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Abstract
Developing advanced luminescent materials that are recognizable under specified conditions provides better opportunity for reliable optical anti-counterfeiting techniques. In this work, to the best of our knowledge, novel GdInO3:Tm,Yb perovskite phosphors with ultrafine sizes and rounded morphologies were successfully synthesized by a facile chemical precipitation route. Two-type perovskites with orthorhombic and hexagonal structures could be obtained by calcining the precursor at 850 and 1100 °C, respectively. Under 980 nm excitation, the two phosphors exhibited cyan-bluish emission at ∼460−565 nm, red emission at 645−680 nm, and near-infrared emission at 770−825 nm arising from 1G4 + 1D2→3H5,6, 3F2,3→3H6, and 3H4→3H6 transitions of Tm3+, respectively, where the hexagonal perovskite phosphor had relatively strong and sharp red emission as well as red-shifted cyan-bluish emission via successive cross relaxations. The Yb3+ sensitizer enhanced the upconversion luminescence via effective Yb3+→Tm3+ energy transfer and the optimal Yb3+ concentrations were 10 at.% for orthorhombic perovskite and 5 at.% for hexagonal one. The upconversion mechanism mainly ascribed to two-photon processes while three-photon was also present. Upon excitation at 254 nm, their down-conversion spectra exhibited broad multibands in the wavelength range of 400−500 nm deriving from combined effects of the defect-induced emission of GdInO3 and the 1D2→3F4 + 4G4→3H6 emissions of Tm3+. The energy transfer from GdInO3 defect level to Tm3+ excitation state was observed for the first time. The unclonable security codes prepared by screen printing from those dual-mode emitting perovskite phosphors were almost invisible under natural light, which had promising potential for anti-counterfeiting application.
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1. Introduction
There is growing interest in perovskite-type compounds with the general chemical formula of ABX3 due to configurational and compositional flexibility of the perovskite octahedral framework [1,2]. In the perovskite family, the rare-earth-type oxide perovskites expressed as ABO3, including A2 + B4 + O3, A3 + B3 + O3, and A + B5 + O3, are the most versatile for a broad range of applications in luminescence, photocatalytic activity, magnetism, piezo-electricity, and energy storage/conversion [3,4]. Herein, ‘A’ as one of the rare-earth ions (RE3+) may be selected from a wide range of the lanthanide family (including Y), which provides a unique opportunity to adjust the structural and physical properties of the system [5]. Diverse rare-earth cationic radii in A site together with B-site cation determine the crystal structure, which can be quantificationally characterized based on the relationship among A, B and O ionic radii (r) given by Goldschmidt’s tolerance factor t = (rA + rO) /[2(rB + rO)]1/2 [6]. An ideal ABO3 perovskite composed of a cubic array of corner-connected BO6 octahedra with a large ‘A’ cation at the body-center position empirically requires the t value between 0.9 and 1.0, where its schematic crystal structure is shown in Fig. S1 of the Supplement 1. However, a further decrease in tolerance factor would reduce the coordination for ‘A’ cation and the space group symmetry to form the orthorhombic (0.75 < t < 0.9) or hexagonal structure (t < 0.75) via the rotation of BO6 polyhedra for structural equilibrium. If the BO6 polyhedra rotates along cubic (111), (010), or (101) direction, rhombohedral R3c, orthorhombic Pnma, or orthorhombic Pbnm structure would be resulted in, respectively [5,7].
Upconversion luminescence is a specific anti-Stoke's process whereby low-energy photons are converted to high-energy ones [8,9]. A trivalent lanthanide-doped upconversion material generally exhibits excellent spectrum characteristics such as near-infrared excitation, good light stability, narrow spectrum, and high quantum yield to be extensively applied in the fields of optical thermometry, biomedical imaging, solar cell, three-dimensional display, fluorescent tags, plastic recycling, optical data storage, and optical anti-counterfeiting [10–17]. Among these applications, the advanced anti-counterfeiting technique is accepting of the luminescent materials, instead of common plasmonic materials, metal-organic-frameworks and quantum dots, owning to the advantages of low cost, non-toxicity, good concealment, low imitation, facile processing, low photobleaching, and high identification and integration [18–20]. In modern society, luminescent markers, holograms, plasmonic labels, magnetic tags, invisible inks, and digital and smart signatures are widely used against counterfeiting of documents, certificates, brands, and pharmaceuticals [21–23]. The key design target for the implementation of technology lies in hard duplication and easy authentication [24–26].
