By Gerd Kästle,* Hans-Gerd Boyen, Frank Weigl, Gunther Lengl, Thomas Herzog, Paul Ziemann,
Silke Riethmüller, Oliver Mayer, Christoph Hartmann, Joachim P. Spatz, Martin Möller, Masaki Ozawa,
Florian Banhart, Michael G. Garnier, and Peter Oelhafen
The preparation of hexagonally ordered metallic nanodots was studied in detail with emphasis on the chemical state of the resulting particles. To obtain these dots, in a first step micellar structures were formed from diblock copolymers in solution. The
reverse micelles themselves are capable of ligating defined amounts of a metal salt within their cores, acting as nanoreactors.
After transfer of the metal-loaded reverse micelles onto a substrate, the polymer was removed by means of different plasmas
(oxygen and/or hydrogen), which also allow the metal salt to be reduced to the metallic state. In this way, ordered arrays of metallic nanodots can be prepared on various substrates. By adjusting the appropriate parameters, the separation and the size of
the dots can be varied and controlled. To determine their purity, chemical state, and surface cleanlinessÐall of which are crucial
for subsequent experiments since nanoscale structures are intrinsically surface dominatedÐin-situ X-ray photoelectron spectroscopy (XPS) and ex-situ transmission electron microscopy (TEM) were applied, also giving information on the formation of
the nanodots.
1. Introduction
Pushed by the ever-growing needs of information technology
for smaller and smaller logic and memory devices on one hand
and scientific curiosity about the behavior of matter on small
length scales on the other, nanoscience has developed into one
of the most active fields of research worldwide. A very attractive feature of these activities is the strong tendency towards
interdisciplinary cooperation. This becomes immediately apparent in the context of the development and testing of various
approaches to preparing nanostructures, often referred to as
the development of nanolithographies. For example, top-down
±
[*] Dr. G. Kästle, Dr. H.-G. Boyen, F. Weigl, Dr. G. Lengl, Dr. T. Herzog,[+]
Prof. P. Ziemann
Abteilung Festkörperphysik, Universität Ulm
D-89069 Ulm (Germany)
E-mail: [email protected]
Dr. S. Riethmüller, O. Mayer, C. Hartmann, Prof. J. P. Spatz,[++]
Prof. M. Möller[+++]
Organische Chemie III, Universität Ulm
D-89069 Ulm (Germany)
Dr. M. Ozawa, Dr. F. Banhart
Zentrale Einrichtung Elektronenmikroskopie, Universität Ulm
D-89069 Ulm (Germany)
Dr. M. G. Garnier, Prof. P. Oelhafen
Institut für Physik, Universität Basel
CH-4056 Basel (Switzerland)
[+] Present address: Infineon Technologies AG, D-81730 München, Germany.
[++] Present address: Institut für Physikalische Chemie, D-69120 Heidelberg,
Germany.
[+++] Present address: Institut für Textilchemie und Makromolekulare Chemie,
RWTH Aachen, D-52056 Aachen, Germany.
[**] The continuous support by the Deutsche Forschungsgemeinschaft (DFG)
within SFB 569 and SP 1072, the Bundesministerium für Bildung und Forschung (BMBF), the Swiss National Science Foundation (NF),02 and the
National Center of Competence in Research (NCCR) ªNanoscale
Scienceº is gratefully acknowledged.
Adv. Funct. Mater. 2003, 13, No. 11, November
DOI: 10.1002/adfm.200304332
procedures such as optical lithography, although offering the
advantage of being parallel processes with the possibility of
defining a high density of structures during a single exposure,
approach their practical limits set by the availability of smallwavelength sources in the deep and extreme UV as well as the
corresponding optical instrumentation. Similarly, electron
beam lithography has its limitations due to proximity effects in
the applied resist and, additionally, is a serial and therefore
slow process. A thorough discussion of these lithographies and
their state-of-the-art can be found in the literature.[1]
A completely different approach, often referred to as bottom-up, relies on the self-organization of larger molecules used
as building blocks for the resulting nanostructures. Well-known
examples are self-assembled monolayers (SAMs), which form
stable and structurally well-defined aggregates on top of various substrates, which can then be used as masks in a subsequent etching process. This approach plays an essential role in
the context of microcontact printing, where SAM-forming
ªinksº are transferred by designed stamps with patterns on the
nanoscale.[2] Due to the large variety of the molecular building
blocks, this technique allows the preparation of many different
periodic and aperiodic patterns. Since, however, contact printing relies on the fabrication of corresponding stamps, its length
scale is limited in the same way as top-down lithography.
