Energy Transport State Resolved Raman for Probing Interface Energy Transport and Hot Carrier Diffusion in Few-Layered MoS₂

Abstract

Quantitative understanding of 2D atomic layer interface thermal resistance (R) based on Raman characterization is significantly hindered by unknown sample-to-sample optical properties variation, interface-induced optical interference, off-normal laser irradiation, and large thermal-Raman calibration uncertainties. In this work, we develop a novel energy transport state resolved Raman (ET-Raman) to resolve these critical issues, and also consider the hot carrier diffusion, which is crucial but has been rarely considered during interface energy transport study. In ET-Raman, by constructing two steady heat conduction states with different laser spot sizes, we differentiate the effect of R and hot carrier diffusion coefficient (D). By constructing an extreme state of zero/negligible heat conduction using a picosecond laser, we differentiate the effect of R and material's specific heat. In the end, we precisely determine R and D without need of laser absorption and temperature rise of the 2D atomic layer. Seven MoS 2 samples (6.6−17.4 nm) on c-Si are characterized using ET-Raman. Their D is measured in the order of 1.0 cm 2 /s, increasing against the MoS 2 thickness. This is attributed to the weaker in-plane electron−phonon interaction in thicker samples, enhanced screening of long-range disorder, and improved charge impurities mitigation. R is determined as 1.22−1.87 × 10 −7 K•m 2 /W, decreasing with the MoS 2 thickness. This is explained by the interface spacing variation due to thermal expansion mismatch between MoS 2 and Si, and increased stiffness of thicker MoS 2 . The local interface spacing is uncovered by comparing the theoretical Raman intensity and experimental data, and is correlated with the observed R variation.

Very fast hot carrier diffusion in unconstrained MoS2on a glass substrate: discovered by picosecond ET-Raman

“…[30][31][32] From another aspect, the mechanical properties of MoS 2 nanosheets can be strongly affected by the properties and morphology of the substrate. Unlike the polished c-Si substrate we used in our previous work, 33 MoS 2 prepared through mechanical exfoliation onto a glass substrate does not follow its nanoscale rough surface but instead is supported by the high points on the substrate. This surface roughness affects the energy coupling of MoS 2 nanosheets and the substrate; therefore, it may cause a local variance in the electrical and mechanical properties of a sample.…”

Nanostructured and Heterostructured 2D Materials for Thermoelectrics

“…[55] These accurate measurements of the bulk crystals provide a baseline for understanding thermal transport in 2D TMDCs. The thermal conductivities of single-and few-layer TMDCs have been reported by a few Raman-based [57][58][59][60][61][62][63] and micro-bridge-based [64][65][66] measurements, but the limited data scattered in a large range (between ～15 and ～100 W/m•K at RT for monolayer MoS2). Meanwhile, careful MD simulations, [68], [50] first-principles [67] and BTE [69] calculations agreed well on the in-plane thermal conductivities of 2D MoS2.…”

Thermal conductance between water and nm-thick WS2: extremely localized probing using nanosecond energy transport state-resolved Raman

“…41 They reported other works that successfully measured the ITR between thin layers of TMD materials and glass or Si substrate. [42][43][44] Raman spectroscopy based techniques have the advantage of being non-contact, non-invasive, and material-specific that leads to higher accuracy of measured parameters.…”

“…Hot carrier transfer inside TMD materials, such as WS2, is well-studied in our previous works. 42,48,52 Regarding the thermal transport in cross-plane direction of WS2 sample, it is assumed that temperature distribution in this direction is uniform. In the thickness direction, heat diffusion length ( L  ) under ns pulsed laser heating can be estimated as:…”