Researchers from the Lawrence Berkeley National Laboratory, University of Nevada, Las Vegas, and other US research centers have developed a new technique to produce bidimensional chiral perovskites, with a 35-fold increase in their capacity do detect right and left circularly polarized light. The indicator value that characterizes differential absorption capacity of such light increased from 3.5 × 10⁻⁴ to 1.2 × 10⁻², one of the highest performance results for this type of materials.
The research teams worked with the 2D perovskite MBA2PbI4 based on lead and iodine. Its structure is characterized by chirality, i.e. it exists in two mirror versions, similar to the right and left hands. Because of this property the material can absorb right and left polarized light differently. This feature is relevant to optical and electronic devices, spintronics and quantum-based technology.
This type of perovskite is usually obtained in a single run: the ingredients are mixed in a solution, spread over the substrate and heated. When this method was used in the experiment it produced a non-homogeneous film with large crystals. The team then split the process into two phases. They first put a layer of lead iodide PbI2 upon the substrate to the thickness of about 90 nm, and then superimposed a solution of the chiral organic sault MBAI. A gel-like layer was temporarily formed between the two substances, and the formation of perovskite started there. After that the film was heated for 10 minutes at 70°C.
In the presence of gel, crystals were growing in a different way. Instead of large grains the system developed multiple elongated nanocrystals tens to about 100 nm wide, positioned in different directions. The researchers also found they can control the shape of crystals by adjusting concentrations of the organic salt. At low concentrations, the crystals were normally shaped as small grans. Optimal concentration produced nanorods, and when concentration was excessively high the crystals developed into larger flakelets.
The resulting film contained fewer defects and had a more homogeneous crystalline structure. In particular, the average charge carrier lifespan increased about 80% – from 0.88 to 1.58 nanoseconds. Calculations have revealed that film defects weaken the material’s internal asymmetry which is essential for differentiated interaction with right and left polarized light. Consequently, with fewer defects the effect was reinforced. The multidirectional positioning of the crystals played a special role in making light-to-material interaction more efficacious.
The researchers managed eventually to significantly improve the optical properties of the perovskite without altering its basic chemical composition – they only changed the crystal formation method. To date the method has only been tested with one type of chiral perovskite and will not be suitable to all similar materials without modification. However, the basic approach to controlling the size, positioning and quality of crystals may be used in future to create materials for opto-electronics, spintronics and quantum-based technology.
