Polypropylene-Functionalized Probes for Scanning Force Microscopy
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Polypropylene-Functionalized Probes for Scanning Force Microscopy L. González-Ronda1, S. L. Kaberline2, E. L. Durieux3 1 Department of Materials Science, Ford Research Laboratory, Dearborn, MI 48124, U.S.A. 2 Department of Chemistry and Environmental Science, Ford Research Laboratory, Dearborn, MI 48124, U.S.A. 3 Department of Physical Chemistry, Flint Ink Research Center, Ann Arbor, MI 48103, U.S.A. ABSTRACT SFM probe tips have been functionalized with polypropylene (PP) layers by immersion into dilute solution. The immersion process was carefully controlled and monitored using a programmable dynamic contact angle analyzer (DCA) equipped with a microbalance and a motor capable of speeds ranging from 2-24 µm/s. Probes were submitted to treatment with ozone prior to coating to eliminate surface contamination, particularly silicone oils, resulting from commercial packaging methods. The chemical composition of the tip surface after the cleaning, coating, and scanning processes was analyzed by Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS). The three-dimensional tip shape and dimensions before and after coating were evaluated by blind reconstruction from images of a columnar titanium thin film using Digital Instruments' Tip Evaluation algorithm. Coating thicknesses of up to 20 nm were calculated, with the thicker coatings showing a larger degree of non-uniformity. INTRODUCTION The functionalization of atomic force microscopy tips allows the study of surface interactions between distinct chemical groups. Chemical force microscopy (CFM), as the technique has become known, has been successfully used to probe adhesive and frictional forces between functional groups such as –COOH and -CH3 [1]. An essential component of CFM methodology is the ability to selectively attach chemical groups to the AFM tip, which has been achieved through the use of functionalized thiols [2] and reactive silanes [3], for example. Here, we discuss the use of nonspecific adsorption to functionalize AFM tips with polypropylene, using a methodology that should be relevant to other polymers as well. EXPERIMENTAL PROCEDURE Probe cleaning and storage Storage vessels for SFM probes were prepared by mounting surgical stainless steel test clips onto glass slides with epoxy. The secured probes were placed onto glass petri dishes and cleaned using an O3 Ozonology ozone generator for 0.5 to 4 h to remove organic contaminants [4,5].
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Tip functionalization Cantilever tips were coated by immersion into dilute solution (0.01-0.1 weight percent polypropylene in toluene). The immersion process was controlled and monitored using a programmable dynamic contact angle analyzer (Cahn DCA 322). Probes were suspended from the DCA microbalance using a test clip and programmed to raise the polymer solution at a speed of 2 µm/s until the cantilever came in contact with the liquid (Figure 1). Movement was stopped once the solution wetted the tip of the cantilever, which was defined as a sudden weight increase greater than 50 µg, as measured by
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