1. Introduction

Persistent phosphors are the materials that exhibit long durations of luminescence after removing the excitation source and are widely used as luminous paints in dial plates, interior accessories, night lighting after extinction and safety signs. The luminescent colors, blue and green are commercially obtained by Eu²⁺-activated alkaline earth aluminates or silicates co-doped with other rare earth ions [1–3]. Though several orange or red persistent phosphors have been reported [4–6], red persistent phosphors with higher brightness and longer luminescence durations comparable to commercialized Eu²⁺-activated persistent phosphors are in high demand.

In addition, red to near-infrared persistent phosphors have been attracted great attention because of their potential application in fluorescent in vivo imaging [7–9]. Fluorescent in vivo imaging is a promising method to visualize biological tissues because it is radioactive-material-free technique. Red to near-infrared luminescence is used because of the high transmittance of these signals through biological tissues [10]. Though many materials such as organic molecules, quantum dots and rare earth doped compounds have been studied [11], one of the common problems of using these materials is the noise due to the excitation light, scattering of light sources and auto fluorescence of biological tissues. By using red to near infrared persistent phosphors, a high signal-to-noise ratio can be obtained because the persistent phosphor is excited before in vivo injection, which prevents the noises mentioned above. In recent years, red to near infrared long persistent phosphors activated by Mn²⁺ [7, 12] or Cr³⁺ [9, 13–15] ions have been reported and in vivo imaging has been demonstrated [7–9, 12, 14].

Mn²⁺-activated phosphors show various luminescent colors from blue-green (490 nm) to red (750 nm) in different hosts. Generally, luminescence of Mn²⁺ ions is due to 3d-3d intra-atomic transition Mn²⁺:⁴T₁→⁶A₁. In the MgGeO₃ lattice, Mn²⁺ in octahedral coordination shows strong red luminescence peaked at 680 nm due to the Mn²⁺:⁴T₁→⁶A₁ transition under ultraviolet (UV) excitation [16] and charge transfer (CT) from Mn²⁺→Mn³⁺ + e^-, which is reported in several hosts such as Zn₂SiO₄, ZnGa₂O₄ and MgSiO₃ [4, 16–18]. Red persistent luminescence in MgGeO₃:Mn²⁺ ceramics and enhanced persistent luminescence by Yb³⁺ co-doping have been reported [16, 19]. Iwasaki et al. discussed the persistent mechanism in MgGeO₃:Mn²⁺-Yb³⁺ as electron-trapping process. The Mn²⁺ is photo ionized by UV irradiation at 254 nm as Mn²⁺→(Mn²⁺)⁺ + e^- (CT transition) and the electron would be trapped on Yb³⁺ as Yb³⁺ + e^-→Yb²⁺.

Similar to Yb, Bi can also take divalent and trivalent states. Due to the large number of possible valence states and strong interaction with the ligand field, Bi exhibits a broad variety of optical properties and potential applications such as white light emitting diodes (white LEDs) [20], luminous paints [21, 22] and optical amplifiers in telecommunication systems [23, 24]. Because the trivalent state is most stable, 6s²→6s6p optical transitions of the trivalent state have been studied in many host materials [25–27]. Divalent state, Bi²⁺ can also be obtained in several crystals under reducing conditions and several kinds of Bi²⁺-activated orange to red phosphors have been reported [20, 28–30]. For persistent luminescence, our group reported that Bi shows white persistent luminescence [21] and also be an efficient co-dopant for the ZnGa₂O₄:Cr³⁺ [31].

Considering Yb effect on the persistent luminescence in MgGeO₃:Mn²⁺ and the similar nature of Yb and Bi that the trivalent state is stable and tend to be divalent state, Bi is a potential element to be an electron trapping center during persistent luminescence in MgGeO₃:Mn²⁺ in the same manner as Yb.

In this paper, we report the red long persistent phosphor MgGeO₃:Mn²⁺-Bi³⁺. By co-doping with Bi, the material showed 30 times stronger persistent luminescence than MgGeO₃ singly doped with Mn. We propose a mechanism of red persistent luminescence in the Mn-Bi co-doped MgGeO₃. The results suggest that the Bi³⁺ ion functions as an electron-trapping center during persistent luminescence.

