Mie Scattering Characteristics of Microsphere Shells Based on Self-Assembled Dense-Packed Gold Nanospheres
Objective In recent years,due to the localized surface plasmon resonance properties of noble metal nanoparticles,they have been widely employed in various optical devices.Meanwhile,dielectric nanostructures with high refractive index are also considered potential candidate materials for high-performance optical devices.However,both metal and dielectric nanostructures have their limitations,which restrict the performance of optical devices.According to the equivalent medium theory,superlattices composed of densely packed noble metal nanoparticles can be equivalent to these dielectric materials,thus providing an opportunity for combining the excellent characteristics of both.Previous studies mainly focus on the optical properties of two-dimensional layered structures and three-dimensional solid spheres,core-shell,and core-island structured superlattices.However,there are no reports on the optical response of self-assembled microsphere shell structures composed of noble metal nanoparticles.Therefore,we focus on the optical response of microsphere shells formed by the self-assembly of gold nanoparticles,demonstrating their rich Mie scattering characteristics.This structure is of great significance for more effective light control and high-performance optical devices.Methods In the experiment,we first synthesize monodisperse and uniformly sized gold nanoparticles using a seed-mediated growth method.Subsequently,we employ a two-step ligand exchange method to coat thiol-terminated polystyrene(PS-SH)onto the surface of the gold nanoparticles.Finally,we prepare the microsphere shell structure using the emulsion self-assembly method.During the emulsification,we employ toluene gold nanosphere suspension with a concentration of 60 nmol/L as the oil phase,while the aqueous phase contains a solution of Pluronic®F-108 with mass fraction of 1％.To characterize the morphology of the microsphere shell,we observe the microsphere shell structure and the arrangement of gold nanoparticles on the microsphere shell surface using scanning electron microscopy(SEM).To study the optical properties of the microsphere shell,we test the scattering spectra of microsphere shells with different sizes using an Olympus dark-field optical measurement system.Meanwhile,the optical scattering properties of the microsphere shell are primarily studied theoretically by the finite-difference time-domain(FDTD)method,with the Mie scattering contributions analyzed based on the multipole expansion method.Results and Discussions We prepare microsphere shell structures composed of densely packed gold nanoparticles to demonstrate microsphere shells with different diameters and defective microsphere shell structures(Fig.2).We verify the highly ordered hexagonal packing arrangement of gold nanoparticles on the microsphere surface and the hollow structure of the microsphere shell,and study their optical responses.Experimental measurements and numerical calculations of single-particle scattering spectra indicate that the optical response of the microsphere shell is similar to that of an equivalent dielectric shell with a high refractive index[Figs.3(a)and 4(a)],with the ability to generate electric and magnetic dipole resonance modes.When the interaction between them satisfies the Kerker condition,directional forward scattering can be formed[Figs.3(c)and 3(d)].Additionally,due to the plasmonic coupling between gold nanoparticles,a larger localized field enhancement can be generated inside the shell structure relative to the dielectric structure[Fig.3(b)].Furthermore,from the multipole expansion results of the scattering spectra of larger-sized microsphere shells,we observe the excitation of higher-order Mie scattering contributions such as annular electric dipoles,electric quadrupoles,and magnetic quadrupoles[Fig.4(b)].Subsequently,we investigate the influence of the layer number in the microsphere shell on its scattering characteristics.The calculation results show that the resonance peak gradually experiences a red shift with the increasing layer number in the microsphere shell[Fig.5(a)].Meanwhile,we experimentally test the dark-field scattering spectra of microsphere shells with diameters of approximately 250 nm,380 nm,430 nm,500 nm,550 nm,and 650 nm to confirm this conclusion.In further studies on the relationship between microsphere shell size and number of layers,we find that as the size of the microsphere shell rises,the shell layer thickness presents a trend of gradual increase(Fig.7).Furthermore,the sensitivity of this structure to changes in environmental refractive index is not significantly different from that of silicon materials with similar structures(Fig.8).Conclusions We prepare microsphere shell structures using the emulsion self-assembly method and research their optical response.Experimental measurements and numerical calculations of the scattering spectra of microsphere shells show that their optical response is similar to that of an equivalent dielectric shell with a high refractive index,with the ability to generate electric and magnetic dipole resonance modes.When the interaction between them satisfies the Kerker condition,directional forward scattering can be formed.Additionally,under small numbers of microsphere layers,the electric and magnetic dipole modes dominate the total scattering spectrum.As the size of the microsphere shell increases,the shell thickness gradually rises,with more higher-order modes excited.These research findings reveal that microsphere shell structures composed of gold nanoparticles possess rich Mie scattering characteristics.Finally,a promising platform is provided for realizing novel nanoscale structures and broad applications in areas such as optical nonlinearity,biosensing,and optical imaging.
Mie scatteringgold nanospheresemulsification self-assemblyequivalent medium theorysurface plasmon
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