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PAPER Qian Wang et al. TiC2: a new two-dimensional sheet beyond MXenes
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Inkjet Fabrication of Highly Efficient Luminescent Eu-Doped ZrO2 Nanostructures Received 00th January 20xx,
Accepted 00th January 20xx DOI: 10.1039/x0xx00000x www.rsc.org/
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A. D. Furasova , V.Ivanovski , A. V. Yakovlev , V. A. Milichko , V. V. Vinogradov and A. V. a Vinogradov * We have demonstrated for the first time an inkjet fabrication of highly efficient luminescent structures based on Eu-doped ZrO2 nanocrystals (3.4±0.3 nm), with refractive index close to the one of the material bulk. The nano particles were sythetysed using a nonhydrolytic method in benzyl alcohol where the particles were post treated using acetic acid, leading to formation of a stable colloid. It was shown that the acetic acid's non-polar methyl group is responsible for its penetration through the hydrophobic layer all the way through to the surface of the ZrO2, leading to the cleavage of the Zr-OCH2C6H5 bond and the formation of surface acetate species and a concomitant decomposition of the zirconia superlattice. Hereby we show a new and efficient universal ink production throuh a multi step process - starting from solvothermal synthesis, dispersion of nanocrystals in water, and adaptation of the rheological parameters of the resulting sols. Eventually, we were able to obtain inks that we used for the production of.optical coatings, monolayer luminescentprotected holography and anti-counterfeiting printing. There structures, obtained at room temperature through inkjet printing, present dense xerogel structures with high optical transparency, high refractive index and more efficient luminescence compared to the non-homogeneous structures produced as a mixture of rare-earth elements and nanocrystals.
Introduction The new tendencies in the methods of material surface deposition dictate state of the art conditions for material's stability and their properties. One of those techniques, which obtained wide popularity in the last years, is the inkjet printing 1 of functionalized nanoparticles (NPs) . The field of application 1 2 include electronic printing , biosensors , anti-counterfeiting 3 applications . In this study, we try to expand the applicability into the direction of optical materials and coatings, however maintaining universality of the developed approaches for other applications. The fluorescent nanoparticles have been known for some time to be optically interesting materials for several reasons. An excellent example is zirconium oxide doped with rare earth 4,5 (RE) ions . Zirconium oxide has a wide transmission window 6 in the visible electromagnetic range (bandgap ≈ 5.0 eV) anda high melting point (Тm= 2715 ⁰С), but is also mechanically stable, making it a perspective candidate material for optical
coatings and different optical structures. Previous works4,7 also mention zirconium oxide as an optimal host for the preparation of materials with high luminescent properties. This is due to its chemical and photochemical inertness, high refraction index (RI) and a low phonon energy (470 cm-1)8, to overcome the phonon decay problem. Most popular, among the contemporary methods of synthesis of Eu-doped zirconia powders are coprecipitation9, sonochemical10 aqueous sol-gel11 and non-aqueous sol-gel4 techniques. In the course of the formation of doped zirconium oxide nanoparticles, the RE ions substitute the Zr4+ ions in the crystal structure. It is however well known that the luminescent properties of Eu3+ ion actually depend on its local structure.12 High crystallinity of the coating possessing luminescent properties is needed for it to show good optical properties when deposited via dip-coating, spin-coating, etc. Generally, it is achieved by an annealing of the coating or the initial material13,14. On the other hand, it is extremely difficult to obtain small enough, highly crystalline particles, without further thermal treatment using the classical methods of wetchemistry15. The best results,among all the known methods for the preparation of doped zirconium oxide, are achieved using the non-aqueous sol-gel synthesis in hermetically closed autoclaves at temperatures of 200 ˚С and higher. In this approach a key role in the formation of the crystal play the solvent, the time and the temperature assolvothermal 16 conditions . In the case of zirconia NPs, solvents such as
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benzyl alcohol and its derivatives also participate in the formation of the morphology of the NPs. This is done trough the formation and attachment of benzyl groups with the surface of NPs, which enables fixed growth of the particles and further results in a quite narrow particle dimension 18 distribution . In this way, highly crystalline NPs are obtained while avoiding the need for calcination of the precipitate or the use of any coating which should prevent the aggregation. This allows polymers to be used as substrates and also avoids the agglomeration and further particle growth in the deposited 4,18 sol . When such NPs, obtained via non-aqueous sol-gel technique, were transferred in a water solutions or deposited on a surface, an agglomeration would occur or a superlattice of NPs 16,18 would form . It is well known that benzyl groups are formed on the nanoparticle surface which are then inter coordinated 17,19 and form complexes with metals . One earlier publication reports on the success of the synthesis of lamellar Re2O3 structures, where benzoate groups were fixed between the lamellas as a result of their coordination on the surface of the nanoparticle and the formation of clysters. The non-aqueous synthesis usually assumes that the obtained nanoparticles can also be dispersed in organic solvents such as 4 chloroform, hexane or toluene, to which surface stabilizers like aliphatic acids, trimethoxy(octyl)silane, di(2ethylhexyl)phosphate, dodecylamine are added. The nanoparticles are then deposited on the surface through the solvent evaporation. The use of non-polar organic solvents however has its drawback in the application of such coatings on substrates made of plastics, polymers or composite materials, which dissolve in different organic solvents. Besides, the mostly used ink in the inkjet printing is actually water based, as a non-flammable and compatible with most of the printing heads and cartridges. This is the reason why in practice the inks are produced as physical mixtures of ions of rare earth elements and hydrosol obtained using aqueous 20 methods . These mixtures however form non-homogeneous in composition surface and possess lower efficiency than the suggested here ink product based on doped NPs. The mentioned drawbacks are the reason why we adopted a complex approach in the hydrosol synthesis. In the course of obtaining zirconia nanoparticles with small dimensions (3.4±0.3 nm) a solvothermal method was employed. It was also possible to obtain high density packaging after the deposition of these particles on the surface, which gives the opportunity of having a refraction index close to the material bulk. Besides, the solvothermal method is ideal for crystal doping with rare earth elements. In this work we doped 3+ zirconium oxide nanocrystals with ions of Eu , so that the obtained material possesses highly efficient luminescence (narrow emission lines and relatively long luminescent decay 4 time), high transparency and high stability .
As previously mentioned, NPs obtained through non-aqueous sol-gel technique are unstable when transferred into water. Thus, we also suggest a method for stabilization through hydrophobic super-lattice decomposition in aqueous medium, using acetic acid and avoiding any other surface active substance for nanoparticle stabilization. The stabilized hidrosol can then be used in inkjet printing technique for the production of stable and universal inks. These inks allow deposition of nanoparticles at room temperatureor by drying at t < 100 ⁰С and obtaining high-grade optical properties without calcination or other treatment. Practical usage of such inks allows production of unique and highly effective coatings, structures and portrayals on a range of surfaces. These coatings are translucent, have a high index of refraction and can luminescent under UV light. Authors hope that the presented here method will find a wide application in different fields of applied optics such as: production of optical elements, optical coatings, monolayer luminescent-protected holography and anti-counterfeiting printing.
