Periodic Reporting for period 1 - IMAGINE (Indirect Magnetic Interactions: Tuning by Electric Field)
Periodo di rendicontazione: 2021-09-01 al 2023-08-31
With silicon-based electronics nearing its limits, spintronics and quantum computing have emerged as technologies promising unprecedented computational power. The fundamental research in this area shows that single metal atoms on well-defined surfaces can serve as qubits, but one major challenge is upscaling. We address this issue by developing protocols for synthesis of large, atomically-ordered arrays of magnetic metal atoms supported on graphene. Graphene is a perfect material for such applications due to its chemical inertness and tunability of charge carrier density by doping. Thus, synthesizing atomically-ordered arrays of magnetic atoms atop graphene opens up fascinating possibilities to design spintronic systems on a technologically relevant support, and also adjust the strength of the magnetic interaction between the individual spin centres. In an envisioned graphene field-effect transistor decorated by an array of magnetic atoms, such tuning of the magnetic interaction could be done in real time. Once fully developed, such a device presents a true dream system for spintronics and quantum computing research, both fundamental and applied.
Within this project, we have made some crucial steps towards this goal. Specifically, we have demonstrated that various 2D magnetic metal-organic frameworks (MOFs) can be synthesized atop graphene supports with different doping levels. We have also demonstrated that these systems are remarkably stable, and even though their synthesis requires ideal conditions of ultrahigh vacuum, they remain stable and atomically-defined even in ambient. Most of the current knowledge of atomically-defined 2D MOFs has been learned on metal supports, therefore we invested a great effort to unravel how the change from metal support to graphene affects the main system parameters, i.e. physical and electronic structure, chemical reactivity, and magnetic ordering. Lastly, we studied how the different doping level of the graphene changes the charge distribution within the MOF. Overall, the main objectives of the project have been fulfilled, and our work clearly contributes to the development of spintronics applications based on 2D metal-organic materials.
Within this project, we have made some crucial steps towards this goal. Specifically, we have demonstrated that various 2D magnetic metal-organic frameworks (MOFs) can be synthesized atop graphene supports with different doping levels. We have also demonstrated that these systems are remarkably stable, and even though their synthesis requires ideal conditions of ultrahigh vacuum, they remain stable and atomically-defined even in ambient. Most of the current knowledge of atomically-defined 2D MOFs has been learned on metal supports, therefore we invested a great effort to unravel how the change from metal support to graphene affects the main system parameters, i.e. physical and electronic structure, chemical reactivity, and magnetic ordering. Lastly, we studied how the different doping level of the graphene changes the charge distribution within the MOF. Overall, the main objectives of the project have been fulfilled, and our work clearly contributes to the development of spintronics applications based on 2D metal-organic materials.
The first crucial task of this project was to master the synthesis of magnetic 2D MOFs atop graphene supports with different doping levels. This was achieved by a multi-technique approach using scanning tunneling microscopy (STM), X-Ray photoemission spectroscopy (XPS) and low energy electron microscopy/diffraction (LEEM/LEED). We have carried out a detailed study targeting three different 2D MOFs consisting of metal atoms (Fe, Ni and Mn) with tetracyanoquinodimethane (TCNQ) linker molecules. We have shown that our synthesis method is universally applicable, as all three metal-organic systems (Fe-TCNQ, Ni-TCNQ and Mn-TCNQ) show very similar structure and overall quality. What is more, we demonstrated that the resulting metal-organic systems are remarkably stable, both thermally and chemically (survive heating to several hundred °C, survive exposure to ambient). This is a crucial prerequisite for their applications and studies outside ultrahigh vacuum. These results have been published (Z. Jakub et al. Nanoscale, 2022, 14, 9507-9515, doi: 10.1039/D2NR02017C) and were presented by oral or poster presentations at six international conferences and seminars.
The second important task was to characterize the properties of the 2D MOFs atop graphene supports. Prior literature studied similar systems in detail on metal supports, and our initial assumption was that these systems should behave very similarly also on graphene. This turned out to be only partially true, and our detailed study comparing Fe-TCNQ on graphene and on gold supports indeed revealed similarity in the main bonding motifs, but significant structural differences in the local coordination environment of the Fe2+ cation. This leads to different occupancy of the individual d-orbitals, and consequently to vastly different properties, both in electronic structure and in chemical reactivity. Overall, we found that the weaker interaction with the graphene support renders the 2D MOFs much more representative of the free-standing models commonly screened in computational studies. Thus, our work shows that synthesis of 2D MOFs on graphene is a convenient way to narrow the gap between experiment and theory, which is a vital requirement for efficient materials research. These findings are summarized in our manuscript currently in revision for publication in the Journal of the American Chemical Society, and were presented by oral or poster presentations at three international conferences and seminars.
