Nonmonotonic thickness-dependence of in-plane thermal conductivity of few-layered MoS2: 2.4 to 37.8 nm

Abstract

Recent first-principles modeling reported a decrease of in-plane thermal conductivity (k) with increased thickness for few layered MoS₂, which results from the change in phonon dispersion and missing symmetry in the anharmonic atomic force constant. For other 2D materials, it has been well documented that a higher thickness could cause a higher in-plane k due to a lower density of surface disorder. However, the effect of thickness on the k of MoS₂ has not been systematically uncovered by experiments. In addition, from either experimental or theoretical approaches, the in-plane k value of tens-of-nm-thick MoS₂ is still missing, which makes the physics on the thickness-dependent k remain ambiguous. In this work, we measure the k of few-layered (FL) MoS₂ with thickness spanning a large range: 2.4 nm to 37.8 nm. A novel five energy transport state-resolved Raman (ET-Raman) method is developed for the measurement. For the first time, the critical effects of hot carrier diffusion, electron–hole recombination, and energy coupling with phonons are taken into consideration when determining the k of FL MoS₂. By eliminating the use of laser energy absorption data and Raman temperature calibration, unprecedented data confidence is achieved. A nonmonotonic thickness-dependent k trend is discovered. k decreases from 60.3 W m⁻¹ K⁻¹ (2.4 nm thick) to 31.0 W m⁻¹ K⁻¹ (9.2 nm thick), and then increases to 76.2 W m⁻¹ K⁻¹ (37.8 nm thick), which is close to the reported k of bulk MoS₂. This nonmonotonic behavior is analyzed in detail and attributed to the change of phonon dispersion for very thin MoS₂ and a reduced surface scattering effect for thicker samples.