Surface polarons in Fe-based spin-crossover complexes – a perspective

Iron-based spin crossover (SCO) complexes have quietly become one of the most versatile families of molecular materials in modern nanoscience, prized above all for their room-temperature bistability between low-spin and high-spin states. That bistability makes them natural candidates for channel materials in molecular transistors and room-temperature spintronic logic, where scalable molecular spin systems could bypass the cumbersome production of high-fidelity entangled states required for quantum information applications. But a new Perspective published in Frontiers in Nanotechnology by Alejandro Martínez Serra of the University of Valencia and Archit Dhingra of the MAX IV Laboratory in Lund argues that the field is missing a crucial piece of the puzzle: the surfaces of these materials, where theory now suggests both small and large polarons may reside.

The central claim of the Perspective is deceptively simple. None of the promising device applications of Fe-based SCO complexes can be truly realized until researchers obtain a solid understanding of what actually terminates their surfaces. As the authors put it, echoing a maxim of interface science, the interface is the device. Any serious attempt at commercializing molecular transistors based on Fe-based SCO systems therefore necessitates a prior understanding of their respective surfaces, because the atomic species lying at the surface strongly influence the surface magnetochemistry of the material.

Deciphering the surface termination of these complexes is far from trivial. These are molecular salts and coordination polymers containing Fe(II) or Fe(III) ions in octahedral ligand fields, and the existing literature shows that ascertaining which molecular moiety caps the crystal is a genuinely hard problem. In a recent pioneering study, Zaz and colleagues resorted to X-ray absorption spectroscopy (XAS) and laboratory-based X-ray photoelectron spectroscopy (XPS) measurements to prove that the surface of the Fe(III) SCO salt [Fe(qsal)2][Ni(dmit)2], where qsal is N(8-quinolyl)salicylaldimine and dmit(2−) is 1,3-dithiol-2-thione-4,5-dithiolato, preferentially terminates in the Ni(dmit)2 moiety. Until that investigation was disseminated, the surface termination of this family of materials lacked full transparency, and the discovery is expected to have significant implications for the fabrication of nanodevices based on this material.

What makes these materials truly intriguing, however, is not just their room-temperature bistability but the physics of how charge moves through them. In SCO complexes containing Fe(II) ions, charge transport is understood to be dictated by polarons, quasiparticles formed when an electron couples to the lattice vibrations around it. Depending on the specific values of the on-site Hubbard interaction and the strength of the ligand-to-metal charge transfer, these materials are theorized to be capable of hosting both small polarons, in which the charge is localized on a single molecular unit, and large polarons, in which the charge drags a extended cloud of lattice distortion behind it.

The theoretical description of large polarons in these systems currently rests on an approximate three-term Fröhlich Hamiltonian, the standard framework dating back to Fröhlich’s 1954 treatment of electrons in lattice fields. In that picture, the momentum of an electron is described by a momentum operator, the momentum and dispersion of phonons are denoted by their respective operators and functions, and the strength of the electron-phonon interaction is given by a coupling term whose specific form is dictated by the system of interest. Creation and annihilation operators associated with a phonon of a given wave number complete the description. It is a rough Hamiltonian, the authors concede, but if the theoretical speculations surrounding the presence of polarons in these systems are experimentally validated, it would represent a significant breakthrough in molecular spintronics, positioning Fe-based SCO materials as promising candidates for room-temperature applications in quantum information science with enhanced functionalities.

The problem is that the surfaces of Fe-based SCO complexes containing Fe(II) and Fe(III) ions in octahedral ligand fields remain clearly understudied, and much more work is needed to unravel the surface termination of these easily switchable spin-state materials. To close that gap in a non-disruptive fashion, Martínez Serra and Dhingra suggest two possible routes. The first is angle-resolved X-ray photoelectron spectroscopy, or ARXPS, in which the photoemission take-off angle is changed to alter the sample probing depth. ARXPS can easily be performed using a standard X-ray Al Kα or Mg Kα anode in any laboratory, making it accessible to virtually any surface science group. The second is photon-energy dependent high-resolution X-ray photoelectron spectroscopy, or HRXPS, which necessitates the use of a synchrotron facility but offers finer control over the depth sensitivity of the measurement.

For detecting the polarons themselves, the authors recommend using synchrotron-based vacuum ultraviolet (VUV) radiation to investigate the highest occupied molecular orbital, also known as the valence band edge, of these materials. Photoemission spectroscopy has already been shown to be one of the reliable methods for the experimental observation of polarons in condensed matter, and the proposal is to hunt for polaronic signatures at the surfaces of the complexes. Any changes in the intensities of the HOMO band and in the work function of the system will manifest as variations in the photoelectron yield and in the position of the secondary electron cutoff on the kinetic energy scale, respectively. Changes to the observed photoemission spectra in the vicinity of the Fermi level would carry physical significance that the authors direct curious readers to explore in the prior literature, given the strict word limit for Perspective articles in the journal.

If spectroscopic signatures pertaining to the existence of polarons in these materials are observed, the authors propose that temperature-dependent electrical transport, optical spectroscopy, and Raman spectroscopy measurements could eventually be used to cross-validate the photoemission spectroscopy data. The standalone applications of polaronic transport are numerous, spanning conducting polymers, oxide interfaces, and electromechanically coupled materials, so if polaronic transport is confirmed in Fe-based SCO systems it could enhance the already well-established versatility of these materials and raise it to another level. Recent direct observations of Fröhlich polaron formation in other nanoscale systems suggest the experimental toolkit is maturing rapidly.

The Perspective, thus, acts as a catalyst intending to push the frontiers of research into the surfaces of this class of materials for applications in room-temperature molecular spintronics. The idea disseminated via the article stems from data collected at the MaxPEEM beamline at the MAX IV Laboratory, a Swedish national user facility supported by the Swedish Research Council, Vinnova, and Formas. The discussions contained in the article are expected to catalyze future investigations into the fabrication of room-temperature nanodevices that exploit polaronic transport in these complexes, turning a family of molecular materials once valued mainly for their switchable spins into potential workhorses of polaronic molecular electronics.

Subject of Research: Surface polarons in Fe-based spin-crossover complexes – a perspective

Article Title: Surface polarons in Fe-based spin-crossover complexes – a perspective

Article References: Serra, A. M., & Dhingra, A. (2026). Surface polarons in Fe-based spin-crossover complexes – a perspective. Frontiers in Nanotechnology, 8, Article 1901345. https://doi.org/10.3389/fnano.2026.1901345

Image Credits: AI Generated

DOI: 10.3389/fnano.2026.1901345

Keywords: polaronic transport, polarons, spin crossover complexes, surface termination, synchrotron science, vacuum ultraviolet (VUV) radiation, Surface, Fe-based, spin-crossover, complexes, perspective, scientific research