WO2005017977A2

WO2005017977A2 - Carbon nanotube bundle probe for scanning probe microscopes

Info

Publication number: WO2005017977A2
Application number: PCT/GB2004/003493
Authority: WO
Inventors: Julie V. Macpherson; Neil Wilson
Original assignee: The University Of Warwick
Priority date: 2003-08-13
Filing date: 2004-08-12
Publication date: 2005-02-24
Also published as: WO2005017977A3; GB0318987D0

Abstract

We describe a probe for use in the imaging of a substrate, said probe comprising a probe body and a probe needle wherein said probe needle comprises a bundle of nanotubes, a process for the manufacture of the probe and uses thereof.

Description

PROBE

This invention relates to a probe for use in the imaging of a substrate, processes for its manufacture and methods of use thereof.

Background to the invention The Atomic Force Microscope (AFM) is used to image substrates in a wide range of technologies, such as in the electronic, telecommunication, biological, chemical, automotive, aerospace industries. Substrates include film coatings, ceramics, composites, glasses, synthetic and biological samples, metals, polymers and semiconductors and the AFM is applied to the study of phenomena such as abrasion, adhesion, cleaning, corrosion, etching, friction, lubrication plating and polishing.

One of the main objectives of the operation of an AFM is to measure the forces (at the atomic level) between a sharp probing tip (which is attached to a cantilever spring) and a sample surface. There are three main classes of interaction between the tip and the sample, these are contact mode, tapping mode and non-contact mode.

The earliest AFM comprised a minute shard of diamond glued onto the end of a minute strip of gold foil. Over time the conventional cantilever was developed, made of silicon (Si) or silicon-nitride (Si₃N₄) with an integrated pyramidal tip and formed by means of a micro-fabrication technique, such as lithography, etching etc. Since the cantilever detects the force between the surface of a specimen and the tip apex, the resolution of the image is determined by the degree of sharpness of the tip end.

The attachment of a multi-walled carbon nanotube (MWNT: diameter 5 - 20 nm) to the side of a micro-fabricated silicon AFM tip using a soft acrylic adhesive (Dai et al, Nature 1996) conferred on the AFM tips both an increased lateral resolution due to the small tube diameter, and the ability to accurately determine the surface topography of sharp recesses as a result of the high aspect ratio. The presence of the tube also meant that the tube could survive repeated tip crashes given its resistance to damage resulting from reversible elastic buckling. In an attempt to increase spatial resolution, Lieber et al. (1998) attached single walled carbon nanotubes (SWNTs: diameter ~ 1 nm) to the AFM tip. SWNTs can be directly grown from the end of a silicon tip (e.g. J. Haftier, C. Cheung and CM. Lieber (1999). hi this method a tip is dipped into a solution containing catalyst particles and then chemical vapour deposition (CVD) is employed to grow the SWNTs from the catalyst particles. This method requires there to be particles, ideally one particle, at the end of the tip. An alternative method which is 100 % successful is referred to as the 'pick-up' method (J. Hafher, C. Cheung, T. Oosterkamp and C. Lieber). In this method CVD is used to grow SWNTs on a Si plate. Some of the tubes stand proud of the surface and when a scanning AFM probe comes into contact with a vertically- aligned tube, the tube is 'picked up' onto the tip of the probe. It is held in place by van der Waals forces.

However it was found that MWNTs and SWNTs were prone to falling off the AFM tip during repeatedly scanning surfaces of specimens, especially under solution, since the carbon nantotube was simply 'stuck' on the protruding end.

An attempt method to improve the attachment of carbon nanotubes to AFM tips is disclosed in US 2003/0001091. This method involves fastening a conductive nanotube to a cantilever by means of a conductive deposit. This deposit is formed by means of building up decomposed-components of organic gases, with the decomposition of these organic gases, such as hydrogen-carbon series gases or organic metallic gases being achieved by means of electron or ion beams.

