Long-lived Photon Upconversion Phosphorescence in RbCaF3:Mn2+,Yb3+ and the Dynamic Color Separation Effect.

The development of luminescence materials with long-lived upconversion (UC) phosphorescence and long luminescence rise edge (LRE) is a great challenge to advance the technology of photonics and materials sciences. The lanthanide ions-doped UC materials normally possess limited UC lifetime and short LRE, restricting direct afterglow viewing in visual images by the naked eye. Here, we show that the RbCaF3:Mn2+,Yb3+ UC luminescence material generates a long UC lifetime of ∼62 ms peaking at 565 nm and an ultralong LRE of ∼5.2 ms. Density functional theory calculations provide a theoretical understanding of the Mn2+-Yb3+ aggregation in the high-symmetry RbCaF3 host lattice that enables the formation of the long-lived UC emission center, superexchange coupled Yb3+-Mn2+ pair. Through screen printing ink containing RbCaF3:Mn2+,Yb3+, the visualized multiple anti-counterfeiting application and information encryption prototype with high-throughput rate of authentication and decryption are demonstrated by the dynamic color separation effect.

Introduction

Luminescent materials with long-lived emission have been extensively investigated over the past decades and attracted increasing attention owing to their fundamental scientific importance and emerging applications, ranging from optoelectronics, bio-imaging, emergency signage, information encryption/decryption, and anti-counterfeiting (Pan et al., 2012, Maldiney et al., 2014, Li et al., 2016, Liu et al., 2017a, Liu et al., 2017b). Using the long-lived luminescent materials in information security, the persistent emission may be adopted for encryption, substantially increasing the difficulty of duplication or decryption and thus providing extra high-level security protection (Zhang et al., 2018, Zhuang et al., 2018, Liu et al., 2018). The conventional inorganic persistent emission materials yield afterglow emission of tens of hours, but they exhibit clear limitations of (1) the long pre-irradiation time to store the energy and (2) the UV or visible light excitation source enabling the inevitable background fluorescence in photoluminescence mode (Li et al., 2016, Wang et al., 2017, Matsuzawa et al., 1996). Therefore, the development of inorganic photon upconverting materials with minimum background interference that demonstrate afterglow emission without long-time pre-irradiation is crucial not only for fundamental research, but also for practical applications (Chen et al., 2018, Dong et al., 2017, Zhou et al., 2015, Gargas et al., 2014). However, the lifetimes of the conventional lanthanide ions doped upconversion (UC) luminescent materials are usually in the range of only several tens of microseconds to a few milliseconds (Auzel, 2004, Liu et al., 2017a, Liu et al., 2017b; Martín-Rodríguez et al., 2013), which does not meet the application requirements for direct viewing in visual images.

In contrast to the relatively short-lived emission of the lanthanide ions, the transition metal Mn2+ normally presents a long lifetime on the order of tens of milliseconds, originated from an inherent parity and spin double forbidden d-d transition [4T1(4G)→6A1(6S)] (Feldmann et al., 2003, Yu et al., 2013, Lin et al., 2014, Vink et al., 2001, Zhou et al., 2018). In addition, the emission of Mn2+ depends strongly on crystal-field environment so that the wavelength-tunable Mn2+ UC emission with a long emission lifetime has been obtained in some Mn2+/Yb3+ codoped structures upon 980-nm laser excitation (Suyver et al., 2005, Song et al., 2016). Nevertheless, because of the serious thermal and concentration quenching effects (Valiente et al., 2000, Martín-Rodríguez et al., 2010), or the aggregation of Mn2+ ions (Song et al., 2015), the Mn2+ UC emission is relatively weak and the emission lifetime is usually limited to less than 50 ms. Besides, the UC emission of Mn2+ has also been realized by constructing core-shell structure NaGdF4:Yb/Tm@NaGdF4:Mn (Liu et al., 2017a, Liu et al., 2017b, Li et al., 2015). However, since the Mn2+ UC emission belongs to a five-photon energy migration UC process in this system, the Mn2+ UC emission is much lower than that of the Tm3+ and the pure Mn2+ UC emission cannot be achieved, suggesting the limited applications.

