Solution-Synthesis and Optical and Magnetic Properties of MnX ₂ Se ₄ (X= Bi, Sb, In) Nanomaterials

Unique properties that emerge at the nanoscale make nanomaterials pivotal in the quest to discover and develop new functionalities. Consequently, it is crucial to adapt existing synthetic strategies or develop alternate ones to access materials traditionally found in bulk form. While significant work has been conducted on the synthesis of binary and ternary chalcogenides, it has primarily focused on a limited class of materials. Herein, we report our efforts to expand the solution-based synthesis of other 2-D and 3-D ternary chalcogenide nanomaterials, with a focus on the MnX 2 Se 4 (X = Bi, Sb, In) family. This class of materials is of great potential for topological, thermo/photoelectric, battery, and catalytic applications. We demonstrate the synthesis of these manganese-based ternary selenides using a two-pot, hot-injection solution-phase synthesis approach. Two distinct formation pathways were observed depending on the choice of the trivalent metal, while the use of octadecene and MnX 2 (X = I, Br) proves key to accessing the target compounds. The MnBi 2 Se 4 and MnSb 2 Se 4 particles were Se-rich as evidenced from surface and bulk composition analysis, requiring a postsynthesis treatment with a dodecanethiol-oleylamine mixture. Diffuse reflectance measurements, along with Kubelka–Munk analysis and electronic structure calculations, revealed that MnBi 2 Se 4 and MnSb 2 Se 4 possess direct band gaps of ∼1.0 and 1.3 eV, respectively, increasing to 1.5 eV for MnIn 2 Se 4 . Temperature-dependent magnetic susceptibility studies showed an absence of long-range magnetic ordering in the 7–300 K range despite AFM coupling suggesting short-range AFM correlations for all three ternary chalcogenides. Insights gained from this research could be applicable to other similar systems and can serve as a guide to precursor and reaction parameter selection.