Magnetic tunable plasmonic coupling of nanoshells enabled by space-unrestricted growth

Abstract

Methods of forming magnetic / plasmonic hybrid structures are disclosed. The methods include synthesizing colloidal magnetic nanoparticles; modifying the magnetic nanoparticles in a solution of polymeric ligands; binding metal seed nanoparticles to the surface of the magnetic nanoparticles; and performing seed-mediated growth on the metal seed nanoparticles by reducing a metal salt in solution to form the magnetic / plasmonic hybrid structures.

Description

[0001] Government Rights

[0002] This invention was made with government support under CHE-1808788 awarded by the National Science Foundation (NSF). The government has certain rights in the invention. TECHNICAL FIELD

[0003] The present disclosure relates generally to magnetically tunable plasmonic coupling of nanoshells enabled by space-free confined growth, and more particularly, it relates to methods of forming, for example, magnetic / plasmonic hybrid structures.

[0004] BACKGROUND

[0005] Resonant scattering of plasmonic nanoparticles has attracted increasing attention due to its emerging applications in flexible transparent displays, optical metasurfaces, smart windows, wearable electronics, and color holograms. These applications rely on strong localized surface plasmon resonances (LSPRs) that can efficiently scatter light of specific wavelengths while being “transparent” at non-resonant wavelengths. Considerable research efforts have been made to develop plasmonic structures with large scattering cross sections and tunable resonance wavelengths by controlling their size, morphology, and chemical environment. In early studies, it has been recognized that large plasmonic nanospheres, especially Au and Ag plasmonic nanospheres, have scattering-dominated LSPRs that can be tuned from the visible to the near-infrared (NIR) region by simply increasing the particle size. Due to the decay of LSPR along the depth from the surface, the scattering efficiency decreases significantly with increasing particle size. For Au nanoparticles, the active skin depth is estimated to be only tens of nanometers, leading to a relatively low scattering efficiency for large solid particles due to the high percentage of inert interior atoms.

[0006] Plasmon nanoshells with noble metal coatings on dielectric cores exhibit significantly higher scattering efficiency and wavelength tunability in LSPR compared to their solid-state counterparts. However, their practical realization is challenged by the lack of robust methods for large-scale production of high-quality nanoshells with tunable resonant scattering and fairly low non-resonant absorption. A conventional approach to obtaining Au nanoshells involves growing them directly from seeds pre-fixed on organic or inorganic cores. Careful control of seed bonding and growth kinetics may be required to avoid self-nucleation, making the method difficult to scale up. A more critical issue is the production of uniform and thin nanoshells, as free growth of multiple seeds results in thick Au shells only through the fusion of large grains, leading to low plasmon activity and significantly broadened extinction due to the presence of high-density grain boundaries. Growth becomes particularly problematic when core sizes are below approximately 100 nm due to the inhomogeneous distribution of metallic seeds (e.g., gold (Au) seeds) and the increased difficulty in controlling growth kinetics on highly curved surfaces. This presents an additional challenge to fully exploring the potential of Au nanoshells, especially in biomedical applications, as the small and thin shells allow for efficient extravasation into tissues or tumors, and they also allow for easier targeting of LSPRs in the visible and NIR regions most relevant to biomedicine.

[0007] Overview

[0008] Considering the above issues, a space-unconstrained growth method that allows for the production of high-quality, for example, gold (Au), silver (Ag), or copper (Cu) nanoshells on relatively small magnetic Fe3O4 cores (e.g., 10 nm to 150 nm) with excellent plasmon polaritonal properties would be desirable. According to an exemplary embodiment, the resulting core-shell nanoparticles can be magnetically assembled into plasmon polaritonal chains, which can exhibit dynamically tunable coupled resonant scattering.

[0009] According to an exemplary embodiment, a soft, deformable, and highly permeable polymer shell is employed to mediate and confine the seed growth of metals (e.g., Au, Ag, or Cu). The deformable polymer shell confines seed growth at its interface with the magnetic core and allows for the tuning of Au, Ag, or Cu growth without creating additional confinement spaces or gaps in templated synthesis. While its high deformability provides space for forming a complete metal shell, the elastic polymer shell inhibits the deposition of metal (e.g., Au, Ag, or Cu) atoms, thus limiting their growth along the radial direction. Furthermore, the high magnetic susceptibility of the core enables the development of a novel approach to actively and reversibly tune the plasmon coupling and resonant scattering of Au, Ag, or Cu nanoshells by magnetically assembling hybrid nanoparticles into plasmon resonance chains, making it a promising material for creating novel transparent displays and anti-counterfeiting devices.

[0010] According to one aspect, a method for forming a magnetic / plasmonic hybrid structure includes: synthesizing colloidal magnetic nanoparticles; modifying the magnetic nanoparticles in a polymer ligand solution; bonding metal seed nanoparticles to the surface of the magnetic nanoparticles; and performing seed-mediated growth on the metal seed nanoparticles by reducing a metal salt in solution to form a magnetic / plasmonic hybrid structure.

[0011] According to another aspect, a method for using a magnetic / plasmonic structure in an anti-counterfeiting device includes: fixing a one-dimensional plasmonic chain having a pre-designed orientation in a solid polymer using a magnetic field; applying polarized light from a first direction to the one-dimensional plasmonic chain, the polarized light exhibiting a first color spectrum; and applying polarized light from a second direction to the one-dimensional plasmonic chain, the polarized light exhibiting a second color spectrum, the first color spectrum being different from the second color spectrum.

[0012] According to another aspect, a magnetic / plasmotropic hybrid structure comprises: a magnetic nanoparticle core; a deformable and permeable polymer ligand shell; and a metal seed attached to the magnetic nanoparticle core, wherein the metal seed is configured to have restricted growth due to the deformation and permeable polymer ligand shell on the magnetic core inhibiting the deposition of metal atoms and limiting the growth of metal atoms in the radial direction. Brief description of the attached diagram

[0014] Figures 1a-1f Figure 1a shows a schematic diagram of seed-mediated growth of Au nanoshells on iron oxide nanospheres. TEM images of Fe3O4 nanospheres (Figure 1b), Fe3O4 / Au@RF (Figure 1c), and Fe3O4@Au@RF nanospheres (Figure 1d) are also shown. The evolution of plasmon resonance peaks during seed growth of Au nanoshells on 125-nm Fe3O4 nanospheres (Figure 1e) and 20-nm Fe3O4 nanospheres (Figure 1f) is also depicted.

[0015] Figures 2a-2f are schematic diagrams of tunable resonant scattering of Au nanoshells. TEM images of Fe3O4@Au@RF nanospheres synthesized using Fe3O4 nanospheres with diameters of 20 nm (Figure 2a), 70 nm (Figure 2b), 125 nm (Figure 2c), and 150 nm (Figure 2d) as cores are shown. Figure 2e is a schematic diagram of the UV-Vis spectra of Fe3O4@Au@RF nanospheres with different core diameters. Figure 2f is a digital image of colloidal dispersions of Fe3O4@Au@RF nanospheres with core diameters of (i) 20 nm, (ii) 70 nm, (iii) 125 nm, and (iv) 150 nm. Figure 2g illustrates the UV-Vis spectra of Au nanoshells thus synthesized after etching away the RF. Inset from left to right in Figure 2g: Fe3O4@Au@RF, Fe3O4@Au, and Fe3O4@Au solutions exposed to a horizontally oriented magnetic field from left to right. The diameter of the Fe3O4 core is 20 nm.

