Electronic and magnetic properties of silicene nanoflakes by first-principles calculations

The geometric, electronic, and magnetic properties of silicene nanoflakes (SiNFs) and corresponding two-dimensional (2D) framework assembled by SiNFs are studied by first-principles calculations. We find that the hexagonal SiNFs exhibit semiconducting behavior, while the triangular SiNFs is magnetic. Although the triangular SiNFs linked directly is antiferromagnetic, the system linked with an odd-number Si chains can exhibit ferromagnetic (FM) behavior, which is ascribed to anti-parallel spin rule on Si atoms, consistent with the Lieb–Mattis criterion. More interestingly, the 2D framework composed of triangular SiNFs linked by a Si atom shows a half-metallic character with an integer magnetic moment. These results provide a better understanding for silicene-based nanoflakes, and expect to pave an avenue to assemble FM silicon materials in spintronics.     

A review on silicene — New candidate for electronics

Silicene-the silicon-based counterpart of graphene-has a two dimensional structure that is responsible for the variety of potentially useful chemical and physical properties. The existence of silicene has been achieved recently owing to experiments involving epitaxial growth of silicon as stripes on Ag(001), ribbons on Ag(110), and sheets on Ag(111). The nano-ribbons observed on Ag(110) were found-by both high definition experimental scanning tunneling microscopy images and density functional theory calculations-to consist of an arched honeycomb structure. Angle resolved photo-emission experiments on these silicene nano-ribbons on Ag(110), along the direction of the ribbons, showed a band structure which is analogous to the Dirac cones of graphene. Unlike silicon surfaces, which are highly reactive to oxygen, the silicene nano-ribbons were found to be resistant to oxygen reactivity.On the theoretical side, recent extensive efforts have been deployed to understand the properties of standalone silicene sheets and nano-ribbons using both tight-binding and density functional theory calculations. Unlike graphene it is demonstrated that silicene sheets are stable only if a small buckling (0.44 Å) is present. The electronic properties of silicene nano-ribbons and silicene sheets were found to resemble those of graphene.Although this is a fairly new avenue, the already obtained outcome from these important first steps in understanding silicene showed promising features that could give a new future to silicon in the electronics industry, thus opening a promising route toward wide-range applications. In this review, we plan to introduce silicene by presenting the available experimental and theoretical studies performed to date, and suggest future directions to be explored to make the synthesis of silicene a viable one.     

Field-modulated low-energy electronic and optical properties of armchair silicene nanoribbons

The tight-binding model including spin–orbit coupling is used to study electronic and optical properties of armchair silicene nanoribbons (ASiNRs) in electric fields. Perpendicular electric field monotonically increases band-gap, the DOS, and absorption frequency and strength. It does not change spin-degeneracy, edge-states, and optical selection rule. However, parallel electric field strongly modulates energy dispersions resulting in oscillatory band-gaps, shift in edge-states, and destruction of spin-degeneracy. It induces more transition channels and constructs new selection rules that exhibits richer optical spectra. Modulations of electronic and optical properties of ASiNRs have strong dependence on the direction of electric field and nanoribbon's geometry.     

First-principles studies of the hydrogenation effects in silicene sheets

Using density functional theory (DFT) with both the generalized gradient approximation (GGA) and hybrid functionals, we have investigated the structural, electronic and magnetic properties of a two-dimensional hydrogenated silicon-based material. The compounds, i.e. silicene, full- and half-hydrogenated silicene, are studied and their properties are compared. Our results suggest that silicene is a gapless semimetal. The coverage and arrangement of the absorbed hydrogen atoms on silicene influence significantly the characteristics of the resulting band structures, such as the direct/indirect band gaps or metallic/semiconducting features. Moreover, it is interesting to see that half-hydrogenated silicene with chair-like structure is shown to be a ferromagnetic semiconductor.     

