Printed Thin Diblock Copolymer Films with Dense Magnetic

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Applications of Polymer, Composite, and Coating Materials

Printed Thin Diblock Copolymer Films with Dense Magnetic Nanostructure Senlin Xia, Lin Song, Wei Chen, Volker Körstgens, Matthias Opel, Matthias Schwartzkopf, Stephan V. Roth, and Peter Muller-Buschbaum ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b06573 • Publication Date (Web): 28 May 2019 Downloaded from http://pubs.acs.org on May 29, 2019

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ACS Applied Materials & Interfaces

Printed Thin Diblock Copolymer Films with Dense Magnetic Nanostructure Senlin Xia†, Lin Song$,†, Wei Chen†, Volker Körstgens†, Matthias Opel‡, Matthias Schwartzkopfξ, Stephan V. Rothξ,‖, Peter Müller-Buschbaum*,†,§ †Technische

Universität

München,

Physik-Department,

Lehrstuhl

für

Funktionelle

Materialien, James-Franck-Straße 1, D-85748 Garching, Germany $Institute

of Flexible Electronics, Northwestern Polytechnical University, West Youyi Road

127, 710072, Xi'an, Shanxi, China ‡Walther-Meissner-Institut,

Bayerische Akademie der Wissenschaften, Walther-Meissner-Str.

8, D-85748 Garching, Germany ξDeutsches

‖KTH

Elektronen-Synchrotron DESY, Notkestrasse 85, D-22603 Hamburg, Germany

Royal Institute of Technology, Department of Fibre and Polymer Technology,

Teknikringen 56-58, SE-100 44 Stockholm, Sweden §Heinz

Maier-Leibnitz Zentrum (MLZ), Technische Universität München, Lichtenbergstr. 1,

D-85748 Garching, Germany

KEYWORDS: hybrid films, diblock copolymer, magnetic nanoparticles, GISAXS, superparamagnetic behavior

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ABSTRACT

Thin hybrid films with dense magnetic structures for sensor applications are printed using diblock copolymer (DBC) templating magnetic nanoparticles (MNPs). To achieve a high density magnetic structure, the printing ink is prepared by mixing polystyrene-blockpoly(methyl methacrylate) (PS-b-PMMA) with a large PS volume fraction and PS selective MNPs. Solvent vapor annealing is applied to generate a parallel cylindrical film morphology (with respect to the substrate), in which the MNP-residing PS domains are well separated by the PMMA matrix and thus, the formation of large MNP agglomerates is avoided. Moreover, the morphologies of the printed thin films are determined as function of the MNP concentration with real and reciprocal space characterization techniques. The PS domains are found to be saturated with MNPs at 1 wt %, at which the structural order of the hybrid films reaches a maximum within the studied range of MNP concentration. As a beneficial aspect, the MNP loading improves the morphological order of the thin DBC films. The dense magnetic structure endows the thin films with a faster superparamagnetic responsive behavior, as compared to thick films where identical MNPs are used, but dispersed inside the minority domains of the DBC.


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1. INTRODUCTION Nanostructured diblock copolymer (DBC) films have received enormous attention during the past decades, owing to the versatility of formulation and wide applicability of the fabrication processes. Attributable to the microphase separation, various periodic nanostructures, such as spheres, cylinders, lamellae and bicontinuous gyroids can be achieved via tuning parameters of the polymer intrinsic properties, like the DBC molecular weight, volume fraction of each block, interaction parameter as well as chain architectures.1-3 Due to such peculiar and appealing properties, DBC films have been widely employed as constructive scaffolds guiding nanoparticles for installing controlled hybrid structures.4-6 Via incorporating various types of functionalized inorganic nanoparticles into a nanostructured polymer matrix, flexible hybrid nanocomposite films with outstanding electrical, optical or magnetic properties can be engineered. Such hybrid films have versatile applications in various fields, such as sensors, solar cells, metal catalysis and magnetic storage devices.7-11 Among these applications, magnetic thin films consisting of polymer matrices and magnetic nanoparticle (MNP) additives have received intense research interest, due to the distinct properties from hard bulk metal materials.12,

13

Nanoscale magnetic particles possess intriguing properties originating

from the quantum-size effect and high surface-to-volume ratio.12, 14 Accordingly, the DBCMNP thin hybrid films with mutual benefits from both, the polymer matrix and the MNPs prove to be promising materials in sensor applications. Regarding the fabrication of MNP-polymer films, one of the biggest challenges is dispersing sufficient MNPs inside the polymer matrices. Due to their different chemistry nature, strong incompatibility exists between inorganic MNPs and organic polymer chains, which make it challenging to achieve the desired localization of the MNPs inside hosting polymer blocks of a DBC matrix. The aggregation of MNPs cannot be avoided due to the magnetic attractive force present among the MNPs. So far, one focus of the related research was oriented towards 3 ACS Paragon Plus Environment


improving the affinity of the MNPs to the polymer matrices. Among the different pathways, one effective approach is the surface modification of the MNPs via grafting of organic ligands. Diverse ligands, such as polymer chains,15-18 silane groups,19 and oleic acid chains2023

were studied for this purpose. Reports showed that the penalty from the surface energy

