A first-principles study on the multiferroicity of semi-modified X2M (X = C, Si; M = F, Cl) monolayers.

The research of two-dimensional multiferroic materials has attracted extensive attention in recent years. In this work, we systematically investigated the multiferroic properties of semi-fluorinated and semi-chlorinated graphene and silylene X2M (X = C, Si; M = F, Cl) monolayers under strain using first principles calculations based on density functional theory. We find that the X2M monolayer has a frustrated antiferromagnetic order, and a large polarization with a high reversal potential barrier. When increasing the applied biaxial tensile strain, the magnetic order remains unchanged, but the polarization flipping potential barrier of X2M gradually decreases. When the strain increases to 35%, although the energy required to flip the fluorine and chlorine atoms is still very high in the C2F and C2Cl monolayers, it goes down to 312.5 meV and 260 meV in unit cells of the Si2F and Si2Cl monolayers, respectively. At the same time, both semi-modified silylenes exhibit metallic ferroelectricity with a band gap of at least 0.275 eV in the direction perpendicular to the plane. The results of these studies show that Si2F and Si2Cl monolayers may become a new generation of information storage materials with magnetoelectric multifunctional properties.

Theoretical investigation of the magnetic and optical properties in a transition metal-doped GaTeCl monolayer.

We investigate the structural, magnetic, electronic and optical properties of a transition metal-doped GaTeCl monolayer, denoted as M@GaTeCl (M = V, Cr, Mn, Fe and Co), by using first-principles calculations. It is found that the magnetic ground state can be regulated by different M elements. In the meantime, the electronic structure is different with the doping of different M metal atoms, and thus the optical absorption changes correspondingly. The electronic calculations of M@GaTeCl suggest that V@GaTeCl, Cr@GaTeCl, Mn@GaTeCl and Fe@GaTeCl are semiconductors and the magnetic ground states are G-type antiferromagnetic (AFM), C-type AFM, A-type AFM and C-type AFM order, respectively, while Co@GaTeCl is a metal and the ground state is ferromagnetic (FM) order. The different magnetic ground states are discussed with the Heisenberg model. The rough estimation of the ferroelectric polarization value of M@GaTeCl suggests that M@GaTeCl still exhibits multiferroicity. The electronic structure is explained by the projected density of states, band structure and decomposed charge of the valence band maximum (VBM) and conduction band minimum (CBM). Simultaneously, the absorption coefficient calculations indicate that M@GaTeCl absorption shows anisotropic properties, as the same as in a pure GaTeCl monolayer, there exists enhanced visible light absorption in these M@GaTeCl monolayers relative to the pure GaTeCl one, which can be interpreted by the anisotropic structure and by the peculiar electronic structure. Thus, we found that the magnetic ground state, the electronic structure, and the absorption coefficient of M@GaTeCl can be tuned by doping different transition metal M atoms, and the ferroelectricity is still retained, which makes M@GaTeCl a potential multifunctional material in spintronics and optics.

Prediction of two-dimensional monolayer C2O2Fe with chiral magnetic and ferroelectric orders.

Low-dimensional multiferroics are highly desired for applications and contain exotic physical properties. Here we predict a two-dimensional material, C2O2Fe monolayer, through Fe intercalation in the graphene oxide monolayer. The crystal stable texture, chiral spin order, and ferroelectric polarization of the C2O2Fe monolayer are theoretically studied by considering the electron on-site Coulomb interaction and spin orbit coupling, which also manifests the ferroelectric polarization and reversal barrier at 30% biaxial tensile strain comparable with the other two-dimensional ferroelectric materials, such as GeS and GeSe. Moreover, first-principles calculations show that the polarization flipping is accompanied by spin orientation reversal, when the ferroelectric polarization is upward to the plane, a clockwise chiral antiferromagnetic ground state is obtained, while when the polarization is downward, the monolayer shows the anticlockwise chiral antiferromagnetic structure. In this sense, a strong electrically controlled magnetism exists in the designed C2O2Fe monolayer film.

A study on the multiferroic properties of semi-hydrogenated X2H (X = C, Si, and Ge) monolayer films.

