- Optical contrast is a normalised difference between the intensities of light reflected by sample and surrounding substrate
- Optical contrast of layered materials changes in a step-like manner with increasing number of layers, allowing accurate thickness estimate for flakes up to 15 layers thick
- Optical contrast measurements work best for materials with strong absorption in the visible range, such as graphene, transition metal mono- and dichalcogenides, deposited onto silicon substrates with 90 or 290 nm dioxide layer, but can be extended to other types of substrates
2DMT: Optical microscopy
- Optical microscopy is an essential tool in 2D materials research due to the small typical size of the samples.
- Three most commonly used microscopy methods are bright-field, dark-field, and photoluminescence microscopy.
- Bright-field microscopy used reflected/transmitted light to produce a magnified image of a sample. Most common applications: flake search (identification of monolayer flakes produced by mechanical exfoliation), thickness measurements through optical contrast measurements, crystal axis direction identification using flake edges.
- Dark-field microscopy relies on the light scatter by the sample and is used to identify inhomogeneous features, such as trapped contamination and edges/steps in thickness.
- Photoluminescence microscopy uses light emitted by the sample under optical excitation. While only applicable to luminescent materials, such as monolayer group-VIB transition metal dichalcogenides, it can provide various information about the sample, including thickness, composition, material and interface quality.