Other bottom-up approaches are based on the techniques of
colloidal chemistry, either permitting the direct preparation of
nanoparticles or using ordered arrays of colloids as masks for a
subsequent preparation process. Examples of direct and indirect processes are, respectively, inorganic semiconductor nanocrystals[3] and ordered domains of densely packed spherical
colloids serving as masks for metal evaporation.[4]
Alternative routes to nanolithography by means of bottomup approaches take advantage of scanning probe microscopy
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(SPM). Among them, techniques such as the manipulation of
single atoms[5] or clusters within an electrochemical environment[6] or the delivery of molecules to a surface using a dippen technique[7] have successfully been established to prepare
nanostructures with feature sizes below 10 nm.
Yet another variant of the bottom-up approach, which is at
the focus of the present article, exploits the self-organization of
polymer blends[8±11] in general or, more particularly, of diblock
copolymers[12±14] on top of chemically homogeneous or heterogeneous surfaces for the formation of lithographic masks.
Masks, however, in any case demand a second process to transfer their pattern onto a substrate either by etching or by depositing material through the windows defined by the mask. For
example, by evaporating or sputtering metals, arrays of metallic nanoislands can be obtained, which due to their extraordinary electronic, magnetic, superconducting, or optical properties are at the center of our interest. Our route to such
nanostructures is much more direct, as will be described in the
following. The basic idea[15,16] is to use spherical reverse micelles, which form when a diblock copolymer consisting of a
hydrophilic and a hydrophobic block is dissolved in an apolar
solvent such as toluene. The core of such reverse micelles can
be loaded with a metal salt ligated by complexation or protonization to the inner polymer block. In a second step, the loaded
micelles are deposited onto a smooth substrate, where they
form a hexagonal array due to their spherical shape, if the deposition process is optimized to result in one single layer of micelles. Thus, the reverse micelles are simply used as carriers for
the metal precursor, exploiting their self-assembly into an ordered array. Next, in a final step, the precursor salt is reduced
to the corresponding metal by exposing the micellar arrangement to a plasma, which simultaneously allows the polymer
matrix to be removed. As will be demonstrated below, this removal is possible without destroying the original order of the
reverse micelles, i.e., the result is an ordered array of metallic
nanodots, the size of which as well as the interdot distance can
be controlled chemically.
After a short introduction to the basic steps of our bottomup approach as already addressed above, emphasis will be
placed on the formation of the metal particles within the micellar core, as well as on chemical processes taking place during
the removal of the polymer matrix by plasma processes or laser
ablation. Additionally, results will be given on the spatial correlation functions of the resulting metallic nanoparticles and
their size distribution.
2. Results and Discussion
2.1. Preparation of Nanoparticles by the Micellar Approach
The preparation of metallic nanoparticles on various substrates is based on the self-assembly of suitable diblock copolymers. First a polystyrene (PS)-block-poly(2-vinylpyridine)
(P2VP) copolymer is dissolved in an apolar solvent such as toluene. As a consequence, spherical reverse micelles are formed
in the solution, the hydrophilic P2VP forming the core and the
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hydrophobic PS the outer shell. Next, a metal salt (e.g.,
HAuCl4) is added to the micellar solution and, while stirring
continues, the salt slowly migrates into the core of the micelles,
where it is bonded by either complex formation or protonization. Finally, at equilibrium, all micelles are loaded with an
equal amount of metal salt (see Fig. 1a). For more details of
this procedure (materials, concentrations, reaction times, etc.)
see the literature.[16]
(a)
(b)
Fig. 1. a) Sketch of the formation of reverse micelles within PS-P2VP diblock copolymer solutions. The core of the micelle can be loaded with a metal precursor
(e.g., HAuCl4). b) The reverse micelles in solution are transferred onto substrates
by dip coating. The polymer is removed and, simultaneously, the metal salt is reduced by applying either an oxygen plasma followed by a hydrogen plasma or by
a single hydrogen plasma step (more details are given below).
