Letter www.acsami.org
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 Fe3O4-coated Ag nanoparticles (NPs) are assembled in large scale (18 × 18 mm2) 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 Fe3O4 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 In particular, a drop-dry method with an external magnetic field taking advantage of the magnetic properties of magnetoplasmonic nanoparticles has been reported.15−17 However, control over the assembly of NPs into predefined superstructure using the drying-mediated assembly is quite difficult. In a previous study, we clarified the mechanism of dryingmediated and magnetic field-induced Fe3O4@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@Fe3O4 magnetoplasmonic NPs were utilized to assemble a linear nanochain array in macroscopic
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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 singleparticle-level studies, attempts to scale up have been made by introducing various assembly methods to apply magnetoplasmonic 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. © 2017 American Chemical Society
Received: May 17, 2017 Accepted: July 11, 2017 Published: July 11, 2017 24433
DOI: 10.1021/acsami.7b06977 ACS Appl. Mater. Interfaces 2017, 9, 24433−24439
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ACS Applied Materials & Interfaces
Figure 1. Characterization of the Ag@Fe3O4 core−shell NPs synthesized with various concentrations of Fe(NO3)3: (A−H) SEM and TEM images of Ag@Fe3O4 NPs synthesized with different shell thicknesses. Scale bars: 1 μm and 100 nm in SEM and TEM images, respectively. (I) Evaluation of the corresponding average sizes of core diameter and shell thickness averaged over 100 nanoparticles. (J) Corresponding UV−visible spectra. (K) Zeta-potential of the Ag@Fe3O4 nanoparticle colloidal solution.
scale. The assembly was induced by combining magnetic fields and electrostatic forces through dip coating process. This assembly method is advantageous to minimize thermal dynamic effects during evaporation process and to scale up the coating area that is limited within drop area in the drop-drying technique. Using this dip-coating technique, 1D nanochains can be fabricated uniformly over whole area of substrate regardless of substrate size under appropriately uniform magnetic field. Moreover, this dip-coating technique could be used for roll-toroll process for continuously fabrication, which would be very promising for industrial applications. Then, collective optical properties, which are derived from integrated optical signal of assembled particles in the array in order to achieve amplified and stronger response, were monitored in longitudinal and transversal modes of plasmon coupling within the nanochain arrays. Furthermore, their optical anisotropic bands were tuned in the range of visible to near-infrared wavelength by controlling IPDs of Ag cores within the nanochains. The experimental data were compared with theoretical calculations and simulations generated by COMSOL simulation software. These tunable magnetoplasmonic nanochains could be applied in the fields of anisotropic optics, nanoscale sensing platforms, etc.17−20 Fe3O4-coated Ag NPs (Ag@Fe3O4) were utilized for optical manipulation through the common alignment method where the core acts as a plasmonic source and the shell as a spacer by variance of its thickness. To control the thickness of the magnetic shell, we varied the initial concentration of Fe(NO3)3, a precursor of Fe3O4. The Fe3O4 shell of the Ag@Fe3O4 NPs is a unique suprastructure previously reported by us,21 because the Fe3O4 shell possesses hundreds of nanocrystals, which is mimicking the supramolecular assemblies that occur in protein, such as viral capsids. Thus, such magnetic structures respond sufficiently well to be aligned along a magnetic field with a fast response time. The saturation magnetization of the Ag@Fe3O4
NP with different shell thicknesses of 41, 44, 50, and 60 nm is 52.7, 55.5, 58.2, and 62.0 emu g−1, respectively. They possess soft ferromagnetism (Mr ≈ 5 emu/g) as shown in the hysteresis curve at 300 K in Figure S1. The anisotropic optical response of the synthesized core−shell nanostructures was measured for each shell thickness. The average diameters of the Ag@Fe3O4 NPs synthesized with 0.062, 0.068, 0.074, and 0.080 mol L−1 of Fe(NO3)3 were determined by SEM images, yielding 141.4 ± 8.3, 150.7 ± 8.3, 164.3 ± 9.9, and 181.2 ± 10.5 nm, respectively, from 300 NPs (Figure 1A−D). The increased shell thickness of Ag@Fe3O4 NPs was clearly confirmed by TEM (Figure 1E−H). Contrary to the consistent average Ag core diameter of ∼60 nm for each sample, the average thickness of the Fe3O4 shell dramatically increased from 41 to 60 nm (Figure 1I). Detailed evaluation of the corresponding size distribution of the silver core and the Fe3O4 shell thickness is shown by histograms in Figure S2A−D. Figure 1J presents the optical properties of the prepared NPs in suspension. Localized surface plasmon resonances (LSPR) of the NPs red-shift as the shell thickness increases. This phenomenon is attributed to increased refractive index of the surrounding medium. The addition of the oxide shell coated on Ag particle can be considered as a change in the effective refractive index of the medium. The shell material and thickness of the shell therefore affect the magnitude of the shift. M. Nieminen et al. reported about the tunability of the SPR peak position without significantly decreasing the intensity of the peak by varying a shell of metal oxides on a spherical Ag nanoparticle.22 In the case of particles with relatively big core radius (d = 60 nm), the SPR peak was effectively tuned by coating with high refractive index oxides (ZrO2, n = 2.14 and TiO2, n = 2.64 at wavelength 700 nm). Meanwhile, the position of the SPR peak showed minor change after coating thickness t > 40 nm of SiO2 shell (n = 1.46 at wavelength 700 nm). Here because of high refractive index of Fe3O4 (n = 2.42 at wavelength 700 nm), SPR peaks 24434
DOI: 10.1021/acsami.7b06977 ACS Appl. Mater. Interfaces 2017, 9, 24433−24439
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Figure 2. 1D assembly of Ag@Fe3O4 NPs: (A) schematic illustration of experimental setup for assembly of Ag@Fe3O4 NPs; (B) resulting photograph of the assembled nanochain array on a cover glass (18 mm × 18 mm); (C) representative dark-field image; and (D) a SEM image of the nanochain array.
