Tailoring multi-dimensional traps towards rewritable multi-level optical

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Tailoring multi-dimensional traps towards rewritable multi-level optical data storage Dong Liu, Lifang Yuan, Yahong Jin, Haoyi Wu, Yang Lv, Guangting Xiong, Guifang Ju, Li Chen, Shihe Yang, and Yihua Hu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b13011 • Publication Date (Web): 02 Sep 2019 Downloaded from pubs.acs.org on September 2, 2019

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ACS Applied Materials & Interfaces

Tailoring multi-dimensional traps towards rewritable multi-level optical data storage Dong Liu,† Lifang Yuan,†, ‡ Yahong Jin,*, †, § Haoyi Wu,† Yang Lv,† Guangting Xiong,† Guifang Ju,† Li Chen,† Shihe Yang,*, §, ‖ Yihua Hu.*, † †

School of Physics and Optoelectronic Engineering, Guangdong University of Technology, WaiHuan Xi Road, No. 100, Guangzhou 510006, China. ‡ Experimental Teaching Department, Guangdong University of Technology, WaiHuan Xi Road, No. 100, Guangzhou 510006, China. § Department of Chemistry, The Hong Kong University of Science and Technology, Kowloon, 999077 Hong Kong, China. ∥

Guangdong Key Lab of Nano-Micro Material Research, School of Chemical Biology and Biotechnology, Shenzhen Graduate School, Peking University, 518055, Shenzhen, China ABSTRACT: In the current “big data” era, the state-of-the-art optical data storage (ODS) has become a front-runner in the competing data storage technologies. As one of the most promising methods for breaking the physical limitation suffered by traditional ones, the advance of optically stimulated luminescence (OSL)-based optical storage technique is now still limited by the simultaneous single-level write-in and read-out in a same spot. In this work, to bridge the data-capacity gap, we report for the first time a novel and promising non-physical multidimensional OSL-based ODS flexible medium for erasable multi-level optical data recording and reading. We tailor multi-dimensional traps with discrete, narrowly distributed energy levels through (multi-)co-doping of selective trivalent rare-earth ions into Eu2+-activated barium orthosilicate. Upon UV/blue light illumination, information can be sequentially recorded in different traps assisted by thermal cleaning with an increase of storage capacity by orders of magnitude, which is addressable individually in the whole domain or bit-by-bit mode without crosstalk by designed thermal/optical stimuli. Remarkably, good data retention and robust fatigue resistance have been achieved in recycle data recording. Insight is forged from charge carrier dynamics and interactions with traps for a universal method of data storage, and proof-of-concept applications are also demonstrated, thereby providing the way to not only rewritable multi-level ODS but also high-security encryption/decryption. Keywords: silicates · optical data storage · solid-state reactions · trap engineering · optically stimulated luminescence INTRODUCTION

storage. Nevertheless, the expansion of storage capacity is still constrained by the reliance on physically adding storage layers.13-19 Optically stimulated luminescence (OSL) relies on stimulated relaxation when the pre-excited material with metastable trapped charge carriers turns back to the thermodynamic equilibrium upon external light irradiation.20-22 The unique feature of controllable storage and release of charge carriers promises application in erasable ODS. This concept was successfully implemented earlier by utilizing OSL of rareearth doped alkaline (-earth) halides or sulfides.23-26 However, the applications were ultimately constrained to only x-ray storage and limited due to the intrinsically inferior moisture resistance. In recent years, rear-earth and transition metal ions doped chemical stable metallic oxy-salts have attracted keen attention due to their excellent OSL for ODS (write-in with UV/blue light). In 2013, Liu et al. reported the superior NIR OSL and optical write-in/read-out from Cr3+-doped LiGa5O8.27 In 2018, Zhuang et al., successfully demonstrated multidimensional ODS via emission intensity/wavelength multiplexing and deep trap depth engineering enabled ODS.28-30 Taking advantage of the ODS property and the

