Domains In Thin Organic Films Studied by Polarized SNOM Investigations

  • Fig. 1: SNOM for polarized investigations. An “-SNOM” from WITEC GmbH (Germany) with a cantilever tip was used.Fig. 1: SNOM for polarized investigations. An “-SNOM” from WITEC GmbH (Germany) with a cantilever tip was used.
  • Fig. 1: SNOM for polarized investigations. An “-SNOM” from WITEC GmbH (Germany) with a cantilever tip was used.
  • Fig. 2: Absorption spectra of the compound in solution and in the spin-cast layer.
  • Fig. 3: SNOM transmission image. One image of the crystalline film detected by polarized SNOM investigations using a special analyzer angle is shown. A set of images with different analyzer angle were taken

We present a SNOM-study of highly-crystalline organic films of an oligomeric polyquater-thiophene (PQT-12). The crystalline structure influences the suitability of films for applications as charge transport in devices as e.g. OFETs.
The absorption of polarized light depends on the orientation of crystallites. So in a SNOM investigation with polarized light the crystallites with different orientation differ in the transmitted intensity. The material (PQT-12) has a very narrow Mw distribution. Roughly 85 mol. % of the material consist of the quarter-thiophene dimer. Due to this fact extended crystallites are formed during film-formation from solution by spin coating. This is clearly indicated by absorption spectroscopy (Fig 2).
As measured by AFM the films are quite smooth (RMS ca 2,5nm) and the average thickness of the film is 50nm. The surface topography observed via AFM differs considerably from the optically detected domain orientation structure in the SNOM transmission images. Crystal domains and their relative orientation are detectable by polarized SNOM investigations. The film is scanned with linearly polarized light. An analyzer is used to change stepwise the detection angle of transmitted light (Fig 3).
The data of polarized SNOM investigations will be a basis for further quantitative calculations of anisotropic physical properties.

References:
[1] P. Pingel, A. Zen, D. Neher, I. Lieberwirth, G. Wegner, S. Allard, U. Scherf,
Appl. Phys. A 95 (2009) 67-72.

Please click here to read the poster as a PDF

This poster was presented on NanoBioView, 6-7 October 2010
http://www.nanobioviews.net

Authors:
B. Stiller*, Bat El Pinchasik, P. Pingel, D. Neher
Uni Potsdam, Inst. f. Physik und Astronomie, Karl-Liebknecht-Straße 24-25, 14476 Potsdam

*corresponding author: busti@uni-potsdam.de

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