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Synthesis of Perovskite Quantum Dots

During the original development of all-inorganic perovskite quantum dots, several synthetic routes were attempted in order to find a process that can reliably produce stable and defect resistant dots. Two major routes to the synthesis have now been developed: room temperate synthesis and synthesis by hot injection.

Perovskite quantum dot ink synthesis
Injection of Cs-oleate into lead precursor during hot injection synthesis of perovskite QD

Synthesis at Room Temperature


The first route can be undertaken at room temperature and in air.

First, stoichiometric amounts of cesium halide (CsX) and lead halide (CsX2) are dissolved in a good solvent such as dimethyl sulfoxide (DMSO) or N,N-dimethylformamide (DMF). Capping ligands such as oleic acid and oleylamine are then added, and the mixture is stirred vigorously.

An aliquot of this mixture is then added to vigorously stirring flask of a poorer solvent (such as toluene). This causes the perovskite quantum dots to precipitate within seconds. The resultant crude reaction mixture can then be separated by centrifuge and the pellet washed to give quantum dots in good yield and with high photoluminescent quantum yields (PLQYs).

This method is good for preparing CsPbBr3 quantum dots but is less reliable at producing QDs containing either chloride or iodide.

Synthesis by Hot Injection


The second, so-called ‘hot injection,’ synthesis firstly requires the preparation of cesium oleate by stirring cesium carbonate and oleic acid in 1-octadecene (ODE) under argon at 150 °C. Separately, lead halide is dried in ODE by heating in vacuo. Capping ligands such as oleic acid and oleylamine are added under argon to fully dissolve the lead halide and the temperature is raised to 140 – 200 °C depending on the desired emission wavelength of the quantum dot.

The cesium oleate solution at 150 °C is then swiftly injected and the mixture stirred for 5–10 seconds before quenching the reaction in an ice water bath.

Quenching the Reaction in an Ice-Water Bath

Separation by centrifuge and washing of the pellet is able to afford the QDs in good yields and with PLQY values approaching unity.

Synthesis of quantum dots containing chloride requires the extra step of adding trioctylphosphine (TOP) to ensure full dissolution of PbCl2 prior to the injection of cesium oleate.

Perovskite QDs can be synthesised from PbCl2, PbBr2, PbI2 or any mixture of PbCl2/PbBr2 or PbBr2/PbI2 to form mixed halide QDs. Alternatively, CsPbBr3 quantum dots can be transformed post-synthesis via ion exchange reaction with chloride or iodide salts.

The entire spectrum of visible light can be emitted by perovskite quantum dots by tuning the reaction temperature and the proportion of halides in the product.

Conclusion


Unlike metal chalcogenide quantum dots, perovskite quantum dots do not use toxic materials like cadmium metal and do not require surface passivation. They are therefore considerably less prone to defects. The available synthetic processes of perovskite quantum dots mean that they can be reliably mass-produced with photoluminescent yields of up to 100%.

We synthesize our new range of perovskite quantum dots using the hot injection method. Cesium lead perovskite quantum dots of chloride/bromide (450 nm, blue), bromide (515 nm, green) and iodide (685 nm, red) are now available.

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