Narrow Results

Favored structures of SiO_n monolayers on Mo(112) surface have been studied using the total energy minimization technique based on DFT semirelativistic approach. In the [SiO₄] complexes, which form the c(2 × 2) silica structure on the Mo(112), the bonding of the Si atoms with the surface is accomplished through the oxygen atoms. The structure with a symmetric position of oxygen atoms has been found to be the most favorable. In this structure, two oxygen atoms occupy bridge-on-row sites on the Mo(112) surface, with Si atoms between them, while oxygen atoms in the troughs appear not in expected threefold sites, but adjust their positions along the middle lines of the troughs. Estimated main phonon frequency and density of states for the symmetric [SiO₄] structure agree well with experimental results.

We have performed Density Functional Theory calculations in the Generalized Gradients Approximation for the (001) surface of the intermetallic compound TiFe. We have focused on the interplay between spin polarization and surface relaxations, and the influence of the chemical species at the surface. We found that the surface shows a magnetic ordering. The magnetic moment at the surface layer depends strongly on the surface termination, being much larger for the case of Fe than Ti termination (3.11 and 0.29 μ_B/atom, respectively). The magnetic moments show an alternating behavior with a slow decaying as we go inside the material. On the other hand, the modification of the atomic positions due to the surface relaxation results in a very small influence on the magnetic moment with respect to the ideal, nonrelaxed configuration.

A systematic study of unrelaxed and relaxed surface characters on the TiO₂ (101) surface has been carried out by first-principles calculations using plane-wave pseudopotential method. We find that O_2c atoms have an inward relaxation of 0.012 Å and Ti_5c atoms have an outward relaxation of 0.155 Å by taking a 24-layer slab with 5 Å vacuum width to consider the atomic relaxations, in good agreement with other theoretical values. The slab thickness has significant effect on the quality of band structure and density of states, and 24-layer slab is sufficient to present the electronic properties of TiO₂ (101) surface. Atomic relaxations result in a large transfer of surface charges from outermost layer to inner layer, and the surface bonds have a rehybridization, which makes the ionization reduce and the covalence increase; we believe that it causes the surface bond shorten. A fine analysis of band structure and density of states of the TiO₂ (101) surface shows that the surface relaxation induces the transformation from semi-metallic to semiconducting characteristic.

In calculating band structure, the local density approximation and density functional theory are widely popular and do reproduce a lot of the basic physics. Regrettably, without some fine tuning, the local density approximation and density functional theory do not generally get the details of the experimental band structure correct, in particular the band gap in semiconductors and insulators is generally found to be too small when compared with experiment. For experimentalists using commercial packages to calculate the electronic structure of materials, some caution is indicated, as some long-standing problems exist with the local density approximation and density functional theory.

The binding energies and related energies of associative desorption for oxygen on close-packed (W(110), Mo(110), and Ru(0001)) and furrowed (W(112), Mo(112), and ) surfaces have been calculated by DFT method with generalized gradient approximation for exchange-correlation potential in the revised-Perdew–Burke–Ernzerhof form. The unified approach allows one for a direct comparison of calculated binding energies for different transition metals and different surface geometries, thus revealing the trends that are essential for catalytic properties of surfaces with adsorbed oxygen layers.

The co-adsorption of carbon monoxide and benzene on Co(0001) has been studied using density functional calculations. We used the ordered surface unit cell. A comparison of the co-adsorption with CO and benzene two-dimensional networks is also given. The electronic structure reveals that the CO orbitals interact with benzene and Co layer. Regarding the bonding, the Co–Co overlap population decrease 18% after benzene adsorption and increase a little after CO adsorption with a net 14.6% decrease in the co-adsorption system. The CO–benzene interaction is shown by the changes in the C–O (CO) and C–H (benzene) bonds.

In this work, the p-type doping of the S_A type stepped Ge(100) surface by a diborane (B₂H₆) gas flow has been simulated by the possible dissociation and adsorption models. The most probable dissociation model of B₂H₆ and adsorption models of the fragments of B₂H₆ on the stepped Ge(100) surface have been determined by the local minimum total energy and/or binding energy calculations based on the Density functional (B3LYP/6-3g) and Hartree–Fock (HF/STO-3g) theories, respectively. The present calculations have shown that, the step region (for both up and down terraces) of the stepped Ge(100) surface has the most attractive sites for BH₃ molecules determined to be the first dissociation fragments of B₂H₆ by an external energy of ~ 1.3 eV. It has been found that, at the first step of the adsorption, BH₃ can dissociate to BH₂ and BH fragments on the stepped Ge(100) surface. While BH₃ and BH₂ products prefer to be attached to a single surface Ge atom, BH is bridged between two adjacent surface Ge atoms. According to the present optimization calculations, the p-type doping process of the stepped Ge(100) surface has started with the adsorption of BH₃ on the electron deficient site (buckled down) of the Ge dimer bond close to the step edge and ended with the substitutional occupation of the Ge site in the layers of the surface by B atom. The beginning of the p-type doping of the stepped Ge(100) surface has been illustrated by the electronic states of B appeared in the optical energy gap of Ge very close to the edge of the HOMO.

