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Sep 4, 2013 - In addition, salt crystals were used as a master to prepare a replica mold ... optical microscopy (OM) (Olympus BX51), atomic force micr...
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Two-Dimensional Micropatterns via Crystal Growth of Na2CO3 for Fabrication of Transparent Electrodes Dong-Eun Lee, SeungJae Go, GyungSeok Hwang, Byung Doo Chin, and Dong Hyun Lee* Department of Polymer Science and Engineering, Dankook University, 152 Jukjeon-ro, Suji-gu, Yongin-si, Gyeonggi-do 448-701, Korea S Supporting Information *

ABSTRACT: The simple and versatile method to generate two-dimensional micropatterns by controlling precisely crystallization of sodium carbonate (Na2CO3) was investigated. Dense clusters of dendrites of salt crystals were homogeneously formed in a large area with an aqueous solution of Na2CO3 during evaporation of water. The dimensions and morphologies of dendritic salt crystals were tuned by changing the growth conditions such as salt concentration, relative humidity, and temperature. Then, 2D micropatterns of salt crystals were directly used as a mask for the deposition of a silver (Ag) layer to fabricate transparent electrodes. After salt crystals were completely dissolved in water, the network of an electrically conductive Ag layer, whose patterns were reversely produced from salt crystals, was generated on glass substrates. In addition, salt crystals were used as a master to prepare a replica mold of poly(dimethylsiloxane) (PDMS) for utilizing the imprinting technique. By imprinting a flexible PDMS mold with Ag inks, Ag micropatterns that were perfectly identical to dendrites of salt crystals were transferred to the other substrate.

1. INTRODUCTION Methods to create patterns at surfaces have many applications in microelectronics, micro-optics, biochip, surface modification, information storage media, and catalysts.1−8 There are two main strategies for the fabrication of patterned surfaces. The top-down approach uses traditionally lithographic techniques, a flow of a multistep process involving exposure of light, development by removing selectively a photoresist layer, and writing by etching inorganic crystals.1 In contrast, the bottomup approach adds selectively atoms or molecules to specific sites and uses self-organization from nature to create patterns.9−13 Self-organization, which is spontaneous and reversible transformation from disorganized to organized states, can produce a variety of well-defined patterns at surfaces. Crystallization is the most typical self-organization in nature and immediately occurs as solid crystals precipitate from a homogeneous solution.12−18 Crystallization can provide numerous well-defined patterns ranging from nanometer to some hundreds of micrometers scale in large area. In addition, the patterns and morphologies also can be finely tailored by controlling growth conditions. Even though micropatterning using crystals could be only realized in a limited region due to the difficulty to generate large-scale patterns, there are some efforts to utilize crystal patterns directly for fabricating microdevices and creating artificial features at surfaces.19−23 Recently, a novel approach merging patterning techniques with conducting materials to pursue intrinsic conductivity of materials and high transparency simultaneously was introduced.22−24 In this case, because only patterned surface imposes enough conductivity to the substrate © 2013 American Chemical Society

while pattern-free surface maintains its transparency, transparent electrodes can be effectively produced. As a consequence, the combination between 2D micropatterns of crystals and conducting materials would be able to offer a potential approach for transparent electrodes. Herein, we describe a bottom-up method to fabricate 2D micropatterns from self-organization of sodium carbonate (Na2CO3) molecules for applying transparent electrodes. As the crystal growth of sodium carbonate was precisely controlled by adjusting growth conditions, its branched (dendritic) crystal structures were uniformly generated on the whole surface of substrates during evaporation of water. These crystal structures were directly utilized as a mask for the deposition of a silver layer. After silver inks were spin-coated, crystal structures were completely removed by rinsing with water due to their high solubility while a silver layer deposited on crystal-free region was remained. Then, unique networks of the silver layer were successfully fabricated and characterized for transparent electrodes. Furthermore, these 2D micropatterns of salt crystals were used as a master for replication with poly(dimethylsiloxane) (PDMS). The flexible replica was utilized for imprinting technique with silver inks.

