Implanting bismuth in color-tunable emitting microspheres of (Y, Tb, Eu)BO3 to generate excitation-dependent and greatly enhanced luminescence for anti-counterfeiting applications

ABSTRACT To prevent counterfeiting, a lot of advanced security technologies have been developed, including luminescent printing. Therefore, the pigments with advanced security features are urgently pursued for luminescence printing-based anti-counterfeiting technology. Here, micron-sized spheres of hexagonal-structured and color-tunable emitting (Y, Tb, Eu, Bi)BO3 have been rapidly synthesized by microwave processing, followed by a proper annealing. Incorporation of Bi3+ greatly enhances the emission intensity of Eu3+ and of Tb3+. Under the excitation at 260 nm, the spheres exhibit UV emission at 330 nm (3P1→1S0 transition of Bi3+), green emission at 546 nm (5D4→7F5 transition of Tb3+) and orange-red emission at 592 nm (5D0→7F1 transition of Eu3+), mainly due to the three energy transfer processes of Bi3+→Tb3+, Bi3+→Eu3+, and Tb3+→Eu3+. However, under the excitation at 230 nm, the Tb3+→Eu3+ energy transfer contributes to the orange-red emission of Eu3+ and the green emission of Tb3+, in the absence of Bi3+ emission. The maximum energy transfer efficiency of Bi3+→ “Eu3+ and Tb3+” and Tb3+→ Eu3+ is 63% and 50% for (Y0.883Tb0.02Eu0.09Bi0.007)BO3. The (Y0.963Tb0.02Eu0.01Bi0.007)BO3 spheres exhibit a distinct excitation-dependent luminescence behavior. Facile switching the excitation wavelength from 260 nm to 230 nm yields emission color changing from orange to green yellow, which possesses the advanced security feature.


Introduction
Recently, counterfeiting has grown to be a serious global threat, which includes the duplication of currencies, electronic products, merchandise, passports, official documents, pharmaceuticals, and so on [1]. Counterfeiting has caused enormous loss to the economy and a constant risk to the health and safety of consumers [1,2]. To prevent counterfeiting, a lot of advanced security technologies have been developed, including radio frequency identification, hologram attachment, isotope tracking and luminescent printing [3]. Among above anticounterfeiting technologies, luminescence printing attracted much attention, because of its low production costs, being eco-friendly and hard to counterfeit [4,5]. Therefore, developing the pigments with advanced security features, such as a single excitable dual emissive luminescence [3] or excitation dependent color tunable luminescence [6,7], is the general trend in luminescence printing-based anti-counterfeiting technologies. Recently, fluorescent patterns based on up-conversion materials have been developed for anti-counterfeiting, because of their high-level security [8][9][10][11].
Yttrium orthoborate of YBO 3 , possessing a hexagonal vaterite-type structure, exhibits low toxicity, high chemical stability, and vacuum ultraviolet transparency [12][13][14][15]. More importantly, the Y site in this structure can be facile substituted with various RE 3+ ions (RE = rare earth). Therefore, YBO 3 is a good host lattice for luminescence studies. Eu 3+ , Tb 3+ , and Eu 3+ /Tb 3+ activated YBO 3 phosphors are the important red, green, and color-tunable emitting phosphors finding wide applications in areas such as fluorescent lamps, white light-emitting diodes, plasma display panels, flat panel displays, and field emission displays [12][13][14][15][16]. It is widely accepted that activator Eu 3+ occupies the Y 3+ site in hexagonal YBO 3 :Eu 3+ with the point symmetry of S 6 , thus the 5 D 0 → 7 F 1 transition of Eu 3+ takes the dominate role [17]. Therefore, the solid-solution phosphor always exhibits orange-red emission at 590 nm. While under exposure to UV light, YBO 3 : Tb 3+ displays the typical green emission at~546 nm ( 5 D 4 → 7 F 5 transition of Tb 3+ ), and color-tunable photoluminescence can be found in solid solution phosphor of Y/Tb/Eu ternary system through an efficient energy transfer from Tb 3+ to Eu 3+ ions [18][19][20]. Recently, it was reported that doping Bi 3+ in rare-earth borates could effectively manipulate the synthetic/thermal stable temperature [21]. In addition, further doping Bi 3+ in Y/Tb/Eu ternary system is a good idea to take advantage of the sensitization effect of Bi 3+ to obtain a single-phase phosphor with color-tunable and greatly improved emission by manipulating the doped concentrations of Bi 3+ , Eu 3+ , and Tb 3+ in YBO 3 [22].
