Magnetic Particle Self-Assembly at Functionalized Interfaces
Apurve Saini, Katharina Theis-Brohl, Alexandros Koutsioubas, Kathryn L. Krycka, Julie Borchers, Max Wolff
We study the assembly of magnetite nanoparticles in water-based ferrofluids in wetting layers close to silicon substrates with different functionalization without and with an out-of-plane magnetic field. For particles of nominal sizes 5 nm, 15 nm and 25 nm, we extract density profiles form neutron reflectivity measurement. We show that self-assembly is only promoted by a magnetic field if a seed layer is formed at the silicon substrate. Such a layer can be formed by chemisorption of activated N-hydroxysuccinimide ester-coated nanoparticles at an (3-aminopropyl)triethoxy silane functionalized surface. Less dense packing is reported for physisorbtion of the same particles at a piranha treated (strongly hydrophilic) silicon wafer and no wetting layer is found for a self-assembled monolayer of octadecyltrichlorosilane (strongly hydrophobic) at the interface. We show that once the seed layer is formed and under an out-of-plane magnetic field further wetting layers assemble. These layers become denser with time, larger magnetic fields, higher particle concentrations and larger moment of the nanoparticles.
Magnetic nanoparticles, Self-assembly, Dipole interactions, Polarized neutron reflectivity, SANS