Surface enhanced plasmon effects by gold nanospheres and nanorods in Langmuir-Blodgett films

Abstract

Hydrophobic gold nanospheres and nanorods were synthesized by a reduction method and a seed-mediated growth method, respectively, and by the surface-modification with 1-dodecanethiol. Langmuir-Blodgett (LB) films of gold nanoparticles were prepared at different surface pressures and two-dimensional ordering of nanoparticles was evaluated. A plasmon band of nanospheres slightly red-shifted from 525nm in water to 543nm in chloroform but it was 634nm at LB film. Plasmon bands at 750 and 515nm of nanorods in water drastically changed to a band at 550nm in chloroform but it appeared at 621 and 1000nm. Moreover, Langmuir monolayer of eiocosanoic acid was transferred on LB films of nanospheres on CaF2 window and transmission surface enhanced infrared absorption spectra (SEIRAS) were measured. A CO stretching vibration band at 1702cm−1 was strongly enhanced compared to other vibration bands due to the selection rule of SEIRAS. That is, the dipole moment of CO stretching mode should be on the same direction as the surface enhanced electric field, which is perpendicular to the gold surface. From the comparison of SEIRAS for poly(amido amine) dendrimer on LB films of nanospheres and nanorods, it is apparent that the enhancement effect of nanorod LB film is higher than that of nanosphere LB film, although the surface coverage and the number density of nanorods in LB film are lower than those of nanosphere LB film, suggesting the effect of hot spots.