Inspecting and tuning electric fields on the nanometer scale offers a great potential in overcoming limitations inherent in assembling nanostructures. Both optical and electronic devices may be improved in performance provided that a quantitative knowledge on the strength and orientation of local (stray) fields is gained. Here we present nanoscale investigations of functional surfaces probing the surface potential and electronic properties of ferroelectric and ultra thin organic films. We developed methodologies that are able to non-invasively track the electric field both above and below interfaces, thus providing insight also into the sample. Hence, interface dipole formation and interface charging directly shows up in potential changes revealing the donor/acceptor characteristics of molecules, as well as the surface charge screening in ferroelectrics. Such inspections are possible using conventional scanning force microscopy operated in sophisticated modes measuring the electrostatic force or the inverse piezoelectric effect. Finally, electric fields are also probed in the optical regime using near-field optical methods. Examples are shown where the strength and frequency of surace plasmon resonances become tunable due to simple nanostructuring of metallic thin films.
The microscopic mechanism of spontaneous polarization and refractive indices in 180° ferroelectric domain walls of tetragonal barium titanate (BaTiO3) is discussed by using a microscopic model. This model is based on the orbital approximation in correlation with the dipole-dipole interaction due to the local field acting on all constituent ions within the domain wall. It is found that the behavior of both the spontaneous polarization and refractive indices depends on the thickness of the domain wall which was varied between 5 and 20 Å. Moreover, the spontaneous polarization shows a hyperbolic tangent dependence for domain walls of a larger thickness and vanishes at the center of the domain wall. The refractive indices suggest the domain wall to act like a biaxial crystal resulting in refractive index profiles of a Gaussian shape for domain walls of approximately 20 Å. This dramatically affects optical transmission through the domain wall specifically for light being polarized parallel to the domain wall.
We report the deposition, characterization, and application of novel optically transparent electrodes, namely ultrathin chromium films and amorphous carbon layers (a-C:H), suitable for replacing ITO and other common materials used so far in electro-optics. The ultrathin layers provide sufficient optical transmission of up to 95% for layer thicknesses of 2 nm and 5 nm for Cr and a-C:H, respectively, showing a flat spectral dependence between 400 and 800 nm. These features are maintained when using these coatings as electrodes on tapered optical fibers as used for scanning near-field optical microscopy (SNOM). We show the successful application of such coated optical tips for ferroelectric domain switching on the nanometer scale.
Scanning tunneling microscopy, although capable of yielding very high resolution on periodic structures, very often provides only moderate resolution on singular features. Our work aims at the use of laser light to improve the identification of individual molecules. We report on scanning tunneling microscopy measurements performed on dye molecules dissolved in a liquid crystal and adsorbed onto highly oriented pyrolytic graphite. Either localized perturbations of the liquid crystal structure with the size of single molecules or more or less extended ordered domains of well resolved dye molecules were reproducibly imaged for several dyes. To study light-induced resonant effects the influence of non-resonant absorption leading to thermal expansion of tip and sample has to be suppressed. Therefore, an electro- optical system was realized using an ArPLU- and a dye laser of different wavelengths power-modulated with a relative phase shift of 180 degree(s). Preliminary results obtained with this setup are presented documenting the efficiency of the compensation.
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