrated PL intensity and the intensity normalized PLE spectra, respectively. The solid line in the inset in part a is the fit of eq 7.

Figure 6. (a) PL decay profiles of the emission at 2.8 eV recorded at the designated temperatures within the range of 78−350 K. The fourth harmonic (266 nm) of a nanosecond Nd:YAG laser is used as an excitation source. The solid lines in part a are the fit of eq 2. (b) Temperature dependence of the fitted values of β and τ. The average decay time ⟨τ⟩ estimated from eq 3 are shown in the inset of part b.

rather from a interband-like transition, as will be discussed again in a later section. To further investigate the emission properties of the blue and orange PL bands in the MgO:Li, we measured the temperature dependence of the PL and PLE spectra. As shown in Figure 4a,b, the PL and PLE spectra of the blue PL emission show a simple decrease in intensity with increasing temperature. On the other hand, the PL and PLE spectra of the orange PL emission exhibits a complex temperature dependence (Figure

5a,b). The energy-integrated intensity of the orange PL band shows a gradual increase with increasing temperature from 78 K, reaches a maximum value at 225 K, and then decreases upon further increase in temperature to 375 K (see the inset in Figure 5a). The corresponding PLE spectra also show a similar irregular temperature dependence, accompanied by a pronounced temperature-induced red shift of the low-energy side of the spectra (Figure 5b) . D


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Figure 7. (a) PL decay profiles of the emission at 2.14 eV recorded at the designated temperatures within the range of 78−380 K. The third harmonic (355 nm) of a nanosecond Nd:YAG laser is used as an excitation source. The solid lines in part a are the fit of eq 2. (b) Temperature dependence of the fitted values of β and τ. The average decay time ⟨τ⟩ estimated from eq 3 are shown in the inset of part b.

where Γ represents the gamma function.51 As shown in the inset of Figure 6b, ⟨τ⟩ shows a constant decrease with increasing temperature. This is intuitively in agreement with the temperature dependence of the decay profiles given in Figure 6a. Figure 7 shows the decay profiles of the orange PL obtained by using the third harmonic (355 nm) of a pulsed Nd:YAG laser. The observed decay profiles can be reasonably fitted with a stretched exponential function, as in the case of the blue PL emission. However, the temperature dependence of τ and β is quite different from that observed for the blue PL. As for the orange PL, τ and β are practically temperature independent in the temperature region from 78 to ∼250 K, yielding the fitted values of τ ∼ 1 × 10−6 s and β ∼ 0.25. At temperatures above ∼250 K, however, the τ and β values show a sudden decrease, eventually resulting in τ ∼ 10−10 s and β ∼ 0.1 at 350 K. The average decay time ⟨τ⟩ shows a similar temperature dependence as that of τ (see the inset of Figure 7b). These results indicate that the recombination scheme responsible for the orange PL emission drastically changes when the temperature of the system is increased beyond 250 K.

3.3. Time-Resolved PL Measurements. We next investigate the decay dynamics of the blue and orange PL emissions observed in the MgO:Li microcrystals. Figure 6a presents the decay profiles for the blue PL band monitored at the peak position in the temperature region from 78 to 350 K under excitation with the fourth harmonic (266 nm) of a pulsed Nd:YAG laser. The blue PL emission decays in a highly nonexponential manner especially at low temperature region below ∼200 K. To get a quantitative measure of the decay behavior we fitted the decay profiles with a Kohlrausch− Williams−Watts (KWW) function, commonly referred to as a stretched exponential function:48 I(t ) = I0 exp( −(t /τ )β )

