Lead-free cesium bismuth bromide perovskite nanocrystals are synthesized via the heating-up method with tailored morphology and optical properties.

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Reconfigurable light absorbers have attracted much attention by providing additional optical responses and expanding the number of degrees of freedom in security applications. Fabry− Perot absorbers based on phase change materials with tunable properties can be implemented over large scales without the need for additional steps such as lithography, while exhibiting reconfigurable optical responses. However, a fundamental limitation of widely used phase change materials such as vanadium dioxide and germanium−antimony−tellurium-based chalcogenide glasses is that they have only two distinct phases; therefore, only two different states of optical properties are available. Here, we experimentally demonstrate active multilevel absorbers that are tuned by controlling the external temperature. This is produced by creating large-scale lithography-free multilayer structures with both undoped and tungsten-doped solution-processed monoclinicphase vanadium dioxide thin films. The doping of vanadium dioxide with tungsten allows for the modulation of the phase-transition temperature, which results in an extra degree of freedom and therefore an additional step for the tunable properties. The proposed multilevel absorber is designed and characterized both numerically and experimentally. Such large-scale multilevel tunable absorbers realized with nanoparticle-based solution fabrication techniques are expected to open the way for advanced thermo-optical cryptographic devices based on tunable reflective coloration and near-infrared absorption.

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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...

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