In order to achieve high-quality homogeneous profile for anti-counterfeiting technology or fingerprint identification, ultrafine and uniform powder phosphors with good luminescence performance are preferred. Yb3+ is widely used as an excellent sensitizer for RE3+ upconversion luminescence due to its large absorption cross-section from 2F7/2 to 2F5/2 transition in the spectral range of 850–1080 nm as well as simple energy level scheme [27]. A matrix material with low phonon vibration would reduce the multi-phonon relaxations to contribute to efficient upconversion luminescence [28]. The GdInO3 perovskites have low phonon energy (∼368–425 cm−1) [29], and thus can be considered as luminous substrate materials by properly doping with activator ions. Hitherto, there has been no reports dealing with the luminescence properties of GdInO3:Tm,Yb perovskite phosphors in the references. In this work, we successfully developed new fluorescent perovskite materials for dual-mode anti-counterfeit applications. The effects of crystal structures on upconversion luminescence as well as the energy transfer from Yb3+ to Tm3+ were studied through a detailed contrastive analysis. More important, the down-conversion luminescence composed of interesting energy transfer from GdInO3 defect level to Tm3+ excitation state was observed for the first time.
2. Experimental details
2.1. Phosphor preparation
The starting raw materials were Gd(NO3)3·6H2O (99.99% purity, Diyang Chemical Co., Ltd., Shanghai, China), Tm(NO3)3·6H2O (99.99% purity, Diyang Chemical Co., Ltd., Shanghai, China), Yb(NO3)3·5H2O (99.99% purity, Diyang Chemical Co., Ltd., Shanghai, China), and In(NO3)3·4.5H2O (99.99% purity, Diyang Chemical Co., Ltd., Shanghai, China). The four nitrate salts were together dissolved in distilled water to prepare a 0.2 mol/L mother liquor according to the stoichiometric ratio of (Gd0.999-xTm0.002Ybx)InO3 (x = 0–0.15) compositions. A 1.5 mol/L NH4HCO3 (∼99.99% purity, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) precipitant was dropwise added to the mother liquor to yield white precipitation under magnetic stirring at room temperature, where the molar ratio of NH4HCO3 to the total cations was 3:1. After aging for 1.5 h, the suspension was repeatedly rinsed with distilled water and ethanol via centrifugal separation to remove byproducts. The white precipitation precursors were dried at 90 °C for more than 12 h and finally calcined in a muff furnace at 850 and 1100 °C for 3 h with a heating rate of 5 °C/min to yield the target products.
2.2 Screen-printing technology
The detailed screen-printing procedures for anti-counterfeit applications could refer to the schematic diagram shown in Fig. 1. The commercial terpineol (AR, Shanghai Macklin Biochemical Technology Co., Ltd., China) and ethyecellulose (CP, Shanghai Macklin Biochemical Technology Co., Ltd., China) according to the mass ratio of 100:7 were well mixed under magnetic stirring at a constant temperature of 60 °C to yield a transparent paste medium [Fig. 1(a)]. After natural cooling, the glue solution and the phosphor were together ground with an agate mortar and a pestle [Fig. 1(b)] to obtain a uniform viscous slurry [Fig. 1(c)]. A printing screen containing the desired pattern [e.g. quick response (QR) codes or labels; Figs. 1(d)] was fixed by the aluminum frame [Fig. 1(e)], and then the preprinting material was placed below the printing screen [Fig. 1(f)]. The as-prepared slurry was poured onto the screen [Fig. 1(g)], followed by printing with a rubber brush to obtain the target pattern [Fig. 1(h)].
