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Synthesis and inkjet printing of aqueous ZnS:Mn nanoparticles
Journal of Luminescence 136 (2013) 100–108
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Synthesis and inkjet printing of aqueous ZnS:Mn nanoparticles Peter D. Angelo n, Rosanna Kronfli, Ramin R. Farnood Department of Chemical Engineering & Applied Chemistry, Pulp & Paper Centre, University of Toronto, 200 College Street, Toronto, Canada M5G3A1
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 August 2012 Received in revised form 2 October 2012 Accepted 24 October 2012 Available online 2 November 2012
Nanoparticles of ZnS doped with Mn, a common photo- and electro-luminescent species, were synthesized in water using a competitive precipitation method. Particle size was controlled by selection of an appropriate stabilizer added during synthesis, 3-mercaptopropionic acid, which also rendered the particles water-dispersible after synthesis and isolation. Primary particle size was 3 nm, with small agglomerates of 10–20 nm in size. The particles were stably dispersed into water at a loading of 2.5 w/ w%. This dispersion formed the basis for an aqueous inkjet ink, containing 1 w/w% ZnS:Mn. The small particle size allowed the nanoparticles to be successfully delivered to several substrates without loss during filtration or jetting. Bright photoluminescence was observed in the printed patterns on some substrates (glass, photo-paper, foil, etc.) but was quenched on other substrates where the ink penetrated into the surface (uncoated paper). The small drop volume (10 pL) allowed for reasonably high-resolution printed patterns to be deposited, albeit with significant surface roughness due to the ‘‘coffee-ring’’ effect. & 2012 Elsevier B.V. All rights reserved.
Keywords: Inkjet printing Nanoparticles Photoluminescence Paper
1. Introduction Inkjet printing is a rapid, in-line, roll-to-roll process capable of producing patterned material in a single unit operation. The advantages of inkjet printing include rapid creation and substitution of custom patterns with no need for fabrication of a new mask or roll, drop-on-demand ink ejection which reduces ink waste to a minimum, high resolution, and so forth [1–6]. However, inkjet technology is primarily limited by the ingrained necessity of using inks themselves; transfer of a desired material into the liquid phase is often a difficult undertaking. An ink formulation often must juggle successful jetting and drop formation, wetting and drying behaviour on the substrate, and ink performance upon jetting [7]. Inkjet printers generally require suspended particle sizes in the sub-micron range, and viscosities significantly higher than those of many common solvents, necessitating special considerations in ink formulation [7,8]. In the case of specialized inks containing materials other than the common dyes and pigments used in printed matter – such as metallic colloids [9–17], carbon nanostructures [18,19], and so forth – the desired functionality (e.g. electrical conductivity) must also be retained after printing. Inkjet printing is particularly well-suited for the deposition of colloidal suspensions and small molecules rather than dissolved polymers, which jet does poorly due to the so-called ‘‘bead-on-a-string’’ effect [4,20]. Therefore, for the
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0022-2313/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2012.10.043
deposition of certain materials, such as semiconductors, inkjet printing of colloidal inorganic semiconductors has become particularly attractive [21–24]. One of such inorganic semiconductors is doped ZnS, a wellknown photo- and electro-luminescent material [25]. Doped ZnS nanoparticles have been prepared in aqueous solution by many groups [26–40] using acetate or chloride salts of the metallic precursors. The inherent facility and safety of aqueous synthesis for this material, rather than the complex methods of synthesizing such colloidal semiconductors such as CdSe/CdS/CdTe [41,42] or PbS [43], makes it an attractive alternative. The most common synthesis method for doped ZnS is referred to as ‘‘competitive precipitation’’ and, as stated above, involves the mixing of dissolved Zn, dopant, and S salts [32]. A schematic of this type of synthesis, described in detail in Ref. [37], is shown in Fig. 1. The reaction results in instantaneous formation of ZnS nuclei, which are surrounded by a layer of anions. This attracts further Zn2 þ and dopant, growing ZnS nanoparticles with integrated dopant clusters. Generally, the use of sodium sulphide as the S2 source results in the formation of a Na þ -rich boundary layer over the anion-rich layer, which prevents initial agglomeration of the particles by electrostatic repulsion [37]. The underlying theory of nanoparticulate nucleation and ripening postulated by La Mer [44] is the basis by which particle size is theoretically arrested in the nanometre range during this synthesis, using control of pH, concentration, and temperature. In our previous work [24], control of these variables alone was not sufficient to yield a large fraction of nanosized particles after redispersion, and treatment with acrylic acid after synthesis as
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101
Zn2+ -
Zn(Ac-)2 H 2O Mn(Ac-)2
Ac Zn2+ 2+ Ac- Zn Mn2+ 2+ Zn AcAc-
S source
S2-
H 2O
S2S2-
S2-
S2-
Ac-
S2-
-
Mn2+
S22-
Mn2+
Zn2+
S2ZnS:Mn
AcZnS ZnS ZnS S2S2ZnS MnS AcAcS2-
Ac
Zn2+ Zn2+
Zn2+
Zn2+
Fig. 1. Competitive precipitation of ZnS:dopant nanoparticles using acetate salts; Mn is an example dopant.
Table 1 Capping agents’ concentrations in Zn2 þ /dopant precursor solution. Capping agent
SHMP PVP Chitosan 3-MPA
Zn2 þ :cap in solution
Source
Literature
This study
10:1 (w:w) Not specified 20:1 (w:w) 1:3 or 1:4 (mol:mol)
10:1 (w:w) 10:1 (w:w) 20:1 (w:w) 1:4 (mol:mol)
suggested in Ref. [26] with only minimally improved dispersity. However, several so-called capping agents have been described as likely candidates for arresting particle growth and inducing either steric or electrostatic hindrance to primary crystallite agglomeration. Some of these agents include certain sulphur-bearing species such as mercaptans [36,45–47], water-soluble polymers like polyvinylpyrrolidone (PVP) [48] or sodium polyphosphates [37], or more exotic materials such as chitosan [49]. In this work, ZnS nanoparticles doped with Mn (ZnS:Mn) were to be dispersed in an inkjet ink. Therefore, they had to be small enough to pass through filtration and jetting (o200 nm), and ideally be dispersible at as high a concentration as possible to deliver them to the substrate in reasonable amounts. A brief comparison of different capping materials was made, after which nanoparticles were added to an aqueous inkjet ink and jetted onto several substrates, including paper, to demonstrate photoluminescent emission.
2. Experimental The synthesis method, using hydrated acetate salts of Zn2 þ and Mn2 þ and Na2S as the sulphur source, has been described in the previous work [24]. Mn2 þ was doped at a level of 1.5 at% to achieve the brightest photoluminescent yield, as has been discussed both in our previous work and by several other authors [32]. The molar ratio of Zn2 þ /S2 was 2:1. These conditions were previously found to yield ZnS:Mn with the highest photoluminescent intensity under UV excitation. However, it has been established that in the synthesis of ZnS nanoparticles, time, pH, and temperature play a role in the development of photoluminescent intensity and the control of particle size [48,50]. More importantly than reaction conditions, the choice of capping agent was expected to have a bearing on the particle size and surface passivation, resulting in changes to luminescent properties. Several different caps referenced in the literature, including sodium hexametaphosphate (SHMP), chitosan, PVP, and 3-mercaptopropionic acid (3-MPA) were used (see Table 1). The synthesis proceeded as described below; all reagents were provided by Sigma-Aldrich Canada, except where otherwise noted.
