Strong red emission in Bi3+ and Mn4+ codoped Mg3.5Ge1.25O6 phosphors applied in optical agriculture

Strong red emission in Bi3+ and Mn4+ codoped Mg3.5Ge1.25O6 phosphors applied in optical agriculture

Author’s Accepted Manuscript Strong red emission in Bi3+ and Mn4+ codoped Mg3.5Ge1.25O6 phosphors applied in optical agriculture Qiying Huang, Weihao ...

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Author’s Accepted Manuscript Strong red emission in Bi3+ and Mn4+ codoped Mg3.5Ge1.25O6 phosphors applied in optical agriculture Qiying Huang, Weihao Ye, Guangqi Hu, Xiaotang Liu www.elsevier.com/locate/jlumin

PII: DOI: Reference:

S0022-2313(18)31841-6 https://doi.org/10.1016/j.jlumin.2019.01.047 LUMIN16245

To appear in: Journal of Luminescence Received date: 8 October 2018 Revised date: 6 January 2019 Accepted date: 24 January 2019 Cite this article as: Qiying Huang, Weihao Ye, Guangqi Hu and Xiaotang Liu, Strong red emission in Bi3+ and Mn4+ codoped Mg3.5Ge1.25O6 phosphors applied in optical agriculture, Journal of Luminescence, https://doi.org/10.1016/j.jlumin.2019.01.047 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Strong red emission in Bi3+ and Mn4+ codoped Mg3.5Ge1.25O6 phosphors applied in optical agriculture

Qiying Huanga, Weihao Yea, Guangqi Hua, Xiaotang Liua,b* a

Guangdong Provincial Engineering Technology Research Center for Optical Agricultural, College of Materials and Energy, South China Agricultural University, b Department of Physics, University of Texas at Arlington, Arlington, TX, 76019, US [email protected]

Abstract Bi3+ and Mn4+ co-activated Mg3.5Ge1.25O6 phosphors were synthesized via a solid-state method in air. Codoping Bi3+ into the matrix leads to effective enhancement of the deep-red emssion (659 nm) compared to Bi3+-free samples. The spectroscopic properties are discussed based on the Bi3+(6s2)→Ge4+(3d10) metal-to-metal charge transfer and energy transfer between Bi3+ and Mn4+. All samples emit deep-red emission ascribed to 2Eg→4A2g transition of Mn4+. We have carried out the X-ray power diffraction (XRD), photoluminescence spectrum (PL), field emission scanning electron microscope (FESEM), fourier transform infrared spectroscopy (FT-IR) and UV-Vis absorption measurement to investigate the influences induced by Bi3+ incorporation.

KEYWORDS: Germanate, bismuth, metal-to-metal charge transfer, energy transfer

1. Introduction The attention of the red-emitting phosphors have been recently raised. To date, due to the abilities to improve the colour rendering index of white LEDs and promote plant growth, red phosphors have widespread practice and extended value in lighting, solar cell, display field, signal indication and plant cultivation[1-16]. Especially, the red-emitting region(600~680nm) is required for plant’s photosynthesis[3, 7, 8]. Thus, it’s significant to synthesize an efficient red phosphor which can convert the harmful UV light and underutilized green light for plant’s growth into red light to improve the photosynthesis and increase the counts of the carotenoids and chlorophyll of plants. The Eu ion has been the most common activator of red phosphors ascribed to f-f(Eu2+) and/or

