Two-Stage Assembly of Mesocrystal Fibers with Tunable Diameters in Weak Magnetic Fields

Controlling the morphology and crystallographic coherence of assemblies of magnetic nanoparticles is a promising route to functional materials. Time-resolved small-angle X-ray scattering (SAXS) was combined with microscopy and scaling analysis to probe and analyze evaporation-induced assembly in levitating drops and thin films of superparamagnetic iron oxide nanocubes in weak magnetic fields. We show that assembly of micrometer-sized mesocrystals with a cubic shape preceded the formation of fibers with a high degree of crystallographic coherence and tunable diameters. The second-stage assembly of aligned cuboidal mesocrystals into fibers was driven by the magnetic field, but the first-stage assembly of the oleate-capped nanocubes was unaffected by weak magnetic fields. The transition from 3D growth of the primary mesocrystals to the second stage 1D assembly of the elongated fibers was related to the size and field dependence of isotropic van der Waals and directional dipolar interactions between the interacting mesocrystals.


Iron oxide nanocube synthesis
The detailed preparation of iron oxide NCs capped by oleic acid (OA) has been described previously. 1 In brief, iron oxide NCs with an edge length of 12.0 ± 0.9 nm (NC120) were synthesized by dissolving 10 mmol of an iron oleate precursor in 50 mL of a 1-octadecene (90%, Sigma-Aldrich) and 2.56 g eicosane mixture. 5 mmol sodium oleate (97%, TCI) and oleic acid (99% TCI) were added afterwards. The mixture was degassed for 30 min at 140 °C, then heated at 3 °C min −1 and refluxed at 325 °C for 25 min for the synthesis of NC120. After reflux, the mixture was cooled down to room temperature. The mother liquor was washed with a toluene/ethanol mixture and centrifuged several times to remove excess organics. The purified dispersion was dried under vacuum to yield a paste with an iron oxide content of 54 % for NC120. For the time-resolved self-assembly experiments, 5.54 mg (NC120) of the NC paste was redispersed in 0.75 mL toluene and 0.25 mL decane to obtain a dispersion with a concentration of 3 mg iron oxide mL −1 . Small amounts of oleic acid were added to the dispersions (1 mg OA mg paste −1 for NC120) and the dispersion was then sonicated for 30 min.

Time-resolved SAXS and video microscopy
We performed time-resolved X-ray scattering experiments at the P03 beamline at DESY, Hamburg, Germany. The dispersion droplet was illuminated with an X-ray beam with a spot size of 40 x 20 µm 2 , a wavelength λ = 0.96 Å, and an exposure time of 0.5 seconds per frame which resulted in a time-resolution of about 0.9 seconds per frame.
The scattered X-rays were detected by a Pilatus 1M detector covering a range of 0.11 < q < 3.59 nm -1 . The 2D data was reduced and radially and azimuthally integrated to a 1D pattern using the program DPDAK. We placed 2-3 µL of a colloidal dispersion in an acoustic levitator (model 13K11, tec5, Oberursel, Germany) using microliter syringes (Hamilton company, USA). A Helmholtz coil with a radius of 75 mm was installed to produce a weak magnetic field of maximal 6 mT, which was vertically oriented. The magnetic field strength was controlled from outside the experimental station, and the maximal field strength was obtained using a voltage of 3 V and a current of about 7 A.
The size of the shrinking droplet was observed with a microscope camera. The videos were processed with VirtualDub to obtain image frames, which were subsequently analyzed with ImageJ to obtain the radii a and c of the ellipsoidal droplets. We calculated the droplet volume by which was then used to

Optical microscopy
We drop-cast 1 µL of a NC120 dispersion onto a 2.5 2.5 mm 2 silicon wafer which × was positioned in the centre of a petri dish and added a 30 µL reservoir of toluene in the vicinity of the substrate to saturate the environment with toluene vapour, which decreased the drying rate of the dispersion droplet. The petri dish was placed in the centre of the Helmholtz magnet, so that the magnetic field was oriented parallel to the substrate surface. The droplet was slowly dried within 30-40 minutes and simultaneously observed by optical microscopy. Images were taken every 0.5 seconds, the Video S1 has a frame rate of 10 frames per second.

Scanning electron microscopy (SEM)
SEM images were taken using a JEOL JSM-7000F (JEOL, U acc = 15 kV, WD = 10 mm) equipped with a Schottky-type field emission gun (FEG). We removed organic residues from the formed mesocrystals by heating the sample in a tube furnace at 2 °C min −1 to 500 °C for 2 h under argon atmosphere.

Transmission electron microscopy (TEM)
TEM micrographs were recorded with a JEOL JEM-2100F equipped with a Schottkytype FEG (Point resolution: 0.19 nm, spherical aberration C s = 0.5 mm) and operated at an accelerating voltage of 200 kV.

Physical properties measurement system/vibrating sample magnetometer (PPMS/VSM)
The magnetic moments of the nanocubes and the assembled mesocrystals were measured as a function of an applied DC magnetic field using the VSM option of a PPMS (Quantum Design, USA). The magnetic properties of dispersed nanoparticles were measured on a diluted dispersion of NC120 in paraffin wax (0.1 wt%). The mesocrystals MC NC120 used for the measurements were grown on a 2.5 2.5 mm 2 Si × wafer using an initial concentration of 5 mg mL −1 . The mesocrystal fibers MCF NC120 used for the measurements were grown on a 2.5 2.5 mm 2 Si wafer in a 6 mT × magnetic field parallel to wafer surface using an initial concentration of 5 mg mL −1 . The magnetic properties were measured parallel and perpendicular to the long axis of the fibers.

Dynamic Light Scattering (DLS) and diffusion length calculation
We conducted DLS measurements on a diluted NC120 dispersion (0.03 mg mL −1 ) in decane. We obtained a diffusion coefficient of D NC120 = 25.6 µm 2 s −1 for NC120 and calculated the diffusion length L of the NCs during the duration of the first assembly stage (t -t MC 19 s) by: We obtained a diffusion length of 132 µm. From the critical NC120 volume fraction 0.045 we estimated the edge length of a cuboidal mesocrystal a MCNC120 that ϕNC,crit = contains all NC120 present in spherical volume with a radius equal to L using the following relation:      Gaussian fits (solid lines) gave the mean fiber widths < w F,SEM > and standard deviations for fibers grown at 1.5 mT (red), 3 mT (blue), and 6 mT (orange). Magnified SEM image of (a). c) SEM image of cuboidal mesocrystals grown on a substrate in the absence of a magnetic field.