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Near-field microscopy with synchrotron radiation

Especially interesting for
  • the chemical industry
  • materials research

In collaboration with the Freie Universität Berlin, a scattering-type Scanning Near-field Optical Microscope (s- SNOM) for the infrared (IR) range has been commissioned, using, for the first time, also broadband synchrotron radiation provided by PTB's electron storage ring Metrology Light Source (MLS) in Berlin-Adlershof. This now allows IR spectroscopic investigations on sample systems to be carried out with a lateral resolution of less than 100 nm in a large spectral range.

Figure: a) Comparison of the nano-FTIR spectra of silicon carbide (SiC) and gold (Au), recorded with broadband synchrotron radiation at the IR beamline of the MLS. The two measurement positions are identified with an “X” in the near-field microscopic image of the sample surface (Figure b, top right). When performing a scan along the white line in b), a lateral resolution of less than 100 nm is achieved at the gold edge (see Figure c, bottom right).

Fourier Transform Infrared (FTIR) spectroscopy is often used for the chemical characterization of organic and inorganic substances or to investigate the conductivity of diverse sample systems. Since the spatial resolving power of this spectroscopic method is, however, limited due to diffraction, FTIR investigations on structures on a sub-micrometre scale are not possible without limitations. The new scattering-type scanning near-field microscope at the MLS is based on the principle of a scanning force microscope and makes it possible to investigate, in addition to the topography, also the optical and the dielectrical properties of the sample, respectively. For this purpose, a sharp gold- or platinum-coated tip – a so-called “near-field probe” – is placed in a focused laser or synchrotron beam. During the subsequent scanning process, the position of the tip remains unchanged, only the sample is moved. Hereby, the nearfield probe, with a typical diameter between 20 nm and 50 nm, acts like an antenna and amplifies the electromagnetic field in the immediate vicinity of the metallic tip, which allows a lateral resolution clearly below 100 nm to be attained. Compared to conventional FTIR methods, this procedure represents a considerable improvement of the local resolution and of the sensitivity.

The main radiation sources used to date for near-field investigations in the IR spectral range have been CO- and CO2 gas lasers. These provide a sufficiently high radiation power, but can only cover the relatively narrow wavelength ranges from 5.2 μm to 6.1 μm and 9.2 μm to 10.8 μm, respectively. In order to extend the wavelength range, ing time of 15 minutes within the uncertainty limit of 0.1 nV/V. The new AC quantum voltmeter is now being optimized by means of on-site tests at the partner's (esz AG) accredited laboratory. With this valuable end-user input, it will be developed to become a fully automated, user-friendly measuring system. The main objective is to reach a relative uncertainty of 2.5 μV/V at 1 kHz. The system will be developed in a modular approach which will allow a future the nearfield microscope was combined with the IR beamline of the MLS, so that a wide wavelength range from the near IR up to the THz range can be used continuously. Hence, it is now possible to perform nano-FTIR spectroscopy with a lateral resolution clearly below the diffraction limit and to characterize surfaces and nanostructures with IR spectroscopy in a wavelength range from, for a start, 1 μm to 20 μm. Later, it is planned to extend the wavelength range even further.

Scientific publication:

P. Hermann, A. Hoehl, A. Patoka, F. Huth, E. Rühl, G. Ulm: Near-field imaging and nano-Fourier-transform infrared spectroscopy using broadband synchrotron radiation. Optics Express 21, 2913 (2013)