2. EXPERIMENTAL SECTION Preparation of Salt Crystals. Sodium carbonate (Na2CO3) was purchased from Samchun Chemical and directly used without further Received: July 15, 2013 Revised: August 31, 2013 Published: September 4, 2013 12259

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Figure 1. SEM images of crystal patterns of sodium carbonate grown from 0.07 wt % aqueous solution at 25 °C and different humidity [(a) 35%, (b) 50%, (c) 65%, and (d) 80%]. The average diameters of crystal patterns of (a), (b), (c), and (d) are 7 μm, 180 μm, 430 μm, and 2 mm, respectively. Fabrication of Polymeric Replica. Poly(dimethylsiloxane) (PDMS, Sylgard 184) was used to replicate 2D micropatterns of salt crystals. The mixture of PDMS resin and curing agent was solutioncast on top of salt crystals and annealed at 60 °C for 3 h. After being cooled to room temperature, the PDMS mold was peeled off.

purification. The certain amount of sodium carbonate was added in purified water to prepare dilute aqueous solutions (0.05−0.3 wt %). Glass plates were purchased from G.S.P Corporation. Before their use, all substrates were cleaned with the mixing solvent of acetone and IPA under ultrasonication for 40 min followed by oxygen (O2) plasma treatment for 10 min to remove any contamination on their surface. 20 μL of sodium carbonate solution with various concentrations was introduced onto the surface of the 1 in. × 1 in. substrate. The crystallization was conducted in a chamber (LAB HOUSE THC-P150) that could precisely control temperature and relative humidity during the experiments. The salt crystal patterns were generated over the whole surface area by evaporation of water from the surface of aqueous salt solution. To characterize crystal patterns of sodium carbonate, optical microscopy (OM) (Olympus BX51), atomic force microscopy (AFM) (Veeco Instruments MMAFM-2/1441EX), and scanning electron microscopy (SEM) (Hitachi S-4800) were utilized. Deposition of a Silver Layer. A silver (Ag) layer was deposited on the crystal patterns of sodium carbonate by the spin-coating method with silver (Ag) inks in which Ag nanoparticles (diameter ∼50 nm) were dispersed in triethylene glycol monoethyl ether at the rate of 2000− 6000 rpm. After the Ag layer was deposited, the specimens were placed on a hot stage for its sintering. Then, samples were immersed into purified water to remove crystals of sodium carbonate. After this process, the Ag layer prepared on the bare glass surface only remained while the Ag layer on crystal patterns were lifted off. Characterization of Transparent Electrodes. For measuring electrical properties of the samples, salt crystals were first prepared on the glass substrates having two ITO electrodes on their both edges. ITO glass which was protected by Kapton tape at both ends (1 cm × 1 cm for each) was immersed in an ITO etchant solution (MA-SO2, DONGWOO FINE-CHEM) at 40 °C. After 3 min, the ITO glass was taken out from the etchant and washed several times with DI water. Then, by releasing the tapes, two electrodes having ITO coated surface at both end edges were prepared. Electrical conductivity of transparent electrodes was measured by a probe station (MS-TECH MST8000C). In addition, optical transparency of electrodes was measured by a UV/vis/NIR spectrometer (PerkinElmer Lambda 950).

3. RESULTS AND DISCUSSION Figure 1 displays SEM images of crystal patterns of sodium carbonate (Na2CO3) produced from different relative humidity at constant concentration (0.07 wt %) and temperature (25 °C) in a chamber. The sufficient amount of Na2CO3 solution was introduced onto the surface until it was completely wet. It is noted that all crystal branches of sodium carbonate exhibited in Figure 1 were grown radially from the center of one crystal structure until these branches approached to adjacent crystal structures which were generated from different centers. These crystal structures, which already existed in aqueous solution, could be formed immediately once the evaporation of water occurred.12,14 Namely, it is found that the crystallization of sodium carbonate encountered the typical mechanism of nucleation and growth during its solidification process. It is also observed that dendrites of salt crystals formed from these four different conditions exhibited various dimensions in width and height of their branches. So, these SEM images indicate that crystal patterns of Na2CO3 can be tuned by the growth condition in which they form. For example, Figure 1a shows that the average diameter of the clusters of Na2CO3 dendrites was 7 μm even though the crystals grew in the same manner that was described previously. Many branches of salt crystals were extended from the center of clusters to their boundaries. Interestingly, pin-like short branches of salt crystals were also formed densely along the main branches grown from the center of the cluster. With relative humidity, the dimensions of dendrites of Na2CO3 were dramatically increased as shown in Figures 1b and 1c while their 12260