f-d(Eu3+) transitions. It’s widely invstigated that many typical matrix activated by Eu ion with red emission,for instance, oxide[9], sulphide[10, 11], nitride[12] , aluminate[13] , molybdate[14] , silicate[15] , ect. However, the shortage of the rare earth(RE) content limits further large-scale productions of RE activated phosphors. Hence, it’s significant to explore efficient RE-free phosphors as alternatives to RE activated phosphors. In recent years, transition-metal(TM) ions serving as luminescence centres is brought attentions. According to other literatures, numbers of TM activated phosphors with high efficiency have been successfully synthesized[16-21]. Mn4+ ions with d3 electron configuration found on octahedral sites in solid. The Mn-3d states are spilt into three- and two- fold degenerate t2g and eg states with a large gap between them. As a result, the three Mn-3d electrons of Mn4+ stay in the majority-spin t2g states and the Mn4+ can be stabilized. Mn4+-doped phosphors exhibit boardband exitation between 300 and 450nm and sharp emission varing in 600 to 740nm. The optical properties of Mn4+-doped materials strongly depend on the local crystal field environment, in other words, the covanlency of the Mn4+-ligand boding. For years, Mn4+-doped germanate has been a typical species phosphor drawing interest of various investigations and improvements[22-29]. Williams introduced the Mn4+-doped magnesium germanate red phosphor with a raw material formular 4MgO:1GeO2:0.01Mn4+ in 1947[30]. The compositon of this material was first determined as Mg28Ge10O48 with some excess MgO in 1970[31]. In the Mg28Ge10O48(MGO) crystal structure, Mg2+ and one-thirds of Ge4+ ions occupy octahedral sites[31, 32], which are Mn4+ ions found on mentioned above. This Mn4+-doped phosphor can be efficiently excited by near-UV with a strong absorbtion peak centred at around 308nm and emits red light centred at around 657nm. Due to its distinct optical properties, it has been applied in LED lighting[33]. In particularly, the emission spectrum of this phosphor well matches the absorbtion spectrum of chlorophylls responsible for photosynthesis. Consequently, this Mn4+-doped magnesium germanate red phosphor has potential to serve as an agricultural sunlight spectrum convertor. With a 6s2 configuration, Bi3+ achieves absorption and emissison as results of energy transitions between the ground state(1S0) and the excited states(3P0,1,2 and 1P1) raised by the 6s2→6s16p1 interconfigurational transitions.[34]. The 1S0→3P0 and 1S0→3P2 transitions are spin-forbidden while the 1S0→3P1 and 1S0→1P1 transitons are spin-allowed. The 1S0→1P1 transiton usually located in far ultroviolet region and not be discussed in detali here. The 1S0→3P1 transition, as a spin-allowed transition, couples and mixes with the 1P1 state. In addtion, the 3P1→3S0 transition is a emtting state[34, 35]. 3P0 is also a emitting state in a isolated Bi3+ ion but it’s a metastable state act as a electron trap and the transition probability from 3P0 is small. On the other hand, Bi3+ has been widely reported for its metal-to–metal charge transfer (MMCT) character between its outer 6s2 configuration and the host metal cations with d0 or d10 configuration.[34-37]. Roy H.P. Awater et al.[35] have put forward a model that an electron transfers from the 1S0 ground state to the bottom of the conduction band of host to explain MMCT taken place in compounds containing Bi3+. Philippe Boutinaud[34] has illustrated different configurations of the energy levels leading to luminescence to clarify the relationship between 1S0→3P1/3P1→3S0 transition and MMCT in Bi3+ doped solids and introduced a model to predict MMCT in oxide compouds doped Bi3+ and d0 or d10 metals (Mn+) . In recent years, numbers of literatures[36, 38-41] about applications based on the MMCT between Bi3+ and d0 mental cations (e.g., Ti4+, Zr4+, Nb5+, W6+ and Mo6+ etc.) have been published. However, reports about MMCT between Bi3+ and d10 metal cations, for instance Ge4+, are relatively rare.

It’s been well known that emission enhancement can be achieved via codoping with Bi3+ which acts as a sensitizer via codoping with a variety of luminescence centres[42-46]. However, the interaction between Bi3+ and Mn4+ doped germanate is rarely reported so far. It’s worth developing designs of efficient phosphors based on germanate codoping Bi3+ and Mn4+. In this paper, Bi3+ and Mn4+ codoped-MGO phosphors have been synthesized via traditional high temperature solid-state method in air. A relatively efficient emission enhancement has been achieved comparing with the Mn4+ doped-MGO. To clarify the reasons of the emission enhancement, analyses about the luminescence properties, crystal structure, mophology, uv-vis absoption, IR spectroscopy, and quantum yield of the products have been carried out.

2. Experimental 2.1 Sample preparation All raw materials were purchased from Alddin Chemical Reagent Company and used directly without further purification. The stoichiometric ratio of raw materials was 4MgO:1GeO2:0.01MnCO3:xBi2O3 (x=0, 0.005, 0.0075, 0.01, 0.0125, 0.015,0.02). Proper amount of these materials were mixed in an agate mortar with addition of ethanol and then triturated thoroughly. The obtained powders were dried by ethanol evaporation and transferred to a corundum crucible, followed by annealing at 1100℃ in an ambient atmosphere with a heating rate of 10℃/min for 13 hours in a muffle furnace.