(2)

where I0 is the PL intensity at time t = 0, τ is a characteristic decay time, and β (0 < β ≤ 1) is a stretching exponent. In general, values of β < 1 correspond to the existence of a broad distribution of decay times.49 Note, however, that fitting is not necessarily perfect especially in the region where the intensity of the emission signal is less than ∼10. This implies that eq 2 cannot represent the entire decay process. If we introduce a double stretched exponential function instead of a single stretched exponential function eq 2, the quality of the fitting will be improved. However, this results in complication of analysis process and difficulties in interpreting the results. Thus, in this work, we employed eq 2 to minimize the number of regression variables and to make the situation simple. Figure 6b demonstrates that τ is almost constant over the whole temperature range except for the low temperature region below ∼100 K, whereas β shows a linear increase in the temperature region from 78 to ∼250 K and then becomes constant at temperatures above ∼250 K. The observed linear increase in β with temperature suggests that the relevant radiative recombination processes are limited by multiple trapping and detrapping events of photoexcited electrons,50 as will be discussed later. Since both τ and β exhibit a different temperature dependence, it is useful to calculate an average decay time ⟨τ⟩ from the fit functions by ⎛τ ⎞ ⎛1⎞ ⟨τ ⟩ = ⎜ ⎟Γ⎜ ⎟ ⎝β⎠ ⎝β⎠

4. DISCUSSION 4.1. Orange PL Emission and Bandgap Reduction. As mentioned earlier, the PLE spectra for the orange emission at 2.14 eV have a sharp signal onset at ∼3 eV and cover a wide energy region up to ∼6 eV. These PLE features imply that the orange PL emission is induced by the interband-like excitation. A similar orange emission has been observed from a layered MgO−LixO−MgO film during injection of accelerated electrons.42 We, however, consider that the observed energy onset at ∼3 eV and the related orange PL do not result from Li2O impurity, which might be deposited during the reaction between B2O3 and Li. This is because Li2O has an excitonic surface state at 4.3 eV,52 which is too high to explain the onset energy of the PLE spectrum shown in Figure 3d. Rather, we suggest that substantial band gap narrowing down to a value as low as ∼3 eV occurs at the surface of the Li-doped MgO structure. To corroborate that the surface states are related to the orange PL, the MgO:Li powders were chemically etched with dilute hydrochloric acid, and we compared the PL spectra before and after the etching process. For chemical etching, the sample powder was soaked and stirred in hydrochloric acid (0.1

(3) E


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The Journal of Physical Chemistry C M) for ∼10 min at room temperature. The soaked powder was filtered and washed with distilled water, followed by drying in an oven overnight at 60 °C for PL measurements. As a result, we found that the chemically etched MgO:Li sample hardly shows the orange PL emission (see the inset in Figure 3d), indicating that the PL results from the emission centers that are present at the surface of the unetched MgO:Li. It should be noted, however, that the orange PL emission is still observed even after the MgO:Li powders are stored in a portable drybox for more than 6 months. This not only implies that the relevant emission centers are stable against water adsorption and are likely to be present at subsurface regions as well, but it also excludes the possibility that the orange PL results from Li2O impurity, which is hygroscopic in nature. If the orange PL is originated from the interband-like excitation, the onset energy of the PLE spectra will shift to lower energies with increasing temperature in a similar manner as observed for the bandgap of semiconductors. One of the often used equations to represent the temperature dependence of semiconductor bandgaps is the following Bose−Einstein expression53 ⎞ ⎛ 2 E (T ) = a − b ⎜ 1 + ⎟ exp(ℏω/kBT ) − 1 ⎠ ⎝

(F(R )hν)2 = C(hν − Eg )

(6)

Figure 9. (hνF(R))2 vs photon energy (hν) curve for determination of the surface optical bandgap energy of the MgO:Li microcrystals.

