At the start of the project, a focus was placed on synthesizing luminescent nanosized building blocks (nanocrystals) from lead halide perovskite materials. Chemical methods were employed in which solubilized metal salts reacted with organic surfactants to produce cube-like nanocrystals with sizes tunable from approximately 5-6 nm to 10-12 nm. A key priority involved carefully controlling reaction conditions (temperature, concentrations, purification) to ensure that the nanocrystals within each batch remained as uniform as possible in size and shape.
After synthesis, these nanocrystals were assembled into microscopic solids, with each “cube” tightly packed, comparable to cartons of milk on a shelf. An early milestone consisted in modifying the nanocrystal synthesis to incorporate various organic surfactants, thus enabling precise control over inter-nanocrystal spacing and order in the resulting solids. This development provided a platform for exploring more complex mixtures of nanocrystals and for conducting advanced structural and optical characterization, particularly regarding how the structure of nanocrystal solids influences collective optical properties. Notably, controlling inter-particle distances is essential for mediating dipole-dipole interactions that underlie phenomena such as superfluorescence.
In parallel, methods were devised to study how light interacts with these nanocrystal solids. Construction of the specialized microscope system was started, incorporating an optical cryostat, allowing precise cooling of nanomaterials to liquid helium temperatures while enabling optical microscopy and spectroscopy. This setup facilitates examinations of light emission from individual nanocrystal solids at various temperatures and under different experimental conditions. Ongoing efforts are extending the system’s capabilities to include further optical characterization tools, such as measurements of emission lifetimes and quantum properties (e.g. photon correlation).
A theoretical investigation was done to study the parameters that affect collective optical properties. Because multiple aspects of nanocrystal solids can be tuned, the parameter space is vast. Theoretical modeling narrows this space by identifying key variables through simplified representations of the underlying physics. Following this approach, a single-excitation superradiance model (a simplified version of superfluorescence) was applied to two-dimensional nanocrystal solids. These 2D structures exhibit similar qualitative behavior to their 3D counterparts yet are easier on computational resources and more time-efficient to explore.
Two central findings emerged from this approach. First, energy disorder caused by variations in the energies of nanocrystals within the assembly was identified as the main cause of the weakening of the superradiance. Second, a strategy for engineering superradiance was formulated: larger nanocrystal solids featuring a higher volume density of emitters are preferable to smaller ones, larger nanocrystals are more robust against energy disorder than smaller ones at the expense of the absolute magnitude of the superradiance. Experimental work is ongoing to validate these predictions.
Midway through the project, different types of photoluminescence were studied in individual nanocrystal solids, leading to a classification into Type I and Type II. Type I photoluminescence is a largely uniform brightness distribution across the nanocrystal solid and minor spectral shifts between edges and center. Type II photoluminescence is highly heterogeneous, featuring noticeable bright spots on the surface or incorporated inside the nanocrystal solids. These inclusions have a lower bandgap and are efficient energy sinks, significantly influencing energy transfer and overall photoluminescence.
These results are important steps towards engineering cooperative light-emitting properties in nanocrystal solids, potentially paving the way for future technological breakthroughs.