[0016] Figures 3a-3h The figures illustrate resonant scattering of Au nanoshells for transparent displays. Figure 3a illustrates the optical properties of Au nanoshells prepared using 70-nm Fe3O4 nanospheres as cores. Figure 3b is a color-coded diagram of the surface (right) of the Au nanoshells (70-nm core and 25-nm shell, left) during plasmon excitation and the far-field radiation pattern of the Au nanoshells excited by 710 nm light. Figure 3c illustrates the corresponding surface Poynting vector (surface arrows) and electric field distribution (color map) of the Au nanoshells. Figure 3d illustrates the fabrication of a PVA / Au composite film for transparent displays. The inner diameter of the gold nanoshells is 70 nm. Figure 3e is a photograph of the composite film under natural light. Figures 3f and 3g are photographs showing a PVA / Au composite film containing hybrid nanoparticles with 70-nm Fe3O4 cores (Figure 3f) and 20-nm Fe3O4 cores (Figure 3g). The letters were generated by a projector and then illuminated onto the film using white light from the top row and monochromatic light from the bottom row. Figure 3h is a photograph of a pure PVA film under the same illumination. Scale bar: 1 cm.

[0017] Figures 4a-4c The figures illustrate plasmon coupling of Au nanoshells achieved by magnetically assembling Fe3O4@Au@RF nanospheres into plasmon chains. Figure 4a shows the extinction spectra of the plasmon chains measured under different field strengths. Figure 4b shows the extinction spectra of the plasmon chains measured at different orientations. Figure 4c is a polarization optical macroscopic (POM) image of Fe3O4@Au@RF nanoparticles under different magnetic fields. The white arrows indicate the direction of the magnetic field. The incident light is horizontally polarized.

[0018] Figures 5a-5dThe diagram illustrates coupled resonant scattering of Au nanoshells achieved by magnetically assembling Fe3O4@Au@RF nanospheres into plasmonic chains. Figure 5a is a schematic diagram of the photolithography process used to fabricate thin films with different chain orientations. Figure 5b shows normal optical microscopy and polarized optical microscopy images of the boundary regions of the plasmonic nanochains with horizontal (left) and vertical (right) orientations. White arrows indicate the polarization of the incident light. The image in the lower right corner is under normal light excitation. Figure 5c is a digital image of the pattern under horizontal polarization (top) and vertical polarization (bottom). Figure 5d is a digital image of the pattern under horizontal polarization (top), vertical polarization (middle), and 45° polarization (bottom). Scale bar: 10 μm (Figure 5b); Figures 5c and 500 μm (Figures 5c and 5d).

[0019] Figures 6a-6h illustrate the conceptual design. Simulated absorption (Figure 6a), scattering (Figure 6b), and extinction (Figure 6c) of the Au nanoshell cross-sections are shown. The thickness of the Au nanoshell is 25 nm. The core size increases from 20 nm to 150 nm. Figures 6d and 6e show simulated atomic scattering cross-sections of Au nanoshells (Figure 6d) and solid Au nanoparticles (Figure 6e) with different diameters. Simulations were performed using Au nanoshells with a thickness of 25 nm. Figure 6f shows the simulated atomic absorption cross-section of the Au nanoshell. Figure 6g shows the figure of merit for scattering by the Au nanoshell. Figure 6h shows the near-field electric field distribution (right) and Poynting vector (left) of the Au nanoshell excited at the indicated resonant wavelength.

[0020] Figures 7a-7d These are TEM images of Fe3O4 nanoparticles at 20 nm (Fig. 7a), 70 nm (Fig. 7b), 125 nm (Fig. 7c), and 150 nm (Fig. 7d).

[0021] Figures 8a-8c The chemical structures of PAA (Fig. 8a) and branched PEI are illustrated. (Fig. 8b) Fig. 8c illustrates the zeta potential of the magnetic nanoparticles before and after PEI modification.

[0022] Figure 9 This is a TEM image of an Au seed crystal.

[0023] Figures 10a-10d The backscattered scanning electron microscopy image of Fe3O4@Au@RF nanospheres with a 125-nm core is shown in Figure 10a. Corresponding elemental mapping and merged images of Au (Figure 10b) and Fe (Figure 10c) are shown in Figure 10d.

[0024] Figures 11a and 11b illustrate typical TEM images (in Figure 11a) and high-magnification TEM images (in Figure 11b) of Fe3O4@Au@RF nanospheres using 125-nm colloidal nanoparticles as the core. Selected areas of the high-resolution images are indicated by the dashed boxes in Figure 11a.

[0025] Figures 12a-12e illustrate simulated extinction cross sections of Au nanoshells with different core sizes (in Figure 12a). Simulated optical cross sections of Au nanoshells with core diameters of 20 nm (Figure 12b), 70 nm (Figure 12c), 125 nm (Figure 12d), and 150 nm (Figure 12e) are also shown.

[0026] Figures 13a-13g illustrate TEM images of Fe3O4@Au nanoparticles prepared by etching away the RF shell in NaOH solution. Core sizes: 20 nm (Figure 13a), 70 nm (Figure 13b), and 125 nm (Figure 13c). Figure 13d illustrates the extinction spectrum of the Fe3O4@Au nanoparticles. Figure 13e illustrates the simulated extinction spectrum of the Fe3O4@Au nanoparticles. UV-Vis spectra of the Au nanoshells thus synthesized before and after RF etching: Figure 13f shows the 20-nm core, and Figure 13g shows the 70-nm core. Inset in Figure 13g: Solutions of Fe3O4@Au@RF and Fe3O4@Au with a 70-nm core under bright and dark fields. Images highlighted by black and red borders show the solutions before and after RF shell etching, respectively.

[0027] Figures 14a-14c The illustration shows the “letters” projected onto a transparent screen by a commercial projector (in Figure 14a). Specifically, the “UCR” in the top row is white. The letters “UCR” in the bottom row are red, green, and blue from left to right. Figures 14b and 14c are photographs showing projected images of PVA / Au composite films prepared with 20nm (Figure 14b) and 70nm (Figure 14c) Fe3O4 nanospheres as cores, used as transparent displays. Scale bar: 1cm.

[0028] Figure 15 This is a schematic diagram of active tuning of plasmon coupling in an Au shell achieved through magnetic assembly of Fe3O4@Au@RF nanoparticles. Specifically, changing the chain orientation relative to light polarization alters the plasmon excitation of the 1D nanochain, which further leads to changes in the chain's scattering properties.

[0029] Figures 16a-16dFigure 16a illustrates the simulated extinction spectra of the plasmon chain at different particle spacings. Figure 16b shows the local electric field distributions at spacings of 5 nm, 20 nm, and 40 nm. The excitation wavelengths from left to right are 750 nm, 710 nm, and 690 nm, respectively. Figure 16c shows the simulated extinction spectra of the plasmon chain at different orientations. The particle spacing is set to 30 nm. Figure 16d shows the local electric field distributions of the plasmon chain at orientations of 0°, 45°, and 90°.

