MILOS IADR

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Self-Assembling Block Copolymers
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Polymers Everywhere in Nanochemistry
Polymers have appeared in various guises in
nanochemistry. Polymers can be homopolymers,
polymer chains built of a single kind of monomer
units polymers also can be copolymers, polymer
chains with a custom-designed architecture made
out of two or more types of monomers, with
unprecedented control over chain length
distribution.
A schematic illustration of anionic living
polymerization, one of several living
polymerization methods
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Block Copolymer Self-Assembly
Block copolymers (di-, tri- and multi-block
copolymers) with immiscible or incompatible
segments or blocks can undergo microphase
separation. At certain chain lengths (volume
fractions), the microphase separation can result
self-organized structures including ordered
spheres, cylinders, lamellae and bi-continuous
structures, in order to minimize the surface free
energy.
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Nanostructured Ceramics
Block copolymer can form templates which length
scale goes well beyond the 10nm limit of most
surfactant-based micellar and liquid crystal
assemblies. An example of this block copolymer
templating approach make use of
poly(isoprene)-b-poly(ethyleneoxide) (PI-PEO) for
synthesizing different aluminosilicate
mesostructures.
The PEO phase of a PI-PEO block copolymer can be
selectively swollen with aluminosi/icate
precursors, resulting in organic-inorganic
composites after condensation
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Nano-Objects
Block copolymers can also be used to form
isolated aluminosilicate nano-objects.
Domains of PEO in self-assembled block copolymers
can be swollen and polymerized with
aluminosilicate precursors. These nanocomposites
can then be dissolved to generate isolated
nano-objects such as spheres, cylinders, and
lamellae
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Block Copolymer Thin Films
The common methods for making block copolymer
thin films include drop casting, dip coating, and
spin coating, the latter usually giving the most
homogenous films. Following deposition, the
films are usually post-treated to increase the
degree of ordering of the periodic mesostructure.
This can be done by thermally annealing for
several days above the order-disorder transition
(the temperature above which the block copolymer
is in a disordered isotropic state), which gives
mobility to the polymer segments and allows a
more extensive phase separation. Annealing in
an atmosphere of solvent vapor makes the polymer
more mobile through a slight solvent swelling,
and has a similar effect as thermal annealing.
A microphase-separated block copolymer film
ordered by controlled solvent evaporation
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Thin films of block copolymers can give different
domain orientations depending on the wetting
characteristics of the substrate
Block copolymer A-B is cast as a thin film on a
substrate. If the substrate wets B
preferentially, the cylinders of A will be
oriented parallel to the substrate so that a
think layer of B separates them from the
substrate. If conversely the A block is wetting,
the cylinders will oriented vertically with the A
domains in contact with the substrate. The
wetting characteristics of a substrate can be
easily controlled by its composition,
hydrophobicity/hydrophilicity, charge, or by
surface treatments.
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Electrical Ordering
One of the most conveniently tuned stimuli are
electric fields. Electrodes can easily be
patterned by various ways, which includes soft
lithography, microtransfer printing, optical
lithography, electron lithography and others. It
is thus quite significant that the ordering of
block copolymers can be directed by electric
fields.
TEM micrographs of block copolymer films annealed
in the presence of an electric field and without
a field, highlighting the electric field
alignment near the electrode seen in the bottom
left parts of the images
The A and B segments of the block copolymer,
being chemically different, have different
dielectric constants and polarizabilities.
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Spatial Confinement of Block Copolymers
Local control over the positional order of
microphase separation can be achieved by spatial
confinement of the block copolymer in
geometrically well-defined surface relief
patterns that have been lithographically sculpted
in a substrate. An impressive example involves
polystyrene-b-poly(methyl methacrylate) in which
the topography of a patterned micromold forces
vertical alignment of hexagonally close-packed
PMMA cylinders in the PS matrix.
The selective removal of the vertically aligned
PMMA cylinders by oxygen plasma etching to create
a hexagonally close-packed array of nanoscale
channels.
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Nano-Epitaxy
Integration of high-resolution lithography and
block copolymer self-assembly can induce
epitaxial self assembly of the domains of a phase
separated block copolymer on a patterned
substrate to minimize formation of defects.
Block copolymer epitaxy can overcome the
intrinsic defect limitation of the self-assembly
process. When commensurability between the period
of the block copolymer and the surface pattern is
within a few percent, the resulting domain
pattern is found to be free of defects, oriented
and registered with the underlying substrate over
large areas.
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Example
A silicon wafer can be inscribed with a chemical
line pattern by high-resolution electron
lithography. If the line pattern has the same
spacing as the block copolymer (commensurate) the
order is perfect, but if the spacing is
non-commensurate the block copolymer domains
appear random
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Block Copolymer Lithography
Block copolymers microphase separate with highly
controlled and tunable characteristic length
scale on the order of 5-50nm, an accuracy much
greater than any available lithography technique.
This can be utilized to lithographically pattern
a substrate with the precision, perfection, and
reproducibility that is demanded in
nanomanufacturaing.
Strategies for performing block copolymer
lithography with a PS-PB block copolymer.
Staining or degradation of the PB block can
enable selective etching of polymer domains and
underlying substrate
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Decorating Block Copolymers
A creative strategy has been employed to organize
nanoparticles into block copolymer derived
nanoscale pits using capillary forces.
A PS-b-PMMA block copolymer with 30 PMMA can be
assembled as a thin film with PMMA cylinders
oriented orthogonal to the substrate. Treatment
with UV-light crosslinks the PS domains,
stabilizing them towards organic solvents, and
degrades the PMMA domains, leaving nanoscale
holes from 10-50nm in diameter after washing with
acetic acid.
Such a nanoporous substrate can then be dipped
into a toluene solution of capped nanocrystals
and withdrawn at a controlled rate. During the
withdrawal, solvent evaporation at the meniscus
results in strong capillary forces, which act to
drive the nanocrystals into the self-assembled
surface patterns.