2.2 Characterization The XRD measurements of synthesized samples were carried out using a Rigaku-Ultima Ⅳ X-ray diffractiometer with Cu-Kα radiation(λ = 0.154056 nm) at 40 kV and 30 mA in a scan range of 10-80º. The morphology of the particles were examined by JSM-7001F field emission scanning electron microscope (FESEM). IR spectra were measured by Thermo Scientific Nicolet iS10 fourier transform infrared spectrometer. UV-Vis absorption spectra were measured using a Shimadzu UV-2550 spectrophotometer. The photoluminescence (PL) of the powder samples was measured by a fluorescence spectrophotometer. Fluorescence at 77kand quantum yield were obtained by Edinburgh FLS980 steady/transient fluorescence spectrometer.

3. Results and discusssions 3.1 XRD XRD patterns with varing raw material’s proportion of Bi ions are shown in Fig. 1. Peaks belong to MgO (JCPDS Card PDF#45-0946) have been marked out by red patches. The XRD pattern of the Bi-free sample indicated a dominating Mg2GeO4 (JCPDS Card PDF#36-1479) phase with excessive MgO. As Bi ions being codoped into the systerm, XRD patterns changed significantly indicating phase of Mg3.5Ge1.25O6 (JCPDS Card PDF#47-0304) occurred even though the concentration of Bi ions is relatively low (x=0.005). As the concentration of Bi ions increasing, peaks of Mg3.5Ge1.25O6 grew obiously, while intensities of peaks belong to excessive MgO reduced. It can be illustrated from Fig. 1 that XRD patterns of samples are approximately consistent with the PDF standard card of Mg3.5Ge1.25O6 and a little amont of MgO when x is up to 0.0075, meanwhile the relative trace of Bi and Mn ions didn’t induce any significant impure peaks.

From Fig. 1, we consider that Bi codped can efficiently accelerates crystal transfer of the systerm from Mg2GeO4 into Mg3.5Ge1.25O6 under a same synthesis condition while the Bi-free sample just forms the first crystal structure. As has been widely reported, Mn4+ is fond of octahedral occupation site. The Mg2GeO4 phase consists of GeO4 tetrahedrons which arrange in a zigzag-chain. Mg atoms occupy two positions in the matrix. Partial Mg ions bond the GeO4 with Mg-O bond to form the zigzag-chain, while the others exist as lone Mg atoms which can be substituted by Mn4+[33]. On the other hand, Ge atoms exist as GeO4 tetrahedron and GeO6 octahedron in the composition of Mg3.5Ge1.25O6. GeO6 octahedrons should be preferencial positions for Mn4+ to occuppy for the equal electrical charge and similar radius (RGe4+= RMn4+= 0.53Å) between Ge4+ and Mn4+[25]. According to Brik[47], the Ge site of GeO6 has the highest symmetry in favor of Mn4+ substitution. In addition, all Mg sites are six coordinated with O forming MgO6 octahedrons in the Mg3.5Ge1.25O6 lattice. Due to the preference of Mn4+ to octahedral sites, Mn4+ would sbustitute the Mg sites despite the unbalanced charge and different radius. Obviously, Mg3.5Ge1.25O6 is a preferencial host for Mn4+ comparing to Mg2GeO4.

Fig. 1 XRD patterns of MGO:xBi3+,0.01Mn4+ at different Bi3+ concentrations: x=0, 0.005, 0.0075, 0.01, 0.0125, 0.015, 0.02.

3.2 PL Spectrums The excitation spectrums with two absorptions peaks monitored at 659 nm are plotted at Fig. 2. As is well known, 4A2→4T1 and 4A2→4T2 are spin-allowed transitions of Mn4+. The stronger absorption peak at the range of near-UV light is corresponding to 4A2→4T1 transition of Mn-d3 configuration. As codoping Bi3+ into the sample, the center of this peak red shift from 294 nm to 308 nm. The other absorption peak centred at 427 nm is corresponding to 4A2→4T2 transition of

Mn-d3 configuration. The centred wavelength of this peak are almost constant in the blue region.

