생물질 모식공정을 이용한 나노구조제어

조규진, 장종관, 정성기, 문일식, 이재석,． 조영선．

순천대학교 공과대학 화학공학과, 전남 순천시 매곡동 315번지

．광주과학기술원 신소재공학과, 광주시 북구 오룡동 1번지

Nanostructure Control Through Biomimetic Process

Gyoujin Cho, Jongkwan Jang, Sungki Chong, Il-Shik Moon,

Jae-Suk Lee,． Young-Sun Cho,．

Department of Chemical Engineering  Sunchon National University, 315 Maegok Sunchon, Chonnam 540-742 Korea

．Department of Materials Science and Engineering, Kwangju Institute of Science and Technology (K-JIST), Kwangju 506-712 Korea

Introduction

We report a new method to control both the nucleation and the growth of SiO2 to form nanopatterns using a tenet biomineralization in nature. Preparing inorganic materials with 3-dimensional and nano-size periodic patterns is the essential steps in designing miniature devices.1 A simple micron-size periodic pattern can be prepared using conventional photolithography.2 On the other hand, complex materials with periodic patterns having sizes below 0.1□ cannot be simply constructed using conventional photolithography. Recently, an alternative approach to attain complex nanostructures has been developed by using microphase separated block copolymer as a template.3 The block copolymer can self-assemble to produce periodic nanodomains, such as lamellar, hexagonal, and spherical phases.4 These self-assembled structures with length scales of 10 - 100 nm are ideally suited to control the nucleation, growth, and morphology of inorganic materials,5 and are used for nanoscale patterning.3 To do this, regioselective nucleation is regulated by the difference in interfacial energy between the reaction medium and different nanodomains of the block copolymer.5 Usually, heterogeneous nucleation is favored for the nanodomains which have a lower interfacial energy. Although these methods were successful for two-dimensional nanopatterning, we found that the influence of the morphology of the block copolymer on the nanostructures becomes weaker as the structures build up, so that the boundaries between the patterns become blurred gradually as the thickness increases. To overcome this problem, we have developed a new method to promote the nucleation and growth of SiO2 regioselectively.

Experimentals

To regioselectively nucleate and grow SiO2 for nanopatterning, we prepared poly(styrene-b-4-vinylpyridine) (PS-b-4PV) with Mw of 20,000 g/mol and polydispersity of 1.1 using anionic polymerization.6 The ratio of PS/4PV was 0.95 calculated from integrating the 1H-NMR peaks of PS and 4PV. A main reason for using PS-b-4PV is that the pyridine rings can be easily protonated, so that there is a large difference in the surface energy between the PS and 4PV blocks. Furthermore, the protonated 4PV can act as acid catalyst sites for the hydrolysis of tetraethoxysilane (TEOS). Thus SiO2 can nucleate onto only the 4PV block and grow from the surface. After the regioselective nucleation, the direction of SiO2 growth can be controlled by using an oil instead of aqueous medium.7

To examine the process of controlled nanopatterning PS-b-4PV was dissolved in methylethyl ketone (1 mg/mL), and films were cast on either carbon coated nickel TEM specimen grids or graphite plates (1 x 1 cm2). The substrate was dipped into solution, and solvent was evaporated under ambient condition, and the dried grid or plate was annealed under nitrogen at 150 ℃ for 3 h. The morphology of the microphase-separated PS-b-4PV on the grid was investigated using TEM.

