Surface-enhanced Raman scattering and gap-mode tip-enhanced Raman scattering investigations of phthalocyanine molecules on gold nanostructured substrates

Abstract

This study reports the use of surface-enhanced Raman scattering (SERS) and tip-enhanced Raman scattering (TERS), both independent and in combination, to investigate Raman enhancement of films under different confinement geometries. The experiments are performed on ultrathin cobalt phthalocyanine (CoPc) films deposited on specially designed SERS structures. The SERS structures are fabricated by electron-beam lithography and contain nanostructured gold films and gold dimer arrays with controlled size and internanocluster distance. Such structures allow investigation of the effects of nanocluster size and internanocluster distance, excitation wavelength, and polarization of light upon the electromagnetic SERS enhancement. Significant enhancement of the Raman scattering by CoPc is observed under 632.8 nm excitation because of the double resonance originating from the energy match between the laser excitation and the localized surface plasmon and electronic transitions in CoPc. The SERS signal of CoPc is further enhanced by decreasing the internanocluster distance. Maximum SERS enhancement occurs when the polarization of the incident light is perpendicular to the dimer axis. Under 514.5 nm excitation, nanostructured gold films give greater enhancement than any of the nanocluster arrays, with the highest enhancement realized using the so-called “gap-mode TERS” wherein the SERS structures are probed in the TERS condition. The TERS experiment is performed using a customized TERS setup and all-metal atomic force microscopy tips custom fabricated. In terms of obtaining the ultimate sensitivity in Raman spectroscopy, further enhancement is achieved by confining the electromagnetic field in a gap between two metallic nanostructures either by using SERS or by combining SERS and TERS.