Both TERS and SERS rely on a localized surface plasmon for increasing the ought-to-be weak Raman signal. The only difference between them is that the sample in SERS has a rough surface that hinders application of a sharp AFM-like tip. TERS, on the other hand, uses a metal-coated tip having some roughness at nanoscale. The “hot spot” theory is very popular in explaining the large enhancement in the signal. That is, the signal from “hot spots” on the surface of the sample dominates the total signal from the sample. This is also reinforced by the fact that the distance between nanoparticles and sample is an important factor in obtaining high Raman signal.