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Simultaneously Excited Downshifting/Upconversion Luminescence from Lanthanide-Doped Core/Shell Fluoride Nanoparticles for Multimode Anticounterfeiting Jing Liu, Hannes Rijckaert, Min Zeng, Katrien Haustraete, Brecht Laforce, Laszlo Vincze, Isabel Van Driessche, Anna M. Kaczmarek,* and Rik Van Deun* According to a recent report, which has been published in 2017, the counterfeiting market was evaluated to be $107.26 billion in 2016 and forecast to reach $206.57 billion by 2021, at a compound annual growth rate of 14.0%.[5] In anticounterfeit prevention the packaging of a product is normally the first line of defense. Many security technologies involving the product packaging have been exploited to prevent counterfeiting, such as barcoding, watermarking/stamping, digital signature standard, box seals. Traditional security methods have been displaced over time due to easy duplication.[6] Luminescence printing promises a very high-level of security due to its designable and tunable properties. IR reactive inks, multifeature inks, pen reactive inks, ultraviolet (UV) fluorescent inks, optically variable inks, and thermochromic inks have already been widespread in banknotes, passports, ID cards, biomimetic microfingerprints, pharmaceutical, and other items.[7–10] For example, the lanthanide europium (Eu2+/Eu3+) is used in Euro banknotes to provide luminescence anticounterfeiting features, which are only visible when placing a banknote under UV light. Chinese RMB banknotes take advantage of lanthanide upconversion luminescence for anticounterfeiting purposes. Also international and national passports have serial numbers with a digital code, which is only visible under UV light.[11,12] Trivalent lanthanides have characteristic sharp excitation and emission peaks, which are virtually independent from the external environment. This can produce multicolor emission when exciting various lanthanide materials into their UV, visible or near-infrared (NIR) excitation bands.[13] There are several types of luminescence conversions: upconversion, downconversion, and downshifting.[14] Downshifting is the process when a photon with higher energy is converted into one photon of lower energy, downconversion is the process where one higher-energy photon is converted into two lower-energy photons, while upconversion is the adverse process, where at least two lower-energy photons are “added up” to obtain one higher-energy photon. Rare-earth fluorides are well known as efficient host matrices for both upconversion and downshifting lanthanide luminescence, due to their low phonon energy, high chemical stability, and low refractive indices.[15] NaYF4 has always been

This work presents a novel anticounterfeiting strategy based on a material changing its emission color in response to a change in the excitation sources—where a single ultraviolet (UV) or near-infrared (NIR) light source are employed or simultaneously using two excitation sources (xenon lamp and NIR laser). Following this approach, various combinations of lanthanide (Ln3+)-doped LiLuF4/LiYF4 core/shell nanoparticles are prepared, providing a promising route to design flexible nanomaterials, as well as already a small library of luminescent materials, which change color when varying the excitation source (UV, NIR or both UV and NIR). Aside from excitation source-dependent color change, these materials additionally show excitation-source power-dependent color change. This work exploits the possibility of developing a new class of multimode anticounterfeit nanomaterials, with excellent performance, which would be almost impossible to mimic or replicate, providing a very high level of security.

1. Introduction The illegal production and sale of goods as well as packaging without authorization all belongs to counterfeiting behaviours.[1,2] Various products such as currency, documents, software, movies, pharmaceutics, automobile parts, electronics or designer goods are being counterfeited everyday. Counterfeiting is a very serious global issue as counterfeiting crimes cause tremendous economic losses (counterfeited banknotes, luxurious goods, electronic products, etc.), and also cause a potential risk for the lives of consumers, as for example, pharmaceutical frauds can cause health damage to patients and fake products often are of low quality.[3,4] The best way to fight counterfeiting is to prevent it, therefore various advanced security technologies are continuously being explored. J. Liu, Dr. H. Rijckaert, M. Zeng, K. Haustraete, B. Laforce, Prof. L. Vincze, Prof. I. Van Driessche, Dr. A. M. Kaczmarek, Prof. R. Van Deun Department of Chemistry Ghent University Krijgslaan 281-S3, B-9000, Ghent, Belgium E-mail: [email protected]; [email protected] The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.201707365.

