In comparison to the single component 2D assemblies,binary nanosheet can bring collective properties from two differentconstituents. Thus, assembly of nanoparticle with different shape andcomposition into one system will enrich 2D assemblies and bring moreinteresting properties and novel structural diversity.
However, an additional influencing factor from the geometricdifference of building blocks will be involved as well, this may result inincreased interparticle interaction complex, increasing the difficulty duringthe assembly process. Although highlyordered 2D binary superlattices have been reported from Au nanoparticle and Fe3O4or LaF3 nanoparticles,102, 103 limitednumbers of pure binary 2D plasmonic assemblies were reported.Schiffrin demonstrated a binary plasmonic monolayerassembled from different size of gold nanoparticles (Figure 2.8a).
104Interestingly, by simply controlling the size ratio and the constituent’spercentage, the particles can be assembled into ordered hexagonal binary array,phase-separated domains, and randomly packed structure. Such results showed thepossibility of assembling highly ordered binary plasmonic system andhighlighted the importance of size and building blocks ratio during assembly.However, the assembled structures are limited to short range, fabrication ofordered binary system with large scale is still needed. Liz-Marzan’s groupreported binary monolayers assembled from gold nanowires and nanosphere ornanorods on a liquid diethyleneglycol(DEG) surface (Figure 2.8b).105Different from the assemblies from single component nanoparticle, nanospheresand nanorods were embedded within nanowires. The incorporation of nanoparticleswithin nanowires was found to be an entropy-driven process.
The density ofnanoparticle can be controlled by adjusting the nanoparticle concentration, andthe interparticle distance can be tuned by nanowires to further control theoverall optical properties of the binary films.Despite the binary system from constituent with different morphology and size,binary assemblies from different particle composition was also reported.106 Thisbinary plasmonic monolayer was fabricated by immobilizing Au and Agnanoparticles the same substrate. Specifically, the substrate was dipped sequentiallyin Au and Ag nanoparticle suspensions, followed with drying and annealing.
Anotherbinary assemblies of Au and Ag nanoparticles were obtained by a simply dropcasting method.44 Bytuning the constituent ratio, a series of binary 2D assemblies were fabricated. However, in both cases, Au and Agnanoparticles located randomly in the assemblies,therefore, these samples showed poor ordering and loosely arrangement with somecracks and defects. Improved packing order of Au and Ag binary 2D assemblieswas reported by Karg et al..107 Theyfabricated a binary plasmonic honeycomb structure from Au and Ag nanoparticle(Figure 2.8c).
They first assembled Ag@Au or Au@Au-PNIPAM particles at theair/water interface. Then successively deposited hexagonally packed Au and Agmonolayers onto the same substrate, forming highly ordered honeycomb lattice. Since Ag@Au-PINIPAM andAu@Au-PINIPAM monolayers showed the same hexagonal arrangements andinterparticle distance, the Au and Ag nanoparticles located complementary to each other after sequentialdeposition.
EDX maps confirmed the results and showed homogenous honeycombarrays and alternative positions of Auand Ag nanoparticles (Figure 2.8c).