Surface morphologies, electronic structures, and Kondo effect of lanthanide(iii)-phthalocyanine molecules on Au(111) by using STM, STS and FET properties for next generation devices

Abstract

The crystal structures of double-decker single-molecule magnets (SMMs) LnPc₂ (Ln = Tb(III) and Dy(III); Pc = phthalocyanine) and non-SMM YPc₂ were determined by using single crystal X-ray diffraction analysis. The compounds are isomorphous to each other. The compounds have metal-centers (M³⁺ = Tb, Dy, and Y) 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 (STM) was used to investigate the compounds adsorbed on a Au(111) surface, deposited by using thermal evaporation in ultra-high vacuum. Both MPc₂ with eight-lobes and MPc with four-lobes, which has lost one Pc ligand, were observed. In the scanning tunneling spectroscopy (STS) 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. In order 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 (OFETs) 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 hole mobility (μ_H) of ∼10⁻⁴ cm² V⁻¹ s⁻¹. This behavior has important implications for the electronic structure of the molecules.