DOI: 10.1002/adfm.201707365

Adv. Funct. Mater. 2018, 1707365

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considered as one of the most efficient rare-earth fluoride materials until Prasad and co-workers reported that LiYF4:Er nanocrystals yielded four times higher quantum yields than hexagonal NaYF4:Er nanoparticles.[16] Chen and co-workers published a report on Ln3+-doped LiLuF4—which has the same crystalline structure as LiYF4—core/shell upconversion nanoparticles, which achieved the highest absolute upconversion quantum yields, 7.6%.[17] These results yielded a growth in interest toward LiYF4 and LiLuF4 lanthanide-doped materials. When LiYF4 or LiLuF4 nanoparticles are synthesized in a hightemperature coprecipitation method, they are well dispersible in organic solvents such as cyclohexane, toluene, and even water after removing the surface ligands by acid treatment. The good dispersibility and formation of stable suspensions in various solvents ensures that such nanoparticles can be efficiently used to prepare printable luminescence inks feasible for use in real anticounterfeiting applications.[18,19] In this paper, we propose a novel strategy for creating multimode anticounterfeiting materials, which would have a very high security level and would be almost impossible to mimic. In this approach we take advantage of simultaneously employing two different excitation wavelengths (and therefore excitation sources) to generate synchronously both downshifting and upconversion luminescence. Selected lanthanidedoped LiLuF4/LiYF4 core/shell nanoparticles show adjustable color changes under different excitation conditions (single UV or NIR light or simultaneously using two excitation sources), and some chosen lanthanide-doped nanomaterials show independent color regardless of the change of excitation sources. These materials, which are easily dispersed in various solvents and therefore highly processable, could allow the development of novel, advanced multimode security technologies. To the best of our knowledge, no such concept has been reported up to date. No materials, where the emission spectra consist of combined downshifting and upconversion lanthanide luminescence have been so far introduced for anticounterfeiting applications. In this work, we have employed three types of lanthanide-doped LiLuF4 nanomaterial as the core, two types of lanthanide-doped LiYF4 nanomaterial as the shell, which resulted in six different combinations in total to prove our concept. Employing other lanthanide combinations in the core and the shell of the nanomaterials, could lead to the development of an extended library of materials, which can further be implemented in anticounterfeiting security technologies.

green and blue upconversion color, respectively. The shells, on the other hand, are made of LiYF4, doped with either the combination Eu3+/Ce3+(a) or Tb3+/Ce3+(b). Excitation with UV light yields red downshifting emission for the Eu3+/Ce3+ shells and green downshifting emission for the Tb3+/Ce3+ shells. The combined core/shell particles 1a, 1b, 2a, 2b, 3a, and 3b, respectively, show very distinct upconversion/downshifting color combinations upon simultaneous excitation with 975 nm NIR laser light and UV light, as shown in Scheme 2. This means that dualsource excitation yields unique color combinations in these materials, enabling their use in anticounterfeiting and security applications. Furthermore, these colors can be tuned by varying the pump power of the 975 nm continuous-wave (CW) laser. In what follows, the structural and morphological characterization of the materials will be discussed, as well as a detailed investigation of the upconversion/downshifting luminescence properties will be commented on. The X-ray diffraction (XRD) patterns of LiLuF4:Er, Yb core 1 and LiLuF4:Er, Yb/LiYF4:Eu, Ce core/shell nanomaterial 1a, both synthesized by a high-temperature coprecipitation method, are shown in Figure S1 of the Supporting Information. All the diffraction peaks are in accordance with the standard tetragonal-phase LiLuF4 (JCPDS No. 027–1251),[17,20] with space group I41/a, and unit parameters a = b = 5.126, c = 10.539. The transmission electron microscopy (TEM) images presented in Figure 1a illustrate that the shape of the LiLuF4:Er, Yb core nanoparticles 1 is roughly spherical, with an average size of 15 nm, which is consistent with the dynamic light scattering (DLS) result of 15.7 nm (see Figure S3a, Supporting Information). In Figure 1d, it can be seen that rhombohedral shaped particles were obtained for the LiLuF4:Er, Yb/LiYF4:Eu, Ce core/ shell nanomaterial 1a, with an average size of 27 nm, which is consistent with the DLS result of 28.2 nm (see Figure S3b, Supporting Information). This means that the thickness of shell is around 12 nm. The corresponding high-resolution transmission electron microscopy (HRTEM) images are shown

