Periodic Reporting for period 1 - WEPOF (Watching Excitons in Photoactive Organic Frameworks)
Reporting period: 2022-09-01 to 2025-02-28
The WEPOF project addresses one of the most pressing challenges of our time: transitioning to an energy economy based on renewable resources. A critical component of this shift is the development of technologies that can efficiently and cost-effectively harvest and store solar energy. While solar energy has immense potential, its intermittent nature poses significant challenges for consistent electricity generation. Artificial photosynthesis (APS) offers a promising solution by using solar energy to produce chemical fuels from water and carbon dioxide. However, the efficiency of APS devices is currently limited due to energy losses associated with exciton recombination before charge separation occurs.
The project aims to tackle this issue by advancing our understanding of excitonic processes in photoactive materials, particularly covalent organic frameworks (COFs). COFs are an emerging class of materials that can be tailored at the atomic level to optimize light absorption and charge transfer, making them ideal candidates for APS. The core objectives of the WEPOF project are twofold: first, to simplify the structural complexity of COFs by developing atomically thin model systems that retain the key functionalities of their bulk counterparts; and second, to pioneer novel imaging techniques capable of observing excitonic states and charge transfer at the atomic scale for the first time.
By achieving these objectives, the WEPOF project expects to significantly improve the efficiency of photoenergy conversion in APS devices. The insights gained could lead to the development of next-generation materials for solar energy harvesting, ultimately contributing to a more sustainable and resilient energy infrastructure. The project’s innovative approach could also have broader implications, influencing the design of other nanomaterials used in various optoelectronic applications.
The project aims to tackle this issue by advancing our understanding of excitonic processes in photoactive materials, particularly covalent organic frameworks (COFs). COFs are an emerging class of materials that can be tailored at the atomic level to optimize light absorption and charge transfer, making them ideal candidates for APS. The core objectives of the WEPOF project are twofold: first, to simplify the structural complexity of COFs by developing atomically thin model systems that retain the key functionalities of their bulk counterparts; and second, to pioneer novel imaging techniques capable of observing excitonic states and charge transfer at the atomic scale for the first time.
By achieving these objectives, the WEPOF project expects to significantly improve the efficiency of photoenergy conversion in APS devices. The insights gained could lead to the development of next-generation materials for solar energy harvesting, ultimately contributing to a more sustainable and resilient energy infrastructure. The project’s innovative approach could also have broader implications, influencing the design of other nanomaterials used in various optoelectronic applications.
1) Design of photoactive 2D-COF model systems.
The ORCA quantum chemistry code has been set up to perform calculations of the geometry and electronic structure of donor and acceptor molecular building blocks. Potential pairs of molecular precursors have been identified, guiding the experimental synthesis.
2) Synthesis of photoactive 2D-COF model systems.
Two-dimensional covalent organic frameworks (2D-COFs) can be fabricated through on-surface synthesis, which makes use of atomically flat surfaces as templates to confine condensation reactions in two dimensions. However, despite on-surface synthesis in ultra-high vacuum (UHV) enables the growth of atomically thin COFs, the resulting degree of crystallinity is typically low. This issue is caused by the irreversibility of covalent bond formation, which limits the synthesis of ordered structures. The formation of crystalline structures requires a mechanism for error correction, where covalent bonds can be broken and reformed. Such approach, called dynamic covalent chemistry, is based by the addition of the reaction by-products (e.g. water), which enable the healing of defects in the frameworks, facilitating the formation of highly crystalline structures. As the water partial pressure is a key parameter to determine the chemical equilibrium of the system, systematic studies are required to identify the conditions which favour the formation of crystalline structures. As water pressures in the millibar range are typically required for dynamic covalent chemistry, conventional X-ray photoelectron spectroscopy (XPS) cannot be used, as this approach is limited to high vacuum conditions. Therefore, near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) experiments have been performed. As result, by tuning the sample temperature and the water pressure in the experimental chamber, controlled formation or dissolution of the framework has been observed by NAP-XPS. For the case of a 2D-boroxine framework, under typical temperatures used for 2D-COF growth (about 120 °C), a pressure of 0.5 mbar is needed to enable covalent bond breaking.
3) On-surface synthesis of two-dimensional frameworks with delocalized electronic states.
Two-dimensional metal-organic frameworks (2D-MOFs) stand out as a fascinating class of atomically thin materials, combining the structural flexibility observed in molecular systems with the ordered crystalline arrangement typical of solid materials. At the heart of their structure is the robust bonding between organic linkers and transition metal centres, a feature that is expected to give rise to delocalized electronic states. Despite such expectation, the precise mechanism through which the band structure in 2D-MOFs emerges from the coupling of electronic states within the building blocks remains a substantial scientific puzzle. We presented a breakthrough in the characterization of the electronic structure of π-conjugated 2D-MOFs. Through a combination of experimental and theoretical methodologies, we provided direct evidence for the emergence of band structure upon hierarchical assembly of the organic linkers and transition metal centres into the 2D-MOF lattice. Our results elucidate the role of intra-layer charge transfer in driving the formation of energy dispersive states.
