Nanowire Probes for High Resolution Combined Scanning

NANO LETTERS

Nanowire Probes for High Resolution Combined Scanning Electrochemical Microscopy − Atomic Force Microscopy

2005 Vol. 5, No. 4 639-643

David P. Burt,† Neil R. Wilson,‡ John M. R. Weaver,§ Phillip S. Dobson,§ and Julie V. Macpherson*,† Departments of Chemistry and Physics, UniVersity of Warwick, CoVentry CV4 7AL, UK, and Department of Electronics and Electrical Engineering, UniVersity of Glasgow, Rankine Building, Oakfield AVenue, Glasgow, G12 8LT, UK Received January 4, 2005; Revised Manuscript Received February 23, 2005

ABSTRACT We describe a method for the production of nanoelectrodes at the apex of atomic force microscopy (AFM) probes. The nanoelectrodes are formed from single-walled carbon nanotube AFM tips which act as the template for the formation of nanowire tips through sputter coating with metal. Subsequent deposition of a conformal insulating coating, and cutting of the probe end, yields a disk-shaped nanoelectrode at the AFM tip apex whose diameter is defined by the amount of metal deposited. We demonstrate that these probes are capable of high-resolution combined electrochemical and topographical imaging. The flexibility of this approach will allow the fabrication of nanoelectrodes of controllable size and composition, enabling the study of electrochemical activity at the nanoscale.

The ability to correlate nanoscale surface structure with (electro)chemical activity is of great importance in the study of a wide range of fundamental processes such as corrosion,1 catalysis,2 crystal growth/dissolution,3 and membrane ion transport.4 Scanning electrochemical microscopy (SECM) enables the electrochemical activity of heterogeneous surfaces to be investigated.5 SECM is performed by scanning an electrode at a defined height above the surface of interest. Typically both the spatial resolution of the technique and the height of the electrode above the surface are of the order of the diameter of the electrode employed. Thus, to achieve high spatial resolution two criteria must be met: (i) a small diameter electrode must be used, and (ii) a means of “sensing” the topography must be integrated into the probe in order to maintain a sufficiently small, and constant, electrode-surface separation. To satisfy the second of these criteria, the development of dual functionality combined scanning electrochemicalatomic force microscopy (SECM-AFM) probes, allowing the simultaneous monitoring of surface topography and electrochemical activity, has gained increasing interest in recent years.6 The combination of SECM and AFM also allows the direct correlation of topographic features with electrochemical activity. SECM-AFM probes have, to date, been used to identify and characterize electrochemically * Corresponding author. E-mail: [email protected] † Department of Chemistry, University of Warwick. ‡ Department of Physics, University of Warwick. § University of Glasgow. 10.1021/nl050018d CCC: $30.25 Published on Web 03/05/2005

© 2005 American Chemical Society

active surface sites,7 visualize and locally modify crystal surfaces,8 and investigate polymer chain structure and dynamics9 and enzyme activity.10 Current SECM-AFM probe fabrication procedures comprise three main approaches: (1) hand fabrication through the insulation of individual flattened and etched metal microwires;7,8,11 (2) production of individual probes incorporating frame-shaped electrodes using micromachining techniques;10,12 and (3) batch fabrication using electron beam lithography.13 However, with each approach there are limitations; for example, hand-fabricated probes vary in electrode geometry from one probe to the next and only operate in contact mode; micromachined probes, at present, measure an average current signal over ca. ∼800 nm electrode; and current batch-fabricated probes have an overall electrode geometry also at the micrometer level. In this letter we present a novel approach to SECM-AFM probe fabrication which solves the problems highlighted above; in particular, this procedure enables the incorporation of a well-defined disk geometry nanoelectrode, with controllable diameter down to tens of nanometers, at the tip of an AFM probe. We have recently shown that by using singlewalled carbon nanotubes (SWNTs) as templates, a metal nanowire can be formed at the apex of an AFM tip;14 here we extend the technique and demonstrate that by coating the nanowire with a conformal insulator, and then cutting off the probe end, it is possible to form a well-defined nanoelectrode at the tip apex.

Figure 1. Schematic of the nanowire electrode fabrication process.

