recongition force microscopy study
Klein, D.C.G.
Citation
Klein, D. C. G. (2004, November 11). Feeling sugar-protein interactions using carbon
nanotubes : a molecular recongition force microscopy study. Retrieved from
https://hdl.handle.net/1887/106077
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Klein, D.C.G.
Chapter 5
Carbon nanotubes as nanometer-sized
probes for AFM
In this chapter, the application of carbon nanotubes as scanning probe tips is discussed. This chapter consists of two parts. The first part (§ 5.1 and 5.2) describes the growth of single-walled carbon nanotubes and the mounting procedure of these nanotubes onto commercial AFM tips. The mechanical behavior of carbon nanotubes as a function of tip radius and length is described. AFM images of IgG are shown in Section 5.3, to illustrate the use of these tips.
The second part (§ 5.4 and 5.5) deals with multi-walled nanotube AFM tips. Fabrication of multi-walled carbon nanotube AFM tips is discussed and high-resolution TEM images are shown. Finally, in § 5.6, the use of single-walled nanotube AFM tips is compared with the use of multi-walled nanotube AFM tips. Advantages and disadvantages of both types of tips are considered.
5.1 Introduction
Binding of a ligand molecule to the end of an AFM tip for MRFM has to be done carefully. First of all, the ligand should have some freedom to move and rotate in order to be able to position itself into the binding pocket of receptors on the surface. A flexible spacer between AFM tip and ligand can provide this freedom. If a high lateral resolution is pursued, the flexible spacer should not be too flexible, as was discussed in Chapter 1. Ideally, the spacer should have a small diameter in order to position the molecule precisely above a binding pocket.
Single-walled carbon nanotubes (SWNTs) have a diameter in the range of 0.5 nm to 3 nm, which seemingly makes them the ideal AFM tip.1,2 However, this small diameter makes the SWNTs a less favorable candidate for scanning rough surfaces, because they are less stiff than multi-walled carbon nanotubes (MWNTs), and lateral bending will decrease the resolution. In the second part of this Chapter (5.4-5), the use of MWNTs is discussed. MWNT tips are more robust, adhere better to the AFM tip and can be mounted while being imaged in an SEM. This makes them more easy to work with and especially more apt for applications in liquid.
5.2 Single-walled carbon nanotube AFM tips
Production: chemical vapor deposition
Chemical vapor deposition is a process in which the source of material that will be grown is provided by a gaseous precursor molecule. For carbon nanotube growth, methane, ethylene etc. can be used as a carbon source. Catalyst particles are used to locally make the decomposition of the gas possible at temperatures (700-850 °C) that are much lower than would be needed if no catalyst were present. For carbon nanotube growth, nanoparticles of transition metals such as Fe, Mo, Co and Ni or composites containing these metals are used3,4. A schematic picture of this process and a TEM image of a SWNT grown from a catalyst particle, both taken from Hafner et al.3, are shown in Figure 1.
The recipe for SWNT growth was taken from Hafner et al.2: a solution of 150 µg/ml ferric nitrate nona-hydrate was made in 2-propanol. Substrates of 1 cm x 1 cm of a silicon wafer were dipped into this solution for 10 s. Then, the pieces were rinsed in hexane for 10 s, and dried in a nitrogen flow. The tube oven was heated to temperatures between 700 and 850 ° C, and a flow of 0.6 l/min of argon and 0.4 l/min of hydrogen was used. Ethylene was let in at 0.5 or 1 ml/min. during 2-20 min. At the end of the growth process, the oven was cooled under argon during 20 min. For a more detailed description see the report of Clausen, Leiden University. 5
The precise growth process is not well understood, but many groups have developed hypotheses for catalytic carbon nanotube growth6. One of the hypotheses is that a layer of graphite is growing on a metal surface, and at a protrusion of the metal a cylindrical graphene sheet is formed. In this way, carbon nanotubes with a controlled diameter can be grown, by using catalyst particles with a well-defined diameter7.
