Biaxial Compressive Strain Research Articles

Analysis of the microscopic mechanism of the stacking stability of h-BN on a metal substrate under external force

Nanoelectromechanical systems (NEMS) based on layered materials often rely on mechanical deformation, which involves relative sliding between layers. And the charge distribution between layers is essential for understanding its mechanical and electromechanical behavior. As a typical layered material, h-BN has a small energy difference between different stacks, so it is prone to phase transition under external force. That is, relative sliding between layers. Therefore, analyzing the effect of external forces on the electronic structure of h-BN is crucial for understanding the mechanical deformation of layered materials. In this work, Cu and Ni are used as substrate materials. On the one hand, the influence of different substrates on the electronic structure and stacking stability of h-BN is studied. On the other hand, the microscopic mechanism of the phase of h-BN on the Cu substrate is analyzed and discussed. Through analysis, the stacking stability of h-BN on a metal substrate depends on the hybrid strength of the metal 3d and h-BN p orbitals. That is the adsorption. In addition, the phase transition of h-BN on the Cu substrate is mainly attributed to the difference in the change rate of the total charge density of the h-BN/Cu interface between the two stacks with the external force. However, neither biaxial tensile nor biaxial compressive strain will cause h-BN to undergo transformation on the Ni substrate.

Design and optimization of stress/strain in GAA nanosheet FETs for improved FOMs at sub-7 nm nodes

Stress/strain engineering techniques are employed to boost the performance of Gate-all-around (GAA) vertically stacked nanosheet field-effect transistors (NSFETs) for 7 nm technology nodes and beyond. In this work, we report on the 3D numerical simulation study of the impacts of source/drain epitaxial and uniaxial strained-SiGe channel stresses on p-type NSFETs. It is shown that the uniaxial strained-SiGe channel improves the drive current by up to 107% due to higher compressive stress while the 3-stack NSFET can achieve an enhancement in drive current even up to 187% using a 30% Ge mole fraction. Furthermore, we compare the multiple stacked channel NSFETs and nanowire FETs (NWFETs) considering different strain techniques. As compared to a 3-stack strained-SiGe NWFET, NSFETs show 27% and 10% enhancements in ION and SS, respectively. Vertically stacked NSFETs are shown to be the best option to improve the hole mobility under biaxial and uniaxial compressive strain rather than NWFETs. We also look at how the Ge mole fraction affects various electrical properties in a uniaxial strained-SiGe channel with shrinking dimensions of scaled NSFETs. It is observed that for a fixed Lg, ION/IOFF ratio, SS and DIBL decrease with the increase in Ge mole fraction.

Tuning of optical performance of CrS2 monolayer using strain engineering

In the present manuscript, the compressive and tensile strain in both uniaxial and biaxial directions are used to investigate the electronic and optical properties of CrS2 monolayer. It is revealed that both types of strain have a significant impact on the bandgap. For tensile strain, the value of Eg keeps decreasing but for compressive strain, the bandgap value first rises to 1.196 eV at 8% of uniaxial strain and then subsequently falls to 1.132 eV at 12% strain. The value of Eg in the biaxial case reaches to 1.75 eV at 6% compressive strain, and subsequently decreases to 0.60 eV at 12% compressive strain. The value of Eg for biaxial tensile strain continues to decrease and reached to 0.11 eV at 12% strain. The value of dielectric function i.e., ε2(ω) changes from 2.97 to 1.82 and then to 1.95 under 8% and 12% uniaxial compressive strain. ε2(ω) also changes from 2.97 to 2.52 and then to 3.30 under 4% and 12% biaxial compressive strain, respectively. A blue as well as a red shift in the absorption edge under compressive and tensile strains, respectively are observed. Our results imply that strain can be a useful tool for altering the optoelectronic properties of CrS2 monolayer.

Oxygen-terminated Ti3C2 MXene as an excitonic insulator

Excitonic insulators originate from the formation of bound excitons (electron–hole pairs) in semiconductors and provide a solid-state platform for quantum many-boson physics. We determined the excitonic insulator phase of Ti3C2O2 monolayer from its indirect quasiparticle band structure and from the precise evaluation of the relative value of the fundamental bandgap vs the momentum-indirect excitonic binding energy. The excitonic insulator is stable over the ±4% range of compressive and tensile biaxial strain. The energy region relevant for the optical absorption is strongly strain-dependent.