2. Results and Discussion In Scheme 1, the different core, shell and core/shell materials discussed in this paper have been schematically represented. As can be seen, the cores are made of LiLuF4, doped with either the combination Er3+/Yb3+ (1) Ho3+/Yb3+ (2) or Tm3+/Yb3+ (3). These cores can be excited with 975 nm NIR laser irradiation, after which they show an intense green,


Scheme 1. Overview of the core, shell, and core/shell materials discussed in this paper.

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Scheme 2. Representation of the multimode luminescent properties of LiLuF4:Ln3+/LiYF4:Ln3+ core/shell nanoparticles.

Figure 1. a) TEM images, b) HRTEM images, and c) selected area electron diffraction (SAED) pattern of LiLuF4:Er, Yb core nanoparticles (material 1). d) TEM images e) HRTEM images, and f) selected area electron diffraction (SAED) pattern of LiLuF4:Er, Yb/LiYF4:Eu, Ce core/shell nanoparticles (material 1a).


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in Figure 1b,e, which exhibit clear lattice fringes with an interplanar spacing of 0.45 nm for the (101) plane of tetragonalphase LiLuF4. The selected area electron diffraction (SAED) ring patterns (Figure 1c,f) also confirm the high crystallinity of both the core and core/shell nanoparticles. The upconversion luminescence properties of different lanthanide-doped LiLuF4 cores 1, 2, and 3 and LiLuF4/LiYF4 core/shell nanomaterials 1a, 2a, and 3a have been illustrated in Figure 2. Under 975 nm CW laser irradiation all the upconversion luminescence spectra show sharp and typical emission peaks, which could be attributed to intra 4f electronic transitions of Er3+, Ho3+, and Tm3+. All the peaks for material 1, 2, and 3 presented in Figure 2 have been assigned to the appropriate upconversion transitions in Table S2 of the Supporting Information. The upconversion luminescence intensity of the core/shell materials 1a, 2a, and 3a have improved ≈3, 6 and 6 times compared to their respective core only samples 1, 2, and 3, because of decreased surface quenching because of the shell coating.[21] From the inset photographs in Figure 2, it can be observed directly that the emitted colors of the core/shell nanoparticles (on the right) are much brighter than those of the core ones (on the left).

The time-resolved upconversion luminescence decay traces of the LiLuF4 core materials 1, 2, and 3 all exhibit a singleexponential decay with fitted lifetimes 54.8, 90.3, and 245.7 µs, respectively (Figure S4, Supporting Information). For the corresponding core/shell nanomaterials 1a, 2a, and 3a, the upconversion luminescence lifetimes are 117.9, 387.4, and 928.1 µs, respectively (Figure S5, Supporting Information). The CIE diagrams in Figure S6 of the Supporting Information present the emission colors of core/shell nanomaterials 1a, 2a, and 3a, under CW laser irradiation at 975 nm, with the pump power changing from 381.6 to 44.65 mW. The corresponding x and y CIE coordinates are presented in Tables S3–S5 of the Supporting Information, respectively. As can be seen, the coordinates do not show a significant shift with increasing laser power and the emitted colors remain largely the same: for the Er3+, Yb3+ sample 1a, and the Ho3+, Yb3+ sample 2a, the emitted color is green, whereas for the Tm3+, Yb3+ sample 3a, it is blue. While the observed colors do not change much, the brightness of the emitted light obviously increases with increasing laser power. The downshifting luminescence spectra excited by UV light have been presented in Figure 3. In the emission spectrum

