Pseudopotential Density Functional Theory Calculations Research Articles

Understanding Acetaldehyde Thermal Chemistry on the TiO2 (110) Rutile Surface: From Adsorption to Reactivity

Plane-wave pseudopotential density functional theory calculations are used to investigate the details of acetaldehyde adsorption and thermal chemistry on the TiO2 (110) rutile surface. The adsorption properties of a single acetaldehyde molecule on models of stoichimetric, reduced, and oxidized TiO2 surfaces are first examined. The experimentally observed shift of the temperature-programmed desorption (TPD) maximum on increasing coverage is rationalized on the basis of the adsorbate to surface bonding mechanism. Examination of the thermal chemistry of acetaldehyde on models of the TiO2 (110) surface allows one to understand the diffusion of acetaldehyde molecules on the surface and the formation of butene, as is experimentally observed. This reaction takes place through a noncatalytic process where the bridge oxygen vacancies play a fundamental role. The molecular details of the process allow one to explain the low amount of butene found in the experiment. The study of the thermal chemistry of acetaldehyde...

Minerals from Macedonia. XXVII: Theoretical and experimental study of the vibrational spectra of endemic nežilovite

Endemic mineral nežilovite was studied by vibrational spectroscopy (IR and Raman), quantum theoretical methods and X-ray powder diffraction. Experimental Raman as well as the higher-frequency region of the infrared spectra of this mineral closely resembles to the corresponding spectra of magnetoplumbite-group minerals, while the appearance of the lower-frequency part of the IR spectra is markedly similar to the corresponding region in Ba-ferrite. Essentially, all bands in the experimental spectra were tentatively assigned. In most cases, the assignments were either supported or implied by the theoretical data obtained on the basis of plane-wave pseudopotential density functional theory (DFT) calculations.

First-Principle Study of Hydroxyl Functional Groups on Pristine, Defected Graphene, and Graphene Epoxide

Oxygen-containing functional groups can be present in considerable amount intentionally or unintentionally on graphene, and a complete reduction of graphene oxide is difficult to achieve. To address the origin of this behavior, we have performed pseudopotential density functional theory calculations to investigate in particular the adsorption of hydroxyl (OH) on perfect and defected graphene, individually and in the presence of other coadsorbed functional groups. We found that hydroxyl groups weakly adsorb on perfect graphene, easily aggregate, also with coadsorbed epoxy groups, and can react with each other with a barrier of about 0.5 eV forming water. Defect sites are more reactive for OH adsorption but play different roles. At variance with single vacancy defects where the OH adsorption is highly dissociative, Stone−Wales defects could stabilize the hydroxyl groups on the graphene basal plane, with a much stronger binding and higher barriers for recombination and water formation than pristine.

On the Origin of the Visible‐Light Activity of Titanium Dioxide Doped with Carbonate Species

Plane-wave-based pseudopotential density functional theory (DFT) calculations are used to elucidate the origin of the high photocatalytic efficiency of carbonate-doped TiO(2). Two geometrically possible doping positions are considered, including interstitial and substitutional carbon atoms on Ti sites. From the optical absorption properties calculations, we believe that the formation of carbonates after doping with interstitial carbon atoms is crucial, whereas the contribution from the cationic doping on Ti sites is negligible. The carbonate species doped TiO(2) exhibits excellent absorption in the visible-light region of 400-800 nm, in good agreement with experimental observations. Electronic structure analysis shows that the carbonate species introduce an impurity state from Ti 3d below the conduction band. Excitations from the impurity state to the conduction band may be responsible for the high visible-light activity of the carbon doped TiO(2) materials.

