Comprehensive study of SrF2 growth on highly oriented pyrolytic graphite (HOPG): Temperature-dependent van der Waals epitaxy

This study explores the molecular beam epitaxy (MBE) growth of SrF2 on highly oriented pyrolytic graphite (HOPG) highlighting the temperature-dependent variations in growth morphology, crystalline structure and electronic properties. The comprehensive characterization of SrF2/HOPG interfaces was carried out using atomic force microscopy (AFM), reflection high-energy electron diffraction (RHEED), ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). The spectroscopy data suggest that the chemical interaction of the fluoride with the substrate is weak at each deposited thickness and temperature of the substrate during the deposition, indicating a growth under a van der Waals epitaxial regime. SrF2 nanostructures deposited on HOPG depict a distinctive bulk-like character, concerning their crystallinity and composition, even at the very initial growth stage. Remarkably, temperature plays a crucial role in driving the growth patterns, moving from coalescence of dendritic islands at room temperature to induce nearly 1D rows along the step-edges of HOPG terraces at higher temperatures (400 °C).