Effects of the edge shape and the width on the structural and electronic properties of silicene nanoribbons

Under the generalized gradient approximation (GGA), the structural and electronic properties are studied for H-terminated silicene nanoribbons (SiNRs) with either zigzag edge (ZSiNRs) or armchair edge (ASiNRs) by using the first-principles projector-augmented wave potential within the density function theory (DFT) framework. The results show that the length of the Si-H bond is always 1.50 Å, but the edge Si-Si bonds are shorter than the inner ones with identical orientation, implying a contraction relaxation of edge Si atoms. An edge state appears at the Fermi level E_F in broader ZSiNRs, but does not appear in all ASiNRs due to their dimer Si-Si bond at edge. With increasing width of ASiNRs, the direct band gaps exhibit not only an oscillation behavior, but also a periodic feature of Δ_3n > Δ_3n+1 > Δ_3n+2 for a certain integer n. The charge density contours analysis shows that the Si-H bond is an ionic bond due to a relative larger electronegativity of H atom. However, all kinds of the Si-Si bonds display a typical covalent bonding feature, although their strength depends on not only the bond orientation but also the bond position. That is, the larger deviation of the Si-Si bond orientation from the nanoribbon axis as well as the closer of the Si-Si bond to the nanoribbon edge, the stronger strength of the Si-Si bond. Besides the contraction of the nanoribbon is mainly in its width direction especially near edge, the addition contribution from the terminated H atoms may be the other reason.     

Electronic and magnetic properties of Pd-Ni multilayers: Study using density functional theory

Electronic and magnetic properties of Pd-Ni multilayers have been studied using VASP method in the framework of the density functional theory (DFT). The calculations performed for different configurations (Pd_n/Ni_m(1 1 1), where n Pd layers are piled up over m Ni layers with n=0 to 4 and n+m=4), reveal that the important magnetic moment of Ni is significantly enhanced according as n increases due to hybridization effects between Pd and Ni mostly localized at the interface. The results also indicate that the Pd atoms are strongly polarized in the studied systems when compared with the pure metal.     

Electronic structure and elastic properties of scandium carbide and yttrium carbide: A first principles study

We have studied the electronic, structural, and elastic properties of scandium carbide and yttrium carbide by means of accurate first principles total energy calculations using the full-potential linearized plane wave method (FP-LAPW). We have used the generalized gradient approximation (GGA) for the exchange and correlation potential. Volume optimization, energy band structure, and density of states (DOS) of the systems are presented. The second order elastic constants have been calculated and other related quantities such as the Zener anisotropy factor, Poisson's ratio, Young's modulus, Kleinman parameter, Debye temperature, and sound velocities have been determined. The band gap calculation shows that YC is relatively more ionic than ScC.     

Tuning the electronic and magnetic properties of germanene by surface adsorption of small nitrogen-based molecules

The structural, energetic and electronic properties of germanene adsorbed with small nitrogen-based molecules, including N₂, NH₃, NO₂ and NO, have been investigated by using first-principles calculations. The results show that all nitrogen-based molecules considered bind much stronger to germanene than to graphene due to the hybridized sp²-sp³ bonding of Ge atoms. The N₂, NO and NO₂ molecules all act as an acceptor, while the NH₃ molecule donates electrons to germanene. We also found sizable band gaps (2–158 meV) are opened at the Dirac point of germanene through N₂, NH₃, and NO₂ adsorptions, but with only slightly destroying its Dirac cone shape. The NO₂ molecule also shows a heavy p-type doping character and makes germanene to be metallic. Moreover, when adsorbed by NO molecule, the germanene can change to be a ferromagnetic half-metal with 100% spin-polarization at the Fermi level. Overall, the different adsorption behaviors of small nitrogen-based gas molecules on germanene provide a feasible way to exploit chemically modified germanene for a wide range of practical applications, such as field-effect transistors, gas sensors and spintronic devices.     

