二次元材料

Graphene on Copperfoil Investigated by Imaging Ellipsometry

SAMPLE AND SETUP:

To cover the increasing demand of high-quality Graphene, CVD grown Graphene upon a Cu-foil substrate seems to be a very promising way. However, traditional methods of quality and quantity control, like optical microscopy, SEM, Raman, either do not work or are too slow on the Cu-foil directly.

Imaging ellipsometry with the QDQRILOPB(3 offers highest lateral ellipsometric resolution down to 1 µm. It combines the sensitivity for thickness and refractive indices measurements of ellipsometry with the benefits of magnification and lateral resolution from conventional optical microscopy.

MEASUREMENT:

The shown ǻ map on the left clearly distinguishes the hexagonal structure of the grown Graphene from the Cu-foil. The shown field of view is measured within one single measurement. By moving the sample, a pattern of the sample can be recorded and stitched automatically by the QDQRILOPB(3. The field of view obtained with the 5x objective was 1 mm X 1.4 mm at an angle of incidence of 60°. A pattern of 1.5 cm X 1 cm on the Cu-foil was done within 2 hours and allowed the identification of structures down to 5 µm.

RESULTS:

- Graphene crystals can be clearly distinguished from Cu-foil on the stitched ǻ map at 390 nm
- Surface coverage of Graphene
- Folds of Graphene induced by the different temperature coefficients during the cooling down can be seen

Hetero-Structure MoS₂ /WSe₂ INVESTIGATED BY IMAGING ELLIPSOMETRY

SAMPLE AND SETUP:

2D-materials often show superior properties compared to their bulk or few-layer materials. The stacking of different mono layered materials do even promise more interesting combinations. MoS₂ and WSe₂ show strong excitonic dominated behaviour. In theory, the stacking shows the opportunity of intra-layer excitons.

Imaging ellipsometry with the QDQRILOPB(3 offers highest lateral ellipsometric resolution down to 1 µm. It combines the sensitivity for thickness and refractive indices measurements of ellipsometry with the benefits of magnification and lateral resolution from optical microscopy. Spectroscopic dispersions can be obtained by varying the wavelength.

MEASUREMENT:

The mechanically exfoliated flakes of MoS₂ and WSe₂ are stacked, that their mono layered regions overlap. The unique feature knife-edge illumination of the QDQRILOPB(3is used to avoid disturbing backside reflections from the thin, transparent glass substrate. The Ȍ map on the left (Ȝ = 450 nm) shows differentiated contrast for both monolayer regions and the hetero-structure. The overlapping area is 2 µm x 6 µm sized. The 20x objective offers high lateral ellipsometric resolution within a field of view of 150 µm x 150 µm. The wavelength is varied from 400 nm to 800 nm. All regions inside the field of view are measured simultaneously.

RESULTS:

- Spectroscopic ǻ Ȍvalues/maps for MoS₂, WSe₂ and the overlapping hetero-structure within one single measurement

- Refractive indices for each regions independently