Chemically-resolved vertical structures with sub-Angstrom resolution
We employ and develop the normal incidence x-ray standing wave technique (NIXSW) to study interfaces between molecular layers and their substrates, and also (internal) interfaces of two-dimensional materials.
The NIXSW technique is based on x-ray diffraction and x-ray photoemission. It relies on the presence of a crystalline substrate in contact to the sample structure and provides ultra-precise structural information with respect to the crystal lattice of the substrate. In particular, adsorption heights, layer distances and other vertical dimensions can be obtained at a precision better than 0.05 Å. Because X-ray reflectivity information and photoemission data are acquired simultaneously, the model-free data analysis is complex. Our open-source analysis program Torricelli automizes the data analysis [1, 2]. In Torricelli, non-dipolar effects in the photoemission process and other geometric parameters (often ignored until now) are systematically taken into account [3]. We have applied NIXSW both to molecules [4, 5, 6, 7, 8, 9, 10, 11] and two-dimensional materials [12, 13, 14, 15]. Recently, we have also applied the NIXSW method to determine the vertical position of dopants in a bulk crystal: Sr dopants in Bi2Se3 are discussed in the context of topological superconductivity, and their precise position in the unit cell appears to be decisive in this context. Considering the inelastic mean free path of the electrons, usually neglected when analyzing NIXSW data, opened the door for the first unambiguous site determination of Sr dopants in Bi2Se3 [16].
Experiments are carried out at the NIXSW beamline I09 at the Diamond Light Source. It is the only photoemission-based XSW beamline worldwide which allows to apply this technique in UHV.