The fabrication of the AFM probes described in US 2003/0001091 is time consuming and complex and involves the deposition of a number of conductive components. A conductive film has to be formed on the cantilever which is then electrically connected to a conductive nanotube by means of the conductive deposit. To further assure electrical conductivity, the conductive film and conductive nanotube is forced to be electrically connected by means of covering over the conductive deposit with a conductive coating film. The use of strong magnetic metals (Fe, Co, Ni) as the conductive substance enables the magnetic property of surfaces of specimens to be characterised. Thus this probe is able to be used in topographical and magnetic force microscopy. There remains a need however for the development of a probe in which a firmly secured nanotube tip overcomes the complex fabrication requirements and stability problems associated with the previously weakly adhered tube tips outlined in the prior art. Furtheπnore there is a need for a probe which can be utilised in a increased range of imaging applications, including the conventional topographical imaging and magnetic force imaging (MFM imaging), but also in ultra-high resolution electrical imaging applications e.g. conductivity imaging (conducting- AFM); electrical field mapping (electrical field microscopy: EFM); and electrochemical imaging which involves using the SWNT as an ultra-small microelectrode.

Summary of the invention

Thus according to a first aspect of the invention there is provided a probe for use in the imaging of a substrate, said probe comprising a probe body and a probe needle wherein said probe needle comprises a bundle of nanotubes.

A bundle of nanotubes, preferably single-walled nanotubes (SWNTs) are attached to the apex of the probe body preferably using the 'pick-up' method as described above. The length, /, of the SWNT protruding from the end of the probe body can vary from a few nanometres to a few microns, although the length / can be reduced if required by electrical etching.

In an alternative preferred embodiment of the invention multi-walled nanotubes (MWNT) are used.

Preferably the bundle comprises from between 2-500 nanotubes, and more preferably still said bundle comprises from between 10-100 nanotubes. The diameter of the bundle is preferably from between 2-5 Onm, and even more preferably from between 5-10mn.

h a further preferred embodiment of the invention the bundle comprises carbon SWNTs. The nanotubes in the bundle are preferably held together by inter-nanoτube bonding, for example by strong van der Waals attraction between nanotubes.

A bundle of SWNTs has increased stability due to the large number of SWNTs touching the tip and displays increased rigidity compared to that of an individual SWNT, which is an important consideration for the production of high aspect ratio nanowire probes . This increase in rigidity can be quantified by estimating the resistance of the end of the SWNT to forces perpendicular to the SWNT axis, i.e. the lateral spring constant ki, and the axial force required to buckle the SWNT, i.e. the Euler buckling load F_b. Modeling a SWNT attached to a probe body as a simple cantilever fixed at one end and free at the other, for a single nanotube of radius r, and Young's modulus Y_nt (1.25 TPa) gives ^ - ajΫ and F_b = π² a/ 1² , where a = Y_nt πr⁴/4 is the flexural rigidity of the SWNT. Thus, for a 2 nm diameter SWNT,

1 /mi long, kj » 3 x 10^"6 N m^"1 and F_b » 0.01 nN, this is too flexible to use for imaging in contact or tapping mode. The increased radius of a probe needle comprising a bundle of nanotubes confers on the probe needle increased rigidity. For example, a 10 nm diameter bundle of SWNTs of 1 μm in length has Fb « 6 nN and ki » 2 x 10^"3 N m^" ^l, and can be used to image in the tapping and contact modes.

Thus in a preferred embodiment of the invention said probe has an F in the range of from 1-100 nN. Even more preferably still said probe has an Fb of 50 nN. When the probe is to be used in the tapping mode, where some flexibility is required, a preferred buckling force of approximately 2 nN would be desired. Alternatively, if the probe is to be used in the contact mode where the probe does not need to have as much flexibility, a buckling force of preferably 50-100 nN would be desired.

In a still preferred embodiment of the invention said probe has a ki in the range of from 0.001-100 N m^"1. Even more preferably still said probe has a ki of 1 N m^"1.

The preferred kj will vary depending on the imaging mode. For instance, in the tapping mode, a low ki is preferred, whilst in the contact mode a high kj is preferred. The parameters of F_b and Ki can be adjusted by varying the length and/or number of the nanotubes within the bundle. The advantages of a bundle of nanotubes include the ease of fabrication using the 'pick-up' technique, the increased stability due to the large number of nanotubes in contact with the tip and an improved topographical resolution due to an individual nanotube protruding from the end of the bundle.

Therefore, in a preferred embodiment of the invention, at least one nanotube protrudes from said bundle. Preferably this protruding nanotube protrudes from a surface of the bundle which is furthest from the probe body and provides a contact surface for said substrate.

The electrical conductivity of the bundle of nanotubes is generally governed by an individual nanotube which protrudes from the bundle and contacts the substrate first. The electrical contact is generally made to the end of the SWNT rather than the side.