In the present study, to design an ideal system fulfilling the applications, the Mn2+-Yb3+ ions are introduced into the highly symmetric fluoride perovskite RbCaF3, in which the Mn2+ and Yb3+ tend to aggregate, enabling the formation of a UC emission center, viz., superexchange coupled Yb3+-Mn2+ pair. Accordingly, an intense Mn2+-based UC luminescence with a long emission lifetime of ∼62 ms has been achieved. Moreover, such a local structure forms an ultralong UC luminescence rise edge of ∼5.2 ms due to the energy transfer from the isolated Yb3+ ion to the exchange coupled Yb3+-Mn2+ pair. As the UC luminescence rise time and UC lifetime of the RbCaF:Mn2+,Yb3+ are one to two orders of magnitude of conventional Yb3+/Er3+ (or Tm3+, or Ho3+) codoped UC materials, both of them have been applied into information encryption and high-level anti-counterfeiting based on the dynamic color separation effect. By using the UC luminescence material RbCaF:Mn2+,Yb3+ to fabricate the screen printing ink, and comparing this ink with the ink containing commercialized UC red/green/blue (RGB) materials, the printing of images with multiple information encryption and a high level anti-counterfeiting application with a fast authentication rate have been demonstrated.

Results and Discussion

The UC materials RbCa1-F3:xMn2+,yYb3+ (0 < x ≤ 0.20; 0 < y ≤ 0.05) (denoted as RCF:xMn2+, yYb3+) have been prepared (see details on the phase structure and morphology characterizations in Figure S1) and the RCF:0.10Mn2+,0.05Yb3+ was selected as an example to study the UC luminescence and UC phosphorescence. Figure 1A presents the room temperature UC spectrum of RCF:0.10Mn2+, 0.05Yb3+. Upon 980-nm laser excitation, the sample exhibits an intense near-infrared to yellow UC emission, and the emission spectrum consists of a broadband emission peaking at 565 nm, corresponding to the d-d transition [4T1(4G)→6A1(6S)] of Mn2+. The concentration-dependent UC emissions of RCF:xMn2+,yYb3+ (x = 0.10–0.30; y = 0.05) show that all samples contain a visible single-band UC emission, and the optimal doping concentration of Mn2+ is determined to be x = 0.10 (see Figure S2A). The color purity of the material RCF:0.10Mn2+, 0.05Yb3+ estimated from the spectrum was determined as ∼98%, showing much higher value than that of the conventional lanthanide-doped commercial UC emission materials (Figure S3). Moreover, such a pure yellow UC emission band is quite close to the highest sensitivity light region of the human eye (Vos, 1978), which is beneficial for the direct visual images as discussed later. On the other hand, the laser pumping is usually required to achieve UC luminescence (Zhong et al., 2015). Interestingly, the yellow UC emission of RCF:Mn2+,Yb3+ can be obtained even upon incoherent light excitation of a 940-nm light-emitting diode (LED) (Figure 1A). Additionally, the corresponding UC emission spectrum is almost completely overlapped with the spectrum excited by a 980-nm laser, meaning that the Mn2+-Yb3+-related UC luminescence process and mechanism pumped by laser were also achieved with incoherent light excitation in this case. When the temperature was gradually decreased from 300 to 77 K, the emission peak positions shifted from 568 to 587 nm, whereas the emission intensity was increased to ∼420% (Figure S4), ascribed to the shrinking of host lattice and the decreased non-irradiative relaxation processes, respectively. Correspondingly, the UC emission lifetime of Mn2+ was increased from 62 ms at 300 K to 138 ms at 77 K (Figure 1B). Such long lifetime and single-exponential decay behavior of Mn2+ at the temperatures of 77–300 K (Figure S4) show that only one type of UC emission centers appeared in RCF:Mn2+,Yb3+. It is worth noting that the UC emission lifetime in RCF:Mn2+,Yb3+ is much longer than that of the previous reported Mn2+-related UC systems (see Table S1). The long UC emission lifetime in RCF:Mn2+,Yb3+ is ascribed to the combined effect from the high symmetry of host lattice RbCaF3 and the un-aggregation characteristics of Mn2+ even at a high Mn2+ concentration (x = 0.30) in this structure, which has been further confirmed by the concentration-dependent single-band visible UC and Stokes emissions in RCF:xMn2+,0.05Yb3+ (x = 0.01–0.30) (Figure S2). This is significantly different from the other similar Mn2+/Yb3+ codoped perovskites AXF3 (A = Cs, Rb, K; X = Mg, Zn, Cd), and in AXF3:xMn2+,0.005Yb3+, the Mn2+ ions would aggregate as the Mn2+ concentration rises to x = 0.20 (Song et al., 2015). Since the Mn2+ UC emission is strong enough with a long (62–138 ms) lifetime, the UC afterglow was observed by the naked eye in RCF:Mn2+,Yb3+ after stoppage of the laser irradiation, as shown in Figure 1C. Specifically, the UC afterglow time observed by the naked eye is longer than 300 ms in the daylight environment, whereas it is even longer than 400 ms in the dark environment. Furthermore, unlike the conventional long afterglow materials depending on the traps and the pre-radiation time (Pan et al., 2012, Li et al., 2016), the UC afterglow time of RCF:Mn2+,Yb3+ is only related with the UC emission lifetime of Mn2+, and thus the UC afterglow was observed by the naked eye even with a 980-nm laser dynamic scanning (Video S1).

