5.2 Microscopy and Spectroscopy
137
Fig. 5.3 Sketch of a micro-spectroscopy apparatus. This schematic representation was chosen
by the author as a typical setup example owing to the fact that it displays his team member’s own
visual interpretation (with very minor editing remarks by the author himself) of the confocal (2 × 4 f
Fourier-space imaging capable) microscopy setup constructed by the author in the initial phase of
2D-materials research for his team. Reproduced under the terms of the CC-BY 3.0 Licence (http://
creativecommons.org/licenses/by/3.0/) from Lippert et al., 2D Mater., “Influence of the substrate
material on the optical properties of tungsten diselenide monolayers”, https://doi.org/10.1088/20531583/aa5b21 [7] Copyright 2017 IOP Publishing Ltd
In such setups, white light from a tungsten lamp or laser light can be used for
different characterisation aims. The respective light beam is focused onto the sample
using a microscope objective (MO) with magnifications typically below 60x and
numerical apertures (NA) as high as 0.7. Here, the working distance of the objective
can be crucial for the use with optical cryostats. Their optical-window thickness and
the sample-to-window distance pose strict limitations regarding the use of highlymagnifying high-NA optics. For optical control, an imaging CCD or CMOS camera
is typically used. For micrometer-precise spatial selection of the sample’s reflected
or emitted signal, an iris aperture or pinhole is placed in the first available focal plane
in the path of the collected beam, resembling the configuration of a conventional
confocal microscope [7]. With a fully-closeable iris, the confocal spatial selection
enables transmission of signal from a sample spot of roughly 1 µm.
One major advantage of imaging 2D flakes under the microscope reveals itself
when placing 2D semiconductors under laser illumination (with protection from
the laser light for the operator). Based on the direct-to-indirect band gap transition
around a layer number of two, the monolayers of common TMDCs reveal themselves
137
Fig. 5.3 Sketch of a micro-spectroscopy apparatus. This schematic representation was chosen
by the author as a typical setup example owing to the fact that it displays his team member’s own
visual interpretation (with very minor editing remarks by the author himself) of the confocal (2 × 4 f
Fourier-space imaging capable) microscopy setup constructed by the author in the initial phase of
2D-materials research for his team. Reproduced under the terms of the CC-BY 3.0 Licence (http://
creativecommons.org/licenses/by/3.0/) from Lippert et al., 2D Mater., “Influence of the substrate
material on the optical properties of tungsten diselenide monolayers”, https://doi.org/10.1088/20531583/aa5b21 [7] Copyright 2017 IOP Publishing Ltd
In such setups, white light from a tungsten lamp or laser light can be used for
different characterisation aims. The respective light beam is focused onto the sample
using a microscope objective (MO) with magnifications typically below 60x and
numerical apertures (NA) as high as 0.7. Here, the working distance of the objective
can be crucial for the use with optical cryostats. Their optical-window thickness and
the sample-to-window distance pose strict limitations regarding the use of highlymagnifying high-NA optics. For optical control, an imaging CCD or CMOS camera
is typically used. For micrometer-precise spatial selection of the sample’s reflected
or emitted signal, an iris aperture or pinhole is placed in the first available focal plane
in the path of the collected beam, resembling the configuration of a conventional
confocal microscope [7]. With a fully-closeable iris, the confocal spatial selection
enables transmission of signal from a sample spot of roughly 1 µm.
One major advantage of imaging 2D flakes under the microscope reveals itself
when placing 2D semiconductors under laser illumination (with protection from
the laser light for the operator). Based on the direct-to-indirect band gap transition
around a layer number of two, the monolayers of common TMDCs reveal themselves