Magnetic Phase Transition in Strained Two-Dimensional CrSeTe Monolayer

Tunable magnetic phase transition in two-dimensional materials is a fascinating subject of research. We perform first-principle calculations based on density functional theory to clarify the magnetic property of CrSeTe monolayer modulated by the biaxial compressive strain. Based on the stable structure confirmed by the phonon calculation, CrSeTe is determined to be a ferromagnetic metal that undergoes a phase transition from a ferromagnetic state to an antiferromagnetic state with nearly 2.75% compressive strain. We identify the stress-magnetism behavior originating from the changes in interactions between the nearest-neighboring Cr atoms (J 1) and the next-nearest-neighboring Cr atoms (J 2). Through Monte Carlo simulation, we find that the Curie temperature of the CrSeTe monolayer is 160 K. The CrSeTe monolayer could be an intriguing platform for the two-dimensional systems and potential spintronic material.

Strain tuning of the electronic structure and optical properties of novel Janus MgBrI monolayer: Insights from first-principles calculations

Using first-principles calculations in the framework of density functional theory (DFT), we systematically carry out the dynamic stability, electronic and optical properties of the Janus MgBrI monolayer and its parent MgI2 and MgBr2 monolayers. Our findings show that these systems have no imaginary frequencies in the entire Brillouin zone (BZ), which means they are very stable. At the equilibrium state, the results indicate that the Janus MgBrI monolayer is a direct band gap insulator with an energy gap of 3.475 eV, which is smaller than the bandgap of MgI2 and MgBr2 monolayers. The studied optical properties including the complex dielectric function, refractive index, reflectivity, extinction coefficient and absorption coefficient as a function of the photon energy have been investigated. The absorption coefficient shows a strong absorption of the light in the ultraviolet (UV) region. We studied how biaxial strain effects the electronic and optical properties of the Janus MgBrI monolayer. When the strain ranged from −10% to 10%, the Janus MgBrI monolayer showed an insulator characteristic with a direct band gap, which changed to an indirect band gap when the biaxial compressive strain was −10%. In addition, the Janus MgBrI displays an absorption of around 105 cm−1 in the ultra-violet (UV) region. Our results highlight an intriguing new two-dimensional (2D) material that could be used in optoelectronic devices and UV absorbers.

Tight-binding analysis of the effect of strain on the band structure of GaN

The effects of strain on the band structure of GaN are investigated by using an empirical tight-binding method. The impacts on its bandgap, carrier effective mass, and group velocity are discussed. By analyzing the orbital components at the top of the valence band, the cause of the variation of the band structure including effective-mass exchange is discussed. Analysis of the average group velocity indicates that tensile uniaxial or compressive biaxial strain may be beneficial for achieving higher breakdown voltage in vertical GaN devices due to the smaller group velocity of the valence band. For the same reason, we also predict higher breakdown voltages due to tensile biaxial strain for horizontal devices.

Enhancement of Polarization Properties of Bulk PbTiO3 by Engineering Strain

The effect of strain (up to 8%) on the polarization of bulk PbTiO3 is clarified, in which the focus is put on the ferroelectric hysteresis loop. The shell model, a powerful simulation tool for calculating electric polarization, is developed. The results obtained show that the ferroelectric hysteresis loop is expanded or shrunk under the strain. Besides, the study also points out two main results which have potential applications in practice. (1) The remanent polarization is enhanced significantly under the compressive biaxial strain (1.68 times). (2) The ferroelectric-paraelectric phase transition can be achieved under the compressive uniaxial strain (at 8%).

Effect of strain on the thermoelectric properties of three‐dimensional PtS2: A first‐principles study

AbstractThe effect of strain on the electronic structure and thermoelectric properties of three‐dimensional (3D) PtS2 was investigated through ab initio calculations. The phonon dispersion curve has no imaginary frequency under biaxial tensile and compressive strains, indicating that the studied structure is stable. All the studied structures show higher Seebeck coefficients (>400 μV/K), and the Seebeck coefficient of p‐type semiconductors is superior to that of n‐type semiconductors. Tensile strain can decrease the lattice thermal conductivity of PtS2, whereas compressive stress has the opposite effect on the lattice thermal conductivity. The smallest lattice thermal conductivity is 26.7 W/m·K under 6% strain when the temperature is 800 K. The maximum ZT values for the p‐type and n‐type semiconductors are 0.17 and 0.48, respectively. This work provides a guide to studying how strain modulates thermoelectric properties of materials; the relaxation time calculation method in this study is believed to be more accurate due to the fact that the influence of the carrier concentration is considered.

First-Principles Study of Structural and Electronic Properties of Monolayer PtX2 and Janus PtXY (X, Y = S, Se, and Te) via Strain Engineering.

