Band Decomposed Charge Density Research Articles

Structural stability and electronic properties of Er nanowire on Si(001)

The silicon surfaces covered with rare earth metals have attracted great interest in recent years due to the promising device applications. Here we report a systematic study on the structural stability and electronic properties of the Er nanowire on Si(001) surface by ab initio calculations. It is shown that Er atomic chains with a double-core odd-membered-ring (5-7-5) structure on Si(001) surface is one of the most stable structures at lower coverage. Total energy calculations show that Er atoms prefer ferromagnetic coupling with a spin moment of 2.04–2.06 μB on Er sites derived from the 4f electrons. Electronic band structure and band-decomposed charge density distribution calculations show that the odd-membered-ring (5-7-5) structure has a semiconducting feature with a small indirect band gap of 0.13 eV. This study provides a new insight for further exploration of self-assembled nanowires of rare-earth metal on silicon surfaces.

Three non-metallic carbon materials with comparable electrical conductivity to metals

Carbon materials with metallic properties are very worthy of research because they possess electrical and thermal conductivity and huge application potential in the manufacturing of electronic equipment, aviation, and industry. By means of first-principles calculations, three metallic carbon allotropes with outstanding physical characteristics were designed in this work. These three carbon phases, denoted as mP20, mP22 and mP24 carbon, contain 20, 22, and 24 carbon atoms in each unit cell, respectively. The relevant enthalpies of mP20, mP22 and mP24 carbon are more favorable than those of P4mmm C24, TY‑carbon and T‑carbon. In addition, the electronic band structures indicate that mP20, mP22 and mP24 carbon are all metallic. Furthermore, their band decomposed charge density also proves that these three carbon materials can conduct electricity in the [001] direction. More interestingly, the electrical conductivities of mP22 and mP24 in the [001] direction are comparable to those of metallic Pt at the same reference temperature of 300 K. The X-ray diffraction patterns have been studied and can be used for future experimental verification.

Breaking the Limitation of Elevated Coulomb Interaction in Crystalline Carbon Nitride for Visible and Near-Infrared Light Photoactivity.

Most near‐infrared (NIR) light‐responsive photocatalysts inevitably suffer from low charge separation due to the elevated Coulomb interaction between electrons and holes. Here, an n‐type doping strategy of alkaline earth metal ions is proposed in crystalline K+ implanted polymeric carbon nitride (KCN) for visible and NIR photoactivity. The n‐type doping significantly increases the electron densities and activates the n→π* electron transitions, producing NIR light absorption. In addition, the more localized valence band (VB) and the regulation of carrier effective mass and band decomposed charge density, as well as the improved conductivity by 1–2 orders of magnitude facilitate the charge transfer and separation. The proposed n‐type doping strategy improves the carrier mobility and conductivity, activates the n→π* electron transitions for NIR light absorption, and breaks the limitation of poor charge separation caused by the elevated Coulomb interaction.

DensityTool: A post-processing tool for space- and spin-resolved density of states from VASP

The knowledge of the local electronic structure of heterogeneous solid materials is crucial for understanding their electronic, magnetic, transport, optical, and other properties. VASP, one of the mostly used packages for density-functional calculations, provides local electronic structure either by projecting the electronic wave functions on atomic spheres, or as a band-decomposed partial charge density. Here, we present a simple tool which takes the partial charge density and the energy eigenvalues calculated by VASP as input and constructs local charge and spin densities. The new data provides a much better spatial understanding than the projection on the atomic spheres. It can be visualized directly in the real space e.g. with Vesta, or averaged along planes spanned by two of the lattice vectors of the periodic unit cell. The plane-averaged local (spin) density of states can be easily plotted e.g. as color-coded data using almost any plotting program. DensityTool can be applied to visualize and understand the local electronic structure of any system calculated with VASP. We expect it to be useful especially for researchers concerned with inhomogeneous systems, such as interfaces, defects, surfaces, adsorbed molecules, or hybrid inorganic-organic composites. Program summaryProgram Title: DensityToolCPC Library link to program files:https://doi.org/10.17632/26mcg4jwvd.1Developer's repository link:https://github.com/llodeiro/DensityToolLicensing provisions: MITProgramming language: FORTRANSupplementary material: A complementary User Manual and examples can be found in the developer's repository link.Nature of problem: Total and local density of states (LDOS) are widely-used quantities for understanding the electronic structure of condensed matter. In VASP, one of the mostly used packages for numerically solving the Kohn-Sham equations of density-functional theory, one does not directly obtain the LDOS. Usually, one works with partial density of states (PDOS), where the wave function is projected onto atomic orbitals. Alternatively, it is possible to calculate the band- and wavevector-dependent charge densities. Together with the band structure, LDOS can be obtained using post-processing tools.Solution method: DensityTool combines the band structure with band- and wavevector-dependent charge densities calculated by VASP to construct LDOS. In addition, spin-resolved LDOS (LSDOS) can be calculated for magnetic systems. Thus obtained LDOS and LSDOS can be further processed with DensityTool. In particular, it can be averaged along chosen spatial directions, which is convenient for example to demonstrate the spatially-resolved electronic structure of inhomogeneous systems.DensityTool can handle all three types of approaches implemented in VASP (non-magnetic, collinear magnetic, non-collinear magnetic including spin-orbit coupling), as well as all types of self-consistent band calculations as DFT (e.g., LDA, GGA, mGGA, BJ, and hybrid exchange-correlation functionals) and HF.For convenience, the output of DensityTool follows the format of VASP (the CHGCAR file), which can be easily used in other programs for visualization. We provide a sample python script to plot averaged L(S)DOS from the calculated data.

