ME382 Experiments in Micro/Nano Science and Engineering Term Project Paper

Introduction

In this era of micro/nano-technology, microscopy techniques become more important in exploring extremely small scale phenomena. AFM (atomic force microscopy) is apparently one of the most widely utilized techniques because it is a very versatile tool for not only measuring the topology of surfaces but also manipulating nanostructures. For example, AFM can be used for lateral arrangements of single atoms or biomolecules to make patterns on a substrate1,2, for tensile tests of carbon nanotubes (CNTs) to study their properties and mechanics3, and for electronic applications to data storage devices to develop ultrahigh-density recording system4.

Meanwhile, CNT has been attracted great attentions from many researchers because of its high strength, stiffness, variable conductivity, and promising optoelectronic properties for flat display panels5. As a result, there has been interest to make application with CNTs such as nano-composites5,6, nano-scale rotational berings7, and hydrogen storage media8.

One of the efforts is made in the area of microscopy studies. Many AFM users realize that the conventional silicon cantilever tip is easily broken when it hits the sample surface by mistakes during the operation. In addition, the fidelity of result is affected by the geometry of the tip so that the topology of high aspect ratio structures often cannot be measured exactly for certain types of pyramidal tips9. To solve these problems, a new technique has been proposed such that the cantilever tip has a protruding CNT from its apex, and then the needle-like CNT works as a probe. Since the mechanical property of CNT is very strong, robust and sensitive both chemically and biologically, along with its high aspect ratio geometry that can improve the measurement resolution, the CNT-probed microscopy brought a breakthrough in the development of microscopy technique9.

The ongoing research in this area can be classified into two major groups: a gluing method9-21 and a growing method22-24. In the former method, a readily available CNT is bonded on the point of the silicon tip. For example, Lieber et al.9 and Dai et al.10 glued CNTs on silicon tips on which acrylic adhesive is coated. This tip could be used to produce oxide nanostructures, showing high wear-resistance11. Similarly, Lieber et al.12 made CNT-probed tips using UV cure adhesive.

Some research groups utilized electrophoresis phenomena13-15 to align CNTs before assembling them on silicon tips. It is reported that Nakayama et al.13,14 used carbon deposition for the attachment whereas Tang et al.15 used AC electric field. Magnetic field could also be used for bonding CNTs16. Lastly, it is reported that CVD (chemical vapor deposition) grown CNTs are transferred to cantilever tips by applying DC electric field17,18.

In the latter, or the growing method, CNTs are directly grown on AFM tips. The idea is first reported by Lieber et al.19 The apex of conventional silicon tip is flattened, and nanopores is created along the tip axis. Then iron catalyst is electrodeposited into the nanopores, and single-walled CNTs are grown on the AFM tips. Later, the same group reported a similar but enhanced fabrication technique, so that the flattening of apex is no more required20. Manalis et al. also performed a similar investigation21.

In this review paper, details of the above-mentioned studies will be discussed in the following four viewpoints: fabrication issues, mechanics of probes during the measurement, resolution enhancements, and disadvantages in each method.

The Gluing Method

Bonding readily available CNTs on the apex of the silicon tips is an intuitive and direct method, even though it is somewhat difficult to implement and control due to small size of CNTs and tips. Many research groups have contributed to attaching CNTs to SPM tips, and their approach can be classified into two categories: one is a gluing method in which CNTs are bonded on the tips with coating adhesive, and the other is an attracting method in which CNTs are induced, aligned by the magnetic or electrical field, and finally pulled onto the vertex of tips. The following paragraphs introduce these methods in detail.

