Red persistent luminescence in rare earth-free AlN:Mn2+ phosphor

Red persistent luminescence in rare earth-free AlN:Mn2+ phosphor

Accepted Manuscript Red persistent luminescence in rare earth-free AlN:Mn2+ phosphor Jian Xu, Nerine J. Cherepy, Jumpei Ueda, Setsuhisa Tanabe PII: DO...

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Accepted Manuscript Red persistent luminescence in rare earth-free AlN:Mn2+ phosphor Jian Xu, Nerine J. Cherepy, Jumpei Ueda, Setsuhisa Tanabe PII: DOI: Reference:

S0167-577X(17)31064-9 http://dx.doi.org/10.1016/j.matlet.2017.07.015 MLBLUE 22857

To appear in:

Materials Letters

Received Date: Revised Date: Accepted Date:

11 May 2017 30 June 2017 3 July 2017

Please cite this article as: J. Xu, N.J. Cherepy, J. Ueda, S. Tanabe, Red persistent luminescence in rare earth-free AlN:Mn2+ phosphor, Materials Letters (2017), doi: http://dx.doi.org/10.1016/j.matlet.2017.07.015

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Red persistent luminescence in rare earth-free AlN:Mn2+ phosphor Jian Xua,*, Nerine J. Cherepyb,*, Jumpei Uedaa, Setsuhisa Tanabea

a

Graduate School of Human and Environmental Studies, Kyoto University, Kyoto, 606-8501, Japan b

Lawrence Livermore National Laboratory, Livermore, CA 94550, United States

Corresponding authors: * E-mail addresses: [email protected], [email protected]

ABSTRACT: We investigated the persistent luminescence (PersL) properties of a rare earth-free Mn2+ doped AlN (AlN:Mn) red phosphor together with a commercial SrAl2O4:Eu2+, Dy3+ green persistent phosphor as a reference. Similar to its photoluminescence (PL) spectrum, the PersL spectrum of the AlN:Mn phosphor exhibited a red emission band centered at 600 nm due to the Mn2+: 4T1(4G)→6A1(6S) transition with a relatively narrow full width at half maximum (FWHM) of 43 nm. The luminance of AlN:Mn powders was 0.65 mcd/m2 at 60 min after ceasing ultraviolet (UV) illumination, and its duration upon 0.32 mcd/m2 could reach over 110 min. An extremely broad thermoluminescence (TL) glow curve was observed ranging from 100 K to 600 K and peaked at around 310 K, indicating a wide trap distribution in this material.

Keywords: Optical materials and properties; Phosphors; Luminescence

1. Introduction In the past decade, nitride phosphors have attracted much attention, particularly for solid-state lighting due to their excellent physical and chemical stability, superior water resistance, and less thermal quenching of luminescence compared with conventional oxide and sulfide phosphors [1-4]. Some of them, such as CaAlSiN3:Eu 2+ [5] and Sr[LiAl3N4]:Eu2+ [6], have achieved a big success as standard red phosphors toward new generation of phosphor-converted white LEDs (pc-wLEDs) with high luminous efficiency and high color rendering index (CRI). Most of the red-emitting nitride phosphors activated by Eu2+ provide broad emission bands extending beyond 750 nm. Mn2+-doped AlN (AlN:Mn) had been known in thin films to give a relatively narrow emission band centered at 600 nm [7]. Recently, Cherepy et al have successfully developed a highly efficient AlN:Mn red phosphor with optimized preparation procedures and Mn2+ doping concentration (0.06 mol%) [8,9]. They also suggested that it can be useful as replacement for Y2O3:Eu3+ in fluorescent lamps, since the AlN:Mn phosphor provides a spectrum and integrated intensity comparable to that of the commercial Y2O3:Eu3+ phosphor under 254 nm excitation, as well as excellent lumen maintenance in fluorescent lamp conditions [8-10]. Possible replacement of the rare earth-based phosphor Y2O3:Eu 3+, with the earth abundant phosphor AlN:Mn is a new option, this work resulting from a US Department of Energy Critical Materials Institute effort to identify replacements for rare earth-based materials important to energy-related applications [11]. Besides, Wang et al reported that AlN:Mn has a great potential for full-color field emission displays (FEDs) due to its high color purity, low current saturation and stable cathodoluminescence (CL) chromaticity [12]. However, in contrast to these “desirable”

luminescence properties shown in AlN:Mn, red persistent luminescence (PersL, a kind of “self-sustained” luminescence that can last for minutes or even hours after ceasing excitation sources) was also observed in this material after ceasing ultraviolet (UV) illumination [13], which is generally “undesirable” and not beneficial to a phosphor’s practical application, especially for FEDs. Nevertheless, there are practical PersL applications for requiring a high-visibility orange-red persistent phosphor, and the few established candidates to choose from provide deep-red luminescence with poor lumen sensitivity, such as MgGeO3:Mn2+-Bi3+ (MGO:Mn-Bi with emission band: 600-800 nm) [14] and ZnGa2O4:Cr3+-Bi3+ (ZGO:Cr-Bi with emission band: 650-800 nm) [15,16]. Therefore, in order to evaluate the PersL properties of AlN:Mn phosphors, three important parameters including PersL spectrum, PersL decay and thermoluminescence (TL) glow curve were investigated.

