Elimination of gallium concentration on focused-ion-beam-fabricated Au/Ag nanorod surface to recover its Raman scattering characteristic

• Au and Au/Ag nanorod arrays are fabricated by focused Ga-ion beam (fibAu _NR and fibAu/Ag_NR) with a tilted angle.
• Surface enhanced Raman scattering (SERS) property in NRs can be recovered by eliminating Ga at the outermost surface.
• The fibAu_NR and fibAu/Ag_NR made by a tilted or small-size Ga-ion beam can increase the effect of SERS.
• Low-Ga-content FIB-fabricated Au and Au/Ag NR arrays are particularly promising for plasmonic nano-devices.

Well-ordered Au and Au/Ag nanorod (NR) arrays were fabricated by focused ion beam (FIB) (fibAu _NR and fibAu/Ag_NR). The gallium (Ga) concentration and lattice damage on the surfaces of fibAu _NR and fibAu/Ag_NR were decreased by tilting Ga+ beam with respect to the substrate (7°) and reducing beam size (from 60 to 7 nm). The effect of residual Ga in fibAu _NR and fibAu/Ag_NR on surface-enhanced Raman scattering (SERS) was verified using low-concentration target molecular probes (<10−7 M) and Raman laser with wavelengths of 633 and 785 nm. The results indicate that FIB preparation with a tilted and small-size beam tends to increase the effect of SERS at the substrate surface. The use of varied molecular probes and laser wavelengths affected the localized electromagnetic field, stimulating electron transfer from the surface of NRs to Raman-active sites. Moreover, the level of residual Ga in Au or Ag matrix mainly contributes to the enhancement of Raman scattering. It is proposed that lower Ga content in fibAu/Ag_NR surface increases the effect of SERS, leads to discrete multipole plasmon modes of electron oscillation along the long axis, and reduces optical losses as well as lattice damage in the original structure.