[0030] Figure 17 These are POM images of the Fe3O4 / Au@RF dispersion under different magnetic fields. The white arrows indicate the direction of the applied magnetic field.

[0031] Figure 18a illustrates the optical properties of the Au shell prepared using 70 nm Fe3O4 nanoparticles as a core. Figure 18b is an epipolar plot showing the angular distribution of scattered light from the Au shell at 738 nm under p-mode and s-mode excitation. Figure 18c illustrates the corresponding surface Poynting vector and electric field distribution of the Au shell. Figure 18d illustrates the fabrication of a PVA / Au composite film for transparent displays. Figure 18e is a photograph of the fabricated film under natural light. Photographs show transparent displays prepared from the PVA / Au composite film (Figure 18f) and a pure PVA film (Figure 18g).

[0032] Detailed Explanation

[0033] The present preferred embodiments of this disclosure will now be described in detail, examples of which are illustrated in the accompanying drawings.

[0034] According to an exemplary embodiment, unconfined growth of metallic nanoshells (e.g., Au, Ag, or Cu nanoshells) with well-defined plasmon polaritonic properties is disclosed, along with active tuning of their plasmon polaritonic coupling via nanoscale magnetic assembly. The seeding of Au occurs uniquely at the hard-soft interface between the Fe3O4 core and the phenolic resin without creating confinement space, representing a versatile and ingenious approach for a variety of core-shell nanostructures. The deformability of the permeable phenolic layer plays a crucial role in tuning the interfacial growth of Au, Ag, or Cu nanoshells. While polymer elasticity inhibits radial deposition of atoms (e.g., Au, Ag, and Cu atoms), their high deformability provides sufficient space for the formation of conformal metallic shells. The coupled magnetic-plasmon polaritonic properties allow for active tuning of plasmon polaritonic coupling and resonant scattering of Au, Ag, or Cu nanoshells by magnetically assembling hybrid nanoparticles into plasmon polariton chains, with potential applications in designing transparent displays and anti-counterfeiting devices.

[0035] According to an exemplary embodiment, active tuning of the scattering of Au nanoshells is designed based on the fact that Au nanoshells can selectively scatter light at specific wavelengths due to LSPR, while remaining almost "transparent" at non-resonant wavelengths. Simulations, according to an exemplary embodiment, show that as the core size of 25-nm thick Au nanoshells increases from 20nm to 70nm, 125nm, and finally 150nm, their resonant scattering redshifts from 590nm to 980nm (Figs. 6a-6d). Compared to solid nanoparticles of the same diameter (Fig. 6e), Au nanoshells exhibit higher scattering efficiency due to enhancements at multiple interfaces and a wider tuning range due to their hybrid plasmon resonance modes. Primary resonant scattering is far from the strongest resonant absorption in the Au nanoshells (Fig. 6f), which facilitates scattering-based applications and leads to reduced optical losses at the resonant frequency. Furthermore, the ratio between scattering efficiency and maximum absorption efficiency gradually increases with core size, and its value is greater than one across a broad spectrum (Fig. 6g). The surface scattering properties of Au nanoshells (left panel in Figure 6h) indicate strong scattering around the entire surface. The electric field in Au nanoshells with larger cores (right panel in Figure 6h) is on a much lower order of magnitude, which further confirms their lower absorption at the resonant wavelength.

[0036] According to an exemplary embodiment, Figure 1a illustrates the unique confined synthesis of Au nanoshells on magnetic nanoparticles. Magnetite (Fe3O4) nanoparticles of different sizes (e.g., from 20 nm to 150 nm) were synthesized using polyacrylic acid (PAA) as end-capping ligands (Figures 7a-7b). Figure 1b shows a typical TEM image of 125-nm iron oxide nanoparticles with uniform size and good dispersion. They exhibit superparamagnetism because each nanoparticle consists of multiple sub-10-nm crystallites. Branched polyethyleneimine (PEI) was attracted to the surface of the PAA-endopeptide nanoparticles via electrostatic interactions, which was confirmed by a significant change in zeta potential from -70 mV to +90 mV (Figure 8a). Then, negatively charged Au seeds (approximately 2 nm, phosphonium tetra(hydroxymethyl)chloride) end-capped with phosphonium tetrachloride were electrostatically adsorbed. Figure 9 Fe3O4 / Au nanoparticles were fixed on the surface of iron oxide to form Fe3O4 / Au nanoparticles for further seed growth. Subsequently, the composite particles were coated with a layer of water-permeable and flexible phenolic resin via catalytic step-growth polymerization of resorcinol and formaldehyde (RF). The thickness of the RF shell was 35 nm (Figure 1c).

[0037] According to the exemplary embodiment, one of the keys to synthesizing uniform Au nanoshells is ensuring that Au atoms are uniformly deposited onto multiple seed crystals and minimizing the self-nucleation of free Au nanoparticles, which can be achieved by maintaining a low reduction rate. For example, H₂O₂ can be used as a mild reducing agent to support the seed growth of Au nanoshells because H₂O₂ has a pH-dependent standard reduction potential: 1.763 V (in acidic solutions) and 0.867 V (in alkaline solutions). This is achieved by forming anions of HO₂. - This makes the reducing agent effect of H2O2 under alkaline conditions possible. Therefore, to prevent Au seed crystals from being etched by H2O2, sodium oleate is added to adjust the pH of the growth solution to slightly alkaline, so that H2O2 has a higher reduction potential than AuCl4. - (+0.93V) and AuCl2 - (+1.15V) Low. Therefore, H2O2 can maintain the continuous, exclusive deposition of Au atoms on multiple seed crystals to form a conformal coating, while carefully moderating its reducing power to minimize self-nucleation during seed crystal growth. Seed crystal growth of Au nanoshells under alkaline conditions occurs as follows:

[0038] H2O2+OH - =HO2 - +H2O (1)

[0039] 3HO2 - +AuCl4 - =Au + 3HO2· + 4Cl - (2)

[0040] Figure 1d shows the morphology of the particles after seed growth, with a complete Au nanoshell approximately 25 nm thick. Backscattered scanning electron microscopy (SEM) images in Figure 10a confirm the core / shell structure of the nanoparticles. Because Au has a much higher atomic number than Fe and O, the Au nanoshell can scatter electrons more efficiently and therefore appears much “brighter” than the Fe3O4 core. Conversely, the RF shell cannot be observed in backscattered mode due to the low atomic numbers and low electron scattering efficiency of C, H, and O atoms. The elemental mapping results in Figures 10b-10d also help explain the core / shell structure of the final product. High-magnification TEM images are presented in Figures 11a and 11b, and the results clearly demonstrate that the Au nanoshell is a conformal coating rather than a colloidal cluster at the Fe3O4-RF interface. Notably, the thickness of the RF shell decreases from an initial 35 nm to 18 nm during seed growth, indicating their expansion due to the continuous deposition of Au atoms.

