418
22 Layered Structures
22.2.1 Phonon Frequency Tunability
A huge database has been established toward the phonon spectroscopy/microscopy of
the layered 2-D structures and their nanoribbons subjecting to various stimulation.
The stimuli include the number-of-layer (n) [38–42], uniaxial strain (ε) [43–45],
mechanical compression (P) [46], thermal excitation (T) [47, 48], defect density and
location [49], substrate interaction [50, 51], hydrogenation or passivation [52, 53],
impurity or doping [54], incident photon polarization [55], edge conditioning [56,
57], etc.
Graphene shows two Raman active modes as do its allotropes. As the first-order
approximation, the G band (~1580 cm
−1 ) was suggested to arise from the in-plane
vibration of the sp
2 bonding network [58]. The 2D band (2680 cm
−1 ) was thought
to be a second-order process of double resonant scattering [59]. In the presence
of undercoordinated defect or edge atoms, the defect-induced D band appears at
frequencies around 1345 cm
−1 with intensity being varied with edge conditions
compared with the D band of bulk diamond featured at 1331 cm
−1 [56, 57]. The
monolayer BP shows the A
1
g , A
2
g and B 2g and the MX 2 shows the A 1g and E
1
2g Raman
active modes.
The phonon behavior of the layered BP, GNR, and MX 2 [17, 60, 61] follows the
same trend but by different amounts under the same excitation. The D/2D bands of
graphene [40, 62] and the A 1g mode of the MoS 2 and the WS 2 [35] undergo a redshift
when the number-of-layer is reduced. For instance, under 514.5 nm light radiation,
the D mode shifts from 1367 to 1344 cm
−1 and the 2D mode changes from 2720
to 2680 cm
−1 when the bulk graphite evolves into the monolayer graphene [41]. In
contrast, a blueshift happens to the G mode that shifts from 1582 to 1587 cm
−1 when
the n is reduced from twenty to one [40, 41]. The B 2g and A 2g modes for BP [63,
64] and the E
1
2g mode for MoS 2 and WS 2 [35] undergo a blueshift as well when
their number-of-layer is reduced. When the n is increased from a few to multiple, the
Raman peaks turn from the dominance of the monolayer component to the dominance
of the bulk component [40]. It is even amazing that the spectral intensity of the D
band is one order higher at the armchair edge than it is at the zigzag edge of the GNR
[56, 57].
Introducing the uniaxial strain to the 2-D substances by bending the layered structures on the elastic substrates can lower the work function to improve their carrier
mobilities and raise their photoelectron emissivity [25, 26, 65]. The uniaxial tensile
strain softens and splits the phonon bands of graphene [43] and MX 2 and narrows
the bandgap of MX 2 [66]. The extent of the phonon band splitting depends on the
strain and its relative direction between the strain and a specific bond of the layered
structures [45, 67]. Under compressive strain, the Raman shift extrapolates the trend
of tension in an opposite direction [68].
Mechanical compression depresses the V-P profile but raises all the phonon frequencies [46, 69–71]. Contrastingly, heating softens the Raman phonon to lower frequencies [72]. Heating from ultra-low to a temperature well-above room-temperature
softens the A
1
g , A
2
g and B 2g modes of BP [70, 71] and the A 1g and E
1
2g phonons of
22 Layered Structures
22.2.1 Phonon Frequency Tunability
A huge database has been established toward the phonon spectroscopy/microscopy of
the layered 2-D structures and their nanoribbons subjecting to various stimulation.
The stimuli include the number-of-layer (n) [38–42], uniaxial strain (ε) [43–45],
mechanical compression (P) [46], thermal excitation (T) [47, 48], defect density and
location [49], substrate interaction [50, 51], hydrogenation or passivation [52, 53],
impurity or doping [54], incident photon polarization [55], edge conditioning [56,
57], etc.
Graphene shows two Raman active modes as do its allotropes. As the first-order
approximation, the G band (~1580 cm
−1 ) was suggested to arise from the in-plane
vibration of the sp
2 bonding network [58]. The 2D band (2680 cm
−1 ) was thought
to be a second-order process of double resonant scattering [59]. In the presence
of undercoordinated defect or edge atoms, the defect-induced D band appears at
frequencies around 1345 cm
−1 with intensity being varied with edge conditions
compared with the D band of bulk diamond featured at 1331 cm
−1 [56, 57]. The
monolayer BP shows the A
1
g , A
2
g and B 2g and the MX 2 shows the A 1g and E
1
2g Raman
active modes.
The phonon behavior of the layered BP, GNR, and MX 2 [17, 60, 61] follows the
same trend but by different amounts under the same excitation. The D/2D bands of
graphene [40, 62] and the A 1g mode of the MoS 2 and the WS 2 [35] undergo a redshift
when the number-of-layer is reduced. For instance, under 514.5 nm light radiation,
the D mode shifts from 1367 to 1344 cm
−1 and the 2D mode changes from 2720
to 2680 cm
−1 when the bulk graphite evolves into the monolayer graphene [41]. In
contrast, a blueshift happens to the G mode that shifts from 1582 to 1587 cm
−1 when
the n is reduced from twenty to one [40, 41]. The B 2g and A 2g modes for BP [63,
64] and the E
1
2g mode for MoS 2 and WS 2 [35] undergo a blueshift as well when
their number-of-layer is reduced. When the n is increased from a few to multiple, the
Raman peaks turn from the dominance of the monolayer component to the dominance
of the bulk component [40]. It is even amazing that the spectral intensity of the D
band is one order higher at the armchair edge than it is at the zigzag edge of the GNR
[56, 57].
Introducing the uniaxial strain to the 2-D substances by bending the layered structures on the elastic substrates can lower the work function to improve their carrier
mobilities and raise their photoelectron emissivity [25, 26, 65]. The uniaxial tensile
strain softens and splits the phonon bands of graphene [43] and MX 2 and narrows
the bandgap of MX 2 [66]. The extent of the phonon band splitting depends on the
strain and its relative direction between the strain and a specific bond of the layered
structures [45, 67]. Under compressive strain, the Raman shift extrapolates the trend
of tension in an opposite direction [68].
Mechanical compression depresses the V-P profile but raises all the phonon frequencies [46, 69–71]. Contrastingly, heating softens the Raman phonon to lower frequencies [72]. Heating from ultra-low to a temperature well-above room-temperature
softens the A
1
g , A
2
g and B 2g modes of BP [70, 71] and the A 1g and E
1
2g phonons of