One way of transferring the reverse micelles from the solution to a substrate is to use standard spin-coating techniques.
This way, the micelles will be distributed randomly on the substrate without being destroyed and the solvent will evaporate
during coating. It turned out, however, that dipping the substrate into the micellar solution and pulling it out at a controlled velocity (5±50 mm/min) is much simpler, faster, and
more reliable. While the substrate is being pulled out, the solvent evaporates more slowly than in the case of spin-coating
and one single layer of micelles is formed, which exhibits a high
degree of hexagonal order, reflecting the packing of the spherical micelles. At this stage the core of the micelles is still loaded
with the metal precursor rather than with a metal. Thus, in order to obtain regularly arranged ªnakedº metal nanodots on
top of a given substrate, additional steps are necessary. Surprisingly, it was found that the removal of the surrounding polymer
as well as the reduction of the metal precursor to a metal can
be accomplished by exposing a ªmonomicellarº layer to different plasmas (oxygen, hydrogen), as shown schematically in
Figure 1b.
Most importantly, it is observed that despite such a chemically aggressive plasma process, the original order of the micellar
arrangement is conserved and the polymer matrix is removed
completely, i.e., hexagonally arranged metallic nanodots are
obtained as demonstrated in Figure 2. Here, atomic force microscopy (AFM) pictures (Fig. 2a) are presented, showing a
single layer of Au-loaded reverse micelles deposited on a silicon substrate by dip coating and, additionally, the same sample
after the polymer had been completely removed by an oxygen
plasma (Fig. 2b). Reasonably good local order of the metalsalt-loaded micelles (Fig. 2a) and of the array of naked metallic
nanoparticles after the plasma process (Fig. 2b) is demonstrat-
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periments indispensable. Adsorption of hydrocarbons due to
sample transport in air would result in the same fingerprint as
residual polymer from incomplete removal of the micelles
when looked at by spectroscopic measurements. Therefore the
samples were exposed to different plasmas (oxygen, hydrogen)
within the preparation chamber of an electron spectrometer
and then transferred in situ to the analysis chamber, allowing
X-ray photoelectron spectroscopy (XPS) to be performed. As
an example of this procedure, in Figure 3 XPS spectra of Auloaded micelles on silicon are presented with emphasis on the
Au 4d, C 1s, and Cl 2s core levels. Here, the sample was ex-
Fig. 2. Gold-loaded micelles on a silicon substrate before ((a), z-scale 30 nm)
and after ((b), z-scale 15 nm) removal of the polymer matrix. Clearly, the order
of the resulting nanoparticles reflects the order of the original micellar array.
Local order is reasonably good as proven by the autocorrelation functions in the
insets. The bottom panels show the results of line scans measured along the white
lines shown.
ed by the autocorrelation functions displayed in the upper right
corners. It should be emphasized that, after removal of the
polymers, AFM images as presented in Figure 2 can be measured at different sample positions representing macroscopic
distances (millimeters), demonstrating the homogeneity of the
micellar approach. Figure 2 also presents line scans (bottom
panels) as measured along the white lines, showing a height of
about 19 nm for the micelles after their deposition on top of a
silicon wafer and a height of 6 nm for the resulting nanoparticles.
