Unveiling highly sensitive Dy3+ doped BaMgAl10O17 phosphor’s thermoluminescence trap features and temperature dependent luminescence for solid state lighting applications

This article explores the structural, morphological, thermoluminescence, and temperature-dependent emission properties of novel Dy3+ doped BaMgAl10O17(BAM-Dy) phosphors. Structural studies reveal that the phosphor has a similar structure to β-alumina. Rod-shaped interlinked surface morphology is a notable characteristic of the BAM-Dy samples. Incorporation of Dy3+ ions lead to the formation of distinct emission peaks (485 nm, 577 nm, and 635 nm) in the PL emission spectra. The effect of dopant concentration on the light output is crucial, and the highest PL intensity is observed for 1.5 mol% of Dy3+ concentration. The potential use of BAM-Dy phosphors for Gamma radiation detection and measurement is tested through thermoluminescence studies, which reveal high sensitivity, low threshold dose values, and exceptional response to low gamma doses, making them suitable for dosimetry applications. The investigation of trap parameters provides useful information about the number of traps, trapping mechanism, and trap energies, which is essential for in-depth study of synthesized phosphors for γ dosimetry application. The study of the effect of temperature on the luminescence behaviour is crucial for the application of phosphors in the light-emitting field. The BAM-Dy samples exhibit excellent thermal stability even at 210 °C, making them suitable for use in the field of White LEDs.


Introduction
Radiation dosimetry plays a critical role in the advancement of scientific and technological fields, as well as in examining the interactions between materials and radiation.Gamma dosimetry offers a wide array of practical uses, including medical diagnostic and therapeutic purposes, industrial processes, and environmental monitoring [1].The significance of dosimetric materials in precisely measuring gamma radiation for the purpose of safeguarding human safety while remaining impervious to incident gamma rays is immeasurable.In recent years, there has been a marked increase in the pursuit of optimal dosimetric materials, leading to extensive research focused on the creation and assessment of specialized materials tailored for gamma dosimetry applications.The search for excellent dosimetric materials has confirmed that phosphors are the potential candidates for gamma radiation detection.Phosphors exhibit remarkable characteristics, such as high sensitivity, ease of use, stability, and low fading properties [2].Phosphors, could convert incident radiation into measurable light signals, making them exceptional candidates for this gamma dosimetry application.The critical factor in achieving accurate and reliable dose measurements lies in the selection of an appropriate phosphor.
Thermoluminescence (TL) can be used to accurately measure the radiation exposure [3].TL Dosimeters use the TL technique to estimate the absorbed dose.TL is connected to the defects in materials.Crystalline defects trap electrons and holes when they migrate from the valence band to the conduction band after being excited by ionising radiation.Thermal activation liberates trapped electrons and holes, leading to the emission of light [3].Thermoluminescence dosimeters are widely used in various fields, including personal dosimetry, medical and environmental monitoring [4].The selection of a suitable host material with rare earth doping is crucial in enhancing the optical and thermoluminescence properties of the phosphor.
Research into various rare earth-activated compounds has recently garnered significant interest.Numerous studies are being conducted on different host matrices, such as aluminates, tungstates, borates, and silicates.Among these, aluminates phosphor has received particular attention due to its exceptional luminous efficiency, superior chemical stability, and long-lasting afterglow.Many scholars have discovered that aluminate-based phosphors possess long lifetimes and exhibit high radiation intensity when doped with rare earth metals.There are studies reported on the structural and photoluminescence properties of BaMgAl 10 0 17 phosphors doped with Ce 3+ [5], Eu 2+ [6][7][8], Cr 3+ [9, 10], Tb 3+ [11].But there has not been any discussion on Dy 3+ doped BaMgAl 10 O 17 phosphor as per our knowledge.Dy 3+ plays a critical role in the optical properties of phosphors.The f-block energy levels of the Dy 3+ ion is intricate, and the transition between them results in sharp line spectra with a 4 F 9 configuration.Dy 3+ ions acting as dopants have two distinct emission bands.One is in the blue region (470-500 nm) arising from the 4 F 9/2 → 6 H 15/2 transition, and the other is in the yellow region (560-600 nm) arising from the 4 F 9/2 → 6 H 13/2 transition.
This article presents a novel investigation into the structural, morphological, thermoluminescent characteristics, and variation of luminescence properties of Dy 3+ doped BaMgAl 10 O 17 (BAM-Dy) phosphors.Specifically, the study explores how the concentration of the dopant and temperature affect the variation of luminescence properties.The research examines the structural characteristics of BAM-Dy phosphors, focusing on the hexagonal crystalline host that resembles the β-alumina structure, which promotes the formation of antisite defects.These defects are critical in determining the material's dosimetry applications.The TL glow curve analysis, linear dose response, and sensitivity are evaluated.In the context of white light emitting applications, it is essential that thermal stability be maintained at elevated temperatures.Therefore, the PL behavior of BAM-Dy samples is explored at high temperatures.The insights gained from this study not only advance the understanding of the material's dosimetric potential but also contribute valuable knowledge to the broader field of luminescence and radiation dosimetry.

