Control of Molecular Orientation and Carrier Transport of Thiophene-Based Semiconducting Polymer via Superparamagnetic Nanoparticles Fe3O4@C-Assisted Magnetic Alignment Method

Realizing the high molecular orientation and structurally ordered microstructure of organic semiconductor polymer thin films is beneficial for enhancing the charge transport of conjugated polymers and achieving high-performance organic electronic devices. In this work, we successfully developed large-area highly aligned films of a thiophene-based polymer, namely poly(2,5-bis(3alkylthiophen-2-yl) thieno [3,2-b] thiophene) (PBTTT), using the magnetic alignment method at a low magnetic field (0.12 T), which was assisted by superparamagnetic nanoparticles Fe3O4@C. The aligned microstructure of the composite films is confirmed by systematic analysis that includes polarized optical microscopy, polarized UV–visible absorption spectroscopy, and an atomic force microscope. Organic field effect transistors based on magnetic aligned composite film exhibit a 2.8-fold improvement in carrier mobility compared with the unaligned films. We hold a formation mechanism that the rapid magnetically induced self-assembly property of Fe3O4@C and its intermolecular interaction with polymer chains are key to the new method of preparing oriented


Introduction
Conjugated polymers are of great interest for a variety of applications, including organic field effect transistors [1,2], organic light-emitting diodes [3,4], and organic photovoltaic cells [5,6], owing to their low temperature, solution processability, and mechanical flexibility, which may enable the fabrication of low-cost and large-area flexible electronic devices. However, most semiconductor polymer-based devices suffer from a low performance, which leads to their limitation in a wide range of applications.
It is already well-established that the charge mobilities are highest along the backbone chain and the π-π stacking directions, but low along the alkyl side chain direction [1,[7][8][9]. Therefore, a key method for improving the charge transport of semiconductor polymers is to align the main chains of the semiconducting polymers [8][9][10]. Aligned films not only enhance the mobilities along the preferred backbone direction, but also provide a unique opportunity to study the interaction between polymer film microstructure and charge transport performance.
So far, several facile and efficient methods to realize oriented polymer thin films have been reported, such as off-center spin coating [11,12], bar coating [13,14], shear coating [15], Chinese brush coating [8], dip coating [1,16], and electric field alignment [17,18]. In addition to these techniques, the magnetic alignment method acts as a tool for tuning molecular alignment for the fabrication of a large-area aligned film in a clean (non-contact) manner, which is more suitable for device applications.
In general, the anisotropy of the magnetic susceptibility of molecules causes anisotropy of the molecular magnetic energy, which leads to magnetic alignment [19][20][21][22]. Nevertheless, mostly conjugated polymers are diamagnetic materials, and their magnitude is typically on the order of −10 −8 to −10 −6 [22][23][24]. Thus, a high magnetic field (>6 T) is usually required to achieve magnetic alignment of polymer molecules [24][25][26][27]. Therefore, a high magnetic field of 45 T was used to align carbon nanotubes [25]. Our previous work demonstrated that the semiconducting polymers of P(NDI2OD-T2) and PBTTT can be aligned to a high degree under an 8 T high magnetic field [26,27]. Due to the limitation of high-magneticfield conditions, the practical application of organic semiconductor polymer growth and structure regulation under a magnetic field has been largely limited. Thus, developing a facile approach that enables the alignment of polymer chains on a large area by using a smaller magnetic field is urgently needed for its practical application.
Smaller magnetic fields have been used to align nonmagnetic microparticles, but only with the assistance of magnetic or superparamagnetic nanoparticles [28][29][30]. This method was previously used to align carbon nanotubes and graphene flakes with a low magnetic field in the range 0.1 to 0.8 T [29][30][31][32]. This is mainly due to van der Waals forces, as the Fe 3 O 4 nanoparticles can easily attach to the surface of carbon nanotubes and graphene flakes to make them more responsive to a magnetic field. However, polymer molecules have difficulty in forming strong interactions with Fe 3 O 4 nanoparticles. Thus, examples of a low-magnetic-field manipulation of the film structure of polymer/Fe 3 O 4 nanohybrids are scarce.
In this work, we report a simple, novel and upscalable method to form a well-oriented poly(2,5-bis(3-alkylthiophen-2-yl) thieno [3,2-b] thiophene) (PBTTT) thin film by applying a low magnetic field (0.12 T) assisted by carbon-encapsulated Fe 3 O 4 superparamagnetic colloidal nanoparticles (Fe 3 O 4 @C). This method mainly relies on the strong response of the superparamagnetic nanoparticles to the magnetic field and the characteristics of rapid magnetically induced self-assembly, as well as a strong interaction with polymer molecules, to improve the orientation and crystallinity of low-magnetic-field-induced polymer films.
More importantly, structure and orientation were characterized by polarized light absorption spectroscopy. We found that the presence of a small amount of Fe 3 O 4 @C nanoparticles in the weak magnetic field greatly improved the degree of chain alignment, and the chain backbones of PBTTT within the composite films are highly aligned along the direction of the applied magnetic field. Furthermore, the degree of chain alignment of the composite films is higher than that of pure polymer films aligned by a high magnetic field (8 T) [26]. We also found that such structural tuning plays a crucial role in the carrier transport of the organic field-effect transistor (OFET) devices.