Effect of the coexistence of active metals and boron vacancies on the performance of 2D hexagonal boron nitride resistance memory

First-principles calculations were carried out to calculate the formation energy, migration barrier and electronic properties of a resistive memory model based on hexagonal boron nitride (h-BN) in the presence of an active metal and a boron vacancy (VB) using density functional theory (DFT). Following the benchmark of the exchange correlation functional and the calculated parameters of monolayer h-BN, a model of a multilayer h-BN vertical stack with distribution states of SW-5577 defects was proposed. For four active metal dopants (Ti, Ag, Cu and Ni), a preference towards substitution sites (S1) with the lowest dopant formation energies (DFEs) was identified, which enhanced the formation of adjacent VB, especially for the nearest neighbour. Furthermore, a low concentration of Ti dopant in the closest location to the initial position of the migration path would drastically reduce the migration barrier of the VB between layers. Finally, Ti dopants with two and three VB neighbours in the same layer significantly improved the conductivity and the formation of conducting channels because of the improvement of charge distribution in the resistance model, which was demonstrated by DOS plots, band-decomposed charge density and Bader charge. Our present work can provide theoretical guidance for the rational design and device optimization of h-BN-based RRAM devices.

First-Principles Study of Bi-Doping Effects in Hg0.75Cd0.25Te.

First-principles calculations based on density functional theory have been performed for exploring the structural and electronic properties of Bi-doped Hg0.75Cd0.25Te (MCT), using the state-of-the-art computational method with the Heyd–Scuseria–Ernzerhof (HSE) of hybrid functional to correct the band gap. Structural relaxations, charge densities, electron localization functions (ELFs), density of states (DOSs), band structures, and band decomposed charge density were obtained to reveal the amphoteric behavior of Bi in Hg0.75Cd0.25Te. The bonding characteristics between Bi and host atoms were discussed by analyzing charge densities and ELFs. The influence of Bi impurity on the electronic structure of Bi-doped Hg0.75Cd0.25Te was also analyzed by the calculated DOSs, band structures, and the band decomposed charge density of the defect band. It has been demonstrated that Bi can show a typical amphoteric substitution effect of group V elements.

La induced Si3 trimer bilayer on the Si(111) surface.

Using first-principles calculations, we identify a robust R30° reconstruction of a Si3 trimer bilayer on the Si(111) surface with a La coverage of 2/3 monolayer. Each surface unit cell contains two Si3 trimers and two La atoms, where the upper Si3 trimer is located just above the lower one with a rotation of about 60°, while two La atoms with different heights are distributed between the Si3 trimers and located on the T4 top site of the Si(111) surface, forming a honeycomb-like network structure. We find that the two La atoms have different valence states, La2+ and La3+, respectively. The high structural stability is attributed to the lower La atom saturating all the three dangling bonds of the upper Si3 trimer, while the higher La atom compensates two electrons to the lower Si3 trimer. The electronic band structure and band-decomposed charge density distribution show a semiconducting characteristic with a small surface band gap of 42 meV. Moreover, simulated STM images show a good structural match with the recent experimental observations.

