[Paper Review] Engineering on-surface spin crossover: spin-state switching in a self-assembled film of vacuum-sublimable functional molecule

Senthil Kumar Kuppusamy, Michał Studniarek+8 more|arXiv (Cornell University)|Jan 1, 2018

Molecular Junctions and Nanostructures65 references66 citations

TL;DR

The paper demonstrates spin-crossover switching in a self-assembled, thin film of a vacuum-subli mable Fe(II) complex on weakly interacting surfaces, showing comparable spin switching in bulk and 10 nm films due to lamellar bilayer–like assembly.

ABSTRACT

Realization of spin crossover (SCO) based applications requires studying of spin state switching characteristics of SCO complex molecules at nanostructured environments especially on-surface. Except for a very few cases, the SCO of a surface bound thin molecular film is either quenched or heavily altered due to (i) strong molecule-surface interactions and (ii) differing intermolecular interactions in films relative to the bulk. By fabricating SCO complexes on a weakly interacting surface such as highly oriented pyrolytic graphite (HOPG) and copper nitride (CuN), the interfacial quenching problem has been tackled. However, engineering intermolecular interactions in thin SCO active films is rather difficult. This work proposes a molecular self-assembly strategy to fabricate thin spin switchable surface bound films with programmable intermolecular interactions. Molecular engineering of the parent complex system [Fe(H$_{2}$B(pz)$_{2}$)$_{2}$(bpy)] (pz = pyrazole, bpy = 2,2'-bipyridine) with a dodecyl (C$_{12}$) alkyl chain yielded a classical amphiphile-like functional and vacuum sublimable charge neutral Fe$^{\ m II}$ complex, [Fe(H$_{2}$B(pz)$_{2}$)$_{2}$(C$_{12}$-bpy)] (C$_{12}$-bpy = dodecyl[2,2'-bipyridine]-5-carboxylate). The bulk powder and 10 nm thin film, on quartz glass/SiO$_{\ m x}$ surface, of the complex showed comparable spin state switching characteristics mediated by similar lamellar bilayer like self-assembly/molecular interactions in both bulk and thin film states. This unprecedented observation augurs well for the development of SCO based applications, especially in molecular spintronics.

Motivation & Objective

• Investigate spin crossover (SCO) behavior of SCO complexes when confined to nanostructured, on-surface environments.
• Develop a molecular self-assembly strategy to control intermolecular interactions in thin SCO films.
• Assess whether thin films retain bulk-like SCO switching characteristics.
• Explore the suitability of the approach for molecular spintronics applications.

Proposed method

• Modify the parent SCO complex [Fe(H2B(pz)2)2(bpy)] by attaching a C12 alkyl chain to form [Fe(H2B(pz)2)2(C12-bpy)], yielding a vacuum-sublimable, amphiphile-like Fe(II) complex.
• Fabricate thin films (10 nm) and bulk powder samples on quartz glass/SiOx to compare SCO behavior.
• Characterize spin-state switching and self-assembly features attributed to lamellar bilayer–like arrangements in both bulk and thin film states.
• Use weakly interacting substrates (HOPG, CuN) to minimize interfacial quenching and preserve SCO activity.

Experimental results

Research questions

• RQ1 Can a self-assembled, vacuum-sublimable SCO complex retain bulk-like spin crossover behavior in thin films?
• RQ2 How do intermolecular interactions in a self-assembled film influence SCO switching on surfaces?
• RQ3 Is spin-state switching in a 10 nm film governed by lamellar bilayer–like assembly similar to bulk powder?
• RQ4 Can this approach enable SCO-based functionality in molecular spintronics?

Key findings

• Bulk powder and 10 nm film exhibit comparable spin-state switching characteristics.
• SCO behavior is mediated by similar lamellar bilayer–like self-assembly and molecular interactions in both states.
• Fabrication on weakly interacting surfaces helps mitigate interfacial quenching of SCO.
• The molecular design yields a classical amphiphile-like functional and preserves spin switching.
• The results support the potential for SCO-based applications in molecular spintronics.