Spectroscopic Study of a Single Crystal of SrAl

Article

Reference

Spectroscopic Study of a Single Crystal of SrAl

O

:Eu

:Dy

DELGADO PEREZ, Teresa, AFSHANI AGHAJARI, Jafar, HAGEMANN, Hans-Rudolf

Abstract

The extraordinary long phosphorescence of SrAl2O4:Eu2+:Dy3+ has been widely studied in powder samples because of its broad range of applications in this form and despite the fact that the bulk material shows higher intensity emission and longer afterglow. However, the investigation of SrAl2O4:Eu2+:Dy3+ crystals that, unlike the powder, do not contain surface defects, would allow a better insight into the mechanism that governs the long-lasting phosphorescence of this co-doped material. Thus, a SrAl2O4:Eu2+:Dy3+ single crystal was studied in detail by absorption spectroscopy and photoluminescence, including a novel estimation of its extinction coefficient. In addition, thermoluminescence measurements and wavelength-dependent quantum efficiency measurements have been performed to improve the understanding of the role of both europium and dysprosium ions in the corresponding persistent phosphorescence mechanism.

DELGADO PEREZ, Teresa, AFSHANI AGHAJARI, Jafar, HAGEMANN, Hans-Rudolf.

Spectroscopic Study of a Single Crystal of SrAl₂O₄:Eu²⁺:Dy³⁺. Journal of Physical Chemistry.

C, 2019, vol. 123, no. 14, p. 8607-8613

DOI : 10.1021/acs.jpcc.8b12568

Available at:

http://archive-ouverte.unige.ch/unige:147125

Disclaimer: layout of this document may differ from the published version.

1 / 1

1 Accepted paper

This paper was published in the Journal of Physical Chemistry C 2019, 123, 14, 8607–8613 https://doi.org/10.1021/acs.jpcc.8b12568

Spectroscopic study of a single crystal of SrAl

O

:Eu

:Dy

Teresa Delgado^a, Jafar Afshani^a, Hans Hagemann^a

aDépartement de Chimie Physique, Université de Genève, Quai Ernest-Ansermet 30, Genève 1211, Switzerland.

Abstract :

The extraordinary long phosphorescence of SrAl2O4:Eu²⁺:Dy³⁺ has been widely studied in powder samples due to its broad range of applications in this form and despite the fact that the bulk material shows higher intensity emission and longer afterglow. However, the investigation of SrAl2O4:Eu²⁺:Dy³⁺

crystals that, unlike the powder do not contain surface defects, would allow a better insight into the mechanism that governs the long-lasting phosphorescence of this co-doped material. Thus, a SrAl2O4:Eu²⁺:Dy³⁺ single crystal was studied in detail by absorption spectroscopy and photoluminescence, including a novel estimation of its extinction coefficient. In addition, thermoluminescence measurements and wavelength dependent quantum efficiencies measurements have been performed to improve the understanding of the role of both europium and dysprosium ions in the corresponding persistent phosphorescence mechanism.

INTRODUCTION

The alkaline earth aluminate SrAl2O4:Eu²⁺:Dy³⁺ exhibits one of the longest afterglows among the broad range of inorganic persistent phosphors,¹ which makes it an excellent candidate for a wide range of applications such as safety emergency signs,² photocatalysis,³ solar cells,⁴ etc. The host structure of SrAl2O4:Eu²⁺:Dy³⁺ is well characterised. The SrAl2O4 crystalizes in a monoclinic system and consist of rings formed by six corner sharing oxide aluminate tetrahedra which form channels within which the Sr²⁺ ions are situated. There are two non-equivalent Sr²⁺ sites with the same coordination number and Sr-O distances but they differ by a slight distortion of their square planes.⁵ In consequence, when the Sr²⁺ ions (r = 126 pm in VIII coordination ) are replaced by Eu²⁺ (r = 125 pm)⁶, the luminescence properties of the two different Eu²⁺sites slightly differ too and two different emissions are observed at around 450 nm and 520 nm at low temperatures.^7-9 Contrary, at ambient temperature the blue emission is thermally quenched. In addition, energy transfer of the excited Eu²⁺ from the blue site to the lower energy Eu²⁺ green site takes place.^7,10 It is worthy to mention that the SrAl2O4:Eu²⁺ system itself shows a significant long phosphorescence. However, when another co-dopant such as Dy³⁺(r = 97 pm) is incorporated into the host by replacing the Sr²⁺ ions a longer-lasting afterglow is observed which lasts up to several hours after the excitation source is removed.^11-13

