Persistent Luminescence Materials for Information Storage Applications

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Trap Depth Engineering of SrSi2O2N2:Ln2+,Ln3+ (Ln2+ = Yb, Eu; Ln3+ = Dy, Ho, Er) Persistent Luminescence Materials for Information Storage Applications Yixi Zhuang, Ying Lv, Le Wang, Wenwei Chen, Tianliang Zhou, Takashi Takeda, Naoto Hirosaki, and Rong-Jun Xie ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17271 • Publication Date (Web): 26 Dec 2017 Downloaded from http://pubs.acs.org on December 26, 2017

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ACS Applied Materials & Interfaces

Trap Depth Engineering of SrSi2O2N2:Ln2+,Ln3+ (Ln2+ = Yb, Eu; Ln3+ = Dy, Ho, Er) Persistent Luminescence Materials for Information Storage Applications Yixi Zhuang,*,1 Ying Lv,1 Le Wang,*,2 Wenwei Chen,1 Tian-Liang Zhou,1 Takashi Takeda,3 Naoto Hirosaki,3 and Rong-Jun Xie*,1,3 1

College of Materials, Xiamen University, Simingnan-Road 422, Xiamen, 361005, P.R. China

2

College of Optical and Electronic Technology, China Jiliang University, Hangzhou 310018,

China 3

Sialon Group, Sialon Unit, National Institute for Materials Science, 1-1 Namiki, Tsukuba,

Ibaraki, 305-0044 Japan

KEYWORDS: advanced optical materials, SrSi2O2N2, persistent luminescence, trap depth engineering, information storage

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ABSTRACT: Deep-trap persistent luminescence materials exhibit unique properties of energy storage and controllable photon release under additional stimulation, allowing for both wavelength and intensity multiplexing to realize high-capacity storage in the next-generation information storage system. However, the lack of suitable persistent luminescence materials with deep traps is the bottleneck of such storage technologies. In this paper, we successfully developed a series of novel deep-trap persistent luminescence materials in the Ln2+/Ln3+-doped SrSi2O2N2 system (Ln2+ = Yb, Eu; Ln3+ = Dy, Ho, Er) by applying the strategy of trap depth engineering. Interestingly, the trap depth can be tailored by selecting different codopants, and it monotonically increases from 0.90 to 1.18 eV in the order of Er, Ho, and Dy. This is well explained by the energy levels indicated in the host-referred binding energy (HRBE) scheme. The orange-redemitting SrSi2O2N2:Yb,Dy and green-emitting SrSi2O2N2:Eu,Dy phosphors are demonstrated to be good candidates of information storage materials, which are attributed to their deep traps, narrow thermoluminescence glow bands, high emission efficiency and excellent chemical stability. This work not only validates the suitability of deep-trap persistent luminescent materials in the information storage applications, but also broadens the avenue to explore such kinds of new materials for applications in anti-counterfeiting and advanced displays.


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■ INTRODUCTION

Optical information storage shows the merits of low energy-consumption, long lifetime, and large capacity, taking an indispensable role in the modern information storage networks.1,2 However, with the fast-growing amount of digital data that is produced every year, the conventional optical information storage technologies (e.g. compact discs, digital video discs, or Blu-ray discs) are facing increasing challenges due to the limit of the two-dimensional spatial resolution. On the other hand, the multiplexing technology provides an exceptional approach to greatly increase the information size within the same recording volume through expanding the two-dimensional planar space to multi-dimensional spaces. Although a few concepts of multiplexed optical recording (e.g. multiplexing of intensity, wavelength, polarization, and time gate) have been proposed,3-8 the practical applications are still difficult because of the lack of suitable optical storage media. Therefore, it is quite essential to develop novel advanced optical materials for the applications of multi-dimensional information storage. Persistent luminescence materials have found widespread applications in glow-in-the-dark,9-12 bio-imaging,13-20 alternating current light emitting diodes (AC-LEDs),21-23 information storage,2426

and anti-counterfeiting,27,28 owing to their storage ability of excitation light and long-lasting

emissions after removal of the excitation sources. The unique properties of energy storage and super-long emission lifetime provide the persistent luminescence materials with prominent advantage over conventional technologies in the above-mentioned applications.29 By applying near-infrared (NIR) persistent luminescence nano-particles as biomarkers in the in vivo imaging, the background interference from tissue auto-fluorescence can be perfectly eliminated, allowing for a remarkably high signal-to-noise ratio and sensitivity for bio-sensing.30 The AC-LEDs



