Date of Degree

2-2014

Document Type

Dissertation

Degree Name

Ph.D.

Program

Chemistry

Advisor

Ilona Kretzschmar

Subject Categories

Chemical Engineering | Mechanics of Materials | Nanoscience and Nanotechnology

Keywords

directed assembly, iron oxide, Janus particle, magnetic thin film, micro-magnetometer, micro-viscometer

Abstract

Magnetic properties are important since they enable the manipulation of particle behavior remotely and therefore provide the means to direct a particle’s orientation and translation. Magnetic Janus particles combine magnetic properties with anisotropy and thus are potential building blocks for complex structures that can be assembled from a particle suspension and can be directed through external fields. In this thesis, a method for the fabrication of three types of magnetic Janus particles with distinct magnetic properties is introduced, the assembly behavior of magnetic Janus particles in external magnetic and electric fields is systematically studied, and two potential applications of magnetic Janus particles are successfully tested.

Janus particles with different magnetic properties are fabricated by varying the deposition rate of iron in an Ar/O2 atmosphere using physical vapor deposition (PVD). The extent of oxidation for each type of iron oxide is precisely controlled by the time it is exposed to the Ar/O2 atmosphere during deposition. Two of the three magnetic Janus particles produced show distinct assembly behavior into staggered and double chain structures, whereas the third shows no assembly behavior under an external magnetic field. The effect of the iron oxide cap thickness (≤ 50 nm) on the Janus particle assembly behavior is studied resulting in a deposition rate diagram that shows the relationship between the assembly behavior and the deposition rate. The cap materials for staggered chain, double chain, and no assembly behavior are assigned as Fe1-xO, Fe3O4, and Fe2O3, respectively, based on optical appearance and physical properties. The assignment is further confirmed by in-depth material characterization with scanning and transmission electron microscopy, atomic force microscopy, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy. The magnetic hardness of the iron oxides is tested using the magneto-optic Kerr effect.

The assembly behavior of Fe3O4-capped Janus particles is studied in overlapping parallel and perpendicular AC electric and magnetic fields. The chains formed by Fe3O4-capped magnetic Janus particles show contraction behavior of ~30%, which suggests their application as an in situ viscometer. The chain contraction rate is found to depend on the viscosity of the liquid as well as the size of Janus particles and an in situ microviscometer is realized. Further, the magnetic dipole-dipole interactions of Fe1-xO and Fe3O4-capped Janus particles are studied by analyzing the particle-particle interaction force and energy during the process of Janus particle doublet formation. Using the magnetic particle interaction energy, the magnetization of each iron oxide cap is determined and found to be in excellent agreement with magnetization values obtained using standard SQUID measurements suggesting the application of magnetic Janus particles as a micro-magnetometer.

In summary, three types of magnetic Janus particles with distinct magnetic properties have been fabricated and show versatile assembly behaviors that make them useful basic building blocks for complex structures and applications. For example, magnetic Janus particles can be used to measure the viscosity of a fluid or the magnetic property of a thin film cap material. It is likely that other interesting applications will emerge, when Janus particles of various sizes and/or patchy particles with magnetic properties are combined and explored.

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