Nanostructured surfaces of metals and polymers by imprinting with nanoporous alumina

Article abstract of DOI:10.1002/zaac.200600358

Anodic oxidation of aluminium surfaces yields nanoporous films which can be applied as masks to structure various materials. Pore widths and lengths can be adjusted by the oxidation conditions. Polymers such as poly(methylmethacrylate) (PMMA), polyethylene (PE), polycarbonate (PC), poly(tetrafluoroethylene) (PTFE), silicone polyester coated surfaces as well as the metals aluminium, iron, nickel, palladium, platinum, copper, silver, zinc, and brass have successfully been nanostructured.Commercially available press devices have been applied using pressures between ca. 100 and 1000 MPa, depending on the hardness of the corresponding material. The structure of the masks is 1:1 transferred onto polymer or metal surfaces. The quality of the imprinted surfaces depends mainly on the quality of the material's surface to be imprinted, since any kind of roughness or pre-structuring is transferred to the new surface. Optical properties of several of the nanostructured surfaces have been investigated as well as the change of the wetting behaviour of PTFE. Pillars on silver surfaces, generated by imprinting, behave like nanoparticles of appropriate size, i. e. they exhibit plasmon resonances in the UV-vis spectra. The wavelengths depend characteristically on the size of the structure units. Imprinted glassy polymers show a different light transmission, compared with non-imprinted surfaces. This effect can be used to improve their reflection behaviour.

Full text of DOI:10.1002/zaac.200600358

Articles
DOI: 10.1002/zaac.200600358
Nanostructured Surfaces of Metals and Polymers by Imprinting with
Nanoporous Alumina
Günter Schmid*, Matthias Levering, and Thomas Sawitowski
Essen/Germany, Institute of Inorganic Chemistry, University of Duisburg-Essen
Received December 18th, 2006.
th
Dedicated to Professor Dieter Fenske on the Occasion of his 65 Birthday
Abstract. Anodic oxidation of aluminium surfaces yields nano-
porous films which can be applied as masks to structure various
materials. Pore widths and lengths can be adjusted by the oxidation
conditions. Polymers such as poly(methylmethacrylate) (PMMA),
polyethylene (PE), polycarbonate (PC), poly(tetrafluoroethylene)
imprinted, since any kind of roughness or pre-structuring is trans-
ferred to the new surface. Optical properties of several of the nano-
structured surfaces have been investigated as well as the change of
the wetting behaviour of PTFE. Pillars on silver surfaces, generated
by imprinting, behave like nanoparticles of appropriate size, i. e.
they exhibit plasmon resonances in the UV-vis spectra. The wave-
lengths depend characteristically on the size of the structure units.
Imprinted glassy polymers show a different light transmission,
compared with non-imprinted surfaces. This effect can be used to
improve their reflection behaviour.
(PTFE), silicone polyester coated surfaces as well as the metals
aluminium, iron, nickel, palladium, platinum, copper, silver, zinc,
and brass have successfully been nanostructured.Commercially
available press devices have been applied using pressures between
ca. 100 and 1000 MPa, depending on the hardness of the corre-
sponding material. The structure of the masks is 1:1 transferred
onto polymer or metal surfaces. The quality of the imprinted surfa-
ces depends mainly on the quality of the material’s surface to be
Keywords: Nanoporous alumina; nanostructured surfaces; poly-
mers; metals; light transmission; wetting behaviour
Introduction
graphy which, for instance, can be chemically etched or
miniaturized by isotropic deformation.
Following Whitesides, five principal techniques to micro/
nanostructure surfaces are available: lithography, self-as-
sembly, controlled deposition, size reduction, and repli-
cation by physical contact [1].Lithographic techniques play
a major role in semiconductor industry [2, 3]. Besides nu-
merous advantages the limit of resolution is given by the
wavelength of the used irradiation. So-called nanolitho-
graphical methods however, open a completely novel door
to nanostructured surfaces, as has impressively been dem-
onstrated by Mirkin [4]. They are not based on the use of
electromagnetic waves as tools, but on writing by means
of an AFM tip. Nanostructured surfaces arising from self-
assembly are becoming increasingly interesting due to the
diversity of self-assembling building blocks. They reach
from simple molecules up to nanoparticles, nanowires,
nanotubes etc.The controlled deposition uses focused laser
and focused ion beams (FIB), cleaved edge overgrowth
The following contribution is dealing with replication
techniques using nanoporous alumina membranes as
stamps. The generation of nanoporous alumina layers on
aluminium surfaces is a long known and frequently used
process [5Ϫ9]. The unique advantage of this technique is
the easy adjustability of pore width and pore length. The
pore width depends on the applied voltage and on the nat-
ure of the polyprotic acid used as electrolyte. The length of
the nanopores is directly related to the anodization time.