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technology using persistent luminescence phosphors to restrain the light flicker effect show great potentials to reduce the power consumption for the AC to DC conversion and the production cost for the required current rectifiers in the commercial LEDs.31 For the application to information storage, the input information is recorded into the persistent luminescence materials by capturing incident photons at energy traps, and the output signal is retrieved by releasing the trapped charge carriers and emitting photons. The output signal contains specific spectral characteristics, thus enabling intensity-multiplexing and wavelength-multiplexing for high-capacity information storage system. For example, the compound BaFBr:Eu2+ with blue emission are widely used as imaging storage media for X-ray irradiation.32 Recently, the Cr3+-doped LiGa5O8 persistent phosphors are discovered to be excellent optical storage media giving NIR emission.24 Nevertheless, only a few persistent luminescence materials have been reported as storage media, and they are not satisfied for the requirements in high-capacity information storage. The small number of persistent luminescence materials for the information storage is dominantly attributed to the commonly adopted trail-and-error method for exploring new materials. For the persistent luminescence materials working at room temperature (RT) such as the glow-in-the-dark or bio-imaging, the primary performance of the materials can be simply evaluated by the intensity and duration of persistent luminescence.33-35 However, in the case of information storage applications, a much larger trap depth, typically in the range of 0.8 ~ 1.6 eV, is required to retain the recorded information at RT for a sufficiently long duration.36-38 These deep-trap persistent luminescence materials give imperceptible persistent luminescence at RT, and thus they are usually neglected without a systematic thermoluminescence (TL) investigation. In order to explore new deep-trap persistent luminescence materials, it is of great importance to acquire a universal strategy that allows to predict the trap depth in a certain host. 4 ACS Paragon Plus Environment

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Recently, Dorenbos proposed the host-referred binding energy (HRBE) and vacuum-referred binding energy (VRBE) schemes, which provide a powerful tool to determine the energy levels of all divalent and trivalent lanthanide dopants in a single compound or in a group of solid-solution materials.39-41 On the basis of the HRBE and VRBE schemes, a material design method, known as band-gap engineering by controlling the locations of conduction band (CB) or valence band (VB), has been adopted to modulate the trap depth in several solid-solution compounds.42-47 However, the band-gap engineering generally shows side effects on the emission efficiency of luminescent center as well as the energy charging efficiency, resulting in a balance dilemma between the trap depth and the emission/charging efficiency.48-50 On the other hand, the trap depth engineering by appropriately choosing trivalent lanthanide codopants as trap centers in a certain host has emerged as an important strategy to achieve required trap depths. Till now, it is applied only to quite a few materials.51-54 In this work, for the first time, we successfully develop novel deep-trap persistent luminescence materials in the material system SrSi2O2N2:Ln2+,Ln3+ by the strategy of trap depth engineering. These persistent luminescence materials exhibit tunable trap depths in the range of 0.90 ~ 1.18 eV, narrow TL glow bands, high emission efficiency and excellent chemical stability, which meet the requirements for the applications in information storage. By using these materials, we demonstrate to store the patterned information through both the projection printing and the transfer printing, and then to retrieve the recorded information by means of the high-temperature thermal stimulation or NIR photo-stimulation. The work indicates that the investigated deep-trap persistent luminescence materials not only show great promise in the information storage, but also open new opportunities for applications in anti-counterfeiting and advanced displays.



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■ EXPERIMENTAL SECTION

Chemicals and Materials. Chemical reagents SrCO3 (3.5N) and SiO2 (4N) were purchased from Sinopharm Corporation; Si3N4 (α phase > 95%) was purchased from UBE Corporation; rare earth oxides Yb2O3(4N), Eu2O3 (4N), Dy2O3 (4N), Ho2O3 (4N), Er2O3 (4N), and Tm2O3 (4N) were supplied by Zhong-Nuo-Xin-Cai (Beijing) Corporation. Al2O3 crucibles (> 99%, Φ25 mm * 15 mm) and BN crucibles (> 98%, Φ25 mm * 15 mm,) were used as sample holders in the sintering process. A commercial red persistent phosphor (Ca,Sr)S:Eu as a reference material was purchased from Luming Technology Group. Synthetic procedures. All the samples were prepared by a two-step high-temperature solidstate reaction method.55 In the first step, stoichiometric SrCO3, SiO2, and Ln2O3 (Ln = Yb, Eu, Dy, Ho, Er, or Tm) were homogeneously mixed and sintered at 1300 ºC for 3 h in a horizontal tube furnace under a mixed gas flow of H2/N2 (5%/95%) to obtain Sr2SiO4:Ln precursors. After cooling down to RT, the obtained powders were ground and further mixed with appropriate amounts of Si3N4. The obtained mixtures were compacted into pellets with a size of Φ10 mm * 2 mm under a uniaxial pressure of 30 kN, then loaded into BN crucibles with lids, and finally calcined at 1550 ºC for 6 h in the horizontal tube furnace under the H2/N2 atmosphere. Without pelletizing the raw materials, phosphor powders were also prepared under the same firing condition. Basically, two types of rare earth elements were codoped into the matrix to obtain SrSi2O2N2:LnA,LnB compounds (hereinafter denoted as SSON:LnA,LnB for simplicity). Here, LnA is Yb or Eu (2 mol%) and LnB is one of species from Dy, Ho, or Er (1 mol%). Samples doped with only LnA or codoped with LnA and two or three species of LnB were also prepared as


