Homogeneous Surface Profiles of Inkjet-Printed Silver Nanoparticle

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Homogeneous Surface Profiles of Inkjet Printed Silver Nanoparticle Films by Regulating Their Drying Micro-Environment Ruiqiang Tao, Honglong Ning, Zhiqiang Fang, Jianqiu Chen, Wei Cai, Yicong Zhou, Zhennan Zhu, Rihui Yao, and Junbiao Peng J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b12793 • Publication Date (Web): 11 Apr 2017 Downloaded from http://pubs.acs.org on April 15, 2017

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The Journal of Physical Chemistry

Homogeneous Surface Profiles of Inkjet Printed Silver Nanoparticle Films by Regulating Their Drying Microenvironment Ruiqiang Tao, Honglong Ning*, Zhiqiang Fang, Jianqiu Chen, Wei Cai, Yicong Zhou, Zhennan Zhu, Rihui Yao*, Junbiao Peng Institute of Polymer Optoelectronic Materials & Devices, State Key Laboratory of Luminescent Materials & Devices, South China University of Technology, P.R. China

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ABSTRACT: Obtaining homogeneous surface profiles of inkjet printed silver nanoparticle films (SNFs) is severely hampered by the classical coffee-ring phenomenon. Traditional approaches to suppress the coffee-ring effect are achieved by improving the uniformity of liquid evaporation rate or introducing inwards Marangoni force during ink drying. However, these existing methods involve extra chemicals, treatments or equipment that will definitely increase the production cost or reduce conductivity. In this study, we demonstrate an inexpensive and efficient method to obtain uniform surface profiles of inkjet printed SNFs by rationally regulating the drying microenvironment. To this end, a number of surface profiles of printed dots and lines, such as concave, convex and flat, were first obtained via this method. The underlying principle of this method was then investigated by analytical calculations based on the simplified vapor diffusion model. Consequently, homogeneous surface profiles of inkjet printed SNFs were achieved on the basis of aforementioned analytical calculations. Our work provides a new possibility to regulate the surface profiles of inkjet printed nanoparticle-based patterns in the form of low cost and efficiency.


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1. INTRODUCTION Inkjet printing technique has attracted great attention as an ambient, direct, cost-effective, noncontact, and promising pattern method to fabricate electrodes of thin film transistors (TFTs) for displays1-8. Silver possesses the highest conductivity9, higher cost-efficiency10-12 than gold, and higher thermal stability compared to copper, showing its potential application in the fabrication of high-performance electrode of TFT by inkjet printing technology13. However, the coffee-ring effect is still a big challenge for fabricating uniform surface profiles of inkjet printed silver electrodes because of the outwards fluid flow inside of liquid materials and the pinning of the contact line14-18. In extreme cases of this morphological phenomenon, no materials remain at the center of metal tracks, which will severely restrict the improving of the lifetime and the performance of electronic devices18-20. Several remarkable discoveries have been utilized to reduce the coffee-ring effect of printed patterns from different perspectives: (a) Modulating the chemistry and formulations of ink by adding surfactant21, dodecanethiol22, a higher boiling point solvent with lower surface tension23, a gelating polymer24, or by changing nanoparticle shape25. (b) Substrate treatments, like surface wettability26, reducing substrate temperature (17 ℃ )27, pre-pattering28 and electrowetting of substrates29. (c) Additional equipment, like evaporation mask with micron-sized holes30 and a dipped tip into a droplet31. However, aforementioned methods are invasive to the printing system and will definitely increase the production cost or reduce the conductivity of printed electrodes. In this paper, homogeneous surface profiles of inkjet printed silver nanoparticle films（SNFs） were first achieved by rationally regulating the drying micro-environment of silver nanoparticle ink. Through adding adjacent patterns, the drying micro-environment of as-printed wet SNFs can be modified that enable the regulation of solvent evaporation rate across the surface of SNFs.


