Magnetic-Field-Induced Assembly of Superparamagnetic Cobalt

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Interface-Rich Materials and Assemblies

Magnetic-Field-Induced Assembly of Superparamagnetic Cobalt Nanoparticles on Substrates and at Liquid-Air Interface Li Tan, Bing Liu, Ulrich Glebe, and Alexander Böker Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b02673 • Publication Date (Web): 03 Nov 2018 Downloaded from http://pubs.acs.org on November 4, 2018

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Magnetic-Field-Induced Assembly of Superparamagnetic Cobalt Nanoparticles on Substrates and at Liquid-Air Interface Li Tan,a, b Bing Liu,c Ulrich Glebe,a,* Alexander Böker a, b,* a Fraunhofer

Institute for Applied Polymer Research IAP, Geiselbergstr. 69, 14476 Potsdam-Golm, Germany

b Lehrstuhl

für Polymermaterialien und Polymertechnologie, Universität Potsdam, 14476 Potsdam-Golm, Germany

c Institute

of Chemistry Chinese Academy of Sciences, 100864, Beijing, China

ACS Paragon Plus Environment

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ABSTRACT: Superparamagnetic cobalt nanoparticles (Co NPs) are an interesting material for self-assembly processes because of their magnetic properties. We investigated the magneticfield-induced assembly of superparamagnetic cobalt nanoparticles and compared three different approaches, namely the assembly on solid substrates, at water-air, and ethylene glycol-air interfaces. Oleic acid- and trioctylphosphine oxide-coated Co NPs were synthesized via a thermolysis of cobalt carbonyl and dispersed into either hexane or toluene. The Co NP dispersion was dropped onto different substrates (e.g., TEM grid, silicon wafer) and onto liquid surfaces. Transmission electron microscopy (TEM), scanning force microscopy (SFM), optical microscopy, as well as scanning electron microscopy (SEM) showed that superparamagnetic Co NPs assembled into 1-D chains in an external magnetic field. By varying the concentration of the Co NP dispersion (1 mg/mL–5 mg/mL) and the strength of the magnetic field (4 mT–54 mT), the morphology of the chains changed. Short, thin and flexible chain structures were obtained at low NP concentration and low strength of magnetic field; while they became long, thick and straight when the NP concentration and the magnetic field strength increased. In comparison, the assembly of Co NPs from hexane dispersion at ethylene glycol-air interface showed the most regular and homogeneous alignment, since a more efficient spreading could be achieved on ethylene glycol than on water and solid substrates.

Keywords: magnetic nanoparticles, superparamagnetic, self-assembly, ethylene glycol-air interface, 1-D chains, magnetic field strength


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INTRODUCTION Magnetic nanoparticles (MNPs) are submicron moieties made of magnetic materials and are one of the most widely investigated types of nanoparticles. The unique properties of MNPs derive from the high surface-to-volume ratio of the nanoscale magnets which differ from their bulk materials. Recently, magnetic nanoparticles have attracted great attention due to their potential application in various fields, such as high-density data storage, magnetic energy storage, drug delivery, biological imaging and medicine.1–7 Concerning the applicability in electronics and magnetic switching, the ordering of magnetic nanoparticles plays an important role. Assembly in an external magnetic field is an effective way to achieve a high degree of ordering. Therefore, understanding the assembly behavior of magnetic nanoparticles in an external magnetic field is very important and also essential to design novel magnetic media, which would have a much higher data storage density than hard drives in current computers.1,8 Up to now, various techniques have been applied to investigate the assembly behavior of magnetic nanoparticles, including drying of a NP dispersion on substrates either by drop-casting or spin-coating,8–13 and assembly at liquid-air interfaces followed by transfer to substrates by Langmuir-Blodgett or Langmuir-Schaefer method.14,15,24,25,16–23 For the drop-casting on a substrate, the slow evaporation of the solvent is necessary to achieve self-assembly of the nanoparticles into well-ordered structures. Thus, the use of a carrier solvent with a high boilingpoint (e.g., dichlorobenzene, octane, toluene) or evaporation under a relatively high degree of solvent saturation atmosphere is needed. In order to obtain a highly ordered and homogeneous film on the substrate, pre-treatment of the substrate (thorough cleaning or chemical modification) is often required. The Langmuir technique is widely used to assemble films of amphiphilic materials including polymers, colloids and nanoparticles at the liquid-air interface.26 A