Electronically, for example one in three SWNTs is metallic whilst the remainder are direct band gap semiconductors. SWNTs have been demonstrated to be ballistic conductors at room temperature and can conduct some tens of microamps current. Thus an individual SWNT protruding from the bundle may be metallic or semi- conducting. For apphcations where robustness, reproducible low resistance electrical properties or specific electrode materials are required it is possible to form metallic nanowire probes or nano-electrodes.

Therefore, in a further preferred embodiment of the invention said probe needle is coated with a metal film. Even more preferably said probe body is coated with a metal film. If an uncoated nanotube bundle is to be used in the imaging applications described above, in order to achieve an electrical connection between the probe needle and the probe body, the later would need to be coated with a metal film prior to the 'picking-up' of the nanotube bundle.

The nanowires can be formed by sputter coating a thin film of metal onto the probe needle and/or probe body. Preferably the thickness of this film is less than 100 nm. Even more preferably said thickness is less than 35nm. During sputtering a substrate is placed in a vacuum chamber with a target of the material to be deposited. A plasma is generated in a passive source gas (e.g. Argon) in the chamber, and ion bombardment is directed towards the target, causing material to be sputtered off the target and condense on the chamber walls and the substrate, isotropic deposition. A strong magnetic field (magnetron) can be used to concentrate the plasma near the target to increase the deposition rate. Despite the high aspect ratio of the probe this results in a uniform coating on both the probe needle and probe tip, which is not possible when depositing the material using an evaporator. Furthermore, the increased rigidity of the bundle of nanotubes, makes the individual nanotubes more resistive to the mechanical and thermal stresses induced by sputtering, enabling the formation of longer and straighter nanowires. As a result of coating the tips with a metal film prior to 'pick-up, an electrical etching procedure, which is used to reduce the length of the nanotubes, becomes more controllable and in general can occur at lower voltages. However, the 'picking-up' of SWNTS onto uncoated tips increases the likelihood of the nanotubes passing directly through the apex of the tip rather than to one side.

In a further preferred embodiment of the invention the metal is an element. In principle any metal element that is compatible with the sputter coating process could be used to form a nanowire. Preferably this element is a transition element, even more preferably this transition element is selected from Group VIII transition elements, and is selected from the group consisting of; Fe, Pt, Ir Co and Ni or mixtures therefore. Alternatively this transition element is selected from Group lb transition elements, for example Au, Ag or mixtures therefore. Alternatively still said the transition element is selected from Group nib transition elements, for example Ti. Even more alternatively still said the transition element is selected from Group Via transition elements, for example Cr.

The bundle may also be coated with non-transition elements, such as Al.

In an alternative embodiment of the invention the metal is an alloy, selected from the group consisting of; AuPd, Pur or mixtures thereof. The ability to produce a variety of metallic nanowires, extends the capability of an AFM probe to conductivity (using for example AuPd, Ptlr coated nanotubes), magnetic (using for example Fe, Co and Ni coated nanotubes) and electrochemical imaging (using for example Pt, Au, Ir coated nanotubes for pH sensing and using for example Ag coated nanotubes for Cl^" potentiometric detection). The probe can also be used to simultaneously record topography whilst imaging in the applications outlined above.

Once metal has been deposited on the probe needle it is significantly stiffer and more robust than the original bundle. Not only is each tube in the bundle securely attached to the probe body by the coating of metal on top of it, but furthermore, the metal adds to the rigidity of the bundle despite its comparatively low Young's modulus.

For example, for a nanowire formed from a metal of Young's modulus Y_met, outer radius R, and with inner SWNT bundle radius r, the resultant flexural rigidity is a = π Y_ntr⁴ +Y_met(R⁴ -r⁴)/4. Thus for a 1 μm long bundle, 10 nm in diameter, a 25 nm thick coating of AuPd (taking Y_AUPU ~ 100 Gpa) results in an increase of k from

2x10^" N m^" to 0.2 N m^" and, although it will not buckle reversibly, an increase of E_t, from 6 nN to 600 nN. For a typical adhesion force of a few nN, far less than F_b, this corresponds to a lateral deflection of the nanowire of 10 - 20 nm, comparable to the probe body radius. By halving the length of the nanowire this is reduced to a few nm, sufficient for the nanowire to be used to image in contact mode.

In a further preferred embodiment of the invention said probe needle coated with a metal film has a Εuler buckling load F_b in the range of from 1 nN-1 mN. In an even more preferred embodiment of the invention said probe needle coated with a metal film has a lateral spring constant kj in the range of from 0.001 Nm^_1-1000N m^"1.

