The role of mid-gap phonon modes in thermal transport of transition metal dichalcogenides

Abstract

We present a comprehensive theoretical study on thermal transport in monolayer transition metal dichalcogenides MX 2 (M: Mo, W; X: S, Se) with various sample sizes. An unusually high anharmonic scattering strength is found in MoSe 2 compared to the other three family members, which arises from its unique phonon band dispersion, specifically the midfrequency phonon branches associated with the vibrations of Se atoms of MoSe 2 . The midfrequency modes almost completely span the gap that exists between the high-frequency phonon branches and the acoustic ones, allowing the former to readily decay into the latter. The resultant high anharmonic scattering gives rise to a short mean free path which makes the room temperature in-plane thermal conductivity in MoSe 2 even lower than WSe 2 when the sample length is larger than 51.5 nm. With varying sample sizes, the ordering of thermal conductivity among the four materials changes as phonon transport transits from the ballistic to diffusive regime, driven by the competition between the phonon frequency spectrum range and the scattering strength. Our work provides a microscopic picture of phonon transport in TMDs and guidance to tailor their thermal conductivities for electronic and thermoelectric applications.

Reduced thermal conductivity of supported and encased monolayer and bilayer MoS2

“…Interestingly, most simulations find the 1L in-plane thermal conductivity to fall in two distinct ranges at 300 K (excluding studies with extremely high or low values), i.e. between 19 to 38 W m −1 K −1 on the low end [26,[33][34][35][36][37][38][39][40][41] and between 82 to 178 W m −1 K −1 on the high end [30,[42][43][44][45][46][47][48][49][50][51][52][53][54][55][56], with no calculations falling between the two (as summarized in supplementary figure S1 (available online at stacks.iop.org/2DM/8/011001/ mmedia)).…”

Evidence of Topological Phase Transition with Excellent Catalytic Activity in the AgCaAs Heusler Alloy: A First-Principles Investigation

“…From Figure 2a,d, we can observe a very small frequency band gap between the acoustic and optical branches, which may indicate that AgCaAs might have low thermal conductivity and high scattering-process rate. 62,63 The vibrations of the Ag (heaviest) and Ca (lightest) atoms in the AgCaAs compound influence the phonon modes in the acoustic and optical regions, respectively. Similar behaviors are reflected in the PhDOS of bulk and 2D AgCaAs.…”