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.
