Scanning probe microscopy methods allow investigating a variety of sample surface properties on a nanometer scale, even down to single molecules. As molecular electronics advances, the characterization of electrical properties becomes more and more important. In research and industry, films made from composite materials and lithographically structured elements have already reached structure sizes down to a few nanometers. Therefore electrical modes play an increasing role for material characterization.

Kelvin probe force microscopy (KPFM) is sensitive to local variations in the work function of materials. The work function depends on the specific material, adsorption layers (e.g. water), oxide layer thickness, dopant concentration, electrostatic charges, surface dipole moments and temperature. Thus the KPFM mode becomes useful to study surfaces made from different materials, e.g. polymer blends, composite materials. Furthermore functional materials and devices (e.g. solar cells) can be investigated which respond to light or to vapours. In order to study light induced charging of surfaces (e.g. solar cells) we have build up a photo-sensitive scanning force microscope which is situated in an environmental chamber (see Figure 1).

With such a setup we were able to study light induced charge separation in polymer functionalized nanorods on a single particle level using Kelvin probe force microscopy. Functionalized nanoparticles were spin coated from toluene on a transparent and conductive substrate (indium tin oxide, ITO). The samples were illuminated by laser light at a wavelength of λ = 488 nm and intensity ~10 W/cm2 through the ITO substrate. Upon light absorption charge separation takes place and the internal potentials in the particle-polymer system changed (Figure 2). This potential changes correspond to a negative charging of the ZnO particle core and a positive charging of the polymeric corona extending over distances up to 200 nm.

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• M.C. Lechmann, S.A.L. Weber, J. Geserick, N. Hüsing, R. Berger and J.S. Gutmann: Investigating Morphology and Electronic Properties of Self-Assembled Hybrid systems for Solar Cells, J. Mater. Chem., 21 7765 - 7770 (2011).
• E. Sengupta, A.L. Domanski, S.A.L. Weber, M.B. Untch, H-J. Butt, T. Sauermann, H.J. Egelhaaf, R. Berger: Photo induced degradation studies of organic solar cell materials using Kelvin Probe Force and Conductive Scanning Force Microscopy J. Phys. Chem. C, 115, 19994–20001 (2011)