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Therefore, concave, convex and flat profiles of inkjet printed silver nanoparticle patterns were obtained. Analytical calculations based on simplified vapor diffusion model were then utilized to better understand the underlying mechanism of this regulation method. Our results open up a new possibility for controlling profiles of inkjet printed patterns, and provide referential significance for new pattern design rules of inkjet printing technology.

2. EXPERIMENTAL SECTION A commercial colloidal ink of silver nanoparticles (50 nm) in triethylene glycol monoethyl ether has been used (DGP-45LT-15C, Advanced Nano Products). The ink concentration is around 45% and the boiling point of the solvent is 256 °C. The prepared conductive ink was ejected by a inkjet printing system (DMP-2831, FUJIFILM Diamtix, USA) onto cleaned glass substrate under ambient conditions (22 °C, 70% relative humidity). The cartridge (DMC-11610, FUJIFILM Diamtix, USA) of the system setup consists of drop-on-demand piezoelectric inkjet nozzles with 21 μm orifices. The diameter of the printed dots on the glass substrate is 50-55 μm. Patterns were printed with the same printing parameters, i.e., falling velocity of 2 m/s, interdrop spacing of 35 μm, and substrate temperature of 25 °C, which is the lowest substrate temperature that can be controlled by the heating equipment, and is the process temperature of evaporation. The influence of drying micro-environment on inkjet printed dots (Figure 1a) and lines (Figure 1b) is explored by adding frame patterns (black frame in Figure 1a and 1b). All the patterns were printed in one shot, followed by drying for 2 hours, after which the printed patterns almost dried up. The printing process only take several seconds, and the drying done in the printer after printing. The experiment was carried out in clean room and the lid of the printer was used to


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avoid air transport through the volume above the substrate. Due to the influence of the solvent vapor provided by adjacent patterns, the drying time is varying with different designed patterns. In order to keep the drying time be the same, high power UV curing equipment (IntelliRay 600 W, Uvitron, USA) was used to ensure all the undried part of the printed patterns dried up in a short time (180 s in this experiment). The profiles of dots and lines were measured by a Dektak 150 surface profiler (Veeco Instruments, Inc). The images of final deposits were obtained by an optical microscopy (Nikon Eclipse E600 POL) with a DXM1200F digital camera.

Figure 1. The establishment of drying micro-environment with different solvent vapor concentration for printed (a) dots and (b) lines.

3 RESULTS AND DISCUSSION 3.1 The Profiles of Inkjet Printed Dots and Lines Regulated by Adjacent Patterns 3.1.1 The Profiles of Inkjet Printed Dots. Inkjet printed silver nanoparticle dots with extremely different profiles were obtained by adding an extra pattern (Figure 1a). The optical microscope image was taken after drying for 45 minutes, and three dots of different distance to frame pattern were denoted as A, B, and C, respectively (Figure 2a). The different color of dots observed in the


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frame pattern is a strong proof of the different evaporation process of the circled dots. We consider that this effect induced by the ink solvent vapor diffusion of the undried frame pattern. The profiles of A, B, and C dot are shown in Figure 2b and Figure 2c, D represents the original dot without regulating drying micro-environment. The convex profile of dot A indicates the success of suppressing the coffee-ring effect via adding extra pattern. As the distance to the frame pattern increased, the profiles of dots are gradually changed from convex to concave. When the distance is too long to regulate the drying micro-environment the coffee-ring behavior appears, just like the profile of dot D.

Figure 2. The effect of the added extra pattern on the profiles of printed dots. (a) The optical microscope image showing inkjet printed silver nanoparticle dots inside and outside the frame pattern. (b) A, B and C are the profiles of dots as circled in (a), and D denotes the profile of an original dot without regulation. (c) The 3D profiles of A, B, C, and D dot. 3.1.2 The Profiles of Inkjet Printed Lines. The profiles of inkjet printed lines (line width of 50 μm in this experiment) can be also regulated by adding extra pattern (Figure 1b).The printed


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result is shown in Figure 3a, three lines (A, B, C) with different locations are circled and their corresponding profiles are demonstrated in Figure 3b. The profiles of line A, B, and C gradually turns from convex to concave when the distance between the line and the frame pattern increases. Figure 3c shows the double-humped surface profile of a wide line (150 μm) caused by the coffeering effect. When a parallel fine line (50 μm) was printed near to the wide line at a distance of approximately 125 μm, the surface profile of the wide line changes (Figure 3d), the height of bump adjacent to narrow line is lower than that of another bump. This phenomenon indicates the affected area and the regulation sensitivity of this technique.

