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Thermoluminescence and near-infrared persistent luminescence in LaAlO3:Mn4+,R (R= Na+, Ca2+, Sr2+, Ba2+) ceramics
Author’s Accepted Manuscript Thermoluminescence and near-infrared persistent luminescence in LaAlO3:Mn4+,R (R= Na+, Ca2+, Sr2+, Ba2+) ceramics Jiaren Du, Olivier Q. De Clercq, Dirk Poelman www.elsevier.com/locate/ceri
PII: DOI: Reference:
S0272-8842(18)32308-3 https://doi.org/10.1016/j.ceramint.2018.08.243 CERI19278
To appear in: Ceramics International Received date: 17 May 2018 Revised date: 13 August 2018 Accepted date: 21 August 2018 Cite this article as: Jiaren Du, Olivier Q. De Clercq and Dirk Poelman, Thermoluminescence and near-infrared persistent luminescence in LaAlO3:Mn4+,R (R= Na+, Ca2+, Sr2+, Ba2+) ceramics, Ceramics International, https://doi.org/10.1016/j.ceramint.2018.08.243 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Thermoluminescence and near-infrared persistent luminescence in LaAlO3:Mn4+,R (R= Na+, Ca2+, Sr2+, Ba2+) ceramics Jiaren Du, Olivier Q. De Clercq, Dirk Poelman* LumiLab, Department of Solid State Sciences, Ghent University, Krijgslaan 281-S1, Ghent, Belgium *Correspondence. [email protected]
Abstract: The near-infrared (NIR) persistent luminescence and thermoluminescence of Mn4+ activated LaAlO3 (LAO) and LAO:Mn4+,R (R= Na+, Ca2+, Sr2+, Ba2+) ceramics were investigated. After irradiation with 335 nm excitation light, deep red and NIR persistent luminescence was obtained in the wavelength region from 650 nm to 750 nm due to the Mn4+ dopants, well within the biological tissue transparency window for in vivo imaging. The afterglow can be attributed to the presence of Mn4+ induced traps in the LAO host. By carrying out a series of thermoluminescence measurements, the formation of a broad range of traps was identified, with two main peaks centered at 0 ℃ and 114 ℃. Na+, Ca2+, Sr2+ or Ba2+ ions were added as co-dopants to optimize the location and energy of the trap levels, improving the NIR persistent luminescence.
Keywords: Perovskites; thermoluminescence; properties.
luminescence; optical materials/
1. Introduction Persistent luminescent materials, also known as long persistent phosphors (LPPs), relate to a particular optical phenomenon through which the light emission can persist for several minutes or hours after the excitation has finished. The first reported LPP was the Bologna stone, which could emit light due to its natural impurities.[1] Since 1996, the most extensively studied materials have been rare earth-doped alkaline earth aluminates, for example, the strontium aluminate based material SrAl2O4:Eu,Dy.[2,3] Persistent luminescent materials have become a major academic research focus in the field of new materials and material science. Based on their inherent optical energy-storing property, LPPs have been applied to optical storage media, security encoding, emergency route signs and medical imaging.[4] There is a special interest and increasing need for deep red or near-infrared (NIR) emitting persistent phosphors, in the spectral region between 650 nm and 950 nm and between 1000 nm to 1350 nm. In these two wavelength ranges, biological tissues show limited absorption and scattering, thus allowing NIR-emitting probes to penetrate deeper than comparable fluorophores emitting visible light.[5-7]
Therefore, LPPs have many features of merit for in vitro
and in vivo imaging applications. One major drawback overcome by LPPs is the presence of autofluorescence emerging in luminescent probes such as up-converting lanthanide nanoparticles, heavy metal based semiconductor quantum dots or organic fluorophores, which all need a continuous, high-power excitation source to exhibit their NIR emitting properties.[4,8-10] With LPPs, a pre-excitation is performed outside the
body and no external irradiation is needed during in vivo imaging. Thus, LPPs with deep red or NIR emission are recognized as suitable materials for this application.[11,12] LPPs consist of an inorganic matrix as a host, doped with activators. The activators can be divalent or trivalent lanthanides, comprising atomic numbers 57 to 71 of the periodic table (such as Eu2+, Eu3+, Dy3+, Ce3+, Nd3+, Sm3+, etc., mainly the so-called Ln3+ or Ln2+ ions),[13] transition metals (Mn2+, Mn4+, Cr3+, Ti4+, etc.) or several main group elements (such as Bi3+). The activator can act as emitter or trap. The trap stores the excitation energy and subsequently releases it gradually. The amount of stored energy and the time scale of energy release thus determines the duration and intensity of the afterglow. Recently, LaAlO3 (LAO) was found to be a possible host for several activators, such as Eu2+/Eu3+.[14] Our group and others reported Mn4+ activated LAO as a NIR emitting phosphor.[15,16] Y. Katayama et al. first reported Cr3+ activated LAO deep red persistent ceramics as a promising material for in vivo imaging.[17] Other Cr3+/Ln (Ln= Sm3+, Ce3+, Eu3+, Tm3+) co-doped LAO ceramics were also investigated and further developed.[18-20] When it comes to NIR emitting luminescence in the LAO host, mainly Mn4+ and Cr3+ ions have been investigated. Both Mn4+ and Cr3+ ions are highly promising NIR emitters and ideal doping candidates in the LAO host and their 3d3 electron configuration allows narrow band emission coming from the spin-forbidden 2Eg 4
4+
→ A2g transition. The LAO:Mn
and LAO:Cr3+ ceramics show persistent luminescence
peaking at the wavelength regions from 650 nm to 750 nm and 720 nm to 780 nm,
respectively.[15,17]
The LAO crystal structure corresponds to the rhombohedral, nearly cubic perovskite structure, which involves a rotation of the AlO6 octahedra with respect to cubic perovskite.[21] Two types of units are found in the crystal structure: AlO6 octahedra and LaO12 polyhedra. The central Al3+ cation is located in AlO6 octahedral units with six-fold oxygen coordination. Both Mn4+ and Cr3+ ions are supposedly substituting for Al3+ in this architecture given the similar ionic radius (RCr3+ = 61.5 pm, RMn4+ = 53 pm, RAl3+ = 53.5 pm, while RLa3+ = 136 pm) and octahedral coordination configuration. To enhance the performance of the NIR persistent luminescence, Tanabe et al. investigated the effect of various lanthanide co-dopants on Cr3+ activated LAO phosphors and Sm3+ was found to be a good co-dopant.[19] For Mn4+ activated LAO ceramics, we recently reported a preliminary screening of the effect of various co-dopants on the persistent luminescence performance of LAO:Mn4+.[15] Co-dopants such as Na+, Ca2+, Sr2+ and Ba2+ ions were found to be beneficial for improving both the steady state luminescence and afterglow intensity. Encouraged by this observation, a detailed investigation of the effects of co-doping on afterglow performance and traps behavior has been carried out.
In this work, LAO:Mn4+ and LAO:Mn4+,R (R= Na+, Ca2+, Sr2+, Ba2+) ceramics were studied using temperature dependent optical charging, fading and afterglow decay experiments and thermoluminescence measurements. NIR persistent luminescence in
the wavelength region from 650 nm to 750 nm was obtained, which can be attributed to the Mn4+ dopants. This work focuses on Mn4+ doped LAO phosphors and their thermoluminescence behavior. The results are interpreted in terms of the nature of traps formed in the perovskite lattice. Investigating the formation of traps and their energy distribution are of fundamental importance, in order to understand the mechanism(s) behind the persistent luminescence.[22] The thermoluminescence study of LAO:Mn4+,R is also beneficial for the development of other deep red or NIR emitting persistent luminescence ceramics.
2. Materials and Methods LAO:Mn powder samples were prepared via high temperature solid state reaction. All the raw chemicals were analytical grade, used without further purification. The used precursors, La2O3 (Sigma Aldrich, Saint Louis, MO, USA, 99.99%) and Al2O3 (Fluka, Schwerte, Germany, 99.5%), were mixed and ground in an agate mortar. The dopant Mn was added as MnO2 (Alfa Aesar, Karlsruhe, Germany, 99.997%). Co-dopants were mixed as Na2CO3 (Alfa Aesar, 99.95%), CaCO3 (Alfa Aesar, 99.95%), SrCO3 (Alfa Aesar, 99.99%) and BaCO3 (Alfa Aesar, 99.95%), respectively. The concentrations of dopants and co-dopants were chosen to make the following chemical composition formula:
LaAlO3:xMn4+
(x=
0.1%,
0.2%,
0.5%,
1%,
2%
and
5%)
and
LaAlO3:0.5%Mn4+,1%R (R= Na+/Ca2+/Ba2+/Sr2+). The molar % concentration is defined with respect to one mole of the host phosphor chemical formula (with respect to the Al
content in LAO).
