Unclonable Security Codes Designed from Multicolor Luminescent

Apr 10, 2017 - Herein, we report the synthesis of multicolor luminescent lanthanide-doped Y2O3 nanorods by hydrothermal method and their usability in ...
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Unclonable Security Codes Designed from Multicolor Luminescent Lanthanide-Doped Y2O3 Nanorods for Anticounterfeiting Pawan Kumar,†,‡ Kanika Nagpal,† and Bipin Kumar Gupta*,† †

Alternative Energy Materials Section, Advanced Materials and Devices Division, and ‡Academy of Scientific and Innovative Research (AcSIR), CSIR−National Physical Laboratory Campus, Dr K S Krishnan Road, New Delhi 110012, India S Supporting Information *

ABSTRACT: The duplicity of important documents has emerged as a serious problem worldwide. Therefore, many efforts have been devoted to developing easy and fast anticounterfeiting techniques with multicolor emission. Herein, we report the synthesis of multicolor luminescent lanthanide-doped Y2O3 nanorods by hydrothermal method and their usability in designing of unclonable security codes for anticounterfeiting applications. The spectroscopic features of nanorods are probed by photoluminescence spectroscopy. The Y2O3:Eu3+, Y2O3:Tb3+, and Y2O3:Ce3+ nanorods emit hypersensitive red (at 611 nm), strong green (at 541 nm), and bright blue (at 438 nm) emissions at 254, 305, and 381 nm, respectively. The SEM and TEM/ HRTEM results reveal that these nanorods have diameter and length in the range of 80−120 nm and ∼2−5 μm, respectively. The two-dimensional spatially resolved photoluminescence intensity distribution in nanorods is also investigated by using confocal photoluminescence microscopic technique. Further, highly luminescent unclonable security codes are printed by a simple screen printing technique using luminescent ink fabricated from admixing of lanthanide doped multicolor nanorods in PVC medium. The prospective use of these multicolor luminescent nanorods provide a new opportunity for easily printable, highly stable, and unclonable multicolor luminescent security codes for anti-counterfeiting applications. KEYWORDS: photoluminescence, lanthanide, nanorods, counterfeiting and anticounterfeiting

1. INTRODUCTION Recently, the duplicity of brands and valuable documents such as diplomas, certificates, and currency is a growing critical issue that has created a serious problem for everyone including companies, government bodies, etc.1,2 Therefore, counterfeiting of documents and certificates such as passports, checks, bond certificate, and currency is a serious problem for every country, whether it is developed, developing, or under developed.3 The modern advancements in science and technology have made counterfeiting an effortlessly feasible job for frauds. Therefore, many efforts are going on worldwide to develop anticounterfeiting techniques for overcoming this serious problem. Different materials are being investigated for printing of advanced security codes for anticounterfeiting techniques with high-end security.3−9 These advanced anticounterfeiting techniques make it hard to duplicate and easy to authenticate.1 In last few decades, luminescent materials that emit visible light upon excitation with ultraviolet (UV) or infrared radiation © 2017 American Chemical Society

have been used as simple luminescent markers, security labels, holograms and invisible ink for printing of security codes or barcodes for the protection against counterfeiting. Numerous materials such as luminescent materials, plasmonic materials, metal−organic-frameworks (MOFs) and quantum dots (both semiconductor as well as carbon-based quantum dots) etc. have been developed for the anticounterfeiting application to combat counterfeiting problem.10 Although various materials are used for anticounterfeiting applications but they have their own problems. For example plasmonic materials are expensive and hence the synthesis or fabrication of plasmonic materials for anticounterfeiting at economic cost is still a challenge. The synthesis of MOF with high yield is still a big challenge for material chemists. The semiconductor quantum dots such as Received: March 8, 2017 Accepted: April 10, 2017 Published: April 10, 2017 14301

DOI: 10.1021/acsami.7b03353 ACS Appl. Mater. Interfaces 2017, 9, 14301−14308

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Figure 1. (a) XRD patterns of Y2O3:Eu3+, Y2O3:Tb3+ and Y2O3:Ce3+ nanorods. (b−d) Rietveld refinement patterns of Y2O3:Eu3+, Y2O3:Tb3+, and Y2O3:Ce3+ nanorods; observed results, calculated results, their difference, and Bragg position. (e) Crystal structure of Y2O3:Eu3+, Y2O3:Tb3+, and Y2O3:Ce3+ nanorods. (f) Schematic diagram for cationic symmetry site in Y2O3.

