Strain-engineered thermophysical properties ranging from band-insulating to topological insulating phases in β-antimonene

The use of strain in semiconductors allows extensive modification of their properties. Due to their robust mechanical strength and flexibility, atomically thin 2D materials are very well suited for strain engineering to extract exotic electronic and thermophysical properties. We investigated the structural, electronic, thermal, and vibrational characteristics along with the phonon and carrier dynamics of β-Sb elemental monolayers for achieving the band-insulating phase at no strain and topological insulating phase at ∼15% biaxial strain. A reduction in stiffness was noticed due to the weakening of the π and σ bonds with strain, leading to anharmonicity in the system. This was further reflected by the drop in lattice thermal conductivity (κl) from 4.5 to 3.1 W m-1 K-1 at 15% strain, i.e., in the topological phase. The appearance of helical edge states at 15% strain and meeting the Z2 invariant criterion confirm the non-trivial topological state. The significant contribution of the out-of-plane A1g vibrational mode was noticed in the topological phase compared with the band-insulating phase. Further, the observed larger reduction in hole lifetime could be attributed to strong scattering near the valence band edge. Importantly, the dominance of the out-of-plane optical modes contributes significantly along the band edges to the topological phase, which is primarily due to the reduced buckling height under strain. Thus, this work emphasizes the microscopic origin of the onset of the topological phase in strained β-Sb monolayers and provides strain-engineered structure-property correlations for better insights.

Orbital distortion and electric field control of sliding ferroelectricity in a boron nitride bilayer

Recent experiments confirm that two-dimensional boron nitride (BN) films possess room-temperature out-of-plane ferroelectricity when each BN layer is sliding with respect to each other. This ferroelectricity is attributed to the interlayered orbital hybridization or interlayer charge transfer in previous work. In this work, we attempt to understand the sliding ferroelectricity from the perspective of orbital distortion of long-pair electrons. Using the maximally localized Wannier function method and first-principles calculations, the out-of-planep_zorbitals of BN are investigated. Our results indicate that the interlayer van der Waals interaction causes the distortion of the Np_zorbitals. Based on the picture of out-of-plane orbital distortion, we propose a possible mechanism to tune the ferroelectric polarization by external fields, including electric field and stress field. It is found that both the polarization intensity and direction can be modulated under the electric field. The polarization intensity of the system can also be controlled by stress field perpendicular to the plane. This study will provide theoretical help in the device design based on sliding ferroelectrics.

Antimonene: a tuneable post-graphene material for advanced applications in optoelectronics, catalysis, energy and biomedicine

The post-graphene era is undoubtedly marked by two-dimensional (2D) materials such as quasi-van der Waals antimonene. This emerging material has a fascinating structure, exhibits a pronounced chemical reactivity (in contrast to graphene), possesses outstanding electronic properties and has been postulated for a plethora of applications. However, chemistry and physics of antimonene remain in their infancy, but fortunately recent discoveries have shed light on its unmatched allotropy and rich chemical reactivity offering a myriad of unprecedented possibilities in terms of fundamental studies and applications. Indeed, antimonene can be considered as one of the most appealing post-graphene 2D materials reported to date, since its structure, properties and applications can be chemically engineered from the ground up (both using top-down and bottom-up approaches), offering an unprecedented level of control in the realm of 2D materials. In this review, we provide an in-depth analysis of the recent advances in the synthesis, characterization and applications of antimonene. First, we start with a general introduction to antimonene, and then we focus on its general chemistry, physical properties, characterization and synthetic strategies. We then perform a comprehensive study on the allotropy, the phase transition mechanisms, the oxidation behaviour and chemical functionalization. From a technological point of view, we further discuss the applications recently reported for antimonene in the fields of optoelectronics, catalysis, energy storage, cancer therapy and sensing. Finally, important aspects such as new scalable methodologies or the promising perspectives in biomedicine are discussed, pinpointing antimonene as a cutting-edge material of broad interest for researchers working in chemistry, physics, materials science and biomedicine.