Role of Coverage and Surface Oxidation Degree in the Adsorption of Acetone on TiO2(110). A Density Functional Study

A systematic study of the adsorption of acetone on a TiO2 (110) surface by means of plane-wave pseudopotential density-functional theory calculations is presented. The relationship between the binding energy of acetone and the degree of surface coverage (up to 0.5 ML) is established on the basis of a partial reduction of the surface by the incoming acetone molecules. This allows one to explain the observed displacement toward lower temperature of the temperature-programmed desorption (TPD) maximum with increasing acetone loading. Adsorption at the bridge oxygen vacancies on supercell models of the reduced surface shows similar properties to that of regular Ti channel sites, in agreement with experimental data that do not indicate a preferential binding at vacancy sites. A high dependency between the degree of reduction of the surface and the acetone binding energies is found, implying that surface models with an excessive mean number of vacancies per unit cell may result in altered properties of this surface. Models of the oxidized TiO2 (110) surface show that coadsorbed oxygen stabilizes acetone via the formation of acetone/O-adatom complexes that, in contrast to acetone, present a barrier for desorption. This barrier allows one to explain the high temperature feature observed in the TPD spectrum for acetone desorption from the oxidized TiO2 (110) surface.

Endohedral nitrogen storage in carbon fullerene structures: Physisorption to chemisorption transition with increasing gas pressure

We present extensive pseudopotential density functional theory (DFT) calculations in order to analyze the structural properties and chemical reactivity of nitrogen molecules confined in spheroidal (C(82)) and tubelike (C(110)) carbon fullerene structures. For a small number of encapsulated nitrogens, the N(2) species exist in a nonbonded state within the cavities and form well defined molecular conformations such as linear chains, zigzag arrays, as well as both spheroidal and tubular configurations. However, with increasing the number of stored molecules, the interaction among the confined nitrogens as well as between the N(2) species and the fullerene wall is not always mainly repulsive. Actually, at high densities of the encapsulated gas, we found both adsorption of N(2) to the inner carbon surface together with the formation of (N(2))(m) molecular clusters. Total energy DFT calculations reveal that the shape of the interaction potential of a test molecule moving within the carbon cavities strongly varies with the number and proximity of the coadsorbed N(2) from being purely repulsive to having short-range attractive contributions close to the inner wall. In particular, the latter are always found when a group of closely spaced nitrogens is located near the carbon cage (a fact that will naturally occur at high densities of the encapsulated gas), inducing the formation of covalent bonds between the N(2) and the fullerene network. Interestingly, in some cases, the previous nitrogen adsorption to the inner surface is reversible by reducing the gas pressure. The calculated average density of states of our considered carbon compounds reveals the appearance of well defined features that clearly reflect the occurring structural changes and modifications in the adsorption properties in the systems. Our results clearly underline the crucial role played by confinement effects on the reactivity of our endohedral compounds, define this kind of materials as nonideal nanocontainers for high density nitrogen storage applications, and must be taken into account when analyzing the diffusion properties of the encapsulated species.

Atomic and Electronic Structure at Au/CdSe Interfaces

By means of plane-wave pseudopotential periodic-supercell density functional theory calculations with a gradient-corrected exchange-correlation functional, we investigated the formation and the electronic structure of thin Au overlayers on CdSe(0001) and CdSe(0001) surfaces. We explored several possible Au/CdSe interfaces, including nonstoichiometric cases in which the very interface layer is mixed, namely, contains atoms of both the metal and the semiconductor. The relative formation energies of the computed model structures indicate that the formation of a very thin Au layer on CdSe surfaces can be epitaxial in the very early deposition stages but only in rather Au-rich conditions. The analysis of the band structures, densities of states, and wave functions for the low-energy interfaces reveals that hybridization occurs between the metal and the semiconductor electron states. This hybridization is confined at the very interface and is not expected to have significant consequences on the plasmonic and excitonic excitations that are appealing for nanotechnology applications of metal-semiconductor nanoparticles.