[0041] The optical properties of the Au nanoshell gradually change during seed growth (Fig. 1e). At 1 minute, a plasmon band appears at 810 nm, indicating successful Au growth within the RF shell. As the reaction progresses, this band first redshifts and then blueshifts to 880 nm, consistent with the formation of a thin shell followed by its gradual thickening. Notably, in addition to the initial plasmon peak at 545 nm, another plasmon peak appears at 675 nm. Furthermore, as shown in Figs. 6a-6h, calculations indicate that the plasmon band at 880 nm originates from strong scattering by the Au nanoshell, while the two bands at shorter wavelengths exhibit absorption-dominated resonances. Using a 20 nm Fe3O4 core, a continuous redshift of a single plasmon band from 525 nm to 600 nm was observed (Fig. 1f). The absence of a plasmon band at 530 nm in the final product suggests the absence of self-nucleation events during seed growth.

[0042] According to the exemplary embodiment, this method can be readily used to produce Au nanoshells with different core sizes. Figures 2a-2d show some Fe3O4@Au@RF nanoparticles synthesized with Fe3O4 cores of 20 nm, 70 nm, 125 nm, and 150 nm and Au nanoshells of 25 nm. A redshift of the plasmon band from 585 nm to 730 nm, 875 nm, and 960 nm was observed in their extinction spectra (Figure 2d), along with a corresponding change in solution color from blue to green and brown (Figure 2f). The measured extinction spectra of the Au nanoshells are consistent with the simulated spectra in Figures 12a-12e, indicating their high quality and well-defined plasmon properties. Considering the need for accessible metallic surfaces in many plasmon applications (especially in biosensing, imaging, and SERS), a reliable etching process was further developed that can be used to remove the RF layer in NaOH solution at 80 °C while preserving the excellent plasmon properties of the hybrid particles. For Au nanoshells containing a 20-nm core, a blue shift from 600 nm to 540 nm was observed, along with a color change from blue to red, due to the decrease in the surrounding refractive index from approximately 1.5 to 1.33 (Fig. 2g). The sharper peaks confirm the removal of RF and the well-dispersed Au nanoshells. Figures 13a-13g Fe3O4@Au nanoparticles can be magnetically separated from the solution when the magnet is placed near it (inset in Figure 2g).

[0043] This robust and flexible synthesis method allows for convenient tuning of the optical scattering of Au nanoshells, providing significant opportunities for the fabrication of scattering-based applications. Simulations demonstrate that Fe3O4@Au nanoparticles with a 70-nm core and a 25-nm shell exhibit resonant scattering much stronger than absorption (Fig. 3a). At the resonant wavelength, the scattering field of the Au nanoshells is similar to the radiation pattern of a typical dipole antenna (Fig. 3b). The Poynting vector and local electric field distribution in Fig. 3c further confirm the wide-angle scattering of the Au nanoshells. To utilize this interesting optical property, the Au nanoshells were incorporated into a transparent polymer matrix, with a final Fe3O4@Au concentration of approximately 0.005 mg / mL, and the mixture was then spin-cast onto a PDMS substrate (Fig. 3d). Upon drying, the film appeared highly transparent under ambient light (Fig. 3e). Notably, when the film is irradiated with strong white light, it appears red (Fig. 3f, top row) when hybrid nanoparticles with a 70-nm core are used, while it appears green (Fig. 3g, top row) when the core is 20-nm. This demonstrates the resonant frequency-dependent selective scattering of the Au nanoshell. In both cases, as... Figures 14a-14c As illustrated, the film was illuminated using a commercial image projector with three white letters in the top row and letters in the three primary colors (blue, green, and red) in the bottom row. In Figure 3f, the film displays red letters (in the top row) when illuminated with white light containing the letters because the Au nanoshells with 70-nm cores have a resonant wavelength at 685nm and preferentially scatter red light. In Figure 3g, the composite film displays green letters because the resonant wavelength of the embedded Au nanoshells (approximately 540nm) is in the green color gamut (500nm to 565nm). More importantly, under monochromatic light illumination, the film displays the corresponding pattern only when the wavelength of the projected light matches the plasmon polariton band. As shown in the bottom rows of Figures 3f and 3g, films prepared from Au nanoshells with 20-nm and 70-nm cores can selectively display green and red letters, respectively, while in cases of mismatch, the letters are not easily noticeable. In contrast, due to the lack of strong scattering, almost no image can be observed on a pure polymer film without an Au nanoshell under the same projected light (Fig. 3h). According to exemplary embodiments, these transparent films could reveal applications of interest, such as anti-counterfeiting devices or information displays.

[0044] The combination of magnetic and plasmon polaritonic properties in the core-shell structure makes it possible to actively tune plasmon coupling by magnetically assembling Au nanoshells into 1D nanochains. Figure 15First, the optical cross-sections of the assembled chains under different interparticle spacings and orientations were calculated. For the plasmon resonance chain containing Au nanoshells with a 50-nm core, a redshift of the coupling band was observed as the interparticle spacing decreased (e.g., from 40 nm to 5 nm), along with a significant increase in the enhancement of the local electric field within the gap (Figs. 16a and 16b). On the other hand, for the nanochain with an interparticle spacing of 30 nm, the peak intensity at approximately 695 nm decreased as the angle between the chain orientation and polarization changed from 0° to 90° (Figs. 16c and 16d). To confirm the plasmon resonance coupling of the Au nanoshells, the extinction spectra of Fe3O4@Au@RF were measured under different magnetic fields. As the field strength increased, the peak position of the coupling band redshifted slightly due to the decrease in interparticle spacing (Fig. 4a). In Fig. 4b, the coupling peak reached its maximum value when a magnetic field parallel to the polarization (0°) of the incident light was applied. The gradual increase in angle weakens the coupling strength, resulting in a decrease in the peak intensity of the coupling band at approximately 725 nm.

[0045] Magnetic assembly and orientation control represent a reliable method for actively tuning coupled resonant scattering of Au nanoshells. As shown in the optical microscopy image in Figure 4c, bright spots are observed only in the absence of any magnetic field, attributed to scattering by individual nanoparticles. When a magnetic field parallel to the light polarization is applied, 1D chains with a bright red color form immediately. This extremely fast response is attributed to the high saturation magnetization of the Fe3O4 core. When the magnetic field is removed, the chains disintegrate due to electrostatic repulsion at the surface of the RF shell, enabling reversible and dynamic tuning of plasmon coupling in the Au nanoshells. Intriguingly, the apparent color of the 1D chains depends on their orientation under linearly polarized light. A bright red color is observed when the chain orientation is parallel to the light polarization. The red color disappears in the other two perpendicular orientations. To further confirm the plasmon origin of the color, control experiments were conducted using nanoparticles before seed growth as building blocks. Figure 17 As shown, no obvious color was observed in the three typical orientations. Therefore, it is reasonable to conclude that the red color of the perceived 1D chain originates from coupled resonant scattering of the Au nanoshell.

[0046] Compared to previous methods for actively tuning the coupling of plasmon nanostructures (e.g., altering the surrounding dielectric or interparticle spacing through chemical and electronic doping), magnetic assembly offers advantages such as rapid response, complete reversibility, and no chemical control. The magnetic anisotropy of the 1D plasmon chains also facilitates controlled orientation of the assemblies. Therefore, color-changing anti-counterfeiting devices can be fabricated by immobilizing plasmon chains with pre-designed orientations within a photocurable polymer (Fig. 5a). As shown in the optical microscopy image in Fig. 5b, under vertically polarized light, the right region appears red, while the left region is brown due to coupled resonant scattering from the Au nanoshell. The colors in these two regions change when the polarizer is rotated to a horizontal position. Furthermore, the color contrast between the two regions disappears at a relative angle of 45° or in the absence of a polarizer because the plasmon excitation of the 1D chain is identical in both cases. Figs. 5c and 5d show two patterns produced by a multi-step photolithography method. The color contrast of the perceived pattern can be easily altered by changing the polarization of the incident light, which could potentially be used for anti-counterfeiting or information encryption.

