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

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1. Introduction: Purpose of CNT AFM tipsSince 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 concepts2.1 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 assembly of 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:

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 atomic configuration 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, FEULER, is exceeded’ 1. This force can be expressed as:

FEULER=π2YIL2

Where Y is the Young’s modulus, I is the bending stress moment over the cross-section of the nanotube and L is the length of the nanotube. When that force is exceeded, the nanotube will start to bend. In this particular

a) b) c)

study, FEULERis estimated to be 5nN, 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 ¿100 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 ().

Figure (): 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 (), 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 (): 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.2. 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.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 ac electrophoresis 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 ().

Figure (): 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. (figure optional). 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 of the nanotubes from the cartridge to the silicon tip using an electric field. 3. 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 could distinguish (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 will be discussed below), either MWNT or SWNT tips would be fabricated.

In this method, shown schematically in Figure (), 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, MFeSO4 and MH 2SO4 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 (C2H4) was added for 10 minutes to grow the nanotubes.

Figure (): 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 FeOx 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.

4. Batch production: CVD and the dielectrophoretic method

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:

Figure (): Schema of the fabrication processThe fabrication process, which is shown schematically in Figure (), 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 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 S i3N4 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 ():

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 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 20045 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 (), 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 (): 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 show 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’.

5. On the threshold of new methods

5.1. The Langmuir-Blodgett method

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 over being 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 obtained 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 6.1, the SWNTs were transferred onto the AFM tip apex and then the tip was dried.

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.

5.2. 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 CNT modified as explained below.

The lateral force constant (k L) of the CNT tip can be expressed as:

k L=r4

L3

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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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 (), 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-nanotubes>.