Synthesis of Mixed-Valent Lanthanide Sulfide Nanoparticles

Abstract

In targeting reduced valent lanthanide chalcogenides, we report the first nanoparticle synthesis of the mixed-valent ferromagnets Eu₃ S₄ and EuSm₂ S₄ . Using divalent lanthanide halides with bis(trimethylsilyl)sulfide and oleylamine, we prepared nanoparticles of EuS, Eu₃ S₄ , EuSm₂ S₄ , SmS_1.9 , and Sm₃ S₄ . All nanoparticle phases were identified using powder X-ray diffraction, transmission electron microscopy was used to confirm morphology and nanoparticle size, and magnetic susceptibility measurements for determining the ordering temperatures and valence. The UV/Vis, Raman and X-ray photoelectron spectroscopies for each phase were compared. Surprisingly, the phase is influenced by the halide and the reaction temperature, where EuCl₂ formed EuS while EuI₂ formed Eu₃ S₄ , highlighting the role of kinetics in phase stabilization. Interestingly, at lower temperatures EuI₂ initially forms EuS, and converts over time to Eu₃ S₄ .

Keywords: lanthanide chalcogenides; magnetism; phase transformation; semiconductors.

Figure 1.

A). PXRD of EuS (1-Cl) nanoparticles from EuCl₂, B). PXRD of Eu₃S₄ (1-I) nanoparticles from EuI₂. C). Transmission electron microscopy micrograph of Eu₃S₄ (1-I) with histogram (D).

Figure 2.

Characterization of ferromagnetic Eu₃S₄. A). M-vs-H of Eu₃S₄ (1-I) taken at T = 2.3 K, M_sat = 45 emu/g (45 Am/kg), H_c = 60 Oe (6 mT), the insert expands the graph to illustrate the small hysteresis. B). UV-visible spectra of Eu₃S₄ (red)-vs-EuS (black). C). XPS of the 3d region in Eu₃S₄ (1-I) nanoparticles.

Figure 3.

Characterization of EuSm₂S₄ nanoparticles (3-I) : A). PXRD, B). XPS, C). TEM and D). Size histogram.

Figure 4.

Properties of EuSm₂S₄. A). Raman spectra (laser was 514 nm) showing a smooth shift of Raman peak frequencies from Eu₃S₄ (2-I) to EuSm₂S₄ (3-I), to Sm₃S₄ (5-I). B). M-vs-H for ferromagnetic EuSm₂S₄ (3-I) at 2 K, Msat = 6.5 emu/g (6.5 Am²/kg), Hc = 14 Oe (1.4 mT), the inset expands the graph to illustrate the small hysteresis. C). TEM micrograph with EDS composition map of EuSm₂S₄ (3-I), Eu (green), Sm (blue), S (red), showing uniform distribution of metals, and Eu rich material. D). UV-visible absorption spectra of EuSm₂S₄ (3-I).

Figure 5.

A. Nanoparticle synthesis using EuI₂ and TMS PXRD over time showing the transformation of EuS (T-I-30) (blue) at 30 mins, to Eu₃S₄ (T-I-90) (red) at 90 mins. B. Summary of phase stabilization: 150°C: yellow = preformed EuS* from [Eu(S₂CNEt₂)₃Phen] at 300°C, heated with TMS forms EuS (1-SSP-TMS), purple = timed experiment of EuI₂ and TMS. 200°C: green = EuS from EuCl₂ (1-Cl), purple = Eu₃S₄ from EuI₂ (2-I). 300°C: Purple: Preformed Eu₃S₄ (2-I-300C) annealed in oleylamine, is not reduced, but remains Eu₃S₄. C. Raman spectroscopy of timed materials showing loss of EuS (T-I-30) and growth of Eu₃S₄ (T-I-90).