Figure 3. Optical properties of Ag@Fe3O4 nanochain arrays: (A) AFM images of nanochain array films with various shell thickness of (A) 41, (B) 44, (C) 50, and (D) 60 nm. Extinction spectra of nanochains with different IPDs: (E) NC135, (F) NC141, (G) NC154, and (H) NC171. (I) Plot of the corresponding LSPR peak wavelengths from NC135 to NC171 as a function of polarization angles, where the dash lines are in the absence of polarization. (J) Experimental normalized extinction spectra of randomly deposited and isolated Ag@Fe3O4 NPs with consistent core size of ca. 60 nm and shell thicknesses of 41, 44, 50, and 60 nm on the cover glass. (K) Comparison plot of LSPR wavelength between randomly isolated NPs and transversal or longitudinal mode of nanochains as a function of average IPDs of nanochains, i.e., λ LSPR of nanochain − λ LSPR of nanoparticle..
still show large redshifts from 590 to 651 nm (Figure. 1J) as increasing the shell thickness from 41.3 to 59.7 nm (Figure 1E−H). These well-controlled NPs have a negative zeta
potential (−35 mV) from coating citrates (Figure 1K), which can be helpful for the magnetic field induced 1D assembly process.23 24435
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the distance between centers of adjacent NPs within the nanochain arrays in the SEM images (Figure S5A−E). Figure 3E−H show UV−visible spectra of the array. In each set of spectra, polarized light with seven different angles in the range of 0−90° was irradiated onto the samples and the spectral shift as a function of polarization angles were plotted in Figure 3I. It was found that the LSPR wavelength from NC135 to NC171 was dependent on not only the IPDs of nanochains, but also the angle of polarized incident light. Such observations are related with near-field and far-field coupling of nanochains. In the series of experiments, the peak absorption wavelength was manipulated in the range of 566−690 nm with control of the polarization angle and the IPDs of the nanochain arrays. To understand the origin of the induced spectral shift manipulated by varying the angles of polarized light and the IPD, the LSPR peaks of longitudinal and transversal mode of NC135 to NC171 were compared with that of individual Ag@ Fe3O4 NPs. The longitudinal and transversal modes of the nanochains stem from parallel and perpendicularly polarized excitation, respectively. The LSPR wavelength of individual Ag@Fe3O4 NPs was acquired by measuring the randomly deposited NPs on the cover glass (Figure 3J). The individual NPs exhibit independent polarization owing to random dispersion (uncoupling was proven by SEM and extinction spectra in Figure S6). As shown in Figure 3K, each longitudinal and transversal mode of the nanochain responded inversely, i.e., the longitudinal mode was red-shifted (λLSPR of nanochain − λLSPR of NPs > 0), whereas the transversal mode was blue-shifted (λLSPR of nanochain − λLSPR of NPs < 0) compared to the LSPR peak of the individual NPs. Moreover, as the IPD is increased, the red shift decreased slightly from 50 to 37 nm, whereas blue shift increased swiftly from −5 nm to −51 nm. The red shift of the longitudinal mode is mainly caused by near field coupling of neighboring plasmonic NPs, and the coupling is weakened at larger separation distances. For that reason, in typical studies of plasmonic NPs on strong near-field coupling, IPD is set to between 1 and 30 nm.27,28 However, the IPD in this study is 135 to 171 nm, which is at least twice as large as the diameter of the silver core, resulting in relatively weakened near-field coupling. Thus, as shown in the red bar of Figure 3K, the variance of the red shift was minor within the range of the IPD. On the other hand, blue shift phenomenon at transversal mode is mainly considered a far-field coupling. The interference of light scattered and rescattered by periodic alignment of silver cores with perpendicularly polarized light shows retardation effects, resulting in an overall blue-shift of extinction spectra.29 Therefore, with such a large IPD, far-field coupling leads to a significant variance of spectral properties (blue bar in Figure 3K). The strongest far-field interaction between Ag cores in nanochains observed in the largest IPD can be highly promising for refractive index sensing applications.30 To better understand the experimental observation of optical properties of nanochain arrays, theoretical calculations were performed by COMSOL Multiphysics version 5.2a incorporated with Wave Optics Modules based on the finite-element method (FEM). For an in-depth analysis of the measured spectra, we start with a single NP and subsequently expand the model to the nanochain with chain length distribution as well as near field enhancement on polarization dependency. Figure 4A shows the geometry of an identical model of a core−shell NP depicted as a double-layered sphere having diameters and shell thicknesses measured from TEM images (Figure 1E−H). The inner layer radius (r) and shell thickness (t) were assumed to