Materials
PBTTT was purchased from 1-Material inc. and used as received. Ferrocene (≥98%) and o-dichlorobenzene (≥99%) was purchased from Alfa Aesar. The hydrogen peroxide (≥30%) and acetone (≥99%) were of analytic grade from the Shanghai Chemical Factory, China. All chemicals were used as received without further purification.

Synthesis of Monodispersed Fe 3 O 4 @C Colloidal Nanoparticles
The synthesis followed a typical procedure, as in previous work [33,34]. Ferrocene (0.3 g) was first dissolved in acetone (30 mL) under continuous mechanical stirring for 30 min, and 1.5 mL of hydrogen peroxide was slowly added to this mixed solution, which was then mechanically stirred for 1 h. The mixture was transferred into a Teflon-lined stainless-steel autoclave with a 50 mL capacity and heated at 240 • C for 72 h. Then, the black product was washed with acetone several times to remove excess ferrocene and dried under vacuum at 50 • C for 5 h for further use.

Preparation of Fe 3 O 4 @C/PBTTT Composite Solution
At first, Fe 3 O 4 @C dispersions were prepared by the Fe 3 O 4 @C powder added into odichlorobenzene (o-DCB) with a concentration of 2 mg/mL. The mixture was then sonicated at 540 W for 30 min. After settling for 1 h, homogeneous dark Fe 3 O 4 @C dispersion was generated from the top portion of the solution. The PBTTT was dissolved in o-DCB to form a solution of 10 mg/mL and blended with Fe 3 O 4 @C dispersion at volume ratio of 10:1. The solution was then sonicated at 50 W for 10 h to yield the composite solution.

Magnetic Alignment
Alignment experiments were performed using a magnet with a 0.12 T magnetic field. The magnet was designed and manufactured by Hefei Hongzhong Science & Technology Company (Hefei, China) and is shown in Figure S1. The aligned films were prepared by casting the Fe 3 O 4 @C/PBTTT composite solution on the Si/SiO 2 substrates under the magnetic field at room temperature (RT). The magnetic field was applied until the solvent was evaporated. The films from the magnetic alignment process were then thermally annealed at 180 • C for 20 min in a N 2 atmosphere and then allowed to cool down to RT slowly in order to remove the remaining solvent and improve the structural order of the films.

Sample Characterization
The X-ray diffraction (XRD) patterns were collected on a Rigaku MiniFlex powder X-ray diffractometer using Cu-Kα radiation (λ = 1.54 Å) over a 2θ range of 20 • -80 • . Static magnetic properties were measured with a superconducting quantum interference device (MPMS3, Quantum Design company, San Diego, CA, USA). The Fourier transform infrared (FT-IR) spectra of samples were taken on a VERTEX-80V spectrometer (Bruker Corporation, Billerica, American) in the range of 500-4000 cm −1 . The Raman spectrum was taken on a Horiba Jobin YvonT64000 Micro-Raman instrument (HORIBA, Ltd., Kyoto, Japan) with the 532 nm line at room temperature. The morphology and structure of the products were analyzed by employing scanning electron microscopy (SEM, Helios NanoLab 600i, FEI Company, Hillsboro, OR, USA). Optical texture and the morphology of the prepared films were probed by a polarized optical microscope as well as a HITACHI-5500M atomic force microscope (AFM, Tokyo, Japan) in the tapping mode, respectively. The film optical anisotropy was examined by a UV-vis spectrophotometer (Shimadzu, UV-3100, Kyoto, Japan) in transmission geometry on the quartz substrates.

