Tuning spin and charge in graphene nanoribbons with atomic precision

Graphene—a single layer of carbon atoms arranged in a honeycomb lattice—has captivated scientists because of its extraordinary electronic and mechanical properties. Its electrons move through the lattice almost as if they were massless, giving graphene exceptionally high electrical conductivity and mobility. However, pristine graphene sheets are not magnetic and their electrons are delocalized across the entire plane, which limits their usefulness in applications that require localized electronic or spin states, such as quantum devices. One promising strategy to overcome this limitation is to sculpt graphene into very narrow strips known as graphene nanoribbons (GNRs). By confining electrons within these nanoribbons, their motion becomes quantized, leading to new electronic behaviours and, under certain conditions, the emergence of magnetic moments at the ribbon edges. In principle, this opens a path toward using graphene for spin-based electronics and quantum information technologies.

Yet, turning that principle into practice has proved difficult. When GNRs are fabricated or studied on metallic substrates, the metal tends to “screen” or wash out their intrinsic electronic structure. The metal’s electrons hybridize with the ribbon’s electronic states, smearing out any discrete energy levels and destroying the subtle balance of interactions that can give rise to localized spins. To harness graphene nanoribbons as controllable quantum objects, researchers need to isolate them from the metal surface while still being able to control their charge—essentially, to decouple and tune them at the same time.

This challenge is precisely what a new piece of work ¹ sets out to solve. The authors demonstrate that by placing atomically precise graphene nanoribbons on a single layer of magnesium oxide (MgO) grown atop a silver (Ag) substrate, they can both preserve the ribbon’s discrete electronic structure and systematically control its charge state. This, in turn, allows them to determine whether the ribbon behaves as a non-magnetic system or hosts a localized spin-½ state—simply by changing its length.

Decoupling and control: The role of MgO

The experimental setup exploits a delicate balance between electrical isolation and coupling. The MgO layer, only one atom thick, acts as an insulating buffer between the ribbon and the metallic silver beneath. It prevents the direct hybridization that would blur the ribbon’s energy levels, yet it remains thin enough to permit some degree of electron tunnelling or electrostatic coupling. In essence, the MgO decouples the graphene nanoribbon from the metal’s electronic sea while still letting electrons move between them in quantized amounts—one by one, rather than continuously.

An important secondary effect of the MgO layer is that it modifies the work function of the silver substrate, effectively lowering the energy required for electrons to leave the metal. This makes it energetically favourable for electrons to transfer into the nanoribbon. Because the ribbon’s states are discrete, it can only accept or donate whole electrons, leading to a series of well-defined charge states. The result is a hybrid system that acts somewhat like a molecular quantum dot: electrons can hop in or out, but always in integer steps, and their energy levels can be probed with exquisite precision.

Probing discrete states with STM and STS

To explore these properties, the researchers used scanning tunnelling microscopy (STM) and scanning tunnelling spectroscopy (STS) under ultrahigh vacuum and cryogenic temperatures. These techniques allow them to visualize individual nanoribbons and measure the local electronic density of states with sub-nanometre resolution. On bare silver, the ribbons show broad, featureless conductance peaks—signatures of strong coupling to the metal. But once transferred onto MgO, the spectra reveal sharp, well-defined resonances separated by tens to hundreds of millielectronvolts. These resonances correspond to quantum well (QW) states—discrete electronic levels arising from quantum confinement within the ribbon.

Even more strikingly, each main resonance is accompanied by a series of weaker satellite peaks, separated by characteristic energy intervals. These features, known as Franck–Condon replicas, arise from the coupling between electronic transitions and vibrational modes of the nanoribbon. Their presence confirms that the electrons are localized and that the coupling to the substrate is sufficiently weak to preserve these fine molecular details. In other words, the MgO layer enables the researchers to observe the intrinsic, molecule-like electronic behaviour of the graphene nanoribbon.

