Electronic transport properties of spin-crossover polymer plus polyaniline composites with Fe3O4 nanoparticles

Adding Fe3O4 nanoparticles to composites of [Fe(Htrz)2(trz)](BF4) spin-crossover polymer and polyaniline (PANI) drives a phase separation of both and restores the molecular structure and cooperative effects of the spin-crossover polymer without compromising the increased conductivity gained through the addition of PANI. We observe an increased on-off ratio for the DC conductivity owing to an enlarged off state resistivity and a 20 times larger AC conductivity of the on state compared with DC values. The Fe3O4 nanoparticles, primarily confined to the [Fe(Htrz)2(trz)](BF4) phase, are ferromagnetically coupled to the local moment of the spin-crossover molecule suggesting the existence of an exchange interaction between both components.


Introduction
Composite materials are multi-component systems whose functionality is governed by the interplay between their constituents.The homogeneity of the compound depends on the materials, the materials interactions, and synthesis and profoundly affects properties.Nanoparticles have shown to promote mixing of immiscible liquids by becoming surfactants due to a reduction of interfacial tension [1] and may lead to a reversible transformation between immiscible and miscible polymer solutions.Adding magnetic nanoparticles to molecular ferroelectrics has enabled the synthesis of multiferroic materials [2][3][4].There are few studies of adding magnetic nanoparticles to spin-crossover molecules, though spin-crossover complexes can form nanoparticles themselves [5][6][7].One such spin-crossover complex is [Fe(Htrz) 2 (trz)](BF 4 ) (Htrz = 1H-1,2,4-triazole, trz − = deprotonated triazolato ligand) [7][8][9][10][11][12] that also has been combined with nanoparticles [13,14].[Fe(Htrz) 2 (trz)](BF 4 ) is characterized by a temperature-dependent spin-state switching causing a change in conductivity [9,[15][16][17][18][19].The spin-crossover transition temperature of this particular molecule is typically (340-360) K yielding a bistability of spin states near room temperature [8][9][10][11][12][15][16][17][18][19][20].Through the addition of polyaniline (PANI) [19,21] or polypyrrole [21,22], the on-state resistance of the resulting homogeneous composite can be reduced to <1 Ω•cm so that smaller molecular devices become possible [23] without long delay times resulting from a high impedance.To understand modifications to cooperative effects governing the bistability between spin states [24] in spin-crossover complexes, a variety of techniques have been employed [25][26][27].While metal substitutions for Fe in [Fe(Htrz) 2 (trz)](BF 4 ) decreases conductivity [18], adding metallic nanoparticles like Fe 3 O 4 may avoid this problem altogether through driving a morphology change.Leveraging the full potential of such multi-component systems and modifications due to the addition of nanoparticles requires a profound understanding of the paramagnetic correlation length and the coupling at the interface between superparamagnetic nanoparticles and polymers.Studies of this kind have been extremely rare [13,14] and it is important to begin to address this deficiency in terms of magnetic and electronic transport characterization in conjunction with structural and chemical microanalysis.
Here, we report the effects of adding superparamagnetic nanoparticles to composite films of [Fe(Htrz) 2 (trz)](BF 4 ) spin-crossover molecules and PANI.Scanning transmission electron microscopy with energy-dispersive x-ray spectroscopy and scanning transmission x-ray spectromicroscopy confirm a phase separation of [Fe(Htrz) 2 (trz)](BF 4 ) and PANI that is driven by the Fe 3 O 4 nanoparticles (1 weight-%), which primarily reside in the former phase.Infrared spectroscopy and magnetometry studies corroborate the restoration of molecular structure and cooperative effects after adding the nanoparticles without compromising the increased conductivity gained through the addition of PANI.In fact, DC and AC resistivity measurements divulge an increased on-off ratio for the DC conductivity by enlarging the resistivity of the high-spin state (off state) and a 20 times larger AC conductivity of the on state compared with DC values.Moreover, x-ray magnetic circular dichroism (XMCD) spectroscopy unveils ferromagnetic coupling between the local moment of [Fe(Htrz) 2 (trz)](BF 4 ) and Fe 3 O 4 nanoparticles necessitating a complex exchange coupling between the disordered components.