Advances in the fabrication of carbon nanotube AFM tips for high aspect ratio combined topographical and electrochemical probes have recently been made. Rinzler et al. coated multiwalled carbon nanotube (MWNT) tips, attached to silicon AFM tips, with a conformal film of Parylene C.15 Selective removal of the polymer from the end of the nanotube was achieved using laser vaporization. Collier et al. used an insulating fluorocarbon coating on a SWNTAFM tip. Removal of the tip end was achieved using electrical pulse etching.16 In each case superior mechanical stability was observed, compared with the uncoated nanotube (as was also observed with metal coated SWNT-AFM tips14). However, for both, neither redox voltammetry was reported for the tip nor electrochemical imaging was demonstrated, and so it is not known how effective the probe is as a nanoelectrode. In addition, both approaches are limited to one electrode material, carbon, and an electrode size defined specifically by the attached diameter of the MWNT or SWNT. A schematic of the nanoelectrode probe fabrication process is given in Figure 1. The process is composed of six steps and starts with a conventional silicon AFM tip. In brief: (1) the probe is thermally oxidized; (2) SWNTs are attached to the tip using the “pick-up” technique;17 (3) a metallic nanowire is formed by sputter coating with Au; (4) a thin, conformal, insulating film of poly(oxyphenylene) is electrodeposited;18 (5) an additional insulating layer of silicon nitride is added by low temperature inductively coupled plasma chemical vapor deposition (ICP CVD) for enhanced stability and longevity; and finally, (6) the end of the nanowire is removed by focused ion beam (FIB) milling, to leave an exposed nanoelectrode. The role of the nanowire in this process is two-fold. First its diameter (controlled by the thickness of metal deposited) defines the diameter of the nanoelectrode. Second, and crucially, it provides a target length for FIB milling. Cutting the nanowire at any position along its length results in a nanoelectrode of similar size and composition. The SWNTs used were grown by catalyzed chemical vapor deposition and attached to the AFM probes by the “pickup” technique, as described previously.19 Multiple SWNTs are attached to each tip in order to increase their stability for the subsequent metal deposition process. In principle, MWNTs, or nanotubes grown directly onto the tip, could also be used. The SWNT-AFM tips are then sputter coated with metal, forming a conducting metal nanowire at the AFM 640

Figure 2. (a) TEM image of a Au nanowire tip after deposition of a poly(oxyphenylene) film. (b) Cyclic voltammetry of a completely insulated nanowire probe in an electrolyte solution containing 10 mM Ru(NH3)63+ and 0.1 M KNO3 at a scan rate of 50 mV s-1. The image width of (a) is 1 µm.

tip apex; the metal increases the mechanical rigidity of the SWNT-AFM tips while the SWNTs anchor the nanowire to the tip. For the work reported here Au is employed as the electrode material, with an adhesive layer of Ti (5 nm), although in principle any material compatible with the sputtering process could be used. Preliminary tests have shown that Pt, Au, and AuPd are all viable and are also compatible with poly(oxyphenylene) electrodeposition. The diameter of the nanowire is set by the thickness of metal deposited and thus can be controlled with great precision. Typically metal nanowires of diameter 50-100 nm were fabricated for this work, although thinner nanowires can be formed. A significant technical challenge for the use of highresolution electrochemical imaging probes is ensuring pinhole free insulation of the probe. Many different insulation procedures were investigated; the most reliable and effective consisted of a three-stage insulation process. The conductive silicon tips are thermally oxidized in step 1 (see Supporting Information for details) to minimize both Faradaic and capacitive currents from any exposed silicon areas on the probe. After formation of the nanowire, the probe is coated in a thin (typically ∼40-80 nm) conformal insulating film of poly(oxyphenylene), formed by electropolymerization of a 1:1 methanol/water-based monomer solution containing 60 mM phenol, 90 mM 2-allylphenol, and 160 mM 2-nbutoxyethanol18 (see Supporting Information for details). A layer of silicon nitride (typically ∼100 nm, but can be varied according to the size of probe we wish to fabricate) is then deposited on either side of the AFM probe using lowtemperature ICP CVD (see Supporting Information for details). This serves not only as an extra insulation layer but also protects against polymer film breakdown in corrosive environments, prolonging the lifetime and stability of the probe. Figure 2a shows a TEM image of a Au nanowire tip after deposition of the poly(oxyphenylene) film. The nanowire diameter is 65 nm, and the polymer film is coated conformally with a thickness of 40 nm. The cyclic voltammetric (CV) response of an insulated nanowire SECM-AFM probe, prior to FIB milling, is shown in Figure 2b, versus an Ag/ AgCl reference electrode. For this measurement, the tip and cantilever were dipped into a solution containing 10 mM Nano Lett., Vol. 5, No. 4, 2005

Figure 4. (a) Linear sweep voltammogram of a nanowire probe in an electrolyte solution containing 10 mM Ru(NH3)63+ and 0.1 M KNO3 at a scan rate of 10 mV s-1. (b) SEM image of a nanowire SECM-AFM probe after silver electrodeposition testing. The image width of (b) is 8.9 µm.

Figure 3. Focused ion beam (FIB) images of a nanowire probe prior to (a) and after (b) FIB machining. A TEM image (c) of a SECM-AFM probe. The image widths are (a) and (b) 1.3 µm and (c) 3.7 µm.

