Multifunctional silicene/CeO2 heterojunctions: Desirable electronic material and promising water-splitting photocatalyst

The first-principles calculations demonstrate that covalently bonded (cb) heterojunction and van der Waals (vdW) heterojunction can coexist in silicene/CeO₂ heterojunctions, due to the different stacking patterns. Especially, the cb heterojunction with band gap of 1.97 eV, forms a type-II heterojunction, exhibits good redox performance and has high-effective optical absorption spectra, thus it is a promising photocatalyst for overall water splitting. Besides, for the vdW heterojunction, the Dirac cone of silicene is well kept on CeO₂ semiconducting substrate, with a considerable energy gap of 0.43 eV, which can be an ideal material in building silicene-based electronic device. These results may open a new gateway in both of nanoelectronic device and energy conversion for silicene/CeO₂ nanocomposites.     

Electronic Properties of Transition‐Metal‐Decorated Silicene

The electronic properties of 3d transition metal (TM)‐decorated silicene were investigated by using density functional calculations in an attempt to replace graphene in electronic applications, owing to its better compatibility with Si‐based technology. Among the ten types of TM‐doped silicene (TM–silicene) studied, Ti‐, Ni‐, and Zn‐doped silicene became semiconductors, whereas Co and Cu doping changed the substrate to a half‐metallic material. Interestingly, in cases of Ti‐ and Cu‐doped silicene, the measured band gaps turned out to be significantly larger than the previously reported band gap in silicene. The observed band‐gap openings at the Fermi level were induced by breaking the sublattice symmetry caused by two structural changes, that is, the Jahn–Teller distortion and protrusion of the TM atom. The present calculation of the band gap in TM–silicene suggests useful guidance for future experiments to fabricate various silicene‐based applications such as a field‐effect transistor, single‐spin electron source, and nonvolatile magnetic random‐access memory.     

Transition metal atom adsorptions on a boron nitride nanocage

We have applied density functional theory within B3LYP and M05 functionals to investigate the chemical functionalization of B₁₂N₁₂ nanocage with 3d transition metal (TM) atoms. Main focuses have been placed on configurations corresponding to the located minima of the adsorbates, corresponding adsorption energies, and the modified electronic properties of the cage. It was shown that there is linear correlation between the adsorption energies of the B3LYP and M05 as the results of M05 are higher than those of B3LYP, about 0.52 eV. Based on calculations, the most stable adsorption site is over the bond shared by a four- and a six-membered ring in the outer surface of cluster, in most cases. Based on the M05 results, the adsorption energies of the Sc, Ti, V, Co, and Fe are relatively high (>1.51 eV) and those of Mn, Ni, and Cu calculated to be in the range of 1.00–1.22 eV. The Cr atom forms a weak bond with a boron atom of the B₁₂N₁₂ cluster, while Zn atom cannot be chemically adsorbed. Charge transfer from metals to cluster ascertained that the B₁₂N₁₂ plays as an electron-trapping center. Inducing certain impurity states within the electron density of states, the TM adsorption significantly reduces the HOMO–LUMO gap of cluster, ranging from 32 to 79 %.     

First-principle studies of Ca-X (X=Si,Ge,Sn,Pb) intermetallic compounds

The structural properties, elastic properties, heats of formation, electronic structures, and densities of states of 20 intermetallic compounds in the Ca-X (X=Si, Ge, Sn, Pb) systems have been systematically investigated by using first-principle calculations. Our computational results indicated that with increasing atomic weight of X, the bulk modulus of Ca-X intermetallic compounds decreases gradually. It was also found that Ca₃₆Sn₂₃ and CaPb are mechanically unstable phases. Results on the electronic energy band and densities of states also indicated that Ca₃Si₄ is an indirect band gap semiconductor with a band gap of 0.598 eV, and Ca₂Si, Ca₂Ge, Ca₂Sn, and Ca₂Pb are direct band gap semiconductors with band gaps of 0.324, 0.265, 0.06, and 0.07 eV, respectively. In addition, it is found that the absolute values of heats of formation for all Ca-X intermetallics are larger than 30 kJ/mol atom.     