The interaction between the H₂ molecule and the PdAg, PdAu, PtAg and PtAu bimetallic dimers deposited on the MgO(100) surface is investigated using density functional theory (DFT). The bimetallic dimers, whose molecular axes are considered to be perpendicular to the support surface, are adsorbed on top of an oxygen atom. Within this adsorption mode, the dimers prefer the orientation in which their Pd or Pt end is closer to the oxygen atom. The Ag and Au ends of the MgO-supported dimers capture the H₂ molecule with small exoenergetic effects. The spontaneous dissociation of H₂ on these ends does not occur. Thus, the MgO support decreases the ability of the dimers to adsorb and dissociate the H₂ molecule. From a catalytic viewpoint, it means that the activity of small bimetallic clusters toward the dissociative adsorption of H₂ is reduced when they are arranged on MgO. On the other hand, the results of our calculations show that the presence of the MgO support strengthens the binding of H atoms inside the PdAu, PtAg and PtAu dimers.

The structural and electronic properties of BN_xAs_1-x alloys have been investigated in the total range of nitrogen by the FP-LAPW method based on DFT within the EV-PW-GGA scheme. The equilibrium lattice constants, bulk moduli, first-order pressure derivatives of the bulk moduli, and cohesive energies have been obtained by total energy calculations of the alloys after both volume and geometry optimizations. The large bowing parameters found for the lattice constants and bulk moduli have demonstrated that the validity of Vegard's linear rule in the definitions of these structural features of the BN_xAs_1-x alloys is broken. The energy bands and the effective masses of the alloys have been calculated as a function of nitrogen concentration. The large bowing displayed by the variation of the energy gaps has indicated the band gap engineering capacity of the BN_xAs_1-x alloys and again in deviations from Vegard's linear rule. The effective electron masses calculated either at the edges of the conduction bands or along the directions approaching the edges of the conduction bands are all found to be small with respect to the effective electron masses in the BAs and BN compounds calculated at the Δ_min and X points, respectively.

This study presents an ab initio investigation of the interaction of Al₅H₁₂ with the alloying transition elements (TM) in the TMAl₅H₁₂ (TM = Ti, V, Fe, Co and Ni). Hydrogen atoms are found to prefer the tetrahedral sites in the fcc Al bulk system with a binding energy that is more energetically favored than octahedral sites by 0.17 eV. Absorbed H atoms in Al increase the lattice constant and decrease the cohesive energy by 13.8% and 32.7%, respectively. The present values of the H–H bond length (0.75 Å) and the dissociation energy of H₂ (4.47 eV) are in good agreement with the previously measured values. The binding energy of H atoms in TiAl₅H₁₂ system is higher than the other studied systems due to the hybridization between s-state of H and d-state of Ti atom. The magnetic moments of Fe in FeAl₅H₁₂ system and Ni in NiAl₅H₁₂ system are enhanced as compared to the dehydrogenated system.

Molecular hydrogen adsorption on MOF-210 was evaluated at the density functional theory level. The most stable H₂ adsorption occurs near the acetenyls in the organic linker, but its binding energy (0.113 eV) is not sufficient to satisfy the minimum value (0.24 eV) required for practical applications. Meanwhile, Li cation-decorated MOF-210 has the average hydrogen adsorption energies of 0.28 eV, and its saturated hydrogen storage capacity reaches 5.35 wt.%.

Effect of light alkali metal (Li and Na) decorated on the C₅₉B heterofullerene for hydrogen storage is considered using DFT-MPW1PW91 method. Results show that Li and Na atoms strongly prefer to adsorb on top of five-member and six-member ring where a carbon atom is replaced by a boron atom. Significant charge transfer from the alkali metal to the C₅₉B compensates for the electron deficiency of C₅₉B and makes the latter aromatic in nature. Corrected binding energies of hydrogen molecule on the alkali-doped C₅₉B using counterpoise method, structural properties and NBO analysis indicate that first hydrogen molecule is adsorbed physically and does not support minimal conditions of DOE requirement. Finally, positive values of binding energies for the adsorption of a second hydrogen molecule show that alkali doped C₅₉B are capable of storing a maximum of one hydrogen molecule.

