Luminescent materials with dual-mode excitation and tunable emission color for anti-counterfeiting applications

Abstract

GdVO₄-based dual-mode phosphors were successfully synthesized via a hydrothermal approach. The X-ray diffraction analysis determined the tetragonal structure as well as I4₁/amd space group of products by comparing with a reference pattern no. ICDD #01-072-0277. The morphology of yielded phosphors was confirmed by transmission electron microscopy and scanning electron microscopy. Detailed spectroscopy analysis revealed tunable luminescence properties with an increasing Yb³⁺ content in series of GdVO₄: x% Yb³⁺, y% Tm³⁺, 5% Eu³⁺ (x = 5, 10, 15, 20; y = 0.1, 0.5, 1) phosphors. For Yb³⁺, Tm³⁺, and Eu³⁺- codoped phosphors we observed bands related to the ¹G₄ → ³H₆ and ¹G₄ → ³F₄ transitions of Tm³⁺ ions, occurred through the cooperative up-conversion mechanism, where two nearby Yb³⁺ ions were involved in near-infrared absorption. Moreover, the GdVO₄: 20% Yb³⁺, 0.5% Tm³⁺, 5% Eu³⁺ showed the most outstanding color tunability from red color (x = 0.6338, y = 0.3172) under UV to blue color (x = 0.2640, y = 0.1988) under NIR excitation, which can be applied in anti-counterfeiting activity.

Introduction

Inorganic materials doped with lanthanide ions (Ln³⁺) play a significant role in many fields of everyday life, based on their numerous applications as lasers, thin film phosphors, in drug delivery, bioimaging, or anti-counterfeiting^{1,2,3,4,5,6,7}. The last-mentioned group consists of several media in which anti-counterfeiting tags are implemented, such as bar codes, inks, holograms, RFID (Radio Frequency Identification). Every aforementioned security approach has its limitations and thus cannot be applied in i.e., clothes or document markings, which may be regularly treated with water, washing agents, or UV radiation. Recently developed cellulose fibers modified- with inorganic phosphors^8,9,10,11,12 are prepared via an environmentally friendly NMMO (N-Methylmorpholine-N-Oxide) method. As an outcome, the so-called Tencel fibers may be used for paper modification or as a part of the fabric. Also, during this rigorous process of fibers preparation, the luminescent modifier used has to outstand with its excellent stability. In our research, we chose GdVO₄-based dual-mode phosphors based on their strong energy absorption as well as the high efficiency of energy transfer processes^13,14,15. Another advantage of vanadate-based phosphors as an alternative type of material versus fluoride-based phosphors is their high thermal stability, beneficial in i.e. light-emitting diode application, in which operating temperature exceeds 100 °C¹⁶. In comparison to the vanadate materials, commonly used in phosphor applications fluoride materials are not only sensitive to high temperatures but also to surface contamination which may accidentally influence spectroscopic properties such as luminescence lifetime or emission color¹⁷.

Contrary to the accidental influence, the intentional impact on the intensity and different luminescence color under UV or NIR excitation is caused by the specified Ln³⁺ dopant concentration. Rare earth-doped materials exhibiting dual-mode luminescence possess vast potential for various applications^18,19,20,21. When a combination of ions capable of absorbing energy with diverse energy values based on their electron structure is used, a unique luminescence color tunability can be achieved. The perfect combination for obtaining tunable luminescence is the system comprising Yb³⁺, Tm³⁺, and Eu³⁺ ions. Here, the NIR excitation of 980 nm can be absorbed by Yb³⁺ and after transferring two or more photons towards Tm³⁺ and Eu³⁺ ions, a visible up-conversion emission is observed, which is highly dependent on Tm³⁺ and Eu³⁺ ratio. Also, under UV irradiation, there is a possibility of direct Eu³⁺ excitation or energy transfer from the orthovanadate matrix to dopant ions—in both cases, a red emission associated with Eu³⁺ is observed^22,23.