A theoretical study on the chirality detection of serine amino acid based on carbon nanotubes with and without Stone-Wales defects

In the present study, the interaction of serine (SER) amino acid (AA) with the pristine and defected carbon nanotubes (CNTs) has been investigated by employing the molecular dynamics (MD) and the density functional theory (DFT) approaches. Furthermore, the potential application of CNTs with and without the Stone-Wales (SW) defects in sensing of SER chirality has been studied. Our results confirm that introducing the chiral l and d SERs (LSER and DSER) exerts a significant effect on the electronic and optical properties of the CNTs with and without the SW defect. According to the MD results, it is observed that for all the structures, the obtained minimum distance is among the SER aliphatic segments and the tube atoms. The calculated structural and electronic properties of pristine and defected CNT are in good agreement with the reported research studies. The results indicate that pyramidalization angles (θ_p) at C atoms are altered in the presence of the SW defects. The overall increment of θ_p suggests that the reactivity has increased at the defective regions. In the case of CNT with one SW defect (CNTSW1), the central C–C bond of the SW defect is the most chemically reactive site. Our results establish that pristine CNT is a semiconductor when the LSER and DSER are adsorbed (with the band gap of 0.30 eV and 0.32 eV, respectively). The LSER-adsorbing CNT with two SW defects (CNTSW2) is a semiconductor with a reduced band gap (0.41 eV), while the DSER-adsorbing CNTSW2 is an n-type semiconductor (with a band gap of 0.70 eV). The optical properties are inferred from the dielectric functions of the CNTs. The most remarkable result belongs to the CNTSW2; the imaginary part of the CNTSW2 dielectric function can sensitively distinguish the chirality of the SER amino acid.

     

Thermal stability,morphology and electronic band gap of Zn(NCN)

The thermal behavior of zinc carbodiimide Zn(NCN) was examined in the temperature range between 200 and 1100 °C in Ar atmosphere. The material starts to partially decompose at about 800 °C. Heat treatment at temperatures beyond 800 °C results in the formation of the byproducts nitrogen-containing bamboo-like multiwall carbon-nanotubes of 20–50 nm in diameter due to a partial decomposition of Zn(NCN) into dicyan (CN)₂, zinc and nitrogen gas followed by the polymerization of the former product to paracyanogen (CN)_n. At 1100 °C, the yield of the residual carbodiimide depends on the dwelling time and the initial amount of powder used for pyrolysis. One hour dwelling at 1100 °C yields ∼50% of the Zn(NCN) separated as pure material. Temperature-induced change in the band structure, namely indirect-to-direct band gap transition, is registered when compared the Zn(NCN) at room temperature with the residual material annealed at 1100 °C. The transition from indirect (E_g = 4.32 eV) to direct band gap (E_g = 4.93 eV) is due to the thermal annealing process which results in healing of crystal defects.     

AMoO4 (A = Mg,Ni) molybdates: Phase stabilities,electronic structures and chemical bonding properties from first principles

Stability of different phases of AMoO₄ (A = Mg, Ni) molybdates versus A–O bonding and the corresponding electronic structures are examined from first principles. The energy-volume equations of state for three forms (β, α, ω), characterized by decreasing volumes in the sequence of Mg and Ni molybdates are established. While NiMoO₄ is energy stabilized in the sequence β → α → ω, an opposite behavior is identified for the Mg molybdate. Charge analysis characterizing ionic Mg²⁺ versus covalent Ni^+1.2 behaviors can explain the trend. Electronic band structure also shows large differences: MgMoO₄ is insulating with a ～2 eV band gap while in a magnetic state, NiMoO₄ is a small gap (～0.2 eV) semi-conductor. Chemical bonding properties show weak Mg and strong Ni bonding with oxygen, while identifying the Mo–O interaction as prevailing.     

First-principles study of a new type nanojunction: Finite length carbon chain inserted into half of a carbon nanotube

Based on first-principles calculations, we investigate the structure and electronic properties of a carbon atomic chain in finite length inserted into half of a single walled carbon nanotube (SWCNT), which we called half chain@SWCNT or more generally HCS. Comparing the optimized structure of HCS with that of the same chiral indices SWCNT and all carbon chain inserted SWCNT, we find that the geometry of the tube in HCS is slightly altered due to the weakly interacting between the inserted chain and the outer tube wall of HCS. Our calculation of band structure indicates that the armchair (5, 5) HCS exhibits metallic character, which is as that of (5, 5) SWCNT and all carbon chain inserted (5, 5) SWCNT. The zigzag (8, 0) and (9, 0) HCSs have small change in the energy gap compared to the corresponding pristine ones. Due to the downshift of conduction bands originating from the carbon chain, the calculation of band structure shows that chiral (6, 4) HCS is a semiconductor system with a small band gap of 0.94 eV, less than 1.125 eV in pristine SWCNT. The studied HCSs with unique structure and electronic property may construct a new generation nanoscale junctions without the usual heptagon–pentagon defect pair considerations.     