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references. The detailed compositions of all the samples used in this study are listed in Table S1, Supporting Information (SI). Structural and optical characterizations. Powder X-ray diffraction (XRD) patterns of the phosphors were recorded by an X-ray diffractometer (Bruker, D8 ADVANCE) with Cu Kα radiation at an interval of 0.02 º with a scanning speed of 10 º/min. Photoluminescence (PL) and photoluminescence excitation (PLE) spectra were measured in a fluorescence spectrophotometer (Edinburgh Instrument Ltd., FLS980). Temperature-dependent PL spectra were measured by using a charge-coupled device (CCD, Ocean Optics, USB2000+) and a cooling/heating stage (Linkam Scientific Instruments, THMS600E). The persistent luminescence decay curves were recorded in a home-made measurement system (Figure S1, SI). A xenon lamp (300 W) or a 254 nm Hg lamp (15W) was used as the excitation source. A filter-attached photomultiplier tube (PMT, Hamamatsu, R928P) and a luminance meter (Evenfine, LM-5) simultaneously monitored the persistent luminescence intensity after ceasing the excitation source. A NIR laser at 980 nm (100 mW) was projected on the surface of the sample as a photo-stimulation source. TL glow curves were recorded via another home-made measurement system (Figure S2, SI). In a typical TL measurement, the sample was first cooled to 80 K and irradiated by the excitation source for 5 min. After the excitation source was removed and waited for 30 s, the sample was heated to 600 K at a certain heating rate to record the TL signals. The obtained TL glow curves were corrected by thermal quenching curves of PL. The above-mentioned home-made measurement systems were driven by LabVIEW-based PC programs. Photographs of the samples were taken with a digital camera (Canon, EOS 5D Mark II) in an all-manual mode. The following parameters were kept constant: ISO value -1000, aperture value -2.8, and white balance -sunlight.



Evaluation of chemical stability. 200 mg of phosphor powders (particle size ~ 10 µm) were immersed in 10 ml deionized (DI) water, kept at RT for different time (from 1 to 16 h), and finally dried in an oven at 80 ºC. Persistent luminescence decay curves of the treated samples were recorded to evaluate their chemical stability against moisture and thermal attacks. Applications in information storage. A pellet sample was covered with a photomask and exposed to UV light to record a specific pattern on the surface. After removing the UV light and the photomask, photon emission from the sample was real-time monitored by a PMT detector (for reading the emission intensity) and a digital camera (for reading the emission color and the luminous pattern). In order to retrieve the recorded information, the sample was either (i) heated to a higher temperature (typically 100 ºC), or (ii) exposed by a 980-nm laser spot (NIR photostimulation). In another set of experiment, phosphor powders were mixed with silica gel precursors (in liquid) in a weight ratio of 1:2 to prepare the fluorescent ink. The fluorescent ink was used to print characters (e.g. Roman letters and Arabic numerals) on an aluminum foil via a transfer printing method. Then the printed ink was dried at 150 ºC for 3 h in an oven. Similar to the above experiment, the printed characters were exposed to UV light beforehand. After UV-off, the emission from the printed characters was monitored at RT or at an elevated temperature.

■ RESULTS AND DISCUSSION

Crystalline phase and electronic structure. The XRD analysis indicates that a pure crystalline phase of SrSi2O2N2 is obtained in all the samples by the two-step sintering method (Figure S3, SI). The crystalline structure of SrSi2O2N2 in the triclinic unit cell has been comprehensively investigated elsewhere.56,57 As schematically depicted in Figure 1a, the SrSi2O2N2 crystal shows a 8 ACS Paragon Plus Environment

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typical layer structure, in which [Si2O2N2]2- anionic groups and Sr2+ cations are alternately arranged. In the [Si2O2N2]2- layers, each N atom is connected to three neighboring Si tetrahedron centers, and the O atoms are exclusively bonded to Si atoms. In the cation layers, the Sr atoms are coordinated to six O atoms and one N atom, showing a coordination number (CN) of 6 + 1. Considering the similar ionic radius, the doped rare earth ions (0.95 ~ 1.20 Å, CN = 7) are incorporated into Sr2+ sites (1.21 Å, CN = 7) in SrSi2O2N2. The incorporated Yb or Eu atoms dominantly exist in the divalent state under the reducing sintering atmosphere and work as luminescence centers. On the other hand, the trivalent state is more stable than the divalent one for Dy, Ho, and Er atoms, and they principally act as trap centers (Figure 1b). The role of the doped rare earth ions is discussed in detail in the following section.