As the society steps into the 21st century-an age of “big data”, the growth of storage capacity is at least 3 times outpaced by the super-exponential increase of data generation.1-4 At this unprecedented pace of growth, the total massive amount of digital data will approach at least 1011 TB by 2025, and such an information explosion forces scientists and engineers to develop new technologies that can dramatically boost the data storage capacity.5,6 Currently, there are primarily three mainstream types of modern information storage techniques, i.e., semiconductor- based, magnetically and optically based ones. Thus far, optical data storage (ODS) shows unparalleled advantages, including fast speed, high storage density, long lifetime, low cost, easy portability and lower power consumption, thereby promising to be the next-generation storage method.6-8 In the past decades, tremendous efforts have been made along this line. For example, multiplexing approaches by exploiting the parameter space of wavelength, polarization, fluorescence intensity/lifetime and spatial domains,9-12 etc., to tackle the challenges of memory cells miniaturization and diffraction limitation, advocating the evolution from 2D to 3D and 5D optical

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by a conventional high-temperature solid-state reaction method. These starting materials were weighed according to the stoichiometric ratio with 8% mol BaF2 used as flux. After mixed and ground thoroughly for 0.5 h, the mixtures were firstly heated at 1000 ℃ for 6 h in ambient atmosphere. Then, the mixtures were calcined again at 1320 ℃ for 4 h in a reductive atmosphere (85% N2 + 15% H2). After natural cooling down to RT (room temperature) and re-ground again, the final products were obtained for the subsequent measurements. The fabrication of flexible phosphor films. The synthesized phosphors were mixed with silica gel precursors (in liquid) in a weight ratio of 1:2 and then stirred slightly to form viscous phosphor slurries. Firstly, the phosphor slurries were carefully cast on corrosion resistant plate to form a flat and smooth surface. Secondly, the baffle-plates is used to control the size (7х7 cm) and the thickness (~1 mm) (Figure S18). Finally, after heating up to 150 ℃ for 5 min in an oven, the flexible phosphor films were fabricated. In this process, the heating rate is slow to remove air bubbles. Characterization. The phase structures of as-prepared samples were characterized by X-ray diffraction (XRD) using an XD-2 powder diffractometer (XRD Beijing, Puxi XD-2) with Cu Kɑ irradiation (1.5406 Å) at a continuous scanning step of 0.02° in the 2 range from 10° to 80°. The morphology, particle size and elemental mapping of the powder samples were characterized by a scanning electron microscope (SEM, Hitach-SU8220 Instruments). Photoluminescence excitation and emission spectra were measured using a FLS-980 fluorescence spectrophotometer (Edinburgh Instruments Ltd, UK) equipped with a photomultiplier tube operating at 400 V, using a 450 W Xe lamp as the excitation source. The TL curves were recorded from RT to 350 ℃ by an SL08-L thermoluminescent dosimeter with a heating rate of 2 ℃ s-1. Prior to the TL measurements, the samples were excited by UV (254 nm) irradiation for 5 min. Multi-level optical data write-in and read-out. The phosphor films were covered by a photomask of specific pattern and irradiated for 5 min by UV light (254 nm). After the removal of the UV light and photomask, the desired information were recorded. To retrieve the multi-level recorded whole domain information, the phosphor films were heated up target temperatures. Moveover, a commercialized microlaser engraving machine also was used to demonstrate the application of optical data storage and readout on prepared phosphor. In this process, 450 nm laser was used as write-in light source, while 980, 808 nm and 635 nm lasers were used as read-out light source to access the information stored in different traps in a bit-by-bit mode. The images of optical information read-out were taken by a commonly home used smartphone (Huawei, Honor 9).