Synthesis and molecular structure
The [Fe(Htrz) 2 (trz)](BF 4 ) spin-crossover polymer and PANI were synthesized following methods reported by Kroeber et al [20], Tang et al [28], and Adams and Hendrickson [29].For [Fe(Htrz) 2 (trz)](BF 4 ), a 3 M solution of 1,2,4-triazole (Alfa Aesar) in anhydrous ethanol (Aldrich) was added to a stirring solution of Fe(BF 4 ) 2 •6H 2 O (Aldrich) in anhydrous ethanol at a rate of 30 µl min −1 .The resulting pink suspension was stirred for an hour at room temperature and was kept overnight at ambient conditions.Then, the supernatant was removed and the pale pink solid was collected via gravity filtration and washed with anhydrous ethanol several times.For the PANI synthesis, a solution of 0.25 M aniline (Fisher Scientific) in 30 ml of water was placed in an ice bath and cooled to near 0 • C while stirring.10 ml of 0.25 M ammonium persulfate (Millipore Sigma) in 1 M HCl (Fisher Scientific) were added to the aniline solution in one step.The reaction mixture was stirred for six hours in the ice bath to promote polymerization.A dark green precipitate was noted after six hours, but the reaction mixture was left to stir overnight at room temperature.The next day, the dark green powder was collected via centrifugation at 4500 rpm for 30 min several times.The resulting PANI was rinsed with 1 M HCl, ethanol, and water.The [Fe(Htrz) 2 (trz)](BF 4 ) plus PANI composite was obtained after careful grounding with mortar and pestle to smooth powders separately.25 mg of PANI (5000 g mol −1 ) were dispersed in 2 ml of water and sonicated to a uniform dispersion upon which 25 mg of [Fe(Htrz) 2 (trz)](BF 4 ) (348.67 g mol −1 ) were added.The resulting suspension was sonicated and the water was allowed to evaporate overnight.Adding 1 weight-% Fe 3 O 4 nanoparticles to the uniform mixture followed by 30 min sonication at room temperature yields the [Fe(Htrz) 2 (trz)](BF 4 ) plus PANI plus Fe 3 O 4 composite.The non-functionalized iron(II,III) oxide nanopowder (97% trace metals) with a particle diameter (50-100) nm was purchased from Millipore Sigma.All reagents and solvents were used as received.

