Hybrid nanoparticle-nanoline plasmonic cavities as SERS substrates with gap-controlled enhancements and resonances.

We present hybrid nanoline-nanoparticle plasmonic substrates which allow easily achievable sub-5 nm gaps and a possibility of large-area fabrication. These substrates--based on plasmonic nanocavities formed by arrays of plasmonic nanoparticle (NP) dimers lying inside periodic metal nanolines (NLs)--can be used as tunable surface enhanced Raman scattering (SERS) substrates due to the tunability of cavity modes in the gap regions. Theoretical studies were conducted, using finite difference time domain (FDTD) modeling, to understand the plasmon resonance tunability as a function of gaps in these hybrid plasmonic substrates. The gaps forming the nanocavities include those between nanolines and nanoparticles (NL-NP) and between two nanoparticles (NP-NP). Our analysis reveals that these gaps play a combined role in tuning the resonance wavelength and the magnitude of electromagnetic field enhancement. Moreover, distinct structure-dependent plasmon resonance peaks are present in addition to material-dependent resonance peaks characteristic to the metal involved. Replacing the spherical particle arrays inside the nanolines with nanorod arrays revealed the possibility of tuning the plasmon resonance in the near-infrared regime. This indicates that there is a possibility of tuning the plasmon resonance wavelength to any region of the visible or near-infrared spectrum by changing the size or shape of the particles assembled inside these plasmonic nanolines.