van der Waals Heterostructures Based on Nanolayered Paramagnetic Tm(II) Compounds and Boron Nitride for Investigating Spin Frustration

The utility of molecular precursors to create thin-film components for constructing van der Waals heterostructures was explored by studying the layered Tm(II) compound [(THF)Cs(μ-η 5:η 5 -Cp′) 3 Tm II ] n (Cp′ = C 5 H 4 SiMe 3 ). This compound, synthesized by the reduction of Cp′ 3 Tm III with metallic cesium, crystallizes as layers of hexagonal nets composed of Tm(II) ions and Cs(I) ions at alternating vertices, bridged by Cp′ ligands. Full characterization of the compound, including SQUID magnetometry, was consistent with 4f 13 Tm(II). Though the triangular orientation of S = 1/2 Tm(II) ions in the layered structure indicated a potential environment for spin frustration, the SQUID data suggest these metal centers lack enough interaction to enforce such behavior. Successful exfoliation of the air- and moisture-sensitive single crystals was achieved in a glovebox at low temperatures to yield thin films. These could be stacked under hexagonal boron nitride to create an air-stable van der Waals heterostructure, which allowed analysis by AFM to determine a thickness of approximately 50 layers for the covered Tm-containing structure.