2.3 Sample characterization
The phase structures of the precursors and the calcination products were identified by X-ray diffraction (XRD; Model D8 Advance, Bruker, Karlsruhe, Germany) using nickel-filtered Cu-Kα as the incident radiation. The chemical composition of precipitation precursor was qualitatively analyzed by Fourier transform infrared spectroscopy (FTIR; Model NICOLET 6700, Thermo Fisher Scientific, Massachusetts, USA). The morphologies of precipitation precursors and calcination products were observed on a field-emission scanning electron microscopy (FE-SEM, Model Phenom Pharos, Thermo Fisher, Eindhoven, Netherlands). The upconversion luminescence spectra together with the fluorescence decay behaviors were characterized using a spectrofluorometer (Model FLS980, Edinburgh Instruments, Edinburgh, UK) under the excitation of a 980 nm laser diode at room temperature. The down-conversion luminescence spectra as well as the fluorescence decay kinetics were measured by a fluorescence spectrophotometer (Model F-4600, Hitachi, Tokyo, Japan) using a 450 W xenon lamp as the excitation source at room temperature.
The crystallite size (dXRD) can be calculated from the full width at half maximum (FWHM) of the XRD diffraction peak using Scherrer’s equation: dXRD = Kλ/(βcosθ), where K is the shape factor (0.89), λ is the wavelength of the X-ray (λ = 0.15406 nm), β is the FWHM, and θ is the Bragg’s angle in degree.
3. Results and discussion
3.1 Phase structures and microscopic morphologies
Figure 2(a) shows XRD patterns of the precursors intended for (Gd0.998-xTm0.002Ybx)InO3 (x = 0 and 0.1) and their calcination products obtained at 850 and 1100 °C. The precursor exhibits typical amorphous state without characteristic peaks. FTIR analysis further indicates that the precursor has a chemical composition of hydrated basic carbonate (Fig. S2 in the Supplement 1). Additionally, TG analysis further reveals that the pyrolytic processes of the precursors undergo dehydration, dehydroxylation, and decarbonization (Fig. S3 in the Supplement 1), while the sample with Yb3+ co-doping causes a far lower final decomposition temperature than the counterpart free of Yb3+ due to the lower alkalinity of Yb3+ than that of Gd3+. After calcination at 850 °C, the products fully crystallize into the orthorhombic perovskites in agreement with the or-GdInO3 standard card (JCPDS No. 21-0335). A 250 °C increase in calcination temperature up to 1100 °C induces phase transformation and the diffraction peaks can be well indexed into the h-GdInO3 standard card (JCPDS No. 14-0150). Observed from the right-hand inset in Fig. 2(a), the XRD peaks shift towards the high angle side after 10 at.% Yb3+ co-doping, since the ionic radius of Yb3+ is much smaller than that of Gd3+ according to lanthanide contraction law leading to lattice contraction. The calculated cell dimensions from Bragg's equation are a = 0.5536 nm, b = 0.5850 nm, c = 0.8071 nm, and V = 0.2614 nm3 for or-GdInO3:Tm; a = 0.5521 nm, b = 0.5814 nm, c = 0.8042 nm, and V = 0.2581 nm3 for or-GdInO3:Tm,Yb; a = b = 0.3657 nm, c = 1.2257 nm, and V = 0.1420 nm3 for h-GdInO3:Tm; and a = b = 0.3645 nm and c = 1.2206 nm, and V = 0.1404 nm3 for h-GdInO3:Tm,Yb. That is to say, the Yb3+ co-doping only slightly shrinks the unit cell, however, the crystal form transformation from orthorhombic to hexagonal structure brings with dramatic cell dimension contraction almost half of the volume. The h-GdInO3:Tm perovskite has a rather larger crystallite size (∼84.2 nm) than the or-GdInO3:Tm counterpart (∼69.8 nm), because the higher calcination temperature promotes the crystallite growth. After 10 at.% Yb3+ co-doping, their corresponding crystallite sizes become smaller (∼52.0 nm for or-GdInO3:Tm,Yb and ∼81.7 nm for h-GdInO3:Tm,Yb), which are mainly attributed to the smaller average cationic radius together with contracted unit cell.
