Anisotropic
Electron–Phonon Coupling in Colloidal Layered TiS<sub>2</sub> Nanodiscs Observed via Coherent Acoustic Phonons
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Abstract
Atomically
thin layered transition metal dichalcogenides with highly anisotropic
structure exhibit strong anisotropy in various material properties.
Here, we report the anisotropic coupling between the interband optical
transition and coherent acoustic phonon excited by ultrashort optical
excitation in a colloidal solution of multilayered TiS<sub>2</sub> nanodiscs. The transient absorption signal from the diameter- and
thickness-controlled TiS<sub>2</sub> nanodiscs dispersed in solution
exhibited an oscillatory feature, which is attributed to the modulation
of the interband absorption peak by the intralayer breathing mode.
However, the signature of the interlayer acoustic phonon was not observed,
while it has been previously observed in noncolloidal exfoliated sheets
of MoS<sub>2</sub>. The dominance of the intralayer mode in modulating
the interband optical transition was supported by the density functional
theory (DFT) calculations of the optical absorption spectra of TiS<sub>2</sub>, which showed the stronger sensitivity of the interband absorption
peak in the visible region to the in-plane strain than to the out-of-plane
strain