Morphology Modulation of Direct Inkjet Printing by Incorporating

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Morphology modulation of direct inkjet printing by incorporating polymer and surfactant into sol-gel ink system Zhennan Zhu, Honglong Ning, Wei Cai, Jinglin Wei, Shangxiong Zhou, Rihui Yao, Xubing Lu, Jianhua Zhang, Zhongwei Zhou, and Junbiao Peng Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00745 • Publication Date (Web): 11 May 2018 Downloaded from http://pubs.acs.org on May 15, 2018

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Morphology modulation of direct inkjet printing by incorporating polymer and surfactant into sol-gel ink system Zhennan Zhu1, Honglong Ning1*, Wei Cai1, Jinglin Wei1, Shangxiong Zhou1, Rihui Yao1*, Xubing Lu2, Jianhua Zhang3, ZhongWei Zhou4, Junbiao Peng1 1

Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent

Materials and Devices, South China University of Technology, Guangzhou 510640, China 2

Institute for Advanced Materials and Guangdong Provincial Key Laboratory of Quantum

Engineering and Quantum Materials, South China Normal University, Guangzhou 510006, China 3

Key Laboratory of Advanced Display and System Applications of Ministry of Education,

Shanghai University, Shanghai 200072, China 4

Skyworth LCD Co.Ltd, Shenzhen, 518108 China

*

Corresponding author email: [email protected] (H.N.); [email protected] (R.Y.)

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Abstract Many methods have been reported to prevent the nonuniformity of inkjet printing structures. Most of them depend on the balance of capillary flow in printing pattern during the evaporation of solvent. However, as relation of evaporation and capillary flow can obviously vary between different ink systems, it is difficult for a method to fit most of the situations. Therefore, it would be a promising way to eliminate any capillary flow before solvent evaporation so that morphology of printing structure will not be affected by evaporation behavior of ink system. In this paper, a novel method of direct inkjet printing uniform metal oxide structure is reported. We introduce a polymer PAM and a surfactant FSO into a sol-gel ink system and the new ink system can gel from printing pattern edge to center as temperature increases due to the cross-linking of the polymer chains. By that means, transport of solute molecules and solvent molecules is limited. Meanwhile, surfactant can ensure that solute in central liquid phase deposit uniformly by enhancing Marangoni flow during gelation process. ZrO2 film with uniform morphology was fabricated by drying and annealing the gelating film and afforded a leakage current density of 7.48×10-7 A.cm-2 at 1 MV and a breakdown field of 1.9 MV.cm-1 at an annealing temperature of 250 °C.

Keywords: Morphology, direct inkjet printing, gel, polymer, surfactant


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Introduction Morphology modulation has been one of the most important issues of inkjet printing electronic devices for its great effect on device performance, especially for thin film devices like TFT or FET.1-6 Liquid drop on substrate ejected by inkjet printer usually exhibits a ring-like structure after drying, which is known as coffee ring stain effect.7 This phenomenon is caused by the outward capillary flow compensating for evaporation loss at the edge of drop and the pinning of three-phase contact line.8, 9 Deposits like that are common in various liquid dispersion systems and thus it is difficult to obtain uniform patterns by inkjet printing. Many methods aiming to modify the outward capillary flow have been reported since the internal mechanism of coffee ring stain effect was revealed, including using ellipsoidal particles,10 drying drops under a porous template with micrometer scale aperture11 and inserting a capillary into the center of the droplet.12 These researches made further efforts to revealing the mechanism of drop evaporation behavior, but few researches applying such methods to device fabrication have been reported yet due to their rigorous preparation requirement. Depinning three-phase contact line (TCL) is another strategy to avoid non-uniform deposit,13 but it will bring difficulty to printing resolution controlling for the sliding edges of the pattern. Enhancing Marangoni flow that generally flows from edge to center can offset the effect of outward capillary flow to realize uniform deposit.14 This method is relatively simple to use and thus easier to be applied to device fabrication. Co-solvents ink system can significantly enhance Marangoni flow during evaporation15 and has been proved effective in device fabrication.16-18 Another way to enhance Marangoni flow is to add surfactant to ink systems.19 This can also be a promising way to fabricate uniform structure in device.20 Almost all of the methods above depend upon the relative balance between all the capillary flows