Figure 3. The effect of the added extra pattern on the surface profiles of printed lines. (a) The optical microscopic image of the frame pattern and printed lines inside and outside of the frame pattern. (b) 3D profiles of the line A, B, and C as circled in (a). (c) The profile of a printed line with an average 150 μm width. (d) A fine line (50 μm) was printed near the wide line (150 μm) to control its profile, the line pitch is 125 μm.


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3.2 Analytical Calculations for the Model of the Surface Profile Regulated by the Drying Micro-Environment. The influence of the added extra patterns on the surface profiles of inkjet printed dots and lines is comparable to most of the coffee-ring suppressing methods. The mechanism of the regulation process was considered then as the varying solvent vapor distribution of different printed patterns, which was called the drying micro-environment in the paper. The analytical calculation followed by based on the simplest case of vapor diffusion aims to demonstrate the rationality of the proposed mechanism. 3.2.1 Drying Processes of Inkjet Printed Patterns with Different Area and Location. As shown in Figure 4a, two types of dots including single or double ink droplets were printed on glass substrate and their mean surface areas are approximately 2530 μm2 and 4300 μm2, respectively. As the drying time reached 20 min, some printed dots at the edges exhibit a shiny and silver appearance due the fully evaporation of organic solvent. In comparison to dots consisting of double ink droplets, printed dots with single ink droplet possess a shorter drying time as a whole (Figure 4b. The evaporation count time started when the first white dot was observed, and the dried dots with larger area kept a lower number ratio during the whole evaporation process. Birdi32 et al demonstrated that the larger the surface area of printed dots, the longer the drying time. The evaporation rate of a sessile droplet is linearly proportional to the radius of the liquidsolid interface. When two droplets merge, the volume of the dots increases by two times, which is larger than the increased ratio of the evaporation rate. Thus, the printed patterns of a larger surface should have a longer drying time than the pattern with a relatively small surface area. However, in this study, an opposite phenomenon was observed, in that the printed dots with two ink droplets demonstrated a faster drying speed than dots with single ink droplet.


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To explain this abnormal phenomenon, the single ink droplet dots array was also printed on glass substrate with a lateral and vertical drop space of 200 μm, respectively (Figure 4c). The area of each printed dot in the array was measured by optical microscope (Figure S1). The printed dots located at the edge of the array demonstrated shortest drying time, while the printed dots in the center of the array showed longest drying time. The increased drying time for the dots from edge to center of the array was further verified by 3D color map surface in Figure 4d. The map indicates a position-dependent solvent evaporation rate of printed dots as described above. We assume this phenomenon attributed to different drying micro-environments of dots in different position.

Figure 4. (a) The visual appearance of printed dots consisting of one or two droplets dried at ambient environment for 20 minutes. (b) The number ratio of the dried dots to wet dots increased as the increase of evaporation time. (c) The visual appearance of printed dots only made of single ink droplet dried at ambient environment for 5 minutes, (d) The drying time of the dots in (c) as a function of their position.


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3.2.2 Analytical Calculation of the Establishment of the Drying Micro-Environment. The influence of the drying micro-environment on profiles, stands in contrast to the profiles obtained by only changing the atmospheric environment30,

32-36

. The reason can be revealed by the

calculation based on the simplified vapor diffusion model (Supporting Information Part B), from which the micro-environment induced by a single line can be expressed as following. c r c  (i.e. c  r 0 ) c0  

(1)

Where 𝜌 is the distance from the center of the line, r is half width of the line, c is the vapor concentration, c0 is the vapor concentration when ρ = r.

