13.2 Two-Dimensional Compound Semiconductors
395
Fig. 13.19 Absorption spectrum of WSe 2 monolayer (derived from reflectance spectrum). X A,B (X A,B ∗ ) denote the
ground state (excited states) of the A- and B-exciton, respectively. X A,T at the low energy side denotes a trion. X C
denotes a further transition discussed further in [1299]. Based on data from [1298]
(a)
(b)
Fig. 13.20 a Top: Optical image of MoS 2 crystal on a silicon substrate with 1.0 and 1.5 µm diameter holes. Bottom:
Photoluminescence intensity image of the same sample. The freely suspended parts of the single layer exhibit the
highest intensity. b Photoluminescence spectra (excited at 532 nm) detected from regions with monolayers and bilayers,
respectively. The arrows denote the positions of the (indirect) transitions for 2–6 layers. Adapted from [1291]
13.2.3 Optical Properties and Valley Polarization
The absorption spectrum of a WSe 2 monolayer is shown in Fig. 13.19 as reported in [1298]; various
excitonic transitions are labeled that are discussed further below. The A- and B-excitons are formed
from the upper and lower state of the spin-split valence band, respectively.
The direct-to-indirect transition of the band structure for monolayer and multiplayer TMDC can
be directly observed from the photoluminescence. For MoS 2 , the single layer exhibits a 2–3 orders of
magnitude higher luminescence intensity (Fig. 13.20a) than any multilayer sample since no phonons
are needed for the optical band-band transition. The direct transition is at about 1.9 eV; the indirect
band gap decreases monotonously and discretely from about 1.6 eV for a bilayer, via that of 3, 4, 5 or
6 layers (Fig. 13.20b) to that of bulk material (E g ≈ 1.3 eV) [1291].
The K- and K’-point pseudo-spin together with the spin-orbit interaction leads to spin-polarized
bands as shown schematically in Fig. 13.22a. If excited with polarized light, optical absorption occurs
between bands of the same spin orientation. In the absence of any inter-valley scattering, optical
395
Fig. 13.19 Absorption spectrum of WSe 2 monolayer (derived from reflectance spectrum). X A,B (X A,B ∗ ) denote the
ground state (excited states) of the A- and B-exciton, respectively. X A,T at the low energy side denotes a trion. X C
denotes a further transition discussed further in [1299]. Based on data from [1298]
(a)
(b)
Fig. 13.20 a Top: Optical image of MoS 2 crystal on a silicon substrate with 1.0 and 1.5 µm diameter holes. Bottom:
Photoluminescence intensity image of the same sample. The freely suspended parts of the single layer exhibit the
highest intensity. b Photoluminescence spectra (excited at 532 nm) detected from regions with monolayers and bilayers,
respectively. The arrows denote the positions of the (indirect) transitions for 2–6 layers. Adapted from [1291]
13.2.3 Optical Properties and Valley Polarization
The absorption spectrum of a WSe 2 monolayer is shown in Fig. 13.19 as reported in [1298]; various
excitonic transitions are labeled that are discussed further below. The A- and B-excitons are formed
from the upper and lower state of the spin-split valence band, respectively.
The direct-to-indirect transition of the band structure for monolayer and multiplayer TMDC can
be directly observed from the photoluminescence. For MoS 2 , the single layer exhibits a 2–3 orders of
magnitude higher luminescence intensity (Fig. 13.20a) than any multilayer sample since no phonons
are needed for the optical band-band transition. The direct transition is at about 1.9 eV; the indirect
band gap decreases monotonously and discretely from about 1.6 eV for a bilayer, via that of 3, 4, 5 or
6 layers (Fig. 13.20b) to that of bulk material (E g ≈ 1.3 eV) [1291].
The K- and K’-point pseudo-spin together with the spin-orbit interaction leads to spin-polarized
bands as shown schematically in Fig. 13.22a. If excited with polarized light, optical absorption occurs
between bands of the same spin orientation. In the absence of any inter-valley scattering, optical