WO2004102582A1 - Carbon nanotube-based probes, related devices and methods of forming the same - Google Patents

Abstract

A device for high resolution probing of a surface includes a support member, and an extension portion attached to a distal end of the support member. A core of the extension portion is electrically connected to the support member. The core is a carbon nanotube in the case of an AFM probe or a ferromagnetic composition in the case of an MFM probe. An electrically insulating sheath layer is disposed on a portion of the core, wherein the tip portion of the core opposite the support member does not include the sheath layer.

Description

CARBON NANOTUBE-BASED PROBES, RELATED DEVICES AND METHODS OF

FORMING THE SAME

FIELD OF THE INVENTION [0001] This invention relates generally to nanoscale probes and related devices formed from nanotubes attached to mounting members, such as cantilever tips.

BACKGROUND OF THE INVENTION [0002] An emerging class of devices for nanotechnology applications are proximity probes of various types. These probes include scanning tunneling microscopes (STM), atomic force microscopes (AFM) and magnetic force microscopes (MFM). While substantial progress has been made in controlling the position of the macroscopic probe to sub-angstrom accuracy and in designing sensitive detection schemes, existing tip designs have led to a number of limiting shortcomings.

[0003] Carbon nanotubes have shown promise in forming such probes. Techniques for attaching multi-walled carbon nanotubes onto standard AFM cantilever tips such that the nanotube becomes the tip probing the sample have been recently disclosed. Since the nanotube itself is electrically conducting, when mounted on an electrically conducting AFM tip the nanotube tip becomes electrically accessible. Since carbon nanotube modified AFM tips were first disclosed, variations in the mounting method, extension to single wall nanotubes and applications exploiting the high nanotube aspect ratio, small tip radius, electrical conductivity and chemical versatility have all been demonstrated. These studies have concentrated on the intrinsic advantages of nanotube modified tips. However a critical look at this teclmology also reveals several problems. In particular, despite the rigidity of the nanotube sidewall, since the nanotube is long and slender the nanotube is highly susceptible to bending and buckling instabilities. [0004] Given the Young's modulus of the column material, the load at which the nanotube buckles can be readily calculated and is referred to as the Euler buckling force. This buckling force depends on the fourth power of the diameter and on the inverse of the length squared of the nanotube. Clearly, a thicker column is more stable against buckling. Moreover, doubling the thickness provides 16 times the load carrying capacity, while a longer column is more prone to buckling as doubling the length lowers the load at which it does so by a factor of 4. Since an AFM uses the change in position of the cantilever to assess the distance to the surface within a feedback loop, when a nanotube buckles it sends the feedback loop into oscillations making imaging unstable.

[0005] Similar to AFM, nanotube buckling is problematic for MFM and related methods. MFM is a technique that measures, non-destructively, the magnetic topography of a sample. It is a non-contact mode of measurement in which the phase or frequency shift or amplitude change in an oscillating magnetic tip, due to interaction between the long range magnetic dipoles force, as the tip is rastered across a magnetic sample surface at a fixed height, is mapped. Digital Instruments, Inc. (Dl), located in Buffalo, NY, is a company that supplies industry standard Scanning Probe Microscopes (SPMs), also known as Atomic Force Microscopes (AFM's). Dl is currently a division of Veeco Corporation. The Dl AFM generally records the MFM data in a two pass measurement that allows the topographic signal to be separated from the magnetic signal. In the first pass, the topography of the surface is mapped. In the second pass the tip is lifted to a user defined height (lift height) from the surface and same line scan in retraced.

[0006] Thus, a nanotube that extends too far off the tip (i.e. is too long for given diameter), cannot be used for AFM or MFM imaging. Regarding AFM probes, some have suggested use of an electric, nano-arc based shortening procedure to remove tube material at the end of the nanotube until its length becomes short enough to provide stable imaging. However, this method results in at least two problems. A first problem is that this method is quite labor intensive because of the need for repeated testing of the stability upon each shortening iteration. The second problem results because the mounting procedure usually provides a bundle of nanotubes with one nanotube sticking out furthest being the nanotube that is used for imaging. If this length is too long for stable imaging, shortening it can be tricky, because removing the end of a single nanotube to the point where there are two or more tubes makes for a less than optimally sharp probe. This arrangement is limiting because the total tip diameter is the analytically significant probe dimension.

BRIEF DESCRIPTION OF THE DRAWINGS [0007] A fuller understanding of the present invention and the features and benefits thereof will be accomplished upon review of the following detailed description together with the accompanying drawings, in which:

[0008] FIG. 1 (a) shows an AFM tip mounted nanotube before being coated with a stiffening polymer layer.

[0009] FIG. 1(b) shows an AFM tip mounted nanotube after being coated with a stiffening polymer layer, according to an embodiment of the invention.

[0010] FIG. 2 is a graph showing the calculated Euler buckling force and ratio of lateral displacement (coated/unco ated) for a coated nanotube as a function of the coating thickness.

[0011] FIG. 3 shows a polymer coated nanotube having an exposed tip after laser exposure to the tip, according to an embodiment of the invention.

