2D-Materials

Imaging spectroscopic ellipsometry is applied to characterize graphene and other 2D materials. CVD grown, exfoliated and epitaxically grown flakes of 2D materials are investigated with the imaging ellipsometer nanofilm_ep4. This compendium addresses the following topics:
• Imaging of monolayers on different substrates
• Determination of the optical dispersion
• Distinction between mono-, bi- or n-layers
• Automatic flakesearch algorithm for identification of monolayers or well-defined thickness regions
• Exploration of hetero structures
• Quality control by detecting defects and special aspects of 2D nanoplates

The enhanced spatial resolution of imaging ellipsometers in the range of 1 µm potentially expands ellipsometry into new areas of microanalysis, microelectronics, and especially exfoliated 2D materials. In a number of papers, Imaging ellipsometry has been applied to characterize graphene flakes of few micrometer size. Ellipsometric contrast micrographs, delta and Psi maps as well as wavelength spectra [1],[2] and single layer steps in multilayer graphene/graphite stacks [3] have been reported. Also Molybdenum disulfide, a layered transition metal dichalcogenide have been reported [4]. As an example, Delta- and Psi maps of a MoS2 flake at different wavelength are displayed. Additional to microscopic characterization of tiny flakes, there is an increasing need of fast methods for quality control of large area graphene samples. This becomes possible by comparing the sample with a similar reference sample. Due to the orientation of the reference (see Fig. 1) it acts as an ideal compensator for all wavelengths. Hence no compensator is required. If the sample is equal to the reference, the outgoing light is linear polarized and can extinguished with a crossed analyzer. If there’s any difference between sample and reference, the light becomes elliptic polarized and the detected light flux increases. In order to maximize the rate of data acquisition, the analyzer is set to an off-nullposition. During the measurement neither the polarizer- nor the analyzer-angle are changed, so the measurement speed is only limited by the intensity of the light source and the processing speed of the spectrometer.

[1] Wurstbauer et al., Appl. Phys. Lett. 97 (2010) 231901

[2] Matkovic et al. J. Appl. Phys. 112 (2012) 123523

[3] Albrektsen O. J. OF Appl. Phys. 111 (2012) 064305