Warad et al. [37] Manzoor et al. [48] Warad et al. [49] Klausch et al. [46], Schrage et al. [47], Zhuang et al. [50]
6 mmol Zn(CH3COO)2d2H2O were dissolved completely in 30 mL H2O. A separate solution containing 3 mmol Na2Sd9H2O dissolved in 120 mL H2O was also prepared and treated with 4 mL pH 10 buffer. Because 3 mmol of ZnS would be formed by this reaction, 0.045 mmol Mn2 þ was required to achieve 1.5 at% doping. 0.045 mmol Mn(CH3COO)2d4 H2O were dissolved in 6 mL H2O and mixed into the Zn2 þ solution. At this point, individual capping agents were added to the Zn2 þ /Mn2 þ solution in the amounts given in Table 1. SHMP and PVP were readily soluble in water; chitosan required the addition of 5 mL glacial acetic acid to the mixture to dissolve. 3-MPA formed an insoluble complex with the Zn2 þ /Mn2 þ ions immediately upon addition; pH was adjusted to dissociate the Zn2 þ –mercaptan complex. Upon adjustment of pH to 10 using 2 M NaOH, as suggested in Refs. [46,50], the Zn2 þ complex became soluble and the precursor solution clarified. It was also noted that high pH is favourable for nanoparticle growth using organic ligands by preventing the protonation of the ligands and their resulting detachment from the particles’ surface [44]. Both solutions were heated to 70 1C. The synthesis was then initiated by adding the Zn2 þ /Mn2 þ /capping agent solution, dropwise from a burette, into the S2 solution, with gentle stirring. In the case of all the capping agents except 3-MPA, the solution immediately turned white as a precipitate formed. The 3-MPAcapped solution remained transparent. The reaction was allowed to proceed at 70 1C for 16 h, after which , the reaction broth was centrifuged at 5100 RPM for 30 min to collect the precipitate. The 3-MPA-capped ZnS:Mn was not isolated by precipitation; instead, the nanoparticles were crashed out of solution using acetone (acetone:broth¼3:1 by volume) and then centrifuged. Isolated nanoparticles were rinsed once with acetone and once with water and dried overnight in air. Crystal structure of the dried particles was observed using a Philips PANalytical PW1830 X-ray diffractometer (XRD). Three different dispersions of the particles were prepared in water. In the first, the particles were redispersed in water at 2.5 w/w% after drying by ultrasonication for 1 h. Samples of this dispersion were diluted to 1 w/w% ZnS:Mn with water before characterization. A second set of samples was treated with thioglycolic acid (TGA) and NaOH in order to improve dispersity
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[51]. Enough TGA was added to a 2.5 w/w% dispersion of ZnS:Mn in water to produce a concentration of 0.023 M TGA, and the pH was adjusted to 9 with 0.5 M NaOH to obtain a transparent dispersion after 1 h sonication in an ice bath. Again, these samples were diluted to 1 w/w% ZnS:Mn before characterization. Photoluminescence (PL) and photoluminescent excitation (PLE) spectra of the nanoparticles in all three suspensions were established using a Perkin–Elmer LS-55 spectrofluorophotometer. Images of PL in dispersed ZnS:Mn were obtained by exciting the suspensions with a UVP UVM-57 ultraviolet lamp with a 302 nm emission wavelength. Particle size was observed in three ways. Firstly, primary crystallite size was estimated from the XRD spectrum using the Scherrer equation: L¼
Kl bcos y
where L is the particle diameter, l is the wavelength of the X-rays ˚ b is the width of the diffraction peak at the Bragg (1.5406 A), angle at half-maximum intensity, K is 0.9 and y is the Bragg angle at which the peak is located [52]. The size of primary crystallites was confirmed using an FEI Tecnai 20 transmission electron microscope (TEM). Finally, the size of dispersed particles in solution was measured using a Malvern Zetasizer Nano ZS dynamic-light-scattering apparatus (DLS). DLS measurements were taken on water-borne, water/TGA/NaOH-borne, and inkborne ZnS:Mn particles. Finally, the ZnS:Mn/TGA/NaOH dispersion was used as the basis for an inkjet ink. The ink was formulated to be jetted on a piezoelectric FUJIFILM-Dimatix DMP2831 inkjet printer with sixteen 10 pL nozzles, according to the guidelines shown in Table 2. Surface tension was controlled by adding isopropanol, and viscosity was increased by adding polyvinylpyrrolidone (PVP, Mw ¼ 1 300 000) and butoxyethanol. The glycol ether also served as a humectant, preventing the ink from drying in the printer nozzles when not jetting. The final formulation (by weight) was 40% ZnS:Mn/TGA/NaOH stock solution, 10% isopropanol, 15% butoxyethanol, 0.1% PVP 1 300 000, with the balance water. These fluid properties of the ink made it suitable for printing on the DMP2831. For printing, the ink was first filtered at 0.2 mm Table 2 Ink properties for jetting on the DMP2831. Ink property Surface tension Viscosity Particle size (D95)
Jettable range
Actual ink properties
20–40 mN/m 2–12 cP o200 nm
32.1 mN/m 12.1 cP Passed 0.2 mm filter
through a nylon syringe filter and then injected into the DMP2831 cartridge. The voltage driving the piezoelectric nozzles was tailored to produce spherical drops with no satellites [20]. Drops were observed using the Drop Watcher camera built in to the DMP2831 printer. The tailored voltage waveform was then applied to the printhead to produce printed patterns, jetting from 1 mm print height onto a preheated (60 1C) substrate. All printed patterns of ZnS:Mn ink droplets were dried on a hot plate in air at 150 1C for 15 min. Drop size upon impact with the substrate was observed by jetting a spread of drops (254 mm apart) onto a cleaned glass slide and examining the drops using a Leica DM-LA microscope. Ideal drop spacing was established by jetting several lines of ink at different drop spacings in increments of 5 mm. The resulting films were observed using the microscope for uniformity. Ideal drop spacing was considered to be when lines were fully merged (no holes) but not overlapping excessively. Using the optimum line spacing, films of 1–5 layers of ink were printed and then characterized for thickness and topography using a Veeco WYKO NT1100 optical profilometer. Finally, patterns of ZnS:Mn ink were printed onto glass and paper to observe print resolution and PL emission.
3. Results and discussion 3.1. Nanoparticle synthesis After producing nanoparticles capped with SHMP, PVP, or chitosan, none of them was able to pass through a 0.2 mm filter, even after dispersion with TGA/NaOH (Fig. 2). It was evident even during synthesis that the particles were forming large agglomerates that were not redispersible—the reaction solution became opaque and white with visible particles upon the addition of the S2 solution. Conversely, using 3-MPA as the capping ligand, with pH buffering and pH adjustment of the Zn2 þ /Mn2 þ /3-MPA solution, a product was formed that showed no white colour or visible agglomeration upon mixing of the Zn2 þ and S2 precursors, remaining transparent (Fig. 2). The pH buffer was added to the reaction mixture to prevent protonation of the mercaptan ligands [29], maintaining dispersion stability. The high-pH environment facilitated deprotonation of the acid, allowing the sulphur-bearing end of the molecule to occupy S2 vacancies within the particle itself, forming a strong passivating layer and a steric barrier to particle agglomeration [36,53]. A schematic of the reaction and products is shown in Fig. 3. 3-MPA-based synthesis directly addressed two major problems encountered previously during ZnS:Mn ink deposition: the retention of nanoparticles in the ink during filtration (and jetting), and the loading of the ink with a high concentration of well-dispersed nanoparticles.
Visible
UV (300 nm)
unfiltered
filtered (0.2 µm)
Fig. 2. ZnS:Mn nanoparticles in water (0.1 w/w%), with different stabilizers: (a) none; (b) chitosan; (c) PVP 10000; (d) SHMP and (e) 3-MPA.
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103
Fig. 3. Finalized synthesis method of water-soluble monodisperse ZnS:Mn quantum dots.
Upon redispersion, the 3-MPA-capped particles had the brightest PL and degree of dispersion, which was retained after filtration (a prerequisite for jetting).
Aside from the strong PL emission visible in the 3-MPA-capped nanoparticles, the successful synthesis of cubic ZnS:Mn was confirmed by XRD (Fig. 4). Broadened peaks at 2y 28.51, 47.51, and 56.61, characteristic of nanosized ZnS particles [34,35] were clearly visible. According to the Scherrer equation, the mean primary crystallite size was 2.7 nm. TEM imaging of the water-borne dispersion of 3-MPA-capped ZnS:Mn (no TGA/NaOH) confirmed the primary crystallite size as approximately 2–3 nm (Fig. 5). The particles were generally clumped into loose agglomerates on the TEM grid, where they had likely grouped during the drying process. The dispersion of the particles in solution was not visible using TEM, so DLS was used to establish the degree of dispersion in solution. Fig. 6 shows the particle sizes in solution of the three dispersions (water, TGA/ NaOH, and ink). DLS suggested that the particles were present as agglomerates in solution, and that the presence of TGA/NaOH did little to improve dispersion, whereas incorporation into an ink compromised dispersion to some extent. The hydrodynamic radii of the dispersed particles were likely somewhat larger than the particles themselves, due to the presence of the 3-MPA ligands. In the case of the ink, the presence of PVP and non-aqueous components may have also led to a degree of agglomeration of primary crystallites, as the nanoparticles were capped with a water-soluble ligand. The solubility of the ligand in either isopropanol or butoxyethanol was not established. The average particle size in the ink solution, therefore, was somewhat larger than the well-dispersed water-borne suspensions. However, the particle size was still smaller than the maximum allowable size (200 nm) for filtration and inkjet printing. This was a marked
Counts
3.2. Particle morphology and size
[111]
[220] [311]
20
25
30
35
40 2θ (°)
45
50
55
60
Fig. 4. XRD spectrum of dried ZnS:Mn nanoparticles, showing cubic crystal structure (inset). Bracketed numbers represent the coordinates corresponding to lattice planes.
contrast to the ZnS:Mn nanoparticles capped with the other agents and those prepared using in-situ polymerized acrylic acid [24], of which barely, if any, passed through a 200 nm filter (qualitatively observed in Fig. 2). Those that did pass through the filter in those cases were generally not emissive at the characteristic 585 nm peak for ZnS:Mn [25]. The dispersity and small size of the 3-MPA-capped particles, even if somewhat agglomerated in the ink, allowed for bright PL before and after filtration and jetting. It may be inferred that the appearance of a white precipitate during synthesis precludes the redispersion and eventual jetting of ZnS nanoparticles, as only the 3-MPA-capped ZnS:Mn’s reaction broth remained transparent during synthesis. Successful redispersion of ZnS for inkjet printing where the broth contained precipitate was only reported by Small et al. [53]; however, a mercaptan-derived acid was also used in this case as a stabilizer. Therefore, the most likely explanation is the improved electrostatic hindrance to
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10 nm 20 nm Fig. 5. TEM micrographs of ZnS:Mn nanoparticles, dried from dispersion in water. (a) Agglomerate, 10–20 nm diameter and (b) primary crystallites, 2–3 nm diameter.