In this work, the structural parameters and electronic properties of PtX2 and Janus PtXY (X, Y = S, Se, and Te) are studied based on the density functional theory. The phonon spectra and the Born criteria of the elastic constant of these six monolayers confirm their stability. All PtX2 and Janus PtXY monolayers show an outstanding stretchability with Young's modulus ranging from 61.023 to 82.124 N/m, about one-fifth that of graphene and half that of MoS2, suggesting highly flexible materials. Our first-principles calculations reveal that the pristine PtX2 and their Janus counterparts are indirect semiconductors with their band gap ranging from 0.760 to 1.810 eV at the Perdew-Burke-Ernzerhof level (1.128-2.580 eV at the Heyd-Scuseria-Ernzerhof level). By applying biaxial compressive and tensile strain, the electronic properties of all PtX2 and Janus PtXY monolayers are widely tunable. Under small compressive strain, PtX2 and Janus PtXY structures remain indirect semiconductors. PtTe2, PtSeTe, and PtSTe monolayers undergo a semiconducting to metallic transition when the strain reaches -6, -8, and -10%, respectively. Interestingly, there is a transition from the indirect semiconductor to a quasi-direct one for all PtX2 and Janus PtXY monolayers when the tensile strain is applied.

Thermodynamics modeling of ZrCo-H systems with biaxial compressive strain during desorption

As known, inferior cycling performance and deteriorated de-/hydrogenation kinetics resulting from by hydrogen-induced disproportionation severely impedes engineering applications of zirconium-cobalt (ZrCo) alloys used in the industry of hydrogen isotope storage. In this work, the first principles calculations are conducted to investigate the thermodynamic behavior of ZrCo hydrides with and without biaxial compressive strain during desorption. Importantly, the whole dehydrogenation process is divided into two steps due to the presence of intermediate ZrCoHx (x = 2, 2.25, 2.50, and 2.75). During Step I, the hydrided phase ZrCoH3 is decomposed into ZrCoHx (ZrCoH3 → ZrCoHx), and then the phase transformation from ZrCoHx to ZrCo (ZrCoHx → ZrCo) occurs in Step II. Afterwards, the equilibrium pressures, enthalpy changes, and entropy changes involved in the reactions are determined through the thermodynamic relations. The results demonstrate that compared to the second step, there is a superiority of dehydrogenation performance for the first step, which is derived from higher equilibrium pressure. Moreover, the equilibrium pressure increases with the decrease of the H content in ZrCoHx because of the intensification of vibrational degrees of freedom of H atoms. Further study concerns on the influence of biaxial strain (-3% - 0%) along [010] and [001] on thermodynamics properties of the ZrCo-H systems, indicating that the strain deteriorates the lattice stability and improves the equilibrium pressure. By contrast, it is more favorable to enhancing the dehydrogenation performance when the compressive strain is beyond 1.5%.

Investigating the magnetic, thermoelectric, and thermodynamic properties of the GeCH3 single-layer considering external magnetic field, doping, and strain

Extensive research is ongoing to improve the performance of thermoelectric and thermodynamic properties of the material because preventing energy waste is vital in modern society. Herein, we study the thermoelectric and thermodynamic properties of the GeCH3 single-layer (SL) under the influence of an external magnetic field, electron doping, and tensile and compressive biaxial strain by using the tight-binding and equilibrium Green’s function method. We found that the electronic heat capacity, magnetic susceptibility, and electronic thermal and electrical conductivity increase by employing an external magnetic field, electron doping, and tensile biaxial strain. However, compressive biaxial strain yields a decrease in thermoelectric and thermodynamic properties. The results of our study show that the GeCH3 SL is paramagnetic. The results presented here that the GeCH3 SL is a suitable alternative for use in thermoelectric, spintronic, and valleytronics devices.

First-principles calculations of improving carrier mobility for [formula omitted]CsPbI3

In the past few years, inorganic perovskite CsPbI3 has attracted great interest due to high conversion efficiency in solar cells. Carrier mobility, as an important physical parameter in optoelectronic devices, directly affected power consumption. In this study, the phonon-limited mobilities of β-CsPbI3 were investigated using Boltzmann transport equation based on first-principles calculations. Using the fully relaxed lattice constants of a = 8.829/c = 6.505 Å, we obtained a mobility of μe,x=151/μe,z = 241 cm2V−1s−1 and μh,x=94/μh,z = 163 cm2V−1s−1. Longitudinal optical phonon associated with the relative vibration between Pb and I atoms was revealed to be the main scattering source, and the scattering mechanism was independent of temperature and carrier type. Biaxial compressive strain of in-plane (ab-plane) decreased mobility, which should be avoided in film growth. Since the exchange correlation functional with GGA-PBE form overestimated the lattice constants, the experimental values were used to correct the results. The band gap was mainly determined by the lattice constant of in-plane, and the mobility was extremely sensitive to the lattice constant in out-of-plane (c-axis). Using experimental lattice constants of a = 8.776/c = 6.214 Å, the mobility was improved to μe,x=503/μe,z = 1084 cm2V−1s−1 and μh,x=373/μh,z = 871 cm2V−1s−1.