Next, we studied the effects of graphene doping on the properties of supported 2D MOFs. We compared the properties of Ni-TCNQ on undoped graphene/Ir(111) with a Ni-TCNQ network synthesized on an n-doped graphene prepared by intercalation of the graphene/Ir(111) system. We clearly identified significant differences in the structure of these two Ni-TCNQ systems, which are most likely linked to the different charge distribution between the 2D MOF and the support. We have also studied the properties of Ni-TCNQ atop n-doped graphene prepared on SiC crystals, and we have explored the possibilities of remote graphene doping by X-Ray or UV irradiation. Lastly, we have studied the magnetic properties of the Ni-TCNQ and Fe-TCNQ networks in collaboration with our partners, and we explored the possibilities of synthesizing multilayer metal-organic structures. As of now, these collaborative experimental efforts and the supporting computational work are still ongoing. Some results were presented in one invited seminar talk, and a part will be published in a currently prepared manuscript. Overall, it is expected that at least two more publications will be published within next year, summarizing our work on graphene doping and properties of multilayer metal-organic frameworks.
The second important task was to characterize the properties of the 2D MOFs atop graphene supports. Prior literature studied similar systems in detail on metal supports, and our initial assumption was that these systems should behave very similarly also on graphene. This turned out to be only partially true, and our detailed study comparing Fe-TCNQ on graphene and on gold supports indeed revealed similarity in the main bonding motifs, but significant structural differences in the local coordination environment of the Fe2+ cation. This leads to different occupancy of the individual d-orbitals, and consequently to vastly different properties, both in electronic structure and in chemical reactivity. Overall, we found that the weaker interaction with the graphene support renders the 2D MOFs much more representative of the free-standing models commonly screened in computational studies. Thus, our work shows that synthesis of 2D MOFs on graphene is a convenient way to narrow the gap between experiment and theory, which is a vital requirement for efficient materials research. These findings are summarized in our manuscript currently in revision for publication in the Journal of the American Chemical Society, and were presented by oral or poster presentations at three international conferences and seminars.
Next, we studied the effects of graphene doping on the properties of supported 2D MOFs. We compared the properties of Ni-TCNQ on undoped graphene/Ir(111) with a Ni-TCNQ network synthesized on an n-doped graphene prepared by intercalation of the graphene/Ir(111) system. We clearly identified significant differences in the structure of these two Ni-TCNQ systems, which are most likely linked to the different charge distribution between the 2D MOF and the support. We have also studied the properties of Ni-TCNQ atop n-doped graphene prepared on SiC crystals, and we have explored the possibilities of remote graphene doping by X-Ray or UV irradiation. Lastly, we have studied the magnetic properties of the Ni-TCNQ and Fe-TCNQ networks in collaboration with our partners, and we explored the possibilities of synthesizing multilayer metal-organic structures. As of now, these collaborative experimental efforts and the supporting computational work are still ongoing. Some results were presented in one invited seminar talk, and a part will be published in a currently prepared manuscript. Overall, it is expected that at least two more publications will be published within next year, summarizing our work on graphene doping and properties of multilayer metal-organic frameworks.
Over the past 15 years, a great effort has been invested into the studies of 2D metal-organic structures on surfaces, and the atomic-scale insights gained are truly impressive. However, most of the work has been done on single-crystal metal surfaces. Moving towards more technologically relevant supports brings new challenges and forces us to rethink some of the basic concepts of on-surface 2D MOFs. Our work goes beyond the state of the art and provides some of the first systematic comparisons of 2D MOFs prepared on prototypical metal supports and on graphene. This allows us to bridge the gap between atomically-resolved experimental work and high-throughput computational screening studies. Moreover, doping of the graphene support allows us to tune the MOF-support charge transfer without changing the physical structure of the MOF-support interface. This opens ways to disentangle the electronic effects from the structural ones, which is impossible on standard metal or oxide supports. Overall, our studies of graphene-supported 2D MOFs bring a new level of deep understanding of the material properties at the atomic scale, which is highly relevant for many applications ranging from quantum computing to single-atom catalysis.