Design and applications of plasmonic superlattice nanomembrane

Building plamonic superlattice nanomembrane enables new class of two-dimensional (2D) optical metamaterials, which are free-standing, one-particle-thick, supracrystalline structures constituted by metallic nanocrystals. The past two decades have witnessed the wet chemical synthesis of plasmon-active nanocrystals, allowing them to serve as “meta-atoms” from the “plasmonic periodic table”. The following challenge lies on rationally group those artificial atoms into 2D plasmonic superlattices. In theory, it is possible to virtually design nanostructures with desired plasmonic properties by selecting nanocrystal types, adjusting soft ligand length, and controlling lattice structures. In practice, however, it has been notoriously challenging to manipulate at will for large-scale, ordered assembly of these meta-atoms. This thesis addresses the existing challenges, specifically, focuses on high-quality synthesis of plasmonic nanocrystal building blocks including gold nanospheres (AuNPs), ultrathin gold nanowires (AuNWs), bimetallic core-shell Au@Ag nanobricks (NBs), following by their programmed assembly with a general polymer-ligand-based approach in conjunction with drying-mediated self-assembly at air-water interface. Three types of novel plasmonic superlattice nanomembranes are prepared, which represent the thinnest possible metallic membrane known to date, including free-standing AuNPs superlattice nanomembrane, giant AuNWs superlattice nanomembranes with mechanical strength, optical transparency and electrical conductivity, and giant superlattice membranes with customizable hybridized plasmon modes and near-field distributions. This thesis begins with a literature review of initial success in growing 1D~3D nanoparticle superlattices by soft ligands. The dominant nanoscale interactions and theoretical model that involved in superlattice construction are firstly outlined. From the view of methodological perspective, pioneering and ongoing research on nanoparticle superlattices are categorized in three types, namely, molecule-based, DNA-based, and polymer-based nanoparticle superlattices. Based on those previously reported design rules, Chapter 3 presents a novel and general method to construct free-standing nanoparticle superlattice nanomembranes using polymers as ligands. The long-range ordered superlattice nanomembranes could be transferred onto arbitrary substrates, and both structural features (interparticle spacing) and functional properties (plasmonic coupling) can be precisely adjusted by tailoring the sizes of the constituent nanoparticles. To further tackle the challenge in fabricating defect-free superlattice membranes at large area, Langmuir-Blodgett techniques is employed to assemble ultraflexible AuNWs into giant mettallic superlattice membrane. Chapter 4 presentes the thinnest possible version of metallic membranes known to date, which is about 2.5 nm thick but with macroscopic lateral dimensions. Such giant metallic nanomembranes are transparent, conductive and mechanically strong, with an optical transmittance of 90-97%, an electrical resistance of ~1142 kΩ sq-1, and a breaking strength of ~14 N m-1 with a typical atomic force microscope probe. Beside the structural and mechanical features, this nanomembrane exhibits multi-angular potential applications, such as in lightweight foldable supercapacitor and electrocatalysis. Beyond structural and functional programmability, Chapter 5 extends the fabrication technique to more complex bimetallic nanobricks. Both experimental evaluation and theoretical simulation demonstrated the customizable plasmon modes and near-field distributions of giant superlattice nanomembrane. Consequently, Raman hot spots could be generated at specific excitation wavelength in a highly predictable way. The NB superlattice membranes are homogeneous in structure, vapor-permeable and mechanically flexible. It can be used as universal and unique SERS substrates with highly uniform Raman hotspot distributions across large area, for rapid and sensitive multi-phase detection of chemical species in air and liquid. Finally, conclusions arising from this thesis have been summarized in Chapter 6, as well as opportunities and perspectives for future focus of this field.