Dynamics of Charges in Super-Long Blacklight Emitting CaB2O4

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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Dynamics of Charges in Super-Long Blacklight Emitting CaBO:Ce Persistent Phosphor 2

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Suchinder K. K Sharma, Marco Bettinelli, Irene Carrasco, Jan Beyer, Richard Gloaguen, and Johannes Heitmann J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 23 May 2019 Downloaded from http://pubs.acs.org on May 23, 2019

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The Journal of Physical Chemistry

Dynamics of Charges in Super-Long Blacklight Emitting CaB2O4:Ce3+ Persistent Phosphor

†

Suchinder K. Sharma,∗,‡,¶ Marco Bettinelli,§ Irene Carrasco,k Jan Beyer,‡ Richard Gloaguen,¶ and Johannes Heitmann‡ ‡Institute of Applied Physics, TU Bergakademie Freiberg, Leipziger Str. 23, 09599 Freiberg, Germany ¶Helmholtz–Zentrum Dresden–Rossendorf, Helmholtz Institute Freiberg for Resource Technology, Chemnitzer Str. 40, 09599 Freiberg, Germany §Luminescent Materials Laboratory, Department of Biotechnology, University of Verona, and INSTM, UdR Verona, Strada Le Grazie 15, 371 34 Verona, Italy kAdvanced Technology Institute, Department of Electrical and Electronic Engineering, University of Surrey, Guildford GU2 7XH, United Kingdom E-mail: [email protected] Phone: +49 (0)37 3139 2212

Abstract The optical and persistent luminescence properties of CaB2 O4 :Ce3+ phosphor are presented. The optical emission for excitation between 250–340 nm wavelength region, is dominated by two bands at 365 and 460 nm. Lifetime measurements suggested that the 365 nm emission band is due to interconfigurational Ce3+ 5d → 4f transitions. Upon excitation with 254 nm UV lamp, a super long persistent luminescence †

A footnote for the title

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was observed for at least 15 h, with an excellent reproducibility, and an emission in the UVA1 region (340–400 nm, blacklight), perfectly suitable for phototherapy application. Initial-rise method was applied on the thermoluminescence glow curves to determine the trap distribution and trap depth. The results suggest that one distinct trap , with activation energy ∼ 0.52 eV, was solely responsible for the persistent luminescence in the CaB2 O4 :Ce3+ phosphor. The other traps had a quasi-continuous distribution, with activation energies between 0.56–1.15 eV. Proposed persistent luminescence, and the thermoluminescence mechanism are elucidated using experimental parameters obtained from the optical and thermoluminescence results, and the theoretically calculated electronic structure of the Ce3+ ion in CaB2 O4 . The lowest of Ce3+ 5d1 level was found to be ∼ 0.97 eV below the conduction band, and the persistent luminescence/thermoluminescence emission was dominated by the radiative transitions between Ce3+ energy levels, 5d → 2 F5/2,7/2 .

1

Introduction

The use of ultraviolet emitting sources for therapy (UV–phototherapy) has been of interest, in the treatment of various dermatological conditions, including cutaneous T-cell lymphoma (CTCL). The phototherapy using UV–light can be divided into four modalities depending upon the light range, and its applications: broadband UVB emission (290–320 nm) for vitiligo and psoriasis treatment; narrowband UVB emission (311–313 nm), and Psoralen (drug) + UVA (PUVA) emission between 320–400 nm for therapeutic effects in psoriasis; and long-wave UVA1 emission (340–400 nm) for treatment of atopic dermatitis via introduction of apoptosis into skin infiltrating helper T–cells. 1 UVB exposure is considered to be the most efficient modality, as it prohibits the rapid accumulation of keratinocytes in psoriasis, and prevents the proliferation of skin infiltrating T-cells in mycosis fungicides (MF). However, UVB (both broadband and narrow-band) is highly erythemogenic which causes DNA damage, immune suppression, and skin cancer. 2 PUVA is capable of clearing moderately 2