Here hv is the photon energy, C a characteristic constant for each semiconductor, and Eg the direct-allowed band gap energy. The Eg value is estimated to be 2.75 eV, which is in fairly agreement with the onset energy of the PLE spectrum observed at 300 K shown in Figure 5b. These results strongly support our assumption that the orange PL results from the interbandlike transition occurring at the surface of the MgO:Li microcrystals. As for pure MgO, the surface interband transition is observed at 6.2 eV.57,58 Thus, the expected surface bandgap energy of the MgO:Li microcrystals is much less than that of pure MgO. In general, the reduction of bandgap energy at the surface is attributed to a large gradient of the Madelung potential at the surface, giving rise to a strong electric field that should produce a Stark mixing of ionic states.57,59 It is hence most likely that the substantial perturbation and modification of the Madelung potential is induced at the MgO:Li surface. In addition, recent theoretical calculations29 predict that Li impurity at the surface of MgO facilitates the formation of oxygen vacancies, resulting in lots of gap states around the Fermi level. These gap states are also expected to contribute to the observed bandgap narrowing, resulting in semiconductorlike surface electronic states. Hence, it can safely be assumed that the semiconductor/insulator heterojunctions with modified band structure are formed at the surface of MgO:Li. These heterojunctions may provide active sites for the adsorption/ reaction of reactants/reaction intermediates,60 giving a possible explanation for the excellent catalytic activity of Li-doped MgO. As for the orange PL emission, its temperature dependence is also worth mentioning. As shown in Figure 5a, the orange PL does not show normal temperature quenching but shows a maximum in intensity at T = 225 K. A similar luminescence temperature antiquenching behavior has also been reported from anion-vacancy-related emissions in wide-gap semiconductors such as ZnO61,62 and Mg3N2.63 We62,63 have recently derived an analytical expression to represent the complex temperature dependence of the vacancy-related emission in terms of the thermally assisted electron transfer from the excited state to the emission state. Provided that the rate constant of the thermally assisted emission process is described by the Arrhenius-type equation with an activation energy EA, the temperature dependence of the emission intensity I(T) is described by63

(4)

where the parameter a, b, and ℏω represent the band gap energy at 0 K, the strength of the electron−phonon coupling, and the average phonon energy, respectively. Since the observed PLE spectra have a low-energy tail, we defined the onset energy as the energy at which the normalized maximum intensity is 0.5 just for comparison purposes (see the inset of Figure 5b). As shown in Figure 8, the temperature dependence

Figure 8. Temperature dependence of the low-energy onset of the PLE spectra for the orange PL bands shown in Figure 5b. The solid line is the fit of the data with eq 4.

of the onset energy is well described by eq 4. The fitted value of ℏω is 40.2 meV, which is in reasonable agreement with a typical optical phonon energy of MgO (ℏω ≈ 50 meV).54 To further estimate the surface bandgap energy of the MgO:Li powder, we carried out the diffuse reflectance measurement. The optical absorbance is approximated from the reflectance data by the Kubelka−Munk function55 as given by (1 − R )2 (5) 2R where R is the diffuse reflectance. We found that the Tauc plot56 shows a linear region in which the following equation is fulfilled (Figure 9): F (R ) =

( ) E

I (T ) ∝

1 + A 2 exp − k AT B

A1 + A 2 exp F

( ) + A exp(− ) E − k AT B

3

ENR kBT

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The Journal of Physical Chemistry C where ENR is an effective activation energy for nonradiative recombination, and Ai (i = 1,2, and 3) are fitting constants. We found that the temperature antiquenching behavior of the orange PL emission is well described by eq 7 (see the solid line in the inset in Figure 5a). The fitted value of EA (120 meV) is less than the half of that of ENR (280 meV), rationalizing that the expected thermally assisted emission dominates over nonradiative recombination at lower temperature regions (T< ∼ 250 K). This scheme is also consistent with the time-resolved PL measurements shown in Figure 7, demonstrating that the decay time is almost constant up to the temperatures of ∼250 K. At temperatures higher than ∼250 K, the thermally assisted emission process will compete with nonradiative recombination, which leads to a decrease in both τ and β. From an analogy of the thermally assisted defect-related emissions in ZnO61,62 and Mg3N2,63 it is probable that the emission centers responsible for the orange PL are oxygen vacancies at the surface or inner-surface of MgO:Li. A possible energy scheme for the orange PL is given in Figure 10.