Optical gain and threshold current density of strained wurtzite GaN/AlGaN quantum dot lasers

In this work, the influences of biaxial compressive and tensile strains on optical gain and threshold current density are investigated theoretically as a function of the side lengths of the quantum box in the GaN/Al0.2Ga0.8N structure by using a model based on the density matrix theory of semiconductor lasers with relaxation broadening. For various side lengths of the quantum box, we compare the spectra gain curves of compressive, tensile-strained, and unstrained structures of the GaN/Al0.2Ga0.8N cubic quantum-dot (QD) laser. The dependence of peak optical gain on carrier density and the modal gain on current density is plotted too for all cases. The results reveal that many enhancements can be made to the laser structure by introducing -0.5% compressive strain: a higher value of optical gain of 18421 cm-1 at L=60A, a lower value of transparency of carrier density of Ntr=0.13*10¨¨19 cm-3and transparency current density of Jtr=26.9 A/cm and a lower threshold current density of Jth= 78.87 A/cm2 at L=100 A.

Finite Element Analysis of Large Plastic Deformation Process of Pure Molybdenum Plate during Hot Rolling

The rare molybdenum resources are being increasingly used in heavy industries. In this study, the common unidirectional and cross hot rolling operations, for pure molybdenum plates, were numerically simulated by using MSC. Marc software. An elastic–plastic finite element model was employed, together with the updated Lagrange method, to predict stress and strain fields in the workpiece. The results showed that there was a typical three-dimensional additional compressive stress (σy> σz > σx) in the deformation zone, while strain could be divided into uniaxial compressive strain and biaxial tensile strain (εy > εx > εz). Tensile stress σx increased with the accumulation of reduction and the decrease in friction coefficient at the edge of the width spread. More importantly, the interlaced deformation caused by cross-commutations, which were helpful in repairing the severe anisotropy created by unidirectional hot rolling. The evolution of the temperature field of pure molybdenum plate was investigated. The surface quenching depth of the pure molybdenum plate was about 1/6 H under different initial temperatures and reductions. In addition, the fundamental reason for the nonuniform distribution of stress and strain fields was the joint influence of rolling stress, contact friction, and external resistance. By comparing the theoretical simulation value of the model with the experimental verification data, we found that the model was aligning well with the actual engineering.

Two dimensional Janus RuXY (X, Y = Br, Cl, F, I, X ≠ Y) monolayers: ferromagnetic semiconductors with spontaneous valley polarization and tunable magnetic anisotropy.

Two-dimensional (2D) ferromagnetic (FM) materials with valley polarization are highly desirable for use in valleytronic devices. The 2D Janus materials have fascinating physical properties due to their asymmetrical structures. In this work, the electronic structure and magnetic properties of Janus RuXY (X, Y = Br, Cl, F, I, X ≠ Y) monolayers are systematically studied using first-principles calculations. RuBrCl, RuBrF, and RuClF monolayers are all FM semiconductors. The valley polarization is present in the band structure and this is determined by the spin orbit coupling (SOC). The valley splitting energy of the RuClF monolayer is as large as 204 meV, with a perpendicular magnetic anisotropy (PMA) energy of 1.918 mJ m-2 and a Curie temperature of 316 K. Therefore, spontaneous valley polarization at room temperature will be seen in the RuClF monolayer. The Curie temperature of the RuBrF monolayer is higher than that of the RuClF, but the magnetic anisotropy energy (MAE) is in-plane magnetic anisotropy (IMA). The valley splitting energy of the RuBrCl monolayer is higher and the PMA energy is lower than that of the RuClF monolayer. The Curie temperature was only 197 K. The valley polarization was modulated in the RuXY monolayers at different biaxial strains, during which the semiconductor properties are still maintained. The PMA of the RuClF and RuBrCl monolayers is enhanced by the biaxial compressive strains, which are mainly attributed to the variation of the (dyz, d2z) orbital matrix elements of the Ru atoms. The MAE of the RuBrF monolayer is tuned from IMA into PMA at a biaxial strain of -6%. These results show an example of a 2D Janus ferrovalley material.

