Biaxial Compressive Strain Research Articles

Prediction of nodal-line semimetals in two-dimensional black phosphorous films

Semimetals are a new kind of quantum materials, in which the conduction and valence bands cross each other near the Fermi level. Based on density-functional theory calculations and symmetry analysis, we propose nodal-line semimetals in layered stacked black phosphorus (BP) films which are designed to have a mirror symmetry lying in the BP layer plane and thus rendering them different from the BP film systems previously studied. A closed nodal-line degenerate band can appear around the Fermi level in the BP films after a biaxial compressive strain is applied. The calculated Z2 number of Z2 = − 1 indicates the robustness of the nodal-line semimetals obtained in the BP films, protected by the in-plane mirror symmetry. Intriguingly, with the increase of the film thickness, a smaller biaxial compressive strain is required to produce the nodal-line semimetals, more accessible in experiments. Our results provide a promising route to carrying out the nodal-line semimetals based on various two-dimensional materials.

A two-dimensional electron gas based on a 5s oxide with high room-temperature mobility and strain sensitivity

The coupling of optical and electronic degrees of freedom together with quantum confinement in low-dimensional electron systems is particularly interesting for achieving exotic functionalities in strongly correlated oxide electronics. Recently, high room-temperature mobility has been achieved for a large bandgap transparent oxide – BaSnO3 upon extrinsic La or Sb doping, which has excited significant research attention. In this work, we report the realization of a two-dimensional electron gas (2DEG) on the surface of transparent BaSnO3 via oxygen vacancy creation, which exhibits a high carrier density of ~7.72 × 1014 cm−2 and a high room-temperature mobility of ~18 cm2·V−1·s−1. Such a 2DEG is rather sensitive to strain and a less than 0.1% in-plane biaxial compressive strain leads to a giant resistance enhancement of ~350% (more than 540 kΩ/□) at room temperature. Thus, this work creates a new path to exploring the physics of low-dimensional oxide electronics and devices applicable at room temperature.

Tuning structural and electronic properties of two-dimensional aluminum monochalcogenides: Prediction of Janus Al2XX′ (X/X′:O,S,Se,Te) monolayers

The realization of ternary, single-layer transition metal dichalcogenides has suggested a promising strategy to develop two-dimensional (2D) materials with alternative features. In this study, we design and investigate Janus aluminum monochalcogenide monolayers, ${\mathrm{Al}}_{2}X{X}^{\ensuremath{'}}$ $(X/{X}^{\ensuremath{'}}=\phantom{\rule{0.28em}{0ex}}\mathrm{O},\phantom{\rule{0.28em}{0ex}}\mathrm{S},\phantom{\rule{0.28em}{0ex}}\mathrm{Se},\phantom{\rule{0.28em}{0ex}}\mathrm{Te})$ by using first-principles methods. Starting from binary constituents, the ternary structures are optimized without any constraint and ground-state configurations are obtained. The stability of these systems is tested by performing phonon spectra analysis and ab initio molecular dynamics simulations and all ${\mathrm{Al}}_{2}X{X}^{\ensuremath{'}}$ monolayers other than AlTeO are confirmed to be dynamically stable. Mechanical properties are examined by calculating Young's modulus and Poisson's ratio and subsequently compared with binary counterparts. Monolayers of ${\mathrm{Al}}_{2}X{X}^{\ensuremath{'}}$ have a brittle character but oxygenation makes them less stiff. The electronic structure is also analyzed and variation of the band gap with the type of chalcogen atoms is revealed. It is found that different from their binary counterparts, ${\mathrm{Al}}_{2}X\mathrm{O}$ monolayers are direct band-gap semiconductors. Additionally, modification of the electronic structure in the presence of biaxial compressive or tensile strain is investigated by taking into account possible indirect-direct band-gap transitions. Our results not only predict stable 2D ternary ${\mathrm{Al}}_{2}X{X}^{\ensuremath{'}}$ structures but also point out them as promising materials for optoelectronic applications.