Results and discussion Synthesis of nanoparticles As mentioned earlier, the non-aqueous sol-gel method appears to be the optimal one in cases when there is a need for NP with a strict morphology, obtained through the control inthe temperature, the time of synthesis and the used solvent. A detailed description on the formation of NP of HfO2 using the solvothermal method of synthesis, where benzyl alcohol 19,21 was used as solvent, is given in the works . The process of NP formation occurs in two stages. The first stage (cf. eq. 1) is a reversible reaction of ligand exchange between propoxide ligand and benzyl alcohol, at room temperature. In the second stage, the increase in the temperature triggers the condensation reaction between the alkoxy groups which leads to formation of Zr-O-Zr bonds and ethers (eq. 2). As mentioned before, the role of the benzyl alcoholasa regulatorof the 18 particle growth is through its coordination on the surface of the NP . 3+ The morphology of the zirconia NPs doped with 11% Eu obtained using non-aqueous sol-gel method is presented in Fig. 1. We would like to stress here, that we will use the term "doping" throughout the text for the process of substitution 4+ 3+ (as will be shown later) of Zr with Eu , even though the molar fraction of the dopant that we used might be considered higher than the one ordinarily connected to this term. According to the XRD, the crystallites between 3.14 and 3.82 nm, depending on the time of the synthesis were formed (Fig. 1b), where dimensions decrease with increasing time. This trend might be in connection with the increase of the 22 nanocrystallites and NPs density as a whole .
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Figure 1.Structural parameters of doped ZrO2 with 11 mol % of Eu. a) XRDpatterns 2 for samples with two days (2d), three days (3d) and six days (6d) of solvothermal treatment; b) Crystallite Size (Scherrer) of the obtained powder material depending on the treatment duration; c) TEM of doped zirconia NPs with HRTEM as insert; d) HRSEM Image of NPs agglomerates.
According to the XRD analysis, the doped zirconium oxide NPs (Fig. 1a) have a cubic crystal structure with a space group Fm3 m (Oh5 – Schoenflies notation). This conclusion follows the comparison with the characteristic peaks (01-078-1303) for pure zirconia, however for temperatures above 2640 ⁰С. This seems incompatible with fact that our measurements were performed at room temperature (25 °C) and that the cubic structure of zirconium oxide is metastable and quickly transforms into monoclinic. However, when 2 or 3 valent ions substitute the sites of Zr4+ in the crystal structure, forming oxygen vacancies, it stabilizes the cubic and tetragonal structures. In has been already shown that 5 % of substitute ions of rare earth elements leads to transformation of the host structure into a cubic one4,18. From the above mentioned we come to a conclusion that the Eu3+ ions really substitute the Zr4+ sites in the structure and are not present as inclusions. This difference plays a particular role in the probability of radiative vs. nonradiative decay mechanisms of the Eu3+ ion.8The TEM images (Fig. 1c) prove small dimensions of the crystallites (3.4±0.3 nm), and also the presence of the high temperature cubic phase (Fig. 1c as insert), which are in accord with the results obtained using XRD and TEM. The measured specific surface area data (ESI Fig. S1), using BET method, corresponds well to the specific surface area calculated for zirconia NP having 3 23 spherical form and density of 6.1 g/cm . In one of the 16 works , authors show that the obtained NP are prone to form superlattices, which is explained through a narrow dimension distribution and coordination of organic groups on the NP surface. In this way, the produced materials possess best starting properties for construction of homogeneous optical coatings close to the bulk material.
It is known that Zr atoms have a tendency to form chelate complexes with benzyl groups playing the role of ligands. Zirconia NP bonding in the solvothermal synthesis is due tothe formation of ordered organic-inorganic structures via π-π stacking. This on one hand allows for high density coatings to be formed, with a refractive index close to the bulk of the material. However, in order for the ink to be used in the inkjets and be printable, it should represent a stable NP hydrosol. The inks are actually stored in the cartridges and dispersed through the jets having diameter of ~21 µm (Dimatix Materials Cartridge, 10 pL), so thatany presence of agglomeration or sedimentation can lead to malfunction of the cartridge and clogging of the jets. This is the reason why distruction of agglomerates and superlattices into separate nanoparticles is a very important task. As mentioned before, resuspension and decomposition of the superlattice is possible using organic solvents and surface stabilizers. We however used different approach, allowing the decomposition of the solid hydrophobic superlattice previously formed through the self-organized layers of benzyl groups on the NP surfaces. As reported in the literature, for an 4 effective decomposition aliphatic acids were used . Inorganic acids like HNO3 H2SO4, HClcannot be applied, because they do not penetrate the hydrophobic layer and get to the NPs surface, as they lack a hydrophobic part. In this work we used acetic acid as an aliphatic acid which was added to the NPs. The acetic acid has a non-polar methyl group which is responsible for its good penetration through the hydrophobic layer to the surface of the ZrO2, which afterward leads to the cleavage of the Zr-OCH2C6H5 bond and formation of surface acetate species and a concomitant decomposition of the zirconia superlattice. It is worth mentioning that acidic acid partially desorbs from the NP surface into the bulk of the sol. As mentioned in the Experimental section, after the separation of the supernatant from the product, the acetic acid was added to the sediment with a constant stirring, which after a time of less than a minute turned the sediment into sol (Fig. 4a). This process is depicted in more details in scheme (3). It should be pointed out that no sol was formed when the experiment was carried out using HNO3 or HCl at different concentration levels. The Raman spectrum of zirconium oxide show bands which are -1 24,25 all in range below 760 cm . The same can be said for its IR spectrum.25 This means that the organic modification of the
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Figure 2.IR-ATR absorbance spectra of Eu-doped ZrO2 NPs; A: as prepared Eu-doped ZrO2 NPs (a), Eu-doped ZrO2 NPs in a sol (b), sum spectrum of BnOH and CH3COONa spectra given for comparison (c); B: sodium acetate (solid) (d), CH3COOH (liquid, glacial) (e), benzyl alcohol (f) given as organic materials used or supposed to form. Spectra were shifted along the y-axis for clarity.
crystals can be easily detected, because all important bands for the identification are above this range. The IR-ATR spectra of the specimens ZrO2 NPs with 11 mol % of Eu, (synthesized using benzyl alcohol), Eu-doped ZrO2 NPs in a sol, (synthesized using benzyl alcohol and treated with acetic acid afterward), sodium acetate, benzoic acid and glacial acetic acid are presented in Fig. 2. The spectra were converted to absorbance and ATR corrected. The spectra in Fig. 2 (a), (b) and (c) were also normalized -1 according to the height of the band at 1551 cm , in order to be able to compare the intensities of the bands between the spectra.