Several mechanisms have been proposed in order to explain the later phenomena. In the very well- known “hole trapping mechanism” elucidated by Matsuzawa et al.,¹⁴ after excitation, the Eu²⁺ ion takes an electron from the valence band (VB) of the host and the Dy³⁺ traps the hole left in the VB, being reduced to Dy⁴⁺. The delayed radiative emission is produced when by thermal activation one electron is promoted from the top of the VB to the Dy⁴⁺ ion which is reduced back to Dy³⁺, and the trapped hole returns to the Eu¹⁺ site. However, the low probability of the “hole trapping process” given the instability

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of Dy⁴⁺ and Eu¹⁺ led other researchers to propose new alternatives. Dorenbos et al.¹⁵ raised the so- called “electron-trapping mechanism” based on the excitation of one electron of Eu²⁺ into the conduction band (CB) of the host to generate Eu³⁺ and the trapping of this electron by the Dy³⁺ with its consequent reduction to Dy²⁺. In this case the electron trap can be thermally released and the recombination with the Eu³⁺ gives rise to the delayed luminescence. Nevertheless, none of these mechanisms explained the intrinsic phosphorescence of the Dy³⁺ free material SrAl2O4:Eu²⁺. Thus, other models based on oxygen vacancies have been proposed by Clabau et al.,¹⁶ in which the electron that is excited from the Eu²⁺ into the CB is trapped by an oxygen vacancy instead. In addition, if there is residual Eu³⁺ not reduced during the synthesis of the doped aluminate, under irradiation, one electron from the VB can be promoted to the Eu³⁺ cations and the subsequent recombination with the VB would give rise to the luminescence at 450 nm. Recently, ab initio calculations have been used to assign the green luminescence of Eu²⁺ to the Sr2 site¹⁷ and calculations based on density functional theory have underlined the importance of the intrinsic defects of the host in the long luminescence mechanism.¹⁸ Nonetheless, although persistent phosphorescence is known from centuries ago,¹⁹ and despite the effort to prove experimentally the above described mechanisms, the existence of traps with diverse origins and the presence of traps distributions rather than single traps enhance the difficulty to expose a single valid mechanism.

Herein, the properties of a single crystal of SrAl2O4:Eu²⁺:Dy³⁺ have been studied in order to contribute to the development of a mechanism that could explain the role of co-dopans in the long-lasting phosphorescence of this kind of materials. SrAl2O4:Eu²⁺:Dy³⁺ single crystal fibers were synthesized for the first time by Jia et al. by laser heated pedestral growth taking advantage of the low vapor preassure of the alkaline earth aluminates at their melting point.^20,21 At that time, in 1998, just after the Matsuzawa mechanism was raised, the new crystal fibers were preliminary characterised by photo and thermoluminescence. Since then few studies have been focused on the investigation of SrAl2O4:Eu²⁺:Dy³⁺ crystals^22,23 but rather powders despite their lower intensity and shorter afterglow, since they can be more easily integrated in pigments or surfaces for applications,² especially in the case of nanoparticles.²⁴ However, the use of nano or microcrystalline powder implies that not only bulk defects are present in the material but also surface defects. Thus, in order to study the mechanism responsible for the long phosphorescence, only thermodynamically relatively stable defects should be present. This is the case of a single crystal whose purity is ensured by the slow growth process used for its synthesis.

Our SrAl2O4:Eu²⁺:Dy³⁺ single crystal characterization includes the estimation of its extinction coefficient by analysing its optical properties and the real concentration of Eu²⁺ within it. The value of the extinction coefficient is up to our knowledge the first one proposed for this kind of material and could serve as a starting point to calculate the real concentration of Eu²⁺ in any sample of the same composition. In addition, thermoluminescence analysis have been performed in order to study the effect of the crystallinity of these samples in the quality of the traps. Besides, wavelength dependence quantum efficiency measurements have been performed on the SrAl2O4:Eu²⁺:Dy³⁺ crystal and compared with a Dy³⁺ free sample so that the contribution of both Dy³⁺ and Eu²⁺ individually to the phosphorescence can be better understood.