at higher temperatures. This temperature dependence can be interpreted in terms of the trap-controlled mechanism.50 According to this mechanism, the emission process is governed by a multiple trapping mechanism of photoexcited electrons. At low temperatures, a random release from the temporary traps occurs at longer times, resulting a dispersive character in the overall emission process. The release processes are thermally activated with increasing temperature, leading to a linear increase in β. At high temperatures, the temporary traps are thermally emptied, and, accordingly, β will depend solely on the sample topology and becomes independent of temperature. Possible candidates for the blue emission center are bulk oxygen vacancies, or F-type centers, in the MgO:Li microcrystals although their electronic states will be influenced by nearby Li ions. As for the F and F+ centers in MgO, we45 have shown that photoionization and the subsequent thermally assisted tunneling motion of trapped carriers will occur during the emission processes, in harmony with the trap-controlled emission mechanism mentioned above. In MgO:Li, however, the electronic states of the F-type centers will be strongly affected by and interacted with the hole states created by Li substitutional defects.39,42 Consequently, defect complexes comprising F-type centers and Li ions will be formed, leading to the modification of the emission energy of the F-type centers. Considering that the present MgO:Li sample exhibits a single and rather symmetric PL peak at 2.81 eV, we suggest that a single defect complex is responsible for the blue PL emission. However, further investigations need to be done to clarify the origin of the emission center responsible for this blue PL.

5. CONCLUSIONS We have prepared the MgO:Li microcrystals using the solid phase reaction between B2O3 and metals of Mg and Li. The resulting MgO:Li microcrystals show the blue (2.8 eV) and orange (2.1 eV) PL emissions under excitation with photons of energies of ∼5 and ∼3 eV, respectively. The blue PL emission is supposed to result from F-type centers in the bulk although their electronic states will be modified by the nearby Li ions incorporated into the MgO structure. From the temperature dependence of the PL decay profiles, we suggest that a multiple trapping of photoexcited electrons is involved in the emission process of the blue PL. On the other hand, the orange PL most likely results from oxygen-vacancy-related centers at the surface and subsurface whose bandgap energy is reduced down to a value of ∼3 eV. Note also that the orange PL shows the temperature antiquenching behavior, similar to the case of the thermally assisted emission induced by interband transitions in wide-gap semiconductors. Thus, we suggest that the orange PL emission is initiated by the surface interband transition, followed by the thermally assisted electron transfer from the excited state to the emission state. This emission scheme is also consistent with the temperature dependence of the PL decay profiles. The present results demonstrate that when MgO is prepared under highly reducing conditions in the presence of Li, its defect-related PL properties are influenced by the Li ions incorporated into the MgO lattice. This is true not only for the bulk but also for the surface, hence shedding light on the effect of aliovalent doping on the electronic structure of MgO.

Figure 10. Model of the surface photoexcitation, thermally assisted electron transfer, and emission processes that are responsible for the orange PL emission in the MgO:Li microcrystals. Surface oxygen vacancy is considered as a probable candidate for the emission center.

4.2. Blue PL Emission. In contrast to the case of the PLE spectrum of the orange PL, the PLE spectrum of the blue PL does not have a sharp low-energy onset (see the solid-line PLE spectrum in Figure 3c), implying that the band edge excitation is not involved in the blue PL emission process. We should note that the blue PL emission characterized by an excitation/ emission couple of 5.1/2.8 eV (240/440 nm) and ∼1 μs decay time has not been observed in the PL spectra of nominally stoichiometric MgO powders.64−69 Hence, this blue PL emission cannot be attributed neither to the low-coordinated surface sites nor to the surface OH species, although both of which generally show broad PL bands in a blue spectral region.64−69 We also found that unlike the case with the orange PL, the intensity of the blue PL is almost unchanged even after chemical etching with dilute hydrochloric acid. This strongly suggests that the emission center responsible for the blue PL emission is present in the bulk of the MgO:Li microcrystals. As in the case of the orange PL, the decay curves of the blue PL show a stretched exponential behavior (Figure 6a). However, the temperature dependence of the stretching index β is quite different between these two PL processes. As for the blue PL emission, β shows an almost linear increase at low temperature region from 78 to ∼250 K and becomes constant G


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Corresponding Author

*(T.U.) E-mail: [email protected]. Telephone: +81 78 803 5681. Fax: +81 78 803 5681. ORCID

Takashi Uchino: 0000-0002-4899-3078 Notes

The authors declare no competing financial interest.

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