[0012] FIG. 4 shows an exemplary MFM probe embodied as polymer coated magnetic metal nanowire probe, according to another embodiment of the invention.

[0013] FIG. 5 shows a TEM image of an intermediate structure used in the formation of MFM probes according to the invention which includes an etched nanotube with resultant nanopore, according to an embodiment of the invention. [0014] FIG. 6 shows a transmission electron microscope (TEM) image of an intermediate structure in the formation of MFM probes, according to an embodiment of the invention.

[0015] FIG. 7 shows a completed MFM nanoprobe according to an embodiment of the invention formed from abrading the intermediate structure shown in FIG. 6 against a micro rough surface to form an MFM tip, the MFM tip including a polymer coating layer disposed on a Co nanowire.

SUMMARY OF THE INVENTION [0016] A device for high resolution probing of a surface includes a support member and an extension portion attached to a distal end of the support member. A core of the extension portion is electrically connected to the support member. The core comprises a carbon nanotube or ferromagnetic composition. An electrically insulating sheath layer is disposed on a portion of the core, wherein a tip portion of the core disposed opposite the support member does not include the sheath layer. The invention can be used to fabricate a wide variety of improved nanoscale probes and related devices, such as for atomic force microscopy (AFM), scanning electron microscopy (SEM), scanning tunneling microscopy (STM) and magnetic force microscopy (MFM). In one AFM probe embodiment, the core comprises a single wall carbon nanotube.

[0017] The electrically insulating sheath layer can be a polymer, or a non-polymer such as silicon dioxide. The length of the tip portion can be less than 0.5 μm. The thickness of the sheath layer can be at least 30 nm, such as 50 nm or 80 nm.

[0018] In another embodiment of the invention, the device is an MFM probe having a core comprising a ferromagnetic composition, h this embodiment, the core can be substantially cylindrically shaped. As used herein, substantially cylindrically shaped is defined as the range in shapes provided by nanotubes. The diameter of the ferromagnetic core can range from 1 nm to 40 nm, such as less than 7 nm, less than 4 nm, or less than 2 nm.

A method of forming a device for probing a surface includes the steps of providing an electrically conductive support member having a carbon nanotube attached thereto, coating a surface of the nanotube with a sheath layer to form a coated nanotube, and selectively removing the sheath layer from a tip portion of the coated nanotube. A laser beam or focused ion beam can be used for the selective removing step. The laser preferably emits polarized radiation, the polarization oriented substantially perpendicular to a length of the nanotube. The selective removing step can include abrasion of the coated nanotube tip against an abrasive surface, wherein the abrasion comprises buckling the tip of the coated nanotube against the abrasive surface, and then translating the coated nanotube tip across the surface.

The method can further include the step of selectively removing at least a portion of the nanotube to form a cylindrical cavity bounded by the remaining sheath layer. In this embodiment, electrochemical etching can be used for selective removal of the nanotube. The method can include the step of depositing a ferroelectric composite core inside the cavity. The ferroelectric composition is preferably deposited using a process comprising electrodeposition. The diameter of the ferroelectric core can be in the range from 1 nm to 40 nm.

DETAILED DESCRIPTION OF THE INVENTION [0019] A device and related methods for forming the same includes a support member, a carbon nanotube attached to the support member, and a stiffening layer, such as a polymer, disposed on a portion of the surface of the nanotube. Although described herein as being polymer, the stiffening layer can more generally be an electrically insulating layer, such as silicon dioxide. A tip portion of nanotube does not include the stiffening layer. The invention can be practiced with either single wall nanotubes (SWNTs) or multi-walled carbon nanotubes (MWNTs). Although, the invention is generally used to form analytical devices, such as AFM or MFM probes, the invention can be used to form other types of devices, such as nanoscale lithography devices.

[0020] The invention solves the nanotube buckling problem without the need to shorten the nanotube. A uniform, electrically insulating layer disposed on the nanotube except for the very end (tip) of the nanotube permits fabrication of extremely small diameter, high performance ultra-microelectrode-based analytic devices. Depending on the needs of a given application, a tip length of a few tens of nanometers (e.g. 20, 30 , 40 , 50 , 60 , 70 , 80 or 90 nm) to several hundred nanometers (e.g. 100, 200, 300, 400, 500, 600, 700, 800, or 900 nm) of polymer or other electrically insulating material can be removed from the nanotube tip. [0021] However, as described in detail below, in the case of a MFM probe according to the invention, following polymer removal from the nanotube tip, the nanotube is at least partially removed to form a nanopore. The nanopore is then filled with a ferromagnetic composition to result in placement of the ferromagnetic nanowire inside the polymer layer. In this MFM embodiment, the polymer layer can coat the entire length of the nanowire in the finished device, with the exception of a substantially 2-dimensional circular nanowire cross section which is exposed for imaging (see FIG. 7). [0022] Besides providing electrical insulation, the coating stabilizes the long and slender nanotubes against compressive buckling. This allows long nanotubes (e.g. 2 um long) having a substantial coating thicknesses (e.g. 120 nm) to provide stable imaging. Depending on the application, the electrically insulating layer coating thicknesses can be varied as desired within a large range, such as being 20 nm, 30 nm, 40 nm, 50 nm, 60 nm, 70 nm, 80 nm, 90 nm, 100 nm, 110 nm, 120 nm, 130 nm, 140 nm, 150 nm, 160 nm, 170 nm, 180 nm, 190 nm, or 200 nm.