Figure 1

UC Photoluminescence and Phosphorescence in Perovskite RbCaF3: Mn2+,Yb3+

(A) UC emission spectra of RCF:0.10Mn2+,0.05Yb3+ upon excitation with a 980-nm laser or incoherent light of a 940-nm LED with the sensitivity curve of the human eye (dotted line) also given as a comparison. The inset shows the UC luminescence pictures of RCF:0.10Mn2+,0.05Yb3+ powder upon 980-nm laser excitation (i) and the lightened UC luminescence LED fabricated with a 940-nm LED chip and RCF:0.10Mn2+,0.05Yb3+ powder (ii).

(B) Temperature dependence of the UC luminescence decay curves of RCF:0.10Mn2+, 0.05Yb3+ between 77 and 300 K.

(C) Steady UC luminescence (0 ms) of RCF:0.10Mn2+, 0.05Yb3+and the time-resolved UC afterglow after stoppage of the laser under dark and daylight environment. (The images are recorded by a digital camera with an exposure time of 1/25 s using the snapshot mode.) The scale bar is 1 cm.

Video S1. UC Dynamic Luminescence of RbCaF3:0.10Mn2+,0.05Yb3+ Powder under 980 nm Laser Fast Scanning, Related to Figure 1

The video shows that the UC luminescence afterglow (or tailed emission) from RbCaF3:0.10Mn2+,0.05Yb3+ can be observed by the naked eyes upon 980-nm laser fast scanning.

