When a crystal is cleaved, a new surface forms that interrupts the regular distribution of ions within the material. The breaking of surface bonds and the alteration of interatomic forces increase the surface stress and reduce the stability of the fresh surface. To overcome this instability, the majority of surfaces undergo a spontaneous geometrical reconstruction, which is generally associated with charge transfer between surface atoms. Unraveling the mechanism responsible for such reconstructions is essential to understanding properties of surfaces, and it helps optimize materials performance in applications such as microelectronics and fuel cells. By combining first principles calculations (University of Vienna, group Cesare Franchini) and surface sensitive experiments (TU Wien, group Ulrike Diebold), an alternative and radically different mechanism for surface reconstructions was found based on charge trapping.
Polarons play a pivotal role in this process. These quasiparticles, which form via the coupling between excess charges and the lattice phonon field, are ubiquitous in polar semiconductors such as oxides. We studied an archetypal polaron material, rutile titanium dioxide, and varied the polaron density at the surface by introducing an increasing number of surface oxygen vacancies. Owing to the repulsive interaction between the polarons, the surface free energy increases until, at a critical amount, the surface transforms.
This polaron-mediated mechanism is likely to be a pervasive phenomenon that could explain structural, electronic, and magnetic reconstructions at surfaces and interfaces of ionic materials. Besides the fundamental interest, surface polarons could be employed to tune surface properties, control surface geometries, and provide a way to facilitate charge transfer in catalytic processes.
Michele Reticcioli, Martin Setvin, Xianfeng Hao, Peter Flauger, Georg Kresse, Michael Schmid, Ulrike Diebold, and Cesare Franchini, "Polaron-Driven Surface Reconstructions",
Phys. Rev. X 7, 031053 (2017)
