Controllable Assembly Of Metal Nanoparticles Modulated By Electrostatic Potential Well And Research Of The Properties Of Assemblies

The localized surface plasmon resonance(LSPR)effect of metal nanoparticles can bring a series of unique optical and electrical properties,including specific scattering,absorption,and local electric field enhancement,thus being widely applied in photoelectrical devices,biochemical sensing,energy conversion,and other fields.Compared with individual metal nanoparticles,the assembly structures of metal nanoparticles with well-defined spatial configurations exhibit more prominent synergistic and coupling effects in addition to the properties of the nanoparticles themselves,such as near-field coupling,Fano resonance,and light focusing beyond the diffraction limit.Several solution-based self-assembly strategies have been well established by using DNA,thiol molecules,or block copolymers as linking molecules,which provides precise structure control.However,it is difficult for such metal nano-assemblies dispersed in solution to be uniformly transferred onto the substrate and then to maintain their original spatial configurations after drying.Therefore,the construction of stable and programmable metal nano-assemblies on substrate surface becomes one of the key issues in studying their properties and expanding their applications.To date,the commonly used strategy is to fabricate templates on the substrate by top-down techniques,and then perform bottom-up assembly of metal nanoparticles into the template with the help of connecting molecules and other driving forces,such as capillary force and electrostatic force.However,template fabrication relies on sophisticated instruments,which means high processing costs and the risk of technology monopoly.Herein,based on the electrostatic interaction in the colloid system,a bowl-shaped electrostatic potential well centered on a positively charged nanoparticle is established on a negatively charged substrate.Utilizing the regulation of the potential well by the substrate potential and the capture of the negative nanoparticles by the potential well,the controllable preparation of various nano-assemblies was achieved on the substrate and their properties were further studied.Detailed research contents include the following three aspects.(1)In terms of theory,the electrostatic potential well model on charged substrates is established based on the colloidal bilayer theory,the electric multipole moment expansion theory,and COMSOL simulations for the directional gathering and capture of charged nanoparticles.Within a reasonable potential interval,the stronger the negative substrate potential,the stronger the gathering and capture effects towards the negatively charged nanoparticles.Thus,we proposed the idea of controllable assembly of nanoparticles by rationed capture of nanoparticles under the control of substrate potential.(2)In terms of the experiment,the feasibility of configuration regulation of nanoassemblies by the substrate potential was verified.Through surface modification and charge inversion,negatively charged substrates with different substrate potentials were prepared.It was found that negatively charged Au nanoparticles could be controllably captured onto the surface of positively charged Au nanoparticles located on negatively charged substrates due to the existence of the potential well,and the substrate potential can reliably regulate the number of the captured nanoparticles.Given the widespread electrostatic interaction in colloidal systems,this flexible and modular strategy can be further extended to the assembly of other colloidal nanoparticles with various morphologies,sizes,and components,enabling the controllable preparation and property tuning of multi-component and multiconfiguration nano-assemblies.(3)Using Au and Ag nanoparticles as building units,a series of dimer structures were prepared by changing the constituent and size of the nanoparticles under suitable substrate potential conditions,followed by the investigation of their significant LSPR co-bonding coupling properties and the application in Surface Enhanced Raman Spectroscopy(SERS)and dark-field information encryption.Owing to the LSPR coupling effect,the dimer structure of Au nanoparticles can generate the "hot spot" region with significantly enhanced local electric field at the particle gap,which enhances the SERS signal of the 4mercaptobenzoic acid(4-MB A)probe molecule significantly.In addition,a variety of goldsilver dimer and gold-gold dimer structures were prepared,realizing the color tuning of darkfield image from blue to red.Furthermore,the regional assembly of multiple nanostructures can be realized on the same substrate utilizing inkjet printing technique and is then applied to micro-area information encryption and recognition,showing great promises in optical encryption,anti-counterfeiting,and other practical applications.