could be adequately curtailed via the molecule groups grafted to the MNPs’ surfaces. The spatial repulsion is increased by the presence of bulky organic chains, which decrease the tendency to an irreversible agglomeration via steric hindrance. Therefore, organic ligands can be preferentially employed, in order to achieve the desired dispersion of MNPs inside polymer hosting matrices. Generally, the integrated properties of the hybrid nanocomposites depend not only on the properties of the individual components, but also on the spatial localization and arrangement of MNPs within the DBC matrix. For instance, Yao et al. fabricated hybrid films embedded with maghemite γ-Fe2O3 nanoparticles under an external magnetic field.24 The maghemite MNPs aligned under the magnetic driving force and formed well-oriented wires. The achieved hybrid films showed superparamagnetic behavior with a magnetic anisotropy.24 Our recent report showed that the orientation of DBC structures guided the alignment of MNPs.25 By using a DBC template containing highly ordered cylindrical structures, oriented perpendicularly on the substrate, the surface-modified MNPs selectively aggregated inside the cylinders and formed vertically elongated clusters. Thus, an additional magnetic anisotropy was achieved.25 In MNP-polymer hybrid systems, polymer nanostructures and particle arrangement are interplaying parameters. The nanostructure of the polymer matrices plays an indispensable role in the corresponding magnetic properties of the hybrid system. Better ordered and more homogeneously distributed MNPs contribute to a more desirable magnetic behavior.25-27 Moreover, by choosing the suitable polymer template, dense magnetic structures can be 4 ACS Paragon Plus Environment

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achieved, which proved to be beneficial for enhancing magnetic properties.28 The MNPs’ localization inside specific hosting domains,29, nanostructures31,

32

30

the influence of MNPs on DBC

and the effect of an external field on the film morphology and the

corresponding magnetic properties have been incestigated.24,

28

However, typically research

focused on thick hybrid films, which had a thickness in the range from several hundred nanometers to micrometers.24-26, 28 Thin hybrid films with a thickness of tens of nanometers were less studied so far. Such thin magnetic polymer films, especially the ones with a high density of MNPs, appear highly promising nowadays for use in modern microelectronics and as magnetic sensor devices. To fabricate such thin magnetic polymer films, several challenges need to be overcome. Due to the unfavorable affinity between polymers and inorganic substrates, dewetting of an initially homogeneous hybrid film may occur,33-35 which prevents application of homogeneous films. Moreover, a suitable film thickness is required, in order to accommodate the MNPs inside the films and to achieve the desired magnetic properties.10 In addition, to gain dense magnetic structures, the nanostructure of the DBC templates needs to be fine-tuned, in order to host as many MNPs as possible, but avoid the formation of largesized MNP clusters and a collapse of the DBC structure. In the present work, thin magnetic polymer films with the thickness of around 35 nm are successfully fabricated by using a printing technique. A cylindrically structured polystyreneblock-poly(methyl methacrylate) (PS-b-PMMA) nanotemplate is used to host the MNPs. Via solvent vapor annealing, a highly-ordered parallel cylindrical morphology is obtained, meaning that the PS matrix contains well-separated, parallel oriented PMMA domains (with respect to the substrate). Due to the affinity to PS chains, the MNPs are preferentially dispersed inside the major PS domains, which endows the thin films with a high magnetic structure density. To follow the nanostructure evolution as function of the MNPs loading, thin films with different MNP concentrations are prepared. The surface and inner nanostructure of 5 ACS Paragon Plus Environment


the thin films are probed via scanning probe microscopy (AFM and SEM) and grazing incidence small-angle X-ray scattering (GISAXS), respectively. Compared to our previously studied thick films where identical MNPs were used,25 the present thin films show enhanced magnetic properties, which suggests an improved potential usage in applications such as sensors.

2. EXPERIMENTAL SECTION 2.1 Materials The diblock copolymer polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) was obtained from the company Polymer Source Inc. The 1H-NMR spectrum of PS-b-PMMA is provided in Figure S1. No further purification was applied. The number average molecular weight of PS-b-PMMA was 66.5 kg/mol with the volume fraction of the PS block as fPS = 0.685. The polydispersity index was 1.13. According to the theoretical phase diagram of DBCs, a cylindrical morphology can be expected.2 Magnetite MNPs (Fe3O4) with oleic acid grafting were purchased from Sigma-Aldrich. MNPs had an average diameter of 10 nm, and were dispersed in toluene with a concentration of 5 mg/ml. 2.2 Sample Preparation Silicon substrates (Si 100, n-type, Silchem) were pre-cleaned following an acid bath cleaning protocol.36 Polymer solutions were prepared using toluene as solvent with a ﬁxed polymer concentration of 7 mg/ml. MNP concentrations (weight ratio between MNPs and DBC) were varied: 0, 0.5, 1, 3, 7, 15 wt%. All hybrid films were fabricated by a printing method (meniscus guided slot-die coating), which was detailed in our previous work.28 All films were post-treated by solvent vapor annealing (SVA) with a solvent mixture containing acetone and 6 ACS Paragon Plus Environment

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tetrahydrofuran (THF) in a volume ratio of 1:1. SVA was carried out in a sealed desiccator at room temperature. Inside the desiccator all samples were place 5 cm above the liquid solvent bath to ensure proper exposure to solvent vapor without condensation. After 10 min, all samples were taken out and dried in air at room temperature. The film characterization techniques and involved parameters are detailed in the supporting information.