Experimental details 2.1. Synthesis
The BaMgAl 10 O 17 :Dy 3+ phosphor was prepared using the combustion method.Initial materials are taken in the nitrate form, and fuel is added to enhance the combustion.The metal nitrates taken are Ba (NO 3 ) 2 (99%), ) and urea (NH 2 CONH 2 , 98%) is used as a fuel.The compositions are mixed according to the stoichiometric ratio.The stoichiometric compositions are calculated using the basic rules of propellant chemistry.All reagents are combined and crushed in an agate mortar for 30 min.The resulting product was transferred to a crucible and heated in a furnace at 600 °C.This will form a white foamy product that is ground into fine powder.The powder obtained after grinding is used for characterizations.The sample notation is given in the table 1.

Characterization techniques
X-ray Diffraction (XRD) studies are performed to confirm the polycrystalline arrangement and phase of the samples.XRD measurement was performed using Rigaku Miniflex 600 (5th gen) with K-α (λ = 1.54 Å) radiation (40 kV, 15 mA) by varying the scanning angle from 10°to 70°at 2°/min.The morphological properties and the elemental composition are carried out using Scanning Electron Microscopy (SEM) and Energy dispersive x-ray Spectroscopy (EDS) (sigma Zeiss instrument).The functional groups are identified using SHIMADZU-IRSpirit Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy (ATR-FTIR).The Photoluminescence (PL) excitation and emission properties are recorded using JASCO-FP 8500 spectrofluorometer.TLD reader 1009I Nucleonix India is used to record TL spectra.Irradiation is carried out Gamma Chamber-5000 and irradiated with 60 Co γ dose ranging from 5 Gy to 5 kGy.The Temperature dependent Photoluminescence (TDPL) spectra is obtained using Agilent Cary Eclipse Fluorescence Spectrophotometer.

Phase analysis
The crystalline phase exploration of BAM-1.5 Dy samples are done using the XRD pattern.The diffraction peaks match with the pure BaMgAl 10 O 17 phosphor [7] (JCPDS number 26-0163).Thus, the samples crystallize in to hexagonal structure with with P63/mmc space group (figure 1).BAM-Dy unit cell has a β-alumina structure, where BaO mirror planes are sandwiched between oxygen spinel blocks (figure 2).The spinel network has two tetrahedral and two octahedral Al sites.One of these sites might be occupied by Mg ion, but the tetrahedral site occupation would give more stability to the lattice [6].Dy 3+ ion occupies one of the Ba sites (Ba 2+ ionic radius = 0.143 nm, Dy 3+ ionic radius = 0.097 nm).The phase purity of BAM-1.5 Dy has been confirmed using Rietveld refinement technique.Figure 3 shows that the experimentally obtained XRD peaks matches with the pure BAM peaks, with χ 2 = 1.9734.The cell parameters were found as a = b = 5.6239 Å, c = 22.670 Å, and α = β = 90°, γ = 120°and volume = 620.960 Å 3 were estimated from refinement results.