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advanced skin lesions of MF. The mechanism of this beneficial effect is unknown, but probably involves covalent photobinding of methoxsalen molecules to pyrimidine bases in DNA at the cellular level, and impaired T-cell function or survival at the tissue level. 3 Nevertheless, non-melanoma and melanoma skin cancer have been reported among psoriasis patients upon long term use of PUVA therapy. 4–7 The last modality, use of fourth modality–UVA1 for atopic dermatitis, has revealed some interesting results, since it has been observed that the T-helper apoptosis which leads to the removal of T-cells from eczematous skin lesions, happens successfully in UVA1 treated patients. 8 UVA1 induces two apoptotic pathways, and offers greater advantage over PUVA and UVB modalities, which provide a single apoptotic pathway. 9 Hence, there is a need to develop efficient UVA1 emitters that can be used for phototherapy, as well as to determine whether this therapy is able to sustain long–term remission. Since the first report on an application of near infra-red (NIR) emitting persistent luminescent (PersL) phosphors, the interest in the development of new PersL materials has grown rapidly, particularly for real–time optical imaging. 10–12 PersL is an optical phenomenon in which the material continues to emit radiation for a significant time after the initial excitation has been seized, with a possibility of tuning emission wavelength between UV and the NIR region, depending upon the host-dopant combinations. 13–16 For hosts containing Ce3+ as a dopant, the PersL can be tuned in the 385-625 nm range as a result of splitting and lowering of the Ce3+ 5d1 band due to interaction with the host crystal field. 17 To exhibit an efficient PersL, the mobility of the electron or its de-localization in the conduction band, is an important condition. Ce3+ –doped alkaline earth aluminates MAl2 O4 (M = Ca, Sr, Ba) have been amongst the most studied Ce3+ -doped materials, with PersL emission for over 3 h. 18 More recently, UV–emitting PersL materials have been explored for their potential multifunctional applications, and several strategies have been presented to develop new emitters. 19–21 However, in these reports, the PersL emission is generally composed of a combination of ultraviolet (UV) and visible (VIS) bands. The contribution in the VIS is

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expected to overlap significantly with an autofluorescence of the living tissues, which limits their real–time use, as they require further optimization to separate two emissions: probe emission vs autofluorescence, see supplementary Figure S1. Hence, new materials with a distinct UV PersL emission, emission < 400 nm, are desired for practical applications. Calcium metaborate (CaB2 O4 ) is an interesting host due to its low hygroscopic nature, and an excellent chemical stability for radiation sensitive applications, such as thermal neutrons detection. 22–24 The overall luminescence properties in the host are expected to be determined by the presence/absence of defects after the introduction of dopant ions into the lattice. To this end, the present work is aimed to study the luminescence behavior of polycrystalline 0.1% Ce3+ –doped CaB2 O4 . By combining techniques like thermoluminescence (TL), photoluminescence (PL), and theoretically calculated vacuum referred binding energy (VRBE) diagrams, we investigate the charge storage and release, as well as the PersL /TL mechanisms in CaB2 O4 :Ce3+ , to understand the reasons for a super long PersL emission within 340–400 nm range (UVA1 emission), that lasts for at least 15 h. We evaluate, and compare our results especially in regard to the emission spectra and the emission duration, with the existing PersL phosphors.

2 2.1

Experimental Sample preparation

0.1% Ce3+ -doped CaB2 O4 was synthesized by solid state synthesis route.

Appropriate

amounts of CaCO3 (Sigma–Aldrich, 99% purity), H3 BO3 (Aldrich, 99.97% purity), and Ce(NO3 )3 · 6H2 O (Aldrich, 99.99% purity) were used as starting materials. Stoichiometric amounts of carbonate (0.5000 g), and nitrate (0.0022 g) were thoroughly mixed and ground with a 3% – excess in weight of boric acid (0.6369 g). The mixture was pelleted under 10 Bar load, and fired at 500 ◦C (heating ramp of 4 ◦C/min) for two hours, and then quenched at room temperature. Afterwards, the materials were ground, pelleted again under 10 Bar load, 4


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treated at 850 ◦C (heating ramp of 4.5 ◦C/min) for five hours, and subsequently quenched to room temperature. Attempts of doping the host with a higher concentration of the dopant (1%), gave rise to impure phases in the X-ray diffraction pattern.

