First-principles screening of single transition metal atoms anchored on two-dimensional C9N4 for the nitrogen reduction reaction.

Compared to the Haber-Bosch process, the electrochemical nitrogen reduction reaction (NRR) can convert N2 into NH3 under ambient conditions, and thus has attracted considerable attention in recent years. However, it remains a challenge to fabricate NRR catalysts with high faradaic efficiency and yield rate. In this work, by systematic first-principles calculations, we investigate the structure, stability and catalytic performance of single metal atoms anchored on porous monolayer C9N4 (M@C9N4) for the electrochemical NRR. A total of 25 transition metals (Sc-Zn, Zr-Mo, Ru-Ag, Hf-Au) were explored, and we screened out four promising systems, i.e., Nb, Ta, Re and W@C9N4, which not only exhibit high catalytic activity with low limiting potentials of -0.3, -0.42, -0.49 and -0.25 V, respectively, but also have superior selectivity that suppresses the competitive hydrogen evolution reaction. The physical origin lies in the coupling between the d orbitals of the transition metals and the 2π* orbital of N2, which activates the N2 molecule and facilitates the reduction process. Our proposed systems are kinetically and thermodynamically stable, which may shed light on future design and fabrication of high-efficiency single atom catalysts for various technologically important chemical reactions.

Topological band transition between hexagonal and triangular lattices with (px,py) orbitals.

By combining tight-binding modelling with density functional theory based first-principles calculations, we investigate the band evolution of two-dimensional (2D) hexagonal lattices with (px,py) orbitals, focusing on the electronic structures and topological phase transitions. The (px,py)-orbital hexagonal lattice model possesses two flat bands encompassing two linearly dispersive Dirac bands. Breaking the A/B sublattice symmetry could transform the model into two triangular lattices, each featuring a flat band and a dispersive band. Inclusion of the spin-orbit coupling and magnetization may give rise to quantum spin Hall and quantum anomalous Hall (QAH) states. As a proof of concept, we demonstrate that half-hydrogenated stanene is encoded by a triangular lattice with (px,py) orbitals, which exhibits ferromagnetism and QAH effect with a topological gap of ∼0.15 eV, feasible for experimental observation. These results provide insights into the structure-property relationships involving the orbital degree of freedom, which may shed light on future design and preparation of 2D topological materials for novel electronic/spintronic and quantum computing devices.

Intrinsic Valley Polarization and High-Temperature Ferroelectricity in Two-Dimensional Orthorhombic Lead Oxide.

Recent years have witnessed a surge of research in two-dimensional (2D) ferroelectric structures that may circumvent the depolarization effect in conventional perovskite oxide films. Herein, by first-principles calculations, we predict that an orthorhombic phase of lead(II) oxide, PbO, serves as a promising candidate for 2D ferroelectrics with good stability. With a semiconducting nature, 2D ferroelectric PbO exhibits intrinsic valley polarization, which leads to robust ferroelectricity with an in-plane spontaneous polarization of 2.4 × 10-10 C/m and a Curie temperature of 455 K. Remarkably, we reveal that the ferroelectricity is strain-tunable, and ferroelasticity coexists in the PbO film, implying the realization of 2D multiferroics. The underlying physical mechanism is generally applicable and can be extended to other oxide films such as ferroelectric SnO and GeO, thus paving an avenue for future design and fabrication of functional ultrathin devices that are compatible with Si-based technology.

Functionalization induced quantum spin Hall to quantum anomalous Hall phase transition in monolayer jacutingaite.

As novel states of quantum matter, quantum spin Hall (QSH) and quantum anomalous Hall (QAH) states have attracted considerable interest in condensed matter and materials science communities. Recently, a monolayer of the naturally occurring mineral jacutingaite (Pt2HgSe3), was theoretically proposed to be a large-gap QSH insulator and experimentally confirmed. Here, based on first-principles calculations and tight-binding modeling, we demonstrate QSH to QAH phase transition in jacutingaite by chemical functionalization with chalogen. We show that two-dimensional (2D) chalogenated jacutingaite, Pt2HgSe3-X (X = S, Se, Te), is ferromagnetic with Curie temperature up to 316 K, and it exhibits QAH effect with chiral edge states inside a sizeable topological gap. The physical mechanism lies in the adsorption induced transformation of the original Kane-Mele model into an effective four-band model involving (px, py) orbitals on a hexagonal lattice, so that the topological gap size can be controlled by spin-orbit coupling strength of the chalogen (0.28 eV for Pt2HgSe3-Te). These results not only show the promise of functionalization in orbital-engineering of 2D functional structures, but also provide an ideal and practical platform for achieving exotic topological phases for dissipationless transport and quantum computing.

Time-Domain Ab Initio Insights into the Reduced Nonradiative Electron-Hole Recombination in ReSe2/MoS2 van der Waals Heterostructure.

Two-dimensional (2D) ReSe2 has attracted considerable interest due to its unique anisotropic mechanical, optical, and exitonic characteristics. Recent transient absorption experiments demonstrated a prolonged lifetime of photoexcited charge carriers by stacking ReSe2 with MoS2, but the underlying mechanism remains elusive. Here, by combining time-domain density functional theory with nonadiabatic molecular dynamics, we investigate the electronic properties and charge carrier dynamics of 2D ReSe2/MoS2 van der Waals (vdW) heterostructure. ReSe2/MoS2 has a type II band alignment that exhibits spatially distinguished conduction and valence band edges, and a built-in electric field is formed due to interface charge transfer. Remarkably, in spite of the decreased band gap and increased decoherence time, we demonstrate that the photocarrier lifetime of ReSe2/MoS2 is ∼5 times longer than that of ReSe2, which originates from the greatly reduced nonadiabatic coupling that suppresses electron-hole recombination, perfectly explaining the experimental results. These findings not only provide physical insights into experiments but also shed light on future design and fabrication of functional optoelectronic devices based on 2D vdW heterostructures.

Large-Scale Synthesis of Strain-Tunable Semiconducting Antimonene on Copper Oxide.

Controlled synthesis of 2D structures on nonmetallic substrate is challenging, yet an attractive approach for the integration of 2D systems into current semiconductor technologies. Herein, the direct synthesis of high-quality 2D antimony, or antimonene, on dielectric copper oxide substrate by molecular beam epitaxy is reported. Delicate scanning tunneling microscopy imaging on the evolution intermediates reveals a segregation growth process on Cu3 O2 /Cu(111), from ordered dimer chains to packed dot arrays, and finally to monolayer antimonene. First-principles calculations demonstrate the strain-modulated band structures in antimonene, which interacts weakly with the oxide surface so that its semiconducting nature is preserved, in perfect agreement with spectroscopic measurements. This work paves the way for large-scale growth and processing of antimonene for practical implementation.