Interaction of CO and NO molecules by different orientations on (BN)_n=3-5 clusters have been studied at the B3LYP/6-311+G* level of theory. Total electronic energies have been corrected for geometrical counterpoise (gCP) and dispersion (D3) energies at the B3LYP/6-31G* level. Formation of a new sigma bond between the gas and (BN)₃ cluster, atom in molecules (AIM) results, density of states spectrums (DOS), molecular electrostatic potential (MEP) surfaces, and visualization of wave function of molecular orbitals in the nearest bonding regions to the Fermi level have confirmed that adsorption of CO by carbon end atom, and NO by nitrogen end atom is covalent in nature, so that the charge transfer is occurred from gas molecule to the cluster.

The electronic band structure, density of states (DOS) and interlayer interaction in Li-intercalated graphene bilayers are studied by means of density functional theory (DFT) calculations. It has been found that for a pristine bilayer, the relative shift of graphene layers from AB stacking configuration, pertinent to a bulk graphite, to AA configuration results in the opening of the bandgap at Fermi level, so that the bilayer becomes a semiconductor. The Li intercalation of the graphene bilayer significantly increases the density of states at Fermi level, which can be considered as an increased metallicity. The electronic density in the space between graphene layers also substantially increases and leads to related increase of the interlayer interaction. We hope that the obtained results of calculations will be useful for various applications of Li-intercalated graphene layers in nanoelectronics.

In the present work, we apply wurtzoids nanocrystals with density functional theory to explain the sensitivity of ZnO nanostructures towards hydrogen and oxygen molecules. Present results of ZnO nanocrystals’ sensing to H₂ and O₂ molecules show a reduction in the energy gap and hence electrical resistivity of ZnO nanocrystals upon attachments of these molecules in agreement with experiment. The results also show that higher temperatures increase the sensitivity of ZnO wurtzoids towards H₂ and O₂ molecules with the maximum sensitivity approximately at 390${}^{\circ }$C and 417${}^{\circ }$C for H₂ and O₂ molecules, respectively, after which it begins to decline according to calculated Gibbs free energy. These temperatures are comparable with experimentally reported operating temperatures of 325${}^{\circ }$C and 350${}^{\circ }$C for the two gases, respectively. The main reaction mechanism is the dissociation of H₂ or O₂ molecules on ZnO nanocrystal surface in which hydrogen and oxygen atoms are attached to neighboring Zn and O surface atoms. The removal of these molecules from the surface is also performed by the formation of H₂ and O₂ molecules prior to their removal from the ZnO nanocrystal surface. Electronic charge transfers to the adsorbed atoms and molecules confirm and illustrate the mechanism mentioned above.

Comparison of structural and electronic properties between pristine and N-doped titanium dioxide-(TiO₂)/molybdenum disulfide (MoS₂) nanocomposites and their effects on the adsorption of thiophene molecule were performed using density functional theory calculations. To correctly estimate the adsorption energies, the van der Waals interactions were taken into account in the calculations. On the TiO₂/MoS₂ nanocomposite, thiophene molecule tends to be strongly adsorbed by its sulfur atom. The five-fold coordinated titanium atom of TiO₂ was found to be an active binding site for thiophene adsorption. The results suggest that the thiophene molecule has not any mutual interaction with MoS₂ nanosheet. The electronic structures of the complex systems are discussed in terms of the density of states and molecular orbitals of the thiophene molecules adsorbed to the TiO₂/MoS₂ nanocomposites. It was also found that the doping of nitrogen atom is conductive to the interaction of thiophene with nanocomposite. Thus, it can be concluded that the interaction of thiophene with N-doped TiO₂/MoS₂ nanocomposite is more energetically favorable than the interaction with undoped nanocomposite. The sensing capability of TiO₂/MoS₂ toward thiophene detection was greatly increased upon nitrogen doping. These processes ultimately lead to the strong adsorption of thiophene on the N-doped TiO₂/MoS₂ nanocomposites, indicating potential applicability of these nanocomposites as novel gas sensors.

