Int J Nanotechnol., Vol 8, Nos 3/4/5, 2011 Luminescent nanomaterials containing rare earth ions for security printing Tran Kim Anh*, Dinh Xuan Loc, Tran Thu Huong and Nguyen Vu Institute of Materials Science, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Cau Giay distr., Hanoi, Vietnam E-mail: kimanh1949@gmail.com E-mail: locdx@ims.vast.ac.vn E-mail: huongtt@ims.vast.ac.vn E-mail: nguyenvu@ims.vast.ac.vn *Corresponding author Le Quoc Minh Institute of Materials Science, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Cau Giay distr., Hanoi, Vietnam and College of Technology, Vietnam National University of Hanoi, 144 Xuan Thuy, Hanoi, Vietnam E-mail: lequocminhvn@gmail.com Abstract: The high-efficiency luminescent nanomaterials with different emission wavelengths of red (YVO4:Eu3+), green (CePO4:Tb3+), ZnS:Mn2+ and YVO4:Eu3+@SiO2 were successfully prepared with different concentrations of Mn and rare earth ions as active centres by chemical synthesis Structure properties were studied It was found that the particle size of our samples was in the range of 10–30 nm Photoluminescent properties were studied under 325, 337, 365 and 370 nm excitations in order to apply in luminescent labels The primary colour components are red and green emission making them very convenient and attractive for screen security printing systems Hundreds of different labels with a size of 1–10 cm2 were prepared by screen-printing as well as inkjet printing By improving the Epson printer, commercial red, green and blue inks were used in the printing application Screen and inkjet printing were deemed good methods for security printing Our products were beautiful, high resolution and withstood tropical weather Keywords: luminescent nanomaterial; Tb3+; Eu3+ security printer; coreshell Reference to this paper should be made as follows: Anh, T.K., Loc, D.X., Huong, T.T., Vu, N and Minh, L.Q (2011) ‘Luminescent nanomaterials containing rare earth ions for security printing’, Int J Nanotechnol., Vol 8, Nos 3/4/5, pp.335–346 Copyright © 2011 Inderscience Enterprises Ltd 335 336 T.K Anh et al Biographical notes: Tran Kim Anh graduated from Hanoi University in 1970, received her PhD in Physics from the Institute of Physics, Polish Academy of Science, Warsaw, Poland, in 1988 She has been an Associate Professor since 1996 She has been the Director of Physics and Chemistry of Advanced Optical Materials Laboratory since 1998 She received the National Award (group) on Sciences and Technology in 2005 She has been a Director of Research since 2009 Her research interests include physics and technology of luminescent nanomaterials containing rare earth ions and nano-structured semiconductors including ZnO and ZnS as well as planar waveguide Dinh Xuan Loc graduated from Hanoi University in 1990, received Master’s Degree in Chemistry in 1999 and has been a Senior Researcher since 2008 He is currently doing his PhD thesis at the Institute of Materials Science, Vietnam Academy of Science and Technology His research interests include nanomaterials containing rare earth and transition metal ions Tran Thu Huong received her Bachelor’s Degree in Chemistry She did her Master’s Degree at the International Training Institute of Materials Science in 1999 She received her Doctorate of Philosophy in Materials Science from the Institute of Materials Science, Vietnam Academy of Science and Technology, in 2007 Her research interests include luminescent nanomaterials containing rare earth ions and planar waveguide She received the Vallet Scholarship 2006–2007 Nguyen Vu received his Master’s Degree from the International Training Institute of Materials Science in 1998, and then he received the PhD in Materials Science from the Institute of Materials Science, Vietnam Academy of Science and Technology, in 2007 His research interests include the physical chemistry of nanomaterials, optoelectronic and photonic application, luminescent nanomaterials containing rare earth ions and up-conversion nanomaterials Le Quoc Minh is an Associate Professor of Physics in the Institute of Materials Science and the Director of Research, Vietnam Academy of Science and Technology He received his PhD in Physical Chemistry, Berlin, Germany He has been the Director of Laboratory of Photochemistry, Imaging and Photonics; Lecturer at the University of Engineering and Technology, Vietnam National University, Hanoi He is the Vice President and Secretary of Society of Materials Science of Vietnam His research interests include materials science, physics and chemistry