165 research outputs found
An analysis of van der Waals density functional components: Binding and corrugation of benzene and C60 on boron nitride and graphene
The adsorption of benzene and C60 on graphene and boron nitride (BN) is
studied using density functional theory with the non-local correlation
functional vdW-DF. By comparing these systems we can systematically investigate
their adsorption nature and differences between the two functional versions
vdW-DF1 and vdW-DF2. The bigger size of the C60 molecule makes it bind stronger
to the surface than benzene, yet the interface between the molecules and the
sheets are similar in nature. The binding separation is more sensitive to the
exchange variant used in vdW-DF than to the correlation version. This result is
related to the exchange and correlation components of the potential energy
curve (PEC). We show that a moderate dipole forms for C60 on graphene, unlike
for the other adsorption systems. We find that the corrugation is very
sensitive to the variant or version of vdW-DF used, in particular the exchange.
Further, we show that this sensitivity arise indirectly through the shift in
binding separation caused by changing vdW-DF variant. Based on our results, we
suggest a concerted theory-experiment approach to assess the exchange and
correlation contributions to physisorption. Using DFT calculations, the
corrugation can be linked to the optimal separation, allowing us to extract the
exchange-correlation part of the adsorption energy. Molecules with same
interfaces to the surface, but different geometries, can in turn cast light on
the role of van der Waals forces.Comment: 16 page
Phonon Knudsen flow in nanostructured semiconductor systems
We determine the size effect on the lattice thermal conductivity of nanoscale
wire and multilayer structures formed in and by some typical semiconductor
materials, using the Boltzmann transport equation and focusing on the Knudsen
flow effect. For both types of nanostructured systems we find that the phonon
transport is reduced significantly below the bulk value by boundary scattering
off interface defects and/or interface modes. The Knudsen flow effects are
important for almost all types of semiconductor nanostructures but we find them
most pronounced in Si and SiC systems due to the very large phonon mean-free
paths. We apply and test our wire thermal-transport results to recent
measurements on Si nanowires. We further investigate and predict size effects
in typical multilayered SiC nanostructures, for example, a
doped-SiC/SiC/SiO layered structure that could define the transport channel
in a nanosize transistor. Here the phonon-interface scattering produces a
heterostructure thermal conductivity smaller than what is predicted in a
traditional heat-transport calculation, suggesting a breakdown of the
traditional Fourier analysis even at room temperatures. Finally, we show that
the effective thermal transport in a SiC/SiO heterostructure is sensitive
to the oxide depth and could thus be used as an in-situ probe of the SiC
oxidation progress.Comment: 29 pages, 9 figures. (Submitted to Journal of Applied Physics
An exchange functional that tests the robustness of the plasmon description of the van der Waals density functional
Is the plasmon description within the non-local correlation of the van der
Waals density functional by Dion and coworkers (vdW-DF1) robust enough to
describe all exchange-correlation components? To address this question, we
design an exchange functional, termed LV-PW86r based on this plasmon
description as well as recent analysis on exchange in the large -regime. In
the regime with reduced gradients smaller than
, dominating the non-local correlation part of the binding energy,
the enhancement factor closely resembles the Langreth-Vosko screened
exchange. In the -regime beyond, dominated by exchange, passes
smoothly over to the revised Perdew-Wang-86 form. Our tests indicate that
vdW-DF1(LV-PW86r) produces accurate separations and binding energies of the S22
data set of molecular dimers as well as accurate lattice constants of layered
materials and tightly-bound solids. These results suggest that vdW-DF1 has a
good plasmon description in the low-to-moderate -regime
Thermal transport in SiC nanostructures
SiC is a robust semiconductor material considered ideal for high-power
application due to its material stability and large bulk thermal conductivity
defined by the very fast phonons. In this paper, however, we show that both
material-interface scattering and total-internal reflection significantly limit
the SiC-nanostructure phonon transport and hence the heat dissipation in a
typical device. For simplicity we focus on planar SiC nanostructures and
calculate the thermal transport both parallel to the layers in a
substrate/SiC/oxide heterostructure and across a SiC/metal gate or contact. We
find that the phonon-interface scattering produces a heterostructure thermal
conductivity significantly smaller than what is predicted in a traditional
heat-transport calculation. We also document that the high-temperature heat
flow across the metal/SiC interface is limited by total-internal reflection
effects and maximizes with a small difference in the metal/SiC sound
velocities.Comment: 15 pages, 4 figure
Stacking and band structure of van der Waals bonded graphane multilayers
We use density functional theory and the van der Waals density functional
(vdW-DF) method to determine the binding separation in bilayer and bulk
graphane and study the changes in electronic band structure that arise with the
multilayer formation. The calculated binding separation (distance between
center-of-mass planes) and binding energy are 4.5-5.0 {\AA} (4.5-4.8 {\AA}) and
75-102 meV/cell (93-127 meV/cell) in the bilayer (bulk), depending on the
choice of vdW-DF version. We obtain the corresponding band diagrams using
calculations in the ordinary generalized gradient approximation for the
geometries specified by our vdW-DF results, so probing the indirect effect of
vdW forces on electron behavior. We find significant band-gap modifications by
up to -1.2 eV (+4.0 eV) in various regions of the Brillouin zone, produced by
the bilayer (bulk) formation.Comment: 11 pages, 7 figures, 2 tables, accepted for publication in Phys. Rev.