3.3. Jetting performance Jetting was stable indefinitely and did not result in nozzle clogging, forming spherical droplets of 10 pL volume (Fig. 8). The ‘‘bead-on-a-string’’ effect, which can be induced by the inclusion of high-MW polymers [7,8,20], i.e. the PVP, was observed. The droplet ‘‘tail’’ did not coalesce into the main droplet; this may have contributed to non-uniformity in the printed films discussed below. Droplets which impacted a glass surface spread to an average diameter of 65 mm, with a pronounced coffee-ring effect, where solids migrated to the edge of the droplets (Fig. 9). This problem
7 6 5 Intensity (a.u.)
agglomeration provided by the polar 3-MPA (or similar) molecule, when compared to that offered by polymeric capping agents. PL emission from the dopant centres in all of the nanoparticles was close to the typical orange–yellow range of 585 nm, where emission from ZnS:Mn has been widely observed. The emission results from the d-electron states in the dopant centres interacting with the s–p electron states in the ZnS host, and the resulting transition of Mn’s 4T1 energetic state to the 6A1 energetic state [59], rather than from excitonic recombination across the host material’s bandgap. As a result, the emission colour from the dopant centres was not shifted by quantization effects, confirming observations made during previous syntheses of ZnS:Mn nanocrystals [60–62]. In fact, considering that typical bulk ZnS:Mn emission peaks around 585 nm [60], the nanoparticles’ emission was somewhat red-shifted in comparison, to 593 nm. This has been hypothetically attributed to a large amount of either surface states or electron–phonon coupling in previous studies [63]. In our previous work [24] and with the other capping agents reported above, where the surface states were not as efficiently passivated (using a polymeric cap), the red-shift was more pronounced. The small differences in peak locations for the ZnS:Mn particles dispersed in water vs. NaOH/TGA vs. ink were attributed to improved passivation by TGA [51]. No blue-shift in the characteristic blue emission (at 430 nm) from S2 vacancies on the nanoparticles’ surface was observed either. Indeed, since the Bohr exciton radius of ZnS:Mn is estimated to be approximately 2.5 nm [54–56,62], the particle size of the ZnS:Mn in this study may have been insufficiently small to observe quantum effects. The location of the PLE peak (at 338 nm) was similar to that of bulk ZnS:Mn. A brief consideration of the UV–visible absorbance of the nanoparticles yielded an estimated bandgap of 3.71 eV; again, similar to that of bulk ZnS:Mn. There may have been a minor blue-shift and bandgap widening in the ZnS host (from a bandgap of 3.68, estimated for bulk ZnS [63] to the measured 3.71 eV). Regardless, the particles remained suitable for jetting due to their relatively small size.Fig. 7
4 3 2 1 0 0.1
1
10
100
Diameter (nm) Fig. 6. DLS scans of ZnS:Mn nanoparticles in (a) water; (b) water/NaOH, TGA dispersant and (c) ZnS:Mn ink.
was not as pronounced once printed lines were deposited, although solids still migrated to the edge of the printed lines. The size of the droplets was not as important as the line spacing in a printed layer, where line overlap induced film roughness. Solids migration led to the formation of ridges where the printed lines overlapped at lower drop spacing. As drop spacing was increased, the films gradually appeared to be more level, as is seen in Fig. 9e. Above 55 mm drop spacing, overlapping and ridges were the least pronounced, but holes began to form in the printed layer where individual drops did not coalesce. Therefore, 55 mm was considered the optimal drop spacing for this ink. For a simple printed pattern of ZnS:Mn, these topographical effects were not particularly relevant; however, if a film was intended for use as a layer in an LED or similar structure, any holes or excessive roughness could compromise device functionality [55]. This exact problem was encountered by Haverinen et al. [23], where it caused localized dimming of a printed LED device. However, excessively closely-spaced lines would cause such a drastic increase in film thickness and roughness that such a device might draw large amounts of current, resulting in an arc or short across the LED. Reducing ink solids content (including polymer and nanoparticles) [55], and selection of solvents in the mixture with similar values of surface tension [57] or boiling point [7] might serve to improve film smoothness and levelling. If the ZnS:Mn ink is to be applied to a permeable substrate, such as textile or paper, such concerns are immaterial. The degree of solids migration occurring during drop deposition and drying was clearly visible in the optical profiles of the printed ZnS:Mn ink (Fig. 10). Ridges at the edges of printed lines were pronounced distinctly, with peaks of almost 1 mm in height
105
Normalized absorbance (unitless)
P.D. Angelo et al. / Journal of Luminescence 136 (2013) 100–108
225
275
325
375
425
475
525
575
625
Wavelength (nm) Fig. 7. PL emission spectra (325 nm excitation) of (a) ZnS:Mn in water; (b) ZnS:Mn in water/NaOH, TGA-stabilized; (c) ZnS:Mn ink. PLE spectrum of ZnS:Mn in water is also shown (d). Inset: (e) ZnS:Mn ink under visible light and (f) under 300 nm UV excitation.
200 µm m
Fig. 8. Drop formation of ZnS:Mn ink from DMP2831 nozzle. Time between each image is 5 ms.
100 µm Fig. 9. Printed ZnS:Mn ink on slide glass. Drop size ¼65 mm (a). Varying drop spacing: (b) 25 mm; (c) 35 mm; (d) 45 mm; (e) 55 mm and (f) 65 mm.
nm 3-D
nm
1300
1100
800
900
600 600 400 600 µm µ 450 µm
300
200 0
0 2-D
10 00 µm
Fig. 10. Optical profiles of printed ZnS:Mn ink on slide glass, 55 mm drop spacing. (a) 1 printed layer and (b) 5 overprinted layers. Arrows on scale bars refer to the location of the glass surface.
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even for single printed layers. Overprinting with subsequent layers widened the ridges without raising them, improving overall film smoothness. However, overlap and levelling issues in the printed films made them unsuitable as smooth films of controlled thickness, as might be required for an electronic device. Establishment of precise film thickness was not possible due to the large variation in feature size between the ridges and ‘‘valleys’’ of the patterns printed on glass. The nearly micron-thick ridge features would certainly be too pronounced to allow for use in an electrically functional layer, which would require film thicknesses in the few hundred nanometre range. Aside from the issues with topography, the resolution of the printed films was quite good, with relatively smooth edges and minimum feature size determined solely by drop size, as is visible in the 2-D profiles in Fig. 10 as well as in Fig. 11. It was found by printing successively narrower features that the smallest feature size that could realistically be printed with a single layer of ink, as expected, was 65 mm (the average drop size). For features incrementally larger than this size, the printer deposited two lines of ink at the programmed drop spacing (55 mm), resulting in features of approximately 110 mm in size with an overlapping ridge identical to those shown in Figs. 9 and 10. This same pattern was repeated, naturally, for larger and larger features—feature sizes were limited by the drop spacing to increase in multiples of 55 mm. This occurred in features printed both parallel and normal to the movement of the printhead, as in either case, the features were composed of at least one line of 65 mm drops, or multiple lines spaced 55 mm apart. In the case of spaces between printed features, the minimum space achievable was found to be 40 mm (on glass) with an intended space of 100 mm in both cases. Space size was not limited by the size of drops or drop
print edge
spacing, so very narrow unprinted channels are technically feasible with this type of printer and ink. Spacing below 100 mm caused the printed regions to merge into a single film. Therefore, the maximum achievable resolution using a ZnS:Mn ink may be controlled by either controlling drop volume (by adjustment of voltage or use of a cartridge with nozzles of different diameter) or by careful substrate selection or treatment to influence spreading, and control drop spacing. However, because the volume of ink per drop remains constant, narrower spacing on substrates which are wetted less would likely lead to thicker films. Successive overprinting and the resulting layering of the ridged regions resulted in better film smoothness, as mentioned above, with an increase in film thickness. The higher concentration of ZnS:Mn nanoparticles on the surface also resulted in brighter PL of printed patterns (Figs. 11 and 12). However, repeated overprinting presented several disadvantages, which naturally included increased material use and film thickness. An unforeseen problem associated with overprinting was the splattering of ink onto the surrounding substrate, as is clearly visible in Fig. 12b. As droplets impacted the wet ink on the surface during subsequent print passes, ink was ejected from the previously deposited film and landed randomly on the substrate surface. This phenomenon led to reduced edge resolution in the printed patterns, to the point where ‘‘hazy’’ or indistinct edges and smearing were visible even at the macro-scale (Fig. 12a, 20 and 50 printed layers). PL intensity was not much improved by more than 10 overprints in any case; an optimal number likely exists for each substrate at which edge resolution is preserved and PL intensity is sufficiently high. PL emission was bright and distinct after jetting of the ink on all of the substrates shown above (glass, ITO PET, and Al foil).
