Tunable magnetic anisotropy and phase transitions in 2D Janus transition-metal chalcogenide halides.

The strain engineering is an effective method to modulate the optical properties of germanium. The biaxial tensile strain has been extensively studied, most of the investigations focusing on biaxial tensile strain with equal in-plane strain at different crystal orientations, namely symmetric biaxial tensile strain. However, the effect of biaxial tensile strain with unequal in-plane strain at different crystal orientations, namely asymmetric biaxial tensile strain, has not been reported. In this paper, we systematically investigate the effect of asymmetric biaxial tensile strain on the band structure of Ge by using first-principle calculation.#br#We firstly calculate and analyze the dependence of band gap on strain for Ge with asymmetric biaxial tensile strain along three low Miller index planes, i.e., (001), (101) and (111). Then, we present the values of band gap and strain for some typical indirect-to-direct bandgap-transition-points under asymmetric biaxial tensile strain. Finally, we analyze the influence of biaxial tensile strain on the valance band structure. For the asymmetric biaxial tensile strain along the (001) plane, the indirect-to-direct band gap transition only occurs when the strain of one orientation is larger than 2.95%. For asymmetric biaxial tensile strain along the (101) plane, the indirect-to-direct band gap transition only occurs when the strain of one orientation is larger than 3.44%. Asymmetric biaxial tensile strain along the (111) plane cannot transform Ge into direct band gap material.#br#For asymmetric biaxial tensile strains along the (001) and (101) plane, the indirect-to-direct band gap transition points can be adjusted by changing the combination of in-plane strain at different crystal orientations. The value of bandgap of direct-band-gap Ge under biaxial tensile strain is inversely proportional to the area variation induced by application of strain. The asymmetric biaxial tensile strain along the (001) plane is the most effective to transform Ge into direct band gap material among the three types of biaxial strains, which are similar to the symmetric biaxial tensile strains.#br#In addition, the symmetric biaxial tensile strain will remove the three-fold degenerate states of valance band maximum, leading to a removal of the degeneracy between one heavy hole band and the light hole band. For biaxial tensile strain along the (001) and (101) plane, the asymmetric biaxial tensile strain could further remove the degeneracy between another heavy hole band and the light hole band.

Using first-principles calculations, we investigate the stability and electronic properties of five isomers of two-dimensional (2D) GeSe monolayer under in-plane strain. Our calculated results show that the five isomers of GeSe monolayer are all stable. It is found that the α-GeSe has a direct band gap, while each of the β-GeSe, γ-GeSe, δ-GeSe and ε-GeSe possesses an indirect band gap. By applying compressive or tensile uniaxial and biaxial strain to the five GeSe isomers, the indirect-to-direct transition in band gap is found. In the α-GeSe, the changes from indirect-to-direct and semiconducting-to-metallic are both found under an applied strain. In the 2D β-GeSe and γ-GeSe, an adjustable range of indirect band gap under strain is found. Moreover, a direct band gap in the δ-GeSe is found separately under the biaxial compression strain of <i>σ</i><i><sub>xy</sub></i> = –2% and <i>σ</i><i><sub>xy</sub></i> = –4%. By applying a tensile strain of 10% along the armchair direction in ε-GeSe, a transition from an indirect to direct band gap occurs. When the tensile strain is continuously increased to 20%, the band structure of ε-GeSe maintains direct character. This direct band gap can be tuned from 1.21 eV to 1.44 eV. When 10% tensile strain is applied along the biaxial direction, the transition in band gap from indirect-to-direct also occurs. Our results indicate that the direct band gap can be tuned from 0.61 eV to 1.19 eV when the tensile strain is increased from 10% to 19% in ε-GeSe.

Two-dimensional (2D) magnetic semiconductors have great promising for energy-efficient ultracompact spintronics due to the low-dimensional ferromagnetic and semiconducting behavior. Here, we predict hexagonal titanium nitride monolayer (h-TiN) to be a ferromagnetic semiconductor by investigating stability, magnetism, and carrier transport of h-TiN using the first-principles calculations. The thermodynamical stability of h-TiN is revealed by phonon dispersion, molecular dynamics simulation and formation energy. The energy band structure shows that h-TiN is a ferromagnetic semiconductor with medium magnetic anisotropy, the magnetic moment of 1μB and the band gaps of 1.33 and 4.42 eV for spin-up and -down channels, respectively. The Curie temperature of h-TiN is estimated to be about 205 K by mean-field theory and not enhanced by the compressive and tensile strains. Higher carrier mobility, in-plane stiffness and conductivity indicate that h-TiN has favorable transport performance. The ferromagnetic semiconducting behavior is robust against the external strains, indicating that h-TiN could be a rare candidate for nanoscale spintronic devices.

