Graphsene as a novel porous two-dimensional carbon material for enhanced oxygen reduction electrocatalysis.

Graphene allotropes with varied carbon configurations have attracted significant attention for their unique properties and chemical activities. This study introduces a novel two-dimensional carbon-based material, termed Graphsene (GrS), through theoretical study. Comprising tetra-, penta-, and dodeca-carbon rings, GrS's cohesive energy calculations demonstrate its superior structural stability over existing graphene allotropes, including graphyne and graphdiyne families. Phonon dispersion analysis confirms GrS's dynamic stability and its relatively low thermal conductivity. All calculated GrS elastic constants meet the Born criteria, ensuring mechanical stability. Ab-initio molecular dynamic simulations show GrS maintains its structure at 300 K. HSE06 calculations reveal a narrow electronic bandgap of 20 meV, with the electronic band structure featuring a highly anisotropic Dirac-like cone due to its intrinsic structural anisotropy along armchair and zigzag directions. Notably, GrS is predicted to offer exceptional catalytic performance for the oxygen reduction reaction, favoring the four-electron reduction pathway with high selectivity under both acidic and alkaline conditions. This discovery opens promising avenues for developing metal-free catalyst materials in clean energy production.

Comprehensive Study of Lithium Diffusion in Si/C-Layer and Si/C3N4 Composites in a Faceted Crystalline Silicon Anode for Fast-Charging Lithium-Ion Batteries.

By using silicon (Si) as an anode of lithium-ion batteries, the capacity can be significantly increased, but relatively large volume expansion limits the application as an efficient anode material. Huge volume expansion of the silicon anode during lithiation, however, leads to cracking and losing its connection with the current collector. This shortcoming can be improved by the deposition of a nanometric carbon- or nitrogen-doped carbon coating on the silicon surface, resulting in Si/C-layer and Si/C3N4 interfaces. In this work, Li+ diffusion in Si/C-layer and Si/C3N4 composite materials along three Si surfaces and various ion pathways were carefully analyzed by using density functional theory and ab initio molecular dynamic (AIMD) simulations. Both Si/C and Si/C3N4 interfaces and three Si surfaces of (100), (110), and (111) were investigated. The formation of nitrogen holes and monatomic carbon binders in the composite increases ion diffusivity and limits volume expansion. Furthermore, the Bader analysis shows that the type and orientation of the surfaces have important effects on ion distribution. The results indicated that the C3N4 composite increases Li+ diffusion in Si (100) from 7.82 × 10-5 to 3.17 × 10-4 cm2/s. The presented results provide a guide for the appropriate design of stable and safe high-energy-density batteries.

The carrier mobility and superconducting properties of monolayer oxygen-terminated functionalized MXene Ti2CO2.

In this study, the carrier mobility of monolayer Ti2CO2 was evaluated by employing the Boltzmann transport equation and superconducting transition temperature (Tc) of Ti2CO2 was determined by utilizing the Migdal and Eliashberg formalism in the first-principles framework. In contrast to previous studies, the results reveal that optical phonons in monolayer Ti2CO2 have dominant roles in scattering processes, which significantly reduce the mobility of carriers. Alongside the rigid band model, the jellium model is implemented to investigate the screening effects on electron-phonon interactions. Based on the jellium model and full-band electron-phonon calculations, the predicted maximum electron mobility at room temperature is 38 cm2 V-1 s-1 in which 80% of the total scattering rate originates from the intra-valley transitions within the M-valleys, indicating the crucial role of the long wavelength phonon wavevectors in scattering processes. On the other hand, for the p-type material, a maximum room temperature mobility of about 285 cm2 V-1 s-1 is calculated, which can be explained by a relatively small effective mass and tiny scattering phase space. Moreover, a maximum Tc of 39 (10) K is obtained for the n-type monolayer Ti2CO2 based on the rigid (jellium) model. Outcomes indicate that the important peaks of α2F(ω) are mainly caused by the optical phonons. The remarkable couplings between the electron states and phonons are related to the non-zero slope of (near the Brillouin zone center) the longitudinal optical branch denoted by Eu caused by the displacements of oxygen and carbon atoms at intermediate and high energy ranges of phonon dispersion, respectively.

A comprehensive investigation of the plasmonic-photocatalytic properties of gold nanoparticles for CO2 conversion to chemicals.