200 µm
Fig. 11. Edge resolution of ZnS:Mn ink on glass: (a) 1 printed layer; (b) 3 printed layers; (c) 6 printed layers; (d) 9 printed layers; (e) 12 printed layers. Print edges are highlighted by black lines. Insets show PL of the samples under UV excitation.
Fig. 12. Multiple layers of ZnS:Mn ink printed on (a) aluminium foil (1–50 layers) and (b) ITO PET (50 layers), showing edge splattering with excessive overprinting.
P.D. Angelo et al. / Journal of Luminescence 136 (2013) 100–108
Background fluorescence from the PET caused a certain amount of ‘‘washing out’’ of the typical orange emission; similar effects were expected on paper. However, on photo-paper, which emits a large amount of background fluorescence, the characteristic emission from ZnS:Mn was brightly visible. Other, more highly absorbent sheets blocked PL emission by allowing the ZnS:Mn nanoparticles to sink into the bulk of the sheet, where they likely penetrated into the porous paper fibres (Fig. 13). Optical brighteners, fibres, fillers, and other paper furnishes may competitively fluoresce as well, obscuring the emission from the ZnS:Mn. The photo-paper and glossy sheets had a thick, dense coating that captured the aqueous ink at the surface [19] and hence retained ZnS:Mn in a location where the UV emission could reach the nanoparticles, and the emitted light would be visible. A similar effect was observed in our previous work, when depositing conductive films on the same paper types: sheets designed to immobilize ink at the surface to preserve colour intensity and brightness retained functional materials there as well, resulting in increased conductivity [58]. Furthermore, as is clearly visible in Fig. 13a, PL background emission from the photo-paper sheet was much weaker than from the other sheets, producing an apparently more intense emission from the ZnS:Mn nanoparticles on that sheet. The high gloss of this sheet may have also reflected the majority of the UV light, rather than absorbing it and fluorescing. Emission at similar brightness is present but overwhelmed somewhat by the more intense background emission on the glossy sheet in Fig. 13b. Therefore, for maximum emission intensity at the desired wavelength, substrates should be chosen for this type of ink to be either impermeable or able to retain the ink primarily at the surface layer, while simultaneously fluorescing as little as possible when interrogated with an exciting laser or UV source.
4. Conclusions Water-soluble ZnS:Mn nanoparticles were prepared using a competitive precipitation method. It was found that the use of acid (3-MPA) as a capping agent, at controlled pH, temperature, and
107
reaction time, was suitable for forming ZnS:Mn nanoparticles of sufficiently small size to be redispersible in an aqueous inkjet ink. Although many other researchers have produced particles with primary crystallite sizes small enough to be incorporated into an ink using other capping agents, only 3-MPA (and similar species, such as TGA, mercaptoethanol, p-thiocresol, and other organosulphur compounds) resulted in dispersed particles which were below the particle size cutoff for the printer of 200 nm. 3-MPA was most suitable for this role because of its (relatively) lower toxicity and increased ease of handling when compared to the other thiols mentioned. The particles produced using this method were nanosized and well-passivated, resulting in bright PL emission very slightly red-shifted to 591–595 nm from the characteristic ZnS:Mn emission wavelength of 585 nm. Effective passivation was achieved by bonding of the sulphur atom in 3-MPA to the S2 vacancy sites, causing primarily orange emission with minimal emission from S2 vacancies in the blue range. Their small size and dispersity were crucial in formulation of an inkjet ink containing a relatively high loading of solids—1 w/w% ZnS:Mn. Inclusion of PVP as a polymeric binder to aid in the formation of contiguous films had only a minor effect on jetting. Print resolution was determined by drop size, which is determined by nozzle diameter. The small size of the particles in the ink would allow for use of nozzles of significantly smaller diameter and resulting in higher resolution. Printing of these nanoparticles produced brightly luminescent films, whose brightness was enhanced by overprinting multiple times. Films were formed on several substrates with no notable reduction in brightness, except in the case of absorbent sheets, which allowed ZnS:Mn nanoparticles to migrate into the sheet bulk where excitation and emission were blocked by fibres. On impermeable substrates, the high solids loading, surface tension gradients between solvents, and rapid evaporation of volatile solvents lead to solids migration into 1 mm ridges at the edges of printed features. A consideration of ink reformulation by adjustment of PVP or ZnS:Mn content, or by inclusion of levelling aids, might alleviate this issue, which could be problematic if such a film were to be applied in an electronic device. The small film thickness required for a DC-driven LED, for example, which would
Visible
UV Fig. 13. ZnS:Mn ink, 10 overprints, printed on (a) photo-paper; (b) glossy paper; (c) inkjet paper and (d) multiuse (high-yield pulp) paper, under visible and UV (300 nm) excitation.
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be a suitable application for ZnS:Mn nanoparticles, would necessitate smooth films with no significant topographical irregularity. However, this work presents a successful means of patterning luminescent nanoparticles on several substrates and the possibility for producing such patterns on the context of optoelectronic devices, with minor refinements. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28]
P. Calvert, Chem. Mater. 13 (2001) 3299. E. Tekin, P. Smith, U. Schubert, Soft Matter 4 (2008) 703. H. Sirringhaus, T. Shimoda, MRS Bull. 28 (11) (2003) 802. E. Tekin, B. De Gans, U. Schubert, J. Mater. Chem. 14 (2004) 2627. B. De Gans, P. Duineveld, U. Schubert, Adv. Mater. 16 (3) (2004) 203. Y. Yoshioka, G. Jabbour, Synth. Met. 156 (2006) 779. S. Magdassi (Ed.), The Chemistry of Inkjet Inks, World Scientific Publishing, Singapore, 2010. Dimatix-Fujifuilm, Dimatix Materials Printer DMP2800 Series User Manual, 2006. A. Dearden, P. Smith, D. Shin, N. Reis, B. Derby, P. O’Brien, Macromol. Rapid Commun. 26 (2005) 315. Y. Wu, Y. Li, B. Ong, J. Am. Chem. Soc. 129 (2007) 1862. T. van Osch, J. Perelaer, A. de Laat, U. Schubert, Adv. Mater. 20 (2008) 343. J. Perelaer, A. de Laat, C. Hendriks, U. Schubert, J. Mater. Chem. 18 (2008) 3209. B. Nguyen, J. Gautrot, M. Nguyen, X. Zhu, J. Mater. Chem. 17 (2007) 1725. S. Jeong, H. Song, W. Lee, Y. Choi, B. Ryu, J. Appl. Phys. 108 (2010) 1. I. Kim, J. Kim, J. Appl. Phys. 108 (2010) 1. H. Jung, S. Cho, J. Joung, Y. Oh, J. Electron. Mater. 36 (9) (2007) 1211. B. Ryu, Y. Choim, H. Park, J. Byun, K. Kong, J. Lee, H. Chang, Colloids. Surf. A 270 (2005) 345. T. Mustonen, K. Kordas, S. Saukko, G. Toth, J. Penttila, P. Helisto, H. Seppa, H. Jantunen, Phys. Status Solidi B 244 (11) (2007) 4336. P. Angelo, R. Farnood., J. Adhes. Sci. Tech. 24 (3) (2010) 643. E. Lee, Microdrop Generation, CRC Press, Boca Raton, 2003. H. Haverinen, R. Myllyla, G. Jabbour, Appl. Phys. Lett. 94 (2009) 073108. V. Wood, M. Panzer, J. Chen, M. Bradley, J. Halpert, M. Bawendi, V. Bulovic, Adv. Mater. 21 (2009) 2151–2155. H. Haverinen, R. Myllyla, G. Jabbour, J. Disp. Tech. 6 (3) (2010) 87. P. Angelo, R. Farnood, J. Exp. Nanosci. 6 (5) (2011) 473. W. Yen, Inorganic Phosphors: Compositions, Preparation and Optical Properties, CRC Press, Boca Raton, 2004. H. Althues, R. Palkovits, A. Rumplecker, P. Simon, W. Sigle, M. Bredol, U. Kynast, S. Kaskel, Chem. Mater. 18 (2006) 1068. T. Igarashi, T. Isobe, M. Senna, Phys. Rev. B 56 (11) (1997) 6444. D. Adachi, T. Hama, T. Toyama, H. Okamoto, J. Mater. Sci.: Mater. Electron. 10 (2007) 1.
