Spontaneous Formation of Two-Dimensional Micropatterns with

Article pubs.acs.org/crystal

Spontaneous Formation of Two-Dimensional Micropatterns with Straight and/or Curving Dendrites through Crystal Growth of Ba(NO3)2 in Polymer Matrix Daijiro Tokutomi, Ryuta Ise, Yuya Oaki, and Hiroaki Imai* Department of Applied Chemistry, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan S Supporting Information *

ABSTRACT: Microscale patterns consisting of two-dimensional (2D) dendrites with trunks and branches 1−3 μm wide were precisely controlled through crystal growth in a thin polymer matrix by the use of a dipping technique. A variety of 2D micropatterns, such as orthogonal lattices, bull’s horns, randomly curving weaves, and aligned dots, were homogeneously formed with an aqueous solution of Ba(NO3)2 and poly(vinyl alcohol) in a wide area ranging over several centimeters on flat and rounded substrates. The micrometric dendritic growth that produced these several patterns was tuned by changing the withdrawal rate and the polymer concentration. The crystallographic orientation of the micropatterns was characterized to discuss on the formation mechanism of the specific morphologies. The curving branches were found to be induced by gradual change in the growth direction through low-angle grain boundaries under a highly diffusion-limited condition. This simple, bottom-up patterning process is applicable for various crystalline materials, including inorganic and organic substances.

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INTRODUCTION Micrometer-scale two-dimensional (2D) patterning has attracted much attention in various academic and industrial fields, including the study of optics,1−5 semiconductors,6−8 surface modification,9,10 and drug delivery systems.11 Top-down technologies, including photolithography, have commonly been applied to fabricate finely designed patterning. In biological systems, various fascinating 2D patterns are spontaneously produced through self-assembly and selforganization on the surfaces of plants,12−14 insects,15−17 and fish18−20 Recently, spontaneous formation of micropatterns, using bioinspired bottom-up processes, has been studied as a low-cost and simple alternative method.21−30 A wide variety of patterns and morphologies can be observed on crystals. Therefore, crystal growth is regarded as an important phenomenon for pattern formation. In general, the patterns and morphologies of crystals vary with growth conditions. The regular polyhedral shape originating from the crystallographic symmetry of the unit cell is formed through kinetic-limited growth, under near-equilibrium conditions. Particular branching morphologies are formed with crystals when growth proceeds under diffusion-limited conditions, © XXXX American Chemical Society

while increasing the driving force or restricting the solute diffusion. Single-crystalline dendrites and isotropic polycrystalline dendrites are obtained under conditions close to equilibrium and far from equilibrium, respectively. The morphological evolution of crystals emerges in organic polymer matrices, such as gels and viscous solutions.31−34 As the polymer density increases, the polyhedral single crystal changes into randomly winding morphologies through regularly branching dendrites. As a consequence, the controlled crystal growth has potential as a bottom-up technique for pattern formation. Various kinds of dendritic patterns were reported with metals and inorganic crystals under diffusion-limited conditions.31−43 However, controllability of the patterns and application for wide-area fabrication have not been achieved by the conventional crystal-growth techniques. In the present article, we report a bottom-up method for producing various micropatterns by dip-coating solutions of dissolved crystal and polymeric species. Here, we achieved Received: March 21, 2013 Revised: April 27, 2013

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dx.doi.org/10.1021/cg400418n | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

micrometric 2D patterning of Ba(NO3)2 in a thin film of poly(vinyl alcohol) (PVA) over entire areas of flat and rounded substrates, in a range of several centimeters, whereas the influence of the polymer matrix on the crystal morphology has been reported in our previous work.31 A wide variety of 2D morphologies, including orthogonal lattices, bull’s horns, randomly curving weaves, and aligned dots were precisely controlled through dendritic and/or curving crystal growth with various parameters, such as the polymer concentration and the withdrawal rate. Moreover, replication of the micropatterns with a resin is achieved for application of the self-organized architectures. The characteristic patterns and the formation mechanism of the dendrites are discussed on the basis of a detailed analysis of crystallographic orientation in the branching and curving morphologies. These results are highly helpful for understanding the formation mechanism and further control of the bottom-up micropatterning through crystal growth.

