The hot carrier diffusion coefficient of sub-10 nm virgin MoS₂: uncovered by non-contact optical probing

We report a novel approach for non-contact simultaneous determination of the hot carrier diffusion coefficient (D) and interface thermal resistance (R) of sub-10 nm virgin mechanically exfoliated MoS₂ nanosheets on c-Si. The effect of hot carrier diffusion in heat conduction by photon excitation, diffusion, and recombination is identified by varying the heating spot size from 0.294 μm to 1.14 μm (radius) and probing the local temperature rise using Raman spectroscopy. R is determined as 4.46-7.66 × 10^-8 K m² W^-1, indicating excellent contact between MoS₂ and c-Si. D is determined to be 1.18, 1.07, 1.20 and 1.62 cm² s^-1 for 3.6 nm, 5.4 nm, 8.4 nm, and 9.0 nm thick MoS₂ samples, showing little dependence on the thickness. The hot carrier diffusion length (L_D) can be determined without knowledge of the hot carrier's life-time. The four samples L_D is determined as 0.344 (3.6 nm), 0.327 (5.4 nm), 0.346 (8.4 nm), and 0.402 μm (9.0 nm). Unlike previous methods that are implemented by making electrical contact and applying an electric field for D measurement, our technique has the advantage of being truly non-contact and non-invasive, and is able to characterize the electron diffusion behavior of virgin 2D materials. Also it points out that hot carrier diffusion needs to be taken into serious consideration in Raman-based thermal property characterization of 2D materials, especially under very tightly focused laser heating whose spot size is comparable to the hot carrier diffusion length.