Spectrum (c) in Fig. 2 is actually a sum of the spectrum of the pure BnOH (Fig. 2(f)) and the spectrum of CH3COONa (Fig. 2(d)). Inspecting the results in Fig. 2 it is possible to say that
Figure 3. Raman spectra of Eu-doped ZrO2 NPs; A: as prepared Eu-doped ZrO2 NPs (a), Eu-doped ZrO2 NPs in a sol (b), sum spectrum of BnOH and CH3COONa spectra given for comparison (c); B: benzyl alcohol (liquid) (d), sodium acetate (solid) (e), CH3COOH (liquid, glacial) (f), given as organic materials used or supposed to form. Spectra were shifted along the y-axis for clarity.
the spectra of ZrO2 with 11 mol % of Eu in a sol (Fig. 2(b)) is almost identical with the spectrum of the sum spectrum in Fig. 2(c), which cannot be said for the spectrum in Fig. 2(a). The equivalence between the spectra is due to the fact that the ZrO2 nanoparticles have attached benzyl alcohol molecules on its surface, and were afterwards treated with acetic acid in order to destroy the superlattice. However, no bands from acetic acid are seen in the spectrum in Fig. 2(b), in particular no band from the characteristic stretching C=O vibration at 1709 cm-1 visible in Fig. 2(e). Thus, the ZrO2 with 11 mol % of Eu particles in a sol are covered mainly with molecules of benzyl alcohol and acetate ions. The non-correspondence of the spectrum of as prepared ZrO2 with 11 mol % of Eu NPs (Fig. 2(a)), to the BnOH (Fig. 2(f)) is difficult to explain, particularly knowing that benzyl alcohol has only been used in the synthesis of Eu-doped ZrO2 NPs. The reason behind this may be the possible transformation of the benzyl alcohol molecules attached on the ZrO2 NPs, since these can play the role of a catalyst. It is known that ZrO2-CeO2 NPs catalyse the oxidation 26 of benzyl alcohol into benzaldehyde. A further proof can be obtained through the inspection of Raman spectra, given in Fig. 3.
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Figure 4. a) Photograph of Eu-doped ZrO2 NPs in water before and after acidic treatment; TEM (b), STEM (c) and STEM EDX (d) Image mapping images of zirconia sol after acidic treatment.
From Fig. 3 it is possible to see that spectra (b) and (c) are almost identical, i.e. the spectrum of ZrO2 with 11 mol % of Eu NPs in a sol is equal to the sum of the benzyl alcohol and sodium acetate Raman spectra, Fig. 3 (d) and (f), respectively. This result is in accord with the result obtained from the IR measurements. In the same line, the Raman spectrum of as prepared ZrO2 with 11 mol % of Eu nanoparticles Fig. 3(a) is different from the benzyl alcohol Fig. 3(d), particularly in the -1 27 region from 1900 to 1250 cm of Raman shift. Here, a very -1 broad band with maximum at 1770 cm is present, something that is not characteristic for benzyl alcohol, but as mentioned previously, can originate from the benzaldehyde, or even ester C = O stretching vibration. In this way, it is possible to make a conclusion that the acetic acid acts on the agglomerates and superlattices of the zirconia nanocrystals by cleavaging the bonds existing between different particles and a formation of a transparent sol in contrast to the starting suspension Fig.4a. In support of the obtained data, the analysis of the morphology of the NPs using TEM (Fig. 4b) and STEM (Fig. 4c) was done, which support the formation of the 3.4±0.3 nm NPs, being separated from one another. The Fig. 4d shows a nicely separated nanocrystalswhich produce the characteristic signal for Zr at the places where nanocrystals are located.
Luminescent properties As discussed earlier, the use of the solvothermal synthesis for the production of doped NPs with highly efficient luminescent properties4,18 is the preferred method. The percentage of europium in the crystal structure of zirconium oxide was 11 mol %, as according to the literature data this ratio corresponds to maximal luminescence quantum yield4,28.
Figure 5. Luminescent characteristics of zirconia NPs. a)The excitation spectra for doped and mixed (11% Eu) zirconia NPs; b)The emission spectra for doped and mixed zirconia (11% Eu) NPs excited at 250 nm (curves for doped 11% Eu NPs with different synthesis duration - 2 days (green), 3 days (red), 6 days (blue)) and 260 nm (for pure 3+ zirconia mixed with Eu ions(cyan)) and room temperature PL decay times for synthesized samples; c)PL decay curves for doped 11% Eu NPs with different synthesis 3+ duration - 2 days (green), 3 days (red), 6 days (blue), NPs mixed with 11% Eu (cyan); d) Photograph of cuvettes with mixed 11% Eu and doped zirconia crystals.
When the solvothermal synthesis method is applied, the ions of europium are directly included in the crystal structure of the 2– metal oxide. The electronic transitions occur from the O ions 3+ to the 4f orbital of the Eu ion, in the so called charge transfer 29 2states (CTS) process, with probability depending on the O 3+ 3+ −Eu bond length, the thermal vibrations of the Eu , etc. Due to the high mechanical resistance of the zirconium oxide crystals and the ion exchange of zirconium with rare earths, the product is characterized with interesting luminescence properties like:narrow emission peaks, relatively long luminescent times and luminescence efficiency. In Fig. 5a the excitation spectra for 11 % mol europium doped ZrO2 (2 days treatment) NPs in DI H2O, corresponding to the emission maximum at 613 nm is shown. A wide range of UV light excitation with one maximum at 250 nm is characteristic for doped NPs. This corresponds to an electronic transition 2– 3+ from 2p orbitals of the O ions to the 4f orbitals of the Eu 4,29 3+ ion. However, when the excitation spectrum of the Eu zirconia NPs mixture is observed, two maxima are present: at 260 nm (being CTS process) and the maximum due to an excitation of the 4f levels (5L6 at λexc = 390 nm).29
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The emission spectrum of the doped NPs, where europium ions exchange crystal sites with the zirconium ions and NPs of pure zirconia physically mixed with europium ions, show substantial differences. When excited with UV light (λex = 250 nm) the doped zirconia samples show bands with clearly resolved maxima (Fig.5 b). The emission band at 613 nm is the 5 7 most intensive and originates from the D0- F2 electric dipole 3+ transition, the intensity of which depends on the Eu local surrounding and the matrix coupling strength. The rest of the 5 7 5 7 emission maxima, originating from D0- F1 (590 nm), D0- F3 5 7 (653 nm) and D0- F4 (700 nm) transitions, are also well separated in the spectrum. On the contrary, however, the emission spectrum of the mixture Eu3+ and zirconia NPs possesses broad and overlapped bands with unresolved maxima. It is possible to detect only the 5D0-7F1, 5D0-7F2 and 5 D0-7F4 transitions. The luminescence decay with time is different for the doped and mixed sample (Fig.5с), calculated for the 5D0-7F2 (613 nm) transition. A clear difference (fig. 5 c) in the rate of the luminescent decay (which for the mixture is higher) can be seen through a complete cease in luminescence for the mixture already after 4 ms, while for the doped specimen it can still be detected after 20 ms. This rate difference between the mixed and doped specimens can be explained through the higher degree of freedom that Eu3+ ions possess in water, in respect to the Eu3+ ions which are fixed in the crystal lattice. The higher degree of freedom increases the probability for non-radiative transitions, and also increases the probability for electron transfer from the excited states of Eu3+ ion to the water molecules. The illustrative results on the effectiveness of the emission are presented in Fig.5d for doped and mixed zirconia NPs. The cuvettes contain samples of zirconia hydrosol (V = 3 ml). The mole ratio of Zr/Eu in the doped sample is 9/1, while the mass concentration of this sample in the cuvette is 0.028 g/ml. The zirconia – Eu3+ sol mixture was obtained in a way that a sol of pure zirconia with mass concentration of 0.025 g/ml was prepared first and 3.9×10-4mol Eu3+ ions were added to it
Figure 6. Droplets formation and deposition on glass surface. a) Drop formation from Dimatix Materials Cartridge with drops speed of 4 m/s; b) Drops on glass surface deposited at temperature 45 °C with 70 µm drops spacing; с) Five wet layers coating on glass with high 35 µm drops spacing deposited at 45 °C.
