INSTITUTE OF PHYSICS PUBLISHING
NANOTECHNOLOGY
Nanotechnology 17 (2006) 1375–1379
doi:10.1088/0957-4484/17/5/035
Controlled direct patterning of V2O5
nanowires onto SiO2 substrates by a
microcontact printing technique
Yong-Kwan Kim1 , Sung Joon Park1 , Jae Pil Koo1 , Dong-Jin Oh2 ,
Gyu Tae Kim2 , Seunghun Hong3 and Jeong Sook Ha1,4
1
Department of Chemical and Biological Engineering, Korea University, Seoul 136-701,
Korea
2
Department of Electrical Engineering, Korea University, Seoul 136-701, Korea
3
Physics and Nano-Systems Institute, Seoul National University, NS50, Seoul 151-742, Korea
E-mail: [email protected]
Received 12 November 2005, in final form 12 January 2006
Published 10 February 2006
Online at stacks.iop.org/Nano/17/1375
Abstract
Vanadium pentoxide (V2 O5 ) nanowires were directly transferred to desired
patterns on SiO2 substrates using the microcontact printing (MCP) technique.
The hydrophilicity of the poly(dimethylsiloxane) (PDMS) stamp exerted a
strong influence on the mechanism of transfer of polar V2 O5 nanowires onto
the substrate. The V2 O5 nanowires were transferred from the relief side of
the hydrophilic stamp, whereas they were transferred from the recess edges
of the hydrophobic one forming agglomerated nanowire patterns on the
substrate. When the hydrophobic stamp was used, the width of the
agglomerated nanowire patterns could be controlled by the concentration of
the nanowire solution as well as by the width of the recess area of the PDMS
stamp. This method allows us to generate nanowire patterns with a
submicrometre line width, which is much smaller than a the few-micrometre
sizes of PDMS stamp patterns. When the hydrophilic stamp with a
small-sized ( the average length of V2 O5 nanowires) pattern was used,
alignment of individual nanowires in the direction of the boundary of the line
pattern was obtained. These results suggest that the transfer mechanism in
the MCP process strongly depends on the wetting interaction between the
stamp and the nanowire ink.
(Some figures in this article are in colour only in the electronic version)
1. Introduction
Extensive attention has been paid to the field of nanofabrication
due to its huge number of potential applications in future
nanodevice technology. Two major concerns which need
to be addressed for practical applications are the synthesis
of useful nanomaterials and the assembly of the synthesized
nanostructures at desired locations on the substrate with precise
orientations.
One-dimensional nanowires have attracted scientific and
technological interest owing to their novel structures and their
4 Author to whom any correspondence should be addressed.
0957-4484/06/051375+05$30.00
potential applications as future nanoelectronic components.
Field-effect transistors [1, 2], crossed junctions [3] and
rotational actuators [4] have been demonstrated. In particular,
V2 O5 nanowires have many intrinsic merits, such as a uniform
geometric cross-section of 1.5 nm × 10 nm [5–9], which might
be important in nanofunctional materials. A heterojunction of
V2 O5 nanowires with carbon nanotubes, field-effect transistors
and actuators as artificial muscles have been demonstrated as
feasible electronic components [8, 9].
A number of new lithographic techniques such as
soft lithography [10, 11], dip-pen lithography [12] and
nanoimprinting [13] have been developed in the last
© 2006 IOP Publishing Ltd Printed in the UK
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Y-K Kim et al
few decades.
Among those techniques, microcontact
printing (MCP) using polymer stamps has been widely
applied to immobilize molecules [11], nanomaterials and
biomaterials [14–16] on desired sites over large areas
of various substrates. In common MCP processes, the
relief sides of the patterned poly(dimethylsiloxane) (PDMS)
stamp transfer non-polar inks such as alkanethiols and
alkyltrichlorosilanes to the substrate. However, polar materials
cannot be transferred nicely using the conventional MCP
technique since polar inks do not wet the common stamp
surfaces, which are usually hydrophobic. Many researchers
have tried to modify the hydrophobic stamp surface to be
more hydrophilic by treating it with oxygen plasma [17, 18]
or ultraviolet in ozone (UVO) [19, 20], making it possible to
pattern polar materials such as PD+
2 complex [17].
