Biaxial Compressive Strain Research Articles

Strain-induced structural, elastic, and electronic properties of 1L-MoS2

The electronic band structures, structural, and elastic properties of monolayer MoS2 under the biaxial strains are investigated using DFT and DFT + U methods. Significant changes in the bond distances, bond angles, electronic structures, and effective mass of electron me* (hole mh*) are observed under biaxial strain. The Bulk modulus decreases (increases) by increasing the tensile (compressive) biaxial strain. The band-gap values of unstrained 1L-MoS2 are estimated as 1.78 (1.81) eV within the GGA (GGA + U) approximations; however, direct band gap varies from 1.74 (1.76) to 1.92 (1.95) eV within a region of 0.3(0.4)% tensile to 1.13(1.11)% compressive strains. Beyond this strain region, direct nature of the band gap becomes indirect, and further increment causes semiconductor to metallic transition. Direct band-gap tuning and observed small effective mass values of electrons and holes carriers under applied strain indicate the enhanced optoelectronic properties in strained monolayer MoS2.Graphical abstract

Analyzing the electronic and optical properties of bulk, unstrained, and strained monolayers of SrS2 by DFT

Density functional theory (DFT) calculations were conducted for the electronic and optical properties of the bulk, unstrained and strained monolayers of SrS2. Electronic band structures of the bulk SrS2 indicated an indirect bandgap with 1.32 eV, whereas the SrS2 monolayer displayed a higher indirect bandgap with 1.95 eV. Biaxial tensile strains of different magnitudes cause complex effects by increasing or decreasing the electronic band gaps of the SrS2 monolayer. In contrast, the various biaxial compressive strains always diminish the bandgap energy of the SrS2 monolayer. Furthermore, both bulk and monolayer SrS2 have individual low-dielectric constants suggesting their possible use in microelectronics. In addition, owing to their high refractive indices and optical conductivities in the infrared (IR) region, unstrained bulk and monolayer SrS2 can be effective materials for both IR and solar cell applications. Also, the observed ultraviolet (UV) absorption peaks of unstrained bulk and monolayer SrS2 indicate their suitability as UV absorbers. Strain-dependent optical properties of the monolayer SrS2 were also evaluated. Under biaxial tensile strains between 5% and 25%, monolayer SrS2 demonstrates optical isotropy in which its optical properties remain almost unchanged. However, biaxial compressive strains larger than 10% caused slight variations in the optical properties of the SrS2 monolayer. Obtained phonon dispersions curves for unstrained bulk and monolayer SrS2 prove the dynamical stability of these compounds. Further, SrS2 monolayers SrS2 monolayers under biaxial compressive strains up to 5% and under biaxial tensile strains up to 15% remain dynamically stable.

Structural, electronic, and magnetic properties of CrTe2

Two-dimensional chromium ditelluride (${\mathrm{CrTe}}_{2}$) is a promising ferromagnetic layered material that exhibits long-range ferromagnetic ordering in the monolayer limit. The formation energies of the different possible structural phases (1T, 1H, 2H) calculated from density functional theory (DFT) show that the 1T phase is the ground state, and the energetic transition barriers between the phases, calculated by the nudged elastic band method, are large, on the order of 0.5 eV. The self-consistent Hubbard $U$ correction parameters are calculated for all the phases of ${\mathrm{CrTe}}_{2}$. The calculated magnetic moment of 1T-${\mathrm{CrTe}}_{2}$ with $\ensuremath{\ge}2$ layers lies in the plane, whereas the magnetic moment of a monolayer is out-of-plane. Band filling and tensile biaxial strain cause the magnetic moment of a monolayer to switch from out-of-plane to in-plane, and compressive biaxial strain in a bilayer causes the magnetic moment to switch from in-plane to out-of-plane. The magnetic anisotropy is shown to originate from the large spin-orbit coupling (SOC) of the Te atoms and the anisotropy of the exchange coupling constants ${J}_{xy}$ and ${J}_{z}$ in an XXZ type Hamiltonian. Renormalized spin wave theory using experimental values for the magnetic anisotropy energy and Curie temperatures provides a range of values for the nearest neighbor exchange coupling.