OFET Fabrication and Characterization
The bottom-gate/bottom-contact (BG/BC) OFET devices were fabricated on heavily doped Si wafers with a 230 nm thermally grown SiO 2 layer. The surfaces of the Si/SiO 2 substrates were photolithographically pre-patterned with inter-digitated electrode arrays of Cr/Au (3 nm/50 nm) as source/drain electrodes. The electrode arrays are characteristic of the channel width (W) of 1.7 mm and channel lengths (L) of 200 µm. The aligned Fe 3 O 4 @C/PBTTT composite films were prepared as previously described. The films were then thermally annealed at 180 • C for 60 min in a N 2 glove-box. The BC/BG OFETs were therefore constructed via the above procedure. The devices were characterized in a N 2 atmosphere using a Keithley 2612A parameter analyzer on a probe station. The field-effect mobility (µ FET ) was calculated in the saturated regime according to the transistor equation: where Ci is the area capacitance of dielectric (15 nF/cm 2 for SiO 2 ,), V G is the gate voltage, and VT is the threshold voltage.

Microstructure of Fe 3 O 4 @C Nanoparticles
The molecular structures of the PBTTT polymer used in this study are shown in Figure 1a. Figure 1b,c show the SEM images of Fe 3 O 4 @C colloidal nanoparticles sample, where we can observe that the sample is composed of many nearly monodisperse spherical particles with a diameter of approximately 100 nm. The XRD pattern of the as-prepared samples is shown in Figure S2. All the diffraction peaks are in accordance with the standard profile of cubic magnetite Fe 3 O 4 (JCPDS file No. 19-0629).
pared samples is shown in Figure S2. All the diffraction peaks are in accordance with the standard profile of cubic magnetite Fe3O4 (JCPDS file No. . No additional peaks are observed, which suggests the high purity of the Fe3O4 phase [34,35]. The Raman profile spectrum of Fe3O4@C nanoparticles ( Figure 1d) exhibits two broad peaks (D-peak at 1310 cm −1 and G-peak at 1600 cm −1 ), which indicate the dominant existence of a carbon phase in the samples [36]. We also performed an FT-IR spectrum analysis of the samples to further confirm the existence of carbon ( Figure 1e). The two broad peaks around 1700 cm −1 and 3410 cm −1 were apparently caused by the stretching vibration of carboxyl that rose from the carboxyl groups on the surface of the nanoparticles. Thus, the nanoparticles exhibit a negative zeta potential due to the carboxyl with a negatively charged group [34,36,37].
It is worth noting that there is no peak corresponding to the carbon shell in the XRD pattern ( Figure S2), which may be the carbon shell prepared by the hydrothermal method in an amorphous state. On the basis of the abovementioned measurement result, the prepared nanoparticles contain a magnetite/carbon core/shell structure with the carboxyl groups on the surface (Figure 1f), and the diameter is approximately 100 nm.

Microstructure of Magnetically Aligned Films
The magnetically aligned Fe3O4@C/PBTTT composite films are prepared on the Si/SiO2 substrates by solution casting from the composite solution under a permanent magnet with a magnetic field of 0.12 T at room temperature. The schematic of the fabrication procedure for the Fe3O4@C/PBTTT composite thin film is also shown in Figure 2a. To visualize the anisotropy of the Fe3O4@C/PBTTT composite film, the film surface is imaged by using polarized optical microscopy (POM). As shown in Figure 2b, the POM images of No additional peaks are observed, which suggests the high purity of the Fe 3 O 4 phase [34,35]. The Raman profile spectrum of Fe 3 O 4 @C nanoparticles (Figure 1d) exhibits two broad peaks (D-peak at 1310 cm −1 and G-peak at 1600 cm −1 ), which indicate the dominant existence of a carbon phase in the samples [36]. We also performed an FT-IR spectrum analysis of the samples to further confirm the existence of carbon (Figure 1e). The two broad peaks around 1700 cm −1 and 3410 cm −1 were apparently caused by the stretching vibration of carboxyl that rose from the carboxyl groups on the surface of the nanoparticles. Thus, the nanoparticles exhibit a negative zeta potential due to the carboxyl with a negatively charged group [34,36,37].
It is worth noting that there is no peak corresponding to the carbon shell in the XRD pattern ( Figure S2), which may be the carbon shell prepared by the hydrothermal method in an amorphous state. On the basis of the abovementioned measurement result, the prepared nanoparticles contain a magnetite/carbon core/shell structure with the carboxyl groups on the surface (Figure 1f), and the diameter is approximately 100 nm.