[0047] In summary, an unconventional synthetic method for Au nanoshells has been developed by confining the seed growth of Au within the hard-soft interface between a Fe3O4 core and a deformable RF shell. The growth of the Au nanoshells does not require confinement of gaps or space, but instead depends on the elastic deformation of the cross-linked RF shell. The Au nanoshells thus prepared exhibit excellent plasmon resonance properties, which can be easily tuned from the visible light region to the NIR region by simply increasing the core size. This space-unconstrained growth can potentially be extended to the synthesis of other core-shell nanostructures, where growth is preferably parallel to rather than perpendicular to the core surface. The well-defined, highly tunable optical properties of the Au nanoshells allow for the successful development of unique transparent displays and anti-counterfeiting colorimetric devices. Furthermore, the nanoscale magnetic assembly strategy offers many advantages in actively tuning the plasmon resonance coupling of nanostructures, including instantaneous response, chemical-free remote control, and complete reversibility.

[0048] Synthesis of Au shell

[0049] Chemicals: All chemicals were used directly without further purification. Ethanol was purchased from Decon Labs. Ferric chloride (III) hexahydrate, tetraethyl orthosilicate (TEOS), polyacrylic acid (PAA, MW = 1800), sodium hydroxide, polyvinylpyrrolidone (PVP, MW = 10000), resorcinol (R), formaldehyde f, 2-hydroxy-2-methylphenylethyl ketone, polyvinyl alcohol (PVA), 2-hydroxy-2-methylphenylethyl ketone, and tetra(hydroxymethyl)phosphonium chloride (THPC) were purchased from Sigma-Aldrich. Ammonium hydroxide and hydrogen peroxide (H2O2) were purchased from Fisher Scientific. Chloroauric acid (III) trihydrate (HAuCl4·3H2O) and ethylene glycol (EG) were from Acros Organics. Acrylamide (AM) and N,N'-methylenebisacrylamide (BIS) were purchased from Fluka. Sodium oleate (NaOL) was purchased from TCI AMERICA. SYLGARD 184 silicone elastomer curing agent and SYLGARD 184 silicone elastomer base were purchased from Dow Silicone Corporation. Acrylamide (AM) and N,N'-methylenebisacrylamide (BIS) were purchased from Fluka.

[0050] Synthesis of Fe3O4 Nanoparticles: Colloidal particles with tunable sizes of Fe3O4 nanoparticles were synthesized. The reaction was carried out in the solution phase at elevated temperatures. A stock solution was prepared by dissolving 50 mmol NaOH in 20 mL of DEG and heating it at 120 °C for 1 hour under nitrogen protection. The solution was maintained at 70 °C as a stock solution. In a typical synthesis, PAA (4 mmol) and FeCl3 (0.4 mmol) were dissolved in DEG (17 mL), and the mixture was heated to 22 °C for 30 min under nitrogen protection. A certain amount of stock solution was rapidly injected. The mixture was heated at 220 °C for another hour. The stock solution volumes used to synthesize Fe3O4 nanoparticles with average diameters of 70 nm, 125 nm, and 150 nm were 1.7 mL, 1.75 mL, and 1.8 mL, respectively. The obtained Fe3O4 nanoparticles were washed several times with deionized water and dispersed in 20 mL of water.

[0051] PEI Modification: For PEI modification, 5 mL (0.25 batch) of an aqueous solution of Fe3O4 nanoparticles was added to 30 mL of PEI solution (20 mg / mL, Mw = 800) under ultrasonic treatment. The mixture was stirred overnight by vortexing. Magnetic stirring is not recommended during PEI modification to avoid magnetic field-induced aggregation. Subsequently, the Fe3O4 nanoparticles were washed three times with water and then dispersed in 5 mL of water.

[0052] Au seed crystal preparation: Add 12 μL of THPC and 250 μL of NaOH (2 M) to 45 mL of water. After stirring for 5 minutes, add 2 mL of HAuCl4. Store the Au seed crystal solution in the dark for further adhesion.

[0053] Au seed attachment: Under ultrasonic treatment, 5 mL of PEI-modified Fe3O4 nanoparticle solution was slowly added to 30 mL of Au seed solution. The solution was stirred and mixed for about 1 hour. Excess Au seeds were removed by centrifugation. The Au seeds were attached to the surface of the Fe3O4 nanoparticles due to electrostatic interaction, forming Fe3O4 / Aus nanoparticles. They were then washed three times with water and then dispersed in 10 mL of water.

[0054] RF Coating: PVP modification was performed before RF coating. In a typical method, 10 mL of Fe3O4 / Aus solution was added to 30 mL of PVP solution (5 mg / mL) under ultrasonic treatment. The solution was stirred overnight by vortexing. The solution was washed three times with water to remove excess PVP. Finally, it was dispersed in 28 mL of water for RF coating. In a catalytic step-growth polymerization, 20 mg R and 28 μL F were added sequentially. 100 μL of ammonia solution (2.8%) was added to the solution. The reaction was first ultrasonicated for 1 hour and then transferred to a 50 mL round-bottom flask. The reaction was maintained at 100 °C for 3 hours to further concentrate the RF resin and increase its crosslinking ratio. After cooling to room temperature, the solution was washed three times with water. The obtained Fe3O4 / Aus@RF was then finally dispersed in 2 mL of water.

[0055] Seed-mediated growth of Au nanoshells: In a typical procedure, 500 μL of PVP (50 mg / mL, Mw = 40000), 100 μL of NaOL (10 mM), 20 μL of HAuCl4 (0.25 M), and 50 μL of H2O2 were sequentially added to 7.5 mL of deionized water. Then, 25 μL of Fe3O4 / Aus@RF solution was added. The reaction was allowed to proceed at room temperature for 30 minutes. The product was washed three times with deionized water and dispersed in deionized water for characterization.

[0056] Etching the RF shell: First, a solution of Fe3O4@Au@RF nanoparticles was dispersed in a 2M NaOH solution. To improve the dispersibility of the colloidal particles, the reaction was carried out in the presence of 0.5M PVP. After incubation overnight at 80°C, the Fe3O4@Au nanoparticles were washed three times with DI water. The removal of RF also caused a blue shift in the plasmon bands of the synthesized Au shell with a 70-nm core from 730 nm to 685 nm (Fig. S8g). Against a bright background, the color perceived in the colloidal dispersion was complementary to the plasmon extinction. Thus, the color changed from green to light blue (the two left images in the inset of Fig. 13g). Notably, against a dark background, both the solutions before and after RF shell removal appeared red (the two right images in Fig. 13g), which is attributed to their strong scattering of red light between 622 nm and 770 nm.

