Complex Surface Chemistry of an Otherwise Inert Solvent Molecule: Tetrahydrofuran on Si(001)

The reaction of tetrahydrofuran (THF), an otherwise inert solvent molecule, on Si(001) was experimentally studied in ultra‐high vacuum. Using scanning tunneling microscopy (STM) and photoelectron spectroscopy at variable temperature, we could both isolate a datively bound intermediate state of THF on Si(001), as well as the final configuration that bridges two dimer rows of the Si(001) surface after ether cleavage. The latter configuration implies splitting of the O?C bond, which is typically kinetically suppressed. THF thus exhibits a hitherto unknown reactivity on Si(001).     

Electronic and optical properties of Sr3X2 (X=N,P, As,Sb and Bi) compounds: first principles study

ABSTRACT

Direct bandgap semiconductors are very essential to fulfil the demand for the advancement in optoelectronic devices. Therefore it is important to predict new potential candidates having such unique features. In current work, Sr₃X₂ (X=N, P, As, Sb and Bi) compounds have been reported for the first time by well trusted FP-APW+lo method. For the better prediction of the energy band gap, mBJ is used alongwith routine generalised gradient approximation (GGA). The results show small and direct energy band gaps at Γ-Γ symmetry points with magnitude in the range from 0.62?eV (Sr₃P₂) to zero energy band gap (Sr₃Bi₂). In partial density of state Sr-d state and X-p state are contributed in the band structure. The compounds show mostly covalent bonding nature. The frequecy dependent optical properties in the linear optical range are also investigated.     

The effect of replacing pnictogen elements on the physical properties of SrMg_2X_2(X=N,P,As,Sb,Bi) Zintl compounds

The effect of replacing the anion from N to Bi down the group in the periodic table is investigated on SrMg_2X_2(X = N,P,As,Sb,Bi).A full potential linearized augmented plane wave plus local orbitals method is used along with different exchange–correlation potentials to obtain the lattice constants,phonons,electronic,and optical properties of the Sr Mg_2X_2(X = N,P,As,Sb,Bi) Zintl compounds.A good agreement is achieved and our calculations are validated by previous experimental and theoretical data.All compounds have shown stable dynamical behavior with gamma centered longitudinal response having no imaginary frequencies.Electronic band structures reveal the semiconducting nature of the compounds.The Pnictogen(X)-p state contributed mainly in the valence band and the Sr-d state forms the conduction of the compounds.Relative charge transfer and low overlapping of the atomic densities indicates the preferable ionic bonding character of these materials.In the optical properties,real and imaginary parts of dielectric function,complex refractive index,birefringence,reflectivity,and optical conductivity are calculated.These compounds can be utilized in the optical and optoelectronic devices.     

Tight-binding molecular dynamics simulation of ZnSe liquid within the local environment dependence

We investigate the structural, electronic and dynamical properties of ZnSe liquid using tight-binding molecular dynamics (TBMD) simulations. We report the TBMD calculations for the solid and liquid forms of the ZnSe compound. To produce more realistic results the TB model includes the local environment dependence in the Hamiltonian matrix at finite temperature for ZnSe. To further demonstrate the efficiency of the TBMD approach, we present results for finite temperature physical properties of ZnSe liquid. We are able to show good agreement with experiment for the atomic mean-squared displacement and melting point.     

Mechanism of linear and nonlinear optical properties of the urea crystal family

First-principles calculations of the second-order optical response functions and the dielectric functions of urea [CO(NH(2))(2)] and some of its derivatives such as monomethylurea (H(2)NCONHCH(3), MMU), and N,N'-dimethylurea (H(3)CHNCONHCH(3), DMU) crystals are performed. On the basis of the density functional theory (DFT) in the local-density approximation (LDA), the highly accurate full-potential projected augmented wave (FP-PAW) method was used to obtain the electronic structure. Over a wide frequency range (0.0-10.0 eV), the dielectric constants and second-harmonic generation (SHG) susceptibilities of the urea crystal family have been obtained, and the results are in good agreement with the experimental values. The origin of the linear and nonlinear optical (NLO) properties of the urea crystal family has been analyzed by coupling the calculated electronic structure and optical spectrum. The prominent spectra of χ((2)) are successfully correlated with the dielectric function ε(ω) in terms of single-photon and double-photon resonances. The virtual electron (VE) and virtual hole (VH) processes have also been performed for the urea crystal family. From the research into the electron deformation density, crystal configuration, substitutional group, and so forth, it is found that the origin of the SHG of the urea crystal family is the charge transfer due to the strong "(?)push-pull" effect along the hydrogen bond, which favors a head-to-tail arrangement of the molecules and enhances the SHG response. The electron-donating substitutional group supplies more electrons to the electron-accepting group, and helps to form large dipoles in molecules. The influence on the NLO properties of the local symmetry of the substitutional group is also discussed in detail.     