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EXPERIMENTAL SECTION

Ba(NO3)2 (Wako Pure Chemical) and poly(vinyl alcohol) (PVA) (Junsei Chemical, Mw = 22000) were used without further purification. Certain amounts of Ba(NO3)2 (1.0−9.0 g, CBN) and PVA (1.0−9.0 g, CPVA) were dissolved in 100 g of purified water at room temperature. Glass slides washed with acetone and purified water were dip-coated with the solution at a withdrawal rate of 1.0−3.0 mm/min. A vessel of the solution was covered with an outer shield to prevent rapid evaporation of water (Figure S1 of the Supporting Information). The coating was performed at 25 °C and at a humidity of ca. 50% in an airconditioned room without airflow. As water evaporated from the substrate with distance from the solution in the shield, crystal growth then proceeded on the glass plate. The crystal morphology was observed with a scanning electron microscope (SEM, Keyence VE-9800) and optical microscopes (Olympus BX51-FL and Keyence VK-9710). Structural analysis for the products was performed using an X-ray diffractometer (XRD, Rigaku MiniFlex II) with Cu Kα radiation and a field-emission transmission electron microscope (FE-TEM, Philips TECNAI-F20). Crystals grown on the glass slides were directly analyzed with XRD by using the θ−2θ scanning method. TEM observation was performed using dendrites grown on a collodion film pasted on a Cu grid by dip coating.

Figure 1. Morphological variations of the Ba(NO3)2 dendritic patterns formed at CBN = 7.0 g/100 g of water with the change of CPVA and withdrawal rate.

straight trunks and curving branches (bull’s horn) (Figure 2, panels g and h), and randomly winding weaves consisting of curving branches (Figure 2, panels i and j). The trunks in all the patterns were grown from the initial line of the air−solution interface on the substrate in the dipping vessel. The orthogonal patterns covering the entire substrate are basically composed of straight trunks and branches parallel and perpendicular to the withdrawing direction, respectively. Similar gridlike morphologies were partially observed through dendritic crystal growth.44,45 However, the patterns shown in the present study are regarded as a 2D single crystal spreading over a range of several centimeters. Increasing the length of the substrate can fundamentally enlarge the size of the single-crystalline orthogonal patterns. The bending trunks were observed in a domain of the orthogonal pattern with a decrease in the withdrawal rate (Figure 2c). The multidomain orthogonal pattern divided by major bending parts was obtained with the change of the growth direction of the branches (Figure 2, panels d−f). The curving branches increased with a decrease in the PVA concentration (Figure 2, panels g and h). Randomly winding weaves consisting of curving branches were formed at relatively low PVA concentrations and low withdrawal rates. The isotropic aspect of the patterns increased with an increase of the bending and curving parts by decreasing the withdrawal rate. The thickness of the trunks and branches, depending on the PVA concentration and the withdrawal rate, was in a range from 100 to 500 nm. As shown in Figure 3 and Table 1, the average width of the trunks and branches in the patterns decreased with decreasing CBN. As reported in the present work, we succeeded in achieving spontaneous 2D patterning on the glass substrate through dendritic crystal growth, with variation in the morphology and size. Similar morphological variation is observed with other polymeric species, such as gelatin. The crystal habit is not influenced by the presence of PVA. Thus, we conclude that PVA limits the ion diffusion and increases the film thickness without a specific chemical interaction with Ba(NO3)2. Crystallinity. Figure 4 shows XRD profiles for five typical patterns. Only {h00} peaks of Ba(NO3)2 were observed for all

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RESULTS AND DISCUSSION Morphological Variations of Ba(NO3)2 Crystals in PVA Matrix. We observed a well-defined front of drying and crystallization around the upper rim of the outer shield. The patterns were homogeneously formed through a downward progression of the drying front over entire areas of the substrates, except for narrow regions near the side and bottom edges. Various dendritic patterns consisting of trunks and branches 1−3 μm wide were formed at CBN = 5.0−9.0 g/100 g of water (Figures 1 and 2). On the other hand, fine particles 1− 2 μm in diameter formed on the glass substrate as shown in Figure S2 of the Supporting Information when CBN was lower than 5.0 g/100 g of water. The solute was not dissolved at CBN higher than 10.0 g/100 g of water. Figure 1 shows the morphology of crystals grown on the substrate in the polymer matrix at CBN = 7.0 g/100 g of water. The micropatterns that depend on the concentration of PVA and the withdrawal rate were classified into five categories: an orthogonal pattern consisting of straight trunks and branches (Figure 2, panels a and b), an orthogonal pattern consisting of straight and bending trunks and branches (Figure 2c), a multidomain orthogonal pattern consisting of straight and bending branches (Figure 2, panels d−f), an orthogonal pattern consisting of B