[0057] Characterization: Extinction spectra were measured using an Ocean Optics HR2000 spectrometer. TEM images were acquired at 120 kV using a Tecnai 12 transmission electron microscope. Dark-field macroscopic images were acquired using a Zeiss AXIO Imager optical microscope. SEM images were acquired using a ThermoFisher Scientific (formerly FEI / Philips) NNS450 scanning electron microscope with a backscattered electron detector. Elemental mapping was performed at 50 kV.

[0058] Fabrication of plasma exciton membranes:

[0059] Preparation of PVA-Au Composite Film: PDMS film serves as the transparent substrate for the PVA-plasmonic composite film. Silicone elastomer curing agent and silicone elastomer base material were thoroughly mixed at a mass ratio of 1:10. The mixture was left to stand under ambient conditions for 2 hours to remove air bubbles from the viscous solution. Then, it was cured at 60°C for 2 hours. A 10% PVA solution was first prepared by dissolving PVA in deionized water under ultrasonic treatment. Then, a certain amount of the PVA solution was added to the Au shell solution at a final concentration of approximately 0.005 mg / mL. The resulting mixture solution was spin-cast onto the PDMS substrate. To form a uniform PVA film, the PDMS substrate was first plasma-treated for 20 minutes. The cast film was then vacuum-dried at room temperature.

[0060] Preparation of the anti-counterfeiting film: 2-Hydroxy-2-methylphenylethyl ketone (AM) was used as a photoinitiator. AM was the monomer, and BIS was the crosslinking agent. In a typical method, 250 mg AM, 14 mg BIS, and 3 μL of 2-hydroxy-2-methylphenylethyl ketone (AM) were added to 1 mL of DEG. The Au shell was first precipitated by centrifugation and then dispersed in the DEG solution. The mixture was sandwiched between glass slides and then exposed to UV light (254 nm) for 1 minute. A photomask with a pre-designed pattern was placed on top of the sample, and a magnetic field (B1) was applied. After the first UV exposure, 1D plasmon resonance chains aligned parallel to the external field were fixed in the uncovered areas. The photomask was then removed, and a second UV exposure was applied to polymerize the remainder of the film under a horizontal magnetic field (B2). Magnetic alignment was achieved by placing the mixture at the center of two identical permanent magnets. The field strength was measured to be 25 mT (250 G).

[0061] Optical properties of Au nanoshells analyzed using the finite element method

[0062] Calculation of the optical cross-section of a single Au nanoshell: The optical cross-section and efficiency are calculated using the finite element method (Comsol Multiphysics). A sphere with a core-shell-shell geometry is modeled to simulate the Fe3O4@Au@RF nanostructure. The refractive index of RF is identified as 1.5. Domains of Au are defined by the built-in "Au" material. Their physical properties (such as wavelength-dependent complex refractive index) are fully described in the Comsol Multiphysics material library. For the Fe3O4 domains, their refractive index is also wavelength-dependent and has complex values, where the real part determines the scattering properties and the imaginary part determines the absorption of the Fe3O4 material. The size dependence of the Au shell is analyzed at the appropriate time (…). Figures 1a-1f The thicknesses of the Au and RF shells are 25 nm and 20 nm, respectively. Conversely, the core diameter increases from 20 nm to 30 nm, 50 nm, 70 nm, 80 nm, 90 nm, 100 nm, 125 nm, and 150 nm. Their scattering (σ) is calculated by solving Maxwell's equations in Comsol Multiphysics. sca ), absorption (σ) abs ) and extinction (σ ext The optical coefficients (Q) of the Au shell, such as the scattering and absorption coefficients, are calculated by dividing the corresponding optical cross-section by the physical cross-section of the Au shell. The correlation between them can be expressed as σ = Qπr. 2 , where r is the radius of the Au shell.

[0063] The coupling between Au nanoshells within the plasmon resonance chain was analyzed: the optical cross-section of the plasmon resonance chain was also calculated using the finite element method. Geometrically, a 1D chain containing Au shells with well-controlled interparticle spacing and orientation was first created in Comsol Multiphysics. Then, σ was calculated. ext And normalize it to many Au shells in the 1D chain.

[0064] Synthesis of Au shell

[0065] Chemicals: All chemicals were used directly without further purification. Ethanol was purchased from Decon Labs. Ferric chloride (III) hexahydrate, tetraethyl orthosilicate (TEOS), polyacrylic acid (PAA, MW = 1800), sodium hydroxide, polyvinylpyrrolidone (PVP, MW = 10000), resorcinol (R), formaldehyde f, 2-hydroxy-2-methylphenylethyl ketone, polyvinyl alcohol (PVA), 2-hydroxy-2-methylphenylethyl ketone, and tetra(hydroxymethyl)phosphonium chloride (THPC) were purchased from Sigma-Aldrich. Ammonium hydroxide and hydrogen peroxide (H2O2) were purchased from Fisher Scientific. Chloroauric acid (III) trihydrate (HAuCl4·3H2O) and ethylene glycol (EG) were from Acros Organics. Acrylamide (AM) and N,N'-methylenebisacrylamide (BIS) were purchased from Fluka. Sodium oleate (NaOL) was purchased from TCI AMERICA. SYLGARD 184 silicone elastomer curing agent and SYLGARD 184 silicone elastomer base were purchased from Dow Silicone Corporation. Acrylamide (AM) and N,N'-methylenebisacrylamide (BIS) were purchased from Fluka.

[0066] Synthesis of Fe3O4 Nanoparticles: Colloidal particles with tunable sizes of Fe3O4 nanoparticles were synthesized. The reaction was carried out in the solution phase at elevated temperatures. A stock solution was prepared by dissolving 50 mmol NaOH in 20 mL of DEG and heating it at 120 °C for 1 hour under nitrogen protection. The solution was maintained at 70 °C as a stock solution. In a typical synthesis, PAA (4 mmol) and FeCl3 (0.4 mmol) were dissolved in DEG (17 mL), and the mixture was heated to 22 °C for 30 minutes under nitrogen protection. A certain amount of stock solution was rapidly injected. The mixture was heated at 220 °C for another hour. The stock solution volumes used to synthesize Fe3O4 nanoparticles with average diameters of 70 nm, 125 nm, and 150 nm were 1.7 mL, 1.75 mL, and 1.8 mL, respectively. The obtained Fe3O4 nanoparticles were washed several times with deionized water and dispersed in 20 mL of water.

[0067] PEI Modification: For PEI modification, 5 mL (0.25 batch) of an aqueous solution of Fe3O4 nanoparticles was added to 30 mL of PEI solution (20 mg / mL, Mw = 800) under ultrasonic treatment. The mixture was stirred overnight by vortexing. Magnetic stirring is not recommended during PEI modification to avoid magnetic field-induced aggregation. Subsequently, the Fe3O4 nanoparticles were washed three times with water and then dispersed in 5 mL of water.

[0068] Au seed crystal preparation: Add 12 μL of THPC and 250 μL of NaOH (2 M) to 45 mL of water. After stirring for 5 minutes, add 2 mL of HAuCl4. Store the Au seed crystal solution in the dark for further adhesion.

[0069] Au seed attachment: Under ultrasonic treatment, 5 mL of PEI-modified Fe3O4 nanoparticle solution was slowly added to 30 mL of Au seed solution. The solution was stirred and mixed for about 1 hour. Excess Au seeds were removed by centrifugation. The Au seeds were attached to the surface of the Fe3O4 nanoparticles due to electrostatic interaction, forming Fe3O4 / Aus nanoparticles. They were further washed three times with water and then dispersed in 10 mL of water.

