Direct observation of lanthanide(III)-phthalocyanine molecules on Au(111) by using scanning tunneling microscopy and scanning tunneling spectroscopy and thin-film field-effect transistor properties of Tb(III)- and Dy(III)-phthalocyanine molecules

The crystal structures of double-decker single molecule magnets (SMM) LnPc₂ (Ln = Tb(III) and Dy(III); Pc = phthalocyanine) and non-SMM YPc₂ were determined by using X-ray diffraction analysis. The compounds are isomorphous to each other. The compounds have metal centers (M ) Tb³⁺, Dy³⁺, and Y³⁺) sandwiched by two Pc ligands via eight isoindole-nitrogen atoms in a square-antiprism fashion. The twist angle between the two Pc ligands is 41.4°. Scanning tunneling microscopy was used to investigate the compounds adsorbed on a Au(111) surface, deposited by using the thermal evaporation in ultrahigh vacuum. Both MPc ₂ with eight lobes and MPc with four lobes, which has lost one Pc ligand, were observed. In the scanning tunneling spectroscopy images of TbPc molecules at 4.8 K, a Kondo peak with a Kondo temperature (T_K) of ∼250 K was observed near the Fermi level (V = 0 V). On the other hand, DyPc, YPc, and MPc₂ exhibited no Kondo peak. To understand the observed Kondo effect, the energy splitting of sublevels in a crystal field should be taken into consideration. As the next step in our studies on the SMM/Kondo effect in Tb-Pc derivatives, we investigated the electronic transport properties of Ln-Pc molecules as the active layer in top- and bottom-contact thin-film organic field effect transistor devices. Tb-Pc molecule devices exhibit p-type semiconducting properties with a hole mobility (μ_H) of ∼10^-4 cm² V^-1 s^-1. Interestingly, the Dy-Pc based devices exhibited ambipolar semiconducting properties with an electron mobility (μ_e) of ∼10^-5 and a μ_H of ∼10^-4 cm² V^-1 s^-1. This behavior has important implications for the electronic structure of the molecules.