Utilizing First-principles Calculations Research Articles

Electronic structures, magnetic properties and band alignments of 3d transition metal atoms doped monolayer MoS2

Utilizing first-principles calculations, the electronic structures, magnetic properties and band alignments of monolayer MoS2 doped by 3d transition metal atoms have been investigated. It is found that in V, Cr, Mn, Fe-doped monolayers, the nearest neighboring S atoms (SNN) are antiferromagnetically polarized with the doped atoms. While in Co, Ni, Cu, Zn-doped systems, the SNN are ferromagnetically coupled with the doped atoms. Moreover, the nearest neighboring Mo atoms also demonstrate spin polarization. Compared with pristine monolayer MoS2, little change is found for the band edges' positions in the doped systems. The Fermi level is located in the spin-polarized impurity bands, implying a half-metallic state. These results provide fundamental insights for doped monolayer MoS2 applying in spintronic, optoelectronic and electronic devices.

Indium passivation impact on HfO2/GaAs interface: A first-principles study

The interface quality between III–V semiconductor and high-k material is critical for improving the performance of the metal-oxide-semiconductor field effect transistors. Utilizing first-principles calculations, we explore the electronic properties of the GaAs/HfO2 interface when indium interfacial atom defects are introduced. The stability of the interfaces will increase with the increasing of the number of indium interfacial atoms. Meanwhile, the band offsets also depend on the number of indium atom substitutions and interstitials. Furthermore, the substitutions suppress the interfacial gap states near the VBM and have the partially passivation, but the effect of interstitial passivation on the interfacial states are limited.

Crystal symmetry lowering in chiral multiferroic Ba3TaFe3Si2O14 observed by x-ray magnetic scattering

Chiral multiferroic langasites have attracted attention due to their doubly-chiral magnetic ground state within an enantiomorphic crystal. We report on a detailed resonant soft X-ray diffraction study of the multiferroic Ba$_3$TaFe$_3$Si$_2$O$_{14}$ at the Fe $L_{2,3}$ and oxygen $K$ edges. Below $T_N$ ($\approx27K$) we observe the satellite reflections $(0,0,\tau)$, $(0,0,2\tau)$, $(0,0,3\tau)$ and $(0,0,1-3\tau)$ where $\tau \approx 0.140 \pm 0.001$. The dependence of the scattering intensity on X-ray polarization and azimuthal angle indicate that the odd harmonics are dominated by the out-of-plane ($\mathbf{\hat{c}}$-axis) magnetic dipole while the $(0,0,2\tau)$ originates from the electron density distortions accompanying magnetic order. We observe dissimilar energy dependences of the diffraction intensity of the purely magnetic odd-harmonic satellites at the Fe $L_3$ edge. Utilizing first-principles calculations, we show that this is a consequence of the loss of threefold crystal symmetry in the multiferroic phase.

Intrinsic magnetism and electronic structure of graphene-like Be3C2 nanoribbons and their Si, Ge analogues: a computational study

Utilizing first-principles calculations, we perform a systematic study on the electronic and magnetic properties of recently proposed graphene-like Be3C2 nanostructures.

Structural stability and the electronic properties of a (SiH)2O-formed siloxene sheet: a computational study.

Very recently, a (SiH)2O-formed siloxene sheet has been synthesized in an experiment [S. Li, et al. J. Mater. Chem. A 2016, 4, 15841]. Utilizing first-principles calculations, we have systematically investigated the structural stability and the physical properties of this (SiH)2O sheet. We have found a dynamically stable structure because it is a puckered one with bent Si-O-Si connections, which leads to an anisotropic elastic behaviour with negative Poisson ratios. The (SiH)2O sheet has a strain-tunable direct band gap whose valence and conduction band edges straddle the redox potentials of water even in the presence of stress (either tensile or compressive). The calculated carrier mobilities are prominently anisotropic: in its zigzag direction the electron mobility reaches a high value of 5.39 × 103 cm2 V-1 s-1, while the hole mobility remains small. These different carrier mobilities permit fast charge separation and transfer, which is crucial for efficient solar-to-electric energy conversion for photovoltaic and water splitting applications. Similar to phosphorene, a typical linear dichroism phenomenon is also present in the (SiH)2O sheet. Due to its outstanding electronic properties, the (SiH)2O sheet is a promising material in the fields of nano-electronics, sustainable energy and fuel generation.

