Raman excitation profiles of hybrid systems constituted by single-layer graphene and free base phthalocyanine: Manifestations of two mechanisms of graphene-enhanced Raman scattering

The ability of single-layer graphene (SLG) to enhance Raman scattering of planar aromatic molecules denoted graphene-enhanced Raman scattering (GERS) is currently the subject of focused interest. We report on manifestations of two mechanisms of GERS in Raman spectra of glass/SLG/free base phthalocyanine (H2Pc) monolayer (ML) hybrid systems: (i) photoinduced charge transfer from SLG Fermi level to LUMO of H2Pc excited at onset of the near IR region, and (ii) modification of resonance Raman scattering of H2Pc in the visible region by SLG-H2Pc interaction resulting into delocalization of the electronic transition over the benzene rings of H2Pc. Glass/SLG/H2Pc hybrid systems with either a bilayer or a monolayer of H2Pc molecules and a graphite/H2Pc (ML) reference system were prepared by a spectrally controlled adsorption-desorption of H2Pc from solution, followed by Raman mapping of samples at excitation wavelengths in the 532-830 nm range, construction of excitation profiles for H2Pc Raman bands of the glass/SLG/H2Pc samples and determination of GERS enhancement factors for the glass/SLG/H2Pc (ML) sample versus the graphite/H2Pc (ML) reference sample (3-24 at 633 nm and 3-19 at 647 nm excitations). Selectivity of the excitation profiles and of the GERS enhancement factors with respect to localization of the vibrational modes within the H2Pc molecule demonstrates involvement of a different resonant electronic transition in each of the two mechanisms of GERS. Copyright (c) 2017 John Wiley & Sons, Ltd.