Optical properties of LaNiO3 films tuned from compressive to tensile strain

Materials with strong electronic correlations host remarkable -- and technologically relevant -- phenomena such as magnetism, superconductivity and metal-insulator transitions. Harnessing and controlling these effects is a major challenge, on which key advances are being made through lattice and strain engineering in thin films and heterostructures, leveraging the complex interplay between electronic and structural degrees of freedom. Here we show that the electronic structure of LaNiO3 can be tuned by means of lattice engineering. We use different substrates to induce compressive and tensile biaxial epitaxial strain in LaNiO3 thin films. Our measurements reveal systematic changes of the optical spectrum as a function of strain and, notably, an increase of the low-frequency free carrier weight as tensile strain is applied. Using density functional theory (DFT) calculations, we show that this apparently counter-intuitive effect is due to a change of orientation of the oxygen octahedra.The calculations also reveal drastic changes of the electronic structure under strain, associated with a Fermi surface Lifshitz transition. We provide an online applet to explore these effects. The experimental value of integrated spectral weight below 2 eV is significantly (up to a factor of 3) smaller than the DFT results, indicating a transfer of spectral weight from the infrared to energies above 2 eV. The suppression of the free carrier weight and the transfer of spectral weight to high energies together indicate a correlation-induced band narrowing and free carrier mass enhancement due to electronic correlations. Our findings provide a promising avenue for the tuning and control of quantum materials employing lattice engineering.

Lattice strain influence on conduction band nonparabolicity in GaAs and InAs: Application to intraband optical absorption in InGaAs-GaAs asymmetric step quantum wells

We use empirical sps*d5 tight-binding calculations to determine the effects of compressive biaxial lattice strain, perpendicular to the [001] crystal direction, in zinc blende GaAs and InAs. Under that approach, we have been able to compute the behavior of quantities such as the average valence band energy, the energy band gap, the conduction band effective mass, and the spin-orbit split-off energy, as functions of the biaxial strain, within a range from 0 to −7%. Expressions governing these dependencies are reported for both materials. With such information at hand it is possible to calculate the variation of the coefficient of conduction band nonparabolicity due to the presence of strain. Also, the outcome for such quantities allows to evaluate the valence band offset in GaAs/InGaAs heterointerfaces as a consequence of the strain appearing from the difference between the lattice constants of the involved materials. Taking advantage of the above mentioned results, we have performed the calculation of confined conduction band states in step-like asymmetric quantum wells of the GaAs/Inx1Ga1-x1As/Inx2Ga1-x2As/GaAs prototype, using a k→⋅p→ formalism that solves the effective mass equation arising from a bi-cuadratic (nonparabolic) dispersion law. We report the calculation of the optical absorption coefficient related with intraband transitions that involve the ground and first excited energy levels. For that purpose, the study takes into account the variation of the layer widths and compositions.

Strain-modulated electronic and magnetic properties of Co2TMAl

Discovery of magnetism in full-Heusler compounds opens up new opportunities due to their potential applications in spintronics. Here, we investigated the electronic and magnetic properties of full-Heusler Co2TMAl (TM = Ti, V, Cr, Mn, and Fe) compounds via strain engineering using density functional theory. It is found that biaxial strains along the ab-plane substantially affects the bond lengths, which further tune the magnetic moment and spin polarization (SP). The reorientation of the hybridization between TM and Co orbitals results in the variation of charge confinement that optimized physical properties. The most striking feature of the present study is the unusual response of Co2CrAl and Co2MnAl as a function of strain. For Co2CrAl, the robust half-metallicity is achieved under the biaxial compressive strains ranging from −1% to −5%, while tensile strain shows a negative impact. In the case of Co2MnAl, a substantial enhancement (decrease) in the SP magnitude is evident for compressive (tensile) strain. Hence, we affirm that strain can be an efficient way to tune the physical properties of Heusler compounds.

Tunable electronic structure and magnetic properties of two-dimensional g-C3N4/Cr2Ge2Te6 van der Waals heterostructures

Recently, it has been proven that the strain, electric field and interlayer distance can effectively modulate the electronic structure and magnetic properties of two-dimensional (2D) van der Waals (vdW) heterostructures, which can facilitate the applications of 2D materials in spintronic devices. Here, the electronic structure and magnetic properties of 2D g-C3N4/Cr2Ge2Te6 vdW heterostructures are investigated by first-principles calculations. The combination of these two monolayers significantly affects their lattice structures. With the increase of biaxial compressive strain ε, the band gap of the heterostructures gradually decreases, where the band gap is 61.4 meV at ε = −2%. At ε = −4% and −6%, the g-C3N4/Cr2Ge2Te6 heterostructures are metallic. At a biaxial compressive strain, the heterostructures show the in-plane magnetic anisotropy (IMA), but the system shows the perpendicular magnetic anisotropy at a biaxial tensile strain. By applying the electric field, the heterostructures show IMA. The band gap of the heterostructures hardly changes by changing the interlayer distance, but the magnetic anisotropy is changed. The spin channel of g-C3N4 inverses at valence band maximum as the interlayer distance increases by 0.75 Å. These results are significant to developing the low-dimensional spintronic devices.