Figure 2. Upconversion luminescence spectra of a) LiLuF4:Er, Yb core nanoparticles (1, black) and LiLuF4:Er, Yb/LiYF4:Eu, Ce core/shell nanoparticles (1a, green). b) LiLuF4:Ho, Yb core nanoparticles (2, black) and LiLuF4:Ho, Yb/LiYF4:Eu, Ce core/shell nanoparticles (2a, green). c) LiLuF4:Tm, Yb core nanoparticles (3, black) and LiLuF4:Tm, Yb/LiYF4:Eu, Ce core/shell nanoparticles (3a, blue) under CW laser excitation at 975 nm. The inset pictures show photographs of the corresponding samples under CW laser excitation at 975 nm (left: core; right: core/shell).


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Figure 3. a) Excitation spectrum of LiLuF4:Er, Yb/LiYF4:Eu, Ce core/shell nanoparticles 1a monitored at 612 nm. b) Emission spectrum of LiLuF4:Er, Yb/LiYF4:Eu, Ce core/shell nanoparticles 1a excited at 390 nm; the inset picture shows a photograph of a cyclohexane suspension of the corresponding sample under laboratory UV lamp excitation (λ = 365 nm). c) Excitation spectrum of LiLuF4:Er, Yb/LiYF4:Tb, Ce core/shell nanoparticles 1b monitored at 542 nm. d) Emission spectrum of LiLuF4:Er, Yb/LiYF4:Tb, Ce core/shell nanoparticles 1b excited at 292 nm; the inset picture shows a photograph of a cyclohexane suspension of the corresponding sample under laboratory UV lamp excitation (λ = 302 nm).

of the LiLuF4:Er, Yb/LiYF4:Eu, Ce core/shell nanomaterial 1a (Figure 3b), the characteristic peaks due to the 5D0 →7FJ transitions of the Eu3+ ion can be seen (with J = 0–4). Tb3+ and Ce3+ ions are often encountered together in luminescent applications, such as green emitting phosphors, because Ce3+ can be efficient at absorbing (part of) the excitation light and transferring its energy to the Tb3+ ion.[22,23] Reasonably, the emission spectrum of LiYF4:Tb3+, Ce3+ material 1b shows only characteristic peaks of Tb3+, and the peaks shown in the excitation spectrum (Figure 3c) could be ascribed to the 5d ← 4f transition of the Ce3+ ion, which indicates very efficient energy transfer.[24] All the peaks for materials 1a and 1b in Figure 3 have been assigned to the appropriate downshifting transitions in Table S6 of the Supporting Information. The inset photograph in Figure 3b shows a cyclohexane suspension of the LiYF4:Eu3+, Ce3+ sample 1a under 365 nm excitation by a laboratory UV lamp. As the 365 nm excitation wavelength of the lamp is rather far away from the 390 nm absorption maximum of the Eu3+ ion, the typical red Eu3+ emission color is not very obvious in this picture, and the violet/blue light from


the lamp scattered by the particles in suspension is more easily seen. On the other hand, the inset photograph in Figure 3d does show the bright green color emitted by material 1b upon 302 nm excitation by a laboratory UV lamp. As this wavelength is quite close to the 292 nm absorption maximum, the material more easily shows its bright green emission. The downshifting luminescence lifetime of the 612 nm emission from the LiLuF4:Er, Yb/LiYF4:Eu, Ce core/shell material 1a under pulsed excitation at 390 nm is 4.11 ms, whereas for the 542 nm emission from the LiLuF4:Er, Yb/LiYF4:Tb, Ce core/shell material 1b under excitation at 292 nm, the decay time is 4.87 ms. Both values are quite long, illustrating that the LiYF4 matrix is a very good low-phonon host for downshifting luminescent lanthanide ions. Both decay profiles have been presented in Figure S7 of the Supporting Information. The absolute downshifting quantum yields of materials 1a, 2a, 3a, 1b, 2b, and 3b have been overviewed in Table S7 of the Supporting Information. To confirm that the peaks in the excitation spectrum in Figure 3c originated from Ce3+, an LiYF4 material singly doped

