A monolayer of copper-based molecular qubits assembled on graphene maintains strong one-dimensional antiferromagnetic coupling – the graphene itself may help strengthen that magnetic interaction.
Study: Antiferromagnetic Chains in a Monolayer of Molecular Qubits Assembled on Graphene. Image Credit: ATK 3D Works/Shutterstock.com
A study published in Small shows that a monolayer of the copper(II) complex [Cu(dttt)2] (Cudttt), assembled on graphene grown on silicon carbide (SiC), preserves both the ordered chain structure and the strong antiferromagnetic behavior associated with the bulk material.
The researchers combined experimental measurements with theory and found that graphene not only supports the copper molecules but also appears to play a part in the magnetic exchange within the layer.
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Electron-spin molecular qubits are attractive for quantum information processing because their properties can be tuned through chemical design. That makes them useful candidates for building systems in which magnetic interactions between spin centers can be controlled with precision.
Antiferromagnetic materials have attracted increasing attention for this work because they offer fast spin dynamics, resistance to external perturbations, high magnetic wave propagation velocities, and the potential for optical control of antiferromagnetic coupling.
For quantum technologies, and for STM-based studies of single spins, it is especially important to create regular arrays of addressable magnetic units on surfaces. Neutral, thermally stable molecular qubits are therefore particularly well suited to ultra-high vacuum (UHV) deposition experiments.
Cudttt is based on the hydrogen-free, sulfur-rich ligand 1,3,2-dithiazole-4-thione-5-thiolate (dttt). Earlier work predicted an upper coherence-time limit of about 300 µs due to its nuclear-spin-depleted environment, although radical impurities in the diamagnetic crystalline host reduce it to about 2 µs.
In its pure bulk form, Cudttt behaves as a one-dimensional antiferromagnet, with exchange couplings reported up to about 100 cm-1. Its flat geometry and stability under UHV also make it a strong candidate for sublimation and surface assembly.
Coupling Cu with Graphene
The team investigated a Cudttt monolayer formed by thermal sublimation onto graphene grown on SiC, hoping to examine the film’s structure, electronic properties, and magnetism.
To do this, they used scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), angle-resolved photoemission spectroscopy (ARPES), synchrotron-based X-ray absorption methods, density functional theory (DFT), and wavefunction-based simulations.
Cudttt and the tetrabutylammonium precursor ligand (TBAdttt) were prepared using previously reported methods. The powders were washed several times with dichloromethane, methanol, and diethyl ether to remove contaminants, then further purified under UHV by stepwise heating to 150 °C.
The graphene substrate was produced by thermal decomposition of SiC(0001) in argon at around 1300 °C and 750 mbar for 10 minutes in a vertical cold-wall reactor.
The growth conditions were refined using machine learning to improve graphene coverage. For deposition, the researchers used a custom-built sublimation cell with a quartz crucible. The air-sensitive powders were handled in an argon glove box, transferred under controlled conditions, and sublimed at 385 K. Deposition rate was monitored with a quartz crystal microbalance.
Cudttt Assembly
The results show that Cudttt assembles into densely packed, well-ordered chains on graphene. STM revealed stripe-like features, while DFT supported a structure with short intermolecular Cu-S···S-Cu contacts, closely resembling those seen in the crystalline bulk phase.
XPS confirmed that the monolayer retained the expected stoichiometry and molecular integrity. The Cu2p spectra were consistent with the Cu2+ oxidation state and did not indicate significant electron transfer from the molecules to the substrate.
ARPES showed that graphene largely kept its intrinsic electronic structure after deposition. The Dirac point shifted only slightly, from about −0.44 eV for bare graphene to −0.46 eV after monolayer deposition, which the authors interpret as evidence of weak or negligible molecule-substrate electronic interaction.
Synchrotron-based X-ray absorption measurements, together with DFT and wavefunction simulations, also confirmed that the molecules lie flat and remain well-ordered on the surface.
The magnetic analysis showed that strong one-dimensional antiferromagnetic coupling persists in the monolayer, with an intrachain exchange interaction of about 50 cm-1.
The study also points to a role for graphene in that magnetic behavior. DFT calculations showed that the intrachain antiferromagnetic coupling is stronger when the substrate is included than in the same molecular arrangement without graphene.
The proposed mechanism involves spin delocalization through overlap between graphene 2pz orbitals and sulfur 3p orbitals, creating a non-negligible through-surface exchange pathway.
Temperature-dependent X-ray magnetic circular dichroism (XMCD) measurements supported the presence of strong antiferromagnetic interactions and were used to estimate an average defect-free chain length of 14.2 ± 1.1 Cu2+ centers in the monolayer.
What This Means for Molecular Qubits
The study shows that a surface-assembled monolayer of molecular qubits can preserve strong antiferromagnetic ordering while remaining electronically compatible with graphene. As such, it is a useful platform for building ordered spin-chain architectures on surfaces without losing the magnetic interactions that matter for future quantum technologies.
It also suggests that graphene is not only an inert support. Instead, it may help reinforce the intermolecular magnetic exchange while still retaining its own electronic character. That relationship between the molecular layer and the substrate could be important for the design of future hybrid quantum materials.
Journal Reference
Santanni, F. et al. (2026). Antiferromagnetic Chains in a Monolayer of Molecular Qubits Assembled on Graphene. Small, e73217. DOI: 10.1002/smll.73217