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Figure 2. SEM images of crystal patterns of sodium carbonate grown from aqueous solutions with various concentrations at constant temperature and relative humidity (25 °C and 65 RH%) [(a) 0.05, (b) 0.07, (c) 0.1, (d) 0.15, (e) 0.2, and (f) 0.3 wt %]. The scale bar of (a) is 100 μm, and the rest are 500 μm. All scale bars of insets are 20 μm.

typical branched shapes were still maintained. The average diameter of crystal clusters reached about 430 μm in Figure 1c. It is noted that crystal structures of sodium carbonate shown in Figure 1d, which were grown at the relative humidity of 80%, are clearly distinct with three samples introduced previously. The clusters of these salt crystals display around 2 mm of diameter while the number of their branches was rather reduced as compared to that of others. When the solution of sodium carbonate is exposed to higher relative humidity, the evaporation rate of water becomes slower so that salt molecules can experience enough time to diffuse and self-organize for crystallization. Therefore, with relative humidity, the sizes of crystal clusters were dramatically increased from 7 μm to larger than 2 mm in diameter as shown in Figure 1 even though their typical dendritic shapes were still maintained. The effect of salt concentration on the dimension of salt crystals was also investigated. Figure 2 exhibits various crystal patterns of sodium carbonate formed at different concentrations of its aqueous solutions varied from 0.07 to 0.3 wt % under controlled condition (65 RH% and 25 °C). According to the results of these SEM experiments, it was found that the

Figure 3. Plot of height and width of its crystal patterns versus its concentration. Insets are SEM images of cross-sectional view and top view of crystal patterns of sodium carbonate. Scale bars are 10 μm.

overall feature of Na2CO3 crystals was still typical branched patterns even though their dimensions were strongly dependent 12261

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Figure 4. SEM images of crystal patterns of Na2CO3 (a, b) and SEM image of (c) Ag layer on crystal patterns and (d) networks of Ag layer after liftoff the crystal patterns. OM (reflection mode) image of networks of an Ag layer generated from two different conditions [(e) 0.15 wt %/25 °C/65 RH% and (f) 0.28 wt %/25 °C/50 RH%] on glass substrates.

ratios between patterned area and pattern-free area of samples were obtained and are shown in the Supporting Information. Because OTS monolayer were only prepared on pattern-free area, we found that the area of OTS monolayer was reduced with the size of salt crystals. In addition, systematic study on the crystallography of sodium carbonate may help a better understanding of the underling crystal growth mechanism and can be suggested as a future direction. From our results, we proposed that dendritic crystal patterns with various heights and widths could take some advantages to prepare electrically conducting patterns for the transparent electrode. Transparent electrodes require both high electrical conductivity arisen from a uniform electrical potential over the surface of substrates and high optical transparency. To enhance the conductivity of electrodes without the fall of its transmittance, micropatterning of conducting materials like gold, silver, graphene, and carbon nanotube, etc., has been applied. Our concept tuning the areas occupied by salt crystals in this study would offer accordingly the unique and versatile method to fabricate transparent electrodes. Scheme 1 illustrates fabrication of transparent electrodes using 2D micropatterns

on the concentration of their aqueous solutions. The effect of salt concentration on the dimension of 2D micropatterns in crystal clusters obtained in Figure 2 was quantitatively investigated by using both AFM and SEM. According to the image analysis of Na2CO3 crystal patterns, the heights of 2D micropatterns of salt crystals were varied from 150 nm to 1.3 μm while their widths were laid on from 2 to 80 μm as shown in Figure 3. Cross-sectional and top views of the dendritic crystal patterns of sodium carbonate grown at 25 °C, 65 RH%, and 0.1 wt % are exhibited in the insets of Figure 3, respectively. These observations are clearly acceptable because the total amount of sodium carbonate deposited on the surface of substrate after evaporation of water was increased with its concentration in aqueous solutions. Moreover, this result also points out that the area covered with dendritic 2D micropatterns of salt crystals can effectively be regulated by means of this method. To confirm the control of the area covered with salt crystals, the contact angle measurement of water drops on samples, which had 2D micropatterns of salt crystals with different dimension, treated with octadecyltrichlorosilane (OTS) monolayer was conducted (Figures S1 and S2). The relative 12262