Magnetic and thermoelectric properties of REMoN3 (RE = La,Ce, Pr,Nd, and Sm): A first-principles study

We investigated the magnetic and thermoelectric properties of REMoN₃ (RE = La, Ce, Pr, Nd, Sm) perovskites using the full potential linearized augmented plane wave (FP-LAPW) method. To overcome the problem of underestimation of electronic interaction, we employed the DFT + U approach to accurately map the electronic structure of these compounds. Our study shows an increasing trend in the magnetic moments with the increasing number of unpaired electrons in RE. Among these compounds, SmMoN₃ possesses a large magnetic moment, which is suitable for applications such as memory devices and sensors. Interestingly, all these perovskites display ferromagnetic behavior except CeMoN₃, which exhibits an antiferromagnetic nature. Furthermore, our analysis indicates n-type thermoelectric behavior in all these materials. The compound, namely PrMoN₃, exhibits a high figure of merit among REMoN₃, which can be improved by modifying the lattice sites.     

Imposing changes of band and spin–orbit gaps in GaNBi

We present first-principles pseudopotential plane-wave calculations to explore the effects of alloying of non conventional III–V compound GaN with bismuth. We found a highly nonlinear reduction of the energy gap of GaN for small Bi composition. Consequently the optical band gap bowing is found extremely important and composition dependent. The stronger contribution is due principally to structural and, to less extent, to charge transfer effects. Moreover, because of strong relativistic effects caused by bismuth, we found a giant bowing for the spin–orbit splitting energy of valence band, by far the largest of any III–V ternary alloys.     

Investigations of physical aspects of spinel ABi2O4 (A=Zn,Cd) oxides via ab-initio calculations

This present study aims to unravel physical properties of spinel oxides in cubic phase represented by general empirical formula of ABi₂O₄ (A=Zn, Cd) using full potential lineralized augmented plane plus local orbital (LAPW+lo) method. The structural aspect of spinel oxides in cubic phase has been investigated by Perdew-Burke-Ernzerhof generalized gradient approximation (PBEsol-GGA). The thermodynamic stability of ABi₂O₄ is established by computing the cohesive energies. On the other hand, born stability standard which is based on Chapin's tensor analysis method revealed that the mechanical stability of the oxide compounds under considerations gives a stable cubic phase. For a better prediction of electronic and thermoelectric properties of these compounds, the modified Tran-Bhala Becke and Johnson (TB-mBJ) potential is used which revels very precise band gap. Other than these investigations, thermoelectric characteristics i.e., Wiedman-Tranz constant, Seebeck coefficient and power factor are calculated and it provides essential data for fabrication of thermoelectric devices.     

Nature of adhesion of condensed organic films on platinum by first-principles simulations

Understanding the nature of the adhesion of an organic liquid on a metal surface is of paramount importance for elucidating the stability and chemical reactivity at these complex interfaces. However, to date, the morphology, layering and chemical properties at organic liquid metal interfaces have been rarely known. Using semi-empirical dispersion corrected density functional theory calculations and ab initio molecular dynamics simulations, we show that carbon tetrachloride and ethanol films confined to a platinum surface alter their intrinsic properties and exhibit interfacial reactivity. A few interface carbon tetrachloride (ethanol) molecules adsorb dissociatively (molecularly) on platinum thanks to the surrounding medium. The adsorption strength of the interfacial molecules is consequently increased in the condensed phase as compared to the gas phase. This remarkable effect is rationalized by an interaction energy decomposition model and an electrostatic potential analysis.