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Research

Functional Adsorbates and Nanodevices

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The ability to adsorb purpose-designed molecules on surfaces where they perform functions such as switching, sensing or catalysis is a key capability in the quest for a future molecular nanotechnology. Molecular switches, that is, molecules with properties that can be bistably and reversibly switched by external stimuli may act as well-defined building blocks that offer the vision of an ultimate integration of functional properties. Retaining the molecular function when adsorbed on surfaces is a challenging task and involves knowledge of electronic and sterical effects. Azobenzene and its derivatives are interesting toy-models to study such effects, because of their well-understood photo-induced isomerization behaviour.

Collaborations:
  • Richard Berndt, University of Kiel, Germany
  • Johannes V. Barth, Technical University Munich, Germany
  • Christof Wöll, Karlsruhe Institute of Technology, Germany

Relevant Publications

  • Dynamics of Spatially Confined Bisphenol A Trimers in a Unimolecular Network on Ag(111)
  • Spin manipulation by creation of single-molecule radical cations
  • Bistability Loss as a Key Feature in Azobenzene (Non-)Switching on Metal Surfaces
  • Broken Symmetry of an Adsorbed Molecular Switch Determined by Scanning Tunneling Spectroscopy
  • Photoswitching in nanoporous, crystalline solids: an experimental and theoretical study of azobenzene linkers incorporated in MOFs
  • Assessing computationally efficient isomerization dynamics: SCF density-functional theory study of azobenzene molecular switching



Dynamics of Molecules at Surfaces

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Explicit atomistic simulation of the dynamics of molecular reactions can help to gain valuable understanding of processes relevant to catalysis, surface chemistry, and surface scattering. Molecular dynamics on metal surfaces are especially interesting due to a variety of quantum and nonadiabatic effects that determine the qualitative and quantitative outcome of reactions. Such effects include non-adiabatic energy transfer between adsorbate motion and substrate electron-hole pair excitations leading to vibrational cooling of adsorbate motion or quantum nuclear effects that facilitate barrier crossings via tunnelling. Our current efforts are in developing methodology to include the above quantum effects in the study of non-equilibrium processes on surfaces.


Collaborations:
  • John C. Tully, Yale University, USA
  • Hua Guo, University of New Mexico, USA
  • Bin Jiang, University of Science and Technology, Hefei, China

Relevant Publications:

  • The Role of Tensorial Electronic Friction in Energy Transfer at Metal Surfaces
  • Ab-initio tensorial electronic friction for molecules on metal surfaces: nonadiabatic vibrational relaxation
  • Mode specific electronic friction in dissociative chemisorption on metal surfaces: H2 on Ag (111)



Computational Surface Spectroscopy

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Surface-sensitive spectroscopic methods such as x-ray photoelectron spectroscopy (XPS), near-edge x-ray absorption fine-structure spectroscopy (NEXAFS) , and, most prominently, Scanning Tunneling Microscopy and Spectroscopy (STM/STS), play a key role in the characterization of adsorbate structure. These methods have become the workhorses for the study of purpose-driven design based on molecular self-assembly and surface functionalization on the nanoscale. Corresponding techniques to simulate the above act as complementary validation methods and support assignment of spectroscopic signatures to structural moieties. We furthermore work on establishing efficient Density-Functional Theory-based methods to calculate electronically excited states of metal-surface adsorbed molecules.

Collaborations:
  • F. Stefan Tautz, FZ Jülich, Germany
  • Michael Gottfried, University of Marburg, Germany

Relevant Publications:

  • Interpretation of X-ray Absorption Spectroscopy in the Presence of Surface Hybridization
  • Assessing computationally efficient isomerization dynamics: ΔSCF density-functional theory study of azobenzene molecular switching
  • Excited-state potential-energy surfaces of metal-adsorbed organic molecules from linear expansion Δ-self-consistent field density-functional theory (ΔSCF-DFT)
  • Thiolate-Bonded Self-Assembled Monolayers on Ni(111): Bonding Strength, Structure and Stability
  • Temperature-dependent templated growth of porphine thin films on the (111) facets of copper and silver
  • X-ray standing wave simulations based on Fourier vector analysis as a method to retrieve complex molecular adsorption geometries

Efficient Approximate Electronic Structure Methods for Hybrid Organic-Inorganic Interfaces

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The typical system sizes involved in the study of structure and dynamics of adsorbates on surfaces pose a challenge to highly accurate first-principles methodology. Correspondingly, there is a significant need for efficient simulation methodology that addresses large time and length scales than currently accessible to Density Functional Theory (DFT) and efficient ab initio molecular dynamics methods. We currently address these challenges by developing global structure prediction methods for complex interfaces and efficient semi-empirical electronic structure methods. Many systems that are currently studied in the context of molecular machinery and nanotechnology are far too large to be studied with DFT and more approximate methods have to be developed.

Collaborations:
  • Karsten Reuter, Technical University Munich, Germany
  • Chiara Panosetti, Technical University Munich, Germany

Relevant Publications:

  • Charge-Population Based Dispersion Interactions for Molecules and Materials
  • Global materials structure search with chemically-motivated coordinates
  • Excited-state potential-energy surfaces of metal-adsorbed organic molecules from linear expansion Δ-self-consistent field density-functional theory (ΔSCF-DFT)

Design and Study of Functional Hybrid Organic-Inorganic Interfaces

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Fundamental to the design of functionalized surfaces, and optically or electronically active hybrid interfaces is the understanding of the underlying interactions between substrate and adsorbate. First principles based molecular simulation provides the necessary atomistic insight into geometry and energetics that determine these composite materials. We actively contribute to the development and assessment of methods that improve the description of long-range dispersive interactions and study the effects of anharmonic mode coupling and finite-temperature on adsorbate structure and energetics.

Collaborations:
  • Alexandre Tkatchenko, University of Luxembourg, Luxembourg
  • Stefan F. Tautz, FZ Jülich, Germany
  • Phil Woodruff, University of Warwick, UK
  • Giovanni Costantini, University of Warwick, UK

Relevant Publications:

  • Thermal and electronic fluctuations of flexible adsorbed molecules: Azobenzene on Ag(111)
  • Many Body Dispersion Effects in the Binding of Adsorbates on Metal Surfaces
  • Temperature-dependent templated growth of porphine thin films on the (111) facets of copper and silver
  • Quantification of finite-temperature effects on adsorption geometries of π-conjugated molecules: Azobenzene/Ag(111)
  • Adsorption structures and energetics of molecules on metal surfaces: Bridging experiment and theory