[0023] Nanotubes according to the invention can also be essentially impervious to take-off angle modifications upon crashing. During a crash, the exposed part of the nanotube may not even buckle since the buckling force is quite high. Even if buckling occurs, the nanotube or magnetic nanowire in the case of MFM probes is embedded in a thick coating layer so upon bouncing back, its take-off angle has been found to not measurably change. Accordingly, probe tips according to the invention are virtually crash proof. This means that all the benefits of nanotube tips are now available without the reliability problem that had previously made them prohibitively expensive due to the need for frequent replacement following crashes. [0024] The insulating coating layer also improves binding of the nanotube to the probe (e.g. AFM) tip, easily permitting imaging in fluids. Although generally described as applied to AFM and MFM, as noted above, the invention is generally applicable to other proximal probe techniques, such as scanning tunneling microscopy, electrostatic force microscopy, and scanning capacitance microscopy.

[0025] The invention provides an alternative to the present conventional method for strengthening probes against buckling which comprises physically shortening the nanotube length. According to the present invention, the nanotube/AFM probe tip is first coated with a conformal stiffening coating, such as a polymer. The high intrinsic resolution of the small diameter bare nanotube probe tip for AFM applications is then recovered by removing the coating from only the tip of the nanotube to expose the nanotube tip, while maintaining the stiffening coating on the remainder of the nanotube.

[0026] The coating and process of application is preferably capable of conformal coverage and nanoscale control of the film thickness. The coating should also provide good mechanical properties and high dielectric strength. In one embodiment, a polymer system in which monomer delivery occurs from the vapor phase can be used. One suitable polymer system is well known in the microelectronics industry by the trade name Parylene. Parylene is a common generic name for a unique series of polymers based on paraxylene. Parylene C shown below is the most widely used dimer, providing a useful combination of properties, plus a very low permeability to moisture, chemicals, and other corrosive gases. Related Paralyne D is closely related to Paralyne C, but instead of a single CI includes a second CI on each aromatic ring.

[0027] The Parylene family of polymers is generated from derivatives of di-para-xylylene (DPX). A solid at room temperature, DPX sublimes with appreciable vapor pressure at temperatures above 80 °C. When this vapor is passed through a high temperature zone (>680 °C) the DPX dimer decomposes to monomer. Upon landing on a surface at temperature below 95 °C the monomer spontaneously polymerizes producing a uniform conformal coating over all exposed surfaces. Parylene C, for example, has good mechanical properties,

16 high dielectric strength (about 2.2 MV/cm), high volume resistivity (8.8x10 ohm-cm) and excellent chemical resistance. One of the few known solvents for Parylene is hot naphthalene. [0028] Figure 1(a) shows a scanning electron microscope (SEM) image of a nanotube tip 110 secured to a conventional AFM probe tip 120 before Parylene coating. Figure 1(b) shows a SEM of the nanotube tip 110 having a Parylene coating 140 (nantube 110 not visible) secured to AFM probe tip 120 which is also coated with Parylene 140. The nanotube tip 110 shown is estimated to have a diameter of about 8 nm, and the Parylene coating 140 to have a thickness of about 100 nm. In the case of a Parylene coating, the coating thickness can be controlled by process time, reducing the dimer load in the sublimation zone, or by reducing the monomer vapor pressure by increased pumping speed in the vacuum pumped deposition chamber.

[0029] The electrically insulating coating provides several benefits besides stabilization against buckling and bending instabilities. Conventionally, nanotube attachment to probe tips is generally weak as it relies on weak van der Waals forces. However, the random nature of the vapor delivered monomer coating process, as described above or similar process, provides a single step, parallel coating process solving that problem as well.

[0030] Since the binding between the nanotube and cantilever tip is exceptionally robust both mechanically and chemically, the nanotube/ AFM tip can be exposed to a wide range of solvents without delamination from the support member. The high structural stability of the coated tip allows high levels of mechanical violence to its end without significant change in the sharpness of the tip, or angle of contact with the surface. Thus, the tips can last much longer than even silicon nitride hardened standard AFM tips.

[0031] Moreover, particles picked up during imaging can be dislodged by raising the scan speed to induce collisions with elevated sample features. This techniques is not generally possible for standard AFM tips without damage to the standard tip. Another aspect of mechanical robustness of probes according to the invention is that a coated nanotube tip can be further sharpened by purposely buckling it via a significant force against an abrasive surface (e.g. nano-crystalline), and abrading its sidewalls to leave a conical endshape. When mounted on an electrically conducting support member (e.g. cantilever tip), since the nanotube is itself an electrical conductor, a nanoscale carbon "fiber" microelectrode is obtained, capable of simultaneous electrochemical and AFM imaging. [0032] The buckling effects can be mathematically modeled for nanotubes both with and without the coating. The expression characterizing compressive buckling of a long, slender,

1 homogeneous column is the Euler buckling formula: F_E = , where F„ is the end load

0.67 L force, E the column Young's modulus, I its area moment of inertia and L its length. The numerical factor derives from the fixed-free boundary conditions (one end clamped at the attachment to the tip, the other free to pivot on the surface). The expression characterizing the bending displacement W of a homogeneous slender rod due to a lateral load force, P, applied

PL³ at its free end, is given by, W = — — .

