All-inorganic lead-free luminescent metal halide nanocrystals (NCs) are very important in optoelectronics, but their applications are limited by the low photoluminescence (PL) efficiency. It is an effective approach via ns²-metal ions doping for tailoring the optical properties of metal halide NCs and expanding their applications.  

However, the PL origin of ns²-metal ion doped metal halides remains controversial and not well understood. Moreover, the controlled synthesis of Sb³⁺-doped 0D In-based halide NCs and the size effect on the optical properties and excited-state dynamics remain unexplored.  

In a study published in Nano Today, the research group led by Prof. CHEN Xueyuan from Fujian Institute of Research on the Structure of Matter (FJIRSM) of the Chinese Academy of Sciences (CAS) reported the controlled synthesis of Sb³⁺-doped 0D Cs₃InCl₆ NCs with tunable sizes and investigated the effect of NC size on the optical properties and excited-state dynamics of Sb³⁺. 

The researchers obtained 0D Cs₃InCl₆: Sb³⁺ NCs with tunable sizes by the controlled synthesis based on a HCl hot-injection method. The as-synthesized NCs exhibit bright green PL with a broad emission band.  

They comprehensively surveyed the effects of NC size and Sb³⁺ concentration on the electronic structures and PL properties of the NCs and the excited-state dynamics of Sb³⁺ by means of temperature-dependent steady-state and transient PL and ultrafast femtosecond transient absorption (fs-TA) spectroscopies.  

Owing to the strong electron-phonon coupling of Sb³⁺ in the spatially confined 0D structure of Cs₃InCl₆, Sb³⁺ ions experienced a dynamic Jahn-Teller distortion in the ³P₁ excited state and an off-center position in the ¹S₀ ground state, which resulted in intense broadband emission of Sb³⁺ from the inter-configurational ³P₁ → ¹S₀ transition with a large Stokes shift and a high PL quantum yield (QY) of 52.3%. 

Furthermore, taking advantage of the hydroscopicity of the NCs, the researchers demonstrated the water-triggered chemical transformation of the NCs from green-emitting Cs₃InCl₆: Sb³⁺ to orange-emitting Cs₂InCl₅·H₂O: Sb³⁺ with a PLQY up to 75.3%, thus revealing the superiority of the Sb³⁺ luminescence in metal halide NCs. 

These findings provide fundamental insights into the excited-state dynamics of Sb³⁺ in 0D In-based halide NCs, thereby laying a foundation for future design of novel lead-free metal halide NCs with tunable structure and switchable PL through ns²-metal ion doping or alloying towards versatile applications such as luminescent chemosensors.