Correlative Raman Imaging and Scanning Electron Microscopy: The Role of Single Ga Islands in Surface-Enhanced Raman Spectroscopy of Graphene

Surface-enhanced Raman spectroscopy (SERS) is a perspective nondestructive analytic technique enabling the detection of individual nanoobjects, even single molecules. In this paper, we have studied the morphology of Ga islands deposited on chemical vapor deposition graphene by ultrahigh vacuum evaporation and local optical response of this system by the correlative Raman imaging and scanning electron microscopy (RISE). Contrary to the previous papers, where only an integral Raman response from the whole ununiformed Ga nanoparticles (NPs) ensembles on graphene was investigated, the RISE technique has enabled us to detect graphene Raman peaks enhanced by single Ga islands and particularly to correlate the Raman signal with the shape and size of these single particles. In this way and by a support of numerical simulations, we have proved a plasmonic nature of the Raman signal enhancement related to localized surface plasmon resonances. It has been found that this enhancement is island-size-dependent and shows a maximum for medium-sized Ga islands. A reasonable agreement between the simulations of the plasmon enhancement of electric fields in the vicinity of Ga islands and the experimental intensities of corresponding Raman peaks proved the plasmonic origin of the observed effect known as SERS.