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with Ce3+ was synthesized. The corresponding excitation and emission spectra have been shown in Figure S8 of the Supporting Information. The emission band due to Ce3+ 5d → 4f transition is located at around 325 nm, and the excitation due to 5d ← 4f transition of the Ce3+ ion is located at around 254 and 287 nm. Compared to the excitation spectrum in Figure 3c, there is a slight red shift in the emission spectrum when the excitation wavelength increases. The LiYF4:Eu, Ce (a) and LiYF4:Tb, Ce (b) materials were used as a downshifting shell for the LiLuF4 upconversion cores 1, 2,

and 3. In the following section, the emission of these combined core/shell nanomaterials will be discussed, upon simultaneous excitation by a fixed power xenon lamp (UV, monochromated at 390 nm) and a CW 975 nm laser (NIR), of which the power was varied from 381.6 to 1.15 mW, in steps of 43.5 mW. The luminescence spectra and corresponding CIE diagrams of the LiLuF4:Er, Yb/LiYF4:Eu, Ce material 1a, the LiLuF4:Ho, Yb/LiYF4:Eu, Ce material 2a, and the LiLuF4:Tm, Yb/LiYF4:Eu, Ce material 3a are shown in Figure 4. For these three materials, it is obvious that the peaks in the spectrum which are due to the

Figure 4. a) Emission spectra of LiLuF4:Er, Yb/LiYF4:Eu, Ce core/shell nanomaterial 1a. c) Emission spectra of LiLuF4:Ho, Yb/LiYF4:Eu, Ce core/ shell nanomaterial 2a. e) Emission spectra of LiLuF4:Tm, Yb/LiYF4:Eu, Ce core/shell nanomaterial 3a under simultaneous excitation at both 390 nm (xenon lamp, fixed power) and 975 nm (CW laser, power varied from 381.6 to 1.15 mW). b,d,f) The corresponding CIE diagrams of (a), (c), and (e).


