Hybrid heterostructures of fe(II) spin crossover nanoparticles in 2D substrates for spintronics applications
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Date
2026-06-16
Authors
Χαρίτος, Αλέξανδρος
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Abstract
Two-dimensional (2D) materials have gathered intense research interest, particularly since 2004, when single-layer graphene (SLG) was isolated, at which point the scientific community reevaluated van-der-Waals solids, since it was now feasible to thoroughly investigate atomically-thin flakes, which were found to be of great interest in terms of their optical response, mainly due to quantum confinement. Over the last 15 years, sufficient experimental data has surfaced, establishing that the immediate environment of atomically-thin crystals has a profound effect on the optical indices of two-dimensional materials, such as Raman-active phonon modes and excitonic emission bands (e.g. excitons, trions, biexcitons), when discussing direct bandgap semiconductors. External perturbations that can cause significant alterations in the aforementioned indicators of 2D materials include controlled mechanical deformation (uniaxial, biaxial), temperature variations, and charge doping effects as well as powerful optical fields. It is possible, however, to quantify external perturbations by introducing phase-change solids to 2D substrates.
To that end, the present Doctoral Thesis, presents a detailed optical investigation of a plethora of advanced molecular materials, which were studied on mainly two fronts. The experimental characterization was focused on Raman spectroscopy on the individual, constituent materials and optical mapping of the synthesized heterostructures, through spatially-resolved Raman and photoluminescence (PL) spectroscopy.
In a nutshell, we fabricated and characterized heterostructures based on CVD-grown Tungsten Disulfide (WS2) flakes, and Fe(II) nanoparticles, widely known in the literature for the metastable magnetic behavior that they exhibit, under the influence of a ligand field, when they form the so-called spin-crossover complexes (SCO).
Initially, we performed Raman investigations on a plethora of different SCO species based on nanoparticles of the system [Fe(NH2trz)3]n(Br2)n, including four GO/Gn-hybrid samples (NP5-8) and a Cu-doped sample (NP9). Subsequently, a thorough spectral study of 1D nanorods of the system [Fe(Htrz)2(trz)]n(BF4)n (NP10) was carried out, both at room temperature and in a controlled-temperature environment, while two different excitation wavelengths (λexc) were used to obtain the measurements. The main difference between the two SCO systems revolves around the fact that, on average, the latter exhibits temperature-dependent magnetic susceptibility loops (hysteresis) approximately 50K above that of the former. Therefore, the second system was preferred to synthesize the SCO/WS2 heterostructure through a simple physical method, referring to single-step spin-coating. We dispersed NP10 in laboratory-grade purity Ethanol, at a concentration of 10mg/mL to use for the spin-coating procedure.
The heterostructure was characterized by spatially-resolved Raman and PL spectroscopy at selected areas of interest, where we had also performed optical mapping in pristine form, for post-deposition comparison purposes.
For the case of the [Fe(NH2trz)3]n(Br2)n nanoparticles, we determined that room-temperature micro-Raman spectroscopy has the capacity to simultaneously detect and trigger the SCO phenomenon to the point of saturation, having also noted relatively consistent state-resolved spectral changes in all of the samples within this group (NP1-8). The detailed Raman study of NP10 nanorods demonstrated that the dynamics of the SCO effect are wavelength-dependent, with 514nm having caused notable frequency shifts in characteristic phonon modes and introduced photothermal contributions in the region within close proximity to the focused beam spot. In the case of heterostructure, we noted relatively weak mechanical loads, traced back to the SCO overlayer, as well as a relative increase in charge density. The hysteretic loop of NP10, now in thin-film form, was heavily impacted, exhibiting inversed heating and cooling branches.
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Keywords
Raman spectroscopy, Photoluminescence spectroscopy, Coordination polymer, SCO/2DM heterostructures, Iron(II) complexes, Spin-crossover nanoparticles as molecular actuators, Spin-crossover nanoparticles, Spin-crossover phenomenon