fabrication of carbon nanotube on silicon (cnt on si) high resolution afm tips: a chronological view
TRANSCRIPT
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Fabrication of carbon nanotube on silicon CNT on Si)
high resolution AFM tips: a chronological view
Mario Peláez Fernández
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Index
1. Introduction: Purpose of CNT AFM tips 3
2.
Some previous concepts 3
3.
Early attempts of fabrication 5
4.
Refining the method: the direct chemical vapour deposition 7
5. Batch production: CVD and the dielectrophoretic method 9
6. On the threshold of new methods 11
7. Conclusion 14
8. Bibliography 15
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1. Introduction: Purpose of CNT AFM tips
Since their discovery in 1991 4, carbon nanotubes (CNTs) have been a discovery of great
interest due to their unique properties and their wide diversity of applications, including
probe tips for scanning probe microscopy (SPM) 1
.
Among other applications in this field, the use of CNT in scanning probe microscopy (mainly
AFM) (CNT/AFM probes) have been proved to display a better high-resolution imaging when
compared to silicon probes, since this kind of probes feature special properties such as a
smaller tube diameter (less than 10 nm, which significantly improves their lateral resolution
as opposed to conventional silicon probes), a higher aspect ratio (between 10 and 1000, which
makes it possible to probe deep and steep features)2 ; a high chemical stability and stiffness
and, what’s most important, an elastic buckling when contacting the sample 4. On top of that,
since molecular structures of CNT are well defined, theoretically it will be possible to make a
batch of nanotube tips where all of them have identical structure and resolution.
.Several methods have been developed over time to fabricate CNTs onto AFM probes in a way
that can be controlled as much as possible. All these fabrication process will start with a classic
silicon AFM tip (a microfabricated silicon or silicon nitride cantilever with an integrated
pyramidal tip 3) and will end with the same tip covered in carbon nanotubes presenting some
kind of arrangement that we will discuss later.
2. Some previous concepts
2.1 Types of nanotubes
When fabricating carbon nanotubes, we can find several kinds of them:
Single-wall nanotubes (SWNT) are tubes of graphite which structure can be visualized as a
layer of graphene which is rolled into a seamless cylinder. They are more pliable than MWNT
but it is harder to make them. They can be twisted, flattened, and bent without breaking.
Multi-wall nanotubes (MWNT) are tubes of graphite that can the form of a coaxial assemblyof SWNT s of different diameter, or of a scroll of a single sheet of graphene. MWNT are easier
to produce in high volume quantities than SWNT. However, the structure of MWNT is less
well understood due to its complexity.
Double wall nanotubes (DWNT) are a midpoint between SWNTs and MWNTs. It is a coaxial
assembly of two SWNTs.
All three kinds are represented in the Figure 2.1:
a) b) c)
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Figure 2.1: a) SWNT. b) DWNT. c) MWNT
2.2 First use of CNT as SPM probes
In 1996, first attempts of SPM probes with CNT were made by Hongjie et al . 1. This research
was made in order to solve the problem of ‘tip crash’ problems due to the use of brittle tips
such as the classical pyramidal silicon AFM tips; as well as the inability to know the atomicconfiguration of the tip during imaging accurately.
One of the main reasons why CNT were so relevant for this purpose is that this kind of tip has
the unusual property of being both stiff and gentle. ‘ It is stiff because there is no bending of
the nanotube at all when it encounters a surface at near-normal incidence until the Euler
buckling force, , is exceeded’ 1. This force can be expressed as:
Where is the Young’s modulus, is the bending stress moment over the cross-section of the
nanotube and is the length of the nanotube. When that force is exceeded, the nanotube will
start to bend. In this particular study, is estimated to be nN, much gentler than the
maximal force a pyramidal silicon tip can exert when tapping a sample if the experiment is
not carefully controlled, which is nN. This is quite important as it can prevent damage
to delicate organic and biological samples 2.
