Surface-enhanced Raman scattering study of organic thin films of 2-octadecyl-7,7,8,8-tetracyanoquinodimethane and its charge transfer films doped with 3,3′,5,5′-tetramethylbenzidine

Abstract

Raman, surface-enhanced Raman scattering (SERS) and ultraviolet–visible–near infrared (UV–vis–NIR) spectra have been measured for Langmuir–Blodgett (LB) films of 2-octadecyl-7,7,8,8-tetracyanoquinodimethane (octadecyl-TCNQ) and mixed-stack charge transfer (CT) films of octadecyl-TCNQ doped with 3,3′,5,5′-tetramethylbenzidine (TMB) deposited on CaF 2 plates and Au-evaporated glass slides. The 488.0 nm-excited Raman spectra of one-layer LB and CT films of octadecyl-TCNQ deposited on the Au-evaporated glass slides show a high signal-to-noise ratio owing to surface-enhanced Raman scattering (SERS). These Raman spectra are resonanced with absorption bands arising from intramolecular excitations of TCNQ molecules. Thus, we suggest that the 488.0 nm-excited Raman spectra of the LB and CT films deposited on the Au-evaporated glass slides are surface-enhanced resonance Raman (SERRS) spectra. The UV–vis–NIR spectrum of the CT film on the Au-evaporated glass slide is largely different from that of the film on the CaF 2 plate. However, their Raman spectra are very close to each other, although the SERRS spectrum of the CT film on the Au-evaporated glass slide is more enhanced than the resonance Raman spectrum of the film on the CaF 2 plate. It seems that the structure of octadecyl-TCNQ and TMB changes little with the substrates, but that the mechanism of CT is perturbed by the interaction between the first layer and the substrate.