In recent years, 2D ferromagnetic semiconductors have attracted much attention because of its potential application in spintronic devices. Using first‐principles calculations, the magnetic and optical properties of intrinsic and chalcogen‐doped VCl3 monolayers are investigated. In contrast to previous work, VCl3 monolayer is proved to be an antiferromagnetic semiconductor rather than a Dirac half‐metal after considering the electronic correlation effect. At a low S concentration x between and , S‐doped VCl3 monolayer forms a ferromagnetic semiconductor with a large bandgap and a strong exchange splitting in both valence and conduction bands. As the doping content x increases above , S‐doped VCl3 monolayer will change to be an anti‐ferromagnetic semiconductor and a non‐magnetic metal successively. Moreover, Se‐ and Te‐doped VCl3 monolayers can also form robust ferromagnetic semiconductors at low doping concentration. In particular, the Curie temperature of Se‐doped VCl3 monolayer can reach 170 K, higher than that of S‐ and Te‐doped VCl3 monolayers. At last, chalcogen‐doped VCl3 monolayers have enhanced optical absorption in the visible regions compared to intrinsic VCl3 monolayer. The results show that chalcogen‐doped VCl3 monolayers have promising potential applications in future spintronic and optoelectronic devices.

While ${\mathrm{SnS}}_{2}$ is an earth-abundant large-band-gap semiconductor material, the indirect nature of the band gap limits its applications in light harvesting or detection devices. Here, using density functional theory in combination with the many-body perturbation theory, we report indirect-to-direct band-gap transition in bulk ${\mathrm{SnS}}_{2}$ under moderate, $2.98%$ uniform biaxial tensile (BT) strain. Further enhancement of the BT strain up to $9.75%$ leads to a semiconductor-to-metal transition. The strain-induced weakening of the interaction of the in-plane orbitals modifies the dispersion as well as the character of the valence- and the conduction-band edges, leading to the transition. A quasiparticle direct band gap of 2.17 eV at the $\mathrm{\ensuremath{\Gamma}}$ point is obtained at $2.98%$ BT strain. By solving the Bethe-Salpeter equation to include excitonic effects on top of the partially self-consistent ${\mathrm{GW}}_{0}$ calculation, we study the dielectric functions, optical oscillator strength, and exciton binding energy as a function of the applied strain. At $2.98%$ BT strain, our calculations show the relatively high exciton binding energy of 170 meV, implying strongly coupled excitons in ${\mathrm{SnS}}_{2}$. The effect of strain on vibrational properties, including Raman spectra, is also investigated. The Raman shift of both in-plane (${E}_{2g}^{1}$) and out-of plane (${A}_{1g}$) modes decreases with the applied BT strain, which can be probed experimentally. Furthermore, ${\mathrm{SnS}}_{2}$ remains dynamically stable up to $9.75%$ BT strain, at which it becomes metallic. A strong coupling between the applied strain and the electronic and optical properties of ${\mathrm{SnS}}_{2}$ can significantly broaden the applications of this material in strain-detection and optoelectronic devices.

Modulation of the electronic structure and magnetism performance of V-doped monolayer MoS2 by strain engineering

Prediction of indirect to direct band gap transition under tensile biaxial strain in type-I guest-free silicon clathrate Si46: A first-principles approach

Half-metal to magnetic semiconductor transition in Mn-doped monolayer Bi2O2Se tuned by strain

Prediction of electronic structure and magnetic anisotropy of two-dimensional MSi2N4 (M = 3d transition-metal) monolayers

The intrinsic ferromagnetism of two-dimensional transition metal carbide Co2C is remarkable. However, its practical application in spintronic devices is encumbered by a low Curie temperature (TC). To surmount this constraint, double transition-metal carbide CoMC (M = Ti, V, Cr, Mn, Fe, Ni) monolayers are constructed with the aim of improving the magnetic properties and Curie temperature of Co2C. The magnetic properties of CoMC monolayers are comprehensively investigated by first-principles calculations and the effects of hole doping and biaxial strain on the magnetic properties of CoMC (M = V, Cr, Mn) monolayers are also studied. The ground states of CoTiC, CoMnC and CoNiC monolayers all favor ferromagnetic ordering, whereas the CoVC and CoCrC monolayers favor antiferromagnetic ordering and the CoFeC monolayer is non-magnetic. Excitedly, the CoMnC monolayer displays a high total magnetic moment of 4.024μB and a TC of 1366 K. Moreover, the control of hole doping can effectively improve the TC of CoVC, CoCrC, and CoMnC monolayers to 680, 1317, 3044 K, respectively. Finally, applying the in-plain biaxial strain, the CoVC monolayer can be transformed into a ferromagnetic semiconductor under a tensile strain of 6%. The TC values of CoVC, CoCrC, and CoMnC monolayers are tuned by biaxial strain to 440, 1334 and 2390 K, respectively. Their TC above room temperature demonstrates that these monolayers have potential applications in spintronic devices. These theoretical investigations provide valuable insights into guiding experimental synthesis endeavors.