In recent years, the research on the physical properties of two-dimensional (2D) materials has attracted much attention. In this paper, the magnetic and ferroelectric (FE) properties of semi-hydrogenated graphene, silylene and germanene X2H (X = C, Si, and Ge) under strain are systematically investigated. The results have shown that X2H is a magnetic FE semiconductor with ferromagnetic (FM) and FE structures, both perpendicular to the plane, a large energy gap, and a high polarization reversal barrier. It is found that both the polarization reversal barrier and the magnitude of FE polarization gradually decrease, but the FM state remains the same, upon gradually increasing the tensile strain. As the tensile strain is increased to 19%, the barriers of the Si2H and Ge2H monolayer films to flip a single valence bond are decreased to 1.123 eV and 0.768 eV, respectively, and the systems still maintain semiconductor characteristics. When the strain is increased to 20%, the films begin to show metallicity in the plane of films, but still have the polarity perpendicular to the plane because of the anisotropy of the band structure. These research results suggest that the magnetoelectric properties of Si2H and Ge2H monolayer films provide the possibility for achieving a new generation of information storage materials.

Pressure effects on the structures and electronic properties of halide perovskite CsPbX3 (X = I, Br, Cl).

Cesium lead halide perovskites as ideal photovoltaic and optoelectronics materials have attracted more and more attention. Here, we investigated the structure and electronic properties of halide perovskite CsPbX3 (X = I, Br, Cl) by particle swarm optimization and first principles methods at hydrostatic pressure. For CsPbI3, the structure phases and corresponding phase transitions of non-perovskite orthorhombic Pnma (non-Pv-Pnma), monoclinic C2/m (C2/m-I), and another different monoclinic phase C2/m (C2/m-II) are found in the pressure range 0-120 GPa. The largest piezochromic effect in the perovskites can be predicted in CsPbI3 under a pressure below 10 GPa. The band gap is reduced under pressure and the C2/m-II phase becomes a metal under pressures larger than 65 GPa. For CsPbBr3 and CsPbCl3, the same phase sequence under pressure, i.e. from perovskite Pnma (Pv-Pnma), to non-Pv-Pnma, C2/m-I, and then to another orthorhombic Cmcm phase, is found. The newly discovered non-Pv-Pnma phase gives an excellent explanation of previous measurements at low pressure for CsPbBr3 and CsPbCl3. Furthermore, the calculations of electronic properties show that pressure is an effective means to tune the band gap. Our calculations and results extend the applications in photovoltaics and optoelectronics of halide perovskites.

Magnetism and spin exchange coupling in strained monolayer CrOCl.

The magnetism and spin exchange coupling of monolayer CrOCl with different strains are investigated systematically using first principles. It is found that the magnetic ground state can be changed from ferromagnetic (FM) to antiferromagnetic (AFM), and the Curie temperature (TC) is enhanced significantly by applying the uniaxial strain along a- or b-axis direction. The variations of spin exchange coupling are explained according to the Goodenough-Kanamori-Anderson (GKA) and Bethe-Slater Interaction (BSI) rules. The strain-dependent magnetic state is mainly attributed to the competition between direct exchange interactions of cation-cation and indirect superexchange ones of cation-anion-cation in monolayer CrOCl. The different competitions in a- and b-axis direction determine the different critical intervals R of magnetic transitions, where R is the distance of the two nearest-neighbor (NN) Cr3+ ions. The AFM-FM transition occurs at R/r3d = 2.9 and 3.75 in a-axis direction, while it happens at R/r3d = 2.65 along b-axis direction. These results indicate that the sensitive relevancy between the external strain and magnetic coupling makes monolayer CrOCl a promising candidate for spintronics.

Emergent multiferroicity and strain-driven metal-semiconductor transitions in LaMnO3/RMnO3 superlattices (R = Pr, Pm, Sm and Gd).

It is known that rare-earth manganites LnMnO3 with Ln = La to Gd are typical Mott insulators favoring the A-type antiferromagnetic (A-AFM) state. Certainly no ferroelectricity can be possible although the alternatively stacked LnO layers are both polar. Nevertheless, under the inspiration that one plus one is more than two, it is appreciated that by combining two components of this manganite series into a superlattice functionality is added. In this work, we construct a (001)-oriented LaMnO3/RMnO3 (R = Pr, Pm, Sm and Gd) superlattice and investigate the possible emergent ferroelectricity by means of first-principles calculations. It is revealed that the lattice matching in these superlattices may generate lattice distortions to each component based on the scenario of hybrid improper ferroelectricity, resulting in spontaneous ferroelectric polarization, which is larger than the traditional type II Ln'MnO3 (Ln' radius is smaller than that of Gd) polarization. In the meantime, the A-AFM state remains the magnetic ground state of these superlattices. Furthermore, it is predicted that the externally imposed in-plane compressive strain can trigger the semiconductor to half-metal transitions accompanying the A-AFM to ferromagnetic (FM) transitions. The present work sheds light on the possibility to design multiferroic materials and functionality by tailoring artificial superlattices/heterostructures from those non-ferroelectric systems, and to design electronic devices by utilizing the electronic transport properties under epitaxial strain.

