The band structure of semiconductors can be strongly perturbed by mechanical strain, giving rise to the possibility of using mechanical deformation to tune their electronic and photonic performance.¹ This principle, dubbed strain engineering, has been used for a long time in semiconductor devices manufacturing. For instance, applying uniaxial or biaxial tensile strain to a silicon transistor makes it possible to enhance electron mobility significantly (or hole mobility).² However, the classical components of these devices are made of bulk inorganic materials whose physical properties are sensitive to downsizing and can only withstand a reduced strain, limiting the further application of strain engineering.

In this context, two-dimensional (2D) semiconductors are presented as an outstanding alternative. Due to the covalent nature of the in-plane bonds and the ultrathin thickness of 2D materials, flexibility, optical transparency, and exceptional mechanical strength are expected, permitting the application of a strain as high as 11% before fracture.³ Additionally, from the electronic point of view, 2D materials can show a wide spectra of behaviours, such as ultrahigh carrier mobility and field-effect switching ratio. All these features make some of them, ideal channels in field-effect transistors (FETs).⁴

In the Research Team of Molecular Materials, at the ICMol (https://www.icmol.es/rtmm/), we had been working for some years with MoS₂, the flagship of 2D semiconducting materials, facing both its chemical modification in solution⁵ and its integration in electronic/spintronic devices by dry methods.⁶ On the other hand, we have a solid experience in designing molecular materials and switchable molecular-based nanostructures.⁷ In this scenario, we envisioned the possibility of developing a self-strainable 2D material, where the induced strain could be applied by a molecular system chemically integrated on the surface of the 2D layer.

Now, in Nature Chemistry, we report the strain engineering of chemically exfoliated MoS₂ (CE-MoS₂), by coupling the 2D layers with core@shell nanoparticles of a coordination compound that exhibit the spin-crossover phenomenon (SCO). SCO is one of the most spectacular examples of molecular bistability and it is displayed by some transition metal complexes. These compounds can exist in two different electronic states, the stable low spin state (LS) or the metastable high spin state (HS), as a function of external stimuli like temperature, light, or pressure. Remarkably, the physical properties of the SCO compounds differ dramatically as a function of the spin state. In particular, due to the filling of antibonding orbitals in the HS, the compound in this spin state displays a larger volume. For this reason, SCO nanoparticles have been widely employed as mechanical actuators and sources of mechanical strain.

Therefore, in this work, we have taken advantage of the tuneable strain applied by SCO nanoparticles grafted on the semiconducting MoS₂, to prepare a self-strainable system. The work begins with developing the SCO/MoS₂ heterostructure via a three-step protocol, starting with the chemical exfoliation of the MoS₂, followed by its functionalization with a molecular linker, to finally attach the SCO nanoparticles through this molecular bridge (Figure 1). 

The obtained SCO/MoS₂ heterostructure displays a spin state-dependent on-off electrical conductivity switching, product of the MoS₂ bandgap modulation. This effect is originated from the different strain applied by the SCO nanoparticles upon the spin transition and can also be induced by light. Even more, this gap tuning can be optically followed. Noticeably, the magnitude of the applied strain can be modified by playing with the degree of coverage and the size of the anchored nanoparticles. Additionally, the new heterostructure permits a simple electrical sensing of the nanoparticles spin state.

Therefore, the reported approach permits the coupling of phase transition nanoparticles with a 2D semiconductor to simultaneously tune its electronic and optical response, becoming a smart heterostructure. This synthetic strategy may be extended to other 2D materials besides MoS₂ and to different SCO systems, opening the way to the preparation of a new family of 2D heterostructures.  You can read more about this work in Nature Chemistry. “Spin-crossover nanoparticles anchored on MoS₂ layers for heterostructures with tunable strain driven by thermal or light-induced spin switching”  https://doi.org/10.1038/s41557-021-00795-y

Have full-text access to a view-only version  by using the following link: https://rdcu.be/cy5S8

Written by R. Torres-Cavanillas and A. Forment-Aliaga.

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4. Radisavljevic B, Radenovic A, Brivio J, Giacometti V, Kis A. Single-layer MoS₂ Nat Nanotechnol. 2011;6(3):147-150.

5. a) Morant-Giner, M., et al. Functionalisation of MoS₂ 2D layers with diarylethene molecules. Mater. Chem. C, 2021, 9, 10975-10984. b) Morant‐Giner, M., et al., Prussian Blue@ MoS₂ Layer Composites as Highly Efficient Cathodes for Sodium‐and Potassium‐Ion Batteries. Advanced Functional Materials, 2018, 28(27), 1706125.

6. Galbiati, M., et al. Path to overcome material and fundamental obstacles in spin valves based on MoS₂ and other transition-metal dichalcogenides. Physical Review Applied, 2019, 12(4), 044022.

7. a) Torres‐Cavanillas, R. et al., Design of Bistable Gold@ Spin‐Crossover Core–Shell Nanoparticles Showing Large Electrical Responses for the Spin Switching. Advanced Materials, 2019, 31(27), 1900039. b) Dugay, J., et al. Sensing of the Molecular Spin in Spin-Crossover Nanoparticles with Micromechanical Resonators. The Journal of Physical Chemistry C, 2019, 123(11), 6778-6786.