Sum frequency generation from alkanethiol capped metallic nanoparticles and vibrational mode specific enhancement in nanoparticle aggregates

Abstract

Vibrational Sum Frequency Generation (VSFG) on gold and silver nanoparticles capped with alkanethiols is studied. Aggregation of nanoparticles is characterized using TEM and SEM methods. VSFG process is enhanced due to the coupling of surface plasmon induced by the visible radiation in gold nanoparticle with vibrational transition of chemisorbed alkanethiol excited by the infrared beam. VSFG spectra show methyl and methylene stretch transitions. The ratio of their intensities varies with changing size of the particles and length of alkane chain. Dramatic change in intensity ratio and overall enhancement of VSFG intensity is observed when aggregation of gold nanoparticles occurs. For the first time we report the mode-specific SFG enhancement, namely, the methyl antisymmetric stretch gains the highest intensity. One possible explanation is that enhancement is caused by the change in SFG selection rules due to the effect of locally inhomogeneous electric field of plasmon. VSFG response from aggregates is significantly depolarized in comparison with response from non-interacting particles. This can be due to the depolarization of plasmon induced in aggregates of metallic nanoparticles. Divergence of VSFG beam from aggregates is stronger than that of the beam from non-interacting particles. This can be attributed to the incoherent nonlinear scattering in aggregates. Potential applications of SFG nanoprobes for imaging, IR radiation conversion, and opto-electronic integrated circuits are discussed.