High-Throughput Computational Screening of Two-Dimensional Semiconductors

High solar-to-hydrogen efficiency in Z-scheme AlN/GaO heterojunctions for visible light water splitting

Hydrogen production from solar energy is an important means to solve the problems of fossil fuel consumption and environmental pollution, and the efficiency of hydrogen production from solar energy is an important indicator. Photocatalytic water decomposition technology driven by solar energy is an ideal way to create clean energy. In this paper, a new Z-scheme AlN/GaO van der Waals heterojunction is proposed. Through first-principles calculations, we have systematically studied the electronic properties and photocatalytic hydrogen production performance of the AlN/GaO heterostructure. The calculation results show that the lattice mismatch rate of the AlN/GaO heterojunction is only 0.48%. At the same time, it not only performs well in terms of thermodynamics, kinetics and mechanical stability, but also has an appropriate band gap of 1.45 eV with an electron mobility of up to 2753.48 cm2 V-1 s-1. Under light irradiation, the transfer of internal photogenerated carriers forms a built-in electric field from AlN to GaO, which forms a typical Z-scheme, and leads to the hydrogen evolution reaction on AlN with strong reduction ability. It is worth noting that the AlN/GaO heterojunction shows a high absorption coefficient in the visible light absorption range and has an excellent solar-to-hydrogen efficiency of 60.1%. These advantages demonstrate that the AlN/GaO heterojunction, as a promising photocatalyst, has significant application potential and offers a novel approach to address the energy crisis and environmental pollution challenges.

Penta-Octa B4C2N3: A New 2D Material for High-Performance Energy Applications

Penta-octagraphene (POG) is a newly suggested two-dimensional carbon allotrope recognized for its distinct configuration and fascinating electronic characteristics. This work presents a new inorganic counterpart of POG, named POG-B4C2N3, designed through density functional theory (DFT) calculations. This new structure exhibits a direct band gap transition at the X-point, measured at 0.32/0.86 eV with PBE/HSE functionals. Mechanical properties were comprehensively assessed, showcasing its Young's modulus (Ymax/Ymin = 157.12/100.84 N/m) and shear modulus (Gmax/Gmin = 83.03/38.09 N/m), alongside Poisson's ratio (νmax/νmin = 0.58/-0.09), indicating that POG-B4C2N3 is an auxetic material. Additionally, Li decoration on this monolayer was studied to investigate its potential to enhance hydrogen storage through physisorption. The Li@POG-B4C2N3 system shows robust physisorption (adsorption energies ranging from -0.35 to -0.19 eV), high hydrogen storage capacity (8.35 wt %), and effective hydrogen desorption dynamics, positioning this novel material as a promising platform for reversible hydrogen storage.

Facing the "Cutting Edge:" Edge Site Engineering on 2D Materials for Electrocatalysis and Photocatalysis

The utilization of 2D materials as catalysts has garnered significant attention in recent years, primarily due to their exceptional features including high surface area, abundant exposed active sites, and tunable physicochemical properties. The unique geometry of 2D materials imparts them with versatile active sites for catalysis, including basal plane, interlayer, defect, and edge sites. Among these, edge sites hold particular significance as they not only enable the activation of inert 2D catalysts but also serve as platforms for engineering active sites to achieve enhanced catalytic performance. Here it is comprehensively aimed to summarize the state-of-the-art advancements in the utilization of edge sites on 2D materials for electrocatalysis and photocatalysis, with applications ranging from water splitting, oxygen reduction, and nitrogen reduction to CO2 reduction. Additionally, various approaches for harnessing and modifying edge sites are summarized and discussed. Here guidelines for the rational engineering of 2D materials for heterogeneous catalysis are provided.

Structure, stability and electronic properties of two-dimensional monolayer noble metals with triangular lattices: Cu, Ag, and Au

First-principles calculations were performed to investigate the structure, stability, and electronic properties of two-dimensional noble metal monolayers, including Cu, Ag, and Au, inspired by the recent synthesis of a two-dimensional gold monolayer. All 2D noble metals exhibit great stability with almost standard equilateral triangular lattice structures. These monolayers can survive 10 ps MD annealing simulations under 300 K. Phonon spectra do not exhibit negative frequencies. The independent elastic constants, Young's modulus, and Poisson's ratio were obtained, indicating that these monolayers are mechanically stable and slightly anisotropic. Additionally, the results of band structures and density of states (DOS) calculations reveal typical metallic electronic properties. The dumbbell-like Fermi surfaces suggest anisotropic electron transport properties. These findings highlight the immense application potential of 2D noble metal monolayers in diverse fields.

Altermagnetism in two-dimensional Ca2RuO4 perovskite

We propose and characterize a novel two-dimensional material, 2D-CRO, derived from bulk calcium-based ruthenates (CROs) of the Ruddlesden-Popper family, Can+1RunO3n+1 (n = 1 and 2). Using density functional theory, we demonstrate that 2D-CRO maintains structural stability down to the monolayer limit, exhibiting a tight interplay between structural and electronic properties. Notably, 2D-CRO displays altermagnetic behavior, characterized by zero net magnetization and strong spin-dependent phenomena, stabilized through dimensionality reduction. This stability is achieved by breaking inversion symmetry along the z-axis, favoring altermagnetic properties even in the absence of van der Waals interactions. Through theoretical models and computational analysis, we explore the altermagnetic behavior of 2D-CRO, both with and without spin-orbit coupling. We identify the spin components that contribute to the altermagnetic character and highlight the potential of 2D-CRO as a promising material for investigating altermagnetic phenomena and topological features.