Figure 7.The characterization of printed drops and coating on holographic films. a) The printed zirconia drop on a polymer holographic paper substrate; b) The comparison of drops thickness created by multiple deposition technique (drop-ondrop) 1,2 and 3 layers with thickness 35, 83 and 165 nm respectively. c) The SEM microphotograph of Inkjet printed structures on a polymer holographic paper with 165 nm thickness. d) The HRSEM of zirconia NPs in a printed layer.
afterwards, coming to Zr/Eu ratio of 8.9/1.1 in the 3 ml of sol. A visual demonstration on the practical effectiveness of the doped sol samples is given in Fig. 5d. The numerical data on the intensity for the investigated samples for the 613 nm transition is given in the supporting information (Table S1).
Inkjet printing Previously we explained the ink composition leading to a stable and uniform print. The highly stable inks are obtained, because the solpossesses a narrow particle dimension distribution and a high zeta potential (Fig.S2). As seen from Fig. 6a, we used one jet printing method with a drop speed of around 4 m/s (the recommended speed is from 6 to 10 m/s). Using square wave voltage curve (Fig.S3) we achieved a stable print in overall 3 h time, including the head cleaning procedure every 120 s. The printing head was placed on 500 µm height above the surface of the substrate. When deposited on a glass surface, the drop obtains a proper form (Fig. 6b) and even coating, with a small edge thickening, caused by the “coffee30 ring” effect . The dimension of the drops deposited on the surface is no more than 40 µm and possess an even central part. It is possible to produce highly precise thickness layer in a multilayer deposition procedure, because the deposited drops have a thickness of ca. 35 nm (Fig.S4). It has to be however taken into account that a simultaneous multi drop deposition (Fig. 6c) can lead to non-even layer deposit, which is not the case when one at a time drop procedure is used, i.e. is coupled with a drying time between drops depositions.
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The Inkjet printing method allows a precise positioning of the droplet on the floppy or hard substrate surface which is a prerequisite for obtaining uniform layers without shattering, even with thickness of several micrometers (Fig.7b). What's more, such coatings are transparent (cf. Fig. 7a), which allows them to be used as coatings on diffraction grids and holograms. Employing the method of consecutive layer drying, it is possible to deposit layer on layer with high precision and thus vary the thickness of the coating. Using drop-on-drop deposition, it is possible to obtain discontinuous thickness change from 35 nm to ≈165 nm (Fig. 7b). Uniform coatings can be produced with high repeatability of the surface (Fig.7c), covering large areas and preserving the particles dimensions in the deposit (Fig. 7d). As a matter of fact, the high packing density of the nanoparticles is the reason for the high transparency and excellent optical properties.
Optical properties of inkjet-printer coatings The results on the refractive index of the printed coatings are depicted in Fig. 8e. For this to be done, the reflectance at normal incidence was investigated as a function of the wavelength for the case of layered structure, where the thin layer is placed between two semi-infinite dielectric media (air and glass). Fringes maxima shown in Figs. 8 a-d, correspond to constructive interference of the reflected radiation with characteristic wavelengths from the layer. In this case, independent on the layer thickness, the reflectance for these wavelengths is obtained through the 2 2 2 relation R=((n1n3-n2 )/(n1n3+n2 )) , where n2 is the refractive
Figure 8. Optical properties of inkjet printed coatings. Reflectance spectrums from Eudoped zirconia with 2 (a), 3 (b) and 6 (c) days of solvothermal synthesis and nondoped zirconia sample (d); e) The calculated refractive index for Eu-doped zirconia with a different preparation time; f) The optical transmittance of Eu-doped zirconia on glass and holographic paper.
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coefficient for the film at the same wavelengths. Using the experimental data for the reflectance R(λ), it is possible to obtain the dispersion relation of the refractive index as a function of the wavenumber Fig.8e, according to the formula n(λ)=√((n1n3(1+√(R(λ))))/(1-√(R(λ)))). The high refraction index (RI) of the ink obtained after 6-days of solvothermalsynthesis of Eu-doped ZrO2 (RI~1.80) in contrast to fused silica (RI=1.48) 32 and the polymeric holographic paper (RI~1.41) speaks on behalf of a high crystallity of the obtained coatings and of a high degree of porosity, that brings the material properties close to its bulk. Depending on the time of the synthesis, the refractive index is increased which proves the decrease in the dimensions of the crystallites (Fig. 1b). It is important to stress that the crystal doping does not only leads to luminescence, but also, due to the much larger atomic mass of the Eu3+ leads to an increase density of crystals (crystals reducing in size) and increase refractive index33. Thus, the methodology of variation of the index of refraction by the process of doping could play a role in the production of optical elements with increased value of the index of refraction where needed (micro-lenses, holography). It is important though to know that such structures show saturation limit, after which is no loner possible to increase the index of refraction.4
Figure 9. Inkjet printed samples on different surfaces. a) The printed by Eu-doped zirconia square on a glass substrate with 1 µm thickness and luminescence under UVlight (b); c) The inkjet printed text (ITMO) with the thickness ~300 nm on a holographic paper in daylight and in UV-light (d); e) Anoriginal Russian ruble banknote under UVlight and inkjet printed by Eu-doped Zirconia text on the banknote (f).
