Proton, UV, and X-ray Induced Luminescence in Tb 3 + Doped LuGd 2 Ga 2 Al 3 O 12 Phosphors

: The well-known solid-state reaction method is used for the synthesis of Tb doped LuGd 2 Ga 2 Al 3 O 12 phosphor. XRD and SEM techniques are used for the phase and structural morphology of the synthesized phosphor. UV, X-ray and proton induced spectroscopy is used to study the luminescence properties. LuGd 2 Ga 2 Al 3 O 12 :Tb 3 + phosphor shows its highest peak in green and blue region. The two major emission peaks correspond to 5 D 3 → 7 F J (at 480 to 510 nm, blue region) and 5 D 4 → 7 F J (at 535 to 565 nm, green region). Green emission is dominant; therefore, it may be used as an e ﬃ cient green phosphor. The absorption spectra of the synthesized material matches well with the spectra of light emitting diodes (LEDs); therefore, it may have applications in LEDs. X-ray spectroscopic study suggests that this phosphor may have uses in medical applications, such as X-ray imaging. The synthesized phosphor exhibits 81% e ﬃ cacy in comparison to the commercial plasma display panel material (Gd 2 O 2 S:Tb 3 + ). The Commission Internationale de l’Eclairage (CIE) chromaticity diagram is obtained for this phosphor. The decay time of ms range is measured for the synthesized phosphor.


Introduction
Rare-earth doped yttrium aluminum garnet (YAG) is a well-known commercially used phosphor due to its brilliant luminescence properties [1,2]. YAG has been studied extensively for the past few decades. LuAG is another host material that has shown promising properties due to its diverse range of properties, such as octahedral and tetrahedral structures, the transparent nature to various types of radiations, high optical quality, better thermal chemical and stability, low temperature for the synthesis, and radiation hardness [3][4][5][6].
Owing to the promising photoluminescence properties of trivalent terbium ion, contributed to green phosphors, has been investigated for last few decades. Green emission of Tb-doped borate phosphor has been investigated [7,8]. Tb doped borates (LiBaB 9 0 15 ) give dominant green luminescent peak at 542 nm [9]. Tb has shown much better luminescence when doped in YAG. LuAGs possess broad peaks of emission and absorption and can be excited by blue LEDs, which make it suitable for light emitting diode (LED) applications [10][11][12].

Experimental
To get the phosphors' final goal, firstly, it is synthesized through solid state reaction method; a well-known technique for the synthesis of phosphors [13,14]. Lutetium oxide (Lu 2 O 3 , 99.998%), gallium oxide (Ga 2 O 3 , 99.99%) gadolinium oxide (Gd 2 O 3 , 99.99%), and aluminum oxide (Al 3 O 12 ) (Sigma-Aldrich, Daegu, South Korea) are weighed according to the balanced chemical equation to get a specified amount of host material LuGd 2 Ga 2 Al 3 O 12 . Different concentrations of activator, Terbium oxide (Tb 2 O 3 , 99.99%), are doped with the host. Duration of ball milling, temperature, heating rate, and cooling rates are optimized for sintering the synthesized material. After measuring mass of powders it is ball milled for 7 h in order to mix it well. For sintering purpose, the mixed powders are kept in the electric furnace. The temperature of the furnace is kept constant at 700 • C for 20 h followed by slow heating and cooling rate of 100 • C/h in the air environment. Finally, fine grinned samples of Tb 3+ doped LuGd 2 Ga 2 Al 3 O 12 phosphors are obtained.

Characterizations
Scanning electron microscopy (SEM) (AIS 2000C, Seron, South Korea) is used to find the shape and size of the grains and the overall morphology of the phosphors. X-ray diffraction (XRD) (Philips XPERT-MED, Amsterdam, Netherlands) is used to find the material's crystallinity. To find luminescent properties of the synthesized material UV and X-ray induced spectroscopy (Beckman DU640 UV/Vis spectrophotometer, Kraemer Boulevard Brea, CA, USA) is used. The chromaticity diagram is obtained for the purpose of studying the white light emission. Proton beam line (45 MeV energy, 2 nA current) passes through 0.2 mm thick aluminum window, which is capping the beam pipe with 5 cm of air, loses energy up to 39 MeV [15].
Using the same spectrometer (QE65000, Ocean optics) the variation of the color with the Tb concentration is obtained using the Commission Internationale de l'Eclairage (CIE) 1931 chromaticity diagram.

