Mn-X (X = F, Cl, Br, I) Co-Doped GeSe Monolayers: Stabilities and Electronic, Spintronic and Optical Properties

GeSe monolayer (ML) has recently attracted much interest due to its unique structure and excellent physical properties that can be effectively tuned through single doping of various elements. However, the co-doping effects on GeSe ML are rarely studied. In this study, the structures and physical properties of Mn-X (X = F, Cl, Br, I) co-doped GeSe MLs are investigated by using first-principle calculations. The formation energy and phonon disspersion analyses reveal the stability of Mn-Cl and Mn-Br co-doped GeSe MLs and instability of Mn-F and Mn-I co-doped GeSe MLs. The stable Mn-X (X = Cl, Br) co-doped GeSe MLs exhibit complex bonding structures with respect to Mn-doped GeSe ML. More importantly, Mn-Cl and Mn-Br co-doping can not only tune magnetic properties, but also change the electronic properties of GeSe MLs, which makes Mn-X co-doped GeSe MLs indirect band semiconductors with anisotropic large carrier mobility and asymmetric spin-dependent band structures. Furthermore, Mn-X (X = Cl, Br) co-doped GeSe MLs show weakened in-plane optical absorption and reflection in the visible band. Our results may be useful for electronic, spintronic and optical applications based on Mn-X co-doped GeSe MLs.

A density functional theory study on the strain modulated electronic and photocatalytic properties of a GaSe monolayer for photocatalytic water splitting and artificial photosynthesis

The strain modulated electronic and photocatalytic properties of GaSe monolayer for photocatalytic water splitting and artificial photosynthesis using DFT study.

A pathway toward high-throughput quantum Monte Carlo simulations for alloys: A case study of two-dimensional (2D) GaSxSe1-x

The study of alloys using computational methods has been a difficult task due to the usually unknown stoichiometry and local atomic ordering of the different structures experimentally. In order to combat this, first-principles methods have been coupled with statistical methods such as the cluster expansion formalism in order to construct the energy hull diagram, which helps to determine if an alloyed structure can exist in nature. Traditionally, density functional theory (DFT) has been used in such workflows. In this paper, we propose to use chemically accurate many-body variational Monte Carlo (VMC) and diffusion Monte Carlo (DMC) methods to construct the energy hull diagram of an alloy system due to the fact that such methods have a weaker dependence on the starting wavefunction and density functional, scale similarly to DFT with the number of electrons, and have had demonstrated success for a variety of materials. To carry out these simulations in a high-throughput manner, we propose a method called Jastrow sharing, which involves recycling the optimized Jastrow parameters between alloys with different stoichiometries. We show that this eliminates the need for extra VMC Jastrow optimization calculations and results in significant computational cost savings (on average 1/4 savings of total computational time). Since it is a novel post-transition metal chalcogenide alloy series that has been synthesized in its few-layer form, we used monolayer GaS_xSe_1-x as a case study for our workflow. By extensively testing our Jastrow sharing procedure for monolayer GaS_xSe_1-x and quantifying the cost savings, we demonstrate how a pathway toward chemically accurate high-throughput simulations of alloys can be achieved using many-body VMC and DMC methods.

2D auxetic material with intrinsic ferromagnetism: a copper halide (CuCl2) monolayer

The discovery of ferromagnetism in monolayer transition metal halides exemplified by CrI₃ has opened a new avenue in the field of two-dimensional (2D) magnetic materials, and more such 2D materials are waiting to be explored. Herein, using an unbiased structure search combined with first-principles calculations, we have identified a novel CuCl₂ monolayer, which exhibits not only intrinsic ferromagnetism but also auxetic mechanical properties originating from the interplay of lattice and Cu-Cl tetrahedron symmetries. The predicted Curie temperature of CuCl₂ reaches ∼47 K, and its ferromagnetism is associated with the strong hybridization between the Cu 3d and Cl 3p states in the configuration. Moreover, upon biaxial tensile strain or carrier doping, the CuCl₂ monolayer can be converted from ferromagnetic to non-magnetic and from half-metal to metal. These properties endow this CuCl₂ monolayer with great potential for applications in auxetic/spintronic nanodevices.