2. Experimental

Polycrystalline ceramics of Mg_(0.998-x)Mn_0.002Bi_xGeO₃ (x = 0, 0.001, 0.005, 0.01, 0.02) were synthesized by solid state reaction. Commercial powders of MgO (99.99%), GeO₂ (99.99%), MnCO₃ (99.999%) and Bi₂O₃ (99.99%) were used as starting materials. Batches of the starting powders were mixed in the presence of ethanol. After drying, the mixture was pressed into pellets measuring 13 mm in diameter and sintered at 1200 °C for 5h in air. As references, non-doped and Bi singly doped polycrystalline ceramics with a composition of Mg_(1-y)Bi_yGeO₃ (y = 0, 0.005) were also synthesized by the same method. Because MgO becomes hydroxide and carbonate, which react with H₂O and CO₂ in air to some extent, the raw material was sintered at 1000 °C for 10 h before being weighed. A cation ratio [Ge]/([Mg] + [Mn] + [Bi]) of 1.05 was adopted to avoid the generation of the Mg-rich impurity phase, Mg₂GeO₄ due to the volatile nature of GeO₂ above 1250 °C.

The crystal phases of the sintered samples were identified by X-ray diffraction (Rigaku, Ultima IV). Diffuse reflectance spectra were measured using a scanning-type spectrophotometer (Shimadzu, UV3600) with a BaSO₄-based integrating sphere. Photoluminescence (PL), photoluminescence excitation (PLE) spectra and time dependent PL intensities were measured using a fluorescence spectrometer (Shimadzu, RF-5300). PL and persistent luminescence spectra of the sample with x = 0.005 were measured by a CCD detector (Ocean Optics, QE65Pro) under 300 nm excitation, which was obtained by combination of a 300-W Xe lamp (Asahi spectra Co., Ltd., MAX-302) and a band pass filter (300 nm). Afterglow decay curves were measured using a radiance meter (KONICA MINOLTA, Glacier X) after 10 min irradiation with UV light over a wavelength range of 250 to 400 nm, which was obtained by the Xe lamp with a mirror module. Thermoluminescence (TL) measurements were performed in a cryostat (Advanced Research Systems, Helitran LT3). The samples were irradiated with UV light for 10 min at 100 K. The heating rate used to obtain the curves was 10 K/min.

3. Results

3.1 X-ray Diffraction

Figure 1 shows the XRD patterns of the obtained MgGeO₃ samples. For all samples, MgGeO₃ of a orthorhombic enstatite structure was obtained almost as a single phase (JCPDS No. 076-1387). No shift in the diffraction peaks was observed in these samples with increasing Bi concentration.

 

Fig. 1 X-ray diffraction patterns of non-doped Mg₁GeO₃ and Mg_(0.998-x)Mn_0.002Bi_xGeO₃ (x = 0, 0.001, 0.005, 0.01, 0.02) samples.

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3.2 Optical properties

3.2.1 Reflectance and fluorescence spectra

Figure 2 shows the diffuse reflectance spectra of the MgGeO₃ samples. The non-doped sample showed an absorption edge at approximately 220 nm with a weak absorption at approximately 300 nm. The Mn singly doped sample (x = 0) showed strong absorption in the range of 250-300 nm due to the CT transition of Mn²⁺ (Mn²⁺→Mn³⁺ + e^-). In the Mn-Bi co-doped samples (x = 0.001, 0.005, 0.01 and 0.02), the absorption intensity in the UV region increased with increasing Bi content, x. These UV absorption bands are due to the intra-atomic transition Bi³⁺:¹S₀→³P₁ and probably due to the CT transitions Bi³⁺→Bi⁴⁺ + e^- and Bi³⁺ + e^-→Bi²⁺. These assignments will be discussed later. In the visible region, no absorption peak was observed in any samples.

 

Fig. 2 Diffuse reflectance spectra of the non-doped MgGeO₃ and MgGeO₃:0.002Mn-xBi (x = 0, 0.001, 0.005, 0.01, 0.02) samples.

