Assembly Behavior of Iron Oxide-Capped Janus Particles in a Magnetic Field Bin Ren, Aleksey Ruditskiy, Jung Hun (Kevin) Song, and Ilona Kretzschmar* Department of Chemical Engineering, City College of New York, City University of New York, 140th St. and Convent Ave., New York, New York 10031, United States * S Supporting Information ABSTRACT: Three types of iron oxide Janus particles are obtained by varying the deposition rate of iron in a 3:1 Ar/O 2 atmosphere during physical vapor deposition. Each type of iron oxide Janus particle shows a distinct assembly behavior when an external magnetic field is applied, i.e., formation of staggered chains, double chains, or no assembly. A detailed deposition rate diagram is obtained to identify the relationship between deposition rate and assembly behavior. The extent of iron oxidation is identified as the key parameter in determining the assembly behavior. In addition, the effects of particle volume fraction, thickness of the iron oxide cap, and assembly time on the final assembly behavior are studied. Cap thickness is shown not to influence the assembly behavior, while particle volume fraction and assembly time affect the chain growth rate and the chain length, but not the overall assembly behavior. The samples are characterized by optical, scanning electron, and atomic force microscopies. 1. INTRODUCTION Recent studies of magnetic Janus particles have focused on the assembly behavior of the following three types of particles: 1 (i) particles with one magnetic hemisphere, 2-5 (ii) fluorescently labeled magnetic particles with a nonmagnetic cap, 6,7 and (iii) Janus particles with a magnetic cap. 8 These magnetic Janus particles are of interest for use in applications due to their addressability by external fields. For example, superparamag- netic hydrogel Janus particles with one magnetic hemisphere have been proposed as ideal candidates for building of three- dimensional hydrogel superstructures with chemically and magnetically tunable complexity for tissue engineering and sensing applications. 3 Magnetically modulated optical nanop- robes (MagMOONs) are fluorescently labeled magnetic particles coated with a nonmagnetic metal cap that emit a fluorescence signal. These MagMOONs can be used to measure chemical concentrations and probe local rheological properties 7 by “blinking” in response to rotating magnetic fields. The functionalization of MagMOONs has resulted in new optical, 9 magnetic, 7 chemical, 10-12 and electrical proper- ties. 10,13 More general, ferromagnetic micro- and nanoparticles have found application in display technology 14,15 and biosensing 4 and as ferro- 16 and magnetorheological fluids. 17 Linear and nonlinear rotations of driven magnetic micro- particles may lead to a new class of physiochemical micro- and nanoparticle sensors that may be of significant biomedical and technological importance. 18 Particles with one magnetic hemisphere are typically fabricated by oil-in-water emulsion, 2 microfluidic, 3 droplet templating, 19 and spin-coating methods. 5 Fluorescently labeled magnetic particles with a nonmagnetic cap and Janus particles with a magnetic cap have been fabricated by vapor deposition 10,13 or metal sputtering. 20,21 These Janus particles can be assembled into various structures under an external electric 1,3,8,22-25 or magnetic field. 2,3,8 Assembly structures reported for magnetic Janus particles have been staggered chains, 2 chainlike or meshlike superstructures, 3 and double and staggered chains. 8 More specifically, Smoukov et al. 8 reported the formation of staggered and double chains for Janus particles with 34 and 8 nm iron caps, respectively. Iron and its oxides are interesting materials because they possess magnetic and electric properties that are closely related to their degree of oxidation. This characteristic makes them viable for applications such as magnetic storage media and spin transport devices, 26 band gap engineering, 27 magneto-rheo- logical fluids, 28 and tunneling magneto-resistance devices. 29 Iron metal is a soft ferromagnetic material, γ-Fe 2 O 3 and Fe 3 O 4 are ferrimagnetic, and α-Fe 2 O 3 and stoichiometric FeO are antiferromagnetic materials. 30 Of these oxides, stoichiometric FeO is not stable at room temperature decomposing into Fe 3 O 4 and Fe below 570 °C. 31 However, stable nonstoichiometric Fe 1-x O has been reported. 27,32-34 Several studies have been performed with the goal of depositing iron oxides with a controlled Fe:O stoichiometric ratio using reactive sputtering in the presence of a 3:1 Ar/O 2 mixture, 30,32-36 reactive laser deposition in the presence of oxygen, 27 and gas-phase deposition. 37 Here, we report the use of physical vapor deposition (PVD) to deposit a thin iron oxide film onto a monolayer of polystyrene particles in order to fabricate Janus particles with a Received: October 11, 2011 Revised: December 9, 2011 Published: December 12, 2011 Article pubs.acs.org/Langmuir © 2011 American Chemical Society 1149 dx.doi.org/10.1021/la203969f | Langmuir 2012, 28, 1149-1156