X-ray Diffraction Analysis
XRD peaks are shown in Figure 1, which gives comparison of peaks of LuGd 2 Ga 2 Al 3 O 12 :Tb 3+ phosphor with Al 5 Gd 3 O 12 (PDF No. 98-002-3849). The peaks of our sample well match with the peaks of the reference material (Al 5 Gd 3 O 12 ). It proved that the phase achieved is the required phase for the synthesized phosphor. It also verifies that the synthesized phosphors are octahedral in structure. It is proved that extra peaks of the reactants are not present in the synthesized phosphors. It proves the single-phase phosphor material as a product. Furthermore, the Bragg's law is used for the calculation of finding lattice constants of the synthesized phosphors [16]. LuGd2Ga2Al3O12 is one of the LuAG phases that is less studied. Therefore, this work consists of synthesis and luminescence study of Tb doped LuGd2Ga2Al3O12 phosphor.

Experimental
To get the phosphors' final goal, firstly, it is synthesized through solid state reaction method; a well-known technique for the synthesis of phosphors [13,14]. Lutetium oxide (Lu2O3, 99.998%), gallium oxide (Ga2O3, 99.99%) gadolinium oxide (Gd2O3, 99.99%), and aluminum oxide (Al3O12) (Sigma-Aldrich, Daegu, South Korea) are weighed according to the balanced chemical equation to get a specified amount of host material LuGd2Ga2Al3O12. Different concentrations of activator, Terbium oxide (Tb2O3, 99.99%), are doped with the host. Duration of ball milling, temperature, heating rate, and cooling rates are optimized for sintering the synthesized material. After measuring mass of powders it is ball milled for 7 h in order to mix it well. For sintering purpose, the mixed powders are kept in the electric furnace. The temperature of the furnace is kept constant at 700 °C for 20 h followed by slow heating and cooling rate of 100 °C /h in the air environment. Finally, fine grinned samples of Tb 3+ doped LuGd2Ga2Al3O12 phosphors are obtained.

Characterizations
Scanning electron microscopy (SEM) (AIS 2000C, Seron, South Korea) is used to find the shape and size of the grains and the overall morphology of the phosphors. X-ray diffraction (XRD) (Philips XPERT-MED, Amsterdam, Netherlands) is used to find the material's crystallinity. To find luminescent properties of the synthesized material UV and X-ray induced spectroscopy (Beckman DU640 UV/Vis spectrophotometer, Kraemer Boulevard Brea, CA, USA) is used. The chromaticity diagram is obtained for the purpose of studying the white light emission. Proton beam line (45 MeV energy, 2 nA current) passes through 0.2 mm thick aluminum window, which is capping the beam pipe with 5 cm of air, loses energy up to 39 MeV [15].
Using the same spectrometer (QE65000, Ocean optics) the variation of the color with the Tb concentration is obtained using the Commission Internationale de l'Eclairage (CIE) 1931 chromaticity diagram.

X-ray Diffraction Analysis
XRD peaks are shown in Figure 1, which gives comparison of peaks of LuGd2Ga2Al3O12:Tb 3+ phosphor with Al5Gd3O12 (PDF No. 98-002-3849). The peaks of our sample well match with the peaks of the reference material (Al5Gd3O12). It proved that the phase achieved is the required phase for the synthesized phosphor. It also verifies that the synthesized phosphors are octahedral in structure. It is proved that extra peaks of the reactants are not present in the synthesized phosphors. It proves the single-phase phosphor material as a product. Furthermore, the Bragg's law is used for the calculation of finding lattice constants of the synthesized phosphors [16].

Scanning Electron Microscopy
The grain shape of LuGd 2 Ga 2 Al 3 O 12 :Tb 3+ (5 mol%) and LuGd 2 Ga 2 Al 3 O 12 :Tb 3+ (1 mol%) powder are shown in Figure 2. These micrographs show similar nature of grains in terms of morphology and shape. The crystallinity and the grain size in the micrometer range of the phosphor are fundamental structural properties to get high luminescence [17]. Figure 2a,b shows that LuGd 2 Ga 2 Al 3 O 12 :Tb 3+ (5 mol%) having small grains with irregular sharp edges, while LuGd 2 Ga 2 Al 3 O 12 :Tb 3+ (1 mol%) having agglomerated large grains, as shown in Figure 2. Since large grains have less chances of reflection, when the sample is exposed to light; therefore, it has better luminescence than the LuGd 2 Ga 2 Al 3 O 12 :Tb 3+ (5 mol%) [18]. LuGd 2 Ga 2 Al 3 O 12 :Tb 3+ phosphors have grain size within the micrometer range, which results in a better luminescence. Phosphors, having grain size of a micrometer, are usually used for X-ray imaging in the medical field. It is research proven that the luminescent properties of LuAG phosphors are affected by size and crystalline nature of phosphors [19,20].

