Thermally Driven (Mo, W)-(S2, Se2) Phonon and Photon Energy Relaxation Dynamics

It is fascinating that heating softens not only the photon bandgap energies but also the A(1g) and E-2g(1) phonon frequencies of the few-layered MX2 (M = Mo, W and X = S, Se) semiconductors, yet a common mechanism behind remains challenging. With the aid of Raman phonon scattering and photon absorption spectrometrics, we have been able to correlate and formulate these observations from the perspective of interatomic bond thermal relaxation. Reproduction of measurements clarified that the photon energy relaxation depends on the response of bond energy to thermal excitation, and the phonon frequency varies with the bond stiffness a function of length and energy. Besides, theoretical matching to experimental observations has derived the Debye temperatures of the specifically layered nanostructures and the atomic cohesive energy of their bulk counterparts as well. The electronegativity difference between the M and X in each case discriminates the rate of their bandgap energy and phonon frequency relaxation.