Magnetization and magnetic interactions
The close proximity of ligands to the Fe metal center and corresponding crystal field effect in [Fe(Htrz) 2 (trz)](BF 4 ) causes orbital hybridization splitting the Fe 3d orbital into t 2g and e g states [34].In the low-spin state, the six electrons pair up in the t 2g orbital (S = 0) leading to diamagnetic behavior [35] at around 300 K [8][9][10][11][12][15][16][17][18][19][20].At 380 K, a spin-crossover transition occurs from the t 2g to the e g orbital (S = 2) that is responsible for paramagnetic properties observed in the magnetic hysteresis curves (figure 2(a)).The latter were obtained for powder samples using superconducting quantum interference device magnetometry with a Quantum Design Magnetic Properties Measurement System.Adding Fe 3 O 4 nanoparticles to [Fe(Htrz) 2 (trz)](BF 4 ) plus PANI yields unsurprisingly a magnetization even in the low-spin state (figure 2(b)), which is characteristic of superparamagnetism [1].Both low-spin and high-spin state reveal a non-vanishing high-field magnetic susceptibility (paramagnetism).The low-spin state paramagnetism suggests an exchange coupling between [Fe(Htrz) 2 (trz)](BF 4 ) and Fe 3 O 4 .The large local moment of Fe in the high-spin state results in a stronger paramagnetic contribution of disordered spin-crossover molecules.Given the weight ratio of 25 : 25 : 0.5 for [Fe(Htrz) 2 (trz)](BF 4 ) plus PANI plus Fe 3 O 4 , we estimate a magnetic susceptibility for the spin-crossover molecules that is 15% of that of pure [Fe(Htrz) 2 (trz)](BF 4 ).The lack of a magnetic hysteresis, i.e. sizable coercive field, suggests a weak, if not absent, magnetostatic interaction between sparsely dense Fe 3 O 4 nanoparticles.In contrast, assemblies of closely packed carboxylated Fe 3 O 4 nanoparticles with separations ≲ 5 nm become ferromagnetic [1].
The role of magnetic intermolecular interactions and cooperative effects is examined in terms of the thermal hysteresis of χ T (the product of magnetic susceptibility and temperature) between low-spin and high-spin state.[Fe(Htrz) 2 (trz)](BF 4 ) possesses a thermal hysteresis with a width of 44 K (figure 3(a)) corroborating cooperative effects due to significant intermolecular interaction.The addition of PANI lowers, in particular, the warming transition temperature (resulting in a smaller hysteresis) and causes an overall less steep transition (figure 3(b)).The corresponding weakening of cooperative effects can be explained by the  increased separation of individual [Fe(Htrz) 2 (trz)](BF 4 ) molecules due to intermixing with PANI.Markedly, the intermolecular coupling is restored through the presence of Fe 3 O 4 nanoparticles resulting in an as wide thermal hysteresis as for pure [Fe(Htrz) 2 (trz)](BF 4 ) that is shifted by 20 K to higher temperatures (figure 3(c)).This modification infers some kind of coupling between Fe 3 O 4 nanoparticles and [Fe(Htrz) 2 (trz)](BF 4 ) mediated by an inherent nanoparticle functionalization and orbital bonding to the spin-crossover molecule and phase separation between the [Fe(Htrz) 2 (trz)](BF 4 ) and PANI, as discussed below.
Even aging, i.e. exposing the samples to ambient conditions for weeks, preserves the thermal hysteresis.For [Fe(Htrz) 2 (trz)](BF 4 ) films, this pertains to a width of 40 K and rectangular shape that coincide with The coupling between [Fe(Htrz) 2 (trz)](BF 4 ) and Fe 3 O 4 was further examined with x-ray absorption and XMCD spectroscopies at beamline 6.3.1 (Advanced Light Source, Berkeley, CA).Using total electron yield and circularly polarized x-rays with an estimated degree of polarization of 0.66, the x-ray absorption spectra of films, obtained by drop-casting a solution of [Fe(Htrz) 2 (trz)](BF 4 ) plus PANI plus Fe 3 O 4 on an highly oriented pyrolytic graphite (HOPG) substrate, were recorded around the Fe L 3,2 edges.The absorption spectra of composites in the low-spin state are unaffected by the presence of a magnetic field and look similar to prior reports on Fe 3 O 4 [36,37] where the 708.1 eV peak is roughly 55% in height of the 709.5 eV peak, indicating primarily trivalent Fe (figures 4(a) and (b)).The pre-edge shoulders unique to Fe 3 O 4 and FeO are also observed.The enhanced x-ray absorption at 708.1 eV is due to an increased high-spin state population near the spin-crossover transition temperature and not unexpected as the applied magnetic field can lower the activation barriers to the high-spin state [38][39][40][41].As evident from the thermal hysteresis (figure 3(c)), a temperature of 380 K is insufficient to completely populate the high-spin state at remanence.This conclusion is supported by a marginal change of the XMCD signal with magnetic field in the high-spin state.The absorption spectra in the low-spin state are identical for magnetic fields in the range of (-1.5~1.5)T/µ 0 .Comparison with literature [42][43][44] reveals a mixture of divalent and trivalent Fe in Fe 3 O 4 since neither the first (707.8eV) nor the second (710.0 eV) dip in the XMCD signal dominates (figure 4(c)).The increase in magnitude at 707.8 eV at elevated temperature originates from the local Fe moment of [Fe(Htrz) 2 (trz)](BF 4 ) and corroborates ferromagnetic coupling between [Fe(Htrz) 2 (trz)](BF 4 ) and Fe 3 O 4 .A similar ferromagnetic coupling was reported for spin-crossover Fe octaethylporphyrin complexes on oxidized nickel and cobalt surfaces [45].
The phase separation directly affects the morphology and homogeneity of the drop-casted films.Scanning electron microscopy with an in-lens detector on [Fe(Htrz) 2 (trz)](BF 4 ) plus PANI plus Fe 3 O 4 (1 weight-%) revealed a continuous inhomogeneous film with significant clustering and percolation (supplementary figure 3).These imperfections become more prominent with increasing nanoparticle concentration and enhance contributions from extrinsic properties that are undesirable for our study.