2.2

Characterization

The phase quality was determined using powder X-ray diffraction (pXRD), measured using a Thermo ARL X’TRA powder diffractometer, in the Bragg-Brentano geometry, with a Cutarget and a Peltier Si (Li) cooled solid-state detector. The diffraction pattern was obtained with a scan speed of 0.03◦ /s in the 5-90◦ 2θ range. The results were analyzed by the Rietveld profile refinement method using FullProf Suite program (Version Dec–2017). The PL emission spectra were measured by excitation at an interval of 5 nm each, between 250– 330 nm, using a laser driven light source Model EQ-99X LDLSTM with a Hyperchromator type SG–600–500 from Mountain Photonics GmbH. The signal was detected using a liquidnitrogen-cooled CCD camera (Princeton Instruments SPEC-10: 100 BR eXcelon) coupled to an Acton SP2560i monochromator. The spectra were corrected for spectral response of the excitation source (normalization to the excitation power), and the detection system. The PersL measurements were performed at room temperature using Risoe TL/OSL Reader DA–20 having 9635Q–type PMT detector, by gluing 2 mg of powder sample on a stainless steel disc using silicone oil. The excitation was performed using 4 W Philipps tube purchased from Opto Mechanical Devices, Germany (power density = 0.196 mWcm−2 at a distance of 15 cm). The TL measurements were performed on the same setup by heating the sample at a linear heating rate of 2 K/s. The PersL/TL emission was recorded on an iCCD camera PI–MAX4–RB, using a LightField software from Princeton Instruments.

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Figure 1: (a) The experimental (obs), and calculated (cal) pXRD patterns for the 0.1% Ce3+ – doped CaB2 O4 phosphor, and (b) orthorhombic-type crystal structure with P nca(#60) space group. Ca represents calcium, B–boron and O–oxygen in the figure. Two joined BO3 groups are presented in a separate rectangle to indicate the shared oxygen atoms in (b).

3 3.1

Results and Discussion Rietveld refinement and crystal structure

The results for the Rietveld refinement of pXRD pattern for 0.1% Ce3+ –doped CaB2 O4 sample, are shown in Figure 1(a). The experimental XRD pattern, and the calculated peak positions are similar, suggesting a single phase of polycrystalline powder. The data could be fitted with χ2 = 2.21. The calculated cell parameters are: a = 6.2185 ˚ A, b = 11.5927 ˚ A, 3 c = 4.2775 ˚ A; α = β = γ = 90◦ , and the calculated volume, V = 308.3682 ˚ A , with an axial

ratios of 0.536 : 1 : 0.369. The cell parameters are similar to the earlier reported values (∆a,

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∆b, ∆c, ∆V < 1%), 25 suggesting high crystallinity of the sample. The unit cell of CaB2 O4 has an orthorhombic-type structure with P nca space group (#60) (Z = 4), Figure 1(b). Each calcium (Ca) atom is surrounded by eight oxygen atoms in the first coordinating sphere, with an average distance of 2.52(16) ˚ A. The boron (B) atom is surrounded by three oxygen (O) atoms forming almost an equilateral triangle, with an average distance of 1.36(2) ˚ A. In each BO3 triangle, two oxygen atoms are shared with other BO3 group, cf. Figure 1(b,rectangle). The BO3 groups are linked together to form endless strings parallel to the crystallographic c − axis. In the host lattice, up on doping, Ce3+ is expected to replace Ca2+ due to their matching ionic radii (Ca2+ = 1.120 ˚ A and Ce3+ = 1.143 ˚ A, in 8-fold coordination). 26

