all-inorganic ternary perovskite nanocrystals (NCs) are widely studied for their promising characteristics as semiconducting materials for optoelectronic applications owing to their high photoluminescence (PL) quantum yields (PLQYs) and narrow emission bandwidths. [3] The optical properties of perovskite NCs can be tuned by tailoring their sizes, compositions, and confined dimensions. [4][5][6][7] In addition to size confinement on the nanoscale, the dimensionality of ternary metal halides can be defined based on the arrangement of the [BX 6 ] octahedral units in the crystal structure. [8,9] Lattice distortion occurs within the metal halide structure depending on the size of the elements or the coordination number of the A cations. This induces either a two-or 1D arrangement, wherein the [BX 6 ] octahedra are linked in two directions or one direction, respectively, or complete isolation of the octahedral units (0D). [10][11][12] Among these, 0D materials have recently attracted considerable attention because their unique optical and electronic properties are distinct from those of 3D metal halide perovskite structures. [13] Specifically, 0D metal halide materials have electronically separated polyhedra or clusters resulting in a large bandgap, low conductivity, and low mobility. [8,14,15] In addition, these materials show enhanced radiative recombination owing to the self-trapping of excitons [16,17] or defects [18,19] in the spatially isolated octahedrons and broadband PL properties attributed to strong exciton-phonon coupling.Several studies have synthesized 0D metal halide structures with isolated metal halide octahedral building blocks, and their intrinsic properties have been investigated by tailoring the composition. For example, 0D Cs 4 PbBr 6 perovskites have been extensively studied because of their unique optical properties such as high PLQY, as well as high thermal, chemical, and photostabilities; however, the origin of the green emission of Cs 4 PbBr 6 is still under debate. [20][21][22][23] Lead-free 0D metal halides have also been reported, containing either main group elements, such as tin, [24,25] bismuth, [26] indium, [27] and antimony, [28] or transition metals, such as copper, [29] manganese, [30] and zinc. [31] These metal halides can be utilized as Herein, the synthesis of novel, highly luminescent, and nearly monodisperse zero-dimensional (0D) cesium lanthanide chloride (Cs 3 LnCl 6 ; Ln = Y, Ce, Gd, Er, Tm, Yb) colloidal nanocrystals (NCs) is reported for the first time. The Cs 3 LnCl 6 NCs are synthesized using a heating-up method and exhibit highly uniform size and shape. The monoclinic-phase Cs 3 LnCl 6 NCs contain completely isolated [LnCl 6 ] 3− octahedral units, resulting in 0D ternary metal halide structures. Therefore, these NCs exhibit deep-blue photoluminescence under ultraviolet excitation, and this photoluminescence can be tuned by changing the lanthanide cations within the [LnCl 6 ] 3− octahedral units. High photoluminescence quantum yields of up to 60% and 90% are observed for...
