Tuning Plasmon Resonance in Magnetoplasmonic Nanochains by Controlling Polarization and Interparticle Distance for Simple Preparation of Optical Filters Y. Song, † V. T. Tran, † and J. Lee* Department of Cogno-Mechatronics Engineering, Pusan National University, Busan 46241, Republic of Korea * S Supporting Information ABSTRACT: Magnetoplasmonic Fe 3 O 4 -coated Ag nanoparticles (NPs) are assembled in large scale (18 × 18 mm 2 ) in order to observe unique modulation of plasmonic coupling and optical tunable application via both external magnetic field and the combination of magnetic dipole and electrostatic interactions of particle-particle and particle-substrate. These large nanochains film exhibits outstanding tunability of plasmonic resonance from visible to near-infrared range by controlling the polarization angle and interparticle distance (IPD). The enormous spectral shift mainly originated from far-field rather than near-field coupling of Ag cores because of the sufficiently large separation between them in which Fe 3 O 4 shell acts as spacer. This tunable magnetoplasmonic film can be applicable in the field of anisotropic optical waveguides, tunable optical filter, and nanoscale sensing platform. KEYWORDS: magnetoplasmonic, plasmon resonance, nanochain, magnetic field assembly, anisotropic optics N anoscale assembly using magnetoplasmonic nanoparticles (NPs) has become a field of increasing interest because of the easy spatial control of NPs by external magnetic fields as well as facile modulation of plasmonic properties. In particular, one-dimensional (1D) nanochain array in a designated macroscale area has shown promise for a variety of potential applications like anisotropic plasmonic waveguides, 1,2 electrical devices, 3,4 and biomedical sensors, 5 because they exhibit unique optical properties, which is attributed to their anisotropic nature, compared to spherical structures that have one typical plasmon mode. 6,7 To manipulate the plasmon coupling not only for magnetoplasmonic but also conventional plasmonic nanochains such as silver or gold, interparticle distance (IPD) between NPs is a crucial parameter that can be modified by controlling inorganic shell thickness or organic surfactants. 5,8,9 For example, when two single NPs are in close proximity, their plasmon resonance splits into two sets of dipolar modes with regard to the direction of the polarization: i.e., a longitudinal mode (polarized light parallel to the long axis of a dimer) and a transversal mode (polarized light perpendicular to the long axis of a dimer). 10 On the basis of these dipole interactions of the dimer, single nanochain multipoles have been recently studied by single particle spectroscopy. 11 In addition to the single- particle-level studies, attempts to scale up have been made by introducing various assembly methods to apply magneto- plasmonic nanostructures to optical devices. Generally, many approaches using top-down lithographs have been developed, but still require complex processes and the use of costly equipment. 12-14 On the other hand, various bottom-up approaches have been introduced to overcome these issues. In particular, a drop-dry method with an external magnetic field taking advantage of the magnetic properties of magneto- plasmonic nanoparticles has been reported. 15-17 However, control over the assembly of NPs into predefined super- structure using the drying-mediated assembly is quite difficult. In a previous study, we clarified the mechanism of drying- mediated and magnetic field-induced Fe 3 O 4 @Au nanochains which is not linear but composed of aggregated NP with length up to hundreds of microns because magnetic moment of the individual NP is too weak to produce significant dipolar interactions against thermal energy. 16 Therefore, to minimize the effect of thermal dynamics during assembly, a new approach for the magnetic field-induced assembly is required. Much of the work on magnetoplasmonic nanochains has been focused on plasmonic shells. 15,17 This is because plasmonic NPs located outside can form a strong electric field and can potentially be applied to various optical devices. However, since the magnetic NPs are located inside of these structures, their magnetic properties can be deteriorated by the shells, and as a result, it is difficult to finely control the NP assembly using magnetic fields, making it difficult to approach detailed optical studies according to various and distinct structural conditions such as the linearity, the orientation and the IPD of nanochains. In this paper, Ag@Fe 3 O 4 magnetoplasmonic NPs were utilized to assemble a linear nanochain array in macroscopic Received: May 17, 2017 Accepted: July 11, 2017 Published: July 11, 2017 Letter www.acsami.org © XXXX American Chemical Society A DOI: 10.1021/acsami.7b06977 ACS Appl. Mater. Interfaces XXXX, XXX, XXX-XXX