Bogdan Benin

Metal-halide based semiconductors have been in the limelight for the past few years as a result of the outstanding performance of devices in a variety of optoelectronic applications utilizing lead-halide perovskites. Lead-free materials based on Sb, Sn, or Bi with a three-dimensional (3D) framework, on the other hand, have yet to provide a true alternative. This thesis instead explores the field of low-dimensional, specifically zero-dimensional (0D), lead-free metal-halides as luminescent materials. These 0D materials contain disconnected metal-halide octahedra, which drastically alters their optoelectronic properties compared to fully connected 3D structures and, prior to 2017, the library of such 0D metal-halides was exceedingly small. This work began with the study of the optical properties of one known yet uninvestigated incongruently melting phase — Cs4SnBr6. This material was found to exhibit broad yet efficient room temperature photoluminescence (RT PL), which occurs as a result of the recombination of self-trapped excitons (STEs). The STE emission in this phase was then found to be compositionally tunable within the Cs4-xAxSn(Br1-yIy)6 (A=Rb,K; x,y≤1) family. The discovery of this and other phases by the community prompted a closer look at the optical properties of various additional Sn-based 0D and 1D materials such as (C4H14N2I)4SnI6 and [C(CH2)3]2SnBr4. In doing so, it became evident that their PL lifetimes were extremely temperature dependent (~ 20 ns/K). This opened the door to using 0D metal-halides as remote-optical thermometric and thermographic luminophores i.e. materials which can be used to optically determine temperature. In addition to this thermal sensitivity, this emission process was found to be intrinsic and incredibly robust with no changes to the PL lifetime observed between synthetic batches or after partial degradation or partial oxidation. These two factors together allowed for a thermometric precision of ±13 mK. Although this was quite impressive, the fact of the matter remained that these are still tin-based materials and they will, inevitably, fully oxidize. This inspired the dimensional reduction of the pnictogen halides to discover new, oxidatively stable 0D materials for remote-optical thermometry. This resulted in the Rb7Bi3-3xSb3xCl16 (x≤1) family of materials, which also exhibit STE PL with a similar thermal sensitivity as the tin-based materials. Furthermore, these structures contain edge-shared octahedral dimers, which were determined to be the source of RT PL and the luminescent properties of structures containing them have not been previously investigated. This work also led to the discovery of a new set of mixed-valent materials with the composition Rb23MIII7SbV2Cl54 (MIII = Bi, Sb). These 0D structures contain octahedra of with xi various oxidation states (3+ and 5+) and exhibit intense colors as a result of intervalent/mixed-valent charge transfer. While non-luminescent even at 12 K, these materials do exhibit relatively high mobility-lifetime products under X-ray illumination, suggesting that the site-to-site tunneling through this structure may provide a potentially useful tool for new X-ray and hard-radiation detector materials. In summary, the work presented here has resulted in several, substantial contributions to the low-dimensional metal-halide community, which include the synthesis and characterization of several new materials as well as the identification and successful demonstration of remote-optical thermometry/thermography as a new application for this class of materials. This dissertation serves as an effective foundation for further research in the field by giving other researchers an overview of the field as well as insights into potentially interesting avenues for investigation, both for materials as well as possible applications