[29] D. Adachi, T. Morimoto, T. Hama, T. Toyama, H. Okamoto, J. Non-Cryst. Solids 354 (2008) 2740. [30] M. Konishi, T. Isobe, M. Senna, J. Lumin. 93 (2001) 1. [31] M. Hwang, I. Oh, J. Kim, Lee, C. Ha, Curr. Appl. Phys. 5 (2005) 31. [32] H. Chander, Mater. Sci. Eng. R49 (2005) 113. [33] N. Karar, S. Raj, F. Singh, J. Cryst. Growth 268 (2004) 585. [34] H. Takahashi, T. Isobe, Jpn. J. Appl. Phys. 44 (4) (2005) 922. [35] D. Mu, Gu, Z. Xu, Mater. Res. Bull. 40 (2005) 2198. [36] W. Vogel, P. Borse, N. Deshmukh, S. Kulkarni, Langmuir 16 (4) (2000) 2032. [37] H. Warad, S. Ghosh, B. Hemtanon, C. Thanachayanont, J. Dutta, Sci. Tech. Adv. Mater 6 (2005) 296. [38] H. Yang, P. Holloway, B. Ratna, J. Appl. Phys. 93 (1) (2003) 586. [39] T. Yu, M. Isobe, J. Senna, Phys. Chem. Solids 57 (4) (1996) 373. [40] Z. Jindal, N. Verma, J. Mater. Sci. 43 (2008) 6539. [41] P.Guyot-Sionnest. Hines, J. Phys. Chem. 100 (1996) 468. [42] D. Talapin, A. Rogach, A. Kornowski, M. Haase, H. Weller, Nano Lett. 1 (4) (2001) 207. [43] S.Musikhin Bakueva, M. Hines, T. Chang, M. Tzolov, G. Scholes, E. Sargent, Appl. Phys. Lett. 82 (17) (2003) 2895. [44] I. Capek, Nanocomposite Structures and Dispersions, Science and Nanotechnology - Fundamental Principles and Colloidal Particles, Elsevier, Amsterdam, 2006. [45] W. Zhang, H.-R. Lee, Appl. Opt. 49 (14) (2010) 2566. [46] A. Klausch, H. Althues, C. Schrage, P. Simon, A. Szatkowski, M. Bredol, D. Adam, S. Kaskel, J. Lumin. 130 (2010) 692. [47] C. Schrage, H. Althues, A. Klausch, D. Adam, S. Kaskel, J. Nanosci. Nanotech. 10 (2010) 4335. [48] K. Manzoor, S. Vadera, N. Kumar, T. Kutty, Mater. Chem. Phys. 82 (2003) 718. [49] H. Warad, S. Ghosh, B. Hemtanon, C. Thanachayanont, J. Dutta. Sci. Tech. Adv. Mater. 6 (2005) 296. [50] J. Zhuang, X. Zhang, G. Wang, D. Li, W. Yang, T. Li, J. Mater. Chem. 13 (2003) 1853. [51] P. Yang, M. Bredol, Res. Lett. Mater. Sci. 2008 (2008) 1. [52] B. Cullity, S. Stock, Elements of X-ray Diffraction, 3rd edition, Prentice-Hall Inc., New Jersey, 2001. [53] A. Small, J. Johnston, N. Clark, Eur. J. Inorg. Chem. (2010) 242. [54] C. Sun, Size and Confinement Effects on Nanostructures, School of Electrics & Electronics Engineering, Singapore, 2005. [55] F. Berger, Semiconductor Materials, CRC Press, Boca Raton, 1996. [56] F. Caruso (Ed.), Colloids and Colloid Assemblies, Wiley, Weinheim, Germany, 2004. [57] A. Tracton, Coatings Technology Handbook, CRC Press, Boca Raton, 2005. [58] P. Angelo, R. Farnood, Proceedings of MRS Spring Meeting G, 2010. [59] K. Karlin, Progress in Inorganic Chemistry V, 54, Wiley-Interscience, New Jersey, 2005. [60] P. Smet, I. Moreels, Z. Hens, D. Poelman, Materials 3 (2010) 2834. [61] R. Bhargava, D. Gallagher, T. Welker, J. Lumin. 60–61 (1994) 275. [62] R. Bhargava, D. Gallagher, Phys. Rev. Lett. 72 (3) (1994) 416. [63] H. Yang, S. Santra, P. Holloway, J. Nanosci. Nanotech. 5 (2005) 1364.
Contents lists available at SciVerse ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Synthesis and inkjet printing of aqueous ZnS:Mn nanoparticles Peter D. Angelo n, Rosanna Kronfli, Ramin R. Farnood Department of Chemical Engineering & Applied Chemistry, Pulp & Paper Centre, University of Toronto, 200 College Street, Toronto, Canada M5G3A1
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 August 2012 Received in revised form 2 October 2012 Accepted 24 October 2012 Available online 2 November 2012
Nanoparticles of ZnS doped with Mn, a common photo- and electro-luminescent species, were synthesized in water using a competitive precipitation method. Particle size was controlled by selection of an appropriate stabilizer added during synthesis, 3-mercaptopropionic acid, which also rendered the particles water-dispersible after synthesis and isolation. Primary particle size was 3 nm, with small agglomerates of 10–20 nm in size. The particles were stably dispersed into water at a loading of 2.5 w/ w%. This dispersion formed the basis for an aqueous inkjet ink, containing 1 w/w% ZnS:Mn. The small particle size allowed the nanoparticles to be successfully delivered to several substrates without loss during filtration or jetting. Bright photoluminescence was observed in the printed patterns on some substrates (glass, photo-paper, foil, etc.) but was quenched on other substrates where the ink penetrated into the surface (uncoated paper). The small drop volume (10 pL) allowed for reasonably high-resolution printed patterns to be deposited, albeit with significant surface roughness due to the ‘‘coffee-ring’’ effect. & 2012 Elsevier B.V. All rights reserved.
Keywords: Inkjet printing Nanoparticles Photoluminescence Paper
1. Introduction Inkjet printing is a rapid, in-line, roll-to-roll process capable of producing patterned material in a single unit operation. The advantages of inkjet printing include rapid creation and substitution of custom patterns with no need for fabrication of a new mask or roll, drop-on-demand ink ejection which reduces ink waste to a minimum, high resolution, and so forth [1–6]. However, inkjet technology is primarily limited by the ingrained necessity of using inks themselves; transfer of a desired material into the liquid phase is often a difficult undertaking. An ink formulation often must juggle successful jetting and drop formation, wetting and drying behaviour on the substrate, and ink performance upon jetting [7]. Inkjet printers generally require suspended particle sizes in the sub-micron range, and viscosities significantly higher than those of many common solvents, necessitating special considerations in ink formulation [7,8]. In the case of specialized inks containing materials other than the common dyes and pigments used in printed matter – such as metallic colloids [9–17], carbon nanostructures [18,19], and so forth – the desired functionality (e.g. electrical conductivity) must also be retained after printing. Inkjet printing is particularly well-suited for the deposition of colloidal suspensions and small molecules rather than dissolved polymers, which jet does poorly due to the so-called ‘‘bead-on-a-string’’ effect [4,20]. Therefore, for the
n
Corresponding author. Tel.: þ1 416 738 3597. E-mail address: [email protected] (P.D. Angelo).
0022-2313/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2012.10.043
deposition of certain materials, such as semiconductors, inkjet printing of colloidal inorganic semiconductors has become particularly attractive [21–24]. One of such inorganic semiconductors is doped ZnS, a wellknown photo- and electro-luminescent material [25]. Doped ZnS nanoparticles have been prepared in aqueous solution by many groups [26–40] using acetate or chloride salts of the metallic precursors. The inherent facility and safety of aqueous synthesis for this material, rather than the complex methods of synthesizing such colloidal semiconductors such as CdSe/CdS/CdTe [41,42] or PbS [43], makes it an attractive alternative. The most common synthesis method for doped ZnS is referred to as ‘‘competitive precipitation’’ and, as stated above, involves the mixing of dissolved Zn, dopant, and S salts [32]. A schematic of this type of synthesis, described in detail in Ref. [37], is shown in Fig. 1. The reaction results in instantaneous formation of ZnS nuclei, which are surrounded by a layer of anions. This attracts further Zn2 þ and dopant, growing ZnS nanoparticles with integrated dopant clusters. Generally, the use of sodium sulphide as the S2 source results in the formation of a Na þ -rich boundary layer over the anion-rich layer, which prevents initial agglomeration of the particles by electrostatic repulsion [37]. The underlying theory of nanoparticulate nucleation and ripening postulated by La Mer [44] is the basis by which particle size is theoretically arrested in the nanometre range during this synthesis, using control of pH, concentration, and temperature. In our previous work [24], control of these variables alone was not sufficient to yield a large fraction of nanosized particles after redispersion, and treatment with acrylic acid after synthesis as
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101
Zn2+ -
Zn(Ac-)2 H 2O Mn(Ac-)2
Ac Zn2+ 2+ Ac- Zn Mn2+ 2+ Zn AcAc-
S source
S2-
H 2O
S2S2-
S2-
S2-
Ac-
S2-
-
Mn2+
S22-
Mn2+
Zn2+
S2ZnS:Mn
AcZnS ZnS ZnS S2S2ZnS MnS AcAcS2-
Ac
Zn2+ Zn2+
Zn2+
Zn2+
Fig. 1. Competitive precipitation of ZnS:dopant nanoparticles using acetate salts; Mn is an example dopant.