3.2

PL excitation and emission

Figure 2 shows the contour plot for the PL emission spectra of CaB2 O4 :Ce3+ . The sliced spectra for excitation, and emission bands, is also presented. The excitation spectra has two bands with maxima at 325 and 255 nm, cf. Figure 2(top). The full–width at half maximum (FWHM) for the two maxima is quite broad in comparison to the values reported earlier. 22,23 On comparison of the excitation spectra of CaB2 O4 :Ce3+ with a similar host SrB2 O4 :Ce3+ , the later possess excitation bands at 206, 252, 274, 306, 322 nm, due to interconfigurational Ce3+ 4f → 5d5−1 transitions, respectively. 27 As both, CaB2 O4 and SrB2 O4 , are characterized by similar crystal structure and bandgap, the crystal-field effects on the Ce3+ ion upon substitution at the Ca2+ /Sr2+ sites, are expected to be similar. Fujimoto et.al. 23 has also reported similar values for excitation bands for CaB2 O4 :Ce3+ . We infer that the results in our PL excitation spectra could not be resolved due to experimental limitations, and we assume that the 325 and 255 nm bands are composed of two intrinsic bands each, which can be tentatively assigned to the interconfigurational Ce3+ 4f → 5d1,2 and 4f → 5d3,4 transitions, respectively, cf. Figure 2(top). The separation between the band maxima for 5d1 and 5d2 levels, which measures the strength of tetragonal crystal–field splitting in 8-fold 7


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Figure 2: PL Contour plot for excitation and emission spectra of the 0.1% Ce3+ –doped CaB2 O4 phosphor. Excitation spectra showing two prominent bands at 255 and 325 nm (top), and emission spectra recorded for excitation between 250-330 nm, showing two distinct bands (right). coordinated Ce3+ ions, is ∼ 1500 cm−1 . This value is much lower than the reported value for YAG:Ce3+ , ∼ 7600 cm−1 , 28 suggesting that the Ce3+ ions experience a considerably weaker crystal-field in the CaB2 O4 lattice. The PL emission spectra for CaB2 O4 :Ce3+ at variable excitation wavelengths, is shown in Figure 2(right). The spectra show two distinguishable emission bands with maxima at 365 nm (27400 cm−1 ), and 460 nm (21740 cm−1 ). Usually, the emission spectra in Ce3+ doped samples are expected to be a doublet due to the spin–orbit splitting of the 4f 1 ground state into 2 F5/2 and 2 F7/2 levels, with a separation of ∼ ±250 2000 cm−1 . 29 The PL emission at 365 and 460 nm for our sample, has a separation of ∼ 5660 cm−1 , which cannot be assigned to Ce3+ alone. To understand the origin of these emission, we measured the decay curves at peak maximum of two emission bands (365 and 460 nm), as shown in Figure 3. The average lifetime (τav. ) was calculated by fitting decay curves in Figure 3 using a single exponential equation. The evaluated τav. were 35±2, and 97±3 ns for emissions at 365 and

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Figure 3: Luminescence decay curves measured at emission (Em.) wavelengths, 365 and 460 nm, for an excitation with 337.1 nm emitting N2 laser. The fitting of decay curves with exponential function is also presented. 460 nm, respectively. From the τav. data, we could infer that the PL emission is the result of charge recombination at two different centers: a) Ce3+ – with much shorter decay time (in the order of ns, typical of Ce3+ 5d → 4f transitions), 30 with emission between 340–400 nm having narrower FWHM (∼ 38 nm), and b) at another recombination centre with much longer decay time, and emission between 400–590 nm with larger FWHM (∼ 85 nm). As discussed in Ref. 27, an emission in the 400-590 nm wavelength range is also observed for undoped sample, suggesting that the defect producing this emission is unaffected up on Ce3+ doping. The defects can be some kind of self-trapped, or trapped exciton emission, or defect emission, as discussed by Fujimoto. 22 Similar reports on the PL emission in Dy3+ doped BaB8 O13 host, have been attributed to the self–trapped emission (STE). 31

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Figure 4: (a) PersL decay curves recorded up on UV–excitation for exposure times, 100, 200 and 300 s; (b) Power density of PersL recorded at a distance of 5 cm from the sample surface upon UV excitation for 100 s;(c) TL glow curves recorded immediately after UV exposure, and after recording PersL for 15 h for various exposure times; and (d) PersL emission spectra at 300 K, and TL emission spectra at 412 and 487 K.

