Lead-free double perovskite materials, A 2 M(I)M′(III)X 6 , have recently attracted attention as environment-friendly alternatives to lead-based perovskites, APbX 3 , because of both rich fundamental science and potential applications. We report band gap tuning via alloying of Cs 2 AgBiCl 6 nanocrystals (NCs) with nontoxic, abundant Na. It results in a series of Cs 2 Na x Ag 1−x BiCl 6 (x = 0, 0.25, 0.5, 0.75, and 1) double perovskite NCs, leading to increase in optical band gap from 3.39 eV (x = 0) to 3.82 eV (x = 1) and 30-fold increment in weak photoluminescence. The tuning of band gap has been further explored by electronic structure calculation under the framework of density functional theory (DFT). The latter confirms that the increase in band gap is due to reduction of Ag contribution near valence band maxima (VBM) on incorporation of Na ion in place of Ag. These alloyed double perovskites can have useful potential applications in optoelectronic devices.

Lead-free halide double perovskites (DPs) have emerged as a stable and greener alternative to very toxic lead-based perovskites owing to their outstanding photophysical properties. Here, a series of Cs 2 (Na x Ag 1−x )BiBr 6 (x = 0, 0.25, 0.5, 0.75, and 1) double-perovskite nanocrystals (NCs) are synthesized, which have not been much explored yet. The formation of highly miscible Cs 2 (Na x Ag 1−x )BiBr 6 solid solutions is favored because of very low lattice mismatch between Cs 2 NaBiBr 6 and Cs 2 AgBiBr 6 . These DP NCs have tunable band gaps, which successively increase with increasing Na/Ag ratio, enabling us to tune their band gap by more than 0.5 eV. The band gap tuning is further confirmed by electronic structure calculations using density functional theory (DFT), which is in line with the band gap estimated from the absorption data. The Cs 2 (Na x Ag 1−x )BiBr 6 (x = 0, 0.25, 0.5) DP NCs are found to be more stable in the ambient conditions than the Cs 2 (Na x Ag 1−x )BiBr 6 (x = 0.75, 1.00) DP NCs.

The layered hybrid double perovskites (LDPs) possess excellent stability and environmental friendliness, which makes them remarkable semiconducting materials. Contrary to significant advancements, the nonfluorescent nature at ambient temperature and pressure is still a major hurdle in this intriguing family. Here, we demonstrate doping of the transition metal cation Mn 2+ in two-dimensional (2D) (PEA) 4 NaInCl 8 (PEA = phenylethylamine) LDP, by a solution-processed crystallization method. This results in broadband emission at ambient conditions and initial contraction followed by expansion of host lattice on increasing dopant concentration. A higher dopant feed ratio in this wide band gap material leads to the absorption at 2.95 eV due to the 6 A 1 ( 6 S) → 4 A 1 ( 4 G) transitions on Mn 2+ centers. First-principles calculations based on density functional theory (DFT) confirm that Mn 2+ in substitutional sites results in lattice contraction while interstitial site Mn 2+ doping leads to lattice expansion. The potential of Mn 2+ to improve optical and magnetic properties of host lattice and a deeper understanding of distribution of Mn 2+ dopant make these LDPs a promising material for emitters for solid-state lighting and magneto-optical applications.

Lanthanide ions (Ln3+) are well-known dopants for controlling the optoelectronic properties of double perovskites (DPs). However, the excitation energy of Ln3+-doped Cs2AgInCl6 being too high (∼250–290 nm) limits its direct excitation by commercial UV light-emitting diodes (≥365 nm). To overcome this challenge, we employed Bi3+ as a sensitizer to induce the emission of Sm3+ at much lower excitation energy in Sm3+–Bi3+ codoped Cs2AgInCl6 DP nanocrystals (NCs). Spectral analysis shows that a trace amount of Bi3+ (∼1%) doping provides dual emission of self-trapped excitons (STEs) and characteristic emissions of Sm3+ assigned to 4G5/2 to 6H J (J = 5/2, 7/2, 9/2, and 11/2) transitions with 368 nm excitation energy. Transient absorption spectroscopic results revealed the existence of nonradiative energy transfer from STE states. Subsequently, we propose a mechanism to explain the formation of energy-transfer channels from STEs to excited states of Sm3+. Our study demonstrates that Bi3+ can efficiently sensitize Sm3+ to modify the optical properties of lead-free DP NCs to expand their luminescence application.