As a more stable and environmentally benign alternative to lead-based halide perovskites, all-inorganic zero-dimensional (0D) metal halides with a unique structure in which isolated metal halide polyhedrons bridged by monovalent alkali metal ions have become a hot material in recent years. The intrinsic 0D electronic structure induces strong exciton localization, leading to characteristic broadband emissions. Besides, by combinations of various metal and halide elements, 0D metal halides have more compositional and structural changes with different coordination polyhedrons compared with the octahedron structure in 3D perovskite. In addition, The luminescence properties of 0D metal halides can also be tuned across the ultraviolet, visible, and near-infrared spectra by incorporating multiple ions. Although 0D metal halides have appealing luminescence properties, the emission modulations of different host doping systems still need to be explored for promising applications.

This thesis begins with a literature review of the synthetic protocols, novel luminescence characteristics, and emerging applications of various all-inorganic 0D metal halides. Diverse configurations of the metal halide polyhedrons and their spatial arrangement provide various opportunities for the formation of 0D metal halides. The 0D metal halides can be classified by the coordination polyhedrons of the B-site metal ions, such as MX₆ octahedron, MX₄ tetrahedron, MX₃ pyramid, and M_mX_n polyhedron dimer. Optical modulation could be realized by incorporating ions into different B-site polyhedrons. Besides, information storage and encryption, thermal sensing, and solid-state lighting enabled by these advanced optical materials will also be discussed.

In Chapter 4, we report a multiexcitonic emission process in Sb³⁺-doped Cs₂ZrC_l6 perovskite crystals due to the intrinsic host self-trapped excitons and dopant-induced extrinsic self-trapped excitons, respectively. Steady-state and transient-state spectroscopy reveal that the host and dopant STEs can be independently charged at specific energies. Density functional theory calculations confirm that the multiexcitonic emission stems from minimal interactions between the host and dopant STEs in the zero-dimensional crystal lattice. By selective excitation of different STEs through precise control of excitation wavelength, we further demonstrate dynamic color tuning in the Cs₂ZrCl₆:Sb³⁺ crystals. The color kinetic feature offers exciting opportunities for constructing multicolor light-emitting devices and encrypting multilevel optical codes.

In general, the inhomogeneous distribution of energy states in different coordinating environments determines the d-orbitals energy level splitting of B-site ions, leading to a variety of optical transition processes. Therefore, we change the B-site ions in Chapter 5 to study the optical properties and the application in thermal sensing. In Chapter 5, we present an investigation of thermo-responsive luminescence in doped Cs₂ZnCl₄ crystals. Density functional theory calculations reveal a thermal elevation of the orbital state of the dopant ions originating from distortion of the local coordination polyhedron, leading to a blueshift of the self-trapped exciton (STE) emissions. By examining a series of Cs₂ZnCl₄ crystals doped with Sb³⁺, Te⁴⁺, Cu⁺, and Mn²⁺, we quantitatively address the polyhedral distortion and its effect on the luminescence properties. Owing to the 0D nature of the host material, we further synthesize Sb³⁺/Mn²⁺ co-doped Cs₂ZnCl₄ with dual emissions that display distinct luminescence responses to thermal stimuli. The advances in these optical materials enable a new tactic to construct tunable thermochromic luminescence for applications such as thermal sensing and optical encryption.

In addition to changing the B-site ions of the host, we further explore the luminescence process of co-doped ions in all-inorganic lead-free metal halides. In Chapter 6, we present a codoping strategy to incorporate Bi³⁺ and Te⁴⁺ emission centers into all-inorganic lead-free Rb₂HfCl₆ perovskite crystals. The as-prepared Rb₂HfCl₆ crystals show bright blue (Bi³⁺), yellow (Te⁴⁺), and white (Bi³⁺/Te⁴⁺) emissions, respectively. The dual-band emission was assigned to [BiCl₅]^2- and [TeCl₆]^2- polyhedron-related self-trapped excitons. In addition, a WLED device based on Bi³⁺/Te⁴⁺ co-doped Rb₂HfCl₆ perovskite crystals with a CIE of (0.312, 0.386), a CCT of 6674K, and a high light efficiency of 96.47 lm W^-1, was fabricated with a UV-LED chip (365 nm). Our findings suggest that Bi³⁺/Te⁴⁺ co-doped Rb₂HfCl₆ crystals are a potential single-phase white light-emitting alternative to lead-free perovskite for next-generation solid-state lightings.