Raman spectral characteristics of 4-aminobenzenethiol adsorbed on ZnO nanorod arrays

Interest in the surface-enhanced Raman scattering (SERS) of 4-aminobenzenethiol (4-ABT) has surged recently. The SERS spectral features are highly dependent on the measurement conditions; a notable example is the appearance of b2-type bands that are not evident in the normal Raman (NR) spectrum. In an effort to discover new information and make any necessary corrections, we measured the Raman spectrum of 4-ABT adsorbed on a semiconducting material that would only enable the chemical enhancement mechanism; accordingly, the Raman spectrum of 4-ABT adsorbed on ZnO nanorods grown on an indium tin oxide substrate was measured for the first time. In the NR spectrum of the zinc salt of 4-ABT, which was taken as a reference, only the a1-type bands of 4-ABT were identified. However, in the surface Raman spectrum of 4-ABT on ZnO, the b2-type bands were also clearly evident, suggesting that the b2-type bands arose owing to its adsorption onto ZnO. The b2-type bands were also observed for 4-ABT analogs adsorbed on ZnO; this confirms that the b2-type bands were not a result of a surface-catalyzed photoreaction. Based on electric-potential and excitation-wavelength dependence studies, the a1- and b2-type bands were attributed to a charge-transfer (CT) transition from the surface defect levels of ZnO to the unoccupied La and Lb states (in Platt's notation) of 4-ABT, respectively; the bands gained intensity via the Herzberg–Teller coupling terms. The enhancement factor associated with the CT transition was estimated to be around 22, suggesting that it is, at best, a moderately effective process.