Results and Discussion

Figure 1A shows a plain TEM image of the resulting thin film after staining with OsO4. The nanodomains of 4PV in the PS-b-4PV appear as dark lamellar periodic nanodomains with 30 nm interspacing.8 After the growth of SiO2 onto the PS-b-4PV template in an aqueous or an oil phase for 10 to 300 min,9 TEM images of the samples were taken without staining. Figure 1B, C, and D show TEM images of SiO2 nanopatterning grown in an aqueous phase with 10, 20, and 300 min growth, respectively. It can be seen that SiO2 starts to nucleate following the nanodomains of 4PV after 10 min (Figure 1B) and completely nucleates onto all of the 4PV nanodomains after 20 min (Figure 1C). The reason for the fast regioselective nucleation onto only the protonated hydrophilic 4PV block is that the interfacial energy between water and the nanodomains of protonated 4PV is lower than that for the PS nanodomains.10 However, because the heterogeneous nucleation of SiO2 was very fast, there is no favored growth direction to decrease interfacial energy. Furthermore, at longer growth time, SiO2 can also nucleate on the PS nanodomains because the nucleation with less selectivity is promoted by longer growth time.  As the result, the patterning becomes blurred after 60 min of growth time, and is completely lost after 300 min (Figure 1D). To check the height of SiO2 nanopatterns of Figure 1C, SiO2 nanopatterns were grown onto a PS-b-4PV coated graphite plate under the same conditions for AFM studies with contact mode (Nanoscope III, Digital Instruments). Based on the cross-sectional anlysis of AFM image (Figure 1E), the height of the nanopatterns were 3 ∼ 4 nm with 30 to 40 nm width. On the other hand, a longer time (60 min) was required to complete regioselective nucleation in an oil phase, and clear nanopatterns could be observed for all samples with 60 to 300 min growth time. As an example, the TEM image of a sample with 300 min growth is shown in Figure 2A, in which the definition of the grown SiO2 pattern is as sharp as the PS-b-PV template (Figure 1A). For further characterization, SiO2 nanopatterns were also grown onto a graphite plate under the same conditions for AFM studies with contact mode. The AFM image (Figure 2B) shows that the nanodomains are similar to those observed from TEM, and much more definitive than those obtained for samples with growth in an aqueous medium (Figure 1E). Based on the cross-sectional analysis of the AFM image (Figure 2C), the height of SiO2 patterning is estimated to be about 5 - 15 nm with 40 nm width for samples prepared with 300 min of growth.

Figure 1. TEM image of stained PS-b-4PV (A)         Figure 2. (A) TEM image of SiO2 on PS-b-4PV

with OsO4, and TEM images of SiO2                     without staining after 300 minute of

on PS-b-4PV without staining after                       immersion into the oil phase.

10 (B), 20 (C), and 300 (D) minutes                     (B) AFM image and (C) cross-sectional

of immersion into aqueous solution                       analysis of SiO2 on a PS-b-4PV

for SiO2 growth. (E) AFM image with                    coated graphite plate prepared under

cross sectional analysis for 20 min                       the same condition as the sample in (A).

growth of SiO2.

The remarkable difference in the nanopatterning growth of SiO2 in the two phases can be explained by the following consideration. Because protonated pyridine rings are hydrophilic, each micro domain of 4PV may be covered by a very thin water layer,11 which does not mix with the surrounding medium in an oil phase. It may be compared to the liquid-flux in both vapor-liquid-solid (VLS)12 and solution-liquid-solid (SLS)13 methods of crystal growth. The thin strips of water film can act as acid catalyst to let SiO2 nucleate regioselectively on the surface of P4V block. After the nucleation, SiO2 can further grow only along the vertical direction because the thin water strips do not mix with oil and stay on top of the SiO2 columns. During the growth, more TEOS from the oil phase is slowly fed into the thin water strips and hydrolize into SiO2 following only the patterns of the 4PV block in the template. This explanation is substantiated by the observation that, without the acid treatment, bulk SiO2 with sizes of a few m grow randomly on the substrates coated with PS-b-4PV.

In summary, we have demonstrated a new method to copy the nanostructures of micro-phase separated copolymer thin films to form silica thin films up to a height of 15 nm with 40 nm width. Detailed studies of morphological changes and growing mechanism relating to changes in interfacial tensions of the medium are being carried out.

Acknowledgment.

The work of GC was supported by the Ministry of Science and Technology (MOST) of Korea.

References

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(7) Contact angles were measured using contact angle micrometer. It was found that the advanced      contact angles for polystyrene and poly-4-vinylpyridine films were 90 and 60 respectively.          After acid treatement of  poly-4-vinylpyridine film, the advanced contact angle was 30 because      of the formation of pyridinium salt.

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(9) Since the surface of 4PV has proton acceptor sites, they can be simply protonated by treating 1          N of HCl  solution for 20 sec. The acid treated PS-b-4PV was rinsed throughly with deionized          water, and then immersed into either an aqueouse or silicone oil solution containing 0.01 M of          TEOS. The immersion times ranged from 10 min to 300 min, and the temperature was kept at 25       C.  After taking out the sample from the aqueouse solution, it was rinsed with stirring deionized         water for 5 min. The samples immersed in the silicone oil solution, n-hexane was  used for rinsing.       All of the SiO2 fabricated PS-b-4PV samples were investigated using TEM without staining.

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