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Figure 3 shows the PL spectra of the Mn singly doped (x = 0) and Mn-Bi co-doped samples (x = 0.001, 0.005, 0.01, 0.02) obtained at excitation wavelength of 300 nm. All samples showed a red luminescence band that peaked at 680 nm due to the Mn²⁺:⁴T₁→⁶A₁ transition. In addition to the luminescence band at 680 nm, the Bi-doped samples (x = 0, 0.001, 0.005, 0.01 and 0.02) showed two luminescence bands that peaked at 360 and 405 nm due to the Bi³⁺:³P₁→¹S₀ transition.

 

Fig. 3 PL spectra of the MgGeO₃: 0.002Mn-xBi (x = 0, 0.001, 0.005, 0.01, 0.02) samples under 300 nm excitation.

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Figure 4 shows the PLE spectra (upper) of the MgGeO₃:0.002Mn-xBi (x = 0, 0.001, 0.005, 0.01 and 0.02) samples monitoring the Mn²⁺ red luminescence and the diffuse reflectance spectra (lower). In the visible range, the PLE spectra showed several bands due to the 3d-3d intra-atomic transitions of Mn²⁺ ion such as ⁶A₁→⁴T₁(~600 nm), ⁶A₁→⁴A₁/⁴E(420 nm) and ⁶A₁→⁴T₂(380 nm). In the UV range, strong broad PLE bands that peaked at approximately 220, 250 and 300 nm were observed in all the samples. The PLE band at 220 nm was assigned to the band-to-band transition, which is consistent with the absorption edge of the non-doped sample. The relative intensity of the PLE band at 220 nm increased with increasing Bi content, x. The Mn singly doped sample showed a broad PLE band in the range of 250-300 nm. That band can be assigned to the CT transition Mn²⁺→Mn³⁺ + e^- [4, 17, 18]. The PLE bands that peaked at approximately 300 nm increased with Bi co-doping. Therefore, this band is assigned to the ¹S₀→³P₁ transition.

 

Fig. 4 PLE spectra (upper) of the MgGeO₃: 0.002Mn-xBi (x = 0, 0.001, 0.005, 0.01, 0.02) samples with monitored Mn²⁺ luminescence at 670 nm. Diffuse reflectance spectra (lower; the same plot shown in Fig. 2) are shown for reference.

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Because two luminescence bands due to Bi³⁺ that peaked at 360 and 405 nm were observed in the Mn-Bi co-doped samples as shown in Fig. 3, the PLE spectra of the Mn-Bi co-doped MgGeO₃ sample with x = 0.005 obtained by monitoring at 360 and 405 nm were also measured and are depicted in Fig. 5(a) along with the PLE spectrum obtained by monitoring Mn²⁺ red luminescence. Figure 5(b) shows the absorption spectra of the non-doped, Mn singly doped (x = 0), Mn-Bi co-doped (x = 0.005) and Bi singly doped samples. Absorption spectra were obtained from the diffuse reflectance spectra by Kubelka-Munk equation. The PLE spectra showed two bands that peaked at 290 and 310 nm by monitoring 360 nm and 405 nm, respectively. On the other hand, the absorption spectrum of the Bi singly doped sample showed a peak at 255 nm. Comparing the PLE spectra of the Bi³⁺ luminescence with the absorption spectra of the Bi singly doped sample (nearly the same as that of the Mn-Bi co-doped sample with x = 0.005), the absorption band at 255 nm does not contribute to Bi³⁺ luminescence. The attribution of this transition will be discussed later.

 

Fig. 5 (a) PLE spectra of the Mn-Bi co-doped MgGeO₃ sample with x = 0.005 monitored at 360 (violet dotted), 405 (blue dashed) and 670 nm (red solid) and (b) absorption spectra of the non-doped, Mn singly doped, Mn-Bi co-doped and Bi singly doped samples.

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3.3.2 Persistent luminescence properties

Figure 6 shows photographs of the MgGeO₃ samples. Under fluorescent lamp illumination, as shown in Fig. 6(a), all samples appear white. Under 254 nm light excitation, the Mn-doped samples shows red luminescence. As shown in Fig. 6(b), the Mn singly doped sample with x = 0 shows the strongest photoluminescence and its intensity decreases with increasing Bi content, x. On the other hand, the persistent luminescence intensity 10 s after shutting off the excitation light is maximized in the Mn-Bi co-doped sample with x = 0.005.