of photoresponsive compounds, luminescence materials, nanomaterials, hybrid and composite for optoelectronics and application in biophotonics Introduction Nanophotonics is an exciting field in nanoscience and nanotechnology It allows the opportunities for basic research and the development of new breakthrough solutions The interaction of light with matter on a nanometre-size scale has had many new developments Nanophotonics is an exciting frontier in nanoscience and technology for chemists, physicists, engineers and biologists as well as biomedical researchers [1] Some interesting topics include the novel synthetic routes and processing of nanomaterials, self-assembled periodic, nanoscales, non-linear optical processes and Luminescent nanomaterials containing rare earth ions for security printing 337 time-resolved and spectrally resolved studies Many report, for example, about the applications of nanoparticles in biophotonics [2], luminescent nanomaterials for biological labelling [3], functionalised europium nanorods for in vitro imaging [4], development of new ink materials based on luminescent nanomaterials for the security labels of printing products such as passport and visa documents has encouraged research in this direction [5] We studied luminescence, energy transfer and up-conversion mechanism of Y2O3 doped with Eu3+, Tb3+, Tm3+, Er3+ and Yb3+ ions [6], and security labels is attractive application Figure shows an image under UV exposure of a European visa for corresponding author, in which security part was made with luminescent materials This paper reports on the preparation of luminescent nanomaterials from YVO4:Eu3+, ZnS:Mn2+, and CePO4:Tb3+ for two important spectral fields at red and green regions Their structures, nanoform and optical properties, especially luminescence behaviour, were shown to depend on preparation conditions and allowed us to obtain a high luminescence efficiency, nanosize and fine distribution in the liquid phase We present also the fabrication capacity of a new ink based on the obtained luminescent nanomaterials for screen or inkjet printing technology Figure Security printing for a European visa (see online version for colours) Experiment 2.1 Sample preparation Preparation of YVO4:Eu3+ nanoparticles by hydrothermal synthesis YVO4:Eu3+ nanomaterials were prepared by hydrothermal method, which is described in detail [7] Y2O3, Eu2O3, HNO3, NH4VO3 and NaOH (analytical reagent of Merck) were used as starting materials and ethanol (Prolabo Chemical reagent) as solvent for the preparation and washing of the YVO4.Eu3+ nanocrystalline powders Y(NO3)3 and Eu(NO3)3 were prepared by dissolving Y2O3 and Eu2O3 in nitric acid The process of the synthesis of YVO4:Eu3+ nanoparticles powders is as follows: the Y(NO3)3.6H2O and Eu(NO3)3.5H2O were dissolved in 15 ml of deionised water, and the solution was poured into a Teflon vessel During stirring, 15 ml of 1M NaOH solution were rapidly added to the solution, which become a white suspension To the well-stirred suspension, Na3VO4.10H2O dissolved in 30 ml of deionised water was added Finally, the reaction mixture pH reached 12.5 by using 1M NaOH solution The Teflon vessel containing the suspension was capped and placed in an autoclave and heated at 200°C for h 338 T.K Anh et al The suspension was centrifuged at 3000 rpm for 10 The supernatant was discarded The precipitate was suspended in 30 ml deionised water and centrifuged again for several times The powder of the nanoparticles was obtained by removing ethanol, which was treated at 60°C for several hours Silica coating of the YVO4:Eu3+ nanoparticles YVO4:Eu3+ nanomaterials were prepared by co-precipitate method The synthesis procedure of YVO4:Eu3+ nanoparticles is as follows: The Na3VO4 powder was completely dissolved in 50 ml of deionised water Y(NO3)3.6H2O, 99.8%, and Eu(NO3)3.5H2O, 99.9%, were added to the solution in a 100 ml round-bottomed flask and this was followed by a magnetic stirring for 120 The pH of the solution after the reaction was in the range of 12–12.5 by using 1M NaOH solution The content ratio of Y3+ and Eu3+ were 0.9/0.1 moles, respectively After that, the solution was poured into a Teflon-lined stainless steel autoclave and heated at 200°C from 20 h to 40 h, and then cooled down slowly to room temperature The resulting products were collected, and centrifuged at 5900 rpm, washed several times using water, and then air-dried at 60°C for several hours ml of an aqueous solution of tetraethylorthosilicate (TEOS) and isopropanol were mixed in