Project RETAIN

Introduction

Project RETAIN ("Routing Energy Transfer via Assembly of Inorganic Nanoplatelets," Grant agreement ID: 794560) is a research fellowship/grant awarded to Dr. Baranov under the umbrella of Marie Sklodowska-Curie Actions (a part of the EU Research and Innovation programme Horizon 2020) to carry out research in Europe. The project was hosted by the Nanochemistry Department at the Italian Institute of Technology in Genova, Italy, had an overall budget of € 168 277,20, and run for two years from July 16, 2018, until July 15, 2020. The official fact sheet for the project is available on the CORDIS website.

This page summarizes the project's original objective and results. The scientific publications resulted from the project are listed at the end of the page and are open to access at the publishers' websites via provided links. The publications and conference materials that resulted from the project are openly accessible through the RETAIN Collection in the Zenodo repository.

The objective of the Project

Investigation of artificial light-harvesting systems is an important part of the European research effort towards the development of sustainable and carbon-free energy sources. This research proposal aims to develop a new type of semiconductor heterostructures – solution-processed nanoplatelet assemblies capable of harvesting and transferring energy of light similar to antenna complexes of photosynthetic organisms. Towards this goal, the research project will use quantum-confined 2D semiconductor nanoplatelets arranged into superplatelet structures by means of colloidal self-assembly. The 2D nanoplatelets, which can be thought of as giant artificial chlorophyll molecules, have superior optical properties and enable the design of hybrid materials with absorption spectrum covering the energy range from ultraviolet to near-infrared by mixing and matching lead halide-based perovskites with II-VI and IV-VI binary semiconductors. The proposal consists of three key parts. First, 2D nanoplatelets with optimized properties are obtained and tuned via chemical synthesis. Second, anisotropic interactions between 2D nanoplatelets of dissimilar materials are exploited for nanoplatelet assembly into heterostructured ribbons and layers. Third, the figures of merit for energy transfer and charge separation in the assemblies are obtained by spectroscopic, photochemical, and photoelectric characterizations. The resulting assemblies would constitute a new class of artificial excitonic materials, expanding the family of optoelectronic heterostructures beyond epitaxially grown semiconductors and mechanically stacked exfoliated 2D materials. The proposed research project combines the strengths of the experienced researcher and capabilities of the host institution in a complementary fashion, assuring mutual benefit from the Fellowship and providing the experienced researcher with opportunities to achieve a high level of professional maturity and significantly expand his career opportunities.

Summary of Main Results

• Summary of the context and overall objectives of the project

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 their 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.

• Work performed from the beginning of the project to the end of the period covered by the report, and main results achieved

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 the Zenodo repository. 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 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.

• Progress beyond state of the art expected results until the end of the project and potential impacts

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 synchrotron 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.

Publications

• Multilayer Diffraction Reveals That Colloidal Superlattices Approach the Structural Perfection of Single Crystals (Open Access)

^†Toso S, ^†Baranov D, Altamura D, Scattarella F, Dahl J, Wang X, Marras S, Alivisatos P, Singer A, Giannini C, Manna L, ACS Nano, 2021, 15, 6243-6256;

An earlier version of the work has been deposited to the ChemRxiv preprint server, preprint no. 13103507 (Accessed on October 19, 2020);

• Aging of Self-Assembled Lead Halide Perovskite Nanocrystal Superlattices: Effects on Photoluminescence and Energy Transfer (Open Access)

Baranov D, Fieramosca A, Yang RX, Polimeno L, Lerario G, Toso S, Giansante C, De Giorgi M, Tan LZ, Sanvitto D, Manna L, ACS Nano, 2020, ASAP;

An earlier version of the work has been deposited to ArXiv preprint server, preprint no. 2008.02853 (Accessed on August 6, 2020);

• Superlattices are Greener on the Other Side: How Light Transforms Self-Assembled Mixed Halide Perovskite Nanocrystals (Open Access)

Brennan MC, Toso S, Pavlovetc IM, Zhukovskyi M, Marras S, Kuno M, Manna L, Baranov D, ACS Energy Letters, 2020, 5, 1465-1473;

• Transforming Colloidal Cs₄PbBr₆ Nanocrystals by Poly(maleic anhydride-alt-1-octadecene) into Stable CsPbBr₃ Perovskite Emitters Through Intermediate Heterostructures (Open Access)

Baranov D, Caputo G, Goldoni L, Dang Z, Scarfiello R, De Trizio L, Portone A, Fabbri F, Camposeo A, Pisignano D, Manna L, Chemical Science, 2020, 11, 3986-3995; (correction)

• Cs₃Cu₄In₂Cl₁₃ Nanocrystals: A Perovskite-Related Structure with Inorganic Clusters at A Sites (Open Access from December 12, 2020)

Kaiukov R, Almeida G, Marras S, Dang Z, Baranov D, Petralanda U, Infante I, Mugnaioli E, Griesi A, De Trizio L, Gemmi M, Manna L, Inorganic Chemistry, 2020, 59, 548-554

• Green-Emitting Powders of Zero-Dimensional Cs₄PbBr₆: Delineating the Intricacies of the Synthesis and the Origin of Photoluminescence (Open Access)

Ray A, Maggioni D, Baranov D, Dang Z, Prato M, Akkerman QA, Goldoni L, Caneva E, Manna L, Abdelhady AL, Chemistry of Materials, 2019, 31 , 7761-7769

• Wide-Angle X-ray Diffraction Evidence of Structural Coherence in CsPbBr₃ Nanocrystal Superlattices (Open Access)

Toso S, Baranov D, Giannini C, Marras S, Manna L, ACS Materials Letters, 2019, 1, 272-276

• Fully Inorganic Ruddlesden-Popper double Cl-I and triple Cl-Br-I Lead Halide Perovskite Nanocrystals (Open Access)

Akkerman QA, Bladt E, Petralanda U, Dang Z, Sartori E, Baranov D, Abdelhady AL, Infante I, Bals S, Manna L, Chemistry of Materials, 2019, 31, 2182-2190

• Investigation into the Photoluminescence Red Shift in Cesium Lead Bromide Nanocrystal Superlattices (Open Access)

Baranov D, Toso S, Imran M, Manna L, The Journal of Physical Chemistry Letters, 2019, 10, 655–660

• Shape-Pure, Nearly Monodispersed CsPbBr₃ Nanocubes Prepared Using Secondary Aliphatic Amines (Open Access)

Imran M, Ijaz P, Baranov D, Goldoni L, Petralanda U, Akkerman QA, Abdelhady AL, Prato M, Bianchini P, Infante I, Manna L, Nano Letters, 2018, 18, 7822-7831