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Three-Dimensional Multilayered Nanostructures from Crosslinkable Block Copolymers Sanghoon Woo,† Hyun Suk Wang,† Youngson Choe,‡ June Huh,† and Joona Bang*,† †
Department of Chemical and Biological Engineering, Korea University, Seoul 136-713, Republic of Korea Department of Chemical Engineering, Pusan National University, Kumjeong-ku, Busan 609-735, Republic of Korea
‡
ABSTRACT: Block copolymer (BCP) lithography has generally been synonymous to one- or two-dimensional single layered lithographic templates as a means to fabricate simple nanoscaled structures. Recently, the rapidly increasing demand for complex nanostructures and the corresponding evolution in BCP lithography have led to three-dimensional (3D) BCP nanostructures, which can be fabricated in various ways such as directed self-assembly or stacking of cross-linked BCP patterns. This review covers the recent advances in the 3D multilayered structures from cross-linkable BCPs, which provide an easy and robust means for integrating various BCP structures into one scaffold. In this case, wetting-optimized adjustment of BCP microdomains at the layer interface plays a critical role in the formation of well-defined 3D multilayer nanostructures.
B
lock copolymer (BCP) lithography, referring to a patterning process via block copolymer self-assembly,1−7 is an intriguing research area due to its spontaneous process controllable with various external fields7−10 and wide range of applications, such as templates for nanoparticles/nanowires,11−15 separation membranes,16,17 nanoreactors,18,19 and microelectronics.20−23 To prepare BCP templates, most studies have focused on the fabrication of simple one-dimensional (1D) or two-dimensional (2D) single layer patterns (i.e., lamellae, hexagonally packed cylinder), which can be controlled by molecular asymmetry of BCPs.24−27 Interest in threedimensional (3D) patterns has rapidly grown in recent years due to the increasing demand for complex patterns and intricate structures desirable in a variety of novel technologies such as functional surface coating, advanced 3D nanolithography, fabrication of complex 3D scaffolds, and so on.28−43 Generally, there are two main approaches to fabricate 3D BCP patterns. One of the approaches uses inherent 3D morphologies (i.e., gyroid, perforated lamellae) directly from BCP self-assembly. In this case, the directed self-assembly (DSA) of BCPs or their blends using geometrical restrictions such as square arrayed spots or posts, and v-shaped grooves, is a usual choice for controlling the orientations or the symmetry types of 3D BCP nano patterns.28−32 Using these methods, various 3D nanostructures such as quadratically perforated lamellae (QPL),28 crossed or junction structures,32 and variously packed structures were fabricated. The other approach is a layer-by-layer stacking of BCP patterns, which can be achieved by various methods: cross-linking of underlying layer,33−38 deposition of protective materials on the underlying layer,39,40 transfer of upper layer onto bottom layer by printing,41 and sequential deposition of BCP patterns by electrospray deposition42 or multiple phtoprocessing steps.43 © XXXX American Chemical Society
Given that the cross-linked underlying layer is not disturbed by any solvent during the stacking process, 3D multilayered structure consisting of layers with different structural characteristics (e.g., symmetry, periodicity) can be readily fabricated by this sequential stacking process. Recently, Yagel et al. demonstrated wide-area fabrication of well-aligned multilayered nanostructures by multiple photoprocessing steps, combined with material conversion into rigid inorganic replicas, resulting in various lattice symmetries that do not natively appear in block copolymers.43 Among these methods, a cross-linking approach offers an easy way for fabricating multilayers that integrate different BCP patterns via sequential layer-by-layer stacking of cross-linked BCP layers. Moreover, these crosslinkable BCPs can be readily designed by incorporating various cross-linkable subgroups into the desirable block and easily synthesized via living radical polymerization.34,35,37,38 In this context, the present paper reviews the latest developments in BCP multilayered films using cross-linkable BCPs as a key material for the layer-by-layer assembly, covering the methodology of stacking process of BCP layers, the structural features of the resultant multilayered BCP structures, and the potential applications using their structural characteristics. The structural features of multilayered BCPs and their related properties can be designed by many variables such as the type of BCP nanostructure in each layer, stacking sequence, and interlayer interaction between different BCP patterns. In particular, the interlayer interactions in the course of layer stacking, which involve not only energetic contribution but also entropic effect associated with an adjustment of BCP Received: December 15, 2015 Accepted: January 28, 2016
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DOI: 10.1021/acsmacrolett.5b00908 ACS Macro Lett. 2016, 5, 287−291