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downshifting Eu3+ emission, do not change in shape, nor intensity, upon variation of the power of the 975 nm CW laser. This concerns mainly the peaks at 588 nm (5D0→7F1), at 612 nm (5D0→7F2), and at 698 nm (5D0→7F4). These contribute a red component to the observed emission color. On the other hand, for the Er3+-containing material 1a, the upconversion luminescence peaks of Er3+, which contribute a green component to the observed emission color, gradually decrease in intensity with decreasing laser power (see Figure 4a). As a result, the combined observed emission color shifts from green to yellow to red, and the coordinates shown in the CIE diagram (Figure 4b) change linearly with varying pump power from 381.6 to 1.15 mW. This means that material 1a shows a power-dependent color change. For LiLuF4:Ho, Yb/LiYF4:Eu, Ce core/shell nanomaterial 2a, the same principle holds for the Eu3+ downshifting luminescence: the intensity and shape of the peaks at 588, 612, and 698 nm do not change with changing 975 nm laser power, whereas the intensity of the peaks corresponding to the Ho3+ upconversion transitions decreases with decreasing laser power (Figure 4c). The corresponding CIE diagram is shown in Figure 4d. As the Ho3+ upconversion emission introduces a green component into the observed color, at high laser power, the observed emission is green, changing to yellow and then to red upon decreasing laser power, somewhat similar to material 1a. For LiLuF4:Tm, Yb/LiYF4:Eu, Ce core/shell nanomaterial 3a, the combined downshifting/upconversion emission spectra with changing laser power are shown in Figure 4e. Again, the red color component from the downshifting Eu3+ luminescence stays unchanged, but now the blue color component from the Tm3+ upconversion decreases with decreasing laser power. This means the observed emission color changes from violet to reddish white, as shown in the CIE diagram in Figure 4f. For the core/shell nanomaterials 1b, 2b, and 3b, containing the Tb3+/Ce3+ downshifting shell, Figure 5 shows the combined downshifting/upconversion luminescence spectra under simultaneous excitation at both 292 nm (xenon lamp) and 975 nm (CW laser) upon changing laser power (again from 381.6 to 1.15, in 43.5 mW steps), as well as the corresponding CIE diagrams. For the LiLuF4:Er, Yb/LiYF4:Tb, Ce core/ shell nanomaterial 1b, the color shift is quite small, since the LiLuF4:Er, Yb core shows green upconversion luminescence, and the LiYF4:Tb, Ce shell also shows green downshifting luminescence. Hence, upon laser power variation, the combined emission color remains green (Figure 5a,b). Similarly, for the LiLuF4:Ho, Yb/LiYF4:Tb, Ce core/shell nanomaterial 2b, the combined emission color also remains green, although the chromaticity shift value is larger than for material 1b (Figure 5c,d). On the other hand, as the combined spectra and CIE diagram for the LiLuF4:Tm, Yb/LiYF4:Tb, Ce core/shell nanomaterial 3b show in Figure 5e,f, there is a significant change in the observed emission color from violet over white to green for this material upon decreasing the 975 nm laser power: whereas the shape and the intensity of the peaks situated at around 542, 585, and 619 nm (corresponding to the 5D4→7F5, 5D4→7F4, and 5D4→7F3 Tb3+ downshifting transitions and contributing a Adv. Funct. Mater. 2018, 1707365

green color component to the observed emission color), remain the same, the Tm3+ upconversion transition peaks located at around 450, 485, 650 nm and between 700 and 750 nm (and mainly contributing a blue color component) decrease in intensity with decreasing laser power. All the peaks for materials 1a, 1b, 1c and 2a, 2b, 2b in Figures 4 and 5 have been assigned to the appropriate transitions in Tables S8 and S9 of the Supporting Information, respectively. The x and y CIE coordinates corresponding to the diagrams from Figure 4b,d,f and Figure 5b,d,f have been summarized in Tables S10–S15 of the Supporting Information. The color stability can be quantifiably described by the chromaticity shift (ΔE) using the following equation[25,26] ∆E =

(ut′ − uo′ )2 + ( v t′ − v o′ )2 + ( w t′ − w o′ )2 (1)

where u′ = 4x/(3 − 2x + 12y), v′ = 9y/(3 − 2x + 12y), and w′ = 1 − u′ − v′. u′ and v′ are the chromaticity coordinates in u′v′ uniform color space, x and y are the chromaticity coordinates in CIE 1931 color space, and o and t are the chromaticity shift at pump power of 381.6 mW compared to the chromaticity at 1.15 mW. The values of the chromaticity shift for all samples have been presented in Table S16 of the Supporting Information. The largest chromaticity shift value (0.3082) was observed for the LiLuF4:Tm, Yb core with the LiYF4: Tb, Ce shell (sample 3b) whereas the smallest chromaticity shift value (0.0107) was observed for the LiLuF4:Er, Yb/LiYF4:Tb, Ce core/shell sample 1b. It should be mentioned here, that normally the chromaticity shift is used to determine the color stability of light-emitting diodes during temperature changes,[25] but here we utilize this equation to reify the color changes in regard to different lanthanide combinations instead.