3. RESULTS AND DISCUSSION In this work, MNP-DBC hybrid magnetic films with various particle concentrations are established by a printing technique, enabling control over the film thickness. Previous studies showed that the incorporation of MNPs can change the orientation of the printed polymer nanostructures.26, 27 Also SVA was reported in literature as a powerful tool to tune the order and orientation of polymer nanostructures.37-39 We use both approaches to control the final architecture of the hybrid films. For this purpose, all films are printed with a thickness close to the periodic interdomain distance of the DBC. Post-treatment is applied to all films in order to reach highly ordered nanostructures. Due to the presence of acetone, which is a good solvent for PMMA,40 a geometry with PMMA cylinders lying parallel to the substrate surface is expected. The periodic interdomain distance (D) can be estimated in the strong segregation limit (SSL) for a polymer melt via 41 2 1 3 6

𝐷 ~ 𝑎𝑁 𝜒

where χ = 0.042 is the Flory-Huggins interaction parameter,42 N = 645 represents the polymerization degree, and a = 0.64 nm shows the statistical segment length.43 Accordingly, a characteristic interdomain distance D of 38 nm is calculated for the pure DBC film. It should



be noted that due to the applied SVA this estimated D value cannot be expected to match exactly the experimental values. 3.1 Film thickness The film thickness as a function of the MNP concentration is presented in Figure 1. As the MNP concentration increases from 0 wt% to 15 wt%, the thickness increases from (35.5 ± 1.7) nm to (58.3 ± 2.1) nm. Since all hybrid films have been fabricated with an identical printing protocol, the increase is directly related to the presence of the MNPs, which change the viscosity of the printed liquid and which need to be accommodated inside the hybrid films. Interestingly, the film thickness evolves in a non-linear fashion. It increases significantly with MNP concentration for concentrations below 7 wt% (stage I in Figure 1), and remains almost constant at higher concentrations (stage II in Figure 1). Differences in the localization of the MNPs can explain such concentration dependence.44 At low concentrations (stage I), the MNPs are located preferentially inside the polymer domains, which results in the expansion of the respective domains and a swelling of the films (thickness increase). At high concentrations (stage II), the MNPs can no longer be accommodated inside the polymer domains due to saturation and form aggregates on the film surface.


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Figure 1. Thickness of MNP-DBC hybrid films at different MNP concentrations (the red solid line is a guide to the eye). Two different stages are identified: Stage I (swelling, light yellow) and stage II (saturation and MNP aggregates formation, yellow).

3.2 Surface structures The nanostructure of the hybrid film surfaces is probed with SEM. Figure 2 shows the SEM images with distinct features at the corresponding MNP concentrations. For the MNP-free film (Figure 2a), a morphology with cylinders parallel to the substrate surface is observed, which matches the assumption based on the volume fraction of the PMMA block (fPMMA = 0.315). The brighter parts correspond to the PS blocks, and the darker parts show the PMMA blocks (also see Figure S2). The SEM image shows that the PMMA cylinders form the classical fingerprint-like morphology, which is commonly achieved in case of having a limited degree of order.45-47 As discussed above, the thickness of the pure DBC film (dm = 35.5 ± 1.7 nm) is quite similar to the theoretically calculated periodic interdomain spacing (D = 38 nm). The relevance of the film thickness with respect to the film morphology was reported for such a confinement situation.43, 46, 48 For thicker films (dm > D), a parallel oriented cylinder morphology can be achieved, because of the surface boundary conditions.48 For thinner films (dm < D), a perpendicular orientation is expected in order to maximize the polymer chains’ conformational entropy.43,

46

In the present work, a parallel orientation is

observed for the pure film with the thickness slightly smaller than the characteristic periodicity. This behavior can result from the SVA post-treatment, in which a PMMAfavorable solvent (acetone) has been used. According to the literature, the interaction parameters between polymer and solvent (χpolymer-solvent) are χPMMA-acetone = 0.29, χPS-acetone = 1.1, χPMMA-THF = 0.88, and χPS-THF = 0.34.40 Therefore, acetone is a selective solvent to the PMMA block, and THF is more neutral to both blocks. When the DBC films are placed in such mixed 9 ACS Paragon Plus Environment


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solvent vapor, the PMMA chains will stretch to maximize their exposed volume. Meanwhile, the strong attraction of the native SiOx layer on the substrate to the PMMA chains will also prevent the formation of a perpendicularly oriented morphology.49 After incorporation of a low concentration of MNPs (0.5 wt %, Figure 2b) the SEM contrast of the whole system is enhanced. Also several small white dots are found being located preferentially on the brighter PS domains on the film surface (also see Figure S3). These dots are MNP agglomerates, which evidence the affinity of the MNPs to the PS domains. To follow the evolution of structural order, two-dimensional fast Fourier transform (2D-FFT) patterns are extracted from the SEM images. Compared with the pure DBC film, in case of the hybrid film a second order ring appears in the FFT pattern (0.5 wt %, shown as the white arrow in Figure 2b). This finding reveals an enhanced degree of order upon MNP loading, which results from the more stretched polymer chains after MNPs incorporation.10,

50

However, incorporation of MNPs has no influence on the general film morphology as compared with our previous investigation, in which the entire morphology turns from a parallel orientation into a perpendicular orientation after addition of small amounts of MNPs.26 This suggests that the influence of the selective acetone vapor dominates the structure orientation over the impact of the MNP during the films’ fabrication in the present work. Therefore, a cylindrical nanostructure with parallel orientation is also found with MNPs.