FTIR analysis
FTIR spectroscopy is a method used to investigate molecular vibrations.In this context, the FTIR transmission spectrum (figure 4) are analyzed to determine the presence of functional groups in the samples.The peak for the O-H stretching is located at 3729 cm −1 , while the Mg-O and Ba-O peaks are attributed to vibrations at 1514 cm −1 and 1363 cm −1 , respectively [12].The β-alumina structure of BAM-Dy phosphor is composed of AlO 4 tetrahedral and AlO 6 octahedral structures, resulting in two types of Al-O vibrations (one for each type of structure).The AlO 4 absorption causes a band to form at 528 cm −1 and 768 cm −1 , while the AlO 6 absorption peak is located at 1007 cm −1 [13].These findings align with the estimated crystal structure and XRD results, as indicated in the table 2 provided for band assignment.

Morphological study and elemental analysis
The morphology of the BAM-1.5 Dy sample can be seen in figure 5(a).A magnified SEM image (figure 5(b)) displays rod-shaped structures, and it is clear that the interlinked crystallites exhibit a high degree of agglomeration [8].EDS analysis in figure 5(c) reveals the presence of Ba, Mg, Al, O, and Dy.The successful incorporation of the dopant and its corresponding host elements is confirmed through EDS spectrum.

Luminescence properties
The emission of any phosphor system is a characteristic of the dopant ion and the host matrix.The luminescence properties of BAM doped Dy 3+ phosphors are studied using the excitation and emission spectra.The excitation spectra are recorded for 578 nm emission (figure 5).The intensity of the spectra is varying with Dy 3+ concentration, and it is maximum for 1.5 mol%.The peaks in figure 6 can be assigned to different transitions: 295 nm ( 6 H 15/2 → 4 D 7/2 ), 325 nm ( 6 H 15/2 → 6 P 3/2 ), 350 nm ( 6 H 15/2 → 6 P 7/2 ), 365 nm ( 6 H 15/2 → 6 P 5/2 ), and 385 nm ( 6 H 15/2 → 4 M 19/2 ) [14-16].Among 5 transitions, 6 H 15/2 → 6 P 7/2 has highest intensity and is chosen as the excitation wavelength for recording emission.The emission spectra (figure 7) were recorded with 350 nm excitation and 3 peaks were observed in blue, yellow and red regions.The transitions are identified as follows, 485 nm ( 4 F 9/2 → 6 H 15/2 ) and 577 nm ( 4 F 9/2 → 6 H 13/2 ) and 635 nm ( 4 F 9/2 → 6 H 11/2 ) [17,18].The yellow emission had the highest intensity, and the emission intensity increased with the concentration of Dy 3+ ion.However, when the concentration of Dy 3+ ion exceeded 1.5 mol%, an intensity drop was observed, which is attributed to concentration quenching.This phenomenon occurs when the energy transfer takes place from one dopant ion to the adjacent ion, causing a decrease in the PL emission.The cause of the concentration quenching is a consequence of the non-radiative transfer of energy among the Dy 3+ ions through exchange interactions,  radiation re-absorption, and multipole-multipole interactions [19,20].If the wavefunctions of donor and acceptor ions overlap either directly or indirectly, the energy transfer is due to exchange interaction.For the energy transfer interaction, the critical distance between the dopants should be within 5 Å limit.On the other hand, radiation reabsorption is observed when the excitation and emission spectra overlap extensively.But, for BAM-1.5 Dy phosphor the overlap is very small (figure 8), and the radiation re-absorption is not the possible quenching mechanism.To validate the energy transfer interaction, the critical distance R c is calculated.
The unit cell volume V = 620.960(Å) 3 , N = 2, and X c = 0.015.R c = 34.06Å (>5 Å), hence the concentration quenching is caused by multipolar interaction.There are various types of multipolar interactions such as dipole-dipole, dipole-quadrupole, quadrupole-quadrupole.The corresponding multipolar interaction in BAM-Ce phosphors can be identified using Dexter's theory [21].PL emission intensity exclusively depends on dopant ion concentration (equation ( 2)).On taking the log and simplifying, we get a straight-line equation where log I x is y-axis, x log is the x-axis and q -3 is the slope (equation ( 3)).The value of θ determines the type of multipolar interaction.For θ = 6, 8, 10 the reactions are dipole-dipole, dipole-quadrupole, and quadrupole-quadrupole respectively.The slope of log(I/x) versus 1/x is drawn (figure 9), and the slope is −1.57655.And θ = 4.73 ∼ 6; hence the dipole-dipole interaction is responsible for energy transfer.here n is the slope of inverse line, n = (x − x e )/(y − y e ), here (x e , y e ) is the epicentre of convergence, here x e = 0.332 and y e = 0.186.For optimum concentration of Dy 3+ , the CCT value is 4358 K which falls in the range of 3500 K to 6500 K and this indicates that prepared phosphor can produce cool light.