3.3

PersL decay, TL glow curves and PersL emission

The PersL decay curves were recorded for 900 min (15 h) at 300 K after an excitation with 254 nm UV–lamp, and for different exposure times (100, 200, 300 s), Figure 4(a). The PersL intensity increases with an increase in the exposure time (the background is shown to compare PersL decay signal/background intensities). The decay curves present a fast initial decrease of about two orders of magnitude up to 250 min, followed by a slow decrease of less than one order of magnitude, maintained until the end of the measurement time. The power density of PersL was also measured at a distance of 5 cm from the samples surface, Figure 4(b). The results suggest that the material has a super–long PersL for at least 15 h. The initial intensities of the present phosphor were compared with other well–known PersL 10


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materials, for example with Sr4 Al14 O25 :Eu2+ ,and ZnGa2 O4 :Mn2+ , supplementary Figure S2. The results suggest that the CaB2 O4 :Ce3+ possess better PersL intensity among compared materials. The defect distribution in the system, and the type of luminescence centre, are important parameters to evaluate the performance and application of a good PersL material. We determined these two parameters by measuring PersL/TL emission spectra, as discussed in the next section. The TL glow curves were measured between 275 and 625 K at a linear heating rate of 2 K/s, both immediately after stopping the excitation, and after recording PersL for 15 h. The glow curves are shown in Figure 4(c). For TL recorded immediately after UV exposure, glow curve is broad with two maxima at 360 and 487 K (please see supplementary Figure S3 with log y-scale for better visualization). For the glow curves measured after recording PersL for 15 h, the maxima were obtained at 412 and 487 K, suggesting that the broad nature of the TL glow curve could be a mixture of different glow peaks, and/or to a system possessing distribution of the traps, which act as charge reservoirs. These traps are explored and discussed in a later sections. The PersL emission spectrum at 300 K, and the TL emission spectra at 412 and 487 K, are presented in Figure 4(d). The emission spectra are broad with emission between 340-400 nm wavelength region, which corresponds to an emission due to Ce3+ interconfigurational 5d1 → 2 F5/2,7/2 transitions, cf. Figure 3. There was no PersL emission in the higher wavelength region, above 400 nm. The TL glow curves were also recorded with and without Hoya UG11 bandpass filter. The obtained glow curves (see supplementary Figure S4), shows < 1% change in the area under the TL glow curve, for a same UV exposure time (100 s). We infer that the final recombination center in PersL and TL is solely Ce3+ , contrary to what was observed in the PL emission spectra, where two emission bands were observed at 365 and 460 nm. The difference in PL and TL/PersL spectra also suggests that the defect excitation band is located much deeper than the lowest of the Ce3+ 5d1 level (> 0.97 eV, value from section 3.7), favoring high probability of the charge recombination at Ce3+ 4f

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band. Further, the 340-400 nm wavelength range is of great interest for UVA1 phototherapy, in which a minimum of 45–60 min of UV–exposure is desired for patients suffering from atopic dermatitis. For this reason, the present material can be potentially interesting for such kind of phototherapies, since it provides much longer PersL emission in the UVA1 region. However, the area of research for UVA1 therapy is still new, and need more detailed sets of experiments with PersL materials.

