In-plane Biaxial Strain Research Articles

Stacking order effects on the electronic and optical properties of GaS/XMoY (X/Y = S, Se, Te) Van der Waals heterostructures: a first-principles study

Two-dimensional (2D) heterostructures formed by van der Waals (vdW) interactions have attracted considerable attention in the fields of electronics and optoelectronics. The stacking order is not only an important method for regulating interlayer interactions, but also an intrinsic property specific to 2D vdW heterostructures. Herein, the GaS/XMoY (X/Y = S, Se, Te) vdW heterostructures are proposed by first-principles calculations. The effects of stacking order (namely, GaS/SMoSe, GaS/SeMoS, GaS/SeMoTe and GaS/TeMoSe) on the electronic properties, light absorption, and photocatalysis of the heterostructures are discussed in detail. We identify stacking order as a dominant pathway for interlayer interactions, and surmise that stacking order effectively regulates dipole moment, mechanical flexibility, carrier mobility, optical absorption coefficient, and photocatalytic water-splitting of GaS/XMoY heterostructures. The in-plane biaxial strain can make the energy gap of each stacking order reach a maximum value, and their photocatalytic performance can also be improved to different degrees. This work analyzes the modulation effect of stacking order on the material properties of GaS/XMoY heterostructures, which provides theoretical clues for the design of efficient and stable optoelectronic devices and photocatalytic water-splitting.

How graphenic are graphynes? Evidence for low-lying correlated gapped states in graphynes.

Graphynes can be structurally envisioned as 2D extensions to graphene, whereby linearly bonded carbon linkages increase the distance between trigonal carbon nodes. Many graphynes have been predicted to exhibit a Dirac-like semimetallic (SEM) graphenic electronic structure, which could potentially make them competitive with graphene for applications. Currently, most graphynes remain as attractive synthetic targets, and their properties are still unconfirmed. Here, we demonstrate that the electronic structure of hexagonal α-graphyne is analogous to that of biaxially strained graphene. By comparison with accurate quantum Monte Carlo results on strained graphene, we show that the relative energetic stability of electronic states in this correlated 2D system can be captured by density functional theory (DFT) calculations using carefully tailored hybrid functionals. Our tuned hybrid DFT approach confirms that α-graphyne has a low energy correlated Mott-like antiferromagnetic insulating (AFI) state, which competes with the SEM state. Our work shows that the AFI-SEM crossover in α-graphyne could be tunable by in-plane biaxial strain. Applying our approach to other graphynes shows that they should also exhibit correlated AFI states, which could be dominant even at zero strain. Calculations using an onsite Coulombic repulsive term (i.e., DFT + U) also confirm the predictions of our hybrid DFT calculations. Overall, our work strongly suggests that graphynes are not as graphenic (i.e., Dirac-like) as often previously predicted by DFT calculations using standard generalized gradient approximation functionals. However, due to the greater electronic versatility (e.g., tunable semiconducting bandgaps and accessible spin polarized states) implied by our study, graphynes could have novel device applications that are complementary to those of graphene.

Coexistence of giant Rashba spin splitting and quantum spin Hall effect in H–Pb–F

Rashba spin splitting (RSS) and quantum spin Hall effect (QSHE) have attracted enormous interest due to their great significance in the application of spintronics. In this work, we theoretically proposed a new two-dimensional (2D) material H–Pb–F with coexistence of giant RSS and quantum spin Hall effec by using the ab initio calculations. Our results show that H–Pb–F possesses giant RSS (1.21 eV⋅Å) and the RSS can be tuned up to 4.16 eV⋅Å by in-plane biaxial strain, which is a huge value among 2D materials. Furthermore, we also noticed that H–Pb–F is a 2D topological insulator (TI) duo to the strong spin–orbit coupling (SOC) interaction, and the large topological gap is up to 1.35 eV, which is large enough for for the observation of topological edge states at room temperature. The coexistence of giant RSS and quantum spin Hall effect greatly broadens the potential application of H–Pb–F in the field of spintronic devices.