2.2. Formation and Chemical State of the Nanoparticles
The previous section gave an outline of the experimental
steps that are involved when metallic nanoparticles are prepared by the micellar approach. The aspects of particle spacing,
size, and homogeneity could be well characterized by AFM
measurements. To study various physical properties of the
nanoparticles, however, more detailed information on all preparational steps is necessary. Since the surface-to-volume ratio
of particles increases drastically when their dimensions are reduced to 1 nm, all properties are very likely influenced by surface effects. Whereas for bulk samples cleanliness and the
chemical state of the surface will affect the properties only to
minor degree, nanostructures or nanoparticles can even be
dominated by the surface conditions. Ultimately this can even
lead to such dramatic effects as a ligand-induced metal-to-insulator transition as shown recently for Au nanoparticles.[17] In
this section detailed studies of the plasma process will be presented. Emphasis is placed on the cleanliness of the method,
the formation of nanoparticles, and their final chemical state.
The plasma experiments on the various samples were carried
out either ex situ (TePla 100-E microwave plasma system) or in
situ, depending on the experimental aspect of interest. Especially the issue of cleanliness of the method made in-situ ex-
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Fig. 3. XPS spectra for Au-loaded micelles on top of a Si substrate showing the
Au 4d, C 1s, and Cl 2s core levels after exposure of the sample to an oxygen
plasma for increasing times. After the final plasma step (30 min), the amount of
remaining carbon atoms is found to be below 1/200 monolayer as estimated from
the corresponding C 1s core line intensity.
posed to an oxygen plasma for increasing times as indicated for
each spectrum. In the as-prepared state (topmost spectrum) a
very weak Au 4d signal and a strong C 1s signal are observed
since the photoelectrons originating from the Au loaded core
are strongly suppressed by the surrounding polymer. The next
two spectra, taken after 5 and 10 min of plasma exposure, respectively, clearly demonstrate that the polymer component is
significantly reduced. After 30 min of oxygen plasma the C 1s
peak and the Cl 2s peak have both disappeared, proving that
the polymer has been completely removed, resulting in naked
Au nanoparticles on top of the silicon substrate. The fact that
the above plasma procedure leads to clean particle surfaces
was exemplified for Au nanoparticles here, but holds for nanoparticles of other materials as well.
If a hydrogen plasma is used instead of oxygen a similar behavior is observed: the polymer is removed and the metal salt
is reduced. In more detail, however, transmission electron microscopy (TEM) images of nanoparticles revealed a significant
difference between samples that were prepared by hydrogen or
oxygen plasma treatments. Whereas formation of nanoparticles
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occurs early in the plasma process for oxygen, the formation is
strongly retarded in the hydrogen plasma. The top row of Figure 4 shows TEM pictures of Au nanoparticles after different
hydrogen plasma exposure times. Even though a significant
Fig. 4. TEM images of Au-loaded micelles that were exposed to hydrogen (a) or
oxygen (b) plasmas (100 W, 0.2 mbar) for different times. The formation of Au
nanoparticles is significantly slower if a hydrogen plasma is used instead of an
oxygen plasma.
amount of polymer is already removed after 2 min, the core
volume still contains a significant number of smaller particles
(dark spots in the picture) rather than just one. It is only after
about 20 min of hydrogen plasma exposure that coalescence of
these tiny metallic fragments leads to one single particle. The
(a)
bottom row of Figure 4 shows a similar sequence for Auloaded micelles exposed to an oxygen plasma. The formation
of the nanoparticles is practically accomplished after 1 or
2 min of plasma exposure even though the polymer matrix is
far from completely removed.