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Figure 1. (a) Crystal structure of SrSi2O2N2 projected on the (001) plane. The layered SiON3 tetrahedra are separated by Sr atoms. (b) Coordination geometry around the Sr atoms. Each Sr atom is surrounded by six O atoms and one N atom. The substitutions of divalent Yb2+/Eu2+ ions in the Sr2+ sites form luminescent centers (L); while the incorporated trivalent Dy3+/Ho3+/Er3+ ions act as trap centers (T).

The PL spectra of SSON:Yb,Dy, SSON:Yb,Ho and SSON:Yb,Er under the 395 nm excitation are depicted in Figure 2a. All of them give a broad emission band centered at 620 nm due to Yb2+: 5d1-4f transition.58 Small dips in the PL spectra are possibly attributed to the reabsorption by the trivalent codopant of LnB, such as Ho3+: 5I8 → 5I5 at 650 nm and Er3+: 4I15/2 → 4F9/2 at 650 nm. Nevertheless, typical 4f-4f sharp emissions are not observed. The SSON:Eu,Dy, SSON:Eu,Ho and SSON:Eu,Er samples show a broad emission band at 540 nm due to Eu2+: 5d1-4f radiative transition (Figure 2b).59,60 The PL spectra indicate the Yb2+ and Eu2+ ions with parity-allowed transitions are the major luminescent centers in the LnA-LnB-codoped samples. The PLE spectra of the SSON:Yb,Dy and SSON:Eu,Dy samples contain multiple bands covering from UV to the blue region (Figure 2c-d). From the PLE spectra, the energy differences between the ground state and the lowest excited state for Yb2+ and Eu2+ in SrSi2O2N2 are 2.71 eV (458 nm) and 2.70 eV (460 nm), respectively. We also investigated the effect of temperature on the PL intensity (Figure 2e-f). The thermal quenching temperature (T50%, the temperature at which the intensity is quenched to 50 % of the maximum) is 340 K for the Yb2+-activated SrSi2O2N2 and 520 K for the Eu2+-activated sample.


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Figure 2. (a-b) PL spectra in SSON:Yb,LnB and SSON:Eu,LnB (LnB = Dy, Ho, or Er). (c-d) PLE spectra in SSON:Yb,Dy and SSON:Eu,Dy. The monitoring wavelength was 620 and 540 nm for SSON:Yb,Dy and SSON:Eu,Dy, respectively. (e-f) Temperature dependence of PL spectra in SSON:Yb,Dy and SSON:Eu,Dy. The integrating PL intensity as a function of temperature from 80 to 600 K is given in the inset. T50% is defined as the temperature at which the intensity is quenched to 50 % of the maximum.



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According to the fundamental absorption threshold of a non-doped compound (Figure S4, SI), the exciton creation energy (Eex) of SrSi2O2N2 is estimated to be 6.18 eV. The band gap energy is obtained by adding the exciton binding energy, which is approximately 8 % of Eex as a rule of thumb.61 Also, the PLE spectrum of Tm3+-doped SrSi2O2N2 was measured, which shows an intense charge transfer band (CTS) at 4.96 eV (Figure S5, SI). Finally, the PLE spectra of a Ce3+doped SrSi2O2N2 sample were recorded to estimate the centroid shift of Ce3+ 5d levels (εc) as well as the Coulomb repulsive force of Eu3+/Eu2+ (U(6,A)) (Figure S6, SI). The estimated value for U(6,A) was 6.45 eV. Based on the above experimental data, we establish the HRBE and VRBE schemes for Ln2+/Ln3+-doped SrSi2O2N2 (Figure 3). The estimated values of the key parameters (Eex, U(6,A), Efd1 of Eu2+, CTS of Tm3+) to construct the schemes are given in the caption of Figure 3. They are also highlighted as single-arrow lines in Figure 3. The HEBE and VRBE schemes provide important information on the energy level locations for the whole family of rare-earth-doped SrSi2O2N2 phosphors. For example, the HRBE scheme indicates that the energy of the electronic transition from 4f to 5d1 state in Yb2+ is 2.71 eV. This value is well consistent with the PLE spectra in SSON:Yb,Dy. Also, the energy gap from the 5d1 state to the bottom of CB is 0.73 and 0.30 eV for Eu2+ and Yb2+ as shown in Figure 3, which explains a higher thermal quenching temperature of SSON:Eu,Dy than SSON:Yb,Dy (Figure 2ef)). Furthermore, we notice that the 4f GS in Dy2+/Ho2+/Er2+ is located at 0.8 ~ 1.2 eV below the bottom of CB and the energy levels monotonically rise in the order of Dy, Ho, and Er (highlighted in Figure 3). If the trap levels due to Ln3+-codoping possess a definite correlation with the GS in Ln2+ as presented in several Eu-Ln-codoped compounds,51,52 the accurately determined GS of Ln2+ in the HRBE scheme may offer useful guidelines to adjust the trap depth by selecting different Ln3+ ions (i.e. trap depth engineering). Specifically, the monotonic increase in the GS of 12 ACS Paragon Plus Environment

