NANO LETTERS
Enhanced Functionality of Nanotube Atomic Force Microscopy Tips by Polymer Coating
2004 Vol. 4, No. 2 303-308
Amol Patil, Jennifer Sippel, Gregory W. Martin, and Andrew G. Rinzler* Department of Physics, UniVersity of Florida, GainesVille, Florida 32611-8440 Received November 20, 2003; Revised Manuscript Received December 15, 2003
ABSTRACT To stabilize nanotube-modified atomic force microscopy tips and extend their functionality, a method for conformal polymer coating of the tips and removal of the polymer from just the probing nanotube end is described. Expressions quantifying stabilization of the tips against buckling and bending due to stresses encountered while imaging are developed. Electrical conductivity of the probes is demonstrated by their use in scanned conductance microscopy, where a substantial sensitivity advantage over standard tips is realized and explained. Further advantages of these electrically insulated (save for the tip) probes are discussed, including their proposed application in bioelectrochemical research.
Introduction. Since carbon nanotube modified atomic force microscopy (AFM) tips were first described,1 variations of the mounting method,2-6 extension to single wall nanotubes,5-8 and applications exploiting the high nanotube aspect ratio,1,9,10 small tip radius,11-17 electrical conductivity,18-20 and chemical versatility21,22 have all been demonstrated. These studies concentrated on the intrinsic advantages of nanotube modified tips; however, a critical look at this technology also reveals some limitations. Despite the remarkable rigidity of the nanotube sidewall, an object so long and slender is subject to bending and buckling instabilities. The buckling instability sets a limit on how long the nanotube can be (for given diameter) and still give stable imaging. The bending instability results in imaging artifacts at step edges where the nanotube tip can be laterally displaced by various proximal forces.23 These instabilities have required that a nanotube attached to an AFM cantilever be shortened (usually by electric arcing1,5) until stable imaging is obtained. Here we describe an attractive alternative to shortening: coating the nanotube/AFM tip with a conformal polymer coating, and recovering the high intrinsic resolution of the bare tip by removing the polymer from only the tip of the nanotube. Such coating provides several benefits. (1) The nanotube is stabilized against buckling and bending instabilities. (2) The binding between the nanotube and cantilever is made exceptionally robust, both mechanically and with respect to a wide range of solvents to which it can be exposed without delaminating from the cantilever. (3) The high structural * Corresponding author. E-mail:
[email protected]. 10.1021/nl0350581 CCC: $27.50 Published on Web 01/08/2004
© 2004 American Chemical Society
stability of the coated tip allows extreme mechanical violence to its end without significant change in the tip’s sharpness, or angle of contact with the surface. This suggests that these tips can last longer than even hard, silicon nitride AFM tips and provides an ancillary benefit: particles picked up during imaging can be dislodged by raising the scan speed to induce collisions with elevated sample features (a method sometimes applied as a last resort with standard AFM tips, which is itself often fatal for the tip). (4) The polymer provides an insulating coating over all but the tip of the nanotube. Since the nanotubes are electrically conducting, if the AFM cantilever to which it is mounted is also conducting, we obtain a nanoscale “carbon fiber” nanoelectrode showing great promise for nanoscale bio and/or electrochemical probing. We describe here the nanotube coating method, calculations quantifying the stiffening as a function of polymer coating thickness, a method of polymer removal from the tip, and a first application taking advantage of some of these features. To obtain a thin, insulating coating over nanotubes mounted on AFM tips with nanoscale control over the film’s thickness we sought a polymer system in which monomer delivery occurs from the vapor phase. A system found to be nearly ideal is one well-known in the microelectronics industry by the trade name Parylene. This 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
Figure 2. Tip mounted nanotube buckling and bending force directions and relevant parameters (defined in the text). Figure 1. AFM tip mounted nanotube before and after (inset) Parylene C coating. Nanotube mounting proceeds as first described by Dai et al.1
DPX dimer decomposes to monomer. Upon landing on a surface at temperature below 95° C the monomer spontaneously polymerizes forming a uniform, conformal coating over all exposed surfaces. Parylene C, which we use, has good mechanical properties, high dielectric strength (∼2.2 MV/cm), high volume resistivity (8.8 × 1016 ohm-cm), and excellent chemical resistance (one of the few known solvents is hot naphthalene). Figure 1 shows scanning electron microscope (SEM) images of a nanotube tip before and after (inset) Parylene coating. The nanotube tip here is estimated to have a diameter of 8 nm and the coating has thickness of ∼126 nm (total probe diameter 260 nm). For the purposes of exposition we show examples with relatively thick deposited coatings. Thinner coatings are produced at will by reducing the dimer load in the sublimation zone. The expression characterizing compressive buckling of a long, slender, homogeneous column is the Euler buckling formula: FE )