Additional evidence for faster formation of Au nanoparticles
within an oxygen plasma than in a hydrogen plasma is provided
by XPS spectra of the Au 4f core levels. This is demonstrated
by Figure 5, where the spin±orbit split Au 4f core level spectra
of Au-salt-loaded micelles are shown after exposure to an oxygen (a) or hydrogen (b) plasma (100 W, 0.2 mbar) for increasing times. The topmost spectra (identical in both panels) represent the as-introduced state of the micelles without any plasma
treatment. Clearly, two different doublets can be distinguished,
indicating different chemical environments of the probed Au
atoms. Based on the observed shift of the binding energy
(ªchemical shiftº) as compared to a Au reference film (lowest
curve), one of these doublets can be attributed to Au atoms in
a highly oxidized state (corresponding to the metal salt[18]), the
other one to metallic Au, which, however, exhibits an offset towards higher binding energies of the order of 1 eV. This offset,
in the present case, strongly points to the presence of single Au
atoms or small clusters (dimers, trimers, etc.[19,20]). During the
photoionization process these clusters are charged and, as a
consequence of their small, size-dependent electrical capacitance, a significant energy loss of the escaping photoelectron
(final-state effect[21]) is observed, leading to the above mentioned offset. Hence, because of the different chemical shifts
(metal salt, metal) observed for the as-prepared monomicellar
film, one has to conclude that the pure metal salt stored within
the micellar cores had already decomposed in part either within the micellar solution (before the dip-coating process) or
(b)
Fig. 5. Au 4f core level spectra of Au-loaded micelles after different plasma treatments performed ex situ. The formation of the nanoparticles is significantly different for preparation by oxygen plasma (a) or hydrogen plasma (b).
Because the XPS measurements were performed several days after the plasma exposure, no additional component
corresponding to Au oxide is observed in the left diagram. In this case, the metastable Au oxide that was present
immediately after the plasma treatment has already decomposed into pure Au and oxygen molecules.
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Adv. Funct. Mater. 2003, 13, No. 11, November
after the dip-coating process due to the exposure of the sample
to ambient conditions.
After exposing the sample for 2 min to either an oxygen (left
side) or a hydrogen (right side) plasma, the doublet corresponding to Au in a highly oxidized state completely disappeared, indicating that the metal salt is already reduced to its
metallic state after such treatments. In the case of oxygen, the
position of the Au 4f lines corresponding to the metallic component is close to what is expected from the Au reference sample (binding energy difference 0.1 eV). This result points to the
formation of single large (10 nm) particles with a correspondingly large capacitance. For the sample treated in the hydrogen
plasma there still exist two different metallic componentsÐone
at the position close to the bulk reference sample and another
one, which is even more pronounced, still showing the energy
shift due to the presence of small clusters. Thus, one has to conclude that only a small fraction of the metal within a micellar
core has formed into a large particle. Rather, many small clusters still reside within the core isolated from each other. It is
only after an extended hydrogen plasma treatment of 20 min
that all the clusters have merged into one single nanoparticle
as monitored by the corresponding Au 4f spectrum, which is
then practically identical to the bulk reference. Thus, both the
TEM images (Fig. 4) and the XPS results (Fig. 5) consistently
reveal that, for the oxygen plasma, the formation of single
nanoparticles occurs on a short time scale whereas, for the hydrogen plasma, the particle formation is significantly slower.
An alternative approach to removing the polymer is excimer
laser ablation (wavelength 193 nm). It has been shown recently,[22] however, that even though the polymer can be removed by laser ablation in the presence of oxygen (air) the formation of hexagonally ordered nanoparticles fails completely.
In this case, only ªberryº-like agglomerates of several fragments are reminiscent of the original micellar array. Coalescence of the metal salt from the micellar core to one single
nanodot could not be achieved by this method.
From these various results one concludes that the preparation of naked metallic nanoparticles can most effectively and
most reliably be achieved by removing the polymer by an oxygen plasma. This way, if the plasma conditions are continuously
controlled, cleanliness and single particle formation are guaranteed.
Since, beside the formation of particles, their chemical state
is of considerable interest, another point has to be addressed
for particles formed within an oxygen plasma. In the case of
Au films, the exposure to an oxygen plasma is well known to
result in the formation of a 3±4 nm thick layer of gold oxide
(Au2O3),[23] which, in turn, can be decomposed into metallic
gold and oxygen upon annealing to about 100 C or by storing
the sample for some days under ambient conditions. Figure 6a
demonstrates a similar behavior for Au nanoparticles that were
partly oxidized during an ex-situ oxygen plasma treatment necessary to remove the polymer matrix. As in the case of Au
films, annealing at 110 C leads to the decomposition of the
oxide, resulting in pure metallic nanoparticles on top of the
substrate.