Disparate In-Plane Thermal Anisotropy in Monolayer B4X4 (X = F, Cl, Br): The Effect of Lone-Pair Electrons and High-Order Phonon Scattering

The halogenated borophene is theoretically found to exhibit excellent performance in metal-oxide-semiconductor field-effect transistors (MOSFETs) benefiting from the anisotropic electronic structure. In consideration of the thermal management of MOSFETs, the thermal transport mechanisms in halogenated borophene need to be explored. In this work, with the combination of first-principles and phonon Boltzmann transport equation, we report disparate in-plane thermal anisotropy in halogenated monolayer B4X4 (X = F, Cl, Br) arising from the key role of lone-pair electrons and high-order phonon scattering. More than 40% reduction in thermal conductivity is found in B4F4 after including four-phonon scattering by greatly suppressing the contribution from the acoustic modes. The resulting thermal conductivities along the zigzag/armchair directions are 47.13/44.50, 85.56/32.99, and 75.93/20.79 W/mK for B4F4, B4Cl4, and B4Br4, respectively. Anomalously, B4F4 demonstrates a nearly isotropic thermal transport behavior, while B4Br4 has the largest thermal anisotropy, with the thermal conductivity along the zigzag direction being three times higher than that along the armchair direction. The significant influence of high-order phonon scattering is rooted in the strong phonon anharmonicity induced by the lone-pair electrons. In B4F4, due to the large electronegativity difference between the B and F atoms, the interaction between the lone-pair electrons and the bonding electrons is enhanced, resulting in a stronger attenuation of the thermal conductivity compared to the other two materials. Furthermore, the weakening of thermal anisotropy in B4F4 originates from the higher phonon group velocity of longitudinal acoustic modes along the armchair direction, which gives rise to an extra contribution to thermal conductivity and makes the armchair thermal conductivity comparable to the zigzag thermal conductivity. Our results extend the understanding of the anisotropic thermal transport behavior and the important role of high-order phonon scattering in two-dimensional materials.

Theoretical Design for Thorium-Containing Two-Dimensional Materials

Actinide elements are characterized by their unique electronic correlations, variable valence states, and localized 5f electrons, leading to unconventional electronic and topological properties in their compounds. The distinctive physical properties of actinide materials are maintained in low-dimensional forms, yet two-dimensional (2D) actinide materials remain largely unexplored due to their scarcity and the experimental challenges posed by their radioactivity. To fill the knowledge gap in 2D actinide materials, we theoretically designed a series of stable thorium-containing 2D materials, including MXenes, chalcogenides, halides, and other compounds with unique structures. These novel thorium-containing 2D materials show excellent thermodynamic, mechanical, and dynamical stability. Their electronic structures and potential applications were investigated. Considering the proper band edge positions, strong visible light absorption, and effective separation of photogenerated electron-hole pairs, ThPSe3 is proposed as a promising photocatalyst for water splitting. Our work significantly expands the 2D actinide materials family, and further opens new avenues for their experimental realization and applications.

Panoramic analysis of 2D dirubidium telluride monolayer benchmarking the DFT approach

Through the DFT computations, the structural, vibrational, electronic, elastic, optical and thermal (thermoelectric, thermodynamic) properties of the two-dimensional Rb2Te monolayer are briefly contemplated. The Perdew-Bruke-Ernzerhof (PBE) form of generalized gradient approximation (GGA) functional in WIEN2k was deployed for the analysis of all these material properties. The trigonally crystallizing monolayer with an indirect band gap of 1.72 eV may be an upright single-layer that suffices distinct applications. 'No negative' phonon bands confirm the dynamical stability of the monolayer. The Rb2Te monolayer has large indirect band gap than Rb2S and Rb2Se. It exhibits mechanical stability with positive elastic constants satisfying the Born-Huang criterion for two-dimensional materials. The absorption coefficient spanning largely in the ultra-violet range makes the monolayer a congruous material for UV applications. Also, the thermoelectric figure of merit for p-type Rb2Te single-layer at room temperature is high (0.67) compared to the analogous series of compounds, that makes the monolayer a viable one for thermoelectric flexibility and experimental synthesis. The monolayer has high hole effective mass and D ratio. The obtained results aids in revealing the outstanding properties and excellent stability of the monolayer. Based on these findings the Rb2Te monolayer paves the way for promising applications in the fields of photovoltaics, thermoelectrics and UV-based applications.

B2C3N monolayers with high theoretical capacity as anode materials for lithium-ion batteries: first-principles calculations

The search for new anode materials with high lithium-ion battery (LIB) capacity has attracted considerable attention due to the increasing need for electrical power. Here, we utilized first-principles calculations to develop a honeycomb-structured B2C3N monolayer, which exhibits an ultra-high Li-ion storage capacity of 2244 mA h g-1 as an anode material for LIBs. Furthermore, the calculations show that the B2C3N monolayer has a comparatively small diffusion barrier of 0.352 eV and a low open-circuit voltage of 0.134 V. The stability of B2C3N has been verified by analyzing phonon dispersion curves, conducting molecular dynamics simulations, and examining elastic constants. We have found an ultra-high capacity and efficient anode material through theoretical design, which provides a theoretical reference for responding to the global energy crisis and promoting clean energy transformation.