 

Fig. 6 Images of the non-doped and 0.002Mn-xBi doped MgGeO₃ (x = 0, 0.001, 0.005, 0.01, 0.02) samples (a) under fluorescent lamp, (b) under 254 nm light excitation and (c) 10 sec after shutting off the excitation lamp.

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Figure 7 shows PL spectrum by 300 nm excitation and persistent luminescence spectrum of the sample with x = 0.005 10 s after shutting off the excitation light. In the PL spectrum, UV and red luminescence bands due to the Bi³⁺:³P₁→¹S₀ and Mn²⁺:⁴T₁→⁶A₁ transitions were observed. In the persistent luminescence spectrum, on the other hand, only the red luminescence band due to the Mn²⁺:⁴T₁→⁶A₁ transition and no luminescence band of Bi³⁺:³P₁→¹S₀ can be observed.

 

Fig. 7 PL spectrum by 300 nm excitation (red solid) and persistent luminescence spectrum (black dashed) 10 s after shutting off the excitation light (x = 0.005).

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The persistent decay curves of red persistent luminescence of the Mn singly doped MgGeO₃ (x = 0) and Mn-Bi co-doped MgGeO₃ (x = 0.005) samples are shown in Fig. 8 and compared with those of SrAl₂O₄:Eu-Dy and ZnGa₂O₄:Cr [31]. The radiance 60 min after shutting off the excitation light was 1.1 × 10⁻³ (mW/Sr/m²) for the Mn singly doped MgGeO₃ (x = 0), 3.1 × 10⁻² (mW/Sr/m²) for the Mn-Bi co-doped MgGeO₃ (x = 0.005), 1.5 × 10⁻² (mW/Sr/m²) for ZnGa₂O₄:Cr³⁺ and 5.8 × 10⁻¹ (mW/Sr/m²) for SrAl₂O₄:Eu³⁺-Dy³⁺. By Bi co-doping, the intensity of red persistent luminescence increased 30-fold compared with that of the Mn singly doped sample.

 

Fig. 8 Decay curves of persistent luminescence of the SrAl₂O₄:Eu-Dy, ZnGa₂O₄:Cr, MgGeO₃:Mn(x = 0), and MgGeO₃:Mn-Bi(x = 0.05) samples.

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Figure 9(a) shows the TL glow curves of the MgGeO₃: Mn-xBi (x = 0, 0.001, 0.005, 0.01 and 0.02) samples. The Mn singly doped sample showed several glow peaks at 170, 240, 310 and 420 K. The peak at 420 K showed the highest TL intensity. The Mn-Bi co-doped samples showed an additional peak at approximately 320 K. The intensity of this glow peak had a maximum in the sample with x = 0.005. For the sample with x = 0.005, the intensity of the peak at 320 K became comparable to that of the peak at 420 K, and for the sample with x greater than 0.01, the peak at 320 K showed the highest intensity. The Bi content dependence of the TL integrated intensity in the range from 260 to 360 K with photographs of the persistent luminescence of the corresponding samples is shown in Fig. 9(b). As shown in Fig. 9(b), the TL integrated intensity and persistent luminescence intensity at room temperature showed the same Bi content dependence.

 

Fig. 9 (a) TL glow curves of the MgGeO₃: 0.002Mn-xBi (x = 0, 0.001, 0.005, 0.01, 0.02) samples after excitation for 10 min by UV light source in the range of 250-400 nm at 100 K with heating rate 10 K/min and (b) (lower): Bi content dependence of the relative TL integrated intensity in the range from 260 to 360 K; (upper): photographs of corresponding samples taken at R. T. 10 s after shutting off 254 nm excitation light.