a 100 ml round-bottomed flask Then, YVO4:Eu3+ nanoparticles were added to the same flask The resulting solution was stirred at ambient temperature for h The obtained mixture was then refluxed for 24 h The grafted colloids were finally purified by ultrafiltration Preparation of ZnS:Mn2+ nanoparticles ZnS:Mn2+ nanomaterials were prepared by co-precipitate method from Zn(CH3COO)2, Mn(CH3COO)2 and (NH4)2Sx with different concentration of Mn of 0.01, 0.025 and 0.05 at% Polyvinyl alcohol (PVA) was used as the polymer matrix Metal solution was mixed with PVA solution with the weight ratio of metal acetate to PVA of 1/3 for about one hour while being stirred at 80°C Then, (NH4)2Sx solution was added drop-wise into the solution above while stirring The emulsion was vigorously agitated using a magnetic stirrer for one hour more The ZnS:Mn2+ nanoparticles were separated from the solution by centrifugation and washed several times in hot water and ethanol The powder of the nanoparticles was obtained by removing the ethanol after being dried at 70–80°C for 2–3 h Preparation of CePO4:Tb3+/LaPO4 core/shell nanoparticles Recently, CePO4:Tb3+ nanorods and nanowires were studied [8,9] In our laboratory, CePO4:Tb3+ core nanoparticles were synthesised as follows: two aqueous solutions of Ce(NO3)3 and Tb(NO3)3 were mixed in an appropriate molar ratio The solution was concentrated by heating at 80°C until the excess free water had evaporated The mixture was dissolved in methanol Then, tris(2-ethylhexyl)phosphate (TEHP) was added to the solution under a vacuum at 80°C, the water and the methanol ingredients contained in the solution were distilled away A freshly prepared solution of crystalline phosphoric acid dissolved in a mixture of trioctylamine and TEHP was added into the metal solution The resulting microemulsion solution was transferred into a stainless Teflon-lined autoclave and heated at 200°C for h The resulting suspension was naturally cooled down to room temperature to get to the CePO4:Tb3+ core nanoparticles Luminescent nanomaterials containing rare earth ions for security printing 339 CePO4:Tb3+/LaPO4 core/shell nanoparticles were synthesised as follows: The solution of La(NO3)3 in TEHP was prepared as same as Ce(NO3)3 solution in TEHP And, the freshly prepared solution of crystalline phosphoric acid dissolved in a mixture of trioctylamine and TEHP was added into the above-mentioned La(NO3)3 solutions Then, this solution was added to one half of the CePO4:Tb3+ product Finally, the resulting microemulsion solution was transferred into a stainless Teflon-lined autoclave and heated at 200°C for h 2.2 Characterisation The YVO4:Eu3+, CePO4:Tb3+, and ZnS:Mn2+ nanoparticles were analysed by the D-5000 X-ray diffractometer (Siemens) with Cu Kα radiation The morphology of the samples was observed by using a transmission electron microscope (TEM, JEM-1010) and a field emission scanning electron microscope (FE-SEM, Hitachi, S-4800) The photoluminescent (PL) measurements were performed at room temperature by using a cw He-Cd laser (325 nm) as the excitation source, a Spectrapro 2300i monochromator (Acton) as the dispersive unit, and a Pixis 256 CCD (Acton) as the detector PL spectra of the nanoparticles were also measured by using a spectrometer system Horiba Jobin Yvon IHR 550 after being excited at 370 nm by a diode Different wavelengths of 254 nm, 277 nm and 365 nm were used for our studies and applications Results and discussion 3.1 Structural and morphological properties The X-ray diffraction pattern of typical ZnS:Mn2+ nanoparticles is given in Figure It can be noted that all of the diffraction peaks could be well indexed to the zinc-blende crystal structure, having prominent (111), (220), and (311) d-spacings In this Mn-doped sample, no additional phase was observed, indicating that manganese was successfully doped in the ZnS crystalline lattice The width of the diffraction lines is broadened because of the small size of the crystallites Figure XRD pattern of ZnS:Mn2+ (see online version for colours) 340 T.K Anh et al A TEM image of ZnS:Mn2+ is presented in Figure Figure shows a TEM image of the YVO4:Eu3+ nanoparticles prepared by hydrothermal synthesis It can be seen from the figures that the YVO4:Eu3+ sample occurs through the aggregation of the nanocrystals exhibiting sizes of about 20 nm Figure TEM image of ZnS:Mn2+ Figure TEM image of YVO4:Eu3+ nanoparticles prepared by hydrothermal synthesis The mean diameter of a nanoparticle corresponds to the diameter of a spherical volume from the FE-SEM