In this work we have used hydrophobic and hydrophilic
stamps to directly transfer polar V2 O5 nanowires onto SiO2
substrates via different transfer mechanisms depending on
the hydrophilicity of the stamp surfaces. In the case of
hydrophobic stamps, the aqueous solutions of negatively
charged [21] V2 O5 nanowires (polar nanowire inks) do not
wet the relief sides but are stored in the recess region of
the stamp. Thus, V2 O5 nanowires were transferred to the
substrate along the edge of the stamp patterns with a line
width of a few hundred nanometres, which is much smaller
than the few-micrometre width of the stamp patterns (edge
transfer mechanism). Up to now, overpressure printing [22]
and discontinuous dewetting [23] methods have been reported
to use the edge transfer mechanism. In addition, we also
transferred V2 O5 nanowires to the SiO2 substrate through the
relief side of the stamp using the relatively hydrophilic stamps
treated with hydrochloric acid (HCl). These results suggest
that the pattern transfer of V2 O5 nanowires on SiO2 substrate
can be controlled by the hydrophilicity of the stamp.
2. Experimental details
The PDMS used as the stamp material in this experiment
was purchased from Dow Corning. Patterned PDMS stamps
were fabricated by a previously reported method [24]. In
addition, unpatterned PDMS stamps were used to check
the hydrophilicity of the stamp surfaces by measuring the
advanced contact angle. Hydrophilic stamps can be prepared
by immersing PDMS stamps in 1 N aqueous solution of HCl
for 1 day. This treatment caused the hydrophobic stamp
surfaces to become more hydrophilic without any damage [25].
After this treatment, the advanced contact angles were changed
from 115◦ to 80◦ . All stamps were cleaned by an ultrasonic
cleaner in isopropyl alcohol, ethanol and deionized water
before inking. This step removes any dust particles which can
non-uniformly trap the nanowire ink on the relief area of the
PDMS stamp.
V2 O5 sols were prepared following a previously reported
recipe [26]. On average, 2 µm long nanowires could
be obtained after 2–2.5 months in sol solution [26]. In
acidic solution, V2 O5 nanowires are known to be covered
with negatively charged hydroxyl (−OH) groups [21]. The
synthesized V2 O5 sols were diluted with deionized water to use
as inks. SiO2 films were grown by immersing p-type Si(100)
substrates in piranar solution (H2 O2 :H2 SO4 = 3:1) at 90 ◦ C
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Figure 1. Schematic diagram depicting the MCP procedure using a
V2 O5 nanowire solution as ink. The PDMS stamp is (a) hydrophobic
as fabricated and becomes (b) hydrophilic after the treatment with
HCl.
for 15 min [27]. After this process, a Si surface is covered with
hydroxyl functional group (−OH). To promote the attractive
forces between OH-covered V2 O5 nanowires and the substrate,
the SiO2 substrate was immersed in 20 mM aqueous solution
of 3-aminopropyl amine (APS) for 4 h. After this treatment,
the SiO2 substrate becomes positively charged with −NH2
groups.
Advanced contact angles were measured on the substrates
and the stamps in each step by using a FACE CA-DT contact
angle meter (FACE, Japan). All atomic force microscope
(AFM) images were obtained with an XE-100 AFM (PSIA,
Korea) in a non-contact mode under ambient conditions. The
tips, with a force constant of 49 N m−1 and a resonance
frequency of 190 Hz, were purchased from NanoWorld (model
#NCHR). All the images were analysed by XEI 1.5 software
(PSIA).