The appropriate stoichiometric amount of precursors was weighed, suspended in ethanol and put in a ZrO2 grinding jar. The starting materials were ground in a Retsch PM 100 Planetary ball mill for 6 h to reduce the particle size and improve the mixing homogeneity of the precursors. After evaporating the remaining ethanol, the mixture was transferred to open alumina crucibles and subsequently heated up to 1550 ℃ in ambient air for 6 h to form the final compound. The employed heating rate was 300 ℃/h, using a tube furnace (ETF30-50/18-S furnace, ENTECH, Ängelholm, Sweden). After synthesis, all samples were allowed to cool naturally inside the tube furnace and the prepared powder samples were ground again manually prior to further characterization.
The
crystal
structures
of
the
studied
LaAlO3:Mn4+
and
LaAlO3:Mn4+,R
(Na+/Ca2+/Ba2+/Sr2+) were verified by Powder X-ray diffraction (XRD) measurements on a Siemens D5000 diffractometer (40 kV, 40 mA, Bruker) using Cu Kα1 radiation (λ = 0.154 nm). The XRD data were collected in the range 2θ from 10◦ to 80◦ at room temperature with a step time of 1.2 second. The reference data for LaAlO3 (JCP2.2CA.No. 00-031-0022) was used to compare with the obtained XRD patterns. Scanning electron microscopy was performed in a Hitachi S-3400 N, equipped with a Thermo Scientific Noran System 7 for energy-dispersive X-ray (EDX) analysis. EDX measurements were performed at a pressure of 25 Pa in order to avoid sample charging,
with an accelerating voltage of 20 kV. Steady state photoluminescence excitation and emission spectra were recorded using an Edinburgh FS920 (Edinburgh Instruments Ltd., Livingston, UK) fluorescence spectrometer with a monochromated 450 W Xe-arc lamp as the excitation source. Afterglow decay profiles were conducted using the above-mentioned fluorescence spectrometer under excitation for 5 min and afterglow decay was monitored at different emission wavelengths. All spectra were automatically corrected for detector response. Afterglow decay curves at elevated temperature were measured with a photosensor amplifier (Hamamatsu C9329, Japan) and a Centronics OSD100-5T silicon photodiode.
TL experiments were performed inside a small home-built vacuum chamber with a well-characterized cooling and heating stage.[23] Thin pressed pellets of samples were in good thermal contact with the heat exchanger by using thermally conductive adhesive. Identical size thin pellet samples with the chemical composition formula LaAlO3:xMn4+ (x= 0.1%, 0.2%, 0.5%, 1%, 2% and 5%) and LaAlO3:0.5%Mn4+, 1%R (R= Na+/Ca2+ /Ba2+ /Sr2+) were used for the TL measurements. Prior to each TL experiment, a thermal cleaning of the traps was conducted by heating up to 220 ℃. The excitation source for the charging was a monochromated 300 W Xenon arc lamp (Oriel Instruments, Stratford, CT, U.S.A.) and a 335 nm wavelength was used for 10 min at 20 ℃. After the optical charging, the sample was kept in the dark for 1 min at 20 ℃ before recording the TL output. TL glow curves were collected using a constant heating rate β (β= 10 ℃/min,
20 ℃/min, 30 ℃/min, 40 ℃/min or 50 ℃/min). The light emitted from the sample during each step (charging, fading, and heating step) was collected and a full emission spectrum was recorded to investigate both spectral shapes and intensities of the emission during the entire measurement. The TL output was guided to a ProEM1600 EMCCD camera attached to an Acton SP2300 monochromator (Princeton Instruments) using an optical fiber. TL glow curve measurements at temperatures in the range from -60 ℃ to 220 ℃ could be performed in an automated way, as demonstrated in previous work.[23,24]
3. Results and Discussion 3.1 Crystal Structure To identify the crystal structure and verify the phase purity of the as-prepared samples, powder X-ray diffraction (XRD) was performed and compared to the standard XRD pattern of LAO (JCP2.2CA.No.00-031-0022). The representative XRD patterns of LAO:0.5%Mn4+ and LAO:0.5%Mn4+,1%R (R= Na+, Ca2+, Sr2+, Ba2+) are shown together with the reference pattern in Figure 1. The optimized concentrations of the dopant Mn and co-dopants R were selected as 0.5% and 1% based on previous work.[15] All the XRD peaks can be exactly indexed to the standard XRD pattern of LAO and the doping of Mn or other co-dopant R does not make any appreciable changes to the LAO host structure. To further confirm the doping of Na+, Ca2+, Ba2+ and Sr2+ ions into the lattice, EDX mapping in scanning electron microscope was performed as shown in
Figure S1-S4. EDX mapping indicates the homogeneous distribution of manganese and other co-dopants and confirms the feasibility of doping in each of the prepared samples.
Figure 1. XRD patterns of LAO:0.5%Mn4+ and LAO:0.5%Mn4+,1%R. Co-dopant R represents Na+, Ca2+, Sr2+ and Ba2+ ions. The standard XRD pattern of LAO is illustrated with red bars. All the XRD patterns are normalized to arbitrary units.
3.2 Photoluminescence and Afterglow Properties The steady-state PL emission spectrum and afterglow decay curves were measured to analyze
the
photoluminescence
and
persistent
luminescence
properties
of
LAO:0.5%Mn4+ and are depicted in Figure S5. The excitation wavelength was 335 nm and the emission peaks are located at 697.5 nm, 704.5 nm, 710.5 nm, 718 nm, 724.5 nm and 731 nm, which are shown as red arrows and are coming from the spin-forbidden 2
Eg→4A2g transition and the vibrational sidebands of the zero-phonon line (ZPL) with
phonon assistance.[15] After exposure to 335 nm excitation for 5 minutes, wavelength-dependent afterglow decay curves of LAO:0.5%Mn4+ were recorded as a function of time by monitoring different discrete emission wavelengths as shown in Figure S5. It indicates that the persistent luminescence spectrum of the LAO:Mn4+ phosphor has a similar intensity distribution to the steady-state emission spectrum. All the afterglow decay curves of the different emission wavelengths have a similar decay behavior as shown in Figure S6. This result indicates that all Mn-ions that contribute to the steady-state PL emission also equally contribute to the afterglow spectrum, and that the afterglow spectrum does not change as a function of time. The afterglow intensity is in the wavelength region from 650 nm to 750 nm with several pronounced peaks, which satisfactorily falls in the wavelength range of the first biological tissue transparency window.
3.3 Thermoluminescence behavior of LAO: Mn4+ as a function of Mn-concentration The nature of the traps plays a vital role in the afterglow properties of LPP compounds. In general, the storage of the irradiation energy by the traps is supposed to be responsible for persistence luminescence. However, the precise mechanisms governing the persistent property in LPPs are still under discussion, and there are different possible trapping sites in different host or activator cases. When Mn4+ ions are doped in the LAO host, appropriate traps are generated and afterglow emission can be observed. It is well known that the Mn4+ ion usually stabilizes in an octahedral site with 6-fold coordination
(in Al3+ site for LAO).[25,26] In the case of LAO:Mn phosphor system, it is believed that the afterglow originates from the trapping sites, induced by Mn on an Al site. The thermoluminescence (TL) behavior is largely dependent on experimental parameters, such as the employed heating rate or excitation dose, etc.[27] The position of a TL peak approximately reveals the depth of a certain trap level and a TL peak refers to a particular trapping center in LPP phosphors. A TL peak situated at a higher temperature is corresponding to a deeper trap level and appropriate trap depths in phosphors are essential for LPP materials. It is generally acknowledged from previous reports that the TL peaks corresponding to the most effective persistent luminescence at room temperature are located slightly above room temperature, in the range of 320 K to 400 K.[28] Shallow traps normally lead to a fast afterglow decay and a short afterglow since the trapped charge carriers (holes or electrons) have a very high escape rate at or even well below room temperature. On the contrary, deep traps have too large binding energy for trapped charge carriers to escape at room temperature, resulting in little or no persistent emission. Shallow or deep traps are comparatively defined depending on the purpose of the application in mind.