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dispersion of the nanoparticles can be dropped onto the liquid phase (water or less polar solvents like glycols) to form a monolayer of the NPs.14,15,24,25,16–23 In addition to the transfer of a monolayer onto various substrates like TEM grids, silica wafers, highly oriented pyrolytic graphite (HOPG), etc., a multilayer can be formed on the substrate by multiple dipping of the substrate. Apart from the unassisted self-assembly, magnetic-field-induced assembly of magnetic nanoparticles provides another method for the assembly of 1-D, 2-D and 3-D superlattices. Due to the rapid response (within one second) and remote control, magnetic-field-induced assembly has been widely investigated.27–30 Among the various types of magnetic NPs, superparamagnetic nanoparticles are most suitable for reversible assembly.31 On the one hand, the usual superparamagnetic NPs have a high saturation magnetization, thus, they exhibit a strong response to the applied magnetic field. On the other hand, their magnetic interaction can be controlled by an external magnetic field: when the magnetic field is applied, they are aligned in the direction of the external magnetic field, after removal of the magnetic field, magnetization drops to zero and they disassemble.32,33 The assembly of magnetic NPs into 1-D, 2-D and 3-D structures is the result of dipole-dipole interaction between the superparamagnetic NPs. When superparamagnetic NPs are under an external magnetic field, they are magnetized. The magnetic dipole moment of superparamagnetic NPs, when the magnetic field is low, can be calculated with the formula: 𝑚 = 𝜒 ⋅ 𝑉 ⋅ 𝐻, where χ, V and H are the particle’s volume susceptibility, particle’s volume and the magnetic field strength, respectively.34,35 When the external magnetic field is strong enough, the magnetic moments of NPs reach their saturation value. The induced magnetic field of one particle can affect any surrounding particle, and the dipole-dipole interaction with the second particle


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depends on the angle of the connecting line between the center of these two particles and the direction of the applied magnetic field.13,15,16 The critical angle where the interaction approaches zero is 54.09°. When the angle is in the range of 0° to 54.09°, they attract each other, and when the angle is between 54.09° to 90°, repulsive interaction dominates.34,35 The local NP concentration and the interaction of NPs are two key parameters that determine the final assembly structures.36–39 1-D chain structures are the simplest ordered structures, which derive from the induced dipole-dipole interaction.33 The magnetic-field-induced assembly can be conducted by deposition of a magnetic nanoparticle dispersion either on substrates or at liquid-air interfaces. Several studies about the magneticfield-induced assembly of magnetic NPs into 1-D superlattices have been reported.40–44 However, most of these reports address the assembly of iron oxide nanoparticles. Cobalt bulk materials have a relatively high saturation magnetization (161 emu/g), compared to magnetite (Fe3O4) (92 emu/g).45 Therefore, our studies focus on the investigation of the magnetic-fieldinduced assembly of Co NPs. Although some studies have been reported on the magnetic assembly behavior of Co NPs on substrates,44–49 the magnetic-field-induced assembly of superparamagnetic Co NPs at the liquid-air interface, to the best of our knowledge, has not been investigated yet. Herein, the assembly behavior of pre-synthesized OA- and TOPO-coated superparamagnetic Co NPs was studied under an applied magnetic field at the ethylene glycol-air interface. For comparison, the magnetic-field-induced assembly behavior on substrates and at the water-air interface was also investigated. The parameters of the concentration of the Co NP dispersion and the strength of the applied magnetic field were varied and their influence on the assembly behavior of the different approaches was analyzed. EXPERIMENTAL SECTION


