Synthesis and photoluminescent characterization of ceramic phosphors Li₂MgGeO₄:Ln³⁺ (Ln³⁺ = Pr³⁺ or Tm³⁺) under different excitation wavelengths

Abstract

In the current work, germanate phosphors Li₂MgGeO₄:Ln³⁺ (Ln = Pr, Tm) have been synthesized and then investigated using luminescence spectroscopy. The X-ray diffraction analysis demonstrate that ceramic compounds Li₂MgGeO₄ containing Pr³⁺ and Tm³⁺ ions crystallize in a monoclinic crystal lattice. Luminescence properties of Pr³⁺ and Tm³⁺ ions have been examined under different excitation wavelengths. The most intense blue emission band related to the ¹D₂ → ³F₄ transition of Tm³⁺ is overlaps well with broad band located near 500 nm, which is assigned to F-type centers. These effects are not evident for Pr³⁺ ions. Ceramic phosphors Li₂MgGeO₄:Ln³⁺ (Ln = Pr, Tm) are characterized based on measurements of the excitation/emission spectra and their decays. The experimental results indicate that germanate ceramics Li₂MgGeO₄ doped with trivalent rare earth ions can be applied as inorganic phosphors emitting orange (Pr³⁺) or blue (Tm³⁺) light.

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Introduction

Germanate ceramics with olivine structure are promising inorganic compounds due to their excellent low-permittivity microwave dielectric properties, solid electrolytic properties for lithium-ion batteries and emission properties for modern visible or infrared photonics. In particular, germanate ceramics belonging to olivine-type family with general chemical formula Li₂MGeO₄, where M = Co,¹ Zn or Mg² are interesting from the scientific and technological points of view. They crystallized into an orthorhombic or monoclinic phase. Also, the reversible phase transformation monoclinic (low-temperature) ↔ orthorhombic (high-temperature) can occur. These ceramic systems are able to accommodate transition metals or rare earth ions and then can be applied as efficient phosphors emitting visible light or near-IR radiation. Recently published studies were limited to inorganic phosphors Li₂ZnGeO₄ (ref. 3 and 4) and Li₂MgGeO₄ (ref. 5 and 6) singly doped with Mn²⁺ emitting blue/green light. Moreover, olivine-type phosphors (Li_x–Mg_1−x)(Sc_x–Mg_1−x)GeO₄ singly doped with Cr³⁺ present tunable near-IR emission and have practical applications as light sources for nondestructive food analysis.⁷ In contrast to transition metals, germanate ceramics with rare earth ions have not been often examined. Inorganic phosphors Li₂SrGeO₄ present enhanced emission through energy transfer process between Ce³⁺ and Tb³⁺/Dy³⁺.⁸ The enhanced persistent blue emission was also realized in Li₂ZnGeO₄ phosphors by rare earth doping (Pr³⁺, Nd³⁺, Gd³⁺).⁹

Among rare earths, the Pr³⁺ and Tm³⁺ ions are interesting optical dopants due to presence of several multicolor 4f–4f luminescent transitions. The increasing demand for crystalline and amorphous host matrices has promoted the researchers to develop various types of ceramic phosphors¹⁰ and inorganic glasses¹¹ doped with Pr³⁺ and Tm³⁺ ions for their potential applications as luminescent materials. Trivalent Pr³⁺ ions can offer luminescence in the blue, green, red and white spectral ranges depending on the kind of host matrices. Luminescence properties of Pr³⁺ ions have been examined under ultraviolet¹² or blue light¹³ irradiation. Most of the inorganic phosphors containing Pr³⁺ ions emit intense orange or red light.^14–16 Special attention has been paid to Pr³⁺ doped phosphors emitting white light.^17–19 Novel optical temperature sensor was also proposed based on emission of Pr³⁺ ions in Ba_0.7Sr_0.3TiO₃.²⁰ In contrast to Pr³⁺, ceramic compounds doped with Tm³⁺ ions are known mainly as a blue-emitting phosphors.^21–24 Thulium doped inorganic phosphors could be also applicable in white light emitting diodes (wLEDs) through Dy³⁺ co-doping.²⁵