TL investigations on LPP materials can help to reveal certain trap distributions, trap depths or the density of traps at a specific level, e.g. via the use of different specialized analysis techniques.[29-31] To investigate the relationship between TL profiles and the concentration of the Mn4+ dopant, TL measurements with different concentrations of
Mn4+ (from 0.1% to 5%) were performed under the same experimental conditions. Excitation was performed at 335 nm excitation for 10 min at -60 ℃, and a waiting time of 1 min. was observed before increasing the temperature at a heating rate of 30 ℃/min. TL glow curves were recorded in the range from -60 ℃ to 220 ℃. The final TL glow curves were plotted versus temperature and obtained by integrating the TL emission spectra over the wavelength range from 650 nm to 750 nm).
Figure 2. (a) TL glow curves with different concentrations of dopants (Mn4+ concentrations of 0.1%, 0.2%, 0.5%, 1%, 2% and 5%). (b) TL emission spectra (low resolution) of LAO:0.5%Mn4+ at different temperatures during heating. TL experiments were measured under the same conditions (After 335 nm excitation
for 10 min at -60 ℃, waiting for 1 min before increasing the temperature at a heating rate of 30 ℃/min.).
As illustrated in Figure 2(a), the positions of the peaks are identical in these TL curves, demonstrating that the trap depths are basically the same in these LAO:Mn4+ samples with different concentrations of dopants. On the other hand, the relative intensities of the TL peaks have a general trend that the intensities of the two main peaks keep rising as the Mn4+ concentrations increase from 0.1% to 0.5%. However, with a further increase of Mn4+ concentration from 0.5% to 5%, the intensities of the TL peaks of LAO:Mn4+ phosphors decrease gradually. For the LAO:5%Mn4+ phosphor, the main peaks in the TL profile nearly disappear most probably due to a severe concentration quenching of the emission. It was found that 0.5% Mn4+ is the optimal doping concentration for TL glow intensity and this concentration also provides the longest afterglow duration and highest steady state PL intensity.[15] The relative intensities of the two TL peaks remain the same as the Mn4+ concentrations increase in the range from 0.1% to 0.5%. However, with a further increase of Mn4+ concentration from 0.5% to 5%, the relative intensities of the two TL peaks of LAO:Mn4+ phosphors change. On the other hand, the two defect centers change differently with the concentration of Mn4+. It also indicates the concentration quenching affects the shallow traps and the deep traps in LAO phosphor in a complex way. The occurrence of two broad glow peaks, centered at 0 ℃ and 114 ℃, implies the presence of two dominant trapping centers in LAO:Mn4+
phosphors. Emission spectra at different temperatures during the TL heating procedure are illustrated in Figure 2(b). It can be seen that the overall shape of the spectra is the same as observed in PL (inset of Figure S5); the fine details of the spectra are missing due to the large spectrometer slit settings, needed to measure the low intensity signals. The TL spectra can be attributed to the spin-forbidden 2Eg→4A2g transition of Mn4+, confirming that Mn4+ acts as the recombination center for the released charges at all temperatures.
3.4 TL procedures for LAO:0.5%Mn4+ In order to study the trap behavior of the phosphors, different TL procedures were carried out for LAO:0.5%Mn4+. Under excitation with a certain wavelength for a period of time is called charging process, which is intended to fill the traps in the phosphor. TL glow curves of LAO:0.5Mn4+ with different excitation times are illustrated in Figure S7. When increasing the excitation time, the intensities of the two peaks gradually increase but the center positions of the two peaks remain the same. The experiments indicate that all trap levels can be filled gradually with longer time. Conversely, if the traps inside the phosphors are completely filled, a longer irradiation time can hardly improve the afterglow performance. In order to fill most of the traps, at least a 10 min-irradiation time is needed for the LAO:Mn4+ phosphor.
The heating rate β (℃/min), an important parameter adopted in TL measurements, requires careful selection. The experimental schematic is shown in Figure 3(a). When gradually increasing the heating rate from 10 ℃/min up to 50 ℃/min, the signal-to-noise ratio (SNR) is improved, the intensity of the peaks increases and the TL peak position shifts slightly to higher temperatures as displayed in Figure 3(b), which is a normal behavior since the traps are emptied in a shorter time when the heating is faster. On the other hand, a higher heating rate may induce the risk of temperature gradients over the samples, leading to an unreliable result.[32] To balance among SNR, the intensity of TL glow peaks and the TL reliability, a moderate heating rate 30 ℃/min was chosen for further experiments.
Figure 3. (a) An illustration of the TL procedure at different heating rates. (b) TL glow curves of LAO:0.5Mn4+ at different heating rates (10 ℃/min, 20 ℃/min, 30 ℃/min, 40 ℃/min or 50 ℃/min).
The effect of different charging temperatures on the TL glow peaks is shown in Figure 4. The charging temperature was set at -60 ℃, -40 ℃, -20 ℃, 0 ℃, 20 ℃, 40 ℃ and 60 ℃ respectively in each TL measurement as seen in Figure 4(a). The heating rate remained constant at 30 ℃/min as discussed above. For different charging temperatures, the same peak centered around 114 ℃ is observed and the broad TL glow curve from -20 ℃ to 60 ℃ already strongly indicates the presence of a range of multiple traps with a continuous distribution of trap levels. Figure 4(b) shows clearly the two main peaks centered at 0 ℃ and 114 ℃ under excitation at -60 ℃ and the two dominant trap levels in LAO:Mn4+ phosphors.
Figure 4. (a) Illustration of the TL procedure at different charging temperatures. (b) TL glow curves of LAO:0.5Mn4+ at different excitation temperatures (-60 ℃, -40 ℃, -20 ℃, 0 ℃, 20 ℃, 40 ℃ or 60 ℃).
In order to find out the contribution of the different traps to the afterglow, the TL curves of LAO:0.5%Mn4+ were recorded with different fading times between charging and TL. Fading experiment (Wait for a period of time in the darkness without changing temperature) is often used to investigate the trap structures. The sample was irradiated at 335 nm for 10 min and placed in a dark chamber at room temperature for different fading times. The TL fading experimental procedure is displayed in Figure 5(a). The fading time was chosen as 10s, 30s, 1 min, 10 min or 100 min. As can be seen from Figure 5(b), the intensity of the TL glow peaks from 20 ℃ to 90 ℃ decreases gradually
with longer fading time and eventually disappears after the removal of the excitation light for 100 min. Usually, the shallow trap releases its charge carriers at a faster ratio so that the energy stored in shallow traps will be emptied at the first stage and then the captured charge carriers in the deeper traps are released at a lower rate. It implies that TL glow peaks from 20 ℃ to 90 ℃ are responsible for the afterglow occurrence at room temperature. However, both the peak position and intensity from the higher temperature side remains intact, centered at 114 ℃, with only a minor reduction. While TL glow peaks from the higher temperature side do apparently not contribute significantly to the afterglow at room temperature, they can make a significant contribution to the release of the stored energy and the afterglow at elevated temperatures.
Figure 5. (a) Illustration of the TL procedure for different fading times. (b) TL glow curves of LAO:0.5Mn4+ for different fading times (10s, 30s, 1 min, 10 min or
100 min).
3.5 Effects of various co-dopants Co-dopants such as Na+, Ca2+, Sr2+ and Ba2+ ions were reported to be beneficial for improving the LAO:Mn4+ luminescence and afterglow intensity.[15] A more in-depth investigation of the effects of co-doping was carried out using a series of the above-mentioned TL measurements. Considering the similar ionic radius and octahedral coordination configuration, the Mn4+ ion is expected to substitute Al3+ in the AlO6 octahedral unit and the co-dopant ion (Na+, Ca2+, Sr2+ or Ba2+) is supposed to replace La3+ in the LaO12 polyhedral unit, as discussed in detail elsewhere.[15,33] Schematics for the doping position with different co-dopants are illustrated in Figure 6(a). Different charging temperatures and fading times were examined for each co-dopant via TL measurements. A series of detailed TL glow curves with different co-dopants are illustrated in Figure S8 and S9.
