22.2 Orbital Hybridization and Structure Configuration
417
with two bonds to the sandwiched M atoms and two electron lone pairs exposing
outwardly to the terminals of the sandwiched main layer. The extent of the orbital
hybridization for the P, S and Se is less obvious than nitrogen and oxygen because
of the orbital order occupancy of the valance electrons but their bond configuration
does show the same trend [28].
The main layers of these materials are composed of (a) one, (b) two and (c) three
sublayers interacting with their next main layers through van der Waals interaction,
which makes the mechanical filtration of these layered structures possible. For the
MX 2 honeycomb-like structure, they can be viewed as a positively charged plane
of the transition-metal M
4+ cations sandwiched between two planes of negatively
charged chalcogen X
2− anions [29]. Each of the sandwiched M
4+ cation bonds to
its four neighboring X
2− anions. The MX 2 layer performs like a giant atom with the
positive M
4+ core and the negative X
2− shells. The MX 2 main layers interact one
another through the short-range X
2− :⇔:X
2− repulsion (the same as the O
2− :⇔:O
2−
super hydrogen-bond in basic solutions [30]) and the slightly longer M
4+ ~ X
2−
Coulomb attraction. Such a set of interactions stabilizes the layered structures.
Figure 22.2 illustrates the atomic dislocations in the Raman-active and the IRactive vibration modes [31–35]:
(1) The transverse optical (TO) mode and the longitudinal optical (LO) Raman
modes shift in frequency with the number-of-layer of the 2-D materials.
(2) The E 2g TO mode for the WX 2 and TiO 2 and the G mode for graphene undergo
blueshift while the A 1g LO mode and the D mode shift to lower frequency as
the feature size is reduced.
Furthermore, ribbons of these layered structures have zigzag- and armchair-edge.
Most importantly, bonds between undercoordinated atoms in the layered structures
are shorter and stronger and the bonds at edges are even shorter and stronger than
they are in the bulk trunks. The bond contraction and the associated core electron
quantum entrapment and valence electron polarization govern the performance of
these undercoordinated systems [36, 37].
Fig. 22.2 Atomic displacements of the four Raman-active modes (subscripted with g) and one
IR-active mode (E 1u ) in the unit cell of the bulk MoS 2 crystal as viewed along the [1000] direction.
Reprinted with copyright permission from [35]
417
with two bonds to the sandwiched M atoms and two electron lone pairs exposing
outwardly to the terminals of the sandwiched main layer. The extent of the orbital
hybridization for the P, S and Se is less obvious than nitrogen and oxygen because
of the orbital order occupancy of the valance electrons but their bond configuration
does show the same trend [28].
The main layers of these materials are composed of (a) one, (b) two and (c) three
sublayers interacting with their next main layers through van der Waals interaction,
which makes the mechanical filtration of these layered structures possible. For the
MX 2 honeycomb-like structure, they can be viewed as a positively charged plane
of the transition-metal M
4+ cations sandwiched between two planes of negatively
charged chalcogen X
2− anions [29]. Each of the sandwiched M
4+ cation bonds to
its four neighboring X
2− anions. The MX 2 layer performs like a giant atom with the
positive M
4+ core and the negative X
2− shells. The MX 2 main layers interact one
another through the short-range X
2− :⇔:X
2− repulsion (the same as the O
2− :⇔:O
2−
super hydrogen-bond in basic solutions [30]) and the slightly longer M
4+ ~ X
2−
Coulomb attraction. Such a set of interactions stabilizes the layered structures.
Figure 22.2 illustrates the atomic dislocations in the Raman-active and the IRactive vibration modes [31–35]:
(1) The transverse optical (TO) mode and the longitudinal optical (LO) Raman
modes shift in frequency with the number-of-layer of the 2-D materials.
(2) The E 2g TO mode for the WX 2 and TiO 2 and the G mode for graphene undergo
blueshift while the A 1g LO mode and the D mode shift to lower frequency as
the feature size is reduced.
Furthermore, ribbons of these layered structures have zigzag- and armchair-edge.
Most importantly, bonds between undercoordinated atoms in the layered structures
are shorter and stronger and the bonds at edges are even shorter and stronger than
they are in the bulk trunks. The bond contraction and the associated core electron
quantum entrapment and valence electron polarization govern the performance of
these undercoordinated systems [36, 37].
Fig. 22.2 Atomic displacements of the four Raman-active modes (subscripted with g) and one
IR-active mode (E 1u ) in the unit cell of the bulk MoS 2 crystal as viewed along the [1000] direction.
Reprinted with copyright permission from [35]