The tip fabrication method for these initial experiments was simple: The bottom section (1-2
µm) of the tip was coated with an acrylic adhesive by sticking it into an adhesive-coated
carbon tape. Then this adhesive-coated tip made contact with the side of a bundle of 5-10
MWNTs while being observed with dark-field optical microscopy 1. Once attached, the
nanotube bundle was pulled from its neighbouring nanotubes leaving a single MWNT at the
end of the tip. An image of these initial tips can be seen in Figure 1.2:
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Figure 1.2: First fabrication of a CNT AFM tip. The selected area corresponds approximately to the coated region of
the silicon tip, the nanotube bundle attached to it and the single MWNT at the end of the tip.
Besides being proved to have an equal or better resolution when compared to silicon tips,
another interesting property showed by these kinds of tips is their great thinness at their apex,
which provides a better imaging at small gaps where a silicon tip would not be accurate
enough. For example, when fabricating these tips a 400 nm wide, 800 nm deep trench etched
in a TiN-coated Si wafer was imaged, confirming this statement. As we can see in Figure 1.3,
the silicon tip did not reach the bottom and imaged a triangular valley, which is consistent
with a imaging artefact caused by the pyramidal shape of the tip. On the other hand, the CNT
tip was able to reach the bottom of the trench and even image the texture of it:
Figure 1.3: a) Image of the abovementioned trench using a Si tip. b) Image using a CNT tip. c)
Texture of the bottom of the trench as imaged by the CNT tip.
3. Early attempts of fabrication
These ‘CNT sticking to Si tip’ methods were the first to be developed for this kind of
fabrication. The main problem with the method used by Hongjie et al. was that, with that
procedure it was difficult to adjust the length of the nanotube tip and to obtain a strong and
reliable attachment. In 1999 Nishijima et al. 2 found a way to improve both of these problems.
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For this fabrication method, previously prepared nanotubes were ultrasonically dispersed in
isopropyl alcohol and then this suspension was centrifuged in order to remove larger particles.
The nanotube solution was thereupon deposited on a 500 µm gap between the knife edges of
two disposable razors, on a glass plate; and they were subsequently aligned by an acelectrophoresis technique. When applying the ac field, the nanotubes were separated from
nanoparticles present in the solution and moved to the knife edges while being oriented
parallel to the electric field. When the solvent evaporated, they remained fixed in that position
by Van der Waals forces, solving one of the problems presented by the previous method.
One of the aligned nanotubes located on the knife edge was transferred to a silicon tip by
applying a dc bias voltage between the two of them, using a SEM to know where the nanotube
had been transferred on the tip.
Finally, the nanotube was attached to the Si tip by amorphous carbon deposition; which was
performed by the dissociation of contaminants (mainly hydrocarbons) by the electron beam of
the SEM mentioned above. An image of the finished tip can be seen in Figure 2.1.
Figure 2.1: CNT/Si tip obtained with this method. The highlighted area indicates where the nanotube tip
is located.
Again, this method had proven to produce tips that reach higher lateral resolutions than the
conventional tips, this time by imaging DNA and comparing the results obtained.
This method has been used for quite some time ever since. For example, it was the method
used in 2006 for the fabrication of SWNT, MWNT and DWNT tips in order to compare their
performance.
Other contemporary early attempts of fabrication included, for example, the creation of a
MWNT cartridge by chemical vapour deposition (CVD) and the subsequent transference ofthe nanotubes from the cartridge to the silicon tip using an electric field.
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4. Refining the method: the direct chemical vapour deposition
Even though some of the initial problems had been solved by this time, there were still some
major issues at stake, some of which could be avoided quite easily. First of all, the use of a SEM
limited the minimum size of the bundles or nanotubes to the minimum size the SEM coulddistinguish (typically between 5 and 10 nm), which affected the lateral resolution of the tip
directly. Second, the attachment time between a nanotube and a tip was relatively long. And
third, there are no well defined and reproducible etching procedures accurate enough to
expose a single nanotube.