Unveiling the hybridization between the Cr-impurity-mediated flat band and the Rashba-split state of the α-Au/Si(111) surface

Adsorption of foreign atoms onto 2D materials can either lead to ordinary electron doping or the emergence of new electronic effects including topology, superconductivity, and quantum anomalous Hall and Kondo states. We have investigated the effect of Cr doping on the electronic structure of the α-Au/Si(111)- surface and its adsorbate-modified family. It has been found that below a critical coverage of ∼0.05 monolayer, Cr adatoms penetrate beneath the Au and topmost Si layers and induce the occupied resonance flat band in the electronic spectrum as revealed by angle-resolved photoelectron spectroscopy. Further deposition of Cr leads to the growth of the 3D islands spoiling the surface homogeneity. Using density functional theory calculations, we have disclosed the effects of Cr doping on the electronic band structure and revealed the nature of hybridization between the Cr-induced magnetic-split band and the Au-induced Rashba-split surface state. We believe that the synthesized 2D phases and electronic effects produced by magnetic atom doping in the ultimate two-dimensional limit will stimulate further investigations related to the highly correlated phases and will find practical applications in nanoelectronic devices.

First principles study of electronic and optical properties and photocatalytic performance of GaN-SiS van der Waals heterostructure

The vertical stacking of two-dimensional materials via van der Waals (vdW) interaction is a promising technique for tailoring the physical properties and fabricating potential devices to be applied in the emerging fields of materials science and nanotechnology. The structural, electronic and optical properties and photocatalytic performance of a GaN-SiS vdW heterostructure were explored using first principles calculations. The most stable stacking configuration found energetically stable, possesses a direct staggered band gap, which is crucial for separating photogenerated charged carriers in different constituents and is efficacious for solar cells. Further, the charge transfer occurred from the SiS to GaN layer, indicating that SiS exhibits p-type doping in the GaN-SiS heterobilayer. Interestingly, a systematic red-shift was observed in the optical absorption spectra of the understudy heterobilayer system. Moreover, the conduction band edge and valence band edge of the monolayers and corresponding heterostructure were located above and below the standard redox potentials for photocatalytic water splitting, making these systems promising for water dissociation for hydrogen fuel production. The results provide a route to design the GaN-SiS vdW heterostructure for the practical realization of next-generation light detection and energy harvesting devices.

Nanomechanics of antimonene allotropes under tensile loading

Monolayer antimonene has drawn the attention of research communities due to its promising physical properties. However, the mechanical properties of antimonene have remained largely unexplored. In this work, we investigate the mechanical properties and fracture mechanisms of two stable phases of monolayer antimonene - β-antimonene (puckered structure) and α-antimonene (buckled structure) - through molecular dynamics (MD) simulations. Our simulations reveal that a stronger chiral effect results in a greater anisotropic elastic behavior in α-antimonene than in β-antimonene. We focus on crack-tip stress distribution using local volume averaged virial stress definition and derive the fracture toughness from the crack-line stress. Our calculated crack tip stress distribution ensures the applicability of linear elastic fracture mechanics (LEFM) for cracked antimonene allotropes with considerable accuracy up to a pristine structure. We evaluate the effect of temperature, strain rate, crack-length, and point-defect concentration on the strength and elastic properties. The tensile strength of antimonene degrades significantly with the increase of temperature, crack length and defect concentration. The elastic modulus is found to be less susceptible to temperature variation but is largely affected by the increase in defects. The strain rate exhibits a power law relationship between strength and fracture strain. Finally, we discuss the fracture mechanisms in the light of crack propagation and establish the relationship between the fracture mechanism and the observed anisotropic properties.