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Figure 2. SEM images of thin films with different MNP concentrations: (a) 0, (b) 0.5, (c) 1, (d) 3, (e) 7, (f) 15 wt%. The brighter parts correspond to the PS blocks, and the darker parts show the PMMA blocks. MNP agglomerates give rise to white spots or domains. The corresponding 2D-FFT patterns are shown as insets in the top right corners of each image.

Further increasing the MNP concentration (1 wt%) leads to a sharper second order ring in the corresponding FFT pattern, showing that an even higher order is achieved. When the concentration reaches 3 wt%, the number of MNP agglomerates increases together with a slight change of the morphology. The initially continuous PMMA phase (darker part) partially changes into isolated islands (small darker dots), which results from a possible morphology transition of the DBC film. This means that the morphology is perturbed at 3 wt% MNPs, although well-ordered as suggested by the second order ring in the FFT image. Moreover, it can be understood as an indication that the PS domains are saturated with MNPs at 1 wt % and higher loading starts to perturb the DBC morphology. From 7 wt% on, the formation of larger MNP clusters on the film surface and the presence of darker dots (PMMA domains) reveals the further alteration of the film morphology. Additionally, the decay of the 2D-FFT 11 ACS Paragon Plus Environment


second order ring demonstrates the loss of order. At the highest probed concentration (15 wt%), no continuous ordered PMMA phases are observed on the film surface. Moreover, only large MNP clusters are found on the film surface. This differs from our previous investigation on MNP-DBC hybrid films, where large areas of the film surface were covered by a layer of MNPs.25, 26 The possible reason may lie in the MNP-hosting domains. In our previous study, the MNPs were located in the minority domains, which had small volume fractions. In the present work, however, the hosting domain is the majority PS matrix, which offers a larger capacity for accommodating the MNPs. Therefore, more MNPs can be dispersed inside the films instead of dwelling on the film surfaces, indicating the possibility to achieve dense magnetic structures for the presently probed thin hybrid films. To quantify the degree of order of the surface structure, power spectral density (PSD) functions are calculated via an azimuthal integration of the intensity distribution of the 2DFFT patterns. The extracted PSD profiles are plotted in Figure 3 with characteristic features being highlighted. Peak I originates from the microphase separation structure of the DBC. With increasing MNP concentration, peak I initially gets slightly narrower and then broadens afterwards, which means that the order is initially enhanced by the addition of MNPs before it decays. From the position of peak I, a periodic interdomain distance of around 36 nm is obtained for the bare DBC film (0 wt%). The slight shift of peak I to smaller q values represents an increase of the interdomain distance due to the addition of MNP. The second order peak (peak II) follows the behavior and its position underlines the order of the PMMA cylinders in the PS matrix in the cylindrical fingerprint morphology. Peak II vanishes at the highest MNP concentration due to a significantly reduced order.


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Figure 3. Power spectral density (PSD) profiles obtained from the FFT patterns of the SEM graphs at different MNP concentrations: 0, 0.5, 1, 3, 7, and 15 wt% (from bottom to top). Peak I originates from the lateral structure of the DBC microphase separation structure and peak II is the related second order peak. All curves are shifted along y axis for better comparison.

Figure 4. AFM topography images of films with different MNP concentrations: (a) 0, (b) 0.5, (c) 1, (d) 3, (e) 7, and (f) 15 wt%.The height color bars are individually adapted for the clarity of the presentation.



To address the topographical information of the hybrid films, all the samples are probed with AFM via tapping mode. The AFM height graphs are displayed in Figure 4 with distinct characteristics at respective MNP concentrations. Generally, all AFM images depict a similar surface morphology as observed from the SEM measurements. The image of the pure DBC film (Figure 4a) shows a parallel cylinder structure with a low local average height of (2.6 ± 0.3) nm. Due to the different heights, the two polymer domains present as different colors in the image. Correlating the morphology to the SEM observation and regarding the PS volume fraction, the brighter part shows the PS block, and the darker part the PMMA block. After incorporating MNPs into the films, the PS domains become even brighter, resulting from the height increase after swelling with MNPs. With increasing MNP concentration, the MNP aggregates appear and increase in size, and the polymer nanostructure evolves following the same fashion as observed in the SEM images and described there. At the highest concentration, an ill-defined surface morphology forms, with the local average height increasing to (9.7 ± 1.3) nm. Based on the MNP diameter of 10 nm, one could speculate that part of the MNPs aggregates are also buried inside the polymer matrix at the highest probed MNP concentration. Similar observations were also found in our previous investigations.24, 28 The root mean square roughness (0.19) of all films is calculated from the AFM data. For the pure DBC film, a local Rrms of (0.19 ± 0.03) nm is found. As concentration increases, Rrms increases but still stays below 1.5 nm, which suggests that flat hybrid films can be prepared from the used printing technique. The roughness of all studied films is detailed in Figure S4. 3.3 Inner structure From AFM and SEM measurements, only the surface morphology can be probed, which is not sufficient to address the influence of MNPs overall. In particular, the dispersion of MNPs inside the hosting polymer matrix cannot be determined. To access the films’ inner structure, 14 ACS Paragon Plus Environment

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grazing incidence small angle X-ray scattering (GISAXS) is performed. With GISAXS, the averaged structure information, such as structure sizes and size distributions, averaged over the illuminated thin film volume can be obtained.51 3.3.1 Lateral nanostructure The 2D GISAXS data are displayed in Figure 5. Distinct features can be clearly observed at different MNP concentrations. To protect the detector from being saturated, a circular beamstop has been placed at the specular peak position in front of the detector. The incidence angle is chosen as 0.35°, so that the material characteristic Yoneda peaks (shown as the red circle in Figure 5a) can be well separated from specular peak.