EXPERIMENTAL

Sample preparation. A single crystal of SrAl2O4:Eu²⁺:Dy³⁺ has been prepared by travelling solvent floating zone. The nominal concentration of Eu²⁺ is 1%. The crystal has been polished until a thickness of 187±1 µm to avoid a saturated absorption. The polishing also led to a very flat surface avoiding internal reflexions (see image in Figure 4).

Absorption spectroscopy. Absorption spectra of the sample at 300 K were recorded with an optical spectrometer (Agilent Cary 5000). To do that, the crystal was mounted on a copper plate with a previously drilled hole of about 0.1 mm in diameter and glued with silver paste.

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Photoluminescence spectroscopy. The room temperature excitation and luminescence spectra were recorded on a Fluorolog 3-22 (Horiba Jobin Yvon), equipped with a water-cooled photo multiplier tube (PMT). For the low temperature measurements, a closed-cycle cryostat (Janis-Sumitomo SHI-4.5) equipped with a programmable temperature controller (Lakeshore Model 331) was used. For the quantum yield measurements an integrated sphere was used (details are given in the supplementary material and Figure S1).

Thermoluminescence. Thermoluminescence (TL) measurements were performed with a home-built system.²⁵ A cartridge (Omega, CSH-101100) was used for the heating and a cold nitrogen gas for the cooling. The excitation source is a LED-light at 445 nm (HL-3WHPL-Roy445, 1.5 W). The temperature was monitored with a temperature sensor (Omega, Type T) and controlled with a PID-controller (Omega, CN7523). A photon counting unit (Hamamatsu, C3863) is used to detect the TL. See details of the measurements in the supporting material.

RESULTS

Optical characterization. The absorption spectrum of a 187 µm crystal of SrAl2O4:Eu²⁺:Dy³⁺ has been recorded at room temperature between 200 and 500 nm (Figure 1a). Two different broad bands at 254 nm and 330 nm are observed. These two bands are associated with the electric dipole allowed 4f–5d transitions. At 254 nm a maximum optical density of 4.07 is found whereas at 330 nm the maximum is slightly lower with a value of 3.68. The strong absorbance of the sample makes rather difficult to distinguish more individual bands in the spectrum. Since the UV-vis absorption spectrophotometer do not discriminate between the transmitted light from the probing wavelength after absorption and the intrinsic strong luminescence of the sample, the luminescence has been cut with different filters as explained in Figure S2.1 and no significant difference has been found in the absorption spectrum. The excitation spectra of the sample at room temperature and 6 K have been recorded between 250 and 500 nm (Figure 1b). The spectra are not exactly the same than the absorption one neither at room temperature nor at low temperature. It presents two broad bands due to the interconfigurational 4f⁷– 4f⁶ 5d¹ transitions between 250 and 460 nm with several maxima at 270 nm, 317 nm, 362 nm and 413 nm, in agreement with previous results observed in powders of the same composition.²⁵ On the other hand the emission spectra of the sample at room temperature and 6 K have been recorded between 400 and 650 nm after excitation at 370 nm (Figure 1c). At room temperature the emission spectrum shows the characteristic broad band centred at 520 nm of Eu²⁺ corresponding to the electric dipole allowed 4f–5d transition. The broadening of this band is due to once again the interconfigurational transition from 4f⁶–5d¹ to the ground state ⁸S7/2 since the electron in the 5d orbital couples strongly with the lattice resulting in a splitting of the energy levels by the crystal field. At 6 K the emission of the blue site appears centered at 455 nm. The latter band is shifted 10 nm upwards respect to the blue site emission in dysprosium free powder samples with the same concentration of europium (Figure S4b).^10,26 A red shift of the Eu blue site emission was already observed by Jia et al.²⁰ in single crystal fibers in the presence of dysprosium although no shift was observed in the presence of dysprosium in powder samples.^8,25 Other studies relate the blue shift to the particle size of the phosphor.²⁷ Furthermore, the presence of trivalent ions within the crystal was studied by selective irradiation at 472 nm (⁶H15/2 – ⁶H7/2 transition of Dy³⁺)²⁸ and 522 nm ( ⁵D1→⁷F0 transition of Eu³⁺),²⁹ where no absorption from the Eu²⁺ is expected. However, the strong emission due to the dipole allowed transitions from Eu²⁺ masks the weaker f-f magnetic (and slightly electric) dipole allowed emission bands from Dy⁺³ and Eu³⁺ (Figures S3a and S3b). It is not known whether the Eu³⁺ ions have been completely reduced to Eu²⁺ during the synthesis of the SrAl2O4:Eu²⁺:Dy³⁺ crystal. Alternatively, the presence of Dy has been confirmed by its characteristic near infrared absorption bands.^30,31 The results are displayed in Figure S3c.