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Dy2+/Ho2+/Er2+ suggests that it could be a good way to gradually modulate the trap depth in SrSi2O2N2.

Figure 3. HRBE (left ordinate) and VRBE (right ordinate) schemes of Ln2+/3+-doped SrSi2O2N2. The 4f ground states (Ln2+:4f) and the lowest 5d excited states (Ln2+:5d) of Ln2+ are labeled by black dots and hollow circles and connected by solid curves. The 4f ground states of Ln3+ (Ln3+:4f) are connected by a dashed curve. The exciton creation energy of the host (Eex: 6.18), the Coulomb repulsive force of Eu3+/Eu2+ (U(6,A): 6.45), the 4f-5d1 transition energy in Eu2+ (Efd1: 2.70), and the CTS energy of Tm3+ (CTS: 4.96) as obtained from experimental results are used to construct the HRBE and VRBE schemes.

Trap depth engineering. The group of three Yb2+-activated samples (marked as 1 2 3 in Figure 4a-b) show almost the same orange body color under natural light (NL) and red-orange emission under the 395 nm excitation (UV). Those Eu2+-activated samples present the similar yellow-green 13 ACS Paragon Plus Environment


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body color and green emission under the 395 nm excitation (4 5 6 in Figure 4a-b). However, when the UV light is turned off, the samples codoped with various trivalent rare earths (LnB) show great differences in the persistent luminescence property. The persistent luminescence in SSON:Yb,Dy after the excitation turn-off is rather weak at RT (Figure 4c-f). While the SSON:Yb,Ho sample gives intense red-orange persistent luminescence (the spectra depicted in Figure 4g, identical with the PL spectra) for a long duration. For the SSON:Yb,Er pellet, extremely bright persistent luminescence is observed in the first several seconds but it quickly decays in a short time. Also, in the group of Eu2+-activated phosphors, we find the similar effect of codopant on the brightness of green persistent luminescence (4 5 6 in Figure 4c-f).

Figure 4. Photographic images of the pellet samples under natural light (a), under UV excitation (b), and 10s (c), 1 min (d), 5 min (e), and 20 min (f) after removal of the excitation source at RT. The samples from [1, 2, 3] and [4, 5, 6] are [SSON:Yb,LnB, LnB = Dy, Ho, Er for 1, 2, 3] and [SSON:Eu,LnB, LnB = Dy, Ho, Er for 4, 5, 6], respectively. The UV excitation sources for 14 ACS Paragon Plus Environment

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SSON:Yb,LnB and SSON:Eu,LnB are 395 nm and 254 nm. (g-h) Persistent luminescence spectra of SSON:Yb,Ho (g) and SSON:Eu,Ho (h). Exposure time of the digital camera is 1/200 s for (a-b) and 2 s for (c-f).

Figure 5. Persistent luminescence decay curves in SSON:Yb,LnB (a) and SSON:Eu,LnB (b) (LnB = Dy, Ho, and Er). The samples are irradiated by a Xe lamp (a) or a Hg lamp dominantly at 254 nm (b) for 20 s before recording. The insets show enlarged parts in the beginning stage.



