5.1 Ceramics
Ceramics are nonmetallic solids, insulators or semiconductors, and include oxides,
non-oxides, and composites. Nanostructure formation of ceramics has been studied
in the Wiesner group mainly by using BCPs as structure-directing agents for
ceramic sol nanoparticles. To that end, ceramic sol nanoparticles prepared by the
sol–gel process are mixed with a BCP solution for self-assembly of BCP/ceramic
nanoparticles. In this way, a variety of nanostructures are accessible by varying
the volumetric ratio of BCP and inorganic sol. The group has extensively studied
aluminosilicates and transition metal oxides.
The early work of the Wiesner group on nanostructured aluminosilicates via
BCP + ceramic nanoparticle SA was reported in 1997, where solutions of polyisoprene-block-polyethylene oxide (PI-b-PEO) and aluminosilicate sol were prepared,
followed by the evaporation of organic solvents [49]. Morphology changes
expected from the phase diagram of di-BCPs were observed on varying the aluminosilicate sol volume fraction in the solutions (see Fig. 8). The selective volumetric
increase of the hydrophilic (PEO) block via the addition of the oxide sols led
to morphology changes in the hybrids because varying the volume fraction of a
block, f, results in morphology changes, as shown in a di-BCP phase diagram (see
morphology changes by varying f at a fixed χN). Thus, a variety of BCP-derived
nanostructures can be obtained by preparing solutions with different compositions
of BCP and inorganic material. The morphology map of PI-b-PEO + aluminosilicate SA summarizes a comprehensive series of experiments in which the aluminosilicate volume fraction, f, was varied for a number of different block copolymer
compositions [14] (see Fig. 8e).
In particular, several morphologies usually elusive in BCP SA were found in
BCP/aluminosilicate hybrids. The plumber’s nightmare and woodpile structures were
observed in AB di-BCP and ABC triblock terpolymer + aluminosilicate SA, respectively [9, 11, 23]. A theoretical study of di-BCP + homopolymer mixtures showed
that the plumber’s nightmare structure can be observed in hybrid systems but may be
less stable than the formation of two distinct phases in a blend [50]. Thus, the
plumber’s nightmare structure observed in BCP + aluminosilicate SA may be a
long-lived metastable structure. Such long-lived metastable structures could be
more pronounced in BCP SA with a sol–gel solution as compared to the SA of a
neat BCP due to structural “locking” during BCP SA and simultaneous aluminosilicate condensation. Such structural locking from sol–gel condensation reactions
incapacitates subsequent annealing processes in BCP + aluminosilicate SA and thus
may limit access to equilibrium structures as well as to better long-range order.
Many oxide semiconductors accessible from sol–gel chemistry such as titania
[51, 52] and niobia [53] can be nanostructured by employing the same strategy
utilized for aluminosilicate hybrids. In particular, thermal crystallization of such
hybrids at a high temperature using CASH chemistries as described in the previous
section leads to the formation of polycrystalline oxide semiconductors with interesting electrical properties [43].
Design and Applications of Multiscale Organic–Inorganic Hybrid Materials. . .
277
Ceramics are nonmetallic solids, insulators or semiconductors, and include oxides,
non-oxides, and composites. Nanostructure formation of ceramics has been studied
in the Wiesner group mainly by using BCPs as structure-directing agents for
ceramic sol nanoparticles. To that end, ceramic sol nanoparticles prepared by the
sol–gel process are mixed with a BCP solution for self-assembly of BCP/ceramic
nanoparticles. In this way, a variety of nanostructures are accessible by varying
the volumetric ratio of BCP and inorganic sol. The group has extensively studied
aluminosilicates and transition metal oxides.
The early work of the Wiesner group on nanostructured aluminosilicates via
BCP + ceramic nanoparticle SA was reported in 1997, where solutions of polyisoprene-block-polyethylene oxide (PI-b-PEO) and aluminosilicate sol were prepared,
followed by the evaporation of organic solvents [49]. Morphology changes
expected from the phase diagram of di-BCPs were observed on varying the aluminosilicate sol volume fraction in the solutions (see Fig. 8). The selective volumetric
increase of the hydrophilic (PEO) block via the addition of the oxide sols led
to morphology changes in the hybrids because varying the volume fraction of a
block, f, results in morphology changes, as shown in a di-BCP phase diagram (see
morphology changes by varying f at a fixed χN). Thus, a variety of BCP-derived
nanostructures can be obtained by preparing solutions with different compositions
of BCP and inorganic material. The morphology map of PI-b-PEO + aluminosilicate SA summarizes a comprehensive series of experiments in which the aluminosilicate volume fraction, f, was varied for a number of different block copolymer
compositions [14] (see Fig. 8e).
In particular, several morphologies usually elusive in BCP SA were found in
BCP/aluminosilicate hybrids. The plumber’s nightmare and woodpile structures were
observed in AB di-BCP and ABC triblock terpolymer + aluminosilicate SA, respectively [9, 11, 23]. A theoretical study of di-BCP + homopolymer mixtures showed
that the plumber’s nightmare structure can be observed in hybrid systems but may be
less stable than the formation of two distinct phases in a blend [50]. Thus, the
plumber’s nightmare structure observed in BCP + aluminosilicate SA may be a
long-lived metastable structure. Such long-lived metastable structures could be
more pronounced in BCP SA with a sol–gel solution as compared to the SA of a
neat BCP due to structural “locking” during BCP SA and simultaneous aluminosilicate condensation. Such structural locking from sol–gel condensation reactions
incapacitates subsequent annealing processes in BCP + aluminosilicate SA and thus
may limit access to equilibrium structures as well as to better long-range order.
Many oxide semiconductors accessible from sol–gel chemistry such as titania
[51, 52] and niobia [53] can be nanostructured by employing the same strategy
utilized for aluminosilicate hybrids. In particular, thermal crystallization of such
hybrids at a high temperature using CASH chemistries as described in the previous
section leads to the formation of polycrystalline oxide semiconductors with interesting electrical properties [43].
Design and Applications of Multiscale Organic–Inorganic Hybrid Materials. . .
277