Growth of centimeter-scale atomically thin MoSfilms by pulsed laser deposition

Abstract

We are reporting the growth of single layer and few-layer MoSfilms on single crystal sapphire substrates using a pulsed-laser deposition technique. A pulsed KrF excimer laser (wavelength: 248 nm; pulse width: 25 ns) was used to ablate a polycrystalline MoStarget. The material thus ablated was deposited on a single crystal sapphire (0001) substrate kept at 700°C in an ambient vacuum of 10-6 Torr. Detailed characterization of the films was performed using atomic force microscopy (AFM), Raman spectroscopy, UV-Vis spectroscopy, and photoluminescence (PL) measurements. The ablation of the MoStarget by 50 laser pulses (energy density: 1.5 J/cm2) was found to result in the formation of a monolayer of MoSas shown by AFM results. In the Raman spectrum, Aand E1peaks were observed at 404.6 cm-1 and 384.5 cm-1 with a spacing of 20.1 cm-1, confirming the monolayer thickness of the film. The UV-Vis absorption spectrum exhibited two exciton absorption bands at 672 nm (1.85 eV) and 615 nm (2.02 eV), with an energy split of 0.17 eV, which is in excellent agreement with the theoretically predicted value of 0.15 eV. The monolayer MoSexhibited a PL peak at 1.85 eV confirming the direct nature of the band-gap. By varying the number of laser pulses, bi-layer, tri-layer, and few-layer MoSfilms were prepared. It was found that as the number of monolayers (n) in the MoSfilms increases, the spacing between the Aand E1Raman peaks (Δf) increases following an empirical relation, Δ f = 26. 45 - 15. 42 1 + 1. 44 n 0. 9 cm-1.