Thermoluminescence Glow curve analysis
TL glow curve is recorded for 10 mg of BAM-Dy samples using 60 Co gamma source.The samples are heated from room temperature to 575K at 2.85 K s −1 .Initially, the optimum sample for Gamma dosimetry application  was selected by measuring the TL intensity values for 0.5, 1, 1.5 and 2 mol% of Dy 3+ ions (figure 11).The TL integrated intensity (area under glow curve) increases up to 1.5 mol% of dopant concentration and decreases beyond that.This can be explained as the effect of concentration quenching.The availability of trap centres differs from one matrix to another.The imperfections of the host, dopants added during synthesis also increase the trap sites.Hence, the luminescence is believed to be enhancing by dopant addition.But, when the dopant concentration crosses a particular limit, the emission is decreasing, since the average distance between neighbouring dopant ions decrease [23].This causes delocalisation of trap energy levels, and the trapping efficiency reduces.There is a detailed description given by Chen et al [24] for concentration quenching based on 3 electron traps and one recombination centre model.The differential equations are solved simultaneously for excitation stage, relaxation stage and the TL read-out stage.This method was employed by Amirouche Bouremani et al to explain concentration quenching in SrY 2 O 4 :Eu 3+ phosphors [25].As it is not the scope of our study, we have not focused on the mechanism of concentration quenching.
The effect of γ rays on BAM-1.5 Dy phosphor was studied by irradiating 10 mg of powder samples with different doses (5 Gy-10 kGy) (figure 12).The emission is increasing with increase in absorbed dose.As the absorbed dose increases, more electrons are excited to the conduction band, and they get trapped in trap centres.Upon heating they recombine with holes at recombination centres.Thus, the increasing TL emission is accountable for the enhancement of trapped electrons and their recombination.

Optimum range of operation
The material used as a dosimeter exhibit is suitable to operate within a dose range where the integrated TL intensity has linear dependency on the absorbed gamma dose.For BAM-1.5 Dy phosphors, figure 12 inset shows a linear trend from 5 Gy-1 kGy doses.Hence the prepared phosphor could be used to detect gamma radiation in this range.

Sensitivity and minimum detectable dose (MDD)
The dosimeter should have high value of sensitivity to detect the incoming radiation.The sensitivity of BAM-1.5 Dy sample is determined by taking the slope of TL response versus the dose graph (figure 13), and the value is 1.54 × 10 7 countsg −1 kGy −1 .The smallest value of the dose that can be detected using the prepared phosphor is calculated using equation ( 5), b s b -the standard deviation of TL reading for 6 samples without irradiation [26].And MDD = 73.2μGy.In some crystals where more than one cation is present, the exchange of cation positions results in a point defect known as antisite formation.When γ rays are incident on the sample, the anions may get displaced from the original position and creates anionic vacancy.Thus, when an electron gets trapped in anionic vacancy, it is known as F + centre.Another type of hole trap is found in inorganic phosphor hosts with O 2− ions.The O 2− ions present near cationic vacancies tend to trap holes, forming O − ions.The possible mechanism resulting in light emission of BAM-Dy is by trapping electrons in F + centres upon gamma ray irradiation [27,28].When heated, these trapped carriers would surmount the traps and recombine with holes via radiative recombination.The number of traps and trap parameters such as trap energy (E), trap lifetime (τ), and escape factor (s).These can be estimated using different methods.We will delve into the CGCD method and Chen's peak shape method in great detail and determine trap features using these methods.