Understanding the interactions between plasmonic gold (Au) nanoparticles and the adsorbate is essential for photocatalytic and plasmonic applications. However, it is often challenging to identify a specific reaction mechanism in the ground state and to explore the optical properties in the excited states because of the complicated pathways of carriers. In this study, photocatalytic reduction of carbon dioxide (CO2) to C1 products (for example, CO and CH4) on the Au(111) nanoparticle (NP) surface was studied based on reaction pathway analysis, adsorbate reactivity, and its ability to stabilize or deactivate the surface. The calculated reaction Gibbs free energies and activation barriers revealed that the first step in CO reduction via a direct hydrogen transfer mechanism on Au(111) is the formation of formyl (*CHO) instead of hydroxymethylidyne (*COH). Furthermore, the size enhanced and symmetry sensitive optical responses of cuboctahedral Au(111) NPs on localized surface plasmon resonance (LSPR) were investigated by using time-dependent DFT (TDDFT) calculations. Although near field enhancement around cuboctahedral Au(111) NPs is only weakly dependent on the morphology of NPs, it was observed that corner sites stabilize *C-species to drive the CO2 reduction to CO. The density of active surface states interacting with the adsorbate states near the Fermi level gradually decreases from the (111) on-top site toward the corner site of the Au(111) NP-CO system, which strongly affects the molecule's binding on catalytic sites and, in particular, electronic excitation. Finally, the spatial distribution of the charge oscillations was determined as a guide for the fabrication of Au NPs with an optimal LSPR response.

Strain engineering of hyperbolic plasmons in monolayer carbon phosphide: a first-principles study.

Natural and tunable in-plane hyperbolic plasmons have so far been elusive, and hence few two-dimensional hyperbolic materials have been theoretically and experimentally discovered. Here, comprehensive first-principles calculations were conducted to study the electronic and plasmonic properties of biaxially strained monolayer carbon phosphide (ß-CP). We found that (i) a compressed ß-CP hosts strong anisotropic Dirac-shaped fermions with robust modulated Fermi velocity, (ii) for biaxial strain of -3% an unprecedented ultra-wide hyperbolic window is extended continuously from terahertz (9 THz) to mid-visible (blue light, 693 THz), (iii) the tunable optical Van Hove singularity as the origin of hyperbolic plasmons in deformed ß-CP is disclosed, (iv) an elliptic to hyperbolic transition in the σ-near-zero regime is demonstrated in terahertz frequencies (9 THz), (v) the propagation angle of the concave wavefront can be actively tuned using biaxial strains, and (vi) hyperbolic dispersion reorientation from one principal axis to another orthogonal one under compressive strains larger than 8% is observed. This study sheds new light on the unique properties of hyperbolic two-dimensional (2D) materials having exotic optoelectronic characteristics which are promising candidates for anisotropic light control with ultimate dexterity in the flat optics.

First principles study on structural, electronic and optical properties of HfS2(1-x)Se2x and ZrS2(1-x)Se2x ternary alloys.

Alloying 2D transition metal dichalcogenides (TMDs) with dopants to achieve ternary alloys is as an efficient and scalable solution for tuning the electronic and optical properties of two-dimensional materials. This study provides a comprehensive study on the electronic and optical properties of ternary HfS2(1-x)Se2(x) and ZrS2(1-x)Se2(x) [0 ≤ x ≤ 1] alloys, by employing density functional theory calculations along with random phase approximation. Phonon dispersions were also obtained by using density functional perturbation theory. The results indicate that both of the studied ternary families are stable and the increase of Selenium concentration in HfS2(1-x)Se2(x) and ZrS2(1-x)Se2(x) alloys results in a linear decrease of the electronic bandgap from 2.15 (ev) to 1.40 (ev) for HfS2(1-x)Se2(x) and 1.94 (ev) to 1.23 (ev) for ZrS2(1-x)Se2(x) based on the HSE06 functional. Increasing the Se concentration in the ternary alloys results in a red shift of the optical absorption spectra such that the main absorption peaks of HfS2(1-x)Se2(x) and ZrS2(1-x)Se2(x) cover a broad visible range from 3.153 to 2.607 eV and 2.405 to 1.908 eV, respectively. The studied materials appear to be excellent base materials for tunable electronic and optoelectronic devices in the visible range.