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Figure 1. a) Room temperature absorption spectrum, b) room temperature and 6 K excitation spectra recorded at 520 nm, c) room temperature and 6 K emission spectra after excitation at 370 nm, d) room temperature excitation spectrum between 700 and 1000 nm recorded at 520 nm and emission spectrum after excitation at 875 nm (photostimulated luminescence).

Estimation of the extinction coefficient of the SrAl2O4:Eu²⁺:Dy³⁺ crystal. The actual concentration of europium in the doped crystal could be slightly different respect to the nominal one used for the synthesis (1% respect to strontium) due to the segregation that depends on the growth conditions.

Thus the real concentration of europium has been investigated as follows: The energy transfer enhancement from the blue site to the green site when increasing the nominal concentration of Eu²⁺

at 10 K has been proved for the SrAl2O4:Eu²⁺:Dy³⁺ powder samples.⁹ The dependence of the intensity ratio between the blue and the green emission with the concentration of Eu²⁺ follows a double exponential behaviour. The intensity ratio between the blue and green emission of the SrAl2O4:Eu²⁺:Dy³⁺ crystal at low temperature shown in Figure 1c has been used to extrapolate the value of the relative concentration of Eu²⁺ respect to the concentration of Sr²⁺ within the crystal. A relative concentration of 1 mol% has been found in agreement with the nominal concentration used for the synthesis (Figure S4). This value has been confirmed by Energy-dispersive X-ray spectroscopy (Figure S5). On the other hand, the molar concentration of Sr²⁺ within the crystal has been calculated by using equation 1:

5 [Sr²⁺](M) = ^Z⁄NA

VC (L) = ^4⁄NA

383.8 ∙10⁻²⁷(L) , (1)

where Z is the number of atoms per unit cell, NA is Avogadro constant and Vc is the unit cell volume calculated with the unit cell parameters from reference 5. A molar concentration of Sr²⁺ of 17.30 M is obtained. Therefore, a molar concentration of Eu²⁺ of 0.173 M is estimated. Thus, considering the later value, the thickness of the crystal and the optical density measured at 254 nm and 330 nm, the extinction coefficient of the SrAl2O4:Eu²⁺:Dy³⁺ crystal at these two wavelengths can be calculated by using equation 2:

ɛ_ (M⁻¹cm⁻¹) =_[Eu2+](M)·thickness (cm) ^OD (2) ɛ_{ = 254 nm}= ^4.07

0.173 M ·187·10⁻⁴ cm= 1258 M⁻¹cm⁻¹ ɛ_{ = 330 nm}= ^3.68

0.173 M ·187·10⁻⁴ cm = 1137 M⁻¹cm⁻¹

An extinction coefficient value of 1258 M^-1 cm^-1 and 1137 M^-1 cm^-1 was found at 254 nm and 330 nm, respectively. Alternatively, a thinner crystal has been analysed and corresponding values of 1320 M^-1 cm^-1 (254 nm) and 1160 M^-1 cm^-1 (330 nm) have been found (Figure S2.2). These values are in good agreement with the results obtained for other Eu²⁺ compounds such as EuCl2 (ε = 1130 ± 56 M^-1cm^-1 ).³²