Figure 5. The calculated solvent vapor concentration between wet lines A and B in x direction, a printed line with 2w in width lies at the center of the two lines. z(c) denotes the diffused solvent vapor concentration from the two lines.


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As shown in Figure 5, the solvent vapor concentration of lines A and B, represented by z(c), is calculated using equation (1), and the results are expressed by the red dash curves in the Figure. The drying micro-environment of a line with width of 2w printed at the center of A and B, can be calculated by simply add the vapor concentration diffused from A and B together, if the vapor distribution rebalancing (from wet lines A and B) was ignored. The combined vapor concentration can be expressed as follows:

c  c  cd   

drc0  d   

(2)

where d is the distance of lines A and B, and the result presented in Figure 5 is solid curve in red. To further explore the effect of the drying micro-environment on the profiles of the printed line at the center of the two lines A and B, the vapor concentration at the center cm and edge ce of line was calculated, respectively

cm 

4rc0 , ce  4rdc0 d d 2  w2

(3)

Thus the vapor concentration difference Δc between the edge and the center of the line is

c  ce  cm 

4drc0 4rc0 4rc0 w2   d 2  w2 d d (d 2  w2 )

(4)

The evaporation rate JD depends upon the vapor concentration near the surface csurf., and can be expressed by

JD 

c

surf

 c  D w

(5)

Accordingly, the evaporation rate difference ΔJD between the edge and the center of the line can be expressed as follows:


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J D  

cD w

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(6)

Consequently, the following equations can be obtained: J D  

cD w

J D 4c0 wD  0 r d ( d 2  w2 ) J D 4rc0 wD(3d 2  w2 )  0 d (d 3  dw2 )2 J D 8rc Dd 3  3 0 2 2 0 w (d  dw )

(7)

(8)

(9)

(10)

Particles are initially driven toward the edge due to the outwards capillary force induced by the evaporation rate difference. As a result of equation (8) to (10), the coffee-ring effect can be suppressed by decreasing d as well as increasing r or w. 3.3.3 Model of Different Profiles Regulated by the Drying Micro-Environment. The silver ink exhibits a slow evaporation rate in ambient environment due to the high boiling point of the solvent, which would be even deteriorate when adjacent patterns exist. The surface temperature gradients become so small that the thermal Marangoni convection is negligible because there is adequate time for heat transfer. Moreover, the Marangoni force induced by surfactants is always opposite to the evaporation induced capillary force. Thus only the capillary force is considered in our proposed model, aiming to clearly express the effect of the micro-environment on the profiles of printed patterns. The Schematic of printed lines regulated and without regulated by the drying microenvironment is shown in Figure 6a. Figure 6b shows the surface profile of printed line dried without regulation, silver nanoparticles migrate along with the outward capillary force induced


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by the larger evaporation rate of edge of the line. The coffee-ring effect can be suppressed by rationally regulating the drying micro-environment, according to the calculated results above. Consequently, concave, flat, and convex profiles of printed lines can be obtained, and vary with the parameters r, d and w, which are capable of changing the evaporation rate difference ΔJD along the surface. Dash curves in black represent the profiles of lines controlled by the drying micro-environment, while the dash curves in blue is the profiles of lines dried in ambient atmosphere. Arrows in blue lines represent initial evaporation rate and arrows in black lines denote the decreased evaporation rate induced by the drying micro-environment. With the decrease of d, and the increase of r or w, the difference in vapor concentration (∆c) between the edge and the center of the line increases, which accordingly leads to the decrease of difference in the evaporation rate (∆JD). Silver nanoparticles, therefore, will migrate from a low evaporation rate area to a high evaporation rate area driven by capillary flow. Thus, the coffee-ring effect happens in Figure 6a and 6c, when ∆JD>0; the uniform profile is obtained in Figure 6d, when ∆JD=0; and the convex profile can be also achieved when ∆JD

Homogeneous Surface Profiles of Inkjet-Printed Silver Nanoparticle

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