Scanning Electron Microscopy
The grain shape of LuGd2Ga2Al3O12:Tb 3+ (5 mol%) and LuGd2Ga2Al3O12:Tb 3+ (1 mol%) powder are shown in Figure 2. These micrographs show similar nature of grains in terms of morphology and shape. The crystallinity and the grain size in the micrometer range of the phosphor are fundamental structural properties to get high luminescence [17]. Figure 2a,b shows that LuGd2Ga2Al3O12:Tb 3+ (5 mol%) having small grains with irregular sharp edges, while LuGd2Ga2Al3O12:Tb 3+ (1 mol%) having agglomerated large grains, as shown in Figure 2. Since large grains have less chances of reflection, when the sample is exposed to light; therefore, it has better luminescence than the LuGd2Ga2Al3O12:Tb 3+ (5 mol%) [18]. LuGd2Ga2Al3O12:Tb 3+ phosphors have grain size within the micrometer range, which results in a better luminescence. Phosphors, having grain size of a micrometer, are usually used for X-ray imaging in the medical field. It is research proven that the luminescent properties of LuAG phosphors are affected by size and crystalline nature of phosphors [19,20].

UV Induced Luminescence of LuGd2Ga2Al3O12:Tb 3+
Excitation and emission transitions with energy levels of Tb 3+ ion are recorded and given in Figure 3. UV-induced excitation and emission spectra are shown in Figure 4. Excitation band of LuGd2Ga2Al3O12:Tb 3+ is observed at 290 nm 7 F6→ 5 D3 transition. Emission band is observed at 378 nm 5 D4→ 7 FJ transition. Where J ranges from 0 to 6.

UV Induced Luminescence of LuGd 2 Ga 2 Al 3 O 12 :Tb 3+
Excitation and emission transitions with energy levels of Tb 3+ ion are recorded and given in Figure 3. UV-induced excitation and emission spectra are shown in Figure 4. Excitation band of LuGd 2 Ga 2 Al 3 O 12 :Tb 3+ is observed at 290 nm 7 F 6 → 5 D 3 transition. Emission band is observed at 378 nm 5 D 4 → 7 F J transition. Where J ranges from 0 to 6.   Figure 4 shows three major excitation peaks at 290 nm, 320 nm, and 378 nm. The major emission peak is observed at 550 nm. The emission spectrum of Tb 3+ doped LuGd2Ga2Al3O12 phosphor by UV light, monitored (at the emission wavelength of 625 nm) is not limited to 550 nm, but it consists of other emission peaks at 380 nm, 420 nm, 480 nm, 580 nm, and 620 nm. Figure 4 also shows the green and blue emission spectrum of LuGd2Ga2Al3O12:Tb 3+ phosphor. The two major emission peaks correspond to 5 D3→ 7 FJ (at 480-510 nm, blue region), 5 D4→ 7 FJ (at 535-565 nm, green region). Green emission is dominant due to the presence of Tb 3+ dopant. A similar emission is published for Tb doped phosphors [21,22]. Figure 5 shows that blue emission (5D3→7FJ) intensity is decreased and green emission (5D4→7FJ) intensity is increased with increasing concentration of Tb 3+ . The intensity ratio (IG/IB) may play the same role as red/orange (IR/IO) intensity ratio of Eu 3+ or yellow/blue (IY/IB) intensity ratio of Dy 3+ . It also describe the symmetry of the local environment around the optically active dopant and covalent/ ionic bonding between Tb 3+ and O 2− . Similar to Eu 3+ or Dy 3+ , the Tb 3+ ions may be used as a spectroscopic probe as well [23,24].