3EI

[0033] These expressions only apply to homogeneous columns or beams, possessing uniform moduli throughout. For a composite beam, consisting of materials A and B having moduli E_A, E , and moments I , I_β, respectively, the general approach is to derive for one of

the materials say, B, its equivalent flexural stiffness product, E_BI'_B, in terms of E_AI_A, and

form a new, equivalent, homogeneous body, B', permitting application of the above homogeneous body expressions. For the case at hand, the coated nanotube may be treated as a solid cylindrical tube with modulus E_N, inner radius R_0j outer radius R^, and area moment

I_N = — (Il_χ - R_Q ) . The coating constitutes a shell having modulus E_c, inner radius R^, outer

radius R_c, and area modulus I_c = — (R_C ⁴ ~ R_N ^A ) • To arrive at the equivalent homogeneous

body a nanotube shell is constructed (over the original nanotube) having inner radius R^, a

variable outer radius R (to be determined), and area moment I = — (R⁴ - R_χ) . To impart to

this shell the same flexural stiffness as the original coating the equality ENI(R,RN)⁼ECIC(RC,RN) is formed and solved for R, thus obtaining

R " = — - (R_C - R_N ⁴ ) - R_N . Finally, for the homogeneous nanotube, having flexural stiffness

^EN equivalent to the original nanotube plus coating, has

F - F — E,

Tχ (^Rc ~ ^RN ^{+ R}N ^■ R which may now be used directly in the

E,

expressions for Euler buckling, F_E, and the lateral displacement, W, above.

[0034] The lateral displacement in beam bending, W, depends upon an additional applied lateral force. It is convenient to instead consider the ratio of tip displacements for a nanotube with and without coating (W_R), for equal applied lateral force. Because of the inverse relation

between W and I, and cancellation of constants, this becomes: W_R = ^N/_p . Both Fg (left y-

/ ¹ N axis)and WR (right y-axis) are plotted in FIG. 2 as a function of Parylene C coating thickness

(modulus E_c = 3.2 GPa) for a typical nanotube (inner diameter 2 nm, outer diameter 8 nm)

for two distinct nanotube lengths of 1 μm and 2 μm.

[0035] It has been found that stable imaging typically requires an F_E greater than about 3

Nn. Unstable imaging with long, uncoated nanotubes, followed by stable imaging upon coating with sufficiently thick layers of Parylene, confirms these calculations.

[0036] Although the coated nanotube clearly has advantages over the uncoated nanotube, resolution would be lost by going from the intrinsic 4 nm tip radius of curvature for the nanotube to 34 nm including the insulating coating. To recover the intrinsic resolution of the nanotube for most applications, the insulating coating layer (e.g. electrically insulating polymer) is preferably generally removed from only the nanotube tip.

[0037] In one embodiment, piezostrictive drive can be used to move the end of the coated nanotube, in towards the center of the diffraction limited focal waist of a 50 mW, CW,

Nd:YAG laser coupled to a high magnification microscope. A critical aspect of achieving fine control over the amount of polymer removed (e.g. to remove approximately 20 nm of polymer from the tip) is the use of polarized laser light with control over the relative orientation of the polarization orientation relative to the nanotube long axis. One of the remarkable properties deriving from the nanoscale diameters of the nanotubes is that they absorb light polarized along their long axis with far greater efficiency than light polarized perpendicular to their long axis.

[0038] Coupled with the high thermal conductivity of the nanotubes makes it very difficult to use light polarized along the nanotube to remove a thermoplastic polymer from the tip. Attempts to do so end up coupling too much of the laser intensity into the nanotube heating it for too much of its length (due to the high thermal conductivity) causing the polymer to melt well away from the focus at the tip. When melted the polymer beads up, under its own surface tension, to retract too far away from the tip. While this may present less of a problem for thermosetting polymers, the high heat transported back via the nanotube may alter the polymer's structure and hence desired properties, such as volume resistivity or mechanical integrity and flexibility.

[0039] A preferred method for removing the polymer from the tip region only is now described. A laser is aligned to have the laser polarization lie within about 10°±5° of the perpendicular to the nanotube long axis. This couples relatively little of the light into the nanotube (although it has been found that coupling some light in is sometimes preferable), removing as little as 100 nm of the polymer from the tip of the nanotube. Figure 3 shows a polymer coated probe tip after polymer removal 300 obtained by laser removal according to the invention. Probe tip 300 is shown secured to a support 330 (e.g. a conventional AFM tip) comprises a portion having the polymer coating 320 over the nanotube and a portion having an uncoated nanotube tip 310 due to the laser processing. [0040] It has been found that by orienting the laser polarization parallel to the nanotube axis the nanotube can readily be cut by the laser beam. This is useful in first mounting the nanotube onto the cantilever support structure. Thus, a nanotube protruding from a tuft of nanotubes can be brought into contact with the support structure near the laser focus and the two now translated to bring the nanotube (at a point near its contact with the tuft) through the laser focus to cut it at that point. This leaves the nanotube attached by relatively weak van der Waals forces to the support. Subsequent polymer coating as described herein makes this attachment robust. As described below, there is also evidence that nanotube cut by this method are sharpened in the process leaving central layers of a multiwalled nanotube protruding further out over outer layers.