Microstructure of Magnetically Aligned Films
The magnetically aligned Fe 3 O 4 @C/PBTTT composite films are prepared on the Si/SiO 2 substrates by solution casting from the composite solution under a permanent magnet with a magnetic field of 0.12 T at room temperature. The schematic of the fabrication procedure for the Fe 3 O 4 @C/PBTTT composite thin film is also shown in Figure 2a. To visualize the anisotropy of the Fe 3 O 4 @C/PBTTT composite film, the film surface is imaged by using polarized optical microscopy (POM). As shown in Figure 2b, the POM images of the magnetically aligned Fe 3 O 4 @C/PBTTT composite film exhibit a homogeneous change in color and brightness, which indicates a uniaxial alignment of the composite film in a large-area. the magnetically aligned Fe3O4@C/PBTTT composite film exhibit a homogeneous ch in color and brightness, which indicates a uniaxial alignment of the composite film large-area. The brighter image was observed at the angle between the magnetic field direc and the polarization axis is 45°, which clearly indicates that the direction of the PB molecules backbone is oriented either parallel or perpendicular to the magnetic fiel rection. However, the POM images of the pure PBTTT film were magnetically ali without Fe3O4@C nanoparticles, which displayed no difference regardless of the rot of the film ( Figure S3). This illustrates the isotropic texture structure of the pure PB film with random distributed molecule chains. Interestingly, we should note that th perparamagnetic Fe3O4@C nanoparticles form micron-scale chain-like structures an highly oriented and uniformly dispersed in the composite film. This phenomenon w discussed in detail later.
In order to deduce the molecular chain direction of PBTTT and the degree of a ment of the composite film, polarized UV-vis absorption spectroscopy was perfor This is mainly because the transition dipole moment of polymers is typically aligned a the backbones; the orientation of the polymer backbone can be estimated by this mea ment. Furthermore, the degree of alignment of the films can be quantified by the dic ratio of the maximum absorbance, R = I∥/I⊥, where I∥ and I⊥ denote the peak absorb in which the applied magnetic field direction is parallel and perpendicular to the dire The brighter image was observed at the angle between the magnetic field direction, and the polarization axis is 45 • , which clearly indicates that the direction of the PBTTT molecules backbone is oriented either parallel or perpendicular to the magnetic field direction. However, the POM images of the pure PBTTT film were magnetically aligned without Fe 3 O 4 @C nanoparticles, which displayed no difference regardless of the rotation of the film ( Figure S3). This illustrates the isotropic texture structure of the pure PBTTT film with random distributed molecule chains. Interestingly, we should note that the superparamagnetic Fe 3 O 4 @C nanoparticles form micron-scale chain-like structures and are highly oriented and uniformly dispersed in the composite film. This phenomenon will be discussed in detail later.
In order to deduce the molecular chain direction of PBTTT and the degree of alignment of the composite film, polarized UV-vis absorption spectroscopy was performed. This is mainly because the transition dipole moment of polymers is typically aligned along the backbones; the orientation of the polymer backbone can be estimated by this measurement. Furthermore, the degree of alignment of the films can be quantified by the dichroic ratio of the maximum absorbance, R = I /I⊥, where I and I⊥ denote the peak absorbance in which the applied magnetic field direction is parallel and perpendicular to the direction of incident light, respectively [1,38]. As shown in Figure 2c, the magnetically aligned Fe 3 O 4 @C/PBTTT composite film shows a stronger absorption at 535 nm when the direction of the applied magnetic field is parallel to the polarization direction of the incident light. This confirms that the chain backbones of PBTTT should be preferentially aligned with the magnetic field direction [38,39].
The composite film has a dichroic ratio of 2.1 at 535 nm, which is higher than that of the aligned pure PBTTT films fabricated by solution casting under 8 T (dichroic ratio = 1.6) as previously reported by us [26]. This suggests that the Fe 3 O 4 @C-nanoparticles-assisted low-magnetic-field alignment method is able to effectively align polymer backbones and fabricate a higher anisotropic polymer film than the film grown without the Fe 3 O 4 @Cnanoparticles-assisted high-magnetic-field alignment method. As a comparison, the pure PBTTT film, which is magnetically aligned without Fe 3 O 4 @C nanoparticles, exhibits a dichroic ratio of 1.06, the isotropic (unaligned) character shown in Figure 2d. This result is consistent with the POM characterization.
We further characterized the surface morphology changes in the films by the tappingmode atomic force microscopy (AFM). As shown in Figure 3, the height images of the magnetically aligned Fe 3 O 4 @C/PBTTT composite film exhibit the formation of fiber structures with a good alignment in the direction of the arrow that corresponds to the magnetic field direction. In contrast, the Fe 3 O 4 @C/PBTTT composite film without magnetic alignment exhibit a disordered surface morphology with small grains.
result is consistent with the POM characterization.
We further characterized the surface morphology changes in the films by the ping-mode atomic force microscopy (AFM). As shown in Figure 3, the height imag the magnetically aligned Fe3O4@C/PBTTT composite film exhibit the formation of structures with a good alignment in the direction of the arrow that corresponds t magnetic field direction. In contrast, the Fe3O4@C/PBTTT composite film without netic alignment exhibit a disordered surface morphology with small grains.
Taken together, the AFM images clearly indicate that the Fe3O4@C-nanoparticle sisted magnetic alignment method allows for a highly oriented polymer film under magnetic field. Compared with the disordered distribution of the Fe3O4@C nanopar without magnetically aligned Fe3O4@C/PBTTT composite films, the Fe3O4@C nano cles formed chain-like structures, which are fixed inside the aligned composite film a regular interparticle spacing under the induction of the applied low magnetic field