By mapping the spatial distribution of these resonances along the ribbon, the authors identify the patterns associated with the lowest and highest quantum well states. These experimental maps match remarkably well with mean-field Hubbard (MFH) model simulations, which take into account both the electronic structure and electron-electron interactions. This agreement allows the team to determine how many electrons occupy the ribbon and whether the system’s ground state is non-magnetic (all electrons paired) or magnetic (one unpaired electron).

A quantum switch controlled by ribbon length

One of the most intriguing outcomes of the study is that the magnetic behaviour of the nanoribbon alternates systematically with its length. When the ribbon contains an even number of repeating molecular units, its frontier electronic levels are completely filled, resulting in a closed-shell, non-magnetic state. When the ribbon has an odd number of units, one electron remains unpaired in the highest occupied state, giving the ribbon a net spin-½ moment. Thus, by adding or removing a single structural unit, the researchers can toggle between a spin-active and a spin-inactive configuration.

This alternation arises from the interplay between quantum confinement and electron–electron interactions. The addition of the MgO layer changes the energy alignment between the ribbon’s discrete levels and the Fermi energy of the substrate, enabling controlled charge transfer. For certain ribbon lengths, this leads to odd electron occupancy and hence an open-shell configuration; for others, the transfer yields an even count and a closed-shell configuration. Spectroscopically, the open-shell ribbons display a small spin-splitting of their frontier levels—a few tens of millielectronvolts—consistent with theoretical predictions from the MFH model.

Interestingly, the overall energy gap of the ribbons does not vary smoothly with length, as one might expect from a simple “particle-in-a-box” model. Instead, it shows non-monotonic behaviour that reflects the discrete transitions between these closed and open electronic shells. Each additional molecular unit slightly shifts the quantization conditions, altering the charge transfer balance and thereby the magnetic state.

Modelling the charge and spin states

To interpret their data quantitatively, the researchers combine two complementary frameworks. The first is a charging model that treats the nanoribbon as a small quantum system in equilibrium with an electron reservoir (the MgO/Ag substrate). The model estimates how many electrons will transfer to the ribbon based on the alignment between the ribbon’s electron affinity, the substrate’s work function, and the electrostatic energy cost of adding charge across the insulating MgO.

The second framework is the mean-field Hubbard model, which accounts for Coulomb repulsion between electrons occupying the same or nearby states. By solving this model self-consistently, the authors calculate the energy levels, spin configurations, and spectral features expected for each charge state. The combined approach reproduces the experimental observations with striking accuracy: as ribbon length increases, the number of excess electrons and the presence or absence of spin alternate in precisely the same sequence seen in the measurements.

The behaviour can be tuned

The implications of this work reach beyond the specific system studied. It demonstrates a clear and reproducible way to modulate charge and spin at the atomic scale in carbon-based nanostructures—without relying on external gates or chemical doping. The MgO layer acts as both a decoupling medium and an intrinsic charge regulator, enabling stable integer charge states and tunable magnetic configurations. This approach could serve as the foundation for building graphene-based spin qubits, where each nanoribbon acts as a controllable spin unit, or for developing ultra-small spintronic components where magnetic and non-magnetic elements are defined by atomic precision.

Moreover, the methodology—using an insulating film on metal to balance decoupling and charge transfer—could be extended to other low-dimensional materials. Similar architectures might allow researchers to explore correlated electron effects, quantum magnetism, or charge localization phenomena in a variety of systems previously inaccessible because of substrate screening.

By combining precise atomic engineering, ultrathin oxide films, and advanced spectroscopic techniques, the researchers have demonstrated a powerful principle: the electronic and magnetic behaviour of graphene nanoribbons can be tuned systematically by their geometry and environment. The MgO layer not only protects the delicate quantum states from the metallic substrate but also provides the subtle electrostatic control needed to add or remove single electrons. The result is a model platform for exploring the boundary between molecular electronics and solid-state quantum physics—where charge, spin, and geometry intertwine at the atomic scale.

Author: César Tomé López is a science writer and the editor of Mapping Ignorance

Disclaimer: Parts of this article may have been copied verbatim or almost verbatim from the referenced research paper/s.