As shown in Figure S8 and Figure S9, co-dopants Ba2+/Sr2+/Ca2+/Na+ lead to the same type of traps as in LAO:Mn4+ phosphors. Often, new trapping centers are created using divalent or trivalent lanthanide co-doping, for example in YPO4:Ce3+,Ln (Ln3+/Ln2+= Nd, Sm, Dy, Ho, Er, Tm) [34], YPO4:Tb3+,Ln3+ (Ln= Nd, Ho, Dy, Sm, Tm) [35], Ca6BaP4O17:Eu2+,Ln3+ (Ln= Dy, Tb, Ce, Gd, Nd) [36] or Mg2GeO4:Mn4+,Ln3+ (Ln= Pr, Er, Nd, Yb) [37]. No obvious additional trapping centers are found in LAO:Mn4+,R (R=
Na+, Ca2+, Sr2+, Ba2+) upon co-doping. Co-doping with Na+/Ca2+ ions has the largest effect, with TL glow peaks becoming much more intense, as seen in Figure S8 and S9.
Figure 6. (a) Schematic for the doping positions in the LAO:Mn4+,R phosphor. Co-dopant ion is Na+/Ca2+/Sr2+/Ba2+. (b) Comparison of TL glow curves from different doping ions with the same TL parameters. (c) Illustration of the temperature dependent excitation measurement using 335 nm irradiation for 30 s at a certain temperature from -60 ℃ to 225 ℃ with a 5 ℃ interval and the steady-state emission spectrum, collected at each temperature platform. (d) Integrated steady-state emission intensities from 650 nm to 750 nm under temperature dependent excitations.
TL glow curves from different doping ions are illustrated and compared in Figure 6(b) and the identical parameters (335 nm excitation for 10 min at -60 ℃, waiting 1 min before TL at a heating rate of 30 ℃/min) were chosen for all the samples. For the afterglow generation at room temperature, too shallow traps (below 20 ℃) or too deep traps (above 140 ℃) contribute little to the release of stored energy at room temperature and are thus not interesting from the point of view of room temperature persistent luminescence. It is expected that the trap range from 40 ℃ to 90 ℃ is responsible for the room temperature afterglow performance in LAO:Mn4+ phosphors. In other words, the traps in the range 40 ℃ - 90 ℃ instead of the two main TL glow curves peaking at 0 ℃ and 114 ℃ may play the leading role in the afterglow occurrence at room temperature. Co-doping with Ba2+/Sr2+ ions increases the intensity of the TL signals but does not create or eliminate any trap levels. On the contrary, co-doping with Na+/Ca2+ ions helps to bring down the low temperature center at 0 ℃, strongly improves the TL output in the effective range 40 ℃-90 ℃ and greatly enhances the intensity of the 114 ℃ peak four-fold. Each co-dopant can be beneficial for increasing the persistent luminescence of LAO:Mn4+ due to the enhancement of TL output in the effective range, which is in agreement with the afterglow results of these co-dopants.[15] In addition, the position of the TL peaking center with each co-dopant at 114 ℃, as displayed in Figure 6(b), shows only little shift. It indicates that no extra defects are formed and only Mn4+ activated traps play an important role in the afterglow behavior of LAO:Mn4+,R phosphors. Apparently, co-doping with Na+/Ca2+/Ba2+/Sr2+ as charge-compensating ions only helps
to increase the number of the effective Mn4+, which has a beneficial effect on improving luminescence output leading to a higher TL signal.
In addition, a temperature dependent excitation measurement was performed for each co-doping sample. The sample was excited with 335 nm for 30 s at a certain temperature from -60 ℃ to 225 ℃ with a 5 ℃ interval and the emission spectrum at each temperature was collected (as shown in Figure 6(c), the heating rate was 10 ℃/min). Figure 6(d) displays the integrated temperature dependent steady state PL intensity (integrated over a steady state PL time scale in the wavelength range from 650 nm to 750 nm). It shows that a decrease of emission intensity gradually happens when increasing temperature, then a notable drop occurs with higher temperature, which is similar to the thermal quenching behavior of the most phosphors for LED application. [38] It is worth mentioning that co-doping with Na+/Ca2+/Ba2+/Sr2+ ions also enhances the steady-state photoluminescence to varying degrees, compared with LAO:Mn4+, which is due to the higher number of effective Mn4+ ions due to the charge compensation.
Figure 7. A comparison between afterglow decay curves of LAO:Mn4+,Na+ phosphor at 20 ℃ and 37 ℃.
Figure 8. (a)-(f) TL glow curves of LAO:Mn4+,Na+ at different elevated charging
temperatures for different fading times (after 335 nm irradiation for 10 min at an elevated charging temperature from 50 ℃ to 90 ℃, fading time was chosen as 1 s or 30 min, and TL curves were collected with a constant heating rate of 30 ℃/min)
LAO:Mn4+,R (R= Na+, Ca2+, Sr2+, Ba2+) phosphors show NIR persistent emission and have a broad range of trapping centers. In the results shown in Figure S10, the samples were first heated up to a certain elevated temperature an optical charging before performing TL measurements. It confirms that there is a wide range of the trapping centers (from 70 ℃ to 130 ℃) in LAO:Mn4+,R phosphors. However, compared to the well-known room temperature LPPs such as SrAl2O4:Eu,Dy, [3] CaAl2O4:Eu,Nd, [39] ZnGa2O4:Cr, [40] Zn3Ga2Ge2O10:Cr, [41] and LiGa5O8:Cr, [42] the NIR persistent luminescence from LAO:Mn4+,R is still largely restricted due to the fact that it contains a large number of deeper traps from 70 ℃ to 130 ℃ (such as the trapping center around 114 ℃). On the other hand, it is worth mentioning that although the TL glow peaks from the higher temperature side do apparently not contribute significantly to the afterglow at room temperature, they can make a contribution to the afterglow at elevated temperatures, for example, the normal human body temperature (known as normothermia, around 37 ℃). Afterglow decay measurements were performed at this elevated temperature (Figure 7). Initially, the decay is faster at 37 °C, but at longer time, the afterglow becomes more intense and shows a much longer tail than observed at 20 °C, corresponding to the liberation of charges from deeper traps. More stored energy
can be released at even higher temperatures, as shown in Figure 8, where fading experiments at different temperatures are shown. The luminescence output at elevated temperature is obviously higher than that at 20 ℃, which leads to a longer afterglow duration time. It indicates that LAO:Mn4+,R (R= Na+, Ca2+, Sr2+, Ba2+) ceramics with NIR emitting persistent luminescence could be of great interest for the application in an environment at elevated temperature. For the application of the material as a persistent phosphor at room temperature, it remains a challenge to fine-tune the trap depths to increase the afterglow performance.
4. Conclusions The NIR persistent emission and the thermoluminescence behavior of Mn4+ activated LAO and LAO:Mn4+,R (R= Na+, Ca2+, Sr2+, Ba2+) ceramics were reported. NIR persistent luminescence in the wavelength region from 650 nm to 750 nm was obtained after 335 nm irradiation, well within the biological tissue transparency window for in vivo imaging. By carrying out a series of thermoluminescence measurements, a broad range of traps was found in LAO:Mn4+ phosphor with two main peaks centered at 0 ℃ and 114 ℃, respectively. Co-doping with Na+, Ca2+, Sr2+ or Ba2+ improves the TL signals in the effective range, and correspondingly enhances the NIR persistent luminescence. LAO:Mn4+,R (R= Na+, Ca2+, Sr2+, Ba2+) phosphor is very promising for applications where persistent luminescence is required at elevated temperature. The combination of afterglow, thermoluminescence and fading experiments is a powerful
method to analyze the trap levels in persistent luminescence compounds. The results show that, while long afterglow can be obtained at elevated temperatures, the trap levels in LAO:Mn4+,R are slightly too deep for room temperature LLP. Efforts are therefore needed to find ways to tune the trap depths, by selection of other appropriate co-dopants or by engineering the host lattice. The present thermoluminescence study of LAO:Mn4+,R is also beneficial for the development of other deep red or NIR emitting persistent luminescence ceramics. Acknowledgments: The authors acknowledge the financial support of the China Scholarship Council (Grant number 201606170077), BOF Cofunding Grant (Ghent University 2018) and the Ghent University’s Special Research Fund (BOF). The authors also thank D. Van der Heggen, Ang Feng and J.J. Joos for assistance with the TL setup and their fruitful discussions.