Table 1 Capping agents’ concentrations in Zn2 þ /dopant precursor solution. Capping agent
SHMP PVP Chitosan 3-MPA
Zn2 þ :cap in solution
Source
Literature
This study
10:1 (w:w) Not specified 20:1 (w:w) 1:3 or 1:4 (mol:mol)
10:1 (w:w) 10:1 (w:w) 20:1 (w:w) 1:4 (mol:mol)
suggested in Ref. [26] with only minimally improved dispersity. However, several so-called capping agents have been described as likely candidates for arresting particle growth and inducing either steric or electrostatic hindrance to primary crystallite agglomeration. Some of these agents include certain sulphur-bearing species such as mercaptans [36,45–47], water-soluble polymers like polyvinylpyrrolidone (PVP) [48] or sodium polyphosphates [37], or more exotic materials such as chitosan [49]. In this work, ZnS nanoparticles doped with Mn (ZnS:Mn) were to be dispersed in an inkjet ink. Therefore, they had to be small enough to pass through filtration and jetting (o200 nm), and ideally be dispersible at as high a concentration as possible to deliver them to the substrate in reasonable amounts. A brief comparison of different capping materials was made, after which nanoparticles were added to an aqueous inkjet ink and jetted onto several substrates, including paper, to demonstrate photoluminescent emission.
2. Experimental The synthesis method, using hydrated acetate salts of Zn2 þ and Mn2 þ and Na2S as the sulphur source, has been described in the previous work [24]. Mn2 þ was doped at a level of 1.5 at% to achieve the brightest photoluminescent yield, as has been discussed both in our previous work and by several other authors [32]. The molar ratio of Zn2 þ /S2 was 2:1. These conditions were previously found to yield ZnS:Mn with the highest photoluminescent intensity under UV excitation. However, it has been established that in the synthesis of ZnS nanoparticles, time, pH, and temperature play a role in the development of photoluminescent intensity and the control of particle size [48,50]. More importantly than reaction conditions, the choice of capping agent was expected to have a bearing on the particle size and surface passivation, resulting in changes to luminescent properties. Several different caps referenced in the literature, including sodium hexametaphosphate (SHMP), chitosan, PVP, and 3-mercaptopropionic acid (3-MPA) were used (see Table 1). The synthesis proceeded as described below; all reagents were provided by Sigma-Aldrich Canada, except where otherwise noted.
Warad et al. [37] Manzoor et al. [48] Warad et al. [49] Klausch et al. [46], Schrage et al. [47], Zhuang et al. [50]
6 mmol Zn(CH3COO)2d2H2O were dissolved completely in 30 mL H2O. A separate solution containing 3 mmol Na2Sd9H2O dissolved in 120 mL H2O was also prepared and treated with 4 mL pH 10 buffer. Because 3 mmol of ZnS would be formed by this reaction, 0.045 mmol Mn2 þ was required to achieve 1.5 at% doping. 0.045 mmol Mn(CH3COO)2d4 H2O were dissolved in 6 mL H2O and mixed into the Zn2 þ solution. At this point, individual capping agents were added to the Zn2 þ /Mn2 þ solution in the amounts given in Table 1. SHMP and PVP were readily soluble in water; chitosan required the addition of 5 mL glacial acetic acid to the mixture to dissolve. 3-MPA formed an insoluble complex with the Zn2 þ /Mn2 þ ions immediately upon addition; pH was adjusted to dissociate the Zn2 þ –mercaptan complex. Upon adjustment of pH to 10 using 2 M NaOH, as suggested in Refs. [46,50], the Zn2 þ complex became soluble and the precursor solution clarified. It was also noted that high pH is favourable for nanoparticle growth using organic ligands by preventing the protonation of the ligands and their resulting detachment from the particles’ surface [44]. Both solutions were heated to 70 1C. The synthesis was then initiated by adding the Zn2 þ /Mn2 þ /capping agent solution, dropwise from a burette, into the S2 solution, with gentle stirring. In the case of all the capping agents except 3-MPA, the solution immediately turned white as a precipitate formed. The 3-MPAcapped solution remained transparent. The reaction was allowed to proceed at 70 1C for 16 h, after which , the reaction broth was centrifuged at 5100 RPM for 30 min to collect the precipitate. The 3-MPA-capped ZnS:Mn was not isolated by precipitation; instead, the nanoparticles were crashed out of solution using acetone (acetone:broth¼3:1 by volume) and then centrifuged. Isolated nanoparticles were rinsed once with acetone and once with water and dried overnight in air. Crystal structure of the dried particles was observed using a Philips PANalytical PW1830 X-ray diffractometer (XRD). Three different dispersions of the particles were prepared in water. In the first, the particles were redispersed in water at 2.5 w/w% after drying by ultrasonication for 1 h. Samples of this dispersion were diluted to 1 w/w% ZnS:Mn with water before characterization. A second set of samples was treated with thioglycolic acid (TGA) and NaOH in order to improve dispersity
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[51]. Enough TGA was added to a 2.5 w/w% dispersion of ZnS:Mn in water to produce a concentration of 0.023 M TGA, and the pH was adjusted to 9 with 0.5 M NaOH to obtain a transparent dispersion after 1 h sonication in an ice bath. Again, these samples were diluted to 1 w/w% ZnS:Mn before characterization. Photoluminescence (PL) and photoluminescent excitation (PLE) spectra of the nanoparticles in all three suspensions were established using a Perkin–Elmer LS-55 spectrofluorophotometer. Images of PL in dispersed ZnS:Mn were obtained by exciting the suspensions with a UVP UVM-57 ultraviolet lamp with a 302 nm emission wavelength. Particle size was observed in three ways. Firstly, primary crystallite size was estimated from the XRD spectrum using the Scherrer equation: L¼
Kl bcos y
where L is the particle diameter, l is the wavelength of the X-rays ˚ b is the width of the diffraction peak at the Bragg (1.5406 A), angle at half-maximum intensity, K is 0.9 and y is the Bragg angle at which the peak is located [52]. The size of primary crystallites was confirmed using an FEI Tecnai 20 transmission electron microscope (TEM). Finally, the size of dispersed particles in solution was measured using a Malvern Zetasizer Nano ZS dynamic-light-scattering apparatus (DLS). DLS measurements were taken on water-borne, water/TGA/NaOH-borne, and inkborne ZnS:Mn particles. Finally, the ZnS:Mn/TGA/NaOH dispersion was used as the basis for an inkjet ink. The ink was formulated to be jetted on a piezoelectric FUJIFILM-Dimatix DMP2831 inkjet printer with sixteen 10 pL nozzles, according to the guidelines shown in Table 2. Surface tension was controlled by adding isopropanol, and viscosity was increased by adding polyvinylpyrrolidone (PVP, Mw ¼ 1 300 000) and butoxyethanol. The glycol ether also served as a humectant, preventing the ink from drying in the printer nozzles when not jetting. The final formulation (by weight) was 40% ZnS:Mn/TGA/NaOH stock solution, 10% isopropanol, 15% butoxyethanol, 0.1% PVP 1 300 000, with the balance water. These fluid properties of the ink made it suitable for printing on the DMP2831. For printing, the ink was first filtered at 0.2 mm Table 2 Ink properties for jetting on the DMP2831. Ink property Surface tension Viscosity Particle size (D95)
Jettable range
Actual ink properties
20–40 mN/m 2–12 cP o200 nm
32.1 mN/m 12.1 cP Passed 0.2 mm filter
through a nylon syringe filter and then injected into the DMP2831 cartridge. The voltage driving the piezoelectric nozzles was tailored to produce spherical drops with no satellites [20]. Drops were observed using the Drop Watcher camera built in to the DMP2831 printer. The tailored voltage waveform was then applied to the printhead to produce printed patterns, jetting from 1 mm print height onto a preheated (60 1C) substrate. All printed patterns of ZnS:Mn ink droplets were dried on a hot plate in air at 150 1C for 15 min. Drop size upon impact with the substrate was observed by jetting a spread of drops (254 mm apart) onto a cleaned glass slide and examining the drops using a Leica DM-LA microscope. Ideal drop spacing was established by jetting several lines of ink at different drop spacings in increments of 5 mm. The resulting films were observed using the microscope for uniformity. Ideal drop spacing was considered to be when lines were fully merged (no holes) but not overlapping excessively. Using the optimum line spacing, films of 1–5 layers of ink were printed and then characterized for thickness and topography using a Veeco WYKO NT1100 optical profilometer. Finally, patterns of ZnS:Mn ink were printed onto glass and paper to observe print resolution and PL emission.