Thermoluminescence measurements. Thermoluminescence (TL) measurements were performed on the crystal after irradiation at 223 K and 293 K with a 445 nm and 395 nm LED-light (see experimental section and Figure S6 for more details). In Figure 2a the results obtained by irradiating at 447 nm are shown. The shape of the curves is rather broad indicating a wide trap distribution. The full width at the half maximum (FWHM) is 113 K after excitation at 223 K and 85 K after excitation at 293 K, whereas the maxima of the afterglow peaks are centred at 328 K and 348 K after excitation at 223 K and 293 K, respectively. This indicates a thermal depletion of traps with increasing temperature in addition to identifying remarkably high ratio of the deep/shallow traps.

In Figure 2b the analogous results obtained by irradiating at 395 nm are shown. As observed before, the strong shape and intensity dependence of the TL curves with the irradiation wavelength is further evident after irradiation at low temperatures.²⁵ Overall a significant decrease of the intensity is observed regardless the irradiation temperature. In addition, the curves are even broader with FWHM values of 143 K and 100 K after excitation at 223 K and 293 K, respectively. Contrary, the maxima of the afterglow peaks are not affected significantly and remain at slightly lower temperatures (320 K after excitation at 223 K, and 343 K after excitation at 293 K).

It is worth mentioning that besides the maxima of the peaks, a shoulder with relatively lower intensity appears at lower temperatures (250 K) when irradiating at 223 K, regardless the irradiation wavelength. In the case of irradiating at 293 K, a shoulder appears at a higher temperature (380 K) relative to the maxima of the peaks. This feature could be associated with two different kind of traps.

More importantly, the maxima observed at very high temperature is promising for a very long phosphorescence since they indicate the presence of very deep traps in the case of crystals. The depth of the traps was estimated with the initial rise analysis (Figure S7). In this method only the low temperature side of the TL curve is considered. Thus, one can assume that the concentration of trapped charge carriers is approximately constant.³³ The analysis leads to a value of around 0.5 eV.

This value is in the optimal range for phosphorescence stablished by Matsuzawa et al.¹⁴ However, it must be taken into account that for a continuous distribution of traps, the depth is only due to the

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shallowest occupied traps in the distribution since only the low-temperature region is analysed. The depth trap obtained for our SrAl2O4: Eu²⁺(1%):Dy³⁺ crystal is the same as the one recently obtained by Chithambo et al.³⁴ In addition, the thermoluminescence curve of the SrAl2O4: Eu²⁺(1%):Dy³⁺ crystal has been measured after different irradiation times at room temperature and only a slight shift of 4 degrees to lower values of the temperature upon longer irradiation time has been found (Figure S8).

The shift of the maximum towards lower temperatures upon longer irradiation time has been attributed by Van den Eeckhout et al.³³ in the case of CaAl2O4:Dy to a system more complex than a single trap obeying first order kinetics. For second order TL re-trapping of the excitations takes place.²¹

Figure 2. Thermoluminescence curve recorded for the SrAl2O4:Eu²⁺(1%):Dy³⁺ crystal after irradiation at a) 447 nm and b) 395 nm at 223 K and 293 K.

Quantum yield measurements. Quantum yield (QY) measurements on solid samples are usually difficult to analyse since the luminescence presents preferred directions instead of being distributed uniformly in the space. This is the case for instance of very thick samples such as powders or crystals where the excitation itself is not uniform. Because of that, integrated spheres which redistribute isotropically the light over its interior surface are used to calculate the QY.³⁵ Thus, the absolute QY of the SrAl2O4:Eu²⁺(1%):Dy³⁺ crystal upon different irradiation wavelengths at room temperature has been measured with a quantum sphere and compared with the results obtained for a dysprosium free SrAl2O4:Eu²⁺(1%)powder sample and for a containing dysprosium SrAl2O4:Eu²⁺(1%):Dy³⁺ powder sample (Figure 3). As observed for the SrAl2O4:Eu²⁺(1%):Dy³⁺ crystal the irradiation at lower energy leads to higher values of the QY. Indeed, at lower energies the probability of an electron of the Eu²⁺ to be excited into the conduction band of the host is lower. Hence, the radiative recombination is favoured above 400 nm. Ueda et al. have reported the onset of photoconductivity in SrAl2O4:Eu²⁺:Dy³⁺ below 400nm.³⁶ This observation can account for the reduced QY below 400nm, as well as the fact that the blue emitting Eu²⁺ which can transfer its energy¹⁰ to the green emitting Eu²⁺ is quenched at room temperature and does not contribute to the intensity of the emission at 520 nm. In order to measure the QY of the