Author Contributions: Dirk Poelman and Jiaren Du conceived and designed the paper, Jiaren Du performed the experiments. All the authors analyzed the data and wrote the paper.
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PII: DOI: Reference:
S0272-8842(18)32308-3 https://doi.org/10.1016/j.ceramint.2018.08.243 CERI19278
To appear in: Ceramics International Received date: 17 May 2018 Revised date: 13 August 2018 Accepted date: 21 August 2018 Cite this article as: Jiaren Du, Olivier Q. De Clercq and Dirk Poelman, Thermoluminescence and near-infrared persistent luminescence in LaAlO3:Mn4+,R (R= Na+, Ca2+, Sr2+, Ba2+) ceramics, Ceramics International, https://doi.org/10.1016/j.ceramint.2018.08.243 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Thermoluminescence and near-infrared persistent luminescence in LaAlO3:Mn4+,R (R= Na+, Ca2+, Sr2+, Ba2+) ceramics Jiaren Du, Olivier Q. De Clercq, Dirk Poelman* LumiLab, Department of Solid State Sciences, Ghent University, Krijgslaan 281-S1, Ghent, Belgium *Correspondence. [email protected]
Abstract: The near-infrared (NIR) persistent luminescence and thermoluminescence of Mn4+ activated LaAlO3 (LAO) and LAO:Mn4+,R (R= Na+, Ca2+, Sr2+, Ba2+) ceramics were investigated. After irradiation with 335 nm excitation light, deep red and NIR persistent luminescence was obtained in the wavelength region from 650 nm to 750 nm due to the Mn4+ dopants, well within the biological tissue transparency window for in vivo imaging. The afterglow can be attributed to the presence of Mn4+ induced traps in the LAO host. By carrying out a series of thermoluminescence measurements, the formation of a broad range of traps was identified, with two main peaks centered at 0 ℃ and 114 ℃. Na+, Ca2+, Sr2+ or Ba2+ ions were added as co-dopants to optimize the location and energy of the trap levels, improving the NIR persistent luminescence.
Keywords: Perovskites; thermoluminescence; properties.
luminescence; optical materials/
1. Introduction Persistent luminescent materials, also known as long persistent phosphors (LPPs), relate to a particular optical phenomenon through which the light emission can persist for several minutes or hours after the excitation has finished. The first reported LPP was the Bologna stone, which could emit light due to its natural impurities.[1] Since 1996, the most extensively studied materials have been rare earth-doped alkaline earth aluminates, for example, the strontium aluminate based material SrAl2O4:Eu,Dy.[2,3] Persistent luminescent materials have become a major academic research focus in the field of new materials and material science. Based on their inherent optical energy-storing property, LPPs have been applied to optical storage media, security encoding, emergency route signs and medical imaging.[4] There is a special interest and increasing need for deep red or near-infrared (NIR) emitting persistent phosphors, in the spectral region between 650 nm and 950 nm and between 1000 nm to 1350 nm. In these two wavelength ranges, biological tissues show limited absorption and scattering, thus allowing NIR-emitting probes to penetrate deeper than comparable fluorophores emitting visible light.[5-7]
Therefore, LPPs have many features of merit for in vitro
and in vivo imaging applications. One major drawback overcome by LPPs is the presence of autofluorescence emerging in luminescent probes such as up-converting lanthanide nanoparticles, heavy metal based semiconductor quantum dots or organic fluorophores, which all need a continuous, high-power excitation source to exhibit their NIR emitting properties.[4,8-10] With LPPs, a pre-excitation is performed outside the
body and no external irradiation is needed during in vivo imaging. Thus, LPPs with deep red or NIR emission are recognized as suitable materials for this application.[11,12] LPPs consist of an inorganic matrix as a host, doped with activators. The activators can be divalent or trivalent lanthanides, comprising atomic numbers 57 to 71 of the periodic table (such as Eu2+, Eu3+, Dy3+, Ce3+, Nd3+, Sm3+, etc., mainly the so-called Ln3+ or Ln2+ ions),[13] transition metals (Mn2+, Mn4+, Cr3+, Ti4+, etc.) or several main group elements (such as Bi3+). The activator can act as emitter or trap. The trap stores the excitation energy and subsequently releases it gradually. The amount of stored energy and the time scale of energy release thus determines the duration and intensity of the afterglow. Recently, LaAlO3 (LAO) was found to be a possible host for several activators, such as Eu2+/Eu3+.[14] Our group and others reported Mn4+ activated LAO as a NIR emitting phosphor.[15,16] Y. Katayama et al. first reported Cr3+ activated LAO deep red persistent ceramics as a promising material for in vivo imaging.[17] Other Cr3+/Ln (Ln= Sm3+, Ce3+, Eu3+, Tm3+) co-doped LAO ceramics were also investigated and further developed.[18-20] When it comes to NIR emitting luminescence in the LAO host, mainly Mn4+ and Cr3+ ions have been investigated. Both Mn4+ and Cr3+ ions are highly promising NIR emitters and ideal doping candidates in the LAO host and their 3d3 electron configuration allows narrow band emission coming from the spin-forbidden 2Eg 4
4+
→ A2g transition. The LAO:Mn
and LAO:Cr3+ ceramics show persistent luminescence
peaking at the wavelength regions from 650 nm to 750 nm and 720 nm to 780 nm,
respectively.[15,17]
The LAO crystal structure corresponds to the rhombohedral, nearly cubic perovskite structure, which involves a rotation of the AlO6 octahedra with respect to cubic perovskite.[21] Two types of units are found in the crystal structure: AlO6 octahedra and LaO12 polyhedra. The central Al3+ cation is located in AlO6 octahedral units with six-fold oxygen coordination. Both Mn4+ and Cr3+ ions are supposedly substituting for Al3+ in this architecture given the similar ionic radius (RCr3+ = 61.5 pm, RMn4+ = 53 pm, RAl3+ = 53.5 pm, while RLa3+ = 136 pm) and octahedral coordination configuration. To enhance the performance of the NIR persistent luminescence, Tanabe et al. investigated the effect of various lanthanide co-dopants on Cr3+ activated LAO phosphors and Sm3+ was found to be a good co-dopant.[19] For Mn4+ activated LAO ceramics, we recently reported a preliminary screening of the effect of various co-dopants on the persistent luminescence performance of LAO:Mn4+.[15] Co-dopants such as Na+, Ca2+, Sr2+ and Ba2+ ions were found to be beneficial for improving both the steady state luminescence and afterglow intensity. Encouraged by this observation, a detailed investigation of the effects of co-doping on afterglow performance and traps behavior has been carried out.
In this work, LAO:Mn4+ and LAO:Mn4+,R (R= Na+, Ca2+, Sr2+, Ba2+) ceramics were studied using temperature dependent optical charging, fading and afterglow decay experiments and thermoluminescence measurements. NIR persistent luminescence in
the wavelength region from 650 nm to 750 nm was obtained, which can be attributed to the Mn4+ dopants. This work focuses on Mn4+ doped LAO phosphors and their thermoluminescence behavior. The results are interpreted in terms of the nature of traps formed in the perovskite lattice. Investigating the formation of traps and their energy distribution are of fundamental importance, in order to understand the mechanism(s) behind the persistent luminescence.[22] The thermoluminescence study of LAO:Mn4+,R is also beneficial for the development of other deep red or NIR emitting persistent luminescence ceramics.
2. Materials and Methods LAO:Mn powder samples were prepared via high temperature solid state reaction. All the raw chemicals were analytical grade, used without further purification. The used precursors, La2O3 (Sigma Aldrich, Saint Louis, MO, USA, 99.99%) and Al2O3 (Fluka, Schwerte, Germany, 99.5%), were mixed and ground in an agate mortar. The dopant Mn was added as MnO2 (Alfa Aesar, Karlsruhe, Germany, 99.997%). Co-dopants were mixed as Na2CO3 (Alfa Aesar, 99.95%), CaCO3 (Alfa Aesar, 99.95%), SrCO3 (Alfa Aesar, 99.99%) and BaCO3 (Alfa Aesar, 99.95%), respectively. The concentrations of dopants and co-dopants were chosen to make the following chemical composition formula:
LaAlO3:xMn4+
(x=
0.1%,
0.2%,
0.5%,
1%,
2%
and
5%)
and
LaAlO3:0.5%Mn4+,1%R (R= Na+/Ca2+/Ba2+/Sr2+). The molar % concentration is defined with respect to one mole of the host phosphor chemical formula (with respect to the Al
content in LAO).
