Yan-ng Li

Single-layer MoS2 as an efficient photocatalyst

The potential application of the single-layer MoS2 as a photocatalyst was revealed based on first-principles calculations. It is found that the pristine single-layer MoS2 is a good candidate for hydrogen production, and its catalysing ability can be tuned by the applied mechanical strain. Furthermore, the p-type doping could make the single layer a good photocatalyst for the overall water splitting.

Pressure-induced superconductivity in CaC2

Carbon can exist as isolated dumbbell, 1D chain, 2D plane, and 3D network in carbon solids or carbon-based compounds, which attributes to its rich chemical binding way, including sp-, sp2-, and sp3-hybridized bonds. sp2-hybridizing carbon always captures special attention due to its unique physical and chemical property. Here, using an evolutionary algorithm in conjunction with ab initio method, we found that, under compression, dumbbell carbon in CaC2 can be polymerized first into 1D chain and then into ribbon and further into 2D graphite sheet at higher pressure. The C2/m structure transforms into an orthorhombic Cmcm phase at 0.5 GPa, followed by another orthorhombic Immm phase, which is stabilized in a wide pressure range of 15.2–105.8 GPa and then forced into MgB2-type phase with wide range stability up to at least 1 TPa. Strong electron–phonon coupling λ in compressed CaC2 is found, in particular for Immm phase, which has the highest λ value (0.562–0.564) among them, leading to its high superconducting critical temperature Tc (7.9∼9.8 K), which is comparable with the 11.5 K value of CaC6. Our results show that calcium not only can stabilize carbon sp2 hybridization at a larger range of pressure but also can contribute in superconducting behavior, which would further ignite experimental and theoretical interest in alkaline–earth metal carbides to uncover their peculiar physical properties under extreme conditions.

Review of two-dimensional materials for photocatalytic water splitting from a theoretical perspective

Two-dimensional (2D) materials have shown extraordinary performances as photocatalysts compared to their bulk counterparts. Simulations have made a great contribution to the deep understanding and design of novel 2D photocatalysts. Ab initio simulations based on density functional theory (DFT) not only show efficiency and reliability in new structure searching, but also can provide a reliable, efficient, and economic way for screening the photocatalytic property space. In this review, we summarize the recent developments in the field of water splitting using 2D materials from a theoretical perspective. We address that DFT-based simulations can fast screen the potential spaces of photocatalytic properties with the accuracy comparable to experiments, by investigating the effects of various physical/chemical perturbations. This, at last, will lead to the enhanced photocatalytic activities of 2D materials, and promote the development of photocatalysis.

New potential super-incompressible phase of ReN2

The structural, elastic, and electronic properties of ReN2 are investigated by first-principles calculations with density functional theory. The obtained orthorhombic Pbcn structure is energetically the most stable structure at ambient pressure. ReN2 is a metallic, super-incompressible solid and presents a rather elastic anisotropy. The estimated Debye temperature and hardness are 735 K and 17.1 GPa, respectively. Its estimated hardness is comparative to that of Si3N4.

Pressure-induced planar N6 rings in potassium azide

The first-principles method and the evolutionary algorithm are used to identify stable high pressure phases of potassium azide (KN3). It has been verified that the stable phase with space group I4/mcm below 22u2005GPa, which is consistent with the experimental result, will transform into the C2/m phase with pressure increasing. These two phases are insulator with anions. A metallic phase with P6/mmm symmetry is preferred above 40u2005GPa, and the N atoms in this structure form six-membered rings which are important for understanding the pressure effect on anions and phase transitions of KN3. Above the studied pressure (100u2005GPa), a polymerization of N6 rings may be obtained as the result of the increasing compactness.

Structural transitions of solid germane under pressure

We present a structural characterization of solid germane (GeH4) under pressure from first-principles calculations. We find that this material undertakes a structural transformation from its low-pressure P21/c phase to high-pressure Cmmm phase at about 15 GPa where insulator-metal transition occurs, followed by two other metallic phases having the P21/m and C2/c structure at up to 200 GPa. Our results indicate that the metallization of GeH4 can be realized through band overlap within the material itself.

