Unprecedented Surface Plasmon Modes in Monoclinic MoO2 Nanostructures

Developing stable plasmonic materials featuring earth-abundant compositions with continuous band structures, similar to those of typical metals, has received special research interest. Owing to their metal-like behavior, monoclinic MoO2 nanostructures have been found to support stable and intense surface plasmon (SP) resonances. However, no progress has been made on their energy and spatial distributions over individual nanostructures, nor the origin of their possibly existing specific SP modes. Here, various MoO2 nanostructures are designed via polydopamine chemistry and managed to visualize multiple longitudinal and transversal SP modes supported by the monoclinic MoO2 , along with intrinsic interband transitions, using scanning transmission electron microscopy coupled with ultrahigh-resolution electron energy loss spectroscopy. The identified geometry-dependent SP energies are tuned by either controlling the shape and thickness of MoO2 nanostructures through their well-designed chemical synthesis, or by altering their length using a developed electron-beam patterning technique. Theoretical calculations reveal that the strong plasmonic behavior of the monoclinic MoO2 is associated with the abundant delocalized electrons in the Mo d orbitals. This work not only provides a significant improvement in imaging and tailoring SPs of nonconventional metallic nanostructures, but also highlights the potential of MoO2 nanostructures for micro-nano optical and optoelectronic applications.