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this thermally cross-linkable BCP, the multilayered nanostructures were fabricated by the repetition of a sequential thermal treatment consisting of self-assembly process at 160 °C and cross-linking of BCB-containing BCP at 250 °C (Figure 1b) for 15 min. The resulting double-layer or triple-layer morphology exhibited well-organized cylindrical domains, perpendicularly extended from the bottom layer to the overlying layers with an excellent pattern registration of cylindrical domains in different layers (Figure 1c). Using a similar process but with lamellae-forming BCP blends, He et al. reported the fabrication of a gradient multilayer film composed of two layers where the domain size of the lamellae in the top layer differs from that in the bottom layer.37 (Figure 2a) The domain size in each layer was separately controlled by mixing the ratio between lamellaeforming PS-b-PMMA and cross-linkable PS (or PMMA) homopolymers into which a small fraction (∼1 mol %) of glycidyl methacrylate (GMA) as a cross-linkable group48 was incorporated. In this case, films were cross-linked by heating at 190 °C for 1 h and different types of vertically gradient nanostructures could be fabricated by controlling the relative content of cross-linkable homopolymers in the overlying layer to that in the underlying layer, as exemplified by Figure 2b,c. As a result, the resultant overcut (PS, brighter region) structures with undercut (removed PMMA, darker region) holes are demonstrated. From a practical viewpoint, it is worth pointing out that such profiles are beneficial to pattern transfer process, since lift-off step can be facilitated by undercut profile that prevents continuous coverage of deposited materials over the substrates and photoresist features. The aforementioned works, while demonstrating excellent pattern matching between stacking layers, employed the BCP patterns having the same type of microdomain, thereby resulting in homogeneous (in terms of pattern type) layers such as “cylinders-on-cylinders” and “lamellae-on-lamellae”. As a further progress, Bang and co-workers have investigated the epitaxial assembly of lamellae-forming PS-b-PMMA on the hexagonally patterned arrays that were formed from crosslinkable, cylinder-forming BCPs (Figure 3).35 To fabricate the cross-linked underlying layer, a small fraction (∼1.5 mol %) of azido group (N3) as a cross-linking group49,50 was introduced to BCPs, namely, P(S-r-(S−N3))-b-PMMA, via RAFT polymerization. In a simple cross-linking reaction, aziridine (major) and diazo (minor) covalent cross-links are easily formed under UVirradiation (within 5 min at 5.3 mW/cm2) or thermal conditions (within 1 min at 250 °C). It should be also noted that such a small amount of cross-linkable group does not alter the wetting condition, as examined in the cross-linkable random copolymers.45,49 The fabricated double layer showed a wettingoptimized adjustment of lamellar microdomains on the hexagonal patterns as follows. First, the overlying lamellae on top of an underlying cross-linked cylindrical layers exhibited the perpendicular orientation to the film direction. Moreover, as seen in Figure 3b, it was observed that the PMMA lamellae were remarkably matched with underlying PMMA cylinders as a result of minimization of interfacial free energy between the two BCP layers (i.e., wetting optimized adjustment). The theoretical analysis based on strong segregation approach on this system suggested that the interfacial energy of such wetting-optimized, perpendicular lamellae on the hexagonal patterns reaches a minimum when the bulk domain spacings of overlying lamellae-forming BCP and underlying cylinderforming BCP are the same. The theory also suggested that
microdomains at the multilayer interfaces, are of critical importance in determining the structural features and their fidelity. As an earlier effort, a multilayered structure with a simple layer configuration, comprised of two or three layers each with the same kind of cross-linkable BCP, has been fabricated and characterized by Kim et al.34 As a cross-linkable BCP, cylinder-forming poly(styrene-r-benzocyclobutene)-bpoly(methyl methacrylate) (P(S-r-BCB)-b-PMMA) was synthesized by reversible addition and fragmentation chain transfer (RAFT) polymerization (Figure 1a). In this case, the benzocyclobutene (BCB) group (∼3 mol %) in the majority block undergoes conrotatory ring-opening reaction due to ring strain and forms o-xylylene at over 180 °C. Through Diels− Alder reactions these species undergo dimerization and immediate cross-linking,44 a reaction that has been extensively used for polymer cross-linking in various studies.45−47 Using
Figure 1. Schematic illustration for (a) cross-linking mechanism of P(S-r-BCB)-b-PMMA BCP and (b) cross-linking process and multilayer registration of the BCP thin films. (c) Cross-sectional SEM images of cross-linked BCP thin films fabricated at two layers (left) and three layers (right). Reproduced with permission from ref 34. Copyright 2008 Royal Society of Chemistry. 288
DOI: 10.1021/acsmacrolett.5b00908 ACS Macro Lett. 2016, 5, 287−291
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Figure 2. (a) Schematic illustration for fabricating bilayer undercut profile via GMA cross-linking. (b) Cross-sectional SEM image of PMMA removed bilayer film with undercut profiles. (c) Top-down SEM image of metal nanowire after lift-off process. Reproduced with permission from ref 37. Copyright 2014 Wiley-VCH.
