Spin-Crossover\nSolids as Phase-Change Materials to\nActuate Strain in 2D MoS<sub>2</sub>

Spin-crossover\n(SCO) transitions in complexes or nanoparticles\ncoupled to two-dimensional (2D) materials, such as graphite and metal\ndichalcogenides, can alter the electronic or optical response of 2D\nnanosheets. However, the mechanism by which the spin transition tunes\nthe properties of the 2D materials remains unclear. Herein, we describe\ntwo 2D/3D heterostructures combining electrochemically exfoliated\nmolybdenum disulfide (MoS<sub>2</sub>) and spin transition crystals\nFe(2-pytrz)<sub>2</sub>Pd(CN)<sub>4</sub>·3H<sub>2</sub>O (2-pytrz\n= 4-(2-pyridyl)-4<i>H</i>-1,2,4-triazole) and Fe(pz)Pd(CN)<sub>4</sub>) (pz = pyrazine), unambiguously demonstrating how spin crossover\ninduces strain in 2D materials leading to altered properties. In both\nthe MoS<sub>2</sub>-Fe(2-pytrz)<sub>2</sub>Pd(CN)<sub>4</sub>·3H<sub>2</sub>O and MoS<sub>2</sub>-Fe(pz)Pd(CN)<sub>4</sub> heterostructures,\nthe MoS<sub>2</sub> photoluminescence emission energy shifted in response\nto the spin-crossover phase transition of the spin-transition crystal\nsubstrates. Importantly, the Raman shifts corresponding to the A<sub>1g</sub> in-plane and E<sub>2g</sub> out-of-plane MoS<sub>2</sub> vibrational modes also correlate with the substrate spin crossover,\nproviding direct evidence of the induced strain. Both the Raman shifts\nand photoluminescence changes of MoS<sub>2</sub> correspond to compressive\nstrain, estimated to be 0.5% for the MoS<sub>2</sub>-Fe(2-pytrz)<sub>2</sub>Pd(CN)<sub>4</sub>·3H<sub>2</sub>O and 0.4% for the MoS<sub>2</sub>-Fe(pz)Pd(CN)<sub>4</sub> heterostructure. These examples\ndemonstrate the viability of using spin-transition solids as phase-change\nmechanical actuators for the switching properties of 2D materials.