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Chemistry

Using spin-crossover metal-organic frameworks to switch two-dimensional materials

Porous chemistry meets solid-state physics

Metal-organic frameworks have long occupied a distinct niche in materials chemistry. Built from metal coordination centres linked by organic bridging molecules into porous, crystalline networks, they have established utility in gas storage, molecular separations, catalysis, and energy storage. Yet their integration with electronically functional solid-state materials has remained largely unexplored. Because coordination frameworks are typically treated as insulating porous matrices rather than electrically active components, solid-state electronic device design has largely evolved without them.

At the same time, the field of two-dimensional quantum materials faces a persistent actuation challenge. Atomic monolayers and few-layer flakes of semimetals, magnetic semiconductors, and superconductors exhibit electronic behaviours that are exceptionally sensitive to external physical perturbations. In conventional device architectures, modifying these states requires static electrostatic gates, chemical doping, or strain applied through external piezoelectric substrates. While electrostatic gating can modulate charge carrier densities, it struggles to manipulate magnetic exchange interactions or induce non-volatile structural transformations without substantial gate voltages. Interfacing two-dimensional materials with dynamic molecular crystals offers a different path, but demonstrating direct, switchable control across multiple physical regimes has proved difficult.

A peer-reviewed study published in Advanced Materials by Carla Boix-Constant and colleagues demonstrates that coordination chemistry can bridge this divide. By integrating a layered spin-crossover metal-organic framework into electrical nanodevices, the researchers showed that molecular phase transitions can serve as local control elements for neighbouring two-dimensional quantum materials. Rather than functioning as a passive porous host, the coordination network acts as an active transducer, converting thermal and optical stimuli into interfacial strain that alters electronic transport, magnetic hysteresis, and superconductivity.

Spin crossover as a nanoscale mechanical actuator

The operative mechanism in this hybrid architecture relies on the phenomenon of spin crossover in transition metal complexes. In specific octahedrally coordinated transition metal systems, the energy gap between the low-spin electronic state and the high-spin electronic state is sufficiently balanced that external stimuli can toggle metal centres between the two configurations. When an iron(II) centre transitions from low spin to high spin, the redistribution of electrons into anti-bonding coordination orbitals lengthens the metal-ligand bonds, expanding the local coordination environment.

Boix-Constant and co-workers embedded this molecular switching behaviour within a layered Hofmann-type coordination framework formulated as {FeII(pyS2Et)2[PtII(CN)4]}. Hofmann-type frameworks feature bimetallic coordination networks where square-planar platinum(II) tetracyanide complexes link octahedrally coordinated iron(II) centres into extended two-dimensional sheets, decorated with axial pyridine-based ligands. Because these sheets stack into layered structures through van der Waals interactions, they can be combined into heterostructures with other two-dimensional crystals.

When the iron centres in {FeII(pyS2Et)2[PtII(CN)4]} undergo a spin transition, the coordinated structural shift across the molecular lattice creates measurable physical deformation. This transition can be activated through thermal changes or through illumination with light. When assembled directly against two-dimensional quantum materials inside van der Waals heterostructures, these lattice alterations transduce mechanical strain directly across the material interface. The coordination network effectively serves as an integrated molecular actuator, delivering targeted strain without requiring external mechanical presses or high-voltage piezoelectric elements.

Controlling conduction, magnetism, and superconductivity

To evaluate how effectively this strain couples to solid-state phenomena, the researchers interfaced the layered spin-crossover framework with three distinct classes of two-dimensional quantum materials: few-layer graphene, magnetic chromium sulfide bromide (CrSBr), and superconducting niobium diselenide (NbSe2).

In devices containing few-layer graphene, the interface translates the framework’s molecular state into a direct transport modulation. The authors showed that the electrical conductivity of graphene can be selectively switched using light. Because the band structure and charge transport of graphene respond to lattice deformation and interfacial environment changes, the light-induced spin transition in the adjacent framework provides an optical mechanism to modulate current flow through the device channel.

The effect proved even more pronounced when interfacing the framework with CrSBr, a layered magnetic semiconductor. In spin-valves fabricated from CrSBr bilayers, pristine material typically lacks magnetic hysteresis. When coupled to the Hofmann-type framework, however, the spin-crossover transition triggers the appearance of magnetic hysteresis. This induced hysteresis enables a tunable, non-volatile zero-field memory state within the device, demonstrating that molecularly generated strain can dynamically reconfigure magnetic exchange pathways in low-dimensional spintronics.

Finally, the researchers fabricated heterostructures incorporating the two-dimensional superconductor NbSe2. Illuminating the device allowed the light-induced spin transition in the overlying framework to modulate both the superconducting critical current and the superconducting transition temperature of the underlying NbSe2 channel. Demonstrating strain-mediated control over conductivity in a semimetal, hysteresis in a magnet, and Cooper pairing in a superconductor establishes the spin-crossover framework as a versatile control element across distinct physical domains.

Engineering hurdles and interface physics

While these demonstrations prove the conceptual viability of using coordination frameworks to manipulate solid-state electronic properties, considerable technical boundaries separate laboratory nanodevices from functional technologies.

A primary consideration is the quality and uniformity of strain transmission across the van der Waals interface. The mechanical coupling described in the study relies on clean, intimate contact between the exfoliated layers. In practice, interfacial contamination, wrinkles, and atomic-scale slip can damp or misdirect the transferred strain, introducing device-to-device variability. Translating these hand-assembled flakes into scalable manufacturing will require deposition or assembly techniques capable of maintaining pristine interfaces across larger surface areas.

A further operational challenge involves the mechanical endurance of the coordination framework. Driving a molecular lattice repeatedly through high-spin and low-spin transitions exerts cyclic mechanical stress on the coordination bonds. Over extended operation, repeated volume changes can promote fatigue, lattice defects, or microcracking within the framework crystal. The authors demonstrated active switching in proof-of-concept devices, but determining the cycle life, fatigue resistance, and long-term chemical stability of thin {FeII(pyS2Et)2[PtII(CN)4]} layers under continuous thermal and optical switching will be critical for any real-world device proposition.

Additionally, device operation remains constrained by the environmental parameters governing the underlying physical phenomena. The conditions required to sustain superconductivity in NbSe2 or trigger specific spin-state transitions impose narrow operating windows. Expanding this molecular-strain architecture towards commercial electronics will necessitate developing coordination frameworks whose spin transitions exhibit broad thermal hysteresis and efficient light-induced switching near ambient room temperatures.

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The R&D takeaway

For research leaders directing programmes in spintronics and functional materials, this work demonstrates that coordination frameworks can provide active mechanical actuation rather than merely acting as passive porous coatings. Development teams working on low-dimensional devices should consider whether stimuli-responsive molecular lattices can replace conventional electrostatic or piezoelectric gating schemes for strain-mediated switching. Prioritising research into interface fatigue, thin-film deposition scalability, and room-temperature framework stability will determine whether this molecular approach matures into practical solid-state memory and switching platforms.

The R&D Innovate desk