Synthesis and Research of Rare Earth Nanocrystal Luminescent Properties for Security Labels Using the Electrohydrodynamic Printing Technique

: YVO 4 :Eu 3 + nanoparticles were successfully synthesized by two methods, namely the sonochemical method and hydrothermal method. The X-ray di ﬀ raction (XRD) patterns showed the tetragonal phase of YVO 4 (JCPDS 17-0341) was indexed in the di ﬀ raction peaks of all samples. The samples synthesized by the sonochemical method had a highly crystalline structure (X-ray di ﬀ raction results) and luminescence intensity (photoluminescence results) than those synthesized by the hydrothermal method. According to the results of ﬁeld emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM), the average size of YVO 4 :Eu 3 + nanoparticles was around 25–30 nm for the sonochemical method and 15–20 nm for the hydrothermal method. YVO 4 :Eu 3 + nanoparticles in the case of the sonochemical method had a better crystalline structure and stronger emissivity at 618 nm. The Eu 3 + ions’ average lifetime in YVO 4 :Eu 3 + at 618 nm emission under 275 nm excitation were at 0.955 ms for the sonochemical method and 0.723 ms for the hydrothermal method. The security ink for inkjet devices contained YVO 4 :Eu 3 + nanoparticles, the binding agent as polyethylene oxide or ethyl cellulose and other necessary solvents. The device used for security label printing was an inkjet printer with an electrohydrodynamic printing technique (EHD). In the 3D optical proﬁlometer results, the width of the printed line was ~97–167 µ m and the thickness at ~9.1–9.6 µ m. The printed security label obtained a well-marked shape, with a size at 1.98 × 1.98 mm.


Introduction
Recently, researchers have paid attention to an inkjet technique in order to produce spare parts, such as electric circuits and bio-sensors [1][2][3][4]. This technique possesses the advantages of reducing steps in preparation and the printing ability in many different substrates, such as conductive, non-conductive, solid or flexible substrates. Besides research on the printing procedure, the most important issue is the preparation of an appropriate ink for a particular application. For that reason, security ink made of rare earth nanoparticles is a current focus as it has a wide application. In 2012, Meruga et al. [5] researched security ink from rare earth nanoparticles using β-NaYF 4 -doped Yb 3+ /Er 3+ and Yb 3+ /Tm 3+ in order to print a security QR code on a paper substrate and PET by Optomec direct-write aerosol jetting. In this research, the thickness of the QR sample printed on the paper substrate was in micrometer and the width in centimeter. A publication by Gupta et al. in 2010 also researched security ink from a rare earth nanoparticle, this time Y 2 O 3 -doped Eu 3+ (Y 2 O 3 :Eu 3+ ), but the printing method was by screen printing devices with a large-size security label, not by inkjet devices [6]. Our research goal is to study the security inkjet technique to enhance its security ability. The utilization of inkjet technology and security ink are emerging in high-security labels for printing money, visas, certificates and military products. There were two keys for the high security requirement: a small delicacy size (related to the printing technique) and strong luminescent intensity of the printed label under UV irradiation (related to optical emissivity of YVO 4 :Eu 3+ nanoparticles in the ink). The luminescent intensity of YVO 4 :Eu 3+ nanoparticles was attributed by many parameters, such as particles size and crystallinity, doped yield of Eu 3+ ions into YVO 4 , etc. A producible method need optimum synergy of these parameters [7][8][9].
In this report, two methods, namely the sonochemical and hydrothermal methods, were experimented with for the YVO 4 :Eu 3+ synthesis. According to a published work, a doped ratio at 5 mol % is the most appropriate ratio for luminescent intensity [9][10][11][12][13][14], which was also our doped ratio of the Eu 3+ ion. YVO 4 :Eu 3+ nanoparticles in the sonochemical method resulted the strongest luminescent intensity, which was used as red phosphor secured ink. Two types of security ink were prepared with different binding agents and solvent ingredients, and lead to different surface tensions. Using the electrohydrodynamic printing technique (EHD) inkjet printer, the line of the security label had width at~97-167 µm and thickness at~3-5 µm. The security labels were printed with various nanoparticle sizes, but the specific size was at 1.98 × 1.98 mm and the minimum distance between the two droplets was controlled at 0.05 µm. The hydrothermal method: The mixture was poured in Teflon sealed tightly and placed in the autoclave oven at 200 • C for one hour.

Preparation of Security Inkjet Ink
Two kinds of security inkjet inks using YVO 4 :Eu 3+ nanoparticles were prepared by mixing YVO 4 :Eu 3+ nanoparticles with a binding agent and solvents in the appropriate ratio (Table 1).