Monolayer MnX and Janus XMnY ( X,Y=S , Se, Te): A family of two-dimensional antiferromagnetic semiconductors

We present first-principles results on the structural, electronic, and magnetic properties of a new family of two-dimensional antiferromagnetic (AFM) manganese chalcogenides, namely monolayer MnX and Janus XMnY (X, Y= S, Se, Te), among which monolayer MnSe was recently synthesized in experiments [\href{https://pubs.acs.org/doi/abs/10.1021/acsnano.1c05532}{ACS Nano 15 (8),13794 (2021)}]. By carrying out calculations of the phonon dispersion and \textit{ab-initio} molecular dynamics simulations, we first confirmed that these systems, characterized by an unconventional strongly coupled bilayer atomic structure (consisting of Mn atoms buckled to chalcogens forming top and bottom ferromagnetic (FM) planes with antiparallel spin orientation) are dynamically and thermally stable. The analysis of the the magnetic properties shows that these materials have robust AFM order, retaining a much lower energy than the FM state even for under strain. Our electronic structure calculations reveal that pristine MnX and their Janus counterparts are indirect-gap semiconductors, covering a wide energy range and displaying tunable band gaps by the application of biaxial tensile and compressive strain. Interestingly, owing to the absence of inversion and time-reversal symmetry, and the presence of an asymmetrical potential in the out-of-plane direction, Janus XMnY become spin-split gapped systems, presenting a rich physics yet to be explored. Our findings provide novel insights in this physics, and highlight the potential for these two-dimensional manganese chalcogenides in AFM spintronics.

Band lineup at hexagonalSixGe1−x/SiyGe1−yalloy interfaces

The natural and true band profiles at heterojunctions formed by hexagonal Si$_x$Ge$_{1-x}$ alloys are investigated by a variety of methods: density functional theory for atomic geometries, approximate quasiparticle treatments for electronic structures, different band edge alignment procedures, and construction of various hexagonal unit cells to model alloys and heterojunctions. We demonstrate that the natural band offsets are rather unaffected by the choice to align the vacuum level or the branch point energy, as well as by the use of a hybrid or the Tran-Blaha functional. At interfaces between Ge-rich alloys we observe a type-I heterocharacter with direct band gaps, while Si-rich junctions are type-I but with an indirect band gap. The true band lineups at pseudomorphically grown heterostructures are strongly influenced by the generated biaxial strain of opposite sign in the two adjacent alloys. Our calculations show that the type-I character of the interface is reduced by strain. To prepare alloy heterojunctions suitable for active optoelectronic applications, we discuss how to decrease the compressive biaxial strain at Ge-rich alloys.

Biaxial strain engineering on the superconducting properties of MgB2 monolayer

The effect of biaxial strain on the superconducting properties of MgB2 monolayer was studied by first-principles calculations. The stability analyses by phonon dispersions show that a biaxial strain of as much as 7% can be applied onto MgB2 without inducing any imaginary frequency. The superconducting property calculations based on the frame of Migdal-Eliashberg theory successfully reproduce the two-gap superconductivity of MgB2. The results show that the tensile biaxial strain can increase the critical temperature of MgB2 by as much as ∼20% while the compressive biaxial strain would decrease the critical temperature by ∼29%. The detailed microscopic mechanism of the biaxial strain effect on the superconducting properties was studied by calculations of electronic structures and phonon dispersions. The increased Tc is a combining result of the increased electron density at the Fermi level and the in-plane boron phonon softening. By means of high-throughput screening of proper substrates, it is found that most of the substrates would result in tensile strain in MgB2 film, some of which can result in tensile strain of higher than 10%, consistent with many experimental works that the MgB2 thin film has increased Tc. The results in this work provide a detailed understanding of the biaxial strain engineering mechanism and demonstrate that biaxial strain engineering can be an effective way of tuning the superconducting properties of MgB2 and other similar materials.

