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Abstract
We have succeeded in preparing gold rods of high aspect ratio ($>$6) and diameter ca. $\approx$32 nm, using anodically grown alumina films as host templates. To evaluate the sample quality, the plasmon resonance spectra of alumina/gold nanoparticle composite films were measured. In general, we found that the plasmon resonance maxima and intensity are dependent on the size, shape, and orientation of the gold nanoparticle.
Furthermore, we have succeeded in orienting such nanoparticles in polyethylene matrices, and confirmed the net orientation via polarization studies in the UV/Visible/Near-IR region of the spectrum. In the polarization studies, we found that the extent of red-shift and damping of the plasmon resonance band are dependent on the size, shape, and orientation of the gold nanoparticle.
The effects of gold particle size, shape, and orientation on the plasmon resonance spectra of polyethylene/oriented gold particle composite films have been compared to the predictions of Rayleigh scattering theory and effective medium theories (Maxwell-Garnett (MG) and Bruggeman (BRG)). From the comparison of the experimental plasmon resonance spectra with the calculated spectra using the above theories, we found that the best model for predicting the experimental plasmon resonance band shape and intensity is the BRG treatment. That finding has implications for what the sample is like.
After we confirmed the net orientation of the nanoparticles in polyethylene matrices, we examined the infrared-transmittance spectra of 3-mercaptopropionic acid (MPA) and 11-mercaptoundecanoic acid (MUA) chemically bonded on such composite films via polarization studies. In general, we found that the IR-transmittance spectra of MPA and MUA show a polarization angle dependence.
We discuss the effects of the gold particle shape, and orientation in the incident field on electromagnetic (EM) enhancements as seen in our oriented gold rod composite studies, in the context of the effective medium type theories for EM enhancements. From these studies, we found that the polarization infrared spectra of these IR-probe species show a polarization angle dependence that is consistent with the effective medium theories for EM spectral enhancements. Our conclusion is that a small enhancement in the intensity of the IR-probe is attributable to an EM enhancement. Large enhancements (ca. 500-fold) seen on previous studies are likely due to chemical effects (such as a charge transfer between the molecule and the metal particle).