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Figure 10(a) shows the PL spectra of the Mn-Bi co-doped (x = 0.005) and Bi singly doped samples obtained by excitation at 300 nm; all samples were heated to 300 °C before measurement to remove traps. The bands at 360 and 680 nm are due to Bi³⁺ and Mn²⁺ respectively as mentioned above. In Fig. 10(b), the PL intensities of the Mn-Bi co-doped (x = 0.005) and Bi singly doped samples obtained by monitoring at 360 and 680 nm are plotted as a function of time. In the Mn-Bi co-doped sample, the intensity of luminescence due to Bi³⁺ at 360 nm decreased, whereas the intensity obtained by monitoring Mn²⁺ luminescence at 680 nm showed an increase. On the other hand, for the Bi singly doped sample, the intensity of Bi³⁺ luminescence at 360 nm remained unchanged. The PL spectra of the Mn-Bi co-doped (x = 0.005) sample were measured by exciting at 300 nm (dotted) and after 3 min continuous irradiation with 300 nm light (red solid) as shown in Fig. 10(a). After 3 min of irradiation, the luminescence band of Bi³⁺ decreased, whereas that of Mn²⁺ increased.

 

Fig. 10 (a) PL spectra of the Mn-Bi co-doped (x = 0.005) (upper, dotted and red solid) and Bi singly doped sample (lower, dashed) at an excitation wavelength of 300 nm. The PL spectra were measured immediately after exciting at 300 nm (dotted) and with 3 min of continuous irradiation with 300 nm light (red solid). (b) Time dependence of the PL intensity of the Mn-Bi co-doped (x = 0.005) and Bi singly doped samples obtained by monitoring at 360 and 680 nm.

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4. Discussion

4.1 Bi incorporation into lattice

Because a single phase of enstatite MgGeO₃ in the Bi₂O₃-doped samples was confirmed by the XRD results, it is suggested that Bi ion is well incorporated into the MgGeO₃ lattice. In the enstatite structure, Mg²⁺ is in six-fold coordination and Ge⁴⁺ is in four-fold coordination [32]. Because the ionic radius of Ge⁴⁺(0.39 Å) in the four-fold coordination is too small for Bi ions, 1.03 Å for the trivalent state and 0.76 Å for the pentavalent state in the six-fold coordination [33], Bi ion may be incorporated into Mg²⁺ sixfold sites (0.72 Å). Considering the charge mismatch between Mg²⁺ and Bi ions, Bi ion is probably incorporated into divalent Mg²⁺ sites as trivalent state.

4.2 Assignments of Bi related electronic transitions

The ground state of Bi³⁺ with 6s² electronic configuration is expressed as ¹S₀. The 6s6p excited states consist of three triplet ³P_J (J = 0, 1, 2) and one singlet ¹P₁. Although the transition from the ground state to the first excited level, ¹S₀→³P₀, is strongly forbidden, transitions from the ¹S₀ ground state to the ³P₁, ³P₂ and ¹P₁ states are observed in optical absorption spectra, and these transitions are called the A-band (strong), B-band (weak) and C-band (very strong), respectively [4].

As indicated by the PLE results, the A-band (¹S₀→³P₁) was observed at 290 and 310 nm in MgGeO₃. On the other hand, in the absorption spectrum, another strong absorption band that peaked at 255 nm was observed, which does not contribute to the Bi³⁺ luminescence. Because the B- (¹S₀→³P₂) and C-bands (¹S₀→¹P₁) can be observed in the PLE spectra [34, 35], the absorption band at 255 nm is not due to the 6s²→6s6p transition. We assume that this transition is probably due to two overlapped CT transitions, Bi³⁺→Bi⁴⁺ + e^- and Bi³⁺ + e^-→Bi²⁺. Generally, such absorption observed in Bi-activated materials is attributed to Bi³⁺→Bi⁴⁺ + e^- [25]. The CT transition of Bi³⁺→Bi⁴⁺ + e^- is a well-known transition in Bi³⁺-doped materials and is called the D-band. Further investigation is necessary to proof this assignment such as temperature dependence of PL. On the other hand, the assignment of the Bi³⁺ + e^-→Bi²⁺ transition will be discussed in the following section regarding the mechanism of persistent luminescence.