structural features of this “lamellae-on-cylinders” provided the resultant film with optically useful properties that can be utilized for the graded-refractive-index (GRIN) antireflection layer for GaN-based light-emitting diode (LED). Owing to the stacking of structurally different layers, the GaN-based LED coated with this double-layered porous film showed an increase in the light extraction efficiency when compared to that coated with a single layer or bare GaN-based LED (Figure 3c). Furthermore, the “lamellae-on-cylinders” system has been extended to create globally aligned line-space patterns via combining with graphoepitaxy technique. For this approach, microgroove patterns were fabricated on the cross-linked hexagonal patterns via conventional photolithography, and they were used as guiding patterns for lamellar alignment (Figure 4a).38 The PS-b-PMMA lamellae were then assembled on the hexagonally patterned layer in the presence of the sidewalls selective for PMMA block. In this case, it was shown that the degree of lamellar alignment was significantly improved compared with the poorer lamellar alignment on the homogeneous neutral layer that was driven only by the graphoepitaxial assembly (Figures 4b,c), indicating that the hexagonally patterned underlying BCP layer can assist the graphoepitaxial assembly of lamellar alignment. Mesoscale density functional theory simulations also support the experimental results in that the graphoepitaxy-driven lamellar alignment is more efficient on the hexagonally patterned layer by reducing a large number of orientation states of the overlying lamellae in the course of graphoepitaxial assembly process. This viewpoint highlights the use of cross-linkable BCPs to fabricate 3D multilayers, as a new and emerging area in BCP thin films. This approach is based on the sequential layer-bylayer process of BCP patterns via epitaxial assembly, providing a facile and robust means for integrating different structural features into a single film. During the formation of BCP multilayers, the basic principle is that the resulting 3D nanostructures are primarily determined by minimization of unfavorable wetting interactions at the film interface, leading to the registration of BCP microdomains between layers. Applications to graphoepitaxial assembly and antireflective GRIN layer coating for GaN LED have demonstrated a
Figure 3. Schematic illustration for (a) the fabrication of BCP multilayered nanostructures. (b) Top-down SEM and TEM images of BCP multilayered nanostructures. (c) Photoluminescence (PL) spectra from bare GaN LED, GaN LEDs that were coated with cylinders and lamellae single layer, and lamellae-on-cylinders multilayer. Reproduced with permission from ref 35. Copyright 2011 American Chemical Society.
