Surface-Enhanced Raman Spectroscopy on Two-Dimensional Networks of Gold Nanoparticle-Nanocavity Dual Structures Supported on Dielectric Nanosieves

We report a facile method for the fabrication of 2-dimensional networks of Au nanoparticle-nanocavity dual structures supported on dielectric nanosieves. The optical extinctions of the as-fabricated network films were found to be dominated by optical transmission, a signature characteristic to nanocavities. The surfaces of the as-fabricated Au films were found to be hydrophobic toward water with a measured contact angle of 125.4 degrees but quite wettable with ethanol with a contact angle of 9.5 at room temperature and are readily convertible to totally hydrophobic or hydrophilic when chemically modified with self-assembled monolayer of molecules with desired functional groups. High quality surface-enhanced Raman spectra (SERS) from a monolayer of self-assembled 4-mercaptobenzoic acid (4-MBA), an electrostatic double layer of 4-mercaptobenzoic acid and rhodamine-6G (4-MBA-R6G), and nonspecifically adsorbed R6G were obtained, respectively, demonstrating that the dual structured network films are active, stable, and uniform as substrates in SERS. Employing the SERS spectra from a monolayer of self-assembled 4-MBA, the enhancement factors of 10(6) and 10(5) were achieved with excitations at 632.8 and 785 nm, respectively. Discrete dipole approximation (DDA) calculations were conducted to examine the electric field intensity distributions on linearly and circularly aggregated nanoparticles, two typical local aggregation patterns of nanoparticles in the as-fabricated network films. The DDA calculations reveal an important relationship between three factors: the generation of the hottest E-field spot on a linear chain aggregate, the position of the extinction peak of the aggregate, and the excitation wavelength used. It was found that the hottest spot on a linear aggregate is generated only when the excitation wavelength is in resonance with the extinction peak of the aggregate. An effective aggregation number scale is proposed to measure the effectiveness of the aggregation of nanoparticles in 1- and 2-dimensions at a given excitation. A hot-spot delocalization picture is proposed for the nanocavities formed by circular aggregation of nanoparticles to account for the observed isotropic SERS on the as-fabricated films.