Structural, thermal and optical properties of ceramics Li₂MgGeO₄ un-doped and doped with Er³⁺ and Ho³⁺ ions were described in our previous work.²⁶ Direct energy band gaps for Li₂MgGeO₄ with Er³⁺ and Ho³⁺ are found to be 5.67 and 5.77 eV and they are comparable to the values reported earlier.^5,6 Further results of the Rietveld refinement confirmed that Li₂MgGeO₄ crystallize in a monoclinic crystal lattice.²⁶ Here, we show preliminary results for ceramics Li₂MgGeO₄ doped with Pr³⁺ and Tm³⁺. In particular, emission properties of rare earths have been analyzed under different excitation wavelengths. The experimental results suggest that germanate ceramics Li₂MgGeO₄ doped with rare earth ions can be applied as inorganic phosphors emitting orange (Pr³⁺) or blue (Tm³⁺) light.

Experimental

Synthesis

Li₂Mg_(100−x)GeO₄:xLn³⁺ (given in molar%), where Ln³⁺ denotes Pr³⁺ or Tm³⁺, in the form of pellets have been synthesized via a conventional high-temperature solid-state reaction method. The concentration of Ln³⁺ was equal to 0.5 mol%. They are referred here as LMG-Pr and LMG-Tm, respectively. High purity initial reagents were Li₂CO₃ (99.997%), MgO (99.99%), GeO₂ (99.99%), and depending on the sample: Pr₂O₃ (99.999%) or Tm₂O₃ (99.999%). The stoichiometric amounts of raw materials in appropriate mass ratios were milled and homogenized thoroughly in an agate mortar for about 1 hour, with ethanol (POCH Basic 96% pure) as a medium. In the next step, grounded samples were calcinated in a non-covered platinum crucible at 1100 °C for 6 h in the air atmosphere to achieve decarbonization. Calcination was divided into two steps: reaching the temperature of 800 °C in 30 minutes, then getting to 1100 °C in 10 minutes. Subsequently, calcinated samples were grounded again and divided into smaller batches. Pellets (10 mm in diameter) were formed using PVA as a binder and cold pressed at 375 MPa. Prepared pellets were subjected to heat treatment to remove the binder at 550 °C for 2 hours (with a heating rate of 3 °C min⁻¹) under ambient air conditions. After cooling down to room temperature, the ceramic samples were sintered in the high-temperature furnace at 1200 °C for 5 h and naturally cooled down to room temperature in a closed furnace. The sintering process includes several steps: rise to 800 °C in one hour, then sintering for 15 minutes, heating up to 1200 °C (9 °C min⁻¹), and sintering for 3 hours.

Methods

In order to study germanate ceramics singly doped with praseodymium and thulium ions, several measurements at room temperature were performed, including XRD, SEM, excitation and photoluminescence spectra, and decay curves. Chromaticity coordinates were also calculated. The nature of the studied germanate ceramics was identified using an X'Pert-Pro diffractometer. All X-ray diffraction measurements were done with a use of the CuK_α1and2 radiation. The microstructure of samples was observed using JSM6480 scanning electron microscope (SEM) along with JSM-7100F TTL LV. Excitation and luminescence measurements were registered using laser equipment consisting of a Photon Technology International (PTI) Quanta-Master 40 (QM40) UV/VIS Steady State Spectrofluorometer coupled with a tunable pulsed optical parametric oscillator (OPO) pumped by the third harmonic of a Nd:YAG laser. The laser system included a double 200 mm monochromator, a xenon lamp as a light source, and a multimode UVVIS PMT R928 (PTI Model 914) detector. The spectra were registered with a resolution of ±0.5 nm. Decay curves with an accuracy of ±0.5 μs were recorded using the PTI ASOC-10 [USB-2500] oscilloscope (Horriba Instruments). It should be also pointed out that the Commission Internationale de I'Eclairage (CIE) chromaticity coordinates (x,y) and chromaticity diagram were calculated from the emission spectra and plotted using Color Calculator software.