Thus the bundle of nanotubes inside the nanowire anchors it firmly to the probe body and also confers increased mechanical and electrical stability.

In a still fiirther embodiment of the invention at least one further film coats the first metal film. Preferably the first film is Ti and the further film comprises a metal element, a metal alloy or mixtures thereof. Ti is used as a first film layer as it can coat SWNTs uniformly in a low film thickness (« 2 nm) and can act as an adhesive layer for the formation of uniform metal nanowires of diameter ≤IO nm.

Alternatively this further film comprises a non-conducting material. The term nonconducting in this sense means that it is not electrically conductive. The insulators can be applied by sputter coating, by CVD or electrochemical deposition. Preferably this non-conducting material is an oxide or nitride, of for example silicon (SiO₂, Si₃N₄) or aluminium (Al₂O₃ or A1N). Alternatively this non-conducting material is a polymer, for example PET, PVC, parylene or phenol 2-allyl phenol.

In a further preferred embodiment of the invention this non-conducting material is selectively removed, for example from the end of the probe needle that provides a contact surface for said substrate. This will produce a reproducible and geometrically well defined 'nano-electrode' probe.

According to a second aspect of the invention there is provided a probe for use in the imaging of a substrate comprising a probe body and a probe needle wherein said probe needle has a Euler buckling load F_b the range of from InN to lOOnN. Preferably still said F is 50nN.

According to a third aspect of the invention there is provided a probe for use in the imaging of a substrate comprising a probe body and a probe needle wherein said probe needle has a lateral spring constant ki in the range from 0.001 N m^"1 to 100 N m^"1. Even more preferably still said ki. is 1 N m^"1.

According to a fourth aspect of the invention there is provide a probe for use in the imaging of a substrate comprising a probe body and a probe needle wherein said probe needle comprises a bundle of nanotubes and said probe needle is coated with at least one substantially polycrystalline metal film.

When a nanotube has been sputter-coated with a metal element, metal alloy or combinations thereof, whilst the metal film is often uniform and continuous, an obvious grain structure is apparent, even when the grain size of the metal/metal alloy, for example AuPd is small. A structural change in the metal/metal alloy from granular to substantially polycrystalline can be induced by passing a large current across the nanotube. This process is referred to as annealing. The resultant structural changes are attributed to the energy dissipated in the nanotube at the grain boundaries of the metal. This effect could be induced by either applying the bias during/- d measurements, or whilst scanning in contact or tapping mode. The resultant nanowires have low resistance and high current carrying capacity.

Preferably, said probe needle is coated with a substantially polycrystalline metal film. Even more preferably said probe body is coated with a substantially polycrystalline metal film. Preferably the thickness of said polycrystalline metal film is less than 35nm.

In a further preferred embodiment of this aspect of the invention the metal is an element. In principle any metal element that is compatible with the sputter coating process could be used to form a nanowire. Preferably this element is a transition element, even more preferably this transition element is selected from Group VIE transition elements, and is selected from the group consisting of; Fe, Pt, Ir Co and Ni or mixtures therefore. Alternatively this transition element is selected from Group lb transition elements, for example Au, Ag or mixtures therefore. Alternatively still said the transition element is selected from Group Illb transition elements, for example Ti. Even more alternatively still said the transition element is selected from Group Via transition elements, for example Cr.

The bundle may also be coated with non-transition elements, such as Al.

In an alternative embodiment of the invention the metal is an alloy, selected from the group consisting of; AuPd, Ptlr or mixtures thereof.

The ability to produce a variety of metallic nanowires, extends the capability of an AFM probe to conductivity (using for example AuPd, Ptlr coated nanotubes), magnetic (using for example Fe, Co and Ni coated nanotubes) and electrochemical imaging (using for example Pt, Au, Ir coated nanotubes for pH sensing and using for example Ag coated nanotubes for Cl^" potentiometric detection). The probe can also be used to simultaneously record topography whilst imaging in the applications outlined above.

According to a fifth aspect of the invention there is provided a process for the manufacture of a probe needle provided with a polycrystalline metal film coating comprising the steps of; i) coating the probe needle with a granular metal; and ii) converting said granular metal with defined grain boundaries into a polycrystalline film.

Preferably said structural conversion is as a result of the dissipation of energy at grain boundaries. Even more preferably still this dissipation of energy is by annealing, preferably by applying a voltage across the metal film, in the range of from 1-10 V, and preferably still applying 3 V.