[0041] Another method for polymer layer removal and contouring the nanotube probe tip is both relatively simple and also highly effective. The tip can be shortened and/or sharpened by buckling the tip against an abrasive surface and then translating the tip across the surface. For example, using the hard, nano-crystalline, sharp facets of a diamond like carbon (DLC) film, the polymer or other insulating layer can be abraded from the tip. Complicit in this approach are the elasticity of both the nanotube and polymer and the very robust attachment of the nanotube to the tip. These properties are necessary because it is desired to remove the polymer not simply from the end of the nanotube, which would leave a blunt tip, but rather more from the sides of the nanotube. The desired final product has a structural contour analogous to a sharpened pencil, hi one embodiment, the tip is driven into an abrasive (e.g.

DLC) substrate as much as about one μm beyond initial contact, forcing the nanotube to

buckle, followed by dragging the tip across the surface in a spiral pattern. [0042] Nanolithography software permits automation of this abrasion process, such as provided by Digital Instruments, Inc., which takes approximately two minutes. The contact angle between the buckled nanotube and surface dictates the angle of abrasion relative to the nanotube axis, and thus the final tip profile. The contact angle, in turn, depends on the initial length of the nanotube and the controlled depth with which the tip is driven in to the surface. It has been found that a circular dragging pattern to be superior to a linear pattern because it leads to a conical end profile, while a spiral is superior to a circle because it avoids repeatedly going over the same peaks of the DLC film, which can cut grooves in the tip leading to tip artifacts when imaging.

[0043] Tapping mode AFM images of single wall carbon nanotube bundles on mica have been collected which demonstrates that the resolution obtained from exemplary probe tips produced using the invention. Images have been obtained and evaluated for a bare nanotube probe, a polymer coated probe, and a polymer coated probe in which the polymer has been DLC abraded from the tip. The image resolution of the DLC abraded probe has been found to be comparable to the bare nanotube probe. By zooming in on acute crossing points of single wall nanotube ropes it is generally found that both the laser sharpened tip and the DLC abraded tips have superior resolution to conventional electrically shortened nanotubes. This is not surprising since the electric field in the electro-shortening method concentrates the field at the sharpest protrusions leaving a more rounded, blunt ended tip. The laser cutting method, in contrast, vaporizes the most exposed outer carbon layers, while leaving intact the best heat sunk, inner layers, yielding a sharpened nanotube tip. The DLC abrasion method sharpens by concentrating its action at the sides of the tip.

[0044] Although laser-based nano-sculpting of tips can be used for certain applications of the invention, the diffraction limited focal spot can be too blunt to permit very many re- sharpening cycles, as the nanotube tip wears with use. h contrast, the abrasion sharpening process can be performed many times. Whenever the tip gets dull, or as is frequently the case in AFM imaging when the tip picks up an adherent particle, it can be abrasively sharpened. Abrasive sharpening can generally be performed in any laboratory, without the need for specialized equipment, requiring only an abrasive film, such as a nano-crystalline abrasive film. Users of AFMs have become accustomed to tips as a disposable commodity, needing to balance the imaging resolution as an ultiasharp tip quickly wears, against the cost of throwing away a commodity costing as much as $40 each. The technology described here will permit users to both have their tip as well as image with it.

[0045] The polymer coated probe tips provide a micro-electrode like character. This aspect has been demonstrated by mounting a nanotube on a gold coated cantilever. Parylene C coated the structure and was exposed to provide electrical contact to a spot on the cantilever substrate. Using translation stages and a high magnification optical microscope the nanotube tip was inserted into the end of a micro-capillary filled with a low concentration iron chloride solution. With a grounded iron wire counter-electrode inserted into the solution from the back end of the capillary, no measurable current was observed to flow at potentials

applied to the nanotube between ±4 V. After DLC abrasion exposing the nanotube tip and

repeating the experiment current was readily detected at the redox potential of the iron chloride, growing an iron particle at the tip. Since one iron atom is deposited for every electron, the iron particle grows very quickly to much larger than the nanotube itself. In another experiment, to make the iron particle much smaller, the leads to the probe were charged to 3 V, then disconnected from the power supply (relying on the capacitance in the wire to provide a limited number of electrons), after which the exposed nanotube end was dipped into the electrolyte.