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in drop during its evaporation, which is a complex model, while gelation of ink system, commonly realized by adding gelating materials, makes morphology more controllable for little capillary flow exists in gels. Schubert et al.21applied a gelating copolymer to stabilize TiO2 particles in ink system. As temperature rising, copolymer becomes insoluble in water, which makes TiO2 particles begin to flocculate, resulting in gelation of the ink. The viscosity of ink system significantly increases, making it difficult for solutes and solvents to transport during this process. Yang et al. applied different kinds of hydrosoluble polymer to solution and eliminated coffee-ring effect by the motion of contact line and improvement of viscosity during evaporation.22 Similar technique has been used in device to fabricate ceramic/polymer composite film with a BST dispersion system.23 Still, no such technique has been applied to sol-gel ink systems in inkjet printing devices. Sol-gel system shows great stability and uniformity, which is worthy of being applied to fabrication of inkjet printing devices.24 Nevertheless, solution containing polymer tends to be viscoelastic fluid, which can remarkably affect the breakup of liquid jets.25 This can surely limit the dosage of polymer in ink systems,26 and extend the gelation time, allowing outward flow to carry more solute to the edge. So realizing uniform deposit of solute in liquid phase until the end of gelation process is demanded. In this paper, by adding a gelating polymer Polyacrylamide (PAM, CAS: 9003-05-8) and a surfactant FSO (a model of Fluorocarbon surfactant, CAS: 65545-80-4) to a basic sol-gel system, a new phase-change ink system was obtained and applied to inkjet printing of ZrO2 film. Gelation behavior of the ink and morphology of printing patterns at various PAM and FSO concentration were studied. Finally, a M-I-M structure capacitor was fabricated to study dielectrical properties of the printing ZrO2 film.


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Results and discussion Gelation and capillary flow model of ink system Polymer chains in solution generally show a curly conformation.27 However, for polymer that has many polarity groups, as molecule chains gradually turn into extended conformation at certain temperatures, cross-linking can occur between molecule chains, forming three-dimensional network structure, which leads to gelation of solution.28 Therefore, by adding a polar polymer, an ink system can gel with the temperature rising and

maintain sol-state at room temperature.

A model of gelation process has been shown in Fig.1. When liquid film is inkjet printed on a substrate at a certain temperature, gelation occurs first at edge of the film due to the higher evaporation rate and being heated more fully. Liquid film keeps gelling from edge to center and liquid phase region keeps shrinking. With the development of gel structure, gelation tends to accelerate. Polymer with lots of polar groups can bond solvent molecules by forming hydrogen bond or bond metal ion by chelating.29 So solutes and solvents are trapped in the net-like structure formed by polymer chains, losing mobility when gelation is over. In this situation, morphology of printing film can be fixed before most of the solvent evaporates and will not be affected by the later heat treatment. Therefore, it can be concluded that the final morphology of printing film depends on the morphology of gelating film formed during gelation process of ink system. During gelation process, capillary flow still exists in liquid phase including outward flow, which tends to transport solute to edge of the region. As the gelation going on, volume of liquid phase keeps shrinking so that TCL of it keeps sliding, which avoids too much solute deposit at the edge to some degree. Besides, the magnitude of driving force of outward capillary flow Qca can be estimated as eq 1 20


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~ ca

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∆ () +

Where σ is surface tension of liquid, µ is the viscosity of liquid, h is the height of droplet, r is the radius of droplet and ∆σ is the difference of the surface tension between center and edge of droplet. So it can be concluded that the gradually forming gel structure in liquid region can improve the viscosity and a higher concentration of polymer can modify outward capillary flow more sufficiently. This can help to obtain uniform deposit during gelation. Meanwhile, surfactant molecules are transported to the edge of liquid phase region by outward capillary flow, which enlarges surface tension gradient, enhancing the Marangoni flow. The magnitude of Marangoni flow can be calculated as eq 2 20 || =

| |

(2)