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Materials. N-hexane (95%) and ethanol (99.5%) were purchased from Th. Geyer, acetonitrile (HPLC grade) from VWR, and ethylene glycol (99.5%) from Carl Roth. Anhydrous 1,2dichlorobenzene (DCB, 99%), cobalt carbonyl Co2(CO)8 containing 1-5% n-hexane as a stabilizer, oleic acid (OA, 99%) and trioctylphosphine oxide (TOPO, 90%) were purchased from Sigma Aldrich. All chemicals were used as received without further purification. Neodymium magnets N45 (45×45×6 mm) were obtained from MagnetMax. Silica wafers were washed by a standard RCA (Radio Corporation of America) cleaning procedure.50 Synthesis of OA- and TOPO-coated Co NPs OA- and TOPO-coated Co NPs with size around 9 nm were prepared as reported before.51 In brief, 100 mg TOPO and 0.1 mL OA were dissolved in 15 mL degassed anhydrous DCB. 540 mg Co2(CO)8 in 3 mL DCB was rapidly injected in the mixture at the reflux temperature of 182 °C. After 15 min, the temperature was reduced to 140 °C, and the reaction mixture stirred for another 15 min. The mixture was allowed to cool to room temperature. The nanoparticle suspension was collected and precipitated into ethanol. The sedimentations were collected by centrifugation, and dispersed in n-hexane or toluene with concentrations ranging from 1–5 mg/mL for the investigation of magnetic-field-induced assembly. Magnetic-field-induced assembly procedures Magnetic-field-induced assembly on substrates by drop-casting A clean silicon wafer (1×1 cm) or carbon-coated TEM grid was placed between two neodymium magnets. The strength of the magnetic field was adjusted to 4 mT, 22 mT or 54 mT by varying the distance between two magnets at 20 cm, 10 cm and 6.8 cm, respectively. OA- and TOPO-


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coated Co NPs were dispersed in toluene at different concentration of 1 mg/mL, 2.5 mg/mL and 5 mg/mL. 10 μL or 4 μL of the respective dispersion was dropped on the silicon wafer or TEM grid. The system was covered with a cap and left for 20 min until toluene was completely evaporated. Magnetic-field-induced assembly at ethylene glycol-air interface A small Teflon well (diameter ≈ 2 cm, height ≈ 1.8 cm) was half-filled with ethylene glycol and different magnetic field strength of 4 mT, 22 mT or 54 mT was applied by changing the distance between two magnets. OA- and TOPO-coated Co NPs were dispersed in n-hexane at different concentration of 1 mg/mL, 2.5 mg/mL and 5 mg/mL. 15 μL of the respective dispersion was carefully dropped on the ethylene glycol surface. The system was left for about 20 min until nhexane was completely evaporated. Afterwards, 500 μL acetonitrile was added carefully along the wall of the Teflon well, so that the film was transferred from the ethylene glycol-air interface to the acetonitrile-air interface. Finally, a silicon wafer or a TEM grid was placed under the floating film, followed by lifting up and drying in air. All the experiments were performed at least three times. Instruments Transmission electron microscopy (TEM) characterization was performed by FEI Titan 80-300 microscope operated at 300 kV. The average size of OA- and TOPO-coated Co NPs and standard deviation were estimated by ImageJ software from a statistics of around 250 NPs. Scanning electron microscopy (SEM) images were obtained by JSM6330F from JOEL operated at the voltage of 5 kV and current of 12 μA. Samples were sputtered with platinum at a thickness of 4 nm. Scanning force microscopy (SFM) images were obtained by Bruker Dimension Icon with


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Tapping Mode and OTESPA tip (k = 42 N/m, fo = 300 kHz). The analysis and processing of the images were done by Nanoscope Analysis (Version 1.5). The optical microscope images were obtained by the optical microscope of the SFM. The magnetic properties of Co NPs were measured using a SQUID magnetometer (MPMS 3, by Quantum Design) at the Quantum Materials Core Lab at the Helmholtz Zentrum Berlin. Magnetization curves were obtained under the fields from 0 – 2 Tesla. The magnetic data were normalized from the sample mass of Co atoms. RESULTS AND DISCUSSION Synthesis of OA- and TOPO-coated Co NPs OA- and TOPO-coated Co NPs were synthesized by thermal decomposition of cobalt carbonyl in 1,2-dichlorobenzene under reflux with OA and TOPO as stabilizers.51 The size and morphology of the nanoparticles was characterized by TEM (Figure 1). The as-prepared monodisperse Co NPs are spherical in shape with an average diameter of 9.0 nm (σ = 4.5 %). A narrow size distribution (σ < 5%) is usually a prerequisite to achieve well-ordered NP superstructures.48 The as-prepared OA- and TOPO-coated Co NPs were superparamagnetic, as already demonstrated in our previous work.51 As shown in Figure S1, the magnetic measurements reveal no hysteresis at 300 K, verifying the superparamagnetic behavior.51


