| AVS 54th International Symposium | |
| Electronic Materials and Processing | Tuesday Sessions |
| Session EM-TuM |
| Session: | Molecular Electronics |
| Presenter: | T.-H. Park, University of Pennsylvania |
| Authors: | M.P. Nikiforov, University of Pennsylvania S. Schneider, Dresden University of Technology, Germany T.-H. Park, University of Pennsylvania U. Zerwek, Dresden University of Technology, Germany C. Loppacher, Dresden University of Technology, Germany L. Eng, Dresden University of Technology, Germany M.J. Therien, University of Pennsylvania D.A. Bonnell, University of Pennsylvania |
| Correspondent: | Click to Email |
In recent years a family of scanning probe microscopy (SPM) techniques has been evolved basing on applying multiple modulations to samples and tips and accessing optical higher-order harmonics. Scanning surface potential microscopy (SSPM or KPFM) based on noncontact atomic force microscopy (nc-AFM) is the first and most basic of the implementations exploiting multiple modulations. SSPM allows the analysis of local work functions on metallic samples and of surface charge distributions on insulators. In addition to the electric modulation, nc-AFM can be extended to optical investigations with the potential to access dielectric functions at surfaces. Of these scanning optical microscopes, so-called scattering near field optical microscopy (S-NSOM) offers the best opportunity for high spatial resolution. In s-NSOM a sharp probe tip is positioned near a surface and illuminated with optical radiation. The electric field is highly enhanced underneath the tip which acts as an optical antenna. Is a sample placed close to the tip, the local dielectric constant of the sample is probed by the tip and may be detected in the far-field scattered light. The strongly nonlinear distance dependence of the near-field may be used to separate its contribution from background signals by so-called higher-harmonic demodulation. In the present study the properties of porphyrin monolayers deposited on highly oriented pyrolytic graphite (HOPG) by evaporation in vacuum are probed. The molecular structure of the monolayer is known from nc-AFM. The polarization dependence of light scattering on the optically active molecules (porphyrin) was measured at four harmonics of the tip oscillation frequency. The contrast differences in the higher harmonics related to the dielectric properties of the molecules is compared with an analytical description of the signal transfer function. The limits in quantifying dielectric function and spatial resolution will be discussed as will the potential to probe dielectric function at the molecular level in the future.