Recent progress in thermoelectric MXene-based structures versus other 2D materials

Thermoelectricity is a next-generation solution for efficient waste heat management. Although various thermoelectric materials exist, there is still a lot of scope for advancement. Recently, two-dimensional (2D) materials, including MXenes, showed promise as thermoelectric materials. The progress of MXenes as magnificent thermoelectric materials is very well established in the form of a tailor-made review. MXenes outstanding thermoelectric activity comes from a unique band structure created from its atomically thin layers and the defective surface of the external layers of atoms. Furthermore, the variety of MXenes chemical composition and MXenes-based nanostructures facilitates the research path based on energy band engineering, optimization, carrier concentration and mobility. The thermoelectric efficiency of MXenes has been mapped over the landscape of other 2D and traditional thermoelectric materials. Meanwhile, MBenes, the latest family member of the flatland, exhibits an incredible diversity of structures with additional crystal symmetries. Owing to the orthorhombic crystal structure, an in-plane structural anisotropy, and hence, the in-plane dependent thermoelectric properties are plausible. As a future prospective, certain strategies that can enhance the thermoelectric performance of MBenes have been presented. In addition, few insights and challenges that have to be considered to overcome the limitations in the thermoelectric field have been debated.Comment: 22 pages, 12 figures, 1 tabl

Origin of bulk uniaxial anisotropy in zinc-blende dilute magnetic semiconductors

It is demonstrated that the nearest neighbor Mn pair on the GaAs (001) surface has a lower energy for the [-110] direction comparing to the [110] case. According to the group theory and the Luttinger's method of invariants, this specific Mn distribution results in bulk uniaxial in-plane and out-of-plane anisotropies. The sign and magnitude of the corresponding anisotropy energies determined by a perturbation method and ab initio computations are consistent with experimental results.Comment: 5 pages, 1 figur

Exact Exchange Scheme in the Parallel r-Space Implementation of the Kohn-Sham Realization of the Density Functional Theory

In this communication, we present the r-space implementation of the Kohn-Sham realization of the density functional theory with the exact exchange functional within the computational algorithm for computers of parallel architecture. In comparison to the standard approach employing the local density functional, the scheme with exact exchange functional requires roughly ten times larger computational burden. The developed parallelization procedure accelerates the computations by a factor of four and six for the exact exchange and the local density functional schemes, respectively. It brings us closer to the treatment of dispersive van der Waals interactions on the fully ab initio level in the large class of systems

Controlling magnetic exchange and anisotropy by non-magnetic ligand substitution in layered MPX3 (M = Ni, Mn; X = S, Se)

Recent discoveries in two-dimensional (2D) magnetism have intensified the investigation of van der Waals (vdW) magnetic materials and further improved our ability to tune their magnetic properties. Tunable magnetism has been widely studied in antiferromagnetic metal thiophosphates MPX3. Substitution of metal ions M has been adopted as an important technique to engineer the magnetism in MPX3. In this work, we have studied the previously unexplored chalcogen X substitutions in MPX3 (M = Mn/Ni; X = S/Se). We synthesized the single crystals of MnPS3-xSex (0 < x < 3) and NiPS3-xSex (0 < x < 1.3) and investigated the systematic evolution of the magnetism with varying x. Our study reveals the effective tuning of magnetic interactions and anisotropies in both MnPS3 and NiPS3 upon Se substitution. Such efficient engineering of the magnetism provides a suitable platform to understand the low-dimensional magnetism and develop future magnetic devices

Optical markers of magnetic phase transition in CrSBr

Here, we investigate the role of the interlayer magnetic ordering of CrSBr in the framework of $\textit{ab initio}$ calculations and by using optical spectroscopy techniques. These combined studies allow us to unambiguously determine the nature of the optical transitions. In particular, photoreflectance measurements, sensitive to the direct transitions, have been carried out for the first time. We have demonstrated that optically induced band-to-band transitions visible in optical measurement are remarkably well assigned to the band structure by the momentum matrix elements and energy differences for the magnetic ground state (A-AFM). In addition, our study reveals significant differences in electronic properties for two different interlayer magnetic phases. When the magnetic ordering of A-AFM to FM is changed, the crucial modification of the band structure reflected in the direct-to-indirect band gap transition and the significant splitting of the conduction bands along the $\Gamma-Z$ direction are obtained. In addition, Raman measurements demonstrate a splitting between the in-plane modes $B^2_{2g}$/$B^2_{3g}$, which is temperature dependent and can be assigned to different interlayer magnetic states, corroborated by the DFT+U study. Moreover, the $B^2_{2g}$ mode has not been experimentally observed before. Finally, our results point out the origin of interlayer magnetism, which can be attributed to electronic rather than structural properties. Our results reveal a new approach for tuning the optical and electronic properties of van der Waals magnets by controlling the interlayer magnetic ordering in adjacent layers.Comment: 33 pages, 15 figure

Exact Exchange Scheme in the Parallel r-Space Implementation of the Kohn-Sham Realization of the Density Functional Theory

In this communication, we present the r-space implementation of the Kohn-Sham realization of the density functional theory with the exact exchange functional within the computational algorithm for computers of parallel architecture. In comparison to the standard approach employing the local density functional, the scheme with exact exchange functional requires roughly ten times larger computational burden. The developed parallelization procedure accelerates the computations by a factor of four and six for the exact exchange and the local density functional schemes, respectively. It brings us closer to the treatment of dispersive van der Waals interactions on the fully ab initio level in the large class of systems

Van Der Waals Density Functionals for Graphene Layers and Graphite

In this communication, we present results of theoretical studies of various systems where Van der Waals interaction plays a considerable role. In the first-principle calculations performed in the density functional theory framework we implement novel functionals accounting for Van der Waals forces and employ to the test cases of graphite and graphene layers. It turns out that this approach provides a solution to the long standing problem of overbinding between graphene layers in bulk graphite, giving the distance between the carbon layers in excellent agreement with experiment. In graphene bilayers, Van der Waals functionals lead to energetic barriers for A-B to A-A ordering of graphene bilayers that are by a factor of two smaller than the barriers obtained with standard functionals. It may be of crucial importance, particularly, if one uses atomistic ab initio methods as a starting point for multi-scale modeling of materials and for determination of effective potentials

Structural and Electronic Properties of Functionalized Graphene

In the present paper, we study the effects of functionalization of graphene with simple organic molecules OH, and $NH_2$, focusing on the stability and band gaps of the structures. We have performed DFT calculations for graphene supercells with various numbers of the attached molecules. We have determined adsorption energies of the functionalized graphene mono- and bilayers, the changes in the geometry, and the band structure. We observe the characteristic effects such as rehybridization of the bonds induced by fragments attached to graphene and opening of the graphene band gap by functionalization. We have also studied the dependence of the adsorption energies of the functionalized graphene on the density of the adsorbed molecules. Our calculations reveal that the -OH and $-NH_2$ groups exhibit the strong cohesion to graphene layers. Further, we determine the critical density of the OH fragments which lead to the opening of the band gap. We also show how to engineer the magnitude of the band gap by functionalizing graphene with $NH_2$ groups of various concentrations