49 research outputs found
Atomic and molecular adsorption on transition-metal carbide (111) surfaces from density-functional theory: A trend study of surface electronic factors
This study explores atomic and molecular adsorption on a number of early
transition-metal carbides (TMC's) by means of density-functional theory
calculations. Trend studies are conducted with respect to both period and group
in the periodic table, choosing the substrates ScC, TiC, VC, ZrC, NbC,
delta-MoC, TaC, and WC and the adsorbates H, B, C, N, O, F, NH, NH2, and NH3.
Trends in adsorption strength are explained in terms of surface electronic
factors, by correlating the calculated adsorption energy values with the
calculated surface electronic structures. The results are rationalized with use
of a concerted-coupling model (CCM), which has previously been applied
succesfully to the description of adsorption on TiC(111) and TiN(111) surfaces
[Solid State Commun. 141, 48 (2007)]. First, the clean TMC(111) surfaces are
characterized by calculating surface energies, surface relaxations, Bader
charges, and surface-localized densities of states (DOS's). Detailed
comparisons between surface and bulk DOS's reveal the existence of
transition-metal localized SR's (TMSR's) in the pseudogap and of several
C-localized SR's (CSR's) in the upper valence band on all considered TMC(111)
surfaces. Then, atomic and molecular adsorption energies, geometries, and
charge transfers are presented. An analysis of the adsorbate-induced changes in
surface DOS's reveals a presence of both adsorbate--TMSR and adsorbate--CSR's
interactions, of varying strengths depending on the surface and the adsorbate.
These variations are correlated to the variations in adsorption energies. The
results are used to generalize the content and applications of the previously
proposed CCM to this larger class of substrates and adsorbates. Implications
for other classes of materials, for catalysis, and for other surface processes
are discussed
Nature of Versatile Chemisorption on TiC(111) and TiN(111) Surfaces
Density-functional calculations on the polar TiX(111) (X = C, N) surfaces
show (i) for clean surfaces, strong Ti3d-derived surface resonances (SR's) at
the Fermi level and X2p-derived SR's deep in the upper valence band and (ii)
for adatoms in periods 1-3, pyramidic trends in atomic adsorption energies,
peaking at oxygen (9 eV). A concerted-coupling model, where adatom states
couple to both kinds of SR's in a concerted way, describes the adsorption. The
chemisorption versatility and the general nature of the model indicate
ramifications and predictive abilities in, e.g., growth and catalysis.Comment: 5 pages, 4 figures, submitted to Physical Review Letters (2006
Nature of Chemisorption on Titanium Carbide and Nitride
Extensive density-functional calculations are performed to understand atomic
chemisorption on the TiC(111) and TiN(111) surfaces, in particular the
calculated pyramid-shaped trends in the adsorption energies for second- and
third-period adatoms. Our previously proposed concerted-coupling model for
chemisorption on TiC(111) is tested against new results for adsorption on
TiN(111) and found to apply on this surface as well, thus reflecting both
similarities and differences in electronic structure between the two compounds.Comment: 7 pages, 4 figures, conference proceeding presented at IWSP-2005
(Polanica Zdoj, Poland, 2005), submitted to Surf. Sci. (2005
A Higher-Accuracy van der Waals Density Functional
We propose a second version of the van der Waals density functional (vdW-DF2)
of Dion et al. [Phys. Rev. Lett. 92, 246401 (2004)], employing a more accurate
semilocal exchange functional and the use of a large-N asymptote gradient
correction in determining the vdW kernel. The predicted binding energy,
equilibrium separation, and potential-energy curve shape are close to those of
accurate quantum chemical calculations on 22 duplexes. We anticipate the
enabling of chemically accurate calculations in sparse materials of importance
for condensed-matter, surface, chemical, and biological physics.Comment: 14 pages, 10 figure
van der Waals density functionals built upon the electron-gas tradition: Facing the challenge of competing interactions
The theoretical description of sparse matter attracts much interest, in
particular for those ground-state properties that can be described by density