Article

Figure 2. Morphological variations of the Ba(NO3)2 dendritic patterns at CBN = 7.0. (a, d, g, and i) Schematic illustration of the pattens throughout the substrate: (a) orthogonal pattern consisting of straight trunks and branches, (d) multidomain orthogonal pattern consisting of straight and bending branches, (g) orthogonal pattern consisting of straight trunks and curving branches, and (i) randomly winding weaves consisting of curving branches. The blue ellipses show the nucleation region. Optical microscope images of the patterns (b, c, e, f, h, and j): (b) CPVA = 7.0 and v = 3.0, (c) CPVA = 7.0 and v = 2.0, (e and f) CPVA = 7.0 and v = 1.0, (h) CPVA = 1.0 and v = 3.0, and (j) CPVA = 1.0 and v = 1.0. CBN and CPVA indicate the initial concentrations (g/100 g of water) of Ba(NO3)2 and PVA, respectively. v indicates the withdrawal rate (mm/min). The black arrows show the withdrawal direction.

Table 1. Variation of the Average Width of Dendritic Patterns with CBN concentration (g/100 g of water) CBN

CPVA

morphology

width (μm)

5.0 7.0 9.0

5.0 5.0 5.0

orthogonal pattern consisting of straight trunks and branches

1.62 ± 0.19 2.77 ± 0.31 2.92 ± 0.53

3.0 7.0

1.0 1.0

orthogonal pattern consisting of straight trunks and curving branches

0.58 ± 0.08 1.26 ± 0.11

substrate, and their growth direction was ⟨100⟩. Thus, the facet observed on the top of the trunks and branches is clearly assigned to {100} (Figure 5d). Moreover, the side surfaces are assignable to {111}. The orthogonal patterns consisting of single-crystalline trunks and branches are regarded as a basic structure of the cubic crystal grown in the polymer matrix. Figure 6a shows TEM image and electron-diffraction patterns for curving branches in the bull’s horn pattern. Although the diffraction spots assigned to {200} are observed at four points in the branch, the ⟨100⟩ direction gradually changes along the curvature. Thus, the curving structure is formed by gradual change in the ⟨100⟩ direction through a low-angle grain boundary under a highly diffusion-limited condition, as illustrated in Figure 6b. The grain boundary was occasionally constricted as shown in Figure 6c. The constricted parts were observed even in the trunks formed with oscillation of the growth rate. Effects of Growth Conditions on Micropatterns. The micropatterns change from regularly branching orthogonal

Figure 3. Optical microscope images of the Ba(NO3)2 dendritic patterns at v = 3.0 mm/min with the change of CBN: (a) CBN = 5.0 and CPVA = 5.0, (b) CBN = 7.0 and CPVA = 5.0, (c) CBN = 9.0 and CPVA = 5.0, (d) CBN = 3.0 and CPVA = 1.0, and (e) CBN = 7.0 and CPVA = 1.0. CBN and CPVA indicate the initial concentrations (g/100 g of water) of Ba(NO3)2 and PVA, respectively. The black arrows show the withdrawal direction.

the patterns, except that no signal was detected from the randomly winding weaves. We observed the electron-diffraction spots assigned to {200} from the orthogonal pattern, as shown in Figure 5a. These results indicate that the {100} faces of the trunks and branches in the orthogonal dendrites (Figure 5, panels b and c) were arranged parallel to the surface of the C



Article

Figure 4. Typical XRD profiles for Ba(NO3)2 dendritic patterns. (a) Orthogonal pattern consisting of straight trunks and branches, (b) orthogonal pattern consisting of straight and bending trunks and branches, (c) multidomain orthogonal pattern consisting of straight and bending branches, (d) orthogonal pattern consisting of straight trunks and curving branches, and (e) randomly winding weaves consisting of curving branches.

Figure 6. Structural analysis of the Ba(NO3)2 curving branches. (a) TEM image and SAED patterns of the curving branches in the bull’s horn pattern. (b) Schematic illustrations of the structure of the curving branches. The red arrows show the growth direction. SEM images of the curving branches having constricted parts. (c) The black arrow shows the withdrawal direction.

