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Scanning probe chemical and topographical microanalysis

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  • Chemistry


The last decade has seen a rapid rise of Scanning Probe Microscopy, SPM, as a prominent and versatile approach for surface studies. SPM instruments are differentiated from the beam-based ones by the fact that they use solid proximal probes for localized analysis. The most commonly used SPM methodology is Atomic Force Microscopy, AFM. In its basic implementation, AFM provides topographical information with nanometer resolution. The most common modifications allow the magnetic, electrostatic, and specific chemical environment to be examined. However, there is no direct way today to perform general chemical analysis with AFM probes. Near-field Scanning Optical Microscopy, NSOM, is another variation of SPM where sharp tapered optical fibers serve dual purposes, being proximal probes of sample topography, and providing the means for localized light delivery for optical studies with sub-wavelength spatial resolution. Again, NSOM itself does not have a general chemical contrast capability. However, the capability to deliver light to localized area opens the way to a multitude of experiments that can be devised using different aspects of light interaction with the sample. This thesis demonstrates several approaches for combined topographical and chemical investigations. Infrared spectroscopy is a sensitive molecular analysis tool. Without scanning proximal probe, IR microscopy has very poor spatial resolution. Enabling methodology for probe fabrication for Near-field Scanning Infrared Microscopy, NSIM, is presented. The efforts in combining NSOM with mass spectrometry, which is probably the most general chemical analysis tool, are outlined. We have demonstrated the possibility of simultaneous topographical and molecular imaging. Another variation of chemical imaging is the combination of SPM and Laser Induced Breakdown Spectroscopy, LIBS. In this method the elemental composition of samples is obtained by analyzing optical emissions from transient plasma plumes formed by intense laser pulses delivered through fiber probes. We have demonstrated the feasibility of this approach. The instrument that we have developed is an attractive complementary tool for established methods of spatial elemental analysis, such as X-ray Fluorescence. Among its attractive features are operation in ambient conditions, minimal requirements for sample preparation, and ease of use.

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