Extending the Possibilities of Near-Field Scanning Optical Microscopy for Simultaneous Topographical and Chemical Force

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Mat. Res. Soc. Symp. Proc. Vol. 584 ©2000 Materials Research Society

be measured, and spectroscopic fingerprints can be obtained. The information obtained by the NSOM is mainly gathered from the topmost surface layer. However, depending on the instrumental configuration, this layer can be 100-200 nm, and information about the chemical functionalities exposed at the interface cannot be obtained directly. This issue is best addressed by chemical force microscopy, in which the probe tip is modified with different molecules so that the tip terminates with well-defined chemical functionalities [1]. The interaction between different surface functionalities can be tailored to give selectivity, thus providing the AFM with chemical sensitivity. To date, no individual technique can provide all these types of information in a single measurement. Our goal is to build a novel hybrid instrument that can incorporate simultaneous topographic, spectroscopic and chemical force imaging of mesoscopic structures on surfaces. In this paper we will describe a systematic method for the preparation of probe tips that are useful in NSOM, but can also be derivatized to provide them with chemical sensitivity. We also report the initial tests in performing chemical force imaging with such probes.

EXPERIMENT Microscope development The microscope we are developing is based on an existing Digital Instruments Nanoscope III, Atomic Force Microscope (AFM), Santa Barbara, CA. The original configuration of the Nanoscope III AFM uses sharp probe tips mounted on microfabricated cantilevers. To allow optical data collection, we have substituted a bent optical fiber for the typical AFM probe tip. The distance sensing mechanism utilizes either the deflection of the cantilever when operating in contact mode, or the amplitude of the cantilever oscillation when using Tapping ModeTM. Optical probe preparation The optical fibers were purchased from SpecTran Specialty Optics Company (Avon, CT). Chemical etching of fibers was done at room temperature without stirring of the etchant solution. All chemicals were purchased from Sigma-Aldrich Canada Ltd. (Oakville, ON). Images of the etched fibers were taken by a Field-emission Scanning Electron Microscope (SEM), Hitachi S-4500. The fibers were sputter coated with gold prior to SEM imaging. Fiber bending was carried out with a PFS-330 Optical Fiber Splicer (Power Technology Inc., Little Rock, Arkansas). The bent probe cantilever section was coated with 100 nm thermally evaporated aluminum. Preparation of test surfaces Test surfaces were prepared by a microcontact printing method [4]. Microscope cover slides, from Fisher Scientific (Unionville, ON), were used as substrates. The slides were coated with 10 nm of titanium and 300 nm of gold by thermal evaporation. Polydimethylsiloxane (Sylgard 184), obtained from Paisley Products Inc. (Scarborough, ON), elastomer stamps bearing a pattern of raised and recessed areas were prepared and inked with millimolar ethanol solution of hexadecane thiol. The stamp was dried with