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Figure 1. TEM images of OA- and TOPO-coated Co NPs in 1,2-dichlorobenzene. Effect of concentration under fixed magnetic field strength When an external magnetic field is applied to superparamagnetic nanoparticles, the induced magnetic dipoles in a nanoparticle can interact with the surrounding particles. Meanwhile, the induced magnetic dipoles can interact with the applied magnetic field. The magnetic field gradient can drive the magnetic nanoparticles to the location with the strongest magnetic field, leading to the redistribution of NPs, and hence change the local NP concentrations

during the magnetic NPs assembly.52-54 Furthermore, the

strength of the applied magnetic field can determine the induced moment in NPs, which affects the NP-NP interaction. Thus, the concentration of the nanoparticles and the applied magnetic field are very important factors which influence the assembly behavior of magnetic nanoparticles. At first, with fixed magnetic field strength, we used three different initial concentrations for the assembly process. Magnetic-field-induced assembly on substrates by drop-casting


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The as-prepared OA- and TOPO-coated Co NPs were dispersed in toluene and dropped onto different substrates, namely silicon wafers and TEM grids, in presence of an external magnetic field, as visualized in Figure 2. For the drop-casting method, the slow evaporation of the solvent plays an important role for the self-assembly into wellordered structures. The OA- and TOPO-coated Co NPs are well dispersable in n-hexane and toluene. In view of the low boiling-point of n-hexane, toluene was chosen as the carrier solvent. Furthermore, the evaporation under a relatively high vapor pressure is also helpful to obtain a uniform film on the substrates. Therefore, the system was covered during the evaporation process. At first, a silica wafer or a TEM grid was placed between two magnets which caused a magnetic field strength of 22 mT. A Co NP dispersion with a concentration of 1 mg/mL or 5 mg/mL, respectively, was then dropped onto the substrates, and the carrier solvent was allowed to evaporate. By varying the concentration of the Co NP dispersion, a different assembly behavior in the magnetic field was observed. SFM and TEM characterization of the resulting structures when using a NP concentration of 1 mg/mL are shown in Figure 3. The Co NPs aligned along the direction of the applied magnetic field and 1D chain structures were observed. These assembled chains were short and flexible and the packed Co NPs showed a low degree of ordering. Because of the low NP concentration, only monolayer and double layer regimes were observed in TEM images (Figure 3c and d). When the concentration of the Co NP dispersion was increased to 5 mg/mL, straight and closely distributed 1-D chains


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were obtained (Figure 4). However, the thickness of these chains varied considerably (Figure 4b and c), which indicated the effect of the initial NP concentration. In this case, NPs packed in the assemblies showed highly ordered structures (Figure 4d and e). In comparison to the mono- and double-layers observed at lower concentration (Figure 3c and d), multilayers were formed at higher concentration. This can be explained by the fact that a higher concentration allows a high fraction of short dipolar chains, which will result in longer and thicker chains under applied magnetic field. This is in agreement with simulations conducted by other groups before.55,56

Figure 2. Illustration of magnetic-field-induced assembly of Co NPs on substrates.


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Figure 3. Magnetic-field-induced assembly of OA- and TOPO-coated Co NPs (1 mg/mL in toluene) on substrates with the applied magnetic field of 22 mT: (a) Optical microscope image; (b) SFM height image; (c, d) TEM images at different magnifications. The direction of the magnetic field is indicated by the white arrow.


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Figure 4. Magnetic-field-induced assembly of OA- and TOPO-coated Co NPs (5 mg/mL in toluene) by drop casting on a silicon wafer with the applied magnetic field of 22 mT: (a) Optical microscope image (the red color in the image is the scattering from the laser of the SFM); (b, c) SFM height images of different areas in (a); (d, e) TEM images at different magnifications. The direction of the magnetic field is indicated by the white arrow. Magnetic-field-induced assembly at ethylene glycol-air interface Water has been widely used as subphase in Langmuir technique to assemble nanoparticles.54-56 Here, the experiments for the magnetic-field-induced assembly at the water-air interface were conducted by dropping the Co NP dispersion carefully on the


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water surface. As illustrated in Figure 5, a glass beaker filled with water was placed between the two magnets which caused the magnetic field strength of 22 mT. Due to the large difference in polarity between water and n-hexane, NPs dispersed in n-hexane could not spread well on the water surface and thus, aggregates formed (Figure S1a). Although Co NP in toluene dispersion exhibited a better spreading on water (Figure S2b), a poor assembly behavior at water-air interface under the external magnetic field was observed (Figure S3 a and b).