Metallic and semiconducting carbon allotropes comprising of pentalene skeletons

The design and synthesis of new 3D carbon allotropes, especially metallic carbon allotropes, have become a focus of study among scientists. In this work, a series of 3D carbon allotropes using the self-assembling C8 skeleton of pentalene as a building block are constructed and named the superpentalene-n family. Based on first-principle calculations, their mechanical and electronic properties are evaluated. These carbon allotropes are energetically more favorable than many other previously reported allotropes, such as synthesized supercubane and T-carbon. The calculated elastic constants and simulated phonon spectra suggest that these new carbon allotropes are dynamically and mechanically stable. These new carbon allotropes exhibit high hardness, with hardness values of 38.20–65.75 GPa, and a bulk modulus of 305–366 GPa. Electronic band and band decomposed charge density calculations indicate that superpentalene-I, superpentalene-II and superpentalene-III are metallic and have one-dimensional conducting channels, whereas superpentalene-IV is a semiconductor. These findings suggest a new strategy for designing new carbon allotropes with an alkene carbon skeleton as a building block.

Facet-Specific Photocatalytic Activity Enhancement of Cu2O Polyhedra Functionalized with 4-Ethynylanaline Resulting from Band Structure Tuning.

Cu2O rhombic dodecahedra, octahedra, and cubes were densely modified with conjugated 4-ethynylaniline (4-EA) for facet-dependent photocatalytic activity examination. Infrared spectroscopy affirms bonding of the acetylenic group of 4-EA onto the surface copper atoms. The photocatalytically inactive Cu2O cubes showed surprisingly high activity toward methyl orange photodegradation after 4-EA modification, while the already active Cu2O rhombic dodecahedra and octahedra exhibited a photocatalytic activity enhancement. Electron, hole, and radical scavenger experiments prove that the photocatalytic charge transport processes have occurred in the functionalized Cu2O cubes. Electrochemical impedance spectroscopy also indicates reduced charge transfer resistance of the functionalized Cu2O crystals. A band diagram constructed from UV–vis spectral and Mott–Schottky measurements reveals significant band energy shifts in all Cu2O samples after decorating with 4-EA. From density functional theory (DFT) calculations, a new band has emerged slightly above the valence band maximum within the band gap of Cu2O, which has been found to originate from 4-EA through band-decomposed charge density analysis. The increased charge density localized on the 4-EA molecule and the smallest electron transition energy to reach the 4-EA-generated band are factors making {100}-bound Cu2O cubes photocatalytically active. Proper molecular decoration represents a powerful approach to improving the photocatalytic efficiency of semiconductors.

Structural and electronic properties of BaSi2(100) thin film on Si(111) substrate

BaSi2, which can be grown on Si(111) substrate by molecular beam epitaxy experimentally, holds great promise for solar-cells. Here, we report a detailed ab initio study on the structural and electronic properties of BaSi2(100) thin films on Si(111) substrate. A high stable interface structure with bond breaking of Si4-tetrahedra at interface is obtained by ab initio molecular-dynamics simulations. We find that the bond breaking of Si4-tetrahedra at interface play a key role to saturate the dangling bonds of Si(111) substrate. Electronic band structures and band-decomposed charge density distributions reveal that such BaSi2(100) thin film structures are semiconductor with an interface band gap of 0.71–0.75 eV and a large surface band gap of 1.17–1.27 eV, closing to the bulk BaSi2 band gap of 1.25 eV. These results provide an excellent explanation for the recent experimental observations on the BaSi2-based thin films on Si(111) substrate.

Structural stability and electronic properties of Sr induced (5×4) reconstruction on Si(111) surface

Honeycomb-chain-channel (HCC) and π-bond Seiwatz chain (SC) are the well-known reconstructions induced by alkaline earth metal on Si(111) surface. Here we identify by ab initio calculations a stable intermediate phase of Sr/Si(111)-(5×4) HCC+SC. This new reconstruction satisfies the electron counting rule with the Sr coverage of 3/10 monolayer. In each (5×4) surface unit cell, six Sr atoms are distributed as evenly as possible in the two channels. One of the channels contains two Sr atoms on the top site T4 and one Sr atom on the hollow site H3, while another channel contains two Sr atoms on site H3 and one on site T4. Electronic band structures and band-decomposed charge density distributions show that there are seven occupied surface states (S1–S5, Σ1 and Σ2) and two unoccupied surface states (S6 and S7) near the Fermi level. Moreover, we find that this new (5×4) surface structure is a semiconductor with a surface band gap of 0.62 eV, determined by the S2 and S6 surface states in honeycomb Si chains.

