Microwave-Assisted Fabrication of Carbon Nanotube AFM Tips

Microwave-Assisted Fabrication of Carbon
Nanotube AFM Tips
Tamara S. Druzhinina, † Stephanie Hoeppener* ,†,‡,§ and Ulrich S. Schubert* ,†,‡,§
† Laboratory of Macromolecular Chemistry and Nanoscience, Eindhoven University of Technology, Den Dolech 2,
5600 MB Eindhoven, The Netherlands, ‡ Dutch Polymer Institute, P.O. Box 902, 5600 AX Eindhoven,
The Netherlands, and § Laboratory of Organic and Macromolecular Chemistry (IOMC), Friedrich-Schiller-University
Jena, Humboldtstrasse 10, D-07743 Jena, Germany
ABSTRACT A new, fast, alternative approach for the fabrication of carbon nanotube (CNT) atomic force microscopy (AFM) tips is
reported. Thereby, the tube material is grown on the apex of an AFM tip by utilizing microwave irradiation and selective heating of
the catalyst. Reaction times as short as three minutes allowed the fabrication of CNT AFM tips in a highly efficient process. This
method represents a promising approach toward a cheaper, faster, and straightforward synthesis of CNT AFM tips.
KEYWORDS Carbon nanotubes, microwave, AFM tip, carbon nanotube synthesis, catalyst
Scanning force microscopy has developed into a standard
tool in material research and represents a frequently
used technique in nearly all fields of science,
including, e.g., chemistry, physics, biology, and others. 1-4
The resolution of this technique is, however, strongly related
to the quality of the available tip material, which limits not
only the lateral resolution but also implies limitations with
respect to the investigation of, e.g., steep edges. 5,6 While
commercially available atomic force microscopy (AFM) tips
are fabricated by using silicon microfabrication techniques
reaching a typical resolution of approximately 10 nm, tailormade
tip layouts have been proposed to improve the tip
performance. The tip quality depends mainly on the dimensions
and shape of the probe, the durability of the tip apex,
and the nature of the interaction between sample and probe.
In this respect, in particular, AFM tips functionalized with a
carbon nanotube (CNT) have attracted considerable attention.
Due to the high Young’s modulus of the CNTs and their
excellent aspect ratio, 7 attempts have been made to use
them as probes for AFM experiments. Not only their unique
mechanical but also their chemical and electronic properties
8-12 open attractive possibilities that might result in
improving imaging performance 13,14 or in measuring the
properties of CNTs. 15 Due to the high resolution of CNT AFM
tips, they can be used to image very fine structures, such as
biological and molecular materials. Several studies have
been performed where CNT AFM tips were used to image
biological materials, such as, DNA or proteins. 16-18 Different
methods have been developed either to directly grow CNTs
on AFM tips 19-23 or to place CNTs on tips. 24-27 The place-
*Corresponding authors. E-mail: s.hoeppener@uni-jena.de (S.H.),
ulrich.schubert@uni-jena.de (U.S.S.). Telephone: +49 (0)3641 948261
(S.H.), +49 (0)3641 948202 (U.S.S.).
Received for review: 06/1/2010
Published on Web: 09/24/2010
pubs.acs.org/NanoLett
ment of the CNT on the AFM tip is usually performed by
using scanning electron microscope (SEM) manipulators,
where individual tubes are picked and stabilized on the AFM
tip with locally deposited carbon. This process is timeconsuming
and requires a rather expensive experimental
infrastructure and is also difficult to be used for a scale-up
of the manufacturing process. Alternatively, the direct growth
of CNTs onto AFM tips can be used. For this purpose
different methods can be utilized, e.g., surface or pore
growth. In particular the chemical vapor deposition (CVD)
is frequently used and yields thin CNTs grown directly on
the tip apex. Besides a relatively fast production time, still
dedicated equipment as well as rather harsh reaction conditions
are required using these conventional CVD approaches.
Due to the fact that all these methods are time-consuming
and costly, there is a demand for alternative methods for
the formation of AFM CNT-tips, which makes them affordable
and allows their use not only for very specialized
applications.
Here we introduce an alternative approach that allows the
fabrication of carbon nanotube AFM probes utilizing the
microwave-assisted growth of CNTs directly on the apex of
a commercially available AFM tip. This approach benefits
from the selective heating of tip mounted catalyst particles
due to a preferential absorption of the microwave irradiation,
which results in a strong, local increase of the temperature
that is sufficient to grow multiwall carbon nanotubes
in the presence of ethanol vapor within very short time
scales of a few minutes only. Different aspects of the
fabrication process are discussed, and an optimized procedure
to fabricate CNT tips is presented.
