No Arabic abstract
Motivated by the recent successful formation of the MoSi2N4 monolayer [Hong et al., Sci. 369, 670 (2020)], the structural, electronic and magnetic properties of MoSi2N4 nanoribbons (NRs) is investigated for the first time . The band structure calculations showed spin-polarization in zigzag edges and a non-magnetic semiconducting character in armchair edges. For armchair-edges, we identify an indirect to direct band gap shift compared to the MoSi2N4 monolayer, and its energy gap increases with increasing NR width. Anisotropic electrical and magnetic behavior is observed via band structure calculations in the zigzag and armchair edges, where, surprisingly, for the one type of zigzag-edges configuration, we identify a Dirac-semimetal character. The appearance of magnetism and Dirac-semimetal in MoSi2N4 ribbon can give rise to novel physical properties, which could be useful in applications for next-generation electronic devices.
Besides its predicted promising high electron mobilities at room temperature, PtSe2 bandgap sensitively depends on the number of monolayers combined by van der Waals interaction according to our calculations. We understand this by using bandstructure calculations based on the density functional theory. It was found that the front orbitals of VBM and CBM are contributed mainly from pz and px+y orbitals of Se which are sensitive to the out-plane and in-plane lattice constants, respectively. The van der Waals force enhances the bonding out-of-plane, which in-turn influences the bonding in-plane. We found that the thickness dependent bandgap has the same origin as the strain dependent bandgap, which is from the change of the front orbital interactions. The work shows the flexibilities of tuning the electronic and optical properties of this compound in a wide range.
The electronic and transport properties of hybrid armchair zigzag nanostructures including U-shaped graphene nanoribbons and patterned nanopores structured graphene were studied using combination of density functional theory and non-equilibrium Green function method. The density of state, electron transmission spectra, and molecular orbitals were analyzed. The obtained results show that GNRs junctions tend to open an energy gap when U-shaped structures were formed due to the formation of quasi-bound states at zigzag edges. The size of U shaped structures has enormous influences on the electron transport of the system. We also considered the effect of corner form of the U-shaped GNRs junctions on energy gap opening. It was found that as some carbon atoms are add to the inner corner, the energy gap in U shaped GNRs significantly changed. For patterned nanopores structured graphene, the calculated results show that patterned nanopores enormous influence on electronic and the transport properties though the GNRs junctions, depending on the shape, size, and the number of nanopores. The study suggests that designed tailored graphene systems based on hybrid armchair zigzag nanostructures can be used to control the energy gap of graphene.
By band engineering the iron chalcogenide Fe(Se,Te) via ab-initio calculations, we search for topological surface states and realizations of Majorana bound states. Proposed topological states are expected to occur for non-stoichiometric compositions on a surface Dirac cone where issues like disorder scattering and charge transfer between relevant electronic states have to be addressed. However, this surface Dirac cone is well above the Fermi-level. Our goal is to theoretically design a substituted crystal in which the surface Dirac cone is shifted towards the Fermi-level by modifying the bulk material without disturbing the surface. Going beyond conventional density functional theory (DFT), we apply the coherent potential approximation (BEB-CPA) in a mixed basis pseudo-potential framework to scan the substitutional phase-space of co-substitutions on the Se-sites. We have identified iodine as a promising candidate for intrinsic doping. Our specific proposal is that FeSe$_{0.325}$I$_{0.175}$Te$_{0.5}$ is a very likely candidate to exhibit a Dirac cone right at the Fermi energy without inducing strong disorder scattering.
There has been tremendous interest in manipulating electron and hole-spin states in low-dimensional structures for electronic and spintronic applications. We study the edge magnetic coupling and anisotropy in zigzag stanene nanoribbons, by first-principles calculations. Taking into account considerable spin-orbit coupling and ferromagnetism at each edge, zigzag stanene nanoribbon is insulating and its band gap depends on the inter-edge magnetic coupling and the magnetization direction. Especially for nanoribbon edges with out-of-plane antiferromagnetic coupling, two non-degenerate valleys of edge states emerge and the spin degeneracy is tunable by a transverse electric field, which give full play to spin and valley degrees of freedom. More importantly, both the magnetic order and anisotropy can be selectively controlled by electron and hole doping, demonstrating a readily accessible gate-induced modulation of magnetism. These intriguing features offer a practical avenue for designing energy-efficient devices based on multiple degrees of freedom of electron and magneto-electric couplings.
Ge with a quasi-direct band gap can be realized by strain engineering, alloying with Sn, or ultrahigh n-type doping. In this work, we use all three approaches together to fabricate direct-band-gap Ge-Sn alloys. The heavily doped n-type Ge-Sn is realized with CMOS-compatible nonequilibrium material processing. P is used to form highly doped n-type Ge-Sn layers and to modify the lattice parameter of P-doped Ge-Sn alloys. The strain engineering in heavily-P-doped Ge-Sn films is confirmed by x-ray diffraction and micro Raman spectroscopy. The change of the band gap in P-doped Ge-Sn alloy as a function of P concentration is theoretically predicted by density functional theory and experimentally verified by near-infrared spectroscopic ellipsometry. According to the shift of the absorption edge, it is shown that for an electron concentration greater than 1x10^20 cm-3 the band-gap renormalization is partially compensated by the Burstein-Moss effect. These results indicate that Ge-based materials have high potential for use in near-infrared optoelectronic devices, fully compatible with CMOS technology.