Residual stress, strain and defects: its effect on the band gap of poly-Ge thin film realized on glass via Au induced layer exchange crystallization process

Residual stress in polycrystalline-Ge thin film realized on glass substrate using Au-induced layer exchange crystallization process is evaluated using x-ray diffraction based technique. The measured stress is found to be tensile in nature, from which we delineate and discuss the extrinsic thermal and intrinsic growth stresses. An in-plane biaxial tensile strain ∼0.15% was estimated to be endured by the polycrystalline-Ge thin film. The narrowing effect that such strain and the crystallization or growth-related defects have on the optical energy band gap of the polycrystalline-Ge thin film is elucidated.

Two dimensional Zr2CO2/H-FeCl2 van 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-FeCl2 heterostructures are calculated by first principles calculations. The 2D Zr2CO2/H-FeCl2 heterostructures are magnetic semiconductor. The electronic structure and magnetic anisotropy of Zr2CO2/H-FeCl2 heterostructure can be regulated by the biaxial strain and external electric field. The band gap and potential difference of Zr2CO2/H-FeCl2 heterostructure can be affected by in-plane biaxial strain. At a compressive strain of −8%, the Zr2CO2/H-FeCl2 heterostructure becomes metallic. All of the Zr2CO2/H-FeCl2 heterostructures are magnetic with in-plane magnetic anisotropy (IMA). The Zr2CO2/H-FeCl2 heterostructure is a semiconductor at the electric field from −0.5 V Å−1 to +0.5 V Å−1. Furthermore, Zr2CO2/H-FeCl2 heterostructure shows IMA at the negative electric field, while it shows perpendicular magnetic anisotropy at the positive electric field. These results show that Zr2CO2/H-FeCl2 heterostructure has potential applications in multifunctionalnanoelectronic devices.

Electronic and photocatalytic properties of ZnO/GaTe heterostructure from first principles calculations

Constructing a novel van der Waals (vdW) heterostructure by integrating diverse two-dimensional (2D) materials is significant to generate the more fascinating performances. In this present contribution, the electronic and photocatalytic properties of ZnO/GaTe heterostructure have been explored using first-principles calculations. The results reveal that the interaction between layers is dominated by the vdW forces. It displays an intrinsic type-II band alignment and a direct band gap of 1.730 eV under hybrid HSE06 functional. The valence band top and conduction band bottom are located at ZnO and GaTe layers, respectively, which guarantees the spatial separation of photogenerated electron-hole pairs and improves the photocatalytic efficiency. Moreover, the band edges all span the energy levels of both the reduction and oxidation potentials of H2O, indicating that its better photocatalytic performance. The modulated photocatalytic properties under varied pH circumstances imply that an acid environment is more favorable to the water splitting process. Furthermore, the band edge alignment exhibits the significant adjustability through varying vertical and in-plane biaxial strains. These theoretical findings indicate that the ZnO/GaTe heterostructure can offer an alternative strategy to design the efficient photocatalysts for water splitting.

Volume-matched piezoelectric LaN/REN superlattices from first-principles

LaN/rare earth nitride (REN) superlattices, having magnetic REN as one of the parent components, are constructed and studied by first-principles calculations. In particular, they are found to be mechanically and dynamically stable with (anti-)ferromagnetic and ferroelectric orderings. We reveal that the volume matching condition is applicable to these superlattices, which results in the elastic constant C33 softening and, when combined with a small c/a value, induces a huge piezoelectric response near the unstrained state. We also show that in-plane biaxial strain can precisely control the nature (indirect or direct) and value of the electronic bandgap. Moreover, the unpaired magnetically active 4f-electrons reduce the c-direction off-centric distortion of the wurtzite structure, making possible the switching of the ferroelectric polarization. This work, therefore, reveals that the volume matching condition also applies to magnetic materials and provides guidance for the design of multiferroic rare-earth nitride superlattices in piezoelectric devices.