[0070] RF Coating: PVP modification was performed before RF coating. In a typical method, 10 mL of Fe3O4 / Aus solution was added to 30 mL of PVP solution (5 mg / mL) under ultrasonic treatment. The solution was stirred overnight by vortexing. The solution was washed three times with water to remove excess PVP. Finally, it was dispersed in 28 mL of water for RF coating. In a catalytic step-growth polymerization, 20 mg R and 28 μL F were added sequentially. 100 μL of ammonia solution (2.8%) was added to the solution. The reaction was first ultrasonicated for 1 hour and then transferred to a 50 mL round-bottom flask. The reaction was maintained at 100 °C for 3 hours to further concentrate the RF resin and increase its crosslinking ratio. After cooling to room temperature, the solution was washed three times with water. The obtained Fe3O4 / Aus@RF was then finally dispersed in 2 mL of water.

[0071] Seed-mediated growth of Au nanoshells: In a typical procedure, 500 μL of PVP (50 mg / mL, Mw = 40000), 100 μL of NaOL (10 mM), 20 μL of HAuCl4 (0.25 M), and 50 μL of H2O2 were sequentially added to 7.5 mL of deionized water. Then, 25 μL of Fe3O4 / Aus@RF solution was added. The reaction was allowed to proceed at room temperature for 30 minutes. The product was washed three times with deionized water and dispersed in deionized water for characterization.

[0072] Etching the RF shell: First, a solution of Fe3O4@Au@RF nanoparticles was dispersed in a 2M NaOH solution. To improve the dispersibility of the colloidal particles, the reaction was carried out in the presence of 0.5M PVP. After incubation overnight at 80°C, the Fe3O4@Au nanoparticles were washed three times with DI water. The removal of RF also caused a blue shift in the plasmon bands of the synthesized Au shell with a 70-nm core from 730 nm to 685 nm (Fig. 13g). Against a bright background, the color perceived in the colloidal dispersion is complementary to the plasmon extinction. Therefore, the color changes from green to light blue (the two left images in the inset of Fig. 13g). Notably, against a dark background, both the solutions before and after RF shell removal appear red (the two right images in Fig. 13g), which is attributed to their strong scattering of red light between 622 nm and 770 nm.

[0073] Characterization: Extinction spectra were measured using an Ocean Optics HR2000 spectrometer. TEM images were acquired at 120 kV using a Tecnai 12 transmission electron microscope. Dark-field macroscopic images were acquired using a Zeiss AXIO Imager optical microscope. SEM images were acquired using a ThermoFisher Scientific (formerly FEI / Philips) NNS450 scanning electron microscope with a backscattered electron detector. Elemental mapping was performed at 50 kV.

[0074] Manufacturing of Plasmon Thermal Membranes

[0075] Preparation of PVA-Au Composite Film: PDMS film serves as the transparent substrate for the PVA-plasmonic composite film. Silicone elastomer curing agent and silicone elastomer base material were thoroughly mixed at a mass ratio of 1:10. The mixture was left to stand under ambient conditions for 2 hours to remove air bubbles from the viscous solution. Then, it was cured at 60°C for 2 hours. A 10% PVA solution was first prepared by dissolving PVA in deionized water under ultrasonic treatment. Then, a certain amount of the PVA solution was added to the Au shell solution at a final concentration of approximately 0.005 mg / mL. The resulting mixture solution was spin-cast onto the PDMS substrate. To form a uniform PVA film, the PDMS substrate was first plasma-treated for 20 minutes. The cast film was then vacuum-dried at room temperature.

[0076] Preparation of the anti-counterfeiting film: 2-Hydroxy-2-methylphenylethyl ketone (AM) was used as a photoinitiator. AM was the monomer, and BIS was the crosslinking agent. In a typical method, 250 mg AM, 14 mg BIS, and 3 μL of 2-hydroxy-2-methylphenylethyl ketone (AM) were added to 1 mL of DEG. The Au shell was first precipitated by centrifugation and then dispersed in the DEG solution. The mixture was sandwiched between glass slides and then exposed to UV light (254 nm) for 1 minute. A photomask with a pre-designed pattern was placed on top of the sample, and a magnetic field (B1) was applied. After the first UV exposure, 1D plasmon resonance chains aligned parallel to the external field were fixed in the uncovered areas. The photomask was then removed, and a second UV exposure was applied to polymerize the remainder of the film under a horizontal magnetic field (B2). Magnetic alignment was achieved by placing the mixture at the center of two identical permanent magnets. The field strength was measured to be 25 mT (250 G).

[0077] Optical properties of Au nanoshells analyzed using the finite element method

[0078] Calculation of the optical cross-section of a single Au nanoshell: The optical cross-section and efficiency are calculated using the finite element method (Comsol Multiphysics). A sphere with a core-shell-shell geometry is modeled to simulate the Fe3O4@Au@RF nanostructure. The refractive index of RF is identified as 1.5. Domains of Au are defined by the built-in "Au" material. Their physical properties (such as wavelength-dependent complex refractive index) are fully described in the Comsol Multiphysics material library. For the Fe3O4 domains, their refractive index is also wavelength-dependent and has complex values, where the real part determines the scattering properties and the imaginary part determines the absorption of the Fe3O4 material. The size dependence of the Au shell is analyzed at the appropriate time (…). Figures 1a-1f The thicknesses of the Au and RF shells are 25 nm and 20 nm, respectively. Conversely, the core diameter increases from 20 nm to 30 nm, 50 nm, 70 nm, 80 nm, 90 nm, 100 nm, 125 nm, and 150 nm. Their scattering (σ) is calculated by solving Maxwell's equations in Comsol Multiphysics. sca ), absorption (σ) abs ) and extinction (σ ext The optical coefficients (Q) of the Au shell, such as the scattering and absorption coefficients, are calculated by dividing the corresponding optical cross-section by the physical cross-section of the Au shell. The correlation between them can be expressed as σ = Qπr. 2 , where r is the radius of the Au shell.

[0079] The coupling between Au nanoshells within the plasmon resonance chain was analyzed: the optical cross-section of the plasmon resonance chain was also calculated using the finite element method. Geometrically, a 1D chain containing Au shells with well-controlled interparticle spacing and orientation was first created in Comsol Multiphysics. Then, σ was calculated. ext And normalize it to many Au shells in the 1D chain.

