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Routing Energy Transfer via Assembly of Inorganic Nanoplatelets

Periodic Reporting for period 1 - RETAIN (Routing Energy Transfer via Assembly of Inorganic Nanoplatelets)

Reporting period: 2018-07-16 to 2020-07-15

The discovery of novel artificial materials that can manipulate the energy of light is essential for the ongoing development of optical and electronic technologies. Project RETAIN (“the project”) set up an ambitious goal of fabricating nanomaterials with a build-in directionality of the energy transfer. Semiconductor nanocrystals, which are tiny particles of inorganic materials (one billionth of a meter in size), were chosen as its building blocks. Nanocrystals of recently discovered cesium lead halide perovskites with a general formula of CsPbX3 (where X stands for a halide anion, such as bromide or iodide) have been explored due to their very efficient light absorption and emission characteristics. The nanocrystal self-assembly from solution, which is like a crystallization of atoms or molecules, was chosen as a low-cost material fabrication strategy. The resulting materials, called nanocrystal assemblies or superlattices, are micron-sized solids composed of ordered nanocrystals close-packed next to each other. Superlattices of CsPbX3 nanocrystals are promising for light-harvesting because they show close similarities with assemblies of pigments found in nature. The project proposed to engineer the energy transfer by a “funnel” principle, according to which the energy of the absorbed photons in a single superlattice would move from a region with a larger bandgap to a region with a smaller bandgap. That principle is a fundamental one and mimics energy transfer in photosynthetic organisms such as plants, algae, and cyanobacteria. The ability to direct energy transfer in man-made materials would open up novel ways of solar energy utilization and may lead to new light sources. The project’s main objectives were to develop a controlled way of nanocrystal assembly into superlattices and to elucidate the spatial, temporal, and efficiency properties of the energy transfer in them. The stated objectives were achieved with minor deviations. Throughout the arc of the project, novel methods of nanocrystal synthesis and assembly have been developed, the structure of the superlattices have been solved, and the conditions for directed energy transfer in them have been established.
The work on the project was divided into three parts (“Synthesis,” “Assemblies,” and “Energy Transfer”) plus dissemination and communication activities, distributed throughout the duration of the project. The “Synthesis” part consisted of preparation, optical and structural characterization of CsPbX3 nanocrystal samples. The nanocrystal samples with the best size and shape uniformity were selected for the superlattice growth by means of self-assembly. Once the superlattices were grown, their structure and basic optical properties were characterized (“Assemblies”). The characterizations were focused on X = Br, I and their mixed compositions as the most promising ones. The results and expertise of the first two parts created the foundation for “Energy Transfer” studies, which consisted of micro-photoluminescence spectroscopy of single superlattices at cryogenic temperatures and the theoretical modeling of the data.
The results and activities of the project were disseminated through peer-reviewed publications and participation in seminars, workshops, and conferences. The project yielded eight publications with two more in preparation (all publications and some conference materials resulted from the project are openly accessible at the RETAIN Collection in Zenodo repository, https://zenodo.org/communities/h2020-msca-if-2017-retain/). Scientific concepts behind the project, as well as its objectives and results, have been communicated through outreach events and social media platforms. The outreach events included an interactive stand “Glow with a Flow” at the European Researchers' Night in Brussels in 2018, an educational laboratory activity “Energy Revolution: It’s All about Nanochemistry” at the Festival of Science 2018 in Genova, and the numerous Family & School Day activities at the host institution and the city of Genova. The outreach and science communication activities were designed and performed together with Ph.D. and Master students from IIT and the University of Genova. A Twitter account @RETAIN_H2020 (https://twitter.com/RETAIN_H2020) was created to engage with a broader community and to communicate the latest results of the project. Several blog posts were published on social media platforms such as Facebook, LinkedIn, and IIT Talk webpage to communicate the results of the project in an informal and accessible way.
Since 2015, CsPbX3 nanocrystals are experiencing a surge of interest due to the promise of low-cost solar cells, artificial lightning, displays, scintillators, and solution-processed lasers. The new knowledge about CsPbX3 nanocrystals and materials based on them is necessary to choose the most promising applications for investment by governmental organizations and industries. Throughout the project, each of its parts produced results that pushed the current knowledge beyond state of the art, and two examples are highlighted below.
First, the structural analysis of CsPbX3 nanocrystal superlattices by x-ray diffraction revealed that they are exceptionally well-ordered solids. Such an order leads to the peculiar x-ray interference effect, which is very sensitive to the structural parameters of the superlattices and enables their precise structural characterization. These findings led to the development of a general methodology for superlattice characterization by means of x-ray diffraction coupled with an open-source data analysis algorithm. It is anticipated that the discovered approach will become an alternative to resource-intensive synchrothron experiments and make the characterization of similar materials accessible to many researchers in academia and industry.
Second, it was found that directed energy transfer initially plays a minor role in the properties of a single CsPbX3 nanocrystal superlattice. However, a fraction of the nanocrystals coalesces into bigger particles over time inside the superlattice. These bigger particles have smaller bandgaps, which turns on the fast and efficient energy transfer: nearly all of the energy of light absorbed by a superlattice ends up funneling into the large particles. The impact of these findings is two-fold. On the one hand, these results challenge recent reports of collective properties in similar materials by providing an alternative explanation. That contributes to a more accurate understanding of the physics of these materials. On the other hand, the aged superlattices are a new example of an artificial nanomaterial with a built-in directional energy transfer. That finding makes them very attractive for applications in artificial photosynthesis and indicates a future research direction worth of investment and study.
Besides the scientific impact, the project substantially impacted the researcher’s career. The new scientific and soft skills acquired over the course of the project increased technical competence and enhanced the preparation of the researcher for an independent career. The communication and dissemination activities resulted from the project contributed to the strengthening of the researcher’s track record. Overall, the project strengthened the researcher’s motivation and prospects to become an independent leader in the design and photophysics of artificial excitonic materials.