Mn-Doped Sr/Si(111)-(3 × 2) HCC Surfaces: Antiferromagnetic Semiconductors for Spintronic Applications.

Manganese and manganese silicide as promising candidates for spintronic applications have attracted great interest in recent years. Here, we adopt Sr-induced Si(111)-(3 × 2) honeycomb-chain channel (HCC) surface as a template and perform a systematical study on the structural stability and magnetic and electronic properties of Mn-doped Sr/Si(111)-(3 × 2) HCC surfaces by ab initio calculations. Our energetic and kinetic results show two robust inserting structures M6 and H4, where Mn atoms are located below the honeycomb Si chain and on the top or hollow site of the Si(111) surface. Their high structural stabilities are attributed to the doped Mn atoms that saturate all the dangling bonds of Si(111) surface. In these two structures, Mn atoms prefer antiferromagnetic coupling with the same local magnetic moment of 3 μB. Electronic band structures and band-decomposed charge density distributions reveal that these two stable surface structures have a semiconducting characteristic with a surface band gap of 0.21-0.28 eV. This work provides an antiferromagnetic system for the possible application in spintronics.

Half-metallic, Magnetic, and Optical Properties for the (001) Surface of Binary Heusler Alloy MgCl3

The half-metallic (HM), magnetic, and optical properties for the (001) surface of binary Heusler alloy MgCl3 were investigated by the first-principles method. The Mg*Mg*-term and Mg*Mg-term had the lower surface energies among the ClCl-term, Mg*Mg*-term, ClMg-term, ClCl*-term, and Mg*Mg-term (001) surfaces, thus, they were the more stable surfaces. The band decomposed charge density showed clearly the valence band maximum was contributed not only by the ClB-3p states but also the ClA-3p and ClC-3p states. The HM character was destroyed in all the five terminations of the (001) surface. The stable Mg*Mg*-term (001) surface had the largest spin polarization − 83.0%. The hydrogen-termination effect showed an effective method to enlarge the spin polarizations for the ClCl-term and ClCl*-term surfaces to produce thin films which had potential applications in spintronics. There were great changes of atomic magnetic moments for Cl atoms in any layer. In energy range of 1.6–3.1 eV for the visible light, the Mg*Mg*-term had superiority in the absorption for purple light.

A new tetragonal superhard metallic carbon allotrope

A new metallic superhard carbon allotrope C5 with five carbon atoms per unit cell is theoretically predicted to be stable at ambient pressure through first-principles calculations combined with unbiased swarm structure searching techniques. This novel carbon allotrope consisting of a mixture of sp2 and sp3 carbon network exhibits excellent mechanical properties with claimed hardness of 58.5 GPa. Results from our calculated strain-stress relationship reveals that the failure mode of crystal lattice for C5 is dominated by shear type in the (010)[101] direction with shear stress magnitude of 74.3 GPa. The calculated electronic band structure of C5 suggests its metallic nature. Detailed analyses of band decomposed charge density show that the metallic nature is contributed by the sp2 carbon atoms, which form conducting pathways along the a- and b-axes.

H-AlN-Mg(OH)2van der Waals bilayer heterostructure: Tuning the excitonic characteristics

Motivated by recent studies that reported the successful synthesis of monolayer $\mathrm{Mg}{(\mathrm{OH})}_{2}$ [Suslu et al., Sci. Rep. 6, 20525 (2016)] and hexagonal $(h\text{\ensuremath{-}})\text{AlN}$ [Tsipas et al., Appl. Phys. Lett. 103, 251605 (2013)], we investigate structural, electronic, and optical properties of vertically stacked $h$-AlN and $\mathrm{Mg}{(\mathrm{OH})}_{2}$, through ab initio density-functional theory (DFT), many-body quasiparticle calculations within the GW approximation and the Bethe-Salpeter equation (BSE). It is obtained that the bilayer heterostructure prefers the $A{B}^{\ensuremath{'}}$ stacking having direct band gap at the $\mathrm{\ensuremath{\Gamma}}$ with Type-II band alignment in which the valance band maximum and conduction band minimum originate from different layer. Regarding the optical properties, the imaginary part of the dielectric function of the individual layers and heterobilayer are investigated. The heterobilayer possesses excitonic peaks, which appear only after the construction of the heterobilayer. The lowest three exciton peaks are analyzed in detail by means of band decomposed charge density and the oscillator strength. Furthermore, the wave function calculation shows that the first peak of the heterobilayer originates from spatially indirect exciton where the electron and hole localized at $h$-AlN and $\mathrm{Mg}{(\mathrm{OH})}_{2}$, respectively, which is important for the light harvesting applications.

