Effect of Scattering Efficiency in the Tip-Enhanced Raman Spectroscopic Imaging of Nanostructures in the Sub-diffraction Limit

The experimental limitations in signal enhancement and spatial resolution in spectroscopic imaging have been always a challenging task in the application of near-field spectroscopy for nanostructured materials in the sub-diffraction limit. In addition, the scattering efficiency also plays an important role in improving signal enhancement and contrast of the spectroscopic imaging of nanostructures by scattering of light. We report the effect of scattering efficiency in the Raman intensity enhancement, and contrast generation in near-field tip-enhanced Raman spectroscopic (TERS) imaging of one-dimensional inorganic crystalline nanostructures of Si and AlN having large variation in polarizability change. The Raman enhancement of pure covalently bonded Si nanowire (NW) is found to be 2 orders of higher in magnitude for the TERS imaging, as compared to that of AlN nanotip (NT) having a higher degree of ionic bonding, suggesting the importance of scattering efficiency of the materials in TERS imaging. The strong contrast generation due to higher signal enhancement in TERS imaging of Si NW also helped in achieving the better resolved spectroscopic images than that of the AlN NT. The study provides an insight into the role of scattering efficiency in the resolution of near-field spectroscopic images.