The tip shape dependence of the STM-induced luminescence

Photon signal enhancement due to surface plasmon coupling becomes an important issue in high-resolution microscopy/spectroscopy, such as scanning tunneling microscopy (STM) induced luminescence, surface/tip enhanced Raman spectroscopy, and so on. Such enhancement is significantly varied by modifying the plasmonic nanocavities. The gap-mode plasmonic nanocavity between an STM tip and surface realizes sub-molecular photon spectroscopy. The plasmonic resonant modes are known to be modified by tip indentation. However, the tip shape dependence of the plasmonic nanocavity remains unclear. In this work, we experimentally present the role of tip shape in surface plasmonic light emission. We have used Ag tips and a Ag(100) substrate, and STM-induced photon signals have been measured from the two opposite sides of the STM tip simultaneously. We found that changing small parts of the tip can differ the photon spectrum significantly due to tip shape asymmetry. This finding can help us to tune the plasmonic photon emission spectra more efficiently and give a new insight by using the symmetric or asymmetric STM tip induced luminescence.