Asymmetry of Inorganic Nanostructures

Abstract

Matured syntheses of inorganic nanocolloids today have allowed production of diverse asymmertic nanoparticles (NP) and even more complex assemblies, introducing a variety of nanomaterials with unique properties. In many cases, the symmetry of the nanomaterials plays the crucial role in determining the electronic structure, optical activity and physical dynamics of the system. However, precise symmetry assignment to nanoscale materials is a challenging task due to their multicomponent nature and multiscale dynamics. Here we explore asymmetry of inorganic nanomaterials to discover fundamental mechanisms and dynamics of previously puzzling properties and develop a new methodology for syntheses and characterization. An example of overlooked asymmetry in nanoscale system was demonstrated with gold nanorod (Au NRs). Direct observations of electrostatic properties of Au NRs via off-axis electron holography and Kelvin force microscopy showed that Au NRs have distinct asymmetry of surface charge density and therefore, behave as non-centrosymmetric structures due to uneven distribution of cetyltrimethylammonium (CTA) moieties capping two ends of NRs. The electrostatic asymmetry of Au NRs and potentially other metallic nanostructures emerging from this study showed a new pathway to explain previously puzzling discrepancies in nonlinear optical (NLO) properties in seemingly centro-symmetric nanostructures. By taking the multicomponent nature of nano-colloids, especially originated from surface ligands, into consideration, we also showed that chiroptically active nanomaterials can be produced by self-assembled processes that are sensitive to subtle anisotropies and ligand-imposed surface of NPs. L-cysteine (L-Cys)– or D-cysteine (D-Cys)–stabilized NPs yields various chiral mesoscale structures including helices and angled conformation of nanoleaves, exhibiting unique chiropitcal properties by combining structural character with their material properties. More interestingly, the handedness of these structures depended solely on the specific Cys enantiomer, thus showing homochiral ensembles. The organic shell can introduce a property-governing asymmetry into nanoscale system, allowing better understanding relationships between geometrical parameters and properties of complex nanomaterials. The experimental and theoretical findings here can be generalized to other nanoscale materials carrying surface stabilizers and opens new possibilities for solution processed metamaterial devices.