Fig. 2 Excitation spectrum of MGO:xBi3+,0.01Mn4+ (x=0, 0.005, 0.0075, 0.01, 0.0125, 0.015, 0.02) monitored at 659 nm. Depending on the d10 electronic configuration of Ge4+, the MMCT taken place in Bi3+ doped MGO host is expressed as Bi3+(6s2)/Ge4+(3d10) →Bi4+(6s1)/Ge3+(4s1)[48, 49]. The dominant modes of energy transfer in samples with Bi3+ incorporation are the MMCT between Bi3+ and the host and transitions between 1S0 and 3P1 states (1S0→3P1/3P1→3S0) which is related to 6s2→6s16p1 of Bi3+. Considering the 3P0 emitting states, the emssion between 3P1, 3P0 and 3S0 is assigned to 3 P1,0→3S0 because 3P1 and 3P0 locate very closely. The top of the valence band consists of Bi3+(6s) and O2-(2p) orbitals, while Bi3+(6p) and Ge4+(3d) orbitals locate at the bottom of the conduction band in Bi3+-doped MGO host. Due to the short Stokes shift, the efficiency of energy transfer in MGO:Bi3+ is high enough to result in luminscence quenching at room temperature. Hence, the MGO:Bi3+ glows inefficiently and energy transfers from Bi3+ to other activators if there are any suitable activators incorporated simultaneously into the host at room temperature. However, the emssion of MGO:Bi3+ can be detected at very low temperature because of the reduction of energy transfer at such a low temperature. Consequently, Fig. 3 presents the emission spectrum of MGO:Bi3+ exicted by 300 nm at 77K and the exictation spectrum of MGO: Mn4+ monitered at 659 nm at room temperature. Compared the two spectrums in Fig. 3, a sinigficant overlap between the emission band of MGO:Bi3+ amd the exicitation band of MGO:Mn4+ can be observed. Therefore, a speculation that effective resonance-types of energy transfer in Bi3+ and Mn4+ codoped-phosphors should be possible. According to above disscussion, optical transitions in Bi3+ and Mn4+ codoped-MGO are explained by schematic diagram illustrated in Fig. 4. Among the energy levels of Mn4+, 2E is the emitting state while the 2T1 and 2T2 states are not presented in Fig. 4. Firstly, electrons of Bi3+ ions

transfer to their 1P1 or 3P1 excited states or MMCT state under exictation of the UV light. Bi3+ ions non-irradiation relax to their MMCT state or 3P1,0 state. Non-radiation relaxation from MMCT state to 3P1,0 state will probably follow. The energy transfer processes between Bi3+ and Mn4+ ions take place via 3P1,0(Bi3+)→2E(Mn4+). On the other hand, Mn4+ ions was excited to their 4T1 state under UV radiation and 4T2 state under blue radiation. Then Mn4+ ions non-radiation relax to their 2 E state. Subsequently, the system returns from the lowest excited state 2E(Mn4+) to the ground state 4A2(Mn4+) via releasing a deep-red emission (659 nm).

Fig. 3 (a) Excitation spectrum of MGO:0.01Mn4+ monitered at 659 nm at room temperature. (b) Emission spectrum of MGO:0.0125Bi3+ monitered at 300 nm at 77K.

Fig. 4 Energy level, electron transitions and energy transfer schematic diagram of Bi3+ and Mn4+

in MGO matrix Fig. 5 shows the emission spectrums excited by 308 nm of the samples. Locations amd shapes of all the emission spectrums are almost constant despite the Bi3+ concentration varying. Each emission spectrums have two peaks of which the stronger one centres at 659 nm and the comparetively lower one centres at 633 nm are corresponding to the spin- and parity-forbidden 2 Eg→4A2g transition of Mn4+. From Fig. 5, emission intensities of samples codoping Bi3+ are efficitently enhanced even though the Bi3+ concentrations are relatively low. As Bi3+ concentration increasing, emssion intensities increase and reach a maximum when x=0.0125. The intensity of 0.0125 Bi3+ codoped sample is about 10.608 times to the Bi3+-free sample. The inset in Fig. 5 displays the intensities of the 659 nm emission of all the samples. In addition, quantum yield (QY) of MGO:0.0125Bi3+,0.01Mn4+ is increased to 49.75% while QY of MGO:Mn4+ is 34.99%. The efficient enhancement of emission intensity partly relates to the cystal transition from Mg2GeO4 to Mg3.5Ge1.25O6 via codoping Bi3+. As the phase changing, more proper sites are available for luminescence centre, then higher emission intensity can be achieved. According to Fig. 1, crystal transition is almost completed when the Bi3+ concentration reaches 0.0075. As seen from Fig. 5, emission intensity achieves further enhancement after the Bi3+ concentration in excess of 0.0075 and the maximum appears when the Bi3+ concentration reaches 0.0125. It further proves interactions between Bi3+ and the Mg3.5Ge1.25O6 matrix. However, codoping Bi3+ ions would give rise to charge imbalances and symmetry distortion of crystal field. In addition, a charge transfer, among the excessive codoping Bi3+, from one Bi3+ to another neighboring Bi3+ forming Bi2+ and Bi4+ is possible. Roy H.P. Awater[35] introduced this charge transfer as intervalence charge transfer (IVCT) which can lead to efficiently quenching of luminescence of Bi3+. As a result, introducing excessive Bi3+ ions into lattice would reduce the emssion intensity.