In order to solve these issues, and given that there was a technique to ‘grow’ nanotubes by
CVD (as seen in the creation of a MWNT cartridge in the last section), a method to ‘grow’
them directly on commercial tips. Furthermore, by changing certain parameters (which willbe discussed below), either MWNT or SWNT tips would be fabricated.
In this method, shown schematically in Figure 3.1, first a flattened area was created at the tip.
With the help of an optical microscope, the apex of the tip was placed in a drop of HF solution
between two wires acting as counter electrodes, and it was subsequently anodized when a
difference of potential is applied between the two electrodes. Then, anodized tips were etched
in a KOH solution. This whole procedure etched 100 nm diameter, 1 µm deep pores.
Then, for MWNT tips, an iron catalyst was electrochemically deposited into the pores, under
de view of an optical microscope. Here, and was used. Later, they were
oxidized and then heated in a furnace at 800°C in a flow of argon and hydrogen, and a flow
of ethylene () was added for 10 minutes to grow the nanotubes.
Figure 3.1: Schema of the fabrication procedure for this CVD method
On the other hand, SWNT tips were prepared in a similar manner but, to favour the growth of
SWNTs, colloidal nanoparticles were used as the CVD catalyst instead of the previous
solution, since the particle size was comparable to the desired nanotube diameter. They were
electrophoretically deposited as well; and the SWNT were grown in similar conditions to those
of MWNT.
5. Batch production: CVD and the dielectrophoretic method
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Up until this point, all possible fabrication methods had been one-by-one procedures, which
was a big issue to fix. The previous CVD method had been used for batch production, but the
main problem with this method was that ‘thermal CVD growth has very little control over the
CNT location, density length and orientation ’ 4. It was really difficult to obtain individual,
well-oriented MWNTs using thermal CVD. The rate of usable tips was thus very low; and even
then, at the end of the fabrication most tips still required a one-at-a-time manipulation to
remove extra CNTs or to shorten the remaining CNTs.
With the objective of avoiding these problems, a bottom-up wafer scale fabrication method
was developed in 2004 4; combining nanopatterning and nanomaterials synthesis with the
silicon cantilever microfabrication technology used for the classical fabrication of tips:
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Figure 4.1: Schema of the fabrication process
The fabrication process, which is shown schematically in Figure 4.1, consisted of 5 major
steps:
First of all, the SOI sample was coated with an electron beam (e-beam) resist (in this case,
PMMA) and this resist was subsequently developed through e-beam lithography, creating
several dots (gaps) on it that would become the tips by the end of the process; as well as
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locating marks in order to achieve this process of nanopatterning one layer in the same place
as the layer before.
Then, a 20 nm Cr or Ti was evaporated on top of the resist and the gaps as a barrier layer and
20 nm Ni was evaporated right after that as a CNT catalyst. Then the sample was submerged
in ethanol to dissolve the PMMA leaving the catalyst deposited on the right space. It was then
covered with a layer of PECVD layer with a thickness of 200 nm since this kind of layer
has been proved to survive harsh dry and wet etching and front-side deep reactive ion etching
(DRIE).
After that, another pattern is made with a photoresist on top of the protective layer in order to
etch the right part of the protective layer and the silicon through dry etching. Following the
same procedure with the photoresist the lower part of the sample was wet etched with a KOH
solution where the oxide acted as an etching stop layer.
Later, the photoresists are dissolved and the protective layer was stripped from the sample. The
oxide below the catalyst was stripped as well in order to make a cantilever.
Finally, CNTs were grown from the defined catalyst spots that were present on the beams we
just had made using plasma enhanced CVD (PECVD).
One of the biggest advantages of this method is that it ‘provides CNT tips that are directly
grown from the silicon cantilevers at the wafer scale, not manually attached or randomly
grown.’4 Another one is that, as we have used e-beam lithography to make the initial holes,
both CNT tips location and diameter are defined by this e-beam process. Finally, their length,
orientation and crystalline quality were controlled by the PECVD. That is because in PECVD an
electric field exists in the plasma and it controls the orientation of the CNTs (parallel to the
electric field). One of the tips obtained can be seen in the following figure:
Figure 4.2: Finished CNT tip through batch CVD fabrication method.