For certain surfactant, |∂σ/∂csur| is a fixed value. So the magnitude of Marangoni flow mainly depends on the local gradient of surfactant concentration |∇csur| and a higher surfactant concentration can enhance Marangoni flow more significantly. This can ensuring uniform deposit of solute till the gelation is completed. In this ink system, polymer and surfactant together can ensure uniform morphology of printing film. Ink system properties and gelation behavior Inks at different PAM and FSO concentration were fabricated to study the effect of the two components on gelation behavior of ink system respectively. Film were inkjet printed using inks mentioned in Table 1 with the same printer parameters. As Fig.2 shows, gelation began at edge of the film at a temperature of 30 °C and gradually spread to the center, which was consistent with the prediction of model. We found that Ink-01 with no PAM gelled at 30 °C for surfactant modules in solution can also form gel structure.30 This phenomenon was beyond our ink system model but didn’t


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affect our research. Gelation rate of Ink-01 was quite low and it took 30 min to complete gelation of film. As concentration of PAM improved, gelation rate improved as well, which indicated that gel structure can form more easily. It can be seen that the obtained film show more uniform morphology as the improvement of PAM concentration, and film with smooth surface was obtained by Ink-03 at polymer concentration of 0.5wt%. It can be concluded that gel structure formed by FSO molecules has weak limitation on transportation of solute and solvent compared to that formed by PAM. Besides, absence and low concentration of PAM can’t provide enough improvement of ink viscosity during gelation, so mobility of solute and solvent in liquid phase will not be limited effectively. Therefore, higher concentration of PAM is conductive to obtaining uniform morphology, which suits the inference of eq1. We also found that concentration of FSO had significantly effect on gelation behavior of inks and morphology of films as shown in Fig.3. Gelation rate of Ink-03, Ink-04 and Ink-05 showed little difference for their same PAM concentration. However, Ink-05 with no FSO showed obvious coffee ring because no enhancing for marangoni flow, which caused excessive deposition at the edge by outward capillary flow before gel structure fully formed. Ink-04 at FSO concentration of 0.05wt% gelled at first which exhibited the predicting gelation behavior. However, with gelation going on, more and more FSO molecules were caught by gel structure and the concentration of FSO in liquid phase gradually decreased, causing non-uniform distribution of solute eventually before gelation was over. It can be seen that the non-uniform region of film printed by Ink-04 was smaller than that of film printed by Ink-05. Uniform deposition is obtained when FSO concentration reached 0.1wt%. It showed that only maintain a high enough FSO concentration can obtain uniform morphology, which conforms to the inference of eq2.


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However, in this research, polymer and surfactant were only used to modulate morphology, thus their effect on the physicochemical properties of ink and product should be avoid as much as possible, which means the dosage of these two components should be as less as possible. Fig.4 shows jetting performance of inks with different polymer concentration. As can be seen from Figure 4 (a), Ink-01 at PAM concentration of 0wt% firstly showed beam split at 30 µs, indicating weakly-viscoelastic property of Ink-01. However, with the introduction of PAM, the Ink-02 and Ink-03 showed more viscoelasticity. In Figure 4 (b), Ink-02 didn’t show beam split until 35 µs, and even at 35 µs, a filament existed at the end of drop. In Figure (c), it can be seen that no obviously beam split was observed at 35 µs, as a result of viscoelasticity of Ink-03. It was obviously that the adding of polymer can significantly delay the breakup of drop filament and have potential impact on printing quality. Therefore, as higher PAM and FSO concentration may bring undesirable change to ink and film properties, Ink-03 at PAM concentration of 0.5wt% and FSO concentration of 0.1wt% was chosen to be the reasonable ink formula for this ink system. Rheological property of Ink-03 is shown in Fig.5. Elastic modulus and viscosity modulus of Ink-03 increased as temperature rose, and elastic modulus exceeded viscosity modulus after temperature reached Tg which is far below the boiling point of ethylene glycol, so little solvent evaporated at this temperature, which meant formation of gel structure.31 So it was proved that gelation of ink system at a relatively low temperature can be realized by adding gelating polymer. Meanwhile, the viscosity of ink system evidently rose after temperature reached Tg, showing that gel structure has limitation on mobility of solvent and solute. Thermal analysis of Ink-03 shown in Fig.6 (a) showed the evaporation behavior of ink system. Evaporation of solvent can be divided into two parts as temperature increased, including one