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The persistent luminescence decay curves in the SSON:Yb,LnB and SSON:Eu,LnB samples were measured at RT to quantitatively compare their persistent luminescence properties (Figure 5). Here we adopt the definition of persistent luminescence decay time (τPersL) as the time when the intensity decays to 0.32 mcd/m2. After irradiated by a Xe lamp for 20 s, the initial intensity of the persistent luminescence in SSON:Yb,Dy is less than 10 mcd/m2 and drops below 0.32 mcd/m2 in several minutes. The SSON:Yb,Ho sample shows excellent red-orange persistent luminescence performance at RT, giving a τPersL of approximately 200 min, 2 times longer than the decay time of the commercial red persistent phosphor (Ca,Sr)S:Eu. For the SSON:Yb,Er sample, the initial intensity is more than 1000 mcd/m2 (the highest value in this group), but the τPersL is less than 50 min because of a fast decay rate. In the case of Eu2+-activated phosphors, the recorded τPersL in the SSON:Eu,Ho pellet reaches at 360 min after irradiated by a 254 nm lamp, suggesting its promising application for glow-in-the-dark. It should be noted that the Xe lamp dominantly at visible light is much less effective for charging the persistent luminescence in SSON:Eu,Ho (τPersL, 25 min, see Figure S7, SI), which may be due to the large energy gap (0.73 eV) from the 5d1 state of Eu2+ to the bottom of the CB. Lastly, the initial intensity of the persistent luminescence in the SSON:Eu,Dy sample is approximately 100 mcd/m2 (1/100 of that in SSON:Eu,Er) and the intensity after irradiation is always much lower than SSON:Eu,Ho and SSON:Eu,Er. In order to reveal the depth of traps introduced by LnB, TL glow curves were systematically investigated. The SSON:Yb sample presents very weak TL bands (Figure S8, SI). These bands may be originated from unknow intrinsic defects or impurities. After codoping with Dy, Ho, or Er, the TL intensity of Yb2+ emission is approximately 100 times higher than that of the SSON:Yb sample (Figure S8, SI), indicating that a large number of traps are created by the LnB-codoping. 16 ACS Paragon Plus Environment

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Figure 6a-b gives the normalized TL glow curves in SSON:Yb,LnB and SSON:Eu,LnB. The two samples codoped with Er (i.e. SSON:Yb,Er and SSON:Eu,Er) show similar TL glow curves with a narrow band with a maximum around 280 K. The peaks gradually shift to approximately 320 K and 380 K in samples codoped with Ho and Dy, respectively. Thus, the peak temperature of the TL glow curves in the SSON:LnA,LnB phosphors is largely determined by the species of codopant (LnB) and independent of the luminescent center (LnA). Interestingly, when two or three species of codopants were involved in the compound (e.g. SSON:Yb,Dy+Ho), the TL glow curves give multiply (two or three) overlapping bands corresponding to those presented in the singly-codoped samples (Figure 6c-f). The multiple TL bands not only verify that the traps for different TL peak temperatures are originated from specific codopants, but also reveal that the various codopants may independently act with the luminescent center and create different traps in the same host.



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Figure 6. TL glow curves of SSON:Yb,LnB and SSON:Eu,LnB. LnB is referred to single element of Dy, Ho, Er in (a-b), two elements in (c-e), and three elements in (f). The heating rate is set as 10 K/min for all the measurements.

The trap depths in the SSON:Yb,LnB and SSON:Eu,LnB (LnB = Dy, Ho, and Er) samples were evaluated by employing the following formula:62,63 ∙ ∙

∙ exp

∙

(1)


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Here, β (K/s) is the heating rate; E (eV) is the trap depth; kB is the Boltzmann constant; Tm (K) is the peak temperature in the TL glow curves; s (s-1) is the frequency factor. By plotting ln(Tm2/β) against 1/(kB·Tm), a straight line with slope E can be determined (i.e. Hoogenstraaten method). Figure 7 shows the heating rate plot for the six samples (the original TL data are given in Figures S9-14, SI). The plotting lines for the samples with the same codopants (LnB) are close and parallel to each other. The trap depths E and frequency factors s as derived from the heating rate plot are compiled in Table 1. As expected, the two Dy-codoped samples show a similar trap depth of 1.17 ~ 1.18 eV, and the trap depths are 1.04 ~1.05 eV and 0.90 eV in the samples codoped with Ho and Er, respectively. It is worth to mention that the trap depths as derived from the TL analysis are in a good agreement with the energy differences between the GS of LnB2+ (LnB = Dy, Ho, and Er) and the bottom of CB as indicated in the HRBE scheme. Accordingly, we confirm the strategy of the trap depth engineering in the Yb2+ and Eu2+-activated SrSi2O2N2, and successfully develop a series of novel deep-trap persistent luminescence materials.



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Figure 7. Heating rate plots of SSON:Yb,LnB and SSON:Eu,LnB (LnB = Dy, Ho, or Er). The heating rates are 2, 5, 10, 20, and 40 K/min for each case. The original TL data are given in Figures S9-14, SI. Table 1. Trap depth E (eV) and frequency factor s (s-1) as derived from the heating rate plots. Sample

E (eV)

s (1014 s-1)

SSON:Yb,Dy

1.17 ± 0.02

1.32 ± 0.04

SSON:Yb,Ho

1.04 ± 0.02

3.83 ± 0.09

SSON:Yb,Er

0.90 ± 0.01

2.45 ± 0.04

SSON:Eu,Dy

1.18 ± 0.01

1.09 ± 0.01

SSON:Eu,Ho

1.05 ± 0.02

2.34 ± 0.09

SSON:Eu,Er

0.90 ± 0.01

1.52 ± 0.02

Also, we take the SSON:Yb,Ho phosphor as an example to evaluate its chemical stability against moisture and thermal attacks (Figure 8a). After immerged in DI water for 16 h and dried out at 80 ºC in air, the persistent luminescence performance in the SSON:Yb,Ho phosphor is hardly changed (Figure 8b). On the contrary, an obvious degradation is found in the commercial red persistent phosphor (Sr,Ca)S:Eu (Figure 8c-d). The excellent chemical stability should be attributed to the rigid network of highly condensed [Si2O2N2]2- layers in the crystalline structure of SrSi2O2N2.