Ru(NH3)63+ with 0.1 M KNO3 added as a supporting electrolyte. Crucially, there is no detectable redox electrochemistry; the currents passed are purely nonfaradaic, proving that there are no pinholes in the insulating film. This was further confirmed by electrodepositing Ag,20 which highlights the presence of pinholes in the insulating film. After insulation the nanoelectrode is revealed by FIB machining, using a beam direction perpendicular to the nanowire and cantilever. Figure 3 shows FIB images of a SECM-AFM probe before (a) and after (b) cutting. Note in Figure 3a the bulbous end of the insulated tip; this has arisen due to the SWNTs kinking in the vicinity of the tip apex during metal deposition. However, through subsequent cutting, such abnormalities can easily be removed. While aligning the probe in the FIB, only low resolution imaging and single scans were employed to minimize damage to the insulating layer. The cut was made by initially imaging the probe and then focusing the beam with a current of ∼10 pA on a selected region until the area had been completely removed. This typically took between 30 s and 3 min (see Supporting Information for details). The TEM image in Figure 3c shows a typical SECM-AFM probe, fabricated in the manner described. The successive layers of Au (∼50 nm), poly(oxyphenylene) (∼90 nm) and outer silicon nitride (∼80 nm) are clearly visible, although the core of SWNTs cannot be resolved here. The disk geometry of the tip apex and a short central nanowire (∼100 nm in length) can also be identified. Figure 4a shows a SEM image of a probe insulated with a film of poly(oxyphenylene). The electrode region has been revealed and subsequent Ag electrodeposition clearly reveals Nano Lett., Vol. 5, No. 4, 2005

that it is only the exposed nanoelectrode - at the tip apex - which is electroactive and able to reduce Ag+ present in solution to Ag. Figure 4b displays a linear sweep voltammogram, recorded at a scan rate of 10 mV s-1, at a SECMAFM probe in a solution containing 10 mM Ru(NH3)63+ in 0.1 M KNO3 supporting electrolyte. In Figure 4b a diffusion-controlled limiting current of ca. 55 pA is clearly evident. FE-SEM and TEM images showed the total diameter of the probe to be ∼400 nm, with the central Au nanowire ∼80 nm in diameter. Given these dimensions, the observed current is less than half the expected steady-state current.21 We surmise that this is as a result of the cutting procedure; it is probable that the nanoelectrode is partially blocked with cutting debris, although this is not resolvable in the TEM or FESEM images taken. We are currently investigating methods to improve the efficacy of the ion mill. However, as shown below, this does not affect the ability of the electrode to image electrochemical surface activity, and the response is very stable over time. The topographical resolution is not limited by the overall diameter of the probe, typically on flat surfaces (roughness ca. 10 nm) the probes image off asperities at the edge of the nanoelectrode insulation and are thus capable of topographical resolution similar to conventional AFM probes (see Supporting Information). Importantly, the electrode geometry remains constant and does not change as a result of imaging. By using nominally contact (spring constant 0.1 N m-1, resonance frequency 10 kHz) and tapping (spring constant 3 N m-1, resonance frequency 75 kHz) silicon tips we have successfully fabricated nanoelectrode probes that image in both tapping and contact mode, in air, and under solution. Figure 5 shows simultaneously acquired tapping mode topography and electrochemical maps of a 2 µm diameter Pt “disk” electrode sealed in glass and embedded in epoxy resin.7,22 Similar results were achieved in contact mode. A Veeco Multimode AFM with Nanoscope IIIa controller was employed inside a Faraday cage. The nanoelectrode probe was fabricated as described above, the core nanowire had a diameter of ∼100 nm, with a total insulation thickness of ∼200 nm. The metal clasp of the AFM fluid cell (Veeco) was used to make electrical contact to the metal-coated AFM probe, which was then insulated with a 50:50 mix of nail varnish and superglue (as described previously)7,8 to ensure 641

Figure 5. Tapping mode AFM height (a) and electrochemical current (b and c) images of a ∼2 µm diameter Pt substrate recorded using a nanowire SECM-AFM probe at a scan rate of 0.5 Hz. The images are 20 µm square.