Figure 1 Growing single walled carbon nanotubes using chemical
Figure 2 shows an AFM height image (a) and an amplitude image (b) recorded in tapping mode (TM) of SWNTs that were grown using chemical vapor deposition. The diameter of the nanotubes is smaller than the particle diameter. It is possible that single nanotubes grow from protrusions on the catalytic particles. Another explanation could be that the reduced particles from which the nanotubes grow are smaller than the oxidized particles, which we see in Figure 2. Very recently, carbon nanotube growth from Ni particles was observed in situ in a TEM8, albeit that the nanofibers were of a diameter of 5 nm. The shape of the Ni particle was found to change during the growth process. Dynamic formation and restructuring of step edges on the crystalline Ni particle appear to promote the formation and growth of graphene layers.
Mounting procedure
One way of mounting nanotubes on AFM tips is the direct growth of SWNTs on AFM tips 9, as is shown in Figure 3 a. Even wafer scale fabrication of SWNT tips for AFM has been demonstrated with this method. 10
b a
Figure 2 Single walled carbon nanotubes grown in a tube oven using the
Another method is growing nanotubes on a surface so that they stand up straight and form a forest of tubes. The nanotubes subsequently are picked up by an AFM tip that scans this surface in tapping mode. As soon as the AFM tip has picked up a nanotube, the z-piezo will make a step, as large as the length of the nanotube part that extends from the AFM tip, as is depicted in Figure 3 b and c. In our lab, we have used the second method.
To conclude: we are able to grow SWNTs on Si wafers, unfortunately without control of the length of the nanotubes. This is crucial for the next step in the process: mounting of SWNTs on AFM tips. For this we need SWNTs that are aligned perpendicularly to the
Figure 3 Two methods for producing AFM tips with carbon nanotubes.
Si wafer, so the SWNTs should form a forest (but with a low density) on the wafer. Most of the nanotubes that we have grown were too long to stand on the surface and they would bend over and lie flat on the surface. We successfully picked up four SWNTs from the sample we made in our group. We have also mounted SWNTs on AFM tips in our lab from a wafer containing SWNTs provided by the Lieber group at Harvard, USA.
Another widely used growth method for carbon nanotubes is arc-discharge, first used by Iijima, Meijo University, Japan.11 This method produces high quality carbon nanotubes with fewer defects (almost defect-free) than the CVD method does. A third method is laser vaporization, which was the method used by Smalley, Rice University, USA.12 We have not used these methods, because with these methods it is not possible to grow single walled carbon nanotubes that are aligned perpendicularly to a surface.
Intermezzo: carbon nanotube mechanics
After the nanotube has been picked up by the AFM tip, the length of the nanotube has to be adjusted. Two criteria can be used to estimate the maximum length for a nanotube with a given diameter that allows for stable imaging. The simplest criterion is that the lateral thermal motion of the nanotube should be smaller than say 0.5-1 nm in order to get AFM images with a high resolution. A criterion that is more stringent when rough surfaces are used is that the lateral bending of the nanotube due to a ‘typical’ adhesion force should be less than 0.5-1 nm.
An estimate for the lateral thermal motion of a carbon nanotube can be made by equating the thermal energy with the potential energy of lateral bending2:
nt B nt
k
T
k
X
=
(5.1)In this equation, Xnt is the thermal motion of the nanotube, kB
Boltzmann’s constant, T the temperature and knt the force constant of
lateral bending. The following approximation from the continuum formula can be used to estimate knt: 2
3 4
4
3
l
Y
r
k
nt=
π
(5.2)Equations 5.1 and 5.2 are valid for SWNTs as well as for MWNTs.
From these two equations, we can calculate that for a nanotube with a diameter of 1 nm and a maximal tip motion of 0.5 nm, the length of the nanotube should be less than 21 nm, as is shown in Table 1. If we allow a maximal tip motion of 1 nm for this nanotube, the length should be less than 33 nm. For a diameter of 2 nm and maximal tip motion of 0.5 nm (1 nm), the maximum nanotube length is 52 nm (83 nm).