Magnetism and hybrid improper ferroelectricity in LaMO3/YMO3 superlattices.

Using first-principles calculations, we investigate the structural, electronic, and magnetic properties of perovskite LaMO3/YMO3 superlattices (M = Cr, Mn, Co and Ni). It is found that ferroelectricity can emerge in LaMO3/YMO3 superlattices (M = Cr, Mn, Co), allowing them to be promising multiferroic candidates, while no ferroelectricity is found in the LaNiO3/YNiO3 superlattice. The electronic structure calculations indicate that the LaCrO3/YCrO3, LaMnO3/YMnO3, and LaCoO3/YCoO3 superlattices are insulators, and their magnetic ground states exhibit G-type antiferromagnetic (AFM), A-type AFM, and G-type AFM order, respectively, while the LaNiO3/YNiO3 superlattice is however a half-metallic ferromagnet. The electronic structure and magnetic ground state are discussed, based on the projected density of states data and Heisenberg model, respectively, and the magnetic phase transition temperature is evaluated based on mean-field theory. In the meantime, the spontaneous ferroelectric polarization of the LaMO3/YMO3 superlattices (M = Cr, Mn, Co) is determined respectively using the Born effective charge model and Berry phase method, and their hybrid improper ferroelectric character is predicted, with the net polarization mainly from the different displacements of the LaO layers and YO layers along the b-axis. It is suggested that alternative multiferroic materials can be obtained by properly designing superlattices that consist of two non-polar magnetic materials but exhibit tunable magnetic ground states and transition temperature and hybrid improper ferroelectricity.

[Detection technology of methane gas concentration based on infrared absorption spectrum].

According to the disadvantages of current methane sensor in coal mine, the infrared methane concentration detection system based on the principle of infrared spectrum absorption was designed using differential absorption technology. In the system single light beam absorbing cell and single light beam and double wavelengths technology are adopted. Differential amplifier circuit serves as the core of faint signal processing circuit that detects the output signal of methane concentration, and linear formula fits the curve of methane concentration and output voltage, which realizes accurate and full range detection of gas concentration. Experiment shows that measurement error is less than 2%, and the system has very high measurement precision and possesses the basis of industrial applications.

Mechanical, Structural and Electronic Properties of CO2 Adsorbed Graphitic Carbon Nitride (g-C3N4) under Biaxial Tensile Strain.

We investigate mechanical, structural and electronic properties of CO2 adsorbed graphitic carbon nitride (g-C3N4) system under biaxial tensile strain via first-principles calculations. The results show that the stress of CO2 adsorbed g-C3N4 system increases and then decreases linearly with the increasing biaxial strain, reaching maximum at 0.12 strain. This is primarily caused by the plane N-C stretching of the g-C3N4. Furthermore, both the Perdew-Burke-Ernzerhof (PBE) and Heyd- Scuseria-Ernzerhof screened hybrid functional (HSE06) band gaps show direct-indirect transitions under biaxial tensile strain and have the maximum also at 0.12 strain. It is found that there is large dipole transition matrix element around Γ point, leading high optical absorption coefficients of the deformed adsorption system, which would be of great use for the applications of new elastic nanoelectronic and optoelectronic devices.

Equibiaxial Strained Oxygen Adsorption on Pristine Graphene, Nitrogen/Boron Doped Graphene, and Defected Graphene.

We report first-principles calculations on the structural, mechanical, and electronic properties of O2 molecule adsorption on different graphenes (including pristine graphene (G-O2), N(nitrogen)/B(boron)-doped graphene (G-N/B-O2), and defective graphene (G-D-O2)) under equibiaxial strain. Our calculation results reveal that G-D-O2 possesses the highest binding energy, indicating that it owns the highest stability. Moreover, the stabilities of the four structures are enhanced enormously by the compressive strain larger than 2%. In addition, the band gaps of G-O2 and G-D-O2 exhibit direct and indirect transitions. Our work aims to control the graphene-based structure and electronic properties via strain engineering, which will provide implications for the application of new elastic semiconductor devices.