Nonlinear elasticity and strain-tunable magnetocalorics of antiferromagnetic monolayer MnPS[formula omitted

The nonlinear elasticity and strain-tunable magnetocaloric effect (MCE) of antiferromagnetic monolayer MnPS3 are demonstrated by multiscale modeling and simulations. A continuum constitutive model expanding the elastic strain energy density by a Taylor series is formulated to describe the nonlinear and large-deformation elasticity of monolayer MnPS3. Fourteen independent elastic constants of the model are determined using the ab initio results. The nonlinear behavior is found to initiate at about 7% strain and nearly isotropic before the rupture. MCE (adiabatic temperature change ΔT and isothermal entropy change −ΔS) of monolayer MnPS3 is found anisotropic due to the anisotropy in the temperature derivative of magnetic susceptibility, i.e. stronger in-plane MCE. An in-plane biaxial strain could improve MCE by a factor of 1.5–2. The maximum −ΔS and ΔT occur at around 10 K and are evaluated as 2.8 μJ m−2 K−1 and 2.2 K, respectively, suggesting monolayer MnPS3 as a potential candidate for low-temperature (¡ 20 K) magnetic refrigeration.

In-Plane Strain Tuned Electronic and Optical Properties in Germanene-MoSSe Heterostructures.

DFT calculations are performed to investigate the electronic and optical absorption properties of two-dimensional heterostructures constructed by Janus MoSSe and germanene. It is found that a tiny gap can be opened up at the Dirac point in both Ge/SMoSe and Ge/SeMoS heterostructures, with intrinsic high-speed carrier mobility of the germanene layer being well preserved. An n-type Schottky contact is formed in Ge/SMoSe, while a p-type one is formed in Ge/SeMoS. Compared to corresponding individual layers, germanene-MoSSe heterostructures can exhibit extended optical absorption ability, ranging from ultraviolet to infrared light regions. The position of the Dirac cone, the Dirac gap value as well as the position of the optical absorption peak for both Ge/SMoSe and Ge/SeMoS heterostructures can be tuned by in-plane biaxial strains. It is also predicted that a Schottky–Ohmic transition can occur when suitable in-plane strain is imposed (especially tensile strain) on heterostructures. These results can provide a helpful guide for designing future nanoscale optoelectronic devices based on germanene-MoSSe vdW heterostructures.

A in-plane biaxial strain tunable electronic structures and magnetic properties of Fe2C monolayer

This work intends to study the effect of a in-plane biaxial strain (ε) on the electronic structures and magnetic properties of monolayer Fe2C by first-principles calculation. The unstrained and strained Fe2C monolayer is thermally, dynamically, and mechanically stable, and unstrained Fe2C monolayer is ferromagnetic metal with the Curie temperature (TC) of 510 K. Introducing the tensile strain, the magnetic moments, spin polarization and magnetic anisotropy energy (MAE) can be significantly enhanced. Furthermore, a phase transition from ferromagnetic to antiferromagnetic ordering occurs at the ε of 12%, −10%. Especially, the result of Monte Carlo simulations demonstrates that the TC decreases with strain and Néel temperature TN increases with the tensile strain. A tensile strain applied on Fe2C monolayer with the value less than 8% facilitates the enhancement of MAE and spin polarization, and maintaining the TC above room temperature. This study provides a theoretical guide for the application of monolayer Fe2C in spintronic devices, and provides a basis for tuning the physical properties to two-dimensional MXene materials.

High Curie temperature and large perpendicular magnetic anisotropy in two-dimensional half metallic OsI3 monolayer with quantum anomalous Hall effect

The 5d transition heavy elements with large spin-orbit coupling (SOC) effect can induce a large magnetic anisotropy energy (MAE) in magnetic materials. Meanwhile, the asymmetric occupation of 5d orbital electrons in transition metal trihalides can also trigger interesting orbital ordering. Here, the electronic structure and magnetic properties of two-dimensional (2D) OsI3 monolayer were investigated by first-principles calculations. The OsI3 monolayer has two orbital orderings with low spin states, where the half-metallic state is an Ising ferromagnet with a Curie temperature (TC) of 208 K and a perpendicular magnetic anisotropy (PMA) of −45.8 meV, but the Mott insulator state has an in-plane magnetic anisotropy (IMA) of 13.7 meV. The half-metallic state included non-self-consistent SOC calculations opens a band gap of 118.3 meV, showing a quantum anomalous Hall effect (QAHE) with a Chern number (C) of −1. A topologically trivial band gap opened in self-consistent SOC calculations, where QAHE of C = −2 can be realized by shifting Fermi surface. The in-plane biaxial strain can significantly tailor the MAE of the half-metallic state, yielding a PMA of −33.0 and −50.7 meV at a compressive strain of 6% and −2%. Meanwhile, TC rises to 275 K at a tensile strain of 6%. The transition between Mott insulator and half-metallic states can also be regulated by strain. These findings suggest that OsI3 monolayer is promising for 2D magnetism and spintronics.