Figure 5. 2D GISAXS data of films with different MNP concentrations: (a) 0, (b) 0.5, (c) 1, (d) 3, (e) 7 and (f) 15 wt %. All intensity patterns are displayed in logarithmic scale with the same intensity scale. The black horizontal and vertical lines are the gaps between modules of the detector. Arrows with different colors indicate the peaks caused by the structure: first order (white), second order (yellow) and third order (orange). The red



circle in figure a shows the position of Yoneda peak of the DBC. The red and white boxes in figure b show the area where the off-detector and horizontal line cuts are extracted, respectively.

From the qy direction, information concerning lateral structure can be achieved. For the MNPfree DBC film, a prominent rod-like pseudo-Bragg peak (at qy = 0.175 nm-1), shown as the white arrow in Figure 5a) is seen together with a second order peak (at qy = 0.317 nm-1), shown as the yellow arrow in Figure 5a) having lower scattering intensity. The pseudo-Bragg peak results from the electron density contrast difference between the PMMA and PS blocks and the protrusions. The existence of the second order peak shows that well-ordered structures seen at the film surface are also present inside the thin film. With low concentration MNP incorporation (0.5 wt%), the intensity of both first and second order peaks is remarkably enhanced, which can be explained via the improved scattering contrast due to the localization of MNPs inside the PS domains. Interestingly, a third order peak (indicated by the orange arrow in Figure 5b), which is weak but still visible, appears at the position of a higher qy value (at qy = 0.469 nm-1). This suggests that an improved structural order is obtained inside the hybrid films after loading with MNPs. Similar scattering features are also observed for films with a MNP concentration of 1 wt% and 3 wt%. At 7 wt% MNPs, the second and third order peaks vanish, and the intensity of the Bragg peak decreases. It almost disappears at 15 wt% MNPs. This absence of side peaks at high MNP concentrations reveals a loss in order inside the hybrid films. To get more details from the scattering data, horizontal line cuts are made at the Yoneda peak position of the DBC in the 2D GISAXS data. All cuts are plotted in Figure 6a together with the corresponding fits, from which lateral structure information can be obtained. Figure 6a depicts the peaks marked with I, II and III as indicated by arrows. The line cuts are fitted 16 ACS Paragon Plus Environment

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using a model including form and structure factors, both of which are assumed to exhibit a Lorentzian size distribution. Peak II with the strongest intensity represents the first order Bragg peak, and originates from the highly ordered microphase-separation structures of the DBC. For the data of the pure DBC film, a cylinder form factor is applied for the fitting. From the modeling, the position of the strong first order peak (Peak II) and weaker second order peak (indicated by the green dashed arrow) are obtained as qy = 0.175 nm-1 and qy = 0.317 nm1.

The ratio of the qy values is 0.552 (close to 1/√3), which matches with the cylinder

geometry assumption. An interdomain distance of 34.9 ± 0.3 nm is obtained for the inner film structure, which deviates slightly from the value of the surface structure as probed with SEM (~ 36 nm). Thus, inner film structure and surface structure of the DBC film might not be fully identical, yet rather closely match.

Figure 6. (a) Horizontal line cuts of the 2D GISAXS data (black triangles) with corresponding fits (red lines) of films with different MNP concentrations (from bottom to top: 0, 0.5, 1, 3, 7 and 15 wt%). For clarity, all curves are shifted along the y axis. Featured peaks are marked as: peak I (contribution of single MNPs), peak II (contribution of DBC structure), and peak III (characteristic MNP-aggregates). The green and red dashed arrows



at the bottom show the second and third order peaks of the DBC structure. Extracted values of (b) the interdomain distance DII displaying the microphase separation structure and of (c) MNP-aggregates distance DIII are plotted as a function of MNP concentration.

Adding a small amount of MNPs into the system contributes to a slightly sharper peak II and a remarkable intensity enhancement of the second order peak (indicated by the dashed green arrow). Moreover, a distinguishable third order peak (shown as dashed red arrow) appears at qy = 0.469 nm-1, which cannot be seen in the data of the MNP-free DBC film. These features demonstrate that moderate MNP loading helps to improve the order of the hybrid system and enhances contrast due to addition of MNPs to the PS domains. To quantify the contribution of loading MNPs to the structural order improvement and contrast enhancement, the intensity increment of both peaks (first and second order peaks) compared with those from the MNPfree film is studied. The results displayed in Figure S5 show that the intensity increase of the second order peak is much higher than that of the first order peak irrespective of MNP concentration. This suggests that loading MNPs contributes more to the structural order than to the contrast enhancement. The position of peak II gradually shifts to lower qy values with increasing MNP concentration due to an increase of the interdomain distance (DII) in the lateral direction. Similar scattering features can also be found for the other samples with concentrations below 7 wt%. Additionally, another broad and weak peak (peak III) is observed in the lower qy region for the MNP-DBC hybrid films. This peak is attributed to the center-to-center spacing of MNP aggregates (DIII). The concentration increase results in the formation of large MNP aggregates with increasing spacing. Thus, a gradual shift of peak III to smaller qy values is observed. At higher MNP concentrations (7 wt% and 15 wt%), both peaks II and III become broader and weaker, and the second and third order peaks start to vanish. This is due to the fact that the order inside the films also decreases, which is 18 ACS Paragon Plus Environment