Ferroelectricity and phase transitions in In2Se3 van der Waals material.

van der Waals layered α-In2Se3 has shown out-of-plane ferroelectricity down to the bilayer and monolayer thicknesses at room temperature that can be switched by an applied electric field. This work addresses the missing theoretical framework through a comprehensive study on the layer-dependent electronic structure, ferroelectricity and the inter-layer interaction of α-In2Se3, by using first-principles density functional theory. Furthermore, surface states and their response to the built-in internal depolarizing field were carefully analyzed. Phase transition and Curie temperatures of 1L α-In2Se3 were studied by employing Monte Carlo and ab initio molecular dynamics simulations. The estimated Curie point is above room temperature, making 1L α-In2Se3 a promising candidate for future ultra-thin ferroelectric devices.

Electronic Transport Properties of Silicane Determined from First Principles.

Silicane, a hydrogenated monolayer of hexagonal silicon, is a candidate material for future complementary metal-oxide-semiconductor technology. We determined the phonon-limited mobility and the velocity-field characteristics for electrons and holes in silicane from first principles, relying on density functional theory. Transport calculations were performed using a full-band Monte Carlo scheme. Scattering rates were determined from interpolated electron-phonon matrix elements determined from density functional perturbation theory. We found that the main source of scattering for electrons and holes was the ZA phonons. Different cut-off wavelengths ranging from 0.58 nm to 16 nm were used to study the possible suppression of the out-of-plane acoustic (ZA) phonons. The low-field mobility of electrons (holes) was obtained as 5 (10) cm2/(Vs) with a long wavelength ZA phonon cut-off of 16 nm. We showed that higher electron (hole) mobilities of 24 (101) cm2/(Vs) can be achieved with a cut-off wavelength of 4 nm, while completely suppressing ZA phonons results in an even higher electron (hole) mobility of 53 (109) cm2/(Vs). Velocity-field characteristics showed velocity saturation at 3 × 105 V/cm, and negative differential mobility was observed at larger fields. The silicane mobility was competitive with other two-dimensional materials, such as transition-metal dichalcogenides or phosphorene, predicted using similar full-band Monte Carlo calculations. Therefore, silicon in its most extremely scaled form remains a competitive material for future nanoscale transistor technology, provided scattering with out-of-plane acoustic phonons could be suppressed.

An implementation of spin-orbit coupling for band structure calculations with Gaussian basis sets: Two-dimensional topological crystals of Sb and Bi.

We present an implementation of spin-orbit coupling (SOC) for density functional theory band structure calculations that makes use of Gaussian basis sets. It is based on the explicit evaluation of SOC matrix elements, both the radial and angular parts. For all-electron basis sets, where the full nodal structure is present in the basis elements, the results are in good agreement with well-established implementations such as VASP. For more practical pseudopotential basis sets, which lack nodal structure, an ad-hoc increase of the effective nuclear potential helps to capture all relevant band structure variations induced by SOC. In this work, the non-relativistic or scalar-relativistic Kohn-Sham Hamiltonian is obtained from the CRYSTAL code and the SOC term is added a posteriori. As an example, we apply this method to the Bi(111) monolayer, a paradigmatic 2D topological insulator, and to mono- and multilayer Sb(111) (also known as antimonene), the former being a trivial semiconductor and the latter a topological semimetal featuring topologically protected surface states.

Ab initio effective deformation potentials of phosphorene and consistency checks.

Electron-phonon interaction in single-layer phosphorene is studied from first principles based on the density functional theory, using finite displacement method. Scattering rates and mobility are numerically evaluated for carriers in the conduction and valence bands. A criterion for the selection of phonon wave-vector in scatterings is proposed. Scattering selection rules are studied, utilizing group theory for the structure with [Formula: see text] space group symmetry. Approximate analytical formulas for scattering rates, adopting the anisotropy intrinsic to phosphorene, are derived, and effective deformation potentials are evaluated by fitting the formulas to numerical scattering rates extracted from ab initio calculations.

Optimization study of third harmonic generation in quantum cascade lasers.

A systematic optimization study of quantum cascade lasers with integrated nonlinearity for third-harmonic generation is performed. To model current transport the Pauli master equation is solved using a Monte Carlo approach. A multi-objective particle swarm optimization algorithm is applied to obtain the Pareto front. Our theoretical analysis indicates an optimized structure with five orders of magnitude increase in the generated third-harmonic power with respect to the reference design. This striking performance comes with a low threshold current density of about 1.6 kA/cm2 and is attributed to double resonant phonon scattering assisted extraction and injection scheme of the laser.