Surface Morphology of a Gold Core Controls the Formation of Hollow or Bridged Nanogaps in Plasmonic Nanomatryoshkas and Their SERS Responses

Surface-enhanced Raman scattering (SERS) probes with a nanometer-sized interior gap between the Au core and shell, also called nanomatryoshkas (NMs), have attracted great interest in SERS-based bioimaging and biosensing. Recently, seed-mediated growth has been shown to be effective for NM synthesis. We found that the structure of nanogaps inside Au NMs depends strongly on the core surface morphology. Specifically, when the initially citrate-stabilized 15 and 35 nm smooth Au cores were further functionalized with 1,4-benzenedithiol (BDT) in the presence of cetyltrimethylammonium chloride (CTAC), the Au shell growth led to the formation of a subnanometer hollow interior gap containing BDT molecules. In contrast, the use of 23 and 35 nm faceted polygonal CTAC-stabilized Au cores for Au shell growth resulted in NMs with small bridged gaps. The formation of incomplete outer shells with one or two nanometer-sized hollow gaps was also observed for 23 nm polygonal cores but not for 35 nm ones. The experimental SERS response from BDT molecules in bridged-gap NMs was an order of magnitude higher than that for hollow-gap NMs. This finding is in agreement with the finite-difference time-domain (FDTD) simulations predicting stronger electromagnetic fields inside nanobridged gaps, as compared to hollow-gap NMs. The major SERS peaks from BDT inside NMs of both types (two sizes, four samples) was an order of magnitude higher than the near-field SERS peaks recorded for the corresponding 15CIT, 23CTAC, 35CIT, and 35CTAC cores alone after surface functionalization with BDT molecules. This observation is explained by a simple dipole approximation (DA) theory, which was developed to estimate the structure- and wavelength-dependent electromagnetic SERS enhancement in the hollow-gap NMs with good accuracy, as confirmed by comparison with exact multilayered Mie (ML Mie) calculations. With a double increase in the core size, the NM SERS response also increased, and the ratio between the major peak intensities of the larger and smaller NMs was about 2. Finally, the calculated SERS spectra of the hollow-gap NMs agree with the data reported for 532, 633, and 785 nm laser excitations. The physical insights acquired from this study open the way for rational design and efficient optimization of new SERS platforms based on electromagnetic field enhancement in subnanometer gaps within plasmonic nanostructures.