160
5 Optical Measurement Techniques
Fig. 5.13 All-optical identification of monolayer WSe 2 with the help of optical contrast analysis
of micrographs and micro-Raman line scans. The measurements are further detailed in [7]. The
microscope images for the measured WSe 2 monolayers deposited on different substrates a–e are
shown together with brightness cross sections (along yellow line, shown in the central row). a, b,
and d Monolayer flake (mechanically exfoliated and) transferred onto the bare substrate material
(labelled), c as-grown monolayers on sapphire (by CVD), e hBN-buffered monolayer on substrate
using the dry-stamping technique for 2D stacking. The last row shows extracted Raman peak
positions obtained along the yellow line by a line scan. Data points at zero in the graphs correspond
to locations, for which no WSe 2 peak was obtained. All monolayers can be identified by the
wave-number below 252 µm −1 . 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
Raman spectra can also be useful for the characterisation of bilayers. No change in
the energetic position is expected for the A
1
g and E
1
1g when comparing both stacking
configurations according to previous density-functional theory (DFT) calculations
and experimental studies on other TMDC-bilayer stacks [103, 104]. However, shear
and breathing modes and out-of-plane modes at the M point [105, 106] are expected
to change. The shifts in these modes that appear at low wave-numbers and the twophonon processes that give rise to the shoulders of the in-plane Raman modes can
be attributed to the predicted change in the interlayer distance and interaction [103].
It was indicated by the Raman measurements in [8] that the stacking type
influences the Raman intensities and energies enough to assume distinguishability between AA’ and AB bilayers, particularly when taking a resonant Raman effect
into account. As the Raman laser can be nearly in resonance with the B exciton,
an enhanced electron–phonon coupling with strong Raman signal is observed as
previously reported [107]. Relative red-shifts of the B exciton based on different
stacking configurations can then cause strong intensity differences of a particular
Raman mode due to a detuning of the resonance Raman effect.
5 Optical Measurement Techniques
Fig. 5.13 All-optical identification of monolayer WSe 2 with the help of optical contrast analysis
of micrographs and micro-Raman line scans. The measurements are further detailed in [7]. The
microscope images for the measured WSe 2 monolayers deposited on different substrates a–e are
shown together with brightness cross sections (along yellow line, shown in the central row). a, b,
and d Monolayer flake (mechanically exfoliated and) transferred onto the bare substrate material
(labelled), c as-grown monolayers on sapphire (by CVD), e hBN-buffered monolayer on substrate
using the dry-stamping technique for 2D stacking. The last row shows extracted Raman peak
positions obtained along the yellow line by a line scan. Data points at zero in the graphs correspond
to locations, for which no WSe 2 peak was obtained. All monolayers can be identified by the
wave-number below 252 µm −1 . 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
Raman spectra can also be useful for the characterisation of bilayers. No change in
the energetic position is expected for the A
1
g and E
1
1g when comparing both stacking
configurations according to previous density-functional theory (DFT) calculations
and experimental studies on other TMDC-bilayer stacks [103, 104]. However, shear
and breathing modes and out-of-plane modes at the M point [105, 106] are expected
to change. The shifts in these modes that appear at low wave-numbers and the twophonon processes that give rise to the shoulders of the in-plane Raman modes can
be attributed to the predicted change in the interlayer distance and interaction [103].
It was indicated by the Raman measurements in [8] that the stacking type
influences the Raman intensities and energies enough to assume distinguishability between AA’ and AB bilayers, particularly when taking a resonant Raman effect
into account. As the Raman laser can be nearly in resonance with the B exciton,
an enhanced electron–phonon coupling with strong Raman signal is observed as
previously reported [107]. Relative red-shifts of the B exciton based on different
stacking configurations can then cause strong intensity differences of a particular
Raman mode due to a detuning of the resonance Raman effect.