Single-Source Precursors for Lanthanide Diselenide Nanosheets

A series of complexes of the type [Ln(Se₂P(phenyl)₂)₃(CH₃CN)_x] (x = 1 or 2) have been synthesized and structurally characterized for Ln = La-Lu (excluding Pm). The complexes are straightforward to prepare and crystallize and are soluble in solvents typically used in nanoparticle synthesis. Solution-phase thermolysis of these complexes formed lanthanide diselenide or LnSe_2-x nanosheets, for Ln = La-Ho, except Eu, which formed EuSe. By contrast, the smallest lanthanides Ln = Er-Lu formed Ln₂Se₃ nanomaterials. The lanthanide dichalcogenides are rare-earth analogues of the transition metal dichalcogenides and exhibit similar properties, including small band gaps, charge density waves, antiferromagnetism, and superconductivity. The lanthanide diselenide nanomaterials exhibited highly anisotropic growth and were phase pure with the exception of CeSe₂ (which also formed a cerium oxide and cerium ultraphosphate). The lateral nanosheet dimensions, based on transmission electron microscopy (TEM), range from ∼50 to 500 nm, and the thickness was found to be ∼4 nm for the thinnest sheets by atomic force microscopy. The nanosheets were further characterized by powder X-ray diffraction, scanning electron microscopy, and Raman spectroscopy. Alloy formation was also demonstrated with the combined solution thermolysis of Sm and Gd complexes to form Sm_1-xGd_xSe_1.8. The alloy was found to have homogeneous composition on the basis of powder X-ray diffraction and TEM.