[0046] Nanotube electrodes may make possible many new applications. For example, imaging in vivo, such as features on a cell wall, locating a feature of interest (e.g. a gated ion channel), and stimulating its opening with either introduction of the appropriate species via a micro-pipette or perhaps by a voltage pulse from the nanotube tip itself, and recording the local current spike from the ion transport through the channel. Alternatively, using a longer exposed length of nanotube, protruding beyond the insulation, the nanotube can be wiggled into the synaptic cleft between neurons and monitoring neurotransmitter release, addition, the inherent stiffness of the nanotube can be used to pierce the cell membrane at various selected locations (selected by imaging) to then monitor the local internal electrochemical activity. While techniques exist which can acquire some information of the sort described here (e.g. patch clamp techniques), none of these provide any spatial information permitting correlation with the local structures. A wide variety of other advanced electrochemical and bio-electrochemical applications will be possible using probes according to the invention. [0047] The nanotube tips can also be chemically functionalized. For example, chemically functionalization can be obtained by dipping the tip into a variety of chemical cocktails. The high chemical resistance of coatings used (e.g. Parylene and similar low reactivity materials) dramatically broadens the range of such permissible cocktails, allowing the entire tip and substrate to be immersed (even at elevated temperatures), without the possibility of detaching the nanotube from the supporting member.

[0048] The invention can also be used to form ultra small magnetic tips suitable for magnetic force microscopy (MFM) probes and related systems, such as MFM-based magnetic bit readers. MFM is a variation of AFM and is an established technique that non- destructively measures the magnetic character of a sample surface having magnetic features. [0049] The MFM components of a deflection sensor, cantilever, and tip are similar to their AFM counterparts. However, in the case of MFM, the cantilever tip is comprised of a ferromagnetic material, such as cobalt or nickel, or alloys of the same and magnetized. Electrons of neighboring atoms in ferromagnetic metals tend to make their little bits of magnetism point in the same direction, thus forming magnetic domains. In a magnetic field, these domains line up with the field making a strong magnet. Once aligned, these materials tend to resist a change in magnetic polarization. As the particle size gets larger, the particle generally provides better resistance against changes in polarization.

[0050] As the probe is scanned across a magnetic sample, the variations in force gradients due to the probe-sample interactions produces an MFM image, or enables a magnetic bit to be "read" in the case of a magnetic bit reader. The general requirements for high image quality MFM probes require the following:

1. The lateral spatial extent of the magnetic tip volume should be as small as possible to minimize the spatial extent of long range dipole tip sample interactions, thus maximizing the image resolution.

2. The magnetic stray field of the tip should be small so as to minimize the influence of the tip on the magnetic structure of the sample.

3. A large magnetic moment, which increases the sensitivity of the MFM.

4. The probes should be magnetically well defined to allow quantitative interpretation of MFM data.

[0051] Thus, ideal MFM probes provide a small ferromagnetic nanowire at the end of a non-magnetic cantilever. The current standard MFM probes are micro fabricated silicon cantilevers, with a pyramidal or conical probe that are sputter coated with a thin layer of magnetic metal, such as cobalt, and a barrier coating such as chromium. Such probes have a resulting tip radius of about 45 nm and show a magnetic resolution of the same order. The lateral size of conventional MFM probe tips thus limits attainable resolution. Due to the resulting conventional MFM probe structure there is a tradeoff between contrast and resolution of the MFM data. Upon decreasing the scan height, in the lift mode, the contrast in the MFM data image increases along with a concomitant increase in the lateral magnetic interaction volume causing reduction in the spatial resolution. Additionally, these conventional MFM probes do not have a well-defined orientation of their magnetic moment leading to difficulties in interpretation of the MFM data.

[0052] Figure 4 shows an exemplary MFM probe embodied as polymer coated magnetic metal nanowire probe 400, according to another embodiment of the invention. Nanowire probe 400 includes a cantilever 415 having a metallized pyramidal (or conical probe) 410 secured thereto. Probe 410 is coupled to and electrically connected to a magnetic nanowire 420. The metal nanowire can be formed from Co, Ni, or other ferroelectric materials, or mixtures thereof. A polymer coating 440 coats the magnetic nanowire 420, except for a tip region 445 where the magnetic nanowire 420 is exposed to the outside world. [0053] Polymer coated ferromagnetic nanowire MFM probes 400 according to the invention substantially satisfy all of the requirements of an ideal MFM probe noted above, such as a laterally small effective magnetic probe (as small as few nanometers), large magnetic moment (due to geometric anisotropy), and well defined magnetic structure. Moreover, there is no significant increase in the lateral interaction volume upon decrease in the scan height. MFM probe tip diameters according to the invention can be equal to or less than 40 nm, such as 35 nm, 30 nm, 25 nm, 20 nm, 15 nm, 10 nm, 9 nm, 8 nm, 7 nm, 6 nm, 5 nm, 4 nm, 3 nm, 2 nm or 1 nm.

[0054] In one embodiment of the invention, one method for forming nanowire MFM probes having enhanced resolution and reliability is provided. A nanotube is attached to the end of an electrically conductive holder, such a conventional electrically conductive AFM probe attached to a cantilever as described above. The nanotube is then coated with an electrically insulating material, such as the polymer Parylene. For example, a MWNT having a diameter of about 6 nm and a length of about 10 μm can be used. The Polymer is then removed from the end of the coated nanotube, such as about 200 nm from the distal end. The nanotube is then selectively removed leaving the polymer attached to the probe having a nearly cylindrical void, where the void dimensions match that of the outer diameter of the nanotube removed.