Indirect-Direct Band Transformation of Few-Layer BiOCl under Biaxial Strain

Being a new two-dimensional layered compounds, the tunable indirect–direct band transformation of BiOCl with different layers can be realized by introducing the biaxial tensile or compressive strains. The band structure and stability of BiOCl with different layers are first researched to clarify the influence of layer numbers. A phase transformation of bilayer BiOCl and metallic characteristic for all are observed under large tensile and compressive strains, respectively. In addition, bond length, interlayer spacing, and band decomposed charge density are calculated to analyze the mechanism behind these phenomena. The results indicate that the band structure transformation is primarily related to the competitions between two kinds of intralayer and interlayer Bi–O bonds and hybridizations between atoms under strains.

The unusual chemical bonding and thermoelectric properties of a new type Zintl phase compounds Ba3Al2As4

Abstract Ba 3 Al 2 As 4 exhibits an unusual anisotropic electrical conductivity, that is, the electrical conductivity along the chain is smaller than those along other two directions. The results is conflict with previous conclusion for Ca 5 M 2 Pn 6 . Earlier studies on Ca 5 M 2 Pn 6 showed that a higher electrical conductivity could be obtained along the chain. The band decomposed charge density is used to explain such unusual behavior. Our calculations indicate the existence of a conductive pathway near the Fermi level is responsible for the electrons transport. Further, the Ba–As bonding of Ba 3 Al 2 As 4 has some degree covalency which is novel for the Zintl compounds.

Vacancy induced Jahn–Teller distortion in silicon and its influence to the electronic structure

Atomic and electronic structures of monovacancy (V1), divacancy (V2) and ring hexavacancy (V6) in crystalline silicon are studied using first-principles calculations in periodic supercells. Our results show that the V6 defect is the most stable among V1, V2 and V6 defects, and the V2-RB structure is a little more stable than the V2-LP structure due to lower vacancy formation energy. Furthermore, it is found that both V1 and V2 undergo the Jahn–Teller (JT) distortion while V6 does not. As a result, V1 and V2 have deep levels in the gap which mainly come from the neighboring atoms to vacancy. V6 has tailing bands in the gap, and so has a more stable electronic structure than V1 and V2. In addition, the JT distortion also reflects in the band decomposed charge density and the difference charge density.

The Electronic Properties of Single-Layer and Multilayer MoS2 under High Pressure

We analyze the changes in the electronic structures of single-layer and multilayer MoS2 under pressure using first-principles methods including van der Waals interactions. For single-layer MoS2, the bond angle is found to control the electronic structure around the band gap under the pressure. For multilayer and bulk MoS2, the changes in electronic structure under pressure are mainly controlled by the coupling of layers. Under pressure, the band gap of single-layer MoS2 changes from direct to indirect, while multilayer MoS2 becomes a band metal. Analysis of the real-space distribution of band-decomposed charge density shows that this behavior can be understood in terms of the different Mo d-electron orbitals making up the states near band gap including those at the top of valence band of Γ and K points and the bottom of conduction band along Λ and at the K point.

Improved thermoelectric performance of CuGaTe2 with convergence of band valleys: a first-principles study

A high value (1.4) of figure of merit (ZT) of CuGaTe2 has been experimentally discovered by T. Plirdpring et al. [Adv. Mater. 24, 3622 (2012)]. In order to further enhance its thermoelectric properties, we investigated its electronic structure and thermoelectric properties by first-principles study. Large band-valley number and high convergence of the bottom conduction bands induced a large Seebeck coefficient and a high electrical conductivity of n-type CuGaTe2. So, for n-type CuGaTe2, the maximum ZT values 2.1 may be found at 950 K by suitable carrier concentration tuning, which results in a 25% increment in the ZT value compared with p-type CuGaTe2. Band decomposed charge density calculations indicate that the transport properties are mainly determined by the Cu and Te atoms at the valence-band maximum, in contrast, transport properties are simultaneously affected by the three kind of atoms at the conduction-band minimum. At high temperature, ab initio molecular dynamics calculations demonstrate that Cu atoms precipitated from their crystal matrices lead to a decrease in thermopower. Along the high symmetry point M, the charge density of all atoms is centrosymmetric. Maybe this centrosymmetric electronic structure leads to the conduction band valley convergence at the M point.