Ferroelectric and strain-tuned MoSe2/Bi2O2Se van der Waals heterostructure with band alignment evolution.

The vertically stacked two-dimensional van der Waals heterostructure (2D vdWH) provides a unique platform for integrating distinctive properties of various 2D materials by functionalizing the interfacial interaction and regulating its band alignment. Herein, we theoretically propose a new MoSe2/Bi2O2Se vdWH material, in which a zigzag-zipper structure of the Bi2O2Se monolayer is constructed to model its ferroelectric polarization and maintain a small interlayer mismatch with MoSe2. The results show a typical unipolar barrier structure with a large band offset in the conduction band and nearly zero offset in the valence band of MoSe2/Bi2O2Se when the ferroelectric polarization of Bi2O2Se is back to MoSe2, in which the electron migration is blocked and holes can migrate unimpeded. It is also found that the band alignment lies between the type-I and type-II heterostructures and band offsets can be flexibly modulated under the joint action of ferroelectric polarization of Bi2O2Se and in-plane biaxial tensile and compressive strains. This work would facilitate the development of multifunctional devices based on the MoSe2/Bi2O2Se heterostructure material.

Strain-induced tunable optoelectronic properties of inorganic halide perovskites APbCl3 (A = K, Rb, and Cs)

Halide perovskites are promising photovoltaic, solar cell, and semiconductor materials. Density-functional theory (DFT) models address compressive and tensile biaxial strain effects on APbCl3, where A = (K, Rb, and Cs). This research shows how A-cation impacts bandgap energy and band structure. The direct bandgap for KPbCl3, RbPbCl3, and CsPbCl3 is found 1.612, 1.756, and 2.046 eV, respectively; increases from A = K to Cs. When spin–orbital coupling (SOC) is introduced, bandgaps in KPbCl3, RbPbCl3, and CsPbCl3 perovskites are reduced to 0.356, 0.512, and 0.773 eV, respectively. More tensile strain widens the bandgap; compressive strain narrows it. Without SOC, the bandgaps of KPbCl3, RbPbCl3, and CsPbCl3 were tuned from 0.486 to 2.213 eV, 0.778 to 2.289 eV, and 1.168 to 2.432 eV, respectively. When the compressive strain is increased, the dielectric constant of APbCl3 decreases (redshift) and increases (blueshift) as the tensile strain is increased. Strain improves APbCl3 perovskite’s optical performance.

Unexpected electro-catalytic activity of the CO reduction reaction on Cr-embedded poly-phthalocyanine realized by strain engineering: a computational study.

The electrochemical conversion of carbon monoxide (CO) into value-added products is highly promising for carbon utilization and CO removal. Based on previous theoretical studies, we computationally explored the effect of strain engineering on electrocatalysis of the CO reduction reaction (CORR) by two-dimensional (2D) transition metal embedded polyphthalocyanines (MPPcs). By calculating the adsorption energy of CO and the free energies of key intermediates on the MPPcs under uniaxial and biaxial strains, it was revealed that only CrPPc under biaxial strain has the potential to exhibit significant enhancement of the catalytic performance. The free energy diagrams of the CORR catalyzed by CrPPc were plotted under specific biaxial strains, where both the optimal reaction pathway and rate-determining step are found to be evidently changed. What's more, the 5% compressive strain imposed on CrPPc results in an ultra-low limiting potential (UL = -0.09 V) with high selectivity on CH4 as the final product, indicating unexpected electro-catalytic activity. Our study clearly elucidates that moderate strain could greatly enhance the electrocatalytic performance of 2D materials in the CORR.

A graphene-like BeS monolayer as a promising gas sensor material with strain and electric field induced tunable response: a first-principles study.

A comprehensive investigation of the gas sensing potential of BeS monolayer has been conducted using DFT calculations. Twelve common pollutant gases: NH3, NO2, NO, CO, CO2, CH4, H2, O2, N2, H2S, H2O and SO2, have been studied. Our analysis reveals defect states in the band structure near the Fermi level and strong hybridization between gas molecule orbitals and the BeS monolayer. We observe higher adsorption energies for NH3 and CO compared to other popular gas sensing materials. The optical properties of CO2 and NO2 adsorbed on the BeS monolayer show increased reflectivity and absorption coefficient in the UV and far infrared region. Tensile strain has minimal impact on adsorption energy, while biaxial compressive strains enhance the gas sensing capability of the BeS monolayer. The application of an electric field offers control over gas adsorption and desorption. We propose the BeS monolayer as a promising candidate for future gas molecule sensing applications due to its high adsorption energy, rapid recovery time, and distinct optical properties.