Download Metal polymer multiscale material formed by arranging

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Metal polymer multiscale material formed by arranging metal nanoparticles
in self patterned spin-on films
P. MIKE GÜNTHER, J. MICHAEL KÖHLER
Techn. Univ. Ilmenau, Institute of Micro- und Nanotechnologies/ Institute for Physics,
Dept. Phys. Chem. and Microreaction Technology, Ilmenau, GERMANY
corresponding author:
Prof. Dr. J. Michael Köhler, Techn. University Ilmenau, Institute of Micro- und Nanotechnologies/ Institute
for Physics, PF 10 05 65, D- 98684 Ilmenau, GERMANY
Abstract: Self-patterned polymer films formed by PEGA/PMMA and PVP/PMMA mixtures were generated during
spin coating of silicon chips. PVP/PMMA films were applied for arranging gold/silver-core/shell nanoparticles on the
film surfaces. The method of spontaneous demixing during spin coating was applied for formation of self-patterned
polymer films. A fast demixing is caused by the fast evaporation of ethanol in the final phase of spin-on deposition.
It results in the formation of polymer domains with a typical size in the lower micrometer range. The application of
aqueous colloidal solution of metal nanoparticles leads to a preferential bonding of the hydrophilic nanoparticles in
the PVP domains of self patterned PMMA/PVP films. By this method patterned surface films of metals could be
generated without application of lithographic techniques
Keywords: nanotechnology, particle manipulation, multi-scale material, self-organization, nanoparticles, gold, silver,
PMMA, polyvinylpyrrolidone
1 Introduction
A specific feature of metal nanoparticles is their
interaction with light. The specific intrinsic energy
states of the particles results in a characteristic light
absorption connected with the plasmonic states of the
particles. The wavelength and intensity of this
absorption as well as light scattering and reemission
are dependent of the nanoparticle material and their
size and shape [2]. Gold and silver are of particular
interest for the application of plasmonic nanoparticles.
On one hand, the noble metal particles are stable in
aqueous colloidal solutions on the other hand the
plasmon absorptions can cover different parts of the
visible spectrum. So, the gold plasmon absorption is in
the range of green light (526 nm) resulting in a red
color of the colloidal solution. Silver nanoparticles
show a characteristic plasmon absorption band at
about 410 nm (violet light) resulting in a yellow color
of colloidal solution. Alloy particles as well as Au/Ag
core/shell particles show plasmon absorption bands in
the whole range between this both values, whereby the
position and the shape of the band can be controlled
by the composition and size of the particles [3].
Reduced energy gaps result from larger particles and
particle aggregates with electronic interactions. This
can be used for detection of interactions between
nanoparticle-labeled molecules [4]. This longer
wavelength absorption - red or purpur - leads to blue
or violet color of the colloidal solutions. The
plasmonic excitation of silver particles and core/shell
The development of nanoparticle-based composite
materials can be used for realization of new functions
in dependence on their specific mechanical, electrical,
chemical or optical properties. These properties are
not only determined by the quality of the components
but are mainly controlled by the distribution of
particles and phases. A high effect of distribution can
be expected if the components differ in their
properties significantly. This is particular the case in
systems consisting of conductors and isolators as
metal/polymer composites.
Metal nanoparticles are of scientific and practical
interest due to their specific electronic and optical
properties as well as their catalytic activity. Metal
nanoparticle materials combine the high electrical
conductivity of metals with the possibility of
controlled electron transfer between them. Isolated
metal nanoparticles can be act as electron
confinements and can be addressed by capacitive
coupled nanoelectrodes. Ultrathin oxide films or
molecular surface films can be used for controlling
electron tunneling between nanoparticles or between a
nanoparticle and a conductive substrate. So, metal
nanoparticles and in particular arrangements of
nanoparticles and polymer molecules are of interest
for the development of nanoelectronic devices [1].
