Shape Induced Symmetry in Self-Assembled Mesocrystals of Iron Oxide Nanocubes

Abstract

Japan Synchrotron Radiation Research Institute (JASRI), Spring-8, Sayo Hyogo 679-5198, Japan b S Supporting Information S elf-assembly of molecules and nanoparticles into tailored structures is a promising strategy for production and design of materials with new functions. [1][2][3][4] The spontaneous organization of the nanoscale building blocks into periodically packed structures is controlled by thermodynamic constraints and specific boundary conditions resulting in systems that commonly span a rich energy landscape.2,5 Modulation of the particle interactions has a pivotal influence on the formed structures and the quality of nanocrystal mesocrystals. [6][7][8] Tuning the magnitude and directionality of the interparticle forces between, e.g., dipolar nanoparticles by applying electrical or magnetic fields may also have a profound effect on the assembly process and the resulting structures of the nanoparticle mesocrystals. 9-13The early work on self-assembly of nanoparticles focused on spheres, with hexagonally packed arrays being produced from monodisperse spheres 14,15 and more recently nanoparticle arrays with complex structures by mixing monodisperse spheres of different sizes. [16][17][18][19][20] Much less experimental work has been reported on the design of suitable conditions for formation of ordered arrays and full characterization of the (three-dimensional) structures formed from nonspherical objects. 21-23Here, we show how superparamagnetic iron oxide nanocubes with a well-defined shape and size can be assembled into wellordered arrays and how the three-dimensional structure can be determined by a combination of grazing incidence small-angle scattering (GISAXS) and electron microscopy. We demonstrate how a small deviation from a perfect cubic geometry of the nanoparticle building blocks can dictate the symmetry of the array and show for the first time that nonspherical nanoparticles can assemble into a highly ordered body-centered tetragonal structure. Energy models that account for the directionality and magnitude of the van der Waals and dipolar interactions as a function of the degree of truncation of the nanocubes were used to evaluate the importance of the directional dipolar forces in the self-assembly process and to predict the occurrence of simple cubic and body-centered tetragonal mesocrystal structures.Iron oxide nanocrystals have attracted much interest for a range of applications because of the combination of tunable magnetic properties and controllable surface properties. 24-28The oleic acid capped nanocrystals were prepared by a modified nonhydrolytic synthesis approach, 9,25 purified into a nanoparticle paste, and dispersed in toluene to a concentration of about 10 14 particles/mL (see Supporting Information for details). Germanium wafers were cleaned by sonication in toluene, followed by ethyl acetate and ethanol (30 min each). Fast evaporation of the carrier solvent was achieved by spin-coating an excess amount of toluene dispersion, at 1000 rpm for 20 s. Evaporation was slowed down...

Magnetic field induced controllable self-assembly of maghemite nanocrystals: From 3D arrays to 1D nanochains

“…Under an applied magnetic field of 0.2-0.3 T, rice-like nanocrystals were initially formed, and large isotropic van der Waals interactions firstly tended to minimize the area of the rice-like nanocrystals, leading to the formation of walnut-like superstructures [31], as was the case with the formation of walnut-like aggregates in the presence of zero magnetic field. In the following stage, the walnut-like superstructures linearly assembled together to form chain-like structures due to the strong magnetic dipole-dipole interactions.…”

Magnetic Nanoparticles: A Subject for Both Fundamental Research and Applications

“…(i) "Drop-casting, " namely, applying a droplet of the MNP dispersion onto a substrate and letting the solvent evaporate [68]. This method often produces very thick 3-dimensional superlattices of relatively high quality.…”

“…By such methods, it is possible to fabricate MNP films of excellent order extending over several micrometers and even involving particles of two or three different sizes (see, e.g., Figure 8). The magnetic properties of such MNP superlattices have been in the focus of many current studies [27,56,68,[72][73][74][75][76]. In most cases, the collective magnetic behavior of the superlattice-being a consequence of dipolar interactionsis intensely investigated.…”

Remanence Plots as a Probe of Spin Disorder in Magnetic Nanoparticles

“…[11][12][13][14][15][16] In fact, interactions are the basis of a large number of nanoparticle-based magnetic materials, e.g., superferromagnets, superspin glasses, artificial spin ice, long range self-assemblies, or ferrofluids. 15,[17][18][19][20][21] Given the crucial importance of interactions in magnetic nanostructures, many direct and indirect approaches have been used to try to quantify them: first order reversal curve (FORC) analysis, 22,23 small angle neutron scattering, SANS, [24][25][26][27] electron holography, 28,29 magnetic force microscopy, 30,31 Lorentz microscopy, 32 Brillouin light scattering, 33 resonant magnetic x-ray scattering 34 and so on. However, one of the most accepted methods to assess interactions is the remanence plots technique (i.e., Henkel or δM plots), [35][36][37] which is routinely used to evaluate interactions between nanoparticles or grains [38][39][40][41][42][43][44][45][46][47][48][49][50][51][52]…”