2D-Materials

Ellipsometry is a sensitive optical method, which has been used
for about a hundred years to derive information about surfaces.
It makes use of the fact that the polarization state of light may
change when the light beam is reflected from a surface. If the surface is
covered by a thin film (or a stack of films), the entire optical system of
film & substrate influences the change in polarization.
It is therefore possible to deduce information about the film properties,
especially the film thickness and optical properties.
Ellipsometry remains a macroanalysis technique, i.e.,
the spot size limits resolution.
Features smaller than the spot are not resolved.

The question is, how to apply ellipsometry to exfoliated 2D materials?

The development of imaging ellipsometry,
which combines the power of ellipsometry with microscopy,
has overcome this limitation. The enhanced spatial resolution of
imaging ellipsometers potentially expands ellipsometry into
new areas of microanalysis, microelectronics, and bioanalytics.

The ep4 is a microscopic imaging spectroscopic ellipsometer with
unique measurement capabilities.
It can provide real-time-ellipsometric enhanced contrast images,
maps of the ellipsometric angles Psi and Delta, or data for very
small regions of interest.
Film thickness and the complex refractive indices of the sample are
determined by computerized optical modeling.

Setup of an imaging ellipsometer including objective and 2D CCD

Ellipsometric enhanced contrast by optimized polarizer and analyzer angle at selected wavelength and angle of incidence.

Regions of Interest (ROI) concept for ellipsometry on smallest flakes

Microscopic mapping of Delta and Psi.