We report the electronic structure and optical properties of the recently synthesized stable two-dimensional carbon allotrope graphdiyne based on first-principles calculations and experimental optical spectrum. Due to the enhanced Coulomb interaction in reduced dimensionality, the band gap of graphdiyne increases to 1.10 eV within the $GW$ many-body theory from a value of 0.44 eV within the density functional theory. The optical absorption is dominated by excitonic effects with a remarkable electron-hole binding energy of over 0.55 eV within the $GW$–Bethe-Salpeter equation calculation. Experimental optical absorption of graphdiyne films is performed, and comparison with the theoretical calculations is analyzed in detail.

A large bulk band gap is critical for the application of quantum spin Hall (QSH) insulators or two-dimensional (2D) topological insulators (TIs) in spintronic devices operating at room temperature (RT). On the basis of first-principles calculations, we predicted a group of 2D TI BiX/SbX (X=H, F, Cl and Br) monolayers with extraordinarily large bulk gaps from 0.32 eV to a record value of 1.08 eV. These giant-gaps are entirely due to the result of the strong spin-orbit interaction related to the p_x and p_y orbitals of the Bi/Sb atoms around the two valleys K and K′ of the honeycomb lattice, which is significantly different from that consisting of the p_z orbital as in graphene/silicene. The topological characteristic of BiX/SbX monolayers is confirmed by the calculated nontrivial Z₂ index and an explicit construction of the low-energy effective Hamiltonian in these systems. We demonstrate that the honeycomb structures of BiX monolayers remain stable even at 600 K. Owing to these features, the giant-gap TIs BiX/SbX monolayers are an ideal platform to realize many exotic phenomena and fabricate new quantum devices operating at RT. Furthermore, biased BiX/SbX monolayers become a quantum valley Hall insulator, exhibiting valley-selective circular dichroism.

Although many prototype devices based on two-dimensional (2D) MoS₂ have been fabricated and wafer scale growth of 2D MoS₂ has been realized, the fundamental nature of 2D MoS₂-metal contacts has not been well understood yet. We provide a comprehensive ab initio study of the interfacial properties of a series of monolayer (ML) and bilayer (BL) MoS₂-metal contacts (metal = Sc, Ti, Ag, Pt, Ni, and Au). A comparison between the calculated and observed Schottky barrier heights (SBHs) suggests that many-electron effects are strongly suppressed in channel 2D MoS₂ due to a charge transfer. The extensively adopted energy band calculation scheme fails to reproduce the observed SBHs in 2D MoS₂-Sc interface. By contrast, an ab initio quantum transport device simulation better reproduces the observed SBH in 2D MoS₂-Sc interface and highlights the importance of a higher level theoretical approach beyond the energy band calculation in the interface study. BL MoS₂-metal contacts generally have a reduced SBH than ML MoS₂-metal contacts due to the interlayer coupling and thus have a higher electron injection efficiency.

Opening a sizable band gap without degrading its high carrier mobility is as vital for silicene as for graphene to its application as a high-performance field effect transistor (FET). Our density functional theory calculations predict that a band gap is opened in silicene by single-side adsorption of alkali atom as a result of sublattice or bond symmetry breaking. The band gap size is controllable by changing the adsorption coverage, with an impressive maximum band gap up to 0.50 eV. The ab initio quantum transport simulation of a bottom-gated FET based on a sodium-covered silicene reveals a transport gap, which is consistent with the band gap, and the resulting on/off current ratio is up to 10⁸. Therefore, a way is paved for silicene as the channel of a high-performance FET.

Monolayer Bi ₂ O ₂ Se is a promising post-silicon-era semiconductor candidate because of its simultaneous excellent device performance and high ambient stability.