the formation of such wetting-optimized lamellae is further assisted by an entropy-type interlayer attraction, namely, nematic interaction51 that is originated from the propensity of polymer chains to lie in the surface of layer boundary. The 289
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(4) Hamley, I. W. Developments in Block Copolymer Science and Technology; Wiley Online Library, 2004. (5) Hawker, C. J.; Russell, T. P. MRS Bull. 2005, 30, 952. (6) Cheng, J. Y.; Ross, C. A.; Smith, H. I.; Thomas, E. L. Adv. Mater. 2006, 18, 2505. (7) Bang, J.; Jeong, U.; Ryu, D. Y.; Russell, T. P.; Hawker, C. J. Adv. Mater. 2009, 21, 4769. (8) Park, C.; Yoon, J.; Thomas, E. L. Polymer 2003, 44, 6725. (9) Li, M.; Coenjarts, C. A.; Ober, C. K., Patternable block copolymers. In Block Copolymers II; Springer, 2005; p 183. (10) Luo, M.; Epps, T. H., III Macromolecules 2013, 46, 7567. (11) Lopes, W. A.; Jaeger, H. M. Nature 2001, 414, 735. (12) Fahmi, A. W.; Stamm, M. Langmuir 2005, 21, 1062. (13) Lin, Y.; Böker, A.; He, J.; Sill, K.; Xiang, H.; Abetz, C.; Li, X.; Wang, J.; Emrick, T.; Long, S.; Wang, Q.; Balazs, A.; Russell, T. P. Nature 2005, 434, 55. (14) La, Y.-H.; Stoykovich, M. P.; Park, S.-M.; Nealey, P. F. Chem. Mater. 2007, 19, 4538. (15) Melde, B. J.; Burkett, S. L.; Xu, T.; Goldbach, J. T.; Russell, T. P.; Hawker, C. J. Chem. Mater. 2005, 17, 4743. (16) Yang, S. Y.; Ryu, I.; Kim, H. Y.; Kim, J. K.; Jang, S. K.; Russell, T. P. Adv. Mater. 2006, 18, 709. (17) Yang, S. Y.; Park, J.; Yoon, J.; Ree, M.; Jang, S. K.; Kim, J. K. Adv. Funct. Mater. 2008, 18, 1371. (18) Sohn, B. H.; Seo, B. H. Chem. Mater. 2001, 13, 1752. (19) Temple, K.; Kulbaba, K.; Power-Billard, K. N.; Manners, I.; Leach, K. A.; Xu, T.; Russell, T. P.; Hawker, C. J. Adv. Mater. 2003, 15, 297. (20) Förster, S.; Antonietti, M. Adv. Mater. 1998, 10, 195. (21) Cheng, J. Y.; Ross, C.; Chan, V. H.; Thomas, E. L.; Lammertink, R. G.; Vancso, G. J. Adv. Mater. 2001, 13, 1174. (22) Jeong, S. J.; Xia, G.; Kim, B. H.; Shin, D. O.; Kwon, S. H.; Kang, S. W.; Kim, S. O. Adv. Mater. 2008, 20, 1898. (23) Black, C. T.; Guarini, K. W.; Zhang, Y.; Kim, H.; Benedict, J.; Sikorski, E.; Babich, I. V.; Milkove, K. R. IEEE Electron Device Lett. 2004, 25, 622. (24) Leibler, L. Macromolecules 1980, 13, 1602. (25) Bates, F. S. Science 1991, 251, 898. (26) Matsen, M. W.; Schick, M. Phys. Rev. Lett. 1994, 72, 2660. (27) Matsen, M. W.; Bates, F. S. Macromolecules 1996, 29, 1091. (28) Daoulas, K. C.; Müller, M.; Stoykovich, M. P.; Park, S.-M.; Papakonstantopoulos, Y. J.; de Pablo, J. J.; Nealey, P. F.; Solak, H. H. Phys. Rev. Lett. 2006, 96, 036104. (29) Chuang, V. P.; Cheng, J. Y.; Savas, T. A.; Ross, C. A. Nano Lett. 2006, 6, 2332. (30) Crossland, E. J.; Kamperman, M.; Nedelcu, M.; Ducati, C.; Wiesner, U.; Smilgies, D.-M.; Toombes, G. E.; Hillmyer, M. A.; Ludwigs, S.; Steiner, U.; Snaith, H. J. Nano Lett. 2009, 9, 2807. (31) Ji, S.; Nagpal, U.; Liao, W.; Liu, C.-C.; de Pablo, J. J.; Nealey, P. F. Adv. Mater. 2011, 23, 3692. (32) KG, A. T.; Gotrik, K. W.; Hannon, A.; Alexander-Katz, C. A.; Ross, C. A.; Berggren, K. K. Science 2012, 336, 1294. (33) Ruiz, R.; Sandstrom, R. L.; Black, C. T. Adv. Mater. 2007, 19, 587. (34) Kim, E.; Shin, C.; Ahn, H.; Ryu, D. Y.; Bang, J.; Hawker, C. J.; Russell, T. P. Soft Matter 2008, 4, 475. (35) Jung, H.; Hwang, D.; Kim, E.; Kim, B.-J.; Lee, W. B.; Poelma, J. E.; Kim, J.; Hawker, C. J.; Huh, J.; Ryu, D. Y.; Bang, J. ACS Nano 2011, 5, 6164. (36) Moon, H.-S.; Shin, D. O.; Kim, B. H.; Jin, H. M.; Lee, S.; Lee, M. G.; Kim, S. O. J. Mater. Chem. 2012, 22, 6307. (37) He, C.; Stoykovich, M. P. Adv. Funct. Mater. 2014, 24, 7224. (38) Jung, H.; Woo, S.; Park, S.; Lee, S.; Kang, M.; Choe, Y.; Son, J. G.; Ryu, D. Y.; Huh, J.; Bang, J. Soft Matter 2015, 11, 4242. (39) Rose, F.; Bosworth, J.; Dobisz, E. A.; Ruiz, R. Nanotechnology 2011, 22, 035603. (40) Kim, J. Y.; Kim, B. H.; Hwang, J. O.; Jeong, S.-J.; Shin, D. O.; Mun, J. H.; Choi, Y. J.; Jin, H. M.; Kim, S. O. Adv. Mater. 2013, 25, 1331.
