5 research outputs found
Distinguishing between Charge-Transfer Mechanisms at Organic/Inorganic Interfaces Employing Hybrid Functionals
When
modeling inorganic/organic interfaces with density functional
theory (DFT), the outcome often depends on the chosen functional.
Hybrid functionals, which employ a fraction of Hartree–Fock
exchange α, tend to give better results than the more commonly
applied semilocal functionals, because they remove or at least mitigate
the unphysical electron self-interaction. However, the choice of α
is not straightforward, as its effect on observables depends on the
physical properties of the investigated system, such as the size of
the molecule and the polarizability of the substrate. In this contribution,
we demonstrate this impact exemplarily for tetrafluoro-1,4-benzoquinone
on semiconducting (copper-I-oxide Cu<sub>2</sub>O) and metallic (Cu)
substrates and explore how the simulated charge transfer depends on
α. We determine the value α* that marks the transition
point between spurious over-localization and over-delocalization of
charges. This allows us to shed light on the interplay between the
value of α* and the physical properties of the interface. We
find that on the inert semiconducting substrate, α* strongly
depends on surface screening. Furthermore, α has a significant
impact on the amount of charge transfer and, in particular, the charge
localization. Conversely, for the adsorption on Cu, α affects
only the amount of transferred charge, but not its localization, which
is a consequence of strong hybridization. Finally, we discuss limitations
to the predictive power of DFT for modeling charge transfer at inorganic/organic
interfaces and explain why the choice of a “correct”
amount of Hartree–Fock exchange is difficult, if not impossible.
However, we argue why simulations still provide valuable insights
into the charge-transfer mechanism at organic/inorganic interfaces
and describe how α can be chosen sensibly to simulate any given
system
Magnetic configurations of open-shell molecules on metals: The case of CuPc and CoPc on silver
For nanostructured interfaces between open-shell molecules and metal surfaces that involve charge transfer upon adsorption, the investigation of molecular magnetic properties is an interesting yet difficult task, because in principle different magnetic configurations with distinct properties can be found. Here, we study the magnetic properties of CuPc-Ag and CoPc-Ag interfaces, which constitute interesting test cases because charge is transferred to the initially open-shell Pc molecules upon adsorption. Using hybrid density functional theory, we examine the stability of the various magnetic configurations occurring at these nanoscale interfaces, as well as for the corresponding gas-phase anions, and compare our findings to those of previous experimental studies. For CuPc-Ag, we identify a high-spin triplet configuration as the most likely configuration at the interface, whereas for CoPc-Ag a quenching of the total magnetic moment is found. Interestingly, such quenching is consistent with two distinctly different interfacial electronic configurations. These important differences in the magnetic properties of CuPc and CoPc on Ag are rationalized by variations in the interaction of their central metal atoms with the substrate. Our work facilitates a deeper understanding of the magnetic configuration and interlinked electronic-structure properties of molecule-metal interfaces. Furthermore, it highlights the necessity of an appropriate choice of methodology in tandem with a detailed evaluation of the different emerging magnetic properties
Numerical Quality Control for DFT-based Materials Databases
Electronic-structure theory is a strong pillar of materials science. Many
different computer codes that employ different approaches are used by the
community to solve various scientific problems. Still, the precision of
different packages has only recently been scrutinized thoroughly, focusing on a
specific task, namely selecting a popular density functional, and using
unusually high, extremely precise numerical settings for investigating 71
monoatomic crystals. Little is known, however, about method- and code-specific
uncertainties that arise under numerical settings that are commonly used in
practice. We shed light on this issue by investigating the deviations in total
and relative energies as a function of computational parameters. Using typical
settings for basis sets and k-grids, we compare results for 71 elemental and 63
binary solids obtained by three different electronic-structure codes that
employ fundamentally different strategies. On the basis of the observed trends,
we propose a simple, analytical model for the estimation of the errors
associated with the basis-set incompleteness. We cross-validate this model
using ternary systems obtained from the NOMAD Repository and discuss how our
approach enables the comparison of the heterogeneous data present in
computational materials databases.Comment: 7 pages, 4 figure
Adsorption Behavior of Nonplanar Phthalocyanines: Competition of Different Adsorption Conformations
Using density functional theory augmented
with state-of-the-art
van der Waals corrections, we studied the geometric and electronic
properties of nonplanar chlorogallium-phthalocyanine GaClPc molecules
adsorbed on Cu(111). Comparing these results with published experimental
data for adsorption heights, we found indications for breaking of
the metal–halogen bond when the molecule is heated during or
after the deposition process. Interestingly, the work-function change
induced by this dissociated geometry is the same as that computed
for an intact adsorbate layer in the “Cl-down” configuration,
with both agreeing well with the experimental photoemission data.
This is unexpected, as the chemical natures of the adsorbates and
the adsorption distances are markedly different in the two cases.
The observation is explained as a consequence of Fermi-level pinning
due to fractional charge transfer at the interface. Our results show
that rationalizing the adsorption configurations on the basis of electronic
interface properties alone can be ambiguous and that additional insight
from dispersion-corrected DFT simulations is desirable
Understanding the adsorption of CuPc and ZnPc on noble metal surfaces by combining quantum-mechanical modelling and photoelectron spectroscopy
10.3390/molecules19032969Molecules1932969-2992MOLE