Phonon interaction with ripples and defects in thin layered molybdenum disulfide

Compared to other extrinsic phonon scattering mechanisms such as surface and interior defects, phonon scattering and lattice thermal resistance due to structural rippling in few-layer two-dimensional (2D) materials are under-examined. Here, the temperature-dependent basal-plane thermal conductivities (κ) of one rippled and four flat molybdenum disulfide (MoS₂) samples are measured using a four-probe thermal transport measurement method. A flat 18 nm thick sample and a rippled 20 nm thick sample show similar peak κ values of 122 ± 17 and 129 ± 19 W m^-1 K^-1, respectively. In comparison, a 32 nm thick flat sample has a peak κ value of only 58 ± 11 W m^-1 K^-1 despite having an increased thickness. The peak thermal conductivities of the five samples decrease with increasing integrated Raman intensity caused by defects in the frequency range of the phonon bandgap in MoS₂. In conjunction with the experimental findings, theoretical calculations of the temperature-, thickness-, strain-, and defect-dependent κ of thin MoS₂ layers reveal the importance of interior defect scattering over scattering from compression-induced ripples and surface defects in these samples. The results further clarify the conditions where ripples are important in determining the basal plane thermal resistance in layered systems.