Investigation of strain and doping on the electronic properties of single layers of C6N6 and C6N8: a first principles study

In this work, by performing first-principles calculations, we explore the effects of various atom impurities on the electronic and magnetic properties of single layers of C6N6 and C6N8. Our results indicate that atom doping may significantly modify the electronic properties. Surprisingly, doping Cr into a holey site of C6N6 monolayer was found to exhibit a narrow band gap of 125 meV upon compression strain, considering the spin-orbit coupling effect. Also, a C atom doped in C6N8 monolayer shows semi-metal nature under compression strains larger than -2%. Our results propose that Mg or Ca doped into strained C6N6 may exhibit small band gaps in the range of 10-30 meV. In addition, a magnetic-to-nonmagnetic phase transition can occur under large tensile strains in the Ca doped C6N8 monolayer. Our results highlight the electronic properties and magnetism of C6N6 and C6N8 monolayers. Our results show that the electronic properties can be effectively modified by atom doping and mechanical strain, thereby offering new possibilities to tailor the electronic and magnetic properties of C6N6 and C6N8 carbon nitride monolayers.

Strain and electric field tuning of semi-metallic character WCrCO2MXenes with dual narrow band gap

Motivated by the recent successful synthesis of double-M carbides, we investigate structural and electronic properties of WCrC and WCrCO2monolayers and the effects of biaxial and out-of-plane strain and electric field using density functional theory. WCrC and WCrCO2monolayers are found to be dynamically stable. WCrC is metallic and WCrCO2display semi-metallic character with narrow band gap, which can be controlled by strain engineering and electric field. WCrCO2monolayer exhibits a dual band gap which is preserved in the presence of an electric field. The band gap of WCrCO2monolayer increases under uniaxial strain while it becomes metallic under tensile strain, resulting in an exotic 2D double semi-metallic behavior. Our results demonstrate that WCrCO2is a new platform for the study of novel physical properties in two-dimensional Dirac materials and which may provide new opportunities to realize high-speed low-dissipation devices.

Strain, electric-field and functionalization induced widely tunable electronic properties in MoS2/BC 3, /C 3 N and /[Formula: see text] van der Waals heterostructures

In this paper, the effect of BC ₃, C ₃ N and [Formula: see text] substrates on the atomic and electronic properties of MoS₂ were systematically investigated using first-principles calculations. Our results show that the MoS₂/BC ₃ and MoS₂/C ₃ N4 heterostructures are direct semiconductors with band gaps of 0.4 and 1.74 eV, respectively, while MoS₂/C ₃ N is a metal. Furthermore, the influence of strain and electric field on the electronic structure of these van der Waals heterostructures is investigated. The MoS₂/BC₃ heterostructure, for strains larger than -4%, transforms it into a metal where the metallic character is maintained for strains larger than -6%. The band gap decreases with increasing strain to 0.35 eV (at +2%), while for strain (>+6%) a direct-indirect band gap transition is predicted to occur. For the MoS₂/C₃N heterostructure, the metallic character persists for all strains considered. On applying an electric field, the electronic properties of MoS₂/C₃N₄ are modified and its band gap decreases as the electric field increases. Interestingly, the band gap reaches 30 meV at +0.8 V/Å, and with increase above +0.8 V/Å, a semiconductor-to-metal transition occurs. Furthermore, we investigated effects of semi- and full-hydrogenation of MoS₂/C₃N and we found that it leads to a metallic and semiconducting character, respectively.

Dirac half-metallicity of Thin PdCl3 Nanosheets: Investigation of the Effects of External Fields, Surface Adsorption and Defect Engineering on the Electronic and Magnetic Properties