Lateral bending of the nanotube can also be caused by adhesion of the nanotube to the side of a particle that is being imaged, and this can disturb high-resolution imaging as well.14 This is especially important for nanotubes that are mounted on the AFM tip under an angle with the surface, as will be shown in Figure 10. Carbon nanotubes have a lateral bending force constant that increases with their radius as r4 (Eq. 5.2), making MWNTs more
appropriate for imaging highly textured surfaces than SWNTs, because they have a larger diameter and thus a higher lateral stiffness. For a nanotube with a diameter of 1 nm and a length of 21 (33) nm, the bending constant is 0.02 (0.005) N/m. Lateral bending resulting from an adhesion force of 100 pN is for this nanotube 5 (20) nm, as is shown in Table 1. For a 2 nm diameter nanotube with a length of 52 (83) nm, the force constant of lateral bending is 0.02 (0.005) N/m, which results in a bending of 5 (20) nm for an adhesion force of 100 pN. For a nanotube with a diameter of 10 nm and a length of 250 nm, the lateral bending for an adhesion force of 100 pN is 1 nm. These numbers are valid for an adhesion force that is applied perpendicularly to the nanotube axis. For smaller angles θ, the net force will be smaller by a factor of sin (θ), as will be the bending, but the bending due to interaction with the surface is likely to increase, proportional to cos (θ), because the nanotube can interact with the surface over a larger length.
Table 1 Carbon nanotube thermal motion, lateral bending constant, and
lateral bending (for an adhesion force of 100 pN) as a function of nanotube diameter and length
Which of the two restrictions that were introduced above is more stringent: that the thermal motion should be less than 1 nm or that the lateral bending should be less than 1 nm? Comparing both bending constants results in the following: only for a lateral adhesion force below 4 pN, the thermal bending restriction is more stringent. This implies that only for very flat surfaces the lateral bending criterion is less important than the thermal motion criterion. In the Appendix about nanotube mechanics, the relationship between nanotube length, radius, buckling force, bending and lateral force constant is plotted.
Figure 4 Amplitude and deflection versus tip-sample distance for an
AFM tip with a nanotube attached. In part a, the nanotube is not touching the surface yet. In part b, it does touch the surface, and the amplitude is reduced. In part c the nanotube buckles, which gives the tip the opportunity to vibrate again and some amplitude is regained. In part d, the apex of the pyramidal silicon AFM tip touches the surface. As the tip retracts from the surface, a similar curve is followed, but usually it lies below the approach curve. Retraction curve is the lower curve.
Nanotube shortening
As was discussed above, the length of the nanotube determines if stable AFM imaging is possible, and it determines the resolution that can be obtained with the nanotube tip. Experimentally, an estimate for the length of the nanotube can be made from force-distance curves taken with this nanotube tip, as is shown in Figure 4.
The cantilever is brought closer to the surface, while tapping amplitude (upper curve) and cantilever deflection (lower curve) are recorded. In part (a), the amplitude of the tapping motion is the free amplitude and the deflection is zero. In part (b), the amplitude is reduced due to interaction between tip and surface (the nanotube taps on the surface) and the cantilever is deflected. If the nanotube is too long, shortening is necessary. If the applied force equals the buckling force (starting point of region c), the tube is buckled and the tapping amplitude is increased, because the tip now has more “space” to oscillate. The deflection levels off, because the energy that is put into the cantilever is transferred into deformation of the nanotube. In the last part (d), the apex of the silicon tip touches the surface, the amplitude is reduced to zero and the cantilever is deflected following Hooke’s law.
Shortening can happen spontaneously by tapping with the nanotube tip on the SWNT sample: the SWNT can slide along the pyramid until it locks itself into a stable position. More or less controlled shortening can be done by applying a voltage pulse between the nanotube tip and a conducting surface, while scanning the tip over the surface in tapping mode. The high field that is created at the tip end presumably results in emission of atoms. Hafner et al. observed constant steps in decrease in length of SWNTs when pulses with a constant voltage were applied.2 We used a thin niobium film that was sputtered on silicon to both provide a conducting surface and, because of the nanometer size grains present in the film, provides a test sample to check if the nanotube is short enough for stable imaging. The length reduction of the nanotube per pulse is again monitored by the steps that the z-piezo makes, as shown in Figure 5.
