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Luminescent Nanocrystals in a Cage for Solar-to-Fuel Conversion

Periodic Reporting for period 1 - NanoINCAGE (Luminescent Nanocrystals in a Cage for Solar-to-Fuel Conversion)

Reporting period: 2016-09-01 to 2018-08-31

One of the biggest challenges of our time is to ensure that the ever-increasing demand for energy does not result in a further increase in the CO2 levels in the atmosphere. Research efforts in renewable energies and energy efficient technologies have been notably increasing in the past ten years, so to decrease CO2 emission while reducing energy utilization. Carbon capture and storage from large industrial sources is one of the most powerful approaches to prevent the CO2 from increasing up to unacceptable levels. In this scenario, the utilization of the sequestrated CO2 by converting it into useful chemicals has a huge potential to contribute reducing the dependence of our society on petroleum. Electrochemical CO2 conversion is emerging as a sustainable technology, especially if the energy generated from intermittent renewable resources (i.e. solar) is employed to power the reactor. The goal of the applicant research is the realization of a photo-electrochemical device that is able to use solar energy to convert CO2 into value-added chemicals, such as ethylene and methane. While much progress has been made, this emerging field is challenged by huge technical and scientific questions.
In natural photosynthesis light absorption and catalysis occur in different sites of the leaf. In a simplified scenario, the energy harvested by the light absorbing pigments is funnelled towards a reaction center through a cycle of reactions called the Calvin cycle. Herein, the idea is to realize an energy conversion device based on Förster Resonance Energy Transfer (FRET) antenna as light absorber. Specifically, A FRET antenna based on the recently discovered all-inorganic perovskite quantum dots (PeQDs) has been chosen as a model system. In this project, the encapsulation with an amorphous alumina matrix deposited by atomic layer deposition (ALD) was successfully implemented as an effective strategy to stabilize PeQDs of different composition against air, water, heat and light. Finally, this project has offered the chance to carry out a highly multidisciplinary and interdisciplinary program and the obtained results on the synthesis and physical characterizations of these novel hybrids in the near future will impact other research fields in chemistry, material science and engineering.
During the last years, a new class of QD has been revealed, known as perovskite QD (PeQD). PeQDs have shown enhanced properties respect to classical chalcogenide QD such as: such as size-and composition dependent bandgap, large absorption coefficients and very high quantum yields. Indeed, PeQDs have been utilized in various electronic devices such as solar cells, LED and lasers showing improved performance. Despite their excellent optical properties, they suffer from a number of drawbacks that hamper their implementation in FRET-based architectures: photo-degradation, fast anion exchange and instability to water and heat. The first part of the project has been dedicated to the resolution of these problems. Solving the stability problem is also of crucial importance not only for their implementation in optoelectronic devices but also when exploring their intrinsic optoelectronic properties, which may require relatively long measurements in air and under the light. In this project, the encapsulation with an amorphous alumina (AlOx) matrix deposited by atomic layer deposition (ALD) was successfully implemented as an effective strategy to stabilize PeQDs of different composition and to avoid problems of anion migration. Specifically, a low-temperature ALD process for the deposition of AlOx on a PeQD thin film has been successfully developed. The AlOx matrix protects the PeQDs from oxygen and moisture in the air, confers them stability in water and prevents sintering, thus improving their stability at high temperature and under light exposure for hours. Additional to stability, ALD is beneficial in fabricating multi-layered QD cascade structures. These results have been published in a scientific publication and disseminated in various conferences.
In the second step to move towards the realization of the proposed device, the energy transfer distance dependence in PeQD bilayer structures and in PeQD/metal catalyst structure has been studied. Data not yet published by the candidate show that an efficient energy transfer is possible in such systems. Moreover in collaboration with a group in the Molecular Foundry (Berkeley) by means of advanced optical spectroscopies, an exciton diffusion length of 200 nm was measured for an ordered PeQD@AlOx monolayer suggesting that PeQD@AlOx nanocomposite are ideal candidate for the realization of the proposed device.
The uniqueness of the results reached during the realization of this project, compared to the others in the literature, is first of all the protection schema proposed for the PeQD that results in the uniformity of the nanocomposites that is ideal for investigating the basic properties of the PeQDs as well as to confer long term stability to the devices in which they are implemented. Indeed the excition diffusion length (how far the generated exciton could travel before recombination) for this type of materials has been successfully measured, observing 10 times improvement respect to classical chalcogenide QDs. Furthermore, this study is of interest for the scientific community working on organic-inorganic perovskites (the most performing material in solar cell to date), which are suffering from similar instability issues.
This project has certainly helped the European society in the various ways. First by allowing the reintegration of a researcher from USA, thereby ensuring that competitive researcher do not stay overseas. Second, by forming an excellent scientist and letting her acquire competitive know-how in an extremely important area with a wide impact on the society. Third, the performed studies and results have certainly a huge impact on different major areas of contemporary research of interest for the European Commission that is the need to create a sustainable society. Moreover, the carried out research on the synthesis of hybrid materials and their integration in a new concept device is of general interest of chemistry, physics, biology and engineering scientific community.
Schematic of NanoINCAGE project