"2D Chalcogenide nanostructures--From topological insulators to plasmonic metamaterials" - Two-dimensional (2D) layered metal chalcogenides exhibit a broad range of electronic properties. They can be poor metals, narrow-gap semiconductors, and the recently discovered topological insulators (TIs). TIs, in particular, have garnered intense interests due to their exotic surface states that are robust against time reversal perturbations and exhibit unique spin-momentum locking property. These surface properties can be greatly enhanced by making chalcogenides into nanostructures. As a rapidly growing research field, 2D chalcogenide nanostructures hold promises for fundamental TI studies as well as energy-related applications such as dissipationless transport, thermoelectric devices, and electrocatalysis. In this talk, I will cover synthesis and transport properties of M2E3 (M=Bi,Sb, E=Se, Te) chalcogenide nanoribbons and nanoplates. In particular, I will discuss weak antilocalization observed in these nanostructures and how magnetic impurities influence the weak antilocalization. I will also demonstrate that ultrathin 2D chalcogenide nanoplates support dielectric photonic and plasmonic modes that are tunable across a broad spectral range, affording a new family of tunable metamaterials.