Phase Engineering on Metastable Lanthanoid Sulfides for Polymorphic Nanocrystal Library

ABSTRACT 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.