Furthermore, it is the almost perfect pore-structure of such
alumina films making this material an ideal tool. The pores
are all running parallel through the layer and when using
pre-structured aluminium surfaces, even highly ordered po-
rous films can be gained. Practically, the pore diameters can
reach from about 5 nm up to ca. 200 nm, exhibiting rather
small size distributions.
In a recent paper we described few examples of imprinted
PMMA [10]. In the following, the successful imprinting of
various transition metals as well as of different polymers
will be described.
(CEO) or shadow evaporation, whereas the method of size
reduction uses structures from conventional microlitho-
*
Prof. Dr. G. Schmid
Results and Discussion
Institut für Anorganische Chemie
Universität Duisburg-Essen
D-45117 Essen
Fax: ϩ49 (0)201 183 4195
E-mail: guenter.schmid@uni-due.de
The imprinting techniques
Different kinds of alumina masks can in principle be used
for the imprinting of polymers and metals: sheets, discs,
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G. Schmid, M. Levering, Th. Sawitowski
Figure 1 a) Sketch of the imprinting device. b) Material flow into the nanopores.
Figure 2 Proof of 1:1 pattern transfer. a) AFM image of an alumina surface with ordered 50 nm pores. b) Imprinted PMMA surface.
Irregular dots from the mask can be identified as holes in the PMMA surface.
foils, and separated alumina membranes. One-sided
anodized foils have the advantage of being more flexible
than thicker discs and so crack formation during the stamp-
ing process can be minimized and reuse becomes possible.
The sketch in Figure 1a shows, how the imprinting process
was performed technically. The alumina mask and the sub-
strate are positioned between the punch and the bottom of
the press device. Figure 1b elucidates the material flow into
the pores of the oxide layer. The penetration depth of the
material and therewith the aspect ratio of the generated pil-
lars depends on pressure and time.
Among the thermoplastics polycarbonate (PC), poly-
ethylene (PE), poly(methylmetacrylate) (PMMA), and poly-
(tetrafluoroethylene) (PTFE) which have been used for im-
printing, only PMMA had to be warmed up slightly above
its glass point to give good results.
Polymers
An important challenge of all imprinting processes is a per-
fect transference of the mask pattern onto the surface to be
nanostructured. Figure 2 shows that this task can indeed
be fulfilled. The pattern of an alumina mask with predomi-
nantly ordered 50 nm pores (a) is perfectly transferred onto
a PMMA surface (b). Besides the generated pillars it can
also be followed from the holes, caused by the defects in
the mask at exactly the same positions. Other samples have
also been tested with same success.
Polycarbonate (PC) can already be imprinted at 200 MPa
at room temperature. As a substrate a 3 mm thick PC disc
was used, imprinted with an alumina mask with 180 nm
pores. The result is shown in Figure 3.
Generally, the material must be softer than the alumina.
If this is not the case at usual temperatures, warming up
can help to improve the conditions.
Besides the hardness of the materials, the quality of their
surfaces is of decisive importance for a successful im-
printing process. Aluminium surfaces, both for generating
alumina films and for being imprinted, can best be
smoothed by electropolishing (see experimental part). The
surfaces of the other metals, namely iron, nickel, palladium,
platinum, copper, silver, zinc and brass, could only be pre-
treated by mechanical polishing with polishing materials of
decreasing grain size.
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Nanostructured Surfaces of Metals and Polymers by Imprinting with Nanoporous Alumina
For practical reasons, imprinting of coatings is of special
relevance, since ordinary technical materials can so be
nanostructured for improvement of the surface properties.
Here we used commercial steel plates which have been co-
ated by inorganic-organic hybrid polymers, namely Silikof-
tal HTL 2 and Silikoftal NS 60, both silicone polyester re-
sins. They are frequently used to coat household utensils.
The successful imprinting of these coatings is dependent on
the hardening steps which, on their side, depend on the tem-
perature. Four hardening steps at 100 °C (1), 160 °C (2),
250 °C (3), and 300 °C (4), have been tested. NS 60 could
best be imprinted after hardening step 4, whereas the more
rigid HTL 2 was best treated after only step 1. Representa-
tive for numerous probes, Figure 5 shows an AFM image
of a NS 60 coating, imprinted with 200 nm pores at
130 MPa after hardening step 4.
Figure 3 AFM image of a structured PC disc (alumina mask with
ϳ 180 nm pores).
180 nm pores in alumina masks have also been used to
imprint Polyethylene (PE). The results correspond mainly
to those of PC.