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Scheme 1. Schematic Illustration of Fabrication of Transparent Electrodes Using Salt Crystal Patterns of Sodium Carbonate (Na2CO3): (a) Na2CO3 Solution Coating, (b) Crystallization of Na2CO3, (c) Spin-Coating of Silver (Ag) Inks on Crystal Patterns of Na2CO3, and (d) Rinsing Crystal Patterns with Water, Respectively

of salt crystals as a mask to produce a conducting layer of silver. At first, the solution of sodium carbonate was applied onto glass substrates (Scheme 1a). To generate branched crystal patterns having desired dimensions, the samples were placed in the chamber maintaining controlled relative humidity and temperature (Scheme 1b). After evaporation of water, silver inks were spin-coated on the substrate with crystal patterns of sodium carbonate grown in the previous step and thermally treated for sintering of silver as shown in Scheme 1c. Then, the samples were rinsed with water to remove salt crystals completely. Finally, networks of a silver layer were fabricated on the whole surface area (Scheme 1d). In addition, it is necessary to claim that branched patterns of sodium carbonate obtained here can also play the role of a mask for the vacuum deposition system of metal layers instead of the solution process of metallic inks (Figure S3). The SEM images in Figures 4a and 4b display crystal patterns of sodium carbonate grown on glass substrate at the growth condition of 0.25 wt %, 25 °C, and 50 RH%. It was observed that boundary of clusters of salt crystals became empty (or porous) and the branches inside clusters were densely packed. Then silver inks, silver nanoparticles dispersed in organic solvent which is poor solvent to salt crystals, were spin-coated on this sample. As the organic solvent of Ag ink was evaporated by heating, silver nanoparticles could contact each other so that form Ag layer through whole areas. As the result, porous areas, which were the boundaries between two different clusters of salt crystals, were filled with Ag inks (Figure 4c). The gap size of porous areas was about 2.75 μm. After lift-off of crystals with water due to their tremendous solubility, silver layers located in the boundaries were only remained as exhibited in Figure 4d and could be regarded as an electrical path for transparent electrodes. As displayed in Figures 4e and 4f (growth conditions: (e) 0.15 wt %/25 °C/65 RH% and (f) 0.28 wt %/25 °C/50 RH%), uniform networks of a silver layer with about 60 nm of thickness were successfully generated on glass substrates. Because reflection mode in OM was utilized to investigate the samples, it was observed that the silver layers became bright regions while bare glass surfaces were relatively dark regions. In Figure 4e, complex 2D micropatterns of an Ag

Figure 5. (a) Photographs and (b) plots of transmittance of (1) single Ag layer, (2) Ag layer on crystal patterns of sodium carbonate, and (3) networks of Ag layer. (c) Current−voltage (I−V) curve of networks of the Ag layer.

layer were obtained from branched crystal patterns in Figure 2d. In contrast, networks of an Ag layer in Figure 4f were produced from the salt crystals having closely packed branches shown in Figure 4a. Because these crystal structures could not provide sufficient empty space, Ag inks could only penetrate into the boundary between crystal clusters. Therefore, networks of thin silver layer can provide sufficient electrical conductivity as well as optical transparency to the substrates. Figure 5a shows photographs of three different samples [(1) a single Ag layer on bare glass, (2) an Ag layer on crystal patterns, and (3) networks of an Ag layer on glass substrates]. By comparing these photographs, it was found that the samples in Figures 5a-(1) and 5a-(2) were distinctly opaque while the Ag networks on glass substrate in Figure 5a-(3) became dramatically transparent after washing salt crystals with water. 12263

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Figure 6. Photographs of (a) salt crystals (master), (b) PDMS replica, (c) PDMS replica filled with Ag inks, and (d) printed Ag patterns on the glass substrate. Inset is SEM image of PDMS replica. Scale bar is 100 μm.