In a further embodiment of the invention said probe body is also provided with a polycrystalline metal film coating.

According to a sixth aspect of the invention there is provided a method of imaging a substrate comprising the steps of; providing a probe comprising a probe body and a probe needle wherein said probe needle comprises a bundle of nanotubes; connecting said probe to image processing equipment; and contacting said probe needle with a substrate to be imaged.

Preferably said probe is used in an imaging application selected from the group consisting of; topographical imaging, conductivity imaging, electric field imaging, magnetic imaging, electrochemical imaging. The probe can be used to simultaneously record topography whilst imaging in the applications outlined above.

In a seventh aspect of the invention there is provided a microscope comprising a probe as herein described, preferably said microscope is an Atomic Force Microscope. Specific embodiments of the invention

Embodiments of the invention will now be described by example only and with reference to the following Figures:

Figure 1: Transmission Electron Microscopy (TEM) images of (a) a bundle of SWNT on an AuPd coated tip, (b) the same tip after sputter coating with AuPd and annealling, and (c) an enlarged view of the nanowire. The image widths are (a) 350 nm, (b) 500 nm, (c) 100 nm.

Figure 2: TEM images of the same part of the same AuPd nanowire before (a) and after (b) annealling. The images are 150 nm square.

Figure 3: Current through the nanowire and deflection of the tip as the probe is brought down towards, 'extend' (black line), and away from, 'retract' (red line) an Au surface. 'A' marks where the probe touches the surface on 'extend', and 'B' where it leaves the surface on 'retract'. The applied bias was 3 V through 1.1 MΩ resistance in series. The spring constant of the tip was measured to be 0.2 N m^"1.

Figure 4: Current- voltage response of an AuPd nanowire probe.

Figure 5: Topography, (a) and (c), and conductivity (b) and (d), images of an Au surface taken with an AuPd nanowire tip after scanning continuously for 4 hrs, (a) and (b), and 5 hrs, (c) and (d). The applied bias was 2 V through 1.1 MΩ series resistance. The images are 3 μm square.

Figure 6: TEM images of an Au nanowire probe (a), (b) and (c) an enlarged view of the nanowire. The image heights are (a) 1.5 μm, (b) 8 μm, and (c) 100 nm. Detailed description of the Figures

A Digital Instruments Multimode Atomic Force Microscope with Nanoscope IIIA controller and Picoforce module was used. TEM images were taken using a JEOL2000FX at 200 kV and a tilt angle of 60°. Sputter coating was performed with a standard Emscope SC500 sputter coater. Single beam Si microfabricated contact tips (Nanosensors 'contact' probes with nominal spring constant 0.07 - 0.4 N m^"1) and tapping tips (Nanosensors 'force modulation' probes with nominal spring constant 1.2 - 5.5 N m^"1) were used. By using shorter nanowires on contact tips, nanowire probes capable of imaging in contact mode were fabricated. For tapping mode applications nanowire probes with lengths up to a few microns could be employed.

Electrical transport through the nanowires was investigated by imaging an Au substrate (evaporated Au on a Si substrate with Ti adhesion layer) in conducting mode where a bias is applied between the tip and substrate whilst the tip is scanned in contact with the surface. The current was measured with a current preamplifier and inputted to the Nanoscope Ilia controller to allow simultaneous acquisition of topography and current images. Simultaneous force - distance, f - d, and current - distance, i - d, measurements were performed using the Nanoscope Ilia controller and Picoforce module to control the distance between the Au surface and the probe whilst externally recording the current, height of the probe above the surface (z position)¹, and deflection of the cantilever (linearly related to the force). The current as a function of applied bias voltage, i - V response, was also measured whilst the nanowire was held stationary in contact with the Au surface. Figure 1

Figure 1 (a) shows a TEM image of a small bundle of SWNTs attached to an AuPd coated contact tip. The bundle, ca. 10 nm in diameter, consists of many SWNT and protudes ca. 600 nm from the end of the tip (taking account of the 60° tilt angle of the TEM). Figure 1 (b) shows the apex of the same tip after sputter coating with 20 - 25 nm AuPd and subsequent annealing, see below. The diameter of the nanowire is measured to be 50 nm whilst the length is 600 nm corresponding to the length of the original tube tip. An enlarged view of the nanowire is shown in Figure 1 (c), its polycrystalline nature is evident as is the uniformity in diameter. By sputter coating with less AuPd (and no Ti or Cr underlayer) we have formed continuous nanowires with diameters as small as 30 nm.

