Use of Multi-Walled Carbon Nanotubes for Conductive Probe Scanning Force Microscopy (CP-SFM)
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USE OF MULTI-WALLED CARBON NANOTUBES FOR CONDUCTIVE PROBE SCANNING FORCE MICROSCOPY (CP-SFM) Kevin B. Stavens and Ronald P. Andres School of Chemical Engineering, Purdue University, W. Lafayette, IN 47907-1283, U.S.A. ABSTRACT Multi-Walled Carbon Nanotubes (MWNTs) mounted on commercial Scanning Force Microscopy (SFM) cantilevers have proven to be excellent probes for high resolution Tapping Mode-Scanning Force Microscopy (TM-SFM). Because of their robust nature and high electrical conductance, MWNTs are also attractive for use in Conductive Probe-Scanning Force Microscopy (CP-SFM). To be used in this application, however, the MWNT must be mounted via a high conductance contact to a conductive cantilever. A technique has been developed that produces such a contact. First, a MWNT is attached using acrylic adhesive to a commercial SFM cantilever that has been vacuum coated with gold. Then, a cap of gold is deposited over the junction between the MWNT and the SFM cantilever via spatially selective electro-deposition. INTRODUCTION The aim of the present research is to take advantage of the unique mechanical and electrical properties of MWNTs to construct a robust probe for CP-SFM. The current technology for CPSFM primarily employs modifying silicon cantilevers via gold plating, diamond plating, or heavy doping of the cantilever [1,2,3]. These are all useful techniques but they do not offer the high lateral resolution, soft compliance, and robustness that a MWNT probe supplies. Multi-Walled Carbon Nanotubes (MWNTs) mounted on commercial Scanning Force Microscopy (SFM) cantilevers have proven to be excellent probes for high resolution Tapping Mode-Scanning Force Microscopy (TM-SFM) [4,5]. The primary technology for adhesion of the MWNT to the cantilever is the acrylic adhesive mounting technique first developed by H. Dai et al. [6]. A TEM micrograph of such a TM-SFM probe produced at Purdue University is shown in Figure 1a. This mounting technique gives rise to two problems. There often is poor mechanical adhesion between the MWNT and the silicon tip. When attempting to image samples in water, extreme care must be taken to avoid contacting the air/water interface with the cantilever tip. The MWNT can penetrate the air/water interface and image submerged samples, however, if the water interface contacts the silicon tip, the wetting action of the water often removes the MWNT from the cantilever. This limits the usefulness of such probes to imaging samples that are submerged at depths no more than the length of the MWNT, which is typically on the order of microns. Secondly there is poor electrical conduction between the MWNT and the cantilever, which limits the probe’s usefulness in CP-SFM. This is because the acrylic adhesive is inherently a poor conductor and because there is poor interfacial contact between the MWNT and the silicon cantilever. The goal of the present work is to develop a technique that improves both the electrical and mechanical contact between the MWNT and the cantilever.
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Figure 1. (a) TEM micro
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