the hydrothermal method and successfully demonstrated their use for anticounterfeiting application. The photoluminescence (PL) and time-resolved photoluminescence (TRPL) spectroscopic techniques were used to explore the spectroscopic features of these nanorods. The Y2O3:Eu3+, Y2O3:Tb3+, and Y2O3:Ce3+nanorods exhibit hyperfine red emission at 611 nm, strong green emission at 541 nm and blue emission 438 nm upon excitation wavelengths of 254, 305, and 381 nm, respectively. The crystal structure analysis of Y2O3:Eu3+, Y2O3:Tb3+, and Y2O3:Ce3+ nanorods were carried out using powder X- ray diffraction and Rietveld refinement. The scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) were used for the morphology and microstructural analysis of nanorods. The PL mapping was performed for the investigation of PL intensity distribution of nanorods. Furthermore, we investigated the utility of these highly luminescent nanorods to design multicolor emitting luminescent ink for anticounterfeiting applications. The obtained spectroscopic results and unclonable multicolor luminescent security codes suggest that these luminescent nanorods have great potential for designing PVC (polyvinyl chloride) medium-based luminescent security ink for anticounterfeiting applications.

CdS, CdSe and CdTe etc have their own problems such as toxicity, broad emission bands, and solubility in harmful solvents. 11 The carbon-based quantum dots have low luminescence quantum yield. But, among all these materials lanthanide based luminescent materials have gained huge attention because these materials have many advantages over other materials such as sharp emission characteristic due to their intra f−f transitions, longer lifetime, both thermal and physical stability, high quantum yield, low photobleaching, high yield synthesis, etc.12,13 The lanthanide ions (Ln3+)-doped nanomaterials have gained great attention because of their exceptional luminescent properties ranging from UV−vis to NIR spectral region which originates from forbidden f−f transitions (produced pure luminescence color with longer lifetime) as well as their wide range of applications such as optoelectronic, display devices, security ink, biomedical, temperature sensor, photovoltaics=, etc.1,11,14−21 Further, the lanthanide-doped nanomaterials can be synthesized by bottom-up methods such as sol− gel, hydrothermal, spray pyrolysis, solvothermal, autocombustion and can be easily scaled up to large amount. Moreover, the size and shape of lanthanide doped nanomaterials can be easily tuned from micrometers to nanometers and spherical nanoparticles to nanorods.10,13,22 The Y2O3 is an attractive host lattice for lanthanide doped nanomaterials which is intensively investigated due to its physical and chemical properties and ease of synthesis of Y2O3-based nanomaterials.11,23 The 1D (one dimensional) lanthanide-doped nanomaterials have been used for anticounterfeiting applications in recent years.11,15The 1D lanthanide-doped nanomaterials are advantageous over their other morphologies for anticounterfeiting applications because of the reduced surface area of former that reduces the amount of material used for security ink. Also, nanorod-shaped nanomaterials are better infiller in cellulose matrix.11,24 Herein, we report the synthesis of lanthanide doped highly luminescent Y2O3:Eu3+, Y2O3:Tb3+, and Y2O3:Ce3+nanorods by

2. EXPERIMENTAL SECTION The precursors; Y2O3(99.99%), Eu2O3(99.99%), Tb2O3(99.99%), Ce2O3(99.99%), HNO3 and N-cetyl-N,N,N-trimethylammonium bromide (C19H42BrN, CTAB) were purchased from Sigma-Aldrich. All reagents were of analytical (AR) grade and used as received without further purification. Deionized water was used throughout the experiments. The hydrothermal method was used for the synthesis of multicolor (red, green and blue) Y2O3:Eu3+ Y2O3:Tb3+, and Y2O3:Ce3+ nanorods. The optimized concentration of Eu3+ ion (15 mol %) was used for the synthesis of red emitting Y1.7O3:Eu0.33+ nanorods as previously reported.16 For the synthesis of Y1.7O3:Eu0.33+ nanorods, the 14302

