  Tip-enhanced Raman spectroscopy (TERS) with sub-nanometer spatial resolution
has been recently demonstrated experimentally. However, the physical mechanism
underlying is still under discussion. Here, we theoretically investigate the
electric field gradient of a coupled tip-substrate system. Our calculations
suggest that the ultra-high spatial resolution of TERS can be partially
attributed to the electric field gradient effect owning to its tighter spatial
confinement and sensitivity to the infrared (IR)-active of molecules.
