Number-of-layer resolved (Mo, W)-(S2, Se2) phonon relaxation.

We unified the number-of-layer dependence of the A1g and the E12g phonon frequencies of the layered MX2 (M = Mo and W; X = S and Se) semiconductors with derivative of quantitative information on the bond relaxation dynamics and clarification of their physical origins. Theoretical reproduction of observations confirms that atomic undercoordination stems the unusual behavior of phonons of the MX2. The difference in electronegativity and ionic radius between the constituent elements resolves the bond nature index m and the relaxation extents of phonon frequencies. It is also clarified that the dimer interaction dictates the blue shift of the E12g phonon but the collective interaction of an atom with its z-neighbors governs the red shift of the A1g phonon dynamics as the number-of-layer reduces. Copyright © 2017 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]