[0055] A preferred method for removing the nanotube involves electrochemical etching of the nanotube in an electrolyte, such as sodium chloride, solution. Other methods for removal will be apparent to those having ordinary skill in the art. Electrical contact can be made to the back end of the nanotube via the AFM tip, cantilever and substrate by an insulated wire. A potential can be applied between the nanotube and a Pt counter electrode to electrochemically etch the nanotube back from its tip back toward the AFM tip. The process can be terminated at any point to leave a portion of the nanotube in the polymer sheath or the nanotube can be etched back to the AFM probe tip.

[0056] Electrodeposition is then preferably used to deposit a ferromagnetic metal in the high aspect ratio cylindrical volume defined by the hollow polymer defined volume. The ferromagnetic metallic coating can be a pure ferromagnetic metal or an alloy, hi this process, the probe tip is immersed in a solution containing ferromagnetic metallic ions, anions and electrolyte. Passage of an appropriate electrical current through the solution results in the deposition of the ferromagnetic metal on the electrically conductive probe which functions as the cathode. Following electrodeposition, the ferromagnetic metal fills substantially the entire cylindrical volume. Monitoring the deposition current provides a signal when the metal deposition has exceeded the end of the void. At that point current begins to rise due the change over to a 3 dimensional growth (as opposed to the essentially 1 dimensional growth within the cylindrical volume. This process results in a bulbous end form of the metal protruding out beyond the polymer. The bulbous Co end may then be removed, such as by running the probe tip against a micro rough surface. The polymer sheath protects the sides of the cobalt or other ferroelectric composition from oxidation. The exposed end of the ferroelectric metal oxidizes to a small depth in the case of Co (generally about 7 nm) before providing an oxygen impermeable layer that prevents further oxidation.

[0057] Although this embodiment of the invention is generally described using MWNTs,

SWNTs may also be used. A SWNT is thinner than a MWNT and provides a typical diameter of about 1 to 1.4 nm and lengths comparable to MWNTs. Accordingly, SWNTs can theoretically provide improved sensitivity and resolution as compared to MWNTs according to the invention.

[0058] Embodied as a MFM reader, the invention can permit the reading of arrays having unprecedented density. In one embodiment, the MFM reader system includes a plurality of

MFM nanowire probes according to the invention.

Examples [0059] The present invention is further illustrated by the following specific example, which should not be construed as limiting the scope or content of the invention in any way.

Preparation of Nanowire MFM probes

Metallization:

[0060] Tapping mode atomic force microscope (AFM) tips with spring constant of 4

N I m and nominal resonant frequency of 80 kHz were UV ashed for fifteen minutes to remove any organic contamination layer. The tips were then transferred into a high vacuum disposition system, where 5 nm of chromium and 40 nm of palladium were sputter deposited under the conditions of 5 mTorr Ar pressure and 12 watts of power.

[0061] Single Multiwalled Carbon Nanotubes (MWNTs), of requisite diameter and orientation, were mounted on the end of the above-described metallized tips by manual manipulation under an optical microscope. Since further processing required a substantial amount of electrochemistry, it was necessary for the contact resistance between the MWNT and the metal be ohmic in nature and have a value as small as possible. The value and nature of the contact resistance was measured and confirmed by dipping the end of the MWNT in a small droplet of mercury and measuring current-voltage curves. Values of the contact resistance that were typically deemed acceptable were values less than approximately 100 kΩ and showing linear current-voltage plots.

Polymer deposition:

[0062] The next step in the process was polymer deposition. The polymer used, Parylene C, has excellent chemical inertness stability and can be put down in conformal layer, was deposited in a chemical vapor deposition (CVD) system. The thickness of the polymer film depends upon the mass of the unpolymerized dimer charge put in, had been found to be a function of the type of substrate. Thus, the amount of polymer put down on the graphite (nanotube) and the transition metal (palladium) was different. To circumvent this problem and ensure a uniform conformal coating of the polymer on the entire probe, polymer was deposited in two-steps.

[0063] hi the first step, a thin layer of polymer, approximately 30 nm in thickness, was put down on the probe. This bonded the nanotube to the tip with sufficient strength to tolerate the subsequent step. To negate the selectivity exhibited by the polymer the entire structure was dipped in A- 174 silane. The probes were then dried in air for fifteen minutes, followed by the second layer of polymer, which was, typically, about 200 nm. [0064] As electrochemical etching of nanotube was the next step in processing, it was necessary for controlled removal of the polymer from the nanotube, to permit exposing the nanotube to the chemical solution. This was achieved by controlled introduction of the tip of the polymer coated MWNT into a tightly focused spot of 50 mW NdYAG laser. This resulted in the removal of polymer, exposing upwards of few tens of nanometer of the nanotube length.