Figure 5. Illustration of the procedure for magnetic-field-induced assembly of Co NPs at water-air interface. Glycol, in comparison with water, has a lower polarity. The static relative dielectric constant for water is 80.10, while it is only 41.40 for ethylene glycol and 31.82 for diethylene glycol.18 Since the as-synthesized nanoparticles were coated with hydrophobic ligands, oleic acid and TOPO, it is difficult to achieve a well spread film on


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the water surface. Thus, glycols are more suitable as subphase for the assembly of hydrophobic NPs compared to water. Since Majetich and Weller used ethylene glycol and diethylene glycol as subphase in the Langmuir trough to obtain closely packed cobalt and cobalt platinum nanoparticle films, respectively,17,18 the assembly of nanoparticles at glycol-air interface has been widely investigated.60,61 Dong et al. found that the solvent evaporation kinetics affected the growth of the film. Using a high boiling-point solvent like toluene led to a random assembly of NPs.61 Thus, in our experiments, ethylene glycol was used as subphase and Co NPs were dispersed in nhexane. As shown in Figure S4, a Teflon well half-filled with ethylene glycol was placed between the two magnets, generating the magnetic field of 22 mT. Co NPs dispersed in n-hexane were carefully dropped onto the liquid surface. A film was formed after the evaporation of n-hexane. Because of the high boiling-point of ethylene glycol, it is very difficult to evaporate the residual ethylene glycol on substrates after transferring the film. Thus, acetonitrile was carefully added along the wall of the Teflon well, and the film was transferred from ethylene glycol-air interface to acetonitrile-air interface. A large area film was obtained by gently placing the substrates under the floating film, followed by lifting up. The film was allowed to dry in air. This process was reported before by Klajn et al., who investigated the self-assembly behavior of magnetite cubic nanoparticles at diethylene glycol-air interface under an external magnetic field, and verified that the transfer of the films to acetonitrile-air interface showed no influence to


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the assemblies, since the assembly process was completed before the addition of acetonitrile.62,63 Figure S4a shows the formation of the film in the Teflon well and Figure S4b shows the film after transfer onto a silicon wafer. The assembly behavior of Co NPs with different concentration at ethylene glycol-air interface was investigated. Figure 6 shows the 1-D chains of Co NPs assembled under the applied magnetic field when using the lowest concentration (1 mg/mL). These chains were sparsely distributed and flexible compared to the assembly from a higher concentration (2.5 mg/mL) shown in Figure 7. Here, the formed 1-D chains assembled in a more close and regular distribution (Figure 7a–c). The crack in Figure 7c probably resulted from the film transfer or drying process, which can be obviously observed in Figure S4b. The 1-D Co NP superstructures were also observed in TEM images (Figure 8d). However, the 1-D chains were not straight along the direction of the applied magnetic field at both concentrations of 1 mg/mL and 2.5 mg/mL, indicating that at such concentrations, the applied magnetic field was not strong enough to align all the induced magnetic moments of Co NPs in a head-to-tail direction, and thermal fluctuation led to the misalignment of NPs (Figure 6 and Figure 7). These results were in agreement with the simulations and theory.64 When the concentration was increased up to 5 mg/mL, a nearly continuous film with almost full coverage of Co NPs was obtained (Figure 8). A large area of chains can be seen in the SEM image in Figure 8d, and the formed 1-D chains were oriented straight along the direction of the applied magnetic field. Thus, as


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conclusion from the assembly behavior, the higher the concentration of the NP dispersion, the straighter are the 1-D chains. This can be explained by two reasons. On the one hand, higher concentration leads to less space for NPs to move and thus, the thermal diffusion is hindered; on the other hand, high concentration of particles shortens the distance between particles, hence, the interaction between particles increases, which results in straighter chain structures.65 At high concentrations, we also observed high packing orders in the aligned assemblies although the crystalline domains showed different orientations (Figure 8f). However, when the concentration of the Co NPs was less than 0.5 mg/mL, no chain structure was found. This is probably because the NPs are randomly distributed with a large distance between the individual particles which leads to reduced interaction between the particles. It is worth to note that the assembly of Co NPs at ethylene glycol-air interface was quite reproducible. The experiments were repeated at least three times and similar results were obtained each time.


