Morphological and structural evolutions of α-MoO3 single crystal belts towards MoS2/MoO2 heterostructures upon post-growth thermal vapor sulfurization

Abstract

We have studied thermal vapor sulfurization (TVS) of α-MoO3 single crystal belts that were grown by chemical vapor deposition and transferred onto sapphire substrates. Micro-Raman scattering spectroscopy reveals an onset of material conversion from α-MoO3 to MoS2/MoO2/α-MoO3 heterostructures at TTVS = 600 °C; the conversion increases with the increase in TTVS, accompanied by monotonically mode hardening of MoO2 phonons up to TTVS = 900 °C; and the α-MoO3 base materials completely disappeared when TTVS is increased to higher than 800 °C, giving rise to thermodynamic stable MoS2/MoO2 heterostructures at TTVS = 900 °C. Layered structures consisted of a network of linear structures with regular in-plane orientation at the bottom and densely packed crystallites on top have also been observed, which is attributable to the effect of the epiready c-plane sapphire substrate. Absorption spectroscopy measurements reveal huge blue shifts in the absorption edge of the MoS2/MoO2 heterostructures when the thickness of the MoO2 layer is reduced, which is attributed to optical interferences at the crystal surfaces.