Morphology Modulation of Direct Inkjet Printing by Incorporating

Article Cite This: Langmuir 2018, 34, 6413−6419

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Morphology Modulation of Direct Inkjet Printing by Incorporating Polymers and Surfactants into a 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 2018.34:6413-6419. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 09/09/18. For personal use only.

†

Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China ‡ Institute for Advanced Materials and Guangdong Provincial Key Laboratory of Quantum Engineering and Quantum Materials, South China Normal University, Guangzhou 510006, China § Key Laboratory of Advanced Display and System Applications of Ministry of Education, Shanghai University, Shanghai 200072, China ∥ Skyworth LCD Co.Ltd, Shenzhen 518108, China ABSTRACT: Many methods have been reported to prevent the nonuniformity of inkjet printing structures. Most of them depend on the balance of the capillary flow in the printing pattern during the evaporation of the solvent. However, as the relation of evaporation and capillary flow can obviously vary among 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 the printing structure will not be affected by the evaporation behavior of the ink system. In this paper, a novel method of direct inkjet printing of a uniform metal oxide structure is reported. We introduce a polymer polyacrylamide and a surfactant FSO into a sol−gel ink system, and the new ink system can gel from the printing pattern edge to center as temperature increases because of the cross-linking of the polymer chains. By that means, transport of solute molecules and solvent molecules is limited. Meanwhile, the surfactant can ensure that the solute in the central liquid phase deposits uniformly by enhancing the Marangoni flow during the gelation process. The 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.

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INTRODUCTION

applying such methods to device fabrication have been reported yet because of their rigorous preparation requirement. Depinning TCL is another strategy to avoid nonuniform deposit,13 but it will bring difficulty to printing resolution controlling for the sliding edges of the pattern. Enhancing the Marangoni flow that generally flows from the edge to the center can offset the effect of the outward capillary flow to realize uniform deposit.14 This method is relatively simple to use and thus easier to be applied to device fabrication. The cosolvent ink system can significantly enhance the Marangoni flow during evaporation15 and has been proved effective in device fabrication.16−18 Another way to enhance the Marangoni flow is to add the surfactant to ink systems.19 This can also be a promising way to fabricate the uniform structure in the device.20

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 thinfilm transistor or field-effect transistor.1−6 A liquid drop on a substrate ejected by an inkjet printer usually exhibits a ring-like structure after drying, which is known as the coffee-ring stain effect.7 This phenomenon is caused by the outward capillary flow, compensating for evaporation loss at the edge of the drop and the pinning of the three-phase contact line (TCL).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 because the internal mechanism of the coffee-ring stain effect was revealed, including using ellipsoidal particles,10 drying drops under a porous template with a micrometer scale aperture11 and inserting a capillary into the center of the droplet.12 These research studies made further efforts to reveal the mechanism of the drop evaporation behavior, but few research studies © 2018 American Chemical Society

Received: March 7, 2018 Revised: May 2, 2018 Published: May 11, 2018 6413

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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 the printing film depends on the morphology of the gelating film formed during the gelation process of the ink system. During the gelation process, the capillary flow still exists in the liquid phase, including the outward flow, which tends to transport the solute to the edge of the region. With the gelation going on, the volume of the liquid phase keeps shrinking so that its TCL keeps sliding, which avoids too much solute deposit at the edge to some degree. Besides, the magnitude of the driving force of outward capillary flow Qca can be estimated as eq 120

Almost all of the methods mentioned above depend upon the relative balance between all the capillary flows in the drop during its evaporation, which is a complex model, while gelation of the ink system, commonly realized by adding gelating materials, makes morphology more controllable for little capillary flow that exists in gels. Schubert et al.21 applied a gelating copolymer to stabilize TiO2 particles in the ink system. As temperature rises, the 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 polymers to the solution and eliminated the coffee-ring effect by the motion of the contact line and improvement of viscosity during evaporation.22 A similar technique has been used in the device to fabricate a 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. The sol−gel system shows great stability and uniformity, which is worthy of being applied to fabrication of inkjet printing devices.24 Nevertheless, the solution containing polymers tends to be viscoelastic fluid, which can remarkably affect the breakup of liquid jets.25 This can surely limit the dosage of polymers in ink systems26 and extend the gelation time, allowing the outward flow to carry more solutes to the edge. Therefore, realizing uniform deposit of the solute in the liquid phase until the end of the 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 the ZrO2 film. The gelation behavior of the ink and morphology of printing patterns at various PAM and FSO concentrations were studied. Finally, an M−I−M structure capacitor was fabricated to study dielectrical properties of the printed ZrO2 film.