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Figure 6. Magnetic-field-induced assembly of OA- and TOPO-coated Co NPs (1 mg/mL in n-hexane) at ethylene glycol-air interface with the applied magnetic field of 22 mT: (a) Optical microscope image (the red color in the picture is the scattering from the laser of the SFM); (b) SFM height image. The direction of the magnetic field is indicated by the white arrow.


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Figure 7. Magnetic-field-induced assembly of OA- and TOPO-coated Co NPs (2.5 mg/mL in n-hexane) at ethylene glycol-air interface with the applied magnetic field of 22 mT: (a) Optical microscope image (the red color in the picture is the scattering from the laser of the SFM); (b, c) SFM height images; (d) TEM image; (e) SEM image. The direction of the magnetic field is indicated by the white arrow.


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Figure 8. Magnetic-field-induced assembly of OA- and TOPO-coated Co NPs (5 mg/mL in n-hexane) at ethylene glycol-air interface with the applied magnetic field of 22 mT: (a) Optical microscope image; (b, c) SFM images: height (b) and phase image (c); (d, e) SEM images at different magnifications; (f) TEM image. The direction of the external magnetic field is indicated by the white arrow. Effect of magnetic field strength on the assembly of Co NPs at fixed concentration When superparamagnetic nanoparticles are exposed to an external magnetic field, their induced moments tend to align along the field direction. The alignment depends on the competition between magnetic interaction induced by the external magnetic field and the fluctuation caused by Néel rotation or Brownian rotation.65 Néel rotation can


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restructure the electronic spins and thus reorient the magnetic moment, while Brownian rotation leads to the rotation of the particle itself in dispersion.66,67 When the magnetic field is weak, the assembled chains are usually short and flexible due to the fluctuation of the induced moments. When the magnetic field increases, the induced moments of the magnetic nanoparticles become larger, and dominate over the fluctuation. Thus, the formed short chains connect to each other and become longer chains; and thicker chains can be obtained through lateral chain-chain interaction. Furthermore, the amount of induced moments in NPs which are oriented in direction of the external field increases with the magnetic field strength, hence, the 1-D chains get straighter.65 Thus, the assembly behavior of Co NPs under different strength of the magnetic field was investigated. Therefore, we fixed the concentration of the nanoparticle dispersion at 2.5 mg/mL and studied the assembly behavior via the two techniques that proved most suitable to obtain ordered films, drop casting and assembly at ethylene glycol-air interface. Magnetic-field-induced assembly on substrates by drop-casting The as-prepared OA- and TOPO-coated Co NPs were dispersed in toluene at a fixed concentration (2.5 mg/mL) and dropped on a silicon wafer similar to the illustration in Figure 2. In contrast to the experiments above, not the concentration, but the magnetic field strength during the drop casting was varied. The applied magnetic field strength was adjusted to 4 mT and 54 mT, respectively. Figure 9a and b show short and worm-


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like chains aligned in the weaker magnetic field, which resulted from the fluctuation of the induced moments of the NPs. However, when the magnetic field was stronger, long and straight chains were formed (Figure 9c and d). Thus, the morphology of the NP assemblies can be clearly affected by varying the strength of the magnetic field.

Figure 9. Magnetic-field-induced assembly of OA- and TOPO-coated Co NPs (2.5 mg/mL in toluene) by drop casting on a silicon wafer: (a, b) the applied magnetic field strength was 4 mT: Optical microscope image (the red color in the picture is the scattering from the laser of the SFM) (a); SFM height image (b); (c, d) the applied magnetic field strength was 54 mT: Optical microscope image (the red color in the


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picture is the scattering from the laser of the SFM) (c); SFM height image (d). The direction of the external magnetic field is indicated by the white arrow.