Inverse Design of 2D Altermagnetic Metal-Organic Framework Monolayers from Hückel Theory of Nonbonding Molecular Orbitals

Altermagnets, characterized by spontaneous spin-splitting without net magnetization, are challenging to realize due to their unique spin group symmetries. Two-dimensional (2D) magnetic metal-organic frameworks (MOFs), with tunable topologies and spins, offer promising platforms for achieving altermagnetism. In this study, we propose a general strategy to create 2D altermagnetic monolayers by bridging Cr with organic ligands exhibiting nonbonding molecular orbitals (NBMOs) based on the Hückel molecular orbital theory and first-principles calculations. Three 2D MOFs, namely, Cr(diz)2, Cr(c-pyr)2, and Cr(f-pid)2 (diz = 1,3-diazete, c-pyr = pyrrolo[3,4-c]pyrrole, f-pid = pyrrolo[3,4-f]isoindole), are constructed using this strategy and exhibit the altermagntic ground state. These MOFs possess the spin point group 24̅1 m 22 and exhibit critical temperatures reaching up to 183 K. Analyses of orbital symmetry and energy levels rationalize the presence of altermagnetism. Our findings highlight the critical role of NBMOs in realizing 2D-MOF-based altermagnets with enhanced critical temperatures.

High Stability, Piezoelectric Response, and Promising Photocatalytic Activity on the New Pentagonal CGeP4 Monolayer

This study introduces the penta-structured semiconductor p-CGeP4 through density functional theory simulations, which possesses an indirect band gap transition of 3.20 eV. Mechanical analysis confirms the mechanical stability of p-CGeP4, satisfying Born-Huang criteria. Notably, p-CGeP4 has significant direct (e 31 = -11.27 and e 36 = -5.34 × 10-10 C/m) and converse (d 31 = -18.52 and d 36 = -13.18 pm/V) piezoelectric coefficients, surpassing other pentagon-based structures. Under tensile strain, the band gap energy increases to 3.31 eV at 4% strain, then decreases smoothly to 1.97 eV at maximum stretching, representing an ∼38% variation. Under compressive strain, the band gap decreases almost linearly to 2.65 eV at -8% strain and then drops sharply to 0.97 eV, an ∼69% variation. Strongly basic conditions result in a promising band alignment for the new p-CGeP4 monolayer. This suggests potential photocatalytic behavior across all tensile strain regimes and significant compression levels (ε = 0% to -8%). This study highlights the potential of p-CGeP4 for groundbreaking applications in nanoelectronic devices and materials engineering.

First-Principles Study of the Heterostructure, ZnSb Bilayer/h-BN Monolayer for Thermoelectric Applications

ZnSb is widely recognized as a promising thermoelectric material in its bulk form, and a ZnSb bilayer was recently synthesized from the bulk. In this study, we designed a vertical van der Waals heterostructure consisting of a ZnSb bilayer and an h-BN monolayer to investigate its electronic, elastic, transport, and thermoelectric properties. Based on density functional theory, the results show that the formation of this heterostructure significantly enhances electron mobility and reduces the bandgap compared to the ZnSb bilayer, thereby increasing its power factor. These findings highlight the potential of the h-BN monolayer-supported ZnSb bilayer heterostructure in thermoelectric applications, where maximizing energy conversion efficiency is essential.

Atomically thin 2D materials for solution-processable emerging photovoltaics

Atomically thin 2D materials, such as graphene and graphene oxide, covalent organic frameworks, layered carbides, and metal dichalcogenides, reveal a unique variability of electronic and chemical properties, ensuring their prospects in various energy generation, conversion, and storage applications, including light harvesting in emerging photovoltaic (ePV) devices with organic and perovskite absorbers. Having an extremely high surface area, the 2D materials allow a broad variability of the bandgap and interband transition type, conductivity, charge carrier mobility, and work function through mild chemical modifications, external stimuli, or combination with other 2D species into van-der-Waals heterostructures. This review provides an account of the most prominent "selling points" of atomically thin 2D materials as components of ePV solar cells, including highly tunable charge extraction selectivity and work function, structure-directing and stabilizing effects on halide perovskite light absorbers, as well as broad adaptability of 2D materials to solution-based manufacturing of ePV solar cells using sustainable and upscalable printing technologies. A special focus is placed on the large potential of the materials discovery and design of ePV functionalities based on van-der-Waals stacking of atomically thin 2D building blocks, which can open a vast compositional domain of new materials navigable with machine-learning-based accelerated materials screening.

Novel two-dimensional molybdenene as a promising electrocatalyst for the nitrogen reduction reaction: a first-principles prediction

The exploration of two-dimensional (2D) metallenes is driven by their noteworthy attributes, encompassing high conductivity and substantial exposure of metal active sites, facilitating the development of nitrogen reduction reaction (NRR) electrocatalysts characterized by a low overpotential and superior selectivity. Here, employing first-principles swarm-intelligence structural search methods, we predict molybdenene as a novel and stable non-precious metallene, featuring a 2-atom-thick structure. Our findings demonstrate that the basal plane of molybdenene showcases remarkable catalytic activity with an overpotential of 0.27 V. Bader charge analysis reveals that the exposed Mo bonded to NxHy groups not only transfers electrons to these groups but also the remaining Mo transfers electrons to NxHy upon the breaking of NN bonds. This electron transfer mechanism contributes to elevated NRR catalytic activity. Our work broadens the scope of metallenes as promising electrocatalysts for the NRR with the expectation that more attention will be paid to emerging metallenes, thus offering a modest contribution to the theoretical exploration, fundamental understanding and practical application of metallenes.

High-Throughput Computing of Janus Chalcogenides as Photocatalysts and Piezoelectric Materials for Overall Water Splitting

The presence of the intrinsic fields in two-dimensional (2D) materials holds promise for photocatalysts, as it diminishes the band gap requirements of 1.23 eV and accelerates the separation of the photogenerated carriers. Inspired by the extensive application in MA2X4 families, we predict Janus ZMXAY derived from MA2X4 materials to introduce intrinsic fields suitable for photocatalysts from 512 candidates. These monolayers also exhibit high mobilities up to ∼104 cm2V-1s-1 with strong anisotropy, and are accompanied by the inherent piezoelectric properties. Notably, all monolayers, except Janus SMoPGeAs, SeMoPSiAs, and SeMoPGeAs, demonstrate suitable band gaps (0.88-1.43 eV) and appropriate band edge positions without the need for any external potential to drive spontaneous overall water splitting. It also demonstrates visible optical absorption capacity and high solar-to-hydrogen conversion efficiency (16.06-41.08%). Our work identifies ideal candidates for multifunctional devices and provides theoretical guidance for future experimental research and application development.