Fig. 5 Emission spectrum of MGO:xBi3+,0.01Mn4+ (x=0, 0.005, 0.0075, 0.01, 0.0125, 0.015, 0.02) excited by 308 nm. The inset provides the 659 nm emission intensities of all samples.

3.3 FESEM Fig. 6 depict the morphology of MGO:0.01Mn4+ and MGO:0.0125Bi3+,0.01Mn4+ with different manificaiton times. Fig. 6 (a), 6 (b) and 6 (c), which are FESEM photographs of MGO:0.01Mn4+, present irregular layer structure with agglomeration composing this sample. It can be described from Fig. 6 (d), 6 (e) and 6 (f) that the morphology of MGO:0.0125Bi3+,0.01Mn4+ is angular cubic-like particles. The estimated average size of this sample is approximately 1 μm. Obviously, Bi3+ codoped crystallized particles through morphology transitions of the samples. Combining XRD patterns and FESEM photographs of samples, Bi2O3, with a melting point at 825 ℃, works as a flux to promote the growth of Mg28Ge10O48 and regularize the mophology of the samples .

Fig. 6 (a, b, c) SEM images of MGO:0.01Mn4+ with manification times of (a)×3000, (b)×8000, (c)×20000. (d, e, f) SEM images of MGO:0.0125Bi3+,0.01Mn4+ with manificaiton times of (d)×3000, (e)×8000, (f)×20000. 3.3 UV-vis Spectrums Fig. 7 presents the UV-vis absorption spectrum of (a) MGO, (b) MGO:0.0125Bi3+, (c) MGO: 0.01Mn4+, (d) MGO:0.0125Bi3+,0.01Mn4+. Comparing to other spectrums, the spectrum of MGO shows weak absorptions in the range of broad spectrum. The (b) cruve of Fig. 7 illustrates doping Bi3+ into the MGO lattice makes significant increament from about 450 nm to ultraviolet region. The absorption spectrums(c,d) of Mn4+ activated samples with shoulders ascribed to 4A2g→4T2g and 4A2g→4T1g transition of Mn4+, are basically corresponding to the descriptions from ref [50, 51]. Compared to Bi3+-free MGO:0.01Mn4+, Bi3+ codoped raises absorption in the rang of about 455 nm through ultraviolet region. The optical band-gap values of the samples can be determined by UV-vis spectrums based on Eq. (1), where α is the absorption coefficient, C is a constant, h is Plank’s constant, ν is frequency and Eg is the optical band gap. The absorption coefficient α increased in proportion to absorbance A according to lambert-beer’s law. Consequently, the values of optical band gap are

estimated by the extrapolation of the linear portion of the plots (Fig. 8) of (Ahν)2 versus hν when (Ahν)2 =0.

α(hν) = C(hν- Eg)1/2 (1) From Fig. 8, values of the optical band-gap energy are 4.628 eV, 3.236 eV, 2.920 eV and 2.861 eV for (a) MGO, (b) MGO:0.0125Bi3+, (c) MGO: 0.01Mn4+ and (d) 3+ 3+ MGO:0.0125Bi3+,0.01Mn4+, respectively. Compared to Bi -free samples, doping Bi into the lattice narrows the optical band-gap of the samples consistent with the presentation in Fig. 8. Additional hybridization of the substituted Bi3+ (6s) and O2(2p), both of which located at the top of the valence band, pushed up the top of the valence band[34] inducing a band-gap narrowing. From Fig. 7 and Fig. 8, Bi3+ codoping shows positive sensitization to the MGO:Mn4+-based phosphor.

