13.2 Two-Dimensional Compound Semiconductors
391
Fig. 13.11 Crystal
structure of two 2D
materials of a hexagonal
BN-type and b MX 2 -type
transition metal
dichalcogenides. The unit
cell is indicated. Left:
Bird’s eye view on the
plane and right: side view.
The greyed area denotes an
elementary cell. Adapted
from [1255], reprinted with
permission, ©2014 RSC
Table 13.1 Properties of various two-dimensional (monolayer) semiconductors, in-plane nearest neighbor distance
d A−B , vertical distance of anions d X−X for MX 2 -type TMDC, band gap E g , exciton binding energy E b
X (exfoliated on
silica) and spin-splitting SOC in conduction and valence band
material
C
BN
MoS 2
MoSe 2
WS 2
WSe 2
d A−B (nm)
0.142
0.144
0.184
0.192
0.184
0.192
d X−X (nm)
0.317
0.334
0.314
0.334
E g (eV)
0
7.3
2.15
2.18
2.41
2.2
E b
X (eV)
2.2
0.31
0.5
0.32
0.5
SOC,CB
(meV)
0
-3
-21
29
36
SOC,VB
(meV)
0
148
184
430
466
13.2.1 Structural Properties
Various two-dimensional configurations are known, the two most prominent that are typical for BN
(1H, hexagonal) and for TMDC (2H, trigonal prismatic, ABA-stacking) respectively, are shown in
Fig. 13.11. A plane-view TEM of the 2H structure of MoS 2 is depicted in Fig. 13.12 [1275]. Other
configurations of the TMDC such as an ABC-like stacking (1T, distorted octahedral) are possible
[1276]. A variation are Janus TMDC of MXY type where the bottom and top layer consist of different
anions [1277]. Two-dimensional alloys can be formed by mixing cations [1279] or anions [1283] (or
both). As example, a STEM image of a Mo 0.47 W 0.53 S 2 monolayer is shown in Fig. 13.12b which shows
the distribution of W and Mo atoms.
Various defects have been investigated for compound 2D semiconductors. For the vacancies in BN
we refer to [1284]. Single sulphur vacancies in MoS 2 are investigated in [1278]. Various antisite defects
in MoS 2 have been identified [1280] (Fig. 13.13). Also extended defects such as dislocations [1275],
grain boundaries [1281] or rotational stacking faults [1282] have been observed.
The phonon spectra of graphene, BN- and BC 2 N-sheets are compared in Fig. 13.14. The spectrum
of BC 2 N is similar to the superposition of the C and BN spectra [1285]. The Raman peaks from MoS 2
show discrete peak energies for thickness of 1–5 or 6 monolayers and then merge closely to the bulk
391
Fig. 13.11 Crystal
structure of two 2D
materials of a hexagonal
BN-type and b MX 2 -type
transition metal
dichalcogenides. The unit
cell is indicated. Left:
Bird’s eye view on the
plane and right: side view.
The greyed area denotes an
elementary cell. Adapted
from [1255], reprinted with
permission, ©2014 RSC
Table 13.1 Properties of various two-dimensional (monolayer) semiconductors, in-plane nearest neighbor distance
d A−B , vertical distance of anions d X−X for MX 2 -type TMDC, band gap E g , exciton binding energy E b
X (exfoliated on
silica) and spin-splitting SOC in conduction and valence band
material
C
BN
MoS 2
MoSe 2
WS 2
WSe 2
d A−B (nm)
0.142
0.144
0.184
0.192
0.184
0.192
d X−X (nm)
0.317
0.334
0.314
0.334
E g (eV)
0
7.3
2.15
2.18
2.41
2.2
E b
X (eV)
2.2
0.31
0.5
0.32
0.5
SOC,CB
(meV)
0
-3
-21
29
36
SOC,VB
(meV)
0
148
184
430
466
13.2.1 Structural Properties
Various two-dimensional configurations are known, the two most prominent that are typical for BN
(1H, hexagonal) and for TMDC (2H, trigonal prismatic, ABA-stacking) respectively, are shown in
Fig. 13.11. A plane-view TEM of the 2H structure of MoS 2 is depicted in Fig. 13.12 [1275]. Other
configurations of the TMDC such as an ABC-like stacking (1T, distorted octahedral) are possible
[1276]. A variation are Janus TMDC of MXY type where the bottom and top layer consist of different
anions [1277]. Two-dimensional alloys can be formed by mixing cations [1279] or anions [1283] (or
both). As example, a STEM image of a Mo 0.47 W 0.53 S 2 monolayer is shown in Fig. 13.12b which shows
the distribution of W and Mo atoms.
Various defects have been investigated for compound 2D semiconductors. For the vacancies in BN
we refer to [1284]. Single sulphur vacancies in MoS 2 are investigated in [1278]. Various antisite defects
in MoS 2 have been identified [1280] (Fig. 13.13). Also extended defects such as dislocations [1275],
grain boundaries [1281] or rotational stacking faults [1282] have been observed.
The phonon spectra of graphene, BN- and BC 2 N-sheets are compared in Fig. 13.14. The spectrum
of BC 2 N is similar to the superposition of the C and BN spectra [1285]. The Raman peaks from MoS 2
show discrete peak energies for thickness of 1–5 or 6 monolayers and then merge closely to the bulk