Towards large area and continuous MoS2 atomic layers via vapor-phase growth: thermal vapor sulfurization

We report on the effects of substrate, starting material, and temperature on the growth of MoS2 atomic layers by thermal vapor sulfurization in a tube-furnace system. With Mo as the starting material, atomic layers of MoS2 flakes are obtained on sapphire substrates while a bell-shaped MoS2 layer, sandwiched by amorphous SiO2, is obtained on native-SiO2/Si substrates under the same sulfurization conditions. An anomalous thickness-dependent Raman shift (A(1g)) of the MoS2 atomic layers is observed in Mo-sulfurizations on sapphire substrates, which can be attributed to the competition between the effects of thickness and the surface/interface. Both effects vary with the sulfurizing temperatures for a certain initial Mo thickness. The anomalous frequency trend of A(1g) is missing when using MoO3 instead of Mo as the starting material. In this case, the lateral growth of MoS2 on sapphire is also largely improved. Furthermore, the area density of the resultant MoS2 atomic layers is significantly increased by increasing the deposition temperature of the starting MoO3 to 700 degrees C; the adjacent ultrathin MoS2 grains coalesce in one or other direction, forming connected chains in wafer scale. The thickness of the so-obtained MoS2 is generally controlled by the thickness of the starting material; however, the structural and morphological properties of MoS2 grains, towards large area and continuous atomic layers, are strongly dependent on the temperature of the initial material deposition, and on the temperature of sulfurization, because of the competition between surface mobility and atom evaporation.