Figure 2

Figure 2 (a) shows a TEM image of part of a SWNT bundle after sputter coating with AuPd. The nanowire formed has an obvious grain structure, though the grain, size is small, as expected for AuPd, and the coating is uniform and continuous. A structural change in AuPd was induced by passing a large current through the AuPd nanowire. Figure 2 (b) shows the same area on the same nanowire after applying a 3 V bias (with a 100 kΩ resistor in series) with the nanowire in contact with a Au surface. The structure of the AuPd alloy has clearly changed from granular to polycrystalline.

Figure 3

Figure 3 shows simultaneous f - d and i - d measurements for an annealed AuPd nanowire, roughly 50 nm in diameter and 250 nm in length, on a contact tip. A 3 V bias was applied through a 1.1 MΩ resistor in series. The cantilever spring constant here was 0.2 N m^"1, as measured by the thermal noise method. The black line shows the response as the probe is lowered towards the surface from right to left, 'extend', and the red line as it is brought away from left to right, 'retract'. 'A' marks the point on the 'extend' curve at which the nanowire first touches the surface, the current immediately jumps to its maximum value and remains there. The observed maximum current of 2.714 μA when in contact with the surface implies a resistance through the nanowire, cantilever and probe of 5 kΩ. 'B' marks the point on the 'retract' curve at which the nanowire loses contact with the surface. The hysteresis between the point at which the nanowire touches the surface on the 'extend' and leaves the surface on 'retract' is indicative of the adhesive forces between the nanowire and Au surface. Note that the current remains constant at its maximum value up until the moment at which the tip leaves the surface, i.e. the current response is independent of the applied force.

Figure 4

Figure 4 shows the i -V response of the nanowire tip, shown in Figures 2(b) and (c), the resistance in series was removed for these measurements. The response is ohmic, with a resistance of only 2.5 kΩ even though the wire is 600 nm long. This puts an upper bound on the resistivity of the nanowire of 30 μΩ cm, although it is likely that some of the 2.5 kΩ resistance is not due to the nanowire itself, but due also to the conducting path to the nanowire through the thin metal film on the tip. Also of interest is the current density through the nanowire. At 0.5 V bias the current through the nanowire is 200 μA corresponding to a current density of 25 x 10⁵ A cm^"2. It is probable that much of the current will be flowing through the bundle of SWNTs at the core of the nanowire. SWNTs are known to be able to sustain current densities in excess of 10⁹ A cm^"2. The SWNT may also provide a thermal sink for heat dissipated in the nanowire due to their high thermal conductivity, greater than 200 W mK^"1.

These results demonstrate the nanowire tips are near ideal conducting probes with low, ohmic, load independent resistance up to the point of contact.

Figure 5

To test the robustness and longevity of the nanowire tips an Au surface was imaged continuously in conducting mode, with a 2V bias applied across a 1.1 MΩ resistance in series. The same annealed AuPd nanowire tip was used as for the i - d response shown in Figure 3. Figure 5 shows topography, (a) and (c), and current, (b) and (d), images taken after 4 hrs, (a) and (b), and 5 hrs, (c) and (d), of continuous scanning. The robustness of the nanowire tip is shown by both the clarity of the topography image after 4 hrs continuous imaging (a), and the current image, (b). The latter demonstrating there is still a good conducting contact between nanowire and surface. Both the topography and current images recorded after 4 hrs are similar to those when imaging commenced.

After 5 hrs of imaging it is clear from the artifacts in the topography that the nanowire has been damaged in some way, although, interestingly the current image still demonstrates a good electrical contact between nanowire and surface. Subsequent TEM imaging showed that the nanowire had reduced in length. However, given that the nanowire is conducting along its entire length despite being worn down it still remains a good conducting tip. This is in marked contrast to a metal coated tip where, when the thin metal coating wears away at the tip, the insulating Si (or Si₃N₄) underneath becomes the point of contact. Clearly to improve the longevity for conductivity imaging tougher metals, such as Ptlr, could be used to form the nanowire Figure 6 i order to demonstrate the generality of this nanowire tip fabrication method we have also formed Au nanowires. Au is particularly useful as an electrode material, and as such a well defined Au nanowire would be an invaluable tool for studying electrochemistry at the nanometre level. Figure 6 shows TEM images of an Au nanowire on a tapping mode tip. The nanowire is 1 μm in length, 80 nm wide at its base, and 50 nm at its end, the difference corresponding to the change in diameter of the SWNT bundle at its core. Although Au has a larger grain size than AuPd, and although there appears to be a weak interaction between Au and SWNT's, the nanowire formed is still continuous although not as uniform as the AuPd nanowires. To improve the uniformity of coating a multiple layer process could be employed; previous work proved that Ti coats SWNT uniformly at low film thickness, as low as 2 nm, and can act as an adhesive layer for the formation of uniform metal nanowires of diameter <10 nm. Future work will focus on a variety of multi-layer nanowires, in particular coating the tip and metallic nanowire in insulator, selectively removed at the end of the nanowire, with the aim of forming reproducible and geometrically well defined 'nano-electrode' probes.