Electronic transport properties
The DC current-voltage I(V) characteristics of [Fe(Htrz) 2 (trz)](BF 4 ) plus PANI are compared with the [Fe(Htrz) 2 (trz)](BF 4 ) plus PANI plus Fe 3 O 4 nanoparticles in figures 6(a) and (b).The temperature-dependent DC I(V) measurements were performed on dried solutions of [Fe(Htrz) 2 (trz)](BF 4 ) plus PANI plus Fe 3 O 4 drop-casted on an organic field-effect transistor template manufactured by the Fraunhofer-Institut für Photonische Mikrosysteme (IPMS).The I(V) curves were recorded with a 4200 A SCS parameter analyzer connected to a Lakeshore cryogenic probe station.Both films possess a higher conductivity than pure [Fe(Htrz) 2 (trz)](BF 4 ) [19,21], and the lower conductivity in the high-spin state is consistent with prior reports [9,16,18,19].The nonlinearity of the conductivity in the low-spin state indicates additional free charge carriers that are added by the Fe 3 O 4 nanoparticles to the conduction channel at high bias voltages.In addition, the resistivity of the high-spin state is markedly enhanced (factor of 2).The combination of all these characteristics indicates a conduction through both PANI and [Fe(Htrz) 2 (trz)](BF 4 ) molecules with magnetite nanoparticles despite the phase separation and inhomogeneity of the films.
This conclusion is corroborated by the AC electronic transport properties.Samples for AC measurements were fabricated by drop-casting a solution of [Fe(Htrz) 2 (trz)](BF 4 ) plus PANI plus Fe 3 O 4 in an interdigitated electrode (Metrohm Dropsens) and stored for more than four weeks in an ultra-high vacuum chamber to degas.The AC I(V) curves were obtained in 2-point in-plane geometry using a Quantum Design DynaCool Physical Properties Measurement System at an excitation frequency of 97.6 Hz.The AC conductivity of [Fe(Htrz) 2 (trz)](BF 4 ) plus PANI plus Fe 3 O 4 exceeds that of pure [Fe(Htrz) 2 (trz)](BF 4 ) [18] by several orders of magnitude (figure 6(c)).In addition, the conductivity in the low-spin state is higher than in the high-spin state without a noticeable dependence on the magnetic bias field (up to 4 T/µ 0 ) and Fe 3 O 4 nanoparticles.A conduction through PANI alone would yield larger, virtually temperature-independent conductivity values.Compared with DC measurements, the AC conductivity in the low-spin state is 20 times larger at the expense of a reduced on-off ratio.This illustrates that the short-range conductivity is larger than the long-range conductivity.

Conclusion
We synthesized composite films of [Fe(Htrz) 2 (trz)](BF 4 ) plus PANI plus Fe 3 O 4 nanoparticles and compared molecular structure, morphology, magnetic properties, and electronic transport to [Fe(Htrz) 2 (trz)](BF 4 ) films with and without PANI.The addition of Fe 3 O 4 nanoparticles causes a phase separation of [Fe(Htrz) 2 (trz)](BF 4 ) and PANI that restores cooperative effects between the spin-crossover molecules.The observation of ferromagnetic exchange coupling between [Fe(Htrz) 2 (trz)](BF 4 ) and Fe 3 O 4 nanoparticles by means of XMCD spectroscopy infers an inherent functionalization of the originally non-functionalized nanoparticles and chemical bonding to the molecules.The use of low-concentration nanoparticles (1 weight-%) increases the on-off ratio of DC conductivity by enlarging the resistivity of the high-spin state (off state) and leads to an AC conductivity of the on state that is 20 times larger than the DC on state.Given the stark morphological differences between [Fe(Htrz) 2 (trz)](BF 4 ) plus PANI with and without Fe 3 O 4 nanoparticles, the preservation of high AC and DC conductivity of the low-spin state is striking.Manipulating cooperative effects through the presence of superparamagnetic nanoparticles may open a new avenue for tailoring electronic transport properties of spin-crossover systems and prospective applications of composite materials in microelectronics, particularly where high resistivity has been a major concern.

Figure 4 .
Figure 4. X-ray absorption (XAS) and x-ray magnetic circular dichroism (XMCD) spectroscopy of [Fe(Htrz)2(trz)](BF4) plus polyaniline plus Fe3O4 films.(a) Absorption spectra in the low-spin (320 K) and high-spin (380 K) state at different normal magnetic bias fields revealing field dependence of high-spin state due to its incomplete population.(b) Absorption spectra taken with circularly polarized light at positive and negative magnetic bias unveiling marginal difference.(c) Corresponding XMCD spectra characteristic of trivalent Fe in Fe3O4 with sizable contribution of high-spin state corroborating ferromagnetic coupling between the two components.

Figure 6 .
Figure 6.Electronic transport properties.DC I(V) curves of (a) [Fe(Htrz)2(trz)](BF4) plus polyaniline and (b) [Fe(Htrz)2(trz)](BF4) plus polyaniline plus Fe3O4 measured in the low-spin (320 K) and high-spin (380 K) state corroborating enhanced on-off ratio of the latter.(c) AC conductivity of [Fe(Htrz)2(trz)](BF4) plus polyaniline plus Fe3O4 in the low-spin (300 K) and high-spin (380 K) state measured at remanence and in the presence of a normal magnetic field.The curves measured with and without a magnetic field are virtually the same and partially overlap.