functional theory (DFT). One proposed approach, the van der Waals density
functional (vdW-DF) method, rests on strong physical foundations and offers
simple yet accurate and robust functionals. A very recent functional within
this method called vdW-DF-cx [K. Berland and P. Hyldgaard, Phys. Rev. B 89,
035412] stands out in its attempt to use an exchange energy derived from the
same plasmon-based theory from which the nonlocal correlation energy was
derived. Encouraged by its good performance for solids, layered materials, and
aromatic molecules, we apply it to several systems that are characterized by
competing interactions. These include the ferroelectric response in PbTiO,
the adsorption of small molecules within metal-organic frameworks (MOFs), the
graphite/diamond phase transition, and the adsorption of an aromatic-molecule
on the Ag(111) surface. Our results indicate that vdW-DF-cx is overall well
suited to tackle these challenging systems. In addition to being a competitive
density functional for sparse matter, the vdW-DF-cx construction presents a
more robust general purpose functional that could be applied to a range of
materials problems with a variety of competing interactions
Towards a working density-functional theory for polymers: First-principles determination of the polyethylene crystal structure
Equilibrium polyethylene crystal structure, cohesive energy, and elastic
constants are calculated by density-functional theory applied with a recently
proposed density functional (vdW-DF) for general geometries [Phys. Rev. Lett.
92, 246401 (2004)] and with a pseudopotential-planewave scheme. The vdW-DF with
its account for the long-ranged van der Waals interactions gives not only a
stabilized crystal structure but also values of the calculated lattice
parameters and elastic constants in quite good agreement with experimental
data, giving promise for successful application to a wider range of polymers.Comment: 4 pages, 3 figure
Influence of van der Waals forces on the adsorption structure of benzene on silicon studied using density functional theory
Two different adsorption configurations of benzene on the Si(001)-(2 x 1) surface, the tight-bridge and butterfly structures, were studied using density functional theory. Several exchange and correlation functionals were used, including the recently developed van der Waals density functional (vdW-DF), which accounts for the effect of van der Waals forces. In contrast to the Perdew-Burke-Ernzerhof (PBE), revPBE, and other generalized-gradient approximation functionals, the vdW-DF finds that, for most coverages, the adsorption energy of the butterfly structure is greater than that of the tight-bridge structure
Van der Waals effect in weak adsorption affecting trends in adsorption, reactivity, and the view of substrate nobility
The ubiquitous van der Waals (vdW) force, particularly discernible in weak adsorption, is studied on noble and transition metals. In calculations with the vdW density functional (DF) [ M. Dion et al., Phys. Rev. Lett. 92, 246401 (2004)], the atomic structure near the adsorption site is systematically varied, including dense fcc(111) surface, adatom, pyramid, and step defects. In weak adsorption the vdW force (i) is shown necessary to account for, (ii) is sizable, (iii) has a strong spatial variation, relevant for adsorption on surface defects, (iv) changes reaction rules, and (v) changes adsorption trends in agreement with experimental data. Traditional physisorption theory is also given support and interpretation
Trends in Atomic Adsorption on Titanium Carbide and Nitride
Extensive density-functional calculations on atomic chemisorption of H, B, C,
N, O, F, Al, Si, P, S, and Cl on the polar TiC(111) and TiN(111) yield similar
adsorption trends for the two surfaces: (i) pyramid-like adsorption-energy
trends along the adatom periods; (ii) strongest adsorption for O, C, N, S, and
F; (iii) large adsorption variety; (iv) record-high adsorption energy for O
(8.4-8.8 eV). However, a stronger adsorption on TiN is found for elements on
the left of the periodic table and on TiC for elements on the right. The
results support that a concerted-coupling model, proposed for chemisorption on
TiC, applies also to TiN.Comment: 5 pages, 4 figures, 2 tables, conference proceeding presented at
ECOSS-23 (Berlin, 2005), submitted to Surf. Sci. (2005