The UC luminescence of RCF:Mn2+,Yb3+ cannot be ascribed to the resonance energy transfer as reported in the Yb3+/Er3+ (or Ho3+, or Tm3+) codoped systems owing to the characteristic excitation/emission spectra of Mn2+ and Yb3+. Except for the above-mentioned energy transfer UC mechanism, the cooperative sensitization can also lead to the Yb3+/Mn2+ UC emission. Under this condition, a cooperative luminescence emission band of Yb3+ centered at ∼490 nm would usually simultaneously appear in the UC emission spectrum. However, the cooperative luminescence of Yb3+ has not been observed either in RCF3:Yb3+ or RCF:Mn2+,Yb3+. Additionally, the occurrence of cooperative sensitization usually needs a relative high power density of the pump laser, whereas the UC emission of Mn2+ is observed in this system even pumped by using a 940-nm LED (incoherent light) excitation (Figure 1A), indicating that the possibility of the cooperative sensitization mechanism for generating Mn2+-UC emission is quite low. Moreover, without observation of the Eu3+ UC emission in RCF:Yb3+,Eu3+ (Figure S5), thus the cooperative sensitization UC mechanism is excluded in this system. Upon 256-nm excitation, the emission spectrum of RCF:0.05Yb3+ consists of two sharp emission peaks at 970/1028 nm, corresponding to the 2F5/2→2F7/2 transition of Yb3+ (Figure 2A). By monitoring the emission wavelengths at 970 and 1,028 nm, a similar excitation band centered at ∼256 nm was obtained, which is ascribed to the Yb3+-O2- charge transfer band (CTB) (Meijer et al., 2010, Yang et al., 2018). For RCF:0.10Mn2+,0.05Yb3+, under 395-nm excitation, the Yb3+ emission at 970/1,028 nm was also obtained in addition to the Stokes emission (∼at 564 nm) of Mn2+. Moreover, the excitation spectrum of Yb3+ contains both the typical excitation peaks (310, 331, 351, 395, 424, and 511 nm) of the d-d transition of Mn2+ and O2--Yb3+ CTB (256 nm) of Yb3+, indicating that the energy transfer from Mn2+ to Yb3+ occurs in RCF:0.10Mn2+,0.05Yb3+. This result was further confirmed by the fact that the emission intensity (Figure S6) and emission lifetime (Figure 2B) decrease in RbCF3:0.10Mn2+,0.05Yb3+ as compared with that of RbCF3:0.10Mn2+. Meanwhile, the excitation spectrum of Mn2+ consists of the typical excitation peaks of the octahedral Mn2+ only (Figure 2A). To give more information about the Mn2+ UC emission, the luminescence decay curves of Yb3+ (1,028 nm) in RCF:0.05Yb3+ and RCF:0.10Mn2+, 0.05Yb3+ were measured (see Figure 2C). It was found that the Mn2+ codoping in RbCF3:Yb3+ would cause the decrease of Yb3+ lifetime, suggesting that the Mn2+ UC emission in RCF:Mn2+,Yb3+ resulted from the Yb3+→Mn2+ energy transfer. However, the emission lines from Yb3+ and the excitation spectrum of Mn2+ have no spectral overlap. It is noted that the emission lifetime (∼70 ms) of Mn2+ in RCF:0.10Mn2+,0.05Yb3+ with 395-nm excitation is a little longer than that of UC emission lifetime (∼62 ms) upon 980-nm laser excitation. Meanwhile, the emission lifetime value of Yb3+ (1,028 nm, 42 ms) upon the characteristic excitation wavelength of Mn2+ (395 nm) is much longer than that of 980-nm laser excitation (2.80 ms). Additionally, the Mn2+ UC emission spectrum shifts to the longer wavelength region as compared with that of the Stokes emission (Figure S7); moreover, the Yb3+ emission (970/1,028 nm) excited by 256-nm excitation shows some differences as compared with that by 395 nm excitation (Figure 2A). These facts strongly indicate that some types of new emission centers appeared in RCF:Mn2+,Yb3+. Furthermore, the UC emission of RCF3:0.10Mn2+,0.05Yb3+ upon 980-nm laser or 940-nm LED excitation belongs to a two-photon UC process (Figure S8) and a long UC luminescence rise time (∼5.2 ms) was observed in this system (Figure 2C). Therefore, a possible UC luminescence mechanism based on isolated Yb3+ ion and the exchange coupled Yb3+-Mn2+ pair is proposed and is given in Figure 2D. Upon 980-nm laser or 940-nm LED excitation, both the isolated Yb3+ and exchange coupled Yb3+-Mn2+ pair are first excited to the state 2F5/2 of Yb3+ and intermediate state of the Yb3+-Mn2+ pair, respectively. Then, the electron at the intermediate state is further excited to the emitting state (Yb3+-Mn2+ pair) through the energy transfer from isolated Yb3+ to Yb3+-Mn2+ pair and finally produces the visible (VIS) UC emission. The UC luminescence processes are provided as follows:

Figure 2

Down Conversion Luminescence Properties and UC Luminescence Mechanism of RbCaF3:Mn2+,Yb3+

(A) Room temperature emission and excitation spectra of RCF:0.10Mn2+,0.05Yb3+ (Mn/Yb) and RCF:0.05Yb3+, and the excitation spectrum of RCF:0.10Mn2+ (Mn) is given for comparison.