Fig. 2. XRD patterns of the precursors intended for (Gd0.998-xTm0.002Ybx)InO3 (x = 0 and 0.1) and their calcination products obtained at 850 and 1100 °C (a) and schematic crystal structures of orthorhombic (b) and hexagonal (c) GdInO3:Tm,Yb perovskites. The inset in the right-hand panel (a) shows the enlarged view of the main (112) diffraction peak.
The schematic crystal structures of orthorhombic and hexagonal GdInO3:Tm,Yb perovskites are shown in Figs. 2(b) and (c). The orthorhombic perovskite comprises tilted, slightly distorted, corner-connected InO6 octahedra and distorted GdO8 polyhedra [Fig. 2(b)], where the Gd and In atoms locate in 8-fold and 6-fold coordination environments, respectively. The hexagonal perovskite is built up by repetitive stacking of InO5 trigonal bipyramids and GdO7 polyhedra [Fig. 2(c)], where the InO5 bipyramids contain two apical and three planar oxygen anions while the Gd and In atoms are in 7-fold and 5-fold coordination surroundings, respectively [30]. The detailed data on ionic radii of Gd3+, In3+, Yb3+, and Tm3+ for 5–8 coordination numbers are listed in Table S1 of the Supplement 1. On the basis of similar ionic radius rule, Yb3+ and Tm3+ dopants reasonably prefer to replace the Gd3+ site for both of the two-type perovskites.
Figure 3 compares the morphologies of (Gd0.998Tm0.002)InO3 and (Gd0.898Tm0.002Yb0.1)InO3 precursors and their perovskite products obtained at 850 °C. The two precursors present ultrafine rounded morphologies [Figs. 3(a) and (b)], where the precursor co-doped with Yb3+ contains much finer size compared with the precursor free of Yb3+. Such a phenomenon is closely associated with the nucleation kinetics during precipitation formation. In water solution, M3+ (M = Ln, In) ions are readily hydrated to [M(H2O)6]3+ and then hydrolyze to form [M(OH)x(H2O)6−x]3−x species [31–33]. However, the NH4HCO3 solution contains various ionic species, such as HCO3−, CO32-, OH−, H+, and NH4+. The precipitation formation highly depends on the degree of hydrolysis of M3+. Regardless of In3+ effect due to its fixed content, the degree of hydrolysis of RE3+ increases with decreasing ionic radius. According to the lanthanide contraction law, the degree of hydrolysis increases in the order Gd3+ < Tm3+ < Yb3+. As a result, more Yb3+ dopants achieve a higher activity of [Yb(OH)x(H2O)6−x]3−x leading to the initial formation of Yb(OH)CO3 in priority to act as crystal nuclei when the NH4HCO3 precipitate is added. Hence, a higher Yb3+ content contributes to a higher nucleation density causing a smaller particle size. The orthorhombic perovskites obtained at 850 °C display rounded morphologies [Figs. 3(c) and (d)]. However, the 10 at.% Yb3+ co-doped sample presents bigger particle size with slightly elongated neck than the counterpart free of Yb3+, because its lower final thermolysis temperature provides more probability for particle growth (Fig. S3 in the Supplement 1). In general, the particle sizes of the two-type perovskites are both ultrafine and relatively homogeneous, which would contribute to uniform luminescence intensity distribution.
Fig. 3. FE-SEM micrographs showing morphologies of (Gd0.998Tm0.002)InO3 (a, c) and (Gd0.898Tm0.002Yb0.1)InO3 (b, d) precursors (a, b) and their calcination products at 850 °C (c, d).