DOI: 10.1021/acsami.7b03353 ACS Appl. Mater. Interfaces 2017, 9, 14301−14308

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Figure 2. (a, b) SEM and high-resolution SEM images of Y2O3:Eu3+ nanorods, respectively. (c) TEM image of Y2O3:Eu3+ nanorods. (d) TEM image of single Y2O3:Eu3+ nanorod; inset shows HRTEM of Y2O3:Eu3+ nanorod. stoichiometric amount of Y2O3 and Eu2O3 were dissolved in a solution containing 10 mL of DI (deionized) water and 5 mL of hydrochloric acid (HCl) under stirring at 80 °C. The one molar solution of N,N,N trimethylammonium bromide (C19H42BrN, CTAB) was prepared in 20 mL DI water. The above prepared solutions were mixed in ethanol solution (1:5 ethanol to water ratio) under vigorous stirring at room temperature. The pH value is a critical parameter for the synthesis of nanorods. The sodium hydroxide solution was added to above solution under stirring to control the pH value of solution. The sodium hydroxide solution was added until the pH value of solution reached at ∼12. Then, the solution was transferred into hydrothermal bomb. The hydrothermal bomb was kept at 185 °C for 10 h in a box furnace. The white precipitate was formed during hydrothermal process. The obtained white precipitate was centrifuged several times with DI water at 5000 rpm and dried at 80 °C in oven. Finally, the powder was heated at 1000 °C for 6 h. Similarly, the Y1.7O3:Tb0.33+ and Y1.7O3:Ce0.33+ nanorods were synthesized for green and blue emission. The gross crystal structure analysis of sample was carried out by using X-ray powder diffraction (XRD) with Bruker AXS D8 Advance X-ray diffractometer, using Cu Kα1 radiation (λ = 1.5406 Å). The absorption spectra were recorded by UV−visible spectrometer (Model 160 UV Shimadzu). The Raman spectra were recorded using Renishaw in-Via Raman spectrometer where 785 nm laser act source of excitation. The surface morphology and elemental analysis were investigated by using Carl ZEISS EVOR-18 equipment at 10 kV operating voltage. The microstructural analysis was examined by using Tecnai G2 S-Twin transmission electron microscope with a field emission gun operating at 300 kV. The photoluminescence (PL spectroscopy was carried out using Edinburgh spectrometer, where xenon lamp and microsecond flash lamp act as sources of excitation. To estimate the absolute luminescence quantum efficiency of nanorod bundles, we have used an integrating sphere equipped with an Edinburgh spectrometer (model FLS900) instrument for measuring the integrated fraction of luminous flux and radiant flux with the standard method.15 The PL mapping of luminescent nanorods was performed by using WITec alpha 300R+ Confocal PL microscope system (WITec GnBH, Ulm, Germany), where 375 nm diode laser acts as a source of excitation.

3. RESULTS AND DISCUSSION The gross structural analysis of Y2O3:Eu3+, Y2O3:Tb3+, and Y2O3:Ce3+ nanorods were carried out using powder X-ray diffraction technique. The XRD patterns of Y2O3:Eu3+, Y2O3:Tb3+, and Y2O3:Ce3+ nanorods are shown in Figure 1a. The XRD patterns demonstrate that the nanorods have cubic structure with space group Ia3̅ (as per JCPDS card no. 43− 1036). For further analysis of crystal structure, the Rietveld refinement of XRD patterns was carried out. The Rietveld structural refinement of Y2O3:Eu3+(red), Y2O3:Tb3+(green) and Y2O3:Ce3+(blue) nanorods was performed using FullProf Suite software.25 The crystal structure of Y2O3 from literature was used as the initial parameters for the Rietveld refinement of Y2O3:Eu3+, Y2O3:Tb3+, and Y2O3:Ce3+ nanorods.26 The results obtained from refinement of Y2O3:Eu3+, Y2O3:Tb3+, and Y2O3:Ce3+ nanorods are illustrated in Figure 1b−d, which exhibit the experimentally observed XRD pattern, calculated XRD pattern, difference between observed and calculated XRD patterns and Bragg positions for the refinement of Y2O3:Eu3+, Y2O3:Tb3+, and Y2O3:Ce3+ nanorods. The refinement plots reveal the good agreement between the observed and calculated XRD patterns as shown in Figure 1b−d. The unit cell parameters obtained from the Rietveld refinement are summarized in Table S1.The estimated unit-cell parameters for Y2O3:Eu3+, Y2O3:Tb3+ and Y2O3:Ce3+ nanorods were a = b = c = 10.6541 Å, a = b = c = 10.6060 Å and a = b = c = 10.6678 Å, respectively, which are comparable to the standard lattice parameters a = b = c = 10.6040 Å. The unit-cell volume of Y 2 O 3 :Eu 3+ , Y 2 O 3 :Tb 3+ , and Y 2 O 3 :Ce 3+ nanorods were 1209.355, 1193.054, and 1214.003 Å3, respectively. The Rietveld refinement results reveal that the Y 2O3:Eu 3+, Y2O3:Tb3+, and Y2O3:Ce3+ nanorods have cubic crystal structure and crystal structure of host lattice Y2O3 was remain unchanged with the doping of Eu3+, Tb3+, and Ce3+ ions. 14303