But, even though this method had accomplished something that no other had before (batch
production of CNT AFM tips), further study showed that, after the examination of a large
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number of tips fabricated under the same conditions, there was ‘a relatively large variation of
the length and a wide angular distribution of the nanotube tips thus formed ’5.
As it’s been explained in section 3, it was possible to create CNT fibrils starting from a solution
of polarisable CNTs and an AC electric field. It is showed in a following study in 2004
5
that,with a method based on dielectrophoresis, it is possible to control and predetermine their
length and orientation.
In this process, a commercial Si AFM probe serves as the working electrode, whereas the
counter electrode is a small metal ring underneath it, with a layer of the CNT solution. Both
SWNT and MWNT were used and can be used for this kind of experiment.
As shown in the Figure 4.3, the fabrication process consisted on raising the counter electrode
(the ring) slowly until the apex of the AFM probe was soaked in the solution.
Figure 4.3: Fabrication procedure for the dielectrophoretic method: the ring, working as the counter electrode and
with a layer of the CNT solution, is approached to the AFM tip until the apex of it is soaked in the solution. Then it is
removed leaving a CNT tip.
CNT tips obtained are aligned along the axis of the Si tip due to the dielectrophoresis force
resulting from the interaction between the induce dipole moments of the CNTs and the
electrical field applied between the Si tip and the ring, that is why all probes seem to have a
very low angular deviation from one to another. The CNT probe is a coagulation of CNT that
end up in a thin fibril. The length of this probe can be controlled by the distance that the ring
was translated under the ac field. This study shows a very uniform result for the CNT probe
length.
On the other hand, the diameter of the CNT tip depends on various experimental parameters,
such as ‘the diameter of the initial nanotube bundle, the concentration of the nanotube
suspension, the voltage applied, and the drawing speed’.
6. On the threshold of new methods
6.1. The Langmuir-Blodgett method
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Apart from the dielectrophoretic method, other novel methods have been appearing over time.
One of the methods that has appeared as an upgrade for the conventional batch CVD method
in order to correct the inaccurate orientation of the nanotubes has been the fabrication
method using the Langmuir – Blodgett technique 6
; as well as having other advantages overbeing able to control the density and thickness as well.
The Langmuir-Blodgett technique is based on the utilisation of a monolayer assembled on top
of a liquid, much like the result of spilling a drop of oil on a lake. These kinds of layers are
called Langmuir layers.
To obtain a SWNT solution for the Langmuir layer, a very specific operation takes place.
According to the study, ‘SWNTs were shortened and carboxylated by chemical oxidation in a
mixture of concentrated sulfuric and nitric acid under sonication. The resulting filtrated
SWNTs were dispersed into 100ml of aqueous surfactant by 1h of sonic agitation. After
sonication, SWNT solution was centrifuged. The supernatant was then carefully decanted. This
SWNT solution was filtered for removal of surfactant. The resulting carboxylated SWNTs were
then reacted with 4-aminothiophenol in the presence of 1-[2-(dimethylanino)propyl]-3-
ethylcarbodiimide hydrochloride and N-hydroxysuccinimide SWNTs to give SWNT-SHs. The
SWNTs obtain ed were dissolved in chloroform.’ 6 .
Then, the AFM tip apex is immerged into a water surface and this solution was spread by
pouring minute droplets on the air/water interface. After the evaporation of the solvent, the
hydrophobic SWNTs bundles remained on the interface and were then compressed by
approaching the barriers on the water surface. Then, through a vertical dipping process
shown in Figure 5.1, the SWNTs were transferred onto the AFM tip apex and then the tip was
dried.
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Figure 5.1: a) Experimental setup for this fabrication method. b) Vertical dipping process that generate the CNT
probe.