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particles with a silver surface came to the focus of
analytical techniques due to the possibility of surface
enhanced Raman scattering [5]. By this method, small
concentrations of organic molecules can be detected
by a non-invasive method if these substances are
adsorbed at the surface of the optically active metal
nanoparticles and modulate their plasmonic behavior
[6].
transparent and non-conductive materials are more
interesting if the plasmonic properties of the
nanoparticles are addressed. So, polymer self
patterning seems to be a better principle of
lithographic-free definition of binding areas for
nanoparticles. Such structures can be generated by
mechanic stress or by precipitation and demixing
effects during the evaporation of solvents [13]. The
application of polymer films with thicknesses in the
order of magnitude of 1 micron is suited for the
generation of polymer patterns with typical length at
the scale of a few microns. Therefore, the possibility
of generation of such self-micro patterned polymer
films and their applicability for modulating the local
binding density of Au/Ag core/shell nanoparticles was
investigated.
For technical purposes, the metal nanoparticles must
be brought into a suited micro environment. This
could either be given by the application of metal
nanoparticles at solid surfaces [7], by incorporation in
micro particles or by integration in a carrier matrix
under formation of a composite material. Polymers are
particular interesting for such incorporation. The
properties of the polymer matrix and in particular the
electron and energy transfer properties can be used for
modulate the optical properties of the incorporated
metal nanoparticles. Vice versa, the integration of
metal nanoparticles into the polymer matrix can be
used for tuning their physical properties [8, 9].
2 Experimental
Synthesis of monodisperse Au/Ag nanoparticles
Metal nanoparticles can be integrated in polymer
matrices if their surface properties are adapted to the
chemical properties of the polymers. Molecular
surface capping of metal nanoparticles [10] is a suited
way for achieving compatibility with the desired
polymer matrix. Alternatively, hydrophilic metal
nanoparticles can be introduced into a polymer matrix
by application of water-soluble polymers [11]. This
method is well suited for generation of particle-doped
polymer thin films made by spinning.
The metal nanoparticles were synthesized in aqueous
solution by application of ascorbic acid as reducing
agent. In a first step gold nanoparticles were prepared
by reduction of tetrachloroauric acid at room
temperature. Therefore, 1 mM solution of HAuCl4 was
fast mixed with a 20 mM solution of ascorbic acid.
The gold nanoparticles are formed in about one
second. The formation is indicated by the appearance
of the typical purple color of the colloidal solution of
metallic gold particles.
A more or less statistical distribution of nanoparticles
is expected if they are immobilized or adsorbed at a
smooth solid surface. Self-patterning is only observed
by particle aggregation during the adsorption process.
Irregular islands of particle aggregates or irregular
network structures are mainly observed in these cases.
Alternatively, the density of immobilized particles can
be controlled by application of lithographically
patterned surfaces. Therefore, polymer films can be
used or functionalized micro areas are applied. So,
gold nanoparticles capped by oligonucleotides of
DNA can be arranged in microsquares by application
of patterned films of complementary oligonucleotides
[12].
For preparation of nanoparticles with silver shell, the
colloidal gold solution was diluted and the final
concentration of ascorbic acid was chosen in the range
between1 and 5 mM. The silver nitrate solution (in the
concentration range between 2 mM and 10 mM) was
added by fast mixing. The comparatively small
concentrations of the reducing agent and the silver salt
results in a slow deposition of silver. The silver
nanoparticle formation is very slowly in the absence
of any metal surfaces. Gold nanoparticles act as
catalytic species for silver deposition. Metallic silver
is formed at the surface of the gold nanoparticles. The
deposition rate is dependent on silver salt
concentration, ascorbic acid concentration, pH,
temperature and addition of effectors adsorbing at the
particle surface [14].
Despite microlithography, the self patterning of thin
films could open a possibility for microlocal
modulation of the binding density of nanoparticles.
Deposition of islands of a certain thin film material by
vacuum methods as evaporation or sputtering as well
as galvanic deposition can be applied for generation of
metal spots on a smooth surface of a substrate. But,
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The complete preparation scheme is shown in fig. 1.
The procedure includes both the preparation of the
core shell metal nanoparticles and the formation of
self-patterned polymer films. The core/shell
nanoparticles and the self-patterned polymer films are
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attached to the PMMA film elements is reduced
quickly. In contrast, a certain swelling of the PVP
matrix and the stronger interaction between the
hydrophilic nanoparticles and the hydrophilic PVP
result in an enrichment of the nanoparticles at the
surface of the PVP film elements.
prepared in two separate process chains. In a final
step, the particles are brought on to the surface of the
patterned polymer film.
