186 research outputs found
Computational design of metal-supported molecular switches: Transient ion formation during light- and electron-induced isomerisation of azobenzene
In molecular nanotechnology, a single molecule is envisioned to act as the
basic building block of electronic devices. Such devices may be of special
interest for organic photovoltaics, data storage, and smart materials. However,
more often than not the molecular function is quenched upon contact with a
conducting support. Trial-and-error-based decoupling strategies via molecular
functionalisation and change of substrate have in many instances proven to
yield unpredictable results. The adsorbate-substrate interactions that govern
the function can be understood with the help of first-principles simulation.
Employing dispersion-corrected Density-Functional Theory (DFT) and linear
expansion Delta-Self-Consistent-Field DFT, the electronic structure of a
prototypical surface-adsorbed functional molecule, namely azobenzene adsorbed
to (111) single crystal facets of copper, silver and gold, is investigated and
the main reasons for the loss or survival of the switching function upon
adsorption are identified. The light-induced switching ability of a
functionalised derivative of azobenzene on Au(111) and azobenzene on Ag(111)
and Au(111) is assessed based on the excited-state potential energy landscapes
of their transient molecular ions, which are believed to be the main
intermediates of the experimentally observed isomerisation reaction. We provide
a rationalisation of the experimentally observed function or lack thereof that
connects to the underlying chemistry of the metal-surface interaction and
provides insights into general design strategies for complex light-driven
reactions at metal surfaces.Comment: 14 pages, 5 figures, submitted to J. Phys. Condens. Matte
Bistability loss as key feature in azobenzene (non-)switching on metal surfaces
Coinage metal adsorbed azobenzene is investigated as prototypical molecular
switch. It is shown that switching capabilities are not just lost due to
excited state quenching, but already due to changes in the ground state
energetics. Electron demanding coadsorbates are suggested as strategy to regain
the switching function.Comment: 8 pages, 3 figure
Assessing computationally efficient isomerization dynamics: Delta-SCF density-functional theory study of azobenzene molecular switching
We present a detailed comparison of the S0, S1 (n -> \pi*) and S2 (\pi ->
\pi*) potential energy surfaces (PESs) of the prototypical molecular switch
azobenzene as obtained by Delta-self-consistent-field (Delta-SCF)
Density-Functional Theory (DFT), time-dependent DFT (TD-DFT) and approximate
Coupled Cluster Singles and Doubles (RI-CC2). All three methods unanimously
agree in terms of the PES topologies, which are furthermore fully consistent
with existing experimental data concerning the photo-isomerization mechanism.
In particular, sum-method corrected Delta-SCF and TD-DFT yield very similar
results for S1 and S2, when based on the same ground-state exchange-correlation
(xc) functional. While these techniques yield the correct PES topology already
on the level of semi-local xc functionals, reliable absolute excitation
energies as compared to RI-CC2 or experiment require an xc treatment on the
level of long-range corrected hybrids. Nevertheless, particularly the
robustness of Delta-SCF with respect to state crossings as well as its
numerical efficiency suggest this approach as a promising route to dynamical
studies of larger azobenzene-containing systems.Comment: 25 pages, 6 figure
Interpretation of X-ray Absorption Spectroscopy in the Presence of Surface Hybridization
X-ray absorption spectroscopy yields direct access to the electronic and
geometric structure of hybrid inorganic-organic interfaces formed upon
adsorption of complex molecules at metal surfaces. The unambiguous
interpretation of corresponding spectra is challenged by the intrinsic
geometric flexibility of the adsorbates and the chemical interactions with the
interface. Density-functional theory (DFT) calculations of the extended
adsorbate-substrate system are an established tool to guide peak assignment in
X-ray photoelectron spectroscopy (XPS) of complex interfaces. We extend this to
the simulation and interpretation of X-ray absorption spectroscopy (XAS) data
in the context of functional organic molecules on metal surfaces using
dispersion-corrected DFT calculations within the transition potential approach.