Adsorption of transition-metal atoms on boron nitride nanotube: a density-functional study

Adsorption of transition atoms on a (8,0) zigzag single-walled boron nitride (BN) nanotube has been investigated using density-functional theory methods. Main focuses have been placed on configurations corresponding to the located minima of the adsorbates, the corresponding binding energies, and the modified electronic properties of the BN nanotubes due to the adsorbates. We have systemically studied a series of metal adsorbates including all 3d transition-metal elements (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) and two group-VIIIA transition-metal elements (Pd and Pt). We found that many transition-metal atoms can be chemically adsorbed on the outer surface of the BN nanotubes and that the adsorption process is typically exothermic. Upon adsorption, the binding energies of the Sc, Ti, Ni, Pd, and Pt atoms are relatively high (>1.0 eV), while those of V, Fe, and Co atoms are modest, ranging from 0.62 to 0.92 eV. Mn atom forms a weak bond with the BN nanotube, while Zn atom cannot be chemically adsorbed on the BN nanotube. In most cases, the adsorption of transition-metal atoms can induce certain impurity states within the band gap of the pristine BN nanotube, thereby reducing the band gap. Most metal-adsorbed BN nanotubes exhibit nonzero magnetic moments, contributed largely by the transition-metal atoms.     

Self-consistent electronic band structures of extended polydiacetylene chains with realistic model side groups

Self-consistent semi-empirical band structure calculations for isolated extended polydiacetylene chains with a variety of realistic model side groups have been performed. For side groups with a CH₂ group next to the chain backbone the predicted band gap Δ ≈ 0.5 eV is substantially independent of the detailed structure of the side group and much smaller than the optical absorption thresholds experimentally observed. These results support the hypothesis that the low energy electronic excitations of these systems are better described by an excitonic, rather than an electronic, band model.     

Behavior of carriers in δ-doped quantum wells under in-plane magnetic fields

Self-consistent electronic structure calculations of δ-doped quantum wells (QW) in the presence of in-plane magnetic fields B up to 20 Tesla are carried out within the frameworks of the effective mass and the local density approximations. QWs composed of two layers of Ga_1-xA1_xAs, separated by a layer of GaAs with a donor δ-doped sheet in the center, are considered. The width of the GaAs layer was varied from 100 to 400 Å. It is shown that the diamagnetic shift increases with the increasing of the GaAs QW width. The magnetic field induces remarkable changes in the energy dispersions of electrons and holes, along an in-plane direction perpendicular to B. The most striking effect occurs in the nature of the band gap of these systems. We found that the valence band displays a double-maximum character instead of a single maximum at the center of the Brillouin zone. © 1996 John Wiley & Sons, Inc.     

Investigation of the silicon concentration effect on Si-doped anatase TiO2 by first-principles calculation

A first-principles calculation based on the density functional theory (DFT) was used to investigate the energetic and electronic properties of Si-doped anatase TiO₂ with various silicon concentrations. The theoretical calculations showed that with Si-doping the valence band and conduction band of TiO₂ became hybrid ones with large dispersion, which could benefit the mobility of the photo-generated carriers. This result is in agreement with the experimental reports. At lower doping levels, the band gap of Si-doped anatase TiO₂ decreases about 0.2 eV. With the increase of silicon concentration, the band gap increases gradually and larger formation energies are required during the synthesis of Si-doped TiO₂.     

Effective band gap reduction of titanium oxide semiconductors by codoping from first‐principles calculations

Doping is an efficient approach to narrow the band gap of TiO₂ and enhance its photocatalytic activity. Here, we perform generalized gradient approximation (GGA)+U calculations to narrow the band gap of TiO₂ by codoping of X (F, N) with transition metals (TM = Fe, Co) to extend the absorption edge to longer visible‐light wavelengths. Our results show that all the codoped systems can narrow the band gap significantly, in particular, (F+Fe)‐codoped system could serve as remarkably better photocatalysts with both narrowing of the band gap and relatively smaller formation energies than those of (F+Co) and (N+TM)‐codoped systems. Our results provide useful guidance for codoped TiO₂ efficient for photocatalytic activity. © 2013 Wiley Periodicals, Inc.     