References Dai et al. Nature 1996, 384.

Hafher J., Cheung C, and Lieber CM. J. Am. Chem. Soc. 1999, 121; 9750.

Hafner J., Cheung C, Oosterkamp T., and Lieber CM. J. Phys. Chem. B 2001, 105; 743.

Claims

1. A probe for use in the imaging of a substrate, said probe comprising a probe body and a probe needle wherein said probe needle comprises a bundle of nanotubes.

2. A probe according to claims 1, wherein said bundle comprises in the range of from 2-500 nanotubes.

3. A probe according to claim 2, wherein said bundle comprises comprises in the range of from 10-100 nanotubes.

4. A probe according to any of claims 1-3, wherein the diameter of said bundle is in the range of from 2-50 nm.

5. A probe according to claim 4, wherein said diameter is in the range of from 10-50 nm.

6. A probe according to any of claims 1-5, wherein each of said nanotubes comprises a single wall.

7. A probe according to claim 1, wherein each of said nanotubes is a carbon nanotube.

8. A probe according to any of claims 1-7, wherein said nanotubes are retained within said bundle by inter-nanotube bonding.

9. A probe according to claim 8, wherein said inter-nanotube bonding comprises van der Waals forces.

10. A probe according to any of claims 1-9, wherein said probe has a Euler buckling load F_b in the range of from 1-100 nN.

11. A probe according to claim 10, wherein F_b is 50 nN.

12. A probe according to any of claims 1-11, wherein said probe has a lateral spring constant k; in the range of from 0.001-100 N m^"1.

13. A probe according to claim 14, wherein A is 1 N m^"1.

14. A probe according to any of claims 1-13, wherein at least one nanotube protrudes from said bundle.

15. A probe according to claim 14, wherein said protruding nanotube protrudes from a surface of the bundle which is furthest from the probe body.

16. A probe according to claims 14 or 15, wherein said protruding nanotube provides a contact surface for said substrate.

17. A probe according to any of claims 1-16, wherein said probe needle is coated with a metal film.

18. A probe according to claim 17, wherein said probe body is coated with a metal film.

19. A probe according to claim 17 or 18, wherein the thickness of said metal film is less than lOOnm.

20. A probe according to claim 19, wherein said thickness is less than 35nrn.

21. A probe according any of claims 17-20, wherein said metal is an element.

22. A probe according to claim 21, wherein said element is a transition element.

23. A probe according to claim 22, wherein said transition element is selected from Group VIII transition elements.

24. A probe according to claim 23, wherein said transition element is selected from the group consisting of; Fe, Pt, Ir, Co and Ni or mixtures therefore.

25. A probe according to claim 22, wherein said transition element is selected from Group lb transition metals.

26. A probe according to claim 25, wherein said transition element is selected from the group consisting of; Au, Ag or mixtures therefore.

27. A probe according to claim 22, wherein said transition element is selected from Group Tub transition metals.

28. A probe according to claim 27, wherein said transition element is Ti.

29. A probe accordmg to claim 22, wherein said transition element is selected from Group Via transition metals.

30. A probe according to claim 29, wherein said transition element is Cr.

31. A probe according to claims any of claims 17 -20, wherein said metal is an alloy.

32. A probe according claim 31, wherein said alloy is selected from the group consisting of; AuPd, Ptlr or mixtures thereof.

33. A probe according to any of claims 17-32, wherein said probe needle has a Euhler buckling load F_b in the range of from 1 nN-1 mN.

34. A probe according to any of claims 17-32, wherein said probe needle has a lateral spring constant k] in the range of from 0.001 m^-lOOO m^"1.