(B) Luminescence decay curves of Mn2+ in RCF:0.10Mn2+ and RCF:0.10Mn2+,0.05Yb3+ and the luminescence decay curve Yb3+ in RCF:0.10Mn2+,0.05Yb3+ upon 395-nm excitation. The lifetime of Yb3+ upon 395-nm excitation is determined by fitting the acquired data with a double exponential function.

(C) Luminescence decay curves of Yb3+ in RCF:0.05Yb3+ and RCF:0.10Mn2+, 0.05Yb3+ upon 980-nm laser excitation and the time-dependent UC luminescence of Mn2+ between 0 and 40 ms.

(D) Ground state absorption (GSA)/energy transfer (ETU) UC luminescence mechanism based on isolated Yb3+ ion and exchange coupled Yb3+-Mn2+ pair.

Different from the previously observed Mn2+ UC emissions in other systems (Valiente et al., 2001, Martín-Rodríguez et al., 2010, Gerner et al., 2004), the UC emission is mainly ascribed to a Ground state absorption (GSA)/ energy transfer (ETU) mechanism with the participation of both the Yb3+ ion and Yb3+-Mn2+ pair. The single-band UC emission characteristics and GSA/ETU UC mechanism of RCF:Mn2+,Yb3+ indicate that the high-efficiency UC luminescence can be obtained via Mn2+/Yb3+ codoping (Suyver et al., 2005). Notably, the UC emission intensity of RCF:Mn2+,Yb3+ is much stronger than that of the Mn2+/Yb3+ codoped similar perovskite fluorides AXF3 (A = Cs, Rb, K; X = Mg, Zn, Cd) (Figure S9). In addition, none of the AXF3:Mn2+,Yb3+ produces UC emission upon 940-nm LED excitation except for RCF:Mn2+,Yb3+ found in this work. Therefore, the UC emission of RCF:Mn2+,Yb3+ upon 940 nm LED excitation is mainly ascribed to the efficient UC emission (Table S2) and the obviously spectral overlap between the absorption of Yb3+ and the emission of the 940-nm LED (Figure S10).