3. Conclusion In this paper, we have introduced a new approach to luminescent materials for anticounterfeiting, more specifically a family of core/shell lanthanide fluoride nanomaterials that combine upconversion and downshifting luminescence. Simultaneous excitation with UV light and 975 nm NIR laser light, yields unique luminescence properties, which allows emission color tuning as a function of laser excitation power, e.g., from green to yellow to orange to red or from violet to white to green. We have presented a proof of concept using an LiLuF4 core material with three different lanthanide dopings (Er/Yb, Ho/ Yb, Tm/Yb) for upconversion, combined with an LiYF4 shell material with two different lanthanide dopings (Eu/Ce, Tb/ Ce) for downshifting luminescence. The upconversion color depends on the core doping and yields either green or blue luminescence, whereas the shell downshifting luminescence color can be red (Eu/Ce) or green (Tb/Ce). This concept allows the development of security codes for anticounterfeiting, where the nanosize of the obtained materials can be beneficial for the formulation of stable suspensions for ink-jet printing for example. This proof of concept can be further elaborated by exploring different core/shell doping combinations and concentrations, creating additional color combinations for enhanced multimodal anticounterfeiting.

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Figure 5. a) Emission spectra of LiLuF4:Er, Yb/LiYF4:Tb, Ce core/shell nanomaterial 1b. c) Emission spectra of LiLuF4:Ho, Yb/LiYF4:Tb, Ce core/shell nanomaterial 2b. e) Emission spectra of LiLuF4:Tm, Yb/LiYF4:Tb, Ce core/shell nanomaterial 3b under simultaneous excitation at both 292 nm (xenon lamp, fixed power) and 975 nm (CW laser, power varied from 381.6 to 1.15 mW). b,d,f) The corresponding CIE diagrams of (a), (c), and (e).

4. Experimental Section Synthesis of LiLuF4:Ln3+ Core Nanoparticles: The LiLuF4:Ln3+ (Ln = Er; Tm; Ho; Yb) core-only nanomaterials 1, 2, and 3 were synthesized via a high-temperature coprecipitation method according to the work of Wang et al.[19] In a typical synthesis of LiLuF4:Er, Yb, 0.25 mmol of LiCl, 0.145 mmol of LuCl3 · 6H2O, 0.005 mmol ErCl3 · 6H2O, and 0.1 mmol YbCl3 · 6H2O were mixed with 4 mL of oleic acid and 6 mL of 1-octadecene in a 50 mL three-neck round-bottom flask. The resulting mixture was heated to 160 °C under N2 flow with constant stirring for 30 min to form a clear solution, and then cooled down to room


temperature. Thereafter, 2.5 mL of methanol solution containing 1 mmol of NH4F was added and the solution was stirred at 50 °C for 30 min for crystal growth. After methanol was evaporated, the resulting solution was heated to 300 °C under N2 flow with vigorous stirring for 60 min. The obtained nanoparticles were precipitated by addition of acetone, collected by centrifugation, washed with acetone several times, and finally redispersed in cyclohexane. Synthesis of LiLuF4:Ln3+/LiYF4:Ln3+ Core/Shell Nanoparticles: In a typical synthesis of LiLuF4:Er, Yb/LiYF4:Eu, Ce core/shell nanoparticles (material 1a), 0.25 mmol of LiCl, 0.175 mmol YCl3 · 6H2O, 0.0375 mmol EuCl3 · 6H2O, and 0.0375 mmol CeCl3 · 7H2O were added to a 50 mL

