Klaus Weishaupt

Scanning Near-field Optical Microscopy in Life Science

Scanning Near-field Optical Microscopy (SNOM) allows optical microscopy with highest spatial resolution beyond the diffraction limit. We present a new microscope set-up which uses micro-machined cantilever SNOM sensors and combines in a unique way the advantages of Scanning Near-field Optical Microscopy, Confocal Scanning Microscopy (CSM) and Atomic Force Microscopy (AFM) in one instrument. Results from measurement at different samples system like histological microtome cuts and fluorescent labelled human chromosomes are shown to demonstrate the capabilities of the new set-up.

Confocal Raman AFM, a powerful tool for the nondestructive characterization of heterogeneous materials

The combination of an atomic force microscope (AFM) and a Confocal Raman Microscope (CRM) has been used to study an emulsion consisting of alkyd and acrylic latexes. Alkyds and acrylics are used as binders in paints and coatings. These materials produce a shiny, hard finish that is highly water-resistant. The high spatial resolution of the AFM enables the morphological characterization of the dried emulsion on the nanometer scale. Furthermore, when operating the AFM in Digital Pulsed Force Mode (DPFM), topographic information and local mechanical properties can be simultaneously recorded, allowing a comparison of the mechanical properties of the emulsion components. Raman spectroscopy provides additional information on the chemical composition of materials. In combination with a confocal microscope, the spatial distribution of the various phases can be determined with a resolution down to 200 nm. Therefore, the topographically different structures observed in AFM images can be associated to the chemical composition by using the Confocal Raman Microscope (CRM).

The power of confocal raman-AFM and raman-SEM (RISE) imaging in polymer research

Polymers play an essential role in modern materials science. Due to the wide variety of mechanical and chemical properties of polymers, they are used in almost every field of application and are still a dynamic area in the development of new materials. For many of these developments knowledge about the morphology and chemical composition of heterogeneous polymeric materials on a sub-micrometer scale is crucial. In the past two decades, AFM (atomic force microscopy) was one of the main techniques used to characterize the morphology and phase separations in thin polymeric films. This measuring technique however relies only on mechanical contrast, leaving research on similar polymers out of reach. The combination of a confocal Raman microscope with an AFM one decade ago provided the ability to unequivocally determine the chemical composition of a material. By acquiring Raman spectra at every image pixel, the high spatial and topographic resolution obtained with an AFM can be directly linked to the chemical information provided by confocal Raman spectroscopy [1,2]. In polymer science, Raman spectra provide quantitative information about various features such as: chemical nature (structural units, type and degree of branching, end groups, additives), conformational order (physical arrangement of the polymer chain), state of the order (crystalline, mesomorphous, and amorphous phases), and orientation (type and degree of polymer chain and side group alignment in anisotropic materials). A milestone in microscopy, achieved last year, is the combination of SEM (scanning electron microscopy) with confocal Raman imaging. For polymer research an SEM provides the morphology and the crystalline structure of polymer composites together with the atomic composition mainly carbon and hydrogen. The combination with confocal Raman imaging allows the allocation of the unique chemical composition of the high resolution structures with diffraction limited resolution.

Characterization of Carbon Nanomaterials with a confocal Raman-AFM

Carbon is known to exist in a number of allotropes which range from single crystalline diamond the hardest of all known materials, to the soft, layer based graphite. The discovery by Novoselov and Geim [1] of a simple method to transfer a single atomic layer of carbon from the c-face of graphite to a substrate suitable for measurements of its electrical and optical properties has led to an increased interest in studying and employing two-dimensional model systems. An overview of electron and phonon properties of graphene and their relationship to the one-dimensional form of carbon known as nanotubes can be found in [2]. The unique chemical, mechanical, electrical, and optical properties of graphene lead to its many application possibilities such as: single molecule detectors, high-strength lowweight new materials, design of new semiconductor devices, etc. An important goal however, is the detection of such angstrom-thick two dimensional sheets and precisely determine the number of layers forming the graphene flake. The aim of this contribution is to show how a confocal Raman AFM can contribute to the characterization of such small materials and devices. In the past two decades, AFM (atomic force microscopy) was one of the main techniques used to characterize the morphology of nano-materials spread on nanometer-flat substrates. From such images it is possible to gain information about the physical dimensions of the material on the nanometer scale, without additional information about their chemical composition, crystallinity or stress state. On the other hand, Raman spectroscopy is known to be used to unequivocally determine the chemical composition of a material. By combining the chemical sensitive Raman spectroscopy with high resolution confocal optical microscopy, the analyzed material volume can be reduced below 0.02 μm 3 , thus leading to the ability to acquire Raman images with diffraction limited resolution from very flat surfaces [3, 4]. The combination of confocal Raman microscopy with Atomic Force Microscopy (AFM) is a breakthrough in microscopy. Using such a combination, the high spatial and topographical resolution obtained with an AFM can be directly linked to the chemical information provided by confocal Raman spectroscopy [5]. An example of a confocal Raman-AFM measurement is illustrated in the figures of this abstract. Fig. 1 shows the AFM topography image of a graphene flake deposited on a Si substrate. This image was recorded in AFM AC mode and it reveals the presence of mono-, biand multi-layers of carbon, as highlighted in the two cross sections. A Raman image was recorded from the same sample area by acquiring a spectral array of 85x50 complete Raman spectra. A typical Raman spectrum of graphene deposited on a Si-substrate is shown in Fig. 2. The thousands of Raman spectra were evaluated using peak fitting algorithms, which are very sensitive to small variations of Raman band position and width. The Raman image presented in Fig. 2 highlights the variations of the G-band within the analyzed graphene. The mono-layer of graphene (brown color) can be clearly discriminated from a graphene bilayer (pink color). In yellow color a flipped over graphene sheet is presented.