The dissociative adsorption of silane and disilane on Si(100)-(2×1)

We investigate the dissociative adsorption of silane and disilane on Si(100)-(2 x 1) using pseudopotential planewave density functional theory calculations. These are important steps in the growth of silicon films. Although silane has been studied computationally in some detail previously, we find physisorbed precursor states for the intradimer and interdimer channels. The silane energetics calculated here are in good agreement with experimental data and previous theoretical estimates and provide us with a useful reference point for our disilane calculations. Disilane has not been studied as intensively as silane. We investigate both silicon-silicon bond cleavage and silicon-hydrogen bond cleavage mechanisms, and for each we investigate intradimer, interdimer, and inter-row channels. As in the case of silane, we also find precursor states in the adsorption path in agreement with molecular beam experiments. The qualitative picture that emerges is that adsorption takes place through a weakly bound precursor state with a transition state to chemisorption that is low lying in energy relative to the gas phase. This is in good agreement with experimental data. However, the calculated energetics are only in fair agreement with experiments, with our transition state to chemisorption being about 0.02 eV above the gas phase while experimentally it is estimated to be approximately 0.28 eV below the gas phase. This suggests that accurate theoretical characterization of these weakly bound precursor states and the adsorption barriers requires further computational work.

Optical Properties of ZnO/ZnS and ZnO/ZnTe Heterostructures for Photovoltaic Applications

Although ZnO and ZnS are abundant, stable, and environmentally benign, their band gap energies (3.44, 3.72 eV, respectively) are too large for optimal photovoltaic efficiency. By using band-corrected pseudopotential density functional theory calculations, we study how the band gap, optical absorption, and carrier localization can be controlled by forming quantum-well-like and nanowire-based heterostructures of ZnO/ZnS and ZnO/ZnTe. In the case of ZnO/ZnS core/shell nanowires, which can be synthesized using existing methods, we obtain a band gap of 2.07 eV, which corresponds to a Shockley-Quiesser efficiency limit of 23%. On the basis of these nanowire results, we propose that ZnO/ZnS core/shell nanowires can be used as photovoltaic devices with organic polymer semiconductors as p-channel contacts.

DFT Description on Electronic Structure and Optical Absorption Properties of Anionic S-Doped Anatase TiO2

Plane-wave-based pseudopotential density functional theory (DFT) calculations are used to characterize the doping effect of S substituting for O in anatase TiO(2). Through band structure calculation, a direct band gap is predicted in TiO(2)(-)(x)S(x). Electronic structure analysis shows that the doping S could substantially lower the band gap of TiO(2) by the presence of an impurity state of S 3p on the upper edge of the valence band. Excitations from the impurity state of S 3p to the conduction band may be responsible for the red shift of the absorption edge observed in the S-doped TiO(2). The band gap lowering and red shift of the absorption edge are found to increase as the sulfur concentration increases.

V@Au12-: An Improved Novel Catalyst for CO Oxidation?

The catalytic properties toward CO combustion of an encapsulated cluster, V@Au12-, have been explored by means of plane-wave pseudopotential density-functional theory calculations. Single adsorption of both O2 and CO as well as coadsorption have been considered. The adsorption energy for the O2 molecule is about 0.3-0.4 eV which limits its use to low temperatures. However, in contrast to what happens for pure gold clusters, this system shows a remarkable capacity to bind a high number of oxygen molecules. Moreover, its icosahedral cluster is able to bind 12 CO molecules, since all of the gold atoms are available. The bond between the metal cluster and the oxygen molecule mainly arises from a charge transfer from the metal toward the pi(g) antibonding O2 orbitals, while in the case of CO molecule, the classical sigma-donation pi-back-donation mechanism is observed. Finally, no coadsorption effects are found when both molecules are adsorbed, the interaction properties between the cluster and the substrates remain basically unaltered.

N2O Decomposition on TiO2 (110) from Dynamic First-Principles Calculations

We have carried out a systematic study of N(2)O dissociation on a TiO(2) (110) surface by means of plane-wave pseudopotential density-functional theory calculations. We have made use of both static and dynamic calculations in order to elucidate N(2)O decomposition mechanisms. We find that dissociation is not favorable on the stoichiometric surface. On the other hand, the presence of oxygen bridging vacancies make the N(2)O decomposition possible. The role of the defective surface is to provide electrons to the adsorbed molecule. We find two channels for decomposition, depending on whether the molecule is adsorbed with the O or the N end of the molecule on a vacancy. The first case is energetically downhill and proceeds spontaneously, leading to N(2) ejection from the surface and vacancy oxidation. The second case relies on the formation of an intermediate bridging configuration of the adsorbed molecule and is hindered by a small energy barrier. In this case, molecule breaking produces N(2) in the gas phase and leaves oxygen adatoms on the surface. We relate our results to recent experimental findings.