On the basis of the comprehensive investigations of up and down-conversion luminescence in RCF:Mn2+,Yb3+, a superexchange coupled Yb3+-Mn2+ pair model has been proposed. During the formation of the Yb3+-Mn2+ pair, a critical distance of ∼5 Å between Mn2+ and Yb3+ was required (Valiente et al., 2000, Dexter, 1953). To evaluate the probability of formation of Yb3+-Mn2+ pair in RCF:Mn2+,Yb3+, the structure optimization based on density functional theory (DFT) was performed. The compound RbCaF3 presents a typical cubic perovskite structure with the space group (No. 221), consisting of a three-dimensional (3D) corner-sharing [CaF6] octahedra network, where Rb+ ions are located at the centers of the tetrakaidecahedron cavity surrounded by [CaF6] octahedron (see Figure S11). Considering the similar ionic radius and close valence states between Mn2+ (0.83 Å) and Ca2+ (1.0 Å), and between Yb3+ (0.87 Å) and Ca2+ (1.0 Å) (Shannon, 1976), both Mn2+ and Yb3+ would substitute for Ca2+ ion in RCF, accompanying the formation of Rb vacancy (VRb') or Ca vacancy (VCa'') or introducing oxygen defect OF' for charge compensation. Figure 3A presents that the formation energy (Ef) for the different charge compensation models (Figure S12) of the Yb3+-doped RCF. It is found that the Ef value for model Y1 shows much lower than others, suggesting that the substitution of Yb3+ for Ca2+ would generate one oxygen defect OF' for charge compensation. Therefore, if both Ca2+ and Yb3+ are homogenously distributed at the Ca2+ sites in RCF, the average distance between Mn2+ and Yb3+ is calculated as ∼10.4 Å in RCF:0.10Mn2+,0.05Yb3+ (Yang et al., 2005). This value is apparently much longer than that of the critical distance (∼5 Å) for superexchange interactions, indicating that the formation probability of the Yb3+-Mn2+ pair would be relatively low in this structure, whereas the Mn2+ UC emission was observed even with a much lower concentration of Mn2+ and Yb3+ in RCF:Mn2+,Yb3+ (Figure S13). Thus, the distributions of Mn2+ and Yb3+ in RCF:Mn2+,Yb3+ were further simulated based on the site occupancy of Mn2+ and Yb3+ above. For a 2 × 2 × 2 supercell of RCF:Mn2+,Yb3+,O2−, there are five possible substitution models for Mn2+ (denoted as YM1-YM5, Figure S14). Figure 3B shows the Ef values of the five different substitution models, and the model YM2 exhibits the lowest Ef value, suggesting the model YM2 is the most stable model among them. Under this model, the nearest Yb3+-Mn2+ distance is of 4.06 Å (Figure 3C). Moreover, the distorted octahedron MnF5O and distorted octahedron YbF5O are connected with each other by sharing with one O2− (Figure 3D); therefore, the superexchange coupled Yb3+-Mn2+ pair is formed in RCF:Mn2+,Yb3+. The linear Yb3+-O2--Mn2+ configuration with a connected angle ∠Yb3+-O2+-Mn2+ of 180° would lead to a large overlap of the orbital wave-functions (Figure 3E) (Gerner et al., 2004). Meanwhile, the existence of oxygen of RCF:Mn2+,Yb3+ was confirmed by the energy dispersive spectroscopy (Figure S15) and the X-ray photoelectron spectroscopy (Figure S16); moreover, the observation of Yb3+-O2- CTB also supports this model (Figure 2A). Hence, the UC luminescence of RCF:Mn2+,Yb3+ is ascribed to the transition of superexchange coupled Yb3+-Mn2+ pair based on the energy transfer from the isolated Yb3+ ion to the Yb3+-Mn2+ pair and the GSA process of the Yb3+-Mn2+ pair. The UC emission lifetime (Figure S17) is a little shorter than the Stokes emission lifetime (Figure 2B) in RCF:0.10Mn2+,0.05Yb3+, corresponding to the distorted crystal field environment for Mn2+ in the Yb3+-Mn2+ pair (see Figure 3D and Table S3) and the additional increase in transition possibility due to the Yb3+-Mn2+ interactions in the Yb3+-Mn2+ pair (Martín-Rodríguez et al., 2010).

Figure 3

Theoretical Simulation of the Mn2+/Yb3+ Distribution in RbCaF3:Mn2+,Yb3+

(A) The formation energy (Ef) for three possible charge compensation models (denoted as Y1, Y2, Y3) of Yb3+ in a 2 × 2 × 2 supercell of RbCaF3:Yb3+.

(B) Ef for five situations in a 2 × 2 × 2 RbCaF3:Mn2+,Yb3+,O2− supercell with two Ca2+ ions substituted by one Mn2+ and one Yb3+ and one F− replaced by one O2− (denoted as YM1-YM5 with different shortest Mn2+-Yb3+ distances after structure optimization).

(C) Model YM1 after structure optimization.

(D) The MnF5O and YbF5O octahedron are connected by sharing with one O2−.

(E) Schematic representation of the most important sigma overlaps between the d orbital of Mn2+ and f orbital of Yb3+ and the p(s) orbital of ligand O2− with a bridging angle of 180°.