Fig. 7 UV-vis absorption spectrum of (a) MGO, (b) MGO:0.0125Bi3+, (c) MGO: 0.01Mn4+, (d) MGO:0.0125Bi3+,0.01Mn4+.

Fig. 8 The band gap values of (a) MGO, (b) MGO:0.0125Bi3+, (c) MGO:0.01Mn4+, (d) MGO:0.0125Bi3+,0.01Mn4+ are determined from the plots of (Ahν)2 versus hν. 3.4 FT-IR Spectrums The FT-IR spectrums of (a) MGO, (b) MGO:0.01Mn4+, (c) MGO:0.0125Bi3+, (d) MGO:0.0125Bi3+,0.01Mn4+ are illustrated in Fig. 9. All plots with a strong band between 3000 and 3730 cm-1 are attributed to the stretching vibration assigned to hydroxyl (O-H) groups are the evidence of moistrue from the environment. A well-defined located at about 1640 cm-1 are attributed to the vibration band of the Mg2+[52]. Bands of Ge-O-Ge and O-Ge-O stretching and deformation modes are in the range between 500 to 1000 cm-1. GeO6 octahedra is indicated by a characteristic peak of Ge-O- at around 750 cm-1 related to bridging oxygen band with a relative weaker peak at around 850 cm-1 related to the stretching mode of the non-bridging oxygen[53, 54]. In this region, peaks belong to plots of samples codoping with Bi3+ are much more intense. This is consistent with the XRD patterns that codoping Bi3+ into the matrix promote the growth of the Mg3.5Ge1.25O6 with more GeO6 sites. The bands below 500 cm-1 attribute to stretching vibrations of the Bi-O bonds[55]. Peak at around 850 cm-1 ,which is related to [BiO6] octahedral units[56], didn’t appear in the spectrum of Bi-free sample. A weak band ,in the range between 671 cm-1 to 710 cm-1 and centred at around 690 cm-1, appears in the plots of all the samples consisted of Bi3+ doped MGO matix while not be observed in the plots of MGO, MGO:0.01Mn4+ and Bi2O3[57]. We supposed this band may be ascribed to linkage vibration of Bi-O-Ge. Comparing the FT-IR spectrum of the Bi3+-free and Bi3+-doped samples, it can be indicated that Bi3+ ions were codoped into the matrix successfully that form Bi-O-Ge bonds and influence vibration of other bonds within the matrix.

Fig. 9 FT-IR spectrum of (a) MGO, (b) MGO:0.01Mn4+, (c) MGO:0.0125Bi3+, (d) MGO:0.0125Bi3+,0.01Mn4+.

Conclusions Highly effective phosphor MGO:Bi3+,Mn4+ with a deep-red emission centered at around 659 nm for optical agriculture was prepared via a tradditional solid-state method. Bi3+ ions codoped into the matrix provibed by Bi2O3 among raw materials. Bi2O3 acts a flux to promote the crystal growth of Mg3.5Ge1.25O6 and regularizes the morphology of the products. The phosphor can be effectively excited in the region of near-UV to blue light with a stronger peak centered at around 308 nm and a weaker one centered at around 427 nm. Codoping Bi3+ and Mn4+ into MGO can largely enhance the emssion compared to MGO:Mn4+. The optimal doping concentrations of Bi3+ and Mn4+ are 0.0125 and 0.01 respectively. Comparing the emission intensity between MGO:0.0125Bi3+,0.01Mn4+ and MGO:0.01Mn4+, the latter achieved more than 10 times enhancement. QY of the samples can be also increased by codoping Bi3+. However, QY of all samples are not very high due to the high-temperature calcination. MMCT about Bi3+(6s2)/Ge4+(3d10) → Bi4+(6s1)/Ge3+(4s1) and energy transfer between Bi3+ and Mn4+ are induced to explain the great emssion enhancement. Moreover, increasement of absorbance, optical band-gap narrowing and changes of the FT-IR spectrum for MGO:Bi3+,Mn4+ also improve positive effects of Bi3+ codoping.

Acknowledgments This work is supported by the National Natural Science Foundations of China (No. 21271074), t he Teamwork Projects funded by the Guangdong Natural Science Foundation (No. S20130300128

42), and the Science and Technology Innovating Project funded by the Guangdong Province (No. 2014B090901045).

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