Temperature and wavelength dependent trap filling in M2Si5N8:Eu (M=Ca, Sr, Ba) persistent phosphors

doi:10.1016/j.jlumin.2011.10.022

Journal of Luminescence

Volume 132, Issue 3, March 2012, Pages 682-689

https://doi.org/10.1016/j.jlumin.2011.10.022 Get rights and content

Abstract

The evaluation of persistent phosphors is often focused on the processes right after the excitation, namely on the shape of the afterglow decay curve and the duration of the afterglow, in combination with thermoluminescence glow curve analysis. In this paper we study in detail the trap filling process in europium-doped alkaline earth silicon nitrides (Ca₂Si₅N₈:Eu, Sr₂Si₅N₈:Eu and Ba₂Si₅N₈:Eu), i.e., how the persistent luminescence can be induced. Both the temperature at which the phosphors are excited and the spectral distribution of the excitation light on the ability to store energy in the phosphors' lattices are investigated. We show that for these phosphors this storage process is thermally activated upon excitation in the lower 5d excited states of Eu²⁺, with the lowest thermal barrier for europium doped Ca₂Si₅N₈. Also, the influence of co-doping with thulium on the trap filling and afterglow behavior is studied. Finally there exists a clear relation between the luminescence quenching temperature and the trap filling efficiency. The latter relation can be utilized to select new efficient 5d–4f based afterglow phosphors.

Highlights

► Orange afterglow in M₂Si₅N₈:Eu(Tm) studied with thermoluminescence spectroscopy. ► Strong influences of excitation wavelength and temperature on trap filling. ► Energy level scheme is presented. ► Relation between trap filling with visible light and thermal quenching behavior.

Introduction

The emission intensity of most luminescent materials drops to zero in less than a second after ending the excitation. Some materials, however, continue emitting light for minutes or even hours after the end of the excitation, a phenomenon known as persistent luminescence or afterglow. This continued light emission without the need for a constant energy input is useful for many applications, the most notable being emergency signage [1], medical imaging [2], dials and displays, and decoration.

Persistent phosphors emitting in the red or orange region of the visible spectrum are strongly desired in emergency signage, for displaying warning or stop signs, and in medical imaging, where the phosphors' emission should be situated in the optical window of biological tissue [2], [3]. Unfortunately, the large majority of known persistent luminescent materials are green (e.g. SrAl₂O₄:Eu,Dy [4]) or blue emitting (e.g. Sr₂MgSi₂O₇:Eu,Dy [5]). The reason for this lack of efficient red persistent phosphors is twofold. Firstly, most persistent materials are based on oxides with Eu²⁺ as the emitting ion [6], and it is difficult to achieve a red-shift in oxides that is large enough to obtain red Eu²⁺ emission [7]. Secondly, the sensitivity of the human eye is lower for red light than it is for blue or green light. This effect is even more prominent at the low light intensity conditions typical for persistent luminescence applications. Hence, for red persistent phosphors to have the same apparent brightness as green or blue ones, they have to be considerably more efficient [8], [9].

The second problem is inherent to human vision and cannot be evaded, but the first one can be tackled by choosing other host materials. For example, red persistent luminescence has been observed in CaS:Eu,Tm [15] and Ca₂SiS₄:Eu,Nd [16], [17]. Recently, the family of alkaline earth nitrido-silicates, M₂Si₅N₈:Eu (M=Ca, Sr, Ba) and their Tm-codoped variants were studied [10]. These materials are much more stable than the aforementioned sulfides and show a high quantum efficiency, which has led to extensive research into their use as conversion phosphors in white LEDs [11], [12], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28]. Recently, orange and reddish persistent luminescence was reported in these compounds, most notably in Ca₂Si₅N₈:Eu,Tm [10], [13], [14], [18], [29], [30]. Key parameters for the steady-state photoluminescence and the persistent luminescence of M₂Si₅N₈:Eu(Tm) are given in Table 1.

A great advantage of these Eu²⁺-doped nitride materials is their very broad excitation spectrum, extending well into the visible range. This is promising for indoor applications where (near-)UV light is usually scarce, especially when lighting is based on phosphor-converted LEDs. However, it does not necessarily mean that the persistent luminescence can also be obtained after exciting with visible light. After all, while the (steady-state) photoluminescence is caused by excited electrons in the Eu ions returning almost immediately to the ground state, the persistent luminescence is determined by acceptor states in the band gap of the material acting as traps for the excited electrons, hereby delaying their return to the ground state [6]. Because of these two distinct processes governing both types of emission, it is not unlikely for the excitation spectrum of the steady-state photoluminescence to differ substantially from that of the persistent luminescence.

