MILOS IADR

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Self-Assembly
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What is Self-Assembly?
Simply speaking, we're talking about collections
of objects that put themselves together. Imagine
holding a box containing a jigsaw puzzle, giving
the box a shake, and peeking inside to find that
the puzzle had assembled itself! While such
behavior would be shocking in a jigsaw puzzle, a
little reflection reveals that it's not so
surprising to find self assembling systems in the
natural world. Biological systems as well as a
variety of inorganic systems exhibit self
assembling (self ordering behavior). Inspired by
these systems scientists from numerous
disciplines including chemistry, physics,
biology, engineering, and mathematics, have begun
to investigate the self assembly process in hopes
of learning to design and control the behavior of
self assembling systems. Much of this work is
motivated by recent advances in micro- and
nanoscale science. Generally speaking,
self-assembly is resulted from three types of
forces capillary forces, electrostatic forces
and magnetic forces.
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Layer-by-Layer Self-Assembly
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Building One Layer at a Time
A thin layer can have very interesting and
desirable properties. For instance, if the layer
is conductive it provides a two-dimensionally
conductive path, while occupying a minimum of
volume. As well, if a layering process is
repeated several times, the buildup can lead to
three-dimensional materials. If we can control
the vertical layering process as well as the
lateral distribution of layers, we can fabricate
a three-dimensional material with control over
shape, dimensions, and composition over all
length scale.
Schematic diagram showing the buildup of
electrostatic multilayers of soluble
polyelectrolytes (above) as well as the
generalization of the procedure to charged
objects such as clusters, sheets and rods (below)
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Organic Polyelectrolyte Multilayers
An example that effectively demonstrates the
power of the layer-by-layer (LbL) approach can be
seen in the self-assembly of redox active organic
polyelectrolyte multilayers. In this case, LbL
solution phase self-assembly of a cationic
polyelectrolyte with an anionic sulfonated
polyelectrolyte creates a redox active multilayer
film with controlled thickness. When deposited
on a conducting substrate, the result is a
multilayer film with electrochemically
addressable polyelectrolytes that has
electrochromic, ion transport, and
electrocatalytic functionality.
Schematic of electron hopping between
polyelectrolyte layers in an LBL film
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Recent examples in the literature have shown that
polyelectrolyte films can indeed be used as
electrochromics, and in some cases outperform the
best electrochromic systems available. In a
particular example, electrochromic films are
assembled from polyhexylviologen (PXV) as the
polycation, and negatively charged
poly(3,4-ethylenedioxythiophene)poly(styrene
sulfonate) (PEDOTPSS) colloids as the polyanion,
onto conductive indium-tin oxide (ITO) glass. At
a negative potential, the colorless dicationic
PXV undergoes a reduction to its deep-blue
colored radical cation, and the PEDOTPSS also
becomes colored due to an undoping of the
conductive state.
Optical performance for PEDOTPSS-PXV multilayers
of 40 and 60 bilayers at different potential
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Thickness Measurement Techniques

• Optical Spectroscopy simply monitoring the
  optical absorption as a function of number of
  layers. The measurement is usually easy but
  sometimes hard to interpret. To accurately
  measure film thickness the observation area as
  well as the extinction coefficient of the
  absorbing group must be precisely known.
• Ellipsometry Ellipsometry uses polarized light
  to probe the dielectric properties of a thin
  film, and can determine thickness, refractive
  index, as well as its roughness and large-scale
  molecular orientation.
• Quartz Crystal Microbalance (QCM) QCM
  measurements are often performed to determine the
  amount of polymer adsorbed in each layering step.
  Quartz is in the family of piezoelectric
  materials, and if subjected to an alternating
  electric field a quartz crystal cut in a
  particular orientation will contract and expand
  with characteristic oscillation frequency. This
  frequency is extremely sensitive to the mass of
  material deposited onto the surface of the
  crystal.

