Self-Assembled Layering of Magnetic Nanoparticles in a Ferrofluid on Silicon Surfaces Katharina Theis-Brö hl,* ,† Erika C. Vreeland, ‡,§ Andrew Gomez, ‡ Dale L. Huber, ‡ Apurve Saini, ∥ Max Wolff, ∥ Brian B. Maranville, ⊥ Erik Brok, ⊥,#,¶ Kathryn L. Krycka, ⊥ Joseph A. Dura, ⊥ and Julie A. Borchers ⊥ † University of Applied Sciences, An der Karlstadt 8, 27568 Bremerhaven, Germany ‡ Sandia National Laboratories, Albuquerque, New Mexico 87185, United States § Imagion Biosystems LLC, Albuquerque, New Mexico 87106, United States ∥ Division for Materials Physics, Uppsala University, 75120 Uppsala, Sweden ⊥ NIST Center for Neutron Research, 100 Bureau Drive, Gaithersburg 20899-6102, United States # Department of Materials Science and Engineering, University of Maryland, College Park, Maryland 20742, United States * S Supporting Information ABSTRACT: This article describes the three-dimensional self- assembly of monodisperse colloidal magnetite nanoparticles (NPs) from a dilute water-based ferrofluid onto a silicon surface and the dependence of the resultant magnetic structure on the applied field. The NPs assemble into close-packed layers on the surface followed by more loosely packed ones. The magnetic field-dependent magnet- ization of the individual NP layers depends on both the rotational freedom of the layer and the magnetization of the adjacent layers. For layers in which the NPs are more free to rotate, the easy axis of the NP can readily orient along the field direction. In more dense packing, free rotation of the NPs is hampered, and the NP ensembles likely build up quasi-domain states to minimize energy, which leads to lower magnetization in those layers. Detailed analysis of polarized neutron reflectometry data together with model calculations of the arrange- ment of the NPs within the layers and input from small-angle scattering measurements provide full characterization of the core/ shell NP dimensions, degree of chaining, arrangement of the NPs within the different layers, and magnetization depth profile. KEYWORDS: magnetite nanoparticles, core/shell nanoparticles, ferrofluid, polarized neutron reflectometry, self-assembly, 3D self-ordering, quasidomains ■ INTRODUCTION Advances in the synthesis of well-defined nanoparticles (NPs) have opened up opportunities for their application in various fields. 1−5 Using the properties of small structures as individual objects is one important aspect of nanotechnology. There is, however, also a high interest in ensembles of NPs to use their collective behavior in functional devices. Ensembles of NPs can have properties that differ from those of individual particles as well as from those of the bulk. 6 Potential applications include the improvement of the mechanical properties of materials 7 or the introduction of new electronic, 8 magnetic, 9−11 photonic, 12 or optical functionalities. 8 Of particular interest are magnetite (Fe 3 O 4 ) NPs developed and engineered for potential biomedical applications (e.g., superparamagnetic relaxometry (SPMR), 13−15 magnetic particle imaging, 16−18 and magnetic hyperthermia 19,20 ) because of their low toxicity, strong response to magnetic fields, and superparamagnetic relaxation. The key to well-ordered NP ensembles is self-assembly. Self- assembly is a low-cost method that provides controllable, simple mechanisms for the arrangement of the NPs into ordered structures, which can be achieved either through the direct interaction of the building blocks or by using a template or external field. 21−24 In the present work, we study the self- assembly of magnetite NPs in a ferrofluid (FF) adjacent to a silicon surface with an applied magnetic field. FFs are colloidal suspensions of magnetic NPs with typical sizes ranging from a few nanometers to several tens of nanometers. To prevent agglomeration, surfactants providing steric repulsion are attached to their surface. In solution, the NPs can form various heterogeneous structures such as linear chains, clusters, closed rings, and branched structures 25−27 depending on their Received: October 4, 2017 Accepted: January 4, 2018 Published: January 4, 2018 Research Article www.acsami.org Cite This: ACS Appl. Mater. Interfaces 2018, 10, 5050-5060 © 2018 American Chemical Society 5050 DOI: 10.1021/acsami.7b14849 ACS Appl. Mater. Interfaces 2018, 10, 5050−5060