5.2 Microscopy and Spectroscopy
141
Fig. 5.6 The value of imaging techniques can be evidenced in another example, which combines
optical micrographs with PL as well as SHG mapping. These methods can be very helpful in
analysing vdW heterostructures, bilayers and monolayers. Here, the first row shows microscope
images for AA’ and AB stacked homobilayers (stacking configurations sketched in the insets) of WS 2
on bare substrates or hBN. The second row displays corresponding false-colour PL images recorded
in a micro-PL (μPL) setup. Since bilayers become indirect semiconductors, their luminescence yield
is drastically lower than the monolayer parts, which show spatial inhomogeneity due to quality
fluctuations. The third row shows raster-scan SHG micrographs, where the symmetry situation
determines the signal strength of the interface-sensitive SHG. The rotational symmetric AA’ bilayer
areas show no signal, whereas the symmetry-broken AB configuration shows increased signal for
the bilayer region. The white dashed lines indicate the edges of the WS 2 layers in the bilayer
structures and are guides to the eyes. Reproduced with permission. [8] Copyright 2019 American
Chemical Society
persive element’s refraction (prism) or diffraction (grating) strengths and the camera
chip’s responsivity, respectively.
Assuming a non-deformation regime under steady exposure with light and in
ambient environment, sample imaging with time integration well displays a specimen’s attributes such as size and orientation (cf. [6–8]). In video mode, even changes
of sample shape and position with time can be observed, whereas temporal resolution
depends on the frame-per-second rate recordable. Combined with pulsed illumination and stroboscopic detection, even fast processes such as membrane vibrations
can be made visible by imaging techniques.
Similarly, spectroscopic information can be gathered in the steady-state regime
under constant cw excitation, or time-averaged under repeated pulsed excitation.
These two cases represent prominent forms of luminescence acquisition from a sample. When exciting the sample optically, one measures PL, when exciting the sample
electrically, then electroluminescence (EL). Indeed, other luminescence types exist,
such as chemiluminscence, cathodoluminescence and so forth. Combining pulsed
141
Fig. 5.6 The value of imaging techniques can be evidenced in another example, which combines
optical micrographs with PL as well as SHG mapping. These methods can be very helpful in
analysing vdW heterostructures, bilayers and monolayers. Here, the first row shows microscope
images for AA’ and AB stacked homobilayers (stacking configurations sketched in the insets) of WS 2
on bare substrates or hBN. The second row displays corresponding false-colour PL images recorded
in a micro-PL (μPL) setup. Since bilayers become indirect semiconductors, their luminescence yield
is drastically lower than the monolayer parts, which show spatial inhomogeneity due to quality
fluctuations. The third row shows raster-scan SHG micrographs, where the symmetry situation
determines the signal strength of the interface-sensitive SHG. The rotational symmetric AA’ bilayer
areas show no signal, whereas the symmetry-broken AB configuration shows increased signal for
the bilayer region. The white dashed lines indicate the edges of the WS 2 layers in the bilayer
structures and are guides to the eyes. Reproduced with permission. [8] Copyright 2019 American
Chemical Society
persive element’s refraction (prism) or diffraction (grating) strengths and the camera
chip’s responsivity, respectively.
Assuming a non-deformation regime under steady exposure with light and in
ambient environment, sample imaging with time integration well displays a specimen’s attributes such as size and orientation (cf. [6–8]). In video mode, even changes
of sample shape and position with time can be observed, whereas temporal resolution
depends on the frame-per-second rate recordable. Combined with pulsed illumination and stroboscopic detection, even fast processes such as membrane vibrations
can be made visible by imaging techniques.
Similarly, spectroscopic information can be gathered in the steady-state regime
under constant cw excitation, or time-averaged under repeated pulsed excitation.
These two cases represent prominent forms of luminescence acquisition from a sample. When exciting the sample optically, one measures PL, when exciting the sample
electrically, then electroluminescence (EL). Indeed, other luminescence types exist,
such as chemiluminscence, cathodoluminescence and so forth. Combining pulsed