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powder samples some considerations to check the reproducibility of the results have been done (see supplementary material and Figure S9). Then, the QY at different irradiation wavelengths has been measured by using 0.5 g of powder for both SrAl2O4:Eu²⁺(1%):Dy³⁺ and SrAl2O4:Eu²⁺(1%). The results are shown in Figure 3. As for the SrAl2O4:Eu²⁺(1%):Dy³⁺ crystal, the lower the irradiation energy the higher the QY value for the powder samples. However, in the case of the SrAl2O4:Eu²⁺(1%) sample the values are higher indicating that somehow the presence of dysprosium in SrAl2O4:Eu²⁺(1%):Dy³⁺ decreases the QY, significantly. This is in agreement with the longer afterglow time observed for the Eu-Dy co-doped samples due to the dysprosium traps and the lower concentration of Eu²⁺ available for radiative recombination.¹³ In addition, the non-radiative release of the dysprosium traps may contribute to the lowering of the QY. In the case of the SrAl2O4:Eu²⁺(1%):Dy³⁺ powder the values are even lower than in the crystal. The later observations reveal that the presence of dysprosium has a strong effect not only in the duration of the phosphorescence but also in the QY of the material.

Very recently, Van der Heggen et al³⁷ have reported that the “trapped charge carriers can be optically detrapped by the excitation light”, which implies that these trapped charge carriers equally absorb very strongly around 350-450 nm. Such influence can be only understood considering a high proportion of europium and dysprosium sites participating in the long persistent phosphorescence mechanism, that is to say, a high number of europium sites being excited. However, the absolute storage capacity of SrAl2O4:Eu²⁺:Dy³⁺ was estimated by Van der Heggen et al.³⁸ by concluding that only at around 1.6 % of the Eu ions participate in the phosphorescence mechanism. On the other hand, it is worthy to say that the lowering of the QY values in the presence of dysprosium is more obvious at lower irradiation wavelengths in the case of the crystal. Under higher irradiation wavelengths the QY values of the SrAl2O4:Eu²⁺(1%):Dy³⁺ crystal and SrAl2O4:Eu²⁺(1%) start to approach being the same under irradiation at 425 nm. That indicates that the dysprosium traps must be closer to the Eu²⁺ blue site since only when this site is excited the dysprosium traps show an effect on the QY.

The highest QY values of the same compound have been previously stablished as 55 %,³⁹ although higher values have been recently observed in the case of cage-like microspheres of SrAl2O4:Eu²⁺ up to 95% and 94% after excitation at 365 nm and 460 nm respectively,⁴⁰ and 28% and 71% in the case of SrAl2O4:Eu²⁺:Dy³⁺ powder samples after excitation at 375 nm and 445 nm respectively at very low excitation intensities.³⁷

In order to verify the reliability of the values obtained with the integrating sphere, the internal QY of the SrAl2O4: Eu²⁺(1%) powder samples has been estimated through lifetimes measurements of the green site emission at 10 K and 300 K after irradiation at 370 nm. From reference 9 lifetimes of 1190 ns and 408 ns were obtained for the mentioned samples at 3 K and 300 K, respectively. Hence, the QY of the sample has been estimated as the ratio between both values as shown in equation 3.

Φint =_k ^kr

r+ k_nr = ^1/τ_1/τⁿ= _τ^τ

n = ^{τ ( 300 K)}_{τ ( 3 K)} (3)

, where τn is the natural radiative lifetime at low temperature in the absence of thermal quenching and τrepresents the total radiative and non-radiative lifetimes taking place due to thermal activation at high temperatures.