Tin Oxide has been explored for gas sensing and humidity sensing. In the present work, pure and copper-doped SnO₂ is synthesized by controlled spray pyrolysis technique. The films are homogeneous throughout. Rietveld analysis confirms the absence of other phases due to doping. The optical property is studied using UV-Visible spectroscopy which shows a change in the band gap with the introduction of the dopant. An elaborate impedance analysis is carried which showed the effect of doping. Cu-doped thin film showed a faster drop in the impedance when exposed to humidity. Significant change in the cole–cole plot is observed indicating better sensitivity with the doped sample in comparison to undoped. Higher humidity level from 80 to 92 RH is studied as it is important to detect the same in air conditioning systems, electronic devices, aviation systems and food processing systems. The equivalent circuit also reveals that the adsorption of water molecules on the surface of the thin films changes the impedance characteristics. Levenberg–Marquardt algorithm is employed for the theoretical calculations and identifying the equivalent circuit. The charge transfer in the doped sample is subjected to lesser grain resistance according to theoretical calculation and experimental results. A density functional approach is employed to study the band structure and explain the influence of Cu doping on pure SnO₂. The calculated result supports the use of Cu as a dopant for better humidity sensing device.

With a hexagonal (honeycomb) network of mono-layered carbon atoms, graphene has demonstrated outstanding electronic properties. This work describes the impact of deliberately introduced single, double and triple carbon vacancies in grapheme monolayer. In addition, these carbon vacancies were then substituted with gold atoms and their influence on the electronic properties of the two-dimensional (2D) graphene layers was investigated. In this regard, a first principle calculation was performed to examine electronic properties and formation energy of 2D graphene layer by applying density functional theory (DFT). Introduction of such defects appeared to increase the stability of the graphene sheet as confirmed by formation energy calculations. Moreover, decrease of formation energy was noticed to be significant with an increase in the number of defects. Band structure calculations described the shifting of localized states from valance to conduction bands which caused the transformation of semiconducting behavior into metallic one on the filling of carbon vacancies by gold atoms. Comparing this behavior with that of partial density of states (PDOS) it was noted that a lot of states existed in the valance band in the case of C-vacancies yielding charge free region around the vacancy. On the other hand, filling of C-vacancies by gold generated a large number of energy states in the conduction band illustrating the accumulation of charges near gold atom. Width of the peak across the Fermi level indicated the accumulative energy of electron to be almost 0.15eV. These calculated DOS and PDOS demonstrated metallic like behavior of the graphene monolayer with typical defect states.

The gas response of metal oxide-based sensors depends strongly on its adsorption properties. To explore the potential sensing capability of pristine and nitrogen modified TiO₂/graphene oxide (GO) heterostructures, the adsorption of NO₂ molecule on the N-doped nanocomposites was investigated using density functional theory (DFT) calculations. Six possible configurations were simulated based on the estimated adsorption energies. The binding sites were located over the oxygen, doped nitrogen and five-fold coordinated titanium atoms of TiO₂. The electronic properties including atomic Mulliken population, projected density of states and molecular orbitals were investigated in detail. The N–O bonds of the NO₂ molecule were significantly increased after the adsorption process. The adsorption of NO₂ molecule on the N-doped nanocomposite is more energetically favorable than the adsorption on the undoped one. The results suggest that NO₂ chemisorbs on the considered nanocomposites. Mulliken population analysis reveals a noticeable charge transfer from the nanocomposite to the molecule, which indicate that NO₂ acts as a charge acceptor. Molecular orbital calculations show that the highest occupied molecular orbitals (HOMOs) of the studied systems were mainly localized on the adsorbed NO₂ molecule. The significant overlaps in the projected density of states (PDOS) spectra of the interacting atoms confirm the formation of chemical bonds between them. There is a direct relationship between the results of charge transfer and sensing responses. N-doped nanocomposites have better sensing response than the undoped ones. The results highlight the possibility to develop innovative highly efficient NO₂ sensors based on novel TiO₂/GO nanocomposites.

A theoretical study was carried out of CO₂ adsorption on Cu_mCo_n(2$\le m+n\le$7) clusters using density functional method. Generally CO₂ are located at top or bridge sites, while CO₂ of Cu₂Co₄CO₂, Co₆CO₂ and Cu₂Co₅CO₂ clusters are absorbed at hollow sites. Co₃CO₂ and CuCo₂CO₂ clusters are more stable than their neighbors, while Cu₂CO₂ and Cu₆CO₂ clusters display stronger chemical stability. After adsorption, CO₂ is activated with the elongation of the C–O bond owing to electrons transfer from Cu–Co clusters to 2${\pi }_u$ anti-bonding orbit of CO₂. More charge transfer often corresponds to longer C–O bond and larger adsorption energy, and the chemical activity is stronger correspondingly. Magnetic and electronic properties are also discussed.