1. The Institute of Optics and Photoelectronic Technique, School of Physics and Electronics, Henan University, Kaifeng 475004, China 2. Zhengzhou Municipal Public Security Bureau, Zhengzhou 450000, China
Abstract：Palladium colloid was obtained via laser ablation under 1 064 nm excitation from an Nd∶YAG laser in redistilled deionized water. The Pd colloid consisted of “chemically pure” Pd nanoparticles, which were free from extraneous ions or other chemicals since no chemical reaction was involved in the preparation. There was no characteristic peak in UV/Vis spectrum of Pd colloid in the region of 200-800 nm. Hence, in contrast to the Au and Ag nanoparticles, the average size and the size distribution of the Pd nanoparticles could not be estimated from their UV/Vis adsorption spectral features. After the laser ablation, one drop (50 μL) of Pd colloid was deposited on the aluminum plate and dried naturally to form the Pd island films. This method resulted in the formation of a rough surface with a large number of separated Pd islands 20 μm in diameter. According to the SEM measurement, Pd nanoparticles with the average diameter of ～200 nm formed Pd island films. Surface-enhanced Raman scattering (SERS) activity of Pd colloid and Pd island films was evaluated by using 4-mercaptopyridine (4MPY) as a probe molecule. The SERS study revealed that Pd island film was a highly efficient SERS-active substrate while there was no SERS signal observed from Pd colloid. The surface enhancement factor of Pd island films for 4MPY was estimated, which could reach values as high as 8.7×104 under 632.8 nm excitation. This value was comparable with the largest value of 104 cited in the literature. The SERS spectra of 4MPY molecules adsorbed on Pd surface showed that 4MPY molecules probably tilted from the Pd nanoparticle surface-via sulphur. By contrast, SERS spectrum of 4MPY adsorbed on Ag island films was recorded and analysed. From SERS data it was inferred that 4MPY molecules assumed the standing up orientation on the silver nanoparticle surface. It could be concluded that the 4MPY molecules were more perpendicular to the silver nanoparticle surface than to the Pd nanoparticle surface.
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