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Figure 3. (a) PLE spectrum of Y2O3:Eu3+ nanorods. (b) PL emission spectrum of Y2O3:Eu3+ nanorods and inset shows CIE-color coordinates. (c) PLE spectrum of Y2O3:Tb3+ nanorods. (d) PL emission spectrum of Y2O3:Tb3+ nanorods; inset shows CIE-color coordinates. (e) PLE spectrum of Y2O3:Ce3+ nanorods. (f) PL emission spectrum of Y2O3:Ce3+ nanorods and inset shows CIE-color coordinates.

Moreover, the various R factors obtained from Rietveld refinement of Y2O3:Eu3+, Y2O3:Tb3+, and Y2O3:Ce3+ nanorods are summarized in Table S2. However, the quality of refinement was checked by calculating the parameter called goodness of fit (GoF). The goodness of fit is defined as GoF = Rwp/Rexp. The GoF must be unity for a perfect refinement. The acquired values of GoF for Y2O3:Eu3+, Y2O3:Tb3+, and Y2O3:Ce3+ nanorods are 1.45, 1.49 and 1.51, respectively (shown in Table S2). The cubic crystal structure of Y2O3 is shown in Figure 1e, where Y3+ ion are replaced by Re3+ = Eu3+, Tb3+ and Ce3+ ions because the ionic radii of Y3+ is very close to Eu3+, Tb3+, and Ce3+ ions.26 The cubic crystal structure of Y2O3 has two cation symmetry sites C3i and C2 as shown in Figure 1f. The oxygen ions are positioned at 48e Wyckoff position. Both the cationic sites are located over to Wyckoff position 8b and 24d with local symmetry C3i and C2, respectively.26 The cation site occupancy is a critical parameter that decides optical and other physical properties. The C3i symmetry site has inversion

symmetry. Therefore, only magnetic dipole transitions are allowed at this site. Whereas symmetry C2 sites do not have inversion symmetry, therefore both electric and magnetic dipole transitions are permitted at this site. Hence, these symmetry sites are mainly responsible for the luminescent emission from Eu3+, Tb3+, and Ce3+ ion-doped Y2O3 host lattice.26,27 The scanning electron microscopy (SEM) was used for the examination of surface morphology of Y2O3:Eu3+ nanorods. The SEM image of Y2O3:Eu3+ nanorods is shown in Figure 2a. The SEM image demonstrates that the as-synthesized nanorods have uniform size distribution. The high-resolution SEM image of Y2O3:Eu3+ nanorods is illustrated in Figure 2b. Figure 2b shows that Y2O3:Eu3+ nanorods have diameter in the range of 80−120 nm and length in the range of 2−5 μm. Moreover, the SEM images of Y2O3:Tb3+ and Y2O3:Ce3+ nanorods are shown in Figure S1 (see Supporting Information). Further, transmission electron microscopy (TEM) and high resolved 14304

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Figure 4. (a) Optical image of Y2O3:Eu3+ nanorods. (b) High-resolution optical image of Y2O3:Eu3+ nanorods. (c) 2D PL mapping image of Y2O3:Eu3+ nanorods.

