Two-dimensional tetragonal AlOX (X = Cl, Br, or I) monolayers with promising photocatalytic performance: first-principles investigations

It is of great significance to search for new two-dimensional materials with excellent photocatalytic water splitting properties. Here, the AlOX (X = Cl, Br, or I) monolayers were constructed to explore their electronic and optical properties as a potential photocatalyst and mechanism of high photocatalytic activity by first principles calculations, for the first time. The results show that the AlOX (X = Cl, Br, or I) monolayers are all dynamically and thermodynamically stable. It is found that the AlOI monolayer exhibits visible optical absorption with a 538 nm absorption band edge, due to its direct band gap of 2.22 eV. Moreover, an appropriate band edge potential ensures its excellent reduction-oxidation (redox) ability. The asymmetry of crystals along different directions results in a noncoplanar HOMO and LUMO as well as an anisotropy effective mass and favors the separation of photogenerated carriers. These findings present the potential of the AlOX (X = Cl, Br, or I) monolayers as photocatalysts.

Prediction of induced magnetism in 2D Ti2C based MXenes by manipulating the mixed surface functionalization and metal substitution computed by xTB model Hamiltonian of the DFTB method

We employed the recently developed density functional tight binding (DFTB) method's Hamiltonian, GFN1-xTB, for modeling the mixed termination in Ti2C MXenes, namely three types of termination by combining -O and -OH, -O and -F, and -F and -OH. We demonstrated that the approach yields reliable predictions for the electronic and magnetic properties of such MXenes. The first highlighted result is that the mixed surface functionalization in Ti2CAxBy MXenes induces spin polarization with diverse magnetic alignments, including ferromagnetism and two types of antiferromagnetism. We further identified the magnetic alignment for the investigated MXene in terms of the compositions of the terminal groups. Moreover, the effect of the transition metal (Ti) substituted by the Sc atom on the electronic and magnetic properties was also investigated. We found that the studied systems maintain the magnetism and the metallic characteristics. A magnetic transition from antiferromagnetic (AFM) to ferrimagnetic (FiM) ordering was found for ScTi15C8F8(OH)8 and ScTi15C8F12(OH)4 compounds. Finally, we proved that incorporating the Sc atom into the lattice of Ti2CO2 and the mixed surface termination in Ti2CAxBy is an effective strategy to induce magnetism. Our study may provide a new potential application for designing MXene-based spintronics.

Enhancing CO2 electroreduction performance through transition metal atom doping and strain engineering in γ-GeSe: a first-principles study

The development of electrocatalysts that exhibit stability, high activity, and selectivity for CO2 reduction reactions (CO2RR) remains a significant challenge. Single-atom catalysts (SACs) hold promise in addressing this challenge due to their high atomic utilization efficiency. In this study, we explore the potential of monolayer γ-GeSe doped with transition metals, referred to as TM@γ-GeSe, for facilitating electrocatalytic CO2RR. Among the 26 TM@γ-GeSe SACs systematically designed, we have identified four stable transition metal catalysts (TM = Rh, Pd, Pt, and Au). Mechanistic investigations into the CO2RR pathways reveal exceptional electrocatalytic activity for Rh@γ-GeSe and Pd@γ-GeSe, with limiting potentials of -0.26 and -0.35 V, respectively. Particularly, Pd@γ-GeSe exhibits outstanding product selectivity toward formic acid. The introduction of strain engineering induces modifications in the catalytic activity and selectivity of Rh@γ-GeSe. Notably, a 1% tensile strain promotes formic acid as the preferred product, thereby improving the specific product selectivity of Rh@γ-GeSe. Conversely, compressive strain reduces CO2RR activity while enhancing the hydrogen evolution reaction, leading to a decrease in CO2RR selectivity. Furthermore, we use the work function as a descriptor to elucidate the underlying mechanism of strain tunability. We hope that our theoretical study will offer valuable insights for the design of catalysts based on γ-GeSe for electrocatalytic CO2RR.

