Surface structure and reactivity of
multi-component oxides at the atomic scale
Subproject P02
Multi-component metal oxides exhibit a plethora of stoichiometry-dependent structural phases at the surface, even if the composition of the bulk is kept the same. The long-term objective of P02 is to unravel the relationship between surface electronic and geometric structure and reactivity, to ultimately tune these materials for energy-related reactions such as the ORR. The project applies the surface science approach. We will grow well-defined, epitaxial perovskite thin films of LSFO and LSMO in a UHV-based PLD/surface science apparatus under tight control of the surface stoichiometry in the first project period. We will determine the coordinates of surface atoms quantitatively using LEED-IV in close collaboration with theoretical groups.
Theoretical models will also help with interpreting atomically-resolved ncAFM/STM images. These images give direct insights into the behavior of polarons in these complex materials and show how adsorbates such as O2, H2O, CO, and CO2 interact with electronic and structural defects. XPS, TPD, and FTIR of these well-defined systems will deliver desorption energies, vibrational frequencies, and spectral fingerprints. These experimental data on well-defined systems will build a bridge when tested under ‘realistic’ environments at high pressure/temperature and in aqueous solutions. They will also serve to validate ML-based theory approaches.
Expertise
Our expertise is experimental surface science. We operate a total of seven ultrahigh-vacuum (UHV) chambers, which contain virtually all main experimental surface science techniques, as well as an (electro-)chemistry lab.
All chambers are equipped with facilities for sample preparation (sputtering/annealing/gas dosing), as well as various growth techniques (e-beam evaporators, Knudsen cells, UHV-compatible sputter deposition, pulsed laser deposition (PLD)).
Analysis techniques used in our research include:
- Scanning Tunneling Microscopy (STM) (in UHV 4K – 300 K, electrochemical STM)
- Atomic Force Microscopy (AFM): UHV-based (q+ sensor) and in the ambient (cantilever-based)
- Low-Energy Electron Diffraction (LEED)
- Reflection High Energy Diffraction (RHEED)
- X-ray Photoelectron Spectroscopy (XPS)
- Ultraviolet Photoelectron Spectroscopy (UPS)
- Auger Electron Spectroscopy (AES)
- Low-energy He+ ion scattering (LEIS)
- Thermal Programmed Desorption Spectroscopy (TPD)
Team

Former Members
Publications
2019

Reticcioli, Michele; Sokolović, Igor; Schmid, Michael; Diebold, Ulrike; Setvin, Martin; Franchini, Cesare
Interplay between Adsorbates and Polarons: CO on Rutile TiO2(110)
Journal ArticleIn: Physical Review Letters, vol. 122, no. 1, pp. 016805, 2019.
Abstract | Links | BibTeX | Tags: P02, P07, pre-TACO
@article{Reticcioli2019,
title = {Interplay between Adsorbates and Polarons: CO on Rutile TiO_{2}(110)},
author = {Michele Reticcioli and Igor Sokolović and Michael Schmid and Ulrike Diebold and Martin Setvin and Cesare Franchini},
doi = {10.1103/physrevlett.122.016805},
year = {2019},
date = {2019-01-09},
journal = {Physical Review Letters},
volume = {122},
number = {1},
pages = {016805},
publisher = {American Physical Society (APS)},
abstract = {Polaron formation plays a major role in determining the structural, electrical, and chemical properties of ionic crystals. Using a combination of first-principles calculations, scanning tunneling microscopy, and atomic force microscopy, we analyze the interaction of polarons with CO molecules adsorbed on the reduced rutile TiO_{2}(110) surface. Adsorbed CO shows attractive coupling with polarons in the surface layer, and repulsive interaction with polarons in the subsurface layer. As a result, CO adsorption depends on the reduction state of the sample. For slightly reduced surfaces, many adsorption configurations with comparable adsorption energies exist and polarons reside in the subsurface layer. At strongly reduced surfaces, two adsorption configurations dominate: either inside an oxygen vacancy, or at surface Ti_{5c} sites, coupled with a surface polaron. Similar conclusions are predicted for TiO_{2}(110) surfaces containing near-surface Ti interstitials. These results show that polarons are of primary importance for understanding the performance of polar semiconductors and transition metal oxides in catalysis and energy-related applications.},
keywords = {P02, P07, pre-TACO},
pubstate = {published},
tppubtype = {article}
}
2017

Reticcioli, Michele; Setvin, Martin; Hao, Xianfeng; Flauger, Peter; Kresse, Georg; Schmid, Michael; Diebold, Ulrike; Franchini, Cesare
Polaron-Driven Surface Reconstructions
Journal ArticleOpen AccessIn: Physical Review X, vol. 7, no. 3, pp. 031053, 2017.
Abstract | Links | BibTeX | Tags: P02, P03, P07, pre-TACO
@article{Reticcioli2017,
title = {Polaron-Driven Surface Reconstructions},
author = {Michele Reticcioli and Martin Setvin and Xianfeng Hao and Peter Flauger and Georg Kresse and Michael Schmid and Ulrike Diebold and Cesare Franchini},
doi = {10.1103/physrevx.7.031053},
year = {2017},
date = {2017-09-25},
urldate = {2017-09-25},
journal = {Physical Review X},
volume = {7},
number = {3},
pages = {031053},
publisher = {American Physical Society (APS)},
abstract = {Geometric and electronic surface reconstructions determine the physical and chemical properties of surfaces and, consequently, their functionality in applications. The reconstruction of a surface minimizes its surface free energy in otherwise thermodynamically unstable situations, typically caused by dangling bonds, lattice stress, or a divergent surface potential, and it is achieved by a cooperative modification of the atomic and electronic structure. Here, we combined first-principles calculations and surface techniques (scanning tunneling microscopy, non-contact atomic force microscopy, scanning tunneling spectroscopy) to report that the repulsion between negatively charged polaronic quasiparticles, formed by the interaction between excess electrons and the lattice phonon field, plays a key role in surface reconstructions. As a paradigmatic example, we explain the (1×1) to (1×2) transition in rutile TiO_{2}(110).},
keywords = {P02, P03, P07, pre-TACO},
pubstate = {published},
tppubtype = {article}
}