Security Printing
The test patterns were printed on a glass substrate by a commercial printer (PS JET 300V, Printing Solution, Seoul, Korea) using the electrohydrodynamic (EHD) inkjet technique.

Characterization
The synthesized YVO 4 :Eu 3+ nanoparticles samples were studied by field emission scanning electron microscopy (SU8010, Hitachi, Tokyo, Japan) and transmission electron microscopy (TEM, JEOL, Tokyo, Japan). The crystalline structure and doping of YVO 4 :Eu 3+ has been analyzed by X-ray diffraction spectroscopy and Raman spectroscopy. The excitation and emission spectra were recorded at room temperature through a Hitachi F-4500 spectrofluorometer (Hitachi, Tokyo, Japan). The luminescence time decay was measured by a DeltaflexTM (Horiba, Kyoto, Japan) with a 275 nm SpectraLED excitation source.
The surface tension of the security ink was analyzed by optical contact angle and the surface tension meter KSV CAM 101 (KSV Instruments, Monroe, CT, USA). The viscosity of the security ink was analyzed by a m-VROC™ VISCOMETER (RheoSense, San Ramon, CA, USA). The printed samples were analyzed by a 3D optical profilometer (Sensofar Metrology, Barcelona, Spain) and UV 20 W mercury lamps of 254 nm (Germicidal lamp, Sankyo Denki Co., Kanagawa, Japan). The ink droplet which jetted out of the printer was imaged by high-speed camera (FASTCAM SA-Z, Photron, Tokyo, Japan).