Revealing the electronic, optical and photocatalytic properties of PN-M2CO2 (P = Al, Ga; M = Ti, Zr, Hf) heterostructures

Using DFT, the electronic structure, optical, and photocatalytic properties of PN (P = Ga, Al) and M₂CO₂ (M = Ti, Zr, Hf) monolayers and their PN-M₂CO₂ van der Waals heterostructures (vdWHs) are investigated. Optimized lattice parameters, bond length, bandgap, conduction and valence band edges show the potential of PN (P = Ga, Al) and M₂CO₂ (M = Ti, Zr, Hf) monolayers in photocatalytic applications, and the application of the present approach to combine these monolayers and form vdWHs for efficient electronic, optoelectronic and photocatalytic applications is shown. Based on the same hexagonal symmetry and experimentally achievable lattice mismatch of PN (P = Ga, Al) with M₂CO₂ (M = Ti, Zr, Hf) monolayers, we have fabricated PN-M₂CO₂ vdWHs. Binding energies, interlayer distance and AIMD calculations show the stability of PN-M₂CO₂ vdWHs and demonstrate that these materials can be easily fabricated experimentally. The calculated electronic band structures show that all the PN-M₂CO₂ vdWHs are indirect bandgap semiconductors. Type-II[-I] band alignment is obtained for GaN(AlN)-Ti₂CO₂[GaN(AlN)-Zr₂CO₂ and GaN(AlN)-Hf₂CO₂] vdWHs. PN-Ti₂CO₂ (PN-Zr₂CO₂) vdWHs with a PN(Zr₂CO₂) monolayer have greater potential than a Ti₂CO₂(PN) monolayer, indicating that charge is transfer from the Ti₂CO₂(PN) to PN(Zr₂CO₂) monolayer, while the potential drop separates charge carriers (electron and holes) at the interface. The work function and effective mass of the carriers of PN-M₂CO₂ vdWHs are also calculated and presented. A red (blue) shift is observed in the position of excitonic peaks from AlN to GaN in PN-Ti₂CO₂ and PN-Hf₂CO₂ (PN-Zr₂CO₂) vdWHs, while significant absorption for photon energies above 2 eV for AlN-Zr₂CO₂, GaN-Ti₂CO₂ and PN-Hf₂CO₂, give them good optical profiles. The calculated photocatalytic properties demonstrate that PN-M₂CO₂ (P = Al, Ga; M = Ti, Zr, Hf) vdWHs are the best candidates for photocatalytic water splitting.

Electronic Nature Transition and Magnetism Creation in Vacancy-Defected Ti2CO2 MXene under Biaxial Strain: A DFTB + U Study

The structural, electronic, and magnetic properties of vacancy defect in Ti₂CO₂ MXene and the effect of strain have been investigated using the density functional tight-binding (DFTB) approach including spin-polarization with Hubbard onsite correction (DFTB + U). The band gap of pure Ti₂CO₂ is ∼1.3 eV, which decreases to ∼0.4 and ∼1.1 eV in the case of C- and O-vacancies, respectively, i.e., the semiconducting behavior is retained. In contrast, Ti₂CO₂ undergoes semiconductor-to-metal transition by the introduction of a single Ti-vacancy. This transition is the result of introduced localized states in the vicinity of the Fermi level by the vacancy. Both Ti- and O-vacancies have zero net magnetic moments. Interestingly, the nonmagnetic (NM) ground state of semiconducting Ti₂CO₂ turns into a magnetic semiconductor by introducing a C-vacancy with a magnetization of ∼2 μ_B/cell. Furthermore, we studied the effect of strain on the electronic structure and magnetic properties of Ti-, C-, and O-vacant Ti₂CO₂. The nature of the band gap in the presence of single O-vacancy remains indirect in both compression and tensile strain, and the size of the band gap decreases. Compression strain on Ti-vacant Ti₂CO₂ changes metal into a direct semiconductor, and the metallic character remains under tensile biaxial strain. In opposition, a semiconductor-to-metal transition occurs by applying a compressive biaxial strain on C-vacant Ti₂CO₂. We also find that the magnetism is preserved under tensile strain and suppressed under compression strain on V_C-Ti₂CO₂. Moreover, we show that double C-vacancies maintain magnetism. Our findings provide important characteristics for the application of the most frequent MXene material and should motivate further investigations because experimentally achieved MXenes always contain point defects.