The electronic structures of vanadate salts: Cation substitution as a tool for band gap manipulation

The electronic structures of six ternary metal oxides containing isolated vanadate ions, Ba₃(VO₄)₂, Pb₃(VO₄)₂, YVO₄, BiVO₄, CeVO₄ and Ag₃VO₄ were studied using diffuse reflectance spectroscopy and electronic structure calculations. While the electronic structure near the Fermi level originates largely from the molecular orbitals of the vanadate ion, both experiment and theory show that the cation can strongly influence these electronic states. The observation that Ba₃(VO₄)₂ and YVO₄ have similar band gaps, both 3.8 eV, shows that cations with a noble gas configuration have little impact on the electronic structure. Band structure calculations support this hypothesis. In Pb₃(VO₄)₂ and BiVO₄ the band gap is reduced by 0.9-1.0 eV through interactions of (a) the filled cation 6s orbitals with nonbonding O 2p states at the top of the valence band, and (b) overlap of empty 6p orbitals with antibonding V 3d-O 2p states at the bottom of the conduction band. In Ag₃VO₄ mixing between filled Ag 4d and O 2p states destabilizes states at the top of the valence band leading to a large decrease in the band gap (E_g=2.2 eV). In CeVO₄ excitations from partially filled 4f orbitals into the conduction band lower the effective band gap to 1.8 eV. In the Ce_1−xBi_xVO₄ (0≤x≤0.5) and Ce_1−xY_xVO₄ (x=0.1, 0.2) solid solutions the band gap narrows slightly when Bi³⁺ or Y³⁺ are introduced. The nonlinear response of the band gap to changes in composition is a result of the localized nature of the Ce 4f orbitals.     

Co-existence of magnetic phases in two-dimensional MXene

This study reports first synthesis of MXene-derived co-existing magnetic phases. New family of two-dimensional (2D) materials such as Ti₃C₂ namely MXene, having transition metal forming hexagonal structure with carbon atoms have attracted tremendous interest now a days. We have reported structural, optical and magnetic properties of un-doped and La-doped Ti₃C₂T_x MXene, synthesized using co-precipitation method. The lattice parameter (LP) calculated for La-MXene are a = 5.36 Å, c = 18.3 Å which are slightly different from the parent un-doped MXene (a = 5.35 Å, c = 19.2 Å), calculated from X-ray diffraction data. The doping of La⁺³ ions shrinks Ti₃C₂T_x layers perpendicular to the planes. The band gap for MXene is calculated to be 1.06 eV which is increased to 1.44 eV after doping of La⁺³ ion that shows its good semiconducting nature. The experimental results and density functional theory (DFT) calculations for magnetic properties of both the samples have been presented and discussed, indicating the co-existence of ferromagnetic-antiferromagnetic phases. The results presented here are novel and is first report on co-existence of magnetic properties of 2D carbides for potential applications in two-dimensional spintronics.     

First-principles study of structural,elastic, electronic and optical properties of perovskites XCaH3 (X = Cs and Rb) under pressure

The stability, structural parameters, elastic constants, electronic and optical properties of perovskites CsCaH₃ and RbCaH₃ were investigated by the density functional theory. The calculated lattice parameters are in agreement with previous calculation and experimental data. The energy band structures, density of states, born-effective-charge and Mulliken charge population were obtained. The perovskites CsCaH₃ and RbCaH₃ present a direct band gap of 3.15 eV and 3.27 eV at equilibrium. The top of the valence bands reflects the s electronic character for both structures. Furthermore, the absorption spectrum, refractive index, extinction coefficient, reflectivity, energy-loss spectrum, and dielectric function were calculated. The origin of the spectral peaks was interpreted based on the electronic structures. The static dielectric constant and refractive index are indeed, inverse proportional to the direct band gap.     