The appropriate stoichiometric amount of precursors was weighed, suspended in ethanol and put in a ZrO2 grinding jar. The starting materials were ground in a Retsch PM 100 Planetary ball mill for 6 h to reduce the particle size and improve the mixing homogeneity of the precursors. After evaporating the remaining ethanol, the mixture was transferred to open alumina crucibles and subsequently heated up to 1550 ℃ in ambient air for 6 h to form the final compound. The employed heating rate was 300 ℃/h, using a tube furnace (ETF30-50/18-S furnace, ENTECH, Ängelholm, Sweden). After synthesis, all samples were allowed to cool naturally inside the tube furnace and the prepared powder samples were ground again manually prior to further characterization.
The
crystal
structures
of
the
studied
LaAlO3:Mn4+
and
LaAlO3:Mn4+,R
(Na+/Ca2+/Ba2+/Sr2+) were verified by Powder X-ray diffraction (XRD) measurements on a Siemens D5000 diffractometer (40 kV, 40 mA, Bruker) using Cu Kα1 radiation (λ = 0.154 nm). The XRD data were collected in the range 2θ from 10◦ to 80◦ at room temperature with a step time of 1.2 second. The reference data for LaAlO3 (JCP2.2CA.No. 00-031-0022) was used to compare with the obtained XRD patterns. Scanning electron microscopy was performed in a Hitachi S-3400 N, equipped with a Thermo Scientific Noran System 7 for energy-dispersive X-ray (EDX) analysis. EDX measurements were performed at a pressure of 25 Pa in order to avoid sample charging,
with an accelerating voltage of 20 kV. Steady state photoluminescence excitation and emission spectra were recorded using an Edinburgh FS920 (Edinburgh Instruments Ltd., Livingston, UK) fluorescence spectrometer with a monochromated 450 W Xe-arc lamp as the excitation source. Afterglow decay profiles were conducted using the above-mentioned fluorescence spectrometer under excitation for 5 min and afterglow decay was monitored at different emission wavelengths. All spectra were automatically corrected for detector response. Afterglow decay curves at elevated temperature were measured with a photosensor amplifier (Hamamatsu C9329, Japan) and a Centronics OSD100-5T silicon photodiode.
TL experiments were performed inside a small home-built vacuum chamber with a well-characterized cooling and heating stage.[23] Thin pressed pellets of samples were in good thermal contact with the heat exchanger by using thermally conductive adhesive. Identical size thin pellet samples with the chemical composition formula LaAlO3:xMn4+ (x= 0.1%, 0.2%, 0.5%, 1%, 2% and 5%) and LaAlO3:0.5%Mn4+, 1%R (R= Na+/Ca2+ /Ba2+ /Sr2+) were used for the TL measurements. Prior to each TL experiment, a thermal cleaning of the traps was conducted by heating up to 220 ℃. The excitation source for the charging was a monochromated 300 W Xenon arc lamp (Oriel Instruments, Stratford, CT, U.S.A.) and a 335 nm wavelength was used for 10 min at 20 ℃. After the optical charging, the sample was kept in the dark for 1 min at 20 ℃ before recording the TL output. TL glow curves were collected using a constant heating rate β (β= 10 ℃/min,
20 ℃/min, 30 ℃/min, 40 ℃/min or 50 ℃/min). The light emitted from the sample during each step (charging, fading, and heating step) was collected and a full emission spectrum was recorded to investigate both spectral shapes and intensities of the emission during the entire measurement. The TL output was guided to a ProEM1600 EMCCD camera attached to an Acton SP2300 monochromator (Princeton Instruments) using an optical fiber. TL glow curve measurements at temperatures in the range from -60 ℃ to 220 ℃ could be performed in an automated way, as demonstrated in previous work.[23,24]
3. Results and Discussion 3.1 Crystal Structure To identify the crystal structure and verify the phase purity of the as-prepared samples, powder X-ray diffraction (XRD) was performed and compared to the standard XRD pattern of LAO (JCP2.2CA.No.00-031-0022). The representative XRD patterns of LAO:0.5%Mn4+ and LAO:0.5%Mn4+,1%R (R= Na+, Ca2+, Sr2+, Ba2+) are shown together with the reference pattern in Figure 1. The optimized concentrations of the dopant Mn and co-dopants R were selected as 0.5% and 1% based on previous work.[15] All the XRD peaks can be exactly indexed to the standard XRD pattern of LAO and the doping of Mn or other co-dopant R does not make any appreciable changes to the LAO host structure. To further confirm the doping of Na+, Ca2+, Ba2+ and Sr2+ ions into the lattice, EDX mapping in scanning electron microscope was performed as shown in
Figure S1-S4. EDX mapping indicates the homogeneous distribution of manganese and other co-dopants and confirms the feasibility of doping in each of the prepared samples.
Figure 1. XRD patterns of LAO:0.5%Mn4+ and LAO:0.5%Mn4+,1%R. Co-dopant R represents Na+, Ca2+, Sr2+ and Ba2+ ions. The standard XRD pattern of LAO is illustrated with red bars. All the XRD patterns are normalized to arbitrary units.
3.2 Photoluminescence and Afterglow Properties The steady-state PL emission spectrum and afterglow decay curves were measured to analyze
the
photoluminescence
and
persistent
luminescence
properties
of
LAO:0.5%Mn4+ and are depicted in Figure S5. The excitation wavelength was 335 nm and the emission peaks are located at 697.5 nm, 704.5 nm, 710.5 nm, 718 nm, 724.5 nm and 731 nm, which are shown as red arrows and are coming from the spin-forbidden 2
Eg→4A2g transition and the vibrational sidebands of the zero-phonon line (ZPL) with
phonon assistance.[15] After exposure to 335 nm excitation for 5 minutes, wavelength-dependent afterglow decay curves of LAO:0.5%Mn4+ were recorded as a function of time by monitoring different discrete emission wavelengths as shown in Figure S5. It indicates that the persistent luminescence spectrum of the LAO:Mn4+ phosphor has a similar intensity distribution to the steady-state emission spectrum. All the afterglow decay curves of the different emission wavelengths have a similar decay behavior as shown in Figure S6. This result indicates that all Mn-ions that contribute to the steady-state PL emission also equally contribute to the afterglow spectrum, and that the afterglow spectrum does not change as a function of time. The afterglow intensity is in the wavelength region from 650 nm to 750 nm with several pronounced peaks, which satisfactorily falls in the wavelength range of the first biological tissue transparency window.
3.3 Thermoluminescence behavior of LAO: Mn4+ as a function of Mn-concentration The nature of the traps plays a vital role in the afterglow properties of LPP compounds. In general, the storage of the irradiation energy by the traps is supposed to be responsible for persistence luminescence. However, the precise mechanisms governing the persistent property in LPPs are still under discussion, and there are different possible trapping sites in different host or activator cases. When Mn4+ ions are doped in the LAO host, appropriate traps are generated and afterglow emission can be observed. It is well known that the Mn4+ ion usually stabilizes in an octahedral site with 6-fold coordination
(in Al3+ site for LAO).[25,26] In the case of LAO:Mn phosphor system, it is believed that the afterglow originates from the trapping sites, induced by Mn on an Al site. The thermoluminescence (TL) behavior is largely dependent on experimental parameters, such as the employed heating rate or excitation dose, etc.[27] The position of a TL peak approximately reveals the depth of a certain trap level and a TL peak refers to a particular trapping center in LPP phosphors. A TL peak situated at a higher temperature is corresponding to a deeper trap level and appropriate trap depths in phosphors are essential for LPP materials. It is generally acknowledged from previous reports that the TL peaks corresponding to the most effective persistent luminescence at room temperature are located slightly above room temperature, in the range of 320 K to 400 K.[28] Shallow traps normally lead to a fast afterglow decay and a short afterglow since the trapped charge carriers (holes or electrons) have a very high escape rate at or even well below room temperature. On the contrary, deep traps have too large binding energy for trapped charge carriers to escape at room temperature, resulting in little or no persistent emission. Shallow or deep traps are comparatively defined depending on the purpose of the application in mind.