Two dimensional Zr2CO2/H-FeCl2van der Waals heterostructures with tunable band gap, potential difference and magnetic anisotropy

Two dimensional (2D) van der Waals (vdW) heterostructures have potential applications in novel low dimensional spintronic devices due to their unique electronic and magnetic properties. Here, the electronic and magnetic properties of 2D Zr₂CO₂/H-FeCl₂heterostructures are calculated by first principles calculations. The 2D Zr₂CO₂/H-FeCl₂heterostructures are magnetic semiconductor. The electronic structure and magnetic anisotropy of Zr₂CO₂/H-FeCl₂heterostructure can be regulated by the biaxial strain and external electric field. The band gap and potential difference of Zr₂CO₂/H-FeCl₂heterostructure can be affected by in-plane biaxial strain. At a compressive strain of -8%, the Zr₂CO₂/H-FeCl₂heterostructure becomes metallic. All of the Zr₂CO₂/H-FeCl₂heterostructures are magnetic with in-plane magnetic anisotropy (IMA). The Zr₂CO₂/H-FeCl₂heterostructure is a semiconductor at the electric field from -0.5 V Å^-1to +0.5 V Å^-1. Furthermore, Zr₂CO₂/H-FeCl₂heterostructure shows IMA at the negative electric field, while it shows perpendicular magnetic anisotropy at the positive electric field. These results show that Zr₂CO₂/H-FeCl₂heterostructure has potential applications in multifunctionalnanoelectronic devices.

Intriguing electronic, optical and photocatalytic performance of BSe, M2CO2 monolayers and BSe-M2CO2 (M = Ti, Zr, Hf) van der Waals heterostructures

Using density functional (DFT) theory calculations, we have investigated the electronic band structure, optical and photocatalytic response of BSe, M₂CO₂ (M = Ti, Zr, Hf) monolayers and their corresponding BSe–M₂CO₂ (M = Ti, Zr, Hf) van der Waals (vdW) heterostructures. Optimized lattice constant, bond length, band structure and bandgap values, effective mass of electrons and holes, work function and conduction and valence band edge potentials of BSe and M₂CO₂ (M = Ti, Zr, Hf) monolayers are in agreement with previously available data. Binding energies, interlayer distance and Ab initio molecular dynamic simulations (AIMD) calculations show that BSe–M₂CO₂ (M = Ti, Zr, Hf) vdW heterostructures are stable with specific stacking and demonstrate that these heterostructures might be synthesized in the laboratory. The electronic band structure shows that all the studied vdW heterostructures have indirect bandgap nature – with the CBM and VBM at the Γ–K and Γ-point of BZ for BSe–Ti₂CO₂, respectively; while for BSe–Zr₂CO₂ and BSe–Hf₂CO₂ vdW heterostructures the CBM and VBM lie at the K-point and Γ-point of BZ, respectively. Type-II band alignment in BSe–M₂CO₂ (M = Ti, Zr, Hf) vdW heterostructures prevent the recombination of electron–hole pairs, and hence are crucial for light harvesting and detection. Absorption spectra are investigated to understand the optical behavior of BSe–M₂CO₂ (M = Ti, Zr, Hf) vdW heterostructures, where the lowest energy transitions are dominated by excitons. Furthermore, BSe–M₂CO₂ (M = Ti, Zr, Hf) vdW heterostructures are found to be potential photocatalysts for water splitting at pH = 0, and exhibit enhanced optical properties in the visible light zones.