Signatures of van der Waals binding: a coupling-constant scaling analysis
The van der Waals (vdW) density functional (vdW-DF) method [ROPP 78, 066501
(2015)] describes dispersion or vdW binding by tracking the effects of an
electrodynamic coupling among pairs of electrons and their associated
exchange-correlation holes. This is done in a nonlocal-correlation energy term
, which permits density functional theory calculation in the
Kohn-Sham scheme. However, to map the nature of vdW forces in the fully
interacting materials system, it is necessary to compensate for associated
kinetic-correlation energy effects. Here we present a coupling-constant scaling
analysis that also permits us to compute the kinetic-correlation energy
that is specific to the vdW-DF account of nonlocal correlations. We
thus provide a spatially-resolved analysis of the total nonlocal-correlation
binding, including vdW forces, in both covalently and non-covalently bonded
systems. We find that kinetic-correlation energy effects play a significant
role in the account of vdW or dispersion interactions among molecules. We also
find that the signatures that we reveal in our full-interaction mapping are
typically given by the spatial variation in the binding
contributions, at least in a qualitative discussion. Furthermore, our full
mapping shows that the total nonlocal-correlation binding is concentrated to
pockets in the sparse electron distribution located between the material
fragments.Comment: 15 pages, 8 figure
Interpretation of van der Waals density functionals
The nonlocal correlation energy in the van der Waals density functional
(vdW-DF) method [Phys. Rev. Lett. 92, 246401 (2004); Phys. Rev. B 76, 125112
(2007); Phys. Rev. B 89, 035412 (2014)] can be interpreted in terms of a
coupling of zero-point energies of characteristic modes of semilocal
exchange-correlation (xc) holes. These xc holes reflect the internal functional
in the framework of the vdW-DF method [Phys. Rev. B 82, 081101(2010)]. We
explore the internal xc hole components, showing that they share properties
with those of the generalized-gradient approximation. We use these results to
illustrate the nonlocality in the vdW-DF description and analyze the vdW-DF
formulation of nonlocal correlation.Comment: 13 pages, 6 figures. Submited to Physical Review
Extent of Fock-exchange mixing for a hybrid van der Waals density functional?
The vdW-DF-cx0 exchange-correlation hybrid design has a truly nonlocal
correlation component and aims to facilitate concurrent descriptions of both
covalent and non-covalent molecular interactions. The vdW-DF-cx0 design mixes a
fixed ratio, , of Fock exchange into the consistent-exchange van der Waals
density functional, vdW-DF-cx. The mixing value is sometimes taken as a
semi-empirical parameter in hybrid formulations. Here, instead, we assert a
plausible optimum average value for the vdW-DF-cx0 design from a formal
analysis; A new, independent determination of the mixing is necessary since
the Becke fit, yielding , is restricted to semilocal correlation and
does not reflect non-covalent interactions. To proceed, we adapt the so-called
two-legged hybrid construction to a starting point in the vdW-DF-cx functional.
For our approach, termed vdW-DF-tlh, we estimate the properties of the
adiabatic-connection specification of the exact exchange-correlation
functional, by combining calculations of the Fock exchange and of the
coupling-constant variation in vdW-DF-cx. We find that such vdW-DF-tlh hybrid
constructions yield accurate characterizations of molecular. The accuracy
motivates trust in the vdW-DF-tlh determination of system-specific values of
the Fock-exchange mixing. We find that an average value best
characterizes the vdW-DF-tlh description of covalent and non-covalent
interactions, although there exists some scatter. This finding suggests that
the original Becke value, , also represents an optimal average
Fock-exchange mixing for the new, truly nonlocal-correlation hybrids. To enable
self-consistent calculations, we furthermore define and test a zero-parameter
hybrid functional vdW-DF-cx0p (having fixed mixing ) and document that
this truly nonlocal correlation hybrid works for general molecular
interactions.Comment: 18 pages, 5 figures, accepted by J. Chem. Phy
- …