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(a)
(b)
Fig. 6. Au 4f core level spectra of Au particles (5 nm and 10 nm) prepared on
diamond (a) and Si (b) using an oxygen plasma to remove the polymer matrix
(bottom spectra). Clearly, two doublets can be identified originating from Au oxide (Au2O3) and metallic Au, respectively. Annealing of the oxidized Au particles
at 110 C (left) leads to the decomposition of the oxidized component, resulting
in pure metallic nanoparticles on the substrate (top curve). The Au oxide can also
be reduced to the metallic state by application of an additional hydrogen plasma
step (b) performed at room temperature (top spectrum).
Since annealing of very small nanoparticles to higher temperatures may not be desirable due to their strongly enhanced vapor pressure as compared to the bulk, another technique has to
be applied to reduce the Au oxide to the pure metal at room
temperature. This can be achieved by a short subsequent hydrogen plasma step, as proven in Figure 6b by again referring
to the corresponding XPS spectra. As will be demonstrated later, such a combination of an oxygen plasma (to form or clean
nanoparticles) and a subsequent additional hydrogen plasma
process (to reduce oxides formed during the oxygen plasma
treatment) even allows the preparation of very reactive transition metal nanoparticles such as Fe, Co, or Ni dots.
2.3. Variation of Particle Size, Spacing, and Substrate
The preparational aspects of the micellar technique are very
important since a well-characterized system is needed if the
properties of an ensemble of nanoparticles is to be studied or if
such an ensemble is intended to serve as a mask for patterning
the substrate. In such cases, it is highly desirable to control the
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size and the distance between the nanoparticles over a broad
range. The size distribution of the nanoparticles should be as
ªmonodisperseº as possible and, depending on the experiment,
different substrates should be compatible with the process. Figure 7 shows AFM images of Au nanoparticles prepared on a
variety of different substrates, all evidence of a high degree of
hexagonal order.
The distance of the resulting nanoparticles is closely related
to the diameter of the micelles and thus can be controlled by
the total length of the diblock copolymer. Figure 8 shows examples of Au nanoparticles that were prepared starting with
three diblock copolymers of different lengths. The nomenclature PS(x)-b-P2VP(y) stands for x monomer units of polystyrene linked to y monomer units of poly(2-vinylpyridine). As a
result, the interparticle distance can be varied between 25 and
140 nm.
The size of the nanoparticles can be adjusted by the amount
of metal salt added to the micellar solution, allowing a variation of the particle diameter between 1 and 15 nm. Figure 9a
shows the actual size of Au nanoparticlesÐas determined by
AFM/TEM measurementsÐprepared with the same diblock
copolymer but different amounts of metal salt. The control of
the particle size by the amount of metal salt added to the micellar solution is confirmed by the linear behavior between the
measured nanoparticle diameter and the diameter calculated
from the amount of added metal salt assuming one spherical
Au particle per micelle.
To test the dispersity of the particle size, distributions of their
diameters have been determined by AFM/TEM. The results
are summarized in Figure 9b. Here, the size distributions of the
present micellar particles are found to be comparable to the
best results found in the literature for colloidal particles[26] or
even significantly narrower than for particles prepared by
evaporation.[24,25] Although it certainly would not be correct to
refer to these nanoparticles as ªmonodisperseº, the described
technique is certainly comparable or even superior to several
other preparation methods developed to produce nanoparticles
in the size range between 1 and 10 nm and additionally offers a
high degree of hexagonal order of the final particle arrangement.
2.4. Particle Shape
Fig. 7. Naked Au nanoparticles prepared on a variety of different substrates.
Fig. 8. AFM images of naked Au nanoparticles on mica. Different diblock copolymers (the numbers represent the monomer units per block) were used to control the interparticle spacing. Particles with a separation
of 80 nm and a size of 12 nm (left), a distance of 25 nm and a size of 3 nm (middle), and a separation of
140 nm and a size of 1 nm (right) are shown.