Usually, rare-earth borates, such as REBO 3 , REB 3 O 6 , and RE 3 BO 6 , were synthesized through typical solidstate reactions at high temperatures or sol-gel technology followed by calcinations [12][13][14][15][16]. Because the final products are always featured large size, irregular shape and significant aggregation, they are limited in application of high-definition displays. However, phosphor particles of spherical shape and uniform size are desirable in high-definition displays to improve the resolution and the overall luminescent performance. The spherical engineering of rare-earth borates is of great importance, but remains a considerable challenge in particle science. Very recently, precursor route is reported as an effective way to synthesize well-dispersed and uniform spheres of rare-earth borates. Colloidal moydite spheres of RE(B(OH) 4 )CO 3 in micron size have been facilely synthesized by a modified homogenous precipitation (HP), which were used as precursors for synthesizing REBO 3 (RE = Y, Gd, Eu) phosphor spheres [14,15]. Subsequently, submicron spheres of RE 3 BO 6 (RE = Eu-Yb, Y) have been converted from their colloidal precursor spheres (amorphous phase form) synthesized via homogeneous precipitation [16]. In this work, micron-sized moydite spheres of M(B(OH) 4 )CO 3 (M = Y, Tb, Eu, Bi) quaternary system have been rapid synthesized by microwave processing, and the precursor converted into (Y, Tb, Eu, Bi)BO 3 color-tunable emitting phosphor by a proper annealing. The effects of Bi 3+ incorporation on crystal structure and luminescent behavior were investigated by detailed characterizations of XRD, FE-SEM, HR-TEM, STEM, PLE/PL spectroscopy, and fluorescence decay analysis. The (Y, Tb, Eu, Bi)BO 3 spheres exhibit an excitation-dependent luminescence, with emission color changing from green-yellow to orange by switching the excitation wavelength. The prepared spheres are the potential luminescent pigment to prevent counterfeiting by generating advanced security feature safety levels.

Synthesis
The starting rare-earth sources are Y 2 O 3 , Tb 4 O 7 , and Eu 2 O 3 all 99.99% pure products purchased from Huizhou Ruier Rare-Chem. Hi-Tech. Co. Ltd (Huizhou, China). Bi(NO 3 ) 3 · 5H 2 O and other reagents are of analytical grade and were purchased from China pharmaceutical group chemical reagent Co. LTD (Shanghai, China). The nitrate solution of RE 3+ was prepared by dissolving the corresponding oxide with a slightly excessive amount of nitric acid, followed by evaporation at~90°C to dryness to remove the superfluous acid. In a typical synthesis, 0.225 mol of H 3 BO 3 , 0.5 mol of urea (CO(NH 2 ) 2 ), and 120 mL of ethylene glycol (EG) were dissolved in the mixed nitrate solution to make a total volume of 1000 mL, which was then homogenized under magnetic stirring at 25°C for 30 min. In all the cases, the total concentration of RE 3+ and Bi 3+ was kept constant at 0.015 mol/L. The mixed transparent solution was heated to~95°C within 10 min in a microwave, and then cooled to~70°C after reacting at~95°C for another 120 min. The resultant colloidal particles were collected via centrifugation, washed with distilled water for three times to remove byproducts, rinsed with absolute ethanol, and then dried in air at 50°C for 24 h. (Y, Tb, Eu, Bi)BO 3 was obtained by calcining the precipitation products under flowing O 2 gas (200 mL/min) at 800°C for 2 h with a heating rate of 10°C/min at the ramp stage.