In this study, we aim to provide a comprehensive understanding of the properties and applications of the luminescent marks based on doped orthovanadates. All of above-mentioned features of the GdVO₄: Yb³⁺, Tm³⁺, Eu³⁺ phosphors synthesized by the feasible hydrothermal approach, designate as perfect for the anti-counterfeiting applications as the color tunability within the same material is difficult to falsify. The study introduces dual-mode orthovanadates as a viable substitute for fluoride-based materials in the realm of anti-counterfeiting tags. Through precise selection of dopant ions and host matrix, the resulting luminophores undergo up-conversion processes and demonstrate robust emission capabilities owing to charge transfer phenomena between O^2- and Eu³⁺ ions. These distinct mechanisms give rise to diverse color emissions and various wavelength excitation. Moreover, the orthovanadate materials exhibit remarkable resilience to harsh environmental conditions, including elevated temperatures, thereby rendering them advantageous by comparison to the aforementioned fluorides, which are more prone to decomposition.

A potential application of the implementation of a orthovanadates modifier into cellulose fibers for paper and fabric markings has been confirmed by the patent application submitted^43,44. By showcasing the real-world application of our material, we establish its potential for practical implementation and highlight its relevance to various industries. Finally, according to our knowledge, this is the first time that Yb³⁺, Tm³⁺, Eu³⁺ dopants are incorporated into the orthovanadate matrix, and its structural followed by spectroscopic properties are determined. It fills a gap in the existing literature by presenting an innovative approach that has practical implications and opens new avenues for future research and development.

Experimental section

Materials

Ammonium metavanadate (NH₄VO₃, Sigma Aldrich, 99.9%), gadolinium(III) oxide (Gd₂O₃, Standford Materials, 99.99%), ytterbium(III) oxide (Yb₂O₃, Standford Materials, 99.99%), thulium(III) oxide (Tm₂O₃, Standford Materials, 99.99%), europium(III) oxide (Standford Materials, 99.99%) and acetic acid (CH₃COOH, POCH, 99,95%) used in the synthesis of the materials.

Methods

A series of GdVO₄: x% Yb³⁺, y% Tm³⁺, 5% Eu³⁺ (x = 5, 10, 15, 20; y = 0.1, 0.5, 1) was obtained in hydrothermal conditions. The concentration and the type of dopants was altered based on our knowledge and the literature in order to observe efficient emission processes^{24,25,26,27,28}. In addition, the concentration of Yb³⁺, Tm³⁺ and Eu³⁺ dopant ions in prominent GdVO₄ host was altered to provide the intense, dual-mode luminescence under UV and NIR irradiation for anticounterfeiting applications. The composition of Ln³⁺ ions used was selected to ensure the emission color dependent on excitation wavelength thus considered material is more difficult to replicate.

The synthesis was performed in Berghof autoclave (max. pressure 200 bar, additional stirring). All of the substrates were used as water solutions. The stoichiometric combination of 0.25 M Ln(CH₃COO)₃ was mixed with the 0.1 M NH₄VO₃ added dropwise under continuous stirring for 30 min. Resultant transparent mixture (pH 4.7) was then transferred to Teflon vessel and put for hydrothermal process under 180 °C for 3 h which yielded with yellow powder. Next, when the autoclave was naturally cooled to room temperature, the product was collected by centrifugation, washed with 1:1 mixture of deionized water and ethanol. Finally, the product was dried at 80ºC for 24 h for further analysis.

Characterization

The structural analysis was conducted with Bruker AXS D8 Advance powder X-ray diffractometer equipped with Johansson's monochromator and Lynx Eye strip detector, whereas the measurements were performed with Cu-K_α1 λ = 15,418 Å radiation within the 10–60 2Θ range, 0.05°/s step size. Morphology of studied materials was investigated with the use of transmission electron microscopy, TEM (JEOL 1400 with acceleration voltage of 80 kV) as well as scanning electron microscopy, SEM (Quanta 250 FEG, FEI equipped with EDAX detector). Luminescence properties were studied in terms of photoluminescence and upconversion luminescence, i.e. under UV or NIR excitation. The former was studied with the use of Hitachi F-7000 spectrofluorimeter, equipped with xenon lamp excitation source. The latter phenomenon was studied in terms of emission, luminescence decay and the number of photons involved in the process, with the use of PIXIS:256E Digital CCD Camera equipped with SP-2156 Imaging Spectrograph (Princeton Instruments), Mixed Domain Oscilloscope—200 MHz—Tektronix MDO3022 as well as the excitation source of CNI NIR 2W LASER 975 nm. All of the spectroscopic measurements were conducted at 293 K.