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Figure 8. (a) Schematic illustration of the experimental procedure for the chemical stability evaluation. (b-c) Variation of persistent luminescence decay curves of SSON:Yb,Ho (b) and the commercial red phosphor (Ca,Sr)S:Eu (c) after immerged in DI water for different times. (d) Persistent luminescence intensity of SSON:Yb,Ho and (Ca,Sr)S:Eu as a function of immersion time. The persistent luminescence intensity was taken at the time-point 10 min after ceasing the excitation source. Applications to information storage. An essential prerequisite for the information storage application by using persistent luminescence materials is the dual nature of the detrapping process (i.e. charge carriers should be frozen at RT and released under larger stimulations), which thus 21 ACS Paragon Plus Environment


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calls for materials having deeper traps than conventional RT persistent luminescence materials. As discussed above, deep traps in the range of 0.8~1.2 eV are evidently introduced into SrSi2O2N2 by codoping Er, Ho, or Dy. Also, the deep-trap persistent luminescence materials exhibit narrow TL glow bands (narrow trap distribution) and high emission efficiency. It indicates that these materials would have great potentials for the applications in information storage. As schematically illustrated in Figure 9a, information in a specific pattern was stored on the surface of pellet samples by UV irradiation passing through a photomask (projection printing method) (ii). After removing the UV light and the photomask, the samples were kept at RT for 60 mins (iii), and finally heated to 100 ºC (iv). Figure 9b-c plots the real-time monitored emission intensity and Figure 9e-f shows the photos of luminous patterns in SSON:Yb,LnB and SSON:Eu,LnB (LnB = Dy, Ho, and Er). The Er-codoped samples (in both cases of Yb2+ and Eu2+activated samples) give the most intense emission in the first several minutes, while the luminous patterns from the Dy-codoped samples are almost unrecognizable in the first 60 mins. When the sample is heated to 100 ºC (from 60 min), the emission is greatly enhanced in all samples. Interestingly, by comparing the emission under the high-temperature thermal stimulation (100 ºC), the emission intensity increases in the order of Er, Ho, Dy. This is the exact opposite to the intensity as detected at RT (in the first several minutes). Therefore, one can conclude that SSON:Yb,Dy and SSON:Eu,Dy are the superior candidates for information storage applications, owing to their sufficiently deep traps that enable to effectively restrain the detrapping at RT and thus ensure intense emissions as optical signals for the information readout. In an extended experiment, we excited the SSON:Eu,Dy sample with 254 nm UV light for 20 s and shielded it in the dark. After keeping it at RT for 15 days, intense emission signals (> 3 mcd/m2) are clearly detected under the high-temperature thermal stimulation (Figure S15, SI). 22 ACS Paragon Plus Environment

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Figure 9. (a) Schematic illustration of the application in information storage using deep-trap persistent luminescence materials. (b-c) Decay curves of the luminous patterns on SSON:Yb,Ln (b) and SSON:Eu,Ln (c). The samples were heated from RT to 100 ºC at 60 min. (d-e) Photographic images of the pellets at RT and 100 ºC. Exposure time of the digital camera is 1.6 s (d) and 0.5 s (e). The photos marked by RT and 100 ºC were taken at approximately 1 min and 61 min after removal of the UV light. Scale bar, 5 mm. The NIR photo-stimulation has been applied as an alternative way to release charge carriers in deep traps.64,65 As shown in Figure 10a, a 980-nm laser spot is projected on the surface of the SSON:Eu,Dy sample from 30 min after removing the UV light and photomask. The laser is driven 23 ACS Paragon Plus Environment


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by a programmed power supply to produce periodic laser output (repeating ON and OFF in every 5 min). When the laser is turned on, we observe a bright emission depicting a part of the recorded pattern inside the laser spot area (Figure 10b). Once the NIR photo-stimulation is turned off, the emission intensity suddenly drops to a low value (Figure 10c). Different from the hightemperature thermal stimulation, the NIR photo-stimulation offers a localized and fast-response approach for the information readout, which shows advantages in the practical applications.