3.2 Upconversion luminescence properties and anti-counterfeit application
Upon 980 nm laser diode excitation, the upconversion luminescence spectra of orthorhombic and hexagonal (Gd0.998-xTm0.002Ybx)InO3 (x = 0–0.15) phosphors as a function of Yb3+ concentration are shown in Figs. 4(a) and (b). The two-type phosphors both exhibit cyan-bluish emission at ∼460−565 nm, red emission at 645−680 nm, and near-infrared emission at 770−825 nm, arising from 1G4 + 1D2→3H5,6, 3F2,3→3H6, and 3H4→3H6 transitions of Tm3+, respectively. The cyan-bluish emission dominates for the two kinds of perovskites. By spectral comparison, the hexagonal perovskite phosphor presents the sharper and stronger red emission relative to the orthorhombic counterpart. Such an interesting phenomenon is correlation-induced coordination environment of Tm3+ in different crystal structures. In or-GdInO3 lattice, Gd atoms locate at the Wyckoff 4c position and the atomic distance of two adjacent Gd is ∼0.3928 nm. However, there are two Gd crystallographic sites in h-GdInO3 matrix, viz., Gd1 at the Wyckoff 4b position and Gd2 at the 2a site. As the atomic number ratio of Gd1 to Gd2 is 2:1, it is reasonable to believe that the probability of Tm3+ substitution for Gd1 site is higher owing to random doping. The atomic distance of two adjacent Gd1 is ∼0.3654 nm while that of two neighbor Gd1 and Gd2 is ∼0.3676 nm, which are both smaller than the situation in or-GdInO3 host. Even if two adjacent Tm3+ luminescent centers have small possibility to be doped within one single cell considering its low doping amount (0.2 at.%), the h-GdInO3 perovskite has a unit cell size almost half of or-GdInO3 (Fig. 2). In either case mentioned above, the closer distance between two Tm3+ activators in h-GdInO3 matrix would cause the significant cross relaxations (CR) [referred to CR1 (3F2 + 3F4 = 1G4 + 3H6) and CR2 (1G4 + 3F4 = 3F2 + 3F4) in Fig. 6], and thus the red emission is significantly enhanced. The upconversion luminescence intensity of the two-type phosphors highly depends on the concentration of the Yb3+ sensitizer. Originally, their cyan-bluish emissions gradually increase to a maximum value with increasing Yb3+ incorporation, while more Yb3+ addition leads to an obvious decline in emission intensity due to the occurrence of luminescence quenching. The optimum Yb3+ concentrations for orthorhombic and hexagonal perovskites are 10 and 5 at.%, respectively.
Fig. 4. Upconversion emission spectra of the orthorhombic (a) and hexagonal (b) (Gd0.998-xTm0.002Ybx)InO3 (x = 0−15%) perovskite phosphors as a function of Yb3+ concentration, spectral comparison of the two-type perovskites at their respective optimum Yb3+ content (c), and CIE chromaticity diagram (d). The orthorhombic and hexagonal crystal systems are obtained by calcining the precursors at 850 and 1100 °C, respectively.
Fig. 5. Upconversion emission spectra (a, b) and linear fittings between the logarithm of luminescence intensity and the logarithm of excitation intensity (c, d) of orthorhombic (a, c) and hexagonal (b, d) crystals under excitation of a 980 nm laser diode as a function of output power from 0.1 to 0.78 W.
Figure 4(c) compares the upconversion spectra of orthorhombic and hexagonal systems at their respective optimum Yb3+ content. The h-GdInO3:Tm,Yb perovskite has a 1.2-fold higher spectral integral area than the or-GdInO3:Tm,Yb counterpart, since the former is attained at a higher temperature to further improve the crystallinity. However, the near-infrared 3H4→3H6 emission intensity of the hexagonal phase, in turn, is weaker than that of the orthorhombic one. This is because the CR3 (1G4 + 3H4 = 1D2 + 3F4) in h-GdInO3 matrix reduces electron population on 3H4 state of Tm3+ (referred to Fig. 6). Although the two optimum phosphors both present strong cyan-bluish emission at ∼460−565 nm, the strongest emission peak locates at ∼478 nm for orthorhombic phase and ∼491 nm for hexagonal one. The reason for the varying emission wavelength is similar to our above-mentioned interpretation on CR1, CR2 and CR3 formation. Along with successive CR1→CR2→CR3 processes (Fig. 6), the ∼491 nm cyan emission for the hexagonal perovskite phosphor is thus enhanced. The Commission International de I’Eclairage (CIE) 1931 chromaticity diagram of the two optimum phosphors is shown in Fig. 4(d). The color coordinates are (0.118, 0.118) and (0.120, 0.219) for the orthorhombic and hexagonal perovskites, respectively. The corresponding integral emitting colors are blue and cyan.