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To mention that the optical transparency (Transmittance > 99 %) is present even in the thicker layers (
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Inkjet Fabrication of Highly Efficient Luminescent Eu-Doped ZrO2 Nanostructures Received 00th January 20xx,
Accepted 00th January 20xx DOI: 10.1039/x0xx00000x www.rsc.org/
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A. D. Furasova , V.Ivanovski , A. V. Yakovlev , V. A. Milichko , V. V. Vinogradov and A. V. a Vinogradov * We have demonstrated for the first time an inkjet fabrication of highly efficient luminescent structures based on Eu-doped ZrO2 nanocrystals (3.4±0.3 nm), with refractive index close to the one of the material bulk. The nano particles were sythetysed using a nonhydrolytic method in benzyl alcohol where the particles were post treated using acetic acid, leading to formation of a stable colloid. It was shown that the acetic acid's non-polar methyl group is responsible for its penetration through the hydrophobic layer all the way through to the surface of the ZrO2, leading to the cleavage of the Zr-OCH2C6H5 bond and the formation of surface acetate species and a concomitant decomposition of the zirconia superlattice. Hereby we show a new and efficient universal ink production throuh a multi step process - starting from solvothermal synthesis, dispersion of nanocrystals in water, and adaptation of the rheological parameters of the resulting sols. Eventually, we were able to obtain inks that we used for the production of.optical coatings, monolayer luminescentprotected holography and anti-counterfeiting printing. There structures, obtained at room temperature through inkjet printing, present dense xerogel structures with high optical transparency, high refractive index and more efficient luminescence compared to the non-homogeneous structures produced as a mixture of rare-earth elements and nanocrystals.
Introduction The new tendencies in the methods of material surface deposition dictate state of the art conditions for material's stability and their properties. One of those techniques, which obtained wide popularity in the last years, is the inkjet printing 1 of functionalized nanoparticles (NPs) . The field of application 1 2 include electronic printing , biosensors , anti-counterfeiting 3 applications . In this study, we try to expand the applicability into the direction of optical materials and coatings, however maintaining universality of the developed approaches for other applications. The fluorescent nanoparticles have been known for some time to be optically interesting materials for several reasons. An excellent example is zirconium oxide doped with rare earth 4,5 (RE) ions . Zirconium oxide has a wide transmission window 6 in the visible electromagnetic range (bandgap ≈ 5.0 eV) anda high melting point (Тm= 2715 ⁰С), but is also mechanically stable, making it a perspective candidate material for optical
coatings and different optical structures. Previous works4,7 also mention zirconium oxide as an optimal host for the preparation of materials with high luminescent properties. This is due to its chemical and photochemical inertness, high refraction index (RI) and a low phonon energy (470 cm-1)8, to overcome the phonon decay problem. Most popular, among the contemporary methods of synthesis of Eu-doped zirconia powders are coprecipitation9, sonochemical10 aqueous sol-gel11 and non-aqueous sol-gel4 techniques. In the course of the formation of doped zirconium oxide nanoparticles, the RE ions substitute the Zr4+ ions in the crystal structure. It is however well known that the luminescent properties of Eu3+ ion actually depend on its local structure.12 High crystallinity of the coating possessing luminescent properties is needed for it to show good optical properties when deposited via dip-coating, spin-coating, etc. Generally, it is achieved by an annealing of the coating or the initial material13,14. On the other hand, it is extremely difficult to obtain small enough, highly crystalline particles, without further thermal treatment using the classical methods of wetchemistry15. The best results,among all the known methods for the preparation of doped zirconium oxide, are achieved using the non-aqueous sol-gel synthesis in hermetically closed autoclaves at temperatures of 200 ˚С and higher. In this approach a key role in the formation of the crystal play the solvent, the time and the temperature assolvothermal 16 conditions . In the case of zirconia NPs, solvents such as
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benzyl alcohol and its derivatives also participate in the formation of the morphology of the NPs. This is done trough the formation and attachment of benzyl groups with the surface of NPs, which enables fixed growth of the particles and further results in a quite narrow particle dimension 18 distribution . In this way, highly crystalline NPs are obtained while avoiding the need for calcination of the precipitate or the use of any coating which should prevent the aggregation. This allows polymers to be used as substrates and also avoids the agglomeration and further particle growth in the deposited 4,18 sol . When such NPs, obtained via non-aqueous sol-gel technique, were transferred in a water solutions or deposited on a surface, an agglomeration would occur or a superlattice of NPs 16,18 would form . It is well known that benzyl groups are formed on the nanoparticle surface which are then inter coordinated 17,19 and form complexes with metals . One earlier publication reports on the success of the synthesis of lamellar Re2O3 structures, where benzoate groups were fixed between the lamellas as a result of their coordination on the surface of the nanoparticle and the formation of clysters. The non-aqueous synthesis usually assumes that the obtained nanoparticles can also be dispersed in organic solvents such as 4 chloroform, hexane or toluene, to which surface stabilizers like aliphatic acids, trimethoxy(octyl)silane, di(2ethylhexyl)phosphate, dodecylamine are added. The nanoparticles are then deposited on the surface through the solvent evaporation. The use of non-polar organic solvents however has its drawback in the application of such coatings on substrates made of plastics, polymers or composite materials, which dissolve in different organic solvents. Besides, the mostly used ink in the inkjet printing is actually water based, as a non-flammable and compatible with most of the printing heads and cartridges. This is the reason why in practice the inks are produced as physical mixtures of ions of rare earth elements and hydrosol obtained using aqueous 20 methods . These mixtures however form non-homogeneous in composition surface and possess lower efficiency than the suggested here ink product based on doped NPs. The mentioned drawbacks are the reason why we adopted a complex approach in the hydrosol synthesis. In the course of obtaining zirconia nanoparticles with small dimensions (3.4±0.3 nm) a solvothermal method was employed. It was also possible to obtain high density packaging after the deposition of these particles on the surface, which gives the opportunity of having a refraction index close to the material bulk. Besides, the solvothermal method is ideal for crystal doping with rare earth elements. In this work we doped 3+ zirconium oxide nanocrystals with ions of Eu , so that the obtained material possesses highly efficient luminescence (narrow emission lines and relatively long luminescent decay 4 time), high transparency and high stability .
As previously mentioned, NPs obtained through non-aqueous sol-gel technique are unstable when transferred into water. Thus, we also suggest a method for stabilization through hydrophobic super-lattice decomposition in aqueous medium, using acetic acid and avoiding any other surface active substance for nanoparticle stabilization. The stabilized hidrosol can then be used in inkjet printing technique for the production of stable and universal inks. These inks allow deposition of nanoparticles at room temperatureor by drying at t < 100 ⁰С and obtaining high-grade optical properties without calcination or other treatment. Practical usage of such inks allows production of unique and highly effective coatings, structures and portrayals on a range of surfaces. These coatings are translucent, have a high index of refraction and can luminescent under UV light. Authors hope that the presented here method will find a wide application in different fields of applied optics such as: production of optical elements, optical coatings, monolayer luminescent-protected holography and anti-counterfeiting printing.