4) Structural characterization of 2D photoactive frameworks and imaging of excitonic processes.
A low-temperature scanning tunnelling/atomic force microscope (LT-STM/AFM) has been successfully installed in January 2024. The microscope enables high-resolution imaging of two-dimensional organic structure on surfaces, being the core WEPOF. The setup has been also coupled with a tunable laser source, where the laser beam will be tightly focused on the tip-sample junction to enable photoexcitation experiments.
The ORCA quantum chemistry code has been set up to perform calculations of the geometry and electronic structure of donor and acceptor molecular building blocks. Potential pairs of molecular precursors have been identified, guiding the experimental synthesis.
2) Synthesis of photoactive 2D-COF model systems.
Two-dimensional covalent organic frameworks (2D-COFs) can be fabricated through on-surface synthesis, which makes use of atomically flat surfaces as templates to confine condensation reactions in two dimensions. However, despite on-surface synthesis in ultra-high vacuum (UHV) enables the growth of atomically thin COFs, the resulting degree of crystallinity is typically low. This issue is caused by the irreversibility of covalent bond formation, which limits the synthesis of ordered structures. The formation of crystalline structures requires a mechanism for error correction, where covalent bonds can be broken and reformed. Such approach, called dynamic covalent chemistry, is based by the addition of the reaction by-products (e.g. water), which enable the healing of defects in the frameworks, facilitating the formation of highly crystalline structures. As the water partial pressure is a key parameter to determine the chemical equilibrium of the system, systematic studies are required to identify the conditions which favour the formation of crystalline structures. As water pressures in the millibar range are typically required for dynamic covalent chemistry, conventional X-ray photoelectron spectroscopy (XPS) cannot be used, as this approach is limited to high vacuum conditions. Therefore, near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) experiments have been performed. As result, by tuning the sample temperature and the water pressure in the experimental chamber, controlled formation or dissolution of the framework has been observed by NAP-XPS. For the case of a 2D-boroxine framework, under typical temperatures used for 2D-COF growth (about 120 °C), a pressure of 0.5 mbar is needed to enable covalent bond breaking.
3) On-surface synthesis of two-dimensional frameworks with delocalized electronic states.
Two-dimensional metal-organic frameworks (2D-MOFs) stand out as a fascinating class of atomically thin materials, combining the structural flexibility observed in molecular systems with the ordered crystalline arrangement typical of solid materials. At the heart of their structure is the robust bonding between organic linkers and transition metal centres, a feature that is expected to give rise to delocalized electronic states. Despite such expectation, the precise mechanism through which the band structure in 2D-MOFs emerges from the coupling of electronic states within the building blocks remains a substantial scientific puzzle. We presented a breakthrough in the characterization of the electronic structure of π-conjugated 2D-MOFs. Through a combination of experimental and theoretical methodologies, we provided direct evidence for the emergence of band structure upon hierarchical assembly of the organic linkers and transition metal centres into the 2D-MOF lattice. Our results elucidate the role of intra-layer charge transfer in driving the formation of energy dispersive states.
4) Structural characterization of 2D photoactive frameworks and imaging of excitonic processes.
A low-temperature scanning tunnelling/atomic force microscope (LT-STM/AFM) has been successfully installed in January 2024. The microscope enables high-resolution imaging of two-dimensional organic structure on surfaces, being the core WEPOF. The setup has been also coupled with a tunable laser source, where the laser beam will be tightly focused on the tip-sample junction to enable photoexcitation experiments.
1. Design and synthesis of photoactive 2D-COF model systems.
The first in-situ observation of dynamic covalent chemistry in 2D-COFs represents a stepstone for the growth of crystalline organic frameworks. This unique approach opens to the possibility to precisely determine relevant parameters, as the kinetic barriers for reversible reaction and the Gibbs free energy, ruling dynamic covalent bond formation.
2. On-surface synthesis and atomic-scale characterization of two-dimensional frameworks with delocalized electronic states.
We identified a notable and robustly dispersive nature of the hybrid states formed in 2D-MOFs, a characteristic that persists regardless of the metallic support. This observation underscores the tunability of the band structure, emphasizing the pivotal role of charge transfer from the substrate. Our findings not only provide a rational behind the band-structure engineering in 2D-MOFs, but also open avenues for innovative applications in the realms of electronics and photonics.
The first in-situ observation of dynamic covalent chemistry in 2D-COFs represents a stepstone for the growth of crystalline organic frameworks. This unique approach opens to the possibility to precisely determine relevant parameters, as the kinetic barriers for reversible reaction and the Gibbs free energy, ruling dynamic covalent bond formation.
2. On-surface synthesis and atomic-scale characterization of two-dimensional frameworks with delocalized electronic states.
We identified a notable and robustly dispersive nature of the hybrid states formed in 2D-MOFs, a characteristic that persists regardless of the metallic support. This observation underscores the tunability of the band structure, emphasizing the pivotal role of charge transfer from the substrate. Our findings not only provide a rational behind the band-structure engineering in 2D-MOFs, but also open avenues for innovative applications in the realms of electronics and photonics.