Under 980 nm laser excitation as a function of output power from 0.1 to 0.78 W, the upconversion fluorescence spectra of the orthorhombic and hexagonal GdInO3:Tm,Yb phosphors at their own optimum Yb3+ concentration are shown in Figs. 5(a) and (b). The emission intensity gradually increases with the augmented pump power. The relationship between emission intensity (Iem) and infrared excitation intensity (IIR) can be given by the equation: Iem ∝ IIRn, where n is the number of infrared photons absorbed per emitted visible photon [34,35]. For the sake of revealing upconversion luminescence mechanism, the n value is fitted by taking the logarithm on both sides of this equation. The resulting n values (equal to the slopes of the fitting lines) of the orthorhombic perovskite are 2.07 for the blue emission and 1.87 for the red emission, whereas those of hexagonal phase are 1.03 for the cyan emission and 1.08 for the red emission [Figs. 5(c) and (d)]. Considering n is an integer, their upconversion processes (UCP) mainly belong to two-photon excitation while three-photon excitation may be also present. The specific examples for upconversion mechanisms are shown in Table 1. It should be noted that the determined relatively small n value for the hexagonal sample (a little greater than 1 and much less than 2) may be due to the saturated excited state.
Table 1. The upconversion mechanisms of two-type or-GdInO3:Tm,Yb and h-GdInO3:Tm,Yb phosphors under 980 nm excitation.
The upconversion luminescence processes in oxides, garnets, halides, or other perovskite systems have been extensively investigated, which provides some references for understanding the upconversion mechanisms of our new phosphors. The energy level diagram is drawn in Fig. 6. The upconversion luminescence primarily comprises ground-state absorption (GSA), excited state absorption (ESA), energy transfer (ET), cooperative sensitization (CS), and CR. The energy of the 980 nm diode laser matches well with the absorption energy level from 2F7/2 ground state to 2F5/2 excitation state of Yb3+ sensitizer, and thus only 4f13 electrons of Yb3+ can be excited onto 2F5/2 energy level upon 980 nm excitation as denoted by 2F7/2 (Yb3+) + hυ980 nm → 2F5/2 (Yb3+) via GSA. The 3F2 state of Tm3+ can be populated by energy transfer from Yb3+ to Tm3+ (ET1) or by ET2 plus nonradioactive relaxation plus ESA, which can be comprehensively expressed as 3F2 (Tm3+) + 3F7/2 (Yb3+) = 3F4 (Tm3+) + 2F5/2 (Yb3+). A part of electrons on 3F2 level decay onto the 3F3 and 3H4 states by nonradioactive relaxation. The red and near-infrared emissions are thus yielded upon back-jumping of those electrons from the 3F2,3 and 3H4 excitation levels to the 3H6 ground state of Tm3+, respectively. Besides, the electrons populate the 1G4 level of Tm3+ via GSA plus two ESA (ESA1 + ESA2) processes [1G4 (Tm3+) + 3F4 (Tm3+) = 3H4 (Tm3+) + 3F2 (Tm3+)] or via CS [22F5/2 (Yb3+) = 1G4 (Tm3+) + 3H6 (Tm3+)] [36], leading to the blue emission upon back-jumping of the electrons to 3H6 ground state of Tm3+ [1G4 (Tm3+) → 3H6 (Tm3+) + hυ478 nm].
Figure 7 exhibits the upconverted fluorescence attenuation behaviors of the two or-GdInO3:Tm,Yb and h-GdInO3:Tm,Yb phosphors for their cyan-bluish emissions under 980 nm laser excitation. The fluorescence lifetime can be calculated by fitting the decay curve with a single exponential equation: I = Aexp(-t/τ) + B, where τ is the fluorescence lifetime, I is the emission intensity, t is the decay time, and A and B are constants [37–41]. The fittings yield τ = 0.31 ± 0.001 ms, A = 6.60 × 103 ± 56.1, and B = 0.59 ± 0.54 for the orthorhombic phase; and τ = 0.69 ± 0.005 ms, A = 3.88 × 103 ± 39.9, and B = -26.20 ± 2.16 for the hexagonal counterpart. The lifetime of the latter is roughly twice as long as the former, the reason of which is closely associated with their different upconversion luminescence processes. For the hexagonal phosphor, the electronic transition undergoes longer pathway induced by successive CR1, CR2, and CR3 processes, which is possibly responsible for the prolonged lifetime.