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segment at lower speed from 25 °C to 100 °C and one at higher speed above 100 °C seen from TG, corresponding with two endothermic peaks in DSC curve. For film fabrication, if gelation of a sol-gel system occurs when a mass of solvent has been evaporated, then gel structure won’t suffer a large local stress. However, in this ink system, as most of solvent haven’t evaporated since the gelation was over, rapid thermal processing may cause violent evaporation of solvent and thus led to large local inhomogeneity of capillary stress, which may result in cracks in film,24, 32 especially for the weak intensity of gel structure formed by polymer chains. As Fig.6 (b) shows, a large number of cracks existed in the film, which was directly annealed after gelation was over. By contrast, no obvious cracks were observed in film dried in oven at 75 °C before annealing, as shown in Fig.6 (c). Therefore, the gelatinous film should be put in oven and dried slowly in an ambient condition for some time to avoid local stress, and then annealed to obtain metal oxide structure. Electrical properties of printing ZrO2 film ZrO2 array was inkjet printed on ITO glass using Ink-03 and annealed at 200 °C, 250 °C, 350 °C and 450 °C. The image of printing array has been shown in Fig.7 (b) and single unit showed uniform morphology seen from Fig.7 (c). It can be seen that annealing temperature had significant impact on film thickness in Fig.7 (a). Crystallization of printing film was identified by XRD and the results were shown in Fig.8. No crystallization was measured except film annealed at 450 °C, these results correspond to the exothermic peak of DSC curve at 430 °C in Fig.6 (a). A capacitor was fabricated to study the dielectrical properties of resulting film. Molybdenum electrode was sputtered on printing ZrO2 array using a shadow mask. The leakage and capacitance characteristics of printing films at different annealing temperature are presented in Fig.9. It can be seen from Fig.9 that the printing dielectric layers were easy to breakdown at a very low electric field,


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except ZrO2 film annealed at 250 °C. For ZrO2 film annealed at 200 °C, though the film was with a large thickness, many organic residue still existing in the gel network and low density together provided a leakage path, which made the film easy to breakdown.33 ZrO2 film annealed at 250 °C showed a low leakage current density at 1 MV, this may be due to the removal of organic residue and the film being dense. No crystallization of the 350 °C annealed ZrO2 film was observed, however, chelating of metal ion by polymer chains decreased the diffusion path of condensation polymerization.29 Besides, combustion of carbonaceous residue can provide heat, which causing local temperature rising.34 These may contribute to form local microcrystal structure or local-ordering structure which may generate leakage path. As for ZrO2 film annealed at 450 °C, the presence of grain boundaries led to the high leakage current.

Conclusions We demonstrated a new method to modulate morphology in direct inkjet printing on ink system aspect. By adding polymer PAM and surfactant FSO and adjusting their concentration, a new gelating sol-gel ink system was obtained and applied to fabricated uniform ZrO2 film by direct inkjet printing. Both PAM and FSO can significantly affected the gelation behavior of ink system and thus affected the morphology of printing film. We concluded that the reasonable concentration of PAM is 0.5wt% and that of FSO is 0.1wt% in this ink system. Film with uniform morphology can be fabricated by simply raising the substrate temperature to 30 °C and waiting for completion of gelation. Film printed by this ink system should be dried at low temperature for 20 h in an ambient condition before high temperature annealing to avoid cracks. Printing ZrO2 film annealed at 250 °C showed good leakage characteristics, which exhibited


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promising application potentiality in dielectrics. As the effect of polymer and surfactant on film microstructure remains unknown, many studies need to be carried out in the future.