Electronic structure and topological features of tin-based binary nanosheets and their hydrogenated/fluorinated derivatives: A first-principles study

Utilizing first-principles calculations, we have investigated the structural, electronic and topological properties of binary SnSi and SnGe nanosheets as well as their H-/F-derivatives. It is found that all these systems have chair-like buckled configurations with robust structural stabilities. Unlike the elemental group-IV sheets, SnSi and SnGe sheets are narrow-band-gap semiconductors, which have a gapped Dirac cone with massive Fermions. Under the strains, a direct-to-indirect band gap transition and a semiconductor-to-metal transition would occur in these systems. Although these binary systems are trivial band insulators with ℤ2=0, their topological features can be altered by the surface decorations. Particularly, the fluorinated SiGe sheet becomes a topological insulator with ℤ2=1, and such non-trivial state can also be found in the strained fluorinated SnSi and hydrogenated SnGe sheets. Large non-trivial SOC band gaps of 0.09–0.17eV are obtained in these 2D topological insulating systems, which will be beneficial for realizing the room-temperature quantum spin Hall effect. Our study demonstrates that binary Sn-based nanomaterials possess tunable electronic and topological properties, which have potential applications in low-power nano-electrics and devices.

GaN as an interfacial passivation layer: tuning band offset and removing fermi level pinning for III-V MOS devices.

The use of an interfacial passivation layer is one important strategy for achieving a high quality interface between high-k and III-V materials integrated into high-mobility metal-oxide-semiconductor field-effect transistor (MOSFET) devices. Here, we propose gallium nitride (GaN) as the interfacial layer between III-V materials and hafnium oxide (HfO2). Utilizing first-principles calculations, we explore the structural and electronic properties of the GaN/HfO2 interface with respect to the interfacial oxygen contents. In the O-rich condition, an O8 interface (eight oxygen atoms at the interface, corresponding to 100% oxygen concentration) displays the most stability. By reducing the interfacial O concentration from 100 to 25%, we find that the interface formation energy increases; when sublayer oxygen vacancies exist, the interface becomes even less stable compared with O8. The band offset is also observed to be highly dependent on the interfacial oxygen concentration. Further analysis of the electronic structure shows that no interface states are present at the O8 interface. These findings indicate that the O8 interface serves as a promising candidate for high quality III-V MOS devices. Moreover, interfacial states are present when such interfacial oxygen is partially removed. The interface states, leading to Fermi level pinning, originate from unsaturated interfacial Ga atoms.

The effect of substrate and external strain on electronic structures of stanene film.

From first-principles calculations, the effects of h-BN and AlN substrates on the topological nontrivial properties of stanene are studied with different strains. We find that the quantum spin Hall phase can be induced in stanene film on a h-BN substrate under a tensile strain of between 6.0% and 9.3% with a stable state confirmed by the phonon spectrum, while for stanene on 5 × 5 h-BN, the quantum spin Hall phase can be preserved without strain. However, for stanene on a AlN substrate, the quantum spin Hall phase cannot be found under compressive or tensile strains less than 10%, while for 2 × 2 stanene on 3 × 3 AlN, the compressive strain needed to induce the quantum spin Hall phase is just 2%. These theoretical results will be helpful in understanding the effect of substrate and strain on stanene and in further realizing the quantum spin Hall effect in stanene on semiconductor substrates.

A comparative first-principles study of martensitic phase transformations in TiPd2 and TiPd intermetallics

Martensitic phase transformations in TiPd2 and TiPd alloys are studied employing density-functional, first-principles calculations. We examine the transformation of tetragonal C11b TiPd2 to the low-temperature orthorhombic phase (C11b → oI6), and the transformation of cubic B2 TiPd under orthorhombic (B2→B19) and subsequent monoclinic transformations (B19→B19′) as the system is cooled. We employ a theoretical approach based on a phenomenological Landau theory of the structural phase transitions and a mean-field approximation for the free energy, utilizing first-principles calculations to obtain the deformation energy as a function of strains and to deduce parameters for constructing the free energy. The predicted transition temperature for the TiPd2 C11b → oI6 transition is in good agreement with reported experimental results. To investigate the TiPd B2→B19 transformation, we employ both the Cauchy–Born rule and a soft-mode-based approach, and elucidate the importance of the coupling between lattice distortion and atomic displacements (i.e. shuffling) in the formation of the final structure. The calculated B2→B19 transition temperature for TiPd alloy agrees well with the experimental results. We also find that there exists a very small but finite (0.0005 eV/atom) energy barrier of B19 TiPd under monoclinic deformation for B19→B19′ structural phase transformation.

Dual Defects of Cation and Anion in Memristive Nonvolatile Memory of Metal Oxides

The electrically driven resistance change of metal oxides, called bipolar memristive switching, is a fascinating phenomenon in the development of next-generation nonvolatile memory alternatives to flash technology. However, our understanding of the nature of bipolar memristive switching is unfortunately far from comprehensive, especially the relationship between the electrical transport and the local nonstoichiometry. Here we demonstrate that the coexistence of anion and cation defects is critical to the transport properties of NiO, one of the most promising memristive oxides, by utilizing first-principles calculations. We find that, in the presence of both nickel and oxygen defects, which must exist in any real experimental systems, carrier concentrations of holes generated by nickel defects can be modulated by the presence or absence of oxygen defects around the nickel defect. Such alternation of local nonstoichiometry can be understood in terms of an oxygen ion drift induced by an external electric field. This implication provides a foundation for understanding universally the nature of bipolar memristive switching in various p-type metal oxides.