Distinct optical and acoustic phonon temperatures in nm-thick suspended WS2: Direct differentiating via acoustic phonon thermal field invariant

Abstract

Under highly focused photon irradiation, the cascading energy transport and resulting temperature difference between optical phonons (OPs) and acoustic phonons (APs) in 2D materials is one critical problem in Raman-based energy transport characterization. Despite reported theoretical and experimental work, this problem remains poorly solved and renders the measurement results invalid since Raman thermometry only measures OP temperature while heat conduction is sustained by APs. Here an AP thermal field invariant is constructed to rigorously distinguish OP and AP temperatures. The Raman-measured counterpart differs from this invariant and reflects the effect of OP-AP temperature difference. It is found that under 0.38 μm radius laser heating, the OP-AP temperature difference is 37.6% of AP temperature rise for a 22 nm suspended WS2, confirming the strong OP-AP thermal non-equilibrium. For the first time, the real thermal conductivity of 2D WS2 is measured based on AP temperature. Furthermore, the energy coupling factor between OPs and APs is determined to ∼1014 Wm−3K−1 under rigorous optical interference consideration. Our AP thermal invariant methodology can be generalized to Raman-based thermal conductivity measurement of suspended 2D materials of arbitrary geometry, and will enable high-level rational material design toward great performance.