isolated deep-trap, the primary applications of OSL materials to data encryption/dynamic multimode anticounterfeiting and stable ODS were also realized.31-33 Just this year, a further study to improve the ODS property has been reported on BaSi2O5:Eu2+, Nd3+ PiG by introducing deep traps within a narrow energy distribution (0.16 eV).34 To solve the signal interference in read-out caused by persistent luminescence, a quasilayer-structure Bi3+-doped Ca3Ga4O9 was prepared, realizing anomalous large difference between persistent luminescence and OSL as well as the improved signal to noise ratio (SNR).35 It is commonly recognized that OSLbased ODS strongly relies on the charge carrier trapping and de-trapping by trap states. In principle, excellent OSL-based ODS must fulfill the following four criterions: (1) multiple traps as non-physical multi-storage layers are necessary for multi-level ODS, which can greatly extend the ODS capacity; (2) a deep trap depth is required to prevent the data loss; (3) discrete traps ought to be crosstalk-free to ensure the integrity of different information stored in different traps; (4) a narrow distribution of trap depth. Although it seems straightforward to design a superior OSL-based ODS medium with high capacity, to date, all previous work, including our own on bright OSL with superior durability from a flexible film for ODS, only focused on a single kind of traps for single-level ODS.36,37 The main hindrance comes from the difficulty in intentional controllability of traps and the insufficient understanding of the nature of the traps. Therefore, developing an OSL material implemented with various suitable discrete traps is expected to hugely boost the storage capacity without physically adding storage layers, and be of theoretical significance. Herein, building on our previous work, we report a convenient method to fine-tailor the traps distribution by reasonable co-doping and managing trivalent lanthanide ions. In this design, discrete traps with narrow distributions are intentionally created by controllable co-doping of Dy3+, Tm3+ and Ho3+ into Eu2+doped Ba2SiO4 (BS) flexible film. Consequently, with the illumination of UV/blue light, multi-level data can be recorded in discrete traps with an increase of storage capacity by orders of magnitude, and then read out sequentially by heating/light stimulation. Long term and robust fatigue resistance of ODS are also achieved owing to the proper trap distribution purposely designed. This comprehensive work is expected to break the physical spatial limitation for high-capacity ODS and shed light on the underlying mechanism. EXPERIMENTAL SECTION Reagents and synthesis of BS:Eu2+, Ln3+ OSL phosphor with multi-dimensional discrete traps. The raw reagents BaCO3 (99%), SiO2 (99%), Eu2O3 (4N), Ln2O3 (4N) and BaF2 were received without further purification. A series of BS:Eu2+, Ln3+ (Ln=Ho, Dy, Tm) phosphors were prepared


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sample BS:Eu2+, the excitation spectrum shows a broad band ranging from 230 to 470 nm, while the emission band covers between 450 and 600 nm ascribable to the 5d-4f electronic dipole-allowed transitions of Eu2+.40-42 All the other co-doped samples present the similar shape but different intensity of excitation and emission indicate that Ln3+ ions play the role of trap centers rather than emission center and the small amount of Ln3+ incorporation almost causes undetectable effect on the crystal field. However, according to vacuum referred binding energy scheme as in Figure S8, it is expected that the codoping of Dy, Ho, Er, Tm and Yb ions have possibility of creating new deep traps.43-46 To demonstrate the speculation, thermoluminescence (TL) curves of BS:Eu with and without Er, Ho, Dy, Tm and Yb were recorded (Figure S9). The un-codoped one shows a broad and negligibly weak TL band due to intrinsic defects. The shape of TL curves of samples BS:Eu, Er and BS:Eu, Yb is similar to that of un-codoped one, because that the incorporations of Er and Yb yielded the undetectable shallowest and deepest traps, respectively. The trap depths and distributions are experimentally estimated according to the equation E=TM/500 (TM is the temperature at the maximum of TL peak),47,48 as listed in Table S1. The separate incorporations of Ho, Dy and Tm created isolated traps with depths of 0.723, 0.841 and 1.02 eV and narrow distributions of 0.073, 0.088 and 0.107 eV, respectively. The experimental trap depths show the similar change trend to the theoretical results but lower values, indicating the underestimation of the calculation method. Therefore, we firstly co-doped various contents of Dy and Tm into BS:Eu separately. The TL intensity shows a great dependence on the codoping contents and the optimum codoping concentrations were determined to be 0.002 and 0.001, respectively (Figure S10-11). Upon the incorporation of Ho into two optimum codoped ones (Figure S12-13), the TL bands show the similar change trend. The TL intensity of shallower traps caused by Ho increases while the counterparts of Dy/Tm decreases continuously with the increasing of Ho, which is mainly attributed to the increase concentration of shallower traps and the interception of electrons by the shallower traps. Multi-level optical data storage and encryption. For the excellent multi-level ODS material, the first and foremost is to investigate the write-in process. Thus the filling dynamics of traps for bi-doped, tri-doped and tetra-doped BS samples are characterized by TL curves after the samples are irradiated by 254 nm for different times. As shown in Figure S14, all doped samples exhibit increase of TL intensity and finally reach the saturation. All doped samples reach to the saturation after approximate 5 min UV irradiation except that samples doped by Eu-Tm and Eu-Tm-Ho need 20 s and 3 min, respectively. Furthermore, the dependence of