3. Results and discussion
Figure 1 shows the different MCP procedures depending on the
hydrophilicity of the PDMS stamp surfaces. The hydrophilic
stamp can be coated with nanowire ink by common inking
methods including immersion in ink solution for 30 min and
blowing with N2 gas, whereas the hydrophobic stamp was
treated to trap the V2 O5 nanowire solution only in the recess
region of the stamp. The aqueous solution of V2 O5 nanowires
was prepared so that the solution did not wet the relief surface
of the hydrophobic stamp and is trapped only in the recess
space, like discontinuous dewetting [28]. In addition, we
immersed the stamp in the nanowire solution vertically and
pulled it out slowly to prevent the additional deposition of
nanowires due to gravitational force as well as the formation
of nanowire drops on the stamp. By evacuating with a rotary
Controlled direct patterning of V2 O5 nanowires onto SiO2 substrates by a microcontact printing technique
(a)
(b)
(a)
(b)
(c)
(d)
(c)
(d)
(e)
(f)
Figure 2. Non-contact mode AFM topography images taken from
V2 O5 nanowire patterns transferred via the edge transfer mechanism
using a hydrophobic PDMS stamp. The index ‘a/b’on the top-right
corners of the AFM images represents the (side length)/(separation)
for square patterns and the (line width)/(separation) for line patterns
on the stamps, respectively. For example, ‘1/1.2’ in (a) represents
the square-shaped recess area of 1 µm × 1 µm, which is separated
by 1.2 µm, whereas ‘1/1’ in (c) represents the 1 µm wide relief lines
separated by 1 µm. (a) and (b) are square patterns as shown in the
insets. The bottom-left corners of the AFM images (a) and (b)
represent the square patterns of the stamp, where white shows the
recessed area. (c) and (d) are line patterns of nanowires. The insets in
(c) and (d) show the cross section of the stamps with a channel width
of 1 and 0.5 µm and a depth of 0.35 and 0.3 µm, respectively. (e) and
(f) are the line profiles of AFM images in (c) and (d), respectively.
pump for 5 min and preserving in air for 20–30 min, a
considerable amount of the solvent was removed from the
recess region. As a result, the solution became concentrated
and receded to the edge of the recess region, as in the recent
report where polar [Mo3 Se−
3 ]∞ molecular wires were selforganized to the corners of the PDMS microchannels [29].
Figure 2 shows the AFM images of V2 O5 nanowire patterns which were generated on the aminopropyl triethoxysilane
(APS)-treated SiO2 surfaces using a hydrophobic stamp. All
the AFM images show that V2 O5 nanowires were successfully
transferred, regardless of the shapes and the sizes of the patterns on the PDMS stamp.
The transferred lines were not composed of separate
individual nanowires but of agglomerated wires with an
average height of 4–6 nm. The line width ranges from 100
to 300 nm depending upon the width of the recess area in the
PDMS stamp, but it was much smaller than the pattern size of
the stamp. This implies that the nanowires were not deposited
Figure 3. (a)–(c) AFM topography images taken in non-contact
mode from V2 O5 nanowire patterns transferred using a hydrophobic
PDMS stamp with various concentrations of V2 O5 nanowire
solution. All scales are identical. The original V2 O5 nanowire sols
were diluted with deionized water to have relative concentrations of
(a) 0.13, (b) 0.25 and (c) 0.33. (d) Shows the change of the
transferred line shape versus the concentration of the V2 O5 nanowire
inks. The filled circles and the open squares represent the width (left
y -axis) and height (right y -axis) of the nanowire patterns,
respectively. Here, the PDMS stamp with a line pattern of 0.5/0.5
was used.
from the relief side of the stamp but from the recess edge of
the stamp. Figures 2(e) and (f) show the line profiles of the
patterns of figures 2(c) and (d), respectively. We found that the
wider recess area produces wider V2 O5 nanowire patterns with
the same concentration of nanowire solution.
Figure 3 shows the AFM images of the V2 O5 line
patterns which were generated on APS-treated SiO2 substrates
using various concentrations of V2 O5 solution. The relative
concentration was measured with respect to the original V2 O5
sol solution. The stamp with a 0.5 µm wide line pattern
separated by 0.5 µm was used. With dilution of V2 O5
solution, the transferred pattern became narrower, as shown in
figures 3(a)–(c). Figure 3(d) summarizes the dependence of the
line width and the height on the concentration of the nanowire
solution. The results show that the height as well as the
width of the pattern increases with increasing concentration.