Results and discussion

Structure and morphology

Behind every specific luminescence feature, there are structural and morphological reasons as well. As seen in Fig. 1, the replacement of 25.5% of Gd³⁺ ions with dopant ions in the host structure did not cause severe lattice distortions since the ionic radius of Eu³⁺ is similar, while the Yb³⁺ and Tm³⁺ radii are smaller than the one of Gd³⁺²⁹. The synthesized compounds are confirmed to be of GdVO₄ I4₁/amd tetragonal zircon-type (ZrSiO₄) structure with the cell parameters of a = b = 7.2126 Å, c = 6.3483 Å, according to the reference pattern no. ICDD #01-072-0277^30,31. The tetragonal and polyhedral structure of GdVO₄ are presented in Fig. 2c. Here, vanadium atom of [VO₄]³⁻ is tetrahedrally coordinated with O²⁻ ions, whereas the Ln³⁺ are surrounded by eight oxygen atoms in a distorted dodecahedron structure³². The presence of sharp, narrow reflexes indicates discussed materials as highly crystalline and bulk. By the fact that none of the additional peaks are observed, synthesized powders are monophased and the replacement of Gd³⁺ by Ln³⁺ dopants was successful. What has to be said, an increased grain size growth as well as excellent crystallinity are caused by inevitable high temperature annealing (900 °C) which was applied to induce the UC luminescence, initially diminished by structure defects, typical for materials synthesized in hydrothermal conditions³³. These features were further confirmed with the use of TEM and SEM methods (Fig. 2a,b). The obtained orthovanadate crystals were observed to be agglomerated, displaying an irregular morphology. The average grain size, determined from the broadest fraction, was found to be approximately 1.5 µm. These observations confirm the microstructural properties of the orthovanadate crystals and provide quantitative data regarding their size distribution illustared on histogram (inset Fig. 2a). Despite the large grain size, it was possible to disperse the powder in water for further use in cellulose fiber modification.

Figure 1

A set of diffraction data for GdVO₄: x% Yb³⁺, 0.5% Tm³⁺, 5% Eu³⁺ materials.

Figure 2

Morphology representation of GdVO₄: 20% Yb³⁺, 0.5% Tm³⁺, 5% Eu³⁺ sample in terms of TEM with histogram presenting size distribution (a) and SEM (b) methods and crystal structure of GdVO₄ I4₁/amd tetragonal zircontype (ZrSiO₄) (c).

Spectroscopic properties

According to the Figs. 3 and 4, GdVO₄: 20% Yb³⁺, 0.5% Tm³⁺, 5% Eu³⁺ sample was chosen amongst the series as the most promising sample thus used for the fabric preparation based not only on its diverse luminescence color under UV and NIR excitation, but also the outstanding UC emission in the visible range.

Figure 3

UC luminescence spectra for GdVO₄: x% Yb³⁺, 0.5% Tm³⁺, 5% Eu³⁺ samples recorded under CW 975 nm excitation.

Figure 4

PLE (dashed line) and PL (solid line) spectra of GdVO₄:x%Yb³⁺, 0.5%Tm³⁺, 5%Eu³⁺ materials.

Photoluminescence

Based on the excitation spectrum in the UV range Fig. 4, the O²⁻–Eu³⁺ charge transfer band (CT) with the maximum at 310 nm was chosen to observe visible luminescence at 621 nm, assigned to the ⁵D₀ → ⁷F₂ transition. What has to be mentioned, the broad CT band is in fact combined of O²⁻–V⁵⁺ and O²⁻–Eu³⁺; however, based on the small difference in between O²⁻ and V⁵⁺, as well as the large charge difference, O²⁻–V⁵⁺ in [VO₄]³⁻ is easier observed³⁴. Also, there are additional weak f–f transitions typical for Eu³⁺ ions in the 200–500 nm range^35,36. Based on the ionic radii difference³⁴, Gd³⁺ is being replaced by Eu³⁺ thus in the GdVO₄: Yb, Eu, Tm system, Eu³⁺ has D_2d symmetry as it is surrounded by eight O^2- ions. Relative intensity of ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F₂ is altered based on the local site symmetry of the Eu³⁺ ions^11,37,38. In this research, the intensity of hypersensitive ⁵D₀ → ⁷F₂ is the highest among the Eu³⁺ emission bands which indicated the low symmetry around Eu³⁺ ions³⁸. What is more, with an increasing Yb³⁺ concentration, the intensity of both excitation and emission curves is decreasing in terms of the Eu³⁺ → Yb³⁺ energy transfer, since the distance between these ions is shortened³⁸. This phenomenon is further confirmed by the calculated Eu³⁺ luminescence lifetime values (Table 1) monitored under 310 nm excitation. With the increasing Yb³⁺ concentration, the energy is migrating from Eu³⁺ excited states to Yb³⁺ ions.