Exploiting the stereoelectronic effects for selective tuning of band edge states of α-SnO: GW quasiparticle calculations

Tuning the electronic structure of materials, and thus their electronic, transport, and optical properties, is of fundamental importance for materials design and optimization. Although alloying is a well-established method for engineering the band gap of semiconductors, strain engineering has emerged as a promising approach to selective tuning of band-edge states. Using a combined density functional theory and $GW$ approach, we show that the highly directional intralayer and interlayer couplings, together with the unusual stereoelectronic effects of the Sn $5s$ lone pair in \ensuremath{\alpha}-SnO, may be exploited to tune, in addition to the band gap, the valence and conduction band-edge states selectively using in-plane and/or out-of-plane strains. Whereas the uniaxial strain along the lattice $c$ direction primarily affects the position of the conduction band edge, the valence band edge is very sensitive to the biaxial $ab$ strain. We also establish a strain electronic phase diagram of \ensuremath{\alpha}-SnO, including the insulator--metal phase transition boundary. It is predicted that a compressive biaxial strain of about 3% or an isotropic pressure of 5 GPa can trigger an insulator--metal transition. The quasiparticle band gap can be widely tuned from 0 to more than 2.0 eV with moderate strains.

Influence of spin–orbit coupling and biaxial strain on the inorganic lead iodide perovskites, APbI3 (A = K, Rb, and Cs)

Recently, inorganic metal halide perovskites, APbI3 (A = K, Rb, and Cs), have attracted increased attention in the research community of photovoltaic technology due to their superior stability and exceptional structural, electronic, and optical properties. In this paper, the influence of biaxial compressive and tensile strains (−6% to +6%) on the optoelectronic properties of APbI3 perovskites is thoroughly investigated using density functional theory, based on first-principles calculations. Additionally, this study identifies the function of the A-cation in the optoelectronic properties of APbI3. We find that the biaxial strain in APbI3 can significantly increase its absorbance, both in the visible and ultraviolet light energy range. Interestingly, the absorption coefficient, dielectric function, and electron loss function are tuned into visible ranges when a compressive strain is applied, whereas these functions move into the ultraviolet region for tensile strain. Furthermore, the electronic band structure shows that the APbI3 compounds are all semiconductors with direct bandgap ranges of 0.92–1.09 eV and 1.9 eV at the R-point. The value of the bandgap decreases to 0.17, 0.25, and 0.32 eV for KPbI3, RbPbI3, and CsPbI3, respectively, when the spin–orbit coupling effect is considered. Moreover, the bandgap of APbI3 shows a decreasing trend and tends towards the metallic condition when compressive strain is applied. In addition, when tensile strain is increased, the bandgap of APbI3 exhibits an increasing trend. Therefore, based on the optical and electronic properties, applying a biaxial strain should make APbI3 compounds suitable for optoelectronic device applications.

Biaxial strain engineering of the electronic and optical properties of Ge2SeS monolayer: Promising for optoelectronic applications

Biaxial strain effects on the electronic and optical properties of the Ge2SeS monolayer were studied using the first-principles calculations in the present work. The band gap calculations show that the G2eSeS monolayer is a semiconductor at equilibrium state with an indirect band gap of 1.47 eV calculated by generalized gradient approximation (GGA) of Perdew Burke Ernzerhof (PBE) functional, which can be modulated by applying the biaxial strain changing from −6% to +6%. The optical properties such as dielectric constant, reflectivity, extinction coefficient, and absorption coefficient rise with the compression strain and reduce with tensile strain. Interestingly, it's confirmed that biaxial compression strain results in a high absorption coefficient in visible light. The anticipated results could make the Ge2SeS monolayer a candidate for optoelectronic applications.

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.

Two dimensional Janus SGaInSe(SeGaInS)/PtSe2 van der Waals heterostructures for optoelectronic and photocatalytic water splitting applications