Electrochemical etching:

[0065] The probe insulated throughout by the conformal polymer coating, except for the exposed carbon nanotube tip, was dipped in 0.01 molar sodium hydroxide solution. Controlled etching of the nanotube was done by applying a square voltage pulse, frequency 50 Hz and V_p-P= 3 volts, with graphite counter electrode. The voltage was applied periodically (30 seconds on time with off time of 2 minutes) so as to allow the etching reaction by products to diffuse out from the nanopore created by the dissolution of the carbon nanotube. Figure 5 shows a TEM image of an intermediate MFM probe structure 500 comprising a Pd coated probe tip 510, having nanoprobe 520 comprising a polymer coating 530 disposed on a nanotube 540 secured to probe 510. Remote from probe 510 nanoprobe 520 includes polymer coating 530 disposed on nanopore 545 formed from the etched nanotube. Although a portion of the nanotube 540 is shown remaining, the entire nanotube 540 can be removed, such as by adjusting the nanotube etch time.

Electrodeposition of Cobalt

[0066] The probe was then transferred from the etching solution into de-ionized water. This ensured that there is an exchange of the sodium hydroxide solution from inside the pore with de-ionized water. The probe was then dipped in a cobalt solution comprising 40gm/L of

CoS0₄lH₂0 and 40 gm/Liter of H₃B0₃ in de-ionized water at 18 MΩ and soaked for 30

minutes. This allows for exchange of the cobalt ions for de-ionized water inside the pore. Electrodeposition was carried out using a potentiostat with a voltage of -0.85 volts applied to the probe, with a platinum counter electrode and Ag/AgCl₂ as a reference electrode. [0067] Figure 6 shows a transmission electron microscope (TEM) image of an intermediate MFM probe structure 600 including an AFM probe tip 610 having a MFM nanotip 605 comprising a polymer 620 coating on cobalt nanowire 625, the cobalt extending past the polymer layer 620 and forming a bulbous Co end 630. The bulbous Co end 630 is then generally removed, such as by running the probe tip against a micro rough surface to produce the completed MFM tip 700 shown in FIG. 7.

[0068] MFM tip 700 shown includes polymer coating layer 720 disposed on a Co nanowire 710. It can be appreciated that MFM tip 700 substantially satisfies all requirements of an ideal MFM probe, such as stability from bending, a spatially small effective magnetic probe (a few nanometers), large magnetic moment (due to geometric anisotropy), and well defined magnetic structure.

[0069] It is to be understood that while the invention has been described in conjunction with the preferred specific embodiments thereof, that the foregoing description as well as the examples which follow are intended to illustrate and not limit the scope of the invention. Other aspects, advantages and modifications within the scope of the invention will be apparent to those skilled in the art to which the invention pertains.

Claims

CLAIMSWe claim:

1. A device for high resolution probing a surface, comprising: a support member; an extension portion attached to a distal end of said support member, wherein a core of said extension portion electrically connected to said support member, said core comprising a carbon nanotube or ferromagnetic composition; and an electrically insulating sheath layer disposed on a portion of said core, wherein a tip portion of said core opposite said support member does not include said sheath layer.

2. The device of claim 1, wherein said core comprises a single wall carbon nanotube, said device comprising an AFM probe.

3. The device of claim 1, wherein said sheath layer comprises a polymer.

4. The device of claim 1, wherein a length of said tip portion is less than 0.5 μm.

5. The device of claim 1, wherein a thickness of said sheath layer is at least 50 nm.

6. The device of claim 1, wherein said core comprises said ferromagnetic composition, said device comprising an MFM probe.

7. The device of claim 6, wherein said core is substantially cylindrically shaped.

8. The device of claim 7, wherein a diameter of said core is in a range from 1 nm to 40 nm.

9. The device of claim 8, wherein a diameter of said core is less than 7 nm.

10. The device of claim 8, wherein a diameter of said core is less than 4 nm.

11. The device of claim 8, wherein a diameter of said core is less than 2 nm.

12. A method of forming a device for probing a surface, comprising the steps of:

providing an electrically conductive support member having a carbon nanotube attached thereto; coating a surface of said nanotube with a sheath layer to form a coated nanotube; and selectively removing said sheath layer from a tip portion of said coated nanotube.

13. The method of claim 12, wherein a laser beam or focused ion beam is used for said selective removing step.

14. The method of claim 13, wherein said laser emits polarized radiation, said polarization oriented substantially perpendicular to a length of said nanotube.

15. The method of claim 12, wherein said selective removing step comprises abrasion of said coated nanotube tip against an abrasive surface, wherein said abrasion comprises buckling said coated nanotube tip against said abrasive surface and then translating said coated nanotube tip across said surface.

16. The method of claim 12, further comprising the step of selectively removing at least a portion of said nanotube to form a cylindrical cavity.

17. The method of claim 16, wherein said selective removal of said nanotube step comprises electrochemical etching.

18. The method of claim 16, further comprising the step of depositing a ferroelectric composite core inside said cavity.

19. The method of claim 18, wherein said depositing step comprises electrodeposition.

20. The method of claim 18, wherein a diameter of said core is in a range from 1 nm to 40 nm.