Making the tip water proof
When an AFM tip with a hydrophobic nanotube is inserted into a droplet and scanned, the nanotube is bent away and can be disrupted from the AFM tip. In order not to lose the nanotube when scanning in liquid, a stable contact between SWNT and tip has to be accomplished. We tried to form a carbide between nanotube and silicon by heating the nanotube tips, but we were not able to form a thin layer of silicon carbide. Either no silicon carbide was present (at
Figure 5 Amplitude versus distance curves before (a) and after (b)
application of 4 pulses of 8V. The estimated length of the part of the nanotube that sticks out from the AFM tip is reduced from more than 90 nm to approximately 50 nm. Again, there is hysteresis present, before and after shortening. Although it is expected that the hysteresis should be reduced after shortening, we did not observe this. But the force curves are not exactly reproduced each time, so it is difficult to compare two curves in detail. The retraction curve is light grey.
a
least it was not detectable in the TEM), or the whole nanotube was covered in carbide. For this reason, we used SWNT tips only in air. An application of SWNT tips is discussed in the next section.
5.3 Imaging antibody molecules with SWNT AFM tips
In order to test the imaging quality of the SWNT AFM tips, single IgG molecules were scanned in tapping mode. Bovine IgG molecules (Sigma-Aldrich) were deposited on mica (4µg/ml in PBS, 10µl during 10 s on mica, rinsed with deionized water, dried in a nitrogen flow) and imaged in air with a SWNT tip. IgG is one of the five types of antibodies and it is made up of three fragments. Two are
b a
9.8 nm
8.8 nm
Figure 6 Bovine IgG molecules on mica, imaged in tapping mode in air
called Fab fragments (A and B) and contain the binding sites for antigens and the third fragment is called Fc fragment. At first, only this fragment has been crystallized (that is why it is called c-fragment) and the molecular structure of this part was determined by X-ray studies15. At present, the whole IgG molecule has been crystallized and the molecular structure from X-ray analysis is shown in Figure 7.
The distance from the center of one IgG fragment to the next is around 10 nm. We measured a distance of approximately 10 nm both with a normal Si AFM tip and with an SWNT tip, as can be seen in Figure 6. In order to be able to separately resolve the three fragments, a very sharp tip with a high aspect ratio is required. We have seen the three fragments separately in AFM images using a normal tip only once, see Figure 6 a. With the SWNT tips, we always resolved the three fragments, an example is shown in Figure 6 b. This is because the SWNT tips have a very high aspect ratio. New high aspect ratio tips may be a good alternative to SWNT tips in experiments where chemical functionalization is not important. But we experienced that these sharp tips become blunt or contaminated during imaging more easily than the nanotube tips.
Surprisingly, the contrast in the phase image turned out to be different on one of the three lobes of the molecule compared to the other two lobes, as can be seen in Figure 8, as was also seen by J. Hafner (unpublished results). The phase contrast does not change when the scan direction is rotated 90 degrees (Figure 8 c and d) or when the scan direction is reversed.
This remarkable phase behavior is an indication of a difference in interaction between the hydrophobic nanotube tip and the Fab fragments as compared to the Fc fragment. For example, the physicochemical properties of the Fab fragments are different from the physicochemical properties on the Fc fragment, 16 which could possibly result in a difference in water layer thickness on the two
types of fragments. This could explain the behavior of the phase in Figure 8.