Hydrogen generation by photocatalytic water splitting is considered as a viable and clean energy source for dealing with energy shortages and environmental pollution challenges. By means of first-principles calculations, the SGaInSe(SeGaInS)/PtSe2 van der Waals (vdWs) heterostructures are confirmed to be energetically, dynamically, and thermally stable, indicating that they have a lot of potential for experimental implementation. The Model-I of SGaInSe/PtSe2 heterostructures possesses type-II indirect band alignment, while the other three heterostructures retain type-I band alignment, which is further tuned to type-II with the application of strain. The charge transfer to SGaInSe/SeGaInS layer from PtSe2 layer generates built-in electric field that effectively resists the recombination of photo-generated electron-hole pairs. At pH = 0, the band edge positions of both heterostructures completely straddle the redox potentials. The Model-I of SeGaInS/PtSe2 heterostructures with biaxial −2% compressive strain makes the band edges to do complete water splitting in natural environment (pH = 7). In the visible range of the irradiating spectrum, our designed heterostructures have enhanced imaginary part of the dielectric function and absorption coefficient up to 105 cm−1. Moreover, with the biaxial compressive (tensile) strains, the blue-shift (red-shift) in absorption spectra is examined. Our study extends the applications of Janus monochalcogenides/PtSe2 vdWs heterostructures and supports to design of more heterostructures-based photocatalysts and optoelectronic devices.

Decoupling the Chemical and Mechanical Strain Effect on Steering the CO2 Activation over CeO2-Based Oxides: An Experimental and DFT Approach.

Doped ceria-based metal oxides are widely used as supports and stand-alone catalysts in reactions where CO2 is involved. Thus, it is important to understand how to tailor their CO2 adsorption behavior. In this work, steering the CO2 activation behavior of Ce–La–Cu–O ternary oxide surfaces through the combined effect of chemical and mechanical strain was thoroughly examined using both experimental and ab initio modeling approaches. Doping with aliovalent metal cations (La3+ or La3+/Cu2+) and post-synthetic ball milling were considered as the origin of the chemical and mechanical strain of CeO2, respectively. Experimentally, microwave-assisted reflux-prepared Ce–La–Cu–O ternary oxides were imposed into mechanical forces to tune the structure, redox ability, defects, and CO2 surface adsorption properties; the latter were used as key descriptors. The purpose was to decouple the combined effect of the chemical strain (εC) and mechanical strain (εM) on the modification of the Ce–La–Cu–O surface reactivity toward CO2 activation. During the ab initio calculations, the stability (energy of formation, EOvf) of different configurations of oxygen vacant sites (Ov) was assessed under biaxial tensile strain (ε > 0) and compressive strain (ε < 0), whereas the CO2-philicity of the surface was assessed at different levels of the imposed mechanical strain. The EOvf values were found to decrease with increasing tensile strain. The Ce–La–Cu–O(111) surface exhibited the lowest EOvf values for the single subsurface sites, implying that Ov may occur spontaneously upon Cu addition. The mobility of the surface and bulk oxygen anions in the lattice contributing to the Ov population was measured using 16O/18O transient isothermal isotopic exchange experiments; the maximum in the dynamic rate of 16O18O formation, Rmax(16O18O), was 13.1 and 8.5 μmol g–1 s–1 for pristine (chemically strained) and dry ball-milled (chemically and mechanically strained) oxides, respectively. The CO2 activation pathway (redox vs associative) was experimentally probed using in situ diffuse reflectance infrared Fourier transform spectroscopy. It was demonstrated that the mechanical strain increased up to 6 times the CO2 adsorption sites, though reducing their thermal stability. This result supports the mechanical actuation of the “carbonate”-bound species; the latter was in agreement with the density functional theory (DFT)-calculated C–O bond lengths and O–C–O angles. Ab initio studies shed light on the CO2 adsorption energy (Eads), suggesting a covalent bonding which is enhanced in the presence of doping and under tensile strain. Bader charge analysis probed the adsorbate/surface charge distribution and illustrated that CO2 interacts with the dual sites (acidic and basic ones) on the surface, leading to the formation of bidentate carbonate species. Density of states (DOS) studies revealed a significant Eg drop in the presence of double Ov and compressive strain, a finding with design implications in covalent type of interactions. To bridge this study with industrially important catalytic applications, Ni-supported catalysts were prepared using pristine and ball-milled oxides and evaluated for the dry reforming of methane reaction. Ball milling was found to induce modification of the metal–support interface and Ni catalyst reducibility, thus leading to an increase in the CH4 and CO2 conversions. This study opens new possibilities to manipulate the CO2 activation for a portfolio of heterogeneous reactions.