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observable by GISAXS. Since MNPs are preferentially dispersed inside the PS blocks, extremely high loading of MNPs leads to an excessive expansion of the PS domains, ending up with a perturbed and less-ordered inner structure. As seen already for the film surfaces with SEM and AFM also inside the films the well-ordered microphase separation structure is destroyed. Moreover, a further peak (peak I) arises with intensity getting stronger as the MNP concentration increases. The position of peak I shows no dependence on the MNP concentration, and a center-to-center distance of (10.0 ± 0.6) nm is extracted from the modeling, which matches the diameter of the MNPs. Combined with the observations in AFM and SEM data, in which large MNPs aggregates are present on the film surfaces at high MNP concentrations, we can explain peak I as structure factor of the closely packed MNPs. Similar phenomenon was also observed in previous investigations on MNP containing hybrid films.26, 52

Values of DII and DIII are extracted from the fitting and plotted as a function of MNP concentration in Figures 6b and 6c. The inter-aggregate spacing DIII increases from (58 ± 5) nm to (214 ± 20) nm with a broad distribution, suggesting a random MNP aggregate distribution having increasing distances by adding more MNPs to the aggregates. Interestingly, the interdomain spacing DII displays a similar trend as observed in the film thickness. It increases strongly with slight loading of MNPs, but stays almost constant at high MNP loading. Such behavior can be explained by a different localization of the MNPs at different concentrations. At lower concentrations, the MNPs selectively disperse well inside the PS domains, resulting in a domain expansion. At high concentrations, the PS domains are saturated, and most of the excess MNPs assemble in aggregates on the film surface. A sort of overloading with MNPs causes the formation of ill-defined structures inside the films. 19 ACS Paragon Plus Environment


Since adding MNPs can improve the order of the hybrid films to some extent, it is interesting to determine the concentration, at which the highest order is achieved. Accordingly, the values of full width at half maximum of peak II are plotted in Figure S6 as a function of MNP concentration. A minimum value is reached between 1 and 3 wt%, showing the optimal concentration from the present experiment. Referring to the observations from the SEM and AFM data, the PS domains are supposed to be saturated with MNPs of 1 wt%. Further loading with MNP causes formation of MNP aggregates. 3.3.2 Vertical nanostructure. To investigate how the vertical (with respect to the substrate) nanostructure is influenced by the incorporation of MNPs, off-detector cuts are made at the position of the microphaseseparation structure (qy = 0.18 nm-1, shown as the red box in Figure 5b). To visualize the structure information, the off-detector cuts are only shown in the most interesting qz range (0.4 nm-1 – 0.9 mn-1) in Figure 7a.


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Figure 7. (a) Off-detector cuts of 2D GISAXS data at qy = 0.18 nm-1. The cuts are only shown in the most interesting qz range, and shifted along the vertical direction for clarity with increasing MNP concentration (from bottom to top: 0 wt%, 0.5 wt %, 1 wt %, 3 wt %, 7 wt % and 15 wt %). Intensity maxima are highlighted as maximum I and II and marked by the solid and dashed red arrows, respectively. (b) Correlated thickness extracted from the intensity modulation dc based on the Bragg equation (blue up-triangles) and dw (blue downtriangles) based on the modified Bragg equation for waveguides, respectively. The measured thickness dm (black squares) is shown for comparison. The red line is plotted as guide to the eyes.

In the off-detector cuts intensity modulations are seen (marked as maximum I and II in Figure 7), which result from resonant diffuse scattering due to a special case of interface correlation.53 While in the MNP-free DBC film the intensity modulation is weak, a slight addition of MNP (< 7 wt%) makes the intensity modulation more pronounced, suggesting the presence of an enhanced long-range interface correlation in these MNP-DBC hybrid films. Further incorporation of MNPs smoothens the intensity oscillation gradually, which results from a rougher film surface with the presence of large-sized MNP clusters being uncorrelated.54 Based on the distance of the adjacent peak positions (maximum I and II shown in Figure 7a), the correlated thickness, meaning the long-range correlation length in the vertical direction, can be calculated. Literature showed that both, simple roughness correlation and waveguide-like behavior can occur in nanocomposite thin films.55 In case of simple roughness correlation, where the resonant diffuse scattering is caused by the coherence of the diffusely scattering intensity, the correlated thickness dc can be determined from an onedimensional Bragg equation56

𝑑𝑐 ≃

2𝜋 ∆𝑞𝑧



In case of waveguide-like behavior, where the interference fringes result from the refraction of the incident waves, the correlated thickness dw can be calculated via the modified Bragg equation55, 57