The microwave-assisted synthesis of CNTs on AFM tips
was performed according to a previously reported method. 30
For the microwave irradiation in a synthetic laboratory single
mode microwave (Emrys Liberator, Biotage) was used.
© 2010 American Chemical Society 4009 DOI: 10.1021/nl101934j | Nano Lett. 2010, 10, 4009–4012

SCHEME 1. Experimental Setup a
a Quartz glass pressure vial with mounted AFM cantilevers, support, and liquid ethanol reservoir. Microwave cavity of the single mode synthetic
microwave. The pressure vial is transferred to the cavity, and microwave irradiation is applied.
Typical irradiation times of 5 min were applied. 31 The
reactions were performed in capped pressure vials which
were loaded with 5 mL of ethanol. The samples were placed
onto a quartz glass support above the liquid level of the
ethanol (Scheme 1).
The setup permits the use of a liquid ethanol reservoir as
the carbon source in the bottom of the vial and a glass
support that affords the placement of the substrate above
the liquid level. This approach was demonstrated to allow
the synthesis of carbon nanotubes under relatively mild
synthetic conditions in rather short time scales of less than
five minutes. To use this approach also to grow CNTs directly
on an AFM tip, a few critical issues had to be addressed. In
particular the question if the conditions to initiate the growth
of CNTs onto the small area of the AFM tip can be matched
had to be investigated.
In the previously reported experiments it was essential
for the successful growth of CNTs onto substrates that a
sufficiently high pressure was generated in the vial, due to
the evaporation of the carbon source, i.e., ethanol.
To obtain the required synthesis conditions the total
power of the microwave was limited to 200 W in the course
of the experiments. Due to the restriction of the maximum
temperature (250 °C) and pressure (21 bar) that can be
generated within the pressure vials, the microwave irradiation
usually stops automatically due to a safety shut down
of the microwave. As a consequence the experimental
conditions had to be adjusted to these limitations. Moreover,
heat dissipation effects had to be taken into consideration.
Therefore, commercially available AFM tips were mounted
onto small pieces (0.5 × 1 cm) of silicon wafer by means of
a conducting silver paste which was used to glue the chip,
to which the cantilevers and the tips are connected, onto the
substrate. This allowed the control of the heat dissipation
from the relatively small area of the tip material itself and,
moreover, permited the convenient handling of the tips.
FIGURE 1. SEM micrographs of the AFM tip before (a) and after
microwave irradiation (b). The whole AFM tip is covered with nickel
acetate catalyst (a) and CNTs after microwave irradiation (b).
In a first experiment, commercially available AFM tips
were just immersed intoa5mMethanoic solution of nickel
acetate (Sigma Aldrich) dissolved in ethanol. Subsequently,
the solvent was allowed to dry, and the AFM tips were
mounted onto the silicon support. Scanning electron microscopy
(SEM) investigations (Quanta 3D FEG, FEI, The
Netherlands) prior to the microwave irradiation revealed that
the catalyst material was homogenously deposited on the
whole tip area (Figure 1a) as indicated by the flake-like
structures on the AFM tip. This nickel acetate covering was
transformed into nickel catalyst particles in the course of the
microwave irradiation by thermal activation, and the individually
formed particles were used as catalyst particles for
the growth of CNTs. Figure 1b depicts a SEM image that was
recorded after the microwave irradiation process, and it is
observed that a homogeneous coating of the tip with CNTs
was obtained. Thus, it could be confirmed that the chosen
irradiation conditions were sufficient to obtain the required
temperature and pressure conditions to form CNTs also on
the AFM tip.
In the next step the optimization of the catalyst deposition
process was addressed to ultimately be able to grow only
individual CNTs on an AFM tip. For this purpose, different
approaches were tested to limit the amount of catalyst
deposition. It was found that this can be achieved best by
© 2010 American Chemical Society 4010 DOI: 10.1021/nl101934j | Nano Lett. 2010, 10, 4009-–4012

FIGURE 2. SEM micrographs of an AFM-tip before (a) and after
microwave irradiation (b).
simple scanning of the tip over a dried solution of nickel
acetate drop casted onto a silicon substrate at higher contact
forces (NTegra Aura AFM, NT-MDT, Russia). In this case a
lower amount of nickel acetate is deposited onto the tip,
which can provide the catalyst seed for the CNT growth.