2. Experimental procedure AlN:Mn powders with 0.06 mol% Mn2+ doping concentration were prepared by solid-state reaction and high temperature sintering method [8]. Commercial aluminum nitride powders (Materion or H.C. Starck; materials used were nominally 33.3% N, 0.1% C, <1% O, <0.005% Fe, with surface area of 2 m2/g) were ground with Mn and Si dopants (Sigma Aldrich) in an agate mortar, placed in boron nitride crucibles, and heated under 10-100 atm nitrogen pressure at 2000°C for 2-4 h, and then annealed in flowing nitrogen at 1650°C. A commercial SrAl2O4:Eu 2+, Dy3+ (SAO:Eu-Dy) powder phosphor (LumiNova®GLL-300FFS) purchased from its inventor (Nemoto & Co., Ltd.,) was used as a reference sample [17]. PersL spectra of the AlN:Mn and SAO:Eu-Dy powder samples were measured by a Si

CCD spectrometer (QE65-Pro, Ocean Optics) after charging by the 254 nm and 365 nm mercury lamp (6 W output) for 5 min, respectively. All the PersL spectra were calibrated by using a standard halogen lamp (SCL-600, Labsphere). The persistent luminescent decay curve of the AlN:Mn phosphor after being excited for 5 min by a 300 W Xe-lamp (MAX-302, Asahi Spectra) with an UV mirror module (250-380 nm, with ~48.1 mW/cm2 illumination power) was measured at 25°C using a PMT detector (R928, Hamamatsu Photonics). In order to monitor the Mn2+ emission, the PMT detector was covered with 580 nm short-cut and 700 nm long-cut filters to filter out all but the Mn2+ luminescence, then the decay curve was calibrated to the absolute luminance (in unit of mcd/m2) using a CCD spectrometer (Glacier X, B&W Tek Inc). The 300 W Xe-lamp with the UV mirror module was used as the excitation

source

for

the

TL wavelength-temperature

two-dimensional

(2D)-plot

measurement. The powder sample was set inside a copper holder fixed with a cryostat (Helitran LT3, Advanced Research Systems) to control temperatures and firstly illuminated by UV light at 100 K for 10 min, then heated at 10 min after ceasing the illumination up to 600 K at a rate of 10 K/min. The Si CCD spectrometer was operated simultaneously with the TL measurement to monitor the TL spectra at different temperatures. Photographs of the powder samples were taken by a digital camera (EOS kiss X5, Canon), and the settings remained constant: exposure time-5 s, ISO value-1600, and aperture value (F value)-5.0.

3. Results and discussion Fig. 1 shows the PersL spectra of AlN:Mn and SAO:Eu-Dy powder samples at 1 min after ceasing the UV excitation, in which the photopic vision of human eyes is also plotted.

The AlN:Mn sample gives a broad emission band ranging from 560 nm to 700 nm due to the Mn2+: 4T1(4G)→6A1(6S) transition with a relatively narrow full width at half maximum (FWHM) of 43 nm, and the peak wavelength is located at ~600 nm. Since the photopic vision shows a maximum response at 555 nm with 683 lm/W, the spectrum overlap between the photopic vision and the luminescence spectrum of the AlN:Mn red phosphor is smaller than that of the SAO:Eu-Dy green phosphor, which exhibits a broad emission band peaked at 520 nm due to the parity allowed Eu 2+: 4f65d1→4f7 transition. The persistent luminescent decay curve monitoring the Mn2+ luminescence in the AlN:Mn sample is shown in Fig. 2a, in which the decay curves of the ZGO:Cr-Bi, MGO:Mn-Bi and SAO:Eu-Dy persistent phosphors are also plotted as references [16,18]. The luminance values at 60 min after ceasing UV illumination are 0.65 mcd/m2 for AlN:Mn (the efficient charging wavelength region for AlN:Mn is located only at around 200-340 nm and peaked at ~290 nm, data not shown here) and 25 mcd/m2 for SAO:Eu-Dy, respectively (the photographs of the two powder samples given in Fig. 2b and c). However, the luminance of AlN:Mn is still superior to that of MGO:Mn-Bi (0.17 mcd/m2) and is comparable to ZGO:Cr-Bi (0.68 mcd/m2) deep-red persistent phosphors [16]. PersL duration to reach a luminance of 0.32 mcd/m2 in AlN:Mn is over 110 min, which is much shorter than that in SAO:Eu-Dy (>30 h) (note that the luminance value 0.32 mcd/m2 is the minimum value commonly used by the safety signage industry). It is also worth noting that, in a previous report by Zhang et al [13], AlN:Mn synthesized at moderate temperature (1750oC) exhibited a decrease in PersL of ~100x within 10 minutes. This is significantly faster than that obtained