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three-neck round-bottom flask containing 4 mL oleic acid and 6 mL 1-octadecene and heated to 160 °C under N2 flow with constant stirring for 30 min to form a clear solution, and then cooled down to 50 °C. Thereafter, 0.25 mmol LiLuF4:Er, Yb cores suspended in 4 mL cyclohexane was injected into the above solution. Thereafter, 2.5 mL of methanol solution containing 1 mmol of NH4F was added and the solution was stirred at 50 °C for 30 min. The solution was heated to 300 °C under N2 flow with vigorous stirring for 60 min, and then cooled down to room temperature. The obtained core/shell particles were precipitated by addition of acetone, collected by centrifugation, washed with acetone several times, and finally redispersed in cyclohexane. Synthesis of LiYF4:15% Ce3+ Nanoparticles: 0.25 mmol of LiCl, 0.2125 mmol of YCl3 · 6H2O, and 0.0375 mmol of CeCl3 · 7H2O were mixed with 4 mL of oleic acid and 6 mL of 1-octadecene in a 50 mL three-neck round-bottom flask. The resulting mixture was heated to 160 °C under N2 flow with constant stirring for 30 min to form a clear solution, and then cooled down to room temperature. Thereafter, 2.5 mL of methanol solution containing 1 mmol of NH4F was added and the solution was stirred at 50 °C for 30 min for crystal growth. After methanol was evaporated, the resulting solution was heated to 300 °C under N2 flow with vigorous stirring for 60 min. The obtained nanoparticles were precipitated by addition of acetone, collected by centrifugation, washed with acetone several times, and finally redispersed in cyclohexane. Characterization: XRD patterns were measured on a Thermo Scientific ARL X’TRA diffractometer at a scanning rate of 1.2° min−1, at the range of 15°–70°. XRF measurements were performed by using an in-house developed µXRF instrument, which is equipped with a monochromatic source (XOS, USA) and SDD detector (e2v, UK). XRF mappings were performed at 10 by 10 points (step size 100 µm) with a live time of 10 s per point. The XRF spectra were analyzed by the AXIL software package.[27] Monte-Carlo simulation aided quantification was used to calculate the relative amounts of lanthanide elements present in the samples.[28] TEM, HRTEM, and SAED were carried out by using a Cs-corrected JEOL JEM2200FS transmission electron microscope operated at 200 kV. The solvodynamic diameter measurements were performed on a Malvern Zetasizer Nano Series. The photoluminescence spectra were recorded on an Edinburgh Instruments FLSP 920 UV–vis–NIR spectrofluorimeter with a continuous-wave 975 nm laser with a maximum output power of 400 mW and a 450W xenon lamp as the steady state excitation source for upconversion and downshifting luminescence measurements, respectively. Identical sample concentrations (0.0625 mmol mL−1) were prepared for all measurements. The measurements were carried out under the same conditions (step size = 1 nm, ∆λem = 1 nm, dwell time 0.2 s, slit width of emission and excitation for downshifting and for the combined upconversion-downshifting systems was 2 and 5 nm, respectively; the slit width of emission for upconversion was 2 nm). Luminescence upconversion decay times were recorded by a Continuum Surelite I-10 Nd:YAG pumped OPO Plus laser, operating at a wavelength of 975 nm, and a pulse frequency of 10 Hz. For downshifting luminescence decay times, a 60 W pulsed Xe lamp, operating at a frequency of 100 Hz was used. Photographs of the observed emission color were taken from samples placed under a Cole-Parmer laboratory UV lamp, which could operate at either 254, 302, or 365 nm. The absolute downshifting quantum yields (QYs) were measured in an integrating sphere with a BENFLEC coating, provided by Edinburgh Instruments. An excitation wavelength of 390 nm was used for Eu3+, and an excitation wavelength of 292 nm was used for Tb3+ to measure the absolute QYs.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.


Acknowledgements This work was funded by the China Scholarship Council. A.M.K. acknowledges Ghent University’s Special Research Fund (BOF) for a Postdoctoral Mandate (Project BOF15/PDO/091). The authors acknowledge Prof. Kristof Van Hecke (Department of Chemistry, Ghent University) for creating Scheme 1 and the TOC figure.

Conflict of Interest The authors declare no conflict of interest.

Keywords anticounterfeiting, excitation-source-dependent color, power-dependent color, simultaneous downshifting and upconversion, single ultraviolet (UV) or near-infrared (NIR) light Received: December 19, 2017 Revised: January 19, 2018 Published online:

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