fabrication of carbon nanotube on silicon (cnt on si) high resolution afm tips: a chronological view

8/9/2019 Fabrication of carbon nanotube on silicon (CNT on Si) high resolution AFM tips: a chronological view

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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)

[1] Dai, Hongjie; Jason H. Hafner; Andrew G. Rinzler; Daniel T. Colbert; and Richard E.

Smalley. “Nanotubes as Nanoprobes in Scanning Probe Microscopy.” Nature 384.6605 (1996):

147-50.

[2] Nishijima, Hidehiro; Satsuki Kamo; Seiji Akita; Yoshikazu Nakayama; Ken I. Hohmura;

Shige H. Yoshimura; and Kunio Takeyasu. “Carbon-nanotube Tips for Scanning Probe

Microscopy: Preparation by a Controlled Process and Observation of Deoxyribonucleic

Acid.” Applied Physics Letters 74.26 (1999): 4061.

[3] Cheung, C. L. “Carbon Nanotube Atomic Force Microscopy Tips: Direct Growth by

Chemical Vapor Deposition and Application to High-resolution Imaging.” Proceedings of the

National Academy of Sciences 97.8 (2000): 3809-813.

[4] Ye, Qi; Alan M. Cassell; Hongbing Liu; Kuo-Jen Chao; Jie Han; and M. Meyyappan. “Large-

Scale Fabrication of Carbon Nanotube Probe Tips for Atomic Force Microscopy Critical

Dimension Imaging Applications.”Nano Letters 4.7 (2004): 1301-308.

[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

Nanotube Probes in Atomic Force Microscopy.” CIRP Annals - Manufacturing Technology 58.1

(2009): 455-58.

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

fabrication of carbon nanotube on silicon (cnt on si) high resolution afm tips: a chronological view

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