The interpretation of experimental results and the elaboration of a detailed understanding of processes in the quantum realm require sophisticated theoretical support. In our research we combine different levels of theory such as density functional theory, semi-empirical and classical approaches, and apply them to realistic atomistic models to gain fundamental insight into experimental findings. A close exchange between theory and experiment helps to validate and improve theoretical models and computational algorithms. On the other hand predictive simulations also inspire novel materials concepts and experimental investigation strategies.
Challenges in the ab initio modeling of surface-supported nanosystems
From a theoretical point of view, an accurate description of the structural and electronic properties of hybrid adsorbate/substrate systems such as a graphene nanoribbon in contact with a substrate poses formidable challenges. For example, we are faced with the choice of an appropriate scheme for including dispersion forces at the DFT level (van der Waals corrections), an issue of particular importance for carbon nanostructures on metallic substrates, where tiny structural changes dramatically affect the electronic and magnetic properties. Another challenge is the accurate computation of properties such as the electronic and optical band gaps, which require going beyond the usual approximations of DFT, e.g. to advanced many-body perturbation theory in the form of the GW approximation. Codes such as YAMBO and BerkeleyGW allow obtaining such properties with good accuracy and efficiency. In many cases, we are testing different algorithmic solutions together with the code developers. Mastering the proper theoretical schemes allows us to follow the details of on-surface reactions and to accurately describe the electronic properties of the systems under investigation for a detailed comparison with experiments.
Large scale molecular dynamics simulations
The understanding of mesoscopic phenomena requires the treatment of up to millions of particles and is thus beyond the reach of purely ab-initio calculations. Semi empirical methods provide the tools to tackle such problems and thus make the simulation of complex and diverse processes in materials science possible. For this kind of simulations we rely on highly efficient codes such as LAMMPS. The length and time scales that are accessible to this class of simulations allows the modeling of a wide range of phenomena such as friction (e.g. in the case of an AFM tip sliding on a molecular crystal substrate, see figure), diffusion and melting at interfaces, formation of amorphous coatings, grain boundaries, or dislocations. The applicability and the transferability of the molecular dynamics approach rely on the use of suitable interaction schemes, namely force fields that allow a prediction of the structure and properties of novel systems. The development and testing of classical potentials is thus an active line of research in our group, for example for metal-ceramic systems or for metal oxides of technological interest.
Modeling of analytical tools
Sophisticated experimental approaches such as low temperature scanning tunneling microscopy (STM) and non-contact atomic force microscopy (nc-AFM) call for reliable but at the same time manageable simulation methods that can rapidly give insight into experimental results. In our research we exploit available simulation schemes for scanning probe microscopy, and test novel approaches in collaboration with developer groups in Switzerland and abroad. One example is tip-functionalized nc-AFM, where we complement a classical probe particle model (collaboration with P. Jelinek, Prague, see figure) with more advanced DFT based schemes that include chemical interactions between the functionalized tip and the substrate. Another important activity is the simulation of core-level spectroscopy data such as from X-ray absorption (NEXAFS) and electron energy loss (EELS) experiments. In collaboration with experimental groups from Empa, the Swiss Light Source and IBM’s Binnig-Rohrer Nano-Center, we apply a variety of methods ranging from DFT approaches able to describe the effect of the core hole on the electronic structure (e.g., cp2k and WIEN2K) to multiple scattering approaches such as FEFF.