Figure 8 Single IgG molecule imaged in TM in air. (a) Height image
(b) Phase image. Images c and d are recorded with the scan direction rotated 90 degrees. (c) Phase image, trace (d) phase image, retrace. All image sizes are a 100 nm x 100 nm software zoom of 500 nm x 500 nm images. Vertical scales: height between 0 and 3 nm, phase between 0 and 20 degrees.
a b
5.4 Multi-walled carbon nanotube AFM tips
Multi-walled carbon nanotubes (MWNTs) consist of multiple graphene sheets rolled up in concentric cylinders. MWNTs have diameters in the range of 2-10 nm, which makes it possible to image them with a Scanning Electron Microscope (SEM), unlike SWNTs, which are too small to be imaged in an SEM.
We used an SEM to mount MWNTs to commercially available AFM tips. MWNTs were grown by the group of Prof. Y. Nakayama in Osaka, Japan. Mounting of the nanotubes was done in the group of Prof. S. Jarvis in Dublin, Ireland, in close collaboration.
Production: arc discharge
The carbon nanotubes were prepared by a conventional arc discharge method and are aligned on a knife edge using an alternating current electrophoresis technique.17
Figure 9 SEM image of the mounting procedure. Upper half: razor
Mounting procedure
MWNTs are mounted on AFM tips in an SEM, while both the knife edge containing the nanotubes and the AFM tip are in the field of view of the SEM, as can be seen in Figure 9. The positions of both the knife edge and the AFM tip are manipulated with pairs of micrometer screws. Using these two independent translation stages,18,19 a nanotube can be positioned onto the side of the pyramid of an AFM tip, as is shown in Figure 9.
The angle between the mounted nanotube and the silicon tip is critical. Ideally, the nanotube tip is oriented perpendicular to the sample surface, as is shown in Figure 10 a. If the angle
between nanotube and silicon tip is too large, as is depicted in Figure 10 b and c, adhesion between the nanotube and the sample will lead to image distortion and loss of resolution, as was discussed in Chapter 5. 2.
We mounted nanotubes on two types of cantilevers: stiff silicon cantilevers (k=2 N/m, f = 78 kHz) and soft, thin silicon nitride cantilevers with a gold coating (k=0.03 N/m, f = 30 kHz). In contrast with stiff cantilevers, mounting a nanotube on the soft cantilevers was quite difficult, because the tip is pushed away very easily by the nanotube.
Making the tip water proof
Focusing the electron beam on the spot where the nanotube touches the pyramid results in local deposition of amorphous carbon. The voltage used for this deposition turned out to be crucial. With a
a b c Figure 10 Mounting angle between nanotube and AFM tip. In (a) the
voltage of 15V, a current of 15 µA and zooming in 100 times, the deposited carbon appeared to be well-attached to the AFM tip and relatively water resistant.
This is a big issue: the wall of the nanotube is hydrophobic and will presumably try to avoid contact with water by bending away from it when inserted into a droplet. If the nanotube is long, it can even snap back onto the pyramid of the AFM tip completely. If the deposited carbon fixes the nanotube well enough, such that the nanotube can not bend at the base of the pyramid, snapping can be prevented. Also, the length of the nanotube should be reduced as much as possible to prevent snapping.
Coating of carbon nanotubes with a polymer (e.g. ethylenediamine) can make them less hydrophobic.20 We have not used this procedure because we need uncovered nanotubes for chemical functionalization as described in Chapter 6. In order to test if the MWNT tips that we produced were water-proof, we dipped them in water and imaged them again in the SEM. MWNTs that were shortened to a length of ≤ 500 nm, and that were “cemented” with amorphous carbon as was described above, survived the water dipping test.
Nanotube shortening
Shortening of the nanotubes to the desired length was done by applying voltage pulses between the AFM tip and the razor blade. In the case of silicon tips, a thin metal layer was deposited on the tips first, in order to establish electrical contact between the nanotube and the tip. A buckle is introduced in the nanotube by pushing it against the razor blade, as is shown in Figure 9. Shortening of the nanotube will happen at the buckle, because there the nanotube is strongly bent, which means that locally the electrical conductivity is not so good. This results in heat development at the buckle site, and the tube will break.
