Session: 16-01-01: Government Agency Student Poster Competition

Paper Number: 150670

150670 - Distinguish Optical and Acoustic Phonon Temperatures of Supported 2d Materials by Nanosecond Time-Resolved Raman Scattering 

Two-dimensional (2D) nanomaterials have unique electronic, optical, and mechanical propertiesand Raman spectroscopy technique is widely used for measuring their thermal transport properties. In Raman-based thermophysical characterization of supported 2D materials, the sample heats up and undergoes Raman excitation when irradiated by photons. If the sample’s bandgap is smaller than the photon energy, it absorbs the laser energy, triggering a cascade of energy transfer among the material’s carriers, including hot carriers, optical phonons (OPs), and acoustic phonons (APs). Initially, electrons absorb the photon energy and move from the valence band to the conduction band, leaving holes in the valence band. These electrons quickly recombine with the holes (in the order of ~ns), transferring energy to the OPs through electron-phonon scattering. The energy then moves from OPs to APs, which have higher group velocity, heat capacity, and thermal conductivity, making them dominant in heat conduction. This creates a significant thermal non-equilibrium between OPs and APs. The 2D sample used in this study is MoS₂, with a bandgap of 1.69 eV, smaller than the laser photon energy of 2.33 eV. The described energy absorption and cascading mechanism thus applies. However, Raman-based techniques assume thermal equilibrium between OPs and APs, which can lead to deviations in measured thermal properties. Here, a new method, based on nanosecond time-resolved energy transport-state resolved (ET) Raman, has been developed to measure the OP-AP thermal non-equilibrium in supported 2D materials. This technique uses a continuous wave (CW) laser and an amplitude-modulated CW laser. The modulation frequency ensures a fixed laser-off time (t_off) of 500 ns, with variable laser-on times (t_on) varies from 20-100 ns. As ton approaches zero, the thermal response mainly reflects the OP-AP temperature difference. The temperature rise of OPs and APs follows ∆T_OP=∆T_AP+∆T_OA. Two supported MoS₂ samples (25 and 10 nm-thick) on a silicon substrate are used to obtain the OP-AP temperature difference. The Raman shift power coefficient (ψ=∂ω/∂P) is measured for CW (ψ_OP) and each amplitude-modulated case (ψ_ns). The ψns versus ton are fitted using ψ_ns=[1-τ ⁄ (t (1-exp⁡(-t ⁄ τ)))]⋅ψ_AP+ψ_OA, which has been obtained based on “interfacial energy transfer” (IET) fitting model. Therefore, three parameters, τ, ψ_AP, and ψ_OA can be obtained. The τ converts to the intrinsic interfacial thermal conductance (ITC) between MoS₂ and silicon, ranging from 0.199~1.46 MW·m^-2·K^-1, increasing with sample thickness. This ITC relies on the true AP temperature rise, avoiding the uncertainty associated with the OP temperature rise. The OP-AP thermal non-equilibrium effect is more significant for smaller laser spot sizes, with ψ_OA / ψ_AP exceeding 120% for the 25 nm-thick MoS₂ sample under a 0.439 μm radius laser, while it is 24% for the same sample under 1.78 μm radius laser. Ignoring this effect can lead to substantial errors in thermal property determination. The IET model isapplicable to a 20× objective lens (with a 1.78 mm laser spot radius), as in-plane heat conduction and hot carrier diffusion become significant with smaller laser spot sizes used in 50× and 100× objective lenses.

Presenting Author: Mahya Rahbar Iowa State University

Presenting Author Biography: A 2nd year PhD student at Iowa State University.

Authors:

Mahya Rahbar Iowa State University
Xinwei Wang Iowa State University