Results and Discussion
The purpose of this study is to provide a synthesis of strong luminescent intensity YVO 4 :Eu 3+ nanoparticles for application in security ink as a luminescent substance. Many factors may affect the light emission of YVO 4 :Eu 3+ nanoparticles (related to the light-emitting mechanism), such as the doping ratio of Eu 3+ ions, the particle crystalline growth and the nanoparticle size. With the same chemical components and ratio (the doping concentration of Eu 3+ was 5 mol %), the different correspondences of YVO 4 :Eu 3+ nanoparticles that were found were due to the two synthesis methods, sonochemical or hydrothermal.
According to Figure 1a-c, there is an appearance of sphere morphology in YVO 4 :Eu 3+ nanoparticles from the hydrothermal method, whereas no sphere was found in the sonochemical method. The YVO 4 :Eu 3+ nanoparticle size of both the sonochemical and hydrothermal methods is mainly at 25-30 nm and 15-20 nm, respectively. The YVO 4 :Eu 3+ nanoparticles size synthesized by two methods is suitable for the preparation of security ink. Figure 1d shows the XRD patterns of the YVO 4 :Eu 3+ nanoparticle crystalline prepared by the two different methods. YVO 4 tetragonal phase (JCPDS, No. 17-0341) represented by all the diffraction peaks confirmed the existence of a crystalline structure in all nanoparticles [15][16][17][18][19][20]. The presence of additional peaks indicated that the Eu 3+ ions have been successfully built into the YVO 4 host lattice. However, the peaks in the sonochemical method have a clear appearance with higher YVO 4 :Eu 3+ nanoparticle crystallinity than when using the hydrothermal method.    EDX spectra of YVO 4 :Eu 3+ nanoparticles synthesized by sonochemical and hydrothermal methods. Figure 3a shows the Raman spectra of undoped and Eu-doped YVO 4 nanoparticles synthesized at different methods. The undoped YVO 4 sample is synthesized by the hydrothermal method with the same chemical ratio as two doped samples. The YVO 4 crystal in the tetragonal structure is attributed to the point group of D 4h . The vibration appearance of complex VO 4 3− and Y 3+ ions in YVO 4 unit cells produce the lowest vibrational frequency at 164 cm −1 as external modes of Raman spectra [15]. The six Raman modes of these crystals were at 260, 379, 488, 813, 839 and 893 cm −1 (internal modes) [15,16]. There is a red shift of 388 cm −1 belonging to the undoped YVO 4 to the weaker wavelength at 379 cm −1 (Figure 3b). Furthermore, the intensity ratio difference between the two typical modes is at 839 and 893 cm −1 (Figure 3c), presenting a significant change between doped and undoped samples. The doping frequently causes the expansion or contraction of the crystal lattice depending on the difference between the ionic radius of the dopant and host cations. The replacement of Y 3+ ions by relatively larger Eu 3+ ions causes a stretching of the host lattice which can decrease the phonon energies causing a shift of the Raman modes [7,11,13,17]. The result of Raman spectra shows that the lattice vibration of the YVO 4 structure is altered due to the occupancy of the Eu ions at Y 3+ sites in the YVO 4 host lattice.   Figure 4a shows the YVO 4 :Eu 3+ luminescence mechanism. There are three major steps in the excitation and emission process of YVO 4 :Eu 3+ under UV radiation. First, the absorption of UV radiation by VO 4 3− groups. Second, the migration of thermal activated energy (comes from the UV excitation source) through the vanadate sub-lattice, causing the transferring of excited energy to Eu 3+ ions. Last, the appearance of strong red emissions due to the de-excitation process of excited Eu 3+ ions [11,13,18,[21][22][23]. Figure 4b shows the correspondence of the excited photoluminescence (PLE) spectra of the 618 nm emissions to Eu 3+ : 5 D 0 → 7 F 2 transition of theYVO 4 : Eu 3+ nanoparticles. There is an intensive broadband consistence, ranging from 260 to 350 nm, which occurred in all excited spectra. The origin of the broadband is supposed by O 2−-V 5+ within the VO 4 3   In Figure 4c, the specific peaks appear at 590, 618, 645 and 690 nm as a result of the 5 D 0 → 7 F 1 (magnetic-dipole transition), 5 D 0 → 7 F 2 (electric-dipole transition), 5 D 0 → 7 F 3 and 5 D 0 → 7 F 4 (excited optical source at approximately 275 nm) transfer. The main emission is at 618 nm by strongest emissivity 5 D 0 → 7 F 2 . In this research, the red emission of YVO 4 :Eu 3+ nanoparticles was analyzed (peak at 618 nm).
At 618 nm, the luminescent intensity of the sample synthesized by the sonochemical method is stronger than the one using the hydrothermal method. This is again confirmed with the XRD patterns above. In the sonochemical method, particles have a better crystallinity due to a high local temperature and pressure during the growing. The result in Figure 4c also indicates the occupying of Eu 3+ ions in the asymmetry inversion center instead of Y 3+ in both methods.
In Figure 5, the room-temperature luminescence decay curves of the 5 D 0 → 7 F 2 transition of Eu 3+ are presented for the samples synthesized in both methods. The excitation wavelength is fixed at 275 nm. The common factors that affect to the decay kinetics behavior are the number of different luminescent centers, defects, energy transfer and host impurities [41]. The raw recorded data for the decay curves of all samples are well-fitted by a double-exponential function, descripted as below: where I is the luminescence intensity at time t. A 1 , A 2 are the fitting parameter and τ is the decay lifetime, respectively. The Eu 3+ ions' average lifetime in the samples synthesized by sonochemical and hydrothermal methods at 618 nm emission under 275 nm excitation are 0.955 and 0.723 ms. The average lifetime increases for the sonochemical method sample. The non-radiative transition by the surface defects and/or crystallinity rate causes the appearance of a decay lifetime difference [8].
The surface effect has a great impact on the properties of the material at the nanometer size. In the nanocrystal, the defects of the lattice commonly appear on the surface. The surface defects cause the decreasing of lifetime and luminescent intensity. With sonochemical method, YVO 4 :Eu 3+ nanoparticles have better crystallization due to a high local temperature and pressure, and the lower surface effect. Furthermore, decay time is measured in both radiative and nonradiative transmission. The radiative decay component is depended on the number of light-emitting activator ions in the nanoparticles [19]. The Eu 3+ -doped ions in the host lattice are light emitting activators, showing the better Eu 3+ ion occupancy to Y 3+ sites in the sample when synthesized by the sonochemical method compared to the hydrothermal method. Due to the stronger luminescent intensity and more compatible nanoparticle size, YVO 4 :Eu 3+ nanoparticles synthesized by the sonochemical method are primarily used in the preparation of security ink. Two type of printing ink are prepared with same mass ratio of YVO 4 :Eu 3+ nanoparticles powder but different solvent ingredients. The ingredients of printing ink are shown in Table 1.
The viscosity and surface tension of printing ink are very important in the preparation of patterns using the inkjet technique due to the huge influence in spraying ink out of a head nozzle [42,43]. The analyzed and calculated parameters of ink I and II are presented in Table 2. All these parameters are analyzed under room temperature. Figure 6 presents the diagram of analyzed surface tension of ink I and ink II. Ink I's surface tension is~46-47 mN/m, whereas ink II's is~38-40 mN/m. The average viscosity of ink I and ink II, in turn, is~119 and 108 cP ( Table 2). The parameters of the prepared inks are in good working range of the inkjet device. The pattern is printed onto a glass substrate by a commercial printer PS JET 300V operated by the electrohydrodynamic (EHD) inkjet technique.  6. The diagram of analyzed ink surface tension. Figure 7 shows the mechanism of EHD technology. The application of electrohydrodynamic inkjet technique with positive pressure and high-voltage in the nozzle is to spray a fine droplet or thin line [44,45]. The EHD technique usage in pattern preparation required the adjustment of many parameters in the printing devices. This is an important step which the direction of the inkjet are performed. The parameter for the printing device has been adjusted after many experiments and established the standard parameter, showed in Table 3.  These parameters are used to print both ink I and ink II. The diameter of the printhead nozzle is one of the most important factors influencing the size of ink droplet when contacting to substrate (related to printed patterns size and sharpness). The smaller the printhead nozzle, the smaller the droplet size. Figure 8 shows the picture of an ink droplet jetted onto a glass substrate, using ink II (Figure 8a) and ink I (Figure 8b). The process of an ink droplet jetted onto the glass substrate is shown in three continuous pictures taken by high speed camera with frame rate set at 1000 frames per second. According to the frame rate, the time for an ink droplet to be jetted onto the substrate (from start to finish) is around 0.002 s. With a diameter of the print head at~15 µm, the droplet size on the substrate is at~10 µm with ink II and~2 µm with ink I. The surface tension difference between ink I and ink II causes this phenomenon (with the same viscosity, the surface tension of ink I being higher than ink II). Due to the characteristic differences, each security ink can be applied for separate security label printings. The purpose of this process is to adjust the printing parameters by observing the ink jetting out of the printhead. In Figure 9a,b, the width of lines printed by ink I was~97-115 µm and by ink IĨ 135-167 µm. All lines are sharp and seamless. Figure 9c,d show a clear appearance of security labels with an actual diameter at 1.98 mm × 1.98 mm (printed by ink I). The printed security label size may vary depending on the label shape. With this result, there is a possibility to print labels of a few hundred micrometers in size (by both ink I and ink II). Downsizing the label may increase the security ability. The continuous improving of the ink and the printing process is contributed to the printing ability on different types of substrate with higher sharpness and a few micrometers size. Figure 9. The micrographs from a 3D optical profilometer of a security label's lines printed by ink I (a) and ink II (b). The micrograph of the security label printed by ink I (c,d) Figure 10 shows the height-width diagram of lines printed by ink I and II. The size of the lines printed by ink I is smaller than those printed by ink II, conforming with the results shown in Figure 8. The thicknesses of the labels are, respectively,~10.8 and 13.5 µm and the widths are~110 µm and 145 µm. Figure 10b presents the image of a label printed in different sizes on a glass substrate under daylight and UV lamp. The labels are nearly transparent under daylight and have a red color luminescence under a UV lamp with a wavelength of 254 nm. The line appearance is clear, sharp, seamless and does not overlap one another at small distances.

