Phase Engineering on Metastable Lanthanoid Sulfides for Polymorphic Nanocrystal Library.

Rare earth (RE)-based nanocrystals (NCs) with rich elemental composition and unique electron configuration are promising for a wide range of important applications. However, their precise creation remains to be a synthetic bottleneck, limited to the ultralow reduction potential, strong oxophilicity, rigid compatibility and in particular regarding phase diversity beyond the thermodynamically stable counterparts. Here we define a transition metal cation-stabilized strategy for the phase-selective synthesis of lanthanoid (Ln) sulfide NCs including monoclinic (m), orthorhombic (o), and trigonal (t) crystal systems. We further quantitatively reveal the phase-controlled morphological and structural motif evolution processes of the metastable frameworks, via a combination of lattice energy-dependent thermodynamic control and facet adsorption-induced kinetic control. An extension of the proposed mechanism constructs phase-designed t/m-homojunction and t/o-heterostructure Ln sulfide NCs with multidimensionality and tunable composition, structures, and interfaces. This generalizable phase-engineering protocol provides guiding principle and synthetic methodology for a RE-based polymorphic nanocrystal library that is otherwise inaccessible by conventional approaches.