Results and discussion Synthesis of nanoparticles As mentioned earlier, the non-aqueous sol-gel method appears to be the optimal one in cases when there is a need for NP with a strict morphology, obtained through the control inthe temperature, the time of synthesis and the used solvent. A detailed description on the formation of NP of HfO2 using the solvothermal method of synthesis, where benzyl alcohol 19,21 was used as solvent, is given in the works . The process of NP formation occurs in two stages. The first stage (cf. eq. 1) is a reversible reaction of ligand exchange between propoxide ligand and benzyl alcohol, at room temperature. In the second stage, the increase in the temperature triggers the condensation reaction between the alkoxy groups which leads to formation of Zr-O-Zr bonds and ethers (eq. 2). As mentioned before, the role of the benzyl alcoholasa regulatorof the 18 particle growth is through its coordination on the surface of the NP . 3+ The morphology of the zirconia NPs doped with 11% Eu obtained using non-aqueous sol-gel method is presented in Fig. 1. We would like to stress here, that we will use the term "doping" throughout the text for the process of substitution 4+ 3+ (as will be shown later) of Zr with Eu , even though the molar fraction of the dopant that we used might be considered higher than the one ordinarily connected to this term. According to the XRD, the crystallites between 3.14 and 3.82 nm, depending on the time of the synthesis were formed (Fig. 1b), where dimensions decrease with increasing time. This trend might be in connection with the increase of the 22 nanocrystallites and NPs density as a whole .
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Figure 1.Structural parameters of doped ZrO2 with 11 mol % of Eu. a) XRDpatterns 2 for samples with two days (2d), three days (3d) and six days (6d) of solvothermal treatment; b) Crystallite Size (Scherrer) of the obtained powder material depending on the treatment duration; c) TEM of doped zirconia NPs with HRTEM as insert; d) HRSEM Image of NPs agglomerates.
According to the XRD analysis, the doped zirconium oxide NPs (Fig. 1a) have a cubic crystal structure with a space group Fm3 m (Oh5 – Schoenflies notation). This conclusion follows the comparison with the characteristic peaks (01-078-1303) for pure zirconia, however for temperatures above 2640 ⁰С. This seems incompatible with fact that our measurements were performed at room temperature (25 °C) and that the cubic structure of zirconium oxide is metastable and quickly transforms into monoclinic. However, when 2 or 3 valent ions substitute the sites of Zr4+ in the crystal structure, forming oxygen vacancies, it stabilizes the cubic and tetragonal structures. In has been already shown that 5 % of substitute ions of rare earth elements leads to transformation of the host structure into a cubic one4,18. From the above mentioned we come to a conclusion that the Eu3+ ions really substitute the Zr4+ sites in the structure and are not present as inclusions. This difference plays a particular role in the probability of radiative vs. nonradiative decay mechanisms of the Eu3+ ion.8The TEM images (Fig. 1c) prove small dimensions of the crystallites (3.4±0.3 nm), and also the presence of the high temperature cubic phase (Fig. 1c as insert), which are in accord with the results obtained using XRD and TEM. The measured specific surface area data (ESI Fig. S1), using BET method, corresponds well to the specific surface area calculated for zirconia NP having 3 23 spherical form and density of 6.1 g/cm . In one of the 16 works , authors show that the obtained NP are prone to form superlattices, which is explained through a narrow dimension distribution and coordination of organic groups on the NP surface. In this way, the produced materials possess best starting properties for construction of homogeneous optical coatings close to the bulk material.
It is known that Zr atoms have a tendency to form chelate complexes with benzyl groups playing the role of ligands. Zirconia NP bonding in the solvothermal synthesis is due tothe formation of ordered organic-inorganic structures via π-π stacking. This on one hand allows for high density coatings to be formed, with a refractive index close to the bulk of the material. However, in order for the ink to be used in the inkjets and be printable, it should represent a stable NP hydrosol. The inks are actually stored in the cartridges and dispersed through the jets having diameter of ~21 µm (Dimatix Materials Cartridge, 10 pL), so thatany presence of agglomeration or sedimentation can lead to malfunction of the cartridge and clogging of the jets. This is the reason why distruction of agglomerates and superlattices into separate nanoparticles is a very important task. As mentioned before, resuspension and decomposition of the superlattice is possible using organic solvents and surface stabilizers. We however used different approach, allowing the decomposition of the solid hydrophobic superlattice previously formed through the self-organized layers of benzyl groups on the NP surfaces. As reported in the literature, for an 4 effective decomposition aliphatic acids were used . Inorganic acids like HNO3 H2SO4, HClcannot be applied, because they do not penetrate the hydrophobic layer and get to the NPs surface, as they lack a hydrophobic part. In this work we used acetic acid as an aliphatic acid which was added to the NPs. The acetic acid has a non-polar methyl group which is responsible for its good penetration through the hydrophobic layer to the surface of the ZrO2, which afterward leads to the cleavage of the Zr-OCH2C6H5 bond and formation of surface acetate species and a concomitant decomposition of the zirconia superlattice. It is worth mentioning that acidic acid partially desorbs from the NP surface into the bulk of the sol. As mentioned in the Experimental section, after the separation of the supernatant from the product, the acetic acid was added to the sediment with a constant stirring, which after a time of less than a minute turned the sediment into sol (Fig. 4a). This process is depicted in more details in scheme (3). It should be pointed out that no sol was formed when the experiment was carried out using HNO3 or HCl at different concentration levels. The Raman spectrum of zirconium oxide show bands which are -1 24,25 all in range below 760 cm . The same can be said for its IR spectrum.25 This means that the organic modification of the
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Figure 2.IR-ATR absorbance spectra of Eu-doped ZrO2 NPs; A: as prepared Eu-doped ZrO2 NPs (a), Eu-doped ZrO2 NPs in a sol (b), sum spectrum of BnOH and CH3COONa spectra given for comparison (c); B: sodium acetate (solid) (d), CH3COOH (liquid, glacial) (e), benzyl alcohol (f) given as organic materials used or supposed to form. Spectra were shifted along the y-axis for clarity.
crystals can be easily detected, because all important bands for the identification are above this range. The IR-ATR spectra of the specimens ZrO2 NPs with 11 mol % of Eu, (synthesized using benzyl alcohol), Eu-doped ZrO2 NPs in a sol, (synthesized using benzyl alcohol and treated with acetic acid afterward), sodium acetate, benzoic acid and glacial acetic acid are presented in Fig. 2. The spectra were converted to absorbance and ATR corrected. The spectra in Fig. 2 (a), (b) and (c) were also normalized -1 according to the height of the band at 1551 cm , in order to be able to compare the intensities of the bands between the spectra.