Presumably, a larger amount of V2 O5 nanowire ink is held in
the recess area of the stamp at the higher concentration of V2 O5
solution. However, the line width was always much smaller
than that of the pattern on the stamp.
Figures 4(a)–(d) show the AFM images of the V2 O5
nanowire patterns transferred onto APS-treated SiO2 substrates
using a hydrophilic PDMS stamp. The hydrophobic PDMS
stamp was treated with HCl to change its surfaces to be
more hydrophilic. It should be noted that the transferred
nanowire patterns are distinctively different from those shown
in figures 2 and 3 where agglomerated V2 O5 nanowire patterns
were formed from the recess region of the stamp with a line
width much narrower than those of the carved patterns of the
stamp. In the case of hydrophilic stamps, the patterns exactly
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Y-K Kim et al
(a)
(b)
(c)
(d)
(e)
(f)
Figure 4. (a)–(d) AFM topography images taken in non-contact
mode from V2 O5 nanowire patterns transferred using a hydrophilic
PDMS stamp. (a), (c) and (d) show the line patterns, and (b) is the
square pattern as shown in the inset. The inset of (a) is the
high-resolution image showing that the individual V2 O5 nanowires
transferred without agglomeration. (e) and (f) are the length
distribution of the adsorbed nanowires shown in (c) and (d),
respectively.
resemble the shapes of the original relief patterns on the stamp.
Also, the individual nanowires are observed to have a height
of 1.5–3 nm. Attractive interaction between the polar V2 O5
nanowire solution and the hydrophilic stamp surface might
hinder the nanowires from becoming entangled. Regardless
of the shapes of the stamp patterns, the line width of the
transferred nanowire patterns was the same as that of the relief
side of the PDMS stamp.
Interestingly, we observed that the nanowires remain
inside the patterned area even when the line width of the
pattern is smaller than the average length of the nanowires.
As shown in figures 4(c) and (d), the V2 O5 nanowires do not
cross the boundary of the patterns, and they are even aligned in
the direction of the boundary of the line patterns without any
external force such as an electric field. Figures 4(e) and (f)
show the length distribution of the adsorbed nanowires in the
images of figures 4(c) and (d), respectively. When the stamp
with a relief width of 2.5 µm was used, the average length
of the adsorbed V2 O5 nanowires was estimated to be ∼2 µm
while it was ∼1 µm with a stamp having a 1.0 µm pattern. The
same V2 O5 sols were used in all of the experiments, where
the average length of the nanowires was 2 µm with a broad
length distribution ranging between 1 and 4 µm [24]. Such
interesting results can be explained in terms of the interaction
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between the nanowires and the stamp during the inking and
blowing steps. Presumably, the relatively long wires from a
weak interaction with the stamp can be easily removed from
the stamp during the N2 blowing procedure. Therefore, only
those wires of length similar to the pattern width would remain
on the stamp, and they would be subsequently transferred to the
substrate. This phenomenon can be used to precisely position
and align nanowires at desired locations on solid substrates.
4. Conclusion
We have directly transferred V2 O5 nanowire patterns to
chemically functionalized SiO2 substrates by the MCP
technique. Controlling the hydrophilicity of the PDMS
stamp allows us to produce two different types of pattern.
In particular, hydrophobic stamps can be used to transfer
agglomerated V2 O5 nanowires via an edge transfer mechanism
with a line width narrower than that of the stamp patterns. The
line width of the agglomerated V2 O5 nanowires depended on
the concentration of the nanowire solution as well as on the
recess width of the PDMS stamp. HCl treatment of the PDMS
stamp caused the stamp surfaces to become more hydrophilic,
and the individual V2 O5 nanowires were transferred from the
relief sides of the stamp. These results can be explained in
terms of the wetting interactions between the V2 O5 nanowire
solution and the PDMS stamp.
Acknowledgments
This work was supported by the Ministry of Science
and Technology (grant no. M10503000187-05M0300-18710),
the Basic Research Program of the Korean Science and
Engineering Foundation (grant no. R01-2005-000-106480 (2005)) and the Korea Research Foundation (grant no. KRF2004-005-C00068).
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