Stability of Stone-Wales defect in two-dimensional honeycomb crystals

We have studied the valence effects on the stability of Stone-Wales (SW) defect in some typical two-dimensional honeycomb crystals containing group-IV, V, and VI elements employing density functional theory. The energetics involved in an in-plane formation process of SW defects in pristine and substitutionally doped materials is simulated. The SW defects are stable and have a rotation angle about 90 degree in the group-IV materials. They may become less stable with a smaller rotation angle in the group-V materials and seem difficult to exist in the group-VI materials. Group-VI doping may help eliminate SW defects while group-IV and V doping might introduce SW defects in some group-VI compounds.

Recent Advances in Two-Dimensional Spintronics

Spintronics is the most promising technology to develop alternative multi-functional, high-speed, low-energy electronic devices. Due to their unusual physical characteristics, emerging two-dimensional (2D) materials provide a new platform for exploring novel spintronic devices. Recently, 2D spintronics has made great progress in both theoretical and experimental researches. Here, the progress of 2D spintronics has been reviewed. In the last, the current challenges and future opportunities have been pointed out in this field.

Few-layer GaSe nanosheet-based broadband saturable absorber for passively Q-switched solid-state bulk lasers

In this paper, few-layer two-dimensional (2D) GaSe nanosheets were fabricated and utilized as broadband saturable absorbers (SAs) for passively Q-switched (PQS) solid-state bulk lasers operating at 1.06 and 1.99 µm. For 1.06 µm laser operation, the maximum average output power, the shortest pulse width, and the largest single pulse energy were determined to be 438 mW, 285 ns, and 2.31 µJ, respectively, while for 1.99 µm PQS laser operation, they were 937 mW, 383 ns, and 9.56 µJ. Our results identified the great potential applications of few-layer 2D GaSe nanosheets for practical optical modulators such as SAs for pulsed laser generation.

Bandgap engineering of few-layered MoS2 with low concentrations of S vacancies

Band-gap engineering of molybdenum disulfide (MoS₂) by introducing vacancies is of particular interest owing to the potential optoelectronic applications. In this work, systematic density functional theory (DFT) calculations were carried out for few-layered 3R-MoS₂ with different concentrations of S vacancies. All results revealed that the defect energy levels introduced on both sides of the Fermi level formed an intermediate band in the band gap. Both the edges of the intrinsic and intermediate bands of the structures with the same type of vacancies were generally closer to the Fermi level, and the gaps decreased as the number of layers increased from 2 to 4. The preferentially formed S vacancies at the top layer and the transition of defect types from point to line led to similar indirect band gaps for 2- and 4-layered 3R-MoS₂ with a low bulk concentration (around 5%) of S vacancies. This is different from most reported results about transition metal dichalcogenide (TMD) materials that the indirect band gap decreases as the number of layers increases and the low concentrations of vacancies show negligible influence on the band gap value.

Band-gap engineering of molybdenum disulfide (MoS₂) by introducing vacancies is of particular interest owing to the potential optoelectronic applications.

Comparative study of structural and electronic properties of GaSe and InSe polytypes

Equilibrium crystal structures, electron band dispersions, and bandgap values of layered GaSe and InSe semiconductors, each being represented by four polytypes, are studied via first-principles calculations within the density functional theory. A number of practical algorithms to take into account dispersion interactions are tested, from empirical Grimme corrections to many-body dispersion schemes. Due to the utmost technical accuracy achieved in the calculations, nearly degenerate energy-volume curves of different polytypes are resolved, and the conclusions concerning the relative stability of competing polytypes drawn. The predictions are done as for how the equilibrium between different polytypes will be shifted under the effect of hydrostatic pressure. The band structures are inspected under the angle of identifying features specific for different polytypes and with respect to modifications of the band dispersions brought about by the use of modified Becke-Johnson (mBJ) scheme for the exchange-correlation potential. As another way to improve the predictions of bandgaps values, hybrid functional calculations according to the HSE06 scheme are performed for the band structures, and the relation with the mBJ results are discussed. Both methods nicely agree with the experimental results and with state-of-the-art GW calculations. Some discrepancies are identified in cases of close competition between the direct and indirect gap (e.g., in GaSe); moreover, the accurate placement of bands revealing relatively localized states is slightly different according to mBJ and HSE06 schemes.