TL investigations on LPP materials can help to reveal certain trap distributions, trap depths or the density of traps at a specific level, e.g. via the use of different specialized analysis techniques.[29-31] To investigate the relationship between TL profiles and the concentration of the Mn4+ dopant, TL measurements with different concentrations of
Mn4+ (from 0.1% to 5%) were performed under the same experimental conditions. Excitation was performed at 335 nm excitation for 10 min at -60 ℃, and a waiting time of 1 min. was observed before increasing the temperature at a heating rate of 30 ℃/min. TL glow curves were recorded in the range from -60 ℃ to 220 ℃. The final TL glow curves were plotted versus temperature and obtained by integrating the TL emission spectra over the wavelength range from 650 nm to 750 nm).
Figure 2. (a) TL glow curves with different concentrations of dopants (Mn4+ concentrations of 0.1%, 0.2%, 0.5%, 1%, 2% and 5%). (b) TL emission spectra (low resolution) of LAO:0.5%Mn4+ at different temperatures during heating. TL experiments were measured under the same conditions (After 335 nm excitation
for 10 min at -60 ℃, waiting for 1 min before increasing the temperature at a heating rate of 30 ℃/min.).
As illustrated in Figure 2(a), the positions of the peaks are identical in these TL curves, demonstrating that the trap depths are basically the same in these LAO:Mn4+ samples with different concentrations of dopants. On the other hand, the relative intensities of the TL peaks have a general trend that the intensities of the two main peaks keep rising as the Mn4+ concentrations increase from 0.1% to 0.5%. However, with a further increase of Mn4+ concentration from 0.5% to 5%, the intensities of the TL peaks of LAO:Mn4+ phosphors decrease gradually. For the LAO:5%Mn4+ phosphor, the main peaks in the TL profile nearly disappear most probably due to a severe concentration quenching of the emission. It was found that 0.5% Mn4+ is the optimal doping concentration for TL glow intensity and this concentration also provides the longest afterglow duration and highest steady state PL intensity.[15] The relative intensities of the two TL peaks remain the same as the Mn4+ concentrations increase in the range from 0.1% to 0.5%. However, with a further increase of Mn4+ concentration from 0.5% to 5%, the relative intensities of the two TL peaks of LAO:Mn4+ phosphors change. On the other hand, the two defect centers change differently with the concentration of Mn4+. It also indicates the concentration quenching affects the shallow traps and the deep traps in LAO phosphor in a complex way. The occurrence of two broad glow peaks, centered at 0 ℃ and 114 ℃, implies the presence of two dominant trapping centers in LAO:Mn4+
phosphors. Emission spectra at different temperatures during the TL heating procedure are illustrated in Figure 2(b). It can be seen that the overall shape of the spectra is the same as observed in PL (inset of Figure S5); the fine details of the spectra are missing due to the large spectrometer slit settings, needed to measure the low intensity signals. The TL spectra can be attributed to the spin-forbidden 2Eg→4A2g transition of Mn4+, confirming that Mn4+ acts as the recombination center for the released charges at all temperatures.
3.4 TL procedures for LAO:0.5%Mn4+ In order to study the trap behavior of the phosphors, different TL procedures were carried out for LAO:0.5%Mn4+. Under excitation with a certain wavelength for a period of time is called charging process, which is intended to fill the traps in the phosphor. TL glow curves of LAO:0.5Mn4+ with different excitation times are illustrated in Figure S7. When increasing the excitation time, the intensities of the two peaks gradually increase but the center positions of the two peaks remain the same. The experiments indicate that all trap levels can be filled gradually with longer time. Conversely, if the traps inside the phosphors are completely filled, a longer irradiation time can hardly improve the afterglow performance. In order to fill most of the traps, at least a 10 min-irradiation time is needed for the LAO:Mn4+ phosphor.
The heating rate β (℃/min), an important parameter adopted in TL measurements, requires careful selection. The experimental schematic is shown in Figure 3(a). When gradually increasing the heating rate from 10 ℃/min up to 50 ℃/min, the signal-to-noise ratio (SNR) is improved, the intensity of the peaks increases and the TL peak position shifts slightly to higher temperatures as displayed in Figure 3(b), which is a normal behavior since the traps are emptied in a shorter time when the heating is faster. On the other hand, a higher heating rate may induce the risk of temperature gradients over the samples, leading to an unreliable result.[32] To balance among SNR, the intensity of TL glow peaks and the TL reliability, a moderate heating rate 30 ℃/min was chosen for further experiments.
Figure 3. (a) An illustration of the TL procedure at different heating rates. (b) TL glow curves of LAO:0.5Mn4+ at different heating rates (10 ℃/min, 20 ℃/min, 30 ℃/min, 40 ℃/min or 50 ℃/min).
The effect of different charging temperatures on the TL glow peaks is shown in Figure 4. The charging temperature was set at -60 ℃, -40 ℃, -20 ℃, 0 ℃, 20 ℃, 40 ℃ and 60 ℃ respectively in each TL measurement as seen in Figure 4(a). The heating rate remained constant at 30 ℃/min as discussed above. For different charging temperatures, the same peak centered around 114 ℃ is observed and the broad TL glow curve from -20 ℃ to 60 ℃ already strongly indicates the presence of a range of multiple traps with a continuous distribution of trap levels. Figure 4(b) shows clearly the two main peaks centered at 0 ℃ and 114 ℃ under excitation at -60 ℃ and the two dominant trap levels in LAO:Mn4+ phosphors.
Figure 4. (a) Illustration of the TL procedure at different charging temperatures. (b) TL glow curves of LAO:0.5Mn4+ at different excitation temperatures (-60 ℃, -40 ℃, -20 ℃, 0 ℃, 20 ℃, 40 ℃ or 60 ℃).
In order to find out the contribution of the different traps to the afterglow, the TL curves of LAO:0.5%Mn4+ were recorded with different fading times between charging and TL. Fading experiment (Wait for a period of time in the darkness without changing temperature) is often used to investigate the trap structures. The sample was irradiated at 335 nm for 10 min and placed in a dark chamber at room temperature for different fading times. The TL fading experimental procedure is displayed in Figure 5(a). The fading time was chosen as 10s, 30s, 1 min, 10 min or 100 min. As can be seen from Figure 5(b), the intensity of the TL glow peaks from 20 ℃ to 90 ℃ decreases gradually
with longer fading time and eventually disappears after the removal of the excitation light for 100 min. Usually, the shallow trap releases its charge carriers at a faster ratio so that the energy stored in shallow traps will be emptied at the first stage and then the captured charge carriers in the deeper traps are released at a lower rate. It implies that TL glow peaks from 20 ℃ to 90 ℃ are responsible for the afterglow occurrence at room temperature. However, both the peak position and intensity from the higher temperature side remains intact, centered at 114 ℃, with only a minor reduction. While TL glow peaks from the higher temperature side do apparently not contribute significantly to the afterglow at room temperature, they can make a significant contribution to the release of the stored energy and the afterglow at elevated temperatures.
Figure 5. (a) Illustration of the TL procedure for different fading times. (b) TL glow curves of LAO:0.5Mn4+ for different fading times (10s, 30s, 1 min, 10 min or
100 min).
3.5 Effects of various co-dopants Co-dopants such as Na+, Ca2+, Sr2+ and Ba2+ ions were reported to be beneficial for improving the LAO:Mn4+ luminescence and afterglow intensity.[15] A more in-depth investigation of the effects of co-doping was carried out using a series of the above-mentioned TL measurements. Considering the similar ionic radius and octahedral coordination configuration, the Mn4+ ion is expected to substitute Al3+ in the AlO6 octahedral unit and the co-dopant ion (Na+, Ca2+, Sr2+ or Ba2+) is supposed to replace La3+ in the LaO12 polyhedral unit, as discussed in detail elsewhere.[15,33] Schematics for the doping position with different co-dopants are illustrated in Figure 6(a). Different charging temperatures and fading times were examined for each co-dopant via TL measurements. A series of detailed TL glow curves with different co-dopants are illustrated in Figure S8 and S9.