[0080] Fabrication of transparent displays:

[0081] According to an exemplary embodiment, a transparent display is fabricated by incorporating an Au shell into a PVA film to utilize the strong scattering of the Au shell. Their scattering properties are first investigated using finite element analysis. As shown in Figure 18a, the peak positions of the measured spectrum of Fe3O4@Au with a 70 nm core are in good agreement with the calculated cross-section. Minor differences at non-resonant wavelengths may be attributed to the broadband absorption of the iron oxide. The calculated cross-section confirms that the resonant scattering of the Au shell is much stronger than its absorption, thus confirming that the Au shell is ideal for transparent displays because it only scatters light at specific wavelengths while remaining "transparent" elsewhere. In the p-mode (Figure 18b), a scattering pattern similar to the Lambertian distribution of an ideal diffuse reflective surface is observed. In the s-mode, however, scattering occurs over a wider range, indicating that the scattering of the Au shell can be observed from a wide angle. The Poynting vector and local electric field distribution are shown in Figure 18c, which further confirm the wide-angle scattering of the Au shell. To incorporate the Au shell into the transparent polymer matrix, a 10% by weight aqueous solution of PVA was added to the Au shell dispersion at a final concentration of approximately 0.005 mg / mL. The mixture was spun-cast onto a PDMS membrane and left under ambient conditions to remove air bubbles and dry completely (Fig. 18d). The digital image of the membrane shown in Fig. 18e indicates the high transparency of the fabricated membrane, as details of the background landscape are observable, with only slight variations in the membrane's appearance, color, and brightness. Fig. 18f shows the transparent membrane in operation with red letters projected onto a screen. The projected image is clearly visible and can be viewed from all directions. In contrast, the pure polymer membrane without the Au shell appears dark and may be almost invisible under the same laser projector due to the lack of scattering (Fig. 18g).

[0082] According to an exemplary embodiment, a method for forming a magnetic / plasmic reticulum hybrid structure includes synthesizing colloidal magnetic nanoparticles; modifying the magnetic nanoparticles in a polymer ligand solution; bonding metal seed nanoparticles to the surface of the magnetic nanoparticles; and performing seed-mediated growth on the metal seed nanoparticles by reducing a metal salt in solution to form the magnetic / plasmic reticulum hybrid structure. The method may further include coating the magnetic nanoparticles containing the surface-attached metal seed nanoparticles with a polymer shell of resorcinol-formaldehyde resin prior to performing seed-mediated growth. Additionally, optionally, the resorcinol-formaldehyde coating is removed from the plasmon structure after seed-mediated growth.

[0083] In an exemplary embodiment, the polymer ligand is polyethyleneimine (PEI). The magnetic nanoparticles comprise iron oxides, such as Fe3O4. The metal seed nanoparticles may comprise gold, silver, or copper. Alternatively, the metal salt may comprise gold, silver, or copper salts.

[0084] According to an exemplary embodiment, the method may further include seeding and confining the growth of metal on magnetic nanoparticles using polymer ligands. The polymer ligands form a deformable and permeable polymer shell on the magnetic nanoparticles, which inhibits the deposition of metal atoms and restricts the growth of metal atoms along the radial direction. Additionally, the magnetic / plasmotropic hybrid structure may consist of multiple magnetic / plasmotropic hybrid structures, which are magnetically assembled into plasmotropic chains in active transparent displays or anti-counterfeiting devices. According to an exemplary embodiment, the synthesis of colloidal particles can be controlled to an outer diameter of 10 nm to 150 nm.

[0085] The foregoing detailed methods for forming magnetic / plasmon hybrid structures, methods for using magnetic / plasmon hybrid structures, and various versions of magnetic / plasmon hybrid structures, representing examples of the methods for forming magnetic / plasmon hybrid structures, methods for using magnetic / plasmon hybrid structures, and examples of magnetic / plasmon hybrid structures disclosed herein. However, the invention is not limited to the precise embodiments and variations described. Various changes, modifications, and equivalents may be made by those skilled in the art without departing from the spirit and scope of the invention as defined in the appended claims. It is expressly intended that all such changes, modifications, and equivalents falling within the scope of the claims are covered by the claims.

Claims

1. A method for forming a magnetic / plasmotropic hybrid structure, comprising: Synthesize colloidal magnetic nanoparticles; The magnetic nanoparticles were modified in a polymer ligand solution; The metal seed nanoparticles are bonded to the surface of the magnetic nanoparticles; as well as Seed-mediated growth was performed on the metal seed nanoparticles by reducing the metal salt in solution to form the magnetic / plasmotropic hybrid structure. It further includes: Prior to performing the seed-mediated growth, magnetic nanoparticles containing surface-attached metal seed nanoparticles are coated with a polymer shell of resorcinol-formaldehyde resin.

2. The method according to claim 1, further comprising: Following the seed-mediated growth, the resorcinol-formaldehyde coating is removed from the magnetic / plasmonic hybrid structure.

3. The method according to claim 1, wherein the polymer ligand is polyethyleneimine, i.e., PEI.

4. The method of claim 1, wherein the magnetic nanoparticles comprise iron oxide.

5. The method according to claim 4, wherein the iron oxide is Fe3O4.

6. The method according to claim 1, wherein the metal seed nanoparticles comprise gold, silver or copper.

7. The method according to claim 1, wherein the metal salt comprises a gold salt, a silver salt, or a copper salt.

8. The method of claim 1, further comprising: The polymer ligand mediates and confines the seed growth of metal on the magnetic nanoparticles, the polymer ligand forming a deformable and permeable polymer shell on the magnetic nanoparticles, which inhibits the deposition of metal atoms and restricts the growth of the metal atoms in the radial direction.

9. The method of claim 1, wherein the magnetic / plasmotropic hybrid structure comprises a plurality of magnetic / plasmotropic hybrid structures, the method further comprising: The plurality of magnetic / plasmonic hybrid structures are magnetically assembled into a plasmonic chain in an active transparent display or anti-counterfeiting device.

10. The method of claim 1, further comprising: The synthesis of the colloidal magnetic nanoparticles was controlled to have an outer diameter of 10 nm to 150 nm.

11. The method of claim 1, further comprising: The magnetic / plasmotropic hybrid structure is utilized in biomedical applications.

12. A method for using a magnetic / plasmotropic hybrid structure prepared according to any one of claims 1-11 in an anti-counterfeiting device, the method comprising: One-dimensional plasmon resonance chains with pre-designed orientations are immobilized in a solid polymer using a magnetic field; The one-dimensional plasmon resonance chain is subjected to polarized light from a first direction, the polarized light exhibiting a first color spectrum; and The one-dimensional plasmon resonance chain is subjected to polarized light from a second direction, the polarized light exhibiting a second chromatogram, which is different from the first chromatogram.

13. The method of claim 12, wherein the first color spectrum and the second color spectrum each include a first color region and a second color region, and wherein the first color region and the second color region of each of the first color spectrum and the second color spectrum are switched from the first direction of polarized light to the second direction of polarized light.

14. The method of claim 12, wherein the first direction and the second direction are one of vertically polarized light and horizontally polarized light.

15. A magnetic / plasmotropic hybrid structure comprising: Magnetic nanoparticle core; Deformable and permeable polymer ligand shells; and The metal seed attached to the magnetic nanoparticle core is configured to have restricted growth due to the deformable and permeable polymer ligand shell on the magnetic nanoparticle core inhibiting the deposition of metal atoms and limiting the growth of the metal atoms in the radial direction. The polymer ligand shell comprises resorcinol-formaldehyde resin.

16. The magnetic / plasmotropic hybrid structure according to claim 15, wherein the magnetic nanoparticle core comprises iron oxide.

17. The magnetic / plasmotropic hybrid structure according to claim 16, wherein the iron oxide is Fe3O4.

18. The magnetic / plasmotropic hybrid structure according to claim 15, wherein the seed crystal of the metal comprises gold, silver or copper.

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