lattices into randomly winding weaves with a decrease in the withdrawal rate and the PVA concentration. The randomness of the dendritic morphology commonly increases with an increase in the influence of the diffusion-limited reaction on the crystal growth. In previous works, an increase in the polymer concentration promoted the formation of disordered morphologies grown under diffusion-limited conditions.31−34 In the present work, the morphological variation can be ascribed to the drying rate of the solution layer coated on the substrate. Basically, a higher concentration is favorable for dendritic growth in a conventional drying system. On the other hand, a lower concentration is favorable for dendritic growth when the crystal patterns are produced by a dip-coating method. The thickness of the coated solution layer is proportional to the two-thirds power of the viscosity of the solution and the withdrawal rate.48 The drying rate increases with a decrease in the thickness of the solution layer. Because the viscosity is reduced by lowering the polymer content in the solution, the drying rate during the dip-coating process increases as the polymer concentration is decreased. (The viscosity of a 1 wt % PVA solution is about one-tenth of that at 8 wt %. The influence of the Ba(NO3)2 concentration on the viscosity is relatively low.) Thus, the bending or curving structures containing low-angle grain boundaries are formed under a highly diffusion-limited condition at a low PVA concentration. Although the thickness of the coated layer also decreases with a decreasing withdrawal rate, a change in the rate did not dominantly promote the formation of curving branches. The change in the withdrawal rate (1−3 mm/min) is smaller than that in the polymer concentration (1−9 g/100 g of water). Thus, the effect of the withdrawal rate is relatively small. On the

Figure 5. Structural analysis of the Ba(NO3)2 orthogonal pattern. (a) TEM image and SAED (selected-area electron diffraction) pattern of the orthogonal pattern. In the SAED pattern, the red circles, yellow rhombi, and green squares are assigned to the electron diffraction spots of {200}, {120}, and {220}, respectively. The diffraction spots of {100} and {210} are not observed because of the extinction rule for Ba(NO3)2 with the Pa3 space group.46,47 The diffraction spots of {h00} and {hk0} from the crystal lattice are absent when h and k are odd, respectively. SEM images of the orthogonal pattern from the (b) overhead and (c) oblique views. The black arrows show the withdrawal direction. (d) Schematic illustration for the facets of the trunks and branches. The red arrows show the growth direction.

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other hand, a decrease in the withdrawal rate increases the isotropy of the crystal growth in the micropatterns. In this system, we observed a well-defined front of drying around the upper rim of the outer shield. The patterns were basically formed through a downward progression of the crystallization front over entire areas of the substrate. However, when the withdrawal rate was relatively low, the substrate was kept in the shield for a longer period. In this case, the progression of the crystallization front with the delayed drying process was not limited downward. Consequently, the isotropic multidomain structure and the randomly winding morphology were formed with an occasional change in the growth direction of the dendrites. Formation of Isolated Patterns. As shown in Figure 7, we obtained the isolated patterns which were based on the

Figure 8. Optical microscope images of the Ba(NO3)2 orthogonal pattern on a glass tube.

insoluble epoxy resin. A mixture of an epoxy resin (Nissin Resin, Crystal Resin II) and its resin hardener was dropped on the micropatterns of Ba(NO3)2 formed on a glass slide. After being aged at 60 °C for 24 h, the hardened resin was peeled from the slide. We then obtained the replicated micropatterns on the resin, as shown in Figure 9. The negative patterns of a

Figure 7. SEM images of the Ba(NO3)2 isolated patterns. (a) The orthogonal pattern consisting of straight trunks and branches after being kept for 24 h under humid conditions and the isolated orthogonal pattern consisting of straight trunks and curving branches after (b) 1 h and (c) 24 h.

orthogonal dendrites and curving dendrites, by aging them in a closed case that contained water droplets. The branches and trunks of the orthogonal dendrites were separated through disassembly of the knags after being kept for 24 h under humid conditions (Figure 7a). The curving branches were changed into several rods 1−10 μm long by the disappearance of the constricted parts for 1 h (Figure 7b), and we finally observed dotted patterns consisting of 1−2 μm units after 24 h (Figure 7c). Ripening of the crystals that composed the trunks and branches occurred with dissolution of the constricted parts in a high-humidity environment. Formation of Micropatterns on a Rounded Surface. Patterning on the rounded surface by top-down technologies is generally difficult. We also dip-coated onto the substrate with a rounded surface as the first approach to applying our work to 3D patterning. The micropatterns of Ba(NO 3 ) 2 were successfully produced on a rounded surface. Figure 8 shows optical microscope images of the patterns formed on a glass tube 1.2 mm in diameter. An orthogonal pattern similar to that on a flat surface was observed. Because the grain boundary was not found in the trunks or branches, this pattern was regarded as a strained single crystal. The strain of the crystal lattice was estimated to be 0.083% from the curvature of the glass tube. Fine particles were produced on a glass tube with a diameter smaller than 500 μm. This means that the dendritic growth cannot be achieved when the strain of the crystal lattice is larger than a certain value. The details of the strain in the curving dendrites will be studied in future work. Replication of Micropatterns with an Epoxy Resin. The micropatterns are not stable in the air because Ba(NO3)2 is water soluble. Thus, we replicated the patterns with an

