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nanotimes
10-02 :: February 2010
News in Brief
Near-Field Microscope //
Yields High Precision Optical Images of an
Organic Semiconductor With 17nm Resolution
S
cientists of the research group of Prof. Dr. Alfred parabolic mirror: This combination yields a perfect
Meixner and Dr. Dai Zhang from the Institute of optical antenna. The gold tip concentrates the light
Physical and Theoretical Chemistry at the University locally into the nanometre sized gap between the tip
of Tübingen have developed a near-field micro- apex and the sample surface and thereby generates
scope that can measure the optical properties of e. g. an optical near field which in turn excites the sam-
semiconductor thin films with a spatial resolution and ple. Vice versa, photons that are generated by the
sensitivity long thought unachievable due to funda- sample inside the near-field area are collected by
mental physical laws (diffraction limit). Both the opti- the tip and the parabolic mirror and directed onto a
cal spectrum and the topography of a surface can be sensitive detector.”
mapped simultaneously with nanometre precision.
The near-field measurements of the semiconductors
The researchers have been recorded tip-enhanced made of diindenoperylene (DIP) molecules revealed
near-field optical images and correlated topographic that the edges of the DIP nano terraces radiate
images of an organic semiconductor film (diinden- stronger than the bulk. These edges are one to three
operylene, DIP) on Si with high optical contrast molecular layers high and appear as bright stripes
and high spatial resolution (17nm) using a parabolic of approximately 17nm. This is due to electron hole
mirror with a high numerical aperture for tip illu- pairs in the semiconductor DIP, so called excitons,
mination and signal collection. The DIP molecular which are induced as well as detected by the near
domain boundaries being one to four molecular field of the tip.
layers (1.5-6nm) high are resolved topographically by
a shear-force scanning tip and optically by simultane- “If our tip was not there, the excitons would main-
ously recording the 6x105 times enhanced photolu- ly decay thermally,” explains Alfred Meixner. “This
minescence (PL). breakthrough could lead to the near-field microscopy
becoming a valuable method for materials research,”
“We obtained nanometre resolution and an optical Reinhard Scholz and Frank Schreiber agree.
luminescence enhancement of up to one million,”
explains Alfred Meixner. “This high enhancement
factor is possible because the tip is in the focus of a