Characterization techniques
Phase identification was performed by X-ray diffractometry (XRD, Model Smart Lab, Rigaku, Tokyo, Japan) operating at 40 kV/40 mA using nickel filtered Cu Kα radiation and a scanning speed of 6.0°2θ/min. Morphologies of the products were observed via field emission scanning electron microscopy (FE-SEM, Model JSM-7001 F, JEOL, Tokyo) and transmission electron microscopy (TEM, Model JEM-2000FX, JEOL, Tokyo). Elemental mapping was performed using scanning transmission electron microscopy (STEM, Model JEM-2000FX, JEOL, Tokyo). X-ray photoelectron spectroscopy (XPS) data were measured using an X-ray photoelectron spectrometer (Model Axis Supra, Kratos Analytical Ltd., Manchester, UK) with monochromatized Al Kα X-ray radiation. The measurements were performed using an ultrahigh vacuum chamber with a base pressure below 3 × 10 −9 Torr at room temperature. The binding energies were calibrated by using C 1 s (284.8 eV) of carbon impurities as reference. Photoluminescence and fluorescence decay of the phosphors were analyzed with an FP-8600 fluorospectrophotometer (Jasco, Tokyo). Fluorescence decay kinetics of Bi 3+ emission was measured at room temperature on a HORIBA scientific modular fluorescence lifetime system (Model DeltaFlex, HORIBA Jobin Yvon IBH Ltd., Scotland).

Morphology and phase structure
Colloidal moydite microspheres of M(B(OH) 4 )CO 3 (M = Y, Tb, Eu, Bi) were facilely and rapidly synthesized by a microwave-assisted homogenous precipitation. Figure 1(a-c) shows FE-SEM morphologies of three typical samples, that are (Y 0.945 Eu 0.05 Bi 0.005 )(B(OH) 4 )CO 3 , (Y 0.973 Tb 0.02 Bi 0.007 )(B(OH) 4 )CO 3 , (Y 0.923 Tb 0.02 Eu 0.05 Bi 0.007 )(B(OH) 4 )CO 3 . The results show that the microspheres with a diameter of 1-2 μm were synthesized in all cases. The XRD patterns in Figure 2(a) indicate that the obtained spheres are identified along with the orthorhombic-structured moydite Y(B(OH) 4 )CO 3 (JCPDS No. 78-2062), and no impurity peaks were observed. Because the ionic sizes of Tb 3+ , Eu 3+ and Bi 3+ (for eightfold coordination) are r Eu 3þ = 0.1066 nm, r Tb 3þ = 0.1040 nm, r Bi 3þ = 0.1170 nm, which are larger than that of Y 3+ (for eightfold coordination, r Y 3þ = 0.1019 nm) [23], incorporation of Tb 3+ , Eu 3+ and Bi 3+ in Y(B(OH) 4 )CO 3 results in larger cell constants than that of Y(B(OH) 4 (Figure 2(b)), indicating that the orthorhombic-structured moydite is the ideal precursor for rare earth orthophosphate, because of the low synthesis temperature and facile and green processing. The resultant particles almost remained the original spherical shape of the precursor (Figure 1(d-f)). However, there are obvious holes on the particle surface ( Figure 1(d-f)), mainly due to the decomposition of hydroxyl and carbonate during the calcination. The light and dark contrast in the TEM image of particles suggests the particle surface is not smooth with holes on the surface (Figure 3(a)). The HR-TEM analysis further confirms that the spheres belong to hexagonal crystal structure (Figure 3(b)). The wellresolved lattice fringes suggest their excellent crystallinity, while the spacing of~0.45 nm corresponds well to the (002) plain of hexagonal structured YBO 3 (d(002) =~0.44 nm, JCPDS No. 74-1929) (Figure 3(b)). Elemental mapping results show that all elements are distributed among the particles according to the particle morphology, indicating that they are homogeneous solid solutions (Figure 3(c-f)). Figure S1 presents highresolution XPS spectra of Tb 3d 3/2 and 3d 5/2 for the Tb containing samples calcined at 800ºC, indicating that Tb 3+ is not oxidized to Tb 4+ during the annealing process in oxygen atmosphere.   [14][15][16]. However, another strong excitation band at 260 nm appeared in the presence of Bi 3+ (x = 0.001-0.01), which is assigned to the 1 S 0 → 3 P 1 transition of Bi 3+ [24,25]. Because the 1 S 0 → 3 P 1 transition of Bi 3+ (260 nm) is more intense than the CTB (230 nm), the 260 nm is the most efficient excitation wavelength for (Y 0.95-x Eu 0.05 Bi x )BO 3 spheres. Under excitation at 260 nm, the emission spectra show emission peaks at~592 nm, 610 and~627 nm,~650 and~673 nm, and~694 and~708 nm, which are assigned to the typical 5 D 0 → 7 F J (J = 1,2,3,4) transition of Eu 3+ ions respectively [26] (Figure 4(b)). Because activator Eu 3+ occupies the Y 3+ site in hexagonal structure with the point symmetry of S 6 , the magnetic dipole 5 D 0 → 7 F 1 transition of Eu 3+ at 592 nm takes the dominate role, rather than the forced electric dipole 5 D 0 → 7 F 2 transition of Eu 3+ ions at~610 and~627 nm [12][13][14][15][16]. However, a broad emission band ranging from 275 nm to 375 nm with the maximum at 330 nm appeared with increasing x from 0 to 0.001, and the emission became more intense at a higher x value (Bi 3+ content). Because of the incorporation of Bi 3+ , stronger emission intensity of Eu 3+ was observed, and the emission intensity at 592 nm increased by 710% with increasing x from 0 to 0.005 (Figure 4 (b)), indicating an efficient energy transfer from Bi 3+ to Eu 3+ . However, further increasing the x value above 0.005 yielded a weakened emission, suggesting the optimized concentration for Bi 3+ ions is 0.005. While under excitation at 230 nm (CTB for Eu 3+ ), the emission spectra only show emission of Eu 3+ , in the absence of Bi 3+ emission (Figure 4(d)), indicating that the energy transfer from CTB to Bi 3+ does not exist. Figure 4(c) shows UV-vis absorption spectra of (Y 0.95 Eu 0.05 )BO 3 and (Y 0.945 Eu 0.05 Bi 0.005 )BO 3 , and an intense transition band of 1 S 0 → 3 P 1 of Bi 3+ ions at 260 nm appeared for Bi 3+containing sample, giving the direct evidence of Bi 3+ →Eu 3+ energy transfer. Figure 4(e) shows PLE spectra of (Y 0.98-x Tb 0.02 Bi x )BO 3 . The PLE spectrum recorded by monitoring the green emission at 546 nm ( 5 D 4 → 7 F 5 transition of Tb 3+ ), exhibits a broad transition band ranging from 220 to 300 nm, with two strong bands centered at~234 nm and~260 nm, which are corresponding to the well-documented 4f 8 →4f 7 5d 1 transition of Tb 3+ [18,27] and 1 S 0 → 3 P 1 transition of Bi 3+ [24,25]. Upon UV excitation at~260 nm, the spheres displayed the typical emissions of Tb 3+ at 491 nm,~546 nm,~586 nm, and~622 nm, which are assigned to the 5 D 4 → 7 F J (J = 6-3) transitions of Tb 3+ [18,27], respectively (Figure 4(f)). Obviously, the green emission at~546 nm ( 5 D 4 → 7 F 5 transition of Tb 3+ ) takes the dominated role. Similarly, incorporation of Bi 3+ also enhances the