# Direct Fabrication of Carbon Nanotubes STM Tips by  Liquid Catalyst-Assisted Microwave Plasma-Enhanced  Chemical Vapor Deposition

Authors: Fa-Kuei Tung; Masamichi Yoshimura; Kazuyuki Ueda
Journal: Journal of Nanomaterials
 (2009)

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## Abstract

Direct and facile method to make carbon nanotube (CNT) tips for scanning tunneling microscopy (STM) is presented. Cobalt (Co) particles, as catalysts, are electrochemically deposited on the apex of tungsten (W) STM tip for CNT growth. It is found that the quantity of Co particles is well controlled by applied DC voltage, concentration of catalyst solution, and deposition time. Using optimum growth condition, CNTs are successfully synthesized on the tip apex by catalyst-assisted microwave-enhanced chemical vapor deposition (CA-MPECVD). A HOPG surface is clearly observed at an atomic scale using the present CNT-STM tip.


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## Body

## 1. Introduction

Carbon nanotube (CNT) [1] has attracted considerable attention
because of their unique and outstanding properties, such as chemical inertness,
small tip radius, high-aspect ratio, mechanical robustness, and high electrical
conductivity. It is demonstrated that the CNTs are suitable as tips of atomic
force microscopy (AFM) [2–4] and scanning
tunneling microscopy (STM) [5, 6]. Numerous
methods to make CNTs tips have been reported: manual attachment of CNTs to the
tip apex using an acrylic adhesive [2],
electron beam-induced deposition (EBID) of amorphous carbon under SEM [7], and direct growth by a chemical vapor deposition
(CVD) technique [8–12]. The CVD
technique is most preferred in terms of mass production in the commercial use.
Although a single CNT growth is ideally suitable for CNT tip in scanning probe
microscopy (SPM), plenty of CNTs are grown over the whole tip by CVD because of the difficulty in positioning
catalyst only at the tip apex [12].In the present
study, we have developed fast- and high-reproducible fabrication method of CNT-STM tips. Liquid
catalyst-assisted microwave plasma-enhanced CVD (MPECVD) has been
used
to fabricate highly aligned CNTs onto the STM tip apex. Catalytic particles are
deposited only at the STM tip apex by this electrochemical method. It is shown
that the quantity of deposited catalyst particles is well
controlled by tip immersion time, applied DC voltage, and solution
concentration. Using an optimized growth condition, a single CNT is successfully grown
onto the STM tip. The performance of the present CNT-STM tip is demonstrated by
the observation of HOPG surface.


## 2. Experimental

A tungsten (W)
tip (0.25 mm in
diameter) for STM was prepared by electrochemical etching in KOH solution
(2.5 N). The catalytic solution was prepared from pure water, liquid ammonia,
2-ethylhexanoicacid, and cobalt chloride with different concentrations as10−1 M, 10−2 M, and 10−3 M. After a standstill for a couple of hours,
the mixture separated into two gravity-stabilized layers,
and the
topmost transparent layer was used as catalytic solution.  The prepared W tip was immersed in the solution
by a micrometer-adjustable apparatus. The different DC voltages (2 V, 4 V, and
8 V) were applied on W tips to control the number of catalytic particles attached
on the tip apex and to investigate aggregation of catalytic particles on W
tips. The, thus, prepared catalyst covered W tip was placed into a
custom-made tip holder in a commercial MPECVD system (CVD-CN-100,
Ulvac Japan, Kanagawa, Japan). The tip holder is designed
so that the damage by plasma can be minimized during CNTs growth. Prior to CNTs
growth, the tips were exposed to hydrogen plasma for the formation of
catalyst particles as well as catalyst activation. Subsequently, methane and
hydrogen gases were introduced for the CNTs growth. The mass flow ratio of CH4:H2 was kept at 1:4. The growth pressure was kept in the range of 1–5 torr. The
microwave generator provides 2.45 GHz, 500 W microwave. The growth time was
5–10 minutes, and a
DC voltage of −200 V was applied to the tip. The temperature of sample holder
during CNTs growth process measured by a K-type thermocouple was about 700C°. Field
emission scanning electron microscopy (FESEM, Hitachi S-4700) was used to characterize
the attached quantity of catalyst particles and the CNTs structures. The
crystallization of CNTs was studied by using a Raman spectroscopy (Jobin Yvon
HR-800) with a 633 (1.96 eV) wavelength laser and the analysis power of 25 mW. STM
measurement of a highly oriented pyrolytic graphite (HOPG) surface was
performed in air to examine the capability of the CNTs tip prepared in
this study.