It was known through previous experimental results that the external magnetic field and electrostatic interaction with the substrate were critical parameters to assemble the structure.23,24 In a previous study, a reverse-structured NP, i.e., plasmonic (Au) shell-coated Fe3O4 NPs (Fe3O4@Au) were assembled into nanochains with lengths on the order of hundreds of micrometers. The magnetic moment of individual Fe3O4@Au NPs was not sufficient to produce significant dipolar interactions for overcoming fluidic turbulence and thermal energy in the solution. It is probable that the coated Au shell reduced the magnetic properties of the core.23 In the present study, the clustered form of the Ag@Fe3O4 NPs contains hundreds of magnetic nanocrystals and the magnetic dipole energy of the NPs is therefore substantial. Concretely, when the magnetic dipolar parameter χ > 3 (χ = U0/kBT where U0 is the maximum magnetic energy at contact, kB is Boltzmann constant and T is temperature), the formation of particle chains is observed.25 Because the cluster of the Ag@Fe3O4 NPs contains hundreds of nanocrystals, χ value of the cluster that is roughly sum of χ of each individual nanoscrystal, could easily reaches the threshold value for the nanochain formation. This allows for direct alignment along a magnetic field to form linear particle chains. Furthermore, an effective dip coating method was used to fabricate 1D structures of magnetic particles on a large surface area by combining electrostatic and magnetic dipole interactions (Figure 2A). Ag@Fe3O4 NPs with a negative surface charge were deposited and aligned on the positively charged surface of a cover glass under an externally applied magnetic field (B ≈ 20 mT). The following forces are considered to greatly affect the assembly process; electrostatic attraction (Fea) between the substrate and NPs, electrostatic repulsion (Fer) and magnetic interaction (Fm) between NPs. In detail, the electrostatic interaction can be expressed as Coulomb’s law, that is Fea = −keq1q2/r2 and Fer = keq21/r2 where ke is Coulomb’s constant q1 and q2 are the signed magnitudes of the charges, the scalar r is the distance between the charges. Magnetic interaction can be simplified as magnetic dipole−dipole interaction, Fm = 3 μ2 (1−3cos2 α)/l4, where μ is the induced magnetic moment, α is the angle between the dipole and the line connecting it with a neighbor dipole, and l is the center-to-center distance between two particles.26 The photo in Figure 2B shows a typical example of the assembled particles on the cover glass (18 × 18 mm2) and the corresponding dark-field and SEM image of the nanochain array are shown in Figure 2C, D. To characterize optical properties of Ag@Fe3O4 nanochains, we measured the extinction of nanochain arrays on a cover glass by a UV−visible spectrophotometer, in which a rotational polarizer was mounted between a light source and the sample in order to monitor polarization dependence (Figure S3). The cover glass was pretreated by PDDA (0.2 wt %) to produce a positively charged surface. The deposited NPs were formed into nanochain arrays toward external magnetic field within a few seconds. Figure 3A−D presents atomic force microscopy (AFM) images of the assembled NP structures with four different shell thicknesses. They show many nanochains assembled through magnetic fluxes, showing that the maximum chain length in our experiment reached hundreds of micrometers (Figure S4). The four different samples of nanochain arrays were named NC135, NC141, NC154, and NC171, where the number depicts average IPD of each sample. The average IPDs plotted in Figure S5F were obtained by measuring 24436
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exploits the shift of the energetically lowest resonance to monitor the plasmonic coupling of metal NPs.31 As shown in Figure 4C, the spectral position of the lowest energy mode in the extinction spectra of a nanochain redshifts with increasing chain length until an “infinite chain limit.” The infinite chain limit was determined as 8 NPs in a nanochain. Beyond this limit, the spectral position plateaus and becomes practically independent of the chain length.9 Therefore, 8 units of NPs in a nanochain were chosen as a representative model for further simulation. Before simulation of the nanochain model, far- and near-field interaction of a dimer was simulated to understand these coupling effects on transversal and longitudinal modes. As the incident electric field is perpendicularly interacting with a dimer NP, the near field coupling is seldom observed but far field coupling is generated, causing a blue shift (Figure S7B, C). For the longitudinal mode, however, remarkable coupling effects occurred in the near field but no interaction in the far field was observed, resulting in red shift of the LSPR wavelength (Figure S7D, E). The single nanochain was also simulated in order to monitor the shift of the lowest energy modes in the calculated polarized extinction spectra as shown in Figure 