Q ca ≈

σh4 ⎛⎜ Δσ ⎞⎟ 1+ 3⎝ σ ⎠ μr

(1)

where σ is surface tension of liquid, μ is the viscosity of liquid, h is the height of the droplet, r is the radius of the droplet, and Δσ is the difference of the surface tension between the center and the edge of the droplet. Therefore, it can be concluded that the gradually forming gel structure in the liquid region can improve the viscosity and a higher concentration of the polymer can modify the outward capillary flow more sufficiently. This can help to obtain uniform deposit during gelation. Meanwhile, surfactant molecules are transported to the edge of the liquid phase region by the outward capillary flow, which enlarges the surface tension gradient, enhancing the Marangoni flow. The magnitude of the Marangoni flow can be calculated using eq 220

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RESULTS AND DISCUSSION Gelation and Capillary Flow Model of the Ink System. Polymer chains in solution generally show a curly conformation.27 However, for the 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 a three-dimensional network structure, which leads to gelation of solution.28 Therefore, by adding a polar polymer, an ink system can gel with rise in the temperature and maintain the sol-state at room temperature. A model of gelation process has been shown in Figure 1. When liquid film is inkjet printed on a substrate at a certain

|∇σ | =

∂σ |∇csur| ∂csur

(2)

For a certain surfactant, |∂σ/∂csur| is a fixed value. Therefore, the magnitude of the Marangoni flow mainly depends on the local gradient of the surfactant concentration |∇csur|, and a higher surfactant concentration can enhance the Marangoni flow more significantly. This can ensure uniform deposit of the solute till the gelation is completed. In this ink system, the polymer and the surfactant together can ensure uniform morphology of the printing film. Ink System Properties and Gelation Behavior. Inks at different PAM and FSO concentrations were fabricated to study the effect of the two components on the gelation behavior of the ink system, respectively. Films were inkjetprinted using inks mentioned in Table 1 with the same printer parameters. As Figure 2 shows, gelation began at the edge of the film at a temperature of 30 °C and gradually spread to the center, which is 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 the gel structure.30 This phenomenon was beyond our ink system model but did not affect our research. The gelation rate of Ink-01 was quite low, and it took 30 min to complete gelation of the film. As the

Figure 1. Gelation process of the printing film using the ink system. 6414

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concentration is required to realize uniform deposit, which conforms to the inference of eq 2. However, in this research, polymers and surfactants were only used to modulate morphology; thus, their effect on the physicochemical properties of the ink and the product should be avoided as much as possible, which means that the dosage of these two components should be as less as possible. Figure 4 shows the jetting performance of inks with different polymer concentrations. As can be seen from Figure 4a, Ink-01 at the PAM concentration of 0 wt % first showed a beam split at 30 μs, indicating a weak viscoelastic property of Ink-01. However, with the introduction of PAM, Ink-02 and Ink-03 showed more viscoelasticity. In Figure 4b, Ink-02 did not show a beam split until 35 μs, and even at 35 μs, a filament existed at the end of the drop. In Figure 4c, it can be seen that no obviously beam split was observed at 35 μs, as a result of viscoelasticity of Ink03. It was obvious that adding the polymer can significantly delay the breakup of the drop filament and have a potential impact on the printing quality. Therefore, as higher PAM and FSO concentration may bring undesirable change to ink and film properties, Ink-03 at the PAM concentration of 0.5 wt % and the FSO concentration of 0.1 wt % was chosen to be the reasonable ink formula for this ink system. Rheological property of Ink-03 is shown in Figure 5. Elastic modulus and viscosity modulus of Ink-03 increased as temperature rose, and the elastic modulus exceeded the viscosity modulus after the temperature reached Tg which is far below the boiling point of ethylene glycol, so little solvent evaporated at this temperature, which meant formation of the gel structure.31 Therefore, it was proved that gelation of the ink system at a relatively low temperature can be realized by adding the gelating polymer. Meanwhile, the viscosity of the ink system evidently rose after the temperature reached Tg, showing that the gel structure has a limitation on the mobility of the solvent and the solute. Thermal analysis of Ink-03 shown in Figure 6a showed the an evaporation behavior of the ink system. Evaporation of the solvent can be divided into two parts as temperature increased, including one segment at lower speed from 25 to 100 °C and one at higher speed above 100 °C, seen from TG, corresponding with two endothermic peaks in the differential scanning calorimetry (DSC) curve. For film fabrication, if gelation of a sol−gel system occurs when a mass of solvent has