On the example of X-ray absorption signatures for the prototypical case of
2H-porphine adsorbed on Ag(111) and Cu(111) substrates, we follow the two main
effects of the molecule/surface interaction on XAS: (1) the substrate-induced
chemical shift of the 1s core levels that dominates in physisorbed systems and
(2) the hybridization-induced broadening and loss of distinct resonances that
dominates in more chemisorbed systems.Comment: 13 pages, 4 figure
Computational design of metal-supported molecular switches : Transient ion formation during light- and electron-induced isomerisation of azobenzene
In molecular nanotechnology, a single molecule is envisioned to act as the basic building block of electronic devices. Such devices may be of special interest for organic photovoltaics, data storage, and smart materials. However, more often than not the molecular function is quenched upon contact with a conducting support. Trial-and-error-based decoupling strategies via molecular functionalisation and change of substrate have in many instances proven to yield unpredictable results. The adsorbate-substrate interactions that govern the function can be understood with the help of rst-principles simulation. Employing dispersion-corrected Density-Functional Theory (DFT) and linear expansion Delta-Self-Consistent-Field DFT, the electronic structure of a prototypical surface-adsorbed functional molecule, namely azobenzene adsorbed to (111) single crystal facets of copper, silver and gold, is investigated and the main reasons for the loss or survival of the switching function upon adsorption are identifed. The light-induced switching ability of a functionalised derivative of azobenzene on Au(111) and azobenzene on Ag(111) and Au(111) is assessed based on the excited-state potential energy landscapes of their transient molecular ions, which are believed to be the main intermediates of the experimentally observed isomerisation reaction. We provide a rationalisation of the experimentally observed function or lack thereof that connects to the underlying chemistry of the metal-surface interaction and provides insights into general design strategies for complex light-driven reactions at metal surfaces
Excited-state potential-energy surfaces of metal-adsorbed organic molecules from Linear Expansion \Delta-Self-Consistent Field Density-Functional Theory (\Delta SCF-DFT)
Accurate and efficient simulation of excited state properties is an important
and much aspired cornerstone in the study of adsorbate dynamics on metal
surfaces. To this end, the recently proposed linear expansion \Delta
Self-Consistent Field (le\Delta SCF) method by Gavnholt et al. [Phys. Rev. B
78, 075441 (2008)] presents an efficient alternative to time consuming
quasi-particle calculations. In this method the standard Kohn-Sham equations of
Density-Functional Theory are solved with the constraint of a non-equilibrium
occupation in a region of Hilbert-space resembling gas-phase orbitals of the
adsorbate. In this work we discuss the applicability of this method for the
excited-state dynamics of metal-surface mounted organic adsorbates,
specifically in the context of molecular switching. We present necessary
advancements to allow for a consistent quality description of excited-state
potential-energy surfaces (PESs), and illustrate the concept with the
application to Azobenzene adsorbed on Ag(111) and Au(111) surfaces. We find
that the explicit inclusion of substrate electronic states modifies the
topologies of intra-molecular excited-state PESs of the molecule due to image
charge and hybridization effects. While the molecule in gas phase shows a clear
energetic separation of resonances that induce isomerization and backreaction,
the surface-adsorbed molecule does not. The concomitant possibly simultaneous
induction of both processes would lead to a significantly reduced switching
efficiency of such a mechanism.Comment: 12 pages, 4 figure
Many-body dispersion effects in the binding of adsorbates on metal surfaces
A correct description of electronic exchange and correlation effects for
molecules in contact with extended (metal) surfaces is a challenging task for
first-principles modeling. In this work we demonstrate the importance of
collective van der Waals dispersion effects beyond the pairwise approximation
for organic--inorganic systems on the example of atoms, molecules, and
nanostructures adsorbed on metals. We use the recently developed many-body
dispersion (MBD) approach in the context of density-functional theory [Phys.
Rev. Lett. 108, 236402 (2012); J. Chem. Phys. 140, 18A508 (2014)] and assess
its ability to correctly describe the binding of adsorbates on metal surfaces.
We briefly review the MBD method and highlight its similarities to
quantum-chemical approaches to electron correlation in a quasiparticle picture.