As both the luminescence rise edge and UC emission lifetime of RCF:0.10Mn2+, 0.05Yb3+(denoted as Y) are obviously longer than those of the commercial red β-NaYbF4:6%Er3+ (denoted as R), green β-NaYF4:20%Yb3+,2%Er3+ (denoted as G), and blue β-NaYF4:18%Yb3+,2%Tm3+ (denoted as B) UC phosphors (Figures 4A and 4B), one can easily separate the two kinds of UC emissions based on the different dynamic color separation effect during the luminescence rise and decay stage, respectively. The unique color separation characteristics may find application in advanced multiple information encryption and the high-level anti-counterfeiting. As a proof of concept, four types of security inks (Y, YR, YG, and YB) (see detail in Table S4) were designed and fabricated by using the as-prepared RCF:0.10Mn2+,0.05Yb3+, the commercial R, G, and B UC materials. By using these security inks, the “SCUT” patterns were printed on the multipurpose paper via the screen-printing technique (Figure S18A). Figure 4C illustrates the luminescence images of the patterns upon steady 980-nm laser (29.59 W/cm2) excitation in a dark environment. It was observed that the “SCUT” patterns printed by Y, YR, YG, and YB inks show intense yellow, orange, yellow-green, and white emission, respectively, matching well with their steady UC emission spectra (Figure S19). The emission color of the “SCUT” patterns can be further tuned by varying the pump power density of the laser (Figure S20 and Table S5). Interestingly, after using a 980-nm laser fast scanning the “SCUT” pattern printed by ink Y, six yellow luminescence “SCUT” patterns were unambiguously observed (see Figure 4D and Video S2), and a similar phenomenon has been also obtained under a daylight environment condition (Figure S21A and Video S3). Furthermore, one can observe the pattern “SCUT” afterglow even with a relative low-power-density (∼2.72 W/cm2) laser fast scanning (Figure S21B). For other inks, upon irradiation with a 980-nm laser dynamic scanning, the letters highlighted with white border show different emitting colors, but five identical yellow UC afterglow “SCUT” patterns can be observed (see Figures 4E–4G and the Videos S4, S5, and S6). Moreover, in the “SCUT” pattern upon 980-nm laser dynamic irradiation, the letter “T” showed red for YR ink, green for YG ink, and blue for YB, whereas the corresponding “SCU” exhibited orange, yellow-green, and white emissions, respectively. This is because the luminescence rise edge of RCF:Mn2+,Yb3+ is significantly longer than that of commercial UC materials and the latter could achieve their steady state more rapidly than the former, which is further validated by the time-resolved UC emission (Figure S19) and the pulse-widths dependence of the UC emissions (Figure S22). These results demonstrate that both the long UC luminescence rise and long UC lifetime of RCF:Mn2+,Yb3+ can be applied in information encryption and show fast decryption characteristics. The information of RCF:Mn2+,Yb3+ and the commercial UC phosphors can be fast distinguished in observing windows II and I, respectively, through the dynamic color separation effect.

Figure 4

Multiple Information Encryption and Anti-counterfeiting Applications Based on the Long UC Lifetime and Ultralong Luminescence Rise Edge of RbCaF3:Mn2+,Yb3+

(A) Room-temperature luminescence decay curve of RCF:0.10Mn2+,0.05Yb3+.

(B) UC luminescence decay curves of commercial red (R), green (G), and blue (B) UC phosphors upon 980-nm laser excitation. Here, the UC emission lifetime is determined by , where I0 and I(t) are the maximum emission intensity and the emission intensity at time t after cutoff the pump laser, respectively.

(C–G) Luminescence images of the “SCUT” pattern printed by inks Y,YR, YG, YB with a 980-nm laser steady excitation (C) or dynamic scanning (D-F correspond to inks Y, YR, YG and YB, respectively).

(H) Using ink Y to print some designed patterns in different areas (1, pattern “1234567890”; 2, pattern “strip”) of a Chinese postcard and the luminescence images under 980-nm laser dynamic scanning. The scale bar is 10 mm. The dynamic luminescence photographs were recorded using a digital camera with an exposure time step of 1/50s. The power density and spot diameter of the used 980-nm laser are 10 W/cm2 and 3.0 mm, respectively.

Video S2. UC Luminescence “SCUT” Patterns Printed with ink Y upon 980 nm Fast Scanning in the Dark Environment, Related to Figure 4

The video shows that six UC luminescence afterglow “SCUT” patterns printed with the ink Y can be observed by the naked eyes in the dark upon 980-nm laser fast scanning.

Video S3. UC Luminescence “SCUT” Patterns Printed with Ink Y upon 980 nm Fast Scanning in the Daylight Environment, Related to Figure 4

The video shows that six UC luminescence afterglow “SCUT” patterns printed with the ink Y can be observed by the naked eyes in the daylight environment after 980-nm laser fast scanning.