CGCD method
The CGCD method is the fitting of TL glow curve using Kitis general order relation [29], using nonlinear curve fitting software.The excellent fit is ensured by the Figure of merit (FOM)<5%.The curve fitting would give the number of deconvoluted peaks, or the number of traps.The trap parameters associated with each peak is extracted ensuring the best fit [30].
where b is the order of kinetics, I m is the highest TL intensity, and E is the trap activation energy.The t and s are calculated using equations (7) and (8).
where β = heating rate, T = storage temperature, and b = order of kinetics.Figure 14 shows the deconvoluted peaks and table 4 gives the trap features of BAM-1.5 Dy samples for 5 Gy-1 kGy gamma dose.Each glow curve has 4 peaks, or 4 traps.The traps at lower temperature are shallower (lower energy), and those at higher temperature are deeper traps (higher energy).With rise in γ dose, the trap energy is increasing.This can be attributed to the creation of deeper traps with increase in absorbed dose.Enhancement of incident energy creates deeper trap levels in the forbidden energy gap, thus more trap centres are available with higher trap depth.Also, the lifetime of the traps located at higher temperature is longer.Hence, it can be concluded that the deeper traps have a longer lifetime, showing less fading [31].

Chen's method
Chen's peak shape method is used for the estimation of trap parameters using the peak shape.It uses the parameters w, t d , and defined as follows (equations ( 9)-( 11)) where T m is the peak temperature with the highest intensity, T 1 and T 2 are the temperatures at a half-width intensity to the lower and higher temperature side of T m .The order of kinetics is determined using μ g [32,33].
First-order kinetics -μ g ∼ 0.42 s-order kinetics-μ g ∼ 0.52 The phosphor powders exhibit emission when the trapped electrons are released by supplying external energy (heat).This is called activation energy (Eα) and is calculated using the equations below, [34,35] Where α = τ, δ, ω.Energy E is the average of E δ, E ω, and E τ .Kinetic parameters are estimated using Chen's peak shape method and are tabulated below in the table 5.
The average activation energy of BAM-1.5 Dy samples is increasing with increase in incident radiation (5 Gy-1 kGy).Also, it should be noted that the energy increases with increase in peak temperature.The lifetime is enhanced with rise in absorbed dose.Identical results are observed in CGCD method as well.Thus, we can confirm the correlation between increase in dose on trap energy and trap lifetime.The electrons move to deeper traps with more lifetime.

