Manipulating the Fundamental Building Blocks of All-Inorganic Metal-Halide Perovskites Toward Novel Electronic Behavior

Metal-halide perovskites are viewed as next-generation semiconductor materials due to their facile solution processability and interesting photophysical and optoelectronic phenomena. The properties of the traditional all-inorganic ABX₃ perovskites (A = Rb⁺, Cs⁺; B = Pb²⁺, Sn²⁺; X = Cl^–, Br^–, I^–) are dominated by the three-dimensional (3D) network of metal-halide [BX₆] octahedra within the crystal structure, as these octahedra serve as both the structural and functional units of the material. Manipulation of these octahedral units thus enables modulation of the perovskite crystal’s electronic structure and optoelectronic properties and is traditionally achieved by (1) tuning the crystal’s dimensionality, and/or (2) varying the crystal’s B-site and X-site elements to form pure-halide and mixed-halide compositions. However, another mode of manipulation is to physically isolate the fundamental octahedral building block within the lattice, as is the case in the zero-dimensional (0D) vacancy-ordered double perovskites of form A₂BX₆. In particular, all-inorganic Cs₂TeX₆ (X = Cl^–, Br^–, I^–) single crystals are an excellent platform for exploring the effect that the isolation of [TeX₆]^2– octahedra in the crystal structure has on structural and electronic properties. Serving as the vibrational centers, the isolated octahedra inform the presence of strong exciton-phonon coupling and anharmonic lattice dynamics, as well as the likelihood of a random distribution of 10 octahedral symmetries within the mixed-halide compositional spaces. Serving as the absorbing and emitting centers, the isolated octahedra exhibit compositionally tunable absorption (1.50-3.15 eV) and emission (1.31-2.11 eV) energies. Due to greater molecular orbital overlap between neighboring octahedra with increasing halide anion size, there is a transition from a more molecule-like electronic structure in Cs₂TeCl₆ and Cs₂TeBr₆ – as expected from the effective 0D nature of these single crystals – to a dispersive electronic structure in Cs₂TeI₆, typical of 3D bulk single crystals. Furthermore, by manipulating the ionic bonding in these crystals, 0D semiconductor perovskite inks of Cs₂TeX₆ are readily produced, in which Cs⁺ cations and [TeX₆]²^– octahedral complex anions are stabilized in polar aprotic solvents without the presence of ligands. The successful stabilization of the fundamental [TeX₆]²^– octahedral molecules in solution creates multifunctional inks with the ability to reversibly transform between the liquid ink and the solid-state perovskite crystalline system in air within minutes, highlighting the crucial role of solvated octahedral complexes toward the rapid formation of phase-pure perovskite structures in ambient conditions. Given that these fundamental octahedral building blocks (with maximum length of 5-6 Å) can be stabilized in solution without ligands (as opposed to colloids, whose stability requires the use of ligands), these inks open the possibility to design new perovskite materials and to probe fundamental energy transfer and electronic properties intrinsic to the octahedral building block.