On-Surface Structural and Electronic Properties of Spontaneously Formed Tb${}_2$Pc${}_3$ Single Molecule Magnets

The single molecule magnet (SMM) bis(phthalocyaninato)terbium (III) (TbPc${}_2$) has attracted steady research attention as an exemplar system for realizing molecule-based spin electronics. In this paper, we report on the spontaneous formation of Tb${}_2$Pc${}_3$ species from TbPc${}_2$ precursors via sublimation in ultrahigh vacuum (UHV) onto an Ag(111) surface. The molecules on the surface are inspected using combined scanning tunneling (STM) and non-contact atomic force microscopies (nc-AFM) at 5 Kelvin. Submolecular resolution and height dependent measurements supported by density functional theory (DFT) calculations unambiguously show the presence of both TbPc${}_2$ and Tb${}_2$Pc${}_3$ species. The synthesis of Tb${}_2$Pc${}_3$ species under UHV conditions is independently confirmed by chemical analysis. The high-resolution AFM imaging allows us to register the orientation of the topmost Pc ligand in both Tb${}_2$Pc${}_3$ and TbPc${}_2$ relative to the underlying Ag(111) surface. Measurements of the electronic structure reveal the selective appearance of a Kondo signature with temperature $\sim$ 30K in the Tb${}_2$Pc${}_3$ species, localized to the Pc ligand lobes. We attribute the presence of the Kondo resonance on select Tb${}_2$Pc${}_3$ molecules to the orientation of internal molecular ligands. High-resolution AFM imaging identifies geometric distortions between Tb${}_2$Pc${}_3$ molecules with and without the Kondo effect, the result of the complex interplay between structural and electronic differences.