The Crystalline Structure of Thin Bismuth Layers Grown on Silicon (111) Substrates.

Bismuth films with thicknesses between 6 and ∼30 nm were grown on Si (111) substrate by molecular beam epitaxy (MBE). Two main phases of bismuth — -Bi and -Bi — were identified from high-resolution X-ray diffraction (XRD) measurements. The crystal structure dependencies on the layer thicknesses of these films were analyzed. -Bi layers were epitaxial and homogenous in lateral regions that are greater than 200 nm despite the layer thickness. Further, an increase in in-plane 2 values showed the biaxial compressive strain. For comparison, -Bi layers are misoriented in six in-plane directions and have -Bi inserts in thicker layers. That leads to smaller (about 60 nm) lateral crystallites which are compressively strained in all three directions. Raman measurement confirmed the XRD results. The blue-sift of Raman signals compared with bulk Bi crystals occurs due to the phonon confinement effect, which is larger in the thinnest -Bi layers due to higher compression.

(Digital Presentation) Characterization of Anisotropic Strain in Anelastic Material By Raman Spectroscopy

Raman spectroscopy has recently been applied for non-destructive characterization of strain in crystalline thin films. It makes use of the numerical value of the mode Grüneisen parameter γ , which equals (minus) the ratio of the relative change in the frequency of a Raman-active vibrational mode and the strain-induced relative change in the unit cell volume. In the presence of in-plane, compressive biaxial strain, aliovalent doped CeO2-films exhibit values of γ which are ~30% smaller than the literature values for the bulk ceramics under isostatic stress. This discrepancy is due in large part to the negative contribution from the anelastic (time-dependent) mechanical properties of oxygen-deficient ceria, thereby raising questions concerning the relationship between Raman mode frequency and anelastic strain. Here we propose a way to "separate" anelastic and elastic contributions to the Grüneisen parameter.500-650nm thick films , x=0.1-0.2, were deposited by RF magnetron sputtering on (100) p-Si wafers (1Ω-cm, tSi=280mm). The films (S-films) were in the fluorite phase with pronounced (111) texture and exhibited biaxial , compressive strain. The in- and out-of-plane lattice parameters (aS || , aS ⊥ ) of the S-films were determined by XRD at different sample tilt angles. The in-plane stress (σbi) in the S-films was determined by profilometry from the change in Si-wafer curvature following film deposition. From σbi and the film biaxial modulus, the elastic component of the in-plane compressive strain (ue ) was calculated (~ -0.2%).Strain-free (relaxed) R-films were obtained by partial substrate removal, forming 2 mm diameter self-supported membranes. Pronounced membrane buckling indicated that residual strain was negligible (aR || ≈ aR ^ ). From the increase in area upon substrate removal, measured with optical profilometry, the total S-film in-plane strain (ut ) was calculated from aS ||/aR-1 ≈ -0.5%. The anelastic strain component may be calculated as ua= ut-ue . The small, anisotropic strain-induced blue shift of the frequency of the F2g -phonon mode for S-films suggests that the F2g mode responds primarily to the elastic component of the strain.As a control, 430nm thick films of mechanically elastic Y2O3 were deposited by sputtering on a Si wafer covered with a 1.5 µm thick PECVD deposited SiO2 layer. This layer supported the Y2O3-films upon membrane formation, but prevented complete strain relaxation. The Y2O3-films were in the double fluorite phase with strong (222) texture and, as with the S-films, were under in-plane compressive strain. The Y2O3 S-film lattice parameters were determined by acquiring XRD-patterns at different sample tilt angles, while membrane in- and out-of-plane lattice parameters were determined with XRD in the transmission and reflection modes, respectively. The values of γ calculated from the Raman frequency of the [Fg +Ag ] peak position were close to the literature values for bulk yttria under isostatic stress.This work should serve as a sample protocol for strain characterization of doped ceria thin films. It supports the concept that the discrepancy between the behavior of Raman modes of isostatically and anisotropically strained doped ceria is related to anelasticity. Our results shed light upon strain characterization by Raman spectroscopy and may lay the foundation for future studies to elucidate selective sensitivity to various strain components.