X-Ray Induced Luminescence Spectroscopy
Emission spectra of LuGd2Ga2Al3O12:Tb 3+ are also observed through X-ray induced spectroscopy with various concentrations of Tb 3+ as shown in the Figure 6.
X-ray spectroscopy shows emission in the range of 300-650 nm. X ray spectroscopy shows highest emission peak at 550 nm, which is the result of UV Spectroscopy. The X-ray induced emission spectrum well matches with that of UV induced emission spectrum and with the published data of X-ray luminescence [25]. Figure 6 shows 1 mol% concentration of Tb 3+ in LuGd2Ga2Al3O12 as  Figure 4 shows three major excitation peaks at 290 nm, 320 nm, and 378 nm. The major emission peak is observed at 550 nm. The emission spectrum of Tb 3+ doped LuGd 2 Ga 2 Al 3 O 12 phosphor by UV light, monitored (at the emission wavelength of 625 nm) is not limited to 550 nm, but it consists of other emission peaks at 380 nm, 420 nm, 480 nm, 580 nm, and 620 nm. Figure 4 also shows the green and blue emission spectrum of LuGd 2 Ga 2 Al 3 O 12 :Tb 3+ phosphor. The two major emission peaks correspond to 5 D 3 → 7 F J (at 480-510 nm, blue region), 5 D 4 → 7 F J (at 535-565 nm, green region). Green emission is dominant due to the presence of Tb 3+ dopant. A similar emission is published for Tb doped phosphors [21,22]. Figure 5 shows that blue emission (5D 3 →7F J ) intensity is decreased and green emission (5D 4 →7F J ) intensity is increased with increasing concentration of Tb 3+ . The intensity ratio (I G /I B ) may play the same role as red/orange (I R /I O ) intensity ratio of Eu 3+ or yellow/blue (I Y /I B ) intensity ratio of Dy 3+ . It also describe the symmetry of the local environment around the optically active dopant and covalent/ ionic bonding between Tb 3+ and O 2− . Similar to Eu 3+ or Dy 3+ , the Tb 3+ ions may be used as a spectroscopic probe as well [23,24].

X-Ray Induced Luminescence Spectroscopy
Emission spectra of LuGd2Ga2Al3O12:Tb 3+ are also observed through X-ray induced spectroscopy with various concentrations of Tb 3+ as shown in the Figure 6.
X-ray spectroscopy shows emission in the range of 300-650 nm. X ray spectroscopy shows highest emission peak at 550 nm, which is the result of UV Spectroscopy. The X-ray induced emission spectrum well matches with that of UV induced emission spectrum and with the published data of X-ray luminescence [25]. Figure 6 shows 1 mol% concentration of Tb 3+ in LuGd2Ga2Al3O12 as the optimized concentration of Tb 3+ . This optimized value of 1 mol% Tb 3+ is shown in Figure 7.

X-ray Induced Luminescence Spectroscopy
Emission spectra of LuGd 2 Ga 2 Al 3 O 12 :Tb 3+ are also observed through X-ray induced spectroscopy with various concentrations of Tb 3+ as shown in the Figure 6.
Crystals 2020, 10, x FOR PEER REVIEW 6 of 10 X-ray spectroscopic study suggests that this phosphor may be used in a medical application, such as X-ray imaging.  Figure 7 shows different concentrations of Tb 3+ (mol%) with relevant maximum emission intensities. The optimized concentrations with maximum intensity for emission peak is given, i.e., 1 mole% of Tb. Since the intensity of green color is more dominant, therefore, this phosphor might be used as green phosphor.  X-ray spectroscopy shows emission in the range of 300-650 nm. X ray spectroscopy shows highest emission peak at 550 nm, which is the result of UV Spectroscopy. The X-ray induced emission spectrum well matches with that of UV induced emission spectrum and with the published data of X-ray luminescence [25]. Figure 6 shows 1 mol% concentration of Tb 3+ in LuGd 2 Ga 2 Al 3 O 12 as the optimized concentration of Tb 3+ . This optimized value of 1 mol% Tb 3+ is shown in Figure 7.

Optimization of Tb 3+ Concentrations
Crystals 2020, 10, x FOR PEER REVIEW 6 of 10 X-ray spectroscopic study suggests that this phosphor may be used in a medical application, such as X-ray imaging.  Figure 7 shows different concentrations of Tb 3+ (mol%) with relevant maximum emission intensities. The optimized concentrations with maximum intensity for emission peak is given, i.e., 1 mole% of Tb. Since the intensity of green color is more dominant, therefore, this phosphor might be used as green phosphor.

Proton Induced Luminescence Spectroscopy
The synthesized LuGd2Ga2Al3O12:Tb 3+ phosphor is excited by three major excitation sources. X-ray spectroscopic study suggests that this phosphor may be used in a medical application, such as X-ray imaging. Figure 7 shows different concentrations of Tb 3+ (mol%) with relevant maximum emission intensities. The optimized concentrations with maximum intensity for emission peak is given, i.e., 1 mole% of Tb. Since the intensity of green color is more dominant, therefore, this phosphor might be used as green phosphor.

Proton Induced Luminescence Spectroscopy
The synthesized LuGd 2 Ga 2 Al 3 O 12 :Tb 3+ phosphor is excited by three major excitation sources. X-ray and proton induced emission spectra are shown in Figure 8 whereas UV-induced emission spectrum is shown in Figure 5. All three emission spectra are very similar and matches well with the literature [26,27]. This comparison shows that the emission properties do not depend on the excitation source.