Electronic properties of Li-doped zigzag graphene nanoribbons

Zigzag graphene nanoribbons (ZGNRs) are known to exhibit metallic behavior. Depending on structural properties such as edge status, doping and width of nanoribbons, the electronic properties of these structures may vary. In this study, changes in electronic properties of crystal by doping Lithium (Li) atom to ZGNR structure are analyzed. In spin polarized calculations are made using Density Functional Theory (DFT) with generalized gradient approximation (GGA) as exchange correlation. As a result of calculations, it has been determined that Li atom affects electronic properties of ZGNR structure significantly. It is observed that ZGNR structure exhibiting metallic behavior in pure state shows half-metal and semiconductor behavior with Li atom.     

Electric and optical properties modulations of armchair silicene nanoribbons by transverse electric fields

Under the influence of the external transverse electric fields, the effective mass and optical properties of armchair-edge silicene nanoribbons (ASiNRs) are investigated using the first-principles based on density functional theory (DFT). The results show that, comparing without the external transverse electric fields, the band gaps decrease monotonously, and the effective masses of the electrons and holes change non-monotonously with the absolute value of the electric fields, respectively. The total density of states (DOS) shows that, under the external electric fields, 9-ASiNR exhibits p-type semiconductor characters. Because of the obvious difference of the imaginary parts between the//x and//y directions, 9-ASiNR shows an optical anisotropy. In//x direction, the peaks of the dielectric function have evident red shift which are all associated with the electrons transition between Si 3p orbit and Si 3p, 3s orbits.     

Electronic structure and magnetism in 4d-transition metal clusters

We analyze the stability of magnetic states obtained within the tight-binding model for cubooctahedral (O_h) and icosahedral (I_h) clusters of early 4d (Y, Zr, Nb, Mo, and Tc) transition metals. Several metastable magnetic clusters are identified which suggests the existence of multiple magnetic solutions in realistic systems. A bulk-like parabolic behavior is observed for the binding energy of O_h and I_h clusters as a function of the atomic number along the 4 d-series. The charge transfer on the central atom changes sign, while the average magnetic moments present an oscillatory behavior as a function of the number of d electrons in the cluster. Our results are in agreement with other theoretical calculations. Received: 20 November 1997 / Received in final form: 9 March 1998 / Accepted: 30 March 1998     

Electronic structure and magnetic properties of YbCuAl

The electronic band structure of YbCuAl has been calculated using the self-consistent full potential nonorthogonal local orbital minimum basis scheme based on the density functional theory. We investigated the electronic structure with the spin–orbit interaction and on-site Coulomb potential for the Yb-derived 4f orbitals to obtain the correct ground state of YbCuAl. The exchange interaction between local f electrons and conduction electrons play an important role in the heavy fermion characters of them. The fully relativistic band structure scheme shows that spin–orbit coupling splits the 4f states into two manifolds, the 4f_7/2 and the 4f_5/2 multiplet.     

Electronic structure and optical properties of Sb2S3 crystal

The electronic and optical properties of Sb₂S₃ are studied using the full potential linearized augmented plane wave (FP-LAPW) method as implemented in Wien2k. In this approach, the alternative form of the generalized gradient approximation (GGA) proposed by Engel and Vosko (EV-GGA) was used for the exchange correlation potential. The calculated band structure shows a direct band gap. The contribution of different bands was analyzed from total and partial density of states curves. Moreover, the optical properties, including the dielectric function, absorption spectrum, refractive index, extinction coefficient, reflectivity and energy-loss spectrum are all obtained and analyzed in detail.     

Strain and chirality effects on the mechanical and electronic properties of silicene and silicane under uniaxial tension

We present first principles theory calculations on the mechanical and electronic properties of silicene and silicane structure under uniaxial tensile strain along different directions. Chirality effect is more significant in the mechanical properties of silicene than those of silicane. Different failure mechanisms are identified. A small band gap (up to 0.8 eV) is developed from zero with silicene structure under uniaxial tension and vanishes before the structure reaches its in-plane ultimate strength. However, a pre-existing band gap (2.39 eV) exists with silicane structure and decreases to zero with the increasing tensile strain without chirality effects.     