RESULTS AND DISCUSSION Phase identification, crystal structure and microstructure. The powder X-ray diffraction (PXRD) patterns of BS:Eu2+, Ln3+ (Ln=Dy, Ho, Tm) were collected and shown in Figure 1a. Compared to the standard data (ICSD 6246), all samples doped with a small amount of rare-earth ions crystallize in space group pmcn (No.62), isotypic with the pure barium orthosilicate, belonging to orthorhombic system structure. Figure 1b shows the crystal structure of the pure barium orthosilicate. The skeleton of BS is constructed by Si-O tetrahedron and two different kinds of Ba-O polyhedrons with Wyckoff position 4c, i.e., 10-coordinated Ba1 and 9-coordinated Ba2.38,39 Considering the ionic radii and valance states, it is excepted that the rare-earth ions (Eu2+, Ln3+) should occupy Ba sites in the host lattice. The scanning electron microscopy (SEM) image in Figure 1c indicates the particle size of 1-5 m. The energy-dispersive x-ray spectroscopy (EDS) elemental mapping technique was carried out. The element mapping images of BS:Eu, Tm, Ho, Dy were shown in Figure 1d, inferring great uniform distribution of Si, O, Ba, Eu, Tm, Ho and Dy. Moreover, the SEM and element mapping images of single doped, co-doped and tri-doped samples shown in Figures S1-6 also give the similar results. Photoluminescence and tailoring multi-dimensional traps. The photoluminescence excitation and emission spectra of all the as-obtained samples BS:Eu2+, Ln3+ are shown in Figure S7 (λem=504 nm, λex=360 nm). For

Figure 1 (a) XRD patterns of the as-prepared BS:Eu2+, BS:Eu2+, Ln3+ (Ln=Ho, Dy, Tm), and the standard pattern of PDF card ICSD 6246. (b) Crystal structure of Ba2SiO4 along a axis and coordination geometry around the Ba and Si atoms. (c) SEM image of BS:0.005Eu2+, 0.001Tm3+, 0.001Ho3+, 0,0005Dy3+ micro-particles. (d) The selected particle (d-1) and elemental mapping images of Si, O, Ba, Eu, Tm, Ho and Dy for the selected BS:0.005Eu2+, 0.001Tm3+, 0.001Ho3+, 0,0005Dy3+ particle (Scale bar: 2 m).


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Figure 2. (a) Schematic illustration of the multi-level ODS from a flexible phosphor (BS:0.005Eu2+, 0.001Tm3+, 0.001Ho3+, 0,0005Dy3+) film on a heating stage. (b) The read-out of the stored multi-level patterns (turnip, rabbit and apple) by heating method. (c) and (d) are the decryption of numbers and letters stored in different trap levels. (e) The top panel is the schematic diagram of the information readout process characterized by TL method. The processes of data in multiple traps are read out one by one. The bottom panel: the red line represents the actual TL curves and the gray line represents the potential TL curves. (f) The repeatability of ODS: the TL signal intensity of each trap (denoted as I, II and III) as a function of cyclic number of 55 cycles. normalized intensity for each TL peak on UV irradiation time is extracted and presented in Figure S15. The dynamics of trap filling for three different traps are significantly modified when tri-doped and tetra-doped. We can find that the filling behaviors of three traps are almost identical, indicating only slight or even no interactions among the three discrete traps. Due to the multi-dimensional traps, multiple data can be stored in multi-layers with the assistance of thermal cleaning. According to the TL curves measured with thermal cleaning method in Figure S16, the first trap I and second trap II are completely emptied with almost negligible interference on the second trap II and third trap III when the temperature is elevated up to 85 and 185 ℃, respectively, which predicates the conceptual feasibility of multi-level optical data storage. According to the +V substitution formula 2Ln +3Ba →2Ln , TL peaks I, 3