Video S4. UC Luminescence “SCUT” Patterns Printed with Ink YR upon 980 nm Fast Scanning in the Dark Environment, Related to Figure 4

The video shows that dynamic color separation effects between the RbCaF3:Mn2+,Yb3+ and the conventional red UC materials can be observed by the naked eyes upon 980-nm laser fast scanning. Specifically, the UC luminescence color from red to orange then to yellow of the “SCUT” patterns has been observed by naked eyes upon 980-nm laser fast scanning.

Video S5. UC Luminescence “SCUT” Patterns Printed with Ink YG upon 980 nm Fast Scanning in the Dark Environment, Related to Figure 4

The video shows that dynamic color separation effects between the RbCaF3:Mn2+,Yb3+ and the conventional green UC materials can be observed by the naked eyes upon 980-nm laser fast scanning. Specifically, the UC luminescence color from green to yellow green then to yellow of the “SCUT” patterns has been observed by naked eyes upon 980-nm laser fast scanning.

Video S6. UC Luminescence “SCUT” Patterns Printed with Ink YB upon 980 nm Fast Scanning in the Dark Environment, Related to Figure 4

The video shows that dynamic color separation effects between the RbCaF3:Mn2+,Yb3+ and the conventional blue UC materials can be observed by the naked eyes upon 980-nm laser fast scanning. Specifically, the UC luminescence color from blue to white then to yellow of the "SCUT" patterns has been observed by naked eyes upon 980-nm laser fast scanning.

Additionally, the potential anti-counterfeiting application using the RCF:Mn2+,Yb3+ phosphor was also demonstrated. As shown in Figure 4H, the patterns of “1234567890” and “strip” were printed on 1 and 2 areas of a Chinese postcard. After drying, it is difficult to recognize the colorless patterns using the naked eye under daylight conditions. However, after using a 980-nm laser fast scanning, yellow afterglow “1234567890” and tailed emission in yellow were observed by the naked eye in areas 1 and 2, respectively. These observations revealed that RCF:Mn2+,Yb3+ material provided a high level anti-counterfeiting application with a fast authentication rate. In addition, the authentication will not be affected by the ambient light.

Conclusion

In summary, we have successfully developed an upconverting RCF:Mn2+,Yb3+ material with a long (∼62–138 ms) lifetime and long luminescence rise edge (∼5.2 ms). One can observe UC afterglow by the naked eye upon 980-nm fast irradiation even under daylight environment with a significant visualized UC afterglow >300 ms. The long-lived UC emission is ascribed to the transition of superexchange coupled Yb3+-Mn2+ pair formed in RCF:Mn2+,Yb3+. The long UC luminescence rise edge is originated from the energy transfer from the isolated Yb3+ to Yb3+-Mn2+ pair. The DFT calculations and spectra analysis verify that the UC emission is attributed to the formation of linear {Yb3+-O2--Mn2+} unit in RCF:Mn2+,Yb3+. Given the single-band emission characteristics and two-photon GSA/ETU energy transfer UC mechanism, the highly efficient UC luminescence can be realized through Mn2+/Yb3+ codoping.

On the basis of the special characters of the ultralong UC luminescence rise edge and long UC emission lifetime of RCF:Mn2+,Yb3+, new types of visualized multiple information encryption and anti-counterfeiting with fast decryption rate were demonstrated. These findings show great promise of RCF:Mn2+,Yb3+ for use in advanced anticounterfeiting and multiple information encryption applications without the need of time-gated setup to separate and decode security data and also provide new insights for the dynamic printing color separation and exploration of advanced photon materials.

Limitations of the Study

The distribution of Mn2+ is important for the UC emission intensity and UC lifetime of the RbCaF3:Mn2+,Yb3+. Further in-depth study about the effects of Mn2+ distributions on the performance of the Mn2+/Yb3+-related UC materials should be carried out. In addition, the stability of the UC material RbCaF3:Mn2+/Yb3+ has not been investigated and is worth investigating in the future, which is critical for the practical application.

Methods

All methods can be found in the accompanying Transparent Methods supplemental file.