Deep-ultraviolet surface plasmon resonance of Al and Al_core/Al₂O_3shell nanosphere dimers for surface-enhanced spectroscopy

The localized surface plasmon resonance properties of Al and Al_core/Al₂O_3shell nanosphere dimers with Al and Al core nanosphere radii of 20 nm and Al₂O₃ shell of 2 nm in the deep-ultraviolet region have been studied using the finite difference time domain method. The extinction spectra and the electric field distribution profiles of the two dimers for various gap distances between two individual nanospheres are compared with those of the corresponding monomers to reveal the extent of plasmon coupling. It is found that with the interparticle distance decreasing, a strong plasmon coupling between two Al or Al_core/Al₂O_3shell nanospheres is observed accompanied by a significant red shift in the extinction spectra at the parallel polarization direction of the incident light related to the dimer axis, while for the case of the perpendicular polarization direction, a weak plasmon coupling arises characterized by a slight blue shift in the extinction spectra. The electric field distribution profiles show that benefiting from the dielectric Al₂O₃ shell, the gap distance of Al_core/Al₂O_3shell nanosphere dimers can be tailored to < 1 nm scale and results in a very high electric field enhancement. The estimated surface-enhanced Raman scattering enhancement factors suggests that the Al_core/Al₂O_3shell nanosphere dimers with the gap of < 1 nm gave rise to an enhancement as high as 8.1 × 10⁷ for interparticle gap = 0.5 nm. Our studies reveal that the Al_core/Al₂O_3shell nanosphere dimers may be promising substrates for surface-enhanced spectroscopy in the deep-ultraviolet region.