Quantitative Characterization of the Anisotropic Thermal Properties of Encapsulated Two-Dimensional MoS₂ Nanofilms

Two-dimensional (2D) semiconductors exhibit unique physical properties at the limit of a few atomic layers that are desirable for optoelectronic, spintronic, and electronic applications. Some of these materials require ambient encapsulation to preserve their properties from environmental degradation. While encapsulating 2D semiconductors is essential to device functionality, they also impact heat management due to the reduced thermal conductivity of the 2D material. There are limited experimental reports on in-plane thermal conductivity measurements in encapsulated 2D semiconductors. These measurements are particularly challenging in ultrathin films with a lower thermal conductivity than graphene since it may be difficult to separate the thermal effects of the sample from the encapsulating layers. To address this challenge, we integrated the frequency domain thermoreflectance (FDTR) and optothermal Raman spectroscopy (OTRS) techniques in the same experimental platform. First, we use the FDTR technique to characterize the cross-plane thermal conductivity and thermal boundary conductance. Next, we measure the in-plane thermal conductivity by model-based analysis of the OTRS measurements, using the cross-plane properties obtained from the FDTR measurements as input parameters. We provide experimental data for the first time on the thickness-dependent in-plane thermal conductivity of ultrathin MoS₂ nanofilms encapsulated by alumina (Al₂O₃) and silica (SiO₂) thin films. The measured thermal conductivity increased from 26.0 ± 10.0 W m^-1 K^-1 for monolayer MoS₂ to 39.8 ± 10.8 W m^-1 K^-1 for the six-layer films. We also show that the thickness-dependent cross-plane thermal boundary conductance of the Al₂O₃/MoS₂/SiO₂ interface is limited by the low thermal conductance (18.5 MW m^-2 K^-1) of the MoS₂/SiO₂ interface, which has important implications on heat management in SiO₂-supported and encased MoS₂ devices. The measurement methods can be generalized to other 2D materials to study their anisotropic thermal properties.