Temperature dependent luminescence (TDPL) studies
It is very crucial to explore the effect of temperature on the luminescence properties of phosphors since the abovementioned study helps to select a suitable matrix for lighting applications [36].Thermal stability is a key factor which is studied in detail.The emission characteristics of phosphors are significantly impacted by temperature fluctuations [37,38].As the temperature rises, various phenomena occur that affect the emission spectra and intensity.One primary effect is the thermal activation of carriers within the phosphor material, which promotes carrier excitation to higher energy states, resulting in changes in the emission spectra and intensity.This phenomenon is often observed as a redshift or blueshift in the photoluminescence (PL) peak position, accompanied by changes in the emission intensity.Furthermore, temperature variations can affect the concentration and mobility of defects within the phosphor lattice, leading to enhanced non-radiative recombination processes and reduced PL efficiency.On the other hand, certain phosphors may exhibit  enhanced emission at specific temperature ranges due to the activation of energy transfer mechanisms or the suppression of competing non-radiative decay pathways [39,40].The TDPL spectra of BAM-1.5 Dy phosphors are recorded for temperature range 30 °C − 210 °C (figure 15).The PL intensity reduces gradually, and the mechanism can be explained using coordinate diagrams and energy levels (figure 16).The ground states and excited states of Dy 3+ and BAM host are given as Dy 3+ GS, host  GS, Dy 3+ ES, and host ES respectively.The excited state of Dy 3+ ion has 3 energy levels 4 F 9/2 , 4 I 15/2 , 4 G 11/2 and the GS levels are 6 H 11/2 , 6 H 13/2 , and 6 H 15/2 .When the samples are excited with 350 nm photons, the electrons from the GS (of both dopant and host) gets excited to ES.The point were the ES of dopant and host intersects is denoted as X, and Y is the point where the GS and ES of host intersects.The electrons in ES of dopant would return to GS via radiative collision giving out photons.There is a great probability that the electrons of host ES overcome the barrier height ΔE 1 , and jump to the ES of Dy 3+ , via electron-phonon coupling.And, they return to GS followed by photon emission.Thus, there is an increase in dopant emission intensity.However, with rise in temperature, the electrons would gain sufficient energy to cross the barrier ΔE 2 and directly tunnel to the GS of host (via point Y).This results in the decrease of electron cross over between ES of host and dopant [41].Hence, the emission intensity is reduced.Even though the emission intensity decreases, it is only 25% of the room temperature intensity (figure 17) Thus it is confirmed that the prepared phosphors exhibit excellent  thermal stability even at elevated temperatures (210 °C).Hence the prepared phosphors are suitable candidates for lighting applications [42].
To verify the thermal stability, the activation energy can be evaluated using the Arrehenius equation ).The CIE diagram does not show an appreciable shift in color coordinates; hence we can confirm that the phosphor sample is thermally stable [43].

Conclusion
Dy 3+ doped BaMgAl 10 O 17 phosphors synthesized via combustion method crystallized into a hexagonal structure.The spinel structure of the samples was confirmed using FTIR spectra, which displayed bands corresponding to AlO 6 and AlO 4 vibrations.The rod-shaped structures of the phosphor samples exhibited highly interlinked morphology.Upon 350 nm excitation, the Dy 3+ doped BAM samples emitted blue (485 nm), yellow (577 nm), and red (635 nm) light.The PL intensity reduced beyond 1.5 mol% of Dy 3+ concentration and the emitted light fell in cool white light region.The concentration quenching was caused by the interaction between electric dipoles of the dopant.For TL spectra, concentration quenching was observed at 1.5 mol% of Dy 3+ , hence, BAM-1.5 Dy samples were chosen for further TL studies.The TL intensity increased with the absorbed dose, and the TL dose response curve was linear from 5 Gy to 1kGy.The high sensitivity and low MDD value suggest the suitability of the prepared samples for gamma dosimetry application.The CGCD method confirmed the presence of four trap centers, and the trap energy increased with the absorbed dose.The trap lifetime increased with the incident dose, indicating that the electrons absorb more energy and get trapped in deeper traps with an increase in incident gamma dose.The results from the CGCD method were validated using Chen's peak shape method.The PL emission intensity gradually reduced with temperature, but BAM-1.5 Dy phosphors showed appreciable thermal stability along with stable emission at elevated temperatures and could be used for potential lighting applications.

kT 1
the intensity at initial temperature, I is the intensity at temperature T, C is a constant, ΔE is the activation energy, and k is the Boltzmann constant.On linearizing the equation, graph (figure18) gives ΔE = 0.164 eV.Further to support the thermal stability property, the CIE coordinates of BAM-1.5 Dy sample for all temperatures is calculated and shown (figure19, and table6

Table 3 .
CIE coordinates and CCT values of BAM-Dy phosphors.

Table 5 .
Trap parameters of BAM-1.5 Dy by Chen's method.

Table 6 .
CIE coordinates and CCT values at different temperatures.