5.5 High-resolution TEM results
MWNTs deposited on a carbon grid have been imaged by Iijima using a voltage of 200 kV, as shown in Chapter 1. Here, we imaged MWNTs that were mounted on an AFM tip, using the same high voltage. We observed that the MWNTs were destroyed when imaged, as is illustrated in Figure 11. Although the resolution in Figure 11 b is higher than in Figure 11 a, the cylindrical concentric structure of the nanotube has disappeared and the nanotube end is not well-defined anymore. We assume that the difference between our TEM results and those of Iijima et al. is caused by the fact that Iijima’s MWNTs could transfer heat to an underlying grid, whereas ours could not, because they are suspended from the AFM tip. As a result, our MWNTs heat up too much and the carbon lattice is destroyed. An interesting article on the influence of electron radiation on the structure of carbon nanotubes appeared this year.22
By using a voltage of only 80 kV, we managed to make high resolution TEM images without destruction of the graphene layers, as is shown in Figure 12.
Open versus closed carbon nanotubes
In Figure 12 a and b, the end region and centre region of one MWNT are shown. Figure 12 c and d are similar images for another MWNT. The inner diameter of the tube in Figure 12 a and b is 2.6 nm, and the outer diameter is 15.3 nm. This tube consists of 18 concentric walls. In Figure 12 c and d, we find a tube with 10 walls, an inner diameter of 2.5 nm, and an outer diameter of 9.3 nm. Neither the nanotube end in Figure 12 a, nor that shown in c has the typical dome-shaped structure that is characteristic for a closed nanotube.
Figure 11 TEM images of the end of a MWNT AFM tip, when too high
an acceleration voltage was used (200 keV). Image (b) was taken 5 minutes after continuous imaging of (a). Images were made by Frans Tichelaar, National Centre for HREM, Delft. Scale bars are 3 nm.
Based on this observation, we conclude that both nanotube ends are partly open. From the TEM images it is clear that some of the walls are closed. A necessary requirement for chemical functionalization of the MWNTs is that the outer tube is open. The more open tubes, the more reactive groups will be present on the MWNT end.
b d a c
Figure 12 TEM images at 80 keV e-beam energy of two MWNTs. In
5.6 Conclusion
Comparing the success rate of producing SWNT AFM tips, which is around one per day, with the success rate of producing MWNT tips, which is roughly one per hour, the choice for MWNT AFM tips is easily made. Only in cases where the small diameter of the SWNTs is necessary, as was the case when imaging IgG molecules (Figure 6), one should go through the enormous effort of making SWNT AFM tips. Interestingly, it is possible to extract the inner tube of a MWNT by an electrical breakdown an manipuation process, in order to obtain a MWNT with an SWNT end, so-called “telescopic” nanotube.23
If a surface that contains a very high density of tall features should be measured, long nanotube tips are required. 24 In this case, SWNTs are far too flexible, as was discussed in 5.2. MWNTs with a large diameter should be used, in order to prevent lateral bending of the nanotube.
Another difficulty with SWNT AFM tips is stable attachment of the nanotube to the tip. This is especially important when using nanotube tips in liquid. We have not managed to make SWNT tips water proof, because silicon-carbide formation between the SWNT and the silicon AFM tip was not successful. For this reason, we applied SWNT AFM tips only in air.
An advantage of MWNTs is that they stick better to AFM tips because the contact area is larger than it is in the case of SWNTs. In addition to the larger contact area, we made MWNT tips even more stable by “cementing” them with amorphous carbon that is deposited locally with the electron beam in the SEM. Using this method, we were able to make water-proof MWNT tips, provided that the MWNTs were shortened to 250 nm or less. By shortening the nanotubes to this length and by optimizing the angle between nanotube and tip, thermal motion and lateral bending of the nanotube were reduced to less than 1 nm. Combining these properties, we were able to produce water-proof nanotube tips apt for high-resolution imaging. These nanotube tips have an end that is partly open, and they are ready for chemical functionalization, which will be described in the next chapter.
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