The chips with self-patterned polymer films with and
without metal nanoparticles were inspected by light
microscopy and by scanning electron microscopy.
Strong optical contrasts were obtained by dark field
microscopy in cause of the high light scattering
efficiency of the metal particles. In SEM graphs
(secondary electron image), the metal nanoparticles
appear as bright spots on the darker polymer
background due to the higher electron density and the
higher yield of secondary electrons of the heavy
metals.
Fig. 1 Scheme for formation of nanoparticle
arrangements on self-patterned polymer films
Self-patterning of polymer films were achieved by
demixing of solutions of two polymers with two
different solvents. The phase behavior and pattern
formation was earlier described in polymer systems
[15, 16]. Here, solutions of polymethylmethacrylate
(PMMA) in toluene (1%), polyethyleneglycol (PEG)
in ethanol (10%) and polyvinylpyrrolidone (PVP) in
ethanol (5%) were used. The mixed polymer solutions
were prepared by mixing the solutions of the pure
polymer by vortexing in an Eppendorf cup.
3 Results and Discussion
The thickness of silver shell on gold cores can be
controlled by the ratio of concentration of colloidal
gold particles and the applied silver ion concentration.
Simply, higher shell thicknesses can be achieved by
application of higher silver salt content in relation to
the applied concentration of gold nanoparticles. The
reducing agent is applied in a certain excess. The
combination of fast nucleation (gold core formation)
and low rate of silver deposition lead to a rather
homogeneous silver deposition and a homogeneous
growth of silver shells. So, Au/Ag core/shell
nanoparticles with comparatively narrow size
distribution can be synthesized. The ratio of gold
cores to silver content determines the final mean
diameter of the obtained core/shell particles (Fig. 2).
The distribution of particle diameter can be kept low
even for high shell thickness.
The polymer films were formed by spin-coating of
silicon chips. Therefore, the mixed polymer solution
was dispensed on to the chip surface. The film was
formed by a final rotation rate of about 3500 rpm. The
process of pattern formation could be monitored
visually by the changing reflection from the chip
surface. During the reduction of film thickness and
starting solvent evaporation the polymer film
remained transparent in the first phase. After a few
seconds, the film became opaque what could be seen
by an increase of intensity of scattered light. In
addition, the final reduction of film thickness, drying
and stabilization of film can be observed by changing
interference patterns during the last phase of spinning.
The dried chips with self-patterned polymer films of
PMMA/PVP mixtures were incubated with the
suspension of Au/Ag core/shell nanoparticles. After
an incubation time of about 15 minutes, the chips were
quickly rinsed by cold water. In case of PEG, the
water rinsing is not applicable, because it leads to a
fast dissolution of the PEG matrix.
Fig. 2 Formation of Au/Ag core/shell nanoparticles
with different shell thickness
The water rinsing causes a fast remove of weakly
adsorbed nanoparticles from the surface of the
hydrophobic polymer. So, the number of particles
The size of the core/shell particles can be tuned
between about 20 nm and 400 nm. The thickness of
the shell is influencing the optical properties of the
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is negligible for viscosity changes in the first phase,
but causes a strong increase in film viscosity when the
film becomes smaller and smaller. This behavior
results in to a fast decrease of centrifugal material
transport during a short time interval. This effect is
used for achieving a high precision and reproducibility
in film thickness during spin coating. Beside viscosity
increase, the solubility conditions can be changed due
to the change of solvent concentrations during the last
phase of spinning. Precipitation of polymer
components or a demixing takes place if one solvent
component with high solvation power has the highest
vapor pressure and the residual solvent alone is not
suited for solvation of the polymer or one component
of a polymer mixture.
metal particles. The gold plasmon absorption at 526
nm is dominating the optical spectrum of the
core/shell nanoparticles if the thickness of the silver
shell is low. Increasing silver shell thickness results in
an increasing dominance of the silver plasmon
absorption at about 410 nm. The color of the colloidal
product solutions after silver deposition shifts from
purple to yellow if the silver-to-gold-ratio increases.