demonstrates the PL emission spectrum of Y2O3:Eu3+ nanorods at excitation wavelength 254 nm. The emission spectrum exhibits hypersensitive red emission at 611 nm, which is ascribed to the 5D0−7F2 radiative transitions in Eu3+ ion. The weak PL emission peaks at 580−601 and 631 nm are attributed to the 5D0−7F1 and 5D0−7F2 radiative transitions, respectively. The weak emission peaks in the range of 580−601 corresponding to the 5D0−7F1 are due to the magnetic dipole transitions. However, the strong emission at 611 nm corresponding to 5D0−7F2 is ascribed to electric dipole transition and its intensity depends upon the local environment of Eu3+ ion (such as crystal field or symmetry of Eu3+ ion).28 Therefore, the emission intensity is high for Eu3+ ions located at C2 symmetry site as discussed above (in Figure 1f). The inset of Figure 3b reveals the CIE color coordinates of Y2O3:Eu3+ nanorods at excitation 254 nm, where x = 0.65 and y = 0.35. The energy transfer and exchange mechanisms depend on the thickness and the density of the lanthanide-doped Y2O3 fluorescent particles and related energy transfer processes can be demonstrated based on the crystal field theory.29−31 A simple model for the energy transfer from host lattice to activator is shown in Figure S6. The PL emission spectra of Y2O3:Eu3+ nanorods at different excitation wavelengths (254, 395, and 467 nm) are illustrated in Figure S7. Figure S7 depicts that the Y2O3:Eu3+ nanorods have maximum PL emission at excitation wavelength of 254 nm. Therefore, excitation wavelength 254 nm is prominent for maximum PL emission. The PLE spectrum of Y2O3:Tb3+ nanorods is shown in Figure 3c. The PLE spectrum of Y2O3:Tb3+ nanorods reveals the broad range excitation spectrum in the range of 250−337 nm having maxima at 305 nm. The PL emission spectrum of Y2O3:Tb3+ nanorods at excitation wavelength 305 nm is illustrated in Figure 3d. The emission spectrum of Y2O3:Tb3+ nanorods exhibits four different emission bands corresponding to 5D4−7Fj (j = 6, 5, 4, 3) electronic transitions in Tb3+ ion. The emission bands at 472−510 nm, 532−565 nm, 575−601 nm and 610−631 nm are attributed to the 5D4−7F6, 5D4−7F5, 5 D4−7F4, and 5D4−7F3 radiative transitions.32 The inset of Figure 3ddemonstrations the CIE color coordinates of Y2O3:Tb3+ nanorods at excitation wavelength 305 nm where x = 0.33 and y = 0.57. The PL emission spectra of Y2O3:Tb3+ nanorods at different excitation wavelengths are illustrated in Figure S8. Figure S8 shows that the Y2O3:Tb3+ nanorods have maximum PL emission at excitation wavelength of 305 nm. Figure 3e shows the PLE spectrum of Y2O3:Ce3+ nanorods at

transmission electron microscopy (HRTEM) were performed for the analysis of microstructure of Y2O3:Eu3+ nanorods. The TEM image of Y2O3:Eu3+ nanorods is shown in Figure 2c. Figure 2d shows the TEM image of individual Y2O3:Eu3+ nanorod. The inset of Figure 2d shows the HRTEM of Y2O3:Eu3+ nanorod. The HRTEM image of Y2O3:Eu3+nanorod clearly demonstrates that synthesized nanorods have wellresolved fringes without any distortion which reveals the good crystallinity of nanorods. The estimated d-spacing of nanorods is ∼0.3 nm which is comparable to the 0.306 nm corresponding to (222) plane (JCPDS card no. 43−1036). The qualitative analysis of tricolor Y2O3:Eu3+, Y2O3:Tb3+, and Y2O3:Ce3+ nanorods was performed by using energy-dispersive X-ray spectroscopy (EDS). The EDS spectrum of Y2O3:Eu3+nanorods is shown in Figure S2. The EDS spectrum nanorods exhibits the presence of Y, O and Eu atoms. Further, the EDS spectrum of Y2O3:Tb3+ anorods is shown in Figure S3, which exhibits of presence of Y, O and Tb atoms. Furthermore, the spectrum of Y2O3:Ce3+nanorods is shown in Figure S4, which reveals of presence of Y, O and Ce atoms inY2O3:Ce3+nanorods. Additionally, the thermal stability of Y2O3:Eu3+ nanorods was examined by the thermogravimetric analysis (TGA). The TGA graph of Y2(OH)3:Eu3+ nanorods is illustrated in Figure S5. The TGA results exhibits that three weight losses in three different time intervals. Initially, the weight loss from 99 to 188 °C attributed to water loss. The weight loss in the range of 268−312 °C corresponds to decomposition of nitrates into hydroxides. The weight loss in the range of 374 °C up to 800 °C is due to the decomposition of hydroxide into oxide. The results obtained are consistent with results reported earlier for same host lattice.11 The spectroscopic properties of Y2O3:Eu3+, Y2O3:Tb3+, and Y2O3:Ce3+nanorods were explored using photoluminescence (PL) techniques. The PL emissions spectra of Y2O3:Eu3+, Y2O3:Tb3+, and Y2O3:Ce3+nanorods exhibit the characteristic emission attributed to electronic transitions of Eu3+, Tb3+, and Ce3+ ions. The photoluminescence excitation (PLE) spectrum of Y2O3:Eu3+ nanorods is shown in Figure 3a. The excitation spectrum shows that the Y2O3:Eu3+ nanorods have broad excitation around 254 nm which is originated from the charge transfer (CT) between O2−−Eu3+ and other excitations peaks in the range of 300−500 nm are due to the f−f transitions within 4F6 electron shell of the Eu3+ ion. The excitation peaks at 395 and 465 nm are due to the 7F0−5L6 and 7F0−5D2 transitions of the Eu 3+ ion, respectively.28 Figure 3b 14305