Enhanced electronic and magnetic properties by functionalization of monolayer GaS via substitutional doping and adsorption

The structural, electronic, and magnetic properties of two-dimensional (2D) GaS are investigated using density functional theory (DFT). After confirming that the pristine 2D GaS is a non-magnetic, indirect band gap semiconductor, we consider N and F as substitutional dopants or adsorbed atoms. Except for N substituting for Ga (N_Ga), all considered cases are found to possess a magnetic moment. Fluorine, both in its atomic and molecular form, undergoes a highly exothermic reaction with GaS. Its site preference (F_S or F_Ga) as substitutional dopant depends on Ga-rich or S-rich conditions. Both for F_Ga and F adsorption at the Ga site, a strong F-Ga bond is formed, resulting in broken bonds within the GaS monolayer. As a result, F_Ga induces p-type conductivity in GaS, whereas F_S induces a dispersive, partly occupied impurity band about 0.5 e below the conduction band edge of GaS. Substitutional doping with N at both the S and the Ga site is exothermic when using N atoms, whereas only the more favourable site under the prevailing conditions can be accessed by the less reactive N₂ molecules. While N_Ga induces a deep level occupied by one electron at 0.5 eV above the valence band, non-magnetic N_S impurities in sufficiently high concentrations modify the band structure such that a direct transition between N-induced states becomes possible. This effect can be exploited to render monolayer GaS a direct-band gap semiconductor for optoelectronic applications. Moreover, functionalization by N or F adsorption on GaS leads to in-gap states with characteristic transition energies that can be used to tune light absorption and emission. These results suggest that GaS is a good candidate for design and construction of 2D optoelectronic and spintronics devices.

Experimental and theoretical investigations on the defect and optical properties of S- and Al-doped GaSe crystals

A combination of experimental and computational methods was performed to investigate the defect and optical properties of S-doped and Al-doped GaSe crystals.

The effect of vacancies and the substitution of p-block atoms on single-layer buckled germanium selenide

This paper investigates the effect of point defects of both hole (Ge, Se) and substitution doping of p-block elements, in single-layer b-GeSe, based on first principles plane wave calculations within spin-polarized density functional theory.

Magnetic modification of GaSe monolayer by absorption of single Fe atom

Fe adsorbed GaSe monolayers are studied systematically using density functional theory. A strong orbit coupling effect between Fe and the vicinal Ga and Se atoms results in a half-metallicity with a 100% spin polarization.

Spin-orbital effects in metal-dichalcogenide semiconducting monolayers

Metal-dioxide & metal-dichalcogenide monolayers are studied by means of Density Functional Theory. For an accurate reproduction of the electronic structure of transition metal systems, the spin orbit interaction is considered by using fully relativistic pseudopotentials (FRUP). The electronic and spin properties of MX₂ (M = Sc, Cr, Mn, Ni, Mo & W and X = O, S, Se & Te) were obtained with FRUP, compared with the scalar relativistic pseudopotentials (SRUP) and with the available experimental results. Among the differences between FRUP and SRUP calculations are giant splittings of the valence band, substantial band gap reductions and semiconductor to metal or non-magnetic to magnetic “transitions”. MoO₂, MoS₂, MoSe₂, MoTe₂, WO₂, WS₂ and WSe₂ are proposed as candidates for spintronics, while CrTe₂, with μ ~ 1.59 μ_B, is a magnetic metal to be experimentally explored.

Engineering the electronic and magnetic properties of d(0) 2D dichalcogenide materials through vacancy doping and lattice strains

We have systematically investigated the effects of different vacancy defects in 2D d(0) materials SnS2 and ZrS2 using first principles calculations. The theoretical results show that the single cation vacancy and the vacancy complex like V-SnS6 can induce large magnetic moments (3-4 μB) in these single layer materials. Other defects, such as V-SnS3, V-S, V-ZrS3 and V-ZrS6, can result in n-type conductivity. In addition, the ab initio studies also reveal that the magnetic and conductive properties from the cation vacancy and the defect complex V-SnS6 can be modified using the compressive/tensile strain of the in-plane lattices. Specifically, the V-Zr doped ZrS2 monolayer can be tuned from a ferromagnetic semiconductor to a metallic/half-metallic material with decreasing/increasing magnetic moments depending on the external compressive/tensile strains. On the other hand, the semiconducting and magnetic properties of V-Sn doped SnS2 is preserved under different lattice compression and tension. For the defect complex like V-SnS6, only the lattice compression can tune the magnetic moments in SnS2. As a result, by manipulating the fabrication parameters, the magnetic and conductive properties of SnS2 and ZrS2 can be tuned without the need for chemical doping.