Experimental Section Preparation of inks Polyacrylamide (Sigma Aldrich, Mn=4×104 g.mol-1) was chosen to be the gelating additives and was dissolved in ethylene glycol. The massing was stirred at 60 °C for 3 h to ensure the polyacrylamide fully dissolved. Then ZrOCl2.8H2O (Rich Joint Chemical) was added as ZrOx precursor with a concentration of 0.3 M and the solution was stirred for 20 min. Finally Fluorocarbon surfactant (FSO) was added to the solution and the solution was stirred for 20 min. Ink formulas with different PAM and FSO concentration are listed in Table.1. The final resulting solution was under ultrasonic oscillation for 5 min to discharge bubbles and then aged for 24 h. Inks were filtered by a filter head with an aperture of 0.5 µm before using. Printing patterns and capacitor fabrication ZrO2 films were printed on glass and indium-tin-oxide (ITO) glass using a piezoelectric inkjet printer (Dimatix DMP 2800) equipped with a 10 pl cartridge. The print head temperature was set to 70 °C and the platen temperature was set to 30 °C. Voltage applied to print head was set to 40 V. Drop spacing was set to 35 µm. The printing patterns concluded a block with a side length of 1 mm and an array consisting of 7×4 such blocks. After the gelation finished, the printing pattern was dried in a lab oven at 75 °C for 20 h and then annealed on a heating stage in air. Annealing temperature was set to 200 °C, 250 °C, 350 °C, 450 °C and annealing time was set to 60 min. Capacitor was fabricated by DC sputtering molybdenum electrode on the printing array using a


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shadow mask. Characterization Jetting and gelation images of inks were observed by the printer camera and captured by the printer software. Rheological properties of ink were tested by rheometer MARS 60 with a testing range of 50 °C~70 °C and a testing speed of 2 °C/min. Thermal properties of ink were tested by STA449C, the testing range was from room temperature to 500 °C and the testing speed was 1 °C/min. Polarization images of printing patterns were obtained from Nikon Eclipse E600 POL. The surface profile and 3D morphology image of printing blocks were investigated by Bruker DektakXT and Veeco NT9300, respectively. Gelation images of inks were observed and captured by the printer camera. Crystallization of printing film was identified by X-ray diffraction (XRD) equipped Cu-kα as X-ray source. The scanning rate was 0.05°/s and the scanning range was from 20° to 70°. The I-V characteristics of printing ZrO2 film under different annealing temperatures was tested by Semiconductor Parameter Analyzer from 0 V to 20 V and 0 V to 25 V.


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Tables Table. 1 PAM and FSO concentration of different ink formulas Sam ple Ink01 Ink02 Ink03 Ink04 Ink05

PAM (wt%) 0

FSO (wt%) 0.1

0.3

0.1

0.5

0.1

0.5

0.05

0.5

0


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Acknowledgements This work was supported by National Key R&D Program of China (No.2016YFB0401504), National Natural Science Foundation of China (Grant.51771074, 51521002 and U1601651), National Science Foundation for Distinguished Young Scholars of China (Grant.51725505), National Key Basic Research and Development Program of China (973 program, Grant No.2015CB655004) Founded by MOST, Guangdong Natural Science Foundation (No.2016A030313459 and 2017A030310028),

Guangdong

Science

and

Technology

Project

(No.2016B090907001,

2016A040403037, 2016B090906002 and 2017A050503002), Guangzhou Science and Technology Project (201804020033), the Project for Guangdong Province Universities and Colleges Pearl River Scholar Funded Scheme(2016).


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19. Still, T.; Yunker, P. J.; Yodh, A. G., Surfactant-induced Marangoni eddies alter the coffee-rings of evaporating colloidal drops. Langmuir the Acs Journal of Surfaces & Colloids 2012, 28, (11), 4984. 20. Kajiya, T.; Kobayashi, W.; Okuzono, T.; Doi, M., Controlling the drying and film formation processes of polymer solution droplets with addition of small amount of surfactants. Journal of Physical Chemistry B 2009, 113, (47), 15460. 21. Berg, A. V. D.; Laat, A. D.; Smith, P. J.; Perelaer, J.; Schubert, U. S., Geometric Control of Inkjet Printed Features Using a Gelating Polymer. Journal of Materials Chemistry 2007, 17, (7), 677-683. 22. Cui, L.; Zhang, J.; Zhang, X.; Huang, L.; Wang, Z.; Li, Y.; Gao, H.; Zhu, S.; Wang, T.; Yang, B., Suppression of the coffee ring effect by hydrosoluble polymer additives. Acs Applied Materials & Interfaces 2012, 4, (5), 2775-2780. 23. Mikolajek, M.; Friederich, A.; Kohler, C.; Rosen, M.; Rathjen, A.; Krüger, K.; Binder, J. R., Direct Inkjet Printing of Dielectric Ceramic/Polymer Composite Thick Films. Advanced Engineering Materials 2015, 17, (9), 1294-1301. 24. Scherer, C. J. B., Sol-Gel Science: The Physics and Chemistry of Sol- Gel Processing. 1990,. 25. Goldin; Michael; Yerushalmi; Joseph; Pfeffer; Robert; Shinnar; Reuel, Breakup of a laminar capillary jet of a viscoelastic fluid. Journal of Fluid Mechanics 2006, 38, (4), 689-711. 26. Johns, A. S.; Bain, C. D., Ink-Jet Printing of High-Molecular-Weight Polymers in Oil-in-Water Emulsions. Acs Appl Mater Interfaces 2017, 9, (27). 27. Fixman, M.; Stockmayer, W. H., Polymer conformation and dynamics in solution. Annual Review of Physical Chemistry 1970, 21, (21), 407-428. 28. Flory, P. J., Molecular Size Distribution in Three Dimensional Polymers. I. Gelation1. J.am.chem.soc 1941, 63, (11), 3083-3090. 29. Koh, C.; Tahir, S.; Sen, A.; Pathak, A.; Pramanik, P., Preparation of nanosized mixed oxide ceramic powders using polyvinyl alcohol and polyhydroxy organic compounds. British Ceramic Transactions 2013, 101, (3), 114-119. 30. Raghavan, S. R., Distinct character of surfactant gels: a smooth progression from micelles to fibrillar networks.