Theoretical insights into Z-scheme BAs/GeC van der Waals heterostructure for high-efficiency solar cell

The urgent need for solar electricity production is critical for ensuring energy security and mitigating climate change. Achieving the optimal optical bandgap and effective carrier separation, essential for high-efficiency solar cells, remains a significant challenge when utilizing a single material. In this study, we design a BAs/GeC heterostructure using density functional theory. Our findings indicate that the BAs/GeC heterostructure exhibits direct bandgap semiconductor characteristics. Notably, the BAs/GeC heterostructure demonstrates excellent optical absorption within the infrared and visible light spectrum. Moreover, significant carrier spatial separation was suggested, facilitated by a Z-scheme pathway. Furthermore, applying biaxial strains revealed that the BAs/GeC heterostructure is unstable under compressive strain. However, the electronic and optical properties can be tuned using tensile biaxial strains. The calculated power conversion efficiency (PCE) of the BAs/GeC heterostructure is approximately 31%, as determined by the Scharber method. Hence, the combination of an appropriate bandgap, substantial carrier separation, and superior photoelectric conversion efficiency positions the BAs/GeC heterostructure as a promising candidate for high-efficiency solar cells.

Structure, Synthetic Pathway, and Properties of Metastable Phosphorus Carbide, P4C3

Due to electronegativity (EN) differences, changing from C3N4 to P4C3 is not as trivial as simply replacing nitrogen by phosphorus in the C3N4 structure. Hence, the nonexistent P4C3 phase is nominally the higher-homologue analogue of the well-known C3N4, but its structure and properties are practically unknown for fundamental reasons. Here we predict, by means of an extensive structure search, three energetically favorable yet metastable P4C3 phases adopting space groups P1̅, Cm, and Cmmm, followed by designing their synthetic routes. Different from the planar structural motifs of C3N4, P4C3 is likely to adopt various "puckered" (as regards the P atoms) forms, mainly related to the EN difference between nitrogen and phosphorus, the difference in atomic size, and also less favorable sp orbital mixing for phosphorus related to primogenic repulsion; hence, the P substructures are characterized by single and fractional P-P bonds with rectangular/linear motifs or those resembling simple-cubic P, including multicenter bonding as a function of the large electron count. As regards the carbon substructures, infinite polyacetylene-like motifs, carbon dimers, and infinite chains of cyclopentadienyl-like and polyphenyl-like units are predicted, with C-C bond orders larger than one. Band structure analyses indicate the metallic character of these three phases. The P1̅ polymorph exhibits a Dirac cone, whereas the Cmmm phase, being composed of metallic "phosphorene" units and infinite polyphenyl units, might show potential in applications, e.g., battery electrodes. Moreover, the monolayer of the P1̅ polymorph should be easy to exfoliate and exhibit a large anisotropy with regard to mechanical and electronic properties. The synthesis of P4C3 defining a new class of IV3V4 compounds should be attempted due to its predicted structural and physicochemical properties.

Two-dimensional functionalized MBene Mg2B3T (T = O, H, and F) monolayers as anode materials for high-performance K-ion batteries

Two-dimensional metal borides have received attention as high performance battery anode materials. During the practical application, the 2D surface terminalization is an inevitable problem. This study employs first-principles calculations to investigate the termination of the Mg2B3 monolayer with O, H, F, and Cl groups. These structures' stabilities are examined through energetic, mechanical, kinetic and thermodynamic stability studies. Electronic property analysis shows that Mg2B3T (T = O, H, F, and Cl) monolayers are all metallic. Calculated results reveal that the Mg2B3O, Mg2B3H, and Mg2B3F monolayers exhibit high K ion storage capacities (up to 826 mA h g-1, 980 mA h g-1, and 804 mA h g-1, respectively), with diffusion barriers of 0.338 eV, 0.490 eV, and 0.507 eV, respectively. More importantly, the calculated in-plane lattice constants of the substrate materials exhibit a minimal variation and the observed volume expansion is almost negligible (less than 0.08%) during the entire potassization process, which is much lower than that of the pristine Mg2B3 monolayer. This structural stability is attributed to the presence of surface functional groups. These results provide helpful insights into designing and discovering other high-capacity anode materials for batteries.

Tuning the Surface Stability and Li/Na Storage of MXenes by Controlling the Surface Termination Coverage

2D transition metal carbides and/or nitrides, MXenes, are a class of widely studied materials with great potential for energy storage applications. The control of surface chemistry is an effective approach for preparing novel MXenes and modifying their electrochemical properties. However, an in-depth and systematic atomic-scale study of the effect of surface termination on MXene stability and electrochemical performance is scarce and thus is highly desired. Here, through high-throughput first-principles calculations, 28 stable chalcogen-functionalized M2CTz (M = V, Nb, and Ta, T = S, Se, and Te) under different chemical environments are identified. The reduction of termination coverage improves electrical conductivity but weakens in-plane stiffness. Intriguingly, based on charge transfer mechanism, the diffusion barrier of lithium/sodium atoms on the M2CTz exhibits a volcano-like relationship with termination coverage, and the ion diffusion channel formed in half termination coverage greatly accelerates lithium ion diffusion and returns to or exceeds sodium ion diffusion rate at full termination coverage. V2CSe2/Nb2CSz not only displays the large lithium/sodium capacity (592/409-466 mAhg-1) but also exhibits low barrier energy and open-circuit voltage, suggesting a promising candidate anode material for lithium/sodium-ion batteries. These findings provide insights into the design and fabrication of MXenes and tuning the electrochemical performance of MXenes by controlling termination coverage.