Figure 2a depicts the deposited material present on the AFM
tip after scanning a small area on the nickel acetate loaded
substrate. The presence of material is very visible at the slope
of the tip, indicating that small amounts of the nickel acetate
were attached to the tip. The prepared tips were subsequently
mounted in the microwave vials, and the CNT
growth was pursued.
Figure 2b shows the conversion of the nickel acetate
material into the catalyst particles, which are observed both
on the tip apex as well as on the slope of the tip itself. It could
be observed that few CNTs are grown from the tip, and in
particular, one CNT protrudes vertically away from the tip.
This CNT has a length of approximately 600 nm and a
diameter of 20 nm. Due to the length, a bending of the CNT
is observed. 28,29 This result demonstrates that it is possible
to obtain reaction conditions within the pressure vial that
permit the growth of individual CNTs due to an effective
compensation of the heat dissipation effects. In previous
experiments, performed to grow individual CNTs onto solid
substrates, it was observed that this represents a challenging
task, which was up to now not sufficiently implemented into
the surface-based synthesis approach. However, still the
amount of catalyst material attached to the tip is not yet well
controlled due to the fact that the AFM tip collects considerable
amounts of nickel acetate during the scanning process
of the dried layer.
AFM force spectroscopy was conducted to further demonstrate
the successful functionalization of the AFM tips with
carbon nanotubes. Therefore, a set of measurements was
performed that included first the recording of an approach
and retraction curve with a CNT-modified AFM tip in the
static AFM mode.
Figure 3a displays a representative curve that clearly
demonstrates the bending of the cantilever away from the
surface, when the CNTs are in contact with the surface and
start to slide away or buckle. Relatively large adhesion forces
suggest in this case that several CNTs are attached to the
cantilever, which have a length of approximately 60 nm, as
estimated from the z-displacement position of the onset of
the bending curve until the typical proportional deflection
of the cantilever is observed. These curves are reproducible
FIGURE 3. Force spectroscopy of CNT functionalized AFM tips. (a)
Representative deflection vs distance plot of a CNT modified tip. (b)
Measurement with the same tip after the CNT material was removed
by appling higher forces. (c) I-V curve of a CNT-modified tip gently
approached onto the substrate.
© 2010 American Chemical Society 4011 DOI: 10.1021/nl101934j | Nano Lett. 2010, 10, 4009-–4012

indicating the stability of the CNTs onto the tip. After this
significantly higher forces were applied onto the tip to
remove the CNT material on purpose. The force spectroscopic
measurements after this process (Figure 3b) indicate
the characteristic deviation of the deflection vs distance
curves, and a significant decrease of the adhesion forces was
observed. Moreover, the characteristic snap-in points are
clearly visible, without any indication for a bending of the
cantilever prior to the contact of the AFM tip.
Additionally, the current-voltage characteristic (Figure
3c) was measured on a CNT-modified metal-coated AFM that
was gently brought into contact with a graphite substrate.
In this case a small significantly reduced conductivity could
be measured compared to nonmodified metal-coated tips.
In conclusion, a powerful process was developed that
allows the direct fabrication of CNT AFM tips utilizing
efficient synthetic conditions generated in a single mode
microwave reactor. It could be demonstrated that the growth
of individual CNTs can be achieved, and the optimization of
the preparation conditions resulted in a promising approach
that enabled the fabrication of CNT AFM tips utilizing
relatively mild synthesis conditions.
In particular the relatively low experimental affords as
well as the fast fabrication times are a general advantage of
the introduced method and provide a promising, cheap
technique to fabricate CNT AFM tips. The deposition of the
catalyst material could be further improved by utilizing
particle picking approaches, e.g., by force vs distance curve
recording, to further increase the controllability of the
presented approach.
Acknowledgment. The authors are grateful for the financial
support of the Dutch Council of Scientific Research
(NWO) by a VICI grant awarded to U.S.S. This research has
been carried out with the support of the Materials and
Interface Chemistry Research Unit (SEM) and the Department
of Chemical Engineering and Chemistry, Eindhoven
University of Technology. Dr. Alexander Alexeev is kindly
acknowledged for help with the conductivity measurements
and for fruitful discussions. We thank the Dutch Polymer
Institute (DPI, technology area HTE) for funding.
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