Large Exciton-Driven Linear and Nonlinear Optical Processes in Monolayers of Nitrogen Arsenide and Nitrogen Antimonide

We demonstrate that atomically thin nitrogen-based binary group V wide band gap indirect semiconductors (β-NX (X = As, Sb)) embody 3-fold valley degenerated band structures and strong linear and nonlinear optical activities. Contrary to NAs, the fundamental optical absorption in NSb is quite resilient toward the temperature variations between 0 and 450 K. In the absence of lattice vibrations, exciton in NSb is, however, less strongly bound (1.30 eV) compared to the tighter case in NAs (1.62 eV), leading to a more delocalized Mott–Wannier-type texture. The inhomogeneous excitonic line widths for both monolayers within these temperatures are within the 100–400 meV range. The most significant nonlinear second harmonic optical coefficients (2ω resonances) in NAs and NSb monolayers are obtained as ∼636 and 270 pm/V at 3.2 eV (387 nm) and 2.6 eV (477 nm), respectively. We also present a detailed analysis of in-plane biaxial strain on these structures and found that tensile strain dynamically stabilizes structures with no loss in absorbance spectra (and does not influence exciton binding energy in NAs). The nonlinear coefficient also significantly improves at the two most important 810 and 1560 nm wavelengths compared to the cases offered by the monolayer transitional metal dichalcogenides. Our analyses originate from a fully ab initio G0W0+Bethe-Salpeter excited-state theory. The temperature-dependent linear absorption spectra are evaluated by including the electron–phonon self-energies, whereas the nonlinear spectra are treated using the modern theory of polarization within the same perturbative approach. Our reference findings provide important advanced characteristics for applications of two-dimensional β-NX (X = As, Sb) materials and should motivate further theoretical and experimental investigations of these interesting materials.

First principles study of biaxially deformed hexagonal buckled XS (X=Ge and Si) monolayers with light absorption in the visible region

In the present letter, we report the structural, electronic and optical properties of hexagonal buckled GeS and SiS monolayers under in-plane biaxial strain. The hybrid functional was adopted to calculate more accurate band gaps. The calculated electronic structures using Heyd–Scuseria–Ernzerhof (Perdew-Burke-Ernzerhof) methods show that both GeS and SiS monolayers display indirect band gap of 3.30 and3.03 eV (2.49 and 2.19 eV), respectively. We show that the buckling height, electronic structure and optical absorption of hexagonal buckled GeS and SiS monolayers can be tuned by applying in-plane biaxial strain. The band gap of SiS monolayer decreases by applying the tensile or compressive strain. The band gap of GeS monolayer reaches its maximum value under -2% strain and it decreases when the structure is more compressed or stretched. Both GeS and SiS monolayers exhibit optical absorption with energy covering ultraviolet light and partial visible light. By increasing the biaxial tensile strain, the absorption in visible region is expanded and enhanced due to the red shift of absorption edge.

First-principles study of two-dimensional HfS2/GaS van der Waals heterostructure for photocatalytic action

Exploring excellent performance of photocatalysts for hydrogen/oxygen evolution reactions is an effective way for solar energy applications. In this work, a HfS 2 /GaS van der Waals heterostructure (vdwH) has been designed to achieve efficient and spontaneous water decomposition. Through first-principles calculation, HfS 2 /GaS vdwH has been proved to be a stable and promising photocatalyst with the following obvious advantages: both water reduction and oxidation potentials are located in the band gap of HfS 2 /GaS vdwH (2.04 eV), ensuring that water splitting can occur. The type-Ⅱ band structure and built-in electric field ensure the effective separation of photoelectron-hole, and the high mobility of electrons and holes can also predict high photocatalytic activity . By calculating the free energy of the oxygen evolution reaction on the surface of GaS and the hydrogen evolution reaction on the surface of HfS 2 of the heterostructure , it's found that the water decomposition reaction can occur spontaneously under light. Furthermore, after 1–6% in-plane biaxial tensile strain was applied, the redox reaction of photocatalytic decomposition water can still occur on the HfS 2 /GaS vdwH, and the application of strain can effectively regulate the optical and electronic properties. Therefore, HfS 2 /GaS vdwH could be a promising candidate material for the photocatalytic decomposition of water. • The spatial separation of photogenerated carriers is realized in type-II HfS 2 /GaS heterostructure. • The electron mobility can be as high as 10 3 cm 2 V −1 s −2 . • Free energy calculation shows that the water decomposition reaction can occur spontaneously under light. • HfS 2 /GaS heterostructure reach high light absorption coefficients of 3.6 × 10 5 cm −1 .