emission intensity of Tb 3+ . The emission intensity at 546 nm increased by 1150% with increasing x from 0 to 0.007 (Figure 4(f)), indicating an efficient energy transfer from Bi 3+ to Tb 3+ . However, further increasing the x value above 0.007 yielded a weakened emission, which is similar to that observed for (Y 0.95-x Eu 0.05 Bi x )BO 3 spheres. Incorporation of Bi 3+ in borate host enhances the absorption of the optical excitation, which can effectively promote the energy transfer from Bi 3+ ions to the activated Eu 3+ and Tb 3+ ions and thus enhancing the emission intensity. However, a higher Bi 3+ concentration would lead to the internal consumption between Bi 3+ ions through energy transfer and nonradiative transition, which makes the energy transfer of Bi 3 + →Eu 3+ and Bi 3+ →Tb 3+ become weak. Therefore, the most intense orange-red emission and green emission were found for (Y 0.945 Eu 0.05 Bi 0.005 )BO 3 and (Y 0.973 Tb 0.02 Bi 0.007 )BO 3 . Similarly, under excitation at 234 nm (4f 8 →4f 7 5d 1 transition of Tb 3+ ), the emission spectra only show emission of Tb 3+ , in the absence of Bi 3+ emission (Figure 4(d)), indicating that the energy transfer from Tb 3+ to Bi 3+ does not exist.

Luminescence behavior of (Y, Tb, Eu, Bi)BO 3 spheres
Usually, Tb 3+ and Eu 3+ need different excitation wavelengths for their respective emissions, and thus mixing two phosphors cannot efficiently tune the mission color (though can). The Tb 3+ /Eu 3+ co-doped borates may have the advantage of emitting tunable colors under one single excitation wavelength, through the energy transfer from Tb 3+ to Eu 3+ . Therefore, (Y 0.973-y Tb 0.02 Eu y Bi 0.007 )BO 3 spheres were investigated here. The PLE spectra recorded by monitoring the orange-red emission at 592 nm ( 5 D 0 → 7 F 1 transition of Eu 3+ ) and the green emission at 546 nm ( 5 D 4 → 7 F 5 transition of Tb 3+ ) exhibit different excitation bands in Figure 5(a,b). Obviously, the 1 S 0 → 3 P 1 transition of Bi 3+ at 260 nm is stronger than the CTB at 230 nm ( Figure 5(a)), but weaker than 4f 8 →4f 7 5d 1 transition of Tb 3+ at 234 nm ( Figure 5(b)), indicating that the energy transfer of Bi 3+ →Eu 3+ is more efficient than that of Bi 3+ →Tb 3+ . Under the excitation of 230 nm and 260 nm, the characteristic emissions of both Tb 3+ and Eu 3+ simultaneously appeared on the PL spectra, with the green Tb 3+ emission at 546 nm ( 5 D 4 → 7 F 5 transition of Tb 3 + ) monotonically decreasing with increasing y (Figure 5(c,  d)), which is conforming to the tendency observed from the PLE spectra. Accordingly, successively stronger Eu 3+ emission at 592 nm ( 5 D 0 → 7 F 1 transition of Eu 3+ ) was seen for the y = 0-0.09 samples. The reduced Eu 3+ emission at y = 0.11 is suggestive of concentration quenching. The above results suggest efficient energy migration in transitioning from Tb 3+ to Eu 3+ . It is interesting to find that the PL spectra exhibit characteristic emissions of 3 P 1 → 1 S 0 transition of Bi 3+ at 330 nm except for the typical emissions of Tb 3+ and Eu 3+ ions under the excitation at 260 nm (Figure 5(d)). The UV emission at 330 nm ( 3 P 1 → 1 S 0 transition of Bi 3+ ) and the green emission at 546 nm ( 5 D 4 → 7 F 5 transition of Tb 3+ ) monotonically decrease with