Charge Transport Studies
The effect of the polymer molecule chain alignment on the charge transport pr ties is investigated by fabricating bottom-gate/bottom-contact (BG/BC) OFETs (Figur We define two main orientations for the OFETs: (1) parallel, where the transistor ch is parallel to the magnetic field direction(µ∥), and (2) perpendicular, where the tran channel is perpendicular to the magnetic field direction (µ⊥). As shown in Figure 4b, Figure S4 in the Supporting Information, in the aligned Fe3O4@C/PBTTT composite based OFET, the hole mobilities are measured to be 2.7 × 10 −2 cm 2 V −1 s −1 and 8.3 × 10 − V −1 s −1 in the parallel and perpendicular directions, respectively, which leads to a ra about 3.2. The hole mobility in the parallel device is enhanced by a factor of 2.8 wi spect to isotropic devices of unaligned pure PBTTT film (9.5 × 10 −3 cm 2 V −1 s −1 ). Taken together, the AFM images clearly indicate that the Fe 3 O 4 @C-nanoparticlesassisted magnetic alignment method allows for a highly oriented polymer film under a low magnetic field. Compared with the disordered distribution of the Fe 3 O 4 @C nanoparticles without magnetically aligned Fe 3 O 4 @C/PBTTT composite films, the Fe 3 O 4 @C nanoparticles formed chain-like structures, which are fixed inside the aligned composite film with a regular interparticle spacing under the induction of the applied low magnetic field.

Charge Transport Studies
The effect of the polymer molecule chain alignment on the charge transport properties is investigated by fabricating bottom-gate/bottom-contact (BG/BC) OFETs (Figure 4a). We define two main orientations for the OFETs: (1) parallel, where the transistor channel is parallel to the magnetic field direction(µ ), and (2) perpendicular, where the transistor channel is perpendicular to the magnetic field direction (µ⊥). As shown in Figure 4b,c and Figure S4 in the Supporting Information, in the aligned Fe 3 O 4 @C/PBTTT composite-film-based OFET, the hole mobilities are measured to be 2.7 × 10 −2 cm 2 V −1 s −1 and 8.3 × 10 −3 cm 2 V −1 s −1 in the parallel and perpendicular directions, respectively, which leads to a ratio of about 3.2. The hole mobility in the parallel device is enhanced by a factor of 2.8 with respect to isotropic devices of unaligned pure PBTTT film (9.5 × 10 −3 cm 2 V −1 s −1 ). R PEER REVIEW 7 of 11 aligned structural features, which favor charge transport along the preferential backbone chain direction and greatly reduce the number of slow and energy-demanding interchain hopping events from the source to the drain electrodes [15,40,41].