𝑑𝑤 ≃

𝜋 ∆𝑞𝑧

Figure 7b shows the correlated thickness calculated with the two different equations as mentioned above. The dc for the pure film is ~ 68 nm, while the dw is ~ 34 nm. Since the thickness of the pure film is determined as (35.5 ± 1.7) nm by profilometry, a correlated interface distance of 68 nm is not possible. Consequently, the simple roughness correlation can be excluded, and the waveguide effect needs to be considered in the thin pure DBC film. This reveals that the thin pure DBC film seems to act as a waveguide in the X-ray scattering measurement. In other words, the incident X-ray wave can be guided to the detector through the film body constrained by the top surface and the substrate. At low MNP concentrations (< 7 wt%), neither of the two correlated thickness values (dc, dw) matches the measured film thickness (dm). Therefore, no simple explanation can be used for clarification. Interestingly, at high concentrations (7 wt% and 15 wt %), the correlated thickness (dc) matches well with the measured film thickness (dm), resulting from the swelling of films caused by MNP incorporation. This suggests that a simple roughness correlation is present for the hybrid films with high MNP loading. 3.4 Magnetic properties The magnetic properties of the thin magnetic films are probed with a superconducting quantum interference device (SQUID). The magnetization as a function of the external


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magnetic field is collected at different temperatures ranging from 2 K to 200 K. To detail the magnetic properties, the influence of temperature and MNP concentrations are addressed. 3.4.1 Influence of temperature Figure 8 shows the data collected at different temperatures for the film containing 1 wt% MNPs. At low temperatures such as 2 K, the hybrid film shows a magnetic hysteresis (Figure 8a). As temperature increases, the hysteresis loop gradually narrows and tends to disappear, which is typical for superparamagnetic behavior.58 Literature show that a single-domain state of the lowest energy configuration is present for MNPs with the diameter below the critical size.59 The magnetic behavior of such MNPs relies on the intrinsic relaxation time of system (τ), the time scale of the applied experimental technique (τe) and the associated energy barrier (∆E).60 In case of τe » τ, MNPs are in the so-called superparamagnetic state, in which no spontaneous magnetization behavior is achieved. In case of τ » τe, magnetic moments of particles are frozen showing a blocked state. The critical diameter of superparamagnetic nanoparticles (DSPM) at a certain temperature (T) can be estimated by:60

𝐷 𝑆𝑃𝑀 ≃ (

150 𝑘𝐵𝑇 ) 𝜋 𝐾

where K denotes the anisotropy, and kB the Boltzmann constant.



Figure 8. Hybrid film with 1 wt% MNPs: (a) Magnetization (M) as a function of the external magnetic field (B) measured at different temperatures. The inset at the bottom right shows the zoom-in area focusing on the hysteresis. (b) Remanence Mr and susceptibility χ extracted from the magnetization curves as a function of temperature. The susceptibility χ is obtained by applying a linear fit at the zero-remanence position. All solid lines are guides to the eyes.

To further interpret the data, remanence Mr and susceptibility χ, following reference28, are extracted from the magnetization curves. The results are plotted as a function of temperature in Figure 8b. Both remanence and susceptibility show a strong temperature dependence with different evolution behavior. With increasing temperature, the remanence initially decreases slightly, but drops progressively afterwards. The susceptibility, which represents the response of magnetic film to external field, shows an increase behavior. At high temperatures, the magnetic film can respond quickly to the external magnetic field. For superparamagnetic materials, there exists a critical temperature, which is called blocking temperature (TB). Below this temperature, magnetic moments are frozen, shown as the hysteresis in the magnetization curves. Above TB, magnetic moments acquire enough energy to flip freely, and a high responsive speed is consequently obtained. Generally, TB can be expressed via the integrated Néel-Brown formula:61 𝜏 = 𝜏0 exp

( ) 𝐾𝑉 𝑘𝐵𝑇

in which τ represents the time for relaxation, τ0 shows the inverse attempt frequency, and KV the activation energy. Since the easy magnetization axes distribute randomly in the present hybrid system, TB could be expressed via the Bean and Livingston model: 62

[ ( )]

2𝐾 𝑇 𝐵𝐶 = 0.48 1― 𝜇0𝑀𝑆 𝑇𝐵

𝑏


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in which BC and MS represent the coercivity and magnetization saturation, respectively. The exponent b equals 0.77 in our system, where particles are oriented in a random way.62 Based on the above equation, therefore, TB can be obtained via fitting the coercivity values extracted from the magnetization curves collected at various temperatures. Figure 9 displays the fitting results. A blocking temperature of (121.6 ± 29.5) K is obtained, which keeps consistent with our previous investigation, in which identical MNPs were employed.25

Figure 9. Coercivity extracted from corresponding magnetization data plotted as a function of temperature. The red line is the fitting via the model shown at the top right.

3.4.2 Influence of MNP concentration. To probe the influence of MNP concentration on the thin films’ magnetic behavior, all films were probed at 2 K and 20 K for comparison. Figure 10a shows the magnetization curves of all hybrid films collected at 2 K. Hysteresis loop exists in all M/B curves due to the low probing temperature (< TB). The frozen magnetic moments result in the observed ferromagnetic behavior. To detail the analysis, values of remanence (Mr), saturation (Ms) and the relative remanence (Mr / Ms) at 2 K and 20 K are extracted from curves in Figure 10a, and plotted in Figure 10b as a function of MNP concentration.