increasing y from 0 to 0.11 ( Figure 5(d)). But the Eu 3+ emission at 592 nm ( 5 D 0 → 7 F 1 transition of Eu 3+ ) became stronger at a higher y value until it above 0.09 ( Figure 5 (d)). It is interesting found that the intensity of Eu 3+ emission under excitation at 260 nm increased faster than that under excitation at 230 nm. The I( 5 D 0 → 7 F 1 )/I( 5 D 4 → 7 F 5 ) intensity ratio (monitoring I 592 /I 546 ) under excitation at 260 nm increased from 0.98 at y = 0.01 tõ 53.5 at y = 0.11, which is much larger than that under excitation at 230 nm ( Figure 5(e)). Thus, the emission color of (Y 0.973-y Tb 0.02 Eu y Bi 0.007 )BO 3 spheres moved faster from green to red region by Eu 3+ doping under excitation at 260 nm which are in good agreement with the results for CIE chromaticity diagram ( Figure 6). The above results also indicate that varying the Eu content from 0 to 0.11 yielded color tunable emission from green to red ( Figure 6). From above results, a probable energy transfer process among Bi 3+ , Tb 3+ , and Eu 3+ ions are demonstrated in Figure 7(a). Under ultraviolet-light (260 nm) excitation, the ground-state electrons of Bi 3+ ions at 1 S 0 level are promoted to the 3 P 1 level. Then, the 3 P 1 → 1 S 0 transition of Bi 3+ contributes to the 330-nm emission, along with the three possible energy transfer processes of Bi 3+ →Tb 3+ , Bi 3+ →Eu 3+ , and Tb 3+ →Eu 3+ , which finally contributes to the green emission of Tb 3+ and orange-red emission of Eu 3+ . However, under the excitation at 234 nm (4f 8 →4f 7 5d 1 transition of Tb 3+ ), the Tb 3+ →Eu 3+ energy transfer contributes to the orange-red emission of Eu 3+ , along with the green emission of Tb 3+ from the 5 D 4 → 7 F J transition.
In order to investigate the energy transfer efficiency, lifetimes of the 330-nm Bi 3+ emission and 546nm Tb 3+ emission were analyzed against the Eu 3+ content (0 ≤ y ≤ 0.11) (Figure 7(b,c)). In the absence of Tb 3+ and Eu 3+ , the lifetime of Bi 3+ is 210 ns (Figure S2), and it decreases to 194 ns in the presence of 2% Tb 3+ (Figure S2), indicating the energy transfer from Bi 3+ to Tb 3+ . The lifetime of Bi 3+ decreased from 194 ns for y = 0 to 11 ns for y = 0.11. The shortened lifetime of Bi 3+ emission at higher Eu content (Figure 7(b) and Figure S2) confirms the energy transfer process from Bi 3+ to Tb 3+ and Eu 3+ . In the absence of concentration quenching, the efficiency (η ET ) of the Bi 3+ → "Eu 3+ and Tb 3+ " energy transfer can be calculated from the fluorescence lifetime with η ET = 1-τ/τ 0 , where τ and τ 0 are the fluorescence lifetime of the Bi 3+ emission in the presence and absence of the acceptor [18,26]. The η TE value, calculated from Figure 7(b), gradually increases from 2% for y = 0.01 to 63% for y = 0.09. In addition, fluorescence decay kinetics of the 546 nm Tb 3+ emission were analyzed against the Eu 3+ content (0 ≤ y ≤ 0.11) through single exponential fitting ( Figure S3). The lifetime of Tb 3+ decreased from 5.12 ms for y = 0 to 1.95 ms for y = 0.11 (Figure 7(c)). The shortened lifetime of Tb 3+ emission at higher Eu content (Figure 7(c)) confirms the energy transfer process from Tb 3+ to Eu 3+ . The η TE value, calculated from Figure 7(c), gradually increases from 9% for y = 0.01 to 50% for y = 0.09.