electrical isolation from the electrolyte solution. The electroactive mediator was 10 mM Ru(NH3)63+ in a supporting electrolyte of 0.1 M KNO3. The Pt substrate electrode was held at a potential of -0.4 V (versus Ag/AgCl) sufficient to reduce Ru(NH3)63+ to Ru(NH3)62+ at a diffusion-controlled rate. The nanoelectrode probe was set at a potential of 0.0 V, sufficient to oxidize the Ru(NH3)62+ generated at the Pt electrode at a diffusion-controlled rate. The current through the nanoelectrode probe was measured by an Ithaco 1211 virtual earth preamplifier and inputted to the AFM. The electrochemical image is thus a map of the steady-state concentration profile of electrogenerated Ru(NH3)62+ diffusing away from the Pt electrode. In the topography image, Figure 5a, not only is the Pt electrode clearly resolved, but also features such as scratches on the insulating glass sheath, indicating the surprisingly high topographic resolution of the probes. The electrochemical current map in Figure 5b shows a high current over the electrode, decreasing to zero away from the electrode, as expected.4b,23 Occasional artifacts occur in the tip current image over the Pt electrode as a consequence of brief electrical contact between the tip and protrusions on the Pt substrate. The high electrochemical spatial resolution can be seen through the close similarity between the topographic shape of the electrode and the electrochemical response, clearly indicating that the electrochemical map is dominated by the size of the Pt substrate electrode rather than the tip electrode. The electrochemical data in Figure 5b is replotted in Figure 5c, using a multibanded color scheme to emphasize the emergence of pure radial diffusion at larger distances from the electrode, even though the electrode itself is clearly not circular. The nanoelectrode probes proved to be very robust, giving consistent topography and electrochemical results over periods of up to 10 h continuous imaging. The data presented in Figure 5b and 5c are as recorded (the images have not been filtered, and no background has been subtracted) and clearly demonstrate the stability of the nanoelectrode over the ∼20 min required to take the scan. The probes are also capable of working in a wide potential range, -1 V to +1 642

V in aqueous solution, using both reductive and oxidative mediators. The separation between the tip and substrate can be controlled accurately in tapping mode using “lift-mode” where the tip first scans the topography and then retraces the same line a set distance, the “lift-height”, above the surface. Preliminary results indicate that there is little, if any, effect of the tip oscillation in tapping mode on the electrochemical current, and that the effect of lift-height on the recorded tip current is as expected from recorded tip approach curves. A further more detailed study on these effects, and other details of the electrochemical imaging, is currently being concluded and will be reported elsewhere. In summary, we have applied a simple but reliable technique for the fabrication of nanoelectrodes at the apex of AFM probes with controllable electrode diameters and well-defined disk geometry. We have demonstrated the high resolution obtained with these probes for simultaneous electrochemical and topographical imaging. We expect this to have a dramatic impact on the application of SECM to the study of electrochemical heterogeneity on the nanoscale. We are currently improving the procedure through refinement of the FIB milling process and further development of the insulating layers. In addition, we are pursuing the fabrication of nanoelectrodes from different electrode materials, such as Pt and Ir, which will extend the use of the probes to a variety of electrochemical processes, and applications such as pH sensing. Acknowledgment. We thank Mr. Steve York (Department of Physics, University of Warwick) for help with the electron microscopy, Mr. Thomas Day (Department of Chemistry, University of Warwick) for experimental assistance, and Prof. Patrick Unwin (Department of Chemistry, University of Warwick) for helpful discussions. We are grateful to Dr. Haiping Zhou (Department of Electronics and Electrical Engineering, University of Glasgow) for help with the silicon nitride deposition. J.V.M. thanks the Royal Society for the award of a University Research Fellowship. We also thank the Leverhulme Trust (PDRA for N.R.W.) and the EPSRC (GR/S24138/01) for funding. Nano Lett., Vol. 5, No. 4, 2005

Supporting Information Available: Details on the thermal oxidation of the silicon probes, deposition of the poly(oxyphenylene) layer and the silicon nitride layer, FIB cutting, and an example of the topographic resolution. This material is available free of charge via the Internet at http:// pubs.acs.org. References (1) (a) Punckt,C.; Bo¨lscher, M.; Rotermund, H. H.; Mikhailov, A. S.; Organ, L.; Budiansky, N.; Scully, J. R.; Hudson, J. L. Science 2004, 305, 1133. (b) Leblanc, P.; Frankel, G. S. J. Electrochem. Soc. 2002, 149, B239. (2) Bell, A. T. Science 2003, 299, 1688. (3) (a) Macpherson, J. V.; Unwin, P. R. Prog. React. Kinet. 1995, 20, 185. (b) Macpherson, J. V.; Unwin, P. R.; Hillier, A. C.; Bard, A. J. J. Am. Chem. Soc. 1996, 118, 6445. (4) (a) Scott, E. R.; White, H. S.; Phipps, B. J. Membr. Sci. 1991, 58, 71. (b) Scott, E. R.; White, H. S.; Phipps, B. Anal. Chem. 1993, 65, 1537. (5) Scanning Electrochemical Microscopy; Bard, A. J., Mirkin, M. V., Eds.; Marcel Dekker: New York, 2001. (6) Gardner, C. E.; Macpherson, J. V. Anal. Chem. 2002, 74, 576A. (7) Macpherson, J. V.; Unwin, P. R. Anal. Chem. 2001, 73, 550. (8) Macpherson, J. V.; Unwin, P. R. Anal. Chem. 2000, 72, 276. (9) Abbou, J.; Anne, A.; Demaille, C. J. Am. Chem. Soc. 2004, 126, 10095.

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