Langmuir Layers of Magnetic Nanoparticles
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Langmuir Layers of Magnetic Nanoparticles Sara A. Majetich, Madhur Sachan, Shihai Kan, Yuhang Cheng, and Julie Gardener Department of Physics, Carnegie Mellon University, Pittsburgh, PA 15213-3890, U.S.A. ABSTRACT Methods to form magnetic nanoparticle monolayers using non-aqueous Langmuir layers are reported. Following a discussion of the driving forces in various self-assembly techniques, we describe how aqueous Langmuir layers can be modified for use in conjunction with oxidationsensitive nanoparticles. Monolayers are formed using Fe and ε–Co nanoparticles, and transferred to carbon-coated transmission electron microscopy grids using the Langmuir-Schaefer method.
INTRODUCTION When a dispersion of monodisperse, surfactant-coated nanoparticles is evaporated on a solid, hydrophobic substrate, the particles tend to pack into arrays (Figure 1). In the dispersion, the particles interact through van der Waals attraction and steric repulsion. When the drying front passes through, capillary forces and disjoining pressures drive the packing of the particles into arrays [1-5]. In addition, entropic forces favor the ordering of hard spheres when the volume packing fraction exceeds 0.49 [6, 7]. Self-assembly is a fault-tolerant process, so that ordering can be maintained around defects, such as vacancies and particles with irregular shapes. The mobility of the particles is critical to ordering. The surfactant tails help to prevent flocculation, even though the particle cores are far denser than the surrounding solvent. If the substrate is hydrophilic, the solvent will not spread evenly, and multilayered arrays will be favored. Even on a hydrophobic surface, once the array has dried, the alkane chains of surfactant molecules on adjacent particles are entangled [8]. This entanglement prevents the particles from moving to lower energy positions, and so defects introduced during self-assembly are locked in place. We seek to extend the length scale of the ordering in nanoparticle arrays from the nanoscale to the microscale and beyond.
Figure 1. Array of 9 nm ε–Co nanoparticles. Note the ordering around the vacancy near the bottom of the array, and the presence of smaller particles near the edges.
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It is possible to form self-assembled arrays by other methods in which the particles retain greater mobility. One of the most intriguing is colloidal crystallization through the gradual addition of a poorly coordinating solvent [9]. [This technique has been applied to grow threedimensional nanoparticle crystals that have smaller separations than in most evaporated arrays, and therefore stronger magnetostatic interactions [10, 11]. However, the nanoparticle crystals typically contain multiple grains. The size of the ordered region is currently determined by the size that the nanoparticle assembly reaches prior to sedimentation. In aqueous dispersions, AC electrophoretic deposition has been used to from ordered Au nanoparticle monolayers by modulating the forces on the electrostatic double layer surrounding the particles
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