Uniform deposition of high-k dielectrics on two-dimensional (2D) crystals is highly desirable for their use in future nano-electronic devices. Here, the surface coverage of the Al 2 O 3 films grown by atomic layer deposition (ALD) was investigated on mechanically-exfoliated MoS 2 , WS 2 , WSe 2 , and h-BN flakes for exploring the deposition kinetics of the Al 2 O 3 films on the 2D crystals. The film coverage followed a decreasing order of WSe 2 > WS 2 > MoS 2 > h-BN, which was mainly determined by the ALD temperature and adsorption energy (E ads ) of the ALD precursor (trimethyl-aluminum) during the initial ALD cycles. The obtained |E ads | values of the precursor on the 2D crystals corresponded well to a van der Waals physisorption energy of 0.05-0.26 eV. Furthermore, the magnitude of the extracted E ads values showed a strong dependence on the induced dipole polarizability of the 2D crystals. The obtained results demonstrate that the surface coverage of the ALD high-k dielectrics can be modulated by choosing the types of the 2D substrates, and could provide a pathway for the integration of high-k dielectrics in 2D crystal-based nano-electronic devices.

Vertical MoO 2 /MoS 2 core−shell structures were synthesized on an amorphous surface (SiO 2 ) by chemical vapor deposition at a high heating rate using a configuration in which the vapor phase was confined. The confined reaction configuration was achieved by partially covering the MoO 3 -containing boat with a substrate, which allowed rapid buildup of the partially reduced MoO 3−x crystals in an early stage (below 680 °C). Rapid temperature ramping to 780 °C enabled spontaneous transition of the reaction environment from sulfur-poor to sulfur-rich, which induced a sequential phase transition from MoO 3−x to intermediate MoO 2 and finally to MoO 2 /MoS 2 core−shell structures. The orthorhombic crystal structure of MoO 3−x contributed to the formation of vertical crystals on the amorphous substrate, whereas the nonvolatility of the subsequently formed MoO 2 enabled layer-by-layer sulfurization to form MoS 2 on the oxide surface with minimal resublimation loss of MoO 2 . By adjustment of the sulfurization temperature and time, excellent control over the thickness of the MoS 2 shell was achieved through the proposed synthesis method.

A low‐temperature one‐step growth method for few‐layer MoS2 using an atomic layer deposition scheme with MoCl5 and H2S precursors is systematically studied by introducing an ultrathin Al seed layer. First, to optimize the deposition conditions, the effects of the deposition (200–420 °C) and MoCl5 canister (100–160 °C) temperatures on the MoS2 growth behavior are investigated. On the SiO2 surface, increasing the deposition temperature reduces the growth rate while favoring more lateral growth. However, an increase in the MoCl5 sublimating temperature, which is beneficial to improve the film quality, sharply reduces the growth rate, probably owing to the pronounced self‐etching effect of MoCl5. To compensate for the reduced deposition rate while maintaining the MoS2 quality, an ultrathin Al seed layer (≈5 nm) is introduced, which promotes the surface adsorption of MoCl5 molecules at an early growth stage according to density functional theory calculations. Thus, a polycrystalline mono‐to‐bilayer MoS2 film with negligible amounts of residual contaminants (particularly Cl and Al) is successfully synthesized using the proposed Al seeding approach.