Thermoluminescence (TL) excitation spectroscopy is a convenient way to study the excitation behavior of the persistent luminescence [31]. In this technique, a TL glow curve measurement is performed for various (monochromatic) excitation wavelengths. The intensity of the peaks appearing in the TL curves yields information on the number of charge carriers trapped inside the material. If a glow peak appears for a specific excitation wavelength, we can conclude that the traps were filled by the excitation light, and hence that persistent luminescence can be induced using this wavelength. If no glow peak appears, the traps could not be filled and no persistent luminescence will be present either, even though the material might be fluorescent for this wavelength. When performing these TL measurements for a large set of (evenly spread) excitation wavelengths, a TL excitation contour plot can be constructed, being a two-dimensional plot of the TL emission intensity as a function of excitation wavelength and temperature. From these data an excitation spectrum can be extracted for the persistent luminescence, the so-called trap filling spectrum, by taking a cross-section at a certain glow peak or by integrating the total TL intensity for each excitation wavelength. This can then be compared to the excitation spectrum of the steady state photoluminescence. In this work we will show that both types of spectra contain the same components, albeit with a different relative intensity. This is related to a thermal activation energy, which is required to fill traps in the phosphor, at least when exciting into the lowest 5d excited state.

Section snippets

Experimental setup

The powders were prepared using a solid state reaction at 1400 °C for 3 h, under a reducing atmosphere (90% N₂, 10% H₂). For the host material, appropriate amounts of M₃N₂ (Alfa Aesar for Ca₃N₂ (99%) and CERAC for Ba₃N₂ (99.7%) and Sr₃N₂ (99.5%)) and α-Si₃N₄ (99.85%, Alfa Aesar) were mixed. To optimize the persistent luminescence, a 2.5% deficit of Ca₃N₂ was used [10]. For the dopants, EuF₃ and (if required) TmF₃ (99.9%, Alfa Aesar) were added to the starting mixture. The powders were prepared

Thermoluminescence excitation contour plots

Fig. 2 shows the thermoluminescence excitation contour plots for M₂Si₅N₈:Eu phosphors (M=Ca, Sr and Ba), without intentionally added co-dopant(s). Each horizontal line in such a contour plot shows the TL glow curve obtained at a specific excitation wavelength. Each vertical cross-section shows a trap filling spectrum (obtained at a specific temperature). As described in Ref. [10], all the M₂Si₅N₈:Eu phosphors show persistent luminescence to some degree, with afterglow intensities in the order

Discussion

From the above presented results, it is clear that thermoluminescence excitation spectroscopy is indeed a versatile technique to study persistent phosphors [31], especially if the temperature can be varied at which the excitation is performed. From the thermoluminescence excitation contour plots shown in Fig. 2, Fig. 3 the trap filling spectra were derived, describing which wavelengths can induce persistent luminescence (Fig. 5).

For all studied M₂Si₅N₈:Eu phosphors, the trap filling spectrum

Conclusions

In this work we used thermoluminescence excitation spectroscopy to study the trap filling behavior in M₂Si₅N₈:Eu(Tm) persistent phosphors, in combination with a study on the influence of temperature during the excitation. We showed that the traps relevant for the afterglow can be filled after excitation into the 5d energy levels of the Eu²⁺ dopants. For excitation into the lower 5d levels, a thermal barrier is present before trapping can occur, while this is not the case upon excitation into

Acknowledgments

PFS is indebted to the Fund for Scientific Research – Flanders (FWO-Vlaanderen) for a Mobility Grant to TU Delft. KVdE is a Research Assistant for the BOF-UGent. Dirk Poelman is kindly acknowledged for fruitful discussions.

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Temperature and wavelength dependent trap filling in M2Si5N8:Eu (M=Ca, Sr, Ba) persistent phosphors

Abstract

Highlights

Introduction

Section snippets

Experimental setup

Thermoluminescence excitation contour plots

Discussion

Conclusions

Acknowledgments

J. Lumin.

J. Alloys Compd.

J. Lumin.

J. Phys. Chem. Solids

J. Solid State Chem.

J. Lumin.

J. Lumin.

J. Alloys Compd.

Radiat. Meas.

J. Lumin.

Radiat. Meas.

Opt. Express

Proc. Natl. Acad. Sci.

ACS Nano

J. Electrochem. Soc.

J. Mater. Sci. Lett.

Materials

Opt. Express

Opt. Express

Materials

Int. J. Appl. Ceram. Technol.

J. Electrochem. Soc.

Temperature and wavelength dependent trap filling in M₂Si₅N₈:Eu (M=Ca, Sr, Ba) persistent phosphors