Spectrum (c) in Fig. 2 is actually a sum of the spectrum of the pure BnOH (Fig. 2(f)) and the spectrum of CH3COONa (Fig. 2(d)). Inspecting the results in Fig. 2 it is possible to say that
Figure 3. Raman spectra of Eu-doped ZrO2 NPs; A: as prepared Eu-doped ZrO2 NPs (a), Eu-doped ZrO2 NPs in a sol (b), sum spectrum of BnOH and CH3COONa spectra given for comparison (c); B: benzyl alcohol (liquid) (d), sodium acetate (solid) (e), CH3COOH (liquid, glacial) (f), given as organic materials used or supposed to form. Spectra were shifted along the y-axis for clarity.
the spectra of ZrO2 with 11 mol % of Eu in a sol (Fig. 2(b)) is almost identical with the spectrum of the sum spectrum in Fig. 2(c), which cannot be said for the spectrum in Fig. 2(a). The equivalence between the spectra is due to the fact that the ZrO2 nanoparticles have attached benzyl alcohol molecules on its surface, and were afterwards treated with acetic acid in order to destroy the superlattice. However, no bands from acetic acid are seen in the spectrum in Fig. 2(b), in particular no band from the characteristic stretching C=O vibration at 1709 cm-1 visible in Fig. 2(e). Thus, the ZrO2 with 11 mol % of Eu particles in a sol are covered mainly with molecules of benzyl alcohol and acetate ions. The non-correspondence of the spectrum of as prepared ZrO2 with 11 mol % of Eu NPs (Fig. 2(a)), to the BnOH (Fig. 2(f)) is difficult to explain, particularly knowing that benzyl alcohol has only been used in the synthesis of Eu-doped ZrO2 NPs. The reason behind this may be the possible transformation of the benzyl alcohol molecules attached on the ZrO2 NPs, since these can play the role of a catalyst. It is known that ZrO2-CeO2 NPs catalyse the oxidation 26 of benzyl alcohol into benzaldehyde. A further proof can be obtained through the inspection of Raman spectra, given in Fig. 3.
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Figure 4. a) Photograph of Eu-doped ZrO2 NPs in water before and after acidic treatment; TEM (b), STEM (c) and STEM EDX (d) Image mapping images of zirconia sol after acidic treatment.
From Fig. 3 it is possible to see that spectra (b) and (c) are almost identical, i.e. the spectrum of ZrO2 with 11 mol % of Eu NPs in a sol is equal to the sum of the benzyl alcohol and sodium acetate Raman spectra, Fig. 3 (d) and (f), respectively. This result is in accord with the result obtained from the IR measurements. In the same line, the Raman spectrum of as prepared ZrO2 with 11 mol % of Eu nanoparticles Fig. 3(a) is different from the benzyl alcohol Fig. 3(d), particularly in the -1 27 region from 1900 to 1250 cm of Raman shift. Here, a very -1 broad band with maximum at 1770 cm is present, something that is not characteristic for benzyl alcohol, but as mentioned previously, can originate from the benzaldehyde, or even ester C = O stretching vibration. In this way, it is possible to make a conclusion that the acetic acid acts on the agglomerates and superlattices of the zirconia nanocrystals by cleavaging the bonds existing between different particles and a formation of a transparent sol in contrast to the starting suspension Fig.4a. In support of the obtained data, the analysis of the morphology of the NPs using TEM (Fig. 4b) and STEM (Fig. 4c) was done, which support the formation of the 3.4±0.3 nm NPs, being separated from one another. The Fig. 4d shows a nicely separated nanocrystalswhich produce the characteristic signal for Zr at the places where nanocrystals are located.
Luminescent properties As discussed earlier, the use of the solvothermal synthesis for the production of doped NPs with highly efficient luminescent properties4,18 is the preferred method. The percentage of europium in the crystal structure of zirconium oxide was 11 mol %, as according to the literature data this ratio corresponds to maximal luminescence quantum yield4,28.
Figure 5. Luminescent characteristics of zirconia NPs. a)The excitation spectra for doped and mixed (11% Eu) zirconia NPs; b)The emission spectra for doped and mixed zirconia (11% Eu) NPs excited at 250 nm (curves for doped 11% Eu NPs with different synthesis duration - 2 days (green), 3 days (red), 6 days (blue)) and 260 nm (for pure 3+ zirconia mixed with Eu ions(cyan)) and room temperature PL decay times for synthesized samples; c)PL decay curves for doped 11% Eu NPs with different synthesis 3+ duration - 2 days (green), 3 days (red), 6 days (blue), NPs mixed with 11% Eu (cyan); d) Photograph of cuvettes with mixed 11% Eu and doped zirconia crystals.
When the solvothermal synthesis method is applied, the ions of europium are directly included in the crystal structure of the 2– metal oxide. The electronic transitions occur from the O ions 3+ to the 4f orbital of the Eu ion, in the so called charge transfer 29 2states (CTS) process, with probability depending on the O 3+ 3+ −Eu bond length, the thermal vibrations of the Eu , etc. Due to the high mechanical resistance of the zirconium oxide crystals and the ion exchange of zirconium with rare earths, the product is characterized with interesting luminescence properties like:narrow emission peaks, relatively long luminescent times and luminescence efficiency. In Fig. 5a the excitation spectra for 11 % mol europium doped ZrO2 (2 days treatment) NPs in DI H2O, corresponding to the emission maximum at 613 nm is shown. A wide range of UV light excitation with one maximum at 250 nm is characteristic for doped NPs. This corresponds to an electronic transition 2– 3+ from 2p orbitals of the O ions to the 4f orbitals of the Eu 4,29 3+ ion. However, when the excitation spectrum of the Eu zirconia NPs mixture is observed, two maxima are present: at 260 nm (being CTS process) and the maximum due to an excitation of the 4f levels (5L6 at λexc = 390 nm).29
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The emission spectrum of the doped NPs, where europium ions exchange crystal sites with the zirconium ions and NPs of pure zirconia physically mixed with europium ions, show substantial differences. When excited with UV light (λex = 250 nm) the doped zirconia samples show bands with clearly resolved maxima (Fig.5 b). The emission band at 613 nm is the 5 7 most intensive and originates from the D0- F2 electric dipole 3+ transition, the intensity of which depends on the Eu local surrounding and the matrix coupling strength. The rest of the 5 7 5 7 emission maxima, originating from D0- F1 (590 nm), D0- F3 5 7 (653 nm) and D0- F4 (700 nm) transitions, are also well separated in the spectrum. On the contrary, however, the emission spectrum of the mixture Eu3+ and zirconia NPs possesses broad and overlapped bands with unresolved maxima. It is possible to detect only the 5D0-7F1, 5D0-7F2 and 5 D0-7F4 transitions. The luminescence decay with time is different for the doped and mixed sample (Fig.5с), calculated for the 5D0-7F2 (613 nm) transition. A clear difference (fig. 5 c) in the rate of the luminescent decay (which for the mixture is higher) can be seen through a complete cease in luminescence for the mixture already after 4 ms, while for the doped specimen it can still be detected after 20 ms. This rate difference between the mixed and doped specimens can be explained through the higher degree of freedom that Eu3+ ions possess in water, in respect to the Eu3+ ions which are fixed in the crystal lattice. The higher degree of freedom increases the probability for non-radiative transitions, and also increases the probability for electron transfer from the excited states of Eu3+ ion to the water molecules. The illustrative results on the effectiveness of the emission are presented in Fig.5d for doped and mixed zirconia NPs. The cuvettes contain samples of zirconia hydrosol (V = 3 ml). The mole ratio of Zr/Eu in the doped sample is 9/1, while the mass concentration of this sample in the cuvette is 0.028 g/ml. The zirconia – Eu3+ sol mixture was obtained in a way that a sol of pure zirconia with mass concentration of 0.025 g/ml was prepared first and 3.9×10-4mol Eu3+ ions were added to it
Figure 6. Droplets formation and deposition on glass surface. a) Drop formation from Dimatix Materials Cartridge with drops speed of 4 m/s; b) Drops on glass surface deposited at temperature 45 °C with 70 µm drops spacing; с) Five wet layers coating on glass with high 35 µm drops spacing deposited at 45 °C.