Thermodynamics, electronic, and magnetic properties of Cr-doped Cr[formula omitted]CoAl: Biaxial ([110]) strain impact

Highly spin-polarized materials are extremely desired for the designing of magnetic tunnel junctions, which have potential utilization in data storage devices. Herein, ab-initio calculations were performed to investigate the thermodynamics, electronic, and magnetic properties of inverse full-Heusler Cr2Co1−xCrxAl (x = 0%, 12.5%, 25%, 37.5%, and 50%) compound under the influence of biaxial strain along the [110]-direction. Formation energies confirm the experimental realization of the structures by assuming the chemical potentials of ions in their respective stable phases and utilizing the Convex Hull analysis. The calculated elastic properties affirm that all the motifs are mechanically stable, ductile, and anisotropic. Moreover, positive frequencies of phonon band structures established the dynamical stability of the systems. The ground state of the unstrained pristine Cr2CoAl system is a ferrimagnetic (FiM) metallic due to strong antiferromagnetic coupling between the Cr1/Co and Cr2 ions. Furthermore, our results revealed that the total magnetic moment and spin-polarization get reduced as the Cr-concentration increases, which endorses the recent experimental observations Srivastava et al., 2020. Strikingly, the undoped structure displayed a transition from FiM to non-magnetic state for biaxial compressive strains along the ab-plane due to strong hybridization between ions, while retaining its FiM nature for tensile strains. On the other hand, all the Cr-doped systems flow towards degeneracy by maintaining their magnetic metallic states as the compressive strain strength increases. However, all the tensile strained structures sustained their magnetic metallic nature. Hence, the present work suggested that the combined effect of doping and strain substantially modulated the physical properties of the Heusler structures.

Strain effects on the behavior of intrinsic point defects within the GaN/AlN interface

Defect behaviors in GaN-based compounds studied by using density functional theory have exhibited widely practical applications for optoelectronic devices. Ga is usually used to solubilize in AlN to release the lattice mismatch and the interface strain. In this study, we investigate the formation energies and diffusion barriers of intrinsic point defects within the GaN/AlN interface. The intrinsic defect with the lowest formation energy is positively charged N vacancy, and negatively charged Ga vacancy is the defect with second lower formation energy. Compared with the case in bulk GaN, the GaN/AlN interface promotes the generation of Ga vacancy and inhibits the generation of N vacancy. Meanwhile, the defect diffusion barrier of N vacancy within the GaN/AlN interface is higher than that in the bulk GaN. The hydrostatic compressive/ tensile strain significantly promotes the generation of these two vacancy defects, while biaxial tensile strain and the biaxial compressive strain slightly promote and inhibit the generation of these two vacancy defects, respectively. These interface and strain effects provide the significant information in understanding the electrical properties of GaN-based devices, especially under the extreme conditions. Our work therefore not only elucidates defect behaviors under different strains in GaN/AlN interface, but also paves the way for understanding and designing promising electronic devices with interface engineering.

Tailoring the electronic and optical properties of ZrS2/ZrSe2 vdW heterostructure by strain engineering

Under framework of first-principles calculation, we construct ZrS2/ZrSe2 heterostructure and explore the effects of the uniaxial and biaxial strains on its electronic and optical properties. Band structure shows the unstrained ZrS2/ZrSe2 heterostructure has a type-II band alignment. The large binding energy and small interface distance indicate there is a chemical combination between ZrS2 and ZrSe2 layers beyond the van der Waals interaction. Due to the redistribution of charge, a built-in electric field from ZrSe2 to ZrS2 is formed on the interface of the ZrS2/ZrSe2 heterostructure. In addition, constructing the ZrS2/ZrSe2 heterostructure can enhance light absorption intensity. Then we investigate the effects of uniaxial and biaxial strains in the range of −8%∼8% on the electronic and optical properties of the ZrS2/ZrSe2 heterostructure. The band gap of the ZrS2/ZrSe2 heterostructure increases (decreases) with the increase of tensile (compressive) strain, this tunable band gap shows its potential applications in electronic devices. The band type is very sensitive to strain, because the type-II alignment can be converted to type-I alignment. In addition, under uniaxial (−4% and −2%) and biaxial (−2%) compressive strains, a red-shift phenomenon is occurred compared to the unstrained ZrS2/ZrSe2 heterostructure, indicating the better optical gas sensitivity and can be used in infrared light detectors. These findings make possible the application of two-dimensional ZrS2/ZrSe2 heterostructure in multifunctional electronic and optoelectronic devices.