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Langmuir the Acs Journal of Surfaces & Colloids 2009, 25, (15), 8382. 31. Winter, H. H.; Chambon, F., Analysis of Linear Viscoelasticity of a Crosslinking Polymer at the Gel Point. Journal of Rheology 1986, 30, (2), 367-382. 32. Scherer, G. W., Sintering of sol-gel films. Journal of Sol-Gel Science and Technology 1997, 8, (1-3), 353-363. 33. Park, J. H.; Yoo, Y. B.; Lee, K. H.; Jang, W. S.; Jin, Y. O.; Chae, S. S.; Hong, K. B., Low-Temperature, High-Performance Solution-Processed Thin-Film Transistors with Peroxo-Zirconium Oxide Dielectric. Acs Applied Materials & Interfaces 2013, 5, (2), 410. 34. Saha, S. K.; Pramanik, P., Synthesis of nanophase PLZT (12/40/60) powder by PVA-solution technique. Nanostructured Materials 1997, 8, (1), 29-36.


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Figures

Fig. 1 Gelation process of printing film using the ink system.


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Fig. 2 Gelation properties of Ink-01, Ink-02 and Ink-03 at different PAM concentration and Cross sectional profiles of corresponding printing film after annealing at 250 °C. The size of printing film was 1 mm×1 mm, and substrate temperature of printer was set to 30 °C.


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Fig. 3 Gelation behavior of Ink-03, Ink-04 and Ink-05 at different FSO concentration and Cross sectional profiles of corresponding printing film after annealing at 250 °C. The size of printing film was 1 mm×1 mm, and substrate temperature of printer was set to 30 °C.


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Fig. 4 Drop ejection of Ink-01, Ink-02 and Ink-03 at PAM concentration of (a) 0wt%, (b) 0.3wt% and (c) 0.5wt% respectively. The strobe delay is 20 µs.


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Fig. 5 Rheological test of Ink-03. G’ is the elastic modulus of Ink-03; G’’ is the viscosity modulus of Ink-03; Tg is the gelation temperature.


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Fig. 6 (a) Thermal analysis of Ink-03. Printing ZrO2 film (b) directly annealed without pre-drying and (c) annealed after drying 20 h in oven.


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Fig. 7 (a) Cross sectional profiles of printing ZrO2 film at different annealing temperature. (b) Printing ZrO2 array on ITO glass. (c) 3D image of single printing ZrO2 unit.


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Fig. 8 XRD analysis of printing ZrO2 film at different annealing temperature.


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Fig. 9 Leakage characteristics of printing ZrO2 film at different annealing temperature from 0 MV.cm-1 to 2.5 MV.cm-1 and leakage characteristics of printing ZrO2 film annealed at 250 °C from 0 MV.cm-1 to 1 MV.cm-1.


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Morphology Modulation of Direct Inkjet Printing by Incorporating

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