Poly(tetrafluoroethylene) (PTFE) is a high temperature
resistant, chemically inert thermoplastic material, but with
only low mechanic stability. It is soft and easy to be de-
formed, tends to creep. Since the available PTFE material
was found to have a rather rough surface structure (AFM),
the flow limit had to be considerably exceeded. The flow
limit of PTFE is between 7 and 20 MPa and so is, com-
pared with the other polymers, rather low. Best results were
obtained at pressures of 130 MPa. The result with an anod-
ized aluminium foil with 50 nm pores is shown in Figure 4.
Still, a rather rough surface can be observed, compared
with the other polymers. Nevertheless, the hydrophobicity
increases remarkably (see section 3).
Figure 5 AFM image of the structure of a NS 60 coating, im-
printed with 200 nm pores at hardening step 4. Height of the pil-
lars: 80-100 nm.
Metals
As mentioned above, the surface of aluminium can best be
pre-treated by electropolishing. For this, aluminium sheets
or discs are anodized in highly concentrated electrolyte
solutions at 75 °C for 10 min., aluminium foils (75 µm) for
only 1 min. at 10 V. Due to the temperature and the electro-
lyte, the formed oxide layer is mainly removed continuously
during its formation. Successive treatment with a mixture
of chromic acid and phosphoric acid dissolves remaining
traces of aluminium oxide. The resulting surface is not ab-
solutely flat, but shows a nanosized roughness which still
can be observed on top of the pillars after imprinting as is
to be seen from Figure 6.
The structure after imprinting consists of pillars of
00Ϫ230 nm in height (from a height profile) and
80Ϫ200 nm in diameter, corresponding with an average as-
Figure 4 AFM image of an imprinted PTFE surface (alumina
mask with 50 nm pores).
2
1
The pillars have a height of 50 Ϫ 60 nm, the aspect ratio
is between 1.0 and 1.5 .
pect ratio of about 1. In Figure 6 the original polishing
structure can still be seen on top of the pillars. This means
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G. Schmid, M. Levering, Th. Sawitowski
Figure 6 a) AFM image of an electropolished aluminium surface. b) AFM image of an imprinted (ϳ 180 nm) aluminium surface, indicating
the roughness of the electropolished aluminium surface on top of the pillars.
that the original surface structure of the electropolished
aluminium at least partially survives.
From this result a considerable influence of the substrate
surface on the imprinting results can be concluded.
In spite of their partially high mechanical stabilities the
surfaces of iron, nickel, palladium and platinum could be
imprinted by alumina masks at 985 MPa (not shown).
Polished copper and silver surfaces could also be success-
fully structured by 120 nm and 50 nm pore masks, respec-
tively, both at 985 MPa. As an example, the result for silver
is shown in Figure 7.
The aspect ratios of the pillars lie between 0.8 and 1.5.
Finally zinc and its copper alloy brass have been tested.
The structure transfer to zinc with 120 nm pores succeeded
quite well already at 500 MPa with aspect ratios like in case
of copper. Polished brass surfaces were imprinted with alu-
mina masks of 200 nm pores at 985 MPa. One of the results
is shown in Figure 8. Besides the usual AFM image, the
Figure 7 AFM image of an imprinted silver foil (alumina mask:
50 nm pores).
Figure 8 a) AFM image of an imprinted brass foil (alumina mask with ϳ 200 nm pores). b) Deduced greyscale shading image.
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Nanostructured Surfaces of Metals and Polymers by Imprinting with Nanoporous Alumina
corresponding greyscale shading image is presented. The
average height of the pillars was measured as 200Ϫ230 nm
corresponding to an aspect ratio around 1.
Table 2 Differences in the transmission of imprinted PMMA
samples compared with non-treated PMMA.
Pore diameters
Difference of
transmission
Difference of
transmission
Difference of
transmission
(720 nm) /%
of the masks /nm
(380؊780 nm) /%
(520؊900 nm) /%
Properties
7
5
2,5 ± 0,7
2,5 ± 0,7
1,3 ± 0,8
4,0 ± 0,5
3,9 ± 3,9
2,2 ± 0,5
0,7 ± 0,5
7,1 ± 0,3
6,6 ± 1,5
2,5 ± 0,3
2,1 ± 0,9
Optical Properties
1
1
1
20
50
80
Our interest in the properties of imprinted polymers was
mainly focused on the optical properties, but also on the
change of the hydrophobic behaviour compared with non-
imprinted surfaces. PMMA is an interesting material to in-
vestigate improvement of light transmission, i. e. to reduce
the reflection of light. In Figure 9 the UV-vis transmission
spectra of some different nanostructured PMMA samples,
compared with a non-treated sample, are shown.