Spatially Dependent Inelastic Tunneling in a Single Metallofullerene

We have measured the elastic and inelastic tunneling properties of isolated Gd@C(82) molecules on Ag(001) using cryogenic scanning tunneling spectroscopy. We find that the dominant inelastic channel is spatially well localized to a particular region of the molecule. Ab initio pseudopotential density-functional theory calculations indicate that this channel arises from a vibrational cage mode. We further show that the observed inelastic tunneling localization is explained by strong localization in the molecular electron-phonon coupling to this mode.

Atomic and electronic structure of theK∕Si(111)3×3R30°−Bchemisorption system

Ab initio plane-wave pseudopotential density functional theory (DFT) calculations have been carried out to determine the atomic and electronic structure of the K/Sis111dI33I3R30°-B adsorption system at 1 /3 monolayer coverage. Chemisorption of the potassium atoms has been found to leave the topology and bonding structure of the Sis111dI33I3R30°-B substrate essentially unchanged. The results also show that the lowestenergy K/Sis111dI33I3R30°-B structures are very similar to the most energetically favorable geometries for the K/Sis111d737 chemisorption system. The minimum energy configuration has been found to be a structure in which the potassium atoms are positioned near the hollow H3 sites, the boron atoms occupy the subsurface S5 positions, and the silicon adatoms are located at the T4 sites. The electronic structure of this lowest-energy K/Sis111dI33I3R30°-B configuration has been found to be metallic.

Band-resolved analysis of nonlinear optical properties of crystalline and molecular materials

Through three case studies, plane wave pseudopotential density functional theory calculations are performed to investigate the mechanism of second harmonic generation (SHG) of the nonlinear optical materials. We want to know how SHG coefficients are affected by: (1) anion substitution in $\mathrm{AgGa}{({\mathrm{S}}_{x}{\mathrm{Se}}_{1\ensuremath{-}x})}_{2}$; (2) the isomeric effect in push-pull benzenes; and (3) the length of $\ensuremath{\pi}$-conjugate chain in push-pull polyenes. A sum-over-states type formalism is used for the evaluation of static SHG coefficients. Orbital contribution to the $\ensuremath{\beta}$ or ${\ensuremath{\chi}}^{(2)}$ can be decomposed on a (energy) level-by-level basis by partially summing only two out of all three band indices. Through such process that we proposed, the dominant orbitals that give major contribution to a SHG process can then be identified and analyzed. These so-called ``band-resolved'' plots of SHG coefficient are sensitive to the variation of structure and configuration of the materials, and can thus be used to study the detailed mechanism of SHG processes in crystals and molecules.

Energetics and structural characterization ofC60polymerization in BN and carbon nanopeapods

As in the case of carbon nanotubes, also boron nitride nanotubes may host arrays of ${\mathrm{C}}_{60}$ molecules and form a nanopeapod (NPP). The observed separation between ${\mathrm{C}}_{60}$ molecules in BN NPP's is consistently shorter than in carbon NPP's, which influences their electronic properties. Here we report on total-energy pseudopotential density functional theory (DFT) calculations for polymerized and nonpolymerized ${\mathrm{C}}_{60}$ chains, and optimize their atomic structures to provide a description of their energetic landscape. A fully polymerized ${\mathrm{C}}_{60}$ chain and a ${\mathrm{C}}_{60}$ dimer are found to be more stable than nonpolymerized ${\mathrm{C}}_{60}$, respectively, by 0.89 and $0.38\phantom{\rule{0.3em}{0ex}}\mathrm{eV}∕{\mathrm{C}}_{60}$. The geometry and energetics of an encapsulated ${\mathrm{C}}_{60}$ chain is not significantly different with respect to the isolated molecule. Encapsulation energies in BN and carbon NPP's are, respectively, 1.56 and $1.67\phantom{\rule{0.3em}{0ex}}\mathrm{eV}∕{\mathrm{C}}_{60}$, which are significantly larger than the calculated activation energy for ${\mathrm{C}}_{60}$ polymerization, supporting the hypothesis that encapsulated ${\mathrm{C}}_{60}$'s in NPP's are partially polymerized. Band structure analysis show that polymerization does not affect the gap width of the ${\mathrm{C}}_{60}$ chain. BN NPP's are semiconductors with a gap width determined by the ${\mathrm{C}}_{60}$. The lowest unoccupied ${\mathrm{C}}_{60}$ states lie just above the Fermi level in metallic carbon NPP's and charge transfert could take place, affecting the ${\mathrm{C}}_{60}$ geometry.

