Optical microscope gets down to nanometre view

Scientists in Germany have developed a near-field microscope that can measure the optical properties of thin films, such as semiconductors for solar cells, with a spatial resolution and sensitivity long thought unachievable because of diffraction.

The research group of Professor Alfred Meixner and Dai Zhang from the Institute of Physical and Theoretical Chemistry at the University of Tübingen uses a gold tip as an ‘optical antenna’ to allow both the optical spectrum and the topography of a surface to be mapped simultaneously with nanometre precision.

“Molecular steps of a semiconductor film appear as distinct bright stripes of approximately 17nm width. We obtained new insights that cannot be obtained with any other method,” claimed Meixner.

In the microscope, a fine gold tip is taken as close as 1nm to the semiconductor surface, where it is illuminated by a tightly focused laser beam.

“We obtained an optical luminescence enhancement of up to one million,” said Meixner. “This high enhancement factor is possible because the tip is in the focus of a parabolic mirror: This combination yields a perfect optical antenna. The gold tip concentrates the light locally into the nanometre sized gap between the tip apex and the sample surface and thereby generates an optical near field which in turn excites the sample. Vice versa, photons that are generated by the sample inside the near-field area are collected by the tip and the parabolic mirror and directed onto a sensitive detector.”

The near-field measurements of the semiconductors made of diindenoperylene (DIP) molecules revealed that the edges of the DIP nano terraces radiate stronger than the bulk. These edges are one to three molecular layers high and appear as bright stripes. This is due to electron hole pairs in the semiconductor DIP, so called excitons, which are induced as well as detected by the near field of the tip. “If our tip was not there, the excitons would mainly decay thermally”, said Meixner.

The scientists believe the work, which was published in the journal Physical Review Letters, could lead to the near-field microscopy becoming a valuable method for materials research. For example, in organic solar cells the semiconductor films used differ greatly from the properties of the single organic molecules of which they consist: edges and defects strongly influence the average film properties.

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