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Assembling Metallopolymers
High molecular weight polyferrocenylsilanes with
redox active iron sites in the backbone are a new
class of inorganic polymers. Various types of
functional groups can be bound to the
cyclopentadienyl rings and silicon atoms in the
polymer backbone. Polyferrocenylsilane
electrolytes provide access to tailored
multilayer films on both planar and curved primed
substrates with potentially interesting
semiconductive, magnetic, chemical- and
bio-sensing, drug delivery, photonic and
lubrication properties.
Synthesis of water-soluble polyferrocenylsilanes
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Polyelectrolyte-Colloid Multilayers
Many naturally occurring and/or synthetic
materials have layered structures. These consist
typically of sheets having strong intra-sheet
covalent bonds, but weak inter-sheet bonds that
can be broken to suspend individual sheet
silicates, metal oxides, metal phosphates and
others in the layering electrostatic
self-assembly process creates a new class of
multilayer polyelectrolytes-inorganic hybrid
materials.
Exfoliation of anionic layered materials with
bulky tetrabutylammonium cations leads to a
stable colloidal suspension, which can be layered
onto positively charged substrates and built into
multilayers
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Graded Composition LbL Films 1
In the context of LbL electrostatic
self-assembly, it is straightforward to introduce
compositional gradients into films by simply
controlling the amount and identity of the
constituents that comprise each bilayer. An
example that colorfully demonstrates the
simplicity and effectiveness of this approach is
the creation of an LbL quantum dot
polyelectrolyte multilayer film that displays a
luminescent nano-rainbow seen by confocal
fluorescent microscopy. This is a nice example
which exploits the synergism between the
size-control achievable for the optical
properties of semiconductor quantum dots with
spatial control of LbL electrostatic
self-assembly.
Confocal microscope image (a) of a cross-section
of an LbL film made of green, yellow, orange and
red CdTe quantum dots. On the bottom (b) is shown
the diameter-dependent band gap in these dots,
which leads to the different fluorescence colors
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Graded Composition LbL Films 2
Another example comprises a graded composition
silver nanocluster-polyelectrolyte film, which
functions through the existence of a refractive
index gradient as a dielectric mirror. These
films were made by hydrogen reduction of an
LbL-assembled poly(allylamine hydrochloride)-poly(
acrylic acid) multilayer, in which silver ions
had been exchanged with the proton of the acrylic
acid groups. The silver atoms so formed
underwent aggregation to silver nanoclusters in
spatially predefined regions of the film to
generate a silver refractive index gradient.
These tailored gradients have been shown to
behave as one-dimensional diffraction gratings.
TEM of a graded composition LbL multilayer where
the dark lines correspond to layers permeated
with silver clusters
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Layering on Curved Surfaces
An important extension of the LbL method on
planar substrates involves the formation of
multilayers of oppositely charged nanoscale
organic and inorganic polyelectrolytes, sheets
and clusters onto the curved surfaces of spheres
and rods of large dimension. This is another
illustration of the power of the technique, since
a surface of any shape can be coated just as
effectively as a planar one. In this way, it is
possible to prepare, for example, surface
functionalized microspheres and hollow capsules
thereof by removing the templating microsphere.
Process used for making hollow capsules by LbL.
Bottom left shows a TEM image of hollow PFS-PSS
spheres made using this process
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Crystal Engineering of Oriented Zeolite Film
Zeolites are very important industrial materials
used for catalysis, separation, detergents and
waste remediation. However, for many
applications these materials are required as
membranes or thin films. The LbL strategy by
definition is thus a great prospect for building
functional zeolite thin films.
Functionalization of zeolite nanocrystal and
glass surfaces (above). Organic polyelectrolytes
used in this work (middle). A sample of possible
stacking designs for zeolite-polyelectrolyte
composites (bottom)
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Zeolite-Ordered Multicrystal Arrays
When we look at electrostatic superlattices
fabricated by LbL deposition, these are rarely
well ordered. Rather, the layers resembled a
tangled mess of strands where a particular
polymer chain can penetrate into several adjacent
layers. Order can be introduced by incorporating
colloidal sheets into the process, and these act
as physical barrier to prevent interpenetration.
SEM image of an ordered zeolite nanocrystal array
templated by an ordered polymer multilayer
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Crosslinked Crystal Arrays
Surface functionalization of single size and
shape crystal building blocks enable them to
self-assemble, organize and bind to suitably
functionalized substrates. In the case of
zeolite crystals, micron scale cubes were
arranged as close-packed monolayers and
multilayers, however, their adhesion to the
substrate was weak because of the small area to
large mass ratio. An elegant way to surmount
this mechanical stability problem involves
chemical crosslinking the crystals through imine
or urethane linkages.
Schematic depicting the crosslinking of
amine-terminated zeolite crystals by dialdehydes
or diisocyanates
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Non-Electrostatic Layer-by-Layer Assembly
Up to this point, almost all the examples deal
with the layering of polyelectrolytes of opposite
charges. The buildup of multilayers is
accomplished by the cooperative action of a large
number of relatively weak interactions (as
compared to covalent bonding), in this case
electrostatic bridges. These bridges can be
replaced by interactions such as hydrogen
bonding, or ligand-receptor interactions, and
even covalent bonds in the case of the ordered
zeolite arrays mentioned before.
Side-by-side assembly of red and blue fluorescent
materials. Fluorescence images using a blue or
red filter clearly displays the formed patterns
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Layer-by-Layer Self-Limiting Reactions
Very much related to chemical vapor deposition
(CVD) is the technique of atomic layer deposition
(ALD). CVD is carried out in the gas phase, while
ALD is usually carried out in solution. ALD
begins by treating a surface with a reactant,
which will saturate all available surface sites.
For instance, if we expose a silica surface to a
solution of silicon tetrachloride in methylene
chloride, the SiCl bonds will react with all
available surface hydroxyl groups to form a
covalently bound monolayer. The surface will
then be capped with SiCl groups instead of SiOH
groups. If this surface will then be dipped into
water-saturated methylene chloride, the water
will react with the surface SiCl bonds thereby
regenerating a hydroxyl-capped surface. If these
steps are repeated, a layer of amorphous silica
is produced.