STM tip-enhanced Raman spectroscopy and the investigation of doped graphene

In this manuscript, we report the integrating a home-built scanning tunneling microscopy runner into a parabolic mirror assisted confocal optical microscope for performing high tip-enhanced Raman spectroscopy on graphene. To achieve an optimized optical excitation of the tip, a radially polarized laser beam is focused at the tip apex via a parabolic mirror. Graphene flakes grown on Cu foil are investigated. A decrease in the 2D peak intensity and increase of the G peak position indicate n-doping of the graphene flakes. The ultraviolet photoemission spectroscopy experiment shows that the underlying Cu foil induces a slight n-doping by raising the Fermi level by about 0.29 eV as compared to the free-standing graphene. An increased electron concentration of about 0.6 × 1013 cm−2 can be derived from the tip-enhanced Raman spectra. A possible origin for the n-doping observed in the tip-enhanced Raman experiments, for example through the transfer of hot electrons from the excited Au tip to the graphene flake, is discussed.