PDMS, which was identical to salt crystals of Figure 6a, were successfully generated as seen in Figure 6b. The enlarged SEM image of negative 2D micropatterns of PDMS mold was seen in the inset of Figure 6b. Then, Ag inks were filled into porous areas of PDMS mold. To remove excess Ag inks from the surface of PDMS mold, the edge of a tilted glass plate was carefully dragged through it (Figure 6c). After that, this flexible mold was contacted to the surface of the glass substrate to transfer Ag inks by imprinting and heated up to 150 °C for sintering of Ag inks. Finally, 2D micropatterns of Ag layer were produced on glass substrate as exhibited in Figure 6d.

Optical transparency of these samples was also measured by a UV/vis/NIR spectrometer. Figure 5b shows transmittance data on three samples obtained from Figure 5a. The transmittance of the sample having networks of an Ag layer interestingly reached around 87% within the experimental range of wavelength as exhibited in the curve (3) of Figure 5b while those of other two samples were less than about 40%. This result is in good agreement with our observation in Figure 5a. To investigate the dependence of transparency on crystal size of sodium carbonate, transmittance of samples obtained from Figures 4e and 4f is given in Figure S4. It was found that Ag networks only formed on the void spaces of boundary between two dendritic crystals could give rise to sufficient transmittance (>80%) for transparent electrodes.25 Furthermore, to investigate electrical property of Ag networks prepared on glass substrates, the relationship between current (I) flowing through Ag networks and applied voltage (V) was measured. For measuring electrical property of the sample, salt crystals were first prepared on the glass substrates having two ITO electrodes on their both edges. Electrical conductivity of transparent electrodes was measured by a probe station. Figure 5c displays room temperature current−voltage (I−V) curves of the sample having the networks of an Ag layer on the surface obtained by rinsing salt crystals with water. This plot in I−V curve clearly exhibits a linear relationship, which indicates that the networks of Ag layers on the glass substrate show remarkably ohmic contact even though the conductivity of Ag networks is relatively lower than that of the single Ag layer (Figure S5). Therefore, it is confirmed that networks of an Ag layer sufficiently can be utilized for fabricating transparent electrodes. The crystals of Na2CO3 were then employed as a master to fabricate the soft mold of poly(dimethylsiloxane) (PDMS). The mixture of PDMS resin and curing agent was poured to the salt crystals on the glass substrates shown in Figure 6a and annealed at 60 °C for 3 h. After its release, negative 2D micropatterns on

4. CONCLUSION In summary, crystallization of sodium carbonate from its aqueous solution was controlled by changing growth conditions. The heights of crystal patterns were varied from 150 nm to 1.3 μm while their widths were laid on from 2 to 80 μm according to our microscopic investigations. Moreover, 2D micropatterns of salt crystals obtained in this study were utilized as a mask to fabricate transparent electrodes. Silver (Ag) inks were deposited on crystal patterns of sodium carbonate formed on glass substrates by the spin-coating method. By rinsing salt crystals with water, Ag layers on the bare surface of glass substrate remained while all crystal patterns were dissolved. Networks of the remaining Ag layers not only enhanced the transmittance (about 87%) but also gave rise to significant electrical conductivity for transparent electrodes. The crystals of Na2CO3 were then employed as a master to fabricate the soft mold of poly(dimethylsiloxane) (PDMS). By the imprinting technique, 2D micropatterns of Ag layers were successfully transferred to the surface of glass substrates.

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ASSOCIATED CONTENT

S Supporting Information *

Details of surface modification of samples with 2D micropatterns of salt crystals, contact angle measurement of water 12264

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drops, and the thermal deposition of silver layer. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Ph +82-31-8005-3589; Fax +82-31-8021-7218 (D.H.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the research fund of Dankook University in 2011. REFERENCES

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