As shown in Fig. 3 and Fig. 5, the PL and PLE results showed two sets of different excitation and emission spectra. Because there are two Mg sites in MgGeO₃, the two sets of PL and PLE spectra can be due to the transitions of the Bi³⁺ ions in the two different Mg sites.

4.3 Origin of long red persistent luminescence by Bi co-doping

Based on the results obtained, we propose a possible schematic energy diagram of Mn-Bi co-doped MgGeO₃ as shown in Fig. 11. Based on the diffuse reflectance results, the band gap energy is approximately 5.64 eV. The strong and broad PLE band in the range of 250-300 nm due to a CT transition is described as a transition from the ground state of Mn²⁺ to the bottom of the conduction band (BCB), described as Mn²⁺→Mn³⁺ + e^-. Therefore, the ground state of Mn²⁺ lies 4.1-4.9 eV below the BCB. The strong absorption at 255 nm was attributed to CT transition, Bi³⁺→Bi⁴⁺ + e^-. Therefore the ground state of Bi³⁺ is located at 4.9 eV below the BCB. Based on the empirical equation proposed by Boutinaud [25] which allows us to estimate the charge transfer energy of the CT transition, Bi³⁺→Bi⁴⁺ + e^-, the CT energy of approximately 4.7 eV was obtained. The estimated value showed reasonable accordance with the experimental result.

 

Fig. 11 Energy level scheme of MgGeO₃:Mn-Bi. Arrows represents the energy or wavelength of each transitions and energy levels labeled by 1 and 2 represent Bi³⁺ energy levels in two different symmetric sites.

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Because the TL peak at 320 K appeared with Bi doping, it is suggested that a Bi-related defect is a trapping center that improves red persistent luminescence at room temperature. In the PLE spectra, Mn²⁺ showed CT transition, Mn²⁺→Mn³⁺ + e^-. This result suggests that a hole may remain at Mn²⁺ as Mn³⁺ and an electron moves into the conduction band under UV irradiation. Therefore, the critical trap for the persistent luminescence is probably Bi-related electron trap. In addition, as shown in Fig. 10, a time-dependent change in the PL intensity was observed in Mn-Bi co-doped MgGeO₃. The increase in Mn²⁺ luminescence is due to the saturation process associated with trapping, which is usually observed in persistent phosphors. On the other hand, the decay in Bi³⁺ luminescence indicates a decrease in the concentration of trivalent state ions, Bi³⁺. This result strongly suggests that the trivalent Bi³⁺ ion functions as an electron trapping center as Bi³⁺ + e^-→Bi²⁺.

Regarding charge balance and ionic radius, Bi incorporation into the MgGeO₃ lattice is described in Kröger-Vink notation by the following defect chemical reaction.

Because additional TL peak appeared at 320 K by Bi doping, it is strongly suggested that $B{i}_{Mg}^{•}$ is the corresponding defect. The trap depth is estimated to be approximately 0.64 eV by the simple equation E_T = T_m/500, where E_T is the trap depth and T_m is the peak temperature of the TL glow curve [33, 34]. Therefore, the defect level due to Bi ion $B{i}_{Mg}^x$ is illustrated in Fig. 11 as lying 0.64 eV below the BCB. When the ground state of Bi²⁺ lies 0.64eV below the BCB, the CT transition from Bi³⁺ to Bi²⁺ at 250 nm (~5 eV) is expected by the difference in the trap depth, 0.64 eV and bandgap energy, 5.64 eV. Thus, the Bi related absorption peaked at 255 nm can be assigned to the CT, i.e., the electron transition from the top of the valence band to the ground state of Bi²⁺ ($B{i}_{Mg}^x$).

5. Conclusion

We report the enhanced red persistent luminescence of a MgGeO₃:Mn phosphor by Bi co-doping. Mn-Bi co-doped MgGeO₃ (x = 0.005) showed 30 times stronger persistent luminescence than a Mn singly doped sample (x = 0). A persistent luminescence mechanism through which Bi functions as an electron trapping center in the reaction $B{i}_{Mg}^{•}+e^{-}\to B{i}_{Mg}^x$was proposed. $B{i}_{Mg}^x$ may be a new defect responsible for the TL glow peaks observed at approximately 300 K.