Measurement of the thermal conductivities of suspended MoS2 and MoSe2 by nanosecond ET-Raman without temperature calibration and laser absorption evaluation

Abstract

Steady state Raman spectroscopy is the most widely used opto-thermal technique for measuring a 2D atomic-layer material's thermal conductivity. It requires the calibration of temperature coefficients of Raman properties and measurement/calculation of the absolution laser absorption in 2D materials. Such a requirement is very laborious and introduces very large measurement errors (of the order of 100%) and hinders gaining a precise and deep understanding of phonon–structure interactions in 2D materials. In this work, a novel nanosecond energy transport state resolved Raman (ns ET-Raman) technique is developed to resolve these critical issues and achieve unprecedented measurement precision, accuracy and ease of implementation. In ns ET-Raman, two energy transport states are constructed: steady state and nanosecond thermal transport and Raman probing. The ratio of the temperature rise under the two states eliminates the need for Raman temperature calibration and laser absorption evaluation. Four suspended MoS₂ (45–115 nm thick) and four suspended MoSe₂ (45–140 nm thick) samples are measured and compared using ns ET-Raman. With the increase of the sample thickness, the measured thermal conductivity increases from 40.0 ± 2.2 to 74.3 ± 3.2 W m⁻¹ K⁻¹ for MoS₂, and from 11.1 ± 0.4 to 20.3 ± 0.9 W m⁻¹ K⁻¹ for MoSe₂. This is attributed to the decreased significance of surface phonon scattering in thicker samples. The ns ET-Raman features the most advanced capability to measure the thermal conductivity of 2D materials and will find broad applications in studying low-dimensional materials.