Regulation of zero-dimensional metal halides by ions with ns2 lone-pair electronic structure derived from ns2

Recently, low-dimensional metal halides have attracted considerable attention for their highly versatile tunability and outstanding broadband luminescence characteristics. Concurrently, there is an increasing number of studies focusing on the modulation of photoluminescence properties via doping, particularly with ${\mathrm{GSP}}^{2+}$ (${\mathrm{Ge}}^{2+}$, ${\mathrm{Sn}}^{2+}$, and ${\mathrm{Pb}}^{2+}$) ions possessing ${\mathrm{ns}}^{2}$ lone-pair electrons (originating from neutral atoms with an ${ns}^{2}{np}^{2}$ electronic configuration), which are considered ideal dopants. However, numerous promising experimental outcomes resulting from doping depend significantly on empirical knowledge and fortuitous circumstances. Comprehensive theoretical research is crucial to effectively guide experimental approaches and minimize resource wastage. In this study, we employ ${\mathrm{GSP}}^{2+}$ ions doped into ${\mathrm{Cs}}_{2}{\mathrm{Zn}\mathrm{Br}}_{4}$ as a model to explore the regulatory mechanisms and principles that govern the effects of various ${\mathrm{GSP}}^{2+}$ ions on zero-dimensional metal halides. Our findings indicate that ${\mathrm{GSP}}^{2+}$ ions doped into ${\mathrm{Cs}}_{2}{\mathrm{Zn}\mathrm{Br}}_{4}$ form $[{\mathrm{MBr}}_{4}{]}^{2\ensuremath{-}}$ (M = $\mathrm{Ge}$, $\mathrm{Sn}$, and $\mathrm{Pb}$) tetrahedra, which undergo deformation in the excited state, leading to self-trapping emissions in initially weak-luminescent ${\mathrm{Cs}}_{2}{\mathrm{Zn}\mathrm{Br}}_{4}$. Additionally, there is a gradual decrease in the transition dipole moment and a continuous increase in emission energy from ${\mathrm{Cs}}_{2}{\mathrm{Zn}\mathrm{Br}}_{4}:\mathrm{Ge}$ to ${\mathrm{Cs}}_{2}{\mathrm{Zn}\mathrm{Br}}_{4}:\mathrm{Sn}$ to ${\mathrm{Cs}}_{2}{\mathrm{Zn}\mathrm{Br}}_{4}:\mathrm{Pb}$. Furthermore, the luminescence lifetime of ${\mathrm{Cs}}_{2}{\mathrm{Zn}\mathrm{Br}}_{4}:\mathrm{Pb}$ is shorter than that of ${\mathrm{Cs}}_{2}{\mathrm{Zn}\mathrm{Br}}_{4}:\mathrm{Ge}$ and ${\mathrm{Cs}}_{2}{\mathrm{Zn}\mathrm{Br}}_{4}:\mathrm{Sn}$, owing to the lower barrier for the singlet to triplet transition in ${\mathrm{Cs}}_{2}{\mathrm{Zn}\mathrm{Br}}_{4}:\mathrm{Pb}$. Our theoretical analysis will facilitate the development of more efficacious doping strategies in experimental settings.