Realizing altermagnetism in two-dimensional metal-organic framework semiconductors with electric-field-controlled anisotropic spin current

Altermagnets exhibit momentum-dependent spin-splitting in a collinear antiferromagnetic order due to their peculiar crystallographic and magnetic symmetry, resulting in the creation of spin currents with light elements. Here, we report two two-dimensional (2D) metal-organic framework (MOF) semiconductors, M(pyz)2 (M = Ca and Sr, pyz = pyrazine), which exhibit both altermagnetism and topological nodal point and line by using first-principles calculations and group theory. The altermagnetic 2D MOFs exhibit unconventional spin-splitting and macroscopic zero magnetization caused by 4-fold rotation in crystalline real space and 2-fold rotation in spin space, leading to the generation and control of anisotropic spin currents when an in-plane electric field ( E ) is applied. In particular, pure spin current with the spin Hall effect occurs when E is applied along the angular bisector of the two spin arrangements. Our work indicates the existence of altermagnetic MOF systems and a universal approach to generate electric-field-controlled spin currents for potential applications in antiferromagnetic spintronics.

Unveiling a New 2D Semiconductor: Biphenylene-Based InN

The two-dimensional (2D) materials class earned a boost in 2021 with biphenylene synthesis, which is structurally formed by the fusion of four-, six-, and eight-membered carbon rings, usually named 4-6-8-biphenylene network (BPN). This research proposes a detailed study of electronic, structural, dynamic, and mechanical properties to demonstrate the potential of the novel biphenylene-like indium nitride (BPN-InN) via density functional theory and molecular dynamics simulations. The BPN-InN has a direct band gap energy transition of 2.02 eV, making it promising for optoelectronic applications. This structure exhibits maximum and minimum Young modulus of 22.716 and 22.063 N/m, Poisson ratio of 0.018 and -0.008, and Shear modulus of 11.448 and 10.860 N/m, respectively. To understand the BPN-InN behavior when subjected to mechanical deformations, biaxial and uniaxial strains in armchair and zigzag directions from -8 to 8% were applied, achieving a band gap energy modulation of 1.36 eV over tensile deformations. Our findings are expected to motivate both theorists and experimentalists to study and obtain these new 2D inorganic materials that exhibit promising semiconductor properties.

Strong mechanical anisotropy and an anisotropic Dirac state in 2D C5N3

Two-dimensional (2D) carbon nitride materials have emerged as a versatile platform for the design of high-performance nanoelectronics, but strong anisotropy in 2D carbon nitrides has rarely been reported. In this work, a 2D carbon nitride with strong anisotropy composed of tetra-, penta-, and hexa-rings (named as TPH-C5N3) is proposed. This TPH-C5N3 exhibits both dynamical and mechanical stability. Furthermore, it also showcases remarkable thermal stability, reaching up to 2300 K, as evidenced by AIMD simulations conducted in an NVT environment utilizing the Nosé-Hoover thermostat. Significantly, TPH-C5N3 demonstrates high anisotropic ratios in its mechanical properties, positioning it as the frontrunner in the current carbon nitride systems. In addition, a Dirac cone with an anisotropic ratio of 55.8% and Fermi velocity of 7.26 × 105 m s-1 is revealed in TPH-C5N3. The nontrivial topological properties of TPH-C5N3 are demonstrated by a non-zero Z2 invariant and topologically protected edge states. Our study offers theoretical insights into an anisotropic 2D carbon nitride material, laying the groundwork for its design and synthesis.

Comparative first principles investigation on the structural, optoelectronic and vibrational properties of strain-engineered graphene-like AlC3, BC3and C3N monolayers

Three cardinal two-dimensional semiconductorsviz., AlC3, BC3and C3N, closely resembling the graphene structure, are intriguing contenders for emerging optoelectronic and thermomechanical applications. Starting from a critical stability analysis, this density functional theory study delves into a quantitative assessment of structural, mechanical, electronic, optical, vibrational and thermodynamical properties of these monolayers as a function of biaxial strain(ε)in a sublinear regime(-2%⩽ε⩽4%)of elastic deformation. The structures with cohesive energies slightly smaller than graphene, manifest exceptional mechanical stiffness, flexibility and breaking stress. The mechanical parameters have been deployed to further cultivate acoustic attributes and thermal conductivity. The hexagonal structures with mixed ionic-covalent molecular bonds have indirect electronic band-gap and work-function acutely sensitive toε. Dispersions of optical dielectric function, energy loss, refractive index, extinction coefficient, reflectivity, absorption coefficient and conductivity are deciphered in the UV-Vis-NIR regime against strain, where particular frequency bands featuring high polarization, dissipation, absorbance or reflectance are identified. Phonon band-structure and density of states testify dynamic stability in the ground state for all systems except the compressed ones. A comprehensive group theoretical analysis is performed to cultivate rotational; infrared and Raman-active modes, and the nature of molecular vibrations is delineated. The red-shifting of phonon bands andE2g/A1gRaman peaks with increasingε, associates estimation of Grüneisen parameter. Finally, strain-induced alterations of thermodynamic quantities such as entropy, enthalpy, free energy, heat capacity and Debye temperature are studied, followed by a molecular dynamics-based stability assessment under canonical ensemble.