Catalytic activity for hydrogen evolution reaction in square phase Janus MoSSe monolayer: A first-principles study

Based on density-functional theory, the catalytic property for hydrogen evolution reaction (HER) of square phase Janus MoSSe monolayer is systematically investigated. Similar to 2H-phase transition metal dichalcogenides, the basal plane of pristine 1S- MX 2 and 1S- MXY ( M : Mo; X/Y : S/Se) are catalytically inert. By introducing vacancy defects, the catalytic performance of the basal plane can be activated. The calculated Gibbs free energy differences (Δ G H ) of non-Janus 1S-MoS 2 and 1S-MoSe 2 with point vacancies reduce to 35 and −105 meV, respectively. Due to the atomic asymmetry of Janus structure, Δ G H of 1S-MoSSe monolayer are −53 and −28 meV for S- and Se-vacancy defect, respectively. Qualitatively, the catalytic activity of 1S-MoSSe and 1S-MoS 2 with S vacancy are comparable while 1S-MoSSe is much better than 1S-MoSe 2 with Se vacancy. Additionally, Δ G H of 1S-MoSSe with Se-vacancy keeps within ±50 meV under a −2%–2% biaxial strain, demonstrating high robustness of catalytic activity against strain. The results indicate the potential of square phase Janus MoSSe monolayer as an option for monolayer HER catalyst. • Basal planes of pristine square phase non-Janus and Janus TMD monolayers are catalytically inert. • Introduction of vacancy defects can activate the basal plane of 1S-MoSSe monolayer. • Δ G H of 1S-MoSSe_VSe and 1S-MoSSe_VS are −0.028 eV and −0.053 eV by DFT calculations, respectively. • Introduction of tensile (compressive) biaxial strain reduces (enlarges) the value of Δ G H . • Within the strain range from −2% to 2%, Δ G H of 1S-MoSSe_VSe keeps in the catalytic window.

Germanene/2D-SiC van der Waals heterobilayer: Structural features and tunable electronic properties

Van der Waals heterobilayer, consisting of different two-dimensional (2D) materials, has recently drawn substantial research attention. We present a detailed investigation of the structure and electronic properties of germanene and 2D silicon carbide (Ge/2D-SiC) van der Waals heterobilayer by means of first-principles calculations under the framework of density functional theory. Three different stacking patterns are predicted for this Ge/2D-SiC heterostructure. All the representative structures offer a direct bandgap of approximately 80–100 meV. The breaking of the sub-lattice symmetry as well as the transfer of charges are perceived as the pivotal effects that open the bandgap for each structure. The compressive bi-axial strain is applied for the further tune of the bandgap, resulting in an alter in the bandgap from 85 to 118 meV. Upon varying the interlayer distance between germanene and 2D-SiC the bandgap can further be tuned. The distribution of space charges of the conduction and valence bands and projected density of states represent that germanene plays the main role in forming the heterobilayers electronic properties, thus suggesting the ability of 2D-SiC as a stable substrate. These outcomes expose that Ge/2D-SiC heterostructure would be an incredible resource for imminent Ge-based high-performance nanoelectronic devices such as quantum computing spintronic devices and nanoscale energy storage devices.