3.4

Reproducibility of PersL and TL

Figure 5: (a) Reproducibility of the PersL decay curves recorded after 50 s UV exposure; and (b) variation in intensity ratio (I/Io ) of the area under the curve for the PersL decay curve and the TL glow curves, respectively. The reproducibility of PersL was verified for 10 consecutive cycles by repeated measurements after UV exposure for 50 s, Figure 5(a). The exposure time of 50 s was chosen to 12


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avoid the PersL saturation possibility, if any. The residual TL glow curves after PersL measurements are shown in supplementary Figure S5. The variation in the intensity ratio (I/Io ) for PersL and TL (area under the curve) is shown in Figure 5(b). Repeated measurements follow a normal distribution, which is characterized by the standard deviation (σ) of the group of the results. From a statistical viewpoint, for all the repeated cycles the PersL and TL results fall within 2σ of the mean, suggesting an excellent reproducibility of the results regardless of the number of measurements. The results also suggest that there was no change in the PersL, and TL sensitivity of the sample after UV exposure + heating cycles, contrary to what has been observed for other TL phosphors. 32

3.5

TL Dose response

PersL and TL are complementary techniques to understand the evolution of traps, and their population. Their population strongly influence the PersL in CaB2 O4 . The contour plot for glow curves with an increasing UV exposure time (1-50 s), is shown in Figure 6(a). For all the exposure times, the glow curve possesses a maximum at 365 K. Earlier, we also observed a glow peak at 487 K for normalized glow curves, cf. Figure 4(c). However, as the intensity ratio for 487 and 360 K glow peaks is very low for an UV exposure time of 5 s, for example I487K /I360K ≈ 0.004, the high temperature TL glow peak can not be observed in the contour plot, please see supplementary Figure S3 for comparison. We also studied the dose response for 360 K TL glow peak by integrating the total area under the glow curve between 330 and 380 K, Figure 6(b). The dose response curve shows a linear increase with an increasing UV exposure time up to 50 s, which could be fitted with a two variable linear equation, with a coefficient of determination, R2 = 0.999. Further increase in the UV exposure time, above 50 s, could not be performed due to saturation of the PMT detector.

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Figure 6: (a) Variation of TL glow curve maximum with increasing UV exposure (1-50 s) along with color scale bar indicating emission intensities; and (b) dose response curve for the glow curve maximum at 365 K. The dose response curve is fitted with a linear equation.

3.6

Trap distribution and activation energy

The TL glow curves were recorded as a function of increasing heating rates between 0.5 K/s to 10.0 K/s, Figure 7(a). As the heating rate increases, the height of the glow peak at 365 K decreases, and temperature maximum (Tm ) shift towards the higher temperature. The shift in the Tm is ∼ 72 K for heating rates between 0.5–10.0 K/s, supplementary Figure S6. The reason for such a decrease in the TL intensity can be explained as follows: for a small heating rate more time is required to reach a desired temperature, such that the electron– hole pairs have sufficient time for recombination as they experience less competition from their counterparts, leading to an increased number of photon emission (higher TL intensity). As the heating rate increases, the escape probability of charges from the trap increases causing higher probability of recombination, and the charge carriers recombine at an elevated 14


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Figure 7: (a) Variation of the TL glow curves with an increasing heating rates; (b) partial cleaning method applied to determine the composition of the TL glow curves, and (c) the comparison of activation energies calculated using IR method and using the formula 25kTm , where k is Boltzmann constant and Tm represents the temperature maximum. temperature with a decrease in area under the TL glow curve. Such decrease in the TL glow curve area is attributed to the thermal quenching effects, whose efficiency increases with an increase in the temperature. Mathematically, the Tm in terms of activation energy (E), frequency factor (s) and heating rate (β), can be written using Equation 1. 33 βE −E = s · exp( ) 2 kTm kTm

(1)

where, k is Boltzmann constant = 8.617 X 10-5 eV/K, and Tm is in kelvin (K). Up on rewriting Equation 1,

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β=

skTm2 −E · exp( ) E kTm

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(2)