Table 1. PAM and FSO Concentration of Different Ink Formulas sample

PAM (wt %)

FSO (wt %)

Ink-01 Ink-02 Ink-03 Ink-04 Ink-05

0 0.3 0.5 0.5 0.5

0.1 0.1 0.1 0.05 0

concentration of PAM improved, the gelation rate improved as well, which indicated that the gel structure can form easily. It can be seen that the obtained film shows more uniform morphology as the improvement of the PAM concentration, and the film with a smooth surface was obtained by Ink-03 at a polymer concentration of 0.5 wt %. It can be concluded that the gel structure formed by FSO molecules has weak limitation on transportation of the solute and the solvent compared to that formed by PAM. Besides, the absence and low concentration of PAM cannot provide enough improvement of ink viscosity during gelation, and so mobility of the solute and the solvent in the liquid phase will not be limited effectively. Therefore, the higher concentration of PAM is conductive to obtain uniform morphology, which suits the inference of eq 1. We also found that the concentration of FSO had a significant effect on the gelation behavior of inks and morphology of films, as shown in Figure 3. The gelation rate of Ink-03, Ink-04, and Ink-05 showed little difference in their PAM concentration. However, Ink-05 with no FSO showed an obvious coffee ring because there was no enhancement of the Marangoni flow, which caused excessive deposition at the edge by the outward capillary flow before the gel structure fully formed. Ink-04 at the FSO concentration of 0.05 wt % gelled at first which exhibited the predicting gelation behavior. However, with gelation going on, more and more FSO molecules were caught by the gel structure and the concentration of FSO in the liquid phase gradually decreased, causing nonuniform distribution of the solute eventually before gelation was over. It can be seen that the nonuniform region of the film printed by Ink-04 was smaller than that of the film printed by Ink-05. Uniform deposition is obtained when the FSO concentration reached 0.1 wt %. It showed that maintaining a high enough FSO

Figure 2. Gelation properties of Ink-01, Ink-02, and Ink-03 at different PAM concentrations and cross-sectional profiles of the corresponding printing film after annealing at 250 °C. The size of the printing film was 1 mm × 1 mm, and the substrate temperature of the printer was set to 30 °C. 6415

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

Figure 6b 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 the film dried in an oven at 75 °C before annealing, as shown in Figure 6c. Therefore, the gelatinous film should be put in an oven and dried slowly in an ambient condition for some time to avoid local stress and then annealed to obtain a metal oxide structure. Electrical Properties of Printing the ZrO2 Film. A ZrO2 array was inkjet-printed on ITO glass using Ink-03 and annealed at 200, 250, 350, and 450 °C. The image of the printing array has been shown in Figure 7b, and a single unit showed uniform morphology, as shown in Figure 7c. It can be seen that the annealing temperature had a significant impact on film thickness in Figure 7a. Crystallization of the printing film was identified by X-ray diffraction (XRD), and the results are shown in Figure 8. No crystallization was measured except the film annealed at 450 °C; these results correspond to the exothermic peak of the DSC curve at 430 °C shown in Figure 6a. A capacitor was fabricated to study the dielectrical properties of the resulting film. A molybdenum electrode was sputtered on printing a ZrO2 array using a shadow mask. The leakage and capacitance characteristics of printing films at different annealing temperatures are presented in Figure 9. It can be seen from Figure 9 that the printing dielectric layers were easy to break down at a very low electric field, except the ZrO2 film annealed at 250 °C. For the ZrO2 film annealed at 200 °C, though the film was with a large thickness, many organic residues still existing in the gel network and low density together provided a leakage path, which made the film easy to break down.33 The ZrO2 film annealed at 250 °C showed a low leakage current density at 1 MV, and this may be because of the removal of the organic residue and the film being dense. No crystallization of the 350 °C annealed ZrO2 film was observed; however, chelating of the metal ion by polymer chains decreased the diffusion path of condensation polymerization.29 Besides, combustion of the carbonaceous residue can provide heat, which causes rise in the local temperature.34 These may contribute to form the local microcrystal structure or the local ordering structure which may generate a leakage path. As for the ZrO2 film, annealed at 450 °C, the presence of grain boundaries led to the high leakage current.