Ϫ0,5 ± 0,7
layer with a low refractive index on the surface than by a
moth eye effect which usually requires structure units of
approx. 300 nm [11, 12]. A frequently applied technique to
reduce reflection, e. g. on glasses, is based on the generation
of a layer with a low refractive index and of defined thick-
ness [13].
Nanostructured silver has been selected to study optical
properties. First, a silver surface with 50 nm pillars as
shown in Figure 7, was compared with a non-structured
surface by means of UV-vis spectroscopy. The presence of
the pillars causes a broad plasmon resonance with a maxi-
mum at 390 nm, as can be seen from Figure 10.
Figure 9 UV-vis transmission spectra of PMMA samples, structu-
red with alumina stamps having pore widths between 70 and
180 nm.
As can be seen, the transparency increases with decreas-
ing structure size. Independent on the structure size, there
exist maxima at 520, 720 and 870 nm. The maximum trans-
parency is at a wavelength of 720 nm. Table 1 summarizes
the data of the transparency measurements, Table 2 informs
on the differences between structured and non-structured
probes.
Figure 10 UV-vis reflection spectra of an imprinted and a non-
imprinted silver foil.
The position of a plasmon signal depends, among others,
on the size of the causing particles. For this reason we inves-
tigated two additional silver surfaces with pillars of 10Ϫ15
and 20Ϫ30 nm in diameter. The respective plasmon signals
were observed at 360 and 375 nm, respectively, confirming
the expected shift to shorter wavelengths with decreasing
size of the structure units. Compared with monodispersed
metal nanoparticles the pillars of the investigated surfaces
have a larger size distribution and so cause broader maxima
in the UV-vis spectra.
Table 1 Transmission of structured and non-structured PMMA
windows.
Pore diameters
of the
masks /nm
Average
Transmission
(380؊780 nm) /%
Average
Transmission
(520؊900 nm) /%
Transmission
(720 nm) /%
Ϫ
91,5 ± 0,2
94,0 ± 0,5
94,0 ± 0,5
92,8 ± 0,6
91,0 ± 0,5
91,8 ± 0,2
95,8 ± 0,3
95,7 ± 0,7
94,0 ± 0,3
92,5 ± 0,5
91,9 ± 0,2
99,0 ± 0,1
98,5 ± 1,3
95,4 ± 0,1
94,0 ± 0,7
7
5
120
150
180
Wetting Properties
Dependent on the structure sizes, improvements of up to
% could be reached.
Since the smallest structures give best results, it can be
assumed that the realization of the observed decrease of
reflection is rather to be explained by the generation of a
PTFE has been selected to test the wettability of nanostruc-
tured polymer surfaces. A characteristic value to express the
hydrophobic or hydrophilic character of a surface is the
contact angle. This is the angle between a surface and the
tangent set to the surface of a water droplet. It depends on
7
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G. Schmid, M. Levering, Th. Sawitowski
the surface tension of the substrate and the structure of
its surface. The larger the contact angle, the larger is the
hydrophobicity of a surface. The quantitative determination
of the contact angle is described by the Young equation
Table 3 Dependence of the contact angle of water droplet on the
structure size of PTFE surfaces in air
Pore diameters of
the masks [nm]
Calculated structural
distances[nm]
Contact angle [°]
[
14]:
Ϫ
Ϫ
112 ± 3
128 ± 5
126 ± 3
132 ± 2
140 ± 5
145 ± 2
146 ± 3
5
7
0
5
106 ± 6
159 ± 9
212 ± 12
265 ± 15
398 ± 23
451 ± 26
σsg Ϫ σsl ϭ σlg · cos(θ)
1
1
00
20
(σsl ϭ surface tension solid/liquid, σsg ϭ surface tension solid/gas,
180
00
2
σlg ϭ surface tension liquid/gas, θ ϭ contact angle solid/liquid)
Theoretical limits are 0° and 180°. 0° corresponds to a
perfect wettability, 180° to a perfect hydrophobicity. Micro/
naostructured hydrophobic surfaces exhibit considerably
larger contact angles than non-structured ones. The
phenomenon is well known from nature. The so-called Lo-
tus effect describes the self-cleaning capability of Lotus
leaves based on their hydrophobic character which is caused
by the waxy nature of the leaves on the one side, and by
their micro- and nanostructured surface on the other side
[
15Ϫ17].
Figure 12 Contact angles of water droplets on PTFE surfaces in
dependence of the structure units.