3. Results and discussion 3.1. Nanoparticle synthesis After producing nanoparticles capped with SHMP, PVP, or chitosan, none of them was able to pass through a 0.2 mm filter, even after dispersion with TGA/NaOH (Fig. 2). It was evident even during synthesis that the particles were forming large agglomerates that were not redispersible—the reaction solution became opaque and white with visible particles upon the addition of the S2 solution. Conversely, using 3-MPA as the capping ligand, with pH buffering and pH adjustment of the Zn2 þ /Mn2 þ /3-MPA solution, a product was formed that showed no white colour or visible agglomeration upon mixing of the Zn2 þ and S2 precursors, remaining transparent (Fig. 2). The pH buffer was added to the reaction mixture to prevent protonation of the mercaptan ligands [29], maintaining dispersion stability. The high-pH environment facilitated deprotonation of the acid, allowing the sulphur-bearing end of the molecule to occupy S2 vacancies within the particle itself, forming a strong passivating layer and a steric barrier to particle agglomeration [36,53]. A schematic of the reaction and products is shown in Fig. 3. 3-MPA-based synthesis directly addressed two major problems encountered previously during ZnS:Mn ink deposition: the retention of nanoparticles in the ink during filtration (and jetting), and the loading of the ink with a high concentration of well-dispersed nanoparticles.
Visible
UV (300 nm)
unfiltered
filtered (0.2 µm)
Fig. 2. ZnS:Mn nanoparticles in water (0.1 w/w%), with different stabilizers: (a) none; (b) chitosan; (c) PVP 10000; (d) SHMP and (e) 3-MPA.
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103
Fig. 3. Finalized synthesis method of water-soluble monodisperse ZnS:Mn quantum dots.
Upon redispersion, the 3-MPA-capped particles had the brightest PL and degree of dispersion, which was retained after filtration (a prerequisite for jetting).
Aside from the strong PL emission visible in the 3-MPA-capped nanoparticles, the successful synthesis of cubic ZnS:Mn was confirmed by XRD (Fig. 4). Broadened peaks at 2y 28.51, 47.51, and 56.61, characteristic of nanosized ZnS particles [34,35] were clearly visible. According to the Scherrer equation, the mean primary crystallite size was 2.7 nm. TEM imaging of the water-borne dispersion of 3-MPA-capped ZnS:Mn (no TGA/NaOH) confirmed the primary crystallite size as approximately 2–3 nm (Fig. 5). The particles were generally clumped into loose agglomerates on the TEM grid, where they had likely grouped during the drying process. The dispersion of the particles in solution was not visible using TEM, so DLS was used to establish the degree of dispersion in solution. Fig. 6 shows the particle sizes in solution of the three dispersions (water, TGA/ NaOH, and ink). DLS suggested that the particles were present as agglomerates in solution, and that the presence of TGA/NaOH did little to improve dispersion, whereas incorporation into an ink compromised dispersion to some extent. The hydrodynamic radii of the dispersed particles were likely somewhat larger than the particles themselves, due to the presence of the 3-MPA ligands. In the case of the ink, the presence of PVP and non-aqueous components may have also led to a degree of agglomeration of primary crystallites, as the nanoparticles were capped with a water-soluble ligand. The solubility of the ligand in either isopropanol or butoxyethanol was not established. The average particle size in the ink solution, therefore, was somewhat larger than the well-dispersed water-borne suspensions. However, the particle size was still smaller than the maximum allowable size (200 nm) for filtration and inkjet printing. This was a marked
Counts
3.2. Particle morphology and size
[111]
[220] [311]
20
25
30
35
40 2θ (°)
45
50
55
60
Fig. 4. XRD spectrum of dried ZnS:Mn nanoparticles, showing cubic crystal structure (inset). Bracketed numbers represent the coordinates corresponding to lattice planes.
contrast to the ZnS:Mn nanoparticles capped with the other agents and those prepared using in-situ polymerized acrylic acid [24], of which barely, if any, passed through a 200 nm filter (qualitatively observed in Fig. 2). Those that did pass through the filter in those cases were generally not emissive at the characteristic 585 nm peak for ZnS:Mn [25]. The dispersity and small size of the 3-MPA-capped particles, even if somewhat agglomerated in the ink, allowed for bright PL before and after filtration and jetting. It may be inferred that the appearance of a white precipitate during synthesis precludes the redispersion and eventual jetting of ZnS nanoparticles, as only the 3-MPA-capped ZnS:Mn’s reaction broth remained transparent during synthesis. Successful redispersion of ZnS for inkjet printing where the broth contained precipitate was only reported by Small et al. [53]; however, a mercaptan-derived acid was also used in this case as a stabilizer. Therefore, the most likely explanation is the improved electrostatic hindrance to
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10 nm 20 nm Fig. 5. TEM micrographs of ZnS:Mn nanoparticles, dried from dispersion in water. (a) Agglomerate, 10–20 nm diameter and (b) primary crystallites, 2–3 nm diameter.
3.3. Jetting performance Jetting was stable indefinitely and did not result in nozzle clogging, forming spherical droplets of 10 pL volume (Fig. 8). The ‘‘bead-on-a-string’’ effect, which can be induced by the inclusion of high-MW polymers [7,8,20], i.e. the PVP, was observed. The droplet ‘‘tail’’ did not coalesce into the main droplet; this may have contributed to non-uniformity in the printed films discussed below. Droplets which impacted a glass surface spread to an average diameter of 65 mm, with a pronounced coffee-ring effect, where solids migrated to the edge of the droplets (Fig. 9). This problem
7 6 5 Intensity (a.u.)
agglomeration provided by the polar 3-MPA (or similar) molecule, when compared to that offered by polymeric capping agents. PL emission from the dopant centres in all of the nanoparticles was close to the typical orange–yellow range of 585 nm, where emission from ZnS:Mn has been widely observed. The emission results from the d-electron states in the dopant centres interacting with the s–p electron states in the ZnS host, and the resulting transition of Mn’s 4T1 energetic state to the 6A1 energetic state [59], rather than from excitonic recombination across the host material’s bandgap. As a result, the emission colour from the dopant centres was not shifted by quantization effects, confirming observations made during previous syntheses of ZnS:Mn nanocrystals [60–62]. In fact, considering that typical bulk ZnS:Mn emission peaks around 585 nm [60], the nanoparticles’ emission was somewhat red-shifted in comparison, to 593 nm. This has been hypothetically attributed to a large amount of either surface states or electron–phonon coupling in previous studies [63]. In our previous work [24] and with the other capping agents reported above, where the surface states were not as efficiently passivated (using a polymeric cap), the red-shift was more pronounced. The small differences in peak locations for the ZnS:Mn particles dispersed in water vs. NaOH/TGA vs. ink were attributed to improved passivation by TGA [51]. No blue-shift in the characteristic blue emission (at 430 nm) from S2 vacancies on the nanoparticles’ surface was observed either. Indeed, since the Bohr exciton radius of ZnS:Mn is estimated to be approximately 2.5 nm [54–56,62], the particle size of the ZnS:Mn in this study may have been insufficiently small to observe quantum effects. The location of the PLE peak (at 338 nm) was similar to that of bulk ZnS:Mn. A brief consideration of the UV–visible absorbance of the nanoparticles yielded an estimated bandgap of 3.71 eV; again, similar to that of bulk ZnS:Mn. There may have been a minor blue-shift and bandgap widening in the ZnS host (from a bandgap of 3.68, estimated for bulk ZnS [63] to the measured 3.71 eV). Regardless, the particles remained suitable for jetting due to their relatively small size.Fig. 7
4 3 2 1 0 0.1
1
10
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Diameter (nm) Fig. 6. DLS scans of ZnS:Mn nanoparticles in (a) water; (b) water/NaOH, TGA dispersant and (c) ZnS:Mn ink.
was not as pronounced once printed lines were deposited, although solids still migrated to the edge of the printed lines. The size of the droplets was not as important as the line spacing in a printed layer, where line overlap induced film roughness. Solids migration led to the formation of ridges where the printed lines overlapped at lower drop spacing. As drop spacing was increased, the films gradually appeared to be more level, as is seen in Fig. 9e. Above 55 mm drop spacing, overlapping and ridges were the least pronounced, but holes began to form in the printed layer where individual drops did not coalesce. Therefore, 55 mm was considered the optimal drop spacing for this ink. For a simple printed pattern of ZnS:Mn, these topographical effects were not particularly relevant; however, if a film was intended for use as a layer in an LED or similar structure, any holes or excessive roughness could compromise device functionality [55]. This exact problem was encountered by Haverinen et al. [23], where it caused localized dimming of a printed LED device. However, excessively closely-spaced lines would cause such a drastic increase in film thickness and roughness that such a device might draw large amounts of current, resulting in an arc or short across the LED. Reducing ink solids content (including polymer and nanoparticles) [55], and selection of solvents in the mixture with similar values of surface tension [57] or boiling point [7] might serve to improve film smoothness and levelling. If the ZnS:Mn ink is to be applied to a permeable substrate, such as textile or paper, such concerns are immaterial. The degree of solids migration occurring during drop deposition and drying was clearly visible in the optical profiles of the printed ZnS:Mn ink (Fig. 10). Ridges at the edges of printed lines were pronounced distinctly, with peaks of almost 1 mm in height
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Normalized absorbance (unitless)
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225
275
325
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Wavelength (nm) Fig. 7. PL emission spectra (325 nm excitation) of (a) ZnS:Mn in water; (b) ZnS:Mn in water/NaOH, TGA-stabilized; (c) ZnS:Mn ink. PLE spectrum of ZnS:Mn in water is also shown (d). Inset: (e) ZnS:Mn ink under visible light and (f) under 300 nm UV excitation.
