Magnetic interactions in open-shell nanographenes
BORIS DOI
Abstract
Metals have become synonymous with conductivity and magnetism, forming the backbone of the electric and digital revolutions that profoundly transformed our society over the past century. Our daily lives are filled with devices containing d- and f-block elements, whose extraction, processing, disposal, and limited availability pose significant environmental concerns. However, in the last 20 years, carbon has emerged as a promising alternative, showing great potential for the realization of sustainable electronic and magnetic applications. The emergence of graphene, a two-dimensional form of carbon with exceptional electrical and thermal conductivity, has marked the beginning of a new chapter in materials science. Its electrons propagate through the honeycomb lattice over micrometer distances with negligible scattering, behaving as massless, relativistic Dirac fermions. Quantum confinement into one-dimensional (1D) and quasi-zero-dimensional (0D) graphene nanostructures, via bottom-up chemical synthesis, enables precise tuning of graphene’s electronic properties. This distinctive relationship between structure and electronic properties renders graphene nanostructures—hereafter referred to as nanographenes—as an ideal platform for realizing intriguing quantum phases of matter, including magnetism.
This thesis investigates the emergence of magnetism in nanographenes, with a detailed examination of the principles governing spin-spin interactions. By combining in-solution and on-surface synthesis techniques under the inert vacuum environment, we achieve nanographenes with inherently “frustrated” topologies that prevent the pairing of all π-electrons, leading to nontrivial magnetic ground states. The structural and electronic properties of this system are explored at the single-molecule level by means of scanning probe techniques.
In the first part of the thesis, the two smallest nanographenes with total spin ground states S = 1/2 and S = 1 are synthesized, serving as prototypical magnetic building blocks. Their magnetic ground states are characterized, leveraging the Kondo effect—where metal electrons in the substrate tend to screen the magnetic impurities—as a probe to reveal the intrinsic magnetic properties. We experimentally prove a new theory that describes the temperature dependence of the conductance anomaly resulting from the Kondo effect, emphasizing the critical role of various broadening mechanisms. The second part of the thesis explores the magnetic coupling of the nanographene building blocks, demonstrating the
experimental realization of both antiferromagnetically and ferromagnetically coupled dimers and trimers. We analyze in detail the various exchange coupling mechanisms that come into play and the role of the substrate in the effective spin-spin interaction. The third and final part of the thesis focuses on the emergence of magnetism in a different class of magnetic nanographenes (known as Kekul´e nanographenes), whose topology does not necessarily lead to unpaired electrons, but the system can feature an openshell singlet or a closed-shell ground state. This duality finds expression in the structure and electronic properties of these compounds and can only be accurately captured through a multi-configurational description of their ground state.
The results presented in this thesis underscore the potential of on-surface synthesis and tip-based manipulation in engineering custom-built molecular quantum systems. Our efforts in determining the relevant spin Hamiltonian parameters of these prototypical spin clusters bear significant implications, particularly for their extension into one-dimensional and two-dimensional spin chains and lattices.
This thesis investigates the emergence of magnetism in nanographenes, with a detailed examination of the principles governing spin-spin interactions. By combining in-solution and on-surface synthesis techniques under the inert vacuum environment, we achieve nanographenes with inherently “frustrated” topologies that prevent the pairing of all π-electrons, leading to nontrivial magnetic ground states. The structural and electronic properties of this system are explored at the single-molecule level by means of scanning probe techniques.
In the first part of the thesis, the two smallest nanographenes with total spin ground states S = 1/2 and S = 1 are synthesized, serving as prototypical magnetic building blocks. Their magnetic ground states are characterized, leveraging the Kondo effect—where metal electrons in the substrate tend to screen the magnetic impurities—as a probe to reveal the intrinsic magnetic properties. We experimentally prove a new theory that describes the temperature dependence of the conductance anomaly resulting from the Kondo effect, emphasizing the critical role of various broadening mechanisms. The second part of the thesis explores the magnetic coupling of the nanographene building blocks, demonstrating the
experimental realization of both antiferromagnetically and ferromagnetically coupled dimers and trimers. We analyze in detail the various exchange coupling mechanisms that come into play and the role of the substrate in the effective spin-spin interaction. The third and final part of the thesis focuses on the emergence of magnetism in a different class of magnetic nanographenes (known as Kekul´e nanographenes), whose topology does not necessarily lead to unpaired electrons, but the system can feature an openshell singlet or a closed-shell ground state. This duality finds expression in the structure and electronic properties of these compounds and can only be accurately captured through a multi-configurational description of their ground state.
The results presented in this thesis underscore the potential of on-surface synthesis and tip-based manipulation in engineering custom-built molecular quantum systems. Our efforts in determining the relevant spin Hamiltonian parameters of these prototypical spin clusters bear significant implications, particularly for their extension into one-dimensional and two-dimensional spin chains and lattices.
Date of Publication
2025
Year of graduation
2024
Theses Type
dissertation
Language(s)
en
Author(s)
Faculty/Graduate School
Access(Rights)
open.access
Primary OA Publication
true