Catechin mediated green synthesis of Au nanoparticles: Experimental and theoretical approaches to the determination HOMO-LUMO energy gap and reactivity indexes for the (+)-epicatechin (2S, 3S)

This work suggests a green method for synthesizing Au nanoparticles (AuNPs) using the aqueous extract of Salix aegyptiaca extract. The mechanism of green synthesized AuNPs was examined by molecular electrostatic potential (MEP) calculations. AuNPs were characterized with different techniques such as Ultraviolet–visible spectroscopy (UV–vis), Fourier-transform infrared spectroscopy (FT-IR) spectroscopy, X-ray diffraction (XRD), and Transmission electron microscopy (TEM). Electrochemical investigation of modified glassy carbon electrode using AuNPs (AuNPs/GCE) shows that the electronic transmission rate between the modified electrode and [Fe (CN)₆]^3?/4? increased. Process of oxidation, energy gap, and chemical reactivity indexes of the (+)-epicatechin (2S,3S) were investigated using electrochemical techniques (cyclic voltammetry (CV) and differential pulse voltammetry (DPV) as well as UV–Visible spectroscopy and compared with quantum mechanical calculations. DPV and CV were used to obtain HOMO energies of the (+)-epicatechin (2S,3S), an optical energy gap was obtained from the UV–Vis spectroscopy. Frontier molecular orbitals analysis (FMO) and reactivity indexes such as chemical hardness (?), electrophilicity (?), electronic chemical potential (μ), electron acceptor power (?⁺), electron donor power (?^?) were determined with functional theory (DFT) calculations. In summary, the HOMO energy obtained from the experimental analyses (E_HOMO (from DPV) = -5.24 eV, and E_HOMO (from CV) = -5.28 eV) has a relative agreement with the HOMO energy calculated by B3LYP/6–31 g (d, p) including the solvent effect (water) (E_HOMO (from B3LYP) = -5.75 eV). Also, UV–Vis spectroscopy gives the bandgap energy equal to 4.31 eV, while the 4.13 eV is calculated by TD-DFT-b3lyp/6–31 + g(d).     

DFT calculation of the electronic and optical properties of Ag2PdO2 from X-ray and neutron crystallographic data

Electronic and optical properties of ternary silver palladium oxide (Ag₂PdO₂) are investigated using density functional theory. Two different possible approximations for the exchange correlation potentials were employed. The X-ray and neutron crystallographic data were optimized by minimization of the forces (1 mRy/a.u.) acting on the atoms. The electronic structure, electron space charge density, chemical bonding and optical dielectric were determined from the relaxed geometry seeking deep insight understanding of this material. Our calculated energy band gap (0.15 eV) shows a good agreement with the experimental value (0.18 eV).     

Drastic changes of electronic structure and crystal chemistry upon oxidation of SnII2TiO4E2 into SnIV2TiO6: An ab initio study

From DFT based calculations establishing energy-volume equations of state and electron localization mapping, the electronic structure and crystal chemistry changes from Sn₂TiO₄ to Sn₂TiO₆ by oxidation are rationalized; the key effect being the destabilization of divalent tin Sn^II towards tetravalent state Sn^IV leading to rutile Sn₂TiO₆ as experimentally observed. The subsequent electronic structure change is highlighted in the relative change of the electronic band gap which increases from ∼1 eV up to 2.2 eV and the 1.5 times increase of the bulk modulus assigned to the change from covalently Sn^II based compound to the more ionic Sn^IV one. Such trends are also confronted with the relevant properties of black Sn^IIO.     

Characterization and photocatalytic activity of Fe- and N-co-deposited TiO2 and first-principles study for electronic structure

Titanium dioxide (TiO₂), co-deposited with Fe and N, is first implanted with Fe by a metal plasma ion implantation (MPII) process and then annealed in N₂ atmosphere at a temperature regime of 400-600 °C. First-principle calculations show that the (Fe, N) co-deposited TiO₂ films produced additional band gap levels at the bottom of the conduction band (CB) and on the top of the valence band (VB). The (Fe, N) co-deposited TiO₂ films were effective in both prohibiting electron-hole recombination and generating additional Fe-O and N-Ti-O impurity levels for the TiO₂ band gap. The (Fe, N) co-deposited TiO₂ has a narrower band gap of 1.97 eV than Fe-implanted TiO₂ (3.14 eV) and N-doped TiO₂ (2.16 eV). A significant reduction of TiO₂ band gap energy from 3.22 to 1.97 eV was achieved, which resulted in the extension of photocatalytic activity of TiO₂ from UV to Vis regime. The photocatalytic activity and removal rate were approximately two-fold higher than that of the Fe-implanted TiO₂ under visible light irradiation.