Quantum capacitance in monolayers of silicene and related buckled materials

Silicene and related buckled materials are distinct from both the conventional two dimensional electron gas and the famous graphene due to strong spin orbit coupling and the buckled structure. These materials have potential to overcome limitations encountered for graphene, in particular the zero band gap and weak spin orbit coupling. We present a theoretical realization of quantum capacitance which has advantages over the scattering problems of traditional transport measurements. We derive and discuss quantum capacitance as a function of the Fermi energy and temperature taking into account electron–hole puddles through a Gaussian broadening distribution. Our predicted results are very exciting and pave the way for future spintronic and valleytronic devices.     

Structural and magnetic properties of double-perovskite Ba2MnMoO6 by density functional theory

Perovskite-like materials which include magnetic elements have relevance due to the technological perspectives in the spintronics industry. In this work, we report the studies of Ba₂MnMoO₆ material by using the density functional theory. The interchange-correlation potential was included through the generalized gradient approximation. Our structural calculations are in agreement with the experimental results which show that the material crystallizes in the 225 space group (Fm3¯m) and has a lattice parameter of about 8070 Å. The density of states study was carried out by considering the up and down spin orientations. Results show that Ba₂MnMoO₆ has a conductor behavior due to dominant Mn spin-up and Mo spin-down contributions. The magnetic moment was calculated to be 2.9 μ_B.     

Electronic properties of transition-metal-atom doped Si cage clusters

We present a density-functional study of electronic structures of convex-caged Si clusters doped with transition-metal (TM) atoms. First, we show the reason for their peculiar geometries in terms of interplay among the electron orbitals of Si and TM atoms. Then we describe the potential ability of the clusters to serve as charge sources to other objects such as Si crystal surfaces. Millennium Research for Advanced Information Technology (MIRAI) Project.     

Electronic properties of InAs/GaAs nanowire superlattices

The electronic properties of strained InAs/GaAs nanowire superlattices are computed using a semi-empirical sp³d⁵s^* tight-binding model, taking strains, piezoelectric fields and image charge effects into account. Strain relaxation appears to be efficient in nanowire heterostructures, but is highly inhomogeneous in thin InAs layers. It digs a well in the conduction band that traps the electrons at the surface of the nanowires. This likely decreases the oscillator strength and might ease the capture of the electrons by nearby surface defects.     

Electronic and magnetic properties of Mn-doped ZnO: Total-energy calculations

Based on the spin generalized gradient approximation (σGGA) of the density functional theory (DFT), the structural, magnetic, and electronic properties of Mn-doped ZnO structure have thoroughly been investigated. It is found that the Mn atom prefers to substitute one of the Zn atoms, producing the energetically most stable configuration for the Mn-doped ZnO structure. Employing the Hubbard potential within the calculations suggests various changes and modifications to the structural, magnetic and electronic properties of the Mn-doped ZnO. Our calculations reveal that the local magnetic moment at the Mn site using the ordinary σGGA functional is 4.84 μ_B/Mn, which is smaller than that evaluated by including the Hubbard potential of 5.04 μ_B/Mn. Overall, the electronic band structure of the system, within the σGGA+U, is half-metallic, with metallic nature for the majority state and semiconducting nature for the minority state. Simulated scanning tunneling microscopy (STM) images for both unoccupied and occupied states indicate siginficant brightness on both Zn and Mn atoms and much brighter protrusions around the O atoms, respectively.     

Electronic properties of arrayed K clusters in zeolite LTA with Si/Al = 1.5

Arrayed cationic K clusters including one 4s-electron in each cluster, i.e., K_m+1 ^m+, were incorporated into α-cages of zeolite LTA with Si/Al=1.5. Although no magnetic phase transition was observed regarding the temperature (T) dependence of magnetic susceptibilities originating from the 4s-electron spins (χ_spin) between 2 and 300 K, the χ_spin-T curve could be fitted by the sum of magnetic susceptibilities based on the Curie-Weiss law and Pauli paramagnetism. A possible explanation of this behavior is the existence of a narrow energy band formed out of 1s-cluster orbitals of arrayed K clusters, and the existence of a finite density of state at the Fermi energy.