4D. The spectra are dependent on the polarization of the incoming radiation which exhibits good agreement with the experimental extinction spectra of NC135 (Figure 3E). Figure 4E shows the near field intensity of the nanochain with the polarized light oriented from parallel to perpendicular to the NP chain axis, which is indicated by red arrows. In longitudinal mode, localization of the near field at junctions between NPs was observed. However, in transversal mode, the localizations at the junctions disappear and exist only at interfaces between the Ag core and Fe3O4 shell. Our study highlights the ability to delicately manipulate the plasmon coupling of magnetoplasmonic NPs. The precisely controllable assembly was realized via the combination of magnetic dipole interaction and electrostatic forces of NPs under the dip-coating, which enables the fabrication of various 1D structures on a large surface area regardless of substrate size under appropriately uniform magnetic field. Moreover, this dipcoating technique could be used for roll-to-roll process for continuously fabrication, which would be very promising for industrial applications. Assembling Ag@Fe3O4 core−shell NPs in this way facilitates observation of their collective optical anisotropy, such as the polarization angle dependence of LSPR wavelength, near field and far field interaction. The LSPR wavelength can be tuned from visible to near-infrared wavelength by controlling polarization angles and IPD. Comparison of spectral properties of longitudinal and transversal mode nanochains with that of individual NPs reveals that the extent of blue shift and red shift increases and decrease respectively as increasing IPD. The approach presented here would offer a general strategy alternative to top-down techniques that enables the fabrication of nanostructures for various applications such as optical filters, optical waveguide, and sensor.
Figure 4. Simulation of near-field distribution and comparison of calculated and experimental extinction spectra: (A) cross-sectional illustration of the geometry of a single Ag@Fe3O4 NP; (B) corresponding electric field distribution of the NP with a red arrow showing the direction of the incident electric field; (C) plot of maximum wavelength of longitudinal mode of Ag@Fe3O4 nanochains with various shell thicknesses as a function of the number of nanoparticles in a nanochain with exponential decay fitting; (D) calculated extinction spectra of a single nanochain consisting of 8 repeated units of Ag@Fe3O4 NPs with core size of 60 nm and shell thickness of 44 nm to simulate the experimental extinction spectra of NC135; (E) corresponding near-field distribution of the NC135 model with the plane containing the axis of the chain, showing the electric field norm (plotted in logarithmic color scale).
be equal to the radius of the silver core and thickness of the Fe3O4 shell, respectively. Figure 4B shows the electric field distribution of a single Ag@Fe3O4 NP (r = 30 nm and t = 60 nm) excited by a linearly incident electric field. At the resonance wavelength (650 nm), dipolar plasmon resonance is observed on the surface of silver core and even on the outer Fe3O4 shell although it was weak. Figure S7A shows calculated extinction spectra of the single Ag@Fe3O4 NP with a fixed diameter of 60 nm of the Ag core and various shell thicknesses corresponding to the TEM data. The measured and calculated extinction spectra show two distinct peaks, the shorter wavelength peak arising from scattering due to the increase in the total particle size of the Ag core coated by a thick shell of Fe3O4. The longer wavelength peak, having the lowest energy mode, stems from the ensemble of absorption and scattering as shown in Figure S7A. The detailed calculation method of absorption and scattering in each resonance peak is described in simulation part in experimental section. The lowest energy mode was chosen as a well-known “plasmon ruler”, which
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b06977. Details on synthesis method of Ag@Fe 3O 4 NP, fabrication process of nanochain arrays, and their 24437
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characterization; simulation method; randomly isolated NPs on the glass substrate; polarization spectroscopy setup; calculated optical characterization of a single and a dimer Ag@Fe3O4 NPs (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
J. Lee: 0000-0002-4563-2883 Author Contributions †
Y.S. and V.T.T. contributed equally to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study was supported by the National Research Foundation of Korea(NRF) Grant funded by the Korean Government (MSIP) (NRF-2016R1A2B4012072), the Commercializations Promotion Agency for R&D Outcomes (COMPA) funded by the Ministry of Science, ICT and Future Planning (MSIP) (2016K000135) and the National Research Foundation of Korea(NRF) grant funded by the Korea government (MSIP) (No. NRF-2017R1A4A1015627).
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