The correspondence between particle size (shell
thickness) and optical properties is well reflected by
the centrifugal sedimentation spectroscopy and SEM
imaging for colloidal product solutions of different
optical properties. Increasing silver content in the
solution for shell deposition results into a shift of the
maximum of particle size band to higher diameter as
well as to an increase in the peak height. The expected
increasing particle diameter is confirmed by the SEM
images (fig. 3). The results of both methods show that
the silver is deposited on present metal cores. The
formation of new nuclei during the silver deposition
can be neglected. A certain population of small metal
particles present in the product solution can be
explained by residual gold cores without silver shell.
Fig. 3 Examples of Au/Ag core/shell nanoparticles with
narrow size distribution: particles possessing different silver
shell size but constant gold core diameter (SEM images of
particles adsorbed on silicon chips (without polymer film);
a-f) increasing silver shell thickness (scale bars: 1 µm)
The formation of self-organized polymer structures is
caused by the phase separation during the spinning
process. The spinning is dominated in the first phase
by a continuous reduction of film thickness of the
liquid film. The polymer solution is transported from
the center to the rim of the substrate by the centrifugal
forces. The comparatively low thickness of the liquid
film and the comparatively high viscosity are
responsible for a laminar liquid motion. This leads to a
higher surface flow velocity in regions of high film
thickness and lower flow velocity at lower film
thickness and, therefore, to a fast reduction of local
thickness differences. This effect causes the high
thickness homogeneity which is normally achieved by
spin-on film formation. The last phase of spinning is
dominated by solvent evaporation and viscosity
increase. The evaporation of solvent during spinning
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Fig. 4 Self patterned binary polymer films for different
PMMA/PEG mass ratios (optical images)
Here, we applied polymer solutions consisting of two
polymer and two solvent components. Each of the
both polymers shows a high solubility in one solvent
and a low in the other one: PVP or PEG was dissolved
in ethanol, PMMA in toluene. The solubility of
PMMA in ethanol is low as well as the solubility of
PVP or PEG in toluene. But, ethanolic solutions of
PVP (or PEG) and toluene solutions of PMMA can be
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mixed and form a homogenous liquid. This viscose
liquid is applied on silicon chips for spin-on film
formation. During the spinning, the concentration of
ethanol is faster reduced than the concentration of
toluene due to the higher ethanol vapor pressure. The
passing of limit concentration of ethanol for
miscibility leads to a phase separation, which can be
observed during the spinning process by an increase of
scattering of light from the film. A toluene-rich phase
containing the PMMA is formed beside a PVP-rich
phase or a PEG-rich phase containing the majority of
the residual ethanol.
network consisting of a phase appearing dark in SEM
and a second phase appearing bright. The feature sizes
of the networks structures are about 10 microns. The
dot-like structures have sizes of about 1 to 2 microns.
A high number of bright dots was found in the darker
matrix area. This pattern could be interpreted as a twophase demixing process. It is assumed that the low
ethanol content leads to an early separation into a
ethanol-rich phase (A) mainly containing PVP and a
toluene-rich phase mainly containing PMMA (B). But
both phases contain a certain part of the other solvent
and the second polymer. The consequence is a second
demixing step inside the previously formed phases if
the residual ethanol is evaporated.
The phase separation takes place in a time interval in
the order of magnitude of a second or less. This time
is much to short for a long-range phase separation.
The already reduced film thickness and the fast further
increase of viscosity by solvent evaporation result in a
microlocal character of the separation process.
Consequently, more or less regular micro pattern with
characteristic dimensions of the single features in the
lower micrometer range and characteristic shapes are
obtained.
Examples of typical demixing patterns in the system
PMMA/PEG are given in fig. 4. Therefore different
ratios of PMMA-solution in toluene and PEG-solution
in ethanol were mixed. The SEM images show bright
areas corresponding with the PEG phases and dark
areas corresponding with the PMMA phase. Dark
islands with an extension of several microns separated
by a network of bright lines were found at low PEGto-PMMA-ratio. The principle character of this
network structure was also found if the PEG-toPMMA-ratio was slightly increased. But, in this case
the size of islands was reduced. The network became
denser. A further increase of PEG-to-PMMA-ratio
resulted in a transition of bright network into a island
structure of bright areas. AFM images show that the
polymer film pattern is also connected with a certain
surface topography. The roughness of the film surface
caused by the demixing process is in the range of
about 15 - 25 nm.