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

Figure 5. Rheological test of Ink-03. G′ is the elastic modulus of Ink03; G″ is the viscosity modulus of Ink-03; Tg is the gelation temperature.

been evaporated, then the gel structure will not 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 the solvent and thus lead to large local inhomogeneity of capillary stress, which may result in cracks in the film,24,32 especially for the weak intensity of the gel structure formed by polymer chains. As 6416

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

Figure 7. (a) Cross-sectional profiles of the printed ZrO2 film at different annealing temperatures. (b) Printing the ZrO2 array on ITO glass. (c) 3D image of the single printed ZrO2 unit.

Figure 8. XRD analysis of the printed ZrO2 film at different annealing temperatures.

Figure 9. Leakage characteristics of the printed ZrO2 film at different annealing temperatures from 0 to 2.5 MV cm−1 and leakage characteristics of the printed ZrO2 film annealed at 250 °C from 0 to 1 MV cm−1.

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CONCLUSIONS We demonstrated a new method to modulate morphology in direct inkjet printing on the 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 the uniform ZrO2 film by direct inkjet printing. Both PAM and FSO can significantly affect the gelation behavior of the ink system and thus affect the

morphology of the printing film. We concluded that the reasonable concentration of PAM is 0.5 wt % and that of FSO is 0.1 wt % in this ink system. The film with uniform morphology can be fabricated by simply raising the substrate temperature to 30 °C and waiting for completion of gelation. The film printed by this ink system should be dried at the low 6417

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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), and the Project for Guangdong Province Universities and Colleges Pearl River Scholar Funded Scheme(2016).

temperature for 20 h in an ambient condition before high temperature annealing to avoid cracks. Printing the ZrO2 film annealed at 250 °C showed good leakage characteristics, which exhibited promising application potentiality in dielectrics. As the effect of polymers and surfactants on the film microstructure remains unknown, many studies need to be carried out in the future.

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EXPERIMENTAL SECTION

Preparation of Inks. PAM (Sigma-Aldrich, Mn = 4 × 104 g mol−1) was chosen to be the gelating additive and was dissolved in ethylene glycol. The mass was stirred at 60 °C for 3 h to ensure the PAM fully dissolved. Then, ZrOCl2·8H2O (Rich Joint Chemical) was added as the ZrOx precursor with a concentration of 0.3 M, and the solution was stirred for 20 min. Finally, a fluorocarbon surfactant (FSO) was added to the solution, and the solution was stirred for 20 min. Ink formulas with different PAM and FSO concentrations 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. The voltage applied to the 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. The annealing temperature was set to 200, 250, 350, and 450 °C and the annealing time was set to 60 min. The capacitor was fabricated by dc sputtering the molybdenum electrode on the printing array using a 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 50−70 °C and a testing speed of 2 °C/min. Thermal properties of the 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 three-dimensional (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 the printing film was identified by XRD equipped Cu-kα as the X-ray source. The scanning rate was 0.05°/s and the scanning range was from 20° to 70°. The I−V characteristics of the printed ZrO2 film under different annealing temperatures were tested by a semiconductor parameter analyzer from 0 to 20 V and 0 to 25 V.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.N.). *E-mail: [email protected] (R.Y.). ORCID

Honglong Ning: 0000-0001-9518-5738 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS 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 6418

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