Enhanced Raman spectra of black dye N719 on GaN nanowires

Gallium Nitride (GaN) substrates have recently been proposed as high performance substrates for surface enhanced Raman (SEAS) measurements. The GaN substrate acted as a template for nano-structuration of the surface, and a noble metal (e.g. silver or gold) supported the electrical fields and surface plasmon resonances deemed necessary for SEAS. However, we demonstrate here that charge transfer conditions can be approached with a selected molecule on pure GaN substrates, and that the surface plasmon resonances of the noble metals are not a prerequisite anymore to obtain SEAS conditions. We use a black dye (N719) adsorbed on GaN bundles of nano-wires for which the charge transfer resonances can contribute enhancement factors up to 10(4). The charge transfer (CT) transition from the GaN substrate valence band (-6.75 eV) to the molecular LUMO (-4.26 eV) is in resonance with the energy of the laser excitation at 488 nm (2.54 eV), and with the molecular resonance within the N719 molecule (omega(Mol) = 2.47 eV).