TERS tip manufacture is currently based either on coating standard AFM tips (fully or partially) with a metallic layer or etching all-metal tips by electrochemistry of bulky metallic wires22. The latter tips are used for STM regulation or Normal/ Shear-force microscopy, once individually mounted on the fork of a quartz resonator.
The most common method today is the metallization (gold or silver) of conventional AFM cantilevers with a thin layer of a few tens of nanometers. The coating is usually done by evaporation8-9, sputtering10, or electroplating11-12. Etching this layer by focused ion beam (FIB) right after the coating could also be used to reduce the antenna effects along the coating itself and increase the enhancement of the electromagnetic field13. The tunability of the resonance wavelength plasmon can be obtained by varying the dimensions of the final apex using FIB nano-machining14, which can be also used for etched bulk metallic wires15. For greater enhancement of AFM-TERS tips, one method is to graft a nanoparticle at the vicinity of the AFM tip. For example, silver nanoparticles might be grown by photoreduction at the end of a cantilever by light irradiation in a chemical solution composed of AgNO3 and reducing agents16. The enhancement efficiency can be tuned by varying the deposition and annealing conditions of the silver nanoparticles17.
The nanoparticle can also have a conical shape which will highly improve the produced enhancement by narrowing the field lines and accumulating charge density at the apex. Metallic nanocones can be obtained with a top-down approach through electronic lithography to make nanoholes in a PMMA layer, metal evaporation to fill these holes (and then forming a conically nanoparticle) and lift-off to release the PMMA18-19. They can also be obtained by a dry etching process of a metal layer, where nanocones will appear below a protective mask20-21.