Results and discussion

The X-ray diffraction patterns of the germanate ceramics Li₂MgGeO₄:Pr³⁺ (LMG-Pr) and Li₂MgGeO₄:Tm³⁺ (LMG-Tm) are presented in Fig. 1. The typical SEM micrographs of the studied ceramic samples are also shown.

Fig. 1 The XRD patterns and SEM images of ceramic phosphors LMG-Pr and LMG-Tm.

The XRD analysis confirmed that all diffraction peaks can be well assigned to ceramic compounds crystallize in a monoclinic lattice with P2₁/n space group, which is isostructural to Li₂ZnGeO₄ (ICDD PDF-4 database – card no 04-015-4929).² Moreover, no additional phases associated to impurities exist compared to pure Li₂MgGeO₄ monoclinic phase.²⁶ It evidently reveals that rare earth doping has no effect on the crystalline structure of germanate ceramics and Pr³⁺ (or Tm³⁺) ions are well entered into the lattice Li₂MgGeO₄. The SEM images illustrate germanate samples LMG-Pr and LMG-Tm, which consist of micrometer-sized and irregular-shaped grains similar to ceramics Li₂MgGeO₄ containing Er³⁺ or Ho³⁺ ions.²⁶ Previous studies for La₃Ga₅SiO₁₄:Pr³⁺ phosphors well demonstrated that the crystalline grains in micron size and irregular morphology can be successfully observed and these effects are related to the high-temperature sintering process.²⁷ In our case, rare earth doping caused a reduction of the grain size compared to the un-doped ceramics Li₂MgGeO₄ crystallized in a regular oval-like shape.²⁶ Moreover, the SEM-BS images for ceramic samples LMG-Pr and LMG-Tm are also presented in Fig. 2. It is an image sensitive to the contrast of the chemical composition. Objects presented in the images also have an even shade of gray. Such contrasts indicate that the components have reacted and only one crystalline phase is formed. There are no other precipitations, phases, or inclusions. The monophasic nature of the created germanate ceramics is confirmed by the X-ray diffraction patterns shown in Fig. 1. Considering all the above arguments, a conclusion can be drawn that designed ceramics have been received.

Fig. 2 Ceramic particles observed with SEM-BE images for ceramic phosphors LMG-Pr and LMG-Tm.

The photoluminescence spectra of the germanate ceramics Li₂MgGeO₄:Pr³⁺ (LMG-Pr) and Li₂MgGeO₄:Tm³⁺ (LMG-Tm) have been examined under different excitation wavelengths. The emission spectra are shown in Fig. 3.

Fig. 3 The photoluminescence spectra of ceramic phosphors LMG-Pr (bottom) and LMG-Tm (top).

The systematic studies indicate that luminescence spectra measured for LMG-Pr depend critically on the excitation wavelengths. The spectra excited between 370 and 433 nm range consist of broad low-intensity emission band centered at about 500 nm. The intensity of band decreases in direction to longer excitation wavelengths up to about 433 nm when broad emission of ceramic host is nearly quenched. When sample was excited above 435 nm, narrow luminescence lines characteristic for rare earth ions occur. The emission bands are related to transitions originating from the ³P₀ and ¹D₂ excited states to the lower-lying states of trivalent praseodymium. The most intense emission transitions of Pr³⁺ are observed under excitation of 450 nm line and correspond to bands located in the blue and reddish-orange spectral ranges.

Completely different situation is observed for ceramics doped with Tm³⁺. The photoluminescence spectra measured under excitation between 355 and 370 nm consist of narrow band corresponding to the ¹D₂ → ³F₄ blue transition of Tm³⁺ ions (λ_p = 460 nm), which is well overlapped with broad emission of ceramic host. The luminescence band of Tm³⁺ ions is the most intense, when the ceramic sample was excited at 365 nm. The band assigned to the ¹G₄ → ³F₄ transition of Tm³⁺ ions was also observed in the red spectral range, but its emission intensity is extremely low.