PTFE surfaces were nanostructured by 50, 120 and
1
70 nm pores. The results are shown in Figure 11. Whereas
a non-imprinted surface causes a contact angle of 112 ± 3°
a), there is an increase of the angle with increasing struc-
ture size: 128 ± 5° (50 nm, b), 140 ± 5° (120 nm, c) and 145
2° (170 nm, d).
the contact surface of the water droplets. The contact sur-
face describes the real wetted area. From the measured con-
tact angles the real contact surfaces can be calculated [18].
Here, they vary from 50 % for the 50 nm structured surface
to 25 % for surfaces imprinted with 200 nm pores. The rea-
son for this can be explained by the geometry of the sur-
faces. With increasing anodization voltage, the number of
pores decreases, whereas the cross sectional areas increase.
Theoretically, the product of number of pores and cross sec-
tional area is constant. If the pillars would have absolutely
plane top surfaces, the real contact surfaces should keep
constant with increasing anodization voltage and constant
structure size, respectively. Uniform contact angles would
result. However, with increasing anodization voltages the
increase of the top areas of the pillars is not adjusted with
the consequence of increasing contact angles. Therefore, the
observed dependence results.
(
±
Conclusions
Nanoporous alumina masks with pore diameters between
50 and 200 nm can be used to imprint various polymer and
metal surfaces, accompanied with a 1:1 transfer of the mask
pattern onto the surfaces. The quality, especially of im-
printed metal surfaces, depends mainly on the quality of
their surfaces. Electropolished aluminium surfaces give bet-
ter results than only mechanically polished surfaces. The
imprinting method with nanoporous alumina masks to
structure surfaces can in principle be extended to many
other surfaces. If the hardness of a material is too high,
increase of temperature allows reasonable pressures. Nanos-
Figure 11 Light microscopy images of water droplets on non-
structured (a) 50 nm (b), 120 nm (c), and 170 nm (d) structured
PTFE surfaces.
The results of all measurements are summarized in Table
3
and in Figure 12.
The distance between the pillars (structural distances),
given in Table 3, and used in Figure 12, is responsible for
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Nanostructured Surfaces of Metals and Polymers by Imprinting with Nanoporous Alumina
tructured surfaces change the physical and chemical proper-
ties dramatically compared with plane surfaces. Hydropho-
bizing is one of the goals of surfaces in connection with
self-cleaning. The value of polymers and coatings can so
be increased. Nanostructured glasses show increased light
transparency and herewith reduced reflection properties, an
important practical field in daily life.
For some experiments alumina films were separated from the alu-
minium sheet or disc. To perform a soft separation, the anodization
was not spontonously finished, but the voltage was reduced in 5 %
steps, each reducing the current to 75 % of the former value. From
6
V on the steps were reduced to 0.3 V until below 1 V. The samples
were then put into a bath consisting of 25 wt.% H SO until the
oxide, accompanied by H evolution, is completely separated (ca.
h).
2
4
2
2
Experimental Section
Polymer imprinting: Anodized aluminium sheets or discs were
used. Some of the samples were warmed up for better imprinting.
To do this, two metal discs on top and bottom of mask and poly-
mer are used. The temperature was controlled with a thermocouple.
Metal imprinting: Anodized aluminium foils or separated films
were used to imprint metal surfaces. A 13 mm hard metal pin of a
KBr press tool was used as stamp, another hard metal tip as a
support, sheathed by a shell of hardened steel. Depending on the
pressure needed for imprinting, a hand press (Perkin Elmer) was
used up to 985 MPa. For higher pressures like in case of nickel, or
for larger areas a 40 t MAN pressure machine was used. Usually,
the maximum pressure was hold for 30 sec.
The following equipment for preparation and investigation of the
samples has been used:
Press: Perkin Elmer, hydraulic hand press 17 t; press tool: Perkin
Elmer 13 mm KBr press tool and MAN 40 t universal press; pol-
ishing device: Struers Rotopol 31 and Bühler Polimet Polisher; con-
tact angle measurements: Dataphysics Contact angle System OCA;
UV-vis spectrometer: Varian Cary 1 Bio; AFM: Digital Instru-
ments Multimode RKM Nanoscope IIIa; SEM: Philips XL 30
SSEG.
Alumina masks: aluminium sheets (99.5 %) of 5 mm · 5 mm · 2 mm
and aluminium discs (99,999 %) of 30 mm diameter and 6 mm
thickness were cleaned in a solution of 84 g K
PO (10 wt.%) at 75Ϫ80 °C for 15 min, followed by electropol-
ishing in a mixture of 1100 mL H PO (60 wt.%) and 700 mL
SO (40 wt.%) at 75 °C for 10 min, using the Al as anode and a
2 2 7
Cr O in 1750 mL
H
3
4
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