Nano-spectroscopic and nanoscopic imaging of single GaN nanowires in the sub-diffraction limit

摘要

The optical spatial resolution limit prohibits a precise spectroscopic characterization of objects at the nanoscale. However, plasmonic assisted microscopy at the nanoscale can overcome the diffraction limit owing to the confinement of the localized electric field. We report the spectroscopic imaging of a single GaN nanowire (NW) of diameter ～200 nm with the help of Au nanoparticle assisted plasmonic tip-enhanced Raman spectroscopy (TERS), well below the diffraction limit of ～750 nm. The challenge in the achievement was that the insignificant value of change of polarizability in the partially ionic Ⅲ-Ⅴ nitrides made the Raman scattering intensity too low for spectroscopic studies. Consequently, the enhancement factors for different observed Raman modes for the wurtzite GaN NW were also found to be significantly low. Spectroscopic imaging of such low scattering efficiency group Ⅲ-nitride single GaN NW is achieved in the sub-diffraction limit by careful selection of the TERS substrate. Similarly, the nanoscopic optical imaging of a single GaN NW of diameter ～10nm using the confined electromagnetic wave is also achieved with the help of the plasmonic assisted near-field scanning optical microscopic technique in the sub-diffraction limit.