3 Block Copolymer-Derived Structures
Because many of the physical properties of materials (e.g., photonic, phononic, and
mechanical properties) greatly rely on the structural characteristics (i.e., symmetry
and structural dimensions), designing the structure of materials is a crucial step for
developing novel materials with a targeted physical property. In contrast to
top-down approaches, bottom-up approaches have limited freedom in the selection
of structures. Among the 230 space groups, only a few tens of space groups
have been observed in BCP SA studies. This limitation mainly originates from
the simple chemical composition of previously studied BCPs. As discussed earlier,
most studies have been conducted on BCPs composed of only a few different
chemical blocks, typically ranging from two to five. This is just the tip of
the iceberg in a whole variety of combinations that can be achieved with BCP
synthesis. However, much effort is needed to synthesize new BCPs with additional
blocks due to synthetic challenges such as quantitative terminal group modifications or solvent exchange for additional polymer growth steps. Furthermore,
the experimental parameter space associated with different block compositions
exponentially expands as the number of blocks increases [7]. A substantial number
of BCP-derived structures have already been discovered, some of which are quite
intriguing and, due to their useful structural characteristics, may be interesting for
future applications. As a first step, it is thus instructive to review the type of
structures found to date in BCP condensed phases.
Bottom-up BCP SA provides a facile route to a variety of nanostructures
including three-dimensional (3D) and quasicrystalline structures, usually challenging in top-down approaches. This section delineates such BCP-derived 1D, 2D,
3D, and quasicrystalline structures. The dimension of structures, D, is defined
by D ¼ 3 À n, where n stands for the number of axes that have continuous
translational order. For example, the lamellar structure shown in Fig. 2 has an
axis of translational order with a nonzero lattice dimension but two axes with
continuous translational order. Thus, the dimension of a lamellar structure is
D ¼ 1 with n ¼ 2.
3.1 One- and Two-Dimensional Structures
Lamellar morphology is a 1D structure as shown in Fig. 2, where two or more
chemically distinct sheets alternate along one axis. Selective removal of a polymer
block leads to the formation of nanosheets [10, 11]. The structure is most frequently
observed in BCP SA because it occupies the widest area, e.g., in a diblock copolymer
phase diagram as shown in Fig. 1b. Controlling the orientation and spatial arrangement of lamellae on 2D substrate planes via graphoepitaxy [12] has attracted much
attention for lithography applications addressing challenging current technical
limitations (e.g., low throughput and poor line edge roughness) found in top-down
Design and Applications of Multiscale Organic–Inorganic Hybrid Materials. . .
265
Because many of the physical properties of materials (e.g., photonic, phononic, and
mechanical properties) greatly rely on the structural characteristics (i.e., symmetry
and structural dimensions), designing the structure of materials is a crucial step for
developing novel materials with a targeted physical property. In contrast to
top-down approaches, bottom-up approaches have limited freedom in the selection
of structures. Among the 230 space groups, only a few tens of space groups
have been observed in BCP SA studies. This limitation mainly originates from
the simple chemical composition of previously studied BCPs. As discussed earlier,
most studies have been conducted on BCPs composed of only a few different
chemical blocks, typically ranging from two to five. This is just the tip of
the iceberg in a whole variety of combinations that can be achieved with BCP
synthesis. However, much effort is needed to synthesize new BCPs with additional
blocks due to synthetic challenges such as quantitative terminal group modifications or solvent exchange for additional polymer growth steps. Furthermore,
the experimental parameter space associated with different block compositions
exponentially expands as the number of blocks increases [7]. A substantial number
of BCP-derived structures have already been discovered, some of which are quite
intriguing and, due to their useful structural characteristics, may be interesting for
future applications. As a first step, it is thus instructive to review the type of
structures found to date in BCP condensed phases.
Bottom-up BCP SA provides a facile route to a variety of nanostructures
including three-dimensional (3D) and quasicrystalline structures, usually challenging in top-down approaches. This section delineates such BCP-derived 1D, 2D,
3D, and quasicrystalline structures. The dimension of structures, D, is defined
by D ¼ 3 À n, where n stands for the number of axes that have continuous
translational order. For example, the lamellar structure shown in Fig. 2 has an
axis of translational order with a nonzero lattice dimension but two axes with
continuous translational order. Thus, the dimension of a lamellar structure is
D ¼ 1 with n ¼ 2.
3.1 One- and Two-Dimensional Structures
Lamellar morphology is a 1D structure as shown in Fig. 2, where two or more
chemically distinct sheets alternate along one axis. Selective removal of a polymer
block leads to the formation of nanosheets [10, 11]. The structure is most frequently
observed in BCP SA because it occupies the widest area, e.g., in a diblock copolymer
phase diagram as shown in Fig. 1b. Controlling the orientation and spatial arrangement of lamellae on 2D substrate planes via graphoepitaxy [12] has attracted much
attention for lithography applications addressing challenging current technical
limitations (e.g., low throughput and poor line edge roughness) found in top-down
Design and Applications of Multiscale Organic–Inorganic Hybrid Materials. . .
265