Tunable electronic properties of two-dimensional C3N/antimonene van der Waals heterostructure

In this work, we construct a C3N/antimonene van der Waals heterostructure to investigate its structural and electronic properties using first-principles calculations. The C3N/antimonene heterostructure exhibits an indirect band gap of 0.143 eV with a type-II band alignment. Electrons transferring from C3N to antimonene layer introduce a build-in electric field which can be used to prevent recombination of the photoexited electron–hole pairs. By applying vertical strain, band gap value of the heterostructure can be tuned in a range from 0 to 0.318 eV. A type-II to type-I band alignment transition occurs at a interlayer distance of sim3.2 Å, and the heterostructure experiences a semiconductor to metal transition with a interlayer distance of sim3.7 Å. Moreover, structural and electronic properties of C3N/antimonene heterostructure show modulation under in-plane biaxial strain. A semiconductor to metal transition takes place when strain reaches −2.0%. Moreover, with the increase of compressive strain, buckling degree of the heterostructure increases, and band gap of the heterostructure increases to 0.645 eV at strain of −5.0%. In addition, band gap value of the heterostructure varies almost linearly with vertical electric field of −0.2–0.2 V Å−1, and type-II band alignment can be maintained in this range. Thus, these results indicate that C3N/antimonene heterostructure has great potential in the field of multifunctional optoelectronic devices.

Enhanced High-Temperature Thermoelectric Performance by Strain Engineering in BiOCl

Semiconductor BiOCl has a layered structure with ultralow lattice thermal conductivity [Q.D. Gibson et al., Science 373, 1017--1022 (2021)] and has potential applications in the field of thermoelectric materials. In the present study, the thermoelectric properties of BiOCl crystals are accurately predicted using the first-principles calculation combined with Boltzmann transport theory. The dimensionless figure of merit ($ZT$) of $p$-type BiOCl is found to be 0.2 at room temperature, reaching 1.1 at 800 K. In addition, applying in-plane biaxial tensile strain ${ϵ}_{xy}$ can lead to a further increase in the value of $ZT$ to 1.9 at 800 K. This means that $p$ BiOCl is an excellent high-temperature thermoelectric material. And this is due to the fact that biaxial strain drastically reduces the lattice thermal conductivity and the shift of the valence band toward the Fermi level can optimize the carrier concentration. Thus, the present work paves a way for the design of adjustable high-temperature thermoelectric materials.

Electric field and strain-induced band-gap engineering and manipulation of the Rashba spin splitting in Janus van der Waals heterostructures

The compositional as well as structural asymmetries in Janus transition metal dichalcogenides (J-TMDs) and their van der Waals heterostructures (vdW HSs) induce an intrinsic Rashba spin-splitting. We investigate the variation of band-gaps and the Rashba parameter in three different Janus heterostructures having AB-stacked Mo$XY$/W$XY$ ($X$, $Y$ = S, Se, Te; $X\neq Y$) geometry with a $Y-Y$ interface, using first-principles calculations. We consider the effect of external electric field and in-plane biaxial strain in tuning the strength of the intrinsic electric field, which leads to remarkable modifications of the band-gap and the Rashba spin-splitting. In particular, it is found that the positive applied field and compressive in-plane biaxial strain can lead to a notable increase in the Rashba spin-splitting of the valence bands about the $\Gamma$-point. Moreover, our \textit{ab-initio} density functional theory (DFT) calculations reveal the existence of a type-II band alignment in these heterostructures, which remains robust under positive external field and biaxial strain. These suggest novel ways of engineering the electronic, optical, and spin properties of J-TMD van der Waals heterostructures holding a huge promise in spintronic and optoelectronic devices. Detailed $\mathbf{k\cdot p}$ model analyses have been performed to investigate the electronic and spin properties near the $\Gamma$ and K points of the Brillouin zone.