Table 1 A set of calculated luminescence lifetime values for GdVO₄: x% Yb³⁺, 0.5% Tm³⁺, 5% Eu³⁺ phosphors under 310 nm excitation.

According to the chromaticity diagram Fig. 5, under 310 nm excitation the outcoming luminescence color is not altered by the incorporated Tm³⁺ ions.

Figure 5

Chromaticity diagram for GdVO₄: x% Yb³⁺, 0.5% Tm³⁺, 5% Eu³⁺ samples under 310 nm with an enlarged inset (a) and CW 975 nm excitation (b).

Upconversion luminescence

It is essential for dual-mode luminescence (ergo inimitable anti-counterfeiting materials) to be intense both under UV and NIR irradiation. According to the upconversion spectrum depicted in Fig. 4, the energy transfer between Eu³⁺ and Tm³⁺ ions is observed as variety of Eu³⁺ and Tm³⁺ emission bands are present in the spectrum. What has to be noted, the population of Eu³⁺ via inefficient phononassisted- Yb³⁺–Eu³⁺ is barely observed; here, Tm³⁺ acts as an energy mediator between the sensitizer (Yb³⁺) and emitter (Eu³⁺)²². According to that, several emission bands associated to the Tm³⁺ and Eu³⁺ are observed in the spectrum, which vary with intensity, namely ¹G₄ → ³H₆ (Tm³⁺, ~ 478 nm), ⁵D₁ → ⁷F₁ (Eu³⁺, ~ 521 nm), ⁵D₁ → ⁷F₂ (Eu³⁺, ~ 552 nm), ⁵D₀ → ⁷F₁ (Eu³⁺, ~ 590 nm), ⁵D₀ → ⁷F₂ (Eu³⁺, ~ 615 nm), ¹G₄ → ³F₄ (Tm³⁺, ~ 650 nm), ⁵D₀ → ⁷F₄ (Eu³⁺, ~ 700 nm) as well as the most intense ³H₄ → ³H₆ (Tm³⁺, ~ 800 nm) band. The last-mentioned transition is observed in NIR region of spectrum thus it does not influence the emission color. What has to be mentioned, throughout changing Yb³⁺ concentration, the chromaticity coordinates of synthesized materials change, according to the Fig. 5 and Table 2. With an increasing Yb³⁺ content, the luminescence color shifts from the red towards purple and blue region of CIE chromaticity diagram. By the gradual substitution of Gd³⁺ by Yb³⁺, the distance between sensitizer and emitters such as Tm³⁺, Eu³⁺ decrease. As mentioned before, the efficiency of Yb³⁺–Eu³⁺ transfer is low thus the Yb³⁺–Tm³⁺ transfer is favored here. In this case, the competitive Tm³⁺–Eu³⁺ absorption is decreased, which is also connected with the lower intensity of Eu³⁺ emission bands in the UC spectrum, as well as the red component of the luminescence color²³. Also, when Yb³⁺–Tm³⁺ is greatly enhanced, the relative intensity between blue and red emissions of Tm³⁺ is increased, which results with the outcoming blue upconversion luminescence^40,41.

Table 2 A set of calculated chromaticity coordinates (x,y) for GdVO₄: x% Yb³⁺, 0.5% Tm³⁺, 5% Eu³⁺ phosphors under 310 nm and 975 nm excitation.