This is a procedure that presents a vast set of advantages. First of all, it can be easily done
simultaneously for a batch of probes and it is reproducible for several batches, since the area
of the Langmuir film can be as large as desired. Second, the Langmuir SWNT films are highly
aligned so they provide a proper orientation of the SWNT at the end of the probe. Third, the
film is continuous and homogeneous which leads to a densely packed coating in the AFM
probe which improves mechanical stability.
The results of the study point out a 70% production rate with well-oriented SWNT probes.
6.2. Electron beam induced Pt deposition method
For this method the probes were fabricated by first attaching the nanotubes to a
nanomanipulator tip with adhesive with the help of an optical microscope. Later, under the
view of a SEM, this nanotube was brought into contact with a Si probe and fixed to the end of
it by deposing platinum through electron beam induced deposition. It is important to remark
that the nanotube needed to be necessarily in contact with the Si probe for this method to
work; and that the energy of the electron beam is not too high to avoid a milling effect that
would reduce Pt deposition. Then, the nanomanipulator tip was brought away and the CNTmodified as explained below.
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The lateral force constant () of the CNT tip can be expressed as:
Therefore, taking into account that the length of a nanotube is much higher than its radius,the nanotube probe should be very flexible in the lateral direction and therefore very likely to
bend at small angles.
With the objective of accurately controlling the alignment of the CNT probe, a focused ion
beam (FIB) process was used. By repeated single scanning of the ion beam imaging, the CNT
probe bent towards the FIB beam direction under every single scan until it was aligned with
the FIB beam direction.
A FIB milling process was used too to shorten the CNT probes accurately, by applying energy
on the C-C chemical bonds of the CNT until they had broken. The carbon atoms were
sputtered and the CNT was cut to its desired length. The effect of both procedures can be seen
in the Figure 5.2
Figure 5.2: Effects of FIB alignment and shortening on a misaligned and too long CNT probe
.6. Conclusion
We have seen some of the most widely known methods for the fabrication of CNT AFM tips,
and its evolution from the very beginning up until more recent times. It should be remarked
that this evolution is not over: This is still an active field of research that keeps evolving and
thus, growing; and that might be a commercially competitive technique for the fabrication of
AFM tips someday.
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Bibliography in chronological order)
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Microscopy: Preparation by a Controlled Process and Observation of Deoxyribonucleic
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[3] Cheung, C. L. “Carbon Nanotube Atomic Force Microscopy Tips: Direct Growth by
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[4] Ye, Qi; Alan M. Cassell; Hongbing Liu; Kuo-Jen Chao; Jie Han; and M. Meyyappan. “Large-
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[5] Tang, Jie; Guang Yang; Qi Zhang; Ahmet Parhat; Ben Maynor; Jie Liu; Lu-Chang Qin; and
Otto Zhou. “Rapid and Reproducible Fabrication of Carbon Nanotube AFM Probes by
Dielectrophoresis.” Nano Letters 5.1 (2005): 11-14.
[6]Lee, Jae-Hyeok; Won-Seok Kang; Bung-Sam Choi; Sung-Wook Choi; and Jae-Ho Kim.“Fabrication of Carbon Nanotube AFM Probes Using the Langmuir – Blodgett
Technique.” Ultramicroscopy 108.10 (2008): 1163-167
[7] Fang, F.z.; Z.w. Xu; G.x. Zhang; and X.t. Hu. “Fabrication and Configuration of Carbon
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Note: All figures have been taken from the mentioned articles except for Figure 1.1, which has
been taken from:
Dumé, Belle. “Scientists Delve Deeper into Carbon Nanotubes.” Physics World , 19 Feb. 2013.
Web. .
http://physicsworld.com/cws/article/news/2013/feb/19/scientists-delve-deeper-into-carbon-nanotubeshttp://physicsworld.com/cws/article/news/2013/feb/19/scientists-delve-deeper-into-carbon-nanotubeshttp://physicsworld.com/cws/article/news/2013/feb/19/scientists-delve-deeper-into-carbon-nanotubeshttp://physicsworld.com/cws/article/news/2013/feb/19/scientists-delve-deeper-into-carbon-nanotubes