Dai et al.10 firstly showed that carbon nanotubes might be used as ideal probes for scanning probe microscope. The tip employed in their research as shown in Fig. 1 has prolonged life-time and can survive from the inadvertent tip crashes due to its excellent mechanical properties. Another advantage of the tip is that it can image the sharp sloping nanostructures on the surface because of the high aspect-ratio of CNTs. To fabricate the tip, an acrylic adhesive thin film (1-10nm thick) is coated on the bottom 1-2μm section of the conventional silicon tip by slightly measuring the surface of adhesive-coated carbon tape with contact mode. Next, this tip is moved under the view of an optical microscope and brought into contact with the side of a bundle of 5-10 multi-walled carbon nanotubes (MWNTs). Then one MWNT is attached on the tip due to the strong glutinosity of the adhesive. Once after the attachment, the nanotube bundle is withdrawn from the cluster of nanotubes with the tip. Typically, the protruding MWNTs on tips are 5-20nm in diameter and 0.25-1μm in length. In addition, the high wear-resistance can be obtained through the production of oxide nanostructures on them11. Similarly, Hafner et al.12 used UV cure adhesive to fabricate CNT-probed tips.

The tapping mode SFM images of a 400 nm wide, 800 nm deep trench pattern on a silicon wafer are presented in Fig. 2. These images are taken by the silicon tip without and with the CNT respectively. In Fig. 2(a), the apparent triangular shape of the trench can be observed due to the pyramidal shape of the tip. To the contrary, the thin, long nanotubes are now able to reach the bottom of the trench such that they can give clear image of the steep slope structure.

Manual assembly procedure is very time-consuming, which cannot be applied in the large-scale CNT-tips fabrication, in addition, the spatial resolution is limited by the larger diameter of MWNTs, since we can only view the thick nanotubes under the optical microscopy. Lieber et al.12 found some isolated, vertically aligned SWNTs stand on the substrate produced by CVD. They attempted to use micro-fabricated silicon tips to pick up individual SWNTs from the substrate. This “pick-up” procedure shown in Fig. 3 is accomplished by imaging the nanotube covered wafers in tapping mode. In order to enhance attachment, the micro-fabricated tips are coated with a layer of UV-cure adhesive at first. After the “pick-up” procedure, the glue is baked in UV light for 30min. The range of SWNT-tips diameters obtained by this method is from 0.9 to 2.8nm. The same literature also introduced how to use electrical etch technique to achieve the great control on the length of SWNTs bonded on tips. A 10-20V, 50-100μs DC pulse are applied between the tip and substrate while imaging a conductive sample. In this procedure, the removal amount from the tip end can be monitored from the change of the sample’s z-position. Approximately, 2-5nm length materials will be consistently etched from the tip end per pulse as shown in Fig. 4. In addition, increasing the pulse amplitude can enlarge the length etched per pulse.

Nanotubes exhibit field-induced phenomena under magnetic fields22. Based on the magnetic properties of CNTs, Hall et al.16 implemented an apparatus in order to introduce an alternative magnetic field onto a AFM silicon probes and a nanotube suspension (Fig. 5). With this apparatus, the CNTs dispersed in the solution are driven by the applied magnetic field to come into contact with the probe tip, and to protrude preferentially along the magnetic field direction. The AFM tip is fasten by cover slip on the glass cylinder platform, which positions the AFM tip about 400μm from the coil tip. Meanwhile, 5mL of MWNTs suspension in dichloromethane are prepared, and this solution is stirred by ultrasonic bath to achieve maximum homogeneity. The suspension is then dropped into the beaker where the tip is installed. In order to generate induced potential on the AFM tip, a 60nm gold film is sputter-coated on the AFM probe. Then an alternative current of 7A at 60Hz is applied to the coil, resulting in an oscillating magnetic field of amplitude B 0 =0.1T. After the coil excitation for 1min, useful CNT-probe tips are obtained with the probability of 50%. In general, these CNTs are found to protrude between 100 and 500 nm from the tip surface and have average 35° deflection angles with respect to the cone axis.