Fig. 5 Self patterned binary polymer PVP/PMMA film
(SEM images): a) polymer mass ratio 3:1, b) polymer mass
ratio 4:1 (formation of substructures)
Similar network structures were also obtained in case
of PMMA/PVP films generated from ethanol/toluene
solutions (fig. 5 A). The transition between a close
network of PVP (coherent bright areas) into isolated
islands (incoherent phase) of PVP was observed, if the
PVP/PMMA-ratio was reduced.
Metal nanoparticles can be brought into polymer
solutions if their surfaces are compatible with the
polymer and the solvent. In this case organic colloidal
solutions of the metal nanoparticle can be formed in
analogy to the colloidal aqueous solutions. The
obtained solutions contain the dissolved polymer
molecules as molecular disperse system forming the
coherent phase and the metal particles forming the
incoherent phase.
In some cases of reduced ethanol content, the
formation of a more complex pattern was found. In
this case a structure hierarchy was formed in the selfpatterned polymer films (fig. 5 B): Small dot-like
structures appear inside the features of the polymer
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The polymer phase separation can be combined with
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groups and functional groups of high affinity to the
nanoparticle surface. Such surfactants allow moving
the metal nanoparticles from the aqueous phase into a
non polar solvent as toluene. Then, the obtained
colloidal solution of metal nanoparticles in toluene
can be mixed with the PMMA solution and the PVP
solution and used for film formation by spinning. The
introduction of nanoparticles and surfactants
influences the conditions of pattern formation in the
phase separation during the spinning process. Any
additive substance influences the viscosity of the
involved liquids, the partial vapor pressures,
miscibilities and interface tensions. So, the obtained
self-organized micro pattern differs with changing
concentrations of solvents and nanoparticles.
matrix deposition of metal nanoparticles if they are
introduced into a solution containing two polymers
with different solubility properties and a suited solvent
mixture. Gold nanoparticles with a polar surface state
can be generated in ethanolic solution. So, they can be
introduced into the PMMA/PVP system by mixing the
ethanolic colloidal solution with an ethanolic solution
of PVP and further mixing with the PMMA solution in
toluene.
A selective deposition of nanoparticles at
selforganized micro spots is due to the hydrophilicity
of the metal nanoparticles. A high asymmetry between
PVP and PMMA areas could be realized if suited
rinsing conditions were applied (fig. 7). It was found
that the gold/silver core/shell nanoparticles prefer the
attachment to the more hydrophilic PVP areas and are
removed from the PMMA areas as expected. High
densities of metal nanoparticles on the PVP phase and
lower density on the PMMA phase can be achieved.
Fig. 6 Nanoparticle aggregation and preferential surface
binding of metal nanoparticles (Au/Ag) on a self-patterned
polymer spin-on films of PVA/PMMA (SEM image)
Figure 6 shows the preferential deposition of Au/Ag
core/shell particles (mean diameter: 240 nm) on a selfpatterned PVP/PMMA film (3.75:1). The highest
density of nanoparticles was found in the central
regions of the PVP areas.
4 Conclusions
The investigations show that self-organized
microstructures in thin polymer films can be applied
for arranging metal nanoparticles on chip surfaces.
The preferential bonding of gold/silver core/shell
nanoparticles is obviously due to the hydrophilic
character of these metal nanoparticles and their
affinity to the hydrophilic phase of the self-patterned
polymer film. The distribution of the metal
nanoparticles on the chip surface can be controlled by
the type of realized polymer patterns. This depends on
the ratio of applied polymers as well as on the applied
solvent mixture. The preferential bonding of metal
nanoparticles is not restricted to small nanoparticles,
but can also be applied for particles up to diameters of
several hundred nanometers.
Fig. 7 Selective surface binding of Au/Ag core/shell
nanoparticles on self-patterned polymer spin-on films
(PMMA/PVP)
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