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5 Optical Measurement Techniques
Fig. 5.1 Two AFM examples showing the topography of TMDC heterostructures. a False-colour
AFM image of a WS 2 /WSe 2 type-II monolayer–monolayer heterostructure on hBN buffer (on
SiO 2 ). Heterostructures comprising monolayer TMDCs are commonly revealed by the peculiar
height steps in AFM line profiles, as shown in b. a,b Reproduced with permission. [2] Copyright
2020 Springer Nature. c High-quality 2D-material stack of an hBN-encapsulated WSe 2 monolayer.
The central area with a few bright spots represents the monolayer with mostly clean interfaces
towards the thin hBN layers (bottom 10 nm, top 30 nm). The total thickness amounts to about
40 nm. These AFM images were recorded with the help of the group of G. Witte at the Physics
Department in Marburg. c Reproduced under the terms of the CC-BY Creative Commons Attribution
4.0 International Licence (http://creativecommons.org/licenses/by/4.0/). [3] Copyright 2020 The
Author(s), published by Springer Nature
files, such as differential-interference-contrast and digital holographic microscopy,
light-sheet microscopy, or multi-photon microscopy.
Owing to the optical resolution’s dependency on the light wavelength, naturally,
limitations are imposed with regard to the minimum object sizes resolvable under a
microscope. In fact, after centuries of use, new inventions allow further penetration
into the sub-micro domain, such as super-resolution microscopy techniques (see for
instance Nobel lecture by S. W. Hell in [1]). Further resolution improvement down
to the scale of an atom is only provided so far by electron microscopes (e.g., STEM,
scanning transmission electron microscopes) and scanning-tunneling microscopes
(STM), whereas scanning-near-field optical microscopy (SNOM) and atomic-force
microscopy (AFM) can reach tens of nanometers in lateral resolution. In addition,
sub-nanometer height profiles become measurable with the use of the AFM’s force
tip (see for instance Fig. 5.1).
Spectroscopy with its many variations has become a fundamental method for the
investigation of all kinds of materials. Particularly, the effects of quantisation can
be easily measured for quantum structures when addressing the optical response
of the system. This can be directly done by luminescence, reflectance, absorption
or transmission studies. While steady-state measurements reveal the energetics of
a system, time-resolved variations of the same spectroscopy methods can provide
5 Optical Measurement Techniques
Fig. 5.1 Two AFM examples showing the topography of TMDC heterostructures. a False-colour
AFM image of a WS 2 /WSe 2 type-II monolayer–monolayer heterostructure on hBN buffer (on
SiO 2 ). Heterostructures comprising monolayer TMDCs are commonly revealed by the peculiar
height steps in AFM line profiles, as shown in b. a,b Reproduced with permission. [2] Copyright
2020 Springer Nature. c High-quality 2D-material stack of an hBN-encapsulated WSe 2 monolayer.
The central area with a few bright spots represents the monolayer with mostly clean interfaces
towards the thin hBN layers (bottom 10 nm, top 30 nm). The total thickness amounts to about
40 nm. These AFM images were recorded with the help of the group of G. Witte at the Physics
Department in Marburg. c Reproduced under the terms of the CC-BY Creative Commons Attribution
4.0 International Licence (http://creativecommons.org/licenses/by/4.0/). [3] Copyright 2020 The
Author(s), published by Springer Nature
files, such as differential-interference-contrast and digital holographic microscopy,
light-sheet microscopy, or multi-photon microscopy.
Owing to the optical resolution’s dependency on the light wavelength, naturally,
limitations are imposed with regard to the minimum object sizes resolvable under a
microscope. In fact, after centuries of use, new inventions allow further penetration
into the sub-micro domain, such as super-resolution microscopy techniques (see for
instance Nobel lecture by S. W. Hell in [1]). Further resolution improvement down
to the scale of an atom is only provided so far by electron microscopes (e.g., STEM,
scanning transmission electron microscopes) and scanning-tunneling microscopes
(STM), whereas scanning-near-field optical microscopy (SNOM) and atomic-force
microscopy (AFM) can reach tens of nanometers in lateral resolution. In addition,
sub-nanometer height profiles become measurable with the use of the AFM’s force
tip (see for instance Fig. 5.1).
Spectroscopy with its many variations has become a fundamental method for the
investigation of all kinds of materials. Particularly, the effects of quantisation can
be easily measured for quantum structures when addressing the optical response
of the system. This can be directly done by luminescence, reflectance, absorption
or transmission studies. While steady-state measurements reveal the energetics of
a system, time-resolved variations of the same spectroscopy methods can provide