22.2 Orbital Hybridization and Structure Configuration
419
WS 2 and MoS 2 as well in the Debye thermal decay fashion [60, 73]. Heating from 0
to 300 K also reduces the excitonic transition energy of the WSe 2 in a similar manner
[74]. Both the bandgap and phonon frequency initially drop at low temperature slowly
and then linearly at high temperatures, being the form of the difference between a
constant and the Debye specific heat integration [75]. The effects of mechanical
compression and thermal excitation hold universal to both the layered and the bulk
materials.
Current understanding of the multifield effect on the phonon frequency shift of
the layered substance is focused on two perspectives. One is the continuum bond
oscillating dynamics correlating the phonon frequency evolution to the stimuli in a
plain function, like the Gibbs free energy. The other is Raman photon scattering and
phonon decaying dynamics with involvement of the electronic contribution and the
geometry of the Brillouin zone. The effect of T on the Raman shift is ascribed as
optical phonon decaying into multiple acoustic components [76–80] and the pressure
effect is described in terms of Grüneisen notion—the slope of the Raman shift with
respect to pressure, ∂ω/∂P or ∂(Lnω)/∂(LnP) [43]. A “two-phonon double-resonance
scattering” is suggested to govern the strain effect on phonon band splitting and
softening [67]. The number-of-layer effect on the Raman shift follows hypothetically
the inverse number-of-layer n
−γ with γ being a tunable parameter [31, 81].
The afore-mentioned empirical approaches could fit to the measurements independently despite multiple freely adjustable parameters involved. However, consistent insight into the “external stimulus—bond relaxation—phonon frequency–
macroscopic property” correlation and finding means for detection remain yet to
be explored. For instance, mechanisms govern the opposite trends of the phonon
frequency shifts under the number-of-layer reduction and the edge-discriminated
photon reflectivity is beyond the scope of available premises.
One may be concerned with what dictates the chemical and physical properties
of the layered structures. According to Pauling [82], the nature of the chemical
bond bridges the structure and property of a crystal and a molecule. Therefore, bond
formation and relaxation and the associated energetics, localization, entrapment,
and polarization of electrons mediate the macroscopic performance of substance
accordingly [28]. Compared with the coordination-resolved electron spectrometrics
[37] that probes the local bond length and energy and the electronic dynamics under
high vacuum, the Raman spectrometry features are very sensitive to various stimuli
in amiable manners, which offers rich information on the local bonding dynamics
under external excitation.
Advancing the phonon spectroscopy to spectrometrics aims at quantitative information that the spectroscopy can hardly offer. The phonon and electron spectrometrics complement each other providing comprehensive information on the performance of bonds and electrons of a system under perturbation. The key challenge is
how to correlate and formulate the spectral feature to the bonding identities and the
way of responding to excitation.
The aim of this section is to show that a combination of the BOLS-LBA [36, 75]
and the phonon spectrometrics could reconcile the (n, ε, T, P) effect on the Raman
419
WS 2 and MoS 2 as well in the Debye thermal decay fashion [60, 73]. Heating from 0
to 300 K also reduces the excitonic transition energy of the WSe 2 in a similar manner
[74]. Both the bandgap and phonon frequency initially drop at low temperature slowly
and then linearly at high temperatures, being the form of the difference between a
constant and the Debye specific heat integration [75]. The effects of mechanical
compression and thermal excitation hold universal to both the layered and the bulk
materials.
Current understanding of the multifield effect on the phonon frequency shift of
the layered substance is focused on two perspectives. One is the continuum bond
oscillating dynamics correlating the phonon frequency evolution to the stimuli in a
plain function, like the Gibbs free energy. The other is Raman photon scattering and
phonon decaying dynamics with involvement of the electronic contribution and the
geometry of the Brillouin zone. The effect of T on the Raman shift is ascribed as
optical phonon decaying into multiple acoustic components [76–80] and the pressure
effect is described in terms of Grüneisen notion—the slope of the Raman shift with
respect to pressure, ∂ω/∂P or ∂(Lnω)/∂(LnP) [43]. A “two-phonon double-resonance
scattering” is suggested to govern the strain effect on phonon band splitting and
softening [67]. The number-of-layer effect on the Raman shift follows hypothetically
the inverse number-of-layer n
−γ with γ being a tunable parameter [31, 81].
The afore-mentioned empirical approaches could fit to the measurements independently despite multiple freely adjustable parameters involved. However, consistent insight into the “external stimulus—bond relaxation—phonon frequency–
macroscopic property” correlation and finding means for detection remain yet to
be explored. For instance, mechanisms govern the opposite trends of the phonon
frequency shifts under the number-of-layer reduction and the edge-discriminated
photon reflectivity is beyond the scope of available premises.
One may be concerned with what dictates the chemical and physical properties
of the layered structures. According to Pauling [82], the nature of the chemical
bond bridges the structure and property of a crystal and a molecule. Therefore, bond
formation and relaxation and the associated energetics, localization, entrapment,
and polarization of electrons mediate the macroscopic performance of substance
accordingly [28]. Compared with the coordination-resolved electron spectrometrics
[37] that probes the local bond length and energy and the electronic dynamics under
high vacuum, the Raman spectrometry features are very sensitive to various stimuli
in amiable manners, which offers rich information on the local bonding dynamics
under external excitation.
Advancing the phonon spectroscopy to spectrometrics aims at quantitative information that the spectroscopy can hardly offer. The phonon and electron spectrometrics complement each other providing comprehensive information on the performance of bonds and electrons of a system under perturbation. The key challenge is
how to correlate and formulate the spectral feature to the bonding identities and the
way of responding to excitation.
The aim of this section is to show that a combination of the BOLS-LBA [36, 75]
and the phonon spectrometrics could reconcile the (n, ε, T, P) effect on the Raman