Combined Scanning Tunneling and Aperture Near-field Microscopy: Tip Modeling
Ilya Sychugov and Hiroo Omi
Materials Science Laboratory
Optical and electrical properties of nanostructures can be addressed using radiation or electrical current as a probe. In general, a near-field type of electromagnetic interaction is necessary for an optical probe to enter nanoscale regime in the spatial resolution domain by overcoming the optical diffraction limit (〜 1 µm). A typical scanning near-field optical microscope (aperture-SNOM) provides such an opportunity both for the excitation and collection of light for spectroscopy applications. However, this instrument utilizes a dielectric fiber tip as an aperture, which makes it unsuitable for electrical measurements. On the other hand, a scanning tunneling microscope (STM), capable of atomic-resolution measurements by electrical current, can also cause luminescence of materials. Here, in order to realize both electrical and optical probing at nanoscale, we combine these two kinds of instruments into a single unit (Fig. 1).
In order to evaluate tip geometry influence on its performance for various operation regimes finite element method (FEM) simulations were carried out. It was found that tip transmittance for SNOM excitation mode depends strongly on tip geometry away from the edge. A variation of nearly two orders of magnitude is a result of constructive/destructive interference of the excitation light (Fig. 2). Furthermore, the role of the opening in protective metal film in light collection efficiency was simulated for the STM-luminescence mode. Based on the present simulations one can choose a suitable tip configuration from the interplay between the instrument collection efficiency and its spatial resolution .
We aim at concurrent optical and electrical characterization of nanostructures (e.g. quantum wells, dots, etc.) with high spatial resolution. In addition, this approach may find its niche where electrical modification with subsequent in situ optical probing is desirable.
 I. Sychugov, H. Omi, T. Murashita, and Y. Kobayashi, Appl. Surf. Sci., in press.
Fig. 1. Schematic representation of the setup. Fig. 2. Calculated tip transmittance.
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