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

Fe-based spin crossover (SCO) complexes are a highly versatile family of materials, especially for room-temperature device applications. In addition to their well-documented applications, this class of systems is now theorized to harbor both small and large polarons. Nevertheless, none of their promising applications can be truly realized until a solid understanding of their respective surfaces is obtained. In this Perspective, recent efforts (i) providing insights into the surface termination of some of the well-studied Fe-based SCO materials, and (ii) exploring the possibility of polaronic transport in these complexes, are discussed. Finally, the use of synchrotron-based vacuum ultraviolet (VUV) radiation in order to detect the possible surface polarons in these systems is also proposed. The discussions contained herein are therefore expected to catalyze future investigations into the fabrication of room-temperature nanodevices that exploit polaronic transport in these complexes.

Scalable molecular transistor logic gates can potentially bypass the long-standing issue revolving around the cumbersome production of high-fidelity entangled states (Clauser et al., 1969; Clauser and Horne, 1974; Bennett, 1995; DiVincenzo, 1995) for use in quantum information and science (Dhingra et al., 2022a). One of the primary challenges in building such molecular transistors is finding the right kind of molecular spin systems that can offer bistability at–or in the vicinity of–room temperature, so that they can be used as channel materials in room-temperature spintronics (Dhingra et al., 2022a). This is exactly where Fe-based spin crossover (SCO) complexes jump into the fray: these systems have garnered great research attention owing to the well-documented room-temperature bistability of their spin states (Holovchenko et al., 2016; Soroceanu et al., 2019; Rubio-Giménez et al., 2020; Martínez Serra et al., 2023). As if their bistability was not lucrative enough, this class of materials is also projected to rely on both small and large polarons for charge transport (Soroceanu et al., 2019; Dhingra and Zaz, 2026). However, from the standpoint of both a solid fundamental understanding and reliable nanodevice fabrication, deciphering the surfaces of these Fe-based SCO materials is indispensable. In other words, since “the interface is the device” (EBSCOhost, 2012), any serious attempt at eventual commercialization of molecular transistors based on Fe-based SCO systems necessitates prior understanding of their respective surfaces.

In this Perspective, some recent efforts shedding light on the surface termination of these Fe-based SCO materials are discussed. The main goal of this article is to show the link between the nature of the surface termination of these systems and the potential existence of the much-coveted surface polarons in these fascinating materials. As far as verifying the presence of these quasiparticles is concerned, using synchrotron-based vacuum ultraviolet (VUV) radiation is proposed as a viable route to detect their associated spectroscopic signatures (Vasiljevic et al., 2026).

Surfaces of Fe-based SCO complexes

A dearth of fundamental understanding of what truly is at the surface of a material of interest means that any sort of research endeavor targeted in the general direction of using that material for device fabrication or interface engineering is destined to be utterly futile. Therefore, this subsection shines light on the peculiarity of surfaces of Fe-based SCO materials by referring to research involving Fe-based SCO complexes with Fe(II) (Niel et al., 2001; Martínez et al., 2010; Zacher et al., 2011; Warner et al., 2013; Shvachko et al., 2016; Rohlf et al., 2018; Ekanayaka et al., 2022; Hao et al., 2022; Martínez Serra et al., 2023; Serra et al., 2023; Yazdani et al., 2023) and Fe(III) (Nihei et al., 2007; Li et al., 2010; Griffin et al., 2011; Siig and Kepp, 2018; Zaz et al., 2025b; 2025a) ions in octahedral ligand fields (as shown in Figure 1).

Based on the existing literature pertaining to the investigation of the surface termination of this class of materials, it is evident that ascertaining this is far from trivial (Shepherd et al., 2013; Zaz et al., 2025b). Nonetheless, in a recent study, Zaz et al. (Zaz et al., 2025b) resorted to X-ray absorption spectroscopy (XAS) and laboratory-based X-ray photoelectron spectroscopy (XPS) measurements (shown in Figure 2) to prove that the surface of their Fe(III) SCO salt [Fe(qsal) 2 Ni(dmit) 2 ], where qsal = N(8quinolyl)salicylaldimine, and dmit 2− = 1,3-dithiol-2-thione-4,5-dithiolato, preferentially terminates in the Ni(dmit) 2 moiety. This fundamental discovery is expected to have significant implications for the fabrication of nanodevices based on this material, as the atomic species lying at the surface strongly influence the surface magnetochemistry (Ballav et al., 2013). Given the lack of full transparency regarding the surface termination of this family of materials until the dissemination of Zaz’s pioneering investigation (Zaz et al., 2025b), it merits well-deserved praise.