The obtained results were interpreted on the basis of the excitation and emission spectra measurements for ceramic samples un-doped and singly doped with Pr³⁺ and Tm³⁺ under selective excitation wavelengths. The excitation spectra were measured under monitoring emission wavelengths at 610 nm (LMG-Pr) and 460 nm (LMG-Tm). Fig. 4 shows the excitation and emission spectra measured for samples LMG, LMG-Pr and LMG-Tm, respectively.

Fig. 4 The excitation and emission spectra of ceramic phosphors undoped (LMG) and doped with Pr³⁺ (LMG-Pr) and Tm³⁺ (LMG-Tm).

Our previous spectroscopic investigations²⁶ revealed that luminescence spectrum measured for un-doped LMG consists of broad band located in the blue/green region with maximum peak emission near 500 nm depending on the excitation wavelength. Broad emission is associated to the occurrence of magnesium in LMG ceramic host. Similar effects were also observed earlier for MgO nanobelts²⁸ and MgO films²⁹ where blue/green emission was identified as defects and oxygen vacancies in magnesium oxide assigned to F-type centers. Thus, the excitation spectrum for un-doped LMG monitored under emission wavelength λ_em = 500 nm shows broad band near 380 nm (Fig. 4).

Characteristic emission bands of Pr³⁺ are well observed, when ceramic sample LMG-Pr was excited by 450 nm line. In this case, sample LMG-Pr was excited directly at ³P₂ state of Pr³⁺ and ceramic host LMG could be also pumped theoretically on the edge of its broad excitation band. However, characteristic broad band assigned to ceramic matrix is not evident in the emission spectrum of LMG-Pr under 450 nm excitation. Considering that fact we suggest that the ceramic host cannot be excited efficiently and the interaction between LMG matrix and Pr³⁺ ions is rather difficult to obtain contrary to sample LMG-Tm. When the excitation line 365 nm was used, ceramic sample LMG-Tm is excited simultaneously at ¹D₂ state of Tm³⁺ ions and ceramic matrix. In this case, the excitation band ¹D₂ (Tm³⁺) is very close to the maximum of excitation band of ceramic host LMG. Thus, the luminescence band due to the ¹D₂ → ³H₆ (Tm³⁺) transition is well overlapped with broad emission characteristic for ceramic host LMG. Therefore, we suggest that the excitation energy may be also transferred from ceramic host LMG to Tm³⁺. The excitation spectra measurements confirms this hypothesis. The excitation spectrum measured under monitoring emission wavelength 460 nm (¹D₂ → ³H₆ luminescent transition of Tm³⁺) consists of two overlapped broad and narrow bands characteristic for ceramic host LMG and electronic transition originating from the ³H₆ ground state to the ¹D₂ state of Tm³⁺, respectively. These effects are not observed for LMG-Pr.

In order to determine the color of emitting radiation for studied ceramic phosphors, the Commission Internationale de I'Eclairage (CIE 1931) chromaticity coordinates (x,y) were calculated from the emission spectra registered in the visible spectral range. The CIE diagrams for ceramic samples LMG-Pr and LMG-Tm varying with the excitation wavelengths are presented in Fig. 5 and 6.

Fig. 5 The CIE chromaticity diagram of ceramic phosphors LMG-Pr.

Fig. 6 The CIE chromaticity diagram of ceramic phosphors LMG-Tm.

According to previous studies, different ceramic phosphors containing praseodymium ions, such as BaNb₂O₆,³⁰ CaWO₄,³¹ and Y₂MoO₆,³² exhibit orange-red emission. However, results obtained for Sr_1.5Ca_0.5SiO₄,³³ Ca₃Y₂Si₃O₁₂,³⁴ and CaSnO₃ (ref. 35) show that chromaticity coordinates determined for system doped with Pr³⁺ ions correspond to green and greenish-blue region.