Luminescence Efficiency
In order to investigate the luminescence efficacy of the synthesized phosphor, the emission spectra of LuGd2Ga2Al3O12:Tb 3+ and commercially available plasma display panel (PDP) material (PDP:Gd2O2S:Tb 3+ ) are compared in Figure 9. The light yields are obtained by integrating the area under the emission curves. All of the parameters, such as slit width, integrated time, beam intensity, and excitation wavelength are kept constant for comparison. This comparison reveals that the light yield of the synthesized phosphor is 81% of that of commercially available PDP phosphor. This encouraging result of luminescence efficacy suggests the potential application of this phosphor in the fields of PDPs and LEDs.

Luminescence Efficiency
In order to investigate the luminescence efficacy of the synthesized phosphor, the emission spectra of LuGd 2 Ga 2 Al 3 O 12 :Tb 3+ and commercially available plasma display panel (PDP) material (PDP:Gd 2 O 2 S:Tb 3+ ) are compared in Figure 9. The light yields are obtained by integrating the area under the emission curves. All of the parameters, such as slit width, integrated time, beam intensity, and excitation wavelength are kept constant for comparison. This comparison reveals that the light yield of the synthesized phosphor is 81% of that of commercially available PDP phosphor. This encouraging result of luminescence efficacy suggests the potential application of this phosphor in the fields of PDPs and LEDs. spectra of LuGd2Ga2Al3O12:Tb and commercially available plasma display panel (PDP) material (PDP:Gd2O2S:Tb 3+ ) are compared in Figure 9. The light yields are obtained by integrating the area under the emission curves. All of the parameters, such as slit width, integrated time, beam intensity, and excitation wavelength are kept constant for comparison. This comparison reveals that the light yield of the synthesized phosphor is 81% of that of commercially available PDP phosphor. This encouraging result of luminescence efficacy suggests the potential application of this phosphor in the fields of PDPs and LEDs.

Decay Time Analysis
The decay time is obtained for LuGd 2 Ga 2 Al 3 O 12 phosphors with different concentrations of Tb 3+ , shown in Figure 10. The decay measurement is done at emission wavelength (550 nm) and excitation wavelength (290 nm). All of the decay curves are fifit with single exponential decay equation.
In this equation, "A" stands for integrated area, I and I o represent intensities at times t and 0, respectively, and τ represents the decay time. The decay time becomes shorter with the increase of Tb 3+ concentration. The decay time analysis is very handy in order to understand the energy transfer mechanism and luminescence quenching of Tb 3+ ions. The investigation of these decay curves clarify that decay time gets shorter if Tb 3+ concentration is decreased from 3 mol% to 1 mol% as mentioned in other articles [23,28]. In other words, we can say that beyond 3 mol% the concentration quenching starts, which in turn delays the emission process. The decay time measured for LuGd 2 Ga 2 Al 3 O 12 :Tb 3+ (1 mol%) is to be between 2.80 ms and 2.90 ms.

Decay Time Analysis
The decay time is obtained for LuGd2Ga2Al3O12 phosphors with different concentrations of Tb 3+ , shown in Figure 10. The decay measurement is done at emission wavelength (550 nm) and excitation wavelength (290 nm). All of the decay curves are fifit with single exponential decay equation.
In this equation, "A" stands for integrated area, I and Io represent intensities at times t and 0, respectively, and τ represents the decay time. The decay time becomes shorter with the increase of Tb 3+ concentration. The decay time analysis is very handy in order to understand the energy transfer mechanism and luminescence quenching of Tb 3+ ions. The investigation of these decay curves clarify that decay time gets shorter if Tb 3+ concentration is decreased from 3 mol% to 1 mol% as mentioned in other articles [23,28]. In other words, we can say that beyond 3 mol% the concentration quenching starts, which in turn delays the emission process. The decay time measured for LuGd2Ga2Al3O12:Tb 3+ (1 mol%) is to be between 2.80 ms and 2.90 ms.

Conclusions
The synthesized phosphor is investigated for its UV, X-ray, and proton induced luminescence. All three kinds of emission spectra are very similar, proving that excitation source has no effect on the emission spectrum. This phosphor shows major emission peaks in green color region and a peak in blue region as well. The green emission enhances at the cost of blue emission with the increase of Tb concentration. Absorption spectra of our material matches well with LEDs spectra; therefore, it may be used for LED applications. Grain size is in the micrometer range, having good luminescence, and may be utilized for X-ray imaging applications. The longer decay time of this phosphor is in milliseconds range, which is suitable for lighting applications. Chromaticity diagram confirms green emission, which is supported by UV and X-ray and proton-induced spectroscopy. X-ray luminescence suggests its applications in X-ray imaging.