2

3

Ba2

II and III are assigned to electron traps of and

Tm

3  Ba2 

different patterns are stored in three different traps. Similarly, the information of numbers and letters also can be stored by the means of photon. The stored information can be released only when the phosphor film is provided with sufficient activation energy (heating or photon). As exhibited in Figure 2b, the whole domain of the recorded patterns is read out individually and clearly at controlled temperatures. Moreover, as shown in Figure 2c and d, the pre-stored numbers: “1” (“二”) stored in all three traps, “2” (“三”) stored in two deep traps, and “3” (“日”) stored only in the deepest traps, can be visualized at elevated temperature. Numbers and letters stored in different traps can be designed for highlevel secure anti-counterfeiting, for which the actual code can be obtained only by the right decoding mode. The processes of data storage and read out individually are characterized by TL curves. As shown in the top of Figure 2e, in each cycle, the flexible film was initially irradiated (write in) by 254 nm light for 5 min at RT (room temperature) with subsequent delay time of 30 s, heated up to 85 ℃ and cooled down to RT. Then, it was re-heated up to 185 ℃ and cooled down to RT. Finally, it was re-heated up to 280 ℃ and cooled down to RT. In such three cycles, TL signal was in situ monitored and recorded, as shown in the bottom panel of Figure 2e. Three TL peaks indicate the possibility of three different kinds of information. The data recording is related to the filling of traps with the optically excited electrons, and the reading relies on release of the trapped electrons backward to the emission center triggered by the external stimulation. A suitable control of the critical temperatures, such as 85 and 185 ℃, is pivotal in

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Ba2

Ho3 

Ba2 

,

Dy 3 

Ba2 

, respectively. Also, the TM-Tstop method is

used as shown in Figure S17a, and the results of three upward sloping lines in Figure S17b demonstrate the narrow energy distributions of each kind of traps.49-52 The multi-level optical data recording using the fabricated flexible phosphor film (BS:Eu, Tm, Ho, Dy@PDMS, Figure S18) is illustrated in Figure 2a i-vi. Firstly, the pattern of an apple is stored by 254 nm light irradiation through a patterned photomask or by 450 nm laser equipped on a programmed stepper motor. After thermal treatment at 185 ℃, the pattern of a rabbit is then recorded. Finally, after treatment at 85 ℃, the pattern of a turnip is recorded. Consequently, three


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extracting the individual information with a high signalto-noise ratio. The repeatability is also tested by rigorously repeatable measurements. As shown in Figure 2f, the repeatability performance of multi-level write-in and read-out is assessed by integrating the intensity of three individual TL peaks in each cycle of measurement. The almost invariable three dot-lines after 55 cycles demonstrate the excellent repeatability. The recycle measurements of the overall TL curves were also recorded (Figure S19a) and the result in Figure S19b further confirms nearly no data loss even after 55 cycles. As an alternative to heating, instead of heating, light with different wavelengths also can trigger the retrieval of different information in various traps with different depths using the bit-by-bit read-out mode. As shown in Figure 3a, the intensity of TL peak I decreases continuously with longer time irradiation by 980 nm