Fig. 7. Fluorescence attenuation curves of the two or-GdInO3:Tm,Yb (a) and h-GdInO3:Tm,Yb (b) phosphors for their cyan-bluish emissions under 980 nm laser excitation.
In order to prevent the valuable documents such as diplomas, certificates and currency from counterfeiting, we employ Tm3+/Yb3+ co-doped GdInO3 phosphors as major materials to prepare unclonable scanning ‘QR’ codes via a screen-printing technique as shown in Fig. 8. Under normal room light, almost invisible ink on a commercial photographic paper is achieved, which is beneficial to hide the enciphered information. Upon 980 nm near-infrared (NIR) irradiation, the ‘QR’ codes are readable. The patterns made from or-GdInO3:Tm3+/Yb3+ and h-GdInO3:Tm3+/Yb3+ phosphors emit blue and cyan light to the naked eyes, respectively, which is well consistent with their upconversion luminescence behaviors and CIE chromaticity diagram (Fig. 4). These pattern stripes become more distinct and glaring as the NIR output power increases. The ‘QR’ code made from the or-GdInO3:Tm,Yb phosphor can be identified by smartphone scanner under above 5 W NIR irradiation, while above 3 W NIR irradiation on the ‘QR’ code prepared from the h-GdInO3:Tm,Yb counterpart is enough for recognition due to its stronger upconversion luminescence intensity (Fig. 4). Apart from optical anti-counterfeiting, the two-type phosphors are also appropriate for fingerprint identification as shown in Fig. S4 of the Supplement 1, from which the characteristics of fingerprint profiles could be clearly seen.
Fig. 8. Schematic scanning ‘QR’ codes prepared by screen printing from or-GdInO3:Tm,Yb (a) and h-GdInO3:Tm,Yb (b) phosphors under 980 nm NIR irradiation as a function of output power from 0 to 9 W.
3.3 Down-conversion luminescence performances and anti-counterfeit application
The aforementioned ‘QR’ codes prepared by the screen printing using or-GdInO3:Tm,Yb and h-GdInO3:Tm,Yb phosphors as major materials are also clearly readable via down-converted emitting blue light under irradiation from a 254 nm UV lamp [Figs. 9(a)−(d)]. In consequence, our materials can realize dual-mode luminescence, which provide more protection against counterfeiting compared with single-mode emitting security codes. In general, the down-converted Tm3+ spectrum presents sharp blue emissions arising from 1G4→3H6 and 1D2→3F4 transitions [42,43]. However, we interestingly find that the spectra of our two-type phosphor samples both exhibit broad multibands in the wavelength range of 400−500 nm upon 254 nm excitation [Fig. 9(e)]. There are different transition mechanisms involved in the down-conversion emission as depicted in the schematic energy level diagram [Fig. 9(f)]. It is well known that In2O3 crystals are always accompanied by oxygen vacancies [44–46]. In our case, the two-type GdInO3 perovskites also inherit such a characteristic. The oxygen vacancies would induce the formation of a series of defect energy levels in the bandgap, which could be classified into near-band-edge (NBE) levels and deep levels. The bandgaps of or-GdInO3 and h-GdInO3 are reported to be ∼5.07 and 5.13 eV, respectively [29]. Although the energy of 254 nm wavelength (∼4.88 eV) is not enough to directly excite electrons in the valence band to the conduction band in GdInO3 bandgap, these electrons can be transferred onto the NBE levels along with remaining holes. A mass of electrons decays onto the deep level, and thus the defect induced purple emission results from the radiative recombination of these electrons with photogenerated holes. As the NBE levels in GdInO3 bandgap locate above the 1D2 and 1G4 states of Tm3+, the rest of electrons easily decay onto the 1D2 level of Tm3+ via energy transfer to yield blue emissions upon back-jumping of the electrons from the 1D2 and 1G4 excitation states to the 3F4 and 3H6 ground