Magnetic-field-induced assembly at ethylene glycol-air interface Here, Co NPs were dispersed into n-hexane at 2.5 mg/mL, dropped on the ethylene glycol surface and transferred to Si-wafers in the same way like before (Figure 5). By changing the strength of the applied magnetic field, the assembly behavior changed considerably. Figure 10a and b show the assembled structures of Co NPs in the weaker magnetic field of 4 mT. Although aligned chains are difficult to identify in Figure 10a, however, at higher magnification of SFM height image displays thin and flexible chains, as shown in Figure 10b. When the magnetic field strength was increased to 54 mT, the formed 1-D chains were thicker and straighter along the direction of the applied magnetic field (Figure 10c and d). This is because the magnetic moments of magnetic NPs get co-directed with the increase of magnetic field and decrease fluctuations, as a result, the flexible thinner chains transform into straighter and thicker 1-D chain structures. This was a similar result compared to the assembly by drop-casting on substrates under different magnetic field strength, however, the films formed at ethylene glycol-air interface were more homogeneous and highly ordered. It is worth to


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note that the tilted angle of NP chains with respect to the magnetic field in Figure 10c and d could be explained by both effects from NP concentration and magnetic field strength. On the one hand, the degree of misalignment decreased when compared with the assemblies of Co NPs of 2.5 mg/mL under the magnetic field of 22 mT (Figure 7), indicating that strong magnetic field led to rigid and straight 1-D structures. On the other hand, the 1-D chains were not as straight as the assemblies of Co NPs of 5 mg/mL under the magnetic field of 22 mT (Figure 8). This might be explained by the reason that high concentration of NP dispersion means more fraction of head-to-tail connection of NPs, which shorten the distance between NPs, and increase the interaction of these short chains, finally, they form long, straight and thick 1-D chains. Therefore, both concentration and magnetic field strength can influence the assembly behavior of Co NPs.


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Figure 10. Magnetic-field-induced assembly of OA- and TOPO-coated Co NPs (2.5 mg/mL in n-hexane) at ethylene glycol-air interface: (a, b) the applied magnetic field strength was 4 mT: Optical microscope image (a); SMF height image (b); (c, d) the applied magnetic field strength was 54 mT: Optical microscope image (c); SFM height image (d). The direction of the external magnetic field is indicated by the white arrow. CONCLUSIONS


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In summary, monodisperse superparamagnetic OA- and TOPO-coated cobalt nanoparticles were synthesized. Well-defined Co NP assemblies were formed under an external magnetic field by using various techniques, namely drop-casting on substrates, assembly at water-air and at ethylene glycol-air interface. By changing the concentration of the nanoparticle dispersion and the strength of the applied magnetic field, the assemblies exhibited different morphologies. TEM, SEM, SFM and optical microscopy showed that the formed 1-D chains were shorter and flexible at either lower concentration of the Co NP dispersion or lower strength of the external magnetic field due to thermal fluctuation. However, by increasing the concentration of the NP dispersion or the strength of the applied magnetic field, long, thick and straight chains were obtained. The comparison of the three methods for the magnetic-field-induced assembly exhibited that the assembly at ethylene glycol-air interface generated films of higher order than the drop-casting on substrates. Nicely ordered films could not be obtained via the assembly at water-air interface as the spreading from hydrophobic solution on water is difficult. Hence, assembly technique, subphase / substrate, carrier solvent and nanoparticle properties have to be adjusted to achieve homogeneously ordered

NP

superstructures.

The

magnetic-field-induced

assembly

of

superparamagnetic cobalt nanoparticles may stimulate people to develop methods for incorporating them into electronic devices.


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ASSOCIATED CONTENT Supporting Information. Magnetization curve, TEM, SFM and optical microscopy images for the magnetic field induced assembly at water-air interface (PDF) AUTHOR INFORMATION Corresponding Author *Email: [email protected]; [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT Li Tan thanks CSC (China Scholarship Council) for a PhD scholarship. This work was supported as Fraunhofer High Performance Center for Functional Integration in Materials.


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Table of Contents / Abstract Graphic


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Magnetic-Field-Induced Assembly of Superparamagnetic Cobalt

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