Atomic and electronic structure of the Si(0 0 1)2 × 1–Li chemisorption system at 1.0 monolayer coverage

Ab initio plane-wave pseudopotential density functional theory (DFT) calculations have been performed to determine the atomic and electronic structure of the Si(0 0 1)2 × 1–Li adsorption system at 1.0 monolayer (ML) coverage. Chemisorption of the lithium atoms is found to result in a minimum energy configuration characterized by symmetric Si dimers in agreement with the results of high-resolution core-level photoelectron spectroscopy. The transition from the asymmetric Si dimers of the clean Si(0 0 1) surface to the symmetric dimers of the Li chemisorbed surface is due to charge transfer from the Li adatoms to the substrate. The dispersion of the occupied electronic surface state bands is found to be in good agreement with the angle-resolved photoemission data. The nature of the lowest energy unoccupied surface state band suggests that silicide formation may occur at coverages greater than 1.0 ML.

Oxygen vacancies on TiO2 (110) from first principles calculations.

We have carried out a systematic study of oxygen vacancy formation on the TiO2 (110) surface by means of plane-wave pseudopotential density-functional theory calculations. We have used models with the mean number of vacancies per surface unit cell being theta=0.25 and theta=0.5. The study comprises several kind of vacancies within the outermost layers of the surface. The use of a suitable set of technical parameter is often essential in order to get accurate results. We find that the presence of bridging vacancies is energetically favored in accordance to experimental data, although the formation of sub-bridging vacancies might be possible at moderate temperatures. Surprisingly, the spin state of the vacancy has little influence on the results. Atomic displacements are also analyzed and found to be strongly dependent on the particular arrangement of vacancies.

Origin of vacancy and interstitial stabilization at the amorphous-crystalline Si interface

Using plane-wave pseudopotential density functional theory calculations, we have investigated the behaviors of neutral interstitials and vacancies at the amorphous-crystalline (a–c)Si interface. A continuous random network model is employed in the construction of defect-free a-c interface structure. We find that both vacancies and interstitials prefer to reside on the amorphous side of the interface. In both cases, the most stable defects occur 3–4Å from the a-c interface. Vacancy stabilization is found to be due to strain relief provided to the substrate lattice while interstitial stabilization is due largely to bond rearrangement arising from interstitial integration into the substrate lattice. We also discuss the effect of the “spongelike” behavior of the amorphous phase toward native defects on ultrashallow junction formation in the fabrication of ever-shrinking electronic devices.

Breakdown of the pseudopotential approximation for magnetic systems: Predicting magnetic quenching at the V(001) surface with spin-dependent pseudopotentials

Both experimentally [R.L. Fink et al., Phys. Rev. B 41, 10 175 (1990)] and using all-electron density-functional theory (DFT) methods, the V(001) surface exhibits little or no magnetization. Very recently, independent pseudopotential DFT calculations demonstrated the breakdown of the pseudopotential approximation, showing large magnetic moments at the surface of a V(001) slab. Here we demonstrate that use of spin-dependent pseudopotentials systematically corrects the inaccuracies of conventional spin-neutral pseudopotentials, producing results consistent with all-electron ones. We also show how the use of the spin-dependent pseudopotentials allows one to achieve a high level of accuracy without the numerical difficulties and cost associated with accurate spin-neutral pseudopotentials.