Hence, when the β is increased, the term on the right hand side also increases, implying that the Tm (temperature maximum of the glow curve) increases with β. In Figure 4(b), we observed that the TL glow curve consists of several overlapping peaks which makes it difficult to determine the activation energy for all the traps. To determine the composition of a complex TL glow curve and E of these traps, we applied a partial– cleaning method, as discussed by Furetta. 32 In this method, the remaining TL glow curves are recorded after increasing the preheat (PH) temperature in a step of 5 K, Figure 7 (b). The shift in the glow curve maximum after partial cleaning step, is marked with an arrow. To these glow curves, we applied two different methods to determine the apparent E values: a) first order of kinetics Equation, E = 25kTm , as discussed elsewhere; 32,34,35 and b) order of kinetics independent initial–rise (IR) method, as described by Garlick and Gibson. 36 In the very initial part of the glow peak, when the TL intensity has risen by no more than 15 % of the maximum intensity, the depletion in the population of the trapped carriers is negligible, and an Arrhenius plot between lnI versus 1/T , produces a straight line with slope -E/k. 33 Thus, we applied IR method in the initial rising part of the TL glow curve with ∼ 15% of the maximum TL intensity. The results of the two methods, which are expected to provide information for the distribution of traps, are compared in Figure 7(c). The E values based on an equation for first order of kinetics, shows a monotonous linear increase, with E values between 0.68 and 1.18 eV, in the temperatures region 300–550 K. From the IR method, the E–values are distributed between 0.52 and 1.15 eV, for the same temperature interval. More interestingly, results from IR method showed a plateau between 300 and 350 K, with E ∼ 0.52 eV. Up on comparing TL glow curves from Figure 4(b), and the activation energy values in Figure 7(c), we infer that the trap with E ∼ 0.52 eV is responsible for PersL in the present phosphor, i.e., the charges stored in this trap are emptied at room temperature, giving PersL. The continuous increase in E for temperatures above 350 K, 16


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suggests a quasi-continuous distribution of traps. Upon comparison of the E values from order of kinetics dependent and independent methods, the IR method appears to provide more realistic values. However, the possibility of slight underestimation of E in IR method cannot be neglected, as it ignores any non–radiative events and hence lead to an evaluation of apparent E values, rather than the real ones. 37

3.7

PersL and TL Mechanism

The vacuum referred binding energy (VRBE) level diagram for all the lanthanoids in CaB2 O4 host, is shown in supplementary Figure S7. The calculations are based on the chemical shift model as described by Dorenbos. 38 It provides an electronic structure of lanthanoids with absolute binding energies relatives to the energies of the electrons at rest. The most important parameters in its construction are the Coulomb repulsion energy (U) for Eu, defined as the energy difference between the ground state of Eu2+ and Eu3+ , U(6,CaB2 O4 ) = 6.76. The other values are the exciton creation energy (E ex ) = 7.2 eV; 39 O2− → Eu3+ charge transfer energy (ECT ) = 4.51 eV; 40 exchange energy (E exch ) = 0.503; 41 the redshift defined as the lowering of the first 4f-5d excitation band compared to free energy of 6.12 eV for Ce3+ , D(1,3+,CaB2 O4 ) = 2.305 eV; and the centroid shift (c ) = 1.68. Based on these input values, the theoretically calculated other values are tabulated in supplementary Table S1. By combining the PL, TL and VRBE results, we now propose the following mechanism for PersL and TL in CaB2 O4 :Ce3+ , Figure 8. Under 254 nm excitation (step 1), the electrons from the ground state of Ce3+ are promoted first to the conduction band where they are delocalized, i.e., free to move. The PersL trap with a trap depth of ∼ 0.52 eV (shown in blue) is filled by the electrons as a first preference (step 2), because the TL intensity of the 365 K trap increases more rapidly with an increasing UV exposure time, as compared to the 487 K trap. However, simultaneous filling of all the trap types cannot be neglected completely. The room temperature provide sufficient energy for electrons in PersL trap to 17



get detrapped back to the conduction band (step 3). These detrapped electrons move back to the lowest Ce3+ 5d1 level by means of a non–radiative relaxation. The Ce3+ 5d1 level is approximately 0.97 eV below the conduction band. From the 5d1 level, a radiative decay to the lowest 4f state leads to a UVA1–PersL emission (step 5).

