[Paper Review] Electrical sensing of the thermal and light induced spin transition in robust contactless spin-crossover/graphene hybrid devices

Miguel Gavara‐Edo, Rosa Córdoba+4 more|arXiv (Cornell University)|Apr 1, 2022

Magnetism in coordination complexes46 references49 citations

TL;DR

This study presents a robust, contactless spin-crossover (SCO)/graphene hybrid device using sublimable [Fe(Pyrz)₂] molecules deposited on CVD-graphene, enabling fast, electrically detectable thermal and light-induced spin transitions. A novel growth protocol isolates the elusive tetragonal polymorph of [Fe(Pyrz)₂], achieving ~100% photo-induced spin transition yield in 5 minutes with a PMMA interlayer, demonstrating exceptional cyclability and performance for molecular spintronics.

ABSTRACT

Hybrid devices based on spin-crossover (SCO)/2D heterostructures grant a highly sensitive platform to detect the spin transition in the molecular SCO component and tune the properties of the 2D material. However, the fragility of the SCO materials upon thermal treatment, light irradiation or contact with surfaces and the methodologies used for their processing have limited their applicability. Here, we report an easily processable and robust SCO/2D hybrid device with outstanding performance based on the sublimable SCO [Fe(Pyrz)2] molecule deposited over CVD-graphene, which is fully compatible with electronics industry protocols. Thus, a novel methodology based on growing an elusive polymorph of [Fe(Pyrz)2] (tetragonal phase) over graphene is developed that allows us to electrically detect a fast and effective light-induced spin transition in the devices (~50% yield in 5 minutes). Such performance can be enhanced even more when a flexible polymeric layer of PMMA is inserted in between the two active components in a contactless configuration, reaching a ~100 % yield in 5 minutes.

Motivation & Objective

• Develop a robust, processable, and electrically detectable spin-crossover (SCO)/2D heterostructure compatible with industrial electronics protocols.
• Overcome limitations of fragile SCO materials under thermal, light, and electrical stress by using sublimable [Fe(Pyrz)₂] molecules and a contactless configuration.
• Achieve fast and efficient light-induced spin transition (LIESST effect) with high yield and electrical readout in a contactless device architecture.
• Demonstrate long-term cyclability and stability of the hybrid devices under repeated thermal and optical cycling.
• Explore the role of interfacial layers (e.g., PMMA) in enhancing photo-induced spin transition efficiency in SCO/graphene heterostructures.

Proposed method

• Fabricated horizontal, four-probe devices by sublimating [Fe(Pyrz)₂] molecules directly onto pre-contacted CVD-graphene substrates under high vacuum.
• Developed a novel sublimation protocol to selectively grow the elusive tetragonal polymorph of [Fe(Pyrz)₂] on graphene, distinct from the commonly known triclinic phase.
• Employed a contactless configuration with a poly(methyl methacrylate) (PMMA) interlayer between graphene and the [Fe(Pyrz)₂] film to decouple electrical and optical coupling.
• Used four-probe electrical transport measurements in a PPMS system to detect changes in graphene’s conductance as a readout of the SCO transition state.
• Performed X-ray absorption spectroscopy (XAS) at the Fe L2,3 edge to quantify the high-spin (HS) fraction and confirm the LIESST effect under cryogenic conditions (2 K) and laser irradiation (532 nm, 35 mW).
• Conducted Raman spectroscopy, AFM, and PXRD/SXRD to characterize the structural, morphological, and crystalline properties of the [Fe(Pyrz)₂] films and their integration with graphene.

Experimental results

Research questions

• RQ1 Can a robust, contactless SCO/graphene heterostructure be fabricated using sublimable [Fe(Pyrz)₂] molecules that enables fast and electrically detectable spin transitions?
• RQ2 Does the selective growth of the metastable tetragonal polymorph of [Fe(Pyrz)₂] enhance the efficiency of the light-induced spin transition (LIESST effect) compared to the triclinic phase?
• RQ3 How does the insertion of a PMMA interlayer affect the photo-induced spin transition yield and electrical response in SCO/graphene devices?
• RQ4 To what extent do the hybrid devices maintain cyclability and stability under repeated thermal and optical cycling?
• RQ5 Can XAS at the Fe L2,3 edge provide quantitative confirmation of the high-spin fraction during the LIESST effect in these devices?

Key findings

• A novel sublimation protocol successfully enabled the selective growth of the elusive tetragonal polymorph of [Fe(Pyrz)₂] on CVD-graphene, which was previously considered SCO-inactive.
• The devices exhibited a fast and effective light-induced spin transition (LIESST effect) with a ~50% yield in 5 minutes when using the tetragonal phase without a PMMA interlayer.
• Inserting a PMMA interlayer between graphene and [Fe(Pyrz)₂] significantly enhanced the photo-induced spin transition, achieving ~100% yield in 5 minutes.
• The hybrid devices demonstrated exceptional robustness, maintaining performance and cyclability over more than one month of repeated thermal and optical cycling.
• XAS measurements confirmed a quantitative increase in the high-spin fraction upon 532 nm laser irradiation at 2 K, with a linear combination analysis of LS and HS reference spectra enabling accurate HS fraction quantification.
• Four-probe electrical transport measurements revealed a reversible and highly sensitive response in graphene’s conductance, enabling reliable electrical detection of both thermal and photo-induced spin transitions near room temperature.