Figure 7.The characterization of printed drops and coating on holographic films. a) The printed zirconia drop on a polymer holographic paper substrate; b) The comparison of drops thickness created by multiple deposition technique (drop-ondrop) 1,2 and 3 layers with thickness 35, 83 and 165 nm respectively. c) The SEM microphotograph of Inkjet printed structures on a polymer holographic paper with 165 nm thickness. d) The HRSEM of zirconia NPs in a printed layer.
afterwards, coming to Zr/Eu ratio of 8.9/1.1 in the 3 ml of sol. A visual demonstration on the practical effectiveness of the doped sol samples is given in Fig. 5d. The numerical data on the intensity for the investigated samples for the 613 nm transition is given in the supporting information (Table S1).
Inkjet printing Previously we explained the ink composition leading to a stable and uniform print. The highly stable inks are obtained, because the solpossesses a narrow particle dimension distribution and a high zeta potential (Fig.S2). As seen from Fig. 6a, we used one jet printing method with a drop speed of around 4 m/s (the recommended speed is from 6 to 10 m/s). Using square wave voltage curve (Fig.S3) we achieved a stable print in overall 3 h time, including the head cleaning procedure every 120 s. The printing head was placed on 500 µm height above the surface of the substrate. When deposited on a glass surface, the drop obtains a proper form (Fig. 6b) and even coating, with a small edge thickening, caused by the “coffee30 ring” effect . The dimension of the drops deposited on the surface is no more than 40 µm and possess an even central part. It is possible to produce highly precise thickness layer in a multilayer deposition procedure, because the deposited drops have a thickness of ca. 35 nm (Fig.S4). It has to be however taken into account that a simultaneous multi drop deposition (Fig. 6c) can lead to non-even layer deposit, which is not the case when one at a time drop procedure is used, i.e. is coupled with a drying time between drops depositions.
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The Inkjet printing method allows a precise positioning of the droplet on the floppy or hard substrate surface which is a prerequisite for obtaining uniform layers without shattering, even with thickness of several micrometers (Fig.7b). What's more, such coatings are transparent (cf. Fig. 7a), which allows them to be used as coatings on diffraction grids and holograms. Employing the method of consecutive layer drying, it is possible to deposit layer on layer with high precision and thus vary the thickness of the coating. Using drop-on-drop deposition, it is possible to obtain discontinuous thickness change from 35 nm to ≈165 nm (Fig. 7b). Uniform coatings can be produced with high repeatability of the surface (Fig.7c), covering large areas and preserving the particles dimensions in the deposit (Fig. 7d). As a matter of fact, the high packing density of the nanoparticles is the reason for the high transparency and excellent optical properties.
Optical properties of inkjet-printer coatings The results on the refractive index of the printed coatings are depicted in Fig. 8e. For this to be done, the reflectance at normal incidence was investigated as a function of the wavelength for the case of layered structure, where the thin layer is placed between two semi-infinite dielectric media (air and glass). Fringes maxima shown in Figs. 8 a-d, correspond to constructive interference of the reflected radiation with characteristic wavelengths from the layer. In this case, independent on the layer thickness, the reflectance for these wavelengths is obtained through the 2 2 2 relation R=((n1n3-n2 )/(n1n3+n2 )) , where n2 is the refractive
Figure 8. Optical properties of inkjet printed coatings. Reflectance spectrums from Eudoped zirconia with 2 (a), 3 (b) and 6 (c) days of solvothermal synthesis and nondoped zirconia sample (d); e) The calculated refractive index for Eu-doped zirconia with a different preparation time; f) The optical transmittance of Eu-doped zirconia on glass and holographic paper.
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coefficient for the film at the same wavelengths. Using the experimental data for the reflectance R(λ), it is possible to obtain the dispersion relation of the refractive index as a function of the wavenumber Fig.8e, according to the formula n(λ)=√((n1n3(1+√(R(λ))))/(1-√(R(λ)))). The high refraction index (RI) of the ink obtained after 6-days of solvothermalsynthesis of Eu-doped ZrO2 (RI~1.80) in contrast to fused silica (RI=1.48) 32 and the polymeric holographic paper (RI~1.41) speaks on behalf of a high crystallity of the obtained coatings and of a high degree of porosity, that brings the material properties close to its bulk. Depending on the time of the synthesis, the refractive index is increased which proves the decrease in the dimensions of the crystallites (Fig. 1b). It is important to stress that the crystal doping does not only leads to luminescence, but also, due to the much larger atomic mass of the Eu3+ leads to an increase density of crystals (crystals reducing in size) and increase refractive index33. Thus, the methodology of variation of the index of refraction by the process of doping could play a role in the production of optical elements with increased value of the index of refraction where needed (micro-lenses, holography). It is important though to know that such structures show saturation limit, after which is no loner possible to increase the index of refraction.4
Figure 9. Inkjet printed samples on different surfaces. a) The printed by Eu-doped zirconia square on a glass substrate with 1 µm thickness and luminescence under UVlight (b); c) The inkjet printed text (ITMO) with the thickness ~300 nm on a holographic paper in daylight and in UV-light (d); e) Anoriginal Russian ruble banknote under UVlight and inkjet printed by Eu-doped Zirconia text on the banknote (f).
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To mention that the optical transparency (Transmittance > 99 %) is present even in the thicker layers (