## 3. Results and Discussion

The aggregation
of catalyst particles on W tip apex was investigated with different
deposition parameters. The effect of deposition time on catalytic
aggregation was shown in Figure1(a), and Figure 1(b) shows SEM images
after deposition for 1 second (a) and 5 seconds (b), with immersing
depth of 0.01 mm,
solution concentration of 0.05 M,
and applied voltage of 4 V. In Figure 1(a), no catalyst particles can be
observed, while apparent catalyst particles are observed in Figure 1(b). This
result suggests that the quantity of catalyst particles on the W
tip increases with deposition time. Figures 1(b) and 1(c) show the effect of applied
voltage on the aggregation of catalytic particles. Other deposition parameters are
kept constant: immersing depth (0.01 mm), catalytic concentration (0.05 M), and deposited time (5 seconds). The
effect of solution concentration on catalytic aggregation was shown in Figures
1(d)–1(f), where other
deposition parameters are kept constant: immersing depth (0.1 mm), applied voltage (2 V),
and deposited
time (5 seconds). The higher solution concentration gives the larger size of deposited
materials.SEM pictures of
catalyst-deposited tungsten tips with different electrochemical conditions. (a)
voltage (4 V), concentration (0.05 M), and time (1 second); (b) 4 V, 0.05 M, and 5 seconds; (c) 8 V, 0.05 M, and 5 seconds; (d) 2 V, 0.001 M, and 5 seconds; (e) 2 V, 0.01 M, and 5 seconds; (d) 2 V, 0.1 M, and 5 seconds.
(a)(b)(c)(d)(e)(f)Co cationic
particles in solvent are accelerated by electronic force forward to
cathode W tip apex. The amount of deposited aggregation particles has been shown to be
positively correlated with Co cationic particles flux, then, the relationship between
relative parameters and cationic particles can be supposed as follows:(1)N∝A∗S∗F,F∝M∗V,where N denotes amount of deposited aggregation
particles, F denotes flux
of ionic particles in unit time, A denotes
deposition area perpendicular
to flux, S denotes deposition
time,
M denotes concentration
of ion solvent, and V denotes applied
bias
voltage.Our experimental results can easily prove
this postulate. Higher applied voltage enhances the more ionic particles passing
through the unit area in a constant time, that is, the ion flux is increasing,
further increases the deposited aggregation particles. In the same
concentration and applied voltage, the ion flux in one unit area is in
constant, then, the longer deposition time allows more ion particles to pass
through the unit area, resulting in the increased deposition of  particles.
Finally, the higher concentration also increases the ion flux in unit area in a
constant time and enhances the particles deposition. Those results allow us to
assume that catalyst particles aggregation on W tip increases with the increasing
of the ionic flux. Thus, the amount of deposited ionic particles can be controlled by the
ion flux and depositing time. Besides, in this comparison, the over-deposition
cases were employed in order to emphasize the relationship between different
applied parameters and the catalyst particles aggregations on
the W
tip. Figure2(a) shows
CNTs grown on the catalyst-coated tungsten tip as shown in Figure 1(d) (2 V,
0.001 M, and 5 seconds). Numerous CNTs are grown on the tip surface, indicating the
presence of catalytic Co particles using the present deposition technique.CNT-STM tips fabricated at the
apex of tungsten tip. (a) CNTs from the deposited tip of Figure1(d). (b) SEM
and ((c), (d)) TEM images grown at the apex with optimum condition (2 V, 0.001 M, and ~0.5 seconds).
(a)(b)(c)(d)Figure2(b)
shows low-magnification SEM image of the W tip apex which was deposited by
applied bias 2 V, catalytic concentration (0.001 M), and deposited time (~0.5 second) after CNT
growth. Figures 2(c) and 2(d) show TEM images of the same tip. The
diameter and length are
~40 nm and ~900 nm, respectively. The number of graphen layers is 20–30. The
CNT growth mechanism is clearly confirmed as tip growth [13],
since the Co particle can be observed at the end of the CNTs. As
previously shown, the CNTs of 50 nm to 80 nm in diameter and ~800 nm to
1000 nm in length can be controlled by growth time [12].Raman
spectra are taken to spectroscopically characterize CNTs grown. The baseline-corrected
Raman spectrum from CNT tip is shown in Figure3. After using three Lorentzian peaks fitting, the center
positions were specified at 1354 cm−1, 1584 cm−1, and 1619 cm−1. The intensity rate and
area rate were 5.47:5.84:1 and 12.51:11.03:1, respectively. The peak
at 1354 cm−1 as
defined as D band [14] and
high D band shows
the amorphous carbon structure similar with active coal. The
peak at 1584 cm−1 is defined as G band, and
it shows the graphite structure of CNTs similar with polycrystalline graphite
powder [14]. The high intensity ID/IG ratio shows
the structure defects of CNTs which were synthesized by catalyst pyrolytic
method [15]. Peak at 1619 cm−1 comes from
amorphous
hydrogenated carbon (a:H-C) [16], which is regarded as
one kind of defective graphite structure.Figure 3
The baseline-treated Raman
spectrum of CNT tip. In addition to graphite structure (G band) and disordered
carbon structure (D band), amorphous hydrogenated carbon structure (a:H-C) is
also observed.In order to
demonstrate the capability of the CNT tip fabricated, highly-ordered pyrolytic graphite (HOPG)
surface
was observed as a test sample. Figure4 shows the STM image taken in air. A honeycomb
structure of graphite was obviously resolved at a bias voltage of 6.2 mV, although
the image is somewhat distorted due to thermal drift. The
corrugation measured is about 0.2 nm, comparable to the value obtained by
conventional STM tips. However, we believe the present CNT tip becomes more
powerful in the characterization of deep trench-like structures because of its
high aspect ratio.Figure 4
STM image of HOPG surface
obtained at a bias voltage of 6.2 mV. Honeycomb structure is clearly resolved.


## 4. Conclusion

We have
succeeded in fabricating the CNT-STM tip by liquid catalyst-assisted MPECVD. Highly aligned CNT
with 30–50 nm in diameter
was
grown on STM tip
apex,
as revealed by FE-SEM and TEM. We have demonstrated the performance of those
STM tips by obtaining atomic images of the HOPG surface. This method allows us
to place and deposit catalytic particles in a controlled way on the tip apex
much easier than conventional methods. It should be noted that even a single
CNT-STM
tip can
be fabricated by our CA-MPECVD method.



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*Source: 612549-2009-03-08.xml*