Symmetry breaking of Au nanospheres confined in 1D nanocylinders: exploring helical assembly by 3D transmission electron microscopy†
A chiral plasmonic nanoparticle (NP) superstructure can serve as a promising scaffold for chirooptical sensors, circular polarizers, and optical metamaterials. The helical architecture, a representative NP assembly, can enhance optical chirality, guided by a symmetry breaking of NPs closely packed in a confined geometry within a cylindrical domain. Herein, we report the helical and zigzag assemblies of polystyrene (PS)-tethered Au NPs confined in cylindrical nanochannels produced using an anodic aluminum oxide (AAO) template. The number of NP layers and the NP packing structures were controlled by the molecular weight of PS (Mn-PS), affecting the surface plasmon resonance characteristics of the hybrid assemblies. The decrease in Mn-PS of NPs showed a strong tendency to form multi-layered, multi-stranded helical arrays rather than a two-layered zigzag array, which closely agrees with previously reported predictions by computational studies. These highly complex hybrid nanoassemblies were elaborated by three-dimensional (3D) transmission electron microscopy (TEM). Even when the resultant assemblies were obtained from the same AAO nanochannel, it was difficult to describe the internal NP arrangements using a conventional TEM since different superstructures were observed depending on the viewing angle of the assemblies. However, representative 3D tomograms reconstructed by a series of two-dimensional projections of the hybrid nanoassemblies indicated that all different packing structures observed by TEM result from the specific assemblies with the same NP configuration. This study presents a useful strategy for providing helical assembly and controlling the plasmonic optical activities of Au NPs and reducing scientific errors originating from a lack of accurate analysis and predication in identifying the relationship between nanostructures and their unique functions.