Structural, electronic, optical, thermoelectric and photocatalytic properties of SiS/MXenes van der Waals heterostructures

Vertical stacking of two-dimensional materials into layered van der Waals heterostructures are recently considered as promising candidates for optoelectronic devices because they can combine advantages of the individual 2D materials. MXenes have emerged an appealing optoelectronic material due to the desirable electronic properties. This paper focuses on the structural, electronic, optical, thermoelectric and photocatalytic properties of SiS, Ti2CO2, Zr2CO2, Hf2CO2 monolayers and their corresponding van der Waals (vdW) heterostructures. These materials and their heterostructures are dynamically stable. All unstrained heterostructures show indirect band gap semiconductors except SiS-Ti2CO2 which exhibits metallic nature. The higher carrier mobility of the metallic, SiS-Ti2CO2 is followed by its least effective mass. The induction of considerable biaxial (compressive and tensile) strain offers metallic characteristics to SiS-Ti2CO2 and SiS-Zr2CO2 while SiS-Hf2CO2 retains its semiconducting nature. SiS-Zr2CO2/SiS- Hf2CO2 show type-I/type-II band alignment which are highly suitable for electronic, optical, nanoelectronic and optoelectronic devices. Additionally, the least effective mass of SiS-Zr2CO2 offers higher carrier mobility. Bader charge analysis revealed that MXenes dissipate the charge carriers to SiS monolayer. Furthermore, a good comparison of thermoelectric performance of all the systems illustrates their significant power factor that is well interpreted at 300 K and 800 K. Several excitonic peaks are observed of the systems in the visible region of solar spectrum. Furthermore, valence (conduction) band edge potentials are calculated to understand the photocatalytic behavior of these systems.

Contrasting effects of mismatch strain on the magnetic behavior of undoped and doped BaFeO3-δ thin films

We investigate the effects of biaxial compressive and tensile strain on the magnetic and structural properties of BaFeO3-δ (BFO), BaFe0.9Ti0.1O3-δ (BFTO) and BaFe0.9Ru0.1O3-δ (BFRO) thin films fabricated on SrTiO3 (001) (STO) and MgO (001). The magnetization measurements show a long range ferromagnetic ordering in compressive strained BFO thin film while an antiferromagnetic ordering with weak ferromagnetism appears in tensile strained BFO thin film. This behavior is ascribed to the difference in oxygen stoichiometry under the two strain states in the BFO thin films. Ti4+ (3d0) and Ru4+ (4d5) ion substitutions at B-site cause an increase in unit cell volume irrespective of the strain type resulting in relaxation of tensile strain and enhancement of compressive strain. This B-site disorder leads to collapse of long range ferromagnetic ordering in BFTO and BFRO thin films grown on STO but an increased ferromagnetic ordering comes along in BFTO and BFTRO thin films on MgO. The contrasting results portray that tensile strain suppresses the effect of cation disorder evident under compressive strain, on the magnetic and structural properties of BFO, BFTO and BFRO thin films.

Applied biaxial strain induced tunable sensing performance of green phosphorene monolayer towards small molecules: A DFT study

The development of sensors for inorganic environmental pollutants is crucial for global public health and environmental ecology. Herein, we highlight the tunable sensing performance of harmful carbon oxides and nitrogen oxides on green phosphorene nanosheet induced by applied external biaxial using van der Waals corrected spin-polarized density functional calculations. The results reveal that the charge transfer and electronic structures are effectively changed only after nitrogen oxides exposure, suggesting the high selectivity of green phosphorene towards nitrogen oxides rather than carbon oxides, exhibiting promising targeting selectivity. Moreover, we find that the interactions between target adsorbates and green phosphorene would be further enhanced upon compressive biaxial mechanical strain. The electronic structures of the nitrogen oxides adsorbed systems would be effectively modulated under applying biaxial strain. Additionally, the calculated recovery time indicates that applied tensile strain remarkably facilities the desorption of the considered gas. Our main findings would provide a new avenue to tune the sensing performance of green phosphorene nanosheet and demonstrate the potential application prospect for green phosphorene nanosheet as a reversible sensor for nitrogen oxides detection.

Laser-induced anharmonicity vs thermally induced biaxial compressive strain in mono- and bilayer MoS2 grown via CVD

We studied the role of biaxial compressive strain in laser-induced anharmonicity effects of mono- and bilayer MoS2 grown by chemical vapor deposition. With the increased laser powers, the A1g phonon mode was strongly affected by the anharmonicity, whereas the E12g mode was negligibly affected by the anharmonic effects, which is attributed to the dominant nature of biaxial compressive strain over the effects of anharmonicity. In the case of the A1g phonon mode, anharmonicity effects dominated over the biaxial compressive strain owing to its large out-of-plane thermal expansion coefficient. The origin of biaxial compressive strain was explained by invoking the difference in linear thermal expansion coefficients of MoS2 and the substrate. Moreover, thermally induced biaxial compressive strain also influenced the exciton emission energy. The role of the substrate thermal conductivity was discussed in the light of anharmonicity of the MoS2 monolayer. In addition, the polarizability of Raman modes was found to be dependent on the second layer arrangement over the monolayer.