The physical principle underlying this method is the attraction between dipoles induced in the CNT and the gold film on the AFM tip. According to the electromagnetism, electrical potential is induced by the change in flux, proportional to . The expectation induced current encircling the tip or CNT, I , is governed by the equation (1):

(1)

Where ω is the magnetic field frequency, ρ is the resistivity of the gold coating or the CNT, L is the average circumferential path length around the tip or the CNT, A current is the total cross-sectional area of the gold coating or the CNT, and A flux is the encircling area through which the magnetic flux passes. The average induced dipole moments can be calculated by the equation (2):

(2)

Which are on the order of 10-11 Am2 for the tip and on the order of 10-23 Am2 for the CNT. The potential energy between the two induced dipoles can be evaluated by equation (3),

(3)

Where μ 0 is the permeability of air, r is the distance between the tip and the CNT, and m tip and m CNT are the dipole moments of the tip and the CNT. This potential should overcome the translational thermal energy in order to attract the CNT onto the tips. Through estimation, they found that the dipole attraction will bigger than the separation force within 1 cube μm domain.

Other research groups studied on the feasibility of fabricating CNT-probed tips by dielectrophoresis phenomena, which is often used to manipulate and assemble colloidal particles. Tang et al.15 demonstrated that room-temperature liquid-phase dielectrophoresis provides an efficient method for the fabrication of CNT-probed tips with controlled length and orientation. The experimental setup uses the silicon tips as the working electrode and a small metal ring as the counter electrode mounted on a translation stage and moving vertically with sub-micrometer resolution. A CCD connected to computer monitors the process. Similar to the above magnetic method, the CNTs are dispersed in de-ionized water, homogenized in an ultrasonic bath before dropping inside the metal ring. A 2-MHz AC field was applied between the two electrodes. The counter electrode is raised slowly until the suspension surface wets the apex of the AFM tip, as shown in Fig. 6. The electrode is then gradually withdrawn until the desired length CNT tip is assembled. The length of the CNT probe (varies from 0.5 to 10μm) is controlled by the distance that the counter electrode moves upward.

The orientations of the CNT with respect to the tip axis obtained by this method are consistent. Figure 7 is the histogram showing the angle distribution measured from 15 successful trials, all of which are within a 12o cone angle. As analyzed before, the alignment of the individual CNT with the Si tip done by the dielectrophoresis force caused by the induced dipole moments on the CNT and the tip. The key factor that contributes to the small deviation of the angle is the misalignment between the probe and the electrical field direction during CNT deposition, which can be corrected by improving the experimental setup.

A. Broude et al.23 presented a method which fabricates the CNT-probed tips by combining the sol- gel technique and dielectrophoresis phenomena. The sol-gel solution of TiO2 is prepared, and the viscosity of the sol is adjusted properly in order to obtain the desired thickness of the sol-gel (SG) deposition covering the tungsten tip and the CNT. CNTs are blended into the SG solution and homogenized the suspension completely. The CNTs is deposited from this solution onto the tungsten tip by the dip-coating method. Figure 8 shows the procedure of the fabrication. The tungsten tip are dipped into the SG solution containing the CNTs, a DC voltage is applied between the tip and a counter-electrode localized near the solution surface during the dip-coating process. After that, tips are pulled from the solution at a low speed, and at the same time, the voltage is gradually reduced to zero to avoid arc formation near the solution surface. CNT-tips are annealed at 350° under an infrared lamp for 15min.

The CNT-tips fabricated by this method demonstrated extremely strong adhesion. This is due to two important reasons. Firstly of all, the individual CNTs bond strongly with each other and adhere to the Si surface. In Fig. 9, high-resolution TEM images show that the apex of the tip is covered by CNTs entangled together into a thin fiber, moreover, the inset magnified image shows breakage of CNTs near the tip apex, which increases the contact area between the CNTs and the tip surface. This is due to the application of voltage during the dip-coating process which causes a sudden arc formation on the CNT apex. The relatively large contact area was believed to exhibit stronger adhesion as compared to the mechanically attached individual CNT. Secondly, the CNT-probed tip is sheathed by a thin layer of TiO2 film during the dip-coating process, which plays an important role of jointing the tip and CNT closely. In addition, the method potentially permits large-scale fabrication, since large numbers of tips may be dipped simultaneously. In their experimental, five tips are fabricated at the same time, and four useful tips are obtained in the best case.