The Mechanism for Magnetic Alignment
To thoroughly explain the abovementioned phenomenon, we conduct further research on the alignment mechanism of the Fe3O4@C-nanoparticles-assisted magnetic alignment method. First, the magnetic measurements are carried out on both Fe3O4@C and PBTTT materials. Figure 5a shows the magnetization curves of PBTTT, and the diamagnetic susceptibilities of 6.5 × 10 −8 emu g −1 Oe −1 are obtained. Such a weak susceptibility indicates that PBTTT cannot be magnetically oriented by the low magnetic field of 0.12 T, which is confirmed by the above structural characterization. In contrast, a significant hysteresis loop in the Fe3O4@C M-H curve reveals the superparamagnetic behavior of the Fe3O4@C (Figure 5b). The saturation magnetization (Ms), remanent magnetization (Mr) and coercivity (Hc) values of the Fe3O4@C at room temperature are 52 emu g −1 Oe −1 , 1.4 emu g −1 Oe −1 and 25 Oe, respectively, which is in accordance with the previously report literature [34].
According to previous studies, because of the magnetic dipole attraction and electrostatic repulsion between particles, one dimensional (1D) chain-like structures based on these superparamagnetic colloidal nanoparticles are formed due to the induction of an external magnetic field [28,34,42,43], which can be distinctly observed in POM and AFM images. The FT-IR spectrum shows the presence of carboxyl groups on the surface of the samples. Previous research has proved that the sulfur atom of the thiophene rings and the hydrogen of the carboxyl group can form an intermolecular hydrogen bond (Figure 5c). The experimental study and density functional theory calculations show that the bond These results support the conclusion that the composite film with an induced alignment provided improved charge transport. Moreover, there is little difference in hole mobility between the unaligned Fe 3 O 4 @C/PBTTT composite film (9.3 × 10 −3 cm 2 V −1 s −1 ) and pure PBTTT film-based devices, which suggests that a small amount of Fe 3 O 4 @C nanoparticles in the composite film do not obviously affect the carrier transport. Thus, it is clear that a higher hole mobility in the aligned composite films clearly originates from the high orientation of the polymer chains. The anisotropy and significant enhancement of hole mobility for the aligned composite-film-based OFETs are mainly due to their highly aligned structural features, which favor charge transport along the preferential backbone chain direction and greatly reduce the number of slow and energy-demanding interchain hopping events from the source to the drain electrodes [15,40,41].

The Mechanism for Magnetic Alignment
To thoroughly explain the abovementioned phenomenon, we conduct further research on the alignment mechanism of the Fe 3 O 4 @C-nanoparticles-assisted magnetic alignment method. First, the magnetic measurements are carried out on both Fe 3 O 4 @C and PBTTT materials. Figure 5a shows the magnetization curves of PBTTT, and the diamagnetic susceptibilities of 6.5 × 10 −8 emu g −1 Oe −1 are obtained. Such a weak susceptibility indicates that PBTTT cannot be magnetically oriented by the low magnetic field of 0.12 T, which is confirmed by the above structural characterization. In contrast, a significant hysteresis loop in the Fe 3 O 4 @C M-H curve reveals the superparamagnetic behavior of the Fe 3 O 4 @C (Figure 5b). The saturation magnetization (Ms), remanent magnetization (Mr) and coercivity (Hc) values of the Fe 3 O4@C at room temperature are 52 emu g −1 Oe −1 , 1.4 emu g −1 Oe −1 and 25 Oe, respectively, which is in accordance with the previously report literature [34]. magnetically induced self-assembly characteristic of Fe3O4@C and its synergetic interaction with the polymer chains are key to new methods for preparing oriented thin films.