here with AlN:Mn synthesized at 2000oC, likely due to improved crystallinity and differences in trace impurities that form shallow electron traps [8]. Fig. 3a shows the TL 2D contour plot of the AlN:Mn powder sample in order to identify what kind of emission bands contribute to the TL glow peak at different temperatures. It indicates that the TL spectrum is composed of only one emission band from Mn2+ centered at ~600 nm, also with increasing temperature (shown in Fig. 3c), no additional emission centers contribute to the TL glow curve. The TL glow curve gives a very broad temperature region from 100 K to 600 K and peaked at ~310 K as shown in Fig. 3b. Since the TL peak temperature is correlated to the energy gap between the bottom of conduction band (CB) and the electron trap, the broad TL glow curve suggests a wide trap distribution responsible for the detrapping process [19]. Although a tunneling process independent of temperature effects may occur in the forbidden band [13,20] instead of the classical electron trapping-detrapping channel through CB [21], further experiments such as low temperature decay after longer wavelength charging (visible light) need to be carried out to confirm this assumption [22].

4. Conclusions In summary, we report PersL properties of a rare earth-free AlN:Mn red phosphor. The spectrum shape of PersL in AlN:Mn is nearly identical to its photoluminescence spectrum due to the Mn2+: 4T1(4G)→6A1(6S) transition centered at ~600 nm. Although the persistent luminance and duration of AlN:Mn after ceasing 254 nm excitation are much lower and shorter than that of the commercial SAO:Eu-Dy green persistent phosphor, partially due to the mismatch between its red emission band and the response curve of photopic vision, the

non-negligible PersL shown in this material should be carefully taken into consideration, especially for potential applications of FEDs, and for applications specifically requiring an orange-red nitride persistent phosphor [23]. Since the as-synthesized AlN:Mn phosphor has not been optimized for its PersL behavior, improvements are likely possible. Further research about this material can be focused on either suppressing its PersL behavior for a higher efficiency and lower afterglow red phosphor or increasing its PersL intensity to produce a new red persistent phosphor with long duration.

Acknowledgements We are grateful to Deborah Schlagel (Ames Laboratory) and LLNL colleagues Zachary Seeley, Nicholas Harvey and Ross Osborne for synthesis of AlN:Mn. This work was performed under the auspices of the U. S. Department of Energy by Lawrence Livermore National Laboratory under contract DE-AC52-07NA27344, and funded by the DOE EERE Critical Materials Institute. It was also financially supported by a Grant-in-Aid for JSPS Fellows (No. 16J09849) and JSPS Grant-in-Aid for Scientific Research on Innovative Areas “Mixed anion” (No. JP16H6441).

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Figure Captions Fig. 1. Normalized PersL spectra of the AlN:Mn2+ and SrAl2O4:Eu2+, Dy3+ phosphors at 1 min after ceasing 254 nm and 365 nm excitation for 5 min, respectively (the response curve of human-eye photopic vision plotted as a reference).

Fig. 2. (a) PersL decay curves of the SrAl2O4:Eu2+, Dy3+ (LumiNova®GLL-300FFS), ZnGa2O4:Cr3+, Bi3+, MgGeO3:Mn2+, Bi3+ and AlN:Mn2+ phosphors as well as photographs of the (b) SrAl2O4 :Eu2+, Dy3+ (c) AlN:Mn2+ powders with the same weight (0.2 g) packed in plastic holders (8×8×5 mm3 ).

Fig. 3. (a) Wavelength-temperature (λ-T) contour plot of the AlN:Mn2+ phosphor (b) TL glow curve (c) TL emission spectrum at 310 K.

Highlights ▶ ▶ ▶ ▶ ▶

2+

AlN:Mn shows red persistent luminescence centered at 600 nm. AlN:Mn2+ exhibits a relatively narrow FWHM of 43 nm. The luminance at 60 min after ceasing 254 nm illumination is 0.65 mcd/m2. The luminance duration upon 0.32 mcd/m2 is over 110 min. A broad thermoluminescence glow curve is observed ranging from 100 K to 600 K.