2.2 Fundamentals of 2D Materials
23
42
Mo
1
H
3
Li
2
He
4
Be
11
Na
12
Mg
19
K
20
Ca
37
Rb
38
Sr
55
Cs
56
Ba
87
Fr
88
Ra
21
Sc
22
Ti
23
V
24
Cr
25
Mg
26
Fe
27
Co
28
Ni
29
Cu
30
Zn
39
Sc
40
Zr
41
Nb
43
Tc
44
Ru
45
Rh
46
Pd
47
Ag
48
Cd
57-71
Ln
72
Hf
73
Ta
74
W
75
Re
76
Os
77
Ir
78
Pt
79
Au
80
Hg
89-103
An
104
Rf
105
Db
106
Sg
107
Bh
108
Hs
109
Mt
110
Ds
111
Rg
112
9
F
10
Ne
17
Cl
18
Ar
35
Br
36
Kr
53
I
54
Xe
85
At
86
Rn
117 118
7
N
8
O
15
P
16
S
33
As
34
Se
51
Sb
52
Te
83
Bi
84
Po
115 116
5
B
6
C
13
Al
14
Si
31
Ga
32
Ge
49
In
50
Sn
81
Tl
82
Pb
113 114
.
I
.
II
.
VII
.
VIII
.
V
.
VI
.
III
.
IV
.
Transition metals
1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0
0.0
0.1
0.2
0.3
0.4
0.5
MoS 2
MoSe 2
S 2
WSe 2
W
Absorbance (a.u.)
Energy (eV)
A
B
C
b
a
Fig. 2.4 Pool of TMDC materials, with the elements in the periodic table (a) highlighted after [114],
from which numerous alloys and heterostructures can be produced through synthesis and stacking,
respectively. b Typical absorption signatures from the four prominent TMDCs (drawn freely after
[115], individual spectra off-set against each other arbitrarily for clarity). The two lower-energetic
peaks correspond to the A and B exciton of the respective material. The tungsten-based TMDCs
show sharper and more pronounced A excitons than their molybdenum-based counterparts, which
instead feature a smaller separation between A and B resonances that arise in TMDCs from a
considerable spin–orbit coupling
TMDCs, with formula MX 2 (M: transition metal, X: chalcogen, see Fig. 2.4a), are
among the most studied non-carbonic compounds. They are usually chemically, thermally and mechanically stable even in the monolayer regime, while some compounds
exhibit considerably faster degradation rates than others. The most prominent monolayer ones are MoS 2 , WS 2 , MoSe 2 and WSe 2 , with their similar and characteristic
spectral features shown in Fig. 2.4b.
Nevertheless, the else semimetal-like graphene has opened up new possibilities
in (twisted) bilayer configuration [8, 84], which enabled the formation of sub-bands
and controllable gaps [125–128], or even the observation of extraordinary behaviours
under “magic” twist angles [107–110].
The TMDC Family—Beyond Graphene
Known as popular 2D semiconductors, the members of the TMDC family exhibit a
wide range of different electrical [82, 129–133] and optical [3, 4, 30, 134] properties,
depending on the polytype of the TMDCs and the number of transition metal delectrons. In fact, TMDCs as bulk crystals had been already studied in the past century
(cf. [130, 135–137], and other layered semiconductors [138]). Even thinned-down
systems (so-to-say few-layer configurations) cleaved with ‘Scotch tape’ were under
investigation in the years after, at that time (see e.g. [139]). Changes in interlayer
coupling, quantum confinement, and symmetry breaking lead to dramatic differences
in the electronic structure and optical properties of single-layer TMDCs compared
with their bulk counterparts.