Our optical investigation indicates that the values of (x,y) for ceramic phosphors containing praseodymium ions strongly depend on the excitation wavelengths. They are changed from green to orange region with increasing excitation wavelength from 370 nm to 450 nm. The intensities of emission lines due to characteristic transitions of Pr³⁺ are the highest for sample under 450 nm excitation. Thus, the chromaticity coordinates are due to the orange region of the CIE diagram. The evaluated chromaticity coordinates were similar to that of YAl₃(BO₃)₄.³⁶

Similar situation is observed for ceramic phosphors doped with thulium ions, where the values of (x,y) also depend critically on the excitation wavelengths. They are changed from blue to green region, when the excitation wavelength increases from 340 nm to 430 nm. Previous studies for ceramic materials containing Tm³⁺ ions demonstrated well that phosphors like LiLaSiO₄ (ref. 37) and BaMoO₄ (ref. 38) can be used in blue light emitters. It was also confirmed by our optical results for ceramic phosphor Li₂MgGeO₄:Tm³⁺ excited at 365 nm, where the intensity of emission band due to the ¹D₂ → ³H₆ transition of thulium is the highest. Thus, the chromaticity coordinates calculated for Li₂MgGeO₄ doped with thulium ions correspond to the blue region of the CIE diagram. The results suggests that Li₂MgGeO₄ host can be successfully used as ceramic phosphors emitting orange (Pr³⁺) or blue (Tm³⁺) light under selective excitation wavelengths.

Luminescence decays from the excited states of rare earths in Li₂MgGeO₄ have been also studied. They are shown in Fig. 7.

Fig. 7 The CIE chromaticity diagram of ceramic phosphors LMG-Tm.

For ceramic sample LMG-Pr luminescence decay curves were measured under excitation 450 nm and monitoring emission wavelength 610 nm. For sample LMG-Tm, the parameters were as follows: λ_exc = 365 nm, λ_em = 460 nm. Decay curves for samples LMG-Pr and LMG-Tm are well-fitted to a mono-exponential decay mode described by the equation I(t) = I₀ × exp(−t/τ), where I(t) and I₀ are the luminescence intensities at time t and t = 0, respectively, while τ is the luminescence lifetime. Luminescence decay curve measured for ceramic host LMG under direct excitation 381 nm and monitoring emission wavelength 500 nm is also mono-exponential. Based on decay curve measurements, luminescence lifetimes for the ceramic host LMG and the excited states of rare earths were determined. For LMG and LMG-Tm (¹D₂ state of Tm³⁺), the measured lifetimes are close to 4.6 μs and 12.7 μs, respectively. Similar to the ¹D₂ (Tm³⁺), the experimental values of emission lifetimes for the ³P₀ state of Pr³⁺ ions in numerous glasses and ceramics are very short and difficult to determine with relatively good accuracy. Here, the lifetime of Pr³⁺ is significantly longer and its value is equal to 124.2 μs. It suggests that luminescence is decayed from the lower-lying state ¹D₂ (Pr³⁺) in Li₂MgGeO₄.

Our previous studies for high-phonon borate glasses doped with Pr³⁺ ions³⁹ indicate that the excitation energy is transferred non-radiatively (multiphonon relaxation MPR) very fast from the ³P₀ state to the lower-lying ¹D₂ state due to high-phonon energies and consequently reddish-orange luminescence corresponding to the ¹D₂ → ³H₄ transition of Pr³⁺ is observed, only. For lower-phonon germanate glasses doped with Pr³⁺ ions⁴⁰ two overlapped radiative transitions, i.e. short-lived ³P₀ → ³H₆ and long-lived ¹D₂ → ³H₄ luminescent transitions of Pr³⁺ are existed in this spectral range. It could be attributed to the lower phonon energies and the increased radiative transition from the excited state ³P₀ (Pr³⁺). This is a consequence of a substantial decrease in nonradiative multiphonon relaxation from the ³P₀ state to the lower-lying ¹D₂ state of Pr³⁺. Further luminescent experiments for LMG-Pr excited selectively at 450 nm (³P₀ state) and 585 nm (¹D₂ state) confirm coexistence of both emission bands of Pr³⁺ ions in Li₂MgGeO₄ under blue pumping. Fig. 8 presents luminescence spectra of Pr³⁺ ions in ceramic host LMG in the orange-red range. The sample was pumped selectively by 450 nm and 585 nm excitation lines, respectively.