Figure 10. (a) Persistent luminescence decay curve of the luminous patterns in the SSON:Eu,Dy pellet with NIR photo-stimulation. A 980-nm NIR laser in a pulsed mode (repeating ON and OFF in every 5 min) was projected on the surface of the sample from 30 to 60 min. Photographs of the sample were taken at 31 (b) and 34 min (c) when the NIR laser was turned ON and OFF, respectively. The laser spot was schematically depicted in a red circle in (i). A decay curve without NIR photo-stimulation was recorded as a reference (black curve).


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Furthermore, we employ the obtained persistent luminescence phosphors to prepare fluorescent ink and print characters on aluminum foils (transfer printing method). As shown in Figure 11a, under UV irradiation, the printed letters “X M U” and numerals “1 2 3” give bright emissions from the luminescent centers of Eu2+ and Yb2+. The emissions totally disappear after the UV light is turned off. However, when we heat the printed characters to 100 ºC, emitting characters of “M” and “2” which are made of Dy-codoped phosphors (SSON:Eu,Dy and SSON:Yb,Dy, respectively) are clearly read in the dark. On the contrary, the other characters made of SSON:Eu and SSON:Yb without deep traps are not identified. The self-emission in the dark, which is triggered by a higher temperature, manifests its potential applications in anti-counterfeiting and advanced displays by using temperature as a decoding key. On the basis of the energy levels determined in the HRBE scheme, the mechanism for the electron trapping and detrapping processes in the deep-trap persistent phosphors SSON:Eu,Dy and SSON:Yb,Dy is proposed (Figure 11b). First of all, under the UV irradiation, electrons from luminescent centers (Yb2+ or Eu2+) are excited to the CB or 5d levels below the CB with small gaps (i). The electrons may migrate freely in the CB and subsequently be captured by traps (ii). The TL analysis indicates that the Dy-codoping creates electron traps in depth of ~1.18 eV. Under the thermal stimulation at RT, the releasing rate from these deep trap is quite low (iii), which is evidenced by the decay curves in Figure 9b-c. Consequently, the deep-trap-containing materials can be used as information storage media. By applying the high-temperature thermal stimulation (iv) or the NIR photo-stimulation (v), the trapped electrons can be rapidly released and finally display the stored information in the form of photon emissions.



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Figure 11. (a) Photographic images of transfer-printed characters by using fluorescent ink. The fluorescent ink used for printing “X, M, and U” are made of the SSON:Eu, SSON:Eu,Dy, and SSON:Eu phosphors; and those for “1, 2, and 3” are SSON:Yb, SSON:Yb,Dy, and SSON:Yb, respectively. Exposure time of the digital camera is 1/100 s for the left photos, and 1/4 s for the middle and right photos. (b) A proposed mechanism for the electron trapping and detrapping processes in the deep-trap persistence luminescence materials. The gray broken line depicts the GS of Ln2+ as determined in the HRBE scheme.


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■ CONCLUSION In summary, we reported the HRBE and VRBE schemes in the Ln2+/Ln3+-doped SrSi2O2N2 compounds for the first time. Inspired by the monotonic variation of the energy level locations, we applied the concept of trap depth engineering in the Ln2+/Ln3+-doped SrSi2O2N2 system to develop deep-trap persistent luminescence materials. We successfully synthesized two groups of novel persistent luminescence materials: orange-red-emitting SrSi2O2N2:Yb,LnB and green-emitting SrSi2O2N2:Eu,LnB (LnB = Dy, Ho, and Er). These materials showed excellent chemical stability and contained a large number of traps with tunable depths in the range of 0.8 ~ 1.2 eV. The trap depth increased in the order of Er, Ho, and Dy, which is well consistent with the energy levels indicated in the HRBE scheme. Furthermore, we demonstrated to use these deep-trap persistent luminescence materials in information storage. Due to the dual nature of freezing excited electrons at RT and releasing them under larger stimulations (high-temperature thermal stimulation or NIR photo-stimulation), the developed SrSi2O2N2:Yb,Dy and SrSi2O2N2:Eu,Dy persistent luminescence materials with the large trap depth showed great promise as optical information storage media.

■ ASSOCIATED CONTENT

Supporting Information. Compositions of the prepared samples; measurement systems for persistent luminescence decay curves and TL glow curves; XRD patterns; Tauc plotting for the non-doped compound; PLE and PL spectra of SSON:Tm; PLE spectra of SSON:Ce; effect of different light source on the



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persistent luminescence decay curves in SSON:Eu,Dy; comparison of the TL glow curves in SSON:Yb and SSON:Yb,LnB; TL glow curves of SSON:Yb,LnB in different heating rates.

■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

■ Acknowledgment This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51572232, 51561135015, 51502254), National Key Research and Development Program (2017YFB0404301). We thank Dr. Z. Shi for compiling the LabVIEW-based programs in the automatic decay curves and TL measurements.


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