The frequencies of Raman active modes of few-layer ZrS3 and ZrSe3 and their dependence on the number of layers have been investigated by Raman scattering spectroscopy and first-principles calculations. We find a considerable downshift of the frequency of the Ag 3 mode with decreasing the number of layers, and a slight downshift of the Ag 5 mode. The same frequency shifts are reproduced by the calculations with remarkable accuracy. We explain the difference in the frequency shifts of the Ag 3 and Ag 5 modes in terms of intrachain, interchain, and interlayer force constants in simple one-dimensional lattice models. We thus show that the large shift of the Ag 3 mode reflects the quasi-one-dimensional structure of ZrS3 and ZrSe3, which does not exist in the more widely studied two-dimensional materials.

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Although single-layer transition-metal dichalcogenides with novel valley functionalities are promising candidate to realize valleytronic devices, the essential understanding of valley depolarization mechanisms is still incomplete. Based on pump-probe experiments performed...