image, with the diameter of the YVO4:Eu3+ nanoparticles being about 15–20 nm 3.2 Photoluminescent (PL) properties Synthesis and luminescence properties of Mn2+-doped ZnS nanocrystals are presented in [10] For ZnS:Mn2+, the 4T1-6A1 transition within the 3d5 configuration of the divalent manganese ion has been studied, peak at 584 nm to compare with ZnS:Tb3+ nanoparticles have two main peaks in the green emission at 517 and 541 nm (5D4-7Fj) PL spectra of CePO4:Tb3+ nanoparticles and CePO4:Tb3+/LaPO4 core/shell nanoparticles under 277 nm excitation are presented in Figure The spectra show narrow emission peaks resulting from 4f-4f transitions within Tb3+ ions, with the most Luminescent nanomaterials containing rare earth ions for security printing 341 intense peak at 543 nm corresponding to the 5D4–7F5 transition Three other peaks are observed at 489 nm (5D4–7F6), 583 nm (5D4 –7F4), 618 nm (5D4–7F3), respectively Comparing PL intensity of CePO4:Tb3+ nanoparticles and CePO4:Tb3+/LaPO4 core/shell nanoparticles, it is noted that the PL intensity of CePO4:Tb3+/LaPO4 core/shell sample is about 20 times higher than that of CePO4:Tb3+ sample Figure PL spectra of CePO4:Tb3+ nanoparticles and CePO4:Tb3+/LaPO4 core/shell nanoparticles under 277 nm excitation (see online version for colours) Figure shows PL spectra under 370 nm excitation of YVO4:Eu3+ nanoparticles prepared by hydrothermal synthesis Under UV excitation, europium-doped YVO4 nanoparticles exhibit strong red luminescence caused by a transition within the f-electron shell of the europium ions From the 5D0 level, there is a magnetic dipole level, which allowed transitions to a sublevel of 7F1, two electric dipole level transitions to the sublevels of 7F2, and three magnetic dipole transitions to sublevels of 7F3 The most intense luminescence lines correspond to the 5D0–7F2 emission (619 nm) in YVO4:Eu3+ nanoparticles similar in bulk materials Figure presents the comparison of the emission spectra change from to at.% Eu3+ in YVO4:Eu3+ nanoparticles Similar to bulk materials, the quantum yield shows a maximum at a doping level at about at.% europium and decreases at higher europium concentration (7 at.%) It is the quenching effect of the luminescence at high concentrations in lanthanide-doped systems This typical property is observed when the distance between europium ions decreases below a critical value Figure PL spectra of YVO4:Eu3+ nanoparticles for 5, 3, 7, at.% from top to bottom under 370 nm excitation (see online version for colours) 342 T.K Anh et al Similar PL spectra of YVO4:Eu3+ and YVO4:Eu3+@SiO2 nanoparticles are presented in Figures and Figure PL spectrum of YVO4:Eu3+ nanoparticles at 200°C for 6h under an excitation wavelength of 325 nm at room temperature (see online version for colours) Figure PL spectrum of YVO4:Eu3+@Silica nanoparticles under an excitation wavelength of 325 nm at room temperature (see online version for colours) The PL spectra of the YVO4:Eu3+ nanoparticles and the YVO4:Eu3+ with core-shell structure nanoparticles displayed red emission with narrow peaks corresponding to the intra-4f (5D0–7Fj, j = 1, 2, 3, 4) transitions of Eu3+ ions The peaks were found at 594 nm (5D0–7F1), 619 nm (5D0–7F2), 652 nm (5D0–7F3), and 702 nm (5D4–7F4), with the strongest emission at 619 nm One can observe from the peak at 619 nm that the PL intensity of the SiO2 coated sample is higher than that of the uncoated one 3.3 Application Nano Y2O3 doped with Er3+, Yb3+ was used to prepare infrared cards [11] Labelled microplaces were printed using screen technique or inkjet technology [12] The CorelDraw, Photoshop and Colour system of RGB (Red, Green, Blue) and CMYK printing system controlled the inkjet machine and produced the labelled images Some different printing samples demonstrate the application (Figure 9) Luminescent nanomaterials containing rare earth ions for security printing 343 Figure Lotus image of Vietnam Airline logo and VIETNAM letter using ZnS:Mn2+ /PVA under UV lamp (see online version for colours) Figure 10 shows solution of nanomaterials from CePO4:Tb3+ under a UV lamp Figure 11 presents a Vietnamese passport and a European visa, printed using commercial inks The nanosolution is of CePO4:Tb3+ with a strong green emission under UV 254 nm excitation The comparison of our samples with commercial luminescent materials shown that our nanomaterials could be used for security printing The application of our nanomaterials could be extended into the security field as well as biology labelling Figure 10 Solution