Ph.D. Thesis
Title Electrostatically Controlled Formation
of Nanocomposite Thin Films with Organic Matrices
Adviser Dr. Murali Sastry
Thesis Committee Dr. Murali Sastry, Materials Chemistry Division,
National Chemical Laboratory (NCL), Pune; Dr. Dominique Langevin,
Laboratoire de Physique des solides, Universite Paris Sud, France;
Dr. C. Manohar, Materials Chemistry Division, Bhaba Atomic Research
Centre (BARC), Bombay; Dr. Ponarathnam, Polymers Chemistry Division,
National Chemical Laboratory, Pune.
Essay
Developing techniques for synthesis and characterization
of nanostructure have become a popular target and one of the grand
challenges in materials synthesis. The remarkable, size-dependent
physicochemical properties that nanoparticles can have, has fascinated
and inspired the research activity in this direction. The techniques
such as microlithography and deposition from the vapor that are extensively
used to fabricate microstructures and devices require substantial
efforts for extension to nanometer range. To meet this challenge,
"engineering up" approach of synthesizing supramolecular assemblies,
molecule by molecule to functioning electronic device taking cue from
nature for use interplay of weak noncovalent bonding interactions,
has become an increasingly attractive prospect. In this approach,
nanoparticles play an important role of being building blocks of future
nanostructures and organization of colloidal particles has been a
subject of great interest, and is also the objective of this thesis.
The kind of construction of nanocomposite thin films illustrated in
my thesis work is essentially based on the use of non-covalent interactions
for growing the thin films, which are self-synthesized.
In the work discussed in the thesis, repulsive electrostatic
interactions were extensively used for stabilizing the colloidal particles
and attractive electrostatic interactions between -NH3+
and -COO- was used to organize them. The motivation for
the study was from the earlier work in our department on spontaneous
organization of thermally evaporated fatty lipid films by an ion-exchange
process. The fascinating part of this work was that such ion exchange
leads to an organized lamellar film structure similar to c-axis oriented
Y-type Langmuir Blodgett films. Recognizing that the principle
of ion exchange is quite general, this approach has been extended
to incorporate charged colloid into oppositely charged fatty lipid
film in my thesis work.We have used the colloidal route, which is
one of the most versatile methods for the synthesis of metal and semiconducting
nanoparticles. The surface modification of colloidal particle surfaces
was achieved using self-assembled monolayers of a bifunctional molecule
which serves the dual purpose of electrostatically stabilizing the
colloidal particles as well as providing a means of anchoring the
particles to charged amphiphiles and organize them in fatty lipid
matrix. Three dimensional self-assembly of n-alkane thiols and various
other aspects of functionalization has been carried out rigorously
in this work resulting in the first successful venture of exploiting
hydrophobic interactions between the long chain fatty lipid molecules
on the curved surface of clusters to form interdigitated bilayer structures.
This strategy has been exploited to derivatize colloidal particles
without the use of bifunctional molecules.
We have made a rigorous and successful attempt to organize
colloidal particles in organic matrices to form nanocomposite thin
films. It is shown that surface-modified colloidal particles can be
viewed as "giant ions" and incorporated into thermally evaporated
fatty lipid thin films by a simple immersion of the films in the colloidal
solution. The cluster incorporation process is driven by the strength
of attractive electrostatic interactions between the charged
polar groups of the film matrix and the surface groups on the colloidal
particles. Using various characterization techniques, it has been
shown that the cluster density in the films can be controlled by simple
variation of the colloidal solution pH. This is the only technique,
which can demonstrate flexibility of controlling cluster density in
organic matrix with simple variation of charge on the surface of clusters
or the head-group of the fatty lipid by altering pH of colloidal solution.
Also this is the unique technique to get very high volume fraction
of colloids in nanocomposite, which is more than twice the values
reported in literature till date.
Influence of colloidal particle concentration, particle
size, solution pH as well as film thickness on the kinetics of cluster
incorporation was obtained by quartz crystal microgravimetry (QCM)
measurements. These results were discussed in terms of a one-dimensional
(1-D) Fickian type diffusion model. It was found that 1-D diffusion
adequately represents the cluster mass uptake kinetics observed using
QCM, and an interesting film thickness and particle size dependence
on the cluster diffusivity was observed. The applicability of this
model to thermally evaporated films, leads to physically meaningful
cluster concentration enhancements at the film-colloidal solution
interface as well as cluster diffusivities. It is also observed that
the cluster diffusivity increases as the cluster size is reduced.
The pH at which maximum cluster incorporation occurs, is strongly
dependent on the cluster size which is a very interesting result indicating
that the nanoscale curvature influences strongly the ionization of
the carboxylic acid groups in the monolayer surrounding the particles.
The kinetics of cluster diffusion has also been studied using FTIR
and in-situ and ex-situ UV-visible spectroscopies. FTIR
spectroscopy is used to look at the fatty lipid matrix in the nanocomposite
to understand coupling of the particles to the organic matrix and
change in the orientation of hydrocarbon chains as clusters get incorporated.
Whereas comparison of in-situ and ex-situ UV-Visible spectroscopy
revels important information about swelling of organic matrix as a
function of cluster size and solution pH and this information with
FTIR studies gives a complete picture of the process of cluster diffusion
in fatty lipid films. More than 30 different nanocomposites of various
metal and semiconducting colloids of varying size with varying charge,
in combination with corresponding fatty lipid organic matrix were
systematically studied in the thesis work and role of specific parameters
governing the film composition was thoroughly exploited.
Formation of nanocomposites of metal colloidal particles
with surface derivatization using interdigitated bilayers was also
studied in great detail as a part of my thesis work. It has been shown
that facile incorporation of colloid with primary monolayer of octadecanethiol
and secondary monolayer of carboxylic acid/amine can be achieved into
thermally evaporated oppositely charged fatty lipid matrix. The advantage
of using this approach is that it gives additional flexibility where
varying the chain length of the secondary monolayer as well as the
pH of the sol can monitor diffusivity and cluster density and eventual
structure of the nanocomposite. The study on this novel technique
of formation of nanocomposites was further continued through an investigation
of the partitioning of carboxylic acid derivatized clusters
of different sizes during a simultaneous incorporation process. It
is observed that inspite of the larger diffusivities of the small
colloidal particles, large particles are incorporated in the amine
matrix. This "reverse" fractionation of the clusters was confirmed
using Transmission Electron Microscopy and discussed in terms of an
electrostatic model and the energetics of distortion of the amine
matrix during cluster incorporation strongly supported by experimental
and theoretical 1-Dimensional diffusion modeling data.
Recent developments in this field seem to indicate that
future research will be directed towards the controlled design of
both nanometer scale architectures and bulk materials built up from
nanosized constituents. In this contest my thesis work where the novel
concept of rationally assembling nanoparticles into organic matrix
with intrinsic advantage of flexibility of controlling the cluster
density is gaining significant importance. One of the most attractive
prospects of this strategy is that technologically important, oxide
and magnetic particles like TiO2, Fe3O4 can be
organized using this technique and this work is currently been pursued
in our lab. Our group has recently demonstrated that biologically
important molecules like proteolytic enzime pepsin can be successfully
encapsulated in the fatty lipid films. In addition to the above, well-defined
superlattice structures can be obtained through alternate deposition
of fatty lipid and incorporating different hydrosols which shows promise
for obtaining alternating metal-semiconductor nanocomposites. We believe
that, this technique has exciting potential for application in chemical
and biological sensors, in addition to advanced materials with novel
optical and electronic properties.