Conclusions
YVO 4 :Eu 3+ nanoparticles were synthesized by sonochemical and hydrothermal methods. According to the analysis results, YVO 4 :Eu 3+ nanoparticles were crystallized in the tetragonal structure and exhibited the strongest luminescence at a wavelength of 618 nm due to the 5 D 0 → 7 F 2 transition. The Eu 3+ ions' average lifetime in the samples synthesized by sonochemical and hydrothermal methods at 618 nm emission under 275 nm excitation were 0.955 and 0.723 ms. The sonochemical method created the high local temperature, which had a positive effect on the YVO 4 :Eu 3+ nanoparticles crystalline formation and Eu 3+ ions into the Y 3+ doping position. Thus, YVO 4 :Eu 3+ nanoparticles synthesized by the sonochemical method possessed a higher crystallinity and better luminescence intensity. The TEM and FE-SEM results showed the average size of the YVO 4 :Eu 3+ nanoparticles was about 23-30 nm under the sonochemical method and about 15-20 nm under the hydrothermal method. YVO 4 :Eu 3+ nanoparticles synthesized by the sonochemical method had a higher luminescent intensity and compatible particle size, which might be used as red phosphor in security ink. The security labels were printed by inkjet printers using the electrohydrodynamic printing technique. The widths of the printed line were~97-167 µm and~3-5 µm were the thicknesses. The properties of the printed security labels (the size at 1.98 mm × 1.98 mm) were a well-sharp shape, daylight invisibility and red emission under UV mercury lamps (wavelength~254 nm). The high-security labels require a small size, shape sharpness and high luminescence. The minimum controllable diameter of an ink droplet on a glass substrate was~2 µm, a good base for printed security labels with high sharpness and micrometer size in the future.