Given the Dirac cone in the band structure of monolayer graphene, charge carriers in graphene are considered relativistic particles, whereas in the semiconducting
counterparts they feature a finite effective mass. The mobility may be also quite
high in the plane of a 2D semiconductor, but the formation of bound electron–hole
pairs can dominate the optical and charge-transport properties of those. With record
23
42
Mo
1
H
3
Li
2
He
4
Be
11
Na
12
Mg
19
K
20
Ca
37
Rb
38
Sr
55
Cs
56
Ba
87
Fr
88
Ra
21
Sc
22
Ti
23
V
24
Cr
25
Mg
26
Fe
27
Co
28
Ni
29
Cu
30
Zn
39
Sc
40
Zr
41
Nb
43
Tc
44
Ru
45
Rh
46
Pd
47
Ag
48
Cd
57-71
Ln
72
Hf
73
Ta
74
W
75
Re
76
Os
77
Ir
78
Pt
79
Au
80
Hg
89-103
An
104
Rf
105
Db
106
Sg
107
Bh
108
Hs
109
Mt
110
Ds
111
Rg
112
9
F
10
Ne
17
Cl
18
Ar
35
Br
36
Kr
53
I
54
Xe
85
At
86
Rn
117 118
7
N
8
O
15
P
16
S
33
As
34
Se
51
Sb
52
Te
83
Bi
84
Po
115 116
5
B
6
C
13
Al
14
Si
31
Ga
32
Ge
49
In
50
Sn
81
Tl
82
Pb
113 114
.
I
.
II
.
VII
.
VIII
.
V
.
VI
.
III
.
IV
.
Transition metals
1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0
0.0
0.1
0.2
0.3
0.4
0.5
MoS 2
MoSe 2
S 2
WSe 2
W
Absorbance (a.u.)
Energy (eV)
A
B
C
b
a
Fig. 2.4 Pool of TMDC materials, with the elements in the periodic table (a) highlighted after [114],
from which numerous alloys and heterostructures can be produced through synthesis and stacking,
respectively. b Typical absorption signatures from the four prominent TMDCs (drawn freely after
[115], individual spectra off-set against each other arbitrarily for clarity). The two lower-energetic
peaks correspond to the A and B exciton of the respective material. The tungsten-based TMDCs
show sharper and more pronounced A excitons than their molybdenum-based counterparts, which
instead feature a smaller separation between A and B resonances that arise in TMDCs from a
considerable spin–orbit coupling
TMDCs, with formula MX 2 (M: transition metal, X: chalcogen, see Fig. 2.4a), are
among the most studied non-carbonic compounds. They are usually chemically, thermally and mechanically stable even in the monolayer regime, while some compounds
exhibit considerably faster degradation rates than others. The most prominent monolayer ones are MoS 2 , WS 2 , MoSe 2 and WSe 2 , with their similar and characteristic
spectral features shown in Fig. 2.4b.
Nevertheless, the else semimetal-like graphene has opened up new possibilities
in (twisted) bilayer configuration [8, 84], which enabled the formation of sub-bands
and controllable gaps [125–128], or even the observation of extraordinary behaviours
under “magic” twist angles [107–110].
The TMDC Family—Beyond Graphene
Known as popular 2D semiconductors, the members of the TMDC family exhibit a
wide range of different electrical [82, 129–133] and optical [3, 4, 30, 134] properties,
depending on the polytype of the TMDCs and the number of transition metal delectrons. In fact, TMDCs as bulk crystals had been already studied in the past century
(cf. [130, 135–137], and other layered semiconductors [138]). Even thinned-down
systems (so-to-say few-layer configurations) cleaved with ‘Scotch tape’ were under
investigation in the years after, at that time (see e.g. [139]). Changes in interlayer
coupling, quantum confinement, and symmetry breaking lead to dramatic differences
in the electronic structure and optical properties of single-layer TMDCs compared
with their bulk counterparts.
Given the Dirac cone in the band structure of monolayer graphene, charge carriers in graphene are considered relativistic particles, whereas in the semiconducting
counterparts they feature a finite effective mass. The mobility may be also quite
high in the plane of a 2D semiconductor, but the formation of bound electron–hole
pairs can dominate the optical and charge-transport properties of those. With record