Figure 8: Proposed PersL and TL mechanism in 0.1% Ce3+ -doped CaB2 O4 phosphor.Exc. represents excitation, and Em. represents emission in the figure. The charges trapped with E in the range 0.56–1.15 eV, cannot be released at room temperature, and require an external stimulation energy. This external energy can be supplied either, by increasing the temperature (for TL), or through resonant optical excitation (optically stimulated luminescence, OSL). We observed another TL glow peak maximum at 487 K, suggesting that at this temperature, the electrons trapped with E = 0.56–1.15 eV, get sufficient energy for detrapping to the conduction band (step 4). From the conduction band, their probability of radiative recombination at Ce3+ 2 F5/2,7/2 , is similar to step 5. The probability of the electrons moving back to the traps after step 2 or 4, can be neglected completely, as the TL glow curve does not shift significantly, for an increasing UV exposure time.

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4

Conclusions

To summarize, the CaB2 O4 :Ce3+ phosphor was prepared by the solid–state synthesis method. It shows broad PL emission bands at 365, and 460 nm. The 365 nm band had decay time of 35 ns, a typical value for interconfigurational Ce3+ 5d → 4f transitions. The determination of the origin of the 460 nm band lies beyond the scope of present article. Upon excitation with a 254 nm UV source, the sample showed a PersL emission in the UVA1 region (340–400 nm), lasting for at least 15 h, which is perfectly suitable for an application in phototherapy. The PersL showed an excellent reproducibility under fixed UV exposure time. The order of kinetics dependent and independent methods were applied in the TL experiments, to determine the apparent E, and trap distribution in the host lattice. One distinct trap is observed at E ∼ 0.52 eV, which is found to be responsible for PersL. Other traps showed a quasicontinuous distribution, with E ∼ 0.56-1.15 eV. Theoretical calculation of the electronic structure of the lanthanoids ions in CaB2 O4 , allows us to infer that the 5d1−3 levels of Ce3+ are below the conduction band, with the lowest 5d1 level located at ∼ 0.97 eV below the conduction band, and the highest 5d4,5 levels are within the conduction band. The charge dynamics in the host lattice is dominated by the recombination process, when compared to the retrapping process.

Associated Content Supporting information The Supporting Information is available free of charge on the ACS Publications website at ..... Emission spectra of tissues in comparison to the PersL emission from different reported phosphors; Comparison of PersL intensities in the first 15 min of measurement time with

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other two well–known phosphors; Semi–log plot for TL glow curves recorded after stopping excitation; TL glow curves recorded with and without Hoya UG11 filter; Reproducibility of glow curves; shift in the TL glow curve with increasing heating rate; VRBE diagram of electronic structure of lanthanoids in divalent and trivalent state, Table for experimental input parameters and calculated energy values from chemical shift model.

Author information Corresponding Author *E.Mail: [email protected] (SKS)

ORCID Suchinder K. Sharma: 0000-0002-8351-4597 Marco Bettinelli: 0000-0002-1271-4241 Irene Carrasco: 0000-0002-7854-4214 Jan Beyer: 0000-0002-1403-395X Richard Gloaguen: 0000-0002-4383-473X

Notes The authors declare no conflict of interest.

Acknowledgement The author thanks EIT Raw Materials for financial support under project ’inSPECtor’ (Grant Number: 16304). The X-Ray diffraction experiments were carried out at the Centro Piattaforme Tecnologiche, Univ. Verona. MB thanks Univ. Verona for financial support 20


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through the FUR scheme. We also thank Erica Viviani, University of Verona, for expert technical assistance.

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TOC graphic P e r s L tim e ( m in )

1 0 0 0

P e rs L tra p

A fte r e x c ita tio n A fte r P e rs L P e rs L d e c a y

3 0 0

3 5 0

4 0 0 4 5 0 5 0 0 T e m p e ra tu re (K )

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5 5 0

B g d 6 0 0

P e r s L In te n s ity ( a r b .u n its )

0 .2 5

T L In te n s ity ( a r b . u n its )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Dynamics of Charges in Super-Long Blacklight Emitting CaB2O4

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