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 mu m to 1.14 mu m (radius) and probing the local temperature rise using Raman spectroscopy. R is determined as 4.46-7.66 x 10(-8) K m(2) W-1, indicating excellent contact between MoS2 and c-Si. D is determined to be 1.18(-0.23)(+0.30), 1.07(-0.26)(+0.37), 1.20(-0.27)(+0.34) and 1.62(-0.23)(+0.30) cm(2) 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 LD is determined as 0.344(-0.036)(+0.041) (3.6 nm), 0.327(-0.043)(+0.052) (5.4 nm), 0.346(-0.042)(+0.046) (8.4 nm), and 0.402(-0.030)(+0.036) mu m (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.