Density functional theory study of tunable electronic and magnetic properties of monolayer BeO with intrinsic vacancy and transition metal substitutional doping

Magnetism in 2D has long been the focus of condensed matter physics due to its important applications in spintronic devices. A particularly promising aspect of 2D magnetism is the ability to fabricate 2D heterostructures with engineered optical, electrical, and quantum properties. Recently, the discovery of intrinsic ferromagnetisms in atomic thick materials has provided a new platform for investigations of fundamental magnetic physics. In contrast to 2D CrI3 and Cr2 Ge2 Te6 insulators, itinerant ferromagnetic Fe3 GeTe2 (FGT), which has a larger intrinsic perpendicular anisotropy, higher Curie temperature (TC ), and relatively better stability, is a promising candidate for achieving permanent room-temperature ferromagnetism through interface or component engineering. Here, it is shown that the ferromagnetic properties of FGT thin flakes can be modulated through coupling with a FePS3 . The magneto-optical Kerr effect results show that the TC of FGT is improved by more than 30 K and that the coercive field is increased by ≈100% due to the proximity coupling effect, which changes the spin textures of FGT at the interface. This work reveals that antiferromagnet/ferromagnet coupling is a promising way to engineer the magnetic properties of itinerant 2D ferromagnets, which paves the way for applications in advanced magnetic spintronic and memory devices.

Strain tailored electronic structure and magnetic properties of Fe-doped Zr8C4T8 (T = F, O) monolayers

The two-dimensional (2D) Janus monolayers are promising in spintronic device application due to their enhanced magnetic couplings and Curie temperatures. Van der Waals CrCl3 monolayer has been experimentally proved to have an in-plane magnetic easy axis and a low Curie temperature of 17 K, which will limit its application in spintronic devices. In this work, we propose a new Janus monolayer Cr2Cl3S3 based on the first principles calculations. The phonon dispersion and elastic constants confirm that Janus monolayer Cr2Cl3S3 is dynamically and mechanically stable. Our Monte Carlo simulation results based on magnetic exchange constants reveal that Janus monolayer Cr2Cl3S3 is an intrinsic ferromagnetic semiconductor with T C of 180 K, which is much higher than that of CrCl3 due to the enhanced ferromagnetic coupling caused by S substitution. Moreover, the magnetic easy axis of Janus Cr2Cl3S3 can be tuned to the perpendicular direction with a large magnetic anisotropy energy (MAE) of 142 μeV/Cr. Furthermore, the effect of biaxial strain on the magnetic property of Janus monolayer Cr2Cl3S3 is evaluated. It is found that the Curie temperature is more robust under tensile strain. This work indicates that the Janus monolayer Cr2Cl3S3 presents increased Curie temperature and out-of-plane magnetic easy axis, suggesting greater application potential in 2D spintronic devices.

Developing novel controllable two-dimensional semiconductor materials is crucial to thin film spintronic devices, which may lead to a revolution of information devices. Recently, the easily cleavable CrTe3 has attracted much attention for studying the magnetic properties of two-dimensional materials. In this paper, we have demonstrated theoretically that an elastic tensile strain can turn the antiferromagnetic coupled single-layer CrTe3 (SL-CrTe3) into a ferromagnetic (FM) system, favoring its potential application in thin film spintronic devices. The FM SL-CrTe3 undergoes a further transition from a semiconductor to a metal under a biaxial tensile strain of 9%. The kinetic stability of SL-CrTe3 under 10% tensile strain is verified by a molecular dynamics simulation at room temperature. We suppose that the strain-dependent magnetic behaviors of SL-CrTe3 resulted from the competition between superexchange and direct interactions. The tunable magnetic and electronic properties of SL-CrTe3 imply immense potential in spintronic device applications.