Strain-dependent electronic and optical properties of boron-phosphide and germanium-carbide hetero-bilayer: A first-principles study

Opto-electronic properties of boron phosphide–germanium carbide (BP/GeC), a new van der Walls hetero-bilayer (HBL) with all possible stacking patterns, are studied under the density functional theory originated first-principles. The dynamical and chemical stabilities of the hetero-bilayer are confirmed by phonon spectra and binding energy. Among the dynamically stable HBLs, HBL 1 has the lowest binding energy with the smallest interlayer spacing of about 3.442 Å. Both values and natures (indirect or direct) of the electronic band structure are highly responsive to the stacking patterns. We have found that HBL 1 is indirect, while HBL 2 and HBL 3 become a direct bandgap at the K high symmetry point. All HBLs show type-II band alignment. Both compressive and tensile biaxial strains on the electronic properties of HBLs have been considered. All the HBLs become a direct bandgap for the compressive strain at 4% and 6%. We have also presented the optical property calculations on the HBLs, namely, the complex dielectric function and absorption properties, showing unique optical properties with significant absorption (5 × 105 cm−1 in HBL 2) in the whole solar spectra compared with their comprising monolayers. Moreover, the strain-dependent optical absorption coefficients with varying photon wavelengths are calculated and the maximum value is attained to be about 6.5 × 105 cm−1 in HBL 2 at 4% compressive strain. Consequently, the optoelectronic properties we have explored in our proposed new hetero-bilayer systems can guide the experimental realization of the hetero-bilayers and effective use in the future photovoltaic applications.

Strain tunable band structure of a new 2D carbon allotrope C568

Recently, C568 has emerged as a new carbon allotrope, which shows semiconducting properties with a band gap around 1 eV and has attracted much attention. In this work, the external strain effects on the electronic properties of C568 have been studied theoretically through first-principle calculations. The numerical results show that while in-plane uniaxial and biaxial strains both reduces the band gap of C568 in case of tensile strain, their effects are quite different in the case of compressive strain. With increasing compressive uniaxial strain, the band gap of C568 first increases, and then dramatically decreases. In contrast, the application of compressive biaxial strain up to –10% only leads to a slight increase of band gap. Moreover, an indirect-to-direct gap transition can be realized under both types of compressive strain. The results also show that the optical anisotropy of C568 can be induced under uniaxial strain, while biaxial strain does not cause such an effect. These results indicate good strain tunability of the band structure of C568, which could be helpful for the design and optimization of C568-based nanodevices.

Topological transition in monolayer blue phosphorene with transition-metal adatom under strain

We carried out first-principles calculations to investigate the electronic properties of the monolayer blue phosphorene (BlueP) decorated by the group-IVB transition-metal adatoms (Cr, Mo and W), and found that the Cr-decorated BlueP is a magnetic half metal, while the Mo- and W-decorated BlueP are semiconductors with band gaps smaller than 0.2 eV. Compressive biaxial strains make the band gaps close and reopen, and band inversions occur during this process, which induces topological transitions in the Mo-decorated BlueP (with strain of −5.75%) and W-decorated BlueP (with strain of −4.25%) from normal insulators to topological insulators (TIs). The TI gap is 94 meV for the Mo-decorated BlueP and 218 meV for the W-decorated BlueP. Such large TI gaps demonstrate the possibility to engineer topological phases in the monolayer BlueP with transition-metal adatoms at high temperature.