of nanomaterials from CePO4:Tb3+ under a UV lamp (see online version for colours) Figure 11 Photo images of the luminescent security on Vietnamese passport and a European visa under UV lamp (see online version for colours) 344 T.K Anh et al 3.4 Future works Our new results will lead to future studies on luminescent nanophotonics We have some results on nanomaterials containing rare earth ions [13,14], hybrid and composite with nanostructures for photonic technology [15], preparation and photonic features of erbium-activated silica-zirconia multilayer films derived from sol gel process [16], preparation and infrared emission of silica–zirconia–alumina doped with erbium for planar waveguide [17], luminescence and up-conversion mechanisms of photonic materials doped with Eu3+, Er3+ and Yb3+ ions [18] and nanomaterials containing rare earth ions for infrared card and planar waveguide application [19] We will try to study some new nanomaterials in possible papers on the subject of functionalised fluorescent oxide nanoparticles: artificial toxins for sodium channel targeting and imaging at the single-molecule level [20], inorganic phosphate nanorods as novel fluorescent labels in cell biology [21], up-conversion properties of nanocrystalline ZrO2:Yb3+, Er3+ phosphors [22], functionalised europium nanorods for in-vitro imaging [4], biological and clinical aspects of lanthanide coordination compounds [23] Our luminescent nanomaterials could be used for some application Nanosolution of YVO4:Eu3+ could be used to recognise viruses such as Rota virus Our up-conversion of Y2O3:Er3+ or Y2O3:Er3+, Yb3+ could be used as infrared cards or for security illuminated by infrared laser 980 nm and to see the green or red colour depending on the concentration of Er3+ and Yb3+ Conclusion YVO4:Eu3+, ZnS:Mn2+, and CePO4:Tb3+ nanomaterials were successfully prepared by hydrothermal and co-precipitate methods The crystal structure and surface of nanomaterials were analysed by X-ray diffraction, TEM and photoluminescence characters were studied by high-resolution photoluminescence with different excited wavelengths The X-ray diffraction and TEM results show that our samples are nanomaterials with particle size about 10–30 nm Photoluminescent spectra were studied in detail They have strong PL intensity and their colour can be modified by concentration of Eu3+, Tb3+ and Mn2+ The promising material for imaging technology, excited by He-Cd laser, YVO4:Eu3+, also be excited by a 370-nm diode The optimal concentration of Eu3+ is at.% ZnS:Mn2+ and YVO4:Eu3+ nanomaterials have been the subject of much research due to their wide application in semiconductor-electronic technologies The influence of concentration has been discussed ZnS:Mn2+, CePO4:Tb3+ and YVO4:Eu3+ were used to prepare red or green labels under 365-nm lamps to control money or 370-nm diode to easily recognise green or red emissions Acknowledgements We are very grateful to Prof Acad Nguyen Van Hieu, National independent research project 2/2/742 DTDL NN 2009-2012, the Vietnam Academy of Science and Technology, Key Laboratory for Electronic Materials and Device, Institute of Materials Science, Vietnam Television (VTV2), Prof Pham Thi Minh Chau, Prof Pham Hong Duong, Cooperman laboratory and IMAG Corp Luminescent nanomaterials containing rare earth ions for security printing 345 References and Notes 10 11 12 13 14 15 16 17 18 Prasad, P.N (2004) Nanophotonics, A John Wiley & Sons, New York Prasad, P.N (2003) Introduction to Biophotonics, Wiley-Interscience, New York Wang, F., Tan, W.B., Zhang, Y., Fan, X and Wang, M (2006) ‘Luminescent nanomaterials for 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nanocrystalline ZrO2:Yb3+, Er3+ phosphors’, J Nanomater., Article ID 16391, pp.1–8 23 Misra, S.N., Gagnani, M.A., Indira, D.M and Shukla, R.S (2004) ‘Biological and clinical aspects of lanthanide coordination compounds’, Bioinorg Chem Appl., Vol 2, Nos 3–4, pp.155–192  ... chemistry of nanomaterials, optoelectronic and photonic application, luminescent nanomaterials containing rare earth ions and up-conversion nanomaterials Le Quoc Minh is an Associate Professor... processes and Luminescent nanomaterials containing rare earth ions for security printing 337 time-resolved and spectrally resolved studies Many report, for example, about the applications of nanoparticles... heated at 200°C for h The resulting suspension was naturally cooled down to room temperature to get to the CePO4:Tb3+ core nanoparticles Luminescent nanomaterials containing rare earth ions for