35. A probe according to any of claims 17-34, wherein at least one further film coats the first metal film.

36. A probe according to claim 35, wherein the further film comprises a metal element, a metal alloy or mixtures thereof.

37. A probe according to claims 35, wherein the further film comprises a nonconducting material.

38. A probe according to claim 37, wherein said non-conducting material is a nitride or oxide of silicon.

39. A probe according to claim 38, wherein said silicon nitride or oxide is selected from the group consisting of; SiO₂, Si₃N₄.

40. A probe according to claim 37, wherein said non-conducting material is an oxide of aluminium.

41. A probe according to claim 40, wherein said aluminium oxide is Al₂O ,

42. A probe according to claim 37, wherein said non-conducting material is a polymer.

43. A probe according to claim 42, wherein said polymer is selected from the group consisting of; PET, PVC, parylene or phenol 2-allyl phenol.

44. A probe according to any of claims 37-43, wherein said non-conducting material is selectively removed.

45. A probe according to claims 44, wherein said non-conducting material is selectively removed from the end of the probe needle that provides a contact surface for said substrate.

46. A probe for use in the imaging of a substrate comprising a probe body and a probe needle wherein said probe needle has a Euler buckling load F_b in a range of from 1-100 nN.

47. A probe according to claim 46, wherein said F is 50 nN.

48. A probe for use in the imaging of a substrate comprising a probe body and a probe needle wherein said probe needle has a lateral spring constant kj in a range of from 0.001-100 N m^"1 .

49. A probe according to claim 48, wherein said k_\ is 1 N m^"1.

50. A probe for use in the imaging of a substrate comprising a probe body and a probe needle wherein said probe needle comprises a bundle of nanotubes and said probe needle is coated with at least one substantially polycrystalline metal film.

51. A probe according to claim 50, wherein said probe needle is coated with a substantially polycrystalline metal film.

52. A probe according to claims 50 or 51, wherein said probe body is coated with a polycrystalline metal film.

53. A probe according to any of claims 50-52, wherein the thickness of said polycrystalline metal film is less than 100 nm.

54. A probe accordmg to claim 53, wherein the thickness of said polycrystalline metal film is less than 35 nm.

55. A probe according to any of claim 50-54, wherein said metal is an element.

56. A probe according to claim 55, wherein said element is a transition element.

57. A probe according to claim 56, wherein said transition element is selected from Group VIII transition elements.

58. A probe according to claim 57, wherein said transition element is selected from the group consisting of; Fe, Pt, Ir, Co and Ni or mixtures therefore.

59. A probe according to claim 56, wherein said transition element is selected from Group lb transition metals.

60. A probe according to claim 59, wherein said transition element is selected from the group consisting of; Au, Ag or mixtures therefore.

61. A probe according to claim 56, wherein said transition element is selected from Group IHb transition metals.

62. A probe according to claim 61, wherein said transition element is Ti.

63. A probe according to claim 56, wherein said fransition element is selected from Group Via transition metals.

64. A probe according to claim 63, wherein said fransition element is Cr.

65. A probe according to any of claims 52-56, wherein said metal is an alloy.

66. A probe according claim 65, wherein said alloy is selected from the group consisting of; AuPd, Ptlr or mixtures thereof.

67. A process for the manufacture of a probe needle provided with a polycrystalline metal film coating comprising the steps of; i) coating the probe needle with a granular metal; and ii) converting said granular metal with defined grain boundaries into a polycrystalline film.

68. A process according to claim 67, wherein said conversion is as a result of the dissipation of energy at grain boundaries.

69. A process according to claim 68, wherein said dissipation of energy is by annealing.

70. A process according to claim 69, wherein said annealing is by applying a voltage across the metal film.

71. A process according to claim 70, wherein said voltage is in the range of from 1 - 10 V.

72. A process according to claims 70 or 71, wherein said voltage is 3 V.

73. A method of imaging a substrate comprising the steps of; i) providing a probe comprising a probe body and a probe needle wherein said probe needle comprises a bundle of nanotubes; ii) connecting said probe to image processing equipment; and iii) contacting said probe needle with a substrate to be imaged.

74. A method of imaging a subsfrate according to claim 73, wherein said imaging is selected from the group consisting of; topographical imaging, conductivity imaging, magnetic imaging, electrochemical imaging.

75. A microscope comprising a probe according to any of claims 1-74.

76. A microscope according to claim 75, wherein said microscope is an Atomic Force Microscope. A probe, process or method as substantially as hereinbefore described with reference to the description and examples.