laser (50 mW, The power density is ~1.6 w/cm2) while the other two TL peaks II and III keep nearly invariable, indicating the data read-out [inset of Figure 3a] from trap I exclusively and no data loss from traps II and III. After the trap I is completely emptied, 980 nm laser is still used but hardly has effected on traps II and III, due to the insufficient activation energy to release the trapped electrons (Figure S20a). Therefore, a higher energy 808 nm laser (50 mW) is utilized to trigger the information stored in trap II. The results in Figure 3b and Figure S20b show significant decrease of TL peak II and almost no change of TL peak III demonstrating that 808 nm laser is suitable for data retrieval from trap II without interference by traps III. To access the information saved in trap III a much higher energy 635 nm laser (10 mW) is utilized for the sample after the elimination of traps I and II. As shown in Figure 3c, the information preserved in trap III is effectively fetched. Thus, different information can be stored and read out in the same spot, realizing multi-dimensional traps for rewritable multilevel optical data storage. The summary of data readout from three different traps characterized by integrated intensity of individual TL peaks are shown in Figure 3d. It is clear that light stimulation (980 nm) with low energy just acts on the traps I with shallow depth and higher energy of light stimulation (808 nm) only has effect on the traps II with medium depth in absence of trap I. Only light stimulation (635 nm) with high energy can realize the data read-out from the deepest traps III, as well as the slight rejuvenation of traps II due to the refilling. For multi-level ODS feature, the durability of the preserved data is also an important factor and should be considered. Therefore, to evaluate the durability performance of data in each trap for the as-obtained material, delay-time-dependent TL curves which indicate data read-out were collected. The tested phosphor BS:Eu, Tm, Ho, Dy was pre-irradiated by 254 nm for 5 min and then the TL measurements were carried out with different subsequent delay time. As shown in Figure 4a, we focused on the dependence of all TL peaks on delay time. In each cycle, the preirradiated sample was heated up to 623 K with different delay time varied from 10 s to 300 min. The intensity of TL peaks I, II and III as a function of delay time are depicted in Figure 4b. It can be seen that the data stored in traps I, II and III show attenuation with fast, medium and slow rate, respectively, because electrons release from the shallower traps just requires lower activation energy in the beginning stage,53,54 and finally approaches stabilization (4.5%, 68.2% and 80.6% TL signal remained for traps I, II and III respectively, at delay time of 5 h). In Figure 4c, to focus on data durability in traps II and III the same procedures were performed except that the TL peak I was pre-eliminated. From Figure 4d, we can see that the TL signal

Figure 3. Multidimensional information read-out by photo-stimulation. (a) The TL spectra of the preirradiated sample (BS:Eu2+, Tm3+, Ho3+, Dy3+, 254 nm for 5 min) after 980 nm light stimulation for different time (0-5 min). (b) The TL spectra of the pre-irradiated sample (BS:Eu2+, Tm3+, Ho3+, Dy3+, 254 nm for 5 min) after 808 nm light stimulation for different time (0-5 min). Before the measurement, the sample was heated at 100 ℃ for 1 min to completely empty the trap I. (c) The TL spectra of the pre-irradiated sample (BS:Eu2+, Tm3+, Ho3+, Dy3+, 254 nm for 5 min) after 635 nm light stimulation for different time (0-5 min). Before the measurement, the sample was heated at 190 ℃ for 1 min to completely empty the trap II. The insets show the read-out of the stored optical information. (d) The variation of integrated TL intensity for each individual TL peaks as the function of stimulation time. The TL intensity integration of 300-375 K, 375-466 K and 466575 K were regarded as optical data read-out for traps I, II and III, respectively. The red, green and blue lines represent the variation of traps I, II and III, respectively.