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Figure 10a shows a TEM image of Ausalt-loaded micelles deposited on top of
a carbon-coated copper grid. The interaction between the carbon film and the
organic shell of the micelles seems to
favor a maximized contact area between
the micelles and the carbon film, resulting in a ªfried eggº-like shape of the micelles. Within the core of the micelles
small dark spots are visible, representing
small Au fragments that form as a result
of the decomposition of the metal salt
induced by exposure to ambient conditions or by the electron beam of the
TEM. Figures 10b and 10c present the
final arrangement of Au nanoparticles
after the oxygen plasma procedure followed by the thermal decomposition of
the primarily formed gold oxide. On top
of sapphire (b) the Au nanoparticles are
in close contact with the substrate
whereas for silicon (c) the nanoparticles
reside on a layer of native silicon oxide.
Obviously, for these two types of substrates the resulting nanoparticles (most
often single domain particles, sometimes
twinned particles) are almost spherical
objects. There might be, however, other
combinations of nanoparticles and substrate materials for which hemispheres
or pancake-like particles could be
formed, depending on the interfacial and
surface energies. It is worth mentioning
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2.5. Other Materials
(a)
Up to now, all the different steps and aspects of the micellar
method have been demonstrated on Au nanoparticles. Apart
from control of the nanoparticle size and interparticle distance,
the versatility of this method to prepare nanoparticles of other
materials is one of the most promising aspects. Gold nanoparticles are very interesting as model systems for many purposes
due to their inertness under ambient conditions. But even Au
nanoparticles can show, for example, chemical properties that
deviate dramatically from the corresponding well-known bulk
behavior.[27] It is easy to imagine that size-dependent effects of
nanoparticles of, for example, ferromagnetic or superconducting materials can lead to even more intriguing effects. Therefore, nanoparticles derived from other materials are highly
desirable. Actually, the concept presented above can also be
applied to prepare ordered arrays of nanoparticles of elements
such as Co, Ni, Fe, Pt, Pd, or Ag. This is demonstrated in Figure 11, which presents some AFM images of dots prepared
from four different metals.
(b)
Fig. 9. a) Measured size of Au nanoparticles versus calculated value as obtained
from a given amount of metal salt that is added to the diblock copolymer solution
and assuming one spherical Au particle per micelle. b) Size distribution of nanoparticles prepared by the micellar approach and other techniques (evaporation
[24,25], colloids [26]).
Fig. 11. AFM images of nanoparticles prepared from different elements. The universality of the micellar approach allows a wide variety of metallic, magnetic, or
oxide particles to be prepared.
Fig. 10. TEM cross-section images of Au-salt-loaded micelles deposited onto a
carbon-coated copper grid (a) and of the final array of Au nanodots prepared on
top of a sapphire substrate (b) and on silicon (c), demonstrating the nearly spherical shape of the resulting particles.
that, in all cases studied so far, a preferential orientation of
the resulting nanoparticles with respect to the substrate crystal structure has not been observed.
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Recalling the oxygen plasma step described above to remove
the polymer matrix and to properly form a single metallic
nanoparticle, it is by no means obvious that pure metallic particles other than gold can be prepared by the micellar technique.
Depending on their size even gold nanodots are oxidized by
the highly reactive species provided by an oxygen plasma.[27]
However, as has been shown above, in this case, one is able to
reduce the oxidized Au nanoparticles to the metallic state by
means of a subsequent hydrogen plasma step. In the following
it will be demonstrated that this method can also be used to
prepare transition-metal-based dots such as Co nanoparticles.[28]
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Figure 12a summarizes XPS spectra of the Co 2p3/2 core level region taken sequentially from bottom to top, the topmost
spectrum representing the results obtained from an in-situ
evaporated clean Co reference film. Starting with an array of
Co-salt-loaded micelles on top of a Si(001) wafer, a strong
chemical shift towards higher binding energies (782.3 eV, spectrum 1) is observed as compared to an in-situ prepared clean
reference film (780.3 eV, solid vertical line), which is induced
by ligating to Cl atoms and, possibly, to OH groups (782.0 eV).
