(611g) Investigation of Interfacial Processes Leading to Anisotropic Silver Nanostructures On 2D Monolayers of Silica Spheres

One-dimensional nanostructures comprising noble metals are promising materials for a range of electronic, photonic, biomedical and catalysis applications. In particular, their anisotropic morphology opens up possibilities for charge or photonic transport as well as providing a means for the self- or directed-orientation of the building blocks during  fabrication of a functional structure e.g. using electrical fields or fluid forces. In order to enable widespread exploitation of one-dimensional nanostructures, there needs to be more effort to develop and understand simple and scalable routes to achieve them. In this contribution we describe how one-dimensional silver nanoparticle assemblies can be formed at a solid-air interface in the absence of explicitly added reducing and templating agents. We show that in this extremely simple but highly sensitive process, dynamic and strongly anisotropic effects occur during aging of a 2D monolayer of silica nanospheres that had been treated with the ammoniacal silver complex (Ag(NH₃)₂⁺). Electron microscopy reveals that 5 – 10 nm silver nanoparticles are arranged in necklace-like assemblies on the silica spheres and up to a certain distance from them on the substrate. We investigate the formation of the nanoparticle necklaces by varying simple parameters of the liquid phase and aging treatments. We also describe attempts to scale-up the process from a few mm² to cm², particularly focussing on the formation of the nanostructures on 2D silica nanoparticle monolayers deposited by dip-coating on silicon wafers. We show that together with the simple chemical treatments, dynamic effects at the silica particle monolayer-air interface can lead to a number of interesting morphologies being produced, included the nanoparticle necklaces seen in the smaller scale work. These findings, as well as studies of the interfacial properties of silica help us to narrow down our  mechanistic understanding of the nanostructure formation by the synergistic chemical behavior of the used materials, namely Ag(NH₃)₂⁺ and amorphous silica particles. In particular, the presence of dissolved silicates, a result of the high pH of the ammoniacal silver complex appear to be critical.

Besides potential uses in transparent electrical conductive coatings, substrates for surface-enhanced Raman spectroscopy (SERS) or for electromagnetic shielding the 1D nanostructures produced by our process  could be particularly interesting for possible novel optical properties of the structures. In the latter case we believe that mesh-coated silica particles formed by their further metallization may display resonances in their magnetization at optical frequencies and thus meets requirements for the formation of, for example, negative index metamaterials.