Spin Crossover Heterostructures for Inducing Strain: Two Contrasting Responses in MoS₂ Nanosheets

Spin-crossover (SCO) materials often undergo large, reversible lattice changes associated with the spin-state transitions. Coupling such phase-transition solids with two-dimensional (2D) semiconductors provides a promising route toward dynamically tunable strain engineering. Here, we investigate induced strain in MoS₂ nanosheets deposited directly onto SCO single-crystal substrates using variable-temperature Raman and photoluminescence (PL) spectroscopy. Two representative SCO systems with distinct structural motifs are examined: the molecular complex [{Fe(NCSe)(py)}₂(μ-bpypz)₂] (1), exhibiting a below room temperature spin crossover with ∼4% volume change, and the Hofmann-like framework [Fe(pyrazine){Au(CN)₂}₂] (2), displaying an above room temperature spin crossover with ∼6% volume change. Despite both substrates undergoing cooperative spin transitions, the MoS₂ layers exhibit fundamentally different optoelectronic responses. In the MoS₂-1 heterostructure, the spin transition induces efficient and uniform strain transfer, leading to a reversible Raman shift of the E_2g¹ mode and a hysteretic photoluminescence peak shift that directly reflects bandgap modulation by strain. In contrast, the MoS₂-2 heterostructure displays an unconventional response characterized by a pronounced hysteresis in PL intensity with negligible PL peak shift, accompanied by a Raman signature indicative of compressive strain that is opposite to expectations from simple elastic coupling. By comparing these systems with previously reported MoS₂-SCO heterostructures, we demonstrate that SCO-induced strain in MoS₂ cannot be predicted solely by the magnitude of the substrate volume change. Instead, the crystallographic orientation, surface chemistry, and interfacial conformity collectively determine whether the MoS₂ response is governed by uniform lattice strain, bandgap modulation, or morphology-mediated recombination processes. These findings establish SCO single crystals as versatile mechanical phase change materials capable of programming distinct optoelectronic functionalities in 2D materials through rational interface design.