Optical Nanoantennas for Tip-Enhanced Raman Spectroscopy
Thiago L. Vasconcelos, Bráulio S. Archanjo, Bruno Oliveira, William F. Silva, Rafael S. Alencar, Cassiano Rabelo, Carlos A. Achete, Ado Jório, Luiz Gustavo Cançado
Abstract
The role of plasmon resonance on the optical efficiency of nanoantennas for tip-enhanced Raman spectroscopy (TERS) is reviewed. Technical details on surface plasmon polaritons (SPP), localized surface plasmon resonance (LSPR), and the plasmon gap mode are provided. Nanotechnology engineering is necessary to adequate the nanoantenna's size, shape and composition to match resonance conditions with the exciting radiation source. Computational simulation guides the development of new types of plasmonic nanoantennas with different materials and morphologies, specially designed to reach target applications. An overview on a recently developed nanoantenna composed by a truncated micropyramidal body with a nanopyramid end is presented. The characteristic length L of the nanopyramid tip is dimensioned to fine-tune LSPR modes, giving rise to the so-called plasmon-tunable tip pyramid (PTTP). The plasmonic properties of this type of probe were investigated by electron energy loss spectroscopy and computational simulations, reveling that PTTPs act as monopole nanoantennas. TERS results obtained with the PTTPs demonstrate the achievement of unprecedent levels of field enhancement mediated by LSPR with excellent reproducibility rate.