How spin state and oxidation number of transition metal atoms determine molecular adsorption: a first-principles case study for NH3

Understanding how the electronic state of transition metal atoms can influence molecular adsorption on a substrate is of great importance for many applications. Choosing NH3 as a model molecule, its adsorption behavior on defected SnS2 monolayers is investigated. The number of valence electrons n is controlled by decorating the monolayer with different transition metal atoms, ranging from Sc to Zn. Density-Functional Theory based calculations show that the adsorption energy of NH3 molecules oscillates with n and shows a clear odd-even pattern. There is also a mirror symmetry of the adsorption energies for large and low electron numbers. This unique behavior is mainly governed by the oxidation state of the TM ions. We trace back the observed trends of the adsorption energy to the orbital symmetries and ligand effects which affect the interaction between the 3σ orbitals (NH3) and the 3d orbitals of the transition metals. This result unravels the role which the spin state of TM ions plays in different crystal fields for the adsorption behavior of molecules. This new understanding of the role of the electronic structure on molecular adsorption can be useful for the design of high efficiency nanodevices in areas such as sensing and photocatalysis.

Hydrogenation driven ultra-low lattice thermal conductivity inβ12borophene

Borophene gathered large interest owing to its polymorphism and intriguing properties such as Dirac point, inherent metallicity, etc but oxidation limits its capabilities. Hydrogenated borophene was recently synthesised experimentally to harness its applications. Motivated by experimental work, in this paper, using first-principles calculations and Boltzmann transport theory, we study the freestandingβ12borophene nanosheet doped and functionalised with hydrogen (H), lithium (Li), beryllium (Be), and carbon (C) atoms at differentβ12lattice sites. Among all possible configurations, we screen two stable candidates, pristine and hydrogenatedβ12borophene nanosheets. Both nanosheets possess dynamic and mechanical stability while the hydrogenated sheet has different anisotropic metallicity compared to pristine sheet leading to enhancement in brittle behaviour. Electronic structure calculations reveal that both nanosheets host Dirac cones (DCs), while hydrogenation leads to shift and enhancement in tilt of the DCs. Further hydrogenation leads to the appearance of additional Fermi pockets in the Fermi surface. Transport calculations reveals that the lattice thermal conductivity changes from 12.51 to 0.22 W m-1 K-1(along armchair direction) and from 4.42 to 0.07 W m-1 K-1(along zigzag direction) upon hydrogenation at room temperature (300 K), demonstrating a large reduction by two orders of magnitude. Such reduction is mainly attributed to decreased phonon mean free path and relaxation time along with the enhanced phonon scattering rates stemming from high frequency phonon flat modes in hydrogenated nanosheet. Comparatively larger weighted phase space leads to increased anharmonic scattering in hydrogenated nanosheet contributing to ultra-low lattice thermal conductivity. Consequently, hydrogenatedβ12nanosheet exhibits a comparatively higher thermoelectric figure of merit (∼0.75) at room temperature along armchair direction. Our study demonstrates the effects of functionalisation on transport properties of freestandingβ12borophene nanosheets which can be utilised to enhance the thermoelectric performance in two-dimensional (2D) systems and expand the applications of boron-based 2D materials.

The coexistence of high piezoelectricity and superior optical absorption in Janus Bi2X2Y (X = Te, Se; Y = Te, Se, S) monolayers

Bismuth chalcogenide and its derivatives have been attracting attention in various fields as semiconductors or topological insulators. Inspired by the high piezoelectric properties of Janus Bi2TeSeS monolayer and the excellent optical absorption properties of the Bi2X3 (X = Te, Se, S) monolayers, we theoretically predicted four new-type two-dimensional (2D) monolayers Janus Bi2X2Y (X = Te, Se; Y = Te, Se, S) using the first principles combined with density functional theory (DFT). The thermal, dynamic, and mechanical stabilities of Janus Bi2X2Y monolayers were confirmed based on ab initio molecular dynamics (AIMD) simulations, phonon dispersion, and elastic constants calculations. Their elastic properties, band structures, piezoelectric, and optical properties were systematically investigated. It was found that Janus Bi2X2Y monolayers have a typical Mexican hat-shaped valence band edge structure and, therefore, have a ring-shaped flat band edge, which results in their indirect band gaps. The results show that Janus Bi2X2Y monolayers are semiconductors with moderate band gaps (0.62-0.98 eV at the HSE + SOC level). After considering the electron-phonon renormalization (EPR), the band gaps are reduced by less than 5% at 0 K under the zero-point renormalization (ZPR) and further reduced by approximately 10% at 300 K. Besides, Janus Bi2X2Y monolayers also exhibit excellent optical absorption properties in the blue-UV light region, with the peak values at the order of 8 × 105 cm-1. Particularly, the Janus Bi2Te2S monolayer was found to exhibit a piezoelectric strain coefficient d11 of up to 20.30 pm V-1, which is higher than that of most of the 2D materials. Our results indicate that Janus Bi2X2Y monolayers could be promising candidates in solar cells, optical absorption, and optoelectronic devices; especially, a Janus Bi2Te2S monolayer can also be an excellent piezoelectric material with great prospects in the fields of mechanical and electrical energy conversion.

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.

Activating two-dimensional semiconductors for photocatalysis: a cross-dimensional strategy

The emerging two-dimensional (2D) semiconductors substantially extend materials bases for versatile applications such as semiconductor photocatalysis demanding semiconductive matrices and large surface areas. The dimensionality, while endowing 2D semiconductors the unique properties to host photocatalytic functionality of pollutant removal and hydrogen evolution, hurdles the activation paths to form heterogenous photocatalysts where the photochemical processes are normally superior over these on the mono-compositional counterparts. In this perspective, we present a cross-dimensional strategy to employ thenD (n= 0-2) clusters or nanomaterials as activation partners to boost the photocatalytic activities of the 2D semiconductors. The formation principles of heterogenous photocatalysts are illustrated specifically for the 2D matrices, followed by selection criteria of them among the vast 2D database. The computer investigations are illustrated in the density functional theory route and machine learning benefitted from the vast samples in the 2D library. Synthetic realizations and characterizations of the 2D heterogenous systems are introduced with an emphasis on chemical methods and advanced techniques to understand materials and mechanistic studies. The perspective outlooks cross-dimensional activation strategies of the 2D materials for other applications such as CO2removal, and materials matrices in other dimensions which may inspire incoming research within these fields.