Excitation dependent emission and prospect of application
The (Y, Tb, Eu, Bi)BO 3 spheres exhibited an excitation-dependent luminescence behavior. Taking (Y 0.963 Tb 0.02 Eu 0.01 Bi 0.007 )BO 3 an example, the phosphor exhibited two kinds of PL spectra under the excitation at various wavelengths (Figure 8(A)). Under the excitation at 260 nm, the sample emits UV emission at 330 nm ( 3 P 1 → 1 S 0 transition of Bi 3+ ), green emission at 546 nm and orange red emission at 592 nm, with the intensity of green emission close to that of orange red emission. Therefore, the sample emits orange color with the CIE coordinates of (0.45, 0.45) (Figure 8(B)). The external and internal quantum efficiencies are~46% and 79%, respectively. However, under the excitation at 230 nm, the sample only emits strong green emission at 546 nm and weak orange red emission at 592 nm, which finally results in green-yellow color emission with the CIE coordinates of (0.37, 0.52) (Figure 8(B)). The external and internal quantum efficiencies are~34% and 65%, respectively. The distinct emission color from orange to green yellow can be achieved by facile switching the excitation wavelength from 260 nm to 230 nm (Figure 8(C)), which indicate that the phosphor is the potential anti-counterfeiting pigment. Recently, some research groups have made efforts to developing dual emissive pigments based on different excitation wavelengths [3,28,29]. Here, the excitation-dependent emitting (Y, Tb, Eu, Bi)BO 3 spheres with advanced security feature would be a member of the luminescent-pigment family to prevent counterfeiting by generating advanced security feature safety levels.

Conclusion
In this work, micron-sized moydite spheres of M(B(OH) 4 )CO 3 (M = Y, Tb, Eu, Bi) ternary and quaternary systems have been rapid synthesized by microwave processing, and the precursor converted into hexagonal-structured and color-tunable emitting phosphor of MBO 3 rare earth borates by a proper annealing. Incorporation of Bi 3+ does not significantly affect the crystal structure of the phosphors, but it greatly enhances the emission intensity of Eu 3+ and of Tb 3+ . Under the excitation at 260 nm ( 1 S 0 → 3 P 1 transition of Bi 3+ ), the (Y, Tb, Eu, Bi)BO 3 spheres exhibit UV emission at 330 nm ( 3 P 1 → 1 S 0 transition of Bi 3+ ), green emission at 546 nm ( 5 D 4 → 7 F 5 transition of Tb 3+ ) and orange-red emission at 592 nm ( 5 D 0 → 7 F 1 transition of Eu 3+ ), mainly due to the three possible energy transfer processes of Bi 3+ →Tb 3+ , Bi 3+ →Eu 3+ , and Tb 3+ →Eu 3+ . However, under the excitation at 234 nm (4f 8 →4f 7 5d 1 transition of Tb 3+ ), the Tb 3+ →Eu 3+ energy transfer contributes to the orange-red emission of Eu 3+ and the green emission of Tb 3+ from the 5 D 4 → 7 F J transition, in the absence of Bi 3+ emission. The maximum energy transfer efficiency of Bi 3+ → "Eu 3+ and Tb 3+ " and Tb 3+ → Eu 3+ is 63% and 50% for (Y 0.883 Tb 0.02 Eu 0.09 Bi 0.007 )BO 3 sample. The (Y 0.963 Tb 0.02 Eu 0.01 Bi 0.007 )BO 3 spheres exhibited a distinct excitationdependent luminescence behavior, with emission color from orange to green yellow being achieved by facile switching the excitation wavelength from 260 nm to 230 nm. The excitation-dependent emitting (Y, Tb, Eu, Bi)BO 3 spheres with advanced security feature would be a member of the luminescentpigment family to prevent counterfeiting by generating advanced security feature safety levels.