In particular, we study the binding properties of xenon,
3,4,9,10-perylene-tetracarboxylic acid (PTCDA), and a graphene sheet adsorbed
on the Ag(111) surface. Accounting for MBD effects we are able to describe
changes in the anisotropic polarizability tensor, improve the description of
adsorbate vibrations, and correctly capture the adsorbate--surface interaction
screening. Comparison to other methods and experiment reveals that inclusion of
MBD effects improves adsorption energies and geometries, by reducing the
overbinding typically found in pairwise additive dispersion-correction
approaches
Assessing mixed quantum-classical molecular dynamics methods for nonadiabatic dynamics of molecules on metal surfaces
Mixed-quantum classical (MQC) methods for simulating the dynamics of
molecules at metal surfaces have the potential to accurately and efficiently
provide mechanistic insight into reactive processes. Here, we introduce simple
two-dimensional models for the scattering of diatomic molecules at metal
surfaces based on recently published electronic structure data. We apply
several MQC methods to investigate their ability to capture how nonadiabatic
effects influence molecule-metal energy transfer during the scattering process.
Specifically, we compare molecular dynamics with electronic friction, Ehrenfest
dynamics, Independent Electron Surface Hopping, and the Broadened Classical
Master Equation approach. In the case of Independent Electron Surface Hopping,
we implement a simple decoherence correction approach and assess its impact on
vibrationally-inelastic scattering. Our results show that simple,
low-dimensional models can be used to qualitatively capture experimentally
observed vibrational energy transfer and provide insight into the relative
performance of different MQC schemes. We observe that all approaches predict
similar kinetic energy dependence, but return different vibrational energy
distributions. Finally, by varying the molecule-metal coupling, we can assess
the coupling regime in which some MQC methods become unsuitable.Comment: 15 pages, 13 figure
Hot-electron effects during reactive scattering of H2 from Ag(111) : the interplay between mode-specific electronic friction and the potential energy landscape
The breakdown of the Born-Oppenheimer approximation gives rise to nonadiabatic effects in gas-surface reactions at metal surfaces. However, for a given reaction, it remains unclear which factors quantitatively determine whether these effects measurably contribute to surface reactivity in catalysis and photo/electrochemistry. Here, we systematically investigate hot electron effects during H2 scattering from Ag(111) using electronic friction theory. We combine first-principles calculations of tensorial friction by time-dependent perturbation theory based on Density Functional Theory and an analytical neural network representation, to overcome the limitations of existing approximations and explicitly simulate mode-specific nonadiabatic energy loss during molecular dynamics. Despite sizable hot-electron-induced energy loss, no measurable nonadiabatic effects can be found for H2 scattering on Ag(111). This is in stark contrast to previous reports for vibrationally excited H2 scattering on Cu(111). By detailed analysis of the two systems, we attribute this discrepancy to a subtle interplay between the magnitude of electronic friction along intramolecular vibration and the shape of the potential energy landscape that controls the molecular velocity at impact. On the basis of this characterization, we offer guidance for the search of highly nonadiabatic surface reactions
Predicting long timescale kinetics under variable experimental conditions with Kinetica.jl
Predicting the degradation processes of molecules over long timescales is a
key aspect of industrial materials design. However, it is made computationally
challenging by the need to construct large networks of chemical reactions that
are relevant to the experimental conditions that kinetic models must mirror,
with every reaction requiring accurate kinetic data. Here we showcase
Kinetica.jl, a new software package for constructing large-scale chemical
reaction networks in a fully-automated fashion by exploring chemical reaction
space with a kinetics-driven algorithm; coupled to efficient machine-learning
models of activation energies for sampled elementary reactions, we show how
this approach readily enables generation and kinetic characterization of
networks containing chemical species and -
reactions. Symbolic-numeric modelling of the generated reaction networks is
used to allow for flexible, efficient computation of kinetic profiles under
experimentally-realizable conditions such as continuously-variable temperature
regimes, enabling direct connection between bottom-up reaction networks and
experimental observations. Highly efficient propagation of long-timescale
kinetic profiles is required for automated reaction network refinement and is
enabled here by a new discrete kinetic approximation. The resulting Kinetica.jl
simulation package therefore enables automated generation, characterization,
and long-timescale modelling of complex chemical reaction systems. We
demonstrate this for hydrocarbon pyrolysis simulated over timescales of
seconds, using transient temperature profiles representing those of tubular
flow reactor experiments.Comment: 56 pages, 13 figure
- …