If an in-situ oxygen plasma is applied for 15 min, the binding
energy position of the Co 2p3/2 line finally shifts to a value of
780.7 eV (spectrum 2), which indicates the complete oxidation
of the Co atoms to Co3O4 in accordance with previous experiments on the plasma oxidation of Co films.[29] After exposure
of the oxidized particles to a hydrogen plasma in a subsequent
step (spectrum 3), the chemical shift of the Co 2p3/2 line due to
oxidation is found to be completely removed except for a small
residual offset towards higher binding energies of the order of
0.2±0.3 eV compared to the Co reference film. This offset, in
the present case, is a consequence of the small electrical capacitance of the metallic Co nanoparticles, resulting from their
charging during the photoionization process (final state effect[30]), as already discussed above.
At this point, the question arises as to the stability of Co
nanoparticles against oxidation by oxygen molecules. To answer this point, pure Co nanoparticles were exposed to an
oxygen pressure of 0.1 mbar for about 100 s and XPS spectra
taken in situ. The results are presented in Figure 12b as spectrum 1. Clearly, the particles are already found to be almost
completely oxidized, as evidenced by the corresponding binding energy of the Co 2p3/2 line. Subsequent exposure to an in-
situ hydrogen plasma for 15 min completely transforms the oxide back into Co metal as demonstrated by spectrum 2. As a
next step, the Co particles were exposed to ambient conditions
for 5 min. From the resulting spectrum (spectrum 3 in
Fig. 12b) and the corresponding binding energy position
(782.1 eV), one concludes that as a result of this treatment the
Co nanoparticles are nearly completely transformed into
Co(OH)2 (782.0 eV)[31] rather than into CoO (781.3 eV)[29] or
Co2O3 (781.3 eV)[32]. However, even then, a subsequent hydrogen plasma treatment under identical conditions as above allows the oxidized particles to be transformed back into metallic
Co, as proven by spectrum 4. It is important to note that
despite all the repeated oxidation and reduction steps the total
intensity of the Co 2p3/2 line remained constant, indicating that
the total amount of Co is conserved during these processes.
Therefore, a sequential combination of oxygen/hydrogen plasma treatments can advantageously be used to produce well-defined, clean, metallic nanoparticles.
3. Conclusion
A detailed description was given of how to prepare metallic
nanoparticles based on the self-organization of inverse micelles
formed from diblock copolymers, which are used as carriers for
the metal precursor. It turned out that this method not only allows the particle size and interparticle distance to be controlled, but additionally yields arrays of nanodots exhibiting a
high degree of hexagonal order. Furthermore, although the
method involves exposure to an oxygen plasma in order to remove the polymer matrix, it can even be applied to prepare
clean pure Co nanoparticles when an additional hydrogen plasma step is included. Thus, this approach,
due to its applicability to various substrates and materials, is a promising preparational tool for obtaining
systems offering new properties as a consequence of
their nanoscale.
Received: January 13, 2003
Final version: June 24, 2003
±
Fig. 12. a) Co 2p3/2 binding energy region acquired from HCoCl2-loaded micelles (spectrum 1)
supported on silicon/silicon oxide after exposure to an in-situ oxygen plasma (spectrum 2) followed by an in-situ hydrogen plasma (spectrum 3). The Co 2p3/2 core level spectrum of a Co
reference sample has been added for comparison (spectrum 4). b) Co 2p3/2 spectra, demonstrating the possibility of switching the chemical state of the Co nanoparticles: 1) 6 nm particles,
oxidized by exposure to 0.1 mbar of oxygen, 2) the same Co particles reduced by applying a
hydrogen plasma, 3) re-oxidation by exposure to ambient conditions, 4) repeated reduction to
pure Co.
860
Ó 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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FULL PAPER
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