Electrical and chemical properties of vacancy-ordered lead free layered double perovskite nanoparticles

In this work we synthesized vacancy-ordered lead-free layered double perovskite (LDP) nanoparticles. This structure consists of two layers of trivalent metal halide octahedra [B(III)X₆]³⁻ separated by a layer of divalent metal [B(II)X₆]⁴⁻ (B is a divalent or trivalent metal). The chemical formula of this structure is based on A₄B(II)B(III)₂X₁₂ where A is Cs, B(III) is Bi, X is Cl and B(II) is a different ratio between Mn²⁺ and Cd²⁺. Well-defined colloidal nanoplates of Cs₄Cd_xMn_1−xBi₂Cl₁₂ were successfully synthesized. These nanoplates show photoluminescence (PL) in the orange to red region that can be tuned by changing the Cd/Mn ratio. High resolution scanning transmission electron microscopy (HR-STEM) and atomic resolution elemental analysis were performed on these lead free LDP nanoplates revealing two different particle compositions that can be controlled by the Cd/Mn ratio. Ultraviolet Photoelectron Spectroscopy (UPS) and scanning tunneling spectroscopy (STS) reveal the band gap structure of these LDP nanoplates. Density functional theory (DFT) calculations show the existence of [MnCl₆]⁴⁻ in-gap states. While the absorption occurs from the valence band maximum (VBM) to the conduction band minimum (CBM), the emission may occur from the CBM to an in-gap band maximum (IGM), which could explain the PL in the orange to red region of these nanoplates. This work provides a detailed picture of the chemical and electronic properties of LDP nanoparticles.