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Figure 4. Durability test of ODS property of the OSL phosphor BS:Eu2+, Tm3+, Ho3+, Dy3+. (a), (c), (e) The durability of optical signal storage in three different traps I, II and III. The top is the profile of actual operating temperature, and the bottom is TL curves of after different delay time. (b) The integrated TL intensity of traps I, II and III after different delay time derived form (a). (d) The integrated TL intensity of traps II and III after different delay time derived form (c). (d) The integrated TL intensity of trap III after different delay time derived form (e). The TL intensity integration of 300-375 K, 375-466 K and 466-575 K were regarded as optical data read-out for traps I, II and III, respectively. Before measurements, all the samples were irradiated by 254 nm for 5 min at RT. attenuations show fast and slow rate, respectively, and finally reach stability (65.8% and 79.1% TL signal remained for traps II and III respectively after delay time for 12 h). Lastly, as shown in Figure 4e, the same measurements were carried out again after the elimination of traps I and II, and we only concerned on trap III The result in Figure 4f gives that the intensity of TL peak III still remains 81.3% of the initial level and keep nearly stable after delay time for 12 h. Overall, according to the fitting results using double-exponential decay model including fast and slow decay processes, the attenuation dynamics of TL peak II is identical for both cases in absence and presence of TL peak I, while that the TL peak III is identical to that of the cases in presence of TL peaks I and II or in absence of TL peak I. It indicates that there are no obvious interactions among three different traps. Finally, as presented in Figure S21, an energy level schematic is constructed to illustrate the multi-level ODS mechanism. In brief, upon UV/blue light, electrons are pumped to 5d states of Eu2+. Apart from

the immediate jumping backward to 4f level along with intrinsic emission, the residual excited electrons transporting in conduction band are captured by the traps, thereby completing multi-level data write-in. Different information stored in different dimension of traps also can be realized through thermal cleaning of shallow traps and then re-filling. Suitable external stimuli, such as thermal and optical stimulations, promote effectively the release of frozen electrons back to the 5d states of Eu2+, and the subsequent emission of 5d-4f transitions then accomplish the data read-out. CONCLUSIONS In conclusion, for the first time a proof-of-concept study to develop an OSL material with multi-dimensional traps for non-physical multi-level ODS has been successfully carried out by the facile incorporation of Dy3+, Tm3+ and Ho3+ into the BS:Eu2+ host lattice. Various traps were intentionally introduced, presenting discrete energy levels of 0.72, 0.84 and 1.02 eV within each


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narrow distributions of 0.073, 0.088 and 0.107 eV. With the assistance of thermal cleaning, different information can be optically stored in the fabricated flexible phosphor film with different trap levels and then addressable level by level. Long term and recycling write-in and read-out measurements demonstrate good data retention and high fatigue resistance in data access. By the judicious combination of the fluorescence, OSL and TL, the overall ODS mechanism has been elucidated and discussed comprehensively. Despite its possible flaw and imperfection, the study of the as-obtained ODS material will help the researchers to gain insight into the complicated nature of traps, make leaps and bounds towards erasable multi-level ODS/high-security anticounterfeiting, and provide a universal method to design new multi-level ODS media.

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ASSOCIATED CONTENT Supporting Information. SEM, elemental mapping, excitation and emission spectra, VRBE scheme, temperature and time-depentent TL glow curves, flexible phosphor films, energy level schematic diagram, trap distribution and depths. AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected] *[email protected] Notes The author declar no competing financial interest. ACKNOWLEDGMENTS The authors acknowledge the financial support from the National Natural Science Foundation of China (Nos. 51972065&51802045); Special Fund for Scientific and Technological Innovation Strategy of Guangdong Province (No. 2018A030310539); Youth Innovation Talent Project of Education Department of Guangdong Province, China (No. 2016KQNCX041); the Shenzhen Peacock Plan (KQTD2016053015544057); and the Guangdong Science and Technology Program (2017B030314002). REFERENCES (1) Martin, H.; Priscila, L. The World\"s Technological Capacity to Store, Communicate, and Compute Information. Sci. 2011, 332, 60-65. (2) Gantz, J.; Reinsel, D. The Digital Universe In 2020: Big Data, Bigger Digital Shadows, and Biggest Growth in the Far East. DC iView: IDC Analyze the future 2012, 1-16. (3) Gu, M.; Li, X.; Cao, Y. Optical Storage Arrays: A Perspective for Future Big Data Storage. Light Sci. Appl. 2014, 3, e177. (4) Trelles, O.; Prins, P.; Snir, M.; Jansen, R. C. Big Data, But Are We Ready?. Nat. Rev. Genet. 2011, 12, 224.


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Tailoring multi-dimensional traps towards rewritable multi-level optical

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