To Dope Mo³⁺ in Colloidal Nanocrystals

Mo³⁺ shows intra-configurational spin-flip (ICSF) d → d electronic transitions, emitting near-infrared-II (NIR-II) radiation. Till date, the emission has been reported from Mo³⁺-doped into bulk host materials, in the powder or single crystalline forms. Here we report the synthesis of Mo³⁺ doping in a colloidal nanocrystal (NC). Colloidal Mo³⁺-doped Cs₂AgInCl₆ double perovskite NCs are prepared using a typical hot-injection method in a non-polar medium. The challenges of poor solubility of molybdenum precursor and instability of low oxidation state of Mo³⁺ in the reaction medium are overcome by a rational design of the synthesis. The resulting NCs have cubic shape with edge length ∼9 nm, and oleylammonium ions as the predominant surface capping ligands. The colloidal synthesis enabled us to fabricate thin films of Mo³⁺-doped NCs by spin-coating technique. Both colloidal NCs and their films show the sharp Mo³⁺ ICSF NIR-II emission around 1105 nm. Temperature dependent (7-300 K) PL spectra and lifetime studies provide mechanistic insights about the spin- and Laporte-selection rules of the ICSF emission from [MoCl₆]^3- octahedral unit. This report of a colloidal NC with Mo³⁺ dopants, and their thin films, is expected to open up research avenues for micro-LEDs and quantum light emitters in NIR-II region.