Conclusions
In summary, we successfully demonstrated a low-cost, upscalable, and simple magnetic alignment method by applying a low magnetic field to produce aligned semiconductor polymers film with assisted by Fe3O4@C nanoparticles. The UV-vis and AFM data show that the chain backbones of PBTTT are preferentially aligned with the magnetic field direction. The aligned microstructure of the composite films leads to enhanced charge carrier mobility along the magnetic field direction. We can expect great potential to further improve mobility by optimizing the polymer chain alignment technique. Moreover, the degree of chain alignment of the composite films is higher than that of the pure polymer films aligned by a high magnetic field. A mechanism for nanoparticles-assisted magnetic alignment is proposed, in which the aligned Fe3O4@C chain-like structures can effectively induce the orientation of polymer chains through intermolecular hydrogen bonds. We believe that the strategy and methodology proposed here can also potentially be applied to other types of polymers to increase the overall performance of future sophisticated organic electronics.
Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1. Figure S1: The photograph of the magnet (0.12 T); Figure S2: The XRD pattern of the Fe3O4@C nanoparticles; Figure S3. The POM images of the magnetically aligned pure PBTTT film; The red arrows denote the magnetic field direction; Figure S4: Output curves of the OFET devices of the magnetically aligned Fe3O4@C/PBTTT composite film with channel current perpendicular to magnetic field direction (a), unaligned spin-coated pure PBTTT film (b) and spin-coated Fe3O4@C/PBTTT composite film (c), respectively.   According to previous studies, because of the magnetic dipole attraction and electrostatic repulsion between particles, one dimensional (1D) chain-like structures based on these superparamagnetic colloidal nanoparticles are formed due to the induction of an external magnetic field [28,34,42,43], which can be distinctly observed in POM and AFM images. The FT-IR spectrum shows the presence of carboxyl groups on the surface of the samples. Previous research has proved that the sulfur atom of the thiophene rings and the hydrogen of the carboxyl group can form an intermolecular hydrogen bond (Figure 5c). The experimental study and density functional theory calculations show that the bond energy of the COOH-S hydrogen bond is as high as 25.92 kJ/mol (0.27 eV) [44][45][46]. Based on these studies, we infer that the kinetic process of the Fe 3 O 4 @C-nanoparticles-assisted low-magnetic-field alignment can be partitioned into two stages (Figure 5d).
In the first stage, the Fe 3 O 4 nanospheres in the mixed solution are quickly selfassembled into 1D chain-like structures induced by the external magnetic field and highly oriented in the direction of the magnetic field. In the second stage, the randomly dispersed PBTTT molecules in the composite solution are held in the surface of Fe 3 O 4 @C chains by intermolecular COOH-S hydrogen bond, i.e., the backbone chains of PBTTT molecules are highly oriented by the Fe 3 O 4 @C chains. As the solvent evaporates, molecular deposition eventually leads to the formation of macroscopically aligned films. In summary, the rapid magnetically induced self-assembly characteristic of Fe 3 O 4 @C and its synergetic interaction with the polymer chains are key to new methods for preparing oriented thin films.

Conclusions
In summary, we successfully demonstrated a low-cost, upscalable, and simple magnetic alignment method by applying a low magnetic field to produce aligned semiconductor polymers film with assisted by Fe 3 O 4 @C nanoparticles. The UV-vis and AFM data show that the chain backbones of PBTTT are preferentially aligned with the magnetic field direction. The aligned microstructure of the composite films leads to enhanced charge carrier mobility along the magnetic field direction. We can expect great potential to further improve mobility by optimizing the polymer chain alignment technique. Moreover, the degree of chain alignment of the composite films is higher than that of the pure polymer films aligned by a high magnetic field. A mechanism for nanoparticles-assisted magnetic alignment is proposed, in which the aligned Fe 3 O 4 @C chain-like structures can effectively induce the orientation of polymer chains through intermolecular hydrogen bonds. We believe that the strategy and methodology proposed here can also potentially be applied to other types of polymers to increase the overall performance of future sophisticated organic electronics.
Supplementary Materials: The following are available online at https://www.mdpi.com/article/ 10.3390/magnetochemistry8060064/s1. Figure S1: The photograph of the magnet (0.12 T); Figure  S2: The XRD pattern of the Fe 3 O 4 @C nanoparticles; Figure S3. The POM images of the magnetically aligned pure PBTTT film; The red arrows denote the magnetic field direction; Figure S4: Output curves of the OFET devices of the magnetically aligned Fe 3 O 4 @C/PBTTT composite film with channel current perpendicular to magnetic field direction (a), unaligned spin-coated pure PBTTT film (b) and spin-coated Fe 3 O 4 @C/PBTTT composite film (c), respectively.