states of Tm3+, respectively. Actually, the energy transfer from GdInO3 defect level to Tm3+ excitation state is observed for the first time. The CIE 1931 chromaticity diagram shows that the color coordinates are (0.22, 0.25) and (0.21, 0.24) for or-GdInO3:Tm,Yb and h-GdInO3:Tm,Yb, respectively, which both fall into the blue color region [Fig. 9(g)]. The result of the chromaticity diagram is consistent with the color of the irradiated anti-counterfeit patterns to the naked eyes. Under 254 nm excitation, the down-converted fluorescence decay kinetics of or-GdInO3:Tm,Yb and h-GdInO3:Tm,Yb phosphors for the 426 nm defect emission and the 470 nm Tm3+ emission are depicted in Fig. 9(h). The fluorescence lifetimes fitted by the single exponential equation are listed in Table 2. The lifetime values of the two-type phosphors are close to each other for each emission wavelength. However, the lifetime value for the 426 nm defect emission is longer relative to that for the 470 nm Tm3+ emission, because relaxing the trapped electrons on the defect levels for luminescence generally needs slow accumulation over time. The determined fluorescence lifetimes for 470 nm Tm3+ emission attained in this work are much longer than the reported values of Tm3+ doped phosphors such as LuVO4:Tm (∼107.08 µs) [42] and (La,Y)2O2S:Tm (∼6.4−6.5 µs) [43], because of the energy transfer from the defect level of GdInO3 to the excitation state of Tm3+ in our case.
Fig. 9. Schematic scanning ‘QR’ codes prepared by the screen printing using or-GdInO3:Tm,Yb (a, b) and h-GdInO3:Tm,Yb (c, d) phosphors as major materials under normal room light (a, c) and 254 nm UV irradiation (b, d), down-converted PL spectra (e), schematic diagram of the possible mechanism for the down-conversion luminescence behavior (f), CIE 1931 chromaticity diagram (g), and fluorescence decay kinetics for 426 and 470 nm emissions (h).
Table 2. Single exponential fitting results from the fluorescence decay curves of or-GdInO3:Tm,Yb and h-GdInO3:Tm,Yb phosphors for 426 and 470 nm emissions.
4. Conclusions
Two-type GdInO3:Tm,Yb perovskite phosphors with orthorhombic and hexagonal structures were prepared to investigate the crystal structural effects on up/down-conversion luminescence behaviors, and the main conclusions were summarized as follows. (1) The precipitation precursor fully crystallized into orthorhombic perovskite at 850 °C, followed by phase transition into hexagonal structure at 1100 °C along with dramatic cell dimension contraction. (2) Compared with the orthorhombic perovskite, the hexagonal phosphor presented relatively strong and sharp red emission as well as red-shifted cyan-bluish emission via successive cross relaxations under 980 nm excitation. The upconversion mechanism mainly ascribed to two-photon processes, where three-photon excitation excitation was also involved for the orthorhombic GdInO3:Tm,Yb phosphors. (3) The defect-induced emission of GdInO3 and the blue emission of Tm3+ jointly contributed to the down-conversion luminescence to result in broad multibands in the wavelength range of 400−500 nm under 254 nm excitation. The energy transfer from defect level of GdInO3 to the excitation state of Tm3+ prolonged the fluorescence lifetime by ∼2.5 s. (4) Our developed dual-mode luminescent materials exhibited potential applications for optical anti-counterfeiting and fingerprint identification.
Funding
Natural Science Foundation of Zhejiang Province (LY23F050007); Regional Joint Fund Project of Liaoning Provincial Department of Science and Technology (2022-YKLH-12); Yingkou Enterprise and Doctoral Entrepreneurship Program Project (QB-2022-02); .
Disclosures
The authors declare no conflicts of interest.
Data availability
The data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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