Multifield-driven bond–phonon–photon performance of layered (Mo, W)–(S2, Se2).

We show the reconciliation of the strain ( ε )-, pressure ( P )-, and layer-number ( N )-dependence of the phonon frequency and photon energy shifts for MX 2 in terms of stimulated bond relaxation, with quantification of the coefficients of bond length and energy relaxation, the bond nature index, force constant, and cohesive energy density. We found that (i) atomic under-coordination shortens and stiffens the M–X bond, which widens the bandgap; (ii) the directional strain governs phonon-frequency splitting and narrowing; and (iii) homogenous mechanical compression induces the opposite effect of stretching. [ABSTRACT FROM AUTHOR]