Figure 4. (a) Schematic illustration of the experimental setup for lamellar alignment by combining chemo- and grapho-epitaxy. (b) SEM image (upper) and image analysis result of PS-b-PMMA lamellae with Mn = 51 kg mol−1 on the hexagonally patterned layers. (c) SEM image (upper) and image analysis result of PS-b-PMMA lamellae with Mn = 51 kg mol−1 on the neutral layers. Reproduced with permission from ref 38. Copyright 2015 Royal Society of Chemistry.
versatile use of this method for BCP multilayer nanostructures. Furthermore, since this method is simply based on the sequential deposition (via spin coating) and cross-linking of underlying layers, one can expect that the number of layers in the BCP multilayers can be readily further extended to generate more complex and integrated 3D nanostructures. We anticipate that this system provides an important platform for welldefined 3D nanostructures having various potential applications such as antireflection coatings, light management layers, transparent electrodes, biomaterial scaffolds, separation membranes, advanced 3D nanolithography, and so on.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was by National Research Foundation of Korea grant funded by the Korea government (MSIP; Nos. 2015R1A2A2A01006008, 2012M3A7B4049863, and 2012M3A7B4035323) and also by the Global Frontier R&D Program (No. 2013M3A6B1078869) on Center for Hybrid Interface Materials (HIM) funded by the Ministry of Science, ICT & Future Planning.
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REFERENCES
(1) Huang, E.; Rockford, L.; Russell, T. P.; Hawker, C. J. Nature 1998, 395, 757. (2) Fasolka, M. J.; Mayes, A. M. Annu. Rev. Mater. Res. 2001, 31, 323. (3) Krausch, G.; Magerle, R. Adv. Mater. 2002, 14, 1579. 290
DOI: 10.1021/acsmacrolett.5b00908 ACS Macro Lett. 2016, 5, 287−291
Viewpoint
ACS Macro Letters (41) Jeong, J. W.; Park, W. I.; Do, L.-M.; Park, J.-H.; Kim, T.-H.; Chae, G.; Jung, Y. S. Adv. Mater. 2012, 24, 3526. (42) Choo, Y.; Hu, H.; Toth, K.; Osuji, C. O. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 247. (43) Majewski, P. W.; Rahman, A.; Black, C. T.; Yager, K. G. Nat. Commun. 2015, 6, 7448. (44) Farona, M. F. Prog. Polym. Sci. 1996, 21, 505. (45) Ryu, D. Y.; Shin, K.; Drockenmuller, E.; Hawker, C. J.; Russell, T. P. Science 2005, 308, 236. (46) Ryu, D. Y.; Wang, J.-Y.; Lavery, K. A.; Drockenmuller, E.; Satija, S. K.; Hawker, C. J.; Russell, T. P. Macromolecules 2007, 40, 4296. (47) Ma, B.; Lauterwasser, F.; Deng, L.; Zonte, C. S.; Kim, B. J.; Fréchet, J. M.; Borek, C.; Thompson, M. E. Chem. Mater. 2007, 19, 4827. (48) Han, E.; Gopalan, P. Langmuir 2010, 26, 1311. (49) Bang, J.; Bae, J.; Lowenhielm, P.; Spiessberger, C.; Given-Beck, S. A.; Russell, T. P.; Hawker, C. J. Adv. Mater. 2007, 19, 4552. (50) Akhrass, S. A.; Gal, F.; Damiron, D.; Alcouffe, P.; Hawker, C. J.; Cousin, F.; Carrot, G.; Drockenmuller, E. Soft Matter 2009, 5, 586. (51) Pickett, G. T.; Witten, T. A.; Nagel, S. R. Macromolecules 1993, 26, 3194.
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