Possible topological states in two dimensional Kagome ferromagnet MnGe

From first principles computation, we propose hexagonal MnGe as a ferromagnetic Kagome candidate. As for three dimensional (3D) MnGe, the nearest, next-nearest and next-next-nearest magnetic exchange interaction parameters are respectively estimated to be 0.4 meV, 7.8 meV and 1.8 meV within the Heisenberg model while the calculated Curie temperature Tc is approaching 115 K based on Monte Carlo (MC) simulation. As for two dimensional (2D) MnGe monolayer, the spin-orbit coupling (SOC) would bring in a tiny gap (~ 1.9 meV) at the Dirac nodes and quantum anomalous Hall (QAH) effect, characterized by the chiral edge states and nonzero Chern number (C = 1). Moreover, the Dirac points can be tuned to the fermi level EF by 1.5 electrons doping or 3.3% compressive biaxial strain. Besides, possible synthetic scheme as well as the mechanical, dynamical and thermal stability are confirmed for both bulk and monolayer MnGe. Thus our work provides a promising Kagome ferromagnet to study intriguing topological physics associated with Dirac fermions towards spintronics application.

Strain Tunable Electronic Band Structure and Magnetic Anisotropy of CrI3 Bilayer

Bilayer (BL) CrI3 attracts major attention among two dimensional (2D) materials due to the exhibition of unique interlayer antiferromagnetic ordering in contrast to other atomically thin films exhibiting long-range ferromagnetic ordering. It is believed that the high temperature monoclinic AB′ stacking plays the key role to induce unusual antiferromagnetic interlayer coupling in BL CrI3. The magnetic states of this system can be effectively controlled by external perturbations like magnetic field, electric field, carrier doping, strain etc. So, it is expected that strain effects can show a distinct behavior in the magnetic transition in BL CrI3. Here, we have studied the electronic and magnetic properties of BL CrI3 under in-plane biaxial and out-of-plane vertical strain from compression to stretch through density functional theory calculations. A compressive biaxial strain around −2.5 can induce a transition from AFM to FM magnetic ground state which continues upto −6 strain. Beyond this point, a second transition from FM to AFM1 state occurs. Detailed analysis of electronic structure indicates a strain induced direct to indirect band gap transition. Magnetic anisotropy energy (MAE) calculations show an out-of-plane easy axis for pristine CrI3 BL and this feature remains unchanged in the entire range of applied strain. It is also found that most of the perpendicular magnetic anisotropy contribution comes from the iodine atoms. Large MAE found in BL CrI3 may find useful applications in high density data storage devices.

First-principles study on the effect of biaxial strain on the carrier lifetime and absorption spectrum redshift of (S, Se, Te) double-doped ZnO

Considering the mismatch of lattice constants between the substrate and the (S, Se, Te) double-doped ZnO system and of the thermal expansion coefficients between the substrate and the doped system, the doped system was subject to external strain. Previous studies have neglected this issue. In the current research, we first applied biaxial strain on the model and then performed density functional theory calculation. The effects of applying different strains on the stability of the doped system, the red shift of the absorption spectrum, the trap effect, and the carrier lifetime were determined. The research results showed that Zn36SSeO34, Zn36STeO34, and Zn36SeTeO34 systems had smaller formation energies and better stability under the Zn-rich condition than under the O-rich condition. All doped systems had negative cohesive energies and good stability. With the synergistic effect of strain and doping, when the biaxial compressive strain of the Zn36STeO34 system was − 5%, the hole–electron separation was good, the trap effect was substantial, the hole life increased, the absorption spectrum had a better red shift, and the oxidation reaction improved. These findings had a certain theoretical guiding effect on photocatalysts for the decomposition of water for oxygen production.