Lithium and Calcium Carbides with Polymeric Carbon Structures

We studied the binary carbide systems Li2C2 and CaC2 at high pressure using an evolutionary and ab initio random structure search methodology for crystal structure prediction. At ambient pressure Li2C2 and CaC2 represent salt-like acetylides consisting of C2(2-) dumbbell anions. The systems develop into semimetals (P3m1-Li2C2) and metals (Cmcm-Li2C2, Cmcm-CaC2, and Immm-CaC2) with polymeric anions (chains, layers, strands) at moderate pressures (below 20 GPa). Cmcm-CaC2 is energetically closely competing with the ground state structure. Polyanionic forms of carbon stabilized by electrostatic interactions with surrounding cations add a new feature to carbon chemistry. Semimetallic P3m1-Li2C2 displays an electronic structure close to that of graphene. The π* band, however, is hybridized with Li-sp states and changed into a bonding valence band. Metallic forms are predicted to be superconductors. Calculated critical temperatures may exceed 10 K for equilibrium volume structures.

The electronic origin of shear-induced direct to indirect gap transition and anisotropy diminution in phosphorene

Artificial monolayer black phosphorus, so-called phosphorene, has attracted global interest with its distinguished anisotropic, optoelectronic, and electronic properties. Here, we unraveled the shear-induced direct-to-indirect gap transition and anisotropy diminution in phosphorene based on first-principles calculations. Lattice dynamic analysis demonstrates that phosphorene can sustain up to 10% applied shear strain. The bandgap of phosphorene experiences a direct-to- indirect transition when 5% shear strain is applied. The electronic origin of the direct-to-indirect gap transition from 1.54 eV at ambient conditions to 1.22 eV at 10% shear strain for phosphorene is explored. In addition, the anisotropy diminution in phosphorene is discussed by calculating the maximum sound velocities, effective mass, and decomposed charge density, which signals the undesired shear-induced direct-to-indirect gap transition in applications of phosphorene for electronics and optoelectronics. On the other hand, the shear-induced electronic anisotropy properties suggest that phosphorene can be applied as the switcher in nanoelectronic applications.

Formation of Nanofoam carbon and re-emergence of Superconductivity in compressed CaC6

Pressure can tune materials electronic properties and control its quantum state, making some systems present disconnected superconducting region as observed in iron chalcogenides and heavy fermion CeCu2Si2. For CaC6 superconductor (Tc of 11.5u2005K), applying pressure first Tc increases and then suppresses and the superconductivity of this compound is eventually disappeared at about 18u2005GPa. Here, we report a theoretical finding of the re-emergence of superconductivity in heavily compressed CaC6. The predicted phase III (space group Pmmn) with formation of carbon nanofoam is found to be stable at wide pressure range with a Tc up to 14.7u2005K at 78u2005GPa. Diamond-like carbon structure is adhered to the phase IV (Cmcm) for compressed CaC6 after 126u2005GPa, which has bad metallic behavior, indicating again departure from superconductivity. Re-emerged superconductivity in compressed CaC6 paves a new way to design new-type superconductor by inserting metal into nanoporous host lattice.

Potential ultra-incompressible material ReN: First-principles prediction

Abstract The structural, elastic and electronic properties of ReN are investigated by first-principles calculations based on density functional theory (DFT). The most stable structure of ReN is a NiAs-like structure, belonging to space group P 6 3 / m m c with a = 2.7472 and c = 5.8180 A. ReN is a metallic ultra-incompressible solid and it exhibits low elastic anisotropy. Its linear incompressibility along the c -axis exceeds that of diamond. Its ultra-incompressibility is attributed to the high valence electron density and strong covalence bondings. Our results indicate that ReN can be used as a potential ultra-incompressible conductor. In particular, a superconducting transition temperature is predicted as T c ≈ 4.8 K for NiAs-like ReN, which agrees well with the available experimental value.