Perovskenes: two-dimensional perovskite-type monolayer materials predicted by first-principles calculations

Inspired by the successful transfer of freestanding ultrathin films of SrTiO3 and BiFeO3 onto various substrates without any thickness limitation, in this study, using density functional theory (DFT), we assessed the structural stability of a group of two-dimensional perovskite-type materials which we call perovskenes. Specifically, we analyzed the stability of 2D SrTiO3, SrZrO3, BaTiO3, and BaZrO3 monolayers. Our simulations revealed that the 2D monolayers of SrTiO3, BaTiO3, and BaZrO3 are at least meta-stable, as confirmed by cohesive energy calculations, evaluation of elastic constants, and simulation of phonon dispersion modes. With this information, we proceeded to investigate the electronic, optical, and thermoelectric properties of these perovskenes. To gain insight into their promising applications, we investigated the electronic and optical properties of these 2D materials and found that they are wide bandgap semiconductors with significant absorption and reflection in the ultraviolet (UV) region of the electromagnetic field, suggesting them as promising materials for use in UV shielding applications. In addition, evaluating their thermoelectric factors revealed that these materials become better conductors of electricity and heat as the temperature rises. They can, hence, convert temperature gradients into electrical energy and transport electrical charges, which is beneficial for efficient power generation in thermoelectric devices. This work opens a new window for designing a novel family of 2D perovskite type materials termed perovskenes. The vast variety of different perovskite compounds and their variety of applications suggest deeper studies on the perovskenes materials for use in innovative technologies.

Two-dimensional AlN/TMO van der Waals heterojunction as a promising photocatalyst for water splitting driven by visible light

In this study, the photocatalytic properties of AlN/TMO heterojunctions formed by coupling MoO2 and WO2 of transition metal oxides with AlN are studied in detail using first-principles calculations with the aim of finding efficient and low-cost photocatalysts for water splitting to produce hydrogen to reduce environmental pollution. The AIMD, phonon spectrum, and elastic constants demonstrated the thermodynamic, kinetic, and mechanical stabilities of the AlN/TMO heterojunction. The results showed that the AlN/MoO2 (1.55 eV) and AlN/WO2 (1.99 eV) heterojunctions have typical type-II energy band arrangements, which can effectively promote the separation of photogenerated electrons and hole pairs. Meanwhile, the AlN/MoO2 heterojunction showed excellent carrier mobilities (electron, 250.05 cm2 V-1 S-1 and hole, 45 467.07 cm2 V-1 S-1), which greatly exceeded those of each component. The AlN/WO2 heterojunction showed an excellent HER (-0.07 eV) performance, which was close to the expected value. For the AlN/WO2 heterojunction, a suitable band gap value, excellent HER, and other properties indicated that it has the potential to become a new candidate for photocatalytic water splitting. Our study enriches the theoretical research of transition metal oxide materials and wide-band gap materials by providing a reference direction for the design of reasonably high-quality photocatalysts.

Super-high carrier mobilities and excellent thermoelectric performances of Tri-Tri group-VA monolayers

High-performance thermoelectric materials in theoretical and experimental research are mostly composed of expensive, scarce, heavy elements and rarely of single light elements, which severely limit their application and development. Based on density functional and semiclassical Boltzmann transport theory, we determine that a stable phosphorene allotrope, named Tri-Tri phosphorene, has super-high electron mobility (23845.29 cm2 V-1 s-1) much higher than those of most two-dimension materials. Moreover, its optimized maximum ZT can reach up to 3.43 at room temperature (4.83 at 500 K and 5.92 at 700 K), exhibiting highly favorable prospects in practical thermoelectric systems. Motivated by the excellent properties of Tri-Tri phosphorene, we further demonstrate the structural stability of Tri-Tri arsenene and Tri-Tri antimonene and predict that the two Tri-Tri structures also have high Seebeck coefficients and electron mobilities. Their lattice thermal conductivities are dramatically decreased compared with Tri-Tri phosphorene. Thus, their predicted thermoelectric performances are also excellent, with maximum ZT values of 4.12 (Tri-Tri arsenene) and 3.54 (Tri-Tri antimonene) at room temperature. The low layer moduli of the three Tri-Tri structures indicate that they have high mechanical flexibility and suitability for current device assemblies. All these desirable properties make Tri-Tri group-VA materials promising for future applications in thermoelectric devices.

A novel two-dimensional NiCl2O8 lattice with negative Poisson's ratio and magnetic modulation

Two-dimensional (2D) materials with simultaneous magnetic semiconducting properties and a negative Poisson's ratio are crucial for fabricating multifunctional electronic devices. However, progress in this area has been generally constrained. Based on first-principles calculations, we engineered a 2D Ni-based oxyhalide with a honeycomb lattice structure. It was observed that the NiCl2O8 monolayer exhibits both high- and low-buckling states in its geometry, along with intrinsic magnetic semiconductor properties in its electronic structure. Importantly, we demonstrated that the magnetic ordering of the NiCl2O8 lattice is susceptible to applied strain, which resulted in a phase transition from paramagnetic to ferromagnetic under biaxial strain. The Curie temperature was also evaluated using Monte Carlo simulations within the Ising model. Additionally, our research uncovered that the 2D NiCl2O8 lattice chain displays a negative Poisson's ratio (NPR) along the z-direction. The triangular hinge structure in its centrosymmetric configuration was identified as the origin of this unique phenomenon. The coexistence of NPR and magnetic phase transition properties in the NiCl2O8 lattice makes it quite promising for applications in nanoelectronic and spintronic devices.