Theoretical routes for current-free magnetization switching induced by joint effects of strain and Dzyaloshinskii–Moriya interaction

Voltage-induced strain is regarded as an energy-efficient choice of tuning spin-dynamics. However, studies on the strain-mediated switching of magnetization in a perpendicular-magnetic-anisotropy layer are few because of the uncertainties that arise from the magnetization oscillation at high strain. In this work, we demonstrate theoretically how to deterministically switch the perpendicular magnetization in an ultrathin magnetic nanodisk by combining biaxial in-plane strain with the Dzyaloshinskii–Moriya interaction (DMI). The magnetization-switching process is carefully investigated under different strains and DMI strengths. The underlying switching mechanism is attributed to the remnant magnetization component, which deviates away from the film plane during the strain-pulse-impulsion period and which is also highly dependent on the DMI. Based on simulation results, a theoretical route for obtaining deterministic switching regarding strain and DMI is established. In this route, the minimum duration of the strain pulse can be shortened to a critical time of 2.5 ns as the strain increases to 7000 ppm at a DMI value of 0.6 mJ/m2. Moreover, nonvolatile and reversible switching between the spin-up and spin-down states of perpendicular magnetization is realized using pulses of biaxial in-plane isotropic strain. This switching occurs via an intermediate skyrmion and shows potential in overcoming the edge-roughness-related pinning that occurs in spin–orbit-torque current-induced switching. This study provides a robust insight into strain-induced current-free magnetization switching, providing a guide for experimental research into the strain-mediated voltage control of memory applications.

Effects of the in-plane uniaxial and biaxial strains on the structural and electronic properties of the monolayer ZrS2: A first-principles investigation

The effects of in-plane uniaxial strain along both armchair direction and zigzag direction as well as symmetrical biaxial strain on the structural and electronic properties of the monolayer ZrS2 have been investigated by using first-principles calculations. It is found that firstly different from the bulk materials, the band gap type of the monolayer ZrS2 at zero strain depends on the odd (even) nature of the selected supercell, i.e. an indirect (direct) band gap is obtained for odd (even) supercell. Second, for three strain schemes the band gap value of the monolayer ZrS2 decreases quickly with increasing compressive strain but increases firstly to the maximum value and then decreases with increasing tensile strain mainly resulted from the variation of the conduction band minimum, and the compressive strain is more effective than the tensile strain in band gap engineering. Thirdly, under biaxial strain the indirect band gap of the monolayer ZrS2 increases from 0.00 eV (metal) at -10% compressive strain to the maximum value of 1.616 eV at 6% tensile strain and then decreases. Fourth, when a uniaxial strain is applied to the monolayer ZrS2 along either armchair or zigzag direction a direct to indirect band gap transition is observed at -10% compressive strain.

Tunable electronic structure and CO2 adsorption of hb-Sb/graphene van der Waals heterostructure

The van der Waals (vdW) heterostructure combining different 2D materials stimulates broad research interests. Novel chemical and electronic properties can be obtained via building vdW heterostructures. In this paper, the CO2 adsorption properties of buckled honeycomb antimonene/graphene (hb-Sb/G) vdW heterostructure are examined by first-principles calculation, which provides theoretical insights into the enhanced electrocatalytic activity for CO2 reduction of hb-Sb/G heterostructure reported in a recent experimental study. Furthermore, by applying in-plane biaxial strains, the tunable Schottky barrier and transition of contact property are observed. As compressive strain decreases or tensile strain increases, the catalytic activity of hb-Sb/G heterostructure is predicted to be enhanced as a result of the increased interlayer coupling. Our work reveals the origin of electrocatalytic activity of hb-Sb/G heterostructure for CO2 reduction and proposes a strain engineering method to enhance its catalytic activity via tuning its electronic structure. This work uncovers promising applications of hb-Sb/G heterostructure in green energy and ultra-thin devices.