200 µm m
Fig. 8. Drop formation of ZnS:Mn ink from DMP2831 nozzle. Time between each image is 5 ms.
100 µm Fig. 9. Printed ZnS:Mn ink on slide glass. Drop size ¼65 mm (a). Varying drop spacing: (b) 25 mm; (c) 35 mm; (d) 45 mm; (e) 55 mm and (f) 65 mm.
nm 3-D
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600 600 400 600 µm µ 450 µm
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200 0
0 2-D
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Fig. 10. Optical profiles of printed ZnS:Mn ink on slide glass, 55 mm drop spacing. (a) 1 printed layer and (b) 5 overprinted layers. Arrows on scale bars refer to the location of the glass surface.
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even for single printed layers. Overprinting with subsequent layers widened the ridges without raising them, improving overall film smoothness. However, overlap and levelling issues in the printed films made them unsuitable as smooth films of controlled thickness, as might be required for an electronic device. Establishment of precise film thickness was not possible due to the large variation in feature size between the ridges and ‘‘valleys’’ of the patterns printed on glass. The nearly micron-thick ridge features would certainly be too pronounced to allow for use in an electrically functional layer, which would require film thicknesses in the few hundred nanometre range. Aside from the issues with topography, the resolution of the printed films was quite good, with relatively smooth edges and minimum feature size determined solely by drop size, as is visible in the 2-D profiles in Fig. 10 as well as in Fig. 11. It was found by printing successively narrower features that the smallest feature size that could realistically be printed with a single layer of ink, as expected, was 65 mm (the average drop size). For features incrementally larger than this size, the printer deposited two lines of ink at the programmed drop spacing (55 mm), resulting in features of approximately 110 mm in size with an overlapping ridge identical to those shown in Figs. 9 and 10. This same pattern was repeated, naturally, for larger and larger features—feature sizes were limited by the drop spacing to increase in multiples of 55 mm. This occurred in features printed both parallel and normal to the movement of the printhead, as in either case, the features were composed of at least one line of 65 mm drops, or multiple lines spaced 55 mm apart. In the case of spaces between printed features, the minimum space achievable was found to be 40 mm (on glass) with an intended space of 100 mm in both cases. Space size was not limited by the size of drops or drop
print edge
spacing, so very narrow unprinted channels are technically feasible with this type of printer and ink. Spacing below 100 mm caused the printed regions to merge into a single film. Therefore, the maximum achievable resolution using a ZnS:Mn ink may be controlled by either controlling drop volume (by adjustment of voltage or use of a cartridge with nozzles of different diameter) or by careful substrate selection or treatment to influence spreading, and control drop spacing. However, because the volume of ink per drop remains constant, narrower spacing on substrates which are wetted less would likely lead to thicker films. Successive overprinting and the resulting layering of the ridged regions resulted in better film smoothness, as mentioned above, with an increase in film thickness. The higher concentration of ZnS:Mn nanoparticles on the surface also resulted in brighter PL of printed patterns (Figs. 11 and 12). However, repeated overprinting presented several disadvantages, which naturally included increased material use and film thickness. An unforeseen problem associated with overprinting was the splattering of ink onto the surrounding substrate, as is clearly visible in Fig. 12b. As droplets impacted the wet ink on the surface during subsequent print passes, ink was ejected from the previously deposited film and landed randomly on the substrate surface. This phenomenon led to reduced edge resolution in the printed patterns, to the point where ‘‘hazy’’ or indistinct edges and smearing were visible even at the macro-scale (Fig. 12a, 20 and 50 printed layers). PL intensity was not much improved by more than 10 overprints in any case; an optimal number likely exists for each substrate at which edge resolution is preserved and PL intensity is sufficiently high. PL emission was bright and distinct after jetting of the ink on all of the substrates shown above (glass, ITO PET, and Al foil).
200 µm
Fig. 11. Edge resolution of ZnS:Mn ink on glass: (a) 1 printed layer; (b) 3 printed layers; (c) 6 printed layers; (d) 9 printed layers; (e) 12 printed layers. Print edges are highlighted by black lines. Insets show PL of the samples under UV excitation.
Fig. 12. Multiple layers of ZnS:Mn ink printed on (a) aluminium foil (1–50 layers) and (b) ITO PET (50 layers), showing edge splattering with excessive overprinting.
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Background fluorescence from the PET caused a certain amount of ‘‘washing out’’ of the typical orange emission; similar effects were expected on paper. However, on photo-paper, which emits a large amount of background fluorescence, the characteristic emission from ZnS:Mn was brightly visible. Other, more highly absorbent sheets blocked PL emission by allowing the ZnS:Mn nanoparticles to sink into the bulk of the sheet, where they likely penetrated into the porous paper fibres (Fig. 13). Optical brighteners, fibres, fillers, and other paper furnishes may competitively fluoresce as well, obscuring the emission from the ZnS:Mn. The photo-paper and glossy sheets had a thick, dense coating that captured the aqueous ink at the surface [19] and hence retained ZnS:Mn in a location where the UV emission could reach the nanoparticles, and the emitted light would be visible. A similar effect was observed in our previous work, when depositing conductive films on the same paper types: sheets designed to immobilize ink at the surface to preserve colour intensity and brightness retained functional materials there as well, resulting in increased conductivity [58]. Furthermore, as is clearly visible in Fig. 13a, PL background emission from the photo-paper sheet was much weaker than from the other sheets, producing an apparently more intense emission from the ZnS:Mn nanoparticles on that sheet. The high gloss of this sheet may have also reflected the majority of the UV light, rather than absorbing it and fluorescing. Emission at similar brightness is present but overwhelmed somewhat by the more intense background emission on the glossy sheet in Fig. 13b. Therefore, for maximum emission intensity at the desired wavelength, substrates should be chosen for this type of ink to be either impermeable or able to retain the ink primarily at the surface layer, while simultaneously fluorescing as little as possible when interrogated with an exciting laser or UV source.
4. Conclusions Water-soluble ZnS:Mn nanoparticles were prepared using a competitive precipitation method. It was found that the use of acid (3-MPA) as a capping agent, at controlled pH, temperature, and
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reaction time, was suitable for forming ZnS:Mn nanoparticles of sufficiently small size to be redispersible in an aqueous inkjet ink. Although many other researchers have produced particles with primary crystallite sizes small enough to be incorporated into an ink using other capping agents, only 3-MPA (and similar species, such as TGA, mercaptoethanol, p-thiocresol, and other organosulphur compounds) resulted in dispersed particles which were below the particle size cutoff for the printer of 200 nm. 3-MPA was most suitable for this role because of its (relatively) lower toxicity and increased ease of handling when compared to the other thiols mentioned. The particles produced using this method were nanosized and well-passivated, resulting in bright PL emission very slightly red-shifted to 591–595 nm from the characteristic ZnS:Mn emission wavelength of 585 nm. Effective passivation was achieved by bonding of the sulphur atom in 3-MPA to the S2 vacancy sites, causing primarily orange emission with minimal emission from S2 vacancies in the blue range. Their small size and dispersity were crucial in formulation of an inkjet ink containing a relatively high loading of solids—1 w/w% ZnS:Mn. Inclusion of PVP as a polymeric binder to aid in the formation of contiguous films had only a minor effect on jetting. Print resolution was determined by drop size, which is determined by nozzle diameter. The small size of the particles in the ink would allow for use of nozzles of significantly smaller diameter and resulting in higher resolution. Printing of these nanoparticles produced brightly luminescent films, whose brightness was enhanced by overprinting multiple times. Films were formed on several substrates with no notable reduction in brightness, except in the case of absorbent sheets, which allowed ZnS:Mn nanoparticles to migrate into the sheet bulk where excitation and emission were blocked by fibres. On impermeable substrates, the high solids loading, surface tension gradients between solvents, and rapid evaporation of volatile solvents lead to solids migration into 1 mm ridges at the edges of printed features. A consideration of ink reformulation by adjustment of PVP or ZnS:Mn content, or by inclusion of levelling aids, might alleviate this issue, which could be problematic if such a film were to be applied in an electronic device. The small film thickness required for a DC-driven LED, for example, which would
Visible
UV Fig. 13. ZnS:Mn ink, 10 overprints, printed on (a) photo-paper; (b) glossy paper; (c) inkjet paper and (d) multiuse (high-yield pulp) paper, under visible and UV (300 nm) excitation.
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be a suitable application for ZnS:Mn nanoparticles, would necessitate smooth films with no significant topographical irregularity. However, this work presents a successful means of patterning luminescent nanoparticles on several substrates and the possibility for producing such patterns on the context of optoelectronic devices, with minor refinements. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28]
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