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Figure 5. (a) Schematic for smart phone with a QR scanning application can read the encoded message “CSIR-NPL” in QR code in normal light and UV light and red, green and blue QR codes under UV light. (b−d) optical images of red, green and blue printed logo of “CSIR-NPL” under UV light, respectively.

terfeiting applications. For the practical application of multicolor nanorods for anticounterfeiting, the commercially available PVC medium (used for commercial screen printing) was used as ink medium. To fabricate security ink, we dispersed the multicolor nanorods in PVC medium. The red, green, and blue inks were fabricated using Y2O3:Eu3+, Y2O3:Tb3+, and Y2O3:Ce3+ nanorods, respectively. The Quick Response (QR) code and “CSIR-NPL” logo were printed on black paper using red, green, and blue inks. The schematic for screen printing process in shown in Figure S9. Figure 5a shows the red, green, and blue QR codes printed on black paper using under UV light. The printed QR do not exhibits any color under normal light as shown in Figure 5a. Therefore, these QR codes are not readable under normal light; however, the QR code shows red, green, and blue colors under UV light. Consequently, the QR codes are readable in UV light as shown in Figure 5a. A part from this, we also printed logo of CSIR-NPL on black paper using red, green and blue inks. The printing logo of CSIR-NPL on black paper was show strong red, green and blue color under UV light as shown in Figure 5b−d. The effect of the substrates on the fluorescent signals of printed logo of CSIR-NPL was also studied in details. Figures S10−S12 show the optical images of red, green, and blue emitting materials (Y2O3:Eu3+, Y2O3:Tb3+,and Y2O3:Ce3+nanorods) based printed logo of “CSIR-NPL” on rough pastel black paper, shining black paper and different origami colors papers under normal light and UV light. The SEM of red printed logo of CSIR-NPL is shown in Figure S13, which clearly shows the presence of nanorods in

emission wavelength of 438 nm. The PL emission spectrum of Y2O3:Ce3+ nanorods at excitation wavelength 381 nm is shown in Figure 3f. The inset of Figure 3f demonstrations the CIE color coordinated of Y2O3:Ce3+nanorods at excitation 381 nm where x = 0.16 and y = 0.08. Further, the estimated quantum efficiencies of Y2O3:Eu3+, Y2O3:Tb3+, and Y2O3:Ce3+ nanorods are 86, 71, and 65%, respectively. The PL intensity distribution is a critical parameter for the use of luminescent materials for any application.33 Therefore, the PL emission intensity distribution in Y2O3:Eu3+ nanorods was investigated using photoluminescence confocal microscopy where 375 nm diode laser was used as source of excitation. Figure 4a exhibits the optical image of Y2O3:Eu3+nanorods. The Figure 4b shows the high resolution optical image of Y2O3:Eu3+ n a n o r o d s . Fu r t h e r P L m a p p i n g o f a bu n c h o f Y2O3:Eu3+nanorods was carried out at region marked redregion in Figure 4b. The 2D (2 Dimensional) image of PL mapping of region marked red in Figure 4b is illustrated in Figure 4c. The Figure 4c reveals that theY2O3:Eu3+ nanorods have uniform PL intensity distribution. Therefore, the obtained spectroscopic and PL mapping results suggest that these highly luminescent nanorods with uniform PL intensity distribution could be highly useful for various applications such as biomedical, optical display devices, and anticounterfeiting applications.11,14,34 The preventing the valuable documents such as diplomas, certificates and currency form counterfeiting is an important task.10,35 Therefore, we demonstrated the use of the multicolors Y2O3:Eu3+,Y2O3:Tb3+, and Y2O3:Ce3+ nanorods for anticoun14306

