Raman Microscopy and Mapping as a Probe for Photodegradation in Surface Relief Gratings Recorded on Layer-by-Layer Films of Congo Red/Polyelectrolyte

Abstract

Raman microscopy, mapping, and surface-enhanced Raman scattering techniques have been applied to investigate the degradation of Congo Red (CR) in a surface relief grating (SRG) recorded on an electrostatically assembled layer-by-layer film of CR alternated with poly(dimethyl diallylammonium chloride) (PDAC). Photoinduced degradation was suggested by a decrease in the Raman bands assigned to CR with a concomitant increase in the background signal. More degradation was observed in the valleys of the SRG, which corresponded to the most illuminated areas. In a line mapping across the SRG, the Raman intensity of CR bands follows the modulation of the grating, which indicates that photodegradation of CR contributes at least partially to the SRG formation. Surface-enhanced resonance Raman scattering (SERRS) indicates that the phenyl ring groups are located at the film surface. Mapping of the Raman signal over many periods of SRG shows that, within the limits of detection, the chromophores appear to be oriented in much the same way throughout the film. They have no preferred orientation in the peaks and valleys of the SRG after the recording process has been completed.