PdCl₃ belongs to a novel class of Dirac materials with Dirac spin-gapless semiconducting characteristics. In this paper based, on first-principles calculations, we have systematically investigated the effect of adatom adsorption, vacancy defects, electric field, strain, edge states and layer thickness on the electronic and magnetic properties of PdCl₃ (palladium trichloride). Our results show that when spin-orbital coupling is included, PdCl₃ exhibits the quantum anomalous Hall effect with a non-trivial band gap of 24 meV. With increasing number of layers, from monolayer to bulk, a transition occurs from a Dirac half-metal to a ferromagnetic metal. On application of a perpendicular electrical field to bilayer PdCl₃, we find that the energy band gap decreases with increasing field. Uniaxial and biaxial strain, significantly modifies the electronic structure depending on the strain type and magnitude. Adsorption of adatom and topological defects have a dramatic effect on the electronic and magnetic properties of PdCl₃. In particular, the structure can become a metal (Na), half-metal (Be, Ca, Al, Ti, V, Cr, Fe and Cu with, respective, 0.72, 9.71, 7.14, 6.90, 9.71, 4.33 and 9.5 μ_B magnetic moments), ferromagnetic-metal (Sc, Mn and Co with 4.55, 7.93 and 2.0 μ_B), spin-glass semiconductor (Mg, Ni with 3.30 and 8.63 μ_B), and dilute-magnetic semiconductor (Li, K and Zn with 9.0, 9.0 and 5.80 μ_B magnetic moment, respectively). Single Pd and double Pd + Cl vacancies in PdCl₃ display dilute-magnetic semiconductor characteristics, while with a single Cl vacancy, the material becomes a half-metal. The calculated optical properties of PdCl₃ suggest it could be a good candidate for microelectronic and optoelectronics devices.

Electronic structures, and optical and photocatalytic properties of the BP–BSe van der Waals heterostructures

The combination of two-dimensional materials in the form of van der Waals (vdW) heterostructures has been shown to be an effective method for designing electronic and optoelectronic equipment.

Control of C3N4 and C4N3 carbon nitride nanosheets’ electronic and magnetic properties through embedded atoms

In the present work, the effect of various embedded atom impurities on tuning electronic and magnetic properties of C₃N₄ and C₄N₃ nanosheets have been studied using first-principles calculations.

Embedding of atoms into the nanopore sites of the C6N6 and C6N8 porous carbon nitride monolayers with tunable electronic properties

Using first-principles calculations, we study the effect of embedding various atoms into the nanopore sites of both C₆N₆ and C₆N₈ monolayers.

Modulating the electro-optical properties of doped C3N monolayers and graphene bilayers via mechanical strain and pressure

The effect of atomic doping on the electronic properties of C₃N monolayer and graphene bilayer is investigated. We found that doped C₃N monolayer and doped graphene bilayer are a direct semiconductor. Our result show that the electronic properties of the studied structures can be modulated by electric field and mechanical strain.

Two-Dimensional SiP, SiAs, GeP and GeAs as Promising Candidates for Photocatalytic Applications

Group IV–V-type layered materials, such as SiP, SiAs, GeP and GeAs, are among the most attractive two-dimensional (2D) materials that exhibit anisotropic mechanical, optical and transport properties. In this short communication, we conducted density functional theory simulations to explore the prospect of SiP, SiAs, GeP and GeAs nanosheets for the water-splitting application. The semiconducting gaps of stress-free SiP, SiAs, GeP and GeAs monolayers were estimated to be 2.59, 2.34, 2.30 and 2.07 eV, respectively, which are within the desirable ranges for the water splitting. Moreover, all the considered nanomaterials were found to yield optical absorption in the visible spectrum, which is a critical feature for the employment in the solar water splitting systems. Our results furthermore confirm that the valence and conduction band edge positions in SiP, SiAs, GeP and GeAs monolayers also satisfy the requirements for the water splitting. Our results highlight the promising photocatalytic characteristics of SiP, SiAs, GeP and GeAs nanosheets for the application in solar water splitting and design of advanced hydrogen fuel cells.

Introducing novel electronic and magnetic properties in C3N nanosheets by defect engineering and atom substitution

Using first-principles calculations the effects of topological defects, vacancies, Stone–Wales and anti-site and substitution of atoms, on the structure and electronic properties of monolayer C₃N are investigated.