Two-dimensional transition metal dichalcogenides (TMDs) are promising low-dimensional materials which can produce diverse electronic properties and band alignment in van der Waals heterostructures. Systematic density functional theory (DFT) calculations are performed for 24 different TMD monolayers and their bilayer heterostacks. DFT calculations show that monolayer TMDs can behave as semiconducting, metallic or semimetallic depending on their structures; we also calculated the band alignment of the TMDs to predict their alignment in van der Waals heterostacks. We have applied the charge equilibration model (CEM) to obtain a quantitative formula predicting the highest occupied state of any type of bilayer TMD heterostacks (552 pairs for 24 TMDs). The CEM predicted values agree quite well with the selected DFT simulation results. The quantitative prediction of the band alignment in the TMD heterostructures can provide an insightful guidance to the development of TMD-based devices.

Surface engineering of transition metal layered double hydroxides (LDHs) provides an efficient way of enhancing their catalytic activity toward the oxygen evolution reaction (OER). However, the underlying mechanism of atomistic doping or heterogeneous interface with foreign atom is still ambiguous. Herein, a case study of NiFe‐LDHs that are homogeneously doped with Ce (CeNiFe‐LDH) and interfaced with Ce(OH)3 (Ce@NiFe‐LDH), which elucidates their electronic modulation, in situ evolution of active site, and catalytic reaction mechanisms by using X‐ray photoelectronic spectroscopy, operando electrochemical Raman spectroscopy, and first‐principles density functional theory (DFT) calculations, is reported. The results indicate that Ce and Fe atoms serve as the electron acceptors and facilitate the coupled oxidation of Ni3+/4+ in NiFe‐LDH, and the activated oxyhydroxide phase of the catalysts exhibits superior catalytic activity for water oxidation. Especially, Ce@NiFe‐LDH shows a stronger electron transfer between the loaded Ce(OH)3 and the matrix, which leads to a better catalytic activity than CeNiFe‐LDH. DFT calculations provide a clear picture with atomistic resolution for charge redistribution in the NiFe‐LDH surface induced by Ce, which eventually leads to the optimal free energy landscape for the enhanced OER catalytic activity.

We investigated the surface potential (V surf ) of exfoliated MoS 2 flakes on bare and Au-coated SiO 2 /Si substrates using Kelvin probe force microscopy. The V surf of MoS 2 single layers was larger on the Au-coated substrates than on the bare substrates; our theoretical calculations indicate that this may be caused by the formation of a larger electric dipole at the MoS 2 / Au interface leading to a modified band alignment. V surf decreased as the thickness of the flakes increased until reaching the bulk value at a thickness of ∼20 nm (∼30 layers) on the bare and ∼80 nm (∼120 layers) on the Au-coated substrates, respectively. This thickness dependence of V surf was attributed to electrostatic screening in the MoS 2 layers. Thus, a difference in the thickness at which the bulk V surf appeared suggests that the underlying substrate has an effect on the electric-field screening length of the MoS 2 flakes. This work provides important insights to help understand and control the electrical properties of metal/MoS 2 contacts.

Semiconductor epitaxy on two-dimensional materials is beneficial for transferrable and flexible device applications. Graphene, due to the absence of permanent electric dipoles, cannot screen the electric field coming from the opposite side surface, allowing remote epitaxy for heteroepitaxy. This study demonstrates remote heteroepitaxy of ZnO microrods (MRs) on the GaN substrate across graphene layers via hydrothermal growth. Even the use of tri-layer graphene yields the remote heteroepitaxial MR arrays. Transmission electron microscopy reveals the remote heteroepitaxial relation between ZnO MRs and the GaN substrate despite the existence of graphene interlayers in between them. Density-functional theory calculations show that charge transfer along the z-direction at graphene/c-GaN possibly attract adatoms leading to remote heteroepitaxy, implying the field permeability of graphene. The ability of graphene to be released from the host substrate is exploited to exfoliate the overlayer MRs and regenerate the substrate.

Vertical and horizontal ZnO microrods are grown on a- and c-plane ZnO across graphene interlayer, owing to charge transfer through graphene, and the remote homoepitaxial microrods were exfoliated for substrate regeneration.