As shown in Figure S8 and Figure S9, co-dopants Ba2+/Sr2+/Ca2+/Na+ lead to the same type of traps as in LAO:Mn4+ phosphors. Often, new trapping centers are created using divalent or trivalent lanthanide co-doping, for example in YPO4:Ce3+,Ln (Ln3+/Ln2+= Nd, Sm, Dy, Ho, Er, Tm) [34], YPO4:Tb3+,Ln3+ (Ln= Nd, Ho, Dy, Sm, Tm) [35], Ca6BaP4O17:Eu2+,Ln3+ (Ln= Dy, Tb, Ce, Gd, Nd) [36] or Mg2GeO4:Mn4+,Ln3+ (Ln= Pr, Er, Nd, Yb) [37]. No obvious additional trapping centers are found in LAO:Mn4+,R (R=
Na+, Ca2+, Sr2+, Ba2+) upon co-doping. Co-doping with Na+/Ca2+ ions has the largest effect, with TL glow peaks becoming much more intense, as seen in Figure S8 and S9.
Figure 6. (a) Schematic for the doping positions in the LAO:Mn4+,R phosphor. Co-dopant ion is Na+/Ca2+/Sr2+/Ba2+. (b) Comparison of TL glow curves from different doping ions with the same TL parameters. (c) Illustration of the temperature dependent excitation measurement using 335 nm irradiation for 30 s at a certain temperature from -60 ℃ to 225 ℃ with a 5 ℃ interval and the steady-state emission spectrum, collected at each temperature platform. (d) Integrated steady-state emission intensities from 650 nm to 750 nm under temperature dependent excitations.
TL glow curves from different doping ions are illustrated and compared in Figure 6(b) and the identical parameters (335 nm excitation for 10 min at -60 ℃, waiting 1 min before TL at a heating rate of 30 ℃/min) were chosen for all the samples. For the afterglow generation at room temperature, too shallow traps (below 20 ℃) or too deep traps (above 140 ℃) contribute little to the release of stored energy at room temperature and are thus not interesting from the point of view of room temperature persistent luminescence. It is expected that the trap range from 40 ℃ to 90 ℃ is responsible for the room temperature afterglow performance in LAO:Mn4+ phosphors. In other words, the traps in the range 40 ℃ - 90 ℃ instead of the two main TL glow curves peaking at 0 ℃ and 114 ℃ may play the leading role in the afterglow occurrence at room temperature. Co-doping with Ba2+/Sr2+ ions increases the intensity of the TL signals but does not create or eliminate any trap levels. On the contrary, co-doping with Na+/Ca2+ ions helps to bring down the low temperature center at 0 ℃, strongly improves the TL output in the effective range 40 ℃-90 ℃ and greatly enhances the intensity of the 114 ℃ peak four-fold. Each co-dopant can be beneficial for increasing the persistent luminescence of LAO:Mn4+ due to the enhancement of TL output in the effective range, which is in agreement with the afterglow results of these co-dopants.[15] In addition, the position of the TL peaking center with each co-dopant at 114 ℃, as displayed in Figure 6(b), shows only little shift. It indicates that no extra defects are formed and only Mn4+ activated traps play an important role in the afterglow behavior of LAO:Mn4+,R phosphors. Apparently, co-doping with Na+/Ca2+/Ba2+/Sr2+ as charge-compensating ions only helps
to increase the number of the effective Mn4+, which has a beneficial effect on improving luminescence output leading to a higher TL signal.
In addition, a temperature dependent excitation measurement was performed for each co-doping sample. The sample was excited with 335 nm for 30 s at a certain temperature from -60 ℃ to 225 ℃ with a 5 ℃ interval and the emission spectrum at each temperature was collected (as shown in Figure 6(c), the heating rate was 10 ℃/min). Figure 6(d) displays the integrated temperature dependent steady state PL intensity (integrated over a steady state PL time scale in the wavelength range from 650 nm to 750 nm). It shows that a decrease of emission intensity gradually happens when increasing temperature, then a notable drop occurs with higher temperature, which is similar to the thermal quenching behavior of the most phosphors for LED application. [38] It is worth mentioning that co-doping with Na+/Ca2+/Ba2+/Sr2+ ions also enhances the steady-state photoluminescence to varying degrees, compared with LAO:Mn4+, which is due to the higher number of effective Mn4+ ions due to the charge compensation.
Figure 7. A comparison between afterglow decay curves of LAO:Mn4+,Na+ phosphor at 20 ℃ and 37 ℃.
Figure 8. (a)-(f) TL glow curves of LAO:Mn4+,Na+ at different elevated charging
temperatures for different fading times (after 335 nm irradiation for 10 min at an elevated charging temperature from 50 ℃ to 90 ℃, fading time was chosen as 1 s or 30 min, and TL curves were collected with a constant heating rate of 30 ℃/min)
LAO:Mn4+,R (R= Na+, Ca2+, Sr2+, Ba2+) phosphors show NIR persistent emission and have a broad range of trapping centers. In the results shown in Figure S10, the samples were first heated up to a certain elevated temperature an optical charging before performing TL measurements. It confirms that there is a wide range of the trapping centers (from 70 ℃ to 130 ℃) in LAO:Mn4+,R phosphors. However, compared to the well-known room temperature LPPs such as SrAl2O4:Eu,Dy, [3] CaAl2O4:Eu,Nd, [39] ZnGa2O4:Cr, [40] Zn3Ga2Ge2O10:Cr, [41] and LiGa5O8:Cr, [42] the NIR persistent luminescence from LAO:Mn4+,R is still largely restricted due to the fact that it contains a large number of deeper traps from 70 ℃ to 130 ℃ (such as the trapping center around 114 ℃). On the other hand, it is worth mentioning that although the TL glow peaks from the higher temperature side do apparently not contribute significantly to the afterglow at room temperature, they can make a contribution to the afterglow at elevated temperatures, for example, the normal human body temperature (known as normothermia, around 37 ℃). Afterglow decay measurements were performed at this elevated temperature (Figure 7). Initially, the decay is faster at 37 °C, but at longer time, the afterglow becomes more intense and shows a much longer tail than observed at 20 °C, corresponding to the liberation of charges from deeper traps. More stored energy
can be released at even higher temperatures, as shown in Figure 8, where fading experiments at different temperatures are shown. The luminescence output at elevated temperature is obviously higher than that at 20 ℃, which leads to a longer afterglow duration time. It indicates that LAO:Mn4+,R (R= Na+, Ca2+, Sr2+, Ba2+) ceramics with NIR emitting persistent luminescence could be of great interest for the application in an environment at elevated temperature. For the application of the material as a persistent phosphor at room temperature, it remains a challenge to fine-tune the trap depths to increase the afterglow performance.
4. Conclusions The NIR persistent emission and the thermoluminescence behavior of Mn4+ activated LAO and LAO:Mn4+,R (R= Na+, Ca2+, Sr2+, Ba2+) ceramics were reported. NIR persistent luminescence in the wavelength region from 650 nm to 750 nm was obtained after 335 nm irradiation, well within the biological tissue transparency window for in vivo imaging. By carrying out a series of thermoluminescence measurements, a broad range of traps was found in LAO:Mn4+ phosphor with two main peaks centered at 0 ℃ and 114 ℃, respectively. Co-doping with Na+, Ca2+, Sr2+ or Ba2+ improves the TL signals in the effective range, and correspondingly enhances the NIR persistent luminescence. LAO:Mn4+,R (R= Na+, Ca2+, Sr2+, Ba2+) phosphor is very promising for applications where persistent luminescence is required at elevated temperature. The combination of afterglow, thermoluminescence and fading experiments is a powerful
method to analyze the trap levels in persistent luminescence compounds. The results show that, while long afterglow can be obtained at elevated temperatures, the trap levels in LAO:Mn4+,R are slightly too deep for room temperature LLP. Efforts are therefore needed to find ways to tune the trap depths, by selection of other appropriate co-dopants or by engineering the host lattice. The present thermoluminescence study of LAO:Mn4+,R is also beneficial for the development of other deep red or NIR emitting persistent luminescence ceramics. Acknowledgments: The authors acknowledge the financial support of the China Scholarship Council (Grant number 201606170077), BOF Cofunding Grant (Ghent University 2018) and the Ghent University’s Special Research Fund (BOF). The authors also thank D. Van der Heggen, Ang Feng and J.J. Joos for assistance with the TL setup and their fruitful discussions.
Author Contributions: Dirk Poelman and Jiaren Du conceived and designed the paper, Jiaren Du performed the experiments. All the authors analyzed the data and wrote the paper.
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