DOI: 10.1021/acsami.7b03353 ACS Appl. Mater. Interfaces 2017, 9, 14301−14308

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Figure 6. (a) Optical image of red printed logo of “CSIR-NPL” under UV light. (b) 2D PL mapping image of the yellow marked region in a image at an excitation wavelength of 375 nm. (c) 2D PL mapping image of the yellow marked region in b at an excitation wavelength of 375 nm.

nanorods. Further, we utilized these multicolor Y2O3:Eu3+, Y2O3:Tb3+, and Y2O3:Ce3+ nanorods for the fabrication of security ink for anticounterfeiting applications. The obtained results suggests that these multicolor nanorods provide a futuristic approach to print multicolor security codes for anticounterfeiting applications, which are hard to duplicate and easy to detect.

printed patterns. The effect of different substrate surface was The PL intensity distribution in red emitting logo of CSIR-NPL was investigated by using PL confocal microscopy. The PL mapping was performed on the yellow parked region in Figure 6a under an excitation wavelength of 375 nm. Figure 6b shows the image of 2D (2-dimensional) PL intensity distribution of yellow marked region in Figure 6a under an excitation wavelength of 375 nm. Figure 6b clearly reveals that the printed pattern has uniform PL intensity distribution. Further, the PL mapping was also performed on yellow marked region in Figure 6b for examine the PL intensity distribution at high resolution. The image of 2D PL intensity distribution of yellow marked in Figure 6b is illustrated in Figure 6c. Figure 6c exhibits that the printed pattern has uniform PL intensity distribution at high resolution. These obtained results divulge that the multicolor luminescent nanorods in single host lattice (Y2O3) using different activators (Eu3+, Tb3+, and Ce3+) can be potentially used for printing of unclonable security codes. The major advantage used of single host lattice, its easily dispersion in PVC medium as well as less expensive as compared to designed multicolor emitting luminescent materials with different host lattices. Because, the suitability of different host lattice in PVC medium is a challenging as well as highly expensive task. Moreover, these multicolor luminescent nanorods excitable with three different wavelengths which include additional security features on valuable documents that provide them more protection against counterfeiting in compared to with range excitable single color emitting security codes. Therefore, these multicolors luminescent nanorods provide new opportunities for fabrication luminescent security ink for screen printing of easy, fast, and multicolor luminescent security codes at economical cost for anticounterfeiting applications

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b03353. SEM images, EDS analysis, TGA analysis, a simple model for the energy transfer, emission spectra of Y2O3:Eu3+ nanorods, schematic for screen printing process, pattern printed on different surfaces under normal of UV light, SEM image of printed pattern, table of unit-cell parameters obtained from the Rietveld refinement, and table of R factors obtained from the Rietveld refinement (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Bipin Kumar Gupta: 0000-0002-0176-0007 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. D. K. Aswal (Director, N.P.L., New Delhi) for his keen interest in the work. The authors are thankful to Prof. O. N. Srivastava (Banaras Hindu University, Varanasi) for his encouragement. Mr. P. Kumar gratefully acknowledges the financial support from University Grant Commission (UGC), Government of India. The authors are grateful to the CSIR-TAPSUN program for confocal PL mapping facility.

4. CONCLUSIONS We have successfully demonstrated the synthesis of multicolor luminescent Y2O3:Eu3+, Y2O3:Tb3+, and Y2O3:Ce3+ nanorods and their use in fabrication of luminescent security ink for anticounterfeiting applications. These multicolor Y2O3:Eu3+, Y2O3:Tb3+, and Y2O3:Ce3+ nanorods exhibit emission at 611, 541, and 438 nm upon excitation wavelengths of 254, 305, and 381 nm, respectively. XRD and Rietveld refinement results exhibit that these nanorods have cubic crystal structure with space group Ia3̅. SEM results reveal that these nanorods have diameter and length in range of 80−120 nm and 2−5 μm, respectively. The PL mapping results exhibit that these nanorods have uniform PL intensity distribution, which also confirms almost uniform substitutions of activators in Y2O3

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ABBREVIATIONS PVC, polyvinyl chloride; Ln3+, lanthanide ions; PL, photoluminescence; GoF, goodness of fit; PLE, photoluminescence excitation; CT, charge transfer 14307

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DOI: 10.1021/acsami.7b03353 ACS Appl. Mater. Interfaces 2017, 9, 14301−14308