π2 EI 0.67 L2
where FE is the end load 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 at its free end, is given by W) 304
PL3 3EI
These expressions apply only to homogeneous columns or beams, possessing uniform modulus throughout. For a layered, composite beam, consisting of materials A and B having moduli EA, EB, and moments IA, IB, respectively, the general approach is to derive for one of the materials say, B, its equivalent flexural stiffness product, EBI′B in terms of EAIA, and form a new, equivalent, homogeneous body, B′, permitting application of the above homogeneous body expressions. In the case at hand (see Figure 2 for illustration of the defined quantities), the nanotube may be treated as a solid cylindrical tube with modulus EN, inner radius Ro outer radius RN, and area moment π IN ) (R4N - R4o) 4 The coating constitutes a shell having modulus EC, inner radius RN, outer radius RC, and area modulus π Ic ) (R4C - R4N) 4 To arrive at the equivalent homogeneous body we construct a nanotube shell (over the original nanotube) having inner radius RN, a variable outer radius R (to be determined), and area moment π I ) (R4 - R4N) 4 To impart to this shell the same flexural stiffness as the original coating we form the equality ENI(R,RN) ) ECIC(RC,RN) Nano Lett., Vol. 4, No. 2, 2004
Figure 3. Calculated Euler buckling force (left axis) and ratio of lateral displacements for a coated to uncoated nanotube (right axis) as a function of coating thickness.
and solve for R, obtaining EC 4 (R - R4N) - R4N EN C
R4 )
Finally, our homogeneous nanotube, having flexural stiffness equivalent to the original nanotube plus coating, has ENI′N ) EN
[
]
π EC 4 (R - R4N) + R4C - R4o 4 EN C
which may now be used directly in the expressions for Euler buckling, FE, and the lateral displacement, W, above. The lateral displacement in beam bending, W, depends on an additional applied lateral force, P. It is convenient to instead consider the ratio of tip displacements for a nanotube with and without coating for equal applied lateral force. Because of the inverse relation between W and I, and cancellation of constants, this becomes WR ) IN/I′N. Both FE and WR are plotted in Figure 3 as a function of Parylene C coating thickness (modulus EC ) 3.2 GPa) for a typical nanotube (inner diameter 2 nm, outer diameter 8 nm) for two distinct lengths of 1 and 2 µm. Despite the relatively low modulus of the polymer (3.2 GPa vs 1000 GPa for the nanotube), the 4th power dependence of the flexural stiffness on the coating thickness quickly dominates, so that by 40 nm of polymer (total diameter 88 nm, including the nanotube) the buckling force has increased by more than a factor of 20. Similarly, the relative lateral displacement for the coated nanotube for equal lateral loads has decreased by over a factor of 20. We have found that stable imaging typically requires an FE greater than ∼3 nN. Note from the plot that prior to the coating a 2 µm length nanotube would be incapable of imaging, while after 40 nm of coating imaging becomes possible. These calculations are consistent with unstable imaging observed with long, as-mounted (uncoated) nanotubes, followed by stable imaging upon coating with a thin layer of Parylene. Nano Lett., Vol. 4, No. 2, 2004
Figure 4. Optical image of a coated nanotube on an AFM tip, indicating the translations relative to the laser focus for polymer removal. The laser focal spot is not visible to the operator (except for light scattered from the nanotube tip); it is merely represented in the figure by the diffuse green spot for exposition.
Such benefits not withstanding, giving up resolution in going from the intrinsic, nominally 4 nm tip radius of curvature for a nanotube, to 44+ nm including the stabilizing coating, is hardly appealing. To recover the intrinsic resolution of the nanotube we have devised methods for polymer removal from its end. In the approach described here we use the high intensity focus of a laser (50 mW, CW, Nd: YAG) coupled into an optical microscope (Axiovert 100, 50×, 0.5 N.A. objective) to vaporize the polymer from the tip. The AFM tip, with its mounted, coated nanotube, is affixed to a precision XYZ stage equipped with piezostrictive drives (Newport, ESA-CXA), for fine motion control. The coated nanotube, which is oriented to lie perpendicular to the microscope optic axis, is lined up in the microscope focal plane to point directly at the laser beam focal waist (Figure 4). The position of the latter is first established relative to a reticule in the eyepiece by reflecting the light from a glass slide, which is subsequently moved out of the field of view. The coated nanotube is translated toward the laser focus until it begins to scatter light from the wings of the Gaussian beam waist. The brightness of this scattered light is used to indicate when the nanotube is best lined up with the beam center (Figure 4), after which the nanotube is translated in incremental (60 nm) steps toward the center of the beam. It is impossible to resolve optically the amount of polymer we typically remove (
