Synthesis of transition metal dichalcogenides and their heterostructures

As compared to the heterostructures based on bulk materials, the heterostructures based on transition metal dichalcogenides (TMDCs) have the advantages of atomically sharp hetero-interface and freedom from lattice-match restriction, which make them promising for nanoscale electronic and photonic devices. In this paper, we systematically investigate the synthesis of TMDC heterostructures using two-step processes via chemical vapor deposition (CVD). We found that if a MoS₂ growth at 730 °C is followed by a WSe₂ growth at 875 °C, the selenium atoms in the precursor will replace the sulfur atoms in MoS₂ during the second step and form MoSe₂/WSe₂ lateral heterostructures. On the other hand, if WSe₂ growth is followed by MoS₂ growth, WSe₂/MoS₂ lateral heterostructures were formed and no ion-exchange was observed. This result indicates that in the two-step processes, low-temperature growth followed by high-temperature growth may result in ion-exchange in the first material, while high-temperature growth followed by low-temperature growth results in epitaxy growth along the edge without ion-exchange. In addition, we have investigated the synthesis of MoS₂ and WSe₂ on sapphire substrates. In contrast to MoS₂ grown on SiO₂/Si substrates where the domains are randomly oriented, the domain edges of MoS₂ grown on sapphire substrates are mostly aligned to the primary directions, which can significantly reduce the amount of grain boundaries when the domains are merged together to form continuous films. This finding will be critical to enable the fabrication of large-scale electronic devices with high-performance and uniform distributions across the wafers based on these materials.