A Janus CrSSe monolayer with interesting ferromagnetism

The search for intrinsic half-metallic ferromagnetic (FM) monolayers with a high Curie temperature (TC), considerable magnetic anisotropy energy (MAE), and multiferroic coupling is key for the development of ultra-compact spintronics. Here, we have identified a new stable FM Janus monolayer, the tetrahedral CrSSe, through first-principles structural search calculations, which not only exhibits very interesting magnetoelectric properties with a high TC of 790 K, a large MAE of 0.622 meV per Cr, and robust half-metallicity, but also shows obvious ferroelasticity with a modest energy barrier of 0.31 eV per atom. Additionally, there appears to be interesting multiferroic coupling between in-plane magnetization and ferroelasticity. Furthermore, by replacing the Se/S atoms in the CrSSe monolayer with S/Se atoms, we obtained two new half-metallic FM CrS2 and CrSe2 monolayers, which also exhibit excellent magnetoelectric properties. Therefore, our findings provide a pathway to design novel multiferroic materials and enrich the understanding of 2D transition metal chalcogenides.

Two-dimensional β-noble-transition-metal chalcogenide: novel highly stable semiconductors with manifold outstanding optoelectronic properties and strong in-plane anisotropy

In this work, five two-dimensional (2D) noble-transition-metal chalcogenide (NTMC) semiconductors, namely β-NX (N = Au, Ag; X = S, Se, Te), were designed and predicted by first-principles simulations. Structurally, the monolayer β-NX materials have good energetic, mechanical, dynamical, and thermal stability. They contain two inequivalent noble-transition-metal atoms in the unit cell, and the N-X bond comprises a partial ionic bond and a partial covalent bond. Regarding the electronic properties, the β-NX materials are indirect-band-gap semiconductors with appropriate band-gap values. They have tiny electron effective masses. The hole effective masses exhibit significant differences in different directions, indicating strongly anisotropic hole mobility. In addition, the coexistence of linear and square-planar channels means that the diffusion and transport of carriers should be anisotropic. In terms of optical properties, the β-NX materials show high absorption coefficients. The absorption and reflection characteristics reveal strong anisotropy in different directions. Therefore, the β-NX materials are indirect-band-gap semiconductors with good stability, high absorption coefficients, and strong mechanical, electronic, transport, and optical anisotropy. In the future, they could have great potential as 2D semiconductors in nano-electronics and nano-optoelectronics.

Structural, mechanical, electronic and optical properties of biphenylene hydrogenation: a first-principles study

Biphenylene networks typically exhibit a metallic electronic nature, while hydrogenation can open the band gap changing it to a semiconductor. This property makes hydrogenated biphenylene a promising candidate for use in semiconductor optoelectronic materials and devices. In this work, three representative configurations of hydrogenated biphenylene, denoted by α, β and γ, were investigated. The structural, mechanical, electronic, and optical properties of these hydrogenated biphenylene configurations were calculated by first-principles calculations. Band gaps with HSE correction were 4.69, 4.42 and 4.39 eV for α, β, and γ configurations, respectively. Among these three configurations, β presents the best electronic performance and special elastic properties (negative Poisson's ratio), while γ exhibits the best elastic properties. In addition, we comprehensively analyze the mechanical properties of these configurations and provide evidence that hydrogenated biphenylene possibly exhibits a negative-Poisson's-ratio along the zigzag and armchair directions when hydrogen atoms are added to biphenylene in certain ways. Furthermore, although the electronic properties of γ are weaker than those of β, they are also excellent. In addition, the binding energies of β and γ are relatively lower, which indicates that β and γ are more stable. Our findings demonstrate that the hydrogenated biphenylene is a promising material with significant application potential in optoelectronic devices.

First-principles study on the structure and electronic properties of M2CSx (M = Sc, Ti, Y, Zr and Hf, x = 1, 2)

Two-dimensional (2D) transition metal carbides/nitrides, known as MXenes, have attracted extensive attention due to their rich elemental composition and diverse surface chemistry. In this study, the crystal structure, electronic, mechanical, and electronic transport properties of M₂CS_x (M = Sc, Ti, Y, Zr, and Hf, x = 1, 2) were investigated by density functional theory (DFT). Our results showed that the studied M₂CS_x except Y₂CS₂ are thermodynamically, dynamically, thermally, and mechanically stable. The p-d hybridization between the M-d state and the C/S-p state of M₂CS is stronger than that of the corresponding M₂CS₂. However, the antibonding state would appear near the Fermi level and thus reduce the thermal stability of the material due to the introduction of sulfur vacancies in the Y-free MXenes studied. In contrast, sulfur vacancies would significantly enhance the bonding states of Y-C and Y-S bonds and improve the stability of Y₂CS_x. This provides an explanation for the experimentally observed formation of non-stoichiometric Ti₂CS_1.2. The room-temperature electron mobilities of semiconductor Sc₂CS (Y₂CS) along the x and y directions were determined to be 232.59 (818.51) and 628.22 (552.55) cm² V^-1 s^-1, and the room-temperature hole mobilities are only 88.32 (1.64) and 61.75 (17.80) cm² V^-1 s^-1. This work is expected to provide theoretical insights for the preparation and application of S-terminated MXenes.