Ultrathin Films of Tin Halide Perovskite on Gold Single Crystals

This thesis investigates the growth, structural characteristics, and stability of ultrathin films of the lead-free perovskite CsSnBr3 (cesium tin bromide) on Au(111) and Au(100) substrates under ultra-high vacuum (UHV) conditions. Motivated by the need for environmentally friendly alternatives to lead-based perovskites in optoelectronic applications, this work employs a combination of scanning tunneling microscopy (STM), low-energy electron
diffraction (LEED), and X-ray photoelectron spectroscopy (XPS) to explore the fundamental properties of CsSnBr3 films.
The study examines the behavior of precursor molecules, CsBr and SnBr2, on both gold surfaces, revealing distinct growth patterns and the influence of substrate symmetry on adsorption and subsequent film formation. Co-deposition of these precursors leads to the formation of CsSnBr3 films, the structure of which is analyzed in detail. A tetramer phase on the (001) surface of CsSnBr3 is confirmed through atomically resolved STM imaging
and supported by density functional theory (DFT) calculations. Furthermore, XPS analysis provides insights into the chemical composition of the films and elucidates the degradation mechanisms upon exposure to air, highlighting the critical role of Sn4+ formation in the degradation process.
The findings demonstrate that UHV-grown CsSnBr3 films initially exhibit a favorable Sn2+ oxidation state but degrade rapidly to include Sn4+ states upon air exposure, underscoring the importance of encapsulation for maintaining stability. This research advances the understanding of lead-free perovskite film growth and stability while offering valuable insights for the development of stable, efficient, and environmentally friendly optoelectronic devices.