Fig. 8 Luminescence spectra of ceramic phosphors LMG-Pr excited at 450 nm (³P₀ state) and 585 nm (¹D₂ state).

The spectrum measured for LMG-Pr under 585 nm excitation shows emission band related to the ¹D₂ → ³H₄ transition of Pr³⁺. Completely different situation is observed for LMG-Pr excited at higher-lying state ³P₀ (Pr³⁺) by 450 nm line, where several emission bands are present. Compared to the results obtained for sample excited at 585 nm, the emission spectrum consists of characteristic band due to the ¹D₂ → ³H₄ transition as well as additional bands corresponding to transitions from the ³P₀ state to the lower-lying states of Pr³⁺. It proves that both ³P₀ and ¹D₂ excited states of Pr³⁺ ions coexist in ceramic host Li₂MgGeO₄ under blue excitation.

Finally, germanate ceramics Li₂MgGeO₄:Ln³⁺ (Ln = Pr, Tm) have been examined for near infrared luminescence. From literature data it is well known that some ceramic materials synthesized in nano- or micrometric scale exhibit efficient luminescence in the second near-infrared window (NIR-II), which is important for numerous biomedical and optical applications.^41–48 Among rare earths, the Pr³⁺ (ref. 49) and Tm³⁺ (ref. 50) ions belong to promising optical dopants emitting radiation in the NIR-II region. However, inorganic phosphors with Pr³⁺ or Tm³⁺ have been rarely investigated for NIR-II luminescence. To the best of our knowledge, they are rather less documented in literature. Our optical studies indicate that both ceramic samples LMG-Pr and LMG-Tm show near-IR emission in the 1350–1650 nm spectral range.

Fig. 9 presents near-infrared luminescence spectra for samples LMG-Pr and LMG-Tm measured under excitation by 450 nm (Pr³⁺) and 808 nm (Tm³⁺) line, respectively. Luminescence spectrum for LMG-Tm consists of NIR band centered at about 1.47 μm, which corresponds to the ³H₄ → ³F₄ transition of Tm³⁺.⁵¹ In contrast to LMG-Tm, three NIR luminescence bands are observed for sample LMG-Pr, which can be assigned to the ¹G₄ → ³H₅, ¹D₂ → ¹G₄ and ³F₃, ³F₄ → ³H₄ transitions of Pr³⁺.⁵² Our preliminary investigations demonstrate that Li₂MgGeO₄:Ln³⁺ (Ln = Pr, Tm) can be also applied as ceramic source emitting radiation in the NIR-II window. These phenomena will be the subject of further investigation.

Fig. 9 Near-IR emission spectra of ceramic phosphors LMG-Pr and LMG-Tm.

Conclusion

Rare earth doped germanate ceramics Li₂MgGeO₄:Ln³⁺ (Ln = Pr, Tm) referred here as LMG-Pr and LMG-Tm have been prepared using solid-state reaction method and then studied using luminescence spectroscopy. The ceramic compounds Li₂MgGeO₄ doped with Pr³⁺ and Tm³⁺ ions crystallize in monoclinic crystal lattice, which was confirmed by X-ray diffraction analysis. Luminescence properties of trivalent rare earth ions have been analyzed under various excitation wavelengths. For sample LMG-Tm, the most intense blue emission band is due to the ¹D₂ → ³F₄ transition of Tm³⁺ and well overlapped with broad band centered at about 500 nm assigned to F-type centers. These effects are not evident for LMG-Pr. Based on measurements of the excitation/emission spectra and their decays, the experimental results for germanate ceramics Li₂MgGeO₄:Ln³⁺ (where Ln = Pr or Tm) are presented and discussed in details. It was suggested that germanate ceramics Li₂MgGeO₄ doped with trivalent rare earth ions can be applied as inorganic phosphors emitting orange (Pr³⁺) or blue (Tm³⁺) light. Further studies suggest that rare earth doped germanate phosphors Li₂MgGeO₄:Ln³⁺ (Ln = Pr, Tm) can be applied as ceramic sources emitting radiation in the second near-infrared window.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The National Science Centre (Poland) supported this work under project No. 2019/35/B/ST5/01924.

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