Lead (Pb) based perovskites with the general formula ABX₃ have attracted significant attention in providing desirable optoelectronic devices, such as solar cells, light-emitting diodes (LEDs), and photocatalysis. However, the high toxicity and low stability of Pb are a big concern for the health and the environment as well as commercialization, leading to extensive research to replace lead with no or less toxic elements [1], [2], [3]. Recent works have been exhibited tin (Sn²⁺) as a promising alternative due to comparable ionic radius and in a group electronic configuration to Pb⁺. Nevertheless, Sn²⁺ is prone to oxidation, easily transforming into Sn⁴⁺ in ambient condition, which creates a destabilized perovskite structure and significantly reduces its functional performance [4]. To overcome this issue, this work explores an alternative strategy by inserting the Sn²⁺ octahedron into a layered double perovskite (LDP) with the formula Cs₄M(II)M(III)₂Cl₁₂. In this LDP structure, Sn²⁺ is placed on the M(II) site, while the M(III) site is occupied by different trivalent metal cations such as Bi³⁺, In³⁺, or Sb³⁺. The two M(III) octahedrons in a unit cell serve as top and bottom protective layers, effectively suppressing the oxidation sensitivity of Sn²⁺ by minimizing the contact with any degradation factors such as oxygen and water molecule leads to the formation of Sn⁴⁺.

Our x-ray diffraction (XRD) and elemental analysis confirm the successful formation of LDP Cs₄M(II)M(III)₂Cl₁₂ nanocrystals (NCs) via hot injection method, showing well-defined and stable crystal structures. The as-synthesized perovskite NCs demonstrate a morphology of hexagonal nanoplatelets with diverse size distribution based on the selected M (III) cations. Specifically, In-based nanoplatelets show an average size of 41.4 nm, while Bi and Sb cases exhibit average sizes of 28.9 nm and 58.8 nm, respectively. The size variance is generated by distinct ionic radii and chemical surroundings of the trivalent metal ions.

Compared to conventional lead-based perovskite, the long term stability test upon the tracking of XRD patterns reveals that these NCs could retain their original structural integrity for more than 100 days, a remarkable improvement over Pb or other Sn(II)-based perovskite, which latter case normally degrades even within hours. In addition, the optical stability of the LDP NCs, as regularly measured by UV-visible absorption spectroscopy, maintained the original feature for over 40 days, indicating a hint of their potential for stable optoelectronic applications.

Besides, this nanomaterial also demonstrates tunable electronic properties by substituting distinct trivalent metal cations at the M(III) site, which means the bandgap could be adjusted. For instance, the presence of In³⁺ shows a relatively wide bandgap at 3.54 eV, indicating a good fit for widescale of bandgap semiconductors. Interestingly, once Sb³⁺ is placed at the M(III) site, the bandgap could be narrowed to 2.09 eV, which is in turn suitable for better light-harvesting. We also investigate the emission properties by measuring the photoluminescence (PL) spectra in the visible range, showing a wide and asymmetric spectral feature in all cases. This is likely due to the formation of self-trapped exciton (STE) states, which have been observed in other reports that show LDP structure. Also, the excited state lifetime of these NCs really depends on the selection of M(III) cations, for which Bi-based LDP NCs reveal the extremely fastest decay time at 18 ps, while In³⁺ and Sb³⁺ cases exhibit decay time 83 ps and 303 ps, respectively.

In summary, this work demonstrates a novel Sn(II)-based LDP NCs with various trivalent metal ions at the M(III) site, showing significantly enhanced structural stability and tunable optical properties, all which make them highly versatile for a wide range of application in future.