Detecting surface polarons

Fe-based SCO materials are not only bistable at room temperature, but they also have the added advantage that charge transport in those containing Fe(II) ions is understood to be dictated by polarons (Soroceanu et al., 2019; Dhingra and Zaz, 2026). What makes these materials truly intriguing in this aspect is the fact that–depending on the specific values of the on-site Hubbard interaction and the strength of the ligand-to-metal charge transfer–they are capable of hosting both small and large polarons (Dhingra and Zaz, 2026). However, as things stand, all we have at our disposal is an approximate three-term Fröhlich Hamiltonian that describes large polarons in these materials. This rough Hamiltonian can be spelled out as follows (Fröhlich, 1954)

As described elsewhere (Dhingra and Zaz, 2026), the momentum of an electron of mass m is described by the momentum operator p ^ e, the momentum of phonons is denoted by k, their dispersion is denoted by ω k, and the strength of the interaction between electrons and phonons is given by V k. The specific forms of these functions are dictated by the system of interest (Grusdt et al., 2017; Houtput and Tempere, 2021). The creation and annihilation operators associated with a phonon of wave number k are given by b ^ k † and b ^ k, respectively.

Therefore, if the theoretical speculations surrounding the presence of polarons in these systems are experimentally validated, this could represent a significant breakthrough in the field of molecular spintronics, as it would position the Fe-based SCO materials as promising candidates for room-temperature applications in quantum information and science, offering enhanced functionalities.

The surfaces of the Fe-based SCO complexes containing Fe(II) and Fe(III) ions in octahedral ligand fields are clearly understudied. Therefore, what Zaz et al. have shown in their recent study is quite praiseworthy. Nevertheless, much more work is needed to unravel the surface termination of these complexes with easily switchable spin states. In light of this, we suggest two possible routes to decipher the surface termination of these materials in a non-disruptive fashion: (i) angle-resolved X-ray photoelectron spectroscopy (ARXPS), where the photoemission take-off angle is changed to alter the sample probing depth (Dhingra et al., 2020; 2021b; 2021a; 2022b; 2023; Di Bernardo, 2021); and (ii) photon-energy dependent high-resolution X-ray photoelectron spectroscopy (HRXPS). The former, i.e., ARXPS, can easily be performed using a standard X-ray Al K α (or Mg K α) anode in any laboratory. In contrast, the latter, i.e., photon-energy-dependent HRXPS, necessitates the use of a synchrotron facility.

Since even the standalone applications of polaronic transport are numerous (Bredas and Street, 1985; Sarkar et al., 2019; Santucci et al., 2025; Tinti et al., 2025), if their presence in these materials is experimentally confirmed, they could enhance the already well-established versatility of Fe-based SCO systems and raise it to another level. Now that photoemission spectroscopy has been shown to be one of the reliable methods for the experimental observation of polarons in condensed matter (Kestler et al., 2026; Vasiljevic et al., 2026), we recommend using synchrotron-based VUV radiation to investigate the highest occupied molecular orbital (HOMO), also known as the valence band edge, of these materials for the experimental detection of polarons at the surfaces of these complexes. Any changes in the intensities of the HOMO band and the work function of the system will manifest as variations in the photoelectron yield and the position of the secondary electron cutoff on the kinetic energy scale, respectively. As a side note, while it would be interesting to discuss the physical significance of these changes to the observed photoemission spectra in the vicinity of the Fermi level (E F), due to the strict word limit for Perspective articles appearing in this journal, we would like to direct the curiosity of readers to Figure 4 of (Vasiljevic et al., 2026). If spectroscopic signatures pertaining to the existence of polarons in these materials are observed, then temperature-dependent electrical transport, optical spectroscopy, and Raman spectroscopy measurements could eventually be used to cross-validate the photoemission spectroscopy data. This 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.