Figure 9. SEM images of the epoxy resins transferring the patterns. The orthogonal pattern consisting of straight trunks and branches from the (a) overhead view, (b) oblique view, and (c) the orthogonal pattern consisting of straight trunks and curving branches.

stable material were easily obtained by a simple technique. The depth of the trenches of the micropatterns was ca. 200 nm, which is comparable to the thickness of the dendritic crystal. Light Diffraction by Micropatterns As a Grating. The micropatterns perform as a light grating due to their micrometric periodic structure. Figure 10 shows the diffraction patterns of a semiconductor laser (λ = 650 nm) by the micropattern. The size and direction of the expanded beam are associated with the micrometric periodicity of the orthogonal crystal pattern. As listed in Table 2, the length of the diffracted beam increases as the periodicity of the branches decreases, in accordance with Bragg’s law. Application to Other Material Systems. As shown in Figure 11, the orthogonal micropatterns of NH4Cl were formed on a glass substrate by using the dip-coating method with an aqueous solution containing PVA as a polymeric agent. As shown in Figure 12, the orthogonal micropatterns of pyrene were formed on a glass substrate by using the dip-coating method with a xylene solution containing polystyrene (PS) as a polymeric agent. The morphology of these crystals changed E



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CONCLUSION We succeeded in the spontaneous fabrication of 2D micrometer-scale patterns consisting of trunks and branches in a range of several centimeters on a glass substrate by using the dip-coating technique. A wide variety of the micropatterns, such as orthogonal lattices, sinuous weaves, and aligned dots, were precisely tuned through crystal growth of Ba(NO3)2 in a thin matrix of poly(vinyl alcohol). A detailed analysis of crystallographic orientation in the branching and curving morphologies was performed for in order to understand the formation mechanism and for further control of the bottom-up micropatterning through crystal growth. Moreover, replication of the micropatterns with a resin was achieved for application of the self-organized architectures. The bottom-up patterning method can be applicable to various crystalline materials, including inorganic and organic substances.

Figure 10. Light diffraction by the Ba(NO3)2 orthogonal pattern as a grating. (a) Optical microscope image of the orthogonal pattern. (b) The pattern of light on the screen. (c) FFT (Fast Fourier Transform) image of the orthogonal pattern. The distances from the laser source to the sample and the screen were 18.5 and 37.0 cm, respectively.

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Table 2. Relationship between the Distance of Branches and the Length of Diffracted Beam distance of branches (μm)

length of diffracted beam (μm)

11.4 18.7 20.4

4.07 3.50 3.25

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ASSOCIATED CONTENT

S Supporting Information *

Schematic illustration of the dip-coating method with an outer shield. SEM images of the fine particles of Ba(NO3)2 formed at CBN = 3.0: CPVA = 3.0 and v = 3.0. 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]. Tel: +81 45 566 1556. Fax: +81 45 566 1551. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by a Grant-in-Aid for Challenging Exploratory Research (Grant 22656158) and a Grant-in-Aid for Scientific Research (Grant 22107010) on Innovative Areas of “Fusion Materials: Creative Development of Materials and Exploration of Their Function through Molecular Control” (Area no. 2206) from the Ministry of Education, Culture, Sports, Science and Technology and by Kato Foundation for the Promotion of Science (R.I.).

Figure 11. Optical microscope images for the orthogonal patterns of NH4Cl at CNC = 7.0: (a) CPVA = 7.0 and v = 3.0, (b) CPVA = 5.0 and v = 3.0, and (c) CPVA = 7.0 and v = 1.0. CNC and CPVA indicate the initial concentrations (g/100 g of water) of NH4Cl and PVA, respectively. v indicates the withdrawal rate (mm/min). The black arrows show the withdrawal direction.

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REFERENCES

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Figure 12. Optical microscope images for the orthogonal patterns of pyrene at CPy = 7.0: (a) CPS = 7.0 and v = 3.0, (b) CPS = 3.0 and v = 3.0, and (c) CPS = 3.0 and v = 1.0. CPy and CPS indicate the initial concentrations (g/100 g of water) of pyrene and PS, respectively. v indicates the withdrawal rate (mm/min). The black arrows show the withdrawal direction.

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Spontaneous Formation of Two-Dimensional Micropatterns with

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