A value of 34% has been obtained for SrAl2O4:Eu²⁺(1%), in a very good agreement with the value obtained by the integrating sphere (35%). Consequently, the wavelength dependence study carried out through this method represents a reliable and fast technique to evaluate the differences between all the samples under study here.

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Figure. 3. Quantum yield measurements performed in the SrAl2O4:Eu²⁺(1%):Dy³⁺ crystal, and SrAl2O4:Eu²⁺(1%) and SrAl2O4:Eu²⁺(1%):Dy³⁺ powder samples at different wavelengths.

CONCLUSIONS

In Figure 4, a summary of the optical properties of the SrAl2O4:Eu²⁺:Dy³⁺ single crystal and a recapitulation of the information obtained in this study are shown. A very thin and high-quality single crystal of SrAl2O4: Eu²⁺(1%): Dy³⁺ has been optically characterised. The absorption and excitation spectra show the broad bands associated with the electric dipole allowed 4f–5d transitions. The emission spectrum at room temperature shows the characteristic broad band centred at 520 nm. At 6 K the emission of the blue site appears centred at 455 nm. The latter band is shifted 10 nm upwards respect to the blue site emission in dysprosium free powder samples. Besides, photoinduced detrapping of the deeper traps by irradiation in the near infrared region has been proven.

On the other hand, the absorption spectrum of the crystal and the estimation of the real Eu²⁺

concentration within the crystal through the intensity ratio of blue and green emission at 10 K allowed to obtain reliable values of the extinction coefficient for any SrAl2O4: Eu²⁺: Dy³⁺ sample. The value of the extinction coefficient has been estimated as 1258 M^-1 cm^-1 and 1137 M^-1 cm^-1 at 254 nm and 330 nm, respectively.

In addition, the thermoluminescence curves of the SrAl2O4: Eu²⁺(1%): Dy³⁺ crystal reveal the presence of very deep traps, which is crucial to get a longer phosphorescence due to the higher thermal energy needed to release them.

Furthermore, the dependence of the quantum yield values of the SrAl2O4: Eu²⁺(1%): Dy³⁺ crystal with the excitation wavelength has been followed. An increase of the QY values has been obtained when increasing the irradiation wavelength since the radiative recombination is favoured when only the green europium site is excited. More importantly, when the dependence is compared with the one of a dysprosium free sample, the QY values differ more at lower irradiation wavelengths when the Eu blue site is excited indicating that the dysprosium traps are situated closer to the blue sites.

These results shed light upon the role of co-dopants in the long- phosphorescence mechanism. In this regard, the analysis of SrAl2O4: Eu²⁺: Dy³⁺ crystals that in addition contained further dopants such as barium or boron^16,41,42 is needed to achieve this goal.

Moreover, our estimation of the extinction coefficient serves as an orientation to calculate the real concentration of europium in any kind of SrAl2O4 samples, which is crucial to rationalise the experimentally observed phosphorescence.

55 50 45 40 35

30 25 20

QY /%

420 410 400 390 380 370 360

_exc /nm SrAl₂O₄:Eu²⁺(1%):Dy³⁺ crystal SrAl₂O₄:Eu²⁺(1%) powder SrAl₂O₄:Eu²⁺(1%):Dy³⁺ powder

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Figure 4. Summary of optical properties of the SrAl2O4:Eu²⁺:Dy³⁺ single crystal and information obtained.

ASSOCIATED CONTENT

Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

Quantum yield measurements of SrAl2O4:Eu²⁺:Dy³⁺ single crystal and powder samples, Luminescence spectra of SrAl2O4:Eu²⁺:Dy³⁺ single crystal and powder samples at different excitations, UV Absorption spectroscopy measurements, Thermoluminescence measurements, Quantum yield measurements of SrAl2O4:Eu²⁺:Dy³⁺ powder samples

AUTHOR INFORMATION Corresponding author.

*Mariateresa.delgadoperez@unige.ch +41 22 37 96106

Author Contributions

All authors have given approval to the final version of the manuscript.

AKNOWLEDGEMENT

We are grateful to Dr. Enrico Giannini for the help during the synthesis of the crystal and to the Swiss National Science Foundation (Project number: 200021-169033), as well the KTI (Project number 25902.1 PFNM-NM) for the financial support.

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