Single-crystal antimonene flakes are observed on sapphire substrates after the postgrowth annealing procedure of amorphous antimony (Sb) droplets prepared by using molecular beam epitaxy at room temperature. The large wetting angles of the antimonene flakes to the sapphire substrate suggest that an alternate substrate should be adopted to obtain a continuous antimonene film. By using a bilayer MoS/sapphire sample as the new substrate, a continuous and single-crystal antimonene film is obtained at a low growth temperature of 200 °C. The results are consistent with the theoretical prediction of the lower interface energy between antimonene and MoS. The different interface energies of antimonene between sapphire and MoS surfaces lead to the selective growth of antimonene only atop MoS surfaces on a prepatterned MoS/sapphire substrate. With similar sheet resistance to graphene, it is possible to use antimonene as the contact metal of 2D material devices. Compared with Au/Ti electrodes, a specific contact resistance reduction up to 3 orders of magnitude is observed by using the multilayer antimonene as the contact metal to MoS. The lower contact resistance, the lower growth temperature, and the preferential growth to other 2D materials have made antimonene a promising candidate as the contact metal for 2D material devices.

Bi-layer graphene is grown directly on sapphire substrates by using ethane as the precursor without the assistance of a metal catalyst. A growth model of graphene flake formation in the furnace, followed by a complete film growth is also proposed. Using the graphene/sapphire sample as the new substrate, scalable MoS 2 films with good layer number controllability can be grown directly on the substrate. After fabricating the MoS 2 /graphene hetero-structures into bottom-gate photo-transistors, a Dirac point shift is observed for the device under the light irradiation condition, which is attributed to the extraction of photo-excited electrons in the MoS 2 layer to the graphene channel. The photo-voltaic response observed for the photo-transistors may provide a potential application of the 2D material hetero-structure in thin-film solar cells.

Similar material characteristics are observed for the elemental 2D material of group-V antimonene prepared by both molecular beam epitaxy and thermal evaporation on MoS 2 surfaces. Group-IV germanene and stanene can also be prepared by using the same thermal evaporation technique. With the well stacked hetero-structures, lattice mismatch does not play an important role for van der Waals epitaxy between 2D materials. The observations of the elemental 2D materials instead of their conventional semiconductor structures on MoS 2 surface demonstrate the preferential 2D structure formation on 2D material surfaces. Compared with the higher growth temperature of germanene, room-temperature growth is observed for stanene. The contact resistance reduction and the integration with the standard metal lift-off procedure on MoS 2 surfaces has made stanene a promising candidate as the contact metal for 2D devices.