Generally, as concentration increases, hybrid films behave in a similar way at both temperatures. Both remanence and saturation increase with increasing concentration, which is due to more freely flipping magnetic moments contributed by increasing number of MNPs. At higher temperature (T = 20 K), a smaller remanence is observed as compared with the measurement at 2K for the same film. This results from the higher thermal energy of the MNPs at high temperature. The same saturation magnetization is obtained at both temperatures, since identical film was probed at respective concentration. The relative remanence Mr/Ms, which can be regarded as the data normalized to particle concentration, is found to be constant and independent of the MNP concentration. One can conclude that no interactions among MNPs are present to influence the magnetic properties.

Figure 10. (a) Magnetic moments versus external magnetic field at 2 K for samples with different MNP concentration. The inset presents the partially magnified hysteresis loops at zero field. (b) The influence of MNP concentration on the remanence, Mr, saturation magnetization, Ms, and relative remanence, Mr / Ms, at 2 K (red) and 20 K (blue). The lines are guides to the eye.

3.4.2 Influence of magnetic structure density.


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Figure 11. Comparison between the magnetic susceptibility of two different films. Blue circles: thin film with 1 wt % of MNPs (present work). Black rectangles: thick film with 2 wt% of MNPs (previous work).25 The data are normalized to the MNP weight ratio and the film thickness. Two solid lines are used to guide the eyes.

To address the benefit of the high magnetic structure density, a comparison of the magnetic susceptibility is made between the present thin film and the thicker films (~ 170 nm) studied in our previous work.25 Identical magnetite NPs were used in the previous investigation, but the MNPs were dispersed inside the minor domains (volume fraction: 0.34). It is worthy to note that the minor domains were saturated with MNPs at 2 wt% and maintained a high structural order.25 Therefore, it is meaningful to compare the two highly ordered MNPsaturated films, namely the thin film containing 1 wt% of MNPs in the present study and the thick film with 2 wt% of MNPs in previous study. The susceptibility (normalized to the film thickness and the MNP weight ratio) of the two films obtained at different temperatures are summarized in Figure 11. At low temperatures, similar values are found for both films, since most of the magnetic moments are frozen below the blocking temperature. As temperature increases, a faster response to the external magnetic field is observed in the thin film than the thicker one. At higher temperatures, the magnetic moments are unblocked and can flip freely. At the same time, the dense magnetic structure contributes more free moment. Therefore, the



normalized number of free magnetic moments in the thin film is larger than that in the thick one at high temperatures, which explains the faster responsive behavior.

4. CONCLUSION In this work, thin films with dense magnetic structures are successfully fabricated using a printing technique. By using the DBC PS-b-PMMA with a large PS volume fraction (0.685), a cylindrically structured template is achieved. Through solvent vapor annealing, a parallel alignment of the PMMA cylinders in the PS matrix is obtained (with respect to the substrate). Since the used MNPs are selective to the PS domains, the fabricated PS-dominated template offers a high capacity to host MNPs. The surface nanostructure is examined by the real-space techniques AFM and SEM as a function of the MNP concentration. As a beneficial effect, upon slight MNPs incorporation, the structural order is significantly enhanced. Further MNP loading leads to the formation of large MNP aggregates located on the film surface. The cylindrical structure is then perturbed and ends up in a disordered ill-defined morphology. The inner structure of thin films is probed with reciprocal-space technique GISAXS, by which the entire thin film can be penetrated by X-rays. Scattering data further proves the order improvement contributed by gentle MNPs loading throughout the thin film. 1 wt% is found to be the critical concentration where the highest structural order is obtained in the hybrid film. From both real and reciprocal-space techniques, the PS domains are evidenced to be saturated with MNPs at 1 wt% with only few aggregates present on the film surface. Therefore, a highly ordered thin film with dense magnetic structure capped by polymer materials is achieved. All hybrid films are studied with SQUID for magnetic property investigation. Results show that all the MNPs-DBC films exhibit a superparamagnetic behavior. Compared


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with our previous work, where identical MNPs were used, the thin films show faster responsive behavior due to the high density of magnetic structure.

AUTHOR INFORMATION Supporting Information Characterization techniques (OM, Profilometry, AFM, SEM, GISAXS and SQUID), 1H-NMR spectrum of PS-b-PMMA, SEM image proving the cylindrical morphology, SEM image showing the localization of MNPs inside PS domains, intensity increment of the first and second order peak, Rrms of all films, and the FWHM as a function of MNP concentration.

Corresponding Author * Email: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by funding from the International Research Training Groups 2022 Alberta/Technical University of Munich International Graduate School for Environmentally Responsible Functional Hybrid Materials (ATUMS), the Excellence Cluster Nanosystems Initiative Munich (NIM), and the Center for NanoScience (CeNS). S. X., L. S. and W. C. acknowledge the China Scholarship Council (CSC). We thank Professor Alexander Holleitner and Peter Weiser for the possibility to carry out SEM and AFM measurements. Parts of this



research were carried out at the light source PETRA III at DESY, a member of the Helmholtz Association (HGF).

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Printed Thin Diblock Copolymer Films with Dense Magnetic

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