2D XBiSe3(X = As, Sb) monolayers with high anisotropic mobility and enhanced optical absorption in visible light region

Configurations and properties of 2D XBiSe3(X = As, Sb) monolayers are predicted by first-principles calculations. Dynamic and thermal stabilities are affirmed by phonon dispersion and ab initio molecular dynamics simulation, respectively. Bandgap and band edges, density of states, optical absorptions, and mobilities are calculated by using the obtained structures. Effects of strain engineering on the band structure and optical absorption are also explored. Results reveal that the AsBiSe3 and SbBiSe3 monolayers have indirect gaps of 1.2 eV (HSE06)/1.43 eV (GW) and 1.1 eV (HSE06)/1.32 eV (GW), respectively. The high electron (approximately 105 cm2V−1s−1) and hole (approximately 104 cm2V−1s−1) mobilities of the XBiSe3 monolayers are observed for zigzag and armchair directions, and obvious anisotropy is observed. The intense optical absorptions in the UV–visible light regions are identified for the two monolayers. Interestingly, the band edge of the AsBiSe3 monolayer can span the redox potential of water under −4% biaxial compressive strain, which implies that this monolayer can be used as a candidate material for photocatalytic water splitting to produce hydrogen. All the results support the assertion that the XBiSe3 monolayers have potential for application in the optoelectronic, photocatalytic, and photovoltaic fields.

Investigation of the electronic and thermoelectric properties of hydrogenated monolayer germanene under biaxial tensile and compressive strains by DFT approach

Abstract Two-dimensional materials possess a great potential in thermoelectric devices. In this work, by using a combination of first-principles calculations and semiclassical Boltzmann transport theory, a systematic investigation of electronic and thermoelectric properties of hydrogenated monolayer germanene under biaxial strain is presented. The results show that in all cases, the thermoelectric performance of the n-type doping on the germanene is substantially superior than that of the p-type doping. In the p-type doping, increasing the tensile strain would result in increasing the power factor, and the strain +3% creates the highest power factor. However, in the n-type doping, increasing the compressive strain increases the power factor, and the strain −6% leads to the highest power factor. Under the strain of −6%, the peak of the power factor for n-type doping equals to 32.3⨯1010 W/K2ms; while this amount equals to 12.2⨯1010 W/K2ms for the pristine case, which in fact has increased as 2.8 times. The mentioned peak will reach 57.21⨯1010 W/K2ms at the temperature 800 K, which is very significant. These results show the enormous potential of hydrogenated monolayer germanene in the thermoelectric industry.

The effects of strain and charge doping on the electronic properties of graphitic C3N5

AbstractInspired by the successful synthesis of a new graphitic C3N5 (termed as g‐C3N5), we systematically investigate its geometry and electronic properties. The layered g‐C3N5 has a nanopore diameter of 13.8 Å. It is a direct semiconductor with a band gap of 0.53 eV. The influence of strains on the electronic properties is considered. When applying uniaxial or biaxial compressive strain, the band gap can decrease until zero, resulting in a semiconductor‐to‐metal transition. The effects of charge doping on the electronic properties are also studied. With the increase of negative charge doping, the band gap becomes narrow until zero, indicating that a semiconductor‐to‐metal transition occurs as well. In addition, the electronic properties of g‐C3N5 can be tuned by both strain and charge doping. Thus, we provide a fundamental understanding of g‐C3N5, and its semiconductor‐to‐metal transition could be possibly experimentally approached by strain and charge doping, extending the electronic usage of g‐C3N5.

Strain‐tunable electronic properties and optical properties of Hf2CO2 MXene

AbstractTwo‐dimensional materials have been extensively applied because of their unusual electronic, mechanical, and optical properties. In this paper, the electronic structure and optical properties of Hf2CO2 MXene under biaxial and uniaxial strains are investigated by the Heys‐Scuseria‐Ernzerhof (HSE06) method. Monolayer Hf2CO2 can sustain stress up to 6.453 N/M for biaxial strain and 3.072 N/M for uniaxial strain. Monolayer Hf2CO2 undergoes the transition from semiconductor to metal under −12% strain whether it is under biaxial or uniaxial strain. With the increasing biaxial compressive strain, the blue shift of Hf‐d, O‐p, and C‐p orbitals in valence band maximum results in the metallization of monolayer Hf2CO2, while the red shift of Hf‐d and O‐p orbitals in conduction band minimum results in the metallization of monolayer Hf2CO2 with increasing uniaxial compressive strain. The analysis of optical properties indicates that uniaxial strain weakens the reflectivity and refractive index of monolayer Hf2CO2 in the visible‐light range. In addition, the effective mass and the charge distribution under biaxial and uniaxial strains are also explored.