In-depth characterization of annealing-induced restructuring processes of doped organic adlayers

Abstract

The physical impact of thermal treatment applied to an organic material hosting alkali-metal atoms can be rather complex. Therefore, especially for ultrathin films, only little is known about annealing-induced effects on the molecular scale. In this work, we study the annealing of a model system consisting of ultrathin layers of potassium-doped 3,4,9,10-perylene tetracarboxylic dianhydride (${\mathrm{K}}_{x}\mathrm{PTCDA}$) on Ag(111) [C. Zwick et al., ACS Nano 10, 2365 (2016)]. We evidence three fundamental categories of annealing-induced structural changes, ascertained for distinct doping ratios $x$ of the as-grown ${\mathrm{K}}_{x}\mathrm{PTCDA}$ phases: order-to-disorder ($x=4$), order-to-order ($x=2$), and disorder-to-order ($x=0.5$) transitions can be demonstrated. We provide an in-depth structural analysis of all well-ordered phases found by combining distortion-corrected low-energy electron diffraction and scanning tunneling microscopy with submolecular resolution. The changes of the epitaxial relations (including epitaxy type, lattice parameters, and unit-cell composition) are extracted with high precision, eventually allowing for an interpretation of the interactions yielding a particular adsorbate layer structure.