Using density functional theory calculations, we have investigated the electronic band structure, optical and photocatalytic response of BSe, M₂CO₂ (M = Ti, Zr, Hf) monolayers and their corresponding BSe–M₂CO₂ (M = Ti, Zr, Hf) van der Waals heterostructures.

Anisotropic strain in epitaxial single-layer molybdenum disulfide on Ag(110)

In this work we prove that ordered single-layer MoS₂ can be grown epitaxially on Ag(110), despite the different crystalline geometry of adsorbate and substrate. A comprehensive investigation of electronic and structural features of this interface is carried out by combining several techniques. Photoelectron diffraction experiments show that only two mirror crystalline domains coexist in equal amount in the grown layer. Angle-resolved valence band photoelectron spectroscopy shows that MoS₂ undergoes a semiconductor-to-metal transition. Low-energy electron diffraction and scanning-tunneling microscopy experiments reveal the formation of a commensurate moiré superlattice at the interface, which implies an anisotropic uniaxial strain of the MoS₂ crystalline lattice of ca. 3% in the [11̄0] direction of the Ag(110) surface. These outcomes suggest that the epitaxial growth on anisotropic substrates might be an effective and scalable method to generate a controlled and homogeneous strain in MoS₂ and possibly other transition-metal dichalcogenides.

Many-body effects in an MXene Ti2CO2 monolayer modified by tensile strain: GW-BSE calculations

MXenes, two-dimensional (2D) layered transition metal carbide/nitride materials with a lot of advantages including high carrier mobility, tunable band gap, favorable mechanical properties and excellent structural stability, have attracted research interest worldwide. It is imperative to accurately understand their electronic and optical properties. Here, the electronic and optical response properties of a Ti₂CO₂ monolayer, a typical member of MXenes, are investigated on the basis of first-principles calculations including many-body effects. Our results show that the pristine Ti₂CO₂ monolayer displays an indirect quasi-particle (QP) band gap of 1.32 eV with the conduction band minimum (CBM) located at the M point and valence band maximum (VBM) located at the Γ point. The optical band gap and binding energy of the first bright exciton are calculated to be 1.26 eV and 0.56 eV, respectively. Under biaxial tensile strains, the lowest unoccupied band at the Γ point shifts downward, while the lowest unoccupied band at the M point shifts upward. Then, a direct band gap appears at the Γ point in 6%-strained Ti₂CO₂. Moreover, the optical band gap and binding energy of the first bright exciton decrease continuously with the increase of the strain due to the increase of the lattice parameter and the expansion of the exciton wave function. More importantly, the absorbed photon flux of Ti₂CO₂ is calculated to be 1.76-1.67 mA cm^-2 with the variation of the strain, suggesting good sunlight optical absorbance. Our work demonstrates that Ti₂CO₂, as well as other MXenes, hold untapped potential for photo-detection and photovoltaic applications.

Strain-tunable electronic structures and optical properties of semiconducting MXenes

The feature of an indirect bandgap of most semiconducting transition metal carbides (MXenes) limits their applications in optoelectronics devices. By means of density functional theory (DFT) calculations, we have found that the transition of indirect-direct bandgap can occur in MXenes with different functional groups and structures under appropriate biaxial strain. The controllable bandgap of MXenes stems from the fact that the electronic states near the Fermi level have different responses to tensile strain. The stress-strain curves and phonon spectra suggest that semiconducting MXenes can maintain their stability during a wide range of strains. Moreover, the optical dielectric constants of MXenes are red-shifted and enhanced continuously via applying tensile strains. The tunable electronic and optical properties of semiconducting MXenes make them promising candidates for the design of optoelectronic devices.

Optoelectronic and solar cell applications of Janus monolayers and their van der Waals heterostructures

Janus monolayers and their van der Waals heterostuctures are investigated by hybrid density functional theory calculations.

N-Functionalized MXenes: ultrahigh carrier mobility and multifunctional properties

Two dimensional (2D) nanomaterials have demonstrated huge potential in wide applications from nanodevices to energy harvesting/storage.