The population of Eu³⁺ and Tm³⁺ excited levels, as well as the energy transfer between these species were studied in terms of luminescence decay. Here, due to the color change, the most important bands are the ones in the blue and red region of spectrum. Based on that, the greatest attention was put to the lifetime of ¹G₄ → ³H₆ (Tm³⁺, ~ 478 nm), ⁵D₀ → ⁷F₂ (Eu³⁺, ~ 615 nm) and ¹G₄ → ³F₄ (Tm³⁺, ~ 650 nm). As shown in Table 3, the luminescence lifetime of Eu³⁺ is decreasing with an increasing Yb³⁺ content. Based on that, the possibility of Tm³⁺–Eu³⁺ is diminished, whereas Yb³⁺–Tm³⁺ transfer is favored. It is also confirmed by the enhanced blue emission of Tm³⁺ since less energy is transferred towards Eu³⁺ site of lattice. Moreover, in order to derive the average number of photons (n) involved in the upconversion process, a laser power dependent luminescence study was performed (Fig. 6). Interestingly, the slopes (n) of ¹G₄ → ³H₆ and ¹G₄ → ³F₄ transitions suggest the involvement of two photons regardless the Yb³⁺ concentration. In the contrary to the common understanding of the ¹G₄ population mechanism, i.e. through threephoton- absorption, meant also as sequential sensitization, in the case of GdVO₄: x% Yb³⁺, 0.5% Tm³⁺, 5% Eu³⁺ a cooperative sensitization is in fact happening. In this mechanism there are two Yb³⁺ ions involved, which absorbs photons in order to promote themselves towards ²F_5/2 excited state. Then, a coupled cluster state of two Yb³⁺ ions formed transfers energy towards Tm³⁺ which results with the population of its ¹G₄ level⁴².

Table 3 A set of calculated luminescence lifetime values for GdVO4: x% Yb³⁺, 0.5% Tm³⁺, 5% Eu³⁺ phosphors under 975 nm excitation.

Figure 6

Laser power density studies for GdVO₄: x% Yb³⁺, 0.5% Tm³⁺, 5% Eu³⁺ samples recorded under CW 975 nm excitation.

To summarize all studied processes, a mechanism of upconversion in GdVO₄: x% Yb³⁺, 0.5% Tm³⁺, 5% Eu³⁺ systems can be proposed in Fig. 7. The whole phenomenon begins under 975 nm CW excitation, when the energy is first absorbed by two nearby Yb³⁺ ions. This results with the promotion of sensitizers from their ²F_7/2–²F_5/2 level. At this point, two simultaneous processes are happening. Due to the formation of coupled Yb³⁺ cluster state, another photon is absorbed and transferred towards ¹G₄, from which 478 nm and 650 nm emissions occur. Also, there is a photon transferred from this level towards Eu^{3+ 5}D₁, where after energy dissipation to ⁵D₀, several emissions associated with Eu³⁺ are observed in the spectrum. What is more, the Tm^{3+ 3}H₅ is populated via photon transfer from Yb^{3+ 2}F_5/2. After nonradiative relaxation to ³H₄, an 800 nm emission is observed.

Figure 7

Energy level diagrams of Yb³⁺, Tm³⁺ and Eu³⁺ ions and possible energy transfer (a) and UC (b) mechanism.

Based on its pristine tunable luminescence properties, sample composed of GdVO₄: 20% Yb³⁺, 0.5% Tm³⁺, 5% Eu³⁺ was chosen for fibers preparation. Then luminescent fibers were used for paper modification and fabric production as an example of anti-counterfeiting application^43,44. Regardless the medium, the outcoming luminescence color as well as its intensity remain unchanged. These properties recommend GdVO₄: 20% Yb³⁺, 0.5% Tm³⁺, 5% Eu³⁺ phosphor for anticounterfeiting applications performed in patent proposal submission. In Fig. 8, an actual luminescence color under different excitation sources is present.

Figure 8

Actual images of GdVO₄: 20% Yb³⁺, 0.5% Tm³⁺, 5% Eu³⁺ powder in daylight (a), under 254 nm UV excitation (b) and 975 nm IR excitation (c).

Conclusions

To conclude, the pre-eminent, spectroscopic properties of GdVO₄: 20% Yb³⁺, 0.5% Tm³⁺, 5% Eu³⁺ define this material as the excellent one for anti-counterfeiting purposes. In our study, we investigated different Yb³⁺ concentration and its influence on structural and spectroscopic properties. With an increased sensitizer content, the upconversion luminescence is more intense whereas its color is tuned towards blue region of spectrum. What is more, in Yb³⁺/Tm³⁺/Eu³⁺ system, ¹G₄ → ³H₆ and ¹G₄ → ³F₄ emissions result from twophoton excitation in terms of cooperative sensitization where two nearby Yb³⁺ ions are involved in IR excitation absorption process. Also, varied luminescence color under different sources of excitation qualified GdVO₄: 20% Yb³⁺, 0.5% Tm³⁺, 5% Eu³⁺ phosphor for anticounterfeiting application. In applied cellulose medium, luminescence color and intensity remained unchanged in comparison to the phosphor in the powder state.